NC0004979_01_Allen_CAP Update 2019_Text_20191231

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Synierra
CORRECTIVE ACTION PLAN
UPDATE
Site Name and Location:
Groundwater Incident No.:
NPDES Permit No.:
NCDEQ CCR Impoundment Ranking:
Date of Report:
Permittee and Current
Property Owner:
Consultant Information:
Latitude and Longitude of Facility:
Allen Steam Station
253 Plant Allen Road
J Belmont, NC 28012-8845
Not Assigned
NC0004979
Low -Risk
December 31, 2019
Duke Energy Carolinas, LLC
526 South Church Street
Charlotte, NC 28202-1803
[phone]
SynTerra Corporation
148 River Street, Suite 220
Greenville, SC 29601-2567
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Corrective Action Plan Update December 31, 2019
Allen Steam Station SynTerra
Note to the Reader from Duke Energy
Duke Energy Carolinas, LLC (Duke Energy) is pleased to submit this groundwater
Corrective Action Plan (CAP) for the Allen Steam Station (Allen) located in Gaston
County, North Carolina. Since 2004, Duke Energy has been engaged in extensive site
investigation activities to comprehensively characterize environmental conditions in
soil, groundwater, surface water, and sediments associated with the presence of coal
combustion residuals (CCR) in and around the Allen coal ash basins and coal piles.
Activities have been performed in compliance with the North Carolina Coal Ash
Management Act of 2014, as amended (CAMA), as well as the United States
Environmental Protection Agency's (USEPA) CCR Rule. In 2018, the North Carolina
Department of Environmental Quality (NCDEQ) ranked the ash basins at Allen as low -
risk pursuant to CAMA.
Thousands of multi -media samples have been collected at the Allen yielding over
191,000 individual analyte results. All of this work has been coordinated with the
NCDEQ, which has provided review, comments, and approvals of plans and reports
related to these activities. This CAP provides the results of these extensive assessment
activities, and presents a robust corrective action program to address groundwater
conditions where concentrations of constituents of interest (COI) are above applicable
regulatory criteria. Closure plan(s) to address the ash basin source areas are submitted
separately.
As detailed in this CAP, we have begun to implement, and will continue implementing,
source control measures at the site, including (i) complete decanting of the ash basins to
remove the hydraulic head, thereby reducing hydraulic gradients, groundwater
seepage velocities, and COI transport potential; and (ii) complete closure of the ash
basins. In addition, we intend to implement a robust groundwater remediation
program that includes actively addressing COI in groundwater above applicable
standards at or beyond the compliance boundary using groundwater extraction
combined with clean water infiltration and removal of the low pH area source
proximate to the coal pile area. These corrective action measures will most effectively
achieve remediation of the groundwater through the use of extraction wells to the
north, northeast, and east of the ash basins and coal piles, and strategically located clean
water infiltration wells. Significantly, groundwater modeling simulations indicate (i)
these measures will control COI at or beyond the compliance boundary; and (ii) at such
time the site -specific considerations detailed within this CAP have been satisfied,
including, but not limited to, securing all required state approvals, installing the

Corrective Action Plan Update December 31, 2019
Allen Steam Station SynTerra
necessary equipment, and commencing full-scale system operation, COI at or beyond
the compliance boundary will meet the remedial objectives in nine years.
This CAP contains over 2,500 pages of technical information that we believe represents
one of the most detailed and well supported corrective action plans ever submitted to
the NCDEQ and forms the basis of the robust groundwater remediation approach
described above. Thousands of labor hours by PhD -level scientists, engineers, and
geologists have been performed to obtain and evaluate the large amount of data
generated at Allen and inform this CAP. This combined effort has enabled a
comprehensive understanding of site conditions, creation of a highly detailed three-
dimensional groundwater flow and solute transport model used to simulate
remediation scenarios, and evaluation and selection of a site -specific corrective action
program for Allen. Duke Energy believes it is also important to provide a science -based
perspective on these extensive studies, which include the following key findings:
• The human health and ecological risk assessments performed for Allen using
USEPA guidance demonstrate that risks to potential human health and
ecological receptors associated with the coal ash basins are not measurably
greater than risks posed by naturally occurring background conditions.
• Ash basin- and coal pile -related constituents have not affected, nor are they
predicted to affect, off -site water supply wells. This has been confirmed by
analytical results from groundwater samples and water level measurements
collected from over 211 monitoring wells over 31 separate monitoring events,
and performing over 175 groundwater and geochemical modeling simulations.
• An additional 23 monitoring wells were installed in late 2019 to further assess
the low pH area and coal pile area.
In addition, even though no off -site wells were impacted, Duke Energy has already
provided owners of surrounding properties within 0.5-mile radius of the ash basin
compliance boundary with either connections to water supplied by the City of Belmont
or water filtration systems under a program approved by the NCDEQ. These alternate
water supplies provide additional peace of mind for our neighbors. Importantly,
ongoing multi -media sampling of the nearby surface water aquatic system, the Catawba
River (Lake Wylie), confirms that this surface water system is healthy with a robust fish
population.
Duke Energy looks forward to proactively implementing this CAP.

Corrective Action Plan Update December 31, 2019
Allen Steam Station SynTerra
EXECUTIVE SUMMARY
(CAP Content Section Executive Summary)
ES.1 Introduction
SynTerra prepared this groundwater Corrective Action Plan (CAP) Update on behalf of
Duke Energy Carolinas, LLC (Duke Energy). The plan pertains to the Allen Steam
Station (Allen, Site, or Station) coal combustion residuals (CCR) ash basins and coal
piles. The Site is located in Gaston County, North Carolina (Figure ES-1). At Allen, the
coal piles are adjacent to and downgradient of the ash basins. The coal piles, therefore,
are considered a component of this CAP Update.
This CAP Update addresses the requirements of Section 130A-309.211(b) of the North
Carolina General Statutes (G.S.), as amended by Coal Ash Management Act (CAMA) of
2014. The CAP Update is consistent with North Carolina Administrative Code (NCAC),
Title 15A, Subchapter 02L .0106 corrective action requirements, and with the CAP
guidance provided by the North Carolina Department of Environmental Quality
(NCDEQ) in a letter to Duke Energy, dated April 27, 2018, and adjusted on September
10, 2019 (Appendix A).
This CAP Update evaluates remedies for constituents of interest (COIs) in groundwater
associated with the Allen ash basins and coal piles, which are considered sources of
COIs. The ash basins include the active ash basin (AAB) and the retired ash basin
(RAB).
Specifically, this CAP Update focuses on constituents detected at concentrations greater
than applicable North Carolina groundwater standards [NCAC, Title 15A, Subchapter
02L, Groundwater Classification and Standards (02L); Interim Maximum Allowable
Concentrations (IMAC); or background values, whichever is greater] at or beyond the
compliance boundary. The COIs were detected in the following areas:
• North and northeast of the RAB and the coal piles
• East of the AAB and RAB
In accordance with G.S. requirements, a CAP for Allen was previously submitted to the
NCDEQ in two parts, as follows:
• Corrective Action Plan Part 1— Allen Steam Station Ash Basins (HDR, 2015b)
• Corrective Action Plan Part 2 (included CSA Supplement 1 as Appendix A) — Allen
Steam Station Ash Basins (HDR, 2016a)
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Allen Steam Station SynTerra
This CAP Update considers data collected through June 2019, with the exception of data
collected through October 2019 related to additional assessment of the coal piles and the
"low pH area", an area where low pH conditions are present within and downgradient
of the RAB.
Ash basin closure is detailed in a separate document prepared by AECOM. Closure
scenarios evaluated in this CAP include closure -in -place and closure -by -excavation.
Therefore, the groundwater remediation alternatives evaluated and recommended in
this CAP Update consider the closure -in -place scenario and closure -by -excavation
scenario. Groundwater modeling simulations consistently indicate the closure -in -place
and closure -by -excavation scenarios have a similar effect on COI concentrations in
groundwater.
Summary of CAP Approach
As stated above, this CAP Update meets the corrective action requirements under G.S.
and Subchapter 02L .0106. The preferred groundwater remediation approach assumes
source control under the ash basin closure -in -place or closure -by -excavation scenarios.
Both closure scenarios provide similar source control by reducing and/or eliminating
further releases of COIs to groundwater. The groundwater remediation approach
presented in this CAP Update can be implemented under either ash basin closure
scenario and can achieve 02L .0202 groundwater quality standards based on
groundwater modeling simulations. The focus of groundwater corrective action at
Allen is reducing COIs to concentrations less than applicable criteria at or beyond the
compliance boundary consistent with Subchapter 02L .0106(e)(4) and to address
Subchapter 02L .106(j). Applicable criteria in this case are defined as the 02L
groundwater standard, IMAC, or background, whichever is greatest, defined as the COI
criterion. If a COI does not have an 02L standard or IMAC, then the background value
defines the COI criteria.
Duke Energy has implemented, or plans to implement, the following multi -component
Corrective Action Plan at Allen:
Source Control Measures
• Completion of ash basin decanting is currently underway and will reduce the
hydraulic head in the dam area, thereby significantly reducing the hydraulic
driving force for potential COI migration in groundwater downgradient of
the basin. As of December 1, 2019, 53,300,000 gallons of water have been
removed from the AAB and the water elevation has decreased by 14.1 feet.
Completion of decanting is projected to occur on or before June 30, 2020.
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Allen Steam Station SynTerra
Groundwater modeling indicates that the average linear velocity of
groundwater in the vicinity of the AAB dam will decrease from 0.1-5.0 feet
per day (ft/day) pre -decanting to 0.01-0.5 ft/day post -decanting.
• Removal of the coal piles when the Station is decommissioned.
Groundwater Remediation Measures
• A robust groundwater remediation approach for Allen is planned. The
approach includes actively addressing COIs in groundwater with
concentrations greater than applicable standards at or beyond the compliance
boundary using groundwater extraction combined with groundwater
infiltration and treatment. Site data and groundwater models were used to
evaluate and optimize an effective remedial approach to reduce COI
concentrations north, northeast, and east of the source areas. The following is
a summary of components of the preferred remediation system that would be
installed in areas north, northeast, and east of the ash basins and coal piles:
o 87 vertical extraction wells
o 76 clean water vertical infiltration wells or 48 clean water vertical
infiltration wells combined with 22 clean water horizontal infiltration
wells
Effectiveness Monitoring Plan (EMP)
• Duke Energy has prepared an effectiveness groundwater monitoring plan as
discussed in Section 6.8 and provided in Appendix O of this CAP Update.
The EMP includes an optimized groundwater monitoring network for the ash
basins and coal piles based on Site -specific COI mobility and distribution.
The EMP is also designed to be adaptable and targets key areas where
changes to groundwater conditions are most likely to occur during corrective
action implementation or basin closure activities. The monitoring plan
includes provisions for a post -closure monitoring program in accordance
with G.S. Section 130A-309.214(a)(4)k.2 upon completion of basin closure
activities.
Details and supporting rationale for these CAP activities are provided in the following
sections.
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Allen Steam Station SynTerra
ES.2 Background
Plant Operations
Operations began at Allen in 1957. Five coal-fired units are operated at the Station. CCR
materials, composed primarily of fly ash and bottom ash, were initially hydraulically
sluiced to the RAB, also referred to as the inactive ash basin, until the AAB was
constructed and placed into operation in 1973. CCR materials were hydraulically
sluiced to the AAB from 1973 to 2019. In 2008, hydraulic sluicing of fly ash was
discontinued and was replaced with a dry fly ash (DFA) handling system. In 2019,
Duke Energy converted to dry handling of bottom ash, and coal ash is no longer placed
in either basin. The Allen ash basins have been operated under a National Pollution
Discharge Elimination System (NPDES) Permit issued by the NCDEQ Division of Water
Resources (DWR).
Pursuant to N.C. General Statute § 130A-309.213(d)(1), a November 13, 2018 letter from
NCDEQ to Duke Energy, documented the classification of the CCR surface
impoundment at Allen as low -risk (Appendix A). The letter cited that Duke Energy has
"established permanent water supplies as required by NCGS 130A-309.211(cl)" and has
"rectified any deficiencies identified by, and otherwise complied with the requirements
of, any dam safety order issued by the Environmental Management
Commission... pursuant to NCGS 143-215.32." The relevant closure requirements for
low -risk impoundments are in N.C. General Statute § 130A-309.214(a)(3), which states
that low -risk impoundments shall be closed as soon as practicable, but no later than
December 31, 2029.
Source Areas
The RAB and AAB are the primary source areas evaluated in this CAP. General
information is provided below for the coal pile area as an additional source area. The
coal pile is adjacent to the RAB, and therefore, the source areas (the AAB, RAB, and coal
pile area) are evaluated together as Source Area 1 within this CAP Update.
Ash Basins
The RAB includes two ash storage areas, two structural fills, and the double -lined RAB
Ash Landfill. Pyrite -rich rocks known as "clinkers" or "mill rejects" have also been
observed to be mixed with ash within in the north-northeast portion of the RAB.
Clinkers were mixed with coal but not combusted as part of the power -generation
process. Pyrite within the clinkers has caused low pH conditions in the subsurface
within and downgradient of the north-northeast portion of the RAB. The area
containing the clinkers and areas downgradient extending toward the main coal pile are
referred to as the "low pH area." The AAB includes three areas of ponded water known
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Allen Steam Station SynTerra
as primary ponds 1, 2, and 3. Each of the ponds were constructed atop ash and
separated with divider dikes. These features within the RAB and AAB are not
considered separate sources but are considered collectively as part of the ash basins
within the corrective action approach.
Mechanical decanting of the AAB was initiated on June 5, 2019. The former operating
elevation of the ash basin ponded water was approximately 635 feet. As of December 1,
2019 approximately 53,300,000 gallons of water have been pumped from the AAB with
a corresponding reduction in hydraulic head of 14.1 feet in elevation. Ash basin
decanting, as part of the ash basin closure process, is scheduled to be complete on or
before June 30, 2020.
Coal Piles
The coal piles are an additional source area evaluated in this CAP.
Coal has been stored north and northeast of the RAB within two separate
piles. Collectively, the coal piles are referred to in this CAP Update as the coal pile area.
The coal pile area is downgradient of the RAB. The live coal pile, located adjacent to the
Catawba River (Lake Wylie), encompasses approximately 2 acres. The main coal pile is
located west of the live coal pile and northeast of the RAB. The main coal pile
encompasses approximately 15 acres. The approximate locations of the coal piles have
remained consistent throughout the operating history of the Site. Minor changes to the
footprint occur depending upon the volume of coal stockpiled on Site, which can vary
substantially throughout the year. Coal is not waste, therefore, the coal piles do not
have waste or compliance boundaries. However, a portion of the main coal pile lies
within the ash basin compliance boundary. The coal piles are not lined. However, in
2018, a lined holding basin was built in the southeast corner of the main coal pile
footprint as part of a water redirect project. It is anticipated that the coal piles will
remain in place until the Station is retired, currently planned for 2024 for Units 1, 2, and
3 and 2028 for Units 4 and 5.
Data from monitoring wells installed downgradient of the coal piles indicate
concentrations of COIs, primarily concentrations of sulfate and total dissolved solids
(TDS), are greater than applicable comparative criteria. Between the RAB and the coal
pile, sulfate and TDS concentrations are less than concentrations in areas downgradient
of the coal piles. This indicates the coal pile area is a separate source of COIs in
groundwater. Therefore, the coal pile area is included as a component of this CAP.
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Pre -Basin Closure Activities
Initial ash basin closure efforts included ceasing all wastewater flows to the ash basins.
To accommodate closure of the ash basins, mechanical decanting (removal) of free
water from the AAB began on June 5, 2019, as required by a Special Order by Consent
(SOC) issued through the North Carolina Environmental Management Commission
(EMC) on April 25, 2018 (EMC SOC WQ S17-009) (Appendix B of Appendix J). The
SOC requires completion of decanting by June 30, 2020. Decanting of free, ponded
water from the ash basin before closure will reduce or eliminate seepage from
constructed or non -constructed seeps. Constructed seeps are seeps on or within the dam
structure that convey wastewater via a pipe or constructed channel to an NPDES-
regulated receiving water. Seeps that do not meet the constructed seep definition are
considered non -constructed seeps. Decanting is considered an important component of
the corrective action strategy because it will significantly reduce the hydraulic head and
gradients, thereby reducing the groundwater flow velocity and COI migration potential
associated with the ash basin. As of December 1, 2019, 53,300,000 gallons of water had
been removed from the AAB and the water elevation had decreased by 14.1 feet.
Basis for CAP Development
A substantial amount of data related to the ash basins, coal pile area, and the general
Allen Site has been collected to date. A summary of the Allen assessment
documentation used to prepare this CAP Update is presented in Table ES-1.
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Allen Steam Station SynTerra
TABLE ES-1
SUMMARY OF ALLEN ASSESSMENT DOCUMENTATION
Comprehensive Site Assessment Report - Allen Steam Station Ash Basin [HDR
Engineering, Inc. of the Carolinas (HDR, 2015a)].
Corrective Action Plan Part 1 - Allen Steam Station Ash Basin (HDR, 2015b).
Corrective Action Plan Part 2 (included CSA Supplement 1 as Appendix A) - Allen Steam
Station Ash Basin (HDR, 2016a).
Comprehensive Site Assessment Supplement 1 - (included in CAP 2 as Appendix A) -
Allen Steam Station Ash Basin (HDR, 2016b).
Comprehensive Site Assessment Supplement 2 - Allen Steam Station Ash Basin (HDR,
2016c).
2018 Comprehensive Site Assessment Update - Allen Steam Station (SynTerra, 2018a).
Preliminary Updated Groundwater Flow and Transport Modeling Report - Allen Steam
Station (FRx, Inc., Falta Environmental, and SynTerra, 2018).
Human Health and Ecological Risk Assessment Summary Update - Allen Steam Station
(SynTerra, 2018b).
Allen Steam Station HB 630 Completion (Duke Energy, 2018).
Ash Basin Pumping Test Report - Allen Steam Station (SynTerra, 2019a).
Surface Water Evaluation to Assess 15A NCAC 02B .0200 Compliance for Implementation
of Corrective Action Under 15A NCAC 02L .0106 (k) and (I) - Allen Steam Station
(SynTerra, 2019b).
2018 CAMA Annual Interim Monitoring Report (SynTerra, 2019c).
Community Impact Analysis of Ash Basin Closure Options at the Allen Steam Station
(Exponent, 2018).
Allen Steam Station Ash Basin Closure Options Analysis - Summary Report (AECOM,
2018).
Updated Background Threshold Values for Groundwater (SynTerra, 2019d).
Prepared by: LWD Checked by: CIS
NCDEQ reviewed the January 31, 2018 Comprehensive Site Assessment (CSA) Update
report, and in a June 11, 2018 letter to Duke Energy, NCDEQ stated that sufficient
information was provided to allow preparation of this CAP Update (Appendix A).
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Allen Steam Station SynTerra
The assessment work referenced in the documents listed in Table ES-1 has resulted in a
very large dataset that has informed the development of this CAP Update. The
following site assessment -related activities have been completed and are summarized in
Table ES-2. The table is current as of June 2019 and does not include additional wells or
sampling events for assessment of the low pH area and coal pile area. However, data
related to these areas are evaluated and included within this CAP Update.
TABLE ES-2
SUMMARY OF ALLEN ASSESSMENT ACTIVITIES
Tasks
Total
Total Monitoring Wells Installed (CAMA and CCR Wells around basins)
211
Groundwater Monitoring Events
31
Groundwater Samples Collected
3,233
Individual Analyte Results
191,551
Off -site Water Supply Well Sampling (Total inorganic analysis) -
Number of Analyses
8,410
Ash Pore Water - Number of Analyses (Total and dissolved)
20,513
Ash Pore Water Sampling Events
27
Surface Water Monitoring Events
13
Surface Water Sample Locations
33
Area of Wetness Sample Events
33
Ash Samples (Within ash basins analyzed for SPLP)
11
Soil Samples Collected
249
Soil Sample Locations
82
Sediment Sample Locations
19
Geotechnical Soil Sample Locations
84
Geochemical Ash, Soil, Partially Weathered Rock, Whole Rock Samples
101
Hydraulic Conductivity Tests (Slug Tests, Pumping Tests, Packer Tests,
FLASH Analysis of Bedrock HPF Data)
186
Groundwater Flow and Transport Simulations
93
PHREEQC Geochemical Simulations
82
Prepared by: LWD Checked by: CIS
Notes:
Data available to SynTerra as of June 2019
FLASH - Flow -Log Analysis of Single Holes
HPF - Heat Pulse Flow
SPLP - Synthetic Precipitation Leaching Procedure
PHREEQC - pH Redox Equilibrium in computer code C
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Allen Steam Station SynTerra
A COI management process was developed by Duke Energy at the request of NCDEQ
to gain understanding of the COI behavior and distribution in groundwater and to aid
in selection of the appropriate remedial approach. The COI management process
consists of three steps:
1. Performing a detailed review of the applicable regulatory requirements of
NCAC, Title 15A, Subchapter 02L
2. Acquiring an understanding of the potential mobility of Site -related COIs in
groundwater based on Site hydrogeology and geochemical conditions
3. Determining the COI distribution related to the ash basins and coal piles under
current or predicted future conditions.
This COI management process is supported by multiple lines of technical evidence,
including empirical data collected at the Site, geochemical modeling, and groundwater
flow and transport modeling. This approach has been used to understand and predict
COI behavior in the subsurface related to the ash basins and coal piles, or to identify
COIs that are naturally occurring. COIs that have migrated at or beyond the
compliance boundary at concentrations greater than 02L, IMAC, and background
values that are related to the ash basins or coal piles would be subject to corrective
action. COIs that are naturally occurring at concentrations greater than the 02L
standard do not require corrective action. Details on the COI management approach are
presented in Section 6.
Groundwater
In conformance with requirements of G.S. Section 130A-309.211, groundwater corrective
action is the main focus of this CAP Update. Groundwater COIs to be addressed with
corrective action are those detected in groundwater at or beyond the compliance
boundary greater than the 02L standard, IMAC, or background concentrations,
whichever is greater.
Soil
Data indicate unsaturated soil COI concentrations are generally consistent with
background concentrations or are less than regulatory screening values. In the few
instances where unsaturated soil COI concentrations are greater than Preliminary Soil
Remediation Goal (PSRG) Protection of Groundwater (POG) standards or background
values, either COI concentrations are generally within the range of the background
dataset concentrations or there are no mechanisms by which the COI could have been
transported from the ash basins to the unsaturated soils. One exception is at a location
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Allen Steam Station SynTerra
north of the coal pile where iron was detected at a concentration at an isolated depth
interval of 2 to 3 feet below ground surface that might indicate potential effects from the
coal pile. However, iron is prevalent naturally in saprolitic soils, so the detected
concentration might be a natural variation in concentration within the subsurface.
Furthermore, iron concentrations at deeper unsaturated soils are less than PSRG POG;
the iron in soil at the 2- to 3-foot interval is not considered a significant source of COIs
in groundwater. Therefore, this CAP Update focuses on remediation of COIs in
groundwater derived from the ash basins and the coal piles.
Risk Assessment
The human health and ecological risk assessments, prepared based on state and federal
guidance, demonstrated no measurable difference in modeled risks to potential human
or ecological receptors compared with background concentrations. The updated risk
assessments of the Allen ash basins and coal piles are presented in Section 6 of this CAP
Update. Data from former water supply wells and the Catawba River (Lake Wylie)
indicate no evidence of risk posed by groundwater migration associated with the ash
basins or coal piles based on evaluation of concentrations of CCR constituents in
environmental media and potential receptors.
Risk Ranking
Also, in accordance with G.S. Section 130A-309.211(cl) of House Bill 630 (2016), Duke
Energy:
• Connected 191 households to the City of Belmont water supply (nine of those
households were already connected to the city of Belmont water supply)
• Installed 10 water treatment systems
• Abandoned three public water supply wells that served 77 households
Of the remaining 12 households/properties that were initially considered eligible by
being within a 0.5-mile radius of the ash basin compliance boundary:
• Two households either opted out of the option to connect to a water treatment
system or did not respond to the offer.
• One household was demolished, but that property will be connected at a future
date.
• Six locations were deemed not eligible because the property did not contain a
household.
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• Three additional locations were associated with a business, church, or school,
which are not eligible for the HB 630 provisions.
Additionally, Duke Energy voluntarily connected two businesses and 23 households to
the City of Belmont water supply that were otherwise not eligible per G.S. Section
130A-309.211(cl).
Connection of households to City of Belmont water supply and installation of water
supply filtration systems, along with certain improvements to the ash basin dams
completed by Duke Energy, resulted in the ash basins being ranked as low -risk.
ES.3 CSM Overview
The Conceptual Site Model (CSM) is a written and graphical representation of the
hydrogeologic conditions and COI interactions specific to the Site and is critical to
understanding the subsurface conditions related to the ash basins and coal pile area.
The updated CSM developed for Allen included in this CAP Update is based on a
United States Environmental Protection Agency (USEPA) document titled
"Environmental Cleanup Best Management Practices: Effective Use of the Project Life
Cycle Conceptual Site Model" (USEPA, 2011). This document describes six CSM stages
for a project life cycle. The CSM is an iterative tool designed to assist in the decision -
making process for Site characterization and remediation as the Site progresses through
the project life cycle and new data becomes available. The current Allen CSM is
consistent with Stage 4 "Design CSM", which allows for iterative improvement of the
Site CSM during design of the remedy while supporting development of remedy design
basis (USEPA, 2011).
Multiple lines of evidence have been used to develop the CSM based on the large data
set generated for Allen. The remedial action evaluation to meet the effectiveness criteria
in the CAP guidance provided by NCDEQ is also based on the updated CSM (NCDEQ,
2018).
The following provides an overview of the updated CSM for the Allen ash basins and
coal piles which forms the basis of this CAP Update. Supporting details for the CSM are
presented in Section 5.0.
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Key conclusions of the CSM include the following:
• No material increases in risk to human health related to the ash basins or coal
piles have been identified. The Site -specific risk assessments of the ash basins
and coal pile indicate no measurable difference between evaluated Site -related
risks and risks imposed by background concentrations. Site -specific risk
assessments indicate there is no evidence of unacceptable risks to human and
ecological receptors exposed to environmental media potentially impacted by
CCR constituents at Allen.
The ash basins and coal pile area do not increase risks to ecological receptors.
The assessment did not indicate an increase of risks to ecological receptors
(benthic invertebrates, fish, mallard, great blue heron, killdeer bird, muskrat,
river otter, robin, red-tailed hawk, meadow vole, and red fox) that might access
surface water and sediments downgradient of the ash basins and coal pile.
• Groundwater from the ash basins and coal pile area has not and does not flow
toward any water supply wells. That conclusion is based on groundwater flow
patterns of over 30 monitoring events using data from 234 monitoring wells
and the upgradient or side -gradient wells relative to the location of water
supply wells in the area. Groundwater data from water supply wells and on -
Site monitoring wells, groundwater elevation measurements from 31 monitoring
events, and groundwater flow and transport modeling results all indicate that
Site COIs are not affecting, and have not affected, water supply wells, most of
which are no longer in use or are abandoned.
The permanent water solution program implemented by Duke Energy
provided city water connections or water filtration systems to owners of
surrounding properties with water supply wells within a 0.5-mile radius of the
ash basin compliance boundary. The hydrogeologic data collected at Allen
confirms that Site -related COIs are not affecting off -Site water supply users.
Modeling predicts that Site -related COIs will not, affect off -Site water supply
users. Nevertheless, in accordance with General Statutes 130A-309.211(cl), Duke
Energy connected 191 households to the City of Belmont water supply (nine of
the 191 households were connected to the City of Belmont water supply prior to
HB 630); installed 10 water treatment systems, and abandoned three public water
supply wells that served 77 households. Of the remaining 12 water supply wells
within a 0.5-mile radius of the ash basin compliance boundary; two (2)
households opted out or were non -responsive to Duke Energy's offer of a
permanent water solution; one (1) household was demolished but would be
connected at a future date; six (6) locations were deemed not eligible because the
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property did not contain a household; and three (3) locations were associated
with a business, church, or school which are not eligible for the HB 630 (2016).
Furthermore, Duke Energy voluntarily connected two businesses and 23
households to the City of Belmont water supply that were otherwise not eligible
per G.S. Section 130A-309.211(cl).
• The hydrogeologic setting of the Allen ash basins and coal pile area limits COI
transport. The Site, located in the Piedmont Physiographic Province, conforms
to the general hydrogeologic framework for sites in the Blue Ridge/Piedmont
area, which are characterized by groundwater flow in a slope -aquifer system
within a local drainage basin with a perennial stream (LeGrand 2004). Predictive
groundwater flow and transport model simulations indicate that ash basin
decanting will affect groundwater flow patterns within the basin by lowering
hydraulic heads in and around the ash basin dam, which will reduce the rate of
COI transport, and provide source control prior to completion of basin closure.
As of December 1, 2019, 53,300,000 gallons water had been removed from the
AAB and the water elevation had decreased by 14.1 feet.
• The physical setting and hydraulic processes control the COI flow pattern
within the ash basins, underlying groundwater system, and downgradient
areas. The ash basins are predominantly a horizontal water flow -through system.
Groundwater enters the upgradient side of the ash basins; it is supplemented by
rainfall infiltration and flows laterally through the middle of the ash basins
under a low horizontal gradient, and then flows downward near the dam. This
flow system results in limited downward migration of COIs into the underlying
soils and saprolite upgradient from the dams. Near the dam, COIs in water
either discharge through the NPDES-permitted outfall or flow downward out of
the basin and under the dam. Beyond the dam, groundwater flows upward
toward the Catawba River (Lake Wylie) discharge zone, limiting downward
migration of COIs to the area proximate to the dam. The exception is near the
northern portion of the RAB, where a relatively small component of the
groundwater flow system flows in a similar manner, but toward the discharge
canal within Duke Energy property. Bedrock wells installed at various depths
within the basin footprint and downgradient of the dam structure support the
flow characteristics and limited COI distribution.
• Horizontal distribution of COIs in groundwater proximate to the basins and
coal pile area is limited to the north and east. The physical extent of constituent
migration north and east of the basins and coal pile area is controlled by
Page ES-13

Corrective Action Plan Update December 31, 2019
Allen Steam Station SynTerra
hydrologic divides, dilution from unaffected groundwater, and the
groundwater -to -surface water discharge zones.
• Geochemical processes stabilize and limit certain constituent migration along
the flow path. Each COI exhibits a unique geochemical behavior related to the
specific constituent partition coefficient (Ka), response to changing geochemical
parameters (i.e., pH and Eh), and sorption capacity of the soil and/or rock. Based
on geochemical modeling:
o Non -conservative, reactive COIs (e.g., strontium) will remain in mineral
phase assemblages that are stable under variable Site conditions north and
east of the basins, demonstrating sorption as an effective attenuation
mechanism. Strontium reactivity is less in the deep and bedrock flow
zones and can be more mobile under lower pH conditions, due to both the
lower sorption affinity of strontium at lower pH values as well as the
increased concentration of other divalent ions (e.g., Ca+2, Mg+2, Co+2,
Mn+2) that might compete with strontium for ion exchange sites.
o Variably reactive COIs (e.g., cobalt, iron, and manganese) can exhibit
mobility, depending on geochemical conditions and availability of
sorption sites.
o Conservative, non -reactive COIs (e.g., boron, sulfate, and TDS) migrate in
groundwater as soluble species and are not strongly attenuated by
reactions with solids but are reduced in concentration with distance
primarily by physical processes such as mechanical mixing (dispersion),
dilution, and diffusion into less permeable zones. Sorption of boron to
clay particles might occur, especially for groundwater with slight alkaline
to alkaline pH values. Maximum boron sorption occurs at pH values from
about 7.5 standard units (S.U.) to 10 S.U., then decreases at pH values
greater than 10 S.U. (EPRI 2005, ATSDR 2010).
The groundwater corrective action strategies evaluated herein consider the
potential for dynamic geochemical conditions under basin closure scenarios,
currently under appeal, and account for potential mobilization of COIs.
• COIs in groundwater are contained within Duke Energy's property. COI
distribution extends from the ash basins toward the Catawba River (Lake Wylie).
In addition to the station property, Duke Energy owns and operates the
Catawba-Wateree Project (Federal Energy Regulatory Commission Project
Number 2232), which includes the Lake Wylie reservoir. The plume associated
with the ash basins has been characterized and is stable to decreasing.
Page ES-14

Corrective Action Plan Update December 31, 2019
Allen Steam Station SynTerra
• Groundwater/surface water interaction has not caused and is not predicted to
cause, COIs at concentrations greater than NCAC, Title 15A Subchapter 02B,
Surface Water and Wetland Standards (02B). Analytical results for surface water
samples collected from the Catawba River (Lake Wylie) indicate that this water
body meets 02B criteria under current conditions. An evaluation of future surface
water quality conditions of basin -related jurisdictional streams was conducted
using a surface water mixing model with closure option model simulation
inputs. The evaluation indicates that no future groundwater COI migration
would result in constituent concentrations greater than applicable 02B surface
water criteria.
• The aquatic systems of the Catawba River adjacent to the Site are healthy
based on multiple lines of evidence including robust fish populations, species
variety and other indicators derived from years of sampling data. Ongoing
sampling and analysis of the Catawba River confirm that the surface waters have
been environmentally healthy and functioning ecosystems for many years. This
finding combined with results of the ecological risk assessment, indicate that
there are no adverse ecological effects to the main surface water systems
proximate to the ash basins or coal pile area.
• Most of the COIs identified in the CSA Update occur naturally in
groundwater, some at concentrations greater than the 02L standard or IMAC.
Groundwater at Allen naturally contains antimony, chromium, cobalt, iron,
manganese, and vanadium at concentrations greater than 02L and/or IMAC
standards. The occurrence of inorganic constituents in groundwater from the
Piedmont Physiographic Province is well documented in the literature. For
example, vanadium has natural background concentrations in all flow zones at
the Site greater than its IMAC value, and iron and manganese have natural
background concentrations in the shallow and deep flow zones greater than
respective 02L values. For the Allen CAP Update, these constituents and others
are evaluated based on Site -specific statistically derived background values, and
on additional lines of evidence to determine whether constituent concentrations
represent migration from the ash basins and the coal pile area, or are naturally
occurring.
These CSM aspects, combined with the updated human health and ecological risk
assessments, provide the basis for the CAP Update pertaining to the Allen ash basins
and the adjacent coal piles.
Page ES-15

Corrective Action Plan Update December 31, 2019
Allen Steam Station SynTerra
ES.4 Corrective Action Approach
Corrective Action Objectives and Zones Requiring Corrective Action
Migration of COIs related to the ash basins and coal piles in groundwater at or beyond
the ash basin compliance boundary occurs in areas to the north, northeast, and east of
the ash basins and coal piles. To satisfy G.S. and maintain compliance with 02L, the
corrective action approach planned for the Site focuses on restoring ash basin -affected
and coal pile -affected groundwater at or beyond the compliance boundary. The
following remedial objectives address the regulatory requirements of NCAC Title 15A
Subchapter 02L for the Allen CAP Update:
• Restore groundwater quality at or beyond the compliance boundary by returning
COIs to the 02L/IMAC groundwater quality standards, or applicable background
concentrations (whichever are greater), or as closely thereto as is economically
and technologically feasible consistent with Subchapter 02L. 0106(a).
• Use a phased CAP approach that includes initial active remediation with
effectiveness monitoring of remedy implementation followed by monitored
natural attenuation (MNA) as provided in Subchapters 02L. 0106(j) and (1).
• If appropriate given future Site conditions, Duke Energy may seek approval of
an alternate plan that does not require meeting groundwater
02L/IMAC/applicable background concentration values after satisfying the
requirements set out in Subchapter 02L .0106(k).
The compliance boundary extent is shown on Figure ES-1. Groundwater
concentrations greater than 02L/IMAC/applicable background concentration values
occur locally beyond the compliance boundary north, northeast, and east of the ash
basins. There is no waste or compliance boundary associated with the coal piles and it is
not subject to CAMA. COI concentrations are less than 02B surface water standards
within the Catawba River (Lake Wylie), which is adjacent to and downgradient of the
compliance boundary.
The area proposed for corrective action is shown on Figure ES-2.
Summary of Source Control and Corrective Measures
It is critical to take into account all of the various activities Duke Energy has performed
and will perform to improve subsurface conditions at Allen related to the ash basins
and coal piles. The remedial program incorporates source control by basin decanting
and closure, coal pile removal, active groundwater remediation, and effectiveness
monitoring. Table ES-3 summarizes the discrete components of the planned corrective
action for COI -affected groundwater.
Page ES-16

Corrective Action Plan Update December 31, 2019
Allen Steam Station SynTerra
TABLE ES-3
COMPONENTS OF SOURCE CONTROL, ACTIVE REMEDIATION, AND
MONITORING
Groundwater
Remedy
Rationale
Component
Source Control Activities
Ash Basin Decanting
Active source remediation by removing ponded water in the AAB.
Decanting will lower the hydraulic head within the AAB and reduce
hydraulic gradients, reducing groundwater seepage velocities and
COI transport potential. Decanting will return the groundwater
flow system to its approximate natural condition, flowing toward
the axis of the perennial stream valley, then east.
Mechanical decanting was initiated on June 5, 2019. As of
December 1, 2019, 53,300,000 gallons of water had been
pumped from the AAB, with a corresponding reduction in hydraulic
head of 14.1 feet in elevation. Completion of decanting is
projected to occur on or before June 30, 2020.
In addition, ash basin decanting will be effective in reducing or
eliminating seeps identified under the Special Order by Consent.
Ash Basin Closure
The ash basin closure -in -place or by closure -by -excavation
scenarios are considered source control activities. Extensive
groundwater modeling indicates that either method results in
similar effects with respect to groundwater remediation. Closure
would include management of the ash storage areas, structural
fills and low pH area within the RAB, and locations of ponded
water within the AAB.
Holding Basin
Construction of the holding basin between the main and live coal
Construction
piles has improved control of stormwater runoff that has had
contact with the coal pile. This stormwater is captured and initially
treated for total suspended solids and pH within the lined holding
basin and then pumped to the lined retention basin for final
treatment prior to discharge at NPDES Outfall 006.
Coal Pile
As part of the decommissioning process, coal will be removed
Decommissioning
from the Site, mitigating a potential source of certain COIs
associated with the coal pile, such as sulfate.
Page ES-17

Corrective Action Plan Update December 31, 2019
Allen Steam Station SynTerra
TABLE ES-3
COMPONENTS OF SOURCE CONTROL, ACTIVE REMEDIATION, AND
MONITORING
Groundwater
Remedy
Rationale
Component
Active Groundwater Remediation Activities
Active Groundwater
Groundwater remediation focused on meeting the stated remedial
Remediation
objectives at and beyond the compliance boundary is planned.
These efforts will focus on areas downgradient (north, northeast
and east) of the ash basins and coal pile area where COIs are
present at concentrations greater than applicable criteria.
To meet the above -referenced CAP objectives, approximately 87
extraction wells and approximately 76 clean water vertical
infiltration wells or approximately 48 clean water vertical
infiltration wells combined with approximately 22 clean water
horizontal infiltration wells are planned for placement in areas to
reduce COI concentrations based on actual Site data and
groundwater modeling simulations.
Institutional Controls and Monitoring
Permanent Water
Groundwater data at the Site indicate that surrounding water
Solution for Water
supply wells have not been affected by Site -related COIs.
Supply Well Users
Nevertheless, Duke Energy connected 191 households to public
within a 0.5-mile
water supply and installed and maintains 10 water filtration
radius of the Ash
systems for occupied households. Duke Energy also abandoned
Basin Compliance
three public water supply wells. Duke Energy's actions were
Boundary and
approved by the NCDEQ, which addressed stakeholder concerns.
Associated Water
Duke Energy maintains these systems on behalf of the property
Filtration System
owners.
Maintenance
Maintain Ownership
ICs in the form of a Declaration of Perpetual Land Use Restrictions
and Institutional
may be requested in the future based on the results of the
Controls (ICs)
groundwater remediation activities.
Consisting of a Land
Use Restriction
Page ES-18

Corrective Action Plan Update December 31, 2019
Allen Steam Station SynTerra
TABLE ES-3
COMPONENTS OF SOURCE CONTROL, ACTIVE REMEDIATION, AND
MONITORING
Groundwater
Remedy
Component
Rationale
Effectiveness
Duke Energy plans to monitor the groundwater to confirm the
Groundwater
corrective action objectives are met and maintained over time.
Monitoring
This monitoring program includes provisions for monitoring COIs
within the compliance boundary as required under NCAC Title
15A. 0107(k)(2). Flow and transport plus geochemical modeling
have been conducted to predict future groundwater conditions
after closure. Effectiveness monitoring will provide data to
validate modeling or provide input for model refinement in the
future. The CAP Update includes a comprehensive review of
groundwater data collected through June 2019 (with additional
data through October 2019) and a plan to optimize the monitoring
program. Within 30 days of CAP approval, Duke Energy would
implement the effectiveness monitoring program.
Provision for
The Allen ash basins, coal pile area, and surrounding areas
Adaptive
constituent a complex site; therefore, Duke Energy believes it is
Management of
important to allow for an adaptive approach during
Groundwater
implementation of this CAP Update. This approach is consistent
Remedies
with the Interstate Technology and Regulatory Council (ITRC)
document titled Remediation Management of Complex Sites
(ITRC, 2017). This approach may include (i) adjustments to the
groundwater remedy, if necessary, based on new data, or if
conditions change; or (ii) an alternate groundwater standard for
boron of 4,000 pg/L (USEPA tap water regional screening level)
pursuant to NCDEQ's authority under 15A NCAC 02L .0106(k).
Prepared by: LWD Checked by: CIS
Page ES-19

Corrective Action Plan Update December 31, 2019
Allen Steam Station SynTerra
Corrective Action at Remediation Zones
The area proposed for groundwater remediation in accordance with 02L requirements
is to the north, northeast, and east of the basins at or beyond the compliance boundary
(Figure ES-2). Multiple potential groundwater remedial technologies were initially
screened as part of the CAP Update to identify the most applicable remedial methods
based on Site -specific hydrogeologic conditions and COIs distribution in groundwater.
After initial screening, the following remedial alternatives were further evaluated in
detail:
• Remedial Alternative 1: Monitored Natural Attenuation
• Remedial Alternative 2: Groundwater Extraction and Treatment
• Remedial Alternative 3: Groundwater Extraction Combined with Targeted Clean
Water Infiltration and Treatment
These remedial alternatives were screened against the following criteria outlined in
Section 6.D.iv. (1-10) of the CAP guidance (NCDEQ, 2018):
• Protection of human health and the environment
• Compliance with applicable federal, state, and local regulations
• Long-term effectiveness and permanence
• Reduction of COI toxicity and mobility, and volume of COI -affected
groundwater
• Short-term effectiveness at minimizing effects on the environment and local
community
• Technical and logistical feasibility
• Time required to initiate
• Predicted time required to meet remediation goals
• Cost
• Sustainability
• Community acceptance
Groundwater modeling simulations were performed to evaluate the effectiveness of the
alternatives and to develop the most effective approach. The results of the analysis
indicate that groundwater remedial Alternative 3: Groundwater Extraction Combined
with Targeted Clean Water Infiltration and Treatment will most effectively achieve the
Page ES-20

Corrective Action Plan Update December 31, 2019
Allen Steam Station SynTerra
remedial objectives presented above. This alternative would include installation of a
system located north, northeast and east of the ash basins and coal pile area consisting
of:
• 87 extraction wells
• 76 clean water vertical infiltration wells or 48 clean water vertical infiltration
wells combined with 22 clean water horizontal infiltration wells
The proposed well layouts are depicted on Figure ES-3a (with vertical wells only) and
Figure ES-3b (with horizontal and vertical wells). It is anticipated the extraction wells
will be screened within the shallow, deep, and bedrock flow zones, with depths ranging
from approximately 65 feet below ground surface (bgs) to 395 feet bgs. It is anticipated
that clean water infiltration vertical wells will be installed in the shallow and transition
zones at depths ranging from 75 feet bgs to 140 feet bgs. If clean water infiltration
horizontal wells are used, they would be installed as overlapping pairs with a deeper
well installed to approximately 80 feet bgs and a shallower well installed to 20 feet bgs.
Raw water from the existing fire suppression system would go through a proposed
treatment process to produce clean infiltration water. The clean infiltration water would
be stored in a proposed tank for conveyance to the infiltration wells via proposed
distribution piping.
The flow and transport model predicts the remediation system will have a total
groundwater extraction flow rate of approximately 970 gallons per minute (gpm). It is
planned that the extracted water will be treated and then discharged through an
existing permitted NPDES outfall location, either Outfall 002 or Outfall 006. Details of
this approach are presented in Section 6.
It is recommended that prior to implementation, pilot testing of the proposed
alternative will be conducted at the areas north, northeast, and east of the ash basins
and coal piles. Pilot testing and treatment tests to be conducted include: 1) groundwater
extraction and clean water infiltration, 2) treatment testing of extraction and clean water
infiltration water. Pilot study results will inform the design of the full-scale system.
Pilot test work plan(s) would be submitted to NCDEQ within 30 days of CAP approval
to fulfill G.S. Section 130A-309.211(b)(3), amended by CAMA. Remedial performance
monitoring will be performed to evaluate remedy effectiveness as described in Section
6.8 of this CAP Update.
Page ES-21

SINPDES
OUTFALL
DUKE ENERGY CAROLINASg �I Y NPDES OUTFALL 006
ALLEN STEAM b
STATION BOUNDARY
DISCHARGE CANAL �
` f 80 NPDES NPDES OUTFALL 0102B
LP OUTFALL 003
STEAM STATION
NPDES OUTFALL 001 COAL PILE AREA
PLAKT : ^ , NPDES OUT 004
NPDES OUTFALL 0021
RETIRED ASH BASIN^
,
ASH LANDFILL RETIRED ASH BASIN
COMPLIANCE BOUNDARY) ASH LANDFILL
_ fix • BOUNDARY
RETIRED ASH BASIN
RETIRED ASH BASIN ASH LANDFILL �/ r
RETIRED ASH BASIN NPDES OUTFALL 008
WASTE BOUNDARY ' ti
NPDES OUTFALL108
m
yVf1,PLfFE-R sue. • NPDES OUTFALL 108Br
o •.
GYPf;[IS'RA �`�
. ACTIVE ASH BASIN
NPDES OUTFALL104
� NPDES OUTFALL103
0
NPDES OUTFALL 002
ACTIVE ASH BASIN p
WASTE BOUNDARY x`
i p
�Iya ke
ASH BASIN
COMPLIANCE J
{mot_ 1 BOUNDARY
NOTE:
�}1
WATER FEATURES DEPICTED WITHIN WASTE BOUNDARIES
OF THE ASH BASINS ON THE 2016 USGS TOPOGRAPHIC MAP
DO NOT REPRESENT CURRENT CONDITIONS. THE CONDITIONS
DEPICTED ARE SIMILAR TO THOSE SHOWN ON THE 1968 AND
q�
1973 USGS TOPOGRAPHIC MAPS OF THE AREA [(1968 WEST
SrG
CHARLOTTE (1:24000) AND 1973 BELMONT (1:24000)].
xr
SOURCE:
v
2016 USGS TOPOGRAPHIC MAP, BELMONT & CHARLOTTE WEST
V
QUADRANGLE, OBTAINED FROM THE USGS STORE AT
https://store.usgs.gov/map-locator.DUKE
p
ENERGY.
WINSTON-SALEM•
CAROLINAS
GASTON
COUNTY
n
,!
FIGURE ES-1
USGS LOCATION MAP
CORRECTIVE ACTION PLAN UPDATE
ALLEN STEAM STATION
BELMONT, NORTH CAROLINA
DRAWN BY: B. YOUNG DATE: 05/02/2019 GRAPHICSCALE
REVISED BY: C. WYATT DATE: 12/08/2019 1,000 o z000 2,000
CHECKED BY: L. DRAGO DATE: 12/08/2019
APPROVED BY: L. DRAGO DATE: 12/08/2019 (IN FEET)
PROJECT MANAGER: C. SUTTELL

NOTES:
1. THE WATERS OF THE US DELINEATION HAS NOT BEEN
APPROVED BY THE US ARMY CORPS OF ENGINEERS AT THE TIME
OF THE MAP CREATION. THIS MAP IS NOT TO BE USED FOR
JURISDICTIONAL DETERMINATION PURPOSES. THE WETLANDS
AND STREAMS BOUNDARIES WERE OBTAINED FROM STREAM AND
WETLAND DELINEATION CONDUCTED BY AMEC FOSTER WHEELER
ENVIRONMENTAL & INFRASTRUCTURE, INC. NATURAL RESOURCE
TECHNICAL REPORT (NRTR) FOR ALLEN STEAM STATION DATED
MAY 29, 2015.
2. ALL BOUNDARIES ARE APPROXIMATE.
3. PROPERTY BOUNDARY PROVIDED BY DUKE ENERGY
CAROLINAS.
4. AERIAL PHOTOGRAPHY OBTAINED FROM GOOGLE EARTH PRO
ON DECEMBER 13, 2018. AERIAL WAS COLLECTED ON MARCH 30,
2018.
5. DRAWING HAS BEEN SET WITH A PROJECTION OF NORTH
CAROLINA STATE PLANE COORDINATE SYSTEM FIPS 3200
(NAD83/2011).
I
1 .
I
I
I
I
I
1
I
h
LEGEND
AREA PROPOSED FOR ACTIVE GROUNDWATER
REMEDIATION
ACTIVE ASH BASIN WASTE BOUNDARY
- - RETIRED ASH BASIN WASTE BOUNDARY
- - ASH BASIN COMPLIANCE BOUNDARY
_ RETIRED ASH BASIN ASH LANDFILL WASTE
BOUNDARY
RETIRED ASH BASIN ASH LANDFILL
- COMPLIANCE BOUNDARY
- - DORS FILLS BOUNDARIES
SITE FEATURE
- DUKE ENERGY CAROLINAS PROPERTY LINE
STREAM (AMEC NRTR 2015)
® WETLAND (AMEC NRTR 2015)
- - - r - - - - . ` ,
I•' ,' r ` , `' ` VPNt PVLEN KOPO
♦P
ASH
STORAGE
ACTIVE
A
GRAPHIC SCALE
ItDUKE 250 0 250 500
ENERGY (IN FEET)
CAROLINAS
DRAWN BY: K. KING DATE: 06/19/2019
REVISED BY: J. KIRTZ DATE: 12/19/2019
APPROVED BY: L. DRAGO DATE: 12/19/2019
CHECKED BY: L. DRAGO DATE: 12/19/2019
T PROGRAM MANAGER: C. SUTiELL
fALLEEAMATION'
1
I
I
I
I
I
I
I
r
/
I
I
I
i
r
LAq
FIGURE ES-2
AREA PROPOSED
FOR CORRECTIVE ACTION
CORRECTIVE ACTION PLAN UPDATE
ALLEN STEAM STATION
BELMONT, NORTH CAROLINA

Corrective Action Plan Update December 2019
Duke Energy Carolinas, LLC - Allen Steam Station SynTerra
Figure ES-3a
Proposed Corrective Action Approach
Well System Layout
(Vertical Wells Only)
Provided in separate electronic figure file as a
large sheet size

Corrective Action Plan Update December 2019
Duke Energy Carolinas, LLC - Allen Steam Station SynTerra
ES-3b
Proposed Corrective Action Approach
Well System Layout
(Vertical and Horizontal Wells)
Provided in separate electronic figure file as a
large sheet size

Corrective Action Plan Update December 2019
Allen Steam Station SynTerra
TABLE OF CONTENTS
SECTION
PAGE
EXECUTIVE SUMMARY.................................................................................................... ES-1
ES.1 Introduction...................................................................................................................
1
ES.2 Background....................................................................................................................4
ES.3 CSM Overview............................................................................................................11
ESA Corrective Action Approach.....................................................................................16
1.0
INTRODUCTION.........................................................................................................1-1
1.1 Background.................................................................................................................1-2
1.2 Purpose and Scope.....................................................................................................1-3
1.3 Regulatory Basis for Corrective Action..................................................................1-4
1.4 List of Considerations by the Secretary for Evaluation of Corrective Action
Plans.............................................................................................................................1-6
1.5 Facility Description....................................................................................................1-7
1.5.1 Location and History of Land Use.....................................................................1-7
1.5.2 Operations and Waste Streams Coincident with the Ash Basins ..................1-9
1.5.3 Overview of Existing Permits and Special Orders by Consent....................1-11
2.0
RESPONSE TO CSA UPDATE COMMENTS........................................................ 2-1
2.1 Facility -Specific Comprehensive Site Assessment (CSA) Comment Letter ......
2-1
2.2 Duke Energy's Response to DEQ Letter.................................................................
2-1
3.0
OVERVIEW OF SOURCE AREAS BEING PROPOSED FOR CORRECTIVE
ACTION......................................................................................................................... 3-1
4.0
SUMMARY OF BACKGROUND DETERMINATIONS......................................4-1
4.1 Background Concentrations for Soil.......................................................................4-2
4.2 Background Concentrations for Groundwater......................................................4-3
4.3 Background Concentrations for Surface Water .....................................................
4-4
4.4 Background Concentrations for Sediment.............................................................
4-5
5.0
CONCEPTUAL SITE MODEL................................................................................... 5-1
5.1 Site Geologic and Hydrogeologic Setting...............................................................5-2
5.1.1 Site Geologic Setting.............................................................................................
5-2
5.1.2 Site Hydrogeologic Setting..................................................................................
5-3
5.1.2.1 Groundwater Flow Direction..........................................................................
5-4
Page i

Corrective Action Plan Update December 2019
Allen Steam Station SynTerra
TABLE OF CONTENTS (CONTINUED)
SECTION
PAGE
5.1.2.2
Groundwater Seepage Velocities..................................................................... 5-7
5.1.2.3
Hydraulic Gradients.......................................................................................5-9
5.1.2.4
Particle Tracking Results..............................................................................
5-10
5.1.2.5
Subsurface Heterogeneities...........................................................................
5-10
5.1.2.6
Bedrock Matrix Diffusion and Flow.............................................................
5-11
5.1.2.7
Onsite and Offsite Pumping Influences........................................................
5-13
5.1.2.8
Ash Basin Groundwater Water Balance.......................................................
5-14
5.1.2.9
Effects of Naturally Occurring Constituents ...............................................
5-16
5.2 Source Area Location...............................................................................................
5-17
5.3 Summary of Potential Receptors...........................................................................
5-17
5.3.1 Surface Water.......................................................................................................5-18
5.3.1.1 Environmental Assessment of Lake Wylie .................................................... 5-19
5.3.2 Availability of Public Water Supply................................................................5-19
5.3.3 Water Supply Wells............................................................................................ 5-19
5.3.4 Surrounding Land Use....................................................................................... 5-20
5.3.5 Future Groundwater Use Area......................................................................... 5-20
5.4 Human Health and Ecological Risk Assessment Results...................................5-20
5.5 CSM Summary......................................................................................................... 5-23
6.0 CORRECTIVE ACTION APPROACH FOR SOURCE AREA 1
(ASH BASINS AND COAL PILE AREA)................................................................6-1
6.1 Extent of Constituent Distribution.......................................................................... 6-6
6.1.1 Source Material within the Waste Boundary .................................................... 6-6
6.1.1.1 Description of Waste Material and History of Placement .............................. 6-6
6.1.1.2 Specific Waste Characteristics of Source Material .......................................... 6-8
6.1.1.3 Volume and Physical Horizontal and Vertical Extent of Source Material... 6-10
6.1.1.4 Volume and Physical Horizontal and Vertical Extent of Anticipated
Saturated Source Material............................................................................ 6-11
6.1.1.5 Saturated Ash and Groundwater.................................................................. 6-12
Page ii

Corrective Action Plan Update December 2019
Allen Steam Station SynTerra
TABLE OF CONTENTS (CONTINUED)
SECTION
PAGE
6.1.1.6 Chemistry within Waste Boundary.............................................................. 6-13
6.1.1.7 Other Potential Source Material................................................................... 6-20
6.1.1.8 Interim Response Actions............................................................................. 6-25
6.1.2 Extent of Constituent Migration beyond the Compliance Boundary ......... 6-27
6.1.2.1 Piper Diagrams............................................................................................. 6-35
6.1.3 Constituents of Interest(COIs).......................................................................... 6-38
6.1.4 Horizontal and Vertical Extent of COIs...........................................................6-45
6.1.4.1 COIs in Unsaturated Soil............................................................................. 6-47
6.1.4.2 Horizontal and Vertical Extent of Groundwater in Need of Restoration ..... 6-50
6.1.5 COI Distribution in Groundwater.................................................................... 6-53
6.1.5.1 Conservative Constituents............................................................................ 6-54
6.1.5.2 Non -Conservative Constituents.................................................................... 6-57
6.1.5.3 Variably Conservative Constituents............................................................. 6-58
6.2 Potential Receptors Associated with Source Area .............................................. 6-60
6.2.1 Surface Waters — Downgradient Within a 0.5-Mile Radius of the Waste
Boundary.............................................................................................................. 6-60
6.2.2 Water Supply Wells............................................................................................ 6-63
6.2.2.1 Provision of Alternative Water Supply......................................................... 6-64
6.2.2.2 Findings of Drinking Water Supply Well Surveys ...................................... 6-66
6.2.3 Future Groundwater Use Areas........................................................................ 6-68
6.3 Human and Ecological Risks..................................................................................
6-69
6.4 Description of Remediation Technologies...........................................................
6-69
6.4.1 Monitored Natural Attenuation........................................................................
6-70
6.4.2 In -Situ Technologies...........................................................................................
6-71
6.4.3 Groundwater Extraction....................................................................................
6-76
6.4.4 Groundwater Treatment....................................................................................
6-82
6.4.5 Groundwater Management...............................................................................
6-85
6.4.6 Technology Evaluation Summary....................................................................
6-91
6.5 Evaluation of Remedial Alternatives....................................................................
6-91
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Corrective Action Plan Update December 2019
Allen Steam Station SynTerra
TABLE OF CONTENTS (CONTINUED)
SECTION
PAGE
6.5.1 Remedial Alternative 1— Monitored Natural Attenuation ........................... 6-91
6.5.1.1 Problem Statement and Remediation Goals .................................................. 6-92
6.5.1.2 Conceptual Model......................................................................................... 6-92
6.5.1.3 Predictive Modeling...................................................................................... 6-94
6.5.2 Remedial Alternative 2 — Groundwater Extraction and Treatment ............ 6-95
6.5.2.1 Problem Statement and Remediation Goals .................................................. 6-95
6.5.2.2 Conceptual Model......................................................................................... 6-96
6.5.2.3 Predictive Modeling...................................................................................... 6-97
6.5.3 Remedial Alternative 3: Groundwater Extraction Combined with
Targeted Clean Water Infiltration and Treatment ......................................... 6-97
6.5.3.1 Problem Statement and Remediation Goals .................................................. 6-98
6.5.3.2 Conceptual Model......................................................................................... 6-98
6.5.3.3 Predictive Modeling.................................................................................... 6-101
6.6 Remedial Alternatives Screening Criteria.......................................................... 6-101
6.7 Remedial Alternatives Criteria Evaluation........................................................ 6-107
6.7.1 Remedial Alternative 1— Monitored Natural Attenuation .........................
6-107
6.7.2 Remedial Alternative 2 — Groundwater Extraction and Treatment ..........
6-111
6.7.3 Remedial Alternative 3 — Groundwater Extraction Combined with
Targeted Clean Water Infiltration and Treatment .......................................
6-115
6.8 Proposed Remedial Alternative Selected for Source Area ...............................
6-120
6.8.1 Description of Proposed Remedial Alternative and Rationale for
Selection..............................................................................................................
6-120
6.8.2 Design Details....................................................................................................
6-125
6.8.2.1 Process Flow Diagrams for All Major Components of Proposed
Remedy........................................................................................................
6-126
6.8.2.2 Engineering Designs with Assumptions, Calculations, and
Specifications...............................................................................................
6-135
6.8.2.3 Permits for Remedy and Schedule...............................................................
6-139
6.8.2.4 Schedule and Cost of Implementation.........................................................
6-140
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Corrective Action Plan Update December 2019
Allen Steam Station SynTerra
TABLE OF CONTENTS (CONTINUED)
SECTION PAGE
6.8.2.5 Measure to Ensure Health and Safety........................................................ 6-141
6.8.2.6 Description of all Other Activities and Notifications being conducted to
Ensure Compliance with 02L, CAMA, and Other Relevant Laws and
Regulations.................................................................................................. 6-141
6.8.3 Requirements For 02L .0106(1) — MNA.......................................................... 6-141
6.8.4 Requirements For 02L .0106(k) — Alternate Standards ................................ 6-141
6.8.5 Sampling and Reporting.................................................................................. 6-142
6.8.5.1 Progress Reports and Schedule................................................................... 6-144
6.8.5.2 Sampling and Reporting Plan During Active Remediation ....................... 6-146
6.8.6 Sampling and Reporting Plan After Termination of Active
Remediation....................................................................................................... 6-150
6.8.7 Proposed Interim Activities Prior to Implementation ................................. 6-151
6.8.8 Contingency Plan.............................................................................................. 6-151
6.8.8.1 Description of Contingency Plan................................................................ 6-151
6.8.8.2 Decision Metrics for Contingency Plan Areas ........................................... 6-152
6.9 Summary and Conclusions...................................................................................6-155
7.0 PROFESSIONAL CERTIFICATIONS...................................................................... 7-1
8.0 REFERENCES................................................................................................................ 8-1
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Corrective Action Plan Update December 2019
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LIST OF FIGURES
Executive Summary
Figure ES-1 USGS Location Map
Figure ES-2 Area Proposed for Corrective Action
Figure ES-3a Proposed Corrective Action Approach Well System Layout (Vertical
Wells Only)
Figure ES-3b Proposed Corrective Action Approach Well System Layout (Vertical
and Horizontal Wells)
1.0 Introduction
Figure 1-1 USGS Location Map
Figure 1-2 Site Layout Map
Figure 1-3 1948 Aerial Photograph
4.0 Summary of Background Determinations
Figure 4-1 Background Sample Location Map
5.0 Conceptual Site Model
Figure 5-1 Conceptual Site Model - Pre -Decanting Conditions
Figure 5-2 LeGrand Slope Aquifer System
Figure 5-3 Generalized Profile of Ash Basin Pre -Decanting Flow Conditions in the
Piedmont
Figure 5-4a
Water Level Map - Shallow Flow Zone - March 13, 2019
Figure 5-4b
Water Level Map - Deep Flow Zone - March 13, 2019
Figure 5-4c
Water Level Map - Bedrock Flow Zone - March 13, 2019
Figure 5-4d
Water Level Map - Shallow Flow Zone - October 2019
Figure 5-4e
Water Level Map - Deep Flow Zone - October 2019
Figure 5-4f
Water Level Map - Bedrock Flow Zone - October 2019
Figure 5-5a
Vector Velocity Map for Pre -Decanting Conditions - Shallow Flow
Zone
Figure 5-5b
Vector Velocity Map for Closure -by -Excavation Conditions - Shallow
Flow Zone
Figure 5-5c
Vector Velocity Map for Closure -in -Place Conditions - Shallow Flow
Zone
Figure 5-6
Map of Surface Waters
Figure 5-7a
Water Supply Well Sample Locations
Figure 5-7b HB 630 Provision of Permanent Water Supply Completion Map
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Corrective Action Plan Update December 2019
Allen Steam Station SynTerra
LIST OF FIGURES (CONTINUED)
6.0 Corrective Action Approach for Source Area 1(Ash Basins and Coal Pile Area)
Figure 6-1
Fly Ash and Bottom Ash Interbedded Depiction
Figure 6-2
General Cross Section A -A'
Figure 6-3
General Cross Section B-B'
Figure 6-4
General Cross Section C-C'
Figure 6-5
General Cross Section D-D'
Figure 6-6
General Cross Section E-E'
Figure 6-7
Saturated Ash Thickness Map for Pre -Decanting and Post -Closure
Conditions
Figure 6-8
Unsaturated Soil Sample Locations and Exceedances
Figure 6-9a
General Cross Section A -A' - Conservative Group - Mean of Boron,
Sulfate and Total Dissolved Solids (TDS)
Figure 6-9b
General Cross Section A -A' - Non -Conservative Group - Mean of
Strontium
Figure 6-9c
General Cross Section A -A' - Variable Group - Mean of Cobalt, Iron
and Manganese
Figure 6-10a
General Cross Section B-B' - Conservative Group - Mean of Boron,
Sulfate and Total Dissolved Solids (TDS)
Figure 6-10b
General Cross Section B-B' - Non -Conservative - Mean of Strontium
Figure 6-10c
General Cross Section B-B' - Variable Group - Mean of Cobalt, Iron
and Manganese
Figure 6-11a
General Cross Section C-C' - Conservative Group - Mean of Boron,
Sulfate and Total Dissolved Solids (TDS)
Figure 6-11b
General Cross Section C-C' - Non -Conservative - Mean of Strontium
Figure 6-11c
General Cross Section C-C' - Variable Group - Mean of Cobalt, Iron
and Manganese
Figure 6-12a
General Cross Section D-D' - Conservative Group - Mean of Boron,
Sulfate and Total Dissolved Solids (TDS)
Figure 6-12b
General Cross Section D-D' - Non -Conservative - Mean of Strontium
Figure 6-12c
General Cross Section D-D' - Variable Group - Mean of Cobalt, Iron
and Manganese
Figure 6-13a
General Cross Section E-E' - Conservative Group - Mean of Boron,
Sulfate and Total Dissolved Solids (TDS)
Figure 6-13b
General Cross Section E-E' - Non -Conservative - Mean of Strontium
Figure 6-13c
General Cross Section E-E' - Variable Group - Mean of Cobalt, Iron
and Manganese
Figure 6-14
Geochemical Water Quality Plots
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Corrective Action Plan Update December 2019
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LIST OF FIGURES (CONTINUED)
6.0 Corrective Action Approach for Source Area 1 (Ash Basins and Coal Pile Area)
(Continued)
Figure 6-15a Ash Pore Water and Groundwater Piper Diagrams
Figure 6-15b Ash Pore Water and Groundwater Piper Diagrams - Low pH Area and
Coal Pile Area
Figure 6-16
Site Layout - Decanting Monitoring Network
Figure 6-17a
Hydrographs - Downgradient
Figure 6-17b
Hydrographs - Upgradient and Southern Ash Basin
Figure 6-17c
Hydrographs - Sidegradient and Northern Ash Basin
Figure 6-17d
Hydrographs - Surface Water
Figure 6-18a
Hydrographs - Holding Basin
Figure 6-18b
Hydrographs - Holding Basin
Figure 6-19a
Isoconcentration Map Boron in Shallow Flow Zone
Figure 6-19b
Isoconcentration Map Boron in Deep Flow Zone
Figure 6-19c
Isoconcentration Map Boron in Bedrock Flow Zone
Figure 6-20a
Isoconcentration Map Sulfate in Shallow Flow Zone
Figure 6-20b
Isoconcentration Map Sulfate in Deep Flow Zone
Figure 6-20c
Isoconcentration Map Sulfate in Bedrock Flow Zone
Figure 6-21a
Isoconcentration Map TDS in Shallow Flow Zone
Figure 6-21b
Isoconcentration Map TDS in Deep Flow Zone
Figure 6-21c
Isoconcentration Map TDS in Bedrock Flow Zone
Figure 6-22a
Isoconcentration Map Strontium in Shallow Flow Zone
Figure 6-22b
Isoconcentration Map Strontium in Deep Flow Zone
Figure 6-22c
Isoconcentration Map Strontium in Bedrock Flow Zone
Figure 6-23a
Isoconcentration Map Cobalt in Shallow Flow Zone
Figure 6-23b
Isoconcentration Map Cobalt in Deep Flow Zone
Figure 6-23c
Isoconcentration Map Cobalt in Bedrock Flow Zone
Figure 6-24a
Isoconcentration Map Iron in Shallow Flow Zone
Figure 6-24b
Isoconcentration Map Iron in Deep Flow Zone
Figure 6-25a
Isoconcentration Map Manganese in Shallow Flow Zone
Figure 6-25b
Isoconcentration Map Manganese in Deep Flow Zone
Figure 6-25c
Isoconcentration Map Manganese in Bedrock Flow Zone
Figure 6-26
Seep and Surface Water Piper Diagrams
Figure 6-27
Pourbaix Diagram For Iron -Water System
Figure 6-28a
Remedial Alternative 3 - Well System Layout - Groundwater
Remediation by Extraction Combined with Clean Water Infiltration
and Treatment (Vertical)
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Corrective Action Plan Update December 2019
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LIST OF FIGURES (CONTINUED)
6.0 Corrective Action Approach for Source Area 1 (Ash Basins and Coal Pile Area)
(Continued)
Figure 6-28b Remedial Alternative 3 - Well System Layout - Groundwater
Remediation by Extraction Combined with Clean Water Infiltration
and Treatment (Horizontal)
Figure 6-28c Remedial Alternative 3 - Conceptual Vertical Clean Water Infiltration
Well Schematic - Remediation by Extraction Combined With Clean
Water Infiltration and Treatment
Figure 6-28d Remedial Alternative 3 - Conceptual Horizontal Well - Remediation
by Extraction Combined With Clean Water Infiltration and Treatment
Figure 6-28e Remedial Alternative 3 - Conceptual Horizontal Well - Remediation
by Extraction Combined With Clean Water Infiltration and Treatment
Figure 6-28f Remedial Alternative 3 - Conceptual Vertical Extraction Well
Schematic - Remediation by Extraction Combined With Clean Water
Infiltration and Treatment
Figure 6-28g Remedial Alternative 3 - Conceptual Trench Detail - Remediation by
Extraction Combined With Clean Water Infiltration and Treatment
Figure 6-28h
Remedial Alternative 3 (Vertical Wells Only) - Groundwater
Remediation by Extraction Combined with Clean Water Infiltration
and Treatment - Simulated Boron Concentrations in All Flow Zones
Figure 6-28i
Remedial Alternative 3 (Vertical Wells Only) - Groundwater
Remediation by Extraction Combined with Clean Water Infiltration
and Treatment - Simulated Sulfate Concentrations in All Flow Zones
Figure 6-28j
Remedial Alternative 3 (Horizontal and Vertical Wells) - Groundwater
Remediation by Extraction Combined with Clean Water Infiltration
and Treatment - Simulated Boron Concentrations in All Flow Zones
Figure 6-28k
Remedial Alternative 3 (Horizontal and Vertical Wells) - Groundwater
Remediation by Extraction Combined with Clean Water Infiltration
and Treatment - Simulated Sulfate Concentrations in All Flow Zones
Figure 6-29
Conceptual Process Flow Diagram - Clean Water Infiltration System
Figure 6-30
Conceptual Process Flow Diagram - Groundwater Extraction System
Figure 6-31
Conceptual Process Flow Diagram - Groundwater Treatment System
Figure 6-32
CAP Implementation Gantt Chart
Figure 6-33
Effectiveness Monitoring Well Network and Flow Paths
Figure 6-34
Work Flow For Effectiveness Monitoring and Optimization
Figure 6-35
Termination of Groundwater Remediation Flow Diagram
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Corrective Action Plan Update December 2019
Allen Steam Station SynTerra
LIST OF TABLES
Executive Summary
Table ES-1 Summary of Allen Assessment Documentation
Table ES-2 Summary of Allen Assessment Activities
Table ES-3 Components of Source Control, Active Remediation, and Monitoring
1.0 Introduction
Table 1-1 Summary of Onsite Incidents
3.0 Overview of Source Areas Proposed for Corrective Action
Table 3-1 Summary of Onsite Facilities
4.0 Summary of Background Determinations
Table 4-1
Background Soil Sample Summary
Table 4-2
Background Values for Soil
Table 4-3
Background Values for Groundwater
Table 4-4
Background Dataset Ranges for Surface Water
Table 4-5
Background Dataset Ranges for Sediment
5.0 Conceptual Site Model
Table 5-1a March 2019 Water Level Measurements and Elevations
Table 5-1b October 2019 Water Level Measurements and Elevations
Table 5-2 Groundwater Balance Summary
Table 5-3 Surface Water Classifications
6.0 Corrective Action Approach for Source Area 1(Ash Basins and Coal Pile Area)
Table 6-1 Boron Concentrations in Groundwater Below Source Area
Table 6-2 Soil PSRG POG Standard Equation Parameters and Values
Table 6-3 Summary of Unsaturated Soil Analytical Results
Table 6-4 Source Area Interim Actions
Table 6-5 Means of Groundwater COIs — January 2018 to June 2019, and October
2019
Table 6-6 COI Management Matrix
Table 6-7 Summary Trend Analysis Results for Monitoring Wells
Table 6-8 Seep Corrective Action Strategy
Table 6-9 Water Supply Well Analytical Results Summary
Table 6-10 NPDES Permit Limits and Anticipated Groundwater Remediation
Parameter Levels
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Corrective Action Plan Update December 2019
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LIST OF TABLES (CONTINUED)
6.0 Corrective Action Approach for Source Area 1 (Ash Basins and Coal Pile Area)
(Continued)
Table 6-11 Feature Irrigation System Setback
Table 6-12 Remedial Technology Screening Summary
Table 6-13 Alternative 3 Groundwater Extraction and Clean Water Infiltration
Well Summary
Table 6-14 Environmental Sustainability Comparisons for Remediation
Alternatives
Table 6-15 Modeled Clean Water Infiltration Well Details
Table 6-16 Modeled Groundwater Extraction Well Details
Table 6-17 Effectiveness Monitoring Plan Elements
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Corrective Action Plan Update December 2019
Allen Steam Station SynTerra
LIST OF APPENDICES
Appendix A Regulatory Correspondence
Appendix B Comprehensive Site Assessment Update Report Review Comments
and Responses
Appendix C Updated Comprehensive Analytical Data Table
Appendix D HB 630 Provision of Water Supply Completion Documentation
Appendix E Human Health and Ecological Risk Assessment
Appendix F Fractured Bedrock Evaluation
Appendix G Updated Groundwater Flow and Transport Modeling Report
Appendix H Geochemical Model Report
COI Management Plan Approach
Appendix I Monitored Natural Attenuation Report
Appendix J Surface Water Evaluation to Assess 15A NCAC 02B .0200
Compliance for Implementation of Corrective Action under 15A
NCAC 02L .0106 (k) and (1) Report
Surface Water Future Conditions Evaluation to Assess 15A NCAC
02B .0200 Compliance for Implementation and Termination of
Corrective Action under 15A NCAC 02L .0106 (k) and (1), and (m)
Report
Appendix K Remedial Alternative Cost Estimate Details
Appendix L Sustainability Calculations
Appendix M Remediation Alternative Summary
Appendix N Proposed Remedial Alternative Design Calculations
Appendix O Effectiveness Monitoring Plan
Appendix P Low pH Area and Coal Pile Area Assessment Technical
Memorandum
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Corrective Action Plan Update December 2019
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LIST OF ACRONYMS
02B
NCDEQ Title 15A, Subchapter 02B. Surface Water and Wetland
Standards
02L
NCDEQ Title 15A, Subchapter 02L. Groundwater Classification and
Standards
AAB
Active Ash Basin
Allen
Allen Steam Station
AOW
Area of Wetness
ASTM
American Society for Testing and Materials
BGS
Below Ground Surface
BTV
Background Threshold Value
CAMA
Coal Ash Management Act
CAP
Corrective Action Plan
CBD
Citrate-Bicarbonate-Dithionite
CCR
Coal Combustion Residuals
CFR
Code of Federal Register
COI
Constituent of Interest
CSA
Comprehensive Site Assessment
CSM
Conceptual Site Model
CY
Cubic Yards
DFA
Dry Fly Ash
DORS
Distribution of Residual Solids
Duke Energy
Duke Energy Carolinas, LLC
DWR
Division of Water Resources
Eh
Oxidation Reduction Potential
EMC
Environmental Management Commission
EMP
Effectiveness Monitoring Program
EPRI
Electric Power Research Institute
FERC
Federal Energy Regulatory Commission
FGD
Flue Gas Desulfurization
G.S.
General Statutes
GPM
Gallons per Minute
HAO
Hydrous Aluminum Oxide
HFO
Hydrous Ferric Oxide
IMAC
Interim Maximum Allowable Concentration
IMP
Interim Monitoring Plan
ISV
In -situ Vitrification
Ka
Partition Coefficient
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Corrective Action Plan Update December 2019
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LIST OF ACRONYMS (CONTINUED)
LEAF
Leaching Environmental Assessment Framework
LRB
Lined Retention Basin
MAROS
Monitoring and Remediation Optimization System
Mg/L
Milligrams per liter
MGD
Million of Gallons per Day
mm
Millimeter
MNA
Monitored Natural Attenuation
NAVD 88
North American Vertical Datum of 1988
NCAC
North Carolina Administrative Code
NCDENR
North Carolina Department of Environment and Natural Resources
NCDEQ
North Carolina Department of Environmental Quality
NORR
Notice of Regulatory Requirements
NPDES
National Pollutant Discharge Elimination System
NRTR
National Resource Technical Report
NTU
Nephelometric Turbidity Units
ORP
Oxidation Reduction Potential
OSWER
Office of Solid Waste and Emergency Response
POG
Protection of Groundwater
PRB
Permeable Reactive Barrier
PSRG
Preliminary Soil Remediation Goal
RAB
Retired Ash Basin
S.U.
Standard Units
SB
Soil Boring
Site
Allen Steam Station
SOC
Special Order by Consent
SPLP
Synthetic Precipitation Leaching Procedure
Station
Allen Steam Station
TDS
Total Dissolved Solids
TOC
Total Organic Carbon
USEPA
United States Environmental Protection Agency
µg/L
Micrograms per Liter
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Corrective Action Plan Update December 2019
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1.0 INTRODUCTION
(CAP Content Section 1)
SynTerra prepared this groundwater corrective action plan (CAP) update on behalf of
Duke Energy Carolinas, LLC (Duke Energy). The plan pertains to the Allen Steam
Station (Allen, Site, or Station) coal combustion residual (CCR) ash basins and adjacent
coal piles. Duke Energy owns and operates Allen, located in Belmont, Gaston County,
North Carolina (Figure 1-1).
In accordance with Section 130A-309.211(b) of North Carolina General Statutes (G.S.), as
enacted by Coal Ash Management Act (CAMA), Duke Energy is submitting this
groundwater CAP Update to prescribe methods and materials to restore groundwater
quality associated with CAMA-regulated units. This CAP Update considers constituent
concentrations detected greater than applicable North Carolina groundwater standards
[NC Administrative Code, Title 15A, Subchapter 02L, Groundwater Classification and
Standards (02L); Interim Maximum Allowable Concentrations (IMAC); or background
values], whichever is greater, at or beyond the compliance boundary.
In accordance with G.S. requirements, a CAP for Allen was previously submitted to the
North Carolina Department of Environmental Quality (NCDEQ) in two parts:
• Corrective Action Plan Part 1 — Allen Steam Station Ash Basin (HDR, 2015b)
• Corrective Action Plan Part 2— Allen Steam Station Ash Basin (HDR, 2016a)
This CAP Update is being submitted to NCDEQ as originally requested in a June 2, 2017
letter from NCDEQ to Duke Energy. In an April 5, 2019 letter to Duke Energy, NCDEQ
issued revised deliverable schedules and requested assessment of additional potential
sources of constituents to groundwater at Allen stating that sources hydrologically
connected to the ash basins are to be assessed and included in an updated CAP. The
coal pile was included as additional source hydrologically connected to the ash basin.
In addition to the CAP Update, Duke Energy is required to submit a CCR Surface
Impoundment Closure Plan to NCDEQ on/before December 31, 2019. Duke Energy is
required to submit final closure plan consistent with the detailed requirements of the
CAMA which is provided under separate cover. This CAP Update has been developed
to be effective with the various closure scenarios determined for the Site.
The CAP content is in accordance with subsequent correspondence between NCDEQ
and Duke Energy, including CAP content guidance issued by NCDEQ on April 27, 2018
and adjusted on September 10, 2019. This CAP Update includes section references to
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Corrective Action Plan Update December 2019
Allen Steam Station SynTerra
the document titled Corrective Action Plan Content for Duke Energy Coal Ash Facilities
(provided in Appendix A), beneath the report section headings and within the text in
parentheses to facilitate the review process.
1.1 Background
(CAP Content Section LA)
A substantial amount of assessment data has been collected for the Allen ash basins,
which include the active ash basin (AAB) and the retired ash basin (RAB), and the
adjacent coal piles to support this CAP Update. Site assessment was performed and the
Allen Comprehensive Site Assessment (CSA) Update Report (SynTerra, 2018a) was
prepared and submitted in accordance with requirements in Subchapter 02L.0106 (g).
The CSA:
• Identified the source(s) and causes of constituent of interest (COIs) in
groundwater.
• Found no imminent hazards to public health and safety.
• Identified receptors and potential exposure pathways.
• Sufficiently determined the horizontal and vertical extent of COIs in soil and
groundwater.
• Determined the geological and hydrogeologic features influencing the
movement, chemical makeup, and physical characteristics of COIs.
NCDEQ provided review of the CSA Update to Duke Energy in a letter dated June 11,
2018 and stated the information provided sufficiently warranted preparation of this
CAP Update (Appendix A). This CAP Update builds on the previous documents to
provide a CAP for addressing the requirements in Subchapter 02L .0106 for corrective
action and the restoration of groundwater quality.
Detailed descriptions of Site operational history, the conceptual Site model (CSM),
physical setting and features, geology/hydrogeology, and findings of the CSA and other
CAMA-related work are documented in the following reports:
• Comprehensive Site Assessment Report —Allen Steam Station Ash Basin (HDR
Engineering, Inc. of the Carolinas (HDR, 2015a)
• Corrective Action Plan Part 1— Allen Steam Station Ash Basin (HDR, 2015b)
• Corrective Action Plan Part 2 (included CSA Supplement 1 as Appendix A) — Allen
Steam Station Ash Basin (HDR, 2016a)
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Corrective Action Plan Update December 2019
Allen Steam Station SynTerra
• Comprehensive Site Assessment Supplement 1— Allen Steam Station Ash Basin (HDR,
2016b)
• Comprehensive Site Assessment Supplement 2 — Allen Steam Station Ash Basin (HDR,
2016c)
• Comprehensive Site Assessment Update — Allen Steam Station (SynTerra, 2018a)
• Ash Basin Pumping Test Report — Allen Steam Station (SynTerra, 2019a)
• Surface Water Evaluation to Assess 15A NCAC 02B.0200 Compliance for
Implementation of Corrective Action Under 15A NCAC 02L.0106 W and (l) —Allen
Steam Station (SynTerra, 2019b)
• 2018 CAMA Annual Interim Monitoring Report (SynTerra, 2019c)
1.2 Purpose and Scope
(CAP Content Section LB)
The purposes of this corrective action approach are the following:
• Restore groundwater affected by the ash basins and coal piles at or beyond the
ash basin compliance boundary to the applicable groundwater standards or as
close to the standards as is economically and technically feasible, consistent with
Subchapter 02L .0106(a).
• Address response requirements contained within 15A North Carolina
Administrative Code (NCAC) 02L .0107(k) for exceedances of standards (1) in
adjoining classified groundwater, (2) presenting an imminent hazard to public
health and safety, and/or (3) in bedrock groundwater that might potentially
affect a water supply well.
• Meet the requirements for corrective action plans specified in G.S. Section 130A-
309.211(b).
The scope of the CAP and this CAP Update is defined by G.S. Section 130A-309.211,
amended by CAMA. The legislation required, among other items, assessment of
groundwater at coal combustion residual impoundments and corrective action in
conformance with the requirements of Subchapter 02L. These corrective action for
restoration of groundwater quality requirements were codified into G.S. Section 130A-
309.211, which was further amended by House Bill 630 to require a provision for
alternate water supply for receptors within 0.5-mile downgradient from the established
compliance boundary.
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Corrective Action Plan Update December 2019
Allen Steam Station SynTerra
Based on conditions and the results from the Site investigations, this CAP Update
develops and compares alternative methods for corrective action and presents the
recommended plan. This CAP Update presents a holistic, multi -component corrective
action approach for groundwater COIs associated with the ash basins and coal piles at
or beyond the compliance boundary north, northeast, and east of the ash basins and
coal piles. Design information and steps necessary for implementation are included in
the CAP Update. Once the CAP is approved by NCDEQ, implementation is planned to
begin within 30 days, as required by the G.S..
1.3 Regulatory Basis for Corrective Action
(CAP Content Section 1.0
Comprehensive groundwater assessment activities, conducted in accordance with a
Notice of Regulatory Requirements (NORR) issued to Duke Energy on August 13, 2014
by the North Carolina Department of Environment and Natural Resources (NCDENR)
(Appendix A), indicate the coal ash basins and the related contiguous units - the coal
piles - have demonstrated that constituent concentrations greater than applicable
regulatory standards are contained within the compliance boundary of the ash basins
with the exception of the areas east of the AAB and north, northeast and east of the RAB
and coal piles.
The regulatory requirements for corrective action at coal combustion residuals surface
impoundments under CAMA are in G.S. Section 130A-309.211(b), (c), and (c1). Section
(b) of G.S. Section 130A-309.211 requires that the CAP shall provide for groundwater
restoration in conformance with the requirements of Subchapter L of Chapter 2 of Title
15A of the North Carolina Administrative Code (15A NCAC Subchapter 02L). In
accordance with G.S. Section 130A-309.211(b)(1), the groundwater CAP shall include, at
a minimum, the following (CAP Content Section 1.C.a):
• A description of all exceedances of the groundwater quality standards, including
any exceedances that the owner asserts are the result of natural background
conditions
• A description of the methods for restoring groundwater in conformance with the
requirements of Subchapter L of Chapter 2 of Title 15A of the NCAC and a
detailed explanation of the reasons for selecting these methods
• Specific plans, including engineering details, for restoring groundwater quality
• A schedule for implementation of the groundwater corrective action plan
• A monitoring plan for evaluating the effectiveness of the proposed corrective
action and detecting movement of any constituent plumes
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Corrective Action Plan Update December 2019
Allen Steam Station SynTerra
• Any other information related to groundwater assessment required by NCDEQ
In addition to CAMA, requirements for CAPs are also contained in Subchapter 02L
.0106(e), (h) and (i).
Section 02L .0106(e)(4) requires implementation of an approved CAP for restoration of
groundwater quality at or beyond the compliance boundary in accordance with a
schedule established by the Secretary.
To comply with 02L .0106(h), CAPs must include (CAP Content Section 1.C.b):
• A description of the proposed corrective action and reasons for its selection
• Specific plans, including engineering details where applicable, for restoring
groundwater quality
• A schedule for the implementation and operation of the proposed plan
• A monitoring plan for evaluating the effectiveness of the proposed corrective
action and the movement of the constituent plume
This CAP Update presents an evaluation of the options available for corrective action
under Subchapter 02L .0106(j), (k), and (1).
• Under paragraph 0), corrective action would be implemented using remedial
technology for restoration of groundwater quality to the standards (02L).
• Under paragraph (k), a request for approval of a corrective action plan may be
submitted without requiring groundwater remediation to the standards (02L) if
the requirements in (k) are met.
• Under paragraph (1), a request for approval of a corrective action plan may be
submitted based on natural processes of degradation and attenuation if the
requirements in (1) are met.
This CAP Update has been prepared in general accordance with the NCDEQ guidance
document titled Corrective Action Plan Content for Duke Energy Coal Ash Facilities which
provides an outline of the technical content and format presented in the NCDEQ's letter
dated September 10, 2019, provided in Appendix A (CAP Content Section 1.C.c).
In addition to this groundwater CAP, the Allen ash basins are subject to closure
requirements under CAMA. Basin closure activities will provide source control within
the ash basin and are considered a component of the overall corrective action for the
site. The Allen ash basins meet the low -risk classification criteria set forth in CAMA for
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CCR surface impoundments. On October 12, 2018, the NCDEQ confirmed that Duke
Energy satisfactorily completed the alternate water provision under CAMA, G.S.
Section 130A-309.211(c1). On November 14, 2018, the NCDEQ confirmed that Duke
Energy rectified certain dam safety deficiencies, reclassifying the ash basins from their
prior draft ranking of "intermediate" to "low -risk". Under CAMA, a low -risk coal
combustion residuals surface impoundment may be closed by excavation, closure -in -
place, or a hybrid approach.
On April 1, 2019, the NCDEQ issued a determination that the Allen coal ash basins are
to be closed using the excavation approach (Appendix A). This decision was
subsequently appealed by Duke Energy. The CAP approach described herein can be
implemented under either scenario.
1.4 List of Considerations by the Secretary for Evaluation of
Corrective Action Plans
(CAP Content Section 1.D.a through g)
Potential targeted active remedial alternatives were developed using the criteria
included in the NCDEQ's CAP Guidance (NCDEQ, 2018). An evaluation of remedial
alternatives was performed based on the following criteria:
• Protection of human health and the environment
• Compliance with applicable federal, state, and local regulations
• Long-term effectiveness and permanence
• Reduction of toxicity, mobility, and volume
• Short-term effectiveness at minimizing impact on the environment and local
community
• Technical and logistical feasibility
• Time required to initiate
• Predicted time required to meet remediation goals
• Cost
• Community acceptance
In the evaluation of CAPs as specified in 02L .0106(i), the criteria includes:
• A consideration of the extent of any violations
• The extent of any threat to human health or safety
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• The extent of damage or potential adverse impact to the environment
• Technology available to accomplish restoration
• The potential for degradation of the constituents in the environment
• The time and costs estimated to achieve groundwater quality restoration
• The public and economic benefits to be derived from groundwater quality
restoration
These 02L .0106(i) criteria form the basis for defining the screening criteria outlined in
Section 6.6 for use in evaluating remedial alternatives in Section 6.7.
In addition, institutional controls [(provided by the restricted designation (RS)] may be
proposed by Duke Energy to limit access to groundwater use (Subchapter 02L .0104).
The RS designation may be requested for areas outside of an established compliance
boundary when groundwater may not be suitable for use as drinking water supply
without treatment. RS designation is a temporary designation and removed by the
NCDEQ Director upon a determination that the quality of the groundwater has been
restored to the applicable standards or when the groundwater has been reclassified by
the NCDEQ. NCDEQ is authorized to designate existing or potential drinking water
(Class GA groundwater) as RS where the Director has approved a CAP, or the
termination of corrective action, that will not result in the immediate restoration of such
groundwater to the standards established in 02L.
1.5 Facility Description
(CAP Content Section LE)
1.5.1 Location and History of Land Use
(CAP Content Section 1.E.a)
Allen is located on the west bank of the Catawba River on Lake Wylie in
Belmont, Gaston County, North Carolina (Figure 1-1). Allen is a five -unit coal-
fired electricity generating station with a combined capacity of 1,155 megawatts.
The station began commercial operations in 1957 with Units 1 and 2, which have
a capacity of 330 megawatts total. Unit 3 (275 megawatts) was placed into
commercial operation in 1959, followed by Unit 4 (275 megawatts) in 1960, and
Unit 5 (275 megawatts) in 1961. Cooling water for Allen is provided by the
Catawba River (Lake Wylie).
The area surrounding Allen generally consists of residential properties,
undeveloped land, and the Catawba River (Lake Wylie) (Figure 1-2).
Topography at the Site ranges from an approximate high elevation of 680 feet
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North American Vertical Datum of 1988 (NAVD 88) near the west and southwest
boundaries of the Site to an approximate low elevation of 570 feet at the shoreline
of the Catawba River (Lake Wylie). The elevation of the Catawba River (Lake
Wylie) near the Site is approximately 565 feet.
The station and supporting facilities lie within a 1,009-acre parcel owned by
Duke Energy. Duke Energy also owns property along the Discharge Canal to the
east and west of South Point Road (NC 273), as shown on Figure 1-1. In addition
to the station property, Duke Energy owns and operates the Catawba-Wateree
Project (Federal Energy Regulatory Commission [FERC] Project Number 2232).
Lake Wylie reservoir is part of the Catawba-Wateree Project and is used for
hydroelectric generation, municipal water supply, and recreation. Based on a
review of available historical aerial photography, the Site consisted of a
combination of agricultural land, rural residential, and woodlands prior to the
development of the Station. Figure 1-3 presents an aerial photograph from 1948
prior to development of the Site.
The ash basins are located south of the power block and are generally bounded
by earthen dams to the east, a natural ridge to the west, north and south, and an
earthen divider dike separating the two basins oriented east -west (Figure 1-2). A
topographic ridge that acts as a groundwater divide that affects regional
groundwater flow is located west of the Site and partially within the western
portions of the Site. South Point Road (NC Highway 273) is situated along parts
of this topographical ridge in the vicinity of the Site. The ridge curves eastward
around the basins to the north and south. Topography to the east of the ridge
generally slopes downward toward the Catawba River (Lake Wylie) to the east.
One exception is in the north central portions of the Station where topography
(including areas north of the RAB) slopes toward the discharge canal. The
discharge canal ultimately flows to the South Fork Catawba River (also known as
the South Fork River). Topography to the west of the topographic ridge generally
slopes downward toward the South Fork River to the west and, locally, toward
the discharge canal.
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1.5.2 Operations and Waste Streams Coincident with the Ash
Basins
(CAP Content Section 1.E.b)
Coal -Related Operational Storage and Waste Streams
Coincident with the Ash Basins
Coal is a highly combustible sedimentary or metamorphic rock typically dark in
coloration and present in rock strata known as coal beds or seams. Coal is
predominantly made up of carbon and other elements such as hydrogen, oxygen,
nitrogen, and sulfur as well as trace metals. The composition of coal makes it
useful as a fossil fuel for combustion processes. Coal results from the conversion
of dead vegetative matter into peat and lignite. The exact composition of coal
varies depending on the environmental and temporal factors associated with its
formation.
The historical specific coal sources used at Allen are bituminous coal from
Northern Appalachia and Central Appalachia. Coal has been delivered to Allen
through rail transportation since operations began. Coal is conveyed via transfer
belts to the station where it is pulverized before being utilized in the powerhouse
boilers.
Coal storage has historically occurred at two separate piles located immediately
south of the power block and north-northeast of the RAB. The live coal pile,
located adjacent to the Catawba River, encompasses approximately 2 acres. The
live coal pile is where coal is staged to feed the boilers within the power block.
The main coal pile is located west of the live coal pile and north and northeast of
the RAB. The main coal pile encompasses approximately 15 acres. The main coal
pile is where coal is stockpiled for longer -term storage. Both coal piles are
unlined and remain active. In 2018, a lined holding basin was built in the eastern
footprint of the main coal pile as part of a water redirect project. Collectively, the
coal piles are referred to in this CAP Update as the coal pile area.
Assessment and corrective action of the coal pile area is within the scope of this
CAP Update. The coal piles are located within the groundwater drainage area of
the ash basins and are downgradient of the RAB (Figure 1-2). Therefore, the
corrective action approach for the coal pile area is included with the corrective
action approach for the ash basins.
Coal ash and other CCR are produced as a result of coal combustion. The smaller
ash particles (fly ash) are carried upward in the flue gas and are captured by air
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pollution control devices, including an electrostatic precipitator. The larger ash
particles (bottom ash) fall to the bottom of the boiler.
Approximately 70 to 80 percent of ash produced during coal combustion is fly
ash (EPRI, 1995). Typically, 65 to 90 percent of fly ash has particle sizes that are
less than 0.010 millimeter (mm). In general, fly ash has a grain size distribution
similar to that of silt. The remaining 20 to 30 percent of ash produced is
considered bottom ash. Bottom ash consists of angular particles with a porous
surface and is normally gray to black in color. Bottom ash particle diameters can
vary from approximately 38 mm to 0.05 mm. In general, bottom ash has a grain
size distribution similar to that of fine gravel to medium sand (EPRI, 1995).
Coal "mill rejects" or "clinkers" have been observed mixed with ash within the
northern portion of the RAB. "Clinkers" or "mill rejects" are rocks that were
mixed with coal that was not combusted as part of the power generation process.
This material at Allen is typically dark gray to black, with angular to subangular
grains ranging in size from coarse sand to pebbles and often contains vugs.
Clinkers can be rich in pyrite and can cause low pH subsurface conditions.
Non -Coal -Related Operational Storage and Waste Streams
Coincident with the Ash Basins
Environmental incidents at Allen have occurred only in the vicinity of the Station
power block area. Incidents that initiated notifications to NCDEQ and
subsequent remediation under NCDEQ's Division of Waste Management mainly
consisted of petroleum or tetrachloroethene (PCE). A summary of the historical
environmental incidents at Allen is provided in Table 1-1. None of these
incidents had an effect on the ash basins and coal pile COI distribution in
groundwater because the Station power block is located in an area considered
downgradient of the ash basin or coal piles (Figure 1-2).
Beneath the powerhouse generating units within the power block, fuel oil
constituents in groundwater related to NCDEQ Incident Number 11186 may
overlap with or lie just beyond the extent of COIs related to the ash basins and
coal piles. A Notice of No Further Action (NFA) was issued by the North
Carolina Department of Waste Management on June 30, 2017 (Appendix A). The
letter indicated that groundwater in the area is not suitable for use as a water
supply. The NFA was conditional upon filing of a Notice of Residual Petroleum
with the Register of Deeds and deed restriction enforced that prohibits
groundwater for use as water supply within the area of affected groundwater. A
December 2018 Free Product Recovery Report prepared by Anchor QEA, included in
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Appendix A, provides additional detail regarding NCDEQ Incident Number
11186 (Anchor QEA, 2019). Like other releases, constituents related to the fuel oil
have been managed by Duke Energy in cooperation with NCDEQ. For Incident
Number 11186, a pump -and -treat recovery system was operated that consisted of
five recovery wells. NCDEQ approved decommissioning of the pump -and -treat
system in August 2018. Data from that recovery system was considered as part of
this CAP Update, although, remaining components of that system are not
included as part of the remedy included in this CAP Update related to the ash
basins or coal pile. However, caution would be used in the area during
implementation and operation of the remedial system associated with the ash
basins and coal piles.
No non -coal -related operations or environmental incidents (releases that
initiated notification to NCDEQ) were identified to have occurred in the vicinity
of or coincident to the source areas of the ash basins or coal piles. Therefore, no
environmental incidents at the Allen, including incident number 11186, are
relevant to this CAP Update and are not included as components of this CAP
Update.
1.5.3 Overview of Existing Permits and Special Orders by
Consent
(CAP Content Section 1.E.0
NPDES Permit
Duke Energy is authorized to discharge wastewater from the Allen ash basin to
the Catawba River (Lake Wylie) (Outfall 002) in accordance with National
Pollutant Discharge Elimination System (NPDES) Permit NC0004979, which
issued by NCDEQ on August 1, 2018. The sources of wastewater for these
outfalls include non -contact cooling water, ash basin discharge, sanitary waste,
cleansing and polishing water, low volume wastes, and storm water from
process areas.
The facility operates the following outfalls (except where subsequently noted,
descriptions of each outfall are quoted from the NPDES permit):
• Outfall 001: Once through cooling water. [Subsequent note: this outfall
discharges to the discharge canal]
• Outfall 002: Ash Basin discharge. This outfall includes domestic wastewater,
storm water from the coal pile area, miscellaneous storm water flows, ash sluice,
wastewater from turbine non-destructive testing, landfill leachate, flue gas
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desulfurization (FGD) blowdown, yard drain sump, water treatment filter
backwash, treated groundwater, laboratory wastes, and the power house sump at
Unit 5. The domestic waste is pretreated by a septic tank. Outfall 002 wastewater
is treated using chemical coagulation, settling, and pH neutralization. Outfall
002 and Outfall 006 might be operational at the same time during the transition
ep riod. [Subsequent notes: Outfall 002 will remain active until the ash basin
system is decanted and dewatered. Outfall 002 currently only receives
water that has been treated for pH within the water treatment plant.
Flows to the AAB ceased on February 9, 2019. Outfall 002 is active during
decanting and would be active during dewatering. This outfall discharges
to the Catawba River.]
• Outfall 002A: Coal yard sump overflow (discharge from coal handling and
storage areas).
• Outfall 002B: Powerhouse sump overflow (floor wash water, boiler blowdown,
water treatment waste, condensates, equipment cooling water, sealing water and
miscellaneous leakage).
• Outfall 003: Miscellaneous equipment non -contact cooling and sealing water
• Outfall 004: Miscellaneous non -contact cooling water, vehicle wash water, and
intake screen backwash
• Outfall 006: Upon completion of construction of the Retention Basin, discharge
domestic wastewater, storm water from the coal pile area, miscellaneous storm
water flows, ash sluice, wastewater from turbine non-destructive testing, landfill
leachate, FGD blowdown, yard drain sump, water treatment filter backwash,
treated groundwater, laboratory wastes, and the power house sump at Unit 5. The
domestic waste is pre-treated by a septic tank. Outfall 006 wastewater is treated
using chemical coagulation, settling, and pH neutralization. Outfall 002 and
Outfall 006 might be operational at the same time during the transition period.
[Subsequent note: Outfall 006 will remain active after the ash basin system
is decanted and dewatered.]
Outfall 007. the emergency spillway of the new Retention Basin. The spillway is
designed for a flood greater than 100-year event. Sampling of this spillway is
waived due to unsafe conditions associated with sampling during an overflow
event.
Outfall 008: the emergency spillway of the retired Ash Pond. The spillway is
designed for a flood greater than 100-year event. Sampling of this spillway is
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waived due to unsafe conditions associated with sampling during an overflow
event.
• Toe Drain Outfalls: 103 (lat. - 35°10.512 ; long. -81 °0.360'), 104 (lat. -
35°10.541; long. - 81 °0.364), 108 (lat. - 35°10.710 ; long - 81 °0.384 '), and
108B (lat. - 35°10.689 ; long - 81 °0.391'): 4 potentially contaminated toe drains.
With the exception of internal outfalls and Outfall 001, each outfall discharges
toward the Catawba River. Outfall 001 discharges to the discharge canal which
flows toward the South Fork River. Outfall 003 discharges in the vicinity of
Outfall 001.
Special Order by Consent
A Special Order by Consent (SOC) was issued to Duke Energy on April 25, 2018,
to address the elimination of seeps from Duke Energy's coal ash basins during
the separate and independent process of ash basin closure (Appendix B of
Appendix J). The SOC provided definition for constructed seeps [seeps that (1)
are on or within the dam structures and (2) convey wastewater via a pipe or
constructed channel directly to a receiving water] or non -constructed seeps
(seeps that do not meet the "constructed seep" definition). Ash basin decanting
now underway is expected to substantially reduce or eliminate the seeps.
The SOC requires Duke Energy to accelerate ash basin decanting. After
completion of decanting, remaining seeps, if not dispositioned in accordance
with the SOC, are to be characterized. After post -decanting seep
characterization, an amendment to the CAP and/or Closure Plan, may be
required to address remaining seeps. The SOC terminates 180 days after
decanting or 30 days after approval of the amended CAP. Mechanical decanting
of the AAB at Allen began on June 5, 2019. As of December 1, 2019, 53,300,000
gallons water have been removed from the AAB and the water elevation has
decreased by 14.1 feet. The SOC requires completion of decanting by June 30,
2020.
Permitted Solid Waste Facilities
The RAB Ash Landfill is under the active NCDENR Division of Water Resources
(DWR) Solid Waste Section Permit No. 3612-INDUS. The RAB Ash Landfill is a
double -lined landfill located within the footprint of the RAB, south of the power
block (Figure 1-2).
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Additional Permits
In addition to NPDES wastewater discharge permit NC0004979, the facility also
holds air permit #03757T45 and a hazardous waste permit NCD043678937 as a
RCRA small quantity generator. Duke Energy is permitted to discharge storm
water to the Catawba River (Lake Wylie) and South Fork River in accordance
with NPDES Permit NCS000546.
The facility held a Distribution of Residual Solids (DORS) Fills Permit No.
WQ0000452, which authorized placement of wastewater residuals (i.e., ash) on
land for certain beneficial uses. Permit No. WQ0000452 was rescinded on
September 2, 2015, but that did not affect the status of DORS projects that had
already been completed. The DORS fills are located within the footprint of the
RAB, west of the RAB Ash Landfill and southwest of the powerhouse (Figure 1-
2).
Erosion and sediment control (E&SC) permits are required for construction and
excavation related activities including general construction projects and
environmental assessment and remediation projects if the area of disturbance is
greater than one acre. Multiple E&SC permits have been obtained for various
projects implemented at the Station, including environmental related projects,
such as well installation and access road construction. Most of the E&SC permits
are closed as the related projects are completed. E&SC permits will continue to
be obtained prior to implementation of additional construction projects, as
appropriate.
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2.0 RESPONSE TO CSA UPDATE COMMENTS
(CAP Content Section 2)
2.1 Facility -Specific Comprehensive Site Assessment (CSA)
Comment Letter
(CAP Content Section 2.A)
On January 31, 2018, Duke Energy submitted a CSA Update to NCDEQ. In a letter from
NCDEQ to Duke Energy dated June 11, 2018, NCDEQ stated that sufficient information
had been provided in the 2018 CSA Update to allow preparation for the CAP Update.
The letter also provided a number of CSA-related comments and items required to be
addressed prior to or as part of the CAP Update submittal (Appendix A).
2.2 Duke Energy's Response to DEQ Letter
(CAP Content Section 2.B and 2.B.a)
Responses to all NCDEQ comments within the June 11, 2018 letter are summarized in
Appendix B. Additional content related to NCDEQ's comments is either included
within sections of this CAP Update or as standalone appendices to this CAP Update,
such as the groundwater modeling reports and surface water evaluation reports.
Activities that directly addressed NCDEQ comments include:
• Additional monitoring wells were installed within the shallow flow layer
beneath the retired and active ash basins to assess vertical distribution of COIs
within the footprint of the basins. Discussion of data acquired from the
monitoring wells beneath the ash basins is presented in Section 6.1.
• Groundwater samples continued to be collected on a quarterly basis as part of
the Allen Interim Monitoring Plan (IMP) after CSA Update submittal. Additional
sampling results augmented the groundwater quality database. Comprehensive
groundwater analytical data are included in Appendix C, Table 1.
• Additional groundwater and soil samples were collected in the vicinity of the
coal piles to assess COI distribution in these areas. Discussion of groundwater
and soil assessment results is presented in Sections 6.1.2 and 6.1.4 and
assessment activities are detailed in Appendix P.
• Additional soil assessment downgradient of the source areas was performed to
further delineate COI distribution. Discussion of soil assessment results is
presented in Section 6.1.4.
• Additional assessment of the Catawba River (Lake Wylie) surface water and
sediment downgradient of the Allen source areas was performed in August 2018.
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Results determined that groundwater migration from beneath the ash basins and
coal pile area has not resulted in exceedances of the NCAC, Title 15A Subchapter
02B, Surface Water and Wetland Standards (02B) in the Catawba River (Lake
Wylie). A report summarizing the sampling, results, evaluation, and conclusions
of the surface water evaluation was submitted to NCDEQ in March 2019 and is
included in Appendix J.
• An evaluation of potential groundwater migration and associated impacts to
surface water under future conditions was conducted. Based on the evaluation,
future groundwater discharge to the Catawba River from areas potentially
affected by the ash basins and coal pile area is not predicted cause COI
concentrations in surface water greater than 02B surface water standards. The
evaluation is presented in Appendix J.
• Background groundwater and soil datasets and background values were
updated to include data through December 2018. Information about background
determinations is presented in Section 4.0. Updated soil background threshold
values (BTVs) are listed on Table 4-2, and updated groundwater BTVs are listed
on Table 4-3.
• The Allen flow and transport model and geochemical model were updated to
incorporate additional assessment data and information. The additional data
helped refine the models so the models better represent current Site conditions
and predict future Site conditions. The flow and transport model report is
provided as Appendix G. The geochemical model report is provided as
Appendix H.
• The Allen CSM was updated to improve understanding of Site conditions and to
support remedy design based upon updated Site data, assessment results, and
model predictions. The updated CSM is presented in Section 5.0.
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3.0 OVERVIEW OF SOURCE AREAS BEING PROPOSED FOR
CORRECTIVE ACTION
(CAP Content Section 3)
The ash basins (RAB and AAB) are the only CAMA-regulated units at the Site. The RAB
and AAB are the primary sources of COIs to groundwater. The only secondary source
located within or adjacent to the ash basins addressed under this CAP Update are the
coal piles. Figure 1-2 shows the location of the ash basins waste boundary and the coal
piles (CAP Content Section 3.A and 3.A.a).
Included within the footprint of the RAB are two ash storage areas, a two structural
fills, the double -lined RAB Ash Landfill. Pyrite rich rocks known as "clinkers" or "mill
rejects" have also been observed to be mixed with ash within in the north-northeast
portion of the RAB. Pyrite within the clinkers has caused low pH conditions in the
subsurface within and downgradient of the north-northeast portion of the RAB.
Clinkers have not been observed outside of the RAB waste boundary. The area
containing the clinkers and areas downgradient extending toward the main coal pile are
referred to as the "low pH area". The AAB includes three ponds known as primary
ponds 1, 2, and 3. These features within the ash basins are comprised of primarily of ash
and therefore, are not considered separate sources but are considered collectively as
part of the ash basins within the corrective action approach.
Other facilities at the Site are not part of the source area addressed herein. A consensus
was reached with the NCDEQ DWR regarding sources not considered for corrective
action as part of this CAP Update was provided in a letter from NCDEQ to Duke
Energy dated April 5, 2019 (Appendix A). Brief descriptions of these facilities, their
status of inclusion or exclusion as part of the source area, and the rationale for inclusion
or exclusion is provided in the Table 3-1 (CAP Content Section 3.B). Corrective action
approach for the ash basins and coal piles is discussed in detail in Section 6.0.
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4.0 SUMMARY OF BACKGROUND DETERMINATIONS
(CAP Content Section 4)
Metals and inorganic constituents, typically associated with CCR material, are naturally
occurring and present in the Piedmont physiographic province of north -central North
Carolina. The metals and inorganic constituents can occur in soil, groundwater, surface
water, and sediment. Background analytical results are used to compare detected
constituent concentration ranges from the source area relative to native conditions.
The statistically derived background values for the site are used for screening of
assessment data collected in areas of potential migration of COIs from a source area. If
the assessment data concentrations are less than background, it is likely COI migration
has not occurred in the area. If the assessment data concentrations are greater than
background, additional lines of evidence are used to determine whether the
concentrations represent migration from a source area. Additional lines of evidence
include, but may not be limited to:
• Evaluation of whether the concentration is within the range concentrations
detected at the Site, or within the range for the region
• Evaluation of whether there is a migration mechanism through the use and
interpretation of hydraulic mapping (across multiple flow zones), flow and
transport modeling, and understanding of the CSM
• Do the concentration patterns represent a discernable plume or migration pattern
• Natural variations in Site geology or geochemical conditions between upgradient
(background locations) and downgradient area
• Are other COIs present at concentrations greater than background.
Allen and nine other Duke Energy facilities (Belews Creek Steam Station, Buck Steam
Station, Cape Fear Steam Electric Plant, Cliffside Steam Station, Dan River Steam
Station, Marshall Steam Station, Mayo Steam Electric Plant, Riverbend Steam Station,
and Roxboro Steam Electric Plant) are situated in the Piedmont physiographic province
of north -central North Carolina. The nine Duke Energy facilities are located within a
150-mile radius from Allen. Statistically derived background values from these facilities
provide a geographic regional background range for comparison. Generally
background values derived from the Piedmont facilities are similar, with some
exceptions.
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As more background data become available, the background values may be updated to
continue to refine the understanding of background conditions. However, these
multiple lines of evidence, and additional steps in the evaluation process, will continue
to be important tools to distinguish between background conditions and areas affected
by constituent migration.
Background sample locations were selected to be in areas that represent native
conditions, not affected by the coal ash basins or additional source areas. A map
showing the background locations for all media including groundwater, surface water,
soil, and sediments are shown in Figure 4-1 (CAP Content Section 4.A). Tables referenced
in this section present the background datasets for each media, statistically calculated
BTVs for soil and groundwater, and background dataset ranges for surface water and
sediment.
Background soil and groundwater locations approved by NCDEQ, as well as
statistically derived BTVs, are detailed in Sections 4.1 and 4.2. BTVs were not calculated
for surface water and sediment; however, background locations for surface water and
sediment were approved by NCDEQ as part of the evaluation of potential groundwater
to surface water impacts (Appendix J) and are detailed in Sections 4.3 and 4.4. The
background surface water and sediment samples were collected at locations at least
1,000 feet upstream of the source areas and associated permitted outfalls.
4.1 Background Concentrations for Soil
The locations of the background soil borings are shown on Figure 4-1. The soil
background dataset with the appropriate protection of groundwater (POG) preliminary
soil remediation goals (PSRGs) and BTVs are included in Appendix C, Table 4 (CAP
Content Section 4.B). Background soils samples were collected from multiple
unsaturated depth intervals (Table 4-1). All samples were collected from depth
intervals greater than one foot above the seasonal high water table. The Allen
background soil boring locations, unsaturated soil depth interval and number of
discrete samples collected from the unsaturated soil depth interval are included in
Table 4-1.
The suitability of each of these locations for evaluating background conditions was
addressed in a technical memorandum (May 26, 2017). In a response letter dated July 7,
2017, NCDEQ approved use of the soil data for determination of BTVs (Appendix A).
Soil BTVs were calculated using data from background unsaturated soil samples
collected February 2015 to August 2017 and in accordance with the Revised Statistical
Methods for Developing Reference Background Concentrations for Groundwater and Soil at
Coal Ash Facilities (HDR and SynTerra, 2017).
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Calculated soil BTVs were submitted to NCDEQ in the Comprehensive Site Assessment
Update — Allen Steam Station, dated January 31, 2018. NCDEQ DWR provided comments
and approval of BTVs in a response letter dated May 14, 2018 (Appendix A). Soil BTVs
for Allen were updated in 2019 and are provided, along with the original soil BTVs
from 2018 for comparison and North Carolina Piedmont soil BTV ranges for
comparison, in Table 4-2 (CAP Content Section 4.B).
The updated BTVs were calculated using data from background unsaturated soil
samples collected February 2015 to August 2017 but the 2019 dataset retained extreme
outlier concentrations when data validation and geochemical analysis of background
groundwater concentrations indicated that those outlying concentrations did not result
from sampling error or laboratory analytical error. The approach used to evaluate
whether extreme outlier concentrations should be retained in background soil datasets
is presented the technical memorandum prepared by Arcadis titled, "Background
Threshold Value Statistical Outlier Evaluation — Allen, Belews Creek, Cliffside, Marshall, Mayo,
and Roxboro Sites" which was included as an attachment to the Updated Background
Threshold Values for Constituent Concentrations in Groundwater (SynTerra, 2019d). The
updated BTVs were calculated in accordance with the Revised Statistical Methods for
Developing Reference Background Concentrations for Groundwater and Soil at Coal Ash
Facilities (HDR and SynTerra, 2017).
4.2 Background Concentrations for Groundwater
The groundwater system beneath the Site is divided into the following three layers to
distinguish the interconnected groundwater system: the shallow flow layer, deep
(transition zone) flow layer, and the bedrock flow layer. The Allen flow zones and
background groundwater monitoring wells installed within each flow layer include:
• Shallow flow zone: AB-12S, BG-1S, BG-2S, BG-3S, BG-4S, CCR-BG-1S, GWA-16S,
GWA-19S, GWA-21S, GWA-23S, GWA-26S
• Deep flow zone: AB-12D, BG-1DA, BG-2D, BG-3D, BG-4D, CCR-BG-IDA, GWA-
16D, GWA-19D, GWA-21DA, GWA-23D, GWA-26D
• Bedrock flow zone: BG-1BR, BG-2BRA-2, BG-4BR, GWA-21BR
The locations of the background monitoring wells are shown on Figure 4-1. The
groundwater background dataset with the appropriate 02L standards, IMAC, and BTVs
is included in Appendix C, Table 1 (CAP Content Section 4.C). The suitability of each of
these locations for background purposes was evaluated in the Updated Background
Threshold technical memorandum (May 26, 2017). Identified groundwater data
appropriate for inclusion in the statistical analysis to determine BTVs was approved by
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Allen Steam Station SynTerra
NCDEQ in a response letter dated July 7, 2017 (Appendix A). NCDEQ DWR provided
further comments and approval of BTVs in a response letter dated October 11, 2017,
provided in Appendix A.
Groundwater BTVs for each groundwater flow zone at Allen were updated in 2019 with
the inclusion of five additional background monitoring wells (BG-1BR, CCR-BG-1S,
CCR-BG-IDA, GWA-19D, and GWA-23D) and are provided, along with the original
2018 groundwater BTVs and North Carolina Piedmont groundwater BTV ranges for
comparison, in Table 4-3 (CAP Content Section 4.C).
The updated background datasets were calculated using concentration data from
background groundwater samples collected from March 2011 to December 2018.
Background values were calculated in accordance with the Revised Statistical Methods for
Developing Reference Background Concentrations for Groundwater and Soil at Coal Ash
Facilities (HDR and SynTerra, 2017). The updated background datasets for each flow
system used to statistically assess naturally occurring concentrations of inorganic
constituents in groundwater are presented in the report Updated Background Threshold
Values for Constituent Concentrations in Groundwater (SynTerra, 2019d) provided to
NCDEQ on June 13, 2019. The updated background dataset for each hydrogeologic
flow zone consists of an aggregate of total (non -filtered) concentration data pooled
across background monitoring wells installed within that flow layer. The use of
groundwater BTVs is currently under appeal.
4.3 Background Concentrations for Surface Water
Background surface water sample locations collected from the Catawba River (Lake
Wylie) are located upstream, or outside potential groundwater impact from the source
area to surface water. Surface water background sample locations are outside of future
groundwater to surface water migration pathways as determined by groundwater
predictive modeling results (Appendix J).
Background surface water sample locations include five locations from the Catawba
River (Lake Wylie), J_2_UP, SW-BG-01, SW-BG-02, SW-BG-03, and SW-U1. Background
surface water sample locations are located upgradient of potential groundwater
influence from the ash basins and other potential source areas, as well as NPDES
Outfall 002. Background surface water sample locations are shown on Figure 4-1.
Background surface water data are used for general comparative purposes. The
analytical results provide a comparative range of naturally occurring constituent
concentrations present at background locations. Background surface water analytical
dataset ranges compared to 02B and United States Environmental Protection Agency
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(USEPA) criteria are included in Table 4-4 (CAP Content Section 4.D). The surface water
background dataset with the appropriate 02B standards is included in Appendix C,
Table 2 (CAP Content Section 4.D).
Background data sets include at most five samples collected from each location. Surface
water samples from background locations have been collected in accordance with
NCDEQ guidance as part of periodic sampling events, which include the
comprehensive sampling event in August 2018 used to assess surface water compliance
for implementation of corrective action under Subchapter 02L .0106 (k) and (1).
Analytical results from background surface water sample locations indicate COI
concentrations are less than 02B standards.
4.4 Background Concentrations for Sediment
All background sediment sample locations are co -located with background surface
water sample locations in the Catawba River (Lake Wylie). Background sediment
sample locations are located upstream, or outside potential groundwater migration
from source areas to sediment. Sediment background sample locations remain outside
of future migration areas as determined by groundwater predictive modeling.
Background sediment sample locations from the Catawba River (Lake Wylie) shoreline
include SW-BG-01, SW-BG-02, and SW-BG-03. Background sediment sample locations
are shown on Figure 4-1.
Background sediment data are used for general comparative purposes. The analytical
results provide a comparative range of naturally occurring constituent concentrations
present at background locations. Background sediment analytical dataset ranges are
presented in Table 4-5 (CAP Content Section 4.E). The sediment background dataset
with the appropriate 02B standards is included in Appendix C, Table 5 (CAP Content
Section 4.E).
Background data sets include one sample collected from each location. Sediment
samples were collected concurrently with a background surface water sample.
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5.0 CONCEPTUAL SITE MODEL
(CAP Content Section 5)
The conceptual site model is a descriptive and illustrative representation of the
hydrogeologic conditions and COI interactions specific to the Site. The purpose of the
CSM pertaining to the Allen ash basins and coal pile area is to provide a current
understanding of the distribution of constituents with regard to the Site -specific
geology/hydrogeology and geochemical processes that control the transport and
potential presence of COIs in various media. This information is also considered with
respect to exposure pathways to potential human and ecological receptors.
The CSM is presented in this section is based on a USEPA document titled
Environmental Cleanup Best Management Practices: Effective Use of the Project Life Cycle
Conceptual Site Model (USEPA, 2011). That document describes six CSM stages for an
environmental project life cycle and is an iterative tool to assist the decision process for
characterization and remediation during the life cycle of a project as new data become
available. The six CSM stages for an environmental project life cycle are described
below:
1. Preliminary CSM Stage - Site representation based on existing data; conducted
prior to systematic planning efforts.
2. Baseline CSM Stage - Site representation used to gain stakeholder consensus or
disagreement, identifies data gaps and uncertainties; conducted as part of the
systematic planning process.
3. Characterization CSM Stage - Continual updating of the CSM as new data or
information is received during investigations; supports remedy decision making.
4. Design CSM Stage - Targeted updating of the CSM to support remedy design.
5. Remediation/Mitigation CSM Stage - Continual updating of the CSM during
remedy implementation; and providing the basis for demonstrating the
attainment of cleanup objectives.
6. Post Remedy CSM Stage - The CSM at this stage is used to support reuse
planning and placement of institutional controls if warranted.
The current Allen CSM is consistent with Stage 4, "Design CSM", which allows for
iterative improvement of the Site CSM during design of the remedy while supporting
development of remedy design basis (USEPA, 2011). A three-dimensional depiction of
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Allen Steam Station SynTerra
the CSM under conditions prior to decanting and basin closure is presented as Figure 5-
1.
Anticipated changes to Site conditions, such as with decanting and basin closure, have
been incorporated into the CSM based on groundwater modeling simulations.
Predicted and observed effects will be compared on an ongoing basis to further refine
the CSM.
5.1 Site Geologic and Hydrogeologic Setting
(CAP Content Section 5.A.a)
5.1.1 Site Geologic Setting
(CAP Content Section 5.A.a)
The groundwater system at the ash basins and coal piles is divided into the
following three flow zones to distinguish the interconnected groundwater
system: the shallow flow zone, deep (transition zone) flow zone, and the bedrock
flow zone. The following is a summary of the natural hydrostratigraphic unit
assessment observations:
Shallow flow zone: Shallow soil includes fill, alluvium, regolith, and
saprolite. Fill material was used in the construction of the ash basin dams
and dikes and generally consists of reworked silts, clays, and sands.
Alluvium consists primarily of gravel with clay and sand, sand with
gravel, and silt. The regolith or residuum is in -place weathered soil that
consists primarily of silt with sand, clayey sand, sandy clay, clay with
gravel, and clayey silts. Saprolite is soil developed by in -place weathering
of rock that retains remnant bedrock structure (such as a planar fabric
associated with relic foliation). Saprolite consists primarily of medium
dense to very dense silty sand, sand silt, sand, sand with gravel, sand with
clay, clay with sand, and clay. Sand particle size ranges from fine to
coarse -grained. Much of the saprolite is micaceous. Shallow zone material
is present across the Site, including background locations. The vertical
thickness of shallow zone material observed ranged from approximately
15 feet to more than 130 feet. Shallow flow layer wells are typically
labeled with an "S," "SA," "PWS," or "SS" designation, although there are
some exceptions where "S" wells are screened in ash.
• Deep flow zone: The deep flow zone (transition zone) consists of a
relatively transmissive zone of partially weathered bedrock encountered
below the shallow zone. Observations of core recovered from this zone
included rock fragments, unconsolidated material, and highly oxidized
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bedrock material. The transition zone thickness ranges from
approximately 0 feet to 15 feet. Deep flow layer wells are typically labeled
with a "D," "DA," or "BRU" designation.
• Bedrock flow zone: Bedrock is defined as lithified solid rock, based on
sample recovery and/or drilling resistance, that is generally slightly
weathered to unweathered and fractured to varying degrees. The primary
types of rock at the Site include quartz -diorite and hornblende andesite
(often referred to as Diabase in boring logs and previous reports)
(Appendix F). Quartz -diorite, the predominant rock type at the Site, is
very light gray to dark gray, fine- to coarse -grained, non -foliated and
massive to foliated. Hornblende andesite is greenish black to very dark
greenish gray, is mostly non -foliated, and is noted as aphanitic to fine-
grained, although it is described as fine- to course -grained in some boring
logs. The principal minerals are plagioclase, quartz, biotite, and amphibole
(Appendix F, Attachment D). Groundwater movement in the bedrock
flow zone occurs in secondary porosity represented by fractures. Based on
the orientations of lineaments and open bedrock fractures near the ash
basins and coal pile area at Allen, horizontal groundwater flow within the
bedrock should occur approximately parallel to the hydraulic gradient,
with no preferential flow direction (i.e., no expected, significant
anisotropy). Allen bedrock fracture orientation and flow profile
characterization data sets support overall fracturing and fracture aperture
decreases with increasing depth, and a general decline in hydraulic
conductivity with increasing depth below the top of bedrock (Appendix
F). Groundwater flow in bedrock fractures is anisotropic and difficult to
predict, and velocities change as groundwater moves between fractures of
varying orientations, gradients, pressure, and size. Bedrock wells are
typically labeled with a "BR", "BRA", "BRL" or "BRLL" designation.
However, a few wells (GWA-5D, CCR-16D and CCR-17D) were identified
with a "D" designation that were initially characterized as deep zone
wells were reevaluated and reclassified as bedrock wells. A detailed
evaluation of bedrock conditions is located in Appendix F (CAP Content
Section 5.A.a.iv).
5.1.2 Site Hydrogeologic Setting
(CAP Content Section 5.A.a)
The groundwater system in the natural materials (shallow/deep/bedrock) at
Allen is consistent with the regolith-fractured rock system and is characterized as
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Allen Steam Station SynTerra
an unconfined, interconnected groundwater
system indicative of the Piedmont
physiographic province.
A conceptual model of groundwater flow in
the Piedmont, which applies to the Site, was
developed by LeGrand (1988, 1989) and Daniel
and Dahlen (2002) (Figure 5-2). The model
assumes a regolith and bedrock drainage basin
with a perennial stream. The model describes
conditions before ash basin construction, but
the general groundwater flow directions are
still relevant under current conditions.
Groundwater is recharged by rainfall
infiltration in the upland areas followed by
discharge to the perennial stream. Flow in the
FIGURE 5-2
LEGRAND SLOPE
AQUIFER SYSTEM
aLooc Avwfm 13wmday m.�Tau�ml.c lhmde
4'�w�+h lfm,�n LC.7 b=on Jnr
................ W, Tyh6,
. .... Frecwree
C- d-Floe Ukerbon
regolith follows porous media principals, while flow in bedrock occurs in
fractures. Rarely does groundwater move beneath a perennial stream to another
more distant stream or across drainage divides (LeGrand 1989).
Topographic drainage divides represent natural groundwater divides within the
slope -aquifer system. The areas between the topographic divides are flow
compartments that are open-ended down slope. Compartmented groundwater
flow, applicable to the Allen ash basin and coal pile areas, is described in detail
in A Master Conceptual Model for Hydrogeological Site Characterization in the
Piedmont and Mountain Region of North Carolina (LeGrand, 2004).
5.1.2.1 Groundwater Flow Direction
(CAP Guidance Section 5.A.a.i)
A groundwater divide located primarily west of the Site and roughly
follows topography along South Point Rd and topographic highs north of
the RAB along Plant Allen Road to the discharge canal and south of the
AAB, near Reese Wilson Road. Groundwater on the basin side of the ridge
flows east toward the basins and the Catawba River (Lake Wylie), while
groundwater on the west side of the ridge flows west, away from the basins
toward the South Fork River or the discharge canal. The hydraulic divides
provides natural hydraulic control of ash basin constituent migration within
the stream valley system. Groundwater flow and transport modeling
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indicates that ash basin decanting and basin closure will make the
groundwater divide more pronounced to the west of the Site (Appendix G).
The ash basins were constructed within former perennial stream valleys.
The ash basin's physical settings are horizontal flow -through water systems
characterized by groundwater movement into the upgradient ends, flowing
generally east through the middle regions, and downward near the dams
and dikes (Figure 5-3). Near the dams, vertical hydraulic gradients,
imposed by hydraulic pressure of water impounded within the basins,
promote downward vertical gradients in the groundwater system. Beyond
the dams, groundwater flows upward. Generally, the physical setting of the
ash basins within former perennial stream valleys limit the horizontal and
vertical migration of constituents to areas near and directly downgradient
of the basin dams. The primary flow path of the groundwater remains in
the stream valley systems that encompass the ash basins. Therefore, areas
upgradient and side -gradient of the basins have groundwater divides that
limit groundwater flow in these directions.
FIGURE 5-3
GENERALIZED PROFILE OF ASH BASIN PRE -DECANTING
FLOW CONDITIONS IN THE PIEDMONT
y PRECIPITATION
RUNOFF EARTHEN
DAM
AM
FLOW h
HEAD
CHANGE
7GROUNDWATERFLOWFERING BASIN
(FORMER STREAM CHANNEL] ­41
FLOW/SEEPAGE BENEATH DAM
Note:
Drawing is not to scale
Water level maps for each groundwater flow zone were constructed from
pre -decanting groundwater elevations, obtained in March 2019 (Figures 5-
4a, 5-4b, and 5-4c). March 2019 water level measurements and elevations
are presented in Table 5-1a. General groundwater flow directions can be
inferred from the water -level contours. The groundwater flow direction for
each flow zone associated with the basins and coal piles is generally from
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Correction Action Plan Update December 2019
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west to east toward the Catawba River (Lake Wylie) along with a small
component that flows north toward the discharge canal. Figures 5-4e, 5-4d
and 5-4f present water level maps focusing on the low pH area and coal pile
area based on water levels from wells installed in late 2019 and
supplemented with existing wells in the vicinity of these areas (Table 5-1b).
Predictive flow and transport model simulations indicate that the cessation
of sluicing and subsequent decanting in the AAB will reduce the potential
for COI transport prior to complete closure of the basins. Model simulations
predict downward migration of groundwater below the dams and dikes
will be limited without the presence of ponded water in the AAB.
The following are conclusions from the groundwater modeling results and
empirical data pertaining to groundwater flow beneath the Site:
• Horizontal groundwater flow velocities in areas with free ponded
water within the AAB are less than those seen upgradient of the ash
basins and below the ash basin dams.
Downward vertical gradients occur just upstream of the ash basin
dams.
• Upward vertical gradients occur beyond or downstream of the dams,
which is the main groundwater discharge zone toward the Catawba
River (Lake Wylie).
Empirical Site data from over 30 monitoring events over multiple seasonal
variations and groundwater flow and transport modeling simulations
support groundwater flow is away from water supply wells and that there
are no exposure pathways between the source areas and the pumping wells
used for water supply in the vicinity of the Site. Domestic and public water
supply wells now connected to water supply from the City of Belmont or
connected to a filtration system are outside, or upgradient of the
groundwater flow system containing the ash basins and coal piles.
Domestic and public water supply wells are not affected by constituents
released from the ash basins and coal piles or by the different closure
scenarios, according to groundwater flow and transport model simulations.
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5.1.2.2 Groundwater Seepage Velocities
(CAP Content Section 5.A.a.i)
Groundwater seepage velocities were calculated for current conditions
using horizontal hydraulic gradients determined from measurements
collected in March 2019 (Table 5-1a). Hydraulic conductivity and effective
porosity (ne) values were taken from the updated flow and transport model
(Appendix G). Calibrated hydraulic conductivity and porosity values for
each flow zone were used to align velocity calculations with model
predictions.
The flow and transport model provided subdivided hydraulic conductivity
zones and a calibrated hydraulic conductivity (K) for each zone and model
flow layer. Simulated hydraulic conductivity values range from 0.01 to 115
feet per day (feet/day) for the shallow flow zone, from 0.04 to 280 feet/day
for the deep flow zone, and from 0.00001 to 11 feet/day for the bedrock flow
zone. Hydraulic conductivity values used in calculating seepage velocity
were selected based on area's location within or proximity to subdivided
hydraulic conductivity zones. The flow and transport model uses estimated
effective porosity values of 20 percent for the shallow flow zone, 10 percent
for the deep flow zone, and 5 percent for the bedrock flow zone (Appendix
G).
The horizontal groundwater seepage flow velocity (vs) can be estimated
using a modified form of the Darcy Equation:
K dh
17S = ne (dl)
Using the March 2019 groundwater elevation data, the horizontal
groundwater flow velocity in the vicinity of the AAB was:
• 0.08 ft/day [(27.9 feet per year (ft/yr)] in the shallow flow zone
• 0.15 ft/day (56.0 ft/yr) in the deep flow zone
• 0.04 ft/day (15.0 ft/yr) in the bedrock flow zone
Using the March 2019 groundwater elevation data, the horizontal
groundwater flow velocity in the vicinity of the RAB was:
• 0.07 ft/day (24.6 ft/yr) in the shallow flow zone
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• 0.09 ft/day (34.2 ft/yr) in the deep flow zone
• 0.02 ft/day (7.5 ft/yr) in the bedrock flow zone
Groundwater modeling predicts groundwater elevation changes associated
with closure activities will change flow velocities and result in a more
pronounced groundwater divide upgradient of the ash basins. As of
December 1, 2019, 53,300,000 gallons of water have been removed from the
AAB. The water elevation in the AAB has decreased by 14.1 feet in response
to decanting, indicating significant water level changes in the AAB have
already occurred. For visualization, velocity vector maps of groundwater
under pre -decanting and future conditions were developed. The pre -
decanting conditions map was created from comprehensive Site data
incorporated into the calibrated flow and transport model. The closure
condition maps were created using predicted flow fields for the closure -by -
excavation or closure -in -place scenarios. Saturated conditions in the shallow
zone are relatively constant across the Site; therefore, the shallow flow zone
was selected for the velocity vector maps.
• Velocity vector map for groundwater in the shallow flow zone under
pre -decanting conditions - Figure 5-5a
• Velocity vector map for groundwater in the shallow flow zone under
Closure -By -Excavation - Figure 5-5b
• Velocity vector map for groundwater in the shallow flow zone under
Closure -In -Place scenario - Figure 5-5c
These depictions illustrate potential future changes in groundwater flow
compared with pre -decanting groundwater flow throughout the Site. Key
conclusions from the predictive model simulation of future ash basin
closure conditions include:
• Hydraulic heads decline.
• Small streams are predicted to return to the former perennial stream
channels beneath both the RAB and AAB.
• Groundwater flow patterns change in response to the newly
developed drainage system that includes the reformed stream
channels. Flow directions within the basins are more prominently
eastward compared to pre -decanting conditions.
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• The groundwater divide shifts westward compared to pre -decanting
conditions. Specifically in the AAB, the groundwater divide moves
approximately 1,000 feet to the west and groundwater flow
directions are eastward across the compliance boundary, toward the
Site, along the western portion of the AAB.
• East of the AAB, velocity vectors under pre -decanting conditions
indicate groundwater velocity is greatest (1-5 ft/day) beneath and
immediately downstream of the dam and flows predominantly east
(Figure 5-5a). Under the closure -by -excavation scenario, the velocity
vectors in that area are greatly reduced (0.1-0.2 ft/day). In the
southern portion of the AAB, the velocity vector directions turn
inward within the AAB, simulating the natural funneling system of
the historical stream valley (Figure 5-5b).
• Velocity vector depictions for current and future conditions indicate
that groundwater flow from the ash basin does not flow in the
direction of residential areas and water supply wells to the west. The
exception is where a few arrows indicate eastward flow from an area
of ponded water in the southwest tip of the AAB which receives
storm water runoff from areas upgradient of the AAB.
5.1.2.3 Hydraulic Gradients
(CAP Content Section 5.A.a.i)
Horizontal hydraulic gradients at the Site were calculated from water levels
collected from various wells located in the vicinity of the source areas. The
water level elevations collected in March 2019 are summarized in Table 5-
la. Based on hydraulic gradient calculations using March 2019 groundwater
elevation data, the average horizontal hydraulic gradients [measured in feet
per foot (ft/ft)] for each flow zone along flow paths are: 0.02 ft/ft (shallow
flow zone), 0.02 ft/ft (deep flow zone), and 0.02 ft/ft (bedrock flow zone).
The flow paths used are roughly depicted by generalized cross -sections B-
B', C-C', and E-E' (Figures 6-3, 6-4, and 6-6). Hydraulic gradients are
circum-neutral and relatively flat across large areas of the ash basins due to
the influence of standing water.
Vertical hydraulic gradients were calculated at clustered wells from the
water level data and the midpoint elevations of the well screens. Within the
AAB, a small upward vertical gradient was observed between the ash pore
water and the deep flow zone at well cluster AB-20S/D (-0.01 ft/ft). Farther
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to the east, a small upward vertical gradient was observed between the ash
pore water and the deep flow zone at well cluster AB-21S/D (0.00 ft/ft). At
the AAB dam, upward gradients were observed at AB-22S/D (-0.07 ft/ft),
AB-22D/BR (401 ft/ft), and AB-22BR/BRL (412). A downward vertical
gradient is indicated in the shallow, deep, and bedrock flow zones on the
upstream side of the ash basin dams based on the groundwater flow and
transport modeling results, which are supported by data from the 234
monitoring wells present at Allen.
Downgradient of the ash basin dams, upward or approximately neutral
gradients are observed at well cluster GWA-3 [GWA-3S/D (-0.01 ft/ft),
GWA-3D/BRA (0.00 ft/ft)]. Artesian conditions are observed in bedrock well
GWA-3BRL. The upward component of groundwater flow to the
groundwater discharge zone minimizes the horizontal extent of constituent
migration downgradient of the ash basins.
Exceptions regarding the CSM flow -through system exist near earthen dikes
found within the basins. Earthen dikes are present in the AAB (separating
the primary ponds), the RAB (separating the RAB from the coal pile area),
and separating the AAB from the RAB. The earthen dikes have a similar
effect on hydraulic heads as the dams do, forcing flow downward rather
than flowing laterally within the basins.
5.1.2.4 Particle Tracking Results
(CAP Content Section 5.A.a.ii)
Particle tracking is not available for Allen.
5.1.2.5 Subsurface Heterogeneities
(CAP Content Section 5.A.a.iii)
The nature of groundwater flow across the Site is based on the character
and configuration of the ash basins relative to specific Site features such as
manmade and natural drainage features, engineered drains, streams, and
lakes, hydraulic boundary conditions, and subsurface media properties.
Natural subsurface heterogeneities at the Site are represented by three flow
zones that distinguish the interconnected groundwater system: the shallow
flow zone, deep flow zone, and the bedrock flow zone. The shallow flow
zone is composed of residual soil/saprolite. Typically, the residual
soil/saprolite is partially saturated and the water table fluctuates within it.
Water movement is generally preferential through the weathered/fractured
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and fractured bedrock of the transition zone where permeability and
seepage velocity is enhanced. Groundwater within the Site area exists
under unconfined, or water table, conditions within the saprolite, transition
zone and in fractures and joints of the underlying bedrock. The saprolite,
where saturated thickness is sufficient, acts as a reservoir for supplying
groundwater to the fractures and joints in the bedrock. The shallow water
table and upper bedrock water -bearing zones are typically interconnected.
However, artesian conditions were observed within the deep/lower bedrock
at GWA-3BRL and indicating some fracture zones are not directly
interconnected with overlying material. At CP-2, no fractures were
observed in bedrock that would yield sufficient water for monitoring.
NORR CSA guidance requires a "site map showing location of subsurface
structures (e.g., sewers, utility lines, conduits, basements, septic tanks, drain
fields, etc.) within a minimum of 1,500 feet of the known extent of
contamination" in order to evaluate the potential for preferential pathways.
Identification of piping near and around the ash basins was conducted by
Stantec in 2014 and 2015, and utilities at the Site were also included on a
2015 topographic map by WSP USA, Inc. (SynTerra, 2018a).
Based on groundwater flow direction at the Site and identified subsurface
underground utilities present at the site, there are no potential preferential
pathways for constituent migration through underground utility corridors.
5.1.2.6 Bedrock Matrix Diffusion and Flow
(CAP Content Section 5.A.a.iv)
Matrix Diffusion Principles
When solute plumes migrate through fractures, a solute concentration
gradient occurs between the plume within the fracture versus the initially
clean groundwater in the unfractured bedrock matrix next to the fracture. If
the matrix has pore spaces connected to the fracture, a portion of the solute
mass will move by molecular diffusion from the fracture into the matrix.
This mass is therefore removed, at least temporarily, from the flow regime
in the open fracture. This effect is known as matrix diffusion (Freeze and
Cherry 1979). When the plume concentrations later decline in the fractures
(e.g., during plume attenuation and/or remediation), the concentration
gradient reverses and solute mass that has diffused into the matrix begins to
diffuse back out into the fractures. This effect is sometimes referred to as
reverse diffusion.
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Matrix diffusion causes the bulk mass of the advancing solute plume in the
fracture to advance slower than would occur in the absence of mass transfer
into the matrix. This effect retards the advance of any solute, including
relatively non -reactive solutes like boron, sulfate, and total dissolved solids
(TDS). The magnitude of plume retardation increases with increasing
plume length, because longer plumes have more contact for diffusion to
transfer solute mass from the fracture to the matrix (Lipson et al, 2005). The
magnitude of plume retardation also increases with increasing matrix
porosity.
If the solute sorbs to solids, the retarding effect increases. Sorption of
solutes that have diffused into the matrix within the matrix occurs on a
much larger surface area than would be the case if the solute mass remained
entirely within the fracture. The combined effect of adsorption on the
fracture surface and adsorption in the matrix further enhances plume
retardation relative to the advance that would occur in the absence of
adsorption. If sorption is reversible, when reverse diffusion occurs the
sorbed mass can desorb and transfer back into the aqueous phase and
diffuse back to the fractures. Solute mass that has been converted into stable
mineral species would not undergo desorption.
Site -Specific Data Pertaining to Matrix Diffusion
Overall, the bedrock hydraulic conductivity and calculated fracture
apertures decrease with increasing depth below the top of rock (Appendix
F). The observed decline in bedrock hydraulic conductivity and hydraulic
aperture with increasing depth is consistent with expectations based on the
literature (Gale, 1982, Neretnieks, 1985 and Snow, 1968), and indicates that
the overall volumetric rate of groundwater flow in the bedrock decreases
with depth (Appendix F).
The available data do not indicate any predominant bedrock fracture sets at
Allen. Overall, a wide range of open fracture dip angles and dip directions
is observed. Based on the orientations of lineaments and open bedrock
fractures, horizontal groundwater flow within the bedrock should occur
approximately parallel to the hydraulic gradient, with no preferential flow
direction (i.e., no expected, significant anisotropy) (Appendix F).
Consistent with this interpretation, the current groundwater flow model for
Allen does not simulate plan -view anisotropy.
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Rock core samples from bedrock locations which represent areas of affected
groundwater migration, north, northeast and east of the ash basins and coal
pile area, and are interpreted to coincide with zones of preferential
groundwater flow were analyzed for porosity, bulk density and thin section
petrography.
The reported matrix porosity values ranged from 0.49 percent to 5.16
percent with an average of 1.88 percent. Bulk density ranged from 2.65
grams per cubic centimeter (g/cm3) to 2.95 g/cm3 with an average of 2.78
g/cm3 (Appendix F). Petrographic evaluation classified all seven samples as
igneous rocks. Five samples are intrusive and two are extrusive. The
plutonic igneous rocks are classified as quartz diorite and tonalite, while the
volcanic igneous rocks are classified as hornblende andesite. The principal
minerals are plagioclase, quartz, biotite, and amphibole. Accessory
minerals include K-feldspar, epidote, pyrite, magnetite, apatite, and sphene.
Many plagioclase crystals have been altered into sericite/illitic clays. Biotite
and amphibole crystals are locally altered into chlorite (Appendix F).
The reported matrix porosity values are within the range of those reported
for crystalline rocks in the literature (Freeze and Cherry, 1979; L6fgren,
2004; Zhou and others, 2008; Ademeso and others, 2012). The presence of
measurable matrix porosity suggests that matrix diffusion contributes to
plume retardation at the Site (Lipson and others, 2005). In addition, the
identification of sericite (a mixture of muscovite, illite, or paragonite
produced by hydrothermal alteration of feldspars) in all of the samples
indicates the bedrock has some capacity to sorb boron and other elements
associated with coal ash. The influences of matrix diffusion and sorption are
implicitly included in the groundwater fate and transport model as a
component of the specific constituent partition coefficient (Ka) term used for
the bedrock layers model.
5.1.2.7 Onsite and Offsite Pumping Influences
(CAP Content Section 5.A.a.v)
No on -Site pumping of groundwater occurs at the Site. Water used at the
Site is derived from the Catawba River (Lake Wylie). Decanting was
initiated on June 6, 2019. As of December 1, 2019, 53,300,000 gallons of
water have been removed from the AAB and the water elevation has
decreased by 14.1 feet.
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Because much of the area surrounding the ash basin is either residential
properties or undeveloped land, potential off -Site pumping influences
would be limited to domestic and public water supply wells. Water supply
wells are outside, upgradient, or side -gradient of the groundwater flow
system containing the ash basins and coal piles. Flow and transport
modeling indicated private water wells within the model area remove only
a small amount of water from the overall hydrologic system (Appendix G).
5.1.2.8 Ash Basin Groundwater Water Balance
(CAP Content Section 5.A.a.vi)
The groundwater system flows from a groundwater divide toward the
Catawba River (Lake Wylie). The groundwater divide is primarily west of
the Site and roughly follows topography along South Point Rd and
topographic highs north of the RAB along Plant Allen Road and south of
the AAB, near Reese Wilson Road. The location of the groundwater divides
defining the edge of the watershed change due to decanting and closure
activities because of changing hydraulic conditions. The flow and transport
model was used to evaluate the ash basins hydraulic conditions prior to
decanting, post -decanting, and post -closure (both closure -in -place and
closure -by -excavation). Each scenario water balance was developed using
the results from the flow and transport model under current and predicted
groundwater simulations (Appendix G). The approximate groundwater
flow budget in the ash basin watershed under each simulated scenario is
summarized in Table 5-2.
Pre -Decanting Conditions Groundwater Water Balance
Under pre -decanting conditions, the watershed area that contributes
groundwater flow toward the basins is estimated at approximately 429
acres.
• Groundwater recharge from the 429-acre watershed is estimated to
be 138 gallons per minute (gpm).
Groundwater recharge from the ash basin ponds is estimated to be
428 gpm, and is the primary water balance component for
groundwater recharge under pre -decanting conditions.
• Groundwater recharge and discharge associated with wells and
septic returns from outside the ash basins are estimated to be 12 gpm
each.
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• Groundwater flow to drainages inside and outside of the ash basins
are estimated to be 66 gpm.
• Groundwater that flows through and immediately under the dam,
toward the southeast of the active ash basin, and north to the
discharge canal and coal pile area is estimated to be a total of 340
gpm. The majority of this total is derived from the flow through and
immediately under the dam (estimated 293 gpm).
Post -Decanting Conditions Groundwater Water Balance
The flow and transport model (Appendix G) was used to evaluate the ash
basins and coal piles hydraulic conditions that would occur after decanting
of the AAB. A water balance was developed for the simulated groundwater
system under post -decanting conditions.
The extent of the watershed and location of groundwater divides during
decanting is expected to remain the same as under pre -decanting
conditions. Under simulated post -decanting conditions, the watershed area
contributing flow towards the basins is estimated at approximately 429
acres.
Groundwater recharge from the watershed recharge area is
estimated to be 172 gpm. This includes 60 gpm from outside of the
ash basins and 112 gpm from the ash basins.
• With minimal changes between pre- and post -decanting conditions,
groundwater recharge associated with wells and septic returns from
outside the ash basins is estimated to be 12 gpm. Discharge
associated with wells and septic returns from outside the ash basins
is estimated to be 13 gpm.
• Groundwater flow to drainages inside and outside of the ash basins
are estimated to be 29 gpm.
• Groundwater that flows through and immediately under the dam,
toward the southeast of the active ash basin, and north to the
discharge canal and coal pile area is estimated to be a total of 137
gpm. The majority of this total is derived from the flow through and
immediately under the dam (estimated 110 gpm).
Decanting the AAB has a large impact on the water balance, reducing the
total groundwater flow through and under the dam to the east by more
than 180 gpm compared to pre -decanting conditions.
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Post -Closure Conditions Groundwater Water Balances
The flow and transport model (Appendix G) was used to evaluate the ash
basins and coal piles hydraulic conditions that would occur after two ash
basin closure options: closure -in -place and closure -by -excavation. A water
balance was developed for the simulated groundwater system under post -
closure conditions.
The extent of the watershed under post closure conditions is expected to be
slightly larger than post -decanting conditions. Under closure -in -place
conditions, the location of the groundwater divide to the west of the Site
shifts to the west approximately 300 feet. Under closure -by -removal
conditions, the location of the groundwater divide to the west of the Site
shifts to the west approximately 700 feet.
Groundwater recharge from the watershed recharge area is
estimated to be 92 gpm for closure -in -place or 172 gpm for closure -
by -excavation.
Groundwater recharge associated with wells and septic returns from
outside the ash basins is estimated to be 15 gpm for closure -in -place
or 17 gpm for closure -by -excavation. Discharge associated with wells
and septic returns from outside the ash basins is estimated to be 15
gpm for closure -in -place or 18 gpm for closure -by -excavation.
• Groundwater discharge from the ash basin ponds is estimated to be
20 gpm for closure -in -place or 129 gpm for closure -by -excavation.
• Groundwater discharge to drainages outside of the ash basins are
estimated to be 6 gpm.
• Groundwater recharge that flows toward the coal pile area is
estimated to be 1 gpm for closure -in -place or 5 gpm for closure -by -
removal.
• Groundwater discharge that flows through and immediately under
the dam, or towards the coal pile area is estimated to be a total of 66
gpm for closure -in -place. For closure -by -excavation, discharge in
these areas is estimated to be a total of 33 gpm.
5.1.2.9 Effects of Naturally Occurring Constituents
(CAP Content Section 5.A.a.vii)
Metals and inorganic constituents, typically associated with CCR material,
are naturally occurring and present in the Piedmont physiographic
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province of North Carolina. The metals and inorganic constituents occur in
soil, bedrock, groundwater, surface water, and sediment. During the Allen
CSA efforts, samples of soil and rock were collected during drilling
activities and analyzed for metals and inorganic constituents. Results
indicate that soil and rock at Allen contain naturally occurring constituents
that are also typically related to CCR material and likely effect the chemistry
of groundwater at the Site. Chromium, cobalt, iron, and manganese are
commonly present in background soil and rock samples at concentrations
greater than the PSRG POG values. Although less common, boron,
selenium, and thallium are present in background soil and rock samples at
concentrations greater than the PSRG POG values (Appendix C, Table 4
and Table 4-2). Analytical results for groundwater at background locations
indicate that antimony, chromium, cobalt, iron, manganese, and vanadium
are present at concentrations greater than 02L/IMAC standards (Appendix
C, Table 1 and Table 4-3).
These results suggest that antimony, chromium, cobalt, iron, manganese,
selenium, thallium, and vanadium may occur naturally in soil, rock, and
groundwater at the Site. Therefore, when applicable, the concentrations of
these constituents at the Site are compared to background values.
5.2 Source Area Location
(CAP Content Section 5.A.b)
The ash basins, located south of the Station, are generally bounded by a natural ridge to
the west and earthen dams and dikes to the north, south, and east (Figure 1-2). The coal
piles, also located south of the Station, are bounded by the RAB to the west and south,
and the Catawba River (Lake Wylie) to the east. South Point Road, located along a
topographic ridge, represents a hydrogeologic divide that affects groundwater flow
within an area approximately 0.5 miles west of the ash basins. Topography to the west
of South Point Road generally slopes downward toward the South Fork Catawba River
to the west and discharge canal to the north.
5.3 Summary of Potential Receptors
(CAP Content Section 5.A.0
G.S. Section 130A-309.201(13) defines receptor as "any human, plant, animal, or structure
which is, or has the potential to be, affected by the release or migration of contaminants. Any
well constructed for the purpose of monitoring groundwater and contaminant concentrations
shall not be considered a receptor." In accordance with the NORR CSA guidance, receptors
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cited in this section refer to public and private water supply wells and surface water
features.
The Site -specific risk assessment conducted for the ash basins and coal piles indicates
no measurable difference between evaluated Site -related risks imposed by background
concentrations (Appendix E). It is determined that there is no identified material
increases in risks to human health related to the ash basins and coal piles. Additionally,
multiple lines of evidence support that groundwater from the ash basin and/or coal pile
area has not and does not flow towards any water supply wells based on groundwater
flow patterns and the location of water supply wells in the area. However, Duke Energy
has implemented a permanent water solution which provides owners of the
surrounding properties with water supply wells within a 0.5-mile radius of the ash
compliance boundary with access to an alternate water supply.
The Site -specific risk assessment conducted for the ash basins and coal piles also
indicates that there is no increase in risks to ecological receptors. The Catawba River
(Lake Wylie) aquatic system that is adjacent to Allen is healthy based on multiple lines
of evidence including robust fish populations, species variety, and other indicators
based on years of sampling data.
5.3.1 Surface Water
The Site is located in the Catawba River watershed. The ash basins and coal piles
are located along the west bank of the Catawba River (Lake Wylie). The South
Fork River is located west of the Site, beyond a topographic and hydrogeologic
divide. Associated North Carolina surface water classifications for the Catawba
River (Lake Wylie) and the South Fork Catawba River are summarized in Table
5-3.
An on -Site surface water intake is used to pump water from the Catawba River
(Lake Wylie) for Station operations (Figure 5-6).
A depiction of surface water features - including wetlands, ponds, unnamed
tributaries, seeps, streams, lakes, and rivers - within a 0.5-mile radius of the ash
basin compliance boundary is provided in Figure 5-6. The surface water
information is provided by the Natural Resources Technical Report (NRTR)
prepared by AMEC Foster Wheeler (Wheeler, 2015). In addition, permitted
outfalls under the NPDES and SOC locations are shown on Figure 5-6. Non -
constructed and dispositioned seep sample locations between the ash basin and
the Catawba River (Lake Wylie) are managed by the SOC and are subject to the
monitoring and evaluation requirements contained in the SOC.
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5.3.1.1 Environmental Assessment of Lake Wylie
The NPDES permit for Lake Wylie and the Allen Steam Station (NPDES No.
NC0004979) requires Duke Energy to conduct weekly to monthly outfall
and instream water quality monitoring at 14 locations including within
Lake Wylie. Trace elements (arsenic, selenium) monitoring in fish muscle
tissue is also conducted annually in accordance with a study plan approved
by NCDEQ.
Lake Wylie has been monitored by Duke Energy since 1959.Over the years,
specific assessments have been conducted for water quality and chemistry
as well as abundance and species composition of phytoplankton,
zooplankton, macroinvertebrates, aquatic macrophytes, fish, and aquatic
wildlife. These assessments have all demonstrated that Lake Wylie has been
an environmentally healthy and functioning ecosystem, and ongoing
sampling programs have been established to ensure the health of these
systems will continue. Furthermore, these data indicate that there have been
no significant effects to the local aquatic systems related to coal ash
constituents over the last 50 years. More information related environmental
health assessments conducted for Lake Wylie, including sampling
programs, water quality and fish community assessments, and fish tissue
analysis, can be found in Appendix E.
5.3.2 Availability of Public Water Supply
Residential potable water supply lines are available to residents within a 0.5-mile
radius of the ash basin compliance boundary. The nearest available residential
water supply line to the Site, provided by the City of Belmont, is located west of
the Site on Southpoint Drive. Section 6.2.2 has a more detailed discussion
regarding water supply within a 0.5-mile radius of the ash basin compliance
boundary.
5.3.3 Water Supply Wells
No public or private drinking water wells or wellhead protection areas were
found to be located downgradient of the ash basins. A total of 290 eligible
households for permanent water supply were identified within the 0.5-mile
radius of the ash basin compliance boundary. These eligible households are
located northwest, west, southwest, and south of the ash basins (Figures 5-7a
and 5-7b). Discussion, with supporting material and data, of alternative water
supply provisions (water filtration systems) provided by Duke Energy for
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surrounding occupied residences and findings of the drinking water supply well
survey are included in Section 6.2.2.
5.3.4 Surrounding Land Use
Land use within the 0.5-mile radius of the ash basin compliance boundary
generally consists of residential properties and undeveloped land in Gaston
County to the north, west and south, and residential properties and undeveloped
land in Mecklenburg County to the east and southeast across the Catawba River
(Lake Wylie). The Catawba River (Lake Wylie) bounds the Site to the east.
5.3.5 Future Groundwater Use Area
Duke Energy owns the land and controls the use of groundwater on the land
downgradient of the ash basins and coal piles at and beyond the predicted area
of potential affected groundwater. Therefore, no future groundwater use areas
are anticipated downgradient of the basins and coal piles.
Under G.S. Section 130A-309.211(cl) (added by House Bill 630), Duke Energy
provided permanent water solutions to all eligible households within a 0.5-mile
radius of the ash basin compliance boundary. It is anticipated that residences
within a 0.5-mile of the ash basin compliance boundary will continue to rely on
groundwater resources for water supply for the foreseeable future. Duke Energy
has a performance monitoring plan in place, with details of the plan outlined in
the Permanent Water Supply — Water Treatment Systems document. Duke Energy
will provide quarterly maintenance of the water treatment systems to include
replenishing expendables (salt for brine tank and neutralizer media) and
providing system checks and needed adjustments. Laboratory samples of pre-
treated and treated water will be collected annually to coincide with system
installation, unless there is evidence the system is not performing properly, in
which case samples will be collected more frequently.
5.4 Human Health and Ecological Risk Assessment Results
(CAP Content Section 5.A.d)
A human health and ecological risk assessment pertaining to Allen was prepared and is
included in Appendix E. The risk assessment focuses on the potential impacts of CCR
constituents from the Allen ash basins and coal piles on groundwater, surface water,
and sediment. Groundwater flow information was used to focus the risk assessment on
areas where exposure of humans and wildlife to CCR constituents could occur.
Primary conclusions of the risk assessment include: 1) there is no evidence of risks to
on -Site or off -Site human receptors potentially exposed to CCR constituents that may
have migrated from the ash basin; and 2) there is no evidence of risks to ecological
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receptors potentially exposed to CCR constituents that may have migrated from the ash
basin. This risk assessment uses analytical results from groundwater, surface water, and
sediment samples collected March 2015 through June 2019.
Evaluation of risks associated with area of wetness (AOW) locations and soil beneath
the ash basin are not subject to this assessment and will be evaluated independent from
the CAP Update. Consistent with the iterative risk assessment process and guidance,
updates to the risk assessment have been made to the original 2016 risk assessment
(HDR, 2016c) in order to incorporate new site data and refine conceptual site models.
The original risk assessment was prepared in accordance with a work plan for risk
assessment of CCR-affected media at Duke Energy sites (Haley & Aldrich, 2015).
The following risk assessment reports have been prepared:
1. Baseline Human Health and Ecological Risk Assessment, Appendix F of the CAP Part
2 (HDR, 2016a)
2. Comprehensive Site Assessment (CSA) Update (SynTerra, 2018a)
3. Human Health and Ecological Risk Assessment Summary Update for Allen Steam
Station, Appendix B of Community Impact Analysis of Ash Basin Closure Options at
the Allen Steam Station (Exponent, 2018)
To help evaluate options for groundwater corrective action, this risk assessment
characterized potential effects on human health and the environment related to
naturally occurring elements, associated with coal ash, present in environmental media.
This risk assessment follows the methods of the 2016 risk assessment (HDR, 2016c) and
is based on NCDENR, NCDEQ, and USEPA risk assessment guidance (NCDENR, 2003;
NCDEQ, 2017; USEPA, 1989; 1991; 1998).
Human health and ecological CSMs were developed and further refined to guide
identification of exposure pathways, exposure routes, and potential receptors for
evaluation. Additional information regarding groundwater flow and the treatment of
source areas other than the ash basin was incorporated into the refinement of CSMs
presented in Appendix E.
Environmental data evaluated in the risk assessment were compared to human health
and ecological screening values. Risk assessment constituents of potential concern
(COPCs) are different than COIs in that COPC are those elements in which the
maximum detected concentration exceeded human health or ecological screening
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values. COPCs are carried forward for further evaluation in the deterministic risk
assessment. Appendix E contains the results of the screening assessment.
No unacceptable risks from exposure to environmental media were identified. Results
of the human health risk assessment indicate the following:
• On -Site groundwater poses no unacceptable risk for the construction worker
under these exposure scenarios.
• On -Site surface water, and sediment pose no unacceptable risk for the trespasser
under these exposure scenarios.
• Exposure to CCR constituents by current and future commercial/industrial
worker, residences is incomplete.
• No evidence of carcinogenic or non -carcinogenic risks associated with the
recreational swimmer, wader, or boater exposure scenarios was identified.
• No evidence of carcinogenic or non -carcinogenic risks associated with the
recreational fisher exposure scenario was identified.
• There is no increase in estimated risks for the subsistence fisher exposure
scenario attributable to the ash basins. Hexavalent chromium concentrations in
upstream surface water samples also resulted in estimated values within
USEPA's range for excess lifetime cancer risk (ELCR). The modeled
concentration of hexavalent chromium in fish tissue is likely overestimated.
Findings of the baseline ecological risk assessment include the following:
Ecological Exposure Area 1:
No hazard quotients (HQs) based on no observed adverse effects levels
(NOAELs) or lowest observed adverse effects levels (LOAELs) were greater than
unity for the mallard duck, great blue heron, and river otter exposed to surface
water and sediments in the Catawba River adjacent to the site (Exposure Area 1).
• Two endpoints, the killdeer and muskrat, had modeled risk results greater than
unity for aluminum and lead based on modeled NOAEL and LOAEL based
HQs. The killdeer had limited modeled (NOAEL based) risk results greater than
unity for copper. The modeled risks are considered negligible based on natural
and background conditions. The exposure models likely overstate risks to
aluminum and lead.
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Ecological Exposure Area 4:
• No HQs based on NOAELS or LOAELs were greater than unity for the American
robin, red-tailed hawk, and red fox exposed to surface water and sediments
located west of the AAB in Exposure Area 4.
• One endpoint, meadow vole, had limited modeled (NOAEL based) risk results
greater than unity for aluminum. The modeled risks are considered negligible
based on natural and background conditions. The exposure models likely
overstate risks to aluminum. The modeled risks are considered negligible based
on natural and background conditions. The exposure models likely overstate
risks to aluminum, lead, and copper.
In summary, there is no evidence of unacceptable risks to human and ecological
receptors exposed to environmental media potentially affected by CCR constituents at
Allen. This conclusion is further supported by multiple water quality and biological
assessments conducted by Duke Energy as part of the NDPES monitoring program.
5.5 CSM Summary
The Allen CSM presented herein describes and illustrates geologic and hydrogeologic
conditions and constituent interactions specific to the Site. The CSM presents an
understanding of the distribution of constituents with regard to the Site -specific
geologic/hydrogeologic and geochemical processes that control the transport and
potential impacts of constituents in various media and potential exposure pathways to
human and ecological receptors.
In summary, the ash basins and coal piles were constructed within former perennial
stream valleys in the Piedmont of North Carolina, and exhibit limited horizontal and
vertical constituent migration, with the predominant area of migration occurring near
and downgradient of the ash basin dams. The upward flow of water into the basins
minimizes downward vertical constituent migration to groundwater immediately
underlying saturated ash in the upgradient ends of the basins. Due to the prevailing
horizontal flow within the ash basins, there is limited vertical flow of ash basin pore
water into the underlying groundwater. The elevated constituent concentrations found
in groundwater near the dams is due to the operating hydraulic head in the basin. The
ponded water in the basin is the most important factor contributing to constituent
migration in groundwater.
Groundwater flow is away from water supply wells and there are no exposure
pathways between ash basins and the pumping wells used for water supply in the
vicinity of the Site, based on empirical Site data from over 30 monitoring events over
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multiple seasonal variations and groundwater flow and transport modeling
simulations. Risk assessment results conclude that there is no identified material
increases in risks to human health related to the ash basins and/or coal piles.
Through ash basin decanting and closure, the hydraulic head and the rate of constituent
migration from the AAB to the groundwater system will be reduced based on basin
hydrogeology described above. Either closure option considered by Duke Energy will
significantly reduce infiltration to the remaining ash, reducing the rate of constituent
migration. Based on future predicted groundwater flow patterns, under post ash basin
closure conditions, and the location of water supply wells in the area, groundwater flow
direction from the ash basins is expected to be further contained within the stream
valleys and continue flowing east of the ash basin footprint, and therefore will not flow
towards any water supply wells.
Multiple lines of evidence have been used to develop this CSM based on the large data
set generated for Allen. This CSM provides the basis for this CAP Update developed for
the Allen ash basins and coal piles to comply with G.S. Section 130A-309.211, enacted by
CAMA.
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6.0 CORRECTIVE ACTION APPROACH FOR SOURCE AREA 1
(ASH BASINS AND COAL PILE AREA)
(CAP Content Section 6)
Groundwater contains varying concentrations of naturally occurring inorganic
constituents. Constituents in groundwater with sporadic and low concentrations greater
than the corresponding standard (02L/IMAC/background value, as applicable) do not
necessarily demonstrate horizontal or vertical distribution of COI -affected groundwater
migration from source areas. Constituents with concentrations greater than
corresponding standards were evaluated to determine if the level of concentration is
present due to source areas. COIs are those constituents identified from the "constituent
management process" described below. This evaluation assisted in identifying if a unit
is subject to corrective action under G.S. Section 130A-309.211 and 15A NCAC 02L
.0106.
A COI Management Plan was developed at the request of NCDEQ to evaluate and
summarize COI concentrations in groundwater at the Site (Appendix H). Results of this
COI Management Plan are used to identify areas that may require corrective action and
to determine appropriate Site -specific mapping of COI concentrations on figures based
on the actual distribution of each COI in Site groundwater.
• Groundwater COIs to be addressed with corrective action are those which
exhibit concentrations in groundwater at or beyond the compliance boundary
greater than the 02L standard, IMAC, or BTV, whichever is highest. Table 6-6
presents the COI management matrix for determining COIs subject to corrective
action at Allen.
• The COI Management Plan is also used to discern constituents at naturally
occurring concentrations greater than 02L that would not be subject to corrective
action. Examples include naturally occurring COIs that do not exhibit a
discernable plume or COIs that have no correlation with other soluble
constituents associated with coal ash or another primary source (e.g., boron or
sulfate).
A three -step process was utilized in the COI Management Plan approach:
• An evaluation of the applicable regulatory context
• An evaluation of the mobility of target constituents
• A determination of the distribution of constituents within Site groundwater
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The primary goal of the COI Management Plan is to utilize science -based evidence to
determine the realistic distribution and behavior of coal ash -related constituents in
groundwater. The COI Management Plan presents multiple lines of evidence used to
understand the actual COI presence in the subsurface at the Site, uses results from the
COI Management Plan approach to identify Site -specific COIs for inclusion for
corrective action planning, and presents the COI mapping approach for the CAP. The
COI Management Plan approach is described in detail in (Appendix H) and
summarized below.
Numerous Site -assessment activities have been completed to date and support the
CSM, as described in Section 5. Data generated from these Site assessment activities
have been considered within the COI Management Plan approach. Components of the
Site assessment activities and data evaluations utilized within the COI Management
Plan include the hydrogeologic setting, groundwater hydraulics, constituent
concentrations, groundwater flow and transport modeling results, geochemical
modeling results, and groundwater geochemical conditions.
Step 1: Regulatory Review
Step 1 of the COI Management Plan process considers the relevant regulatory
references listed in Appendix H. The regulatory analysis starts with the current
COI list identified in the CSA Update (SynTerra, 2018a) and 2019 IMP submitted
by Duke Energy, March 20, 2019, and approved by NCDEQ April 4, 2019
(Appendix A). COI concentrations were screened against their respective COI
criterion defined as the maximum of the 02L groundwater quality standard,
IMAC, and background. COI concentrations were screened against their
respective COI criterion for groundwater monitoring locations at or beyond the
compliance boundary. Groundwater COI concentrations used in the screening
are based on a calculated central tendency value (mean) including data from
2018 through the 2nd quarter of 2019. Arithmetic mean COI concentrations were
calculated when the range in COI concentrations was less than one order of
magnitude. A geometric mean COI concentration was calculated when the range
in COI concentrations was greater than one order of magnitude.
NCDEQ recommended use of a lower confidence limit (LCL95) concentration
rather than the central tendency value (Appendix H). LCL95 concentrations were
calculated for each COI and the LCL95 concentration for the sample with the
highest COI LCL95 concentration is provided in Table 1 of the COI Management
Approach (Appendix H) for comparison to the maximum COI mean
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concentration. Table 2 of the COI Management Approach (Appendix H)
provides a comparison of the maximum COI central tendency concentrations
compared with the maximum COI LCL95 concentration for wells located at or
beyond the compliance boundary for the Allen Steam Station, Belews Creek
Steam Station, Cliffside Steam Station, Marshall Steam Station, Mayo Steam
Electric Plant, and Roxboro Steam Electric Plant Sites. The COI LCL95
concentrations were typically lower than the COI central tendency value with
very few exceptions. The number of wells exceeding COI criteria using the COI
LCL95 concentration was typically equal to or less than the number of wells
exceeding COI criteria using the COI central tendency concentration. There were
no increases in the number of wells exceeding COI criteria for the Site when
comparing the LCL95 to the COI criterion and the number of exceedances was
typically less for LCL95. Use of the COI central tendency concentrations in the
COI Management Plan process provides conservative estimate of the extent of
COI in Site groundwater.
Step 2: COI Mobility
Step 2 of the COI Management Plan process evaluates the COI mobility to
identify hydrogeologic and geochemical conditions and relative COI mobility
based on:
• Review of regulatory agency and peer -reviewed literature to identify
general geochemical characteristics of COI,
• Analysis of empirical data and results from geochemical and flow and
transport modeling conducted for the Site, and
• Identification of COI -specific mobility as conservative (non -reactive), non -
conservative (reactive), or variably reactive COIs based on results from
geochemical modeling (Appendix H).
Site -specific groundwater geochemical conditions that may affect COI transport
and distribution are described in Table 1 of the COI Management Approach
(Appendix H).
Step 3: COI Distribution
Step 3 of the COI Management Plan process evaluates the relative presence of
COI in Site groundwater. Descriptions of the horizontal and vertical distribution
of COI with mean concentrations above their respective COI criterion at and
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beyond the compliance boundary are summarized in Table 1 of the COI
Management Approach (Appendix H) and provided in more detail in Table 6-6.
The COI Management Plan approach considers the distribution of COI on a Site -
wide basis. These distributions are used for planning appropriate corrective
action as well as determining which COI to map on figures.
Primary descriptions of COI distributions include plume -like distributions for
relatively mobile COI such as boron and sulfate and isolated location(s) for COI
that do not exhibit plume -like distributions. Boron and sulfate are the COIs with
the most plume -like distributions. Some COIs with isolated exceedances of COI
criteria are not associated with the boron plume and these exceedances are
described in more detail in Table 6-6 to place these exceedances within the
context of the Site CSM.
Rationale for inclusion or exclusion of COI from mapping on figures in the 2019
CAP Update is based on the horizontal and vertical distribution of COI with
concentrations greater than their respective COI criterion. All wells that have
COI mean concentration(s) greater than the COI criterion are listed in Table 6-6.
Outcome of COI Management Plan Process
Constituents with concentrations greater than the COI criterion beyond the compliance
boundary were grouped by geochemical behavior and mobility. A comprehensive
evaluation (i.e., means and groupings) of available data was used to demonstrate
constituent distribution and correlation with other soluble constituents associated with
coal ash, and to evaluate the spatial occurrence with a discernable COI plume in the
direction of groundwater flow downgradient of the source area. This evaluation
emphasizes the depiction of those constituents that have migrated downgradient of the
source area, in the direction of groundwater flow at concentrations greater than the COI
criterion with a discernable plume that correlates with other soluble constituents.
COI were assigned to mobility categories based on geochemical modeling results and
information derived from peer -reviewed literature. COI mobility categories are based
on the concept of conservative versus non -conservative COI introduced by NCDEQ in
the January 23, 2019 CAP content guidance document. The use of three mobility
categories for COI was first introduced during in -person COI Management meetings
held with NCDEQ in September 2019 for the Allen, Marshall, Mayo, and Roxboro Sites.
Based on geochemical modeling results, COI mobility categories were expanded from
conservative versus non -conservative to include the following:
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• Conservative, Non -Reactive COI: Antimony, boron, sulfate, TDS. Geochemical
model simulations support that these constituents would transport
conservatively (Kd values <1 liter per kilogram [L/kg]) as soluble species under
most conditions, and that the mobility of these COIs will not change significantly
due to current geochemical conditions or potential geochemical changes related
to remedial actions.
• Non -Conservative, Reactive COI: Beryllium, cadmium, total chromium,
strontium, vanadium. Geochemical model simulations support that these
constituents are subject to significant attenuation in most cases and have high Kd
values indicating the mobility of these COIs is unlikely to be geochemically
affected by current geochemical conditions or potential geochemical changes
related to remedial actions.
• Variably Reactive COI: Arsenic, cobalt, hexavalent chromium, iron, manganese,
molybdenum, nickel, selenium, thallium. Geochemical model simulations, and
resulting Ka values, support these constituents may be non -reactive or reactive in
relation to geochemical changes and are dependent on the pH and oxidation
reduction potential (Eh) of the system. The sensitivity of these COIs to the
groundwater pH and Eh indicates that these constituents could respond to
natural changes, such as water level fluctuations imposed by seasonality, or
decanting and source control activities that have the potential to change the
groundwater pH or Eh.
As discussed in the CSA Update (SynTerra, 2018a) and the 2018 CAMA Annual Interim
Monitoring Report (SynTerra, 2019c), not all constituents with results greater than
background values can be attributed to the ash basin or another source area. Naturally
occurring groundwater contains varying concentrations of inorganic constituents.
Sporadic and low -concentration occurrences of these constituents in the groundwater
data do not necessarily demonstrate horizontal or vertical distribution of COI -affected
groundwater migration from the ash basin [and other source areas, as appropriate].
COI Management Plan Summary
A three -step process was utilized for the COI Management Plan approach considering
the regulatory context, the mobility of constituents, and the distribution of constituents
within Site groundwater. A comprehensive, multiple lines of evidence approach was
followed utilizing extensive Site data. The COI Management Plan approach
incorporated numerous components of the Site CSM in a holistic manner. Clear
rationale was provided for every step of the COI Management process.
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For the regulatory review portion of the COI Management Plan, mean COI
concentrations were compared with COI criteria to identify COI that exceeded their
respective COI criterion. Use of the COI central tendency concentrations in the COI
Management Plan process was shown to provide a conservative estimate of the extent
of COI in Site groundwater. Exceedance ratio values indicate COI concentrations that
exceed COI criteria are typically within one order of magnitude (ER <10) to two orders
of magnitude (ER <100) above the COI criterion.
Results of the COI Management Plan evaluation were used to identify COI for mapping
on figures in the CAP Update. COIs to be mapped include boron, cobalt, iron,
manganese, strontium, sulfate, and total dissolved solids. The following COls have no
exceedances of COI criteria or have isolated exceedances without a discernable plume,
at or beyond the compliance boundary: antimony, arsenic, beryllium, cadmium, total
chromium, hexavalent chromium, lithium, molybdenum, nickel, selenium, thallium,
and vanadium. These constituents will not be mapped on figures in the 2019 CAP
Update.
Results of the COI Management Plan evaluation were also used to identify areas that
require groundwater corrective action as described in Section 6.1.3 based on the actual
distribution of each COI in Site groundwater.
6.1 Extent of Constituent Distribution
(CAP Content Section 6.A)
This section provides an in-depth review of constituent characteristics associated with
source area 1 (the ash basins and coal piles) and the mobility, distribution, and extent of
constituent migration within, at, and beyond the point of compliance.
6.1.1 Source Material within the Waste Boundary
(CAP Content Section 6.A.a)
Waste boundaries are shown on Figure 1-2. An overview of the material within
the ash basins is presented in the following subsections. Although there is no
waste boundary associated with the coal piles, a description of material within
the coal piles is included in Section 6.1.1.7.
6.1.1.1 Description of Waste Material and History of
Placement
(CAP Content Section 6.A.a.i)
The ash basins consist of two impoundments created with the construction
of the dike located north of the RAB, the dam located along the west bank
of the Catawba River (Lake Wylie), and the dike located between the RAB
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and AAB. The RAB was constructed in 1957 and encompasses
approximately 132 acres. The AAB was constructed in 1973 and
encompasses approximately 169 acres. The Allen ash basins have been
operated under a NPDES Permit issued by the NCDEQ DWR.
CCR materials, composed primarily of fly ash and bottom ash, were initially
deposited in the unlined RAB (south of the Station) via sluice lines. A
water/ash slurry was discharged from sluice lines in the northern portion of
the RAB. Pyrite -rich rocks known as "clinkers" or "mill rejects" have been
observed within the north-northeast portion of the RAB. Clinkers were
mixed with coal but were not combusted as part of the power generation
process. Although documentation was not available, this material was
likely historically managed with ash and placed in the RAB via sluicing.
Pyrite within the clinkers has caused low pH conditions in the subsurface
within and downgradient of the north-northeast portion of the RAB, and
this area is referred to as the "low pH area" further discussed in Section
6.1.1.7.
Due to the RAB's diminishing capacity, the AAB was constructed south of
the RAB. After the AAB began operation in 1973, coal ash was sluiced there
and no longer deposited in the RAB.
Duke Energy excavated ash from the northern portion of the AAB to
provide capacity for sluiced ash and the future construction of the primary
ponds. Approximately 300,000 cubic yards of ash was excavated and placed
in unlined ash storage and structural fill areas, which are located within the
footprint of the RAB (Figure 1-2). The ash storage and structural fill areas
were constructed under Duke Energy's DORS Permit issued by NCDENR
DWQ.
The double -lined RAB Ash Landfill (Solid Waste Section Permit No. 3612-
INDUS) was constructed on top of the RAB. The RAB Ash Landfill is
located east of the DORS fills, along the east dam and north of the dike
between the RAB and AAB. The landfill is permitted to receive CCR
materials, including fly ash, bottom ash, boiler slag, mill rejects, and FGD
waste. In addition to these CCR materials, the landfill is permitted to
receive non -hazardous sandblast material, limestone, coal, carbon, sulfur
pellets, cation and anion resins, sediment from sumps, and cooling tower
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Allen Steam Station SynTerra
sludge. The approximate boundary of the RAB Ash Landfill is shown on
Figure 1-2.
The unlined AAB, located in the southern portion of the Site, received CCR
materials from 1973 to 2019. While it was operating, sluice lines discharged
a water/ash slurry into the northern portion of the basin. Primary ponds 1,
2, and 3 were constructed in 2004. Primary ponds 2 and 3 have been used
for settling purposes. Ponded water occurs in three general areas in the
AAB:
• Primary ponds 1, 2, and 3
• Southwest portion of the AAB
• Southeast portion of the AAB
Allen was modified for dry fly ash (DFA) handling in 2008. Since DFA
handling began at Allen, only de minimus quantities of fly ash were sent to
the AAB on occasion upon system start-up. Bottom ash was sluiced to the
AAB until January 2019 when the facility converted to a dry bottom ash
collection system. In February 2019, all sluicing to the AAB was stopped.
Removal of free water, known as decanting or mechanical decanting, of the
AAB began June 5, 2019 and has continued.
6.1.1.2 Specific Waste Characteristics of Source Material
(CAP Content Section 6.A.a.ii)
Source characterization was performed through the completion of soil
borings, installation of monitoring wells, and collection and analysis of
associated solid matrix and aqueous samples. Source characterization was
performed to identify the physical and chemical properties of the ash in the
source areas. The source characterization involved determining physical
properties of ash, identifying the constituents present in ash, measuring
concentrations of constituents in the ash pore water, and performing
laboratory analyses to estimate constituent concentrations from leaching of
ash.
Thirty-three (33) borings (AB-20S/D, AB-211), AB-23BRU, AB-241), AB-25SL,
AB-271), AB-281), AB-291), AB-301), AB-331), AB-341), AB-351), AB-361), AB-
371), AB-381), AB-39S, SB-1, SB-3, SB-4, SB-5, SB-6, SB-8, SB-9, and SB-10
through SB-18) were advanced within the ash basin waste boundaries to
obtain ash samples for chemical analyses (Figure 1-2). Wells clusters AB-40
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Correction Action Plan Update December 2019
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through AB-44 were co -located with adjacent borings SB-10, SB-11, SB-12,
SB-17, and SB-18). Borings were advanced through the main earthen dams
(AB-22D, AB-26D, AB-31D, and AB-32S) without encountering ash, and
three borings at the AB-9 location drilled through the dike (one of which
was advanced into bedrock) did not encounter ash. Ash was not observed
in borings outside the ash basin waste boundaries.
The hydraulically sluiced deposits of ash consisted of interbedded fine- to
coarse -grained fly ash and bottom ash materials. Ash was generally
described as gray to dark gray, non -plastic, loose to medium density, dry to
wet, fine- to coarse -grained sandy silt texture. Physical properties analyses
(grain size, specific gravity, and moisture content) were performed on ash
samples from the ash basins and measured using American Society for
Testing and Materials (ASTM) methods. Ash is generally characterized as a
non -plastic silty (medium to fine) sand or silt. Ash exhibits a lower specific
gravity compared to soil, with six values reported ranging from 1.9 (AB-
25BR) to 2.4 (AB-35S). Moisture content of the ash samples ranges from 22.3
percent to 57.8 percent.
Within an ash basin, ash typically contains interbedded layers of fly ash and
bottom ash as a result of the varying rates and pathways of bottom ash and
fly ash settlement. A depiction of the typical interbedded nature of fly ash
and bottom ash within an ash basin, as seen from an ash boring photograph
(Figure 6-1). Layers of bottom ash are typically more permeable than layers
of fly ash due to the coarser grain size of bottom ash.
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Correction Action Plan Update December 2019
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FIGURE 6-1
FLY ASH AND BOTTOM ASH INTERBEDDED DEPICTION
L
SOIL DATA
.
aB_
O
AB 31
3.W9
Gre1-sJry 8-co SAND(SG-262s2
Q
Buies
A33
d0.OJ20
2tre. &_fi u a'y TCS -265q
p
Bmn.
7E
MW-12
5.0.6.2
Bedmah bxewn fi. u.dy SMT CS —2.214)
O
Bu g
N -12
30. 233
LiaheprySamdy SB.F (SO-2.68M1}
V
Bviq
l -13
2.3 .33
1 m"&dew. B em yMT M-2.4})
6.1.1.3 Volume and Physical Horizontal and Vertical
Extent of Source Material
(CAP Content Section 6.A.a.iii)
Based on CCR inventory data provided by Duke Energy as of July 31, 2019
and upon a surface comparison calculation, performed within AutoCAD
Civil 3D, comparing the approximate pre -development topography to the
existing topographic and bathymetric survey, the approximate volume of
source material in the basins 16,231,500 tons, or 13,526,250 cubic yards. This
estimate includes the DORS Fills, but excludes the double -lined RAB Ash
Landfill. Based on borings located within the RAB and AAB, the maximum
depth of CCR within the ash basins is estimated to be approximately 58 feet
which was observed within a boring (AB-35) on an elevated structural fill
area within the RAB. Volume and physical horizontal and vertical extent of
ash material within the basins under pre -decanting conditions as cross-
section transects, from west to east, are presented in Figures 6-2 through 6-
6. The horizontal limits of source material is depicted by the waste
boundaries as shown on Figure 1-2.
The live coal pile encompasses approximately 2 acres. The main coal pile
encompasses approximately 15 acres. While the footprint of the coal piles
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Allen Steam Station SynTerra
remains relatively constant, the volume of coal changes due to the on -going
operations of the Station.
6.1.1.4 Volume and Physical Horizontal and Vertical
Extent of Anticipated Saturated Source Material
(CAP Content Section 6.A.a.iv)
Volume and physical horizontal and vertical extent of saturated ash
material under pre -decanting conditions, within the basins in plan -view is
presented in Figure 6-7. Water levels of ash pore water wells under pre -
decanting conditions ranged from 0.5 feet to 39 feet below grade surface in
the RAB, and 5 feet to 8 feet below grade surface in the AAB. The large
discrepancy in water levels in the RAB and AAB is due to the wells installed
within the DORS Fills. The maximum saturated ash thickness under pre -
decanting conditions was estimated to be 40 feet in the RAB and 70 feet in
the AAB. The estimates use the approximated bottom of ash from the flow
and transport model simulation (Appendix G) and simulated hydraulic
heads. Due to ponded water being present in the AAB, estimates of
saturated ash in the eastern portion of the AAB are likely overestimated.
The greatest volume of saturated ash follows the former stream valleys in
the central portions of the basins (Figure 6-7).
Decanting of the AAB was initiated in June 2019. As of December 1, 2019,
53,300,000 gallons of water has been decanted and the corresponding pond
water elevation has decreased by 14.1 feet, significantly reducing areas of
saturated ash. Under closure -in -place conditions, the range of potential
saturated ash thickness in the RAB is estimated to be between 0 feet to 20
feet and between 0 feet to 20 feet in the AAB. The estimates use the
approximated bottom of ash from the flow and transport model simulation
(Appendix G) and simulated hydraulic heads. The greatest volume of
saturated ash remains within the former stream valleys in the central
portions of the basins, as it was under pre -decanting conditions (Figure 6-
7). Under closure -in -place conditions, the ash/source material within the
low pH area is estimated to be unsaturated.
Under the closure -by -excavation option, all of the ash in the ash basins
would be excavated, and therefore, no saturated ash would remain in the
ash basin footprints.
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Correction Action Plan Update December 2019
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6.1.1.5 Saturated Ash and Groundwater
(CAP Content Section 6.A.a.v)
Based on the trend analysis results, the thickness of saturated ash remaining
in place following closure (closure -in -place only) will have limited to no
adverse effect on future groundwater quality. Layered ash within the basin
has resulted in relatively low vertical hydraulic conductivity, reducing the
potential for downward flow of pore water into underlying residual
material. The CSM indicates that the flow -through ash basin system should
result in low to non -detectable COI concentrations in groundwater
underlying saturated ash within the basin except near the dam where
downward vertical hydraulic gradients are observed. Boron is the CCR
constituent most indicative of COI transport in groundwater from the
source area as it has a minimal Ka value and has a discernable plume
pattern. Using boron data to indicate COI distribution potentially related to
the ash basins, the generalized flow -through system is consistent with Site -
specific data as summarized in the Table 6-1.
Of the 15 well locations within the RAB, 12 demonstrate minimal (less than
700 µg/L and below the 02L groundwater standard) to non -detectable boron
concentrations consistent with the flow -through system, which suggests
there is no correlation between the thickness of saturated ash and the
underlying groundwater quality. As shown in Table 6-1, and discussed in
Section 5.0, exceptions to the CSM are near earthen dikes found within the
basins (AB-25, AB-27, and AB-33). The earthen dikes have a similar effect on
hydraulic heads as the dams, forcing flow downward rather than flowing
laterally within the basin.
A technical memorandum, titled Saturated Ash Thickness and Underlying
Groundwater Boron Concentrations —Allen, Belews Creek, Cliffside, Marshall,
Mayo, and Roxboro Sites (Arcadis, 2019a), conducted linear regression
analyses to evaluate the relationships between saturated ash thickness and
concentrations of boron in ash pore water and underlying groundwater.
The linear regression analysis was conducted using analytical data from
Piedmont ash basins, including data from Allen.
The statistical evaluation was performed using a dataset which included 88
monitoring wells completed in shallow, transition, and bedrock
groundwater zones directly beneath ash basins and 57 ash pore water
monitoring wells completed in saturated ash. Linear regression results
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Correction Action Plan Update December 2019
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indicated that 86% of the groundwater monitoring locations below
saturated ash locations have less than 02L concentrations of boron in
groundwater. Exceptions to this relationship occur for select groundwater
wells located near ash basin dikes and dams. This is due to the downward
vertical hydraulic gradient in these areas, which enhances migration of
COIs.
Under pre -decanting conditions, the analysis demonstrates saturated ash
and ash pore water are not significantly contributing COI concentrations to
underlying groundwater except near dikes and dams, where downward
vertical gradients exist. Pre -decanting conditions represent the greatest
opportunity for COI migration to occur, not because of the volume of
saturated ash, but because of the existing ash basin hydraulic head and the
downward vertical hydraulic gradient near the dam. Under post -decanting,
the hydraulic head of the ash basin will be reduced, therefore reducing the
downward vertical gradient occurring near the dam and the rate of
constituent migration from the ash basin to the groundwater system.
Decanting the basin to reduce the vertical hydraulic gradient is the most
important factor to limit further constituent migration in groundwater.
6.1.1.6 Chemistry within Waste Boundary
(CAP Content Section 6.A.a.vi)
Analytical sampling results associated with material from within the ash
basin waste boundary are included in the following appendix tables or
appendices:
• Ash solid phase: Appendix C, Table 4 (CAP Content Section
6.A.a.vi.1.1)
• Ash and soil synthetic precipitation leaching procedures (SPLP):
Appendix C, Table 6 (CAP Content Section 6.A.a.vi.1.2)
• Ash Leaching Environmental Assessment Framework: Appendix H,
Attachment C (CAP Content Section 6.A.a.vi.1.3)
• Soil: Appendix C, Table 4 (CAP Content Section 6.A.a.vi 1.4)
• Ash pore water: Appendix C, Table 1 (CAP Content Section
6.A.a.vi.1.6)
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Correction Action Plan Update December 2019
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Ash Solid Phase and Synthetic Precipitation Leaching
Potential
(CAP Content Section 6.A.a.vi.1.1 and 6.A.a.vi.1.2)
Ash samples collected inside the ash basin waste boundaries were analyzed
for total extractable inorganics using EPA Methods 6010/6020. For
information purposes, ash samples were compared to soil background
values and PSRGs for POG. The ash analytical data do not represent soil
conditions outside of or beneath the ash basins. Concentrations of arsenic,
barium, boron, chromium, and selenium were greater than soil background
concentrations and the PSRG POGs (Appendix C, Table 4).
In addition, 35 ash samples collected from borings completed within the ash
basins were analyzed for leachable inorganics using SPLP EPA Method
1312 (Appendix C, Table 6). The purpose of the SPLP testing is to evaluate
the potential for leaching of constituents that might result in concentrations
greater than the 02L standards or IMACs. SPLP analytical results are
compared with the 02L standard or IMAC comparative values to evaluate
potential source contribution; the data do not represent groundwater
conditions.
The results of the SPLP analyses indicated antimony, arsenic, chromium,
cobalt, iron, manganese, selenium, thallium, and vanadium have the
potential to leach at concentrations greater than the 02L/IMAC or shallow
groundwater background values, which are not comparative criteria for ash
pore water, but comparing these results to these values may indicate
potential to affect the shallow flow zone beneath the ash.
In the low pH area of the RAB, 22 of the 35 ash samples were collected from
five of nine borings for laboratory analysis including SPLP. SPLP results
from sample in this area were similar to other locations within the ash
basins, however, cadmium and nickel were additional constituents detected
to have the potential to leach at concentrations greater than the 02L/IMAC
or shallow groundwater background values in one or more samples. These
additional constituents are also detected within the compliance boundary in
groundwater immediately downgradient of the low pH area. Low pH
conditions and abundant sulfate concentrations in this area are contributing
to the leaching potential of solids in this area. Notably, SPLP results from
soil samples collected beneath indicate the that only cobalt, manganese and
nickel at SB-17 have potential to leach to groundwater at concentrations
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greater than 02L/IMAC or shallow groundwater background. Additional
detail regarding the analysis of and leaching potential of constituents in this
area is included in the geochemical model (Appendix H). Additional
discussion of the investigation of the low pH area conducted in the third
quarter of 2019 are included in Appendix P and subsequent sections of this
CAP Update.
Ash Leaching Environmental Assessment Framework
(CAP Content Section 6.A.a.vi.1.3)
Ash samples were analyzed for extractable inorganics, including hydrous
ferric oxide (HFO)/hydrous aluminum oxide (HAO), using the Citrate-
Bicarbonate-Dithionite (CBD) method. Leaching environmental assessment
framework (LEAF) is a leaching evaluation framework for estimating
constituent release from solid materials. Leaching studies of consolidated
ash samples from the Allen ash basins were conducted using two LEAF
tests, EPA methods 1313 and 1316. The data are presented and discussed in
the geochemical modeling report in Appendix H.
Leaching test results, using USEPA LEAF method 1316, indicate that, even
for conservative COIs such as boron, the leachable concentration of boron
present in ash from Allen is considerably lower than the total boron
concentration (Appendix H, Attachment C). Allen data indicate that there is
a process by which the COIs might become stable within the ash and would
make the COI unavailable for leaching. The exact mechanisms of this
process are unknown, however, literature suggests that incorporating COIs,
such as boron, into the silicate mineral phases is a potential mechanism
(Appendix H, Attachment C). The leaching behavior of several COIs as a
function of pH, examined using USEPA LEAF method 1313, demonstrated
that for anionic COIs, the leaching increased with increasing pH and the
cationic COIs showed the opposite trend (Appendix H, Attachment C).
Soil Beneath Ash
(CAP Content Section 6.A.a.vi 1.4 and 6.A.a.vi 1.5)
Soil samples within the ash basin waste boundaries include samples
collected from beneath the ash basins and samples collected from the fill
material within the ash basin dams and dikes. Soil samples beneath the ash
basin were both mostly saturated. Unsaturated soil samples within the
waste boundaries were collected at eight locations. Temporary soil borings
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Correction Action Plan Update December 2019
Allen Steam Station SynTerra
(SB) were used for soil sample collection purposes (i.e., no monitoring wells
were installed at these locations).
Constituents considered for soil evaluation were limited to constituents
identified as COIs for Allen source areas since soil impacts would be related
to the interactions from the source areas to the underlying soils and
groundwater, which may migrate beyond the source areas. The range of
constituent concentrations in soils within the waste boundary, along with a
comparison with soil background values and North Carolina PSRG POG
standards, whichever is greater, is provided in Appendix C, Table 4. For
constituents lacking an established target concentration for soil remediation
(i.e., sulfate), the following equation was used in general accordance with
the references in Subchapter 02L .0202 to calculate a POG value.
C.il = Cgw (kd + (ew + O.H')IPb]df
Where necessary, the PSRG POG values were calculated using laboratory
testing and physical soil data for effective porosity (0.3) and dry bulk
density (1.6 kg/L) prepared in part for flow and transport modeling for the
Site. Soil water partition coefficients (Ka) were obtained from the
Groundwater Quality Signatures for Assessing Potential Impacts from Coal
Combustion Product Leachate (EPRI, 2012). Soil PSRG POG standard equation
parameters and values used in the equation above are outlined on Table 6-
2. The resulting PSRG POG calculated value for sulfate was 1,438 mg/kg
(Appendix C, Table 4).
Saturated soil and rock is considered a component of the groundwater flow
system and can serve as a source for groundwater COIs at the Site. The
potential leaching and sorption of constituents in the saturated zone is
included in the flow and transport and geochemical model evaluations
(Appendix G and H) by continuously tracking the COI concentrations over
time in the saprolite, transition zone, and bedrock materials throughout the
models. Historical transport models simulate the migration of COIs through
the soil and rock from the ash basins and coal piles, and these results are
used as the starting concentrations for the predictive simulations.
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Correction Action Plan Update December 2019
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Unsaturated soil and rock is considered a potential secondary source to
groundwater. Constituents present in unsaturated soil or partially saturated
soil (vadose zone) have the potential to leach into the groundwater system
if exposed to favorable geochemical conditions for chemical dissolution to
occur. Unsaturated fill and/or soil within the waste boundaries were
collected from borings associated with AB-22, AB-26, AB-31, AB-32, AB-34,
SB-1, SB-2, and SB-4 (Figure 6-8).
Analytical results for unsaturated soil data within the waste boundary can
be found on Table 6-3. Unsaturated soil sample AB-32S (33.5-35) has
concentrations of cobalt (82.3 mg/kg) greater than PSRG for POG or
background values, whichever is greater. Unsaturated soil sample AB-34D
(14-14) has concentrations of arsenic (39.8 mg/kg) and selenium (3.7 mg/kg)
greater than PSRG for POG or background values, whichever is greater.
Unsaturated soil sample SB-1 (3-5) is stratigraphically above the ash, but
has concentrations of arsenic (44.5 mg/kg) greater than PSRG for POG or
background values, whichever is greater. Unsaturated soil sample SB-4 (25-
26.5) has concentrations of boron (45.7 mg/kg) greater than PSRG for POG
or background values, whichever is greater. While these values are greater
than PSRG for POG or background values, they all fall within the Piedmont
background range (Table 4-2) with the exception of the cobalt
concentrations at AB-32S (33.5-35) and arsenic concentrations at SB-1 (3-5).
The maximum detected cobalt concentration at from an NCDEQ-approved
background location at a Piedmont site is 81.68 mg/kg and the maximum
detected arsenic concentration at from an approved background location at
a Piedmont site is 43.13 mg/kg. The exceedances at AB-32S (33.5-35) and SB-
1 (3-5) are negligibly (3%) greater than the Piedmont background range.
Additionally, source control and ash basin closure activities will lower
water elevation in these areas, reducing the potential for leaching
constituents into the groundwater system. No other unsaturated soil
samples within the waste boundaries had concentrations greater than PSRG
POG or background values.
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One unsaturated soil sample within the waste boundaries has been
analyzed for leachable inorganics using SPLP procedures EPA Method
1312. SB-1 (3-5), classified as fill in the boring log, is located in the ash
storage area of the RAB stratigraphically above the ash. Several COIs have
the potential to leach at concentrations greater than the 02L/IMAC.
However, wells downgradient (AB-35 cluster) of SB-1 do not exceed
02L/IMAC or Site -wide background values for any COIs.
Ash Pore Water
(CAP Content Section 6.A.a.vi.1.6 and 6.A.a.vi.3)
The ash basins are wastewater treatment systems. Water within the ash
basins is not groundwater; therefore, comparison to 02L/IMAC/background
values is not appropriate. Ash pore water data is provided for general
purposes only in Appendix C, Table 1. Figures 6-9a through 6-13c represent
ash pore water constituent distribution in cross-section from west to east.
For further discussion of geochemical trends within the ash pore water, see
Appendix H, Section 2. All ash pore water sample locations are shown on
Figure 1-2, and analytical results are provided in Appendix C, Table 1.
One ash pore water monitoring well and three groundwater monitoring
wells located in areas that could be sensitive to changing conditions from
ash basin closure activities, including decanting, were selected for
monitoring water elevation and geochemical parameters. Water elevations
are monitored with pressure transducers and geochemical parameters,
including pH, oxidation reduction potential (ORP) and specific
conductivity, are monitored using multi parameter (or geochemical) sondes.
Locations monitored with multi parameter sondes are depicted on Figure 6-
14, and include:
• AB-21SL: ash pore water monitoring well located in the southern
portion of the AAB (data has not been retrieved from transducers at
this location as access became restricted due safety concerns with ash
stability after decanting began)
• AB-21SS: shallow flow zone monitoring well located in the southern
portion of the AAB, stratigraphically below ash pore water
monitoring well AB-21SL (data has not been retrieved from
transducers at this location as access became restricted due safety
concerns with ash stability after decanting began)
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• AB-26S: shallow flow zone monitoring well located within the dam
of the AAB, downgradient of primary pond 3
• GWA-3S: shallow flow zone monitoring well located downgradient
of the AAB and primary pond 3
Hydrographs and geochemical water quality parameter time series plots for
each location are included on Figure 6-14.Observations of water elevation
and multi parameter records from monitored locations include:
• Ash pore water and shallow flow zone monitoring wells within the
waste boundary show a response to ash basin decanting by reduced
water elevation levels (Figure 6-14).
• Shallow flow zone monitoring wells within the AAB dam and
directly downgradient of the AAB dam show a response to ash basin
decanting by reduced groundwater elevation levels (Figure 6-14).
• Geochemical parameters located within the waste boundary (AB-
21SL and AB-21SS) are generally stable and do not show significant
shifts or variability in records since ash basin decanting commenced
(Figure 6-14). This suggests geochemical conditions have remained
stable under changing conditions at locations within the waste
boundary. Data spikes seem to occur around dates where samples
were collected or data retrieval/maintenance was conducted on the
geochemical sondes.
• Geochemical parameters located at or beyond the waste boundary
(AB-26S and GWA-3S) show somewhat sporadic results potentially
due to on -going sampling efforts or data retrieval/sonde
maintenance at the Site, as previously mentioned (Figure 6-14). Clear
trends in the data are not yet apparent, with the exception of a slight
increase in conductivity values at GWA-3S. The increasing trend
appears to be consistent with the trends prior to reduction of water
levels, therefore it is unlikely related to ash basin decanting and
could reflect natural variability.
In general ash pore water and groundwater geochemical parameters appear
stable under changing site conditions. Ash pore water pH and ORP do not
appear to be significantly affected by lowering the ash basin pond's water
level, and therefore represent stable conditions in which an increase in
constituent dissolution and mobility is unlikely to occur. Additionally,
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groundwater pH and ORP, monitored beneath and downgradient of the ash
basin, are unaffected by even larger reductions in water levels, indicating
stable geochemical conditions in which constituent dissolution and mobility
are unlikely to occur.
Ash Pore Water Piper Diagrams
(CAP Content Section 6.A.a.vi.2)
Piper diagrams can be used to differentiate water sources in hydrogeology
(Domenico and Schwartz 1998). Piper diagrams of ash pore water
monitoring data (Figures 6-15a and 6-15b) are used to assess the relative
abundance of major cations (e.g., calcium, magnesium, potassium, and
sodium) and major anions (e.g., chloride, sulfate, bicarbonate, and
carbonate). Data used for the piper diagrams include ash pore water data
between January 2018 and June 2019 with a charge balance between -10 and
10 percent. Additional data collected as recent as October 2019 were
included on the piper diagrams on Figure 6-15b to include the recently
installed low pH area and coal pile area assessment wells.
• The Piper diagrams indicate ash pore water is characterized by two
water types, calcium -bicarbonate to calcium sulfate and that ash pore
water is similar within the RAB and AAB. These results are similar
to findings in a 2006 Electric Power Research Institute (EPRI) study
of 40 ash leachate water samples collected from 20 different coal ash
landfills and impoundments which characterized bituminous coal
ash leachate as calcium -magnesium -sulfate water type (EPRI, 2006).
• Ash pore water samples at Allen are not sodium -rich, which is
different than typical subbituminous coal ash leachate evaluated by
EPRI which was found to be a sodium -calcium -sulfate water type
(EPRI, 2012).
6.1.1.7 Other Potential Source Material
(CAP Content Section 6.A.a.vii)
Other potential source material is coal within the coal piles and pyrite -rich
rocks observed to be mixed with ash in the north-northeast portion of the
RAB. The pyrite rich rocks are known as "clinkers" or "mill rejects" that
were mixed with coal but not combusted as part of the power generation
process and placed with ash within the RAB. Pyrite within the clinkers has
caused low pH conditions in the subsurface within and downgradient of
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the north-northeast portion of the RAB, and this area is referred to as the
"low pH area".
A focused investigation into these two areas was performed in 2019 to
further evaluate previous assessment results that indicated potential
influence to groundwater from these source areas. Details of the 2019
investigation into these areas, including boring logs, screening results, and
sampling techniques, are included in Appendix P. Evaluation of the results
are included within this CAP Update.
Low pH Area
The "low pH area" is a wooded area within the northern portion of RAB
waste boundary directly upgradient (west) of the main coal pile (Figure 1-
2). In this area, ash, ash pore water, and groundwater pH values are
approximately 4.2 standard units (S.U.) or less, which is less than other
areas of the Site, including within ash pore water in other areas within the
AAB and RAB. Ash pore water and groundwater elsewhere at the Site
typically ranges from approximately 5 S.U. to 8 S.U. Based on field
screening of pH, the area where low pH conditions are present
encompasses approximately 3-acres within a wooded tract of land within
the northern portion of RAB waste boundary directly upgradient (west) of
the main coal pile. Boring logs and groundwater quality data from wells
installed in this area indicate coal "mill rejects" or "clinkers" were placed in
this area within the RAB. Clinkers have not been identified beyond the RAB
waste boundary.
A focused investigation into the low pH area was conducted voluntarily in
the third quarter of 2019. The investigation included the installation of nine
borings and 13 monitoring wells. Details of the investigation of the low pH
area, including the executed sampling plan, field screening results, well
construction details and boring logs are provided in Appendix P.
Laboratory analytical results are included in Appendix C, Table 1. Based on
field screening, 22 samples from 5 borings were collected for laboratory
analysis. Low pH conditions are isolated to areas generally near the waste
boundary, specifically at borings SB-11, SB-12, SB-14 through SB-17.
Horizontally, this area encompasses approximately 130,000 square feet.
Vertically, this area extends to an average depth of approximately 20 feet
below ground surface.
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The low pH conditions within the RAB are causing low pH conditions in
groundwater immediately downgradient of the low pH area source area as
indicated by measurements from monitoring wells GWA-6S, CCR-4S, CCR-
5S and CCR-6S. As a result, several constituents not typically detected in
other areas of the Site, have solubilized and been transported, primarily
within the shallow flow zone groundwater. Lesser effects of the low pH
conditions have been observed in the deep flow zone and groundwater data
indicates that the bedrock flow zone has not been affected by the low pH
area.
Constituents likely caused by the low pH conditions include arsenic,
beryllium, cadmium, calcium, nickel, selenium, and thallium. These
constituents are not detected in groundwater at concentrations greater than
applicable regulatory or background values elsewhere at the Site. These
listed constituents are not detected beyond the compliance boundary.
Monitoring wells upgradient, side -gradient, and farther downgradient of
the low pH area have pH values ranging from 5 to 6.5 S.U. Concentrations
of the aforementioned constituents in upgradient and side -gradient wells
are orders of magnitude less than respective concentrations found
immediately downgradient of the low pH area and the concentrations of
these constituents are less than applicable regulatory values (02L, IMAC, or
background). Several of these constituents would otherwise be less mobile
under pH conditions observed elsewhere on the Site. Sulfate concentrations
observed in wells downgradient of the low pH area may also be derived
from the pyrite -rich clinkers.
Coal Pile Area
Coal stored on -Site is a not a waste product and therefore, is not regulated
under G.S. Section 130A-309.211, enacted by CAMA. Therefore, no
compliance or waste boundaries are associated with the coal piles, although
a portion of the RAB compliance boundary encompasses the southwest
portion of the main coal pile.
In an April 5, 2019, letter to Duke Energy, NCDEQ listed and requested
assessment of additional potential sources of constituents to groundwater at
Allen stating that sources hydrologically connected to the ash basins
(including the ash basins) are to be assessed and included in an updated
CAP. The coal pile area was included as an additional source hydrologically
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connected to the ash basins. The coal pile area is adjacent to, and
downgradient of, the RAB.
Coal has been stored on -Site within the same general footprint since
operations began in 1957. There are two adjacent but separate coal storage
areas. The "live coal pile", located adjacent to the Catawba River (Lake
Wylie), encompasses approximately 2 acres. The "main coal pile" is located
west of the active coal pile and north-northeast of the RAB. The main coal
pile encompasses approximately 15 acres.
The volume of coal within the coal piles fluctuates. Typically more coal is
stored on -Site when demand for power is greater and the Station is
producing power. However, other factors such as logistics and economics
can determine how much coal is stored on -Site at any given time. Volume
within the live coal pile is more consistent than the main coal pile. Typically
the volume of coal in the live coal pile is approximately 9,800 cubic yards
(cy) (assuming a radius of 25 yards stacked conically 15 yards high). The
volume of coal within the main coal pile is notably greater, but much more
variable.
The base of the coal piles are unlined, at land surface grade, and drain water
away from the piles. Historical storm water runoff in the vicinity of the coal
piles was channelized between the RAB and the south end of the main coal
pile to a ponded area and managed through capture within a coal yard
sump and yard drain sump. The storm water was then pumped from the
sumps to the AAB for treatment and discharge through NPDES Outfall 002.
To improve storm water management in the area of the coal piles, a lined
holding basin was constructed in 2018 within the eastern footprint of the
main coal pile and west of the live coal pile. Construction of the holding
basin was associated with a water redirect project which is a component of
ash basin closure (Figure 1-2). Storm water runoff in this area is now
captured within the holding basin for pretreatment and then pumped to the
lined retention basin for further treatment before discharge through NPDES
Outfall 006.
To facilitate construction of the holding basin, temporary extraction wells
were used to lower the water table. Extraction well pumping rates and
water levels were monitored in select wells in the vicinity of the holding
basin during construction. Results of these monitoring activities are
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considered in the flow and transport model and were considered in the
development of this CAP Update. Transducers were installed to monitor
water levels in 24 monitoring wells in the vicinity of the holding basin. The
extraction well network was already functioning at the time of transducer
installation. Dewatering data (i.e., gallons treated) was provided by Duke
Energy. Hydrographs depicting the impact the extraction wells had on the
surrounding monitoring wells are included as Figures 6-18a and 6-18b. As
anticipated, monitoring wells located at CCR-7, CCR-8, and CCR-9 were the
most responsive to the dewatering activities.
The coal piles at Allen are exposed to erosion, oxidation, and precipitation.
An estimated 50-95% of precipitation becomes runoff from coal piles (Davis
and Boegly, 1981). Leachate from coal piles tend to be acidic, with pH
values as low as 2 to 3 S.U. At Allen, acidic conditions occur upgradient of
the coal piles in the area of the AB-40 cluster to the GWA-6 and CCR-6
clusters, likely due in part to the mill rejects associated with the low pH
area. Downgradient of the coal piles at Allen, low pH conditions are not
observed. Chemical reactions occur at coal piles when water and oxygen is
introduced to pyrite commonly in coal. The chemical reaction typically
results in iron and sulfate in solution, which is consistent with the values
seen in monitoring wells in the vicinity of the coal piles at Allen. Sulfate and
low pH are potential indicator constituents of coal pile impact (EPRI, 2019).
Sulfate, a conservative constituent at Allen, is observed above 02L in the
low pH area and in the area of the coal piles. Other constituents of note
commonly associated with coal piles are aluminum, calcium, magnesium,
potassium, and sodium (EPRI, 2019). These constituents have been
observed in monitoring wells downgradient of the coal piles at
concentrations greater than background and are within the footprint of the
sulfate plume. The remedial actions put forth in this CAP Update address
sulfate as well as the constituents listed above.
Sulfate concentrations are greatest upgradient/southwest of the coal pile
area at well CCR-06S, near the low pH area. Sidegradient of the main coal
pile, and farther downgradient of the low pH area at CP-01S, sulfate
concentrations are less than at CCR-06S. Farther downgradient, at CP-02S,
sulfate concentrations are greater than at CP-01S. Similarly, sulfate
concentrations at GWA-30S, located at the upgradient edge of the main coal
pile, are less than concentrations at CP-2S. This distribution of COI
concentrations from the low pH area and areas upgradient of the coal piles
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to areas downgradient of coal piles indicates the coal piles are an additional
source of COIs, separate from the low pH area. However, there is overlap of
areas where groundwater has been affected by the RAB and the coal piles
and therefore these areas are grouped together as a single source area with
this CAP Update.
The coal piles are anticipated to be used until the Station is
decommissioned. Decommissioning may play an important role in the
corrective action, as coal pile removal would not only eliminate the coal
piles as a source, but also would improve access for implementation of an
active remedy within the footprint of the coal piles.
6.1.1.8 Interim Response Actions
(CAP Content Section 6.A.a.viii)
Interim response actions to date include ash basin decanting, source area
stabilization, and construction of the lined holding basin as summarized in
Table 6-4.
Ash Basin Decanting
(CAP Content Section 6.A.a.viii.1)
Ash basin decanting commenced on June 6, 2019 and is expected to be
ongoing through June 2020. Decanting is a form of active source
remediation by removing ponded water in the AAB, which is considered a
critical component of reducing constituent migration from the ash basins.
Reduction of constituent migration occurs through decanting by
significantly reducing the hydraulic head and gradients, thereby reducing
the groundwater seepage velocity and COI transport potential.
Prior to ash basin closure, the operating level of ponded water in the AAB
was maintained at approximately 634 feet. Flow and transport modeling
simulations indicate decanting will lower hydraulic heads within and
around the ash basins, flow directions within the basins will be more
prominently eastward, and flow velocities will be reduced.
Four ponded water points from the AAB and 42 monitoring well locations
in the vicinity of the AAB were selected for monitoring water elevations
using pressure transducers to record changing site conditions from ash
basin decanting (Figure 6-16). Ponded water, ash pore water, and
groundwater decanting network hydrographs, using water elevations
recorded between March 2019 (May 2019 for ponded water) through
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August 2019 (September for miscellaneous monitoring wells and ponded
water locations) are depicted on Figures 6-17a through 6-17d. Observations
from hydrographs include:
• By mid -August 2019, no ponded water was present within primary
pond 1. Primary pond 1 is primarily a stormwater retention pond.
• Primary ponds 2 and 3 appear to by hydrologically connected with
each other and the main ash basin pond in the AAB.
• The ponded water in the southwestern finger of the AAB appears to
be more influenced by precipitation than decanting. However, the
levels of all surface waters associated with the AAB have decreased
since the start of decanting efforts at Allen.
• By December 2019, water level in the ash basin pond has decreased
by 14.1 feet since decanting started. Note the water elevations
displayed on Figure 6-17d are not current to December 1, 2019.
• All groundwater monitoring locations show a response to ash basin
decanting by reduced groundwater elevation levels (Figures 6-17a
through 6-17d, with the exception of AB-10. This is likely due to the
close proximity of the AB-10 well cluster to the Catawba River (Lake
Wylie).
• Groundwater monitoring wells south of the ash basin (e.g. CCR-
20S/D, CCR-21S, and CCR-22S) and within the ash basin (e.g. AB-
20S/D, AB-21S/SL/SS/BR, AB-22S/D/BR, and AB-26S/D) show the
largest degree of response from decanting by greatest reduction in
water (Figures 6-17a through 6-17d).
Source Area Stabilization
(CAP Content Section, 6.A.a.viii.2)
In an August 22, 2016 correspondence, NCDEQ provided a notice of
deficiencies related to the ash basin dams including the need for spillway
repair, installation of a new principal spillway, vegetation/tree removal, and
slope repair (Appendix A). In response, Duke Energy undertook activities
in 2016 to correct the deficiencies. In letters provided in Appendix A,
repairs and deficiencies were approved by NCDEQ.
Lined Holding Basin Construction
In the fourth quarter of 2018, Duke Energy installed extraction wells to
dewater the area between the main and live coal piles for construction of
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the lined holding basin. Dewatering continued until construction was
completed in the first quarter of 2019. Water level measurements were
recorded during dewatering from several nearby wells. Measurements
collected from nearby wells are included as Figures 6-18a and 6-18b. The
results indicate that monitoring wells in the vicinity of the dewatering
system were affected by dewatering activities. The results of the dewatering
activities were used to calibrate the flow and transport model based on
observed extraction rates and cone of depression areas.
The holding basin has an area of approximately 3.5 acres and is 5 feet deep.
The basin was constructed with approximately 8 feet thick concrete walls
and lined with HDPE. The holding basin receives water from the coal yard
and sump in the live coal pile and then is pumped to the lined retention
basin for further treatment before discharge via NPDES Outfall 006.
Control and management of storm water runoff from the coal pile area was
improved by construction of a lined holding basin between the main and
live coal piles. Improved handling of the storm water in this area impedes
migration of COIs potentially derived from contact with the coal stored in
this area.
6.1.2 Extent of Constituent Migration beyond the Compliance
Boundary
(CAP Content Section 6.A.b)
This section is an overview of COI occurrences beyond the point of compliance.
The point of compliance at Allen is the ash basin compliance boundary (Figure 1-
2). The compliance boundary for groundwater quality at the Site is defined in
accordance with Title Subchapter 02L .0107(a) as being established at either 500
feet from the waste boundary or at the property boundary, whichever is closer to
the waste. The coal piles do not have an associated waste boundary or
compliance boundary.
Analytical sampling results associated with the source areas (ash basins and coal
piles) for each media are included in the following tables and appendix tables:
Soil: Appendix C, Table 4 and Table 6-3 (CAP Content Section 6.A.b.ii.1)
• Groundwater: Appendix C, Table 1 and Table 6-5 (CAP Content Section
6.A.b.ii.2)
Seeps: Appendix C, Table 3 and Table 6-8 (CAP Content Section 6.A.b.ii.3)
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Surface water: Appendix C, Table 2 and Appendix J (CAP Content Section
6.A.b.ii.4)
• Sediment: Appendix C, Table 5 (CAP Content Section 6.A.b.ii.5)
Soil Constituent Extent
(CAP Content Section 6.A.b.ii.1)
Data indicate unsaturated soil COI concentrations at or beyond the compliance
boundary are generally consistent with background concentrations. In the few
instances where unsaturated soil COI concentrations are greater than PSRG POG
standards or Site -specific background values, either COI concentrations are
generally within range of concentrations represented by larger background
dataset that includes 16 other Duke Energy sites across the Piedmont or there are
no mechanisms by which the COI could have been transported from the ash
basin to the unsaturated soils. Adjacent to the coal pile, one sample at CP-2 from
2 to 3 feet below ground surface had an iron concentration that may indicate
effects from the coal pile. However, iron is prevalent naturally in saprolitic soils,
so the detected concentration at the location adjacent to the coal piles may be
natural variations in concentrations within the subsurface (Table 6-3, Figure 6-8).
Horizontal and vertical extent of COI concentrations in soil is discussed further
in Section 6.1.4.
Groundwater Constituent Extent
(CAP Content Section 6.A.b.ii.2)
The ash basin compliance boundary extends 500 feet beyond the ash basin waste
boundary, or to the property boundary, whichever is closer. Groundwater
concentrations greater than 02L/IMAC/applicable background concentration
values occur locally at or beyond the compliance boundary in two general areas:
1. North and northeast of the RAB and coal piles
2. East of the ash basin dams
The Catawba River (Lake Wylie) bounds the Site to the east. The Catawba River
(Lake Wylie) is a groundwater discharge zone that limits the horizontal transport
of constituents downgradient of the source areas. And due to the limited
presence and mobility of most constituents in the groundwater system, COI
concentrations in groundwater have not caused, and will not cause, current
surface water quality standards to be exceeded (Appendix J).
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The maximum extent of COI -affected groundwater migration for all flow zones
is represented by boron and sulfate concentrations greater than the 02L standard.
Boron has migrated from the ash basins to areas east of the basins at
concentrations greater than the 02L standard, at or beyond the compliance
boundary. The boron plume is bounded by the Catawba River (Lake Wylie),
which is the approximately 100 to 200 feet beyond the RAB waste boundary, and
approximately 100 to 200 feet beyond the AAB waste boundary. Boron has not
migrated at or beyond the point of compliance in any other areas.
Sulfate has migrated from the ash basins and coal piles to areas north and
northeast of these source areas at concentrations greater than the 02L standard, at
or beyond the compliance boundary. Like boron, the sulfate plume is also
bounded by the Catawba River (Lake Wylie) to the east of the ash basins and coal
piles. To the north of the RAB and coal piles, sulfate concentrations are greater
than applicable regulatory or background values in the shallow, deep, and
bedrock flow zones. Bedrock wells were among the wells installed in the third
quarter of 2019 downgradient of the coal pile area. However, at CP-2BR, only
minor fractures were observed throughout the borehole to a depth of 275 feet
below ground surface. Packer tests and slug tests were performed at several
depth intervals during drilling. None of the fractures yielded sufficient water for
monitoring. Therefore, the borehole was abandoned and COIs are considered
delineated at the base of the transition (deep) zone in this area. Sulfate and TDS
concentrations greater than 02L standards detected in the initial round of
sampling in October 2019 from newly installed well clusters GWA-27, GWA-28S,
and GWA-29 confirm the distribution of sulfate and TDS simulated in the
groundwater flow and transport models (Appendix G). The model simulates
that sulfate and TDS concentrations in groundwater greater than 02L standards
downgradient of GWA-28 and GWA-29 wells are limited to areas beneath the
power block units and adjacent switchyard and bound by the discharge canal
and Catawba River (Lake Wylie).
Other constituents, including cobalt, iron, manganese, strontium, and TDS, have
concentrations greater than their respective groundwater regulatory standards at
or beyond the compliance boundary. Of these constituents, concentrations
greater than regulatory standards are at locations where boron and/or sulfate
concentrations are greater than 02L standards. Several COIs (cadmium
beryllium, nickel, selenium and thallium) are only observed at concentrations
greater than 02L/IMAC or background in the vicinity of the low pH area west of
the main coal pile. Elsewhere, COIs (i.e., chromium) at concentrations greater
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than applicable comparative values are isolated and do not exhibit a discernable
plume with other occurrences of the same COIs at concentrations greater than
applicable comparative values.
Section 6.1.3 includes a detailed matrix evaluation and rationale of groundwater
constituents requiring corrective action, and Section 6.1.4 provides
isoconcentration maps and cross sections depicting groundwater flow and
constituent distribution in groundwater at or beyond the compliance boundary
(CAP Content Section 6.A.b.i).
Seep Constituent Extent
(CAP Content Section 6.A.b.ii.3)
Seeps at Allen are subject to the monitoring and evaluation requirements
contained in the SOC. The SOC states that the effects from non -constructed seeps
should be monitored. Attachment A to the SOC identifies the following seeps:
• Non -constructed seeps to be monitored — S-2, S-5, S-6, S-7, and S-10
• Non -constructed seeps dispositioned — S-1 and S-9
• Constructed seeps to be monitored per terms of the NPDES Permit — S-3,
S-4, S-8, and S-8B
The SOC defines dispositioned:
1. The seep is dry for at least three consecutive quarters;
2. The seep does not flow to waters of the State;
3. The coal ash basin no longer impacts the seep for all COIs over four
consecutive sampling events;
4. An engineering solution has eliminated the seep.
Table 6-8 provides a summary of seep general location and approximate flow
rate. Seeps at Allen are contained within well-defined channels or pipes.
Therefore, potential COIs related seep flow are constrained in localized areas
along the channel or at the discharge of pipes. Surface water sampling conducted
downstream of seep channels, near the point where the channels confluence with
the Catawba River, demonstrated that flow from seeps has not caused
constituent concentrations greater than 02B standards in the river. Surface water
samples that were collected near the confluence of the seeps with the Catawba
River are shown on Figure 1-2 and included SW-AB-3 (located upgradient of the
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seeps), SW-AB-4 (located between S-8 and S-8B), SW-AB-5 (located between S-7
and S-6), SW-AB-6 (located downgradient of S-5), SW-AB-7 (located upgradient
of S-3 and S-4), and SW-D1 and SW-DG-1 (located downgradient of S-2, S-3, and
S-4). Analytical results for these samples are included in Appendix C, Table 2
and evaluated in Appendix J.
Surface Water COI Extent
(CAP Content Section 6.A.b.ii.4)
Surface water samples have been collected from NCDEQ-approved locations
from the Catawba River (Lake Wylie) to confirm groundwater downgradient of
the source areas has not resulted in surface water concentrations greater than 02B
water quality standards. Surface water samples were collected to evaluate acute
and chronic water quality values. Surface water samples were also collected at
background locations (upgradient of potential migration areas) within the
Catawba River (Lake Wylie). Analytical results were evaluated with respect to
02B water quality standards and background data. All of this data confirms that
there are no surface water quality exceedances related to the Allen ash basins or
coal piles. Surface water conditions is further discussed in Section 6.2.1 and the
full report for Allen surface water current conditions can be found in
Appendix J.
Additionally, environmental assessments of Lake Wylie have all demonstrated
that Lake Wylie has been an environmentally healthy and functioning ecosystem,
and ongoing sampling programs have been established to ensure the health of
this system will continue. Furthermore, these data indicate that there have been
no significant effects to the local aquatic systems related to coal ash constituents
over the last 50 years. More information related environmental health
assessments conducted for Lake Wylie, including sampling programs, water
quality and fish community assessments, and fish tissue analysis, can be found in
Appendix E.
Sediment Constituent Extent
(CAP Content Section 6.A.b.ii.5)
Each sediment sample location is co -located with surface water or tributary
stream seep sample locations (Figure 1-2). Similar to saturated soils and
groundwater, sediment is considered a component of the surface water system,
and the potential leaching and sorption of constituents in the saturated zone is
related to water quality. Because no regulatory standards are established for
sediment inorganic constituents, both background sediment COI concentration
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ranges and co -located surface water sample results are considered in this
sediment evaluation. Table 4-5 presents constituent ranges of background
sediment datasets. Analytical results for sediment samples are provided in
Appendix C, Table 5.
Assessment of COIs in sediment from surface waters, including the Catawba
River (Lake Wylie) and seeps, was conducted through a comparison evaluation
between sediment sample COI analytical results, from one-time grab samples,
and COI concentration ranges from background sediment datasets. Samples
collected from the Catawba River (Lake Wylie) were compared with background
dataset ranges from the Catawba River (Lake Wylie). No background sediment
locations from tributary streams available, therefore maximum COI
concentrations from the Catawba River (Lake Wylie) background sediment
results are used to compare sediment sample results collected from tributary
streams.
Eleven sediment samples have been collected from the Catawba River (Lake
Wylie). Downstream sediment sample locations (Figure 1-2) include eight
locations downstream of the source areas, along the bank of the Catawba River
(Lake Wylie) (SW-AB-1, SW-AB-2, SW-AB-S, SW-AB-7, SW-CP-2, SW-DG-2, SW-
IAB-1, and SW-IAB-4).
Of the eight sediment samples co -located with surface water sample locations in
the Catawba River (Lake Wylie), six samples had at least one COI concentration
greater than the maximum detected concentrations in background sediment. COI
concentrations from the Catawba River (Lake Wylie) downstream sediment
samples greater than background concentrations include boron, cobalt, iron,
manganese, strontium, and sulfate. Surface water samples co -located with the
sediment samples collected from the Catawba River (Lake Wylie) have COI
concentrations less than 02B surface water standards and are generally within
surface water background COI concentration ranges. A summary of the results is
provided below:
• Sediment sample SW-CP-2, located east of the live coal pile, had cobalt,
iron, manganese, and strontium concentrations greater than background.
The cobalt, iron, manganese, and strontium concentrations in sediment are
less than background values for soil. Therefore, it is likely that the COI
concentrations observed in sediment at SW-CP-2 occur naturally.
Furthermore, sediment sample SW-IAB-1, collected downstream of SW -
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CP-2, had no COI concentrations greater than background indicating
constituent concentrations at SW-CP-2 are localized.
• Sediment sample SW-AB-1, located east of primary pond 3, has COI
concentrations greater than background concentrations of boron, cobalt,
iron, manganese, strontium, and sulfate. Each of these concentrations are
less than soil background values, except for iron, which is a common
element in the Piedmont. This indicates these COI concentrations
observed at SW-AB-1 are likely attributable to background. Sediment
sample SW-IAB-4, collected upstream of SW-AB-1, has a strontium
concentration slightly greater than Site background concentrations, but
less than soil background values. This indicates that strontium
concentrations at SW-IAB-4 are likely naturally occurring and further
indicates that strontium concentrations at SW-AB-1 are also naturally
occurring. Sediment sample SW-AB-2, collected downstream of SW-AB-1,
has an iron concentration slightly greater than Site background
concentrations, but less than background soil values. This indicates that
iron concentrations at SW-AB-2 are likely naturally occurring and that
iron concentrations greater than background values at SW-AB-1 are
localized.
• Sediment sample SW-AB-7, located east of the AAB ponded water, has an
iron concentration slightly greater than background concentrations, but
less than background soil values. This indicates that iron concentrations at
SW-AB-7 are likely naturally occurring. Furthermore, sediment sample
SW-AB-S, collected upstream of SW-AB-7, and SW-DG-2, collected
downstream of SW-AB-7, have no COI concentrations greater than
background concentrations. This indicates the iron concentration observed
at SW-AB-7 greater than background is localized.
As evaluated in the risk assessment (Appendix E), there is no evidence that
sediments in the Catawba River adjacent to the Site pose an increased risk to on -
Site or off -Site human receptors or ecological receptors. Additionally, Duke
Energy has monitored Lake Wylie since 1959. Assessments such as water quality,
chemistry, and general species composition have demonstrated that Lake Wylie
is an environmentally healthy and functioning ecosystem. These assessments
indicated that there have been no significant effects to Lake Wylie related to coal
ash constituents over the last 50 years. Therefore, no corrective action for
sediment in the Catawba River is planned at this time.
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Sediment Collected from Seeps
Sediment samples were collected at seven locations within flow paths or
channels at seep sample locations (Figure 1-2). Of the seven sediment samples (S-
1 through S-7), four samples have at least one COI concentration greater than the
maximum detected concentrations in background sediment. Constituents in seep
sediment samples detected greater than background concentrations include
boron, cobalt, manganese, and strontium. A summary of the results is provided
below:
Sediment sample S-1 had a concentration of manganese detected greater
than background concentrations, but less than background soil values. As
stated in the SOC, no CCR impacts were noted via sampling of surface
water flow and Seep S-1 has been dispositioned per the SOC. Therefore,
the observed manganese concentration is likely naturally occurring.
Sediment sample S-2 had concentrations of boron, cobalt, manganese, and
strontium detected greater than background sediment
concentrations. Seep S-2 is regulated by the SOC and flow ceased in
response to decanting. Therefore flow across the sediments at S-2 has been
minimized to occasional storm water. Therefore, the S-2 is now soil.
Boron, cobalt, manganese, and strontium concentrations at S-2 are less
than background values for soil. This indicates these constituent
concentrations are attributable to background. Furthermore, samples
collected downstream of S-2 in the Catawba River do not indicate
influence from the sediment or soil. Sediment sample SW-S-2, collected
within the channelized seep downgradient from S-2, was part of the
groundwater to surface water assessment. Sediment sample SW-S-2 has
concentrations of cobalt, manganese, and strontium detected greater than
sediment background concentrations, but less than soil background
values. This indicates these constituent concentrations may be attributable
to background. As previously mentioned, seep S-2 is regulated by the
SOC. Flow has ceased at S-2 due to decanting. Sediments collected
downstream within the Catawba River do not indicate influence from the
sediments at SW-S-2.
Sediment sample S-3 has a concentration of strontium detected greater
than sediment background concentrations, but less than background soil
values. This indicates strontium concentrations may be attributable to
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background. Seep S-3 is included in the NPDES Permit as Toe Drain
Outfall 103 and is monitored under the NPDES permit.
Sediment sample S-4 has concentrations of boron, cobalt, manganese, and
strontium detected greater than sediment background concentrations, but
less than soil background values. This indicates these constituent
concentrations may be attributable to background. Seep S-4 is included in
the NPDES Permit as Toe Drain Outfall 104 and is monitored under the
NPDES permit.
• Sediment samples S-5, S-6, and S-7 have no COI concentrations detected
greater than background concentrations. Seeps S-5, S-6, and S-7 are
managed by the SOC.
After completion of decanting, seeps covered by the SOC, are to be characterized
for determination of seep disposition. The SOC defines dispositioned: 1) the seep
is dry for at least three consecutive quarters; 2) the seep does not flow to waters
of the State; 3) the coal ash basin no longer impacts the seep for all COIs over
four consecutive sampling events; 4) an engineering solution has eliminated the
seep. After seep characterization, an amendment to the CAP, may be required to
address non-dispositioned seeps. Additional discussion of potential correct
action for seeps is included in Section 6.8.1.
6.1.2.1 Piper Diagrams
(CAP Content Section 6.A.b.iii)
Piper diagrams can be used to differentiate water sources in hydrogeology
by assessing the relative abundance of major cations (i.e., calcium,
magnesium, potassium, and sodium) and major anions (i.e., chloride,
sulfate, bicarbonate, and carbonate) in water.
Groundwater Piper Diagrams
Piper diagrams of groundwater monitoring data from shallow, deep, and
bedrock flow zones are included on Figures 6-15a and 6-15b. Monitoring
locations included on Figures 6-15a and 6-15b include
upgradient/background locations, locations within the waste boundary, and
locations downgradient of the source areas. Data used for the piper
diagrams on Figure 6-15a include groundwater data between January 2018
and April 2019 with a charge balance between -10 and 10%. Additional data
collected as recent as October 2019 was included on the piper diagrams on
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Figure 6-15b to include the recently installed low pH area and coal pile area
assessment wells. Evaluation of the piper diagrams indicates the following:
• Background groundwater at Allen is generally low in chloride and
sulfate and bicarbonate rich, and classified as a range from calcium -
bicarbonate to calcium -sodium plus potassium -bicarbonate type
water. Background groundwater has a lesser proportion of
bicarbonate, calcium and sulfate proportions and a larger range of
chloride proportions compared to ash pore water.
Shallow groundwater tends have a broader range of calcium,
chloride, bicarbonate, and sodium plus potassium compared to deep
and bedrock groundwater.
Groundwater samples from downgradient locations generally fall
between calcium -bicarbonate type water and calcium -sulfate type
water and have a broader range of bicarbonate proportion compared
to background locations.
• Groundwater at downgradient locations that plot similarly to
background are typically at locations where boron is detected at
concentrations less than the 02L or background, such as the wells
south of the AAB and in deep and bedrock flow zones on the western
side of the basins.
• Groundwater downgradient of the basins where boron
concentrations are greater than or, close to the 02L, plot on the piper
diagrams similar to ash pore water and are more sulfate- and
calcium -rich, and with a wider range of bicarbonate. This indicates
influence from the ash basins by mixing of groundwater and ash
pore water.
• Wells GWA-7D and GWA-6BR plot similar to typical ash pore water,
however boron is not detected in these locations. This indicates
groundwater may be influenced by another source, such as the coal
pile. Piper diagrams could not be made for GWA-6S and GWA-61)
due to charge balance differences beyond acceptable limits (10%).
Groundwater samples from within and downgradient of the low pH
area (Figure 6-15b) tend to group based on flow zone. Shallow flow
zone samples generally plot in the 'affected' or 'potential mixing'
zones. Deep flow zone samples plot relatively evenly throughout the
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three zones, 'affected', 'potential mixing', and 'generally unaffected'.
Bedrock flow zone samples plot in either 'generally unaffected' or
'potential mixing', with CP-6BR as the exception (plots in 'affected').
Groundwater samples from the deep flow zone within the low pH
area (Figure 6-15b) plot as generally unaffected, which agrees with
the data (Table 6-5) and the CSM.
• As noted above, CP-6BR plots in the 'affected' area of the Piper
diagram. Data indicate CP-6BR is unimpacted by the source areas,
with strontium as the only exceedance of comparative criteria.
Concentrations of strontium at CP-6BR are within the Site
background range.
Seep and Surface Water Piper Diagrams
(CAP Content Section 6.A.b.iii)
Piper diagrams of seep and Catawba River (Lake Wylie) surface water
monitoring data (Figure 6-26) are used to assess the relative abundance of
major cations (e.g., calcium, magnesium, potassium, and sodium) and major
anions (e.g., chloride, sulfate, bicarbonate, and carbonate) in surface water.
Data used for the piper diagrams include most recent available seep and
surface water data (Appendix C, Tables 2 and 3) with a charge balance
between -10 and 10 percent. From ash pore water and groundwater piper
diagrams (Figures 6-15a and 6-15b), areas identified where ash pore water
tends to plot is noted as "affected"; areas that show potential mixing with
affected water is noted as "potential mixing", and areas that are similar to
background (or native) water quality are noted as "generally unaffected".
Based on sample location, the samples group predictably.
Seeps and surface waters at the Site are predominantly characterized
as calcium -sulfate waters with a few seep samples plotting between
calcium -sulfate and calcium -bicarbonate.
• Free water within the AAB tends to plot with higher proportions of
sulfate, chloride, calcium, and magnesium, just as ash pore water
generally does.
• Seeps downgradient of the ash basins are primarily characterized by
two water types, calcium -bicarbonate to calcium -sulfate, similar to
ash pore water.
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• A seep located southeast of the AAB (S-02) is magnesium -chloride
type water. This indicates mixing of background water with ash
basin -influenced water within the wet area downgradient of seepage
from the AAB.
A seep located south of the AAB (S-01) plots as calcium -bicarbonate,
which suggests background groundwater influence. This seep has
been dispositioned because water quality samples do not indicate
influence from the ash basin.
Samples from the Catawba River (Lake Wylie) plot between calcium -
sodium plus potassium -bicarbonate and calcium -chloride waters.
These results plot similar to surface water samples analyzed from
Lake Norman in Iredell County, North Carolina upstream of Allen
(USGS, 2008).
6.1.3 Constituents of Interest (COIs)
(CAP Content Section 6.A.c)
This CAP Update evaluates the extent of and remedies for COIs associated with
the Allen ash basins and coal piles that are at or beyond the compliance
boundary to the north, northeast, and east of the source areas detected at
concentrations greater than regulatory criteria or background values, whichever
is greater.
Site -specific COIs were developed by evaluating groundwater sampling results
with respect at concentrations greater than regulatory criteria or background
values, whichever is greater and additional regulatory input/requirements. The
distribution of constituents in relation to the source areas, co -occurrence with
CCR indicator constituents such as boron and sulfate, and migration directions
based on groundwater flow direction are considered in determination of COIs.
The following list of COIs was developed as part of the CSA Update for Allen
(SynTerra, 2018a):
• Antimony • Molybdenum
• Arsenic • Nickel
• Beryllium • pH
• Boron • Selenium
• Cadmium • Strontium
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• Chromium (Hexavalent) • Sulfate
• Chromium (Total) • Thallium
• Cobalt
• Iron
• Manganese
• TDS
• Vanadium
Subsequent sampling and analysis for USEPA CCR Rule compliance indicated
lithium is an additional COI at Allen. Hexavalent chromium was included as a
COI in the CSA Update at the request of NCDEQ (Appendix A).
Soil
(CAP Content Section 6.A.c.i.1)
Unsaturated soil at or near the compliance boundary is considered a potential
secondary source to groundwater. Constituents, if present in unsaturated soil or
partially saturated soil (vadose zone), have the potential to leach into the
groundwater system if exposed to favorable geochemical conditions for chemical
dissolution to occur. Constituents considered for unsaturated soil evaluation
were the same constituents identified as COIs for the ash basins and coal piles,
since soil impacts, if present, would be related to ash pore water interaction to
the underlying soils within the basins, groundwater migration at or beyond the
ash basins, and coal pile runoff.
Allen samples of background soil and rock media at Allen indicate that some
naturally occurring constituents that are also typically related to CCR material
and likely affect the chemistry of groundwater at the Site, are present at
concentrations greater than the PSRGs POG values (Table 4-2). Constituents with
background values greater than PSRGs POG values include total chromium,
cobalt, iron, manganese, selenium, and thallium.
Data indicate unsaturated soil COI concentrations are generally consistent with
background concentrations or are less than regulatory screening values (Table 6-
3). In the few instances where unsaturated soil COI concentrations are greater
than PSRG POG standards or background values, COI concentrations are within
range of background dataset concentrations or there are no mechanisms by
which the COI could have been transported from the ash basin to the
unsaturated soils. Horizontal and vertical extent of COI concentrations in soil,
and reasons why no necessary corrective action for soils is identified at the Site,
is discussed further in Section 6.1.4.
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Groundwater
(CAP Content Section 6.A.c.i.2)
A measure of central tendency analysis (means analysis) of groundwater
constituent data (January 2018 to June 2019) was conducted and means were
calculated to support the analysis of groundwater conditions and to provide a
basis for defining the extent of the COI migration beyond the compliance
boundary. The mean analysis method was selected to capture the central
tendency (arithmetic mean, geometric mean, and median) of constituent
concentrations, which may vary over orders of magnitude. A single sample
result might not be an accurate representation of the concentrations observed
over several months to years of groundwater monitoring. Evaluating constituent
plume geometries with central tendency data minimizes the potential for
incorporating occasions when COIs are reported at concentrations outside of the
typical concentration range, and potentially greater, or substantially less than
enforceable groundwater standards. Previous Site assessment mapping based on
single COI concentrations for each well might have overrepresented or
underrepresented areas affected by the ash basins by posting a single data set on
maps and cross -sections that might have included isolated data anomalies.
NCDEQ (October 24, 2019, Appendix A) recommended the use of a lower
confidence limit (LCL95) rather than the central tendency value. LCL95
concentrations were calculated for each COI. The LCL95 concentration for the
sample with the highest COI LCL95 concentration is provided for comparison to
the COI mean concentration in Table 1 of the technical memorandum titled COI
Management Plan Approach —Allen Creek Steam Station (Arcadis, 2019b) included
within Appendix H. The mean COI concentration is typically higher than the
LCL95 concentration, and therefore, is more conservative for comparison to the
COI criterion.
The mean of up to six quarters of valid data was calculated for each identified
COI to analyze groundwater conditions and define the extent of COI migration
beyond the compliance boundary. At a minimum, four quarters of valid data
were used for calculating means, however, if fewer than four quarters of valid
data were available, the most recent valid sample result was reported. For use in
calculating means, non -detect values were assigned the laboratory reporting
limit and estimated (J-flag) values were treated as the value reported. Procedures
for excluding data from calculating means are based on USEPA's National
Functional Guidelines (USEPA, 2017), published research about leaching of
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elements from coal combustion fly ash (Izquierdo and others, 2012), and
professional judgement.
The following steps outline the approach followed in calculating central
tendency values for constituent concentrations in groundwater:
1. If the maximum analytical value divided by the minimum value for each
constituent was greater than or equal to 10 (i.e. the data set ranges over
an order of magnitude), the geometric mean of the analytical values was
used.
2. If the maximum analytical value divided by the minimum value for each
constituent was less than 10 (i.e. the data set range is within an order of
magnitude), the arithmetic mean was used.
3. The median of the data was used for records that contain zeros or
negative values (e.g., total radium). Negative values were set to zero
prior to calculating the median concentration.
4. If the dataset mode (most common) was equal to the RL, and the
geometric mean or mean value was less than or equal to the dataset's
mode, the value was reported as "<RL" (e.g. the reporting limit for boron
is 50 µg/L; for wells with geometric mean or mean analysis
concentrations less than 50 µg/L the mean analysis result would be
shown as "<50").
Sample results were excluded from calculations for the following conditions:
• Duplicate sampling events for a given location and date. The parent
(CAMA) sample was retained
• Turbidity was greater than 10 Nephelometric Turbidity Units (NTUs)
• pH was greater than 10 S.U. Data with pH greater than 10 S.U. might
suggest well grout impacts
• Data flagged as unusable (RO qualified)
• Data reported as non -detect with a reporting limit greater than the
normal laboratory reporting limit
Table 6-5 presents the mean analysis results of the COI data using groundwater
monitoring sampling results from January 2018 to June 2019. Where means could
not be calculated, the most recent valid sample was evaluated to determine
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whether the sample result is an appropriate representation of the historical
dataset. Data from Table 6-5 are used in evaluating COI plume geometry in the
vicinity of the ash basins and coal pile area.
Constituent Management Approach
As discussed in the beginning of Section 6, a 'COI management process' was
developed by Duke Energy at the request of NCDEQ to gain understanding of
the COI behavior and distribution in groundwater distribution and to select the
appropriate remedial approach. Details of the COI management approach are
provided in Appendix H. In general, the COI management process consists of
three steps:
1. Performing a detailed review of the applicable regulatory requirements of
NCAC, Title 15A, Subchapter 02L
2. Understand the potential mobility of site -related COIs in groundwater
based on Site hydrogeology and geochemical conditions
3. Determine the COI distribution related to the ash basins and coal piles
under current or predicted future conditions.
The management process uses a matrix evaluation [Table 6-6 (CAP Content
Section 6.A.c.i.2)]
This COI management process is supported by multiple lines of evidence
including empirical data collected at the Site, geochemical modeling, and
groundwater flow and transport modeling. This approach has been used to
understand and predict COI behavior in the subsurface related to the ash basins
and coal pile area or COIs that are naturally occurring. COIs that have migrated
beyond the compliance boundary at concentrations greater than 02L, IMAC and
background that are related to an ash basin would be subject to corrective action.
COIs that are naturally occurring at concentrations greater than 02L, IMAC and
background do not require corrective action.
Using the constituent management process, 12 of the 19 inorganic groundwater
constituents (not including pH) identified in the CSA (CSA Update, 2018a),
exhibit mean concentrations that are currently less than background values, the
02L standard, or IMAC at or beyond the compliance boundary, or have few
concentrations greater than comparison criteria but with no discernable COI
plume characteristics (e.g. antimony in the deep flow zone). These 12
constituents include:
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• Antimony
• Lithium
• Chromium
• Molybdenum
• Arsenic
• Nickel
• Beryllium
• Selenium
• Cadmium
• Thallium
• Chromium
• Vanadium
• Chromium VI
These constituents are not expected to migrate distances that would present risk
to potential receptors or beyond the compliance boundary, and are predicted,
based on geochemical modeling, to remain at stable concentrations, typically less
than background values, the 02L standard, or IMAC. Arsenic, beryllium,
cadmium, lithium, nickel, selenium, thallium and vanadium are not detected at
locations beyond the compliance boundary. Of these, arsenic, beryllium,
cadmium, calcium, nickel, selenium, and thallium are only detected in the low
pH area.
As shown in Table 6-6, concentrations of antimony, hexavalent chromium, total
chromium, and molybdenum occur at or beyond the compliance boundary
greater than comparative criteria in one or more groundwater monitoring wells.
Antimony, hexavalent chromium, total chromium, and molybdenum are not
considered corrective action COIs due to the following rationale:
Antimony at concentrations greater than comparative criteria at or beyond
the compliance boundary is present in only one groundwater monitoring
well, CP-1D. Therefore there is no discernable plume of antimony.
Furthermore, antimony concentrations at CP-1D do not exceed the Site -
specific background value ranges at Allen, indicating the antimony
concentration is attributable to background and not the source areas.
Hexavalent chromium is not detected in ash pore water at concentrations
greater than comparative criteria at Allen, therefore, the ash basins are not
a source of groundwater concentrations detected in areas in the vicinity of
the Site. None of the wells surrounding the coal pile have concentrations
greater than applicable criteria for hexavalent chromium, indicating the
coal pile is not a source of hexavalent chromium in groundwater.
Additionally, of the 13 monitoring wells (AB-1R, AB-21), AB-6A/R, AB-
10D, AB-11D, GWA-1D, GWA-2D, GWA-7S/D, GWA-8S, and CCR-
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26D/BR) where hexavalent chromium was detected at concentrations
greater than comparative criteria, the concentrations were less than the
range of hexavalent chromium concentrations within the background
dataset that includes 16 other Duke Energy sites across the Piedmont
province. Further, numerous water supply wells upgradient and beyond a
hydrologic divide exceed comparative criteria for hexavalent chromium.
Therefore, the detected concentrations in these 13 wells are likely
attributable to naturally occurring concentrations and natural variation
within the region and not the ash basins and coal piles.
• Total chromium is not found in ash pore water above comparative criteria
and therefore, is considered not attributable to the ash basins at Allen.
None of the wells surrounding the coal pile have concentrations greater
than applicable criteria for total chromium, indicating the coal pile is not a
source of total chromium in groundwater. Total chromium has been
detected in only one monitoring well (AB-6A) at or beyond the
compliance boundary at concentrations greater than the comparative
criteria for total chromium. However, the concentrations are less than the
flow zone specific background value range at Allen, therefore there is no
discernable plume attributable to the ash basin or coal piles.
Molybdenum at concentrations greater than comparative criteria at or
beyond the compliance boundary is present in only two groundwater
monitoring wells (CCR-11S and GWA-4BRL). Although the these two well
clusters are adjacent to one another, the wells with detections greater than
comparative values are in separate flow zones indicating there is no
discernable plume of molybdenum attributable to the adjacent ash basins.
Furthermore, none of the wells surrounding the coal pile have
concentrations greater than applicable criteria for molybdenum, indicating
the coal pile is not a source of molybdenum in groundwater. While
greater than the comparative criteria, molybdenum concentrations at
CCR-11S and GWA-4BRL are less than the Site specific background value
ranges at Allen. Therefore, the detected concentrations of molybdenum in
these two wells are likely attributable to naturally occurring
concentrations and natural variation within the region and not the ash
basins and coal piles.
Radionuclides radium and uranium have been monitored periodically since
2016. Uranium has not been detected in any wells at a concentration greater than
the EPA MCL of 0.03 micrograms per milliliter (µg/ml). Total radium has not
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been detected in recently any current wells at a concentration greater than the
EPA MCL of 5 picocuries per liter (pCi/L). Historical sample results from wells
CCR-31), CCR-3DA, and CCR-11D had total radium concentrations greater than
5 pCi/L. Of these wells, CCR-31) and CCR-11D were replaced because elevated
pH values likely due to grout impacts from well installation, and the analytical
results were inconsistent, indicating those results are not reliable. The remaining
location, CCR-3DA, which replaced CCR-3D, had a single detection of total
radium greater than 5 pCi/L in April 2017, but concentrations have since been
less than 5 pCi/L after seven additional monitoring events. Therefore, radium
and uranium, if present in groundwater at Allen, are at concentrations less than
applicable regulatory criteria and not considered COIs.
The remaining seven COIs exhibit mean concentrations greater than background
values, 02L standards, or IMACs with plume characteristics downgradient of the
ash basins and coal pile area at or beyond the compliance boundary. These
constituents are as follows:
• Boron • Strontium
• Cobalt • Sulfate
• Iron • TDS
• Manganese
As discussed in the CSA Update (SynTerra, 2018a), and the 2018 CAMA Annual
Interim Monitoring Report (SynTerra, 2019c), not all constituents with results
greater than background values can be attributed to the ash basins or coal piles.
Naturally occurring groundwater contains varying concentrations of inorganic
constituents. Sporadic and low -concentration occurrences of constituents in
groundwater data do not necessarily demonstrate horizontal or vertical
distribution of COI -affected groundwater migration from the ash basins or coal
pile area.
6.1.4 Horizontal and Vertical Extent of COIs
(CAP Content Section 6.A.d)
The COIs at Allen have been sufficiently delineated horizontally and vertically in
groundwater based on sampling and analysis data collected from 234 monitoring
wells present at the site and flow and transport modeling. The majority of COIs
are either present below their applicable standards, do not exhibit discernable
plumes, or have migrated a limited distance from the ash basins and coal piles in
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groundwater. In fact, the presence of COIs downgradient of the ash basins waste
boundary is limited to between approximately 50 and 1,600 feet and
approximately 500 feet from the coal piles. Furthermore, an evaluation of site
data indicates that COI presence in groundwater decreases with depth.
Supporting information for these findings are presented in the COI management
evaluation presented in Section 6.1.3 and in Appendix H.
Boron, a conservative (non -reactive) constituent, is the main COI that is present
in Site groundwater in a discernable plume related to the ash basins, although
boron concentrations decline below its 02L standard within approximately 500
feet beyond the ash basins waste boundary. Boron typically has greater
concentrations in CCR than in native soil and is relatively soluble and mobile in
groundwater (Chu, 2017). Sulfate, also a conservative constituent that is
relatively soluble in groundwater, is the main COI that is present in Site
groundwater in a discernable plume related primarily to the coal piles, although
sulfate concentrations are modeled to decline below its 2L standard within
approximately 750 feet beyond the coal piles. Non -conservative and variable
constituents have smaller, and generally isolated, plume geometries relative to
boron and sulfate because of their high Ka values and reactivity, which reduce
their mobility. Therefore, the maximum extent of the 02L boron plume (700 µg/L)
and sulfate plume [250 milligrams per liter (mg/L)] was used to determine the
maximum extent of COI -affected groundwater migration. Additional constituent
concentrations identified as being greater than their respective groundwater
regulatory standards or background values, and are associated with COI -affected
groundwater migration from the ash basins and coal pile area, are confined
within the extent of the 02L boron and sulfate plume at the Site. Therefore, the
boron (700 µg/L) and sulfate (250 mg/L) plumes were used to define the
maximum extent of COI -affected groundwater migration.
Since naturally occurring COIs might be present at concentrations greater than
Site -specific background values, isoconcentration maps of primary CCR
indicator COIs (e.g., boron, sulfate, and TDS) are most representative of the
groundwater COI plume extent in three-dimensional space. The horizontal
extent of COI -affected groundwater migration in each flow layer is depicted by
the boron (Figures 6-19a through 6-19c), sulfate (Figures 6-20a through 6-20c),
and TDS (Figures 6-21a through 6-21c) isoconcentration maps. The background
and 02L boron, sulfate, and TDS plumes generally represent the maximum extent
of COI -affected groundwater migration in each flow layer.
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Isoconcentration maps and cross -sections use groundwater analytical data to
spatially and visually define areas where groundwater COI concentrations are
greater than the respective constituent background values and/or 02L/IMAC. In
areas where data is not available, flow and transport model results for boron,
sulfate, and TDS were interpreted and included within the isoconcentration
maps. The model predictions are conservative; the model over -predicts the actual
groundwater concentrations in some isolated areas.
Mean data of groundwater COI monitoring sampling results from January 2018
to June 2019 provide an understanding of groundwater flow dynamics and
direction to define the horizontal and vertical extent of the COI plume.
Horizontal extent of the COI plume is depicted on isoconcentration maps for
boron (Figures 6-19a through 6-19c), sulfate (Figures 6-20a through 6-20c), TDS
(Figures 6-21a through 6-21c), strontium (Figures 6-22a through 6-22c), cobalt
(Figures 6-23a through 6-23c), iron (Figures 6-24a and 6-24b), and manganese
(Figures 6-25a through 6-25c). COI concentrations for COIs representative of
each geochemical grouping (conservative, non -conservative, and variable)
discussed in Section 6.1.5 are shown on five cross -sectional depictions of the Site.
Cross-section A -A' (Figures 6-9a through 6-9c) is oriented north to south and
displays the areas downgradient of the coal pile area and ash basins. Cross-
section B-B' (Figures 6-10a through 6-10c) is oriented west to east and displays
the RAB footprint, coal pile area, topography, and depth of saturated ash in the
RAB. Cross-section C-C' (Figures 6-11a through 6-11c) is oriented west to east
and displays the RAB footprint, topography, and depth of saturated ash in the
RAB. Cross-section D-D' (Figures 6-12a through 6-120 is oriented west to east
and displays the northern AAB footprint, topography, free water in the primary
ponds, and depth of saturated ash in the basin. Cross-section E-E' (Figures 6-13a
through 6-13c) is oriented west to east and displays the southern portion of the
AAB footprint, topography, depth of saturated ash in the basin, and free water
near the dam.
Beyond the compliance boundary, the maximum extent of COI -groundwater
affected by the ash basins and coal pile area occurs north, northeast, and east of
the ash basins and coal piles.
6.1.4.1 COIs in Unsaturated Soil
(CAP Content Section 6.A.d.i)
Unsaturated soil at or beyond the compliance boundary has potential to be
a secondary source to groundwater. Constituents present in unsaturated
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soil or partially saturated soil (vadose zone) have the potential to leach into
the groundwater system if exposed to favorable geochemical conditions for
chemical dissolution to occur. Therefore, constituents considered for
unsaturated soil evaluation as related to the ash basins and coal piles were
the same constituents identified as COIs in groundwater related to the ash
basins and coal piles.
Unsaturated soil samples at or beyond the compliance boundary were
collected from well installation activities and an additional soil sampling
event in April 2018. The sampling event in April 2018 was conducted to
further delineate unsaturated soils based on CSA Update comments made
by NCDEQ (Appendix A). Unsaturated soils samples at or beyond the
compliance boundary include samples collected from AB-9, AB-10, AB-11,
CCR-23, CCR-26, CP-1, CP-2, CP-3, CP-4, GWA-1, GWA-2, GWA-3, GWA-5,
GWA-6, GWA-9, GWA-9, GWA-15, GWA-28, and GWA-29 (Figure 6-8). An
evaluation of the potential nature and extent of COIs in unsaturated soil at
or beyond the waste boundary was conducted by comparing unsaturated
soil concentrations with PSRG POG or background values, whichever is
greater [(Table 6-3) (CAP Content Section 6.A.d.i)]. PSRG POG standards
were calculated for sulfate (1,438 mg/kg) (Table 6-3).
Data indicate unsaturated soil COI concentrations are generally consistent
with background concentrations or are less than regulatory screening
values. In the few instances where unsaturated soil COI concentrations are
greater than PSRG POG or background values, COI concentrations are
generally within the range of the background dataset compiled from
Piedmont sites or there are no mechanisms by which the COI could have
been transported from the ash basins to the unsaturated soils, indicating the
observed concentrations occur naturally.
At two locations and at isolated depth intervals at GWA-27 and CP-2, iron
(and manganese at CP-2) concentrations in soil are greater than PSRG POG,
Site -specific background values and the larger range of background
concentrations from the dataset that includes other sites within the
Piedmont. However, the iron and manganese concentrations in these
isolated areas may further indicate natural variability in concentrations for
these COIs. The concentrations isolated to the specific depth intervals at
these locations do not reveal concentration trends indicating clear affects
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from the source areas, with the potential exception of iron detected at CP-2
from 2 to 3 feet below ground surface (bgs).
At GWA-27, iron concentrations detected slightly greater than the Piedmont
background range were detected in samples from 14-15 feet bgs and 19-20
feet bgs. However, concentrations in shallower soil from 10-11 feet bgs are
less than PSRG POG or background values. This indicates there is not a
source of constituents from a shallow source near the surface, which could
be indicative of Station operations or waste management. Furthermore, iron
concentrations in groundwater at this location are less than regulatory
criteria. This indicates that relatively little iron is being transported with
groundwater that has passed beneath the RAB and that iron in
groundwater is not, and has not been, the source of iron in soil at the depth
intervals of 14-15 feet and 19-20 feet bgs, even if groundwater levels in this
area were to have fluctuated as high as 14 feet historically. Therefore, it is
likely that the iron concentrations detected at 14-15 feet bgs and 19-20 feet
bgs at GWA-27 are naturally occurring.
At CP-2, iron concentrations detected at a depth interval of 2-3 feet bgs may
be a result of runoff from the coal pile or other historical operations.
However, iron is prevalent naturally in saprolitic soils, so the detected
concentration may be a natural variation in concentration within the
subsurface. Furthermore, the iron at concentrations greater than Piedmont
background is isolated to this shallow depth interval and is, therefore, not
considered a significant source of COIs in groundwater. Manganese
detected at a concentration greater than the Piedmont background at CP-2
is isolated to a depth interval from 5 to 6 feet bgs. Manganese, like iron, is
prevalent naturally in saprolitic soils, so the detected concentration may be
a natural variation in concentration within the subsurface. Similar to iron
concentrations at GWA-27, the isolated depth interval of manganese
concentrations at CP-2 indicates there is not a source of constituents from a
shallow source near the surface, which could be indicative of Station
operations or waste or coal management. Manganese concentrations in
unsaturated soil at depths below 6 feet are less than background
concentrations. This indicates groundwater transport of manganese from
depths below to the 5- to 6-foot interval is not the mechanism for the
presence of manganese at CP-2, even if historical water levels were higher
than they are currently. Because there is no mechanism for transport of
manganese to the 5- to 6-foot depth interval at CP-2 from potential source
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areas, the manganese at this location and depth interval is likely naturally
occurring.
Active remediation is planned along with source control measures and
decommissioning in the areas of both GWA-27 and CP-2. The contingency
plan generally describes an adaptive approach to address unsaturated soil
in the future, if needed, following decommissioning and closure that could
address iron concentrations at CP-2, if necessary. Therefore, no corrective
action for soils is planned at this time and this CAP Update focuses on
remediation of COIs in groundwater derived from the ash basins and the
coal piles.
6.1.4.2 Horizontal and Vertical Extent of Groundwater in
Need of Restoration
(CAP Content Section 6.A.d.ii)
This section discusses the horizontal and vertical extent of groundwater in
need of restoration in areas north, northeast, and east of the source areas.
Groundwater is not in need of restoration adjacent to the ash basins to the
northwest, west, and south due to the lack of COI concentrations greater
than applicable standards in these areas.
Eastern Extent of COI -Affected Groundwater
East of the dam along the Catawba River (Lake Wylie), downgradient of the
ash basins, the COI plume at or near the compliance boundary is defined by
boron at concentrations greater than 02L (Figures 6-19a through 6-19c). The
extent of affected groundwater transport related to hydraulic conditions is
supported by the following observations (Figures 6-9a through 6-13c,
Figures 6-19a through 6-25c):
Mean analysis of boron from groundwater monitoring wells in the
western portion of the ash basins indicates concentrations are
generally less than background (non -detect), suggesting the influx of
background groundwater from upgradient. This supports the flow -
through setting of the CSM.
Mean analysis of boron from groundwater monitoring wells along a
flow transect within the AAB at AB-25S (ash pore water), AB-25SL
(ash pore water), and AB-25SS (shallow flow zone) indicate
concentrations are greater than 02L in all three wells. These wells are
located on a basin divider dike between primary pond 2 and primary
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pond 3. As discussed in the CSM, downward migration of COIs is
observed at the ash basin dams and at these divider dikes.
Mean analysis of boron from groundwater monitoring wells AB-22S
and AB-32S indicates concentrations are less than background (<50
µg/L). Groundwater monitoring wells AB-22S and AB-32S are
located in the dam east of the basins, centrally located within the
groundwater plume. These wells are located in areas with
downward (positive) vertical gradients due to the dams effect on
groundwater. Due to the vertical gradients, shallow wells in these
areas are generally unaffected by COIs as flow is downward below
the dams. The deep and upper bedrock flow zones in these areas
generally indicate boron values greater than the 02L standard. This
observation is consistent with the CSM. However, boron
concentrations in deep/lower bedrock wells are less than 02L,
indicating limited downward migration of COIs.
• Boron concentrations are greater than 02L east of the dam along the
Catawba River (Lake Wylie) in some locations. Installation of
monitoring wells farther downgradient is not possible due the
proximity of the river. The flow and transport model indicates boron
concentrations at these locations extend beneath the river
approximately 200 feet from the shoreline.
• Deep and bedrock flow zones have similar plume geometries east of
the ash basins. Generally, COI concentrations decrease with depth,
depicting the upward gradient downstream of the dams as
groundwater discharges to the Catawba River, as detailed in Section
5.0.
Mean analysis of boron concentrations north of the RAB beyond the
compliance boundary are delineated by CP-5S for the shallow zone,
CP-04D for the deep zone, and CP-6BR for the bedrock zone.
Mean analysis of boron from groundwater monitoring wells GWA-
02S/D indicate concentrations are less than background values. These
wells delineate the boron plume horizontally and vertically to the
south.
• The deep/lower bedrock wells GWA-3BRL, GWA-4BRL, GWA-5BRL,
GWA-5BRL and AB-10BRL have delineated the boron plume
downgradient of the ash basins.
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• Surface water sampling from the Catawba River (Lake Wylie)
adjacent to the Site confirms that there are no surface water quality
exceedances related to the Allen ash basins or coal piles.
Northern and Northeastern Extent of COI -Affected
Groundwater
North, northeast, and east of the RAB and coal pile area, the COI plume at
or beyond the compliance boundary is defined by sulfate and TDS at
concentrations greater than 02L. This area encompasses northern portions
of the RAB (including the low pH area) and the coal pile area. Boron is also
present at concentrations greater than the 02L beyond the compliance
boundary north of the RAB, but to lesser extents than sulfate and TDS. The
extent of affected groundwater transport related to hydraulic conditions is
supported by the following observations and shown on Figures 6-20a
through 6-21c:
The sulfate and TDS plume is delineated horizontally to the east as it
is bound by the Catawba River (Lake Wylie).
The sulfate and TDS plume is delineated horizontally and vertically
to the south by wells located at CCR-7S/D and CP-6S/D/BR. Notably,
the plume associated with sulfate concentrations greater than 02L
comingles minimally with the plume associated with boron to the
south.
• Sulfate and TDS detected at concentrations greater than 02L
standards in the shallow, deep, and bedrock flow zones north of the
RAB and coal piles confirm the distribution of sulfate and TDS
simulated in the groundwater flow and transport models (Appendix
G). The model indicates these constituents at concentrations greater
than 02L are limited to the north to areas beneath the power block
units and adjacent switchyard and bound by the discharge canal to
the west and Catawba River (Lake Wylie) to the east.
• Flow and transport model simulations indicate the sulfate and TDS
transport north of the RAB and coal piles is limited due to advective
flow from upgradient areas north of the power block where
topography is higher and groundwater flow is east southeast toward
the power block and Catawba River (Lake Wylie).
• Shallow, deep and bedrock flow zones have similar sulfate and TDS
plume geometries north and northeast of the RAB and coal piles.
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This indicates a well-connected, unconfined flow system between the
flow zones. However, at CP-2BR, no significant fractures in bedrock
were encountered which indicates limited to no potential for COIs to
be transported into bedrock in this area. The minor fractures that
were encountered yielded insufficient water for monitoring.
Therefore, COIs are considered delineated at the base of the
transition (deep) zone in the area surrounding the CP-2 well cluster.
Additionally, sulfate and TDS are not detected at concentrations
greater than 02L values in bedrock groundwater immediately north
and northeast of the RAB at GWA-6BRA or GWA-6BRL. This
indicates limited interconnection of the shallow and deep flow zones
with bedrock groundwater in this area but that downward transport
of COIs occurs farther downgradient from the ash basins. This also
indicates sulfate and TDS concentrations north and northeast of the
RAB and coal pile are derived, in part, from the coal piles.
Based on available data, the plume characterized by sulfate, TDS and
boron is stable and bound within the Site.
Other COIs detected at concentrations greater than applicable
regulatory criteria in this area are within the footprint of sulfate and
TDS plumes, although cobalt concentrations in the shallow and deep
flow zones extend slightly more westerly than sulfate, but remain
within the bounds of the Site.
Surface water sampling from the Catawba River (Lake Wylie)
adjacent to the Site confirms that there are no surface water quality
exceedances related to the Allen ash basins or coal piles.
6.1.5 COI Distribution in Groundwater
(CAP Content Section 6.A.e)
As part of the COI management process and geochemical modeling (Appendix
H) constituents with concentrations greater than the 02L standard, IMAC, or
background values beyond the compliance boundary were grouped by
geochemical behavior and mobility. A comprehensive evaluation (i.e. mean
analysis and groupings) of available data was used to demonstrate constituent
distribution in groundwater to evaluate the spatial occurrence with a discernable
plume in the direction of groundwater flow downgradient of the ash basins and
coal pile area. The groupings of constituents that were mapped and are
considered for corrective action are as follows:
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• Conservative, non -reactive constituents: boron, sulfate, and TDS.
Geochemical model simulations support that these constituents would
transport conservatively (Ka values <1 L/kg) as soluble species under most
conditions, and that the mobility of these COIs will not change
significantly due to current geochemical conditions or potential
geochemical changes related to remedial actions.
• Non -conservative, reactive constituent: strontium. Geochemical model
simulations support that this constituent is subject to significant
attenuation in most cases, especially in the shallow flow zone and has
high Ka values indicating the mobility of this COI is unlikely to be
geochemically affected by current geochemical conditions or potential
geochemical changes related to remedial actions. Strontium reactivity is
less in the deep and bedrock flow zones and can be more mobile under
lower pH conditions, due to both the lower sorption affinity of strontium
at lower pH values as well as the increased concentration of other divalent
ions (e.g., Ca+2, Mg+2, Co+2, Mn+2) that may compete with strontium for
ion exchange sites.
Variably reactive constituents: cobalt, iron, and manganese. Geochemical
model simulations, and resulting Ka values, support these constituents
may be non -reactive or reactive in relation to geochemical changes and are
dependent on the pH and Eh of the system. The sensitivity of these COI to
the groundwater pH and Eh indicates that these constituents could
respond to natural changes under current conditions, such as water level
fluctuations imposed by seasonality, and decanting or source control
activities that have the potential to change the groundwater pH or Eh.
COIs identified in the CSA that are not mapped in this CAP Update are not only
limited within the compliance boundary, but are further limited to isolated areas
within the compliance boundary. In fact, several COIs (cadmium beryllium,
nickel, selenium and thallium) are only observed at concentrations greater than
02L/IMAC or background in the vicinity of the low pH area west of the main coal
pile.
6.1.5.1 Conservative Constituents
(CAP Content Section 6.A.e.i)
Boron, sulfate, and TDS mean isoconcentration maps, cross sections and
groundwater flow and transport modeling support the following
observations regarding the extent of COI -affected groundwater represented
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by these conservative (non -reactive) constituents (Figures 6-9a, 6-10a, 6-11a,
6-12a, 6-13a, and 6-19a through 6-21c; Appendix G):
• Shallow, deep, and to a lesser extent bedrock flow zone groundwater
COI plumes east and north of the ash basins and coal pile area
extend to or beyond the compliance boundary, and to the Catawba
River (Lake Wylie). However, there are no COI concentrations in
surface water within the Catawba River (Lake Wylie) greater than
applicable 02B standards. Furthermore, model simulations indicate
boron concentration decline to less than the 02L within
approximately 600 feet of the ash basin waste boundary and sulfate
and TDS concentrations decline to less than applicable groundwater
standards within approximately 1,900 feet of the ash basins waste
boundary and 750 feet of the coal piles.
• Shallow and deep flow zone groundwater COI plumes have
relatively similar COI plume geometries. This supports a connected,
unconfined flow system between the shallow and deep flow zones.
Bedrock groundwater with concentrations greater than applicable
regulatory criteria is limited to the upper fracture zones and to areas
adjacent to the Catawba River (Lake Wylie) near and beneath the ash
basin dams, within, at, and/or beyond the compliance boundary.
Bedrock groundwater is less connected to the upper flow zones, as
indicated by unique COI plume geometry compared with COI plume
geometry of the shallow and deep flow zones.
• The maximum extent of COI -affected groundwater migration for all
flow zones is represented by boron, sulfate, and TDS and the
distribution of other constituents are limited to smaller areas within
the boron, sulfate, and TDS plumes.
Plume Behavior and Stability
(CAP Content Section 6.A.e.i.1)
Mann -Kendall trend analysis was performed using conservative constituent
(boron, sulfate, and TDS) datasets for ash pore water and groundwater
wells within the waste boundary, between the waste boundary and
compliance boundary, and downgradient the source area, at or beyond the
compliance boundary (Table 6-7). Trend analysis and results were prepared
by Arcadis U.S. Inc. and are included as Attachment A in Appendix I.
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The analysis was performed using analytical results for samples collected
from 2004 through 2019, for COIs identified in the 2018 CSA Update (Table
6-7). Trend analysis results are presented where at least four samples were
available and frequency of detection was greater than 50%. Statistically
significant trends are reported at the 95% confidence level. The analysis of
constituent concentrations through time produced six possible results:
1. Statistically significant, decreasing concentration trend (D)
2. Statistically significant, increasing concentration trend (I)
3. Greater than 50% of concentrations were non -detect (ND).
4. Insufficient number of samples to evaluate trend (n < 4) (NE)
5. No significant trend, and variability is high (NT)
6. Stable. No significant trend, and variability is low (S)
A total of 2,540 data sets were evaluated for trends. Excluding the NE
trends described above, 90% of the remaining data sets had statistically
significant decreasing trends, stable trends, no trends, or greater than 50%
non -detect concentrations. Only 10% of the trends were statistically
increasing. Excluding both NE and ND trends described above, 86% of the
remaining data sets had statistically significant decreasing trends, stable
trends, or no trends. (Appendix I).
Ash pore water and groundwater wells within the waste boundary
generally have no trends, stable trends, or decreasing trends, suggesting
limited changing conditions and the plume is stable. Mann -Kendall results
for ash pore water and groundwater within the waste boundary indicate the
following:
• Approximately 80% of ash pore water trend results indicate stable
trends, no trends, non -detect, or decreasing trends for conservative
constituents (boron, sulfate, TDS) (Table 6-7).
Approximately 65% of groundwater trend results indicate stable
trends, no trends, non -detect, or decreasing trends for conservative
constituents (Table 6-7).
• In the shallow flow zone, increasing trends for conservative
constituents are limited to AB-25SS, which is located on an earthen
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dike (Table 6-7). Section 5.0 details the exceptions to the CSM
regarding earthen dams/dikes.
• Similar to the shallow flow zone, increasing trends for conservative
constituents in the deep and bedrock flow zones is generally limited
to locations within earthen dams/dikes (Table 6-7).
• The data indicate overall stability and improvement in groundwater
COI concentrations within the ash basins.
Trend analyses of groundwater monitoring wells north, east, and south of
the source areas near or beyond the compliance boundary indicate the
following:
• Approximately 65% of trends results for groundwater wells at or
beyond the compliance boundary indicate stable trends, no trends,
non -detect, or decreasing trends for conservative constituents (Table
6-7).
Only 17% of trend results for groundwater wells at or beyond the
compliance boundary have increasing trends for conservative
constituents (Table 6-7).
Wells with increasing COI concentration trends are generally located east of
the source areas, along the Catawba River (Lake Wylie). The areas with
increasing COI concentration trends are generally located within the areas
planned for groundwater remedial actions (Section 6.8)
6.1.5.2 Non -Conservative Constituents
(CAP Content Section 6.A.e.ii)
Strontium isoconcentration maps, cross sections and the geochemical model
support the following observations regarding the extent of COI -affected
groundwater represented by this non -conservative (reactive) constituent,
for which there is no 02L standard or IMAC value (Figures 6-9b, 6-10b, 6-
11b, 6-12b, 6-13b, and 6-22a through 6-22c; Appendix H):
• Strontium within all flow zones exhibits a plume -like distribution of
concentrations greater than background similar to the vertical and
horizontal extent of conservative constituents at the Site. However,
there are no COI concentrations in surface water within the Catawba
River (Lake Wylie) greater than applicable 02B standards.
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• The extent of strontium at concentrations greater than background
north of the RAB and coal pile is likely localized to areas beneath the
power block and the switchyard and bound by the discharge canal to
the west and Catawba River (Lake Wylie) to the east. The limits of
strontium extents in this area are likely due to the same factors
limiting the extent of sulfate and TDS concentrations which is
advective flow from upgradient areas north of the power block
where topography is higher and groundwater flow is east southeast
toward the power block and Catawba River (Lake Wylie).
• Strontium is unlike other COIs in that the broadest distribution
across the monitoring well network is within the bedrock flow zone.
Strontium concentrations are less than background in shallow wells
in the vicinity of the AAB, but are greater than background in the
deep and bedrock flow zones. Strontium concentrations are also less
than background in the shallow zone in the western portion of the
RAB but concentrations are greater than background in the deep and
bedrock flow zones in this area. Furthermore, strontium has a more
limited distribution of concentrations greater than background
northeast of the RAB and coal piles compared to the deep and
bedrock flow zones in this area. This indicates the distribution of
strontium at concentrations greater than background in each flow
zone is not likely derived, at least entirely, from the ash basins and/or
coal piles because there is no clear concentration gradient from
greater to lesser from the source areas to deeper flow zones. The
distribution of strontium in the vicinity of the AAB may be entirely
naturally occurring. This also indicates that site -specific background
comparative values are not representative of the range of naturally
occurring strontium concentrations.
6.1.5.3 Variably Conservative Constituents
Cobalt, iron, and manganese isoconcentration maps, cross sections, and the
geochemical model support the following observations regarding the extent
of COI -affected groundwater represented by these variable constituents
(Figures 6-9c, 6-10c, 6-11c, 6-12c, 6-13c, and 6-23a through 6-25c; Appendix
H):
• Localized plume -like distributions of cobalt, iron, and manganese
above the IMAC standard and/or background values occur in
shallow and deep flow layers north and northeast of the RAB and
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southeast of the AAB. The variably reactive constituent plumes are
located within localized areas of the footprint of the conservative
constituent plumes. The distribution of these constituents is limited
to the eastern portion of the ash basins along the Catawba River and
north and northeast of the RAB and coal pile area. Concentrations to
west of these areas are less then applicable comparative values
(IMAC or background). Furthermore, there are no COI
concentrations in surface water within the Catawba River (Lake
Wylie) greater than applicable 02B standards.
• Variable constituents at concentrations greater than comparative
criteria in bedrock are limited to cobalt detected at three locations,
AB-04BR, GWA-06BRA, GWA-29BR and manganese, also at GWA-
29BR. At AB-04BR, multiple water level measurements and sample
analytical results indicate that cobalt concentrations slightly greater
than the IMAC in bedrock are naturally occurring and not derived
from the ash basins because the well is upgradient of the ash basins;
are in an area where upward vertical hydraulic gradients are
observed, consistent with the CSM; and concentrations in the shallow
and deep flow zones are less than the shallower flow zones so there
is no concentration gradient trend that indicates cobalt is migrating,
or has migrated, from the ash basins downward through, the shallow
and deep flow zones to bedrock. At GWA-6BRA, cobalt
concentrations greater than the IMAC are also likely naturally
occurring. Although cobalt concentrations in the shallow zone at this
location (GWA-06S) are notably greater than IMAC and
concentrations within bedrock, due to influence from the RAB/low
pH area, cobalt concentrations in the deep zone (between the shallow
and bedrock) are less than the IMAC. This indicates cobalt may not
be migrating from the shallow zone downward through the deep
zone into bedrock at this location and that the cobalt occurs naturally
at the observed concentrations. Cobalt concentrations within the
lower bedrock well at this location (GWA-06BRL) are less than the
IMAC, which indicates that cobalt concentrations greater than IMAC
in bedrock are limited to the upper bedrock. At GWA-29BR, initial
sample results indicate cobalt and manganese concentrations slightly
greater than applicable comparative values may be a result of
transport from the coal pile area as cobalt and manganese
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concentrations in the shallow and deep flow zones are also greater
than applicable comparative values.
6.2 Potential Receptors Associated with Source Area
(CAP Content Section 6.B)
CSA and ongoing monitoring data confirm that affected groundwater is limited to
between 50 to 1,300 feet immediately downgradient of the ash basins and coal pile area
and COI -affected groundwater is limited to Duke Energy property. COI -affected
groundwater from the ash basins and coal pile area does not reach any water supply
wells, and modeling indicates this will remain the case in the future, although a portion
of the simulated boron plume extends beneath the western portion of Catawba River
(Lake Wylie). Therefore, potential receptors are limited to the Catawba River (Lake
Wylie).
6.2.1 Surface Waters — Downgradient Within a 0.5-Mile Radius
of the Waste Boundary
(CAP Content Section 6.B.a)
A depiction of surface water features — including wetlands, ponds, unnamed
tributaries, seeps, streams, lakes, and rivers — within a 0.5-mile radius of the ash
basin compliance boundary, along with permitted outfalls under the NPDES and
the SOC locations are shown on Figure 5-6 (CAP Content Section 6.B.a.i and
6.B.a.ii). The 0.5-mile radius from the ash basin compliance boundary, for which
data is evaluated and depicted on figures including surface water, is greater than
the required 0.5-mile radius from the waste boundary and is consistent with the
drinking water well and receptor surveys. The ash basins and coal piles are
located along the west bank of the Catawba River (Lake Wylie). The South Fork
River is located west of the Site, beyond a topographic and hydrogeologic divide.
Associated North Carolina surface water classifications for the Catawba River
(Lake Wylie) and the South Fork Catawba River are summarized in Section 5.3.1
and Table 5-3 (CAP Content Section 6.B.a.iii).
For groundwater corrective action to be implemented under 15A NCAC .02L
.0106(k), groundwater discharge to surface water cannot result in exceedances of
standards for surface waters contained in 02B. Surface water constituents with
02B standards include: arsenic, barium, beryllium, cadmium, chloride, chromium
(hexavalent and trivalent), copper, fluoride, lead, mercury, nickel, nitrate and
nitrite, selenium, silver, sulfate, total dissolved solids, thallium, total hardness,
and zinc.
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Surface water samples were collected from the Catawba River. Samples were
also collected from a small channel that flows from a seep -fed wet area southeast
of the AAB (seep/AOW S-2). Decanting has since resulted in no flow being
present within that channel. The samples were collected to confirm groundwater
downgradient of the ash basins and coal piles has not resulted in surface water
concentrations greater than 02B water quality standards. A map of surface water
sample locations for groundwater discharge to surface water evaluation is
included in Appendix K (CAP Content Section 6.B.a.iv). Surface water samples
were collected, using division approved protocols, to evaluate acute and chronic
water quality values. Surface water samples were also collected at background
locations (upgradient of potential migration areas) within the Catawba River.
Analytical results were evaluated with respect to 02B water quality standards
and background data.
Comparisons of surface water data with the applicable USEPA National
Recommended Water Quality Criteria for Protection of Aquatic Life, Human
Health and/or Water Supply (USEPA, 2015; 2017a; 2017b) was also conducted on
surface water samples from the Catawba River. As stated by the USEPA, these
criteria are not a regulation, nor do they impose a legally -binding requirement.
Therefore, comparisons with these criteria are only for situational context. The
constituents that have corresponding USEPA criteria but do not have 02B criteria
are alkalinity, aluminum, antimony, iron and manganese. All concentrations of
alkalinity, aluminum, antimony, iron and manganese in downstream samples
were either non -detect (i.e. antimony) or concentrations were generally
comparable to background concentrations.
The surface water samples were collected in accordance with NCDEQ DWR
Internal Technical Guidance: Evaluating Impacts to Surface Water from
Discharging Groundwater Plumes - October 31, 2017. The full report for Allen
groundwater discharge to surface water and the evaluation of surface waters to
evaluate compliance with 15A NCAC 02B .0200 was submitted to NCDEQ in
March 2019. Surface water data has been reevaluated as a result of surface water
quality standards updated by NCDEQ on June 6, 2019. The revised report is
provided in Appendix J.
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General findings of the evaluation of current surface water quality conditions at
Allen include:
Groundwater migration from beneath the ash basins and coal pile area has
not resulted in exceedances of the 02B surface water quality standards in
the Catawba River.
• Previously identified seeps are deemed covered by North Carolina
Environmental Management Commission (EMC) SOC WQ S17-009.
Surface Water - Future Conditions Evaluation
An evaluation of potential future groundwater migration to surface water was
conducted to identify areas where further evaluation might be warranted. For
areas of potential future groundwater migration to surface water, a mixing
model approach was used for the evaluation of future surface water quality
conditions. Flow and transport modeling results were used to determine where
groundwater migration from the ash basins might intersect surface water in the
future. Predictive groundwater modeling using boron as a proxy for COI plume
migration demonstrated areas within the Catawba River east of the ash basins
could potentially be influenced by future groundwater migration. A
groundwater to surface water mixing model approach was used to determine the
potential surface water quality in the future groundwater discharge zones.
Constituents assessed in the predictive model include those that were identified
as COIs in the 2018 CSA Update (SynTerra, 2018a) with 02B standards. The full
report for Allen groundwater discharge to surface water under future conditions
can be found in Appendix J.
General findings of the evaluation of future surface water conditions in potential
groundwater discharge areas include:
• The surface water mixing model evaluation confirms that predicted
resultant constituent concentrations in applicable surface waters are less
than 02B surface water standards. Therefore, the criteria for compliance
with 02B is met, allowing potential corrective action under 15A NCAC
02L .0106 (k), (1), or (m).
• Modeling scenarios illustrate the maximum extent of the affected
groundwater plume occurs under current conditions. Simulations of
future conditions indicate the plume extent will continue to decrease as
unaffected groundwater migrates from the upgradient direction.
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• The current and predicted transport extent of COIs potentially derived
from the ash basins does not extend toward the South Fork Catawba
River. That conclusion is based on groundwater sample results, water
level elevation measurements used to determine groundwater flow
direction, and groundwater modeling simulation.
As future hydraulic head elevations decline, groundwater flow velocities
slow and resemble a pre -ash basin flow direction and magnitude.
Seeps currently governed by the SOC that remain and are not dispositioned 90
days after completion of decanting would be characterized for determination of
corrective action applicability. Where applicable, and accounting for seep
jurisdictional status, corrective action planning at that time would occur.
The full report for Allen groundwater discharge to surface water under future
conditions can be found in Appendix J.
Based on current and future surface water evaluations, along with relevant
media assessments, no COIs require remediation in surface water at Allen.
6.2.2 Water Supply Wells
(CAP Content Section 6.B.b)
A total of 290 eligible households for permanent water supply were identified
within the 0.5-mile radius of the ash basin compliance boundary. These eligible
households are located northwest, west, southwest, and south of the ash basins
(Figures 5-7a and 5-7b). All of the private water supply wells are located either
upgradient or side -gradient of the ash basins and coal pile area.
No public or private drinking water wells or wellhead protection areas were
found to be located downgradient of the ash basins as discussed in Section 5.3.3.
This finding has been supported by sampling and evaluation of results from
several water supply wells (Table 6-9 and Figure 5-7a), over 30 groundwater
sampling events of monitoring wells on -Site (Appendix C, Table 1), a review of
public records, an evaluation of historical groundwater flow direction data, and
results of groundwater flow and transport modeling (Appendix G). The location
and information pertaining to water wells located upgradient or side -gradient of
the Site, within 0.5 miles of the ash basin compliance boundary, were included in
drinking water supply well survey reports (HDR, 2014a; 2014b).
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6.2.2.1 Provision of Alternative Water Supply
(CAP Content Section 6.B.b.i)
Although results from local water supply well testing do not indicate effects
from the source areas at Allen, water supply wells identified within the 0.5-
mile radius from ash basin compliance boundary have been offered
alternate water supply, per G.S. Section 130A-309.211(cl) requirements.
A property eligibility was contingent that the property did not include:
• A business
• A church
• A school
• An empty lot
As of August 1, 2018, Duke Energy:
connected 191 households to the City of Belmont water supply (nine
of those households were already connected to the city of Belmont
water supply),
• installed 10 water treatment systems, and
• abandoned three public water supply wells that served 77
households in accordance with G.S. Section 130A-309.211(cl) of HB
630 (2016).
Water supply wells were abandoned if requested by the property owner
after the home was connected to the municipal water supply. If the owner
wanted to keep the well for non -potable use, a spigot was installed per City
of Belmont and Gaston County regulations. City of Belmont required a
backflow prevention device be installed for each home that kept their well,
so a large number of owners decided to have their well abandoned. No
wells were abandoned due to necessity related to water quality.
Abandonment was done solely at the owner's request. Water supply well
abandonment records are provided in Appendix D.
Of the remaining 12 households that were initially considered eligible by
being within a 0.5-mile radius of the ash basin compliance boundary:
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• two either opted out of the option to connect to a water treatment
system or did not respond to the offer,
• one household was demolished but will be connected at a future
date,
• six locations were deemed not eligible because the property did not
contain a household, and
• three additional locations were associated with a business, church, or
school which are not eligible for the HB630.
City of Belmont water supply lines were installed along the following
roadways:
• Michael Dominick Drive
•
White Ibis Lane
• Reese Wilson Road
•
Wildlife Road
• Reese Wilson Road Extension
•
Idlewood Lane
• Nutall Oak Lane
•
Midwood Lane
• Sawtooth Oak Lane
•
Warren Drive
• Bell Post Road
•
Southpoint Drive
• Dana Michelle Court
•
Mitchell Street
• Wing Point Drive
•
Lake Mist Drive
• Shorewood Place
•
Lake Breeze Lane
• Egret Ridge
•
Canal Road
Additionally, Duke Energy voluntarily connected two businesses and 23
households to the City of Belmont water supply that were otherwise not
eligible per G.S. Section 130A-309.211(cl).
On August 1, 2018, Duke Energy provided completion documentation to
NCDEQ to fulfill the requirements of FIB 630. NCDEQ provided
correspondence, dated October 11, 2018, to confirm that Duke Energy
satisfactorily completed the alternate water provision under CAMA, G.S.
Section 130A-309.211(cl) at Allen. Both documents are provided in
Appendix D.
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Figure 5-7b (CAP Content Section 6.B.b.i) shows the private and public water
supply well locations with reference to water treatments systems installed
and to be installed, along with vacant parcels and residential properties that
have decided to either opt out of the water treatment system program or
did not respond to the offer. Where provided, Duke Energy maintains the
systems on behalf of the property owners.
6.2.2.2 Findings of Drinking Water Supply Well Surveys
(CAP Content Section 6.B.b.ii)
The location and relevant information pertaining to water wells located
upgradient or side -gradient of the facility, within 0.5-miles of the
compliance boundary, were included in the survey reports. Results from
surveys conducted to identify potential receptors for groundwater,
including public and private water supply wells and surface water features
within a 0.5-mile radius of the ash basin compliance boundary, have been
reported to NCDEQ:
• Drinking Water Well and Receptor Survey — Allen Steam Station Ash
Basin (HDR, 2014a)
• Supplement to Drinking Water Welland Receptor Survey —Allen Steam
Station Ash Basin (HDR, 2014b)
• Comprehensive Site Assessment Report — Allen Steam Station Ash Basin,
(HDR, 2015a)
The surveys identified four public supply wells within 0.5-mile radius of
the ash basin compliance boundary (Figures 5-7a and 5-7b). The three
public water supply wells closest to the ash basins were abandoned in 2018.
Two of the public water supply wells (Heather Glen/Highland) were
located approximately 0.35 miles west and upgradient of the ash basins.
These wells were located in the vicinity of a groundwater divide. One
public water supply well (South Point Landing) was located approximately
0.2 miles west and beyond the groundwater divide from the ash basins. The
farthest public water supply well (River Lakes S/D) from the ash basins is
located approximately 0.5 miles west of the ash basins and beyond a
groundwater divide (South Point Road). This well was not abandoned and
remains in use.
As documented in the 2018 CSA Update, NCDEQ arranged for independent
analytical laboratories to collect and analyze water samples in the first part
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of 2015 from private wells identified during the well survey, if the owner
agreed to have their well sampled. NCDEQ collected and analyzed a total
of 216 samples from 159 private monitoring wells within a 0.5-mile radius
of the Allen ash basin compliance boundary from 2015 through 2017.
Table 6-9 (CAP Content Section 6.B.b.ii) provides tabulated results for the
NCDEQ sampling results as well as identified exceedances of 02L
Standards, IMACs, and bedrock background values, as well as a well -by -
well summary of COI exceedances and characterization. The exceedance
evaluation compares bedrock background values since it is assumed area
water supply wells are installed within the bedrock, which is typical for
water supply wells in the Piedmont. Although some of the water supply
wells may be installed in the lower transition zone. Groundwater
concentrations of boron and sulfate, which are constituents that
conservatively indicate influence from the ash basins or coal pile, are not
detected in the vicinity of the water supply wells and are only detected in
bedrock monitoring wells at locations adjacent to the Catawba River, and
approximately 2,500 feet from the closest water supply well.
Major findings from the water supply well evaluation include:
• All water supply wells are outside of the boron and sulfate plumes as
defined on the conservative isoconcentration contour maps for all
flow zones (Figures 6-19a through Figure 6-21c).
All water supply wells to the west, northwest, and south are
upgradient or sidegradient of the ash basins (Figures 5-7a and 5-7b).
Groundwater modeling simulations indicate that as source control
(decanting) continues, the hydraulic divide will be more pronounced
to the west of the basins, therefore the water supply wells west of the
ash basins will become further isolated from the basins (Figure 5-5a
through 5-5c, Appendix G).
• Five of the seven COIs that have been identified as useful for
mapping to indicate areas for corrective action were present in water
supply wells at values greater than 02LAMAC or background,
whichever is greater including: cobalt, iron, strontium, sulfate, and
TDS; however, the presence of these COIs in the water supply wells
are not associated with the ash basins or coal piles based on the local
hydrogeology and distribution of COIs described above.
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• 95 water supply wells sampled demonstrated concentrations of
chromium (VI) greater than background values. No discernable
plume associated with the ash basins or coal piles was identified.
This finding has been confirmed by more than 30 consecutive
groundwater monitoring events.
75 water supply wells sampled demonstrated concentrations of
strontium greater than background values. No discernable plume
associated with the ash basins or coal piles was identified. This
finding has been confirmed by more than 30 consecutive
groundwater monitoring events.
Six water supply wells sampled demonstrated concentrations of
vanadium greater than background values. No discernable plume
associated with the ash basins or coal piles was identified. This
finding has been confirmed by more than 30 consecutive
groundwater monitoring events.
One public water supply well sampled demonstrated a
concentrations of cobalt greater than IMAC, iron background values,
sulfate 02L, thallium IMAC, and TDS 02L. No discernable plumes
associated with the ash basins or coal piles were identified. This
finding has been confirmed by more than 30 consecutive
groundwater monitoring events.
6.2.3 Future Groundwater Use Areas
(CAP Content Section 6.B.c)
Duke Energy owns the land and controls the use of groundwater on the land
downgradient of the ash basins and coal pile area within and beyond the
predicted area of potential groundwater COI influence. Therefore, no future
groundwater use areas are anticipated downgradient of the basins or coal pile
area.
Based on future predicted groundwater flow patterns, under post ash basin
closure conditions, and the location of water supply wells in the area,
groundwater flow direction from the ash basin is expected to be further
contained within the stream valley and continue flowing east of the ash basin
footprint, and therefore will not flow towards any water supply wells
[(Appendix G) (CAP Content Section 6.B.c.ii)].
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6.3 Human and Ecological Risks
(CAP Content Section 6.0
Updated human health and ecological risk assessments were prepared for Allen
consistent with the CAP content guidance. The updated risk assessments incorporate
results from surface water, sediments, and groundwater samples collected March 2015
through June 2019. Primary conclusions from the risk assessments include:
1. there is no evidence of risks to on -Site or off -Site human receptors potentially
exposed to CCR-related constituents that may have migrated from the ash basins
or coal pile area; an
2. there is no evidence of risks to ecological receptors potentially exposed to CCR-
related constituents that have migrated from the ash basins or coal pile area.
These conclusions are further supported by multiple water quality and biological
assessments conducted by Duke Energy as part of the NDPES monitoring program. A
more detailed discussion regarding human health and ecological risk associated with
the ash basins and coal piles can be found in Section 5.4. An update to the Allen
human health and ecological risk assessment is included in Appendix E.
6.4 Description of Remediation Technologies
This section provides supplemental information beyond CAP Content guidance to
introduce groundwater remediation technologies and considers a range of individual
groundwater remediation technologies that may be used to formulate comprehensive
groundwater remediation alternatives for consideration at Allen. The most feasible
remedial options identified will form the basis, in whole or in part, for the remedial
alternatives evaluated in Section 6.7. Groundwater remediation technologies will be
evaluated based upon two primary criterion:
• Can a technology be effective when addressing one or more Site -specific COI?
• Can a technology be feasibly implemented under Site -specific conditions and be
effective?
The remedial alternative screening includes the criteria in the NCDEQ CAP Guidance
(April 27, 2018). Technologies that are clearly not workable under Site conditions will
not be carried forward. Technologies that have potential application will be retained for
further consideration. Technologies retained for further consideration might be used to
formulate comprehensive groundwater remedial alternatives in Section 6.5.
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6.4.1 Monitored Natural Attenuation
Monitored natural attenuation (MNA) is a groundwater remedy that relies on
natural processes to reduce constituent concentrations in groundwater over time.
The primary objective of an MNA strategy is to identify and quantify natural
attenuation processes specific to a site and demonstrate that those processes will
reduce constituent concentrations in groundwater to levels that are less than
regulatory standards (USEPA, 1999; NCDEQ, 2017).
MNA processes potentially applicable to inorganic constituents include:
• Dispersion • Sorption • Biological stabilization
• Dilution • Radioactive decay • Chemical stabilization
• Transformation • Phyto-attenuation
Dilution from recharge to groundwater, mineral precipitation, and COI
adsorption will occur over time and distance from the source area, thereby,
reducing COI concentrations through attenuation. MNA can be used in
combination with other remediation technologies such as source control. Routine
monitoring of select locations for COI concentrations is used to confirm the
effectiveness of the approach.
The USEPA does not consider MNA to be a "no action" option. Source control
and long-term monitoring are fundamental components of any MNA remedy.
Furthermore, MNA is an alternative means of achieving remediation objectives
that might be appropriate for specific, well -documented site circumstances
where its use will satisfy applicable statutory and regulatory requirements
(USEPA, 1999).
The USEPA, as shown below, considers MNA to be in -situ (USEPA, 1999):
The term "monitored natural attenuation", as used in this Directive, refers to
the reliance on natural attenuation processes (within the context of a carefully
controlled and monitored site cleanup approach) to achieve site -specific
remediation objectives within a time frame that is reasonable compared to that
offered by other more active methods. The "natural attenuation processes"
that are at work in such a remediation approach include a variety of physical,
chemical, or biological processes that, under favorable conditions, act without
human intervention to reduce the mass, toxicity, mobility, volume, or
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concentration of contaminants in soil or groundwater. These in -situ processes
include biodegradation, dispersion; dilution, sorption; volatilization... "
MNA is compared with other viable remediation methods during the remedy
selection process. MNA should be selected only if it will meet site remediation
objectives within a timeframe that is reasonable compared to that offered by
other methods (USEPA, 1999). A contingency remedy should be proposed at the
time MNA is selected to be a site remedy (NCDWM, 2000).
The NCDEQ and USEPA have guidance documents that prescribe the
investigative and analytical processes required for an MNA demonstration.
NCAC 02L provides additional requirements for MNA implementation. USEPA
developed a tiered approach to support evaluation and, if appropriate, selection
of MNA as a remedial technique (USEPA, 2007). Three decision tiers require
progressively greater site information and data to assess the potential
effectiveness of MNA as a remedy for inorganic constituents in groundwater.
MNA will be retained for further consideration at Allen, as groundwater COIs
do not pose an unacceptable risk to human health or the environment under
conservative exposure scenarios and a source control measure will be
implemented that eliminate or mitigate the source of CCR constituents in
groundwater. The MNA evaluation for the technical applicability at Allen is
provided in Appendix I.
6.4.2 In -Situ Technologies
Groundwater remediation technologies that are implemented in -situ, or in place,
are discussed here.
Low Permeability Barriers
When used for the purpose of groundwater remediation, low permeability
barriers (LPBs) are structures constructed in -situ to redirect or contain
groundwater flow. Materials used to construct LPBs are either impermeable
(e.g., steel sheet pile) or have a permeability that is at least two orders of
magnitude less than the permeability of the saturated media that comprises a
targeted groundwater flow path. For this reason, LPBs are typically keyed into a
natural barrier to groundwater flow such as a competent confining unit (e.g.,
aquitard) or bedrock to prevent groundwater from flowing under the LPB.
LPBs can be used to redirect groundwater away from a potential receptor,
redirect groundwater away from a source area, or redirect COI laden
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groundwater towards a groundwater extraction system or in -situ groundwater
treatment system (e.g., permeable reactor barrier). The design and technique
used to construct a LPB typically depends upon the length of the LPB, the depth
to a competent confining layer or bedrock, and cost considerations. Sheet piling,
trenching, and vertical drilling are the most common means to construct a LPB.
Sheet piling and trenching are typically limited to depths of approximately 50
feet whereas installation of a LPB using drilling techniques can achieve depths
greater than 50 feet. For this reason, construction of a LPB at Allen would
involve installation by means of drilling because bedrock is approximately 100
feet below ground surface downgradient of the ash basins.
Construction of a LPB at Allen would involve drilling to competent bedrock and
injecting bentonite or grout into fractured bedrock, the transition zone, and
possibly into saprolite flow zones. Keying the LPB into a natural barrier to
groundwater flow such as a competent confining unit (e.g., aquitard) or bedrock
cannot be achieved with certainty due to the complex Piedmont geology present
at Allen. Installation of an effective low permeability barrier to depths
approaching 100 feet is technically feasible but would be technically challenging
and costly. For these reasons, LPB technology was not retained for further
consideration.
Groundwater Infiltration and Flushing
Groundwater flushing by infiltration can be accomplished by many methods
including vertical wells, horizontal wells, and infiltration galleries.
In -situ groundwater flushing involves infiltration or injection of clean water into
groundwater to accelerate flushing of target constituents. Constituents
mobilized by flushing would be captured by an extraction well. Flushing can
enhance natural constituent transport mechanisms such as advection, dispersion,
and molecular diffusion. This technology is potentially applicable to a broad
range of constituents. Furthermore, in -situ flushing has potential applicability at
almost any depth. However, successful implementation is site -specific. Factors
affecting the effectiveness include the degree of subsurface heterogeneity, the
variability of hydraulic conductivity, and the organic content of soil. Suitability
testing of the clean water source and pre -design collection of data is important
for most sites where this technology might be considered.
Flushing of relatively mobile and unreactive constituents like boron can be
accomplished using clean water.
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In -situ infiltration can also be used to enhance conventional pump and treat
technology at locations with limited natural recharge or low permeability. The
introduction of clean water into groundwater enhances groundwater flow by
increasing the hydraulic gradient between the point of injection and the point of
extraction or discharge. Addition of clean water can mobilize COIs, such as
boron, and enhance the hydraulic gradient to improve hydraulic capture of COIs
(USEPA, 1996).
Groundwater flushing is a technology that has possible application at Allen to
enhance the capture of mobile constituents. Furthermore, groundwater flushing
can be used to facilitate the addition of amendments for treating groundwater
that is acidic when compared to the pH range of groundwater in site background
wells. Increasing the pH of locally acidic groundwater towards neutrality might
also immobilize some constituents that are soluble under acidic conditions.
Groundwater flushing will be retained for further consideration.
Encapsulation
Encapsulation technologies act to prevent waste materials and constituents from
coming into contact with potential leaching agents such as water. Materials used
to encapsulate a waste must be both chemically compatible with the waste and
inert to common environmental conditions such as rain infiltration, groundwater
flow, and freeze/thaw cycles (USEPA, 2002). Waste materials can generally be
encapsulated in three ways: microencapsulation, macroencapsulation or in -situ
vitrification (ISV).
Microencapsulation involves mixing the waste together with the encasing
material before solidification occurs. Macroencapsulation involves pouring the
encasing material over and around a larger mass of waste, thereby enclosing it in
a solidified block. Grout, sulfur polymer stabilization/solidification, chemically
bonded phosphate ceramic encapsulation, and polyethylene encapsulation are
examples of the techniques that have used to improve the long-term stability of
waste materials (USEPA, 2002). ISV involves the use of electrical power to heat
and melt constituent laden soil and buried wastes (e.g., ash). ISV uses an array of
electrodes inserted into the ground. Electrical power is applied to the electrodes
which establishes an electric current through the soil. The electric current
generates sufficient heat (>2500°F) to melt subsurface soil and waste
materials. The molten material cools to form a hard monolithic, chemically inert
crystalline glass -like product with low leaching characteristics (USEPA, 1994).
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Encapsulation technologies are not carried forward for further evaluation for the
following reasons:
• The area and depth requiring groundwater remediation is greater than
feasible for this technology, which is best implemented in areas of limited
size or extent.
• The varied geological conditions pose the unlikelihood that the
performance of an implemented technology will be uniform.
Permeable Reactive Barrier
The USEPA defines a permeable reactive barrier (PRB) as being:
An emplacement of reactive media in the subsurface designed to intercept a
contaminant plume, provide a flow path through the reactive media, and
transform the contaminant(s) into environmentally acceptable forms to attain
remediation concentration goals down -gradient of the barrier (USEPA, 1997).
Construction of PRBs involves emplacement of reactive media below the ground
surface for the purpose of treating groundwater containing dissolved COIs. The
PRB media is designed to be more hydraulically conductive than the saturated
media surrounding the PRB so that groundwater will flow through the PRB
media with little resistance. The depth and breadth of PRBs are oriented
perpendicular to groundwater flow direction so that the PRB will intercept
groundwater targeted for treatment. Design of the PRB thickness takes into
account groundwater velocity and the need to provide sufficient groundwater
residence and contact time for constituents to react with PRB media. PRBs can be
installed as permanent or semi -permanent treatment units. The PRB reactive
media in a permanent treatment unit is designed to remain emplaced over the
needed timeframe whereas the reactive media in a semi -permanent treatment
unit is designed to be replaced periodically once it is spent.
Two of the most common PRB designs are the continuous wall and the "funnel
and gate". The continuous wall design involves the installation of a trench
downgradient of a constituent plume that is oriented perpendicular to
groundwater flow. The funnel and gate configuration involves construction of
two LPBs that redirect groundwater flow towards the PRB. This allows for a
smaller PRB design and treatment of a greater volume of groundwater. A design
factor for both designs is the ability for the PRB be keyed in a low permeability
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confining layer or in bedrock to minimize the potential for groundwater
underflow beneath the PRB.
Media commonly used in PRBs for the treatment of inorganic COIs includes
zero -valence iron (ZVI), apatite, zeolites, and materials used to affect
groundwater pH. The mechanisms that take inorganic constituents out of
solution includes adsorption, ion exchange, oxidation-reduction, or precipitation.
ZVI (FeO) is an effective reducing agent; donates an electron (FeO --+ Fe+2 + 2e-).
ZVI particles can remove divalent metallic cations through reductive
precipitation, surface adsorption, complexation, or co -precipitation with iron
oxyhydroxides. ZVI has been used to treat cationic metals such mercury (Hg+2),
nickel (Ni+2), cadmium (Cd+2), and lead (Pb+2) (USEPA, 2009).
Apatite is a media used in PRBs to treat groundwater for the removal of certain
metals in solution including lead, cadmium, and zinc. Apatite refers to a group
of crystalline phosphate minerals; namely, hydroxylapatite, fluorapatite and
chlorapatite. Apatite IITM is an amorphous form of a carbonated hydroxy-apatite
that has random nanocrystals of apatite embedded in it. The apatite nanocrystals
are capable of precipitating various phosphate phases of metals and
radionuclides. Apatite II is also an efficient non-specific surface adsorber
(Wright et al., 2003).
Zeolite is any of a large group of minerals consisting of hydrated
aluminosilicates of sodium, potassium, calcium, and barium. Zeolites have large
internal surface areas capable of treating inorganics by both adsorption and
cation exchange.
Limestone and materials containing limestone such as recycled cement can be
used as a PRB medium for raising the pH of acidic groundwater like that are
found in mine runoff (Indraratna et al., 2010).
Sulfate reduction facilitated by naturally occurring bacteria has been shown to
effectively treat acidic to net alkaline groundwater containing dissolved heavy
metals, including aluminum, in a variety of situations. The chemical reactions are
facilitated by the bacteria desulfovibrio. This is a well -proven technology often
used to treat acidic runoff from historic mining operations.
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The ability to maintain adequate reactive reagent concentrations at depth over an
extended period of time is a significant operational and performance
consideration. Permeable reactive barriers are not carried forward. Reasons
include:
• Detected concentrations of aluminum, iron, and manganese dissolved in
groundwater could react with, and clog, treatment areas, diminishing the
hydraulic conductivity through the PRB.
• There is recent favorable data suggesting that the technology might be
effective in reducing some coal ash -related constituents, however, PRB
technology is not well suited to treat boron.
• It would not be economically, or in some areas technically, feasible to
construct a PRB along the entire length and depth of the affected areas
beyond the compliance boundary.
6.4.3 Groundwater Extraction
Groundwater extraction is often used when remediating mobile constituents in
groundwater. Groundwater extraction can be used to withdraw affected
groundwater from the subsurface for the purpose of reducing the mass of one or
more target constituent(s) in an aquifer. Groundwater extraction can be used to
hydraulically contain affected groundwater and mitigate groundwater
constituent migration. Groundwater extraction can be conducted using a variety
of methods that are discussed in the following sub -sections.
Vertical Extraction Wells
A vertical well is the most common design for groundwater extraction. Drilling
techniques used to install vertical groundwater extraction wells range from
direct push technology, to hollow stem auger, mud rotary, air rotary, sonic drill
rigs, and other methods. Groundwater extraction wells can be designed and
screened in unconsolidated saturated media such as sand, saprolite, alluvium,
transition zone, fractured bedrock, silts, and clays. Alternatively, groundwater
extraction wells installed in bedrock can be completed as open -hole borings.
Low yielding aquifers can be problematic for vertical extraction wells. Relatively
close spacing of vertical wells might be necessary to capture a constituent plume
if the aquifer yield is low. Enhanced yield can be accomplished through injection
or infiltration of water upgradient of the wells to increase the availability of
water and hydraulic head, or fracking. Alternatively, low yielding wells can be
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effective through intermittent pumping to remove sorbed constituents with each
pump cycle.
Pump options include submersible pumps and centrifugal pumps depending
upon the anticipated yield, depth to water and well diameter. Shallow
centrifugal pumps (shallow well jet pumps) can be used in small diameter wells
where the groundwater level and desired pumping level is relatively shallow
(less than 25 to 30 feet below the ground surface). Submersible pumps or' deep
well jet pumps' can be used to extract groundwater from larger diameter wells
with deeper groundwater levels. Deep well jet pumps have the advantage of
mechanical equipment above grade and power only needs to be provided to a
few pump stations rather than to every well as with submersible pump systems.
All require routine maintenance of the pumps, vaults, piping and well screens to
sustain desired performance.
Groundwater modeling conducted at Allen indicates that vertical groundwater
extraction wells can produce sufficient yield for effective constituent mass
removal without supplemental measures. Pilot testing could be used to verify
and adapt the design to accommodate actual flow rates. Therefore, the use of
vertical groundwater extraction wells is retained for further consideration.
Horizontal/Angular Extraction Wells
Horizontal groundwater extraction wells offer advantages over vertical
groundwater extraction wells when access is difficult or to reduce the number of
system elements requiring maintenance. For example, horizontal wells can be
installed below buried utilities, buildings, and similar surface or near surface
features. Also, horizontal wells can be more efficient and effective when
remediating constituent plumes distributed over a large area within a relatively
thin flow zone. Fewer horizontal wells would be required under this scenario
compared with the number of vertical wells that might be required to achieve
similar remediation goals. Furthermore, recovery efficiency might be increased
relative to vertical wells due to the ability of a single horizontal well to contact a
larger horizontal area, particularly where the horizontal aquifer transmissivity is
greater than the vertical transmissivity.
Installation of a directionally drilled well involves the use of an auger bit that can
be steered in three dimensions. The progress of direction boring installations is
precisely monitored to avoid subsurface obstructions and to install the well as
designed. Tracking accuracy generally decreases with increasing depth of
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installation. Site hydrogeologic and geologic conditions can also affect tracking
accuracy.
Directionally drilled horizontal wells can be completed as blind holes (single -end
completion) or surface-to-surface holes (double -end completion). Single -end
holes involve one drill opening, with drilling and well installation taking place
through this single opening. Borehole collapse might be more likely in single -
ended drilling since the hole is left unprotected between drilling and reaming
and between reaming and casing installation. An additional complication
associated with single -ended completion involves the precise steering of reaming
tools required to match the original borehole path. In contrast, double -end holes
are typically easier to install since reaming tools and well casing can be pulled
backward from the opposite opening, and the hole does not have to be left open.
Materials used for horizontal wells are typically the same or similar as those used
for vertical wells. Factors to consider in the choice of the well screen and casing
materials to be used with horizontal wells include axial strength, tensile strength,
and flexibility (Miller, 1996).
Angle drilled wells are constructed in the same way as a vertical well with the
exception that the drill rig mast is positioned at an angle that is purposely not
plumb. The drilling mast angle and the targeted drilling depth will determine
horizontal offset of the well screen and submersible pump from the location
where drilling was initiated. Otherwise, angled wells function in the same
manner as vertical wells.
Groundwater modeling conducted at Allen indicates that groundwater vertical
extraction wells can produce sufficient yield for purposes of hydraulic
containment and/or constituent mass removal. Vertical extraction wells are
deemed more cost effective. The use of horizontal or angular groundwater
extraction wells is not retained for further consideration.
Extraction Trenches
Shallow horizontal groundwater extraction (collection or intercept) trenches can
be installed in areas near surface waters where groundwater might discharge.
These trenches can be utilized to prevent groundwater from discharging into
surface waters and can be effective in lowering or managing the water table.
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Trenches might be used as temporary installations to intercept and monitor
subsurface flow or can be retained as a permanent installation. Trenches must be
deep enough to tap and provide an outlet for ground water that is in shallow,
permeable strata or in water -bearing sand. The spacing of trenches varies with
soil permeability and drainage requirements.
Extraction trenches function similar to horizontal wells but are installed with
excavation techniques. They can be cost-effective to construct at shallow depths
(less than or equal to 35 feet below ground surface) using conventional
equipment. Trenches can be installed to depths of approximately 50 feet below
ground surface using specialty equipment. Horizontal collection trenches are
usually not cost-effective for deeper installations or bedrock applications.
Horizontal collection trenches do have the advantage of generally having lower
operations and maintenance costs compared with the costs of multiple vertical
wells.
Extraction trenches will not be considered at Allen because the thickness of
saprolite between the ash basin dams and the Catawba River is approximately
100 feet and effectiveness would be limited.
Hydraulic Fracturing
The effectiveness of groundwater extraction systems can sometimes be improved
in low permeability formations, including bedrock, with the use of fracturing
techniques.
Pneumatic fracturing involves injection of highly pressurized air into
consolidated sediments to extend existing fractures and create a secondary
network of fissures and channels. Similarly, hydraulic fracturing involves the
use of high pressure water to extend existing fractures and create a secondary
network of fissures and channels.
Hydraulic fracturing generally involves the application of high pressures to
propagate existing fractures or to create fractures following fracture nucleation.
When hydraulic fracturing is applied to unconsolidated materials, a disk -shaped
notch that serves as the starting point for the fracture is created using high
pressure water to cut into the formation. This is followed by pumping a slurry of
water, sand, and a thick gel at high pressure into the borehole to propagate the
fracture. The residual gel biodegrades and the resultant fracture is a highly
permeable sand -filled lens that might be as large as 60 feet in diameter (USEPA,
1995).
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The presence of COIs in the bedrock groundwater at Allen is limited compared
to the distribution and concentrations of COIs in saprolite and transition zone
groundwater. The use of hydraulic fracturing to enhance remediation of bedrock
groundwater is not considered further because the extent of COIs in bedrock
groundwater is limited and COIs in bedrock groundwater might be addressed as
effectively using more conventional means.
Phytoremediation
Phytoremediation involves the use of plants and trees as a means to extract
groundwater. Water uptake by trees is used for plant growth and metabolism.
Water uptake by plants and trees is ultimately released into the atmosphere via
the pore -like structures on the leaves called stoma. Water on the leaves
evaporates into the atmosphere. The loss of water by plants and trees is called
transpiration. The amount of water transpired by plants, and therefore water
uptake by plants, is a function of:
• Plant type: Plants that are native to and regions must conserve water and
therefore transpire less than plants that are native to wet regions.
• Temperature: Transpiration rates increase with increasing temperature
and decrease with decreasing temperatures.
• Relative humidity: Transpired water on plant leaves evaporate at a faster
rate when the relative humidity is low and that results in a
correspondingly higher transpiration rate. The opposite is true when the
relative humidity is high.
• Wind and air movement: Increased movement of air around a plant will
increase the rate of transpiration by plants
• Availability of soil moisture: Plants can sense when soil moisture is
lacking and will reduce their transpiration rate.
The growth rate of selected plant species and the growing season can be limiting
factors for the effectiveness of this technique. Maintenance can be long term and
require, in most cases, fertilizing, regular monitoring, and harvesting.
Phytoremediation using TreeWell® technology involves the installation of a 3 to
5 foot diameter boring to a target depth, typically a flow zone containing COIs.
A Root Sleeve TM liner and aeration tubing are installed from ground surface to
target depth. The boring is backfilled with soil that might include reactive
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media. If filled with reactive media, the tree well would serve as a PRB as well as
a means to promote phytoremediation.
A tree is planted within the tree well followed by placement of plastic cover over
the soil surrounding the tree. The plastic cover minimizes infiltration of
precipitation into the tree well. The tree well design forces the tree to draw
water from the targeted depth via the Root Sleeve TM liner. Groundwater is also
drawn through reactive media, if present. Consequently, the tree and the tree
well are capable of uptake of some COIs and serve as a means of groundwater
treatment and enhanced natural attenuation.
Ground cover plants stabilize soil/sediment and control hydraulics. In addition,
densely rooted groundcover plants and grasses can also be used to remediate
constituents. Phytoremediation groundcovers are one of the more widely used
applications and have been applied at various bench- to full-scale remediation
projects. Furthermore, in the context of this document, phytoremediation
groundcovers are vegetated systems typically applied to surface soils as opposed
to TreeWells which are targeted to deep soil and/or groundwater. The typical
range of effectiveness for phytoremediation groundcovers is 1-2 feet bgs;
however, depths down to 5 feet have been reported as within the range of
influence under some situations (ITRC, 2009)
Constructed treatment wetlands are manmade wetlands built to remove various
types of pollutants that may be present in water that flows through them. They
are constructed to recreate, to the extent possible, the structure and function of
natural wetlands, which is to act as filters. Wetlands are ideally suited to this
role. They possess a rich microbial community in the sediment to effect the
biochemical transformation of pollutants, they are biologically productive, and
most importantly, they are self-sustaining.
Metals are removed in constructed wetlands by a variety of mechanisms
including the following. Settling and sedimentation achieve efficient removal of
particulate matter and suspended solids. The chemical process that results in
short-term retention or long-term immobilization of constituents is sorption.
Sorption includes the combined processes of adsorption and absorption.
Chemical precipitation involves the conversion of metals in the influent stream to
an insoluble solid form that settles out. (ITRC, 2003)
Phytoremediation technology can be also be used as a means to treat extracted
groundwater. Aquaculture treatment technologies have been applied to the
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treatment of water. Those using aquatic plants, have been demonstrated capable
treatment of metals and other non-metal elements including boron and arsenic
(USEPA, 1982).
Phytoremediation technology can be used to extract groundwater; however,
phytoremediation is not capable of achieving extraction rates necessary to
achieve groundwater remediation within reasonable timeframes. Although,
phytoremediation is not retained for consideration for groundwater corrective
action, phytoremediation could be an effective surface water protection
supplement to a groundwater management system. At Allen, phytoremediation
technology could be applied as corrective measure to address low flowing seeps
east and southeast of the ash basin dam, if those seeps are not dispositioned after
completion of decanting. The use of phytoremediation is retained for further
consideration for potential seep remediation.
6.4.4 Groundwater Treatment
Several technologies exist for treatment of extracted groundwater to remove or
immobilize constituents ex -situ, or above ground. The following technologies are
used for treatment of extracted groundwater. These groundwater treatment
technologies are scalable for small to large flow rates.
pH Adjustment
Adjustment of the pH of extracted groundwater, if required prior to discharge, is
a proven technology. NPDES permitted discharges will impose specific limits on
the pH of discharged wastewater. The existing NPDES permitted outfalls at
Allen maintain a pH between 6.0 and 9.0 S.U. Facilities and equipment to adjust
the pH of wastewater to satisfy NPDES discharge requirements are in -place at
Allen.
The pH adjustment of extracted groundwater is anticipated. Background values
for pH in shallow (saprolite) groundwater at the Allen Site have been reported as
low as 4.5 S.U. Also, field measured pH of groundwater samples collected from
58 downgradient groundwater monitoring wells were less than 6.0 S.U. during
2018 and 2019. This treatment technology will be retained for further
consideration.
Precipitation
Precipitation of metals and other inorganic constituents has been used
extensively to treat extracted groundwater. The process involves the conversion
of soluble (dissolved) constituents to insoluble particulates that will precipitate.
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The insoluble particles are subsequently removed by physical methods such as
clarification or filtration. The process might involve adjustment of the
wastewater pH and/or reduction -oxidation (redox) potential or Eh (volts). The
stability of soluble and insoluble metals and metal complexes is commonly
illustrated in Pourbaix diagrams (pH vs Eh).
As illustrated in the Pourbaix diagram
(Figure 6-27) at right, iron is soluble
(aqueous or aq) at a pH of
approximately 3.5 S.U. or less under
aerobic conditions (Eh > 0 V). If the pH
is increased, ferric (Fe+3) iron will react
to form insoluble (solid or s) complexes
and precipitate out of solution, provided
that the Eh remains between 0.75 and 1.5
V. Adjustment of groundwater pH and
Eh can be used to remove other metals
FIGURE 6-27
POURBAIX DIAGRAM FOR
IRON -WATER SYSTEM
Simplified Pourbaix diagram
for iron -water system at 77°F (25°C)
E,►
2.0
1.6
1.2
0.8
0.4
including cadmium, chromium, copper, 0.0
nickel, and zinc. Flocculation is another .0.4
method that can be used to remove -0.8
inorganics from an aqueous waste -1 2
stream.
Precipitation technology might be
k m
HcO4' (ay)
Fe'+ (ail)
e_ cl
FezO
Fe'+ (aq) C
11
b c
Fe (S)
0_ 2 4 6 8 10 12 14 PH
https://rsteyn.wordpress.com/pourbaix -diagrams
warranted to treat, or pretreat, extracted
groundwater to satisfy NPDES permitted discharge limits. Precipitation
technologies are retained for further consideration. Dissolved constituent
precipitation technology equipment is readily available.
Ion Exchange
Ion exchange processes are reversible chemical reactions that can be used for the
removal of dissolved ions from solution and replacing them with other similarly
charged ions. The ion exchange medium might consist of a naturally occurring
material such as zeolites or a synthetic resin with a mobile ion attached to an
immobile functional acid or base group. Mobile ions held by the ion exchange
resin are exchanged with solute or target ions in the waste stream having a
stronger affinity to the functional group.
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Ion exchange resins can be cation resins or anion resins of varying strength. Ion
exchange resins are generally classified as being:
• Strong acid cation (SAC) resins.
• Weak acid cation (WAC) resins.
• Strong base anion (SBA) resins.
• Weak base anion (WBA) resins.
Over time, a resin can become saturated with the targeted or competing ions.
Breakthrough might occur when a resin becomes saturated. The possibility of
breakthrough is evident when effluent concentrations of the targeted metal ion
steadily increases over time and approach influent concentrations. Ion resins
should be replaced or regenerated before breakthrough occurs. Ion selective
born resins are available and do not have the same competition considerations.
However, capacity and regeneration are still potential limitations and key design
parameters.
Regeneration is laborious and requires safe handling of concentrated chemical
reagents and waste. The first step in the co -flow regeneration process
(regenerant is introduced via ion exchange bed influent) is to backwash the
system with water. The regenerant solution is introduced to drive off ions and
restores the resin capacity to about 60 to 80 percent of the total resin ion exchange
capacity. Sodium hydroxide is a commonly used regenerant for WBA resins;
weaker alkalis such as ammonia (NH3) and sodium carbonate (Na2CO3) can also
be used (SAMCO, 2019).
When sufficient contact time has passed, a slow water rinse is applied to the
resin bed to push the regenerant solution throughout the resin and subsequently
remove the regenerant from the system. The regenerant should be retained for
proper management. The slow rinse is followed by a fast "raw' water rinse to
verify water quality requirements are being met.
A limitation of this technology is that there must be a feasible and economical
method to dispose of the regeneration effluent. An additional challenge could be
groundwater influent streams that might have geochemical characteristics that
result in interference in the ion exchange process. Because of these challenges ion
exchange is not retained for further consideration.
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Membrane Filtration
There are a number of permeable membrane filtration technologies that can be
utilized to remove metals and other constituents from extracted groundwater.
The most common is reverse osmosis. Microfiltration, ultrafiltration, and
nanofiltration are also permeable membrane filtration technologies that are used
less frequently.
All four technologies use pressure to force influent water through a permeable
membrane. Permeable membrane filtration technologies are selected and
designed so that influent water can pass through the membrane while target
constituents are filtered (retained) by the membrane. The permeable membrane
filtration technologies discussed differ in the size of the molecules filtered and
the pressures needed to allow permeate to pass through the membranes.
Permeable membrane filtration technologies can filter one or more target
constituents simultaneously and achieve low effluent concentrations. However,
permeable membrane filtration technologies are also very susceptible to fouling
and often require a pretreatment step. They can also generate a high
concentration reject effluent which might require additional treatment prior to
management. These technologies typically have high capital costs.
Membrane filtration at Allen is not carried forward for further evaluation for the
following reasons:
Extracted groundwater is not expected to be greater than permit discharge
limits.
Pretreatment and a high volume of reject effluent that requires additional
treatment prior to management make this technology costly and high
maintenance.
6.4.5 Groundwater Management
Extracted groundwater must be managed or used as supplemental process water
prior to discharge. The disposition of extracted groundwater is discussed in the
following sections.
National Pollutant Discharge Elimination System (NPDES)
Permitted Discharge
The Allen Steam Station has an NPDES permit (NC0004979) that authorizes the
discharge of specific waste streams to the Catawba River via NPDES Outfalls 002
and 006. Outfalls 002 and 006 are authorized to discharge storm water from the
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coal pile area, miscellaneous storm water flows, ash sluice water, landfill
leachate, yard drain sump, treated groundwater and other waste streams. When
necessary, these waste streams are treated using chemical coagulation, settling,
and/or pH neutralization to satisfy Outfall 002 and 006 NPDES discharge
requirements, which are summarized on Table 6-10. Outfall 002 is located in the
southern end of the Site east of the AAB. Outfall 006 is on the north end of the
plant and receives the discharge from the lined retention basin (LRB). Both of
these outfalls will be considered for the management of extracted groundwater.
Management of extracted groundwater utilizing the NPDES discharge system
will be retained for further consideration.
Publicly Owned Treatment Works (POTW)
This groundwater management option involves the discharge of extracted
groundwater to a sewer that discharges to the local POTW. The feasibility of this
management option depends on a number of factors including:
• The proximity of the nearest sewer line relative to the groundwater
extraction system.
• The available capacity of a POTW to accept a new waste stream.
• The suitability of a groundwater waste stream on POTW operations.
• Capital costs, pretreatment requirements, and management fees.
The City of Belmont wastewater treatment plant (WWTP) is located at 298
Parkdale Drive in Belmont, NC 28012 or about 2.5 miles north of Allen on the
western shoreline of the Catawba River. The City of Belmont Water Distribution
and Sewer Collections Department is responsible for sewer distribution lines to
the Belmont WWTP. A sewer line that discharges to the Belmont WWTP extends
to Plant Allen Road (Miller, 2019a).
The City of Belmont WWTP has the following limits on their influent (Miller,
2019b):
• Daily flow rate: 5 million of gallons per day (MGD).
• pH: minimum 6.00 S.U. maximum 9.00 S.U.
The maximum monthly influent flow rate is 2.226 MGD. Consequently, it
appears that the City of Belmont WWTP has approximately 2.0 MGD of available
treatment capacity. However, the Belmont sewer use ordinance (SUO) states that
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the City of Belmont WWTP will not accept groundwater unless specifically
authorized by the POTW director (SUO §52.060(b)(13)). Total flow rates required
for treatment may be greater than 1.3 MGD as discussed in Section 6.8.2. It is
unlikely that the City of Belmont WWTP will allocate most of its available
capacity to a single industrial user.
Discharge of extracted groundwater to the City of Belmont WWTP is not
retained for further consideration at this time. Management of extracted
groundwater via NPDES Outfall 002 or through the lined retention basin and
NPDES Outfall 006 are considered better options.
Non -Discharge Permit/Infiltration Gallery
Disposition of treated groundwater by way of infiltration into underlying
groundwater involves the construction of an infiltration gallery to receive and
distribute the treatment effluent or wastewater. Discharge of wastewater by way
of an infiltration gallery must not result in a violation of 02L groundwater
standards. Consequently, groundwater treatment must reliably produce an
effluent waste stream that does not result in groundwater violation set by the 02L
standard.
The construction and use of infiltration galleries are permitted under 15A NCAC
02T .0700. The effectiveness of an infiltration system depends in large part on the
type of soils, or classification of soils, receiving the wastewater. Annual
hydraulic loading rates shall be based on in -situ measurement of saturated
hydraulic conductivity in the most restrictive horizon for each soil mapping unit.
United States Department of Agriculture (USDA) soil map of the Site indicates
that over half of the native soil is Udorthents, loamy (USDA, 2019). The capacity
of the most limiting layer of this soil type to transmit water is described as
ranging from a very low to high (0.00 to 1.98 inchesihour) capacity.
Before extracted water could be recycled for infiltration gallery use, inorganic
constituents, including boron, chloride, cobalt, manganese among others, would
have to be treated. Treatment would have to be sufficient so wastewater recycled
to the groundwater system would not result in constituent concentrations greater
than 02L groundwater standards. Treatment of conservative and variably
conservative constituents could result in complicated systems with significant
operation and maintenance efforts. Therefore, the use of infiltration galleries to
dispose of treated groundwater is not retained for further consideration.
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Non Discharge Permit/Land Application
Land application of groundwater involves the distribution of extracted
groundwater onto land to irrigate the vegetative cover and supplying the
vegetative cover with nutrients beneficial for growth. The vegetative cover can
include grasses, tree wells, wetland species, native species of trees and shrubs,
and ornamental trees and shrubbery.
The primary focus of groundwater remediation efforts is to reduce boron
concentrations beyond the anticipated compliance boundary to acceptable levels.
Consequently, extracted groundwater would be expected to contain boron.
Boron is essential for plant growth. More specifically, boron in soil must be
continuously delivered to growing tissues through roots and vascular tissues to
maintain cell wall biosynthesis and optimal plant development (Takano, 2006).
Boron is also essential for plant nitrogen assimilation, for the development of
root nodules in nitrogen -fixing plants, and for the formation of polysaccharide
linkages in plant cell walls (Park, 2002). If extracted groundwater is land
applied, boron would be made available for plant uptake.
Extracted groundwater could be used to irrigate more than 300 acres of planted
vegetative cover following the implementation of source control measures. Land
application of extracted groundwater would occur within the compliance
boundary. A large scale irrigation system could be used to apply thousands of
gallons of water onto the vegetative cover daily. Of the water applied, much of it
would be lost to evaporation, particularly during sunny dry periods. Likewise,
water taken up by vegetation would be lost by way of plant transpiration. All
remaining water would either infiltrate into the soil or migrate downslope to
wetland areas via surface water runoff.
Land application of extracted groundwater must comply with 15A NCAC 02T —
Waste Not Discharged To Surface Waters. Duke Energy would submit an
application for a non -discharge permit in accordance with 15A NCAC 02T .0105 -
.0109. General permits can be effective for up to eight years. General permits
issued pursuant to 15A NCAC 02T shall be considered individual permits for
purposes of Compliance Boundaries established under 15A NCAC 02L .0107.
Permitted facilities shall designate an Operator in Responsible Charge and a
back-up operator as required by the Water Pollution Control System Operators
Certification Commission.
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Application of wastewater to the ground surface or surface irrigation of
wastewater is governed by 15A NCAC 02L .0500 - Wastewater Irrigation Systems.
Requirements under this subsection include:
• A soil scientist must prepare a soil report that evaluates receiving soil
conditions and who makes recommendations for loading rates of liquids
and wastewater constituents.
• A hydrogeologic report must be prepared by a licensed geologist, soil
scientist, or professional engineer for industrial waste treatment systems
with a design flow of over 25,000 gallons per day.
• The applicant must prepare a Residuals Management Plan.
• Each facility shall provide flow equalization with a capacity of 25 percent
of the daily system design flow unless the facility uses lagoon treatment.
• Management areas shall be designed to maintain one -foot vertical
separation between the seasonal high water table and the ground surface.
• Automatically activated irrigation systems shall be connected to a rain or
moisture sensor to prevent irrigation during precipitation events or wet
conditions that would cause runoff.
Setback requirements for irrigation sites (15A NCAC 02T .056) are summarized
in Table 6-11.
The DWR might require monitoring and reporting to characterize the waste
(extracted groundwater) and its effect upon surface water, ground water, or
wetlands.
Land application of extracted groundwater could be used as a means to maintain
the vegetative cover that would be established following implementation of
source control measures. However, the designated area would have to be able to
take continuous flow during both dry and wet seasons, which would not be
practical. Additionally, unless the vegetation is harvested, boron uptake will be
returned to the soil and aquifer upon death and decomposition of the plant
matter. Therefore, land application is not retained as an alternative means for
management of extracted wastewater.
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Beneficial Reuse
Beneficial reuse of extracted groundwater involves the evaluation of existing
Station water demand and the repurposing of extracted groundwater to satisfy a
need for water. Beneficial reuse of extracted groundwater can do the following:
• Provide an alternative to groundwater treatment.
• Reduce reliance on sources of non -potable water required for plant
operations.
• Reduce the need and capacity for wastewater treatment.
A NCDENR 2017 Annual Water Use Report for the Allen Steam Station indicated
that water was withdrawn from the Catawba River every day in 2017. The
average daily withdrawal in a given month ranged from 99.5 million gallons per
day (MGD) to 624.8 MGD. The average daily discharge in a given month ranged
from 99.4 to 624.3 MGD (NCDENR, 2018).
Beneficial Reuse: Fire Protection
A limited amount of extracted groundwater might be used to supplement or
supply water stored for fire suppression within Station operations. However, the
need for fire suppression water is limited, storage is problematic and would not
justify the effort and expense to substitute extracted groundwater for fire
suppression water obtained from the Catawba River (Lake Wylie).
Beneficial Reuse: Non -Contact Cooling Water
Extracted groundwater might be used to supplement or supply makeup water
used for non -contact cooling within Station operations. The alkalinity of
groundwater could pose potential scaling problems for some
applications. However, certain groundwater constituents including the
constituents that comprise alkalinity would be diluted by non -contact cooling
water obtained from the Catawba River. Use of extracted groundwater for non -
contact cooling water is not retained, but might be reconsidered in the future.
Beneficial Reuse: Dust Suppression and Truck Wash
A limited amount of extracted groundwater can possibly be used for dust
suppression during implementation of source control measures. Similarly,
extracted groundwater can possibly be used for washing the tires of haul trucks
leaving the ash basins during implementation of source control measures. The
use of extracted groundwater for dust suppression and truck washing would be
confined within ash basin limit of ash management. However, the need for dust
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suppression and truck wash water is limited and would not justify the effort and
expense to substitute extracted groundwater for dust suppression and truck
wash water obtained from the Catawba River (Lake Wylie). Therefore, beneficial
use of the water is not retained for further consideration.
6.4.6 Technology Evaluation Summary
A summary of the remedial technologies presented above and the rationale for
either retaining or rejecting a specific technology is presented in Table 6-12.
In conclusion, remedial technologies retained for further consideration include,
MNA, in -situ technology groundwater flushing, and vertical extraction wells.
Groundwater treatment technologies retained include pH adjustment and
precipitation. These technologies were retained to meet NPDES permit discharge
limits which was the only technology retained for management of extracted
groundwater. Phytoremediation was also retained for potential corrective action
of seeps. No beneficial reuse technology is retained at this time.
6.5 Evaluation of Remedial Alternatives
(CAP Content Section 6.D)
Technologies evaluated and retained for consideration as discussed in Section 6.4 were
used to formulate the following three groundwater remedial alternatives to remediate
groundwater affected by the ash basins or coal piles at Allen:
• Remedial Alternative 1: Groundwater remediation by MNA
• Remedial Alternative 2: Groundwater remediation by extraction
• Remedial Alternative 3: Groundwater extraction combined with targeted clean
water infiltration/injection and treatment
These groundwater remedial alternatives are detailed in the following subsections.
Information to address CAP Content Section 6.D.a.iv is provided in Sections 6.6 and 6.7.
6.5.1 Remedial Alternative 1 — Monitored Natural Attenuation
(CAP Content Section 6.D.a)
Alternative 1 is the use of MNA to address groundwater COI concentrations that
are at or beyond the ash basin compliance boundary. Under this alternative and
based on flow and transport model simulations, the boron and sulfate
groundwater plumes would naturally attenuate to less than the 02L standard in
approximately 320 years under the closure by excavation scenario and
approximately 310 years under the closure -in -place scenario. A detailed
comprehensive analysis of MNA is provided in Appendix I.
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6.5.1.1 Problem Statement and Remediation Goals
(CAP Content Section 6.D.a.i)
A limited number of constituents in groundwater associated with the Allen
ash basins and coal piles occur at or beyond the compliance boundary to the
north, northeast, and east of the source areas at concentrations detected
greater than applicable 02L standards, IMAC, or background values,
whichever is greater. Remediation goals are to restore groundwater quality
at or beyond the compliance boundary by returning COIs to acceptable
concentrations (02L/IMAC or background, whichever is greater), or as
closely thereto as is economically and technologically feasible consistent
with 15A NCAC 02L. 0106(a). In the future, alternative standards may be
proposed as allowed under 02L .0106(k). This approach is considered
reasonable given the documented lack of human health or ecological risk at
Allen (CAP Content Section 6.D.a.i.2).
The following groundwater COIs to be addressed by corrective action are
identified (Table 6-6) and discussed in Section 6.1.3: boron, cobalt, iron,
manganese, strontium, sulfate, and TDS (CAP Content Section 6.D.a.i.1).
These are the COIs that exhibit a discernable plume associated with the ash
basins and/or coal piles.
More extensive discussion of the CSM can be found in Section 5.0,
discussion of flow and transport modeling in Appendix G, and discussion
of geochemical modeling in Appendix H.
6.5.1.2 Conceptual Model
(CAP Content Section 6.D.a.ii)
Based on the CSM (Section 5.0) and flow and transport modeling results
(Appendix G), the groundwater COIs are hydraulically controlled within
the topographic drainage basin downgradient of the source areas.
Source control is a primary component of MNA as a remedial strategy. Ash
basin decanting commenced on June 6, 2019, and is scheduled to be
completed by June 30, 2020. Decanting is a form of active source control by
removing ponded water in the ash basin, which is considered a critical
component of reducing constituent migration from the ash basins. As of
December 1, 2019, 53,300,000 gallons of water has been decanted and the
corresponding pond water elevation has decreased by 14.1 feet. After
decanting and basin closure, the groundwater divides that control the
migration of COI will become more pronounced and located farther from
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the basins. Decanting and basin closure will reduce the potentiometric head
that contributes to the downward vertical gradient upstream of the ash
basins dams. A lower downward gradient would reduce downward COI
migration and enhance MNA effectiveness. As a result, constituent
concentration reductions through natural attenuation processes are
anticipated following decanting.
For the coal pile area, the conceptual site model would change following
decommissioning of the Station. Infiltration of water through the coal piles
into the subsurface is a source of groundwater COIs underlying the main
and live coal piles. This source would be eliminated from the Site as part of
decommissioning. MNA effectiveness would be enhanced following
complete elimination of coal from the main and live coal piles.
The following three chemical natural attenuation mechanisms are an
effective corrective action approach north, northeast and east of ash basins
and coal pile area because they aid in stabilizing control of reactive and
variable reactive COIs cobalt, iron, manganese, and strontium in
groundwater by the following processes:
• Sorption: Chemical attachment of electrochemically charged ions to
charged receptors in the subsurface media
• Precipitation: Removal of a COI from a dissolved state in
groundwater by incorporation into the matrix of a solid such as a
mineral or an amorphous mass
• Ion Exchange: Incorporation of an ion into the crystal structure of a
matrix mineral or amorphous solid
The following five physical natural attenuation mechanisms are also an
effective corrective action approach north, northeast, and east of the ash
basins and coal pile area because they control the migration and
distribution of all or some COIs, particularly boron, sulfate, and TDS, in
groundwater by the following processes:
• Dilution: Reduce COI concentrations through mixing with
unaffected groundwater
• Dispersion: Reduce COI concentrations through variability of the
flow velocity and concentration gradients
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• Transfer to surface water: Reduce COI concentrations through
mixing and flushing with surface water to further reduce the
potential for concentrations to be greater than 02B standards
Groundwater flow control within the stream valley system: Control
COI migration within hydraulic divide boundaries south, west and
north of the ash basins and coal pile area
• Phyto-attenuation: Uptake of the COIs by plants or organisms
More information on one or more effective natural attenuation mechanism
for reducing the concentration of the COI in groundwater can be found in
Appendix I, Table ES-1.
Currently, COIs in groundwater do not pose an unacceptable risk to human
health or the environment under conservative exposure scenarios and, if
implemented alone, MNA would not pose an unacceptable risk to human
health or the environment in the future. Source control and groundwater
monitoring would verify protection of human health and the environment
and to confirm model predictions. The applicable technologies that would
support this alternative include groundwater monitoring wells within the
former source area and near the former waste boundary, along
downgradient flow transects, at a potential future point of compliance, in
sentinel areas prior to receptors, and near the maximum predicted extent of
migration. There are 234 monitoring wells installed associated with the ash
basins and coal piles. A majority of the wells have dedicated sampling
equipment and an approved interim monitoring plan is in place. A subset of
these monitoring wells could be immediately used for monitoring the
effectiveness of Alternative 1.
6.5.1.3 Predictive Modeling
(CAP Content Section 6.D.a.iii)
Predictive modeling has been conducted to estimate when boron and
sulfate concentrations would be reduced to 02L standards using MNA
alone (primarily relying on natural attenuation by dilution). The
simulations indicate boron and sulfate concentrations would naturally
attenuate to less than the 02L standard in approximately 310 to 320 years.
The flow and transport modeling report that provides the predictions for
boron and sulfate is presented in Appendix G. Similarly, a geochemical
modeling report is presented in Appendix H. The geochemical modeling
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report describes the natural attenuation of the constituents that have
multiple natural attenuation mechanisms, in addition to dilution.
6.5.2 Remedial Alternative 2 — Groundwater Extraction and
Treatment
(CAP Content Section 6.D.a)
Alternative 2 consists of groundwater extraction and treatment as a remedial
alternative for the areas north, northeast and east of the ash basins and coal piles
at or beyond the compliance boundary. This alternative provides technology for
groundwater capture (i.e. extraction) to address Site specific COIs. Under this
alternative, compliance will be achieved in an excess of 500 years from system
startup and operation.
6.5.2.1 Problem Statement and Remediation Goals
(CAP Content Section 6.D.a.i)
Constituents in groundwater associated with the Allen ash basins and coal
piles occur at or beyond the compliance boundary to the north, northeast,
and east of the source areas at concentrations detected greater than
applicable 02L standards, IMAC, or background values, whichever is
greater. Remediation goals are to restore groundwater quality at or beyond
the compliance boundary by returning COIs to acceptable concentrations
(02L/IMAC or background, whichever is greater), or as closely thereto as is
economically and technologically feasible consistent with 15A NCAC 02L
.0106(a). In the future, alternative standards may be proposed as allowed
under 02L .0106(k). This approach is considered reasonable given the
documented lack of human health or ecological risk at Allen (CAP Content
Section 6.D.a.i.2).
The following groundwater COIs to be addressed by corrective action are
identified (Table 6-6) and discussed in Section 6.1.3: boron, cobalt, iron,
manganese, strontium, sulfate, and TDS (CAP Content Section 6.D.a.i.1).
These are the COIs that exhibit a discernable plume associated with the ash
basins and/or coal piles.
The conceptual model and predictive modeling discussions summarize the
foundations for development of the groundwater extraction and treatment
alternative. More extensive discussion of the CSM can be found in Section
5.0, discussion of flow and transport modeling in Appendix G, and
discussion of geochemical modeling in Appendix H.
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6.5.2.2 Conceptual Model
(CAP Content Section 6.D.a.ii)
The applicable technologies that comprise Alternative 2 include:
• 87 extraction wells to the north, northeast and east of the ash basins
and coal pile area
• Pumps, associated piping, and control systems
• Discharge piping and structure
• pH adjustment or other treatment systems
The flow and transport model predicts a total groundwater extraction flow
rate of approximately 970 gpm. The number of extraction wells is estimated
based on flow and transport modeling results.
The system design includes a large number of extraction wells to be
completed into the shallow bedrock to allow full drawdown within the
shallow and deep (transition zone) flow zones. Depths of extraction wells
are dependent on the contacts between the shallow, deep and bedrock flow
zones and fractures within the bedrock. As a result, extraction well depths
range from approximately 65 feet bgs to 395 feet bgs in the design.
Based on the CSM (Section 5.0) and flow and transport modeling results
(Appendix G), the groundwater COIs are hydraulically controlled within
the topographic drainage basin downgradient of the source areas. The
following groundwater COIs subject to corrective action are identified and
discussed in Section 6.1.3.
The distribution of conservative COIs (boron, sulfate and TDS) represents
the area of maximum COI distribution at or beyond the compliance
boundary and is the focus of corrective action. Focusing remedial action
selection on addressing the mobile COIs will also address the reactive COIs
as they will follow the same flow path but with greater attenuation. With
some exceptions, other COIs have generally not migrated horizontally or
vertically in the shallow, deep, and bedrock flow zones appreciably from
the source areas, and are not expected to do so due to constituent
geochemical characteristics and Site geochemical and hydrogeologic
conditions as detailed in Appendix H.
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It is expected that extracted water would be treated and discharge through
the existing NPDES Outfall 002 or the existing LRB through NPDES Outfall
006 based on currently available groundwater data and the current permit.
If necessary, a treatment method other than pH control would be selected
based on the quantity and quality of the extracted groundwater.
A preliminary summary of groundwater data and current discharge permit
limits is presented in Table 6-10.
6.5.2.3 Predictive Modeling
(CAP Content Section 6.D.a.iii)
A groundwater extraction system would hydraulically control and remove
COI mass at or beyond the compliance boundary. A groundwater extraction
system would reduce the mass and concentrations of COIs in Site
groundwater. The permeability of the formations might limit extraction
flow rates. Simulated groundwater extraction flow rates per well are
approximately 13 gpm for combined saprolite/deep wells and 4 gpm for
bedrock wells.
The flow and transport model predicts the maximum extent of the COI
plume at any point in time will be approximately 1,500 feet beyond the
compliance boundary as indicated by sulfate downgradient from the RAB.
For boron, the maximum extent of the simulated plume at any point in time
will be approximately 300 feet beyond the compliance boundary.
Simulations indicate that boron and sulfate concentrations in groundwater
would meet the 02L standards at the compliance boundary in excess of 500
years after ash basin closure.
6.5.3 Remedial Alternative 3: Groundwater Extraction
Combined with Targeted Clean Water Infiltration and
Treatment
(CAP Content Section 6.D.a)
Alternative 3 consists of groundwater extraction combined with clean water
infiltration (i.e. groundwater recharge) and treatment for the areas north,
northeast, and east of the ash basins and coal pile area at or beyond the
compliance boundary. This alternative provides technology for groundwater
control and capture (i.e. infiltration and extraction) to address Site specific COIs.
Under this alternative, compliance is achieved in approximately 10 years from
system startup and operation.
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6.5.3.1 Problem Statement and Remediation Goals
(CAP Content Section 6.D.a.i)
Constituents in groundwater associated with the Allen ash basins and coal
pile area occur at or beyond the compliance boundary to the north,
northeast and east of the source areas at concentrations detected greater
than applicable 02L standards, IMAC, or background values, whichever is
greater. Remediation goals are to restore groundwater quality at or beyond
the compliance boundary by returning COIs to acceptable concentrations
(02L/IMAC or background, whichever is greater), or as closely thereto as is
economically and technologically feasible consistent with 15A NCAC 02L.
0106(a). In the future, alternative standards may be proposed as allowed
under 02L .0106(k). This approach is considered reasonable given the
documented lack of human health or ecological risk at Allen (CAP Content
Section 6.D.a.i.2).
The following groundwater COIs to be addressed by corrective action are
identified (Table 6-6) and discussed in Section 6.1.3: boron, cobalt, iron,
manganese, strontium, sulfate, and TDS (CAP Content Section 6.D.a.i.1).
These are the COIs that exhibit a discernable plume associated with the ash
basins and/or coal piles.
The conceptual model and predictive modeling discussions summarize the
foundations for development of the groundwater extraction and treatment
alternative. More extensive discussion of the CSM can be found in Section
5.0, discussion of flow and transport modeling in Appendix G, and
discussion of geochemical modeling in Appendix H.
6.5.3.2 Conceptual Model
(CAP Content Section 6.D.a.ii)
The applicable technologies that comprise this alternative include:
• 87 extraction wells
• 76 vertical clean water infiltration wells or approximately 48 vertical
clean water infiltration wells combined with approximately 22
horizontal clean water infiltration wells
• Pumps, associated piping, and control systems
• Infiltration and discharge piping and structure
• pH adjustment or other treatment systems, if necessary
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The proposed designs and well locations are shown on Figures 6-28a and 6-
28b. Horizontal infiltration wells may be used in place of some vertical
infiltration wells depending on access to the coal pile area and the power
block area. Vertical wells would be preferred because vertical wells: are
more adaptable for changing site conditions; have similar design
components with the majority of the Site and other locations; cost less to
install; have more predictable performance; and are easier to maintain.
However, flow and transport model simulations indicate that the time to
achieve 02L compliance boron and sulfate would be the same using either
vertical or horizontal infiltration wells.
To achieve compliance with 02L, the flow and transport model predicts a
total groundwater extraction flow rate of approximately 970 gpm and a
infiltration flow rate of approximately 380 gpm if only vertical clean water
infiltration wells are used or approximately 230 gpm (vertical wells) and
175 gpm (horizontal wells) if vertical and horizontal clean water infiltration
wells are used.
The number of extraction and clean water infiltration wells is estimated
based on flow and transport modeling results (Appendix G). Table 6-13
summarizes the systems groundwater extraction well and clean water
infiltration well information. Raw water used for infiltration would be
obtained from the Catawba River using an intake currently used for Station
operations. The plant water system would be accessed from the existing
distribution system. Pressure for delivery of the water to infiltration wells
would be obtained by connecting the infiltration system tank located on the
Site.
The system design includes a large number of extraction wells to be
completed into the shallow bedrock to allow full drawdown within the
shallow and deep (transition zone) flow zones. Depths of extraction wells
are dependent on the contacts between the shallow, deep and bedrock flow
zones and fractures within the bedrock. As a result, extraction well depths
range from approximately 65 feet bgs to 395 feet bgs in the design.
The system design also includes a large number of clean water infiltration
wells to be completed into the shallow and deep (transition zone) flow
zones. Depths of extraction wells are dependent on the contacts between the
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shallow, deep and bedrock flow zones. As a result, infiltration well depths
range from approximately 75 feet bgs to 138 feet bgs in the design.
Horizontal wells, if used, would be installed at two depths at each location.
One well would be installed in the shallow flow zone and another would be
installed deeper within the transition zone. Installing horizontal wells at
two different depths increases the surface area for flushing of constituents
and, as indicated by modeling, also prevents downward migration of
constituents that could result if only shallow horizontal wells are installed.
Based on the CSM (Section 5.0) and flow and transport modeling results
(Appendix G), the groundwater COIs are hydraulically controlled within
the topographic drainage basin downgradient of the ash basins and coal
pile area. The following groundwater COIs subject to corrective action are
identified and discussed in Section 6.1.3: boron, cobalt, iron, manganese,
strontium, sulfate, and TDS. These are the COIs that exhibit a discernable
plume associated with the ash basins and/or coal piles.
The distribution of conservation COIs (boron, sulfate and TDS) represents
the area of maximum COI distribution at or beyond the compliance
boundary and is the focus of corrective action. Because this alternative
provides hydraulic control and capture of boron and sulfate, the most
mobile COIs, it addresses all of the targeted COIs, because reactive and
variably reactive COIs will follow the same flow path but with greater
attenuation. With some exceptions, other COIs have generally not migrated
horizontally or vertically in the shallow, deep, and bedrock flow zones
appreciably from the source area, and are not expected to do so due to
constituent geochemical characteristics and Site geochemical and
hydrogeologic conditions.
It is expected that infiltrations water obtained from the Catawba River
would be treated for pH and suspended solids using pH adjustment
technology and flocculation technology. It is expected that extracted water
would be treated and discharge through the existing NPDES Outfall 002
based on currently available groundwater data and the current permit. If
necessary, a treatment method other than pH control would be selected
based on the quantity and quality of extracted groundwater.
A preliminary summary of groundwater data and current discharge permit
limits is presented in Table 6-10.
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6.5.3.3 Predictive Modeling
(CAP Content Section 6.D.a.iii)
A clean water infiltration and extraction system would result in localized
groundwater flow control and increase the rate of mass removal. While the
permeability of the formations will limit flow, the additional volume of
groundwater created by clean water infiltration will increase the
effectiveness of the system, flushing the system with clean infiltration water
and reducing COI concentrations. Simulated groundwater extraction flow
rates per well are approximately 13 gpm for combined saprolite/deep wells
and 4 gpm for bedrock wells. The flow and transport report (Appendix G)
and geochemical modeling report (Appendix H) provide detailed
predictions, descriptions, and explanations of the effects of clean water
infiltration and extraction.
The flow and transport model predicts the maximum extent of the COI
plume at any point in time will be approximately 1,500 feet beyond the
compliance boundary as indicated by sulfate downgradient from the RAB.
For boron, the maximum extent of the simulated plume at any point in time
will be approximately 300 feet beyond the compliance. Simulations indicate
that boron and sulfate concentrations in groundwater would meet 02L
standards at the compliance boundary approximately 10 years after
implementation of the remedial system.
6.6 Remedial Alternatives Screening Criteria
(CAP Content Section 6.D.a.iv)
This section provides supplemental information beyond the CAP content guidance to
describe the screening criteria used to evaluate groundwater remediation alternatives at
Allen. The screening criteria used to evaluate technologies and alternatives for
groundwater corrective action are described below. These screening criteria are based
on the criteria outlined in 15A NCAC 02L .0106(i) and 40 Code of Federal Register
(CFR) 300.430. The source of the screening criteria descriptions is 40 CFR 300.430. These
screening criteria will be used in evaluating the three remedial alternatives identified in
Section 6.5.
• Protection of human health and the environment
• Compliance with applicable regulations
• Technical and logistical feasibility
• Time required to initiate and implement corrective action alternative
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• Short-term effectiveness
• Long-term effectiveness and permanence
• Reduction of toxicity, mobility, and volume
• Time required to achieve remediation goals
• Community acceptance
Additional considerations for remedial alternative evaluations include:
• Adaptive site management and remediation considerations
• Sustainability
Protection of Human Health and the Environment
The Updated Human and Ecological Risk Assessments report (Appendix E) has
determined that there are no imminent hazards to public health and safety or the
environment associated with coal ash basin or coal ash constituents in Site soil and
groundwater. The updated risk assessment indicates acceptable risk and no exposure
to residential receptors at or near the ash basin (no completed exposure pathways). The
assessment did not result in an increase of risks to ecological receptors (mallard duck,
great blue heron, muskrat, river otter, kill deer bird) exposed to surface water and
sediments associated with the ash basins or coal pile area. Regardless, potential
corrective measures are being evaluated for regulatory compliance.
Technologies and remedial alternatives are evaluated to determine whether they can
achieve regulatory compliance within a reasonable timeframe, without detriment to
human health and the environment.
Compliance with Applicable Regulations
Technologies and alternatives are herein evaluated to assess compliance with applicable
federal and state environmental laws and regulations. These include:
• CAMA (NC SB 729, Subpart 2)
• Groundwater Standards (NCAC, Title 15A, Subchapter 02L)
• Well construction and maintenance standards (NCAC Title 15A Subchapter 02C)
• NPDES (40 CFR Part 122)
• Sediment erosion and control (NCAC Title 15A Chapter 04)
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Technical and Logistical Feasibility
The ease or difficulty of implementing technologies and alternatives are assessed by
considering the following types of factors as appropriate:
• Technical feasibility, including technical difficulties and unknowns associated
with the construction and operation of a technology, the reliability of the
technology, ease of undertaking additional remedial actions, and the ability to
monitor the effectiveness of the remedy
• Administrative feasibility, including activities needed to coordinate with
agencies, and the ability and time required to obtain any necessary approvals
and permits
• Availability of services and materials, including the availability of adequate off -
Site treatment, storage capacity, and management capacity and services; as well
as the availability of necessary equipment and specialists, and provisions to
ensure any necessary additional resources
Time Required to Initiate and Implement Corrective Action
Alternative
The time required to initiate and fully implement a groundwater remedial action takes
into consideration the following activities, if applicable:
• Source control measures
• Bench -scale testing, if needed
• Treatability testing
• Pilot -testing
• Hydraulic conductivity testing
• Groundwater remedial alternative system design
• Permitting
• System Installation
• Startup
These activities might be requisite to finalize the system design, attain regulatory
approval, or initiate construction. Therefore, these activities might dictate the time it
takes to initiate and fully implement a remedial alternative.
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Short-term Effectiveness
The short-term effects of alternatives are assessed considering the following:
• Short-term risks that might be posed to the community during implementation
• Potential impacts on workers during implementation and the effectiveness of
mitigation
• Potential environmental effects during implementation and the effectiveness of
mitigation
• Time until protection is achieved
Long-term Effectiveness and Permanence
Technologies and alternatives are assessed for long-term effectiveness in reducing COI
concentrations and permanence in maintaining those reduced concentrations in
groundwater, along with the degree of certainty that technologies will be successful.
Factors considered, as appropriate, include the following:
• Magnitude of residual risk remaining from untreated material remaining at the
conclusion of remedial activities. The characteristics of the residuals should be
considered to the degree that they could affect long-term achievement of
remediation goals, considering their volume, toxicity, and mobility. Since there
is no current risk, the potential for a remedial technology to increase potential
risk to a receptor is considered in the evaluation process.
• Adequacy and reliability of controls as a means of evaluating alternatives in
addition to managing residual risk.
Reduction of Toxicity, Mobility, and Volume
The degree to which technologies employ recycling or treatment that reduces toxicity,
mobility, or volume will be assessed, including how treatment is used to address the
principal risks posed at the Site. Factors considered, as appropriate, include the
following:
• The treatment or recycling processes the technologies employ and constituents
that will be treated
• The mass of COIs that will be destroyed, treated, or recycled
• The degree of expected reduction in toxicity, mobility, or volume
• The degree to which the treatment is irreversible
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• The type and quantity of residuals that will remain after treatment, considering
the persistence, toxicity, and mobility of such substances and their constituents
• The degree to which treatment reduces the inherent hazards posed by risks at the
Site
Predicted Time Required to Achieve Remediation Goals
This criterion includes the estimated time necessary to achieve remedial action
objectives. This includes time required for permitting, pilot scale testing, design
completion and approval, and implementation of approved remedies.
Cost
The costs of construction and long-term costs to operate and maintain the technologies
and alternatives are considered. Costs that are grossly excessive compared to overall
effectiveness may be considered as one of several factors used to eliminate alternatives.
Alternatives that provide effectiveness and implementability similar to that of another
alternative by employing a similar method of treatment or engineering control, but at
greater cost, may be eliminated.
Community Acceptance
This assessment considers likely support, concerns, or opposition from community
stakeholders about the alternatives. This assessment might not be fully informed until
comments on the proposed plan are received. However, some general assumptions of
how an alternative would be accepted by the community can be made.
Adaptive Site Management and Remediation Considerations
Remediation alternatives are evaluated to determine whether an adaptive site
management process would address challenges associated with meeting remedial
objectives. Adaptive site management is the process of iteratively reviewing site
information, remedial system performance, and current data to determine whether
adjustments or changes in the remediation system are appropriate. The adaptive site
management approach may be adjusted over the site's life cycle as new site information
and technologies become available. This approach is particularly useful at complex sites
where remediation is difficult and may require a long time, or where NCDEQ approves
alternate groundwater standards for COIs, such as 4,000 µg/L for boron, pursuant to its
authority under 15A NCAC 02L .0106(k). Duke Energy might request alternate
standards for ash basin- and coal pile -related constituents, including boron, as allowed
under 15A NCAC 02L .0106(k). Alternate standards are appropriate at Allen given the
lack of human health and ecological risks at the Site. Factors included in this evaluation
include:
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• Potential to hinder use of alternative or contingency technologies later
• Suitability to later modifications or synergistic with other technologies
• Information that could be gained from technology implementation to improve
the Site Conceptual Model and better inform future remediation decision -making
• Ability to adjust and optimize the technology based on performance data
• Suitability for implementation in a sequential remedial action strategy
• Flexibility to implement optimization without significant system modifications
Sustainability
In accordance with sustainability corporate governance documents integral to Duke
Energy and guidance provided by the USEPA, analysis of the sustainability of the
remedial alternatives proposed in this CAP Update was identified as an important
element to be completed as part of remedy selection process described herein.
Sustainable site remediation projects maximize the benefit of cleanup activities through
reductions of the footprint of selected remedies, while preserving the effectiveness of
the cleanup measures.
The USEPA, along with ASTM International, developed the Standard Guide to Greener
Cleanups - ASTM E2893, which was utilized during the evaluation process as part of
the remedial alternative selection effort. ASTM E2893 describes a process to evaluate
and implement cleanup activities in order to reduce the footprint of remediation
projects. Two primary approaches are described in the document: a qualitative Best
Management Practices (BMP) process and quantitative evaluation. Quantitative
evaluation was utilized for remedy selection in this CAP Update.
As stated in the ASTM standard, during the remedial selection process, "... the user
considers how various remedial options may contribute to the environmental footprint.
Conducting a quantitative evaluation at this phase of the remedial alternative selection
process provides stakeholders with information to help identify environmental
footprint reduction opportunities for all alternatives that are protective of human health
and the environment, comply with applicable environmental regulations and guidance,
and meet project objectives (ASTM, 2016)."
Each remedial alternative has been assessed using SiteWiseTM, a public domain tool for
evaluating remediation projects based on the overall footprint. SiteWiseTM estimates
collateral impacts through several quantitative sustainability metrics. The output data
from SiteWiseTM that can be utilized for remedial alternative comparison includes
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greenhouse gases, energy usage, and criteria air pollutants (including sulfur oxides,
oxides of nitrogen, and particulate matter), water use, and resource consumption. The
assessment quantified impacts associated with activities expected to occur during the
remedial alternative construction phase, system operations where applicable and long-
term monitoring.
Two core elements of the USEPA's Greener Cleanup principles were not quantified
through the use of the SiteWiseTM tool, as part of the alternatives evaluation: water
consumption and waste generation. The analysis tool is set up to quantify the footprint
of municipal water use and the accompanying discharge of wastewater for treatment to
a publicly owned treatment works (POTW). The remediation activities proposed in the
CAP Update do not use municipal water or discharge to a POTW, thereby making that
input inapplicable for the calculation. Due to the difficulty of estimating reliable
quantities of waste generated during construction the input was considered too
uncertain to use as a criteria. These two elements were set aside as less -relevant to
remedy selection for the purposes of this CAP Update than the other quantifiable data
points available. For the quantitative evaluation of alternatives discussed here, the
primary assessments for consideration during sustainability screening are CO2, NOx,
SOx, PM10 and energy usage.
Results of these sustainability evaluations are presented and discussed in the detailed
analysis sections of the specific alternatives (Section 6.7).
6.7 Remedial Alternatives Criteria Evaluation
(CAP Content Section 6.D.a.iv)
Groundwater remediation Alternatives 1, 2 and 3 were formulated in Section 6.5 using
groundwater remediation technologies evaluated and retained for consideration
in Section 6.4. The criterion for conducting detailed analysis of each groundwater
remedial alternative are presented and explained here in Section 6.7. The groundwater
remediation alternatives formulated in Section 6.5 will undergo detailed comparative
analysis in the following subsections. A summary of the remediation alternative
detailed analysis is also included in Appendix M.
6.7.1 Remedial Alternative 1 — Monitored Natural Attenuation
Protection of Human Health and the Environment
(CAP Content Section 6.D.a.iv.1)
There is no measurable difference between evaluated Site risks and risks
indicated by background concentrations; therefore, no material increases in risks
to human health related to the source areas have been identified. The
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groundwater corrective action is being planned to address regulatory
requirements. The risk assessment identified no current human health or
ecological risk associated with groundwater downgradient of the source areas.
Water supply wells are located upgradient and sidegradient of the source areas
and an alternate water supply has been provided to those who selected this
option. Surface water quality standards downgradient of the COI -affected
plumes are also met.
Based on the absence of receptors, it is anticipated that MNA would continue to
be protective of human health and the environment because modeling results
indicate COI concentrations will diminish with time. Natural attenuation
mechanisms will reduce COI concentrations, and model predictions indicate that
no existing water supply wells would be impacted. After decanting and closure
the hydraulic divide to the north, west, and south of the ash basins would be
more pronounced and groundwater flow would continue toward the Catawba
River.
Compliance with Applicable Regulations
(CAP Content Section 6.D.a.iv.2)
MNA would comply with applicable regulations assuming the conditions
provided in 02L can be achieved. State and federal groundwater regulations
allow for MNA as an acceptable remediation program if regulatory requirements
are met. The following are the applicable 02L regulations:
(l) Any person required to implement an approved corrective action plan for a non -
permitted site pursuant to this Rule may request that the Director approve such a
plan based upon natural processes of degradation and attenuation of contaminants. A
request submitted to the Director under this Paragraph shall include a description of
site specific conditions, including written documentation of projected groundwater
use in the contaminated area based on current state or local government planning
efforts; the technical basis for the request; and any other information requested by the
Director to thoroughly evaluate the request. In addition, the person making the
request must demonstrate to the satisfaction of the Director: (1) that all sources of
contamination and free product have been removed or controlled pursuant to
Paragraph (fl of this Rule; (2) that the contaminant has the capacity to degrade or
attenuate under the site -specific conditions; (3) that the time and direction of
contaminant travel can be predicted with reasonable certainty; (4) that contaminant
migration will not result in any violation of applicable groundwater standards at any
existing or foreseeable receptor; (5) that contaminants have not and will not migrate
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onto adjacent properties, or that: (A) such properties are served by an existing public
water supply system dependent on surface waters or hydraulically isolated
groundwater, or (B) the owners of such properties have consented in writing to the
request; (6) that, if the contaminant plume is expected to intercept surface waters, the
groundwater discharge will not possess contaminant concentrations that would
result in violations of standards for surface waters contained in 15A NCAC 2B
.0200, (7) that the person making the request will put in place a groundwater
monitoring program sufficient to track the degradation and attenuation of
contaminants and contaminant by-products within and down gradient of the plume
and to detect contaminants and contaminant by-products prior to their reaching any
existing or foreseeable receptor at least one year's time of travel upgradient of the
receptor and no greater than the distance the groundwater at the contaminated site is
predicted to travel in five years, (8) that all necessary access agreements needed to
monitor groundwater quality pursuant to Subparagraph (7) of this Paragraph have
been or can be obtained, (9) that public notice of the request has been provided in
accordance with Rule .0114(b) of this Section; and (10) that the proposed corrective
action plan would be consistent with all other environmental laws.
Appendix I includes a detailed evaluation of the applicability of Alternative 1:
MNA as a remedial alternative for the Site.
Long-term Effectiveness and Permanence
(CAP Content Section 6.D.a.iv.3)
MNA would be an effective long-term technology, assuming source control and
institutional controls (such as an RS designation) for the affected area. Natural
attenuation mechanisms are understood and have been documented
(Appendix I). Once equilibrium conditions of COI concentrations less than 02L
standards are achieved, it is unlikely that the concentrations would increase.
Implementation of MNA will not result in increased residual risk as current
conditions and predicted conditions do not indicate unacceptable risk to human
health or environment. Additionally, Duke Energy connected 191 households to
City of Belmont water supply, fitted 10 households with water treatment
systems, and abandoned three public water supply wells that served 77
households. These water supply improvements were made within a 0.5-miles of
the ash basin compliance boundary in accordance with General Statutes 130A-
309.211(c1) of House Bill 630 (2015). Furthermore, Duke Energy voluntarily
connected two businesses and 23 households to the City of Belmont water
supply that were otherwise not eligible per G.S. Section 130A-309.211(cl).
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Institutional controls (provided by the restricted designation) to limit access to
groundwater may also be implemented.
The adequacy and reliability of this approach would be documented with the
implementation and maintenance of an effectiveness monitoring program to
identify variations from the expected conditions. If factors that are not known at
this time were to affect the attenuation process in the future, alternative
measures could be taken. Monitoring will be in place to evaluate progress and
allow sufficient time to implement changes.
Reduction of Toxicity, Mobility, and Volume
(CAP Content Section 6.D.a.iv.4)
While the COIs are inorganic and cannot be destroyed, they exist in the aquifer
as molecules that interact with the natural components of the matrices to prevent
mobility and toxicity to receptors. MNA can reduce aqueous concentrations
while increasing solid phase concentrations and can therefore, under certain
geochemical conditions, reduce COI plume concentrations, volume, and mass.
There are no treatment or recycling processes involved with MNA as well as no
residuals.
Short-term Effectiveness
(CAP Content Section 6.D.a.iv.5)
The stability and limited areal extent of the COI plume, along with the lack of
unacceptable current risk to human and ecological receptors indicates current
conditions are protective. Therefore, the technology is effective in the short-term.
There are 234 monitoring wells installed associated with the ash basins and coal
pile area. Although some within the immediate areas of the basins will have to
be abandoned as part of closure, monitoring wells along the waste boundary and
at select downgradient areas will remain to monitor natural attenuation in the
short-term.
Technical and Logistical Feasibility
(CAP Content Section 6.D.a.iv.6)
A majority of the 234 monitoring wells have dedicated sampling equipment and
an approved interim monitoring plan is in place. A subset of these monitoring
wells could be immediately used for MNA purposes. Therefore, the technology
could be implemented easily. Other than the abandonment of select wells within
the ash basins from closure, no construction is required to implement this option.
Implementation of an MNA program is a well-defined process, with established
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requirements for sampling, laboratory analysis, reporting, performance review,
and communication of findings to stakeholders.
Time Required to Initiate and Implement Corrective Action
Technologies and Alternatives
(CAP Content Section 6.D.a.iv.7)
The time required for implementation of an MNA program could be as
immediate as approval of the approach since an extensive monitoring well
network already exists. Procedures for collection, analysis, and communication
of results are also established and currently in place.
6.7.2 Remedial Alternative 2 — Groundwater Extraction and
Treatment
Protection of Human Health and the Environment
(CAP Content Section 6.D.a.iv.1)
There is no measurable difference between evaluated Site risks and risks
indicated by background concentrations; therefore, no material increases in risks
to human health related to the source areas have been identified. The
groundwater corrective action is being planned to address regulatory
requirements. The risk assessment identified no current human health or
ecological risk associated with groundwater downgradient of the source areas.
Water supply wells are located upgradient and side -gradient of the source areas
and an alternate water supply has been provided to those who selected this
option. Surface water quality standards downgradient of the COI -affected
plumes are also met.
Based on the absence of receptors, it is anticipated that groundwater extraction
would create conditions that continue to be protective of human health and the
environment because the COI concentrations will diminish with time. By
extracting COI mass within the existing COI plumes, which are not affecting
receptors, active groundwater extraction would further protect human health
and the environment. Therefore, water supply wells would remain unaffected by
COIs related to the source areas.
Compliance with Applicable Regulations
(CAP Content Section 60.a.iv.2)
Groundwater extraction only and treatment would comply with applicable
regulations. Those regulations would include: CAMA, groundwater standards,
and extraction well installation and permitting. Discharge of extracted water
would be in compliance with appropriate discharge requirements, such as pH or
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other COI limitations in the NPDES permit and proper operation and
maintenance of an effectiveness monitoring system.
Activities will also be in compliance with applicable regulations with proper
operation and maintenance of an effectiveness monitoring system.
Long -Term Effectiveness and Permanence
(CAP Content Section 6.D.a.iv.3)
Groundwater extraction may contribute to effective and permanent achievement
of groundwater standards. Although, as indicated by the modeling results for
this alternative, extraction flow rates would be low after basin decanting and
source control measures have been implemented. However, it still can provide a
benefit through hydraulic capture and mass removal, which are significant
factors in achieving remedial objectives. If factors that are not known at this time
were to affect the remediation process in the future, alternative measures could
be taken to modify the remedial approach.
Reduction of Toxicity, Mobility, and Volume
(CAP Content Section 6.D.a.iv.4)
Although the COIs are inorganic and cannot be destroyed, a groundwater
extraction system would help reduce COI concentrations and, therefore, toxicity,
mobility, and volume of COI -affected groundwater. Groundwater extraction
would remove constituent mass from the area of regulatory concern. The
extracted groundwater would be appropriately treated and discharged according
to applicable regulatory requirements. It is anticipated that extracted
groundwater would be discharged through the NPDES permitted Outfall 002.
Analysis of predicted specific COI concentrations and mass in extracted
groundwater during conceptual design of the remediation system may be
completed to further assess compliance with discharge regulatory requirements.
Treatment technologies for extracted groundwater would be further evaluated
after NCDEQ approves the CAP Update and after pilot testing for the proposed
extraction system is complete.
Short-term Effectiveness
(CAP Content Section 6.D.a.iv.5)
The stability and limited extent of the COI plume, along with the absence of
completed exposure pathways, indicates there are no short-term effects on the
environment, workers or the local community. While there are areas with COI
concentrations greater than 02L concentrations, the areas are not presenting
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unacceptable short-term risks. Hydraulic capture of groundwater would occur as
soon as the groundwater extraction system is placed into service.
Technical and Logistical Feasibility
(CAP Content Section 6.D.a.iv.6)
Installation of the proposed a groundwater extraction system would require
significant efforts in planning, designing, and execution of site preparation. The
extensive layout of groundwater remediation system wells, piping, and
treatment system components, as well as site topography and other access
constraints such as power block infrastructure pose significant challenges to
constructability. However, with early awareness of the aforementioned
complexities and effective communications between the design, implementation
and project management teams, successful construction of the system would be
anticipated. If necessary, vertical extraction wells could be installed in phases as
access becomes available as the Station is decommissioned.
Time Required to Initiate and Implement Corrective Action
Technologies and Alternatives
(CAP Content Section 6.D.a.iv.7)
Design and installation of the system could be completed in approximately two
to three years after CAP approval.
Predicted Time Required to Meet Remediation Goals
(CAP Content Section 6.D.a.iv.8)
Time to achieve the remediation goal of reducing the concentration of boron
beyond the compliance boundary to levels less than the 02L standard was
estimated by predictive flow and transport modeling. Modeling results for this
alternative, for both the east and northeast areas, predict that the extraction flow
rate per well would be approximately 13 gpm for combined saprolite/deep wells
and 4 gpm for bedrock wells. These simulated flow rates are reasonably similar
to the flow rates of approximately 5 gpm obtained during dewatering for
construction of the holding basin within the footprint of the coal pile. The
simulated flow rates are greater than the observed flow rates because the
simulated extraction wells extend deeper into zones with greater yield. The flow
and transport simulations predict that boron concentrations in groundwater
would meet the boron and sulfate 02L standards at the compliance boundary in
excess of 500 years after system startup and operation. Thus, groundwater
remediation under this alternative would be slow compared with that of
Alternative 3.
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Cost
(CAP Content Section 6.D.a.iv.9)
The estimated costs for this alternative have not been fully developed. However,
due to the increase in materials and equipment required, the capital cost and
annual cost would be more than Alternative 1 and less than Alternative 3.
Because Alternative 3 requires the additional material and equipment for clean
water infiltration, the capital and operating cost would be greater than
Alternative 2. Despite this, the significantly longer lifetime of the Alternative 2
system operating indicates that the life cycle costs would likely be the largest of
the three alternatives.
Community Acceptance
(CAP Content Section 6.D.a.iv.10)
It is expected that there will be positive and negative sentiment about
implementation of a groundwater extraction only and treatment system. No
landowner is anticipated to be affected. The affected property is owned by Duke
Energy. It is anticipated that the extracted groundwater would be discharged
through a NPDES permitted outfall that flows toward the Catawba River and
that the discharge would be treated as necessary to meet all permit limits. An
expanded groundwater extraction system which addresses potential COI plume
expansion across the entire east and northeast perimeter of the ash basins and
coal pile area might improve public perception. Until the final Site remedy is
developed and comments are received and reviewed, assessment of community
acceptance will not be fully known.
It is anticipated that groundwater extraction and treatment would generally
receive more positive community acceptance than MNA under Alternative 1
since it involves more active measures to attempt physical extraction of COI
mass from groundwater. This alternative would likely be perceived as more
robust than MNA in addressing groundwater impacts even if modeling predicts
essentially the same effects between MNA and groundwater extraction.
Adaptive Site Management and Remediation Considerations
Groundwater extraction using conventional well technology is an adaptable
process. It can be easily modified to address changes to COI plume configuration
or COI concentrations. Individual well pumping rates can be adjusted or
eliminated or additional wells can be installed to address COI plume changes.
Also, while it is not expected, treatment of the system discharge can be modified
to address changes in COI concentrations or permit limits.
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Sustainability
The footprint for Alternative 2 was quantified based on energy use and
associated emissions, during the construction phase (e.g., material quantities and
transportation), active remediation activities (e.g., groundwater pumping and
treatment) and groundwater monitoring activities (e.g., transportation). The
results of the footprint calculations for Alternative 2 are summarized in Table 6-
14. A summary of sustainability calculations for Alternative 2 can be found in
Appendix L.
The footprint of Alternative 2 is the most emission -intensive remedial alternative
being considered. This alternative would use extensive materials for construction
to install 87 extraction wells and energy during operations; therefore, generating
a similar environmental footprint as Alternative 3. Opportunities for system
optimization and energy savings could be pursued throughout the remediation
timeframe, as conditions change and component technologies possibly evolve to
lessen the environmental footprint. However, the significantly -longer operational
window of Alternative 2, when compared to Alternative 3, would be the
predominant factor contributing to an environmental footprint.
6.7.3 Remedial Alternative 3 — Groundwater Extraction
Combined with Targeted Clean Water Infiltration and
Treatment
Protection of Human Health and the Environment
(CAP Content Section 6.D.a.iv.1)
There is no measurable difference between evaluated Site risks and risks
indicated by background concentrations; therefore, no material increases in risks
to human health related to the source areas have been identified. The
groundwater corrective action is being planned to address regulatory
requirements. The risk assessment identified no current human health or
ecological risk associated with groundwater downgradient of the source areas.
Water supply wells are located upgradient and sidegradient of the source areas
and an alternate water supply has been provided to those who selected this
option. Surface water quality standards downgradient of the COI -affected
plumes are also met.
Based on the absence of receptors, it is anticipated that groundwater extraction
would create conditions that continue to be protective of human health and the
environment because the COI concentrations will diminish with time.
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By extracting COI mass within the existing COI plumes, which are not affecting
receptors, active groundwater extraction would further protect human health
and the environment. While the permeability of the formations will limit flow,
the additional volume of infiltration water created will increase the effectiveness
of the system in enhancing COI mass movement for extraction. Therefore, water
supply wells would remain unaffected by COIs related to the source areas.
Compliance with Applicable Regulations
(CAP Content Section 6.D.a.iv.2)
Clean water infiltration, extraction and treatment would comply with applicable
regulations. Those regulations would include: CAMA, groundwater standards,
infiltration and extraction well installation and permitting. Discharge of
extracted water would be in compliance with appropriate discharge
requirements, such as pH or other COI limitations in the NPDES permit and
proper operation and maintenance of an effectiveness monitoring system. If the
water supply for clean water infiltration wells is from a surface water source,
additional permitting may be required.
Activities will also be in compliance with applicable regulations with proper
operation and maintenance of an effectiveness monitoring system.
Long-term Effectiveness and Permanence
(CAP Content Section 6.D.a.iv.3)
Clean water infiltration and extraction will contribute to effective and permanent
achievement of groundwater standards by facilitating movement of impacted
groundwater such that the COI plume is hydraulically controlled and COI mass
is more effectively removed as predicted by modeling results.
The adequacy and reliability of this approach would be documented with the
implementation of an effectiveness monitoring program that would identify
variations from the expected outcome. If factors that are not known at this time
were to affect the remediation process in the future, alternative measures could
be taken to modify the remedial approach.
Reduction of Toxicity, Mobility, and Volume
(CAP Content Section 6.D.a.iv.4)
Although the COIs are inorganic and cannot be destroyed, a groundwater
extraction combined with clean water infiltration would help reduce COI
concentrations and, therefore, toxicity, mobility, and volume of COI -affected
groundwater. Groundwater extraction combined with clean water infiltration
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would remove constituent mass from the area of regulatory concern. The
extracted groundwater would be appropriately treated and discharged according
to applicable regulatory requirements. It is anticipated that extracted
groundwater would be discharged through the NPDES permitted Outfall 002.
Analysis of predicted specific COI concentrations and mass in extracted
groundwater during conceptual design of the remediation system may be
completed to further assess compliance with discharge regulatory requirements.
Treatment technologies for clean water infiltration and extracted groundwater
will be evaluated after NCDEQ approves the CAP Update and after pilot testing
for the proposed extraction system is complete.
Short-term Effectiveness
(CAP Content Section 6.D.a.iv.5)
The stability and limited extent of the COI plume, along with the absence of
completed exposure pathways, indicates there are no short-term effects on the
environment, workers or the local community. While there are areas with COI
concentrations greater than 02L concentrations, the areas are not presenting
unacceptable short-term risks. Hydraulic control and capture of groundwater
would occur as soon as the groundwater extraction and clean water infiltration
system is placed into service.
Technical and Logistical Feasibility
(CAP Content Section 6.D.a.iv.6)
Installation of the proposed clean water infiltration and extraction system would
require significant efforts in planning, designing, and execution of site
preparation. The extensive layout of groundwater remediation system wells,
piping, and treatment system components, as well as site topography and other
access constraints such as power block infrastructure access constraints pose
significant challenges to constructability. However, with early awareness of the
aforementioned complexities and effective communications between the design,
implementation and project management teams, successful construction of the
system would be anticipated.
Time Required to Initiate and Implement Corrective Action
Technologies and Alternatives
(CAP Content Section 6.D.a.iv.7)
Design and installation of the system could be completed in approximately two
to three years after CAP approval.
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Time Required to Meet Remediation Goals
(CAP Content Section 6.D.a.iv.8)
Time to achieve the remediation goal of reducing the concentration of boron and
sulfate beyond the compliance boundary to levels less than the 02L standards
was estimated by predictive flow and transport modeling. Modeling results for
this alternative, for both the east and northeast areas, predict that the extraction
flow rate per well would be approximately 13 gpm for combined saprolite/deep
wells and 4 gpm for bedrock wells. The model simulates infiltration rates of
combined shallow and deep zone wells to be 5 gpm. These simulated flow rates
are reasonably similar to the flow rates of approximately 5 gpm obtained during
dewatering for construction of the holding basin within the footprint of the coal
pile. The simulated flow rates are greater than the observed flow rates because
the simulated extraction wells extend deeper into zones with greater yield. Due
to increased flow rates and soil flushing from the infiltration wells, the flow and
transport model predicts that boron and sulfate concentrations in groundwater
would meet 02L standards at the compliance boundary approximately 10 years
after system startup, a considerably shorter time frame than Alternatives 1 or 2.
Cost
(CAP Content Section 6.D.a.iv.9)
The increase in materials and equipment required, the capital cost and annual
cost would be significantly more than Alternative 1. Relative to Alternative 2,
additional material and equipment would be required for clean water
infiltration, therefore the capital and also the operating cost would be greater
than Alternative 2. Despite this, the significantly less lifetime of the Alternative 3
system operating indicates that the life cycle costs would be the least of the three
alternatives.
Community Acceptance
(CAP Content Section 6.D.a.iv.10)
It is expected that there will be positive and negative sentiment about
implementation of a clean water infiltration and extraction system. No
landowner is anticipated to be affected. The affected property is owned by Duke
Energy. It is anticipated that the extracted groundwater would be discharged
through a NPDES permitted outfall that flows toward the Catawba River and
that the discharge would be treated as necessary to meet all permit limits. An
expanded groundwater extraction system which addresses potential COI plume
expansion across the entire east and northeast perimeter of the ash basins and
coal pile area may improve public perception. Until the final Site remedy is
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developed and comments are received and reviewed, assessment of community
acceptance will not be fully known.
It is anticipated that groundwater extraction combined with clean water
infiltration and treatment under would generally receive more positive
community acceptance than MNA under Alternative 1 since it involves more
active measures to attempt physical extraction of COI mass from groundwater
and would likely be perceived as more robust than MNA.
Alternative 3 could receive more positive community acceptance than
Alternative 2 because it involves additional measures to reduce COI
concentrations and enhances extraction of COI mass from groundwater which
would result in compliance with 02L sooner than Alternative 2.
Adaptive Site Management and Remediation Considerations
Clean water infiltration and extraction using conventional well technology is an
adaptable process. It can be easily modified to address changes to COI plume
configuration or COI concentrations. Individual well infiltration and pumping
rates can be adjusted or eliminated or additional wells can be installed to address
COI plume changes. Also, while it is not expected, treatment of the system
discharge can be modified to address changes in COI concentrations or permit
limits.
Sustainability
The footprint of Alternative 3 was quantified based on energy use and associated
emissions, during the construction phase (e.g., material quantities and
transportation), active remediation activities (e.g., groundwater pumping and
treatment) and groundwater monitoring activities (e.g., transportation). The
results of the footprint calculations for Alternative 3 are summarized in Table 6-
14. A summary of sustainability calculations for Alternative 3 can be found in
Appendix L.
The footprint of Alternative 3 is the second -most energy -intensive of the
remedial alternatives being considered. Alternative 1 (MNA) requires
significantly less materials and energy than Alternative 3 and is therefore
characterized by a dramatically smaller footprint. Alternative 3 presents lower,
but generally comparable, footprint metrics when measured against Alternative
2. Compared to Alternative 2, Alternative 3 utilizes the same number of
extraction wells (87) and either 76 additional vertical clean -water infiltration
wells or 48 vertical clean -water infiltration wells combined with 22 horizontal
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clean -water infiltration wells, generating a higher material -related footprint for
the construction phase. However, the reduced timeframe of remediation system
operation for Alternative 3 (estimated for this calculation as 8 years) when
compared to Alternative 2 (estimated for this calculation as 500 years) produces
air emissions approaching the levels of Alternative 2.Opportunities for system
optimization and energy savings could be pursued throughout the remediation
timeframe, as conditions change and component technologies possibly evolve.
6.8 Proposed Remedial Alternative Selected for Source Area
(CAP Content Section 6.E)
Based on the alternatives detailed analysis using criteria presented in Section 6.7, the
favored remedy for groundwater remediation is Alternative 3, Groundwater Extraction
Combined with Targeted Clean Water Infiltration and Treatment.
To comply with 15A NCAC 02L .0106(h), corrective action plans must contain the
following items, which are included in the following subsection:
• A description of the proposed targeted corrective action and reasons for its
selection.
• Specific plans, including engineering details where applicable, for restoring
groundwater quality.
• A schedule for the implementation and operation of the proposed plan.
• A monitoring plan for evaluating the effectiveness of the proposed corrective
action and the movement of the COI plume.
Each of these corrective action plan components are included in the following
subsections.
6.8.1 Description of Proposed Remedial Alternative and
Rationale for Selection
(CAP Content Section 6.E.a)
The selected remedy for groundwater remediation, Alternative 3, is intended to
provide the remedial technology that has demonstrated to provide the most
effective means for restoration of groundwater quality at or beyond the
compliance boundary by returning COIs to acceptable concentrations (02L/IMAC
or background, whichever is greater), or as closely thereto as is economically and
technologically feasible, consistent with 15A NCAC 02L. 0106(a), and to address
15A NCAC 02L .0106(j) (CAP Content Section 6.E.a.i).
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The groundwater remediation system includes 87 vertical extraction wells and
either approximately 76 vertical clean water infiltration wells or approximately
48 vertical clean water infiltration combined with approximately 22 horizontal
clean water infiltration wells.
The groundwater remediation system also includes all associated piping and
controls, and, as necessary, treatment facilities for both clean water infiltration
and extraction water. Figure 6-28a provides a conceptual layout of the proposed
groundwater extraction combined with clean water infiltration remediation
system showing vertical infiltration wells. Figure 6-28b provides a conceptual
layout of the proposed groundwater extraction combined with clean water
infiltration remediation system showing horizontal infiltration wells in place of
some vertical infiltration wells. Model results predict the 02L standard of 700
µg/L for boron and 250 mg/L for sulfate will be achieved at the Allen ash basin
compliance boundary approximately 10 years after system startup and operation
(Figures 6-28h through 6-28k).
All three groundwater remedial alternatives evaluated contribute to continued
protection of human health and the environment, however, a the approach of
groundwater extraction combined with clean water infiltration and treatment
appears to be the most practical solution given the predicted time frames for 02L
compliance. Rationale for selections follows, and is based off multiple lines of
evidence, including empirical data collected at Allen, geochemical modeling, and
groundwater flow and transport modeling.
Alternative 1 relies on natural attenuation processes and, while there is evidence
to suggest that natural attenuation is occurring, one or more levels of the MNA
tiered analysis did not meet evaluation criteria for selecting the groundwater
remedial alternative, including:
Predicted timeframe to achieve applicable criteria at the compliance
boundary is approximately 460 years after basin closure by excavation
and 270 years after basin closure by closure -in -place, which does not meet
the criteria of achieving the standards at a timeframe similar to more
active remedies.
• Currently boron and sulfate are greater than 02L standards in bedrock at
or beyond the compliance boundary. Boron concentrations greater than
the 02L standard are predicted based on the groundwater model to occur
in bedrock in the future, at or beyond the compliance boundary east of ash
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basins. Sulfate concentrations greater than the 02L standard are predicted
to occur in bedrock in the future, at or beyond the compliance boundary
north and east of the coal pile area.
More detail on the results from the MNA tiered analysis and why MNA alone is
not an appropriate corrective action solution at this time can be found in
Appendix I. MNA may be an appropriate polishing remedy in the future.
Alternative 2 and Alternative 3, remediation systems represent an adaptable
approach. The system could be modified relatively easily if conditions change.
The addition of wells or adjusting well pumping schemes can be readily
accomplished. Although groundwater extraction from Alternative 2 and
Alternative 3 involves a verified remedial technology for groundwater capture
and provides a long-term and permanent approach, Alternative 3 is a more
robust system.
The flow rate predicted for Alternative 2 is insufficient to restore ash basin -
affected groundwater at or beyond the compliance boundary within a reasonable
(i.e. approximately 30 years) timeframe, and therefore does not meet the Duke
Energy's corrective action goals. The additional volume of groundwater created
by recharge from clean water infiltration has the ability to increase the flushing
capacity of the system with clean water and reducing COI concentrations,
significantly increasing the effectiveness of the remediation system. Alternative
3, groundwater extraction and clean water infiltration, is projected to satisfy
remedial action objectives in a shorter timeframe (approximately 10 years)
relative to Alternative 2 (greater than 500 years). Alternative 3 includes clean
water infiltration wells, with groundwater infiltration rates of approximately 5
gpm per well for vertical wells and 7 gpm per well for horizontal wells, for a
total system infiltration rate ranging from approximately 335 to 345 gpm. The
extraction rate per well for Alternative 3 is approximately 13 gpm for combined
saprolite/deep wells and 4 gpm for bedrock wells, for a total system extraction
rate of approximately 950 gpm. Comparatively, Alternative 2 relies on
technology where extraction rates are limited to the groundwater formation's
natural flow rates, without the additional volume of water from clean water
infiltration wells to increase flushing capacity. The extraction rate per well for
Alternative 2 is approximately 10 gpm for combined saprolite/deep wells and 5
gpm for bedrock wells, for a total system extraction rate of approximately 650
gpm. By supplementing the natural groundwater system with clean water
infiltration, extraction rates increases and therefore, increase the effectiveness of
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the remediation system and reduce the timeframe to meet compliance by more
than 250 years.
Additionally, Alternative 2 does not restore ash basin -affected groundwater at or
beyond the compliance boundary by returning COI concentrations to the
groundwater quality standards, or applicable background concentrations
(whichever are greater), or as closely thereto as is economically and
technologically feasible consistent with 15A NCAC 02L. 0106(a). An extraction
only and treatment system would have to maintain operation for a longer period
of time, relative to Alternative 3, which adds a substantial operation and
maintenance (O&M) cost and lessens the economically feasibility.
Although Alternatives 2 and 3 generate a larger environmental footprint in the
sustainability analysis than MNA, the footprint of a groundwater remediation
system is still small in comparison to other elements of the ash basin closure
process. During design phases of the groundwater remediation project,
opportunities for energy efficiency and reduction of the project environmental
footprint can be evaluated. Potential duplication of intensive construction efforts
should be considered.
Relative to Alternative 2, Alternative 3 would accelerate removal of COI mass
from the groundwater system, reducing the groundwater plume footprint to
within the compliance boundary, and achieve compliance within a shorter
timeframe as is economically and technologically feasible. Therefore, Alternative
3 is the favored remedial alternative for implementation at Allen. The long-term
effectiveness would be documented through an effectiveness monitoring
program detailed in Section 6.8.5.
Seep Corrective Action
As stated in the SOC, ash basin decanting is expected to substantially reduce or
eliminate the seeps. Groundwater corrective action discussed in this CAP Update
and ash basin closure would also reduce or eliminate the seeps. After completion
of decanting, remaining seeps (constructed and non -constructed), if not
dispositioned in accordance with the SOC, would be characterized for
determination of disposition. After seep characterization, an amendment to the
CAP and/or Closure Plan, may be required to address remaining seeps. Duke
Energy is prepared to address those seeps through corrective action sufficient to
protect public health, natural resources, and the environment. Seeps that have
the potential to not be fully dispositioned post -decanting are listed on Table 6-8
and discussed in terms of with corrective action strategies, sequentially below. In
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summary, decanting, ash basin closure, and groundwater extraction are the
anticipated corrective action strategies to address each of the seeps and no
additional corrective action is planned at this time.
Non constructed seep S-02, located southeast of the AAB, is covered by the SOC.
As of November 2019, decanting has substantially reduced flow at this location
such that overland flow infiltrates prior to reaching the Catawba River (Lake
Wylie). The last reported flow at this location was during SOC-related sampling
during August 2019. During that time flow was minimal (approximately 0.07
cubic feet per second). No flow or standing water was observed during
inspections in October and November 2019. This indicates that decanting has
been an effective corrective measure and that it may be appropriate for 5-02 to be
dispositioned. Duke Energy will continue to monitor S-02 and if flow resumes
unexpectedly sometime in the future, additional corrective measures, such as
phytoremediation technology, would be considered.
The NPDES Permit includes constructed seep S-3 (identified as Toe Drain Outfall
103) and constructed seep S-4 (identified as Toe Drain Outfall 104), which are
east of the AAB. As permitted outfalls, corrective action is not necessary for these
former seeps. However, in the future, additional corrective action to manage
potential flow from these seeps may be considered proactively. As of November
2019, flow has been reduced, but not yet ceased, in response to decanting from
the AAB. Continued decanting and basin closure may cause these outfalls to
cease flowing and therefore be dispositioned, but retained as permitted outfalls.
Groundwater extraction in the vicinity of these outfalls, planned as part of this
CAP Update may also cause flow to cease. If flow continues, additional
corrective action could include construction of a sump along the flow path of the
seeps to capture flow that could be managed with extracted groundwater. In the
interim, Duke Energy will to continue to monitor the outfalls in accordance with
the NPDES permit.
Non -constructed seeps S-5, S-6, and S-7 emerge intermittently below the
ordinary high water mark. Flow, if present, is often concealed beneath the Lake
Wylie water surface. Flow was observed as recently as November 2019 from S-5
and S-6, but was concealed, if present, at S-7. Source control measures (i.e.
decanting and ash basin closure) are anticipated to reduce potential for flow to
continue. Corrective action planned as part of this CAP Update includes
installation of extraction wells in the vicinity of these seeps. Groundwater
modeling indicates extraction wells would reduce or eliminate these seeps. Duke
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Energy plans to continue to monitor these seeps and if continued decanting,
source control, and groundwater corrective action measures are not effective in
ceasing flow, then additional corrective measures would be considered, such as
capturing flow for management with extracted groundwater or
phytoremediation technologies.
The NPDES Permit also includes constructed seep S-8 (identified as Toe Drain
Outfall 108) and constructed seep S-8B (identified as Toe Drain Outfall 108B),
which are east of the AAB. As permitted outfalls, corrective action is not
necessary for these former seeps. Pre -decanting, flow in these outfalls was
minimal at less than 0.001 cubic feet per second with the last reported flow
observed during SOC-related sampling during February 2019. No flow or
standing water was observed during inspections in October and November 2019.
This indicates that decanting has been an effective corrective measure and that it
would be appropriate for the seeps S-8 and S-8B to be dispositioned, but
maintained as NPDES outfalls. Additionally, basin closure and groundwater
extraction wells planned as part of this CAP Update may also reduce the
potential for flow to resume at these outfalls. Duke Energy will continue to
monitor these locations in accordance with the NPDES Permit and if flow
resumes unexpectedly sometime in the future, additional corrective measures
would be considered.
Flow at seep S-10, located north of primary pond 1 of the AAB, is currently
captured within a French drain system and managed for treatment within the
lined retention basin. Decanting has reduced flow and flow is anticipated to be
further reduced or ceased as decanting continues. S-10 is encompassed within
the area included to be a part of ash basin closure. Basin closure would eliminate
the seep. Duke Energy plans to continue to monitor 5-10 in accordance with the
SOC. If flow increases substantially and unexpectedly in the future prior to
implementation of additional closure activities, additional corrective measures
would be considered.
Final corrective action plans for constructed and non -constructed seeps that are
not dispositioned post -decanting will be proposed in an amendment to this CAP
Update and submitted based on the schedule outlined in the SOC.
6.8.2 Design Details
(CAP Content Section 6.E.b)
Design of the proposed clean water infiltration and extraction system would
require a pilot test (i.e., installation of a portion of the system) to facilitate
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refinement of the final system design. A pilot test work plan will be prepared to
facilitate implementation of the system. As part of this process, the groundwater
flow and transport model will likely be refined to determine the final number
and locations of system wells. As the pilot testing and design process evolves,
refinements to the systems and timeframe, including a potential reduction in the
time needed to achieve compliance may occur compared to the model
predictions presented in this CAP.
The intent of the design would be to maximize pore volume exchange (i.e.
groundwater flushing) and establish groundwater flow control and capture in
areas downgradient of the ash basins and coal pile area. Basic installation
components of the recommended alternative include:
• 87 extraction wells and appurtenances
• 76 clean water vertical infiltration wells and appurtenances or 48 clean
water vertical infiltration wells combined with 22 clean water horizontal
infiltration wells and appurtenances
• Well vault and wellhead piping, fittings, and instrumentation
• A system to control water level within each groundwater extraction well
• Groundwater extraction system discharge piping
• Groundwater physical chemical treatment
• Clean water infiltration pre-treatment system
• Clean infiltration water distribution system
• Electric power supply
• Groundwater remediation telemetry system
Conceptual process flow diagrams for infiltration, extraction, and treatment
systems are provided on Figures 6-29 through 6-31. The detailed design elements
presented below may be adjusted based on a final technical review.
6.8.2.1 Process Flow Diagrams for All Major Components
of Proposed Remedy
(CAP Content Section 6.E.b.i)
Conceptual process flow diagrams for extraction and treatment systems are
provided on Figures 6-29 through 6-31.
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Below is a 10-step process for remedy design considerations and
implementation of major components, including design assumptions,
calculations, and specifications where applicable at the conceptual design
stage.
Site Preparation (Step 1 — Create Access)
Installation of the proposed clean water infiltration and extraction system
would require significant efforts in planning, designing, and execution of
site preparation. The extensive layout of groundwater remediation system
wells, piping, and treatment system components, as well as site topography
and access constraints pose significant challenges to constructability.
Furthermore, installation of groundwater extraction wells between the coal
pile areas and the Catawba River would be challenging due to the presence
of buried utilities and above ground infrastructure (e.g., railroad tracks) the
area. However, with early awareness of the aforementioned complexities
and effective communications between the design, implementation and
project management teams, successful construction of the system would be
anticipated.
Safe access roads for mobile construction equipment (e.g., drill rigs), as well
as long-term operation and maintenance needs, will likely require extensive
clearing, grubbing, grading, and access improvement.
A certain level of flexibility regarding well placement is expected to be
required due to site conditions encountered during construction. Prior to
construction and following the pump tests, an assessment of the precise
locations of wells would be made in collaboration with the modeler. If the
model predictions are not affected, relocation from the predetermined
location due to terrain or other site -specific constraints would expedite
construction.
Land disturbance will require E&SC to be implemented and likely reviewed
and approved by a regulatory agency. Adaptable E&SC should be planned
to limit project delays by avoiding formal modifications of plans.
Decommissioning would include removal of the coal pile. It is anticipated
that the coal piles will remain in place until the Station is retired, currently
planned for 2024 for Units 1, 2, and 3 and 2028 for Units 4 and 5. Removal of
the coal piles would allow access for installation of vertical wells within the
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footprint of the coal piles. However, horizontal wells could be installed
within the footprint of the main coal pile prior to decommissioning.
Pilot Tests (Step 2 — To Finalize Design)
A pilot test would involve installation of a portion of the planned system to
evaluate how the system performs and to make initial progress towards
remediation at the same time. The results of the pilot test would be used to
refine and scale up the final design thereby maximizing the likelihood of
successful operation in the field. Clean water infiltration tests would be
conducted to determine the infiltration rates of clean water infiltration wells
screened within or across saprolite, transition zone, and bedrock flow
zones.
Extraction pilot test wells will be screened within or across a flow zone
similar to model simulations to the extent feasible.
Pilot test results will be used to:
• Determine site -specific well yields for each flow zone
• Validate predictive flow and transport modeling
• Refine predictive flow and transport modeling, as needed
• Confirm groundwater extraction well capture zones in the saprolite
and transition zone flow zones beyond available data
• If warranted, make adjustments to the groundwater extraction
system design
• If warranted, make design adjustments to conveyance for infiltration
water
• If warranted, make design adjustments to the groundwater treatment
system
Clean water infiltration test wells will be screened within or across flow
zones, similar to model simulations to the extent feasible. Groundwater
infiltration test results will be used to:
• Determine site -specific well infiltration rates
• Validate predictive flow and transport modeling
• Verify predictive flow and transport modeling
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• If warranted, make adjustments to the clean water infiltration system
design
• If warranted, make design adjustments to conveyances for
infiltration groundwater
• If warranted, make design adjustments to the clean water infiltration
treatment system
The extraction and clean water infiltration wells used for testing would be
included in the final groundwater remediation system design.
Clean Water Infiltration and Extraction Well Design (Step 3
— Install Wells)
(CAP Content Section 6.E.b.i)
The preliminary design for the groundwater remediation system includes
installation of approximately 87 extraction wells and either approximately
76 vertical clean water infiltration wells or approximately 48 vertical clean
water infiltration wells combined with approximately 22 horizontal clean
water infiltration wells and (Figures 6-28a and 6-28b). The clean water
infiltration and extraction wells would be installed to the north, northeast
and east of the ash basins and coal pile area. The locations are based on
predicted COI plume configuration, with the intent of capturing
groundwater to create groundwater flow control, COI mass removal, and
reduced migration of potentially mobile COIs. The predicted effects of the
wells are defined in detail in flow and transport modeling results
(Appendix G).
Extraction wells would be completed in the shallow, deep and bedrock flow
zones to depths ranging from approximately 65 feet bgs to 365 feet bgs.
Vertical clean water infiltration wells would be completed in the shallow
and deep zones to depths ranging from 75 to 138 feet bgs. Horizontal wells,
if included, would be completed in the shallow zone at an approximate
depth of 20 feet bgs and within or near the deep zone at an approximate
depth of 80 feet bgs. Groundwater infiltration and extraction wells would
be installed by a North Carolina licensed well driller in accordance with
NCAC 15A, Subchapter 2C — Well Construction Standards, Rule 108
Standards of Construction: Wells Other Than Water Supply (15A NCAC
02C .0108). Modeled clean water infiltration well details are provided on
Table 6-15. Modeled extraction well details are provided on Table 6-16.
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The clean water infiltration and extraction wells might be drilled using
hollow stem auger, air percussion/hammer, sonic methods, or a
combination thereof. The drilling method would depend on Site conditions
and well type (e.g., vertical or horizontal). All materials and installations
would be in accordance with 15A NCAC 02C. Completed wells would be 6
inches in diameter to facilitate the installation of pumps and
instrumentation (e.g., level control) in groundwater extraction wells. For
vertical wells, the top of the sand pack would extend to approximately 2
feet above the top of well screens overlain by a bentonite well seal at least 2
feet thick installed on top of the sand pack and a neat cement grout with 5
percent bentonite would be placed on top of the bentonite well seal and
would fill the remaining well annulus to within 3 feet of the ground surface.
Typical well construction schematics for infiltration and extraction wells are
included as Figures 6-28c through 6-28f.
If horizontal wells are used for infiltration, the wells would be installed by
certified North Carolina well driller as double- or single -ended horizontal
infiltration wells in the footprint of the coal pile as shown conceptually on
Figures 6-28d and 6-28e. A typical horizontal environmental well is
installed at an angle approximately minus 12 degrees from horizontal
(Ellington-DTD, 2004). The equipment would be set up at a distance such
that the boring at an angle that is predetermined and would reach the point
of beginning of the screen at the target depth of the screen. A directional
pilot bore smaller than the diameter of the well would be installed using a
navigational system, such as a wireline navigation system. Drilling fluid
would be used for cutting the borehole and stabilizing the borehole wall
until the well materials are installed. Surface seals would be installed in the
annulus at both ends, and the well will be developed. One end of the well
would be capped with a water -tight seal. The well head will be completed
in a manner similar to the vertical injection wells. (Ellington-DTD, 2019).
WeH Head Configuration (Step 4 — Construct Well Heads)
(CAP Content Section 6.E.b.i)
The proposed extraction and clean water infiltration well vaults would be
precast concrete with aluminum access doors that include a drainage
channel. The concrete enclosures would be finished below grade and the
piping and fittings in the enclosures would be Type 304 stainless steel to
reduce risk of damage during O&M. Due to the location of plant
infrastructure and utilities, any of the extraction wells and some of the
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infiltration wells will be located in internal plant roads. Plant personnel
have indicated that the vehicular traffic on these roads are passenger cars
and light trucks travelling at speeds of 30 miles per hour or less. Well heads
that cannot be protected by bollards must have enclosures meeting the
appropriate H2O loading.
Any above ground piping would be insulated and heat traced. The piping
would transition from the Type 304 stainless steel to high density
polyethylene (HDPE) at a flange near the opening where the HDPE pipe
leaves the enclosure. The buried sections of pipe would be fusion -welded
HDPE (Figure 6-25g).
The enclosures would have a 2-inch drain with a compression cap for
controlled release of rainwater or condensate. A water level sensor would
be mounted on the wall of the enclosure approximately 6-inches above the
floor. Should water accumulate to that level, the extraction pump or
infiltration water would be stopped and an alarm would be sent to the
operator, who could ascertain the cause of the high water level.
Clean Water Infiltration Wells (Step 4A)
(CAP Content Section 6.E.b.i)
An HDPE distribution header would convey clean water from the
infiltration water treatment system to each clean water infiltration well
(Figure 6-28c). A seal at the top of the well through which the clean water
infiltration pipe and wiring would enter the well, would be designed to be
leak free.
The hydraulic head at each clean water infiltration well would be controlled
by a pressure control valve. Ten -feet of water (4.34 pounds per square in
gauge) is the infiltration pressure used in the predictive groundwater flow
and transport model, but the pressure could be increased or decreased to
achieve performance objectives. Operation of the clean water infiltration
wells would comply with 15A NCAC 02C.0225. Infiltration pressures and
rates would be determined based on the hydraulic conductivity of the strata
receiving the clean water.
The amount of water flowing into the clean water infiltration well would be
measured by a flow rate and flow totalizing meter. At startup, a ball valve
at the top of the well would be opened to allow water to displace the air in
the well and system piping. Also, pressure transducers installed at the top
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of each clean water infiltration well would monitor well head pressures
(Figure 6-28c).
Other appurtenances in the piping system would include a pressure gauge,
ball valves to isolate piping for maintenance, and a solenoid valve that
would close to stop the flow of infiltration water in the event high water
level in the vault.
Operational parameters, such as infiltration flow rate, totalized infiltration
flow, and well head pressure, as well as critical malfunctions such as
accumulation of water in the well vault would be transmitted to the
groundwater remediation system owner via telemetry system.
Extraction Wells (Step 4B)
(CAP Content Section 6.E.b.i)
A pump would be installed in each groundwater extraction well. If the
water level in the well is above the top water level switch, the pump would
run to pump the water to lower water level switch, which would cause the
pump shut off. The flow of extracted groundwater from the submersible
pump would be measured using a flow rate and flow totalizer meter before
being conveyed to groundwater discharge piping for treatment and
management (Figure 6-30). Other appurtenances in the piping system
would include:
• a check valve to prevent back flow into the well,
• a sampling port, a pressure gauge to indicate the pressure generated
by the pump,
• ball valves to isolate piping for maintenance,
• and a flow control valve such as a stainless steel globe or gate valve
(Figure 6-30)
Operational parameters, such as flow and water level, and critical
malfunctions, such as accumulation of water in the well vault, would be
transmitted via telemetry system to inform the system operator of the status
in the well and enclosure.
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Clean Water Infiltration Water Treatment (Step 5 — Build
Infiltration Treatment)
(CAP Content Section 6.E.b.i)
Water used for clean water infiltration will be obtained from the Catawba
River. Water supplied to the clean water infiltration wells is non -potable
water that is suitable for infiltration as part of the remediation process and
not for consumption.
The existing raw water intake would be used. The intake is located along
the west bank of the Catawba River on the east side of Duke Energy
property near the power block. Raw water would travel through the
existing plant distribution system to an infiltration water treatment plant.
The treatment system would condition the water prior to storage and
distribution to the clean water infiltration wells.
The Catawba River is a dynamic source of water and would provide water
of varying quality. Treatment would address suspended particulates and
total dissolved solids (TDS) and biological growth (e.g., algae and bacteria)
that would be present in raw river water. The 02L standard for TDS is 500
mg/L.
A modular flocculation, settling, and filtration treatment process may be
used to reduce TDS to concentrations less than 500 mg/L and to disinfect the
river water. A polymer and a disinfectant (e.g., sodium hypochlorite) would
be added to raw river water in a rapid mix tank. The polymer would
flocculate with TDS and the disinfectant would kill waterborne bacteria and
algae. Treated water and flocculant would flow from the rapid mix tank to
a modular sedimentation tank where the flocculant and particulates would
settle. Sedimentation tank effluent would undergo filtration to remove
suspended flocculant and particulates. The filtered water would be
pumped to a holding tank where clean infiltration water would be stored
prior to distribution to the clean water infiltration wells. Water leaving the
holding tank might undergo dechlorination (e.g., sulfur dioxide or sodium
metabisulfite) as it enters the clean water infiltration water distribution
system (Figure 6-31).
Parallel treatment processes would facilitate infiltration system operation
and maintenance and should achieve optimal runtime and
performance. Individual system components (e.g., vertical turbine pumps,
equalization tanks, modular treatment system or transfer pumps) could be
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operated singularly or in parallel and achieve 100 percent groundwater
infiltration capacity. Liquid waste materials generated as a result of
maintenance (e.g., filter backwash or wash water) would be directed to the
plant drain system for treatment and management. The equalization tanks,
treatment system, transfer pumps, and holding tank would be housed in an
enclosed structure to prevent exposure to prevailing weather conditions.
Groundwater Extraction Water Treatment (Step 6 —
Address Groundwater Treatment)
(CAP Content Section 6.E.b.i)
Extracted groundwater would be treated by a proposed groundwater
treatment plant or the LRB located at the Site. The proposed groundwater
treatment process is expected to consist of pH adjustment. The pH
adjustment system would consist of chemical addition for the purpose of
changing the pH to meet permit limits established at NPDES Outfall 002. It
should be considered the existing pH adjustment system, provided by
Evoqua used to dewater the AAB, or the LRB could assist or eliminate the
need for the proposed groundwater treatment plant. If so, a modified
treatment method could be selected based on the quantity and quality of the
extracted groundwater.
Clean Water Infiltration Well Distribution System (Step 7 —
Conceptual Clean Water Infiltration System Considerations)
The purpose of the clean water infiltration distribution system is to convey
water from the Catawba River to the infiltration water treatment system
and to convey water from the infiltration water treatment system to the
clean water infiltration wells. The distribution lines would be constructed
with blowoffs so that the system may be flushed to remove buildup on
piping walls.
An existing water intake would convey water through the existing fire
suppression system to the clean infiltration water treatment plant for
treatment and storage. A storage tank would be elevated above the injection
wells to create positive hydraulic head via gravity to maintain positive
pressures at the clean water infiltration wells. Clean infiltration water
would be conveyed from the storage tank through a pipe system to the
clean water infiltration wells. Pressure regulating valves would be installed
at each clean water infiltration well to control groundwater infiltration rate.
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Based upon predictive groundwater flow and transport modeling, the
infiltration flow rate per well would be approximately 5 gpm for combined
saprolite/deep wells.
Groundwater Extraction Well Discharge Piping (Step 8 —
Conceptual Extraction System Considerations)
The proposed groundwater extraction system would consist of 87
groundwater extraction wells (Figures 6-28a and 6-28b). Based upon
predictive groundwater flow and transport modeling, extraction flow rate
per well would be approximately 13 gpm for combined saprolite/deep wells
and 4 gpm for bedrock wells. These simulated flow rates are reasonably
similar to the flow rates of approximately 5 gpm obtained during
dewatering for construction of the holding basin within the footprint of the
coal pile. The simulated flow rates are greater than the observed flow rates
because the simulated extraction wells extend deeper into zones with
greater yield. In total, the estimated volume of extracted groundwater is
approximately 955 to 972 gpm. At that rate, the maximum daily volume
would be approximately 1.3 MGD.
Each of the groundwater extraction wells would discharge into one of a
series of below or above ground pump stations. The pump stations would
operate off of internal level controls and have redundancy built in them for
operation and maintenance. Extracted groundwater would be pumped to
the proposed groundwater treatment plant for treatment and discharge.
6.8.2.2 Engineering Designs with Assumptions,
Calculations, and Specifications
(CAP Content Section 6.E.b.ii)
Pipelines (Step 9 — Pipeline Specifics)
(CAP Content Section 6.E.b.ii)
High density polyethylene (HDPE) piping will be used for water
conveyance in all areas where buried piping will be installed. Polyvinyl
Chloride (PVC) and/or Ductile Iron Pipe (DIP) may be used for gravity
sewer and where unusual circumstances occur. Water conveyance will
include:
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• Groundwater pumped from extraction wells and conveyed to the
physical -chemical wastewater treatment system
• Surface water pumped from the Catawba River and conveyed to the
clean water infiltration water treatment system
• Infiltration water treatment system effluent to clean water infiltration
wells
HDPE piping will conform to standard HDPE pipe specifications such as
the following:
• ASTM F714, "Standard Specification for Polyethylene (PE) Plastic
Pipe (DR -PR) Based on Outside Diameter,"
• ASTM D3035,"Standard Specification for Polyethylene (PE) Plastic
Pipe (DR -PR) Based on Controlled Outside Diameter."
• ANSI/AWWA C906, "Polyethylene (PE) Pressure Pipe and Fittings,
4" to 63", for Water Distribution and Transmission."
• Cell Classification PE445574C per ASTM D3350
• Plastics Pipe Institute (PPI) TR-4 Listing as PE4710 / PE3408
• Hydrostatic Design Basis 1,600 psi @ 73°F (23°C) and 1,000 psi @
140°F (60°C) per ASTM D2837
Fittings will be molded from HDPE compound having cell classification
equal to or exceeding the compound used in the pipe manufacture to ensure
compatibility of polyethylene resins. Substitution may be allowed for
approved material with use of flanged joint sections.
Heat fusion welding of the piping and fittings would be in accordance with
Duke Procedure Number: CCP-ENGSTD-NA-QA-004, "Quality Assurance
and Quality Control of HDPE Pipe Butt Fusion Joints Revision 3," July 8,
2019. Only qualified operators trained in Duke Energy's HDPE fusion
standards would be allowed to perform fusion welding.
Flanged connections would be in accordance with Duke Procedure
Number: CCP-ENGSTD-NA-QA-005, "Requirements for Installation of
Polyethylene Flanged Joints Revision Number 0," August 5, 2019.
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The locations of the HDPE piping systems for extraction and infiltration
water are generally in low traffic areas. The HDPE piping will be typically
installed below grade in 3-foot deep excavated trenches constructed with
compacted granular bedding material. The trenches will be backfilled with
a minimum of 2-feet of excavated native soil and compacted. Pipe in areas
with regular traffic of more than two axles will be installed in trenches
designed to comply with AWWA M-55, "PE Pipe — Design and Installation"
or an approved alternative design.
The design flow rate is approximately 350 gpm for the clean water
infiltration system and 950 gpm for the groundwater extraction system.
Infiltration water distribution lines would connect to each clean water
infiltration well. Likewise, each groundwater extraction well will be
connected the groundwater extraction system to convey extracted
groundwater to the groundwater extraction treatment plant. Preliminary
calculations pertaining to the piping design (e.g., pipe sizing, pressures,
flow, friction losses, etc.) are provided in Appendix N.
Localized collection tanks and pumps or pump stations might be integrated
into the piping system to allow for independent operation of various
segments of the system.
Hydrostatic leak testing in accordance with the most current edition of
Handbook of Polyethylene Pipe, or an approved alternate method, will be
performed and passed prior to the piping being placed into operation.
Pipe Network Calculations (Step 10 — Pipeline Headloss
Calculations)
(CAP Content Section 6.E.b.ii)
The extraction and clean water infiltration networks for the proposed
alternative were designed using Pipe Flow° Expert. Pipe Flow® Expert is a
software used to determine volumetric flow rates, pressure in pipes, friction
losses, pump head, and other information. The calculated outputs and
graphically represented conceptual network layouts are found in
Appendix N.
The extraction network consists of approximately 87 extraction wells with
lines for conveyance and branching pipes providing connections to the
wells. The network operates via gravity and pump flow, collecting the
majority of the flow from the extraction wells and conveying under
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pressure from a common collection point to the groundwater extraction
treatment system. The network was evaluated by generating a model with
well elevations and depths, pipe lengths, etc. Once these values were
incorporated, the calculations were performed using the model to
determine the nature of flow in the network and to ensure that the desired
movement in the pipe system was occurring. After the flow through the
system was verified, pipe diameters and required pump head outputs were
calculated. The calculation outputs took into account the interacting flows
in the system, pipe cleanouts for periodic jetting, and frictional losses from
fittings and pipes to provide evidence of the efficacy of the proposed pipe
network layout design.
The infiltration network consists of either approximately 76 vertical clean
water infiltration wells or approximately 48 vertical clean water infiltration
wells combined with approximately 22 horizontal clean water infiltration
wells. Clean water infiltration wells flow via gravity from an elevated
infiltration tank. The infiltration network was evaluated similarly to the
extraction network; however, due to the operation under gravity flow from
an elevated tank, the network was designed to be operated without
conveyance or infiltration pumps. Accordingly, the calculations performed
using the model were to determine the pipe diameters and the required
elevation of the infiltration water tank.
Telemetry System Design
The groundwater remediation system would be managed using telemetry
system that would enable remote monitoring and operational capabilities.
The telemetry system would be designed to meet the system owner O&M
requirements.
Electrical Design
It is unlikely that existing electrical capacity in the vicinity of the proposed
groundwater remediation system would be sufficient to provide electrical
power to pumps, the clean water infiltration water treatment system, and
other power requirements. Additional electrical capacity is anticipated to
meet groundwater remediation system power requirements.
System Operation and Maintenance Issues
The effectiveness of the system will be dependent on maintaining adequate
infiltration and extraction flow rates through the wells, and stable water
levels, for an extended period of time. This will necessitate effective
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operation and maintenance of the wells. As described in this section and in
the Contingency Plan (Section 6.8.8), each well will be equipped with a
control and monitoring system and monitored continuously by the control
system, and an alert sent if the water level falls outside the prescribed
range. Adjustments to pumping operations can be made if the root cause of
the alert is determined to be system performance.
Another factor in maintaining the effectiveness of the wells will be
monitoring and maintaining the well screens to prevent a loss of efficiency
due to mineral and/or biological fouling. If well performance monitoring
indicates a decrease in flow rate, the well will be inspected for fouling and
the screens will be cleaned as appropriate. Additionally, cleanouts will be
installed on pipes to facilitate periodic maintenance, preventing mineral
scaling or biological fouling on the conveyance pipe network.
In addition to well performance monitoring and maintenance, other system
elements, such as pumps controls, will receive routine maintenance in
accordance with the manufacturer's recommendations.
6.8.2.3 Permits for Remedy and Schedule
(CAP Content Section 6.E.b.iii)
The design documents would provide the necessary plans and
specifications for procurement and construction purposes. This would
include Site layout drawings, plans and profiles, well enclosure details,
trench and discharge piping outlet details, well construction schematics,
piping and instrumentation diagrams/drawings and complete equipment,
materials and construction specifications.
Permit applications that may be needed for the proposed remedy include:
• Erosion and Sediment Control permit
• In Situ Groundwater Remediation Injection Well permit
• NPDES Storm Water permit
• Water Withdrawal and Transfer registration
• Wetlands permit
The schedule for obtaining permits is based off the project implementation
schedule as discussed in Section 6.8.6 and presented on Figure 6-32.
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6.8.2.4 Schedule and Cost of Implementation
(CAP Content Section 6.E.b.iv)
An implementation schedule for the proposed corrective action is provided
in Figure 6-32. The exact timeline of the schedule milestones is dependent
on various factors, including NCDEQ review and approval, permitting,
weather, and field conditions.
Duke Energy will provide construction reports monthly from the beginning
of construction until construction is complete and Duke Energy assumes
full responsibility for operation of the groundwater remediation system.
Reporting will include:
• Health and Safety/Man Hours
• Tasks completed the prior month
• Problems affecting schedule (e.g., inclement weather)
• Measures taken to achieve construction milestones (e.g., increase
number of drilling crews)
• Contingency actions employed, if any
• Tasks to be completed by next reporting period
• Provide updated schedule/Gantt chart
Duke Energy progress reports would be submitted to NCDEQ monthly.
The cost estimate for Alternative 3 is based on capital costs for design and
implementation, and the operations, maintenance (O&M) and monitoring
costs, including well redevelopment and replacement on an annual basis.
The design costs include work plans, design documents and reports
necessary for implementation of the alternative. Implementation costs
include procurement and construction.
O&M costs are based on annual routine labor, materials and equipment to
effectively conduct monitoring, routine annual and 5-year reporting, and
routine and non -routine maintenance costs.
A detailed cost estimate for this Alternative is provided in Appendix K.
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6.8.2.5 Measure to Ensure Health and Safety
(CAP Content Section 6.E.b.v)
There is no measurable difference between evaluated Site risks and risks
indicated by background concentrations; therefore, no material increases in
risks to human health related to the source areas have been identified. The
groundwater corrective action is being planned to address regulatory
requirements. The risk assessment identified no current human health or
ecological risk associated with groundwater downgradient of the source
areas. Water supply wells are located upgradient and/or sidegradient of the
source areas and eligible households have been offered an alternate water
supply. Surface water quality standards downgradient of the COI -affected
plume are also met. Based on the absence of receptors, it is anticipated that
groundwater extraction would create conditions that continue to be
protective of human health and the environment because the COI
concentrations will diminish with time.
6.8.2.6 Description of all Other Activities and
Notifications being conducted to Ensure
Compliance with 02L, CAMA, and Other Relevant
Laws and Regulations
(CAP Content Section 6.E.b.vi)
This CAP Update is for the ash basins and the additional source area
hydrologically connected to the ash basins, the coal piles, as identified in
NCDEQs April 5, 2019 letter (Appendix A). The CAP Update addresses the
requirements of G.S. Section 130A-309.211(b), complies with NCAC 15A
Subchapter 02L. 0106 corrective action requirements, and follows the CAP
guidance provided by NCDEQ in a letter to Duke Energy.
6.8.3 Requirements For 02L .0106(I) — MNA
(CAP Content Section 6.E.c)
The requirements for implementing corrective action by MNA, under 02L
.0106(1), are provided in Section 6.7.1 and Appendix I. MNA is not applicable at
this time for Allen as described in Section 6.8.1.
6.8.4 Requirements For 02L .0106(k) — Alternate Standards
(CAP Content Section 6.E.d)
Regulation 02L .0106(k), states that a request may be made for approval of a
corrective action plan that uses standards other than the 02L groundwater
quality standards. Duke Energy may request alternate standards for ash basin -
related constituents, including boron, as allowed under 15A NCAC 02L .0106(k).
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Alternate standards are appropriate at Allen given the lack of human health and
ecological risks at the Site. G.S. Section 130A, Article 9, Part 8 allows risk -based
remediation as a clean-up option where the use of remedial actions and land use
controls can manage properties safely for intended use. Risk -based corrective
action is where constituent concentrations are remediated to an alternative
standard based on the actual posed risks rather than applicable background -
levels or regulatory standards. The requirements for implementing corrective
action by remediating to alternate standards, under 02L .0106(k), are as follows:
• Sources are removed or controlled;
• Time and direction of contaminant travel can be predicted with reasonable
certainty;
• COIs have and will not migrate onto adjacent properties unless specific
conditions are met (i.e., alternative water sources, written property owner
approval, etc.);
• Standards specified in Rule .0202 of this Subchapter will be met at a
location no closer than one year time of travel upgradient of an existing or
foreseeable receptor, based on travel time and the natural attenuation
capacity of subsurface materials or on a physical barrier to groundwater
migration that exists or will be installed by the person making the request;
If contaminant plume is expected to intercept surface waters, the
groundwater discharge will not possess contaminant concentrations that
would result in violations of standards for surface waters contained in
15A NCAC 02B .0200;
Public notice of the request has been provided in accordance with Rule
.0114(b) of this Section; and
Proposed corrective action plan would be consistent with all other
environmental laws
The alternative groundwater clean-up values may be used to aid in risk
management decisions at Allen.
6.8.5 Sampling and Reporting
(CAP Content Section 6.E.e)
An effectiveness monitoring plan (EMP) has been developed as part of this CAP
consistent with 02L. 0106(h)(4). The EMP is designed to monitor groundwater
conditions at Allen and document progress towards the remedial objectives over
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time. This plan is designed to be adaptive over the project life cycle and can be
modified as the groundwater remediation system design is prepared, completed,
or evaluated for termination.
Duke Energy implemented an IMP after the plan was that was submitted to
NCDEQ on October 23, 2018 and subsequent additional modifications were
agreed upon between Duke Energy and NCDEQ. The IMP includes the locations
of groundwater wells sampled quarterly and semiannually.
The EMP is required by G.S. Section 130A-309.211(b)(1)(e). The IMP will be
replaced by the EMP upon NCDEQ approval of the CAP Update. Either
submittal of the EMP, or the pilot test work plan and permit applications (as
applicable), will fulfill G.S. Section 130A-309.209(b)(3).
The EMP, presented in Appendix O, is designed to be adaptable and target key
areas where changes to groundwater conditions are most likely to occur due to
corrective action and ash basin closure activities. The EMP will be used to
evaluate progress towards remediation. EMP key areas for monitoring are based
on the following considerations:
• Include background locations
• Include designated flow paths
• Within areas of observed or anticipated changing Site conditions, and/or
have increasing constituent concentration trends
• Will effectively monitor COI plume stability and model simulation
verification
• The EMP will be used to evaluate progress towards remediation
EMP elements including well systems, locations, frequency, parameters,
schedule and reporting evaluation are summarized below and outlined on Table
6-17. Effectiveness monitoring well locations are illustrated on Figure 6-33. The
EMP will be implemented 30 days after CAP approval, and will continue until
there is a total of three years of data confirming COIs are below applicable
standards at or beyond the compliance boundary, at which time a request for
completion of active remediation will be filed with NCDEQ. If applicable
standards are not met, the EMP will continue and transition to post -closure
monitoring, if necessary.
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After ash basin closure and following ash basin closure certification, a post -
closure groundwater monitoring plan (PCMP) will be implemented at the Site
for a minimum of 30 years in accordance with G.S. Section 130A-309.214(a)(4)k.2.
If groundwater monitoring results are below applicable standards at the
compliance boundary for three years, Duke Energy may request completion of
corrective action in accordance with G.S. Section 130A-309.214(a)(3)b. If
groundwater monitoring results are above applicable standards, the PCMP will
continue. An EMP work flow and optimization process is outlined on a flow
chart on Figure 6-34.
Optimization of the plan to help determine the remedy's performance,
appropriate number of sample locations, sampling frequency, and laboratory
analytes, and statistical analysis to evaluate the plume stability conditions would
be conducted during EMP review periods. The optimization process would be
conducted using software designed to improve long-term groundwater
monitoring programs such as Monitoring and Remediation Optimization System
(MAROS).
6.8.5.1 Progress Reports and Schedule
(CAP Content Section 6.E.e.i)
After groundwater remediation implementation, evaluation of Site
conditions, groundwater transport rates, and plume stability would be
based on quantitative rationale using statistical, mathematical, modeling, or
empirical evidence. Existing data from historical monitoring and pilot
testing would be used to provide baseline information prior to groundwater
remediation implementation. Schedule and reporting of system quantitative
evaluations, review and optimization would include:
Annual Reporting Evaluation: The EMP will be evaluated annually
for optimization and adaption for effective long-term observations,
using a data -need rationale for each location. The annual evaluation
would include a comparison of observed concentrations compared to
model predictions and an evaluation of statistical concentration
trends, such as the Mann -Kendall test.
Results of the evaluation would be reported in annual monitoring
reports and are proposed to be submitted to NCDEQ annually. The
reports would include the following:
o Laboratory reports on electronic media,
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o Tables summarizing the past year's monitoring events,
o Historical data tables,
o Figures showing the historical data versus time for the
designated monitoring locations and parameters,
o Figures showing sample locations,
o Statistical analysis (Mann -Kendall test) of data to determine if
trends are present, if performed,
o Identification of exceedances of comparative values,
o Groundwater elevation contour maps in plan view and
isoconcentration contour maps in plan view for one or more of
the prior year's sampling events (as mutually agreed upon by
Duke Energy and NCDEQ),
o Any notable observations related to water level fluctuations or
constituent concentration trends attributable to extraction
system performance or water table drawdown, and
o Recommendations regarding modifications to the Plan
• 5-Year Review: Similar to annual evaluation and reporting, the EMP
would be re-evaluated and modified as part of each 5-year review
period as adaptive or, if necessary, additional corrective actions are
implemented or water quality observations warrant adjustments of
the plan. The annual evaluation would include elements of the
annual evaluation, plus updated background analysis, confirmation
of risk assessment, evaluation of statistical concentration trends,
analytical result comparison and model verification. If needed, flow
and transport models could be updated as part of the 5-year review
process to refine future predictions and the associated routine data
needed to confirm the predictions.
Optimization of the monitoring network could be evaluated if the remedy is
determined to be effective or when conditions re -stabilize after the
implementation of closure or, if necessary, additional corrective action
implementation. Optimization of the monitoring network could include a
lesser monitoring frequency and/or parameter list. Flow and transport
model predictions indicate very slow changes in conservative (boron)
concentrations will occur over time. Geochemical model predictions
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indicate very little or much slower changes in the remaining COI
distributions will occur. Therefore, a monitoring frequency consistent with
these predictions would be proposed following confirmation of the models
through site data.
If necessary, modifications to the corrective action approach would be
proposed to achieve compliance within the target timeframe.
A flow diagram for effectiveness monitoring plan work and optimization is
depicted on Figure 6-34.
6.8.5.2 Sampling and Reporting Plan During Active
Remediation
(CAP Content Section 6.E.e.ii)
Groundwater Monitoring Network
EMP monitoring will provide a comprehensive monitoring strategy that (1)
monitors the performance and effectiveness of the selected remedial
alternative, (2) can provide adequate areal (horizontal) and vertical
coverage to monitor plume status at or beyond the compliance boundary
and with regard to potential receptors, and (3) confirm flow and transport
and geochemical model predictions. This monitoring would be
implemented north and northwest of the ash basin (Figure 6-33). EMP
groundwater well monitoring network objectives are outlined below:
• Compliance with 02L
• Measure and track the effectiveness of the proposed clean water
infiltration and extraction system
• Monitor plume status at or beyond the compliance boundary
(horizontally and vertically)
• Verify predictive model simulations
• Verify no unacceptable impact to downgradient receptors
• Verify attainment of remedy objectives through validated model
simulations
• Identify new potential releases of constituents into groundwater
from changing site conditions
• Monitor approved background locations
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The EMP would include 66 groundwater monitoring wells (Table 6-17).
Several of the existing monitoring wells at the site might be abandoned
from ash basin closure and related construction activities. In the event that
closure activities extend to the proposed EMP well locations, the layout of
wells would be modified, if necessary.
Groundwater Monitoring Flow Paths - Trend Analysis
The monitoring network will provide adequate horizontal and vertical
coverage in the area of groundwater remediation to monitor:
• Changes in groundwater quality as Site conditions change (e.g.,
groundwater remediation effects, ash basin closure commences),
• Transport rates, and
• Plume stability
Horizontal and vertical coverage would be provided by using groundwater
monitoring wells located downgradient of the source areas within the
corrective action area. To monitor performance, groundwater monitoring
wells are located within the area of corrective action at specific intervals or
as close as possible from the source area to a receptor as illustrated in
Figure 6-33.
Multi parameters sondes would be installed in wells along the primary flow
paths in the active remedy area (Figure 6-33). Wells that are, but are not
shown as installed along primary flow paths in Figure 6-33 include AB-
10S/D/BR/BRL and GWA-5S/D/BRA/BRL. Table 6-17 provides a detailed
list of monitoring wells to be included in the EMP, along with wells
proposed to have multi parameter sondes installed. Daily monitoring of
changes in groundwater quality on a real-time basis using multi -parameter
sondes and telemetry technology would allow continuous monitoring and
evaluation of geochemical conditions. Geochemical conditions, monitored
using pH and Eh, would be compared to geochemical modeling results to
evaluate changes that could potentially affect the mobility (Ka) of reactive
and variably -reactive COIs. Water levels would also be monitored by the
multi -parameter sondes to verify simulated changes to groundwater flow
from groundwater remediation, and during and after ash basin closure.
Having groundwater quality and water level data readily available will
increase the response time to implement contingencies if field parameters
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significantly deviate from predicted responses. Contingency plans are
included in Section 6.8.8.
Plume stability evaluation would be based primarily on results of trend
analyses. Trend analyses might be conducted using Mann -Kendall trend
test. The Mann -Kendall trend test is a non -parametric test that calculates
trends based on ranked data and has the flexibility to accommodate any
data distribution and is insensitive to outliers and non -detects. The test is
best used when large variations in the magnitude of concentrations may be
present and may otherwise influence a time -series trend analysis.
Trend analysis would be conducted using data from EMP geochemically
non -reactive, conservative constituents (Table 6-17). These constituents
include boron, chloride, and TDS, and best depict the areal extent of the
plume and plume stability and physical attenuation, either from active
remedy or natural dilution and dispersion.
Trend analysis of designated groundwater monitoring flow path wells
(Figure 6-33) would be part of the decision metrics for determining
termination of the active remedy.
Sampling Frequency
Multiple years of quarterly and semiannual monitoring data are available
for use in trend analysis and to establish a baseline to evaluate corrective
action performance. The monitoring plan sampling frequency is based on
semi-annual sampling events to be consistent with other groundwater
monitoring performed at the Site.
Semi-annual monitoring following implementation of corrective action is
recommended for the 66 monitoring wells to be included in the EMP. Over
four years of quarterly monitoring data are available for existing wells,
which will be used to supplement trend analysis and to establish a baseline
to evaluate corrective action performance.
Newly installed wells to be added to the EMP would be monitored by
quarterly sampling events. Quarterly sampling would target locations of
proposed newly installed wells with fewer than four quarters of data.
Quarterly monitoring of parameters outlined on Table 6-17 is proposed for
newly installed wells.
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Quantitative evaluations would also determine additional data needs (i.e.,
increased sampling frequency) for refining statistical and empirical model
development. Additional monitoring described in the contingency plan
would be implemented if significant geochemical condition changes are
identified that could result in mobilization of reactive or variably -reactive
COIs.
Sampling and Analysis Protocols
EMP sampling and analysis protocol will be similar to the existing IMP with
some adjustment for anticipated changing site conditions. Detailed
protocols are presented in Appendix O. Samples would be analyzed by a
North Carolina certified laboratory for the parameters listed in Table 6-17
as summarized below. Laboratory detection limits for each constituent are
targeted to be at or less than applicable regulatory values (i.e., 02L or
IMAC).
• Groundwater Quality Parameters: Based on the constituent
management approach, 7 constituents warrant corrective action at
the Site, and are included as groundwater quality parameters to be
monitored as part of the EMP. These constituents are as follows:
o Boron o Strontium
o Cobalt o Sulfate
o Iron o Total Dissolved Solids
o Manganese
Geochemically conservative, non -reactive constituents boron, sulfate,
and TDS best depict the areal extent of the groundwater plume.
Analyses of these constituents will be used to monitor plume
stability and physical attenuation from groundwater flushing and
extraction, by comparing monitoring results with flow and transport
model simulations.
Changing geochemical conditions that could cause sorption or
precipitation/co-precipitation mechanisms that might affect mobility
of non -conservative and variable constituents would be evaluated
using multi parameter sonde data.
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• Groundwater Field Parameters: The following six field parameters
will be monitored to confirm that monitoring well conditions have
stabilized prior to sample collection and to evaluate data quality:
water level, pH, specific conductance, temperature, dissolved
oxygen, and oxidation reduction potential. For remedy performance
monitoring, these parameters will be measured daily by a multi -
parameter sondes installed in each flow path monitoring well and
used to evaluate geochemical conditions from remedy effectiveness.
Major cations and anions would be analyzed to evaluate monitoring
data quality (electrochemical charge balance). These include
alkalinity, bicarbonate alkalinity, aluminum, calcium, iron,
magnesium, manganese, nitrate + nitrite, potassium and sodium.
Total organic carbon (TOC), ferrous iron, and sulfate analyses are
also proposed as monitoring parameters. TOC is recommended to
help determine if an organic compound is contributing to TDS, and
ferrous iron and sulfate to monitor potential dissolution of iron
oxides and sulfide precipitates as an indicator of changing conditions
related to corrective action. These parameters are indicated on Table
6-17 as water quality parameters.
6.8.6 Sampling and Reporting Plan After Termination of Active
Remediation
(CAP Content Section 6.E.e.iii)
Termination of the proposed remedial alternative will be consistent with and
implemented in accordance with NCDEQ Subchapter 02L .0106(m). A flow chart
of the decision metrics, request, and review timeline for termination is outlined
on Figure 6-35 (CAP Content Section 6.E.e.iii.1). This process will provide
stakeholders an opportunity to evaluate terminating the system, as appropriate,
in the vicinity of the well or wells where groundwater restoration completion is
being evaluated.
Trend analysis described in Section 6.8.5 would be part of the decision metrics
for determining termination of the active remedy (CAP Content Section
6.E.e.iii.1.A and B). Groundwater remediation effectiveness monitoring will
transition to the attainment monitoring phase when NCDEQ determines that the
remediation monitoring phase is complete at a particular well or area.
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6.8.7 Proposed Interim Activities Prior to Implementation
(CAP Content Section 6.E.f)
In accordance with requirements of G.S. Section 130A-309.211(b)(3),
implementation of the proposed corrective action will begin within 30 days of
NCDEQ approval of the CAP Update.
Prior to pilot testing, the clean infiltration water will be sampled for geochemical
and physical parameters for baseline conditions to evaluate the potential for
biofouling and plugging of the clean water infiltration well screens. During pilot
testing, extracted groundwater will be collected and analyzed for geochemical
parameters consistent with the NPDES permit.
Additional interim activities to be conducted prior to implementation of the
corrective action remedy include:
• Implementation of the EMP within 30 days of CAP approval
• Submittal of permit and registration applications to NCDEQ, as applicable
6.8.8 Contingency Plan
(CAP Content Section 6.E.g)
The purpose of the Contingency Plan is to monitor changes in conditions and
operations to effectively reach the remedial action objectives. The contingency
plan addresses operations, groundwater conditions and performance.
The Contingency Plan would be defined in greater detail as design elements of
the system are finalized. A groundwater monitoring program to measure and
track the effectiveness of the proposed comprehensive clean water infiltration
and extraction system is described in Section 6.8.2. This plan is designed to be
adaptive and can be modified as the groundwater remediation system design is
prepared, completed, or evaluated for termination.
6.8.8.1 Description of Contingency Plan
(CAP Content Section 6.E.g.i)
The contingency plan addresses the following areas:
• Operations (including infiltration and extraction wells, pumping,
piping, electrical, and controls)
• Groundwater quality
• Groundwater levels
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• Groundwater treatment
• Comparison to predicted concentrations and water levels
A health and safety plan and an operations manual will be prepared. The
health and safety plan will deal with management of spills and other
unplanned releases and the operation manual will address operational
training including backup personnel, emergency response training, and
reporting to appropriate authorities.
6.8.8.2 Decision Metrics for Contingency Plan Areas
(CAP Content Section 6.E.g.ii)
This section outlines decision metrics and possible contingency actions in
support of a resilient groundwater corrective action strategy.
Operations
A computer control telemetry system would be installed with the system to
provide timely information to the Site Operator regarding key operational
features, particularly infiltration and extraction well water levels and flow
rates. The control system will be tied into a remote monitoring station to
alert key personnel as to the nature and urgency of the issue. The system
would be programmed with expected values for measured parameters.
Alerts would be sent when actual values are outside the programmed
range. Based on the alerts, the functional problem would be evaluated and
repairs or replacement of faulty equipment will be completed. The expected
duration of operations will exceed the life expectancy of most of the
mechanical equipment that will comprise the system so ongoing
replacement of equipment will be part of the operations and maintenance
program.
Several aspects of the monitoring system would be used to optimize system
operations, including:
Maintaining target flow rates and water levels for each well is
important to minimize the potential for loss of clean water
infiltration water and extracted groundwater flow control. Each well
would be monitored continuously by the control system, with all
data being recorded, and an alert sent if the flow rate or water level is
outside the prescribed range. In addition to automated systems, each
element of the system will be physically inspected and maintained as
part of a routine operations and maintenance program.
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Leak detection systems could detect possible leaks related to
pumping, piping and/or wells, and the respective element of the
system could be shut down and a message will be immediately sent
to the operator and to backup personnel. The potential leak will be
inspected and repaired prior to restarting the system element.
Continuous monitoring of key parameters would help maintain
proper operation of the system, if pH adjustment or other water
treatment technology is employed. Variances between prescribed
ranges will alert the operator and other key personnel and may result
in automatic system shut down.
• Routine documented inspections of key components of the system
would be done by the operator to track system status and system
performance.
• System maintenance schedules would be established to track system
performance. System elements will be maintained in accordance with
manufacturer's recommendations, which will be contained in a
system Operation and Maintenance (O&M) Manual. Corrective
measures, performed by appropriately skilled personnel, will be
taken if mechanical issues are identified during routine maintenance
monitoring.
Groundwater Quality
The EMP includes a primary network of wells that will provide focused
monitoring in critical areas following corrective action implementation.
After each sampling event, data will be entered into a comprehensive data
base system. Trend analyses will be conducted, spatially and temporally, to
evaluate COI plume changes. If groundwater quality field parameters or
constituent concentrations significantly deviate from predicted responses, a
focused investigation will be conducted to determine if the variation is due
to system performance or other factors. Based on this analysis, possible
responses could include adding or abandoning infiltration or extraction
wells, or changing flow rates or target water levels.
To assess the effectiveness of changes, or to determine if the unexpected
data trends are temporary, increased monitoring frequency or additional
monitoring locations may be conducted.
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If subsequent results continue to show non-conformance, a more
comprehensive assessment and corrective action plan for the specific non-
conformance may be completed and implemented.
Groundwater Levels
Water levels in selected EMP monitoring wells will be monitored using
downhole instrumentation until Site conditions have stabilized. Water level
data will be evaluated as part of the ongoing monitoring. Technical
evaluations will include spatial and temporal trend analyses, drawdown
calculations, and flow and transport model refinement to reflect current
conditions, as needed. If results conclude that water levels are not similar to
predicted patterns a focused investigation will be conducted that could
include adjusting system pumping rates, refining the flow and transport
model for infiltration and extraction rates, adding monitoring wells to the
EMP monitoring network for greater resolution, installation of monitoring
wells in key areas, and/or other activities.
If subsequent results from ongoing investigation continue to show non-
conformance, a corrective action response with suggested approaches to
determine possible reasons for the non-conformance would be
implemented until resolution is achieved.
Groundwater Treatment
If extracted groundwater treatment is required prior to discharge through a
permitted outfall, evaluation of that system will be part of the routine
monitoring program.
If a treatment system is not meeting performance standards or if trends
suggest performance is not optimal, an analysis of the trends and an
assessment of the system will be completed and corrective measures
implemented. Changes could be the result of changing influent
characteristics.
Comparison to Predicted Concentrations and Water Levels
Many aspects of the proposed remediation approach are based on modeling
and predicted groundwater conditions. As remedial efforts begin, hydraulic
conditions change, and additional groundwater data are collected, the
models will be updated. However, as conditions change, especially at the
beginning of the process there maybe deviations from existing data trends
and model predictions. The models will be updated to reflect changing
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conditions, as necessary, and changes in predicted results will be analyzed
to determine if the remedial approach needs to be modified to effectively
address the changes.
Given that groundwater infiltration is an element of the system, there is a
potential that soil might become saturated near the ground surface, with the
potential to create surface discharges. If this occurs, reducing infiltration
rates by adjusting water -level controllers at wells near the area or increasing
the extraction system would be used to control surficial saturation.
6.9 Summary and Conclusions
This CAP Update meets the corrective action requirements under G.S. and Subchapter
02L.0106 and to addresses Subchapter 02L.01060). This CAP Update proposes a remedy
for COIs in groundwater associated with the Allen coal ash basins and coal piles that
are beyond the Site's compliance boundary to the north, northeast, and east of these
source areas. This CAP Update provides:
• A groundwater remediation approach that can be implemented under either
closure scenario (closure -in -place or closure -by -excavation).
• A screening process of multiple potential groundwater corrective action
alternatives that would address areas requiring corrective action.
• A selection and description of the favored corrective action groundwater
remedy: Alternative 3, Groundwater Extraction Combined with Clean Water
Infiltration and Treatment.
• Specific plans, including engineering design details, for restoring groundwater
quality.
• A schedule for the implementation and operation of the corrective action
strategy.
• A monitoring plan for evaluating the performance and effectiveness of corrective
action groundwater remedy, and its effect on the restoration of groundwater
quality.
• Planned activities prior to full-scale implementation including pilot testing in
selected areas and water treatment testing. Pilot test work plan(s) will be
submitted to NCDEQ within 30 days of CAP approval to fulfill G.S. Section
130A-309.211(b)(3).
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Correction Action Plan Update
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7.0 PROFESSIONAL CERTIFICATIONS
(CAP Content Section 7)
December 2019
Certification for the Submittal of a Corrective Action Plan
Responsible Party and/or Permittee: ❑uke Energy Carolinas, LLC
Contact Person: Paul Draovitch
Address: 525 South__ hurt Street
City: Charlotte State: NC Zip Code: 28202
Site Name: Allen Steam Station
Address: 253 Plant Allen Road
City: Belmont State: NC Zip Code: 28012
Groundwater Incident Number: Not Assigned
SynTerra
We, Christo her Sutteli Professional Geologist and James Clemmer a Professional Engineer for
SynTerra Car oratio (firm or company of employment) do hereby certify that the Information
Indicated herein is as part of the required Corrective Action Plan (CAP) and that to the best of our
knowledge of the data, assessments, conclusions, recommendations and other associated materials
are correct, complete and accurate.
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Correction Action Plan Update December 2019
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8.0 REFERENCES
(CAP Content Section 8)
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