NC0004961_RBSS CAP Part I_Appx C_Final_20151116

Appendix C
UNCC Groundwater
Flow and Transport
Model

This page intentionally left blank

Draft Groundwater Flow and Transport Model
Riverbend Steam Station
Gaston County, NC
Investigators:
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

Contributors:

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

UNC Charlotte / Lee College of Engineering
Department of Civil and Environmental Engineering
EPIC Building 3252
9201 University City Blvd.
Charlotte, NC 28223

Revised November 12, 2015
TABLE OF CONTENTS
1 Introduction ............................................................................................................................ 1 
1.1 General Setting and Background ................................................................................... 1 
1.2 Study Objectives ............................................................................................................. 1 
2 Conceptual Model .................................................................................................................. 2 
2.1 Geology and Hydrogeology (HDR 2015) ........................................................................ 2 
2.2 Groundwater Flow System ............................................................................................. 3 
2.2.1 Ash Basin (HDR 2015) ............................................................................................ 3 
2.2.2 Ash Storage Area (HDR 2015) ................................................................................ 4 
2.2.3 Cinder Storage Area (HDR 2015) ........................................................................... 4 
2.3 Hydrostratigraphic Layer Development (HDR 2015) ...................................................... 5 
2.4 Hydrologic Boundaries ................................................................................................... 6 
2.5 Hydraulic Boundaries ..................................................................................................... 6 
2.6 Sources and Sinks .......................................................................................................... 6 
2.7 Water Balance ................................................................................................................ 6 
2.8 Modeled Constituents of Interest .................................................................................... 6 
2.9 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 
4.2 GMS MODFLOW Version 10 ......................................................................................... 9 
4.3 Model Domain and Grid ................................................................................................ 10 
4.4 Hydraulic Parameters ................................................................................................... 11 
4.5 Flow Model Boundary Conditions ................................................................................. 11 
4.6 Flow Model Sources and Sinks .................................................................................... 11 
4.7 Flow Model Calibration Targets .................................................................................... 12 
4.8 Transport Model Parameters ........................................................................................ 12 
4.9 Transport Model Boundary Conditions ......................................................................... 13 
4.10 Transport Model Sources and Sinks ............................................................................ 13 
4.11 Transport Model Calibration Targets ............................................................................ 13 
5 Model Calibration ................................................................................................................. 13 
5.1 Flow Model Residual Analysis ...................................................................................... 13 
5.2 Flow Model Sensitivity Analysis .................................................................................... 14 
5.3 Transport Model Calibration and Sensitivity ................................................................. 14 
6 SIMULATION OF CLOSURE SCENARIOS ........................................................................ 15 
6.1 Existing Conditions Scenario ........................................................................................ 16 
6.2 Ash Basin Cap-in-Place ................................................................................................ 16 
6.3 Excavation Scenario ..................................................................................................... 17 
7 Transport MODEL Parameter sENSITIVITY ....................................................................... 18 
8 SUMMARY AND CONCLUSIONS ...................................................................................... 19 
8.1 Model Assumptions and Limitations ............................................................................. 19 
8.2 Model Predictions ......................................................................................................... 20 
9 REFERENCES .................................................................................................................... 21 

TABLES
Table 1. Description of MODFLOW and MT3DMS Input Packages Utilized
Table 2. Hydraulic Conductivity in the Model
Table 3. Effective Porosity in the Model
Table 4. Observed vs. Predicted Hydraulic Head (ft msl)
Table 5. Transport Model Calibration Results
Table 6. Predicted Advective Travel Time

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 Areas 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/M2 Layers (Model Layers 5-7)
Figure 10 Modeled Hydraulic Head (feet) vs. Observed Hydraulic Head (feet)
Figure 11 Hydraulic Head (feet) in M2 Saprolite Layer (Model Layer 7)
Figure 12 Hydraulic Head (feet) in North-South Cross Section (C-C’) through Primary and
Secondary Ash Basins
Figure 13 Hydraulic Head (feet) in East-West Cross Section (C-C’) through Primary and
Secondary Ash Basins
Figure 14 Particle Tracking Results (see Table. 6 for Advective Travel Times)
Figure 15 Existing Conditions Scenario - 2015 Predicted Antimony (µg/L) in Shallow Groundwater
Zone
Figure 16 Existing Conditions Scenario - 2015 Predicted Antimony (µg/L) in Deep Groundwater
Zone
Figure 17 Existing Conditions Scenario - 2015 Predicted Antimony (µg/L) in Bedrock
Groundwater Zone
Figure 18 Existing Conditions Scenario - 2115 Predicted Antimony (µg/L) in Shallow
Groundwater Zone
Figure 19 Existing Conditions Scenario - 2115 Predicted Antimony (µg/L) in Deep Groundwater
Zone
Figure 20 Existing Conditions Scenario - 2115 Predicted Antimony (µg/L) in Bedrock
Groundwater Zone
Figure 21 Existing Conditions Scenario – Predicted Antimony (µg/L) in Downgradient
Monitoring Wells Completed in Shallow, Deep and Bedrock Groundwater Zones
Figure 22 Existing Conditions Scenario - 2015 Predicted Chromium (µg/L) in Shallow
Groundwater Zone
Figure 23 Existing Conditions Scenario - 2015 Predicted Chromium (µg/L) in Deep
Groundwater Zone
Figure 24 Existing Conditions Scenario - 2015 Predicted Chromium (µg/L) in Bedrock
Groundwater Zone
Figure 25 Existing Conditions Scenario - 2115 Predicted Chromium (µg/L) in Shallow
Groundwater Zone
Figure 26 Existing Conditions Scenario - 2115 Predicted Chromium (µg/L) in Deep
Groundwater Zone
Figure 27 Existing Conditions Scenario - 2115 Predicted Chromium (µg/L) in Bedrock
Groundwater Zone
Figure 28 Existing Conditions Scenario – Predicted Chromium (µg/L) in Downgradient
Monitoring Wells Completed in Shallow, Deep and Bedrock Groundwater Zones
Figure 29 Existing Conditions Scenario - 2015 Predicted Sulfate (µg/L) in Shallow Groundwater
Zone
Figure 30 Existing Conditions Scenario - 2015 Predicted Sulfate (µg/L) in Deep Groundwater
Zone
Figure 31 Existing Conditions Scenario - 2015 Predicted Sulfate (µg/L) in Bedrock
Groundwater Zone
Figure 32 Existing Conditions Scenario - 2115 Predicted Sulfate (µg/L) in Shallow Groundwater
Zone
Figure 33 Existing Conditions Scenario - 2115 Predicted Sulfate (µg/L) in Deep Groundwater
Zone
Figure 34 Existing Conditions Scenario - 2115 Predicted Sulfate (µg/L) in Bedrock
Groundwater Zone
Figure 35 Existing Conditions Scenario – Predicted Sulfate (µg/L) in Downgradient Monitoring
Wells Completed in Shallow, Deep and Bedrock Groundwater Zones
Figure 36 Cap-in-Place Scenario - 2015 Predicted Antimony (µg/L) in Shallow Groundwater
Zone
Figure 37 Cap-in-Place Scenario - 2015 Predicted Antimony (µg/L) in Deep Groundwater Zone
Figure 38 Cap-in-Place Scenario - 2015 Predicted Antimony (µg/L) in Bedrock Groundwater
Zone
Figure 39 Cap-in-Place Scenario - 2115 Predicted Antimony (µg/L) in Shallow Groundwater
Zone
Figure 40 Cap-in-Place Scenario - 2115 Predicted Antimony (µg/L) in Deep Groundwater Zone
Figure 41 Cap-in-Place Scenario - 2115 Predicted Antimony (µg/L) in Bedrock Groundwater
Zone
Figure 42 Cap-in-Place Scenario – Predicted Antimony (µg/L) in Downgradient Monitoring
Wells Completed in Shallow, Deep and Bedrock Groundwater Zones
Figure 43 Cap-in-Place Scenario - 2015 Predicted Chromium (µg/L) in Shallow Groundwater
Zone
Figure 44 Cap-in-Place Scenario - 2015 Predicted Chromium (µg/L) in Deep Groundwater
Zone
Figure 45 Cap-in-Place Scenario - 2015 Predicted Chromium (µg/L) in Bedrock Groundwater
Zone
Figure 46 Cap-in-Place Scenario - 2115 Predicted Chromium (µg/L) in Shallow Groundwater
Zone
Figure 47 Cap-in-Place Scenario - 2115 Predicted Chromium (µg/L) in Deep Groundwater
Zone
Figure 48 Cap-in-Place Scenario - 2115 Predicted Chromium (µg/L) in Bedrock Groundwater
Zone
Figure 49 Cap-in-Place Scenario – Predicted Chromium (µg/L) in Downgradient Monitoring
Wells Completed in Shallow, Deep and Bedrock Groundwater Zones
Figure 50 Cap-in-Place Scenario - 2015 Predicted Sulfate (µg/L) in Shallow Groundwater Zone
Figure 51 Cap-in-Place Scenario - 2015 Predicted Sulfate (µg/L) in Deep Groundwater Zone
Figure 52 Cap-in-Place Scenario - 2015 Predicted Sulfate (µg/L) in Bedrock Groundwater Zone
Figure 53 Cap-in-Place Scenario - 2115 Predicted Sulfate (µg/L) in Shallow Groundwater Zone
Figure 54 Cap-in-Place Scenario - 2115 Predicted Sulfate (µg/L) in Deep Groundwater Zone
Figure 55 Cap-in-Place Scenario - 2115 Predicted Sulfate (µg/L) in Bedrock Groundwater Zone
Figure 56 Cap-in-Place Scenario – Predicted Sulfate (µg/L) in Downgradient Monitoring Wells
Completed in Shallow, Deep and Bedrock Groundwater Zones
Figure 57 Excavation Scenario - 2015 Predicted Antimony (µg/L) in Shallow Groundwater Zone
Figure 58 Excavation Scenario - 2015 Predicted Antimony (µg/L) in Deep Groundwater Zone
Figure 59 Excavation Scenario - 2015 Predicted Antimony (µg/L) in Bedrock Groundwater
Zone
Figure 60 Excavation Scenario - 2115 Predicted Antimony (µg/L) in Shallow Groundwater Zone
Figure 61 Excavation Scenario - 2115 Predicted Antimony (µg/L) in Deep Groundwater Zone
Figure 62 Excavation Scenario - 2115 Predicted Antimony (µg/L) in Bedrock Groundwater
Zone
Figure 63 Excavation Scenario – Predicted Antimony (µg/L) in Downgradient Monitoring Wells
Completed in Shallow, Deep and Bedrock Groundwater Zones
Figure 64 Excavation Scenario - 2015 Predicted Chromium (µg/L) in Shallow Groundwater
Zone
Figure 65 Excavation Scenario - 2015 Predicted Chromium (µg/L) in Deep Groundwater Zone
Figure 66 Excavation Scenario - 2015 Predicted Chromium (µg/L) in Bedrock Groundwater
Zone
Figure 67 Excavation Scenario - 2115 Predicted Chromium (µg/L) in Shallow Groundwater
Zone
Figure 68 Excavation Scenario - 2115 Predicted Chromium (µg/L) in Deep Groundwater Zone
Figure 69 Excavation Scenario - 2115 Predicted Chromium (µg/L) in Bedrock Groundwater
Zone
Figure 70 Excavation Scenario – Predicted Chromium (µg/L) in Downgradient Monitoring Wells
Completed in Shallow, Deep and Bedrock Groundwater Zones
Figure 71 Excavation Scenario - 2015 Predicted Sulfate (µg/L) in Shallow Groundwater Zone
Figure 72 Excavation Scenario - 2015 Predicted Sulfate (µg/L) in Deep Groundwater Zone
Figure 73 Excavation Scenario - 2015 Predicted Sulfate (µg/L) in Bedrock Groundwater Zone
Figure 74. Excavation Scenario - 2115 Predicted Sulfate (µg/L) in Shallow Groundwater Zone
Figure 75 Excavation Scenario - 2115 Predicted Sulfate (µg/L) in Deep Groundwater Zone
Figure 76 Excavation Scenario - 2115 Predicted Sulfate (µg/L) in Bedrock Groundwater Zone
Figure 77 Excavation Scenario – Predicted Sulfate (µg/L) in Downgradient Monitoring Wells
Completed in Shallow, Deep and Bedrock Groundwater Zones
Figure 78 Antimony Sensitivity – Predicted Variation in Antimony (µg/L) in well GWA-2S due to
Increased and Decreased Transport Parameters, Longitudinal Dispersivity and
Effective Porosity
Figure 79 Antimony Sensitivity – Predicted Variation in Antimony (µg/L) in well GWA-2BR due
to Increased and Decreased Transport Parameters, Longitudinal Dispersivity and
Effective Porosity
Figure 80 Antimony Sensitivity – Predicted Variation in Antimony (µg/L) in well GWA-9S due to
Increased and Decreased Transport Parameters, Longitudinal Dispersivity and
Effective Porosity
Figure 81 Antimony Sensitivity – Predicted Variation in Antimony (µg/L) in well GWA-6D due to
Increased and Decreased Transport Parameters, Longitudinal Dispersivity and
Effective Porosity
Figure 82 Antimony Sensitivity – Predicted Variation in Antimony (µg/L) in well GWA-9BR due
to Increased and Decreased Transport Parameters, Longitudinal Dispersivity and
Effective Porosity
Figure 83 Chromium Sensitivity – Predicted Variation in Chromium (µg/L) in well GWA-2S due
to Increased and Decreased Transport Parameters, Longitudinal Dispersivity and
Effective Porosity
Figure 84 Chromium Sensitivity – Predicted Variation in Chromium (µg/L) in well GWA-9S due
to Increased and Decreased Transport Parameters, Longitudinal Dispersivity and
Effective Porosity
Figure 85 Chromium Sensitivity – Predicted Variation in Chromium (µg/L) in well GWA-9D due
to Increased and Decreased Transport Parameters, Longitudinal Dispersivity and
Effective Porosity
Figure 86 Sulfate Sensitivity – Predicted Variation in Sulfate (µg/L) in well GWA-2S due to
Increased and Decreased Transport Parameters, Longitudinal Dispersivity and
Effective Porosity
Figure 87 Sulfate Sensitivity – Predicted Variation in Sulfate (µg/L) in well GWA-2BR due to
Increased and Decreased Transport Parameters, Longitudinal Dispersivity and
Effective Porosity
Figure 88 Sulfate Sensitivity – Predicted Variation in Sulfate (µg/L) in well GWA-9D due to
Increased and Decreased Transport Parameters, Longitudinal Dispersivity and
Effective Porosity
Figure 89 Sulfate Sensitivity – Predicted Variation in Sulfate (µg/L) in well GWA-9BR due to
Increased and Decreased Transport Parameters, Longitudinal Dispersivity and
Effective Porosity

1

1 INTRODUCTION
Duke Energy owns the Riverbend Steam Station (RBSS), located on a 341 acre tract near
Mount Holly in Gaston County, North Carolina. RBSS began operation as a coal-fired
generating station in 1929 and was retired in April 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, coal ash residue was disposed of in the station’s ash
basin system (HDR Figure 2-1)1.
1.1 General Setting and Background
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. 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 (CT) units at RBSS from 1969 until October 2012.
The CT units could be fired by natural gas or oil and were located to the west of the coal-fired
units. Refer to HDR Figure 2-4 for a map of site features
As described in the RBSS Comprehensive Site Assessment (CSA) (HDR 2015), site
groundwater exists in alluvium, soil, soil/saprolite and bedrock and is consistent with a two-layer
regolith-fractured rock system (HDR Figure 5-3). The saturated zone is an unconfined,
connected system without confining layers (HDR 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 the Catawba River (the main hydrological
feature at the site).
The topography at RBSS ranges from elevation of 786 feet near the south edge of the property
near Horseshoe Bend Beach Road to elevation of 646 feet at the northern boundary with
Mountain Island Lake. The site slopes from south to north with an elevation difference of 140
feet over 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.
1.2 Study Objectives
The objectives of this study are to:
 construct a preliminary groundwater flow and contaminant transport model for the RBSS
 use the models to enhance the present conceptual model of the groundwater flow
system, with the understanding that additional efforts will be required to refine the model
as new data are collected

1 Please refer to the Comprehensive Site Assessment Report, Riverbend Steam Station Ash Basin,
August 2015 (HDR) for referenced HDR figures and tables.
2

 identify data gaps and conceptual model issues that can be used to create a more
robust model that better represents the natural groundwater flow system and physical
processes affecting groundwater flow and contaminant transport at the RBSS,
 simulate present conditions and predict future concentrations of modeled constituents of
interest (COI) for the closure scenario options that are simulated, and
 use the modeling results to compare the effectiveness of simulated closure scenario
options that were selected and considered for the site. Also, the models were used to
predict the time required to achieve compliance with state groundwater standards. This
time was delineated for monitoring wells downgradient of the ash basins and also for the
entire model domain.
The study consisted of three main activities: 1) creation of a steady-state flow model that
represents the current understanding of site conditions, 2) development of a transport model
that is qualitatively calibrated and can reasonably predict 2015 COI concentrations based on a
simplified representation of the ash basin/ash storage area sources and operational history of
these areas, and 3) perform predictive simulations of the selected closure scenario options.
The simulated closure scenario options for RBSS include; an “existing conditions” scenario with
ash sources left in place, “cap-in-place” scenario with ash left in the ash basins and ash storage
areas and the ash covered by low-permeability caps, and an “excavation” scenario with the
accessible ash removed from the RBSS. The ash removal means that future sources of
groundwater contamination in the unsaturated zone are also removed. Note that other Duke
Steam Station sites evaluated another closure scenario option called “Duke’s planned” scenario,
but in this case the excavation scenario at RBSS is Duke’s planned option. The excavation and
removal of ash is the selected process for the RBSS.
2 CONCEPTUAL MODEL
The site conceptual model used for the groundwater flow model is based on information that is
provided in the RBSS Comprehensive Site Assessment (CSA) Report (HDR 2015). The CSA
contains extensive detail regarding the site conceptual model, and this same model was used to
develop the RBSS groundwater flow and contaminant transport models. Following is a summary
of relevant information from the CSA regarding the RBSS site conceptual model including
geology/hydrogeology, groundwater flow system, ash basin and ash storage area sources, site
hydrostratigraphy, hydrologic/hydraulic boundaries, groundwater sources and sinks, water
budget, and COI.
2.1 Geology and Hydrogeology (HDR 2015)
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 (HDR 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
3

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. It is a two-medium system as 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 (TZ), or deep groundwater zone that 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 TZ, as typically
the permeability of the TZ 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 the Catawba River/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 the Catawba River. The groundwater contour maps constructed for the CSA depict these
patterns (HDR Figures 6-5 to 6-7).
2.2 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 (Ash Basin Secondary Cell). From the water table, groundwater
moves vertically and downward through unconsolidated material (residual soil/saprolite) into the
TZ, then fractured bedrock. The mean annual recharge to shallow groundwater aquifers in the
Piedmont ranges from 4.0 to 9.7 inches per year (Daniel 2001).
The historical and current information regarding the RBSS Ash Basin System assembled by
HDR (2015) was used in developing the conceptual site model and ultimately the numerical
groundwater flow models. Refer to HDR Figures 2-1 and 2-2 for locations of the RBSS Ash
Basin System components.
2.2.1 Ash Basin (HDR 2015)
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 HDR Figure 2-2. The Ash Basin Primary Cell is impounded
by an earthen embankment dam, referred to as Dam #1 (Primary), located on the west side of
the Primary Cell. The Ash Basin Secondary Cell is impounded by an earthen embankment dam,
referred to as Dam #2 (Secondary), located along the northeast side of the Secondary Cell. The
toe areas for both dams are in close proximity to Mountain Island Lake.
The intermediate (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
4

dredge pond was located south of the Ash Basin 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 and has a maximum pond elevation of 724 feet.
The Primary Cell contains approximately 1.9 million cubic yards of ash. The surface area of the
Secondary Cell is 28 acres, with a maximum pond elevation of 714 feet. The Secondary Cell
contains approximately 700,000 cubic yards of ash. The full pond elevation of Mountain Island
Lake is 646.8 feet.
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 are conveyed through sluice lines into the Primary Cell. Discharge from the
Primary Cell to the Secondary Cell is 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.
2.2.2 Ash Storage Area (HDR 2015)
The Ash Storage Area is located near the Ash Basin Primary Cell (Figure 2-2). The footprint of
the Ash Storage Area covers 29 acres and it 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.
2.2.3 Cinder Storage Area (HDR 2015)
The Cinder Storage Area is located west/southwest of the Ash Basin Primary Cell, and
northwest of the Ash Storage Area (HDR 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 (began in 1958). It is estimated that the Cinder Storage Area contains 300,000 tons of
ash.
5

2.3 Hydrostratigraphic Layer Development (HDR 2015)
Residual soil consists of clayey sand (Unified Soil Classification is SC), silty sand (SM), silty
sand with gravel (SM), micaceous silty sand (SM), and gravel with silt and sand (GP). 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 through 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 re-distributed 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.
Based on the CSA, the groundwater system is consistent with the regolith-fractured bedrock
system. To define the hydrostratigraphic units, the following classification system was used and
is based on Standard Penetration Testing (N) 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 TZ and BR (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 TZ just in case it extended into the next core run, which when
6

reviewed alone might have met the BR criterion, because of potential core loss or
fractured/jointed rock with indications of water movement (iron/manganese staining).
The above layers designations (M1, M2, TZ, and BR) are used on the geologic cross-sections
with transect locations shown on HDR 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. The
ranges for hydrostratigraphic layer properties measured at RBSS are provided in HDR Tables
11-7 through 11-11.
2.4 Hydrologic Boundaries
The major discharge location for the groundwater system at RBSS is the Catawba River and
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.
2.5 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).
2.6 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 2015). 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.
2.7 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 the Catawba
River 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.
2.8 Modeled Constituents of Interest
As defined in the CSA, the metals, compounds (or constituents) that were identified in the
groundwater assessment plans for sampling and analysis (HDR 2015) are potential COI. The
following criteria were used to determine if a COI required modeling: if the constituent exceeded
regulatory groundwater standards (15A NCAC 02L.0202 groundwater standard or IMAC),
formed a continuous and identifiable plume in groundwater, and is traceable back to the source
(i.e., existed in porewater) - the constituent was then deemed a COI for transport modeling. Per
the CSA, the metals and trace metals detected in ash basin porewater are; antimony, arsenic,
boron, cobalt, iron, manganese, pH, sulfate, thallium, TDS, and vanadium. Antimony
concentrations were elevated in shallow monitoring wells in the ash basins and ash storage
7

area. Antimony concentrations were elevated in deep monitoring wells associated with the ash
storage area, the ash basins, and north of the cinder storage area. Chromium concentrations
were elevated in shallow monitoring adjacent to the ash storage area and northwest of the ash
basin. Chromium concentrations were elevated in deep monitoring wells located beneath the
ash basin, north of the cinder storage area, and in the background wells. Sulfate concentrations
were elevated in the shallow monitoring wells northwest of the cinder storage area. Sulfate
concentrations in some wells were elevated in the deep monitoring wells northwest of the cinder
storage area and south of the ash storage area. Sulfate was not detected in monitoring wells
installed in bedrock. Since antimony, chromium and sulfate form continuous plumes in
groundwater and exist in porewater, transport modeling was performed for these COIs.
2.9 COI Transport
The COI entered 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 Primary 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. 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, and
lakes. The information assembled in the conceptual site model is translated into its numerical
equivalent from which a solution is generated by MODFLOW.
8

3.2 Model Description
The specific MODFLOW package chosen is NWT - a Newton formulation of MODFLOW-2005
that is specifically designed for improving the stability of solutions involving drying and re-
wetting under water table conditions (Niswonger, et al. 2011). The numerical code selected for
the transport model is MT3DMS (Zheng and Wang 1999). MT3DMS is multi-species 3-
dimensional 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 3D “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 three dimensional surfaces representing contacts between
hydrostatigraphic units with properties provided in HDR Tables 11-7 through 11-11.
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 the RBSS, simplified
elevation contours were digitized along the river channels to depress the surface a small
amount below water level.
2) Pre-construction Surface
Elevation contours of the original ground surface were digitized in CAD from engineering
drawings supplied by Duke Energy. These data were imported into GIS, and georeferenced.
These contours were trimmed to the areas underlying ash basins, dams, 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-
9

construction surface was then created using the combination of original surface elevations,
2014 survey elevations, and 2010 LiDAR elevations.
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) 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 ft x 20 ft grid across the area to be
modeled, hybrid algorithm was used with inverse distance weighted two (2) and triangulation
weighted one (1) 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.
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 3D MODFLOW model.

10

Step 1. Creating raster files for the model layer
‐ 3 surface layers (pre-construction, pre-construction with dike, and existing surface including dike and ash)
using GIS and AutoCAD
‐ 2 subsurface layers (transition (TZ) and bedrock (BR)) by converting 3D 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. 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 3D grid model
‐ Converting the solids to 3D grid model using boundary matching
‐ Mapping the conceptual model to 3D 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
‐ Soil-water partitioning coefficient (Kd) from the lab experiments
‐ Recharge concentrations
Step 9. Performing model simulations
‐ Model scenarios - Existing conditions, Remove ash

4.3 Model Domain and Grid
The model domain encompasses the RBSS site, including a section of the Catawba River 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 the Catawba River to the north, east, and west;
 a drainage feature to the east; and
11

 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 2-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 (HDR
2015).
There are 10 (total) 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 in 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
(HDR 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.
4.5 Flow Model Boundary Conditions
The boundary of the model domain was selected to coincide with physical hydrologic
boundaries at the Catawba River/Mountain Island Lake, drainage features, and no flow
boundaries at topographic divides (Section 2.4 and Figure 4). At the Catawba River/Mountain
Island Lake, 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 in Figure 4. It
is assumed that surface drainage is ultimately conveyed to an outfall at the Catawba River.
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 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 4 inches/year (Figure 5).
12

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 the ash, ash
dike, and shallow zone (S/M1/M2); 23 wells screened in the transition zone; and six wells
screened in the fractured bedrock. The observations wells in shallow, deep and bedrock
groundwater zones are shown in Figures 6 to 8.
4.8 Transport Model Parameters
The calibrated, steady-state flow model was used to apply flow conditions for the transport
model at the Ash Basin Primary and Secondary Cells (HDR Figures 2-1 and 2-2) where the
elevated concentrations of antimony, chromium and sulfate were detected during the June/July
2015 sampling event. Although their approximate dates of operation are known, the sluiced ash
loading histories for these locations are not available. A constant concentration boundary
condition was applied in ash (model layers 2-4) at the Ash Basin Primary and Secondary Cells
and Ash Storage Area from the date when the ash basin was placed in service. The relevant
model parameters are the constant boundary concentration and the linear sorption coefficient or
Kd for sorptive constituents (including chromium).
The conceptual transport model specifies that COIs enter the model from the shallow saturated
zone in the ash basin and beneath the ash storage areas. When the measured Kd values are
applied in the numerical model to COIs migrating from the source zones, some COIs do not
reach the downgradient observation wells where it was observed in June 2015 at the end of the
simulation period. The most appropriate method to calibrate the transport model in this case is
to lower the Kd values until an acceptable agreement between measured and modeled
concentrations is achieved. Thus, an effective Kd value results that likely represents the
combined result of intermittent activities over the service life of the ash basin and storage areas.
These may include pond dredging, dewatering for dike construction, and ash grading and
placement.
Kd values for the COIs were applied as follows:
 Antimony: 0.1 ml/g
 Chromium: 0.1 ml/g
 Sulfate: conservative (sorption not modeled)
Sorption was used as a transport model calibration parameter (see Section 5.3). The bulk
density used in the model is 2.65 grams/cubic centimeter for ash, saprolite, TZ, and bedrock
materials.
The velocity of COI 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, dike, and S/M1/M2
layers, which is within the range of estimated values from the CSA (HDR 2015). For the TZ and
bedrock groundwater zone, the porosities applied were 0.05 and 0.005, respectively (see Table
3). Dispersivity quantifies the degree to which mechanical dispersion of COI occurs in
groundwater. The dispersivity values of 80 feet, 8 feet, and 0.8 feet (longitudinal, transverse
horizontal, transverse vertical) were applied in the model. Dispersivity is a physical property of
13

the aquifer medium and is normally a fraction of the field scale problem (e.g., plume length),
commonly 10% (Zheng and Bennett [2002]).
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 dispersivity, should
be less than two (Zheng and Bennett [2002]). Directly beneath the ash basin system, shallow
groundwater flow is vertical downward. In this case, the grid Peclet number criteria will not be
met due to the relatively small value for vertical dispersivity and the relatively large grid spacing,
or thickness of the model layers at depth. The effect of numerical oscillation on modeled
concentration and mass transport is indeterminate. However, any effect is considered to be
limited as vertical groundwater flow transitions to horizontal over a short distance beneath the
ash basin system.
4.9 Transport Model Boundary Conditions
The transport model boundary conditions are zero concentration where water leaves the model.
Initial concentrations and concentrations in recharge are zero. No background concentration is
specified. The ash ponds and ash storage areas are represented by a constant concentration
boundary condition.
4.10 Transport Model Sources and Sinks
The Ash Basin Primary and Secondary Cells and Ash Storage Area are the sources for COI in
the model (HDR 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 and Ash 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 HDR 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 4). The
observation data from this single point in time were used as a flow model calibration data set.
The locations of the observation wells are provided in Figures 6 through 8. The initial trial-and-
error calibration assumed homogeneous conditions in each model layer.
Throughout the flow model calibration process, the assumption of a homogeneous transition
zone was retained, and its horizontal conductivity was assigned to give a reasonable, initial
model result for the water table and water level observations. Recharge was also fixed at
reasonable values early in the calibration process, and then refinements were made by
adjusting hydraulic conductivity in the upper layers (S/M1/M2) by zones within the layers as
shown in Figure 9. The basis for delineating the zones within layers was to obtain the best
14

calibration using values within the range of measurements made during the CSA. The model is
most sensitive to recharge and hydraulic 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 4 and Figure 10. 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 4. 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 = 5.3 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 7.48%. When the NRMSE is < 9%, the model calibration is considered
satisfactory.
The hydraulic head contours for the M2 Saprolite Layer (model layer 7) in the calibrated model
is shown in Figure 11. Figures 12 and 13 are cross sections of hydraulic head through the Ash
Basin Primary and Secondary Cells. Overall, groundwater in the shallow aquifer, transition
zone, and fractured bedrock at the site flows to the north, northeast, and northwest and
discharges to the Catawba River/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 parameter 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. 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 and transition zones, horizontal flow is dominant groundwater in all three groundwater
zones away from the ash basins.
5.3 Transport Model Calibration and Sensitivity
For transport model calibration, the constant source concentrations (resulting from contaminant
flux to groundwater from the ash basins and ash storage area sources) were adjusted to match
the concentrations in target wells as closely as possible. The constant source concentrations
15

and Kd were adjusted during transport model calibration. The transport model calibration results
are shown in Table 5.
Particle tracking was performed during model calibration to determine if advective travel times
are reasonable. Particles were placed at wells located near the Catawba River and also near
the southern model boundary. The particle tracks are shown in Figure 14 and predicted
advective travel times are provided in Table 6.
The initial concentrations and transport modeling results for antimony in the shallow, deep and
bedrock groundwater zones are shown in Figures 15 to 21 (existing conditions in 2015 and
2115). From the source zones at the ash basins and ash storage area, antimony migrated
horizontally and vertically throughout the shallow zone and into the transition zone and fractured
bedrock. The general direction of antimony migration from the sources zones was to the north,
northwest and northeast. The highest antimony concentrations exiting at the model boundary
are downgradient of the Ash Basin Secondary Cell. All of the groundwater zones exited the
model and discharged to the Catawba River.
The initial concentrations and transport modeling results for chromium in the shallow, deep and
bedrock groundwater zones are shown in Figures 22 to 28 (existing conditions in 2015 and
2115). From the source zones at the ash basins and ash storage area, chromium migrated
horizontally and vertically in portions of the shallow zone and into the transition zone, but has
remained below the NCDEQ 2L2 (2L) for chromium. The general direction of chromium
migration from the sources zones is to the north, northwest and northeast. None of the
groundwater zones exited the model.
The initial concentrations and transport modeling results for sulfate in the shallow, deep and
bedrock groundwater zones are shown in Figures 29 though 35 (existing conditions in 2015 and
2115). From the source zones at the ash basins and ash storage area, sulfate migrated
horizontally and vertically throughout the shallow zone and into the transition zone and fractured
bedrock. The general direction of sulfate migration from the sources zones is to the north,
northwest and northeast. All groundwater zones discharged to the Catawba River. The
predicted concentrations of sulfate in all three groundwater zones are below the 2L limit of 250
mg/L.
6 SIMULATION OF CLOSURE SCENARIOS
Three closure scenarios were modeled for the RBSS: an “existing conditions” scenario with ash
sources left in place, “cap-in-place” scenario with ash left in the ash basins and ash storage
areas and the ash covered by low-permeability caps, 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 becomes
available in order to improve its accuracy and reduce its uncertainty. The model domain
developed for existing conditions was applied without modification for the existing and cap-in-
place scenarios.

2 North Carolina Groundwater Rules; Title 15A, Subchapter 02L of the NC Administrative Code.

16

6.1 Existing Conditions Scenario
Concentration contours and concentration breakthrough curves are referenced to 1958, since
that is the year that the Ash Basin became effective. Figures 15 through 17 show initial
antimony concentrations in the shallow, deep and bedrock groundwater zones (2015). Figures
18 through 20 show predicted antimony concentrations in the same zones in 2115 (100 years
post-closure scenario implementation). Figure 21 shows predicted antimony concentrations in
monitoring wells GWA-2, MW-15 and MW-6. The model predictions are projected-out 250 years
(2015 to 2265).
For GWA-2 (north of the Ash Basin Primary Cell), the predicted antimony in all groundwater
zones remains above the 2L standard throughout the modeling period. For MW-15 (located
north of the Ash Basin Secondary Cell), antimony in shallow and deep groundwater zones is
predicted to remain below the 2L standard, but groundwater in the bedrock zone will exceed the
2L standard. For MW-6 (on the western edge of the Ash Basin Primary Cell), antimony
concentrations remain above the 2L standard in all groundwater zones. Antimony exits the
model domain and discharges to the Catawba River/Mountain Island Lake to north, northwest,
and northeast in all groundwater zones. The highest concentrations of antimony leave the
model northeast of the Ash Basin Secondary Cell. Note that the model predicts that under
existing conditions that antimony will remain above the 2L standard in groundwater after 250
years.
Figures 22 through 24 show initial chromium concentration for the shallow, deep and bedrock
groundwater zones (2015). Figures 25 through 27 show predicted antimony concentrations in
the same zones in 2115 (100 years post-closure scenario implementation). Figure 28 presents
the predicted chromium concentrations at monitoring wells GWA-2, MW-15, and MW-6. For
GWA-2, the concentration of chromium in the shallow and deep zones increases and remains
above the 2L standard. For MW-15, located north of the Ash Basin Secondary Cell, no impact is
predicted. For MW-6 (on the western edge of the Ash Basin Primary Cell), chromium is
predicted to remain below the 2L standard until 2045 and at that time groundwater in the
shallow and deep zones will exceed the 2L standard (and continue to rise). Note that after 250
years, antimony remains above the 2L standard. Groundwater in the shallow zone exits the
model domain to the northwest and northeast and discharges into the Catawba River.
Groundwater in the deep zone exits the model to the northwest into the Catawba River.
Figures 29 through 31 show initial (2015) predicted sulfate concentrations in the shallow, deep
and BR zones, respectively. Figures 32 through 34 show the predicted sulfate concentrations in
the same zones in 2115. Figure 35 presents the predicted sulfate concentrations at monitoring
wells GWA-2, MW-15, and MW-6. As shown in Figure 35, the predicted concentration of sulfate
in the wells remains below the 2L standard in all groundwater zones. However, sulfate remains
above the 2L standard in the other areas of the model domain even after 250 years.
Groundwater containing dissolved sulfate exits the model and discharges to the Catawba River.
6.2 Ash Basin Cap-in-Place
In the Cap-in-Place Scenario, the ash basins and ash storage areas are covered by low-
permeability caps to prevent the infiltration of water into the subsurface. In this scenario, all of
the ash is covered by a cap. This closure scenario was simulated by reducing groundwater
17

recharge to zero where caps are installed. The affects of installing the caps and reducing
recharge include lowering the water table by 28 to 40 feet in the center of the Ash Basin Primary
Cell and 41 to 44 feet in the center of the Ash Basin Secondary Cell. . The model uses the
predicted concentration from the 2015 calibration as the starting concentration for this model
scenario.
Figures 36 through 38 show initial antimony concentrations in the shallow, deep and bedrock
groundwater zones (2015). Figures 39 through 41 show predicted antimony concentrations in
the same zones in 2115. In Figure 42, antimony in GWA-2 is predicted to decrease and
concentrations fall below the 2L limit in 2040 within the bedrock groundwater zone and in
shallow and deep groundwater zones in 2070. For MW-15, located north of the Ash Basin
Secondary Cell, the shallow and deep groundwater zones will remain below the 2L standard
and the bedrock groundwater zone will drop below the 2L limit by 2020. For MW-6, antimony in
the shallow and deep zones will continue to drop over time to reach the 2L limit in 2215; the
bedrock zone will remain below the 2L limit.
Figures 43 through 48 show the initial (2015) and predicted (2115) chromium concentrations in
the shallow, deep and bedrock groundwater zones In Figure 49, chromium transport from the
three source zones are graphed to show predicted concentrations over time in three observation
wells; GWA-2, MW-15 and MW-6. Predicted chromium at the site will remain within the basin
footprint, although concentrations will increase in GWA-2 in shallow and deep groundwater
zones to above the 2L standard for chromium between 2020 and 2115, and in MW-6 between
2095 and 2135. Chromium will not impact MW-15.
Figures 50 through 55 show the initial (2015) predicted sulfate concentrations (2115) in the
shallow, deep and bedrock groundwater zones, respectively. In Figure 56, sulfate transport from
the three source zones are graphed to show predicted concentrations over time in three
observation wells; GWA-2, MW-15 and MW-6. In all scenarios, the concentration of sulfate is
less than the 2L standard in groundwater and continues to decrease over time.
6.3 Excavation Scenario
The Excavation model simulates the effects of removing the ash basins, the dikes, and ash
storage areas at the beginning of this scenario. In the model, source zone concentrations at the
ash basins and ash storage areas are set to zero while recharge is applied at the same rate as
other surrounding areas. Groundwater flow beneath the ash basin is affected by this scenario as
the basins are completely drained. In the model, non-sorptive COIs will move downgradient at
the pore velocity of groundwater and will be completely displaced by the passage of a single
pore water volume of clean water. Sorptive COI migration will be retarded relative to the
groundwater pore velocity as they are desorbed by clean water. . The model uses the predicted
concentration from the 2015 calibration as the starting concentration for this model scenario.
Figures 57 through 62 show the initial (2015) and predicted (2115) antimony concentrations in
the shallow, deep and bedrock groundwater zones, respectively. In Figure 63, antimony in
GWA-2 is predicted to increase slightly until 2025 and then decrease steadily, falling below the
2L standard beginning in 2070 for all groundwater zones. For MW-15 (located north of the Ash
Basin Secondary Cell), the shallow and deep groundwater zones will remain below the 2L limit
for antimony and the bedrock groundwater zone will decrease to the 2L limit by 2055. For MW-6
18

(on the western edge of the Ash Basin Primary Cell), antimony will increase slightly until 2025
then decrease steadily over time in all groundwater zones and in 2070 be below the 2L
standard. In all scenarios, antimony will almost not be detectable by 2155.
Figures 64 through 69 show the initial (2015) and predicted (2115) chromium concentrations in
the shallow, deep and bedrock groundwater zones In Figure 70, chromium transport from the
three source zones are graphed to show predicted concentrations over time in GWA-2, MW-15
and MW-6. Chromium concentrations will increase in the shallow and deep groundwater zones
in GWA-2 and exceed the 2L limit in 2020 in the shallow groundwater zone and 2060 in the
deep groundwater zone. Chromium will remain above the 2L through 2260. Chromium will not
be detectable in MW-15 in any zone. In MW-6, chromium concentrations will increase to levels
above the 2L limit in 2060 in shallow groundwater and 2080 in deep groundwater. Chromium
will continue to rise through 2265. Chromium in groundwater will exit the model and discharge to
the Catawba River by 2115.
Figures 71 through 76 show initial (2015) predicted (2115) sulfate concentrations in the shallow,
deep and bedrock groundwater zones As shown in Figure 77, sulfate concentrations at wells
GWA-2, MW-15, and MW-6 will remain in the groundwater in all three zones, but remain below
the 2L standard.
7 TRANSPORT MODEL PARAMETER SENSITIVITY
Sensitivity scenarios were run to the evaluate changes in model predictions when key transport
parameters are varied. The reason for the sensitivity analysis is to determine the parameters
with the greatest influence on modeling results and potential connections between parameters.
The existing conditions scenario was used as the basis for all sensitivity runs. The resultant
sensitivity was examined by comparing the change in predicted constituent concentration at 100
years at selected downgradient wells for each transport parameter tested.
The effective porosity of a porous medium is the porosity that is available for continuous fluid
flow through the material. The effective porosity primarily affects the velocity at which fluid can
move through a material where a smaller value increases fluid velocity and a greater value
decreases fluid velocity. The assigned effective porosity for ash, alluvium, dike materials, and
saprolite is 0.2, while the TZ is 0.05 and bedrock is 0.005. The sensitivity of simulated
constituent transport to this parameter was examined by running the model with a 2X multiplier
(both increasing and decreasing).
In addition, the sensitivity of the longitudinal dispersivity is an empirical factor which quantifies
how much contaminants stray away from a path parallel to the primary direction of groundwater
flow, or how much of the contaminant moves faster than or slower than the average
groundwater velocity. All transport models were assigned a uniform longitudinal dispersivity
value of 80 feet. The sensitivity of this parameter was examined by running the model with a
25% increase (longitudinal dispersivity = 100 feet) and 25% decrease (longitudinal dispersivity =
60 feet).
The Kd relates the adsorbed constituent concentration to the concentration of the constituent
dissolved in water. Non-reactive or conservative ions, which at this site include antimony and
19

sulfate, are assumed to have a Kd of zero and therefore move at advective flow rates (average
linear groundwater velocity). The movement of reactive ions, such as chromium is retarded by
sorption. In the initial condition and closure scenarios, the Kd value used for this constituent is 1
milliliter/gram. The sensitivity of simulated constituent transport to this parameter was examined
by running the model with a 10X increase and 10X decrease.
The results of the longitudinal dispersivity sensitivity runs were examined in selected
downgradient wells and are shown in Figures 78 through 89. Increasing or decreasing
longitudinal dispersivity showed little consistency with increasing or decreasing well
concentrations after 100 years. In the selected downgradient wells, antimony concentrations
(Figures 78 through 82) after 100 years were not sensitive to a 25% increase or decrease in
longitudinal dispersivity This equates to a change of 2% when compared to the antimony
concentrations in the existing conditions scenario after 100 years. Chromium concentrations
showed to be the most sensitive to longitudinal dispersivity changes (Figures 83 through 85) as
a 25% longitudinal dispersivity increase or decrease resulted in an average concentration
change of 8 µg/L. This equates to a change of 9% when compared to the chromium
concentrations in the existing conditions scenario after 100 years. The sulfate concentrations
are not sensitive to longitudinal dispersivity changes (Figures 86 through 89) as a 25% increase
or decrease resulted in a change of 2% from the existing conditions after 100 years. The
direction in which sulfate concentrations changed was least correlated with the direct of change
in longitudinal dispersivity. At the selected downgradient wells, a 25% increase in longitudinal
dispersivity resulted in sulfate concentration changes as high as 8,220 µg/L, while a 25%
decrease resulted in sulfate concentration changes as high as 9,980 µg/L.
8 SUMMARY AND CONCLUSIONS
The study consisted of three main activities: 1) creation of a steady-state flow model that
represents the current understanding of site conditions (2015), 2) development of a transport
model that is qualitatively calibrated and can reasonably predict 2115 COI concentrations based
on a simplified representation of the ash basin/ash storage area sources and operational history
of these areas, and 3) perform predictive simulations of the closure scenario options. The
modeling results were used to compare the effectiveness of closure scenario options and to
predict the time required to achieve compliance with state groundwater standards.
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/July 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.
 MOFLOW 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 site.
20

 The model was calibrated by adjusting the constant source concentrations at the Ash
Basins and Ash Storage Area to reasonably match 2015 COI concentrations in
groundwater.
 For the purposes of numerical modeling and comparing closure scenarios, it is
assumed that the selected closure scenario will be completed in 2015.
 Predictive simulations were performed and steady-state flow conditions were assumed
from the time that the ash basins and ash storage area were placed in service through
the current time until the end of the predictive simulations (2265).
 COI source zone concentrations at the Ash Basins and Ash Storage Area were
assumed to be constant with respect to time for transport model calibration.
 The uncertainty in model parameters and predictions has not been quantified;
therefore the error in the model predictions is not known. It is assumed the model
results are suitable for a relative comparison of closure scenario options.
 Since the Catawba River is modeled as a constant head boundary in the numerical
model, it will not be possible to assess the affects 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 for monitoring wells located downgradient from the Ash Basin are
summarized as follows:
 For the existing conditions scenario, the model predicts that steady-state
concentrations of antimony will exceed the 2L standard at the compliance boundary in
2015 with the exception of both shallow and deep groundwater zones at MW-15.
Chromium will exceed the 2L standard in both shallow and deep groundwater zones
by 2075 and continue to increase through the modeling period; however, chromium will
not impact MW-15. Sulfate will not exceed the 2L standard at any time during the
modeling period.
 For the cap-in-place scenario, model predictions show that antimony will exceed the
2L standard at the compliance boundary from 2015 to 2070. Predicted chromium at
the site will remain within the basin footprint, although concentrations will increase in
GWA-2 in shallow and deep groundwater zones to above the 2L standard for
chromium between 2030 and 2090, and in MW-6 between 2095 and 2135. Chromium
will not impact MW-15. The model predicts that sulfate will not exceed the 2L standard
at the compliance boundary at any time during the modeling period.
 For the excavation scenario, the model predicts that antimony will decrease to reach
the 2L standard in monitoring wells at the compliance boundary in 2070. Chromium will
exceed the 2L standard in the shallow groundwater zone in 2020 and deep
groundwater zone in 2055 and remain above the 2L standard through the end of the
period modeled, with the exception of MW-15. Chromium will not be detectable in MW-
15 in any zone. Sulfate does not exceed the 2L standard at any time during the
modeling period.
21

 The model results suggest that the excavation scenario is sufficient for remediation of
antimony, sulfate and possibly other conservative COI in the groundwater at the
RBSS, but this closure scenario option will not be sufficient for chromium as the model
predictions indicate that the 2L will not be attainable within 250 years.
9 REFERENCES
Daniel, C.C., III, 2001, stimating 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.
Haven, W.T. Introduction to the North Carolina Groundwater Recharge Map-Groundwater
Circular Number 19, North Carolina Department of Environment and Natural Resources Division
of Water Quality, Groundwater Section.
HDR. Comprehensive Site Assessment Report, Riverbend River Steam Station Ash Basin,
August 2015.
LeGrand, H. E. 2004. A Master Conceptual Model for Hydrogeological Site Characterization in
the Piedmont and Mountain Region of North Carolina, A Guidance Manual, North Carolina
Department of Environment and Natural Resources Division of Water Quality, Groundwater
Section.
Niswonger, R.G., Panday, S., and Ibaraki, M. 2011. MODFLOW-NWT, A Newton formulation for
MODFLOW-2005: U.S. Geological Survey Techniques and Methods 6-A37, 44 p.
Zheng, C. and Bennett, G. 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.

TABLES

Table 1. Description of 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

Table 2. Hydraulic Conductivity in the Model

Model
Layers
Hydrostratigraphic
Unit
Measured
Value
Range1
Calibrated Model Value
Horizontal
Hydraulic
Conductivity
(feet/day)
Horizontal Hydraulic
Conductivity (feet/day)
Vertical Hydraulic
Conductivity (feet/day)
1 - 4 Ash 17.13-0.59 3.126 0.313
2 - 4 Dike 0.41-0.04 0.0028 0.0028

5 – 6

7

M1-Saprolite

M2-Saprolite
7.45-0.15

6.42-0.09
S-1 0.426 0.0426
S-2 0.092 0.0092
S-3 0.568 0.0568
S-4 0.034 0.0085
S-5 0.092 0.0092
S-6 0.092 0.0092
S-7 2.273 0.2273
S-8 0.568 0.0568
S-9 0.092 0.0092
S-10 0.284 0.0284
8 TZ 1.08-0.04 0.043 0.0085
9 – 10 BR 0.46-0.01 0.048 0.048
1Range = geometric mean +/- one standard deviation (see HDR Tables 11-7 to 11-11)

Table 3. Effective Porosity in the Model 

Model
Layer
Hydrogeologic
Unit Effective Porosity
1 - 3 Ash Material 0.1
4 - 6 Dike and Ash Storage
Material 0.1
5 M1 – Saprolite
and Alluvium 0.1
6 M1 – Saprolite
and Alluvium 0.1
7 M2 - Saprolite 0.1
8 Transition Zone 0.05
9 – 10 Bedrock 0.005

Table 4. Observed vs. Predicted Hydraulic Head (ft msl)

Well Name Model Layer Obs. Head
(ft msl)
Pred. Head
(ft msl)
Square Error
(S.E.) ft2
AB-3S 2 706.31 704.05 9.54
AB-4S 2 706.23 701.72 50.65
AB-5S 2 707.41 700.29 7.51
AB-1S 4 685.76 692.88 3.85
AB-5SL 4 706.78 700.02 5.09
AB-6S 4 686.48 698.07 75.31
AB-7S 4 714.18 717.57 20.35
C-1S 4 653.31 653.25 74.25
AB-8S 5 656.54 659.71 50.74
AS-1S 5 678.16 681.15 45.76
AS-2S 5 694.09 694.03 157.83
AS-3S 5 713.24 709.12 134.35
AS-3SA 5 713.01 709.02 16.82
C-2S 5 656.34 656.59 4.69
GWA-10S 5 643.83 645.15 11.50
GWA-22S 5 713.28 707.43 1.48
GWA-23S 5 705.93 703.92 10.04
GWA-4S 5 671.45 677.82 0.10
GWA-5S 5 710.28 714.32 8.95
GWA-6S 5 715.44 716.41 117.13
GWA-7S 5 670.07 673.13 0.00
Well Name Model Layer Obs. Head
(ft msl)
Pred. Head
(ft msl)
Square Error
(S.E.) ft2
GWA-8S 5 671.35 663.09 79.52
GWA-9S 5 648.23 652.00 16.96
AB-7I 6 713.43 711.26 15.91
AS-3D 6 712.53 703.61 0.00
GWA-1S 6 661.09 663.97 6.75
GWA-2S 6 645.12 648.14 0.06
GWA-3S 6 646.22 645.57 1.62
GWA-3SA 6 645.22 645.41 1.76
AB-2D 7 678.98 676.24 0.90
AB-7D 7 713.23 709.13 8.27
AB-8D 7 658.43 659.65 130.02
C-2D 7 653.46 656.06 34.23
GWA-22D 7 708.8 697.40 106.15
GWA-3D 7 645.55 645.44 135.23
GWA-4D 7 671.25 676.12 4.06
GWA-6D 7 714.75 712.36 10.29
GWA-7D 7 670.83 672.21 9.14
GWA-9D 7 653.77 652.20 0.01
AB-1D 8 666.34 669.43 0.43
AB-3D 8 700.26 698.30 0.04
AB-5D 8 677.04 685.66 23.73
AS-1D 8 675.37 675.68 40.54
AS-2D 8 695.48 684.66 16.33
GWA-23D 8 704.85 693.22 5.69
GWA-8D 8 671.07 662.27 0.93
MW-15D 8 643.72 644.61 2.67
MW-9D 8 647.92 646.94 1.90
AB-4D 9 703.00 694.32 9.36
AB-6BRU 9 663.6 676.16 77.51
GWA-10D 9 643.89 645.16 68.23
GWA-1D 9 662.06 661.11 2.48
GWA-23BR 9 703.22 692.92 14.22
GWA-7BR 9 670.98 672.61 0.31
MW-15BR 9 643.83 644.39 0.79
MW-7BR 9 711.72 705.74 35.80
MW-9BR 9 648.68 647.23 2.11
GWA-2BR 10 645.22 648.43 0.96
Maximum 715.44 Sum S.E.1670.81 ft2 
Minimum 643.72 Avg S.E.0.76 ft2 
Max - Min 71.72 Sqrt(Avg S.E)5.37ft2 

Table 5. Transport Model Calibration Results

COI Source Area Monitoring
Well
Measured
Concentration
(µg/L)
Predicted
Concentration
(µg/L)
Antimony
Ash Basin Primary Cell
and Ash Storage Area
Constant Concentration = 5 µg/L
AB-6S 1.7 4
GWA-2S 0.5 U 3
MW-9 0.5 U 3
MW-9D 2.6 3
AB-6BRU 0.29 J 2
GWA-2BR 0.25 J 3
MW-9BR 1.3 3
Ash Basin Secondary
Cell
Constant Concentration = 5 µg/L
AB-1S 2.8 5
AB-2S 3.4 4
GWA-1S 0.22 J 0
GWA-10S 0.5 U 3
AB-1D 1.5 4
AB-2D 0.85 4
GWA-1D 2.6 0
GWA-10D 0.5 U 4
Chromium
Ash Basin Primary Cell
and Ash Storage Area
Constant Concentration = 1,000 µg/L
GWA-2S 1 J+ 6
GWA-9S 0.36 J+ 0.7
MW-1S 0.5 UJ 15.4
GWA-9D 0.4 J+ 0.3
MW-1D 0.66 J+ 3
GWA-2BR 3.4 0
GWA-9BR NA 0
Ash Basin Secondary
Cell
Constant Concentration = 1,000 µg/L
MW-5S 1.6 0
MW-11SR 1.9 J 0
MW-13 0.4 J+ 0
MW-15 4.9 0
MW-5D 2.4 J+ 0
MW-11DR NA 0
MW-15D 3.5 0
MW-15BR 4.2 0
Sulfate Ash Basin Primary Cell
and Ash Storage Area
Ash Basin Primary Cell Constant Concentration
= 208,000 µg/L
Ash Storage Area Constant Concentration =
559,000 µg/L
GWA-2S 36,100 118,358
GWA-9S 26,500 71,758
MW-1S 1,200 J- 173,403
GWA-9D 33,200 72,947
COI Source Area Monitoring
Well
Measured
Concentration
(µg/L)
Predicted
Concentration
(µg/L)
MW-1D 86,100 164,261
GWA-2BR 39,500 111,227
GWA-9BR NA 75,644
Ash Basin Secondary
Cell
Constant Concentration = 88,900 µg/L
MW-5S 10,200 68,649
MW-11SR 37,600 1,440
MW-13 15,700 66,231
MW-15 29,000 5,642
MW-5D 44,600 68,581
MW-11DR 36,000 2,121
MW-15D 40,100 7,439
MW-15BR 39,500 40,615

Table 6. Predicted Advective Travel Time 

Groundwater
Zone Monitoring Well
Advective Travel
Time to Model
Boundary (years)1
Shallow
AB-1S 100
AB-3S 569
GWA-1S 218
GWA-2S 4
GWA-3S 3
GWA-8S 22
GWA-9S 12
GWA-10S 10
GWA-22S 441
Deep
AB-1D 95
AB-3D 538
GWA-1D 192
GWA-3D 165
GWA-8D 186
GWA-9D 14
GWA-10D 10
GWA-22D 662
MW-9D 617
MW-15D 48
Bedrock
GWA-2BR 209
GWA-9BR 124
MW-9BR 688
MW-15BR 155
1Computed travel time over 3-D flow path using flow terms from the groundwater flow model

FIGURES

Fi
g
u
r
e

1
.

C
o
n
c
e
p
t
u
a
l

G
r
o
u
n
d
w
a
t
e
r

F
l
o
w

M
o
d
e
l
/
M
o
d
e
l

D
o
m
a
i
n

Fi
g
u
r
e

2
.

M
o
d
e
l

D
o
m
a
i
n

No
r
t
h
-
S
o
u
t
h

C
r
o
s
s

S
e
c
t
i
o
n

(
A
-
A
’
)

T
h
ro
u
g
h

P
r
i
m
a
r
y

a
n
d

S
e
c
o
n
d
a
r
y

A
s
h

B
a
s
i
n
s

Fi
g
u
r
e

3
.

M
o
d
e
l

D
o
m
a
i
n

E
a
s
t
-
W
e
s
t
C
r
o
s
s

S
e
c
t
i
o
n

(
B
-
B
’
)

T
h
r
o
u
g
h

P
r
im
a
r
y

a
n
d

S
e
c
o
n
d
a
r
y

A
s
h

B
a
s
i
n
s

Fi
g
u
r
e

4
.

N
u
m
e
r
i
c
a
l

Mo
d
e
l

B
o
u
n
d
a
r
y

C
o
n
d
i
t
i
o
n
s

Fi
g
u
r
e

5
.

M
o
d
e
l

R
e
c
h
a
r
g
e

A
r
e
a
s

an
d

C
o
n
t
a
m
i
n
a
n
t

S
o
u
r
c
e

Z
o
n
e
s

(
C
o
n
s
t
a
n
t

C
o
n
c
e
n
t
r
a
t
i
o
n

C
e
l
l
s
)

Fi
g
u
r
e

6
.

O
b
s
e
r
v
a
t
i
o
n

W
e
l
l
s

i
n

S
h
a
l
l
o
w

G
r
o
u
n
d
w
a
t
e
r

Z
o
n
e

Fi
g
u
r
e

7
.

O
b
s
e
r
v
a
t
i
o
n

W
e
l
l
s

i
n

D
e
e
p

G
r
o
u
n
d
w
a
t
e
r

Z
o
n
e

Fi
g
u
r
e

8
.

O
b
s
e
r
v
a
t
i
o
n

W
e
l
l
s

i
n

B
e
d
r
o
c
k

G
r
o
u
n
d
w
a
t
e
r

Z
o
n
e

Fi
g
u
r
e

9
.

H
y
d
r
a
u
l
i
c

C
o
n
d
u
c
t
i
v
i
t
y

Z
o
n
a
t
i
o
n

i
n

S
/
M
1
/
M
2

L
a
y
e
r
s

(
M
o
d
e
l

L
a
y
e
r
s

5
-
7
)

Fi
g
u
r
e

1
0
.

M
o
d
e
l
e
d

H
y
d
r
a
u
l
i
c

H
e
a
d

(
f
e
e
t
)

v
s
.

O
b
s
e
r
v
e
d

H
y
d
r
a
u
l
i
c

H
e
a
d

(
f
e
e
t
)

Fi
g
u
r
e

1
1
.

H
y
d
r
a
u
l
i
c

H
e
a
d

(
f
e
e
t
)

i
n

M
2

S
a
p
r
o
l
i
t
e

L
a
y
e
r

(
M
o
d
e
l

L
a
y
e
r

7
)

Fi
g
u
r
e

1
2
.

H
y
d
r
a
u
l
i
c

H
e
a
d

(
f
e
e
t
)

i
n

N
o
r
t
h
-
S
o
u
t
h

C
r
o
s
s

S
e
c
t
i
o
n

(
C
-
C
’
)

t
h
r
o
u
g
h

P
r
i
m
a
r
y

a
n
d

S
e
c
o
n
d
a
r
y

A
s
h

B
a
s
i
n
s

Fi
g
u
r
e

1
3
.

H
y
d
r
a
u
l
i
c

H
e
a
d

(
f
e
e
t
)

i
n

E
a
s
t
-
W
e
s
t

C
r
o
s
s

Se
c
t
i
o
n

(
C
-
C
’
)

t
h
r
o
u
g
h

P
r
i
m
a
r
y

a
n
d

S
e
c
o
n
d
a
r
y

A
s
h

B
a
s
i
n
s

Fi
g
u
r
e

1
4
.

P
a
r
t
i
c
l
e

T
r
a
c
k
i
n
g

R
e
s
u
l
t
s

(
s
e
e

T
a
b
l
e
.

6

f
o
r

A
d
v
e
c
t
i
v
e

T
r
a
v
e
l

T
i
m
e
s
)

Fi
g
u
r
e

1
5
.

E
x
i
s
t
i
n
g

C
o
n
d
i
t
i
o
n
s

S
c
e
n
a
r
i
o

-

2
0
1
5

P
r
e
d
i
c
t
ed

A
n
t
i
m
o
n
y

(
µ
g
/
L
)

i
n

S
h
a
l
l
o
w

G
r
o
u
n
d
w
a
t
e
r

Z
o
n
e

Fi
g
u
r
e

1
6
.

E
x
i
s
t
i
n
g

C
o
n
d
i
t
i
o
n
s

S
c
e
n
a
r
i
o

-

2
0
1
5

P
r
e
d
i
ct
e
d

A
n
t
i
m
o
n
y

(
µ
g
/
L
)

i
n

D
e
e
p

G
r
o
u
n
d
w
a
t
e
r

Z
o
n
e

Fi
g
u
r
e

1
7
.

E
x
i
s
t
i
n
g

C
o
n
d
i
t
i
o
n
s

S
c
e
n
a
r
i
o

-

2
0
1
5

P
r
e
d
i
c
t
ed

A
n
t
i
m
o
n
y

(
µ
g
/
L
)

i
n

B
e
d
r
o
c
k

G
r
o
u
n
d
w
a
t
e
r

Z
o
n
e

Fi
g
u
r
e

1
8
.

E
x
i
s
t
i
n
g

C
o
n
d
i
t
i
o
n
s

S
c
e
n
a
r
i
o

-

2
1
1
5

P
r
e
d
i
c
t
ed

A
n
t
i
m
o
n
y

(
µ
g
/
L
)

i
n

S
h
a
l
l
o
w

G
r
o
u
n
d
w
a
t
e
r

Z
o
n
e

Fi
g
u
r
e

1
9
.

E
x
i
s
t
i
n
g

C
o
n
d
i
t
i
o
n
s

S
c
e
n
a
r
i
o

-

2
1
1
5

P
r
e
d
i
ct
e
d

A
n
t
i
m
o
n
y

(
µ
g
/
L
)

i
n

D
e
e
p

G
r
o
u
n
d
w
a
t
e
r

Z
o
n
e

Fi
g
u
r
e

2
0
.

E
x
i
s
t
i
n
g

C
o
n
d
i
t
i
o
n
s

S
c
e
n
a
r
i
o

-

2
1
1
5

P
r
e
d
i
c
t
ed

A
n
t
i
m
o
n
y

(
µ
g
/
L
)

i
n

B
e
d
r
o
c
k

G
r
o
u
n
d
w
a
t
e
r

Z
o
n
e

Figure 21. Existing Conditions Scenario – Predicted Antimony (µg/L) in Downgradient
Monitoring Wells Completed in Shallow, Deep and Bedrock Groundwater Zones

Fi
g
u
r
e

2
2
.

E
x
i
s
t
i
n
g

C
o
n
d
i
t
i
o
n
s

S
c
e
n
a
r
i
o

-

2
0
1
5

P
r
e
d
i
c
t
e
d

C
h
r
o
m
i
u
m

(
µ
g
/
L
)

i
n

S
h
a
l
l
o
w

G
r
o
u
n
d
w
a
t
e
r

Z
o
n
e

Fi
g
u
r
e

2
3
.

E
x
i
s
t
i
n
g

C
o
n
d
i
t
i
o
n
s

S
c
e
n
a
r
i
o

-

2
0
1
5

P
r
e
d
i
c
t
e
d

C
h
r
o
m
i
u
m

(
µ
g
/
L
)

i
n

D
e
e
p

G
r
o
u
n
d
w
a
t
e
r

Z
o
n
e

Fi
g
u
r
e

2
4
.

E
x
i
s
t
i
n
g

C
o
n
d
i
t
i
o
n
s

S
c
e
n
a
r
i
o

-

2
0
1
5

P
r
e
d
i
c
t
e
d

C
h
r
o
m
i
u
m

(
µ
g
/
L
)

i
n

B
e
d
r
o
c
k

G
r
o
u
n
d
w
a
t
e
r

Z
o
n
e

Fi
g
u
r
e

2
5
.

E
x
i
s
t
i
n
g

C
o
n
d
i
t
i
o
n
s

S
c
e
n
a
r
i
o

-

2
1
1
5

P
r
e
d
i
c
t
e
d

C
h
r
o
m
i
u
m

(
µ
g
/
L
)

i
n

S
h
a
l
l
o
w

G
r
o
u
n
d
w
a
t
e
r

Z
o
n
e

Fi
g
u
r
e

2
6
.

E
x
i
s
t
i
n
g

C
o
n
d
i
t
i
o
n
s

S
c
e
n
a
r
i
o

-

2
1
1
5

P
r
e
d
i
c
t
e
d

C
h
r
o
m
i
u
m

(
µ
g
/
L
)

i
n

D
e
e
p

G
r
o
u
n
d
w
a
t
e
r

Z
o
n
e

Fi
g
u
r
e

2
7
.

E
x
i
s
t
i
n
g

C
o
n
d
i
t
i
o
n
s

S
c
e
n
a
r
i
o

-

2
1
1
5

P
r
e
d
i
c
t
e
d

C
h
r
o
m
i
u
m

(
µ
g
/
L
)

i
n

B
e
d
r
o
c
k

G
r
o
u
n
d
w
a
t
e
r

Z
o
n
e

Figure 28. Existing Conditions Scenario – Predicted Chromium (µg/L) in Downgradient
Monitoring Wells Completed in Shallow, Deep and Bedrock Groundwater Zones

Fi
g
u
r
e

2
9
.

E
x
i
s
t
i
n
g

C
o
n
d
i
t
i
o
n
s

S
c
e
n
a
r
i
o

-

2
0
1
5

P
r
e
d
i
c
t
e
d

S
u
l
f
a
t
e

(
µ
g
/
L
)

i
n

S
h
a
l
l
o
w

G
r
o
u
n
d
w
a
t
e
r

Z
o
n
e

Fi
g
u
r
e

3
0
.

E
x
i
s
t
i
n
g

C
o
n
d
i
t
i
o
n
s

S
c
e
n
a
r
i
o

-

2
0
1
5

P
r
e
d
i
c
t
e
d

S
u
l
f
a
t
e

(
µ
g
/
L
)

i
n

D
e
e
p

G
r
o
u
n
d
w
a
t
e
r

Z
o
n
e

Fi
g
u
r
e

3
1
.

E
x
i
s
t
i
n
g

C
o
n
d
i
t
i
o
n
s

S
c
e
n
a
r
i
o

-

2
0
1
5

P
r
e
d
i
c
t
e
d

S
u
l
f
a
t
e

(
µ
g
/
L
)

i
n

B
e
d
r
o
c
k

G
r
o
u
n
d
w
a
t
e
r

Z
o
n
e

Fi
g
u
r
e

3
2
.

E
x
i
s
t
i
n
g

C
o
n
d
i
t
i
o
n
s

S
c
e
n
a
r
i
o

-

2
1
1
5

P
r
e
d
i
c
t
e
d

S
u
l
f
a
t
e

(
µ
g
/
L
)

i
n

S
h
a
l
l
o
w

G
r
o
u
n
d
w
a
t
e
r

Z
o
n
e

Fi
g
u
r
e

3
3
.

E
x
i
s
t
i
n
g

C
o
n
d
i
t
i
o
n
s

S
c
e
n
a
r
i
o

-

2
1
1
5

P
r
e
d
i
c
t
e
d

S
u
l
f
a
t
e

(
µ
g
/
L
)

i
n

D
e
e
p

G
r
o
u
n
d
w
a
t
e
r

Z
o
n
e

Fi
g
u
r
e

3
4
.

E
x
i
s
t
i
n
g

C
o
n
d
i
t
i
o
n
s

S
c
e
n
a
r
i
o

-

2
1
1
5

P
r
e
d
i
c
t
e
d

S
u
l
f
a
t
e

(
µ
g
/
L
)

i
n

B
e
d
r
o
c
k

G
r
o
u
n
d
w
a
t
e
r

Z
o
n
e

Figure 35. Existing Conditions Scenario – Predicted Sulfate (µg/L) in Downgradient Monitoring
Wells Completed in Shallow, Deep and Bedrock Groundwater Zones

Fi
g
u
r
e

3
6
.

C
a
p
-
i
n
-
P
l
a
c
e

S
c
e
n
a
r
i
o

-

2
0
1
5

P
r
e
d
i
c
t
e
d

A
n
t
i
m
o
n
y

(
µ
g
/
L
)

i
n

S
h
a
l
l
o
w

G
r
o
u
n
d
w
a
t
e
r

Z
o
n
e

Fi
g
u
r
e

3
7
.

C
a
p
-
i
n
-
P
l
a
c
e

S
c
e
n
a
r
i
o

-

2
0
1
5

P
r
e
d
i
c
t
ed

A
n
t
i
m
o
n
y

(
µ
g
/
L
)

i
n

D
e
e
p

G
r
o
u
n
d
w
a
t
e
r

Z
o
n
e

Fi
g
u
r
e

3
8
.

C
a
p
-
i
n
-
P
l
a
c
e

S
c
e
n
a
r
i
o

-

2
0
1
5

P
r
e
d
i
c
t
ed

A
n
t
i
m
o
n
y

(
µ
g
/
L
)

i
n

B
e
d
r
o
c
k

G
r
o
u
n
d
w
a
t
e
r

Z
o
n
e

Fi
g
u
r
e

3
9
.

C
a
p
-
i
n
-
P
l
a
c
e

S
c
e
n
a
r
i
o

-

2
1
1
5

P
r
e
d
i
c
t
e
d

A
n
t
i
m
o
n
y

(
µ
g
/
L
)

i
n

S
h
a
l
l
o
w

G
r
o
u
n
d
w
a
t
e
r

Z
o
n
e

Fi
g
u
r
e

4
0
.

C
a
p
-
i
n
-
P
l
a
c
e

S
c
e
n
a
r
i
o

-

2
1
1
5

P
r
e
d
i
c
t
ed

A
n
t
i
m
o
n
y

(
µ
g
/
L
)

i
n

D
e
e
p

G
r
o
u
n
d
w
a
t
e
r

Z
o
n
e

Fi
g
u
r
e

4
1
.

C
a
p
-
i
n
-
P
l
a
c
e

S
c
e
n
a
r
i
o

-

2
1
1
5

P
r
e
d
i
c
t
ed

A
n
t
i
m
o
n
y

(
µ
g
/
L
)

i
n

B
e
d
r
o
c
k

G
r
o
u
n
d
w
a
t
e
r

Z
o
n
e

Figure 42. Cap-in-Place Scenario – Predicted Antimony (µg/L) in Downgradient Monitoring
Wells Completed in Shallow, Deep and Bedrock Groundwater Zones

Fi
g
u
r
e

4
3
.

C
a
p
-
i
n
-
P
l
a
c
e

S
c
e
n
a
r
i
o

-

2
0
1
5

P
r
e
d
i
c
t
e
d

C
h
r
o
m
i
u
m

(
µ
g
/
L
)

i
n

S
h
a
l
l
o
w

G
r
o
u
n
d
w
a
t
e
r

Z
o
n
e

Fi
g
u
r
e

4
4
.

C
a
p
-
i
n
-
P
l
a
c
e

S
c
e
n
a
r
i
o

-

2
0
1
5

P
r
e
d
i
c
t
e
d

C
h
r
o
m
i
u
m

(
µ
g
/
L
)

i
n

D
e
e
p

G
r
o
u
n
d
w
a
t
e
r

Z
o
n
e

Fi
g
u
r
e

4
5
.

C
a
p
-
i
n
-
P
l
a
c
e

S
c
e
n
a
r
i
o

-

2
0
1
5

P
r
e
d
i
c
t
e
d

C
h
r
o
m
i
u
m

(
µ
g
/
L
)

i
n

B
e
d
r
o
c
k

G
r
o
u
n
d
w
a
t
e
r

Z
o
n
e

Fi
g
u
r
e

4
6
.

C
a
p
-
i
n
-
P
l
a
c
e

S
c
e
n
a
r
i
o

-

2
1
1
5

P
r
e
d
i
c
t
e
d

C
h
r
o
m
i
u
m

(
µ
g
/
L
)

i
n

S
h
a
l
l
o
w

G
r
o
u
n
d
w
a
t
e
r

Z
o
n
e

Fi
g
u
r
e

4
7
.

C
a
p
-
i
n
-
P
l
a
c
e

S
c
e
n
a
r
i
o

-

2
1
1
5

P
r
e
d
i
c
t
e
d

C
h
r
o
m
i
u
m

(
µ
g
/
L
)

i
n

D
e
e
p

G
r
o
u
n
d
w
a
t
e
r

Z
o
n
e

Fi
g
u
r
e

4
8
.

C
a
p
-
i
n
-
P
l
a
c
e

S
c
e
n
a
r
i
o

-

2
1
1
5

P
r
e
d
i
c
t
e
d

C
h
r
o
m
i
u
m

(
µ
g
/
L
)

i
n

B
e
d
r
o
c
k

G
r
o
u
n
d
w
a
t
e
r

Z
o
n
e

Figure 49. Cap-in-Place Scenario – Predicted Chromium (µg/L) in Downgradient Monitoring
Wells Completed in Shallow, Deep and Bedrock Groundwater Zones

Fi
g
u
r
e

5
0
.

C
a
p
-
i
n
-
P
l
a
c
e

S
c
e
n
a
r
i
o

-

2
0
1
5

P
r
e
d
i
c
t
e
d

S
u
l
f
a
t
e

(
µ
g
/
L
)

i
n

S
h
a
l
l
o
w

G
r
o
u
n
d
w
a
t
e
r

Z
o
n
e

Fi
g
u
r
e

5
1
.

C
a
p
-
i
n
-
P
l
a
c
e

S
c
e
n
a
r
i
o

-

2
0
1
5

P
r
e
d
i
c
t
e
d

S
u
l
f
a
t
e

(
µ
g
/
L
)

i
n

D
e
e
p

G
r
o
u
n
d
w
a
t
e
r

Z
o
n
e

Fi
g
u
r
e

5
2
.

C
a
p
-
i
n
-
P
l
a
c
e

S
c
e
n
a
r
i
o

-

2
0
1
5

P
r
e
d
i
c
t
e
d

S
u
l
f
a
t
e

(
µ
g
/
L
)

i
n

B
e
d
r
o
c
k

G
r
o
u
n
d
w
a
t
e
r

Z
o
n
e

Fi
g
u
r
e

5
3
.

C
a
p
-
i
n
-
P
l
a
c
e

S
c
e
n
a
r
i
o

-

2
1
1
5

P
r
e
d
i
c
t
e
d

S
u
l
f
a
t
e

(
µ
g
/
L
)

i
n

S
h
a
l
l
o
w

G
r
o
u
n
d
w
a
t
e
r

Z
o
n
e

Fi
g
u
r
e

5
4
.

C
a
p
-
i
n
-
P
l
a
c
e

S
c
e
n
a
r
i
o

-

2
1
1
5

P
r
e
d
i
c
t
e
d

S
u
l
f
a
t
e

(
µ
g
/
L
)

i
n

D
e
e
p

G
r
o
u
n
d
w
a
t
e
r

Z
o
n
e

Fi
g
u
r
e

5
5
.

C
a
p
-
i
n
-
P
l
a
c
e

S
c
e
n
a
r
i
o

-

2
1
1
5

P
r
e
d
i
c
t
e
d

S
u
l
f
a
t
e

(
µ
g
/
L
)

i
n

B
e
d
r
o
c
k

G
r
o
u
n
d
w
a
t
e
r

Z
o
n
e

Figure 56. Cap-in-Place Scenario – Predicted Sulfate (µg/L) in Downgradient Monitoring Wells
Completed in Shallow, Deep and Bedrock Groundwater Zones

Fi
g
u
r
e

5
7
.

E
x
c
a
v
a
t
i
o
n

S
c
e
n
a
r
i
o

-

2
0
1
5

P
r
e
d
i
c
t
e
d

A
n
t
i
m
o
n
y

(
µ
g
/
L
)

i
n

S
h
a
l
l
o
w

G
r
o
u
n
d
w
a
t
e
r

Z
o
n
e

Fi
g
u
r
e

5
8
.

E
x
c
a
v
a
t
i
o
n

S
c
e
n
a
r
i
o

-

2
0
1
5

P
r
e
d
i
c
t
e
d

A
n
t
i
m
o
n
y

(
µ
g
/
L
)

i
n

D
e
e
p

G
r
o
u
n
d
w
a
t
e
r

Z
o
n
e

Fi
g
u
r
e

5
9
.

E
x
c
a
v
a
t
i
o
n

S
c
e
n
a
r
i
o

-

2
0
1
5

P
r
e
d
i
c
t
e
d

A
n
t
i
m
o
n
y

(
µ
g
/
L
)

i
n

B
e
d
r
o
c
k

G
r
o
u
n
d
w
a
t
e
r

Z
o
n
e

Fi
g
u
r
e

6
0
.

E
x
c
a
v
a
t
i
o
n

S
c
e
n
a
r
i
o

-

2
1
1
5

P
r
e
d
i
c
t
e
d

A
n
t
i
m
o
n
y

(
µ
g
/
L
)

i
n

S
h
a
l
l
o
w

G
r
o
u
n
d
w
a
t
e
r

Z
o
n
e

Fi
g
u
r
e

6
1
.

E
x
c
a
v
a
t
i
o
n

S
c
e
n
a
r
i
o

-

2
1
1
5

P
r
e
d
i
c
t
e
d

A
n
t
i
m
o
n
y

(
µ
g
/
L
)

i
n

D
e
e
p

G
r
o
u
n
d
w
a
t
e
r

Z
o
n
e

Fi
g
u
r
e

6
2
.

E
x
c
a
v
a
t
i
o
n

S
c
e
n
a
r
i
o

-

2
1
1
5

P
r
e
d
i
c
t
e
d

A
n
t
i
m
o
n
y

(
µ
g
/
L
)

i
n

B
e
d
r
o
c
k

G
r
o
u
n
d
w
a
t
e
r

Z
o
n
e

Figure 63. Excavation Scenario – Predicted Antimony (µg/L) in Downgradient Monitoring Wells
Completed in Shallow, Deep and Bedrock Groundwater Zones

Fi
g
u
r
e

6
4
.

E
x
c
a
v
a
t
i
o
n

S
c
e
n
a
r
i
o

-

2
0
1
5

P
r
e
d
i
c
t
e
d

C
h
r
o
m
i
u
m

(
µ
g
/
L
)

i
n

S
h
a
l
l
o
w

G
r
o
u
n
d
w
a
t
e
r

Z
o
n
e

Fi
g
u
r
e

6
5
.

E
x
c
a
v
a
t
i
o
n

S
c
e
n
a
r
i
o

-

2
0
1
5

P
r
e
d
i
c
t
e
d

C
h
r
o
m
i
u
m

(
µ
g
/
L
)

i
n

D
e
e
p

G
r
o
u
n
d
w
a
t
e
r

Z
o
n
e

Fi
g
u
r
e

6
6
.

E
x
c
a
v
a
t
i
o
n

S
c
e
n
a
r
i
o

-

2
0
1
5

P
r
e
d
i
c
t
e
d

C
h
r
o
m
i
u
m

(
µ
g
/
L
)

i
n

B
e
d
r
o
c
k

G
r
o
u
n
d
w
a
t
e
r

Z
o
n
e

Fi
g
u
r
e

6
7
.

E
x
c
a
v
a
t
i
o
n

S
c
e
n
a
r
i
o

-

2
1
1
5

P
r
e
d
i
c
t
e
d

C
h
r
o
m
i
u
m

(
µ
g
/
L
)

i
n

S
h
a
l
l
o
w

G
r
o
u
n
d
w
a
t
e
r

Z
o
n
e

Fi
g
u
r
e

6
8
.

E
x
c
a
v
a
t
i
o
n

S
c
e
n
a
r
i
o

-

2
1
1
5

P
r
e
d
i
c
t
e
d

C
h
r
o
m
i
u
m

(
µ
g
/
L
)

i
n

D
e
e
p

G
r
o
u
n
d
w
a
t
e
r

Z
o
n
e

Fi
g
u
r
e

6
9
.

E
x
c
a
v
a
t
i
o
n

S
c
e
n
a
r
i
o

-

2
1
1
5

P
r
e
d
i
c
t
e
d

C
h
r
o
m
i
u
m

(
µ
g
/
L
)

i
n

B
e
d
r
o
c
k

G
r
o
u
n
d
w
a
t
e
r

Z
o
n
e

Figure 70. Excavation Scenario – Predicted Chromium (µg/L) in Downgradient Monitoring Wells
Completed in Shallow, Deep and Bedrock Groundwater Zones

Fi
g
u
r
e

7
1
.

E
x
c
a
v
a
t
i
o
n

S
c
e
n
a
r
i
o

-

2
0
1
5

P
r
e
d
i
c
t
e
d

S
u
l
f
a
t
e

(
µ
g
/
L
)

i
n

S
h
a
l
l
o
w

G
r
o
u
n
d
w
a
t
e
r

Z
o
n
e

Fi
g
u
r
e

7
2
.

E
x
c
a
v
a
t
i
o
n

S
c
e
n
a
r
i
o

-

2
0
1
5

P
r
e
d
i
c
t
e
d

S
u
l
f
a
t
e

(
µ
g
/
L
)

i
n

D
e
e
p

G
r
o
u
n
d
w
a
t
e
r

Z
o
n
e

Fi
g
u
r
e

7
3
.

E
x
c
a
v
a
t
i
o
n

S
c
e
n
a
r
i
o

-

2
0
1
5

P
r
e
d
i
c
t
e
d

S
u
l
f
a
t
e

(
µ
g
/
L
)

i
n

B
e
d
r
o
c
k

G
r
o
u
n
d
w
a
t
e
r

Z
o
n
e

Fi
g
u
r
e

7
4
.

E
x
c
a
v
a
t
i
o
n

S
c
e
n
a
r
i
o

-

2
1
1
5

P
r
e
d
i
c
t
e
d

S
u
l
f
a
t
e

(
µ
g
/
L
)

i
n

S
h
a
l
l
o
w

G
r
o
u
n
d
w
a
t
e
r

Z
o
n
e

Fi
g
u
r
e

7
5
.

E
x
c
a
v
a
t
i
o
n

S
c
e
n
a
r
i
o

-

2
1
1
5

P
r
e
d
i
c
t
e
d

S
u
l
f
a
t
e

(
µ
g
/
L
)

i
n

D
e
e
p

G
r
o
u
n
d
w
a
t
e
r

Z
o
n
e

Fi
g
u
r
e

7
6
.

E
x
c
a
v
a
t
i
o
n

S
c
e
n
a
r
i
o

-

2
1
1
5

P
r
e
d
i
c
t
e
d

S
u
l
f
a
t
e

(
µ
g
/
L
)

i
n

B
e
d
r
o
c
k

G
r
o
u
n
d
w
a
t
e
r

Z
o
n
e

Figure 77. Excavation Scenario – Predicted Sulfate (µg/L) in Downgradient Monitoring Wells
Completed in Shallow, Deep and Bedrock Groundwater Zones

Figure 78. Antimony Sensitivity – Predicted Variation in Antimony (µg/L) in well GWA-2S due to
Increased and Decreased Transport Parameters, Longitudinal Dispersivity and
Effective Porosity

Figure 79. Antimony Sensitivity – Predicted Variation in Antimony (µg/L) in well GWA-2BR due to
Increased and Decreased Transport Parameters, Longitudinal Dispersivity and
Effective Porosity

Figure 80. Antimony Sensitivity – Predicted Variation in Antimony (µg/L) in well GWA-9S due to
Increased and Decreased Transport Parameters, Longitudinal Dispersivity and
Effective Porosity

Figure 81. Antimony Sensitivity – Predicted Variation in Antimony (µg/L) in well GWA-6D due to
Increased and Decreased Transport Parameters, Longitudinal Dispersivity and
Effective Porosity

Figure 82. Antimony Sensitivity – Predicted Variation in Antimony (µg/L) in well GWA-9BR due to
Increased and Decreased Transport Parameters, Longitudinal Dispersivity and
Effective Porosity

Figure 83. Chromium Sensitivity – Predicted Variation in Chromium (µg/L) in well GWA-2S due
to Increased and Decreased Transport Parameters, Longitudinal Dispersivity and
Effective Porosity

Figure 84. Chromium Sensitivity – Predicted Variation in Chromium (µg/L) in well GWA-9S due
to Increased and Decreased Transport Parameters, Longitudinal Dispersivity and
Effective Porosity

Figure 85. Chromium Sensitivity – Predicted Variation in Chromium (µg/L) in well GWA-9D due
to Increased and Decreased Transport Parameters, Longitudinal Dispersivity and
Effective Porosity

Figure 86. Sulfate Sensitivity – Predicted Variation in Sulfate (µg/L) in well GWA-2S due to
Increased and Decreased Transport Parameters, Longitudinal Dispersivity and
Effective Porosity

Figure 87. Sulfate Sensitivity – Predicted Variation in Sulfate (µg/L) in well GWA-2BR due to
Increased and Decreased Transport Parameters, Longitudinal Dispersivity and
Effective Porosity

Figure 88. Sulfate Sensitivity – Predicted Variation in Sulfate (µg/L) in well GWA-9D due to
Increased and Decreased Transport Parameters, Longitudinal Dispersivity and
Effective Porosity

Figure 89. Sulfate Sensitivity – Predicted Variation in Sulfate (µg/L) in well GWA-9BR due to
Increased and Decreased Transport Parameters, Longitudinal Dispersivity and
Effective Porosity