NC0038377_01_Mayo CAP Update 2019_Text_20191231

synTerra
CORRECTIVE ACTION PLAN
UPDATE
Site Name and Location:
Mayo Steam Electric Plant
10660 Boston Road
Roxboro, North Carolina 27574
Groundwater Incident No.:
Not Assigned
NPDES Permit No.:
NCO038377
NCDEQ CCR Impoundment Ranking:
Low -Risk
Date of Report:
December 31, 2019
Permittee and Current
Duke Energy Progress, LLC
Property Owner:
410 South Wilmington Street
Raleigh, North Carolina 27601
704-382-3853
Consultant Information:
SynTerra Corporation
148 River Street
Greenville, South Carolina>>�KH•,.
[phone] �� Qom•.••
Latitude and Longitude of Facility:
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Kathy Webb, NC LG 1328
Project
Director

Corrective Action Plan Update December 2019
Mayo Steam Electric Plant SynTerra
Note to the Reader from Duke Energy
Duke Energy Progress, LLC (Duke Energy) is pleased to submit this groundwater
Corrective Action Plan (CAP) for the Mayo Steam Electric Plant (Mayo) located in
Person 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 Mayo 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 Mayo as low -risk pursuant to
CAMA.
Thousands of multi -media samples have been collected at Mayo yielding over 73,000
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.
Significantly, groundwater quality data confirm, based on one year of quarterly
monitoring results, that constituents of interest (COI) identified at Mayo do not exceed
the applicable 02L Standards at or beyond the ash basin compliance boundary.
Accordingly, groundwater corrective action under 15A NCAC 02L.0106 is not triggered.
However, we have implemented, or will implement, source control measures at the Site,
including (i) complete ash basin decanting to remove the hydraulic head, thereby
mitigating the risk of potential COI migration into groundwater; (ii) complete ash basin
closure; and (iii) continued operation of the dam toe -drain water collection system, as
necessary and permitted, to reduce COI concentrations in surface water and in
groundwater proximate to the system. Closure plans to address the ash basin source
area are submitted separately.
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 evaluation 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 Mayo and inform this CAP.
This combined effort has enabled a comprehensive understanding of site conditions and
creation of a highly detailed three-dimensional groundwater flow and solute transport
model used to simulate future conditions. Duke Energy believes it is also important to

Corrective Action Plan Update December 2019
Mayo Steam Electric Plant SynTerra
provide a science -based perspective on these extensive studies, which include the
following key findings:
• The human health and ecological risk assessments performed for Mayo 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 79 monitoring wells over 41 separate monitoring events, and
performing over 213 groundwater and geochemical modeling simulations.
In addition, even though no off -site wells were impacted, Duke Energy has already
provided owners of surrounding properties within 0.5-mile radius of the ash
compliance boundary with water filtration systems under a program approved by the
NCDEQ. These alternate water supplies provide additional peace of mind for our
neighbors.
Duke Energy looks forward to proactively implementing this CAP.

Corrective Action Plan Update December 2019
Mayo Steam Electric Plant SynTerra
Executive Summary
(CAP Content Section Executive Summary)
ES.1 Introduction
SynTerra prepared this groundwater corrective action plan (CAP) update on behalf of
Duke Energy Progress, LLC (Duke Energy). The plan pertains to the Mayo Steam
Electric Plant (Mayo, Plant, or Site) coal combustion residuals (CCR) surface
impoundment (ash basin) in Person County, North Carolina (Figure ES-1).
In accordance with North Carolina General Statutes (G.S.) Section 130A-309.211(b),
amended by the 2014 North Carolina Coal Ash Management Act (CAMA), Duke
Energy is required to submit a CAP for the restoration of groundwater in conformance
with the requirements of North Carolina Administrative Code (NCAC), Title 15A,
Subchapter 02L. 0106 (02L). Analytical data obtained over one year of quarterly
monitoring indicate the Mayo ash basin is currently in compliance with 02L
groundwater quality standards; therefore, groundwater corrective action under 15A
NCAC 02L .0106 is not required at this time for the Mayo ash basin.
This CAP Update addresses the requirements of CAMA and is prepared consistent with
the CAP guidance provided by the North Carolina Department of Environmental
Quality (NCDEQ) in a letter to Duke Energy, dated September 10, 2019 (Appendix A).
This CAP Update evaluates groundwater associated with the Mayo ash basin.
Specifically, this CAP focuses on constituent concentrations detected greater than
applicable North Carolina groundwater standards [02L; Interim Maximum Allowable
Concentrations (IMAC); or background threshold values, whichever is greater], and
verifying decreasing groundwater concentrations during decanting and subsequent
closure of the basin.
In accordance with G.S. Section 130A-309.211, amended by CAMA, a CAP for Mayo
was previously submitted to the NCDEQ in two parts:
• Corrective Action Plan Part 1— Mayo Steam Electric Plant
(SynTerra, 2015b)
• Corrective Action Plan Part 2 —Mayo Steam Electric Plant (SynTerra, 2016a)
This CAP Update considers data collected through June 2019.
Ash basin closure is detailed in a separate document prepared by AECOM (AECOM,
2019). Closure scenarios include a closure -in -place and closure -by -excavation scenario.
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Mayo Steam Electric Plant SynTerra
Closure scenarios (e.g., source control) will be effective in addressing the ash basin
source area which is an important part of the overall corrective action strategy.
Groundwater modeling simulations consistently indicate the different closure scenarios
would have a similar effect on the concentrations of unit -specific constituents of interest
(COI) in groundwater.
Summary of CAP Approach
This CAP Update is prepared to meet requirements under CAMA Section 309.211(b)
and includes documentation that supports groundwater quality does not exceed
applicable 02L groundwater quality standards at or beyond the ash basin compliance
boundary. Applicable criteria in this case are defined as the 02L groundwater standard,
interim maximum allowable concentration (IMAC), or background, whichever is
greatest. If a constituent does not have an 02L standard or IMAC, then the background
value defines the constituent criteria. Therefore, groundwater corrective action under
15A NCAC 02L.0106 is not required at this time for the Mayo ash basin.
ES.2 Background
Plant Operations
Mayo is a coal-fired steam station owned and operated by Duke Energy that generates
electrical power for thousands of customers in North Carolina. Mayo operations began
in 1983 with a single coal-fired unit which remains in operation. CCR materials,
composed primarily of fly ash and bottom ash, were initially deposited in the ash basin
by hydraulic sluicing operations. In November 2013, Mayo converted to a dry ash
system in which 90 percent of generated CCR was handled dry with final system
upgrades completed in 2016. CCR generated at Mayo has been handled dry since 2016.
Dry CCR was placed in the Duke Energy Roxboro Steam Electric Plant Industrial
Landfill (PN 7302-INDUS) until the on -Site Industrial Landfill (Monofill; PN 7305-
INDUS) began operation in November 2014. The Mayo ash basin has operated under a
National Pollution Discharge Elimination System (NPDES) Permit issued by the
NCDEQ Division of Water Resources (DWR) since initial Plant operations began.
Pursuant to N.C. General Statute (G.S.) Section 130A-309.213(d)(1), NCDEQ has
determined that the CCR surface impoundment at Mayo has met the conditions for
low -risk classification as described in the above statute by establishing permanent water
supplies and rectifying any dam deficiencies (Holman to Draovitch, November 13, 2018;
Appendix A). Relevant closure requirements for low -risk impoundments are found 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.
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Corrective Action Plan Update December 2019
Mayo Steam Electric Plant SynTerra
Source Area
The Mayo ash basin is the source area evaluated in this CAP. The ash basin includes the
CCR surface impoundment (ash basin), the flue gas desulfurization (FGD) forward
flush pond and the FGD settling pond. The ash basin contains ash generated from the
Plant's historic coal combustion. The FGD forward flush pond was originally used in
the bioreactor treatment process. The bioreactor has since been decommissioned and
the FGD forward flush pond is inactive. The FGD settling pond historically received the
stream of FGD blowdown water as well as leachate from the monofill. The FGD ponds
were constructed within the footprint of the ash basin; however, both ponds were
constructed with an engineered liner system.
Additional Adjacent Source Area
There are no additional adjacent source areas that influence the ash basin COI plume.
On April 5, 2019, NCDEQ provided correspondence that included a list of primary
sources to be included for the submittal of CSAs and CAPS for each station and a
schedule of submittal dates for the reports. For Mayo, the letter indicated that the CAP
may include the active coal storage pile area. On October 11, 2019, Duke Energy
submitted a request to the NCDEQ to assess the coal pile storage area separate from the
ongoing evaluation of and preparation of this CAP Update for the ash basin. This
request was based on detailed information including an initial evaluation of assessment
findings to date for the coal storage pile area. The request was approved on November
13, 2019. This CAP Update does not include information pertaining to any additional
source areas at Mayo.
Pre -Basin Closure Activities
To accommodate closure of the ash basin, decanting (removal) of free water from the
basin began on June 27, 2019 as required by a Special Order by Consent (SOC) issued
through North Carolina Environmental Management Commission (EMC) on August 16,
2018 (EMC SOC WQ S18-005). The SOC requires completion of decanting by December
31, 2020. Decanting of free, ponded water from the ash basin before closure is expected
to reduce or eliminate seepage from constructed and/or non -constructed seeps.
Decanting is considered an important component of closure because it will reduce the
hydraulic head and vertical gradient near the ash basin dam, thereby reducing the
constituent migration potential associated with the ash basin. Decanting is scheduled to
be complete on or before December 31, 2020. As of December 1, 2019, approximately
124,200,000 gallons of water had been removed from the ash basin and the water
elevation decreased by about 7 feet.
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Basis for CAP Development
A substantial amount of data related to the ash basin and general Mayo Site has been
collected to date. A summary of Mayo assessment documentation used to prepare this
CAP Update is presented in Table ES-1.
TABLE ES-1
SUMMARY OF MAYO ASSESSMENT DOCUMENTATION
Comprehensive Site Assessment Report - Mayo Steam Electric Plant (SynTerra, 2015a)
Corrective Action Plan Part 1 - Mayo Steam Electric Plant (SynTerra, 2015b).
Corrective Action Plan Part 2 - Mayo Steam Electric Plant (SynTerra, 2016a).
Comprehensive Site Assessment Supplement 1 - Mayo Steam Electric Plant
(SynTerra, 2016b).
Comprehensive Site Assessment Update - Mayo Steam Electric Plant (SynTerra, 2017b)
Updated Groundwater Flow and Transport Modeling Report for Mayo Steam Electric Plant
(FRx, Inc., SynTerra, and Falta Environmental, 2019).
Human Health and Ecological Risk Assessment Summary Update - Mayo Steam Electric
Plant (SynTerra, 2019e).
Ash Basin Pumping Test Summary Report - Mayo Steam Electric Plant (SynTerra, 2019a).
Surface Water Evaluation to Assess 15A NCAC 02B - Mayo Steam Electric Plant
(SynTerra, 2019b).
2018 CAMA Annual Interim Monitoring Report (SynTerra, 2019c).
Community Impact Analysis of Ash Basin Closure Options at the Mayo Steam Electric Plant
(Exponent,2018)
Mayo Plant Ash Basin Closure Options, Groundwater Modeling and Community Impact
Analysis (Duke Energy/AECOM, 2018)
Updated Background Threshold Values for Constituent Concentrations in Groundwater
(SynTerra, 2019d)
Mayo Steam Station HB 630 Provision of Permanent Water Supply Completion
Documentation (Duke Energy (Draovitch) to NCDEQ (Holman), August 30, 2018.
Prepared by: JAW Checked by: PWA
The NCDEQ reviewed the 2017 Comprehensive Site Assessment (CSA) Update report
(SynTerra, 2017b), and in a May 7, 2018, letter, stated that sufficient information was
provided to allow preparation of this CAP Update (Appendix A).
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Mayo Steam Electric Plant SynTerra
The assessment work referenced in the documents listed in Table ES-1 has resulted in a
very large dataset that has informed the development of this CAP Update. As of June
2019, the following data collection and analysis activities have been completed:
TABLE ES-2
SUMMARY OF MAYO ASSESSMENT ACTIVITIES
(THROUGH JUNE 2019)
Tasks
Total
Total Monitoring Wells Installed (CAMA and CCR Wells around basin)
79
Groundwater Monitoring Events
41
Groundwater Samples Collected
868
Individual Analyte Results
73,505
Off -Site Water Supply Well Sampling (Total inorganic analysis) - Number
of Analyses
536
Ash Pore Water - Number of Analyses (Total and dissolved)
4,149
Ash Pore Water Sampling Events
16
Surface Water Monitoring Events
22
Surface Water Sample Locations
9
Area of Wetness Sample Events
21
Ash Samples (Within ash basin analyzed for SPLP)
6
Soil Samples Collected
138
Soil Sample Locations
67
Sediment Sample Locations
15
Geotechnical Soil Sample Locations
20
Geochemical Ash, Soil, Partially Weathered Rock, Whole Rock Samples
71
Hydraulic Conductivity Tests (Slug Tests, Pumping Tests, Packer Tests,
FLASH Analysis of Bedrock HPF Data)
58
Groundwater Flow & Transport Simulations
49
PHREEQC Geochemical Simulations
164
Notes:
Data available to SynTerra as of June 2019
SPLP - Synthetic Precipitation Leaching Procedure
FLASH - Flow -Log Analysis of Single Holes
HPF - Heat Pulse Flow
PHREEQC - pH Redox Equilibrium in computer code C
Prepared by: JAW Checked by: PWA
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Mayo Steam Electric Plant SynTerra
A constituent management process was developed by Duke Energy at the request of
NCDEQ to gain a thorough understanding of the constituent behavior and distribution
in Site groundwater and to aid in identification of COIs related to the Mayo ash basin
that may require corrective action. The constituent management process consists of
three steps:
1. Perform a detailed review of the applicable regulatory requirements under
NCAC, Title 15A, Subchapter 02L
2. Understand the potential mobility of ash basin -related constituents in
groundwater based on Site hydrogeology and geochemical conditions
3. Determine constituent distribution at the Mayo ash basin under pre -decanting or
predicted future conditions.
This constituent 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
constituent behavior in the subsurface related to the ash basin or constituents that are
naturally occurring. The constituent management process was utilized to identify COIs
related to the Mayo ash basin that may require corrective action. Constituents 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.
Constituents that are naturally occurring at concentrations greater than the 02L
standard do not require corrective action. Boron is the only constituent observed in
concentrations greater than 02L, IMAC, or background with a discernable plume;
therefore, is the only COI identified for the Mayo ash basin. Details on the constituent
management approach are presented in Section 6.0.
Groundwater
Analytical data obtained over one year of quarterly monitoring indicate COI
concentrations have been less than applicable 02L standards in groundwater samples
collected from monitoring wells at or beyond the compliance boundary. Therefore, the
ash basin is in compliance with 02L requirements and a CAP prepared under 02L is not
required. However, G.S. Section 130A-309.211, amended by CAMA, requires submittal
of a CAP and this document is intended to fulfill that obligation.
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Soil
Analytical results for soil samples indicate unsaturated soil constituent concentrations
in the vicinity of the ash basin are consistent with background concentrations, or are
less than Preliminary Soil Remediation Goal (PSRG) Protection of Groundwater (POG)
standards. Therefore, no COIs are identified and corrective action for soil is not
required.
Risk Assessments
Human health and ecological risk assessments were prepared for the Mayo ash basin
based on state and federal guidance. The human health risk assessment completed for
the Mayo Site found no evidence of risks to human receptors. The ecological risk
assessment found no measurable differences in modeled risks associated with surface
water and sediment compared with background concentrations. Data from off -site
water supply wells and Crutchfield Branch indicate no evidence of increased risk posed
by groundwater migration associated with the ash basin based on evaluation of
concentrations of CCR constituents in environmental media and potential receptors.
The updated risk assessments for the Mayo ash basin are presented in Section 5.4 and
Appendix E of this CAP Update.
Risk Ranking
In accordance with G.S. Section 130A-309.211(cl), Duke Energy installed 16 water
filtration systems at occupied residences or businesses within a half -mile of the Mayo
ash basin compliance boundary. Installation of filtration systems, along with certain
improvements to the Mayo ash basin dam completed by Duke Energy, resulted in the
ash basin being ranked as low risk.
ES.3 CSM Overview
The Conceptual Site Model (CSM) is a written and graphical representation of the
hydrogeologic conditions and constituent interactions specific to the Site and is critical
to understanding the subsurface conditions related to the ash basin. The updated CSM
developed for Mayo included in the CAP Update is based on Environmental Cleanup Best
Management Practices: Effective Use of the Project Life Cycle Conceptual Site Model (USEPA,
2011). This document describes six CSM stages for a project life cycle. The CSM is an
iterative tool designed to assist in the decision -making process for characterization and
remediation as the site progresses through the project life cycle and new data becomes
available. The current Mayo CSM is consistent with Stage 3, "Characterization CSM
Stage", consistent with the six stages of CSM development (USEPA, 2011). The findings
of Stage 3, Characterization for Mayo conclude that the Mayo ash basin is in compliance
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Mayo Steam Electric Plant SynTerra
with 02L and does not require corrective action. Anticipated changes to Site conditions,
such as ash basin decanting and closure, will be incorporated in the future into the
CSM. Predicted and observed effects will be compared on an ongoing basis to refine the
understanding of groundwater flow and constituent transport relative to potential
receptors.
Based on the large data set generated for Mayo, multiple lines of evidence have been
evaluated to develop the CSM. The following provides an overview of the updated
CSM which forms the basis of this CAP. Supporting details for the CSM are presented
in Section 5.0.
Key conclusions of the CSM include the following:
No risks to human health related to the ash basin were identified. The
ecological risk assessment conducted for the ash basin area indicates no
measurable difference between evaluated risks and background
concentrations. Ash basin risk assessments indicate incomplete exposure
pathways and no risk to commercial/industrial worker, residences, and
recreational receptors near the ash basin.
The ash basin does not cause increased risks to ecological receptors. The
updated risk assessment no evidence of unacceptable risks to ecological
receptors (mallard duck, great blue heron, killdeer bird, muskrat, river otter, bald
eagle, American robin, meadow vole, red-tailed hawk, red fox) that may access
surface water and sediments associated with Crutchfield Branch.
• Groundwater from the ash basin has not and does not flow towards any water
supply wells based on groundwater flow patterns, the location of water supply
wells in the area, and evaluation of groundwater analytical data. Groundwater
data from water supply wells and 79 on -Site monitoring wells, groundwater
elevation measurements from 41 monitoring events, and groundwater flow and
transport modeling results all indicate that ash basin -related groundwater
constituents are not affecting, and have not affected, water supply wells.
The permanent water solution program implemented by Duke Energy
provided owners of surrounding properties with water supply wells within a
0.5-mile radius of the ash basin compliance boundary with water filtration
systems. The hydrogeologic data collected at Mayo confirms that Site -related
COIs have not and are not affecting off -Site water supply users. Modeling
predicts that groundwater constituents related to the ash basin will not affect off -
Site water supply users. Nevertheless, Duke Energy installed 16 water filtration
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systems at surrounding occupied residences in accordance with G.S. Section
130A-309.211(cl). One eligible household was non -responsive to the offer.
• The hydrogeologic setting of the Mayo ash basin 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 the groundwater flow patterns within the basin by lowering hydraulic
heads and gradients in and around the ash basin dam, which will reduce the
groundwater seepage velocity and provide source reduction prior to completion
of basin closure. Lower hydraulic heads and the cessation of sluicing into the ash
basin combine to cause the maximum extent of the boron plume to remain
within the compliance boundary for modeled closure scenarios. As of December
1, 2019, approximately 124,200,000 gallons of water had been removed from the
ash basin and the water elevation decreased by about 7 feet.
• The physical setting and hydraulic processes control the flow pattern within
the ash basin, underlying groundwater system, and downgradient areas. The
ash basin is a horizontal water flow -through system. Groundwater enters the
upgradient portion 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 under the dam. This flow system results in
limited downward migration of constituents into the thin, underlying saprolite
upgradient from the dam. A localized effect to the overall flow -through water
system occurs just south of the ponded water in the ash basin where ash was
"stacked" for dewatering prior to transport for beneficial reuse. The effect of this
ash "stack out" area results in a slight variation of the horizontal flow -through
concept as there is slight downward vertical migration of groundwater under the
area. Downward flow is limited to the surficial and transition zone as bedrock
upward vertical gradients prevent downward flow below the transition zone.
Near the dam, constituents in water either discharge through the constructed
seeps or flow under the dam. Beyond the dam, groundwater flows upward
toward the Crutchfield Branch discharge zone, limiting downward migration of
constituents to the area proximate to the dam. Hydraulic and water quality data
from bedrock wells installed at various depths just downgradient of the dam
structure support the flow characteristics and limited constituent distribution.
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Horizontal distribution of constituents in groundwater proximate to the ash
basin is limited. The extent of ash basin -related groundwater constituents is
within the compliance boundary based on the last 4 quarters of groundwater
data collected.
• Vertical distribution of constituents in groundwater proximate to the ash basin
is limited. Shallow bedrock boron concentrations greater than the 02L are
limited to areas near the ash basin waste boundary. Boron concentrations in
bedrock decrease with depth, indicating vertical migration is generally limited to
shallow bedrock (top 40 feet of rock). The deep bedrock groundwater monitoring
wells just outside of the waste boundary have boron concentrations less than or
just above the laboratory reporting limit (50 µg/L).
• Geochemical processes stabilize and limit constituent migration along the
groundwater flow path. Each constituent exhibits unique geochemical behavior
related to the specific constituent partition coefficient (Ka), response to changing
geochemical parameters (i.e., pH and Eh), and sorption capacity of the soil and/or
rock matrix. The geochemical mechanisms that stabilize (i.e., attenuate)
constituents and limit migration are sorption, ion exchange, and precipitation/co-
precipitation. Based on geochemical modeling:
o Non -conservative, reactive constituents (e.g., arsenic) will remain in
mineral phase assemblages that are stable under variable Site conditions,
demonstrating sorption as an effective attenuation mechanism.
o Variably reactive constituents (e.g., manganese) can exhibit mobility
depending on geochemical conditions and availability of sorption sites.
o Non -reactive, conservative constituents (e.g., boron) 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. Sorption of boron to clay particles might occur,
especially for groundwater with slightly alkaline to alkaline pH values.
Geochemical processes are a key factor in limiting downgradient constituent
migration beyond the ash basin waste boundary as indicated by the absence of
discernable plumes downgradient of the ash basin for non -conservative and
variably reactive constituents.
• Constituent concentrations in groundwater greater than 02L standards are
contained within Duke Energy's property and within the compliance
boundary. This finding is supported by the groundwater sampling and analysis
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Mayo Steam Electric Plant SynTerra
results collected from 79 monitoring wells over 41 monitoring events. This
finding is further supported by groundwater flow and transport modeling
results that predict boron will not exceed groundwater standards at or beyond
the compliance boundary in the future.
• Groundwater/surface water interaction has not and is not predicted to cause
constituent concentrations greater than North Carolina Administrative Code,
Title 15A, Subchapter 02B, Surface Water and Wetland Standards (02B).
Analytical results for surface water samples collected from Crutchfield Branch
indicate that this water body meets 02B standards. Further, an evaluation of
future surface water quality conditions of basin -related jurisdictional streams
was conducted using a surface water mixing model with closure option model
simulation inputs. The evaluation found that future groundwater migration
would not result in violations of 02B surface water standards. An engineered
system was installed to capture flow from each ash basin toe drain and direct it
to the ash basin and wastewater treatment system to reduce constituent
contributions to Crutchfield Branch. Flow and transport model predictions
indicate the decanting of the water in the ash basin will lower water levels in the
area and reduce the volume of water towards the engineered seeps. The
collection of water from engineered seeps and discharge to a permitted outfall is
anticipated to remain in place until determined to no longer be necessary under
the provisions of the Special Order by Consent (SOC).
• Most of the constituents identified in the CSA Update occur naturally in
groundwater, some at concentrations greater than the 02L standard and/or
IMAC. Groundwater at Mayo naturally contains arsenic, barium, chromium,
cobalt, iron, manganese, molybdenum, strontium, sulfate, total dissolved solids
(TDS), and vanadium. 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 value. For the Mayo CAP, vanadium is evaluated
based on its Site -specific statistically derived background value and additional
lines of evidence to determine if constituent concentrations represent migration
from the ash basin or are naturally occurring.
• The boron groundwater plume for all three flow zones is stable or decreasing.
Results from the Mann -Kendall statistical trend analysis indicate that
concentrations of boron are stable or decreasing. Further, flow and transport
model simulations demonstrate that boron concentrations in any flow zone will
not be greater than the 02L standard beyond the compliance boundary for pre -
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Corrective Action Plan Update December 2019
Mayo Steam Electric Plant SynTerra
decanting conditions or any closure scenario and demonstrates that the boron
plume will contract towards the south.
These CSM aspects, combined with the updated human health and ecological risk
assessments, provide the basis for this CAP Update developed for the Mayo ash basin
to comply with G.S. Section 130A-309.211, amended by CAMA.
ES.4 Corrective Action Approach
Corrective Action Objectives
Migration of ash basin -sourced constituents in groundwater does not extend to or
beyond the compliance boundary. Therefore, the approach planned for Mayo focuses
on verifying decreasing groundwater concentrations predicted by modeling. The
following objective addresses the regulatory requirements of CAMA for the Mayo CAP:
• Monitor groundwater quality to verify that ash basin -sourced constituents
remain below standards.
Additional Measures Eliminating Risk
Duke Energy owns the property downgradient from the Mayo ash basin dam to the
North Carolina/Virginia state line. Ownership of the property allows Duke Energy to
control activities, thereby managing risks for future property use. As a proactive
corrective action measure, on August 23, 2019, Duke Energy purchased the
approximately 56-acre parcel positioned on the north side of Mayo Lake Road. Duke
Energy -owned property bordered the acquired parcel on the west, south, and east sides.
As a result of this acquisition, the ash basin compliance boundary has been revised. The
compliance boundary now extends further to the north beyond Mayo Lake Road and
500 feet from the entire waste boundary (Figure ES-1).
Summary of Source Control and Corrective Measures
It is critical to take into account all of the various activities Duke Energy has/will
perform to improve subsurface conditions at Mayo related to the ash basin. The
corrective action program incorporates source control by basin decanting and closure
followed by confirmatory monitoring. Table ES-3 presents the discrete components of
the planned corrective action for groundwater.
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TABLE ES-3
COMPONENTS OF SOURCE CONTROL, ACTIVE REMEDIATION, AND
MONITORING
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 near the ash basin
dam, reducing groundwater seepage velocities and
constituent transport potential. Decanting will return the
groundwater flow system to its more natural condition.
Decanting was initiated on June 27, 2019. Decanting is
scheduled to be complete on or before December 31, 2020.
As of December 1, 2019, approximately 124,200,000 gallons
of water had been removed from the ash basin and the
water elevation decreased by about 7 feet.
In addition, ash basin decanting is expected to be effective
in reducing or eliminating seeps identified under the Special
Order by Consent.
Toe Drain Collection
An engineered seep collection system captures flow from the
toe drains (constructed seeps) and directs flow back into the
ash basin and wastewater treatment system.
Ash Basin Closure
Closure -in -place or by closure -by -excavation are considered
source control activities. Extensive groundwater modeling
indicates that either method results in similar effects with
respect to future groundwater conditions.
Institutional Controls and Monitoring
Permanent Water Solution
Groundwater data at the Site indicates that surrounding
for Water Supply Well Users
water supply wells are not and have not been affected by
within a 0.5-mile radius of
Site -related COIs. Nevertheless, installation and
the Coal Ash Basin
maintenance by Duke Energy of water filtration systems for
Compliance Boundary and
16 occupied households has been completed and approved
Associated Water Filtration
by the NCDEQ to address current and future stakeholder
System Maintenance
concerns. Duke Energy maintains these systems on behalf of
the property owners.
Maintain Ownership and
Duke Energy owns the land downgradient of the ash basin
Institutional Controls (ICs)
and controls its use. The parcel located north of the ash
basin and the Mayo Plant has been purchased by Duke
Energy allowing Duke Energy to control activities on the
property, thereby managing risks to property users
downgradient of the ash basin to the North Carolina/Virginia
state line. Duke Energy ownership of property mitigates
potential future risk by controlling or eliminating potential
exposure pathways associated with Site -related COIs.
Page ES-13

Corrective Action Plan Update December 2019
Mayo Steam Electric Plant SynTerra
TABLE ES-3
COMPONENTS OF SOURCE CONTROL, ACTIVE REMEDIATION, AND
MONITORING
Remedy Component Rationale
Confirmation Monitoring Duke Energy will monitor groundwater to confirm that
Plan (CMP) concentrations at the compliance boundary remain in
compliance with 02L and to compare with model -predicted
changes in boron plume during and after basin closure. The
CAP Update includes a comprehensive review of
groundwater data collected through June 2019 and a plan to
optimize the monitoring program. Within thirty (30) days of
CAP approval, Duke Energy would implement the
Confirmation Monitoring Plan. The CMP monitoring schedule
will be consistent with the Interim Monitoring Plan (IMP) to
maintain seasonal data correlation.
Prepared by: JAW Checked by: PWA
Monitoring Program During and Post -Source Control
Basin decanting and closure processes will have a beneficial effect on the Site
hydrogeology, groundwater flow direction, gradients, and velocity. As an example, the
modeled net groundwater flow rate from the ash basin across the dam footprint into the
downgradient area is predicted to decrease from 18 gallons per minute (gpm) under
pre -decanting conditions to 7 gpm after decanting, primarily due to the reduction in the
hydraulic head in the ash basin (detailed information is provided in Section 5.1.2.8).
The effects of decanting and closure are being monitored by pressure transducers to
record water level changes and geochemical sondes to record geochemical changes near
the dam, immediately downgradient of the ash basin. Monitoring of groundwater
chemistry supports continued evaluation of changing conditions due to decanting and
closure activities.
Flow and transport modeling results predict boron constituent concentrations will
decrease over time primarily due to dilution from recharge. Geochemical model data
indicate conditions, primarily pH and redox potential (Eh), will remain stable, resulting
in stable reactive constituent concentrations. The predictive model results will be
verified through evaluation of the confirmatory monitoring data.
Contingency for Corrective Action
If the confirmatory groundwater monitoring data indicates that additional groundwater
corrective action is warranted, an updated CAP may be prepared to address these
conditions in accordance with applicable regulations.
Page ES-14

z'
DUKE ENERGY PROGRESS-
s41 - PROPERTY LINE �: .�d r d•'
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BOUNDARY
60' RIGHT-OF-WAY NORTH CAROLINA-VIRGINIA STATE LINE
{'--�• .'; 1 HALIFAX COUNTY (APPROXIMATE)
PERSON COUNTY
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QUADRANGLE, QUAD ID: 36078E8, OBTAINED FROM THE USGS STORE AT
https://store.usgs.gov/map-locator.
PERSON FIGURE ES-1
DUKE COUNTY USGS LOCATION MAP
ENERGY CORRECTIVE ACTION PLAN UPDATE
PROGRESS WINSTON-SAL EM MAYO STEAM ELECTRIC PLANT
RA`EIGH • ROXBORO, NORTH CAROLINA
CHARLOTTE •
DRAWN BY: A. ROBINSON DATE: /0/2019 GRAPHIC SCALE
REVISED BY: A. ROBINSONBINS 12
DATE: 12/O6/2019 1,000 0 1.000 2.000
A MTA.�.�... CHECKED BY: P. WYLIE DATE: 12/06/2019
,�r 1 � rd APPROVED BY: J. WYLIE DATE: 12/O6/2019 (IN FEET)
www.synterracorp.com PROJECT MANAGER: J. WYLIE

Corrective Action Plan Update December 2019
Mayo Steam Electric Plant SynTerra
TABLE OF CONTENTS
SECTION
PAGE
EXECUTIVE SUMMARY.................................................................................................... ES-1
1.0 INTRODUCTION.........................................................................................................1-1
1.1. Background..........................................................................................................1-2
1.2 Purpose and Scope..............................................................................................1-3
1.4 List of Considerations by the Secretary for Evaluation of Corrective Action
Plans......................................................................................................................1-6
1.5 Facility Description.............................................................................................1-6
1.5.1 Location and History of Land Use............................................................1-6
1.5.2 Operations and Waste Streams Coincident with the Ash Basin ...........
1-8
1.5.3 Overview of Existing Permits and Special Orders by Consent ............1-9
2.0 RESPONSE TO CSA UPDATE COMMENTS IN SUPPORT OF CAP
DEVELOPMENT...........................................................................................................
2-1
2J. Facility -Specific Comprehensive Site Assessment Comment Letter from
NCDEQ.................................................................................................................
2-1
2.2 Duke Energy's Response to NCDEQ CSA Comment 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
4.3 Table of Background Concentrations for Surface Water ...............................
4-4
4.4 Table of 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.........................................................................5-4
5.1.2.1 Groundwater Flow Direction..............................................................
5-4
5.1.2.2 Groundwater Seepage Velocities........................................................5-7
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Corrective Action Plan Update December 2019
Mayo Steam Electric Plant SynTerra
TABLE OF CONTENTS (CONTINUED)
5.1.2.3 Hydraulic Gradients............................................................................. 5-9
5.1.2.4 Particle Tracking Results....................................................................5-10
5.1.2.5 Subsurface Heterogeneities...............................................................
5-11
5.1.2.6 Bedrock Matrix Diffusion and Flow.................................................5-12
5.1.2.7 Onsite and Offsite Pumping Influences...........................................5-14
5.1.2.8 Ash Basin Groundwater Balance ......................................................
5-14
5.1.2.9 Effects of Naturally Occurring Constituents...................................5-16
5.2
Source Area Location........................................................................................5-17
5.3
Summary of Potential Receptors.....................................................................5-17
5.3.1 Public and Private Water Supply Wells .................................................
5-17
5.3.2 Availability of Public Water Supply .......................................................
5-18
5.3.3 Surface Water.............................................................................................
5-18
5.3.4 Future Groundwater Use Area................................................................
5-19
5.4
Human Health and Ecological Risk Assessment Results ............................
5-19
5.5
CSM Summary...................................................................................................
5-21
6.0
CORRECTIVE ACTION APROACH FOR MAYO ASH BASIN ........................
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 Source
Material...................................................................................................
6-4
6.1.1.4 Volume and Physical Horizontal and Vertical Extent of Anticipated
Saturated Source Material....................................................................
6-4
6.1.1.5 Saturated Ash and Groundwater.......................................................
6-5
6.1.1.6 Chemistry Within Waste Boundary ...................................................
6-7
6.1.1.7 Other Potential Source Material........................................................
6-11
6.1.1.8 Interim Response Actions.................................................................. 6-12
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Corrective Action Plan Update December 2019
Mayo Steam Electric Plant SynTerra
TABLE OF CONTENTS (CONTINUED)
6.1.2 Extent of Constituent Migration beyond the Compliance Boundary 6-14
6.1.2.1
Piper Diagrams....................................................................................6-20
6.1.3
Constituents of Interest (COIs)................................................................
6-22
6.1.4
Horizontal and Vertical Extent of COIs.................................................
6-32
6.1.4.1
COIs in Unsaturated Soil...................................................................
6-32
6.1.4.2
Horizontal and Vertical Extent of Groundwater in Need of
Restoration...........................................................................................
6-32
6.1.5
COI Distribution in Groundwater...........................................................
6-35
6.1.5.1
Conservative Constituents.................................................................
6-36
6.1.5.2
Non -Conservative Constituents.......................................................
6-38
6.1.5.3
Variably Reactive Constituents.........................................................6-38
6.2
Receptors Associated with Ash Basin............................................................
6-39
6.2.1
Surface Waters — Downgradient within a 0.5-Mile Radius of the Waste
Boundary.....................................................................................................
6-39
6.2.2
Water Supply Wells...................................................................................
6-41
6.2.2.1
Provision of Alternative Water Supply ...........................................
6-42
6.2.2.2
Findings of Drinking Water Supply Well Surveys ........................
6-43
6.2.3
Future Groundwater Use Areas..............................................................
6-45
6.3
Human and Ecological Risks...........................................................................
6-45
6.4
Evaluation of Remedial Alternatives..............................................................
6-46
6.5
Proposed Remedial Alternatives Selected for the Ash Basin ......................
6-46
6.5.1
Description of Proposed Remedial Alternative ....................................
6-46
6.5.2
Design Details of Proposed Remedial Alternative ...............................
6-46
6.5.3
Monitored Natural Attenuation Requirements ....................................
6-47
6.5.4
Requirements for 02L.0106 Rule..............................................................
6-47
6.5.5
Sampling and Reporting...........................................................................
6-47
6.5.5.1
Confirmation Monitoring Plan .........................................................
6-47
6.5.6
Interim Activities Prior to Implementation ...........................................
6-52
Page iii

Corrective Action Plan Update December 2019
Mayo Steam Electric Plant SynTerra
TABLE OF CONTENTS (CONTINUED)
6.5.7 Contingency Plan.......................................................................................6-52
6.5.7.1 Description of Contingency Plan......................................................6-53
6.5.7.2 Decision Metrics for Implementing Contingency Plan ................. 6-53
6.6 Ash Basin Summary..........................................................................................6-54
7.0 PROFESSIONAL CERTIFICATION.........................................................................7-1
8.0 REFERENCES................................................................................................................ 8-1
Page iv

Corrective Action Plan Update December 2019
Mayo Steam Electric Plant SynTerra
LIST OF FIGURES
Executive Summary
Figure ES-1 USGS Location Map
1.0 Introduction
Figure 1-1 USGS Location Map
Figure 1-2 Site Layout Map
Figure 1-3 1951 Aerial Photograph
4.0 Summary of Background Determinations
Figure 4-1 Background Sample Locations
5.0 Conceptual Site Model
Figure 5-1 Conceptual Site Model Pre -Decanting Conditions
Figure 5-2 LeGrand Slope Aquifer System
Figure 5-3 General Profile of Ash Basin Pre -Decanting Flow Conditions in the
Piedmont
Figure 5-4a Water Level Map - Surficial Flow Zone - April 2019
Figure 5-4b Water Level Map - Transition Flow Zone - April 2019
Figure 5-4c Water Level Map - Bedrock Flow Zone - April 2019
Figure 5-5a Flow Velocity Vectors and Magnitudes for Pre -Decanting Conditions
Figure 5-5b Flow Velocity Vectors and Magnitudes for Closure -by -Excavation
Scenario
Figure 5-5c Flow Velocity Vectors and Magnitudes for Closure -in -Place Scenario
Figure 5-6 HB630 Provision of Permanent Water Supply Completion Map
Figure 5-7 Map of Surface Waters
6.0 Corrective Action Approach for Ash Basin
Figure 6-1 Fly Ash and Bottom Ash Interbedded Depiction
Figure 6-2 General Cross Section A -A' - Ash Basin
Figure 6-3 Saturated Ash Thickness Map — Pre -Decanting and Closure -in -Place
Scenarios
Figure 6-4 General Cross Section A -A' - Ash Basin - Boron
Figure 6-5 Site Layout - Decanting Monitoring Network
Figure 6-6 Geochemical Water Quality Plots
Figure 6-7 Ash Pore Water and Groundwater Piper Diagrams
Figure 6-8a Hydrographs — Within Ash Basin
Page v

Corrective Action Plan Update December 2019
Mayo Steam Electric Plant SynTerra
LIST OF FIGURES (CONTINUED)
6.0 Corrective Action Approach for Ash Basin (Continued)
Figure 6-8b
Hydrographs — Within and North of Ash Basin
Figure 6-8c
Hydrographs — North of Ash Basin
Figure 6-9
Unsaturated Soil Sample Locations and Exceedances
Figure 6-10
Seep and Surface Water Quality Piper Diagrams
Figure 6-11a
Map of Boron Distribution - Surficial Zone
Figure 6-11b
Map of Boron Distribution - Transition Zone
Figure 6-11c
Map of Boron Distribution - Bedrock Zone
Figure 6-12
Confirmation Monitoring Plan Network
Figure 6-13
Confirmation Monitoring Plan Work Flow Diagram
Page vi

Corrective Action Plan Update December 2019
Mayo Steam Electric Plant SynTerra
LIST OF TABLES
Executive Summary
Table ES-1 Summary of Mayo Assessment Documentation
Table ES-2 Summary of Mayo Assessment Activities (Through June 2019)
Table ES-3 Components of Source Control, Active Remediation, and Monitoring
3.0 Overview of Source Areas Proposed for Corrective Action
Table 3-1 Summary of Onsite Potential Additional Source Facilities
4.0 Summary of Background Determinations
Table 4-1 Background Soil Sample Summary
Table 4-2 Background Values for Soil
Table 4-3 Background Values for Groundwater
Table 4-4 Background Dataset Ranges for Surface Water
Table 4-5 Background Dataset Ranges for Sediment
5.0 Conceptual Site Model
Table 5-1 April 2019 Water Level Measurements and Elevations
Table 5-2 Groundwater Balances Summary
Table 5-3 Surface Water Classification
6.0 Corrective Action Approach for Ash Basin
Table 6-1 Boron Concentrations in Groundwater Below Source Area
Table 6-2 Source Area Interim Actions
Table 6-3 Soil PSRG POG Standard Equation Parameters and Values
Table 6-4 Summary of Unsaturated Soil Analytical Results
Table 6-5 Seep Corrective Action Strategy
Table 6-6 Means of Groundwater Constituents - January 2018 to April 2019
Table 6-7 Constituent Management Matrix
Table 6-8 July 2018 - July 2019 Boron Concentrations Near or Beyond
Compliance Boundary - Downgradient of Ash Basin
Table 6-9 Summary Trend Analysis Results for Groundwater Monitoring Wells
Table 6-10 Water Supply Well Analytical Results Summary
Table 6-11 Confirmation Monitoring Plan Elements
Page vii

Corrective Action Plan Update December 2019
Mayo Steam Electric Plant SynTerra
LIST OF APPENDICES
Appendix A Regulatory Correspondence
Appendix B Comprehensive Site Assessment Update Report Review Comments
and Responses
Appendix C
Updated Comprehensive Analytical Data 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
Surface Water Evaluation to Assess 15A NCAC 02B .0200 Compliance
for Implementation of Corrective Action under 15A NCAC 02L .0106
(k) and (1) Report
Surface Water Future Conditions Evaluation in Accordance with
Compliance to 15A NCAC 02B .0200
Appendix J
Ash Basin Groundwater Confirmation Monitoring Plan
Page viii

Corrective Action Plan Update December 2019
Mayo Steam Electric Plant SynTerra
LIST OF ACRONYMS
µg/L
micrograms per liter
02B
North Carolina Administrative Code, Title 15A,
Subchapter 02B, Surface Water and Wetland Standards
02L
North Carolina Administrative Code, Title 15A, Subchapter
02L, Groundwater Classification and Standards
AOW
Area of Wetness
CAMA
Coal Ash Management Act of 2014
CAP
Corrective Action Plan
CBD
Citrate-Bicarbonate-Dithionite
CCR
Coal Combustion Residuals
Closure Plan
CCR Surface Impoundment Closure Plan
CMP
Confirmation Monitoring Plan
COI
Constituent of Interest
COPC
Constituent of Potential Concern (risk assessment)
CSA
Comprehensive Site Assessment
CSM
Conceptual Site Model
cy
cubic yard
Duke Energy
Duke Energy Progress, LLC
DWM
NCDEQ Division of Waste Management
DWR
NCDEQ Division of Water Resources
Eh
Redox Potential
EMC
Environmental Management Commission
EMP
Effectiveness Monitoring Plan
FGD
Flue Gas Desulfurization
FLASH
Flow -Log Analysis of Single Holes
ft/day
feet per day
ft/ft
feet by foot
ft/yr
feet per year
g/cm3
grams per cubic centimeter
gpm
gallons per minute
G.S.
NC General Statute
HAO
Hydrous Aluminum Oxide
HFO
Hydrous Ferric Oxide
HPF
Heal Pulse Flowmeter
HQ
Hazard Quotient
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Corrective Action Plan Update December 2019
Mayo Steam Electric Plant SynTerra
LIST OF ACRONYMS (CONTINUED)
ICs
Institutional Controls
IMAC
Interim Maximum Allowable Concentrations
IMP
Interim Monitoring Plan
J-flag
Laboratory estimated concentration.
Ka
Partition Coefficient
kg/L
kilograms per liter
L/kg
liters per kilogram
LCL
lower confidence limit
LOAEL
lowest observed adverse effects levels
Mayo, Plant, or Site
Mayo Steam Electric Plant
mm
millimeter
NA
Not applicable
NAVD88
North American Vertical Datum of 1988
NC
North Carolina
NCAC
North Carolina Administrative Code
NCDEQ
North Carolina Department of Environmental Quality
ND
non -detect (less than the laboratory reporting limit)
ne
Effective Porosity
NOAEL
no observed adverse effects levels
NORR
Notice of Regulatory Requirements
NPDES
National Pollution Discharge Elimination System
NRTR
Natural Resources Technical Report
NTUs
Nephelometric Turbidity Units
ORP
Oxidation Reduction Potential
POG
Protection of Groundwater
PSRGs
Preliminary Soil Remediation Goals
RO qualified
data are unusable
RC
Rural Conservation
RL
Reporting Limit
s.u.
Standard Units
SOC
Special Order by Consent
SPLP
Synthetic Precipitation Leaching Procedures
SWS
Solid Waste Section
TDS
Total Dissolved Solids
TOC
Total organic carbon
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Corrective Action Plan Update December 2019
Mayo Steam Electric Plant
SynTerra
LIST OF ACRONYMS (CONTINUED)
USEPA United States Environmental Protection Agency
USGS U.S. Geological Survey
Page xi

Corrective Action Plan Update December 2019
Mayo Steam Electric Plant SynTerra
1.0 INTRODUCTION
(CAP Content Section 1)
SynTerra prepared this groundwater corrective action plan (CAP) Update on behalf of
Duke Energy Progress, LLC (Duke Energy). This CAP Update pertains to the Mayo
Steam Electric Plant (Mayo, Plant, or Site) coal combustion residual (CCR) surface
impoundment (ash basin). Duke Energy owns and operates Mayo, located in Roxboro,
Person County, North Carolina (Figure 1-1).
In accordance with North Carolina General Statutes (G.S.) Section 130A-309.211 (b),
amended by the 2014 North Carolina Coal Ash Management Act (CAMA), Duke
Energy is required to submit a CAP for the restoration of groundwater in conformance
with the requirements of 02L. Analytical data obtained over one year of quarterly
monitoring indicate the Mayo ash basin is currently in compliance with 02L
groundwater quality standards; therefore, groundwater corrective action under 15A
NCAC 02L .0106 is not required at this time for the Mayo ash basin.
This CAP Update addresses the requirements of CAMA and is prepared consistent with
the CAP guidance provided by the North Carolina Department of Environmental
Quality (NCDEQ) in a letter to Duke Energy, dated September 10, 2019 (Appendix A).
This CAP Update evaluates groundwater associated with the Mayo ash basin.
Specifically, this CAP focuses on constituent concentrations detected greater than
applicable North Carolina groundwater standards [02L; Interim Maximum Allowable
Concentrations (IMAC); or background threshold values, whichever is greater], at or
beyond the compliance boundary.
In accordance with G.S. Section 130A-309.211(b), a CAP for Mayo was previously
submitted to the NCDEQ in two parts:
• Corrective Action Plan Part 1 —Mayo Steam Electric Plant (SynTerra, 2015b)
• Corrective Action Plan Part 2 —Mayo Steam Electric Plant (SynTerra, 2016a).
This CAP Update is being submitted to NCDEQ as originally requested in a June 2,
2017, letter from NCDEQ to Duke Energy. In letters to Duke Energy dated April 5, 2019
and May 9, 2019, NCDEQ provided revised CAP deliverable schedules and requested
assessment of additional potential sources of constituents to groundwater
(Appendix A).
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Corrective Action Plan Update December 2019
Mayo Steam Electric Plant SynTerra
This CAP Update includes section references to the document, Corrective Action Plan
Content for Duke Energy Coal Ash Facilities (provided in Appendix A), beneath report
section headings and within text in brackets to facilitate the review process. The CAP
content and submittal schedule are in accordance with correspondence between
NCDEQ and Duke Energy, including CAP content guidance issued by NCDEQ on
April 27, 2018 and adjusted on September 10, 2019 (Appendix A). In addition to the
CAP Update, Duke Energy will submit a CCR Surface Impoundment Closure Plan
(Closure Plan) to NCDEQ on/before December 31, 2019 under separate cover. Duke
Energy will submit final closure plans consistent with the detailed requirements of G.S.
Section 130A-309.214. This CAP Update has been developed to be effective with any
closure scenarios determined for the Site.
1.1 Background
(CAP Content Section 1.A)
A substantial amount of assessment data has been collected for the Mayo ash basin to
support this CAP Update. Site assessment was completed and the Comprehensive Site
Assessment (CSA) Update Report (SynTerra, 2017b) was submitted in accordance with
requirements in 15A NCAC 02L.0106 (g). The CSA:
• Identified CCR-related constituents present in groundwater in the ash basin area
at concentrations greater than applicable regulatory standards.
• Found no imminent hazards to public health and safety.
• Identified no receptors and no significant exposure pathways.
• Sufficiently determined the horizontal and vertical extent of CSA identified
constituents in soil and groundwater.
• Determined the geological and hydrogeological features influencing the
movement, chemical makeup, and physical characteristics of CSA identified
constituents.
NCDEQ provided review of the CSA Update to Duke Energy in a letter dated May 7,
2018 and stated that sufficient information was provided to warrant preparation of this
CAP Update (Appendix A). This CAP Update builds on previous documents to provide
a CAP for addressing the requirements of CAMA which requires restoration of
groundwater in conformance with the requirements of 02L.
Detailed descriptions of Site operational history, the CSM, physical setting and features,
geology/hydrogeology, and findings of the CSA and other CAMA-related work are
documented in this CAP Update and in the following reports:
Page 1-2

Corrective Action Plan Update December 2019
Mayo Steam Electric Plant SynTerra
• Comprehensive Site Assessment Report — Mayo Steam Electric Plant
(SynTerra, 2015a)
• Corrective Action Plan Part 1— Mayo Steam Electric Plant (SynTerra, 2015b)
• Corrective Action Plan Part 2 — Mayo Steam Electric Plant (SynTerra, 2016a)
• Comprehensive Site Assessment Supplement 1 — Mayo Steam Electric Plant
(SynTerra, 2016b)
• Comprehensive Site Assessment Update — Mayo Steam Electric Plant
(SynTerra, 2017b)
• Ash Basin Pumping Test Report — Mayo Steam Electric Plant (SynTerra, 2019a)
• Surface Water Evaluation to Assess 15A NCAC 02B.0200 Compliance for
Implementation of Corrective Action Under 15A NCAC 02L.0106 W and (l)
(SynTerra, 2019b)
• 2018 CAMA Annual Interim Monitoring Report — Mayo Steam Electric Plant
(SynTerra, 2019c)
Reports and other documents submitted to NCDEQ that pertain to the calculation and
approval of background threshold values (BTVs) for Site media are presented and
described in Section 4.0.
1.2 Purpose and Scope
(CAP Content Section 1.B)
The purpose of this Mayo CAP Update is to:
• Meet the requirements for corrective action plans specified in G.S. Section 130A-
309.211 (b) prepared in accordance with NCDEQ CAP guidance
• Provide supporting evidence that groundwater quality does not exceed
applicable 15A NCAC 02L.0202 groundwater quality standards at or beyond the
ash basin compliance boundary
• Present an optimized groundwater monitoring network for the ash basin based
on ash basin -related constituent mobility and distribution
The scope of the CAP and this CAP Update is defined by G.S. Section 130A-309.211,
amended by CAMA. The CAMA legislation required, among other items, assessment of
groundwater at coal combustion residual impoundments and corrective action in
conformance with the requirements of 15A NCAC 02L. These corrective actions for
restoration of groundwater quality requirements were codified into G.S. Section 130A-
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Corrective Action Plan Update December 2019
Mayo Steam Electric Plant SynTerra
309.211 which was further amended by House Bill 630 to require a provision for
alternate water supply for receptors within 0.5-half mile downgradient from the
established compliance boundary.
Based on Site conditions and assessment results, the CAP purpose is to evaluate the
need for corrective action, other than source control through closure, and present a
recommended plan. Migration of ash basin -sourced constituents in groundwater does
not extend past the compliance boundary of the Mayo ash basin. Therefore, the CAP
Update for Mayo focuses on constituent concentrations detected greater than applicable
North Carolina groundwater standards [02L; Interim Maximum Allowable
Concentrations (IMAC); or background threshold values, whichever is greater], and
verifying decreasing groundwater concentrations during decanting and subsequent
closure of the basin. Once the CAP is approved by NCDEQ implementation is planned
to begin within 30 days as required in G.S. Section 309.211 (b)(3).
1.3 Regulatory Basis for Corrective Action
(CAP Content Section 1.0
The regulatory requirements for corrective action at CCR surface impoundments are
found in G.S. Section 130A-309.211(b), (c), and (c1). Section (b) of G.S. 130A-309.211
requires that the CAP shall provide for groundwater restoration in conformance with
the requirements of Subchapter L of Chapter 2 of Title 15A of the North Carolina
Administrative Code (15A NCAC 02L). In accordance with 130A-309.211(b), the
groundwater CAP shall include, at a minimum, the following
(CAP Content Section 1.C.a):
• A description of all exceedances of the groundwater quality standards, including
any exceedances that the owner asserts are the result of natural background
conditions
• A description of the methods for restoring groundwater in conformance with the
requirements of Subchapter L of Chapter 2 of Title 15A of the NCAC and a
detailed explanation of the reasons for selecting these methods
• Specific plans, including engineering details, for restoring groundwater quality
• A schedule for implementation of the groundwater corrective action plan
• A monitoring plan for evaluating the effectiveness of the proposed corrective
action and detecting movement of any constituent plumes
• Any other information related to groundwater assessment required by NCDEQ
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Corrective Action Plan Update December 2019
Mayo Steam Electric Plant SynTerra
In addition to CAMA, requirements for CAPS are also contained in 15A NCAC 02L
.0106(e), (h) and (i).
Section 02L .0106(e)(4) requires implementation of an approved CAP for restoration of
groundwater quality at or beyond the compliance boundary in accordance with a
schedule established by the Secretary.
In a letter dated December 18, 2009, NCDEQ informed Duke Energy that the North
Carolina Attorney General's Office clarified how corrective action requirements apply
to facilities permitted prior to December 30, 1983. The Attorney General determined
that facilities exceeding groundwater standards, permitted under G.S. Section 143-215.1,
and permitted prior to December 30, 1983, fall under 15A NCAC 02L .0106(c). The letter
then stated that this clarification gives Duke Energy the option to seek approval of a
CAP that does not require remediation to groundwater standards [15A NCAC 02L
.0106(k)] or may allow natural attenuation by natural processes [15A NCAC 02L
.0106(1)]. Therefore, where applicable, the CAP Update presents an evaluation of the
options available for corrective action under 15A NCAC 02L .0106(j), (k), and (1).
• Under paragraph 0), corrective action would be implemented using remedial
technology for restoration of groundwater quality to the standards (02L).
• Under paragraph (k), a request for approval of a corrective action plan may be
submitted without requiring groundwater remediation to the standards (02L) if
the requirements in (k) are met.
• Under paragraph (1), a request for approval of a corrective action plan may be
submitted based on natural processes of degradation and attenuation if the
requirements in (1) are met.
This CAP Update has been prepared in general accordance with the NCDEQ guidance
document titled Corrective Action Plan Content for Duke Energy Coal Ash Facilities which
provides an outline of the technical content and format presented in the NCDEQ's letter
dated September 10, 2019, provided in Appendix A (CAP Content Section 1.C.c).
In addition to this groundwater CAP, the Mayo ash basin is subject to closure
requirements under CAMA. Basin closure activities will provide source control within
the ash basin and are considered a component of the overall corrective action for the
Site. Importantly, the Mayo ash basin meets the low -risk classification criteria set forth
in CAMA for CCR surface impoundments. On November 14, 2018, NCDEQ confirmed
that Duke Energy had established permanent water supplies for surrounding properties
by August 31, 2018, and rectified certain dam safety deficiencies, reclassifying the ash
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basin from its prior draft ranking of "intermediate" to "low -risk." Under G.S. Section
130A-309.214, a low -risk CCR surface impoundment may be closed by excavation,
closure -in -place, or a hybrid approach.
Ash basin closure is detailed in a separate document (AECOM, 2019); therefore, this
CAP Update considers multiple ash basin closure scenarios. Closure by cap -in -place or
closure -by -excavation will be effective in addressing the ash basin source area which is
an important part of the overall corrective action strategy. Groundwater modeling
simulations indicate the different closure options have a similar effect on COI
concentrations in groundwater.
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),
now NCDEQ (Appendix A), confirm groundwater quality does not exceed applicable
15A NCAC 02L .0202 groundwater quality standards at or beyond the ash basin
compliance boundary; therefore, groundwater corrective action under 15A NCAC 02L
.0106 is not required at this time for the Mayo ash basin. The CAP requirements noted
in Section 1.C.b. are not applicable nor presented in this CAP Update (CAP Content
Section 1.C.b and 1.C.c).
1.4 List of Considerations by the Secretary for Evaluation of
Corrective Action Plans
(CAP Content Section 1.D)
This CAP Update meets the corrective action requirements under G.S. Section 130A-
309.211, amended by CAMA. Groundwater quality data confirms that constituents
identified at the Mayo Site do not exceed applicable 15A NCAC 02L .0202 groundwater
quality standards at or beyond the ash basin compliance boundary; therefore,
groundwater corrective action under 15A NCAC 02L.0106 is not required for the Mayo
ash basin at this time.
1.5 Facility Description
(CAP Content Section LE)
1.5.1 Location and History of Land Use
(CAP Content Section 1.E.a)
Mayo is located on the northwest side of Mayo Reservoir in Roxboro, Person
County, North Carolina (Figure 1-1). Mayo is a single -unit coal-fired electricity
generating plant. The Mayo ash basin was completed in October 1982 and power
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generation began in 1983. Mayo Reservoir was created to provide cooling water
for Plant operations.
The overall topography of the Site generally slopes toward the east (Mayo
Reservoir) and northeast (Crutchfield Branch). The Site is roughly bisected by US
Highway 501 with the majority of the Site — including the power block, the ash
basin, and most of its operational features — located east of US Highway 501.
The portion of the Site west of US Highway 501 contains the operational
industrial landfill (Monofill; Permit #7305-INDUS) and a haul road that connects
the Monofill with the operational portion of the Plant (Figure 1-2). The eastern
portion of the Site, excluding Mayo Reservoir, encompasses 460 acres. Mayo
Reservoir encompasses 2,880 acres with a normal water elevation of
approximately 433 feet (North American Vertical Datum of 1988 [NAVD 88]).
The ash basin is the dominant feature on the portion of the property east of US
Highway 501. The ash basin is bounded on the west by US Highway 501 and on
the east by a railroad line. Ridges east, west, and south of the ash basin act as
groundwater divides that provide control of groundwater migration to within
the former Crutchfield Branch stream valley.
The Mayo generating station and supporting facilities lie within property owned
by Duke Energy. Based on a review of available historical and aerial
photography, the Site consisted of a combination of agricultural land, rural
residential, and woodlands prior to the impoundment of Mayo Creek for the
formation of Mayo Reservoir and development of the Plant. Figure 1-3 presents
an aerial photograph from 1951 prior to development of the Plant and
construction of Mayo Reservoir (CAP Content Section 1.E.a).
Land use within the 0.5-mile radius of the ash basin compliance boundary
generally consists of undeveloped, forested land with infrequent rural
residential. The entire area encompassed by the Mayo Plant and surrounding
properties is zoned by Person County as RC (Rural Conservation). Properties
located within a 500-foot radius of the Mayo ash basin compliance boundary are
all contained within the Site (Figure 1-2).
Properties adjacent to the Site are located in Person County, North Carolina, and
Halifax County, Virginia. The closest residences to the east of the Site are along
the easternmost shore of Mayo Lake. Undeveloped land borders the Site to the
north. Several residences are located just outside the Site boundaries to the south
and northwest (in North Carolina and in Virginia). Louisiana Pacific
Corporation, located south and west of the Mayo property boundary, opposite
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the Plant's entrance road, is a manufacturing facility that produces oriented
strand board. The facility has been in operation since 1994.
1.5.2 Operations and Waste Streams Coincident with the Ash
Basin
(CAP Content Section 1.E.b)
Coal -Related Operations 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 comprised 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 Mayo through rail transportation since Plant operations
began. Coal storage has historically occurred at the Site's coal pile storage area
located immediately southeast of the ash basin (CAP Content Section 5.A.b)
(Figure 1-2). The approximate location of the coal pile has remained consistent
throughout operation of the Plant, with minor changes to the footprint
depending on the volume of coal stored on Site. The coal pile is not lined. Surface
water runoff from the area was historically directed to the ash basin; however, a
lined coal pile retention basin system was constructed in 2018 to capture runoff.
Coal is stored in the coal pile storage area then conveyed via transfer belts to the
coal handling facilities where it is pulverized before being utilized in the boilers.
Coal ash and other CCRs are produced from the combustion of coal. The smaller
ash particles (fly ash) are carried upward in the flue gas and are captured by an
air pollution control device. 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 (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
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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).
Non -Coal -Related Operations and Waste Streams Coincident
with the Ash Basin
No non -coal related operations or environmental incidents (i.e., releases that
initiated notifications to NCDEQ) are known to have occurred at Mayo in the
vicinity of, or coincident to, the ash basin. The power plant area is separate from
the ash basin; operations and/or environmental incidents associated with that
area would have no effect on ash basin groundwater.
1.5.3 Overview of Existing Permits and Special Orders by
Consent
(CAP Content Section 1.E.0
NPDES Permit
Duke Energy is authorized to discharge wastewater from the Mayo ash basin to
Mayo Reservoir (Outfall 002) in accordance with NPDES Permit NC0038377,
which was renewed by NCDEQ on July 13, 2018. The facility has two permitted
outfalls in the current NPDES discharge permit. The sources of wastewater for
these outfalls include non -contact cooling water, ash basin discharge, sanitary
waste, cleansing and polishing water, low volume wastes, and storm water from
process areas.
The facility operates the following outfalls (except where subsequently noted,
descriptions below are excerpted directly from the NPDES permit):
• Outfall 001: Cooling Tower System. Less than once per year, the cooling towers
and circulating water system are drained by gravity and discharged directly to
Mayo Reservoir.
Outfall 002: Ash Pond Treatment System. Discharged directly to Mayo
Reservoir. The ash basin receives coal pile runoff, storm water runoff, cooling
tower blowdown, and various low -volume wastes. Internal Outfalls 008 and 009
are discharged into the ash basin.
• Internal Outfall 008: Cooling tower blowdown is discharged directly to the ash
basin. Cooling tower blowdown is indirectly discharged to Mayo Reservoir via the
ash pond treatment system (Outfall 002).
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Internal Outfall 009: Discharge from the flue gas desulfurization (FGD)
blowdown treatment system. FGD wastewater is treated via thermal evaporation.
Outfall 009 is inactive.
Outfall 002A: Upon completion of construction, discharge from the new lined
retention basin (LRB). (The recently completed LRB received the first water
from the Plant on June 7, 2019 and discharged to Outfall 002 on July 8,
2019.) The basin will receive wastes from holding cell (vacuumed sediments and
solids), coal pile runoff, storm water runoff, cooling tower blowdown, and various
low -volume wastes such as boiler blowdown, oily waste treatment,
was from the water treatment processes including reverse -osmosis
wastewater, plant area wash down water, equipment heat exchanger water,
groundwater, occasional piping leakage from limestone slurry and FGD system,
chemical metal cleaning waste, and treated domestic wastewater. Wastewater
from this outfall discharges to Mayo Reservoir via Outfall 002.
Internal Outfall 002B: Yard sump overflows (contain all wastes routed to the
new retention basin). Wastewater from this outfall discharges to Mayo Reservoir
via Outfall 002.
• Internal Outfall 011: Domestic wastewater plant. Wastewater from this outfall
discharges to Mayo Reservoir via Outfall 002.
Outfalls 004, 005, 006c, 006d, and 006e: These former storm water outfalls
primarily contain storm water and groundwater with some additional dust
suppression irrigation and cooling tower drift. These outfalls discharge to Mayo
Reservoir. (Outfalls 004 and 005 have been rerouted upstream of Outfall
002. Outfall 006c, 006d, and 006e were abandoned in early 2018 and no
longer flow to Mayo Reservoir.)
Mayo Plant is also authorized to discharge stormwater to Mayo Reservoir in
accordance with NPDES Permit NCS000580.
Special Order by Consent
A Special Order by Consent (SOC) was issued to Duke Energy on August 15,
2018 (Appendix A) to address seeps from the Mayo ash basin 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).
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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, will 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 Mayo began June 27, 2019. The
SOC requires completion of decanting by December 31, 2020.
Permitted Solid Waste Facilities
Mayo has an industrial landfill (Monofill) permitted by the NCDEQ Division of
Waste Management (DWM), Solid Waste Section (SWS) (Permit No. 7305-
INDUS) The Monofill is located west of US Highway 501 within a groundwater
drainage area separate from the ash basin. Mayo CCRs that are not beneficially
re -used are placed in the Monofill.
Air Quality/Hazardous Waste
Mayo holds a Title V air quality operating permit (#03478T47) and a hazardous
waste permit (NCD000830612) as a RCRA small quantity generator.
Erosion and Sediment Control Permits
Erosion and Sediment Control (E&SC) permits are obtained for construction -
related activities where the area of disturbance is greater than one acre. Multiple
E&SC permits have been obtained for various projects adjacent to the ash basin
including assessment field activities and wastewater redirection to initiate ash
basin closure. E&SC permits are opened and closed as related projects commence
and complete. 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 Comment
Letter from NCDEQ
(CAP Content Section 2.A)
On October 31, 2017, Duke Energy submitted a CSA Update to NCDEQ (SynTerra,
2017b). In a letter to Duke Energy dated May 7, 2018, NCDEQ stated that sufficient
information had been provided in the CSA Update to allow preparation of a CAP. The
letter also provided CSA-related comments and items required to be addressed prior to
or as part of the CAP submittal (Appendix A).
On June 7, 2018, NCDEQ Raleigh Regional Office (RRO) submitted an email with the
subject: Draft comprehensive review comments for Mayo and Roxboro. The email provided
an attachment with additional draft CSA Update comments (Appendix A).
2.2 Duke Energy's Response to NCDEQ CSA Comment Letter
(CAP Content Section 2.B and 2.B.a)
Responses to each of the NCDEQ comments within the May 7, 2018 letter and in the
June 7, 2018 email attachment are summarized in Appendix B. Additional content
related to NCDEQ's comments is either included within sections of this CAP Update or
as standalone appendices to this document, such as the groundwater modeling reports
and surface water evaluation reports. Additional content location is referenced in the
response summary, as applicable.
Activities that directly addressed NCDEQ comments concerning the Mayo CSA Update
include:
• Groundwater samples continued to be collected on a quarterly basis as part of
the Mayo Interim Monitoring Plan (IMP) after CSA Update submittal. Additional
sampling results augmented the groundwater quality database. Comprehensive
groundwater analytical data are included in Appendix C, Table 1.
• Additional soil assessment, including assessment of soil surrounding the ash
basin perimeter and downslope of the ash basin, was performed. Discussion of
soil assessment results are presented in Section 6.0.
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• Additional assessment of Crutchfield Branch surface water and sediment
downgradient of the Mayo ash basin was performed during in April/May 2018.
A report summarizing the sampling, results, evaluation, and conclusions of the
surface water evaluation was submitted to NCDEQ on March 21, 2019 and is
included in Appendix I.
• An evaluation of potential groundwater migration and associated effects on
surface water under future conditions was conducted and the results of the
evaluation are presented in Appendix I.
• Background groundwater and soil datasets and BTVs were updated to include
data through December 2018. Information about background determinations is
presented in Section 4.0. Updated soil BTVs are listed on Table 4-2, and updated
groundwater BTVs are listed on Table 4-3.
• The Mayo flow and transport model and geochemical model were updated to
incorporate additional assessment data and information. The additional data
helped refine the models. The flow and transport model report is provided as
Appendix G. The geochemical model report is provided as Appendix H.
• The Mayo CSM was updated to improve understanding of Site conditions 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)
As previously described, the Mayo ash basin is currently in compliance with 02L
groundwater quality standards; therefore, groundwater corrective action under 15A
NCAC 02L .0106 is not required at this time for the Mayo ash basin. A general Site
layout is presented as Figure 1-2 (CAP Content Section 3.A and 3.A.a).
Sources Not Connected to the Ash Basin to be Addressed in
Subsequent CSAs
(CAP Content Section 3.B)
Coal Pile Storage Area
In a letter dated April 5, 2019 (Appendix A) (CAP Content Sections 3.B), NCDEQ stated
the Mayo CAP Update may include the coal storage pile area. Based on initial
evaluation of assessment findings for the coal storage pile area, Duke Energy submitted
a request to the NCDEQ on October 11, 2019 to assess the area independent from the
ongoing evaluation and preparation of this ash basin CAP Update. The request was
approved on November 13, 2019 (Appendix A), a work plan was approved on
December 10, 2019 (Appendix A), and the coal storage pile area is being evaluated
independently of the ash basin with a due date for the CSA report to be mutually
agreed upon by NCDEQ and Duke Energy.
Gypsum Storage Pad Area
In a letter dated April 5, 2019 (Appendix A) (CAP Content Sections 3.B), NCDEQ stated
that a CSA for the Mayo gypsum storage pad, a potential secondary source not
associated with the ash basin, is due for submittal by March 31, 2020. However, the
gypsum storage pad area may be hydrologically related to the east side of the coal pile
storage area, so it may be appropriate to evaluate and report findings pertaining to both
units at the same time. Evaluation of the gypsum storage pad area is ongoing.
Low Volume Ponds
In a letter dated April 5, 2019 (Appendix A) (CAP Content Sections 3.B), NCDEQ stated
that a CSA for the low volume ponds, a potential secondary source not associated with
the ash basin, is due for submittal by March 31, 2020. On June 26, 2019 Duke Energy
requested that NCDEQ review the request for evaluation of the NPDES-permitted lined
low volume waste ponds. On July 19, 2019 NCDEQ submitted a letter to Duke Energy
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(Appendix A) determining that an evaluation of the low volume ponds is not required
at this time.
A brief description of Mayo potential additional sources, their status of inclusion or
exclusion as part of the ash basin source area, and the rationale for inclusion or
exclusion is provided in Table 3-1 (CAP Content Section 3.B).
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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 at background locations not affected by Mayo ash basin
operations. Additionally, CCR-related constituents are naturally occurring and present
in the Piedmont physiographic province of north -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 ash basin 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 constituents from a source
area. If the assessment data concentrations are less than background, it is likely
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 of concentrations
detected at the Site, or within the range for the geographic region
• Evaluation of whether there is a migration mechanism such as through 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 area
• Consideration of other constituents present at concentrations greater than
background values.
Mayo and eight other Duke Energy facilities (Allen Steam Station, Belews Creek Steam
Station, Buck Steam Station, Cape Fear Steam Electric Plant, Cliffside Steam Station,
Dan River Steam Station, Marshall Steam Station, Mayo Steam Electric Plant, and
Riverbend Steam Station) are situated in the Piedmont physiographic province of north -
central North Carolina. The nine Duke Energy facilities are located within a 220-mile
radius from Mayo. Statistically derived background values from these facilities provide
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a geographic regional background range for comparison. Generally, background values
derived from the Piedmont facilities are similar, with some exceptions.
As more background data become available, the background threshold values (BTVs)
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 Mayo ash basin. Background locations for all media
including groundwater, surface water, soil, and sediments are shown on a map
presented as Figure 4-1 (CAP Content Section 4.A). Tables referenced in this section
present background datasets for each media, statistically calculated BTVs for soil and
groundwater, and background dataset ranges for surface water and sediment.
Background soil and groundwater locations approved by NCDEQ, as well as
statistically derived BTVs, are detailed in Section 4.1 and Section 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 migration to surface water (Appendix I) and are detailed in Section 4.3
and Section 4.4.
4.1 Background Concentrations for Soil
The soil background dataset with the appropriate preliminary soil remediation goals
(PSRGs) for protection of groundwater (POG) and BTVs for constituents is provided in
Appendix C, Table 4 (CAP Content Section 4.B). The locations of the background soil
borings are shown on Figure 4-1. The background soil dataset includes samples
collected from multiple unsaturated depth intervals (Table 4-1). Samples were collected
from depth intervals greater than one foot above the seasonal high water table.
In a letter dated July 7, 2017 (Appendix A), NCDEQ approved locations and use of soil
data for determination of BTVs. Soil BTVs related to COIs at Mayo 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)
and submitted to NCDEQ in an Updated Background Threshold Values for Soil Technical
Memorandum, dated August 25, 2017 (Appendix A). NCDEQ DWR provided comments
and partial approval of BTVs in response letters dated September 1, 2017 and May 14,
2018 and final approval May 23, 2019 (Appendix A).
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Soil BTVs at Mayo were updated in 2019 in accordance with the Revised Statistical
Methods for Developing Reference Background Concentrations for Groundwater and Soil at
Coal Ash Facilities (HDR and SynTerra, 2017). 2018 BTVs and newly calculated 2019 soil
BTVs for Mayo are provided in Table 4-2 (CAP Content Section 4.B). Soil BTV ranges for
Duke Energy stations located in the North Carolina Piedmont are also included in
Table 4-2. The updated BTVs were calculated using data from background unsaturated
soil samples collected February 2015 to July 2017, but the 2019 dataset retained extreme
outlier concentrations when data validation and geochemical analysis of background
groundwater concentrations indicated that those outlying concentrations did not result
from sampling error or laboratory analytical error. The approach used to evaluate
whether extreme outlier concentrations should be retained in background soil datasets
is presented the technical memorandum prepared by Arcadis titled, "Background
Threshold Value Statistical Outlier Evaluation — Allen, Belews Creek, Cliffside, Marshall, Mayo,
and Roxboro Sites,". which was provided as an attachment to the Updated Background
Threshold Values for Constituent Concentrations in Groundwater (SynTerra, 2019d). No
additional background samples have been collected since submittal of the CSA Update
(SynTerra, 2017b); therefore, the soil background dataset is current.
4.2 Background Concentrations for Groundwater
The groundwater background dataset with the appropriate 02L/IMAC/BTVs is
provided in Appendix C, Table 1(CAP Content Section 4.C). The 2018 and 2019
groundwater BTVs for Mayo are provided in Table 4-3 (CAP Content Section 4.0 and
5.A.a.vii). Groundwater BTV ranges for Duke Energy stations located in the North
Carolina Piedmont are also included in Table 4-3. The use of updated groundwater
BTVs is currently under appeal.
The groundwater system at the ash basin is divided into the following three flow zones
to distinguish the interconnected groundwater system: the surficial flow zone
(comprised of wells completed in either saprolite or alluvial material), transition zone
flow zone, and the bedrock flow zone. Mayo flow zones and background groundwater
monitoring wells installed within each flow zone include:
• Surficial: MW-12S (saprolite)
• Transition zone: BG-02, MW-12D
• Bedrock: BG-01, MW-13BR, MW-14BR, CCR-102BR-BG
The locations of the background monitoring wells are shown on Figure 4-1. The
occurrence of saturated surficial material is limited at Mayo, resulting in only one viable
background well screened in the surficial flow zone (MW-12S). The suitability of each of
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these locations for background purposes was evaluated in a SynTerra technical
memorandum dated May 26, 2017 (Appendix A) (SynTerra, 2017a). Groundwater data
appropriate for inclusion in the statistical analysis to determine BTVs was approved by
NCDEQ in a response letter dated July 7, 2017, provided in Appendix A.
Groundwater BTVs at Mayo were initially calculated and submitted to NCDEQ in an
Updated Background Threshold Values for Groundwater Technical Memorandum (August 16,
2017). NCDEQ DWR provided comments and approval of BTVs in a response letter
dated May 14, 2018 (Appendix A) with only four exceptions (hexavalent chromium in
transition zone and bedrock; sodium in bedrock; and vanadium in bedrock). Additional
detailed information concerning the four values that were not initially approved were
addressed in a conference call with DWR, Duke Energy, and SynTerra on July 10, 2019
and summarized in a technical memorandum and associated statistical evaluation files
provided to DWR via email on July 11, 2018 (Appendix A).
Groundwater BTVs in each groundwater flow zone at Mayo were updated in 2019 with
the approved inclusion of one additional background monitoring well. CCR-102BR-BG,
a background well screened in the bedrock flow zone, is used to establish background
groundwater concentrations for the CCR compliance program. The data from this well
was deemed usable and has been included in the update of BTVs provided herein.
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) using data from background groundwater samples
collected from 2010 to December 2018. The updated background datasets for each flow
system used to statistically assess naturally occurring concentrations of inorganic
constituents in groundwater are presented in the report Updated Background Threshold
Values for Constituent Concentrations in Groundwater (SynTerra, 2019d) provided to
NCDEQ on June 13, 2019 (Appendix A). The updated background data set for each
hydrogeologic flow zone consists of an aggregate of total (non -filtered) concentration
data pooled across the background monitoring wells installed within that flow layer.
4.3 Table of Background Concentrations for Surface Water
The surface water background dataset with the appropriate 02B surface water criteria
for constituents is provided in Appendix C, Table 2 (CAP Content Section 4.D).
Comparative ranges for Mayo background surface water analytical results compared to
02B and USEPA criteria are included in Table 4-4 (CAP Content Section 4.D).
Background surface water sample locations are located upstream from, or outside of,
potential groundwater migration from the source area to surface water (Figure 4-1).
Groundwater predictive modeling shows that surface water background sample
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locations are outside of the potential for groundwater -to -surface water migration and,
therefore, will not be affected by groundwater constituents.
Background surface water sample locations include two locations from an unnamed
stream that flows into Mayo Reservoir and two locations in unnamed tributaries to
Bowe's Branch. There are no upstream (of the ash basin) locations on Crutchfield
Branch to serve as a direct background comparison. Surface water sample locations are
shown on Figure 4-1 and summarized below based on surface water body and spatial
distribution relative to the source area.
• Three locations in an unnamed stream that flows into Mayo Reservoir southeast
of the Plant and the ash basin: SW-REF1, SW-REF2, and S-6
• Two locations in unnamed streams that flow into Bowe's Branch and are located
west of US Highway 501: SW-BB1 and SW-BB2
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 collected samples. Surface water samples from
background locations have been collected in accordance with NCDEQ guidance as part
periodic sampling events. Surface water samples have been collected
contemporaneously with scheduled CAMA groundwater monitoring. Surface water
samples are collected from background locations at the same time as downstream
surface water samples are collected to provide additional comparative data.
Background sampling included the comprehensive sampling event in April/May 2018
used to assess surface water to determine if corrective action is required under
02L .0106.
4.4 Table of Background Concentrations for Sediment
The sediment background dataset is provided in Appendix C, Table 5 and sediment
background dataset ranges are included in Table 4-5 (CAP Content Section 4.E).
Background sediment sample locations are co -located with background surface water
sample locations in the unnamed streams. Background sediment sample locations are
located upstream and outside potential groundwater migration from the source area to
sediment. Groundwater predictive modeling shows that sediment background sample
locations remain outside future areas of groundwater migration from the ash basin.
Background sediment sample locations include:
• SW-REF1, SW-REF2, SW-BB1, SW-BB2, and S-6
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Background sediment data are used for general comparative purposes. The analytical
results provide a comparative range of naturally occurring constituent concentrations
present at background locations. Background data sets include one sample collected
from each location. Sediment samples were collected concurrently with a background
surface water sample.
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5.0 CONCEPTUAL SITE MODEL
(CAP Content Section 5)
The Conceptual Site Model is a descriptive and illustrative representation of
hydrogeologic conditions and constituent interactions specific to the Site. The Mayo ash
basin CSM provides a current understanding of the distribution of constituents with
regard to the Site -specific geological/hydrogeological and geochemical processes that
control the transport and potential presence of COIs in various media. The CSM also
supports evaluation of potential exposure pathways to human and ecological receptors.
The CSM presented in this section is based on the United States Environmental
Protection Agency (USEPA) document titled Environmental Cleanup Best Management
Practices: Effective Use of the Project Life Cycle Conceptual Site Model (USEPA, 2011). That
document describes six CSM stages for an environmental project life cycle. The CSM is
an iterative tool to assist in the decision process for characterization and potential
remediation during the life cycle of an environmental project as new data become
available. The six CSM stages for an environmental project life cycle are described
below:
1. Preliminary CSM Stage — Site representation based on existing data; conducted
prior to systematic planning efforts.
2. Baseline CSM Stage — Site representation used to gain stakeholder consensus or
disagreement, identifies data gaps and uncertainties; conducted as part of the
systematic planning process.
3. Characterization CSM Stage — Continual updating of the CSM as new data or
information is received during investigations; supports remedy decision making.
4. Design CSM Stage — Targeted updating of the CSM to support remedy design.
5. Remediation/Mitigation CSM Stage — Continual updating of the CSM during
remedy implementation; 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 Mayo ash basin CSM is consistent with Stage 3, Characterization CSM, as
indicated above. The findings of Stage 3, Characterization conclude that the Mayo ash
basin is in compliance with 02L and does not require corrective action. Therefore, the
CSM will not move forward in the progression of CSM development as outlined by the
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USEPA (2011). A three-dimensional depiction of the CSM under conditions prior to
decanting and basin closure is presented as Figure 5-1.
Anticipated changes to Site conditions, such as 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. Long-
term Site monitoring and periodic evaluation of Site conditions will be incorporated
into the CSM to support documentation and future Site planning needs.
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 Mayo ash basin is divided into three
hydrostratigraphic zones to distinguish the interconnected groundwater system:
the surficial, transition zone, and the bedrock. The following is a summary of the
natural hydrostratigraphic zone assessment observations in the ash basin
vicinity:
• Surficial: Surficial zone soil includes regolith (dominated by saprolite),
alluvium, and fill material. The surficial zone at Mayo is relatively thin
and mostly unsaturated based on field observations during drilling and
well installation activities. The regolith, in -place soil that develops by
weathering, consists primarily of sandy loam, with layers of loamy sand,
loam, and clay. Mineralogical analyses indicate that clay minerals
comprise the bulk portion of Site soils, followed by quartz, feldspars, and
amphiboles in order of decreasing abundance. The regolith is dominated
by saprolite, the in -situ weathering product of parent rock; therefore, for
the purpose of this evaluation discussion of the surficial zone will refer to
saprolite and alluvium materials.
Saprolite
Topographic highs tend to exhibit thinner soil-saprolite zones, while
topographic lows typically contain thicker soil-saprolite zones.Saprolite
thickness at the Site ranged from not present to more than 50 feet at
upgradient well pair MW-12; that thickness of saprolite. However, based
on field observations during drilling activities the 50 feet of saprolite at
MW-12 is an exception beneath the Site. Saprolite beneath the power
plant area of the Site and the northern, eastern, and western parts of the
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Site is almost entirely unsaturated. Saturated saprolite is encountered
more frequently in the southern portion of the Site.
Alluvium
Alluvium found along Crutchfield Branch was about 7 feet thick and
directly overlies saprolite. Alluvium and saprolite are referred to herein as
one single unit due to the limited extent of alluvium, the general lack of
saturated saprolite, and the interaction of groundwater with surface
water.
Surficial flow zone wells are typically labeled with an "S" designation.
Transition zone: The transition zone consists of a relatively transmissive
zone of partially weathered bedrock encountered below the saprolite flow
zone. Observations of core recovered from this zone included rock
fragments, unconsolidated material, and highly oxidized bedrock
material. Both saturated and unsaturated conditions occur in the
transition zone at Mayo. Transition zone flow zone wells are typically
labeled with a "D" designation.
• Bedrock zone: Bedrock is defined as sound rock, based on sample
recovery and/or drilling resistance, and limited weathering. The main rock
types in the immediate vicinity of the ash basin are granitic gneiss
interbedded with tonalite, phyllite, and mica schist. The principal
minerals are plagioclase, quartz, biotite, and muscovite (Appendix F,
Attachment D). Groundwater movement in the bedrock flow zone occurs
in secondary porosity represented by fractures in the bedrock. Bedrock
fractures encountered tend to be small and sparse with low bulk hydraulic
conductivity. The majority of water -producing fracture zones were
encountered within the top 40 feet of competent rock. Mayo bedrock
fracture orientation and flow profile characterization data sets supports
field observations of small fracture apertures that are mildly productive
(Appendix F). Based on the predominant orientations of lineaments and
bedrock fractures, horizontal groundwater flow within the bedrock is
expected to occur preferentially toward the general north-northeast
direction, and parallel to the hydraulic gradient. (Appendix F). Bedrock
flow zone wells are typically labeled with a "BR," "BRL," or "BRM"
designation.
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5.1.2 Site Hydrogeologic Setting
(CAP Content Section S.A.a)
The groundwater system in the natural materials (surficial/transition
zone/bedrock) is consistent with the regolith-fractured rock system and is
characterized as an unconfined, interconnected groundwater system indicative of
the Piedmont physiographic province.
A conceptual model of groundwater
flow in the Piedmont, which applies to
the Mayo Site, was developed by
LeGrand (1988), (1989) and Daniel and
Dahlen (2002) (Figure 5-2). The model
assumes a regolith and bedrock drainage
basin with a perennial stream. The
model describes conditions before ash
basin construction, but 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 a perennial stream. Flow
in the regolith follows porous media
behavior, while flow in bedrock occurs
FIGURE 5-2
LEGRAND SLOPE
AQUIFER SYSTEM
in fractures. Rarely does groundwater move beneath a perennial stream to
another more distant stream or across drainage divides (LeGrand, 1989).
Topographic drainage divides represent natural groundwater divides within the
slope -aquifer system. The areas between the topographic divides are flow
compartments that are open-ended down slope. Compartmented groundwater
flow, applicable to the 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 Content Section 5.A.a.i)
Hydraulic divides south, east, and west of the ash basin provide natural
hydraulic control of ash basin constituent migration within the stream
valley system, with the predominant direction of groundwater flow to the
north. A groundwater divide is located west of the ash basin represented by
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a topographical ridge approximated by US Highway 501. Groundwater
divides are represented by topographical ridges south and east of the ash
basin that run through the power plant and along the northern portion of
the railroad. Groundwater on the ash basin side of each ridge flows toward
the ash basin while groundwater on the opposite side of the ridge flows
away from the ash basin.
The ash basin was constructed within a former perennial stream valley. The
ash basin's physical setting is a horizontal flow -through water system with
groundwater migration into the upgradient end, flowing north through the
middle regions, and migrating downward near the dam (Figure 5-3). Near
the dam, vertical hydraulic gradients, imposed by hydraulic pressure of
basin water, promote downward vertical gradients in the groundwater
system. Beyond the dam, groundwater flows upward toward Crutchfield
Branch. Generally, the physical setting of the ash basin within a former
perennial stream valley limits the horizontal and vertical migration of
constituents to areas near and directly downgradient of the basin's dam.
The primary flow path of the groundwater remains in the basin's former
stream valley system. Therefore, areas upgradient and side -gradient of the
ash basin have groundwater divides that limit groundwater flow in those
directions.
A localized effect to the overall flow -through water system occurs just south
of the ponded water in the ash basin. A review of historical aerial
photographs shows that in early 2006 sluiced ash in the built-up ash "delta"
area (formed from historic sluicing) had begun to be mechanically moved
and stacked in the area of exposed ash south of the present-day ponded
water (Google Earth Pro, 2018a). The ash was "stacked" for dewatering
prior to transport for beneficial reuse. This area is commonly referred to as
the ash "stack out" or "harvest" area (Figure 1-2). This activity continued
until about 2009 and the area has since remained topographically higher
than the surrounding ash basin. Historical photographs also indicate that
the last time the entire ash basin was covered with water was prior to mid-
2006 (Google Earth Pro, 2018b). Around that time, sluicing was moved to
infill the western portion of the basin. After mid-2006, the southern portion
of the basin fills in and is emergent. Beginning in mid-2008, sluicing was
being redirected to the northern portion of the basin (Google Earth Pro,
2018c) (Google Earth Pro, 2018d).
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The effect of this ash "stack out" out area is a localized one that results in a
slight variation of the horizontal flow -through concept. In this area of the
ash basin, within the ash basin boundary, there is slight downward vertical
migration of groundwater within the overall ash basin flow -through
system. Downward flow is limited to the saprolite and transition zone as
bedrock upward vertical gradients prevent downward flow below the
transition zone.
FIGURE 5-3
GENERAL PROFILE OF ASH BASIN PRE -DECANTING FLOW
CONDITIONS IN THE PIEDMONT
RUNOFF
GROUNDWATER FLOW
ENTERING BASIN
(FORMER STREAM CHANNEL)
`1•
Note:
Drawing is not to scale
PRECIPITATION
EARTHEN
ASH DAM
FLOW
HEAD
CHANGE
.. .. CI nuI ICCCDn F_C ¢CNC�nII
Water -level maps for each groundwater flow zone were constructed from
groundwater measurements collected in April 2019 (Figure 5-4a through
Figure 5-4c). April 2019 water level measurements and elevations are
presented in Table 5-1. General groundwater flow directions can be
inferred from the water -level contours. Groundwater flow directions
developed from water -level elevations measured in the surficial, transition
zone, and bedrock wells indicate groundwater flow from the ash basin is
generally to the north toward the Crutchfield Branch stream valley. This
flow direction is also approximately parallel with the predominant bedrock
fracture strike and mapped lineaments (Appendix F).
The following are conclusions pertaining to groundwater flow beneath the
Site in the vicinity of the ash basin:
• 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.
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• Downward vertical gradients generally occur just upstream of the
ash basin dam with limited exceptions in the area of exposed ash
(ash "stack out" area) in the southern portion of the ash basin.
• Upward vertical gradients occur beyond or downstream of the dam,
which is the main groundwater discharge zone.
Using empirical Site water elevation data, groundwater flow and transport
modeling simulations support groundwater flow is away from water
supply wells and there are no exposure pathways between the groundwater
flow -through the ash basin and the pumping wells used for water supply in
the vicinity of the Mayo ash basin. Domestic and public water supply wells
are upgradient or outside of the groundwater flow system containing the
ash basin. Domestic and public water supply wells are not affected by
constituents released from the ash basin or by the different closure options
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 using horizontal hydraulic
gradients determined from pre -decanting water level measurements
collected in April 2019 (Table 5-1). Hydraulic conductivity (K) and effective
porosity (ne) values were taken from the updated flow and transport model
(Appendix G). Calibrated hydraulic conductivity and porosity values for
each flow zone were used to align velocity calculations with model
predictions.
The flow and transport model provided subdivided hydraulic conductivity
zones and a calibrated hydraulic conductivity (K) for each zone and model
flow layer. Simulated hydraulic conductivity values were 1.0 foot per day
(ft/day) for the surficial zone, 1.0 ft/day for the transition zone, and ranged
from 0.03 to 0.005 ft/day for the bedrock zone. Hydraulic conductivity
values used in calculating seepage velocity were selected based on the
area's location within or proximity to subdivided hydraulic conductivity
zones. The flow and transport model uses an estimated effective porosity
(ne) of 20 percent for the surficial zone and the transition zone and 5 percent
for the bedrock zone (Appendix G).
The horizontal groundwater seepage flow velocity (vs) can be estimated
using a modified form of the Darcy Equation:
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K dh
vs ne (dl)
Using the April 2019 groundwater elevation data, the calculated horizontal
groundwater flow velocity in the vicinity of the ash basin was:
• 0.08 ft/day (approximately 30 ft/yr) in the surficial zone
• 0.03 ft/day (approximately 10 ft/yr) in the transition zone
• 0.02 ft/day (approximately 9 ft/yr) in the bedrock zone
Groundwater modeling predicts groundwater elevation changes associated
with closure activities will change flow velocities and result in a more
pronounced stream valley system within the ash basin footprint. The pre -
decanting conditions map was created from comprehensive Site data
incorporated into the calibrated flow and transport model. The closure
condition maps were created using predicted flow fields for the closure -by -
excavation and closure -in -place scenarios in the transition zone. Saturated
conditions in the surficial flow zone are limited across the ash basin area
and flow in bedrock is limited to small, sparse fractures. Additionally, the
transition zone has the highest geometric mean hydraulic conductivity;
therefore, the transition zone was selected for the velocity vector maps to
represent the primary groundwater zone.
• Velocity vector map for groundwater in the transition zone under
pre -decanting conditions - Figure 5-5a
• Velocity vector map for groundwater in the transition zone under
closure -by -excavation - Figure 5-5b
• Velocity vector map for groundwater in the transition zone under
closure -in -place scenario - Figure 5-5c
The velocity vector maps illustrate potential future changes in groundwater
flow compared to pre -decanting groundwater flow throughout the ash
basin area of the Site. Key conclusions from the predictive model simulation
of pre -decanting and post -closure groundwater flow conditions include:
• Hydraulic heads within the ash basin decline after decanting which
causes hydraulic gradients to increase within the ash basin footprint,
slightly decrease near the dam, but remain relatively unchanged
downgradient of the ash basin dam under both closure scenarios.
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• Small streams are predicted to return to the former perennial stream
channels within the ash basin footprint under both closure scenarios.
• Groundwater flow patterns outside of the basin remain similar to
pre -decanting conditions for both closure scenarios.
• North of the ash basin, velocity vectors under pre -decanting
conditions indicated groundwater velocity is greatest (0.5 to 1.0
ft/day) beneath and immediately downstream of the eastern side of
the ash basin dam and flows predominantly north (Figure 5-5a).
• Under future conditions, the velocity vector directions within the ash
basin footprint turn toward the former Crutchfield Branch stream
channel; limited change from pre -decanting Site conditions is
observed north of the ash basin (Figure 5-5b and Figure 5-5c). The
velocity vectors illustrating the natural flow system of the historical
stream valley increase from pre -decanting velocities to 0.2 -1.0
ft/day under the closure -by -excavation scenario (Figure 5-5b) and to
0.01- 0.2 ft/day under the closure -in -place scenario (Figure 5-5c).
5.1.2.3 Hydraulic Gradients
(CAP Content Section 5.A.a.i)
Within the ash basin waste boundary, hydraulic gradients are primarily
neutral (flat or nearly flat) across large areas beneath the ash basin due to
the influence of ash basin ponded water. On the downgradient side of the
ash basin dam, the approximate average horizontal hydraulic gradients
(measured in feet/foot) in each flow zone were: 0.01 ft/ft (surficial), 0.01 ft/ft
(transition zone), and 0.07 ft/ft (bedrock) based on hydraulic gradient
calculations using April 2019 (pre -decanting) groundwater elevation data.
Calculated horizontal gradients are consistent with gradients calculated
from previous monitoring events, including data presented in the 2018
CAMA Annual Interim Monitoring Report (SynTerra, 2019c). The calculated
horizontal hydraulic gradients generally align with groundwater flow
velocity magnitudes presented on velocity vector maps presented in
Section 5.1.2.
Vertical hydraulic gradients were calculated in clustered wells from the
water level data and the midpoint elevations of the well screens. Within the
ash basin, small vertical gradients between ash pore water and underlying
material ranged from -0.001 ft/ft (ABMW-03/-03S) to 0.03 ft/ft (ABMW-04X/-
04D). Positive vertical gradients indicate downward flow and negative
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vertical gradients indicate upward flow. In the southernmost portion of the
basin, where the ash "stack out" area is located, there is a downward
vertical gradient between ash pore water and the transition zone (0.03 ft/ft
at ABMW-04X/-04D). This is a localized effect in this area, and downward
migration is limited to the transition zone by the slightly upward vertical
gradient from the bedrock to transition zone (-0.004 ft/ft at ABMW-
04D/ABMW-04BR). On the immediate upstream side of the ash basin dam,
a downward vertical gradient is indicated in the surficial, transition zone,
and bedrock flow zones based on the groundwater flow and transport
modeling results (Appendix G).
Downstream of the dam, groundwater flows upward toward the
Crutchfield Branch discharge zone, limiting downward migration of
constituents to the area in close proximity to the dam. Below the ash basin
dam, a strong upward gradient was observed between the upper flow
zones (surficial/transition zone) and bedrock exhibited by well pair CCR-
1045/BR (-0.11 ft/ft). Bedrock wells MW-104BRM and MW-104BRL were
artesian under pre -decanting conditions. Artesian conditions are
attributable to the wells' location near the ash basin dam and an unnamed
tributary to Crutchfield Branch, a groundwater discharge zone.
5.1.2.4 Particle Tracking Results
(CAP Content Section 5.A.a.ii)
As discussed in the CSA Update (SynTerra, 2017b), a numerical capture
zone analysis was conducted for Mayo to evaluate potential effects of
upgradient water supply pumping wells. The analysis was done using
MODPATH, a "particle tracking" model that interfaces with the
MODFLOW flow model. MODPATH was used to trace groundwater flow
lines around pumping wells to indicate where the water being pumped
from the well originates. The analysis for Mayo indicates that well capture
zones from wells located to the northwest and southeast of the Mayo Plant
are limited to the immediate vicinity of the well head and do not extend
toward the ash basin. Results indicate that none of the particle tracks
originating in the ash basin moved into the well capture zones.
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5.1.2.5 Subsurface Heterogeneities
(CAP Content Section S.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
man-made 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 the three
flow zones that distinguish the interconnected groundwater system: the
surficial, the transition zone, and the bedrock. The occurrence of saturated
surficial zone is limited in the vicinity of the ash basin at Mayo. Where
saturated conditions occur, the surficial zone materials are 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 in the vicinity of the ash basin exists under
unconfined, or water table, conditions within the surficial and transition
zones and in fractures of the underlying bedrock. The surficial water table
and shallow bedrock water -bearing zones are interconnected. The surficial
zone, where saturated thickness is sufficient, acts as a reservoir for
supplying groundwater to the fractures in the bedrock in upland areas of
the watershed. Based on the orientations of lineaments and open bedrock
fractures near the ash basin at Mayo, horizontal groundwater flow within
the bedrock should occur approximately parallel to the hydraulic gradient
with preferential flow towards the north and the Crutchfield Branch stream
valley (Appendix F).
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 basin was conducted by
Stantec in 2014 and 2015 and utilities around the Site were also included on
a 2015 topographic map by WSP USA, Inc. (SynTerra, 2017b). Due to the
isolation of the ash basin from the Plant area, subsurface utilities in the
Plant area are not expected to be major flow pathways. The depth to
groundwater below the majority of the ash basin is much greater than
would be anticipated for installation of subsurface utilities; therefore, the
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likelihood of underground utilities being preferential pathways, other than
at the dam seepage structures, is not anticipated.
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
unaffected 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
& 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 boron. The magnitude of plume
retardation increases with increasing plume length, because longer plumes
have more contact for diffusion to transfer solute mass from the fracture to
the matrix (Lipson, Kueper, & Gefell, 2005). The magnitude of plume
retardation also increases with increasing matrix porosity.
If the solute sorbs to solids, the retarding effect increases. Sorption of solutes
that have diffused into the matrix 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.
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Site -Specific Data Pertaining to Matrix Diffusion
Overall, the hydraulic conductivity values and calculated fracture hydraulic
apertures in the bedrock downgradient of the ash basin dam are relatively
low (Appendix F). These data suggest that the bedrock is not likely to serve
as a significant preferential flow zone for groundwater downgradient of the
ash basin. This interpretation is supported by the limited presence and
vertical extent of boron at concentrations greater than the 02L standards in
bedrock groundwater, which is generally limited to approximately the top
40 feet of bedrock (Appendix F).
Based on predominant orientations of lineaments identified from aerial
photographs and topographic maps, and bedrock fractures measured in -
situ using borehole televiewer, horizontal groundwater flow within the
bedrock would be expected to occur preferentially toward the general
north-northeast direction (the predominant strike direction of bedrock
fractures) (Appendix F). The current groundwater flow model for the ash
basin area does not include plan -view anisotropy, but the simulated flow
directions in the bedrock are generally aligned with the predominant flow
direction interpreted based on measured fracture orientations.
Rock core samples were selected from two locations which represent the
hydrogeologic conditions downgradient of the ash basin dam were
analyzed for porosity, bulk density, and thin section petrography. The
reported matrix porosity values ranged from 0.46 percent to 4.97 percent
with an average of 2.11 percent. Bulk density ranged from 2.59 grams per
cubic centimeter (g/cm3) to 2.71 g/cm3 with an average of 2.67 g/cm3
(Appendix F). Petrographic evaluation classified all samples as tonalite
(igneous rocks) based on relative abundance of minerals (i.e., quartz, alkali
feldspar, and plagioclase). The principal minerals are plagioclase, quartz,
biotite, and muscovite (Appendix F). The plagioclase crystals present in the
samples were extensively altered into sericite (a mixture of muscovite, illite,
or paragonite produced by hydrothermal alteration of feldspars).
The reported matrix porosity values are within the range of those reported
for crystalline rocks in the literature (Freeze & Cherry, 1979). The presence
of measurable matrix porosity suggests that matrix diffusion contributes to
plume retardation at the Site (Lipson, Kueper, & Gefell, 2005). Additionally,
the presence of sericite indicates the bedrock has some capacity to sorb
boron and other elements associated with coal ash. The influences of matrix
diffusion and sorption are implicitly included in the groundwater flow and
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transport model as a component of the constituent partition coefficient (Ka)
term used for the bedrock layers model.
5.1.2.7 Onsite and Offsite Pumping Influences
(CAP Content Section S.A.a.v)
There is no onsite or offsite pumping in the vicinity of the ash basin;
therefore, there are no influences to groundwater flow and direction related
to pumping activities.
5.1.2.8 Ash Basin Groundwater Balance
(CAP Content Section5.A.a.vi)
The ash basin is located within a single watershed and groundwater flow
system. 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). Each scenario water
balance was developed using the results from flow and transport model
pre -decanting and predicated future groundwater simulations. The
approximate groundwater flow budget in the ash basin watershed under
each simulated scenario is summarized in Table 5-2.
Pre -Decanting Conditions Groundwater Balance
Under pre -decanting conditions, the watershed area contributing flow
toward the basin is estimated at approximately 516 acres.
• Groundwater recharge from the watershed recharge area of 516 acres
is estimated to be 142 gpm. This includes 129 gpm from the 367 acres
outside of the ash basin and 13 gpm from the 147 acres of the ash
basin.
• Ponded water upstream of the ash basin dam discharges
approximately 62 gpm to the ash basin flow -through system.
• Water discharge from the groundwater system by streams outside the
ash basin is approximately 63 gpm.
• Groundwater that flows through and under the dam is estimated to be
18 gpm.
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Post -Decanting Groundwater Balance
The flow and transport model (Appendix G) was used to evaluate the ash
basin hydraulic conditions that would occur after decanting of the ash
basin. A water balance was developed for the simulated groundwater
system under post -decanting conditions.
The extent of the ash basin watershed during decanting is expected to
remain the same as under pre -decanting conditions. Under simulated post -
decanting conditions, the watershed area contributing flow toward the
basin is estimated at approximately 516 acres.
• Groundwater recharge from the watershed recharge area of 516 acres is
estimated to be 163 gpm. This includes 134 gpm from the 369 acres
outside of the ash basin and 29 gpm from the 147 acres of the ash basin.
• The decanting drains inside the ash basin represent the decanting
system to remove ponded water in the ash basin. Water discharge by
decanting drain is approximately 106 gpm.
• Water discharge from the groundwater system by streams outside the
ash basin is approximately 50 gpm.
• Groundwater that flows through and under the dam is estimated to be
7 gpm.
Decanting the ash basin has a moderate effect on flow through and under
the dam to the north. The estimated flows are reduced from 18 gpm prior to
decanting to 7 gpm after decanting of the ponded water in the ash basin.
Post -Closure Groundwater Balances
The flow and transport model was used to evaluate the ash basin hydraulic
conditions that would occur after two ash basin closure options: closure -in -
place and closure -by -excavation. A water balance was developed for the
simulated groundwater system under post -closure conditions. The extent of
the ash basin portion of the watershed under post closure conditions is
expected to be larger than the post -decanting conditions. The approximate
watershed area is 384 acres under closure -in -place conditions, and 394 acres
under closure -by -excavation conditions.
• Groundwater recharge from areas outside of the ash basin footprint is
estimated to be 137 gpm for closure -in -place and 126 gpm for closure -
by -excavation.
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• Groundwater recharge from the area within the ash basin footprint is
affected by the closure option. Closure -in -place reduces groundwater
recharge from within the ash basin footprint from 29 gpm post -
decanting to 0 gpm post -closure because of the impermeable final
cover system. Closure -by -excavation increases groundwater recharge
within the ash basin footprint from 29 gpm post -decanting to 53 gpm
post -closure.
• Under closure -in -place conditions, drains inside the ash basin
represent the drain system under the final cover system to control the
groundwater elevation. Estimated groundwater discharge to the drain
system is approximately 81 gpm.
• Under closure -by -excavation conditions, drains inside the ash basin
represent the streams that potentially re-form within the excavated ash
basin footprint after closure. Estimated groundwater discharge to the
streams is approximately 131 gpm.
Water discharge from the groundwater system by streams outside the
ash basin is approximately 46 to 48 gpm, depending on the selected
closure option.
5.1.2.9 Effects of Naturally Occurring Constituents
(CAP Content Section 5.A.a.vii)
Metals and inorganic constituents, typically associated with CCR material,
are naturally occurring and present in the Piedmont physiographic
province of north -central North Carolina. The metals and inorganic
constituents occur in soil, bedrock, groundwater, surface water, and
sediment. During the Mayo 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 Mayo contain naturally
occurring constituents that are also typically related to CCR material and
likely affect the chemistry of groundwater at the Site. Chromium, cobalt,
iron, manganese, and vanadium were present in background soil and rock
samples at concentrations greater than the PSRG POG values (Table 4-2).
Beryllium, cadmium, chromium, and nickel were present in the transition
zone (partially weathered rock) at background location MW-12D at
concentrations greater than the Site soil BTVs (SynTerra, 2017b).
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These results suggest that beryllium, cadmium, chromium, cobalt, iron,
manganese, nickel, and vanadium may occur naturally in groundwater at
the Site. Analytical results for groundwater at background locations
indicate that iron, manganese, and vanadium are present at concentrations
greater than 02L/IMAC standards (Table 4-3). Therefore, the downgradient
concentrations of these constituents are compared to background values.
Downgradient iron, manganese, and vanadium concentrations are within
background concentration ranges.
5.2 Source Area Location
(CAP Content Section 5.A.b)
The ash basin is generally bounded by an earth dam to the north, US Highway 501 to
the west, the plant entrance road to the south, and the railroad to the east (Figure 1-2).
US Highway 501 and the railroad, generally located along topographic ridges, represent
hydrogeologic divides that affect groundwater flow within an area approximately 0.5
miles northwest, west, and south of the ash basin. Topography of the ash basin area
generally slopes downward toward the Crutchfield Branch stream valley system.
5.3 Summary of Potential Receptors
(CAP Content Section 5.A.c)
G.S. Section 130A-309.201(13), amended by CAMA, defines receptor as "any human,
plant, animal, or structure which is, or has the potential to be, affected by the release or
migration of contaminants. Any well constructed for the purpose of monitoring groundwater
and contaminant concentrations shall not be considered a receptor." In accordance with the
NORR CSA guidance, receptors cited in this section refer to public and private water
supply wells and surface water features.
5.3.1 Public and Private Water Supply Wells
Groundwater from beneath the ash basin has not and will not flow towards any
water supply wells based on the CSM, groundwater flow patterns prior to and
post ash basin closure (either scenario), the location of water supply wells in the
area, and evaluation of groundwater analytical data. Assessment activities
including groundwater data from water supply wells and on -Site monitoring
wells, groundwater elevation measurements, and groundwater flow and
transport modeling results all indicate that ash basin related constituents are not
affecting, and will not affect, water supply wells.
No public or private drinking water wells or wellhead protection areas were
found to be located downgradient of the ash basin. A total of 22 private water
supply wells were identified within the 0.5-mile radius of the ash basin
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compliance boundary (Figure 5-6). Most of these water supply wells are located
to the south and upgradient of the Site, centered near Mullins Lane; northwest
and upgradient of the Site, on the North Carolina side (south) of the North
Carolina/Virginia state line; and northwest and upgradient of the Site, on the
Virginia side (north) of the state line. Discussion with supporting material and
data, of alternative water supply provisions (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.
5.3.2 Availability of Public Water Supply
A municipal water line (City of Roxboro) is present along US Highway 501
(Boston Road) toward the south of the Plant. The water line does not extend
north of the intersection of Boston Road and the Plant entrance road. The line
supplies potable water to the Mayo Plant. No municipal water lines serve the
area north of the Site (along Mayo Lake Road).
5.3.3 Surface Water
Mayo is located within the Roanoke River Basin. Surface water bodies within 0.5
mile of the ash basin, and the associated North Carolina surface water
classifications, are indicated on Figure 5-7 and summarized in Table 5-3.
The only surface water intake located in the vicinity of Mayo is the Duke Energy
intake used to supply water from the Mayo Reservoir for Mayo Plant operations.
The intake location is shown on Figure 5-7.
A depiction of surface water features including wetlands, ponds, unnamed
tributaries, seeps, streams, lakes, and rivers within a 0.5-mile radius of the
compliance boundary of the ash basin is provided in Figure 5-7. Surface water
information is provided from the Natural Resources Technical Report (NRTR)
prepared by Amec Foster Wheeler (Amec Foster Wheeler, 2014). In addition,
NPDES-permitted outfalls and locations covered by the SOC are shown on
Figure 5-7. Non -constructed and dispositioned seep sample locations located
north of the ash basin are managed by the SOC and are subject to monitoring and
evaluation requirements contained in the SOC.
Downgradient streams (Crutchfield Branch and its tributaries) are groundwater
discharge zones. The Crutchfield Branch stream system is the only surface water
feature downgradient and within 0.5-mile of the Mayo ash basin. Surface water
samples were collected from locations within Crutchfield Branch where
groundwater flowing from the ash basin might cause constituent concentrations
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greater than 02B water quality standards. 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. No constituent concentrations greater than 02B surface water standards
were identified in Crutchfield Branch. Sample locations for the surface water
evaluation are included on Figure 5-7. The full report for evaluation of Mayo
groundwater discharge to surface water and the evaluation of surface waters to
evaluate compliance with 15A NCAC 2B .0200 was submitted to NCDEQ on
March 23, 2019. A copy of the report is provided in Appendix I.
5.3.4 Future Groundwater Use Area
Duke Energy owns the property downgradient from the Mayo ash basin dam to
the North CarolinaNirginia state line as shown on Figure 1-2. Ownership of the
property allows Duke Energy to control activities; thereby, managing risks for
future property use. No future groundwater use areas are anticipated
downgradient of the basin.
5.4 Human Health and Ecological Risk Assessment Results
(CAP Content Section 5.A.d)
A human health and ecological risk assessment pertaining to Mayo was prepared and is
included in Appendix E. The risk assessment focuses on the potential effects of CCR
constituents from the Mayo ash basin on groundwater, surface water, and sediment.
Groundwater flow information was used to focus the risk assessment on areas where
exposure of humans and wildlife to CCR constituents could occur. Primary conclusions
of the risk assessment include: 1) there is no evidence of risks to on -Site or off -Site
human receptors potentially exposed to CCR constituents that may have migrated from
the ash basin; and 2) there is no evidence of risks to ecological receptors potentially
exposed to CCR constituents that may have migrated from the ash basin. This risk
assessment uses analytical results from groundwater, surface water, and sediment
samples collected March 2015 through June 2019.
Mayo Reservoir is not affected by groundwater flow from the ash basin; therefore, there
is no exposure of CCR constituents to humans and wildlife using Mayo Reservoir.
Evaluation of risks associated with areas of wetness (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 (SynTerra,
2016a) in order to incorporate new Site data and refine conceptual site models. The
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original risk assessment was prepared in accordance with a work plan for risk
assessment of CCR-affected media at Duke Energy sites (Haley and 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 (SynTerra, 2016a)
2. Comprehensive Site Assessment (CSA) Update (SynTerra, 2017b)
3. Human Health and Ecological Risk Assessment Summary Update for Mayo Steam
Electric Plant, Appendix B of Community Impact Analysis of Ash Basin Closure
Options at the Mayo Steam Electric Plant (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 (SynTerra, 2016a)
and is based on (NCDENR DWM, 2003), (NCDEQ, 2017), and USEPA risk assessment
guidance (USEPA, (1989); (1991); (1998)).
Human health and ecological conceptual site models were developed and further
refined to guide identification of exposure pathways, exposure routes, and potential
receptors for evaluation. Additional information regarding groundwater flow and the
treatment of source areas other than the ash basin was incorporated into the refinement
of CSMs presented in Appendix E.
Environmental data evaluated in the risk assessment were compared to human health
and ecological screening values. Risk assessment constituents of potential concern
(COPCs) are different than COIs in that COPCs 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. Appendix E contains the results of the screening assessment.
No unacceptable risks from exposure to environmental media were identified. Results
of the human health risk assessment indicate the following:
• On -site groundwater poses no unacceptable risk for the construction worker
under these exposure scenarios.
• On -site surface water and sediment pose no unacceptable risk for the trespasser
under these exposure scenarios.
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• Exposure to CCR constituents by current and future commercial/industrial
worker, residences and recreational receptors is incomplete.
Findings of the baseline ecological risk assessment include the following:
• No hazard quotients (HQ) based on no observed adverse effects levels
(NOAELs) or lowest observed adverse effects levels (LOAELs) were greater than
unity for the mallard duck, great blue heron, river otter, bald eagle, American
robin, red-tailed hawk, and red fox exposed to surface water and sediments in
the Crutchfield Branch exposure area.
• Three endpoints, muskrat, meadow vole, and killdeer, had limited modeled risk
results greater than unity for aluminum and total chromium.
• The modeled risks are considered negligible based on natural and background
conditions. The exposure models likely overstate risks to aluminum and total
chromium.
In summary, there is no evidence of unacceptable risks to human and ecological
receptors exposed to environmental media potentially affected by CCR constituents at
Mayo.
5.5 CSM Summary
The Mayo CSM presented herein describes and illustrates geologic and hydrogeologic
conditions and constituent interactions specific to the Mayo ash basin. 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 effects of constituents in various media and potential exposure
pathways to human and ecological receptors.
In summary, the ash basin was constructed within a former perennial stream valley in
the Piedmont of North Carolina, and exhibits 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. A localized area of downward vertical migration
occurs in the south-central portion of the ash basin under the "stack out" area. Due to
the prevailing horizontal flow within the ash basin, there is limited vertical flow of ash
basin pore water into the underlying groundwater, except under the ash "stack out"
area. Downward vertical migration in the "stack out" area is limited to the surficial and
transition zone. The elevated constituent concentrations found in groundwater near the
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ash basin 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.
Empirical Site data from over 41 monitoring events over multiple seasonal variations
and groundwater flow and transport modeling simulations support groundwater flow
is away from water supply wells. Additionally, there are no exposure pathways
between the ash basin and the pumping wells used for water supply in the vicinity of
the Mayo ash basin.
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 option considered by Duke Energy will
significantly reduce infiltration to the remaining ash, reducing the rate of constituent
migration. Based on future predicted groundwater flow patterns, under post ash basin
closure conditions, and the location of water supply wells in the area, groundwater flow
direction from the ash basin is expected to be further contained within the stream valley
and continue flowing north of the ash basin footprint and not towards any water
supply wells.
Multiple lines of evidence have been used to develop the CSM based on the large data
set generated for Mayo. The CSM provides the basis for this CAP Update developed for
the Mayo ash basin to comply with G.S. Section 130A-309.211, amended by CAMA.
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6.0 CORRECTIVE ACTION APROACH FOR MAYO ASH BASIN
(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 unit. Constituents with concentrations above corresponding
standards were evaluated to determine if the level of concentration is present due to the
source unit. Constituents of interest (COI) are those constituents identified from the
"constituent management process' described below and are specific to individual
source unit(s), not the Site. This evaluation assisted in identifying if a unit is subject to
corrective action under G.S. 130A-309.211 and 15A NCAC 02L .0106.
A constituent management process was developed by Duke Energy at the request and
conditional acceptance of NCDEQ (NCDEQ letter dated October 24, 2019, Appendix A),
to gain a thorough understanding of constituent behavior and distribution in Site
groundwater and to aid in identifying unit -specific COIs. The constituent management
process consists of three steps:
1. Perform a detailed review of the applicable regulatory requirements under
NCAC, Title 15A, Subchapter 02L
2. Understand the potential mobility of unit -related constituents in groundwater
based on Site hydrogeology and geochemical conditions
3. Determine the constituent distribution at the unit under pre -decanting and
predicted future conditions.
Multiple lines of evidence including empirical data collected at the Site, geochemical
modeling, and groundwater flow and transport modeling support this constituent
management process. The management process uses a matrix evaluation to identify
those constituents that have migrated downgradient of the source unit, in the direction
of groundwater flow at concentrations greater than 02L/IMAC/background value with a
discernable plume. The matrix evaluation considers the following per constituent:
• Regulatory criteria
• Site and Piedmont background values
• Maximum mean constituent concentrations
• Exceedance ratios
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• Number and distribution of wells at or beyond the compliance boundary with
constituent concentrations greater than criterion
• Constituent presence in ash pore water at concentrations greater than criterion
• Constituent geochemical mobility
This approach has been used to identify unit -specific COIs that have migrated from the
Mayo ash basin and may require corrective action. The results of the constituent
management process (described in detail in Section 6.1.3) identify one unit -specific
groundwater COI for the Mayo ash basin: boron. No constituents in unsaturated soil
were present in concentrations greater than the corresponding standard (PSRG POG or
background value); therefore, no soil COIs were identified for the Mayo ash basin.
COIs identified for the ash basin, that have migrated beyond the compliance boundary
at concentrations greater than 02LAMAC/background value are subject to corrective
action. Analytical data obtained over one year of quarterly monitoring indicate
concentrations of boron, the Mayo ash basin COI, have been less than applicable 02L
standards in groundwater samples collected from monitoring wells at or beyond the
compliance boundary of the Mayo ash basin. Therefore, the ash basin is in compliance
with 02L requirements and corrective action under 02L is not required.
6.1 Extent of Constituent Distribution
This section provides an in-depth review of constituent characteristics associated with
the Mayo ash basin and the mobility, distribution, and extent of constituent migration
within, at, and beyond the point of compliance.
6.1.1 Source Material Within the Waste Boundary
(CAP Content Section 6.A.a)
An overview of the material within the ash basin is presented in the following
subsections.
6.1.1.1 Description of Waste Material and History of
Placement
(CAP Content Section 6.A.a.i)
Solids deposited in the Mayo ash basin are mostly CCR materials,
composed primarily of fly ash and bottom ash. CCR was primarily
conveyed to the ash basin by combining with water to produce a slurry that
could be transported via piping, otherwise known as sluicing. Sluicing of
CCR to the ash basin began when the Plant became operational in 1983.On
May 15, 2013, the Plant converted to a dry ash handling system that only
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required sluicing when there was a shutdown of the dry ash handling
system (approximately 90 percent of CCR was handled dry). Dry ash
handling system upgrades were complete in December 2016 which
eliminated the need for periodic sluicing of CCR. No CCR has been placed
in the ash basin since December 2016.
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 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
area. 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.
Thirteen (13) borings (AB-1, AB-2, AB-3, AB-4, ABMW-1, ABMW-2/BR/BRL,
ABMW-3/S, and ABMW-4/D/BR) were advanced within the ash basin waste
boundary to obtain ash samples for chemical analyses (Figure 1-2). Ash was
encountered in the borings to varying depths. Ash was not observed
outside the ash basin waste boundary in any other borings completed for
this assessment.
The hydraulically sluiced deposits of ash consisted of interbedded fine -to
coarse -grained fly ash and bottom ash material. Ash was generally
described in field observations as gray to dark gray, non -plastic, loose to
medium density, dry to wet, fine- to course -grained sandy silt texture.
Physical properties analyses (grain size, specific gravity, and moisture
content) were performed on six ash samples from the ash basin and
measured using ASTM methods. Fly ash is generally characterized as a
moderately dense silty fine sand or silt. Bottom ash is generally
characterized as a loose, poorly graded (fine- to coarse -grained) sand
(Figure 6-1). Compared with soil, ash exhibits a lower specific gravity with
values reported from 2.2 (AB-2) to 2.7 (ABMW-3) (SynTerra, 2017). Moisture
content of the ash samples ranges from 1.4 percent (ABMW-1) to 41.6
percent (AB-2) (SynTerra, 2017b).
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FIGURE 6-1
FLY ASH AND BOTTOM ASH INTERBEDDED DEPICTION
SOIL DATA
SAaiLE
DEP'fat
O
B—S
AHM A
36 9
Cnry sileyR — SeL (SG-2 674)
O
0
Hmng
2,._g
AHMV. _3
Mu.'t2
C 0A2 O
SO-62
Hm.sn 8c pfe) [v sandy Q .1Y (SG— 65.1)
ReddisA brmm B-saudp SIFT (SG-3 71s)
O
Hnwg
hnc'-1_
5o.v-52.3i
tigLt gry• n. sandy SLLY(SU-2684)
V
Hemg
3 43
2.5475
Cney & Mowo S sead)' SILT (5fi — 2654)
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 5.5 million cubic yards (cy) (AECOM, 2019). Based
on borings located within the ash basin, the maximum depth of CCR within
the ash basin is estimated to be approximately 80 feet. Volume and physical
horizontal and vertical extent of ash material within the basin as cross-
section transect (A -A') along the centerline, from south to north, are
presented in Figure 6-2 and Figure 6-3. Given the compact size and
configuration of the Mayo ash basin, only one cross-section is sufficient to
characterize hydrogeological conditions in the ash basin area of the Site.
6.1.1.4 Volume and Physical Horizontal and Vertical
Extent of Anticipated Saturated Source Material
(CAP Content Section 6.A.a.iv)
Volume and physical horizontal and vertical extent of saturated ash
material under pre -decanting conditions, within the basin in plan -view, is
presented in Figure 6-3. Ash is thickest in areas that coincide with the
former stream valleys in the southern portion of the basin. Thinner areas of
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ash extend out to the boundaries of the basin. A lesser amount of ash is
present in other areas of the basin currently covered by ponded water
(Figure 6-3). Water levels of ash pore water wells indicated that ash within
the basin, prior to decanting, was saturated at depths of 0 feet to 14 feet
below grade surface, yielding approximately 63 feet of saturated ash in the
thickest monitoring well location, in the south-central portion of the ash
basin near the ash "stack out" area. The estimates use the approximated
bottom of ash from the flow and transport model simulations and simulated
hydraulic heads (Appendix G). Due to the presence of ponded water in the
ash basin, estimates of saturated ash in the northern portion of the ash basin
are likely overestimated.
Ash basin decanting was initiated in June 2019. As of December 1, 2019,
124,200,000 million gallons of water has been decanted and the
corresponding pond water elevation has decreased by 7.1 feet thereby
reducing areas of saturated ash. Under closure -in -place conditions, the
range of anticipated saturated ash thickness is between 0 feet to 62 feet with
the greatest volume of saturated ash remaining in the location of the former
stream flow channel (Figure 6-3). Closure -in -place simulated saturated ash
thickness is based on approximated bottom of ash from the flow and
transport model simulation and simulated hydraulic heads (Appendix G).
Model simulations incorporated an engineered underdrain system.
Under the closure -by -excavation closure option, all of the ash in the ash
basin would be removed; 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 (closure -in -place only) will have limited to no
adverse effect on future groundwater quality. Layered ash within the basin
has resulted in relatively low vertical hydraulic conductivity, further
reducing the potential for downward flow of pore water into underlying
residual material. The CSM indicates that the flow -through ash basin
system should result in low to non -detectable constituent concentrations in
groundwater underlying saturated ash within the ash basin except under
the ash "stack out" area and near the dam where downward vertical
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hydraulic gradients are observed. Using boron data, the generalized CSM is
consistent with Site -specific data as summarized in Table 6-1.
In summary, the data from three well cluster locations within the ash basin
demonstrate boron concentrations consistent with the CSM. Boron
concentrations greater than the 02L standard (700 µg/L) were limited to the
surficial and transition zone in the ash "stack out" area where well clusters
ABMW-4D/BR/X and ABMW-3/S are located. Each bedrock well exhibits
non -detectable boron concentrations, indicating downward migration
under the ash "stack out" area is limited to the surficial and transition zone
by a slight upward vertical gradient in the bedrock. Additionally, the data
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) presented linear regression analyses
to evaluate the relationships between saturated ash thickness and
concentrations of boron in ash pore water and underlying groundwater.
The linear regression analysis was conducted using analytical data from
Piedmont ash basins, including data from Mayo.
The statistical evaluation was performed using a dataset which included 89
monitoring wells completed in surficial, transition, and bedrock
groundwater zones directly beneath ash basins and 54 ash pore water
monitoring wells completed in saturated ash. Linear 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 and other areas where ash has
been "stacked" — whether for "harvesting" or placement in an unlined
landfill. This is due to the downward vertical hydraulic gradient in these
areas, which enhances migration of constituents.
Under pre -decanting conditions, the analysis demonstrates saturated ash
and ash pore water are not significantly contributing constituent
concentrations to underlying groundwater except in localized areas where
downward vertical gradients exist. Pre -decanting conditions represent the
greatest opportunity for constituent migration to occur, not because of the
volume of saturated ash, but because of the existing ash basin hydraulic
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head and the downward vertical hydraulic gradient near the dam. Under
post -decanting conditions, the hydraulic head of the ash basin will be
reduced. Therefore, the downward vertical gradient occurring near the dam
will be reduced and the rate of constituent migration from the ash basin to
the groundwater system will be less. 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)
• 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 EPA Methods 6010/6020. For
information purposes, ash samples were compared to soil background
values and PSRGs for POG. The ash analytical data do not represent soil
conditions outside of or beneath the ash basin. Concentrations of arsenic,
chromium, cobalt, iron, and vanadium in ash samples were greater than
concentrations of the same constituents in soil background samples
(Appendix C, Table 4).
In addition, six ash samples collected from borings completed within the
ash basin were analyzed for leachable inorganic constituents using synthetic
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precipitation leaching procedures (SPLP) EPA Method 1312 (Appendix C,
Table 6). The purpose of 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. Analyses indicated that
concentrations of antimony, arsenic, chromium, iron, manganese, nitrate,
thallium, and vanadium, in the SPLP laboratory setting, were greater than
the 02L/IMAC comparative values.
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 Mayo ash basin were conducted using
two LEAF tests, EPA methods 1313 and 1316 (USEPA, (2012a); (2012b)). The
data are presented and discussed in the Geochemical Modeling Report
in Appendix H, Attachment C.
Leaching test results, using USEPA LEAF method 1316, indicate that, even
for conservative constituents such as boron, the leachable concentration of
boron present in ash from Mayo is considerably lower than the total boron
concentration (Appendix H, Attachment C). Mayo data indicate that there
is a process by which the constituents might become stable within the ash
and would make the constituents unavailable for leaching. The exact
mechanisms of this process are unknown; however, literature suggests that
incorporating constituents, such as boron, into the silicate mineral phases is
a potential mechanism (Appendix H, Attachment C). The leaching behavior
of several constituents as a function of pH, examined using USEPA LEAF
method 1313, demonstrated that for anionic constituents, the leaching
increased with increasing pH and the cationic constituents showed the
opposite trend (Appendix H, Attachment C).
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Soil Beneath Ash
(CAP Content Section 6.A.a.vi 1.4 and 6.A.a.vi 1.5)
Soil was collected from borings beneath the ash basin within the waste
boundary at locations AB-3, ABMW-1, ABMW-2BR, and ABMW-3S
(Figure 1-2). Soil/Saprolite was not encountered at other assessment
locations within the ash basin. Soil samples obtained from beneath the ash
basin were saturated. Saturated soil and rock is considered a component of
the groundwater flow system and can serve as a source for groundwater
constituents at the Site. The potential leaching and sorption of constituents
in the saturated zone is included in the flow and transport and geochemical
model evaluations (Appendix G and Appendix H) by continuously
tracking the constituent concentrations over time in the surficial, transition
zone, and bedrock materials throughout the models. Historical transport
models simulate the migration of constituents through the soil and rock
from the ash basin, and these results are used as the starting concentrations
for the predictive simulations. No saturated soils beneath the ash basin have
been analyzed for leachable inorganics using SPLP procedures EPA
Method 1312.
Ash Pore Water
(CAP Content Section 6.A.a.vi.1.6 and 6.A.a.vi.3)
The Mayo ash basin is a NPDES-permitted wastewater treatment unit.
Water within the ash basin is wastewater; therefore, isoconcentration maps
were not prepared for ash pore water and comparison to
02L/IMAC/background values is not appropriate. Ash pore water samples
have been analyzed in accordance with the Interim Monitoring Plan (IMP)
from five monitoring wells screened in the ash pore water of the Mayo ash
basin. Ash pore water sample locations are shown on Figure 1-2 and
analytical results are provided in Appendix C, Table 1. Figure 6-4
represents ash pore water boron distribution in cross section (A -A') from
south to north. This cross-section represents the greatest physical horizontal
and vertical extent of volume of source material within the ash basin
(ABMW-2/BR/BRL and ABMW-3/S). Ash pore water concentrations are
provided for general purposes only and are not compared to 02L/IMAC and
background reference values because it is not groundwater. Discussion of
geochemical trends within the Mayo ash basin pore water is included in
Appendix H, Section 2.
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Three 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 geochemical parameters and water
elevation. Geochemical parameters (pH, oxidation reduction potential
(ORP), and specific conductivity) are monitored using multi -parameter (or
geochemical) sondes. The multi -parameter sondes are equipped with
pressure transducers to monitor water elevations. Locations monitored with
multi -parameter sondes are depicted on Figure 6-5 and include:
ABMW-3: ash pore water well located in the central portion of the
ash basin footprint
CCR-103D: transition zone well located downgradient of the ash
basin on the northwest side of the ash basin dam
CCR-105D: transition zone well located downgradient of the ash
basin, west of the east toe drain of the dam.
Hydrographs and geochemical water quality parameter time series plots for
each location are included on Figure 6-6. Observations of water elevation
and multi -parameter records from monitored locations include:
The ash pore water monitoring location (ABMW-3) within the ash
basin waste boundary shows a response to ash basin decanting by
reduced groundwater elevation levels of approximately 2 feet
(Figure 6-6).
• The transition zone monitoring location at the base of the western
side of the ash basin dam (CCR-103D) shows a response to ash basin
decanting by reduced groundwater elevation levels of approximately
2 feet (Figure 6-6). CCR-103D is located on the far western edge of
the ash basin dam. It is expected that the first responses to decanting
in downgradient wells will be in the wells on the edges of the ash
basin dam because of their elevation relative to the ash basin pond
surface and the Crutchfield Branch stream valley.
The transition zone monitoring location near the east toe drain (CCR-
105D) shows minimal reduction in groundwater elevation levels
(Figure 6-6). CCR-105D is located near the east toe drain, where
response to decanting is expected to be delayed given the proximity
to the Crutchfield Branch stream valley.
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Geochemical parameters pH and ORP do not show significant shifts
or variability in records since ash basin decanting commenced
(Figure 6-6). This suggests geochemical conditions have remained
stable under changing Site conditions at locations within the waste
boundary and downgradient of the source area.
Ash pore water and groundwater geochemical parameters appear stable
under changing Site conditions. Ash pore water pH and ORP do not appear
to be significantly affected by lowering the ash basin'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 ORP, monitored beneath and downgradient of the ash basin, are
unaffected by reductions in water levels, indicating stable geochemical
conditions in which constituent dissolution and mobility are unlikely to
occur.
Ash Pore Water Piper Diagrams
(CAP Content Section 6.A.a.vi.2)
Piper diagrams can be used to differentiate water sources in hydrogeology
(Domenico & Schwartz, 1998). Piper diagrams of ash pore water monitoring
data (Figure 6-7) are used to assess the relative abundance of major cations
(e.g., calcium, magnesium, potassium, and sodium) and major anions (e.g.,
chloride, sulfate, bicarbonate, and carbonate) in water. Data used for the
piper diagrams include ash pore water data collected between January 2018
and April 2019 with a charge balance between -10 and 10 percent.
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, 2012). However, the ash pore water for the Mayo ash
basin does not follow this generalization as the results plot in the same area
on the piper diagram as Site background and side -gradient wells
(Figure 6-7).
6.1.1.7 Other Potential Source Material
(CAP Content Section 6.A.a.vii)
Coal Storage Pile Area
The Mayo coal storage pile area is identified as a potential source of
constituent migration to groundwater. The coal storage pile area located
south of the ash basin and west of the power plant is being evaluated
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independent of the Mayo ash basin. The Assessment Report for this
potential source is due to NCDEQ on a 2020 date to be determined.
Gypsum Storage Pile Area
The Mayo gypsum storage pile area is identified as a potential source of
constituent migration to groundwater. The gypsum storage pile area,
located south of the ash basin and west of the power plant, is being
evaluated independent of the Mayo ash basin. The Assessment Report for
this potential source is due to NCDEQ on a 2020 date to be determined.
6.1.1.8 Interim Response Actions
(CAP Content Section 6.A.a.viii)
Interim response actions conducted to date are summarized in Table 6-2.
Ash Basin Decanting
(CAP Content Section 6.A.a.viii.1)
Ash basin decanting commenced on June 27, 2019, and is expected to be
ongoing through most of 2020. 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 basin. Reduction of constituent migration occurs through decanting by
significantly reducing the hydraulic head within the ash basin and
hydraulic gradients in and under the ash basin dam, thereby reducing the
groundwater seepage velocity near the dam and further limiting constituent
transport potential.
Three ponded water locations within the ash basin fingers and 20
groundwater monitoring wells located within and north of the basin were
selected for monitoring water elevations using pressure transducers to
record changing Site conditions from ash basin decanting (Figure 6-5).
Ponded water and groundwater decanting network hydrographs, using
water elevations recorded between March 2019 (April 2019 for ash basin
fingers only) through September 2019, are depicted on Figure 6-8a through
Figure 6-8c. Observations from hydrographs include:
• As of December 1, 2019, the water level in the ash basin pond
decreased by 7.1 feet (Figure 6-8a). Note the water elevations
displayed on the figure are not current to December 1, 2019.
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Ash basin finger water levels on average decreased by approximately
one foot (Figure 6-8a). The minimal drawdown of water levels
observed in the ash basin fingers suggests the areas are only weakly
connected to the ponded water area.
Groundwater monitoring locations show a response to ash basin
decanting as indicated by reduced groundwater elevation levels
(Figure 6-8a through Figure 6-8c). Comparison of precipitation
trends observed throughout the monitoring period and decreases in
groundwater elevations during decanting confirms that lowered
groundwater levels are related to decanting and not to
seasonal/precipitation effects.
• Groundwater monitoring wells located on the far edges of the ash
basin dam (CCR-103BR and CCR-108BR) show the largest response
from decanting with the greatest reduction in water levels relative to
wells north of the dam (Figure 6-8a through Figure 6-8c).
Toe Drain Collection System
(CAP Content Section 6.A.a.viii.1)
Two engineered toe drains were designed and installed during ash basin
dam construction which was completed in 1982. The toe drains are outlets
for an engineered drain system constructed within the dam. The toe drains
collection system was installed to capture flow from the base of the ash
basin and route the flow back into the ash basin. The collection system
includes sump boxes, pumps, level switches, piping, primary and
secondary power, and system logic controls. The collection systems
operated intermittently from September 2016 to August 2018. Continuous
operation of the system was implemented in August 2018 when the current
NPDES permit went into effect.
Source Area Stabilization
(CAP Content Section, 6.A.a.viii.2)
The Mayo ash basin dam was not subject to NCDEQ Dam Safety Order 16-
01 (August 22, 2016) and had no deficiencies noted by NCDEQ inspection
(Holman to Draovitch, November 13, 2018; Appendix A). Duke Energy
voluntarily completed several repairs to the ash basin dam and ash basin
piping as routine and preventative measures to mitigate potential future
erosion and improve dam conditions including:
• Minor erosion repair
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• Remove stumps and woody vegetation
• Regrade and compact to restore embankment to uniform slopes
• Vegetation established to stabilize disturbed areas
• Abandon existing riser pipe from basin to forebay
• Construct new equalization spillway pipes leading from ash basin
into forebay and construct other related improvements
Project closeout summaries for this work are provided in Appendix A.
6.1.2 Extent of Constituent Migration beyond the Compliance
Boundary
(CAP Content Section 6.A.b)
There are no constituent concentrations in soil, groundwater, or surface water
associated with the Mayo ash basin greater than applicable regulatory criteria at
or beyond the compliance boundary based on monitoring results from four
consecutive quarterly monitoring events. The compliance boundary for
groundwater quality at the ash basin is defined in accordance with Title 15A
NCAC 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.
Analytical sampling results associated with the Mayo ash basin for each media
are included in the following tables and appendix tables:
• Soil: Appendix C, Table 4 and Table 6-4 (CAP Content Section 6.A.b.ii.1)
• Groundwater: Appendix C, Table 1 and Table 6-6 (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 K (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)
Based on the following unsaturated soil evaulation, there are no ash basin -
related consituent concentrations greater than the corresponding standard (PSRG
POG or background value); therefore, there are no constituents in soil that
require corrective action at Mayo. Unsaturated soil at or beyond the waste
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boundary is considered a potential seconday source to groundwater.
Constituents present in unsaturated soil or paritally saturated soil (vadose zone)
have the potential to leach into the groundwater system if exposed to favorable
geochemical conditions for chemical dissolution. Possible effects from the ash
basin to soils would be a result of ash pore water interaction with underlying
soils within the basin and groundwater migration beyond the basin. Therefore,
constituents considered for soil evaluation were limited to the constituents
identified as groundwater COIs in the CSA Update (SynTerra, 2017b) for Mayo
ash basin (arsenic, barium, boron, chromium, chromium (VI), cobalt, iron,
manganese, molybdenum, strontium, sulfate, TDS, and vanadium).
For constituents lacking an established target concentration for soil remediation
(e.g., sulfate), the following equation was used in general accordance with the
reference in 15A NCAC 02L .0202 to calculate a POG value using Site -specific
data.
Csoii = Cg. Ikd + (6w + 0,,H')IPb]df
Of the consituents evaluated, sulfate was the only constituent that required the
calculation of a Site -specific PSRG POG value. The PSRG POG value was
calculated using laboratory testing and physical soil data for effective porosity
(0.3) and dry bulk density (1.6 kilograms per liter [kg/L]) prepared in part for
flow and transport modeling for the Site. Soil water partition coefficients (Kd)
were obtained from the Groundwater Quality Signatures for Assessing Potential
Impacts from Coal Combustion Product Leachate (EPRI, 2012). The resulting PSRG
POG calculated value for sulfate was 1,438 mg/kg (Table 4-2). A summary of the
parameters and values used to calculated the PSRG POG for sulfate is provided
in Table 6-3. The range of constituent concentrations in unsaturated soils, along
with a comparison with soil background values and North Carolina PSRG POG
standards (NCDEQ February 2018), whichever is greater, is provided in
Table 6-4.
Unsaturated soil samples at or beyond the waste boundary were collected from
soil borings and during well installation activities. Soil samples were collected
from locations up-, side-, and downgradient of the ash basin (Figure 6-9). In
response to the CSA Update (SynTerra, 2017b), NCDEQ requested additional
evaluation of shallow soil surrounding, especially downgradient to, the ash basin
to determine the degree of possible effects from historical CCR management at
Mayo. Unsaturated soil samples surrounding the ash basin waste boundary and
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north of the ash basin dam were collected in April 2019. An evaluation of the
potential nature and extent of constituents in unsaturated soil beyond the waste
boundary was conducted by comparing unsaturated soil concentrations with
background values or PSRG POG standards, whichever is greater (Table 6-4).
Constituents detected at concentrations greater than either the background value
or the PSRG POG in unsaturated soil samples (depth in feet) near or beyond the
waste boundary include (Table 6-4):
• Chromium: BGSB-8 (8-9)
• Manganese: BGSB-9 (14-15)
Although greater than the background values and PSRG POG, chromium and
manganese detections are at background locations (Figure 6-9).
Data indicate unsaturated soil constituent concentrations at or beyond the
compliance boundary are consistent with background concentrations or are less
than regulatory screening values (Table 6-4). Therefore, there are no constituents
in soil related to the Mayo ash basin.
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 associated with the Mayo ash basin greater than
02L/IMAC/applicable background concentration values occur north of the ash
basin dam and are contained within the compliance boundary.
The maximum extent of ash basin -affected groundwater migration for all flow
zones is represented by boron concentrations greater than the 02L standard. The
boron plume is contained to the area immediately downgradient of the ash basin
dam and within the compliance boundary.
Section 6.1.3 includes a detailed matrix evaluation and rationale of groundwater
constituents requiring corrective action, and Section 6.1.4 provides
isoconcentration maps and cross sections depicting groundwater flow and
constituent distribution in groundwater at or beyond the compliance boundary
(CAP Content Section 6.A.b.i).
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Seep Constituent Extent
(CAP Content Section 6.A.b.ii.3)
Seeps at Mayo 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-1A, S-2A, S-2B, S-8, and 5-10
• Non -constructed seeps dispositioned — S-3, S-4, S-5, S-6, S-7, and S-9
• Constructed seeps - S-1, S-2
The SOC defines dispositioned as:
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 affects the seep for all COIs over four
consecutive sampling events;
4. An engineering solution has eliminated the seep.
Non-dispositioned seeps, where monitoring has indicated the presence of CCR
affects, are located within the compliance boundary and include: S-1, S-2, S-2B, S-
8, and 5-10 (Figure 5-7). Table 6-5 provides a summary of seep general location
and approximate flow rate. Seeps at Mayo are contained within well-defined
channels. Therefore, potential constituent migration related to seep flow is
constrained in localized areas along the channel.
Similar to groundwater, the extent of seeps affected by ash basin -related
constituents is indicated by boron concentrations observed in seeps
downgradient of the ash basin. However, boron does not have an established 02B
standard; therefore, boron data is used for informational purposes only to
identify the extent of ash basin -related constituents in downgradient seeps. The
most recent valid boron data collected from seeps between January 2018 and
June 2019 are included on Figure 5-7. Surface water sampling conducted
downstream of seep channels, at the point of the channels' confluence with
surface water receptors (i.e., Crutchfield Branch), demonstrated that flow from
seeps has not caused constituent concentrations greater than 02B standards.
Analytical results for seeps are included in Appendix C, Tables 2 and 3.
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Surface Water Constituent Extent
(CAP Content Section 6.A.b.ii.4)
Surface water samples have been collected from Crutchfield Branch to confirm
groundwater downgradient of the ash basin has not resulted in surface water
concentrations greater than 02B water quality standards; therefore, there are no
surface water constituents related to the Mayo ash basin that require corrective
action. Surface water samples were collected to evaluate acute and chronic water
quality values. Surface water samples were also collected at background
locations (upgradient of the source area) upstream of the Plant and the ash basin.
Analytical results were evaluated with respect to 02B water quality standards
and background data. Surface water conditions are further discussed in
Section 6.2.1 and the full report for Mayo surface water current conditions can be
found in Appendix I.
As discussed above, the extent of surface water affected by ash basin -related
constituents is indicated by boron concentrations observed in surface water
receptors downgradient of the ash basin. However, boron does not have an
established 02B standard; therefore, boron data is used for informational
purposes only to identify the extent of ash basin -related constituents in
downgradient surface water receptors. Surface water boron concentrations are
included on Figure 5-7.
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 per water body. Analytical results for all sediment samples are
provided in Appendix C, Table 5.
Assessment of constituents in sediment from surface waters, including
Crutchfield Branch, unnamed tributaries to Bowe's Branch, unnamed feeder
stream to Mayo Reservoir, and seeps, was conducted through a comparison
evaluation between sediment sample constituent analytical results, from one -
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time grab samples, and constituent concentration ranges from background
sediment datasets. Samples collected from Crutchfield Branch were compared
with background dataset ranges from upgradient surface water bodies.
Sediments Collected from Crutchfield Branch
Eleven sediment samples were collected from locations downgradient of the ash
basin dam within the Crutchfield Branch stream system. Sediment sample
locations are co -located with surface water sample locations (Figure 5-7).
No sediment sample from locations at or beyond the compliance boundary (SW-
CB1, SW-CB2, SW-CB3, and SW-CB4) had constituent concentrations greater
than the range of background values. Based on the sediment evaluation, there
are no constituents in sediment that require corrective action. Sediment analytical
results (Appendix C, Table 5) are compared to results from six background
locations upgradient of the ash basin and PSRG POG standards.
Sediments Collected from Seeps
Sediment samples from seep locations immediately downgradient of the ash
basin dam and within the compliance boundary were analyzed and compared to
results from surface water sediment background locations upgradient of the ash
basin. There are no seep sediment background locations as there no identified or
non-dispositioned seeps upgradient of the ash basin.
S-1: Arsenic and iron sediment concentrations are greater than
background concentrations (Table 4-5 and Appendix C, Table 5). The S-1
sediment sample was collected from the bottom of the concrete structure
for the engineered west toe drain and is located within the waste
boundary. Therefore, no corrective action is required.
• S-2: Constituent concentrations are consistent with sediment background
ranges (Table 4-5 and Appendix C, Table 5). Additionally, the S-2
sediment sample was collected from the bottom of the concrete structure
for the engineered east toe drain and is located within the waste
boundary. Therefore, no corrective action is required.
• S-2B: Constituent concentrations are consistent with sediment background
ranges (Table 4-5 and Appendix C, Table 5).
• S-3: Chromium concentration is greater than background values (Table 4-
5 and Appendix C, Table 5). Chromium is not present in ash pore water
at concentrations greater than regulatory standards. Chromium in
sediment at the S-3 location is not associated with the Mayo ash basin.
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Additionally, S-3 is located immediately downstream from the engineered
east toe drain and is well within the compliance boundary. Therefore, no
corrective action is required.
• S-4: The arsenic and chromium concentrations are greater than
background values (Table 4-5 and Appendix C, Table 5). S-4 is located
immediately downstream from the engineered west toe drain and is well
within the compliance boundary. Therefore, no corrective action is
required.
• S-8: Cobalt and manganese concentrations were greater than sediment
background concentrations (Table 4-5 and Appendix C, Table 5). The
cobalt concentration observed at S-8 is less than the Mayo soil background
value (Table 4-2 and Appendix C, Table 4). S-8 is located on a
topographic high on the eastern edge of the ash basin dam. The material
located at S-8 is representative of soil rather than sediment and, therefore,
comparison to soil background values is appropriate. Manganese
concentrations are greater than PSRG POG, sediment background, and
soil background values. S-8 is located just beyond the waste boundary,
well within the compliance boundary. Additionally, the seep at S-8 is
expected to cease flowing in response to ash basin decanting. Therefore,
no corrective action is required.
6.1.2.1 Piper Diagrams
(CAP Content Section 6.A.b.iii)
Piper diagrams can be used to differentiate water sources in hydrogeology
by assessing the relative abundance of major cations (i.e., calcium,
magnesium, potassium, and sodium) and major anions (i.e., chloride,
sulfate, bicarbonate, and carbonate) in water.
Groundwater Piper Diagrams
Piper diagrams of groundwater monitoring data from surficial zone,
transition zone, and bedrock zone background locations and downgradient
of the ash basin (Figure 6-7) are used to assess the relative abundance of
major cations (e.g., calcium, magnesium, potassium, and sodium) and
major anions (e.g., chloride, sulfate, bicarbonate, and carbonate) in
groundwater. Data used for the piper diagrams include groundwater data
results from sampling between January 2018 and April 2019 with a charge
balance between -10 and 10 percent.
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• Majority of the groundwater wells in the surficial zone, transition
zone, and shallow bedrock flow zone located immediately
downgradient of the ash basin dam (CCR-103S/D/BR, CCR-104S/BR,
CCR-105S/D/BR, CCR-106BR, CCR-107BR, CCR-108BR, CCR-109BR,
and CW-2) plot with higher proportions of chloride and sulfate,
which is consistent with the literature for groundwater affected by
ash pore water (EPRI, 2012) (Figure 6-7). However, boron
concentrations in seven of those 13 wells are less than the 02L
standard with three wells showing non -detectable boron
concentrations (CCR-104BR, CCR-108BR, and CCR-109BR),
indicating little to no influence from ash pore water (Appendix C,
Table 1).
The distribution of results on the piper diagrams in Figure 6-7
indicate no conclusion can be made regarding effects on
groundwater from the ash basin based on relative abundance of
major cations and anions.
Seep and Surface Water Piper Diagrams
Piper diagrams of seep and surface water monitoring data are included on
Figure 6-10. 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 percent. As discussed in Section 6.1.1, ash pore water
from the Mayo ash basin does not plot on piper diagrams in an area that is
distinguishable from background groundwater. Therefore, the data shown
on Figure 6-10 cannot be used to make inferences regarding potential effects
to surface water from the ash basin at Mayo. General observations from the
seep and surface water piper diagrams include:
• Surface water samples from background or reference locations (S-6, SW-
REF1, SW-REF2, SW-BB2, and S-9) plot together in a cluster that is
distinguishable from the downgradient surface water from Crutchfield
Branch (S-3, SW-CB1, SW-CB2, SW-CB3, SW-CB4) (Figure 6-10). Surface
water sample, SW-CBT1, which was collected from a tributary to
Crutchfield Branch north of the ash basin, plots with the background
locations (Figure 6-10).
• Seeps S-1, S-2, S-2B, and S-8 are all located immediately downgradient of
the ash basin dam. The seeps plot on the piper in the area that is typical
for CCR leachate "affected" water (Figure 6-10); however, the ash pore
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water in the Mayo ash basin does not plot in this area (Figure 6-7).
Additionally, each of these seeps is covered by the SOC.
6.1.3 Constituents of Interest (COIs)
(CAP Content Section 6.A.0
This CAP Update evaluates the extent of COIs associated with the Mayo ash
basin detected at concentrations greater than applicable regulatory criteria.
Mayo ash basin -related constituents were developed by evaluating groundwater
sampling results with respect to the presence of concentrations greater than
regulatory criteria or background values, whichever is greater and additional
regulatory input/requirements. The distribution of constituents in relation to the
ash management areas, co -occurrence of CCR indicator constituents such as
boron, and migration direction based on groundwater flow direction are
considered in determination of Mayo ash basin -related constituents.
The following list of ash basin -related constituents has been developed for Mayo
(SynTerra, 2017b) and serve as the foundation of the constituent management
process used to identify COIs associated with the Mayo ash basin:
• Arsenic
• Barium
• Boron
• Chromium (Total)
• Chromium (Hexavalent)
• Cobalt
• Iron
Soil
(CAP Content Section 6.A.c.i.1)
• Manganese
• Molybdenum
• pH
• Strontium
• Sulfate
• Total Dissolved Solids (TDS)
• Vanadium
No constituents were detected greater than PSRG POG or background values at
sample locations downgradient of the ash basin. Therefore, there are no COIs for
soil related to the Mayo ash basin.
Groundwater
(CAP Content Section 6.A.c.i.2)
A measures of central tendency analysis of the groundwater constituent data
(January 2018 to April 2019) was conducted and means were calculated to
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support the analysis of groundwater conditions to provide a basis for defining
the extent of the constituent migration at or beyond the compliance boundary. A
measure of central tendency analysis captures the appropriate measure of central
tendency (arithmetic mean, geometric mean, or median) for each dataset of
constituent concentrations. Constituent concentrations in a single well may 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 constituent plume geometries with
central tendency data minimizes the potential for incorporating occasions where
constituents 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
constituent concentrations for each well might have overrepresented or
underrepresented areas affected by the ash basin by posting a single data set on
maps and cross -sections that might have included isolated data anomalies.
The mean of up to six quarters of valid data was calculated for each identified
constituent to analyze groundwater conditions and define the extent of
constituent migration at or beyond the compliance boundary. At a minimum,
four quarters of valid data were used for calculating means; however, if fewer
than four quarters of valid data were available, the most recent valid sample
result was reported. Less than four quarters of valid data were not available
either because the well was recently installed or sample results from one or more
quarters were excluded. For use in calculating means, non -detect (ND) values
were assigned the laboratory reporting limit (RL) and estimated (J-flag) values
were treated as the reported value. Procedures for excluding data from
calculating means are based on USEPA's National Functional Guidelines
(USEPA, (2017a); (2017b)), published research about leaching of elements from
coal combustion fly ash (Izquierdo & Querol, 2012), and professional judgement.
The following steps outline the approach followed in calculating central
tendency values for constituent concentrations in groundwater:
1. If the maximum analytical value divided by the minimum value for each
constituent was greater than or equal to 10 (i.e., the data set ranges over an
order of magnitude), the geometric mean of the analytical values was
used.
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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.
4. If the dataset mode (most common) is equal to the RL, and the 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 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.) Data with pH greater than 10
s.u. might suggest well grout impacts.
• Data flagged as unusable (RO qualified)
• Data reported as non -detect with a RL greater than the normal laboratory
RL
For each constituent at Mayo, the arithmetic mean was determined to be the
most appropriate measure of central tendency. Table 6-6 presents the calculated
means of the constituent data using groundwater monitoring sampling results
from January 2018 to April 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 6-6 are used
in evaluating constituent plume geometry near the ash basin.
Constituent Management Approach
A COI Management Plan was developed at the request of NCDEQ to evaluate
and summarize constituent concentrations in groundwater at the Site. Results of
this COI Management Plan are used to identify areas that may require corrective
action and to determine appropriate Site -specific mapping of constituent
concentrations on figures based on the actual distribution of each constituent in
Site groundwater.
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• Groundwater COIs to be addressed with corrective action are those which
exhibit concentrations in groundwater at or beyond the compliance
boundary greater than the 02L standard, IMAC, or BTV, whichever is
highest. Table 6-7 presents the constituent management matrix for
determining COIs subject to corrective action at Mayo.
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 constituents that
do not exhibit a discernable plume or constituents that have no correlation
with other soluble constituents associated with coal ash or another
primary source (e.g., boron).
A three -step process was utilized in the COI Management Plan approach:
1. An evaluation of the applicable regulatory context
2. An evaluation of the mobility of target constituents
I 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 constituent 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, described in Section 5.0 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 constituent list identified in the CSA Update (SynTerra,
2017b) and 2019 IMP submitted by Duke Energy, March 20, 2019, and
approved by NCDEQ April 20, 2019. Constituent concentrations were
screened against their respective constituent criterion defined as the
maximum of the 02L groundwater quality standard, IMAC, and
background. COI concentrations were screened against their respective
constituent criterion for groundwater monitoring locations at or beyond
the compliance boundary. Groundwater constituent concentrations used
in the screening are based on a calculated central tendency value (mean)
including data from 2018 through the second quarter of 2019.
NCDEQ (NCDEQ, 2019) recommended use of a lower confidence limit
(LCL95) concentration rather than the central tendency value. LCL95
concentrations were calculated for each constituent and the LCL95
concentration for the sample with the highest COI LCL95 concentration is
provided in Table 1 of the COI Management Plan (Appendix H) for
comparison to the maximum constituent mean concentration. Table 2 of
the COI Management Plan (Appendix H) provides a comparison of the
maximum constituent central tendency concentrations compared with the
maximum constituent LCL95 concentrations for wells located at or
beyond the compliance boundary for the Allen Steam Station, Belews
Creek Stream Station, Cliffside Steam Station, Marshall Steam Station,
Mayo Steam Electric Plant, and Roxboro Steam Electric Plant sites. The
constituent LCL95 concentrations were typically lower than the
constituent central tendency value with very few exceptions. The number
of wells exceeding constituent criteria using the constituent LCL95
concentration was typically equal to or less than the number of wells
exceeding constituent criteria using the constituent central tendency
concentration. There were no increases in the number of wells exceeding
constituent criteria for the Site when comparing the LCL95 to the
constituent criterion and the number of exceedances was typically less for
LCL95. Use of the constituent central tendency concentrations in the COI
Management Plan process provides a conservative estimate of the extent
of constituents in Site groundwater.
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Step 2: COI Mobility
Step 2 of the COI Management Plan process evaluates the constituent
mobility to identify hydrogeologic and geochemical conditions and
relative constituent mobility based on:
• Review of regulatory agency and peer -reviewed literature to
identify general geochemical characteristics of constituents
• Analysis of empirical data and results from geochemical and flow
and transport modeling conducted for the Site
• Identification of constituent -specific mobility as conservative (non -
reactive), non -conservative (reactive), or variably reactive based on
results from geochemical modeling (Appendix H)
Site -specific groundwater geochemical conditions that may affect
constituent transport and distribution are described in Table 1 of the COI
Management Plan (Appendix H).
Step 3: COI Distribution
Step 3 of the COI Management Plan process evaluates the relative
presence of constituents in Site groundwater. Descriptions of the
horizontal and vertical distribution of constituents with mean
concentrations above their respective COI criterion at and beyond the
compliance boundary are summarized in Table 1 of the COI Management
Plan (Appendix H) and provided in more detail in Table 6-7 (CAP Content
Section 6.A.c.i.2). The COI Management Plan approach considers the
distribution of constituents on a Site -wide basis. These distributions are
used for planning appropriate corrective action, if necessary, as well as
determining which constituents to map on figures.
Primary descriptions of constituent distributions include plume -like
distributions for relatively mobile constituents such as boron and isolated
location(s) for constituents that do not exhibit plume -like distributions.
Boron is the constituent with the most plume -like distribution. Some
constituents with isolated exceedances of constituent criteria are not
associated with the boron plume and these exceedances are described in
more detail in Table 6-7 to place these exceedances within the context of
the Site CSM.
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Rationale for inclusion or exclusion of constituents from mapping on
figures in the 2019 CAP Update is based on the horizontal and vertical
distribution of constituents with concentrations greater than their
respective constituent criterion. All wells that have constituent mean
concentration(s) greater than the constituent criterion are listed in
Table 6-7.
Outcome of COI Management Plan Process
Constituents with concentrations greater than the constituent 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 constituent 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 constituent criterion
with a discernable plume that correlates with other soluble constituents.
Constituents were assigned to mobility categories based on geochemical
modeling results and information derived from peer -reviewed literature.
Constituent mobility categories are based on the concept of conservative
versus non -conservative constituents introduced by NCDEQ in the
January 23, 2019 CAP content guidance document. The use of three
mobility categories for constituents 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, constituent mobility categories were expanded from
conservative versus non -conservative to include the following:
• Conservative, Non -Reactive constituents: [boron and TDS]
Geochemical model simulations support that these constituents
would transport conservatively (Ka values <1 liter per kilogram
[L/kg]) as soluble species under most conditions, and that the
mobility of these constituents will not change significantly due to
current geochemical conditions or potential geochemical changes
related to remedial actions.
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• Non -Conservative, Reactive constituents: [arsenic and chromium]
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 constituents is unlikely to be
geochemically affected by current geochemical conditions or
potential geochemical changes related to remedial actions.
• Variably Reactive constituents: [barium, hexavalent chromium,
cobalt, iron, manganese, molybdenum, strontium, sulfate, and
vanadium] 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 constituents to the
groundwater pH and Eh indicates that these constituents could
respond to natural changes, such as water level fluctuations
imposed by seasonality, or to decanting and source control
activities that have the potential to change the groundwater pH or
Eh.
As discussed in the CSA Update (SynTerra, 2017b) and the 2018 CAMA
Annual Interim Monitoring Report (SynTerra, 2019c), not all constituents
with results greater than background values can be attributed to the ash
basin or another source area. Naturally occurring groundwater contains
varying concentrations of inorganic constituents. Sporadic and low -
concentration occurrences of these constituents in the groundwater data
do not necessarily demonstrate horizontal and vertical distribution of
COI -affected groundwater migration from the ash basin.
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
constituent concentrations were compared with constituent criteria to
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identify constituents that exceeded their respective constituent criterion.
Use of the constituent central tendency concentrations in the COI
Management Plan process was shown to provide a conservative estimate
of the extent of constituents in Site groundwater. Exceedance ratio values
indicate constituent concentrations that exceed constituent criteria are
typically within one order of magnitude (exceedance ratio <10) above the
constituent criterion.
Of 13 inorganic groundwater constituents (not including pH) identified in
the CSA (SynTerra, 2017b), eight constituents (arsenic, barium, chromium,
chromium (VI), cobalt, iron, sulfate, and vanadium) exhibit mean
concentrations that are currently less than the 02L standard, IMAC, or
background value near or beyond the compliance boundary, with no
discernable plume characteristics (Table 6-7). These constituents are not
expected to migrate beyond the compliance boundary and are predicted,
based on geochemical modeling, to remain at concentrations less than the
02L standard, IMAC, or background values. Therefore, arsenic, barium,
chromium, chromium (VI), cobalt, iron, sulfate, and vanadium are not
retained as COIs for this CAP Update.
Of the remaining five constituents, four (manganese, molybdenum,
strontium, and TDS) exhibit mean concentrations greater than the 02L,
IMAC, or background values at isolated wells downgradient of the ash
basin near or beyond the compliance boundary. However, as clearly
indicated in Table 6-6 and described below, detected concentrations are
isolated and sporadic occurrences that do not represent a discernable
plume and are not associated with groundwater migration from the ash
basin.
• The manganese mean concentration exceeds the background values
at one well near or beyond the compliance boundary (MW-3BR);
however, the mean is within the Site and regional background
ranges (Table 6-6 and Table 6-7). No wells beyond the compliance
boundary downgradient of the ash basin have manganese mean
concentrations greater than the background values. Additionally,
the distribution of manganese mean concentrations does not exhibit
a discernable plume and some of the highest concentrations on Site
are side -gradient to the ash basin (Table 6-7). Therefore,
manganese is not retained as a COI for this CAP Update.
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• Molybdenum mean concentrations that are greater than Site
background values near or beyond the compliance boundary are
observed only in wells MW-16D and MW-16BR (Table 6-6).
Molybdenum is not observed in concentrations above background
immediately downgradient of the ash basin and its distribution
does not exhibit a discernable plume
(Table 6-7). Therefore, the mean concentrations of molybdenum
observed in MW-16D and MW-16BR are not attributed to
groundwater migration from the ash basin. Molybdenum is not
retained as a COI for this CAP Update.
• Strontium mean concentrations greater than background values are
observed in one well (MW-16S) near or beyond the compliance
boundary (Table 6-6). The strontium mean concentration in MW-
16S is within Site and regional background ranges (Table 6-7).
Furthermore, the distribution of strontium does not exhibit a
discernable plume. Therefore, the strontium observed in MW-16S is
not attributed to groundwater migration from the ash basin.
Strontium is not retained as a COI for this CAP Update.
• TDS is present at concentrations greater than the 02L standard at
two wells downgradient of the ash basin near or beyond the
compliance boundary (MW-3BR and CCR-109BR) (Table 6-6). TDS
is not present in ash pore water in concentrations above the 02L
standard and the TDS distribution downgradient of the ash basin
does not exhibit a discernable plume (Table 6-7). TDS
concentrations observed near and on the compliance boundary in
MW-3BR and CCR-109BR are not attributed to groundwater
migration from the ash basin. Therefore, TDS is not retained as a
COI for this CAP Update.
Boron is the only CSA identified COI retained for evaluation in this CAP
Update as it is a key indicator of constituent migration (i.e., groundwater
affected by the ash basin pore water) and exhibits a discernable plume
associated with the ash basin.
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6.1.4 Horizontal and Vertical Extent of COIs
(CAP Content Section 6.A.d)
The maximum extent of affected groundwater migration from the ash basin is
north of the ash basin along the Crutchfield Branch stream valley based on
sampling and analysis data from 79 monitoring wells present at the Site. The
plume geometry is largely shaped by Site hydraulic conditions associated with
the basin, basin dam, free water within the basin, and the Crutchfield Branch
system north of the basin as detailed in Section 5.0.
Boron, a conservative (non -reactive) constituent, continues to be a key indicator
of ash basin -affected groundwater migration and plume characteristics
associated with the ash basin. Boron typically has greater concentrations in CCR
than in native soil and is relatively soluble and mobile in groundwater (Chu,
Panzion, & Bradley, 2017). The maximum extent of the 02L boron plume (700
µg/L) represents the maximum extent of ash basin -affected groundwater
migration.
6.1.4.1 COIs in Unsaturated Soil
(CAP Content Section 6.A.d.i)
There are no ash basin -related soil COIs identified for Mayo; therefore, this
section is not applicable.
6.1.4.2 Horizontal and Vertical Extent of Groundwater in
Need of Restoration
(CAP Content Section 6.A.d.ii)
Based on groundwater sampling data from the four recent consecutive
quarters, there are no COI concentrations greater than 02L standards
associated with constituent migration from the ash basin at or beyond the
compliance boundary; therefore, groundwater corrective action associated
with the ash basin is not required. The most recent boron concentration and
the historical maximum boron concentration for wells near and beyond the
compliance are presented in Table 6-8. This section will focus on the
horizontal and vertical extent of boron, the only ash basin -associated COI,
north and downgradient of the ash basin dam.
The horizontal extent of affected groundwater migration in each flow layer
is depicted on the boron plume maps (Figure 6-11a through Figure 6-11c).
The 02L boron plume and background boron plume represent a maximum
extent of ash basin -affected groundwater migration in each flow layer. The
02L boron plume and background boron plume depicted on Figure 6-11a
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through Figure 6-11c were generated from the flow and transport model
and informed by boron mean concentration data. The model predictions are
conservative and may over -predict the extent of boron distribution in
groundwater. The vertical extent of the boron -affected groundwater
migration is shown on generalized cross-section A -A' (Figure 6-4). As
indicated on Figure 6-4 and Figure 6-11a through Figure 6-11c, the
maximum extent of ash basin -affected groundwater occurs north of the ash
basin but does not extend beyond the compliance boundary.
Boron plume maps and cross-section support the following observations
regarding the extent of affected groundwater:
Mean concentrations of boron from ash pore water monitoring wells
(ABMW-01, ABMW-02, ABMW-03, and ABMW-04) are greater than
02L standards (Table 6-6).
Mean concentrations of boron from saprolite (ABMW-03S) and
transition zone (ABMW-04D) groundwater monitoring wells in the
southern portion of the ash basin are greater than 02L standards
(Table 6-6).
Mean concentrations of boron in bedrock groundwater monitoring
wells (ABMW-02BR and ABMW-02BRL) in the southern portion of
the ash basin are less than background (non -detect), supporting the
flow -through with limited downward migration CSM discussed in
Section 5.0 (Table 6-6).
Maximum boron concentrations in groundwater within the ash basin
waste boundary are 1,320 µg/L in the saprolite (ABMW-03S) and
3,242 µg/L in the transition zone (ABMW-04D) (Table 6-6). Boron is
below the reporting limit (50 µg/L) in bedrock within the ash basin
waste boundary.
Concentrations of boron from groundwater surficial, transition zone,
and shallow bedrock monitoring wells immediately north
(downgradient) of the ash basin dam are greater than 02L standards
(Table 6-6). Groundwater monitoring wells CCR-103S/D/BR,
CCR104S/BR, CCR-105S/D/BR, and CCR107BR are located at or near
the ash basin waste boundary. The CCR-103S/D/BR, CCR-104S/BR,
and CCR-105S/D/BR clusters each show increasing concentrations of
boron with depth. The wells are located in areas of upward
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(negative) vertical gradients due to the effect of the ash basin ponded
water upgradient of the dam as discussed in the CSM. However, the
vertical extent of boron downgradient of the ash dam is limited to the
shallow bedrock.
• Boron concentrations in bedrock at or near the ash basin waste
boundary are limited to the top 40 feet of rock (less than 75 feet
below ground surface). The deep bedrock groundwater monitoring
wells MW-103BRM/BRL, MW-104BRM/BRL, and MW-105BRM/BRL
have boron concentrations less than the laboratory reporting limit (50
µg/L). Wells MW-107BRM/BRL have boron concentrations slightly
greater than the laboratory reporting limit: 83 µg/L at MW-107BRM
and 68 µg/L at MW-107BR (Appendix F). Boron concentrations in
bedrock decrease with depth indicating vertical migration is
generally limited to shallow bedrock (top 40 feet of rock).
Bedrock boron concentrations that are greater than the 02L standard
are limited to those located at or near the ash basin waste boundary.
Groundwater monitoring well CW-2D, a shallow bedrock well
located on the compliance boundary, has a mean concentration of
boron less than the 02L standard but greater than Site background
(273 µg/L) (Table 6-6). Bedrock monitoring wells at or beyond the
compliance boundary (CW-4, MW-3BR, and MW-16BR) all have
mean concentrations of boron less than laboratory reporting limit
(non -detect) (Table 6-6).
The surficial and transition zone flow zone boron plumes are within
the compliance boundary and have relatively similar geometries
(Figure 6-11a and Figure 6-11b). This supports the interpretation that
these two zones are hydraulically connected. Differences between the
groundwater boron plumes are related to hydraulic conditions; the
saprolite has limited saturated thickness across the Site.
The mean concentration of boron at the compliance boundary (CW-2)
is slightly greater than the 02L standard (720 µg/L) (Table 6-6).
Further downgradient at MW-16D, the mean concentration of boron
is less than the laboratory reporting limit (non -detect). The mean
concentration of boron at monitoring well CW-3 is non -detect. CW-3
is on the compliance boundary, north of the ash basin, and outside of
the Crutchfield Branch stream valley.
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• Mean concentration of boron from MW-16S, a groundwater
monitoring well screened in the surficial flow zone beyond the
compliance boundary, is less than the 02L standard (167 µg/L) (Table
6-6). The boron mean concentration inside the compliance boundary
at MW-3 is greater than the 02L standard (1049 µg/L) (Table 6-6).
Saturated saprolite north of the ash basin is limited to the Crutchfield
Branch stream valley; therefore, the boron plume in the surficial flow
zone is also limited to the area near Crutchfield Branch.
• Flow and transport modeling has demonstrated that when
Crutchfield Branch is not free flowing and is impounded due to the
presence of beaver dams, locally losing stream conditions occur.
When these conditions occur, surface water recharges the shallow
alluvium in which MW-16S is screened. Under these conditions,
detected groundwater concentrations of boron in MW-16S reflect
surface water concentrations of boron.
• The maximum mean boron concentration beyond the compliance
boundary in the surficial groundwater is 167 µg/L (MW-16S)
(Table 6-6). Boron concentrations in the transition zone and bedrock
zones beyond the compliance boundary are below the laboratory
reporting limit.
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 (Appendix H), constituents identified in the CSA Update
(SynTerra, 2017b) as related to the ash basin were grouped by geochemical
behavior and mobility. 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. The
evaluation grouped constituents into three mobility groups: conservative (non -
reactive), non -conservative (reactive), and variably reactive. As discussed in
Section 6.1.3, boron is the only constituent retained as a COI; therefore,
discussion of non -conservative and variably reactive constituents is not
applicable for Mayo.
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6.1.5.1 Conservative Constituents
(CAP Content Section 6.A.e.i)
Boron plume maps and cross -sections support the following observations
regarding the extent of COI -affected groundwater represented by
conservative constituents:
• Surficial, transition zone, and bedrock flow zone groundwater boron
plumes are within the compliance boundary.
• The Surficial, transition zone, and bedrock flow zone groundwater
boron plumes have relative similar plume geometries
(Figure 6-11a through Figure 6-11c). This supports a connected,
unconfined flow system between the surficial, transition zone, and
bedrock flow zones.
Empirical data from the Site indicates boron concentrations greater
than 02L standards are limited to the top 40 feet of bedrock
(Figure 6-4).
Boron distribution in groundwater has been horizontally delineated
downgradient of the ash basin. Boron delineation is demonstrated by
detected constituent concentrations that are less than regulatory
standard or are not detected from groundwater monitoring wells
CCR-104BR, CCR-109BR, CW-3, CW-4, MW-2, and MW-16S/D/BR
(Figure 6-11a through Figure 6-11c).
Boron distribution in groundwater has been vertically delineated
downgradient of the ash basin. Boron delineation is demonstrated by
field screening and laboratory analytical results of the deep bedrock
evaluation that indicate boron concentrations are less than regulatory
standard or are not detected in groundwater monitoring wells MW-
103BRL/BRM, MW-104BRL/BRM, MW-105BRL/BRM, and MW-
107BRL/BRM (Appendix F).
• In general, conservative constituents like boron are expected to
migrate in groundwater as soluble species and are not strongly
attenuated by reactions with solids but are reduced in concentration
with distance by physical processes such as mechanical mixing
(dispersion), dilution, and diffusion into less permeable zones.
However, the presence of sericite as a weathering product in the
bedrock indicates that the bedrock has some capacity to sorb boron
(Appendix F).
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Plume Behavior and Stability
(CAP Content Section 6.A.e.i.1)
The boron plume is stable or decreasing in the surficial, transition zone, and
bedrock flow zones.
Mann -Kendall trend analysis was performed on a select set of wells within
the waste boundary, between the waste boundary and compliance
boundary, and near or beyond the compliance boundary. Mayo recently
began continual operation of two seep collection systems, decanting of the
ash basin, and cessation of all wastewater flows to the ash basin. Recent
monitoring well analytical results, which are representative of pre -
decanting conditions, were utilized for this trend analysis. The analysis was
performed using analytical results for boron from samples collected from
January 2018 through June 2019 (Table 6-9).
Trend analysis results are presented where at least four samples were
available and frequency of detection was greater than 50 percent.
Statistically significant trends are reported at the 95 percent confidence
level. The analysis of constituent concentrations through time produced six
possible results:
1. Statically significant, decreasing concentration trend
2. Statically significant, increasing concentration trend
3. Greater than 50 percent of concentrations were non -detect
4. No significant trend, and variability is high
5. Stable. No significant trend, and variability is low
Ash pore water and groundwater wells within the waste boundary with
detectable boron have stable or no boron concentration trends, suggesting
limited changing conditions (Table 6-9).
Groundwater monitoring wells north of the ash basin, between the waste
boundary and compliance boundary include CCR-103S/D/BR, CCR-
104S/BR, CCR-105S/D/BR, CCR-106BR, CCR-107BR, CCR-108BR, MW-2,
and MW-3. Groundwater monitoring wells north of the ash basin and near
or beyond the compliance boundary include CCR-109BR, CW-2/D, CW-3,
CW-4, MW-3BR, MW-16S/D/BR. Mann -Kendall results for groundwater
wells north of the ash basin include:
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• Trend analysis results for groundwater monitoring wells in the
surficial flow zone indicate that all but one well (MW-3) exhibit a
stable trend in boron concentrations (Table 6-9). Boron concentrations
in MW-3 do not exhibit a statistically significant trend. Therefore, the
overall boron plume within the surficial flow zone is characterized as
"stable."
Trend analysis results for groundwater monitoring wells in the
transition zone flow zone within the compliance boundary exhibit
stable boron concentration trends (Table 6-9). However, the transition
zone monitoring wells near or beyond the compliance boundary
exhibit a decreasing trend in boron concentration (CW-2) or do not
have detectable boron concentrations (CW-3 and MW-16D). Therefore,
the overall boron plume within the transition zone flow zone is
characterized as "decreasing."
Only 4 of the 15 bedrock wells used in the trend analysis have boron
concentrations greater than the reporting limit (50 µg/L) (Table 6-9).
Three of the bedrock monitoring wells are located immediately
downgradient of the ash basin dam (CCR-103BR, CCR-105BR, and
CCR-107BR). CCR-103BR exhibits an increasing trend in boron
concentrations. CCR-105BR exhibits a stable trend in boron
concentrations. CCR-107BR exhibits a decreasing trend in boron
concentrations. Boron concentration trends in bedrock near the
compliance boundary (CW-2D) are stable. Therefore, the overall boron
plume within the bedrock flow zone is characterized as "stable."
6.1.5.2 Non -Conservative Constituents
(CAP Content Section 6.A.e.i)
There are no non -conservative COIs associated with the ash basin at Mayo;
therefore, this section is not applicable.
6.1.5.3 Variably Reactive Constituents
(CAP Content Section 6.A.e.i)
There are no variably reactive COIs associated with the ash basin at Mayo;
therefore, this section is not applicable.
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6.2 Receptors Associated with Ash Basin
(CAP Content Section 6.B)
CSA and ongoing monitoring data confirm that affected groundwater is limited to
Duke Energy property. Ash basin -affected groundwater does not reach any water
supply wells and modeling indicates this will remain the case in the future. Therefore,
potential receptors are limited to the Crutchfield Branch stream system.
6.2.1 Surface Waters - Downgradient within a 0.5-Mile
Radius of the Waste Boundary
(CAP Content Section 6.B.a)
A depiction of surface water features - including wetlands, ponds, unnamed
tributaries, seeps, streams, lakes, and rivers - within a 0.5-mile radius of the ash
basin compliance boundary, along with permitted outfalls under the NPDES and
the SOC locations are shown on Figure 5-7 (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 from the waste boundary and is consistent with the drinking water well
and receptor surveys. Associated North Carolina surface water classifications for
Crutchfield Branch, two unnamed tributaries to Bowed Branch, and an unnamed
feeder stream to Mayo Reservoir are summarized in Section 5.3.1 and depicted
on Figure 5-7 (CAP Content Section 6.B.a.iii).
For groundwater corrective action to be implemented under Subchapter .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, TDS, thallium, total
hardness, and zinc.
Surface water samples were collected from locations within Crutchfield Branch
to confirm groundwater downgradient of the ash basin has not resulted in
surface water concentrations greater than 02B water quality standards. Surface
water locations sampled for groundwater discharge to surface water evaluation
are shown on Figure 5-7 (CAP Content Section 6.B.a.iv). Surface water samples
were collected, using DWR-approved protocols, to evaluate acute and chronic
water quality values. Surface water samples were also collected at background
locations (upgradient of the source area) upstream of the Plant and the ash basin.
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Analytical results were evaluated with respect to 02B water quality standards
and background data.
Comparisons of surface water data with the applicable USEPA National
Recommended Water Quality Criteria for Protection of Aquatic Life, Human
Health, and/or Water Supply (USEPA, (2017a); (2017b)) were conducted on
surface water samples from Crutchfield Branch. As stated by the USEPA, these
criteria are not a regulation, nor do they impose a legally -binding requirement.
Therefore, comparisons with these criteria are only for situational context. The
constituents that have corresponding USEPA criteria but do not have 02B criteria
are alkalinity, aluminum, antimony, iron, and manganese. All concentrations of
alkalinity, aluminum, antimony, and iron were either non -detect (i.e., antimony)
or concentrations were comparable to background concentrations. Analytical
results for manganese the two downstream Crutchfield Branch sample locations
are greater than the USEPA values and background concentrations. However,
the concentrations of manganese observed in Crutchfield Branch are greater than
the groundwater monitoring wells in the area downgradient of the ash basin;
therefore, the surface water concentrations of manganese are not attributable to
groundwater to surface water interaction.
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 Mayo groundwater
discharge to surface water and the evaluation of surface waters to evaluate
compliance with 15A NCAC 2B .0200 was submitted to NCDEQ on March 23,
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 I.
General findings of the evaluation of surface water quality conditions at Mayo
include:
• Groundwater migration from the ash basin source area has not resulted in
violations of the 02B surface water quality standards in Crutchfield
Branch.
• An engineered seep collection system captures flow from the toe drains
and directs flow back into the ash basin and wastewater treatment system.
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Surface Water — Future Conditions Evaluation
An evaluation of potential future groundwater migration to surface water was
conducted to identify areas where further evaluation might be warranted. For
areas of potential future groundwater migration to surface water, a mixing
model approach was used for the evaluation of future surface water quality
conditions. Flow and transport modeling results were used to determine where
groundwater migration from the ash basin might intersect surface water in the
future. Predictive groundwater modeling, using boron as a proxy for plume
migration, identified the area to the northeast of the ash basin (specifically
Crutchfield Branch) could potentially be influenced by future groundwater
migration. A groundwater to surface water mixing model approach was used to
determine the future potential surface water quality in Crutchfield Branch.
Constituents assessed in the predictive model include those that were identified
as constituents related to the ash basin in the 2017 CSA Update (SynTerra,
2017b). The full report for Mayo groundwater discharge to surface water under
future conditions can be found in Appendix I.
General findings of the evaluation of future effects on surface water from
groundwater discharge at Mayo include:
The surface water mixing model evaluation confirms that predicted future
resultant constituent concentrations in applicable surface waters are less
than 02B surface water standards. Therefore, the criterion for compliance
with 02B is met.
Seeps currently governed by the SOC that remain and are not
dispositioned 90 days after completion of decanting would be
characterized for determination of corrective action applicability. Where
applicable, and accounting for seep jurisdictional status, corrective action
planning would occur at that time.
6.2.2 Water Supply Wells
(CAP Content Section 6.B.b)
A total of 22 private water supply wells were identified within the 0.5-mile
radius of the ash basin compliance boundary (Figure 5-6). Most of these wells are
associated with residences located to the south and upgradient of the Site,
centered near Mullins Lane; residences located northwest and upgradient of the
Site, on the south/North Carolina side of the North CarolinaNirginia state line;
and residences located northwest and upgradient of the Site, on the
northNirginia side of the state line.
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No public or private drinking water wells or wellhead protection areas were
identified downgradient of the ash basin. No public supply wells were identified
within a 0.5-mile radius of the ash basin compliance boundary as discussed in
Section 5.4. This finding has been 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). 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 testing do not indicate effects
from the ash basin, water supply well owners identified within the 0.5-mile
radius from the ash basin compliance boundary were offered alternate
water supply in accordance with General Statute 130A-309.211(cl).
Of the 17 eligible connections, one did not respond to the offer. Duke
Energy installed 16 water filtration systems at surrounding occupied
residences, including one business. Duke Energy is providing ongoing
maintenance for these systems.
On August 30, 2018, Duke Energy provided completion documentation to
NCDEQ to fulfill the requirements of House Bill 630. NCDEQ provided
correspondence, dated October 12, 2018, to confirm that Duke Energy
satisfactorily completed the alternate water provisions under G.S. Section
130A-309.211(cl) at Mayo. Both documents are provided in Appendix D.
The private and public water supply well locations with reference to water
treatments systems installed, vacant parcels, and residential properties that
opted not to respond to the offer are indicated on Figure 5-6 (CAP Content
Section 6.B.b.i). As discussed in Section 5.0, all of the private water supply
wells are located either upgradient or side -gradient of the ash basin (in
separate drainage systems).
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6.2.2.2 Findings of Drinking Water Supply Well Surveys
(CAP Content Section 6.B.b.ii)
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. 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 Mayo ash basin compliance
boundary, have been reported to NCDEQ:
Drinking Water Well and Receptor Survey — Mayo Steam Electric Plant
(SynTerra, 2014a)
Supplement to Drinking Water Well and Receptor Survey —Mayo Steam
Electric Plant (SynTerra, 2014b)
• Update to Drinking Water Welland Receptor Survey —Mayo Steam
Electric Plant (SynTerra, 2016c)
A total of 22 private water supply wells within the 0.5-mile radius of the ash
basin compliance boundary were identified as part of the required well
survey. Most of these water supply wells are located to the south and
upgradient of the Site, centered near Mullins Lane; northwest and
upgradient of the Site, on the North Carolina side (south) of the North
Carolina/Virginia state line; and northwest and upgradient of the Site, on
the Virginia side (north) of the state line. Unoccupied rural properties are
located north of the Site and downgradient of the ash basin (Figure 5-6)
An Environmental Data Resources, Inc. (EDR, 2015) report for the nearby
Louisiana Pacific Corporation site west of US Highway 501 indicated that
Bethel Hill Baptist Church, located approximately 0.5 miles south and
upgradient of the Site at 201 Old US Hwy 501 (Roxboro, North Carolina),
maintains a public water supply provided by a groundwater well.
Table 6-10 (CAP Content 6.B.b.ii) provides tabulated results for the
NCDENR and Duke Energy sampling results as well as identified
groundwater constituent concentrations greater than 02L standards,
IMACs, and bedrock background values. A well -by -well summary and
evaluation of groundwater analytical data is presented in Table 6-10. The
evaluation compares bedrock background values as it is assumed area
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water supply wells are installed within the bedrock, which is typical for
water supply wells in the Piedmont.
The major findings from the water supply well evaluation include:
• All water supply wells are outside the boron plume as defined on the
boron plume maps for all flow zones (Figure 6-11a through
Figure 6-11c).
• All water supply wells to the south are upgradient of the ash basin
(Figure 5-4c).
• All water supply wells to the northwest are upgradient of the ash
basin (Figure 5-4c).
• Boron, the only COI exhibiting a discernable plume related to the ash
basin, was not detected above the laboratory reporting limit in any of
the water supply wells sampled (Table 6-10).
• Strontium was detected in three of the wells at concentrations greater
than background values. Each of these wells are located northwest of
the ash basin, upgradient of the ash basin and separated from the ash
basin by a hydraulic divide (Figure 5-5a). Additionally, no
discernable strontium plume associated with the ash basin was
identified. Therefore, the strontium in these wells is not attributed to
the Mayo ash basin.
Manganese was detected in one well at concentrations greater than
background values but is located south and upgradient of the ash
basin (Figure 5-4c). Additionally, no discernable manganese plume
associated with the ash basin was identified. Therefore, manganese
in this well is not attributed to the Mayo ash basin.
• Vanadium was detected in one well at a concentration greater than
background values but is located north of the ash basin and
separated by a natural hydraulic divide (Figure 5-5a). Additionally,
no discernable vanadium plume associated with the ash basin was
identified. Vanadium in this well is not attributed to the Mayo ash
basin.
A numerical capture zone analysis for the Mayo Site was conducted
to evaluate potential effects of upgradient water supply pumping
wells. The analysis for Mayo indicated that well capture zones from
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wells located to the northwest and southeast of Mayo are limited to
the immediate vicinity of the well head and do not extend toward the
ash basin. None of the particle tracks originating in the ash basin
moved into the well capture zones (SynTerra, 2017b).
This evaluation and the detailed evaluation results presented in the CSA
Update (SynTerra, 2017b) indicate no effect on water supply wells from the
Mayo ash basin. Furthermore, based on flow and transport modeling, no
future effects to water supply wells are predicted.
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 within and beyond the predicted area of potential
groundwater influence from the ash basin. Therefore, no future groundwater use
areas are anticipated downgradient of the basin.
It is anticipated that private and public properties within a 0.5-mile radius of the
ash basin compliance boundary will continue to rely on groundwater resources
for water supply for the foreseeable future; therefore, Duke Energy will provide
periodic maintenance of the provided water treatment systems for each property
that accepted the alternative water supply [(Figure 5-6) (CAP Content Section
6.B.c.i)].
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 Crutchfield Branch stream valley 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 Mayo
consistent with the CAP content guidance. The Human Health and Ecological Risk
Assessments conducted for the Mayo ash basin concluded:
1. there is no evidence of risks to on -Site or off -Site human receptors potentially
exposed to CCR constituents that may have migrated from the ash basin; and
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2. there is no evidence of risks to ecological receptors potentially exposed to CCR
constituents that may have migrated from the ash basin.
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 Mayo human health and
ecological risk assessment is included in Appendix E.
6.4 Evaluation of Remedial Alternatives
(CAP Content Section 6.D)
This section is not applicable for the Mayo ash basin. Analytical data obtained over one
year of quarterly monitoring indicate the Mayo ash basin is currently in compliance
with 02L groundwater quality standards; therefore, groundwater corrective action
under 15A NCAC 02L .0106 is not required at this time for the Mayo ash basin.
6.5 Proposed Remedial Alternatives Selected for the Ash Basin
(CAP Content Section 6.E)
This section is not applicable for the Mayo ash basin. Analytical data obtained over one
year of quarterly monitoring indicate the Mayo ash basin is currently in compliance
with 02L groundwater quality standards; therefore, groundwater corrective action
under 15A NCAC 02L .0106 is not required at this time for the Mayo ash basin.
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). Although groundwater concentrations do not exceed
the 02L standard of 700 µg/1 for boron at or beyond the ash basin compliance boundary,
Mayo could be approved for alternate standards given the lack of human health and
ecological risks at the Site.
6.5.1 Description of Proposed Remedial Alternative
(CAP Content Section 6.E.a)
This section is not applicable for the Mayo ash basin.
6.5.2 Design Details of Proposed Remedial Alternative
(CAP Content Section 6.E.b)
This section is not applicable for the Mayo ash basin.
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6.5.3 Monitored Natural Attenuation Requirements
(CAP Content Section 6.E.0
This section is not applicable for the Mayo ash basin.
6.5.4 Requirements for O2L.O1O6 Rule
(CAP Content Section 6.E.d)
This section is not applicable for the Mayo ash basin.
6.5.5 Sampling and Reporting
(CAP Content Section 6.E.e)
Sampling and analysis of groundwater and surface water associated with the
Mayo ash basin is conducted in accordance with an Interim Monitoring Plan
(IMP). As defined in NCDEQ correspondence, Facility Interim Monitoring Plans
Networks and Sampling Requirements (December 21, 2016; Appendix A), the IMP
was implemented to facilitate completion of the CSA and CAP for Mayo.
Implementation of the IMP commenced in the second quarter of 2017. Additional
modifications to the plan were approved by NCDEQ on April 4, 2019
(Appendix A). Analytical results from IMP sampling are submitted to NCDEQ
quarterly.
6.5.5.1 Confirmation Monitoring Plan
(CAP Content Section 6.E.e)
Sampling and analysis of groundwater and surface water associated with
the Mayo ash basin is conducted in accordance with an IMP. As defined in
NCDEQ correspondence, Facility Interim Monitoring Plans Networks and
Sampling Requirements (December 21, 2016), the IMP was implemented to
facilitate completion of the CSA and CAP for Mayo. Implementation of the
IMP commenced in the second quarter of 2017. Additional modifications to
the plan were approved by NCDEQ on April 4, 2019. Analytical results
from IMP sampling are submitted to NCDEQ quarterly.
An Effectiveness Monitoring Plan (EMP) is required by G.S. Section 130A-
309.211 (b)(1)(e) for evaluating the effectiveness of proposed corrective
action. Analytical data obtained over one year of quarterly monitoring
indicate the Mayo ash basin is currently in compliance with 02L
groundwater quality standards; therefore, groundwater corrective action
under 15A NCAC 02L .0106 is not required at this time for the Mayo ash
basin. Because corrective action is not required, an EMP is not required.
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The Mayo ash basin is in compliance with 02L at this time; therefore, Duke
Energy requests that the IMP be replaced by a Confirmation Monitoring
Plan (CMP). The CMP, presented in Appendix J, is designed to be
adaptable and target key areas where changes to groundwater conditions
are most likely to occur throughout the ash basin closure process. CMP key
areas for monitoring are based on the following considerations:
• Include background locations
• Include designated flow paths
• Within areas of observed or anticipated changing Site conditions,
and/or have increasing constituent concentration trends
• Monitor constituent plume stability and verify model simulation
CMP elements including well systems, locations, frequency, parameters,
schedule, and reporting are summarized below and outlined on Table 6-11.
Confirmation monitoring well locations are illustrated on Figure 6-12. The
CMP will be implemented within 30 days of CAP approval and will
continue until there is a total of three years of data confirming that COIs are
below applicable standards at or beyond the compliance boundary, at
which time a request for termination of CMP sampling will be filed with
NCDEQ. If applicable standards are not met, the CMP will continue and
transition to post -closure monitoring, if necessary.
After ash basin closure and following ash basin closure certification, a Post -
Closure Groundwater Monitoring Plan (PCMP) will be developed and
implemented 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.
A conceptual flow diagram for CMP and PCMP elements is depicted on
Figure 6-13.
Reporting and Schedule
(CAP Content Section 6.E.e.i)
Groundwater corrective action is not required for the Mayo ash basin;
therefore, "effectiveness" progress reports and schedule and a sampling
and analysis plan during remediation are not applicable.
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During basin closure, evaluation of Site conditions and constituent plume
stability would be based on quantitative rationale using statistical,
mathematical, modeling, or empirical evidence. Existing data from
historical monitoring would be used to provide baseline information.
Schedule and reporting of confirmation monitoring data, including plan
review and optimization, while the CMP is active, would include:
Annual Reporting Evaluation: The data collected as part of the CMP will
be evaluated annually. The evaluation will include a summary of annual
groundwater monitoring results, evaluation of statistical concentration
trends, comparison of observed concentrations to model predictions,
evaluation of 02L compliance, and recommendation for plan adjustments, if
applicable.
Results of the evaluation would be reported in annual monitoring reports
submitted to NCDEQ. The reports will include:
• Laboratory reports on electronic media,
• Tables summarizing the past year's monitoring events,
• Historical data tables,
• Figures showing sample locations,
• Figures showing the historical data versus time for the designated
monitoring locations and parameters with emphasis on those
constituents identified as part of the constituent management process
(Section 6.1.3),
• Statistical analysis (Mann -Kendall test) of data to determine if trends
are present,
• 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 changing Site
conditions, and
• Recommendations regarding adjustments to the CMP, if needed.
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Sampling and Evaluation
(CAP Content Section 6.E.e.ii)
The CMP is a comprehensive monitoring plan that integrates multiple
monitoring systems designed for key areas of the Site with unique
characteristics or monitoring requirements.
Groundwater Monitoring Systems
The Mayo CMP monitoring system will (1) monitor Site conditions, (2)
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.
Groundwater Monitoring Flow Paths - Trend Analysis
The CMP will provide adequate horizontal and vertical coverage to
monitor:
Changes in groundwater quality as Site conditions change (e.g., ash
basin closure commences and groundwater flow and transport
conditions respond)
• Transport rates
• Constituent plume stability
The monitoring network includes wells along primary groundwater flow
paths. Groundwater monitoring wells are located as indicated in
Figure 6-12 and described below:
1. Background locations
2. 500 feet downgradient of waste boundary (compliance boundary)
3. 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 will be installed in 10 wells along the primary
groundwater flow paths (Figure 6-12). Monitoring of changes in
groundwater quality on a real-time basis using multi -parameter sondes and
telemetry technology will allow continuous monitoring and evaluation of
geochemical conditions. Geochemical conditions, monitored using pH and
Eh, will be compared, as needed, to geochemical modeling results to
evaluate changes that could potentially affect the mobility (Ka) of reactive
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and variably -reactive COIs. The multi -parameter sondes also monitor water
levels which will be used to verify simulated changes to groundwater flow.
Groundwater quality and water level data will increase the response time to
implement contingencies if field parameters significantly deviate from
predicted responses. A contingency plan is included in Section 6.5.7 of this
CAP Update.
Plume stability evaluation will be based primarily on results of trend
analyses. Trend analyses will 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 could otherwise influence a time -series trend analysis.
Mann -Kendall trend tests will be conducted using data from CMP
(geochemically nonreactive, conservative constituents). For Mayo, boron
best depicts the areal extent of the plume and plume stability and physical
attenuation as described in the constituent management approach
(Section 6.1.3). The test would be performed in accordance with Statistical
Analysis of Groundwater Monitoring Data at RCRA Facilities — Unified Guidance
(USEPA, 2009).
Sampling Frequency
Sampling for the CMP will be semi-annually. Multiple years of quarterly
and semi-annual monitoring data are available for use in trend analysis and
to establish a baseline to evaluate corrective action performance. Therefore,
semi-annual sampling at locations defined in the CMP will provide
adequate analytical data to monitor plume stability. Quantitative
evaluations will determine if additional data is necessary (i.e., increased
sampling frequency) for refining statistical and empirical model
development. Additional monitoring described in the contingency plan will
be implemented if significant geochemical condition changes are identified
that could result in mobilization of reactive or variably -reactive COIs.
Sampling and Analysis Protocols
(CAP Content Section 6.E.e.ii)
CMP sampling and analysis protocol will be similar to the existing IMP and
could be adjusted in the future based on further analysis. Detailed protocols
are presented in the CMP document (Appendix J). Samples will be
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analyzed by a North Carolina -certified laboratory for the parameters listed
in Table 6-11 as summarized below. Laboratory detection limits for each
constituent are targeted to be at or less than applicable regulatory values
(i.e., 02L, IMAC, background).
Groundwater quality confirmation monitoring parameters:
Conservative constituent analyses of boron will be conducted to
monitor groundwater conditions using designated wells along the
groundwater flow paths. Boron was selected because it is non -
reactive to changing geochemical conditions and encompasses the
areal extent of the plume. Physical attenuation mechanisms of
dilution and dispersion will 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: 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 ORP
• Additional geochemical parameters: Cations and anions will 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.
6.5.6 Interim Activities Prior to Implementation
(CAP Content Section 6.E.f)
This section is not applicable for the Mayo ash basin.
6.5.7 Contingency Plan
(CAP Content Section 6.E.g)
This section is not applicable for the Mayo ash basin. Because no remediation
system will be installed, there is no remediation system that could have
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insufficient performance. However, Duke Energy has developed the contingency
plan described below that identifies conditions that trigger further evaluation.
6.5.7.1 Description of Contingency Plan
(CAP Content Section 6.E.g.i)
If evaluation of analytical data obtained and evaluated in accordance with
the CMP identify that a more active approach to groundwater corrective
action is potentially warranted, an evaluation will be conducted to
determine if additional data is needed to validate conditions (e.g., more
frequent sampling, additional parameters, additional monitoring
location(s), etc.) and determine if the Mayo ash basin CAP should be
updated to evaluate corrective action approaches and technologies.
6.5.7.2 Decision Metrics for Implementing Contingency
Plan
(CAP Content Section 6.E.g.ii)
Potential corrective approach evaluation is warranted if:
Changing groundwater quality conditions downgradient of the ash
basin represented by an increase of a COI concentration over four
consecutive monitoring events.
• Changing surface water quality conditions downgradient of the ash
basin represented by an increase of a COI concentration over four
consecutive monitoring events.
• Site conditions measurably different than predictive model
simulations, including geochemical condition changes which could
result in mobilization of reactive and variably -reactive COIs.
Potential remedial alternatives considered would be screened against the
following criteria outlined in 15A NCAC 02L .0106(i):
• 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 effects on the environment
and local community
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Corrective Action Plan Update December 2019
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• Technical and logistical feasibility
• Time required to initiate
• Predicted time required to meet remediation goals
• Cost
• Sustainability
• Community acceptance
6.6 Ash Basin Summary
Groundwater corrective action is not required by 02L for the Mayo ash basin because
there are no exceedances of ash basin -derived constituents in groundwater beyond the
compliance boundary. Multiple lines of evidence provided in this CAP Update indicate
that the groundwater plume originating from the ash basin, represented by boron, does
not currently, nor is it predicted to, extend beyond the compliance boundary.
Although active groundwater corrective action is not required, a CMP is proposed. The
CMP is designed to provide systematic evaluation of groundwater conditions at and
beyond the compliance boundary and to identify changing conditions that may warrant
attention. The CMP will begin within 30 days of CAP Update approval.
Duke Energy's preferred groundwater remediation approach assumes source control
through either basin closure -in -place or closure -by -excavation. Source control measures
are separate from the CAP Update and do not affect the preferred groundwater
remediation approach.
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Correction Action Plan Update December 2019
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7.0 PROFESSIONAL CERTIFICATION
(CAP Content Section 7)
Certification for the Submittal of a Corrective Action Plan
Responsible Party and/or Permittee: Qij-ke-EneMy Progress. LLC
Contact Person: Paul Draovitch
Address: 526 South Church Street
City: Charlotte State: NC Zip Code: 28202
Site Name: Mayo Steam E12ctric Plant
Address: 10660 Boston Road
City: Roxboro State: NC Zip Code: 27574
Groundwater Incident Number (if applicable): NA/Coal Ash Management Act CAP
I, Jerry A. Wylie, a Professional Geologist for SynTerra Corporation do hereby certify that the
information indicated below is enclosed as part of the required Corrective Action Plan (CAP) and that
to the best of my knowledge the data, assessments, conclusions, recommendations and other
associated materials are correct, complete and accurate.
Swom to and wbKftW
before me this
1 �+ofb.c
DARNELL B. DELLINGER
my Canrlli�� &#m 1TJam
(Affix Seal and Signature)
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. 41�
Proj Manager
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Correction Action Plan Update December 2019
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8.0 REFERENCES
(CAP Content Section 8)
AECOM. (2019). Ash Basin Closure Plan Report, Mayo Steam Electric Plant, December
31, 2019.
Amec Foster Wheeler. (2014). Natural Resources Technical Report.
Arcadis. (2019). Saturated Ash Thickness and Underlying Groundwater Boron
Concentrations - Allen, Belews Creek, Cliffside, Marshall, Mayo, and Roxboro
Sites, March 20, 2019.
Chu, J., Panzion, P., & Bradley, L. J. (2017). An Approach to Using Geochemical
Analysis to Evaluate the Potential Presence of Cal Ash Constituents in Drinking
Water. 2017 World of Coal Ash (WOCA). Lexington, KY.
Daniel, C. C., & Dahlen, P. R. (2002). Preliminary hydrogeologic assessment and study plan
for a regional ground -water resource investigation of the Blue Ridge and Piedmont
provinces of North Carolina. Raleigh, North Carolina: U.S. GEOLOGICAL SURVEY
Water -Resources Investigations Report 02-4105.
Domenico, P. A., & Schwartz, F. W. (1998). Physical and chemical hydrogeology (Vol. 44).
New York: Wylie.
Duke Energy/AECOM. (2018). Mayo Steam Electric Plant Ash Basin Closure Options,
Groundwater Modeling and Community Impact Analysis.
EDR. (2015, August 5). EDR Radius Map, Report with GeoCheck. Inquiry Number:
4375544.2s. (L. P. Corporation, Ed.) Envirommnental Data Resources, Inc.
EPRI. (1995). Coal ash disposal manual: Third edition. Palo Alto, CA: Electric Power
Research Institute, TR-104137.
EPRI. (2012). Groundwater Quality Signatures for Assessing Potential Impacts from Coal
Combustion Product Leachate. Palo Alto, CA: EPRI.
Exponent. (2018). Community Impact Analysis of Ash Basin Closure Options at the
Mayo Steam Electric Plant.
Freeze, R. A., & Cherry, J. A. (1979). Groundwater. Englewood Cliffs, NJ: Prentice -Hall.
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Correction Action Plan Update December 2019
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FRx, Inc., SynTerra, and Falta Environmental. (2019, December). Updated Groundwater
Flow and Transport Modeling Report for Mayo Steam Electric Plant, December 31,
2019.
Google Earth Pro. (2018a). Mayo Steam Electric Plant, February 25, 2006.
Google Earth Pro. (2018b). Mayo Steam Electric Plant, June 30, 2006.
Google Earth Pro. (2018c). Mayo Steam Electric Plant, June 17, 2008.
Google Earth Pro. (2018d). Mayo Steam Electric Plant, May 30, 2009.
Haley and Aldrich. (2015). Report on Risk Assessment Work Plan for CAMA Sites,
Duke Energy - November 2015.
HDR and SynTerra. (2017). Statistical Methods for Developing Reference Background
Concentrations for Groundwater and Soil at Coal Ash Facilities. HDR Engineering,
Inc. and SynTerra Corporation.
Izquierdo, M., & Querol, X. (2012). Leaching behaviour of elements from coal
combustion fly ash: An overview. International Journal of Coal Geology, 94. 54-56.
doi:10.1016/j . coal.2011.10.006
LeGrand, H. (1988). Region 21, Piedmont and Blue Ridge. In: J. Black, J. Rosenshein, P.
Seaber, ed. Geological Society of America, 0-2, (pp. 201-207).
LeGrand, H. (1989). A conceptual model of ground water settings in the Piedmont
region, in groundwater in the Piedmont. In: Daniel C., White, R., Stone, P., ed.
Ground Water in the Piedmont of the Eastern United States (pp. 317-327). Clemson,
SC: Clemson University.
LeGrand, H. (2004). A master conceptual model for hydrogeological site
characterization in the Piedmont and Mountain Region of North Carolina: A
guidance manual. North Carolina Department of Environment and Natural Resources,
Division of Water Quality, Groundwater Section, Raleigh, NC, 55.
Lipson, D., Kueper, B., & Gefell, M. (2005). Matrix diffusion -derived plume attenuation
in fractured bedrock. Groundwater, 43(pp. 30-39), No. 1.
NCDENR DWM. (2003). Guidelines for Performing Screening Level Ecological Risk
Assessments Within the North Carolina Division of Waste Management.
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Correction Action Plan Update December 2019
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NCDEQ. (2017). Technical Guidance for Risk -Based Environmental Remediation of
Sites.
NCDEQ. (2019). October 24, 2019 letter from Mr. J. Gregson, NCDEQ to Mr. P.
Draovitch, Duke Energy, titled, Approach to Managing Constituents of Interests
for Purposes of Corrective Action Plans.
SynTerra. (2014a). Drinking Water Well and Receptor Survey - Mayo Steam Electric Plant -
September 2014. Roxboro, NC.
SynTerra. (2014b). Supplement to Drinking Water Well and Receptor Survey - Mayo Steam
Electric Plant - November 2014. Roxboro, NC.
SynTerra. (2015a). Comprehensive site assessment report - Mayo Steam Electric Plant -
September 2, 2015. Roxboro, NC.
SynTerra. (2015a). Comprehensive Site Assessment Report - Mayo Steam Electric Plant -
September 2, 2015. Roxboro, NC.
SynTerra. (2015b). Corrective Action Plan - Part 1: Mayo Steam Electric Plant - December 1,
2015. Roxboro, NC.
SynTerra. (2015b). Corrective Action Plan Part 1 - Mayo Steam Electric Plant. September
2015.
SynTerra. (2016a). Corrective Action Plan Part 2 - Mayo Steam Electric Plant - February 29,
2016.
SynTerra. (2016b). Comprehensive Site Assessment Supplement 1 - Mayo Steam Electric
Plant.
SynTerra. (2016c). Update to drinking water well and receptor survey - Mayo Steam Electric
Plant - September 2016. Roxboro, NC.
SynTerra. (2017a). Up -to -Date Background Groundwater Data Technical Memorandum,
May 26, 2017.
SynTerra. (2017b). 2017 Comprehensive Site Assessment Update - October 31, 2017.
SynTerra. (2019a). Ash Basin Pumping Test Summary Report, January 28, 2019.
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SynTerra. (2019b). Surface Water Evaluation to Assess 15A NCAC 02B - Mayo Steam Electric
Plant, March 21, 2019.
SynTerra. (2019c). 2018 CAMA Annual Interim Monitoring Report - Mayo Steam Electric
Plant, April 30, 2019.
SynTerra. (2019d). Updated Background Threshold Values for Constituent Concentrations in
Soil and Groundwater, June 2019.
SynTerra. (2019e). Human Health and Ecological Risk Assessment Summary Update - Mayo
Steam Electric Plant, December 2019.
USEPA. (1989). Risk assessment guidance for superfund, volume 1: Human health evaluation
manual, part A. Washington, D.C.: Office of Emergency and Remedial Response,
U.S. Environmental Protection Agency. EPA/540/1-89/002.
USEPA. (1991). Risk Assessment Guidance for Superfun: Volume 1 - Human Health
Evaluation Manual (Part B, Development of Risk -based Preliminary Remediation
Goals). Office of Emergency and Remedial Response, EPA/540IR-92/003.
USEPA. (1998). Guidelines for ecological risk assessment. In Risk Assessment Forum.
Washington, DC: Office of Research and Development, Federal Register
63(93):26846-26924.
USEPA. (2009). Statistical Analysis of Groundwater Monitoring Data at RCRA Facilities
- Unified Guidance. EPA 530-R-09-007.
USEPA. (2011). Environmental Cleanup Best Management Practices: Effective Use of the
Project Life Cycle Conceptual Site Model. Washington. doi:12 pp, 2 MB, 2011, 542-17-
11-011
USEPA. (2012a). Method 1313 - Liquid -Solid partitioning as a function of extract pH
using parallel batch extraction procedure. Test methods for evaluating solid
waste: Physical/Chemical methods. SW0846, 3rd.
USEPA. (2012b). Method 1316 - Liquid -Solid Partitioning as a Function of Liquid -to -
Solid Ratio in Solid Materials Using a Parallel Batch Procedure. Test methods of
Evaluating Solid Waste: Physical/Chemical Methods. SW-846, 3rd.
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USEPA. (2017a). National recommended water quality criteria for aquatic life. Retrieved
October 20, 2017, from EPA: https://www.epa.gov/wgc/national-recommended-
water-quality-criteria-aquatic-life-criteria-table#altable
USEPA. (2017b). Secondary Drinking Water Regulations: Guidance for Nuisance Chemicals.
Retrieved October 2017, from EPA:
https://www.epa.gov/dwstandardsregulations/secondary-drinking-water-
standards-guidance-nuisance-chemicals
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