NC0003468_5. DRSS CAP Part 2_Appx B_FINAL_20160210

Appendix B
Groundwater Flow and
Transport Model

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Memorandum

January 29, 2016

TO: Ed Sullivan and Tyler Hardin
FROM: Bruce Hensel
SUBJECT: DAN RIVER MODEL REVIEW

Summary
EPRI has reviewed the Dan River model report and files provided by Duke Energy, HDR
Engineering, and the University of North Carolina-Charlotte. The review was performed by
John Ewing (Intera), with input by Chunmiao Zheng and myself. Based on this review, it is
our opinion that, subject to the caveat below, this model is set-up and meets its flow and
transport calibration objectives sufficiently to meet its final objective of predicting effects of
corrective action alternatives on groundwater quality. The caveat associated with this opinion
is:
• Constant heads used to represent some boundaries may provide an unmitigated source
of water for simulation of any corrective action alternatives that potentially involve
pumping near those boundaries. If corrective actions involve pumping, drains may be
a more appropriate boundary condition choice for the Dan River and service settling
water pond that use constant head boundaries.

Specific Comments
Model Report, Setup, and Calibration
a) Is the objective/purpose of modeling clearly defined?
Yes. The objective and purpose of the modeling is clearly defined in Section 1.2 as
consisting of three main activities: development of a calibrated steady-state flow model of
current conditions, development of a historical transient model of constituent transport
calibrated to current conditions, and predictive simulations of different corrective action
options.
b) Is the site description adequate?
Yes. Section 1.1 of the report provides a description of the site that is adequate for
evaluating the model for both flow and transport purposes.
c) Is the conceptual model well described with appropriate assumptions?
Yes. The conceptual model section contains subsections discussing the geology and
hydrogeology, the groundwater flow system, hydrologic boundaries, hydraulic boundaries,
sources and sinks, water budget, modeled constituents of interest, and constituent transport.
The CSA report is referenced in describing aquifer properties. The method used to estimate
Dan River Model Review
January 29, 2016
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recharge is discussed along with references. The rationale behind the choice of constituents
to be modeled is discussed.
d) Is the numerical model properly set up (steady state or transient; initial condition;
boundary conditions; parameterization; etc.)?
i) Appropriateness of the simulator
The use of MODFLOW-NWT and MT3DMS to simulate groundwater flow and contaminant
transport, respectively, are both appropriate for the modeling objectives and represent an
industry standard in choice of simulators.
ii) Discretization: temporal and spatial (x-y notably z)
The spatial discretization for this model appears more than adequate to delineate differences
in simulated heads and concentrations for all reasonable calibration and predictive purposes.
For transport simulations, the Courant Number and the Grid Peclet Number can be used to
determine whether discretization is numerically appropriate for a given model. In this model,
transport time steps are automatically calculated by the simulator based on a specified
Courant Number constraint of 1. This ensures that temporal discretization is appropriate for
the spatial discretization of the model.
The Grid Peclet Number (Pe_grid = ∆l/α) can be used to evaluate whether numerical
dispersion dominates constituent transport based on spatial discretization and physical
dispersion. Grid Peclet numbers preferably less than 2 and no more than 10 are generally
recommended to minimize numerical dispersion.
The lateral discretization (∆x = ∆y = 20 feet) for the entire model area, coupled with the
longitudinal dispersivity of 80 feet used in this model results in a Grid Peclet number of 0.25
which is adequate. The transverse lateral dispersivity of 8 feet results in a Grid Peclet
number of approximately 2.5 which is also adequate.
The vertical discretization is variable in contrast to the horizontal discretization. Because a
considerable portion of the early transport in the model is vertical, special consideration of
this discretization is warranted. The maximum cell thicknesses (∆z) in the model is 49 feet.
The transverse vertical dispersivity (αv) is 8 feet in the model. This results in a maximum
grid Peclet number (Pe_grid = ∆z/αv) equal to 49/8 = 6. This indicates that numerical
dispersion is likely to be smaller or, at most, the same order of magnitude as physical
dispersion even in the vertical dimension and is likely to play a minor role in simulated
results of constituent transport.
iii) Hydrologic framework – hydraulic properties
Hydraulic properties were grouped by material type (ash, dike, upper unconsolidated zone,
transition zone, and fractured bedrock zone) which are based on the hydrostratigraphy of the
site described in the CSA report. The hydrostratigrapic layers were further subdivided into
zones by which the hydraulic properties were additionally varied for better calibration to
observed water levels. This approach is reasonable.
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The report notes that properties were kept within the ranges from the CSA which, strictly
speaking, is not accurate. In some cases, the calibrated properties extended beyond the CSA
ranges; however, they are not inconsistent with the CSA ranges.
The vertical/horizontal anisotropy in hydraulic conductivities ranges from 0.0001 to 1. Most
zonal anisotropies seem very reasonable but the endpoint values are somewhat extreme for
the scale of the model. These could either be reevaluated, or additional explanation as to
why the very small and very large anisotropy values were necessary for calibration could be
included.
iv) Boundary conditions
The rate of areal recharge originating as precipitation is based on ranges for the Piedmont
ranges from a referenced study. This is appropriate. Flow to and from surface water
boundaries such as the Dan River to the south and a service settling water pond to the west
are represented by constant head boundaries. The drain package was used to simulate an
unnamed stream along the eastern model boundary. Beneath these surficial boundaries, a
groundwater divide is assumed which is represented as no flow. A topographical
groundwater divide defines the northern boundary which is represented as no flow. These
boundaries are reasonable.
The recharge in the shallow flow system conceptualized as primarily discharging to the Dan
River along the southern boundary and the unnamed stream and the service settling water
pond along the eastern and western boundaries of the model. The constant heads may also
provide an unmitigated source of water for simulation of any corrective action alternatives
that potentially involve pumping near those boundaries, however, personal communication
with the HDR/UNCC team indicates that corrective actions involving pumping are unlikely.
If corrective actions do involve pumping, drains may be a more appropriate boundary
condition choice for the unnamed streams that use constant head boundaries.
v) Initial conditions in transient simulations
The flow model is run in steady state, where initial conditions are not relevant apart from
numerical convergence, which was achieved. The transient transport model assumes a zero
concentration in 1956 shortly after the beginning of plant operations with the ash ponds
acting as a constant concentration sources thereafter. These initial conditions are reasonable.
vi) Convergence criteria and mass balance errors
The head tolerance in the NWT packages of 1e-3 is more than adequate in ensuring head
precision for the purposes of the model. The flow mass balance discrepancy in the
MODFLOW Listing file of 0.00 percent is more than adequate in ensuring minimal flow
mass balance errors. The cumulative concentration mass balance discrepancy from
MT3DMS is consistently less than 1e-2 percent for the transport simulation which is more
than adequate in ensuring minimal constituent mass balance errors.
e) Is the calibration done properly and adequately?
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For flow calibration to heads, the Normalized Root Mean Square of the Error is 5.5%
percent, which is below the industry standard of 10 percent (i.e., below 10 percent is
acceptable). The plot of simulated vs. observed heads shows both positive and negative
residuals with very little bias in the residuals.
Comparison of simulated and observed concentrations for each of the COIs generally shows
very good agreement. The notable exceptions are the highest observed Selenium and Sulfate
concentrations which are simulated as very low. The wells in which these high
concentrations are observed, AS-10D and GWA-8D for Selenium and Sulfate, respectively,
are both deep and upstream of the ash basin. In any case, the region between the ash basin
and the Dan River reflects reasonable agreement between simulated and observed
concentrations for all COIs and the few upstream discrepancies are unlikely to impact the
model predictions.
i) Property/boundary condition correlation – parameter bounds
Recharge was fixed early during the calibration process and hydraulic parameters were
adjusted zonally during calibration. The hydraulic properties were consistent with the ranges
of measured values from the CSA although several of the calibrated values extended beyond
the measured range. A hierarchy in recharge rates, whereby infiltration was higher in the ash
basins than the areas outside the basins, was enforced. Based on the recharge package file,
the recharge rates were 13.14 inches per year within the ash basins and 6.5 inches per year
outside the ash basins as shown in Figure 5. This approach appears reasonable.
ii) Discretization of calibration parameters
Hydraulic properties were grouped by material type (ash, dike, upper unconsolidated zone,
transition zone, and fractured bedrock zone) and then further differentiated by zones. This is
a reasonable approach. Recharge is grouped by infiltration within the ash basins and outside
the basins. This is also a reasonable approach.
iii) Appropriateness of target as a metric of simulation objectives (e.g., calibrating
primarily to heads when transport is the primary purpose)
The flow model calibration to observed heads is adequate. The concentrations of all COIs
are adequately calibrated for the area between the ash basin and the Dan River and can be
used to assess corrective actions in that area.
f) Is the sensitivity analysis conducted and if so, correctly?
i) Sensitivity Analysis approach (look for parameters which may be insensitive to flow
but not to transport)
A flow model sensitivity analysis was conducted whereby recharge rates and hydraulic
conductivities in the shallow and transition zones were perturbed up and down and the
change to the Normalized Root Mean Square of the Error was observed. This shows the
sensitivity of the calibration objective function to six flow model parameters.
No quantitative transport model sensitivity analysis was presented. The report mentions that
a decrease in sorption and an increase in dispersivity both lead to an increase in the spatial
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extent of the modeled concentrations. This is consistent with expectations. However, the
report also mentions that an increase in porosity, results in an increase in the spatial extent of
the modeled concentrations. This is not consistent with expectations and should be checked
or additional explanation is necessary.
Model Files:
a) Can the model be run with the input files provided by the developer?
Yes.
b) Do the model results match those presented in the report?
i) Independent check of input data vs. conceptual model/report
Model inputs are consistent with what is described in the report with the exception of Figure
12 which appears different than the simulated results presented in Table 3.
ii) Check of water balance vs. conceptual model
The MODFLOW listing file indicates that 99.6% of the inflow to the flow model enters as
recharge with the remaining 0.4% of inflow entering through constant head boundaries. 98%
of the outflow exits through constant heads representing the Dan River and the service
settling water pond along the western boundary. The remaining 2% of the outflow exits
through drains representing an unnamed stream along the eastern site boundary. This is all
consistent with the conceptual model.
iii) Independent check of model results vs. those reported
Independently generated model results are consistent with what is described in the report.

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Groundwater Flow and Transport Model
Dan River Steam Station
Rockingham County, NC
Investigators:

HDR Engineering, Inc.
440 S. Church St, Suite 1000
Charlotte, NC 28202
Contributors:

William G. Langley, Ph.D., P.E.
Dongwook Kim, Ph.D.
UNC Charlotte / Lee College of Engineering
Department of Civil and Environmental Engineering
EPIC Building 3252
9201 University City Blvd.
Charlotte, NC 28223

February 5, 2016

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Groundwater Flow and Transport Model
Dan River Steam Station
TABLE OF CONTENTS
1 Introduction ............................................................................................................................ 1
1.1 General Setting and Background ................................................................................... 1
1.2 Third Party Review ......................................................................................................... 2
2 Conceptual Model .................................................................................................................. 2
2.1 Geology and Hydrogeology ............................................................................................ 2
2.2 Hydrostratigraphic Layer Development .......................................................................... 3
2.3 Ash Basin and Ash Storage Areas ................................................................................. 3
2.4 Groundwater Flow System ............................................................................................. 5
2.5 Hydrologic Boundaries ................................................................................................... 5
2.6 Hydraulic Boundaries ..................................................................................................... 5
2.7 Water Sources and Sinks ............................................................................................... 6
2.8 Water Budget .................................................................................................................. 6
2.9 Modeled Constituents of Interest .................................................................................... 6
2.10 Constituent Transport ..................................................................................................... 6
3 Computer Model .................................................................................................................... 7
3.1 Model Selection .............................................................................................................. 7
3.2 Model Description ........................................................................................................... 7
4 Groundwater Flow and Transport Model Construction .......................................................... 8
4.1 Model Hydrostratigraphy ................................................................................................ 8
4.2 GMS MODFLOW Version 10 ......................................................................................... 9
4.3 Model Domain and Grid ................................................................................................ 11
4.4 Hydraulic Parameters ................................................................................................... 11
4.5 Flow Model Boundary Conditions ................................................................................. 12
4.6 Flow Model Sources and Sinks .................................................................................... 12
4.7 Flow Model Calibration Targets .................................................................................... 12
4.8 Transport Model Parameters ........................................................................................ 12
4.9 Transport Model Boundary Conditions ......................................................................... 14
4.10 Transport Model Sources and Sinks ............................................................................ 14
4.11 Transport Model Calibration Targets ............................................................................ 14
5 Model Calibration to Current Conditions .............................................................................. 15
5.1 Flow Model Residual Analysis ...................................................................................... 15
5.2 Flow Model Sensitivity Analysis .................................................................................... 15
5.3 Transport Model Calibration and Sensitivity ................................................................. 16
5.4 Advective Travel Times ................................................................................................ 17
6 Simulations of Closure Scenarios ........................................................................................ 17
6.1 Existing Conditions Scenario ........................................................................................ 17
6.2 Excavation Scenario ..................................................................................................... 17
7 Closure Scenario Results .................................................................................................... 18
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Groundwater Flow and Transport Model
Dan River Steam Station
7.1 Antimony ....................................................................................................................... 18
7.2 Arsenic .......................................................................................................................... 19
7.3 Boron ............................................................................................................................ 19
7.4 Chromium ..................................................................................................................... 19
7.5 Cobalt ........................................................................................................................... 19
7.6 Hexavalent Chromium .................................................................................................. 20
7.7 Selenium ....................................................................................................................... 20
7.8 Sulfate .......................................................................................................................... 20
7.9 Thallium ........................................................................................................................ 20
7.10 Vanadium ..................................................................................................................... 21
8 Summary ............................................................................................................................. 21
8.1 Model Assumptions and Limitations ............................................................................. 21
8.2 Model Predictions ......................................................................................................... 22
9 References .......................................................................................................................... 22

ATTACHMENTS
TABLES
Table 1. MODFLOW and MT3DMS Input Packages Utilized
Table 2. Model Hydraulic Conductivity
Table 3. Observed vs. Predicted Hydraulic Head
Table 4. Flow Parameter Sensitivity Analysis
Table 5. Transport Model Calibration Results
Table 6. Predicted Advective Travel Time to Boundary

FIGURES
Figure 1 Conceptual Groundwater Flow Model/Model Domain
Figure 2 Model Domain Cross section A-A’
Figure 3 Model Domain Cross section B-B’
Figure 4 Flow Model Boundary Conditions
Figure 5 Recharge and Constant Concentration Boundary Condition
Figure 6 Shallow Monitoring Wells
Figure 7 Deep Monitoring Wells
Figure 8 Bedrock Monitoring Wells
Figure 9 Hydraulic Conductivity Zonation in Shallow Model Layers
Figure 10 Hydraulic Conductivity Zonation in Deep Model Layer
Figure 11 Hydraulic Conductivity Zonation in Bedrock Model Layers
Figure 12 Measured versus Modeled Water Levels
Figure 13 Potentiometric Head Contour Map
Figure 14 Potentiometric Head Cross Section C-C’
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Groundwater Flow and Transport Model
Dan River Steam Station
Figure 15 Potentiometric Head Cross section D-D’
Figure 16 Particle Tracking Results
Figure 17 Predicted Antimony in Monitoring Well AB-10S
Figure 18 Predicted Antimony in Monitoring Well AB-30S
Figure 19 Predicted Antimony in Monitoring Well GWA-6S
Figure 20 Initial (2015) Antimony Concentrations in Shallow Groundwater Zone
Figure 21 Initial (2015) Antimony Concentrations in Deep Groundwater Zone
Figure 22 Initial (2015) Antimony Concentrations in Bedrock Groundwater Zone
Figure 23 Existing Conditions Scenario 1 - 2115 Predicted Antimony Concentrations in Shallow
Groundwater Zone
Figure 24 Existing Conditions Scenario 1 - 2115 Predicted Antimony Concentrations in Deep
Groundwater Zone
Figure 25 Existing Conditions Scenario 1 - 2115 Predicted Antimony Concentrations in Bedrock
Groundwater Zone
Figure 26 Excavation Scenario 2 - 2115 Predicted Antimony Concentrations in Shallow
Groundwater Zone
Figure 27 Excavation Scenario 2 - 2115 Predicted Antimony Concentrations in Deep
Groundwater Zone
Figure 28 Excavation Scenario 2 - 2115 Predicted Antimony Concentrations in Bedrock
Groundwater Zone
Figure 29 Predicted Arsenic in Monitoring Well AB-10S
Figure 30 Predicted Arsenic in Monitoring Well AB-30S
Figure 31 Predicted Arsenic in Monitoring Well GWA-6S
Figure 32 Initial (2015) Arsenic Concentrations in Shallow Groundwater Zone
Figure 33 Initial (2015) Arsenic Concentrations in Deep Groundwater Zone
Figure 34 Initial (2015) Arsenic Concentrations in Bedrock Groundwater Zone
Figure 35 Existing Conditions Scenario 1 - 2115 Predicted Arsenic Concentrations in Shallow
Groundwater Zone
Figure 36 Existing Conditions Scenario 1 - 2115 Predicted Arsenic Concentrations in Deep
Groundwater Zone
Figure 37 Existing Conditions Scenario 1 - 2115 Predicted Arsenic Concentrations in Bedrock
Groundwater Zone
Figure 38 Excavation Scenario 2 - 2115 Predicted Arsenic Concentrations in Shallow
Groundwater Zone
Figure 39 Excavation Scenario 2 - 2115 Predicted Arsenic Concentrations in Deep Groundwater
Zone
Figure 40 Excavation Scenario 2 - 2115 Predicted Arsenic Concentrations in Bedrock
Groundwater Zone
Figure 41 Predicted Boron in Monitoring Well AB-10S
Figure 42 Predicted Boron in Monitoring Well AB-30S
Figure 43 Predicted Boron in Monitoring Well GWA-6S
Figure 44 Initial (2015) Boron Concentrations in Shallow Groundwater Zone
Figure 45 Initial (2015) Boron Concentrations in Deep Groundwater Zone
Figure 46 Initial (2015) Boron Concentrations in Bedrock Groundwater Zone
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Groundwater Flow and Transport Model
Dan River Steam Station
Figure 47 Existing Conditions Scenario 1 - 2115 Predicted Boron Concentrations in Shallow
Groundwater Zone
Figure 48 Existing Conditions Scenario 1 - 2115 Predicted Boron Concentrations in Deep
Groundwater Zone
Figure 49 Existing Conditions Scenario 1 - 2115 Predicted Boron Concentrations in Bedrock
Groundwater Zone
Figure 50 Excavation Scenario 2 - 2115 Predicted Boron Concentrations in Shallow
Groundwater Zone
Figure 51 Excavation Scenario 2 - 2115 Predicted Boron Concentrations in Deep Groundwater
Zone
Figure 52 Excavation Scenario 2 - 2115 Predicted Boron Concentrations in Bedrock
Groundwater Zone
Figure 53. Predicted Chromium in Monitoring Well AB-10SL
Figure 54. Predicted Chromium in Monitoring Well AB-30S
Figure 55. Predicted Chromium in Monitoring Well GWA-6S
Figure 56 Initial (2015) Chromium Concentrations in Shallow Groundwater Zone
Figure 57 Initial (2015) Chromium Concentrations in Deep Groundwater Zone
Figure 58 Initial (2015) Chromium Concentrations in Bedrock Groundwater Zone
Figure 59 Existing Conditions Scenario 1 - 2115 Predicted Chromium Concentrations in Shallow
Groundwater Zone
Figure 60 Existing Conditions Scenario 1 - 2115 Predicted Chromium Concentrations in Deep
Groundwater Zone
Figure 61 Existing Conditions Scenario 1 - 2115 Predicted Chromium Concentrations in
Bedrock Groundwater Zone
Figure 62 Excavation Scenario 2 - 2115 Predicted Chromium Concentrations in Shallow
Groundwater Zone
Figure 63 Excavation Scenario 2 - 2115 Predicted Chromium Concentrations in Deep
Groundwater Zone
Figure 64 Excavation Scenario 2 - 2115 Predicted Chromium Concentrations in Bedrock
Groundwater Zone
Figure 65 Predicted Cobalt in Monitoring Well AB-10S
Figure 66 Predicted Cobalt in Monitoring Well AB-30S
Figure 67 Predicted Cobalt in Monitoring Well GWA-6S
Figure 68 Initial (2015) Cobalt Concentrations in Shallow Groundwater Zone
Figure 69 Initial (2015) Cobalt Concentrations in Deep Groundwater Zone
Figure 70 Initial (2015) Cobalt Concentrations in Bedrock Groundwater Zone
Figure 71 Existing Conditions Scenario 1 - 2115 Predicted Cobalt Concentrations in Shallow
Groundwater Zone
Figure 72 Existing Conditions Scenario 1 - 2115 Predicted Cobalt Concentrations in Deep
Groundwater Zone
Figure 73 Existing Conditions Scenario 1 - 2115 Predicted Cobalt Concentrations in Bedrock
Groundwater Zone
Figure 74 Excavation Scenario 2 - 2115 Predicted Cobalt Concentrations in Shallow
Groundwater Zone
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Groundwater Flow and Transport Model
Dan River Steam Station
Figure 75 Excavation Scenario 2 - 2115 Predicted Cobalt Concentrations in Deep Groundwater
Zone
Figure 76 Excavation Scenario 2 - 2115 Predicted Cobalt Concentrations in Bedrock
Groundwater Zone
Figure 77 Predicted Hexavalent Chromium in Monitoring Well GWA-10D
Figure 78 Initial (2015) Hexavalent Chromium Concentrations in Shallow Groundwater Zone
Figure 79 Initial (2015) Hexavalent Chromium Concentrations in Deep Groundwater Zone
Figure 80 Initial (2015) Hexavalent Chromium Concentrations in Bedrock Groundwater Zone
Figure 81 Existing Conditions Scenario 1 - 2115 Predicted Hexavalent Chromium
Concentrations in Shallow Groundwater Zone
Figure 82 Existing Conditions Scenario 1 - 2115 Predicted Hexavalent Chromium
Concentrations in Deep Groundwater Zone
Figure 83 Existing Conditions Scenario 1 - 2115 Predicted Hexavalent Chromium
Concentrations in Bedrock Groundwater Zone
Figure 84 Excavation Scenario 2 - 2115 Predicted Hexavalent Chromium Concentrations in
Shallow Groundwater Zone
Figure 85 Excavation Scenario 2 - 2115 Predicted Hexavalent Chromium Concentrations in
Deep Groundwater Zone
Figure 86 Excavation Scenario 2 - 2115 Predicted Hexavalent Chromium Concentrations in
Bedrock Groundwater Zone
Figure 87 Predicted Selenium in Monitoring Well AB-10S
Figure 88 Predicted Selenium in Monitoring Well AB-30S
Figure 89 Predicted Selenium in Monitoring Well GWA-6S
Figure 90 Initial (2015) Selenium Concentrations in Shallow Groundwater Zone
Figure 91 Initial (2015) Selenium Concentrations in Deep Groundwater Zone
Figure 92 Initial (2015) Selenium Concentrations in Bedrock Groundwater Zone
Figure 93 Existing Conditions Scenario 1 - 2115 Predicted Selenium Concentrations in Shallow
Groundwater Zone
Figure 94 Existing Conditions Scenario 1 - 2115 Predicted Selenium Concentrations in Deep
Groundwater Zone
Figure 95 Existing Conditions Scenario 1 - 2115 Predicted Selenium Concentrations in Bedrock
Groundwater Zone
Figure 96 Excavation Scenario 2 - 2115 Predicted Selenium Concentrations in Shallow
Groundwater Zone
Figure 97 Excavation Scenario 2 - 2115 Predicted Selenium Concentrations in Deep
Groundwater Zone
Figure 98 Excavation Scenario 2 - 2115 Predicted Selenium Concentrations in Bedrock
Groundwater Zone
Figure 99 Predicted Sulfate in Monitoring Well AB-10S
Figure 100 Predicted Sulfate in Monitoring Well AB-30S
Figure 101 Predicted Sulfate in Monitoring Well GWA-6S
Figure 102 Initial (2015) Sulfate Concentrations in Shallow Groundwater Zone
Figure 103 Initial (2015) Sulfate Concentrations in Deep Groundwater Zone
Figure 104 Initial (2015) Sulfate Concentrations in Bedrock Groundwater Zone
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Groundwater Flow and Transport Model
Dan River Steam Station
Figure 105 Existing Conditions Scenario 1 - 2115 Predicted Sulfate Concentrations in Shallow
Groundwater Zone
Figure 106 Existing Conditions Scenario 1 - 2115 Predicted Sulfate Concentrations in Deep
Groundwater Zone
Figure 107 Existing Conditions Scenario 1 - 2115 Predicted Sulfate Concentrations in Bedrock
Groundwater Zone
Figure 108 Excavation Scenario 2 - 2115 Predicted Sulfate Concentrations in Shallow
Groundwater Zone
Figure 109 Excavation Scenario 2 - 2115 Predicted Sulfate Concentrations in Deep
Groundwater Zone
Figure 110 Excavation Scenario 2 - 2115 Predicted Sulfate Concentrations in Bedrock
Groundwater Zone
Figure 111 Predicted Thallium in Monitoring Well AB-10S
Figure 112 Predicted Thallium in Monitoring Well AB-30S
Figure 113 Predicted Thallium in Monitoring Well GWA-6S
Figure 114 Initial (2015) Thallium Concentrations in Shallow Groundwater Zone
Figure 115 Initial (2015) Thallium Concentrations in Deep Groundwater Zone
Figure 116 Initial (2015) Thallium Concentrations in Bedrock Groundwater Zone
Figure 117 Existing Conditions Scenario 1 - 2115 Predicted Thallium Concentrations in Shallow
Groundwater Zone
Figure 118 Existing Conditions Scenario 1 - 2115 Predicted Thallium Concentrations in Deep
Groundwater Zone
Figure 119 Existing Conditions Scenario 1 - 2115 Predicted Thallium Concentrations in Bedrock
Groundwater Zone
Figure 120 Excavation Scenario 2 - 2115 Predicted Thallium Concentrations in Shallow
Groundwater Zone
Figure 121 Excavation Scenario 2 - 2115 Predicted Thallium Concentrations in Deep
Groundwater Zone
Figure 122 Excavation Scenario 2 - 2115 Predicted Thallium Concentrations in Bedrock
Groundwater Zone
Figure 123 Predicted Vanadium in Monitoring Well AB-10SL
Figure 124 Predicted Vanadium in Monitoring Well AB-30S
Figure 125 Predicted Vanadium in Monitoring Well GWA-6S
Figure 126 Initial (2015) Vanadium Concentrations in Shallow Groundwater Zone
Figure 127 Initial (2015) Vanadium Concentrations in Deep Groundwater Zone
Figure 128 Initial (2015) Vanadium Concentrations in Bedrock Groundwater Zone
Figure 129 Existing Conditions Scenario 1 - 2115 Predicted Vanadium Concentrations in
Shallow Groundwater Zone
Figure 130 Existing Conditions Scenario 1 - 2115 Predicted Vanadium Concentrations in Deep
Groundwater Zone
Figure 131 Existing Conditions Scenario 1 - 2115 Predicted Vanadium Concentrations in
Bedrock Groundwater Zone
Figure 132 Excavation Scenario 2 - 2115 Predicted Vanadium Concentrations in Shallow
Groundwater Zone
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Groundwater Flow and Transport Model
Dan River Steam Station
Figure 133 Excavation Scenario 2 - 2115 Predicted Vanadium Concentrations in Deep
Groundwater Zone
Figure 134 Excavation Scenario 2 - 2115 Predicted Vanadium Concentrations in Bedrock
Groundwater Zone

ACRONYMS
2L Standard Title 15A NCAC Subchapter 2L.0202 North Carolina Groundwater Quality
Standards
3-D three-dimensional
CAMA North Carolina Coal Ash Management Act of 2014
CAP corrective action plan
CCR coal combustion residuals
COI constituent of interest
CSA comprehensive site assessment
DHHS North Carolina Department of Health and Human Services
DRSS Dan River Steam Station
DRCCS Dan River Combined Cycle Station
EPRI Electric Power Research Institute
HSL health screening level
IMAC interim maximum allowable concentration
NRMSE normalized root mean square error
PPBC proposed provisional background concentration
RMS root mean squared

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Dan River Steam Station

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viii
Groundwater Flow and Transport Model
Dan River Steam Station
1 INTRODUCTION
The purpose of this study is to predict the groundwater flow and constituent transport that may
occur as a result of excavation of ash from the site. This study consists of three main activities:
development of a calibrated steady-state flow model of site conditions observed in June and
July 2015, development of a historical transient model of constituent transport calibrated to
current conditions, and predictive simulations of the source removal operation.
1.1 General Setting and Background
Duke Energy owns and formerly operated the Dan River Steam Station (DRSS), located on a
380-acre tract adjacent to the Dan River in Rockingham County near Eden, North Carolina. The
three-unit station began commercial operation in 1949 with operation of a single coal-fired unit
(Unit 1), with a second unit (Unit 2) being added in 1950. A third unit was added by 1955 and
resulted in a total installed capacity of 276 megawatts. All three coal-fired units, along with three
28 megawatt oil-fired combustion turbine units, were retired in 2012. The Dan River Combined
Cycle Station (DRCCS) a 620 megawatt combined cycle natural gas facility was constructed at
the site and began operations in December 2012. Coal combustion residuals (CCR) from DRSS
operations were disposed in the ash basin system located northeast of the station, as shown on
Comprehensive Site Assessment (CSA) Report Figure 2-1 1(HDR 2015a).
The natural topography at the DRSS site generally slopes from northwest to southeast and
ranges from an approximate high elevation of 606 feet near the northern property boundary just
west of Edgewood Road to an approximate low elevation of 482 feet at the interface with the
Dan River. Ground surface elevation varies about 124 feet over an approximate distance of 0.7
miles. Surface water drainage generally follows site topography and flows from the northwest to
the southeast across the site except where drainage patterns have been modified by the ash
basins or other construction. A site features map is shown on CSA Report Figure 2-4.
Regional groundwater flow in the vicinity of the DRSS is south/southeast toward the Dan River.
Groundwater in the shallow aquifer under the DRSS ash basin and under the ash storage areas
flows horizontally to the southeast and discharges to the Dan River. This flow direction is away
from the direction of the nearest public or private water supply wells. The Dan River serves as a
lower hydrologic boundary for groundwater within the shallow aquifer, and serves as a
discharge feature for shallow groundwater flow from the ash basin.
The North Carolina Coal Ash Management Act of 2014 (CAMA) Section §130A-309.209(b)
requires implementation of corrective action for the restoration of groundwater quality in
accordance with Subchapter L of Chapter 2 of Title 15A of the North Carolina Administrative
Code (T15A NCAC 02L) and requires the submittal of a corrective action plan (CAP) for each
regulated facility no later than 180 days after submittal of the CSA. In conjunction with
decommissioning activities and in accordance with CAMA requirements, Duke Energy will
permanently close the DRSS ash basin by August 1, 2019. Closure of the DRSS ash basin was
defined in CAMA as excavation of ash from the site, and beneficial reuse of the material or
1 Please refer to the Comprehensive Site Assessment Report, Dan River Steam Station Ash Basin, August 2015 (HDR 2015a) for
more information and referenced CSA Report figures and tables.
1

Groundwater Flow and Transport Model
Dan River Steam Station
relocation to a lined structural fill or landfill. As part of the DRSS ash basin closure process,
Duke Energy submitted a coal ash excavation plan to state regulators in November 2014. As
part of the CAP, groundwater modeling was performed to evaluate groundwater flow as well as
fate and transport of COIs for existing and post-excavation conditions at the site.
1.2 Third Party Review
The calibrated flow and transport model third-party peer review team was coordinated by the
Electric Power Research Institute (EPRI) and included Dr. Chunmiao Zheng from the University
of Alabama, James Rumbaugh from Environmental Simulations, Inc., and experienced
modelers from Intera, Inc. The reviewers were provided with the conceptual site model, the Dan
River Comprehensive Site Assessment (CSA) Report (HDR 2015a), a draft model report, and
digital model input and output files, allowing them to reconstruct the model for independent
review. During the course of the review, the reviewers communicated with the modelers to
better understand how the model was developed and calibrated. As a result of these
communications, the model was modified and recalibrated, which allowed the reviewers to
conclude that the model was constructed and calibrated sufficiently to achieve its primary
objective of comparing the effects of closure alternatives on nearby groundwater quality. In
addition, the reviewers identified limitations with the model, which are included in the discussion
of model limitations later in this documentation.
After EPRI acceptance of the initial Dan River groundwater model, the model was improved
using water level results from monitoring wells at the Dan River site. These changes did not
affect the model structure or boundaries and did not deviate from EPRI guidelines. EPRI
reviewed the refined model and confirmed the model continued to meet its flow and transport
calibration objectives sufficiently to meet its final objective of predicting effects of corrective
action alternatives on groundwater quality.
2 CONCEPTUAL MODEL
The site conceptual model for DRSS is primarily based on the CSA Report. The CSA Report
contains extensive detail and data related to most aspects of the site groundwater model used
in this study.
2.1 Geology and Hydrogeology
The DRSS site is underlain by rocks of the Pine Hall and Cow Branch Formations. Alluvial and
terrace deposits consisting of unconsolidated sand, silt, and clay with occasional sub-rounded to
well-rounded pebbles occur along the Dan River and major tributaries. Near surface geology at
the DRSS site is comprised of two interconnected layers, or mediums: 1) residual soil/saprolite
and weathered fractured rock (regolith) overlying 2) fractured bedrock (CSA Report Figure 5-3).
The regolith layer is a thoroughly weathered and structureless residual soil that occurs near the
ground surface with the degree of weathering decreasing with depth. The residual soil grades
into saprolite, a coarser grained material that retains the structure of the parent bedrock.
Beneath the saprolite, partially weathered/fractured bedrock occurs with depth until sound
bedrock is encountered. A transition zone at the base of the regolith has been interpreted to be
present in many areas of the Piedmont including the Dan River Basin. The transition zone
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consists of partially weathered/fractured bedrock and lesser amounts of saprolite that grade into
bedrock and may serve as a conduit for rapid flow and transmission of water.
The groundwater system is a two-medium system that mirrors that of the geology, and is
generally restricted to the local drainage basin. Typically, the residual soil/saprolite is partially
saturated and the water table fluctuates within it. Water movement is generally preferential
through the weathered/fractured bedrock of the transition zone (i.e., zone of higher horizontal
permeability). The character of the system results from the combined effects of the rock type,
fracture system, topography, and weathering. Topography exerts an influence on both
weathering and the opening of fractures, while the weathering of the crystalline rock modifies
both transmissive and storage characteristics. Ranges of hydrostratigraphic layer properties
measured at DRSS are provided in CSA Report Tables 11-6, 11-7, and 11-8.
2.2 Hydrostratigraphic Layer Development
Soil conditions encountered in the borings showed minimal variation across the site. Residual
soil consists of clayey sand, silty sand, silty sand with gravel, micaceous silty sand, and gravel
with silt and sand. The following materials were encountered during the CSA and are consistent
with material descriptions from previous site exploration studies.
Ash – Ash was encountered in borings advanced within the ash basin and ash storage areas,
as well as in some borings advanced through the pond perimeter and intermediate dam. Ash
several inches thick was encountered in one location within the ash dredge area located
between Ash Storage 1 and Ash Storage 2. Ash was generally described as gray to dark bluish
gray with a silty to sandy texture, consistent with fly ash and bottom ash.
Fill – Fill material generally consisted of re-worked materials from the DRSS site. The base of
filled areas was difficult to distinguish from in-place soil/saprolite.
Alluvium – Alluvium is unconsolidated soil and sediment that has been eroded and redeposited
by streams and rivers.
Residuum (Residual Soils) – Residuum is the in-place soil that develops by weathering.
Saprolite/Weathered Rock – Saprolite is soil developed by in-place weathering of rock.
Partially Weathered/Fractured Rock – Partially weathered (slight to moderate) and/or highly
fractured rock was encountered below refusal (auger, casing advancer, etc.).
Bedrock – Sound rock in boreholes, generally slightly weathered to fresh and relatively
unfractured.
2.3 Ash Basin and Ash Storage Areas
Historical and current information about the DRSS ash basin system compiled by HDR during
the CSA was used to form a conceptual understanding that was developed into a numerical
groundwater flow model. Refer to CSA Report Figures 2-1 and 2-2 for locations of ash basin
system components described below.
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2.3.1 Ash Basin
The current ash basin system was constructed in three phases beginning in 1956 and ending in
1976. The original ash basin was constructed in 1956 with a footprint that lies within the current
primary cell. The dam for the original basin was constructed as a homogenous sandy silt
embankment with an approximate crest elevation of 525 feet. In 1968 and 1969, the ash basin
was expanded to a footprint the size of the current primary and secondary cells by constructing
another dam with a crest elevation of approximately 530 feet. Based on a review of historical
construction drawings, it appears the dams were constructed of earthen materials borrowed
from within the ash basin footprint. At that time, the ash basin was a single impoundment. In
1976, the ash basin was divided into the current primary and secondary cells by construction of
an interior dike on top of existing ash. This divider dike was constructed as an earthen
embankment at elevation approximately 530 feet. The embankment for what is now the primary
cell was raised to 540 feet.
The area contained within the waste boundary for the primary and secondary cells currently
extends over an area of approximately 53 acres. The primary cell extent is approximately 28.1
acres and contains approximately one million tons of ash material. The primary cell has a crest
elevation of approximately 540 feet and has an impoundment surface area of approximately
21.8 acres. The secondary cell extent is approximately 15.3 acres and contains approximately
200,000 tons of ash material. The secondary cell has a crest elevation of approximately 530 feet
and has a current impoundment surface area of approximately 12.2 acres. The elevation of the
Dan River adjacent to the ash basin is approximately 482 feet.
During operation of the coal-fired units, the ash basin received variable inflows of fly ash, bottom
ash, pyrites, stormwater runoff (including runoff from the coal pile), cooling water, powerhouse
floor drains, sanitary waste effluent, station yard drainage sump, and boiler chemical cleaning
wastes. Flow was historically routed from the primary cell to the secondary cell through a
concrete discharge tower. Since February 2, 2014, all inflows into the ash basin have been
routed directly to the secondary cell. Effluent from the secondary cell is routed to the Dan River
via a concrete discharge tower located in the secondary cell. The water surface in both the
primary and secondary cells is controlled by the use of stop logs.
2.3.2 Ash Storage 1
The ash storage area identified as Ash Storage 1 is located north of the primary and secondary
cells on an upland area of the site. The unit is bounded by a topographic rise to the north, filling
former drainages of the rise to the south. The surface area within the waste boundary for Ash
Storage 1 encompasses approximately 20.1 acres. The CCR and soil material stored in the Ash
Storage 1 footprint was placed during several projects between 1994 and 2010 and totals
approximately 1.1 million tons.
Ash was previously dredged to the southernmost portion of Ash Storage 1, and free liquids were
allowed to gravity drain to the topographically lower dredge pond located between the dry ash
storage areas. Once dewatered, ash was hauled and placed dry in Ash Storage 1 and Ash
Storage 2 and a vegetated soil cap covers the storage area footprint.
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2.3.3 Ash Storage 2
The ash storage area identified as Ash Storage 2 is located southwest of Ash Storage 1 and on
the southwest slope of a former ridge on the site. The cell is bounded by a topographic rise to
the north, filling a former drainage at the southwest base of the former ridge. The area
contained within the waste boundary for Ash Storage 2 encompasses approximately 13.4 acres.
The CCR and soil material in Ash Storage 2 was placed in 1994 and during 1997-1998 and
totals approximately 372,000 tons.
Ash Storage 2 was created in a similar manner as Ash Storage 1, using ash removed from the
ash basin. A vegetated soil cap covers the surface of Ash Storage 2, and the footprint this
capped surface is currently used as a laydown area when needed for various projects at the
site.
2.4 Groundwater Flow System
Groundwater is recharged by infiltration where the ground surface is permeable, including the
dikes and ash of the ash basin system where exposed at the ground surface. After infiltrating
the ground surface, water in the unsaturated zone percolates downward to the unconfined water
table, except where ponded water conditions exist on a portion on the secondary cell of the ash
basin system. From the water table, groundwater moves downward and laterally through
unconsolidated material (residual soil/saprolite) into the weathered, fractured rock, then into
fractured bedrock.
Mean annual recharge in the Piedmont ranges from 4.0 to 9.7 inches per year (Daniel 2001).
However, given that it has not been measured or estimated in the CSA or other studies of the
site, recharge to the ash basin system has been estimated by considering it as a calibration
parameter in the groundwater flow model.
Groundwater under the DRSS site flows horizontally generally toward the south and discharges
to the Dan River. Lesser amounts of groundwater discharge to unnamed streams and the
service settling water ponds to the east and west of the site.
In accordance with the Piedmont slope aquifer system (LeGrand 2004), fractured density in
bedrock decreases with depth, limiting deep groundwater flow as shown on CSA Report Figure
5-5.
2.5 Hydrologic Boundaries
The major discharge locations for the groundwater system at DRSS, the Dan River, and
unnamed streams and the service settling water ponds to the east and west, serve as
hydrologic boundaries. Local ditches and streams also serve as local, shallow hydrologic
boundaries. In the flow model, these features are treated as internal water sinks as described in
Section 4.5.
2.6 Hydraulic Boundaries
The groundwater flow system at the DRSS study area does not contain impermeable barriers or
boundaries with the exception of bedrock at depths where fracture density is minimal. Natural
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groundwater divides exist along topographic divides, but are a result of local flow conditions as
opposed to barriers.
2.7 Water Sources and Sinks
Recharge, including that from the ash basins, is the major source of water to the groundwater
system. Most of this water discharges to the hydrologic boundaries described above.
The CSA receptor survey identified three private water supply wells and no public water supply
wells within the 0.5 mile radius of the ash basin compliance boundary. The CSA Report
indicates that DRSS ash basin is not considered to be within the capture zone or zone of
influence of any extraction well.
2.8 Water Budget
Over an extended period of time, the rate of water inflow to the study area is equal to its rate of
outflow. That is, there is no change in groundwater storage. Water enters the groundwater
system through recharge and ultimately discharges to the Dan River, unnamed streams and the
service settling water ponds to the east and west, and other small-scale discharge locations.
Recharge to the ash basin represents the summation of precipitation, evaporation,
evapotranspiration, plant wastewater discharge, and discharge through the outlet structures.
2.9 Modeled Constituents of Interest
As defined in the CSA, constituents are those chemicals or compounds that were identified in
the approved groundwater assessment plans for sampling and analysis. If a constituent
exceeded its respective regulatory standard or screening level in the medium in which it was
found, the constituent was then termed a constituent of interest (COI). The CSA found the
following COIs in the ash porewater at DRSS: antimony, arsenic, barium, beryllium, chromium,
cobalt, iron, lead, manganese, thallium, and vanadium.
Antimony, arsenic, boron, chromium, hexavalent chromium, cobalt, selenium, sulfate, thallium,
and vanadium were considered in the transport simulations. Sulfate and boron occur at elevated
levels in water infiltrating from ash basin systems, and are considered very mobile in
groundwater as these constituents do not readily precipitate or adsorb to soils. Antimony,
arsenic, chromium, hexavalent chromium, cobalt, selenium, thallium, and vanadium, also occur
at elevated levels in infiltrating water, but are readily adsorbed to commonly occurring soil types.
2.10 Constituent Transport
Chemical constituents enter the ash basin system in the dissolved phase and solid phase of the
station’s wastewater discharge. Some constituents are also present in native soils and
groundwater beneath the basin. In the ash basin, constituents may incur phase changes
including dissolution, precipitation, adsorption, and desorption. Dissolved phase constituents
may incur these phase changes as they are transported in groundwater flowing downgradient
from the basin. In the fate and transport model, chemical constituents enter the basin in the
dissolved phase by specifying a steady state concentration in the ash porewater. Phases
changes (dissolution, precipitation, adsorption, and desorption) are collectively taken into
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account by specifying a linear sorption coefficient Kd. The accumulation and subsequent release
of chemical constituents in the ash basin over time is a complex process. In the conceptual fate
and transport model, it was assumed that the entry of constituents into the ash basin is
represented by a constant concentration in the saturated zone of the basin, which is continually
flushed by infiltrating recharge from above.
At the ash storage areas, leachable constituents enter the dissolved phase during transient
infiltration events through the stored ash. Infiltration with constituents in solution occurs
downward through the stored ash and underlying, unsaturated soils and finally into groundwater
at the water table. Dissolved phase constituents may also incur phase changes as they migrate
through the stored ash and native soils to the water table. In the conceptual fate and transport
model, it was assumed that the entry of constituents into groundwater beneath the ash storage
areas is represented by a constant concentration at the water table beneath the ash storage
areas, which is continually flushed by infiltrating recharge from above.
The following approach was used for transport modeling:
• A physical-type modeling approach was used, as site-specific geochemical conditions
are not understood or characterized at the scale and extent required for inclusion in the
model.
• The flux of contaminant mass from the ash sources is not quantified, so it is not included
in the conceptual site model or represented in the numerical model. As such, a simplified
approach was used and the entry of constituents was represented in the model using a
constant concentration in the saturated zone of the basin (which is continually flushed by
water moving through the porous media). The constant concentration cells for various
source areas within the ash basin were determined during transport model calibration
based on observed detections in downgradient monitoring wells.
• The retardation effects of the COI (e.g., by adsorption onto solid surfaces) were
collectively taken into account by specifying a linear soil-water partitioning coefficient
(Kd).
3 COMPUTER MODEL
3.1 Model Selection
The computer code MODFLOW solves the system of equations that quantify the flow of
groundwater in three dimensions. MODFLOW simulates steady-state and transient flow, as well
as confined and water table conditions. Additional components of groundwater can be
considered including pumping wells, recharge, evapotranspiration, rivers, streams, springs, and
lakes. The information assembled in the conceptual site model is translated into its numerical
equivalent from which a solution is generated by MODFLOW.
3.2 Model Description
The specific MODFLOW package chosen for the study was NWT - a Newton formulation of
MODFLOW-2005 that is specifically designed for improving the stability of solutions involving
drying and re-wetting under water table conditions (Niswonger, et al. 2011). The numerical code
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selected for the transport model is MT3DMS (Zheng and Wang, 1999). MT3DMS is multi-
species three-dimensional (3-D) transport model that can simulate advection,
dispersion/diffusion, and chemical reaction of COIs in groundwater flow systems and has a
package that provides a link to the MODFLOW codes. The MODFLOW-NWT and MT3DMS
input packages used to create the groundwater flow and transport model are provided in
Table 1 along with a brief description of their use.
4 GROUNDWATER FLOW AND TRANSPORT MODEL CONSTRUCTION
The flow and transport model for the study was developed through a multi-step process. First, a
3-D model of the site hydrostratigraphy was constructed based on historical site construction
drawings and field data. Once the model domain was determined, a 3-D steady-state
groundwater model based on the hydrostratigraphy and the site conceptual model was
produced. Flow parameters, assigned to the numerical grid, were adjusted during the steady-
state flow model calibration process. Once the steady-state flow model provided a reasonable
calibration was calibrated, a transient transport simulation for the selected COIs was developed,
and then calibrated by adjusting transport parameters and revising flow parameters to best
match the observed 2015 concentrations and water levels in selected monitoring wells.
Three terrain surface models for DRSS were created using geographic information systems
(GIS) software: 1) current existing surface, 2) pre-construction surface without ash and ash
basin dikes, and 3) pre-construction surface with dikes but without ash. An interpolation tool in
ArcGIS 10.3 software was used to generate the terrain surfaces as a raster datasets with 20-
foot cells. Each surface was created to cover the extent of the groundwater model domain.
4.1 Model Hydrostratigraphy
The model hydrostratigraphy was developed using historical site construction drawings and
borehole data to construct 3-D surfaces representing contacts between hydrostratigraphic units
with properties provided in CSA Report Tables 11-6 through 11-10.
4.1.1 Existing Ground Surface
Topographic and bathymetry elevation contours and spot elevations were produced from
surveys conducted in 2014. Since these surveys did not cover the entire model extent, elevation
data extracted as spot elevations from the North Carolina Floodplain Mapping Program’s 2010
LiDAR elevation data were used for the areas surrounding the surveys. At DRSS, simplified
elevation contours were digitized along the river channels to depress the surface a small
amount below water level.
4.1.2 Pre-construction Surface
Elevation contours of the original ground surface were digitized in CAD from engineering
drawings supplied by Duke Energy. These data were imported into GIS, and georeferenced.
These contours were trimmed to the areas underlying ash basins, dams, and dikes and ash
storage areas. The source data used in the existing surface were then replaced by the original
surface data where there was overlap. Elevation data from coal storage areas were removed.
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The pre-construction surface was then created using the combination of original surface
elevations, 2014 survey elevations, and 2010 LiDAR elevations.
4.1.3 Pre-construction Surface with Dikes
Surface models of the ash basin dams and dikes were constructed from crest elevations as
determined from the 2014 survey and slopes given on the engineering drawings. Only the
sections of the dams and dikes facing the ash basins were modeled in this way. The 2014
survey data were used for dike/dam crests and outwardly facing surfaces. These surfaces were
merged with the pre-construction surface. These GIS data sets were exported into formats
readable by RockWorks and GMS MODFLOW.
4.1.4 3-D Hydrostratigraphic Grids
The natural materials in the CSA boreholes and existing boreholes were assigned a
hydrostratigraphic layer using the above classification scheme and judgment and the borehole
data entered into RockWorks 16™ for 3-D modeling. In the portions of the area to be modeled
for which borehole data is not available, dummy boreholes were used to extend the model to the
model boundaries. These boreholes were based on the hydrostratigraphic thickness of the
existing boreholes and the elevation of the existing boreholes based on the assumption that the
hydrostratigraphic layers are a subdued replica of the original topography of the site and
geologic judgment.
A grid of the pre-construction ground surface (described above) was used to constrain the
modeling of the natural layers. For gridding the data on a 20 foot x 20 foot grid across the area
to be modeled, a hybrid algorithm was used with inverse distance weighted two , triangulation
weighted one , and declustering, smoothing, and densifying subroutines. The declustering
option is used to remove duplicate points and de-cluster clustered points. The option creates a
temporary grid with a z-value assigned based on the closest data point to the midpoint of a
voxel. The smoothing option averages the z-values in a grid based on a filter size. For this
modeling, the z-value is assigned the average of itself and that of the eight nodes immediately
surrounding it. One smoothing pass is made. The densify option adds additional points to the
xyz input by fitting a Delaunay triangulation network to the data and adding the midpoint of each
triangle to the xyz input points. The net result is that the subsequent gridding process uses more
control points and tends to constrain algorithms that may become creative in areas of little
control. Only one densification pass is made. The completed model grids were exported in
spreadsheet format for use in the groundwater flow and transport model.
4.2 GMS MODFLOW Version 10
The conceptual model approach to construct a MODFLOW simulation in GMS MODFLOW
consists of employing GIS tools in a map module to develop a conceptual model of the site
being modeled. The location of sources/sinks, layer parameters (such as hydraulic conductivity),
and all other data necessary for the simulation can be defined at the conceptual model level.
Once this model is complete, the grid is generated and the conceptual model is converted to the
grid model and all of the cell-by-cell assignments are performed automatically
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The following table presents the sequence of the steps used for the groundwater modeling.
Steps 1 through 6 describe the creation of 3-D MODFLOW model.
Step 1. Creating raster files for the model layer
- Three surface layers (pre-construction, pre-construction with dike, and existing
surface including dike and ash) using GIS and AutoCad
- Two subsurface layers (transition zone and bedrock ) by converting 3-D scatter data
Step 2. Creating the raster catalog to group the raster layers
- Assigning horizons and materials for each layer
Step 3. Creating horizon surfaces (i.e., triangulated irregular network (TIN)) from raster data
- Used exiting surface and bedrock rasters
Step 4. Building solids from the raster catalog and TINs
- Used raster data for the top and bottom elevations of the solids
Step 5. Creating the conceptual model
- Building model boundary, specified head boundary, and drain
- Defining zones and assigning hydraulic conductivity and recharge rate
- Importing observation wells and surface flow data
Step 6. Creating the MODFLOW 3-D grid model
- Converting the solids to 3-D grid model using boundary matching
- Mapping the conceptual model to MODFLOW 3-D grid
Step 7. Flow model calibration/sensitivity analysis
- Initializing the MODFLOW model
- Steady-state calibration with the trial-and error method
- Parameters: hydraulic conductivity and recharge rate
- Used observation well and surface flow data
Step 8. Setting the transport model (MT3DMS)
- Species
- Stress periods
- Porosity and dispersion coefficient
- Distribution coefficient (Kd) from the lab experiments
- Recharge concentrations
Step 9. Performing model simulations
- Model scenarios – 1) Existing Conditions, 2) Excavation

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4.3 Model Domain and Grid
The model domain encompasses the DRSS site, including a section of the Dan River and all
site features relevant to the assessment of groundwater. The model domain extends beyond the
ash management areas to hydrologic boundaries such that groundwater flow and COI transport
through the area is accurately simulated without introducing artificial boundary effects.
The bounding rectangle around the model domain extends 5,000 feet north to south and 6,000
feet east to west and has a grid consisting of 255,442 active cells (Figure 1). In plan view, the
DRSS model domain is bounded by the following hydrologic features of the site:
• the northern shore of the Dan River to the south,
• the unnamed stream to the east,
• the presumed groundwater divide along a topographic divide north of the site, and
• the unnamed stream and the service settling water pond to the west.
The domain boundary was developed by manually digitizing two-foot LiDAR contours in
ARCMAP.
The lower limit of the model domain coincides with an assumed maximum depth of water
yielding fractures in bedrock. This was assumed to be 80 feet below the base of the transition
zone across the site upper limit based on a review of boring logs contained in the CSA Report.
There are a total of 12 model layers divided among the identified hydrostratigraphic units to
simulate curvilinear flow with a vertical flow component (Figures 2 and 3). The units are
represented by the model layers listed below:
• Model layers 1 through 3 Ash Material
• Model layers 4 through 6 Dike and Ash Material
• Model layer 7 Saprolite and Alluvium where present
• Model layer 8 Saprolite
• Model layer 9 Saprolite
• Model layer 10 Transition Zone
• Model layers 11 and 12 Fractured Bedrock
In the groundwater flow model, fractured bedrock was simulated as an equivalent porous
medium. The materials comprising each layer and typical layer thicknesses are shown in the
north-south and east-west cross-sections through the active/inactive ash basin areas on Figure
2 and Figure 3.
4.4 Hydraulic Parameters
Horizontal and vertical hydraulic conductivities, which are specific for each hydrostratigraphic
unit, are the primary determinants of groundwater flow for a given set of boundary conditions
and sources and sinks, including recharge. Field measurements of these parameters from CSA
Report Tables 11-6 through 11-10 provided guidance for their selection during the flow model
calibration. Values assigned to the model are provided in Table 2.
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4.5 Flow Model Boundary Conditions
Boundary conditions for the DRSS flow model are one of two types: constant head or drain. No-
flow boundaries of the site have no prescribed boundary in the model. The outer boundary of
the model domain was selected to coincide with physical hydrologic boundaries at rivers and
drainage features, and no flow boundaries at topographic divides (Figure 4).
At the model boundary defined by the Dan River and the cooling water settling pond, a constant
head boundary was applied at those layers above fractured bedrock (model layers 11 and 12)
with bottom elevations below the water surface (Figure 4). The head was interpolated digitally
from photogrammetry recorded in 2014.
Drain boundaries were applied at the unnamed streams east and west of the site (Figure 5).
Drain boundaries were applied with a conductance calculated using the hydraulic conductivity of
the adjacent model layer, an assumed bed thickness of one foot, and width equal to the model
cell width. It was assumed that this surface drainage is ultimately conveyed to an outfall at the
Dan River.
Constant heads used to represent the Dan River may provide an unmitigated source of water
for simulation of any corrective action alternatives that potentially involve pumping near those
boundaries. If corrective actions involve pumping, drain boundaries may be a more appropriate
boundary condition selection than constant heads for this boundary.
4.6 Flow Model Sources and Sinks
Recharge is the only source considered in the model. (Drainage boundaries are described
above as flow boundary conditions.) Mean annual recharge in the Piedmont ranges from 4.0 to
9.7 inches per year (Daniel 2001). Recharge applied to the area of the model outside the ash
basins was assigned a value of 6.5 inches per year. Recharge within the ash basins was
calculated using Darcy’s Law based on the area of the ash basin, the approximate depth of
water or saturated ash, and the range of measured hydraulic conductivity values within the ash
and fill. The calculation provided a narrow range of recharge values, with a value of 13.14
inches per year applied to the ash basins. Recharge in the model is shown in Figure 5.
No pumping wells or other sources and sinks were identified in the CSA Report.
4.7 Flow Model Calibration Targets
The steady state flow model calibration targets are the 50 water level observations made in
June 2015. These wells include 16 wells screened in the ash, ash dike, and shallow zone, 20
wells in the transition zone, and 14 wells in fractured bedrock. Observations were assigned by
layer as shown in Table 3.
4.8 Transport Model Parameters
The calibrated, steady-state flow model was used to apply flow conditions for the transport
models at the primary and secondary cells of the ash basin and the ash storage areas (CSA
Report Figures 2-1 and 2-2). Although their approximate dates of operation are known, the
sluiced ash loading histories for these locations are not available. In order to calibrate the
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transport model to existing conditions, constant concentration source zones were applied at the
ash model layers in the ash basin cell and the layers where the water table occurs beneath the
ash storage areas starting from the date when each was placed in service (Figure 5). The
relevant input parameters were the constant concentration at the source zone, the linear
sorption coefficient (Kd) for sorptive constituents (including all COIs, except for sulfate), the
effective porosity, and the dispersivity tensor.
The conceptual transport model specifies that COIs enter the model from the shallow saturated
source zones in the ash basins. The range of Kd values applied was derived from UNCC
laboratory measured values and adjusted to achieve calibration in the model. The most
appropriate method to calibrate the transport model was to use the lower limit of measured Kd
values to produce an acceptable agreement between measured and modeled concentrations.
Thus, an effective Kd value results that likely represents the combined result of intermittent
activities over the service life of the ash basin. These may include pond dredging, dewatering
for dike construction, and ash grading and placement. This approach is expected to produce
conservative results, as sorbed constituent mass is released and transported downgradient.
The Kd values for the COIs were applied as follows:
• Antimony: 20 mL/g
• Arsenic: 55 mL/g
• Boron: 1 mL/g
• Chromium: 20 mL/g
• Cobalt: 220 mL/g
• Hexavalent Chromium: 15 mL/g
• Selenium: 31 mL/g
• Sulfate: conservative (sorption not modeled)
• Thallium: 290 mL/g
• Vanadium: 25 mL/g
The bulk density used in the model is 2.65 grams/cubic centimeter for saprolite, deep, and
bedrock materials, and 2.12 grams/cubic centimeter for ash and dike materials.
The velocity of COIs in groundwater is directly related to the effective porosity of the porous
medium. A single effective porosity value of 0.10 was assigned to the ash and dike layers. For
the S/M1 layers, an effective porosity value of 0.05 was assigned. For the M2 layer, an effective
porosity value of 0.10 was assigned. The effective porosity values are within the range of
estimated values from the CSA Report. For the transition zone and fractured bedrock, the
porosities applied were 0.01 and 0.001, respectively.
Dispersivity quantifies the degree to which mechanical dispersion of COIs occurs in advecting
groundwater. Dispersivity values of 80 feet, 8 foot, and 8 foot (longitudinal, transverse
horizontal, transverse vertical) were applied in this model. Traditionally, dispersivity is estimated
to be some fraction of the scale, or plume length (Zheng and Bennett, 2002). The commonly
applied estimate is 10% of the observation scale. In order to avoid artificial oscillation in the
numerical solution to the advection dispersion equation, the grid Peclet number, or the ratio of
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Dan River Steam Station
grid spacing to longitudinal and transverse dispersivity, and the ratio of layer thickness to
vertical dispersivity, should be less than two and no more than 10 to minimize numerical
dispersion (Zheng and Bennett 2002). The longitudinal and transverse dispersivity results in grid
Peclet numbers of 0.25 and 2.5, which is within the acceptable range. The vertical discretization
is more variable than the horizontal discretization with model layers 7 and 8 and fracture
bedrock layers at approximately 20 to 40 feet, and the transition zone on the order of 10 feet or
less. The variable vertical discretization results in grid Peclet numbers of 0.625 to 5, which is
within the acceptable range.
4.9 Transport Model Boundary Conditions
The transport model boundary conditions have an initial concentration of zero where water
leaves the model. The background concentration used as initial concentrations for each COI is
specified as the proposed provisional background concentration (PPBC) identified in the DRSS
CAP Part 1 (HDR, 2015b). Recharge does not have a specified concentration.
The background concentrations for the COIs applied as initial concentrations are as follows:
• Antimony: 1 µg/L/g
• Arsenic: 1 µg/L
• Boron: 50 µg/L
• Chromium: 5 µg/L/g
• Cobalt: 0.5 µg/L
• Hexavalent Chromium: 0.089 µg/L
• Selenium: 1 µg/L
• Sulfate: 31,000 µg/L
• Thallium: 0.2 µg/L
• Vanadium: 1 µg/L
4.10 Transport Model Sources and Sinks
The ash basin primary and secondary cells and the ash storage areas are the sources for COIs
in the model. These sources are modeled as fixed concentrations in the layers at the water table
of these areas and their constant concentrations are shown in Table 5. The source values are
based on the ash porewater sample results provided in CSA Report Tables 7-5 and 7-6.
The transport model sinks correspond to the constant head and drain boundaries of the flow
model. Water and COI mass are removed as they enter cells comprising these boundaries.
4.11 Transport Model Calibration Targets
The calibration targets are the measured COI concentrations for June/July 2015 shown in CSA
Report Tables 7-5, and 10-9 to 10-11. Specific calibration target wells are highlighted in Section
5.3 of this study.
14
Groundwater Flow and Transport Model
Dan River Steam Station
5 MODEL CALIBRATION TO CURRENT CONDITIONS
5.1 Flow Model Residual Analysis
The flow model was calibrated to the water level observations taken during July 2015 in the
shallow, deep, and bedrock wells (Table 3). The water table elevations in the ash basin cells
and selected drainage features and depressions across the site were also considered as
calibration targets. The observation data from this single point in time were used as a flow
model calibration data set.
The locations of the observation wells are provided in Figures 6, 7, and 8. Throughout the flow
model calibration process, the horizontal conductivity was assigned based on slug test and
packer test results to give a reasonable, initial model result for the water table and water level
observations, as shown on Figures 9, 10, and 11, and in Table 2. Recharge was also fixed at
reasonable values early in the calibration process, then refinements were made by adjusting
hydraulic conductivity with the model by zones as shown in Figures 9, 10, and 11. The goal of
delineating the zones in this way was to obtain the best local calibration using hydraulic
conductivity values within the range of measurements made during the CSA.
The calibrated flow model hydraulic conductivity parameters are provided in Table 2. Measured
and modeled water levels (post-calibration) are compared in Table 3 and on Figure 12. The
calibrated flow model is considered to represent steady-state flow conditions for the site and the
ash basin system under long-term, average conditions. This assumption should be verified as
additional data are collected from the existing and any additional monitoring wells.
The square root of the average square error (also referred to as the root mean squared error, or
RMS error) of the modeled versus observed water levels for wells gauged in June 2015 is also
provided in Table 3. The model calibration goal is an RMS error less than 10% of the change in
head across the model domain. The ratio of the average RMS error to total measured head
change is the normalized root mean square error (NRMSE). The NRMSE of the calibrated
model is 5.5%.
Contours of hydraulic heads for the calibrated steady-state flow model are shown for the
shallow groundwater zone (model layer 8) in Figure 13. From the upland area of the site,
groundwater in the shallow, deep, and fractured bedrock zones flows to the southwest, south,
and southeast. In the shallow zone, head contours reflect the zones of variable hydraulic
conductivity. All groundwater flow ultimately discharges to the Dan River, the unnamed stream
to the east, and the unnamed stream and the service settling water pond to the west. Head
gradients steepen in the direction of the discharge boundaries. Most groundwater flow passing
beneath and originating from the ash basins and ash storage areas discharges to the Dan
River. The remainder discharges to the unnamed stream to the east. Figures 14 and 15 show
the water table cross sections across the site.
5.2 Flow Model Sensitivity Analysis
Sensitivity of the flow model was considered by varying selected parameters by the amounts
shown in Table 4 above and below their respective calibration values and calculating the
15
Groundwater Flow and Transport Model
Dan River Steam Station
NRMSE for comparison with the calibration value. The model was more sensitive to decreasing
horizontal hydraulic conductivity (Kh) of the transition zone relative to the shallow zone. A
slightly lower NRMSE error was noted for increased Kh in both zones. Decreasing recharge
inside the ash basin yielded a slightly lower NRMSE, while changes in recharge outside the ash
basin a much higher NRMSE. The model NRMSE was relatively insensitive to changes in
shallow zone vertical hydraulic conductivity.
5.3 Transport Model Calibration and Sensitivity
For the transport model calibration, the calibration parameters consisted of the constant source
concentrations, porosity and the linear sorption coefficient (Kd) for each COI. These parameters
were adjusted to minimize residual concentrations in target wells. Applying the low end of
measured sorption coefficients (Kd) for sorptive COIs was essential to achieve a reasonable
calibration as discussed in Section 4.8. Dispersivity and porosity changes can also be applied
for transport calibration, but their simultaneous effects on each COI must be considered. The
model assumed initial concentrations within the groundwater system for all COIs at the
beginning of operations approximately 59 years ago. A source term matching the ash porewater
concentrations for each COI was applied within the active ash basin and the ash storage areas
at the start of the calibration period. The source concentrations were adjusted within the range
of ash porewater concentrations to match measured values in the downgradient monitoring
wells that had exceedances of the 2L Standard 2 or interim maximum allowable concentration
(IMAC) for each COI in June 2015. Monitoring wells with measured values below the 2L
Standard or IMAC for each COI were also observed in the model during calibration, as it is also
important to not overestimate the spatial or vertical extent of measured COIs. During transport
model calibration, the flow model parameters were also modified within measured values as
needed to provide a better constituent calibration. This iterative process provided a better flow
and transport calibration as the spatial extent of elevated constituents provides insight into
groundwater flow directions and velocities.
The calibration results comparing measured versus predicted model concentrations are
provided in Table 5 for the modeled COIs. Table 5 also shows the calibration source
concentrations in the active ash basin and the ash storage areas. The locations of the
monitoring wells are provided in Figures 6, 7, and 8.
These calibration parameters were used in the transport model to simulate the initial (2015)
concentrations in the shallow, deep, and bedrock groundwater flow zones of each COI for the
excavation scenario.
A detailed sensitivity analyses for porosity, dispersivity, and sorption were not completed as part
of CAP Part 2 as informal analysis indicated that sensitive parameters did not change due to
revisions to the model parameters. A decrease in the sorption coefficient (Kd) resulted in an
increase in the spatial extent in the modeled concentrations from the source areas. An increase
in porosity and dispersivity also resulted in an increase in the spatial extent in the modeled
concentrations from the source areas.
2 North Carolina Groundwater Rules; Title 15A, Subchapter 02L of the NC Administrative Code.
16

Groundwater Flow and Transport Model
Dan River Steam Station
5.4 Advective Travel Times
Particle tracking was performed during model calibration to determine if advective travel times
are reasonable. Particles were placed in the shallow zone at wells located near the Dan River,
downgradient of the ash storage area and also near the northern model boundary. The particle
tracks are shown in Figure 16 and predicted advective travel times are provided in Table 6.
6 SIMULATIONS OF CLOSURE SCENARIOS
The groundwater model, calibrated for flow and constituent fate and transport under existing
conditions, was applied to evaluate two ash basin closure scenarios at the DRSS: 1) Existing
Conditions and 2) Excavation (ash removal). Being predictive, these simulations produce flow
and transport results for conditions that are beyond the range of those considered during model
calibration. Thus, the model should be recalibrated and verified over time as new data become
available in order to improve its accuracy and reduce its uncertainty.
The model domain developed for existing conditions was applied without modification for the
Existing Conditions scenario. For the Excavation scenario, model layers containing ash in the
ash basin were made inactive. The flow parameters for this model were identical to the Existing
Conditions scenario, except for the removal of ash related layers, and the same recharge being
applied to the ash basin as the remainder of the site.
6.1 Existing Conditions Scenario
The Existing Conditions scenario consists of modeling each COI using the calibrated model for
steady-state flow and transient transport under the existing conditions across the site to
estimate when steady-state concentrations would be reached across the site and at the
compliance boundary. COI concentrations can only remain the same or increase initially for this
scenario with source concentrations being held at their constant value over all time. Thereafter,
their concentrations and discharge rates remain constant. This scenario represents the worst
case in terms of groundwater concentrations on and off site, and COIs discharging to surface
water at steady state.
The time to achieve a steady-state concentration plume depends on the source zone location
relative to the compliance boundary and its loading history. Source zones close to the
compliance boundary will reach steady-state sooner. The time to steady-state concentration is
also dependent on the sorptive characteristics of each COI. Sorptive COIs will be transient for a
longer time period as their peak breakthrough concentration travels at a rate that is less than the
groundwater pore velocity. Lower effective porosity will result in shorter times to achieve steady
state for both sorptive and non-sorptive COIs. Lower total porosity will result in longer times for
sorptive COIs.
6.2 Excavation Scenario
In the Excavation scenario, ash from the ash basin and ash storage areas is removed and
transported offsite. In the model, the constant concentration sources and ash above and below
the water table are removed. This scenario assumes recharge rates become equal to rates
surrounding the ash basins (6.5 inches per year). Starting from the time that excavation is
17
Groundwater Flow and Transport Model
Dan River Steam Station
complete, COIs already present in the groundwater continue to migrate downgradient as clean
water infiltrates from ground surface and recharges the aquifer at the water table. The COIs are
flushed from the saturated zone beneath the source areas. COI migration is retarded relative to
the ash porewater velocity as sorptive COIs adsorb to the soil/rock surfaces. The model uses
the predicted concentration from the 2015 calibration as the initial concentration.
7 CLOSURE SCENARIO RESULTS
Closure scenario results are presented as predicted concentration vs. time plots in
downgradient monitor wells and as groundwater concentration maps for each modeled COI in
Figures 17 through 134, as discussed in the following sub-sections. Groundwater concentration
maps were generated based on groundwater zones and include the various model layers as
follows:
• Shallow groundwater zone – model layers 1 through 9 (S/M1, M2)
• Deep groundwater zone – model layer 10 (D, transition zone)
• Bedrock groundwater zone – model layers 10 and 11 (BR)
Concentration contours and concentration breakthrough curves are all referenced to a time zero
that represents the time the closure action was implemented, which for the purposes of
modeling is assumed to be 2016. Concentration contours and concentration breakthrough
curves are referenced to 1957, since that is the year that the ash basin became effective.
Constituent concentrations were analyzed at three downgradient monitoring wells: AB-10S/SL,
AB-30S and GWA-6S (Figure 6) for all COIs, except hexavalent chromium. Hexavalent
chromium concentrations were analyzed at downgradient monitoring well GWA-10D (Figure 7).
Monitoring wells AB-10S/SL are located between the primary and secondary ash basins,
downgradient of the ash storage areas and the northern extent of the ash basins. Monitoring
well AB-30S is located directly downgradient of the secondary ash basin on the southern side of
the earthen embankment, while GWA-6S is located at the southwestern toe of the primary ash
basin embankment. Monitoring well GWA-10D is located south of the ash storage area and
north of the ash basins. All of these wells are directly downgradient from the ash basins or ash
storage areas and upgradient of the ash basin compliance boundary at the Dan River.
7.1 Antimony
Figures 17 to 19 show predicted antimony concentrations versus time at downgradient
monitoring wells for both scenarios. The concentration versus time curves show that under the
Existing Conditions scenario, antimony exceeds the IMAC of 1 µg/L in 2015 at all three
monitoring wells. Figures 20 to 22 show initial (2015) antimony concentrations in the shallow,
deep and bedrock groundwater zones. In 2115, antimony is predicted to exceed the IMAC at the
compliance boundary under the Existing Conditions scenario (Figures 23 to 25). Under the
Excavation scenario (Figures 26 to 28), antimony concentrations remain above the 2L Standard
at the compliance boundary after 100 years in all groundwater zones.
18
Groundwater Flow and Transport Model
Dan River Steam Station
7.2 Arsenic
Figures 29 to 31 show predicted arsenic concentrations at downgradient monitoring wells under
both scenarios. The concentration versus time curves show that under existing conditions
(2015), arsenic exceeds the 2L Standard (10 µg/L) at AB-10S and is below the 2L Standard at
AB-30S and GWA-6S. Figures 32 to 34 show initial (2015) arsenic concentrations in the
shallow, deep and bedrock groundwater zones, and Figures 35 to 37 show predicted
concentrations after 100 years (2115). Arsenic is not predicted to exceed the groundwater
standard in 2115 at the compliance boundary under the Existing Conditions scenario or the
Excavation scenario (Figures 38 to 40) in any groundwater layer.
7.3 Boron
Figures 41 through 43 show predicted boron concentrations at downgradient monitoring wells
for both scenarios. The concentration versus time curves show that under the Existing
Conditions scenario, boron reaches steady-state concentrations shortly after 2015 at all three
monitor wells and remains below the 2L Standard of 700 µg/L. Figures 44 to 46 show initial
(2015) boron concentrations in the shallow, deep and bedrock groundwater zones; the predicted
concentrations in 2115 are presented in Figures 47 to 49. After 100 years, boron is predicted to
exceed the 2L Standard at the compliance boundary under the Existing Conditions scenario
within all groundwater zones. Under the Excavation scenario (Figures 50 to 52), boron
concentrations decrease below the 2L Standard at the compliance boundary within
approximately 50 years in all groundwater zones.
7.4 Chromium
Figures 53 to 55 show predicted chromium concentrations at downgradient monitoring wells for
both scenarios. The concentration versus time curves show that, chromium remains below the
2L Standard of 10 µg/L under both closure scenarios for the duration of the modeling period.
Figures 56 to 58 show initial (2015) chromium concentrations in the shallow, deep and bedrock
groundwater zones. After 100 years, chromium is not predicted to exceed the 2L Standard at
the compliance boundary under either the Existing Conditions scenario or the Excavation
scenario (Figures 59 to 64).
7.5 Cobalt
Figures 65 through 67 show predicted cobalt concentrations versus time at downgradient
monitoring wells for both scenarios. The concentration versus time curves for all three
downgradient monitor wells show that cobalt is generally elevated above the IMAC of 1 µg/L for
both scenarios. Figures 68 to 70 show initial (2015) cobalt concentrations in the shallow, deep
and bedrock groundwater zones. The predicted cobalt concentrations in 2115 are presented in
Figures 71 to 73. After 100 years, cobalt is predicted to exceed the IMAC at the compliance
boundary under existing conditions in all groundwater zones. Under the Excavation scenario
(Figures 74 to 76), cobalt is predicted to exceed the IMAC within the shallow and deep
groundwater zones, but not in the bedrock groundwater zone.
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Dan River Steam Station
7.6 Hexavalent Chromium
Figure 77 shows predicted hexavalent chromium concentrations at GWA-10D downgradient of
the ash storage area for both scenarios. The concentration versus time curves show that under
the Existing Conditions scenario, hexavalent chromium remains above the North Carolina
Department of Health and Human Services (DHHS) health screening level (HSL) groundwater
standard of 0.07 µg/L. Figures 78 to 80 show initial (2015) hexavalent chromium concentrations
in the shallow, deep and bedrock groundwater zones. After 100 years, hexavalent chromium is
predicted to exceed the DHHS HSL standard at the compliance boundary under both the
Existing Conditions scenario and the Excavation scenario (Figures 81 to 86).
7.7 Selenium
Figures 87 through 89 show predicted selenium concentrations versus time at downgradient
monitoring wells for both scenarios. The concentration versus time curves show that selenium
remains in a steady state at background concentrations well under the 2L Standard in both
scenarios. Figures 90 to 92 show initial (2015) selenium concentrations in the shallow, deep and
bedrock groundwater zones. After 100 years, selenium is not predicted to exceed the 2L
Standard of 20 µg/L at the compliance boundary in any groundwater zone under both the
Existing Conditions and Excavation scenarios (Figures 93 to 98).
7.8 Sulfate
Figures 99 through 101 show predicted sulfate concentrations versus time at downgradient
monitoring wells for both scenarios. The concentration versus time curves for all three
monitoring wells show that sulfate remains at a steady state well below the 2L Standard of
250,000 µg/L for both scenarios. Figures 102 to 104 show initial (2015) sulfate concentrations in
the shallow, deep and bedrock groundwater zones and an exceedence at the compliance
boundary at the eastern unnamed stream. The predicted sulfate concentrations in 2115 are
presented in Figures 105 to 107. After 100 years, sulfate is predicted to exceed the 2L Standard
at the compliance boundary under the Existing Conditions scenario at the eastern unnamed
stream within all groundwater zones. Under the Excavation scenario (Figures 108 to 110),
sulfate is predicted to fall below the 2L Standard at the compliance boundary in all groundwater
zones.
7.9 Thallium
Figures 111 through 113 show predicted thallium concentrations versus time at downgradient
monitoring wells for both scenarios. The concentration versus time curves show that thallium
remains elevated above the IMAC of 0.2 µg/L at two of the three downgradient monitoring wells
for the Existing Conditions scenario and above the IMAC at AB-10S for the Excavation scenario.
Figures 114 to 116 show initial (2015) thallium concentrations in the shallow, deep and bedrock
groundwater zones. After 100 years, thallium is predicted to exceed the IMAC at the compliance
boundary under both the Existing Conditions and the Excavation scenario (Figures 117 to 122)
within all groundwater zones.
20
Groundwater Flow and Transport Model
Dan River Steam Station
7.10 Vanadium
Figures 123 through 125 show predicted vanadium concentrations versus time at downgradient
monitoring wells for both scenarios. The concentration versus time curves show that vanadium
remains elevated above the IMAC of 0.3 µg/L for both scenarios throughout the modeling
period. Figures 126 to 128 show initial (2015) vanadium concentrations in the shallow, deep and
bedrock groundwater zones. After 100 years, vanadium is predicted to exceed the IMAC at the
compliance boundary under both the Existing Conditions and Excavation scenarios (Figures
129 to 134) within all groundwater zones.
8 SUMMARY
8.1 Model Assumptions and Limitations
The model assumptions include the following:
• The steady-state flow model was calibrated to hydraulic heads measured at observation
wells in June 2015 and considered the ash basin water level. The model is not calibrated
to transient water levels over time, recharge, or river flow. A steady-state calibration
does not consider groundwater storage and does not calibrate the groundwater flux into
adjacent surface water bodies.
• MODFLOW simulates flow through porous media and groundwater flow in the bedrock
groundwater zone is via fractures in the bedrock. A single domain MODFLOW modeling
approach for simulating flow in the primary porous groundwater zones and bedrock was
used for contaminant transport at the DRSS site.
• The model was calibrated by adjusting the constant source concentrations at the ash
basins and ash storage area to reasonably match 2015 COI concentrations in
groundwater.
• For the purposes of numerical modeling and comparing closure scenarios, it is assumed
that the selected closure scenario will be completed in 2016.
• Predictive simulations were performed and steady-state flow conditions were assumed
from the time the ash basins and ash storage area were placed in service through the
current time until the end of the predictive simulations (2265).
• COI source zone concentrations at the inactive and active ash basins and ash storage
area were applied uniformly within each source area and assumed to be constant with
respect to time for transport model calibration.
• The uncertainty in model parameters and predictions has not been quantified; therefore,
the error in the model predictions is not known. It is assumed the model results are
suitable for a relative comparison of closure scenario options.
• Since the Dan River is modeled as a constant head boundary in the numerical model, it
will not be possible to assess the effects of pumping wells or other groundwater sinks
that are near the river.
• The model does not account for varying geochemical conditions such as pH and redox
potential that could affect COI mobility and change modeling results.
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Dan River Steam Station
8.2 Model Predictions
The model predictions are summarized as follows:
• Antimony – Concentrations are predicted to exceed the 2L Standard/IMAC at the
compliance boundary under both the Existing Conditions and Excavation scenarios.
• Arsenic, chromium, and selenium– Predicted concentrations indicate arsenic, chromium,
and selenium will not exceed their respective 2L Standard at the compliance boundary
under either scenario.
• Boron and sulfate – Concentrations of these COIs are predicted to exceed the 2L
Standard at the compliance boundary under the Existing Conditions scenario but not
under the Excavation scenario. The model shows the Excavation scenario will result in
the rapid reduction of both boron and sulfate. Among the COIs, sulfate and boron are
similar in that both are considered conservative; that is, neither has a strong affinity to
attenuate nor adsorb to soil/rock surfaces. As a result, the model predicts similar
behavior for both of these COIs, and other COIs with low Kd values—rapid and nearly
complete reduction under the Excavation scenario.
• Cobalt – Cobalt concentrations are predicted to exceed the 2L Standard at the
compliance boundary in all groundwater zones under the Existing Conditions scenario.
Under the Excavation scenario, cobalt is predicted to exceed the 2L Standard at the
compliance boundary in the shallow and deep groundwater zones, but not the bedrock
groundwater zone.
• Hexavalent chromium – Predicted concentrations of hexavalent chromium are expected
to exceed the DHHS HSL at the compliance boundary under both scenarios.
• Vanadium – Concentrations of thallium and vanadium are expected to exceed the IMAC
at the compliance boundary under both scenarios.
• The background concentrations for vanadium and hexavalent chromium used in the
model are above their respective standards
9 REFERENCES
Daniel, C.C., III. 2001. Estimating ground-water recharge in the North Carolina Piedmont for
land use planning [abs.], in 2001 Abstracts with Programs, 50th Annual Meeting, Southeastern
Section, April 5-6, 2001: Raleigh, N.C., The Geological Society of America, v. 33, no. 2, p. A-80.
HDR, 2015a. Comprehensive Site Assessment Report, Dan River Steam Station Ash Basin,
August 2015.
HDR, 2015b. Corrective Action Plan Part 1. Dan River Steam Station Ash Basin, November
2015.
LeGrand, H. E. 2004. A Master Conceptual Model for Hydrogeological Site Characterization in
the Piedmont and Mountain Region of North Carolina, A Guidance Manual, North Carolina
Department of Environment and Natural Resources Division of Water Quality, Groundwater
Section.
22
Groundwater Flow and Transport Model
Dan River Steam Station
Niswonger, R.G., Panday, S., and Ibaraki, M., 2011. MODFLOW-NWT, A Newton formulation
for MODFLOW-2005: U.S. Geological Survey Techniques and Methods 6-A37, 44 p.
Zheng, C. and Bennett, G. 2002. Applied Contaminant Transport Modeling Second Edition,
Wiley Interscience. 2002.
Zheng, C. and P. Wang., 1999. MT3DMS, A modular three-dimensional multi-species transport
model for simulation of advection, dispersion and chemical reactions of contaminants in
groundwater systems, Documentation and Users Guide, U.S. Army Engineer Research and
Development Center Contract Report SERDP-99-1, Vicksburg, MS, 202 p.

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24

Appendix B
Groundwater Flow and
Transport Model
Attachments
 Tables
 Figures

Attachments provided in electronic format
on CAP Part 2 CD only.

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Groundwater Flow and Transport Model
Dan River Steam Station

TABLES
Table 1 MODFLOW and MT3DMS Input Packages Utilized
Table 2 Model Hydraulic Conductivity
Table 3 Observed vs. Predicted Hydraulic Head
Table 4 Flow Parameter Sensitivity Analysis
Table 5 Transport Model Calibration Results
Table 6. Predicted Advective Travel Time to Boundary

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Dan River Steam Station

Table 1 MODFLOW and MT3DMS Input Packages Utilized
MODFLOW Input Package Description
Name (NAM) Contains the names of the input and output files used in the
model simulation and controls the active model program
Basic (BAS) Specifies input packages used, model discretization, number of
model stress periods, initial heads and active cells
Discretization (DIS)
Contains finite-difference grid information, including the number
and spacing of rows and columns, number of layers in the grid,
top and bottom model layer elevations and number of stress
periods
Specified Head and
Concentration (CHD)
Specifies a head and/or a concentration that remains constant
throughout the simulation
Drain (DRN) Acts as a “drain” to remove water from the groundwater system.
Simulates drainage areas within the model
Recharge (RCH) Simulates areal distribution of recharge to the
groundwater system
Newton Solver (NWT) Contains input values and the Newton and matrix
solver options
Upstream Weighting (UPW) Replaces the LPF and/or BCF packages and contains the input
required for internal flow calculations
Flow Transfer Link File (LMT) Used by MTDMS to obtain the location, type, and flow rates of
all sources and sinks simulated in the flow model
MT3DMS Input Package Description
Flow Transfer Link File (FTL) Reads the LMT file produced by MODFLOW
Basic Transport Package (BTN) Reads the MODFLOW data used for transport simulations and
contains transport options and parameters
Advection (ADV) Reads and solves the selected advection term
Dispersion (DSP) Reads and solves the dispersion using the explicit finite-
difference formulation
Source and Sink Mixing (SSM) Reads and solves the concentration change due to sink/source
mixing using the explicit finite-difference formulation
Chemical Reaction (RCT) Reads and solves the concentration change due to chemical
reactions using the explicit finite-difference formulation
Generalized Conjugate Gradient
(GCG) Solver
Solves the matrix equations resulting from the implicit solution of
the transport equation

Groundwater Flow and Transport Model
Dan River Steam Station

Table 2 Model Hydraulic Conductivity
Model
Layer
Hydrostrati-
graphic Unit
Measured Value
Range1 Calibrated Model Value
Horizontal
Hydraulic
Conductivity
(ft/day)
Horizontal
Hydraulic
Conductivity (ft/day)
Vertical Hydraulic
Conductivity (ft/day)
1-3 Ash 9.12-0.05 2.000 0.4
4-6 Dike 7.86-0.22 0.284 0.009
7 Alluvium 0.6-0.22
Z-1 0.0300 0.0100
Z-2 0.1370 0.1000
Z-3 0.2000 0.0100
Z-4 0.2840 0.0090
Z-5 0.4000 0.0100
8 M1 14.29-0.0033
Z-6 0.4000 0.0004
Z-7 1.7050 0.1705
Z-8 2.0000 0.4000
9 M2 0.84-0.0042
Z-9 2.4000 0.5000
Z-10 2.8420 0.2842
Z-11 10.0000 0.0010
10 Transition Zone 3.42-0.09
Z-12 0.0500 0.0010
Z-13 0.5500 0.0500
Z-14 3.6000 0.3000
Z-15 12.5000 0.0800
11-12 Bedrock 1.67-0.06
Z-16 0.1000 0.1000
Z-17 0.1000 0.0100
Z-18 0.5500 0.0500
Z-19 0.8000 0.1000
Z-20 1.1000 0.2000
1Range = geometric mean +/- one standard deviation (see CSA Report Tables 11-6 to 11-11)

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Table 3 Observed vs. Predicted Hydraulic Head
Well Name Model Layer Measured WL
(ft MSL)
Model WL
(ft MSL)
Residual
(ft MSL)
AB-25S 7 496.57 496.91 -0.34
AB-10S 8 520.32 505.56 14.76
AB-30S 8 514.18 505.08 9.10
AB-5S 8 522.06 520.91 1.15
AS-12S 8 548.95 556.14 -7.19
BG-5S 8 507.29 517.72 -10.43
GWA-10S 8 523.07 530.41 -7.34
GWA-11S 8 540.35 543.15 -2.80
GWA-12S 8 572.49 571.21 1.28
GWA-3S 8 552.96 551.35 1.61
GWA-4S 8 516.36 507.06 9.30
GWA-6S 8 487.95 490.97 -3.02
GWA-7S 8 527.08 525.47 1.61
GWA-8S 8 568.52 574.78 -6.26
GWA-9S 8 581.5 589.46 -7.96
AB-10SL 9 498.84 500.96 -2.12
AB-10D 10 492.14 500.93 -8.79
AB-25D 10 491.51 495.74 -4.23
AB-30D 10 490.6 495.89 -5.29
AB-5D 10 521.56 520.97 0.59
AS-10D 10 570.06 571.18 -1.12
AS-2D 10 528.32 533.11 -4.79
AS-4D 10 544.67 544.22 0.45
AS-6D 10 551.25 555.11 -3.86
AS-8D 10 545.13 546.99 -1.86
BG-5D 10 507.17 517.77 -10.60
GWA-10D 10 520 530.33 -10.33
GWA-11D 10 541.45 540.76 0.69
GWA-12D 10 564.31 569.03 -4.72
GWA-3D 10 549.82 550.07 -0.25
GWA-4D 10 499.95 507.40 -7.45
GWA-6D 10 483.26 486.03 -2.77
GWA-7D 10 526.15 525.26 0.89
GWA-8D 10 567.75 567.85 -0.10
GWA-9D 10 580.37 581.54 -1.17
MW-318D 10 536.72 540.60 -3.88
AB-25BR 12 486.12 495.30 -9.18
Continued on next page

Groundwater Flow and Transport Model
Dan River Steam Station

Table 3 Observed vs. Predicted Hydraulic Head (continued)
Well Name Model Layer Measured WL
(ft MSL)
Model WL
(ft MSL)
Residual
(ft MSL)
AB-30BR 12 489.9 495.42 -5.52
AB-35BR 12 508.98 513.97 -4.99
AS-8BR 12 545.39 549.22 -3.83
MW-22BR 12 486.5 490.82 -4.32
MW-301BR 12 570.64 566.38 4.26
MW-303BR 12 557.85 559.54 -1.69
MW-306BR 12 544.34 544.72 -0.38
MW-308BR 12 521.08 521.03 0.05
MW-310BR 12 521.48 522.67 -1.19
MW-311BR 12 495.88 494.87 1.01
MW-314BR 12 510.45 513.37 -2.92
MW-315BR 12 525.57 531.42 -5.85
MW-317BR 12 547.75 551.43 -3.68
SSE 1475.35
Max 581.5 OBS 50
Min 483.26 MSE 5.43
Max-Min 98.24 NRMSE 0.0553
Notes:
1. S or SL – shallow groundwater zone
2. D – deep groundwater zone
3. BR – bedrock groundwater zone
4. WL – water level
5. ft – feet
6. MSL – mean sea level
7. SSE – sum of squared error
8. OBS – number of observations
9. MSE – mean squared error
10. NRMSE – normalized root mean squared error

Groundwater Flow and Transport Model
Dan River Steam Station

Table 4 Flow Parameter Sensitivity Analysis
Parameter
Calibrated Calibrated +20% Calibrated -20%
NRMSR
(Head) NRMSR (Head) % NRMSR (Head) %
Shallow
Zone Kh
0.0550
0.0510 -7.27 0.0660 20.00
Shallow
Zone Kv 0.0550 0.00 0.0560 1.82
Transition
Zone Kh 0.0520 -5.45 0.0770 40.00
Transition Zone Kv 0.0540 -1.82 0.0570 3.64
Recharge
ex. Ash Basin 0.0970 76.36 0.0670 21.82
Recharge
Ash Basin 0.0580 5.45 0.0540 -1.82
Notes:
1. NRMSR – normalized root mean squared error
2. Kh – horizontal hydraulic conductivity
3. Kv – vertical hydraulic conductivity

Groundwater Flow and Transport Model
Dan River Steam Station

Table 5 Transport Model Calibration Results
COI Monitoring
Well
Measured
Concentration
(µg/L)
Predicted
Concentration
(µg/L)
Antimony
Ash Basin Constant Concentration Range = 1 - 12 µg/L
Ash Storage Area Concentration Range = 1 - 12 µg/L
Sorption Coefficient [Kd] = 20 mL/g
AB-10S 1.30 9.71
AB-10SL 9.80 9.65
AB-15S 6.70 7.04
AB-25BR 1.00 1.07
AB-25D 1.70 1.59
AB-25S 1.60 2.63
AB-30BR 0.50 1.06
AB-30D 1.50 1.65
AB-30S 2.60 2.41
AB-35BR 0.48 1.00
AB-5D 0.50 1.00
AS-10D 2.90 0.99
AS-12S 0.23 0.95
AS-4D 0.50 1.00
AS-6D 0.24 1.00
AS-8D 1.00 1.00
BG-5D 0.50 0.99
BG-5S 0.50 0.98
GWA-11S 0.18 0.93
GWA-12D 1.00 0.98
GWA-3D 1.00 0.99
GWA-6S 2.50 2.54
GWA-8D 0.71 1.00
GWA-9D 0.50 1.00
MW-23BR 1.21 1.00
MW-311BR 0.30 1.01
MW-314BR 0.47 1.00
MW-318D 1.00 0.99
MW-308BR 0.24 1.00
GWA-10D 1.00 0.98
MW-303BR 0.25 1.00
MW-317BR 2.10 1.00
Continued on next page

Groundwater Flow and Transport Model
Dan River Steam Station

Table 5 Transport Model Calibration Results (continued)
COI Monitoring
Well
Measured
Concentration
(µg/L)
Predicted
Concentration
(µg/L)
Arsenic
Ash Basin Constant Concentration Range = 10 – 400 µg/L
Ash Storage Area Concentration Range = 2.5 - 10 µg/L
Sorption Coefficient [Kd] = 55 mL/g
BG-5D 0.50 1.00
BG-5S 0.28 0.99
AB-5S 145.00 146.47
AB-10S 233.00 167.58
AB-10SL 9.60 6.55
AB-15S 442.00 364.41
AB-25S 96.40 96.00
AB-10D 0.68 1.01
AB-25BR 2.60 1.13
AB-25D 0.45 5.37
AB-30BR 0.83 1.00
AB-30D 0.41 1.00
AB-30S 0.50 1.53
AB-35BR 11.70 13.14
AB-5D 8.50 8.65
MW-23BR 6.36 1.00
GWA-8D 1.40 1.00
GWA-9D 2.30 1.00
GWA-6S 2.20 2.31
GWA-12D 3.71 0.99
AS-8D 1.24 1.75
AS-4D 1.80 2.06
AS-6D 2.00 1.00
AS-10D 8.90 1.00
MW-10 0.50 1.20
MW-10D 0.50 1.04
MW-11 0.14 1.01
MW-11D 0.25 1.00
MW-22D 0.41 1.00
MW-22S 0.94 0.99
MW-23D 0.27 1.00
MW-9 0.00 1.01
MW-9D 0.21 1.01
Continued on next page

Groundwater Flow and Transport Model
Dan River Steam Station

Table 5 Transport Model Calibration Results (continued)
COI Monitoring
Well
Measured
Concentration
(µg/L)
Predicted
Concentration
(µg/L)
Boron
Ash Basin Constant Concentration Range = 300 – 800 µg/L
Ash Storage Area Concentration Range = 100 - 900 µg/L
Sorption Coefficient [Kd] = 1 mL/g
AB-10S 330.00 313.06
BG-5D 50.00 44.98
BG-5S 50.00 43.63
AB-10D 290.00 338.12
AB-25BR 180.00 635.51
AB-25D 840.00 689.06
AB-30BR 320.00 236.21
AB-30D 250.00 270.84
AB-30S 290.00 289.76
AB-35BR 270.00 401.24
AB-5D 500.00 695.76
MW-23BR 50.00 44.65
GWA-8D 50.00 42.70
GWA-9D 50.00 45.42
GWA-6S 610.00 635.39
GWA-12D 50.00 40.11
AS-8D 50.00 97.91
AS-4D 810.00 747.02
AS-6D 37.00 41.39
AS-10D 29.00 36.26
AS-12S 1000.00 234.38
MW-306BR 920.00 787.72
MW-318D 1310.00 589.15
MW-311BR 690.00 578.61
MW-314BR 500.00 431.77
MW-10 460.00 294.48
MW-10D 460.00 305.86
MW-11 230.00 243.87
MW-11D 87.00 249.48
MW-22D 670.00 595.53
MW-22S 510.00 593.88
MW-23D 50.00 37.36
MW-9 28.00 773.84
MW-9D 1000.00 714.85
Continued on next page

Groundwater Flow and Transport Model
Dan River Steam Station

Table 5 Transport Model Calibration Results (continued)
COI Monitoring
Well
Measured
Concentration
(µg/L)
Predicted
Concentration
(µg/L)
Chromium
Ash Basin Constant Concentration Range = 5 - 50 µg/L
Ash Storage Area Concentration Range = 5 - 25 µg/L
Sorption Coefficient [Kd] = 20 mL/g
BG-5D 0.38 4.97
BG-5S 2.60 4.89
AB-10S 1.30 5.14
AB-10SL 5.10 5.31
AB-15S 50.00 43.70
AB-25S 0.47 5.00
AB-25BR 30.10 5.00
AB-25D 0.96 5.00
AB-30BR 2.20 5.00
AB-30D 2.70 4.99
AB-30S 0.99 4.99
AB-35BR 3.80 5.00
AB-5D 3.30 5.00
MW-23BR 1.00 5.00
GWA-8D 3.20 5.00
GWA-9D 2.90 5.00
GWA-6S 3.60 4.86
GWA-12D 2.87 4.91
AS-8D 1.90 5.00
AS-4D 3.30 10.53
AS-6D 0.47 5.00
AS-10D 4.10 4.96
AS-12S 1.10 4.76
MW-318D 2.80 4.96
MW-311BR 3.30 5.00
MW-314BR 5.00 5.00
GWA-11S 15.60 4.67
GWA-3D 17.70 4.97
Continued on next page

Groundwater Flow and Transport Model
Dan River Steam Station

Table 5 Transport Model Calibration Results (continued)
COI Monitoring
Well
Measured
Concentration
(µg/L)
Predicted
Concentration
(µg/L)
Hexavalent
Chromium
Ash Basin Constant Concentration Range = 0.089 - 1 µg/L
Ash Storage Area Concentration Range = 0.089 µg/L
Sorption Coefficient [Kd] = 15 mL/g
AB-25D 0.04 0.09
AB-30D 1.38 0.09
AB-30S 0.03 0.09
AB-35BR 0.87 0.31
MW-23D 0.09 0.09
AS-8D 0.01 0.09
AS-4D 0.01 0.09
AS-10D 4.51 0.09
GWA-10D 0.11 0.10
Cobalt
Ash Basin Constant Concentration Range = 5 - 34 µg/L
Ash Storage Area Concentration Range = 34 µg/L
Sorption Coefficient [Kd] = 220 mL/g
AB-10S 8.50 9.04
AB-10SL 1.80 8.80
AB-15S 34.10 34.00
AB-25BR 0.27 0.50
AB-25D 0.30 0.50
AB-25S 0.16 0.50
AB-30BR 0.29 0.50
AB-30D 0.30 0.55
AB-30S 29.00 15.77
AB-35BR 0.26 0.50
AB-5D 1.00 0.98
AS-10D 0.50 0.50
AS-12S 2.20 0.50
AS-4D 4.00 0.67
AS-6D 1.10 0.50
AS-8D 1.00 1.13
BG-5D 1.00 0.50
BG-5S 2.10 0.50
GWA-11S 3.10 0.49
GWA-12D 0.00 0.50
GWA-3D 1.02 0.50
Continued on next page

Groundwater Flow and Transport Model
Dan River Steam Station

Table 5 Transport Model Calibration Results (continued)
COI Monitoring
Well
Measured
Concentration
(µg/L)
Predicted
Concentration
(µg/L)
Cobalt
(cont.)
GWA-6S 1.30 0.60
GWA-8D 0.26 0.50
GWA-9D 1.20 0.50
MW-23BR 1.00 0.50
MW-311BR 0.84 0.50
MW-314BR 0.34 0.50
MW-318D 13.00 0.79
MW-308BR 0.60 0.50
GWA-10D 3.45 0.50
AB-10D 1.10 0.50
AS-2D 2.10 0.62
GWA-3S 12.20 0.49
GWA-4S 2.70 0.50
GWA-7D 2.80 0.50
GWA-7S 17.70 0.50
GWA-8S 9.70 0.50
GWA-9S 9.30 0.50
MW-315BR 2.20 0.50
MW-10 0.28 0.50
MW-10D 0.57 0.50
MW-11 29.20 17.19
MW-11D 0.22 1.07
MW-22D 0.15 0.50
MW-22S 0.37 0.50
MW-23D 0.25 0.50
MW-9 6.40 0.50
MW-9D 1.50 0.50
Selenium
Ash Basin Constant Concentration Range = 1.2 - 12.2 µg/L
Ash Storage Area Concentration Range = 1.2 µg/L
Sorption Coefficient [Kd] = 220 mL/g
AB-10S 0.24 1.05
AB-10SL 1.10 1.05
AB-15S 12.20 9.90
AB-25BR 0.64 1.00
AB-25D 0.50 1.00
AB-25S 0.74 1.00
Continued on next page

Groundwater Flow and Transport Model
Dan River Steam Station

Table 5 Transport Model Calibration Results (continued)
COI Monitoring
Well
Measured
Concentration
(µg/L)
Predicted
Concentration
(µg/L)
Selenium
(cont.)
AB-30BR 0.50 1.00
AB-30D 0.50 1.04
AB-30S 0.50 1.12
AB-35BR 0.67 1.00
AB-5D 0.50 1.10
AS-10D 35.30 1.00
AS-12S 0.50 0.97
AS-4D 0.50 1.00
AS-6D 0.50 1.00
AS-8D 1.00 1.00
BG-5D 0.50 1.00
BG-5S 0.50 0.99
GWA-10D 1.00 0.99
GWA-11S 0.50 0.94
GWA-12D 1.00 0.99
GWA-3D 1.00 1.00
GWA-6S 0.50 0.98
GWA-8D 1.10 1.00
GWA-9D 0.50 1.00
MW-23BR 1.00 1.00
MW-303BR 0.33 1.00
MW-308BR 0.50 1.00
MW-311BR 0.50 1.00
MW-314BR 0.50 1.00
MW-317BR 0.75 1.00
MW-318D 1.03 1.03
Sulfate
Ash Basin Constant Concentration Range = 31,000 - 120,000 µg/L
Ash Storage Area Concentration Range = 31,000 - 80,000 µg/L
Sorption Coefficient [Kd] = 0 mL/g
AB-10S 24500 45346
AB-10SL 29800 45466
AB-15S 39300 42868
AB-25BR 99000 89104
AB-25D 83000 89352
AB-25S 74700 79794
AB-30BR 96700 62428
Continued on next page

Groundwater Flow and Transport Model
Dan River Steam Station

Table 5 Transport Model Calibration Results (continued)
COI Monitoring
Well
Measured
Concentration
(µg/L)
Predicted
Concentration
(µg/L)
Sulfate
(cont.)
AB-30D 71500 75838
AB-30S 81500 81155
AB-35BR 129000 101406
AB-5D 45200 65928
AS-10D 43900 1201
AS-12S 71700 34307
AS-4D 44900 65734
AS-6D 15100 739
AS-8D 25000 39054
BG-5D 39400 2
BG-5S 41300 2
GWA-11S 4400 20333
GWA-12D 16000 0
GWA-3D 6200 3835
GWA-6S 10900 22949
GWA-8D 368000 321
GWA-9D 4200 2
MW-23BR 12000 0
MW-311BR 53000 45359
MW-314BR 95800 75924
MW-318D 91000 68503
MW-308BR 323000 140073
MW-21D 274000 216721
MW-10 26800 46557
MW-10D 41300 49833
MW-11 48100 84084
MW-11D 70000 86462
MW-22D 84100 77539
MW-22S 800 74863
MW-23D 12000 0
MW-9 29800 71151
MW-9D 87300 72278
Continued on next page

Groundwater Flow and Transport Model
Dan River Steam Station

Table 5 Transport Model Calibration Results (continued)
COI Monitoring
Well
Measured
Concentration
(µg/L)
Predicted
Concentration
(µg/L)
Thallium
Ash Basin Constant Concentration Range = 0.2 - 3.4 µg/L
Ash Storage Area Concentration Range = 0.2 µg/L
Sorption Coefficient [Kd] = 290 mL/g
AB-10S 0.54 0.85
AB-10SL 0.00 0.83
AB-15S 3.40 1.34
AB-25BR 0.00 0.20
AB-25D 0.02 0.20
AB-25S 0.04 0.20
AB-30BR 0.00 0.20
AB-30D 0.00 0.20
AB-30S 0.02 0.20
AB-35BR 0.00 0.20
AB-5D 0.02 0.20
AS-10D 0.00 0.20
AS-12S 0.00 0.20
AS-4D 0.03 0.20
AS-6D 0.00 0.20
AS-8D 0.00 0.20
BG-5D 0.00 0.20
BG-5S 0.02 0.20
GWA-11S 0.00 0.20
GWA-12D 0.00 0.20
GWA-3D 0.00 0.20
GWA-6S 0.50 0.22
GWA-8D 0.02 0.20
GWA-9D 0.00 0.20
MW-23BR 0.55 0.20
MW-311BR 0.02 0.20
MW-314BR 0.02 0.20
MW-318D 0.00 0.20
MW-308BR 0.00 0.20
GWA-10D 0.26 0.20
Continued on next page

Groundwater Flow and Transport Model
Dan River Steam Station

Table 5 Transport Model Calibration Results (continued)
COI Monitoring
Well
Measured
Concentration
(µg/L)
Predicted
Concentration
(µg/L)
Vanadium
Ash Basin Constant Concentration Range = 3 - 254 µg/L
Ash Storage Area Concentration Range = 3 - 14 µg/L
Sorption Coefficient [Kd] = 25 mL/g
AB-10S 0.62 10.58
AB-10SL 14.60 10.46
AB-15S 254.00 207.60
AB-25BR 5.50 1.17
AB-25D 0.69 2.95
AB-25S 13.30 14.00
AB-30BR 0.91 1.04
AB-30D 0.86 1.51
AB-30S 0.00 2.31
AB-35BR 1.80 1.27
AB-5D 0.77 2.06
AS-10D 15.30 0.99
AS-12S 1.10 0.96
AS-4D 4.80 4.05
AS-6D 0.69 1.00
AS-8D 1.88 2.61
BG-5D 0.00 1.00
BG-5S 2.60 0.98
GWA-11S 0.52 0.94
GWA-12D 5.65 0.99
GWA-3D 0.00 1.00
GWA-6S 6.50 3.53
GWA-8D 5.20 1.00
GWA-9D 1.80 1.00
MW-23BR 1.38 1.00
MW-311BR 0.89 1.07
MW-314BR 1.30 1.25
MW-318D 0.65 1.44
MW-308BR 0.84 1.04
GWA-10D 3.36 1.14
MW-303BR 3.70 1.02
MW-10 0.00 1.68
MW-10D 0.00 1.28
Continued on next page

Groundwater Flow and Transport Model
Dan River Steam Station

Table 5 Transport Model Calibration Results (continued)
COI Monitoring
Well
Measured
Concentration
(µg/L)
Predicted
Concentration
(µg/L)
Vanadium
(cont.)
MW-11 2.60 1.18
MW-11D 0.00 1.09
MW-22D 0.33 2.17
MW-22S 0.74 3.09
MW-23D 0.00 1.00
MW-9 2.30 10.82
MW-9D 11.10 4.81
Notes:
1. mL/g – milliliters per gram
2. µg/L – micrograms per liter

Groundwater Flow and Transport Model
Dan River Steam Station

Table 6. Predicted Advective Travel Time to Boundary
Groundwater Zone Monitoring Well Advective Travel Time
to Boundary (days)
Shallow
MW-21S 110
AB-30S 168
MW-10 117
MW-9S 52
GWA-11S 544
GWA-10S 604
GWA-9S 2,157
Deep
MW-21D 118
AB-30D 8
MW-10D 3
MW-9D 5
GWA-11D 68
GWA-10D 48
GWA-9D 426
Bedrock
AB-30BR 6
MW-22BR 15
MW-315BR 69
MW-317BR 268
MW-301BR 401
Note:
Computed travel time over 3-D flow path using flow and transport terms from the groundwater flow
and transport model.

Groundwater Flow and Transport Model
Dan River Steam Station

FIGURES
Figure 1 Conceptual Groundwater Flow Model/Model Domain
Figure 2 Model Domain Cross section A-A’
Figure 3 Model Domain Cross section B-B’
Figure 4 Flow Model Boundary Conditions
Figure 5 Recharge and Constant Concentration Boundary Condition
Figure 6 Shallow Monitoring Wells
Figure 7 Deep Monitoring Wells
Figure 8 Bedrock Monitoring Wells
Figure 9 Hydraulic Conductivity Zonation in Shallow Model Layers
Figure 10 Hydraulic Conductivity Zonation in Deep Model Layer
Figure 11 Hydraulic Conductivity Zonation in Bedrock Model Layers
Figure 12 Measured versus Modeled Water Levels
Figure 13 Potentiometric Head Contour Map
Figure 14 Potentiometric Head Cross Section C-C’
Figure 15 Potentiometric Head Cross section D-D’
Figure 16 Particle Tracking Results
Figure 17 Predicted Antimony in Monitoring Well AB-10S
Figure 18 Predicted Antimony in Monitoring Well AB-30S
Figure 19 Predicted Antimony in Monitoring Well GWA-6S
Figure 20 Initial (2015) Antimony Concentrations in Shallow Groundwater Zone
Figure 21 Initial (2015) Antimony Concentrations in Deep Groundwater Zone
Figure 22 Initial (2015) Antimony Concentrations in Bedrock Groundwater Zone
Figure 23 Existing Conditions Scenario 1 - 2115 Predicted Antimony Concentrations in Shallow
Groundwater Zone
Figure 24 Existing Conditions Scenario 1 - 2115 Predicted Antimony Concentrations in Deep
Groundwater Zone
Figure 25 Existing Conditions Scenario 1 - 2115 Predicted Antimony Concentrations in Bedrock
Groundwater Zone
Figure 26 Excavation Scenario 2 - 2115 Predicted Antimony Concentrations in Shallow
Groundwater Zone
Figure 27 Excavation Scenario 2 - 2115 Predicted Antimony Concentrations in Deep
Groundwater Zone
Figure 28 Excavation Scenario 2 - 2115 Predicted Antimony Concentrations in Bedrock
Groundwater Zone
Figure 29 Predicted Arsenic in Monitoring Well AB-10S
Figure 30 Predicted Arsenic in Monitoring Well AB-30S
Figure 31 Predicted Arsenic in Monitoring Well GWA-6S
Figure 32 Initial (2015) Arsenic Concentrations in Shallow Groundwater Zone
Figure 33 Initial (2015) Arsenic Concentrations in Deep Groundwater Zone
Figure 34 Initial (2015) Arsenic Concentrations in Bedrock Groundwater Zone
Figure 35 Existing Conditions Scenario 1 - 2115 Predicted Arsenic Concentrations in Shallow
Groundwater Zone

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 36 Existing Conditions Scenario 1 - 2115 Predicted Arsenic Concentrations in Deep
Groundwater Zone
Figure 37 Existing Conditions Scenario 1 - 2115 Predicted Arsenic Concentrations in Bedrock
Groundwater Zone
Figure 38 Excavation Scenario 2 - 2115 Predicted Arsenic Concentrations in Shallow
Groundwater Zone
Figure 39 Excavation Scenario 2 - 2115 Predicted Arsenic Concentrations in Deep Groundwater
Zone
Figure 40 Excavation Scenario 2 - 2115 Predicted Arsenic Concentrations in Bedrock
Groundwater Zone
Figure 41 Predicted Boron in Monitoring Well AB-10S
Figure 42 Predicted Boron in Monitoring Well AB-30S
Figure 43 Predicted Boron in Monitoring Well GWA-6S
Figure 44 Initial (2015) Boron Concentrations in Shallow Groundwater Zone
Figure 45 Initial (2015) Boron Concentrations in Deep Groundwater Zone
Figure 46 Initial (2015) Boron Concentrations in Bedrock Groundwater Zone
Figure 47 Existing Conditions Scenario 1 - 2115 Predicted Boron Concentrations in Shallow
Groundwater Zone
Figure 48 Existing Conditions Scenario 1 - 2115 Predicted Boron Concentrations in Deep
Groundwater Zone
Figure 49 Existing Conditions Scenario 1 - 2115 Predicted Boron Concentrations in Bedrock
Groundwater Zone
Figure 50 Excavation Scenario 2 - 2115 Predicted Boron Concentrations in Shallow
Groundwater Zone
Figure 51 Excavation Scenario 2 - 2115 Predicted Boron Concentrations in Deep Groundwater
Zone
Figure 52 Excavation Scenario 2 - 2115 Predicted Boron Concentrations in Bedrock
Groundwater Zone
Figure 53. Predicted Chromium in Monitoring Well AB-10SL
Figure 54. Predicted Chromium in Monitoring Well AB-30S
Figure 55. Predicted Chromium in Monitoring Well GWA-6S
Figure 56 Initial (2015) Chromium Concentrations in Shallow Groundwater Zone
Figure 57 Initial (2015) Chromium Concentrations in Deep Groundwater Zone
Figure 58 Initial (2015) Chromium Concentrations in Bedrock Groundwater Zone
Figure 59 Existing Conditions Scenario 1 - 2115 Predicted Chromium Concentrations in Shallow
Groundwater Zone
Figure 60 Existing Conditions Scenario 1 - 2115 Predicted Chromium Concentrations in Deep
Groundwater Zone
Figure 61 Existing Conditions Scenario 1 - 2115 Predicted Chromium Concentrations in
Bedrock Groundwater Zone
Figure 62 Excavation Scenario 2 - 2115 Predicted Chromium Concentrations in Shallow
Groundwater Zone
Figure 63 Excavation Scenario 2 - 2115 Predicted Chromium Concentrations in Deep
Groundwater Zone

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 64 Excavation Scenario 2 - 2115 Predicted Chromium Concentrations in Bedrock
Groundwater Zone
Figure 65 Predicted Cobalt in Monitoring Well AB-10S
Figure 66 Predicted Cobalt in Monitoring Well AB-30S
Figure 67 Predicted Cobalt in Monitoring Well GWA-6S
Figure 68 Initial (2015) Cobalt Concentrations in Shallow Groundwater Zone
Figure 69 Initial (2015) Cobalt Concentrations in Deep Groundwater Zone
Figure 70 Initial (2015) Cobalt Concentrations in Bedrock Groundwater Zone
Figure 71 Existing Conditions Scenario 1 - 2115 Predicted Cobalt Concentrations in Shallow
Groundwater Zone
Figure 72 Existing Conditions Scenario 1 - 2115 Predicted Cobalt Concentrations in Deep
Groundwater Zone
Figure 73 Existing Conditions Scenario 1 - 2115 Predicted Cobalt Concentrations in Bedrock
Groundwater Zone
Figure 74 Excavation Scenario 2 - 2115 Predicted Cobalt Concentrations in Shallow
Groundwater Zone
Figure 75 Excavation Scenario 2 - 2115 Predicted Cobalt Concentrations in Deep Groundwater
Zone
Figure 76 Excavation Scenario 2 - 2115 Predicted Cobalt Concentrations in Bedrock
Groundwater Zone
Figure 77 Predicted Hexavalent Chromium in Monitoring Well GWA-10D
Figure 78 Initial (2015) Hexavalent Chromium Concentrations in Shallow Groundwater Zone
Figure 79 Initial (2015) Hexavalent Chromium Concentrations in Deep Groundwater Zone
Figure 80 Initial (2015) Hexavalent Chromium Concentrations in Bedrock Groundwater Zone
Figure 81 Existing Conditions Scenario 1 - 2115 Predicted Hexavalent Chromium
Concentrations in Shallow Groundwater Zone
Figure 82 Existing Conditions Scenario 1 - 2115 Predicted Hexavalent Chromium
Concentrations in Deep Groundwater Zone
Figure 83 Existing Conditions Scenario 1 - 2115 Predicted Hexavalent Chromium
Concentrations in Bedrock Groundwater Zone
Figure 84 Excavation Scenario 2 - 2115 Predicted Hexavalent Chromium Concentrations in
Shallow Groundwater Zone
Figure 85 Excavation Scenario 2 - 2115 Predicted Hexavalent Chromium Concentrations in
Deep Groundwater Zone
Figure 86 Excavation Scenario 2 - 2115 Predicted Hexavalent Chromium Concentrations in
Bedrock Groundwater Zone
Figure 87 Predicted Selenium in Monitoring Well AB-10S
Figure 88 Predicted Selenium in Monitoring Well AB-30S
Figure 89 Predicted Selenium in Monitoring Well GWA-6S
Figure 90 Initial (2015) Selenium Concentrations in Shallow Groundwater Zone
Figure 91 Initial (2015) Selenium Concentrations in Deep Groundwater Zone
Figure 92 Initial (2015) Selenium Concentrations in Bedrock Groundwater Zone
Figure 93 Existing Conditions Scenario 1 - 2115 Predicted Selenium Concentrations in Shallow
Groundwater Zone

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 94 Existing Conditions Scenario 1 - 2115 Predicted Selenium Concentrations in Deep
Groundwater Zone
Figure 95 Existing Conditions Scenario 1 - 2115 Predicted Selenium Concentrations in Bedrock
Groundwater Zone
Figure 96 Excavation Scenario 2 - 2115 Predicted Selenium Concentrations in Shallow
Groundwater Zone
Figure 97 Excavation Scenario 2 - 2115 Predicted Selenium Concentrations in Deep
Groundwater Zone
Figure 98 Excavation Scenario 2 - 2115 Predicted Selenium Concentrations in Bedrock
Groundwater Zone
Figure 99 Predicted Sulfate in Monitoring Well AB-10S
Figure 100 Predicted Sulfate in Monitoring Well AB-30S
Figure 101 Predicted Sulfate in Monitoring Well GWA-6S
Figure 102 Initial (2015) Sulfate Concentrations in Shallow Groundwater Zone
Figure 103 Initial (2015) Sulfate Concentrations in Deep Groundwater Zone
Figure 104 Initial (2015) Sulfate Concentrations in Bedrock Groundwater Zone
Figure 105 Existing Conditions Scenario 1 - 2115 Predicted Sulfate Concentrations in Shallow
Groundwater Zone
Figure 106 Existing Conditions Scenario 1 - 2115 Predicted Sulfate Concentrations in Deep
Groundwater Zone
Figure 107 Existing Conditions Scenario 1 - 2115 Predicted Sulfate Concentrations in Bedrock
Groundwater Zone
Figure 108 Excavation Scenario 2 - 2115 Predicted Sulfate Concentrations in Shallow
Groundwater Zone
Figure 109 Excavation Scenario 2 - 2115 Predicted Sulfate Concentrations in Deep
Groundwater Zone
Figure 110 Excavation Scenario 2 - 2115 Predicted Sulfate Concentrations in Bedrock
Groundwater Zone
Figure 111 Predicted Thallium in Monitoring Well AB-10S
Figure 112 Predicted Thallium in Monitoring Well AB-30S
Figure 113 Predicted Thallium in Monitoring Well GWA-6S
Figure 114 Initial (2015) Thallium Concentrations in Shallow Groundwater Zone
Figure 115 Initial (2015) Thallium Concentrations in Deep Groundwater Zone
Figure 116 Initial (2015) Thallium Concentrations in Bedrock Groundwater Zone
Figure 117 Existing Conditions Scenario 1 - 2115 Predicted Thallium Concentrations in Shallow
Groundwater Zone
Figure 118 Existing Conditions Scenario 1 - 2115 Predicted Thallium Concentrations in Deep
Groundwater Zone
Figure 119 Existing Conditions Scenario 1 - 2115 Predicted Thallium Concentrations in Bedrock
Groundwater Zone
Figure 120 Excavation Scenario 2 - 2115 Predicted Thallium Concentrations in Shallow
Groundwater Zone
Figure 121 Excavation Scenario 2 - 2115 Predicted Thallium Concentrations in Deep
Groundwater Zone

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 122 Excavation Scenario 2 - 2115 Predicted Thallium Concentrations in Bedrock
Groundwater Zone
Figure 123 Predicted Vanadium in Monitoring Well AB-10SL
Figure 124 Predicted Vanadium in Monitoring Well AB-30S
Figure 125 Predicted Vanadium in Monitoring Well GWA-6S
Figure 126 Initial (2015) Vanadium Concentrations in Shallow Groundwater Zone
Figure 127 Initial (2015) Vanadium Concentrations in Deep Groundwater Zone
Figure 128 Initial (2015) Vanadium Concentrations in Bedrock Groundwater Zone
Figure 129 Existing Conditions Scenario 1 - 2115 Predicted Vanadium Concentrations in
Shallow Groundwater Zone
Figure 130 Existing Conditions Scenario 1 - 2115 Predicted Vanadium Concentrations in Deep
Groundwater Zone
Figure 131 Existing Conditions Scenario 1 - 2115 Predicted Vanadium Concentrations in
Bedrock Groundwater Zone
Figure 132 Excavation Scenario 2 - 2115 Predicted Vanadium Concentrations in Shallow
Groundwater Zone
Figure 133 Excavation Scenario 2 - 2115 Predicted Vanadium Concentrations in Deep
Groundwater Zone
Figure 134 Excavation Scenario 2 - 2115 Predicted Vanadium Concentrations in Bedrock
Groundwater Zone

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 1 Conceptual Groundwater Flow Model/Model Domain

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 2 Model Domain Cross section A-A’

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 3 Model Domain Cross section B-B’

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 4 Flow Model Boundary Conditions

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 5 Recharge and Constant Concentration Boundary Condition

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 6 Shallow Monitoring Wells

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 7 Deep Monitoring Wells

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 8 Bedrock Monitoring Wells

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 9 Hydraulic Conductivity Zonation in Shallow Model Layers

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 10 Hydraulic Conductivity Zonation in Deep Model Layer

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 11 Hydraulic Conductivity Zonation in Bedrock Model Layers

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 12 Measured versus Modeled Water Levels
Observed vs. Modeled Target Values
480
502
524
546
568
590
480 502 524 546 568 590
Mo
d
e
l

H
e
a
d

(
f
e
e
t
)
Observed Head (feet)
Layer 7
Layer 8
Layer 9
Layer 10
Layer 12

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 13 Potentiometric Head Contour Map

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 14 Potentiometric Head Cross Section C-C’

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 15 Potentiometric Head Cross section D-D’

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 16 Particle Tracking Results
Note: Figure is representative of particle tracking for all flow layers

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 17 Predicted Antimony in Monitoring Well AB-10S

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 18 Predicted Antimony in Monitoring Well AB-30S

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 19 Predicted Antimony in Monitoring Well GWA-6S

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 20 Initial (2015) Antimony Concentrations in Shallow Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Antimony IMAC value = 1 µg/L
3. Antimony PPBC = 1 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 21 Initial (2015) Antimony Concentrations in Deep Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Antimony IMAC value = 1 µg/L
3. Antimony PPBC = 1 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 22 Initial (2015) Antimony Concentrations in Bedrock Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Antimony IMAC value = 1 µg/L
3. Antimony PPBC = 1 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 23 Existing Conditions Scenario 1 - 2115 Predicted Antimony Concentrations in Shallow Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Antimony IMAC value = 1 µg/L
3. Antimony PPBC = 1 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 24 Existing Conditions Scenario 1 - 2115 Predicted Antimony Concentrations in Deep Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Antimony IMAC value = 1 µg/L
3. Antimony PPBC = 1 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 25 Existing Conditions Scenario 1 - 2115 Predicted Antimony Concentrations in Bedrock Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Antimony IMAC value = 1 µg/L
3. Antimony PPBC = 1 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 26 Excavation Scenario 2 - 2115 Predicted Antimony Concentrations in Shallow Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Antimony IMAC value = 1 µg/L
3. Antimony PPBC = 1 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 27 Excavation Scenario 2 - 2115 Predicted Antimony Concentrations in Deep Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Antimony IMAC value = 1 µg/L
3. Antimony PPBC = 1 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 28 Excavation Scenario 2 - 2115 Predicted Antimony Concentrations in Bedrock Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Antimony IMAC value = 1 µg/L
3. Antimony PPBC = 1 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 29 Predicted Arsenic in Monitoring Well AB-10S

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 30 Predicted Arsenic in Monitoring Well AB-30S

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 31 Predicted Arsenic in Monitoring Well GWA-6S

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 32 Initial (2015) Arsenic Concentrations in Shallow Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Arsenic 2L Standard = 10 µg/L
3. Arsenic PPBC = 1 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 33 Initial (2015) Arsenic Concentrations in Deep Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Arsenic 2L Standard = 10 µg/L
3. Arsenic PPBC = 1 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 34 Initial (2015) Arsenic Concentrations in Bedrock Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Arsenic 2L Standard = 10 µg/L
3. Arsenic PPBC = 1 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 35 Existing Conditions Scenario 1 - 2115 Predicted Arsenic Concentrations in Shallow Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Arsenic 2L Standard = 10 µg/L
3. Arsenic PPBC = 1 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 36 Existing Conditions Scenario 1 - 2115 Predicted Arsenic Concentrations in Deep Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Arsenic 2L Standard = 10 µg/L
3. Arsenic PPBC = 1 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 37 Existing Conditions Scenario 1 - 2115 Predicted Arsenic Concentrations in Bedrock Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Arsenic 2L Standard = 10 µg/L
3. Arsenic PPBC = 1 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 38 Excavation Scenario 2 - 2115 Predicted Arsenic Concentrations in Shallow Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Arsenic 2L Standard = 10 µg/L
3. Arsenic PPBC = 1 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 39 Excavation Scenario 2 - 2115 Predicted Arsenic Concentrations in Deep Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Arsenic 2L Standard = 10 µg/L
3. Arsenic PPBC = 1 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 40 Excavation Scenario 2 - 2115 Predicted Arsenic Concentrations in Bedrock Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Arsenic 2L Standard = 10 µg/L
3. Arsenic PPBC = 1 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 41 Predicted Boron in Monitoring Well AB-10S

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 42 Predicted Boron in Monitoring Well AB-30S

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 43 Predicted Boron in Monitoring Well GWA-6S

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 44 Initial (2015) Boron Concentrations in Shallow Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Boron 2L Standard = 700 µg/L
3. Boron PPBC = 50 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 45 Initial (2015) Boron Concentrations in Deep Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Boron 2L Standard = 700 µg/L
3. Boron PPBC = 50 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 46 Initial (2015) Boron Concentrations in Bedrock Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Boron 2L Standard = 700 µg/L
3. Boron PPBC = 50 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 47 Existing Conditions Scenario 1 - 2115 Predicted Boron Concentrations in Shallow Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Boron 2L Standard = 700 µg/L
3. Boron PPBC = 50 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 48 Existing Conditions Scenario 1 - 2115 Predicted Boron Concentrations in Deep Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Boron 2L Standard = 700 µg/L
3. Boron PPBC = 50 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 49 Existing Conditions Scenario 1 - 2115 Predicted Boron Concentrations in Bedrock Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Boron 2L Standard = 700 µg/L
3. Boron PPBC = 50 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 50 Excavation Scenario 2 - 2115 Predicted Boron Concentrations in Shallow Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Boron 2L Standard = 700 µg/L
3. Boron PPBC = 50 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 51 Excavation Scenario 2 - 2115 Predicted Boron Concentrations in Deep Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Boron 2L Standard = 700 µg/L
3. Boron PPBC = 50 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 52 Excavation Scenario 2 - 2115 Predicted Boron Concentrations in Bedrock Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Boron 2L Standard = 700 µg/L
3. Boron PPBC = 50 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 53. Predicted Chromium in Monitoring Well AB-10SL

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 54. Predicted Chromium in Monitoring Well AB-30S

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 55. Predicted Chromium in Monitoring Well GWA-6S

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 56 Initial (2015) Chromium Concentrations in Shallow Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Chromium 2L Standard = 10 µg/L
3. Chromium PPBC = 5 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 57 Initial (2015) Chromium Concentrations in Deep Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Chromium 2L Standard = 10 µg/L
3. Chromium PPBC = 5 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 58 Initial (2015) Chromium Concentrations in Bedrock Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Chromium 2L Standard = 10 µg/L
3. Chromium PPBC = 5 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 59 Existing Conditions Scenario 1 - 2115 Predicted Chromium Concentrations in Shallow Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Chromium 2L Standard = 10 µg/L
3. Chromium PPBC = 5 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 60 Existing Conditions Scenario 1 - 2115 Predicted Chromium Concentrations in Deep Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Chromium 2L Standard = 10 µg/L
3. Chromium PPBC = 5 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 61 Existing Conditions Scenario 1 - 2115 Predicted Chromium Concentrations in Bedrock Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Chromium 2L Standard = 10 µg/L
3. Chromium PPBC = 5 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 62 Excavation Scenario 2 - 2115 Predicted Chromium Concentrations in Shallow Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Chromium 2L Standard = 10 µg/L
3. Chromium PPBC = 5 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 63 Excavation Scenario 2 - 2115 Predicted Chromium Concentrations in Deep Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Chromium 2L Standard = 10 µg/L
3. Chromium PPBC = 5 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 64 Excavation Scenario 2 - 2115 Predicted Chromium Concentrations in Bedrock Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Chromium 2L Standard = 10 µg/L
3. Chromium PPBC = 5 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 65 Predicted Cobalt in Monitoring Well AB-10S

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 66 Predicted Cobalt in Monitoring Well AB-30S

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 67 Predicted Cobalt in Monitoring Well GWA-6S

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 68 Initial (2015) Cobalt Concentrations in Shallow Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Cobalt IMAC value = 1 µg/L
3. Cobalt PPBC = 0.5 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 69 Initial (2015) Cobalt Concentrations in Deep Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Cobalt IMAC value = 1 µg/L
3. Cobalt PPBC = 0.5 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 70 Initial (2015) Cobalt Concentrations in Bedrock Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Cobalt IMAC value = 1 µg/L
3. Cobalt PPBC = 0.5 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 71 Existing Conditions Scenario 1 - 2115 Predicted Cobalt Concentrations in Shallow Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Cobalt IMAC value = 1 µg/L
3. Cobalt PPBC = 0.5 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 72 Existing Conditions Scenario 1 - 2115 Predicted Cobalt Concentrations in Deep Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Cobalt IMAC value = 1 µg/L
3. Cobalt PPBC = 0.5 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 73 Existing Conditions Scenario 1 - 2115 Predicted Cobalt Concentrations in Bedrock Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Cobalt IMAC value = 1 µg/L
3. Cobalt PPBC = 0.5 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 74 Excavation Scenario 2 - 2115 Predicted Cobalt Concentrations in Shallow Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Cobalt IMAC value = 1 µg/L
3. Cobalt PPBC = 0.5 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 75 Excavation Scenario 2 - 2115 Predicted Cobalt Concentrations in Deep Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Cobalt IMAC value = 1 µg/L
3. Cobalt PPBC = 0.5 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 76 Excavation Scenario 2 - 2115 Predicted Cobalt Concentrations in Bedrock Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Cobalt IMAC value = 1 µg/L
3. Cobalt PPBC = 0.5 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 77 Predicted Hexavalent Chromium in Monitoring Well GWA-10D

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 78 Initial (2015) Hexavalent Chromium Concentrations in Shallow Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Hexavalent chromium DHHS HSL value = 0.07 µg/L
3. Hexavalent chromium PPBC = 0.089 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 79 Initial (2015) Hexavalent Chromium Concentrations in Deep Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Hexavalent chromium DHHS HSL value = 0.07 µg/L
3. Hexavalent chromium PPBC = 0.089 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 80 Initial (2015) Hexavalent Chromium Concentrations in Bedrock Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Hexavalent chromium DHHS HSL value = 0.07 µg/L
3. Hexavalent chromium PPBC = 0.089 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 81 Existing Conditions Scenario 1 - 2115 Predicted Hexavalent Chromium Concentrations in Shallow Groundwater
Zone
Notes:
1. µg/L = micrograms per liter
2. Hexavalent chromium DHHS HSL value = 0.07 µg/L
3. Hexavalent chromium PPBC = 0.089 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 82 Existing Conditions Scenario 1 - 2115 Predicted Hexavalent Chromium Concentrations in Deep Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Hexavalent chromium DHHS HSL value = 0.07 µg/L
3. Hexavalent chromium PPBC = 0.089 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 83 Existing Conditions Scenario 1 - 2115 Predicted Hexavalent Chromium Concentrations in Bedrock Groundwater
Zone
Notes:
1. µg/L = micrograms per liter
2. Hexavalent chromium DHHS HSL value = 0.07 µg/L
3. Hexavalent chromium PPBC = 0.089 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 84 Excavation Scenario 2 - 2115 Predicted Hexavalent Chromium Concentrations in Shallow Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Hexavalent chromium DHHS HSL value = 0.07 µg/L
3. Hexavalent chromium PPBC = 0.089 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 85 Excavation Scenario 2 - 2115 Predicted Hexavalent Chromium Concentrations in Deep Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Hexavalent chromium DHHS HSL value = 0.07 µg/L
3. Hexavalent chromium PPBC = 0.089 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 86 Excavation Scenario 2 - 2115 Predicted Hexavalent Chromium Concentrations in Bedrock Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Hexavalent chromium DHHS HSL value = 0.07 µg/L
3. Hexavalent chromium PPBC = 0.089 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 87 Predicted Selenium in Monitoring Well AB-10S

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 88 Predicted Selenium in Monitoring Well AB-30S

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 89 Predicted Selenium in Monitoring Well GWA-6S

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 90 Initial (2015) Selenium Concentrations in Shallow Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Selenium 2L Standard = 20 µg/L
3. Selenium PPBC 1 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 91 Initial (2015) Selenium Concentrations in Deep Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Selenium 2L Standard = 20 µg/L
3. Selenium PPBC 1 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 92 Initial (2015) Selenium Concentrations in Bedrock Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Selenium 2L Standard = 20 µg/L
3. Selenium PPBC 1 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 93 Existing Conditions Scenario 1 - 2115 Predicted Selenium Concentrations in Shallow Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Selenium 2L Standard = 20 µg/L
3. Selenium PPBC 1 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 94 Existing Conditions Scenario 1 - 2115 Predicted Selenium Concentrations in Deep Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Selenium 2L Standard = 20 µg/L
3. Selenium PPBC 1 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 95 Existing Conditions Scenario 1 - 2115 Predicted Selenium Concentrations in Bedrock Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Selenium 2L Standard = 20 µg/L
3. Selenium PPBC 1 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 96 Excavation Scenario 2 - 2115 Predicted Selenium Concentrations in Shallow Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Selenium 2L Standard = 20 µg/L
3. Selenium PPBC 1 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 97 Excavation Scenario 2 - 2115 Predicted Selenium Concentrations in Deep Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Selenium 2L Standard = 20 µg/L
3. Selenium PPBC 1 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 98 Excavation Scenario 2 - 2115 Predicted Selenium Concentrations in Bedrock Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Selenium 2L Standard = 20 µg/L
3. Selenium PPBC 1 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 99 Predicted Sulfate in Monitoring Well AB-10S

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 100 Predicted Sulfate in Monitoring Well AB-30S

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 101 Predicted Sulfate in Monitoring Well GWA-6S

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 102 Initial (2015) Sulfate Concentrations in Shallow Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Sulfate 2L Standard = 250,000 µg/L
3. Sulfate PPBC = 31,000 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 103 Initial (2015) Sulfate Concentrations in Deep Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Sulfate 2L Standard = 250,000 µg/L
3. Sulfate PPBC = 31,000 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 104 Initial (2015) Sulfate Concentrations in Bedrock Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Sulfate 2L Standard = 250,000 µg/L
3. Sulfate PPBC = 31,000 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 105 Existing Conditions Scenario 1 - 2115 Predicted Sulfate Concentrations in Shallow Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Sulfate 2L Standard = 250,000 µg/L
3. Sulfate PPBC = 31,000 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 106 Existing Conditions Scenario 1 - 2115 Predicted Sulfate Concentrations in Deep Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Sulfate 2L Standard = 250,000 µg/L
3. Sulfate PPBC = 31,000 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 107 Existing Conditions Scenario 1 - 2115 Predicted Sulfate Concentrations in Bedrock Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Sulfate 2L Standard = 250,000 µg/L
3. Sulfate PPBC = 31,000 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 108 Excavation Scenario 2 - 2115 Predicted Sulfate Concentrations in Shallow Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Sulfate 2L Standard = 250,000 µg/L
3. Sulfate PPBC = 31,000 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 109 Excavation Scenario 2 - 2115 Predicted Sulfate Concentrations in Deep Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Sulfate 2L Standard = 250,000 µg/L
3. Sulfate PPBC = 31,000 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 110 Excavation Scenario 2 - 2115 Predicted Sulfate Concentrations in Bedrock Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Sulfate 2L Standard = 250,000 µg/L
3. Sulfate PPBC = 31,000 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 111 Predicted Thallium in Monitoring Well AB-10S

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 112 Predicted Thallium in Monitoring Well AB-30S

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 113 Predicted Thallium in Monitoring Well GWA-6S

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 114 Initial (2015) Thallium Concentrations in Shallow Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Thallium IMAC value = 0.2 µg/L
3. Thallium PPBC = 0.2 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 115 Initial (2015) Thallium Concentrations in Deep Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Thallium IMAC value = 0.2 µg/L
3. Thallium PPBC = 0.2 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 116 Initial (2015) Thallium Concentrations in Bedrock Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Thallium IMAC value = 0.2 µg/L
3. Thallium PPBC = 0.2 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 117 Existing Conditions Scenario 1 - 2115 Predicted Thallium Concentrations in Shallow Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Thallium IMAC value = 0.2 µg/L
3. Thallium PPBC = 0.2 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 118 Existing Conditions Scenario 1 - 2115 Predicted Thallium Concentrations in Deep Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Thallium IMAC value = 0.2 µg/L
3. Thallium PPBC = 0.2 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 119 Existing Conditions Scenario 1 - 2115 Predicted Thallium Concentrations in Bedrock Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Thallium IMAC value = 0.2 µg/L
3. Thallium PPBC = 0.2 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 120 Excavation Scenario 2 - 2115 Predicted Thallium Concentrations in Shallow Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Thallium IMAC value = 0.2 µg/L
3. Thallium PPBC = 0.2 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 121 Excavation Scenario 2 - 2115 Predicted Thallium Concentrations in Deep Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Thallium IMAC value = 0.2 µg/L
3. Thallium PPBC = 0.2 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 122 Excavation Scenario 2 - 2115 Predicted Thallium Concentrations in Bedrock Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Thallium IMAC value = 0.2 µg/L
3. Thallium PPBC = 0.2 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 123 Predicted Vanadium in Monitoring Well AB-10SL

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 124 Predicted Vanadium in Monitoring Well AB-30S

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 125 Predicted Vanadium in Monitoring Well GWA-6S

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 126 Initial (2015) Vanadium Concentrations in Shallow Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Vanadium IMAC value = 0.3 µg/L
3. Vanadium PPBC = 1 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 127 Initial (2015) Vanadium Concentrations in Deep Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Vanadium IMAC value = 0.3 µg/L
3. Vanadium PPBC = 1 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 128 Initial (2015) Vanadium Concentrations in Bedrock Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Vanadium IMAC value = 0.3 µg/L
3. Vanadium PPBC = 1 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 129 Existing Conditions Scenario 1 - 2115 Predicted Vanadium Concentrations in Shallow Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Vanadium IMAC value = 0.3 µg/L
3. Vanadium PPBC = 1 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 130 Existing Conditions Scenario 1 - 2115 Predicted Vanadium Concentrations in Deep Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Vanadium IMAC value = 0.3 µg/L
3. Vanadium PPBC = 1 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 131 Existing Conditions Scenario 1 - 2115 Predicted Vanadium Concentrations in Bedrock Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Vanadium IMAC value = 0.3 µg/L
3. Vanadium PPBC = 1 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 132 Excavation Scenario 2 - 2115 Predicted Vanadium Concentrations in Shallow Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Vanadium IMAC value = 0.3 µg/L
3. Vanadium PPBC = 1 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 133 Excavation Scenario 2 - 2115 Predicted Vanadium Concentrations in Deep Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Vanadium IMAC value = 0.3 µg/L
3. Vanadium PPBC = 1 µg/L

Groundwater Flow and Transport Model
Dan River Steam Station

Figure 134 Excavation Scenario 2 - 2115 Predicted Vanadium Concentrations in Bedrock Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Vanadium IMAC value = 0.3 µg/L
3. Vanadium PPBC = 1 µg/L