NC0004987_01_MSS_CAP Update 2019_Text_20191231

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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:
Marshall Steam Station
8320 East Carolina Highway 150
Terrell, NC 28682
Not Assigned
NC0004987
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
[phone]
N 35.59778IMh„8A.965
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Corrective Action Plan Update December 2019
Marshall 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 Marshall Steam Station (MSS) located in Catawba
County, North Carolina. Since 2010, 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 MSS coal ash basin. 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 basin at the MSS as low -risk pursuant to CAMA.
Thousands of multi -media samples have been collected at the MSS yielding over
190,000 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 area are submitted
separately.
As detailed in this CAP, Duke Energy has begun to implement, and will continue
implementing, source control measures at the site, including (i) complete ash basin
decanting to lower the hydraulic head within the ash basin and decrease hydraulic
gradients, reducing groundwater seepage velocities and COI transport potential; and
(ii) complete ash basin closure, as well as closure of adjacent ash management areas. 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 a combination of groundwater extraction and
clean water infiltration. These corrective action measures will most effectively achieve
remediation of the groundwater through the use of groundwater extraction wells along
the ash basin dam and to the east and north of the dam; and (ii) clean water infiltration
wells to the north of the ash basin dam and east of the ash basin. Significantly,
groundwater modeling simulations indicate (i) these measures will address 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 necessary equipment, and commencing full-

Corrective Action Plan Update December 2019
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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 the MSS 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 the MSS. 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 the MSS
using USEPA guidance demonstrate that risks to potential human health and
ecological receptors associated with the coal ash basin are not measurably
greater than risks posed by naturally occurring background conditions.
• Ash basin -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 180 monitoring wells over 30 separate monitoring events, and
performing over 200 groundwater and geochemical modeling simulations.
In addition, even though no off -site wells were affected, Duke Energy has already
provided owners of surrounding properties within 0.5-mile radius of the ash
compliance boundary with permanent water solutions through either connection to
public water supply or installation of 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 systems, including Lake Norman, confirm that these surface
water systems are healthy with robust fish populations.
Duke Energy looks forward to proactively implementing this CAP.

Corrective Action Plan Update December 2019
Marshall Steam Station SynTerra
EXECUTIVE SUMMARY
(CAP Content Section Executive Summary)
ES.1 Introduction
SynTerra prepared this groundwater corrective action plan (CAP) on behalf of Duke
Energy Carolinas, LLC (Duke Energy). The plan pertains to the Marshall Steam Station
(MSS, Plant, or Site) coal combustion residuals (CCR) ash basin in Catawba County,
North Carolina (Figure ES-1). For MSS, the following additional sources are considered
adjacent to the ash basin and are components of the CAP: Dry Ash Landfills (Phase I
and Phase II), photovoltaic (PV) structural fill, access road structural fill, the Industrial
Landfill (ILF) No. 1 subgrade structural fill, coal pile, and gypsum pad.
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 MSS ash basin and adjacent additional sources listed above, which
are considered sources of COIs, that are at or beyond the compliance boundary to the
east of the ash basin. Specifically, this CAP Update focuses on constituent
concentrations detected 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.
In accordance with G.S. requirements, a CAP pertaining to MSS was previously
submitted to the NCDEQ in two parts, as follows:
• Corrective Action Plan Part 1— Marshall Steam Station Ash Basin (HDR, 2015b)
• Corrective Action Plan Part 2 (included CSA Supplement 1 as Appendix A) — Marshall
Steam Station Ash Basin (HDR, 2016b)
This CAP Update considers data collected through May 2019.
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Ash basin closure is detailed in a separate document prepared by AECOM. Closure
scenarios include a closure -in -place and closure -by -excavation scenario. Therefore, the
groundwater remediation alternatives evaluated and recommended in this CAP Update
consider both the closure -in -place scenario and closure -by -excavation scenario.
Groundwater modeling simulations indicate the closure -by -excavation and closure -in -
place scenarios would 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.
Section 130A-309.21 1 (b) and Subchapter 02L .0106. The preferred groundwater
remediation approach assumes source control through either basin closure -in -place or
closure -by -excavation. The groundwater remediation approach presented in this CAP
Update can be implemented under either scenario. The focus of groundwater corrective
action at the MSS 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 .0106(j). Applicable criteria in this case are defined as the 02L
groundwater standard, interim maximum allowable concentration (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 the MSS:
Source Control Measures
• Ash basin decanting is currently underway and will reduce the hydraulic head
and gradients in the area of the dam, thereby significantly reducing the hydraulic
driving force for potential COI migration in groundwater. As of December 1,
2019, approximately 128,400,000 gallons water have been removed from the ash
basin and the water elevation has decreased by 7.3 feet. Groundwater modeling
indicates that the average linear velocity of groundwater will decrease from a
range of 0.01 to 5 feet per day (ft/day) under pre -decanting conditions to 0.01 to 1
ft/day post -decanting.
• Completion of ash basin closure activities.
Groundwater Remediation Measures
• A robust groundwater remediation approach is planned for the MSS that
includes actively addressing COIs in groundwater with concentrations greater
than applicable standards at or beyond the compliance boundary using a
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combination of groundwater extraction and treatment along with clean water
infiltration. Site data and groundwater models were used to evaluate and
optimize an effective remedial approach to reduce COI concentrations east of the
ash basin. The following is a summary of components of the preferred
remediation system that would be installed east of the ash basin:
o 66 new groundwater extraction wells along the buttress of the ash basin
dam and to the east of the basin towards the unnamed tributary of Lake
Norman
o 24 vertical clean water infiltration wells between the ash basin and
unnamed tributary located east of the ash basin
o Groundwater treatment, as needed, to meet discharge criteria
Effectiveness Monitoring Plan (EMP)
• Duke Energy has prepared an effectiveness groundwater monitoring plan, which
is discussed in Section 6.8 and provided in Appendix O of this CAP Update.
This EMP includes an optimized groundwater monitoring network for the ash
basin based on site -specific COI mobility and distribution. The EMP is designed
to be adaptable and would target key areas where changes to groundwater
conditions are most likely to occur due to corrective action implementation or
basin closure activities. The plan includes provisions for a post -closure
monitoring program in accordance with G.S. Section 130A-309.214(a)(4)k upon
completion of basin closure activities.
Details and supporting rationale for these CAP activities are provided in the following
sections.
ES.2 Background
Plant Operations
MSS began electrical power generation operations in 1965. The station currently
operates four coal-fired steam units. CCR materials, composed primarily of fly ash and
bottom ash, were initially deposited in the ash basin by hydraulic sluicing operations. In
1984, fly ash sluicing was replaced with a dry fly ash handling system. In early 2019, a
dry bottom ash collection system became active. All bottom ash and fly ash is currently
handled dry. The MSS ash basin has operated under a National Pollutant Discharge
Elimination System (NPDES) Permit issued by the NCDEQ Division of Water Resources
(DWR) since initial operations began.
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Pursuant to G.S. Section 130A-309.213(d)(1), a November 13, 2018 letter from NCDEQ to
Duke Energy, documented the classification of the CCR surface impoundment at MSS
as low -risk (Appendix A). The letter cited that Duke Energy has "established
permanent water supplies as required by G.S. Section 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 G.S. Section 143-215.32."
The relevant closure requirements for low -risk impoundments are in G.S. Section 130A-
309.214(a)(3), which states low -risk impoundments shall be closed as soon as
practicable, but no later than December 31, 2029.
Additional Adjacent Source Areas
The closed Dry Ash Landfills are located adjacent to the east (Phase I) and northeast
(Phase II) portions of the ash basin. In December 1983, the North Carolina Department
of Environment and Natural Resources (NCDENR) Division of Waste Management
(DWM) issued an initial permit (Permit No. 1804-INDUS) to operate the Dry Ash
Landfills. Phase I consists of approximately 14.5 acres and approximately 522,000 cubic
yards (cy) of fly ash, which was placed from September 1984 through March 1986.
Phase II consists of approximately 46 acres and approximately 4,064,000 cy of fly ash,
which was placed from March 1986 through 1999. The landfill units are unlined and
were closed with a soil cover system.
The photovoltaic farm structural fill (PV Structural Fill), located adjacent to and
partially on top of the northwest portion of the ash basin, was constructed of fly ash
under the structural fill rules found in 15A NCAC 13B .1700 et seq., and bottom ash,
under Duke Energy's Distribution of Residuals Solids (503 Exempt) Permit Number
WQ0000452, which was issued by NCDENR Division of Water Quality (DWQ).
Placement of dry ash in the PV Structural Fill began in October 2000. The PV Structural
Fill covers approximately 83 acres and contains approximately 5,410,000 cy of ash. The
PV Structural Fill is unlined and was closed with a soil cover system in February 2013.
The access road structural fill, adjacent to the ash basin waste boundary south of the PV
Structural Fill, was constructed of fly ash under the structural fill rules found in 15A
NCAC 13B .1700 et seq. The access road structural fill covers approximately 2.5 acres
and contains approximately 128,000 cy of ash outside of the ash basin waste boundary.
Construction of the unlined structural fill began in 1997 and was completed in 1998.
The subgrade for portions of the Industrial Landfill No. 1 (ILF, Permit No. INDUS-1812)
was constructed of fly ash under the structural fill rules found in 15A NCAC 13B .1700
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et seq. The subgrade structural fill, which contains approximately 726,000 cubic yards of
ash, was closed with a soil cover in 2013. The ILF was constructed over portions of this
unlined structural fill and the northern reach of the ash basin.
Coal is stored south of the ash basin, immediately north of the steam station, on
approximately 35 acres. The coal pile is unlined. However, in 2018, lined holding basins
were built west and east of the coal pile as part of a water redirect project. These
Retention Basins receive coal pile storm water runoff collected from the coal pile
through a concrete -lined perimeter ditch and associated collection trench.
Gypsum, a byproduct of the coal combustion process, is stored on an approximately
3.5-acre lined concrete pad located west of the coal pile and retention basin.
These additional source areas are located within the same groundwater drainage
system as the ash basin. Therefore, COIs that have the potential to migrate in
groundwater from these additional source areas at or beyond the ash basin compliance
boundary and above regulatory criteria are addressed as part of the CAP for the ash
basin.
Pre -Basin Closure Activities
To accommodate closure of the ash basin, decanting (removal) of free water from the
basin began with the removal of stop logs (gravity -feed) on July 16, 2019, as required by
a Special Order by Consent (SOC) issued through North Carolina Environmental
Management Commission (EMC) on April 25, 2018 (EMC SOC WQ S17-009, Appendix
B of Appendix J). Mechanical decanting (pumping) of free water from the basin
commenced on September 13, 2019. The SOC requires completion of decanting by
March 31, 2021. Decanting of free, ponded water from the ash basin before closure is
expected to 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, 128.4 million gallons of water have been removed from the ash
basin and the elevation of the ponded water within the basin has decreased by 7.3 feet.
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Basis for CAP Development
A substantial amount of data related to the MSS ash basin and adjacent source areas has
been collected to date. A summary of the MSS assessment documentation used to
prepare this CAP Update is presented in Table ES-1.
TABLE ES-1
SUMMARY OF MSS ASSESSMENT DOCUMENTATION
Comprehensive Site Assessment Report - Marshall Steam Station Ash Basin [HDR
Engineering, Inc. of the Carolinas (HDR, 2015a)]
Corrective Action Plan Part 1 - Marshall Steam Station Ash Basin (HDR, 2015b)
Corrective Action Plan Part 2 (included CSA Supplement 1 as Appendix A) - Marshall Steam
Station Ash Basin (HDR, 2016b)
Comprehensive Site Assessment Supplement 2 - Marshall Steam Station Ash Basin (HDR,
2016a)
Comprehensive Site Assessment Update - Marshall Steam Station (SynTerra, 2018a)
Preliminary Updated Groundwater Flow and Transport Modeling Report - Marshall Steam
Station (Falta Environmental, SynTerra, and FRx, Inc., 2018)
Human Health and Ecological Risk Assessment Summary Update - Marshall Steam Station
(SynTerra, 2018b)
Community Impact Analysis of Ash Basin Closure Options at the Marshall Steam Station
(Exponent, 2018)
Marshall Steam Station HB 630 Provision of Permanent Water Supply Completion
Documentation (Duke Energy, 2018)
Closure Options Analysis (AECOM, 2018)
Ash Basin Pumping Test Report - Marshall Steam Station (SynTerra, 2019a)
Estimating Partition coefficient (Ka) for Modeling Boron Transport Using EPQ Method 1316 -
Marshall Steam Station (SynTerra, 2019b)
Surface Water Evaluation to Assess 15A NCAC 02B - Marshall Steam Station (SynTerra,
2019c)
2018 CAMA Annual Interim Monitoring Report (SynTerra, 2019d)
Updated Background Threshold Values for Constituent Concentrations in Groundwater
(SynTerra, 2019e)
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NCDEQ reviewed the 2018 Comprehensive Site Assessment (CSA) Update, and in an
August 17, 2018 letter to Duke Energy, NCDEQ stated that sufficient information was
provided to allow preparation of this CAP Update (Appendix A).
The assessment work referenced in the documents listed in Table ES-1 has resulted in a
large dataset that has informed the development of this CAP Update. As of September
2019, the following data collection and analyses activities have been completed and are
summarized in Table ES-2 below:
TABLE ES-2
SUMMARY OF MSS ASSESSMENT ACTIVITIES
Tasks
Total
Total Monitoring Wells Installed (CAMA, CCR wells around ash basin)
186
Groundwater Monitoring Events
32
Groundwater Samples Collected
2,858
Individual Analyte Results
190,632
Off -Site Water Supply Well Sampling (Total inorganic analysis) - Number of
Analyses
2,616
Ash Pore Water - Number of Analyses (Total and dissolved)
19,154
Ash Pore Water Sampling Events
17
Surface Water Monitoring Events
16
Surface Water Sample Locations
29
Area of Wetness Sample Events
11
Ash Samples (Within ash basin analyzed for SPLP)
14
Soil Samples Collected
437
Soil Sample Locations
97
Sediment Sample Locations
12
Geotechnical Soil Sample Locations
44
Geochemical Ash, Soil, Partially Weathered Rock, Whole Rock Samples
103
Hydraulic Conductivity Tests (Slug Tests, Pumping Tests, Packer Tests,
FLASH Analysis of Bedrock HPF Data)
119
Groundwater Flow & Transport Simulations
124
PHREEQC Geochemical Simulations
84
Notes:
Data available to SynTerra as of September 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
Prepared by: BER Checked by: WCG
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A constituent 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 under
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 at the MSS ash basin and adjacent source areas
under current and predicted future conditions.
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 basin and adjacent source areas or 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 an ash basin or an adjacent source 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 6.1.
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 that exhibit concentrations in groundwater at or beyond the
compliance boundary greater than the 02L standard, IMAC, or background
concentrations, whichever is greatest.
Soil
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 in downgradient locations are greater than
Preliminary Soil Remediation Goal (PSRG) Protection of Groundwater (POG) standards
or background values, concentrations are within range of Piedmont background values
or there are no mechanisms by which the COIs could have migrated from the ash basin
or adjacent sources to the unsaturated soils. Furthermore, these COI occurrences are not
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present in groundwater at the same location greater than applicable regulatory criteria.
For these reasons, the soil concentrations do not warrant consideration as potential
secondary source of constituents to the groundwater. Therefore, the CAP Update
focuses on remediation of groundwater associated with the ash basin and adjacent
source areas.
Risk Assessment
The human health and ecological risk assessments were prepared using standard
USEPA methods and demonstrate no measurable difference in modeled risks to
potential human or ecological receptors compared with background concentrations. The
updated risk assessments for the MSS are presented in Section 5.4 and Appendix E of
this CAP Update. Data from water supply wells and Lake Norman indicate no evidence
of increased risk posed by groundwater migration associated with the ash basin and
adjacent source areas based on evaluation of concentrations of CCR constituents in
environmental media and potential receptors.
Risk Ranking
In accordance with G.S. Section 130A-309.211(cl) of House Bill 630 (2016), Duke Energy
connected 62 households to public water supply and installed three water filtration
systems at occupied residences within a 0.5-mile radius of the ash basin compliance
boundary. Additionally, Duke Energy voluntarily provided permanent water solutions
to six properties, including businesses and churches, within a 0.5-mile radius of the MSS
compliance boundary that were otherwise not eligible per G.S. Section 130A-
309.211(c1). Provision of permanent water supply and installation of filtration systems,
along with certain improvements to the ash basin dam, resulted in the MSS ash basin
being classified 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 basin and adjacent source
areas. The updated CSM developed for the MSS included in this CAP Update is based
on a U.S. Environmental Protection Agency (USEPA) document titled "Environmental
Cleanup Best Management Practices: Effective Use of the Project Life Cycle Conceptual
Site Model" (EPA, 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 become available. The current MSS CSM is consistent with Stage 4
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"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 the MSS. The remedial action evaluation to meet the effectiveness
criteria in the CAP guidance provided by NCDEQ is also based on the updated CSM
(NCDEQ, 2019).
The following provides an overview of the updated CSM for the MSS ash basin and
adjacent source areas, which forms the basis of this CAP Update. Supporting details for
the CSM are presented in Section 5.
Key conclusions of the CSM include the following:
• No material increases in risks to human health have been identified related to
the ash basin and adjacent source areas. The Site -specific risk assessment
indicates no measurable difference between evaluated Site -related risks and
risks imposed by background concentrations. Site -specific risk assessments
indicate incomplete exposure pathways and no risk to residential receptors near
the ash basin and adjacent source areas (no complete exposure pathways).
The ash basin and adjacent source areas do not cause an increase in risks to
ecological receptors. The assessment did not indicate an increase of risks to
aquatic wildlife receptors (mallard duck, great blue heron, bald eagle, and river
otter) evaluated for the Lake Norman exposure area. Two receptors had limited
modeled risk with hazard quotients (HQ) greater than 1.0: the muskrat (7.2) and
killdeer (4.6). However, the modeled risks are considered negligible based on
natural and background conditions.
• Groundwater from the ash basin area has not and does not flow toward any
water supply wells based on groundwater flow patterns and the location of
water supply wells in the area around the Site, and evaluation of groundwater
analytical data. Groundwater data collected from water supply wells and on -
Site monitoring wells, groundwater elevation measurements from 32 monitoring
events, and groundwater flow and transport modeling results all indicate that
Site COIs are not affecting, and have not affected, water supply wells.
• The permanent water solution implemented by Duke Energy provides
qualified owners of surrounding properties with water supply wells within a
0.5-mile of the ash basin compliance boundary with access to the public water
supply or water filtration systems. The hydrogeologic data collected at MSS
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confirms that Site -related COIs have not affected off -Site water supply users.
Groundwater modeling predicts that Site related COIs will not affect off -Site
water supply users. Nevertheless, Duke Energy connected 62 households to
public water supply and installed three water filtration systems at surrounding
occupied residences in accordance with G.S. Section 130A-309.211(cl). Six
additional properties, including businesses and churches, within a 0.5-mile
radius of the MSS compliance boundary were also provided permanent water
solutions by Duke Energy, although they did not meet the eligibility
requirements outlined in G.S. Section 130A-309.211(cl).
• The hydrogeologic setting at the MSS ash basin and adjacent source areas
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 hydraulic gradients, thereby reducing the rate of COI transport prior to
completion of basin closure. As of December 1, 2019, 128.4 million gallons of
water have been removed from the ash basin and the elevation of the ponded
water within the basin has decreased by 7.3 feet.
• The physical setting and hydraulic processes control the COI flow pattern
within the ash basin, underlying groundwater system, and downgradient
areas. The ash basin is predominantly a horizontal water flow -through system.
Groundwater enters the upgradient side of the ash basin; it is supplemented by
rainfall infiltration and flows laterally through the middle of the ash basin 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
saprolite upgradient from the dam. 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 Lake
Norman (e.g., discharge zone), limiting downward migration of COIs to the area
near the dam. Exceptions occur at 3 of the 16 well clusters installed within the
basin, where COIs are detected in groundwater underlying the ash basin.
Outside of the ash basin, near the southern portion of the closed Dry Ash
Landfill (Phase II), landfilled dry ash has resulted in the leaching of COIs to the
underlying bedrock.
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• Horizontal distribution of COIs in groundwater east of the MSS ash basin is
limited spatially. The physical extent of constituent migration to the east of the
basin is controlled by 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
partition coefficient (Kd), response to geochemical parameters (i.e., pH and
reduction -oxidation potential [Eh]), and sorption capacity of the soil and/or rock
matrix. Based on geochemical modeling, the following observations can be made:
o Non -conservative, reactive COIs (e.g., beryllium, chromium, and
vanadium) will remain in mineral phase assemblages that are stable under
variable Site conditions, demonstrating sorption as an effective
attenuation mechanism.
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, lithium, and sulfate) 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 slightly
alkaline to alkaline pH values. Maximum boron sorption occurs at pH
values between 7.5 standard units (S.U.) and 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 closure -by -excavation and
closure -in -place scenarios and account for potential mobilization of COIs.
• COIs in groundwater are contained within Duke Energy's property. COI
distribution extends from the ash basin toward Lake Norman and the unnamed
tributary east of the ash basin. Flow and transport model simulations predict
COI migration in groundwater below adjacent portions of Lake Norman are
limited; however, bedrock fracture orientation data suggest that the simulated
eastward extent of COI migration may be over -predicted. The groundwater
concentrations predicted below Lake Norman are a result of the hydraulic heads
created by the impounded ash basin free water. This groundwater eventually
ES-12

Corrective Action Plan Update December 2019
Marshall Steam Station SynTerra
discharges to the overlying surface water, where COIs are reduced in
concentration by physical processes such as mechanical mixing (dispersion) and
dilution. Flow and transport groundwater model predictions indicate basin
decanting will lower the hydraulic head within the ash basin and reduce COI
transport.
• Groundwater/surface water interaction has not resulted in exceedances of
North Carolina Administrative Code, Title 15A, Subchapter 02B, Surface Water
and Wetland Standards (02B) in Lake Norman. The downgradient stream
(unnamed tributary east of the basin) and Lake Norman are groundwater
discharge zones that limit the horizontal transport of constituents downgradient
of the basin. Due to the limited presence and mobility of most constituents in the
groundwater system, the groundwater associated with the ash basin generally
has not caused, and will not cause, current surface water quality standards in
Lake Norman to be exceeded. However, under seasonal low -flow conditions,
elevated hardness has been reported at SOC seep S-1 in the tributary east of the
ash basin and Phase I landfill. Duke Energy is actively addressing this occurrence
in compliance with the SOC.
The aquatic systems (unnamed tributary and Lake Norman) surrounding the
MSS ash basin are healthy based on multiple lines of evidence including
robust fish populations, species variety and other indicators based on years of
sampling data. Lake Norman 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 Norman has been
an environmentally healthy and functioning ecosystem, and ongoing sampling
programs have been established to ensure the health of the 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 60 years.
• Most of the COIs identified in the CSA Update occur naturally in groundwater
at concentrations greater than the 02L standard or IMAC. Groundwater at MSS
naturally contains barium, chromium, cobalt, iron, manganese, total radium, and
vanadium at concentrations greater than their respective 02L standard or IMAC.
The occurrence of inorganic constituents in groundwater of 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. Therefore, vanadium is evaluated based on its Site-
ES-13

Corrective Action Plan Update December 2019
Marshall Steam Station SynTerra
specific, statistically derived background value, and additional lines of evidence
to determine whether constituent concentrations represent migration from the
ash basin and/or an additional source area, or are naturally occurring. The same
consideration is given for other COIs with naturally occurring concentrations
greater than applicable regulatory criteria.
These CSM aspects, combined with the updated human health and ecological risk
assessments, provide the basis for the corrective action plan developed for the ash basin
and adjacent source areas.
ES.4 Corrective Action Approach
Corrective Action Objectives and Zones Requiring Corrective Action
Migration of COIs related to the ash basin in groundwater at or beyond the compliance
boundary occurs in localized areas to the east of the ash basin. Groundwater corrective
action was also evaluated for the adjacent source areas. However, because they lie
within the drainage network of the ash basin, and groundwater flow from these areas
and the ash basin is southeastward, groundwater from the adjacent source areas would
be captured through the groundwater remediation system east of the ash basin. To
satisfy G.S. and maintain compliance with 02L, the corrective action approach planned
for the Site will focus on restoring ash basin- and adjacent source area -affected
groundwater at or beyond the compliance boundary. The following remedial objectives
address the regulatory requirements of NCAC Title 15A Subchapter 02L for the MSS
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).
ES-14

Corrective Action Plan Update December 2019
Marshall Steam Station SynTerra
The ash basin compliance boundary is displayed on Figure ES-1. Groundwater
concentrations greater than 02L/IMAC/applicable background concentration values
occur locally at or beyond the compliance boundary in two areas:
1. In the limited area east of the compliance boundary, between the ash basin and
unnamed tributary
2. Along limited areas downgradient of the ash basin dam, within the compliance
boundary (coincides with the Lake Norman shoreline)
The areas of proposed groundwater corrective action under either closure scenario are
shown on Figure ES-2.
Summary of Source Control and Corrective Measures
It is critical to take into account all various activities Duke Energy has/will perform to
improve subsurface conditions at MSS related to the ash basin and adjacent source
areas. The remedial program incorporates source control by basin decanting and
closure, active groundwater remediation and effectiveness monitoring. Table ES-3
summarizes the discrete components of the planned corrective action for COI -affected
groundwater.
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Corrective Action Plan Update December 2019
Marshall Steam Station SynTerra
TABLE ES-3
COMPONENTS OF SOURCE CONTROL, ACTIVE REMEDIATION, AND
MONITORING
Groundwater Remedy Component
Rationale
Source Control Activities
Ash Basin Decanting
Active source remediation by removing ponded
water in the ash basin. Decanting will lower the
hydraulic head within the basin 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.
Decanting was first initiated on July 16, 2019
with the removal of stop logs from the outfall.
Mechanical decanting commenced on
September 13, 2019. As of December 1, 2019,
128.4 million gallons of water have been
removed from the ash basin and the elevation
of the ponded water within the basin has been
reduced by 7.3 feet. Decanting is required to be
complete on or before March 31, 2021.
In addition, ash basin decanting is expected to
be effective in reducing or eliminating seeps
identified in the SOC.
Ash Basin Closure
The ash basin closure -in -place scenario or
closure -by -excavation scenario are considered
source control/removal activities. Extensive
groundwater modeling indicates that either
method results in similar effects with respect to
groundwater remediation.
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Corrective Action Plan Update December 2019
Marshall Steam Station SynTerra
TABLE ES-3
COMPONENTS OF SOURCE CONTROL, ACTIVE REMEDIATION, AND
MONITORING
Groundwater Remedy Component
Rationale
Source Control Activities
Closure of Adjacent Additional Sources
Closure -in -place of the PV Structural Fill and
Dry Ash Landfill Phase II will reduce the
potential of COI migration from these sources.
Installation of an impermeable cover system will
prevent infiltration of precipitation through
these sources and reduce COI leaching potential
to underlying groundwater. Additionally, due to
the unique hydrogeologic setting and close
proximity to Waters of the US, the Dry Ash
Landfill Phase I is proposed to be excavated.
The Structural Fill Access Road will also be
removed as part of Ash Basin Closure (under
closure -by -excavation), or capped with an
impermeable cover system under the closure -
in -place scenario. The Industrial Landfill No.1
Structural Fill Subgrade will be capped with a
geosynthetic liner when the landfill is expanded.
Active Groundwater Remediation Activities
Active Groundwater Remediation
Groundwater remediation focuses on meeting
the remedial objectives at the compliance
boundary. These efforts will focus near the
basin dam area and areas north of the dam
toward the unnamed tributary east of the basin
where COIs are present at concentrations
greater than applicable criteria.
To meet the above -referenced CAP objectives,
66 extraction wells and 24 clean water
infiltration wells are planned to be placed in
areas to reduce COI concentrations based on
actual site data and groundwater modeling
simulations.
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Corrective Action Plan Update December 2019
Marshall Steam Station SynTerra
TABLE ES-3
COMPONENTS OF SOURCE CONTROL, ACTIVE REMEDIATION, AND
MONITORING
Groundwater Remedy Component
Rationale
Institutional Controls and Monitoring
Permanent Water Solution for Water
Groundwater data at the Site indicates that
Supply Well Users within a 0.5-mile
surrounding water supply wells have not been
radius of the Ash Basin Compliance
and are not affected by Site -related COIs.
Boundary and Associated Water
However, Duke Energy installed 62 connections
Filtration System Maintenance
to public water supply and 3 water filtration
systems for qualifying occupied households. Six
additional properties, including businesses and
churches, within a 0.5-mile radius of the MSS
compliance boundary were also provided
permanent water solutions by Duke Energy,
although they did not meet the eligibility
requirements outlined in G.S. Section 130A-
309.211(ci). Duke Energy's actions were
approved by NCDEQ, which addresses
stakeholder concerns. Duke Energy maintains
the water filtration systems on behalf of the
residents.
Maintain Ownership and Institutional
ICs in the form of a Declaration of Perpetual
Controls (ICs) Consisting of a Land Use
Land Use Restrictions may be requested in the
Restriction
future based on the results of the groundwater
remediation activities.
Effectiveness Groundwater Monitoring
Duke Energy plans to monitor the groundwater
to confirm the corrective action objectives are
met and maintained over time. 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 future
model refinement. The CAP Update includes a
comprehensive review of groundwater data
collected through June 2019 and a plan to
optimize the monitoring program. Within 30
days of CAP approval, Duke Energy would
implement the effectiveness monitoring
program.
ES-18

Corrective Action Plan Update December 2019
Marshall Steam Station SynTerra
TABLE ES-3
COMPONENTS OF SOURCE CONTROL, ACTIVE REMEDIATION, AND
MONITORING
Groundwater Remedy Component
Institutional Controls and Monitoring
Provision for Adaptive Management of
Groundwater Remedies
Rationale
The MSS ash basin and surrounding area is a
complex site; therefore, Duke Energy believes it
is important to allow for an adaptive approach
during implementation of this CAP. This
approach is consistent with the Interstate
Technology and Regulatory Council (ITRC)
document titled Remediation Management of
Complex Sites (ITRC, 2017). This approach may
include adjustments to the groundwater
remedy, if necessary, based on new data, or if
conditions change.
Prepared by: BDW Checked by: WCG
Corrective Action at Remediation Zones
The areas proposed for groundwater remediation in accordance with 02L requirements
are east of the ash basin beyond the compliance boundary and downgradient of the
dam (Figure ES-2). A wide variety of potential groundwater remedial technologies
were initially screened as part of this CAP Update to identify the most applicable
remedial methods based upon site specific hydrogeologic conditions and COI
distribution in groundwater. After initial screening, the following remedial alternatives
were further evaluated in detail:
Remedial Alternative 1: Monitored Natural Attenuation
0 Remedial Alternative 2: Groundwater extraction, clean water infiltration, and in -
situ treatment with chemical amendments
• Remedial Alternative 3: Groundwater extraction and clean water infiltration
These remedial alternatives were further screened against the following criteria
outlined in Section 6.D.iv. (1-10) of the CAP guidance (NCDEQ, 2019):
• Protection of human health and the environment
• Compliance with applicable federal, state, and local regulations
• Long-term effectiveness and permanence
ES-19

Corrective Action Plan Update December 2019
Marshall Steam Station SynTerra
• 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 Alternative 3: Groundwater extraction combined with clean water
infiltration will most effectively achieve the remedial objectives presented above. The
well layout is illustrated on Figure ES-3 and consists of:
• Sixty-six (66) groundwater extraction wells along the buttress of the dam and to
the east and north of the dam
• Twenty-four (24) clean water infiltration wells to the north of the dam and east of
the ash basin
It is recommended that prior to implementation, pilot testing of the proposed
alternatives will be conducted. Pilot testing and treatment tests to be conducted include:
1) groundwater extraction, 2) clean water infiltration, and 3) treatment testing of water
for clean water infiltration. Pilot study results will inform the design of the full-scale
system. Planned activities prior to full-scale implementation, where either submittal of
the remedial performance monitoring plan (i.e., effectiveness monitoring plan), or the
pilot test work plan and permit applications (as applicable) will be submitted to
NCDEQ within 30 days of CAP approval to fulfill G.S. Section 130A-309.211(b)(3).
Duke Energy will also be addressing additional primary sources, including the Dry Ash
Landfill Phase I and Phase II (INDUS-1804) and the PV Structural Fill, with NCDEQ
Division of Waste Management (DWM) in separate submittals. The Dry Ash Landfill
Phase I (INDUS-1804) is proposed to be excavated and the PV Structural Fill and Dry
Ash Landfill Phase II are proposed for additional closure measures including
ES-20

Corrective Action Plan Update December 2019
Marshall Steam Station SynTerra
installation of a geosynthetic liner and cover system. Installation of an impermeable
cover system on the PV Structural Fill and Dry Ash Landfill Phase II will prevent
infiltration of precipitation through these sources and reduce COI leaching potential to
underlying groundwater. These are source control measures that will assist
groundwater corrective action downgradient of these facilities.
As a further source control measure, Duke Energy proposes to excavate the Dry Ash
Landfill Phase I due to the unique hydrogeologic setting and close proximity to surface
water receptors. The land space could provide additional room for groundwater
remediation infrastructure or corrective action plan modification, if deemed necessary,
without interfering with ash basin closure or site operations. Vertical migration of COIs
beneath and downgradient of the Dry Ash Landfill Phase I is not limited or intercepted
by the flow -through ash basin system, as described in the updated CSM herein.
Excavation of the Dry Ash Landfill Phase I will remove the source and reduce
additional migration of COIs.
ES-21

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CORRECTIVE ACTION PLAN UPDATE
MARSHALL STEAM STATION
TERRELL, NORTH CAROLINA
DRAWN BY: B.YOUNG DATE: 05/15/2019 GRAPHIC SCALE
REVISED BY: B. YOUNG DATE: 12/10/2019 1,000 0 1,000 2,000
CHECKED BY: E. WEBSTER DATE: 12/10/2019
--------
APPROVED BY: B. WILKER DATE: 12/10/2019
PROJECT MANAGER: B. WILKER (IN FEET)

1
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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 MCKIM & CREED MARCH
2016.
2. ALL BOUNDARIES ARE APPROXIMATE
3. PROPERTY BOUNDARY PROVIDED BY DUKE ENERGY
CAROLINAS.
4. AERIAL PHOTOGRAPHY OBTAINED FROM GOGGLE EARTH PRO
ON JULY 26, 2018. IMAGE COLLECTED ON MARCH 30, 2018.
5. DRAWING HAS BEEN SET WITH PROJECTION OF NORTH
CAROLINA STATE PLANE COORDINATE SYSTEM FIPS 3200 (NAD83).
DUKE 750 0
RAPHICSC7LE
50 1,500
It ENERGY® (IN FEET)
CAROLINAS
DRAWN BY: B. YOUNG DATE: 08/27/2019
'47 REVISED BY: C. WYATT DATE: 12/18/2019
CHECKED BY: E. WEBSTER DATE: 12/18/2019
APPROVED BY: B. WILKER DATE: 12/18/2019
synTerra PROJECT MANAGER: B. WILKER
www.synterracorp.com
ROW
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Wk
LAKE -
NORMAN
LEGEND
AREA PROPOSED FOR ACTIVE GROUNDWATER
REMEDIATION
- ASH BASIN COMPLIANCE BOUNDARY
ASH BASIN WASTE BOUNDARY
LANDFILL BOUNDARY
STRUCTURAL FILL BOUNDARY
- LANDFILL COMPLIANCE BOUNDARY
DUKE ENERGY CAROLINAS MARSHALL
STEAM STATION SITE BOUNDARY
1 STREAM (MCKIM & CREED)
® WETLAND (MCKIM & CREED)
FIGURE ES-2
AREAS PROPOSED FOR CORRECTIVE ACTION
CORRECTIVE ACTION PLAN UPDATE
MARSHALL STEAM STATION
TERRELL, NORTH CAROLINA
I.

Corrective Action Plan Update December 2019
Marshall Steam Station SynTerra
Figure ES-3
Proposed Corrective Action Approach
Provided in separate electronic figure file as a
large sheet size

Corrective Action Plan Update December 2019
Marshall Steam Station SynTerra
TABLE OF CONTENTS
SECTION
PAGE
EXECUTIVE SUMMARY.................................................................................................... ES-1
ES.1 Introduction............................................................................................................ ES-1
ES.2 Background.............................................................................................................
ES-3
ES.3 CSM Overview.......................................................................................................
ES-9
ESA Corrective Action Approach..............................................................................
ES-14
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 Basin ....................1-9
1.5.3 Overview of Existing Permits and Special Orders by Consent....................1-10
2.0 RESPONSE TO CSA UPDATE COMMENTS IN SUPPORT OF CAP
DEVELOPMENT...........................................................................................................
2-1
2.1 Facility -Specific Comprehensive Site Assessment (CSA) Comment Letter
.....2-1
2.2 Duke Energy's Response to NCDEQ 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
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Corrective Action Plan Update December 2019
Marshall Steam Station SynTerra
TABLE OF CONTENTS (CONTINUED)
SECTION PAGE
5.1.2.1
Groundwater Flow Direction.....................................................................5-4
5.1.2.2
Groundwater Seepage Velocities...............................................................5-6
5.1.2.3
Hydraulic Gradients....................................................................................5-9
5.1.2.4
Particle Tracking Results...........................................................................
5-12
5.1.2.5
Subsurface Heterogeneities.......................................................................5-12
5.1.2.6
Bedrock Matrix Diffusion and Flow ........................................................
5-13
5.1.2.7
Onsite and Offsite Pumping Influences ..................................................
5-16
5.1.2.8
Ash Basin Water Balance...........................................................................5-16
5.1.2.9
Effects of Naturally Occurring Constituents ..........................................
5-19
5.2 Source Area Location..............................................................................................5-19
5.3 Summary of Potential Receptors..........................................................................5-20
5.3.1 Surface Water.......................................................................................................5-20
5.3.1.1 Environmental Assessment of Lake Norman.........................................5-21
5.3.2 Availability of Public Water Supply................................................................5-21
5.3.3 Water Supply Wells............................................................................................
5-21
5.3.4 Future Groundwater Use Area.........................................................................
5-22
5.4 Human Health and Ecological Risk Assessment Results..................................5-22
5.5 CSM Summary........................................................................................................
5-25
6.0 CORRECTIVE ACTION APPROACH FOR SOURCE AREA 1 (ASH BASIN
AND ADJACENT SOURCE AREAS).......................................................................
6-1
6.1 Extent of Constituent Distribution.........................................................................
6-2
6.1.1 Source Material Within the Waste Boundary ...................................................
6-2
6.1.1.1 Description of Waste Material and History of Placement ......................6-2
6.1.1.2 Specific Waste Characteristics of Source Material...................................6-3
6.1.1.3 Volume and Physical Horizontal and Vertical Extent of
SourceMaterial.............................................................................................
6-5
6.1.1.4 Volume and Physical Horizontal and Vertical Extent of Anticipated
Saturated Source Material...........................................................................
6-5
6.1.1.5 Saturated Ash and Groundwater...............................................................
6-6
6.1.1.6 Chemistry within Waste Boundary...........................................................
6-7
6.1.1.7 Other Potential Source Material...............................................................
6-13
6.1.1.8 Interim Response Actions.........................................................................
6-16
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Corrective Action Plan Update December 2019
Marshall Steam Station SynTerra
TABLE OF CONTENTS (CONTINUED)
SECTION
PAGE
6.1.2 Extent of Constituent Migration beyond the Compliance Boundary ......... 6-18
6.1.2.1 Piper Diagrams........................................................................................... 6-23
6.1.3 Constituents of Interest(COIs).......................................................................... 6-25
6.1.4 Horizontal and Vertical Extent of COIs........................................................... 6-35
6.1.4.1 COIs in Unsaturated Soil........................................................................... 6-37
6.1.4.2 Horizontal and Vertical Extent of Groundwater in Need
ofRestoration.............................................................................................. 6-38
6.1.5 COI Distribution in Groundwater.................................................................... 6-40
6.1.5.1 Conservative Constituents........................................................................
6-41
6.1.5.2 Non -Conservative Constituents...............................................................
6-44
6.1.5.3 Variably Conservative Constituents........................................................
6-44
6.2 Potential Receptors Associated with Source Area .............................................
6-45
6.2.1 Surface Waters — Downgradient Within 0.5-Mile Radius of the Waste
Boundary..............................................................................................................
6-46
6.2.2 Water Supply Wells............................................................................................
6-48
6.2.2.1 Provision of Alternative Water Supply ...................................................
6-49
6.2.2.2 Findings of Drinking Water Supply Well Surveys ................................
6-50
6.2.3 Future Groundwater Use Areas........................................................................
6-51
6.3 Human and Ecological Risks.................................................................................
6-52
6.4 Description of Remediation Technologies..........................................................
6-52
6.4.1 Monitored Natural Attenuation........................................................................
6-53
6.4.2 In -Situ Technologies...........................................................................................
6-54
6.4.3 Groundwater Extraction....................................................................................
6-59
6.4.4 Groundwater Treatment....................................................................................
6-65
6.4.5 Groundwater Management...............................................................................
6-69
6.4.6 Technology Evaluation Summary....................................................................
6-74
6.5 Evaluation of Remedial Alternatives...................................................................
6-75
6.5.1 Remedial Alternative 1— Monitored Natural Attenuation (MNA) .............
6-75
6.5.1.1 Problem Statement and Remediation Goals...........................................6-76
6.5.1.2 Conceptual Model......................................................................................
6-76
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Corrective Action Plan Update December 2019
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TABLE OF CONTENTS (CONTINUED)
SECTION
PAGE
6.5.1.3 Predictive Modeling...................................................................................6-77
6.5.2 Remedial Alternative 2 - Groundwater Extraction, Infiltration and In -Situ
Treatment.............................................................................................................
6-77
6.5.2.1 Problem Statement and Remediation Goals ...........................................
6-81
6.5.2.2 Conceptual Model......................................................................................
6-82
6.5.2.3 Predictive Modeling...................................................................................6-83
6.5.3 Remedial Alternative 3 - Groundwater Extraction and Clean Water
Infiltration............................................................................................................
6-83
6.5.3.1 Problem Statement and Remediation Goals...........................................6-86
6.5.3.2 Conceptual Model......................................................................................
6-87
6.5.3.3 Predictive Modeling...................................................................................6-88
6.6 Remedial Alternative Screening Criteria.............................................................6-89
6.7 Remedial Alternatives Criteria Evaluation.........................................................
6-95
6.7.1 Remedial Alternative 1- Monitored Natural Attenuation ...........................
6-95
6.7.2 Remedial Alternative 2: Groundwater Extraction, Infiltration and In -Situ
Treatment - Compliance in the Midterm ......................................................
6-100
6.7.3 Remedial Alternative 3: Groundwater Extraction and Clean Water
Infiltration..........................................................................................................
6-109
6.8 Proposed Remedial Alternative Selected for Source Area ..............................
6-117
6.8.1 Description of Proposed Remedial Alternative and Rationale for
Selection..............................................................................................................
6-117
6.8.2 Design Details....................................................................................................
6-119
6.8.2.1 Process Flow Diagrams for all Major Components of
ProposedRemedy....................................................................................
6-120
6.8.2.2 Engineering Designs with Assumptions, Calculations and
Specifications.............................................................................................
6-127
6.8.2.3 Permits for Remedy and Schedule.........................................................6-130
6.8.2.4 Schedule and Cost of Implementation..................................................
6-131
6.8.2.5 Measures to Ensure Health and Safety .................................................
6-132
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................................................................................................
6-132
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Corrective Action Plan Update December 2019
Marshall Steam Station SynTerra
TABLE OF CONTENTS (CONTINUED)
SECTION
PAGE
6.8.3 Requirements of 02L .0106(1) - MNA.............................................................
6-133
6.8.4 Requirements for 02L .0106(k) -Alternate Standards .................................
6-133
6.8.5 Sampling and Reporting..................................................................................
6-134
6.8.5.1 Progress Reports and Schedule..............................................................
6-135
6.8.5.2 Sampling and Reporting Plan During Active Remediation...............
6-137
6.8.6 Sampling and Reporting Plan After Termination of Active
Remediation.......................................................................................................
6-141
6.8.7 Proposed Interim Activities Prior to Implementation .................................
6-142
6.8.8 Contingency Plan..............................................................................................
6-142
6.8.8.1 Description of Contingency Plan...........................................................
6-143
6.8.8.2 Decision Metrics for Contingency Plan Areas ......................................
6-143
6.9 Summary and Conclusions..................................................................................6-146
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 Areas Proposed for Corrective Action
Figure ES-3 Proposed Corrective Action Approach
1.0 Introduction
Figure 1-1
USGS Location Map
Figure 1-2
Site Layout Map
Figure 1-3
1962 Topographic Survey
Figure 1-4
1950 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 (May 2019)
Figure 5-4b
Water Level Map - Deep Flow Zone (May 2019)
Figure 5-4c
Water Level Map - Bedrock Flow Zone (May 2019)
Figure 5-5a
Velocity Vector Map for Pre -Decanting Conditions - Deep Flow Zone
Figure 5-5b
Velocity Vector Map for Closure -by -Excavation Conditions - Deep
Flow Zone
Figure 5-5c
Velocity Vector Map for Closure -in -Place Conditions - Deep Flow
Zone
Figure 5-6 Map of Surface Waters
Figure 5-7 Water Supply Well Sample Locations
Figure 5-8 HB 630 Provision of Permanent Water Supply Completion Map
6.0 Source Area Evaluation - Active Ash Basin
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'
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Corrective Action Plan Update December 2019
Marshall Steam Station SynTerra
LIST OF FIGURES (CONTINUED)
Figure 6-6
Saturated Ash Thickness Map for Pre -Decanting and Post -Closure
Conditions
Figure 6-7a
General Cross Section A -A' - Conservative Group - Mean of Boron,
Chloride, and TDS
Figure 6-7b
General Cross Section A -A' - Non -Conservative Group - Mean of
Strontium, Thallium and Total Radium
Figure 6-7c
General Cross Section A -A' - Variable Group - Mean of Cobalt, Iron,
and Manganese
Figure 6-8
Ash Basin Decanting Monitoring Network
Figure 6-9
Geochemical Water Quality Plots
Figure 6-10a
Hydrographs - Ash Basin and Vicinity
Figure 6-10b
Hydrographs - Ash Basin and Vicinity
Figure 6-10c
Hydrographs - Ash Basin and Vicinity
Figure 6-10d
Hydrographs — Ash Basin Ponded Water
Figure 6-11
Water Quality Piper Diagrams
Figure 6-12
Surface Water Quality Piper Diagrams
Figure 6-13a
Isoconcentration Map - Boron in Shallow Flow Zone
Figure 6-13b
Isoconcentration Map - Boron in Deep Flow Zone
Figure 6-13c
Isoconcentration Map - Boron in Bedrock Flow Zone
Figure 6-14a
Isoconcentration Map - Chloride in Deep Flow Zone
Figure 6-14b
Isoconcentration Map - Chloride in Bedrock Flow Zone
Figure 6-15
Isoconcentration Map - Cobalt in Shallow Flow Zone
Figure 6-16a
Isoconcentration Map - Iron in Deep Flow Zone
Figure 6-16b
Isoconcentration Map - Iron in Bedrock Flow Zone
Figure 6-17a
Isoconcentration Map - Lithium in Shallow Flow Zone
Figure 6-17b
Isoconcentration Map - Lithium in Deep Flow Zone
Figure 6-17c
Isoconcentration Map - Lithium in Bedrock Flow Zone
Figure 6-18a
Isoconcentration Map - Manganese in Shallow Flow Zone
Figure 6-18b
Isoconcentration Map - Manganese in Deep Flow Zone
Figure 6-18c
Isoconcentration Map - Manganese in Bedrock Flow Zone
Figure 6-19a
Isoconcentration Map - Total Radium in Deep Flow Zone
Figure 6-19b
Isoconcentration Map - Total Radium in Bedrock Flow Zone
Figure 6-20a
Isoconcentration Map - Strontium in Shallow Flow Zone
Figure 6-20b
Isoconcentration Map - Strontium in Deep Flow Zone
Figure 6-20c
Isoconcentration Map - Strontium in Bedrock Flow Zone
Figure 6-21a
Isoconcentration Map - TDS in Shallow Flow Zone
Figure 6-21b
Isoconcentration Map - TDS in Deep Flow Zone
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Corrective Action Plan Update December 2019
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LIST OF FIGURES (CONTINUED)
Figure 6-21c
Isoconcentration Map - TDS in Bedrock Flow Zone
Figure 6-22
Isoconcentration Map - Thallium in Shallow Flow Zone
Figure 6-23
Unsaturated Soil Sample Exceedances
Figure 6-24
Simplified Pourbaix Diagram
Figure 6-25
Simulated Boron Concentrations In All Flow Zones - Remedial
Alternative 1, MNA
Figure 6-26
Conceptual Groundwater Remedial System Layout - Alternative 2
Figure 6-27
Extraction Well Schematic
Figure 6-28
Conceptual Process Flow Diagram - Water Infiltration Galleries
Figure 6-29
Simulated Boron Concentrations in All Flow Zones, Remedial
Alternative 2
Figure 6-30
Groundwater Remedial System Layout - Alternative 3
Figure 6-31
Clean Water Infiltration Well Schematic
Figure 6-32
Simulated Boron Concentrations in All Flow Zones, Remedial
Alternative 3
Figure 6-33
Conceptual Process Flow Diagram - Clean Water Infiltration
Figure 6-34
Conceptual Process Flow Diagram - Groundwater Extraction System
Figure 6-35
CAP Implementation GANTT Chart
Figure 6-36
Effectiveness Monitoring Well Network and Flow Paths
Figure 6-37
Work Flow for Effectiveness Monitoring and Optimization
Figure 6-38
Work Flow for Termination of Groundwater Remediation Program
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Corrective Action Plan Update December 2019
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LIST OF TABLES
Executive Summary
Table ES-1 Summary of MSS Assessment Documentation
Table ES-2 Summary of MSS 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 Area Proposed for Corrective Action
Table 3-1 Summary of Onsite Facilities
4.0 Summary of Background Determination
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 5-1
May 2019 Water Level Measurements and Elevations
Table 5-2
Horizontal Hydraulic Gradients and Flow Velocities
Table 5-3
Vertical Hydraulic Gradients
Table 5-4
Groundwater Balance Summary
Table 5-5
Surface Water Classification
6.0 Source Area Evaluation — Ash Basin
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 - February 2018 to May 2019
Table 6-6
COI Management Matrix
Table 6-7
Summary Trend Analysis Results for Groundwater 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)
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
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LIST OF APPENDICES
Appendix A Regulatory Correspondence
Appendix B Comprehensive Site Assessment Update Report Review Comments
and Responses
Appendix C Updated Comprehensive Analytical Data Tables
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)
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), (1),
and (m)
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 2019 Additional Assessments
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Corrective Action Plan Update December 2019
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LIST OF ACRONYMS
02B NCAC Title 15A, Subchapter 02B. Surface Water and Wetland
Standards
02L NCAC Title 15A, Subchapter 02L. Groundwater Classification and
Standards
AOW area of wetness
aq
aqueous
ASTM
ASTM International
AWWA
American Water Works Association
BTV
Background Threshold Value
bgs
below ground surface
BMP
Best Management Practices
BR
bedrock flow zone
CAMA
Coal Ash Management Act of 2014
CAP
Corrective Action Plan
CBD
citrate-bicarbonate-dithionite
CCR
Coal Combustion Residuals
CERCLA
Comprehensive Environmental Response, Compensation and Liability
Act
CFR
Code of Federal Regulations
COI
Constituent of Interest
CSA
Comprehensive Site Assessment
CSM
Conceptual Site Model
cy
cubic yards
D
deep flow zone
Duke Energy
Duke Energy Carolinas, LLC
DWM
Division of Waste Management
DWQ
Division of Water Quality
DWR
Division of Water Resources
E&SC
Erosion and Sediment Control
Eh
reduction -oxidation potential (volts)
EMC
Environmental Management Commission
EMP
Effectiveness Monitoring Program
EPRI
Electric Power Research Institute
ELCR
excess lifetime cancer risk
OF
degrees Fahrenheit
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Corrective Action Plan Update December 2019
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LIST OF ACRONYMS (CONTINUED)
FGD
flue gas desulfurization
FLASH
Flow -Log Analysis of Single Holes
ft
feet
GCL
geosynthetic clay liner
gpm
gallons per minute
G.S.
General Statutes
GWPS
groundwater protection standard
HAO
hydrous aluminum oxide
HDPE
high density polyethylene
HFO
hydrous ferric oxide
HPF
heat pulse flowmeter
HQ
hazard quotient
IAP
Interim Action Plan
IC
institutional control
IDW
investigation derived waste
ILF
Industrial Landfill
IMAC
Interim Maximum Allowable Concentration
IMP
Interim Monitoring Plan
ITRC
Interstate Technology and Regulatory Council
ISV
in -situ vitrification
k
hydraulic conductivity
Kd
partition coefficient
kg/yr
kilograms per year
lbs
pounds
L/kg
liter per kilogram
LEAF
leaching environmental assessment framework
LLDPE
linear low -density polyethylene
LOAEL
least observed adverse effects level
LPB
low permeability barrier
LRB
lined retention basin
LTM
long term monitoring
MAROS
Monitoring and Remediation Optimization System
mg/kg
milligrams per kilogram
MGD
million gallons per day
mm
millimeter
MNA
monitored natural attenuation
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Corrective Action Plan Update December 2019
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LIST OF ACRONYMS (CONTINUED)
MSS
Marshall Steam Station
MW
megawatt
NCAC
North Carolina Administrative Code
NCDHHS
North Carolina Department of Health and Human Services
NCDENR
North Carolina Department of Environment and Natural Resources
NCDEQ
North Carolina Department of Environmental Quality
ne
effective porosity
NOAEL
no observed adverse effects level
NORR
Notice of Regulatory Requirements
NPDES
National Pollutant Discharge Elimination System
NPV
net present value
NRTR
Natural Resources Technical Report
NTU
Nephelometric Turbidity Units
O&M
operation and maintenance
OEES
Occupational and Environmental Epidemiology Section
OFA
other federal agency
ORP
oxidation-reduction potential
PE
polyethylene
Plant/Site
Marshall Steam Station
PBTV
Provisional Background Threshold Value
PPE
personal protection equipment
PPI
Plastic Pipe Institute
PRP
potential responsible party
POG
protection of groundwater
POTW
publically owned treatment works
psi
pounds per square inch
PRB
permeable reactive barrier
PSRG
preliminary soil remediation goal
PV
Photovoltaic
PVC
poly -vinyl chloride
RCRA
Resource Conservation and Recovery Act
RQD
rock quality designation
RS
restricted designation
S
shallow flow layer
SAC
strong acid cation
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Corrective Action Plan Update December 2019
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LIST OF ACRONYMS (CONTINUED)
SAP
sampling and analysis plan
SBA
strong base anion
SOC
Special Order of Consent
SPLP
synthetic precipitation leaching procedure
SPP
Storm Water Permitting Program
SSL
statistically significant level
S.U.
standard units
TDS
total dissolved solids
TOC
total organic carbon
TSS
total suspended solids
TZ
transition zone
µg/L
micrograms per liter
UIC
underground injection control
USDA
U.S. Department of Agriculture
USEPA
U.S. Environmental Protection Agency
USGS
U.S. Geological Survey
WAC
weak acid cation
WBA
weak base anion
ZVI
zero-valent iron
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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 Marshall Steam
Station (MSS, Plant, or Site) coal combustion residual (CCR) ash basin and adjacent
source areas. Duke Energy owns and operates MSS, located in Terrell, Catawba
County, North Carolina (Figure 1-1).
In accordance with Section 130A-309.211(b) of the North Carolina General Statutes
(G.S.), as amended by Coal Ash Management Act of 2014 (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
evaluation considers constituent concentrations detected greater than applicable North
Carolina groundwater standards [NC Administrative Code (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.
In accordance with G.S. requirements, a CAP for MSS was previously submitted to the
North Carolina Department of Environmental Quality (NCDEQ) in two parts:
• Corrective Action Plan Part 1— Marshall Steam Station Ash Basin (HDR, 2015b)
• Corrective Action Plan Part 2 (included Comprehensive Site Assessment Supplement 1
as Appendix A) —Marshall Steam Station Ash Basin (HDR, 2016b)
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 CAP deliverable schedules and also requested assessment of
additional potential sources of constituents to groundwater at MSS, stating that sources
hydrologically connected to the ash basin are to be assessed and included in an updated
CAP. The coal pile and gypsum storage pad areas were identified as additional sources
hydrologically connected to the ash basin.
In addition to the CAP Update, Duke Energy is required to submit a CCR Surface
Impoundment Closure Plan (Closure Plan) to NCDEQ on/before December 31, 2019
under separate cover. This CAP Update has been developed to be effective with the
various closure scenarios developed for the Site.
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Corrective Action Plan Update December 2019
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CAP content and submittal schedule are 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 the document titled, Corrective Action Plan Content for Duke
Energy Coal Ash Facilities (provided in Appendix A), beneath report section headings
and within text in to facilitate the review process.
1.1 Background
(CAP Content Section 1.A)
A substantial amount of assessment data has been collected for the MSS ash basin and
contiguous additional source areas to support this CAP Update. The Comprehensive
Site Assessment (CSA) Update Report, dated January 31, 2018 (SynTerra, 2018a), was
performed in accordance with requirements in 15A NCAC 02L .0106 (g). The CSA:
• Identified the source(s) and causes of constituents 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 hydrogeological 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 April 26,
2018, and stated the information provided sufficiently warranted preparation of this
CAP Update. This CAP Update builds on previous documents to provide a CAP for
addressing the requirements in 15A NCAC 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 — Marshall Steam Station Ash Basin (HDR
2015a)
• Corrective Action Plan Part 1 — Marshall Steam Station Ash Basin (HDR, 2015b)
• Corrective Action Plan Part 2 (included CSA Supplement 1 as Appendix A) —
Marshall Steam Station Ash Basin (HDR, 2016a)
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Corrective Action Plan Update December 2019
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• Comprehensive Site Assessment Supplement 2 — Marshall Steam Station Ash Basin
(HDR, 2016b).
• Comprehensive Site Assessment Update — Marshall Steam Station Ash Basin
(SynTerra, 2018a).
• Ash Basin Pumping Test Report — Marshall Steam Station (SynTerra, 2019a)
• Estimating Partition Coefficient (Kd) for Modeling Boron Transport Using EPA Method
1316 —Marshall Steam Station (SynTerra, 2019b)
• Surface Water Evaluation to Assess 15A NCAC 02B.0200 Compliance for
Implementation of Corrective Action Under 15A NCAC 02L .0106 W and (l) — Marshall
Steam Station (SynTerra, 2019c)
• 2018 CAMA Annual Interim Monitoring Report (SynTerra, 2019d)
1.2 Purpose and Scope
(CAP Content Section 1.B)
The purposes of this corrective action plan approach are the following:
• Restore groundwater affected by the ash basin and adjacent source areas 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,
in accordance with Subchapter 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 the
MSS.
• Address response requirements contained within 15A 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 found 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 actions 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
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Corrective Action Plan Update December 2019
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alternate water supply for receptors within 0.5 mile downgradient from the established
compliance boundary.
Based on conditions and 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 basin and adjacent sources at or beyond
the compliance boundary, to the east of the ash basin. Initial 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 LQ
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) and multiple subsequent regulatory requests, indicate the ash basin and
the related adjacent units have contributed to constituent concentrations in
groundwater greater than applicable regulatory standards beyond the ash basin
compliance boundary.
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). G.S.
Section 130A-309.211(b) 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
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Corrective Action Plan Update December 2019
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• A monitoring plan for evaluating the effectiveness of the proposed corrective
action and detecting movement of any constituent plumes
• Any other information related to groundwater assessment required by NCDEQ
In addition to CAMA, requirements for CAPS are also contained in Subchapter 02L
.010(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 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).
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Corrective Action Plan Update December 2019
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The MSS ash basin meets the low -risk classification criteria set forth in CAMA for CCR
surface impoundments. On October 12, 2018, the NCDEQ confirmed that Duke Energy
satisfactorily completed the alternate water provisions under G.S. Section 130A-
309.211(c1). On November 13, 2018, the NCDEQ confirmed that Duke Energy rectified
prior dam safety deficiencies, reclassifying the ash basin from its prior draft ranking of
"intermediate" to "low -risk." A low -risk coal combustion residuals surface
impoundment may be closed by excavation, closure -in -place, or a hybrid approach.
Ash basin closure is detailed in a separate document prepared by AECOM. Closure
scenarios include a closure -in -place and closure -by -excavation scenario. 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 include the following:
• A consideration of the extent of any violations
• The extent of any threat to human health or safety
• The extent of damage or potential adverse impact to the environment
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Corrective Action Plan Update December 2019
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• 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 might not be suitable for use as drinking water supply
without treatment. RS designation is a temporary designation and is 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 1.E)
1.5.1 Location and History of Land Use
(CAP Content Section LE.a)
MSS is located on the west bank of Lake Norman on NC Highway 150 E near the
town of Terrell, Catawba County, North Carolina (Figure 1-1). MSS is a four -unit
coal-fired electricity generating plant with a combined capacity of approximately
2,090 megawatts (MW). Operation of Unit 1 (350 MW) began in 1965, and
operation of Unit 2 (350 MW) began in 1966. Operation of Unit 3 (648 MW) began
in 1969, and operation of Unit 4 (648 MW) began in 1970. Cooling water for MSS
is provided by Lake Norman, which was created to serve this purpose.
The area surrounding MSS generally consists of residential properties,
undeveloped land, and Lake Norman (Figure 1-2). Natural topography at the
Site generally slopes downward from an approximate high elevation of 885 feet
North American Vertical Datum of 1988 (NAVD88) along ridges west and north
of the basin to an approximate low elevation of 775 feet at the base of the ash
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Corrective Action Plan Update December 2019
Marshall Steam Station SynTerra
basin dam. Downstream of the dam, Lake Norman encompasses approximately
32,000 acres at a full pond elevation of 760 feet, with an average elevation of 756
feet.
The station and supporting facilities lie within a 1,446-acre parcel owned by
Duke Energy. Based on a review of available historical aerial photography, the
Site consisted of a combination of agricultural land and woodlands prior to the
impoundment of the Catawba River for the formation of Lake Norman. Figure 1-
3 presents a 1962 topographic survey map depicting the area of the MSS Site
prior to its development and construction of Lake Norman. Figure 1-4 presents
an aerial photograph taken in 1950 prior to development of the Site and
construction of Lake Norman (CAP Content Section 1.E.a).
The MSS ash basin, approximately 394 acres in size, is located north of the
station, and is generally bounded by an earthen dam and natural ridges to the
west (Sherrills Ford Road) and north (Island Point Road). Sherrills Ford Road
and Island Point Road, located along topographic ridges, represent hydrologic
divides that affect groundwater flow within an area approximately one mile
west, north and northeast of the ash basin (CAP Content Section 5.b) (Figure 1-2).
Topography to the east of Sherrills Ford Road generally slopes downward
toward Lake Norman to the southeast. Topography along Island Point Road, to
the north and northeast of the ash basin generally slopes downward toward Lake
Norman to the southeast.
Land use within the 0.5-mile radius of the ash basin compliance boundary
generally consists of undeveloped land and Lake Norman to the east,
undeveloped land and residential properties located to the north and west,
portions of MSS (outside the compliance boundary), undeveloped land, and
residences to the south, and commercial properties to the southeast along North
Carolina Highway 150.
The Catawba County zoning map indicates that the majority of the properties
fronting Sherrills Ford and Island Point Roads are zoned Residential (R20 or
R30). The Duke Energy property is zoned General Industrial (GI). No significant
change in land use surrounding MSS is anticipated.
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1.5.2 Operations and Waste Streams Coincident with the Ash
Basin
(CAP Content Section 1.E.b)
Coal -Related Operational Storage and Waste Streams
Coincident with the Ash Basin
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.
Coal has arrived at MSS through rail transportation since operations began. Coal
storage has historically occurred at the Site's coal pile located immediately north
of the powerhouse and south/adjacent to the ash basin (Figure 1-2). Coal is
conveyed via transfer belts to the station where it is pulverized before being used
in the boilers.
NCDEQ identified the coal pile as a potential additional source area adjacent to
the ash basin. The coal pile is not regulated under CAMA; however, assessment
and characterization was conducted, and the findings are incorporated into this
CAP.
Coal ash and other CCRs are produced from coal combustion. The smaller ash
particles (fly ash) are carried upward in the flue gas and are captured by an air
pollution control device, such as an electrostatic precipitator. The larger ash
particles (bottom ash) fall to the bottom of the boiler.
Approximately 70 percent to 80 percent of ash produced during coal combustion
is fly ash (Electric Power Research Institute [EPRI] 1995). Typically, 65 percent 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 percent 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).
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Non -Coal -Related Operational Storage and Waste Streams
Coincident with the Ash Basin
The gypsum storage pad, which is lined and located southeast of the ash basin,
has also been identified by NCDEQ as a potential additional source area adjacent
to the ash basin. The gypsum pad is not regulated under CAMA; however,
potential effects are considered adjacent to the ash basin. Therefore, assessment
and characterization were completed and the findings incorporated into this
CAP. Results of the assessment conducted at the lined gypsum storage pad
indicate no impacts to underlying soil or groundwater as a result of gypsum
storage and operation. Therefore, the gypsum storage pad is not being carried
forward for corrective action in this CAP Update.
Environmental incidents (i.e., releases) have occurred at MSS that initiated
notifications to NCDEQ. The historical incidents most often consisted of minor
releases of petroleum constituents near the intake canal or around the steam
station. A summary of historical on -site environmental incidents at MSS is
provided in Table 1-1. None of these incidents were near the ash basin nor had
an effect on the ash basin COI distribution in groundwater. No non -coal related
operations or environmental incidents (i.e., releases that initiated notification to
NCDEQ) are known to have occurred within the vicinity of the ash basin;
therefore, no environmental incidents at MSS are relevant to this CAP 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 / Special Order by Consent
Duke Energy is authorized to discharge wastewater from the MSS ash basin to
Lake Norman (Outfall 002) in accordance with National Pollutant Discharge
Elimination System (NPDES) Permit NC0004987, which was renewed by
NCDEQ on May 1, 2018. The sources of wastewater 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:
• Outfall 001: Once -through cooling water and intake screen backwash.
• Outfall 002: Treated wastewater from the ash settling basin (consisting of
metal cleaning wastes, coal pile runoff, ash transport water, storm water,
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low volume wastes, landfill leachate, and flue gas desulfurization (FGD)
wet scrubber wastewater).
• Outfall 005: Discharge from the new lined retention basin. Basin will
accept wastes from holding basin (coal pile runoff), ash transport water,
various sumps, storm water runoff, FGD wastewater, and various low
volume wastes such as boiler blowdown, oily waste treatment, wastes/
backwash from the water treatment processes, plant area wash down
water, equipment heat exchanger water, landfill leachate, and ash
transport water.
• Outfalls 002A and 002B: Yard sump overflows.
• Outfall 007: The emergency spillway of the ash basin. The spillway is
designed for a flood greater than a 100-year event. Sampling of this
spillway is waived due to unsafe conditions associated with sampling
during an overflow event.
• Internal Outfall 001 / 001A: Yard sump (wastewater from the yard sump
2, the yard sump 3, the fly ash silo yard sump, and storm water)
discharging to the retention basin.
• Internal Outfall 003: Non -contact cooling water from the induced draft
fan control house to the intake for cooling water pumps.
• Internal Outfall 004: Treated FGD wet scrubber wastewater, and
storm water to the ash settling basin. (Note: this outfall has been abandoned
and is no longer active)
• Internal Outfall 006: Treated FGD wet scrubber wastewater to the
retention basin. During the transition period, both outfalls (004 and 006)
can be discharging.
• Internal Outfall 010 from Holding Basin: Coal pile runoff, and storm
water to the retention basin.
A Special Order by Consent (SOC) was issued to Duke Energy on April 18, 2018,
to address the elimination of seeps from Duke Energy's ash basins during the
separate and independent process of ash basin closure. 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 is now underway and is
expected to substantially reduce or eliminate discharge from the seeps.
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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. Basin decanting at
MSS via gravity flow began on July 16, 2019 with the removal of stop logs from
the outlet structure. Mechanical decanting (pumping) began on September 13,
2019. Since the commencement of decanting, as of December 1, 2019, 128.4
million gallons of water have been removed from the ash basin and the elevation
of the ponded water within the basin has decreased by 7.3 feet. The SOC requires
completion of decanting by March 31, 2021.
Permitted Solid Waste Facilities
There are three solid waste permits associated with MSS:
Permit 1804-INDUS-1983, which includes:
• Dry Ash Landfill (Phase I)
• Dry Ash Landfill (Phase II)
• Construction & Demolition (C&D) Landfill
• Asbestos Landfill
2. Permit 1812-INDUS-2008 (Industrial Landfill No. 1)
3. Permit 1809-INDUS- [Flue Gas Desulfurization (FGD) Residue Landfill]
The double -lined Industrial Landfill No. 1 (ILF) is located north and upgradient
of the ash basin. The C&D, Asbestos, and unlined Dry Ash Landfill Phase II are
proximal to each other, adjacent to the northern portion of the ash basin. The
unlined Dry Ash Landfill Phase I is located immediately east and downgradient
of the ash basin. The closed FGD Residue Landfill is located upgradient and west
of the southern portion of the ash basin (Figure 1-1). The FGD Landfill was
constructed with an engineered, single -liner system and was capped with a 40-
mil linear low -density polyethylene (LLDPE) geomembrane, geocomposite
drainage layer, and two feet of final cover soil. The closed Dry Ash Landfills
(Phase I and Phase II), constructed of fly ash generated from MSS, are located
within the ash basin groundwater drainage system and are addressed as part of
this CAP Update.
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Additional Permits
In addition to NPDES wastewater discharge permit NC0004987 and solid waste
permits (as mentioned above), the facility also holds air permit #03676T57, and a
hazardous waste permit NCD043678879 as a Resource Conservation and
Recovery Act (RCRA) small quantity generator.
The facility is subject to federal NPDES storm water discharge permit
requirements per 40 Code of Federal Regulations (CFR) §122.26 (b)(14)(vii). MSS
received a separate NPDES industrial storm water discharge permit
(NCS000548), effective May 15, 2015, from the North Carolina Division of
Energy, Mineral, and Land Resources Storm Water Permitting Program (SPP).
The facility discharges to Lake Norman, a class WS-IV, B, CA water in the
Catawba River Basin. Storm water discharges are subject to the monitoring
requirements specified in Permit No. NCS000548.
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 IN SUPPORT OF CAP
DEVELOPMENT
(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 August 17, 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 submittal (Appendix A).
2.2 Duke Energy's Response to NCDEQ Letter
(CAP Content Section 2.B and 2.B.a)
Responses to all NCDEQ comments within the August 17, 2018 letter are summarized
in Appendix B. Duke Energy received additional, informal comments to the CSA
Update Report from the NCDEQ Mooresville Regional Office (MRO) which are also
addressed in Appendix B. Additional content related to NCDEQ's comments is either
included within section of the CAP Update or as standalone appendices to this CAP
Update, such as the groundwater modeling report and surface water evaluation report.
Activities that directly addressed NCDEQ comments include:
• Groundwater samples continued to be collected on a quarterly basis as part of
the MSS Interim Monitoring Plan (IMP). Additional sampling results augmented
the groundwater quality database. Comprehensive groundwater analytical data
are included in Appendix C, Table 1.
• Since the CSA Update submittal, additional assessments have been completed
including additional well installations, pumping tests, bedrock evaluation
(including geophysical borehole surveys), groundwater to surface water
interaction, soil sampling, slug testing, geochemical modeling and associated
sampling, and groundwater flow and transport modeling. The results of these
assessments have been used to provide additional supporting information for
this CAP Update. The assessment reports have either been previously submitted
to NCDEQ or are attached as appendices to this report.
• Characterization of fractured bedrock based on additional evaluation of
lineaments, the bedrock fracture system, and the bedrock matrix was conducted.
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A report summarizing the evaluation and implications for bedrock groundwater
flow and transport is included in Appendix F.
• Additional assessment of Lake Norman surface water and sediment was
performed in August 2018. There were no constituent concentrations greater
than 02B surface water standards attributable to the groundwater plume(s). 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 Lake Norman from areas potentially affected by
the ash basin and adjacent source areas are not predicted to cause COI
concentrations in surface water greater than 02B surface water standards. The
evaluation is presented in Appendix J.
• Background values for soil and groundwater were updated. Information about
background determinations is presented in Section 4.0.
• The MSS flow and transport model and geochemical model were updated to
incorporate additional assessment data and information. The models were used
to evaluate current and predicted future Site conditions. The flow and transport
model report is provided as Appendix G. The geochemical model report is
provided as Appendix H.
• The MSS 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 MSS ash basin is the only CAMA-regulated unit at the Site. Additional primary
sources located within or adjacent to the ash basin and considered in this CAP include:
• Closed Dry Ash Landfills (Phase I and Phase II)
• Photovoltaic (PV) structural fill
• Structural fill beneath Industrial Landfill No. 1
• Access road structural fill
• Coal pile
• Gypsum pad
CAMA defines CCR surface impoundments as topographic depressions, excavations, or
diked areas formed primarily of earthen materials, without a base liner, and that meet
other criteria related to design, usage, and ownership (G.S. Section 130A-309.201).
The CCR surface impoundment (ash basin) at MSS and the adjacent sources are the
focus of this CAP Update. A certification that 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). A summary 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 Table 3-1 (CAP
Content Section 3.B).
Results of the assessment conducted at the lined gypsum storage pad indicate no
impacts to underlying soil or groundwater as a result of gypsum storage and operation.
Therefore, the gypsum storage pad is not being carried forward for corrective action in
this CAP Update.
The closed Dry Ash Landfills (Phase I and Phase II) are under NCDEQ DWM
regulatory oversight and are monitored on a semiannual basis. The PV Structural Fill is
inspected on a yearly basis by NCDEQ DWM. Groundwater sampling data indicate
constituents similar to COIs identified from CAMA groundwater monitoring of the ash
basin (e.g., boron) are present in groundwater beneath and within a limited horizontal
extent of the landfill and structural fill footprints. Duke Energy is proposing to excavate
the Dry Ash Landfill Phase I (INDUS-1804). Excavation of the Dry Ash Landfill Phase I
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will remove the source and reduce additional migration of COIs east of the ash basin
toward the unnamed tributary. The Dry Ash Landfill Phase II (INDUS-1804) and the PV
Structural Fill are proposed for additional closure measures including installation of a
geosynthetic liner and cover system. Installation of an impermeable cover system on the
Dry Ash Landfill Phase II and PV Structural Fill will prevent infiltration of precipitation
through these sources and reduce COI leaching potential to underlying groundwater.
These are source control measures that will assist groundwater corrective action
downgradient of these facilities.
The additional primary sources listed above generally lie within the ash basin
compliance boundary. A very limited portion of the southwest corner of the PV
Structural Fill lies beyond the ash basin compliance boundary. A combination of
historical groundwater data (MW-12S/D) and additional wells installed in 2019 (PVSF-4
cluster) confirm there are no COI concentrations greater than 02L, IMAC, or
background, whichever is greatest (Appendix C, Table 1); therefore, MNA is proposed
as a viable remedial alternative between the compliance boundary and Duke Energy
property boundary (see Appendix I). Similarly, coal pile and gypsum pad assessment
results indicate no impacts from these identified sources to underlying soil or
groundwater at or beyond the ash basin compliance boundary. Groundwater flow from
beneath any of these features is predicted to flow within the flow fields of the ash basin.
Therefore, any corrective actions identified for the ash basin compliance boundary
would also address COIs potentially related to the facilities identified above.
Groundwater flow is not predicted to migrate to the north and west beyond the
compliance boundary in the future. The corrective action approach for the ash basin
and adjacent source areas is discussed in detail in Section 6.5.
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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 central North
Carolina. The metals and inorganic constituents 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
• Evaluation of concentration patterns (i.e., do the patterns represent a discernable
plume or migration pattern?)
• Consideration of natural variations in site geology or geochemical conditions
between upgradient (background locations) and downgradient areas
• Consideration of other constituents present at concentrations greater than
background.
The MSS and nine other Duke Energy facilities (Allen Steam Station, Belews Creek
Stream Station, Buck Steam Station, Cape Fear Steam Electric Plant, Cliffside Steam
Station, Dan River 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 an
approximate 150-mile radius from MSS. 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 Site coal ash basin or adjacent source areas. Background
locations for all media, including groundwater, surface water, soil, and sediments are
illustrated in Figure 4-1 (CAP Content Section 4.A). Tables referenced in this section
present background datasets for each media, statistically calculated background
threshold values (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 Section 4.3 and 4.4.
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 background values is included in Appendix C,
Table 4 (CAP Content Section 4.B). Background soils samples were collected from
multiple unsaturated depth intervals that were greater than one foot above the seasonal
high water table elevation. The MSS background soil boring locations, unsaturated soil
depth interval and number of discrete samples collected from the unsaturated soil
depth interval are provided in Table 4-1.
The suitability of each of these locations for evaluating background conditions was
addressed in a technical memorandum (May 26, 2017). Soil data appropriate for
inclusion in the statistical analysis to determine background values was approved by
NCDEQ in a response letter dated July 7, 2017. Additional soil samples were collected
from background soil borings in August 2017 to satisfy the minimum number of soil
samples for statistical calculation of BTVs as required by NCDEQ in a letter dated April
28, 2017.
Soil background values related to COIs at MSS were calculated using unsaturated
background soil data collected from May 2015 to March 2017 and submitted to NCDEQ
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in the Comprehensive Site Assessment Update — Marshall Steam Station, dated January 31,
2018. NCDEQ DWR provided comments and approval of BTVs in a response letter
dated June 15, 2018 (Appendix A). 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).
Soil BTVs were updated in 2019 and are provided, along with the previously approved
soil BTVs and North Carolina Piedmont soil background ranges for comparison, in
Table 4-2 (CAP Content Section 4.B). The updated 2019 BTVs were calculated using data
from approved background unsaturated soil samples collected June 2015 to April 2017,
however the 2019 dataset retained extreme outlier concentrations when data validation
and geochemical analysis of background groundwater concentrations indicated that
those previously identified 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, Cliftside, Marshall, Mayo, and Roxboro Sites, ", which was
provided as an attachment to the Updated Background Threshold Values for Constituent
Concentrations in Groundwater (SynTerra, 2019e). 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 aquifer system: the shallow flow layer, deep (transition
zone) flow layer, and the bedrock flow layer. Background groundwater monitoring
wells installed within each flow zone include:
• Shallow flow zone: BG-1S, BG-2S, BG-3S, GWA-4S, GWA-5S, GWA-6S, GWA-8S,
GWA-125, MS-10, MW-4
• Deep flow zone: BG-1D, GWA-4D, GWA-5D
• Bedrock flow zone: BG-1BRA, BG-2BR, BG-3D, BG-3BR, GWA-6D, GWA-8D,
GWA-12BR, MW-4D
The locations of the background monitoring wells are shown on Figure 4-1. The
suitability of each of these locations for background purposes was evaluated in the
Updated Background Threshold Values for Groundwater technical memorandum (May 26,
2017). Groundwater data appropriate for inclusion in the statistical analysis to
determine BTVs was approved by NCDEQ in a response letter dated July 7, 2017.
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NCDEQ DWR provided further comments and approval of BTVs in a response letter
dated October 11, 2017, provided in Appendix A.
Groundwater BTVs in each flow zone at MSS were updated in 2019 and are provided,
along with the original groundwater BTVs for comparison, in Table 4-3. The updated
BTVs were calculated using concentration data from background groundwater samples
collected from 2010 (beginning of compliance monitoring) to December 2018 (SynTerra,
2019e). 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). No additional background groundwater
monitoring wells have been added to the monitoring well network. Three wells (BG-3D,
GWA-6D, GWA-8D) were historically included in the background dataset for the deep
flow layer. However, after a thorough review of monitoring well construction logs, it
was determined that these three wells are screened within the bedrock flow zone.
Therefore, these wells were included in the bedrock flow zone background dataset. The
updated background datasets for each flow zone 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, 2019e) 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 background datasets contained more than the required minimum of 10
valid sample data (Appendix C, Table 1) (CAP Content Section 4.C).
Both sets of BTVs from 2018 and 2019, in addition to ranges of background
concentrations collected at similar sites in the Piedmont hydrogeological province, are
used for understanding natural background conditions at the Site and are provided for
comparative purposes in Table 4-3 (CAP Content Section 4.0 and 5.A.a.vii).
4.3 Background Concentrations for Surface Water
The Site is located in the Catawba River watershed along the western shoreline of Lake
Norman in Catawba County. The ash basin designated effluent outfall is approximately
100 feet downgradient from the base of the ash basin dam where it discharges to Lake
Norman (NPDES Outfall 002).
Background surface water sample locations for MSS 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 from the source area as determined by groundwater predictive
modeling results.
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Background surface water sample locations include two streams upgradient of the ash
basin on Duke Energy property, and two locations in Lake Norman east and upstream
of MSS. Background surface water sample locations are shown on Figure 4-1. Locations
are summarized as follows based on surface water body and spatial distribution relative
to the source area:
• Lake Norman sample locations upstream of potential groundwater migration to
surface water from the ash basin area: SW-105, SW-106
• Minor streams upgradient of the ash basin, northwest of potential groundwater
migration to surface water from the ash basin: SW-7, SW-8
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 data sets from each
location include data from five or more samples. 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 15A NCAC 02L .0106 (k) and (1). Analytical results from background
surface water sample locations indicate all constituent concentrations are less than 02B
standards, with the exception of dissolved oxygen at SW-105 and temperature at SW-
106. Background surface water analytical results compared with 02B and USEPA
criteria are included in Table 4-4 (CAP Content Section 4.D).
4.4 Background Concentrations for Sediment
All background sediment sample locations are co -located with background surface
water sample locations in the minor streams upgradient of the ash basin and Lake
Norman. Background sediment sample locations are located upstream, or outside
potential groundwater migration from the source areas to sediment. Sediment
background sample locations remain outside of future migration areas as determined
by groundwater predictive modeling.
Background sediment sample locations are shown on Figure 4-1 and include:
• Lake Norman: SW-105, SW-106
• Minor streams: SW-7, SW-8
Background sediment data are used for general comparative purposes. The analytical
results provide a comparative range of naturally occurring constituent concentrations
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present at background locations. Background data sets include one sample collected
from each location. Sediment samples were collected concurrently with a background
surface water sample. Background sediment analytical results are presented in Table 4-
5 (CAP Content Section 4.E). Analytical results for sediment samples are included in
Appendix Q Table 5 (CAP Content Section 4.E).
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5.0 CONCEPTUAL SITE MODEL
(CAP Content Section 5)
The Conceptual Site Model (CSM) 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 MSS ash basin and adjacent source areas 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 presented in this section is based on an U.S. Environmental Protection Agency
(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 in the
decision -making process for characterization and remediation during the life cycle of a
project as new data becomes 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 MSS 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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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 basin and adjacent source areas is divided
into the following three layers 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, regolith, and saprolite. Fill
material, used in the construction of the ash basin dam, generally
consisted of reworked silts, clays, and sands. The range of fill thickness
observed in the ash basin main dam was 18 feet to 65 feet. 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 relict foliation).
Saprolite consists primarily of medium dense to very dense silty sand,
sandy silt, sand, sand with gravel, sand with clay, clay with sand, and
clay. Sand particle size ranges from fine- to coarse -grained. The thickness
of saprolite/weathered rock observed was in the range of less than 10 feet
to more than 80 feet. Shallow flow layer wells are typically labeled with an
/IS// or "DU" 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 significantly weathered, fractured bedrock
encountered below the shallow zone. The deep flow layer at the Site is
varied in thickness and depth. Observations of core recovered from this
zone included rock fragments, unconsolidated material, and highly
oxidized bedrock material. Some "D" wells were completed in fractured
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bedrock, and were re-classified as bedrock wells for data evaluation, as
documented in the 2018 CAMA Annual Interim Monitoring Report
(SynTerra, 2019d). Deep flow layer wells are typically labeled with a "D"
designation.
Bedrock flow zone — Based on sample recovery, bedrock is defined as
sound rock that is generally slightly weathered to unweathered and
fractured to varying degrees. The primary rock types in the immediate
vicinity of the ash basin are several varieties of gneiss, granite, and schist.
Groundwater movement in the bedrock flow zone occurs in secondary
porosity represented by fractures. The majority of water -producing
fracture zones are found within the top 50 feet of competent rock. Water -
bearing fractures in bedrock are often only mildly productive. The
bedrock hydraulic conductivity and overall volumetric rate of
groundwater flow at the Site also decreases 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. In areas where a preferential
fracture set exists, groundwater flow is anisotropic and occurs
preferentially parallel to the predominant strike of bedrock fractures.
Bedrock wells are typically labeled with a 'BR", 'BRL", "BRLL", or
"BRLLL" designation. A detailed evaluation of bedrock conditions is
presented 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 MSS is
consistent with the regolith-fractured
rock system and is characterized as an
unconfined, interconnected aquifer
system indicative of the Piedmont
Physiographic Province.
A conceptual model of groundwater
flow in the Piedmont, which is
applicable to the MSS Site, was
developed by LeGrand (1988, 1989) and
Harned and Daniel (1992) (Figure 5-2).
The model assumes a regolith and
FIGURE 5-2
LEGRAND SLOPE
AQUIFER SYSTEM
BEEF srslF„[
_.i
S"e "A.qLei, B... d-y and TVS,.ploe D-&
------- Dvch-gc Bm Wy
- - - - - - - Campar.neas (C7 So,a,d�y
............•... Water Table
Cacoatdw�er F]ow �e«n
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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 pre -decanting conditions. Groundwater is recharged by
rainfall infiltration in the upland areas followed by discharge to the perennial
stream. Flow in the 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 approximately coincide with 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 ash basin, 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)
Groundwater divides are located west and north of the Site, concurrent
with topographic ridges along Sherrills Ford Road to the west and Island
Point Road to the north. Groundwater within this flow compartment flows
toward the southeast (Lake Norman). This flow compartment contains the
MSS ash basin and the additional adjacent sources. The topographically
controlled flow direction provides natural hydraulic control of constituent
migration from the ash basin and adjacent sources within the stream valley
system, with the predominant direction of groundwater flow being from the
northwest to the southeast from the ash basin toward Lake Norman.
The ash basin was constructed within a former perennial stream valley. The
ash basin's physical setting is a flow -through system with groundwater
movement into the upgradient end, flowing laterally through the middle
regions and downward near the dam (Figure 5-3). Near the dam, vertical
hydraulic gradients, imposed by hydraulic pressure of basin surface water,
promote downward vertical gradients into the groundwater system.
Beyond the dam, groundwater flows upward. Generally, the physical
setting of the ash basin and adjacent sources within a perennial stream
valley limits the horizontal and vertical migration of constituents to areas
near and directly downgradient of the dam. The primary flow path of the
groundwater remains in the stream valley system that encompasses the ash
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basin. Therefore, adjacent source areas upgradient and side -gradient of the
basin have groundwater divides that limit groundwater flow in these
directions. Groundwater in the vicinity of the adjacent source areas beyond
the ash basin waste boundary, such as the coal pile and gypsum pad, flows
toward the former perennial stream valley that encompasses the ash basin.
Exceptions to the typical groundwater flow pathway occur at MSS in the
vicinity of the Phase I Landfill where the hydraulic head of the operational
basin induced groundwater flow to the north across a small topographic
ridge, toward an unnamed tributary. The reduction of the head in the basin
will result in the groundwater flow direction returning to the natural flow
direction toward the southeast.
FIGURE 5-3
GENERALIZED PROFILE OF ASH BASIN PRE -DECANTING FLOW
CONDITIONS IN THE PIEDMONT
PRECIPITATION
RUNOFF
ASH
-------------
FLOW
GROUNDWATER FLOW'
ENTERING BASIN
{FORMER STREAM CHANNEL}}
Note: Drawing is not to scale
Water level surface maps for each groundwater flow zone were constructed
from pre -decanting groundwater elevations obtained in May 2019 (Figures
5-4a, 5-4b and 5-4c). May 2019 water level measurements and elevations are
presented in Table 5-1. General groundwater flow directions can be
determined from the water level contours. Groundwater flow directions
developed from water -level elevations measured in the shallow, deep and
bedrock wells indicated groundwater flow at the Site is generally from
upland areas to the north and west into the ash basin flow compartment
and then outward to the east/southeast toward Lake Norman.
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Predictive flow and transport model simulations indicate that the cessation
of sluicing and subsequent decanting in the ash basin will reduce the
potential for COI transport prior to complete closure of the basin. Model
simulations predict downward migration of groundwater below the dam
east of the ash basin will be limited without the presence of ponded water
in the basin.
The following are conclusions pertaining to groundwater flow beneath the
Site:
Horizontal groundwater flow velocities in areas with free ponded
water within the ash basin are less than those seen upgradient of the
ash basin and below the ash basin dam.
• Downward vertical gradients occur just upstream of the ash basin
dam.
• Upward vertical gradients occur beyond or downstream of the dam,
which is the main groundwater discharge zone.
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 groundwater exposure pathways between the source area and the
pumping wells used for water supply in the vicinity of the Site. Domestic
water supply wells now connected to public water supply or connected to a
filtration system are outside, or upgradient of the groundwater flow system
containing the ash basin and adjacent source areas. Domestic and public
water supply wells are not affected by constituents released from the source
area or by the different closure scenarios, according to groundwater flow
and transport model simulations.
5.1.2.2 Groundwater Seepage Velocities
(CAP Content Section 5.A.a.i)
Groundwater seepage velocities were calculated for pre -decanting
conditions using horizontal hydraulic gradients determined from pre -
decanting water level measurements collected in May 2019 (Table 5-2).
Hydraulic conductivity (I) and effective porosity (n,) values were taken
from the updated flow and transport model (Appendix G). Calibrated
conductivity and porosity values for each flow zone were used to align
velocity calculations with model predictions.
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The flow and transport model provided subdivided hydraulic conductivity
zones and a calibrated hydraulic conductivity for each flow zone and model
flow layer. Simulated hydraulic conductivity values were 1.0 feet per day
(ft/day) for the shallow flow zone, 1.5 ft/day for the deep flow zone, and 0.7
ft/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 by model flow layer.
The flow and transport model uses estimated effective porosity values of
thirty percent for the shallow and deep flow zone, and one percent for the
bedrock flow zone (Appendix G).
The horizontal groundwater seepage flow velocity (v,) can be estimated
using a modified form of the Darcy Equation:
K dh
_
vs ne (dl
Using the May 2019 groundwater elevation data, the average horizontal
groundwater flow velocity in the vicinity of the ash basin is:
• 0.06 ft/day (approximately 21 ft/yr) in the shallow flow zone
• 0.09 ft/day (approximately 34 ft/yr) in the deep flow zone
• 0.86 ft/day (approximately 315 ft/yr) in the bedrock flow zone
The bedrock seepage velocities presented in Table 5-2 are approximately
one order of magnitude greater than the shallow and deep flow zone
seepage velocities, because the bedrock effective porosity value derived
from the flow and transport model (0.01) is one order of magnitude less
than the corresponding value for the shallow and deep flow zones (0.3).
More detail on fractured bedrock at MSS is provided in Appendix F.
Groundwater modeling predicts groundwater elevation changes associated
with closure activities will change localized flow velocities and result in a
more pronounced groundwater divide upgradient, north and west of the
ash basin. As of December 1, 2019, the elevation of free water in the ash
basin has decreased by 7.3 feet in response to gravity and mechanical
decanting efforts, indicating significant water level changes in the basin
have already occurred.
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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 future condition maps were created using
predicted flow fields for the closure -by -excavation and closure -in -place
scenarios. Saturated conditions in the deep zone are relatively constant
across the Site; therefore, the deep flow zone was selected for the velocity
vector maps.
• Velocity vector map for groundwater in the deep flow zone under
pre -decanting conditions - Figure 5-5a
• Velocity vector map for groundwater in the deep flow zone under
closure -by -excavation scenario - Figure 5-5b
Velocity vector map for groundwater in the deep flow zone under
closure -in -place scenario - Figure 5-5c
These depictions illustrate potential future changes in groundwater flow
compared to pre -decanting groundwater flow throughout the Site. Key
conclusions from the predictive model simulation of future ash basin
closure conditions include the following:
North of the ash basin, velocity vectors under pre -decanting
conditions (Figure 5-5a), closure -by -excavation (Figure 5-5b) and
closure -in -place conditions (Figure 5-5c) indicate groundwater
velocity is greatest upgradient (north) of the basin near the ILF (0.2 to
0.5 ft/day) and east of the Dry Ash Landfill Phase I (0.1 to 1.0 ft/day).
Groundwater flows predominately southward in the direction of the
ash basin.
Northwest of the basin, the velocity vectors under pre -decanting
conditions (Figure 5-5a), closure -by -excavation (Figure 5-5b) and
closure -in -place conditions (Figure 5-5c) indicate a groundwater flow
direction from PV Structural Fill area toward the ash basin with a
flow velocity that generally ranges from 0.01 ft/day to 0.1 ft/day, with
smaller areas of increased velocities up to 0.2 ft/day. Groundwater
flows east in the general direction of the ash basin.
• East of the basin, downgradient of the ash basin and adjacent
sources, model simulations indicate a general decrease in
groundwater velocity toward surface water receptors after decanting
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compared to pre -decanting conditions. General velocities under pre -
decanting conditions east of the basin range from 0.01 ft/day to 1.0
ft/day (Figure 5-5a); predicted velocities closure -by -excavation east
of the basin generally range from 0.01 ft/day to 0.5 ft/day (Figure 5-
5b) and closure -in -place east of the basin generally range from 0.001
ft/day to 1.0 ft/day. Groundwater east of the basin flows in the
general direction of the unnamed tributary.
Within the basin, the velocity vectors under pre -decanting conditions
(Figure 5-5a), closure -by -excavation (Figure 5-5b) and closure -in -
place conditions (Figure 5-5c) indicate that groundwater generally
flows southward and southeasterly with a flow velocities generally
ranging from 0.001 ft/day to 0.5 ft/day (pre -decanting conditions),
from 0 ft/day to 1.0 ft/day (closure -by -excavation), and 0 ft/day to 0.2
ft/day (closure -in -place).
• Velocity vectors depictions for pre -decanting and future post -closure
scenarios support that groundwater flow from the ash basin is
consistent with historic hydrology of the slope -aquifer system of
LeGrand (1988, 1989) and does not, and will not, flow in the direction
of residential areas and water supply wells to the west and north.
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 vicinities of the ash basin, PV
Structural Fill, and Dry Ash Landfills (Phase I and Phase II), coal pile and
gypsum storage pad. The water level elevations collected in May 2019 are
summarized in Table 5-1. Based on hydraulic gradient calculations using
May 2019 groundwater elevation data, the average horizontal hydraulic
gradients (measured in feet/foot) for each flow zone is: 0.02 ft/ft (shallow
flow zone), 0.02 ft/ft (deep flow zone), and 0.01 ft/ft (bedrock flow zone)
(Table 5-2). Hydraulic gradients are circum-neutral across large areas of the
ash basin 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 (Table 5-3).
Within the ash basin, a small upward vertical gradient occurred between
the ash pore water and the deep flow zone at well pair AB-12SL/-12D (-0.06
ft/ft). To the southeast, downstream (east) of the ash basin dam, an upward
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gradient between the deep and bedrock flow zones was observed at well
cluster AB-1D/-1BR (-0.12 ft/ft). This well cluster also indicated a smaller
upward gradient between AB-11) and AB-1S (-0.03 ft/ft). Additionally, the
lower bedrock wells at this well cluster (AB-1BRL/-1BRLL/-1BRLLL) are
free -flowing artesian wells. Artesian conditions, indicating upward
hydraulic gradients, were also encountered at AB-2 during the deep
bedrock evaluation (see Appendix F for more detail). These findings
support the conceptual site model, as described above, where there is
upward flow immediately downgradient of the ash basin dam. A
downward vertical gradient is expected, with support from flow and
transport modeling, to be present in the shallow, deep, and bedrock flow
zones on the upstream side of the ash basin dam.
The Dry Ash Landfill Phase I is located on a narrow topographic ridge east
of the ash basin. Under pre -decanting conditions in May 2019 the vertical
gradient in the area was generally downward. A downward vertical
gradient of 0.12 ft/ft occurred between the shallow and deep flow zones at
well pair AL-1S/-01D. In comparison, the deep and bedrock pairing at this
well cluster (AL-1D/-1BR) indicated a small upward gradient of -0.05 ft/ft.
Well cluster GWA-11S/-11D/-11BR lies to the southeast of the AL-1 cluster,
between the landfill boundary and Lake Norman. The vertical hydraulic
gradient between the shallow and deep flow zone (GWA-11S/-11D) was
downward (0.03 ft/ft). Additionally, a very slight downward hydraulic
gradient (0.01 ft/ft) was observed between the deep and bedrock flow zones
(GWA-11D/-11BR). The trends of downward groundwater flow in this area
are due to the small topographical ridge that lies between the landfill and
Lake Norman.
To the north of the ash basin is the Dry Ash Landfill Phase II. Upward
groundwater flow gradients exist at the southern side of the landfill
between the shallow and deep flow zones as well as between deep and
bedrock flow zones. The well pair AL-2S/-2D indicated a gradient of -0.03
ft/ft between the shallow and deep flow zones. AL-2D/-2BR indicated a
downward vertical gradient of -0.03 ft/ft between the deep and bedrock
flow zones. The water level elevations from this well cluster also indicated
downward groundwater flow in the lower bedrock at this location with the
gradient at AL-2BR/-2BRL being 0.07 ft/ft and 0.01 ft/ft at AL-2BRL/-2BRLL.
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At the north side of the Dry Ash Landfill Phase II, groundwater flow was
almost exclusively downward. At the northeastern corner of the landfill at
the well pair AL-3S/-3D, the vertical hydraulic gradient was found to be
0.02 ft/ft between the ash pore water and deep flow zones. Likewise, the
gradient between the deep and bedrock flow zones (AL-3D/-3BR) was 0.04
ft/ft. On the northwestern side of the landfill is the well cluster AL-4D/-
4BR/-4BRL. The gradient between the deep flow zone and bedrock (AL-4D/-
4BR) was 0.03 ft/ft downward, or neutral compared to the upward flow
observed in the bedrock between AL-04BR/-04BRL (-0.16 ft/ft). The
overwhelmingly downward flow gradient in this portion of the landfill
creates the potential for migration of constituents from the landfill into the
groundwater system with migration toward the ash basin.
The PV Structural Fill is located to the northwest corner of the MSS Site. The
groundwater gradient in this area is downward, as predicted by the CSM
due to the majority of the footprint located outside of the former stream
valleys encompassed by the ash basin. Four well clusters (PVSF-1 through
PVSF-4), installed in 2019 to evaluate this area, indicate neutral to
downward hydraulic gradients ranging from 0.00 ft/ft (PVSF-2S/-2D) to 1.19
ft/ft (PVSF-4D/-BR). Slightly upward gradients were observed at PVSF-3
between PVSF-3S/-3D (-0.02 ft/ft) and PVSF-3S/-3BR (-0.01 ft/ft), which may
limit COI migration with depth. At the request of NCDEQ, one lower
bedrock (greater than 130 feet bgs) well is being installed at the PVSF-2
cluster in December 2019. Additional information on vertical hydraulic
gradients in this area will be available at a later date.
Downgradient of the PV Structural Fill, in the vicinity of the structural fill
access road, the hydraulic gradient between ash pore water and bedrock
(AB-6S/-6D) was 0.03 ft/ft downward; the bedrock hydraulic gradient (AB-
6D/-6BRA) was also slightly downward, 0.01 ft/ft. The constituent
migration in groundwater from this area is also toward the ash basin flow
compartment (CAP Content Section 5.A.a.iii).
The coal pile is located south of the ash basin. Similar to other adjacent
source areas that lie outside of the ash basin waste boundary, downward
hydraulic gradients are predominant. Three well clusters were installed
along the perimeter of the coal pile in March 2019. Vertical hydraulic
gradients in these wells range from 0.00 ft/ft (CP-2S/-2D) to 0.01 ft/ft (CP-
1S/-1D and CP-3S/-3D). At the request of NCDEQ, one bedrock well (CP-
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1BR) was installed at the CP-1 cluster on the northeast corner of the coal pile
in November 2019. This location lies within the ash basin compliance
boundary. Additional results of this evaluation will be available in 2020;
however, the corrective actions presented herein account for potential
impacts from the coal pile.
The gypsum storage pad lies immediately west of the coal pile, also south of
the ash basin. Three well clusters were installed along the perimeter of the
gypsum pad in March 2019. Downward vertical hydraulic gradients were
calculated north of the gypsum pad (0.01 ft/ft at GP-2S/-2D). Vertical
gradients at GP-1S/-1D and GP-3S/-3D, south and southeast of the gypsum
pad, are considered neutral.
5.1.2.4 Particle Tracking Results
(CAP Content Section 5.A.a.ii)
Particle tracking is not available for Marshall.
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 basin 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
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 shallow
water table and bedrock water -bearing zones are interconnected. The
saprolite, where saturated thickness is sufficient, acts as a reservoir for
supplying groundwater to the fractures and joints in the bedrock.
Based on the orientations of lineaments and open bedrock fractures at MSS,
horizontal groundwater flow within the bedrock should occur
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approximately parallel to the hydraulic gradient, with no preferential flow
direction (Appendix F). Consistent with this interpretation, the current
groundwater flow model for MSS does not simulate plan -view anisotropy.
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 included on a 2015
topographic map by WSP USA, Inc. and presented on Figure 4-1 of the CSA
Update (CSA Update, 2018).
Based on groundwater flow direction at MSS, subsurface utilities are not
viewed as potential preferential pathways for COI migration, as Lake
Norman serves as the lower hydraulic boundary for groundwater flow from
ash basin and other potential source areas at the Site.
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.
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 chloride and boron. The magnitude of
plume retardation increases with increasing plume length, because longer
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plumes have more contact for diffusion to transfer solute mass from the
fracture to the matrix (Lipson et al 2006). 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
The bedrock beneath the MSS site is crystalline, and consists of and granite,
diorite, gneiss and schist. Solid samples of unfractured metamorphic rock
and plutonic igneous rock have low porosities - rarely larger than 2%. In
general, crystallite rock porosity is much lower than that of sedimentary
rocks. Granite has primary (i.e., matrix) porosity in the range of 0.05 to 1%
Freeze and Cherry (1979). Pankow and Cherry (1996) cite a representative
granite porosity of 0.6%. Ademeso et al (2012) reported matrix porosity
values between 0.03 and 0.16% for a variety of crystalline rocks. L6fgren
(2004) measured matrix porosity values between 0.16 and 0.48% for 75
unfractured granite, mafic volcanic, and metamorphosed granite samples.
Zhou et al (2008) reported crystalline rock matrix porosity values between
0.3 and 4.1 %.
The predominant bedrock fracture set near the ash basin at MSS strikes
northeast -southwest, consistent with the results of the lineament evaluation;
this fracture set dips to the southeast. The mean strike of open fractures at
each location was approximately N37°E. The mean dip angle of open
fractures at the logged locations was approximately 30 degrees toward the
southeast. The inferred groundwater flow direction from water level
elevations measured in wells across the site is consistent with the mean
orientations of the fractures. The most abundant secondary fracture set is
nearly horizontal. Fewer cross -cutting fractures were also observed, with
various orientations. In general, groundwater flow at the Site is interpreted
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to be anisotropic in plan view, with higher hydraulic conductivity toward
the northeast -southwest (parallel to the Lake Norman shoreline), and lower
hydraulic conductivity toward the northwest -southeast (perpendicular to
the Lake Norman shoreline) (Appendix F).
Overall, the bedrock hydraulic conductivity at the Site 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 and Neretnieks, 1985), and indicates that
the overall volumetric rate of groundwater flow in the bedrock decreases
with depth (Appendix F).
Rock core samples from bedrock locations which represent areas of affected
groundwater migration, south and southeast of the ash basin 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 0.83 percent to 5.82
percent with an average of 2.66 percent. Bulk density ranged from 2.607
grams per cubic centimeter (g/cm3) to 2.752 g/cm3 with an average of 2.696
g/cm3 (Appendix F). Petrographic evaluation classified all samples as
igneous rocks. Based on the relative abundances of minerals (quartz, alkali
feldspar, and plagioclase), the igneous rocks were classified as granodiorite,
tonalite, monzonite, and quartz monzonite. Plagioclase crystals are
extensively or locally altered into sericite/illitic clays in all of the thin section
samples. The illitic clays are present in some moldic pores and fractures
(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). The influences of
matrix diffusion and sorption are implicitly included in the groundwater
fate and transport model as a component of the constituent partition
coefficient (Ka) term used for the bedrock layers model.
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5.1.2.7 Onsite and Offsite Pumping Influences
(CAP Content Section 5.A.a.v)
Current onsite pumping within the groundwater flow system containing
the ash basin is ongoing with ash basin decanting. Mechanical decanting
(pumping) was initiated on September 13, 2019. As of December 1, 2019,
128,400,000 gallons of water have been removed from the ash basin and the
water elevation of free water within the basin has decreased by 7.3 feet.
Effects of interim actions on the groundwater system are discussed more in
Section 6.1.1.8.
Because much of the area surrounding the ash basin is comprised of
residential properties, farm land, or undeveloped land, potential offsite
pumping influences would be limited to domestic and public water supply
wells. These water supply wells are outside, or upgradient of the
groundwater flow system containing the ash basin. 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 Water Balance
(CAP Content Section 5.A.a.vi)
The ash basin and adjacent source areas are located within a single
watershed and groundwater flow system. 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 basin hydraulic conditions
prior to decanting, post -decanting and post -closure (both closure -in -place
and closure -by -excavation). The estimated approximate groundwater flow
budget in the ash basin watershed under each scenario is summarized in
Table 5-4. Each scenario water balance was developed for using the results
from flow and transport model current and predicted groundwater
simulations (Appendix G). Under each simulation, an estimated 2 gpm of
recharge was attributed to domestic septic return, and 2 gpm of discharge
was attributed to domestic water usage (pumping).
Groundwater flow and transport modeling simulations indicate
groundwater velocities in the vicinity of the ash basin will decline as the
basin is decanted and closed. The model estimates discharge to Lake
Norman downgradient of the ash basin footprint is reduced from
approximately 217 gpm during pre -decanting conditions to approximately
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117 gpm after closure. Discussion of the water balance to support these
estimates is provided below.
Pre -Decanting Conditions Water Balance
(CAP Content Section 5.A.a.vi)
Under pre -decanting conditions, the watershed area that contributes
groundwater flow toward the basin is estimated to be approximately 1,327
acres. Removing the areas that do not contribute recharge to the
groundwater system (capped or lined) and the free water surface of the ash
basin pond, the remaining area is approximately 1,161 acres.
• Groundwater recharge from the watershed area of 1,161 acres is
estimated to be 482 gallons per minute (gpm). This includes 346 gpm
of direct recharge to the watershed and 136 gpm of direct recharge to
the basin. The ash basin pond accounts for 80 gpm of recharge.
Approximately 270 gpm are removed by the modeled drains within
the ash basin (e.g., finger lakes and canals) and 75 gpm are removed
by modeled drains outside of the ash basin (e.g., streams and
ditches).
Groundwater that flows through and immediately under the dam,
and then discharges to surface water downstream of the dam, is
estimated to be 217 gpm.
Post -Decanting Conditions Water Balance
(CAP Content Section 5.A.a.vi)
A water balance was developed for the simulated groundwater system
under post -decanting conditions (Table 5-4). Groundwater recharge to the
watershed totals approximately 518 gpm. Approximately 346 gpm of
recharge occurs to the watershed outside of the ash basin and 170 gpm of
recharge occurs directly to the basin. Discharge from the watershed can be
categorized as follows: ash basin water (16 gpm); drainage inside the ash
basin (284 pgm); drainage outside the ash basin (66 gpm); flow through and
under the dam (150 gpm). The estimated groundwater discharge flow rate
to Lake Norman is reduced by 67 gpm from the pre -decanting simulations.
Post -Closure Conditions Water Balances
(CAP Content Section 5.A.a.vi)
Changes to hydraulic conditions at the Site are predicted due to decanting
and closure activities. The flow and transport model was used to evaluate
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the ash basin hydraulic conditions that would occur after decanting and
closure of the ash basin under both closure -in -place and closure -by -
excavation scenarios (Table 5-4). The estimates presented below are subject
to uncertainty related to the subsurface hydraulic conductivity distribution,
but are useful in understanding potential general hydrogeological
conditions at the Site.
Under a closure -in -place scenario, capping of the ash would prevent direct
recharge to the ash basin. Recharge would occur only to the watershed
outside of the ash basin (336 gpm).
• Capping of the basin would also preclude flow removed from the
ash basin pond.
Ditches and streams that form within the ash basin footprint are
simulated as drains. Drains also include flow removed within the
footprint of the former ponded water (pore water) behind the dam.
These features discharge a combined 158 gpm to the natural surface
water drainage network within the ash basin flow -through system.
• Drainage outside the basin accounts for 43 gpm.
• Groundwater that flows through and immediately under the dam,
and then discharges to the surface water downstream of the dam, is
estimated to be 135 gpm.
Under a closure -by -excavation scenario, ash is removed and portions of the
basin would revert to an open water pond. Recharge would occur by direct
recharge to the basin (112 gpm) and direct recharge to the watershed
outside of the ash basin (308 gpm).
• Ditches and streams that form within the former ash basin footprint
are simulated as drains. Drains also include flow removed within the
footprint of the former ponded water (pore water) behind the dam.
These features discharge a combined 127 gpm to the natural surface
water drainage network within the ash basin flow -through system.
• Drainage outside the basin accounts for 20 gpm.
• The open water pond would drain approximately 157 gpm.
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• Groundwater that flows through and immediately under the dam,
and then discharges to the surface water downstream of the dam, is
estimated to be 117 gpm.
5.1.2.9 Effects of Naturally Occurring Constituents
(CAP Content Section 5.A.a.vi1)
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 occur in soil, bedrock, groundwater, surface water, and
sediment. During the CSA assessment, samples of soil and rock were
collected during drilling activities and analyzed for metals and inorganic
constituents. Results indicate that soil and rock at the MSS contain naturally
occurring constituents that are also typically related to CCR material and
likely affect the chemistry of groundwater at the Site.
Samples of background soil indicate that naturally occurring constituents,
which are also typically related to CCR material, likely affect the chemistry
of groundwater at the Site and are present at concentrations greater than the
PSRGs POG values. Constituents with background values greater than
PSRGs POG values include arsenic, barium, chromium (total), cobalt, iron,
manganese, nickel, selenium and thallium (Table 4-2).
Samples of background groundwater indicate that naturally occurring
constituents, which are also typically related to CCR material, are naturally
present at concentrations greater than 02L standard or IMAC values.
Constituents with background values greater than regulatory criteria
include barium, chromium (total), cobalt, iron, manganese, radium (total),
and vanadium (Table 4-3). Therefore, the downgradient concentrations of
these constituents in groundwater are compared to background.
5.2 Source Area Location
(CAP Content Section 5.A.b)
The ash basin, located east of Sherrills Ford Road and to the north of the MSS, is
generally bounded by an earthen dam and a natural ridge to the northeast, Island Point
Road to the north and Highway 150 to the south, beyond the supporting station
infrastructure (Figure 1-2). Sherrills Ford Road and Island Point Road, located along
topographic ridges, represent hydrologic divides that affect groundwater flow within
an area approximately one mile west, north and northeast of the ash basin. Topography
to the east of Sherrills Ford Road generally slopes downward, across the area of the ash
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basin and adjacent source areas, towards Lake Norman to the southeast. Topography
along Island Point Road to the north and northeast of the ash basin of generally slopes
downward toward Lake Norman to the southeast.
5.3 Summary of Potential Receptors
(CAP Content Section S.A.c)
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 Notice of Regulatory
Requirements (NORR) CSA guidance (Appendix A), receptors 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 basin and adjacent source areas
indicates no measurable difference between evaluated Site -related risks and risks
imposed by background concentrations. It is determined that there is no identified
material increases in risks to human health related to the ash basin and adjacent source
areas. Additionally, multiple lines of evidence support that groundwater from the ash
basin 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 that provides
qualifying owners of surrounding properties with water supply wells within a 0.5-mile
radius of the ash compliance boundary with a permanent water solution (either water
filtration systems or connection to the municipal supply).
The site -specific risk assessment conducted for the ash basin also indicates that there is
no increase in risks to ecological receptors. The Lake Norman aquatic systems
surrounding the MSS are 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 basin is located
along the west bank of Lake Norman (former Catawba River). North Carolina
surface water classifications for Lake Norman are summarized on Table 5-6. The
surface water intake for MSS plant use is located in Lake Norman at the southern
end of the intake canal (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. Surface water information
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is provided from the Natural Resources Technical Report (NRTR) (AMEC, 2015).
In addition, NPDES-permitted outfalls and locations covered by the SOC are
shown on Figure 5-6. Non -constructed and dispositioned seep sample locations
between the ash basin and Lake Norman are managed by the SOC and are
subject to the monitoring and evaluation requirements contained in the SOC.
5.3.1.1 Environmental Assessment of Lake Norman
The National Pollutant Discharge Elimination System (NPDES) permit for
the Marshall Steam Station (NPDES No. NC0004987) requires Duke Energy
to conduct weekly to quarterly outfall and instream water quality
monitoring at 13 locations, including two locations within Lake Norman.
Trace elements (arsenic, selenium) monitoring in fish muscle tissue is also
conducted annually in accordance with a study plan approved by the
NCDEQ.
Lake Norman 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 Norman has
been an environmentally healthy and functioning ecosystem, and ongoing
sampling programs have been established to ensure the health of the 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 60 years. More information related to
environmental health assessments conducted for Lake Norman, 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
Catawba County owns the public water system serving the area around MSS but
does not operate it. The City of Hickory, through contract with Catawba County,
provides operations, maintenance, and management of the municipal water
system, and anyone connected to the system becomes a customer of the City of
Hickory. Section 6.2.2 presents 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 basin. This finding has been
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supported by field observations, a review of public records, an evaluation of
historical groundwater flow direction data and results of groundwater flow and
transport modeling (Appendix G). A total of 127 private water supply wells and
one public supply well were initially identified within the 0.5-mile radius of the
ash basin compliance boundary (Figure 5-7). Most of these water supply wells
are located north and west of the ash basin, along Sherrills Ford Road and Island
Point Road. Additional discussion, with supporting material and data, of
alternative water supply provisions (public supply or water filtration systems)
provided by Duke Energy for surrounding occupied residences and findings of
the drinking water supply well survey are included in Section 6.2.2. Figure 5-8
illustrates properties within the 0.5 mile radius of the ash basin compliance
boundary with reference to water treatment systems installed, along with vacant
parcels and residential properties whose owners have decided to either opt out
of the water treatment system program or did not respond to the offer.
5.3.4 Future Groundwater Use Area
Duke Energy owns the land and controls the use of groundwater on the land
downgradient of the ash basin area within and beyond the predicted area of
potential groundwater COI influence. Therefore, no future groundwater use
areas are anticipated downgradient of the ash basin and adjacent source areas.
Under G.S. Section 130A-309.211(cl), 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 these residences will continue to rely
on municipal water or 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, Performance Monitoring Plan (Duke Energy, 2017). 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 MSS was prepared and is
included in Appendix E. The risk assessment focuses on the potential impacts of CCR
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constituents from the MSS ash basin and adjacent source areas 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 source area; and 2) there is no evidence of risks to
ecological receptors potentially exposed to CCR constituents that may have migrated
from the source area. This risk assessment uses analytical results from groundwater,
surface water, and sediment samples collected March 2015 through June 2019.
Evaluation of risks associated with AOW locations and soil beneath the ash basin are
not subject to this assessment and will be evaluated independent from the CAP.
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, 2016c)
2. Comprehensive Site Assessment (CSA) Update (SynTerra, 2018a)
3. Human Health and Ecological Risk Assessment Summary Update for Marshall Steam
Station, Appendix B of Community Impact Analysis of Ash Basin Closure Options at
the Marshall 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, 2003; NCDEQ, 2017; and USEPA risk assessment guidance
(USEPA, 1989; 1991a; 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.
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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
values. COPCs are carried forward for further evaluation in the deterministic risk
assessment. Constituents remaining as a result of the screening were carried forward in
the baseline 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.
• Exposure to CCR constituents by current and future commercial/industrial
worker, trespasser, and residences is incomplete.
• No evidence of carcinogenic or non -carcinogenic risks was identified in relation
to the recreational swimmer, wader or boater exposure scenarios associated with
Lake Norman.
• No evidence of carcinogenic or non -carcinogenic risks was identified in relation
to the recreational fisher exposure scenario associated with Lake Norman.
• No evidence of material increase in carcinogenic risks related to the subsistence
fisher exposure scenario is attributable to the ash basin. Hexavalent chromium
concentrations in upstream surface water samples also resulted in modeled
excess lifetime cancer risk (ELCR) within USEPA's target risk range. Modeled
concentration of hexavalent chromium in fish tissue is likely overestimated.
• Potential non -carcinogenic risks from consumption of fish containing cobalt for
the subsistence fisher were identified. Cobalt concentrations in upstream surface
water samples resulted in similar modeled results. The subsistence fisher
exposure scenarios overestimate risks based on exposure model assumptions.
Findings of the baseline ecological risk assessment include the following:
• No hazard quotients (HQs) based on no observed adverse effects levels
(NOAELs) or least observed adverse effects levels (LOAELs) were greater than
unity (1) for the aquatic wildlife receptors (mallard duck, great blue heron, bald
eagle, and river otter) exposed to surface water or sediments from the Lake
Norman area.
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• Two endpoints, muskrat and killdeer had limited modeled risk results greater
than unity for aluminum. The killdeer had limited NOAEL based modeled risk
results greater than unity for barium, total chromium, copper, and selenium.
• The modeled risks are considered negligible based on natural and background
conditions. The exposure models likely overstate risks to aluminum, barium,
total chromium, copper, and selenium.
In summary, there is no evidence of unacceptable risks to human and ecological
receptors exposed to environmental media potentially affected by CCR constituents at
MSS. 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 MSS CSM presented herein describes and illustrates 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 geological/hydrogeological
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 basin and adjacent source areas were constructed within a former
perennial stream valley 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 dam. The upward flow of water into
the basin minimizes downward vertical constituent migration to groundwater
immediately underlying saturated ash in the upgradient ends of the basin. Due to the
prevailing horizontal flow within the ash basin, there is limited vertical flow of ash
basin pore water into the underlying groundwater. The elevated constituent
concentrations found in groundwater near the dam are 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 the ash basin and the pumping wells used for water supply in the
vicinity of the MSS Site based on empirical Site data from over 30 monitoring events
over 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 basin and adjacent source areas.
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Through ash basin decanting and closure, the hydraulic head and the rate of constituent
migration from the ash basin to the groundwater system will be reduced based on basin
hydrogeology described above. Either closure scenario 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 basin is expected to be further contained within the stream valley
and continue flowing south and southeast of the ash basin footprint, and therefore will
not flow towards any water supply wells.
Multiple lines of evidence have been used to develop the CSM based on the large data
set generated for MSS. The CSM provides the basis for this CAP Update developed for
the MSS ash basin and adjacent source areas to comply with G.S. Section 130A-309.211,
amended by CAMA.
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6.0 CORRECTIVE ACTION APPROACH FOR SOURCE AREA 1
(ASH BASIN AND ADJACENT SOURCE AREAS)
(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 the source area (ash basin and adjacent sources). Constituents with
concentrations above corresponding standards were evaluated to determine if the level
of concentration is present due to the source area. Constituents of interest (COI) 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 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. A detailed review of the applicable regulatory requirements under NCAC, Title
15A, Subchapter 02L and identification of areas where constituent concentrations
were greater than the applicable criteria
2. An evaluation of the potential mobility of ash basin -related constituents in
groundwater based on Site hydrogeology and geochemical conditions using
results from the geochemical model (Appendix H)
3. An analysis of constituent distribution downgradient of the ash basin under pre -
decanting and predicted future conditions
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 basin and adjacent source areas 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
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action. A detailed description of the COI management process and results are presented
in Section 6.1.3.
6.1 Extent of Constituent Distribution
This section provides an in-depth review of constituent characteristics associated with
source area 1 and the mobility, distribution and extent of constituent migration within,
at, and beyond the point of compliance. As identified in Section 3, source area 1
includes the MSS ash basin and additional primary sources located within or adjacent to
the ash basin, including:
• Closed Dry Ash Landfills (Phase I and Phase II)
• PV Structural Fill
• Structural fill beneath Industrial Landfill No. 1
• Access road structural fill
• Coal pile
• Gypsum pad
Due to the site hydrogeology as described in the CSM (Section 5), potential effects from
the above listed units to groundwater would be addressed by the groundwater
remedies proposed herein.
Results of the assessment conducted at the gypsum storage pad indicate no impacts to
underlying soil or groundwater as a result of gypsum storage and operation (Appendix
C, Table 1 and Table 4). Therefore, the gypsum storage pad is not being carried forward
for corrective action in this CAP Update.
6.1.1 Source Material Within the Waste Boundary
(CAP Content Section 6.A.a)
An overview of the material within the MSS ash basin and adjacent source areas
is presented in the following subsections. Waste boundaries are shown on Figure
1-2. Although there is no waste boundary associated with the coal pile, a
description of material within the coal pile, along with other adjacent source
areas, are 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 MSS ash basin, approximately 394 acres in size, is located north of the
station. The ash basin consists of a single cell impounded by the main
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earthen dam located on the south end of the ash basin (Figure 1-2). The
ash basin dam is an earthen embankment armored with rip rap on both
the upstream and downstream faces of the dam. The perimeter of the
basin is mostly unaltered and well -vegetated with the exception of the
dam and a small shoreline section on the east (emergency spillway) that
are armored with rip rap. The crest of the dam, which contains an access
road, is raised about 10 feet higher than the ash basin water level. A 500-
foot compliance boundary encircles the ash basin and is generally co -
located with the property boundary on the western edge of the Site and
Lake Norman shoreline on the east.
CCR materials, composed primarily of fly ash and bottom ash, were
initially deposited in the unlined ash basin via sluice lines beginning in
1965. Fly ash precipitated from flue gas and bottom ash collected in the
bottom of the boilers was sluiced to the ash basin using conveyance water
drawn from Lake Norman. In 1984, MSS converted from a wet fly ash
handling system to a dry fly ash handing system. Since 1984, fly ash has
been disposed in the on -Site landfills. Bottom ash continued to be sluiced
to the ash basin until early 2019 when the facility converted to a dry
bottom ash collection system. All CCR material is currently handled dry.
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.
The physical and chemical properties of coal ash are determined by
reactions that occur during the combustion of the coal and subsequent
cooling of the flue gas. The hydraulically sluiced deposits of coal ash within
the basin consist of interbedded fine- to coarse -grained fly ash and bottom
ash materials. Fly ash is generally characterized as a moderately dense silty
fine-grained sand or silt. Bottom ash is generally characterized as a loose,
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Corrective Action Plan Update December 2019
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poorly graded (fine- to course -grained) sand. Ash was generally described
in field observations 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 ten ash samples from the ash basin using
ASTM International methods. Compared with soil, ash exhibits a lower
specific gravity with a reported value of 2.164 (AB-71)). Moisture content of
the ash samples ranges from 19.8 percent to 86.7 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. Figure 6-1 provides a depiction of the typical
interbedded nature of fly ash and bottom ash within an ash basin, as seen
from an ash boring photograph. Layers of bottom ash are typically more
permeable than layers of fly ash due to the coarser grain size of bottom ash.
FIGURE 6-1
FLY ASH AND BOTTOM ASH INTERBEDDED DEPICTION
Particle Size Distribution Report
sots DATA
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Corrective Action Plan Update December 2019
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6.1.1.3 Volume and Physical Horizontal and Vertical
Extent of Source Material
(CAP Content Section 6.A.a.iii)
Based on topographic and bathymetric surveys, the ash basin is estimated
to contain approximately 14,033,000 cy of CCR (AECOM, 2019). Horizontal
extent of CCR is represented by the ash basin waste boundary, landfill and
structural fill waste boundaries as shown on Figure 1-2. Based on boring
logs located within the ash basin, the maximum depth of CCR within the
ash basin is estimated to be approximately 85 feet. The volume and physical
horizontal and vertical extent of ash material within the basin under pre -
decanting conditions are illustrated on a cross-section transect A -A' (Figure
6-2) along the centerline of the basin, from northwest to southeast. Volume
and physical vertical and horizontal extent of ash material in the southern
portion of the basin, and across the basin (west to east), are presented in
Figure 6-3 (B-B'), Figure 6-4 (C-C') and Figure 6-5 (D-D'). Additional details
on waste materials contained within source areas adjacent to the ash basin
are presented in Section 6.1.1.7.
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 material under pre -
decanting and post -closure (closure -in -place) conditions, within the basin is
presented on Figure 6-6.
Closure -in -place simulated potential saturated ash thickness is based on
closure model results with an underdrain system installed (Appendix G).
Under ash basin closure by closure -in -place, the range of potential saturated
ash thickness is between a few feet to 50 feet with greatest volume of
saturated ash remaining in the south central portion of the ash basin near
the dam (Figure 6-6). The horizontal extent of potential saturated ash under
post -closure conditions generally mimics, to a lesser extent, pre -decanting
conditions. The majority of potential saturated ash would remain along the
former stream channels contained within the ash basin (Figure 6-6).
However, the vertical extent of potential saturated ash would be
significantly reduced from pre -decanting conditions under a closure -in -
place scenario. Across the basin, saturated ash thicknesses would be
reduced by approximately 10 to 20 feet (Figure 6-6). Under the closure -by -
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Corrective Action Plan Update December 2019
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excavation scenario, all of the ash in the ash basin would be excavated, and
therefore, no saturated ash would remain in the ash basin footprint.
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 (under a closure -in -place scenario) will have
limited to no adverse effect on future groundwater quality. Layered ash
within the basin has resulted in relatively low vertical hydraulic
conductivity, further reducing the potential for downward flow of pore
water into underlying residual material. The CSM indicates that the
horizontal flow -through ash basin system results in low to non -detectable
COI concentrations in groundwater underlying saturated ash within the
basin except in the vicinity of 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 basin, the generalized horizontal
flow -through system is consistent with Site -specific data as summarized in
Table 6-1.
In summary, 24 of 27 monitoring wells screened beneath the ash basin
demonstrate low (< 700 µg/L) 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 (Table 6-1).
A technical memorandum, titled Saturated Ash Thickness and Underlying
Groundwater Boron Concentrations — Allen, Belews Creek, Cliffside, Marshall,
Mayo, and Roxboro Sites (Arcadis, 2019), used two statistical methods (Mann -
Kendall and linear regression trend analysis) to evaluate correlations
between groundwater boron concentrations and saturated ash thickness,
and between groundwater boron concentrations and ash pore water boron
concentrations. The linear regression analysis was conducted using
analytical data from Piedmont ash basins, including data from MSS.
The statistical evaluation was performed using a dataset which included 89
monitoring wells completed in shallow, transition, and bedrock
groundwater zones directly beneath ash basins and 54 ash pore water
monitoring wells completed in saturated ash at Piedmont sites. Linear
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Corrective Action Plan Update December 2019
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regression results indicated that 87% 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.
At Marshall, shallow groundwater boron concentrations were positively
correlated with saturated ash thickness (groundwater boron concentrations
increased with increasing saturated ash thickness). For all groundwater
zones, boron concentrations were negatively correlated with ash pore water
concentrations (groundwater boron concentrations decreased with
increasing ash pore water boron concentrations) (Arcadis, 2019). The
positive correlation between groundwater boron concentrations and
saturated ash thickness suggest that boron concentrations in groundwater
will decrease as saturated ash thickness decreases due to decanting of the
ash basin. Data demonstrate that concentrations for other, less mobile,
constituents are also low below saturated ash.
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 synthetic precipitation leaching procedures (SPLP): Appendix C,
Table 6 (CAP Content Section 6.A.a.vi.1.2)
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Corrective Action Plan Update December 2019
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• 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)
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 boundary were analyzed
for total extractable inorganics using USEPA Methods 6010/6020. For
information purposes, ash samples were compared to soil background
values and PSRG POGs. The ash analytical data do not represent soil
conditions outside of or beneath the ash basins. Concentrations of arsenic,
boron, chromium, molybdenum, and selenium in ash samples were greater
than soil BTVs and the PSRG POGs (Appendix C, Table 4).
In addition, thirteen ash samples collected from borings completed within
the ash basin and additional sources were analyzed for leachable inorganic
constituents using synthetic precipitation leaching procedures (SPLP)
USEPA 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 or IMAC comparative values
to evaluate potential source contribution; the data do not represent
groundwater conditions. The results of the SPLP analyses indicated that
concentrations of antimony, arsenic, barium, boron, chromium, cobalt, iron,
lead, manganese, nickel, selenium, thallium, and vanadium were greater
than the 02L or IMAC comparative value.
Ash Leaching Environmental Assessment Framework
(CAP Content Section 6.A.a.vi.1.3)
Ash samples were analyzed for extractable metals analysis, 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 MSS ash basin were conducted using
two LEAF tests, USEPA methods 1313 and 1316 (USEPA, 2012a, b) and
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Corrective Action Plan Update December 2019
Marshall Steam Station SynTerra
(SynTerra, 2019b). The data are presented and discussed in the Geochemical
Model Report presented in Appendix H, Attachment C.
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 MSS is considerably lower than the total boron
concentration (Appendix H, Attachment C). The MSS 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 (Boyd, 2002). 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 boundary include samples collected
from beneath the ash basin and samples collected from the fill material
within the ash basin dam. Soil samples beneath the ash basin were
saturated, including those obtained from borings associated with AB-3D,
AB-4SL, AB-5D, AB-7D, AB-81), AB-10D, AB-11D, AB-13D, AB-14D, AB-
15D, AB-16D, AB-18D, AB-20D, GWA-1S/BR, SB-1, SB-2, SB-3, SB-7, SB-10,
SB-11, SB-13, SB-14, SB-15. Temporary soil borings ("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 the MSS ash basin since soil impacts would be related
to the source area interaction to the underlying soils and groundwater,
which may migrate beyond the ash basin. The range of constituent
concentrations in saturated soils within the waste boundary, along with a
comparison with soil background values and North Carolina PSRG POG
standards (NCDEQ February 2018), whichever is greater, is provided in
Appendix C, Table 4. For constituents lacking an established target
concentration for soil remediation (e.g., chloride and sulfate), the equation
presented in Table 6-2 was used in general accordance with the references
in the NCDEQ PSRG, May 2019 to calculate a POG value.
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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). The resulting PSRG POG values
were calculated for chloride (938 milligrams per kilogram [mg/kg]) and for
sulfate (1,438 mg/kg).
Saturated soil is considered a component of the groundwater flow system.
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.
Unsaturated soil 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 soil samples within the ash basin waste boundary include
samples collected from the fill material within and near the ash basin dam
at the AB-2, CCR-5 and GWA-1 locations (Figure 1-2). Constituent
concentrations from unsaturated soil samples within the waste boundary
[AB-2S (6-7), CCR-5 (0.5-1), (2-3), GWA-1BR (8-10), (14-15.5), (18-20)] were
compared with the North Carolina PSRG POGs or background values
(Table 6-3). COI concentrations from the unsaturated soils within the waste
boundary are not greater than the PSRG POGs or background values,
whichever is greater.
The range of constituent concentrations in soils within and beyond the
waste boundary, along with a comparison with background values and the
PSRG POGs, is provided in Appendix C, Table 4. Soil SPLP constituent
concentrations within the waste boundary, along with a comparison to
02L/IMAC, for comparative purposes only, is provided in Appendix C,
Table 6.
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Corrective Action Plan Update December 2019
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Ash Pore Water
(CAP Content Section 6.A.a.vi.1.6 and 6.A.a.vi.3)
The ash basin is a permitted waste water treatment system. Water within
the ash basin is not groundwater; therefore, isoconcentration maps were not
prepared for ash pore water and comparison to 02L/IMAC/background
values is not appropriate. Ash pore water data is presented in Appendix C,
Table 1. Figures 6-7a, 6-7b, and 6-7c represent ash pore water distribution in
cross section (A -A') from northwest to southeast across the ash basin.
Means of ash pore water concentrations are provided for general purposes
only. 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.
Two groundwater monitoring wells located in areas that could be sensitive
to changing Site 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
illustrated on Figure 6-8, and include:
MW-7S: shallow flow zone monitoring well located east of the dam,
between the ash basin and Lake Norman
CCR-13S: shallow flow zone monitoring well located between the ash
basin and Dry Ash Landfill Phase I
17 shallow groundwater monitoring wells and six isolated surface water
bodies within the basin were fitted with pressure transducers to monitor
water level changes before, during and after decanting of the ash basin.
AB-10S
AB-10SL
AB-12S
AB-12SL
AB-21 S
AB-2S
A134S
AB-4SL
AL-1S
CCR-9S
CCR-11S
CCR-12S
CCR-14S
AB-3S
AB-3D
AB-5S
AB-8S
AB-9S
SG-1
SG-2
SG-3
SG-4
SG-5
SG-6
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Corrective Action Plan Update December 2019
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It should be noted that ash pore water monitoring well AB-3S has been
observed to be dry as a result of basin decanting; therefore, the pressure
transducer was decontaminated and relocated to the bedrock flow zone
well at the same location, AB-31). Two of the ponded water monitoring
points installed in isolated bodies of free water within the southwestern
portion of the ash basin (SG-2 and SG-3) have been removed due to ash
basin operations (Figure 6-8). Geochemical water quality and hydrograph
time series plots for each location are included on Figures 6-9 and 6-10a
through 6-10d. Observations of water elevation and multi -parameter
records from monitored locations include:
• Ash pore water, shallow, and deep flow zone monitoring wells
within the waste boundary show a response to ash basin decanting
by reduced water elevation levels (Figure 6-10a through 6-10c).
Monitoring locations at areas of ponded water within the ash basin
indicate a response to decanting by reduced water elevations (Figure
6-10d).
Geochemical parameters located within the waste boundary (CCR-
13S and MW-7S) show very slight variability in records since ash
basin decanting commenced (Figure 6-9). This suggests geochemical
conditions have remained stable under changing conditions at
locations within the waste boundary.
In general, ash pore water and groundwater geochemical parameters
appear stable under changing site conditions. Ash pore water pH and Eh 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,
groundwater pH and Eh, 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 of ash pore water monitoring data and groundwater
monitoring data from shallow, deep and bedrock background locations and
locations downgradient and adjacent to the ash basin are presented on
Figure 6-11. Data used for the Piper diagrams include ash pore water data
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Corrective Action Plan Update December 2019
Marshall Steam Station SynTerra
and groundwater data between January 2018 and May 2019 with charge
balance errors less than 10 percent. Data were excluded from inclusion in
the Piper diagrams if pH values were greater than 8.5 S.U. and turbidity
values greater than 10 Nephelometric turbidity units (NTUs).
Ash pore water results tend to plot with higher proportions of sulfate,
chloride, calcium, and magnesium, which is generally characteristic of ash
pore water (EPRI, 2006). At MSS, ash pore water samples generally follow
this generalization (Figure 6-11).
6.1.1.7 Other Potential Source Material
(CAP Content Section 6.A.a.vii)
Two unlined ash landfill units, referred to as the Dry Ash Landfills
(NCDEQ Division of Solid Waste Permit No. 1804-INDUS), are located
adjacent to the east (Phase I) and northeast (Phase II) portions of the ash
basin (Figure 1-2). Phase I contains approximately 522,000 cy of fly ash,
which was placed from September 1984 through March 1986. Placement of
ash in the Phase II areas began around March 1986 and was completed in
1999. Phase II contains approximately 4,064,000 cy of fly ash. The landfill
units are unlined and were closed with a soil cover system.
The PV Structural Fill was constructed of fly ash, under the structural fill
rules found in 15A NCAC 13B .1700 et seq., and bottom ash, under Duke
Energy's Distribution of Residuals Solids (503 Exempt) Permit Number
WQ0000452, which was issued by NCDENR Division of Water Quality
(DWQ), and is located adjacent to and partially on top of the northwest
portion of the ash basin (Figure 1-2). The PV Structural Fill, used for
renewable energy research and production, contains a solar panel field on
the southern portion of the structural fill unit. Placement of dry ash in the
structural fill began in October 2000. The PV Structural Fill covers
approximately 83 acres and contains approximately 5,410,000 cy of ash. The
structural fill is unlined and was closed with a soil cover system in February
2013.
The access road structural fill, adjacent to the ash basin waste boundary,
and south of the PV Structural Fill, was constructed of fly ash under the
structural fill rules found in 15A NCAC 13B .1700 et seq (Figure 1-2). The
access road structural fill covers approximately 2.5 acres and contains
approximately 127,982 cubic yards of ash. Construction of the unlined
structural fill road began in 1997 and was completed in 1998.
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Corrective Action Plan Update December 2019
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The subgrade for portions of the ILF No. 1 was constructed of fly ash under
the structural fill rules found in 15A NCAC 13B .1700 et seq. The subgrade
structural fill, which contains approximately 726,000 cy of ash, was closed
with a soil cover in 2013. The ILF No. 1 was constructed over portions of
this unlined structural fill and the northern reach of the ash basin (Figure 1-
2).
Duke Energy will be addressing additional primary sources, including the
Dry Ash Landfill Phase I and Phase II (INDUS-1804) and the PV Structural
Fill, with NDCEQ DWM in separate submittals. The Dry Ash Landfill
Phase I (INDUS-1804) is proposed to be excavated and the Dry Ash Landfill
Phase II (INDUS-1804) and PV Structural Fill are proposed for additional
closure measures including installation of a geosynthetic liner and cover
system. Excavation of the Dry Ash Landfill Phase I will remove the source
and reduce potential additional migration of COIs from the facility.
Installation of an impermeable cover system at the PV Structural Fill and
Dry Ash Landfill Phase II will prevent infiltration of precipitation through
these sources and reduce COI leaching potential to underlying
groundwater.
As a further source control measure, Duke Energy proposes to excavate the
Dry Ash Landfill Phase I due to the unique hydrogeologic setting and close
proximity to surface water receptors. The land space could provide
additional room for groundwater remediation infrastructure or corrective
action plan modification, if deemed necessary, without interfering with ash
basin closure or site operations. Vertical migration of COIs beneath and
downgradient of the Dry Ash Landfill Phase I is not limited or intercepted
by the flow -through ash basin system, as described in the updated CSM.
Excavation of the Dry Ash Landfill Phase I will remove the source and
reduce additional migration of COIs. Decanting of the ash basin will also
significantly reduce the hydraulic gradients within the basin and COI
migration potential.
In an April 5, 2019, letter to Duke Energy, NCDEQ listed and requested
assessment of additional potential sources of constituents to groundwater at
Marshall stating that sources hydrologically connected to the ash basin are
to be assessed and included in an updated CAP. The coal pile and gypsum
storage pad areas were included as additional sources hydrologically
connected to the ash basin.
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Corrective Action Plan Update December 2019
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The gypsum storage pad, which contains gypsum that is generated as a
byproduct of generating coal, is approximately 4 acres and has a synthetic
liner installed beneath the concrete pad. Gypsum is sold for beneficial reuse
off -Site. Results of the assessment conducted at the lined gypsum storage
pad indicate no impacts to underlying soil or groundwater as a result of
gypsum storage and operation. Therefore, the gypsum storage pad is not
being carried forward for corrective action in this CAP Update.
Coal stored on -Site is a not a waste product and therefore, is not regulated
under North Carolina General Statutes (G.S.), as amended by CAMA.
Therefore, no compliance or waste boundaries are associated with the coal
pile. Coal has arrived at MSS through rail transportation since operations
began. Coal is, and has historically been, stored at the Site's unlined coal
pile located immediately north of the powerhouse and south/adjacent to the
ash basin, on approximately 35 acres (Figure 1-2). Coal is conveyed via
transfer belts to the station where it is pulverized before being used in the
boilers.
To improve storm water management in the area of the coal pile, lined
holding basins were built in 2018 west and east of the coal pile. These
retention basins receive coal pile storm water runoff collected from the coal
pile through a concrete -lined perimeter ditch and associated collection
trench. Construction of the retention basins was associated with water
redirect efforts to reroute storm water flows from the ash basin to the new
Lined Retention Basins installed in 2018 (Figure 1-2). The reroute of storm
water flows was completed to assist in ash basin closure.
The coal pile is 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. Chemical reactions occur at coal piles when water and
oxygen is introduced to pyrite commonly found in coal. The chemical
reaction typically results in iron and sulfate in solution, which is consistent
with the values seen in the northern corner of the coal pile (CP-1S/D).
Sulfate and low pH are potential indicator constituents of coal pile impact
(EPRI, 2019). Low pH (average 4.0 S.U.) and elevated sulfate (350 mg/L) are
present in shallow groundwater off the northern corner of the coal pile (CP-
1S). These results also coincide with elevated concentrations of beryllium,
cobalt, iron, manganese, sulfate, TDS, and thallium in CP-1S. In the deep
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Corrective Action Plan Update December 2019
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groundwater flow zone at CP-11), concentrations of cadmium, cobalt,
manganese, sulfate, and TDS are consistently greater than applicable
comparative criteria. Although this location is within the ash basin
compliance boundary, one additional bedrock well (CP-1BR) was installed
in 2019 to delineate these COIs off the northern corner of the coal pile. These
COIs are delineated horizontally downgradient by the CCR-4 and CCR-5
well clusters off the buttress of the ash basin dam. Furthermore,
groundwater remedies presented herein account for any potential impacts
as a result of coal handling and storage at the coal pile.
6.1.1.8 Interim Response Actions
(CAP Content Section 6.A.a.viii)
Interim response actions performed to date include active decanting of the
ash basin, provision of permanent water supplies to qualifying households,
and stabilization of the ash basin dam, as summarized on Table 6-4.
Ash Basin Decanting
(CAP Content Section 6.A.a.viii.1)
Ash basin decanting via gravity flow commenced on July 16, 2019,
mechanical decanting commenced on September 13, 2019. Decanting is a
form of active source remediation by removing ponded water in the ash
basin, 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 mechanical decanting, the elevation of ponded water in the ash
basin was 789 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.
Water elevations were monitored using pressure transducers to record
changing site conditions from ash basin decanting at the following locations
(Figure 6-8):
• 18 groundwater monitoring wells located within and around the
basins (17 currently active due to dry conditions at AB-3S).
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Corrective Action Plan Update December 2019
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• 6 locations within ponded water located in the ash basin (4 currently
active due to site operations).
Ponded water and groundwater decanting network hydrographs, using
water elevations recorded between May 2019 (sondes deployed in June,
2019) through October 2019 are depicted on Figures 6-10a through 6-10d.
Observations from hydrographs include:
• As of December 1, 2019, the water level in the free water in the ash
basin has decreased by approximately 7.3 feet since the
commencement of decanting (Figure 6-10d). Note the water
elevations displayed on Figure 6-10d are not current to December 1,
2019.
• In September 2019, pressure transducer located in AB-3S was
relocated to AB-31) due to dry conditions present in AB-3S.
Isolated areas of ponded water within the ash basin have decreased
on average by approximately one to three feet (Figure 6-10d). In SG-2
and SG-3, an increase in water level was observed due to site
operations at associated ponded water. These monitoring points
were subsequently removed due to site operations.
• All groundwater monitoring locations show a response to ash basin
decanting through a reduction in shallow groundwater elevations
(Figures 6-10a through 6-10c).
Source Area Stabilization
(CAP Content Section 6.A.a.viii.2)
In December 2015, NCDEQ issued a draft risk classification for the MSS ash
basin as "intermediate," requiring closure by December 31, 2024. Duke
Energy subsequently made the required improvements to the dam pursuant
to G.S. Section 130A-309.213(d)(1), including repairs/improvements to the
overflow spillway. Improvements specifically consisted of hard armoring of
the downstream slope of the dam and emergency spillway with concrete
and/or riprap, and replacement of the principal spillway structure. NCDEQ
provided correspondence, dated November 13, 2018, to confirm that Duke
Energy rectified prior dam safety deficiencies, reclassifying the ash basin
from its prior draft ranking of "intermediate" to "low -risk" (Appendix A).
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6.1.2 Extent of Constituent Migration beyond the Compliance
Boundary
(CAP Content Section 6.A.b)
This section is an overview of constituent occurrences beyond the ash basin
compliance boundary. 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 Dry Ash Landfills Phase I and
Phase II and ILF also have compliance boundaries approximately 250 feet from
the landfill waste boundaries. The ash basin compliance boundary and landfill
compliance boundary overlap in areas north (ILF) and east (Dry Ash Landfill
Phase I) of the ash basin waste boundary (Figure 1-2). Groundwater constituent
migration from the ash basin and landfills, along with the other adjacent source
areas outlined in Section 3, is comingled and indiscernible.
Analytical sampling results associated with the ash basin and adjacent source
areas for each media are included in the following tables and appendices:
• 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 (CAP Content Section 6.A.b.ii.3)
• 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 concentrations are generally consistent with
background concentrations or are less than PSRG POG or background,
whichever is greater (Table 6-3). In the few exceptions, these soil concentrations
are:
• generally within range of Piedmont background dataset concentrations
(Table 4-2),
• delineated vertically by groundwater constituent concentrations less than
applicable regulatory criteria in the corresponding monitoring well,
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indicating the soil concentrations are not a secondary source of
constituents to the groundwater, and/or
• lacking transport mechanisms by which the constituent could have
migrated from the source area to the unsaturated soils.
Near the ash basin compliance boundary, east of the Dry Ash Landfill Phase I,
concentrations of arsenic and selenium were detected slightly greater than the
PSRG POG and background values (Table 6-3). However, these constituents are
not present in groundwater at the same location greater than applicable
regulatory criteria (Appendix C, Table 1); therefore, these soil concentrations do
not warrant consideration as potential secondary source of constituents to the
groundwater. For these reasons, no soil COIs were identified for the MSS and no
soil concentrations are identified for corrective action at the Site.
Horizontal and vertical extent of constituent 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 east of the ash basin
towards the unnamed tributary and the cove of Lake Norman.
The maximum extent of CCR-affected groundwater migration for all flow zones
is generally represented by boron concentration greater than the 02L standard.
Boron has migrated east of the ash basin towards the tributary and Lake
Norman, at or beyond the compliance boundary. There is very limited land area
between the ash basin compliance boundary and Lake Norman. These surface
waters are groundwater discharge zones that limit the horizontal transport of
constituents downgradient of the basin. However, constituent concentrations in
groundwater have not caused, and will not cause, current surface water quality
standards to be exceeded (Appendix J). Other areas of constituent migration,
beyond the 02L boron plume, occur along the southern portion of the ash basin
dam and consist of variably reactive constituents (e.g., cobalt).
Section 6.1.3 includes a detailed matrix evaluation and rationale of groundwater
constituents requiring corrective action, and Section 6.1.4 provides
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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 MSS 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-1, S-2, S-4
• Non -constructed seeps dispositioned — S-3
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.
Non-dispositioned seeps, where monitoring conducted has indicated the
presence of CCR effects, include: S-1 and S-2 (Figure 5-6). Seeps at MSS are
contained within well-defined channels. Therefore, potential constituent
migration related to seep flow are constrained in localized areas along the
channel. Dry conditions have been consistently observed at seep S-2 and S-4 in
2019, likely a result of ash basin decanting. Surface water sampling conducted
downstream of non-dispositioned seep S-1, in Lake Norman, demonstrates that
flow from S-1 has not caused constituent concentrations greater than 02B
standards in the reservoir. Analytical results for these samples are included in
Appendix C, Table 2 and Table 3.
Surface Water Constituent Extent
(CAP Content Section 6.A.b.ii.4)
Surface water samples were collected from Lake Norman to confirm
groundwater downgradient of the ash basin has not resulted in surface water
concentrations greater than 02B water quality standards. A map of all surface
water sample locations for groundwater discharge to surface water evaluation is
included in Appendix J. Surface water samples were collected to evaluate acute
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and chronic water quality values. Surface water samples were also collected at
background locations (upstream of potential migration areas) within Lake
Norman and minor streams upgradient of the source area. Analytical results
were evaluated with respect to 02B water quality standards and background
data. Surface water conditions is further discussed in Section 6.2.1 and the full
report for the MSS surface water current conditions can be found in Appendix J.
Additionally, environmental assessments of Lake Norman have all demonstrated
that Lake Norman has been an environmentally healthy and functioning
ecosystem, and ongoing sampling programs have been established to ensure the
health of the 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 60 years. More information related to environmental
health assessments conducted for Lake Norman, 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)
All sediment sample locations are 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 constituent
concentration 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 all sediment samples are provided in
Appendix C, Table 5.
Assessment of constituents in sediment from surface waters, including Lake
Norman and seeps, was conducted through a comparison evaluation between
sediment sample analytical results, from one-time grab samples, and constituent
concentration ranges from background sediment datasets. Samples collected
from Lake Norman and associated streams were compared with background
dataset ranges from the respective surface water body.
As stated above, there are no regulatory standards established for inorganic
constituents in sediment. The surface water evaluations conducted as part of the
CAP Update (Appendix J) have concluded there are no concentrations greater
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than the 02B surface water quality standards, nor are there predicted to be under
future conditions. Additionally, the updated risk assessment, provided in
Appendix E, concludes that there is no evidence of unacceptable risks to human
and ecological receptors exposed to environmental media potentially affected by
CCR constituents at MSS. This conclusion is further supported by multiple water
quality and biological assessments conducted by Duke Energy as part of the
NDPES monitoring program.
For these reasons, concentrations of constituents in sediment at the MSS do not
warrant corrective action. Observations below are for comparative and
informative purposes only.
Sediments Collected from Lake Norman
Six sediment samples have been collected from Lake Norman. Sediment sample
locations (Figure 1-2) included:
• Upstream Areas of Lake Norman (background) — SW-105, SW-106
Downgradient Areas of Lake Norman (four locations) — SW-101, SW-102,
SW-103, SW-104 (immediate areas downgradient of affected groundwater
plume)
Of the four downgradient sediment samples collected along the bank of Lake
Norman, all four samples have constituent concentrations greater than the
maximum detected concentration in background sediment including boron,
chloride, cobalt, iron, manganese, strontium, and thallium. However, these
detections do not warrant corrective action. The surface water evaluations
conducted as part of the CAP Update (Appendix J) have concluded there are no
concentrations greater than the 02B surface water quality standards, nor are there
predicted to be under future conditions. Additionally, the updated risk
assessment, provided in Appendix E, concludes that there is no evidence of
unacceptable risks to human and ecological receptors exposed to environmental
media potentially affected by CCR constituents at MSS. This conclusion is further
supported by multiple water quality and biological assessments conducted by
Duke Energy as part of the NDPES monitoring program.
Sediments Collected from Seeps
Sediment samples have been collected from the unnamed tributary of Lake
Norman east of the ash basin and former active seep S-2. No flow has been
consistently observed at seep S-2, downgradient of the ash basin dam, since
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commencement of ash basin decanting. Sediment sample locations (Figure 1-2)
included:
• Seep S-2 - S-02
• Unnamed tributary (seep S-1) - SW-06, SW-109, SW-110
Concentrations of chromium, cobalt, iron, manganese, and selenium in sediment
at seep locations were greater than the maximum detected concentration in
background sediment. However, these detections do not warrant corrective
action. The surface water evaluations conducted as part of the CAP Update
(Appendix J) have concluded there are no concentrations greater than the 02B
surface water quality standards, nor are there predicted to be under future
conditions. Additionally, the updated risk assessment, provided in Appendix E,
concludes that there is no evidence of unacceptable risks to human and
ecological receptors exposed to environmental media potentially affected by
CCR constituents at MSS. This conclusion is further supported by multiple water
quality and biological assessments conducted by Duke Energy as part of the
NDPES monitoring program.
After completion of decanting, all seeps, constructed and non -constructed and if
not dispositioned in accordance with the SOC, are to be characterized post -
decanting for determination of seep disposition by the decanting process. 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. If a seep is dispositioned, no
corrective action for the location would be evaluated. After seep characterization,
an amendment to the CAP, may be required to address non-dispositioned seeps.
6.1.2.1 Piper Diagrams
(CAP Content Section 6.A.b.iii)
Piper diagrams can be used to graphically differentiate water sources in
hydrogeology (Domenico and Schwartz 1998) 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 background locations and locations downgradient and adjacent to
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the ash basin (in addition to ash pore water) are presented on Figure 6-11.
Data used for the Piper diagrams include groundwater and ash pore water
data between January 2018 and May 2019 with charge balance errors less
than 10 percent. Data were excluded from inclusion in the Piper diagrams if
pH values were greater than 8.5 S.U. and turbidity values greater than 10
Nephelometric turbidity units (NTUs).
The distribution of results on the Piper diagrams in Figure 6-11 indicate
groundwater in all three flow zones considered generally unaffected by the
source area contains relatively lower proportions of sulfate, chloride,
calcium, and magnesium. Unaffected groundwater trends toward
containing greater sodium and potassium content than ash pore water data,
which tend to plot with higher proportions of sulfate, chloride, calcium, and
magnesium.
Seep and Surface Water Piper Diagrams
Piper diagrams of ponded source water, seeps, and Lake Norman surface
water monitoring data (Figure 6-12) are used to assess the relative
abundance of major cations (i.e., calcium, magnesium, potassium, and
sodium) and major anions (i.e., 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, Table 2) with a charge
balance between -10 and 10%. From ash pore water and groundwater piper
diagrams (Figure 6-11), 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".
• Areas displaying influence from COI -affected groundwater
("affected") include locations near the dam, south and southeast of
the ash basin (SW-01, SW-10, SW-103, and SW-104). Seep location S-1
(former NPDES Seep Outfall 101) shows historical signs of mixing
ash pore water.
• Areas displaying "potential mixing" with COI -affected groundwater
include locations near the Outfall 007 and unnamed tributary, to the
east of the ash basin (SW-101, SW-102, and SW-110).
• Surface water locations that are unaffected from ash pore water
include SW-7, SW-8, SW-11, SW-105, and SW-106.
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Although the groupings displayed on the Piper diagrams may indicate
influence from COI -affected groundwater, COI concentrations in Lake
Norman surface water remain, and are predicted to remain, less than 02B
surface water quality standards.
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 ash basin and adjacent source areas that are at or beyond the ash basin
compliance boundary 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 area, co -occurrence with
CCR indicator constituents such as boron and sulfate, and migration directions
based on groundwater flow direction are considered in determination of
groundwater COIs.
The following list of COIs has been developed for Marshall, which represents
COIs presented in the CSA Update (SynTerra, 2018a), in addition to COIs added
for federal regulatory consideration (lithium) and to accommodate NCDEQ
requests (hexavalent chromium, total radium) (Appendix B):
• Antimony
•
Lithium
• Arsenic
•
Manganese
• Barium
•
Molybdenum
• Beryllium
•
Nickel
• Boron
•
Radium (Total)
• Cadmium
•
Selenium
• Chloride
•
Strontium
• Chromium (Hexavalent)
•
Sulfate
• Chromium (Total)
•
Total Dissolved Solids (TDS)
• Cobalt
•
Thallium
• Iron
•
Vanadium
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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 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 basin, since soil impacts
would be related to ash pore water interaction to the underlying soils within the
basin and groundwater migration at or beyond the ash basin.
Samples of background soil indicate that naturally occurring constituents, which
are also related to CCR material, likely affect the chemistry of groundwater at the
Site and are present at concentrations greater than the PSRGs POG values.
Constituents with background values greater than PSRGs POG values include
arsenic, barium, chromium (total), cobalt, iron, manganese, nickel, selenium and
thallium (Table 4-2).
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 the constituent is not present in
groundwater at the same location greater than applicable regulatory criteria
(Appendix C, Table 1); therefore, these soil concentrations do not warrant
consideration as potential secondary source of constituents to the groundwater.
Furthermore, there is a lack of transport mechanisms by which the COI could
have migrated from the source area 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.
Groundwater
(CAP Content Section 6.A.c.i.2)
A measure of central tendency analysis of groundwater COI data (February 2018
to May 2019) was conducted and means were calculated to support the analysis
of groundwater conditions to provide a basis for defining the extent of the COI
migration at or beyond the compliance boundary. A measure of central tendency
analysis was completed to capture the appropriate measure of central tendency
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(arithmetic mean, geometric mean, or median) for each dataset of constituent
concentrations. Constituent concentrations in a single well might vary over
orders of magnitude; therefore, a single sample result might not be an accurate
representation of the concentrations observed over several months to years of
groundwater monitoring. Evaluating COI plume geometries with central
tendency data minimizes the potential for incorporating occasions where 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 areas affected by the ash basin by posting a
single data set on maps and cross -sections that might have included isolated data
anomalies.
NCDEQ recommended use of a lower confidence limit (LCL95) rather than the
central tendency value (Appendix A). 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 — Marshall
Steam Station (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. If less than four quarters of valid data were
available, the most recent valid sample result was reported. For calculating
geomeans, non -detect values were assigned the laboratory reporting limit and
estimated (J-flag) values were treated as the reported value. Procedures for
excluding data from calculating geomeans are based on USEPA's National
Functional Guidelines (USEPA, 2017a, 2017b), published research about leaching
of elements from coal combustion fly ash (Izquierdo, 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
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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) is equal to the RL, and the geometric
mean or mean value is 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 standard units (S.U.) (for antimony, arsenic,
chromium, molybdenum, selenium, and vanadium only)
• 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 of the COI data using groundwater
monitoring sampling results from February 2018 to May 2019. Where means
could not be calculated, the most recent valid sample was evaluated to determine
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 basin.
Constituent Management Approach
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
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concentrations on figures based on the actual distribution of each COI in Site
groundwater. Table 6-6 presents the COI management matrix for determining
COIs subject to corrective action at Marshall.
• 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.
• 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 COI 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
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 shown in Table ES-2. 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.
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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 Interim Monitoring Plans (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 A).
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 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 two COI that had increases in the number of wells exceeding
COI criteria; one well for boron and one well for chloride (Appendix H).
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Chloride had an increase from two wells exceeding COI criterion based on
the central tendency concentration to three wells exceeding COI criterion
based on the LCL95 concentration. The additional well exceeding COI
criterion based on the LCL95 concentration is AB-01BR. The LCL95 and
the central tendency concentrations are 258 and 240 mg/L, respectively,
compared to the COI criterion of 250 mg/L (Appendix H). Boron had an
increase from seven wells exceeding COI criterion to eight wells exceeding
COI criterion. The additional well exceeding COI criterion based on the
LCL95 concentration is MW-06S. The LCL95 and the central tendency
concentrations are 723 and 405 mg/L, respectively, compared to the COI
criterion of 700 mg/L (Appendix H). The LCL95 concentration is based on
the entire period of record while the COI central tendency concentration is
based on data from 2018 through June 2019. Boron concentrations range
from a maximum of 4,450 mg/L in September 2016 to 150 mg/L in May
2019 (most recent sample included in the evaluation). Boron
concentrations have a statistically significant decreasing trend at MW-06S
and the most recent sample (150 mg/L) was below the COI criterion
(Attachment A of Appendix I). AB-01BR and MW-06S are located within
the area of planned corrective action for the Site (Figure ES-4). 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).
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Step 3: COI Distribution
Step 3 of the COI Management Plan process evaluates the relative
presence of COIs in Site groundwater. Descriptions of the horizontal and
vertical distribution of COIs with mean concentrations above their
respective COI criterion at and 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 COIs on a Site -wide basis. These
distributions are used for planning appropriate corrective action as well as
determining which COIs 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 COIs that do not exhibit plume -like distributions. Boron is
the COI 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
COIs 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
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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:
Conservative, Non -Reactive COI: antimony, boron, chloride, lithium,
sulfate and 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, chromium (total), strontium,
and 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, barium, cadmium, chromium (VI), cobalt,
iron, manganese, molybdenum, nickel, selenium, radium (total), and
thallium. Geochemical model simulations, and resulting Kd 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 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, 2019d), 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
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or vertical distribution of COI -affected groundwater migration from the ash
basin and adjacent source areas.
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.
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 within one order of magnitude (ER
<10) to two orders of magnitude (ER <100) above the COI criterion.
Using the COI management process (Appendix H), 5 of 22 inorganic
groundwater COIs exhibit mean concentrations that are currently less than
background values, 02L standard, or IMAC at or beyond the compliance
boundary, and therefore do not warrant corrective action at the Site (Table 6-6).
These five constituents include:
• Arsenic • Chromium (VI)
• Cadmium • Nickel
• Chromium (Total)
These constituents are not expected to migrate distances at or beyond the
compliance boundary or migrate distances that would present risk to potential
receptors, and are predicted, based on geochemical modeling, to remain at stable
concentrations, typically less than background values, 02L standard, or IMAC.
One exception to this conclusion might be enhanced mobility of pentavalent
arsenic if Eh values are sufficiently high to allow such species to persist
(Appendix H). However, the proposed remedial alternative would account for
capture of dissolved constituents in groundwater.
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The remaining 17 COIs exhibit mean concentrations greater than background
values, 02L standard, or IMAC downgradient of the ash basin at or beyond the
compliance boundary. These constituents warrant corrective action and include:
• Antimony • Molybdenum
• Barium • Selenium
• Beryllium • Strontium
• Boron • Sulfate
• Chloride • Thallium
• Cobalt • Total Dissolved Solids (TDS)
• Iron • Total Radium
• Lithium • Vanadium
• Manganese
As discussed in the CSA Update (SynTerra, 2018a) and the 2018 CAMA Annual
Interim Monitoring Report (SynTerra, 2019d), not all constituents with results
greater than background values can be attributed to the ash basin. 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.
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,
chloride, cobalt, iron, lithium, manganese, thallium, total dissolved solids, and
total radium (Appendix H). The following COI have no exceedances of COI
criteria or have isolated exceedances without a discernable plume, at or beyond
the compliance boundary: antimony, arsenic, barium, beryllium, cadmium, total
chromium, hexavalent chromium, molybdenum, nickel, selenium, sulfate, and
vanadium. These constituents will not be mapped on figures in the 2019 CAP
Update.
6.1.4 Horizontal and Vertical Extent of COIs
(CAP Content Section 6.A.d)
The COIs at the MSS have been delineated horizontally and vertically in
groundwater based on sampling and analysis data collected from 186 monitoring
wells present at the Site. At the request of NCDEQ, two additional bedrock wells
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have been installed in December 2019 to vertically delineate COIs detected at the
northern corner of the coal pile (CP-1BR) and east of the PV Structural Fill (PVSF-
2BRL) (Appendix P). Both of these locations are within the ash basin compliance
boundary. The results of these assessments will be available at a later date.
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
basin in groundwater. Furthermore, an evaluation of Site data indicates that COI
presence in groundwater decreases with depth (Appendix F). Supporting
information for these findings are presented in the COI management evaluation
presented in Section 6.1.3 and detailed in Appendix H.
Boron, a conservative (nonreactive) constituent, is the main COI that is present in
Site groundwater in a discernable plume. Boron typically has greater
concentrations in CCR than in native soil and is relatively soluble and mobile in
groundwater (Chu, et. al., 2017). Chloride, lithium, and TDS are also conservative
constituents; however, these constituents display reduced discernable COI
plume geometries compared to boron. 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 basin, are generally coincident within the
extent of the 02L boron plume at the Site. Non -conservative and variably
reactive constituents have smaller plume geometries, generally consisting
isolated and sporadic detections, relative to boron because of their high Ka values
and reactivity, which reduce their mobility in groundwater.
Since naturally occurring COIs might be present at concentrations greater than
Site -specific BTVs, isoconcentration maps of primary CCR indicator COIs (i.e.,
boron, chloride, lithium, and TDS) are generally most representative of the
groundwater COI plume extent in three-dimensional space.
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.
Mean data of groundwater COI monitoring sampling results from February 2018
to May 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
(Figures 6-13a through 6-22). Vertical extent of the COI plume is represented
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with mean concentrations displayed on cross -sectional depictions of the Site.
Cross-section A -A' (Figures 6-7a through 6-7c) is oriented northwest to southeast
and displays the general north -south basin footprint topography and depth of
saturated ash in the former Holdsclaw Creek channel and free water near the
dam.
Beyond the compliance boundary, the maximum extent of COI -groundwater
affected by the ash basin occurs in the limited area east of the ash basin towards
the unnamed tributary.
6.1.4.1 COIs in Unsaturated Soil
(CAP Content Section 6.A.d.i)
Based on the unsaturated soil evaulation, there are no constituents in soil
associated with the source area that require corrective action at the MSS.
Unsaturated soil at or beyond the compliance boundary 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. Therefore, constituents
considered for unsaturated soil evaluation as related to the ash basin and
adjacent source areas were the same constituents identified as COIs in
groundwater related to the source areas.
An evaluation of the potential nature and extent of COIs in unsaturated soil
beyond the waste boundary was conducted using data from well
installation activities and an additional soil sampling event in April 2019.
The sampling event in April 2019 was conducted to better delineate
unsaturated soils based on CSA Update comments made by NCDEQ
(Appendix B). Unsaturated soil samples near or beyond the compliance
boundary include samples collected from AB-2S, AL-1, CCR-5, CCR-9, CP-
3D, GP-1D, GP-31), GWA-1BR, BGSB-GWA-2, GWA-2DA, GWA-7, ILF-21),
MW-10, MW-14BR, PVSF-3BR, and PVSF-4D. Unsaturated soil analytical
results (Appendix C, Table 4) are compared to background values or PSRG
POGs, whichever is greater (Table 6-3). COIs in saturated soil are
considered and evaluated as part of the groundwater flow system, separate
from this evaluation.
Constituents detected at concentrations greater than either background
values or the PSRG POG standard in unsaturated soil samples (depth),
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upgradient or downgradient of the ash basin, are presented on Figure 6-23,
and include:
• Arsenic: AL-11) (32-34)
• Barium: B-11 (6.5-7.5), GP-31) (2-3)
• Cobalt: B-11 (6.5-7.5), GP-31) (5-10)
• Iron: D-11 (0-1), PVSF-3BR (2-4), (6-8)
• Manganese: B-11 (6.5-7.5), GP-31) (5-10)
• Selenium: AL-11) (32-34)
• Sulfate: GP-11) (0-5)
• Thallium: B-11 (6.5-7.5)
Although greater than a comparative criteria, these concentrations were
generally within the range of concentrations detected in soil samples from
upgradient and/or background locations (Appendix C, Table 4).
Additionally, all unsaturated soil samples with values reported greater than
the PSRG POG standard or background values are vertically delineated by
groundwater constituent concentrations less than applicable regulatory
criteria in the corresponding monitoring well (Appendix C, Table 1).
Furthermore, there is a lack of transport mechanisms by which the COI
could have migrated from the source area to the unsaturated soils. For these
reasons, the soil concentrations do not warrant consideration as potential
secondary source of constituents to the groundwater.
Because all unsaturated soil concentrations are generally within range of
background soil concentrations, and all soil concentrations are delineated
by groundwater concentrations, which indicates there is no potential
secondary source to groundwater from leaching of soil, and the lack of
transport mechanisms, additional soil sampling is not warranted and no soil
exceedances are identified for corrective action at the Site.
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 east of the ash basin. Groundwater is not in
need of restoration adjacent to the ash basin to the south, west, and north
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due to the lack of COIs above applicable standards in these areas. A limited
number of COIs in groundwater are present at or beyond the compliance
boundary to the east of the MSS ash basin. Additional detail for this area is
provided below.
Eastern Extent of COI -Affected Groundwater
Boron, chloride, lithium, and TDS mean concentrations near the compliance
boundary support the following observations regarding the eastern extent
COI -affected by the ash basin groundwater:
The extent of COIs within the shallow and deep flow zones east of
the ash basin have relatively similar geometries and extend beyond
the compliance boundary toward the tributary to the east. This
supports the interpretation that these two zones are hydraulically
connected. Chloride, lithium, and TDS plumes are contained within
the extent of the 02L boron plume.
COI -affected groundwater within bedrock is relatively limited
compared to the shallow and deep plume geometry, and contained
within the extent of the shallow and deep groundwater COI plumes.
This supports the limited vertical migration of COIs described in the
CSM. The eastern extent of COI -affected groundwater in bedrock is
limited to immediately downstream of the dam (AB-1), beneath the
Dry Ash Landfill Phase I towards the tributary (AL-1), and beneath
the Dry Ash Landfill Phase II (AL-2).
• Shallow, deep and bedrock COI -affected groundwater at
concentrations greater than 02L standards is horizontally limited to
the area east of the ash basin towards the unnamed tributary and
immediately downstream of the dam. The plume is delineated to the
north by GWA-7S/D, in the upper portion of the draw towards the
tributary. The 02L boron plume is delineated to the south by MW-
8S/D and the additional wells further south along the dam.
• The vertical extent of COI -affected groundwater in bedrock has been
adequately delineated (Figures 6-7a through 6-7c). East of the ash
basin towards the tributary, the vertical extent of COIs is delineated
by AL-1BRL (205 feet into bedrock), GWA-11BR (10 feet into
bedrock), MW-14BRL (230 feet into bedrock), and GWA-15D - the
deep flow zone well furthest downgradient toward the shoreline.
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Immediately downstream of the dam, the vertical extent of the 02L
boron plume is delineated by AB-1BRLLL (320 feet into bedrock).
The vertical extent of COI -affected groundwater beneath the Dry Ash
Landfill Phase II, the area of the deepest known CCR-affected
groundwater, is delineated by AL-2BRLLL (345 feet into bedrock).
The groundwater COI plume shape relates to hydraulic conditions
associated with the flow -through system described in the CSM (Section 5).
Upward and neutral gradients limit COI migration from the ash pore water
to groundwater below ash and below the basin, except near the dam where
a downward vertical hydraulic gradient promotes downward COI
migration in groundwater and beneath additional source areas beyond the
footprint of the ash basin, where the hydraulics described in the CSM might
not apply.
Downgradient of the dam, groundwater flows upward toward the
discharge zone (Lake Norman), limiting downward migration of COIs to
the area adjacent to the dam. The extent of COI -affected groundwater east
of the dam is limited by hydraulic conditions in that area. Below the ash
basin dam, a strong upward gradient is observed between the bedrock wells
and the shallow flow zone at well cluster AB-1 (-0.01 to -0.06 ft/ft; Table 5-
3). Three of the four bedrock wells at this location are artesian wells. The
hydraulic head of AB-1BR is approximately 0.02 feet below top of the flush -
mounted casing.
6.1.5 COI Distribution in Groundwater
(CAP Content Section 6.A.e)
As step two of the constituent management process and the geochemical
modeling evaluation, COIs were grouped by geochemical behavior and mobility
(Section 6.1.3 and Appendix H). An evaluation (i.e., mean analysis and mobility
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 direction downgradient of the ash basin and
adjacent source areas (Table 6-6). The evaluation grouped constituents into three
mobility groups: conservative (non -reactive), non -conservative (reactive), and
variably reactive.
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6.1.5.1 Conservative Constituents
(CAP Content Section 6.A.e.i)
Isoconcentration maps for conservative COIs boron and TDS display
simulated plumes from the groundwater flow and transport model
(Appendix G) to provide insight where empirical data is not available. The
model outputs are modified where empirical data can refine model
assumptions. The transport model calibration targets are boron and TDS
concentrations measured in 181 monitoring wells in the first quarter of
2019. All sampled wells are included in the calibration. However, more
recent wells and data that have been collected since that timeframe were not
included in the updated model calibration process. Fall 2019 data from
newly installed wells suggest the model predictions are conservative; the
model over -predicts the actual groundwater concentrations in some isolated
areas.
Isoconcentration maps for boron (Figures 6-13a-c), chloride (Figures 6-14a-
b), lithium (Figures 6-17a-c), and TDS (Figures 6-21a-c) mean
isoconcentration maps and cross section (Figure 6-7a) support the following
observations regarding the extent of COI -affected groundwater represented
by these conservative constituents:
• The conservative COI plumes for the shallow and deep flow zones
have relatively similar plume geometries, with boron generally
representing the greatest extent of COI migration.
• COI migration east of the ash basin and Dry Ash Landfill Phase I
represents the leading edge of the COI -affected groundwater plume
beyond the ash basin compliance boundary.
• The extent of COI -affect groundwater plumes within bedrock
groundwater are generally reduced in comparison to shallow and
deep groundwater isoconcentration maps. This indicates limited
vertical migration of COIs, further supporting the CSM (Section 5).
• COI -affected groundwater in bedrock is horizontally limited to wells
at four clusters beneath the basin (AB-6, AB-10, AB-12, PVSF-2),
beneath the Dry Ash Landfill Phase I (AL-1) and Phase II (AL-2), east
of the basin towards the tributary (MW-14), and east of the ash basin
dam (AB-1). The areas above, beyond or near the ash basin waste
boundary, were specifically targeted for an evaluation of
groundwater flow within the deep bedrock (Appendix F). Vertical
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delineation of COIs at these locations was achieved in 2019 through
the installation of deep bedrock monitoring wells (Appendix C,
Table 1).
Based on the results of the coal pile and PV Structural Fill
assessments (Appendix P), one additional bedrock well is being
installed at two locations for vertical delineation: northern corner of
the coal pile (CP-1BR) and eastern perimeter of the PV Structural Fill
(PVSF-2BRL). Results of the additional assessments will be available
at a later date. The deepest extent of the bedrock groundwater plume
is beneath the Dry Ash Landfill (Phase II), where COIs are present
approximately 200 feet into bedrock (AL-2BRLL). These
concentrations were delineated vertically (boron <50 µg/L) with the
installation of AL-2BRLLL, screened approximately 345 feet into
bedrock (Appendix F).
• Boron concentrations, which best -represent CCR-affected
groundwater migration, are vertically and horizontally bounded
downgradient of the basin, beyond the compliance boundary, by
either discharge zones or concentrations less than applicable
regulatory criteria. COI -affected groundwater delineation is
demonstrated by detected constituent concentrations that are less
than regulatory standard or are not detected from groundwater
monitoring wells GWA-7S/D, GWA-10S/D, AL-1BRL, GWA-11BR,
GWA-15D, MW-6D, CCR-9DA, MW-10S/D, AB-1BRLLL, MW-7D,
and MW-8S/D. The downgradient groundwater discharge zones (i.e.,
surface water receptors) limit COI migration.
The maximum extent of COI -affected groundwater migration for all flow
zones is represented by boron, with the exception of sporadic exceedances
of variably reactive COIs (e.g., cobalt, lithium, manganese) along the
southern portion of the dam. Chloride and TDS concentrations identified as
being greater than their respective groundwater regulatory standards are
associated with COI -affected groundwater migration from the ash basin but
are generally confined within the extent of the 02L boron plume.
Plume Behavior and Stability
(CAP Content Section 6.A.e.i.1)
Mann -Kendall trend analysis was performed using conservative constituent
(boron, lithium and TDS) datasets for ash pore water and groundwater
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wells within the waste boundary, between the waste boundary and
compliance boundary, and 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.
The analysis was performed using analytical results for samples collected
from 2011 through 2019, for COIs requiring corrective action (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. Statically significant, decreasing concentration trend (D)
2. Statically 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 1,628 trends were evaluated. Excluding the NE and ND trends
described above, 80% of the remaining trends were statistically decreasing,
stable or had no trend. Only 13% of the trends were statistically increasing
(Appendix I).
Groundwater wells within the waste boundary generally had no trends or
stable trends, suggesting limited changing conditions and that the
groundwater plume is stable. Mann -Kendall results for ash pore water and
groundwater within the waste boundary indicate the following:
• Over 50% of ash pore water trend results indicate no trends for
conservative constituents (i.e. boron, chloride, lithium and TDS) and
approximately 30% of trend results indicate stable trends for these
conservative constituents (Table 6-7).
• The data indicate overall improvements in groundwater COI
concentrations.
• For shallow groundwater, increasing trends for TDS are grouped
along the dam (wells CCR-05S, MW-08S, and MW-09S).
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• In the deep and bedrock flow zones, boron and TDS are the most
prevalent constituents with increasing concentration trends. These
wells tend to be grouped along the dam and near the Dry Ash
Landfill (Phase 1).
• Wells with increasing COI concentration trends are generally located
within the areas planned for groundwater remedial actions or are
located in upgradient areas, which will also be addressed by the
selected groundwater remedy (Section 6.8).
6.1.5.2 Non -Conservative Constituents
(CAP Content Section 6.A.e.ii)
Strontium mean isoconcentration maps (Figures 6-20a-c) and cross section
(Figure 6-7b) support the following observations regarding the extent of
COI -affected groundwater represented by this non -conservative
constituent:
The extent of strontium concentrations greater than background in
all three groundwater flow zones are similar, with a reduced extent
in bedrock, a trend similar to the distribution of conservative COIs
(i.e., concentration less than background at downgradient bedrock
well GWA-11BR). The horizontal extent is generally contained within
the ash basin waste boundary, beneath the Dry Ash Landfill Phase I
and Phase II, and localized areas east of the basin toward the
unnamed tributary.
6.1.5.3 Variably Conservative Constituents
Cobalt (Figure 6-15), iron (Figures 6-16a-b), manganese (Figures 6-18a-c),
radium (total) (Figures 6-19a-b), and thallium (Figure 6-22) isoconcentration
maps and cross section (Figure 6-7c) support the following observations
regarding the extent of COI -affected groundwater represented by these
variably reactive constituents:
• Contours of the variably reactive COIs within the shallow flow zone
indicate concentrations greater than applicable regulatory criteria are
sporadic, but are generally limited to the following areas: southern
portion of ash basin (north of coal pile) and southern portion of dam,
east of the ash basin and Dry Ash Landfill Phase I toward the
unnamed tributary, east of PV Structural Fill, and beneath Dry Ash
Landfill Phase 11(AL-2). These localized plumes generally coincide
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with the extent of conservative COIs, with the exception of the
sourthern portion of the dam, where boron concentrations are less
than the 02L standard.
• Deep flow zone groundwater COI plumes are limited to the
following areas: primarily within the former perennial stream valley
within the ash basin, PV Structural Fill, as well as isolated
occurrences beyond Dry Ash Landfills (Phase I and Phase II).
The greatest extent of variably reactive COIs in the deep flow zone is
represented by manganese concentrations, which coincide with the
areas mentioned above for the shallow zone (southern ash basin, ash
basin dam, and east of the Dry Ash Landfill Phase I). Total radium is
only detected above background in two deep flow zone wells - both
east of the Dry Ash Landfill Phase I (AL-11) and GWA-15D).
• The extent of iron, manganese, and total radium concentrations in
bedrock groundwater greater than applicable regulatory criteria
generally overlap, with affected areas typically including: beneath
the Dry Ash Landfill Phase II (AL-2) and central ash basin (AB-6, AB-
10, AB-12), east of the Dry Ash Landfill Phase I (AL-1), and east of
the basin dam (AB-1). Similar to conservative COIs, the COI -affected
groundwater distribution is limited compared to the distribution
within shallow and deep groundwater (i.e., manganese).
6.2 Potential Receptors Associated with Source Area
(CAP Content Section 6.B)
CSA results indicate COI -affected groundwater has migrated to localized areas
immediately downgradient of the MSS ash basin. COI -affected groundwater is limited
to Duke Energy property. Flow and transport simulations predict limited migration in
areas beneath Lake Norman immediate to the shoreline. In addition, these predictive
model simulations may overestimate the extent of the COI migration beneath the lake
because the predominant strike of bedrock fractures - and anticipated groundwater
flow - is parallel to the shoreline rather than perpendicular to it. COI -affected
groundwater from the ash basin does not reach any water supply wells, and modeling
indicates this will remain the case in the future. Therefore, potential receptors are
limited to Lake Norman and the unnamed tributary east of the ash basin. As the
updated human health and risk assessment concluded, there is no evidence of
unacceptable risks to human and ecological receptors exposed to environmental media
potentially affected by CCR constituents at MSS (Appendix E). This conclusion is
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further supported by multiple water quality and biological assessments conducted by
Duke Energy as part of the NDPES monitoring program.
6.2.1 Surface Waters - Downgradient Within 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 is provided in 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, is greater than the required 0.5-
mile radius of the waste boundary and is consistent with the water supply well
survey. Surface water information is provided from the NRTR (AMEC, 2015). In
addition, NPDES-permitted outfalls and locations covered by the SOC are shown
on Figure 5-6. Non -constructed and dispositioned seep sample locations between
the ash basin and Lake Norman are managed by the SOC and are subject to the
monitoring and evaluation requirements contained in the SOC. Non -constructed
seeps, currently covered under the SOC, that have the potential to not be fully
dispositioned post -decanting are listed on Table 6-8. No constructed seeps are
present at the MSS.
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 15A NCAC 02B .0200 (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.
Surface water samples were collected from Lake Norman to confirm
groundwater downgradient of the ash basin has not resulted in surface water
concentrations greater than 02B water quality standards. A map of all surface
water sample locations for groundwater discharge to surface water evaluation is
included in Appendix J (CAP Content Section 6.B.a.iv). Surface water samples
were collected to evaluate acute and chronic water quality values. Surface water
samples were also collected at background locations (upstream of potential
migration areas) within Lake Norman and minor streams upgradient of the
source area. Analytical results were evaluated with respect to 02B water quality
standards and background data.
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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; 2018a; 2018b) was conducted on
surface water samples from background locations, the unnamed tributary and
Lake Norman. 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 (Appendix J).
Antimony was not detected in any of the surface water samples tested.
Conversely, manganese was detected above the USEPA criterion at all locations,
including background locations in Lake Norman, indicating that it is endemic to
the area. Alkalinity and iron exceeded the USEPA criteria for the samples from
the on -site background streams, but not in the surface water samples from the
tributary, seep or lake. However, the few background exceedances for alkalinity
were generally comparable to the screening criterion. Aluminum exceeded the
USEPA criteria in the on -site background streams, the Lake Norman shoreline,
and AOW seep location.
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 MSS groundwater
discharge to surface water and the evaluation of surface waters to evaluate
compliance with 15A NCAC 02B .0200 was submitted to NCDEQ on March 22,
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.
General findings of the evaluation of current surface water quality conditions at
MSS include:
• Groundwater migration from the ash basin source area has not resulted in
exceedances of the 15 NCAC 02B surface water quality standards in Lake
Norman.
• Previously identified seeps are deemed covered by Special Order by
Consent EMC SOC WQ S17-009 (SOC).
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Surface Water — Future Conditions Evaluation
Based on current and future surface water evaluations, along with relevant
media assessments, no COIs require remediation in surface water at MSS. 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 basin might intersect surface water in the
future. A groundwater to surface water mixing model approach was used to
determine the potential surface water quality in the future groundwater
discharge zones. The full report for MSS 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 evaluation demonstrates that under the closure -by -excavation
scenario and the closure -in -place scenario, future groundwater migration
from the ash basin would not result in constituent concentrations in Lake
Norman or the unnamed tributary east of the ash basin greater than 02B
surface water standards. The criteria for compliance with 02B is met,
allowing further evaluation of potential corrective action under 15A
NCAC 02L .0106 (k) or (1).
Because this evaluation demonstrates that predicted resultant constituent
concentrations in surface waters are less than 02B surface water standards,
the results and conclusions of this evaluation support future corrective
action termination under 15A NCAC 02L .0106 (m).
6.2.2 Water Supply Wells
(CAP Content Section 6.B.b)
A total of 127 private water supply wells and one public supply well were
initially identified within the 0.5-mile radius of the ash basin compliance
boundary (Figure 5-7). Most of these water supply wells are located north and
west of the ash basin, along Sherrills Ford Road and Island Point Road.
No public or private drinking water wells or wellhead protection areas were
found to be located downgradient of the ash basin, as discussed in Section 5.4.
This finding has been supported by field observations, a review of public
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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 supply wells located upgradient or side -gradient
of the facility, within 0.5 miles of the ash basin compliance boundary, were
included in drinking water supply well survey reports.
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 area at MSS, water supply wells identified within the 0.5-
mile radius from the ash basin compliance boundary have been offered
alternate water supply, per G.S. Section 130A-309.211(cl) requirements. No
sampled water supply wells were deemed impacted by COI -affected
groundwater. Appendix C, Table 1 summarizes analytical results for supply
wells associated with the Site.
• Property eligibility was contingent that the property did not include:
• A business
• A church
• A school
• Connection to the public water supplier
• An empty lot
Of the 127 private water supply wells, Duke Energy identified a total of 79
eligible households near MSS qualifying for a permanent water solution. Of
the 79 eligible households, 14 either opted out of the option to connect to a
water treatment system or did not respond to the offer. Duke Energy
installed water filtration systems on 3 households, and 62 households were
connected to the public water supplier by Duke Energy in accordance with
G.S. Section 130A-309.211(cl).
Additionally, Duke Energy voluntarily provided permanent water solutions
to six properties, including businesses and churches, within a 0.5-mile of the
MSS compliance boundary that were otherwise not eligible per G.S. Section
130A-309.211(cl).
On August 31, 2018, Duke Energy provided completion documentation to
NCDEQ to fulfill the requirements of House Bill 630. NCDEQ provided
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correspondence, dated October 12, 2018, to confirm that Duke Energy
satisfactorily completed the alternate water provisions under G.S. Section
130A-3099.211(cl) at MSS. Both documents are provided in Appendix D.
Figure 5-8 illustrates properties within the 0.5 mile radius of the ash basin
compliance boundary with reference to water treatment systems installed,
along with vacant parcels and residential properties whose owners have
decided to either opt out of the water treatment system program or did not
respond to the offer. On -going periodic maintenance is provided by Duke
Energy for the filtration system in accordance with the Permanent Water
Supply — Water Treatment Systems, Performance Monitoring Plan (Duke Energy
2017). Figure 5-7 shows the private and public water supply well locations
and NCDEQ sample numbers.
6.2.2.2 Findings of Drinking Water Supply Well Surveys
(CAP Content Section 6.B.b.ii)
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 Supply Well and Receptor Survey, Marshall Steam Station
Ash Basin (HDR, 2014a)
• Supplement to Drinking Water Supply Well and Receptor Survey, Marshall
Steam Station Ash Basin (HDR, 2014b)
• Comprehensive Site Assessment Supplement 2, Marshall Steam Station
(HDR, 2016b),
• 2018 Comprehensive Site Assessment Update, Marshall Steam Station
(SynTerra, 2018a)
As documented in the 2018 CSA Update (SynTerra, 2018a), NCDEQ
coordinated sampling of private water supply wells identified within a half -
mile radius of the ash basin compliance boundary from February to October
in 2015. NCDEQ performed sampling and analysis of the water supply
wells identified within the 0.5 mile radius of the ash basin compliance
boundary, if the owner agreed to have their well sampled. No sampled
water supply wells were determined to be impacted by COI -affected
groundwater. Analytical results for supply wells associated with the Site are
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discussed in Section 5.3.3 and included in Table 6-9 (CAP Content Section
6.B.b.ii and 6.B.b.iii) and Appendix C, Table 1 (CAP Content Section 6.B.b.11).
No public or private drinking water wells or wellhead protection areas
were found to be located downgradient of the ash basin. This finding has
been supported by field observations, a review of public records, 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 compliance boundary, were included in the survey
reports noted below.
The initial survey identified four public water supply wells within a 0.5-
mile radius of the ash basin compliance boundary; however, one of those
wells is not currently in use. Two water supply wells classified as transient,
non -community are located at the Midway Restaurant and Marina and The
Old Country Church. Both of these wells are located west and upgradient of
the MSS ash basin. The Catawba County Environmental Health Department
had records for one public water supply well owned by Duke Energy,
which is not used for consumption.
A total of 127 private water supply wells were initially identified within the
0.5-mile radius of the ash basin compliance boundary. Most of these water
supply wells are located north and west of the ash basin, along Sherrills
Ford and Island Point Roads (Figure 5-7). All of the private water supply
wells are located either upgradient or side -gradient of the ash basin.
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 basin area within and beyond the predicted area of
potential groundwater COI influence. Therefore, no future groundwater use
areas are anticipated downgradient of the ash basin and adjacent source areas.
It is anticipated that residences within a 0.5-mile radius of the ash basin
compliance boundary will continue to rely on municipal water or groundwater
resources for water supply for the foreseeable future; therefore, Duke Energy will
provide periodic maintenance of the provided water treatment systems for each
household that accepted the alternative water supply [(Figure 5-8) (CAP Content
Section 6.B.c.i)].
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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 south/southeast of the
ash basin footprint, and therefore will not flow towards any water supply wells
[(Appendix G) (CAP Content Section 6.B.c.ii)].
6.3 Human and Ecological Risks
(CAP Content Section 6.0
Updated human health and ecological risk assessments were prepared for the MSS
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 assessment updates include:
1. The ash basin does not cause an increase in risks to potential human receptors
located on -Site or off -Site.
2. The ash basin does not cause an increase in risks to ecological receptors.
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 basin can be found in Section 5.4. An update to the MSS human health and
ecological risk assessment is included in Appendix E.
6.4 Description of Remediation Technologies
This section provides supplemental information beyond the CAP content guidance to
introduce groundwater remediation technologies and considers a range of individual
technologies that might be used to formulate comprehensive groundwater remediation
alternatives for consideration at MSS. 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
criterions:
• Can a technology be effective when addressing one or more site -specific COIs?
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
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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 6.7.
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 below regulatory
standards (USEPA, 1999).
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,
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chemical, or biological processes that, under favorable conditions, act without
human intervention to reduce the mass, toxicity, mobility, volume, or
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
(NCDEQ, 2017). 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 is retained for further consideration at MSS because groundwater COIs do
not pose unacceptable risk to human health or the environment under
conservative exposure scenarios and a source control measure will be
implemented that eliminates or mitigates the source of CCR constituents in
groundwater. The MNA evaluation for the technical applicability at MSS is
provided in Appendix I.
6.4.2 In -Situ Technologies
Groundwater remediation technologies implemented in -situ, or in -place, are
discussed below.
Low Permeability Barriers
When used for 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 lower 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.
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LPBs can be used to redirect groundwater away from a potential receptor,
redirect groundwater away from a source area, or redirect COI laden
groundwater towards a groundwater extraction system or in -situ groundwater
treatment system (e.g., permeable reactive barrier). The design and technique
used to construct an 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 an LPB.
Sheet piling and trenching are typically limited to depths of approximately 50
feet whereas installation of an LPB using drilling techniques can achieve depths
greater than 50 feet. For this reason, construction of an LPB at MSS would
involve installation by means of drilling because bedrock is approximately 80
feet below the ground surface at locations downgradient and east of the MSS ash
basin.
Construction of an LPB at MSS would involve drilling to competent bedrock and
injecting bentonite or cement 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 the MSS. Installation of an effective low permeability barrier to depths
approaching 80 feet would be technically challenging and costly. Another
drawback to the implementation of an LPB over a large area is that it could cause
groundwater to mound behind the barrier. This could increase the gradient and
induce COIs to migrate downward, or result in groundwater flow around the
barrier, possibly resulting in the migration of COIs into other areas of the Site.
For these reasons, LPB technology will not be retained for further consideration.
Groundwater Infiltration and Flushing
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
influencing the effectiveness include the degree of subsurface heterogeneity, the
variability of hydraulic conductivity, and 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.
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In -situ flushing 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 infiltration 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.
Groundwater flushing by infiltration can be accomplished by many methods
including vertical wells, horizontal wells, and infiltration galleries. Groundwater
flushing is a technology that has possible application at MSS to enhance the
capture of mobile constituents. Groundwater flushing is 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 been 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 waste (e.g., ash). ISV
uses an array of electrodes that are 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). Two additional considerations associated with this technology are
permanence of the reaction product insolubility and the ability to distribute
reactants sufficiently to ensure adequate contact with the COIs.
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Contact between the encasing material and affected media could pose a
challenge in the transition zone and fractured rock formations. It is difficult to
ensure that encasing material are uniformly distributed in transition zone and
fractured bedrock to assure adequate encapsulation of affected media.
Microencapsulation and ISV would not be feasible for the areas south and
southeast of the ash basin that would need to be encapsulated, due to the size
and depths of the areas requiring groundwater remediation.
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.
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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 and oriented perpendicular to
groundwater flow. The funnel and gate configuration involves construction of
two LPBs that redirect groundwater flow toward 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 to be keyed into a low
permeability confining layer or into 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 organic materials used to affect
groundwater Eh and pH. The mechanisms that take inorganic constituents out of
solution include adsorption, ion exchange, oxidation-reduction, or precipitation.
ZVI (FeO) is an effective reducing agent as it readily donates electrons to receptor
molecules or constituents (Fe° --* 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 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 media for raising the pH of acidic groundwater, like the pH found
in mine runoff (Indraratna 2010).
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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.
It would be technically challenging and cost prohibitive to construct an effective
PRB in saprolitic/transition zone material up to 80 feet thick. PRB technology
would be better suited to treat coal ash constituents that are less mobile and
more reactive than boron.
The ability to maintain adequate reactive amendment concentrations at depth
over an extended period of time is also a significant operational and performance
consideration.
Elevated concentrations of non -target metals constituents dissolved in
groundwater (e.g., aluminum) can become problematic because they might
precipitate within the treatment zone. The barrier could also become clogged and
a large reduction in hydraulic conductivity could occur. Given the depth of these
barriers, in -situ rehabilitation of the reactive media is considered infeasible;
therefore, walls would have to be reconstructed on a periodic basis to address
clogging or effectiveness of the chemical amendments. Given these limitations,
constructed wall PRB technology will not be considered for application at the
MSS Site.
A PRB, however, could also be implemented by the infiltration of chemical
amendments into the subsurface through a grid of closely spaced vertical
boreholes. This approach would emplace reactive chemicals in contact with
affected groundwater so that treatment could occur in -situ as the groundwater
migrates through the zone of infiltration. Should replenishment of the
amendments be necessary in the future for continued groundwater treatment
effectiveness, additional boreholes would be installed for the infiltration of the
additional amendments. The placement of chemical amendments through drilled
boreholes as a permeable reactive barrier is retained for further consideration to
treat reactive COIs along the southern end of the dam at the MSS Site.
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
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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
infiltration of clean water upgradient of the wells to increase the availability of
water and hydraulic head. Alternatively, low yielding wells can be 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 (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 needs to be provided to only 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 for MSS indicates that vertical groundwater
extraction wells can produce sufficient yield for effective constituent mass
removal without supplemental measures. The use of vertical groundwater
extraction wells is retained for further consideration.
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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 are 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 to
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 groundwater transmissivity is
greater than the vertical transmissivity.
Installation of a directionally drilled horizontal well involves a drill bit that can
be steered in three dimensions. The progress of directional boring installations is
precisely monitored to avoid subsurface obstructions and to install the well as
designed. Tracking accuracy generally decreases with increased depth of
installation. Site hydrogeologic conditions can affect tracking accuracy during
drilling.
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 to 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
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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.
Installation through transition zones of saprolite and partially weathered rock
can be challenging. Horizontal wells can be more costly to install as compared to
vertical or angular wells, but can often replace more than one vertical well.
Horizontal or angular wells could be used at MSS to effect remediation beneath
areas not accessible from land surface (e.g., beneath the ash basin spillway).
Groundwater modeling conducted for MSS indicates that vertical groundwater
extraction wells can produce sufficient yield for the 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.
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 bgs) 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.
Shallow trenches are easy to install and can be an effective surface water
protection supplement to a groundwater management system. If applied at MSS,
trench technology effectiveness would be limited if used for the purpose of
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groundwater extraction. The thickness of saprolite/transition zone downgradient
of the ash basin is up to approximately 80 feet below ground surface. The use of
horizontal extraction trenches will not be retained for further consideration.
Hydraulic Fracturing
The effectiveness of groundwater extraction systems can sometimes be improved
in low permeability formations, including bedrock, with the use of hydraulic
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 or polymers 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 with high-
pressure water to cut into the formation. Pumping of a slurry of water and sand
in a thick gel at high pressures, into the borehole propagates the fractures.
Proppants are typically well-rounded, very coarse -grained quartz sand. The
polymer is then broken or biodegrades and is pumped out of the formation. The
proppants remain in place to keep the fractures open. The resultant fracture is a
permeable sand -filled lens that might be as large as 60 feet in diameter (USEPA,
1995).
The presence of COIs in the bedrock groundwater at MSS is limited compared to
the distribution and concentrations of COIs in the saprolite and transition zone
groundwater, therefore the use of hydraulic fracturing to enhance remediation of
bedrock groundwater is not retained for further consideration.
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 the following:
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• 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
result in an increase in the rate of transpiration by the plant.
• 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 tree well technology involves the installation of a 3- to 5-
foot diameter boring to a target depth, typically a flow zone containing COIs. A
Root SleeveTM liner and aeration tubing are installed from ground surface to
target depth. The boring is backfilled with soil that might include reactive 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 (at land surface) followed by placement of a
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
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to Tree Wells which are targeted to deep soil and/or groundwater. The typical
range of effectiveness for phytoremediation groundcovers is 1-2 feet below
ground surface (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 contaminants 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
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. The
effectiveness of phytoremediation in terms of water removal and COI uptake
will vary depending on the season of the year and the depth of affected
groundwater. Therefore, phytoremediation is not retained for consideration for
groundwater extraction at this time, but may be reconsidered in the future for
areas accessible through the use of Tree Wells.
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
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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. Permitted discharges to Lake Norman will impose specific
limits on the pH of discharged wastewater. The existing NPDES permitted
outfalls at MSS maintain a pH from 6.0 to 9.0 standard units (S.U.). Facilities and
equipment to adjust the pH of wastewater to satisfy NPDES discharge
requirements are currently in -place at MSS.
Groundwater monitoring has indicated that the pH from some monitoring wells
is outside of these permit limits. With means to adjust pH already in place, it is
assumed that the pH of extracted groundwater can be adjusted to meet the
existing NPDES permit limits prior to discharge.
The means and technology needed to adjust the pH of extracted groundwater is
well established and available at the Site. This treatment technology is retained
for consideration in the future, if needed; however, has not been incorporated
into a proposed remedial alternative at this time.
Precipitation
Precipitation of metals and other inorganic
constituents has been used extensively in
treating affected groundwater. The process
involves the conversion of soluble
(dissolved) constituents to insoluble
particulates that will precipitate. 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 Eh (volts). The stability of soluble
and insoluble metals and metal complexes
is commonly illustrated in Pourbaix
diagrams (pH vs Eh) (Figure 6-24).
FIGURE 6-24
Simplified Pourbaix diagram
for iron -water system at 77°F (25°C)
E0
2.0
1.6
1.2
0.8
0.4
0.0
-0.4
-0.8
-1.2
Fe'+ (aq )
W+(aq)
m
F004' (aq)
FC203
C
li Fi'a 4 i
b
Fc t+} ;
0 2 4 6 8 18 12 14 PH
https://rsteyn.wordpress.com/pourbaix-dia gra ms
As illustrated in the Pourbaix diagram, iron is soluble [aqueous (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
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and precipitate out of solution, provided that the redox potential (Eh) remains
between 0.75 and 1.5 V. Adjustment of groundwater pH and Eh can be used to
remove other metals including cadmium, chromium, copper, nickel, and zinc.
Flocculation is another method that can be used to remove inorganics from an
aqueous waste stream. This technology involves adding a flocculent to extracted
water and then removing (through sedimentation or filtration) formed
particulates to reduce concentrations, such as total suspended solid (TSS).
Precipitation technology might be warranted as a means to treat, or pretreat,
extracted groundwater to satisfy NPDES permitted discharge limits. Extracted
groundwater is not expected to cause violations of the NPDES permit when
discharged; therefore, precipitation technologies are not retained for further
consideration.
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.
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 increase over time and approach influent concentrations. Ion resins
should be replaced or regenerated before breakthrough occurs. Ion selective
boron resins are available and do not have the same competition considerations.
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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 disposal. 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 may have geochemical characteristics that
result in interference in the ion exchange process. Because of these challenges ion
exchange is not retained for further consideration.
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 can achieve low effluent concentrations.
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However, permeable membrane filtration technologies are also susceptible to
fouling and often require a pretreatment step. They can also generate a high
concentration reject effluent that might require additional treatment prior to
disposal. These technologies typically have high capital and operational costs.
Membrane filtration at MSS is not carried forward for further evaluation for the
following reasons:
• Extracted groundwater is not expected to be greater than permit discharge
limits.
The need for pretreatment and the high volume of reject effluent that
requires additional treatment prior to disposal. These factors make the
implementation of this technology costly and it requires 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 MSS has an NPDES permit (NC0004987) that authorizes the discharge of
certain waste streams to Lake Norman. When MSS added the primary and
secondary Lined Retention Basin (LRB), the Yard 1a Sump and other significant
changes, NCDEQ issued a modified permit that became effective in May 2018.
Outfall 005 is associated with the LRBs, which have capacity for the extracted
groundwater. Outfall 002 is permitted to discharge water from the ash basin,
during decanting and dewatering (removing the interstitial water from the ash).
The NPDES permit states:
When the facility commences the ash pond/ ponds dewatering, the facility shall treat
the wastewater discharged from the ash pond/ponds using physical -chemical
treatment, if necessary, to assure state Water Quality Standards are not contravened
in the receiving stream. Duke Energy shall notify DWR NPDES Permitting and
DWR Mooresville Regional Office, in writing, within seven calendar days of
installing additional physical -chemical treatment at this Outfall.
A summary of the NPDES limitations for discharge through Outfall 002 during
dewatering and for Outfall 005 is presented in Table 6-10. Anticipated
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concentrations of COIs in extracted groundwater is not expected to exceed
NPDES permit parameters.
Discharge of extracted groundwater utilizing NPDES Outfalls 002 or 005 is a
viable option that is 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 disposal fees
The City of Newton's Clark Creek Wastewater Treatment Plant (WWTP) is
located at 1407 McKay Farm Rd, Newton, NC 28658, or about 19 miles west of
the MSS Site. The plant is permitted for 5 MGD of wastewater. The average daily
flow from the Marshall plant in 2018 was 2.38 MGD. Total flow rates required for
treatment may be greater than 0.94 MGD as discussed in Section 6.5. It is
unlikely that the City of Newton's WWTP will allocate a significant portion of its
available capacity to a single industrial user.
Given the relatively high costs for construction of sewer piping and lift stations,
and ongoing monthly sewer use charges, discharge of extracted groundwater to
the City of Newton wastewater treatment plant is not retained for further
consideration at this time.
Non -Discharge Permit/Infiltration Gallery
Management 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 extracted water by
way of an infiltration gallery must not result in concentrations greater than 02L
groundwater standards. Consequently, groundwater treatment must reliably
produce an effluent waste stream that does not result in a groundwater violation
set by the 02L standard.
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The construction and use of infiltration galleries are permitted under 15A NCAC
02T .0700. The effectiveness of an infiltration system will depend 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.
U.S. Department of Agriculture (USDA) soil map of MSS indicates that a
majority of the native soils fall into the following classifications (USDA, 2019):
• Cecil sandy loam (CaB, CaC, and CaD)
• Pacolet gravelly fine sandy loam (PcC)
• Madison-Udorthents Complex (MkF4)
• Udorthents (Ud), loamy and clayey
The capacity of the most limiting layer of Cecil, Pacolet and Madison-Udorthent
loams to transmit water is described as being moderately high to high (0.57 to
1.98 inches/hour). The capacity of the most limiting layer of this soil type (clayey
Udorthents) is described as very low to high (0.00 to 0.98 inches/hour).
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.
Non -Discharge Permit/Land Application
Land application of groundwater involves the application 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 at 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
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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 existing and
possibly future 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 lowland 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.
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 make 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
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• Each facility shall provide flow equalization with a capacity of 25 percent
of the daily system design flow unless the facility uses lagoon treatment
• Discharge 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
on Table 6-11.
The DWR might require monitoring and reporting to characterize the extracted
groundwater and its effect on surface water, ground water, or wetlands.
Land application of extracted groundwater could be used as a means to maintain
the vegetative cover that will 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 groundwater.
Beneficial Reuse
Beneficial reuse of extracted groundwater involves the evaluation of existing
MSS 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 Station
operations
• Reduce the need and capacity for wastewater treatment
A NCDEQ 2018 Annual Water Use Report for the MSS indicated that water was
withdrawn from Lake Norman every day in 2018. The average daily withdrawal
in a given month ranged from 724.2 million gallons per day (MGD) to 1312.4
MGD. The average daily discharge in a given month ranged from 723.6 MGD to
1310.8 MGD (NCDEQ, 2019).
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Beneficial Reuse: Fire Protection
A limited amount of extracted groundwater might be used to supplement or
supply water stored for fire suppression within MSS 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 Lake Norman. Therefore, beneficial reuse of
groundwater for fire protection is not retained for further consideration.
Beneficial Reuse: Non -Contact Cooling Water
Extracted groundwater was considered as a supplement or supply of makeup
water to the non -contact cooling process for MSS operations. MSS has a once -
through non -contact cooling system that pulls water from Lake Norman and
discharges the water through Outfall 001. Challenges with this beneficial reuse
include making a physical connection to the once -through cooling system to
supply the extracted groundwater, and potential issues with extracted
groundwater alkalinity. The alkalinity of extracted groundwater could pose
potential scaling problems for some applications, although certain constituents
that comprise alkalinity would be diluted by non -contact cooling water obtained
from Lake Norman. The use of groundwater to supplement non -contact cooling
water at MSS is not retained for further consideration.
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 basin during implementation of source control measures. The
use of extracted groundwater for dust suppression and truck washing would be
confined within the limits of the ash basin. However, the need for dust
suppression and truck wash water is limited. The effort and expense to substitute
extracted groundwater for other sources of clean water for dust suppression and
truck washing is not justified. 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 on Table 6-12.
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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
Site groundwater.
• Remedial Alternative 1: Groundwater Remediation by MNA
• Remedial Alternative 2: Groundwater Extraction, Infiltration and In -Situ
Treatment
• Remedial Alternative 3: Groundwater Extraction and Clean Water Infiltration
These groundwater remedial alternatives are presented and described in the following
subsections. Information to address CAP Content Section 6.D.a.iv is provided in Section
6.6 and Section 6.7.
6.5.1 Remedial Alternative 1 — Monitored Natural Attenuation
(MNA)
(CAP Content Section 6.D.a)
Alternative 1 is the use of MNA as a remedial alternative to address
groundwater COI concentrations at or beyond the ash basin compliance
boundary that are at actionable concentrations relative to regulatory standards.
The MSS site has undergone the extensive hydrogeologic characterization
necessary to evaluate natural attenuation processes and rates. Site -specific
groundwater data including saturated media within the saprolite, transition
zone, and bedrock flow zones has been collected at MSS for MNA evaluation. A
comprehensive analysis of MNA is provided in Appendix I.
MNA would involve the construction of 12 new monitoring wells to replace
wells that would be abandoned during implementation of basin closure and
source control measures. These replacement monitoring wells would be installed
along geochemical transects to monitor groundwater concentration trends in the
footprint of the ash basin. There is an extensive groundwater monitoring system
that is associated with the ash basin and adjacent source areas (Figure 1-2). 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.
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6.5.1.1 Problem Statement and Remediation Goals
(CAP Content Section 6.D.a.i)
A limited number of CCR constituents in groundwater associated with the
MSS ash basin and and adjacent source areas occur at or beyond the
compliance boundary to the east of the ash basin 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) (CAP Content Section 6.D.a.i.2). 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 the MSS.
The following groundwater COIs to be addressed by corrective action are
identified (Table 6-6) and discussed in Section 6.1.3: antimony, barium,
beryllium, boron, chloride, cobalt, iron, lithium, manganese, molybdenum,
selenium, strontium, sulfate, thallium, TDS, total radium, and vanadium.
The majority of these COIs only occur at concentrations greater than
applicable regulatory criteria in limited, and often isolated, locations and do
not exhibit a discernable plume.
Detailed evaluations of constituent occurrence are presented in Section 6.1.
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)
Aspects of the conceptual site model will change following source control,
which is ongoing with decanting of the ash basin. The source of COIs in
groundwater will be substantially reduced compared to existing conditions
following decanting of the ash basin and closure. As of December 1, 2019,
128,400,000 gallons of water had been decanted at the MSS ash basin.
Decanting will reduce the potentiometric head responsible for the
downward vertical gradient behind the ash basin dam. A lower downward
gradient would reduce downward COI migration. As a result, constituent
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concentration reductions through natural attenuation processes are
anticipated following decanting.
Currently, COIs in groundwater do not pose an unacceptable risk to human
health or the environment under conservative exposure scenarios.
Residential properties located north, west, and south of the ash basin are
situated in topographically higher areas than the ash basin. The residential
properties are located beyond the topographic divides that control flow
from the basin. Therefore, the groundwater flow direction is locally toward
the ash basin (to the east) and away from residential properties. If
implemented alone, Groundwater Remedial Alternative 1: MNA would not
pose an unacceptable risk to human health or the environment.
More information on one or more of the effective natural attenuation
mechanisms for reducing the concentration of the COIs in groundwater can
be found in Appendix I.
6.5.1.3 Predictive Modeling
(CAP Content Section 6.D.a.iii)
Predictive modeling has been conducted to estimate when boron
concentrations would be reduced to 02L standards using MNA alone
(primarily relying on natural attenuation by dilution). The simulations
indicate boron concentrations would naturally attenuate to less than the 2L
standard in approximately 700 years after basin closure (Figure 6-25). The
extended timeframe to reach 2L is a result of relying on natural processes
(e.g., sorption, precipitation, ion exchange, advection, dispersion, and
dilution) to act on the COIs. No active remedy is employed under MNA to
enhance the groundwater remedy. The flow and transport modeling report
that provides the predictions for boron is presented in Appendix G.
Similarly, a geochemical modeling report is presented in Appendix H. It
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,
Infiltration and In -Situ Treatment
(CAP Content Section 6.D.a)
Groundwater Remedial Alternative 2 involves a multi -technology approach in
two areas at MSS to address groundwater COI concentrations at or beyond the
ash basin compliance boundary that are at actionable concentrations relative to
regulatory standards. Area 1 is designated as the downgradient area east of the
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ash basin along the dam and toward the unnamed tributary to Lake Norman.
Area 2 is designated as the distal southern end of the ash basin dam and area
immediately east of the coal pile. Concentrations of COIs, other than boron (e.g.,
cobalt), are greater than applicable regulatory criteria in this area. Targeted COIs
include: cobalt, lithium, manganese, strontium and thallium.
Predictive flow and transport modeling was conducted to conceptually design
the groundwater remedial approach. The modeling was used to simulate the
response of boron plume concentrations under a variety of groundwater
extraction scenarios. Under this alternative, compliance is achieved in
approximately 30 years from system start-up. The applicable technologies that
comprise Alternative 2 are outlined below:
Area 1:
33 vertical groundwater extraction wells in the vicinity of the northern
portion of the ash basin dam and the northeast tributary to Lake Norman
where concentrations of boron in the saprolite, transition zone and
bedrock exceed 02L standards
• 7.3 acres of shallow infiltration galleries installed along the northeast
portion of the ash basin, between the basin and the tributary
• Pumps, associated piping, and control systems
Figure 6-26 illustrates the proposed locations of the extraction wells and
infiltration gallery in Area 1.
The extraction wells would be completed in the transition (deep) zone and
bedrock. Modeled screen intervals range from 125 to 165 feet bgs for 24 wells and
from 230 to 245 ft bgs for 9 deeper wells. Estimated total flow from the extraction
wells is 314 gpm (approximately 9.5 gpm per well). The groundwater extraction
rate is based on predictive flow and transport modeling, which assumes a 50
percent extraction well efficiency. The extraction wells would be constructed as
6-inch inner -diameter wells with stainless steel wire -wrapped screens. Typical
construction details for the vertical wells are presented in Figure 6-27.
Hydraulic conductivity and infiltration tests would be conducted to determine
the yields of the extraction and infiltration wells in applicable flow zones for each
technology. Hydraulic conductivity test results would be used to size pumps
with the appropriate horsepower and capacity. Pumps, discharge piping,
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pressure gauges, flow totalizers, check valves, flow control valves, and telemetry
hardware would be included in a design package following hydraulic
conductivity test evaluations.
Each extraction well would be piped to a manifold that will direct extracted
groundwater to an equalization tank. Groundwater from the individual
extraction wells is combined in the equalization tank. This results in a blending
of the constituent concentrations and water parameters (e.g., pH, dissolved
oxygen, oxygen -reduction potential, and alkalinity). This provides more
predictable and consistent water quality and flow to the treatment system or
discharge point, as compared to the potential range of values from individual
groundwater extraction wells.
A transfer pump would draw extracted groundwater from the equalization tank.
Extracted groundwater would be discharged to Lake Norman through an
NPDES outfall, likely Outfall 005 or 002.
The purpose of the infiltration of water into the shallow subsurface northeast of
the dam is to flush and mobilize boron from upper flow zones for capture by the
extraction well network and to help reduce the overall boron concentration in
groundwater below the 02L standard. Predictive modeling estimates that the
total area of the infiltration galleries is 7.3 acres. Total injected water is
anticipated to be 46 gpm (0.067 MGD), which is approximately 6.3 gpm per acre.
The predictive flow and transport model assumes a 10% loss to
evapotranspiration for the infiltration gallery. Water suitable for infiltration
could be withdrawn from Lake Norman and treated, as appropriate, prior to
infiltration.
For each acre of the infiltration system, shallow (approximately 3-feet deep)
trenches would be dug. Perforated piping would then be installed and bedded in
clean gravel aggregate to enhance infiltration. The infiltration piping would be
connected to distribution piping with associated valves, flow, and pressure
meters. Each acre (cell) of the infiltration galleries would be independently
monitored and flow adjusted accordingly. Figure 6-28 presents a conceptual
diagram of the water infiltration galleries.
Area 2:
COI concentrations other than boron are greater than comparative regulatory
criteria in the southern portion of the ash basin dam. Targeted COIs include:
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cobalt, lithium, manganese, strontium, and thallium. Remedial Alternative 2
would include the installation of a PRB to address the occurrence of these COIs
above applicable standards.
Due to the relatively shallow depth of bedrock in this area of the Site and the
heterogeneous nature of the fill material used to construct the on -lapping
buttress of the ash basin dam, excavation/construction of a PRB as a backfilled
trench is not recommended. A PRB, however, could be implemented through the
installation of 143 infiltration borings for in -situ groundwater treatment along
the ash basin dam using chemical amendments. Fill (comprised of boulders, blast
remnants, etc.) occupies the area of the proposed infiltration wells. Direct push
technology is likely not a viable option for placement of chemical amendments
due to the heterogeneous fill encountered along the buttress of the dam. Drilled
boreholes would be a preferable alternative. The projected completion zones,
number and depth of infiltration borings are outlined below:
• Shallow/Saprolite (15 to 35 feet bgs):
0 100 feet length / 10 feet spacing =11 infiltration borings
0 300 feet length / 10 foot spacing = 31 infiltration borings
Transition Zone/Upper Bedrock (75 to 100 feet bgs):
0 1,000 feet total length / 10 feet spacing =101 infiltration borings
The type of chemical amendment and application has been estimated in
consultation with Peroxychem, a specialty chemical manufacturer and maker of
MetaFixTM
a) Initial vendor estimates of MetaFixTM to treat the shallow Saprolite, upper
bedrock over the estimated lengths outline above are greater than 60 tons
b) Vendor recommendation for emplacement chemical amendments is direct
push borings (one-time use) in saprolite/fill east of the dam and open
borehole in deep flow zone (upper bedrock)
c) Based on results of vendor experience and a desk -top study, boring
spacing recommended to be 5-15 feet (staggered). Therefore a 10 foot
spacing was used in the Alternative 2 design
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The design for Alternative 2 is based on flow and transport modeling results
(Appendix G), in addition to vendor consultation regarding chemical
amendments for in -situ treatment.
6.5.2.1 Problem Statement and Remediation Goals
(CAP Content Section 6.D.a.i)
A limited number of CCR constituents in groundwater associated with the
MSS ash basin and adjacent source areas occur at or beyond the compliance
boundary to the east of the ash basin 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) (CAP Content Section 6.D.a.i.2). 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 the MSS.
The following groundwater COIs to be addressed by corrective action are
identified (Table 6-6) and discussed in Section 6.1.3: antimony, barium,
beryllium, boron, chloride, cobalt, iron, lithium, manganese, molybdenum,
selenium, strontium, sulfate, thallium, TDS, total radium, and vanadium.
The majority of these COIs only occur at concentrations greater than
applicable regulatory criteria in limited, and often isolated, locations and do
not exhibit a discernable plume.
The conceptual model and predictive modeling discussions summarize the
foundations for development of the groundwater extraction combined with
clean water infiltration 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.
Periodic monitoring of Site groundwater is an important part of any
remedial alternative. Twelve new/replacement monitoring wells would be
installed during implementation of Alternative 2. These wells would be
incorporated into the established Site -wide groundwater monitoring
network to evaluate the performance and effectiveness of the groundwater
remediation.
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6.5.2.2 Conceptual Model
(CAP Content Section 6.D.a.ii)
The conceptual model and predictive modeling discussions summarize the
foundations for development of the groundwater infiltration and extraction
alternative. More extensive discussion of the CSM can be found in Section
5, discussion of flow and transport modeling in Appendix G, and
discussion of geochemical modeling in Appendix H.
Affected groundwater associated with the ash basin and adjacent source
areas, including the coal pile, ILF subgrade structural fill, Phase II Landfill
and PV Structural Fill areas, flows down -gradient to the ash basin and any
COIs are comingled with the ash basin plume. Thus, a remedy designed to
address affected groundwater from the ash basin will also address
groundwater from the adjacent source areas.
Groundwater Remedial Alternative 2, along with source control, will
change certain aspects of the conceptual site model. Simulations of
groundwater extraction along the Lake Norman shoreline predict that the
current hydraulic gradient toward the lake would be reversed, inducing
lake water infiltration into the groundwater system. Standing water in the
ash basin would be decanted under any source control scenario being
considered. When removed, the potentiometric head responsible for the
downward vertical gradient behind the ash basin dam would be reduced.
A lower downward gradient will reduce downward COI migration. As of
December 1, 2019, 128,400,000 gallons of water had been decanted at the
MSS ash basin.
This remedial alternative addresses conservative COIs (e.g., boron, chloride,
sulfate, TDS) through groundwater extraction along the ash basin dam and
eastern roadway between the basin and tributary to Lake Norman.
Alternative 2 would address additional variably reactive constituents
through in -situ treatment along the southwestern portion of the ash basin
dam.
Currently, COIs in groundwater do not pose unacceptable risk to human
health or the environment under conservative exposure scenarios. If
implemented, Groundwater Remedial Alternative 2 would not pose
unacceptable risk to human health or the environment.
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6.5.2.3 Predictive Modeling
(CAP Content Section 6.D.a.iii)
Site -specific data has been incorporated into Alternative 2 modeling and
used to predict when boron concentrations outside the compliance
boundary would satisfy 02L standards. Predictive modeling assumed that
Alternative 2 was fully implemented concurrent with ash basin closure,
beginning in year 2020. The simulated boron plume would recede within
the 500-foot compliance boundary in approximately 30 years from system
start-up (Figure 6-29). The time frame to achieve compliance for boron is
significantly improved over Alternative 1, MNA where boron
concentrations greater than the 02L standard are predicted to extend
beyond the compliance boundary for up to 700 years (Section 6.5.1).
However, no unacceptable risks to human health or the environment were
identified under Alternative 1. The flow and transport modeling report
(Appendix G) and geochemical modeling report (Appendix H) provide
detailed predictions, descriptions, and explanations of the effects of
groundwater extraction.
The combined groundwater flow rate for this extraction system is predicted
to be 314 gpm or 0.45 MGD. This combined groundwater extraction rate is
based on predictive flow and transport modeling, which assumes a 50
percent well efficiency. Hydraulic conductivity tests would be conducted
during the design phase to determine actual groundwater extraction rates.
The predictive flow and transport model assumes a 10% loss to
evapotranspiration for the infiltration gallery. Infiltration tests would be
conducted during the design phase to determine actual clean water
infiltration rates.
6.5.3 Remedial Alternative 3 — Groundwater Extraction and
Clean Water Infiltration
(CAP Content Section 6.D.a)
Groundwater Remedial Alternative 3 involves a multi -technology approach in
two areas at MSS. This remedial alternative is designed to control migration of
dissolved phase COIs at or beyond the ash basin compliance boundary that are
at actionable concentrations relative to regulatory standards. Area 1 is designated
as the northern portion of the ash basin dam and the northeast tributary to Lake
Norman. Area 2 is designated as the southern end of the ash basin dam.
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Predictive flow and transport modeling was conducted to conceptually design
the groundwater remedial approach. The modeling was used to simulate the
response of boron plume concentrations under a variety of groundwater
extraction scenarios. Under this alternative, compliance is achieved in
approximately 9 years from start-up. Concentrations of COIs, other than boron
(e.g., cobalt), are greater than applicable regulatory criteria in the southern
portion of the ash basin dam. Targeted COIs include: cobalt, lithium, manganese,
strontium and thallium. The network of groundwater extraction wells is
designed to capture these COIs through active pumping. The spacing, depths,
and extraction rates of extraction wells placed along the southern portion of the
dam are comparable to other areas north and east of the dam, where the flow
and transport model indicates effective remediation of mobile COIs.
The applicable technologies that comprise Alternative 3 are outlined below:
Areas 1 and 2 -
A network of 66 vertical groundwater extraction wells would be installed
along the lower buttress area of the ash basin dam, from the southern end
toward the northern end, and to the northeast between the tributary to
Lake Norman and the ash basin
• 24 vertical clean water infiltration wells would be installed along the
northeast portion of the ash basin, between the basin and the tributary
• Pumps, associated piping, and control systems
Figure 6-30 illustrates the proposed locations of the extraction wells and
infiltration wells in Areas 1 and 2. Table 6-13 presents a summary of remediation
components included in Alternative 3.
The groundwater extraction wells would be completed in the transition (deep)
zone and bedrock; modeled screen intervals range from 145 to 245 feet bgs.
Estimated total flow from the extraction wells is 652 gpm (approximately 9.9
gpm per well). The groundwater extraction rate is based on predictive flow and
transport modeling, which assumes a 50 percent well efficiency. The extraction
wells will be constructed as 6-inch inner -diameter wells. Typical construction
details for the vertical extraction wells are presented in Figure 6-27.
Hydraulic conductivity and infiltration tests would be conducted to determine
the yields of the extraction and infiltration wells in applicable flow zones for each
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technology. Hydraulic conductivity test results would be used to size pumps
with the appropriate horsepower and capacity. Pumps, discharge piping,
pressure gauges, flow totalizers, check valves, flow control valves, and telemetry
hardware would be included in a design package following hydraulic
conductivity test evaluations.
Each extraction well would be piped to a manifold that will direct extracted
groundwater to an equalization tank. Groundwater from the individual
extraction wells is combined in the equalization tank. This results in a blending
of the constituent concentrations and water parameters (e.g., pH, dissolved
oxygen, reduction -oxidation potential, and alkalinity). This provides more
predictable and consistent water quality and flow to the treatment system or
discharge point, as compared to the potential range of values from individual
groundwater extraction wells.
A transfer pump would draw extracted groundwater from the equalization tank.
Extracted groundwater would be discharged to Lake Norman through an
NPDES outfall, likely Outfall 005 or 002.
The purpose of the clean water infiltration northeast of the dam is to flush and
mobilize boron from upper flow zones for capture by the extraction well network
and to help reduce the overall boron concentration in groundwater to below the
02L standard. Predictive modeling estimates that the total flow rate of infiltrated
water is 285 gpm (0.41 MGD), which is an average of approximately 11.8 gpm
per well. The groundwater infiltration rate is based on predictive flow and
transport modeling, which assumes a 25 percent infiltration well efficiency.
Water suitable for infiltration could be withdrawn from Lake Norman and
treated, as appropriate, prior to infiltration.
Water distribution piping would be installed in trenches with electrical and
effluent water piping. The infiltration well piping at each well head would be
connected to distribution piping with associated valves, flow and pressure
meters. Each well would be independently monitored and flow adjusted
accordingly. Typical construction details for the vertical clean water infiltration
wells are presented in Figure 6-31.
The design for Alternative 3 is based on flow and transport modeling results
(Appendix G).
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6.5.3.1 Problem Statement and Remediation Goals
(CAP Content Section 6.D.a.i)
A limited number of CCR constituents in groundwater associated with the
MSS ash basin and adjacent source areas occur at or beyond the compliance
boundary to the east of the ash basin 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) (CAP Content Section 6.D.a.i.2). 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 the MSS.
The following groundwater COIs to be addressed by corrective action are
identified (Table 6-6) and discussed in Section 6.1.3: antimony, barium,
beryllium, boron, chloride, cobalt, iron, lithium, manganese, molybdenum,
selenium, strontium, sulfate, thallium, TDS, total radium, and vanadium.
The majority of these COIs only occur at concentrations greater than
applicable regulatory criteria in limited, and often isolated, locations and do
not exhibit a discernable plume.
The conceptual model and predictive modeling discussions summarize the
foundations for development of the groundwater extraction combined with
clean water infiltration 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.
Periodic monitoring of Site groundwater is an important part of any
remedial alternative. Twelve new/replacement monitoring wells would be
installed during implementation of Alternative 3. These wells would be
incorporated into the established Site -wide groundwater monitoring
network to evaluate the performance and effectiveness of the groundwater
remedial alternative. In addition, a routine program of extraction and
infiltration well performance monitoring and well
rehabilitation/redevelopment would be implemented during system
operation to maintain system effectiveness.
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6.5.3.2 Conceptual Model
(CAP Content Section 6.D.a.ii)
The conceptual model and predictive modeling discussions summarize the
foundations for development of the groundwater extraction and clean water
infiltration alternative. More extensive discussion of the CSM can be found
in Section 5, discussion of flow and transport modeling in Appendix G,
and discussion of geochemical modeling in Appendix H.
Affected groundwater beneath the northern basin areas and adjacent source
areas, including the coal pile, ILF subgrade structural fill, Phase II Landfill
and PV Structural Fill areas flows downgradient to the ash basin and any
COIs are comingled with the ash basin plume. Thus, a remedy designed to
address affected groundwater from the ash basin will also address
groundwater from the northern basin area.
Groundwater Remedial Alternative 3, along with source control, will
change certain aspects of the conceptual site model. Simulations of
groundwater extraction along the Lake Norman shoreline predict that the
current hydraulic gradient toward the lake would be reversed, inducing
lake water infiltration into the groundwater system. Standing water in the
ash basin would be decanted under any source control scenario being
considered. When removed, the potentiometric head that is producing the
downward vertical gradient behind the ash basin dam will be reduced. A
decreased downward gradient will reduce the rate of downward COI
migration. As of December 1, 2019, 128,400,000 gallons of water had been
decanted at the MSS ash basin.
Remedial Alternative 3 addresses conservative, non -conservative and
variably reactive COIs through groundwater extraction along the ash basin
dam and eastern roadway between the basin and tributary to Lake Norman.
Clean water infiltration along the eastern roadway would flush boron from
the unsaturated zone where it can be captured by the groundwater
extraction wells.
Currently, COIs in groundwater do not pose an unacceptable risk to human
health or the environment under conservative exposure scenarios. If
implemented, Groundwater Remedial Alternative 3 would not pose an
unacceptable risk to human health or the environment.
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6.5.3.3 Predictive Modeling
(CAP Content Section 6.D.a.iii)
Site -specific data has been incorporated into Alternative 3 modeling and
used to predict when boron concentrations outside the compliance
boundary would satisfy 02L standards. Predictive modeling assumed that
Alternative 3 was fully implemented concurrent with ash basin closure,
beginning in year 2020. The simulated boron plume would recede within
the 500-foot compliance boundary in approximately 9 years from start-up
(Figure 6-32). The material differences between Alternatives 2 and 3 are (1)
the number of groundwater extraction wells and associated total extraction
rate, (2) infiltration of clean water via vertical wells as opposed to shallow
infiltration galleries, and (3) active groundwater extraction along the
southern portion of the dam buttress as opposed to in -situ treatment.
When compared to Alternative 2 (approximately 30 years), the estimated
time frame for achieving compliance for boron under Remedial Alternative
3 (approximately 9 years) is an improvement. It is a significant
improvement over Alternative 1 (MNA) where boron concentrations greater
than the 02L standard are predicted to extend beyond the compliance
boundary for up to 700 years. However, no unacceptable risks to human
health or the environment were identified under Alternative 1. The flow
and transport modeling report is presented in Appendix G, and
geochemical modeling report is presented in Appendix H. Both of these
reports provide detailed descriptions, predictions, and explanations of the
effects of groundwater remediation under Alternative 3.
The combined groundwater flow rate for this extraction system is predicted
to be 652 gpm or 0.94 MGD. This combined groundwater extraction rate is
based on predictive flow and transport modeling, which assumes a 50
percent well efficiency. Table 6-16 presents detailed extraction well design
based on modeled parameters.
Predictive modeling estimates that the total flow rate of infiltrated clean
water is 285 gpm (0.41 MGD), which is an average of approximately 11.8
gpm per well. The groundwater infiltration rate is based on predictive flow
and transport modeling, which assumes a 25 percent infiltration efficiency
to account for well skin effects. Table 6-15 presents detailed infiltration well
design based on model parameters. Hydraulic conductivity tests would be
conducted during the design phase to determine actual groundwater
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extraction rates. Infiltration tests would be conducted during the design
phase to determine actual clean water infiltration rates.
6.6 Remedial Alternative Screening Criteria
(Supplemental Information for 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
the MSS. Each groundwater remedial alternative formulated and discussed in Section
6.5 has undergone detailed comparative analysis using the screening criterion described
below. These screening criteria are based upon the criteria outlined in 15A NCAC 02L
.0106(i), 40 CFR 300.430, and Guidance for Conducting Remedial Investigations and
Feasibility Studies Under CERCLA (USEPA, 1988). The screening criteria are as follows:
• Protection of human health and the environment
• Compliance with applicable regulations
• Short-term effectiveness
• Long-term effectiveness and permanence
• Reduction of toxicity, mobility, and volume
• Technical and logistical feasibility
• Time required to initiate and implement corrective action
• Time required to achieve remediation goals
• Cost
• Community acceptance
Additional considerations for remedial alternative evaluations include:
• Adaptive site management and remediation considerations
• Sustainability
Protection of Human Health and the Environment
Protection of human health and the environment is paramount in the evaluation of any
remedial alternative. Technologies and remedial alternatives are evaluated to
determine whether they can achieve regulatory compliance within a reasonable time
frame, without detriment to human health and the environment. Remedial alternatives
that are not protective of human health and the environment should be rejected from
consideration solely on this basis.
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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)
• CCR (40 CFR § 257.96)
• Well construction and maintenance standards (NCAC Title 15A Subchapter 02C)
• NPDES (40 CFR Part 122)
• Sediment erosion and control (NCAC Title 15A Chapter 04)
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 disposal 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
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• Hydraulic conductivity testing
• Groundwater remedial alternative system design
• Permitting
• System installation
• System startup
These activities might be requisite to finalize the system design, attain regulatory
approval, or initiating construction. Therefore, these activities might dictate the time it
takes to initiate and fully implement a remedial alternative.
Short-term Effectiveness
The short-term impacts of alternatives will be assessed considering the following:
• Protection of the community during implementation of the proposed remedial
action
• Protection of workers during implementation of the proposed remedial action
• Potential environmental impacts during implementation of the proposed
remedial action and the effectiveness of measures taken to mitigate potential
environmental impacts
• Consideration of short-term responsiveness, increasing or decreasing
concentrations during start-up and implementation
• Timeframe to achieve performance criteria
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.
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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
• What type and quantity of residuals will remain
• 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
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 operation and maintenance of 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. Likewise, the fiscal benefit of a remedial alternative
having relatively low capital costs might be offset by relatively high and long-term
operation and maintenance (O&M) costs.
Community Acceptance
This assessment considers probable 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.
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Adaptive Management
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 may request alternate standards
for ash basin -related constituents, including boron, as allowed under 15A NCAC 02L
.0106(k). Alternate standards are appropriate at the MSS given the lack of human health
and ecological risks at the Site. Factors included in this evaluation include:
• 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
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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
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 criterion. 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, PMio, and energy usage related to materials procurement, installation and
operation.
Results of these sustainability evaluations are presented and discussed in the detailed
analysis sections of the specific alternatives (Section 6.7). Assumptions and parameters
used in the sustainability calculations are presented in Appendix L.
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6.7 Remedial Alternatives Criteria Evaluation
(CAP Content Section 6.D.a.iv)
Groundwater remediation alternatives 1, 2 and 3, as described in Section 6.5, were
formulated based on the groundwater remediation technologies that were evaluated
and retained for consideration per Section 6.4. The criteria for each groundwater
remedial alternative were presented in Section 6.6. Detailed comparative analysis of the
groundwater remediation alternatives are presented 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 ash basin have been identified (Appendix E). 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 ash basin. This
conclusion is further supported by multiple water quality and biological
assessments conducted by Duke Energy as part of the NDPES monitoring
program. Water supply wells are located upgradient of the ash basin and
permanent water solutions have been provided to those who selected this option.
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.
Compliance with Applicable Regulations
(CAP Content Section 6.D.a.iv.2)
Alternative 1 can be fully implemented in compliance with applicable laws and
regulations. As it pertains to the selection of a groundwater remedy, the North
Carolina Coal Ash Management Act of 201415A NCAC 13B .1636 states that the
selected remedy will:
1) Be protective of human health and the environment
2) Attain approved groundwater protection standards
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3) Control the source(s) of releases to reduce or eliminate, to the maximum
extent practicable, further releases of constituents into the environment
that may pose a threat to human health or the environment
4) Comply with standards for management of wastes as specified in Rule
.1637(d)
As stated in Section 6.8.1, MNA would be protective of human health and the
environment. MNA would eventually satisfy groundwater protection standards
while being protective of human health and the environment going forward. The
only waste generated by MNA would be investigation derived wastes (IDW)
such as soil cuttings during the installation of monitoring wells and purge water
generated during groundwater sampling. IDW can be managed in compliance
with applicable management standards.
MNA would be conducted with the goal of achieving the 02L standards (15A
NCAC 02L) at the compliance boundary. Groundwater remedial alternatives 2
or 3 would serve as a contingency groundwater remedy if MNA is later
determined to be ineffective.
Samples of Lake Norman surface water immediately downgradient of the source
area have been tested and comply with applicable 15A NCAC 02B standards
(Appendix J). As demonstrated in the surface water future conditions
evaluation, future groundwater migration from the source area under either
closure -in -place or closure -by -excavation scenarios would not result in
constituent concentrations at greater than 02B surface water standards in the
unnamed tributary or Lake Norman (Appendix J).
New MNA monitoring well installations must satisfy applicable requirements of
NCAC Title 15A Subchapter 02C, Well Construction Standards, including 15A
NCAC 02C .0108 (Standards of Construction) and 15A NCAC 02C .0112 (Well
Maintenance).
Compliance with applicable regulations should not materially affect the
implementability, effectiveness, or cost of Groundwater Remedial Alternative 1.
Appendix I includes a detailed evaluation of the applicability of Alternative 1:
MNA as a remedial alternative for the Site.
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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).
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 installed a permanent water
solution (municipal water or filtration systems) at 65 households within a half -
mile of the ash basin compliance boundary in accordance with G.S. Section 130A-
309.211(c1). Furthermore, institutional controls (provided by the RS [restricted]
designation) to limit access to groundwater use are proposed.
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)
COIs 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 is an extensive network of monitoring wells associated with the ash basin.
Groundwater monitoring parameters and the monitoring frequency would be
used to evaluate changes in groundwater quality and effectiveness of the
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remedial alternative through inter -well and intra-well comparisons. Although
some wells within the immediate area of the basin will have to be abandoned
and replaced as part of closure, monitoring wells along the waste boundary and
at select downgradient areas would remain to monitor natural attenuation in the
short-term.
Technical and Logistical Feasibility
(CAP Content Section 6.D.a.iv.6)
Groundwater Remedial Alternative 1 is technically feasible and readily
implementable. Construction of Alternative 1 would involve the installation of
approximately 12 MNA groundwater monitoring wells following completion of
source control measures. The wells would be installed along geochemical
transects to monitor constituent concentration trends within the footprint of ash
basins because some existing wells would be removed during closure.
Installation of groundwater monitoring wells is routine. It would involve a
utility clearance of the area where monitoring wells will be installed. All
groundwater monitoring wells would be installed by a licensed driller.
Afterwards, each well installation would be surveyed for location and elevation.
Material requirements, material availability, and the availability of specialized
services (e.g., licensed drillers, licensed surveyors) and labor are readily
available. Implementation of MNA would not involve direct permitting. Once
implemented, MNA would involve long-term groundwater monitoring and
reporting. Otherwise, there are no "operations" associated with MNA.
MNA relies on natural attenuation processes, which would provide reliable
results as long as the geochemistry (e.g., pH and Eh) within the footprint of the
ash basin achieves equilibrium while taking into account variability attributed to
seasonality. However, natural attenuation processes could be affected by shifts in
Site geochemistry beyond seasonal variability. An MNA effectiveness
monitoring program (EMP) would be developed to assess the effectiveness of
Alternative 1 and monitor key geochemical parameters within the ash basin
footprint going forward.
Time Required to Initiate and Implement Corrective Action
Technologies and Alternatives
(CAP Content Section 6.D.a.vi.7)
The time required for implementation of an MNA program could be as
immediate as the approval of an approach since an extensive monitoring well
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network already exists. Procedures for collection, analysis, and communication
of results are also established and currently in place.
Predicted Time Required to Meet Remediation Goals
(CAP Content Section 6.D.a.iv.8)
The flow and transport model predicts that concentrations of COIs would meet
02L standards at the compliance boundary in approximately 700 years after ash
basin closure (assumed as year 2032 in the model). This estimate is based on
boron reaching a concentration of 700 µg/L at the existing compliance boundary
(Figure 6-25).
Cost
(CAP Content Section 6.D.a.iv.9)
The cost estimate for this groundwater remediation alternative is based on
capital costs for design and implementation including the installation of 12 new
monitoring wells. The design costs include work plans, design documents and
reports necessary for implementation of the alternative. Implementation costs
include procurement and construction.
Costs to implement, operate, and manage the MSS MNA program would include
annual costs and expenses associated with routine O&M, labor and materials to
perform groundwater sampling. Costs also included routine labor for annual and
5-year reporting. A summary of the estimated costs for this alternative are
provided in Appendix K.
Community Acceptance
(CAP Content Section 6.D.a.iv.10)
It is expected that there will be positive and negative sentiment about
implementation of an MNA program. Community stakeholders might consider a
700-year time frame to achieve remediation goals for boron to be unacceptable.
Community stakeholders with concerns regarding the capital and near -term
O&M costs associated with the three alternatives may favor a less costly
alternative. Until the final Site remedy is developed and comments are received
and reviewed, assessment of community acceptance will not be fully known.
MNA as a remedial alternative would be protective of human health and the
environment. Consistent with the USEPA Office of Solid Waste and Emergency
Response (OSWER) Directive 9200.4-17P (April 21, 1999) the use of MNA "does
not imply that EPA or the responsible parties are 'walking away' from cleanup or
financial responsibility at a site."
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Adaptive Site Management and Remediation Considerations
MNA is an adaptable process and can be an effective tool in identifying the need
for alternative approaches if unexpected changes in Site conditions occur. An
MNA program would not hinder or preempt the use of other remedial
approaches in the future if conditions change. In fact, an effectiveness monitoring
program is an essential part of any future remedial strategy. An MNA
effectiveness monitoring program would provide information about changing
Site conditions during and after source control measures.
Groundwater Remedial Alternative 1 is readily amenable to contingencies or
modifications if it is later determined that MNA is an inadequate remedy, or that
supplemental initiatives are necessary to enhance MNA performance.
Sustainability
Sustainability analysis was completed as described in Section 6.6. The footprint
was quantified based on energy use and associated emissions, during the
construction phase (e.g., material quantities and transportation) and
groundwater monitoring activities (e.g., transportation). The results of the
footprint calculations for MNA are summarized in Table 6-14. A summary of
sustainability calculations for Alternative 1 can be found in Appendix L.
The footprint of Alternative 1 is the least energy -intensive of the remedial
alternatives being considered, providing reduced, comparative footprint metrics
in overall energy use and across all air emission parameters. Alternative 1
utilizes significantly fewer resources during construction and throughout the
cleanup timeframe when compared to the other alternatives.
6.7.2 Remedial Alternative 2: Groundwater Extraction,
Infiltration and In -Situ Treatment — Compliance in the
Midterm
Protection of Human Health and the Environment
(CAP Content Section 6.D.a.iv.1)
Groundwater Remedial Alternative 2 is protective of human health and the
environment. Groundwater COIs do not pose an unacceptable risk to potential
receptors under conservative risk assessment exposure scenarios (Appendix E).
Alternative exposure scenarios are not anticipated as long as Duke Energy owns
and controls the property where groundwater COIs exist and institutional
controls (e.g., 15A NCAC 02L) remain in place. Furthermore, Lake Norman
surface water immediately downgradient of the ash basin have been tested and
comply with applicable 15A NCAC 02B standards (Appendix J). This conclusion
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is further supported by multiple water quality and biological assessments
conducted by Duke Energy as part of the NDPES monitoring program.
The updated human health and ecological risk assessment concluded there is no
evidence of unacceptable risks to human and ecological receptors exposed to
environmental media potentially affected by CCR constituents at MSS
(Appendix E). Potential risks to human health and the environment are within
acceptable levels prescribed by the USEPA.
Compliance with Applicable Regulations
(CAP Content Section 6.D.a.iv.2)
Alternative 2 can be implemented in compliance with applicable laws and
regulations. Those regulations would include: CAMA, groundwater standards,
infiltration and extraction well installation and permitting. Waste generated by
Alternative 2 would include IDW (e.g., soil cuttings, purge water) and extracted
groundwater. IDW can be managed in compliance with applicable management
standards.
Alternative 2 would be conducted with the goal of achieving 02L groundwater
standards (15A NCAC 02L) beyond the compliance boundary. Monitoring well
and groundwater extraction well installations must satisfy applicable
requirements of NCAC Title 15A Subchapter 2C, Well Construction Standards,
including 15A NCAC 02C .0108 (Standards of Construction) and 15A NCAC 02C
.0112 (Well Maintenance). Permits would be needed for groundwater withdrawal
and surface water withdrawals from Lake Norman greater than 100,000 gallons
per day.
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.
Current requirements for a certified wastewater treatment plant operator for the
influent to Outfall 002 would probably satisfy any future requirements for
pretreatment/treatment of extracted groundwater prior to discharge via a
permitted outfall.
Infiltration of chemical amendments along the southern portion of the ash basin
dam would affect in -situ treatment of variably reactive COIs in groundwater.
The infiltration of water into the shallow subsurface upgradient of the dam will
aid in flushing COIs from unsaturated soils. Underground infiltration of water,
or water with chemical amendments, must comply with 15A NCAC 02C .0225
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(Groundwater Remediation Wells). Any amendments infiltrated into groundwater
approved by the North Carolina Department of Health and Human Services
(NCDHHS). A risk assessment evaluation must be completed and submitted to
the Occupational and Environmental Epidemiology Section (OEES) of the
Division of Public Health within NCDHHS for any amendment that is not
already approved by NCDHHS.
Compliance with applicable regulations should not affect the implementability,
effectiveness, or cost of Alternative 2.
Long-term Effectiveness and Permanence
(CAP Content Section 6.D.a.iv.3)
Groundwater 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 effectively
removed as predicted by modeling results. Flow and transport modeling
indicates that implementation of Groundwater Remedial Alternative 2, in
conjunction with source control measures, would achieve 02L compliance for
boron within approximately 30 years from system start-up. Furthermore, the
mass of boron and related COI concentrations would be permanently reduced by
groundwater extraction. In -Situ treatment would be used to address variably
reactive COIs near the southern portion of the ash basin dam. Natural
attenuation mechanisms would further reduce COI concentrations following the
shutdown of the groundwater extraction system.
Coal ash constituents within the compliance boundary do not pose an
unacceptable risk to human health since there are no complete routes for
potential exposure. Construction of water supply wells is prohibited within the
compliance boundary of an individually permitted disposal system (15A NCAC
02L .0107 (d)). Groundwater monitoring will continue at the compliance
boundary in accordance with 02L.
The risk to human health and the environment is within acceptable levels
prescribed by the USEPA. The risk to human health and the environment is also
expected to decrease over time following implementation of Alternative 2.
Performance monitoring would be conducted in accordance with the 02L
standard, or applicable federal regulations. Institutional controls, including 15A
NCAC 02L .0107(d), restrict activities that could result in exposure to
groundwater COIs.
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NPDES discharge requirements are protective of human health and the
environment. Extracted groundwater discharged via NPDES Outfall 005 or 002
would comply with applicable discharge requirements and would not pose an
unacceptable risk to human health and the environment.
Reduction of Toxicity, Mobility, and Volume
(CAP Content Section 6.D.a.iv.4)
Implementation of Alternative 2 would help reduce COI concentrations and,
therefore, the toxicity, mobility, and volume of affected groundwater by
groundwater extraction and altering groundwater chemistry in the south dam
area with a PRB.
Constituents most amenable to groundwater extraction are the conservative/non-
reactive COIs followed by variably reactive constituents. Groundwater extraction
would have the least effect on non-conservative/reactive constituents. These
constituents would be best addressed in -situ in the PRB.
Groundwater underlying the ash basin footprint also would be subject to the
influences of natural attenuation. The mechanisms that naturally attenuate the
concentrations of CCR inorganic constituents are dilution, dispersion, advection,
sorption (including ion exchange and precipitation) and phyto-attenuation.
The volume of groundwater containing COIs at concentrations greater than
groundwater standards would be reduced over a measurable time frame. For
example, Groundwater Remedial Alternative 2 would permanently reduce the
concentrations and areal extent of the boron plume in groundwater as defined by
the 02L standard (700 µg/L).
The in -situ treatment of variably reactive COIs by chemical amendments will not
reduce the volume of these constituents in the subsurface but will sequester them
and make the COIs unavailable to advect with groundwater towards the natural
discharge in Lake Norman.
Short-term Effectiveness
(CAP Content Section 6.D.a.iv.5)
The stability and limited areal extent of the COI plume, along with the absence of
complete exposure pathways, indicates there are no short-term impacts to 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.
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Implementation of Groundwater Remedial Alternative 2 would be protective of
communities adjacent to and near MSS. Installation of groundwater monitoring
wells, groundwater extraction wells, discharge lines, collection tanks and related
infrastructure are straight forward and routine tasks that can be conducted
safely. Groundwater COIs do not pose an unacceptable risk to potential receptors
under conservative risk assessment exposure scenarios (Appendix E).
Extracted groundwater would be managed via NPDES discharge. Compliance
with NPDES Permit NC0004987 should make discharges of extracted
groundwater protective of potential on -Site and off -Site receptors.
Hydraulic capture of groundwater near the groundwater extraction wells would
be aided by the infiltration galleries and would occur soon after the groundwater
extraction system is placed into service. Also, the advancement of the boron
plume beyond the compliance boundary would be mitigated as long as
hydraulic capture is sustained. The infiltration of chemical amendments along
the southern portion of the ash basin dam would help to immobilize variably
reactive COIs as they advect through the zone of treatment with groundwater.
Technical and Logistical Feasibility
(CAP Content Section 6.D.a.iv.6)
Groundwater Remedial Alternative 2 is technically feasible; however,
implementation presents challenges. Direct push technology is likely not a viable
option for placement of chemical amendments due to the heterogeneous fill
encountered along the buttress of the dam. Drilled boreholes would be a
preferable alternative. However, the estimated 143 drilled boreholes for
emplacement of chemical amendments are not considered the most efficient
technology to implement remediation in this area. Installation of the proposed
groundwater extraction system, clean water infiltration, and in -situ treatment
and 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. 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. However, due to the
implementability challenges with in -situ treatment mentioned above, this
alternative is not considered the best alternative to achieve remediation goals.
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Alternative 2 would be implemented concurrent with source control measures.
Activities contemplated to implement Alternative 2 are routine with respect to
the maturity of technologies used, material requirements and availability, and
the availability of specialized services (e.g., licensed drillers, electricians) and
labor. Similarly, Remedial Alternative 2 is technically implementable with
respect to the suitability and availability of extraction well installation locations
and associated infrastructure. Most of the extraction well installations will be
between the northern end of the ash basin dam and the access road, and between
the ash basin and tributary to Lake Norman. Infiltration borings will be located
along the southern portion of the dam.
Implementation of Remedial Alternative 2 can be achieved administratively.
Obtaining an Underground Injection Control (UIC) permit to infiltrate water or
water with chemical amendments pre -approved by NCDHHS should be a
straightforward process. Likewise, obtaining groundwater and surface water
withdrawal permits should be readily achievable. The NPDES permit may
require modifications to allow for the discharge of groundwater.
Consideration for dam safety is paramount; however, it appears that there are
ample locations on the lower buttress of the dam for the safe installation of
extraction wells, infiltration borings and associated utilities. No well installations
or construction will occur on the dam or lower buttress area without first
obtaining the requisite permits from Duke Energy and North Carolina
Environmental Quality Energy, Mineral and Land Resources.
In the area where in -situ remediation is proposed, fill (comprised of boulders,
blast remnants, etc.) is beneath the surface. The extent of that fill is not known,
but it is a major consideration in using direct push to insert the amendment into
the area on a very close grid (e.g., 10 foot spacing).
The area where infiltration is to be implemented slopes severely. This slope
poses access challenges for construction. There is a challenge in construction of
the shallow infiltration galleries proposed for this area in getting the water to
infiltrate vertically rather than travel with the slope to Lake Norman. Also,
significant excavation would be required for installation of the shallow
infiltration galleries. Disturbing this steeply sloping area raises questions about
the unacceptable transport of sediment into Lake Norman.
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Time Required to Initiate and Implement Corrective Action
Technologies and Alternatives
(CAP Content Section 6.D.a.iv.7)
Groundwater extraction and treatment proposed under Remedial Alternative 2
would be implemented concurrent with source control measures (basin closure).
Hydraulic conductivity tests would be conducted to validate groundwater yields
predicted by flow and transport modeling. Hydraulic conductivity test results
would be used to refine predictive modeling and adjustments would be made to
the groundwater remedial system design, if warranted.
Bench scale testing will be used to screen the effectiveness of MetaFixTM or other
amendments to treat variably reactive COIs near the south end of the ash basin
dam. A pilot test would be conducted following the selection of a chemical
amendment based on bench scale testing results. Design of the groundwater
extraction system proposed under Alternative 2 would be finalized afterwards.
Hydraulic conductivity tests, preparation of the final design, preparation of bid
documents, and submission of bid documents to prospective bidders would be
accomplished following NCDEQ approval of the CAP Update. No other
prerequisites, such as permitting, bench scale testing, and pilot testing, are
anticipated to delay initiation of Groundwater Remedial Alternative 2. Full-scale
operation of the groundwater remediation system would be implemented
following completion of construction, start-up, break-in, and NCDEQ approval.
Predicted Time Required to Meet Remediation Goals
(CAP Content Section 6.D.a.iv.8)
Groundwater extraction, infiltration, and in -situ treatment under Alternative 2
would be implemented concurrently with ash basin closure. 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 to be approximately 30 years following full
implementation of Remedial Alternative 2.
Cost
(CAP Content Section 60.a.iv.9)
Costs to implement, operate, and manage Groundwater Remedial Alternative 2
would include expenses associated with the design, permitting and construction
management for the installation of 12 new monitoring wells, 33 groundwater
extraction wells, 7.3 acres of clean water infiltration galleries, and 143 borings for
the infiltration of chemical amendments.
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Construction costs also include trenching for utilities and piping of infiltration
water and extracted groundwater, equalization tanks, and piping infrastructure
for the transfer of water to a permitted outfall. O&M expenses would include
costs to operate the groundwater infiltration/extraction system, groundwater
monitoring program, extraction system performance monitoring, and periodic
reporting for a 30-year period.
Costs for implementation, O&M, monitoring, and reporting for Groundwater
Remedial Alternative 2 are provided in Appendix K.
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 system. No landowner is
anticipated to be affected. It is anticipated that the extracted groundwater would
be discharged through a NPDES permitted outfall that flows to Lake Norman
and that the discharge would meet all permit limits. A groundwater extraction
system that addresses potential COI plume expansion across the entire southern
perimeter of the ash basin and east to the unnamed tributary may improve
public perception of the groundwater remedy.
It is anticipated that groundwater extraction under Alternative 2 would generally
receive more positive community acceptance than MNA under Alternative 1
since Alternative 2 involves more active measures to attempt in -situ treatment
and physical extraction of COI mass from groundwater and would likely be
perceived as more robust than MNA.
It is possible that some community stakeholders might have concerns with
potential exposure to discharged groundwater via NPDES permit. Assurances
that any means of groundwater management will be permitted and monitored
by NCDEQ should alleviate stakeholder concerns. Stakeholder concerns should
be further alleviated when they understand that extracted groundwater would
undergo treatment, if necessary, and that constituent concentrations in the
discharged groundwater would be within permitted limits. Until the final Site
remedy is developed and comments are received and reviewed, assessment of
community acceptance will not be fully known.
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
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or COI concentrations. Individual well pumping rates can be adjusted or
eliminated or additional wells can be installed to address COI plume changes.
Following full-scale implementation, it will be important to evaluate
Groundwater Remedial Alternative 2 performance to determine if operational
changes could result in greater efficiencies or shorter remediation time frames.
For example, additional extraction wells could be added to the remedial system
to augment pumping or selected groundwater extraction wells could be
repurposed and converted into clean water infiltration wells to augment
infiltration provided by the infiltration galleries.
Groundwater Remedial Alternative 2 is readily amenable to other contingencies.
The quantity of chemical amendments to be injected as a PRB along the southern
portion of the ash basin dam will be estimated in the remedial design. A
contingency would include whether there will be a need to have the
amendments replenished one or more times to remain effective. However,
bench -scale and field -scale pilot studies, as appropriate, prior to full-scale field
implementation will help to quantify these uncertainties and assure that the
remedy will be successful.
Sustainability
Sustainability analysis was completed as described in Section 6.6. The footprint
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 Remedial 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. Alternative 1 (MNA) requires significantly less materials and
energy than Alternative 2 and is therefore characterized by a dramatically
smaller footprint. Conversely, Alternative 2 generates a dramatically larger
footprint than Alternative 3. Compared to Alternative 3, Alternative 2 utilizes 33
fewer extraction wells, does not employ 24 clean water infiltration wells, but
does propose the use of a 7.3-acre infiltration gallery and the in -situ placement of
approximately 60 tons of reactive media through 143 drilled boreholes. The
additional remediation system components required by Alternative 2 will
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generate higher material -related footprint emissions for the construction phase
than Alternative 3. Additionally, the increased timeframe of remediation system
operation for Alternative 2 (30 years) when compared to Alternative 3 (9 years)
produces air emissions more than five times the levels of Alternative 3. The
quantitative analysis of the footprints of the remedial alternatives under
consideration for this CAP Update indicates Alternative 2 to be the least
sustainable option.
6.7.3 Remedial Alternative 3: Groundwater Extraction and
Clean Water Infiltration
Protection of Human Health and the Environment
(CAP Content Section 6.D.a.iv.1)
Groundwater Remedial Alternative 3 is protective of human health and the
environment. Groundwater COIs do not pose an unacceptable risk to potential
receptors under conservative risk assessment exposure scenarios (Appendix E).
Alternative exposure scenarios are not anticipated as long as Duke Energy owns
and controls the property where groundwater COIs exist and institutional
controls (e.g., 15A NCAC 02L) remain in place. Furthermore, Lake Norman
surface water immediately downgradient of the ash basin has been tested and
comply with applicable 15A NCAC 02B standards (Appendix J). This conclusion
is further supported by multiple water quality and biological assessments
conducted by Duke Energy as part of the NDPES monitoring program. If
implemented, Alternative 3 would be protective of human health and the
environment.
The updated human health and ecological risk assessment concluded that there
is no evidence of unacceptable risks to human and ecological receptors exposed
to environmental media potentially affected by CCR constituents at MSS
(Appendix E). Potential risks to human health and the environment are within
acceptable levels prescribed by the USEPA.
Compliance with Applicable Regulations
(CAP Content Section 6.D.a.iv.2)
Remedial Alternative 3 can be fully implemented in compliance with applicable
laws and regulations. Those regulations would include: CAMA, groundwater
standards, extraction and infiltration well installation and permitting. Waste
generated by Alternative 3 would include IDW (e.g., soil cuttings, purge water)
and extracted groundwater. IDW can be managed in compliance with applicable
management standards.
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Alternative 3 would be conducted with the goal of achieving 02L groundwater
standards (15A NCAC 02L) beyond the compliance boundary. Monitoring well
and groundwater extraction/clean water infiltration well installations must
satisfy applicable requirements of NCAC Title 15A Subchapter 2C, Well
Construction Standards, including 15A NCAC 02C .0108 (Standards of
Construction) and 15A NCAC 02C .0112 (Well Maintenance). Permits would be
needed for groundwater withdrawal and surface water withdrawals from Lake
Norman greater than 100,000 gallons per day.
Discharge of extracted water would be in compliance with appropriate discharge
requirements, such as pH or other COI limitations in the NPDES permit.
However, the NPDES permit may need to be modified to allow for the discharge
of groundwater through one of the outfalls. Any current requirements for a
certified wastewater treatment plant operator for the influent to Outfall 002
would probably satisfy any future requirements for pretreatment/treatment of
extracted groundwater prior to discharge via a permitted outfall.
The infiltration of clean water into the subsurface upgradient of the dam will aid
in flushing COIs from unsaturated soils. Underground infiltration of water must
comply with 15A NCAC 02C .0225 (Groundwater Remediation Wells).
Compliance with applicable regulations should not affect the implementability,
effectiveness, or cost of Alternative 3.
Long-term Effectiveness and Permanence
(CAP Content Section 6.D.a.iv.3)
Groundwater extraction and clean water infiltration will contribute to be an
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 effectively removed as predicted by modeling results.
Flow and transport modeling indicates that implementation of Groundwater
Remedial Alternative 3 in conjunction with anticipated source control measures
(basin closure) will achieve 02L compliance for boron within approximately 9
years after the remedial system is placed into service. Furthermore, the mass of
concentrations of boron will be permanently reduced as a consequence of
groundwater extraction and clean water infiltration.
Coal ash constituents within the compliance boundary should not pose a risk to
human health since there should be no complete routes for potential exposure.
Construction of water supply wells is prohibited within the compliance
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boundary of an individually permitted disposal system (15A NCAC 02L .0107
(d)). Groundwater monitoring will continue at the compliance boundary in
accordance with 02L.
The risk to human health and the environment is within acceptable levels
prescribed by the USEPA. The risk to human health and the environment is
expected to decrease over time following implementation of Alternative 3.
Performance monitoring would be conducted in accordance with 02L the
standard, or applicable federal regulations. Institutional controls, including 15A
NCAC 02L .0107(d), restrict activities that could result in exposure to
groundwater COIs.
NPDES discharge requirements are protective of human health and the
environment. Extracted groundwater discharged via NPDES Outfall 005 or 002
must comply with applicable discharge requirements and will not pose an
unacceptable risk to human health and the environment.
Reduction of Toxicity, Mobility, and Volume
(CAP Content Section 6.D.a.iv.4)
Implementation of Alternative 3 would help reduce COI concentrations and,
therefore, the toxicity, mobility, and volume of affected groundwater by active
groundwater extraction and clean water infiltration.
Constituents most amenable to groundwater extraction are those that are
conservative/non-reactive COIs followed by variably reactive constituents.
Groundwater extraction would have the least effect on non-conservative/reactive
constituents.
Groundwater underlying the ash basin footprint also would be subject to the
influences of natural attenuation. The mechanisms that naturally attenuate the
concentrations of CCR inorganic constituents are dilution, dispersion, advection,
sorption (including ion exchange and precipitation) and phyto-attenuation.
The volume of groundwater containing COIs at concentrations greater than
groundwater standards would be reduced over a measurable time frame. For
example, Groundwater Remedial Alternative 3 would permanently reduce the
concentrations and areal extent of the boron plume in groundwater as defined by
the 02L standard (700 µg/L).
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Short-term Effectiveness
(CAP Content Section 6.D.a.iv.5)
The stability and limited areal extent of the COI plume, along with the absence of
complete exposure pathways, indicates there are no short-term impacts to 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.
Implementation of Groundwater Remedial Alternative 3 would be protective of
communities adjacent to and near MSS. Installation of groundwater monitoring
wells, groundwater extraction wells, clean water infiltration wells, discharge
lines, collection tanks and related infrastructure are straight forward and routine
tasks that can be conducted safely. Groundwater COIs do not pose an
unacceptable risk to potential receptors under conservative risk assessment
exposure scenarios (Appendix E).
Regardless, remediation worker exposure to COIs in groundwater should be
minimal since they would be wearing personal protective equipment (PPE) if
there is the potential for exposure to COIs in ash, soil, or groundwater.
Extracted groundwater will be managed via NPDES discharge. Compliance with
NPDES Permit NC0004987 should make discharges of extracted groundwater
protective of potential on -Site and off -Site receptors.
Hydraulic capture of groundwater near the groundwater extraction wells would
be aided by the vertical infiltration wells and would occur soon after the
groundwater extraction system is placed into service. Also, the advancement of
the boron plume beyond the compliance boundary would be mitigated as long as
hydraulic capture is sustained.
Technical and Logistical Feasibility
(CAP Content Section 6.D.a.iv.6)
Groundwater Remedial Alternative 3 is technically feasible and implementable
with some challenges. 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.
However, with early awareness of the aforementioned complexities and effective
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communications between the design, implementation and project management
teams, successful construction of the system would be anticipated.
Alternative 3 might be implemented concurrent with source control measures
(basin closure). Activities contemplated to implement Alternative 3 are routine
with respect to the maturity of technologies used, material requirements and
availability, and the availability of specialized services (e.g., licensed drillers,
electricians) and labor. Similarly, Remedial Alternative 3 is technically
implementable with respect to the suitability and availability of extraction well
and infiltration well installation locations and associated infrastructure.
Implementation of Remedial Alternative 3 can be achieved administratively.
Obtaining a UIC permit to inject clean water should be a straightforward process.
Likewise, obtaining groundwater and surface water withdrawal permits should
be readily achievable. Modification to the NPDES permit to allow the discharge
of groundwater, should be a straightforward process.
Consideration for dam safety is paramount; however, it appears that there are
ample locations on the lower buttress of the dam for the safe installation of
groundwater extraction wells and associated utilities. No well installations or
construction will occur on the dam or lower buttress area without first obtaining
the requisite permits from Duke Energy and North Carolina Environmental
Quality Energy, Mineral and Land Resources.
In the area along the southern portion of the dam, rock, likely excavated or
blasted during construction of the levee (dam) was used as fill. This may require
drilling equipment used for bedrock drilling to install the extraction wells.
The area where infiltration is to be implemented slopes severely. This slope
poses access challenges for construction. There is a challenge in construction of
the vertical infiltration wells, but the challenge is not insurmountable. Also, the
disturbance from construction activities should be manageable.
Time Required to Initiate and Implement Corrective Action
Technologies and Alternatives
(CAP Content Section 6.D.a.iv.7)
Groundwater extraction proposed under Remedial Alternative 3 can be
implemented concurrently with source control measures (basin closure). Some
aspects of the alternative, (e.g., hydraulic conductivity tests, design and
permitting) could be started upon approval of the CAP, with some construction -
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related activities phased in during ash basin closure. Hydraulic conductivity
tests would be conducted to validate groundwater yields predicted by flow and
transport modeling. Hydraulic conductivity test results would be used to refine
predictive modeling, and adjustments would be made to the groundwater
extraction system design if warranted.
Design of the groundwater extraction system proposed under Alternative 3
would be finalized following completion of hydraulic conductivity tests. Pre -
design testing, preparation of the final design, preparation of bid documents, and
submission of bid documents to prospective bidders could be accomplished
within 22 months following NCDEQ approval of the CAP. No other
prerequisites, such as permitting, are anticipated that would delay initiation of
Groundwater Remedial Alternative 3. Full-scale operation of the groundwater
extraction and clean water infiltration system could be accomplished within 14
months following the selection of a contractor.
Time Required to Achieve Remedial Goals
(CAP Content Section 6.D.a.iv.8)
Groundwater extraction and clean water infiltration performed under
Alternative 3 can be fully implemented concurrent with the ash basin closure.
Time to achieve the remediation goal of reducing the concentration of boron and
variably reactive COIs beyond the compliance boundary to levels less than the
02L standard was estimated by predictive flow and transport modeling to be 9
years after full implementation of Remedial Alternative 3.
Cost
(CAP Content Section 6.D.a.iv.9)
Costs to implement, operate, and manage Groundwater Remedial Alternative 3
would include expenses associated with the design, permitting and construction
management for the installation of 12 new monitoring wells, 66 groundwater
extraction wells, and 24 vertical clean water infiltration wells.
Construction costs also include trenching for utilities and piping of infiltration
water and extracted groundwater, equalization tanks, and piping infrastructure
for the transfer of water to permitted outfall. O&M expenses would include costs
to operate the groundwater infiltration/extraction system, groundwater
monitoring program, extraction system performance monitoring, and periodic
reporting for a 30-year period.
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Costs for implementation, O&M, monitoring, and reporting for Remedial
Alternative 3 are provided in Appendix K.
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 system. No landowner is
anticipated to be affected. It is anticipated that the extracted groundwater would
be discharged through a NPDES permitted outfall that flows to Lake Norman
and that the discharge would meet all permit limits. A groundwater extraction
system that addresses potential COI plume expansion across the entire southern
perimeter of the ash basin and east to the unnamed tributary may improve
public perception.
It is anticipated that groundwater extraction and clean water infiltration under
Alternative 3 would generally receive more positive community acceptance than
MNA under Alternative 1 since Alternative 3 involves more active measures to
attempt physical extraction of COI mass from groundwater and would likely be
perceived as more robust than MNA. Additionally, it is anticipated that
Alternative 3 may receive more positive community acceptance than Alternative
2 because Alternative 3 will not involve the infiltration of large quantities of
chemical amendments into the subsurface for the purposes of in -situ treatment of
COIs. The estimated remedial timeframe to reach the 02L standard for boron
from implementation of the full system is significantly shorter for the
implementation of Alternative 3 (9 years) than for Alternative 2 (30 years) and
Alternative 1 (700 years).
It is possible that some community stakeholders might have concerns with
potential exposure to discharged groundwater via NPDES permit. Assurances
that any means of groundwater management will be permitted and monitored
by NCDEQ should alleviate stakeholder concerns. Stakeholder concerns should
be further alleviated when they understand that extracted groundwater would
undergo treatment, if necessary, and that constituent concentrations in the
discharged groundwater would be within permitted limits. Until the final Site
remedy is developed and comments are received and reviewed, assessment of
community acceptance will not be fully known.
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
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or COI concentrations. Individual well pumping rates can be adjusted or
eliminated or additional wells can be installed to address COI plume changes.
Following full-scale implementation, it will be important to evaluate
Groundwater Remedial Alternative 3 performance to determine if operational
changes could result in greater efficiencies or shorter remediation time frames.
For example, additional extraction wells could be added to the remedial system
to augment pumping or selected groundwater extraction wells could be
repurposed and converted into clean water infiltration wells to augment the
infiltration and flushing provided by the network of infiltration wells.
Sustainability
Sustainability analysis was completed as described in Section 6.6. The footprint
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 emission -intensive remedial
alternative being considered. Alternative 1 (MNA) requires significantly less
materials and energy than Alternative 3 and is therefore characterized by a
smaller footprint. Alternative 3 presents lower energy -consumption metrics
when measured against Alternative 2. Alternative 3 utilizes twice the extraction
wells (33) than Alternative 2 and a clean water infiltration system consisting of 24
wells not planned for Alternative 2. However, Alternative 2 utilizes a 7.3-acre
clean water infiltration gallery and the in -situ placement of approximately 60
tons of reactive media, through drilled boreholes, which Alternative 3 does not
employ. As a result, Alternative 3 will generate a lower material -related
environmental footprint for the construction phase. Additionally, the shorter
timeframe of remediation system operation for Alternative 3 (9 years) when
compared to Alternative 2 (30 years) requires energy usage and produces air
emissions far less than the levels of Alternative 2. The quantitative analysis of the
footprints of the remedial alternatives under consideration for this CAP Update
indicates Alternative 3 to be the second -most sustainable option after MNA.
Opportunities for system optimization and energy savings could be pursued
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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 rankings presented in Section
6.7 and summarized in Appendix M, the favored remedy for groundwater remediation
is Alternative 3, Groundwater Extraction and Clean Water Infiltration.
To comply with 15A NCAC 02L .0106(h), corrective action plans must contain the
following the following items, which are included in the following subsections:
Specific plans, including engineering details where applicable, for restoring
groundwater quality
• A schedule for the implementation and operations of the proposed plan
• A monitoring plan for evaluating the effectiveness of the proposed corrective
action and the movement of the COI plume
6.8.1 Description of Proposed Remedial Alternative and
Rationale for Selection
(CAP Content Section 6.E.a)
The favored 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). 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 the MSS.
Groundwater Remediation Alternatives 1, 2, and 3 are all protective of human
health and the environment and will comply with applicable regulations.
Alternatives 1 and 3 are readily implementable. Portions of Remediation
Alternative 2 would have difficulty during implementation due to the steep
embankment where the infiltration galley would have to be installed.
Additionally, in -situ infiltration of chemical amendments in the dam buttress is
anticipated to be difficult to the heterogeneity of the fill area. Groundwater
Remediation Alternative 1, MNA, was not selected because it does restore ash
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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. In contrast, Groundwater Remediation
Alternative 2 is capable of achieving 02L compliance for boron and variably
reactive COIs within approximately 30 years and Alternative 3 is capable of
achieving compliance within approximately 9 years from implementation of the
full system, assuming implementation is concurrent with source control
measures (basin closure). Remediation Alternative 2 was not selected due to the
anticipated difficulties with implementation of clean water infiltration galleries
and in -situ application of chemical amendments.
Groundwater extraction and clean water infiltration proposed for Remediation
Alternative 3 is an adaptable approach and less costly to implement than
Alternative 2. The remedial system could be modified relatively easily if
conditions change. The addition of wells, or adjusting well pumping schemes,
can be readily accomplished. Treatment of extracted groundwater prior to
discharge could be implemented if future permit requirements are required. The
long-term effectiveness of Remedial Alternative 3 would be documented through
an effectiveness monitoring program.
Groundwater extraction and clean water infiltration via a well network generates
a larger footprint in the sustainability analysis over MNA (Alternative 1) but has
a lower footprint than Remedial Alternative 2, which also includes in -situ
treatment with chemical amendments and a longer timeframe to meet remedial
objectives. The footprint of Alternative 3, however, is 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 footprint can be evaluated.
Source control measures would mitigate the source of CCR COIs to groundwater
and proposed Groundwater Remedial Alternative 3 would mitigate the
migration of groundwater COIs beyond the compliance boundary. Groundwater
Remedial Alternative 3 would reduce boron concentrations until groundwater
remediation objectives are achieved.
Seep Corrective Action
As stated in the SOC, ash basin decanting is expected to substantially reduce or
eliminate the seeps. After completion of decanting, remaining seeps, if not
dispositioned in accordance with the SOC, would be characterized for
determination of disposition. After seep characterization, an amendment to the
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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. Non -constructed
seeps, currently covered under the SOC, that have the potential to not be fully
dispositioned post -decanting are listed on Table 6-8. No constructed seeps are
present at the MSS. In summary, decanting, ash basin closure, and the proposed
groundwater remediation alternative are the anticipated corrective action
strategies to address each of the seeps.
Seep S-01 is located in the unnamed tributary east of the ash basin. As of
December 2019, decanting has not observably reduced flow at this location. A re-
assessment of this seep was conducted between September and November 2019
as a result of hardness levels greater than the interim action level (200 mg/L)
established by the SOC. In accordance with the SOC, Duke Energy is conducting
monthly monitoring of this seep. The findings of this re -assessment were
submitted to NCDEQ in November 2019. The proposed remedial alternative
within this CAP Update is expected to address water quality at this location, as
groundwater extraction wells would be designed to maintain a water elevation
less than the receiving waters (i.e., the unnamed tributary and Lake Norman).
Groundwater flow and transport modeling simulations of groundwater
extraction predict that the current hydraulic gradient toward the unnamed
tributary would be reversed, inducing lake water infiltration into the
groundwater system. Therefore, it could be expected that this location be
dispositioned via dry conditions under the proposed remedial approach.
Seeps 5-02 and S-04 are located east of the ash basin dam toward Lake Norman.
Since the commencement of decanting, there has been no observable flow at
these locations. This indicates that decanting has been an effective corrective
measure and that it may be appropriate for 5-02 and 5-04 to be dispositioned in
accordance with the SOC.
Final corrective action plans for non -constructed seeps that are not dispositioned
post -decanting will be proposed in an amendment to this CAP Update, as
needed, 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
refinement of the final system design. A pilot test work plan will be prepared to
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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 Update.
The intent of the design would be to maximize pore volume exchange and
establish groundwater control in areas downgradient of the ash basin. Basic
aspects of the Alternative 3 call for installation of:
• 24 clean water infiltration wells and flow appurtenances
• 66 extraction 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
• Clean water infiltration pre-treatment system
• Piping to transfer water from the infiltration water supply to the
infiltration well system
• Clean water distribution system
• Electric power supply
• Groundwater remediation telemetry system
6.8.2.1 Process Flow Diagrams for all Major Components
of Proposed Remedy
(CAP Content Section 6.E.b.i)
A conceptual process flow diagram for clean water infiltration is shown on
Figure 6-33 and a process flow diagram for a groundwater extraction
system is shown on Figure 6-34. The detailed design elements presented
below may be adjusted based on a final technical review.
Site Preparation (Step 1 — Create Access)
Installation of the proposed groundwater extraction and clean water
infiltration 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
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well as haul road access constraints pose significant challenges to
constructability. However, with early awareness of the implementability
challenges 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 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 hydraulic conductivity test(s), 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, anticipated to include
somewhat extensive tree and brushy vegetation removal and grubbing, will
require erosion and sedimentation control (ESC) to be implemented and
likely reviewed and approved by a regulatory agency. Adaptable ESC
should be planned to limit project delays by avoiding formal modifications
of plans.
Pilot Test (Step 2a — 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. Extraction pilot test wells will be screened
within or across a flow zone similar to model simulations to the extent
feasible. Clean water infiltration tests would be conducted to determine the
rates of groundwater infiltration wells screened across the saprolite,
transition zone, and bedrock flow zones. The number of wells and their
locations would be specified in the pilot test work plan.
Pilot test results will be used to:
• Determine site -specific well yields for each flow zone
• Validate predictive flow and transport modeling
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• 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 conveyances for extracted
groundwater
• 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. Clean water
infiltration test results will be used to:
• Determine site -specific well infiltration rates
• Validate predictive flow and transport modeling
• Refine predictive flow and transport modeling, as needed
• If warranted, make adjustments to the groundwater infiltration
system design
• If warranted, make design adjustments to conveyances for
infiltration groundwater
• If warranted, make design adjustments to the infiltration water
treatment system
The extraction and infiltration wells used for testing would be included in
the final groundwater remediation system design.
Infiltration Water Quality and Treatment (Step 2b — To
Finalize Design)
The Marshall facility does not have the capacity in the existing intake
system to provide the 285 gpm that is projected for infiltration. The CAP
included a proposed location for the new surface water intake to provide
the water for infiltration based on the information that was available at that
time. However, there is limited information on the quality of water from
Lake Norman at the proposed location that will become infiltration water.
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Prior to completing the design phase for the corrective action on
groundwater at Marshall, the location for the new surface water intake will
be evaluated and the location may be changed based on topography, depth
of water in the lake, or other pertinent conditions at the site. Also, when the
location of the surface water intake is finalized, an evaluation of the surface
water quality at that location will be performed.
Based on the water quality and bench scale treatability studies, technologies
for infiltration water treatment will be evaluated. The potential treatment
technologies for infiltration water include, but are not limited to, the
following:
• pH adjustment
• precipitation
• filtration (i.e., sand filtration, reverse osmosis), and
• ion exchange
Clean Water Infiltration and Extraction Well Design (Step 3
— Install Wells)
The preliminary design for Groundwater Remedial Alternative 3 includes
66 extraction wells and 24 clean water infiltration wells. The new extraction
wells would be installed along the southern portion of the dam, along north
end of the ash basin dam, and along the eastern access road, between the
ash basin and the tributary to Lake Norman (Figure 6-30). 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 the flow and transport modeling results.
Clean water infiltration wells along the eastern access road will be used to
flush residual COIs from shallow soils to the saturated portions of the
aquifer where they can be captured by the extraction well network.
All groundwater extraction and clean water infiltration wells would be
installed by a North Carolina licensed well driller in accordance with North
Carolina Administrative Code Title 15A, Subchapter 2C — Well Construction
Standards, Rule 108 Standards of Construction: Wells Other Than Water
Supply (15A NCAC 02C .0108).
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The 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. Completed wells would
be 6 inches in diameter to facilitate the installation of pumps and
instrumentation (e.g., level control) in groundwater extraction wells. The
top of the sand pack would extend to approximately 2 feet above the top of
well screens. A bentonite well seal at least 2 feet thick would be installed on
top of the sand pack. 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. All materials and
installations would be in accordance with 15A NCAC 02C. Typical well
construction schematics are included for extraction wells (Figure 6-27) and
infiltration wells (Figure 6-31).
Infiltration Wells (Step 4A)
The clean water for infiltration would be stored in a tank near the well
system and an HDPE distribution header would convey clean water from
the infiltration water treatment system to each infiltration well (Figure 6-
33). A seal at the top of the well through which the clean water infiltration -
pipe and wiring would enter the well and would be designed to be leak
free.
The hydraulic head at each clean water infiltration well would be controlled
by a pressure control valve. The predictive flow and transport model
assumed 0 pounds per square in gauge (psig) as the infiltration pressure,
but the pressure could be increased or decreased to achieve performance
objectives. The amount of water flowing into the 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
of each infiltration well would monitor well head pressures (Figure 6-31).
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 rate, totalized infiltration flow,
and well head pressure, as well as critical malfunctions such as
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accumulation of water in the well vault would be transmitted to the
groundwater remediation system owner via telemetry system.
Extraction Wells (Step 4B)
A pump would be installed in each groundwater extraction well. Selection
of pump type (e.g., electric submersible or pneumatic) would be determined
in the final design. 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 pump would be measured using a flow rate and flow totalizer
meter before being conveyed to groundwater discharge piping for
treatment and discharge (Figure 6-27). 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 6-27).
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.
The collection system would consist of gravity sewers, eight duplex pump
stations, and force main pipes to convey flow. Above ground piping, tanks,
and pumps should be equipped with heating and insulation to prevent
freezing in cold conditions.
Clean Water Infiltration Water Treatment (Step 5 — Build
Infiltration Treatment)
Water used for clean water infiltration will be obtained from a water source
such as Lake Norman. If the water quality is not suitable for infiltration, the
groundwater would be treated in a modular treatment system if suspended
solids are the only concern (Figure 6-33). The equalization tanks and the
modular treatment systems would be located in the proximity of the
infiltration system near the production well. The treatment system would
condition the water, as necessary, prior to storage and distribution to the
infiltration wells.
A modular flocculation, settling, and filtration treatment process may be
used to reduce total suspended solids (TSS) to concentrations, if necessary.
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A polymer could be added to the raw water in a rapid mix tank. The
polymer would flocculate with total suspended solids (TSS). Treated water
and flocculent would flow from the rapid mix tank to a modular
sedimentation tank where the flocculent and particulates would
settle. Sedimentation tank effluent would undergo filtration to remove
suspended flocculent and particulates. The filtered water would be
pumped to a holding tank where infiltration water would be stored prior to
distribution to the infiltration wells.
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
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 a
wastewater treatment plant. 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)
Extracted groundwater would flow to an equalization tank and then be
conveyed to a water treatment system to address low pH and other COIs, as
appropriate. Initially, the groundwater would propose to be discharged
with the water from dewatering the ash basin. The pH would be adjusted in
an existing system and the water would then be discharged through the
permitted outfalls. Extracted groundwater would undergo any treatment
processes necessary to satisfy applicable NPDES discharge requirements.
Decanting of the ash basin is to be complete by March 2021. Prior to that
time, options would be evaluated based on the actual groundwater quality
and quantity. The options would include, but are not limited to, transfer to
the new LRB, continue to operate the existing system for pH adjustment, or
adding a new treatment system for extracted groundwater.
Clean Water Infiltration Well Distribution System (STEP 7 —
Conceptual Infiltration System Considerations)
The purpose of the clean water infiltration distribution system is to convey
water to the infiltration water treatment system and to convey water from
the treatment system to the infiltration wells. The distribution system
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design would have features similar to a drinking water distribution system.
For example, distribution lines would be constructed with pressure relief
valves so that the system may be flushed to remove buildup on piping
walls.
Clean water would be transferred from the source to a treatment and
storage plant. A booster pump would convey water from the storage tanks
and provide the hydraulic head to the infiltration well network to maintain
sufficient pressures to reach infiltration wells. Pressure regulating valves
would be installed at each infiltration well to control infiltration rates.
Groundwater Extraction Well Discharge Piping (STEP 8 —
Conceptual Extraction System Considerations)
The proposed groundwater extraction system would consist of 66
groundwater extraction wells. Based upon predictive groundwater flow
and transport modeling, the groundwater extraction wells would generate
on average 9.9 gpm of extracted groundwater per well or about 652 gpm of
extracted groundwater collectively.
Each of the groundwater extraction wells would discharge into one of a
series of headers. Extracted groundwater in these headers then would flow
by gravity to one of several tanks. The collected groundwater in these tanks
would be pumped to a conveyance line ultimately discharging to a
groundwater treatment plant.
6.8.2.2 Engineering Designs with Assumptions,
Calculations and Specifications
(CAP Content Section 6.E.b.ii)
Pipelines (STEP 9 — Pipeline Specifics)
High density polyethylene (HDPE) piping will be used for water
conveyance in all areas where buried piping will be installed. Water
conveyance will include:
Groundwater pumped from extraction wells and conveyed to an
NPDES permitted outfall
Surface water pumped from the clean water source and conveyed to
a infiltration water treatment system
• Infiltration water treatment system effluent to infiltration wells
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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.
The locations of the HDPE piping systems for extraction 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.
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The design flow rate is 285 gpm for the clean water infiltration system and
652 gpm for the groundwater extraction system. Infiltration water
distribution lines would connect to each well the clean water infiltration
system. Likewise, each groundwater extraction well will be connected to a
header that ultimately conveys extracted groundwater to a groundwater
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)
The extraction and clean water infiltration networks for the proposed
alternative were designed using Pipe Flow° Expert. Pipe Flow® Expert is a
software package 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
presented in Appendix N.
The extraction network consists of 66 extraction wells with trunk lines for
conveyance and branching pipes providing connections to the wells. The
network ultimately operates in gravity flow. 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 and frictional losses from fittings and pipes to provide
evidence of the efficacy of the proposed pipe network layout design.
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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 66 submersible pumps, the small transfer pump in the collection
well, 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 would be dependent on maintaining
adequate extraction flow rate through the wells, and stable water levels, for
an extended period of time. This will necessitate effective operation and
maintenance of the wells. As described above 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. Additionally, cleanouts will be installed on
pipes to facilitate periodic maintenance, preventing mineral scaling or
biological fouling on the conveyance pipe network.
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.
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
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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 Stormwater permit
• Right -of -Way (ROW) encroachment agreement with North Carolina
Department of Transportation
• Water Withdrawal and Transfer registration
• Wetlands permit
The schedule for obtaining permits is based off the project implementation
schedule as discussed in Section 6.8.2.4 and presented on Figure 6-35.
6.8.2.4 Schedule and Cost of Implementation
(CAP Content Section 6.E.b.iv)
A Gantt chart (Figure 6-35) is provided for outlining a general timeline of
implementation tasks following CAP Update submittal. 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
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• Tasks to be completed by next reporting period
• Provide updated schedule/Gantt chart
Duke Energy progress reports would be submitted to NCDEQ monthly.
A detailed cost estimate for this Alternative is provided in Appendix K. The
cost estimate is based on capital costs for design and implementation, and
the operations, maintenance (O&M) and monitoring costs. 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.
6.8.2.5 Measures 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 ash basin 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 ash basin.
Water supply wells are located upgradient of the ash basin and alternate
water supplies or water supply filtration systems have been provided to
those who selected this option. 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 basin and the adjacent additional sources 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.
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6.8.3 Requirements of 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.
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).
Alternate standards are appropriate at the MSS 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
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• Proposed corrective action plan would be consistent with all other environmental
laws.
Adaptive site management allows iterative review of site information and data to
determine whether changing site conditions warrant adjustments to site
management and monitoring approaches. Adaptive site management
approaches may be adjusted over the site's life cycle as new information and
technologies become available. This approach is particularly useful at complex
sites where changes in site conditions may require an extended period of time or
where NCDEQ approves alternate groundwater standards for COIs, such as
4,000 µg/l for boron, pursuant to its authority under G.S. Section 15A NCAC 02L
.0106(k).
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 the MSS and document progress towards the remedial objectives
over time. This plan is designed to be adaptive and can be modified as the
groundwater remediation system design is prepared, completed, or evaluated for
termination.
Duke Energy implemented an Interim Monitoring Plan (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 section G.S.130A-309.209(b)(3).
The EMP, presented in Appendix O, is designed to be adaptable and would
target key areas where changes to groundwater conditions are most likely to
occur due to corrective action and ash basin closure activities. EMP key areas for
monitoring are based on the following considerations:
• Include background locations
• Include designated flow paths
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• 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 include reporting evaluation and schedule, groundwater
monitoring well systems, sampling protocol, frequency, and parameters (Table
6-17). Effectiveness monitoring well locations are depicted on Figure 6-36. Thirty
days after CAP approval, the EMP will be implemented at the Site 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.
After ash basin closure and following closure certification, a post -closure
groundwater monitoring plan equivalent to the long-term groundwater
monitoring system well locations, parameters, and sampling frequency would 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 for three consecutive years, Duke Energy may request
termination of the PCMP in accordance with G.S. Section 130A-309.214(a)(3)b.
An EMP work flow and optimization process is outlined on a flow chart
presented on Figure 6-37.
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 will be
conducted during EMP review periods. Optimization evaluation 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 COI plume stability would be
based on quantitative rationale using statistical, mathematical, modeling, or
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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:
• Laboratory reports on electronic media,
• Tables summarizing the past year's monitoring events,
• Historical data tables,
• Figures showing the historical data versus time for the
designated monitoring locations and parameters,
• Figures showing sample locations,
• Statistical analysis (Mann -Kendall test) of data to determine if
trends are present, if performed,
• Identification of exceedances of comparative values,
• 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),
• Any notable observations related to water level fluctuations or
constituent concentration trends attributable to extraction
system performance or water table drawdown, and
• Recommendations regarding adjustments 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
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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. 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 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.
6.8.5.2 Sampling and Reporting Plan During Active
Remediation
(CAP Content Section 6.E.e.ii)
Groundwater Monitoring Network
EMP monitoring will be conducted in coordination with required federal
regulatory groundwater monitoring to provide an integrated and
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 with regard
to potential receptors, and (3) confirm flow and transport and geochemical
model predictions. This monitoring would be implemented east of the ash
basin (Figure 6-36). EMP groundwater well monitoring network objectives
are outlined below:
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• Compliance with 02L
• Measure and track the effectiveness of the proposed clean water
infiltration and extraction system
• Monitor plume status (horizontally and vertically)
• Verify predictive model simulations
• Verify no unacceptable impact to downgradient receptors
• Verify attainment of active remedy objectives through validated
model simulations
• Identify new potential releases of constituents into groundwater
from changing site conditions
• Monitor approved background locations
The EMP would include 98 monitoring wells (Table 6-17). Several of the
existing monitoring wells at the site might be abandoned from ash basin
and landfill closure and related construction activities. In the event that
closure activities extend to the proposed well locations, the layout of wells
would be modified, if necessary.
Groundwater Monitoring Flow Paths - Trend Analysis
The monitoring program will provide adequate horizontal and vertical
coverage to monitor:
• Changes in groundwater quality as Site conditions change (e.g.,
groundwater extraction expands, ash basin closure commences, and
the immediate groundwater flow and transport conditions respond),
• Transport rates, and
• Plume stability.
Horizontal and vertical coverage would be provided by using groundwater
monitoring wells located along three primary groundwater flow paths
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 6-36 and described below:
1. At or near the source area
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2. At waste boundary
3. 250 feet downgradient from waste boundary. If the waste boundary
and compliance boundary are located sufficiently close to evaluate
COI trends over time, this interval location would not be monitored.
4. 500 feet downgradient of waste boundary (CAMA compliance
boundary)
5. No less than one year travel time upgradient of receptor or potential
receptor and no greater than the distance groundwater is expected to
travel in five years
Multi parameters sondes would be installed in 14 wells along the three
primary flow paths in the remedy area (Figure 6-36). 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
significantly deviate from predicted responses. Contingency plans are
included in Section 6.8.8 of the CAP Update.
Plume stability evaluation would be based primarily on results of trend
analyses. Trend analyses may 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.
Mann -Kendall trend tests would be conducted using data from EMP
geochemically nonreactive, conservative constituents. 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. The test would be performed in
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accordance with USEPA Guidance for Statistical Analysis of Groundwater
Data (USEPA 2009).
Trend analysis of designated groundwater monitoring flow path wells
(Figure 6-36) 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 comprehensive integrated monitoring plan
sampling frequency is based on semi-annual sampling events to be
consistent with long-term monitoring under applicable federal regulations.
Semi-annual monitoring following implementation of corrective action is
recommended for the 98 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.
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 the EMP (Appendix O). Samples would be
analyzed by a North Carolina certified laboratory for the parameters listed
in Table 6-17. Laboratory detection limits for each constituent are targeted
to be at or less than applicable regulatory values (i.e., 02L, IMAC, or 02B).
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• Groundwater Quality Parameters: Conservative constituent
analyses of boron, chloride, and TDS would be conducted, in
addition to parameters listed on Table 6-17, to monitor corrective
action performance using the designated wells along the
groundwater flow paths. These constituents were selected because
they are generally non -reactive to changing geochemical conditions
and encompass the areal extent of the plume. Physical attenuation
mechanisms of dilution and dispersion would be evaluated by
comparing monitoring results with flow and transport model
simulations. Changing geochemical conditions that could cause
sorption or precipitation/co-precipitation mechanisms would be
evaluated using multi parameter sondes.
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, 15A NCAC 02L .106 (m). A flow chart of the
request and review timeline for termination is outlined on Figure 6-38 (CAP
Content Section 6.E.e.iii.1). Completion of this phase might also provide
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stakeholders with an opportunity to evaluate terminating the system, as
appropriate, near 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 of the Site.
6.8.7 Proposed Interim Activities Prior to Implementation
(CAP Content Section 6.E.f)
In accordance with requirements of CAMA 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 will 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 extraction and clean water
infiltration system is described in Section 6.8.5. The plan is adaptive and can be
modified as the final design is prepared.
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6.8.8.1 Description of Contingency Plan
(CAP Content Section 6.E.g.i)
The contingency plan addresses the following areas:
• Operations (including extraction and infiltration wells, pumping,
piping, electrical, and controls)
• Groundwater quality
• Groundwater levels
• Groundwater treatment
• Comparison to predicted concentrations and water levels
A health and safety plan and an operations manual will be prepared as part
of the design. The health and safety plan address management of spills and
other unplanned releases and the operations 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 remote telemetry system would be installed to monitor the groundwater
extraction, infiltration, and treatment system. The telemetry system would
be tied into a remote monitoring station that can be accessed by key
personnel responsible for operation and maintenance of the groundwater
remedial system. The telemetry system would alert key personnel if
malfunctions or an emergency condition arises.
Several aspects of the monitoring system would be used to maintain safe
and effective operations of the extraction and infiltration wells, and
treatment system:
• Processes for maintenance of effective operation of each extraction
and infiltration well include target flow rates and water levels for
each well. Each well would be monitored continuously by the control
system, with data being recorded, and an alert sent if the flow rate or
water level is outside the prescribed range. In addition to automated
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systems, each element of the system would be physically inspected
and maintained as part of a routine operations and maintenance
program.
If a leak in the groundwater extraction or infiltration system is
detected by the telemetry system, the affected portion of the system
will be shut down, and an alert 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.
If pH adjustment or other water treatment technology is employed,
continuous monitoring of key parameters would be used to maintain
proper operation of the system. Variances between prescribed ranges
would alert the operator and other key personnel and might result in
an automatic system shutdown.
• The operator inspection schedule, completion, and notes for key
systems would be documented.
• A system maintenance schedule would be established for effective
operation. System elements would be maintained in accordance with
manufacturer's recommendations, included in an Operation and
Maintenance (O&M) Manual. Corrective measures, performed by
appropriately skilled personnel, would 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.
Data is maintained in a comprehensive database system following each
sampling event. 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. Possible responses could include
adding or removing extraction or infiltration 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 might 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 might 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 pre -
decanting 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.
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 might be deviations from existing data trends
and model predictions. The models are anticipated to be updated to reflect
changing conditions, as necessary, and changes in predicted results would
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be analyzed to determine if the remedial approach needs to be modified to
effectively address the changes.
Since clean water infiltration is an element of the remedial approach, 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 or increasing the extraction system would be used to control surficial
saturation.
6.9 Summary and Conclusions
This CAP Update proposes remedies for COIs in groundwater associated with the MSS
ash basin that are at or beyond the compliance boundary to the south and southeast of
the ash basin. This CAP Update provides:
• A screening and ranking process of multiple potential groundwater corrective
action alternatives that would address areas south and southeast of the ash basin
where affected groundwater has migrated at or beyond the Site's compliance
boundary.
• Additional source areas adjacent to the ash basin are being addressed through
the closure plan (structural fill access road and ILF structural fill subgrade),
groundwater remediation system (coal pile, Dry Ash Landfill Phase II, and PV
Structural Fill) and/or through additional enhanced closure efforts with NCDEQ
DWM (Dry Ash Landfill Phase I and Phase II, and PV Structural Fill).
• A selection and description of the proposed targeted corrective action
Alternative 3, Groundwater Extraction and Clean Water Infiltration.
• Specific plans, including engineering details where applicable, for restoring
groundwater quality.
• An EMP for evaluating the performance and effectiveness of the proposed
corrective action and its effect on the movement of the affected groundwater
plume. The EMP uses an optimized groundwater monitoring system with
multiple groundwater flow paths in the area of corrective action that would
monitor geochemical and physical conditions.
• A schedule for the implementation and operation of the proposed groundwater
corrective action strategy.
• Planned activities prior to full-scale implementation include pilot testing in
selected areas. Pilot test work plan(s) will be submitted to NCDEQ within 30
days of CAP Update approval to fulfill G.S. Section 130A-309.211(b)(3).
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Corrective Action Plan Update
December 2019
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7.0 PROFESSIONAL CERTIFICATIONS
(CAP Content Section 7)
Certification for the Submittal of a Corrective Action Plan
Responsible Party and/or Permittee: puke Energy Carolinas, LLC
Contact Person: Paul Draovitch
Address: 526 South Church Street
City: Charlotte State: NC Zip Code: 28202-1803
Site Name: Marshall Steam Station
Address: 8320 East Carolina Highway 150
City: Terrell State: NC Zip Code: 28682
Groundwater Incident Number (not applicable - Coal Ash Management Act CAP)
SynTerra
We, Brian D. Wilker a Professional Geologist and James E. Clemmer, a Professional
Engineer for SynTerra Corporation (firm or company of employment) do hereby certify that
the information contained herein is part of the required Corrective Action Plan (CAP) and
that to the best of our knowledge the data, assessments, conclusions, recommendations
and other associated materials are correct, complete and accurate.
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Corrective Action Plan Update December 2019
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8.0 REFERENCES
(CAP Content Section 8)
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SynTerra, 2018b. Human Health and Ecological Risk Assessment Summary Update -
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for Implementation of Corrective Action Under 15A NCAC 02L .0106 (k) and (1) -
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