NC0004961_5. RBSS CAP Part 2_Appx B_FINAL_20160212

Appendix B
Groundwater Flow and
Transport Model

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Memorandum

February 5, 2016
TO: Ed Sullivan and Tyler Hardin
FROM: Bruce Hensel
SUBJECT: RIVERBEND MODEL REVIEW

Summary
EPRI has reviewed the Riverbend model report and files provided by Duke Energy, HDR
Engineering, Inc., and the University of North Carolina Charlotte. The review was performed
by James Rumbaugh (Environmental Simulations, Inc.), with input by Chunmiao Zheng and
myself. Based on this review, it is our opinion that, subject to the caveats 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
caveats associated with this opinion are:
• 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, consider an
alternative to the constant heads below layer 5 representing the Catawba River.
• Concentrations were added to the model via constant concentration boundaries. Use
of this boundary condition may make it difficult for the model to simulate capping as
a remedial strategy. However, since the closure scenarios include existing conditions
and ash removal, this type of boundary condition is justified.

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 aquifer system
framework, the groundwater flow system, hydrologic boundaries, hydraulic boundaries,
sources and sinks, water budget, modeled constituents of interest, and constituent transport.
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The CSA report is referenced in describing aquifer properties. 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 is a constant 20 ft, which is 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 ft) for the entire model area, coupled with the
longitudinal dispersivity of 80 feet file results in a Grid Peclet number of 0.25 in the entire
model area. The transverse lateral dispersivity of 8 ft results in a Grid Peclet number of 2.5,
which is well within the desired range.
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. Grid cells beneath the ash pond indicate cell thicknesses (∆z)
in layers 5 through 7 range from about 20 to 45 ft. The transverse vertical dispersivity (αv) is
8.0 ft in the model. This results in a grid Peclet number (Pe_grid = ∆z/αv) up to a maximum
value of about 5.6, which is well within the desired range.
iii) Hydrologic framework – hydraulic properties
Hydraulic properties were grouped by material type (ash, dike, saprolite, alluvium, transition
zone, and bedrock) which are based on slug tests and the hydrostratigraphy of the site
described in the CSA report. This approach is reasonable.
iv) Boundary conditions
The model boundary conditions consist of constant heads for the Catawba River and drains
for the seeps and low areas. Recharge is applied in 2 zones at rates of 21.5 inches per year
for the ash basins and 6.5 inches per year for the rest of the model. No-flow boundaries
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represent surface divides, which are assumed to also coincide with groundwater divides.
These are all standard modeling practice.
The transport model used constant concentration boundaries to input mass/concentration
from the ash basins. These boundaries were placed in layers 1 to 4. Recharge was not used to
add concentration to the model. This selection of transport model boundary conditions has
two potential ramifications:
• If there is a grid position within the simulated source area where one or more cells
are dry in all source layers, then recharge from infiltration through the ash basin in
these areas will enter the model with a concentration of zero, which does not agree
with the conceptual model for this facility. This situation does occur in the southern
part of the ash basin. However, the model was adjusted by placing constant
concentration cells in layer 5 below the ash basin so that these areas would continue
to be a source of the COIs.
• The placement of constant concentration cells beneath the ash pond will inhibit the
model’s ability to simulate capping, because lateral flow through these constant
concentration cells will continue to add mass/concentration to the model even
though leaching and infiltration may be greatly reduced by the cap. Simulation of
other corrective actions, other than excavation, will also by affected because the
concentration added to the model will not be representative of the capped facility.
The only corrective action modeled currently is ash removal so this concern in not
an issue at present.

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
different initial concentration for each component simulated. 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 0.0 percent is more than adequate in ensuring minimal flow mass
balance errors. The cumulative concentration mass balance discrepancy from MT3DMS is
less than or equal to 0.01 percent for all constituents (antimony, arsenic, boron, chromium,
sulfate, thallium, and vanadium). These mass balance errors are more than adequate in
ensuring minimal constituent mass balance errors.
e) Is the calibration done properly and adequately?
For flow calibration to heads, the Mean Absolute Error over the range in observed values is
6.5 percent, which is below the industry standard of 10 percent. The scaled RMS error and
Standard Deviation are about 8.2 percent, also below the standard of 10 percent. In addition,
there is no significant spatial bias in the distribution of residuals.
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The transport calibration is reasonable given the uncertainty involved with this type of
modeling. In general, wells with high concentrations show high simulated concentrations for
boron, sulfate, thallium, and vanadium. Likewise, wells showing low levels (or below
detection) also exhibit low predicted concentrations in the model. The remaining
constituents (antimony, arsenic, and chromium) did not have as good of match to measured
data. In general, it is difficult to achieve strong concentration match for large numbers of
modeled constituents, particularly if some are affected by geochemical reactions that are not
simulated by MT3DMS. However, the constituents with good concentration match will
enable use of the model to evaluate its stated objective.
f) Discretization of calibration parameters
Hydraulic properties were grouped by material type (ash, dike, saprolite, alluvium, transition
zone, and bedrock). This is a reasonable approach. Recharge is grouped by infiltration
within the ash basins and outside the basins. This is also a reasonable approach.
i) 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. Several plots also show simulated
constituent concentrations in the context of several observed concentrations. Detailed tables
of observed and simulated concentrations are also provided to document the transport
calibration.
g) 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, horizontal
hydraulic conductivities, and vertical hydraulic conductivities were perturbed up and down
by 20%. It is stated that water levels are most sensitive to horizontal hydraulic conductivity
of the shallow aquifer, decreased recharge outside the ash basins, and increased recharge in
the ash basins. The results are depicted in Table 4. This is a reasonable method for
illustrating flow model sensitivity.
Transport sensitivity analysis was not performed.
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.
ii) Check of water balance vs. conceptual model
The conceptual model does not present a comprehensive numerical water balance. Water
enters the model as recharge from precipitation and the recharge rates noted in the conceptual
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model are consistent with those in the model. The constant heads along the Catawba River
are primarily an exit boundary which is consistent with the conceptual water balance. Drains
representing tributary streams and wetlands remove the remaining recharged water.
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
Riverbend Steam Station
Gaston 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

Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin
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 ................................................................................. 4
2.3.1 Ash Basin ................................................................................................................ 4
2.3.2 Ash Storage Area .................................................................................................... 5
2.3.3 Cinder Storage Area ................................................................................................ 5
2.4 Groundwater Flow System ............................................................................................. 5
2.5 Hydrologic Boundaries ................................................................................................... 6
2.6 Hydraulic Boundaries ..................................................................................................... 6
2.7 Groundwater Sources and Sinks .................................................................................... 6
2.8 Water Balance ................................................................................................................ 6
2.9 Modeled Constituents of Interest .................................................................................... 6
2.10 COI Transport ................................................................................................................. 7
3 Computer Model .................................................................................................................... 7
3.1 Model Selection .............................................................................................................. 7
3.2 Model Description ........................................................................................................... 8
4 Groundwater Flow and Transport Model Construction .......................................................... 8
4.1 Model Hydrostratigraphy ................................................................................................ 8
4.1.1 Existing Ground Surface ......................................................................................... 8
4.1.2 Pre-construction Surface ......................................................................................... 8
4.1.3 Pre-construction Surface with Dikes ....................................................................... 9
4.1.4 3-D Hydrostratigraphic Grids ................................................................................... 9
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
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Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin
4.8 Transport Model Parameters ........................................................................................ 12
4.9 Transport Model Boundary Conditions ......................................................................... 13
4.10 Transport Model Sources and Sinks ............................................................................ 14
4.11 Transport Model Calibration Targets ............................................................................ 14
5 Model Calibration ................................................................................................................. 14
5.1 Flow Model Residual Analysis ...................................................................................... 14
5.2 Flow Model Sensitivity Analysis .................................................................................... 15
5.3 Transport Model Calibration and Sensitivity ................................................................. 16
5.4 Advective Travel Times ................................................................................................ 17
6 Simulation of CLosure Scenarios ........................................................................................ 17
6.1 Existing Conditions Scenario ........................................................................................ 17
6.2 Excavation Scenario ..................................................................................................... 17
7 Closure Scenario Results .................................................................................................... 18
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 Sulfate .......................................................................................................................... 20
7.8 Thallium ........................................................................................................................ 20
7.9 Vanadium ..................................................................................................................... 20
7.10 Potential Groundwater Monitoring Locations at One Year’s Advective Travel Time
Upgradient of Catawba River/Mountain Island Lake ............................................................... 20
8 Summary ............................................................................................................................. 21
8.1 Model Assumptions and Limitations ............................................................................. 21
8.2 Model Predictions ......................................................................................................... 22
9 References .......................................................................................................................... 22

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
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Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin
Table 5 Transport Model Calibration Results
Table 6 Predicted Advective Travel Times

FIGURES
Figure 1 Conceptual Groundwater Flow Model/Model Domain
Figure 2 Model Domain North-South Cross Section (A-A’) Through Primary and Secondary Ash
Basins
Figure 3 Model Domain East-West Cross Section (B-B’) Through Primary and Secondary Ash
Basins
Figure 4 Numerical Model Boundary Conditions
Figure 5 Model Recharge and Contaminant Source Zones (Constant Concentration Cells
Figure 6 Observation Wells in Shallow Groundwater Zone
Figure 7 Observation Wells in Deep Groundwater Zone
Figure 8 Observation Wells in Bedrock Groundwater Zone
Figure 9 Hydraulic Conductivity Zonation in S/M1 Layers (Model Layers 5-6)
Figure 10 Hydraulic Conductivity Zonation in M2 Layers (Model Layer 7)
Figure 11 Hydraulic Conductivity Zonation in Transition Zone Layers (Model Layer 8)
Figure 12 Hydraulic Conductivity Zonation in Bedrock Layers (Model Layers 9-10)
Figure 13 Hydraulic Head (feet) in M1 Layer (Model Layer 6)
Figure 14 Particle Tracking Results
Figure 15 Predicted Antimony in Monitoring Well MW-3S
Figure 16 Predicted Antimony in Monitoring Well MW-5S
Figure 17 Predicted Antimony in Monitoring Well MW-6S
Figure 18 Initial (2015) Antimony Concentrations in Shallow Groundwater Zone
Figure 19 Initial (2015) Antimony Concentrations in Deep Groundwater Zone
Figure 20 Initial (2015) Antimony Concentrations in Bedrock Groundwater Zone
Figure 21 Existing Conditions Scenario - 2115 Predicted Antimony in Shallow Groundwater
Zone
Figure 22 Existing Conditions Scenario - 2115 Predicted Antimony in Deep Groundwater Zone
Figure 23 Existing Conditions Scenario - 2115 Predicted Antimony in Bedrock Groundwater
Zone
Figure 24 Excavation Scenario - 2115 Predicted Antimony in Shallow Groundwater Zone
Figure 25 Excavation Scenario - 2115 Predicted Antimony in Deep Groundwater Zone
Figure 26 Excavation Scenario - 2115 Predicted Antimony in Bedrock Groundwater Zone
Figure 27 Predicted Arsenic in Monitoring Well MW-3S
Figure 28 Predicted Arsenic in Monitoring Well MW-5S
Figure 29 Predicted Arsenic in Monitoring Well MW-6S
Figure 30 Initial (2015) Arsenic Concentrations in Shallow Groundwater Zone
Figure 31 Initial (2015) Arsenic Concentrations in Deep Groundwater Zone
Figure 32 Initial (2015) Arsenic Concentrations in Bedrock Groundwater Zone
Figure 33 Existing Conditions Scenario - 2115 Predicted Arsenic in Shallow Groundwater Zone
Figure 34 Existing Conditions Scenario - 2115 Predicted Arsenic in Deep Groundwater Zone
Figure 35 Existing Conditions Scenario - 2115 Predicted Arsenic in Bedrock Groundwater Zone
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Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin
Figure 36 Excavation Scenario - 2115 Predicted Arsenic in Shallow Groundwater Zone
Figure 37 Excavation Scenario - 2115 Predicted Arsenic in Deep Groundwater Zone
Figure 38 Excavation Scenario - 2115 Predicted Arsenic in Bedrock Groundwater Zone
Figure 39 Predicted Boron in Monitoring Well MW-3S
Figure 40 Predicted Boron in Monitoring Well MW-5S
Figure 41 Predicted Boron in Monitoring Well MW-6S
Figure 42 Initial (2015) Boron Concentrations in Shallow Groundwater Zone
Figure 43 Initial (2015) Boron Concentrations in Deep Groundwater Zone
Figure 44 Initial (2015) Boron Concentrations in Bedrock Groundwater Zone
Figure 45 Existing Conditions Scenario - 2115 Predicted Boron in Shallow Groundwater Zone
Figure 46 Existing Conditions Scenario - 2115 Predicted Boron in Deep Groundwater Zone
Figure 47 Existing Conditions Scenario - 2115 Predicted Boron in Bedrock Groundwater Zone
Figure 48 Excavation Scenario - 2115 Predicted Boron in Shallow Groundwater Zone
Figure 49 Excavation Scenario - 2115 Predicted Boron in Deep Groundwater Zone
Figure 50 Excavation Scenario - 2115 Predicted Boron in Bedrock Groundwater Zone
Figure 51 Predicted Chromium in Monitoring Well MW-3S
Figure 52 Predicted Chromium in Monitoring Well MW-5S
Figure 53 Predicted Chromium in Monitoring Well MW-6S
Figure 54 Initial (2015) Chromium Concentrations in Shallow Groundwater Zone
Figure 55 Initial (2015) Chromium Concentrations in Deep Groundwater Zone
Figure 56 Initial (2015) Chromium Concentrations in Bedrock Groundwater Zone
Figure 57 Existing Conditions Scenario - 2115 Predicted Chromium in Shallow Groundwater
Zone
Figure 58 Existing Conditions Scenario - 2115 Predicted Chromium in Deep Groundwater Zone
Figure 59 Existing Conditions Scenario - 2115 Predicted Chromium in Bedrock Groundwater
Zone
Figure 60 Excavation Scenario - 2115 Predicted Chromium in Shallow Groundwater Zone
Figure 61 Excavation Scenario - 2115 Predicted Chromium in Deep Groundwater Zone
Figure 62 Excavation Scenario - 2115 Predicted Chromium in Bedrock Groundwater Zone
Figure 63 Predicted Cobalt in Monitoring Well MW-3S
Figure 64 Predicted Cobalt in Monitoring Well MW-5S
Figure 65 Predicted Cobalt in Monitoring Well MW-6S
Figure 66 Initial (2015) Cobalt Concentrations in Shallow Groundwater Zone
Figure 67 Initial (2015) Cobalt Concentrations in Deep Groundwater Zone
Figure 68 Initial (2015) Cobalt Concentrations in Bedrock Groundwater Zone
Figure 69 Existing Conditions Scenario - 2115 Predicted Cobalt in Shallow Groundwater Zone
Figure 70 Existing Conditions Scenario - 2115 Predicted Cobalt in Deep Groundwater Zone
Figure 71 Existing Conditions Scenario - 2115 Predicted Cobalt in Bedrock Groundwater Zone
Figure 72 Excavation Scenario - 2115 Predicted Cobalt in Shallow Groundwater Zone
Figure 73 Excavation Scenario - 2115 Predicted Cobalt in Deep Groundwater Zone
Figure 74 Excavation Scenario - 2115 Predicted Cobalt in Bedrock Groundwater Zone
Figure 75 Predicted Hexavalent Chromium in Monitoring Well MW-9D
Figure 76 Initial (2015) Hexavalent Chromium Concentrations in Shallow Groundwater Zone
Figure 77 Initial (2015) Hexavalent Chromium Concentrations in Deep Groundwater Zone
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Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin
Figure 78 Initial (2015) Hexavalent Chromium Concentrations in Bedrock Groundwater Zone
Figure 79 Existing Conditions Scenario - 2115 Predicted Hexavalent Chromium in Shallow
Groundwater Zone
Figure 80 Existing Conditions Scenario - 2115 Predicted Hexavalent Chromium in Deep
Groundwater Zone
Figure 81 Existing Conditions Scenario - 2115 Predicted Hexavalent Chromium in Bedrock
Groundwater Zone
Figure 82 Excavation Scenario - 2115 Predicted Hexavalent Chromium in Shallow Groundwater
Zone
Figure 83 Excavation Scenario - 2115 Predicted Hexavalent Chromium in Deep Groundwater
Zone
Figure 84 Excavation Scenario - 2115 Predicted Hexavalent Chromium in Bedrock Groundwater
Zone
Figure 85 Predicted Sulfate in Monitoring Well MW-3S
Figure 86 Predicted Sulfate in Monitoring Well MW-5S 3
Figure 87 Predicted Sulfate in Monitoring Well MW-6S
Figure 88 Initial (2015) Sulfate Concentrations in Shallow Groundwater Zone
Figure 89 Initial (2015) Sulfate Concentrations in Deep Groundwater Zone
Figure 90 Initial (2015) Sulfate Concentrations in Bedrock Groundwater Zone
Figure 91 Existing Conditions Scenario - 2115 Predicted Sulfate in Shallow Groundwater Zone
Figure 92 Existing Conditions Scenario - 2115 Predicted Sulfate in Deep Groundwater Zone
Figure 93 Existing Conditions Scenario - 2115 Predicted Sulfate in Bedrock Groundwater Zone
Figure 94 Excavation Scenario - 2115 Predicted Sulfate in Shallow Groundwater Zone
Figure 95 Excavation Scenario - 2115 Predicted Sulfate in Deep Groundwater Zone
Figure 96 Excavation Scenario - 2115 Predicted Sulfate in Bedrock Groundwater Zone
Figure 97 Predicted Thallium in Monitoring Well MW-3S
Figure 98 Predicted Thallium in Monitoring Well MW-5S
Figure 99 Predicted Thallium in Monitoring Well MW-6S
Figure 100 Initial (2015) Thallium Concentrations in Shallow Groundwater Zone
Figure 101 Initial (2015) Thallium Concentrations in Deep Groundwater Zone
Figure 102 Initial (2015) Thallium Concentrations in Bedrock Groundwater Zone
Figure 103 Existing Conditions Scenario - 2115 Predicted Thallium in Shallow Groundwater
Zone
Figure 104 Existing Conditions Scenario - 2115 Predicted Thallium in Deep Groundwater Zone
Figure 105 Existing Conditions Scenario - 2115 Predicted Thallium in Bedrock Groundwater
Zone
Figure 106 Excavation Scenario - 2115 Predicted Thallium in Shallow Groundwater Zone
Figure 107 Excavation Scenario - 2115 Predicted Thallium in Deep Groundwater Zone
Figure 108 Excavation Scenario - 2115 Predicted Thallium in Bedrock Groundwater Zone
Figure 109 Predicted Vanadium in Monitoring Well MW-3S
Figure 110 Predicted Vanadium in Monitoring Well MW-5S
Figure 111 Predicted Vanadium in Monitoring Well MW-6S
Figure 112 Initial (2015) Vanadium Concentrations in Shallow Groundwater Zone
Figure 113 Initial (2015) Vanadium Concentrations in Deep Groundwater Zone
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Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin
Figure 114 Initial (2015) Vanadium Concentrations in Bedrock Groundwater Zone
Figure 115 Existing Conditions Scenario - 2115 Predicted Vanadium in Shallow Groundwater
Zone
Figure 116 Existing Conditions Scenario - 2115 Predicted Vanadium in Deep Groundwater Zone
Figure 117 Existing Conditions Scenario - 2115 Predicted Vanadium in Bedrock Groundwater
Zone
Figure 118 Excavation Scenario - 2115 Predicted Vanadium in Shallow Groundwater Zone
Figure 119 Excavation Scenario - 2115 Predicted Vanadium in Deep Groundwater Zone
Figure 120 Excavation Scenario - 2115 Predicted Vanadium in Bedrock Groundwater Zone
Figure 121 Water Level Drawdown at Hypothetical Pumping Wells between Ash Basin Waste
Boundary and Mountain Island Lake

ACRONYMS
2L Standard North Carolina groundwater standards as specified in T15A NCAC 02L
3-D three-dimensional
CAP Corrective Action Plan
COI constituent of interest
CSA comprehensive site assessment
EPRI Electric Power Research Institute
HSL health screening level
IMAC interim maximum allowable concentration
MW megawatt
NCDHHS North Carolina Department of Health and Human Services
NRMSE normalized root mean square error
PPBC proposed provisional background concentration
RBSS Riverbend Steam Station
RMS root mean squared
TZ transition zone

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Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin
1 INTRODUCTION
The purpose of this study is to predict the groundwater flow and constituent transport that will
occur as a result of different possible closure actions at the site. This study consists of three
main activities: development of a calibrated steady-state flow model of site conditions observed
in June/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 the Riverbend Steam Station (RBSS), located on a 341 acre tract located
adjacent to the Mountain Island Lake portion of the Catawba River near Mount Holly in Gaston
County, North Carolina. RBSS is a former seven-unit coal-fired electricity generating facility with
a capacity of 454 megawatts (MW). The station began commercial operation in 1929 with coal-
fired Units 1-4. Units 5-7 began operating in 1952-1954 and all of the coal-fired units were
located in a single power plant. Units 1-3 were retired from service in the 1970s, and Units 4-7
ceased operation on April 1, 2013. Initially, coal ash residue from the coal combustion process
was deposited in a cinder storage area onsite. Following installation of precipitators and a wet
sluicing system in 1957, coal ash residue was disposed of in the station’s ash basin system
located adjacent to the station and Mountain Island Lake as shown on Comprehensive Site
Assessment (CSA) Report Figure 2-1)1(HDR 2015a).During its final years of operation, the plant
was considered a cycling station and was brought online to supplement energy supply during
peak demand periods. Duke Energy also operated four combustion turbine units at RBSS from
1969 until October 2012. The combustion turbine units could be fired by natural gas or oil and
were located to the west of the coal-fired units. Refer to CSA Report Figure 2-4 for a map of site
features.
As described in the CSA Report, site groundwater exists in alluvium, soil, soil/saprolite, and
bedrock and is consistent with a two-layer regolith-fractured rock system (CSA Report Figure 5-
3). The saturated zone is an unconfined, connected system without confining layers (CSA
Report Figure 5-5) that is underlain by a massive meta-plutonic complex. The groundwater flow
at the site is radial and groundwater flows to the north, east, and west and ultimately discharges
to Mountain Island Lake (the main hydrological feature at the site).
The topography at RBSS ranges from an approximate high elevation of 786 feet near the south
edge of the property near Horseshoe Bend Beach Road to approximate low elevation of 646
feet at the northern boundary with Mountain Island Lake. The site slopes from south to north
with an elevation difference of about 140 feet over an approximate distance of 3,500 horizontal
feet. The natural drainage at the site follows the topography and generally flows from the south
to the north, except where the natural drainage patterns have been modified by the existing ash
basins, ash storage area, and other RBSS features.
1 Please refer to the Comprehensive Site Assessment Report, Riverbend Steam Station, August 2015 (HDR 2015a) for more
information and references CSA Report tables and figures.
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Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin
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
Cliffside 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 RBSS groundwater model, the model was refined to
incorporate post-CSA data. These changes did not affect the model structure or boundaries and
did not deviate from EPRI guidelines. An independent review of the refined RBSS model was
conducted by EPRI and found that the model was sufficient to meet the objective of predicting
effects of corrective action alternatives on groundwater quality
2 CONCEPTUAL MODEL
The site conceptual model used for the groundwater flow model is based primarily on
information provided in the CSA Report. The CSA contains extensive detail and data related to
most aspects of the site groundwater model used in this study.
2.1 Geology and Hydrogeology
The RBSS is located within the area of the Charlotte terrane, which is a tectonostratigraphic
terrane that has been defined in the southern and central Appalachians. The Charlotte terrane is
in the western portion of the larger Carolina superterrane (CSA Report Figure 5-1). The
northwest side of the Charlotte terrane is in contact with the Inner Piedmont zone along the
northwest boundary of the Central Piedmont suture. This area’s higher metamorphic grade and
potential tectonism along its boundaries distinguishes it from the Carolina terrane (to the
southeast).
At RBSS, the fractured bedrock is overlain by a mantle of unconsolidated material called
regolith. The regolith includes residual soil and saprolite zones and, where present, alluvial
deposits. The saprolite (a product of chemical weathering of the underlying bedrock) is typically
composed of clay and coarser granular material, and reflects the texture and structure of the
rock from which it was formed.
The groundwater system at the RBSS is a two-medium system that is restricted to a local
drainage basin. The bulk of groundwater occurs in a system of interconnected layers; residual
soil/saprolite (shallow groundwater zone), and weathered rock overlying the fractured crystalline
rock (i.e., bedrock groundwater zone). The weathered rock is called the transition zone, or deep
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Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin
groundwater zone, and exists between the shallow and bedrock groundwater zones. Typically,
the residual soil/saprolite is partially saturated and the water table fluctuates within it.
Preferential groundwater flow occurs within the transition zone, as typically the permeability of
the transition zone exceeds other zones. Also, shallower bedrock is more permeable than
deeper bedrock as the fracture density decreases with depth.
Generally, the groundwater flow at the site can be categorized as radial flow, with groundwater
flowing from the south across the site to the north, northwest, and northeast and ultimately
discharging to Mountain Island Lake. Groundwater in the southwest portion of the site (beneath
the ash storage area) flows to the northwest below the cinder storage area to Mountain Island
Lake. The groundwater contour maps constructed for the CSA Report depict these patterns
(CSA Report Figures 6-5 to 6-7).
2.2 Hydrostratigraphic Layer Development
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 site
exploration 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 the dikes. Ash was generally described as gray to dark gray, non-
plastic, loose to medium dense, dry to wet, fine to coarse-grained.
• Fill – Fill material generally consisted of re-worked silts, clays, and sands that were
borrowed from one area of the site and redistributed to other areas. Fill was generally
classified as silty sand, clay with sand, clay, and sandy clay on the boring logs. Fill was
used in the construction of dikes, and as cover for ash storage area.
• Alluvium –Alluvium encountered in borings during the project was classified as clay and
sand with clay. In some cases alluvium was logged beneath ash.
• Residuum (Residual soils) – Residuum is the in-place weathered soil that consists
primarily of silt with sand, clayey sand, sandy clay, clay with gravel, and clayey silts.
Residuum varies in thickness and was relatively thin compared to the thickness of
saprolite.
• Saprolite/Weathered Rock – Saprolite is soil developed by in-place weathering of rock
that retains remnant bedrock structure. Saprolite consists primarily of medium dense to
very dense silty sand, sandy silt, sand, sand with gravel, sand with clay, clay with sand,
and clay. Sand particle size ranges from fine to coarse grained. Much of the saprolite is
micaceous.
• Partially Weathered/Fractured Rock – Partially weathered (slight to moderate) and/or
highly fractured rock encountered below auger refusal.
• Bedrock – Resistant rock in boreholes was generally slightly weathered to fresh and
relatively unfractured.
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Based on the CSA, the groundwater system is consistent with the regolith-fractured bedrock
system. The material designations shown below (M1, M2, TZ, and BR) are used on the geologic
cross-sections with transect locations in the CSA (CSA Report Figure 11-1). The ash, fill and
alluvial layers are represented by A, F, and S, respectively, on cross sections and in tables in
the CSA Report. The ranges for hydrostratigraphic layer properties measured at RBSS are
provided in CSA Report Tables 11-7 through 11-11. To further define the hydrostratigraphic
units, the following classification system was used based on standard penetration testing
values, recovery (REC), and rock quality designation (RQD) collected during drilling and logging
of boreholes.
The ash, fill and alluvial layers are as encountered at the site. The natural system (except
alluvium) includes the following layers:
M1 – Soil/Saprolite: N<50
M2 – Saprolite/Weathered Rock: N>50 or REC<50%
TZ – Transition Zone: REC>50% and RQD<50%
BR – Bedrock: REC>85% and RQD>50%.
Rock core runs that fell between the values for transition zone and Bedrock (REC<85% and
RQD>50% or REC>85% and RQD<50%) were assigned a hydrostratigraphic layer based on a
review of the borehole logs, rock core photographs, and geologic judgment. The same review
was performed to determine the thickness of the transition zone just in case it extended into the
next core run, which when reviewed alone might have met the bedrock criterion, because of
potential core loss or fractured/jointed rock with indications of water movement (iron/manganese
staining).
2.3 Ash Basin and Ash Storage Areas
2.3.1 Ash Basin
The unlined ash basin is located 2,400 feet to the northeast of the power plant and adjacent to
Mountain Island Lake, as shown on CSA Report Figure 2-2. The ash basin primary cell is
impounded by an earthen embankment dam located on the west side of the ash basin; the
secondary cell by an earthen embankment dam along the northeast side. The toe areas for both
dams are in close proximity to Mountain Island Lake. An intermediate dam (or divider dike) was
constructed in 1979 and consists of soil on top of the existing ash in the basin. This construction
allows hydraulic communication between the primary and secondary cells. Borrow areas within
the ash basin are depicted in some RBSS drawings, but are omitted from others. Based on
information provided by Duke Energy, a dredge pond was located south of the primary cell,
possibly in the current ash storage area. Between 1993 and 2000, the dredged ash was allowed
to dry, and it is possible that some ash was moved offsite.
The ash basin primary cell covers 41 acres, has a maximum pond elevation of 724 feet, and
contains approximately 1.9 million cubic yards of ash. The secondary cell covers 28 acres, has
a maximum pond elevation of 714 feet, and contains approximately 700,000 cubic yards of ash.
The full pond elevation of Mountain Island Lake is 646.8 feet.
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The ash basin system operated as an integral part of the site’s wastewater treatment system.
This system received inflows from the ash removal system, station yard drain sump, and
stormwater flows. During RBSS operations, inflows to the ash basin were variable due to the
cyclical nature of station operations. The inflows from the ash removal system and the station
yard drain sump were conveyed through sluice lines into the primary cell. Discharge from the
primary cell to the secondary cell was conveyed through a concrete discharge tower located
near the divider dike.
Although the RBSS station is retired, wastewater effluent from other non-ash-related stations
flows to the ash basin. Also, the water may be conveyed from the ash basin secondary cell
(through a concrete discharge tower) to Mountain Island Lake. The concrete discharge tower
drains through a 30-inch diameter corrugated metal pipe into a concrete-lined channel that flows
to Mountain Island Lake. The pond level in the ash basin is controlled by the use of concrete
stop logs and discharge from the basin has not occurred in over a year.
2.3.2 Ash Storage Area
The ash storage area is located near the ash basin primary cell (CSA Report Figure 2-2). The
footprint of the ash storage area covers 29 acres and contains approximately 1.5 million tons of
ash. The ash storage area was constructed during two ash basin clean-out projects; the first in
the 2000-2001 timeframe, and the second in late 2006 to early 2008. The clean-out projects
were performed to provide additional capacity in the ash basins.
The ash storage area is unlined and has a 1.5 to 2 feet thick soil/vegetation cover that is
maintained. The stormwater runoff from the ash storage area is routed to the cinder storage
area.
2.3.3 Cinder Storage Area
The cinder storage area is located west/southwest of the ash basin primary cell, and northwest
of the ash storage area (CSA Report Figure 2-2). The cinder storage area is located in a
triangular area northeast of the coal pile and covers 13 acres. Following initial station operation
in 1929 and prior to initial ash basin operation, bottom ash (cinders) generated as part of the
coal combustion process were deposited in the cinder storage area (and other areas near the
coal pile). This area was also used for ash storage prior to the use of the wet ash sluicing
system which began in 1957. It is estimated that the cinder storage area contains 300,000 tons
of ash.
2.4 Groundwater Flow System
Groundwater recharge occurs from precipitation infiltration into the subsurface where the ground
surface is permeable, and, at RBSS, this includes permeable ash within the ash basin system
and dikes that contain the basins (where exposed). After infiltrating the ground surface, water in
the unsaturated zone percolates downward to the water table, except where ponded water
creates a groundwater mound as in the secondary cell of the ash basin. From the water table,
groundwater moves laterally and downward through unconsolidated material (residual
soil/saprolite) into the transition zone, then into fractured bedrock. The mean annual recharge to
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Groundwater Flow and Transport Model
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shallow groundwater aquifers in the Piedmont ranges from 4.0 to 9.7 inches per year (Daniel
2001).
Historical and current information regarding the RBSS ash basin system assembled by HDR
during the CSA was used in developing the conceptual site model and ultimately the numerical
groundwater flow models. Refer to CSA Report Figures 2-1 and 2-2 for locations of the RBSS
ash basin system components.
2.5 Hydrologic Boundaries
The major discharge location for the groundwater system at RBSS is Mountain Island Lake, the
main hydrologic boundary that exists at the site. Local ditches, drainages, and streams also
serve as shallow hydrologic boundaries. These smaller features are treated as internal water
sink terms and represented as drain boundary conditions in the flow model.
2.6 Hydraulic Boundaries
The groundwater flow system in the RBSS study area does not contain impermeable barriers or
boundaries, with the exception of deep bedrock where fracture density is minimal. Natural
groundwater divides exist along topographic divides, but the groundwater divides are result of
local flow conditions (as opposed to flow barriers).
2.7 Groundwater Sources and Sinks
Recharge, including that to the ash basins, is the major source of water to the groundwater
system. Most of this water discharges to the hydrologic boundaries described above. One
private well exists within a half mile radius of the RBSS (HDR 2015a). There are no public water
supply wells near the site. The area of the model domain is not considered to be within a
capture zone or zone of influence of any groundwater extraction well.
2.8 Water Balance
Over an extended period of time, the rate of water inflow into the RBSS study area is equal to
the rate of outflow out of the area. That is, there is no change in groundwater storage. Water
enters the groundwater system through recharge and ultimately discharges to Mountain Island
Lake and small-scale discharge locations. Recharge to the ash basins 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 Report, constituents are those chemicals or compounds that were
identified in the approved groundwater assessment plans for sampling and analysis (HDR,
2015a). 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 COI. The COIs in ash basin
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porewater at RBSS are: antimony, arsenic, boron, chromium 2, cobalt, hexavalent chromium,
iron, lead, manganese, sulfate, thallium, and vanadium.
Antimony, arsenic, boron, chromium, cobalt, hexavalent chromium, 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, cobalt, hexavalent chromium, thallium, and vanadium, also occur at
elevated levels in infiltrating water, but are readily adsorbed to commonly occurring soil types.
2.10 COI Transport
Constituents enter the ash basin system in the dissolved phase and solid phase as components
of wastewater discharge. Some constituents are also naturally present in native soils and in
groundwater beneath the ash basin. The accumulation and subsequent release of chemical
constituents in the ash basin over time is complex. In the ash basin, constituents may incur
phase changes via dissolution, precipitation, chemical reactions and sorption/desorption
processes and mass is exchanged between the phases. The dissolved phase constituents may
undergo some of these processes as they are transported in groundwater and flow
downgradient from the ash basin.
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 cover the ash
basin cells and ash storage area sources and the concentrations for each of these
sources were determined during transport model calibration.
• 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 (3-D). MODFLOW can simulate 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,
2 “Unless otherwise noted, references to chromium in this document indicate total chromium.
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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 is NWT - a Newton formulation of MODFLOW-2005
specifically designed for improving the stability of solutions involving drying and re-wetting under
water table conditions (Niswonger and Ibaraki 2011). The numerical code selected for the
transport model is MT3DMS (Zheng and Wang 1999). MT3DMS is multi-species 3-D transport
model that can simulate advection, dispersion/diffusion, and chemical reaction of the COI 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 models, and a brief description of their use, are provided in Table 1.
4 GROUNDWATER FLOW AND TRANSPORT MODEL CONSTRUCTION
The flow and transport model was developed through a multi-step processes. First, a 3-D solids
model of site hydrostratigraphy was constructed based using site construction and topographic
data and field data. Next, the model domain was determined, from which a numerical value 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 was calibrated, a
steady-state transport simulation for the selected COI was calibrated by adjusting transport
parameters to nearly match the observed concentrations in selected monitoring wells.
Three terrain surface models for RBSS 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 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 hydrostatigraphy was developed using historical site construction drawings and
borehole data to construct 3-D surfaces representing contacts between hydrostatigraphic units
with properties provided in CSA Report Tables 11-7 through 11-11.
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 RBSS, 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.
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Riverbend Steam Station Ash Basin
These contours were trimmed to the areas underlying ash basins, dams, 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. 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, 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 closet 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.
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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.
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 existing 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 3-D MODFLOW 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
- Kd from the lab experiments
- Recharge concentrations
Step 9. Performing model simulations
- Model scenarios – 1) Existing conditions scenario, 2) Excavation scenario

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4.3 Model Domain and Grid
The model domain encompasses the RBSS site, including a section of Mountain Island Lake
and all site features relevant to the assessment of groundwater. Figure 1 shows the conceptual
groundwater flow model and model domain. The model domain extends beyond the ash
management areas to hydrologic boundaries so groundwater flow and COI transport through
the area is accurately simulated without introducing artificial boundary effects.
The bounding rectangle around the model domain extends 5,600 feet north to south and 7,900
feet east to west and has a grid consisting of 265,155 active cells. In plan view, the RBSS
model domain is bounded by the following hydrologic features of the site:
• the southern shore of Mountain Island Lake to the north, east, and west;
• a drainage feature to the east; and
• the presumed topographic groundwater divide south of the site approximated by the
route of Horseshoe Bend Beach Road.
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.
There are a total of 10 model layers divided among the identified hydrostratigraphic units to
simulate curvilinear flow with a vertical flow component. The units are represented by the model
layers listed below:
• Model layers 1 through 3 Ash material
• Model layers 2 through 4 Dike and ash storage material
• Model layer 5 M1 Saprolite and alluvium where present
• Model layer 6 M1 Saprolite
• Model layer 7 M2 Saprolite
• Model layer 8 Transition zone
• Model layers 9 and 10 Fractured bedrock
The materials comprising each layer and typical layer thickness are shown in the north-south
and east-west cross sections through the ash basin on Figures 2 and 3.
4.4 Hydraulic Parameters
Horizontal hydraulic conductivity and the ratio of horizontal to vertical hydraulic conductivity,
which are specific for each hydrostratigraphic unit, are the primary determinants of groundwater
flow for a given set of boundary conditions. Measurements of these parameters from the CSA
(CSA Report Tables 11-7 through 11-11) provide guidance for the flow model calibration. The
hydraulic conductivity values used in the model are provided in Table 2.
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4.5 Flow Model Boundary Conditions
The boundary of the model domain was selected to coincide with physical hydrologic
boundaries at Mountain Island Lake, drainage features, and no flow boundaries at topographic
divides (Section 2.4 and Figure 4). At the Mountain Island Lake boundary, constant head
boundaries were applied to those layers above fractured bedrock with bottom elevations below
the water surface interpolated from photogrammetric surveys.
Drainage features and other low areas act as shallow hydrologic boundaries. Physically they
represent seep locations where the water table intersects the ground surface and groundwater
is discharged. Internal drain boundaries were applied at these locations as shown on Figure 4. It
is assumed that surface drainage is ultimately conveyed to an outfall to the Mountain Island
Lake.
4.6 Flow Model Sources and Sinks
Recharge is the only groundwater source considered in the model. Other than those described
above, no other groundwater sources or sinks were identified. The mean annual recharge in the
Piedmont ranges from 4.0 to 9.7 inches per year (Daniel 2001), and the recharge applied in the
model is 21.5 inches/year for the ash basins and 6.5 inches/year across the rest of the model
domain (Figure 5).
4.7 Flow Model Calibration Targets
The steady-state flow model calibration targets are the 58 static water level measurements
measured in June/July 2015. The observation wells included 29 wells screened in ash, ash dike,
and shallow zone (S/M1/M2); 23 wells screened in the transition zone; and six wells screened in
fractured bedrock. The observations wells in shallow, deep and bedrock groundwater zones are
shown on Figures 6 through 8.
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
transport model to existing conditions, constant concentration source zones were applied at the
ash model layers in the ash basin cells and the layers where the water table occurs beneath the
ash storage area 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 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 basin. 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.
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Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin
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.
Kd values for the COIs were applied as follows:
• Antimony: 12 mL/g
• Arsenic: 250 mL/g
• Boron: 2 mL/g
• Chromium: 15 ml/g
• Cobalt: 2 mL/g
• Hexavalent chromium: 2 mL/g
• Sulfate: conservative (sorption not modeled)
• Thallium: 73 mL/g
• Vanadium: 15 mL/g
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. 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 foot 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 ten percent 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
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 M1/M2 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 the proposed provisional background concentration (PPBC) identified in the
RBSS Corrective Action Plan (CAP) Part 1 (HDR, 2015b). Exceptions are hexavalent chromium
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Riverbend Steam Station Ash Basin
and vanadium where 0.07 µg/L and 0.9 µg/L were used, respectively. Note that recharge does
not have a specified concentration.
The background concentrations for the COIs applied as initial concentrations are as follows:
• Antimony: 1 µg/L
• Arsenic: 1 µg/L
• Boron: 50 µg/L
• Chromium: 5 µg/L
• Cobalt: 3 µg/L
• Hexavalent chromium: 0.07 µg/L
• Sulfate: 970 µg/L
• Thallium: 0.2 µg/L
• Vanadium: 0.9 µg/L
4.10 Transport Model Sources and Sinks
The ash basin and ash storage area are the sources for COI in the model (CSA Report Figure
ES-1). During the transport model calibration and the existing conditions scenario, the sources
were modeled as constant concentration cells in the saturated ash layers of the ash basin
primary and secondary cells, ash storage area and cinder storage area (Figure 5). Note that the
transport model sinks correspond to the constant head and drain boundaries in the flow model.
The groundwater and COI mass are removed as they enter the model grid cells comprising
these boundaries.
4.11 Transport Model Calibration Targets
The calibration targets are the measured antimony, chromium and sulfate concentrations for the
June/July 2015 sampling event as shown in CSA Report Tables 7-5 and 10-6.
5 MODEL CALIBRATION
5.1 Flow Model Residual Analysis
The flow model was calibrated to the water level measurements (or hydraulic head
observations) measured in June/July 2015 in shallow, deep, and bedrock wells (Table 3). 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 on Figures 6 through 8. The initial trial-and-
error calibration assumed homogeneous conditions in each model layer.
Throughout the flow model calibration process, recharge was fixed at reasonable values early in
the calibration process, and then calibration adjustments were made by adjusting hydraulic
conductivity in the S/M1 layers (see Figure 9 for calibrated values), M2 layer (Figure 10),
transition zone layer (Figure 11), and bedrock layers (Figure 12). The basis for delineating the
zones within layers was to obtain the best calibration using values within the range of
measurements made during the CSA. The model is most sensitive to recharge and hydraulic
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Riverbend Steam Station Ash Basin
conductivity in the upper, residual soil/saprolite layers (S/M1/M2). Both within and outside the
ash basins, the model was less sensitive to vertical hydraulic conductivity.
The calibrated flow model parameters are provided in Table 2. The measured and modeled
water levels (post-calibration) are compared in Table 3. The calibrated flow model is steady-
state and assumed to represent long term and average flow conditions for the site. This
assumption should be verified as additional data are collected from the monitoring wells and
additional monitoring wells are installed.
The square root of the average square error (also called the root mean square error, or RMS
error) is provided in Table 3. The model calibration goal is RMS error less than 10% of the
change in head across the model domain; in this case the model calibration goal = 7.1 feet. The
calibration goal was met as the RMS error = 5.82 feet. 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 8.11%.
The hydraulic head contours for the M2 saprolite layer (model layer 6) in the calibrated model
are shown on Figure 13. Overall, groundwater in the shallow aquifer, transition zone, and
fractured bedrock at the site flows to the north, northeast, and northwest and discharges to
Mountain Island Lake. Note that steep hydraulic gradients are associated with the dikes
surrounding the ash basins. The hydraulic gradient is shallow in the lowlands to the east and
west of the ash basins.
5.2 Flow Model Sensitivity Analysis
It is important to understand the sensitivity of the flow model parameters on the predicted
hydraulic head field, so that the affects of changing the parameters are known. The sensitivity of
flow model parameters was tested by varying a subset of parameters by 20% above and below
the values used in the calibrated flow model. The sensitivity was evaluated by re-running the
model and comparing the NRMSE for each simulation (Table 4). A smaller NRMSE indicates
that the model is better calibrated (i.e., model-predicted hydraulic head values better match the
actual or observed values). Using this approach, it was determined that NRMSE is maximized
and the flow model is most sensitive to positive or negative changes in horizontal hydraulic
conductivity in the shallow aquifer, followed by decreased recharge outside of the ash basins.
The least sensitive flow model parameter tested (minimized NRMSE) was decreased recharge
within the ash basins. Also, the model was unaffected by changing the vertical hydraulic
conductivity in the shallow groundwater zone and transition zone, horizontal flow is dominant
groundwater in all three groundwater zones away from the ash basins.
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 on 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.
15
Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin
5.3 Transport Model Calibration and Sensitivity
For the transport model calibration, the constant source concentrations were adjusted to
minimize residual concentrations in target wells. Applying the low end of measured sorption
coefficients 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.
For the transport model calibration, the calibration parameters consisted of the constant source
concentrations, porosity and the Kd for each COI. These parameters were adjusted to minimize
residual concentrations in target wells. The model assumed initial concentrations within the
groundwater system for all COIs at the beginning of ash basin operations approximately 58
years ago. A source term matching the pore water 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 pore water concentrations to match
measured values in the downgradient monitoring wells that had exceedances of the North
Carolina groundwater standard (2L Standard 3) or were greater than the Interim Maximum
Allowable Concentration (IMAC 4) 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 on 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.
Detailed sensitivity analyses for porosity, dispersivity, and sorption were not completed as part
of CAP Part 2 as informal sensitivity analysis indicated that sensitive parameters did not change
due to revisions to the model parameters. A decrease in the 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.
3 North Carolina Groundwater Rules; Title 15A, Subchapter 02L of the NC Administrative Code.
4 Appendix #1 of 15A NCAC Subchapter 02L Classifications and Water Quality Standards Applicable to The
Groundwaters of North Carolina, lists Interim Maximum Allowable Concentrations (IMACs). The IMACs were issued
in 2010 and 2011; however, NCDENR has not established a 2L Standard for these constituents as described in 15A
NCAC 02L.0202(c). For this reason, IMACs noted in this report are for reference only.
16

Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin
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 Mountain
Island Lake, downgradient of the ash storage area and also near the northern model boundary.
The particle tracks are shown on Figure 14 and predicted advective travel times are provided in
Table 6.
6 SIMULATION OF CLOSURE SCENARIOS
Two closure scenarios were modeled for the RBSS: an Existing Conditions scenario with ash
sources left in place and an Excavation scenario with the accessible ash removed from the
RBSS. Being predictive, these simulations produce flow and transport results for conditions that
are beyond the range of those considered during the 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 developed for existing conditions was applied without modification for the Existing
Conditions scenario. For the Excavation scenario, the model layers containing ash in the ash
basin (layers 1-4) were made inactive. The flow parameters for this model are identical to the
Existing Conditions scenario, except for the removal of ash related layers and the same
recharge rate was 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 are reached at the Compliance Boundary. COI
concentrations can only remain the same or increase for this scenario with source
concentrations being held at their constant value over all time. Thereafter, the concentrations
and discharge rates remain constant. This scenario represents the most conservative case in
terms of groundwater concentrations on and off site, with 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 reach a 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
groundwater pore velocity. Use of lower effective porosity values will result in shorter times to
achieve steady state for both sorptive and non-sorptive COIs. The use of lower total porosity will
result in longer times for sorptive COIs (e.g., arsenic and chromium).
6.2 Excavation Scenario
In the Excavation scenario, ash from the ash basin and ash storage area is removed and
transported off-site. In the model, the constant concentration sources and ash above and below
17
Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin
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
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 pore water 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 of the nine
modeled COI on Figures 15 through 120, as discussed in the following sub-sections.
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 2015. 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: MW-3S,
MW-5S, and MW-6S (Figure 6) for all COIs, except hexavalent chromium. Hexavalent
chromium concentrations were analyzed at downgradient monitoring well MW-9D (Figure 7).
Monitoring well MW-3S is located north of the primary and secondary ash basins and
approximately 150 feet from the ash basin waste boundary and 120 feet from the Mountain
Island Lake. Monitoring well MW-5S is located directly downgradient of the secondary ash basin
approximately 60 feet from the ash basin waste boundary on the eastern side of the earthen
embankment within 100 feet of the Mountain Island Lake. Monitoring well MW-6S is also located
near the ash basin embankment and within 100 feet of the Mountain Island Lake. Monitoring
well MW-9D is located northwest of the ash storage and cinder storage areas (downgradient)
and also cross-gradient from the ash basin Primary Cell. Each of these wells is directly
downgradient from either the ash basin, ash storage area, or cinder storage area and
upgradient of the groundwater discharge locations at the Mountain Island Lake and ash basin
Compliance Boundary.
7.1 Antimony
Figures 15 through 17 show predicted antimony concentrations at downgradient monitoring
wells for Existing Conditions and Excavation scenarios. The concentration versus time curves
show that antimony is predicted to be greater than the IMAC (1 µg/L) at MW-3S, MW-5S and
MW-6S and remain above the IMAC for more than 100 years at all three monitoring wells
modeled. Figures 18 to 20 show that initial (2015) antimony concentrations in shallow, deep and
bedrock groundwater zones are greater than the IMAC. Figures 21-26 indicate antimony will
continue to be greater than the IMAC at the Mountain Island Lake after 100 years in all
groundwater zones under both the Existing Conditions and Excavation scenarios.
18
Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin
7.2 Arsenic
Figures 27 through 29 show predicted arsenic concentrations at the downgradient monitoring
wells for both scenarios. These figures show that predicted arsenic concentrations do not
exceed the 2L Standard (10 µg/L) at the monitoring wells. Figures 30 to 32 show initial (2015)
arsenic concentrations do not exceed the 2L Standard at the Mountain Island Lake. In 2115,
arsenic is not predicted to exceed the 2L Standard at the Mountain Island Lake (Figures 33 to
35). For the Excavation scenario (Figures 36 to 38), arsenic remains less than the 2L Standard
at the Mountain Island Lake.
7.3 Boron
Figures 39 through 41 show predicted boron concentrations at downgradient monitoring wells
for the modeled scenarios. The concentration versus time curves show that under the Existing
Conditions scenario, boron concentrations will increase after 2015, but in all cases remain
below the 2L Standard of 700 µg/L. Figures 42 to 44 show initial (2015) boron concentrations do
not exceed the 2L Standard in any groundwater zones at the Mountain Island Lake. The
predicted boron concentrations for Existing Conditions scenario and Excavation scenario are
presented on Figures 45 to 50. After 100 years, boron does not exceed the 2L Standard at the
Mountain Island Lake in either scenario in any groundwater zone.
7.4 Chromium
Figures 51 through 53 show predicted chromium concentrations at the downgradient monitoring
wells for both scenarios. The concentration versus time curves show that chromium
concentrations at the three monitoring wells will remain below the 2L Standard of 10 µg/L
throughout the modeled period. Figures 54 to 56 show initial (2015) chromium concentrations in
the shallow, deep and bedrock groundwater zones; chromium does not exceed the 2L Standard
at Mountain Island Lake. The predicted concentrations for 2115 (Existing Conditions scenario)
are presented on Figures 57 to 59; chromium does not exceed the 2L Standard at Mountain
Island Lake. The same results are observed for the Excavation scenario (Figures 60 to 62) –
chromium does not exceed the 2L Standard.
7.5 Cobalt
Figures 63 through 65 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 remains elevated above the IMAC of 1 µg/L.
Figures 66 to 68 show initial (2015) cobalt concentrations. Cobalt is elevated above the IMAC in
shallow, deep and bedrock groundwater zones. The predicted 2115 cobalt concentrations for
the Existing conditions scenario are presented on Figures 69 to 71, while the Excavation
scenario results are on Figures 72 to 74. After 100 years, cobalt is predicted to exceed the
IMAC at Mountain Island Lake for both scenarios within the shallow, deep, and bedrock
groundwater zones.
19
Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin
7.6 Hexavalent Chromium
Figure 75 shows predicted hexavalent chromium concentrations at MW-9D downgradient of the
ash storage area for both scenarios. The concentration versus time curves show hexavalent
chromium remaining above the NCDHHS HSL groundwater standard of 0.07 µg/L throughout
the modeled period. Figures 76 to 78 show initial (2015) hexavalent chromium concentrations
above the NCDHHS HSL in the shallow, deep and bedrock groundwater zones. The predicted
concentrations for the Existing Conditions and Excavation scenarios are presented on Figures
79 to 84. After 100 years, hexavalent chromium is predicted to be greater than the NCDHHS
HSL at Mountain Island Lake in both scenarios in all groundwater zones.
7.7 Sulfate
Figures 85 through 87 show predicted sulfate concentrations at downgradient monitoring wells.
The concentration versus time curves for all three monitoring wells show that sulfate remains
below the 2L Standard of 250,000 µg/L for both model scenarios. Figures 88 to 90 show initial
(2015) sulfate concentration below the 2L Standard in the shallow, deep, and bedrock
groundwater zones. The predicted sulfate concentrations for the Existing Conditions and
Excavation scenarios are presented on Figures 91 to 96. After 100 years, sulfate is not
predicted to exceed the 2L Standard at Mountain Island Lake for either scenario in any
groundwater zone.
7.8 Thallium
Figures 97 through 99 show predicted thallium concentrations at downgradient monitoring wells.
The concentration versus time curves show that thallium remains above the IMAC of 0.2 µg/L at
the three downgradient monitoring wells for both the Existing Conditions and Excavation
scenarios. Figures 100 to 102 show initial (2015) cobalt concentrations in the shallow, deep and
bedrock groundwater zones. After 100 years, thallium is predicted to be greater than the IMAC
at Mountain Island Lake under both the Existing Conditions scenario (Figures 103-105) and the
Excavation scenario (Figures 106 to 108) in all groundwater zones.
7.9 Vanadium
Figures 109 through 111 show predicted vanadium concentrations at downgradient monitoring
wells. The concentration versus time curves show that vanadium remains elevated above the
IMAC of 0.3 µg/L for both Existing Conditions and Excavation scenarios. Figures 112 to 114
show initial (2015) vanadium concentrations in the shallow, deep and bedrock groundwater
zones. After 100 years, vanadium is predicted to be greater than the IMAC at Mountain Island
Lake under both the Existing Conditions scenario (Figures 115-117) and the Excavation
scenario (Figures 118 to 120) in all groundwater zones.
7.10 Potential Groundwater Monitoring Locations at One Year’s Advective Travel Time
Upgradient of Catawba River/Mountain Island Lake
Particle tracking was performed using the excavation steady-state flow field to identify potential
groundwater monitoring locations located one year’s advective travel time upgradient of
Mountain Island Lake, the receiving surface water body for the site. Note that groundwater from
20
Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin
the ash basin, ash storage area, and cinder storage area sources discharge to Mountain Island
Lake and in some cases do not reach a Compliance Boundary. The monitoring locations are
shown on Figure 121 and are labeled extraction wells.
A simulation was performed to demonstrate the steady-state effects of pumping the six wells at
three gallons per minute and the area influenced by pumping (as represented by the drawdown
contours shown in the figure). Note that, over time, the pumping the wells would capture a
portion of the groundwater in the shallow zone that has been impacted by the ash basin and
other source areas. However, a more detailed modeling analysis is needed to predict recovery
rates and design an efficient pumping recovery system.
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 RBSS.
• 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 was completed in 2015.
• 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 and through to the end of the predictive simulations (2265).
• COI source zone concentrations at the ash basin 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.
21
Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin
• Since Mountain Island Lake 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.
8.2 Model Predictions
The model predictions are summarized as follows:
• The model predicts that under Existing Conditions and Excavation scenarios antimony,
cobalt, thallium, and vanadium will be greater than their respective IMAC at Mountain
Island Lake. Also, hexavalent chromium is predicted to be greater than the NCDHHS
HSL at Mountain Island Lake. For these COIs, the background concentrations used for
modeling is also above their respective IMAC or NCDHHS HSL, so the actual impact of
the site sources on groundwater quality is unknown.
• Model predictions do not show that COI concentrations will be effectively reduced by ash
removal under the Excavation scenario. The COIs that are predicted to exceed 2L
Standard, or be greater than the IMAC or NCDHHS HSL, will not achieve compliance
with the standards within the time period modeled (2015-2265).
• The model predicts that under Existing Conditions and Excavation scenarios arsenic,
boron, chromium and sulfate will not exceed their respective 2L Standard at Mountain
Island Lake.
• 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.
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. Comprehensive Site Assessment Report, Riverbend River Steam Station Ash Basin,
August 2015a.
HDR, 2015b. Corrective Action Plan Part 1. Riverbend Steam Station Ash Basin. November,
2015.
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.
22
Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin
Zheng, C. and Bennett, G. 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.
23

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

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

This page intentionally left blank
Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin
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 Times

Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin
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
Riverbend Steam Station Ash Basin
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 - 4 Ash 17.13 - 0.59 2.000 0.900
2 - 4 Dike 0.41 - 0.04 0.003 0.003
5 - 6 M1 7.45 – 0.15
Z-18 0.110 0.100
Z-20 0.200 0.100
Z-21 0.400 0.200
Z-24 0.568 0.057
Z-30 1.000 1.000
Z-36 3.000 1.000
7 M2 6.42 – 0.09
Z-8 0.030 0.030
Z-19 0.110 0.100
Z-21 0.400 0.200
Z-24 0.568 0.057
Z-27 0.750 0.500
Z-34 2.000 1.000
Z-35 2.000 0.200
Z-37 5.000 0.600
8 Transition Zone 49.6 - 0.02
Z-23 0.500 0.200
Z-28 0.800 0.300
Z-31 1.500 0.600
9 – 10 Bedrock 1.23 - < 0.002
Z-2 0.003 0.003
Z-12 0.050 0.005
Z-26 0.600 0.060
1Range = geometric mean +/- one standard deviation (see CSA Report Tables 11-7 to 11-11)

Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin
Table 3 Observed vs. Predicted Hydraulic Head
Well Name Model
Layer
Observed
Hydraulic Head
(ft MSL)
Predicted
Hydraulic Head
(ft MSL)
Residual
(ft)
AB-7S 4 714.18 711.27 2.91
C-1S 4 653.31 656.06 -2.75
AB-8S 5 656.54 654.30 2.24
AS-1S 5 678.16 678.23 -0.07
AS-2S 5 694.09 695.08 -0.99
AS-3S 5 713.24 707.65 5.59
AS-3SA 5 713.01 707.62 5.39
C-2S 5 656.34 659.20 -2.86
GWA-10S 5 643.83 646.55 -2.72
GWA-22S 5 713.28 705.81 7.47
GWA-23S 5 705.93 703.92 2.01
GWA-4S 5 671.45 677.70 -6.25
GWA-5S 5 710.28 710.75 -0.47
GWA-6S 5 715.44 710.36 5.08
GWA-7S 5 670.07 664.56 5.51
GWA-8S 5 671.35 659.45 11.90
GWA-9S 5 648.23 651.18 -2.95
AB-7I 6 713.43 707.96 5.47
AS-3D 6 712.53 706.83 5.70
GWA-1S 6 661.09 656.71 4.38
GWA-2S 6 645.12 650.33 -5.21
GWA-3S 6 646.22 646.93 -0.71
GWA-3SA 6 645.22 646.81 -1.59
AB-2D 7 678.98 658.00 20.98
AB-7D 7 713.23 706.70 6.53
AB-8D 7 658.43 656.04 2.39
C-2D 7 653.46 657.99 -4.53
GWA-22D 7 708.80 704.34 4.46
GWA-3D 7 645.55 646.78 -1.23
GWA-4D 7 671.25 676.93 -5.68
GWA-6D 7 714.75 709.79 4.96
GWA-7D 7 670.83 664.75 6.08
GWA-9D 7 653.77 651.37 2.40
AB-3D 8 700.26 693.05 7.21
Continued on next page

Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin
Table 3. Observed vs. Predicted Hydraulic Head (continued)
Well Name Model
Layer
Observed
Hydraulic Head
(ft MSL)
Predicted
Hydraulic Head
(ft MSL)
Residual
(ft)
AB-5D 8 677.04 682.45 -5.41
AS-1D 8 675.37 679.11 -3.74
AS-2D 8 695.48 693.42 2.06
GWA-23D 8 704.85 701.64 3.21
GWA-8D 8 671.07 659.42 11.65
MW-15D 8 643.72 646.23 -2.51
MW-9D 8 647.92 648.19 -0.27
AB-4D 9 703.00 692.04 10.96
AB-6BRU 9 663.60 670.44 -6.84
GWA-10D 9 643.89 645.83 -1.94
GWA-1D 9 662.06 656.43 5.63
GWA-23BR 9 703.22 701.22 2.00
GWA-7BR 9 670.98 666.66 4.32
MW-15BR 9 643.83 645.82 -1.99
MW-7BR 9 711.72 704.55 7.17
MW-9BR 9 648.68 648.09 0.59
GWA-2BR 10 645.22 652.26 -7.04
SSE 1727.13
Max 715.44 OBS 51
Min 643.72 RMS 5.82
Max-Min 71.72 NRMSE 0.0811

Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin
Table 4 Flow Parameter Sensitivity Analysis
Parameter
Calibrated Calibrated +20% Calibrated -20%
NRMSR (Head)
(ft)
NRMSR (Head)
(ft) % NRMSR (Head)
(ft) %
Shallow
Zone Kh
0.0810
0.1040 28.40 0.0760 -6.17
Shallow
Zone Kv 0.0810 0.00 0.0810 0.00
Transition
Zone Kh 0.0880 8.64 0.0760 -6.17
Transition Zone
Kv 0.0810 0.00 0.0810 0.00
Recharge
ex. ash basin 0.0800 -1.23 0.1050 29.63
Recharge
ash basin 0.0700 -13.58 0.1080 33.33

Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin
Table 5 Transport Model Calibration Results
COI Monitoring
Well
Measured
Concentration
(µg/L)
Predicted
Concentration
(µg/L)
Antimony
Ash Basin Constant Concentration Range = 1 - 22 µg/l
Ash Storage Area Concentration Range = 1 – 10 µg/l
Cinder Storage Area Concentration Range = 1 µg/l
Sorption Coefficient [Kd] = 12 ml/g
AB-1D 6.3 1.46
AB-1S 3.2 4.19
AB-2D 1 1.82
AB-2S 3.1 3.15
AB-3BR 0 1
AB-3D 0.22 1
AB-4D 1.2 1
AB-5D 1.2 1.19
AB-6BRU 0.32 1.1
AB-6S 1.8 3
AB-7D 7.5 1.42
AB-7I 1 4.29
AB-8D 0 1
AB-8S 0 1
AS-1D 3 1
AS-1S 0.26 1
AS-2D 1 1
AS-2S 0.21 1
AS-3D 1.6 1.93
C-2D 0 1
C-2S 0 1
GWA-10D 1.1 1
GWA-10S 0.5 1.19
GWA-1D 3.2 0.99
GWA-1S 0.28 0.97
GWA-20D 0 1
GWA-20S 0.22 1
GWA-22D 1.4 1
GWA-22S 2.2 1
GWA-23BR 0 1
GWA-23D 0 1
Continued on next page

Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin
Table 5 Transport Model Calibration Results (continued)
COI Monitoring
Well
Measured
Concentration
(µg/L)
Predicted
Concentration
(µg/L)
Antimony
(cont.)
GWA-23S 0.24 1
GWA-2BR 0.24 1
GWA-2S 0 0.98
GWA-3D 0 0.97
GWA-3SA 0.77 0.97
GWA-3SA 0.77 0.97
GWA-4D 0 1
GWA-4S 0 0.97
GWA-5D 0 1
GWA-6D 0.25 1
GWA-6D 0.25 1
GWA-7BR 0.96 1
GWA-7D 0.17 1
GWA-7S 0 0.98
GWA-8D 0 1
GWA-8S 0 0.98
GWA-8S 0 0.98
GWA-9BR 4.5 1.35
GWA-9D 0.5 2.07
GWA-9S 0.5 2.53
MW-11DR 0.59 0.98
MW-15BR 0.25 1
MW-15D 0.74 1
MW-1S 0.5 1
MW-2D 0.5 1
MW-3D 0.5 1.12
MW-3S 0.5 1.47
MW-4D 0 1.23
MW-5D 0 1.34
MW-5S 0 2.48
MW-6D 0 2.57
MW-6S 0 3.88
MW-7BR 0.86 1
MW-8D 0.21 1
MW-8S 0.83 1
Continued on next page

Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin
Table 5 Transport Model Calibration Results (continued)
COI Monitoring
Well
Measured
Concentration
(µg/L)
Predicted
Concentration
(µg/L)
Antimony
(cont.)
MW-9BR 2.3 1
MW-9D 2.7 1
Arsenic
Ash Basin Constant Concentration Range = 1 - 25 µg/l
Ash Storage Area Concentration Range = 1 – 1.5 µg/l
Cinder Storage Area Concentration Range = 1 µg/l
Sorption Coefficient [Kd] = 250 ml/g
GWA-9BR 10.70 1.00
GWA-20S 3.30 1.00
AB-7D 2.20 1.00
AS-1S 1.40 1.50
GWA-2BR 1.30 1.00
C-2D 1.30 1.00
MW-15D 1.00 1.00
GWA-6D 1.00 1.00
GWA-6D 1.00 1.00
AS-3D 0.97 1.00
GWA-5D 0.96 1.00
C-2S 0.95 1.00
GWA-1D 0.93 1.00
GWA-22D 0.91 1.00
GWA-23D 0.90 1.00
AB-1D 0.84 1.00
AB-6S 0.82 25.00
MW-15BR 0.78 1.00
AB-2D 0.72 1.04
MW-9BR 0.69 1.00
AS-1D 0.68 1.00
AS-2D 0.66 1.00
GWA-3D 0.65 1.00
AB-4D 0.65 1.00
GWA-20D 0.64 1.00
AB-3D 0.49 1.00
GWA-7D 0.48 1.00
AS-2S 0.42 1.00
MW-8D 0.32 1.00
Continued on next page

Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin
Table 5 Transport Model Calibration Results (continued)
COI Monitoring
Well
Measured
Concentration
(µg/L)
Predicted
Concentration
(µg/L)
Arsenic
(cont.)
GWA-10D 0.29 1.00
GWA-7BR 0.29 1.00
MW-9D 0.28 1.00
GWA-2S 0.28 1.00
GWA-23BR 0.28 1.00
AB-5D 0.25 1.00
AB-6BRU 0.24 1.00
GWA-3SA 0.23 1.00
GWA-3SA 0.23 1.00
MW-7BR 0.23 1.00
GWA-4D 0.22 1.00
AB-2S 0.20 2.29
GWA-7S 0.20 1.00
AB-8D 0.17 1.00
AB-1S 0.17 2.83
GWA-9D 0.13 1.00
GWA-4S 0.00 1.00
MW-1S 0.00 1.00
GWA-23S 0.00 1.00
MW-8S 0.00 1.00
AB-8S 0.00 1.00
GWA-22S 0.00 1.00
MW-11DR 0.00 1.00
GWA-1S 0.00 0.99
MW-2D 0.00 1.00
MW-3D 0.00 1.00
MW-3S 0.00 1.00
AB-7I 0.00 1.22
GWA-10S 0.00 1.00
MW-4D 0.00 1.00
MW-5D 0.00 1.00
MW-5S 0.00 1.01
MW-6D 0.00 1.03
MW-6S 0.00 2.59
GWA-9S 0.00 1.18
Continued on next page

Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin
Table 5 Transport Model Calibration Results (continued)
COI Monitoring
Well
Measured
Concentration
(µg/L)
Predicted
Concentration
(µg/L)
Arsenic
(cont.)
GWA-8S 0.00 1.00
GWA-8S 0.00 1.00
GWA-8D 0.00 1.00
Boron
Ash Basin Constant Concentration Range = 200 – 2,400 µg/l
Ash Storage Area Concentration Range = 300 – 2,200 µg/l
Cinder Storage Area Concentration Range = 200 - 300 µg/l
Sorption Coefficient [Kd] = 2 ml/g
AB-5SL 2,400.00 2,400.00
AB-6S 45.00 742.34
AB-7S 320.00 300.00
C-1S 300.00 200.00
AB-8S 220.00 289.13
AS-1S 2,200.00 2,200.00
AS-2S 0.00 300.00
C-2S 170.00 200.00
GWA-10S 170.00 190.05
GWA-22S 0.00 55.75
GWA-23S 0.00 165.65
GWA-4S 0.00 47.09
GWA-7S 0.00 46.88
GWA-8S 240.00 132.72
GWA-9S 200.00 233.34
AB-1S 0.00 295.58
AB-2S 0.00 248.89
AB-7I 0.00 169.49
AS-3D 27.00 154.20
GWA-1S 0.00 45.23
GWA-2S 460.00 122.27
GWA-3SA 0.00 67.32
MW-1S 300.00 257.53
MW-3S 260.00 194.82
MW-5S 330.00 243.79
MW-6S 230.00 267.01
MW-8S 0.00 49.51
AB-2D 160.00 215.11
Continued on next page

Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin
Table 5 Transport Model Calibration Results (continued)
COI Monitoring
Well
Measured
Concentration
(µg/L)
Predicted
Concentration
(µg/L)
Boron
(cont.)
AB-7D 0.00 105.19
AB-8D 440.00 260.84
C-2D 0.00 137.28
GWA-20S 0.00 50.04
GWA-22D 0.00 50.51
GWA-3D 34.00 68.00
GWA-4D 0.00 49.80
GWA-6D 0.00 49.45
GWA-7D 0.00 48.23
GWA-9D 260.00 225.14
AB-1D 65.00 220.11
AB-3D 0.00 121.29
AB-5D 150.00 1,291.64
AS-1D 0.00 607.41
AS-2D 28.00 79.85
GWA-20D 0.00 50.11
GWA-23D 0.00 50.68
GWA-5D 0.00 49.91
GWA-8D 110.00 117.82
MW-15D 170.00 47.44
MW-2D 0.00 51.38
MW-3D 260.00 156.94
MW-4D 270.00 259.49
MW-5D 380.00 190.22
MW-6D 320.00 247.54
MW-8D 0.00 50.01
MW-9D 0.00 69.70
AB-3BR 0.00 109.56
AB-4D 0.00 55.65
AB-6BRU 480.00 939.04
GWA-10D 140.00 103.08
GWA-1D 33.00 46.63
GWA-22BR-A 0.00 50.06
GWA-23BR 0.00 50.23
GWA-7BR 0.00 49.37
Continued on next page

Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin
Table 5 Transport Model Calibration Results (continued)
COI Monitoring
Well
Measured
Concentration
(µg/L)
Predicted
Concentration
(µg/L)
Boron
(cont.)
GWA-9BR 210.00 208.44
MW-11DR 320.00 45.84
MW-15BR 260.00 48.15
MW-7BR 0.00 49.84
MW-9BR 0.00 55.68
GWA-2BR 0.00 96.07
Chromium
Ash Basin Constant Concentration Range = 4 - 50 µg/l
Ash Storage Area Concentration Range = 25 - 45 µg/l
Cinder Storage Area Concentration Range = 25 µg/l
Sorption Coefficient [Kd] = 15 ml/g
AS-1D 903 5.39
GWA-20D 186 5
GWA-22D 174 5
GWA-20S 133 5
GWA-23D 55 5
AB-3D 51 5.95
AB-1D 49.2 5
AS-2S 44.8 45
GWA-9BR 40.6 5.09
MW-9D 27 5
GWA-6D 24.8 5
GWA-6D 24.8 5
AB-7D 24.7 5.28
MW-7BR 24.7 5
GWA-5D 24.5 5
AS-3D 23.5 8.35
AS-2D 22.5 5.07
GWA-23BR 15.6 5
AB-2D 13 5.91
GWA-1S 11.6 4.87
GWA-2BR 7.4 5
AB-4D 7.4 5
GWA-8S 6.4 5.05
AS-1S 6.1 25
GWA-1D 6.1 4.98
Continued on next page

Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin
Table 5 Transport Model Calibration Results (continued)
COI Monitoring
Well
Measured
Concentration
(µg/L)
Predicted
Concentration
(µg/L)
Chromium
(cont.)
GWA-10D 6.1 5
GWA-7BR 5.8 5
AB-5D 5.3 7.91
MW-15BR 4.9 5
GWA-8D 4.2 5.01
MW-8D 4.1 5
MW-9BR 4 5
AB-8S 3.8 4.23
GWA-7S 3.8 4.9
MW-2D 3.6 5
MW-15D 3.5 4.98
GWA-10S 3.5 4.93
GWA-7D 3.3 4.99
GWA-23S 3.2 8.92
GWA-2S 2.8 4.94
MW-5D 2.7 5
C-2D 2.5 6.61
MW-11DR 2.5 4.94
AB-2S 2.5 6
AB-6BRU 2.4 6.36
GWA-4D 2.1 5
GWA-4S 2.1 4.87
GWA-22S 1.9 4.85
MW-6D 1.9 5.06
AB-1S 1.8 5
MW-5S 1.7 4.97
MW-3D 1.7 4.99
AB-8D 1.4 4.61
MW-4D 1.4 5
AB-6S 1.2 50
GWA-3D 1.1 5.03
AB-7I 0.9 7.6
MW-8S 0.71 5
C-2S 0.64 25
MW-1S 0.57 4.53
Continued on next page

Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin
Table 5 Transport Model Calibration Results (continued)
COI Monitoring
Well
Measured
Concentration
(µg/L)
Predicted
Concentration
(µg/L)
Chromium
(cont.)
GWA-3SA 0.53 5
GWA-3SA 0.53 5
MW-6S 0.5 5.15
MW-3S 0.44 4.96
GWA-9D 0.39 5.58
GWA-9S 0.32 5.73
Cobalt
Ash Basin Constant Concentration Range = 3 - 70 µg/l
Ash Storage Area Concentration Range = 13 µg/l
Cinder Storage Area Concentration Range = 13 - 15 µg/l
Sorption Coefficient [Kd] = 2 ml/g
AB-5SL 0.25 13
AB-6S 0.31 13
C-1S 102 15
AB-8S 0.55 12.99
AS-1S 5.4 13
AS-2S 12.4 13
C-2S 39.7 15
GWA-10S 10 9.55
GWA-22S 1.3 2.66
GWA-23S 3.3 7.99
GWA-4S 1.7 1.16
GWA-7S 1 2.37
GWA-8S 66.8 52.49
GWA-9S 50.1 46.15
AB-1S 15.9 14.96
AB-2S 17.1 45.41
AB-7I 1.6 2.53
AS-3D 0.16 10.11
GWA-1S 2.1 0.72
GWA-2S 0.34 9.77
GWA-3SA 18.4 7.2
MW-1S 61 42.92
MW-3S 0.14 8.3
MW-5S 1.9 14.84
MW-6S 0.2 27.93
Continued on next page

Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin
Table 5 Transport Model Calibration Results (continued)
COI Monitoring
Well
Measured
Concentration
(µg/L)
Predicted
Concentration
(µg/L)
Cobalt
(cont.)
MW-8S 0.23 1.62
AB-2D 0.15 43.13
AB-7D 0.5 2.29
AB-8D 0.73 12.9
C-2D 0.5 13.61
GWA-20S 12.7 1.36
GWA-22D 3.4 1.67
GWA-3D 1.1 7.15
GWA-4D 0.52 1.19
GWA-6D 0.3 0.88
GWA-7D 0.63 2.2
GWA-9D 0.58 47.79
AB-1D 2.9 14.79
AB-3D 0.18 9.05
AB-5D 0.5 11.7
AS-1D 0.18 9.66
AS-2D 0.2 7.18
GWA-20D 1.9 1.44
GWA-23D 0.5 1.85
GWA-5D 0.21 0.96
GWA-8D 0.7 52.64
MW-15D 0.5 0.86
MW-2D 0.14 3.06
MW-3D 0.5 7.98
MW-4D 0.67 14.4
MW-5D 0.24 14.76
MW-6D 0.5 29.17
MW-8D 1.1 1.14
MW-9D 0.5 11.36
AB-3BR 1.64 8.7
AB-4D 0.18 6.92
AB-6BRU 1.8 13.9
GWA-10D 0.19 8.36
GWA-1D 0.55 0.74
GWA-22BR 1 1.38
Continued on next page

Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin
Table 5 Transport Model Calibration Results (continued)
COI Monitoring
Well
Measured
Concentration
(µg/L)
Predicted
Concentration
(µg/L)
Cobalt
(cont.)
GWA-23BR 0.5 1.67
GWA-7BR 0.5 2.05
GWA-9BR 0.75 46.61
MW-11DR 0.5 0.74
MW-15BR 0.5 0.87
MW-7BR 0.5 0.92
MW-9BR 0.5 9.57
GWA-2BR 0.5 9.66
Hexavalent
Chromium
Ash Basin Constant Concentration Range = 0.07 - 66 µg/l
Ash Storage Area Concentration Range = 5 - 66 µg/l
Cinder Storage Area Concentration Range = 5 µg/l
Sorption Coefficient [Kd] = 2 ml/g
C-2D 2.53 3.04
C-2S 0.27 5
GWA-6D 20.6 0.07
MW-11DR 1.68 0.06
MW-9BR 4.2 2.52
MW-9D 22.1 8.48
Sulfate
Ash Basin Constant Concentration Range = 1,000 – 60,000 µg/l
Ash Storage Area Concentration Range = 5,000 – 230,000 µg/l
Cinder Storage Area Concentration Range = 60,000 µg/l
Sorption Coefficient [Kd] = no sorption
AB-1D 22,800 30,166
AB-1S 690 30,083
AB-2D 22,700 38,154
AB-2S 1,000 35,147
AB-3D 144,000 125,850
AB-4D 13,100 33,824
AB-5D 56,300 58,690
AB-6BRU 83,500 52,148
AB-6S 15,700 60,000
AB-7D 46,500 31,279
AB-7I 6,300 34,258
AB-8D 136,000 125,324
AB-8S 132,000 134,753
Continued on next page

Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin
Table 5 Transport Model Calibration Results (continued)
COI Monitoring
Well
Measured
Concentration
(µg/L)
Predicted
Concentration
(µg/L)
Sulfate
(cont.)
AS-1D 21,300 199,363
AS-1S 231,000 230,000
AS-2D 26,000 10,367
AS-2S 5,200 5,000
AS-3D 18,000 18,828
C-2D 1,900 113,016
C-2S 60,400 60,000
GWA-10D 25,900 21,120
GWA-10S 31,000 21,700
GWA-1D 8,700 1,936
GWA-1S 0 1,949
GWA-20D 376,000 28,160
GWA-20S 980 21,423
GWA-22D 23,500 4,663
GWA-22S 0 2,835
GWA-23BR 18,000 4,789
GWA-23D 10,600 4,840
GWA-23S 0 3,828
GWA-2BR 39,500 1,619
GWA-2S 36,100 1,609
GWA-3D 1,310,000 49,689
GWA-3SA 1,420,000 49,414
GWA-4D 4,400 38,885
GWA-4S 84,700 36,063
GWA-5D 5,000 1,542
GWA-6D 15,500 1,409
GWA-7BR 21,500 12,734
GWA-7D 32,300 12,006
GWA-7S 530 11,886
GWA-8D 23,800 36,026
GWA-8S 36,000 35,758
GWA-9D 33,200 33,657
GWA-9S 26,500 33,183
MW-11DR 36,000 1,365
MW-15BR 39,500 3,645
MW-15D 40,100 3,659
MW-1S 1,200 9,708
Continued on next page

Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin
Table 5 Transport Model Calibration Results (continued)
COI Monitoring
Well
Measured
Concentration
(µg/L)
Predicted
Concentration
(µg/L)
Sulfate
(cont.)
MW-2D 720 4,293
MW-3D 26,700 18,974
MW-3S 29,900 18,959
MW-4D 30,900 29,472
MW-5D 44,600 29,533
MW-5S 10,200 29,488
MW-6D 34,600 31,231
MW-6S 36,700 31,082
MW-7BR 7,200 1,396
MW-8D 0 3,467
MW-8S 580 2,355
MW-9BR 18,000 59,743
MW-9D 27,600 58,751
Thallium
Ash Basin Constant Concentration Range = 0.48 µg/l
Ash Storage Area Concentration Range = 0.2 µg/l
Cinder Storage Area Concentration Range = 0.2 µg/l
Sorption Coefficient [Kd] = 73 ml/g
AB-5SL 0.04 0.48
C-1S 0.06 0.48
AB-8S 0.02 0.38
AS-1S 0.08 0.2
AS-2S 0.11 0.2
C-2S 0.09 0.2
GWA-10S 0 0.2
GWA-22S 0.02 0.2
GWA-23S 0 0.2
GWA-4S 0 0.2
GWA-7S 0 0.2
GWA-8S 0.03 0.2
GWA-9S 0.04 0.22
AB-1S 0 0.26
AB-2S 0.07 0.25
AB-7I 0.05 0.21
AS-3D 0 0.2
GWA-1S 0 0.2
GWA-3SA 0 0.2
Continued on next page

Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin
Table 5 Transport Model Calibration Results (continued)
COI Monitoring
Well
Measured
Concentration
(µg/L)
Predicted
Concentration
(µg/L)
Thallium
(cont.)
MW-1S 0 0.28
MW-3S 0 0.2
MW-5S 0 0.2
MW-6S 0 0.27
MW-8S 0.03 0.2
AB-2D 0.06 0.2
AB-7D 0 0.2
AB-8D 0 0.23
C-2D 0 0.2
GWA-20S 0.09 0.2
GWA-22D 0 0.2
GWA-3D 0 0.2
GWA-4D 0 0.2
GWA-6D 0 0.2
GWA-7D 0.02 0.2
GWA-9D 0 0.2
AB-1D 0.06 0.2
AB-3D 0.03 0.2
AB-5D 0 0.2
AS-1D 0 0.2
AS-2D 0 0.2
GWA-20D 3.2 0.2
GWA-23D 0 0.2
GWA-5D 0 0.2
GWA-8D 0 0.2
MW-15D 0 0.2
MW-2D 0 0.2
MW-3D 0 0.2
MW-4D 0 0.2
MW-5D 0 0.2
MW-6D 0 0.21
MW-8D 0 0.2
MW-9D 0 0.2
AB-3BR 0 0.2
AB-4D 0 0.2
Continued on next page

Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin
Table 5 Transport Model Calibration Results (continued)
COI Monitoring
Well
Measured
Concentration
(µg/L)
Predicted
Concentration
(µg/L)
Thallium
(cont.)
AB-6BRU 0 0.2
GWA-10D 0 0.2
GWA-1D 0 0.2
GWA-22BR 0 0.2
GWA-23BR 0.08 0.2
GWA-7BR 0 0.2
GWA-9BR 0 0.2
MW-11DR 0 0.2
MW-7BR 0.06 0.2
MW-9BR 0.02 0.2
GWA-2BR 0 0.2
Vanadium
Ash Basin Constant Concentration Range = 5 - 200 µg/l
Ash Storage Area Concentration Range = 50 - 200 µg/l
Cinder Storage Area Concentration Range = 50 µg/l
Sorption Coefficient [Kd] = 15 ml/g
AB-5SL 151 200
AB-6S 25.2 200
C-1S 4 50
AB-8S 0.79 4.56
AS-1S 1.6 50
AS-2S 3.9 50
C-2S 0.52 50
GWA-10S 0.62 1.79
GWA-22S 0 0.95
GWA-23S 0 10.77
GWA-4S 0.84 0.88
GWA-7S 3.5 0.88
GWA-8S 0 2.09
GWA-9S 0.52 18.3
AB-1S 1.8 21.01
AB-2S 3.8 24.22
AB-7I 0 26.79
AS-3D 18.2 17.51
GWA-1S 7.9 0.88
GWA-2S 2.3 1.05
Continued on next page

Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin
Table 5 Transport Model Calibration Results (continued)
COI Monitoring
Well
Measured
Concentration
(µg/L)
Predicted
Concentration
(µg/L)
Vanadium
(cont.)
GWA-3SA 1.3 1.18
MW-1S 0 3.7
MW-3S 0.6 3.45
MW-5S 0 9.29
MW-6S 0.35 21.02
MW-8S 0.32 0.9
AB-2D 32.4 14.11
AB-7D 40.5 3.64
AB-8D 2.7 3.03
C-2D 10.9 4.84
GWA-20S 23 0.9
GWA-22D 18.5 0.9
GWA-3D 4.6 1.23
GWA-4D 2.4 0.9
GWA-6D 28 0.9
GWA-7D 12.1 0.9
GWA-9D 2.8 13.25
AB-1D 20 2.98
AB-3D 11.5 5.12
AB-5D 12.5 13.77
AS-1D 14.4 1.85
AS-2D 34.6 0.99
GWA-20D 2.1 0.9
GWA-23D 34.1 0.9
GWA-5D 13.9 0.9
GWA-8D 5.5 1.15
MW-15D 22.4 0.9
MW-2D 4.7 0.9
MW-3D 6.2 1.42
MW-4D 1.1 1.87
MW-5D 8.6 2.3
MW-6D 3.8 10.82
MW-8D 4.1 0.9
MW-9D 12.8 0.9
AB-3BR 32.1 1.77
Continued on next page

Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin
Table 5 Transport Model Calibration Results (continued)
COI Monitoring
Well
Measured
Concentration
(µg/L)
Predicted
Concentration
(µg/L)
Vanadium
(cont.)
AB-4D 24 0.9
AB-6BRU 18.6 6.97
GWA-10D 6.8 0.9
GWA-1D 21 0.9
GWA-22BR 4.33 0.9
GWA-23BR 5.3 0.9
GWA-7BR 10.9 0.9
GWA-9BR 67.5 3.44
MW-11DR 2.1 0.89
MW-15BR 2.2 0.9
MW-7BR 2.1 0.9
MW-9BR 7.8 0.9
GWA-2BR 11.8 0.9

Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin
Table 6 Predicted Advective Travel Times
Groundwater
Zone Monitoring Well
Predicted Advective Travel
Time to Model Boundary
(days)1
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
MW-9BR 153
GWA-2BR 58
GWA-23D 470
MW-7BR 7,125
GWA-7BR 384
1Computed travel time over 3D flow path using flow terms from the groundwater flow model

Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin
Figures
Figure 1 Conceptual Groundwater Flow Model/Model Domain
Figure 2 Model Domain North-South Cross Section (A-A’) Through Primary and Secondary Ash
Basins
Figure 3 Model Domain East-West Cross Section (B-B’) Through Primary and Secondary Ash
Basins
Figure 4 Numerical Model Boundary Conditions
Figure 5 Model Recharge and Contaminant Source Zones (Constant Concentration Cells
Figure 6 Observation Wells in Shallow Groundwater Zone
Figure 7 Observation Wells in Deep Groundwater Zone
Figure 8 Observation Wells in Bedrock Groundwater Zone
Figure 9 Hydraulic Conductivity Zonation in S/M1 Layers (Model Layers 5-6)
Figure 10 Hydraulic Conductivity Zonation in M2 Layers (Model Layer 7)
Figure 11 Hydraulic Conductivity Zonation in Transition Zone Layers (Model Layer 8)
Figure 12 Hydraulic Conductivity Zonation in Bedrock Layers (Model Layers 9-10)
Figure 13 Hydraulic Head (feet) in M1 Layer (Model Layer 6)
Figure 14 Particle Tracking Results
Figure 15 Predicted Antimony in Monitoring Well MW-3S
Figure 16 Predicted Antimony in Monitoring Well MW-5S
Figure 17 Predicted Antimony in Monitoring Well MW-6S
Figure 18 Initial (2015) Antimony Concentrations in Shallow Groundwater Zone
Figure 19 Initial (2015) Antimony Concentrations in Deep Groundwater Zone
Figure 20 Initial (2015) Antimony Concentrations in Bedrock Groundwater Zone
Figure 21 Existing Conditions Scenario - 2115 Predicted Antimony in Shallow Groundwater
Zone
Figure 22 Existing Conditions Scenario - 2115 Predicted Antimony in Deep Groundwater Zone
Figure 23 Existing Conditions Scenario - 2115 Predicted Antimony in Bedrock Groundwater
Zone
Figure 24 Excavation Scenario - 2115 Predicted Antimony in Shallow Groundwater Zone
Figure 25 Excavation Scenario - 2115 Predicted Antimony in Deep Groundwater Zone
Figure 26 Excavation Scenario - 2115 Predicted Antimony in Bedrock Groundwater Zone
Figure 27 Predicted Arsenic in Monitoring Well MW-3S
Figure 28 Predicted Arsenic in Monitoring Well MW-5S
Figure 29 Predicted Arsenic in Monitoring Well MW-6S
Figure 30 Initial (2015) Arsenic Concentrations in Shallow Groundwater Zone
Figure 31 Initial (2015) Arsenic Concentrations in Deep Groundwater Zone
Figure 32 Initial (2015) Arsenic Concentrations in Bedrock Groundwater Zone
Figure 33 Existing Conditions Scenario - 2115 Predicted Arsenic in Shallow Groundwater Zone
Figure 34 Existing Conditions Scenario - 2115 Predicted Arsenic in Deep Groundwater Zone
Figure 35 Existing Conditions Scenario - 2115 Predicted Arsenic in Bedrock Groundwater Zone
Figure 36 Excavation Scenario - 2115 Predicted Arsenic in Shallow Groundwater Zone
Figure 37 Excavation Scenario - 2115 Predicted Arsenic in Deep Groundwater Zone
Figure 38 Excavation Scenario - 2115 Predicted Arsenic in Bedrock Groundwater Zone
Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin
Figure 39 Predicted Boron in Monitoring Well MW-3S
Figure 40 Predicted Boron in Monitoring Well MW-5S
Figure 41 Predicted Boron in Monitoring Well MW-6S
Figure 42 Initial (2015) Boron Concentrations in Shallow Groundwater Zone
Figure 43 Initial (2015) Boron Concentrations in Deep Groundwater Zone
Figure 44 Initial (2015) Boron Concentrations in Bedrock Groundwater Zone
Figure 45 Existing Conditions Scenario - 2115 Predicted Boron in Shallow Groundwater Zone
Figure 46 Existing Conditions Scenario - 2115 Predicted Boron in Deep Groundwater Zone
Figure 47 Existing Conditions Scenario - 2115 Predicted Boron in Bedrock Groundwater Zone
Figure 48 Excavation Scenario - 2115 Predicted Boron in Shallow Groundwater Zone
Figure 49 Excavation Scenario - 2115 Predicted Boron in Deep Groundwater Zone
Figure 50 Excavation Scenario - 2115 Predicted Boron in Bedrock Groundwater Zone
Figure 51 Predicted Chromium in Monitoring Well MW-3S
Figure 52 Predicted Chromium in Monitoring Well MW-5S
Figure 53 Predicted Chromium in Monitoring Well MW-6S
Figure 54 Initial (2015) Chromium Concentrations in Shallow Groundwater Zone
Figure 55 Initial (2015) Chromium Concentrations in Deep Groundwater Zone
Figure 56 Initial (2015) Chromium Concentrations in Bedrock Groundwater Zone
Figure 57 Existing Conditions Scenario - 2115 Predicted Chromium in Shallow Groundwater
Zone
Figure 58 Existing Conditions Scenario - 2115 Predicted Chromium in Deep Groundwater Zone
Figure 59 Existing Conditions Scenario - 2115 Predicted Chromium in Bedrock Groundwater
Zone
Figure 60 Excavation Scenario - 2115 Predicted Chromium in Shallow Groundwater Zone
Figure 61 Excavation Scenario - 2115 Predicted Chromium in Deep Groundwater Zone
Figure 62 Excavation Scenario - 2115 Predicted Chromium in Bedrock Groundwater Zone
Figure 63 Predicted Cobalt in Monitoring Well MW-3S
Figure 64 Predicted Cobalt in Monitoring Well MW-5S
Figure 65 Predicted Cobalt in Monitoring Well MW-6S
Figure 66 Initial (2015) Cobalt Concentrations in Shallow Groundwater Zone
Figure 67 Initial (2015) Cobalt Concentrations in Deep Groundwater Zone
Figure 68 Initial (2015) Cobalt Concentrations in Bedrock Groundwater Zone
Figure 69 Existing Conditions Scenario - 2115 Predicted Cobalt in Shallow Groundwater Zone
Figure 70 Existing Conditions Scenario - 2115 Predicted Cobalt in Deep Groundwater Zone
Figure 71 Existing Conditions Scenario - 2115 Predicted Cobalt in Bedrock Groundwater Zone
Figure 72 Excavation Scenario - 2115 Predicted Cobalt in Shallow Groundwater Zone
Figure 73 Excavation Scenario - 2115 Predicted Cobalt in Deep Groundwater Zone
Figure 74 Excavation Scenario - 2115 Predicted Cobalt in Bedrock Groundwater Zone
Figure 75 Predicted Hexavalent Chromium in Monitoring Well MW-9D
Figure 76 Initial (2015) Hexavalent Chromium Concentrations in Shallow Groundwater Zone
Figure 77 Initial (2015) Hexavalent Chromium Concentrations in Deep Groundwater Zone
Figure 78 Initial (2015) Hexavalent Chromium Concentrations in Bedrock Groundwater Zone
Figure 79 Existing Conditions Scenario - 2115 Predicted Hexavalent Chromium in Shallow
Groundwater Zone
Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin
Figure 80 Existing Conditions Scenario - 2115 Predicted Hexavalent Chromium in Deep
Groundwater Zone
Figure 81 Existing Conditions Scenario - 2115 Predicted Hexavalent Chromium in Bedrock
Groundwater Zone
Figure 82 Excavation Scenario - 2115 Predicted Hexavalent Chromium in Shallow Groundwater
Zone
Figure 83 Excavation Scenario - 2115 Predicted Hexavalent Chromium in Deep Groundwater
Zone
Figure 84 Excavation Scenario - 2115 Predicted Hexavalent Chromium in Bedrock Groundwater
Zone
Figure 85 Predicted Sulfate in Monitoring Well MW-3S
Figure 86 Predicted Sulfate in Monitoring Well MW-5S 3
Figure 87 Predicted Sulfate in Monitoring Well MW-6S
Figure 88 Initial (2015) Sulfate Concentrations in Shallow Groundwater Zone
Figure 89 Initial (2015) Sulfate Concentrations in Deep Groundwater Zone
Figure 90 Initial (2015) Sulfate Concentrations in Bedrock Groundwater Zone
Figure 91 Existing Conditions Scenario - 2115 Predicted Sulfate in Shallow Groundwater Zone
Figure 92 Existing Conditions Scenario - 2115 Predicted Sulfate in Deep Groundwater Zone
Figure 93 Existing Conditions Scenario - 2115 Predicted Sulfate in Bedrock Groundwater Zone
Figure 94 Excavation Scenario - 2115 Predicted Sulfate in Shallow Groundwater Zone
Figure 95 Excavation Scenario - 2115 Predicted Sulfate in Deep Groundwater Zone
Figure 96 Excavation Scenario - 2115 Predicted Sulfate in Bedrock Groundwater Zone
Figure 97 Predicted Thallium in Monitoring Well MW-3S
Figure 98 Predicted Thallium in Monitoring Well MW-5S
Figure 99 Predicted Thallium in Monitoring Well MW-6S
Figure 100 Initial (2015) Thallium Concentrations in Shallow Groundwater Zone
Figure 101 Initial (2015) Thallium Concentrations in Deep Groundwater Zone
Figure 102 Initial (2015) Thallium Concentrations in Bedrock Groundwater Zone
Figure 103 Existing Conditions Scenario - 2115 Predicted Thallium in Shallow Groundwater
Zone
Figure 104 Existing Conditions Scenario - 2115 Predicted Thallium in Deep Groundwater Zone
Figure 105 Existing Conditions Scenario - 2115 Predicted Thallium in Bedrock Groundwater
Zone
Figure 106 Excavation Scenario - 2115 Predicted Thallium in Shallow Groundwater Zone
Figure 107 Excavation Scenario - 2115 Predicted Thallium in Deep Groundwater Zone
Figure 108 Excavation Scenario - 2115 Predicted Thallium in Bedrock Groundwater Zone
Figure 109 Predicted Vanadium in Monitoring Well MW-3S
Figure 110 Predicted Vanadium in Monitoring Well MW-5S
Figure 111 Predicted Vanadium in Monitoring Well MW-6S
Figure 112 Initial (2015) Vanadium Concentrations in Shallow Groundwater Zone
Figure 113 Initial (2015) Vanadium Concentrations in Deep Groundwater Zone
Figure 114 Initial (2015) Vanadium Concentrations in Bedrock Groundwater Zone
Figure 115 Existing Conditions Scenario - 2115 Predicted Vanadium in Shallow Groundwater
Zone
Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin
Figure 116 Existing Conditions Scenario - 2115 Predicted Vanadium in Deep Groundwater Zone
Figure 117 Existing Conditions Scenario - 2115 Predicted Vanadium in Bedrock Groundwater
Zone
Figure 118 Excavation Scenario - 2115 Predicted Vanadium in Shallow Groundwater Zone
Figure 119 Excavation Scenario - 2115 Predicted Vanadium in Deep Groundwater Zone
Figure 120 Excavation Scenario - 2115 Predicted Vanadium in Bedrock Groundwater Zone
Figure 121 Water Level Drawdown at Hypothetical Pumping Wells between Ash Basin Waste
Boundary and Mountain Island Lake

Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin

Figure 1 Conceptual Groundwater Flow Model/Model Domain
Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin
Figure 2 Model Domain North-South Cross Section (A-A’) Through Primary and Secondary Ash Basins
Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin
Figure 3 Model Domain East-West Cross Section (B-B’) Through Primary and Secondary Ash Basins
Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin
Figure 4 Numerical Model Boundary Conditions
Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin
Figure 5 Model Recharge and Contaminant Source Zones (Constant Concentration Cells
Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin
Figure 6 Observation Wells in Shallow Groundwater Zone
Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin

Figure 7 Observation Wells in Deep Groundwater Zone
Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin

Figure 8 Observation Wells in Bedrock Groundwater Zone
Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin
Figure 9 Hydraulic Conductivity Zonation in S/M1 Layers (Model Layers 5-6)
Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin
Figure 10 Hydraulic Conductivity Zonation in M2 Layers (Model Layer 7)
Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin
Figure 11 Hydraulic Conductivity Zonation in Transition Zone Layers (Model Layer 8)
Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin

Figure 12 Hydraulic Conductivity Zonation in Bedrock Layers (Model Layers 9-10)
Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin

Figure 13 Hydraulic Head (feet) in M1 Layer (Model Layer 6)
Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin

Figure 14 Particle Tracking Results
Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin

Figure 15 Predicted Antimony in Monitoring Well MW-3S
Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin

Figure 16 Predicted Antimony in Monitoring Well MW-5S
Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin

Figure 17 Predicted Antimony in Monitoring Well MW-6S
Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin

Figure 18 Initial (2015) Antimony Concentrations in Shallow Groundwater Zone
Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin

Figure 19 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
Riverbend Steam Station Ash Basin

Figure 20 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
Riverbend Steam Station Ash Basin

Figure 21 Existing Conditions Scenario - 2115 Predicted Antimony 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
Riverbend Steam Station Ash Basin

Figure 22 Existing Conditions Scenario - 2115 Predicted Antimony 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
Riverbend Steam Station Ash Basin

Figure 23 Existing Conditions Scenario - 2115 Predicted Antimony 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
Riverbend Steam Station Ash Basin

Figure 24 Excavation Scenario - 2115 Predicted Antimony 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
Riverbend Steam Station Ash Basin

Figure 25 Excavation Scenario - 2115 Predicted Antimony 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
Riverbend Steam Station Ash Basin

Figure 26 Excavation Scenario - 2115 Predicted Antimony 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
Riverbend Steam Station Ash Basin

Figure 27 Predicted Arsenic in Monitoring Well MW-3S
Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin

Figure 28 Predicted Arsenic in Monitoring Well MW-5S
Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin
.
Figure 29 Predicted Arsenic in Monitoring Well MW-6S

Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin

Figure 30 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
Riverbend Steam Station Ash Basin

Figure 31 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
Riverbend Steam Station Ash Basin

Figure 32 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
Riverbend Steam Station Ash Basin

Figure 33 Existing Conditions Scenario - 2115 Predicted Arsenic 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
Riverbend Steam Station Ash Basin

Figure 34 Existing Conditions Scenario - 2115 Predicted Arsenic 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
Riverbend Steam Station Ash Basin

Figure 35 Existing Conditions Scenario - 2115 Predicted Arsenic 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
Riverbend Steam Station Ash Basin

Figure 36 Excavation Scenario - 2115 Predicted Arsenic 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
Riverbend Steam Station Ash Basin

Figure 37 Excavation Scenario - 2115 Predicted Arsenic 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
Riverbend Steam Station Ash Basin

Figure 38 Excavation Scenario - 2115 Predicted Arsenic 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
Riverbend Steam Station Ash Basin

Figure 39 Predicted Boron in Monitoring Well MW-3S
Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin

Figure 40 Predicted Boron in Monitoring Well MW-5S
Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin

Figure 41 Predicted Boron in Monitoring Well MW-6S
Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin

Figure 42 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
Riverbend Steam Station Ash Basin

Figure 43 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
Riverbend Steam Station Ash Basin

Figure 44 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
Riverbend Steam Station Ash Basin

Figure 45 Existing Conditions Scenario - 2115 Predicted Boron 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
Riverbend Steam Station Ash Basin

Figure 46 Existing Conditions Scenario - 2115 Predicted Boron 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
Riverbend Steam Station Ash Basin

Figure 47 Existing Conditions Scenario - 2115 Predicted Boron 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
Riverbend Steam Station Ash Basin

Figure 48 Excavation Scenario - 2115 Predicted Boron 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
Riverbend Steam Station Ash Basin

Figure 49 Excavation Scenario - 2115 Predicted Boron 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
Riverbend Steam Station Ash Basin

Figure 50 Excavation Scenario - 2115 Predicted Boron 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
Riverbend Steam Station Ash Basin

Figure 51 Predicted Chromium in Monitoring Well MW-3S
Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin

Figure 52 Predicted Chromium in Monitoring Well MW-5S
Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin

Figure 53 Predicted Chromium in Monitoring Well MW-6S
Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin

Figure 54 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
Riverbend Steam Station Ash Basin

Figure 55 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
Riverbend Steam Station Ash Basin

Figure 56 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
Riverbend Steam Station Ash Basin

Figure 57 Existing Conditions Scenario - 2115 Predicted Chromium 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
Riverbend Steam Station Ash Basin

Figure 58 Existing Conditions Scenario - 2115 Predicted Chromium 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
Riverbend Steam Station Ash Basin

Figure 59 Existing Conditions Scenario - 2115 Predicted Chromium 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
Riverbend Steam Station Ash Basin

Figure 60 Excavation Scenario - 2115 Predicted Chromium 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
Riverbend Steam Station Ash Basin

Figure 61 Excavation Scenario - 2115 Predicted Chromium 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
Riverbend Steam Station Ash Basin

Figure 62 Excavation Scenario - 2115 Predicted Chromium 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
Riverbend Steam Station Ash Basin

Figure 63 Predicted Cobalt in Monitoring Well MW-3S
Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin

Figure 64 Predicted Cobalt in Monitoring Well MW-5S
Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin

Figure 65 Predicted Cobalt in Monitoring Well MW-6S
Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin

Figure 66 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 = 3 µg/L
Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin

Figure 67 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 = 3 µg/L
Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin

Figure 68 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 = 3 µg/L
Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin

Figure 69 Existing Conditions Scenario - 2115 Predicted Cobalt in Shallow Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Cobalt IMAC value = 1 µg/L
3. Cobalt PPBC = 3 µg/L
Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin

Figure 70 Existing Conditions Scenario - 2115 Predicted Cobalt in Deep Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Cobalt IMAC value = 1 µg/L
3. Cobalt PPBC = 3 µg/L
Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin

Figure 71 Existing Conditions Scenario - 2115 Predicted Cobalt in Bedrock Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Cobalt IMAC value = 1 µg/L
3. Cobalt PPBC = 3 µg/L
Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin

Figure 72 Excavation Scenario - 2115 Predicted Cobalt in Shallow Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Cobalt IMAC value = 1 µg/L
3. Cobalt PPBC = 3 µg/L
Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin

Figure 73 Excavation Scenario - 2115 Predicted Cobalt in Deep Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Cobalt IMAC value = 1 µg/L
3. Cobalt PPBC = 3 µg/L
Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin

Figure 74 Excavation Scenario - 2115 Predicted Cobalt in Bedrock Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Cobalt IMAC value = 1 µg/L
3. Cobalt PPBC = 3 µg/L
Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin

Figure 75 Predicted Hexavalent Chromium in Monitoring Well MW-9D
Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin

Figure 76 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
Riverbend Steam Station Ash Basin

Figure 77 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
Riverbend Steam Station Ash Basin

Figure 78 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
Riverbend Steam Station Ash Basin

Figure 79 Existing Conditions Scenario - 2115 Predicted Hexavalent Chromium 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
Riverbend Steam Station Ash Basin

Figure 80 Existing Conditions Scenario - 2115 Predicted Hexavalent Chromium 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
Riverbend Steam Station Ash Basin

Figure 81 Existing Conditions Scenario - 2115 Predicted Hexavalent Chromium 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
Riverbend Steam Station Ash Basin

Figure 82 Excavation Scenario - 2115 Predicted Hexavalent Chromium 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
Riverbend Steam Station Ash Basin

Figure 83 Excavation Scenario - 2115 Predicted Hexavalent Chromium 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
Riverbend Steam Station Ash Basin

Figure 84 Excavation Scenario - 2115 Predicted Hexavalent Chromium 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
Riverbend Steam Station Ash Basin

Figure 85 Predicted Sulfate in Monitoring Well MW-3S
Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin

Figure 86 Predicted Sulfate in Monitoring Well MW-5S 3
Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin

Figure 87 Predicted Sulfate in Monitoring Well MW-6S
Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin

Figure 88 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 = 970 µg/L
Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin

Figure 89 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 = 970 µg/L
Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin

Figure 90 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 = 970 µg/L
Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin

Figure 91 Existing Conditions Scenario - 2115 Predicted Sulfate in Shallow Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Sulfate 2L Standard = 250,000 µg/L
3. Sulfate PPBC = 970 µg/L
Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin

Figure 92 Existing Conditions Scenario - 2115 Predicted Sulfate in Deep Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Sulfate 2L Standard = 250,000 µg/L
3. Sulfate PPBC = 970 µg/L
Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin

Figure 93 Existing Conditions Scenario - 2115 Predicted Sulfate in Bedrock Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Sulfate 2L Standard = 250,000 µg/L
3. Sulfate PPBC = 970 µg/L
Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin

Figure 94 Excavation Scenario - 2115 Predicted Sulfate in Shallow Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Sulfate 2L Standard = 250,000 µg/L
3. Sulfate PPBC = 970 µg/L
Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin

Figure 95 Excavation Scenario - 2115 Predicted Sulfate in Deep Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Sulfate 2L Standard = 250,000 µg/L
3. Sulfate PPBC = 970 µg/L
Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin

Figure 96 Excavation Scenario - 2115 Predicted Sulfate in Bedrock Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Sulfate 2L Standard = 250,000 µg/L
3. Sulfate PPBC = 970 µg/L
Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin

Figure 97 Predicted Thallium in Monitoring Well MW-3S
Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin

Figure 98 Predicted Thallium in Monitoring Well MW-5S
Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin

Figure 99 Predicted Thallium in Monitoring Well MW-6S
Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin

Figure 100 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
Riverbend Steam Station Ash Basin

Figure 101 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
Riverbend Steam Station Ash Basin

Figure 102 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
Riverbend Steam Station Ash Basin

Figure 103 Existing Conditions Scenario - 2115 Predicted Thallium 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
Riverbend Steam Station Ash Basin

Figure 104 Existing Conditions Scenario - 2115 Predicted Thallium 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
Riverbend Steam Station Ash Basin

Figure 105 Existing Conditions Scenario - 2115 Predicted Thallium 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
Riverbend Steam Station Ash Basin

Figure 106 Excavation Scenario - 2115 Predicted Thallium 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
Riverbend Steam Station Ash Basin

Figure 107 Excavation Scenario - 2115 Predicted Thallium 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
Riverbend Steam Station Ash Basin

Figure 108 Excavation Scenario - 2115 Predicted Thallium 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
Riverbend Steam Station Ash Basin

Figure 109 Predicted Vanadium in Monitoring Well MW-3S
Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin

Figure 110 Predicted Vanadium in Monitoring Well MW-5S
Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin

Figure 111 Predicted Vanadium in Monitoring Well MW-6S
Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin

Figure 112 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 = 29.9 µg/L
Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin

Figure 113 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 = 29.9 µg/L
Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin

Figure 114 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 = 29.9 µg/L
Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin

Figure 115 Existing Conditions Scenario - 2115 Predicted Vanadium in Shallow Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Vanadium IMAC value = 0.3 µg/L
3. Vanadium PPBC = 29.9 µg/L
Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin

Figure 116 Existing Conditions Scenario - 2115 Predicted Vanadium in Deep Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Vanadium IMAC value = 0.3 µg/L
3. Vanadium PPBC = 29.9 µg/L
Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin

Figure 117 Existing Conditions Scenario - 2115 Predicted Vanadium in Bedrock Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Vanadium IMAC value = 0.3 µg/L
3. Vanadium PPBC = 29.9 µg/L
Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin

Figure 118 Excavation Scenario - 2115 Predicted Vanadium in Shallow Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Vanadium IMAC value = 0.3 µg/L
3. Vanadium PPBC = 29.9 µg/L
Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin

Figure 119 Excavation Scenario - 2115 Predicted Vanadium in Deep Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Vanadium IMAC value = 0.3 µg/L
3. Vanadium PPBC = 29.9 µg/L
Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin

Figure 120 Excavation Scenario - 2115 Predicted Vanadium in Bedrock Groundwater Zone
Notes:
1. µg/L = micrograms per liter
2. Vanadium IMAC value = 0.3 µg/L
3. Vanadium PPBC = 29.9 µg/L
Groundwater Flow and Transport Model
Riverbend Steam Station Ash Basin

Figure 121 Water Level Drawdown at Hypothetical Pumping Wells between Ash Basin Waste Boundary and
Mountain Island Lake
Notes:
1. µg/L = micrograms per liter
2. Vanadium IMAC value = 0.3 µg/L
3. Vanadium PPBC = 29.9 µg/L