NC0038377_CSA October 2017 Report_20171101

2017 Comprehensive Site Assessment Update October 2017
Mayo Steam Electric Plant SynTerra

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Explanation of altered or items not initialed:

Item 1. The CSA was specifically designed to assess the coal ash
management areas of the facility. Sufficient information is
available to prepare the groundwater corrective action plan for
the ash management areas of the facility. Data limitations are
discussed in Section 11.0 of the CSA report. Continued
groundwater monitoring at the Site is planned.
Item 2. Imminent hazards to human health and the environment have
been evaluated. The NCDEQ data associated with nearby water
supply wells is provided herein and is being evaluated.
Item 5. The groundwater assessment plan for the CSA as approved by
NCDEQ was specifically developed to assess the coal ash
management areas of the facility for the purposes of developing
a corrective action plan for groundwater. Other areas of possible
contamination on the property, if noted, are anticipated to be
evaluated separately.

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EXECUTIVE SUMMARY
ES.1 Source Information
Duke Energy Progress, LLC (Duke Energy) owns and operates the Mayo Steam Electric
Plant (the Mayo Plant, Plant, or Site), located in Person County, near Roxboro, North
Carolina. The Comprehensive Site Assessment (CSA) update was conducted to refine
and expand the understanding of subsurface conditions and evaluate the extent of
impacts from historical management of coal ash. This CSA update contains an
assessment of site conditions based on a comprehensive interpretation of geologic and
sampling results from the initial site assessment and geologic and sampling results
obtained subsequent to the initial assessment.
The Mayo Plant began operations in 1983 and is presently in service. At the Site, there is
a single ash basin located northwest of the Plant. The ash basin, constructed with an
earthen dike, is approximately 140 acres and contains ash generated from the Plant’s
historic coal combustion. Discharge from the ash basin via Outfall 002 to Mayo Lake is
permitted by the NCDEQ Division of Water Resources (NCDEQ-DWR) under National
Pollutant Discharge Elimination System (NPDES) Permit NC0038377. A lined coal
combustion residuals (CCR) landfill is located to the west of the Plant on the west side
of US Highway 501 (Boston Road). No other areas of coal ash are known to exist at the
Site.
CCR was managed at the Plant’s on-site ash basin and transported via wet sluicing until
2013 when the Mayo Plant converted to a dry ash system in which 90 percent of
generated CCR was dry. After the conversion, wet sluicing was conducted only when
there was a shutdown of the dry fly ash transport system. Final system upgrades were
completed in October 2016 and all CCR collection is now dry. Prior to November 2014,
dry ash was hauled to and disposed of in the lined landfill located at the nearby
Roxboro Steam Electric Plant (near Semora, North Carolina). Since November 2014,
CCR from the Mayo Plant has been placed in a newly constructed on-site Industrial
landfill (monofill; Permit #7305) at the Mayo Site.
Assessment results indicate the thickness of CCR in the portion of the ash basin not
covered with water is approximately 13 to 66 feet. Assessment findings determined that
CCR accumulated in the ash basin is the source of impact to groundwater. The inferred
general extent of constituent migration from the ash basin based on evaluation of
constituent concentrations greater than both water quality standards and background is
shown on Figure ES-1. A detailed evaluation of constituent migration is included in the
CSA update report.
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ES.2 Initial Abatement and Emergency Response
Duke Energy has not conducted emergency responses because groundwater impacts
from the ash basin do not present an imminent and substantial threat to the
environment requiring emergency action. Regarding initial abatement, as previously
described, upgrades to the dry fly ash handling system were completed in October 2016
and all CCR collection is now dry. In preparation for ash basin closure, new retention
basins and wastewater treatment systems are being designed and constructed.
ES.3 Receptor Information
In accordance with North Carolina Department of Environmental Quality (NCDEQ)
direction, CSA receptor survey activities include listing and depicting all water supply
wells (public or private, including irrigation wells and unused wells) within a 0.5-mile
radius of the ash basin compliance boundary.
ES.3.1 Public Water Supply Wells
According to public records, Bethel Hill Baptist Church, located approximately
0.5 miles south (upgradient) of the Plant at 201 Old US Highway maintains a
public water supply provided by a groundwater well.

ES.3.2 Private Water Supply Wells
No private water supply wells are located within 0.5 mile downgradient of the
ash basin compliance boundary. Drinking water is obtained from approximately
20 private groundwater wells by residences within a 0.5-mile radius of the Mayo
Plant compliance boundary and located to the south and northwest the Plant,
upgradient of the predominant groundwater flow direction and upgradient of
the ash basin. In 2015, NCDEQ coordinated sampling of three of the private
water supply wells, and in 2017, Duke Energy collected samples from eight
additional private water supply wells. Available analytical data for the private
wells generally show detected concentrations below statistically derived
background concentrations and geochemistry (cation/anion distribution) not
attributable to CCR impacts. The supply well data should be interpreted
cautiously because well construction and equipment (e.g., galvanized piping,
pump components) may influence analytical results. The water supply wells are,
without exception, located upgradient of the Mayo Plant ash basin, and
modeling has demonstrated that well capture zones are limited to the immediate
vicinity of the well head and do not extend toward the ash basin.
The land directly downgradient of the ash basin and the Duke property line is
undeveloped; therefore, no water supply wells are located north and
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downgradient of the groundwater plume within the survey radius. The updated
Corrective Action Plan (CAP) will address any potential future risk to future
groundwater receptors.

ES.3.3 Surface Water Bodies
The Site is located in the Roanoke River Basin. Although the Site is located near
Mayo Lake, groundwater influenced by the ash basin primarily flows toward
and discharges to the Crutchfield Branch stream valley, a small stream located
north of the ash basin and near the northern property boundary. There is no
surface water intake located in Crutchfield Branch.
ES.3.4 Human and Ecological Receptors
A baseline human health and ecological risk assessment was performed in 2016
as a component of the CAP, Part 2 (SynTerra, 2016a), concluding that no
unacceptable risks to humans resulted from hypothetical exposure to
constituents detected in the ash basin and south creek (upstream) areas.
Based on review and analysis of groundwater and surface water data collected
since completing the human health and ecological risk assessment in 2016, there
is no evidence of potential risks to humans and wildlife at the Mayo Site. This
update to the human health and ecological risk assessment supports a risk
classification of “Low”.
ES.3.5 Land Use
Land use surrounding the Mayo Plant includes rural, rural residential,
agricultural, and forest land. Duke Energy-owned and maintained land borders
the Mayo Plant to the west, east (Mayo Lake), and north (with the exception of
two undeveloped parcels). A small residential area borders the Site to the south-
southwest. Property within 500 feet of the Mayo Plant compliance boundary is
owned by Duke Energy with the exception of the undeveloped parcels located
due north of the northern Plant boundary along Mayo Lake Road. No change in
land use surrounding the Mayo Site is currently anticipated.
ES.4 Sampling/Investigation Results
The comprehensive site assessment included evaluations of the hydrogeological and
geochemical properties of soil and groundwater at multiple depths and distances from
the ash basin.
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ES.4.1 Background Concentration Determinations
Naturally occurring background concentrations were determined using
statistical analysis for both soil and groundwater. Statistical determinations of
provisional background threshold values (PBTVs) were performed in strict
accordance with the revised Statistical Methods for Developing Reference Background
Concentrations for Groundwater and Soil at Coal Ash Facilities (statistical methods
document) (HDR and SynTerra, 2017). The current background monitoring well
network consists of wells installed within three flow zones – surficial, transition
zone, and fractured bedrock. Background datasets for each flow system used to
statistically determine naturally occurring concentrations of inorganic
constituents in soil and groundwater are provided herein. As of September 1,
2017, NCDEQ approved a number of the statistically derived background values;
however, others are still under evaluation and thus considered preliminary at
this time. Background results may be greater than the PBTVs due to the limited
valid dataset currently available. The statistically derived background threshold
values will continue to be adjusted as additional data becomes available.
ES.4.2 Nature and Extent of Contamination
Site-specific groundwater constituents of interest (COIs) were developed by
evaluating groundwater sampling results with respect to 2L/IMAC and PBTVs,
and additional regulatory input/requirements. The distribution of constituents in
relation to the ash basin, co-occurrence with CCR indicator constituents such as
boron, and likely migration directions based on groundwater flow direction are
considered in determination of groundwater COIs.
The following list of groundwater COIs has been developed for Mayo:
Arsenic Manganese
Barium Molybdenum
Boron pH
Chromium (hexavalent) Strontium
Chromium (total) Sulfate
Cobalt Total Dissolved Solids (TDS)
Iron Vanadium

Wells monitoring the surficial, transition zone, and bedrock flow units were
installed beneath the ash basin. Wells completed in the saprolite or transition
zone beneath the ash basin have PBTV and 2L exceedances for arsenic, barium,
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boron, cobalt, iron, manganese, molybdenum, strontium, TDS and vanadium (a
number of which only occur in the transition zone). For the most recent
monitoring event (March-April 2017), bedrock monitoring wells installed within
the ash basin indicate only strontium is detected greater than the PBTV. There is
no 2L standard for strontium. The remaining constituents were not detected in
the bedrock below the ash basin at concentrations greater than PBTV and 2L.
At Mayo, boron, manganese, and strontium are key indicators of CCR
groundwater impacts. Boron is detected at concentrations greater than the 2L
standard beneath and downgradient (north-northeast) of the ash basin. Boron is
not detected in background groundwater. Manganese and strontium are
detected at concentrations greater than PBTV.
The area farthest downgradient at which boron, manganese, and strontium are
detected at a concentration greater than applicable 2L and PBTVs is interpreted
as the leading edge of the CCR-derived plume moving downgradient from the
source area. For the surficial flow unit, boron, manganese, and strontium were
detected in monitoring wells screened in Crutchfield Branch alluvium
downgradient of the ash basin. In the transition zone and bedrock flow units,
boron and strontium are detected above PBTVs in downgradient wells closest to
the northern property line. Manganese was detected above PBTVs in a bedrock
well near the compliance boundary. Boron was not detected in groundwater
from wells located approximately 1,000 feet downgradient of the property. The
leading edge of the transition zone and bedrock plume is interpreted to be
at/near the northern property line.
The surficial and transition zone flow units at Mayo, though impacted, are not
vertically extensive. Impact to the bedrock flow unit is present in the upper 50 to
75 feet of fractured bedrock. The vertical extent of the plume is represented by
groundwater concentrations in bedrock wells beneath and downgradient of the
ash basin.
ES.4.3 Maximum Contaminant Concentrations (Source
Information)
The source area at Mayo Plant includes CCR material and pore water
accumulated in the ash basin. Ash pore water samples collected from wells
installed within the ash basins and screened in the ash layers have been
monitored since 2015. The concentrations of detected constituents have been
relatively stable with minor fluctuations. The ash basin is a permitted
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wastewater system; therefore, comparison of pore water within the wastewater
treatment residuals (ash) to 2B or 2L/IMAC is not required.
Soil samples collected below the ash/soil interface from three locations within the
ash basin indicate only arsenic concentrations in one soil sample both exceeded
the calculated soil PBTV and the NCDEQ Preliminary Soil Remediation Goals
(PSRG) Protection of Groundwater (POG) value. Several other constituents
exceeded the PBTV only.
ES.4.4 Site Geology and Hydrogeology
The subsurface at the Mayo Site is comprised of a surficial unit (soil, fill and
reworked soil, alluvium, and saprolite), a transition zone, and fractured bedrock.
The transition zone is comprised of partially weathered rock that is gradational
between saprolite and competent bedrock. The bedrock is dominantly granitoid
gneiss and mica gneiss with minor mica schist and phyllite. Shallow bedrock is
fractured; however, only mildly productive fractures (providing water to wells)
were observed within the top 50 – 75 feet of bedrock. The majority of fractures
are relatively small (e.g., close and tight) and appear to be limited in connectivity
between borings. Yields from pumping or packer testing are low. Groundwater
exists under unconfined or water table conditions throughout the Site. For the
most part, saturated conditions are limited to secondary fractures within the
underlying bedrock. Saturated conditions in saprolite and the transition zone are
limited and sporadic across the Site.
The hydrogeologic characteristics of the ash basin environment are the primary
control mechanisms on groundwater flow and constituent transport. The basin
acts as a bowl-like feature toward which groundwater flows from the northwest,
west, south, and east. Groundwater flows from the highest topographic portion
of the Site (near the Plant entrance road) to the north and northeast. The flow of
ponded water within the ash basin is controlled laterally by groundwater flow
that enters the basin and is controlled downgradient (north-northeast) by the ash
basin dam and the National Pollutant Discharge Elimination System (NPDES)
outfall/discharge (east side of basin). The head created by the impounded water
in the ash basin creates a slight mounding effect that influences the groundwater
flow direction in the immediate vicinity of the basin. East of the ash basin, there
is a groundwater divide that separates the Crutchfield Branch flow regime from
the Mayo Lake flow regime.
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The stream valley in which the ash basin was constructed is a distinct slope-
aquifer system in which flow of groundwater into the ash basin and out of the
ash basin is restricted to the local flow regime. Groundwater and surface water
flow from the ash basin is funneled into the small valley formed by Crutchfield
Branch and flows north-northeast from the ash basin into the Crutchfield Branch
valley, which flows north of the property and into Mayo Creek.
ES.5 Conclusions and Recommendations
The investigation described in the CSA presents the results of the assessments required
by the Coal Ash Management Act (CAMA) and 2L. The ash basin CCR material pore
water was determined to be a source of the groundwater contamination. The
assessment investigated the Site hydrogeology, determined the direction of
groundwater flow from the ash basin, and determined the horizontal and vertical extent
of impacts to groundwater and soil sufficient to proceed with preparation of a CAP.
Only one soil sample from below the ash basin contained arsenic at a concentration
greater than PBTV and the PSRG POG value. Strontium was also detected in the same
soil sample at concentrations greater than the PBTV. Arsenic is not detected in
groundwater at concentrations greater than the 2L standard beyond the ash basin waste
boundary. There is no PSRG POG for strontium. Strontium is present above the
groundwater PBTVs beyond the compliance boundary in the surficial aquifer only. No
other COIs were detected in soil beneath the ash basin at concentrations greater than
both the PBTV and POG. Shallow soil impacts are anticipated to be addressed through
basin closure and the CAP.
Boron is the primary constituent detected in groundwater at concentrations greater than
background and the 2L standard near or beyond the compliance boundary. Manganese
and strontium above the respective PBTVs also appear to be indicators of impact to
groundwater. The interpreted extent of boron concentrations greater than the 2L
standard is near the compliance boundary in the surficial and transition flow zones. The
boron concentration is less than 2L standard in the bedrock flow unit near the
compliance boundary. The interpreted extent of manganese concentrations greater than
the PBTV and 2L standard is beyond the compliance boundary in the surficial and
bedrock flow zones; however, the manganese concentration is less than the PBTV
within the transition zone at the compliance boundary. The interpreted extent of
strontium concentrations greater than the PBTV extends beyond the compliance
boundary only within the surficial flow zone.
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Mayo Plant’s ash basin is currently designated as “Intermediate” risk under CAMA,
meaning that closure of the ash basin is required by 2024. The updated evaluation of
risks has determined no imminent risk to human health or the environment due to
groundwater, surface water, or sediment impacts. The private water supply wells
located near the plant are not located in the ash basin groundwater flow path. A "Low"
risk classification and closure via a cap-in-place scenario are considered viable.
A preliminary evaluation of groundwater corrective action alternatives is included in
this CSA to provide insight into the CAP preparation process. For Mayo, the primary
source control (closure) methods anticipated to be evaluated in the CAP are:
Dewater the ash within the basin and cap the residuals with a low permeability
engineered cover system to minimize infiltration;
Excavate the ash to remove the source of the COIs from the groundwater flow
system; and
Potentially some combination of the above.
The source control (closure) options will be evaluated in the CAP to determine the most
technically and economically feasible means of removing or controlling the ash and ash
pore water as a source to the groundwater flow system. The evaluation will include
predictive groundwater modeling to evaluate the cost-benefit associated with various
options. For basin closure, preliminary modeling indicates ash dewatering and
reduction of the amount of water migrating from the basin to groundwater will have
the greatest positive impact on groundwater and surface water quality downgradient of
the ash basin. A well-designed capping system can be expected to minimize ongoing
migration to groundwater after dewatering.

In addition to source control measures, the CAP will evaluate measures to address
groundwater conditions associated with the ash basin. Groundwater corrective action
by monitored natural attenuation (MNA) is anticipated to be a remedy further
evaluated in the CAP. As warranted, a number of viable groundwater remediation
technologies such as phytoremediation, groundwater extraction, or hydraulic barriers
may be evaluated based upon short-term and long-term effectiveness,
implementability, and cost. Results of the evaluation, including groundwater fate and
transport modeling, and geochemical modeling, will be used for remedy selection in the
CAP.
148 RIVER STREET, SUITE 220
GREENVILLE, SOUTH CAROLINA 29601
PHONE [phone]
www.synterracorp.com
PROJECT MANAGER:
LAYOUT:
DRAWN BY:
JERRY WYLIE
DATE:ADAM FEIGL
ES1 - MAYO
10/09/2017
10/29/2017 2:37 PM P:\Duke Energy Progress.1026\00 GIS BASE DATA\Mayo\Map_Docs\CSA_Supplement_2\3D SCM\Mayo_3D_ES1.dwg
FIGURE ES-1
APPROXIMATE EXTENT OF IMPACTS
MAYO STEAM ELECTRIC PLANT
DUKE ENERGY PROGRESS, LLC
ROXBORO, NORTH CAROLINA
PROGRESS
MAYO
LAKE
MAYO LAKE ROAD
BOSTON RD. (HIGHWAY 501)1981 C & D
LANDFILL
MAYO STEAM
ELECTRIC PLANT
SURFICIAL
TRANSITION ZONE
VISUAL AID ONLY -
DEPICTION NOT TO SCALE
NORTH
BEDROCK
CRUTCHFIELD
BRANCH
ASH
BASIN
ASH BASIN WASTE BOUNDARY
GENERALIZED GROUNDWATER FLOW DIRECTION
APPROXIMATE LANDFILL WASTE BOUNDARY
NOTE:
1.OCTOBER, 2016 AERIAL PHOTOGRAPHY OBTAINED FROM GOOGLE EARTH PRO ON
SEPTEMBER 27, 2017, DATED JUNE 13, 2016.
2.STREAM FROM WSP SURVEY, 2014.
3.GENERALIZED GROUNDWATER FLOW DIRECTION BASED ON APRIL 10, 2017 WATER LEVEL
DATA.
4.PROPERTY BOUNDARY PROVIDED BY DUKE ENERGY.
5.GENERALIZED AREAL EXTENT OF MIGRATION REPRESENTED BY NCAC 02L EXCEEDANCES.
STREAM WITH FLOW DIRECTION
RESIDENTIAL UNIT
DUKE ENERGY PROPERTY BOUNDARY
LEGEND
AREA OF CONCENTRATION IN GROUNDWATER
ABOVE NC2L (SEE NOTE 5)
INFERRED AREA OF CONCENTRATION IN GROUNDWATER
ABOVE NC2L (SEE NOTE 5)
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TABLE OF CONTENTS
SECTION PAGE
CERTIFICATION PAGE
ES.1 SOURCE INFORMATION ....................................................................................... ES-1
ES.2 INITIAL ABATEMENT AND EMERGENCY RESPONSE ................................ ES-2
ES.3 RECEPTOR INFORMATION .................................................................................. ES-2
ES.3.1 Public Water Supply Wells ................................................................................... ES-2
ES.3.2 Private Water Supply Wells ................................................................................. ES-2
ES.3.3 Surface Water Bodies ............................................................................................. ES-3
ES.3.4 Human and Ecological Receptors ........................................................................ ES-3
ES.3.5 Land Use .................................................................................................................. ES-3
ES.4 SAMPLING/INVESTIGATION RESULTS .......................................................... ES-3
ES.4.1 Background Concentration Determinations ...................................................... ES-4
ES.4.2 Nature and Extent of Contamination .................................................................. ES-4
ES.4.3 Maximum Contaminant Concentrations (Source Information) ...................... ES-5
ES.4.4 Site Geology and Hydrogeology ......................................................................... ES-6
ES.5 CONCLUSIONS AND RECOMMENDATIONS ................................................ ES-7
1.0 INTRODUCTION ......................................................................................................... 1-1
1.1 Purpose of Comprehensive Site Assessment ........................................................ 1-1
1.2 Regulatory Background ........................................................................................... 1-2
Notice of Regulatory Requirements (NORR) ............................................... 1-2 1.2.1
Coal Ash Management Act Requirements .................................................... 1-3 1.2.2
1.3 Approach to Comprehensive Site Assessment ..................................................... 1-4
NORR Guidance ................................................................................................ 1-4 1.3.1
USEPA Monitored Natural Attenuation Tiered Approach ........................ 1-5 1.3.2
ASTM Conceptual Site Model Guidance ....................................................... 1-5 1.3.3
1.4 Technical Objectives ................................................................................................. 1-5
1.5 Previous Submittals .................................................................................................. 1-6

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TABLE OF CONTENTS
SECTION PAGE
2.0 SITE HISTORY AND DESCRIPTION ..................................................................... 2-1
2.1 Site Description, Ownership and Use History...................................................... 2-1
2.2 Geographic Setting, Surrounding Land Use, Surface Water Classification ..... 2-2
2.3 CAMA-related Source Areas ................................................................................... 2-5
2.4 Other Primary and Secondary Sources .................................................................. 2-6
2.5 Summary of Permitted Activities ........................................................................... 2-6
2.6 History of Site Groundwater Monitoring .............................................................. 2-8
Ash Basin Voluntary Groundwater Monitoring .......................................... 2-8 2.6.1
Ash Basin NPDES Groundwater Monitoring ............................................... 2-8 2.6.2
Ash Basin CAMA Groundwater Monitoring................................................ 2-9 2.6.3
Landfill Groundwater Monitoring ................................................................. 2-9 2.6.4
2.7 Summary of Assessment Activities ........................................................................ 2-9
2.8 Summary of Initial Abatement, Source Removal or
Other Corrective Action .......................................................................................... 2-9
3.0 SOURCE CHARACTERISTICS ................................................................................. 3-1
3.1 Coal Combustion and Ash Handling System ....................................................... 3-1
3.2 General Physical and Chemical Properties of Ash............................................... 3-2
3.3 Site-Specific Coal Ash Data ..................................................................................... 3-4
4.0 RECEPTOR INFORMATION ..................................................................................... 4-1
4.1 Summary of Receptor Survey Activities................................................................ 4-2
4.2 Summary of Receptor Survey Findings ................................................................. 4-3
Public Water Supply Wells .............................................................................. 4-4 4.2.1
Private Water Supply Wells ............................................................................ 4-4 4.2.2
4.3 Private Water Well Sampling .................................................................................. 4-6
4.4 Numerical Well Capture Zone Analysis ............................................................... 4-8
4.5 Surface Water Receptors .......................................................................................... 4-8
5.0 REGIONAL GEOLOGY AND HYDROGEOLOGY ............................................... 5-1
5.1 Regional Geology ...................................................................................................... 5-1
5.2 Regional Hydrogeology ........................................................................................... 5-3

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TABLE OF CONTENTS
SECTION PAGE
6.0 SITE GEOLOGY AND HYDROGEOLOGY ............................................................ 6-1
6.1 Site Geology ............................................................................................................... 6-2
Soil Classification .............................................................................................. 6-2 6.1.1
Rock Lithology .................................................................................................. 6-3 6.1.2
Structural Geology ............................................................................................ 6-5 6.1.3
Soil and Rock Mineralogy and Chemistry .................................................... 6-6 6.1.4
6.2 Site Hydrogeology .................................................................................................... 6-6
Hydrostratigraphic Layer Development ....................................................... 6-7 6.2.1
Hydrostratigraphic Layer Properties ............................................................. 6-7 6.2.2
6.3 Groundwater Flow Direction .................................................................................. 6-8
6.4 Hydraulic Gradient ................................................................................................. 6-10
6.5 Hydraulic Conductivity ......................................................................................... 6-11
6.6 Groundwater Velocity ............................................................................................ 6-11
6.7 Contaminant Velocity ............................................................................................. 6-12
6.8 Slug Test and Aquifer Test Results ...................................................................... 6-13
6.9 Fracture Trace Study Results ................................................................................. 6-14
7.0 SOIL SAMPLING RESULTS ...................................................................................... 7-1
7.1 Background Soil Data ............................................................................................... 7-1
7.2 Facility Soil Data ....................................................................................................... 7-2
8.0 SEDIMENT RESULTS ................................................................................................. 8-1
8.1 Sediment/Surface Soil Associated with AOWs .................................................... 8-1
8.2 Sediment in Major Water Bodies ............................................................................ 8-2
9.0 SURFACE WATER RESULTS .................................................................................... 9-1
9.1 Discussion of Results for Constituents Without Established 2B Standards ..... 9-3
9.2 Comparison of Exceedances of 2B Standards ....................................................... 9-3
9.3 Discussion of Surface Water Results ...................................................................... 9-4

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TABLE OF CONTENTS
SECTION PAGE
10.0 GROUNDWATER SAMPLING RESULTS ............................................................ 10-1
10.1 Background Groundwater Concentrations ......................................................... 10-2
Background Dataset Statistical Analysis ..................................................... 10-3 10.1.1
Piper Diagrams (Comparison to Background) ........................................... 10-6 10.1.2
10.2 Downgradient Groundwater Concentrations..................................................... 10-6
Monitoring Wells Beneath Ash Basin .......................................................... 10-6 10.2.1
Monitoring Wells Downgradient of Ash Basin .......................................... 10-8 10.2.2
Monitoring Wells in Separate Flow Regime ............................................... 10-9 10.2.3
Monitoring Wells East of Rail Line (Separate Flow Regime) ................... 10-9 10.2.4
Piper Diagrams (Comparison to Downgradient/ Separate 10.2.5
Flow Regime) ................................................................................................. 10-10
10.3 Site-Specific Exceedances (Groundwater COIs) ............................................... 10-11
Provisional Background Threshold Values (PBTVs) ............................... 10-11 10.3.1
Applicable Standards ................................................................................... 10-11 10.3.2
Additional Requirements ............................................................................. 10-12 10.3.3
Mayo Plant COIs ........................................................................................... 10-13 10.3.4
11.0 HYDROGEOLOGICAL INVESTIGATION .......................................................... 11-1
11.1 Plume Physical and Chemical Characterization ................................................ 11-1
Plume Physical Characterization .................................................................. 11-1 11.1.1
Plume Chemical Characterization ................................................................ 11-4 11.1.2
11.2 Pending Investigation(s) ...................................................................................... 11-17
12.0 RISK ASSESSMENT .................................................................................................. 12-1
12.1 Human Health Screening Summary .................................................................... 12-2
12.2 Ecological Screening Summary ............................................................................. 12-4
12.3 Private Well Receptor Assessment Update ......................................................... 12-4
12.4 Risk Assessment Update Summary ..................................................................... 12-5

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TABLE OF CONTENTS
SECTION PAGE
13.0 GROUNDWATER MODELING RESULTS........................................................... 13-1
13.1 Summary of Fate and Transport Model Results................................................. 13-2
Flow Model Construction .............................................................................. 13-3 13.1.1
Transport Model Construction ..................................................................... 13-8 13.1.2
Summary of Flow and Transport Modeling Results To Date ................ 13-11 13.1.3
13.2 Summary of Geochemical Model Results ......................................................... 13-13
Model Construction ...................................................................................... 13-13 13.2.1
Summary of Geochemical Model Results To Date................................... 13-16 13.2.2
13.3 Summary of Groundwater to Surface Water Evaluation ................................ 13-18
14.0 SITE ASSESSMENT RESULTS ................................................................................ 14-1
14.1 Nature and Extent of Contamination ................................................................... 14-1
14.2 Maximum COI Concentrations ............................................................................. 14-4
14.3 Contaminant Migration and Potentially Affected Receptors ........................... 14-6
15.0 CONCLUSIONS AND RECOMMENDATIONS ................................................. 15-1
15.1 Overview of Site Conditions at Specific Source Areas ...................................... 15-1
15.2 Revised Site Conceptual Model ............................................................................ 15-1
15.3 Interim Monitoring Program ................................................................................. 15-3
IMP Implementation ....................................................................................... 15-3 15.3.1
IMP Reporting ................................................................................................. 15-4 15.3.2
15.4 Preliminary Evaluation of Corrective Action Alternatives............................... 15-4
CAP Preparation Process ............................................................................... 15-5 15.4.1
Summary .......................................................................................................... 15-7 15.4.2
16.0 REFERENCES ............................................................................................................... 16-1

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LIST OF FIGURES
Executive Summary
Figure ES-1 Approximate Extent of Impact

1.0 Introduction
Figure 1-1 Site Location Map

2.0 Site History and Description
Figure 2-1 Mayo Plant Vicinity Map
Figure 2-2 1968 USGS Topographic Map
Figure 2-3 1948 Aerial Photograph
Figure 2-4 1981 Aerial Photograph
Figure 2-5 1993 Aerial Photograph
Figure 2-6 2008 Aerial Photograph
Figure 2-7 NPDES Flow Diagram
Figure 2-8 Site Layout Map

3.0 Source Characteristics
Figure 3-1 Known Sample of Ash for Comparison
Figure 3-2 Elemental Composition for Bottom Ash, Fly Ash, Shale, and
Volcanic Ash
Figure 3-3 Coal Ash TCLP Leachate Concentration Ranges Compared to
Regulatory Limits

4.0 Receptor Information
Figure 4-1 USGS Map with Water Supply Wells
Figure 4-2 Water Supply Well Locations and Data
Figure 4-3 Water Supply Well Capture Zone

5.0 Regional Geology and Hydrogeology
Figure 5-1 Regional Geologic Map
Figure 5-2 Piedmont Slope-Aquifer System

6.0 Site Geology and Hydrogeology
Figure 6-1 Generalized Geologic Map
Figure 6-2 General Cross Section A-A'
Figure 6-3 General Cross Section B-B'
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LIST OF FIGURES (CONTINUED)
Figure 6-4 Generalized Water Level Map - November 2016
Figure 6-5 Surficial Water Level Map - November 2016
Figure 6-6 Transition Zone Water Level Map - November 2016
Figure 6-7 Bedrock Water Level Map - November 2016
Figure 6-8 Generalized Water Level Map - June 2017
Figure 6-9 Surficial Water Level Map - June 2017
Figure 6-10 Transition Zone Water Level Map - June 2017
Figure 6-11 Bedrock Water Level Map - June 2017
Figure 6-12 Potential Vertical Gradient Between Shallow and Deep Zones
Figure 6-13 Fracture Trace Analysis

7.0 Soil Sampling Results
Figure 7-1 Potential Secondary Source Soil Analytical Results

9.0 Surface Water Results
Figure 9-1 Piper Diagram - Surface Water and AOWs

10.0 Groundwater Sampling Results
Figure 10-1 Piper Diagram – Surficial Groundwater
Figure 10-2 Piper Diagram - Transition Zone Groundwater
Figure 10-3 Piper Diagram – Bedrock Groundwater

11.0 Hydrogeological Investigation
Figure 11-1 Isoconcentration Map - Arsenic in Surficial Groundwater
Figure 11-2 Isoconcentration Map - Arsenic in Transition Zone Groundwater
Figure 11-3 Isoconcentration Map - Arsenic in Bedrock Groundwater
Figure 11-4 Isoconcentration Map - Barium in Surficial Groundwater
Figure 11-5 Isoconcentration Map - Barium in Transition Zone Groundwater
Figure 11-6 Isoconcentration Map - Barium in Bedrock Groundwater
Figure 11-7 Isoconcentration Map - Boron in Surficial Groundwater
Figure 11-8 Isoconcentration Map - Boron in Transition Zone Groundwater
Figure 11-9 Isoconcentration Map - Boron in Bedrock Groundwater
Figure 11-10 Isoconcentration Map - Calcium in Surficial Groundwater
Figure 11-11 Isoconcentration Map - Calcium in Transition Zone Groundwater
Figure 11-12 Isoconcentration Map - Calcium in Bedrock Groundwater
Figure 11-13 Isoconcentration Map - Chloride in Surficial Groundwater
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LIST OF FIGURES (CONTINUED)
Figure 11-14 Isoconcentration Map - Chloride in Transition Zone Groundwater
Figure 11-15 Isoconcentration Map - Chloride in Bedrock Groundwater
Figure 11-16 Isoconcentration Map - Chromium (VI) and Chromium (Total) in
Surficial Groundwater
Figure 11-17 Isoconcentration Map - Chromium (Total and Hexavalent) in
Transition Zone Groundwater
Figure 11-18 Isoconcentration Map - Chromium (Total and Hexavalent) in
Bedrock Groundwater
Figure 11-19 Isoconcentration Map - Cobalt in Surficial Groundwater
Figure 11-20 Isoconcentration Map - Cobalt in Transition Zone Groundwater
Figure 11-21 Isoconcentration Map - Cobalt in Bedrock Groundwater
Figure 11-22 Isoconcentration Map - Iron in Surficial Groundwater
Figure 11-23 Isoconcentration Map - Iron in Transition Zone Groundwater
Figure 11-24 Isoconcentration Map - Iron in Bedrock Groundwater
Figure 11-25 Isoconcentration Map - Manganese in Surficial Groundwater
Figure 11-26 Isoconcentration Map - Manganese in Transition Zone
Groundwater
Figure 11-27 Isoconcentration Map - Manganese in Bedrock Groundwater
Figure 11-28 Isoconcentration Map - Molybdenum in Surficial Groundwater
Figure 11-29 Isoconcentration Map - Molybdenum in Transition Zone
Groundwater
Figure 11-30 Isoconcentration Map - Molybdenum in Bedrock Groundwater
Figure 11-31 Isoconcentration Map - Strontium in Surficial Groundwater
Figure 11-32 Isoconcentration Map - Strontium in Transition Zone Groundwater
Figure 11-33 Isoconcentration Map - Strontium in Bedrock Groundwater
Figure 11-34 Isoconcentration Map - Sulfate in Surficial Groundwater
Figure 11-35 Isoconcentration Map - Sulfate in Transition Zone Groundwater
Figure 11-36 Isoconcentration Map - Sulfate in Bedrock Groundwater
Figure 11-37 Isoconcentration Map - Total Dissolved Solids in Surficial
Groundwater
Figure 11-38 Isoconcentration Map - Total Dissolved Solids in Transition Zone
Groundwater
Figure 11-39 Isoconcentration Map - Total Dissolved Solids in Bedrock
Groundwater
Figure 11-40 Isoconcentration Map - Vanadium in Surficial Groundwater
Figure 11-41 Isoconcentration Map - Vanadium in Transition Zone Groundwater
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LIST OF FIGURES (CONTINUED)
Figure 11-42 Isoconcentration Map - Vanadium in Bedrock Groundwater
Figure 11-43 Isoconcentration Map - pH in Surficial Groundwater
Figure 11-44 Isoconcentration Map - pH in Transition Zone Groundwater
Figure 11-45 Isoconcentration Map - pH in Bedrock Groundwater
Figure 11-46 Concentration Versus Distance From Source pH, Boron, Strontium,
Sulfate, TDS, Arsenic Constituents
Figure 11-47 Concentration Versus Distance From Source Barium, Chromium
(VI), Chromium, Cobalt, Iron, Manganese
Figure 11-48 Concentration Versus Distance From Source Molybdenum,
Vanadium, Radium
Figure 11-49 Arsenic Analytical Results - Cross Section C-C'
Figure 11-50 Barium Analytical Results - Cross Section C-C'
Figure 11-51 Boron Analytical Results - Cross Section C-C'
Figure 11-52 Calcium Analytical Results - Cross Section C-C'
Figure 11-53 Chloride Analytical Results - Cross Section C-C'
Figure 11-54 Chromium (Hexavalent and Total) Analytical Results -
Cross Section C-C'
Figure 11-55 Cobalt Analytical Results - Cross Section C-C'
Figure 11-56 Iron Analytical Results - Cross Section C-C'
Figure 11-57 Manganese Analytical Results - Cross Section C-C'
Figure 11-58 Molybdenum Analytical Results - Cross Section C-C'
Figure 11-59 pH Analytical Results - Cross Section C-C'
Figure 11-60 Strontium Analytical Results - Cross Section C-C'
Figure 11-61 Sulfate Analytical Results - Cross Section C-C'
Figure 11-62 Total Dissolved Solids Analytical Results - Cross Section C-C'
Figure 11-63 Vanadium Analytical Results - Cross Section C-C'

12.0 Screening-Level Risk Assessment
Figure 12-1 Exposure Areas - Human Health Risk Assessment
Figure 12-2 Exposure Areas - Ecological Risk Assessment

14.0 Site Assessment Results
Figure 14-1 Time Versus Concentration - pH in Surficial Zone
Figure 14-2 Time Versus Concentration - pH in Transition Zone
Figure 14-3 Time Versus Concentration - pH in Bedrock
Figure 14-4 Time Versus Concentration - Arsenic in Surficial Zone
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LIST OF FIGURES (CONTINUED)
Figure 14-5 Time Versus Concentration - Arsenic in Transition Zone
Figure 14-6 Time Versus Concentration - Arsenic in Bedrock
Figure 14-7 Time Versus Concentration - Barium in Surficial Zone
Figure 14-8 Time Versus Concentration - Barium in Transition Zone
Figure 14-9 Time Versus Concentration - Barium in Bedrock
Figure 14-10 Time Versus Concentration - Boron in Surficial Zone
Figure 14-11 Time Versus Concentration - Boron in Transition Zone
Figure 14-12 Time Versus Concentration - Boron in Bedrock
Figure 14-13 Time Versus Concentration - Chromium (Total and Hexavalent) in
Surficial Zone
Figure 14-14 Time Versus Concentration - Chromium (Total and Hexavalent) in
Transition Zone
Figure 14-15 Time Versus Concentration - Chromium (Total and Hexavalent) in
Bedrock
Figure 14-16 Time Versus Concentration - Cobalt in Surficial Zone
Figure 14-17 Time Versus Concentration - Cobalt in Transition Zone
Figure 14-18 Time Versus Concentration - Cobalt in Bedrock
Figure 14-19 Time Versus Concentration - Iron in Surficial Zone
Figure 14-20 Time Versus Concentration - Iron in Transition Zone
Figure 14-21 Time Versus Concentration - Iron in Bedrock
Figure 14-22 Time Versus Concentration - Manganese in Surficial Zone
Figure 14-23 Time Versus Concentration - Manganese in Transition Zone
Figure 14-24 Time Versus Concentration - Manganese in Bedrock
Figure 14-25 Time Versus Concentration - Molybdenum in Surficial Zone
Figure 14-26 Time Versus Concentration - Molybdenum in Transition Zone
Figure 14-27 Time Versus Concentration - Molybdenum in Bedrock
Figure 14-28 Time Versus Concentration - Sulfate in Surficial Zone
Figure 14-29 Time Versus Concentration - Sulfate in Transition Zone
Figure 14-30 Time Versus Concentration - Sulfate in Bedrock
Figure 14-31 Time Versus Concentration - Total Dissolved Solids in Surficial
Zone
Figure 14-32 Time Versus Concentration - Total Dissolved Solids in Transition
Zone
Figure 14-33 Time Versus Concentration - Total Dissolved Solids in Bedrock
Figure 14-34 Time Versus Concentration - Vanadium in Surficial Zone
Figure 14-35 Time Versus Concentration - Vanadium in Transition Zone
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LIST OF FIGURES (CONTINUED)
Figure 14-36 Time Versus Concentration - Vanadium in Bedrock
Figure 14-37 Time Versus Concentration - Strontium in Surficial Zone
Figure 14-38 Time Versus Concentration - Strontium in Transition Zone
Figure 14-39 Time Versus Concentration - Strontium in Bedrock
Figure 14-40 Groundwater Concentration Trend Analysis – Arsenic in All Flow
Layers
Figure 14-41 Groundwater Concentration Trend Analysis – Barium in All Flow
Layers
Figure 14-42 Groundwater Concentration Trend Analysis – Boron in All Flow
Layers
Figure 14-43 Groundwater Concentration Trend Analysis – Calcium in All Flow
Layers
Figure 14-44 Groundwater Concentration Trend Analysis – Chloride in All Flow
Layers
Figure 14-45 Groundwater Concentration Trend Analysis - Chromium (VI and
Total) in All Flow Layers
Figure 14-46 Groundwater Concentration Trend Analysis – Cobalt in All Flow
Layers
Figure 14-47 Groundwater Concentration Trend Analysis - Iron in All Flow
Layers
Figure 14-48 Groundwater Concentration Trend Analysis – Manganese in All
Flow Layers
Figure 14-49 Groundwater Concentration Trend Analysis – Molybdenum in All
Flow Layers
Figure 14-50 Groundwater Concentration Trend Analysis – Strontium in All
Flow Layers
Figure 14-51 Groundwater Concentration Trend Analysis – Sulfate in All Flow
Layers
Figure 14-52 Groundwater Concentration Trend Analysis - Total Dissolved
Solids in All Flow Layers
Figure 14-53 Groundwater Concentration Trend Analysis – Vanadium in All
Flow Layers
Figure 14-54 Groundwater Concentration Trend Analysis – pH in All Flow
Layers
Figure 14-55 Groundwater Concentration Trend Analysis – Arsenic in Surface
Water
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LIST OF FIGURES (CONTINUED)
Figure 14-56 Groundwater Concentration Trend Analysis - Barium in Surface
Water
Figure 14-57 Groundwater Concentration Trend Analysis - Boron in Surface
Water
Figure 14-58 Groundwater Concentration Trend Analysis – Calcium in Surface
Water
Figure 14-59 Groundwater Concentration Trend Analysis – Chloride in Surface
Water
Figure 14-60 Groundwater Concentration Trend Analysis - Chromium (IV and
Total) in Surface Water
Figure 14-61 Groundwater Concentration Trend Analysis – Cobalt in Surface
Water
Figure 14-62 Groundwater Concentration Trend Analysis - Iron in Surface Water
Figure 14-63 Groundwater Concentration Trend Analysis – Manganese in
Surface Water
Figure 14-64 Groundwater Concentration Trend Analysis - Molybdenum in
Surface Water
Figure 14-65 Groundwater Concentration Trend Analysis - Strontium in Surface
Water
Figure 14-66 Groundwater Concentration Trend Analysis - Sulfate in Surface
Water
Figure 14-67 Groundwater Concentration Trend Analysis - Total Dissolved
Solids in
Surface Water
Figure 14-68 Groundwater Concentration Trend Analysis - Vanadium in Surface
Water
Figure 14-69 Groundwater Concentration Trend Analysis – pH in Surface Water
Figure 14-70 Comprehensive Groundwater Data
Figure 14-71 Comprehensive Surface Water / Area of Wetness Data
Figure 14-72 Comprehensive Soil and Sediment Data

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LIST OF TABLES
2.0 Site History and Description
Table 2-1 Well Construction Data
Table 2-2 NPDES Groundwater Monitoring Requirements

3.0 Source Characteristics
Table 3-1 Ash, Rock, and Soil Composition
Table 3-2 Physical Properties of Ash
Table 3-3 Mineralogy of Ash
Table 3-4 Whole Rock Metal Oxide Analysis of Ash
Table 3-5 Whole Rock Elemental Analysis of Ash

6.0 Site Geology
Table 6-1 Physical Properties of Soil
Table 6-2 Chemical Properties of Soil
Table 6-3 Soil, Sediment, and Ash Analytical Methods
Table 6-4 Ash Pore Water, Groundwater, Surface Water, and Seep
Analytical Methods
Table 6-5 Historical Water Level Data
Table 6-6 Horizontal Hydraulic Gradient and Flow Velocities
Table 6-7 Vertical Hydraulic Gradients
Table 6-8 In-situ Hydraulic Conductivity Results
Table 6-9 Vertical Hydraulic Conductivity of Undisturbed Soil Samples

7.0 Soil Sampling Results
Table 7-1 Provisional Background Threshold Values for Soil
Table 7-2 Secondary Soil Evaluation

10.0 Groundwater Sampling Results
Table 10-1 Background Groundwater Results
Table 10-2 Groundwater Provisional Background Threshold Values

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LIST OF TABLES (CONTINUED)
13.0 Groundwater Modeling Results
Table 13-1 Summary of Kd Values From Batch and Column Studies

15.0 Conclusions and Recommendations
Table 15-1 Groundwater Interim Monitoring Program Analytical Methods
Table 15-2 Interim Groundwater Monitoring Plan Sample Locations

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LIST OF APPENDICES
Appendix A Regulatory Correspondence
NCDEQ Expectations Document (July 18. 2017)
Completed NCDEQ CSA Update Expectations Check List – Mayo Steam
Electric Plant
Zimmerman To Draovitch (September 1, 2017)
NCDEQ Background Dataset Review (July 7, 2017)
Revised Interim Monitoring Plans for 14 Duke Energy Facilities
(October 19, 2017)
NCDENR NORR Letter (August 13, 2014)
1981 C&D Landfill

Appendix B Comprehensive Data Table
Comprehensive Data Table Notes
Table 1 Groundwater Analytical Results
Table 2 Surface Water Results
Table 3 AOW Results
Table 4 Soil and Ash Results
Table 5 Sediment Results
Table 6 SPLP Results

Appendix C Site Assessment Data
CSA Data Reports (Physical)
UNCC Soil Sorption Report
UNCC Soil Sorption Report Addendum
Slug Test Results

Appendix D Receptor Surveys
Drinking Water Well And Receptor Survey – Mayo Steam Electric Plant
(SynTerra, September 2014)
Supplement To Drinking Water Well And Receptor Survey – Mayo Steam
Electric Plant (SynTerra, November 2014)
Update To Drinking Water Well And Receptor Survey – Mayo Steam
Electric Plant (SynTerra, September 2016)
Dewberry Potable Water Programmatic Evaluation
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LIST OF APPENDICES

Appendix E Supporting Documents
STANTEC Draft Report
WSP Drawings

Appendix F Boring Logs and Well Construction Diagrams

Appendix G Methodology
Duke Energy Low Flow Sampling Plan (June 10, 2015)
Assessment Methodology

Appendix H Background Statistical Evaluation Report

Appendix I Lab Reports
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LIST OF ACRONYMS
2B NCDEQ Title 15A, Subchapter 2B. Surface Water and Wetland
Standards
2L NCDEQ Title 15A, Subchapter 2L. Groundwater Classification and
Standards
ADD Average Daily Dose
AOW Areas of Wetness
ASTM American Society for Testing and Materials
C&D Construction and Demolition
CAMA Coal Ash Management Act
CAP Corrective Action Plan
CCR Coal Combustion Residuals
CFR Code of Federal Register
COI Constituent of Interest
COPC Constituents of Potential Concern
CSA Comprehensive Site Assessment
CSM Conceptual Site Model
CP&L Carolina Power & Light
DEP Duke Energy Progress, LLC
DO Dissolved Oxygen
DPT Direct Push Technology
DWR Division of Water Resources
EDR Environmental Data Resources, Inc.
EDXRF Energy Dispersive X-ray Diffraction
EMP Effectiveness Monitoring Plan
EPC Exposure Point Concentration
FGD Flue Gas Desulfurization
GAP Groundwater Assessment Work Plan, as amended
HAO Hydroxide Phases of Aluminum
HMO Hydroxide Phases of Manganese
HFO Hydroxide Phases of Iron
HI Hazard Index
IHSB Inactive Hazardous Site Branch
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LIST OF ACRONYMS (CONTINUED)
IMAC Interim Maximum Allowable Concentrations
IMP Interim Monitoring Plan
LIDAR Light Detection and Ranging
LOAEL lowest-observed-adverse-effect level
MCL Maximum Contaminant Level
NCAC North Carolina Administrative Code
NCDEQ North Carolina Department of Environmental Quality
NORR Notice of Regulatory Requirements
NCDEQ-DWM North Carolina Department of Environmental Quality – Division of
Waste Management
NCDEQ-DWR North Carolina Department of Environmental Quality – Division of
Water Resources
NPDES National Pollutant Discharge Elimination System
NTU Nephelometric Turbidity Unit
NURE National Uranium Resource Evaluation
OSB Oriented Strand Board
Plant/Site Mayo Steam Electric Plant
PBTV Provisional Background Threshold Value
PMCL Primary Maximum Contaminant Level
POG Protection of Groundwater
PSRG North Carolina Preliminary Soil Remediation Goals
PTO Permit to operate
PWR Partially Weathered Rock
RBC Risk-Based Concentration
SCM Site Conceptual Model
SMCL Secondary Maximum Contaminant Level
SPLP Synthetic Precipitation Leaching Procedure
TCLP Toxicity Characteristic Leaching Procedure
TDS Total Dissolved Solids
TOC Total Organic Carbon
TRV Toxicity Reference Values
USEPA United States Environmental Protection Agency
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LIST OF ACRONYMS (CONTINUED)
UNCC University of North Carolina - Charlotte
USGS United States Geological Survey
UTL upper tolerance limit

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1.0 INTRODUCTION
Duke Energy Progress, LLC (Duke Energy, DEP) owns and operates the Mayo Steam
Electric Plant (the Mayo Plant, Plant, or Site) located at 10660 Boston Road in Roxboro,
Person County, North Carolina (Figure 1-1). The Mayo Plant began operations in 1983
with a single 727 megawatt capacity generating coal-fired unit. Coal combustion
residuals (CCR) have historically been managed in the Plant’s on-site ash basin (surface
impoundment). CCR were initially deposited in the ash basin by hydraulic sluicing
operations. CCR was managed at the Plant’s on-site ash basin and transported via wet
sluicing until 2013 when the Mayo Plant converted to a dry ash system in which 90
percent of CCR was dry. Final system upgrades were completed in October 2016; all
CCR collection is dry. Beginning in November 2014, CCR from the Plant has been
managed in a newly constructed on-site Industrial landfill (monofill) permitted by the
North Carolina Department of Environment Quality (NCDEQ)1 Division of Waste
Management (NCDEQ-DWM) Permit 7305. Discharge from the ash basin to Mayo Lake
is permitted by the NCDEQ Division of Water Resources (NCDEQ-DWR) under
National Pollutant Discharge Elimination System (NPDES) Permit NC0038377.
1.1 Purpose of Comprehensive Site Assessment
This Comprehensive Site Assessment (CSA) update was conducted to refine and
expand the understanding of subsurface geologic/hydrogeologic conditions and
evaluate the extent of impacts from historical management of coal ash in the ash basin.
This CSA update contains an assessment of Site conditions based on a comprehensive
interpretation of geologic and sampling results from the initial Site assessment and
geologic and sampling results obtained subsequent to the initial assessment and has
been prepared in coordination with Duke Energy and NCDEQ in response to requests
for additional information, including additional sampling and assessment of specified
areas.
This CSA update was prepared in conformance to the most recently updated CSA table
of contents provided by NCDEQ to Duke Energy on September 29, 2017. In response to
a request from NCDEQ for an updated CSA report, this submittal includes the
following information. The NCDEQ Expectations Document (July 18, 2017) and the
completed NCDEQ CSA Update Expectations Check List are included in Appendix A:

1 Prior to September 18, 2015, the NCDEQ was referred to as the North Carolina Department of Environment and
Natural Resources (NCDENR). Both naming conventions are used in this report, as appropriate.
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Review of baseline assessment data collected and reported as part of CSA
activities;
A summary of NPDES and Coal Ash Management Act (CAMA) groundwater
monitoring information;
A summary of potential receptors including results from water supply wells;
A description and findings of additional assessment activities conducted since
submittal of the CSA Supplement report(s);
An update on background concentrations for groundwater and soil;
Definition of horizontal and vertical extent of CCR constituents in soil and
groundwater based upon NCDEQ approved background concentrations; and
An update to human health and ecological risk assessment to evaluate the
existence of imminent hazards to public health, safety and the environment.
1.2 Regulatory Background
The CAMA of 2014 directs owners of CCR surface impoundments in North Carolina to
conduct groundwater monitoring, assessment, and remedial activities, if necessary. The
CSA was performed to collect information necessary to evaluate the horizontal and
vertical extent of impacts to soil and groundwater attributable to CCR source area(s),
identify potential receptors, and screen for potential risks to those receptors.
Notice of Regulatory Requirements (NORR) 1.2.1
On August 13, 2014, North Carolina Department of Environment and Natural
Resources (NCDENR) issued a Notice of Regulatory Requirements (NORR) letter
notifying Duke Energy that exceedances of groundwater quality standards were
reported at 14 coal ash facilities owned and operated by Duke Energy. Those
groundwater quality standards are part of 15A NCAC 02L (2L) .0200
Classifications and Water Quality Standards Applicable to the Groundwaters of
North Carolina. The NORR stipulated that for each coal ash facility, Duke Energy
was to conduct a CSA. The NORR also stipulated that before conducting each
CSA, Duke was to submit a Groundwater Assessment Work Plan (GAP or Work
Plan) and a receptor survey. In accordance with the NORR requirements, a
receptor survey was performed to identify all receptors within a 0.5-mile radius
(2,640 feet) of the ash basin compliance boundary, and a CSA was conducted for
each facility. The NORR letter is included in Appendix A.
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Coal Ash Management Act Requirements 1.2.2
The CAMA of 2014 — General Assembly of North Carolina Senate Bill 729
Ratified Bill (Session 2013) (SB 729) requires that ash from Duke Energy coal
plant sites located in North Carolina either (1) be excavated and relocated to fully
lined storage facilities or (2) go through a classification process to determine
closure options and schedule. Closure options can include a combination of
excavating and relocating ash to a fully lined structural fill, excavating and
relocating the ash to a lined landfill (on-site or off-site), and/or capping the ash
with an engineered synthetic barrier system, either in place or after being
consolidated to a smaller area on-site.
As a component of implementing this objective, CAMA provides instructions for
owners of coal combustion residuals surface impoundments to perform various
groundwater monitoring and assessment activities. Section §130A-309.209 of the
CAMA ruling specifies groundwater assessment and corrective actions, drinking
water supply well surveys and provisions of alternate water supply, and
reporting requirements as follows:
(a) Groundwater Assessment of Coal Combustion Residuals Surface Impoundments. –
The owner of a coal combustion residuals surface impoundment shall conduct
groundwater monitoring and assessment as provided in this subsection. The
requirements for groundwater monitoring and assessment set out in this
subsection are in addition to any other groundwater monitoring and assessment
requirements applicable to the owners of coal combustion residuals surface
impoundments.
(1) No later than December 31, 2014, the owner of a coal combustion residuals
surface impoundment shall submit a proposed Groundwater Assessment Plan
for the impoundment to the Department for its review and approval. The
Groundwater Assessment Plan shall, at a minimum, provide for all of the
following:
a. A description of all receptors and significant exposure pathways.
b. An assessment of the horizontal and vertical extent of soil and
groundwater contamination for all contaminants confirmed to be present
in groundwater in exceedance of groundwater quality standards.
c. A description of all significant factors affecting movement and transport
of contaminants.
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d. A description of the geological and hydrogeological features influencing
the chemical and physical character of the contaminants.
2) The Department shall approve the Groundwater Assessment Plan if it
determines that the Plan complies with the requirements of this subsection
and will be sufficient to protect public health, safety, and welfare; the
environment; and natural resources.
(3) No later than 10 days from approval of the Groundwater Assessment Plan,
the owner shall begin implementation of the Plan.
(4) No later than 180 days from approval of the Groundwater Assessment Plan,
the owner shall submit a Groundwater Assessment Report to the Department.
The Report shall describe all exceedances of groundwater quality standards
associated with the impoundment.
1.3 Approach to Comprehensive Site Assessment
This CSA has been performed to meet NCDEQ requirements associated with potential
site remedy selection. The following components were utilized to develop the
assessment.
NORR Guidance 1.3.1
The NORR requires that site assessment provide information to meet the
requirements of 2L .0106 (g). This regulation lists the items to be included in site
assessments conducted pursuant to Paragraph (c) of the rule. These requirements
are listed below and referenced to the applicable sections of this CSA.
15A NCAC 02L .0106(g) Requirement CSA Section(s)
(1) The source and cause of contamination Section 3.0
(2) Any imminent hazards to public health and safety, as defined
in G.S. 130A-2, and any actions taken to mitigate them in
accordance with Paragraph (f) of this Rule
Sections ES.2 and 2.8
(3) All receptors and significant exposure pathways Sections 4.0 and 12.0
(4) The horizontal and vertical extent of soil and groundwater
contamination and all significant factors affecting contaminant
transport
Sections 7.0, 8.0, and
14.0
(5) Geological and hydrogeological features influencing the
movement, chemical, and physical character of the
contaminants
Sections 6.0, 11.0, and
15.0

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USEPA Monitored Natural Attenuation Tiered Approach 1.3.2
The assessment data is compiled in a manner to be consistent with “Monitored
Natural Attenuation of Inorganic Contaminants in Groundwater” (EPA/600/R-
07/139) (USEPA, October 2007). The tiered analysis approach discussed in this
guidance document is designed to align site characterization tasks to reduce
uncertainty in remedy selection. The tiered assessment data collection includes
information to evaluate:
Active contaminant removal from groundwater and dissolved plume
stability,
The mechanisms and rates of attenuation,
The long-term capacity for attenuation and stability of immobilized
contaminants, and
Anticipated performance monitoring needs to support the selected
remedy.
ASTM Conceptual Site Model Guidance 1.3.3
The American Society for Testing and Materials (ASTM) E1689-95 generally
describes the major components of conceptual site models, including an outline
for developing models. To the extent possible, this guidance was incorporated
into preparation of the Site Conceptual Model (SCM). The SCM is used to
integrate Site information, identify data gaps, and determine whether additional
information is needed at the Site. The model is also used to facilitate selection of
remedial alternatives and effectiveness of remedial actions in reducing the
exposure of environmental receptors to contaminants (ASTM, 2014).
1.4 Technical Objectives
The rationale for CSA activities fall into one of the following categories:
Determine the range of background groundwater quality from pertinent geologic
settings (horizontal and vertical) across a broad area of the Site.
Evaluate groundwater quality from pertinent geologic settings (horizontal and
vertical extent of CCR leachate constituents).
Establish perimeter (horizontal and vertical) boundary conditions for
groundwater modeling.
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Provide source area information including ash pore water chemistry, physical
and hydraulic properties, CCR thickness, and residual saturation within the ash
basin.
Address soil chemistry in the vicinity of the ash basin (horizontal and vertical
extent of CCR leachate constituents in soil) compared to background
concentrations.
Determine potential routes of exposure and receptors.
Compile information necessary to develop a groundwater Corrective Action Plan
(CAP) protective of human health and the environment in accordance with 2L.
1.5 Previous Submittals
Detailed descriptions of the Site operational history, the Site conceptual model, physical
setting and features, geology/hydrogeology, and results of the findings of the CSA and
other CAMA-related work are documented in full in the following:
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).

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2.0 SITE HISTORY AND DESCRIPTION
An overview of the Mayo Steam Electric Plant setting and operations is presented in the
following sections.
2.1 Site Description, Ownership and Use History
The Mayo Plant is a coal-fired electricity-generating facility in north-central North
Carolina. The Plant is in the northeastern corner of Person County, north of the City of
Roxboro. The address of the Mayo Plant is 10660 Boston Road (US Highway 501),
Roxboro, North Carolina. The Carolina Power & Light (CP&L) Mayo Plant became fully
operational in June 1983. CP&L merged with Florida Progress Corporation in 2000 to
become Progress Energy Inc. Progress Energy merged with Duke Energy in July 2012.
The Mayo Plant began coal-fired power production in 1983. A scrubber system is
currently in place to reduce emissions from coal combustion. At the Site, there is a
single ash basin located northwest of the Plant. The ash basin, which contains ash
generated from the Plant’s historic coal combustion, is approximately 140 acres in size,
is constructed with an earthen dike. A 500-foot compliance boundary for the routine
groundwater monitoring well network encircles the ash basin, except on the
northeastern edge of the Site where the compliance boundary is co-located with the
boundary of the Site. No other areas of coal ash, other than possible de minimis
quantities, are known to exist at the Site other than in the lined landfill described below.
CCR was managed at the Plant’s on-site ash basin and transported via wet sluicing until
2013. That year, the Mayo Plant converted to a dry ash system in which 90 percent of
CCR was dry. After the conversion, wet sluicing was conducted when there was a
shutdown of the dry fly ash collection system. Final system upgrades were completed
in October 2016; all CCR collection is dry. Until recently, dry ash had been hauled and
disposed in the lined landfill located at the nearby Roxboro Steam Electric Plant (near
Semora, North Carolina). Beginning in November 2014, CCR from the Mayo Plant has
been placed in a newly constructed on-site industrial landfill (monofill) (NCDEQ DWM
Permit No. 7305) at the Mayo Site.
A Flue Gas Desulfurization (FGD) system is active at the Mayo Plant. The FGD system
directs flue gas into an absorber where limestone (calcium carbonate) slurry is sprayed.
Sulfur dioxide in the flue gas reacts with the limestone slurry to produce calcium
sulfate, or gypsum. The system reclaims the unreacted limestone slurry to be reused in
the absorber. A small blowdown stream is used to maintain the chloride concentration
in the reaction tank. There are two FGD ponds at the Mayo Site. The FGD Forward
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Flush Pond was originally used in the bioreactor treatment process. The bioreactor has
been decommissioned and the FGD Forward Flush Pond is inactive; it no longer
receives the back-flush of the bioreactor. The FGD Settling Pond receives the stream of
FGD blowdown water as well as leachate from the monofill. From the FGD settling
pond the water is pumped to the thermal evaporator system. The thermal evaporator
system is a process that creates a clean distillate and brine. The brine solution is mixed
with fly ash that is placed in the on-site monofill. The clean distillate is used in
absorber make-up water. The FGD ponds were constructed within the footprint of the
ash basin; however, both ponds are constructed with an engineered liner system.
2.2 Geographic Setting, Surrounding Land Use, Surface Water
Classification
The Mayo Plant is situated in a rural area in the northeast corner of Person County,
approximately 10 miles north of Roxboro, North Carolina and approximately 13 miles
south of South Boston, Virginia on US Highway 501 (Boston Road; Figure 1-1). A
description of the physical setting of the Mayo Site is described in the following
sections.

Geographic Setting
The primary operational portion of the Site is situated east of US Highway 501 and
includes the power plant, the majority of Plant operational facilities and equipment, the
ash basin, and Mayo Lake (Figure 2-1). The Site’s northern property line extends to the
North Carolina/Virginia state line. The overall topography of the Site generally slopes
toward the east (Mayo Lake aka Mayo Reservoir) and northeast (Crutchfield Branch).
The Site is roughly bisected by US Highway 501. The majority of the Site - including the
power plant, the ash basin, and most of its operational features - is located east of US
Highway 501 and extends to the eastern shore of Mayo Lake. The property is bounded
to the north by the North Carolina/Virginia state line. Mayo Lake Road runs west to
east from the intersection with US Highway 501 to NC county road 1504. The portion of
the Site located west of US Highway 501 contains the recently operational monofill and
a haul road that connects the monofill with the operational portion of the Mayo Plant. A
railroad spur bisects the western portion of the property. RT Hester Road also cuts
across the northeastern portion of the western property. The eastern portion of the Site,
excluding Mayo Lake, encompasses 460 acres. Mayo Lake encompasses 2,880 acres with
a normal water elevation of 432.3 feet 2.

2The datum for all elevation information presented in this report is NAVD88.
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The terrestrial portion of the Site (east) is roughly bisected by a railroad line that
supplies the Plant. The ash basin is situated to the northwest of the railroad line, and the
power plant and majority of supporting operational features (e.g., coal pile, cooling
towers, administrative buildings, substation, etc.) are situated southeast of the railroad
line. Forested areas encircle the southeast portion of the property to the southwest,
south, east, and northwest. Mayo Lake borders the entire eastern portion of this part of
the Site.
The ash basin is the dominant feature on the portion of the property northwest of the
railroad line and east of US Highway 501. Heavily wooded land surrounds the ash
basin with the exception of the wastewater treatment facility on the southeast side of
the ash basin.
A construction and demolition (C&D) landfill is located east of the ash basin. The
permitted landfill (NCDEQ DWM Permit No. 73-B) was used to contain debris
generated during the construction of the Plant in the early 1980s. The landfill permit
was reopened briefly in the 1990s for a new project; however, the project was cancelled
(Appendix A). No additional material was placed in the landfill, and the permit was
closed again. The landfill is covered with thick vegetative undergrowth and trees.
A large area west of the former C&D landfill, east of the railroad, and south of Mayo
Lake Road, has been cleared for construction of a new FGD retention basin. Similarly, a
large area on the east side of the Plant haul road, north of the rail line, and just east of
the ash basin, has been cleared for the construction of a lined retention pond.
Construction activities are ongoing for both locations.
Surrounding Land Use
Properties located within a 500-foot radius of the Mayo Plant ash basin compliance
boundary are all contained within the Site, with the exception of an undeveloped parcel
located due north of the northern Plant boundary along Mayo Lake Road. Properties
adjacent to the Site are located in Person County, North Carolina and Halifax County,
Virginia. Undeveloped land occurs on the Site west of US Highway 501, with the
exception of the Mayo monofill. 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 (North Carolina and Virginia). Louisiana Pacific Corporation, located south
and west of the Mayo property boundary, at 10475 Boston Road (Roxboro, North
Carolina) opposite the Mayo Plant’s entrance road, is a manufacturing facility that
produces oriented strand board (OSB). The facility has been in operation since 1994.
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Review of a 1968 topographic map (Figure 2-2) and aerial photograph from 1948
(Figure 2-3) shows the property occupied by forested areas, large cultivated agricultural
fields, and a few farms/homes. An aerial photograph from 1981 (Figure 2-4) shows the
Plant under construction and Mayo Lake filling with water. The Mayo Plant ash basin
dam had not yet been built. In a 1993 aerial photograph (Figure 2-5), the Plant is
operational and appears much as it does now. At that time, ash was being actively
sluiced into the ash basin, which was largely open water. The 1993 photograph shows
that the majority of sluiced ash was discharged to the basin on the northwest side of the
railroad line. An obvious “ash delta” with active sluicing is noted in the photographs. A
2008 photograph (Figure 2-6) indicates that the Site appears almost identical to present
day, with mostly wooded areas surrounding the ash basin and power plant areas. In
that photograph, the ash basin is approximately half open water and half ash.
Meteorological Data
The Site lies within the Piedmont region of the southeastern United States and exhibits a
humid, subtropical climate type (NOAA, 2013). More specifically, the Site lies in the
northern Piedmont of North Carolina where the mean annual temperature is about 58
degrees Fahrenheit (F) and average annual precipitation is approximately 44 inches
(State Climate Office, 2017). A weather data station maintained by the state is located
just east of the city of Roxboro and about 11 miles south of the Mayo Plant. The mean
annual temperature recorded in Roxboro is 56.9 degrees F with a minimum annual
temperature average of 25.7 degrees F in January and a maximum annual temperature
average of 87.8 degrees F in July. Precipitation in Roxboro averages 46.6 inches
annually, slightly higher than the average for the northern Piedmont (State Climate
Office, 2017).
Surface Water Classification
The Site is located within the Roanoke River Basin. Major surface water bodies in the
vicinity of the Mayo Plant are described in this section. There is no groundwater
influence from the ash basin on the surface water bodies described with the possible
exception of Crutchfield Branch (Section 9.0).
Mayo Lake is the dominant feature on the eastern portion of the Site. Mayo Lake was
formed when Mayo Creek was dammed, and it now encapsulates the majority of the
reach of Mayo Creek as well as a number of smaller streams that flowed into Mayo
Creek prior to the lake formation. The 2,800-acre lake is maintained by the earthen dam
located at the northern end of the lake. The dam is approximately 100 feet high. Water
level is controlled by a spillway located on the eastern end of the lake/dam. A
diversion/drawdown pipe (72-inch diameter) also discharges water at the base of the
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dam on the bottom of the eastern end of the dam. The water flowing from this pipe
merges with the water from the spillway to reform Mayo Creek. Mayo Creek flows
north and then northwest and merges with Crutchfield Branch flowing northeast. The
merged Mayo Creek flows north and merges with Hyco River just south of Cluster
Springs, Virginia. The Hyco River flows northeast and merges with the Dan River just
upstream of Kerr Reservoir (Roanoke River).
Mayo Lake and the portion of Mayo Creek that flows into Mayo Lake are classified as
WS-V waters. Mayo Creek, as it flows from the Mayo Lake dam to the Virginia state
line is classified as “C”. Crutchfield Branch, from its source to the Virginia state line, is
classified as “B”. One other “blue line” stream is present on the Site and is designated as
Bowes Branch. The stream is classified as “C” waters and flows through the western
portion of the Site north toward the state line (NCDENR, 2015). Several intermittent
streams are also present on the Site. A channel associated with NPDES Outfall 002
originates on the east side of the ash basin and flows southeast, under the railroad line,
and into Mayo Lake.
2.3 CAMA-related Source Areas
CAMA provides for groundwater assessment of CCR surface impoundments defined as
topographic depressions, excavations, or diked areas formed primarily of earthen
materials, without a base liner, and that meet other criteria related to design, usage, and
ownership (Section 130A-309.201). At Mayo, the groundwater assessment was
conducted for the ash basin CCR surface impoundment.
The Mayo Plant ash basin is located in the northern portion of the Site, northwest of the
railroad line that effectively bisects the Plant. The ash basin is impounded by an earthen
embankment system approximately 2,300 feet long, with a dam height of 110 feet and a
crest height elevation of 479.8 feet. The entire basin area is approximately 140 acres and
contains approximately 6,600,000 tons of CCR material (Duke Energy, 2017). Borings
installed in the ash basin encountered ash from 13.5 feet to 66.1 feet in thickness.
Roughly 40 percent of the ash basin is currently covered with standing water. No other
ash storage facilities have been identified on the Site other than the permitted monofill.
The Mayo Plant ash basin was constructed by damming Crutchfield Branch.
Examination of historic United States Geological Survey (USGS) aerial photography and
topographic maps (Figure 2-2 to Figure 2-6) indicates that the Site was heavily wooded
with some open pasture prior to use for ash settling. Three tributaries that formed
Crutchfield Branch are present within heavily wooded areas in a 1948 aerial photograph
(Figure 2-3). A 1981 historical aerial photograph (Figure 2-4) shows the Plant under
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construction and the dam at Mayo Lake under construction and water being
impounded. However, the ash basin dam has not yet been constructed nor has Site
clearing for the ash basin been initiated. Construction of the ash basin was complete by
October 1982, and the Mayo Plant began operations in June 1983.
The ash basin dam is an earthen embankment armored with rip rap on the basin side
and on the downslope base of the dam. The perimeter of the basin is mostly unaltered
and well-vegetated with the exception of the ash basin dam and a small shoreline
section on the east (near the forebay) that are armored with rip rap. The ash basin dam
and dam access road are raised about 10 feet higher than the ash basin water level. The
Mayo Plant ash basin was constructed with two engineered toe drains located at the
base of the dam. On the southeast side of the ash basin, water enters a holding lagoon or
forebay for water quality treatment and flows into the forebay through a 48-inch pipe,
riser, and decant pipe through the forebay embankment. Discharge from the forebay is
controlled by a concrete overflow weir. Discharge from the forebay passes over the weir
and flows through NPDES Outfall 002, through a discharge canal, and into Mayo Lake.
15A NCAC 02L .0106 (f)(4) requires secondary sources that could be potential
continuing sources of possible impact to groundwater be addressed in the CAP. At the
Mayo Site, the soil located below the ash basin could be considered a potential CAMA-
related secondary source. Information to date indicates that the thickness of soil
impacted by ash would generally be limited to the depth interval near the ash/soil
interface.
2.4 Other Primary and Secondary Sources
CSA activities included an assessment of the horizontal and vertical extent of
constituents related to ash management areas observed at concentrations greater than
2L, Interim Maximum Allowable Concentrations (IMAC) or PBTVs. If the CSA indicates
constituent exceedances are related to sources other than the ash basin, those sources
will be addressed as part of a separate process in compliance with the requirements of
2L.
2.5 Summary of Permitted Activities
The NPDES program regulates wastewater discharges to surface waters. The Mayo
Plant is permitted to discharge wastewater under NPDES Permit NC0038377, which
authorizes discharge from the facility to Mayo Lake in accordance with effluent
limitations, monitoring requirements, and other conditions set forth in the permit. Ash
basin is referred to as “ash pond” in the Plant’s NPDES permit. The most recent NPDES
permit for the Mayo Plant became effective on November 1, 2009, and expired in March
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2012. NPDES flow diagrams for the Mayo Plant are included as Figure 2-7. A permit
renewal request has been submitted to NCDENR and is pending so the Site continues to
operate under the administratively extended permit.
NPDES permit NC0038377 authorizes two discharges to Mayo Lake. Outfall 001
discharges cooling tower water and circulating water system discharge water. Outfall
002 is comprised of a number of streams including internal Outfall 008 (cooling tower
blowdown); internal Outfall 009 (FGD blowdown); ash transport water; coal pile runoff;
and other sources including water from wastewater treatment processes. Nine
stormwater outfalls are also authorized for the Mayo Plant.
Surface water monitoring from Mayo Lake has been conducted at the Site since the
Plant began operations. In its current configuration, the monitoring plan includes
sampling for water quality, water chemistry, phytoplankton, chlorophyll, and zebra
mussels on an alternating calendar month frequency. Fisheries sampling is conducted
four times each year, trace element sampling is conducted once per year, and an aquatic
vegetation survey is conducted once per year.
In addition to surface water monitoring, the NPDES permit requires groundwater
monitoring. Permit Condition A (6) Attachment X, Version 1.0, dated March 17, 2011,
lists the groundwater monitoring wells to be sampled, the parameters and constituents
to be measured and analyzed, and the requirements for sampling frequency and results
reporting. Details are provided in Section 2.6.
A C&D landfill is present on the Site. That landfill is located to the east of the ash basin
(Figure 2-4). The permitted landfill (NCDEQ DWM; Permit No. 73-B) was used to
contain debris generated during the construction of the Mayo Plant in the early 1980s.
The landfill permit was reopened briefly in the 1990s for a new project; however, the
project was cancelled, no additional placement of material occurred in the landfill, and
the permit was closed again.
As previously mentioned in the report, beginning in November 2014, CCR from the
Mayo Plant has been placed in a newly constructed on-site monofill (NCDEQ DWM;
Permit No. 7305). NCDEQ DWM issued a permit to operate (PTO) the monofill on July
10, 2014. The monofill is located on an upland area west of the main Plant area (Figure
2-1). Phase 1 consists of 31 acres out of a possible total 104-acre proposed landfill
footprint. The capacity of Phase 1 is 1,592,000 cubic yards. The monofill was constructed
with a double high-density polyethylene liner with leak detection, groundwater
monitoring, and leachate collection systems.
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2.6 History of Site Groundwater Monitoring
The location of the ash basin voluntary and compliance monitoring wells, CSA wells,
the approximate ash basin waste boundary, and the compliance boundary are shown in
Figure 2-8. Construction details for Site monitoring wells are provided in Table 2-1. At
Mayo, CAMA monitoring wells are designated with an S, D, BR, or BRL identifier.
These designations correspond to the flow unit in which the well is screened. “S” refers
to the surficial flow layer (alluvium and saprolite); “D” refers to the poorly weathered
rock within a transition zone between the surficial flow zone and bedrock; and “BR”
and “BRL” refer to the bedrock flow zone (relatively unfractured rock). The following
sections discuss groundwater monitoring activities associated with CCR conducted
prior to CSA activities through current CAMA-related monitoring activities.
Groundwater monitoring results are presented in Section 10.0.
Ash Basin Voluntary Groundwater Monitoring 2.6.1
Four monitoring wells were voluntarily installed in 2008 by Duke Energy for the
purpose of groundwater monitoring. Voluntary well MW-1 has subsequently
been renamed “BG-1” and is used as a background well for the scheduled
NPDES groundwater compliance monitoring. Voluntary wells MW-2 and MW-3
are located downgradient of the ash basin dam. Voluntary well MW-4 is located
in a sidegradient/upgradient position just east of the ash basin. Wells MW-2,
MW-3, and MW-4 are not included in NPDES-related monitoring but have been
included in groundwater assessment activities associated with the CSA.
Ash Basin NPDES Groundwater Monitoring 2.6.2
The NPDES permit requires routine groundwater monitoring. Permit Condition
A (6) Attachment XX, Version 1.0, dated March 17, 2011, lists the groundwater
monitoring wells to be sampled, the parameters and constituents to be measured
and analyzed, and the requirements for sampling frequency and results
reporting. Permit Condition A (6) Attachment XX also provides requirements for
well location and well construction. Groundwater monitoring events related to
the NPDES permit for the Site are conducted three times per year. The ash basin
has a network of 10 monitoring wells which includes (BG-1 (background), BG-2
(background), CW-1, CW-1D, CW-2, CW-2D, CW-3, CW-4, CW-5, and CW-6).
Duke Energy initiated routine compliance boundary monitoring for the ash basin
in December 2010. Currently, Duke Energy conducts routine NPDES
groundwater compliance boundary monitoring during April, July, and
November each year for the parameters listed in Table 2-2.

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Ash Basin CAMA Groundwater Monitoring 2.6.3
Thirty-four (34) groundwater wells were installed as part of this assessment
(Figure 2-8). Groundwater monitoring wells (MW-12S, MW-12D, and MW-14BR)
were installed and are considered background wells. Fourteen groundwater
monitoring wells (MW-3BR, MW-5BR, MW-7D, MW-7BR, MW-8BR, MW-9BRL,
MW-13BR, MW-16S, MW-16D, MW-16BR, MW-18D, MW-18BR, MW-19D, and
MW-19BR) and eight piezometers (MW-6BR, MW-8S, MW-8D, MW-9BR, MW-
10BR, MW-11BR, MW-15BR, MW-17BR) were installed at locations outside of the
perimeter of the ash basin. Nine groundwater monitoring wells (ABMW-1,
ABMW-2, ABMW-2BR, ABMW-2BRL, ABMW-3, ABMW-3S, ABMW-4, ABMW-
4D, and ABMW-4BR) were installed at four locations in the ash basin.
Compliance monitoring wells BG-1, BG-2, CW-1, CW-1D, CW-2, CW-2D, CW-3,
CW-4, CW-5, and CW-6, as well as previously installed wells MW-2, MW-3, and
MW-4 were also used for this assessment.
Landfill Groundwater Monitoring 2.6.4
Duke Energy conducts routine solid waste landfill compliance monitoring in
accordance with NCDEQ DWM Permit No. 7305; however, that monitoring is
not subject to CAMA, and is not detailed herein. The monofill, as previously
described, is located west of the US Highway 501 in a separate groundwater flow
regime, and is not influenced by the ash basin.
2.7 Summary of Assessment Activities
With the exception of the voluntary and compliance well installation/groundwater
monitoring described above, no other known CCR-related groundwater investigations
or site assessments were conducted at Mayo Plant prior to implementation of the GAP
(SynTerra, 2014c).
2.8 Summary of Initial Abatement, Source Removal or Other
Corrective Action
No abatement or source removal activities have been conducted at the Mayo Plant
related to the ash basin other than discontinuing the discharge of ash and sluice water.
As previously indicated, CCR was historically managed at the Plant’s on-site ash basin
and transported via wet sluicing. In 2013, the Mayo Plant converted to a dry ash
collection system. Additional upgrades to the dry fly ash handling system were
completed in October 2016 and all CCR collection is now dry. In preparation for ash
basin closure, new retention basins and wastewater treatment systems are being
designed and constructed.
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3.0 SOURCE CHARACTERISTICS
For purposes of this assessment, the source area is defined by the ash waste boundary
as depicted on Figure 2-8. For the Mayo Site, source areas include the ash management
areas comprised of the Ash Basin and two FGD ponds.
3.1 Coal Combustion and Ash Handling System
Coal ash is produced from the combustion of coal. The coal is dried, pulverized, and
conveyed to the burner area of a boiler. The smaller particles produced by coal
combustion, referred to as fly ash, are carried upward in the flue gas and are captured
by an air pollution control device, such as an electrostatic precipitator. The larger
particles of ash that fall to the bottom of the boiler are referred to as bottom ash or boiler
slag.
After Plant operations began in 1983, CCR was historically managed at the Plant’s on-
site ash basin and transported via wet sluicing. In 2013, the Mayo Plant converted to a
dry ash system in which 90 percent of CCR was dry. Final system upgrades were
completed in October 2016 so that all CCR collection is dry. Prior to 2014, dry ash had
been hauled and disposed in the industrial landfill located at the nearby Roxboro Steam
Electric Plant (near Semora, North Carolina). Beginning in November 2014, CCR from
the Mayo Plant has been placed in the on-site Mayo monofill.
The Mayo Plant began coal-fired power production in 1983. At the Site, there is a single
ash basin located northwest of the Plant that contains ash generated from the Plant’s
historic coal combustion. The ash basin is approximately 140 acres in size, is constructed
with an earthen dike, and contains approximately 6,600,000 tons of CCR (Duke Energy,
2017). The ash basin dam is an earthen embankment armored with rip rap on the basin
side and on the downslope base of the dam. The perimeter of the basin is mostly
unaltered and well-vegetated with the exception of the ash basin dam and a small
shoreline section on the east (near the forebay) that are armored with rip rap. The ash
basin dam and dam access road are raised about 10 feet higher than the ash basin water
level. Roughly one-half of the ash basin is covered with standing water.
A FGD system is active at the Mayo Plant. The FGD system directs flue gas into an
absorber where limestone (calcium carbonate) slurry is sprayed. Sulfur dioxide in the
flue gas reacts with the limestone slurry to produce calcium sulfate, or gypsum. The
system reclaims the unreacted limestone slurry to be reused in the absorber. A small
blowdown stream is used to maintain the chloride concentration in the reaction tank.
There are two lined FGD ponds at the Mayo Site. The liner system is comprised of -
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from bottom to top – fill, a geocomposite liner, and a 60-mil HDPE liner. The FGD
Forward Flush Pond was originally used in the bioreactor treatment process. The
bioreactor has been decommissioned and the FGD Forward Flush Pond is inactive; it no
longer receives the back-flush of the bioreactor. The FGD Settling Pond receives the
stream of FGD blowdown water as well as leachate water from the monofill. From the
FGD settling pond the water is pumped to the thermal evaporator system. The thermal
evaporation system is a process that creates a clean distillate and brine. The brine
solution is mixed with fly ash that is placed in the on-site monofill. The clean distillate
is used in absorber make-up water.
3.2 General Physical and Chemical Properties of Ash
Coal ash consists of fly ash and bottom ash produced from the combustion of coal. The
physical and chemical properties of coal ash are determined by reactions that occur
during the combustion of the coal and subsequent cooling of the flue gas.
Physical Properties
Approximately 70 percent to 80 percent of the ash produced during coal combustion is
fly ash (EPRI, 1993). Typically 65 percent to 90 percent of fly ash has particle sizes that
are less than 0.010 millimeter (mm). In general, fly ash has a grain size distribution
similar to that of silt. The remaining 20 percent to 30 percent of ash produced is
considered bottom ash. Bottom ash consists of angular particles with a porous surface
and is normally gray to black in color. Bottom ash particle diameters can vary from
approximately 38 mm to 0.05 mm. In general, bottom ash has a grain size distribution
similar to that of fine gravel to medium sand (EPRI, 1995). Physical properties of ash are
on Table 3-1.
Based on published literature not specific to Mayo, the specific gravity of fly ash ranges
from 2.1 to 2.9, and the specific gravity of bottom ash typically ranges from 2.3 to 3.0.
The permeability of fly ash and bottom ash vary based on material density, but would
be within the range of a soil with a similar gradation and density (EPRI, 1995).
Chemical Properties
The specific mineralogy of coal ash varies based on many factors including the chemical
composition of the coal, which is directly related to the geographic region where the
coal was mined; the type of boiler where the combustion occurs (i.e., thermodynamics
of the boiler); and air pollution control technologies employed.
The overall chemical composition of coal ash resembles that of siliceous rocks from
which it was derived, particularly shale. Oxides of silicon, aluminum, iron, and calcium
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make up more than 90 percent of most siliceous rocks, soils, fly ash, and bottom ash.
Other major and minor elements (sulfur, sodium, potassium, magnesium, titanium)
make up an additional 8 percent, while trace constituents account for less than one
percent. The following constituents are considered to be trace elements: arsenic, barium,
cadmium, chromium, lead, mercury, selenium, copper, manganese, nickel, lead,
vanadium, and zinc (EPRI, 2010).
The majority of fly ash particles are glassy spheres mainly composed of amorphous or
glassy aluminosilicates, crystalline matter, and carbon. Figure 3-1 presents a
photograph of ash collected from the ash basin at Duke Energy's Cliffside Steam Station
(considered representative of the ash at the Mayo Plant) showing a mix of fly ash and
bottom ash at 10 µm and 20 µm magnifications. The glassy spheres can be observed in
the photograph. The glassy spheres are generally immune to dissolution. During the
later stages of the combustion process and as the combustion gases are cooling after
exiting the boiler, molecules from the combustion process condense on the surface of
the glassy spheres. These surface condensates consist of soluble salts [e.g., calcium
(Ca2+) and sulfate (SO2-)], metals [e.g., copper (Cu) and zinc (Zn)], and other minor
elements [e.g., boron (B), selenium (Se), and arsenic (As)] (EPRI, 1994).
The major elemental composition of fly ash (approximately 95 percent by weight) is
composed of mineral oxides of silicon, aluminum, iron and calcium. Oxides of
magnesium, potassium, titanium and sulfur comprise approximately 4 percent of fly
ash by weight (EPRI, 1995). Trace elemental composition of fly ash typically is
approximately one percent by weight and may include arsenic, antimony, barium,
boron, cadmium, chromium, copper, manganese, mercury, nickel, lead, selenium,
silver, thallium, zinc, and other elements. For comparison, Figure 3-2 shows the
elemental composition of fly ash and bottom ash compared with typical values for shale
and volcanic ash. Table 3-2 shows the bulk composition of fly ash and bottom ash
compared with typical values for soil and rock. In addition to these constituents, fly ash
may contain unburned carbon. Bituminous coal ash typically yields slightly acidic to
alkaline solutions (pH of 5 to 10) on contact with water.
The geochemical factors controlling the reactions associated with leaching of ash are
complex. Factors such as the chemical speciation of the constituent, solution pH,
solution-to-solid ratio, and other factors control the chemical concentration of the
resultant solution. Constituents that are held on the glassy surfaces of fly ash such as
boron, arsenic, and selenium may initially leach more readily than other constituents.
As noted in Table 3-2, aluminum, silicon, calcium, and iron represent the larger
fractions of fly ash by weight. Calcium and iron may limit the release of arsenic by
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forming calcium-arsenic precipitates. Formation of iron hydroxide compounds may also
sequester arsenic and retard or prevent release of arsenic to the environment. Similar
processes and reactions may affect other constituents of concern; however, certain
constituents such as boron and sulfate will likely remain highly mobile.
In addition to the variability that might be seen in the mineralogical composition of the
ash, based on different coal types, different age of ash in the basin, etc., it is anticipated
that the chemical environment of the ash basin varies over time, distance, and depth.
EPRI (2010) reports that 64 samples of coal combustion products (including fly ash,
bottom ash, and flue gas desulfurization residue) from 50 different power plants were
subjected to United States Environmental Protection Agency (USEPA) Method 1311
Toxicity Characteristic Leaching Procedure (TCLP) leaching and no TCLP result
exceeded the TCLP hazardous waste limit (EPRI, 2010). Figure 3-3 provides the results
of that testing.
3.3 Site-Specific Coal Ash Data
Source characterization was performed to identify the physical and chemical properties
of the ash in the ash basin. The source characterization involved development of
selected physical properties of ash, identifying the constituents found in ash, measuring
concentrations of constituents present in the ash pore water, and performing laboratory
analyses to estimate constituent concentrations resulting from the leaching process. The
physical and chemical properties evaluated as part of this characterization will be used
to better understand impacts to soil and groundwater from the source area and will also
be utilized as part of groundwater model development in the CAP.
Source characterization was performed through the completion of soil borings,
installation of monitoring wells, and associated solid matrix and aqueous sample
collection and analysis. Characterization of the ash basin was accomplished by
completing five borings and installing nine monitoring wells in three phases. The first
phase included borings that were installed using direct push technology (DPT) and
continuous sample recovery. Each boring (AB-1, AB-2, AB-2, and AB-4) was advanced
to the bottom of the basin. A second phase was conducted to collect samples of soil,
saprolite, and bedrock beneath the ash, and install groundwater monitoring wells in
various water-bearing horizons beneath and within the ash basin (ABMW wells). A
third phase of work was completed to verify the lack of impact to deeper groundwater
directly beneath the ash basin. This “data gap” effort resulted in the installation of deep
monitoring well ABMW-2BRL.
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The initial assessment effort employing the DPT methods was undertaken to collect
samples of ash and to determine the bottom of the deposited ash. A smaller, lighter DPT
drilling rig was used so that conditions could be observed during transport and drilling
activities and to determine if it was safe to move larger, heavier drilling equipment onto
the ash basin. It was determined that the larger rotary sonic rigs could be employed if
restricted to the so-called “ash harvest” area. This portion of the ash basin is where ash
was removed from the basin and “stacked” for dewatering prior to transport for
beneficial reuse. With the exception of ABMW-1, ash borings and wells were completed
in the ash harvest area. Ash and soil samples were collected from each boring for
physical and chemical testing in accordance with GAP Section 7.1.1 (SynTerra, 2014c)
and as Site conditions allowed. Laboratory results of ash samples are presented in
Appendix B, Table 4.
The thickness of ash in the areas of investigation varied. Borings installed in the ash
basin encountered ash from 13.5 feet to 66 feet in thickness. The contact between ash
and underlying soils was distinct in each boring as seen in Photograph 3-1. Physical
intrusion of ash into the underlying soils appeared to have been negligible.

Physical Properties of Ash
Physical properties (grain size, specific gravity, and moisture content) and mineralogy
determinations were performed on samples from the ash basin. Physical properties
were measured using ASTM methods and mineralogy was determined by X-ray
diffraction (Appendix C). Bottom ash is generally characterized as a loose, poorly
graded (fine- to coarse-grained) sand. Fly ash is generally characterized as a moderately
dense silty fine sand or silt. Compared to soil, fly ash exhibits a lower specific gravity.
Photograph 3-1. GeoProbe core from AB-2 (left) and AB-1 (right) showing one inch
diameter sample in acetate tube. Soil (left of arrows) contact with saturated ash (right) is
distinct with little vertical migration of ash.

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Mineralogy analyses on three ash samples indicate that the ash is predominantly
mullite, with quartz, and one sample indicates 0.5 percent biotite content. Mullite is an
aluminosilicate mineral (Al6Si2O13) that is rare in nature but common in artificial melts
(Hurlbut, 1971). Presumably, the mullite formed from naturally occurring micas and
clays in the coal-fired boiler. Quartz (SiO2) is the primary mineral in most natural sand
deposits. Biotite (K(Mg,Fe) 3(AlSi3O10)(F,OH)2) is a main silicate found in coal (Table 3-
3; (Zhang, 2013). Three ash samples were tested by Energy Dispersive X-Ray
Fluorescence (EDXRF) for metal oxides (Table 3-4) and a suite of elements (Table 3-5).
The sample was comprised primarily of silicon dioxide (SiO2), aluminum oxide (Al2O3),
and iron oxide (Fe2O3). Cerium, copper, tin, and zinc were the trace metals detected in
highest concentrations in the sample.
Chemical Properties of Ash
Thirteen samples of ash were collected and analyzed for total constituent concentrations
and total organic carbon (TOC). Concentrations of arsenic, chromium, cobalt, iron, and
vanadium were reported above soil North Carolina Preliminary Soil Remediation Goals
(PSRG) for Industrial Health and/or Protection of Groundwater (POG) for ash samples
collected within the ash basin.
Six samples were prepared using the USEPA Synthetic Precipitation Leaching
Procedure (SPLP) (Appendix B, Table 6) and the leachate was analyzed. The SPLP was
designed to more closely approximate leaching from a material by rainwater. SPLP
leachate analytical results are compared to 2L and/or IMAC for reference purposes only.
Those results do not represent groundwater samples; therefore, comparison to 2L
and/or IMAC is not required. The SPLP analyses indicated leachate concentrations
greater than 2L or IMAC in one or more samples for antimony, arsenic, chromium,
iron, manganese, nitrate, thallium, and vanadium. Notably, boron was not detected in
ash leachate above the 2L concentration. The SPLP is not intended to mimic complete
leaching processes and results are not necessarily indicative of resultant concentrations
in groundwater. Further, of the detected constituents in SPLP leachate data from the
ash, cobalt, iron, manganese, and vanadium are prevalent in samples from background
soil locations. Of these, cobalt and iron do not appear to readily leach from ash.
Manganese and vanadium appear to leach equally from ash and natural soils.
Chemistry of Ash Pore Water
Pore water refers to water samples collected from wells installed within the ash basin
and screened in the ash layer. Four pore water monitoring wells were completed in the
ash basin (ABMW-1 through ABMW-4; Figure 2-8). Since installation of the wells in
early 2015, the wells have been sampled seven times including the second quarter of
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2017 as part of the CAMA monitoring program. Due to unsafe accessibility after heavy
precipitation events, ABMW-1 has been sampled only six times.
Pore water analytical results are compared to 2L and/or IMAC for reference purposes
only. The ash basin is a permitted wastewater system; therefore, comparison of pore
water within the wastewater treatment residuals (ash) to 2B or 2L/IMAC is not
required. Eleven analytes (antimony, arsenic, barium, boron, cobalt, iron, manganese,
pH, total dissolved solids (TDS), thallium, and vanadium) were detected above the
corresponding 2L or IMAC in one or more pore water samples (Appendix B, Table 1).
Species of radium and uranium were detected in pore water from each well; however,
only groundwater from ABMW-1 showed a concentration of total uranium which
exceeded a comparison criterion. Concentrations of detected constituents in ash pore
water have been relatively stable; although, decreases in iron and manganese have been
noted in ABMW-2 and increases in iron and manganese have been noted in ABMW-4.
Piper diagrams have been prepared for groundwater results from Mayo Plant
monitoring wells and are discussed in more detail in Section 10.0.
Relative redox conditions were determined using an Excel® workbook for identifying
redox processes in ground water (Jurgens, McMahon, Chapelle, & Eberts, 2009). This
workbook allows a standardized method to identify and describe the redox state of
groundwater. Ash pore water from the basin is anoxic (ABMW-3, Fe(III) process),
mixed (anoxic; ABMW-1, Fe(III)-SO4 process), and mixed (oxic/anoxic; ABMW-2 and
ABMW-4, O2-Fe(III)-SO4 process).
Ash pore water, as well as groundwater beneath the ash basin, is predominantly
characterized as calcium-bicarbonate type water, typical of shallow fresh groundwater
but atypical of coal ash leachate. For comparative purposes, a 2006 EPRI study of 40 ash
leachate water samples collected from 20 different coal ash landfills and impoundments
characterized bituminous coal ash leachate as calcium-magnesium-sulfate water type
and subbituminous coal ash leachate as sodium-calcium-sulfate water type. Most of the
ash pore water and groundwater within the ash basin exhibits neutral or slightly basic
pH as contrasted with the slightly acidic groundwater present in background,
sidegradient, and many downgradient wells.
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4.0 RECEPTOR INFORMATION
Section §130A-309.201(13) of the 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 (including irrigation wells and unused wells) and surface water features.
Additional receptors (described in Section 12.0) were evaluated as part of the risk
assessment related to the CSA effort.
The NORR CSA receptor survey guidance requirements include listing and depicting
water supply wells, public or private, including irrigation wells, and unused wells
(other than those that have been properly abandoned in accordance with 15A NCAC 2C
.0100) within a minimum of 1,500 feet of the known extent of contamination. In
NCDEQ’s June 2015 response to Duke Energy’s proposed adjustments to the CSA
guidelines, NCDEQ DWR acknowledged the difficulty with determining the known
extent of contamination at this time. DWR stated that it expected all drinking water
wells located 2,640 feet (0.5-mile) downgradient from the established compliance
boundary to be documented in the CSA reports as specified in the CAMA requirements.
The approach to the receptor survey in this CSA includes listing and depicting all water
supply wells (public or private, including irrigation wells, and unused wells) within a
0.5-mile radius of the ash basin compliance boundary (Appendix D).
Properties located within a 0.5-mile radius of the ash basin compliance boundary
include residences located to the south and upgradient of the Site, centered around
Mullins Lane; residences located northwest and upgradient of the Site, on the
south/North Carolina side of the North Carolina/Virginia state line; and residences
located northwest and upgradient of the Site, on the north/Virginia side of the state line.
Unoccupied rural properties are located north of the Site and downgradient of the ash
basin (Figure 4-1). A municipal water line is present along US Highway 501 (Boston
Road) toward the south of the Plant. The water line does not extend north of the Plant
entrance road. No municipal water lines serve the area north of the Site (along Mayo
Lake Road).
The NORR CSA guidance requires that subsurface utilities be mapped within 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
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topographic map by WSP USA, Inc. (Appendix E). Culverts, pipes, and miscellaneous
subsurface utilities were also located on the Mayo Site. Due to the isolation of the ash
basin from the Plant area, subsurface utilities in the Plant area are not expected to be
major contaminant 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 likelihood of underground utilities being preferential pathways,
other than at the dam seepage structures, is not anticipated.
4.1 Summary of Receptor Survey Activities
Surveys to identify potential receptors including public and private water supply wells
(including irrigation wells and unused or abandoned 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 Well and Receptor Survey – Mayo Steam Electric
Plant (SynTerra, 2016c)
These reports are included as Appendix D.
The Drinking Water Well and Receptor Survey – Mayo Steam Electric (SynTerra, 2014a)
included results of a review of publicly available data from NCDEQ Division of
Environmental Health, NC OneMap GeoSpatial Portal, DWR Source Water Assessment
Program (SWAP) online database, county geographic information system,
Environmental Data Resources, Inc. (EDR) records review, the USGS National
Hydrography Dataset, as well as a vehicular survey along public roads located within
0.5-mile radius of the ash basin compliance boundary.
The Supplement to Drinking Water Well and Receptor Survey- Mayo Steam Electric Plant
(SynTerra, 2014b) supplemented the initial report with additional information obtained
from questionnaires completed by property owners who own property within the 0.5-
mile radius of the ash basin compliance boundary. The report included a sufficiently
scaled map showing the coal ash facility location, the boundary of the Site, the waste
and compliance boundaries, all monitoring wells listed in the NPDES permit and the
approximate location of identified water supply wells. A table presented available
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information about identified wells including the owner's name, address of the well with
parcel number, construction and usage data, and the approximate distance from the
compliance boundary.
The Update to Drinking Water Well and Receptor Survey – Mayo Steam Electric Plant
(SynTerra, 2016c) included review of the most recent available state, county and other
resources and an additional field reconnaissance to observe potential water supply
wells near a 0.5 mile radius of the established Mayo Plant ash basin compliance
boundary. No additional wells were identified near the survey areas that were not
reported in the 2014 surveys. Consistent with findings from previous surveys, no public
or private drinking water wells or wellhead protection areas are located downgradient
of the ash basin.
4.2 Summary of Receptor Survey Findings
The City of Roxboro provides and maintains a municipal water supply line that extends
north from Roxboro along US Highway 501. The line supplies potable water to the
Mayo Plant. The line terminates on US Highway 501 at the Mayo Plant.
No public or private drinking water wells or wellhead protection areas were found to
be located downgradient of the ash basin. This finding was supported by field
observations and a review of public records. Based on the known groundwater flow
direction, none of the wells identified in the water well survey are located
downgradient of the ash basin. The location and relevant information pertaining to
suspected water wells located upgradient of the facility, within 0.5 miles of the ash
basin compliance boundary, were included in the survey reports as required by the
NORR.
As required by G.S. 130A-309.211(c1) of House Bill 630 (HB630), Duke Energy evaluated
the feasibility and costs of providing a permanent replacement water supply to eligible
households. Households were eligible if any portion of a parcel of land crossed the 0.5-
mile compliance line described in House Bill 630 and if the household currently was
using well water or bottled water (under Duke Energy’s bottled water program) as the
drinking water source. Undeveloped parcels were identified but were not considered
“eligible” because groundwater wells are not currently utilized as a drinking source. A
Potable Water Programmatic Evaluation (Dewberry, 2016); (Appendix D) was conducted.
That evaluation included a survey of eligible households, a preliminary engineering
evaluation, a cost estimate, and a schedule. The evaluation report also included a listing
of, and relevant information about, households and properties within the survey area,
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as well as, maps depicting property locations, including those properties for which a
replacement water supply will be provided.
Public Water Supply Wells 4.2.1
An EDR 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 of the Site at 201 Old US Hwy 501 (Roxboro, North Carolina),
maintains a public water supply provided by a groundwater well.
Private Water Supply Wells 4.2.2
The fractured bedrock aquifers in the north-central Piedmont, including in the
rural areas surrounding the Mayo Plant, are commonly used for water supply
purposes. Occupied properties southwest and northwest of the Site are not
serviced by the municipal water supply line. Drinking water is obtained from
private groundwater wells by residents of the following: residences on or near
Mullins Lane located just south of the Mayo Plant; one business and one
residence northwest of Boston Road and the Mayo Plant just south of the state
line; and several residences along the east and west sides of US Highway 501
(Huell Matthews Highway) in Virginia, just north of the state line (Figure 4-1).
Several efforts have been made to locate and document the presence of and
information related to private water supply wells in the vicinity of the Mayo
Plant. The September 2014 Drinking Water Well and Receptor Survey report
indicated that no public or private drinking water wells or wellhead protection
areas were located within the 0.5-mile off-set from the compliance boundary;
however, private water supply wells have been identified within or in close
proximity to the 0.5-mile off-set. SynTerra’s November 2014 report
supplemented the initial report with information obtained from questionnaires
sent to owners of property within the 0.5-mile radius of the compliance
boundary. The questionnaires were designed to collect information regarding
whether a water well or spring is present on the property, its use, and whether
the property is serviced by a municipal water supply. If a well is present, the
property owner was asked to provide information regarding the well location
and construction information. The results from the questionnaires indicated that
as many as 22 wells might be located within 0.5 miles of the compliance
boundary for the Site (reported wells, observed wells, and possible wells).
The November 2014 and September 2016 update reports included a sufficiently
scaled map showing the ash basin location, the facility property boundary, the
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waste and compliance boundaries, all monitoring wells, and the approximate
location of identified water supply wells. A table presented available information
related to identified wells, including: the owner's name, the address of the well
location with parcel number, construction and usage data, and the approximate
distance of the well from the compliance boundary.
There are three inactive water supply wells on the Site (informally designated as
DEP-1, DEP-2, and DEP-3; Figure 2-8). These wells were likely drilled in the late
1970s or early 1980s (and possibly 1990s in the case of DEP-1) to supply water to
the Plant during construction and early start-up operations (DEP-2 and DEP-3).
DEP-1 was drilled to provide potable water to the Plant picnic area (no longer in
use). An evaluation of the wells was conducted during Site assessment activities.
Pumps and piping were removed, and the depths of the well and static water
levels were measured. A downhole camera was used in an attempt to video-log
the entire well to obtain well-specific data. The video logging effort was only
partly successful because the wells have been idle for many years and a
significant biofilm has formed and coated the walls of the borehole. This biofilm
causes very turbid and murky water. In spite of the poor condition of the well
water, relevant information about the wells was obtained (table below). It
appears, based on the nature of the rock viewed with the downhole camera, that
the wells were drilled using a pneumatic air hammer. It is noteworthy that the
wells have been out of service for a number of years, even decades; therefore,
they are not currently influencing groundwater flow.
WELL
DESIGNATION
TOTAL
WELL
DEPTH
(feet bgs)
DEPTH OF
SURFACE
CASING
(feet bgs)
WATER
LEVEL
(feet bgs;
July 2015) WELL INFORMATION
DEP-1 130 36 10.40
Casing: 6-inch steel; Pump: 1.5
HP (household type);
Discharge Pipe: 1-inch (black poly)
DEP-2 238 21 15.65
Casing: 6-inch steel; Pump: 5 HP
(Diamond; installed 10/31/1995);
Pump Set Depth: 215 ft bgs;
Discharge Pipe: 1.25-inch
(galvanized)
DEP-3 250 21 21.23
Casing: 6-inch steel; Pump: 5 HP
(Gould); Pump Set Depth: 227 ft
bgs; Discharge Pipe: 2-inch
(galvanized)

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4.3 Private Water Well Sampling
NCDEQ coordinated sampling of six private water supply wells within a half-mile
radius of the ash basin compliance boundary from March to May in 2015. Water
samples were collected from three of the six wells identified by property owners. Only
one well owner reported a well.
Duke Energy collected samples from eight additional private water supply wells in
2017. The three original wells sampled by NCDEQ were not resampled by Duke
Energy. Tabulated results, provided by Duke Energy, for the NCDEQ and Duke Energy
sampling efforts along with exceedances of 2L Standards, IMACs, and/or other
reporting limits are included in Appendix B, Table 1. The locations of the sampled
wells and summary analytical results are included on Figure 4-2.
A review of the analytical data for the private water supply wells indicated several
constituents were detected above 2L or IMACs including pH (two wells), iron (three
wells), lead (one well), manganese (three wells), and vanadium (eight wells).
Concentrations of analyzed constituents exceeded the respective Provisional
Background Threshold Values (PBTV) for a number of private water supply wells (high
turbidity data/values are excluded) including:
Alkalinity (1 well)
Barium (2 wells)
Calcium (1 well)
Chromium (hexavalent) (4 wells)
Copper (10 wells)
Lead (7 wells)
Manganese (1 well)
Molybdenum (1 well)
Potassium (2 wells)
Selenium (1 well)
Sodium (1 wells)
Strontium (2 wells)
Sulfate (2 wells)
TDS (2 wells)
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Vanadium (1 well)
Zinc (7 wells)
The exceedances of PBTVs in private water wells were further evaluated. First, the
PBTVs have been developed using groundwater data from a set of three background
wells from a geographically limited area, all located on the Mayo Plant. These wells are
located within about one-tenth of square mile of each other. The geochemical data from
these wells may not be representative across the broader area encompassed by the 11
private water supply wells (spread across approximately 1.5 square miles). For
example, if the Mayo dataset were compared with the PBTVs for the nearby and similar
Roxboro Plant (e.g., similar geologic setting), only 10 of the 16 constituents listed above
would exceed a background value in groundwater. Second, well construction may
influence analytical results. For example, galvanized pipe could yield high zinc
concentrations and brass components in wells pumps and valves can be a source of
lead. Information concerning well construction and piping materials is important to
have before attributing detections of ash-related constituents solely to the geochemistry
of the groundwater. Third, there is very limited information available about the wells
(e.g., date of installation, drilling method, well depth, casing length, pump set depth,
etc.). Many private water supply wells in this part of the Piedmont are open-hole rock
wells. A shallow surface casing is installed and then the well is drilled to a depth that
may be as shallow as 40 or 50 feet or as deep as several hundred feet. When a
groundwater sample is collected, it is unknown from what part of the bedrock aquifer
the groundwater is drawn. Groundwater geochemistry in fractured bedrock aquifers
can be quite variable.
A fourth reason for considering the apparent exceedances of PBTVs in groundwater is
that, as previously described, private water wells in bedrock are typically installed as
open-hole wells. Care must be taken when comparing geochemical data from these
wells and comparing them to background concentrations derived from carefully drilled
and installed groundwater monitoring wells with machine-slotted wells screens, proper
filter pack installation, proper well development, and specific sample collection
procedures employed.
Fifth, groundwater flow in the area around Mayo is consistent with the model of
groundwater flow in the Piedmont as described in Section 5.2. This conceptual model of
groundwater flow (LeGrand, 1988) (LeGrand, 1989) describes each distinct Piedmont
surface drainage basin as similar to adjacent basins with the conditions generally
repetitive from basin to basin. Within a basin, movement of groundwater is generally
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restricted to the area extending from the drainage divide to a perennial stream (Slope-
Aquifer System). Each distinct basin and slope-aquifer system may limit the area of
influence of wells. In the case of the private wells located upgradient of the Mayo Plant,
the wells are situated in distinct drainage basins/slope-aquifer systems separate from
the Plant area and the ash basin. Further, groundwater flow from the source (ash basin)
is to the north-northeast and the closest private wells are situated to the north-
northwest, northwest, and south-southeast of the ash basin.
Finally, the geochemical signature of groundwater from the private wells was
compared with the signature of groundwater from the source area using Piper
diagrams (a graphical representation of major water chemistry using two ternary plots
and a diamond plot showing the relative percentage of major cations and major anions
in a sample). The geochemical nature of groundwater from the sampled private wells is
very different from ash pore water and from groundwater beneath the basin (discussed
in Section 10.0). Data from five of the wells was plotted because data with a charge
balance greater than 10 percent were omitted. Four of the wells (MY-03, MY-1000, MY-
1003, and MY-2003) plot consistently with deep bedrock background well MW-13BR
and typical, deep groundwater geochemical types. MY-1002 results suggest a sodium
bicarbonate water type, indicative of ion exchange, possibly indicating that water
treatment is associated with that well.
4.4 Numerical Well Capture Zone Analysis
In December 2015, a numerical capture zone analysis for the Mayo Site was conducted
to evaluate potential impact of upgradient water supply pumping wells. The analysis
employed MODPATH to interface with the MODFLOW flow model. MODPATH is a
“particle tracking” model that traces groundwater flow lines from a starting position.
MODPATH was used to trace groundwater flow lines around pumping wells to
indicate where the water being pumped from the well originates (i.e., well capture zone
analysis). 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. None of the particle tracks
originating in the ash basin moved into the well capture zones (Figure 4-3).
4.5 Surface Water Receptors
The Site is located in the Roanoke River Basin. Although the Site is located near Mayo
Lake, groundwater influenced by the ash basin flows toward Crutchfield Branch, a
small stream located north of the ash basin and near the northern Site property. There is
no surface water intake in Crutchfield Branch.
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5.0 REGIONAL GEOLOGY AND HYDROGEOLOGY
North Carolina is divided into distinct regions by portions of three physiographic
provinces: the Atlantic Coastal Plain, the Piedmont, and the Blue Ridge. The Mayo Plant
is situated in the Piedmont physiographic province of north-central North Carolina. The
Piedmont is generally characterized by mature, well-rounded hills and rolling ridges
cut by small streams and drainages. However, in areas with thinner regolith, like in the
immediate vicinity of the Mayo Plant, the relief is more rugged with incised streams
that occur as the rate of subsurface weathering fails to keep pace with the rate of
erosion.
The Piedmont in North Carolina is several hundred feet higher than in neighboring
South Carolina and Virginia due to the Cape Fear Arch, an uplift feature that trends
roughly along the Cape Fear River and continues through the Piedmont into the
Appalachian Mountains (Rogers, 2006). The resulting geomorphology results in river
flow to the north or south instead of east (Rogers, 2006). Elevations in the area of the
Mayo Plant range from 600 feet in the extreme southwest portion of the Plant property
(near the Plant entrance along Boston Road) to 360 feet in the Crutchfield Branch stream
area (on the north side of the Plant).
The following sections contain a synopsis of geologic and hydrogeologic characteristics
for the area. This section does not provide an exhaustive list or summary of the many
important geologic research efforts that have been published on the region. This section
provides summary information from research.
5.1 Regional Geology
The Plant is located near the contact between two regional zones of metamorphosed
rocks: the Carolina Slate Belt (often referred to as Carolina terrane) on the east and the
Charlotte Belt (or Charlotte terrane) to the west (Figure 5-1). The majority of the Mayo
Site, including the largest portion of the ash basin and Mayo Lake, is situated in the
Carolina terrane (Dicken, Nicholson, Horton, Foose, & Mueller, 2007). The Carolina
terrane includes volcanic and sedimentary rocks metamorphosed to lower greenschist
facies (Butler & Secor, 1991). The metamorphic rocks have been intruded by coarse-
grained granitic rocks and have been subjected to regional structural deformation
(Rogers, 2006). For more than a century, the character and genesis of the rocks within
these regional metamorphic belts has been the subject of intense study and efforts to
describe the mineral resources of the area and the geologic character of the area in
tectonic, structural, and litho-stratigraphic terms.
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In a work co-published by the Virginia Geological Survey and the North Carolina
Geological and Economic Survey in 1917, Laney traced the history of investigations in
and around the so-called Virgilina Ore District from the early 1700s, through the 19th
century, and into the early 20th century (Laney, 1917). The Virgilina District was an
important copper ore producing area located within the Carolina Slate Belt of North
Carolina and Virginia. Laney (1917) provides detailed petrographic, mineralogical,
stratigraphic, and structural geologic descriptions of the rocks and geologic formations
observed and mapped in the Virgilina District noting the broad array of igneous and
metamorphic rock types present.
Carpenter (1976) provided an update on North Carolina’s metallic mineral industry
associated with Carolina Slate Belt rocks as well as a descriptive summary of the variety
of rocks encountered. Carpenter made note of the relatively “higher rank metamorphic
rocks (that) occupy the northeast corner” of Person County. Carpenter also described
the geology of the mines in Person County, and his description of the geology
encountered at a mine located several miles east of the present-day location of the Mayo
Plant noted the presence of chlorite-sericite schist and phyllite with “stringers of
epidote, calcite, and quartz” as well as evidence of shearing and faulting within the
schist (Carpenter, 1976). These descriptions are consistent with rocks encountered
during field work (Section 6.0 Site Geology and Hydrogeology).
In 1973, Glover and Sinha (1973) published research results on the evolution of the rocks
of the central Piedmont of North Carolina and Virginia (Glover & Sinha, 1973). Their
extensive mapping work indicated that the present-day Mayo Plant is located
near/along a contact between felsic and intermediate volcanic rocks (Hyco Formation)
and “tuffaceous epiclastic rocks and reworked tuffs” that commonly present as phyllitic
siltstone, phyllite, and other minor occurrences of conglomerate and pyroclastics
(Aaron Formation). The authors set forth their hypothesis for a major deformational
event called the Virgilina deformation that produced major structures in the central
Piedmont. Glover and Charles Harris revisited the Virgilina deformation theory with a
summary of the hypothesis and additional detail and geologic mapping updates
concerning the stratigraphic and petrologic character of the rocks of the north-central
Piedmont in the Roxboro, North Carolina/Virgilina, Virginia area (Harris & Glover,
1985). The Geologic Map of North Carolina (1985) describes the area of the Mayo Plant
as underlain by felsic meta-volcanic rock interbedded with mafic and intermediate
volcanic rock, meta-argillite and meta-mudstone (NCDNRCD, 1985).
In 1991, Butler and Secor (1991) presented an exhaustive summary of geologic research
on the rocks of the Central Piedmont of North Carolina with a subsection pertaining to
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the genesis, stratigraphy, rock associations, rock ages, and structural relationships of the
northern Carolina slate belt. Researchers in the 1990s began to refer to the slate belt as
Carolina “terrane” or the Carolina “zone” in terms of the tectonostratigraphic
relationships between the rock assemblages (Butler & Secor, 1991). The Carolina zone
was described as “a lower greenschist to amphibolite sequence of felsic and mafic
volcanics and metasedimentary rocks intruded by felsic to mafic plutonic rocks of
various ages” (Wilkins, Shell, & Hibbard, 1995). Foliations within the metamorphic
rocks of the Carolina zone are described as steeply dipping along upright folds. These
rock types and rock structures are contrasted with the higher metamorphic grades and
generally gently dipping foliations and recumbent folds in the Piedmont zone (Wilkins,
Shell, & Hibbard, 1995).
Hibbard and his co-authors (1998) describe the north-northeast trending Hyco shear
zone as a terrane boundary separating the Piedmont (and locally, the Charlotte and
Milton terranes) from the Carolina terrane” (Hibbard, Shell, Bradley, & Wortman, 1998).
The Hyco shear zone extends to the east about 8 kilometers into Carolina terrane rocks –
and into the vicinity of the Mayo Plant – and is defined as the hanging wall of the fault
zone. Confirming the earlier mapping of Glover and Sinha (1973) and Harris and
Glover (1985), the rocks of the eastern edge of the Hyco shear zone occur along the
western edge of the Mayo Plant area and are described as “dominantly greenschist
facies felsic volcanic and volcaniclastic rocks with subordinate intermediate and mafic
components” (Glover & Sinha, 1973); (Harris & Glover, 1985). The authors provide a
detailed analysis of the structural nature of the Hyco shear zone and state their position
concerning the timing and large-scale regional tectonic implications of their work
(Hibbard, Shell, Bradley, & Wortman, 1998).
Since 2000, additional research and contributions to the relevant scientific literature on
the Carolina terrane have continued to refine a model of the complicated tectonic,
stratigraphic, and lithographic character and interrelationships of the region, notably
Hibbard, Stoddard, Secor, and Dennis (2002); Bowman (2010); Pollock, Hibbard, and
Sylvester (2010); and Bowman, Hibbard, and Miller (2013).
5.2 Regional Hydrogeology
The upper portions of rocks in the Piedmont are typically fractured and weathered and
are covered with unconsolidated material known as regolith. The regolith includes
residual soil and saprolite zones and, where present, alluvium. Saprolite is formed by
in-situ chemical weathering of bedrock. It is typically composed of clay and coarser
granular material and reflects the texture and structure of the parent rock. For example,
the weathering products of granitic rocks are quartz-rich and sandy textured. Rocks
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poor in quartz but rich in feldspar and ferro-magnesium minerals form a more clayey
saprolite. The degree of weathering decreases with depth, and partially weathered rock
(PWR) is commonly present near the top of the bedrock surface. The transition zone
from the regolith and the PWR and competent bedrock is often gradational and difficult
to differentiate.
Groundwater flow systems in the Piedmont are comprised of two interconnected
hydrogeologic units: (1) residual soil/saprolite and weathered fractured rock (regolith
and PWR) overlying (2) fractured crystalline bedrock (Heath, 1980); (Harned & Daniel,
1992). The regolith layer is a weathered and structureless residual soil that occurs near
ground surface with the degree of weathering decreasing with depth. Residual soil
grades into saprolite. Beneath the saprolite, partially weathered/fractured bedrock
occurs with depth until competent bedrock is encountered. This mantle of residual soil,
saprolite, and weathered/fractured rock (transition zone) is a hydrogeologic unit that
covers and crosses various types of rock (LeGrand, 1988). This layer serves as the
principal storage reservoir and provides a granular medium through which the
recharge and discharge of water from the underlying fractured rock occurs (Harned &
Daniel, 1992).
A transition zone at the base of the regolith is present in many areas of the Piedmont.
The zone consists of partially weathered/fractured bedrock and lesser amounts of
saprolite that grades into competent bedrock and has been described as “being the most
permeable part of the system, even slightly more permeable than the soil zone” (Harned
& Daniel, 1992). The zone thins and thickens within short distances and its boundaries
may be difficult to distinguish. Where present, the zone may serve as a conduit of rapid
flow and transport of impacted groundwater (Harned & Daniel, 1992).
Daniel and Dahlen (2002) provide a summary of the nature and occurrence of
groundwater in fractured rock. Within the fractured crystalline bedrock, fracture
apertures, connectivity, etc. control groundwater movement and storage capacity. The
bedrock is broken and displaced by faults and shear zones, some of which extend for
miles. Joints, rock breaks without accompanying displacement, are common, and the
joints typically occur in groups oriented in preferred directions. Weathering and
erosion have resulted in fracturing in the form of stress-relief fractures, as well as
expansion of existing fractures, and it is through these fractures that groundwater
flows. Planes and bedding of metamorphic foliation, as well as breaks and folds in
these rocks, are areas of higher permeability (Daniel & Dahlen, 2002).
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LeGrand’s (1988; 1989) conceptual model of the groundwater setting in the Piedmont
incorporates the two-medium system described above into a single feature that is useful
for the description of groundwater conditions. That feature is the surface drainage
basin that contains a perennial stream (LeGrand, 1988). In general terms, each surface
drainage basin is similar to adjacent basins and the conditions are generally repetitive
from basin to basin. Within a basin, movement of groundwater is generally restricted to
the area extending from the drainage divides to a perennial stream (Slope-Aquifer
System; (LeGrand, 1988); (LeGrand, 1989); Figure 5-2). Freeze and Witherspoon’s (1967)
model for regional groundwater flow centers on a large regional discharge area that
will receive water from a groundwater basin except for shallow discharges into smaller
perennial streams located closer to sub-regional recharge areas (Freeze & Witherspoon,
1967). Shallow groundwater near perennial streams will discharge into that stream.
The crests of water table undulations represent natural groundwater divides within a
slope-aquifer system and may limit the area of influence of wells or contaminant
plumes located within their boundaries depending on the depth of the impacted
groundwater. The concave topographic areas between the topographic divides may be
considered as flow compartments that are open-ended down slope. Therefore, in most
cases in the Piedmont, the groundwater system is a two-medium system (LeGrand,
1988) restricted to the local drainage basin. Groundwater within the area exists under
unconfined (water table) conditions within the saprolite, PWR/transition zone, and in
the fractures and joints of the underlying bedrock. The water table and bedrock
aquifers are often interconnected. Typically, the residual soil/saprolite is partially
saturated and the water table fluctuates within it. The saprolite and PWR/transition
zone acts as a reservoir for water supply to the fractures and joints in the underlying
bedrock.
Groundwater recharge in the Piedmont is derived entirely from infiltration of local
precipitation. Groundwater recharge occurs in areas of higher topography (i.e., hilltops)
and groundwater discharge occurs in lowland areas bordering surface water
bodies, wetlands, and floodplains (LeGrand, 2004). Average annual precipitation in the
Piedmont ranges from 40 inches to 50 inches with a minimum of about 30 inches and a
maximum of about 80 inches. Mean annual recharge in the Piedmont ranges from
about 4 inches to under 10 inches (Daniel & Dahlen, 2002).
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6.0 SITE GEOLOGY AND HYDROGEOLOGY
Geology beneath the Mayo Plant can be classified into three units. Regolith (surficial
soils, fill and reworked soil, alluvium along the Crutchfield Branch stream valley, and
saprolite) is the shallowest geologic unit. Saprolite is mostly thin (ranging from
nonexistent to around 25 feet deep) and almost entirely unsaturated. This
generalization is not consistent for the southern, upland parts of the Site where a thick,
saturated saprolite zone is present (e.g., MW-12 well pair) nor for certain locations
beneath the ash basin. The thin to nonexistent saprolite zone across the central and
northern portion of the Site is due to extensive excavation and reworking of surficial
materials during Plant construction. A transition zone of partially weathered rock
underlies the regolith (where present, the saprolite is the lowest portion of the regolith)
and is generally continuous throughout the Mayo Plant area. However, the transition
zone at the Mayo Site is comprised mostly of partially weathered rock that is
gradational between saprolite and competent bedrock. The change from partially
weathered rock to the third unit, competent bedrock, is subjective, and at the Mayo Site,
is defined by subtle changes in weathering, secondary staining and mineralization, core
recovery, and the degree of fracturing in the rock. Typically, mildly productive
fractures (providing water to wells) were observed within the top 50 feet to 75 feet of
competent rock.
In general, three hydrogeologic units or zones of groundwater flow can be described for
the Site. The zone closest to the surface is the shallow or surficial flow zone
encompassing saturated conditions, where present, in the residual soil, saprolite, or
alluvium beneath the Site. A transition zone, encountered below the surficial zone and
above the bedrock, is characterized primarily by partially weathered rock of variable
thickness. The transition zone is not consistently saturated across the Site. The bedrock
flow zone occurs below the transition zone and is characterized by the storage and
transmission of groundwater in water-bearing fractures.
Site investigations included installation of soil borings, collection of soil and rock cores,
groundwater monitoring wells, borings in and through the ash basin, and installation of
wells for the sampling of ash pore water. Physical and chemical properties of soil
samples collected from the borings and wells are presented in Tables 6-1 and 6-2,
respectively. The analytical methods used with solid and aqueous samples are
presented in Table 6-3 and Table 6-4. Table 2-1 summarizes the well construction data
for CAMA-related wells and piezometers at the Site. Strategic locations for anchoring
flow path transects were selected. Boring logs for CAMA-related monitoring
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installations are included in Appendix F. Primary technical objectives for the sampling
locations included: the development of additional background data on groundwater
quality; the determination of horizontal and vertical extent of impact to soil and
groundwater; and the establishment of perimeter boundary conditions for groundwater
modeling that will be used to develop a CAP.
The Mayo Plant ash basin occupies the former stream valley of Crutchfield Branch. The
basin acts as an elongated bowl-like feature with groundwater flowing to the basin
from all sides, except from the northeast, which is the discharge side of the basin.
Groundwater flows north-northeast from the ash basin into the small valley formed by
Crutchfield Branch. Crutchfield Branch flows north off of the Site property into
Virginia. Mayo Lake affects groundwater on the east side of the Plant and acts as a
groundwater discharge area. Groundwater flows from the highest topographic portion
of the Site (near the Plant entrance road) to the north and northeast. US Highway 501
crests a ridge just west of the western edge of the ash basin (near MW-13BR) and bisects
groundwater flow to the west and east.
6.1 Site Geology
The regional geologic setting for the Mayo Plant is described in Section 5.0. A geologic
map for the Mayo Plant area is included as Figure 6-1.
The subsurface at the Site is composed of regolith (including residual soils, fill and
reworked soils, alluvium, and saprolite), transition zone, and bedrock. Each zone was
not encountered at every boring location. Subsurface conditions varied with
topography, parent rock, and Site infrastructure. Alluvium was observed at two
locations along Crutchfield Branch (MW-16S and SB-7), with a maximum thickness of
7.5 feet.
The dominant rock types observed were granitoid gneiss and mica gneiss with minor
mica schist and phyllite. The metavolcanic rocks observed were largely felsic with
some mafic metavolcanics and meta-argillites interbedded. In general, metamorphic
grading increases to the southwest and west of Mayo Plant, with plutonic volcanic
bodies observed to the west.
Soil Classification 6.1.1
Regolith was encountered from a depth range of a few inches to 66 feet bgs at the
Site. A distinct organic soil horizon was rarely seen and, where present, was
often only a few inches thick. Grain size analyses of soils indicate that the major
soil texture at the Site is sandy loam, with layers of loamy sand, loam, and clay.
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Mineralogical analyses indicate the presence of quartz, feldspars, amphibole,
chlorite, and a range of clay minerals (chiefly illite) in Site soils.
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 mineralogical partitioning varied from one location to another.
The regolith was dominated by saprolite, the in-situ weathering product of
parent rock, and the mineralogical composition of the saprolite varied with
subsurface lithology. At MW-12D, a background well with 66 feet of saprolitic
regolith, the mineral assemblage consisted of predominantly clay and felsic
minerals with 52.5 percent illite, 14.6 percent kaolinite, 22.6 percent quartz, and
10.0 percent feldspars. In contrast, the mineralogical data at ABMW-2BR, in the
saprolitic regolith below the ash basin shows the presence of significantly more
mafic components with: 33.5 percent quartz, 32.8 percent amphibole, 15.8 percent
feldspars, and 17.8 percent clay minerals
(smectite and mullite). The composition
differences are evidence of the change in
parent rock from largely mica gneiss on
the western portion of the Site and more
mafic metavolcanics and meta-argillites
to the east. Saprolite often contains relict
structure from the parent rock, retaining
directional properties and permeability
(Daniel & Dahlen, 2002). Photograph 6-1
illustrates the relict structure observed in
saprolite from the MW-12S/12D
background location.
Rock Lithology 6.1.2
The Mayo Plant is located near the contact between the Carolina and Charlotte
(or Milton) terranes, within the Hyco Shear Zone (Hibbard, Shell, Bradley, &
Wortman, 1998). Charlotte terrane rock formations consisted of felsic mica and
granitoid gneiss interbedded with hornblende gneiss and phyllites. The Carolina
terrane formations consist of metamorphosed dacitic to rhyolitic flows and tuffs
interbedded with mafic and intermediate metavolcanic rock, meta-argillite, and
meta-mudstone. The contact has a general northeast–southwest trend with the
Charlotte terrane on the west and the Carolina terrane on the East. The
Photograph 6-1: MW-12D (39.0 ft
below ground surface (BGS))
Saprolite showing relict structure.

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metamorphosed rocks have been intruded by coarse-grained granitic rocks and
have been subject to regional structural deformation (Rogers, 2006).
On the west side of the terrane contact, gneiss consisted of fine to coarse mica
gneiss, mica schist, and granitoid layers. Hornblende gneiss in the region is
found in close association with this felsic gneiss and schist as narrow and
irregular areas and dike-like bands (Laney, 1917). An irregular hornblende
gneiss zone within the felsic mica gneiss was observed at the MW-5BR location
as illustrated in Photograph 6-2.

East of the terrane contact, the subsurface lithology consists of granitoid gneiss,
light gray, dark gray and distinctively green meta-argillite and metavolcanic rock
zones. Chemical analysis indicates an increase in chlorite, biotite and muscovite
in rock samples taken from the northeastern portion of the Site.
Chlorites and micas are minerals found in the greenschist facies, and according
to Butler and Secor (1991), the Carolina terrane consists of volcanic and
sedimentary rocks metamorphosed to lower greenschist facies. Photograph 6-3
illustrates a zone of metavolcanic rock of the greenschist facies encountered at
MW-7BR, northeast of the Plant. At the terrane contact, shear stress is evident by
mylonitic texture identified at several boring locations. Photograph 6-4
exemplifies the mylonitic texture observed in some Mayo Plant bedrock.

Photograph 6-2: MW-5BR (42.0-44.5 ft BGS). Sharp contact between
felsic and mafic gneiss.

Photograph 6-3: MW-7BR (57.0-67.0 ft BGS) Greenschist facies rock to
65.7 ft BGS sharp contact with granitic gneiss.

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Structural Geology 6.1.3
Bedrock underlying the Mayo Plant is a combination of Neoproterozoic- Early
Paleozoic, metaigneous-dominated Charlotte (or Milton) and Carolina terranes.
The metamorphic bedrock is associated with the Hyco Shear Zone, which is an
east-northeast to north-northeast trending structure that separates the
infrastructural Charlotte terrane from the suprastructural Carolina terrane
(Figure 5-1). The shear zone contains Charlotte terrane rocks in the footwall to
the west and Carolina terrane rocks in the hangingwall to the east (Hibbard,
Shell, Bradley, & Wortman, 1998).
The younger Charlotte terrane contains plutonic rocks that intrude a suite of
metaigneous rocks of the amphibolite facies with minor phyllite, mica schist, and
quartzite (Hibbard et al., 1998, 2002). According to Dennis and Shervais (1991,
1996), “On the basis of geochemical studies, the metamafic complexes have been
interpreted as arising from suprasubduction zone magmas related to an episode
of arc-lifting.”
The older Carolina terrane is generally lower-grade metaigneous and associated
metasedimentary rocks, of the greenschist to amphibolite facies. Four sequences
comprise the bulk of the Carolina terrane, the Virgilina sequence, the Albemarle
sequence, the South Carolina sequence, and the Cary sequence. The Virgilina
sequence is the oldest (633 Ma to 612 Ma) of the four. It underlies the portion of
the Carolina terrane along the Hyco Shear Zone and includes the Hyco, Aaron,
and Virgilina Formations (Wortman, Samson, & Hibbard, 2000); (Hibbard,
Stoddard, Secor, & Dennis, 2002). The Hyco Formation includes a thick felsic-
intermediate metavolcanic base overlain by metaclastic turbidites of the Aaron
Formation and capped by metabasalt of the Virgilina Formation (Glover &
Sinha, 1973); (Harris & Glover, 1988). In 1995, Samson, Hibbard, and Wortman
published research results indicating that the Virgilina “sequence is composed of
Photograph 6-4: MW-5BR (44.5-47.0 ft BGS). Mafic bedrock with
mylonitic texture with calcite stringers.

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juvenile, largely mantle-derived crust and likely represents a mature arc built
upon an oceanic substrate” (Samson, Hibbard, & Wortman, 1995).
At the Mayo Site, fractures were observed within the bedrock at each bedrock
core hole. The majority of fractures were relatively small (e.g., close and tight)
and appeared to be limited in connectivity between borings. Yields from
pumping or packer testing were low. Mylontic textures were observed in some
bedrock cores, most notably, in MW-8BR. A zone of highly fractured bedrock
was encountered within the competent rock at this location, possibly indicating a
local zone of faulting or shear. This zone also yielded small amounts of
groundwater.
Soil and Rock Mineralogy and Chemistry 6.1.4
Mineralogy and chemistry of the soil and rock encountered are presented in
Section 7.0.
6.2 Site Hydrogeology
According to LeGrand (2004), the soil/saprolite regolith and the underlying fractured
bedrock represent a composite water-table aquifer system. The regolith provides the
majority of water storage in the Piedmont province, with porosities that range from 35
percent to 55 percent (Daniel & Dahlen, 2002). Calculated porosities specific to the Site
(21 percent to 54 percent) are consistent with this range. Two major factors that
influence the behavior of groundwater in the vicinity of the Site include the thickness
(or occurrence) of saprolite/regolith and the hydraulic properties of underlying bedrock.
Thickness of the regolith is directly related to topography, type of parent rock, and
weathering. Topographic highs typically exhibit to thinner soil-saprolite zones, while
topographic lows typically exhibit thicker soil-saprolite zones.
Saprolite thickness at the Mayo Plant ranges from zero to 66 feet (MW-12 location).
LeGrand (2004) makes the generalization that gneiss and schist, which are dominant
rock types at the Site, yield thicker soils and moderate to relatively high fracture
densities compared with the densities of unaltered igneous rocks such as granite.
According to Daniel and Dahlen (2002), foliated rocks such as schist provide planes of
weakness that facilitate fracturing at the onset of weathering. This weathering process
can produce a relatively transmissive zone. Massive igneous/meta-igneous parent rocks
such as granitic gneiss that do not provide tightly spaced planes of weakness and are
less susceptible to secondary porosity development due to weathering. Hydrogeologic
conditions encountered above these rocks revealed less-distinct transition zones than
those at mica schist locations. Porosity of the regolith is directly influenced by parent
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rock type based on susceptibility to weathering. As weathering advances to formation
of clays from mica content, the relative permeability will be reduced.
Hydrostratigraphic Layer Development 6.2.1
Hydrostratigraphic units were identified using the framework described by
LeGrand (2004) where the soil/saprolite regolith and the underlying fractured
bedrock represent a composite water-table aquifer system. Continuous core
drilling techniques were employed to continually observe the subsurface for
saturated zones, weathered in-situ material, and characteristics of underlying
parent rock that may contribute to a water-bearing zone. Borings were advanced
to a depth of 50 feet beyond the top of competent bedrock to define water-
bearing zones within, adjacent to, and underlying the ash basin. Determination
of regolith saturation, transition zone thickness, and potential well yield were
made by the field geologist.
Four hydrostratigraphic units were identified at the Mayo Site. Those units
include the ash pore water (confined to the area of the ash basin); the shallow or
surficial zone (alluvium, residual soil, fill/reworked soil, and saprolite); a
transition zone between surficial materials and competent bedrock; and fractured
bedrock. A description of each is provided in the following section.
Hydrostratigraphic Layer Properties 6.2.2

Ash Pore Water
The ash pore water unit consists of saturated ash material. Ash depths range
from a few feet to approximately 55 feet within the ash harvesting area portion of
the ash basin, where approximately 50 feet of ash is saturated.
Shallow/Surficial Zone
The shallow/surficial flow zone consists of regolith (soil/saprolite) and alluvial
material. Thickness of regolith is directly related to topography, type of parent
rock, and geologic history. Topographic highs tend to exhibit thinner soil-
saprolite zones, while topographic lows typically contain thicker soil-saprolite
zones. Alluvium found along Crutchfield Branch was about 7 feet thick and
directly overlies saprolite. 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, is an exception beneath the Site. Saprolite beneath the power plant
area of the Site and the northern, eastern, and western parts of the Site is almost
entirely unsaturated. Saturated saprolite is encountered more frequently in the
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southern portion of the Site. 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. Wells within
the shallow flow zone that are installed within alluvial and surficial (shallow)
wells contain an “S” designation.
Transition Zone
The transition zone consists of a relatively transmissive zone of partially
weathered bedrock. 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 wells are labeled with a “D” designation.
Fractured Bedrock
The fractured bedrock unit occurs within competent bedrock. Bedrock at the
Mayo Plant is dominated by granitic gneiss interbedded with phyllite, mica
schist and mafic metavolcanic rocks. The majority of water producing fracture
zones was found within 50 to 75 feet of the top of competent rock. Water-bearing
fractures encountered are only mildly productive (providing water to wells).
Bedrock wells are labeled with a “BR” designation.
Two transects were selected to illustrate flow path conditions in the vicinity of the ash
basin. Section A-A’ is a transverse section through the ash basin, perpendicular to
groundwater flow, in relation to the adjacent areas to the northwest and southeast
(Figure 6-2). Section B-B’ illustrates conditions upgradient (south) and along the flow
path within the ash basin, then downgradient/downstream along the Crutchfield
Branch stream valley (Figure 6-3).
6.3 Groundwater Flow Direction
As recognized in previous investigations and through compliance monitoring activities,
groundwater flow is generally to the north-northeast in the direction of the Crutchfield
Branch stream valley. At the Mayo Plant, it is appropriate to combine the flow zones
into one generalized flow map. In large portions of the subsurface beneath the Site, the
surficial flow zone and the transition zone are not saturated, and the shallow bedrock is
the first and only zone where groundwater is encountered. Further, where there are
saturated conditions in either regolith or the transition zone, the difference between the
water levels in wells in those zones, as compared with the level in adjacent bedrock
wells, is miniscule. Horizontal flow dynamics dominate over vertical flow.
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Water levels were measured in Site wells and piezometers within a 24-hour period on
June 19-20, 2017 and November 2-3, 2016 to provide water-level elevation data for wet
and dry seasons (respectively) at the various flow systems observed at the Site (Table 6-
5). The wet and dry seasons for Mayo were established based on precipitation data
from the State Climate Office of North Carolina website (2015) that indicates relatively
more precipitation typically occurs during the spring of each year and less precipitation
during the fall of each year. Groundwater flow directions and the overall morphology
of the potentiometric surface vary little from the “dry” to “wet” seasons. Water levels
do fluctuate up and down with significantly increased or decreased precipitation, but
the overall groundwater flow directions do not change due to seasonal changes in
precipitation.
Water level elevation data indicate a north-northeast flow direction in the multiple
zones of saturation toward the Crutchfield Branch floodplain (Figures 6-4 through 6-
11). The Mayo Plant ash basin occupies the former stream valley of Crutchfield Branch.
The basin acts as an elongated bowl-like feature toward which groundwater flows from
the northwest, west, south, and east. A small topographic high is present along the
eastern side of the ash basin, and groundwater is somewhat radial away from this
feature. Groundwater flow east of the railroad line, which is constructed along a
natural ridge, is to the east and toward Mayo Lake.
Groundwater flows north-northeast from the ash basin into the small valley formed by
Crutchfield Branch. Crutchfield Branch flows north off of the Site into Virginia. Mayo
Lake influences groundwater on the east side of the Plant acting as a groundwater
discharge area. Groundwater flows from the highest topographic portion of the Site
(near the Plant entrance road) to the north and northeast. US Highway 501 follows the
crest of a ridge just west of the western edge of the ash basin and influences flow on this
portion of the Site.
The groundwater flow system at the Site serves both to store and provide a means for
groundwater movement. The porosity of the regolith is largely controlled by pore
space (primary porosity), whereas in bedrock porosity is largely controlled by the
number, size and interconnection of fractures. As a result, the effective porosity in the
regolith is normally greater than in the bedrock and thus the quantity of groundwater
flow will be greater in the regolith. At the Mayo Site, saturated regolith was observed
in only a few wells, and the regolith is the least transmissive of the flow zones. The
majority of groundwater across the Site appears to flow through the transition zone and
bedrock. Downgradient of the Mayo Plant, groundwater gradients in the shallow flow
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zone are affected by man-made features (rail cuts, basins, stormwater run-off ditches)
and the ash basin.
6.4 Hydraulic Gradient
Horizontal hydraulic gradients based on the June 2017 water-level measurements in the
various hydrogeologic zones vary across the Site (Table 6-6). Hydraulic gradients,
vertical and horizontal, have been calculated along transects that follow cross section
B–B’. Horizontal hydraulic gradients were derived by calculating the difference in
hydraulic head over the length of the flow path between two wells with similar well
construction (e.g., wells within the same water-bearing unit). The following equation
was used to calculate horizontal hydraulic gradient:
i = dh / dl i = hydraulic gradient; dh = difference between two hydraulic heads (measured in feet); and dl = flow path length between the two wells (measured in feet).
Generally horizontal gradients along the southern portion of the Site (B-B’) range from
0.017 feet/feet to 0.018 feet/feet. Horizontal gradients along the northern end of the Site
(B-B’) range from 0.03 feet/feet to 0.034 feet/feet. The hydraulic gradient in the northern
portion of the Plant is likely due to the much higher relief between this area and the ash
basin dam.
Vertical hydraulic gradients were calculated by taking the difference in groundwater
elevation in a deep and shallow well pair over the difference in total well depth of the
deep and shallow well pair. A positive output indicates downward flow and a negative
output indicates upward flow. Vertical gradients at select well pairs have been
calculated and are presented on Table 6-7 and visually presented in Figure 6-12.
Throughout the Site, vertical gradients in saprolite, transition zone and bedrock wells
are near equilibrium indicating that there is no distinct horizontal confining layer
beneath the Mayo Plant. The approximate range of hydraulic gradient varies from
0.2682 feet/feet to -0.1333 feet/feet. Generally, there is recharge (downward gradients)
on the northeast portion of the Site and discharge (upward gradients) to the west in and
around the ash basin, with the exception of recharge at the ABMW-2 well cluster.
Upward vertical gradients from bedrock, as groundwater from the west, south, and east
recharge the groundwater beneath the basin into the former Crutchfield Branch stream
valleys, reduces the potential for downward migration of constituents of interest (COI)
into bedrock.
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Beyond the ash basin area, the area near the MW-8 cluster has the greatest downward
gradient of 0.2689 feet/feet. Relatively strong upward gradients occur near the MW-18
cluster and CW-3/MW-3BR pairs. The greatest upward gradient is at MW-18D and
MW-18BR, with a gradient of -0.0627 feet/feet.
6.5 Hydraulic Conductivity
Hydraulic conductivity values for the various hydrogeologic zones in which the wells
are screened varied Site-wide as determined by the slug test method conducted in
accordance with GAP Section 7.1.4 (Table 6-8). Slug test field and analytical methods
are discussed in Section 6.8.
Hydraulic conductivity values for wells screened in saprolite have a geometric mean of
1.48 x 10-4 cm/sec. Hydraulic conductivity values recorded for wells screened in the
transition zone have a geometric mean of 3.31 x 10-4 cm/sec. These measurements reflect
the dynamic nature of the transition zone, where hydrologic properties can be heavily
influenced by the formation of clays and other weathering by-products. Hydraulic
conductivity results for bedrock wells across the Site have a geometric mean of 6.66 x
10-5 cm/sec. The hydraulic conductivity measurements in bedrock wells can be regarded
as a generalized representation of the localized bedrock fractures in specific areas of a
well cluster.
6.6 Groundwater Velocity
To calculate the velocity that water moves through a porous medium, the specific
discharge, or Darcy flux, is divided by the effective porosity, ne . The result is the
average linear velocity or seepage velocity of groundwater between two points.
Groundwater flow velocities for the surficial and transition flow zones were calculated
using Darcy's Law equation which describes the flow rate or flux of fluid through a
porous media by the following formula: 𝐕𝐬=𝐊𝐢/ ne Vs = seepage groundwater velocity;
K = hydraulic conductivity;
i = the horizontal gradient; and
ne = effective porosity

Effective porosities were calculated using laboratory testing and physical soil data
presented in Table 6-1 and estimated on a Fetter-Bear diagram (Johnson, 1967). This
technique provides a simple method to estimate specific yield; however, there are
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limitations to this method that may not provide an accurate determination of the
specific yield of a single sample (Robson, 1993). Groundwater velocities calculated for
the four flow paths described in Section 6.2.2 range from 14.5 feet per year to 25.4 feet
per year (Table 6-6). For each flow zone, the geometric mean from the calculated
hydraulic conductivity from slug tests was utilized to compute velocity (Appendix C).
From background well MW-12S to the ash basin ABMW-3S, there is a horizontal
gradient of 0.017 feet/feet with a velocity of 14.5 feet/year; from the same surficial ash
basin well (ABMW-3S) to the surficial downgradient well (MW-16S) the horizontal
gradient is 0.034 feet/feet and a velocity of 23.3 feet/year. The larger velocity indicated
from the ash basin to the Crutchfield Branch floodplain is due to the potentiometric
head created by the ash basin being constructed over 100 feet higher than the stream
channel into which it flows. Velocities measured across the Plant in the transition zone
range from 21.2 feet/year to 25.4 feet/year.
At Mayo, groundwater movement in the bedrock flow zone is due primarily to
secondary porosity represented by fractures in the bedrock. Primary (matrix) porosity
is negligible; therefore, it is not technically appropriate to calculate groundwater
velocity using effective porosity values and the method presented above. Bedrock
fractures encountered at Mayo tend to be isolated with low interconnectivity. Further,
hydraulic conductivity values measure the fractures immediately adjacent to a well
screen, not across the distance between two bedrock wells. Groundwater flow in
bedrock fractures is anisotropic and difficult to predict, and velocities change as
groundwater moves between factures of varying orientations, gradients, pressure, and
size. For these reasons, bedrock groundwater velocities calculated using the seepage
velocity equation are not representative of actual Site conditions and were not
calculated. For additional information on the movement of groundwater around and
downgradient of the ash basin over time, refer to discussion concerning groundwater
fate and transport modeling (Section 13.0).
6.7 Contaminant Velocity
The degree of migration, retardation, and attenuation of constituents in the subsurface
is a function of physical and chemical properties of the media through which the
groundwater passes. Contaminant velocity depends on factors such as the rate of
groundwater flow, the effective porosity of the aquifer material, and the soil-water
partitioning coefficient, or Kd term. Soil samples were collected and analyzed for grain
size, total porosity, soil sorption (Kd), and anions/cations to provide data necessary for
completion of a fate and transport model.
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Constituents enter the ash basin system in both dissolved and solid phases, and those
constituents may undergo phase changes that include dissolution, precipitation,
adsorption, and desorption. Dissolved phase constituents may undergo these phase
changes as they are transported in groundwater flowing through the basin. Phase
changes are collectively addressed by specifying a linear soil-groundwater partitioning
coefficient (sorption coefficient [Kd]). In the fate and transport model, the entry of
constituents into the ash basin is represented by a constant concentration in the
saturated zone (pore water) of the basin, and is continually replaced by infiltrating
recharge from above. Laboratory Kd terms were developed by University of North
Carolina – Charlotte (UNCC) researchers via column testing of 14 site-specific samples
of soil. The methods used by UNCC and Kd results obtained from the testing are
presented in Appendix C. The Kd data were used as an input parameter to evaluate
constituent fate and transport through the subsurface.

Boron is relatively mobile in groundwater and is associated with low Kd values. This is
primarily because boron is mostly inert, has limited potential for sorption, and lacks an
affinity to form complexes with other ions. In general, the low Kd measured for boron
allows the constituent to move with groundwater. The higher Kd values measured for
other constituents, like arsenic and cobalt, are consistent with the observed, limited
migration of these constituents. Constituents like cobalt and arsenic have much higher
Kd values and will move at a much slower velocity than groundwater as it sorbs onto
surrounding soil. It should be noted that the fractured bedrock flow system is highly
heterogeneous in nature and low permeability zones predominate. Geochemical
mechanisms controlling the migration of constituents are discussed further in Section
13.0. Groundwater modeling to be performed for the updated CAP will include
discussion of contaminant velocities for the modeled constituents.
6.8 Slug Test and Aquifer Test Results
As previously discussed, hydraulic conductivity values for the various hydrogeologic
zones in which the wells are screened varied Site-wide as determined by the slug test
method conducted in accordance with GAP Section 7.1.4 (Table 6-8). Slug test field and
analytical methods are included in Appendix G, and results are presented in
Appendix C.
Slug tests were conducted for all wells installed for the 2015 groundwater assessment
except in cases where there was not a sufficient amount of water in the well for the test.
Hydraulic conductivity results for the slug tests are summarized below. Where
multiple tests were conducted for the same well, the geometric mean result is used.
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Saturated alluvium sediment is scarce throughout the Site and consists of fine to course
sand mixed with fine to course gravel. One well, MW-16S, was screened across the
saturated alluvium and the hydraulic conductivity is 6.09 x 10-6 cm/sec.
Slug test data was analyzed for CAMA and other regulatory wells were screened across
the surficial, transition, and bedrock zones throughout the Site. Hydraulic conductivity
values for wells screened in saprolite have a geometric mean of 1.48 x 10-4 cm/sec.
Hydraulic conductivity values recorded for wells screened in the transition zone have a
geometric mean of 3.31 x 10-4 cm/sec. Hydraulic conductivity results for bedrock wells
across the Site have a geometric mean of 6.66 x 10-5 cm/sec.
Infiltration tests using Guelph permeameters were not performed because the
groundwater model developer indicated that those data would not be needed because
slug test data were available. Shelby tube samples were collected at nine locations and
were used for vertical hydraulic conductivity tests, each conducted on media from five
distinct zones: saprolite, residual soil, ash, alluvium, and fill from the toe of the ash
basin (Table 6-9). The vertical conductivities were calculated to be, on average, three to
four orders of magnitude smaller than the horizontal results. These data indicate that
lateral groundwater flow predominates over vertical flow at the Site.
6.9 Fracture Trace Study Results
Fracture trace analysis is a remote sensing technique used to identify lineaments on
topographic maps and aerial photography that may correlate to locations of bedrock
fractures exposed at the earth’s surface. Although fracture trace analysis is a useful tool
for identifying potential fracture locations, and by extension, potential preferential
pathways for infiltration and flow of groundwater near a site, results are not definitive.
Lineaments identified as part of fracture trace analysis may or may not correspond to
actual locations of fractures exposed at the surface, and if fractures are present, it cannot
be determined from fracture trace analysis whether these are open or healed.
Strongly linear features at the earth’s surface are commonly formed by weathering
along steeply dipping to vertical fractures in bedrock. Morphological features such as
narrow, sharp-crested ridges, narrow linear valleys, linear escarpments, and linear
segments of streams otherwise characterized by dendritic patterns are examples. Linear
variations in vegetative cover are also sometimes indicative of the presence of exposed
fractures, though in many cases these result from unrelated human activity or other
geological considerations (e.g., change in lithology).
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Straight (as opposed to curvilinear) features are commonly associated with the presence
of steeply dipping fractures. Curvilinear features in some cases are associated with
exposed moderately-dipping fractures, but these also can be a result of preferential
weathering along lithologic contacts, or along foliation planes or other geologic
structure. As part of this study, only strongly linear features were considered, as these
are far more commonly indicative of steeply dipping or vertical fractures.
The effectiveness of fracture-trace analysis in the eastern United States, including in the
Piedmont, is commonly hampered by the presence of dense vegetative cover, and often
extensive land-surface modification owing to present and past human activity. Aerial-
photography interpretation is most affected, as identification of small-scale features can
be rendered difficult or impossible in developed areas.
Available geologic maps for the area were consulted prior to performance of aerial-
photography and topographic-map interpretation, to identify lithologies and geologic
structure in the area that can control fracture occurrence and orientations. Fracture trace
analysis was performed in the vicinity of the Site, and no major faults or shear zones
were identified (Figure 6-13). The fracture trace analysis indicates a predominance of
southwest-northeast trending features, especially the former Crutchfield Branch stream
valley directly below the ash basin. Typical of the Piedmont, joint sets perpendicular to
the southwest-northwest trend were also prevalent. Minor north-south trending
features were noted. The observations were corroborated by direct field observations
and mapping of surface exposures in the area around Mayo Plant and Mayo Lake.
Measured (using a Brunton pocket transit) joint set orientations and dominant foliation
trends in rocks near the Plant ranged from approximately N30E to N65E. Secondary
joint sets were measured with northwest-southeast and north-south trend lines.

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7.0 SOIL SAMPLING RESULTS
Soils samples were collected and tested in accordance with GAP (Section 7.1.1) and the
analytical methods for testing soil are summarized in Table 6-3. Soils test data is
included in Appendix C. Soil borings were conducted in upgradient and downgradient
areas of the ash basin to collect soil samples from the unsaturated zone and the zone of
saturation for these areas (Figure 2-8). Physical property testing of soil and saprolite
indicate that Site soils are predominantly sand- and silt-sized (sandy loam) with some
coarse sandy loams, silty loams, and minor clay inclusions (Table 6-1). Mineralogical
analysis of soil samples indicate clay minerals (illite, smectite, and kaolinite) comprise
the bulk portion of Site soils, along with quartz, feldspars, and amphiboles (Table 6-2).
7.1 Background Soil Data
Four upgradient borings (SB-1, SB-2, SB-3, and SB-4) and groundwater monitoring wells
(MW-10BR, MW-11BR, MW-12S, MW-12BR, and MW-14BR) were originally installed
for use as background wells and borings.
A background soil dataset based on the 2015 CSA data was provided to NCDEQ on
May 26, 2017 for consideration of background soil concentrations. Additionally, the
revised Statistical Methods for Developing Reference Background Concentrations for
Groundwater and Soil at Coal Ash Facilities (statistical methods document) (HDR and
SynTerra, 2017) was provided to NCDEQ as a basis for determination. On July 7, 2017,
NCDEQ provided a response letter for each Duke Energy coal ash facility that
identified soil and groundwater data appropriate for inclusion in the statistical analysis
to determine PBTVs for both media. NCDEQ requested that Duke Energy collect a
minimum of 10 valid background samples, rather than the previously planned eight
that was provided, prior to the determination of PBTVs for each constituent. In
addition, soil samples meeting the following criteria are considered valid for use in
statistical determinations of PBTVs:
Sample was collected from a location that is not impacted by coal combustion
residuals or coal associated materials.
Sample was collected from a location that is not impacted by other potential
anthropogenic sources of constituents.
Sample was collected from the unsaturated zone, greater than one foot (ft) above
the seasonal high water table elevation.
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NCDEQ determined samples collected from several locations/depths do not meet
NCDEQ Inactive Hazardous Site Branch (IHSB) Guidance requirements; therefore, they
are not appropriate for use in determining PBTVs. The background soil dataset
included laboratory reporting limits for antimony and thallium above the NCDEQ IHSB
PSRG Protection of Groundwater values (dated October 2016). NCDEQ requested the
values for antimony and thallium be reported below the PSRG Protection of
Groundwater values.
To address these requirements, additional soil samples were collected from background
locations on July 18, 2017 (Figure 2-8). Boring logs associated with the additional soil
samples are included in Appendix F. The updated background dataset was screened
for outliers prior to statistical determinations. Soil PBTVs were submitted to NCDEQ
and partially approved on September 1, 2017 (NCDEQ, September 2017; Appendix
A). PBTV values were accepted for all constituents except copper, iron, manganese,
sodium, and thallium. PBTVs were recalculated and concurrence on values was
achieved in a meeting on October 13, 2017 between NCDEQ, Duke Energy, and
SynTerra. PBTVs for soil constituents were computed and are provided in Table 7-1. A
background summary report for soils is included as Appendix H.

7.2 Facility Soil Data
Soil samples were collected during CSA monitoring well installations. Comparison of
soil analytical results with background is discussed below based on the area of the Site.
Soil Beneath Ash Basin
The contact between the ash and underlying soils in the ash basin borings was visually
distinct. There was no visible evidence of substantial migration of ash into underlying
soils or mixing of ash with those soils.
Five soil samples were collected from three boring/monitoring well locations below the
ash basin. Arsenic, beryllium, barium, boron, calcium, chromium, and strontium
exceeded the PBTV in at least one soil sample (Appendix B, Table 4). Chromium was
the only constituent detected in a concentration that exceeded the Industrial PSRG. Six
constituent concentrations (arsenic, chromium, cobalt, iron, manganese, and vanadium)
exceeded the POG in the soil below the ash harvesting area (dewatered portion of the
ash basin).
SPLP was used to determine the ability of simulated rainwater to leach site-specific
constituents out of the soil to groundwater. The 2L/IMAC standards are used for
reference only of SPLP data; SPLP test results do not represent groundwater; therefore,
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comparison to 2L/IMAC is not required. The SPLP analyses revealed that only
chromium, cobalt, iron, manganese, and vanadium leach from soils beneath the ash
basin at concentrations that exceed the 2L/IMAC. Further, those exceedance
concentrations were only in one location beneath the ash stack area (AMBW-02BR).
Cobalt, iron, manganese, and vanadium appear to be ubiquitous across the Mayo Site in
soils regardless of location (e.g., beneath ash, upgradient, downgradient) and tend to
leach in concentrations that are often greater than the 2L/IMAC in the leachate even for
soils not beneath the ash basin.
Soil Beyond Waste Boundary and Within Compliance Boundary
Soil samples were collected from soil borings and monitoring wells outside of the ash
basin waste boundary (downgradient, sidegradient). Arsenic and chromium exceeded
the Industrial PSRG in these soil samples. Detected concentrations of chromium, cobalt,
iron, manganese, and vanadium in multiple soil samples exceeded the PSRG POG.
Detected concentrations of selenium (three samples) and thallium (four samples)
exceeded the PSRG POG in one soil sample each. SPLP results for soils beyond the
waste boundary indicate that cobalt, iron, manganese, and vanadium readily leach from
natural soils. Leaching of chromium from natural soils is inconsistently observed.
Detected soil concentrations of beryllium, boron, cadmium, calcium, chloride,
chromium, copper, lead, manganese, mercury, nickel, strontium, thallium, and
vanadium exceeded a background soil PBTV in at least one sample. Detections of COIs
in soil were sporadic and inconsistent and did not indicate a source of soil impact
beyond the ash basin waste boundary.
Comparison of PWR and Bedrock Results to Background
Three samples were collected from the transition zone or bedrock and analyzed as soil
samples. MW-12D (88.5–90) was collected from weathered mica schist. MW-13BR (52-
54) was silt with sand and rock fragments collected within the transition zone. MW-
16BR (54.5-55.5) was dark gray silt collected from within a fracture zone in phyllite.
Chromium exceeded the Industrial PSRG and POG for each of three samples. Cobalt,
iron, manganese, and vanadium exceeded the PSRG POG in all three samples.
Beryllium, cadmium, chromium, and nickel concentrations in MW-12D (88.5-90)
exceeded the PBTV. Boron and calcium exceeded the PBTV in MW-16BR (54.5-55.5).
Secondary Sources
For soil samples beneath the ash basin, only arsenic and strontium concentrations in
one soil sample exceeded both the calculated soil PBTV and the NCDEQ Preliminary
Soil Remediation Goals (PSRG) Protection of Groundwater (POG) value. Strontium,
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which does not have a specific PSRG POG, was also detected in the same soil sample at
concentrations greater than the PBTV. Arsenic is not detected in groundwater at
concentrations greater than the 2L beyond the ash basin waste boundary. Strontium is
present above the groundwater PBTVs beyond the compliance boundary in the surficial
aquifer only. No other COIs were detected in soil beneath the ash basin at
concentrations greater than both a PBTV and POG value.
Analysis of soil analytical data presented in Appendix B, Table 4 and Table 7-2 shows
that only in a limited extent have COIs from the source mobilized and sorbed onto soils
beneath the ash basin. Arsenic and strontium were detected at concentrations greater
than their respective PBTV or PSRG, whichever is higher. Figure 7-1 shows the
exceeding data in relation to the ash basin. Although there are limited exceedances of
PBTVs and/or PSRGs in soil beneath the ash basin, the distribution of COIs does not
appear widespread beneath the ash basin area.
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8.0 SEDIMENT RESULTS
Sediment samples were collected during the CSA from 10 locations beyond the
perimeter of the ash basin (Figure 2-8). Sediment analytical results are presented in
Appendix B, Table 5.
8.1 Sediment/Surface Soil Associated with AOWs
Seven sediment sample locations were co-located with designated Areas of Wetness
(AOWs). For some of these locations, solid material was collected at or near the point of
emergence or flow of water. In most cases, the solid material that was collected was
actually surface soil over which water originating at the AOW was flowing. For sample
locations S-3 and SW-CB1, sediment was collected from the bottom of the channel. A
description of the AOW and the results of sediment analysis are provided below:
S-1: Engineered toe drain system (west toe drain) draining into a concrete
structure. Sediment was collected from the bottom of the structure. Arsenic,
chromium, iron, and manganese concentrations exceeded the PSRG POG. Boron
was also detected in this sediment sample. Arsenic, boron, iron, and zinc
concentrations exceeded the respective soil PBTV.
S-2: Engineered toe drain system (east toe drain) draining into a concrete
structure. Sediment was collected from the bottom of the structure. Chromium,
cobalt, iron, manganese, and vanadium concentrations exceeded the POG. There
were no concentrations that exceeded a PBTV.
S-2B: Adjacent to the S-2 structure in an area of rip rap at the toe of the east side
groin of the ash basin dam. Flow appears to originate several hundred feet
upslope within the groin. Sediment was collected from beneath/around rip rap.
There were no concentrations that exceeded a PBTV. Cobalt, iron, and
manganese concentrations exceeded the PSRG POG.
S-3: Located approximately halfway between the east toe drain (S-2) and Mayo
Lake Road in the former Crutchfield Branch valley and downstream of the
confluence of water originating from S-1 area, S-2 area, and S-8. Chromium,
cobalt, iron, manganese, and vanadium concentrations exceeded the PSRG POG
only. Chromium and lead concentrations exceeded the respective PBTVs.

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S-4: Adjacent to S-1 in a wide, shallow channel comprised mostly of flow from
S-1. Water was typically stagnant with abundant iron oxide floc and algae
growth. Sediment collected from channel. Arsenic, chromium, cobalt, iron,
manganese, and vanadium concentrations exceeded both the PSRG POG. Boron
was also detected in this sediment sample. Arsenic, boron, iron, and manganese
concentrations exceeded the respective PBTVs.
S-8: Located in hillside adjacent to ash basin dam. Flow emerges from hillside at
several points, merges with diffuse flow along hillside into a single “channel,”
and flows downhill. Sediment collected from inundated surface soil at the point
of water emergence. Chromium, cobalt, iron, manganese and vanadium
exceeded the POG but only manganese exceeded a PBTV.
SW-CB1: Located in the former Crutchfield Branch valley downstream of S-3 at
the culvert under Mayo Lake Road. Chromium, cobalt, iron, manganese, and
vanadium concentrations exceeded the POG; however, when compared to soil
PBTVs, none of the detected concentrations exceeded the respective PBTVs.
The detected concentrations of shallow, sediment/inundated surface soils that are
laterally restricted in areal extent do not indicate a source of impact to groundwater.
8.2 Sediment in Major Water Bodies
Three sediment sample locations were located in surface water bodies. For these
locations, sediment was collected from the bottom of the stream channel. As described
below, two of the locations are upstream reference locations and one of the locations is
downstream of the ash basin. A description of the sediment sampling location and the
results of sediment analysis are provided below:
SW-REF1: Reference location in wooded area south of the power plant and
upstream from the ash basin. This location is downstream of NPDES permitted
stormwater outfall 010. This location is upstream from S-6. The stream is
approximately 3 feet wide with a silt and cobble bottom substrate. Chromium,
iron, manganese, and vanadium concentrations exceeded the POG; however,
when compared to soil PBTVs, none of the detected concentrations exceeded the
respective PBTVs.
S-6: Reference location in wooded area south of the power plant and upstream
from the ash basin. This location is downstream of NPDES permitted stormwater
outfall 010. The stream is approximately 6 feet wide with a silt, floc, and cobble
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bottom substrate. Chromium, cobalt, iron, manganese, and vanadium
concentrations exceeded the POG; however, when compared to soil PBTVs, none
of the detected concentrations exceeded the respective PBTVs.
SW-CB2: Located in Crutchfield Branch north of the Mayo Plant on privately
owned property. Chromium, cobalt, iron, manganese, and vanadium
concentrations exceeded the POG; however, when compared to soil PBTVs, none
of the detected concentrations exceeded the respective PBTVs.
The detected concentrations of COIs in sediment from water bodies do not indicate a
source of impact to groundwater. In fact, exceedances of PSRGs in upstream sediment
samples mirror those of samples taken downstream of the ash basin. None of the
detected constituents, with the exception of chromium in S-3, exceeded a PBTV for soil.
Detected concentrations for sediment from reference locations are similar, and in some
cases higher than, those from downstream locations.

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9.0 SURFACE WATER RESULTS
The Mayo Plant ash basin is located in the central part of the Site. The ash basin receives
surface water runoff and groundwater recharge from upland areas that are northwest,
west, south, and southeast of the ash basin. Groundwater from the ash basin discharges
downgradient of the ash basin dam into the former Crutchfield Branch valley. Flow
from the west toe drain (S-1) follows a small channel to the north (S-4) and then east
where it merges with the channel from the east toe drain (S-2). Several AOWs, in
addition to the toe drains (S-1 and S-2), emerge beneath or near the ash basin dam (S-
1A, S-2A, S-2B, S-8, and S-10). Flow from AOWs S-2A and S-2B merge with the flow
from the east toe drain in the channel downstream of the toe drain. Flow from S-8
merges with the flow originating from the east toe drain prior to it merging with the
flow originating at the west toe drain. The combined flow re-forms (represented by
sample S-3) and flows north toward and beneath Mayo Lake Road (sample location SW-
CB1). S-10 has only been noted to be flowing one time and has been dry during each
subsequent observation/sampling attempt. Flow from S-10 would enter the system just
upstream from AOW location SW-CB1.
Crutchfield Branch reforms north of the Plant property boundary and flows through a
heavily beaver-impounded channel toward the Virginia state line (surface water sample
SW-CB2). A small intermittent tributary to Crutchfield Branch forms north of the 1981
C&D Landfill and flows north beneath Mayo Lake Road (SW-CBT1), eventually
merging with Crutchfield Branch off-site.
Aqueous samples discussed within the following sections include three distinct types:
1) ash basin wastewater, 2) AOWs, and 3) named surface waters. For the scope of this
CSA, it is only appropriate to compare named surface waters to NCDENR Title 15A,
Subchapter 02B Surface Water and Wetland Standards (2B) because AOWs, wastewater
and wastewater conveyances (effluent channels) are evaluated and governed wholly
separate in accordance with the NPDES Program administered by NCDEQ DWR. This
process is on-going in a parallel effort to the CSA and subject to change.
Surface water and AOW analytical results are included in Appendix B, Table 2, and
Table 3. The surface water sample locations are included on Figure 2-8.
Ash Basin Water Samples
Sample S-5 was collected directly from the water column in the ash basin adjacent to the
wastewater treatment area and prior to water flowing into the forebay. Water in the ash
basin is a combination of groundwater that may have discharged into the basin, water
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column that is in contact with ash present in the bottom of the ash basin, local storm
water runoff, cooling tower blowdown, water sourced from permitted internal outfalls,
and treated wastewater. Ash sluicing was discontinued in October 2016.
Area of Wetness (AOW) Sample Locations
Eleven AOWs have been identified and sampled routinely for monitoring purposes.
The Mayo Site is inspected semi-annually for the presence of existing and potentially
new AOWs along the former Crutchfield Branch valley and along and downgradient of
the ash basin. Inspections include observations of the ash basin along the toe of the
dam; areas below full pond elevation for the ash basin; between the ash basin and
receiving waters; and drainage features associated with the basin including engineered
channels. Per the interim administrative agreement, these inspections are governed by
a Discharge Identification Plan (DIP) until the NPDES permit is issued.
These AOWs include:
Two engineered toe drains (S-1 and S-2) at the base of the ash basin dam,
Four AOW locations in/around the ash basin (S-1A, S-2B, S-8, and S-10),
Three AOW locations within the channels formed from the combined flow of the
toe drain outfalls (S-3, S-4, SW-CB1),
One location from an intermittent flow in a flow regime distinct from the ash
basin (SW-CBT1), and
One location upstream/upgradient of the ash basin (S-9).
Surface Water Sample Locations
Three surface water samples were also collected and analyzed. SW-CB2 was collected
from Crutchfield Branch on private property north of the Plant property boundary and
at the North Carolina/Virgina state line. SW-REF1 and S-6 were collected from the
southern portion of the Mayo Site, upstream and upgradient from the ash basin, from a
small stream that originates south of the coal pile area, flows beneath the Plant entrance
road, and eventually to Mayo Lake. This stream channel is associated with NPDES
stormwater outfall 010. SW-REF1 is located about 100 feet upstream of where a
previously identified, but now seasonally dry, AOW (S-7) merged with the small
stream. S-6 is situated several hundred feet upstream from the point that the stream
enters Mayo Lake. These two locations were intended for use as reference locations for
other Site surface water locations.
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NCDEQ Sample Locations
NCDEQ directed the sampling and analysis of AOWs in March 2014. The locations and
analytical results from this sampling event were provided by NCDEQ to Duke Energy
and are assumed to be accurate. Assessment samples collected during the CSA were
analyzed for a more exhaustive constituent list than those collected in 2014 by NCDEQ
and Duke Energy (for example, cobalt was not analyzed during the 2014 sampling
efforts).
9.1 Discussion of Results for Constituents Without Established 2B
Standards
A 2B has not been established for a number of constituents. A summary of these results
for COIs without 2B standards follows. The results from surface water location SW-CB2
are compared to the upgradient, reference surface water data from locations SW-6 and
SW-REF1. The background surface water concentrations have not been statistically
derived or approved by NCDEQ and are for discussion purposes only.
Boron is detected in SW-CB2 but not in reference locations.
Strontium was detected in both SW-CB2 and in reference location SW-REF1;
however, the detected concentrations in SW-CB2 were generally two to three
times higher than in SW-REF1. Other constituents are higher in SW-CB2 surface
water than in the reference locations (e.g., TDS, cobalt, iron, manganese);
although, not several times higher.
Chromium was only detected in a reference location (SW-REF1).
Aluminum and vanadium concentrations were higher in the reference locations
than in the downstream surface water location (SW-CB2).
9.2 Comparison of Exceedances of 2B Standards
The following surface water location occurs in Crutchfield Branch and sample results
are compared to 2B (Class B) values.
SW-CB2
Surface water sample results from upstream, reference locations that eventually flow to
Mayo Lake are compared to 2B (Class WS-V) values.
S-6, SW-REF1
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A review of historic surface water sample data indicates that only turbidity and
dissolved oxygen (DO) concentrations have exceeded a 2B standard. Specifically, the
historic data indicate the following 2B exceedances:
S-6: DO (1 of 8 results); Turbidity (1 of 8 results)
SW-REF1: DO (3 of 6 results)
SW-CB2:
• DO (3 of 6 results)
• Dissolved copper was reported above the 2B in one sample collected/
analyzed in April 2017.
9.3 Discussion of Surface Water Results
As previously described, prior to construction of the ash basin, Crutchfield Branch was
a perennial stream that originated about 1,000 feet southwest of the current ash basin
footprint. The ash basin now encapsulates the former headwaters of Crutchfield Branch
and two smaller, intermittent streams that flowed into Crutchfield Branch.
Groundwater underlying the ash basin flows north-northeast and eventually through
the base of the dam. Several AOWs and discharges from engineered toe drains emerge
from the area around the ash basin dam and contribute to the volume of water flowing
downstream of the dam in the former Crutchfield Branch stream valley. The reformed
Crutchfield Branch flows northeast from Mayo Lake Road toward the state line.
Crutchfield Branch, its valley, and its tributaries are groundwater discharge zones
downgradient of the ash basin.
The current AOW and surface water data reflect that the majority of the flow in
Crutchfield Branch is associated with engineered drainage from the ash basin
immediately below the dam and other natural seepage of ash basin water. The
groundwater in the area in deeper flow zones near the receiving stream generally
contains constituent concentrations less than those of the receiving waters.
Boron concentrations are greatest proximate to the engineered toe drains. Boron
concentrations in Crutchfield Branch decrease from the Mayo Lake Road to the state
line. Manganese concentrations are similarly consistent immediately downstream of the
ash basin and begin to decline downstream; however, at the most downstream surface
water location (SW-CB2), manganese concentrations increase.
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Piper diagrams, graphical representations of major water chemistry using two ternary
plots and a diamond plot, for AOWs and surface water are included as Figure 9-1. One
of the ternary plots shows the relative percentage of major cations in individual water
samples, and the other shows the relative percentage of the major anions. The
geochemical nature of AOW and surface water samples projected on the diagrams
coincides closely with monitoring wells MW-3 and MW-16S, both installed within the
alluvium of Crutchfield Branch. The geochemical signatures of surface water from
upgradient, reference locations also coincide closely with each other and are distinct
from AOWs/surface water downstream of the ash basin. Piper diagrams for
groundwater monitoring wells are presented and discussed in more detail in Section
10.0.
Additional surface water sampling will be performed and an evaluation of potential
impacts of groundwater on surface water will be presented in the CAP.
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10.0 GROUNDWATER SAMPLING RESULTS
This section provides a summary of groundwater analytical results for the most recent
monitoring event (2Q2017; March-April 2017) and discussion of historical data results
and trends. A comprehensive table with all media analytical sample results is provided
in Appendix B. As indicated on the comprehensive data table, at the request of
NCDEQ, the groundwater results have been marked to indicate data points excluded
based on a measured turbidity greater than 10 NTUs; high pH values that may indicate
possible grout intrusion into the well screen; and data that may be auto-correlated
because it was collected within 60 days of a previous sampling event. The most recent
(March-April 2017) valid data collected is presented on the pertinent maps.
Two limited rounds of sampling and analysis were conducted in March 2015 and June
2015. They were included in the initial CSA report (SynTerra, 2015a). In addition, the
following monitoring events have been completed:
Comprehensive Round – September 2015 (reported in CAP, Part 1)
Comprehensive Round – December 2015 (reported in CAP, Part 2)
Comprehensive Round – January 2016 (reported in CSA Supplement 1)
Limited Round (background wells; wells located along flow transect) – April
2016 (reported in CSA Supplement 1)
Limited Round (background wells; wells located along flow transect) – July 2016
Limited Round (background wells; wells located along flow transect) –
September 2016
Comprehensive Round – November 2016
Comprehensive Round – February 2017
Comprehensive Round – April 2017
Groundwater sampling methods were in general accordance with the procedures
described in the GAP (SynTerra, 2014c) and included in Appendix G. Analytical data
reports are included in Appendix I. A background summary report for groundwater is
included as Appendix H.
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10.1 Background Groundwater Concentrations
Locations for background monitoring wells installed in 2015 for the initial CSA field
effort were chosen based on the information available including the previously installed
NPDES monitoring well network, horizontal distance from the waste boundary, the
relative topographic and groundwater elevation difference compared to the ash basin
surface water. After the background wells were installed and a sufficient number of
samples were collected, statistical analysis was used to confirm the analytical results
represented background conditions.
The following monitoring wells have been approved by NCDEQ as background
monitoring wells (Zimmerman to Draovitch, July 7, 2017; Appendix A). Background
monitoring wells are depicted on Figure 2-8.
MW-12S – Surficial
BG-2 – Transition Zone
MW-12D – Transition Zone
BG-1 – Bedrock
MW-13BR – Bedrock
MW-14BR - Bedrock
Monitoring wells BG-1 (transition zone) and BG-2 (bedrock) were installed prior to
2015. Samples have been collected from these wells since 2010, and both wells are
currently used as background wells for NPDES and other monitoring programs. Both
wells are hydraulically upgradient of the ash basin. Monitoring well pair MW-12S and
MW-12D is located approximately 2,500 feet upgradient of the ash basin. Geologic
conditions encountered at the MW-12 well pair were different than conditions
encountered elsewhere at the Site. Saprolite encountered at MW-12 (approximately 70
feet) was the thickest observed at the Site and the saprolite graded into highly
weathered mica schist before grading into weathered gneiss. This portion of the Site
was not subjected to significant grading and reworking, and the “natural” thickness of
saprolite is still intact.
Monitoring well MW-13BR is upgradient of the ash basin on the west side of US
Highway 501. MW-13BR was installed on the edge of a topographic draw that extends
from the westernmost arm of the ash basin, only about 750 feet to the east. The well
was to be used to investigate the potential of a fracture zone presenting itself as a linear
topographic draw. However, based on groundwater elevation data, it is apparent that
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MW-13BR is upgradient of influence from the ash basin and serves as a background
bedrock monitoring location. MW-14BR is located on the northwest portion of the Site
east of US Highway 501. The hydraulic gradient from MW-14BR to the ash basin is
downward approximately 25 feet across the 750 feet of horizontal elevation change.
Monitoring wells MW-5BR, MW-10BR, and MW-11BR were originally proposed as Site
background wells but have been removed from use as background well. MW-11BR is
screened in a rock type that was only encountered at this drilling location, namely a
hard, dark green hornblende, chlorite-bearing gabbro. There has been a steady increase
in pH since installation in 2015; therefore, the well screen is thought to have been
impacted by grout during the well installation process. Bedrock samples from the well
screen zone were analyzed for paste pH which indicated a pH of 9.30 for the bedrock.
This indicates that the pH values of groundwater measured in the field were likely a
result of grout incursion. Therefore, the analytical data for this well is no longer used;
however, the well is retained for use as a piezometer only.
Background Dataset Statistical Analysis 10.1.1
The revised background groundwater datasets and statistically determined
PBTVs are presented below. The current background monitoring well network
consists of wells installed within three flow zones – surficial, transition zone, and
fractured bedrock. Well locations are presented on Figure 2-8. For groundwater
datasets with less than 10 valid samples available for determination of PBTVs, no
formal upper tolerance limit (UTL) statistics were run and the PBTV for a
constituent and groundwater flow system were computed to be either:
The highest value, or
If the highest value is above an order of magnitude greater than the
geometric mean of all values, then the highest value should be considered
an outlier and removed from further use and the PBTV is computed to be
the second highest value.
This procedure applies to the dataset for the surficial flow zone. NCDEQ
requested that the updated background groundwater dataset exclude data from
the background data set due to one or more of the following conditions:
Sample pH is greater than or equal to 8.5 standard units unless the
regional NCDEQ office has determined an alternate background threshold
pH for the Site.
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Sample turbidity is greater than or equal to 10 NTUs.
Result is a statistical outlier identified for background sample data
collected through second quarter 2017.
Sample collection occurred less than a minimum 60 days between
sampling events.
Non-detected results are greater than 2L/IMAC.
Statistical determinations of PBTVs were performed in accordance with the
revised Statistical Methods for Developing Reference Background Concentrations for
Groundwater and Soil at Coal Ash Facilities (statistical methods document) (HDR
and SynTerra, 2017).
Background datasets provided to NCDEQ on May 26, 2017 were revised based
on input from NCDEQ in the July 7, 2017 correspondence. The revised
background datasets for each flow system used to statistically determine
naturally occurring concentrations of inorganic constituents in groundwater are
provided in Table 10-1. The following sections summarize the refined
background datasets along with the results of the statistical evaluations for
determining PBTVs.
Shallow/Surficial Flow Unit
One well, MW-12S, monitors background groundwater quality within the
surficial (saprolite) flow zone. Eleven samples have been collected from MW-12S
since June 2015. Samples collected less than 60 days between sampling events,
statistical outliers, and invalid data have been omitted from use in statistical
determinations which has resulted in six valid samples for the surficial flow
zone. Additional samples will be collected from MW-12S to achieve the
minimum requirement of 10 valid samples for determining PBTVs. Only iron,
cobalt, manganese, and vanadium currently have a PBTV greater than the
2L/IMAC.
Transition Zone Flow Unit
Two wells, BG-2 and MW-12D, monitor background groundwater quality within
the transition zone. The transition zone background groundwater dataset meets
the minimum requirement of 10 samples for all constituents except radionuclides
(radium-226, radium-228, uranium-233, uranium-234, uranium-236, and
uranium-238). PBTVs for radionuclides were computed to be either the
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maximum value, or, if the maximum value was above an order of magnitude
greater than the geometric mean of all values, the second highest value. PBTVs
were calculated for all remaining constituents monitored within the transition
zone using formal UTL statistics. Only iron, manganese, and vanadium currently
have a PBTV greater than the 2L/IMAC for the transition zone unit.
Fractured Bedrock Flow Unit
Three wells (BG-1, MW-13BR, and MW-14BR) monitor background groundwater
quality within the fractured bedrock. NCDEQ identified four samples collected
from BG-1 and two samples collected from MW-13BR that were collected less
than 60 days between sampling events and are not appropriate for use in
statistical determinations. The background bedrock dataset satisfies the
minimum requirement of 10 samples to use formal UTL statistics to derive
PBTVs for each constituent monitored within fractured bedrock. Only iron,
cobalt, manganese, and vanadium currently have a PBTV greater than the
2L/IMAC for the bedrock flow unit.
Summary
The calculated groundwater PBTVs were less than their applicable 2L/IMAC for
every constituent within each of the three flow units except:
Cobalt: PBTVs of 1.02 µg/L (surficial); 1.19 µg/L (bedrock) versus IMAC
of 1 µg/L.
Iron: PBTVs of 385 µg/L (surficial); 1,319 µg/L (transition zone); 2,550
µg/L (bedrock) versus 2L of 300 µg/L.
Manganese: PBTVs of 253 µg/L (surficial); 298 µg/L (transition zone); 544
µg/L (bedrock) versus 2L of 50 µg/L.
Vanadium: PBTVs of 0.974 µg/L (surficial); 5.88 µg/L (transition zone);
5.52 µg/L (bedrock) versus IMAC of 0.3 µg/L.
Groundwater PBTVs were calculated for the following constituents that do not
have a 2L standard, IMAC or Federal Maximum Contaminant Level (MCL)
established: alkalinity, bicarbonate, calcium, carbonate, magnesium, methane,
potassium, sodium, sulfide, and TOC.
Background threshold values will continue to be evaluated and adjusted over
time as additional background data becomes available.
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Piper Diagrams (Comparison to Background) 10.1.2
A Piper diagram is a graphical representation of major water chemistry using
two ternary plots and a diamond plot. One of the ternary plots shows the relative
percentage of major cations in individual water samples and the other shows the
relative percentage of the major anions. The apices of the cation plot are calcium,
magnesium, and sodium plus potassium. The apices of the anion plot are sulfate,
chloride, and carbonates. The two ternary plots are projected onto the diamond
plot to represent the major ion chemistry of a water sample. The ion composition
can be used to classify groundwater of particular character and chemistry into
sub-groups known as groundwater facies. Percentages of major anions and
cations are based on concentrations expressed in meq/L (EPRI, 2006). Plots of
pore water, surficial, transition, and bedrock groundwater including background
locations are shown on Figure 10-1, Figure 10-2, and Figure 10-3, respectively.
Historical data from neighboring Orange County serves as a useful comparison
to those wells identified as background in this CSA (Cunningham & Daniel,
2001). In Orange County, the groundwater is characterized as circumneutral Ca-
HCO3. At Mayo, background wells in the surficial zone and transition zone fall
within the same region as those studied in Cunningham and Daniel, supporting
their selection as background. In the bedrock flow zone at Mayo, three
background wells show Ca-HCO3 waters, while MW-14BR falls outside of the
range given by Cunningham and Daniel and is dominated by Na-HCO3.
10.2 Downgradient Groundwater Concentrations
The following is a summary of groundwater analytical data for areas around the Mayo
ash basin. The comprehensive groundwater analytical data table is included as
Appendix B, Table 1.
Monitoring Wells Beneath Ash Basin 10.2.1
Monitoring wells ABMW-2BR, ABMW-2BRL, ABMW-3S, ABMW-4D, and
ABMW-4BR were installed beneath the ash basin. Since monitoring began in
2015, these wells have one or more detected concentrations greater than PBTVs
for the following COIs:
pH: ABMW-2BR (PBTV); ABMW-2BRL (PBTV); ABMW-3S (2L); ABMW-
4BR (PBTV); ABMW-4D (PBTV/2L)
Arsenic: ABMW-2BR (PBTV); ABMW-2BRL (PBTV); ABMW-3S (PBTV);
ABMW-4BR (PBTV); ABMW-4D (PBTV/2L)
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Barium: ABMW-2BR (PBTV); ABMW-3S (PBTV); ABMW-4BR (PBTV);
ABMW-4D (PBTV/2L)
Boron: ABMW-3S (PBTV/2L); ABMW-4D (PBTV/2L)
Cobalt: ABMW-3S (PBTV/ IMAC); ABMW-4D (PBTV/ IMAC)
Hexavalent Chromium: ABMW-2BR (PBTV); ABMW-2BRL (PBTV);
ABMW-3S (PBTV); ABMW-4BR (PBTV); ABMW-4D (PBTV)
Iron: ABMW-2BR (2L); ABMW-2BRL (2L); ABMW-3S (PBTV/2L); ABMW-
4BR (2L); ABMW-4D (PBTV/2L)
Manganese: ABMW-2BR (2L); ABMW-2BRL (2L); ABMW-3S (PBTV/2L);
ABMW-4BR (2L); ABMW-4D (PBTV/2L)
Molybdenum: ABMW-4D (PBTV)
Strontium: ABMW-2BR (PBTV); ABMW-2BRL (PBTV); ABMW-3S
(PBTV); ABMW-4BR (PBTV); ABMW-4D (PBTV)
Sulfate: ABMW-4BR (PBTV)
TDS: ABMW-2BR (PBTV); ABMW-3S (PBTV); ABMW-4BR (PBTV/2L);
ABMW-4D (PBTV)
Vanadium: ABMW-2BR (IMAC); ABMW-2BRL (IMAC); ABMW-3S
(IMAC); ABMW-4BR (IMAC); ABMW-4D (PBTV/ IMAC)
COI concentrations in the bedrock wells beneath the ash basin (ABMW-2BR,
ABMW-2BRL, and ABMW-4BR) have lower concentrations than wells completed
in the surficial and transition zone flow units under the ash basin. Since
monitoring began in 2015, the bedrocks wells have had periodic 2L/IMAC
exceedances for iron, manganese, TDS, thallium, and vanadium and one or more
concentrations greater than PBTVs for pH, arsenic, barium, hexavalent
chromium, strontium, sulfate, and TDS. Since their first sampling event in 2015,
the wells completed in the saprolite or transition zone (ABMW-3S and ABMW-
4D) have had periodic exceedances above 2L/IMAC for pH, antimony, arsenic,
barium, boron, cobalt, iron, manganese, TDS, thallium, and vanadium. In the
March-April 2017 sampling event, 2L/IMAC exceedances, for all wells beneath
the ash basin, were pH, arsenic, barium, boron, cobalt, iron, manganese, TDS,
and vanadium.

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Monitoring Wells Downgradient of Ash Basin 10.2.2
Monitoring wells CW-2, CW-2D, MW-3, MW-16S, MW-16D, and MW-16BR are
located directly downgradient of the ash basin in or near the Crutchfield Branch
valley. These wells have one or more detected concentrations greater than
PBTVs and/or 2L/IMAC as listed below:
pH: CW-2 (2L); CW-2D (2L); MW-3 (2L); MW-16S (2L); MW-16D (PBTV);
MW-16BR (PBTV/2L)
Arsenic: MW-16BR (PBTV)
Barium: CW-2 (PBTV); MW-3 (PBTV); MW-16S (PBTV)
Boron: CW-2 (PBTV/2L); CW-2D (PBTV); MW-3 (PBTV/2L); MW-16S
(PBTV)
Chromium: MW-16S (PBTV)
Hexavalent Chromium: MW-3 (PBTV)
Cobalt: CW-2D (IMAC); MW-16S (PBTV/IMAC)
Iron: MW-16S (PBTV/2L); MW-16D (2L); MW-16BR (2L)
Manganese: CW-2 (PBTV/2L); CW-2D (PBTV/2L); MW-3 (PBTV/2L); MW-
16S (PBTV/2L); MW-16D (2L); MW-16BR (2L)
Molybdenum: MW-3 (PBTV); MW-16D (PBTV); MW-16BR (PBTV)
Strontium: CW-2D (PBTV); MW-3 (PBTV); MW-16S (PBTV)
Sulfate: CW-2 (PBTV); CW-2D (PBTV); MW-3 (PBTV); MW-16S (PBTV);
MW-16D (PBTV)
TDS: MW-3 (PBTV); MW-16S (PBTV)
Vanadium: CW-2 (IMAC); CW-2D (IMAC); MW-3 (IMAC); MW-16S
(IMAC); MW-16D (IMAC); MW-16BR (IMAC)
In the March-April 2017 sampling event, 2L/IMAC exceedances for the
downgradient monitoring wells in or near the Crutchfield Branch valley were
noted only for pH, boron, iron, manganese, and vanadium.
Monitoring wells CW-3, CW-4, CW-6, MW-2, and MW-3BR are also located
downgradient of the ash basin, although outside of the Crutchfield Branch
stream valley. MW-6BR was categorized as a piezometer (water level only) after
development proved unsuccessful due to low groundwater yield. These
downgradient wells have one or more historic concentrations greater than PBTVs
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for pH, chromium, cobalt, iron, manganese, sulfate, and TDS. For the March-
April 2017 sampling event, 2L/IMAC exceedances were noted for pH, iron,
manganese, TDS, and vanadium.
Monitoring Wells in Separate Flow Regime 10.2.3
Monitoring wells MW-7D, MW-7BR, MW-8BR, MW-9BR, and MW-9BRL are
located east of the ash basin between the ash basin and Mayo Lake. Recently
reviewed groundwater elevation data from construction-related piezometer
installations for the future FGD Pond confirm these wells to be in a separate flow
regime from the ash basin, as the ridge under the railroad serves as a
groundwater divide. MW-9BR continues to have turbidity greater than 10 NTUs
for every sampling event, resulting in excluded analytical data. As a result,
NCDEQ requested a replacement well deeper into bedrock, and the conversion
of MW-9BR into a piezometer. MW-9BRL was installed on February 2, 2017.
Due to development time prior to sampling, groundwater analytical data was
not available for MW-9BRL at the time of this report. MW-7D and MW-7BR are
in the footprint of the future FGD Pond requiring abandonment prior to
construction activities. The well pair was sampled a total of five times, with the
last sampling event on November 4, 2016. NCDEQ DWR was consulted and
approved abandonment of the well pair. The wells were abandoned on June 12,
2017. MW-7D, MW-7BR, and MW-8BR have one or more concentrations greater
than PBTVs for pH, barium, hexavalent chromium and total chromium,
manganese, molybdenum, strontium, sulfate, and TDS. In the March-April
sampling event, MW-8BR was the only well sampled in this area, and 2L/IMAC
exceedances were pH, iron, manganese, vanadium.
Monitoring Wells East of Rail Line (Separate Flow 10.2.4
Regime)
Monitoring wells MW-18D, MW-18BR, MW-19D, and MW-19BR were installed
northeast of the Plant, between the existing FGD Pond and Mayo Lake, to assess
the east side of the railroad line, east of the existing FGD ponds. A groundwater
divide roughly bisects the Site property from southwest to northeast. In general
terms, the divide appears to be positioned roughly beneath the railroad line.
Groundwater flow further to the east has been demonstrated to flow east of the
divide into the Mayo Lake flow regime and flow to the west is toward the former
C&D landfill area, the tributary on the east side of the landfill, and the ash basin.
A monitoring well cluster (MW-18D/BR) was installed directly across the
railroad tracks from the FGD ponds (east) between the rail line and the stream
that receives NPDES outfall 002 flow from the ash basin. A second well cluster
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(MW-19D/BR) was installed further to the east and closer to Mayo Lake. CW-1
and CW-1D are compliance wells, in the same area, across the NPDES 002
outfall. These wells have one or more concentrations greater than PBTVs and/or
2L/IMAC for the following COIs:
pH: CW-1 (PBTV/2L); MW-18D (2L)
Barium: MW-18BR (PBTV)
Hexavalent Chromium: MW-18BR (PBTV); MW-19BR (PBTV)
Chromium: CW-1D (PBTV/2L)
Iron: MW-19D (PBTV/2L); MW-19BR (2L)
Manganese: CW-1D (2L); MW-18BR (PBTV/2L); MW-19D (PBTV/2L);
MW-19BR (PBTV/2L)
Molybdenum: CW-1D (PBTV)
Strontium: MW-18BR (PBTV); MW-19D (PBTV); MW-19BR (PBTV)
Sulfate: CW-1 (PBTV); MW-18D (PBTV); MW-19D (PBTV); MW-19BR
(PBTV)
TDS: MW-18BR (PBTV); MW-19D (PBTV/2L); MW-19BR (PBTV/2L)
Vanadium: CW-1 (IMAC); CW-1D (IMAC); MW-18D (IMAC); MW-18BR
(IMAC); MW-19D (IMAC); MW-19BR (IMAC)

Piper Diagrams (Comparison to Downgradient/ 10.2.5
Separate Flow Regime)
The Piper Diagrams (Figures 10-1 to 10-3) display water chemistry for pore water
and downgradient wells. Groundwater samples from downgradient wells
generally indicate calcium-bicarbonate type water, typical of shallow fresh
groundwater. A few exceptions to this characterization include CW-2, with a
geochemical signature which is representative of calcium-sulfate type water, and
background well MW-14BR, with a signature representative of sodium-
bicarbonate type water indicative of deep groundwater. As previously described
in Section 3.3, pore water at the Mayo Site is atypical when compared to other
ash leachate sources. As such, comparison of monitoring well data within the
Piper diagram is limited in its usefulness as similar characteristics are also
observed in Mayo Site pore water, unimpacted groundwater, and potentially
impacted groundwater.
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10.3 Site-Specific Exceedances (Groundwater COIs)
Site-specific COIs were developed by evaluating groundwater sampling results with
respect to PBTVs, applicable regulatory standards, and additional regulatory
input/requirements. The approach to determining those constituents which should be
considered constituents of interest (COI)s for the purpose of this assessment is
discussed in the following section.
Provisional Background Threshold Values (PBTVs) 10.3.1
As presented in 2L .0202 (b)(3) — “Where naturally occurring substances exceed
the established standard, the standard shall be the naturally occurring
concentration as determined by the Director” — the following report was
provided to NCDEQ: Statistical Methods for Developing Reference Background
Concentrations for Groundwater and Soil at Coal Ash Facilities (HDR and SynTerra,
2017). NCDEQ (July 7, 2017) addressed each Duke Energy coal ash facility and
identified soil and groundwater data appropriate for inclusion in the statistical
analysis to determine BTVs and PBTVs. A revised and updated technical
memorandum that summarized revised background groundwater datasets and
statistically determined PBTVs for the Mayo Plant was submitted to NCDEQ on
August 16, 2017. A list of NCDEQ-approved groundwater PBTVs were provided
to Duke Energy on September 1, 2017 (Zimmerman to Draovitch; Appendix A).
Each of the proposed groundwater PBTVs, as proposed, was accepted.
Applicable Standards 10.3.2
As part of CSA activities at the Site, sampling and analysis for inorganic
constituents has been conducted for coal ash, ponded water in the ash basin, ash
pore water, AOW, surface water, sediment, soil, and groundwater
downgradient/sidegradient of the ash basin and in background areas. Based on
comparison of those sampling results from the multiple media to background
values and applicable regulatory values, potential lists of COIs were developed
in the 2015 CSA, CAPs, and CSA Supplement.
For the purpose of developing the groundwater COIs, constituent exceedances in
downgradient groundwater of PBTVs and 2L or IMAC are considered a primary
focus. Additionally, NCDEQ requested that hexavalent chromium be included as
a COI at each CAMA-related site due to public interest and receptor wells.
Molybdenum and strontium do not have 2L or IMACs established; however,
these constituents are considered potential COIs with regards to CCR and are
evaluated as potential COIs for the Site at the request of NCDEQ.
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The following constituents do not have a 2L standard, IMAC, or Federal MCL
established: alkalinity, aluminum, bicarbonate, calcium, carbonate, magnesium,
methane, potassium, sodium, sulfide, and TOC. Results from these constituents
are useful in comparing water conditions throughout the Site. For example
calcium is listed as a constituent for detection monitoring in Appendix III to 40
CFR Part 257. Although these constituents will be used to compare and
understand groundwater quality conditions at the Site, because there are no
associated 2L, IMACs, or MCLs, these constituents are not evaluated as potential
COIs for the Site.
Additional Requirements 10.3.3
NCDEQ requested that figures be included in the CSA that depict groundwater
analytical results for the constituents in 40 CFR 257, Appendix III detection
monitoring and 40 CFR 257, Appendix IV assessment monitoring (CCR Rule)
(USEPA, 2015). Detection monitoring constituents in 40 CFR 257 Appendix III
are:
Boron
Calcium
Chloride
Fluoride (limited historical data at this Site, not on assessment
constituent list)
pH
Sulfate
TDS
Constituents for assessment monitoring listed in 40 CFR 257
Appendix IV include:
Antimony
Arsenic
Barium
Beryllium
Cadmium
Chromium
Cobalt
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Fluoride (limited historical data at this Site, not on assessment
constituent list)
Lead
Lithium (not analyzed)
Mercury
Molybdenum
Selenium
Thallium
Radium 226 and 228 combined
Aluminum, copper, iron, manganese, and sulfide were originally included in the
Appendix IV constituents in the draft rule; USEPA removed these constituents in
the final rule. Therefore, these constituents are not included in the listing above;
however, they are included as part of the current Interim Monitoring Plan (IMP;
Section 15.3). NCDEQ requested that vanadium be included as a COI.
Mayo Plant COIs 10.3.4
Exceedances of comparative values, the distribution of constituents in relation to
the ash basin, comparison with background concentrations, co-occurrence with
CCR indicator constituents such as boron, and likely migration directions based
on groundwater flow direction are considered in determination of groundwater
COIs. A constituent exceedance in an outlying area with no co-occurrence of
boron or similar CCR-related constituent would likely not be considered a reason
to list the constituent as a COI. A constituent exceedance based on a single
sampling event when previous results indicate a concentration trend below
comparative values would likely not indicate inclusion as a COI. Based on site-
specific conditions, observations, and findings, the following list of COIs has
been developed for the Mayo Plant:

Arsenic
Barium
Boron
Chromium (total)
Chromium (hexavalent)
Cobalt
Iron
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Manganese
Molybdenum
pH
Strontium
Sulfate
TDS
Vanadium
Table 10-2 lists the COIs and other constituents at the Mayo Site along with the
established PBTVs and associated 2L/IMACs.
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11.0 HYDROGEOLOGICAL INVESTIGATION
Results from the hydrogeological assessment of the Mayo Site, summarized in this
section, are primary components of the SCM.3 Plume physical and chemical
characterization is detailed below for each groundwater COI. The horizontal and
vertical extent of constituent concentrations is presented on isoconcentration maps and
cross sections. These descriptions and depictions are based primarily on the most recent
comprehensive groundwater sampling event (March-April 2017).
11.1 Plume Physical and Chemical Characterization

Plume Physical Characterization 11.1.1
The groundwater plume is defined as locations (in three-dimensional space)
where groundwater quality is impacted by the ash basin. Other COIs (defined in
Section 10.0) are used to help refine the extent and degree to which areas are
impacted by groundwater from the ash basin. The comprehensive groundwater
data table (Appendix B) and an understanding of groundwater flow dynamics
and direction (Section 6.2.3, Figures 6-4 to 6-11) were used to define the
horizontal and vertical extent of the plume. As discussed in Section 13.2
(Geochemical Modeling), not all constituents with PBTV exceedances can be
attributed to the ash basin. Naturally occurring groundwater contains varying
concentrations of alkalinity, aluminum, bicarbonate, cadmium, carbonate,
copper, lead, magnesium, methane, nickel, potassium, sodium, TOC, and zinc.
Sporadic and low-concentration exceedances of these constituents in the
groundwater data do not necessarily demonstrate horizontal or vertical
distribution in groundwater that indicates impact from the ash basin.
Isoconcentration Maps
The horizontal extent of the plume in each flow unit is interpreted in
concentration isopleth maps (Figures 11-1 to 11-45). These maps use valid

3 Pursuant to the CCR rule, owners and operators of CCR units must install the required groundwater
monitoring system; develop the required groundwater sampling and analysis program to include
selection of the statistical procedures to be used for evaluating groundwater monitoring data; and begin
detection monitoring, which requires owners and operators to have a minimum of eight samples for
each well and begin evaluating groundwater monitoring data for statistically significant increases over
background levels for the constituents listed in Appendix III of 40 C.F.R. Part 257. These data need not
be posted to Duke Energy’s publicly accessible Internet site until such time the annual groundwater
monitoring and corrective action report required under the CCR rule becomes due. Although a portion
of these data was utilized in this assessment for refinement of constituent distribution, these data are not
included in this report because it was not public information as of the date of its completion.
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groundwater analytical data to spatially and visually define areas where
groundwater concentrations are above the respective constituent PBTV and/or
2L/IMAC.
The leading edge of the plume, the furthest downgradient edge, is represented
by groundwater concentrations in the wells in each flow unit. In the bedrock
flow unit, boron is detected in downgradient well CW-2D (located near the
northern property line). In bedrock well MW-16BR, located approximately 1,000
feet downgradient of CW-2D and the property line, boron is not detected. The
leading edge of the bedrock plume is interpreted to be near the northern
property line. Similarly, in the transition zone unit, groundwater from
monitoring well CW-2 contains detectable concentrations of boron while MW-
16D, located about 1,000 feet downgradient, does not. Surficial well MW-3 is
located several hundred feet south of the property line between the property line
and the ash basin dam. MW-16S is located downgradient approximately 1,500
feet from MW-3. Both wells are screened in alluvium. Boron has been detected in
both wells.
Figures 11-7 to 11-9 depict the horizontal extent of boron in downgradient
groundwater. The background contour line in the surficial unit encompasses the
perimeter of the ash basin, extends beyond the Duke Energy property boundary
to the north, and ends just south of the state line. In the transition zone, the
background contour line mimics the general shape of the plume in the surficial
unit except in the downgradient direction where the extent of impact is between
the property line and the MW-16 well cluster. In bedrock, the background
contour encompasses only the northeast half of the ash basin and also extends
just northeast of the property line, but the background contour remains south of
the MW-16 well cluster. As described in Section 6.0, there is no hydrogeologic
confining unit at Mayo; therefore, under these unconfined conditions,
groundwater moves freely across each unit.
Concentration versus Distance Plots
Figure 11-46 to 11-48 depicts concentration versus distance graphs from the
source along the plume centerline for COIs. While PBTV values could not be
distinguished on these graphs because values differ by flow unit, the graphs
show constituent concentrations in source areas and downgradient and aid in
understanding plume distribution. Concentrations of each COI represent March-
April 2017 conditions. The wells used are consistent for each constituent
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represented. The graphs demonstrate that COI concentrations decrease from the
source area to downgradient locations.
Vertical Extent Cross-Sections
The vertical extent of the plume down the centerline is depicted in the cross-
sectional views of the Site (Figures 11-49 to 11-63). Cross-section C-C’ is a more
detailed and focused depiction of cross-section B-B’, and focuses on the ash basin
and directly downgradient. Wells along the centerline plume flow path are
depicted. These wells represent source area and downgradient locations relative
to the ash basin.
COIs have been contoured in the cross-sectional depictions. Constituent
isopleths reflect values above the PBTV and the 2L/IMAC standard, as
applicable.
The surficial and transition zone flow units at Mayo are not vertically extensive
(less than 10 feet thick for surficial; generally less than 30 feet thick for transition
zone). The well screens in the CAMA wells accurately monitor groundwater
conditions and impact to the groundwater flow zones for the surficial and
transition zone units. ABMW-4D is in the transition zone below the ash basin
and contains detectable concentrations of boron (between 3,470 – 5,090 ug/L).
Other COI constituent concentrations also exceed 2L/IMAC and PBTV values;
therefore, the transition zone beneath the basin appears to be impacted.
The vertical extent of the plume is best represented by groundwater
concentrations in bedrock wells beneath and downgradient of the ash basin.
Deep bedrock well ABMW-2BRL contains no boron or manganese concentrations
above 2L or PBTV, respectively. ABMW-2BR and ABMW-4BR are shallower
bedrock wells beneath the ash basin, and groundwater from these wells has the
same general absence of CCR-related constituents at concentrations that exceed
PBTVs or the 2L/IMAC. Upward vertical gradients as groundwater from the
west, south, and east recharge the groundwater beneath the basin reduce the
potential for downward migration of COIs into bedrock.
As groundwater under the ash basin flows northeast toward the ash basin dam,
the hydraulic impact of the ash basin dam and the hydraulic head exerted by the
ash basin water forces groundwater downward into the bedrock, which increases
hydraulic pressure in the bedrock aquifer. In general, the pressures in the
bedrock just downgradient of the base of the dam become greater than in the
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transition zone or surficial aquifers resulting in upward vertical gradients. A
strong upward gradient exists along the western end of the dam and gradually
changes (eastward) to a downward or neutral gradient in the former Crutchfield
Branch valley. This is expected as discharging groundwater and surface water
reach hydraulic equilibrium. As groundwater migrates in the downgradient
direction, groundwater enters the system from upgradient recharge areas to the
west and east.
In summary, the horizontal and vertical extent of the plume has been defined.
Monitoring wells across the Site are appropriately placed and screened to the
correct elevations to monitor groundwater quality. Monitoring wells installed for
other regulatory programs 4 have added additional details about the orientation
and extent of the downgradient plume and have helped refine an understanding
of the vertical and horizontal distribution of the plume.
Plume Chemical Characterization 11.1.2
Plume chemical characterization is detailed below for each COI. Analytical
results are based on the March-April 2017 groundwater sampling event. The
range of detected concentrations is presented with the number of detections for
the sampling event. Descriptions of the COIs identified for the Mayo Site are also
provided. PBTVs and 2L/IMACs are included in Appendix B, Table 1. Pore
water (source) concentrations are discussed in Section 3.0.
Arsenic
Detected Range: 1.06 µg/L – 26.9 µg/L; Number of Detections/Total Samples: 7/33
Concentrations in 5 samples (1 surficial; 4 bedrock) exceeded the PBTV.
Concentrations in 1 sample (transition zone beneath ash basin) exceeded
the 2L of 1 µg/L.

4 Pursuant to the CCR rule, owners and operators of CCR units must install the required groundwater
monitoring system; develop the required groundwater sampling and analysis program to include
selection of the statistical procedures to be used for evaluating groundwater monitoring data; and begin
detection monitoring, which requires owners and operators to have a minimum of eight samples for
each well and begin evaluating groundwater monitoring data for statistically significant increases over
background levels for the constituents listed in Appendix III of 40 C.F.R. Part 257. These data need not
be posted to Duke Energy’s publicly accessible Internet site until such time the annual groundwater
monitoring and corrective action report required under the CCR rule becomes due. Although a portion
of these data was utilized in this assessment for refinement of constituent distribution, these data are not
included in this report because it was not public information as of the date of its completion
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Arsenic is a trace element in the crust, with estimated concentrations ranging
from less than one mg/kg in mafic igneous rocks to 13 mg/kg in clay-rich rocks
(Parker, 1967). It occurs in multiple valence states (As5+, As3+, and As3-). Arsenic in
coal occurs primarily in pyrite (iron sulfide, with arsenic replacing iron in the
crystal structure) (Finkelman, 1995). Arsenic condenses on fly ash as arsenate
(As5+) (Goodarzi, Huggins, & Sanei, 2008). Leaching tests on ash indicate that
trace quantities of up to 50 percent of the arsenic present can be leached. In
addition to the solubility of the source, the concentration of calcium and presence
of oxides appear to limit the mobility of arsenic (Izquierdo & Querol, 2012). The
USEPA estimates that the amount of natural arsenic released into the air as dust
from the soil is approximately equal to the amount of arsenic released by all
human activities (EPRI, 2008a).
Barium
Detected Range: 6 µg/L – 890 µg/L; Number of Detections/Total Samples: 38/33
Concentrations in 7 samples (3 surficial; 2 transition zone; 2 bedrock)
exceeded the PBTV.
Concentrations in 1 sample (transition zone beneath the ash basin)
exceeded the 2L of 700 µg/L.
Historic barium detected concentrations align with the primary path of
groundwater flow transect including historic exceedances greater than the
PBTV and 2L, including in downgradient surficial and transition zone
groundwater.
Barium is a naturally occurring component of minerals that are found in small,
but widely distributed amounts in the earth’s crust (Kunesh, 1978); (Miner, 1969).
Two forms of barium, barium sulfate (barite) and barium carbonate (witherite)
are often found in nature as ore deposits. Barium enters the environment
naturally through the weathering of rocks and minerals. Anthropogenic releases
are associated primarily with industrial processes.
Barium is sometimes found naturally in drinking water and food. However,
because the dominant naturally occurring barium compounds (barium sulfate
and barium carbonate) have a low to moderate solubility in water under most
conditions, the amount of barium found in drinking water is typically small.
Barium compounds such as barium acetate, barium chloride, barium hydroxide,
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barium nitrate, and barium sulfide dissolve more easily in water than barium
sulfate and barium carbonate, but because they are not commonly found in
nature, the latter two compounds do not usually occur in drinking water unless
the water is contaminated by barium compounds that are released from waste
sites (EPRI, 2008b).
Barium is naturally released into the air by soils as they erode and is released
into the soil and water by eroding rocks. Barium released into the air by human
activities comes mainly from barium mines, metal production facilities, and
industrial boilers that burn coal and oil (EPRI, 2008b). The leachability of barium
has been found to be relatively independent of pH but is controlled instead by
the presence of calcium, with which it competes for sulfate (Fruchter, Rai, &
Zachara, 1990). In an overview of leachability studies found in the International
Journal of Coal Geology, the mobility of barium typically ranged from 0.02 percent
to 2 percent (Izquierdo & Querol, 2012).
Regional metamorphic grade greenschist to upper amphibolite in the Piedmont’s
King’s Mountain Belt contains deposits of barium sulfate (barite). Barium is
especially common as concretions and vein fillings in limestone and dolostone,
which are not common geologic facies in North Carolina; however, at various
times in the past century and a half, the Carolinas have been major producers of
barite (USEPA, 2017a) .
In a statistical summary of groundwater quality in North Carolina, the
Superfund Research Program at the University of North Carolina (UNC)
analyzed 1,898 private well water samples in Gaston and Mecklenburg Counties.
The samples were tested by the North Carolina State Laboratory of Public Health
from 1998-2012. This study found an average barium concentration of 50 µg/L.
No samples exceeded the 2,000 µg/L Primary Maximum Contaminant Level
(PMCL) for barium (NCDHHS, 2010a).
Boron
Detected Range: 179 µg/L – 3,470 µg/L; Number of Detections/Total Samples:
6/33
Concentrations in 6 samples (3 surficial; 2 transition zone; 1 bedrock)
exceeded the PBTV.
Concentrations in 4 samples exceeded the 2L of 700 µg/L.
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Boron exceeds the PBTV and 2L in surficial and transition zone
groundwater beneath the ash basin.
Boron is a trace element in the crust, with estimated concentrations ranging from
as little as 1 mg/kg in mafic igneous rocks to hundreds of milligrams per
kilogram in clay rich rocks (Parker, 1967). It occurs only in the trivalent form
(B3+) and is concentrated in sedimentary rocks (Urey & Mem, 1953). This
observation indicates that a mechanism exists to concentrate boron in minerals
because the oceans could dissolve all of the boron estimated to be present in the
crust (Fleet, 1965). Fleet (1965) presents both biogenic and mineralogical
processes to account for the preferential concentration of boron in the crust.
Boron is a micronutrient (Goldberg, 1997) that is concentrated in plant tissue,
including the plants from which coal formed.
While boron is relatively abundant on the earth’s surface, boron and boron
compounds are relatively rare in all geological provinces of North Carolina.
Natural sources of boron in the environment include volatilization from
seawater, geothermal vents, and weathering of clay-rich sedimentary rocks.
Total contributions from anthropogenic sources are less than contributions from
natural sources. Anthropogenic sources of boron include agriculture, refuse, coal
and oil burning power plants, by-products of glass manufacturing, and sewage
and sludge disposal (EPRI, 2005).
Because boron is associated with the carbon (fuel) in coal, it tends to volatilize
during combustion and subsequently condense onto fly ash as a soluble borate
salt (Dudas, 1981). Boron leaches readily (up to 50 percent of total present) and
rapidly from fly ash (Cox, Lundquist, Przyjazny, & Schmulbach, 1978). Boron is
considered a marker COI for coal ash because boron is rarely associated with
other types of industrial waste products.
Boron is the primary component of a few minerals including tourmaline, a rare
gem mineral that forms under high temperature and pressure (Hurlbut, 1971).
The remaining common boron minerals, including borax that was mined in the
Mojave desert, in Boron, California, form from the evaporation of seawater in
deposits known as evaporites. For this reason, boron mobilized into the
environment will remain in solution until incorporation into plant tissue or
adsorption by a mineral.
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Fleet (1965) describes sorption of boron by clays as a two-step process. Boron in
solution is likely to be in the form of the borate ion (B(OH)4-). The initial sorption
occurs onto a charged surface. Observations that boron does not tend to desorb
from clays indicates that it migrates rapidly into the crystal structure, most likely
in substitution for aluminum. Goldberg et al. (1996) determined that boron
sorption sites on clays appear to be specific to boron. For this reason, there is no
need to correct for competition for sorption sites by other anions in transport
models (Goldberg, Forster, Lesch, & Heick, 1996).
Goldberg (1997) lists aluminum and iron oxides, magnesium hydroxide, clay
minerals, calcium carbonate (limestone), and organic matter as important
sorption surfaces in soils (Goldberg, 1997). Boron sorption on oxides is
diminished by competition from numerous anions. Boron solubility in
groundwater is controlled by adsorption reactions rather than by mineral
solubility. Goldberg concludes that chemical models can effectively replicate
boron adsorption data over changing conditions of boron concentration, pH, and
ionic strength.
Chromium
Detected Range: 1.3 µg/L – 9.58 µg/L; Number of Detections/Total Samples: 8/33
Concentrations in 1 sample (transition zone) exceeded the PBTV.
No exceedances of the 2L of 10 µg/L were observed.
Chromium is a blue-white metal found naturally occurring in combination with
other substances. It occurs in rocks, soils, plants, and volcanic dust and gases
(EPRI, 2008c). Background concentrations of chromium in groundwater
generally vary according to the media in which they occur. Most chromium
concentrations in groundwater are low averaging less than 1.0 µg/L worldwide.
Chromium tends to occur in higher concentrations in felsic igneous rocks (such
as granite and metagranite) and ultramafic igneous rocks; however, it is not a
major component of the igneous or metamorphic rocks found in the North
Carolina Piedmont or the Blue Ridge (Chapman, Cravotta, III, Szabo, & Lindsey,
2013)
In a statistical summary of groundwater quality in North Carolina, the
Superfund Research Program at UNC analyzed 1,898 private well water samples
in Gaston and Mecklenburg Counties. The samples were tested by the North
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Carolina State Laboratory of Public Health from 1998 to 2012. The average
chromium concentrations were 5.1µg/L and 5.2 µg/L in Gaston and Mecklenburg
Counties respectively.
Hexavalent Chromium
Detected Range: 0.028 µg/L – 4.4 µg/L; Number of Detections/Total Samples:
18/33
Concentrations in 4 samples (2 surficial; 2 bedrock) exceeded the PBTV.
No exceedances of the 2L of 10 µg/L were observed.
Chromium can also occur in the +III oxidation state, depending on pH and redox
conditions. Cr (VI) is the dominant form of chromium in shallow aquifers where
aerobic conditions exist. Cr(VI) can be reduced to Cr(III) by soil organic matter,
S2- and Fe2+ ions under anaerobic conditions often encountered in deeper
groundwater. Major Cr(VI) species include chromate (CrO4 2-) and dichromate
(Cr2O7 2-) which precipitate readily in the presence of metal cations (especially
Ba2+, Pb2+, and Ag+). Chromate and dichromate also adsorb on soil surfaces,
especially iron and aluminum oxides. Cr(III) is the dominant form of chromium
at low pH.
Chromium mobility depends on sorption characteristics of the soil, including
clay content, iron oxide content, and the amount of organic matter present.
Chromium can be transported by surface runoff to surface waters in its soluble
or precipitated form. Soluble and unadsorbed chromium complexes can leach
from soil into groundwater. The leachability of Cr(VI) increases as soil pH
increases. Most of chromium released into natural waters is particle associated,
however, and is ultimately deposited into the sediment (Smith, Means, Chen, &
others, 1995).
Cobalt
Detected Range: 1.59 µg/L – 5.9 µg/L; Number of Detections/Total Samples: 4/33
Concentrations in 4 samples (2 surficial; 1 transition zone; 1 bedrock)
exceeded the PBTV and IMAC of 1 µg/L.
Cobalt is a base metal that exhibits geochemical properties similar to iron and
manganese, occurring as a divalent and trivalent ion. Cobalt can also occur as
Co1-. In terms of distribution in the crust, all three metals exhibit a strong affinity
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for mafic igneous and volcanic rocks and deep-sea clays (Parker, 1967). Cobalt
occurs in clay minerals and substitutes into the pyrite crystal structure. There is
also evidence that it is organically bound in coal (Finkelman, 1995). Izquierdo
and Querol (2012) report limited leaching of cobalt from coal, attributing this
observation to incorporation into iron oxide minerals. The concentration of cobalt
in surface and groundwater in the United States is generally low— between 1
and 10 parts of cobalt in 1 billion parts of water (parts per billion; ppb) in
populated areas. The concentration may be hundreds or thousands times higher
in areas that are rich in cobalt containing minerals or in areas near mining or
smelting operations. In most drinking water, cobalt levels are less than 1 to 2 ppb
(USGS, 1973). Cobalt is compared to IMAC since no 2L standard has been
established for this constituent by NCDEQ.
Iron
Detected Range: 11 µg/L – 58,800 µg/L; Number of Detections/Total Samples:
28/33
Concentrations in 5 samples (2 surficial; 2 transition zone; 1 bedrock)
exceeded the PBTV.
Concentrations in 17 samples exceeded the 2L of 300 µg/L.
Iron exceeds the PBTV and 2L in surficial and transition zone
groundwater beneath the ash basin.
Iron is a naturally occurring element that may be present in groundwater from
the erosion of natural deposits (NCDHHS, 2010b). A 2015 study by NCDEQ
(Summary of North Carolina Surface Water Quality Standards 2007-2014) found
that while concentrations vary regionally, “iron occurs naturally at significant
concentrations in the groundwaters of NC,” with a statewide average
concentration of 1,320 µg/L. Iron is estimated to be the fourth most abundant
element in the Earth’s crust at approximately 5 percent by weight (Parker, 1967).
Only Oxygen (46.60 weight percent), silicon (27.72 weight percent), and
aluminum (8.13 weight percent) occur in higher concentrations. Iron occurs in
divalent (ferrous, Fe2+), trivalent (ferric, Fe3+), hexavalent (Fe6+), and Fe2- oxidation
states. Iron is a common mineral-forming element, occurring primarily in mafic
(dark colored) minerals including micas, pyrite (iron disulfide), and hematite
(iron oxide), as well as in reddish-colored clay minerals.
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Clay minerals and pyrite are common impurities in coal. Under combustion
conditions in a coal-fired boiler, clay minerals would be dehydrated to mullite or
gibbsite, possibly liberating iron, and pyrite would oxidize to hematite or
magnesioferrite. Research summarized by Izquierdo and Querol (2012) indicates
that iron leaching from coal ash is on the order of one percent of the total iron
present due to the low pH required to solubilize iron minerals. Despite the low
apparent mobilization percentage, iron is often one of the COIs detected in the
highest concentrations in ash pore water. Ferric iron is soluble at pH less than 2
at typical surface conditions (25°C and one atmosphere total pressure (Schmidt,
1962). For this reason, dissolved iron in surficial waters is typically oxidized to
the trivalent state resulting in formation of ferric iron oxide flocculation that
exhibits a characteristic reddish tint.
Manganese
Detected Range: 6 µg/L – 6,960 µg/L; Number of Detections/Total Samples: 29/33
Concentrations in 12 samples (3 surficial; 2 transition zone; 7 bedrock)
exceeded the PBTV.
Concentrations in 25 samples exceeded the 2L of 50 µg/L.
Manganese exceeds the PBTV and 2L in surficial and transition zone
groundwater beneath the ash basin.
Manganese is a naturally occurring silvery-gray transition metal that resembles
iron but is more brittle and is not magnetic. It is found in combination with iron,
oxygen, sulfur, or chlorine to form manganese compounds. High manganese
concentrations are associated with silty soils, and sedimentary, unconsolidated,
or weathered lithologic unit and low concentrations are associated with non-
weathered igneous bedrock and soils with high hydraulic conductivity
(Gillespie, 2013), (Polizzotto, et al., 2015). Manganese is most readily released to
the groundwater through the weathering of mafic or siliceous rocks (Gillespie,
2013). When manganese-bearing minerals in saprolite, such as biotite, are
exposed to acidic weathering, the metal can be liberated from the host mineral
and released to groundwater. It then migrates through pre-existing fractures
during the movement of groundwater through bedrock. If this aqueous-phase
manganese is exposed to higher pH in the groundwater system, it will
precipitate out of solution. This results in preferential pathways becoming
“coated” in manganese oxides and introduces a concentrated source of
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manganese into groundwater (Gillespie, 2013). Manganese(II) in suspension of
silt or clay is commonly oxidized by microorganisms present in soil, leading to
the precipitation of manganese minerals (ATSDR, 2012). Roughly 40 percent to
50 percent of North Carolina wells have manganese concentrations higher than
the state drinking water standard (Gillespie, 2013). Concentrations are spatially
variable throughout the state, ranging from less than 0.01 mg/L to more than 2
mg/L. This range of values reflects naturally derived concentrations of the
constituent and is largely dependent on the bedrock’s mineralogy and extent of
weathering (Gillespie, 2013).
Manganese is estimated to be the 12th most abundant element in the crust (0.100
weight percentage, (Parker, 1967)). Manganese exhibits geochemical properties
similar to iron with Mn7+, Mn6+, Mn4+, Mn3+, Mn2+, and Mn1- oxidation states.
Manganese substitutes for iron in many minerals. Similar to iron, manganese
leaching from coal ash is limited to less than 10 percent of the total manganese
present due to the low pH required to solubilize manganese minerals (Izquierdo
& Querol, 2012). Despite the low apparent mobilization percentage, manganese
can be detected in relatively high concentrations in ash pore water.
Molybdenum
Detected Range: 1.02 µg/L – 1,560 µg/L; Number of Detections/Total Samples:
22/37
Concentrations in 8 samples exceeded the PBTV.
Molybdenum exceeds the PBTV in pore water and transition zone
groundwater beneath the ash basin.
Molybdenum detected in groundwater downgradient of the ash basin
including exceedances of PBTV in transition zone and bedrock
groundwater, including off-site.
Molybdenum is a trace element that exists predominantly as Mo(IV) and Mo(VI).
As a free metal, it is silvery gray, although it does not occur in this form in
nature. It is mined for use in alloys. Molybdenum commonly forms oxyanions in
groundwater that are affected by redox and pH (Ayotte, Gronbert, & Apodaca,
2011). Molybdenum has been observed to leach less from coal cleaning rejects in
acidic than neutral conditions, unlike many other metals (Jones & Ruppert, 2017).
Molybdenum has been shown to become more mobile in procedures that use
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deionized water as a leachant, which may be similar to actual disposal conditions
unlike many other coal ash elements that are more mobile when subjected to
weak acid (Jones & Ruppert, 2017).
Strontium
Detected Range: 27 µg/L – 2,320 µg/L; Number of Detections/Total Samples:
33/33
Concentrations in 12 samples (4 surficial; 2 transition zone; 6 bedrock)
exceeded the PBTV. Strontium does not have a 2L or IMAC.
Strontium exceeds the PBTV in surficial, transition zone, and bedrock
groundwater beneath the ash basin.
Strontium is a soft silver-yellow alkaline earth metal. It is highly chemically
reactive and forms a dark oxide layer when it interacts with air. It is chemically
similar to Ca and replaces Ca or K in igneous rocks in minor amounts. Strontium
is generally present in low concentrations in surface waters but may exist in
higher concentrations in some groundwater (Hem, 1985).
Strontium is present as a minor coal and coal ash constituent. Strontium has been
observed to leach from coal cleaning rejects more in neutral conditions than
acidic, unlike many other metals (Jones & Ruppert, 2017). It has been shown to
behave conservatively in surface waters downstream of coal plants (Ruhl, et al.,
2012).
Sulfate
Detected Range: 0.27 µg/L – 62 µg/L; Number of Detections/Total Samples: 37/37
Concentrations in 17 samples (2 surficial; 7 transition zone; 8 bedrock)
exceeded the PBTV.
No exceedances of the 2L of 250 µg/L were observed.
Sulfate is a naturally occurring substance found in minerals, soil, and rocks. It is
present in ambient air, groundwater, plants, and food. Primary natural sources
of sulfate include atmospheric deposition, sulfate mineral dissolution, and
sulfide mineral oxidation. The principal commercial use of sulfate is in the
chemical industry. Sulfate is discharged into water in industrial wastes and
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through atmospheric deposition (USEPA, 2003). Anthropogenic sources include
coal mines, power plants, phosphate refineries, and metallurgical refineries.
Sulfate has a Secondary Maximum Contaminant Level (SMCL), and no
enforceable maximum concentration set by the USEPA. Ingestion of water with
high concentrations of sulfate may be associated with diarrhea, particularly in
susceptible populations, such as infants and transients (USEPA, 2012). However,
adults generally become accustomed to high sulfate concentrations after a few
days. It is estimated that about 3 percent of the public drinking water systems in
the United States may have sulfate concentrations of 250 mg/L or greater (Miao,
Brusseau, Carroll, & others, 2012). Sulfate is on the list of enforced regulated
contaminates that may cause cosmetic effects or aesthetic effects in drinking
water (USEPA, 2017a).
TDS
Detected Range: 51 mg/L – 730 mg/L; Number of Detections/Total Samples: 33/33
Concentrations in 12 samples (3 surficial; 2 transition zone; 7 bedrock)
exceeded the PBTV.
Concentrations in 6 samples exceeded the 2L of 500 mg/L.
TDS exceeds the PBTV in surficial, transition zone, and bedrock
groundwater beneath the ash basin.
Groundwater contains a wide variety of dissolved inorganic constituents as a
result of chemical and biochemical interactions between the groundwater and
the elements in the soil and rock through which it passes. TDS mainly consist of
cation and anion particles (e.g., calcium, chlorides, nitrate, phosphorus, iron,
sulfur, and others) that can pass through a 2 micron filter (USEPA, 1997). TDS is
therefore a measure of the total amount of dissolved ions in the water, but does
not identify specific constituents or explain the nature of ion relationships. TDS
concentrations in groundwater can vary over many orders of magnitude and
generally range from 0 – 1,000,000 µg/L. The ions listed below are referred to as
the major ions as they make up more than 90 percent of the TDS in groundwater.
TDS concentrations resulting from these constituents are commonly greater than
5,000 µg/L (Freeze & Cherry, 1979).

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Sodium (Na+)
Magnesium (Mg2+)
Calcium (Ca2+)
Chloride (Cl-)
Bicarbonate (HCO3-)
Sulfate (SO4 2-)
Minor ions in groundwater include: boron, nitrate, carbonate, potassium,
fluoride, strontium, and iron. TDS concentrations resulting from minor ions
typically range between 10 – 1,000 µg/L (Freeze & Cherry, 1979). Trace
constituents make up an even smaller portion of TDS in groundwater and
include: aluminum, antimony, arsenic, barium, beryllium, cadmium, chromium,
cobalt, lead, manganese, nickel, selenium, thallium, vanadium, and zinc among
others. TDS concentrations resulting from trace constituents are typically less
than 100 µg/L (Freeze & Cherry, 1979). In some cases, contributions from
anthropogenic sources can cause some of the elements listed as minor or trace
constituents to occur as contaminants at concentration levels that are orders of
magnitude above the normal ranges indicated above.
TDS in water supplies originate from natural sources, sewage, urban and
agricultural run-off, and industrial wastewater. Salts used for road de-icing can
also contribute to the TDS loading of water supplies. Concentrations of TDS from
natural sources have been found to vary from less than 30 mg/L to as much as
6,000 mg/L. Water containing more than 2,000 – 3,000 mg/L TDS is generally too
salty to drink (the TDS of seawater is approximately 35,000 mg/L) (Freeze &
Cherry, 1979). Reliable data on possible health effects associated with the
ingestion of TDS in drinking water are not available (WHO, 1996). TDS is on the
list of “National Secondary Drinking Water Regulations” (NSDWRs) which are
non-enforced regulated contaminates that may cause cosmetic effects or aesthetic
effects in drinking water (USEPA, 2017b).
Vanadium
Detected Range: 0.308 µg/L – 56.7 µg/L; Number of Detections/Total Samples:
22/37
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Concentrations in 2 samples (1 surficial; 1 transition zone) exceeded the
PBTV.
Concentrations in 17 samples exceeded the IMAC of 0.3 µg/L.
Vanadium exceeds the PBTV and IMAC in transition zone, and bedrock
groundwater beneath the ash basin.
Vanadium is estimated to be the 22nd most abundant element in the crust (0.011
weight percent, (Parker, 1967). Vanadium occurs in four oxidation states (V5+,
V4+, V3+, and V2+). It is a common trace element in both clay minerals and plant
material.
The National Uranium Resource Evaluation (NURE) program was initiated by
the Atomic Energy Commission in 1973 with a primary goal of identifying
uranium resources in the United States (Smith S. M., 2016). The
Hydrogeochemical and Stream Sediment Reconnaissance program (initiated in
1975) was one component of NURE. Planned systematic sampling of the entire
United States began in 1976 under the responsibility of four Department of
Energy national laboratories. Samples were collected from 5,178 wells across
North Carolina. Of these, the concentration of vanadium was equal to or higher
that the former IMAC of 0.0003 mg/L in 1,388 well samples (27 percent).
pH
Detected Range: 4.9 – 7.9
Concentrations in 8 samples (1 surficial; 3 transition zone; 3 bedrock)
exceeded the PBTV.
Concentrations in 12 samples exceeded the 2L.
The pH scale is used to measure acidity or alkalinity. A pH value of 7 indicates
neutral water. A value lower than the USEPA-established SMCL range (<6.5
Standard Units) is associated with a bitter, metallic tasting water, and corrosion.
A value higher than the SMCL range (>8.5 Standard Units) is associated with a
slippery feel, soda taste, and deposits in the water (USEPA, 2017b).
In a statistical summary of groundwater quality in North Carolina, the
Superfund Research Program at UNC analyzed 618 private well water samples
for pH in Cleveland and Rutherford Counties. The samples were analyzed by the
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North Carolina State Laboratory of Public Health from 1998 – 2012. This study
found that 16.9 percent of wells in Cleveland County and 20.3 percent of wells in
Rutherford County had a pH result outside of the USEPA’s SMCL range.
11.2 Pending Investigation(s)
Additional metal oxy-hydroxide phases of iron (HFO) and aluminum (HAO) data are
needed to support geochemical modeling conducted as part of the CAP. Soil and rock
samples from previously installed borings or from additionally drilled boreholes along
the primary groundwater flow transect will be used. The samples will be located:

Background/Upgradient
Directly beneath ash basin
Downgradient location, north of the ash basin
The samples will be collected at vertical intervals that coincide with nearby well screen
elevations. Analysis results of collected samples will be used to improve input
parameters for the updated geochemical model.
In accordance with 15A NCAC 02L.0106( k)(5) and (l)(6), the CAP may include an
evaluation of whether groundwater migrating downgradient of the ash basin may have
contaminant concentrations that would result in violations of standards for surface
water. One means of accomplishing this objective is to collect surface water samples to
document existing conditions. Another method will be to conduct groundwater to
surface water modeling. It is anticipated that documenting current conditions through
the collection of the additional surface water samples in Crutchfield Branch will be
coordinated with NCDEQ guidance.

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12.0 RISK ASSESSMENT
A baseline human health and ecological risk assessment was performed as a component
of CAP, Part 2 (SynTerra, 2016a). The 2016 risk assessment characterized potential risks
to humans and wildlife exposed to coal ash constituents present in environmental
media for the purpose in aiding corrective action decisions. Implementation of
corrective action is intended to achieve future Site conditions protective of human
health and the environment, as required by CAMA.
This update to the risk assessment evaluates groundwater and surface water results
collected since the 2016 risk assessment (November 2015 to June 2017) in order to
confirm or update risk conclusions in support of remedial action. Data used in the 2016
risk assessment included groundwater samples from March 11, 2015 through
September 11, 2015, surface water samples from August 27, 2014 through October 13,
2015, and AOW water samples from May 12, 2015 through October 12, 2015. AOW soil
and sediment samples were collected May 12 and 13, 2015. This risk assessment update
uses sampling locations described in Attachment A of the 2016 risk assessment
(SynTerra, 2016a). As previously noted, AOW locations are outside the scope of this
risk assessment because AOWs, wastewater, and wastewater conveyances (effluent
channels) are evaluated and governed wholly separate in accordance with the NPDES
Program administered by NCDEQ DWR. This process is on-going in a parallel effort to
the CSA and subject to change. No new sediment or soil samples, other than for
background evaluations, have been collected that are applicable to the risk assessment;
therefore, risk estimates associated with those media have not been re-evaluated.
As part of the 2016 risk assessment, human health and ecological conceptual site models
(CSMs) were developed to guide identification of exposure pathways, exposure routes,
and potential receptors for evaluation in the risk assessment. The CSMs (CAP, Part 2;
Figures 2-2 and 2-4) describe the sources and potential migration pathways through
which groundwater beneath the ash basin may have transported coal ash-derived
constituents to other environmental media (receiving media) and, in turn, to potential
human and ecological receptors. Exposure scenarios and exposure areas were
presented in detail in the 2016 CAP, Part 2 risk assessment.
This risk assessment update included the following:
Identification of maximum constituent concentrations for groundwater and
surface water;
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Inclusion of new groundwater and surface water data to derive overall average
constituent concentrations for exposure areas;
Comparison of new maximum constituent concentrations to the risk assessment
human health and ecological screening values;
Comparison of new maximum constituent concentrations to human health Risk-
Based Concentrations (RBC); and
Incorporation of new maximum constituent concentrations into wildlife Average
Daily Dose (ADD) calculations for comparison to ecological Toxicity Reference
Values (TRVs).
Evaluation of new groundwater and surface water data and the influence on the 2016
risk assessment conclusions are summarized below by exposure areas (Figures 12-1 and
12-2) at the Mayo Plant.
12.1 Human Health Screening Summary
On-site Groundwater – Surficial Aquifer
Groundwater sample locations used in the human health assessment of the surficial
aquifer include: ABMW-3S, MW-3 and MW-16S. These wells were evaluated because
they represent the potential trespasser/worker exposure area as determined in the 2016
risk assessment. Groundwater analytical results are included in Appendix B, Table 1.
Dissolved thallium (0.45 micron filter) detected at a concentration of 0.227 µg/L in
sample ABMW-3S collected on December 1, 2015 exceeded the human health screening
value of 0.2µg/L. This reported detection is likely anomalous, as it was the only
thallium detection in the surficial aquifer dataset of 23 samples, and is less than the total
thallium concentration (<0.2 µg/L) analyzed in the same sample. No potential risks to
humans exposed to groundwater from the surficial aquifer were identified.
On-site Groundwater – Bedrock and Transition Zone Aquifer
Groundwater sample locations used in the human health assessment of the bedrock and
transition zone aquifer include: ABMW-2BR, ABMW-4BR, ABMW-4D, CW-1/1D
through CW-6, MW-2, MW-3BR, MW-4, MW-7D and MW-16D. Data for the
groundwater sample locations are included in Appendix B, Table 1. One thallium
(total) detection of 0.21 µg/L collected on December 1, 2015 in sample ABMW-2BR
exceeded the human health screening value of 0.2 µg/L. This reported detection is
likely anomalous, as it was the only thallium detection in the bedrock and transitional
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zone aquifer dataset of 128 samples. Additionally, as the reported thallium
concentration is 0.01 µg/L greater than the screening value, it is considered
inconsequential to overall potential risks.
No maximum constituent concentrations exceeded either human health screening
values or RBCs. Thus, no potential for risks to humans exposed to groundwater from
the transition zone aquifer were identified.
Crutchfield Branch – Surface Water
One surface water sample location (SW-CB2) was evaluated in the Crutchfield Branch
area (Figure 12-1). Data for the surface water sample location is included in Appendix
B, Table 2.
The 2016 risk assessment identified potential risks under a hypothetical recreational and
subsistence fisher scenario exposed to cobalt in fish tissue modeled from surface water
concentrations. The risks were likely overestimated because of very conservative
assumptions in the exposure models. New surface water data indicate that cobalt
concentrations have decreased since completing the 2016 risk assessment. Cobalt was
not detected (<1 µg/L) April 7, 2017 in SW-CB2, indicating that potential for
unacceptable health risks under the hypothetical fisher scenario are not likely when
additional data are incorporated into the assessment.
Detected concentrations of cobalt in surface water samples exceeded the 2L; however,
2L exceedances were also noted in upgradient reference location (SW-REF1). Except for
one high turbidity sample, detected cobalt concentrations in surface water did not
exceed the groundwater provisional background concentrations. Additionally, there is
not evidence of cobalt in groundwater downgradient of the ash basin. These
observations indicate that the presence and concentrations of cobalt in SW-CB2 is likely
naturally occurring or from sources other than the ash basin.
No evidence of risks to humans exposed to surface water in the downgradient
Crutchfield Branch area was identified.
“South Creek” – Surface Water
One surface water sample location (S-06) is a reference location for the current CSA.
Data for the surface water sample location are included in Appendix B, Table 2.
No evidence of risks to humans exposed to surface water in the south creek area was
identified.
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12.2 Ecological Screening Summary
Crutchfield Branch (off-site) – Surface Water
One surface water sample location is used in the ecological assessment of Crutchfield
Branch (SW-CB2). Data for the surface water sample are included in Appendix B,
Table 2. The 2016 risk assessment resulted in no potential risks to wildlife from
exposure to Crutchfield Branch surface water.
Dissolved copper (0.45 micron filter) was detected in one of five surface water samples
collected from location SW-CB2 (4.14 µg/L in April 2017). Total copper reported in this
sample was below detection, or less than 1 µg/L, and turbidity in the sample was
elevated at 25.1 NTU. The reported detection of dissolved copper is likely anomalous
due to being greater than the total copper concentration and is not considered further
with respect potential ecological risks. No potential unacceptable risks to wildlife
exposed to surface water in Crutchfield Branch were identified.
“South Creek” – Surface Water
One surface water sample location (S-06) was used in the ecological assessment of the
South Creek area. This location is a reference location for the current CSA. Data for the
surface water sample location are included in Appendix B, Table 2.
No potential risks to wildlife exposed to surface water in the south creek area were
identified.
12.3 Private Well Receptor Assessment Update
An independent study was conducted that evaluated 2015 groundwater data collected
from 3 private drinking water wells within close proximity (<0.5 miles) of the Mayo
Steam Electric Plant and 14 private drinking water wells within a 2 to 10 mile radius of
the Mayo Steam Electric Plant (CAP 2, Section 5.7; Haley & Aldrich, 2015). Pertinent
observations presented in the study included:
Boron and arsenic were not detected in private wells sampled by NCDEQ;
Calcium, sulfate, vanadium, and hexavalent chromium detected in the private
well samples were less than their respective background threshold values; and
Groundwater flow paths from the Mayo plant are away from residential areas.
The Haley & Aldrich report concluded that the constituents detected in the private
wells sampled by NCDEQ are consistent with regional background and do not indicate
impact from constituents derived from coal ash.
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Recent (2017) results from off-site water supply wells indicate that constituent
concentrations are less than 2L or less than PBTV for bedrock wells, with the exception
of one manganese and one vanadium detection. Manganese was detected in samples
collected from bedrock wells at 586 µg/L, compared to the bedrock PBTV of 544 µg/L,
and vanadium was detected at 6.2 µg/L, compared to the bedrock PBTV of 5.52 µg/L.
Based on these observations, there are no indications that potential for risks to off-site
residences exposed to groundwater exist.
12.4 Risk Assessment Update Summary
Based on review and analysis of groundwater and surface water data, there is no
evidence of risks to humans and wildlife at the Mayo Site attributed to CCR constituent
migration in groundwater from the ash basin. This update to the human health and
ecological risk assessment supports a risk classification of “Low”.

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13.0 GROUNDWATER MODELING RESULTS
Groundwater flow, transport, and geochemical models are being developed to simulate
movement of COIs through the subsurface to support the evaluation and design of
remedial options at the Site. The models will provide insights into:
1. COI Mobility: Geochemical processes affecting precipitation, adsorption and
desorption onto solids will be simulated based on analytical data and
thermodynamic principles to predict partitioning and mobility in groundwater.
2. COI Movement: Simulations of the groundwater flow system will be combined
with estimates of source concentrations, sorption, effective porosity, and
dispersion to predict the paths and rates of constituent movement at the field
scale.
3. Scenario Screening: The flow, transport, and geochemical models will be
adjusted to simulate how various ash basin closure design options and
groundwater remedial technologies will affect the short-term and long-term
distribution of COIs.
4. Design: Model predictions will be used to help design basin closure and
groundwater corrective action strategies in order to achieve compliance with
PBTVs and/or 2L in a reasonable cost and time frame.
The groundwater flow model linked with the transport model will be used to establish
transport predictions that best represent observed conditions at the Site particularly for
the constituents, such as boron, that tend to be negligibly affected by geochemical
processes. The geochemical model information will provide insight into the complex
processes that influence constituent mobility, which will be used to refine constituent
sorption within the transport model. Once the flow, transport, and geochemical models
for the Site accurately reproduce observed Site conditions, they can be used as
predictive tools to evaluate the conditions that will result from various remedial options
for basin closure (No Change, Cap-in-Place, or Ash Removal) and potential subsequent
passive or active groundwater remedial technologies.
The site-specific groundwater flow and transport models and the site-specific
geochemical models are currently being updated for use in the CAP. The CAP will
further discuss the purpose and scope of both the groundwater and geochemical
models and will detail model development, calibration, assumptions and limitations.
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The CAP will also include a detailed remedial option evaluation based on observed
conditions and the results of predictive modeling. The evaluation of the potential
remedial options will include comparisons of predictive model results for long-term
source concentration and plume migration trends toward potential receptors. The
model predictions will be used in combination with other evaluation criteria to develop
the optimal approach for basin closure and groundwater remediation.
The following sections provide a brief summary of modeling efforts completed to date.
13.1 Summary of Fate and Transport Model Results
A groundwater flow and transport model was developed to gain an understanding of
COI migration after closure of the ash basin at the Mayo Plant. The initial groundwater
model in the CAP, Part 1 (SynTerra, 2015b) included a calibrated steady-state flow
model of June 2015 conditions, a calibrated historical transient model of constituent
transport to June 2015 conditions, and three potential basin closure scenarios. Those
basin closure simulation scenarios included:
No change in Site conditions (basin remains open, as is)
Cap-in-place
Ash removal (excavation)
The initial model used boron and arsenic as primary modeling constituents. As part of
the CAP, Part 2 (SynTerra, 2016a) the model was revised to include manganese as a
constituent for evaluation of future simulations. Additionally, the model predictive time
was extended from 30 years to 100 years.
The revised model in the CAP, Part 2 (SynTerra, 2016a) included a calibrated steady-
state flow model of June 2015 conditions, a calibrated historical transient model of
constituent transport to June 2015 conditions, and two potential basin closure scenarios.
Those basin closure simulation scenarios included:
No change in Site conditions (basin remains open, as is)
Cap-in-place
The flow and transport model is currently being updated as a part of the final CAP and
will include: development of a calibrated steady-state flow model that includes data
available through November 2017, development of a historical transient model of
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constituent transport, and predictive simulations of basin closure plus groundwater
corrective action scenarios. The updated fate and transport model will consider arsenic,
boron, and possibly additional COIs that are hydraulically driven. Predictive
simulations will have simulation times that continue until modeled COIs concentrations
are below the 2L standard at the compliance boundary.
The following sections provide a brief summary of the groundwater modeling that was
presented in the CAP, Part 2 and a general outline for the updated modeling effort. The
summary of the groundwater modeling presented in the CAP, Part 2 was compiled to
address specific questions regarding model set-up and calibration. A complete updated
groundwater flow and transport model report is being developed and will be submitted
as part of the updated CAP.
The model was developed using the MODFLOW-NWT version (Niswonger, Panday, &
Motomu, 2011). This version provides improved numerical stability and accuracy for
modeling problems within a variable water table. The improved numerical stability
and accuracy can provide better estimates of the water table fluctuations that result
from ash basin operating conditions and potential closure and groundwater corrective
action activities.
MT3DMS was used to simulate fate and transport of selected COIs. MT3DMS uses the
groundwater flow field from MODFLOW to simulate 3D advection and dispersion of
the dissolved COIs, including the effects of retardation due to the soil matrix adsorption
of COIs.
Flow Model Construction 13.1.1
The flow and transport model was built through a series of steps. The first step
was to build a three-dimensional (3D) model of the Site hydrostratigraphy based
on the SCM. The next steps were to determine the model dimensions and the
construction of the numerical grid. The numerical grid was then populated with
flow parameters, which were calibrated in the steady-state flow model. Once the
flow model was calibrated, the flow parameters were used to develop a transient
model of the historical flow patterns at the Site. The historical flow model was
then used to provide the time-dependent flow field for the constituent transport
simulations.
Generally, the model geometry will not be substantially modified for the
updated model. Hydraulic parameters such as hydraulic conductivity values
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may be adjusted within reasonable site-specific ranges to achieve hydraulic head
calibration error below 10 percent.
Flow Model Domain and Grid Layers
The model has dimensions of approximately three miles-by-three miles, with the
ash basin at the center of the model domain. The model domain was rotated 31˚
clockwise so that the model boundaries were parallel with the ash basin dam.
The shortest distance between the ash basin and a model boundary is
approximately one mile.
The hydrostratigraphic model consists of six units: ash, saprolite, transition
zone, upper bedrock (upper fractured rock), middle bedrock (middle fractured
rock), and lower bedrock (lower rock). The units were determined by
interpolating boring log data from historical data, the CSA, and the CAP reports.
The hydrostratigraphic model was developed using “Solids” in GMS and was
subdivided into five solids. A computational mesh (numerical grid) was then
developed based on these solids: ash, saprolite, transition zone, fractured rock,
and rock.
The numerical grid consists of rectangular blocks arranged in columns, rows,
and layers. There are 171 columns, 232 rows, and 15 layers. The maximum
width of the columns and rows is 100 feet. The size of the grid blocks is
approximately 50 feet by 50 feet in the vicinity of the ash basin. The horizontal
dimension of some of the grid blocks is as small as 25 feet in the vicinity of the
dams. The grid consists of 15 layers representing the six hydrostratigraphic
units. It is expected that the updated model will use similar grid spacing.

Hydrostratigraphic layer Grid layer
Ash 1-4
Saprolite 5
Transition zone 6-7
Upper fractured rock 8-10
Middle fractured rock 11-12
Lower Rock 13-15
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Flow Model Boundary Conditions
The south, east, and north boundaries of the model were set to a specified head
within the transition zone, which is a reasonable approximation of Site
conditions. The specified head boundaries along the upland areas were
terminated within a few hundred feet of the locations of streams or lakes that
crossed those boundaries. Lakes are defined as specified heads. Streams are
defined as drain-type boundaries. No-flow boundaries were set along
groundwater divides. A no-flow boundary was also set beneath the transition
zone to the base of the model.
Sources and Sinks
Water can enter the model or leave the model through the use of sources and
sinks. MODFLOW uses point sources/sinks as well as areal sources/sinks. Point
sources/sinks include rivers, wells, drains, and general head. Areal sources/sinks
considered are limited to recharge.
Source (Recharge)
Model recharge sources include:
Recharge that is applied to the ash basin (7.9 inches/year) and
zero recharge applied to the inundated portion of the ash basin.
(The recharge on exposed ash was assumed to be 0.0018 ft/d, the
same as in upland areas. This is because the shallow water table
would have increased evaporation, while the lack of vegetation
would have decreased evapotranspiration on the ash basin
compared to the upland area. As a result, without field data it
was difficult to assess how the recharge on the ash basin would
have differed from the recharge on the uplands.)
Rainwater that infiltrates in the upland areas (6.5 inches/year)
(Precipitation in developed areas of the Site (set to near zero;
assumes most will run off). Large areas of ponded water, such as
the ash basin and Mayo Lake, were represented as specific head
boundaries and recharge was set to zero.)

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Constant head boundaries
(Lakes were represented as specified head boundaries with the
head set to their stage. This includes Mayo Lake and the ash
basin. The stage of the ash basin was set to 480 feet based on
LIDAR data and a surveying point. The stage of Mayo Lake was
432 feet.)
Model Sinks (Drains)
Model sinks include:
Streams within the model domain
The former stream valley of Crutchfield Branch
Engineered toe drains located at the base of the ash basin dam
AOWs at the toe of the dam that feed smaller, unnamed
tributaries to Crutchfield Branch
Ash basin water diverted into a forebay, which is discharged
through NPDES Outfall 002 into Mayo Lake.
Water Supply Wells
Approximately 21 domestic wells were previously identified within one-half
mile of the Site (SynTerra, 2014a). The average daily use for domestic wells was
set at a discharge of approximately 350 gallons per day (USEPA, 2017c).
Hydraulic Conductivity
The horizontal hydraulic conductivity and the horizontal-to-vertical hydraulic
conductivity anisotropy ratio (anisotropy) are the main variable hydraulic
parameters in the model. The distribution of those parameters is based primarily
on the model hydrostratigraphy, with some local variations. The values can be
adjusted during the calibration process to provide a best fit for observing water
levels in wells. Initial estimates of parameters were based on literature values,
results of slug and core testing, and simulations performed using a preliminary
flow model.

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Streams and Lake Hydraulic Parameters
Mayo Lake and the ash basin were represented as specified head boundaries.
The stage of the ash basin was set to 480 feet based on LIDAR data from 2012
and surveying data. The stage of Mayo Lake was set to 432 feet and was also
based on NCDOT LIDAR data from 2012.
The outflow channel from the ash basin to the forebay was represented as
specified head. Survey data of the outflow channel heads were collected and
applied to the model that allows this engineered channel to exchange water with
the groundwater system.
Flow Model Calibration Targets
The steady state flow model calibration data for June 2015 were presented in the
CAP, Part 2. In the final CAP, calibration target data will be incorporated by
taking the mean of the hydraulic head data for each well and applying a
standard deviation to reflect the seasonal changes in the hydraulic heads.
Hydraulic head data will include measurements until November 2017.
Mass Balance
The previous model had a mass balance error of well below 1%. The updated
model will have a similar numerical accuracy.
Flow Model Sensitivity Analysis
A parameter sensitivity analysis for the preliminary calibrated model showed the
highest degree of sensitivity to upland recharge and hydraulic conductivities (in
the transition zone and saprolite stratigraphic units). The model was only
weakly sensitive to the hydraulic conductivities of the ash, deep bedrock, and
hydraulic conductivity of the dams and to the pumping rate of the domestic
wells. Since no major elements within the model are to be changed, there is no
need to perform additional sensitivity testing.
Particle Tracking
A primary concern is the potential impact to domestic and public wells from
COIs migrating from the Site. The final calibrated groundwater flow model will
be used to assess potential impacts by considering pumping from domestic and
public wells, if any, within the model domain.

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Flow Model Assumptions and Limitations
The groundwater model is currently being updated/refined and assumptions
and limitations will be subject to change. Based on the preliminary modeling
results, the assumptions and limitations included the following:
The steady-state flow model was calibrated to hydraulic heads
measured in monitoring wells in June 2015. The model was not
calibrated to transient water levels over time, recharge, or stream flow.
MODFLOW simulates flow through porous media. A single domain
MODFLOW modeling approach for simulating flow in the primary
porous groundwater zones and bedrock was used for contaminant
transport. Flow in fractured bedrock is simulated using the equivalent
porous media approximation.
For the purposes of numerical modeling and comparing predictive
scenarios, it was previously assumed that basin closure would be
completed in 2015. A similar assumption will be used in the updated
model.
Predictive simulations were performed and steady-state flow conditions
were assumed from the time that the ash basin was placed in service
through the current time until the end of the predictive simulations
(2045).
The uncertainty in model parameters and predictions has not been
quantified; therefore, the error in model predictions is not known. It was
assumed the model results are suitable for a relative comparison of
closure scenario options.
Residential wells for which well construction records could not be
obtained were assumed to be completed in the upper bedrock.
Transport Model Construction 13.1.2
Modular 3-D Transport Multi-Species (MT3DMS) is being used to simulate
constituent transport. MT3DMS simulates 3D advection and dispersion of the
dissolved COIs, including the effects of retardation due to the soil matrix
adsorption of COIs based on flow fields established by MODFLOW. The initial
model used boron and arsenic as primary modeling constituents. The updated
fate and transport modeling will focus on arsenic, boron, and possibly additional
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COIs that are hydraulically driven. Other constituents will be considered in
geochemical modeling.
Transport Model Parameters
The key transport model parameters (besides the flow field) are the constituent
source concentrations in the ash basin and the constituent soil-water distribution
coefficients (Kd). Secondary parameters are the longitudinal, transverse, and
vertical dispersivity, and the effective porosity.
Transport Model Boundary Conditions
In the current model, the transport model boundary conditions are “no flow” on
the exterior edges of the model. Infiltrating rainwater is assumed to be clean and
enters with zero concentration from the top of the model. Contaminants are
assumed to leave the model when they reach a drain or are removed by flow that
enters a constant head boundary.
In the current model, the concentrations from the June 2015 sampling event were
set as boundary conditions within the ash basin. These values will be updated to
use the concentration data up through the November 2017 sampling event.
Transport Model Sources and Sinks
Transport model sources include:
The ash basin is considered the source of COIs in the model. The
sources are simulated by applying a constant COI concentration within
the cells of the ash basin and were applied to layers 1 through 4 which
represent the ash. This allows infiltrating water to carry dissolved
constituents from the ash pore water into the groundwater underneath
the ash basin. Chemical analyses from four wells were used to
characterize the distribution of COI concentration within the ash basin,
and the source concentration is used as a calibration parameter in the
transport simulations
The concentration in the vicinity of ABMW-4 in Layer 5 to reflect the
geometry of the ash basin.
As the COIs migrate beneath and away from the coal ash, zones of soil
and fractured rock may become impacted. These impacted zones can
serve as secondary sources and are fully accounted for in the transport
models. For simulations that involve ash excavation, the constant
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concentration sources in the ash zones are removed, but the secondary
sources in the impacted soil and fractured bedrock remain. The
longevity of these secondary sources depends on the COI Kd and on the
degree of flushing by infiltration and groundwater flow.
Transport model sinks include:
Lakes
Streams
Other AOW and engineered drains
Transport Model Calibration Targets and Sensitivity
The initial transport model calibration targets were COI concentrations measured
in monitoring wells in June 2015. The updated model calibration targets will
include COIs concentrations measured in monitoring wells in 2017. Constituents
considered for the next fate and transport model will include boron, arsenic and
possibly other COIs. COIs not amenable to simulation in the fate and transport
model will be addressed in the geochemical model.
Transport Simulation
The updated model will be calibrated to include data through November 2017
and will extend until modeled COI concentrations are below the 2L standard at
the compliance boundary. The following is a summary of the basin closure
options modeled:
No Action – Leave the ash basin as is to evaluate whether groundwater
quality would be restored by natural attenuation under current
conditions.
Cap-in-Place – Grade the ash and place an engineered low permeability
cover system to reduce infiltration of surface water. This scenario
assumes that the ash under the cap will be dewatered.
Ash Removal – Remove the ash from the basin. This scenario assumes
that the ground surface would be restored to its initial grade (prior to
construction of the ash basin).
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The distribution of recharge, locations of drains, and distribution of material will
be modified to represent the different basin closure options. The results of these
simulations will be included as part of the updated CAP submittal.
Summary of Flow and Transport Modeling Results To 13.1.3
Date
The simulated June 2015 concentration distributions described in the CAP, Part 1
(SynTerra, 2015b) were used as initial conditions in a predictive simulation of
future flow and transport at the Site and modeled arsenic and boron. Predictive
simulations of future flow and transport for manganese (SynTerra, 2016a) were
added and predictive simulations of future flow and transport for both boron
and manganese under the “no action” and cap-in-place scenarios were run for a
100-year projection.
No Action
In the CAP, Part 1 report (SynTerra, 2015b) simulated arsenic concentrations in
saprolite beneath the ash increased in 2045 compared to the 2015 values. The
distribution of arsenic in the saprolite in 2015 is patchy, and by 2045 the space
between many of the patches has filled in. The bottom of the transition zone and
underlying rock are less than the 2L standard in 2045, according to the
simulations. It should be noted the Kd value for arsenic was on the low side of the
range of lab values and the simulation over-estimated several of the observed
concentrations during calibration.
The CAP, Part 1 (SynTerra, 2015b) indicates boron concentrations delineated by
the 2L contour are larger in the simulations from 2045 than they are in 2015, but
the differences are relatively small. The leading edge of the boron plume is on
the south side of Mayo Lake Road in 2015, and 30 years later, the leading edge
has moved from the south to the north side of the road, according to the
simulations. The groundwater model as reported in the CAP, Part 2 (SynTerra,
2016a) indicates simulated 2115 boron concentrations delineated by the 2L
contour are similar in 2045 simulations. The 2115 boron simulations within the
transition zone and bedrock are slightly larger than the 2045 simulations. The
2115 boron plume simulations show the edge of the plume within the saprolite
and transition zone is south of Mayo Lake Road. The simulated 2115 boron
bedrock plume has two locations where the edge of the plume has migrated
slightly north of Mayo Lake Road.
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The groundwater model as reported in the CAP, Part 2 (SynTerra, 2016a)
indicates that the observed concentrations of manganese in the ash basin would
result in a plume of manganese extending along Crutchfield Branch. This plume
would occur in a setting where manganese was common as a natural constituent
of groundwater. The simulations indicate that approximately ¾ of the wells
where manganese exceeded the 2L standard represent naturally occurring
manganese that is not included in the model. The other ¼ of the affected wells
are located in a region influenced by the manganese plume in the simulations.
The simulation is able to approximate some of the observed concentrations in the
vicinity of Crutchfield Branch and beneath the ash basin. There are six wells
where both the observed and simulated concentrations are above the 2L
standard. The simulations indicate that some or all of the manganese observed
in water from these wells can be explained as having a source from within the
ash basin. Concentrations at 19 wells are above the 2L standard and the
simulations indicate a concentration of 0. The manganese in the water from
these 19 wells cannot be explained as having a source from within the ash basin.
Concentrations in water from seven wells are predicted to be below the 2L
standard.
Cap-In-Place
In the CAP, Part 1 report (SynTerra, 2015b), simulated arsenic concentrations in
the saprolite beneath the ash basin slightly increased from 2015 to 2045 and are
less than the No Action scenario. According to the simulations, arsenic is below
the 2L standard in the transition zone and fractured bedrock. It should be noted
the Kd value for arsenic was on the low side of the range of lab values and the
simulation over-estimated several of the observed concentrations during
calibration.
Under a cap-in-place (engineered capping system) closure scenario, the model
results indicate a stabilization of boron plume geometry and a reduction of boron
concentrations in the surficial zone by 2030 (SynTerra, 2015b). Likewise, for the
transition zone, plume extent and concentrations decrease by 2030 and remain
stable through 2045. For the fractured bedrock, the plume “footprint” begins to
retreat south away from the compliance boundary by 2030 and continues to
retreat south by 2045. Beginning with the 2045 simulation and continuing until
2115 (100 years), the simulations indicate continued boron concentration
reductions and additional plume retreat by several hundred feet by 2115 for all
units. For all three hydrogeologic units, no 2L exceedances of boron beyond the
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compliance boundary are indicated by the simulation results. Results in the
February 2016 updated fate and transport model indicate that the leading edge of
the boron plume recedes from 2045 to 2115 within the saprolite, transition zone,
and fractured bedrock layers up to 300 feet within the vicinity of Mayo Lake
Road.
Model results indicate that the leading edge of the highest manganese
concentration having a source within the ash basin has begun to recede
approximately 30 years after basin closure (2045). More significantly, after 100
years (2115), the manganese plume in exceedance of the 2L is 100 feet within the
compliance boundary.
The results of the CAP, Part 2 modeling did not substantially alter the
conclusions presented in CAP, Part 1.
13.2 Summary of Geochemical Model Results
The Mayo Site geochemical model investigates how variations in geochemical
parameters affect movement of constituents through the subsurface. The geochemical
site conceptual model (SCM) will be updated as additional data and information
associated with Site constituents, conditions, or processes are developed. The
geochemical modeling approach presented in the following sections was developed
using laboratory analytical procedures and computer simulations to understand the
geochemical conditions and controls on groundwater concentrations at the Mayo Plant
in order to predict how remedial action and/or natural attenuation may occur at the Site
and avoid unwanted side effects. The final geochemical model will be presented in the
updated CAP.
Model Construction 13.2.1
The geochemical model in the CAP, Part 2 (SynTerra, 2016a) included:
EH-pH (Pourbaix) diagrams showing potential stable chemical phases of
the aqueous electrochemical system, calibrated to encompass conditions at
the Site;
Sorption model where the aqueous speciation and surface complexation
are modeled using the USGS geochemical modeling program pH, redox,
equilibrium model written in C language (PHREEQC);
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Simulations of the anticipated geochemical speciation that would occur
for each COI in the presence of adsorption to soils and in response to
changes in redox potential (EH)and pH; and
Attenuation calculations where the potential capacity of aquifer solids to
sequester constituents of interest were estimated.
Laboratory Determination of Distribution Coefficient
SynTerra retained researchers from the UNCC to determine site-specific
distribution coefficients (Kd) for the primary hydrostratigraphic units. The UNCC
Soil Sorption Evaluation and Addendum to the UNCC Soil Sorption Evaluation
reports are provided in Appendix C. Selected soil samples were analyzed using
batch and column experiments to determine Kd values for COIs (Table 13-1). In
addition to these analyses, metal oxy-hydroxide phases of iron (HFO),
manganese (HMO), and aluminum (HAO) in soils were measured. HFO, HMO,
and HAO are considered to be the most important surface reactive phases for
cationic and anionic constituents in many subsurface environments (Ford, W., &
Puls, 2007). Quantities of these phases in soil can thus be considered a proxy for
the presence of ferrihydrite (HFO) and gibbsite (HAO) which can be used to
model COI sorption capacity for a given soil (Dzombak & Morel, 1990);
(Karamalidis & Dzombak, 2010).
Geochemical Model Construction
To examine the sorption behavior of multiple ions of interest in the subsurface
environment surrounding coal-fired power plants, a combined aqueous
speciation and surface complexation model was developed using the USGS
geochemical modeling program PHREEQC. Equilibrium constants for aqueous
speciation reactions were taken from the USGS WATEQ4F database. This
database contained the reactions for most elements of interest except for
antimony, chromium, cobalt, and vanadium. Constants for aqueous reactions
and mineral formation for these elements were taken from the MINTEQ v4
database which is also issued with PHREEQC. The constants were checked to
provide a self-consistent incorporation into the revised database. The source of
the MINTEQ v4 database is primarily the well-known NIST 46 database (Martell
& Smith, 2001). Sorption reactions were modeled using a diffuse double layer
surface complexation model. For self-consistency in the sorption model, a single
database of constants was used as opposed to searching out individual constants
from literature. The diffuse double layer model describing ion sorption to HFO
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and HAO was selected for this effort (Dzombak & Morel, 1990); (Karamalidis &
Dzombak, 2010).
Geochemical Controls on COI
As described in previous geochemical model reports (SynTerra, 2015b; SynTerra,
2016a), pH, EH, and solubility are the primary geochemical parameters affecting
constituent mobility. In the updated geochemical model planned to be
submitted in 2018 as part of the CAP, hydraulically significant flow transects will
be used to evaluate the conceptual model of COI mobility in the subsurface.
Trends in the concentrations of each COI along the transect will be compared
with the model output to verify that the conceptual and qualitative models can
predict COI behavior. The model will then be used to evaluate the potential
impacts of remediation activities. The model will relate the COI concentrations
observed in groundwater along flow transects to key geochemical parameters
influencing constituent mobility (i.e., EH, pH, and saturation/solubility controls).
Geochemical Model Assumptions
Several key assumptions will be applied to the planned geochemical modeling
effort:
1) The thermochemical sorption constant reactions describe ion sorption
to ferrihydrite and gibbsite (HFO and HAO).
2) The model will use the same or more conservative site density
assumptions as those used by Dzombak and Morel (1990) and
Karamalidis and Dzombak (2010) to constrain the surface sites.
HAO and HFO (i.e., gibbsite and ferrihydrite) are used as the primary reactive
minerals due to the availability of surface complexation reactions. Differences
between the sorption behaviors at a specific Site will be primarily due to 1)
differences in the pH, EH, and ion concentrations, and 2) differences in the
extractable iron and aluminum concentrations from site-specific solids.
Additional reactive minerals will be incorporated into the model as needed on a
site-specific basis.
Updated Geochemical Model Development
The updated geochemical site investigation to accompany the CAP will develop
parameters for each aquifer or geologically derived flow zone (geozone) by
considering the bulk densities, porosities, and hydraulic gradients used in the
fate and transport model. Additionally, the potential effects on aquifer
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characteristics resulting from the construction of a future retention basin and
flue-gas desulfurization pond will be evaluated. These parameters are used to
constrain the sorption site concentrations in the model input and will be
incorporated in the 1-D ADVECTION model to accompany the capacity
simulations. The objective of these capacity simulations is to determine the mass
balance on iron and aluminum sorption sites when simulating flow through a
fixed region. Input and initial iron and aluminum concentrations will be fixed
based on site-specific data. Thus, the model will be able to simulate the stability
of the HFO and HAO phases assumed to control constituent sorption.
The final geochemical model report will include a site specific discussion of:
The model description,
The purpose of the geochemical model,
Modeling results with comparison to observed conditions,
COI sensitivity to pH, EH, iron/aluminum oxide content, and
Model limitations.
The updated geochemical modeling will also present multiple methods of
determining constituent mobility at the Site. Aqueous speciation, surface
complexation, and solubility controls will be presented in the revised report.
These processes will be modeled using:
Pourbaix diagrams created with the Geochemist Workbench v10 software
using site-specific minimum and maximum constituent concentrations.
PHREEQC’s combined aqueous speciation and surface complexation
model and the 1-D ADVECTION function to gain a comprehensive
understanding of current geochemical controls on the system and evaluate
how potential changes in the geochemical system might affect constituent
mobility in the future.
Summary of Geochemical Model Results To Date 13.2.2
The geochemical model considers changes in oxidation state for all redox active
constituents of interest (arsenic, chromium, cobalt, iron, manganese, selenium,
sulphur, and vanadium) and changes in chemical speciation for all constituents.
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PHREEQC model predicts:
Arsenic - As(V) as the dominant oxidation state of arsenic. As(III) is the
dominant species measured in ground waters. This is due to the stronger
sorption of As(V). This results in the relatively lower Kd values predicted for
arsenic in the model.
Boron - Boron exists only in the B(III) oxidation state. PHREEQC predicted Kd
values for boron are low. Predicted values are slightly lower, but generally
consistent, with the values chosen for reactive transport modeling and those
measured in batch laboratory experiments.
Chromium - Cr(III) is the dominant oxidation state which. Cr(III) readily sorbs
to mineral surfaces as the pH increases. Cr (VI) however, is weakly sorbing and
decreases sorption as the pH increases. Kd for Cr (III) is relatively high while the
Kd for Cr (VI) is relatively low.
Manganese - Manganese is predominantly present as Mn2+. Sorption of Mn(II) is
generally weak and yields low Kd values. Manganese bearing soil minerals could
occur given sufficiently high manganese concentrations and high pH/ EH
conditions which may play a role on controlling the movement of manganese in
the subsurface.
Cobalt - The dominant cobalt species predicted by the PHREEQC model is Co2+
redox potential exhibits relatively little influence on Co2+. Overall, cobalt is
expected to exhibit minimal transport in these systems (high Kd) relative to more
mobile species.
Selenium – The geochemical behavior of selenium is highly dependent on the EH
of the groundwater. The PHREEQC model predictions show Se(IV) as the
dominant species under approximately neutral pH conditions but the fraction of
Se(VI) increases with increasing pH and EH. Overall the range of Kd values
predicted by PHREEQC agrees with the values determined experimentally from
batch sorption tests.
Vanadium - Vanadium can exist in multiple oxidation states including V(III),
V(IV), and V(V) under the groundwater conditions at the Site. The majority of
vanadium is expected to exist as pentavalent V(V) which exhibits moderate
sorption. Predicted Kd values are highly dependent on the pH of the
groundwater.
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13.3 Summary of Groundwater to Surface Water Evaluation
For the scope of this CSA, it is only appropriate to compare named surface waters to 2B
because AOWs, wastewater, and wastewater conveyances (effluent channels) are
evaluated and governed wholly separate in accordance with the NPDES Program
administered by NCDEQ DWR. This process is on-going in a parallel effort to the CSA
and subject to change.
Prior to construction of the ash basin, Crutchfield Branch was a perennial stream that
originated about 1,000 feet southwest of the current ash basin footprint. The ash basin
now encapsulates the former headwaters of Crutchfield Branch and two smaller,
intermittent streams that flowed into Crutchfield Branch. Groundwater underlying the
ash basin flows north-northeast and beneath and through the base of the ash basin dam
and into the former Crutchfield Branch stream valley. Two discharges from engineered
toe drains emerge from the area around the ash basin dam and contribute to the volume
of water flowing downstream of the dam. Surface water flows northeast beneath Mayo
Lake Road, onto privately-owned property where it merges with another small
tributary that originates on the Mayo Site, and then flows north into Virginia.
Consistent with LeGrand’s model for groundwater flow and discharge in the Piedmont
(LeGrand, 2004) the Crutchfield Branch stream valley and its tributaries are
groundwater discharge zones downgradient of the ash basin. Groundwater data
collected to date and groundwater flow and transport modeling performed in CAP,
Part 2 indicate that surface water in the area downgradient of the ash basin is
influenced by groundwater discharge.
As described in Section 9.0, boron concentrations are greatest near and proximate to the
engineered toe drains and decrease from the Mayo Lake Road to the state line.
Manganese concentrations are similarly consistent immediately downstream of the ash
basin and begin to decline downstream; however, at the most downstream surface
water location (SW-CB2), manganese concentrations increase.

Groundwater modeling shows that surface water in the area downgradient of the ash
basin is influenced by groundwater discharge. The CAP will include updated modeling
and use those results, in conjunction with existing data as needed, to determine if the
proposed corrective action will result in exceedance of surface water quality standards

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in Crutchfield Branch. Closure and bulk dewatering of the ash basin is expected to
reduce groundwater contribution to surface water. Basin closure modeling will be used
to predict groundwater to surface water contribution. The updated CAP will evaluate if
corrective action following basin closure will be necessary to mitigate a potential
exceedance of surface water quality standards.
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14.0 SITE ASSESSMENT RESULTS
14.1 Nature and Extent of Contamination
The site assessment described in the CSA presents the results of investigations required
by CAMA and 2L regulations. The ash basin pore water was determined to be a source
of impact to groundwater. The site assessment investigated the Site hydrogeology,
determined the direction of groundwater flow from the ash basin, and determined the
horizontal and vertical extent of impacts to groundwater and soil sufficient to proceed
with preparation of a CAP.
Constituents of Interest
COIs in groundwater identified as being associated with the Mayo Plant ash basin
include arsenic, barium, boron, chromium, hexavalent chromium, cobalt, iron,
manganese, molybdenum, pH, strontium, sulfate, TDS, and vanadium. Groundwater
COIs migrate laterally and vertically into and through surficial regolith, the
regolith/bedrock transition zone, and shallow bedrock. The surficial zone at the Mayo
Site is generally thin and unsaturated. The transition zone, where saturated, and
shallow bedrock generally contain the first occurrence of groundwater. Constituent
migration in groundwater occurs at variable rates depending on a number of physical
and chemical conditions and properties (e.g., constituent sorption properties, redox
state, pH, hydraulic conductivity, etc.). Some COIs, such as boron, readily solubilize
and migrate with minimal retention. In contrast, some COIs such as arsenic readily
adsorb to aquifer materials, do not readily solubilize, and thus are relatively immobile.

Hydrogeologic Conditions
Site hydrogeologic conditions were evaluated by installing and sampling groundwater
monitoring wells and piezometers; conducting in-situ hydraulic tests; sampling soil for
physical and chemical testing; and sampling surface water, AOW, and sediment
samples. Monitoring wells were completed in each hydrostratigraphic unit. The
groundwater flow system serves to store and provide a means for groundwater
movement. The porosity of the regolith is largely controlled by pore space (primary
porosity); whereas, in bedrock, the effective porosity is largely secondary and controlled
by the number, size, and interconnection of fractures. 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. The majority of groundwater flow across the Site appears to flow through
the transition zone and upper bedrock.
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Four hydrostratigraphic units were identified at the Mayo Plant and were evaluated
during the CSA. A detailed description of each unit is provided in Section 6.2.2.
Ash – The ash pore water unit consists of saturated ash material.
Shallow/Surficial – The shallow/surficial unit consists of soil, saprolite, and
alluvial material that overlie the transition zone with bedrock.
Transition Zone – The transition zone flow unit lies directly above competent
bedrock and is a zone of partially weathered bedrock.
Fractured Bedrock – The majority of water-producing fracture zones were found
within 50 - 75 feet of the top of competent bedrock.
The Mayo Plant ash basin occupies the former stream valley of Crutchfield Branch. The
basin acts as a bowl-like feature toward which groundwater flows from the northwest,
west, south, and east. Groundwater flows north-northeast from the ash basin into the
small valley formed by Crutchfield Branch. Groundwater flows from the highest
topographic portion of the Site (near the Plant entrance road) to the north and
northeast. The ash basin was formed when the Crutchfield Branch valley was dammed.
The flow of ponded water within the ash basin is controlled laterally by groundwater
flow that enters the basin from the east, south, and west and is controlled downgradient
(north-northeast) by the ash basin dam and the NPDES outfall/discharge. The head
created by the ash pore water creates a slight mounding effect that influences the
groundwater flow direction in the immediate vicinity of the ash basin. East of the ash
basin, there is a groundwater divide that separates the Crutchfield Branch flow regime
from the Mayo Lake flow regime.
There are few substantive differences in water level among wells completed in the
different flow zones across the Site (shallow/surficial, transition zone, bedrock), and
lateral groundwater movement predominates over vertical movement. The vertical
gradients are near equilibrium across the Site, indicating that there is no distinct
horizontal confining layer beneath the Mayo Plant. The horizontal gradients, hydraulic
conductivity, and AOW velocities indicate that most of the groundwater transport
occurs through the transition zone and bedrock, as most of the regolith encountered is
largely unsaturated.
Groundwater flow directions and the overall morphology of the potentiometric surface
vary little from “dry” to “wet” seasons. Water levels do fluctuate up and down with
significantly increased or decreased precipitation, but the overall groundwater flow
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direction does not change due to seasonal changes in precipitation. Horizontal
gradients along the southern portion of the Site range from 0.017 feet/feet to 0.018
feet/feet. Horizontal gradients along the northern end of the Site range from 0.03
feet/feet to 0.034 feet/feet. The hydraulic gradient in the northern portion of the Site is
influenced by the higher relief. Generally, upward vertical gradients predominate on
the west side of the Site, including the ash basin area, while downward (recharge)
gradients are more prevalent in the northeast portion of the property. Upward vertical
gradients from bedrock, as groundwater from the west, south, and east recharge the
groundwater beneath the basin into the former Crutchfield Branch valley, reduces the
potential for downward migration of COIs into bedrock.
Horizontal and Vertical Extent of Impact
The groundwater plume is defined as locations (in three-dimensional space) where
groundwater quality is impacted by the ash basin. Naturally occurring groundwater
contains varying concentrations of a number of constituents (e.g., alkalinity, aluminum,
magnesium, sodium, zinc, etc.). Sporadic and low-concentration exceedances of these
constituents in the groundwater data do not necessarily demonstrate distribution of
groundwater that has likely been impacted by the ash basin. The leading edge of the
plume, the furthest downgradient edge, is represented by groundwater concentrations
greater than PBTVs in the wells in each flow unit.
Boron is the primary CCR-derived constituent in groundwater at Mayo and is detected
at concentrations greater than the PBTV and 2L standard beneath and downgradient
(north-northeast) of the ash basin. Boron is not detected in background groundwater.
Boron, in its most common forms, is soluble in water, and boron has a very low Kd
value, making the constituent highly mobile in groundwater. Therefore, the
presence/absence of boron in groundwater provides a close approximation of the
distribution of CCR-impacted groundwater. The detection of boron at concentrations in
groundwater greater than the PBTV best represents the leading edge of the CCR-
derived plume moving downgradient from the source area (ash basin). At Mayo,
boron is detected at concentrations greater than the 2L standard beneath and
downgradient (north-northeast) of the ash basin. Boron is not detected in background
groundwater.
At Mayo, manganese and strontium detections in groundwater also indicate impact.
The area farthest downgradient at which boron, manganese, and strontium are detected
at a concentration greater than applicable PBTVs is interpreted as the leading edge of
the CCR-derived plume moving downgradient from the source area.
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In the bedrock flow unit, boron is detected (does not exceed 2L) in downgradient well
CW-2D (located on Mayo property, near the northern property line). In bedrock well
MW-16BR, located approximately 1,000 feet downgradient of CW-2D and the property
line, boron is not detected. The leading edge of the bedrock plume is interpreted to be
at/near the northern property line. Similarly, in the transition zone unit, groundwater
from monitoring well CW-2 contains detectable concentrations of boron that exceed 2L,
while MW-16D, located about 1,000 feet downgradient, boron is not detected. Surficial
wells MW-16S and MW-3 are screened in Crutchfield Branch alluvium. MW-3 (located
on Mayo property several hundred feet south of the property line between the property
line and the ash basin dam) contains detectable concentrations of boron that exceed 2L,
while at MW-16S (located downgradient approximately 1,500 feet from MW-3), boron is
detected at a lower concentration (does not exceed 2L).
The surficial and transition zone flow units at Mayo – beneath and downgradient of the
ash basin – are impacted by CCR-derived constituents; however, these units are not
vertically extensive. Impact to the bedrock flow unit is confined, approximately, to the
top 50 – 75 feet of fractured bedrock. The vertical extent of the plume is represented by
groundwater concentrations in bedrock wells beneath and downgradient of the ash
basin. ABMW-2BRL, drilled to a depth of 180 feet bgs, contains no boron or manganese
concentrations above 2L or the PBTV. ABMW-2BR and ABMW-4BR are other shallower
bedrock wells beneath the ash basin, and groundwater from these wells has the same
absence of CCR-related constituents at concentrations that exceed 2L/IMAC or PBTVs.
As groundwater under the ash basin flows northeast toward the ash basin dam, the
hydraulic impact of the ash basin dam and the hydraulic head exerted by the ash basin
water forces groundwater downward into the bedrock which increases hydraulic
pressure in the bedrock aquifer. Wells completed in surficial, transition zone, and
bedrock proximate to the north side of the ash basin dam are impacted by COIs. As
groundwater and the plume migrate in the downgradient direction, unimpacted
groundwater enters the system from upgradient recharge areas to the west and east,
mitigating the concentration of some COIs (e.g., boron).
14.2 Maximum COI Concentrations
Changes in COI concentrations over time are included as time-series graphs (Figures
14-1 through Figure 14-39). The maximum historical detected COI concentrations in
groundwater for ash pore water or wells directly beneath the ash basin and non-ash
basin groundwater are included below. Also listed is the range of PBTVs for the
surficial, transition zone, and bedrock flow units:
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Arsenic – Ash Basin: 1,020 µg/L (ABMW-2); Outside Basin: 2.05 µg/L (MW-
16BR); PBTV range: Not Detected in background samples for all three units.
Barium – Ash Basin: 1,280 µg/L (ABMW-1); Outside Basin: 178 µg/L (CW-2);
PBTV range: 19 µg/L – 97 µg/L.
Boron - Ash Basin: 9,200 µg/L (ABMW-2); Outside Basin: 1,050 µg/L (MW-3);
PBTV range: Not Detected in background samples for all three units.
Chromium – Ash Basin: 1.39 µg/L (ABMW-4D); Outside Basin: 53 µg/L (CW-1);
PBTV range: 3.23 µg/L – 7 µg/L.
Chromium (hexavalent) – Ash Basin: 6 µg/L (ABMW-3); Outside Basin: 19.4
µg/L (MW-11BR); PBTV range: 0.088 µg/L – 1.26 µg/L. The Ash Basin value is
questionable because the highest total chromium value is less than 6 µg/L.
Cobalt – Ash Basin: 821 µg/L (ABMW-1); Outside Basin: 10.3 µg/L (MW-16S);
PBTV range: Not detected (transition zone) – 1.19 µg/L.
Iron – Ash Basin: 72,100 (ABMW-4D); Outside Basin: 11,700 µg/L (MW-5BR);
PBTV range: 385 µg/L – 2,550 µg/L.
Manganese – Ash Basin: 6,960 (ABMW-4D); Outside Basin: 2,680 µg/L (MW-
8BR); PBTV range: 253 µg/L – 544 µg/L.
Molybednum – Ash Basin: 1,880 µg/L (ABMW-2); Outside Basin: 60.4 µg/L
(MW-16BR); PBTV range: Not detected (transition zone) – 13.1 µg/L.
pH - Ash Basin: 5.2 (ABMW-3S) – 9.6 (ABMW-2); Outside Basin: 4.9 (MW-4) – 9.3
(MW-16BR); PBTV range: 4.9 – 7.3.
Strontium - Ash Basin: 4,640 µg/L (ABMW-1); Outside Basin: 1,100 µg/L (MW-
7BR); PBTV range: 25 µg/L – 418 µg/L.
Sulfate – Ash Basin: 80 mg/L (ABMW-4BR); Outside Basin: 90 mg/L (MW-10BR);
PBTV range: 1.6 mg/L – 18 mg/L.
TDS – Ash Basin: 700 mg/L (ABMW-4D); Outside Basin: 810 mg/L (MW-19BR);
PBTV range: 85 mg/L – 430 mg/L.
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Vanadium – Ash Basin: 314 µg/L (ABMW-1): Outside Basin: 5.9 µg/L (MW-12S)’
PBTV range: 0.974 µg/L – 5.88 µg/L.
14.3 Contaminant Migration and Potentially Affected Receptors
Contaminant Migration
The groundwater flow system at the Site serves both to store and provide a means for
groundwater movement. The porosity of the regolith is largely controlled by pore
space (primary porosity), whereas in bedrock porosity is largely controlled by the
number, size and interconnection of fractures. As a result, the effective porosity in the
regolith is normally greater than in the bedrock and thus the quantity of groundwater
flow will be greater in the regolith. At the Mayo Plant, saturated regolith was observed
in only a few wells, and the regolith is the least transmissive of the flow zones. The
majority of groundwater across the Site appears to flow through the transition zone and
bedrock.
The pore water in the ash basin is the source of constituents detected above PBTVs or 2L
in groundwater samples in the vicinity of the ash basin. Pore water analytical results are
compared to 2L and/or IMAC for reference purposes only. The ash basin is a permitted
wastewater system; therefore, comparison of pore water within the wastewater
treatment residuals (ash) to 2B or 2L/IMAC is not required. Gradients measured within
the ash basin support the interpretation that ash pore water mixes with
shallow/surficial groundwater and migrates downward into the transition zone.
Continued vertical migration of groundwater downgradient of the ash basin is also
evidenced by detected constituent concentrations. Ash basin constituents become
dissolved in groundwater that flows in response to hydraulic gradients. Groundwater
migrates under diffuse flow conditions in the surficial aquifer in the direction of the
prevailing gradient. As constituents enter the transition zone and fractured bedrock
flow systems, the rate of constituent transport has the potential to increase.
Groundwater flow is the primary mechanism for migration of constituents to the
environment.
At Mayo, groundwater movement in the bedrock flow zone is due primarily to
secondary porosity represented by fractures in the bedrock. Primary (matrix) porosity
is negligible; therefore, it is not technically appropriate to calculate groundwater
velocity using effective porosity values. Bedrock fractures encountered at Mayo tend to
be isolated with low interconnectivity. Further, hydraulic conductivity values measure
the fractures immediately adjacent to a well screen, not across the distance between two
bedrock wells. Groundwater flow in bedrock fractures is anisotropic and difficult to
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predict, and velocities change as groundwater moves between factures of varying
orientations, gradients, pressure, and size.
The hydrogeologic characteristics of the ash basin environment are the primary control
mechanisms on groundwater flow and constituent transport. The basin acts as a bowl-
like feature toward which groundwater flows from all directions except from the north.
The stream valley in which the ash basin was constructed is a distinct slope-aquifer
system in which flow of groundwater into the ash basin and out of the ash basin is
restricted to the local flow regime. Groundwater and surface water flow from the ash
basin is funneled into the small valley formed by Crutchfield Branch. Boron, strontium,
and manganese are present in groundwater downgradient of the ash basin on the Site
near or at the compliance boundary in concentrations that exceed the 2L or PBTV.
Figures 14-40 to 14-54 graphically depict the most recent available (March-April 2017)
valid COI groundwater analytical concentrations for monitoring wells. Figures 14-55 to
14-69 show the most recent COI surface water and AOW concentrations. The figures
are colored-coded to visually depict whether analytical concentrations seem to be
increasing, decreasing, stable, or a trend could not be determined.
Recent concentrations of COIs in groundwater, surface water, and AOWs are provided
on Figures 14-70 and 14-71. Recent concentrations of COIs in solid media, as well as
available geochemical properties of soils, are provided on Figure 14-72.
Potentially Affected Receptors
A baseline human health and ecological risk assessment was performed in 2016 as a
component of the CAP, Part 2 (SynTerra, 2016a), concluding that no unacceptable risks
to humans resulted from hypothetical exposure to constituents detected in the ash basin
area. Based on review and analysis of groundwater and surface water data collected
since completing the human health and ecological risk assessment in 2016, there is no
evidence of potential risks to humans and wildlife at the Mayo Site.
Water Supply Wells
Results from private water supply wells did not indicate human health risks to off-site
residents potentially exposed to groundwater associated with the ash basin. In
addition, no public or private drinking water wells, supply wells, or wellhead
protection areas were found to be located downgradient of the ash basin.
Samples were collected from 11 private water supply wells located upgradient of the
Mayo Plant ash basin to the northwest along US Highway 501, northwest in Virginia
along US Highway 501, and south of the Plant around Mullins Lane. NCDEQ
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coordinated sampling of three private water supply wells in 2015. Duke Energy
collected samples from eight additional private water supply wells in 2017. A review of
the analytical data for the private water supply wells indicated several constituents
were detected above 2L or IMACs, including pH (two wells), iron (three wells), lead
(one well), manganese (three wells), and vanadium (eight wells). In addition,
concentrations of other COIs exceeded the respective PBTVs for a number of these
private water supply wells. However, these data (2L, IMAC and PBTV exceedances)
should be interpreted with caution for the reasons described below:
PBTVs were developed using groundwater data from a set of three background
wells all located on the Site. These wells are located within about 0.1 square miles
of one another. The geochemical data from these wells may not be representative
across the broader area encompassed by the 11 private water supply wells
(spread across approximately 1.5 square miles).
There is very limited information available about the sampled wells. Well
construction equipment such as pipes, pumps, and fittings may influence water
quality.
A numerical capture zone analysis for the Mayo Site was conducted to evaluate
potential impact of upgradient water supply pumping wells. The analysis
indicated that 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. None of the particle tracks originating in the ash
basin moved into the well capture zones. Further, groundwater flow direction
relative to the ash basin has been demonstrated to be to the north-northeast. The
private wells around the Mayo Plant are all located upgradient and away from
the direction of groundwater flow.
The geochemical signature of groundwater from the supply wells was compared
with the signature of groundwater from the source area using Piper diagrams.
The geochemical nature of groundwater from the sampled supply wells is very
different from ash pore water and from groundwater beneath the basin.
It is concluded that there is no impact to the supply wells that are located upgradient
from the ash basin/Mayo Plant. The land directly downgradient of the ash basin and the
Duke property line is undeveloped. Therefore, no water supply wells are located north
and downgradient of the groundwater plume within the survey radius.
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No surface water intakes, other than the intake used to pump cooling tower make-up
water from Mayo Lake for the Mayo Plant operations, are located in the vicinity of
Mayo Plant either in another part of Mayo Lake or in Crutchfield Branch.
Surface Water
Three surface water samples were collected and analyzed, including one downstream of
the ash basin and two reference, upstream locations. Surface water sample data
indicates that only turbidity and DO concentrations exceeded a 2B criteria. Boron was
detected in the downstream location (SW-CB2) but not in the reference locations.
Strontium was detected in both SW-CB2 and in reference location SW-REF1; however,
the detected concentrations in SW-CB2 were generally two to three times higher than in
SW-REF1. Other constituents are higher in SW-CB2 surface water than in the reference
locations (e.g., TDS, cobalt, iron, manganese); although, not several times higher.
The majority of the flow in the former Crutchfield Branch stream valley is associated
with engineered drainage from the ash basin immediately below the dam and other
natural seepage of ash basin water which is evaluated and governed wholly separate in
accordance with the NPDES Program administered by NCDEQ DWR. The groundwater
in the area in deeper flow zones near the receiving stream generally contains
constituent concentrations less than those of the receiving waters.
Additional surface water sampling will be completed and an evaluation of potential
impacts of groundwater on surface water will be presented in the CAP.
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15.0 CONCLUSIONS AND RECOMMENDATIONS
A discussion of preliminary corrective action alternatives that may be appropriate to
consider during the updated CAP development are presented in this section.
15.1 Overview of Site Conditions at Specific Source Areas
CCR material and pore water in the ash basin were determined to be a source of impact
to groundwater. Boron is the primary constituent detected in groundwater at
concentrations greater than background and the 2L near or beyond the compliance
boundary; however, manganese and strontium also appear to be indicators of impact to
groundwater. The interpreted extent of boron concentrations greater than the 2L
standard is near the compliance boundary in the surficial and transition flow zones. The
boron concentration is less than 2L standard in the bedrock flow unit near the
compliance boundary. The interpreted extent of manganese concentrations greater than
the PBTV and 2L standard is beyond the compliance boundary in the surficial and
bedrock flow zones; however, the manganese concentration is less than the PBTV
within the transition zone at the compliance boundary. The interpreted extent of
strontium concentrations greater than the PBTV extends beyond the compliance
boundary only within the surficial flow zone.

15.2 Revised Site Conceptual Model
Site Conceptual Models (SCMs) are developed to be a representation of what is known
or suspected about a site with respect to contamination sources, release mechanisms,
transport, and fate of those contaminants. SCMs can be a written and/or be a graphic
presentation of site conditions to reflect the current understanding of the site, identify
data gaps, and be updated as new information is collected. SCMs can be used to
develop an understanding of the different aspects of site conditions, such as a
hydrogeologic conceptual site model to help understand the site hydrogeologic
conditions affecting groundwater. SCMs can also be used in a risk assessment to
understand contaminant migration and pathways to receptors.
In the initial Site conceptual hydrogeologic model presented in the GAP (SynTerra,
2014c), the geological and hydrogeological features influencing the movement,
chemical, and physical characteristics of contaminants were related to the Piedmont
hydrogeologic system present at the Site. A preliminary SCM was developed from data
generated during previous assessments, existing groundwater monitoring data, and
CSA activities.
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The Mayo Plant has a single ash basin which contains ash generated from the Plant’s
historic coal combustion. The ash basin is approximately 140 acres in size and is
constructed with an earthen dike. CCR was managed at the Plant’s on-site ash basin
and transported via wet sluicing until 2013. The ash basin was constructed with two
engineered toe drains located at the base of the dam. In addition, ash basin water is
diverted into a holding lagoon or forebay for water quality treatment and eventually
discharged through NPDES Outfall 002 into Mayo Lake. Borings installed in the ash
basin encountered ash from 13.5 feet to 66.1 feet in thickness. Roughly 40 percent of the
ash basin is covered with standing water. Assessment findings determined that CCR
accumulated in the ash basin is the primary source of impact to groundwater. As
previously discussed, residual concentrations of some COIs in soil beneath the ash
basin may indicate limited impact to soil beneath the ash basin.
The Mayo Plant ash basin occupies the former stream valley of Crutchfield Branch. The
basin acts as an elongated bowl-like feature toward which groundwater flows from the
northwest, west, south, and east. A small topographic high is present along the eastern
side of the ash basin, and groundwater is somewhat radial away from this feature.
Groundwater flow east of the railroad line, which is constructed along a natural ridge,
is to the east and toward Mayo Lake. Groundwater flows north-northeast from the ash
basin into the small valley formed by Crutchfield Branch. Crutchfield Branch flows
north off of the Site into Virginia.
Site-specific groundwater COIs were developed by evaluating groundwater sampling
results with respect to 2L/IMAC and PBTVs, and additional regulatory
input/requirements. The distribution of constituents in relation to the ash basin, co-
occurrence with CCR indicator constituents such as boron, and likely migration
directions based on groundwater flow direction were considered in determination of
groundwater COIs.
Wells monitoring the surficial, transition zone, and bedrock flow units were installed
beneath the ash basin. Wells completed in the saprolite or transition zone beneath the
ash basin have PBTV and 2L exceedances for arsenic, barium, boron, cobalt, iron,
manganese, molybdenum, strontium, TDS and vanadium (a number of which only
occur in the transition zone). Bedrock monitoring wells installed within the ash basin
indicate only strontium detected greater than the background concentration.
Boron is a key indicator of CCR groundwater impacts. Manganese and strontium also
indicate impact at Mayo. Boron is detected at concentrations greater than the 2L
beneath and downgradient of the ash basin. The area downgradient at which boron,
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manganese, and strontium are detected at a concentration greater than PBTVs is
interpreted as the leading edge of the CCR-derived plume moving downgradient from
the source area. For the surficial flow zone, boron, manganese, and strontium were
detected in monitoring wells screened in Crutchfield Branch alluvium downgradient of
the ash basin and north of the Plant property boundary. The leading edge of the
transition zone and bedrock plume is interpreted to be at/near the northern Plant
property line. The surficial and transition zone flow units at Mayo, though impacted,
are not vertically extensive. Impact to the bedrock flow unit is present in the upper 50 to
75 feet of fractured bedrock.
Surface water data reflect that the majority of the flow in the former Crutchfield Branch
stream valley is associated with engineered drainage from the ash basin. Boron
concentrations are greatest proximate to the engineered toe drains and decrease toward
the state line. Manganese concentrations are similarly consistent immediately
downstream of the ash basin and begin to decline downstream; however, at the most
downstream surface water location, manganese concentrations increase.

The SCM will continue to be refined following evaluation of the completed
groundwater models to be presented in the CAP and additional information obtained in
subsequent data collection activities.
15.3 Interim Monitoring Program
An Effectiveness Monitoring Program (EMP) is required by CAMA §130A-309.209
(b)(1)e. The EMP for the Mayo Plant is anticipated to begin once the basin closure and
groundwater CAP have been implemented. In the interim, an IMP has been developed
at the direction of NCDEQ. The CAP, and a proposed EMP, will be submitted at a
future date.
IMP Implementation 15.3.1
An IMP has been implemented in accordance with NCDEQ correspondence
(NCDEQ, October 19, 2017; Appendix A) that provided an approved “Revised
Interim Monitoring Plans.” Sampling will be conducted quarterly until approval
of the CAP or as otherwise directed by NCDEQ. Groundwater samples will be
collected using low-flow sampling techniques in accordance with the Low Flow
Sampling Plan, Duke Energy Facilities, Ash Basin Groundwater Assessment Program,
North Carolina, June 10, 2015 (Appendix G) conditionally approved by NCDEQ in
a June 11, 2015 email with an attachment summarizing its approval conditions.
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Samples will be analyzed by a North Carolina certified laboratory for the
parameters listed in Table 15-1. The table includes targeted minimum detection
limits for each listed constituent. Analytical parameters and detection limits for
each medium were selected so the results could be used to evaluate the
effectiveness of a future remedy, conditions within the aquifer that may
influence the effectiveness of the remedy, and migration of constituents related
to the ash basin. Laboratory detection limits for each constituent are targeted to
be at or below applicable regulatory values (i.e., 2L, IMAC, or 2B).
Monitoring wells and surface water locations that will be sampled and
monitored as part of the IMP, as approved in NCDEQ correspondence (NCDEQ,
October 19, 2017; Appendix A), are included in Table 15-2.
IMP Reporting 15.3.2
Currently, data summary reports comprised of analytical results received during
the previous month are submitted to NCDEQ on a monthly basis. In addition,
NCDEQ directed that an annual IMP report be submitted by April 30 of the
following year of data collection. The reports shall include materials that
provide “an integrated, comprehensive interpretation of site conditions and
plume status.” The initial report was to be submitted to NCDEQ no later than
April 30, 2018; however, the October 19, 2017 correspondence provides that the
required date for an annual monitoring report will be extended to a date in 2018
to be determined later.
15.4 Preliminary Evaluation of Corrective Action Alternatives
Closure of the ash basin is required by 2024 under CAMA (Intermediate Risk). The
updated risk assessment (Section 12.0) has determined there is no imminent risk to
human health or the environment due to groundwater, surface water, or sediment
impacts. In three ash basin locations where soil samples could be collected, analytical
results indicate a few, limited detections of COIs above PBTVs. If needed, groundwater
and surface water can be remediated over time using a variety of approaches and
technologies. Groundwater modeling has indicated closure by excavation compared to
a cap-in-place closure does not substantively accelerate groundwater clean-up. For
basin closure, bulk dewatering and reduction of infiltrating water will have the greatest
positive impact on groundwater and surface water quality downgradient of the ash
basin. Closure design can augment an overall groundwater corrective action scenario
including cap-in-place or active groundwater remediation which will be evaluated in
the CAP. Therefore, a “low” groundwater risk classification is recommended.
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This preliminary evaluation of corrective action alternatives is included to provide
insight into the updated CAP preparation process. The preliminary evaluation is based
on data available and the current understanding of regulatory requirements for the Site.
It is assumed a source control measure of either implementation of an engineering
capping system (cap-in-place) to minimize infiltration, or excavation of the ash within
the ash basin, or a combination of the two, will be designed following completion of the
risk classification process. Groundwater currently presents minimal , if any, risk to
receptors. A “Low” risk classification and closure via a cap-in-place scenario are
considered viable. Potential groundwater remedial strategies are being considered as
part of the closure design.
CAP Preparation Process 15.4.1
The CAP preparation process is designed to identify, describe, evaluate, and
select remediation alternatives with the objective of bringing groundwater
quality to levels that meet applicable standards, to the extent that the objective is
economically and technologically feasible, in accordance with 2L .0106
Corrective Action. Sections (h), (i), and (j) regarding CAP preparation read as
follows:
(h) Corrective action plans for restoration of groundwater quality, submitted pursuant
to Paragraphs (c), (d), and (e) of this Rule shall include:
(1) A description of the proposed corrective action and reasons for its selection;
(2) Specific plans, including engineering details where applicable, for restoring
groundwater quality;
(3) A schedule for the implementation and operation of the proposed plan; and
(4) A monitoring plan for evaluating the effectiveness of the proposed corrective
action and the movement of the contaminant plume.
(i) In the evaluation of corrective action plans, the Secretary shall consider the extent
of any violations, the extent of any threat to human health or safety, the extent of
damage or potential adverse impact to the environment, technology available to
accomplish restoration, the potential for degradation of the contaminants in the
environment, the time and costs estimated to achieve groundwater quality
restoration, and the public and economic benefits to be derived from groundwater
quality restoration.
(j) A corrective action plan prepared pursuant to Paragraphs (c), (d), or (e) of this
Rule shall be implemented using a remedial technology demonstrated to provide the
most effective means, taking into consideration geological and hydrogeological
conditions at the contaminated site, for restoration of groundwater quality to the
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level of the standards. Corrective action plans prepared pursuant to Paragraphs (c)
or (e) of this Rule may request an exception as provided in Paragraphs (k), (l), (m),
(r), and (s) of this Rule.
To meet these requirements and to provide a comprehensive evaluation, it is
anticipated that the CAP will include:
Corrective action objectives and evaluation criteria
Technology assessment
Formulation of remedial action alternatives
Analysis, modeling, selection, and description of selected remedial action
alternative(s)
Conceptual design elements, including identification of pre-design testing
such as pilot studies
Monitoring requirements and performance metrics
Implementation schedule
The following Site conditions significantly limit the effectiveness of a number of
possible technologies.
The COIs in groundwater flow primarily through the transition zone and
upper fractured bedrock.
The formations are very heterogeneous with anisotropic flow conditions.
The preliminary screening of potential groundwater corrective action included:
Source control by capping in place or excavation, and monitored natural
attenuation, will be vital components to the CAP.
Groundwater migration barriers. The lateral extent potentially required,
along with the depth and heterogenitity of the transition zone and
bedrock, may limit the feasibility of this technology.
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In situ chemical immobilization. This technology has not been
demonstrated to be effective for the primary COI, boron. It may be
applicable for other COIs.
Permeable reactive barrier. Similar to in situ chemical immobilization,
permeable reactive barrier technology has not been demonstrated to be
effective for boron.
Groundwater extraction. Groundwater extraction appears to be most
likely and could potentially be a viable choice as a key element of
groundwater corrective action in combination with source control and
MNA. However, further analysis is required and will be addressed in the
updated CAP.
Potentially viable options will be further evaluated in the CAP with updated fate
and transport and geochemical modeling.
Summary 15.4.2
This preliminary evaluation of corrective action alternatives is intended to
provide insight into the revised CAP preparation process, as outlined in 2L. It is
based on data available and the current regulatory requirements for the Site. It
addresses potentially applicable technologies and remedial alternatives.
Potential approaches are based on the currently available information about Site
hydrogeology and COIs. In general, three hydrogeologic units or zones of
groundwater flow can be described for the Site: shallow/surficial zone, transition
zone, and bedrock flow zone. The Site COIs include a list of common coal ash
related constituents such as boron and manganese.
If required, the potentially applicable technologies to supplement source control
and MNA include groundwater extraction technologies such as conventional
vertical wells, angle-drilled and horizontal wells. All of these extraction
technologies could be augmented with fracturing of the bedrock formation.
Migration barriers, in situ chemical immobilization, and permeable reactive
barriers are also identified as potentially applicable remedial action alternatives.
In the event that extracted groundwater may require treatment prior to
discharge, several water treatment technologies for the relevant COIs would be
evaluated, including pH adjustment, metals precipitation, ion exchange,
permeable membranes, and adsorption technologies.
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The CAP will further evaluate basin closure options to reduce the potential impacts to
human health or the environment; short- and long-term effectiveness, implementability,
and potential for attenuation of contaminants; time and cost to achieve restoration;
public and economic benefits; and compliance with applicable laws and regulations.

The CAP evaluation process will be used to determine which approach, or combination
of approaches, will be most effective. Modeling will also be used to evaluate the
various options prior to selection.

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16.0 REFERENCES
ASTM. (2014). E1689-95: Standard Guide for Developing Conceptual Site Models for
Contaminated Sites.
ATSDR. (2012). Toxicological profile for Manganese. Atlanta: U.S. Department of Health
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