NC0038377_Mayo_Appendix G_20191231

Corrective Action Plan Update December 2019
Mayo Steam Electric Plant
APPENDIX G
SynTerra
UPDATED GROUNDWATER FLOW AND TRANSPORT
MODELING REPORT

UPDATED GROUNDWATER FLOW AND TRANSPORT
MODELING REPORT
FOR
MAYO STEAM ELECTRIC PLANT
ROXBORO, NORTH CAROLINA
DECEMBER 2019
PREPARED FOR
/DUKE
ENERGY
PROGRESS
DUKE ENERGY PROGRESS., LLC
INVESTIGATORS
LAWRENCE C. MURDOCH, PH. D. - FRx, INC.
BONG YU, PH. D. - SYNTERRA CORPORATION
REGINA GRAZIANO, M.S. - SYNTERRA CORPORATION
RONALD W. FALTA, PH.D. - FALTA ENVIRONMENTAL LLC

Updated Groundwater Flow And Transport Modeling Report December 2019
Mayo Steam Electric Plant, Roxboro, North Carolina
EXECUTIVE SUMMARY
This groundwater flow and transport model report provides basic model development
information and simulations of basin closure designs for the Mayo Steam Electric Plant
(Mayo, Site, Plant). The Site is owned and operated by Duke Energy Progress, LLC
(Duke Energy) and is located near Roxboro, in Person County, North Carolina.
Operations at the Site began in 1983 with operation of a single coal-fired unit.
Wastewater and coal combustion residuals (CCRs) have been managed in an ash basin
located northwest of the power plant. Inorganic compounds in the wastewater and ash
have dissolved and migrated in groundwater downgradient of the ash basin within the
compliance boundary.
Numerical simulations of groundwater flow and transport have been calibrated to pre -
decanting conditions and used to evaluate different ash basin closure scenarios being
considered. The predictive simulations presented herein are not intended to represent a
final detailed closure design. These simulations use conceptual designs that are subject
to change as the closure plans are finalized. The simulations are intended to show the
key characteristics of groundwater flow and mobile constituent transport that are
expected to result from the closure actions. It should be noted that, for groundwater
modeling purposes, a reasonable assumption was made about initiation dates for each
of the closure scenarios. The assumed dates were based on information that is currently
evolving and might vary from dates provided in contemporary documents. The
potential variance in closure dates presented in the groundwater model is
inconsequential to the results of the model. This modeling report is intended to provide
basic model development information and simulations of conceptual basin closure
designs. The groundwater is in compliance with North Carolina Administrative Code,
Title 15A, Subchapter 02L, Groundwater Classification and Standards (02L), and
therefore, groundwater corrective action simulations are not required.
The model simulations were developed using flow and transport models MODFLOW
and MT3DMS. Boron was the constituent of interest (COI) selected to estimate the time
to achieve compliance because it is mobile in groundwater and tends to have the largest
extent of migration. The less mobile, more geochemically controlled constituents (i.e.
arsenic, selenium, chromium) would follow the same flow path as boron, but to a lesser
extent. The less mobile geochemically controlled constituents do not have discernable
plumes and are modeled separately using a geochemical model.
This report describes refinements that have improved the accuracy and resolution of
details in the model of the Mayo site since previous versions (SynTerra, 2015b, 2016,
Page ES-1

Updated Groundwater Flow And Transport Modeling Report December 2019
Mayo Steam Electric Plant, Roxboro, North Carolina
2018). The model includes recent revisions to the designs of the closure scenarios
developed by AECOM. The model includes data from new deep wells located
downgradient of the dam, and data from new wells near the coal pile, the FGD Settling
Basin (Waste Water Treatment Basin), and elsewhere. The grid has been refined to
include new data that describes the depths of contacts; this refinement improves
resolution of the hydrostratigraphy. A comprehensive dataset (through 2nd quarter of
2019) of hydraulic heads and boron concentrations was used to recalibrate the model.
Results of the simulations indicate that boron concentrations in groundwater greater
than the 02L standard remain within the compliance boundary. The maximum extent
of the simulated boron plume occurs in year 2020 when it is predicted to be
approximately 130 feet inside of the compliance boundary. Dropping the hydraulic
head in the ash basin in year 2020 by decanting and subsequent closure will reduce the
subsurface flow from the basin, which causes the extent of boron in groundwater to
recede inward from the compliance boundary.
The simulations include an evaluation of two closure scenarios, closure by excavation
and closure in place. The results of the simulations are summarized by the distributions
of the maximum boron concentration predicted for approximate years 2050 and 2200
(Figure ES-1), and by the time series of boron concentrations at two representative
locations downgradient of the ash basin (Figure ES-1 and Figure ES-2). The year 2050
simulation predicts results several decades after closure, whereas the year 2200 shows a
long-term prediction.
Results of the simulations show that the extent where the maximum boron
concentration in groundwater is greater than the 02L standard remains contained
within the compliance boundary for both closure scenarios in approximate year 2050
and both closure scenarios in approximate year 2200 (Figure ES-1). The simulations
indicate that boron concentrations increase to a maximum of approximately 130 µg/L at
the compliance boundary (Location 2 in Figure ES-2). Boron concentrations in
groundwater reach a maximum concentration of approximately 1,000 µg/L
downgradient of the dam and within the compliance boundary at year 2020.
Concentrations decrease sharply in the downgradient direction and are less than the
02L standard at Location 1 (Figure ES-1).
Data from recent ash basin pumping tests and new deep bedrock wells near the ash
basin dam were included in this revision of the model. The pumping test data indicate
that the hydraulic conductivity of the ash decreases with depth in the vicinity of the
Page ES-2

Updated Groundwater Flow And Transport Modeling Report December 2019
Mayo Steam Electric Plant, Roxboro, North Carolina
pumping test wells. It was assumed that this general distribution is representative of
the hydraulic conductivity of the ash throughout the basin.
Four pairs of deep bedrock wells were installed downgradient of the dam during 2019.
The depths of the shallow well in each pair ranged from 125 feet to 240 feet, whereas the
depths of the deep well in each pair ranged from 250 feet to 351 feet. Boron
concentrations in the deep bedrock wells were less than the laboratory reporting limit.
Data from the new wells indicate no deep boron migration in groundwater in this
region at the Mayo site where it would most likely occur. This confirms results from
the model simulations.
The model simulations indicate that there are no exposure pathways associated with the
groundwater flow below or downgradient of the ash basin. Water supply wells are
outside, or upgradient of, the groundwater flow system that contains the ash basin.
Groundwater migration of constituents from the ash basin does not affect water supply
wells under pre -closure conditions, nor in the future under the different closure
scenarios simulated.
Page ES-3

APPROXIMATELY 20 YEARS
POST -CLOSURE -BY -EXCAVATION
Location 1
Location 2
r
,
J
�
APPROXIMATELY 170 YEARS
POST -CLOSURE -BY -EXCAVATION
Location 1
." Location 2
LEGEND
BORON 700 - 4,000 pg/L
BORON > 4,000 Ng/L
PROPOSED LANDFILL
ASH BASIN WASTE BOUNDARY
— - — - ASH BASIN COMPLIANCE BOUNDARY
NOTES:
ALL BOUNDARIES ARE APPROXIMATE
CLOSURE -IN -PLACE SCENARIO IS ESTIMATED TO BE COMPLETED BY YEAR
2026. CLOSURE -BY -EXCAVATION SCENARIO IS ESTIMATED TO BE
COMPLETED BY YEAR 2031.
AERIAL PHOTOGRAPHY OBTAINED FROM ESRI ONLINE ON JUNE 10, 2019.
AERIAL WAS COLLECTED ON FEBRUARY 6, 2017.
DRAWING HAS BEEN SET WITH A PROJECTION OF NORTH CAROLINA STATE
PLANE COORDINATE SYSTEM FIPS 3200 (NAD83).
APPROXIMATELY 20 YEARS
POST -CLOSURE -IN -PLACE
Location 1
�•� • ♦. ♦ Location 2
I
%40 ♦
•Ilk
I ■
■. 1
' I
1
• r .
APPROXIMATELY 170 YEARS
POST -CLOSURE -IN -PLACE
Location 1
�. - Location 2
%' DUKE GRAPHIC SCALE
990 0 990 1,980
ENERGYI (IN FEET)
DRAWN BY: R.YU DATE: 10/25/2019
REVISED BY: R. KIEKHAEFER DATE: 12/18/2019
CHECKED BY: P. ALTMAN DATE: 12/18/2019
APPROVED BY: J. WYLIE DATE: 12/18/2019
synTerm PROJECT MANAGER: J. WYLIE
www cvntarrarnrn rnm
FIGURE ES-1
COMPARISON OF SIMULATED MAXIMUM
BORON CONCENTRATIONS IN ALL
NON -ASH LAYERS AFTER CLOSURE
UPDATED GROUNDWATER FLOW AND TRANSPORT
MODELING REPORT
MAYO STEAM ELECTRIC PLANT
ROXBORO, NORTH CAROLINA

Location 1
1,200
J
�
1,000
O
r- 800
c�
L
v 600
O
V
400
0
L
0
m
200
0
800
700
600
500
400
300
200
100
0
O
r�
Ol
r-I
O O O O
O
N N N N
Year
Location 2
O O O O
N r- N r-
O O r-I -1
N N N N
Year
Reference location 1 is located downgradient of the dam.
Reference location 2 is located at the compliance boundary.
DUKE
DRAWN BY: R. YU DATE: 10/25/2019
FIGURE ES-2
SUMMARY OF SIMULATED MAXIMUM BORON
41*5
ENERGY
REVISED BY: W. PRATER DATE: 12/12/2019
CONCENTRATIONS IN ALL NON -ASH LAYERS AS
PROGRESS
CHECKED BY: P. ALTMAN DATE: 12/12/2019
FUNCTIONS OF TIME FOR THE TWO CLOSURE
APPROVED BY: J. WYLIE DATE: 12/12/2019
PROJECT MANAGER: J. WYLIE
SCENARIOS
UPDATED GROUNDWATER FLOW AND
TRANSPORT MODELING REPORT
MAYO STEAM ELECTRICAL PLANTROXBORO,
synTerra
www.synterracorp.com
NORTH CAROLINA

Updated Groundwater Flow And Transport Modeling Report December 2019
Mayo Steam Electric Plant, Roxboro, North Carolina
TABLE OF CONTENTS
SECTION
PAGE
EXECUTIVE SUMMARY.................................................................................................... ES-1
1.0 Introduction..................................................................................................................1-1
1.1 General Setting and Background........................................................................1-1
1.2 Objectives................................................................................................................1-3
2.0 Conceptual Model........................................................................................................2-1
2.1
Aquifer System Framework.................................................................................2-1
2.2
Groundwater Flow System..................................................................................
2-2
2.3
Hydrologic Boundaries.........................................................................................2-3
2.4
Hydraulic Boundaries...........................................................................................2-4
2.5
Sources and Sinks..................................................................................................
2-4
2.6
Water Budget.........................................................................................................2-5
2.7
Modeled Constituents of Interest........................................................................2-5
2.8
Constituent Transport...........................................................................................2-5
3.0 Computer Model..........................................................................................................3-1
3.1
Model Selection......................................................................................................3-1
3.2
Model Description.................................................................................................3-1
4.0 Groundwater Flow And Transport Model Construction
..................................... 4-1
4.1
Model Domain and Grid......................................................................................
4-1
4.2
Hydraulic Parameters...........................................................................................
4-3
4.3
Flow Model Boundary Conditions.....................................................................4-4
4.4
Flow Model Sources and Sinks............................................................................4-4
4.5
Flow Model Calibration Targets.........................................................................4-6
4.6
Transport Model Parameters...............................................................................
4-6
4.7
Transport Model Boundary Conditions.............................................................4-9
4.8
Transport Model Sources and Sinks...................................................................
4-9
4.9
Transport Model Calibration Targets...............................................................
4-10
5.0 Model Calibration To Pre -Decanting Conditions .................................................
5-1
5.1
Flow Model Calibration........................................................................................5-1
5.2
Flow Model Sensitivity Analysis.........................................................................5-5
5.3
Historical Transport Model Calibration.............................................................
5-5
5.4
Transport Model Sensitivity Analysis................................................................
5-6
Page i

Updated Groundwater Flow And Transport Modeling Report December 2019
Mayo Steam Electric Plant, Roxboro, North Carolina
6.0 Predictive Simulations Of Closure Scenarios........................................................ 6-1
6.1 Interim Models with Ash Basin Ponded Water Decanted (Approximate Year
2021-2026 or 2021-2031)........................................................................................................6-2
6.2 Closure -by -Excavation Scenario.......................................................................... 6-3
6.3 Conclusions............................................................................................................ 6-7
7.0 References......................................................................................................................7-1
Page ii

Updated Groundwater Flow And Transport Modeling Report December 2019
Mayo Steam Electric Plant, Roxboro, North Carolina
LIST OF FIGURES
Figure ES-1 Comparison of simulated maximum boron concentrations in all non -ash
layers after closure
Figure ES-2 Summary of simulated maximum boron concentrations in all non -ash
layers as functions of time for the two closure scenarios
Figure 1-1 USGS location map
Figure 4-1 Numerical model domain
Figure 4-2 Fence diagram of the 3D hydrostratigraphic model used to construct the
model grid
Figure 4-3 Model computational grid
Figure 4-4 Hydraulic conductivity estimated from slug tests performed in coal ash at
14 sites in North Carolina
Figure 4-5 Hydraulic conductivity estimated from slug tests performed in saprolite at
10 Piedmont sites in North Carolina
Figure 4-6 Hydraulic conductivity estimated from slug tests performed in the
transition zone at 10 Piedmont sites in North Carolina
Figure 4-7 Hydraulic conductivity estimated from slug tests performed in the
bedrock at 10 Piedmont sites in North Carolina
Figure 4-8 Distribution of model recharge zones
Figure 4-9 Model surface water features
Figure 4-10 Water supply wells in model area
Figure 5-1a Model hydraulic conductivity zones in ash layer 1
Figure 5-1b Model hydraulic conductivity zones in ash layer 2
Figure 5-1c Model hydraulic conductivity zones in ash layer 3
Figure 5-1d Model hydraulic conductivity zones in ash layer 4
Figure 5-1e Model hydraulic conductivity zones in ash layer 5
Figure 5-1f Model hydraulic conductivity zones in ash layer 6
Figure 5-1g Model hydraulic conductivity zones in ash layer 7
Figure 5-1h Model hydraulic conductivity zones in ash layer 8
Figure 5-1i Model hydraulic conductivity zones in saprolite layer 9
Figure 5-1j Model hydraulic conductivity zones in saprolite layer 10
Figure 5-1k Model hydraulic conductivity zones in saprolite layer 11
Figure 5-11 Model hydraulic conductivity zones in transition zone layer 12
Figure 5-1m Model hydraulic conductivity zones in transition zone layer 13
Figure 5-1n Model hydraulic conductivity zones in upper fractured rock layer 14
Figure 5-10 Model hydraulic conductivity zones in upper fractured rock layer 15
Figure 5-1p Model hydraulic conductivity zones in upper fractured rock layer 16
Page iii

Updated Groundwater Flow And Transport Modeling Report December 2019
Mayo Steam Electric Plant, Roxboro, North Carolina
LIST OF FIGURES (CONTINUED)
Figure 5-1q Model hydraulic conductivity zones in lower fractured rock layers 17 and
18
Figure 5-1r Model hydraulic conductivity zones in bedrock layers 19 through 21
Figure 5-2a Ash basin dam cross -sections
Figure 5-2b Observed and simulated volumetric flowrates through and below the dam
Figure 5-3 Comparison of observed and computed from the calibrated steady state
flow model
Figure 5-4 Simulated pre -decanting hydraulic heads in the transition zone
Figure 5-5 Simulated local ash basin groundwater flow system in transition zone
prior to decanting
Figure 5-6 Boron source zones for historical transport model
Figure 5-7 Simulated maximum boron concentrations in all non -ash modeling layers
prior to decanting
Figure 6-1 Simulated hydraulic heads in the transition zone after decanting
Figure 6-2 Simulated local ash basin groundwater flow system in transition zone
after decanting
Figure 6-3 Simulated maximum boron concentrations in all non -ash layers 5.5 years
after decanting when closure -in -place is completed
Figure 6-4 Simulated maximum boron concentrations in all non -ash layers 10 years
after decanting when closure -by -excavation is completed
Figure 6-5 Closure -by -excavation design used in simulations (from AECOM, 2019a)
Figure 6-6 Simulated local ash basin groundwater flow system in transition zone
after closure -by -excavation
Figure 6-7 Simulated maximum boron concentrations in all non -ash layers after
closure -by -excavation
Figure 6-8 Reference locations for time series datasets
Figure 6-9 Summary of maximum boron concentration in all layers as functions of
time and stratigraphic layer for the closure -by -excavation scenario
Figure 6-10 Closure -in -place design used in simulations (from AECOM, 2019b)
Figure 6-11 Simulated local ash basin groundwater flow system in transition zone
after closure -in -place
Figure 6-12 Simulated maximum boron concentrations in all non -ash layers after
closure -in -place
Figure 6-13 Summary of maximum boron concentration in all layers as functions of
time and stratigraphic layer for the closure -in -place scenario
Figure 6-14 Comparison of simulated maximum boron concentrations in all non -ash
layers after closure
Page iv

Updated Groundwater Flow And Transport Modeling Report December 2019
Mayo Steam Electric Plant, Roxboro, North Carolina
LIST OF FIGURES (CONTINUED)
Figure 6-15 Summary of maximum boron concentration in all layers as functions of
time and stratigraphic layer for both closure scenarios
LIST OF TABLES
Table 5-1 Observed, computed, and residual heads for the calibrated flow model
Table 5-2 Calibrated hydraulic conductivity parameters
Table 5-3 Water balance on the ash basin groundwater flow system for pre -
decanting conditions
Table 5-4 Flow model sensitivity analysis
Table 5-5 Ash basin boron source concentrations (µg/L) used in historical transport
model
Table 5-6 Observed and computed boron (µg/L) concentrations in monitoring wells
Table 5-7 Transport model sensitivity to the boron Ka values
Table 6-1 Water balance on the ash basin groundwater flow system for decanted
conditions
Page v

Updated Groundwater Flow And Transport Modeling Report December 2019
Mayo Steam Electric Plant, Roxboro, North Carolina
1.0 INTRODUCTION
Duke Energy Progress, LLC. (Duke Energy) owns and operates the Mayo Steam Electric
Plant (Site, Plant) located near Roxboro, in Person County, North Carolina. Operations
at the Site began in 1983 with a single coal-fired unit. Coal combustion residuals (CCRs)
have been stored in an ash basin located northwest of the power plant, and more
recently in an industrial CCR landfill 1.5 miles to the west. Inorganic compounds in the
wastewater and ash have dissolved and migrated in groundwater downgradient of the
ash basin within the compliance boundary. Preliminary numerical simulations of
groundwater flow and transport have been calibrated to 2019 pre -decanting conditions
and have been used to evaluate different scenarios being considered for closure of the
ash basin. The methods and results of those simulations are described in this report.
1.1 General Setting and Background
The ash basin, approximately 2,300 feet long and 110 feet high, was constructed with an
earthen dam built across previous headwaters of Crutchfield Branch, a small creek that
flows to the northeast. The ash basin covers approximately 140 acres and contains
approximately 6,900,000 tons of CCR material (Duke Energy, October 31, 2014). The
northeastern half of the ash basin is open water, and much of the southwestern half of
the basin is exposed ash. Woodland surrounds most of the ash basin, with the
exception of a wastewater treatment facility on the southeastern side. A CCR landfill
1.5 miles west of the ash basin is currently used to store ash generated by the plant.
Mayo Reservoir is a 2,800-acre lake to the east of the Site (Figure 1-1). The power plant
area is at an elevation of approximately 520 feet, the ash basin ponded water elevation
was historically maintained at approximately 480 feet, and the elevation of Mayo
Reservoir is 432 feet.
Water that previously flowed into the ash basin is diverted into a Lined Retention Basin
(LRB) for water quality treatment via a newly updated (2018) treatment system.
Treated wastewater is discharged through National Pollutant Discharge Elimination
System (NPDES) Outfall 002 on the eastern side of the basin and into Mayo Reservoir.
Seepage from the ash basin dam occurs through two engineered drains located at the
base of the dam and covered under a Special Order by Consent issued by the North
Carolina Department of Environmental Quality (NCDEQ).
The Site is underlain by weathered saprolite derived from fractured metamorphic rocks
typical of the Piedmont Physiographic Province (Trapp and Horn, 1997). Saprolite
thickness ranges to as much as to 66 feet, but saprolite thickness is less than 25 feet in
most locations at this Site. Saprolite is saturated at lower elevations in valleys but is
Page 1-1

Updated Groundwater Flow And Transport Modeling Report December 2019
Mayo Steam Electric Plant, Roxboro, North Carolina
either unsaturated or partially saturated in the uplands. A transition zone
approximately 20 feet thick occurs between the saprolite and underlying rock. The
degree of weathering decreases with depth (SynTerra, 2015a). The water table in the
areas upland of the basin is typically in the fractured rock. Alluvium occurs along
Crutchfield Branch, downgradient of the ash basin.
The groundwater flow and transport model of the Mayo site has been under
development since 2015. The development process began with a steady-state
groundwater flow model and a transient model of constituent transport that were
calibrated to field observations resulting from site assessment activities in 2015. The
first set of simulations was completed in November 2015 (SynTerra, 2015b) and revised
in February 2016 (SynTerra, 2016). Additional wells drilled in 2016 were used to refine
the spatial resolution of data. Those data were used to recalibrate and refine the flow
and transport model in February 2017. The model was refined and re -calibrated in 2018.
Data from additional wells were included in the 2018 calibration. Averaged hydraulic
head values over the period of record for each well were used in the 2018 model
calibration. Earlier calibrations used hydraulic head data made at the time of
calibration, because the records were too short to develop meaningful average values.
Using average head values avoids potential bias caused by seasonal variations in head.
The 2018 calibration effort used constituent concentration data measured for samples
obtained in September 2017 and November 2017 (SynTerra, 2018).
The earlier models were refined to create a new calibrated model in 2019. The grids
have been revised significantly, including matching the ash bottom elevation to the
updated surface data provided by AECOM, and revising the contact between saprolite
and transition zone, and between the transition zone and bedrock, to boring logs from
existing wells and new deep bedrock wells. The 2019 flow model calibration used
historically averaged hydraulic heads based on water elevation data collected from
monitoring wells until the 2nd quarter of 2019. New hydraulic head and boron
concentration data collected from deep bedrock wells near the ash basin dam, and from
wells next to the new FGD Settling Basin (Waste Water Treatment Basin), the coal pile
area, and elsewhere, were incorporated for calibration. The hydraulic conductivity
setup was revised to reflect the results of groundwater pumping tests conducted in the
ash basin in September 2018. The 2019 boron calibration used boron concentration data
measured for samples obtained through the 2nd quarter of 2019 (SynTerra, 2019c).
Page 1-2

Updated Groundwater Flow And Transport Modeling Report December 2019
Mayo Steam Electric Plant, Roxboro, North Carolina
1.2 Objectives
The overall objectives of the groundwater flow and transport modeling are to predict
the performance of the two closure scenarios, and to guide decisions during the
selection of closure actions. The flow and transport models have been undergoing a
process of continuous improvement and refinement by including new field data. The
continuous improvement process is designed to increase the accuracy and reliability of
the performance predictions.
The objective of this report is to describe the results of the most recent refinement of the
flow and transport models. These models were developed in early and mid-2019 using
data through the 2nd quarter of 2019. Furthermore, a goal is to present results of
simulations of boron transport in all flow zones.
The predictive simulations shown in this report are not intended to represent a final
detailed closure design. These simulations use conceptual designs that are subject to
change as the closure plans are finalized. The simulations are intended to show the key
characteristics of groundwater flow and mobile constituent transport that are expected
to result from the closure actions.
The predictive simulations in this report are an evaluation of the closure scenario
designs and assumptions as of October 2019. Closure designs might have been refined
since then, due in part to results from the simulations. The predictive simulations in this
report characterize the most recent closure design available when the model was
developed, but the design process is ongoing and some aspects of the simulations might
differ from the most current closure design.
Page 1-3

Updated Groundwater Flow And Transport Modeling Report December 2019
Mayo Steam Electric Plant, Roxboro, North Carolina
2.0 CONCEPTUAL MODEL
The Mayo conceptual site model is based on Comprehensive Site Assessment reports
(CSA reports) pertaining to the Mayo Site (SynTerra, 2015a and 2017) and Corrective
Action Plan reports (CAP reports, SynTerra, 2015b, 2016, and 2019c). The reports
contain extensive detail and data related to the site conceptual model.
2.1 Aquifer System Framework
The aquifer system at the Mayo site is unconfined and includes three main
hydrostratigraphic units: a saprolite/transition zone, upper bedrock, and lower bedrock
(Legrand, 1988). The saprolite/transition zone consists of partially to thoroughly
weathered bedrock (SynTerra, 2015a). The saprolite/transition zone is underlain by
fractured metamorphic rock. The degree of fracturing is spatially variable and
generally decreases with depth. Vertical and horizontal fracture zones can cause
localized zones of high permeability within the rock (Legrand, 1988; Miller, 1990). The
permeability of the rock represented by the bedrock monitoring well screened intervals
is moderate, and it is inferred that the fracture density and hydraulic conductivity
decrease downward (Legrand, 1988). The monitoring wells were installed to intercept
water -bearing fractures sufficient for groundwater sample collection. The permeability
of the bedrock matrix is less than the permeability of the screened intervals that
represent water -bearing fractures.
The saprolite/transition zone is saturated in the vicinity of streams and lakes where
groundwater is discharging, but it is unsaturated in most upland areas. The water table
occurs in the fractured bedrock in most upland areas. The thickness of the saprolite has
been observed as much as 66 feet, but the saprolite thickness is generally less than 25
feet at this Site. Alluvium is observed at two locations north of the basin along the
Crutchfield Branch, and the maximum thickness at those two locations is 7.5 feet.
The pre -decanting water level in the ash basin was approximately 480 feet. The ash
surface is approximately 490 feet at the southwestern end of the basin. The elevation of
Crutchfield Branch below the dam is approximately 390 feet.
Hydraulic conductivity was determined in the field using slug tests and pumping tests.
Five slug tests were conducted in four wells completed in the ash. Hydraulic
conductivity based on slug tests spans two orders of magnitude, from 0.05 feet per day
(ft/d) to 4.1 ft/d, and the geometric mean is 0.37 ft/d. Two pumping tests were
conducted in ash. Hydraulic conductivity, based on the analytical solution for the
pumping tests, ranges from 0.05 ft/d to 1.45 ft/d; the numerical model indicates the
Page 2-1

Updated Groundwater Flow And Transport Modeling Report December 2019
Mayo Steam Electric Plant, Roxboro, North Carolina
horizontal hydraulic conductivity ranges from 0.05 ft/d to 8 ft/d and the vertical
hydraulic conductivity ranges from 0.01 ft/d to 1.6 ft/d (SynTerra, 2019b).
The hydraulic conductivity of the saprolite is 0.02 ft/d to 2.9 ft/d with a geometric mean
of 0.42 ft/d. The hydraulic conductivity of the transition zone spans a broad range from
0.02 ft/d to 102.3 ft/d with a geometric mean of 0.94 ft/d. These measurements reflect
the variability of the transition zone, where hydrologic properties occur due to spatial
variations in the degree of weathering. The hydraulic conductivity of the fractured
bedrock also spans a broad range, from 0.0005 ft/d to 5.7 ft/d, and with a geometric
mean of 0.19 ft/d. The greater values were likely measured at wells that intersected
larger fracture zones in the rock.
2.2 Groundwater Flow System
The groundwater system is recharged by rain and from water that infiltrates through
the ash basin. The average value of recharge was estimated from the map of recharge in
North Carolina by Haven (2003) and from analyzing stream hydrographs. A shapefile
of the recharge map by Haven (2003) was enlarged and features of the Site were
superimposed. Colors on the map were compared with colors on the legend because
quantitative data from the file were unavailable. This indicated that recharge was in the
range of 4 inches to 12 inches per year (10 centimeters to 30 centimeters per year) in the
watershed draining into the ash basin (Figure 1-1).
Recharge was also estimated by analyzing the hydrograph from nearby Hyco Creek.
The hydrograph from this stream was selected because it includes sufficient data for
analysis and it is from a setting similar to that of the Site. No hydrograph data were
available from Crutchfield Branch, which flows into Mayo Creek just north of the Site.
The flow in Hyco Creek was obtained from measurements made at the United States
Geological Survey (USGS) 02077200 gauging station near Leasburg, NC. The gauging
station, in Caswell County approximately 20 miles southwest of the Site, measures flow
from a watershed covering approximately 45.9 miles2 to the south. The analysis was
conducted using 11 years of data starting in January 2002. The hydrograph was
analyzed by separating stormflow and baseflow using the method described by the
Institute of Hydrology (1980). This method of hydrograph separation is widely used by
the USGS and others. To estimate the recharge required to produce the observed
baseflow, the separated hydrograph components were analyzed using methods
described by Mau and Winter (1997) and Rutledge and Mesko (1996). Recharge was
estimated on a monthly basis and then averaged over the time period of the dataset.
This resulted in an estimate of recharge that ranges from 3 to 7 inches per year,
depending on how the recharge is assumed to occur.
Page 2-2

Updated Groundwater Flow And Transport Modeling Report December 2019
Mayo Steam Electric Plant, Roxboro, North Carolina
Recharge, estimated using the hydrograph from Hyco Creek, was generally less than
that shown on the map by Haven (2003), although the ranges from the two methods
overlap. The hydrograph method is based on a water balance in the vicinity of the Site,
so it should provide reliable average values, but the method does not provide spatial
resolution smaller than the watershed scale. The data from Haven (2003) provides
better spatial resolution than the spatial resolution of the hydrograph method, but it is
difficult to verify these data using local hydrologic data, such as stream hydrographs.
The discrepancy was resolved by using a value of 0.0018 ft/d (approximately 7.9 inches
per year), which is within the range of values predicted by both methods. Further, it
was assumed that recharge was negligible in the vicinity of the power plant itself. This
is because infiltration is expected to be less in the lined retention basin area, power
block area, other ancillary areas, and in the low -permeability dams. Specific values
assumed for recharge are provided in Section 4.4.
The ash basin at the Mayo occupies the former stream valley of Crutchfield Branch, a
headwater stream that flows northeast. The basin was created by building a dam across
the stream valley. A small stream flows into the ash basin, and groundwater is inferred
to discharge into the basin from uplands to the west, south, and east of the basin. The
surface watershed that drains into the basin is bounded on the east and south by a
divide with drainage into Mayo Reservoir. It is bounded on the west by a divide along
US Highway 501 (SynTerra, 2015a). The Crutchfield Branch stream originates at the
base of the ash basin dam and flows north toward Virginia (Figure 1-1). Water levels in
the wells completed in the ash basin were similar to the pre -decanting ponded water
level. The water levels ranged from 480 to 485 feet in the ash basin wells, with heads
that increased toward the south (Figure 1-1).
The distribution of hydraulic head suggests that groundwater is discharging into the
upstream end of the ash basin from the uplands to the west, south, and east. The pre -
decanting water level near the dam was several tens of feet higher than the water level
in wells along Crutchfield Branch. This large head difference will decrease following
decanting.
2.3 Hydrologic Boundaries
The major discharging locations for the shallow water system serve as hydrologic
boundaries to the shallow groundwater system. These include lakes and streams.
Page 2-3

Updated Groundwater Flow And Transport Modeling Report December 2019
Mayo Steam Electric Plant, Roxboro, North Carolina
2.4 Hydraulic Boundaries
The shallow groundwater system does not appear to contain impermeable barriers or
boundaries in the study area, but it does include hydraulic boundaries between zones of
different hydraulic conductivity. The fracture density, and thus the hydraulic
conductivity, has been shown to generally decrease with depth in metamorphic rock.
This will result in blocks of unfractured rock where the hydraulic conductivity is less.
However, isolated fractures that result in hydraulic conductivities that are great enough
to sustain flow might occur, but the locations of these fractures are difficult to predict.
It was assumed that the rock was essentially impermeable below the depth of the
bottom modeled layer, so a no -flow lower boundary was used to represent this
condition.
2.5 Sources and Sinks
Recharge is the major source of water in the uplands and ash basin. Most of the
groundwater discharges into streams and lakes, as outlined above. Groundwater
discharges into the ash basin and flows as pore water through the ash basin. As a
result, the ash basin pore water acts as both a source of, and sink for, groundwater. A
source is defined herein as the place where water enters the groundwater system, and a
sink is defined herein as the place where water leaves the system.
Approximately 21 domestic water supply wells have been identified within a half -mile
of the Site (SynTerra, 2014), and they were included as sinks in the groundwater model
simulations. The average daily water use in North Carolina is 60 gallons to 70 gallons
per person (Treece et. al., 1990; USGS,1987; USGS, 1995); therefore, a well providing
water for a family of four people was estimated to be pumped at approximately 280
gallons per day. Measurement data on the discharge rate from the wells was
unavailable. The wells are situated in distinct drainage basins/slope-aquifer systems
separate and upgradient relative to groundwater flow from the Plant area and the ash
basin. Current and future simulated boron distributions indicate flow from the ash
basin is away from the water supply wells.
Residential sanitary waste water is disposed of through septic systems in the vicinity of
the Mayo site, which causes much of the water that is pumped from the aquifer to
infiltrate into the vadose zone through septic drain fields. Radcliffe et al. (2006) studied
septic drain fields in Georgia and found that 91 percent of the water used by a
household was discharged to the septic drain field. This corresponds to a consumptive
use of 9 percent. This is consistent with the data presented by Treece, et al. (1990), who
conclude that consumptive use is less than 6 percent. Daniel et al. (1997) developed a
groundwater model of the Indian Creek watershed in North Carolina, which used the
Page 2-4

Updated Groundwater Flow And Transport Modeling Report December 2019
Mayo Steam Electric Plant, Roxboro, North Carolina
analysis of Treece et al. (1990) to characterize pumping and septic return rates. Septic
systems were considered a source of water in the model.
2.6 Water Budget
The long-term average rate of water inflow to the study area is equal to the rate of water
outflow from the study area. Water enters the groundwater system through recharge,
typically derived from rain but also including septic return, and leaves by discharge of
surface water and wells. The magnitude of components of the water budget are
difficult to constrain using field data at the Site, but water budget details derived from
the groundwater model are provided in Section 5.0 and Section 6.0.
2.7 Modeled Constituents of Interest
Not including pH, 13 groundwater constituents of interest (COIs) were identified in the
CSA Update — arsenic, barium, boron, chromium, chromium VI, cobalt, iron,
manganese, molybdenum, strontium, sulfate, TDS, and vanadium (SynTerra, 2017).
Boron is the only constituent that shows a discernable plume associated with
groundwater migration from the ash basin; therefore, boron is the only COI modeled.
2.8 Constituent Transport
The COIs that are present in the coal ash can dissolve into the ash pore water. As water
infiltrates through the basin, water containing COIs can enter the groundwater system
through the bottom of the ash basin. Once in the groundwater system, the COIs are
transported by advection and dispersion, subject to retardation due to adsorption to
solids. During transport, dilution occurs within the groundwater system. If the COIs
reach a hydrologic boundary or water sink, they are removed from the groundwater
system and enter the surface water system where they are further diluted.
Page 2-5

Updated Groundwater Flow And Transport Modeling Report December 2019
Mayo Steam Electric Plant, Roxboro, North Carolina
3.0 COMPUTER MODEL
3.1 Model Selection
The numerical groundwater flow model was developed using MODFLOW (McDonald
and Harbaugh, 1988), a three-dimensional (3D) finite difference groundwater model
created by the USGS. The chemical transport model is the Modular 3-D Transport
Multi -Species (MT3DMS) model (Zheng and Wang, 1999). MODFLOW and MT3DMS,
widely used in industry and government, are considered industry standards. The
models were assembled using the Aquaveo GMS 10.3 graphical user interface
(http://www.aquaveo.com/).
3.2 Model Description
MODFLOW uses Darcy's law and the conservation of mass to derive water balance
equations for each finite difference cell. MODFLOW considers 3D transient
groundwater flow in confined and unconfined heterogeneous systems, and it can
include dynamic interaction with pumping wells, recharge, evapotranspiration, rivers,
streams, springs, lakes, and swamps.
Several versions of MODFLOW have been developed since its inception. This study
uses the MODFLOW-NWT version (Niswonger, et al., 2011). The NWT version of
MODFLOW provides improved numerical stability and accuracy for modeling
problems with variable water tables. That improved capability is helpful in the present
work where the position of the water table in the ash basin can fluctuate depending on
the conditions under which the basin is operated and potential closure action activities.
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 COI
adsorption to the soil matrix.
Page 3-1

Updated Groundwater Flow And Transport Modeling Report December 2019
Mayo Steam Electric Plant, Roxboro, North Carolina
4.0 GROUNDWATER FLOW AND TRANSPORT MODEL
CONSTRUCTION
The flow and transport model of the Site was created through a series of steps:
• Step 1: Build a 3D model of the Site hydrostratigraphy based on field data.
• Step 2: Determine the model domain and construction of the numerical grid.
• Step 3: Populate the numerical grid with flow parameters.
• Step 4: Conduct calibration round 1, which includes adjustment of the numerical
grid.
• Step 5: Develop a transient model of constituent transport development.
• Step 6: Conduct calibration round 2 to ensure the flow model matches the
observed heads and the transient model reproduces the observed plumes.
4.1 Model Domain and Grid
The initial steps in the model grid generation process were the determination of the
model domain and the construction of a 3D hydrostratigraphic model. The model has
dimensions of 2.5 miles by 2.8 miles (Figure 4-1). The model domain was rotated 310
clockwise so boundaries of the model were parallel with the ash basin dam. The
shortest distance between the ash basin and a model boundary is approximately 1 mile.
The ground surface of the model was interpolated from USGS NED n37w0791/3 arc -sec
20131 degree IMG dataset obtained from http://viewer.nationalmap.gov/viewer/. The
elevations for the top of the ash basin were modified using more recent surveying data
from the WSP USA Aerial Topographic Survey from May 2015. The ash basin ponded
water was represented using a large hydraulic conductivity, which was distributed
within the computational grid based on bathymetric data. The hydrostratigraphic
model consists of six units: ash, saprolite, transition zone, upper fractured rock, lower
fractured rock, and bedrock. The hydrostratigraphic model was developed using
"Solids" in GMS (Figure 4-2). Five solids were created and then subdivided after the
computational mesh was developed. The solids include ash, saprolite, transition zone,
fractured rock, and bedrock. The lower contact between the ash basin and the
underlying saprolite was assumed to be the ground surface prior to construction of the
ash basins. An electronic file describing this surface was created by digitizing a
preconstruction topographic map. The digitized points were interpolated to create a
continuous surface representing the preconstruction ground surface, which was used as
the contact between the ash and the underlying saprolite. This provided a surface that
was consistent with the borehole observations except at ABMW-4 where the contact
Page 4-1

Updated Groundwater Flow And Transport Modeling Report December 2019
Mayo Steam Electric Plant, Roxboro, North Carolina
was 20 feet lower. The elevations for the bottom of ash basin were further refined based
on an updated surface elevation file provided by AECOM in 2019. The current surface
is consistent with the borehole observation at ABMW-4. The lateral extent of the ash
was determined from maps in the CSA report (SynTerra, 2015a).
The contacts between the saprolite, transition zone, and underlying bedrock were
determined by interpolating data measured in borings described in the CSA reports and
historical data. This produced two surface maps, one showing the contact between
saprolite and transition zone and the other showing the contact between transition zone
and top of fractured bedrock.
The methodology outlined above for creating a geologic model was done so the
interpolated contacts would approximately follow the ground surface between
boreholes, which is consistent with the expectations based on the hydrogeology of the
Piedmont region (e.g. LeGrand, 1988; Miller, 1990). The upper fractured zone is
approximately 100 feet thick, based on general field observations and data from boring
logs interpretation.
The numerical grid consists of rectangular blocks arranged in columns, rows, and
layers. There are 244 columns, 222 rows, and 21 layers in the grid, containing a total of
737,119 active cells (Figure 4-3). The maximum width of the columns and rows is 100
feet. The size of the grid blocks is refined in the vicinity of the ash basin. The
horizontal dimension of some of the grid blocks is as small as 20 feet near the dam. The
21 layers represent the hydrostratigraphic units. The model grid was set up to conform
to the contacts from the solids. The model grid layers correspond with the
hydrostratigraphic layers as follows:
Hydrostratigraphic Layer
Grid Layer
Ash
1-8
Saprolite
9-11
Transition zone
12-13
Upper fractured rock
14-16
Lower fractured rock
17-18
Bedrock
19-21
Grid layers 1-8 are inactive outside of the region of the ash basin as determined from
the CSA report. Grid layers 1-21 are set as inactive in the eastern, southern, and
western corners of the model domain (Figure 4-1 and Figure 4-3).
Page 4-2

Updated Groundwater Flow And Transport Modeling Report December 2019
Mayo Steam Electric Plant, Roxboro, North Carolina
4.2 Hydraulic Parameters
The horizontal hydraulic conductivity and the horizontal to vertical hydraulic
conductivity anisotropy ratio are the main hydraulic parameters in the model. The
distribution of these parameters is based primarily on the model hydrostratigraphy,
with additional horizontal and vertical variation. Most of the hydraulic parameter
distributions in the model were heterogeneous across a model layer. The geometries
and parameter values of the heterogeneous distributions were determined largely
during the flow model calibration process. Initial estimates of parameters were based
on literature values; results of slug tests, pumping tests, and core tests; and simulations
performed using a preliminary flow model. The hydraulic parameter values were
adjusted during the flow model calibration process described in Section 5.0 to provide a
best fit to observed water levels in observation wells. Slug test data from hundreds of
wells at the Duke Energy coal ash basin sites in North Carolina and pumping test data
from six Duke Energy coal ash basin sites are shown in Figure 4-4 through Figure 4-7.
The hydraulic conductivity of coal ash measured at 14 sites in North Carolina ranges
over 4 orders of magnitude, with a geometric mean value of approximately 1.8 ft/d. Ash
hydraulic conductivity values estimated by interpreting slug test data at Mayo range
from 0.05 ft/d to 4.1 ft/d. Two pumping tests were performed in the ash basin at Mayo
to help refine the value of this parameter. One test was performed by pumping a well
screened at the bottom of the ash, and another with a well screened in the middle of the
ash. Hydraulic conductivity based on analytical and numerical solutions ranges from
0.05 ft/d to 8 ft/d.
Pumping tests were also conducted in the ash basins at five other Duke sites. Those
pumping tests were analyzed using parameter estimation methods with analytical
solutions and with site -specific numerical models. The results are summarized in
Figure 4-4 (SynTerra, 2019a and 2019b).
The hydraulic conductivities from hundreds of slug tests performed in saprolite wells at
10 Piedmont sites range over 5 orders of magnitude and have a geometric mean value
of 0.9 ft/d (Figure 4-5). Slug tests performed at wells completed in saprolite at Mayo
indicate that hydraulic conductivity ranges from 0.02 ft/d to 2.9 ft/d.
Transition zone hydraulic conductivities from hundreds of slug tests at 10 Piedmont
sites range over 5 orders of magnitude, with a geometric mean value of 0.9 ft/d
(Figure 4-6). The measured values at Mayo range from 0.02 ft/d to 102.3 ft/d.
Fractured bedrock hydraulic conductivities from hundreds of slug tests at 10 Piedmont
sites in North Carolina (Figure 4-7) range over more than 6 orders of magnitude, with a
Page 4-3

Updated Groundwater Flow And Transport Modeling Report December 2019
Mayo Steam Electric Plant, Roxboro, North Carolina
geometric mean value of 0.3 ft/d. The measured values at Mayo range from 0.0005 ft/d
to 5.7 ft/d.
Hydraulic conductivity data obtained from slug tests and pumping tests conducted on
wells at the Mayo site span smaller ranges than the overall dataset, and in some cases
the geometric mean values are quite different (Figure 4-4 to Figure 4-7). For example,
the geometric mean hydraulic conductivity for coal ash from the slug tests at Mayo site
is approximately 0.4 ft/d, whereas it is approximately 1.8 ft/d over the entire dataset.
However, the datasets from Mayo, and from all the sites, indicate that hydraulic
conductivity varies spatially by several orders of magnitude due to heterogeneities.
4.3 Flow Model Boundary Conditions
The outer lateral boundary conditions for the saprolite are almost entirely specified
head, with small areas of no -flow locally. Boundaries on the east side of the model
include parts of Mayo Reservoir, which were held at specified head. The boundaries on
the south, east, and north sides of the model are independent of definitive hydrologic
features. A specified head boundary condition with the head set in layer 12, which is
the middle of the transition zone, was used along these boundaries. This boundary
condition forces the water table to be in the transition zone along these boundaries,
which is a reasonable approximation of the expected conditions. Streams or lakes that
intersect the external boundary are defined by their own boundary conditions (as either
general head or drain -type boundaries). The specified head boundary condition
extends along the upland areas and is terminated within a few hundred feet of the
locations of streams or lakes. This creates short intervals of no -flow conditions between
streams or lakes and the uplands. No -flow conditions are assumed along the boundary
beneath layer 12.
4.4 Flow Model Sources and Sinks
The sources and sinks of groundwater within the model domain consist of recharge,
lakes, streams, and groundwater pumping.
Recharge is a key hydrologic parameter in the model (Figure 4-8). As described in
Section 2.2, the recharge rate for upland areas in the vicinity of the Site is assumed to be
0.0018 ft/d (7.9 inches per year). The recharge on exposed ash is 0.0009 ft/d, which is
lower than the recharge for upland areas. This is because fine-grained ash material and
thick vegetation are assumed to have increased evapotranspiration and reduced
infiltration compared with what has occurred in the upland area. The recharge is
assumed to be zero in the regions around ash basin ponded water and Mayo Reservoir
that serve as groundwater discharge zones. Recharge over the coal ash is assumed to be
twice that of the ambient recharge. The recharge rate in the vicinity of the power plant
Page 4-4

Updated Groundwater Flow And Transport Modeling Report December 2019
Mayo Steam Electric Plant, Roxboro, North Carolina
is assumed to be small due to the large areas of roof and pavement. A value of 0.0001
ft/d is assumed. The recharge beneath the lined retention pond, stormwater holding
basin, former FGD pond, and the FGD Settling Basin (Waste Water Treatment Basin) are
assumed to be 0.0001 ft/d. Recharge on the low -permeability dam is set to 0.0001 ft/d,
which is consistent with the low hydraulic conductivity assumed for the dam.
The magnitude and distribution of recharge was held constant during the model
calibration process. In steady-state groundwater flow, hydraulic heads are affected by
the ratio of recharge to hydraulic conductivity, so recharge was held constant and
hydraulic conductivity was modified during calibration.
Mayo Reservoir and the ash basin ponded water are represented as general head
boundaries with the head set to their stage (Figure 4-9). The stage of Mayo Reservoir is
assumed to be 432 feet. The stage of the pre -decanting ash basin ponded water is set to
approximately 480 feet based on Light Detection and Ranging (LiDAR) data and a
surveying point. The ponded water level in the ash basin fingers is assumed to be
maintained by recharge from groundwater, precipitation, sluiced water and
evaporation.
Streams are represented as Type 3 boundary conditions, called "drains' in MODFLOW
(Figure 4-9). The elevation of the streams is equal to, or a few feet lower than, the
ground surface elevation determined from the LiDAR data. The drain conductance is
set to 100 ft2/day, a relatively large value that will cause negligible head loss. The
conductance was not adjusted during calibration.
The ash basin is represented by simulating the pre -decanting ponded water areas as
general head boundaries and applying recharge on the exposed ash (Figure 4-8). This
approach treats the ash basin as a hydrogeologic component in the model. The
hydrologic conditions when the ash was below the level of the water early in the life of
the basin differs from recent conditions. However, the hydraulic head data in the ash
were slightly above the level of the water, resulting in an assumption that the
groundwater flow system surrounding the basin, prior to ash placement, would have
been similar to that of the basin containing ash. As a result, the hydrologic conditions
in the ash basin were assumed to be constant through the life of the basin.
The outflow channel is represented as a general head boundary (Figure 4-9), with the
stage set to the ground surface elevation. This engineered channel can either gain water
from, or lose water to, the groundwater system.
Page 4-5

Updated Groundwater Flow And Transport Modeling Report December 2019
Mayo Steam Electric Plant, Roxboro, North Carolina
Little information is available about the water supply wells in the model area, other
than their locations, which are shown in Figure 4-10 (SynTerra, 2015a). Most of the
wells are probably open holes in the upper few 100 feet of bedrock. The actual locations
of the open intervals of the wells are unknown, so it is assumed that all the wells were
screened in grid layer 16 (upper fractured rock) in the model. The pumping rates from
the wells are unknown, but it is assumed that the wells were pumped at 280 gallons per
day, which is average water use for a family of four (Treece et al., 1990; USGS, 1987;
USGS, 1995). Septic return is assumed to be 94 percent of the pumping rate, based on
Treece et al. (1990); Daniel et al. (1997); and Radcliffe et al. (2006). The septic return is
injected into layer 11 (saprolite) in the model.
4.5 Flow Model Calibration Targets
The flow model steady-state calibration targets were determined by averaging water
level measurements made through the 2nd quarter of 2019 in 113 observations wells.
This does not include the deep bedrock wells constructed in the fall of 2018, because the
heads equilibrate slowly after sampling and reliable head measurements were
unavailable. In general, wells with an ABMW designation are screened in ash, those
with an S designation are screened in saprolite, those with a D designation are screened
in the transition zone, and those with a BR designation are screened in the upper
bedrock, with a few exceptions.
The water levels used for calibration were determined by taking the average value for
head data. Water levels are expected to vary on an annual period due to seasonal
changes in recharge. These fluctuations in water level are not simulated because the
flow model is steady state. The average water level values are the best available
estimates of the steady state. Water -level data have been recorded quarterly (seasonal)
for up to 10 years. Most records span more than one year, so seasonal fluctuations are
accommodated by averaging the head values.
4.6 Transport Model Parameters
The transport model obtains the distribution of groundwater volumetric flux (Darcy
velocity) from a MODFLOW simulation that has been calibrated to Site conditions. The
transport simulation starts in April 1983 and extends through the 2nd quarter of 2019.
Operations at the power plant began in 1983, and it is assumed that the basin was filled
with water at that time. The flow model assumes that the ash basin filled with water
quickly and the heads were maintained at the same level over time. As a result, a
steady-state calibration to the pre -decanting conditions was used to simulate water flow
during transport.
Page 4-6

Updated Groundwater Flow And Transport Modeling Report December 2019
Mayo Steam Electric Plant, Roxboro, North Carolina
The key transport model parameters (besides the flow field) are the boron source
concentration in the ash basin and the boron soil -water distribution coefficients (Ka).
Secondary parameters are the longitudinal, transverse, and vertical dispersivity, and the
effective porosity. The source concentrations were estimated from recently measured
ash pore water concentrations in monitoring wells. Source concentrations of the boron
are held constant at the specified levels in the ash layers during historical transport
simulations, but they are allowed to vary in time during the predictive simulations that
follow.
The numerical treatment of adsorption in the model requires special consideration
because part of the system is a porous media (the ash, saprolite, and transition zone)
with a relatively high porosity, whereas the bedrock is a fractured media with very low
matrix porosity and permeability. As a result, transport in the fractured bedrock occurs
almost entirely through the fractures. The MODFLOW and MT3DMS flow and
transport models used here simulate fractured bedrock as an equivalent porous media.
With this approach, an effective hydraulic conductivity is assigned to the fractured rock
zones so that it produces the correct Darcy flux (volume of water flowing per area of
rock per time) for a given hydraulic gradient. However, because the water flows almost
entirely through the fractures, this approach requires that a small effective porosity
value (0.05 or less) be used for the transport calculations to compute a realistic pore
velocity. The velocity of a COI, Vc, is affected by both the porosity, 0, and the
retardation factor, R, as:
V = V
OR (la)
where the retardation factor is computed internally in the MT3DMS code using a
conventional approach:
R=1+pbK a
0 (lb)
and V is the volumetric flux (Darcy velocity), pb is the bulk density and Ka is the
distribution coefficient assuming linear equilibrium sorption. The retardation factor for
boron in fractured rock is expected to be in the same range as R for porous media.
However, it is apparent from (1b) that R can become large if 0 is reduced and Ka is held
constant. This is unrealistic, and it is the reason why a small Ka value is assigned to the
bedrock, where the effective porosity is due to the fractures, and is low. This reduction
of Ka is justified on physical grounds because COIs in fractured rock interact with only
Page 4-7

Updated Groundwater Flow And Transport Modeling Report December 2019
Mayo Steam Electric Plant, Roxboro, North Carolina
a small fraction of the total volume in a grid block, whereas COIs in porous media are
assumed to interact with the entire volume. The Ka for boron in the bedrock layers of
the model was reduced by scaling it to the bedrock porosity. This causes the
retardation factor in the fractured rock to be similar to R in the saprolite and transition
zone.
Ash leaching tests were performed on seven samples from the Mayo ash basin using the
U.S. Environmental Protection Agency (USEPA) Leaching Environmental Assessment
Framework (LEAF) Method 1316. The data were analyzed to develop a Ka (partition
coefficient) value for boron in the coal ash. Ka of boron in ash in laboratory data ranged
from 0.1 milliliters per gram (mL/g) to 0.5 ml/g with a geometric mean value of 0.21
mL/g. Linear adsorption Ka values for boron were measured in the laboratory using
core materials from the coal ash and native aquifer materials (Langley et al., 2015). In
general, the measured Ka values are greatly variable, and the variability within a given
material type is similar to, or greater than, the variability between different materials.
The values of Ka used in the model are based on initial estimates of Ka measured in the
laboratory, which were then adjusted to improve the match of field data. A value of Ka
= 0.42 mL/g is used for ash in the model, which is within the range of the leaching test
results. The modeling approach for the predictive simulations of future boron transport
allows the boron concentration in the ash to vary with time in response to flushing by
groundwater. Using a Ka value that is derived from ash leaching tests ensures that the
simulated boron transport from the ash consistent with experimental observations.
The Ka values for the boron outside of the ash basin were treated as calibration
parameters. Boron is expected to be mobile, and to have low Ka and retardation values.
Values of Ka for boron in the weathered zone and fractured rock were reduced to
maintain a consistent retardation factor with depth. As a result, Ka = 0.12 mL/g is
assumed for layers 9-13 and Ka = 0.02 mL/g for layers 14-21.
It was assumed the effective porosity was generally uniform within a grid layer and
decreases with depth based on the hydrogeologic conceptual model.
Layer
Effective Porosity
1-8
0.3
9-13
0.2
14-18
0.05
19-21
0.01
Page 4-8

Updated Groundwater Flow And Transport Modeling Report December 2019
Mayo Steam Electric Plant, Roxboro, North Carolina
The longitudinal dispersivity was assigned a value of 20 feet. The transverse
dispersivity is set to 2 feet. The vertical dispersivity is set to 2 feet in the unconsolidated
layers and 0.2 feet in the bedrock layers. The effective porosity is assumed to decrease
with depth from 0.3 in the ash, to 0.2 in the saprolite and transition zone, to 0.05 in the
fractured bedrock, and to 0.01 in the deep bedrock. It is assumed the effective porosity
was uniform within a grid layer. The dry bulk density of the porous media was
assumed to be 1.6 gram per milliliter.
4.7 Transport Model Boundary Conditions
The transport model boundary conditions are specified as no -flow conditions on the
exterior edges of the model except where specified head boundaries exist, where the
transport boundary conditions are specified as a concentration of zero. The infiltrating
rainwater is assumed to be free from boron and enters the top of the model with zero
concentration. All of the drains and general head boundary water bodies have a fixed
concentration of zero. COIs are assumed to leave the model when they arrive at a drain
or are removed by flow that enters a general head boundary.
The initial condition for the transport model (in 1983) is zero concentration of boron in
groundwater. No background concentrations are considered. The concentration in the
ash basin is assumed to rapidly increase to the observed concentrations at the start of
the simulation.
4.8 Transport Model Sources and Sinks
The wastewater and ash in the ash basin is the source of boron in the model. The
sources are simulated by holding the boron concentration constant in cells located
inside the ash basins. This allows infiltrating water to carry dissolved constituents from
the ash into the groundwater system.
Soil and rock affected by boron below the ash basin can be a secondary source to
groundwater. This potential is fully accounted for in the model by continuously
tracking the boron concentrations in time in the saprolite, transition zone, and rock
materials throughout the model. The historical transport model simulates the migration
of boron through the soil and rock from the ash basin, and these results are used as the
starting concentrations for the predictive simulations. Therefore, even if all of the coal
ash is excavated, the transport model predicts soil beneath the ash will have an ongoing
effect on groundwater.
Chemical analyses from four wells were used to characterize the distribution of boron
concentration within the ash basin. The concentration observed in the wells was
assumed to represent the concentration near the wells within the ash basin throughout
Page 4-9

Updated Groundwater Flow And Transport Modeling Report December 2019
Mayo Steam Electric Plant, Roxboro, North Carolina
the simulation. This resulted in a patch -like distribution of concentration within the ash
basins.
The outflow channel flowing southeast from the ash basin is represented as general
head boundary in the model of pre -decanting conditions. As a result of this
assumption, it is possible that water would flow from this engineered feature into the
groundwater. It was assumed that the concentration in the surface water at this
location was zero. This assumption had no bearing on the results, however, because the
outflow channel gained groundwater.
The transport model sinks are lakes and streams. As groundwater enters these features,
it is removed along with any dissolved constituent mass. Similarly, if water containing
a constituent were to encounter an extraction well, the constituent would be removed
with the water.
4.9 Transport Model Calibration Targets
The transport model calibration targets are boron concentrations measured in 76
monitoring wells through the 2nd quarter of 2019.
Page 4-10

Updated Groundwater Flow And Transport Modeling Report December 2019
Mayo Steam Electric Plant, Roxboro, North Carolina
5.0 MODEL CALIBRATION TO PRE -DECANTING CONDITIONS
5.1 Flow Model Calibration
The flow model was calibrated in stages starting with a model that assumed
homogeneous conditions in most hydrostratigraphic layers. Calibration was done by
manual adjustments of parameters. As the effort continued, some formations were
given different properties in different layers. Calibration was done by seeking the
simplest configuration of parameters that matched the observed hydrogeologic
conditions and geologic conditions. The calibration was initiated using the geologic
model to define the geometry of hydrogeologic units and assigning hydraulic
conductivities typical of the region. The parameter estimation software PEST was then
used to minimize the residual between predicted and observed heads during calibration
of the original model. This resulted in reasonably close matches during calibration of
the original model. The calibration was further refined by adjusting hydraulic
conductivities manually to improve the fit between model and data.
Heterogeneities
The next step was to infer heterogeneities that could reduce the residuals. The model
over -predicted the heads at several wells in upland areas. It was inferred that these
wells intersected, or were near zones of, relatively high permeability that were broad
enough to extend toward a nearby stream or lake. This configuration reduced the head
in the well, and the hydraulic conductivity of the zone was increased until either the
head was reduced sufficiently, or an upper limit of hydraulic conductivity was reached
(Figures 5-la-r).
The occurrence of the zones of high hydraulic conductivity as shown in Figures 5-la-r
was inferred at the Mayo site based on head observations, but this inference is based on
a geologic style known from other locations. Flat -lying zones of interconnected
fractures several hundred feet or more across were described in crystalline rock at the
USGS Mirror Lake research site (e.g. Tiedeman et al., 2001), and similar fracture zones
have been recognized at other fractured rock sites that have been studied in detail. It
was assumed that the fractures zones in the model were shaped like flat -lying layers
several hundred feet in maximum dimension, similar to those described by Tiedeman et
al. (2001). Some of the inferred zones of high hydraulic conductivity are elongated, and
those zones are interpreted to be vertical facture zones.
A zone of low permeability inferred to occur to the southeast of the ash basin
corresponds to a ridge where large boulders of recrystallized quartz were observed in
the field. Similar quartz -forming ridges are known elsewhere in the Piedmont where
Page 5-1

Updated Groundwater Flow And Transport Modeling Report December 2019
Mayo Steam Electric Plant, Roxboro, North Carolina
they can form zones of low hydraulic conductivity (e.g. Snipes et al., 1984), presumably
because the quartz fills pore space and resists weathering. This zone is intersected by
MW-4 and MW-6BR. The recharge rate for both wells is extremely slow, an indication
that the wells were completed in low -permeability bedrock zones. The recharge rate
was too slow to obtain meaningful estimates of hydraulic conductivity using slug tests.
Other low permeability ridges were inferred during calibration.
Ash Basin Dam
The model grid was refined to improve the representation of the ash basin dam. This
included identifying a low -permeability zone that represents a cutoff on the upstream
faces of the dam, and several high -permeability zones that represents internal drains
(Figure 5-2a). These features and the dimensions of the dam are consistent with
available drawings describing the dam.
The locations and hydraulic conductivities of the zones composing the dam were
adjusted during calibration to match both the hydraulic heads in the vicinity of the
dam, and the flow rates of water observed in the vicinity of the dam. Flow rates were
measured at two weirs at the base of the dam, and at the confluence of two small
streams draining land below the dam. These flow rates were the only flow rates
available for calibration.
The results of the calibration of the dam indicate flow rates that are within several
gallons per minute (gpm) of the flow rates observed at weirs on the east and west sides
of the dam (S-1 and S-2). The simulated flow rate is approximately one-third to two-
thirds of the flow rates observed at the stream confluence (Figure 5-2b). The larger
observed flow in the stream could be a result of contributions from interflow, which are
not included in the model, or by uncertainty in flow rate measurements.
Flow Balance and Residuals
The final calibrated flow model has the following flow balance:
Flow balance in steady state model in ft3/d
Feature
Input
Output
General/Constant Head
89,427
130,183
Recharge
305,336
0
Wells
0
785
Septic field
739
0
Drains (streams)
0
264,534
Total
395,503
395,503
Notes: Cubic feet per day = ft3/d
Page 5-2

Updated Groundwater Flow And Transport Modeling Report December 2019
Mayo Steam Electric Plant, Roxboro, North Carolina
The difference between the input and output is 0.008 ft3/d, which is a flow balance error
of less than 2x10-6. This small volume balance error confirms that a valid solution to the
head distribution has been obtained. The major input to the model is from recharge
with a lesser amount from general head boundaries. The constant head boundaries
creating input to the model are where groundwater is flowing into the model from the
boundaries around the periphery. The ash basin ponded water is also a general head
input. The output is split between groundwater discharging to constant/general head
boundaries and drains. The major constant head sinks are Mayo Reservoir and ash
basin ponded water, which account for about half the flow going to streams in the
model. Less than 1 percent of the water input is removed through water supply wells.
The flow of water supplied by septic fields is slightly less than the water removed by
the water supply wells.
The final calibrated flow model has a mean head residual of -0.17 feet and a root mean
squared head residual of 3.58 feet. The total span of head measurements ranged from
366 feet to 556 feet. Using this range to normalize the residual gives a normalized root
mean square error of 1.88 percent. A comparison of the observed and simulated water
levels is listed in Table 5-1, and the observed and simulated levels are cross -plotted in
Figure 5-3. Table 5-2 lists the best -fit hydraulic parameters from the calibration effort.
The residual between observed and simulated heads at the monitoring wells is shown
in Figure 5-4.
Hydraulic Conductivity
The calibrated hydraulic conductivity of the ash is 7 ft/d in the upper ash and 2 ft/d in
the lower ash (Table 5-2). These values are consistent with estimated hydraulic
conductivities based on slug tests and pumping tests at Mayo (Figure 4-4). The
calibrated hydraulic conductivity of the saprolite and transition zones is 1.0 ft/d, which
is similar to the geometric mean conductivity based on all the slug tests for saprolite
and transition zone. The hydraulic conductivity of the upper fractured rock is 0.03 ft/d,
and it decreases to 0.005 ft/d with depth. Hydraulic conductivities of the dam are
shown in Figure 5-2a.
The calibrated values of hydraulic conductivity are consistent with values from the slug
tests conducted in the ash, saprolite, transition zone, and upper fractured rock
(Figure 4-4 through Figure 4-7). Additional slug tests were conducted in 2019 at depths
below the upper fractured rock hydrostratigraphic unit (SynTerra, 2019c). The
hydraulic conductivity ranges from 10-5 to 0.24 ft/d with a geometric mean of 0.001 ft/d,
which is consistent with the value used in the model for lower fractured rock and
competent bedrock (0.005 ft/d).
Page 5-3

Updated Groundwater Flow And Transport Modeling Report December 2019
Mayo Steam Electric Plant, Roxboro, North Carolina
Hydraulic Head Distribution
The calibrated model predicts the highest hydraulic head occurs south of the ash basin
and the lowest heads occur on Crutchfield Branch and Mayo Creek along the northern
edge of the model. The heads in the ash basin are in the mid -range between these
extremes, and this is consistent with the basin resembling a generic flow -through lake,
with higher heads on the west, south and east sides, and lower heads to the north
(Figure 5-4 and Figure 5-5).
The distribution of hydraulic head can be used to infer the direction of groundwater
flow by assuming the horizontal hydraulic conductivity is isotropic and by ignoring
effects of heterogeneities. These assumptions imply that the groundwater flow
direction is perpendicular to the hydraulic head gradient. Within the constraints of
these assumptions, the groundwater flow directions are inferred to be to the north
within the upland region south of the basin, and discharge into the southern end of the
ash basin (Figure 5-5).
Potential Receptor Wells and the Ambient Groundwater Flow System
Water supply wells in the general vicinity of the Mayo ash basin are located along
groundwater divides and upgradient of the groundwater flow systems containing the
ash basin. This is important because the groundwater flow system effectively isolates
water that contacts ash from water supply wells in the vicinity. For example,
approximately 12 water supply wells are located east of U.S. Highway 501 in the area of
Mullins Road south of the ash basin. These wells are on a piezometric high to the south
of the divide marking the ash basin groundwater flow system, and some water supply
wells are outside the area (Figure 5-5). Approximately nine wells are located northwest
of the Mayo site along U.S. 501. These wells are on a piezometric ridge north of the
divide marking the ash basin flow system. A well northeast of the Mayo site is also
outside of the ash basin groundwater flow system.
The regional groundwater flow system in the vicinity of the Mayo site has effectively
isolated groundwater exposed to ash from water supply wells, according to the
simulations. The regional groundwater flow system is affected only slightly by closure,
so it is expected to continue to isolate the ash basin water from the water supply wells
in the future.
Water Balance
A water balance for the vicinity of the ash basin was determined from the results of the
calibrated model. The water budget analysis identified a local groundwater flow
system in the vicinity of the ash basin. The local flow system is assumed to be bounded
by a groundwater divide that extends from one end of the dam and wraps around the
Page 5-4

Updated Groundwater Flow And Transport Modeling Report December 2019
Mayo Steam Electric Plant, Roxboro, North Carolina
watershed to the other end of the dam (Figure 5-5). This definition implies that
groundwater cannot enter the local ash basin flow system, but it can leave the system
by flowing through or beneath the dam. The local groundwater flow system is
assumed to be bounded by divides in the uplands outside of the ash basin and
terminate along the axis of the dam (Figure 5-5). Zones were defined within GMS and
the Zone Budget tool in MODLFOW was used to determine components of the water
balance. The results were edited slightly to be consistent with the definition of the flow
system.
The results indicate that recharge on the uplands of 129 gpm is the major inflow into the
system. Recharge to the exposed ash adds another 13 gpm to the system.
Approximately 62 gpm discharges to the ash basin ponded water, and 63 gpm is
discharged to streams in the uplands. Flow through and under the dam removed 18
gpm from the system. The water balance for pre -decanting conditions is summarized in
Table 5-3.
5.2 Flow Model Sensitivity Analysis
A parameter sensitivity analysis was performed on the calibrated model by
systematically increasing and decreasing the main parameters by factors of either 2 or
0.5 from their calibrated values. Table 5-4 shows the results of the analysis, expressed in
terms of the normalized root mean square error (NRMSE) for each simulation. The
baseline NRMSE is 0.0188.
The flow model shows the highest degree of sensitivity to the upland recharge and to
the hydraulic conductivities of the transition zone and saprolite stratigraphic units. The
saprolite and transition zone were saturated beneath the ash basins and in the vicinity
of Crutchfield Branch, and this accounts for the calibration sensitivity. The NRMSE was
moderately sensitive to the hydraulic conductivity of bedrock. The NRMSE was only
weakly sensitive to the hydraulic conductivities of the ash and the fractured bedrock
(Table 5-4).
The NRMSE of the flow model is weakly sensitive to the hydraulic conductivity of the
dams. The heads of wells downgradient of the dam are quite sensitive to the hydraulic
conductivity of the dam. However, the hydraulic conductivity of the dam is quite low,
and changing this value by a factor of 2 has little effect on the overall residual.
5.3 Historical Transport Model Calibration
The transport simulations use a steady state flow model based on pre -decanting
conditions. This assumes that the water level in the ash basin has been maintained at
approximately 480 feet since operations at the Site began in 1983.
Page 5-5

Updated Groundwater Flow And Transport Modeling Report December 2019
Mayo Steam Electric Plant, Roxboro, North Carolina
Concentrations in wells in the ash basin were used to set boundary conditions in the
model, as shown in Figure 5-6 and Table 5-5. Some of the wells where boron was
detected are in areas where the predicted concentration gradients are steep; therefore,
small changes in location result in significant changes in concentration. This is one
factor that explains the differences between predicted and observed concentrations
(Table 5-6).
The primary area of predicted boron groundwater concentrations greater than the 02L
standard occurs beneath the ash basin dam. The longest plume of boron is predicted to
occur beneath the eastern side of the dam. The plume flows under the dam and extends
to approximately 700 feet along Crutchfield Branch from the edge of the water in the
ash basin (Figure 5-7). The predicted front of the plume is approximately 130 feet within
the compliance boundary. No occurrences of concentrations greater than the 02L
standard are predicted to occur beyond the compliance boundary. The plume beneath
the dam includes concentrations greater than 4,000 µg/L in the transition zone and
shallow rock. Concentrations in the range of 700 to 4,000 µg/L occur in deeper rock
beneath and on the upgradient side of the dam in the simulation. Boron concentrations
greater than 700 µg/L extend down to approximately 250 feet below the dam.
Four pairs of deep bedrock wells were drilled downgradient of the dam. The depths of
the shallow well in each pair ranged from 125 feet to 240 feet and the depths of the deep
well in each pair ranged from 250 feet to 351 feet. Boron concentrations were less than
or at detection limits in the deep bedrock wells. Data from the new wells indicate no
deep boron downgradient of the dam at the Mayo site. This confirms results from the
simulations.
Boron concentrations greater than 02L standard also occur beneath the ash basin on the
upgradient side of the basin, as shown in Figure 5-7. Those concentrations result from
recharge through the exposed ash in the southwestern end of the basin into the
groundwater. It is predicted to occur at much shallower depths compared to the plume
beneath the ash basin dam. Boron concentrations greater than 700 µg/L extend to within
or slightly below the transition zone. Data from wells indicate no boron in deep
bedrock, which is consistent with the model prediction.
5.4 Transport Model Sensitivity Analysis
A parameter sensitivity analysis was conducted to evaluate the transport model
sensitivity to Kd, which is considered a key parameter affecting transport. Kd is assumed
to be uniform across each grid layer and to vary with depth, as described in Section 4.6
and Table 5-3. The sensitivity analysis was performed on the calibrated transport
model by systematically increasing and decreasing boron Kd values by a factor of 5 from
Page 5-6

Updated Groundwater Flow And Transport Modeling Report December 2019
Mayo Steam Electric Plant, Roxboro, North Carolina
the calibrated values (Table 5-7). The model was then run for the revised Kd values, and
the NRMSE was calculated and compared to the NRMSE for the calibrated model.
The calibrated transport model simulates concentrations that reasonably match most of
the observed concentrations (Table 5-6) with an overall NRMSE of 1.71 percent.
Reducing Kd by multiplying by a factor of one -fifth increases NRMSE to 2.55 percent,
whereas increasing Kd by 5x increases NRMSE to 3.50 percent (Table 5-7). This indicates
that the value of Kd used in the model is near optimal. It also suggests that the average
difference between simulated and observed concentrations is only moderately affected
by changes in Kd.
Page 5-7

Updated Groundwater Flow And Transport Modeling Report December 2019
Mayo Steam Electric Plant, Roxboro, North Carolina
6.0 PREDICTIVE SIMULATIONS OF CLOSURE SCENARIOS
Once calibrated to pre -decanting conditions, the model was used to predict future
constituent distribution. This process involved a sequence of two simulations:
1. Decanted conditions
2. Closure action
The simulation of decanted conditions involves accounting for transport from the time
when decanting began to the time when the closure is complete. Decanting of the ash
basin ponded water began in June 2019. The decanting is assumed to be completed by
the end of 2020.
After decanting, it is assumed the basin closure activities will begin and will continue
for several years. It is assumed that the closure -by -excavation construction can be
completed in approximately 10 years (2031), and that the closure -in -place construction
can be completed in approximately 5.5 years (2026).
The second step involves simulating transport processes after basin closure for several
centuries or longer. Two different closure scenarios were evaluated:
• Closure -by -excavation: removing the ash by excavation
• Closure -in -place: covering the ash with a low -permeability cap
The compliance boundary for the two closure scenarios is assumed to be the same as the
current compliance boundary. The distribution of recharge, locations of drains, and
distribution of material were modified to represent the different closure scenarios. For
example, the recharge was modified from what is shown in Figure 4-8. The hydraulic
head distribution was recalculated and the transport was simulated for each scenario.
The simulated basin closure designs changed the hydraulic head in the vicinity of the
ash basin as the engineered designs interacted with the hydrogeologic conditions. This
interaction altered the groundwater flow and the transport of dissolved compounds.
In the previous model report (SynTerra, 2018), hydraulic conductivity in ash was set to
the geometric mean value based on slug test measurements from 14 ash basins in North
Carolina (Section 4.2). The single hydraulic conductivity is expected to be an
approximation of the properties of the ash at the Mayo site.
Two pumping tests were conducted in the Mayo ash basin in September 2018 to refine
the coal ash conductivity evaluation. Parameter estimation using analytical solutions
Page 6-1

Updated Groundwater Flow And Transport Modeling Report December 2019
Mayo Steam Electric Plant, Roxboro, North Carolina
ranges from 0.05 to 1.45 (SynTerra, 2019a). Parameters estimated using a numerical
forward model indicates the horizontal hydraulic conductivity ranges from 0.05 ft/d to 6
ft/d for one test and 0.2 ft/d to 8 ft/d for the other; numerical solution for vertical
hydraulic conductivity range from 0.01 ft/d to 0.6 ft/d for one test and 0.02 ft/d to 1.6
ft/d for the other (SynTerra, 2019b). Horizontal and vertical hydraulic conductivities
generally decrease with depth. Based on the pumping tests results, ash in the 2019
model is vertically divided into two zones. A horizontal conductivity of 7 ft/d, which
matches the average value of the maximum numerical solutions, is assigned to the top
four layers. Hydraulic conductivity in the lower four layers is maintained at 2 ft/d,
which is the geometric mean value based on slug tests at all sites and is also generally
consistent with the analysis of the pumping tests. A vertical anisotropy of 10 is used,
which is consistent with the pumping tests and is the same as was used in the previous
model.
6.1 Interim Models with Ash Basin Ponded Water Decanted
(Approximate Year 2021-2026 or 2021-2031)
This simulation represents an interim period after the ponded water is decanted, but
before excavation or cover system construction is completed. This simulation predicts
the initial conditions for the closure simulations. Simulations began by extending the
analysis of pre -decanting conditions to December 31, 2020, when decanting is assumed
to be completed. The simulations of pre -decanting conditions are outlined above. The
interim scenario simulations, which begin in January 2021, assume that the ash basin
water is decanted and then maintained with a head level of 425 feet along the
upgradient side of the dam. It assumes the recharge rate used for the exposed ash in
the pre -decanting condition model (0.0009 ft/d) occurs uniformly over the entire ash
basin. Groundwater will discharge into the ash during the interim period, and it is
assumed that drains will be constructed within the basin to capture the groundwater
discharge (Figure 6-1). Modifications to the flow system include a change in the water
elevation from 480 to 425 feet in the main ash basin ponded water area (Figure 6-2).
Water balance analysis shows that approximately 106 gpm of groundwater discharges
to the decanting drain inside the ash basin. Flow that goes through and under the dam
decreases from 18 gpm under pre -decanting conditions to 7 gpm under decanted
conditions (Table 6-1).
Results from the Interim scenario simulation
The extent of boron in groundwater is predicted to decrease during the interim period.
This is demonstrated by a reduction in size of the region of predicted concentrations
greater than 4,000 µg/L (Figure 6-3 and Figure 6-4) from the pre -decanting conditions
(Figure 5-7). The maximum horizontal extent of boron at concentrations greater than
Page 6-2

Updated Groundwater Flow And Transport Modeling Report December 2019
Mayo Steam Electric Plant, Roxboro, North Carolina
02L is reduced along Crutchfield Branch during the interim period by up to 100 feet.
The extent of the plume shrinks during the interim period as the flow system changes in
response to the reduction in head in the ash basin.
6.2 Closure -by -Excavation Scenario
Design of the closure -by -excavation option was provided by AECOM in October 2019
(AECOM, 2019a, Figure 6-5). The boundary conditions, recharge, and geometry were
adjusted to represent excavation and the simulations were run to evaluate how the
boron concentration in groundwater changed with time.
Model Setup
The closure -by -excavation scenario is simulated by assuming the ash is removed and
acts neither as a source of COIs, nor as a component of the hydrogeologic flow system.
This is represented in the model by making the hydraulic conductivity in layers 1-8
very high (~100 ft/d).
It is assumed that groundwater discharges where the groundwater table intersects
topographic drainages. This is represented by assuming the topography was graded to
the ground surface before the ash was deposited and by including "drain" boundary
conditions along the axes of topographic drainages. The "drains" that were added to
the model are shown as green lines in Figure 6-6. The dam is breached at the location of
the original stream channel. A detention basin is constructed across the breached dam
area, and is represented by a drain boundary condition with bottom elevation set at 387
feet. It is assigned a greater conductance to simulate the operation status under normal
condition that no ponded water exists in the detention basin (Figure 6-6). The area to
the east side of a potential new on -Site landfill is regraded into a channel according to
the design drawings and is also represented using a drain boundary condition.
The excavated coal ash would be placed into a new on -Site lined landfill potentially
sited over a portion of the western side of the ash basin. The recharge is assumed to be
uniformly distributed and equal to the average value of 0.0018 ft/d used elsewhere in
the model, except over the lined landfill where 10-7 ft/d is used, and the location of the
remaining dam, where 0.0001 ft/d is used. The hydraulic conductivity in the dam was
estimated to be low, and this was assumed to reduce the recharge occurring in the area
of the dam.
The transport model is set up using flows from the groundwater model. The
distribution of boron concentrations in 2031 from the interim scenario simulations
(Figure 6-4) is used as the initial concentration conditions.
Page 6-3

Updated Groundwater Flow And Transport Modeling Report December 2019
Mayo Steam Electric Plant, Roxboro, North Carolina
The simulation was started in 2031 and run for 1,000 years. Results were saved every
few years early in the simulation and the interval between the saved results increased
with time. This causes the temporal resolution to be finer at early time than it is later in
the simulation when changes are expected to be slow.
Results
The results show that excavation significantly reduces and changes the distribution of
the hydraulic head in the ash basin. In general, the hydraulic head contours wrap
around the topographic drainages (Figure 6-6). This pattern indicates that groundwater
flow converges on flowing streams that develop in the area exposed by excavating ash.
Horizontal hydraulic head gradients during the closure -by -excavation scenario are
significantly greater than pre -decanting conditions, according to the model. The pattern
of groundwater flow directions outside of the basin during the closure -by -excavation
scenario is similar to the pattern u pre -decanting conditions (Figure 6-6).
Spatial distribution after closure -by -excavation
The boron plume along Crutchfield Branch remains within the compliance boundary in
all flow zones and continues receding through the simulation period (Figure 6-7).
Boron concentrations inside the ash basin decrease sharply over several decades, but it
takes longer for concentrations to decrease to less than 02L under the footprint of the
remaining dam and the lined landfill (Figure 6-7).
The model results show concentrations of boron in both the saturated zone and the
overlying unsaturated zone. Transport through the unsaturated zone is downward and
limited by the infiltration rate. The infiltration rate over parts of the model underlying
the former dam was set to 0.0001 ft/d, and the rate over the lined landfill was set to 10-7
ft/d, which are several orders of magnitude lower than ambient recharge. This low
recharge accounts for the persistence of boron in the unsaturated zone at concentrations
greater than 02L in the saprolite in approximate year 2200, for example (Figure 6-7).
Boron will be mobilized horizontally only when it reaches the saturated zone.
Time series for closure -by -excavation
Time series of concentrations were determined at two locations to provide a finer
temporal resolution than can be depicted in the maps. One location (Location 1) is at
the confluence of tributaries to the Crutchfield Branch, and the other location (Location
2) is where Crutchfield Branch intersects the compliance boundary (Figure 6-8). Time
series were created for maximum boron concentration for all layers, and concentrations
in the saprolite, transition zone, upper fractured bedrock, and lower fractured bedrock
hydrostratigraphic layers (Figure 6-9). Concentrations in competent bedrock at these
two locations are less than detection according to the simulation.
Page 6-4

Updated Groundwater Flow And Transport Modeling Report December 2019
Mayo Steam Electric Plant, Roxboro, North Carolina
The boron concentration at Location 1 reaches a maximum of approximately 1,000 µg/L
in the upper fractured bedrock and a maximum of approximately 820 µg/L in the
transition zone (Figure 6-9). Concentrations are greater than the 02L standard of 700
µg/L prior to year 2000, and the boron concentration remains greater than the 02L until
soon after the start of decanting during the interim period (in 2021 simulations).
Concentrations quickly drop to less than 02L during the interim period, and the
maximum concentration in all layers is approximately 390 µg/L at the start of the
closure -by -excavation scenario (Figure 6-9).
The time series of concentration at Location 2 is similar in shape to that at Location 1,
except the magnitudes are much smaller with a maximum boron concentration of
approximately 130 µg/L (Figure 6-9).
Closure -in -Place Scenario
Simulations of the closure -in -place scenarios use results from the decanting model as
initial conditions, and were run to evaluate how boron concentrations in groundwater
decrease to less than the 02L standard.
Model Set Up
Simulations of the closure -in -place scenarios were created by modifying the distribution
of recharge and flow boundary conditions, as well as the conditions in the transport
analysis. The design for the closure -in -place was obtained from plans developed by
AECOM in January 2019 (AECOM, 2019b). It is assumed that the recharge flux over the
covered area would be 10-7 ft/d based on estimates of cover performance (Figure 6-10).
Drains are assumed to be constructed in the ash along the centerline of the ditches in
the closure cap system. Elevations of the drains are assumed to be approximately 5 feet
below the grade of the cap system. The drains slope to the north over the southern end
of the basin, and they slope to the east in the northern end of the basin. A drain extends
eastward out of the ash basin to Mayo Reservoir to represent the route of the
stormwater system.
It is assumed that water from the drains would be collected, treated and discharged as
required by the NPDES permit. The use of "drain" boundary conditions assumed the
actual drains had idealized behavior. For example, a "drain" boundary condition
allows water to flow from ash into the drain, but it prevents water from flowing from a
drain into ash. The analysis also ignores head losses that might occur due to localized
pore clogging. It also ignores head losses due to flow along the drain. These
assumptions are consistent with a goal of evaluating transport characteristics during the
Page 6-5

Updated Groundwater Flow And Transport Modeling Report December 2019
Mayo Steam Electric Plant, Roxboro, North Carolina
closure -in -place scenario, but additional calculations beyond those shown here will be
needed to fully evaluate the hydraulic performance of drains in the ash.
The transport model was set up using flows from the groundwater model. The
distribution of boron concentrations in 2026 from the interim simulation was used as
the initial concentration conditions. The simulation was started in 2026 and run for
approximately 1,000 years. Results were saved every few years early in the simulation,
and the interval between the saved results increased with time. This causes the
temporal resolution to be finer at early times than it is later in the simulation when
changes are expected to be slow.
Results
The hydraulic heads are altered slightly compared to the heads during pre -decanting
conditions as a result of reducing recharge on the covered area during closure, and
including drains in the ash basin (Figure 6-11). Groundwater flow directions outside of
the ash basin are similar to the directions during pre -decanting conditions. Flow within
the southern end of the ash basin is northward, and the pore water flow converges on
the northeastern part of the basin at the location of the surface water drain. Pore water
flows under and through the dam in the closure -in -place scenario, according to the
simulated head contours (Figure 6-11).
Spatial distribution during closure -in -place
Boron concentrations greater than the 02L standard in groundwater are within the
compliance boundary before closure -in -place starts, and remains within the compliance
boundary throughout the simulated period (Figure 6-12).
Boron concentrations beneath the southern end of the basin decrease over the first
decades after closure -in -place construction, and are mostly less than the 02L standard
by approximate year 2200 (Figure 6-12). Boron concentrations in groundwater decrease
more slowly beneath the dam.
The limited areas occur where the boron concentration is predicted to be greater than
4,000 µg/L (Figure 6-12), primarily at shallow depths, which are mostly above the water
table and in zones where the hydraulic conductivity is assumed to be small (0.02 ft/d).
These factors limit groundwater flow that would reduce the concentration.
Time series during Closure -in -Place
Time series of boron concentration were created at Locations 1 and 2 to show maximum
boron concentration in all layers, and concentrations in the saprolite, transition zone,
upper fractured rock, and lower fractured rock hydrostratigraphic layers (Figure 6-13).
Page 6-6

Updated Groundwater Flow And Transport Modeling Report December 2019
Mayo Steam Electric Plant, Roxboro, North Carolina
The results show that the concentration increases with time and is predicted to be
greater than the 02L standard from approximately 1990 to 2020 at Location 1. Boron
concentrations decrease sharply as a result of decanting during the interim period, and
they are approximately 600 µg/L at the start of the closure -in -place option.
Concentrations at Location 1 continue to decrease with time and are less than 100 µg/L
by approximate year 2060.
The boron time series up to the start of the closure -in -place scenario (approximate year
2026) are identical to the time series during the closure -by -excavation scenario (Figure
6-9). Concentrations in the closure -in -place model decrease slower than those in the
closure -by -excavation model at Location 1, possibly because they are maintained by
flow from the ash basin through and under the dam.
Boron concentrations at Location 2 are almost identical to the time series during the
closure -by -excavation scenario (Figure 6-9), with a maximum level of approximately
130 µg/L (Figure 6-13).
6.3 Conclusions
The following conclusions are based on the results of the groundwater flow and
transport simulations.
• The revised 2019 model predicts boron concentrations in groundwater greater
than the 02L standard extend a maximum of 370 feet from the waste boundary
along Crutchfield Branch. The leading edge of the plume is 130 feet away from
the compliance boundary.
• Ash basin ponded water decanting and initiation of closure actions will affect the
groundwater flow field within the ash basin. Under these future conditions, the
boron 02L concentration contour will recede after the start of decanting. Boron
concentrations greater than the 02L standard remain within the 500-foot
compliance boundary throughout the duration of the simulations of both the
closure -by -excavation and closure -in -place scenarios (Figure 6-14). Simulated
boron distributions for the two closure scenarios are very similar primarily due
to decanting and maintaining a lower head within the ash basin after the closure.
Groundwater flow directions outside of the ash basin and dam are largely
unaffected by closure.
• Time series of boron concentrations at two reference locations for the different
closure scenarios show similar trends of concentration change. Concentrations at
Location 1 are greater than the 02L standard between approximate year 1990 and
2000, and decrease to less than 02L standard before the start of closure due to
Page 6-7

Updated Groundwater Flow And Transport Modeling Report December 2019
Mayo Steam Electric Plant, Roxboro, North Carolina
decanting. As the boron concentrations diminish further during closure, small
differences in concentrations occur between the two closure scenarios
(Figure 6-15). Location 2 at the compliance boundary showed similar boron
concentrations for the two closure scenarios, with maximum concentration of 130
µg/L (Figure 6-15).
• Water supply wells are outside, or at the headwaters, of the groundwater flow
system containing the ash basin. Water supply wells are not affected by
constituents released from the ash basin or by the different closure scenarios.
• Data from ash basin pumping tests were used to refine the hydraulic
conductivity setup for the coal ash. New deep bedrock wells near the ash basin
dam confirmed the model prediction that boron concentrations greater than 02L
do not occur in deeper bedrock.
• Closure -by -excavation and closure -in -place actions result in similar boron
transport predictions at the compliance boundary. Boron concentrations greater
than 02L remain within the compliance boundary in both cases.
Page 6-8

Updated Groundwater Flow And Transport Modeling Report December 2019
Mayo Steam Electric Plant, Roxboro, North Carolina
7.0 REFERENCES
AECOM, 2019a, Mayo Steam Electric Plant Closure by Excavation Ash Basin Closure
(Draft 100% Permit Set), Final Closure Grading Plan 1, November 6, 2019
AECOM, 2019b, Mayo Steam Electric Plant Final Closure Plan Updates, 2019 Closure
Plan (Draft 100%), Final Cover Grading Plan, November 6, 2019
Daniel, C.C., Douglas G. Smith, and Jo Leslie Eimers, 1997, Hydrogeology and
Simulation of Ground -Water Flow in the Thick Regolith-Fractured Crystalline
Rock Aquifer System of Indian Creek Basin, North Carolina, USGS Water -Supply
2341.
Duke Energy. http://www.duke-energy.com/pdfs/duke-energy-ash-metrics.pdf
(Updated Oct. 31, 2014).
HDR and SynTerra, 2017. Statistical Methods for Developing Reference Background
Concentrations for Groundwater and Soil at Coal Ash Facilities. HDR
Engineering, Inc. and SynTerra Corporation.
Haven, W. T., 2003. Introduction to the North Carolina Groundwater Recharge Map.
Groundwater Circular Number 19. North Carolina Department of Environment
and Natural Resources. Division of Water Quality, 8 p.
Institute of Hydrology,1980. Low flow studies. Institute of Hydrology, Wallingford, UK.
Langley, W.G., J. Daniels, and S. Oza, 2015. Sorption Evaluation of the. Roxboro Steam
Electric Plant. Charlotte Department of Civil and Environmental Engineering,
report prepared for SynTerra.
Legrand, H., 1988. Piedmont and Blue Ridge. Back, W., J. Rosenshein, and P. Seaber,
eds. 1988. Hydrogeology: The Geology of North America 0-2: The Decade of
North American Geology. Boulder, Colorado: Geological Society of America.
Geological Society of America. p. 201-208.
Mau, D.P. and Winter, T.C., 1997. Estimating ground -water recharge from streamflow
hydrographs for a small mountain watershed in a temperate humid climate, New
Hampshire, USA. Groundwater, 35(2), p. 291-304.
Page 7-1

Updated Groundwater Flow And Transport Modeling Report December 2019
Mayo Steam Electric Plant, Roxboro, North Carolina
McDonald, M.G. and A.W. Harbaugh, 1988. A Modular Three -Dimensional Finite -
Difference Ground -Water Flow Model, U.S. Geological Survey Techniques of
Water Resources Investigations, book 6, 586 p.
Miller, J.A., 1990. Ground Water Atlas of the U.S. South Carolina and vicinity (HA 730-
G). USGS. http://pubs.usgs.gov/ha/ha730/ch_9/index.html.
Niswonger, R.G., S. Panday, and I. Motomu, 2011. MODFLOW-NWT, A Newton
formulation for MODFLOW-2005, U.S. Geological Survey Techniques and
Methods 6-A37.
Radcliffe, D.E., L.T. West, L.A. Morris, and T. C. Rasmussen., 2006. Onsite Wastewater
and Land Application Systems: Consumptive Use and Water Quality, University
of Georgia.
Rutledge, A.T. and Mesko, T.O., 1996. Estimated hydrologic characteristics of shallow
aquifer systems in the Valley and Ridge, the Blue Ridge, and the Piedmont
physiographic provinces based on analysis of streamflow recession and base flow.
US Geological Survey. No. 1422-B, p. B1-B58.
Snipes, D.S., Burnett, L.L., Wylie, J.A., Sacks, L.A., Heaton, S.B., Dalton, G.A., and Israel,
B.A., 1984. Indicators of ground -water quality and yield for a public water
supply in rock fracture zones of the piedmont: Clemson, S.C., Water Resources
Research Institute Report 115, 80 p.
SynTerra, 2014. Drinking Water Well and Receptor Survey for Mayo Steam Electric Plant,
NPDES Permit# NC0038377. September 30, 2014.
SynTerra, 2015a. Comprehensive Site Assessment Report, Mayo Steam Electric Plant,
Roxboro, NC. September 2, 2015.
SynTerra, 2015b. Corrective Action Plan, Part 1 — Mayo Steam Electric Plant, Roxboro,
NC. December 1, 2015.
SynTerra, 2016. Corrective Action Plan, Part 2 — Mayo Steam Electric Plant, Roxboro, NC.
February 29, 2016.
SynTerra, 2017, 2017 Comprehensive Site Assessment Update, Mayo Steam Electric Plant.
October 31, 2017.
Page 7-2

Updated Groundwater Flow And Transport Modeling Report December 2019
Mayo Steam Electric Plant, Roxboro, North Carolina
SynTerra, 2018, Preliminary Updated Groundwater Flow and Transport Modeling
Report for Mayo Steam Electric Plant, Roxboro. November 2018, Revised March
2019.
SynTerra, 2019a, Ash Basin Pumping Test Report for Mayo Steam Electric Plant - Duke
Energy Progress, LLC - Roxboro, North Carolina. January 28, 2019.
SynTerra, 2019b, Pumping Test Numerical Simulation Report for Mayo Steam Electric
Plant, Roxboro, NC. to be submitted.
SynTerra, 2019c, Corrective Action Plan Update Mayo Steam Electric Plant - Duke
Energy Progress, LLC - Roxboro, North Carolina. December 2019.
Tiedeman, C.R. and P.A. Hsieh, 2001. Assessing an open hole aquifer test in fractured
crystalline rock. Ground Water, v. 39, n.1, p. 68-78.
Trapp, H. and M.A. Horn, 1997. Ground Water Atlas of the U.S. North Carolina and
vicinity (HA 730-L). USGS. http:Hi2ubs.usgs.gov/ha/ha730/ch 1/L-text4.html.
Treece, M.W, Jr., Bales, J.D., and Moreau, D.H., 1990. North Carolina water supply and
use, in National water summary 1987 Hydrologic events and water supply and
use: U.S. Geological Survey Water -Supply Paper 2350, p. 393-400.
USGS, 1987. North Carolina Water Supply and Use, in "National Water Summary 1987 -
Hydrologic Events and Water Supply and Use". USGS Water -Supply Paper 2350,
p. 393-400.
USGS, 1995. North Carolina; Estimated Water Use in North Carolina, 1995. USGS Fact
Sheet FS-087-97.
WSP USA, May 8, 2015, Aerial Topographic Survey Mayo Plant.
Zheng, C. and P.P. Wang, 1999. MT3DMS: A Modular Three -Dimensional Multi -
Species Model for Simulation of Advection, Dispersion and Chemical Reactions
of Contaminants in Groundwater Systems: Documentation and User's Guide,
SERDP-99-1, U.S. Army Engineer Research and Development Center, Vicksburg,
MS.
Page 7-3

Updated Groundwater Flow And Transport Modeling Report December 2019
Mayo Steam Electric Plant, Roxboro, North Carolina
FIGURES

DUKE ENERGY PROGRESS �— CRUTCHFIELD T
� J-. PROPERTY LINE BRANCH �. �
ASH BASIN COMPLIANCE
BOUNDARY
�^� o 60' RIGHT-OF-WAY �1 NORTH CAROLINA-VIRGINIA STATE LINE
HALIFAX COUNTY (APPROXIMATE)
•PERSON COUNTYRSO�j Cd
\ _
Jam` .o ° ,�((� '� = = -' - _ _ � �.�+�.-•
DAMV:
APPROXIMATE ASH BASIN
WASTE BOUNDARY
- - 100' HWY 501
` ' RIGHT-OF-WAY
1 L _•c6FGD
\ 100' RAILROAD 1 SETTLING ..
\; RIGHT-OF-WAY ii BASIN
_CCP MONOFILL�� 1' - j APPROXIMATE 1981 .(WASTE WATER/
1 1 FGD /- ' yl - C&D LANDFILL TREATMENT
PONDS AREA (CLOSED) BASIN)
LINED RETENTION / /1 \� LAKE _ -
r� BASIN AREA �� / OUTFLOW '
G, APPROXIMATE ASH BASIN ) POWER PLANT `
WASTE BOUNDARY
I' / GYPSUM STORAGE �G
1 ` COAL PAD AREA
�^;``` I1li PILE
RAGE
i� AREA _\l �.
GROUNDWATER DIVIDE
- �..Q cif• �� I � � ,��� + � r
WATER SUPPLY WELLS �♦ �� I� C; .�'
SOURCE:
2016 USGS TOPOGRAPHIC MAP, CLUSTER SPRINGS
QUADRANGLE, QUAD ID: 36078E8, OBTAINED FROM THE USGS STORE AT
https://store.usgs.gov/map-locator.
DUKE PERSON
COUNTY
ENERGY W,NSTON-SA EM
PROGRESS _R^
FIGURE 1-1
USGS LOCATION MAP
UPDATED GROUNDWATER
FLOW AND TRANSPORT MODELING
REPORT
MAYO STEAM ELECTRIC PLANT
ROXBORO, NORTH CAROLINA
DRAWN BY: A. ROBINSON
DATE: 05/02/2019
GRAPHICSCALE
REVISED BY: R. KIEKHAEFER
DATE: 12/13/2019
1,100 0 1,100
CHECKED BY: P. ALTMAN
DATE: 12/13/2019
(IN FEET)
APPROVED BY: J. WYLIE
DATE: 12/13/2019
PROJECT MANAGER:J. WYLIE

lz �i • '•
1�
ir•♦ ,
,nr 1
V ♦ �
LEGEND
e WATER SUPPLY WELLS
r-^ DAM
GROUNDWATER DIVIDE
ASH BASIN WASTE BOUNDARY
- - - - ASH BASIN COMPLIANCE BOUNDARY
- - - - RIGHT-OF-WAY (DUKE ENERGY PROPERTY)
DUKE ENERGY PROGRESS MAYO PLANT SITE
BOUNDARY
FLOW AND TRANSPORT MODEL BOUNDARY
NOTES:
ALL BOUNDARIES ARE APPROXIMATE.
PROPERTY BOUNDARY PROVIDED BY DUKE ENERGY PROGRESS.
AERIAL PHOTOGRAPHY OBTAINED FROM ESRI ONLINE ON JUNE 10, 2019.
AERIAL WAS COLLECTED ON FEBRUARY 6, 2017.
DRAWING HAS BEEN SET WITH A PROJECTION OF NORTH CAROLINA STATE
PLANE COORDINATE SYSTEM FIPS 3200 (NAD83).
('DUKE
ENERGY
PROGRESS
1116rip
synTena
GRAPHIC SCALE
1,250 0 1,250 2,500
(IN FEET)
DRAWN BY: R.YU DATE: 10/25/2019
REVISED BY: B. ELLIOTT DATE: 12/13/2019
CHECKED BY: P. ALTMAN DATE: 12/13/2019
APPROVED BY: J. WYLIE DATE: 12/13/2019
PROJECT MANAGER: J. WYLIE
www.svnterracorD.COM
FIGURE 4-1
NUMERICAL MODEL DOMAIN
UPDATED GROUNDWATER FLOW AND TRANSPORT
MODELING REPORT
MAYO STEAM ELECTRIC PLANT
ROXBORO, NORTH CAROLINA

Feet U.S. Surve
0 1000
2000
LEGEND
ASH
SAPROLITE
- TRANSITION ZONE
UPPER BEDROCK
BEDROCK
DUKE
DRAWN BY: R. YU
DATE: 10/25/2019
FIGURE 4-2
FENCE DIAGRAM OF THE 3D
ENERGY®
REVISED BY: W. PRATER
DATE: 12/12/2019
HYDROSTRATIGRAPHIC MODEL
PROGRESS
CHECKED BY: P. ALTMAN
DATE: 12/12/2019
USED TO CONSTRUCT THE MODEL GRID
APPROVED BY: J. WYLIE
PROJECT MANAGER: J. WYLIE
DATE: 12/12/2019
UPDATED GROUNDWATER FLOW AND
TRANSPORT MODELING REPORT
'
MAYO STEAM ELECTRICAL PLANT
ROXBORO, NORTH CAROLINA
synTerra
www.synterracorp.com

I

1.0
"•
W
liffts DUKE
*'ENERGY
PROGRESS
16'
synTen-a
0.2
1 m
0.001 0.01 0.1 1
K (ft/d)
DRAWN BY: R. YLI
DATE: 10/25/2019
REVISED BY: W. PRATER
DATE: 12/12/2019
CHECKED BY: P. ALTMAN
DATE: 12/12/2019
APPROVED BY: J. WYLIE
DATE: 12/12/2019
PROJECT MANAGER: J. WYLIE
www.synterracorp.com
10 100
O All Sites
♦ Mayo slug test
Mayo pumping test
analytical solution
O Mayo pumping test
numerical solution
O Model
Analytical and numerical
solutions for a coal ash
pumping test at Mayo
are included and show
agreement with the slug
test values.
FIGURE 4-4
HYDRAULIC CONDUCTIVITY ESTIMATED FROM SLUG TESTS PERFORMED
IN COAL ASH AT 14 SITES IN NORTH CAROLINA
UPDATED GROUNDWATER FLOW AND TRANSPORT MODELING REPORT
MAYO STEAM ELECTRICAL PLANT
ROXBORO, NORTH CAROLINA

1.0
F:
W.
MIA
0.2
0.0 +-
0.001
Ira]
0.01 0.1 1 10 100 1000
o All Piedmont Sites
0 Mayo slug test
♦ Model
K (ft/d)
11(fts DUKE DRAWN BY: R. YLI DATE: 10/25/2019 FIGURE 4-5
ENERGY REVISED BY: W. PRATER DATE: 12/12/2019 HYDRAULIC CONDUCTIVITY ESTIMATED FROM SLUG TESTS PERFORMED
PROGRESS CHECKED BY: P. ALTMAN DATE: 12/12/2019 IN SAPROLITE AT 10 PIEDMONT SITES IN NORTH CAROLINA
APPROVED BY: J. WYLIE DATE: 12/12/2019
PROJECT MANAGER: J. WYLIE UPDATED GROUNDWATER FLOW AND TRANSPORT MODELING REPORT
'7 MAYO STEAM ELECTRICAL PLANT
synTerra www.synterracorp.com
ROXBORO, NORTH CAROLINA

1.0
.A
0.001 0.01 0.1 1 10 100
o All Piedmont Sites
L Mayo slug test
O Model
K (ft/d)
�' DUKE DRAWN BY: R. YU DATE: 10/25/2019 FIGURE 4-6
ENERGY REVISED BY: W. PRATER DATE: 12/12/2019 HYDRAULIC CONDUCTIVITY ESTIMATED FROM SLUG TESTS PERFORMED
PROGRES CHECKED BY: P. ALTMAN DATE: 12/12/2019 IN THE TRANSITION ZONE AT 10 PIEDMONT SITES IN NORTH CAROLINA
APPROVED BY: J. WYLIE DATE: 12/12/2019
PROJECT MANAGER: J. WYLIE UPDATED GROUNDWATER FLOW AND TRANSPORT MODELING REPORT
10 1 MAYO STEAM ELECTRICAL PLANT
$Terra www.synterracorp.com
ROXBORO, NORTH CAROLINA

liffts DUKE
*'ENERGY
PROGRESS
16'
synTen-a
1.0
A•
M.
MIA
0.2
0.0 /
0.00001 0.0001
o All Piedmont Sites
A Mayo slug tests
♦ Model
mew
0.001 0.01 0.1 1 10 100 Each model value
corresponds to main
background values in
K (ft/d) the model layer intervals
used for calibration.
DRAWN BY: R. YLI
DATE: 10/25/2019
REVISED BY: W. PRATER
DATE: 12/12/2019
CHECKED BY: P. ALTMAN
DATE: 12/12/2019
APPROVED BY: J. WYLIE
DATE: 12/12/2019
PROJECT MANAGER: J. WYLIE
www.synterracorp.com
FIGURE 4-7
HYDRAULIC CONDUCTIVITY ESTIMATED FROM SLUG TESTS PERFORMED
IN THE BEDROCK AT 10 PIEDMONT SITES IN NORTH CAROLINA
UPDATED GROUNDWATER FLOW AND TRANSPORT MODELING REPORT
MAYO STEAM ELECTRICAL PLANT
ROXBORO, NORTH CAROLINA

LEGEND
RECHARGE RATE
- 0.0005 ft/d
0.001 ft/d
- 0.002 ft/d
0.003 ft/d
0.004 ft/d
FLOW AND TRANSPORT MODEL
NOTES:
ALL BOUNDARIES ARE APPROXIMATE.
AERIAL PHOTOGRAPHY OBTAINED FROM ESRI ONLINE ON JUNE 10, 2019.
AERIAL WAS COLLECTED ON FEBRUARY 6, 2017.
DRAWING HAS BEEN SET WITH A PROJECTION OF NORTH CAROLINA STATE
PLANE COORDINATE SYSTEM FIPS 3200 (NAD83).
tEDUKE
ERGY
PROGRESS
0
synTerra
GRAPHIC SCALE
1,200 0 1,200 2,400
(IN FEET)
DRAWN BY: R. YU DATE: 10/25/2019
REVISED BY: B. ELLIOTT DATE: 12/13/2019
CHECKED BY: P. ALTMAN DATE: 12/13/2019
APPROVED BY: J. WYLIE DATE: 12/13/2019
PROJECT MANAGER: J. WYLIE
FIGURE 4-8
DISTRIBUTION OF MODEL RECHARGE ZONES
UPDATED GROUNDWATER FLOW AND TRANSPORT
MODELING REPORT
MAYO STEAM ELECTRIC PLANT
ROXBORO, NORTH CAROLINA

LEGEND
DRAINS
FLOW AND TRANSPORT MODEL BOUNDARY
EXTERNAL BOUNDARY
GENERAL HEAD BOUNDARIES
NOTES:
ALL BOUNDARIES ARE APPROXIMATE.
STREAMS ARE REPRESENTED AS DRAINS. LAKES, ASH BASIN WATER, AND
THE OUTFLOW CHANNELARE REPRESENTED AS GENERAL HEAD
BOUNDARIES WITH ASTAGE EQUAL TO THE SURFACE WATER OR ASH BASIN
WATER ELEVATION. THE HEAD IS SPECIFIED TO BE IN THE MIDDLE OF THE
TRANSITION ZONE ALONG THE EXTERNAL BOUNDARY IN UPLAND AREAS.
AERIAL PHOTOGRAPHY OBTAINED FROM ESRI ONLINE ON JUNE 10, 2019.
AERIAL WAS COLLECTED ON FEBRUARY 6, 2017.
DRAWING HAS BEEN SET WITH A PROJECTION OF NORTH CAROLINA STATE
PLANE COORDINATE SYSTEM FIPS 3200 (NAD83).
It
ENERGY
PROGRESS
1116rip
synTena
GRAPHIC SCALE
1,200 0 1,200 2,400
(IN FEET)
DRAWN BY: R.YU DATE: 10/25/2019
REVISED BY: R. KIEKHAEFER DATE: 12/13/2019
CHECKED BY: P. ALTMAN DATE: 12/13/2019
APPROVED BY: J. WYLIE DATE: 12/13/2019
PROJECT MANAGER: J. WYLIE
www.svnterracorD.COM
FIGURE 4-9
MODEL SURFACE WATER FEATURES
UPDATED GROUNDWATER FLOW AND TRANSPORT
MODELING REPORT
MAYO STEAM ELECTRIC PLANT
ROXBORO, NORTH CAROLINA

II 1
77 1,
77 ��
10
1
'v •------�'l.N
LEGEND
WATER SUPPLY WELLS
ASH BASIN WASTE BOUNDARY
— - — - ASH BASIN COMPLIANCE BOUNDARY
— - — - RIGHT-OF-WAY (DUKE ENERGY PROPERTY)
— - DUKE ENERGY PROGRESS MAYO PLANT SITE
BOUNDARY
FLOW AND TRANSPORT MODEL BOUNDARY
NOTES:
ALL BOUNDARIES ARE APPROXIMATE.
PROPERTY BOUNDARY PROVIDED BY DUKE ENERGY PROGRESS.
AERIAL PHOTOGRAPHY OBTAINED FROM ESRI ONLINE ON JUNE 10, 2019.
AERIAL WAS COLLECTED ON FEBRUARY 6, 2017.
DRAWING HAS BEEN SET WITH A PROJECTION OF NORTH CAROLINA STATE
PLANE COORDINATE SYSTEM FIPS 3200 (NAD83).
.�-----�
44*DUKE
ENERGY
PROGRESS
1116rip
synTena
,r
v14.2
r y J
Z !
GRAPHIC SCALE
1,200 0 1,200 2,400
(IN FEET)
DRAWN BY: R.YU DATE: 10/25/2019
REVISED BY: B. ELLIOTT DATE: 12/13/2019
CHECKED BY: P. ALTMAN DATE: 12/13/2019
APPROVED BY: J. WYLIE DATE: 12/13/2019
PROJECT MANAGER: J. WYLIE
www.svnterracorD.COM
FIGURE 4-10
WATER SUPPLY WELLS IN MODEL AREA
UPDATED GROUNDWATER FLOW AND TRANSPORT
MODELING REPORT
MAYO STEAM ELECTRIC PLANT
ROXBORO, NORTH CAROLINA

LEGEND
HYDRAULIC CONDUCTIVITY
DUKE
t'ENERGY
1,200 GRAPHIC SCALE
0 1,200 2,400
(IN FEET)
FLOW AND TRANSPORT MODEL BOUNDARY
PROGRESS
DRAWN BY: R. YU DATE: 10/25/2019
REVISED BY: B. ELLIOTT DATE: 12/13/2019
NOTES:
CHECKED BY: P. ALTMAN DATE: 12/13/2019
APPROVED BY: J. WYLIE DATE: 12/13/2019
ALL BOUNDARIES ARE APPROXIMATE.
synTerra
PROJECT MANAGER: J. WYLIE
ZONES SHOWN WERE USED TO DEFINE HORIZONTAL HYDRAULIC
www.synterracorp.com
CONDUCTIVITY AND HORIZONTAL TO VERTICAL ANISOTROPY IN THE
MODEL. HYDRAULIC CONDUCTIVITY VALUES AND RATIOS OF HORIZONTAL
FIGURE 5-la
TO VERTICAL ANISOTROPY FOR MODEL LAYERS ARE LISTED IN TABLE 5-2.
MODEL HYDRAULIC CONDUCTIVITY ZONES IN ASH LAYER 1
AERIAL PHOTOGRAPHY OBTAINED FROM ESRI ONLINE ON JUNE 10, 2019.
UPDATED GROUNDWATER FLOW AND TRANSPORT
AERIAL WAS COLLECTED ON FEBRUARY 6, 2017.
MODELING REPORT
DRAWING HAS BEEN SET WITH A PROJECTION OF NORTH CAROLINA STATE
PLANE COORDINATE SYSTEM FIPS 3200 (NAD83).
MAYO STEAM ELECTRIC PLANT
ROXBORO, NORTH CAROLINA

LEGEND
HYDRAULIC CONDUCTIVITY
FLOW AND TRANSPORT MODEL
NOTES:
ALL BOUNDARIES ARE APPROXIMATE
ZONES SHOWN WERE USED TO DEFINE HORIZONTAL HYDRAULIC
CONDUCTIVITY AND HORIZONTAL TO VERTICAL ANISOTROPY IN THE
MODEL. HYDRAULIC CONDUCTIVITY VALUES AND RATIOS OF HORIZONTAL
TO VERTICAL ANISOTROPY FOR MODEL LAYERS ARE LISTED IN TABLE 5-2.
AERIAL PHOTOGRAPHY OBTAINED FROM ESRI ONLINE ON JUNE 10, 2019.
AERIAL WAS COLLECTED ON FEBRUARY 6, 2017.
DRAWING HAS BEEN SET WITH A PROJECTION OF NORTH CAROLINA STATE
PLANE COORDINATE SYSTEM FIPS 3200 (NAD83).
�' DUKE GRAPHIC SCALE
1,200 0 1,200 2,400
ENERGY(IN FEET)
PROGRESS DRAWN BY: R.YU DATE: 10/25/2019
,�Vjp REVISED BY: B. ELLIOTT DATE: 12/13/2019
CHECKED BY: P. ALTMAN DATE: 12/13/2019
APPROVED BY: J. WYLIE DATE: 12/13/2019
s mTerra PROJECT MANAGER: J. WYLIE
www.svnterracorD.com
FIGURE 5-lb
MODEL HYDRAULIC CONDUCTIVITY ZONES IN ASH LAYER 2
UPDATED GROUNDWATER FLOW AND TRANSPORT
MODELING REPORT
MAYO STEAM ELECTRIC PLANT
ROXBORO, NORTH CAROLINA

LEGEND
HYDRAULIC CONDUCTIVITY
DUKE
t'ENERGY
1,200 GRAPHIC SCALE
0 1,200 2,400
(IN FEET)
-
FLOW AND TRANSPORT MODEL BOUNDARY
PROGRESS
DRAWN BY: R. YU DATE: 10/25/2019
REVISED BY: B. ELLIOTT DATE: 12/13/2019
NOTES:
CHECKED BY: P. ALTMAN DATE: 12/13/2019
APPROVED BY: J. WYLIE DATE: 12/13/2019
ALL BOUNDARIES ARE APPROXIMATE.
synTerra
PROJECT MANAGER: J. WYLIE
ZONES SHOWN WERE USED TO DEFINE HORIZONTAL HYDRAULIC
www.synterracorp.com
CONDUCTIVITY AND HORIZONTAL TO VERTICAL ANISOTROPY IN THE
MODEL. HYDRAULIC CONDUCTIVITY VALUES AND RATIOS OF HORIZONTAL
FIGURE 5-1c
TO VERTICAL ANISOTROPY FOR MODEL LAYERS ARE LISTED IN TABLE 5-2.
MODEL HYDRAULIC CONDUCTIVITY ZONES IN ASH LAYER 3
AERIAL PHOTOGRAPHY OBTAINED FROM ESRI ONLINE ON JUNE 10, 2019.
UPDATED GROUNDWATER FLOW AND TRANSPORT
AERIAL WAS COLLECTED ON FEBRUARY 6, 2017.
MODELING REPORT
DRAWING HAS BEEN SET WITH A PROJECTION OF NORTH CAROLINA STATE
PLANE COORDINATE SYSTEM FIPS 3200 (NAD83).
MAYO STEAM ELECTRIC PLANT
ROXBORO, NORTH CAROLINA

LEGEND
HYDRAULIC CONDUCTIVITY
�' DUKE
ENERGY(IN
GRAPHIC SCALE
1,200 O 1,200 2,400
FEET)
FLOW AND TRANSPORT MODEL
PROGRESS
DRAWN BY: R. YU DATE: 10/25/2019
REVISED BY: B. ELLIOTT DATE: 12/13/2019
NOTES:
CHECKED BY: P. ALTMAN DATE: 12/13/2019
APPROVED BY: J. WYLIE DATE: 12/13/2019
ALL BOUNDARIES ARE APPROXIMATE.
synTerra
PROJECT MANAGER: J. WYLIE
ZONES SHOWN WERE USED TO DEFINE HORIZONTAL HYDRAULIC
www.synterracorp.com
CONDUCTIVITY AND HORIZONTAL TO VERTICAL ANISOTROPY IN THE
MODEL. HYDRAULIC CONDUCTIVITY VALUES AND RATIOS OF HORIZONTAL
FIGURE 5-ld
TO VERTICAL ANISOTROPY FOR MODEL LAYERS ARE LISTED IN TABLE 5-2.
MODEL HYDRAULIC CONDUCTIVITY ZONES IN ASH LAYER 4
AERIAL PHOTOGRAPHY OBTAINED FROM ESRI ONLINE ON JUNE 10, 2019.
UPDATED GROUNDWATER FLOW AND TRANSPORT
AERIAL WAS COLLECTED ON FEBRUARY 6, 2017.
MODELING REPORT
DRAWING HAS BEEN SET WITH A PROJECTION OF NORTH CAROLINA STATE
PLANE COORDINATE SYSTEM FIPS 3200 (NAD83).
MAYO STEAM ELECTRIC PLANT
ROXBORO, NORTH CAROLINA

LEGEND
HYDRAULIC CONDUCTIVITY
�' DUKE
ENERGY(IN
GRAPHIC SCALE
1,200 O 1,200 2,400
FEET)
FLOW AND TRANSPORT MODEL
PROGRESS
DRAWN BY: R.YU DATE: 10/25/2019
REVISED BY: B. ELLIOTT DATE: 12/13/2019
NOTES:
CHECKED BY: P. ALTMAN DATE: 12/13/2019
APPROVED BY: J. WYLIE DATE: 12/13/2019
ALL BOUNDARIES ARE APPROXIMATE.
synTerra
PROJECT MANAGER: J. WYLIE
ZONES SHOWN WERE USED TO DEFINE HORIZONTAL HYDRAULIC
www.synterracorp.com
CONDUCTIVITY AND HORIZONTAL TO VERTICAL ANISOTROPY IN THE
MODEL. HYDRAULIC CONDUCTIVITY VALUES AND RATIOS OF HORIZONTAL
FIGURE 5-le
TO VERTICAL ANISOTROPY FOR MODEL LAYERS ARE LISTED IN TABLE 5-2.
MODEL HYDRAULIC CONDUCTIVITY ZONES IN ASH LAYER 5
AERIAL PHOTOGRAPHY OBTAINED FROM ESRI ONLINE ON JUNE 10, 2019.
UPDATED GROUNDWATER FLOW AND TRANSPORT
AERIAL WAS COLLECTED ON FEBRUARY 6, 2017.
MODELING REPORT
DRAWING HAS BEEN SET WITH A PROJECTION OF NORTH CAROLINA STATE
PLANE COORDINATE SYSTEM FIPS 3200 (NAD83).
MAYO STEAM ELECTRIC PLANT
ROXBORO, NORTH CAROLINA

LEGEND
HYDRAULIC CONDUCTIVITY
�' DUKE
ENERGY(IN
GRAPHIC SCALE
1,200 O 1,200 2,400
FEET)
FLOW AND TRANSPORT MODEL
PROGRESS
DRAWN BY: R. YU DATE: 10/25/2019
REVISED BY: B. ELLIOTT DATE: 12/13/2019
NOTES:
CHECKED BY: P. ALTMAN DATE: 12/13/2019
APPROVED BY: J. WYLIE DATE: 12/13/2019
ALL BOUNDARIES ARE APPROXIMATE.
synTerra
PROJECT MANAGER: J. WYLIE
ZONES SHOWN WERE USED TO DEFINE HORIZONTAL HYDRAULIC
www.synterracorp.com
CONDUCTIVITY AND HORIZONTAL TO VERTICAL ANISOTROPY IN THE
MODEL. HYDRAULIC CONDUCTIVITY VALUES AND RATIOS OF HORIZONTAL
FIGURE 5-lf
TO VERTICAL ANISOTROPY FOR MODEL LAYERS ARE LISTED IN TABLE 5-2.
MODEL HYDRAULIC CONDUCTIVITY ZONES IN ASH LAYER 6
AERIAL PHOTOGRAPHY OBTAINED FROM ESRI ONLINE ON JUNE 10, 2019.
UPDATED GROUNDWATER FLOW AND TRANSPORT
AERIAL WAS COLLECTED ON FEBRUARY 6, 2017.
MODELING REPORT
DRAWING HAS BEEN SET WITH A PROJECTION OF NORTH CAROLINA STATE
PLANE COORDINATE SYSTEM FIPS 3200 (NAD83).
MAYO STEAM ELECTRIC PLANT
ROXBORO, NORTH CAROLINA

LEGEND
HYDRAULIC CONDUCTIVITY
FLOW AND TRANSPORT MODEL BOUNDARY
NOTES:
ALL BOUNDARIES ARE APPROXIMATE
ZONES SHOWN WERE USED TO DEFINE HORIZONTAL HYDRAULIC
CONDUCTIVITY AND HORIZONTAL TO VERTICAL ANISOTROPY IN THE
MODEL. HYDRAULIC CONDUCTIVITY VALUES AND RATIOS OF HORIZONTAL
TO VERTICAL ANISOTROPY FOR MODEL LAYERS ARE LISTED IN TABLE 5-2.
AERIAL PHOTOGRAPHY OBTAINED FROM ESRI ONLINE ON JUNE 10, 2019.
AERIAL WAS COLLECTED ON FEBRUARY 6, 2017.
DRAWING HAS BEEN SET WITH A PROJECTION OF NORTH CAROLINA STATE
PLANE COORDINATE SYSTEM FIPS 3200 (NAD83).
�' DUKE GRAPHIC SCALE
1,200 0 1,200 2,400
ENERGY(IN FEET)
PROGRESS DRAWN BY: R.YU DATE: 10/25/2019
REVISED BY: B. ELLIOTTDATE: 12/13/2019
CHECKED BY: P. A -MAN DATE: 12/13/2019
APPROVED BY: J. WYLIE DATE: 12/13/2019
s mTerra PROJECT MANAGER: J. WYLIE
www.svnterracorD.com
FIGURE 5-lg
MODEL HYDRAULIC CONDUCTIVITY ZONES IN ASH LAYER 7
UPDATED GROUNDWATER FLOW AND TRANSPORT
MODELING REPORT
MAYO STEAM ELECTRIC PLANT
ROXBORO, NORTH CAROLINA

LEGEND
HYDRAULIC CONDUCTIVITY
�' DUKE
ENERGY(IN
GRAPHIC SCALE
1,200 O 1,200 2,400
FEET)
FLOW AND TRANSPORT MODEL BOUNDARY
PROGRESS
DRAWN BY: R. YU DATE: 10/25/2019
REVISED BY: B. ELLIOTT DATE: 12/13/2019
NOTES:
�Vjp
CHECKED BY: P. ALTMAN DATE: 12/13/2019
APPROVED BY: J. WYLIE DATE: 12/13/2019
ALL BOUNDARIES ARE APPROXIMATE.
synTerra
PROJECT MANAGER: J. WYLIE
ZONES SHOWN WERE USED TO DEFINE HORIZONTAL HYDRAULIC
www.synterracorp.com
CONDUCTIVITY AND HORIZONTAL TO VERTICAL ANISOTROPY IN THE
MODEL. HYDRAULIC CONDUCTIVITY VALUES AND RATIOS OF HORIZONTAL
FIGURE 5-1 h
TO VERTICAL ANISOTROPY FOR MODEL LAYERS ARE LISTED IN TABLE 5-2.
MODEL HYDRAULIC CONDUCTIVITY ZONES IN ASH LAYER 8
AERIAL PHOTOGRAPHY OBTAINED FROM ESRI ONLINE ON JUNE 10, 2019.
UPDATED GROUNDWATER FLOW AND TRANSPORT
AERIAL WAS COLLECTED ON FEBRUARY 6, 2017.
MODELING REPORT
DRAWING HAS BEEN SET WITH A PROJECTION OF NORTH CAROLINA STATE
PLANE COORDINATE SYSTEM FIPS 3200 (NAD83).
MAYO STEAM ELECTRIC PLANT
ROXBORO, NORTH CAROLINA

LEGEND
HYDRAULIC CONDUCTIVITY
FLOW AND TRANSPORT MODEL BOUNDARY
NOTES:
ALL BOUNDARIES ARE APPROXIMATE
ZONES SHOWN WERE USED TO DEFINE HORIZONTAL HYDRAULIC
CONDUCTIVITY AND HORIZONTAL TO VERTICAL ANISOTROPY IN THE MODEL.
HYDRAULIC CONDUCTIVITY VALUES AND RATIOS OF HORIZONTAL TO
VERTICAL ANISOTROPY FOR MODEL LAYERS ARE LISTED IN TABLE 5-2.
AERIAL PHOTOGRAPHY OBTAINED FROM ESRI ONLINE ON JUNE 10, 2019.
AERIAL WAS COLLECTED ON FEBRUARY 6, 2017.
DRAWING HAS BEEN SET WITH A PROJECTION OF NORTH CAROLINA STATE
PLANE COORDINATE SYSTEM FIPS 3200 (NAD83).
('DUKE
ENERGY
PROGRESS
1116rip
synTena
GRAPHIC SCALE
1,200 0 1,200 2,400
(IN FEET)
DRAWN BY: R.YU DATE: 10/25/2019
REVISED BY: R. KIEKHAEFER DATE: 12/20/2019
CHECKED BY: P. ALTMAN DATE: 12/20/2019
APPROVED BY: J. WYLIE DATE: 12/20/2019
PROJECT MANAGER: J. WYLIE
www.svnterracorD.COM
FIGURE 5-li
MODEL HYDRAULIC CONDUCTIVITY ZONES IN
SAPROLITE LAYER 9
UPDATED GROUNDWATER FLOW AND TRANSPORT MODELING
REPORT
MAYO STEAM ELECTRIC PLANT
ROXBORO, NORTH CAROLINA

LEGEND
HYDRAULIC CONDUCTIVITY
FLOW AND TRANSPORT MODEL BOUNDARY
NOTES:
ALL BOUNDARIES ARE APPROXIMATE
ZONES SHOWN WERE USED TO DEFINE HORIZONTAL HYDRAULIC
CONDUCTIVITY AND HORIZONTAL TO VERTICAL ANISOTROPY IN THE MODEL.
HYDRAULIC CONDUCTIVITY VALUES AND RATIOS OF HORIZONTAL TO
VERTICAL ANISOTROPY FOR MODEL LAYERS ARE LISTED IN TABLE 5-2.
AERIAL PHOTOGRAPHY OBTAINED FROM ESRI ONLINE ON JUNE 10, 2019.
AERIAL WAS COLLECTED ON FEBRUARY 6, 2017.
DRAWING HAS BEEN SET WITH A PROJECTION OF NORTH CAROLINA STATE
PLANE COORDINATE SYSTEM FIPS 3200 (NAD83).
('DUKE
ENERGY
PROGRESS
1116rip
synTena
GRAPHIC SCALE
1,200 0 1,200 2,400
(IN FEET)
DRAWN BY: R.YU DATE: 10/25/2019
REVISED BY: R. KIEKHAEFER DATE: 12/20/2019
CHECKED BY: P. ALTMAN DATE: 12/20/2019
APPROVED BY: J. WYLIE DATE: 12/20/2019
PROJECT MANAGER: J. WYLIE
www.svnterracorD.COM
FIGURE 5-1j
MODEL HYDRAULIC CONDUCTIVITY ZONES IN
SAPROLITE LAYER 10
UPDATED GROUNDWATER FLOW AND TRANSPORT MODELING
REPORT
MAYO STEAM ELECTRIC PLANT
ROXBORO, NORTH CAROLINA

LEGEND
HYDRAULIC CONDUCTIVITY
FLOW AND TRANSPORT MODEL BOUNDARY
NOTES:
ALL BOUNDARIES ARE APPROXIMATE
ZONES SHOWN WERE USED TO DEFINE HORIZONTAL HYDRAULIC
CONDUCTIVITY AND HORIZONTAL TO VERTICAL ANISOTROPY IN THE MODEL.
HYDRAULIC CONDUCTIVITY VALUES AND RATIOS OF HORIZONTAL TO
VERTICAL ANISOTROPY FOR MODEL LAYERS ARE LISTED IN TABLE 5-2.
AERIAL PHOTOGRAPHY OBTAINED FROM ESRI ONLINE ON JUNE 10, 2019.
AERIAL WAS COLLECTED ON FEBRUARY 6, 2017.
DRAWING HAS BEEN SET WITH A PROJECTION OF NORTH CAROLINA STATE
PLANE COORDINATE SYSTEM FIPS 3200 (NAD83).
('DUKE
ENERGY
PROGRESS
1116rip
synTena
GRAPHIC SCALE
1,200 0 1,200 2,400
(IN FEET)
DRAWN BY: R.YU DATE: 10/25/2019
REVISED BY: R. KIEKHAEFER DATE: 12/20/2019
CHECKED BY: P. ALTMAN DATE: 12/20/2019
APPROVED BY: J. WYLIE DATE: 12/20/2019
PROJECT MANAGER: J. WYLIE
www.svnterracorD.COM
FIGURE 5-1k
MODEL HYDRAULIC CONDUCTIVITY ZONES IN
SAPROLITE LAYER 11
UPDATED GROUNDWATER FLOW AND TRANSPORT MODELING
REPORT
MAYO STEAM ELECTRIC PLANT
ROXBORO, NORTH CAROLINA

LEGEND
HYDRAULIC CONDUCTIVITY
FLOW AND TRANSPORT MODEL BOUNDARY
NOTES:
ALL BOUNDARIES ARE APPROXIMATE
ZONES SHOWN WERE USED TO DEFINE HORIZONTAL HYDRAULIC
CONDUCTIVITY AND HORIZONTAL TO VERTICAL ANISOTROPY IN THE MODEL.
HYDRAULIC CONDUCTIVITY VALUES AND RATIOS OF HORIZONTAL TO
VERTICAL ANISOTROPY FOR MODEL LAYERS ARE LISTED IN TABLE 5-2.
AERIAL PHOTOGRAPHY OBTAINED FROM ESRI ONLINE ON JUNE 10, 2019.
AERIAL WAS COLLECTED ON FEBRUARY 6, 2017.
DRAWING HAS BEEN SET WITH A PROJECTION OF NORTH CAROLINA STATE
PLANE COORDINATE SYSTEM FIPS 3200 (NAD83).
('DUKE
ENERGY
PROGRESS
1116rip
synTena
GRAPHIC SCALE
1,200 0 1,200 2,400
(IN FEET)
DRAWN BY: R.YU DATE: 10/25/2019
REVISED BY: R. KIEKHAEFER DATE: 12/20/2019
CHECKED BY: P. ALTMAN DATE: 12/20/2019
APPROVED BY: J. WYLIE DATE: 12/20/2019
PROJECT MANAGER: J. WYLIE
www.svnterracorD.COM
FIGURE 5-11
MODEL HYDRAULIC CONDUCTIVITY ZONES IN
TRANSITION ZONE LAYER 12
UPDATED GROUNDWATER FLOW AND TRANSPORT MODELING
REPORT
MAYO STEAM ELECTRIC PLANT
ROXBORO, NORTH CAROLINA

LEGEND
HYDRAULIC CONDUCTIVITY
FLOW AND TRANSPORT MODEL BOUNDARY
NOTES:
ALL BOUNDARIES ARE APPROXIMATE
ZONES SHOWN WERE USED TO DEFINE HORIZONTAL HYDRAULIC
CONDUCTIVITY AND HORIZONTAL TO VERTICAL ANISOTROPY IN THE MODEL.
HYDRAULIC CONDUCTIVITY VALUES AND RATIOS OF HORIZONTAL TO
VERTICAL ANISOTROPY FOR MODEL LAYERS ARE LISTED IN TABLE 5-2.
AERIAL PHOTOGRAPHY OBTAINED FROM ESRI ONLINE ON JUNE 10, 2019.
AERIAL WAS COLLECTED ON FEBRUARY 6, 2017.
DRAWING HAS BEEN SET WITH A PROJECTION OF NORTH CAROLINA STATE
PLANE COORDINATE SYSTEM FIPS 3200 (NAD83).
('DUKE
ENERGY
PROGRESS
1116rip
synTena
GRAPHIC SCALE
1,200 0 1,200 2,400
(IN FEET)
DRAWN BY: R.YU DATE: 10/25/2019
REVISED BY: R. KIEKHAEFER DATE: 12/20/2019
CHECKED BY: P. ALTMAN DATE: 12/20/2019
APPROVED BY: J. WYLIE DATE: 12/20/2019
PROJECT MANAGER: J. WYLIE
www.svnterracorD.COM
FIGURE 5-1m
MODEL HYDRAULIC CONDUCTIVITY ZONES IN
TRANSITION ZONE LAYER 13
UPDATED GROUNDWATER FLOW AND TRANSPORT MODELING
REPORT
MAYO STEAM ELECTRIC PLANT
ROXBORO, NORTH CAROLINA

LEGEND
HYDRAULIC CONDUCTIVITY
FLOW AND TRANSPORT MODEL BOUNDARY
NOTES:
ALL BOUNDARIES ARE APPROXIMATE
ZONES SHOWN WERE USED TO DEFINE HORIZONTAL HYDRAULIC
CONDUCTIVITY AND HORIZONTAL TO VERTICAL ANISOTROPY IN THE MODEL.
HYDRAULIC CONDUCTIVITY VALUES AND RATIOS OF HORIZONTAL TO
VERTICAL ANISOTROPY FOR MODEL LAYERS ARE LISTED IN TABLE 5-2.
AERIAL PHOTOGRAPHY OBTAINED FROM ESRI ONLINE ON JUNE 10, 2019.
AERIAL WAS COLLECTED ON FEBRUARY 6, 2017.
DRAWING HAS BEEN SET WITH A PROJECTION OF NORTH CAROLINA STATE
PLANE COORDINATE SYSTEM FIPS 3200 (NAD83).
('DUKE
ENERGY
PROGRESS
1116rip
synTena
GRAPHIC SCALE
1,200 0 1,200 2,400
(IN FEET)
DRAWN BY: R.YU DATE: 10/25/2019
REVISED BY: R. KIEKHAEFER DATE: 12/20/2019
CHECKED BY: P. ALTMAN DATE: 12/20/2019
APPROVED BY: J. WYLIE DATE: 12/20/2019
PROJECT MANAGER: J. WYLIE
www.svnterracorD.COM
FIGURE 5-1n
MODEL HYDRAULIC CONDUCTIVITY ZONES IN UPPER
FRACTURED ROCK LAYER 14
UPDATED GROUNDWATER FLOW AND TRANSPORT MODELING
REPORT
MAYO STEAM ELECTRIC PLANT
ROXBORO, NORTH CAROLINA

LEGEND
HYDRAULIC CONDUCTIVITY
FLOW AND TRANSPORT MODEL BOUNDARY
NOTES:
ALL BOUNDARIES ARE APPROXIMATE
ZONES SHOWN WERE USED TO DEFINE HORIZONTAL HYDRAULIC
CONDUCTIVITY AND HORIZONTAL TO VERTICAL ANISOTROPY IN THE MODEL.
HYDRAULIC CONDUCTIVITY VALUES AND RATIOS OF HORIZONTAL TO
VERTICAL ANISOTROPY FOR MODEL LAYERS ARE LISTED IN TABLE 5-2.
AERIAL PHOTOGRAPHY OBTAINED FROM ESRI ONLINE ON JUNE 10, 2019.
AERIAL WAS COLLECTED ON FEBRUARY 6, 2017.
DRAWING HAS BEEN SET WITH A PROJECTION OF NORTH CAROLINA STATE
PLANE COORDINATE SYSTEM FIPS 3200 (NAD83).
('DUKE
ENERGY
PROGRESS
1116rip
synTena
GRAPHIC SCALE
1,200 0 1,200 2,400
(IN FEET)
DRAWN BY: R.YU DATE: 10/25/2019
REVISED BY: R. KIEKHAEFER DATE: 12/20/2019
CHECKED BY: P. ALTMAN DATE: 12/20/2019
APPROVED BY: J. WYLIE DATE: 12/20/2019
PROJECT MANAGER: J. WYLIE
www.svnterracorD.COM
FIGURE 5-10
MODEL HYDRAULIC CONDUCTIVITY ZONES IN UPPER
FRACTURED ROCK LAYER 15
UPDATED GROUNDWATER FLOW AND TRANSPORT MODELING
REPORT
MAYO STEAM ELECTRIC PLANT
ROXBORO, NORTH CAROLINA

LEGEND
HYDRAULIC CONDUCTIVITY
FLOW AND TRANSPORT MODEL BOUNDARY
NOTES:
ALL BOUNDARIES ARE APPROXIMATE
ZONES SHOWN WERE USED TO DEFINE HORIZONTAL HYDRAULIC
CONDUCTIVITY AND HORIZONTAL TO VERTICAL ANISOTROPY IN THE MODEL.
HYDRAULIC CONDUCTIVITY VALUES AND RATIOS OF HORIZONTAL TO
VERTICAL ANISOTROPY FOR MODEL LAYERS ARE LISTED IN TABLE 5-2.
AERIAL PHOTOGRAPHY OBTAINED FROM ESRI ONLINE ON JUNE 10, 2019.
AERIAL WAS COLLECTED ON FEBRUARY 6, 2017.
DRAWING HAS BEEN SET WITH A PROJECTION OF NORTH CAROLINA STATE
PLANE COORDINATE SYSTEM FIPS 3200 (NAD83).
('DUKE
ENERGY
PROGRESS
1116rip
synTena
GRAPHIC SCALE
1,200 0 1,200 2,400
(IN FEET)
DRAWN BY: R.YU DATE: 10/25/2019
REVISED BY: R. KIEKHAEFER DATE: 12/20/2019
CHECKED BY: P. ALTMAN DATE: 12/20/2019
APPROVED BY: J. WYLIE DATE: 12/20/2019
PROJECT MANAGER: J. WYLIE
www.svnterracorD.COM
FIGURE 5-1p
MODEL HYDRAULIC CONDUCTIVITY ZONES IN UPPER
FRACTURED ROCK LAYER 16
UPDATED GROUNDWATER FLOW AND TRANSPORT MODELING
REPORT
MAYO STEAM ELECTRIC PLANT
ROXBORO, NORTH CAROLINA

LEGEND
HYDRAULIC CONDUCTIVITY
FLOW AND TRANSPORT MODEL BOUNDARY
NOTES:
ALL BOUNDARIES ARE APPROXIMATE
ZONES SHOWN WERE USED TO DEFINE HORIZONTAL HYDRAULIC
CONDUCTIVITY AND HORIZONTAL TO VERTICAL ANISOTROPY IN THE MODEL.
HYDRAULIC CONDUCTIVITY VALUES AND RATIOS OF HORIZONTAL TO
VERTICAL ANISOTROPY FOR MODEL LAYERS ARE LISTED IN TABLE 5-2.
AERIAL PHOTOGRAPHY OBTAINED FROM ESRI ONLINE ON JUNE 10, 2019.
AERIAL WAS COLLECTED ON FEBRUARY 6, 2017.
DRAWING HAS BEEN SET WITH A PROJECTION OF NORTH CAROLINA STATE
PLANE COORDINATE SYSTEM FIPS 3200 (NAD83).
('DUKE
ENERGY
PROGRESS
1116rip
synTena
GRAPHIC SCALE
1,200 0 1,200 2,400
(IN FEET)
DRAWN BY: R.YU DATE: 10/25/2019
REVISED BY: R. KIEKHAEFER DATE: 12/20/2019
CHECKED BY: P. ALTMAN DATE: 12/20/2019
APPROVED BY: J. WYLIE DATE: 12/20/2019
PROJECT MANAGER: J. WYLIE
www.svnterracorD.COM
FIGURE 5-1q
MODEL HYDRAULIC CONDUCTIVITY ZONES IN LOWER
FRACTURED ROCK LAYERS 17 AND 18
UPDATED GROUNDWATER FLOW AND TRANSPORT MODELING
REPORT
MAYO STEAM ELECTRIC PLANT
ROXBORO, NORTH CAROLINA

LEGEND
HYDRAULIC CONDUCTIVITY
FLOW AND TRANSPORT MODEL BOUNDARY
NOTES:
ALL BOUNDARIES ARE APPROXIMATE
ZONES SHOWN WERE USED TO DEFINE HORIZONTAL HYDRAULIC
CONDUCTIVITY AND HORIZONTAL TO VERTICAL ANISOTROPY IN THE MODEL.
HYDRAULIC CONDUCTIVITY VALUES AND RATIOS OF HORIZONTAL TO
VERTICAL ANISOTROPY FOR MODEL LAYERS ARE LISTED IN TABLE 5-2.
AERIAL PHOTOGRAPHY OBTAINED FROM ESRI ONLINE ON JUNE 10, 2019.
AERIAL WAS COLLECTED ON FEBRUARY 6, 2017.
DRAWING HAS BEEN SET WITH A PROJECTION OF NORTH CAROLINA STATE
PLANE COORDINATE SYSTEM FIPS 3200 (NAD83).
('DUKE
ENERGY
PROGRESS
1116rip
synTena
GRAPHIC SCALE
1,200 0 1,200 2,400
(IN FEET)
DRAWN BY: R.YU DATE: 10/25/2019
REVISED BY: R. KIEKHAEFER DATE: 12/20/2019
CHECKED BY: P. ALTMAN DATE: 12/20/2019
APPROVED BY: J. WYLIE DATE: 12/20/2019
PROJECT MANAGER: J. WYLIE
www.svnterracorD.COM
FIGURE 5-1r
MODEL HYDRAULIC CONDUCTIVITY ZONES IN
BEDROCK LAYERS 19 - 21
UPDATED GROUNDWATER FLOW AND TRANSPORT MODELING
REPORT
MAYO STEAM ELECTRIC PLANT
ROXBORO, NORTH CAROLINA

Iw7 MT
Column 100
Column 110
Column 125
Column 135

S-1 �♦
S-3 ♦♦
S-2 ,♦♦
♦
'♦♦
Measured
Measured flow
Simulated flow
ID
Description
flow 2017
2019 (9pm)
(9pm)
(9pm)
S-1
West Side Weir
3.9
4
8.2
S-2
East Side Weir
1.8
1.5
3.2
S-3
Stream confluence
28
53
18.6
LEGEND
DUKE
GRAPHIC SCALE
120 0 120 240
DAME
tN E RGY
(IN FEET)
DRAINS
PROGRESS
DRAWN BY: R.YU DATE: 10/25/2019
REVISED BY: R. KIEKHAEFER DATE: 12/16/2019
ASH BASIN WASTE BOUNDARY
��
CHECKED BY: P. ALTMAN DATE: 12/16/2019
APPROVED BY: J. WYLIE DATE: 12/16/2019
— - — - ASH BASIN COMPLIANCE BOUNDARY
synTerra
PROJECT MANAGER: J. WYLIE
www.synterracorp.com
NOTES:
FIGURE 5-2b
ALL BOUNDARIES ARE APPROXIMATE.
OBSERVED AND SIMULATED VOLUMETRIC FLOWRATES
THROUGH AND BELOW THE DAM
FLOWS ARE GIVEN IN GALLONS PER MINUTE (GPM).
UPDATED GROUNDWATER FLOW AND TRANSPORT
AERIAL PHOTOGRAPHY OBTAINED FROM ESRI ONLINE ON JUNE 10, 2019.
MODELING REPORT
AERIAL WAS COLLECTED ON FEBRUARY 6, 2017.
MAYO STEAM ELECTRIC PLANT
DRAWING HAS BEEN SET WITH A PROJECTION OF NORTH CAROLINA STATE
ROXBORO, NORTH CAROLINA
PLANE COORDINATE SYSTEM FIPS 3200 (NAD83).

z
{A
s
O
520
ow
..l
ail
360
360 400 440 480 520 560
Observed heads (ft)
DUKE DRAWN BY: R. YLI DATE: 10/25/2019 FIGURE 5-3
ENERGY REVISED BY: W. PRATER DATE: 12/12/2019 COMPARISON OF OBSERVED AND COMPUTED HEADS FROM THE
PROGRESS CHECKED BY: P. ALTMAN DATE: 12/12/2019 CALIBRATED STEADY STATE FLOW MODEL
APPROVED BY: J. WYLIE DATE: 12/12/2019
PROJECT MANAGER: J. WYLIE UPDATED GROUNDWATER FLOW AND TRANSPORT MODELING REPORT
MAYO STEAM ELECTRICAL PLANT
synTerra www.synterracorp.com
ROXBORO, NORTH CAROLINA

� 0 1
I
i
,s2o
cn •
o � •
•
� • I
•
S�0
•
530
S40
O •
so
570
2 MULLINS LN
o
1
NOTES:
ALL BOUNDARIES ARE APPROXIMATE.
CONTOUR INTERVAL IS 10 FEET. HEAD IS SHOWN FOR MODEL LAYER 13.
RESIDUALSARE EQUAL TO PREDICTED HEAD - OBSERVED HEAD.
THE SIZE OF THE COLOR BAR FOR THE RESIDUAL INDICATES THE VALUE OF
THE RESIDUAL. THE ERROR BAR SYMBOL INDICATES THE SIZE FORA
RESIDUAL OF 9 FEET.
AERIAL PHOTOGRAPHY OBTAINED FROM ESRI ONLINE ON JUNE 10, 2019.
AERIAL WAS COLLECTED ON FEBRUARY 6, 2017.
DRAWING HAS BEEN SET WITH A PROJECTION OF NORTH CAROLINA STATE
PLANE COORDINATE SYSTEM FIPS 3200 (NAD83 AND NAVD88).
LEGEND
ERROR BAR RESIDUALS < 9 ft
AT EACH MONITORING WELL 9 -18 ft
• MONITORING WELLS
HYDRAULIC HEAD (FEET)
ASH BASIN WASTE BOUNDARY
— - ASH BASIN COMPLIANCE BOUNDARY
%' �DUKE ' ENERGY
PROGRESS
1116rip
synTena
GRAPHIC SCALE
610 0 610 1,220
(IN FEET)
DRAWN BY: R. YU DATE: 10/25/2019
REVISED BY: B. ELLIOTT DATE: 12/16/2019
CHECKED BY: P. ALTMAN DATE: 12/16/2019
APPROVED BY: J. WYLIE DATE: 12/16/2019
PROJECT MANAGER: J. WYLIE
FIGURE 5-4
SIMULATED PRE -DECANTING HYDRAULIC HEADS IN THE
TRANSITION ZONE
UPDATED GROUNDWATER FLOW AND TRANSPORT
MODELING REPORT
MAYO STEAM ELECTRIC PLANT
ROXBORO, NORTH CAROLINA

S,
a.
,...�
o-
a
I
{1
I
a �
� 1 �
490
I
NOTES:
ALL BOUNDARIES ARE APPROXIMATE. ARROWS INDICATE INFERRED
DIRECTION ONLY, NOT MAGNITUDE.
CONTOUR INTERVAL IS 10 FEET. HEADS ARE SHOWN IN MODEL LAYER 13.
STREAMS ARE SHOWN AS DRAINS.
AERIAL PHOTOGRAPHY OBTAINED FROM ESRI ONLINE ON JUNE 10, 2019.
AERIAL WAS COLLECTED ON FEBRUARY 6, 2017.
DRAWING HAS BEEN SET WITH A PROJECTION OF NORTH CAROLINA STATE
PLANE COORDINATE SYSTEM FIPS 3200 (NAD83 AND NAVD88).
Pit
N
LEGEND
FLOW WITHIN LOCAL SYSTEM
— FLOW OUTSIDE LOCAL SYSTEM
GROUNDWATER LEAVING LOCAL SYSTEM
ie WATER SUPPLY WELLS
DRAINS
GROUNDWATER DIVIDE
HYDRAULIC HEAD (FEET)
ASH BASIN PONDED WATER
\ _ - ASH BASIN WASTE BOUNDARY
l - - ASH BASIN COMPLIANCE BOUNDARY
DUKE 610 GRAPHIC SCALE
0 610 1,220
ENERGY(IN FEET)
PROGRESS DRAWN BY: R.YU DATE: 10/25/2019
REVISED BY: B. ELLIOTT DATE: 12/16/2019
CHECKED BY: P. ALTMAN DATE: 12/16/2019
APPROVED BY: J. WYLIE DATE: 12/16/2019
synTena PROJECT MANAGER: J. WYLIE
www.svnterracorD.COM
FIGURE 5-5
SIMULATED LOCAL ASH BASIN GROUNDWATER FLOW
SYSTEM IN TRANSITION ZONE PRIOR TO DECANTING
UPDATED GROUNDWATER FLOW AND TRANSPORT
MODELING REPORT
MAYO STEAM ELECTRIC PLANT
ROXBORO, NORTH CAROLINA

LEGEND
ASH BASIN WASTE BOUNDARY
- - - - ASH BASIN COMPLIANCE BOUNDARY
= BORON SOURCE ZONES
NOTES:
ALL BOUNDARIES ARE APPROXIMATE.
NUMBER LABELS CORRESPOND TO CONCENTRATION DATA IN TABLE 5-5.
AERIAL PHOTOGRAPHY OBTAINED FROM ESRI ONLINE ON JUNE 10, 2019.
AERIAL WAS COLLECTED ON FEBRUARY 6, 2017.
DRAWING HAS BEEN SET WITH A PROJECTION OF NORTH CAROLINA STATE
PLANE COORDINATE SYSTEM FIPS 3200 (NAD83).
('DUKE
ENERGY
PROGRESS
1116rip
synTena
GRAPHIC SCALE
450 0 450 900
(IN FEET)
DRAWN BY: R.YU DATE: 10/25/2019
REVISED BY: B. ELLIOTT DATE: 12/11/2019
CHECKED BY: P. ALTMAN DATE: 12/11/2019
APPROVED BY: J. WYLIE DATE: 12/11/2019
PROJECT MANAGER: J. WYLIE
www
FIGURE 5-6
BORON SOURCE ZONES FOR
HISTORICAL TRANSPORT MODEL
UPDATED GROUNDWATER FLOW AND TRANSPORT
MODELING REPORT
MAYO STEAM ELECTRIC PLANT
ROXBORO, NORTH CAROLINA

1
i FC.FNn
BORON 700 — 4,000 pg/L
BORON > 4,000 fag/L
ASH BASIN WASTE BOUNDARY
— - — - ASH BASIN COMPLIANCE BOUNDARY
NOTES:
ALL BOUNDARIES ARE APPROXIMATE.
AERIAL PHOTOGRAPHY OBTAINED FROM ESRI ONLINE ON JUNE 10, 2019.
AERIAL WAS COLLECTED ON FEBRUARY 6, 2017.
DRAWING HAS BEEN SET WITH A PROJECTION OF NORTH CAROLINA STATE
PLANE COORDINATE SYSTEM FIPS 3200 (NAD83).
r
It
ENERGY
PROGRESS
1116rip
synTena
3r-
GRAPHIC SCALE
450 0 450 900
(IN FEET)
DRAWN BY: R.YU DATE: 10/25/2019
REVISED BY: B. ELLIOTT DATE: 12/11/2019
CHECKED BY: P. ALTMAN DATE: 12/11/2019
APPROVED BY: J. WYLIE DATE: 12/11/2019
PROJECT MANAGER: J. WYLIE
www
FIGURE 5-7
SIMULATED MAXIMUM BORON CONCENTRATIONS IN ALL
NON -ASH LAYERS PRIOR TO DECANTING
UPDATED GROUNDWATER FLOW AND TRANSPORT
MODELING REPORT
MAYO STEAM ELECTRIC PLANT
ROXBORO, NORTH CAROLINA

I
LEGEND
DRAINS
HYDRAULIC HEAD (FEET)
ASH BASIN WASTE BOUNDARY
- — - ASH BASIN COMPLIANCE BOUNDARY
NOTES:
ALL BOUNDARIES ARE APPROXIMATE.
CONTOUR INTERVAL IS 10 FEET. HEADS ARE SHOWN FOR MODEL LAYER 13.
AERIAL PHOTOGRAPHY OBTAINED FROM ESRI ONLINE ON JUNE 10, 2019.
AERIAL WAS COLLECTED ON FEBRUARY 6, 2017.
DRAWING HAS BEEN SET WITH A PROJECTION OF NORTH CAROLINA STATE
PLANE COORDINATE SYSTEM FIPS 3200 (NAD83 AND NAVD88).
E
t�ENERGY
DUKE 450 GRAPHIC SCALE
0 450 900
(IN FEET)
PROGRESS DRAWN BY: R.YU DATE: 10/25/2019
1410 REVISED BY: D. WHATLEY DATE: 12/11/2019
CHECKED BY: P. ALTMAN DATE: 12/11/2019
APPROVED BY: J. WYLIE DATE: 12/11/2019
WnTerm PROJECT MANAGER: J. WYLIE
www.synterracorp.com
FIGURE 6-1
SIMULATED HYDRAULIC HEADS IN THE TRANSITION ZONE
AFTER DECANTING
UPDATED GROUNDWATER FLOW AND TRANSPORT
MODELING REPORT
MAYO STEAM ELECTRIC PLANT
ROXBORO, NORTH CAROLINA

a
Em
�570 � � 1p
LEGEND
0 WATER SUPPLY WELLS
—IIP- FLOW WITHIN LOCAL SYSTEM
FLOW OUTSIDE LOCAL SYSTEM
GROUNDWATER LEAVING LOCAL SYSTEM
_ GROUNDWATER DIVIDE
HYDRAULIC HEAD (FEET)
DRAINS
ASH BASIN WASTE BOUNDARY
ASH BASIN COMPLIANCE BOUNDARY
J
NOTES:
ALL BOUNDARIES ARE APPROXIMATE. ARROWS INDICATE INFERRED DIRECTION
ONLY, NOT MAGNITUDE.
CONTOUR INTERVAL IS 10 FEET. HYDRAULIC HEAD CONTOURS ARE SHOWN FOR
MODEL LAYER 13.
AERIAL PHOTOGRAPHY OBTAINED FROM ESRI ONLINE ON JUNE 10, 2019. AERIAL
WAS COLLECTED ON FEBRUARY 6, 2017.
DRAWING HAS BEEN SET WITH A PROJECTION OF NORTH CAROLINA STATE PLANE
COORDINATE SYSTEM RIPS 3200 (NAD83 AND NAVD88).
(� DUKE GRAPHIC SCALE
610 0 610 1,220
4 ENERGY (IN FEET)
PROGRESS DRAWN BY: R.YU DATE: 10/25/2019
REVISED BY: D. WHATLEY DATE: 12/16/2019
CHECKED BY: P. ALTMAN DATE: 12/16/2019
APPROVED BY: J. WYLIE DATE: 12/16/2019
WnTerm PROJECT MANAGER: J. WYLIE
www.synterracorp.com
FIGURE 6-2
SIMULATED LOCAL ASH BASIN GROUNDWATER FLOW SYSTEM
IN TRANSITION ZONE AFTER DECANTING
UPDATED GROUNDWATER FLOW AND TRANSPORT MODELING
REPORT
MAYO STEAM ELECTRIC PLANT
ROXBORO, NORTH CAROLINA

LEGEND
BORON 700 - 4,000 IJg/L
BORON > 4,000 Ng/L
ASH BASIN WASTE BOUNDARY
- - - - ASH BASIN COMPLIANCE BOUNDARY
NOTES:
ALL BOUNDARIES ARE APPROXIMATE.
AERIAL PHOTOGRAPHY OBTAINED FROM ESRI ONLINE ON JUNE 10, 2019.
AERIAL WAS COLLECTED ON FEBRUARY 6, 2017.
DRAWING HAS BEEN SET WITH A PROJECTION OF NORTH CAROLINA STATE
PLANE COORDINATE SYSTEM FIPS 3200 (NAD83).
19 M-WIT'lqm
tEDUKE
ERGY
PROGRESS
116vip
synTerra
GRAPHIC SCALE
450 0 450 900
(IN FEET)
DRAWN BY: R. YU DATE: 10/25/2019
REVISED BY: D. WHATLEY DATE: 12/13/2019
CHECKED BY: P. ALTMAN DATE: 12/13/2019
APPROVED BY: J. WYLIE DATE: 12/13/2019
PROJECT MANAGER: J. WYLIE
FIGURE 6-3
SIMULATED MAXIMUM BORON CONCENTRATIONS IN ALL NON -
ASH LAYERS 5.5 YEARS AFTER DECANTING
WHEN CLOSURE -IN -PLACE IS COMPLETED
UPDATED GROUNDWATER FLOW AND TRANSPORT MODELING
REPORT
MAYO STEAM ELECTRIC PLANT
ROXBORO, NORTH CAROLINA

/ I
1
,I
l I _
♦ �I fd;
.N
LEGEND
BORON 700 — 4,000 fag/L
BORON > 4,000 Ng/L
ASH BASIN WASTE BOUNDARY
— - — - ASH BASIN COMPLIANCE BOUNDARY
NOTES:
ALL BOUNDARIES ARE APPROXIMATE.
AERIAL PHOTOGRAPHY OBTAINED FROM ESRI ONLINE ON JUNE 10, 2019.
AERIAL WAS COLLECTED ON FEBRUARY 6, 2017.
DRAWING HAS BEEN SET WITH A PROJECTION OF NORTH CAROLINA STATE
PLANE COORDINATE SYSTEM FIPS 3200 (NAD83).
t DUKE ERGY
PROGRESS
1116vip
synTerra
GRAPHIC SCALE
450 0 450 900
(IN FEET)
DRAWN BY: R. YU DATE: 10/25/2019
REVISED BY: D. WHATLEY DATE: 12/13/2019
CHECKED BY: P. ALTMAN DATE: 12/13/2019
APPROVED BY: J. WYLIE DATE: 12/13/2019
PROJECT MANAGER: J. WYLIE
FIGURE 6-4
SIMULATED MAXIMUM BORON CONCENTRATIONS IN ALL NON -
ASH LAYERS 10 YEARS AFTER DECANTING
WHEN CLOSURE -BY -EXCAVATION IS COMPLETED
UPDATED GROUNDWATER FLOW AND TRANSPORT MODELING
REPORT
MAYO STEAM ELECTRIC PLANT
ROXBORO, NORTH CAROLINA

Muv Ceee.NLM ' A
___ 1 + f - _..rf.�-•s"?ssi; ;'�•'J� - why -'_ _ -- - -- LscENo
I
jjjj 1 •�� eiiy ...��-``��ti.�}�.tiy'
- .. 'i ���: k5 �_`- tt
\ + �•� r - ��a.mw®^ � •A� ~ � — — — — rm-nrcwul�� aws Ps*r]
�- . . _ [n5x xrir_ - �.. �_a _��/���:JJ).�j�/�/�
// �
, - ��:••.'`~.� ! f� y�l .-.n..l��~~ - A A' l�r ��S7 �fjj f.'/!.� I��F �11 �1:
�/ �F.� l lief'••.': M1�/j`I/ __� _ rwawrsm o.wu
.w-L.�rs�ex .........
•; '. /:, cwamw ou gNeru
,�t�\� •.:. 'r. - T``'�•+ '` � � Q
_
+i lr({�,s_'
r� `� 4 �� ljlir` l �1•„� ��' --` , , •R • a�iSL��m r . r
M-__ � �, r \'uttti%Il' ti:•;. .i� y
V n ..:� y
y/^ `aV�- �` • l ` _' ,.. '../ /� I REFERENCE
f /�
1][-. _ L,INaFILL FEE RV]£111N 1 �' S IL -• ! 1 �' -
JII�� r � 6ENQilLL NOTES, ❑RAWlMO 1 � -�� � / ti_1 i_�lI ���� - memea5m,s-xnrRm er rem •an aux vwn�'
1 r Y - N0. M4Y S999A03ANi7 _ rt -`Ij 11 •`_ I •, ltt 5 5 \
r�r� ���_.��� ` _ - � fJj Nn� P ll/� 1\ 1''. •1��. �,���rt .� ,y'L..�•� !.woa.m-mvacwmm oe�rrv�r�w'.e eruct, 6
'
f `rl : r rA
rx ���=- rs-.. 1 r.ro.�• 5 l4
•r'
_ 1
r '' 1tr tiJ . Ilr?- "`_~� �- ��., y. may. ,-��' F. `��rr' �I.�� ��«.m..mr...�.•.,.�..m,w.w.•wr,..�,.�
_]1,g_�=
AJyr�ilr"
..-
' l ! f I` '` % f `�i'fl ..'_•• I I - .� _ .. TwicY vuxr.Pcwwr
't 'jy: �1r• ��1''+�17 'YAra
1
_ ,� _ AE6HNOit smeLweo neoem corrsm�e'rw
�:
_ ...-.-Tyy
�t
h� 1 1 ` � �l'I:� �5�•_. : yy�i ~ � `. ���:L` .. - �''�✓ �� i.. /IafdAL WHEIai%Fn
'�:' }y` � r r C �_ - ' .! J l•'. SyF �� .5 'S : lC,.. %.C; :J_����
p
a8
f
Y.' ..4;
DRAFT
ff y qB g l
FINAL CLOSURE BRADN16 PLAN t
` ISSUES FOR APPROVAL Nn
k DUKE
4
ir •- u MAY ceeu.aasma m
DUKE
DRAWN BY: R. YU DATE: 10/25/2019
FIGURE 6-5
ENERGY.
REVISED BY: W. PRATER DATE: 12/12/2019
CLOSURE -BY -EXCAVATION DESIGN
PROGRESS
CHECKED BY: P. ALTMAN DATE: 12/12/2019
USED IN SIMULATIONS (FROM AECOM, 2019a)
APPROVED BY: J. WYLIE DATE: 12/12/2019
PROJECT MANAGER: J. WYLIE
UPDATED GROUNDWATER FLOW AND TRANSPORT MODELING REPORT
MAYO STEAM ELECTRICAL PLANT
�+
/7
WmTerra
ROXBORO, NORTH CAROLINA
www.synterracorp.com

LEGEND
WATER SUPPLY WELLS
FLOW WITHIN LOCAL SYSTEM
FLOW OUTSIDE LOCAL SYSTEM
GROUNDWATER LEAVING LOCAL SYSTEM
DRAINS
PROPOSED LANDFILL
GROUNDWATER DIVIDE
HYDRAULIC HEAD (FEET)
ASH BASIN WASTE BOUNDARY
ASH BASIN COMPLIANCE BOUNDARY
0
I♦
I� x
�f'
ALL BOUNDARIES ARE APPROXIMATE. ARROWS INDICATE INFERRED DIRECTION
ONLY, NOT MAGNITUDE.
CONTOUR INTERVAL IS 10 FEET. HYDRAULIC HEAD CONTOURS ARE SHOWN FOR
MODEL LAYER 13.
AERIAL PHOTOGRAPHY OBTAINED FROM ESRI ONLINE ON JUNE 10, 2019. AERIAL WAS
COLLECTED ON FEBRUARY 6, 2017.
DRAWING HAS BEEN SET WITH A PROJECTION OF NORTH CAROLINA STATE PLANE
COORDINATE SYSTEM FIPS 3200 (NAD83 AND NAVDSS).
%' DUKE GRAPHIC SCALE
610 0 610 1,220
ENERGY(IN FEET)
PROGRESS DRAWN BY: R.YU DATE: 10/25/2019
1410 REVISED BY: D. WHATLEY DATE: 12/16/2019
CHECKED BY: P. ALTMAN DATE: 12/16/2019
APPROVED BY: J. WYLIE DATE: 12/16/2019
synTerm PROJECT MANAGER: J. WYLIE
www.synterracorp.com
FIGURE 6-6
SIMULATED LOCAL ASH BASIN GROUNDWATER FLOW SYSTEM
IN TRANSITION ZONE AFTER CLOSURE -BY -EXCAVATION
UPDATED GROUNDWATER FLOW AND TRANSPORT MODELING
REPORT
MAYO STEAM ELECTRIC PLANT
ROXBORO, NORTH CAROLINA

APPROXIMATELY 20 YEARS
POST -CLOSURE -BY -EXCAVATION
APPROXIMATELY 120 YEARS
POST-C LOS U RE -BY -EXCAVATION
14xr]=1z111
BORON 700 - 4,000 pg/L
BORON > 4,000 IJg/L
PROPOSED LANDFILL
ASH BASIN WASTE BOUNDARY
— - — - ASH BASIN COMPLIANCE BOUNDARY
NOTES:
ALL BOUNDARIES ARE APPROXIMATE.
CLOSURE -BY -EXCAVATION SCENARIO IS ESTIMATED TO BE COMPLETED BY
YEAR 2031.
AERIAL PHOTOGRAPHY OBTAINED FROM ESRI ONLINE ON JUNE 10, 2019.
AERIAL WAS COLLECTED ON FEBRUARY 6, 2017.
DRAWING HAS BEEN SET WITH A PROJECTION OF NORTH CAROLINA STATE
PLANE COORDINATE SYSTEM FIPS 3200 (NAD83).
APPROXIMATELY 70 YEARS
POST -CLOSURE -BY -EXCAVATION
� % .
APPROXIMATELY 170 YEARS
POST -CLOSURE -BY -EXCAVATION
tDUKE
ENERGY
PROGRESS
10
synTerra
GRAPHIC SCALE
990 0 990 1,980
(IN FEET)
DRAWN BY: R.YU DATE: 10/25/2019
REVISED BY: D. WHATLEY DATE: 12/18/2019
CHECKED BY: P. ALTMAN DATE: 12/18/2019
APPROVED BY: J. WYLIE DATE: 12/18/2019
PROJECT MANAGER: J. WYLIE
FIGURE 6-7
SIMULATED MAXIMUM BORON CONCENTRATIONS
IN ALL NON -ASH LAYERS AFTER
CLOSURE -BY -EXCAVATION
UPDATED GROUNDWATER FLOW AND TRANSPORT
MODELING REPORT
MAYO STEAM ELECTRIC PLANT
ROXBORO, NORTH CAROLINA

LEGEND
REFERENCE LOCATIONS
CRUTCHFIELD BRANCH DRAIN
ASH BASIN WASTE BOUNDARY
- — - ASH BASIN COMPLIANCE BOUNDARY
NOTES:
ALL BOUNDARIES ARE APPROXIMATE.
AERIAL PHOTOGRAPHY OBTAINED FROM ESRI ONLINE ON JUNE 10, 2019.
AERIAL WAS COLLECTED ON FEBRUARY 6, 2017.
DRAWING HAS BEEN SET WITH A PROJECTION OF NORTH CAROLINA STATE
PLANE COORDINATE SYSTEM FIPS 3200 (NAD83).
Location
AiL
1
486)
DUKE GRAPHIC SCALE
450 0 450 900
ENERGY(IN FEET)
PROGRESS DRAWN BY: R.YU DATE: 10/25/2019
REVISED BY: D. WHATLEY DATE: 12/11/2019
CHECKED BY: P. ALTMAN DATE: 12/11/2019
APPROVED BY: J. WYLIE DATE: 12/11/2019
synTena PROJECT MANAGER: J. WYLIE
www.synterracorp.com
FIGURE 6-8
REFERENCE LOCATIONS FOR TIME SERIES DATASETS
UPDATED GROUNDWATER FLOW AND TRANSPORT
MODELING REPORT
MAYO STEAM ELECTRIC PLANT
ROXBORO, NORTH CAROLINA

1,200
J
1,000
c
800
co
L
y
600
U
i
0
v
400
0
Y
L
0
�� I
0
O O
r- N
O) O
-I fV
Reference location 1 is located downgradient of the dam.
800
700 — —
oIo
= 600
0
500
L
400
u° 300
0
C 200
m
100
0
O
O
�
Ol
N
O
c-I
N
Reference location 2 is located at the compliance boundary
Location 1
O O O
� N r-
O c-I r -I
N N N
Year
Location 2
Saprolite, depth = 8 ft.
Transition Zone, depth = 21 ft.
Upper bedrock, depth = 53 ft.
Bedrock, depth = 113 ft.
Max. all layer
— 2L Std = 700 ug/L
O O O
r- N n
O r-I ­1
N N N
Year
41*5DUKE
DRAWN BY: R. YU DATE: 10/25/2019
FIGURE 6-9
SUMMARY OF MAXIMUM BORON
ENERGY
REVISED BY: W. PRATER DATE: 12/12/2019
CONCENTRATION IN ALL LAYERS AS FUNCTIONS
PROGRESS
CHECKED BY: P. ALTMAN DATE: 12/12/2019
OF TIME AND STRATIGRAPHIC LAYER FOR THE
APPROVED BY: J. WYLIE DATE: 12/12/2019
PROJECT MANAGER: J. WYLIE
CLOSURE -BY -EXCAVATION SCENARIO
UPDATED GROUNDWATER FLOW AND
'
TRANSPORT MODELING REPORT
MAYO STEAM ELECTRICAL PLANT
S)/f"1Terra
www.synterracorp.com
ROXBORO, NORTH CAROLINA

WAY *DOLMAN K• Al
V i � 7r t [ 4.' 4 �r � El r.r- :rr .: .+E':•�1 Y14 _ �r.f /+nrs
1' 7'}J
:: ���� �r� I�'�f.�1 1 ��•r� i��.�' mt i �- ww .[awe
� f, 1. � •` �--, � : �� '�' : ! � � � „ ti \ �~ _ ,I.o,«,44�,�4 � w.,.�.,a.4. �..ti..,.4R a...
f�'-i'+ i `ram `♦ `:. 1 wx� �•�..`�...a"' m
•' �•11 �. � --•li�l.•. \� r - wwrif4drrww wawc+wue.snnewa[nm.aw,e� [R
I �- � l •� E r�„r ' � .:'1 - � f�.. , � } YI: ocu..wa. eo.e.we.mm.mwcno. �
f r 4 4 ~
i 5^� y� Lti- �_ .:: Po.ar �` - � ,.•�CINea, u,s Cv,c rrxwu,or,ofr
IOt FER mwsnlnenLr.^' j
.1...�1...�.. ��
{ - I-'7 ' r 1 r it ~ INM FFLAL rOVER GRO.W6PLM
of
'L cc
�[a dmr x•�r
ro
£ f +I 4 � 4k wly }Y � .' � •a-0 ��OfYIC FOR iM�RC'7�l
r e ` w rwr csn.awam -A
DUKE DRAWN BY: R. YU DATE: 10/25/2019 FIGURE 6-10
ENERGY REVISED BY: W. PRATER DATE: 12/12/2019 CLOSURE -IN -PLACE DESIGN
PROGRES CHECKED BY: P. ALTMAN DATE: 12/12/2019 USED IN SIMULATIONS (FROM AECOM, 2019b)
APPROVED BY: J. WYLIE DATE: 12/12/2019
PROJECT MANAGER: J. WYLIE UPDATED GROUNDWATER FLOW AND TRANSPORT MODELING REPORT
MAYO STEAM ELECTRICAL PLANT
synTena www.synterracorp.com
ROXBORO, NORTH CAROLINA

00
00
40
l �
S00
� • I .y C•
NOTES:
S40
SSO
ALL BOUNDARIES ARE APPROXIMATE. ARROWS INDICATE INFERRED DIRECTION
ONLY, NOT MAGNITUDE.
CONTOUR INTERVAL IS 10 FEET. HYDRAULIC HEAD CONTOURS ARE SHOWN FOR
S60 � -
MODEL LAYER 13.
AERIAL PHOTOGRAPHY OBTAINED FROM ESRI ONLINE ON JUNE 10, 2019. AERIAL
WAS COLLECTED ON FEBRUARY 6, 2017.
G'1
DRAWING HAS BEEN SET WITH A PROJECTION OF NORTH CAROLINA STATE PLANE
6
COORDINATE SYSTEM RIPS 3200 (NAD83 AND NAVD88).
(' DUKE
GRAPHIC SCALE
600 0 600 1,200
ENERGY
(IN FEET)
PROGRESS
DRAWN BY: R. YU DATE: 10/25/2019
��
-
REVISED BY: D. WHATLEY DATE: 12/16/2019
CHECKED BY: P. ALTMAN DATE: 12/16/2019
LEGEND
APPROVED BY: J. WYLIE DATE: 12/16/2019
PROJECT MANAGER: J. WYLIE
WATER SUPPLY WELLS
synTena
wwws nterracor .com
FLOW WITHIN LOCAL SYSTEM
-� FLOW OUTSIDE LOCAL SYSTEM
FIGURE 6-11
GROUNDWATER LEAVING LOCAL SYSTEM
SIMULATED LOCAL ASH BASIN GROUNDWATER FLOW SYSTEM IN
DRAINS
TRANSITION ZONE AFTER CLOSURE -IN -PLACE
- GROUNDWATER DIVIDE
UPDATED GROUNDWATER FLOW AND TRANSPORT MODELING
REPORT
HYDRAULIC HEAD (FEET)
MAYO STEAM ELECTRIC PLANT
ASH BASIN WASTE BOUNDARY
ROXBORO, NORTH CAROLINA
ASH BASIN COMPLIANCE BOUNDARY

APPROXIMATELY 20 YEARS
APPROXIMATELY 70 YEARS
POST -CLOSURE -IN -PLACE
POST -CLOSURE -IN -PLACE
r ■
r
r ■
� r
APPROXIMATELY 120 YEARS
APPROXIMATELY 170 YEARS
POST -CLOSURE -IN -PLACE
POST -CLOSURE -IN -PLACE
.�
.�
Al
r
■
r
r ■
r
r
LEGEND
%' DUKE
GRAPHIC SCALE
990 0 990 1,980
ENERGY(IN
FEET)
BORON 700 - 4,000 IJg/L
PROGRESS
DRAWN BY: R. YU DATE: 10/25/2019
BORON > 4,000 IJg/L
REVISED BY: D. WHATLEY DATE: 12/18/2019
CHECKED BY: P. ALTMAN DATE: 12/18/2019
APPROVED BY: J. WYLIE DATE: 12/18/2019
ASH BASIN WASTE BOUNDARY
synTerra
PROJECT MANAGER: J. WYLIE
— - — - ASH BASIN COMPLIANCE BOUNDARY
www.synterracorp.com
FIGURE 6-12
NOTES:
SIMULATED MAXIMUM BORON CONCENTRATIONS IN
ALL BOUNDARIES ARE APPROXIMATE.
ALL NON -ASH LAYERS AFTER CLOSURE -IN -PLACE
CLOSURE -IN -PLACE SCENARIO IS ESTIMATED TO BE COMPLETED BY YEAR
UPDATED GROUNDWATER FLOW AND TRANSPORT
2026.
MODELING REPORT
AERIAL PHOTOGRAPHY OBTAINED FROM ESRI ONLINE ON JUNE 10, 2019.
MAYO STEAM ELECTRIC PLANT
AERIAL WAS COLLECTED ON FEBRUARY 6, 2017.
ROXBORO, NORTH CAROLINA
DRAWING HAS BEEN SET WITH A PROJECTION OF NORTH CAROLINA STATE
PLANE COORDINATE SYSTEM FIPS 3200 (NAD83).

Location 1
1,200
Saprolite, depth = 6 ft.
1,000 Transition Zone, depth = 15 ft.
Upper bedrock, depth = 46 ft.
3 800
Bedrock, depth = 106 ft.
z; 600 Max. all layer
M
L
— 2L Std = 700 ug/L
a�
= 400
0
V
0 200
0
m
0
0 0 0 0 0
rn o o rr*4-I
r-I N N N N
Reference location 1 is located downgradient of the dam. Year
Location 2
800
se
700
Tr
600
J
b
UI
0 500
BE
0
400
M
L
cu 300
_ _ 21.
c
0
V 200
c
0
0 100
m
0
0 0
0
� N
n
ri N
N
Reference location 2 is located at the compliance boundary.
Year
Simulated boron concentrations at location 2 are close to zero in
layers 15 and 18.
0 0
N r-
r-I r-I
N N
41*5DUKE
DRAWN BY: R. YU
DATE: 10/25/2019
SUMMARFIGURE 6-13
Y OF MAXIMUM BORON
ENERGY
REVISED BY: W. PRATER
DATE: 12/12/2019
CONCENTRATION IN ALL LAYERS AS FUNCTIONS
PROGRESS
CHECKED BY: P. ALTMAN
DATE: 12/12/2019
OF TIME AND STRATIGRAPHIC LAYER FOR THE
APPROVED BY: J. WYLIE
DATE: 12/12/2019
CLOSURE -IN -PLACE SCENARIO
PROJECT MANAGER: J. WYLIE
UPDATED GROUNDWATER FLOW AND
'
TRANSPORT MODELING REPORT
MAYO STEAM ELECTRICAL PLANT
S)/f"1Terra
www.synterracorp.com
ROXBORO, NORTH CAROLINA

APPROXIMATELY 20 YEARS
POST -CLOSURE -BY -EXCAVATION
APPROXIMATELY 170 YEARS
POST-C LOS U RE -BY -EXCAVATION
.•-
LEGEND
BORON 700 - 4,000 Ng/L
BORON > 4,000 Ng/L
PROPOSED LANDFILL
ASH BASIN WASTE BOUNDARY
— - — - ASH BASIN COMPLIANCE BOUNDARY
NOTES:
ALL BOUNDARIES ARE APPROXIMATE
CLOSURE -IN -PLACE SCENARIO IS ESTIMATED TO BE COMPLETED BY YEAR
2026. CLOSURE -BY -EXCAVATION SCENARIO IS ESTIMATED TO BE
COMPLETED BY YEAR 2031.
AERIAL PHOTOGRAPHY OBTAINED FROM ESRI ONLINE ON JUNE 10, 2019.
AERIAL WAS COLLECTED ON FEBRUARY 6, 2017.
DRAWING HAS BEEN SET WITH A PROJECTION OF NORTH CAROLINA STATE
PLANE COORDINATE SYSTEM FIPS 3200 (NAD83).
APPROXIMATELY 20 YEARS
POST -CLOSURE -IN -PLACE
APPROXIMATELY 170 YEARS
POST -CLOSURE -IN -PLACE
%' DUKE GRAPHIC SCALE
990 0 990 1,980
ENERGY(IN FEET)
PROGRESS DRAWN BY: R.YU DATE: 10/2512019
REVISED BY: D. WHATLEY DATE: 12/18/2019
CHECKED BY: P. ALTMAN DATE: 12/18/2019
APPROVED BY: J. WYLIE DATE: 12/18/2019
,5 nM erra PROJECT MANAGER: J. WYLIE
www cvntarrarnrn rom
FIGURE 6-14
COMPARISON OF SIMULATED MAXIMUM BORON
CONCENTRATIONS IN ALL NON -ASH LAYERS
AFTER CLOSURE
UPDATED GROUNDWATER FLOW AND TRANSPORT
MODELING REPORT
MAYO STEAM ELECTRIC PLANT
ROXBORO, NORTH CAROLINA

Location 1
1,200
1,000
J
800
i
- -
- -
- - - - -
-
- - - -
- - - - -
0
L
600
Closure-by-Excavatio n
c
a�
r-
400
Closure -in -Place
0
u
O
200
— — 2L Std = 700 ug/L
0
m
0
0
0 0
0 0
-1
M
rl
o o
N N
N N
Reference location 1 is located
downgradient of the dam. Year
Location 2
800
700
----
600
Closure -by -Excavation
J
=L
500
Closure -in -Place
e
0
400
i
—
— 2L Std = 700 ug/L
C
300
c�
c
0
200
c
100
O
ca
0
0 0
n N
Ol O
rl N
Reference location 2 is located at
the compliance boundary.
0 0 0
I� N
O r-I
N N N
Year
41*5DUKE
DRAWN BY: R. YU DATE: 10/25/2019
SUMMARFIGURE 6-15
Y OF MAXIMUM BORON
ENERGY
REVISED BY: W. PRATER DATE: 12/12/2019
CONCENTRATION IN ALL LAYERS AS FUNCTIONS
PROGRESS
CHECKED BY: P. ALTMAN DATE: 12/12/2019
OF TIME AND STRATIGRAPHIC LAYER FOR BOTH
APPROVED BY: J. WYLIE DATE: 12/12/2019
PROJECT MANAGER: J. WYLIE
CLOSURE SCENARIOS
UPDATED GROUNDWATER FLOW AND
TRANSPORT MODELING REPORT
MAYO STEAM ELECTRICAL PLANT
S)/flTerra
www.synterracorp.com
ROXBORO, NORTH CAROLINA

Updated Groundwater Flow And Transport Modeling Report December 2019
Mayo Steam Electric Plant, Roxboro, North Carolina
TABLES

Updated Groundwater Flow And Transport Modeling Report December 2019
Mayo Steam Electric Plant, Roxboro, North Carolina
TABLE 5-1 OBSERVED, COMPUTED, AND RESIDUAL HEADS FOR
THE CALIBRATED FLOW MODEL
Well
Observed Head (ft)
Computed Head (ft)
Residual Head (ft)
ABMW-1
480.10
482.00
-2.07
ABMW-2
482.93
483.33
-0.74
ABMW-2BR
482.65
483.30
-0.98
ABMW-2BRL
482.27
483.37
-1.42
ABMW-3
482.99
483.62
-1.00
ABMW-3S
482.98
483.60
-1.00
ABM W-4
484.66
484.57
-0.21
ABMW-4BR
485.11
484.60
0.21
ABM W-4D
484.61
484.56
-0.25
BG-1
508.98
510.37
-1.32
BG-2
509.87
517.06
-7.13
CCR-101D-BG
511.60
511.33
0.27
CCR-101S-BG
500.77
496.26
4.51
CCR-102BR-BG
503.20
502.53
0.67
CCR-103BR
472.79
473.66
-0.83
CCR-103D
471.51
473.57
-2.02
CCR-103S
470.22
473.13
-2.87
CCR-104BR
407.08
408.02
-2.14
CCR-104S
402.96
407.67
-5.31
CCR-105BR
379.97
381.33
-1.36
CCR-105D
381.25
381.22
0.03
CCR-105S
379.72
381.14
-1.42
CCR-106BR
375.90
374.84
1.05
CCR-107BR
435.53
425.62
9.82
CCR-108BR
471.59
465.37
6.03
CCR-109BR
397.58
397.71
0.08
CW-1
471.51
468.57
-1.76
CW-1D
471.41
468.74
-2.25
CW-2
375.49
371.52
3.97
CW-2D
374.89
371.53
3.36
CW-3
420.70
424.88
-3.37
CW-4
428.75
433.58
-4.81
CW-5
500.32
500.50
-0.17
CW-6
449.21
448.59
0.61
MW-2
434.57
441.16
-5.28
Page 1

Updated Groundwater Flow And Transport Modeling Report December 2019
Mayo Steam Electric Plant, Roxboro, North Carolina
TABLE 5-1 OBSERVED, COMPUTED, AND RESIDUAL HEADS FOR
THE CALIBRATED FLOW MODEL
Well
Observed Head (ft)
Computed Head (ft)
Residual Head (ft)
MW-3
377.69
372.84
4.85
MW-3BR
420.82
424.94
-3.34
M W-4
491.74
488.70
3.00
MW-5BR
500.27
500.68
-0.40
MW-6BR
449.60
448.76
0.84
MW-7BR
444.61
449.09
-4.12
MW-7D
443.11
449.12
-5.60
M W-8BR
431.40
437.38
-5.78
MW-8D
433.29
437.20
-3.72
MW-8S
437.02
437.04
0.17
MW-9BR
467.15
464.49
2.60
MW-9BRL
466.82
465.17
1.57
MW-10BR
500.15
498.14
2.23
M W-11 BR
490.59
491.44
-0.85
MW-12D
556.33
552.33
4.00
MW-12S
555.93
552.20
3.73
MW-13BR
498.87
499.08
-0.20
MW-14BR
502.90
505.15
-2.24
MW-15BR
403.13
403.45
-0.21
MW-16BR
366.47
368.64
-2.17
MW-16D
366.65
368.57
-1.92
MW-16S
366.38
367.40
-1.02
MW-17BR
457.81
459.94
-1.91
MW-18BR
495.00
491.17
1.52
MW-18D
491.82
494.19
-4.14
MW-19BR
485.39
482.76
0.73
MW-19D
486.66
484.28
0.23
CPA-1 BR
495.35
496.49
-1.13
CPA-1 D
493.55
496.76
-3.20
CPA-2BR
499.85
499.95
-0.09
CPA-2D
500.28
499.28
1.00
CPA-3BR
512.83
512.63
0.20
CPA-3 D
513.83
512.09
1.74
CPA-4D
521.14
519.75
1.39
CPA-5BR
522.63
524.27
-1.64
Page 2

Updated Groundwater Flow And Transport Modeling Report December 2019
Mayo Steam Electric Plant, Roxboro, North Carolina
TABLE 5-1 OBSERVED, COMPUTED, AND RESIDUAL HEADS FOR
THE CALIBRATED FLOW MODEL
Well
Observed Head (ft)
Computed Head (ft)
Residual Head (ft)
CPA-5D
523.34
524.44
-1.09
CPA-5S
523.97
524.56
-0.59
CPA-7D
505.28
505.12
0.21
FGD-1BR
461.46
454.23
7.36
FGD-1D
460.87
464.67
-3.66
FGD-2BR
444.44
448.55
-3.94
FGD-2D
460.80
465.61
-4.62
FGD-3BR
459.84
454.24
5.75
FGD-3D
461.74
467.83
-6.02
FGD-4BR
461.74
453.29
8.72
FGD-5BR
442.23
446.47
-3.98
FGD-5D
442.04
446.69
-4.36
FGD-6BR
445.55
449.55
-3.72
FGD-6D
448.19
449.66
-1.14
FGD-7BR
454.84
452.77
2.35
FGD-7D
455.67
454.80
1.13
FGD-7S
454.69
456.77
-1.84
FGD-8BR
460.12
453.69
6.61
FGD-9BR
460.65
459.08
1.76
FGD-10BR
464.46
463.41
1.21
FGD-11 BR
440.95
442.33
-1.14
FGD-11D
440.22
442.12
-1.65
FGD-12BR
436.25
438.68
-2.23
FGD-12D
435.06
438.37
-3.10
FGD-13BR
435.45
435.23
0.39
FGD-13D
434.02
434.85
-0.65
FGD-14BR
454.94
437.18
17.91
LRB-1
500.64
500.53
0.12
LRB-2
482.42
487.12
-4.62
LRB-3
515.19
517.46
-2.21
P-1
442.51
437.15
5.86
P-1A
438.65
438.26
0.93
P-2
412.54
415.80
-3.23
P-3
394.38
397.75
-3.38
P-3A
395.97
396.88
-0.92
Page 3

Updated Groundwater Flow And Transport Modeling Report December 2019
Mayo Steam Electric Plant, Roxboro, North Carolina
TABLE 5-1 OBSERVED, COMPUTED, AND RESIDUAL HEADS FOR
THE CALIBRATED FLOW MODEL
Well
Observed Head (ft)
Computed Head (ft)
Residual Head (ft)
P-4
415.50
418.07
-2.63
AP-6
482.34
481.82
0.27
AP-6L15
482.21
481.78
0.18
AP-60
482.15
481.74
0.17
AP-6M15
482.28
481.76
0.26
AP-6M3
482.05
481.71
0.09
AP-6U 15
482.22
481.78
0.17
AP-61J3
482.09
481.74
0.08
Prepared by: RY Checked by: PWA
Notes:
ft - feet in NAVD 88 (North American Vertical Datum of 1988)
Page 4

Updated Groundwater Flow And Transport Modeling Report December 2019
Mayo Steam Electric Plant, Roxboro, North Carolina
TABLE 5-2
CALIBRATED HYDRAULIC CONDUCTIVITY PARAMETERS
Unit
Grid layer
Kh (ft/d)
Kh/Kv
Upper Ash
1-4
7
10
Lower Ash
5-8
2
10
Saprolite
9-11
1
1
Transition zone
12-13
1
1
Upper fractured rock
14-16
0.03
1
Lower fractured rock
17-18
0.005
1
Bedrock
19-21
0.005
1
Prepared by: RY Checked by: PWA
Notes:
Kn - horizontal hydraulic conductivity
ft/d - feet per day
Kn/K - horizontal hydraulic conductivity divided by vertical hydraulic conductivity
Page 5

Updated Groundwater Flow And Transport Modeling Report December 2019
Mayo Steam Electric Plant, Roxboro, North Carolina
TABLE 5-3
WATER BALANCE ON THE ASH BASIN GROUNDWATER FLOW SYSTEM
FOR PRE -DECANTING CONDITIONS
Water balance components
Flow in
Flow out
(gPm)
(gPm)
Direct recharge to the ash basin
13
Direct recharge to watershed outside of ash
129
basin
Ash basin ponds
62
Drainage outside of the ash basin
63
Flow through and under the dam
18
Notes•
gpm - gallon per minute
Prepared by: RY Checked by: PWA
Page 6

Updated Groundwater Flow And Transport Modeling Report December 2019
Mayo Steam Electric Plant, Roxboro, North Carolina
TABLE 5-4
FLOW MODEL SENSITIVITY ANALYSIS
Parameter
0.5x calibrated
Calibrated
2x calibrated
Kh in upper ash (7 ft/d)
1.90%
1.88%
1.89%
Kh in lower ash (2 ft/d)
1.89%
1.88%
1.88%
Kh in saprolite (1 ft/d)
2.12%
1.88%
2.01%
Kh in TZ (1 ft/d)
2.53%
1.88%
2.56%
Kh in upper fractured rock (0.03 ft/d)
1.92%
1.88%
1.95%
Kh in lower fractured rock (0.005 ft/d)
1.92%
1.88%
1.89%
Kh in bedrock (0.005 ft/d)
1.91%
1.88%
2.08%
Regional recharge (0.0018 ft/d)
4.74%
1.88%
5.41%
Prepared by: RY Checked by: PWA
Notes:
Parameters are multiplied by 0.5 or 2 and the NRMSE is calculated.
Results are expressed as normalized root mean square error (NRMSE) of the simulated and observed heads.
Kh - horizontal hydraulic conductivity
ft/d - feet per day
Page 7

Updated Groundwater Flow And Transport Modeling Report December 2019
Mayo Steam Electric Plant, Roxboro, North Carolina
TABLE 5-5
ASH BASIN BORON SOURCE CONCENTRATIONS (Ng/L) USED IN
HISTORICAL TRANSPORT MODEL
Date
Area #1
Area #2
Area #3
Area #4
Area #5
Area #6
1983-2019
5500
5000
5000
8600
2000
5000
Prepared by: RY Checked by: PWA
Notes:
Location of each source zone is identified in Figure 5-6.
pg/L - micrograms per liter
Page 8

Updated Groundwater Flow And Transport Modeling Report December 2019
Mayo Steam Electric Plant, Roxboro, North Carolina
TABLE 5-6 OBSERVED AND COMPUTED BORON (Ng/L)
CONCENTRATIONS IN MONITORING WELLS
Well Name
Observed Boron (Ng/L)
Computed Boron (Ng/L)
ABMW-1
5090
5000
ABM W-2
8600
8600
ABMW-2BR
20
10
ABMW-2BRL
0
0
ABM W-3
1960
2000
ABM W-3S
1340
1288
ABMW-4
5540
5000
ABMW-4BR
38
1
ABM W-4D
3040
3143
BG-1
0
0
BG-2
0
0
CCR-101 D-BG
0
0
CCR-101S-BG
0
0
CCR-102BR-BG
0
0
CCR-103BR
2700
2191
CCR-103D
1580
1216
CCR-103S
495
526
CCR-104BR
0
294
CCR-104S
192
385
CCR-105BR
853
1258
CCR-105D
789
758
CCR-105S
353
590
CCR-106BR
88
96
CCR-107BR
1060
1013
CCR-108BR
0
112
CCR-109BR
0
1
Page 9

Updated Groundwater Flow And Transport Modeling Report December 2019
Mayo Steam Electric Plant, Roxboro, North Carolina
TABLE 5-6 OBSERVED AND COMPUTED BORON (Ng/L)
CONCENTRATIONS IN MONITORING WELLS
Well Name
Observed Boron (Ng/L)
Computed Boron (Ng/L)
CW-1
0
0
CW-1D
0
0
CW-2
407
323
CW-2D
211
225
CW-3
0
5
CW-4
0
104
CW-5
0
0
CW-6
0
0
MW-2
0
313
MW-3
899
612
MW-3BR
0
0
MW-4
0
0
MW-5BR
0
0
MW-7BR
0
0
MW-7D
0
0
MW-8BR
0
0
MW-9BR
0
0
MW-9BRL
0
0
MW-103BRL
0
4
MW-103BRM
0
69
MW-104BRL
0
0
MW-104BRM
0
1
MW-105BRL
0
0
MW-105BRM
0
1
MW-107BRL
64
0
MW-107BRM
45
0
Page 10

Updated Groundwater Flow And Transport Modeling Report December 2019
Mayo Steam Electric Plant, Roxboro, North Carolina
TABLE 5-6 OBSERVED AND COMPUTED BORON (Ng/L)
CONCENTRATIONS IN MONITORING WELLS
Well Name
Observed Boron (Ng/L)
Computed Boron (Ng/L)
MW-10BR
0
0
MW-11BR
0
0
MW-12D
0
0
MW-12S
0
0
MW-13BR
0
0
MW-14BR
0
0
MW-16BR
21
0
MW-16D
0
0
MW-16S
50
0
MW-18BR
0
0
MW-18D
0
0
MW-19BR
0
0
MW-19D
0
0
CPA-1BR
41
0
CPA-1D
69
0
CPA-2BR
0
0
CPA-2D
0
0
CPA-3BR
0
0
CPA-3 D
48
0
CPA-4D
39
0
CPA-5BR
0
0
CPA-5D
55
0
CPA-5 S
197
0
CPA-7D
559
0
Notes:
Data collected through May 2019.
pg/L - micrograms per liter
Prepared by: RY Checked by: PWA
Page 11

Updated Groundwater Flow And Transport Modeling Report December 2019
Mayo Steam Electric Plant, Roxboro, North Carolina
TABLE 5-7 TRANSPORT MODEL SENSITIVITY TO THE
BORON Kd VALUES
Well
Boron
g/L)
Boron model
calibrated
Model, low
Ka
Model, high
Kd
NRMSE
1.71%
2.55%
3.50%
ABMW-1
5090
5000
5000
5000
ABMW-2
8600
8600
8600
8600
ABMW-2BR
20
44
10
0
ABMW-2BRL
0
0
0
0
ABM W-3
1960
2000
2000
2000
ABMW-3S
1340
1422
1288
779
ABM W-4
5540
5000
5000
5000
ABMW-4BR
38.3
4
1
0
ABM W-4D
3040
3487
3143
2220
BG-1
0
0
0
0
13G-2
0
0
0
0
CCR-101 D-BG
0
0
0
0
CCR-101S-BG
0
0
0
0
CCR-102BR-BG
0
0
0
0
CCR-103BR
2700
2319
2191
1281
CCR-103D
1580
1429
1216
437
CCR-103S
495
819
526
35
CCR-104BR
0
393
294
52
CCR-104S
192
531
385
88
CCR-105BR
853
1805
1258
108
CCR-105D
789
1088
758
75
CCR-105S
353
851
590
55
CCR-106BR
88
166
96
13
CCR-107BR
1060
1023
1013
846
CCR-108BR
0
121
112
43
CCR-109BR
0
14
1
0
CW-1
0
0
0
0
CW-1D
0
0
0
0
CW-2
407
332
323
211
CW-2D
211
234
225
135
Page 12

Updated Groundwater Flow And Transport Modeling Report December 2019
Mayo Steam Electric Plant, Roxboro, North Carolina
TABLE 5-7 TRANSPORT MODEL SENSITIVITY TO THE
BORON Kd VALUES
Well
Boron
g/L)
Boron model
calibrated
Model, low
Kd
Model, high
Kd
NRMSE
1.71%
2.55%
3.50%
CW-3
0
34
5
0
CW-4
0
208
104
0
CW-5
0
0
0
0
CW-6
0
0
0
0
MW-2
0
869
313
3
MW-3
899
780
612
242
MW-3BR
0
4
0
0
MW-4
0
0
0
0
MW-5BR
0
0
0
0
MW-7BR
0
0
0
0
MW-7D
0
0
0
0
MW-8BR
0
0
0
0
MW-9BR
0
0
0
0
MW-9BRL
0
0
0
0
MW-103BRL
0
57
4
0
MW-103BRM
0
363
69
0
MW-104BRL
0
0
0
0
MW-104BRM
0
10
1
0
MW-105BRL
0
2
0
0
MW-105BRM
0
56
1
0
MW-107BRL
64
1
0
0
MW-107BRM
45
6
0
0
MW-10BR
0
0
0
0
MW-11BR
0
0
0
0
MW-12D
0
0
0
0
MW-12S
0
0
0
0
MW-13BR
0
0
0
0
MW-14BR
0
0
0
0
MW-16BR
21
0
0
0
MW-16D
0
0
0
0
Page 13

Updated Groundwater Flow And Transport Modeling Report December 2019
Mayo Steam Electric Plant, Roxboro, North Carolina
TABLE 5-7 TRANSPORT MODEL SENSITIVITY TO THE
BORON Kd VALUES
Well
Boron
(pg/L)calibrated
Boron model
Model, low
Kd
Model, high
Kd
NRMSE
1.71%
2.55%
3.50%
MW-16S
50
0
0
0
MW-18BR
0
0
0
0
MW-18D
0
0
0
0
MW-19BR
0
0
0
0
MW-19D
0
0
0
0
CPA-1BR
41
0
0
0
CPA-1D
69
0
0
0
CPA-2BR
0
0
0
0
CPA-2D
0
0
0
0
CPA-3BR
0
0
0
0
CPA-3D
48
0
0
0
CPA-4D
39
0
0
0
CPA-5BR
0
0
0
0
CPA-5D
55
0
0
0
CPA-5S
197
0
0
0
CPA-7D
559
0
0
0
Prepared by: RY Checked by: PWA
Notes•
Boron concentrations are shown for the calibrated model, and for models where the Kd is increased by a factor of 5
and decreased by a factor of 1/5.
Kd - soil -water distribution coefficients
pg/L - micrograms per liter
Page 14

Updated Groundwater Flow And Transport Modeling Report December 2019
Mayo Steam Electric Plant, Roxboro, North Carolina
TABLE 6-1
WATER BALANCE ON THE ASH BASIN GROUNDWATER FLOW SYSTEM
FOR DECANTED CONDITIONS
Water balance components
Flow in
(gpm)
Flow out
(gpm)
Direct recharge to the ash basin
29
Direct recharge to watershed outside of ash basin
134
Decanting drain inside ash basin
106
Drainage outside of the ash basin
50
Flow through and under the dam
7
Notes•
gpm - gallon per minute
Prepared by: RY Checked by: PWA
Page 15