NC0003468_DRSS CAP Part I_Report_20151112

F)�
Corrective Action Plan Part I
Dan River Steam Station Ash Basin
Site Location:
Groundwater Incident No.:
NPDES Permit No.
Permittee and Current
Property Owner:
Consultant Information:
Latitude and Longitude of
Facility:
Report Date:
Dan River Steam Station
900 South Edgewood Road
Eden, NC 27288
Not Assigned
NC0003468
Duke Energy Carolinas, LLC
526 South Church St
Charlotte, NC 28202
704.382.3853
HDR Engineering, Inc. of the Carolinas
440 South Church St, Suite 900
Charlotte, NC 28202
704.338.6700
36° 29' 32" N, 79° 42' 59" W
November 12, 2015

Corrective Action Plan Part I
Dan River Steam Station Ash Basin

i
Contents
Executive Summary ...................................................................................................................................... 1
ES-1 Introduction ................................................................................................................................ 1
ES-2 Background Concentrations and Regulatory Exceedances ...................................................... 3
ES-3 Site Conceptual Model .............................................................................................................. 7
ES-4 Modeling .................................................................................................................................. 10
ES-5 Summary and Recommendations ........................................................................................... 12
1 Introduction ........................................................................................................................................ 15
1.1 Site History and Overview ....................................................................................................... 16
1.1.1 Site Location, Acreage, and Ownership ..................................................................... 16
1.1.2 Site Description .......................................................................................................... 16
1.2 Permitted Activities and Permitted Waste ............................................................................... 17
1.3 History of Site Groundwater Monitoring .................................................................................. 17
1.4 Summary of Comprehensive Site Assessment ....................................................................... 17
1.5 Receptor Survey ...................................................................................................................... 18
1.5.1 Surrounding Land Use ............................................................................................... 18
1.5.2 Findings of Drinking Water Supply Well Survey Conducted per the Coal Ash
Management Act of 2014, N.C. Gen. Stat. SS130A-309-200 et seq. ........................ 18
1.6 Summary of Screening Level Risk Assessment ..................................................................... 19
1.7 Geological/Hydrogeological Conditions .................................................................................. 19
1.8 Results of the CSA Investigations ........................................................................................... 20
1.9 Regulatory Requirements........................................................................................................ 20
1.9.1 CAMA Requirements.................................................................................................. 20
1.9.2 Standards for Site Media ............................................................................................ 22
2 Background Concentrations and Regulatory Exceedances .............................................................. 23
2.1 Purpose ................................................................................................................................... 23
2.2 Groundwater ............................................................................................................................ 24
2.2.1 Background Wells and Concentrations ...................................................................... 24
2.2.2 Porewater Exceedances of 2L or IMACs ................................................................... 27
2.2.3 Groundwater Exceedances of 2L Standards or IMACs ............................................. 28
2.2.4 Radionuclides in Groundwater ................................................................................... 30
2.3 Seeps ...................................................................................................................................... 31
2.4 Surface Water ......................................................................................................................... 32
2.5 Sediments ............................................................................................................................... 33
2.6 Soils ......................................................................................................................................... 34
2.6.1 Background Soil and Concentrations ......................................................................... 34
2.6.2 Soil Exceedances of NC PSRGs for POGs ............................................................... 35
2.7 PWR and Bedrock ................................................................................................................... 37
2.8 COI Screening Evaluation Summary ...................................................................................... 37
2.9 Interim Response Actions ....................................................................................................... 38
2.9.1 Source Control ........................................................................................................... 38
2.9.2 Groundwater Response Actions ................................................................................ 39
3 Site Conceptual Model ...................................................................................................................... 40
3.1 Site Hydrogeologic Conditions ................................................................................................ 40
Corrective Action Plan Part I
Dan River Steam Station Ash Basin

ii
3.1.1 Hydrostratigraphic Units ............................................................................................. 40
3.1.2 Hydrostratigraphic Unit Properties ............................................................................. 41
3.1.3 Potentiometric Surface – Shallow and Deep Flow Layers ......................................... 41
3.1.4 Potentiometric Surface – Bedrock Flow Layer ........................................................... 41
3.1.5 Horizontal and Vertical Hydraulic Gradients .............................................................. 42
3.2 Site Geochemical Conditions .................................................................................................. 44
3.2.1 COI Sources and Mobility in Groundwater ................................................................. 44
3.2.2 Geochemical Characteristics ..................................................................................... 51
3.2.3 Source Area Geochemical Conditions ....................................................................... 53
3.2.4 Mineralogical Characteristics ..................................................................................... 53
3.3 Correlation of Hydrogeologic and Geochemical Conditions to COI Distribution ..................... 54
4 Modeling ............................................................................................................................................ 55
4.1 Groundwater Modeling ............................................................................................................ 55
4.1.1 Model Scenarios ......................................................................................................... 55
4.1.2 Calibration of Models.................................................................................................. 56
4.1.3 Kd Terms ..................................................................................................................... 56
4.1.4 Flow and COI Transport Model Sensitivity Analysis .................................................. 57
4.1.5 Flow Model ................................................................................................................. 57
4.1.6 Fate and Transport Model .......................................................................................... 58
4.2 Groundwater - Surface Water Interaction Modeling ................................................................ 63
4.2.1 Mixing Model Approach .............................................................................................. 63
4.2.2 Surface Water Model Results .................................................................................... 64
4.3 Refinement of SCM ................................................................................................................. 65
5 Summary and Recommendations ..................................................................................................... 66
6 References ........................................................................................................................................ 68

Corrective Action Plan Part I
Dan River Steam Station Ash Basin

iii
Tables
2-1 Initial COI Screening Evaluation
2-2 Background Groundwater Concentrations for the DRSS Site: Ranges of Samples with
Turbidity <10 NTU
2-3 Ash Basin Porewater Results for COIs Compared to 2L Standards or IMACs and
Frequency of Exceedances
2-4 Groundwater Results for COIs Compared to PPBCs, 2L Standards, or IMACs, and
Frequency of Exceedances
2-5 Radionuclide Concentrations
2-6A NCDEQ Seep Results for COIs Compared to 2B Standards or USEPA Criteria and
Frequency of Exceedances
2-6B Seep Results for COIs Compared to 2L Standards or IMACs and Frequency of
Exceedances
2-7 Surface Water Results for COIs Compared to Upgradient Surface Water Concentrations,
2B or USEPA Standards, and Frequency of Exceedances
2-8 Sediment COIs Compared to NC PSRGs for POG and Frequency of Exceedances
2-9 Proposed Provisional Background Soil Concentrations
2-10 Soil Results for COIs Compared to NC PSRGs for POG, Background Concentrations,
and Frequency of Exceedances
2-11 Updated COI Screening Evaluation Summary
3-1 Vertical Gradients – Shallow and Deep Well Pairs
3-2 Vertical Gradients – Deep and Bedrock Pairs
4-1 Mixing Zone Sizes and Percentages of Upstream River Flows
4-2 Unnamed East Tributary Calculated Surface Water Concentrations
4-3 Dan River Calculated Surface Water Concentrations

Figures
1-1 Site Location Map
1-2 Site Layout Map
1-3 Compliance and Voluntary Monitoring Wells
1-4 Monitoring Well and Sampling Location
1-5 Seep and Surface Water Sample Locations
1-6 Receptor Survey Map
1-7 Site Vicinity Map
2-1 Groundwater Analytical Results Map
2-2 Groundwater, Surface Water and Seep Analytical Results
2-3 Soil and Sediment Analytical Results
3-1 Site Conceptual Model – 3-D View
3-2 Site Conceptual Model – Cross Section A-A’
3-3 Water Table Surface Map – “S” Wells
3-4 Potentiometric Surface Map – “D” Wells
3-5 Potentiometric Surface Map – “BR” Wells
3-6 Vertical Gradients Map – “S” and “D” Wells
3-7 Vertical Gradients Map – “D” and “BR” Wells
Corrective Action Plan Part I
Dan River Steam Station Ash Basin

iv
Appendices
A Regulatory Correspondence
B Background Well Analysis
C UNCC Groundwater Flow and Transport Model
D UNCC Soil Sorption Evaluation
E Surface Water Modeling Methods

Acronyms and Abbreviations
µg/L micrograms per liter
2B Standards North Carolina Surface Water Quality Standards
2L Standards NCAC Title 15A, Subchapter 2L.0202
BG background
bgs below ground surface
CAMA North Carolina Coal Ash Management Act of 2014
CAP Corrective Action Plan
CCR Coal Combustion Residuals
cfs cubic feet per second
COI Constituent of Interest
COPC Contaminant of Potential Concern
CSA Comprehensive Site Assessment
DRCCS Dan River Combined Cycle Station
DRSS Dan River Steam Station
DWR NCDEQ Division of Water Resources
ft foot / feet
IMAC Interim Maximum Allowable Concentration
mg/kg milligrams per kilogram
MW megawatt
NC PSRGs North Carolina Preliminary Soil Remediation Goals
NCAC North Carolina Administrative Code
NCDENR North Carolina Department of Environment and Natural Resources
NCDEQ North Carolina Department of Environmental Quality
NPDES National Pollutant Discharge Elimination System
POG Protection of Groundwater
PPBC Proposed Provisional Background Concentration
SCM Site Conceptual Model
SU Standard Unit
TDS total dissolved solids
TZ transition zone
UNCC University of North Carolina at Charlotte
USEPA U.S. Environmental Protection Agency
Work Plan Groundwater Assessment Work Plan
Corrective Action Plan Part I
Dan River Steam Station Ash Basin

1
Executive Summary
ES-1 Introduction
ES-1.1 Regulatory Background
Duke Energy Carolinas, LLC (Duke Energy) owns and formerly operated the Dan River Steam
Station (DRSS), located on the Dan River in Rockingham County near Eden, North Carolina.
DRSS began operation as a coal-fired generating station in 1949 and was retired from service in
2012. The Dan River Combined Cycle Station (DRCCS) natural gas generating facility was
constructed at the site and began operations in 2012. Historically, coal ash residue from
DRSS’s coal combustion process was disposed of in an ash basin located northeast of the
station and adjacent to the Dan River. Discharge from the ash basin is currently permitted by
the North Carolina Department of Environmental Quality (NCDEQ)1 Division of Water
Resources (DWR) under the National Pollutant Discharge Elimination System (NPDES) Permit
NC0003468.
The North Carolina Coal Ash Management Act of 2014 (CAMA) directs owners of coal
combustion residuals (CCR) surface impoundments in North Carolina to conduct groundwater
monitoring, assessment, and remedial activities, if necessary. A groundwater assessment work
plan (Work Plan) for DRSS was submitted to NCDENR on December 31, 2014 and approved on
February 16, 2015. A Comprehensive Site Assessment (CSA) was performed to collect
information necessary to determine horizontal and vertical extent of impacts to soil and
groundwater attributable to CCR source area(s), identify potential receptors, and screen for
potential risks to those receptors. The DRSS CSA report was submitted to NCDENR on August
14, 2015 (HDR 2015).
CAMA also requires the submittal of a Corrective Action Plan (CAP) for each regulated facility
no later than 180 days after submittal of the CSA. Duke Energy and NCDEQ mutually agreed to
a two-part CAP submittal, with Part I being submitted within 90 days of submittal of the CSA and
Part II being submitted no later than 180 days after submittal of the CSA.
The purpose of CAP Part I is to provide background information, a brief summary of the CSA
findings, a brief description of the site geology and hydrogeology, a summary of the previously
completed receptor survey, a detailed description of the site conceptual model, and results of
the groundwater flow and transport model and groundwater to surface water interaction model.
The CAP Part II will include alternative methods for achieving groundwater quality restoration,
conceptual plans for recommended corrective actions, implementation schedule and a plan for
future monitoring and reporting. The risk assessment will be submitted under a separate cover
with the CAP Part II submittal.

1 Prior to September 18, 2015, the NCDEQ was referred to as the North Carolina Department of Environment and
Natural Resources (NCDENR). Both naming conventions are used in this report, as appropriate.
Corrective Action Plan Part I
Dan River Steam Station Ash Basin

2
ES-1.2 Summary of CSA
Based on the CSA findings, the source and cause of certain exceedances in areas of the DRSS
site is the coal ash contained in the ash basin and ash storage areas. The cause of these
exceedances is leaching of constituents from the coal ash into the underlying soil and
groundwater. However, certain groundwater, surface water, and soil standards were also
exceeded due to naturally occurring constituents found in the subsurface as discussed in
section 2.0.

If a constituent concentration exceeded the North Carolina Groundwater Quality Standards, as
specified in T15A NCAC .0202L (2L Standards) or Interim Maximum Allowable Concentration
(IMAC)2, NC PSRGs of POG, or 2B Standards or EPA Criteria, it has been designated as a
“Constituent of Interest” (COI).

 The CSA found exceedances in soil for the following constituents: iron, manganese,
arsenic, chromium, and selenium.
 The CSA found exceedances in groundwater for the following constituents: antimony,
arsenic, boron, chromium, cobalt, iron, manganese, sulfate, thallium, TDS, and
vanadium.
 The CSA found exceedances in surface water for the following constituents: aluminum,
arsenic, copper, lead and vanadium

15A NCAC 02L .0106(g)(2) requires the site assessment to identify any imminent hazards to
public health and safety and the actions taken to mitigate them in accordance with Paragraph (f)
of .0106(g). The CSA found no imminent hazards to public health and safety; therefore, no
actions to mitigate imminent hazards are required. However, corrective action at the DRSS site
is required to address soil and groundwater contamination resulting from the source areas. In
addition, a plan for continued monitoring of select monitoring wells and parameters/constituents
will be implemented following NCDEQ’s approval.

ES-1.3 Receptor Survey
Properties located within a 0.5-mile (2,640-foot) radius of the DRSS ash basin compliance
boundary are located in and southeast of Eden, in Rockingham County, North Carolina. The
majority of the land is undeveloped property and land use is typical of rural property. Residential
properties are located north and northwest of the ash basin compliance boundary within the 0.5-
mile radius.
Three water supply wells and one spring were identified within the 0.5-mile radius of the DRSS
compliance boundary during the receptor survey. Information evaluated as part of the CSA
indicated that the identified water supply wells and spring would not be impacted as they are
either hydraulically upgradient or across the Dan River from the ash basin.

2 Appendix #1 of 15A NCAC Subchapter 02L Classifications and Water Quality Standards Applicable to The
Groundwaters of North Carolina, lists Interim Maximum Allowable Concentrations (IMACs). The IMACs were issued
in 2010 and 2011; however, NCDENR has not established a 2L Standard for these constituents as described in 15A
NCAC 02L.0202(c). For this reason, IMACs noted in this report are for reference only.
Corrective Action Plan Part I
Dan River Steam Station Ash Basin

3
ES-1.4 Summary of Screening Level Risk Assessment
A screening level human health and ecological risk assessment was performed as a component
of the CSA Report (HDR 2015). Each screening level risk assessment identified the exposure
media for human and ecological receptors. Human health and ecological exposure media
includes potentially impacted groundwater, soil, surface water, and sediments.
The human health exposure routes associated with the evaluated pathways for the site include
ingestion, inhalation, and dermal contact of environmental media. Potential human receptors
under a current or hypothetical future use include construction/outdoor workers, off-site
residents, recreational users and trespassers. The ecological exposure routes associated with
the evaluated pathways for the site include dermal contact/root absorption/gill uptake and
ingestion of environmental media. Potential ecological receptors under a current or hypothetical
future use include aquatic, riparian, and terrestrial biota.
The screening level risk assessment will continue to be refined consistent with risk assessment
protocol in parallel with the CAP Part II schedule.
ES-2 Background Concentrations and Regulatory
Exceedances
HDR compared background and source area concentrations to the relevant regulatory
standards (for each medium) to determine site-specific background concentrations for each
COI. During the CSA, the source areas were defined as the ash basin (Primary and Secondary
Cells) and the ash storage areas. Source characterization was performed to identify physical
and chemical properties of ash, ash basin surface water, porewater, and ash basin seeps. The
analytical results for source characterization samples were compared to 2L Standards or
IMACs, NC PSRGs of POG, and 2B Standards or EPA criteria for the purpose of identifying
COIs that may be associated with potential impacts to soil, groundwater, sediment, and surface
water from the source areas.

COIs were also found to be naturally occurring in groundwater samples collected at background
and upgradient monitoring wells and in soil samples collected from locations that were not
impacted by ash. Examples of naturally occurring COIs include cobalt, iron, lead, manganese,
thallium, TDS, and vanadium. The occurrence of these COIs in area potentially impacted by ash
require careful examination to determine whether their presence in source areas is naturally
occurring or can be attributed to ash handling and storage.

ES-2.1 Proposed Provisional Background Groundwater Concentrations
Because COIs can be both naturally occurring and related to the source areas, the choice of
monitoring wells used to establish background concentrations is important in determining
whether releases have occurred from the source areas. The determination of whether or not a
monitoring well is a suitable background well is based on the following:

Corrective Action Plan Part I
Dan River Steam Station Ash Basin

4
 The topographic location of the well with respect to the source areas (distance from
source areas and located hydraulically upgradient of source areas)
 Stratigraphic unit being monitored
 Screened intervals of well relative to source water elevation
 Direction of groundwater flow in the region of the well relative to source areas

Proposed Provisional Background Concentrations (PPBCs) for groundwater were calculated
utilizing the background concentrations limited to samples collected from wells with turbidity less
than 10 NTU. PPBCs represent either the highest detected value or the highest laboratory
reporting limit as observed in the background monitoring well. Background concentrations
reported for newly installed CSA background wells were not utilized for this statistical analysis
due to turbidity values greater than 10 NTU’s in many cases. Well development and sampling
will continue and these concentrations will be incorporated into statistical background analysis
once a sufficiently robust data set with turbidity less than 10 NTU’s has been obtained. A
detailed analysis of DRSS background groundwater concentrations are provided in Appendix
B.
Note that COIs identified in the CSA were based on one sampling event and that the PPBCs
presented below are provisional values. If PPBC is greater than the 2L Standard or IMAC, the
PPBC will be used for comparison, conversely, if the PPBC is less than the 2L Standard or
IMAC, the 2L Standard or IMAC will be used. The PPBCs will be updated as more data
becomes available with input from NCDEQ. Groundwater PPBCs for the DRSS site are
presented in the table below.
Proposed Provisional Background Groundwater Concentrations
Constituent
Proposed Provisional Background
Groundwater Concentrations (µg/L)
(Turbidity <10 NTU)
Antimony 1
Arsenic 1
Beryllium 0.2
Boron 50
Chromium 5
Cobalt 0.5
Hexavalent Chromium 0.09
Iron 10
Lead 10
Manganese 63
pH 6.65 to 6.78 SU
Radium-226 and Radium-228 (combined) 2.357
Selenium 1
Sulfate 31,000
Thallium 0.2
TDS 280,000
Vanadium 1
Notes:
1. µg/L = micrograms per liter
2. SU = Standard Units
Corrective Action Plan Part I
Dan River Steam Station Ash Basin

5
ES-2.2 Proposed Provisional Background Soil Concentrations
Because some constituents are naturally occurring in soil and are present in the source areas,
establishing background concentrations is important for determining whether releases have
occurred from the source areas.
Soil proposed provisional background concentrations were calculated for those constituents
analyzed in background soil borings, as shown in below. The methodology followed ProUCL
Technical Guidance, Statistical Software for Environmental Applications for Data Sets with and
without Nondetect Observations (USEPA 2013). The following table provides the 95% upper
tolerance limit (UTL) calculated for these constituents. These 95% UTL values are the PPBCs
for site soils.
Proposed Provisional Background Soil Concentrations
Constituent Site Range (mg/kg)
Proposed Provisional
Background Soil Concentrations
(95% UTL) (mg/kg)
Aluminum 7,360 to 38,900 40,400
Antimony <5.1 to <7.6 NS
Arsenic 3.1 to 30.6 23
Barium 34.2 to 242 281
Beryllium 0.59 to 3.9 3.33
Boron <13.4 to 63.1 63.9
Cadmium 0.44 to <0.91 NS
Calcium 77.4 to 39,100 118,100
Chloride 168 to <372 NS
Chromium 6.4 to 187 144
Cobalt 7.5 to 42.6 36
Copper 2.8 to 79.5 79.5
Iron 12,200 to 95,900 88,550
Lead 8.2 to 31.3 29.9
Magnesium 924 to 19,400 38,120
Manganese 82.7 to 5,170 4,150
Mercury 0.0047 to 0.04 0.041
Molybdenum 1.6 to 22.6 13.5
Nickel 6.7 to 52.2 59.7
Nitrate <25.8 to <37.2 NS
pH (field) 4.5 to 8.6 SU NS
Potassium 168 to 2,730 3,060
Selenium 3.9 to 7.8 NS
Sodium 160 to <378 2190
Strontium 1.9 to 257 569
Sulfate 145 to <372 NS
Thallium <5.1 to <7.6 NS
TOC 457 to 23,800 50,150
Vanadium 8.4 to 75.7 78.3
Zinc 23.5 to 203 217
Notes:
1. NS = no statistic calculated due to too few detections
2. mg/kg = milligrams per kilogram; SU = Standard Units
3. UTL – Upper Tolerance Limit

Corrective Action Plan Part I
Dan River Steam Station Ash Basin

6
ES-2.3 Updated COI Screening Evaluation Summary
The table below summarizes COIs (by geomedium) and identifies those that require further
evaluation to determine if they require possible corrective action. The COIs were based on 2L
Standards, IMAC, and 2B Standards for respective aqueous media and PSRGs for POG of
solid/soil like media. Further evaluation will be performed (as applicable) by developing 3-D
groundwater model for these COIs in Section 4.  
Updated COI Screening Evaluation Summary
Potential
COI COIs by Media COI To Be
Further
Assessed
in
Section 4
Note Solid/
Aqueous Ash Pore
Water
Ground
-water
Surface
Water Seeps Sediment Soil PWR/
Bedrock
Aluminum √ - - √ - - - - √
Antimony - - √ - - - - - √
Arsenic √ √ √ √ √ √ √ √
Barium - - - - - - - √ √
Boron - - √ - - - - - √
Chromium - √ √ √ - - √ - √
Cobalt √ √ √ - √ - √ √ √
Copper - - - - - - - - - 1
Iron √ √ √ √ - √
Lead - - - - - - - - - 1
Manganese √ √ √ - √ - √ - √
pH - - √ - - - - - √
Selenium - - √ - √ - - - √
Sulfate - - √ - - - - - √
Thallium - √ - - - - - - - 2
TDS √ - √ - - - - - √
Vanadium - - √ - √ - √ - √
Notes:
1. Only identified in SW samples, but was not included as COI due to comparison to upgradient sample locations.
2. Identified as COI in groundwater, but was not included due to no exceedance of PPBCs developed for the site.

ES-2.4 Interim Response Actions
In conjunction with decommissioning activities and in accordance with CAMA requirements,
Duke Energy will permanently close the DRSS ash basin by August 1, 2019. Closure of the
DRSS ash basin was defined in CAMA as excavation of ash from the site, and beneficial reuse
of the material or relocation to a lined structural fill or landfill. As part of the DRSS ash basin
closure process, Duke Energy submitted a coal ash excavation plan to state regulators in
November 2014. The excavation plan details a multiphase approach for removing coal ash from
the site with an emphasis on the first 12 to 18 months of activities. During Phase I of the
excavation, an estimated 1.2 million tons of material will be excavated from the Primary and
Secondary Cells and/or Ash Storage Areas. This material is planned to be taken to the
Maplewood (Amelia) Landfill in Jetersville, Virginia. Duke Energy is currently permitting an on-
site landfill that will be located in the vicinity of Ash Storage 1. The landfill will be lined and
Corrective Action Plan Part I
Dan River Steam Station Ash Basin

7
construction is projected to be completed by June 2017. The permit-to-construct application was
submitted to the NCDEQ Division of Waste Management in the third quarter of 2015. The
construction schedule will depend upon receipt of the required permits.
Duke Energy will begin moving coal ash within 60 days after receiving necessary permits and
approvals. Dewatering of the ash basins will begin along with project planning for later phases
to identify storage options for the remaining ash on the plant property. Ash impoundments will
be closed by August 1, 2019.
Based on the results of the CSA impacted groundwater has not migrated beyond the Duke
Energy property boundary north, west, and south of the DRSS site. East of the ash basin,
impacted groundwater has migrated to a property recently acquired by Duke Energy. Iron,
manganese, sulfate, TDS, and vanadium were detected above their respective 2L Standards or
IMACs in the groundwater sample collected from well GWA-15D, which is located on the
recently acquired property. These exceedances are being further evaluated as part of ongoing
groundwater assessment at the DRSS site. The results of surface water sampling indicate no
impacts from groundwater flow into the Dan River.
ES-3 Site Conceptual Model
The purpose of the SCM is to evaluate areal distribution of COIs with regard to site-specific
geological/ hydrogeological and geochemical properties at the DRSS site. The SCM was
developed utilizing data and analysis from the CSA (HDR 2015).
ES-3.1 Geological/Hydrogeological Properties
Seven hydrostratigraphic units were identified as part of the CSA:
 Ash (A)
 Fill (F)
 Alluvium (S)
 Residuum (M1)
 Saprolite/Weathered Rock (M1/M2)
 Partially Weathered/Fractured Rock (TZ)
 Bedrock (BR)
These units are part of the regolith-fractured rock system, which is characterized as an
unconfined, connected aquifer system. The groundwater system is divided into three flow layers
within the connected aquifer system: shallow, deep, and bedrock. In general, groundwater flow
for all three flow layers is from the northern extent of the DRSS property boundary to the
south/southeast toward the Dan River, with the exception of localized mounding around Ash
Storage 1 in the shallow flow layer.
Horizontal hydraulic gradients were derived for the shallow, deep, and bedrock flow layers by
calculating the difference in hydraulic heads over the length of the flow path between two wells
with similar well construction (e.g., both wells having 15-foot screens within the same water-
bearing unit). Applying this methodology to wells installed during the CSA yields the following
average horizontal hydraulic gradients (measured in feet/foot):
Corrective Action Plan Part I
Dan River Steam Station Ash Basin

8
 Shallow – 0.030 feet/foot
 Deep – 0.029 feet/foot
 Bedrock – 0.037 feet/foot

Vertical hydraulic gradients were calculated for twenty well pair locations by taking the
difference in groundwater elevation in a deep and shallow well pair over the difference in well
depth measured at the midpoint of the deep and shallow well pair screens. Based on review of
the results, the vertical gradients of groundwater are generally downward across the site.
Sixteen of the 20 well pairs exhibited a downward gradient ranging from -0.002 foot/foot to -
1.864 foot/foot.
Comparison of vertical gradients between shallow and deep flow layers:

 Potential downward gradient is generally exhibited across the site within the shallow and
deep flow layers with the exceptions detailed below.
 Potential upward gradient was observed on the northern portion of the site north of Ash
Storage 1, which is influencing groundwater flow on this portion of the site.
 Potential upward gradient was also observed immediately downgradient of the
secondary cell of the ash basin and west of the unnamed tributary to the east. This
location exhibits potential upward flow due to hydraulic influences of the ash basin and
the unnamed tributary.

Comparison of vertical gradients deep and bedrock flow layers:

 A potential upward gradient was observed at AB-30D/BR located on the dam of the
secondary cell. This upward flow is due to hydraulic influence of both the ash basin and
proximity of the Dan River.
 A potential downward gradient was observed at AB-25D/BR located on the dam of the
southern most portion of the primary cell. This downward flow is generally observed
across the site, including the source areas.
 Background well vertical gradient is inconclusive at this time as the deep well was dry.

ES-3.2 Site Geochemical Conditions
COI sources at the DRSS site consist of the ash storage areas (Ash Storage 1 and Ash Storage
2) and the primary and secondary Cells of the ash basin. These source areas are subject to
different processes that generate leachate migrating into the underlying soil layers and into the
groundwater. For example, the ash storage areas would generate leachate as a result of
infiltration of precipitation, while the ash basin would generate leachate based on the contact
with ponded water elevation in the basin. The periodic discharging of water to the ash basin
would likely affect the leachate constituents and concentrations over time.
The location of ash, precipitation, and process water in contact with ash is the most significant
control on geochemical conditions. COIs would not be present in groundwater or soils at levels
above background concentrations without ash-to-water contact. Once leached by precipitation
or process water, COIs enter the soil-to-water-to-rock system and their concentration and
Corrective Action Plan Part I
Dan River Steam Station Ash Basin

9
location are controlled by the principles of COI transport in groundwater. Water-to-rock-to-soil
interaction is also responsible for the natural occurrence of constituents in background water
quality locations.
After leaching has occurred, the distribution and concentrations of COIs in groundwater
depends upon factors such as how the dissolved concentrations are transported through the
soil/rock media, the composition of the soil/rock media in the flow path, and the geochemical
conditions present along those flow paths. A constituent may be removed from groundwater and
onto mineral surfaces of the aquifer media through one of the three types of sorption processes:
 Adsorption – solutes are held at the water/solid as a hydrated species,
 Absorption – solutes are incorporated into the mineral structure at the surface,
 Ion Exchange – when an ion becomes sorbed to a surface by changing places with a
similarly charged ion.

These processes result in a decrease of the concentration and therefore the mass of the
constituent as it is removed from the groundwater onto the solid material. The effect of these
processes for a particular constituent can be expressed by the distribution coefficient (or
partition coefficient) Kd. Kd relates the quantity of the adsorbed constituent per unit mass of solid
to the quantity of the constituent remaining in solution.
Laboratory determination of Kd was performed by UNCC on samples of soil material collected
during the CSA well installation program. Solid samples were tested in flow through columns to
measure the adsorption of COIs at varying concentrations. For the DRSS site, 12 column tests
and 22 batch tests were conducted. The methods used by UNCC and Kd results obtained from
the testing are presented in Appendix E. The Kd data was used as an input parameter to
evaluate contaminant fate and transport through the subsurface at the DRSS site, as described
in greater detail in Section 4.1.
ES-3.3 Correlation of Hydrogeologic and Geochemical Conditions to COI
Distribution
Based on results of sampling and analysis performed during CSA activities, the following are
groundwater COIs s the DRSS site: arsenic, boron, chromium, cobalt, iron, manganese, sulfate,
TDS, and vanadium. Cobalt, iron, manganese, and vanadium exceed 2L or IMACs and PPBCs
in source areas. Antimony and selenium were isolated occurrences and need further evaluation
as to source of impacts. Arsenic, boron, chromium, sulfate and TDS are attributable to ash
handling at the site.

The areas of 2L Standards exceedances of arsenic, boron, chromium, sulfate, and TDS are
within or downgradient to the sources indicating that physical and geochemical processes
beneath the DRSS site inhibit lateral migration of the COIs. Discharge of groundwater from
shallow and deep flow layers into surficial water bodies, in accordance with LeGrand’s slope-
aquifer system characteristic of the Piedmont, is evident northwest of the Secondary Cell where
COI concentrations in excess of 2B Standards were detected in the tributary that discharges to
the Dan River. Vertical migration of COIs were observed in select well clusters (S, D, and BR)
Corrective Action Plan Part I
Dan River Steam Station Ash Basin

10
and is likely influenced by infiltration of precipitation and/or ash basin water, where applicable,
through the shallow and deep flow layers into underlying fractured bedrock.
ES-4 Modeling
Groundwater flow, groundwater fate and transport, and groundwater to surface water modeling
was conducted to evaluate COI migration and potential impacts following closure of ash basin
system and ash storage areas at the DRSS site.
UNCC was retained as a sub-consultant to HDR and they developed a site-specific, 3-D,
steady-state groundwater flow and fate and transport model for the DRSS site using
MODFLOW and MT3DMS. The groundwater flow and fate and transport model is based on the
SCM presented in Section 3 and incorporates site-specific data obtained during the CSA and
subsequent data collection. The objective of the groundwater modeling effort was to simulate
steady-state groundwater flow conditions for the DRSS site, and simulate transient transport
conditions in which COIs enter groundwater via the ash basin system over the period it was in
service. These model simulations serve to:
 Predict groundwater elevations in the ash and underlying groundwater flow layers for the
proposed closure scenarios.
 Predict concentrations of the COIs at the compliance boundary or other downgradient
locations of interest over time,
 Estimate the groundwater flow and constituent loading to adjacent downgradient
unnamed tributaries and the Dan River.

The area, or domain, of the simulation included the DRSS ash basin system and areas of the
site that have been impacted by COIs above 2L Standards or IMACs. The model was
developed in accordance with NCDENR DWQ’s Groundwater Modeling Policy dated May 31,
2007. Details of the groundwater modeling are presented in Appendix C.
ES-4.1 Overview of Modeling Work
The model domain encompassed the DRSS site, including a section of the Dan River and all
site features relevant to the assessment of groundwater. The model domain extended beyond
the ash management areas to hydrologic boundaries such that groundwater flow and COI
transport through the area is accurately simulated without introducing artificial boundary effects.
In plan view, the DRSS model domain was bounded by the following hydrologic features of the
site:
 The northern shoreline of the Dan River to the south,
 The unnamed tributary to the east,
 The groundwater boundary along a topographic divide north of the site, and
 The unnamed tributary and service water settling pond to the west.

A total of 12 model layers were divided among the identified hydrostratigraphic units to simulate
curvilinear flow with a vertical flow component. The model domain includes the necessary
Corrective Action Plan Part I
Dan River Steam Station Ash Basin

11
geologic units, hydrologic features, and flow boundaries to encompass the source(s) of COIs
and simulate velocity and direction of COIs travel to potential off-site receptors.
ES-4.2 Model Scenarios
The following ash basin closure scenarios were modeled for the DRSS site:
 Existing Conditions (EC)
 Cap-in-Place (CIP)
 Excavation (EX)
Model scenarios utilized steady-state groundwater flow conditions established during model
calibration and transient transport of COIs identified in Sections 2 and 3 for further analysis.
Each COI was modeled individually using the transient transport model for a period of 250
years. This time period was selected to determine the rate of natural attenuation and COI
concentrations at the compliance boundary.
ES-4.3 Model Conclusions
ES-4.3.1 Flow Model
The model results indicate that groundwater in the shallow aquifer, transition zone, and
fractured bedrock flows radially from the northern extent of the property to drainage features
east and west of the site, and to the southeast toward the Dan River. These findings are
consistent with groundwater potentiometric surface maps and interpreted groundwater flow
directions presented in the CSA report. Moreover, the model indicates that groundwater flow
originating from the ash basin system starts vertically downward then moves horizontally at
depth before discharging as base flow to the Dan River.
ES-4.3.2 Fate and Transport Model
Existing Conditions Scenario
The results of the EC scenario indicated that concentrations for all modeled COIs increase or
reach steady state conditions above 2L Standards or IMACs at the compliance boundary during
the modeled period.
Cap-in-Place Scenario
On-site model concentrations of arsenic, cobalt, thallium and vanadium decrease, but remain
above the 2L Standards or IMACs at the compliance boundary. Boron, chromium, hexavalent
chromium, and sulfate are removed from the model domain under this scenario.
Excavation Scenario
Boron and sulfate (non-sorptive COIs), will be displaced from the system at the Dan River within
50 years under this scenario. Arsenic, chromium, cobalt, thallium and vanadium (sorptive COIs)
concentrations decreased in each flow layer in downgradient wells and was removed from the
model domain within 100 years of completion of the EX closure, thus concentrations of the
above constituents will be below 2L Standards at the compliance boundary.
Corrective Action Plan Part I
Dan River Steam Station Ash Basin

12

ES-4-4 Groundwater-Surface Water Interaction Modeling
Groundwater model output from the fate and transport modeling discussed in Section 4.1 (i.e.,
groundwater volume flux and concentrations of COI with exceedances of the 2L Standards) was
used as input for the surface water assessment in the adjacent Dan River receiving water.
Given that the Dan River is unidirectional and groundwater discharge mixes with upstream river
flow, a mixing calculation was used to assess potential surface water quality impacts.
ES-4.4.1 Overview of Modeling
The relatively simple morphology of receiving waters adjacent to the DRSS makes the site
amenable to the Mixing Model Approach. For this approach, river flow data from USGS or other
suitable gages were analyzed to determine upstream river design flows and assess compliance
with NCDEQ surface water criteria, including determination of 1Q10, 7Q10, and mean annual
river flows.
For each groundwater COI that discharges to surface waters at a concentration exceeding the
2B Standards, the appropriate dilution factor and upstream (background) concentration were
applied to determine the surface water concentration at the edge of the applicable mixing zone.
This concentration was then compared to the applicable water quality criteria to determine
surface water quality standard (WQS) compliance.
ES-4.4.2 Surface Water Quality Results
The mixing model calculations compare relatively well with the observed surface water
concentrations at station SW-3 (near the mouth of the unnamed east tributary) and at stations
SW-6 and SW-7 (in the Dan River adjacent to the DRSS). Some of the differences can be
attributed to using the maximum observed groundwater concentration for the mixing model
calculations. The mixing model results indicate that all water quality standards are attained at
the edge of the mixing zones except for arsenic in the unnamed east tributary for the human
health standard based on the observed concentration at station SW-3.
ES-5 Summary and Recommendations
Based on the data presented herein, and the analysis of these data, Duke Energy provides the
following conclusions and recommendations:

 Duke Energy submitted a receptor survey to the NCDENR in September 2014, and
subsequently submitted a supplement to the receptor survey in November 2014. Three
private water supply wells, one private water supply spring, not currently in use, and
several tributaries to the Dan River were identified within a 0.5-mile radius of the ash
basin compliance boundary. All three water supply wells are located more than 2,000
feet away from the Dan River ash basin compliance boundary and are either upgradient
or across the Dan River from the ash basin system. No information gathered as part of
this assessment suggests that water supply wells or the spring located within the 0.5-
mile radius of the compliance boundary are impacted, or have the potential to be
impacted, by the Dan River ash basin system.
Corrective Action Plan Part I
Dan River Steam Station Ash Basin

13
 PPBCs were calculated for soil and groundwater at the site and are presented in
Section 2. Note that for the DRSS site, the PPBCs were calculated using historical
groundwater quality data from NPDES compliance wells. At the request of NCDEQ,
groundwater results that were obtained with turbidity greater than 10 NTU were removed
from the statistical population. Given that the DRSS site has only one background
compliance well, the removal of these data resulted in an insufficient amount of
qualifying data (less than 8 data points) for statistical evaluation. Therefore, PPBCs were
developed by selecting the highest concentration of a given constituent across the range
of concentrations observed for that constituent in the qualifying events. PPBCs will be
refined as additional data sets are obtained from subsequent sampling events collected
from the background wells.
 COIs were selected for groundwater fate and transport modeling, in part, based on
comparison of constituent concentrations in background wells versus non-background
wells. As mentioned above, PPBCs were developed in background wells exclusive of
data collected with turbidity greater than 10 NTU. Note that data obtained from non-
background (e.g., cross- or down-gradient) wells was not eliminated to the same turbidity
criteria, even though the analytical results for selected constituents can be biased
upward due to the effects of turbidity. As such, the list of COIs to be carried forward in
CAP Part II will be modified, if warranted, as additional groundwater quality data is
obtained and the possible effects of turbidity on the analytical results are evaluated.
 Groundwater samples collected during the CSA and subsequent monitoring events were
analyzed for total and dissolved phase constituents to evaluate potential effects of
turbidity. While dissolved concentrations are not approved by the NCDEQ for use in
compliance well evaluations, they do provide meaningful data in areas of the DRSS site
where elevated turbidity is driven by site-specific geologic conditions. A more detailed
constituent-specific and location-specific analysis of turbidity and its effect on
groundwater quality results will be completed in CAP Part II to refine the list of COIs to
be remediated (if necessary).
 Review of the groundwater modeling yields the following:
o Existing Conditions Scenario - The results of the EC scenario indicated that
concentrations for all modeled COIs increase or reach steady state conditions
above 2L Standards or IMACs at the compliance boundary during the modeled
period.
o Cap-in-Place Scenario – On-site model concentrations of arsenic, cobalt,
thallium and vanadium decrease, but remain above the 2L Standards or IMACs
at the compliance boundary. Boron, chromium, hexavalent chromium, and sulfate
are removed from the model domain under this scenario.
o Excavation Scenario - The simulation fails to reach steady state concentrations
after 250 years. The results of the EX scenario indicated that concentrations of
arsenic, boron, chromium, hexavalent chromium, cobalt, sulfate, thallium, and
vanadium are removed from the model domain under this scenario.
Concentrations of the above constituents will be below 2L Standards at the
compliance boundary.
 Groundwater flow rates and concentrations of COIs from the groundwater model were
used to determine if 2L exceedances would result in exceedances of 2B surface water
Corrective Action Plan Part I
Dan River Steam Station Ash Basin

14
standards in the Dan River and in the tributary stream located east of the ash basin. The
groundwater-surface water interaction was performed using maximum concentrations of
COIs and those results show arsenic exceeds 2B Standards in the eastern unnamed
tributary. The mixing model results indicate that all water quality standards are attained
at the edge of the mixing zones except for arsenic in the unnamed east tributary for the
human health standard based on the observed concentration at station SW-3.
 Data gaps identified as part of the CSA will be assessed and information collected as
part of that assessment will be included in the CSA supplement to be provided after CAP
Part II.

The following recommendations have been made to address areas needing further assessment:

 Geochemical modeling of the DRSS site will be completed and submitted under cover of
the CAP Part II. The geochemical model results coupled with the groundwater flow, fate
and transport and surface water-groundwater models will enhance the understanding of
the processes taking place in the subsurface and ultimately aid in choosing the most
appropriate remedial action for the site. The geochemical model is key to understanding
mobility of iron, manganese, and TDS since it cannot adequately be modeled using
MODFLOW/MT3DMS.
 Additional sampling for radiological parameters on major flow paths is needed to perform
a more comprehensive assessment of radionuclides from source areas and
downgradient wells along major flow paths.
 Models should be updated with results from second round sampling at the DRSS site
and will be included in CAP Part II.
 Two additional background well sampling events are scheduled before the end of 2015.
These additional results will be utilized for statistical analysis of background
concentrations at the DRSS site and will be included in CAP Part II.
Corrective Action Plan Part I
Dan River Steam Station Ash Basin

15
1 Introduction
Duke Energy Carolinas, LLC (Duke Energy) owns and formerly operated the Dan River Steam
Station (DRSS), located on the Dan River in Rockingham County near Eden, North Carolina.
DRSS began operation as a coal-fired generating station in 1949 and was retired from service in
2012. The Dan River Combined Cycle Station (DRCCS) natural gas generating facility was
constructed at the site and began operations in 2012. Historically, coal ash residue from
DRSS’s coal combustion process was disposed of in an ash basin located northeast of the
station and adjacent to the Dan River. Discharge from the ash basin is currently permitted by
the North Carolina Department of Environmental Quality (NCDEQ)3 Division of Water
Resources (DWR) under the National Pollutant Discharge Elimination System (NPDES) Permit
NC0003468.
The North Carolina Coal Ash Management Act of 2014 (CAMA) directs owners of coal
combustion residuals (CCR) surface impoundments in North Carolina to conduct groundwater
monitoring, assessment, and remedial activities, if necessary. A groundwater assessment work
plan (Work Plan) for DRSS was submitted to NCDENR on September 25, 2014, followed by a
revised Work Plan on December 30, 2014. The revised Work Plan was conditionally approved
by NCDENR on February 16, 2015. A Comprehensive Site Assessment (CSA) was performed
to collect information necessary to determine horizontal and vertical extent of impacts to soil and
groundwater attributable to CCR source area(s), identify potential receptors, and screen for
potential risks to those receptors. The DRSS CSA report was submitted to NCDENR on August
14, 2015 (HDR 2015).
CAMA also requires the preparation of a Corrective Action Plan (CAP) for each regulated facility
within 270 days of approval of the Work Plan (90 days within submittal of the CSA report). Duke
Energy and NCDEQ mutually agreed to a two-part CAP submittal, with Part I being submitted
within the original CAP due date, and Part II being submitted 90 days thereafter (Appendix A).
The purpose of this CAP Part I is to provide background information, a brief summary of the
CSA findings, a brief description of the site geology and hydrogeology, a summary of the
previously completed receptor survey, a detailed description of the site conceptual model, and
results of the groundwater flow and transport model and groundwater to surface water model.
The CAP Part II will include the remainder of the CAP, alternative methods for achieving
groundwater quality restoration, conceptual plans for recommended corrective actions,
implementation schedule and a plan for future monitoring and reporting. The risk assessment
will be submitted under a separate cover with the CAP Part II submittal.

3 Prior to September 18, 2015, the NCDEQ was referred to as the North Carolina Department of Environment and
Natural Resources (NCDENR). Both naming conventions are used in this report, as appropriate.
Corrective Action Plan Part I
Dan River Steam Station Ash Basin

16
1.1 Site History and Overview
1.1.1 Site Location, Acreage, and Ownership
The DRSS site is located on the north bank of the Dan River in Eden, Rockingham County,
North Carolina, and occupies approximately 380 acres of land (Figure 1-1).
Duke Energy purchased the former Hopkins Property (Parcel Number 110532) on June 4, 2015
and the former Norfolk Southern Railway Company Property (Parcel Number 110531) on June
23, 2015. Both of these properties are located on the north-northeast side of the DRSS site.
With the acquisition of these two parcels, Duke Energy’s property boundary at the DRSS site
has changed and a new compliance boundary for the north-northeastern portion of the site will
be proposed under a separate cover. Additional compliance wells will be installed on this portion
of the site after the new compliance boundary has been established and in consultation with
NCDEQ. A site layout map is shown on Figure 1-2.
1.1.2 Site Description
DRSS is a former coal-fired electricity generating facility along the Dan River approximately 4.5
miles from the city of Eden, North Carolina. Construction commenced in 1948 and the station
began commercial operation in 1949 with a single coal-fired unit. A second unit was added in
1950 and a third unit was added by 1955, resulting in a total installed capacity of 276 megawatts
(MW). All three coal-fired units, along with three 28 MW oil-fired combustion turbine units, were
retired in 2012. Construction of the DRCCS, a 620 MW combined cycle natural gas facility,
commenced in December 2009 and commercial operations began on December 10, 2012.
The natural topography at the DRSS site generally slopes from northwest to southeast, ranging
from an approximate high elevation of 606 feet near the northern property boundary just west of
Edgewood Road to an approximate low elevation of 482 feet at the interface with the Dan River.
Ground surface elevation varies about 124 feet over an approximate distance of 0.7 miles.
Surface water drainage generally follows site topography and flows from the northwest to the
southeast across the site except where drainage patterns have been modified by the ash basin
or other construction.
The DRSS ash basin is located adjacent to the Dan River and consists of a Primary Cell, a
Secondary Cell, and associated embankments and outlet works, as further described in the
CSA report (HDR 2015). The ash basin is impounded by earthen dams and an earthen/ash
divider dam separates the Primary Cell from the Secondary Cell. Water levels within the Primary
Cell have historically fluctuated approximately 11 feet from October 1984 until April 2013,
ranging from approximately 526 to 537 feet. Since the DRSS facility was retired, process water
and storm water inflow to the ash basin system have been rerouted from the Primary Cell to the
Secondary Cell, resulting in decreased water levels in the Primary Cell. Water levels within the
Secondary Cell have historically fluctuated approximately 8 feet from October 1984 until April
2013, ranging from approximately 518 to 526 feet. The ash storage areas are located
topographically upgradient of the ash basin and consist of Ash Storage 1, Ash Storage 2, a
former dredge pond, and associated dredge dikes. The ash storage areas contain ash that was
previously removed from the ash basin.
Corrective Action Plan Part I
Dan River Steam Station Ash Basin

17
1.2 Permitted Activities and Permitted Waste
Duke Energy is authorized to discharge wastewater that has been adequately treated and
managed to the surface waters of North Carolina or to a separate storm sewer system in
accordance with NPDES Permit NC0003468, which was renewed on March 1, 2013 and expires
on April 30, 2017 (http://portal.ncdenr.org/c/document_library/get_file?uuid=38c490a7-5b93-
4540-a593-d8ae3014d724&groupId=38364).
The NPDES permit authorizes discharges in accordance with effluent limitations monitoring
requirements and other conditions set forth in the permit. A detailed description of NPDES and
surface water sampling requirements, along with the associated NPDES site flow diagram, is
provided in the CSA report.
An inactive (closed) asbestos and land clearing and inert debris landfill (LCID), Permit 79B-
LCID, is located on the west side of Edgewood Road.
1.3 History of Site Groundwater Monitoring
Duke Energy has implemented voluntary and NPDES permit-required compliance groundwater
monitoring at the DRSS site since 1993, in accordance with NPDES Permit NC0003468. From
1993 to 2010, voluntary groundwater monitoring was performed twice annually around the
DRSS ash basin with analytical results submitted to NCDENR DWR. In October 2010, new
compliance monitoring wells were installed and the previously installed monitoring wells became
part of the voluntary monitoring program. Additional groundwater monitoring was required
beginning in March 2011 with the frequency of sampling and parameters to be analyzed
outlined in the NPDES permit.
From January 2011 through May 2015, the compliance groundwater monitoring wells at the
DRSS site have been sampled three times per year for a total of 14 times as part of sampling
required in the NPDES permit. The location of the ash basin voluntary and compliance
monitoring wells, the approximate ash basin waste boundary, and the compliance boundary are
shown on Figure 1-3. The compliance boundary for groundwater quality at the DRSS site is
defined in accordance with Title 15A NCAC 02L .0107(a) as being established at either 500 feet
from the waste boundary or at the property boundary, whichever is closer to the waste
boundary. A detailed description of NPDES and voluntary groundwater monitoring programs
and results is provided in the CSA report.
1.4 Summary of Comprehensive Site Assessment
The CSA for the DRSS site began in February 2015 and was completed in August 2015. Sixty-
three groundwater monitoring wells and three soil borings were installed/advanced as part of the
assessment to characterize the ash, soil, rock, and groundwater at the DRSS site (Figure 1-4).
Seep, surface water, and sediment samples were also collected (Figure 1-5). In addition,
hydrogeological evaluation testing was performed on newly installed wells.
Information obtained during the CSA was utilized to determine existing background constituent
concentrations, source related constituents concentrations and to evaluate the horizontal and
Corrective Action Plan Part I
Dan River Steam Station Ash Basin

18
vertical extent of impacts to soil and groundwater at the site. If a constituent4 concentration
exceeded the North Carolina Groundwater Quality Standards, as specified in T15A NCAC
.0202L (2L Standards) or Interim Maximum Allowable Concentration (IMAC)5, it was designated
in the CSA as a “Constituent of Interest” (COI). In addition, the CSA presented information from
a receptor survey completed in 2014 and a screening level human health and ecological risk
assessment. Additional details of the CSA findings are discussed in following sections.
1.5 Receptor Survey
Duke Energy submitted a receptor survey to NCDENR (HDR 2014a) in September 2014,
followed by a supplement to the receptor survey (HDR 2014b) in November 2014. The purpose
of the receptor surveys was to identify drinking water wells within a 0.5-mile (2,640-foot) radius
of the DRSS ash basin compliance boundary. The supplemental information was obtained from
responses to water supply well survey questionnaires mailed to property owners within the
required distance requesting information on the presence of water supply wells and well usage.
A detailed description of the receptor surveys is provided in the CSA report. Results of the
receptor survey are detailed on Figure 1-6.
1.5.1 Surrounding Land Use
Properties located within a 0.5-mile radius of the DRSS ash basin compliance boundary are
located in and southeast of Eden, in Rockingham County, North Carolina. The majority of the
land is undeveloped property as shown on Figure 1-7 and land use is typical of rural areas.
Residential properties are located north and northwest of the ash basin compliance boundary
within the 0.5-mile radius.
The public drinking water supply source for the DRSS site and surrounding area is the Dan
River above its confluence with the Smith River, which is located approximately 2.5 miles
upstream of the DRSS site. The City of Eden provides potable water to the DRSS site and
surrounding area within the city of Eden, North Carolina. Dan River Water, Inc. provides potable
water outside the city limits of Eden in Rockingham County.
1.5.2 Findings of Drinking Water Supply Well Survey Conducted per the Coal
Ash Management Act of 2014, N.C. Gen. Stat. SS130A-309-200 et seq.
Three water supply wells and one spring were identified within the 0.5-mile radius of the DRSS
compliance boundary during the receptor survey. Information evaluated as part of the CSA
indicated that the identified water supply wells and spring would not be impacted as they are
located either hydraulically upgradient or across the Dan River from the ash basin. NCDEQ has
not conducted independent sampling of the identified drinking water supply wells.

4 Constituents are elements, chemicals, or compounds that were identified in the approved Work Plan for sampling
and analysis, and include antimony, arsenic, boron, chromium, cobalt, iron, manganese, selenium, thallium,
vanadium, sulfate, and total dissolved solids (TDS). 5 Appendix #1 of 15A NCAC Subchapter 02L Classifications and Water Quality Standards Applicable to The
Groundwaters of North Carolina, lists Interim Maximum Allowable Concentrations (IMACs). The IMACs were issued
in 2010 and 2011; however, NCDENR has not established a 2L Standard for these constituents as described in 15A
NCAC 02L.0202(c). For this reason, IMACs noted in this report are for reference only.
Corrective Action Plan Part I
Dan River Steam Station Ash Basin

19
1.6 Summary of Screening Level Risk Assessment
A screening level human health and ecological risk assessment was performed as a component
of the CSA Report (HDR 2015). Each screening level risk assessment identified the exposure
media for human and ecological receptors. Human health and ecological exposure media
includes potentially impacted groundwater, soil, surface water, and sediments.
The human health exposure routes associated with the evaluated pathways for the site include
ingestion, inhalation, and dermal contact of environmental media. Potential human receptors
under a current or hypothetical future use include construction/outdoor workers, off-site
residents, recreational users and trespassers. The ecological exposure routes associated with
the evaluated pathways for the site include dermal contact/root absorption/gill uptake and
ingestion of environmental media. Potential ecological receptors under a current or hypothetical
future use include aquatic, riparian, and terrestrial biota.
The screening level risk assessment will continue to be refined consistent with risk assessment
protocol in parallel with the CAP Part II schedule.
1.7 Geological/Hydrogeological Conditions
DRSS and its associated ash basin system are located in the Dan River Triassic Basin, one of
several northeast-trending Triassic basins that occur within the Piedmont Province. The Dan
River Basin is characterized by sandstone, mudstone, siltstone, and conglomerates. Alluvial and
terrace deposits consisting of unconsolidated sand, silt, and clay with occasional sub-rounded to
well-rounded pebbles occur along the Dan River and major tributaries.
The hydrogeologic regime in the Dan River Basin is characterized by fractured, bedded
sedimentary sequences underlying soil and saprolite. Groundwater may occur under both
unconfined, water table conditions (similar to most Piedmont crystalline sites) and confined
conditions. Groundwater flow has both local and regional components with shallow groundwater
discharging locally to nearby streams (and some movement downward into the deeper flow
system) and deeper groundwater flow toward points of regional discharge, that are generally
higher order stream courses (Venkatakrishnan and Gheorghiu 2003). Both shallow and deep
groundwater systems generally flow in a direction similar to the topographic gradient.
Based on the site investigation completed for the CSA, the groundwater system in the natural
materials (alluvium, soil, soil/saprolite, and bedrock) at the DRSS site is consistent with the
regolith-fractured rock system and is an unconfined, connected aquifer system without confining
layers. The DRSS site groundwater system is divided into three layers referred to as shallow,
deep (transition zone [TZ]), and bedrock flow layers to distinguish the flow layers within the
connected aquifer.
Two unnamed tributaries of the Dan River are located along the eastern and western sides of
the DRSS site. A surface water divide between these two tributaries generally runs north to
south along South Edgewood Road. Contours for the groundwater surface generally mimic the
site surface topography, such that east of South Edgewood Road groundwater generally flows
toward the unnamed tributary located on the east side of the property and toward the ash basin,
Corrective Action Plan Part I
Dan River Steam Station Ash Basin

20
ultimately discharging into the Dan River. Similarly, groundwater on the western side of South
Edgewood Road generally flows toward the unnamed tributary located on the west side of the
property or towards the Dan River.
1.8 Results of the CSA Investigations
Groundwater exceedances were identified at the DRSS site during the CSA investigation.
These groundwater impacts are a result of both naturally occurring conditions and from CCR
material contained in the ash basin and ash storage areas.
The horizontal and vertical extent of source-related soil contamination was also identified during
the CSA, with the exception of off-site areas east and north of Ash Storage 1. Where soil
impacts were identified beneath the ash basin, the vertical extent of contamination beneath the
ash/soil interface is generally limited to the uppermost soil sample collected beneath the ash.
Groundwater contamination at the site attributable to ash handling and storage was delineated
during the CSA activities with the following exceptions:
 Vertical extent in the vicinity of Ash Storage 1
 Horizontal and vertical extent north of Ash Storage 1
 Groundwater-surface water interaction north of the Secondary Cell
The geologic conditions present beneath the ash basin and ash storage areas impede the
vertical migration of contaminants. The direction of contaminant transport is generally to the
south/southeast toward the Dan River, and not toward other off-site receptors.
Additional details pertaining to the horizontal and vertical extent of soil and groundwater impacts
at the DRSS site are detailed in the CSA report.
Surface water impacts were identified in the eastern unnamed tributary that flows to the Dan
River located downgradient of the secondary cell of the ash basin.
Background monitoring wells contained naturally occurring metals and other constituents at
concentrations that exceeded their respective regulatory standards or guidelines. These
included chromium, cobalt, iron, manganese, and vanadium. The CSA report did not propose
provisional background concentrations for soil, groundwater, and surface water COIs identified
in the CSA; however, these concentrations are discussed in Section 2 of this CAP Part I.
1.9 Regulatory Requirements
1.9.1 CAMA Requirements
CAMA Section §130A-309.209 requires implementation of corrective actions for the restoration
of groundwater quality. Analysis and reporting requirements are as follows:
(b) Corrective Action for the Restoration of Groundwater Quality. - The owner of a coal
combustion residuals surface impoundment shall implement corrective action for the restoration
of groundwater quality as provided in this subsection. The requirements for corrective action for
the restoration of groundwater quality set out in the subsection are in addition to any other
Corrective Action Plan Part I
Dan River Steam Station Ash Basin

21
corrective action for the restoration of groundwater quality requirements applicable to the
owners of coal combustion residuals surface impoundments.
(1) No later than 90 days from submission of the Groundwater Assessment Report
required by subsection (a) of this section, or a time frame otherwise approved by the
Department not to exceed 180 days from submission of the Groundwater
Assessment Report, the owner of the coal combustion residuals surface
impoundment shall submit a proposed Groundwater Corrective Action Plan to the
Department for its review and approval. The Groundwater Corrective Action Plan
shall provide restoration of groundwater in conformance with the requirements of
Subchapter L of Chapter 2 of Title 15A of the North Carolina Administrative Code.
The Groundwater Corrective Action Plan shall include, at a minimum, all of the
following:
a. A description of all exceedances of the groundwater quality standards,
including any exceedances that the owner asserts are the result of natural
background conditions.
b. A description of the methods for restoring groundwater in conformance with
requirements of Subchapter L of Chapter 2 of Title 15A of the North Carolina
Administrative Code and a detailed explanation of the reasons for selecting
these methods.
c. Specific plans, including engineering details, for restoring groundwater quality.
d. A schedule for implementation of the Plan.
e. A monitoring plan for evaluating effectiveness of the proposed corrective
action and detecting movement of any contaminant plumes.
f. Any other information related to groundwater assessment required by the
Department.
(2) The Department shall approve the Groundwater Corrective Action Plan if it
determines that the Plan complies with the requirements of this subsection and will
be sufficient to protect public health, safety, and welfare; the environment; and
natural resources.
(3) No later than 30 days from the approval of the Groundwater Corrective Action Plan,
the owner shall begin implementation of the Plan in accordance with the Plan’s
schedule.
As required under the CAMA, Duke Energy is currently closing the DRSS ash ponds in
accordance with the Dan River Steam Station Coal Ash Excavation Plan, which was submitted
by Duke Energy to the NCDENR on November 13, 2014 (Duke Energy 2014). Once all
necessary permits are received ash will actively be removed from the facility. Duke Energy is
anticipated to complete closure of the DRSS ash ponds by August 1, 2019.
Based on the results of soil and groundwater samples collected beneath the ash basins and the
ash storage areas, some residual contamination will remain after excavation; however, the
degree of contamination and the persistence of this contamination over time cannot be
determined at this time. The CAMA requires that corrective action be implemented to restore
groundwater quality where the CSA documents exceedances of groundwater quality standards.
Corrective Action Plan Part I
Dan River Steam Station Ash Basin

22
1.9.2 Standards for Site Media
Groundwater sample analytical results were compared to North Carolina Groundwater Quality
Standards found in the North Carolina Administrative Code Title 15A, Subchapter 2L.0202 (2L
Standards) or the Interim Maximum Allowable Concentrations (IMACs) established by NCDEQ
pursuant to 15A NCAC 02L.0202(c). The IMACs were issued in 2010, 2011, and 2012, however
NCDEQ has not established 2L Standards for these constituents as described in 15A NCAC
02L.0202(c). For this reason, IMACs noted in this report are for reference only.
Surface water sample analytical results were compared to the appropriate North Carolina
Surface Water Quality Standards (2B Standards), selected from a list of standards published by
NCDENR dated April 22, 2015 and applicable U.S. Environmental Protection Agency (USEPA)
National Recommended Water Quality Criteria. A two-step process was employed in this
assessment. First, if surface water bodies receiving surface water discharge from the ash basin
are classified for drinking water use, the standards designated as 'water supply' were used.
Next, this value was compared to the lowest 'aquatic life' value and the lower of these two
values was used in comparison tables included herein addressing surface water quality
(http://portal.ncdenr.org/web/wq/ps/csu, accessed on October 17, 2015).
Soil sample analytical results were compared to North Carolina Preliminary Soil Remediation
Goals (NC PSRGs) ‘new format’ tables for Protection of Groundwater (POG) exposures
(updated March 2015). Sediment sample analytical results were also compared to NC PSRGs
for POG.

Corrective Action Plan Part I
Dan River Steam Station Ash Basin

23
2 Background Concentrations and Regulatory
Exceedances
As part of the CSA, groundwater samples were collected between June 1, and June 30, 2015,
from background locations, beneath the ash basin, beneath the ash storage area, and from
locations outside the DRSS waste boundary. Groundwater samples were also collected from
pre-existing voluntary and compliance wells on the site. Data obtained from this sampling event
were presented in the CSA report and are summarized in Section 1.5.2 of this CAP Part I. The
purpose of this section is to present proposed provisional background concentrations (PPBCs)
for COIs per affected geomedium;6 discuss the nature and extent of COI impacts to geomedia
with regard to PPBCs and applicable regulatory standards or guidelines (i.e., 2L Standards,
IMACs, 2B Standards); and determine which COIs will be retained for further evaluation.
The purpose of this section is to compare background, downgradient, and source area
constituent concentrations to applicable regulatory standards or guidelines to determine if
constituent exceedances are attributable to the source area.
2.1 Purpose
Some COIs identified in the CSA are present in background and upgradient monitoring wells
and may be naturally occurring, and thus require examination to determine whether their
presence downgradient of the source area is naturally occurring or potentially attributed to the
source area. In this section, the list of COIs may be narrowed down based on a review of source
characterization data, background concentrations, and exceedances beneath and downgradient
of the source areas.
Note that COIs identified in the CSA were based on one sampling event and that the PPBCs
presented in the subsections below are provisional values. The PPBCs will be updated as more
data becomes available with input from NCDEQ. The COIs identified in the CSA report are
organized by geomedium and presented in Table 2-1.

6 The term “geomedium” is used in this section to describe a naturally occurring material (e.g., soil, groundwater,
surface water, or sediment) that was evaluated via analytical testing. Geomedia do not include waste material.
Corrective Action Plan Part I
Dan River Steam Station Ash Basin

24
Table 2-1. Initial COI Screening Evaluation
Potential
COI CSA COIs by Media COI To Be
Further
Assessed
in CAP I
Solid/
Aqueous Ash Pore-
water
Ground-
water
Surface
Water Seeps Sediment Soil PWR/
Bedrock
Aluminum √ √ Yes
Antimony √ √ Yes
Arsenic √ √ √ √ √ √ √ Yes
Barium √ - - - - - √ √ Yes
Beryllium - - - - - - - - No
Boron √ √ √ - - - - - Yes
Cadmium - - - - - - - - No
Chloride - - - - - - - - No
Chromium - √ √ - - √ - - Yes
Cobalt √ √ √ - √ √ √ √ Yes
Copper - - - √ - √ - - Yes
Iron √ √ √ - √ - √ √ Yes
Lead - - - √ - √ - - Yes
Manganese √ √ √ - √ - √ √ Yes
Mercury - - - - - - - - No
Nickel - - - - - - - - No
Nitrate - - - - - - - - No
pH - √ √ - - - - - Yes
Selenium √ - √ - - - √ √ Yes
Sulfate - - √ - - - - - Yes
Thallium - - √ - - √ - - Yes
Vanadium √ √ √ √ √ - √ √ Yes
Zinc - - - - - √ - - Yes
TDS - - √ - - - - - Yes
Note: COI exceedance based on 2L Standard, IMAC, or 2B Standard for respective Aqueous media and PSRGs for
solid/soil like media.

2.2 Groundwater
2.2.1 Background Wells and Concentrations
Because COIs can be both naturally occurring and related to the source areas, the choice of
monitoring wells used to establish background concentrations is important in determining
whether releases have occurred from the source areas. The determination of whether or not a
monitoring well is a suitable background well is based on the following:
 The topographic location of the well with respect to the source areas (distance from
source areas and located hydraulically upgradient of source areas)
 Stratigraphic unit being monitored
 Screened intervals of well relative to source water elevation
 Direction of groundwater flow in the region of the well relative to source areas
Corrective Action Plan Part I
Dan River Steam Station Ash Basin

25
Wells that have been determined to represent background conditions are compliance monitoring
well MW-23D and CSA background (BG) monitoring wells MW-23BR, BG-5S, and BG-5D,
GWA-12S and GWA-12D (Figure 2-1). BG-1D was installed as a background well during the
CSA; however, a groundwater sample was not collected during the CSA because the well was
dry.
Note that analytical results for samples collected with turbidity readings greater than 10
Nephelometric Turbidity Units (NTU) were not included in the PPBC calculations. However, the
evaluation of COIs in this CAP does consider analytical data with measured turbidity greater
than 10 NTU. Duke Energy acknowledges that use of these data in comparison to PPBCs
represents a conservative approach that warrants additional evaluation on a well-by-well and
constituent-by-constituent basis. That level of evaluation was not possible using the limited data
set acquired under the time constraints specified in CAMA. In addition, porewater and
groundwater sample results (other than background) which were collected during the CSA with
turbidity readings greater than 10 NTU were utilized in the contaminant fate and transport
modeling discussed in Section 4. This should be taken into consideration when evaluating the
results of the fate and transport model and considering the risk classification for the DRSS site.
Background groundwater concentrations for the DRSS site, PPBCs, and regional background
data are presented in Table 2-2. Background concentrations reported at DRSS are limited to
samples collected from wells with turbidity less than 10 NTU. PPBCs represent either the
highest detected value or the highest laboratory reporting limit as observed in compliance
monitoring well MW-23D. Background concentrations identified for new background wells are
presented for reference only; these concentrations will be incorporated into statistical
background analysis once a sufficiently robust data set has been obtained.
The Regional Background Groundwater Concentrations present publicly available groundwater
data from various local regions, including NURE data collected form within 20 miles of the
DRSS, county-wide background data collected from Rockingham County, where DRSS is
located, and state-wide data collected from throughout North Carolina. The 2-10 Private Well
Data field provides the range of values found for each constituent sampled in private wells
owned by Duke employees living between 2 and 10 miles from the DRSS waste boundary. The
2-10 Private Well results are provided for reference only due to the lack of well construction
data, hydrostratigraphic data, and detailed geological context for the sample locations in
question A detailed analysis of DRSS background groundwater concentrations are provided in
Appendix B.
If the PPBC is greater than the 2L Standard or IMAC, the PPBC will be used for comparison;
conversely, if the PPBC is less than the 2L Standard or IMAC, the 2L Standard or IMAC will be
used.

Corrective Action Plan Part I
Dan River Steam Station Ash Basin

26
Table 2-2. Background Groundwater Concentrations for the DRSS Site: Ranges of
Samples with Turbidity <10 NTU
Constituent
Regional
Background
Groundwater
Concentrations
(µg/L)
2-10 Private Well
Data (May 2015-
August 2015)
(µg/L)
MW-23D -
Compliance Well
Background
Groundwater
Concentrations
(2010 to 2015)
(µg/L)
New
Background
Wells
Groundwater
Concentrations
(June 2015)
(µg/L)
Proposed
Provisional
Background
Groundwater
Concentrations
(µg/L)
Antimony* <6
(North Carolina) <1 to 1.4 <1 <0.5 to 1.16 1
Arsenic
0.5 to 20
(Rockingham
County)
<1 to 14.1 <1 < 1 to 6.29 1
Beryllium* Not reported <0.2 to <1 <0.2 <1 0.2
Boron Not Reported <5 to <50 <50 <50 50
Chromium
0.5 to 40
(Rockingham
County)
<0.5 to <5 <5 <1 5
Cobalt* 1 to 2 <0.5 to <1 <0.5 <1 0.5
Hexavalent
Chromium Not reported <0.03 to <0.06 0.089J No data 0.09
Iron
25 to 27,270
(Rockingham
County)
<10 to 118 <10 <10 to 386 10
Lead
2.5 to 115
(Rockingham
County)
<1 to 4.06 <10 <0.1 to <1 10
Manganese
Below detect to
785.8
(20-mile radius
from site)
<5 to 110 63 31 to 920 63
pH
7.01 to 8.5 SU
(20-mile radius
from site)
5.73 to 8.02 SU 6.65 to 6.78 SU 5.45 to 7.47 SU 6.65 to 6.78 SU
Radium-226
Radium-228
(combined)
Not reported Not reported Not reported 1.22 to 2.357
pCi/L
2.357
pCi/L
Selenium
2.5 to 26
(Rockingham
County)
<0.5 to <1 <1 <0.5 to <1 1
Sulfate
29,000 (mean
value)
(Dan River-
Danville Area
Mesozoic Basin)
960 to 10,000 30,000 to 31,000 12,000 to 41,000 31,000
Thallium*
<1
(Blue Ridge
Mountain and
Piedmont
aquifers)
<0.1 to <0.2 <0.2 <0.1 to <0.2 0.2
TDS Not reported 50,000 to
150,000
200,000 to
280,000
110,000 to
270,000 280,000
Vanadium*
0.1 to 1.2
(20-mile radius
from site)
<300 to 1,330 <1 <0.3 to 2.75 1
Notes:
1. µg/L = micrograms per liter; SU = Standard Units; pCi/L = picocuries per liter
2. < indicates concentration less than laboratory reporting limit.
3. J = Estimated concentration; J+ = Estimated concentration, biased high
Corrective Action Plan Part I
Dan River Steam Station Ash Basin

27
Notes (cont’d):
4. * Indicates 2L Standard not established for constituent; therefore, IMAC used for screening criteria.
5. Regional groundwater concentration data is from NURE data in a 20-mile radius from the site for all constituents
contained in the NURE database. DHHS county-level data was subsequently used for all constituents available.
Remaining constituents for which there is no NURE or DHHS data were pulled from the most spatially relevant,
publicly available sources. Further source information is found in section 10.1 of the DRSS CSA report.
6. Reported range for COIs cobalt, and vanadium is from an NPDES sampling event in May 2015 and a CSA
sampling event in June 2015. Only June 2015 data were provided in the CSA report.
7. Reported compliance monitoring well (MW-23D) concentration ranges for hexavalent chromium, cobalt, and
vanadium are from an NPDES sampling event in April 2015 and/or the June 2015 CSA sampling event. Only
June 2015 data were provided in the CSA report. These constituents were historically not analyzed for as part of
the NPDES sampling program.
8. See Appendix B for determination of PPBCs.

2.2.2 Porewater Exceedances of 2L or IMACs
Porewater refers to water samples collected from monitoring wells installed in the ash basins
and ash storage areas screened within the ash layer. HDR does not consider porewater results
to represent groundwater, but the results are compared to 2L Standards or IMACs for purposes
of discussion. Porewater results are representative of source characterization data with respect
to contamination at the site that is attributed to ash handling and storage. Note that porewater
was not evaluated for remediation in this CAP Part I because porewater will be eliminated
during ash basin closure activities.
Porewater results for COIs, along with a comparison to applicable regulatory standards or
guidelines, are provided in Table 2-3.
Table 2-3. Ash Basin Porewater Results for COIs Compared to 2L Standards or IMACs
and Frequency of Exceedances
COI 2L Standard or
IMAC (µg/L)
Groundwater
Concentrations
Exceeding 2L Standards
or IMACs (µg/L)
Number of Samples
Exceeding 2L Standards or
IMACs/Number of Samples
Antimony* 1 0.94J+ to 11.6 4/5
Arsenic 10 96.4 to 442 4/5
Barium 700 810 1/5
Chromium 10 50 1/5
Cobalt* 1 1.8 to 34.1 4/5
Hexavalent
Chromium 0.07 0.678 1/3
Iron 300 5,800 to 25,100 4/5
Lead 15 77.4 1/5
Manganese 50 72 to 1,500 5/5
Thallium* 0.2 0.54 to
Vanadium* 0.3 0.62J to 254 5/5
Notes:
1. µg/l - micrograms per liter
2. * Indicates 2L Standard not established for constituent; therefore, IMAC used for screening criteria.

Corrective Action Plan Part I
Dan River Steam Station Ash Basin

28
2.2.3 Groundwater Exceedances of 2L Standards or IMACs
Groundwater contamination at the site that is attributed to ash handling and storage was
delineated during the CSA activities with the following exceptions:
 Vertical extent in the vicinity of Ash Storage 1,
 Horizontal and vertical extent north of Ash Storage 1, and
 Groundwater-surface water interaction north of the secondary cell.

Additional assessment is planned to address the exceptions noted above. These exceptions
were identified as data gaps in the CSA. Information gathered from additional assessment will
be submitted under a separate cover.
Groundwater results for COIs, along with a comparison to applicable regulatory standards or
guidelines, are provided in Table 2-4. Groundwater sample locations and analytical results are
depicted on Figure 2-1.
Table 2-4. Groundwater Results for COIs Compared to PPBCs, 2L Standards, or IMACs,
and Frequency of Exceedances
COI
Proposed
Provisional
Background
Concentrations
(µg/L)
2L Standard or
IMAC (µg/L)
Groundwater
Concentrations
Exceeding 2L
Standards or
IMACs (µg/L)
Number of
Samples
Exceeding 2L
Standards or
IMACs/Number of
Samples
Beneath the Ash Basin
Antimony* 1 1 1.5 to 2.6 5/23
Arsenic 1 10 11.7 to 442 3/23
Boron 50 700 700 to 820 3/23
Chromium 5 10 30.1 1/23
Cobalt* 0.25 1 1.1 to 44.7 4/23
Hexavalent
Chromium 0.089 0.07 0.871J to 4.51 3/7
Iron 3,111 300 377 to 36,400 13/23
Manganese 566 50 110 to 2,200 18/23
pH 5.47 to 8.5 SU 6.5 to 8.5 SU 4.92 to 9.12 SU 7/23
TDS 300,000 500,000 844,000 1/23
Vanadium* <1 0.3 0.33 to 11.1 18/23
Within and Beneath the Ash Storage Areas
Antimony* 1 1 2.9 1/12
Boron 50 700 810 to 1,310 4/12
Chromium 5 10 24.8 1/12
Cobalt* 0.25 1 1.1 to 13.0 6/12
Hexavalent
Chromium 0.089 0.07 No Exceedances 0/2
Iron 3,111 300 385 to 4,480 11/12
Manganese 566 50 270 to 1,800 11/12
Corrective Action Plan Part I
Dan River Steam Station Ash Basin

29
COI
Proposed
Provisional
Background
Concentrations
(µg/L)
2L Standard or
IMAC (µg/L)
Groundwater
Concentrations
Exceeding 2L
Standards or
IMACs (µg/L)
Number of
Samples
Exceeding 2L
Standards or
IMACs/Number of
Samples
Selenium 1 20 35.3 1/12
pH 5.47 to 8.5 SU 6.5 to 8.5 SU 5.51 to 10.67 SU 4/12
TDS 300,000 500,000 530,000 1/12
Vanadium* <1 0.3 0.69J to 15.3 12/12
Beyond the Waste Boundary
Antimony* 1 1 2.1 to 2.5 3/24
Chromium 5 10 11.0 to 17.7 4/24
Cobalt* 0.25 1 1.02 to 17.7 14/24
Hexavalent
Chromium 0.089 0.07 0.105 1/3
Iron 3,111 300 307 to 21,200 17/24
Manganese 566 50 94 to 3,700 21/24
pH 5.47 to 8.5 SU 6.5 to 8.5 SU 5.39 to 11.44 SU 13/24
Sulfate 46,000 250,000 307,000 to
416,000 3/24
Thallium* 1 0.2 0.256 to 0.5 2/24
TDS 300,000 500,000 690,000 to
873,000 3/24
Vanadium* <1 0.3 0.3 to 8.4 20/24
At or Beyond the Compliance Boundary
Chromium 5 10 13.8 1/10
Cobalt* 0.25 1 1.2 to 17.7 7/10
Iron 3,111 300 310 to 5,100 9/10
Manganese 566 50 120 to 1,000 10/10
pH 5.47 to 8.5 SU 6.5 to 8.5 SU 5.39 to 6.4 SU 6/10
Sulfate 46,000 250,000 307,000 1/10
TDS 300,000 500,000 705,000 1/10
Vanadium* <1 0.3 0.67J to 5.65 9/10
Notes:
1. µg/L = micrograms per liter
2. SU = Standard Units
3. J = Estimated concentration
4. < indicates concentration less than laboratory reporting limit.
5. * Indicates 2L Standard not established for constituent; therefore, IMAC used for screening criteria.
Observations related to groundwater COIs at DRSS are:
 Of the COIs listed in Table 2-4, chromium, hexavalent chromium, cobalt, iron,
manganese, pH, thallium, and vanadium exceeded their respective 2L Standards,
IMACs, or North Carolina Department of Health and Human Services (NCDHHS)
recommendations for hexavalent chromium in both background wells, and wells within
and downgradient of the compliance boundary. Chromium exceedances were limited to
the ash basin and ash storage areas. Thallium exceeded its IMAC in one background
Corrective Action Plan Part I
Dan River Steam Station Ash Basin

30
location and one source area location. Hexavalent chromium was analyzed in select
groundwater monitoring wells (primarily along presumed groundwater flow paths) at the
site and will need further evaluation as to prevalence at the DRSS site.
 Based on the PPBCs provided in Section 2.2.1, iron, manganese, pH, and thallium
concentrations should be compared to the PPBCs and not their respective 2L Standards
or IMAC.
 Antimony and selenium exceeded their respective 2L Standards or IMACs and PPBCs in
isolated areas within the ash basin and ash storage areas.
 Antimony exceedances were detected in wells within and downgradient of ash
management areas; however, it was also detected above the IMAC at MW-23BR, which
is a background well. Antimony is naturally occurring in some sedimentary rock
formations.
 Selenium concentrations were detected above the 2L Standard at only one location in
Ash Storage 1 (AS-10D).
 Due to the isolated occurrence of antimony and selenium, further assessment is
necessary to determine if these COIs are attributable to ash handling activities at the
site; therefore, these constituents cannot be ruled out as COIs as part of this CAP Part I.

Of the COIs listed above, boron, pH, sulfate, and TDS are considered to be detection monitoring
constituents and are listed in 40 CFR 257 Appendix III of the USEPA’s Hazardous and Solid
Waste Management System; Disposal of Coal Combustion Residuals from Electric Utilities
(CCR Rule). The USEPA considers these constituents to be potential indicators of groundwater
contamination from CCR as they move rapidly through the surface layer, relative to other
constituents, and thus provide an early detection of whether contaminants are migrating from
the CCR unit. Additional details regarding the CCR Rule and applicable constituents can be
found in the CSA report (HDR 2015).
2.2.4 Radionuclides in Groundwater
Radionuclides may be present in groundwater from natural sources (e.g., soil or rock). The
USEPA regulates various radionuclides in drinking water. The following radionuclides were
analyzed as part of the CSA: radium-226, radium-228, natural uranium, uranium-233, uranium-
234, and uranium-236. Ten monitoring wells (AB-10S/SL/D, AB-25S/D/BR, MW-11, MW-11D,
and BG-5S/D) were sampled for these analytes, and results of this analysis are presented in
Table 2-5.
Corrective Action Plan Part I
Dan River Steam Station Ash Basin

31
Table 2-5. Radionuclide Concentrations
Radionuclide USEPA Reporting
Limit Standard*
Background
Concentrations
Source Area
Porewater
Concentrations
Source Area
Groundwater
Concentrations
Radium-226 5 pCi/L
(combined)
1.22 to 2.357
pCi/L
(combined)
1.07 to 1.58 pCi/L
(combined)
<1 J to 5.47 pCi/L
(combined) Radium-228
Natural Uranium 30 µg/L 0.000135J to
0.00113 µg/L
0.00166 to
0.00343 µg/L
0.000197 J to
0.0016 µg/L
Uranium-233 30 µg/L
(combined)
<0.0005 µg/L
(combined)
<0.00005 µg/L
(combined)
< 0.00005 µg/L
(combined) Uranium-234
Uranium-236
Notes:
1. pCi/L = Picocuries per liter
2. µg/L = micrograms per liter
3. J = Estimated concentration
4. < indicates concentration less than laboratory reporting limit.
5. * USEPA Reporting Limit Standard for uranium of 30 µg/L assumes combined concentration for all
isotopes.

As shown in the table, concentrations of radium-226 and radium-228 are slightly higher in
background concentrations as compared to those reported in the source area porewater, but
concentrations in the source area groundwater are higher than both the background and
porewater concentrations. Natural uranium was reported higher in the source area than
concentrations reported in the background. Uranium-223, uranium-234, and uranium-236 were
not reported above the laboratory reporting limit at any of the locations sampled. Based on a
review of available radiological data, additional data for radionuclides at site is needed for a
more comprehensive assessment.
2.3 Seeps
Seep samples collected from locations near an unnamed tributary along the eastern property
boundary and downgradient of the secondary cell (S-1 through S-3) and the southwestern toe of
the primary cell dam (S-4) were compared to the 2L Standards or IMACs. Seep samples were
also collected from four NCDEQ identified seeps (INFSW009, CSSW001, CCSW002OUT, and
DRRC001) and are compared to 2B Standards due to influence of surface water at these
locations. S-1 through S-3 and INFSW009 were dry at time of sampling. There is no background
comparison available for seeps since they are all hydraulically and topographically
downgradient of the ash basin and ash storage areas.
NCDEQ seep results for COIs, along with a comparison to 2B Standards or USEPA Criteria are
provided in Table 2-6A. Seep results for COIs and comparison to 2L Standards or IMACs are
provided in Table 2-6B. Seep sample locations and analytical results are shown on Figure 2-2.
Corrective Action Plan Part I
Dan River Steam Station Ash Basin

32
Table 2-6A. NCDEQ Seep Results for COIs Compared to 2B Standards or USEPA Criteria
and Frequency of Exceedances
COI
2B Standard or
USEPA Guidance
(µg/L)
NCDEQ Seep
Concentrations Exceeding
2B Standards or USEPA
Criteria (µg/L)
Number of Samples
Exceeding 2B
Standards or
USEPA Criteria/
Number of Samples
Cobalt* 4 No Exceedance 0/3
Iron 1,000 No Exceedance 0/3
Manganese 100 160 to 2,200 3/3
Vanadium* NS No Exceedance 0/3
Notes:
1. µg/l - micrograms per liter
2. * Indicates 2B Standard not established for constituent; therefore, USEPA criteria used.

Table 2-6B. Seep Results for COIs Compared to 2L Standards or IMACs and Frequency of
Exceedances
COI 2L Standard or
IMACs (µg/L)
Seep Concentrations
Exceeding 2L Standards or
IMACs (µg/L)
Number of Samples
Exceeding 2L
Standards or IMACs/
Number of Samples
Cobalt* 1 4.88 1/1
Iron 300 5,720 1/1
Manganese 50 829 1/1
Vanadium* 0.3 8.06 1/1
Notes:
1. µg/l - micrograms per liter
2. * Indicates 2L Standard not established for constituent; therefore, IMAC used for screening criteria.

The following seep COIs will be considered for corrective action:
 Cobalt
 Iron
 Manganese
 Vanadium

2.4 Surface Water
Surface water samples were obtained during the CSA from locations along the Dan River and
two tributary streams that flow north to south along the eastern and western property boundary.
Surface water concentrations were compared to the applicable 2B Standards for Class C
waters. In the absence of a 2B Standard, constituent concentrations were compared to USEPA
National Recommended Water Quality Criteria.
Surface water results for COIs, compared to upgradient surface water concentrations (SW-5)
and applicable regulatory standards, are provided in Table 2-7. Surface water sample locations
and analytical results are depicted on Figure 2-2. Where surface water sample exceedances of
Corrective Action Plan Part I
Dan River Steam Station Ash Basin

33
total constituents existed, the dissolved constituent result did not exceed the applicable 2B
Standards.
Table 2-7. Surface Water Results for COIs Compared to Upgradient Surface Water
Concentrations, 2B or USEPA Standards, and Frequency of Exceedances
COI
Concentrations
Exceeding 2B
Standards (µg/L)
2B Standard
or USEPA
Criteria
(µg/L)
Upgradient
Surface Water
Concentrations
(µg/L)
Number of Samples
Exceeding 2B
Standard/Number of
Samples
Dan River Surface Water
Lead 0.88 and 0.97 0.54 <1.0 to 0.64 2/2
Tributary Surface Water
Aluminum* 127 and 27,600 87 90 2/2
Arsenic 63.7 10 <1.0 to 0.22J 1/2
Chromium 33.6 24 <1.0 to 0.8 1/2
Cobalt* 15.5 3 <1.0 to 0.32J 1/2
Copper 27.3 2.7 1.3 to 4.62J+ 1/2
Lead 20.1 0.54 <1.0 to 0.64 1/2
Notes:
1. µg/L = micrograms per liter
2. J = estimated concentration
3. J+ = estimated concentration, biased high
4. < indicates concentration less than laboratory reporting limit.
5. * Indicates USEPA National Recommended Water Quality Criteria used for constituent.

Based on a comparison of surface water concentrations at SW-3 (tributary northeast of
secondary cell) to surface water concentrations at SW-5 (upgradient tributary), the following
COIs will be considered for corrective action:
 Aluminum
 Arsenic
 Chromium

Copper exceeded its 2B Standard in SW-3 and SW-5. Lead exceeded its 2B Standard at, SW-6,
SW-7, and SW-8. Based on exceedances in background locations within the tributary and the
Dan River, copper and lead are not considered COIs in surface water. There were no other
COIs identified above the laboratory reporting limit in the Dan River surface water samples
collected at SW-6 and SW-7.
2.5 Sediments
Sediment samples were collected at the same time as each of the surface water samples (SW-3
through SW-8) and seep samples (S-1 through S-3) with the exception of S-4. In the absence of
NCDEQ sediment criteria, the sediment sample results were compared to the NC PSRGs for
POG.
Corrective Action Plan Part I
Dan River Steam Station Ash Basin

34
Seep and tributary sediment results for COIs, along with a comparison to NC PSRGs for POG
are provided in Table 2-8. Sediment sample locations and analytical results are depicted on
Figure 2-3.
Table 2-8. Sediment COIs Compared to NC PSRGs for POG and Frequency of
Exceedances
COI
Concentrations Beneath
Ash Basin Exceeding
NC PSRGs for POG
(mg/kg)
NC PSRGs for POG
(mg/kg)
Number of Samples
Exceeding NC PSRGs
for POG/Number of
Samples
Arsenic 7J to 285 5.8 3/9
Cobalt 3.9J 17.1 0.9 8/9
Iron 5,850 to 32,000 150 9/9
Manganese 133 to 3,880 65 9/9
Vanadium 6.2J to 58.8 6 9/9
Notes:
1. mg/kg = milligrams per kilogram
2. J = Estimated concentration
Cobalt, chromium, iron, manganese, and vanadium exceeded their respective NC PSRGs for
POG; however, based on exceedances in upgradient locations within the tributary and Dan
River, these constituents are not considered to be COIs in sediment. Arsenic will be considered
as a COI for corrective action.
2.6 Soils
2.6.1 Background Soil and Concentrations
Because some constituents are naturally occurring in soil and are present in the source areas,
establishing background concentrations is important for determining whether releases have
occurred from the source areas. Boring locations that have been determined to represent
background conditions (see Section 2.2.1) from which background soil samples were collected
include BG-1D, BG-5S/D, GWA-9S/D, GWA-12S/D and SB-1 through SB-3 (Figure 1-4).
Samples shallower that 5 feet below ground surface (bgs) were not included in the population of
background samples because of a concern that aerial deposition of dust may have impacted
these shallow soils. Site geology was reviewed to determine that the soils were from the same
geologic formations and thus could be pooled as a single population. PWR and bedrock
samples were not included in the calculations for soil background statistics because the
mineralogy may be different. There were not enough samples collected in PWR and bedrock to
support the development of separate background statistics for these solid matrices.
Soil PPBCs (i.e., the 95% upper tolerance limit [UTL]) were calculated for those constituents
analyzed in background soil borings, as shown in Table 2-9. The methodology followed ProUCL
Technical Guidance, Statistical Software for Environmental Applications for Data Sets with and
without Nondetect Observations (USEPA 2013). A detailed method review, statistical
evaluation, and results for the PPBCs are included in Appendix B.
Corrective Action Plan Part I
Dan River Steam Station Ash Basin

35
Table 2-9. Proposed Provisional Background Soil Concentrations
Constituent Number of
Samples
Number of
Detections
Range
(mg/kg)
Proposed Provisional
Background Soil
Concentrations
(95% UTL) (mg/kg)
Aluminum 23 23 7,360 to 38,900 40,400
Antimony 23 0 <5.1 to <7.6 7.6*
Arsenic 23 12 3.1 to 30.6 23
Barium 23 23 34.2 to 242 281
Boron 23 5 <13.4 to 63.1 63.9
Cadmium 23 1 0.44 to <0.91 0.91*
Calcium 23 20 77.4 to 39,100 118,100
Chloride 23 1 168 to <372 372*
Chromium 23 23 6.4 to 187 144
Cobalt 23 23 7.5 to 42.6 36
Copper 23 23 2.8 to 79.5 79.5
Iron 23 23 12,200 to 95,900 88,550
Lead 23 23 8.2 to 31.3 29.9
Magnesium 23 23 924 to 19,400 38,120
Manganese 23 23 82.7 to 5170 4,150
Mercury 23 17 0.0047 to 0.04 0.041
Molybdenum 23 9 1.6 to 22.6 13.5
Nickel 23 23 6.7 to 52.2 59.7
Nitrate 23 0 <25.8 to <37.2 37.2*
pH (field) 23 23 4.5 to 8.6 4.5 to 8.6*
Potassium 23 23 168 to 2,730 3,060
Selenium 23 2 3.9 to 7.8 7.8*
Sodium 23 4 160 to <378 2,190
Strontium 23 23 1.9 to 257 569
Sulfate 23 3 145 to <372 372*
Thallium 23 0 <5.1 to <7.6 7.6*
TOC 23 23 457 to 23,800 50,150
Vanadium 23 23 8.4 to 75.7 78.3
Zinc 23 23 23.5 to 203 217
Notes:
1. mg/kg = milligrams per kilogram
2. UTL – upper tolerance limit
3. * Value shown is highest detection or highest ND. Too few detections to develop UTL.

2.6.2 Soil Exceedances of NC PSRGs for POGs
The horizontal and vertical extent of soil contamination at the site attributed to ash handling and
storage was delineated in the CSA (HDR 2015) with the exception of horizontal and vertical
extent north of Ash Storage 1. Additional assessment is planned to address the exceptions
noted above. These exceptions were identified as data gaps in the CSA and will be addressed
under a separate cover. Soil results for COIs, along with a comparison to NC PSRGs for POG,
soil PPBCs, and background concentrations are provided in Table 2-10. Soil sample locations
and analytical results are depicted on Figure 2-3.
Corrective Action Plan Part I
Dan River Steam Station Ash Basin

36
Table 2-10. Soil Results for COIs Compared to NC PSRGs for POG, Background
Concentrations, and Frequency of Exceedances
COI
Concentrations
Exceeding NC
PSRGs for
POG (mg/kg)
NC PSRGs
for POG
(mg/kg)
Background
Soil
Concentrations
(mg/kg)
Soil PPBCs
(mg/kg)
Number of
Samples
Exceeding NC
PSRGs/Number
of Samples
Beneath Ash Basin
Arsenic 3.6 to 71.8 5.8 3.9J to 20.0 23 5/23
Chromium 11.4 to 54.1 3.8 15.5 to 111 144 23/23
Cobalt 5.5J to 26.3 0.9 5.6J to 42.6 36 23/23
Iron 6,400 to 36,400 150 16,500 to
95,900 88,550 23/23
Manganese 76.3 to 1,010 65 82.7 to 2,840 4,150 18/23
Selenium 2.8J to 6.75 2.1 <5.4 to <7.2 7.8 4/23
Vanadium 8.1 to 125 6 16 to 54.4 78.3 23/23
Beneath the Ash Storage Areas
Arsenic 3.7J to 39.9 5.8 3.9J to 20.0 23 3/18
Chromium 7.6 to 60.9 3.8 15.5 to 111 144 18/18
Cobalt 1.7J to 49.6 0.9 5.6J to 42.6 36 18/18
Iron 9,660 to 53,500 150 16,500 to
95,900 88,550 18/18
Manganese 139 to 1,600 65 82.7 to 2,840 4,150 17/18
Selenium 3.0J to 6.75 2.1 <5.4 to <7.2 7.8 4/18
Vanadium 11.5 to 48.3 6 16 to 54.4 78.3 18/18
Beyond the Waste Boundary
Arsenic 3.2J to 30.6 5.8 3.9J to 20.0 23 5/40
Chromium 6.4 to 67.1 3.8 15.5 to 111 144 40/40
Cobalt 3.5J to 33.3 0.9 5.6J to 42.6 36 40/40
Iron 5,260 to 63,300 150 16,500 to
95,900 88,550 40/40
Manganese 97.7 to 5,170 65 82.7 to 2,840 4,150 40/40
Selenium 2.7J to 7.8 2.1 <5.4 to <7.2 7.8 14/40
Vanadium 8.4 to 75.7 6 16 to 54.4 78.3 40/40
At or Beyond the Compliance Boundary
Arsenic 3.2J to 30.6 5.8 3.9J to 20.0 23 5/16
Chromium 6.4 to 187 3.8 15.5 to 111 144 16/16
Cobalt 9.3 to 27.3 0.9 5.6J to 42.6 36 16/16
Iron 12,200 to
63,300 150 16,500 to
95,900 88,550 16/16
Manganese 267 to 5,170 65 82.7 to 2,840 4,150 16/16
Selenium 2.7 to 10.2 2.1 <5.4 to <7.2 7.8 7/16
Vanadium 8.4 to 75.7 6 16 to 54.4 78.3 16/16
Notes:
1. mg/kg = milligrams per kilogram
2. J = Estimated concentration
3. < indicates concentration less than laboratory reporting limit.

Corrective Action Plan Part I
Dan River Steam Station Ash Basin

37
With the exception of selenium, these COIs appear in one or more of the background soil boring
locations at concentrations exceeding the NC PSRGs for POG. Selenium exceeds its NC
PSRGs for POG and is attributable to ash handling and storage at the site.
The following COIs exceed soil PPBCs and will be considered COIs for corrective action:
 Arsenic
 Chromium
 Cobalt
 Manganese
 Selenium
 Vanadium

2.7 PWR and Bedrock
As requested by NCDEQ, samples of PWR and bedrock were obtained from rock cores during
the CSA, pulverized, and analyzed for ash-related constituents as unconsolidated material.
While these analyses provide some insight into constituent concentrations in the bedrock flow
layer, these data are not representative of in-situ PWR/bedrock conditions. Once pulverized,
hydraulic (e.g., porosity) and geochemical properties of the PWR/bedrock are changed. For this
reason, further evaluation of constituents in solid matrix PWR or bedrock will not be conducted.
2.8 COI Screening Evaluation Summary
Table 2-11 summarizes COIs (by geomedium) identified in Sections 2.1 through 2.7 and
identifies those that require further evaluation to determine if they require possible corrective
action. Further evaluation will be performed (as applicable) by developing 3-D groundwater
model (Section 4) for these COIs.
Corrective Action Plan Part I
Dan River Steam Station Ash Basin

38
Table 2-11. Updated COI Screening Evaluation Summary
Potential
COI COIs by Media COI To Be
Further
Assessed
in
Section 4
Note Solid/
Aqueous Ash Pore
Water
Ground-
water
Surface
Water Seeps Sediment Soil
Aluminum √ - - √ - - - √
Antimony - - √ - - - - √
Arsenic √ √ √ √ √ √ √
Barium - - - - - - - √
Boron - - √ - - - - √
Chromium - √ √ √ - - √ √
Cobalt √ √ √ - √ - √ √
Copper - - - - - - - - 1
Iron √ √ √ √ √
Lead - - - - - - - - 1
Manganes
e √ √ √ - √ - √ √
pH - - √ - - - - √
Selenium - - √ - √ - - √
Sulfate - - √ - - - - √
Thallium - √ - - - - - - 2
TDS √ - √ - - - - √
Vanadium - - √ - √ - √ √
Notes:
1. Only identified in SW samples, but was not included as COI due to comparison to upgradient sample
locations.
2. Identified as COI in groundwater, but was not included due to no exceedance of PPBCs developed for the
site.

2.9 Interim Response Actions
2.9.1 Source Control
No interim response actions are necessary at the DRSS site because there are no identified
imminent hazards to human health or the environment.
In conjunction with decommissioning activities and in accordance with CAMA requirements,
Duke Energy plans to permanently close the DRSS ash basin by August 1, 2019. Closure of the
DRSS ash basin was defined in CAMA as excavation of ash from the site, and beneficial reuse
of the material or relocation to a lined structural fill or landfill. As part of the DRSS ash basin
closure process, Duke Energy submitted a coal ash excavation plan to state regulators in
November 2014. The excavation plan details a multiphase approach for removing coal ash from
the site with an emphasis on the first 12 to 18 months of activities.
During Phase I of the excavation, an estimated 1.2 million tons of material will be excavated
from the primary and secondary cells and/or ash storage areas. This material is planned to be
taken to the Maplewood (Amelia) Landfill in Jetersville, Virginia. An alternate landfill, Atlantic
Landfill in Waverly, Virginia, was identified to accept material excavated in the event of any
Corrective Action Plan Part I
Dan River Steam Station Ash Basin

39
issues at the Maplewood Landfill. Ash will be transported by rail car to the landfill and
construction of a rail loading system to accommodate this transport of ash is currently in
progress. Should the situation arise that rail car transportation is disrupted, truck transportation
will be utilized. Subsequent phase(s) will remove the remaining ash at the site. Duke Energy is
currently permitting an on-site landfill that will be located in the approximate footprint of Ash
Storage 1. The landfill will be lined and construction is projected to be completed by June 2017.
The permit-to-construct application was submitted to the NCDEQ Division of Waste
Management in the third quarter of 2015. The construction schedule will depend upon receipt of
the required permits.
Duke Energy will begin moving coal ash within 60 days after receiving necessary permits and
approvals. Dewatering of the ash basins will begin along with project planning for later phases
to identify storage options for the remaining ash on the plant property. Ash impoundments will
be closed by August 1, 2019.
2.9.2 Groundwater Response Actions
Based on the results of CSA activities, impacted groundwater has not migrated beyond the
Duke Energy property boundary of the DRSS site. A data gap was identified in the CSA as
groundwater monitoring well GWA-15D on the eastern edge of Duke Energy’s property had
concentrations of iron, manganese, sulfate, TDS and vanadium detected above their respective
2L or IMACs. These exceedances are being further evaluated as part of ongoing groundwater
assessment at the DRSS site. Furthermore, the results of surface water sampling indicate no
impacts from groundwater flow into the Dan River.
COIs listed in Section 2.7 are further evaluated in Sections 3 and 4 to continue refining the list
of COIs that will be addressed in CAP Part II. As part of CAP Part II, a Monitored Natural
Attenuation (MNA) assessment will be completed and additional model refinement will be
conducted, which may eliminate additional COIs from further evaluation. COIs that remain after
this process will be considered for potential corrective action(s).
Corrective Action Plan Part I
Dan River Steam Station Ash Basin

40
3 Site Conceptual Model
The Site Conceptual Model (SCM) is an interpretation of processes and characteristics
associated with hydrogeologic conditions and COI interactions at the site. The purpose of the
SCM is to evaluate areal distribution of COIs with regard to site-specific geological/
hydrogeological and geochemical properties at the DRSS site. The SCM was developed
utilizing data and analysis from the CSA (HDR 2015). The sources and areas with 2L Standard
or IMAC exceedances of COIs attributable to ash handling are illustrated in the 3-D SCM
presented in Figure 3-1 and in cross-sectional view in Figure 3-2.
3.1 Site Hydrogeologic Conditions
Site hydrogeologic conditions were evaluated through the installation and sampling of 63
monitoring wells. The wells were screened within the shallow, deep, and bedrock flow layers
beneath the site. Additional information obtained during in-situ testing (packer testing) and slug
testing was also utilized to evaluate site conditions. A fracture trace analysis was performed for
the DRSS site, as well as on-site/near-site geologic mapping, to further understand the site
geology in support of the SCM.
3.1.1 Hydrostratigraphic Units
The following materials were encountered during the groundwater assessment site exploration
and are consistent with material descriptions from previous site exploration:
 Ash (A) – Ash was encountered in borings advanced within the ash basin and ash
storage areas, as well as in some borings advanced through the pond perimeter and
intermediate dam. Ash several inches thick was encountered in one location within the
ash dredge area located between Ash Storage 1 and Ash Storage 2. Ash was generally
described as gray to dark bluish gray with a silty to sandy texture, consistent with fly ash
and bottom ash. The range of ash thickness observed at the DRSS site was 0 to 79 feet.
 Fill (F) – Fill material generally consisted of re-worked materials from the DRSS site.
The base of filled areas was difficult to distinguish from in-place soil/saprolite. The range
of fill thickness observed at the DRSS site was 0 to 31 feet.
 Alluvium (S) – Alluvium is unconsolidated soil and sediment that has been eroded and
redeposited by streams and rivers. The range of alluvium thickness observed at the
DRSS site was 0 to 29 feet.
 Residuum (M1) – Residuum is the in-place soil that develops by weathering. The range
of residuum thickness observed at the DRSS site was 0 to 45 feet.
 Saprolite/Weathered Rock (M1/M2) – Saprolite is soil developed by in-place
weathering of rock. The range of saprolite/weathered rock observed at the DRSS site
was 0 to 22 feet.
 Partially Weathered/Fractured Rock (TZ) – Partially weathered (slight to moderate)
and/or highly fractured rock was encountered below refusal (auger, casing advancer,
etc.). The range of transition zone thickness observed at the DRSS site was 0 to 20 feet.
Corrective Action Plan Part I
Dan River Steam Station Ash Basin

41
 Bedrock (BR) – Bedrock is slightly weathered to fresh and relatively unfractured sound
rock that was encountered in boreholes. The maximum depth that borings extended into
bedrock was 67 feet.
Based on the site investigation conducted as part of the CSA, the groundwater system in the
natural materials (alluvium, soil, soil/saprolite, and bedrock) is consistent with the regolith-
fractured rock system and is characterized as an unconfined, connected aquifer system. The
groundwater system beneath the DRSS site is divided into the following three layers to
distinguish the connected aquifer system: the shallow flow layer, deep flow layer, and bedrock
flow layer. Hydrostratigraphic units are shown on cross sections presented in the CSA report.
3.1.2 Hydrostratigraphic Unit Properties
Material properties utilized within the groundwater flow and transport model are total porosity,
effective porosity, specific yield, and specific storage. These properties were developed from
laboratory testing of ash, fill, alluvium, and soil/saprolite and are presented in the CSA report.
Specific yield/effective porosity was determined for a number of samples within the A, F, S, M1,
and M2 hydrostratigraphic units to provide an average and range of expected values.
These properties were obtained through in-situ permeability testing (falling head, constant head,
and packer testing where appropriate); slug tests in completed monitoring wells; and laboratory
testing of undisturbed samples (ash, fill, soil/saprolite). Results from these tests were utilized to
develop the groundwater flow and fate and transport model further discussed in Section 4.
3.1.3 Potentiometric Surface – Shallow and Deep Flow Layers
The shallow and deep flow layers were defined by data obtained from the shallow and deep
groundwater monitoring wells (S and D wells, respectively). In general, groundwater within the
shallow and deep flow layers flows from the northern extent of the DRSS site property boundary
south and southeast toward the Dan River. However, in the area north of Ash Storage 1,
groundwater elevation data indicate the presence of a groundwater divide extending from MW-
12 east to GWA-1. To the north of this divide, localized groundwater within the shallow and
deep flow layers flow north, away from the DRSS site and toward an unnamed tributary that
flows to the Dan River. The potentiometric surfaces for shallow and deep flow layers, based on
groundwater level measurements obtained on September 2, 2015, are illustrated on Figures 3-
3 and 3-4.
3.1.4 Potentiometric Surface – Bedrock Flow Layer
The bedrock flow layer was defined by data obtained from the bedrock groundwater monitoring
wells (BR wells). In general, groundwater within the bedrock flow layer is consistent with
observed flow directions in the shallow and deep flow layers. The potentiometric surface for the
bedrock flow layer, based on groundwater lever measurements obtained on September 2, 2015,
is illustrated on Figure 3-5.

Corrective Action Plan Part I
Dan River Steam Station Ash Basin

42
3.1.5 Horizontal and Vertical Hydraulic Gradients
3.1.5.1 Horizontal Hydraulic Gradients
Horizontal hydraulic gradients were derived for the shallow, deep, and bedrock flow layers by
calculating the difference in hydraulic heads over the length of the flow path between two wells
with similar well construction (e.g., both wells having 15-foot screens within the same water-
bearing unit). Applying this equation to wells installed during the CSA yields the following
average horizontal hydraulic gradients (measured in foot/foot):
 Shallow flow layer: 0.030
 Deep flow layer: 0.029
 Bedrock flow layer: 0.037
3.1.5.2 Vertical Hydraulic Gradients
Vertical hydraulic gradients were calculated (Tables 3-1 and 3-2) for 20 well pair locations by
taking the difference in groundwater elevation in a deep and shallow well pair over the
difference in well depth measured at the midpoint of the deep and shallow well pair screens. A
positive value indicates potential upward flow (higher hydraulic head with depth) and a negative
value indicates potential downward flow (lower hydraulic head with depth). Based on review of
the results, the vertical gradients of groundwater are generally downward across the site.
Sixteen of the 20 well pairs exhibited a downward gradient ranging from -0.002 foot/foot to -
1.864 foot/foot. Vertical hydraulic gradients between the shallow and deep flow layers are
depicted on Figure 3-6 and the vertical gradients between the deep and bedrock flow layers are
depicted on Figure 3-7.
Corrective Action Plan Part I
Dan River Steam Station Ash Basin

43
Table 3-1. Vertical Gradient Calculations for Shallow/Deep Well Pairs
Shallow Well Deep Well Vertical Gradient (ft/ft) Note
AB-10S AB-10D -0.490 --
AB-25S AB-25D -0.100 --
AB-30S AB-30D -0.693 --
AB-5S AB-5D -0.023 --
BG-5S BG-5D -1.864 --
GWA-10S GWA-10D -0.214 --
GWA-12S GWA-12D -0.410 --
GWA-1S GWA-1D 0.004 --
GWA-2S GWA-2D -0.103 --
GWA-3S GWA-3D -0.188 --
GWA-4S GWA-4D -0.330 --
GWA-6S GWA-6D -0.182 --
GWA-7S GWA-7D -0.026 --
GWA-8S GWA-8D -0.053 --
GWA-9S GWA-9D -0.038 --
MW-10 MW-10D N/A Dry
MW-11 MW-11D N/A No Measurement was taken
MW-12 MW-12D -0.002 --
MW-20S MW-20D 0.051 --
MW-21S MW-21D 0.333 --
MW-22S MW-22D N/A Dry
MW-9S MW-9D N/A Dry
Notes:
1. Vertical Gradients = ∆WE/ABS(∆MSE), where ∆ implies deep to shallow, WE is water elevation, and
MSE is mid-screen elevation.
2. N/A implies a lack of sufficient information to satisfy the above formula; a note is included for
explanation.
3. Positive gradient implies upward flow.
4. Depth to Water measurements taken on 9/2/15.

Table 3-2. Vertical Gradient Calculations for Deep/Bedrock Well Pairs
Shallow Well Deep Well Vertical Gradient (ft/ft) Note
AB-25BR AB-25D -0.079 --
AB-30BR AB-30D 0.281 --
MW-22BR MW-22D N/A Dry
MW-23BR MW-23D N/A Artesian
Notes:
1. Vertical Gradients = ∆WE/ABS(∆MSE), where ∆ implies bedrock to deep, WE is water elevation, and
MSE is mid-screen elevation.
2. N/A implies a lack of sufficient information to satisfy the above formula; a note is included for
explanation.
3. Positive gradient implies upward flow.
4. Depth to Water measurements taken on 9/2/15.

Corrective Action Plan Part I
Dan River Steam Station Ash Basin

44
Comparison of vertical gradients between shallow and deep flow layers:
 Potential downward gradient is generally exhibited across the site within the shallow and
deep flow layers with the exceptions detailed below.
 Potential upward gradient was observed on the northern portion of the site north of Ash
Storage 1, which is influencing groundwater flow on this portion of the site.
 Potential upward gradient was also observed immediately downgradient of the
secondary cell of the ash basin and west of the unnamed tributary to the east. This
location exhibits potential upward flow due to hydraulic influences of the ash basin and
the unnamed tributary.

Comparison of vertical gradients deep and bedrock flow layers:
 A potential upward gradient was observed at AB-30D/BR located on the dam of the
secondary cell. This upward flow is due to hydraulic influence of both the ash basin and
proximity of the Dan River.
 A potential downward gradient was observed at AB-25D/BR located on the dam of the
southern most portion of the primary cell. This downward flow is generally observed
across the site, including the source areas.
 Background well vertical gradient is inconclusive at this time as the deep well was dry.
3.2 Site Geochemical Conditions
The site geochemical conditions as described below were incorporated in the fate and transport
modeling discussed further in Section 4. The SCM will be updated as additional data and
information associated with COIs and site conditions are developed. The following site
geochemical conditions were evaluated for site-specific COIs as identified in Section 2.8.
3.2.1 COI Sources and Mobility in Groundwater
3.2.1.1 COI Sources
The overall chemical composition of coal ash resembles that of siliceous rocks from which it
was derived, particularly shale. Oxides of silicon, aluminum, iron, and calcium make up more
than 90 percent of most siliceous rocks, soils, fly ash, and bottom ash. Other major and minor
elements (sulfur, sodium, potassium, magnesium, titanium) make up an additional 8 percent,
while trace constituents account for less than 1 percent. The following constituents are
considered to be trace elements: arsenic, barium, cadmium, chromium, lead, mercury,
selenium, copper, manganese, nickel, lead, vanadium, and zinc (EPRI 2010).
COI sources at the DRSS site consist of the ash storage areas (Ash Storage 1 and Ash Storage
2) and the primary and secondary cells of the ash basin. These source areas are subject to
different processes that generate leachate migrating into the underlying soil layers and into the
groundwater. For example, the ash storage areas would generate leachate as a result of
infiltration of precipitation, while the ash basin would generate leachate based on the contact
with ponded water elevation in the basin. The periodic discharging of water to the ash basin
would likely affect the leachate constituents and concentrations over time. Additionally, it has
been identified that ash management practices alter the concentration range of COIs in ash
Corrective Action Plan Part I
Dan River Steam Station Ash Basin

45
leachate, and that certain groups of COIs are more prevalent in landfill versus pond disposal
scenarios (EPRI and USDOE 2004).
The location of ash, precipitation, and process water in contact with ash is the most significant
control on geochemical conditions. COIs would not be present in groundwater or soils at levels
above background without ash-to-groundwater contact. Once leached by precipitation or
process water, COIs enter the soil-to-groundwater-to-rock system and their concentration and
location are controlled by the principles of COI transport in groundwater. Groundwater-to-rock-
to-soil interaction is also responsible for the natural occurrence of COIs in background water
quality locations.
3.2.1.2 COI Mobility in Groundwater
After leaching has occurred, the distribution and concentrations of COIs in groundwater
depends upon factors such as how the dissolved concentrations are transported through the
soil/rock media, the composition of the soil/rock media in the flow path, and the geochemical
conditions present along those flow paths.
The two processes main involved in transport the dissolved concentrations in groundwater flow
are advection and dispersion. Advection is the movement of dissolved and colloidal COIs by
groundwater flow and is the primary mechanism for movement of a dissolved concentration.
The rate of advection can be described by Darcy flow. The second process that affecting the
location and concentration of inorganic COIs in groundwater flow is mechanical dispersion. This
mixing process happens as groundwater undergoes tortuous paths of various lengths to arrive
at the same location; some water moves faster than other water, which causes longitudinal and
lateral spreading of plumes. Dispersion is scale dependent and increases with plume length and
groundwater flow velocity. The third process involved in the transport of a dissolved
concentration is molecular diffusion. Molecular diffusion occurs when particles spread due to
molecular motion, as in stagnant water. When mechanical dispersion and molecular diffusion
processes are combined, the resultant mixing factor is called hydrodynamic dispersion.
Hydrodynamic dispersion is a scale-dependent phenomenon. There is greater mixing
opportunity over long distances than over short distances, so the hydrodynamic dispersion is
greater for long distances. Advection, dispersion, and diffusion can cause the movement of
concentrations of a COI to over a site and can also cause decreases in concentrations over
distances and time, without consideration of other geochemical processes.
Retardation of a constituent relative to an initial concentration can occur due to adsorption,
absorption, or ion exchange. Which of these three processes occur and the degree to which
they occur depends on factors such as the properties of the solute, the properties of the
soil/rock media, and the geochemical conditions.
Inorganic COIs have a varying propensity to interact with the mineral and organic matter
contained in aquifer media. Depending on the constituent and the mechanism of interaction, the
retention of a COI to the soil or aquifer material and removal of the COI from groundwater may
be a non-reversible or a reversible condition.
In some cases the degree of retardation or attenuation of a COI to the aquifer media may be so
great that the COI will not be mobile and will not transport. In these cases, attenuation may
Corrective Action Plan Part I
Dan River Steam Station Ash Basin

46
result in reduction of COI concentrations to acceptable levels before reaching the point of
compliance or receptors. For other COIs or conditions, the degree of retardation or attenuation
media may be weaker.
3.2.1.3 COI Distribution in Groundwater
The spatial distribution for COIs detected in groundwater samples collected at the DRSS site is
detailed below. For the purposes of this discussion, the shallow flow layer includes the analytical
results reported in the shallow (S) wells, the deep flow layer includes the analytical results
reported in the deep (D) wells, and the bedrock flow layer includes the analytical results
reported in the bedrock (BR) wells.
 Antimony - Antimony was not detected above IMAC in the shallow aquifer above the
laboratory reporting limit. Exceedances in the deep flow layer were restricted to areas
north of Ash Storage 1 and along the downgradient extent of the ash basin. Antimony
was also detected in the bedrock flow layer above the IMAC Standard in MW-23BR
(background bedrock well), MW-317BR, and AB-25BR. Due to the isolated occurrence
of this constituent and its occurrence in background wells, further evaluation is needed
to determine if it is attributable to ash handling at the site.  

Note that turbidity was recorded above 10 NTU at GWA-2D located north of Ash Storage
1 and at AB-10D located in the divider dike between the primary and secondary cells of
the ash basin. The pH was recorded outside the range of the 2L Standard at these
locations. Dissolved concentrations were reported at or within one order of magnitude of
the total concentrations, thus exceedances of antimony are likely not attributable to
turbidity or pH.

 Arsenic – In general, concentrations of arsenic in the shallow and deep flow layers were
localized in the ash storage areas and ash basin, and the concentration varied within
those areas. The only concentration of arsenic detected above the 2L Standard in the
bedrock flow layer was isolated to AB-35BR, located within the Secondary Cell. The
arsenic impacts are attributable to ash handling at the DRSS site.

A review of turbidity, pH and dissolved concentrations for locations with arsenic
exceedances did not indicate exceedances were attributable by turbidity or pH.

 Boron –Boron exceeded the 2L Standard in the shallow and deep flow layers. The
greatest concentrations were found in Ash Storage 2. The only boron exceedances of
the 2L Standard in the bedrock flow layer were isolated to MW-306BR in Ash Storage 1.
The boron impacts are attributable to ash handling at the DRSS site.

Note that turbidity was recorded above 10 NTU at MW-9 located within the southern
portion of the primary cell dam and at MW-318D located in Ash Storage 2. At MW-318D,
pH was recorded outside the range of the 2L Standard. Dissolved concentrations were
reported at or within one order of magnitude of the total concentrations. Exceedances of
boron are not attributable to turbidity or pH.
Corrective Action Plan Part I
Dan River Steam Station Ash Basin

47

 Chromium –Chromium exceeded the 2L Standard in the shallow and deep flow layers
were detected around the perimeter of Ash Storage 1. Concentrations of chromium in
the bedrock flow layer were localized in Ash Storage 2. The chromium impacts may be
attributable to ash handling at the DRSS site due exceedances observed in monitoring
wells downgradient of ash management areas.

Note that turbidity was recorded above 10 NTU at GWA-1S located north of Ash Storage
1 and MW-315BR in Ash Storage 1. At GWA-1S, pH was recorded outside the range of
the 2L Standard. Dissolved concentrations at GWA-1S is within one order of magnitude
of the total concentration and MW-315BR is two orders of magnitude higher than the
total concentration. Exceedances at MW-315BR may be attributable to turbidity.

 Cobalt – Concentrations of cobalt above the IMAC varied in the shallow and deep flow
layer and were distributed across the site. Concentrations of cobalt above the IMAC in
the bedrock flow layer were isolated to the MW-315BR in Ash Storage 2. Cobalt
concentrations are attributable to naturally occurring conditions at the DRSS site, with
the exception of elevated concentrations above both 2L and PPBCs below the ash basin
and ash storage areas.

Note that turbidity was recorded above 10 NTU at the following locations:
o GWA-1S/D (north of Ash Storage 1)
o MW-12/MW-12D (northwest of Ash Storage 1)
o GWA-9S/D (northwest of ash storage areas)
o MW-315BR (Ash Storage 2)
o MW-318D (Ash Storage 2)
o MW-9/MW-9D (southern portion of the primary cell dam)
o GWA-6S (southern most portion at toe of the primary cell dam)
o AB-10D (divider dike between primary and secondary cells of ash basin)

pH was recorded outside the range of the 2L Standard for the following locations
mentioned above:
o GWA-1S
o MW-12/MW-12D
o GWA-9S/D
o MW-318D
o AB-10D

Dissolved concentrations for cobalt exceedances were at or within one order of
magnitude higher than the total concentrations and exceedances are likely not
attributable to turbidity.

 Iron – Concentrations of iron above the 2L Standard varied in both the shallow, deep,
and bedrock flow layers across the site. Iron concentrations are attributable to naturally
Corrective Action Plan Part I
Dan River Steam Station Ash Basin

48
occurring conditions at the DRSS site, with the exception of elevated concentrations
above both 2L and PPBCs below the ash basin and ash storage areas.

Note that turbidity was recorded above 10 NTU at the following locations:
o GWA-1S/D (north of Ash Storage 1)
o MW-12/MW-12D (northwest of Ash Storage 1)
o GWA-9S/D (northwest of ash storage areas)
o AS-8D/BR (Ash Storage 2)
o MW-315BR (Ash Storage 2)
o MW-318D (Ash Storage 2)
o MW-9/MW-9D (southern portion of the primary cell dam)
o GWA-6S (southern most portion at toe of the primary cell dam)
o MW-311BR (primary cell of ash basin)

pH was recorded outside the range of the 2L Standard for the following locations
mentioned above:
o GWA-1S
o MW-12/MW-12D
o GWA-9S/D
o MW-318D

Dissolved concentrations for iron exceedances were less the reporting limit (<50 µg/L) at
GWA-1S and GWA-9S/D. Dissolved concentrations at MW-315BR was within one order
of magnitude higher than the total concentrations and MW-318D was two orders of
magnitude higher than the total concentrations. With the exception of MW-315BR and
MW-318D, exceedances are likely not attributable to turbidity.

 Manganese – Concentrations of manganese above the 2L Standard varied in both the
shallow, deep, and bedrock flow layers across the site. Manganese concentrations are
attributable to naturally occurring conditions at the DRSS site, with the exception of
elevated concentrations above both 2L and PPBCs below the ash basin and ash storage
areas.
Note that turbidity was recorded above 10 NTU at the following locations:
o GWA-1S/D (north of Ash Storage 1)
o MW-12/MW-12D (northwest of Ash Storage 1)
o GWA-9S/D (northwest of ash storage areas)
o AS-8D/BR (Ash Storage 2)
o MW-315BR (Ash Storage 2)
o MW-318D (Ash Storage 2)
o MW-9/MW-9D (southern portion of the primary cell dam)
o GWA-6S (southern most portion at toe of the primary cell dam)
o MW-311BR (primary cell of ash basin)

Corrective Action Plan Part I
Dan River Steam Station Ash Basin

49
pH was recorded outside the range of the 2L Standard for the following locations
mentioned above:
o GWA-1S
o MW-12/MW-12D
o GWA-9S/D
o MW-318D

Dissolved concentrations for manganese exceedances were at or within one order of
magnitude higher than the total concentrations. Manganese exceedances are likely not
attributable to turbidity.

 pH –pH levels outside the 2L Standards range varied in the shallow, deep, and bedrock
flow layers across the site. Elevated pH ranges are attributable to naturally occurring
conditions at the DRSS site.
 Sulfate –Sulfate exceeded the 2L Standard in the shallow and deep flow layers and was
isolated to GWA-8D and MW-21D, located immediately downgradient of the secondary
cell. Sulfate exceeded the 2L Standard in the bedrock flow layer and was isolated to
MW-308BR and GWA-5BR, located within the Secondary Cell and immediately
downgradient of the ash basin. The sulfate impacts are attributable to ash handling at
the DRSS site.
A review of turbidity, pH and dissolved concentrations for locations with arsenic
exceedances did not indicate exceedances were attributable by turbidity or pH.

 TDS – Concentrations of TDS above the 2L Standard followed the same pattern as
sulfate concentrations for the shallow and deep flow layers. In the bedrock flow layer,
concentrations of TDS were localized within Ash Storage 1 and the secondary cell. TDS
concentrations increased in the direction of groundwater flow. The TDS impacts are
attributable to ash handling at the DRSS site.
A review of turbidity, pH and dissolved concentrations for locations with arsenic
exceedances did not indicate exceedances were attributable by turbidity or pH.

 Thallium – Concentrations of thallium above the IMAC in the shallow and deep flow
layers were localized in Ash Storage 2 and decreased with groundwater flow direction.
An isolated concentration was detected at GWA-6D, located immediately downgradient
of the primary cell. Thallium concentrations above the IMAC in the bedrock flow layer
were isolated to background well MW-23BR, located near the western-most property
boundary. Thallium concentrations are attributable to naturally occurring conditions at
the DRSS site, due to concentrations observed in background well locations.
A review of turbidity, pH and dissolved concentrations for locations with arsenic
exceedances did not indicate exceedances were attributable by turbidity or pH.

Corrective Action Plan Part I
Dan River Steam Station Ash Basin

50
 Vanadium – Concentrations of vanadium above IMAC varied in the shallow, deep, and
bedrock flow layers, and were widespread across the site. Vanadium concentrations are
attributable to naturally occurring conditions at the DRSS site, with the exception of
elevated concentrations above both 2L and PPBCs below the ash basin and ash storage
areas.
Note that turbidity was recorded above 10 NTU at the following locations:
o GWA-1S/D (north of Ash Storage 1)
o GWA-2D (northeast of Ash Storage 1)
o MW-12/MW-12D (northwest of Ash Storage 1)
o GWA-9S/D (northwest of ash storage areas)
o AS-8D/BR (Ash Storage 2)
o MW-315BR (Ash Storage 2)
o MW-318D (Ash Storage 2)
o MW-9/MW-9D (southern portion of the primary cell dam)
o GWA-6S (southern most portion at toe of the primary cell dam)
o MW-311BR (primary cell of ash basin)
o AB-10D(divider dike between the primary and secondary cells of the ash basin)

pH was recorded outside the range of the 2L Standard for the following locations
mentioned above:
o GWA-1S
o GWA-2D
o MW-12/MW-12D
o GWA-9S/D
o MW-318D
o AB-10D

Dissolved concentrations for vanadium exceedances less than the reporting limit for
GWA-1S/D, GWA-2D, GWA-6S, GWA-9S/D, MW-12, and MW-12D. Dissolved
concentrations for the remaining locations discussed above were at or within one order
of magnitude higher than the total concentrations. It is important to note that the
reporting limit for vanadium is higher than it’s IMAC. Vanadium exceedances are likely
not attributable to turbidity.

In summary, cobalt, iron, manganese, pH, thallium and vanadium exceedances are naturally
occurring and were detected in background wells above 2L Standards or IMACs as well as
proposed provisional background concentrations. Based on review of available data, these
constituents were observed across the site and correlation to ash management areas is
inconclusive. Arsenic, boron, chromium, sulfate and TDS exceedances were detected in wells
within the footprint of the ash storage areas and ash basin and in wells within the compliance
boundary downgradient of these ash management areas. Further evaluation for antimony and
beryllium is warranted due to their isolated occurrences to determine if they are attributable to
ash handling at the site.
Corrective Action Plan Part I
Dan River Steam Station Ash Basin

51
3.2.2 Geochemical Characteristics
3.2.2.1 Cations/Anions
Classification of the geochemical composition of groundwater aids in aquifer characterization
and SCM development. As groundwater flows through the aquifer media, the resulting
geochemical reactions produce a chemical composition that can be used to characterize
groundwater that may differ in composition from groundwater from a different set of lithological
and geochemical conditions. This depiction is typically performed using Piper diagrams to
graphically depict the distribution of the major cation and anions of groundwater samples
collected at a particular site.
In general, the groundwater and surface water at the DRSS site is predominantly rich in
calcium, magnesium, and bicarbonate with the exception of downgradient groundwater
monitoring wells, which trend closer to a calcium-, magnesium- and sulfate-rich geochemical
composition. Piper diagrams were generated as part of the DRSS CSA to compare the cation
and anion composition of groundwater, ash basin porewater, surface water, and seeps.
Evidence of mixing of ash basin porewater and groundwater can be seen in the piper diagrams
presented in the CSA report.
3.2.2.2 Redox Potential
When elements dissolve in water they are often ionized or charged. Sometimes this is because
the compounds are ionic solids, like salt (NaCl) and when dissolved will form negatively charged
(anions - Cl-) and positively charged (cations - Na+) ions. Other times elements change charge
because they gain or lose electrons from their outer shells. These are reduction-oxidation or
‘redox’ reactions. An example of that is oxidized ferric iron (+3 charge) transforming to ferrous
iron (+2 charge) by accepting an electron. Redox reactions are balanced, every electron donor
has to have an electron receptor.
Redox reactions such as the iron example above can greatly influence the presence of
contaminants in water. Mobility and transport of iron and manganese may be controlled by its
oxidation state. As seen from the speciation sampling discussed below, and the redox
measurement activities, we can expect reduced zones to have higher mobility of most species.
In highly reducing conditions the formation of sulfides such as pyrite can also control the
mobility of iron and manganese. The iron-manganese redox system deserves attention in this
hydrogeologic setting because of its prevalence and the indications of variable redox conditions.
As iron and manganese precipitate due to redox reactions these solids adsorb and attenuate
many COIs. Reductive dissolution of iron oxyhydroxides release adsorbed material.
Determination of the reduction/oxidation (redox) condition of groundwater is an important
component of groundwater assessments, and helps to understand the mobility, degradation,
and solubility of constituents. It is problematic that field measurement of redox conditions using
probes or single redox couples are very difficult to conduct accurately. At the DRSS site, the
approach taken was to measure multiple redox couples and dissolved gasses in addition to
probe measurements. Just as pH reactions are limited by compounds that buffer changes in pH,
the presence of high concentrations of redox couples causes the solution to be poised near a
Corrective Action Plan Part I
Dan River Steam Station Ash Basin

52
certain redox potential. At Dan River anoxic/mixed is the predominant redox category and
ferrous iron/ferrous sulfate are the predominant redox processes.
3.2.2.3 Solute Speciation
For compliance purposes, inorganic solute concentrations are expressed most often as
concentration of the chemical element. In nature, those elements each form a large range of
inorganic species. These species can be present due to a change in valance state (oxidation-
reduction state of the element) as in the case with ferrous (Fe(II)) and ferric (Fe(III)) iron. The
species can also reflect formation of a compound, such that Fe(II)+2 and Fe(OH)2 (aqueous) are
two species formed from the total amount of iron available. Speciation is important for
understanding the fate and transport of COIs as species react differently. Select wells were
sampled for chemical speciation analyses of arsenic (III), arsenic (V), chromium (VI), iron (II),
iron (III), manganese (II), manganese (IV), selenium (IV), and selenium (VI).
Speciation analysis revealed that observed anoxic/mixed redox conditions are reflected in the
speciation of redox-sensitive species. Reduced As(III), Fe(II), Mn(II) and Se(IV) are present in
all fourteen groundwater samples collected for speciation analysis (with exception of one non-
detect for Se(IV)). Speciation results are documented in the CSA. In two of three samples As(III)
is the dominant species. This is significant in that As(III) tends to react less with aquifer media
than As(V); oxidation of arsenic would improve sorption and attenuation of arsenic. The
presence of reduced species COIs at significant concentrations in wells tested indicates that
consideration of speciation is necessary in evaluation of corrective measures.
3.2.2.4 Kd (Sorption) Testing and Analysis
As described in section 3.2.1.2, a constituent may be removed from groundwater and onto
mineral surfaces of the aquifer media through one of the three types of sorption processes:
 Adsorption – solutes are held at the water/solid as a hydrated species,
 Absorption – solutes are incorporated into the mineral structure at the surface,
 Ion Exchange – when an ion becomes sorbed to a surface by changing places with a
similarly charged ion.

These processes result in a decrease of the concentration and therefore the mass of the
constituent as it is removed from the groundwater onto the solid material. The effect of these
processes for a particular constituent can be expressed by the distribution coefficient (or
partition coefficient) Kd. Kd relates the quantity of the adsorbed constituent per unit mass of solid
to the quantity of the constituent remaining in solution.
Laboratory determination of Kd was performed by UNCC on 12 site-specific samples of soil (add
list of wells here), or transition zone material. Solid samples were tested in flow through columns
to measure the adsorption of COIs at varying concentrations. For the DRSS site, 12 column
tests and 22 batch tests were conducted. The methods used by UNCC and Kd results obtained
from the testing are presented in Appendix E. The Kd data was used as an input parameter to
evaluate contaminant fate and transport through the subsurface at the DRSS site, as described
in greater detail in Section 4.1.
Corrective Action Plan Part I
Dan River Steam Station Ash Basin

53
3.2.3 Source Area Geochemical Conditions
COIs will predominantly be immobilized in the groundwater by adsorption and precipitation.
Constituents dissolve while ash receives precipitation and those constituents leach into
groundwater. Mobility of constituents is affected by sorption characteristics of each respective
constituent. Geochemical modeling of COIs will provide a better understanding of geochemical
conditions/processes and their effect on COI mobility in groundwater. Geochemical modeling
was not completed as part of this CAP Part I, but plan for geochemical modeling is discussed in
further detail in Section 4.
3.2.3.1 Ash Storage Area
Ash Storage 1
Within Ash Storage 1, groundwater was encountered at approximately 64.0 feet below ground
surface (ft bgs) during installation of CSA monitoring wells. Refusal was encountered between
approximately 82.5 and 98.0 ft below ground surface (bgs) around the center of Ash Storage 1.
Groundwater is approximately 20 to 30 ft above refusal. Thus, ash below Ash Storage 1 is
saturated and there is no unsaturated soil buffer.
Ash Storage 2
Groundwater was encountered at approximately 30.0 ft bgs during installation of CSA
monitoring wells. Refusal was encountered between approximately 38.0 to 45.0 ft bgs. There is
approximately 5 to 10 ft of unsaturated soil below the ash in Ash Storage 2.
3.2.3.2 Ash Basin – Primary and Secondary Cells
Ash within the ash basin was encountered to a depth of approximately 45 ft bgs and refusal was
encountered at approximately 65 ft bgs, which allows for approximately 20 ft of soil below the
ash. Groundwater was encountered between 15 and 35 ft bgs, which is between 10 to 20 ft
above refusal, causing ash to be saturated. Pond level fluctuation also affects COI mobility due
to increased dissolution of COIs into the groundwater thus increasing COI concentrations with
increased pond levels.
3.2.4 Mineralogical Characteristics
Soil and rock mineralogy and chemical analyses were completed during the CSA and were
presented in the CSA report.
The dominant minerals in soils at the DRSS site are quartz, feldspar (both alkali and plagioclase
feldspars), and muscovite/illite. These soils exhibit a higher degree of weathering and show an
increase in kaolinite with higher percentage amorphous phase (lacking distinct crystalline
structure). Other minerals identified include chlorite, biotite, calcite, dolomite,
hornblende/amphibole, pyrite, ilmenite, goethite, gypsum, and smectite/chlorite. The major
oxides in the soils are SiO2 (41.59% - 75.49%), Al2O3 (13.12% - 34.56%), and Fe2O3 (2.61% -
11.02%). Major transition zone minerals are quartz, feldspar, muscovite/vermiculite/illite,
kaolinite, chlorite, and smectite. The major oxides are SiO2 (50.4% - 67.9%), Al2O3 (16.2% -
25.5%), and Fe2O3 (8.5% - 15.1%).
Corrective Action Plan Part I
Dan River Steam Station Ash Basin

54
These highly weathered Piedmont soils, saprolite, and rock contain high percentages of clay
minerals and hydrous metal oxides and oxyhydroxides. These geologic materials are very fine-
grained and have a large surface area compared to their volume. They are also chemically
reactive, and the attenuation of inorganic compounds by clays and oxides has been a subject of
intense study for over 100 years.
Soil formation typically results in the loss of common soluble cations and the accumulation of
quartz and clay. Feldspars are hydrolyzed to clays. Concentrations of COIs during the
weathering and soil development on the lithologies noted above are negligible other than for a
potential increase in vanadium and cobalt from diabase weathering. Soil chemistry results do
not show marked deviation from normal crustal abundances at the DRSS site. Accordingly, the
residual soils do not appear to contribute significantly to the COI exceedances in the soils.
3.3 Correlation of Hydrogeologic and Geochemical
Conditions to COI Distribution
Based on results of sampling and analysis performed during CSA activities, the following are
groundwater COIs s the DRSS site: arsenic, boron, chromium, cobalt, iron, manganese, sulfate,
TDS, and vanadium. Cobalt, iron, manganese, and vanadium exceed 2L or IMACs and PPBCs
in source areas. Antimony and selenium were isolated occurrences and need further evaluation
as to source of impacts. Arsenic, boron, chromium, sulfate and TDS are attributable to ash
handling at the site. The sources and areas with 2L Standards exceedances of these COIs as
well as other DRSS site features are illustrated the 3-D SCM presented in Figure 3-1 and in
cross-sectional view on Figure 3-2.
On Figure 3-1, the areas of 2L Standards exceedances of arsenic, boron, chromium, sulfate,
and TDS are within or downgradient to the sources indicating that physical and geochemical
processes beneath the DRSS site inhibit lateral migration of the COIs. Discharge of
groundwater from shallow and deep flow layers into surficial water bodies, in accordance with
LeGrand’s slope-aquifer system characteristic of the Piedmont, is evident northwest of the
Secondary Cell where COI concentrations in excess of 2B Standards were detected in the
tributary that discharges to the Dan River. Vertical migration of COIs were observed in select
well clusters (S, D, and BR) and is likely influenced by infiltration of precipitation and/or ash
basin water, where applicable, through the shallow and deep flow layers into underlying
fractured bedrock.
Refinement of this SCM, as it pertains to groundwater fate and transport modeling, is discussed
in Section 4.3. Furthermore, the SCM will continue to evolve as additional data becomes
available during supplemental site investigation activities. 

Corrective Action Plan Part I
Dan River Steam Station Ash Basin

55
4 Modeling
Groundwater flow and fate and transport, and groundwater to surface water models were
conducted to evaluate COI migration and potential impacts following closure of ash basin
system and ash storage areas at the DRSS site. Section 4 summarizes the modeling and
results.
4.1 Groundwater Modeling
UNCC developed a site-specific, 3-D, steady-state groundwater flow and fate and transport
model for the DRSS site using MODFLOW and MT3DMS. The groundwater flow and fate and
transport model is based on the SCM presented in Section 3 and incorporates site-specific data
obtained during the CSA and subsequent data collection. The objective of the groundwater
modeling effort was to simulate steady-state groundwater flow conditions for the DRSS site, and
simulate transient transport conditions in which COIs enter groundwater via the ash basin
system over the period it was in service. These model simulations serve to:
 Predict groundwater elevations in the ash and underlying groundwater flow layers for the
proposed closure scenarios.
 Predict concentrations of the COIs at the compliance boundary or other downgradient
locations of interest over time,
 Estimate the groundwater flow and constituent loading to adjacent downgradient
unnamed tributaries and the Dan River.

The area, or domain, of the simulation included the DRSS ash basin system and areas of the
site that have been impacted by COIs above 2L Standards or IMACs. The model was
developed in accordance with NCDENR DWQ’s Groundwater Modeling Policy dated May 31,
2007. Details of the groundwater modeling are presented in Appendix C. Figures and tables
referenced throughout Section 4.1 are provided in Appendix C.
4.1.1 Model Scenarios
The following ash basin closure scenarios were modeled for the DRSS site:
 Existing Conditions (EC)
 Cap-in-Place (CIP)
 Excavation (EX)
Model scenarios utilized steady-state groundwater flow conditions established during model
calibration and transient transport of COIs identified in Sections 2 and 3 for further analysis.
Each COI was modeled individually using the transient transport model for a period of 250
years. This time period was selected to determine the rate of natural attenuation and COI
concentrations at the compliance boundary.
Corrective Action Plan Part I
Dan River Steam Station Ash Basin

56
4.1.2 Calibration of Models
The groundwater flow model was calibrated to steady-state flow conditions using water level
observations taken at the site during June 2015 in shallow, deep, and bedrock wells. The
transient transport of COIs was calibrated to COI measurements from June 2015 for each
constituent. The transient transport models were calibrated to COIs measurements above the
2L Standards or IMACs by introducing a constant source for each COI (ash basin system) at the
start of the ash basin operation and running the model until June 2015. A detailed account of
the flow and the transient transport model calibrations are included in Appendix C. Ranges of
measured hydrogeologic properties from the CSA were used as a guide for selecting model
values during calibration. The groundwater flow model was calibrated by adjusting model
parameters within the upper and lower bounds of measured hydrogeologic parameters at the
site, including:
 The hydraulic conductivity distribution within each flow layer within the basin (e.g., ash,
dike, upper unconsolidated zone, TZ, and fractured bedrock zone);
 The infiltration rate applied to the water table of the ash basin system;
 The net infiltration due to precipitation applied to other areas of the site;
 The variation of measured pore water COI concentrations;
 The effective porosity of each model layer; and
 The Kd value of each COI.

Calibration results indicate the model adequately represents steady-state groundwater flow
conditions at the site and meets transport calibration objectives. An independent review of the
calibrated DRSS model was conducted by the Electric Power Research Institute (EPRI) and
found that the model sufficiently met the final objective of predicting effects of corrective action
alternatives on groundwater quality. The EPRI review included the arsenic and boron transport
calibrations, which provide a sorptive and non-sorptive COI, respectively. The EPRI review of
the calibrated DRSS model is provided in Appendix C.
4.1.3 Kd Terms
COIs enter the ash basin system in the dissolved and solid phases of the station’s wastewater
discharge. Some COIs are also present in native soils and groundwater beneath the basin. In
the ash basin system, constituents may incur phase changes including dissolution, precipitation,
adsorption, and desorption. Dissolved phase constituents may incur these phase changes as
they are transported in groundwater flowing downgradient from the basin. In the fate and
transport model, chemical constituents enter the basin in the dissolved phase by specifying a
steady-state concentration in the ash porewater. Phase changes (dissolution, precipitation,
adsorption, and desorption) are collectively taken into account by specifying a linear soil-
groundwater partitioning coefficient (sorption coefficient [Kd]). The accumulation and subsequent
release of chemical constituents in the ash basin system over time is a complex process. In the
conceptual fate and transport model, it was assumed that the entry of constituents into the ash
basin and ash storage area was represented by a constant concentration in the saturated zone
of the basin, which is continually removed by infiltrating recharge from above.
Corrective Action Plan Part I
Dan River Steam Station Ash Basin

57
As previously discussed in Section 3.2.2.4, laboratory Kd terms were developed by UNCC via
column testing of 12 site-specific samples of soil, or PWR from the TZ. The methods used by
UNCC and Kd results obtained from the testing are presented in Appendix D. The Kd data was
used as an input parameter to evaluate contaminant fate and transport through the subsurface
at the DRSS site. Note that Kd characteristics were each represented by an isotherm from which
the sorption coefficient Kd, with units of ml/gram is calculated. Sorption studies on soil samples
obtained during the CSA at DRSS indicate that the COIs Kds for native soils surrounding the ash
basin and ash storage areas are higher than the values used in modeling.
4.1.4 Flow and COI Transport Model Sensitivity Analysis
Sensitivity of the groundwater flow model was evaluated by varying key model assumptions by
a percentage above and below their respective calibration values and calculating the normalized
root mean square error (NRMSE) for comparison with the calibration value (Appendix C).
Based on this approach, the groundwater flow model was most sensitive to horizontal hydraulic
conductivity of the shallow flow layer, followed by recharge to areas beyond the ash basin
system, and hydraulic conductivity of the TZ. The model was less sensitive to vertical hydraulic
conductivity beyond the ash basin system, where the dominant groundwater flow direction is
horizontal. The elevation of the water table within the ash basin system is particularly sensitive
to recharge, although the effect on site-wide NRMSE is limited.
Sensitivity of the COI transport models was evaluated by varying key model assumptions by a
percentage above and below their respective calibration values for porosity, dispersivity and Kd.
The transport model was most sensitive to porosity and Kd, where a decrease in these
parameters increased the velocity of COIs moving through the groundwater system. Dispersivity
was less sensitive, where an increase in dispersivity increased the length of the COI plume
initially, but did not result in an increase of CI concentration at distance as quickly as a reduction
in porosity or Kd.
4.1.5 Flow Model
The groundwater model, calibrated for flow and constituent fate and transport under existing
conditions, was applied to evaluate closure scenarios at DRSS. 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 model accuracy and reduce uncertainty.

The model domain developed for existing conditions was applied without modification for the EC
and CIP scenarios. The EC scenario is used as a comparison to other scenarios if closure
scenarios were not completed. The assumed recharge was modified and the constant source
concentration was removed in the other two scenarios. In the CIP scenario, recharge within the
ash basin was reduced from 6 in/year to 0 in/year to represent capping of the ash basin system
and ash storage areas.

For the EX scenario, the primary and secondary cells of the ash basin, the ash basin dikes, and
the ash storage areas were removed. The flow parameters for this model were identical to the
Corrective Action Plan Part I
Dan River Steam Station Ash Basin

58
EC scenario except for the removal of ash related layers, and the same recharge applied over
the remainder of the model domain (i.e.; 5 in/year).

4.1.5.1 Existing Conditions Scenario
The EC scenario served as the basis of comparison to the CIP and EC scenarios.
4.1.5.2 Cap-in-Place Scenario
The CIP scenario results were used to estimate groundwater levels in the ash basin system
subsequent to placement of an engineered geosynthetic soil cap (assuming the ash will remain
in its current position). The model indicated that groundwater levels in the primary and
secondary model cells were not uniform and decreased by approximately 1 to 2 feet. The
placement of an engineered cap does not appear to adequately lower the water table in the
primary and secondary cells below ash without the use of engineering controls such as
extractions wells, cut-off walls, etc. Note that this flow model did not assume such engineering
controls were in place.
4.1.5.3 Excavation Conditions
The EX scenario simulated complete removal of the ash layers in the model and therefore is
incapable of estimating resulting groundwater levels in the ash basin system.
4.1.6 Fate and Transport Model
4.1.6.1 Existing Conditions
The EC scenario used the calibrated groundwater flow and transport model and extended the
time period from the end of the calibration period (present day) to 250 years into the future. The
COIs modeled included arsenic, boron, chromium, cobalt, sulfate, thallium and vanadium. No
changes were made to the model assumptions for this scenario and it was used as a baseline
for comparison of the other model scenarios discussed below.
The time to achieve a steady state COI concentration depends on where the particular COI
plume is located relative to the compliance boundary, its loading history and if it is sorptive or
non-sorptive. Source areas close to the compliance boundary will reach steady state
concentrations sooner than those farther away. The time to achieve steady state concentrations
is also dependent on the sorptive characteristic of each COI. Sorptive COIs will be transient at a
rate that is less than the groundwater pore velocity. Lower effective porosity will result in shorter
time periods to achieve steady state concentrations for both sorptive and non-sorptive COIs.
The results of the EC scenario indicated that concentrations for all modeled COIs increase or
reach steady state conditions above 2L Standards or IMACs during the modeled period.
4.1.6.2 Cap-In-Place
The CIP scenario simulated the effects of capping the ash basin system and ash storage areas
at the beginning of the scenario. In the model, recharge and source area concentrations from
the ash basin system and ash storage areas were set to zero. Under this scenario, groundwater
flow rates are lower (compared to the EC scenario) due to reduced groundwater velocities due
Corrective Action Plan Part I
Dan River Steam Station Ash Basin

59
to the reduction in recharge and the reduction of the groundwater table beneath the capped
areas.
Under the CIP scenario, on-site model concentrations of arsenic, cobalt, thallium and vanadium
decrease, but remain above the 2L Standards or IMACs. Boron, chromium, hexavalent
chromium, and sulfate are completely removed from the model domain under this scenario.
Details are provided below:
 Arsenic: concentrations decreased in each flow layer, but remained above the 2L
Standard within 100 years of completion of the CIP closure scenario.
 Boron: concentrations decreased in each flow layer in downgradient wells and was
removed from the model domain within 35 years of completion of the CIP closure
scenario.
 Chromium: concentrations decreased in each flow layer in downgradient wells and was
removed from the model domain within 25 years of completion of the CIP closure
scenario.
 Hexavalent chromium: concentrations decreased in each flow layer in downgradient
wells and was removed from the model domain within 15 years of completion of the CIP
scenario.
 Cobalt: concentrations decreased in each flow layer, but remain above IMAC in all wells
within 100 years of completion of the CIP closure scenario.
 Sulfate: concentrations decreased in each flow layer in downgradient wells and was
removed from the model domain within 15 years of completion of the CIP closure.
 Thallium: concentrations decreased in each flow layer in downgradient wells and was
removed from the model domain within 100 years of completion of the CIP closure.
 Vanadium: concentrations decreased in each flow layer in downgradient wells and was
removed from the model domain within 100 years of completion of the CIP closure.
4.1.6.3 Excavation
The EX scenario simulate the effects of removing the ash basins, dikes, and ash storage areas
at the beginning of this scenario. In the model, source area concentrations from the ash basin
and ash storage areas are set to zero while recharge is applied at the same rate as the
surrounding area. Groundwater flow beneath the ash basin is affected by this scenario as the
basins are completely drained.
Under the EX scenario, the simulation fails to reach steady state concentrations after 250 years.
The results of the EX scenario indicate the following:
 Arsenic: The deep flow layer has the highest arsenic concentrations at the ash basin, but
concentrations decrease in downgradient wells and was removed from the model
domain within 100 years of completion of the EX closure.
 Boron: concentrations decreased in each flow layer in downgradient wells and was
removed from the model domain within 45 years of completion of the EX closure.
 Chromium: concentrations decreased in each flow layer in downgradient wells and was
removed from the model domain within 45 years of completion of the EX closure.
Corrective Action Plan Part I
Dan River Steam Station Ash Basin

60
 Hexavalent chromium: concentrations decreased in each flow layer in downgradient
wells and was removed from the model domain within 15 years of completion of the EX
closure.
 Cobalt: concentrations decreased in each flow layer in downgradient wells and was
removed from the model domain within 100 years of completion of the EX closure.
 Sulfate: concentrations decreased in each flow layer in downgradient wells and was
removed from the model domain within 15 years of completion of the EX closure.
 Thallium: concentrations decreased in each flow layer in downgradient wells and was
removed from the model domain within 100 years of completion of the EX closure.
 Vanadium: concentrations decreased in each flow layer in downgradient wells and was
removed from the model domain within 100 years of completion of the EX closure.
4.1.6.4 Key Model Assumptions
The key model assumptions and limitations include, but are not limited to, the following:
 The steady-state groundwater flow model is calibrated to a single water levels measured
at monitoring wells in June 2015 and is not calibrated to transient water levels, recharge
or river flow. A steady-state flow calibration does not calculate storage within the
groundwater system and does not calibrate the groundwater flux into adjacent water
bodies.
 Steady-state groundwater flow conditions are assumed from the time that the ash basin
system and ash storage area were placed in service through the simulation period.
 COI source concentrations and recharge in the ash basin system and ash storage areas
are assumed to be constant with respect to time.
 The model is configured to simulate groundwater flow and transport conditions within the
DRSS and is not able to simulate offsite water level or COI transport conditions.
 The river is represented by a constant head, which would not allow the stage of the river
to change during pumping remedial strategies.
 Boron and sulfate are assumed to be non-sorbing, or conservative.
 Sorption coefficients for arsenic, chromium, hexavalent chromium, cobalt, thallium, and
vanadium were applied in the model to allow for transport model calibration consistent
with the conceptual transport model and measured COI concentrations.
 The model does not account for varying geochemical conditions such as pH and redox
potential that could affect COI mobility.
Refer to Appendix C for additional details regarding model assumptions.
4.1.6.5 Proposed Geochemical Modeling Plan
Geochemical Modeling at DRSS
Data obtained during the CSA and subsequent interpretation, determination of groundwater
flow, fate and transport modeling parameters, and the results of that effort have led to
improvements in the Site Geochemical Conceptual model to constrain future geochemical
modeling. It has been determined that:
Corrective Action Plan Part I
Dan River Steam Station Ash Basin

61
 Site specific groundwater flow and fate and transport is as hypothesized; it is operating
within the regional flow and transport process in the Triassic Basin.
 Kd values can be calculated from batch and column adsorption isotherms for all COIs at
the DRSS site.
 The groundwater model can be constrained by site-specific Kd values.
 Kd values within the observed site-specific range (determined in the laboratory) were
used successfully to improve fate and transport model calibration.
 The dominant attenuation processes, as initially hypothesized, are adsorption to hydrous
metal oxides (hydrous ferric oxides (HFO), hydrous manganese oxides (HMO), hydrous
aluminum oxides (HAO)) and clay minerals. Hydrous metal oxides and clay minerals are
abundant in the soil and TZ and it is assumed that concentrations increase with the
degree of bedrock weathering.
 There are correlations between COI concentrations and HFO, HMO, HAO and clay
minerals.
 There is variability in pH and redox conditions across the site; significant enough that pH
and redox influences on COI attenuation should be evaluated.

The binding of COIs to HFO, HMO, HAO and clay minerals is known to be pH and redox
sensitive. Under certain redox and pH conditions HFO, HMO, and HAO may be stable, may
dissolve, or may be actively precipitating. Clay mineral sorption is sensitive to pH and ionic
strength (for example TDS). Sensitivity analysis of COI attenuation at DRSS under variable
conditions is warranted. As previously discussed in the CSA the appropriate manner to conduct
the sensitivity analysis for pH, Eh, and TDS is the use of geochemical models. Other sensitivity
analysis can use more standard Geographical Information System (GIS) tools. The following
sensitivity analysis will be conducted to support the Kd values used in fate and transport
modeling: Mineralogical Stability, Spatial Variability in Retardation, and COI Adsorption under
Variable pH and Redox Conditions.
Mineral Stability
The software program Geochemists Workbench will be used to create Pourbaix plots for HFO,
HMO, and HAO and observed clay minerals. The plots will be created using site-specific
chemistry and will indicate, as in the example below, if minerals are stable under observed and
postulated conditions. In the figure below, by plotting observed dissolved oxygen vs pH we can
see that some waters from an example site favor precipitation of Fe(OH)3 (HFO).
Spatial Variability in Retardation
This process uses GIS retardation equation to evaluate the spatial variability in attenuation
capacity and strength. Retardation represents the combined attenuation effect of reactive area
(porosity and bulk density) and Kd. Use of GIS allows overlay of a retardation function at multiple
depths to evaluate correlation and sensitivity to other measured parameters such as HFO or
clay mineral content.
Corrective Action Plan Part I
Dan River Steam Station Ash Basin

62

COI Adsorption under Variable pH and Redox Conditions
As SCM refinement continues it has been identified that the dominant attenuating processes are
highly sensitive to pH and redox values and variability. Geochemical modeling will be used to
evaluate sensitivity by:
 Using PHREEQC to determine the redox and pH changes that take place under source
term conditions of capping (cessation of O2 delivery by recharge and adjustment to that
new dynamic equilibrium, draining and change in water/rock ratio). These results will be
used to determine if there are changes in leachate chemistry, or if the changes in
leachate affect mobility outside the ash.
 Under the observed variability in pH and redox, and postulated changes in pH and
redox, evaluate the sensitivity of Kd to these conditions. With quantitative mineralogy and
reactive surface area inputs site specific sample attenuation can be simulated in
PHREQC using surface complexation subroutines. Surface complexation is analogous
to Kd, but allows the sensitivity of pH and TDS on adsorption to be modeled.
 Similarly, PHREEQC can intrinsically calculate redox conditions and speciation. The
output of PHREEQC simulations on the effect of change on surface sorption properties
will also provide knowledge of what the expected distribution of species (e.g.,
As(III)/As(V)) would be under those same changed conditions.
 The proposed geochemical modeling effort will provide information related to the
attenuation sensitivity resulting from changes in site conditions using site-specific data,
GIS tools, and geochemical models.
Corrective Action Plan Part I
Dan River Steam Station Ash Basin

63
4.2 Groundwater - Surface Water Interaction Modeling
Groundwater-surface water interactions were completed using groundwater model output and a
surface water mixing model approach to evaluate potential surface water impacts of COIs in
groundwater as they discharge to surface water bodies adjacent to the DRSS site.
4.2.1 Mixing Model Approach
Groundwater model output from the fate and transport modeling discussed in Section 4.1 (i.e.,
groundwater volume flux and concentrations of COI with exceedances of the 2L Standards or
IMACs) were used as inputs for the surface water assessment in the adjacent Dan River
receiving waters. Given that the Dan River is unidirectional and groundwater discharge mixes
with upstream river flow, a mixing calculation was used to assess potential surface water quality
impacts. A summary of this approach and NCDEQ’s mixing zone regulations is presented
below.
 Mixing Model Approach – This approach includes the effects of upstream flow on
mixing and dilution of the groundwater plume within an allowable mixing zone. The
results from this analysis provide information on constituent concentration as a function
of the mixing zone distance from the groundwater input to the adjacent water body.
 Mixing Zone Regulations – A mixing zone is defined in the NCDEQ water quality
standards (Subchapter 2B, Section .0100) as “a region of the receiving water in the
vicinity of a discharge within which dispersion and dilution of constituents in the
discharge occurs and such zones shall be subject to conditions established in
accordance with 15A NCAC 2B .0204(b)”.
 Additional details on mixing zones provided in 15A NCAC 2B .0204(b) are as follows:
A mixing zone may be established in the area of a discharge in order to provide
reasonable opportunity for the mixture of the wastewater with the receiving
waters. Water quality standards shall not apply within regions defined as mixing
zones, except that such zones shall be subject to the conditions established in
accordance with this Rule. The limits of such mixing zones shall be defined by
the division on a case-by-case basis after consideration of the magnitude and
character of the waste discharge and the size and character of the receiving
waters. Mixing zones shall be determined such that discharges shall not:
o Result in acute toxicity to aquatic life [as defined by Rule .0202(1)] or prevent
free passage of aquatic organisms around the mixing zone;
o Result in offensive conditions;
o Produce undesirable aquatic life habitat or result in a dominance of nuisance
species outside of the assigned mixing zone; or
o Endanger the public health or welfare.
Although the NCDEQ mixing zone regulations are typically applied to point source discharges,
the free zone of passage provision was used in this surface water assessment. Mixing zone
sizes and percentages of upstream river design flows used for assessing compliance with
applicable water quality standards as presented in Section 1 are provided in Table 4-1.
Corrective Action Plan Part I
Dan River Steam Station Ash Basin

64
Table 4-1. Mixing Zone Sizes and Percentages of Upstream River Flows
Criteria Mixing Zone Size Percent of Design River Flow1
Acute Aquatic Life 10% of River Width or 15 feet 10% of 1Q10
Chronic Aquatic Life 50% of River Width or 75 feet 50% of 7Q10
Human Health 50% of River Width or 75 feet 50% of 7Q10
Human Health 100% of River Width or 150 feet 100% of Annual Mean
Note:
1. The 1Q10 flow is the lowest one-day average flow that occurs (on average) once every 10 years. The
7Q10 flow is the lowest seven-day average flow that occurs (on average) once every 10 years
(USEPA 2013). Mean annual flow is the long-term average annual flow based on complete annual flow
records.

Using the mixing model approach, output from the groundwater model (i.e., flow and COI
concentrations) were used in the mixing calculation to determine COI concentrations in the
adjacent water body from the point of discharge. These surface water results were compared to
applicable surface water quality standards to determine compliance at the edge of the mixing
zones.
Development of the mixing model inputs required additional data for upstream river flow and
COI concentrations. These data were obtained from the U.S. Geological Survey (USGS) and
USEPA data sources in addition to site-specific surface water quality data collected as part of
the CSA.
4.2.2 Surface Water Model Results
The calculated surface water COI concentrations in the unnamed east tributary (at the
confluence with the Dan River) and in the Dan River downstream from the DRSS are presented
in Tables 4-2 and 4-3. The stream flows, groundwater flows, and COI concentrations presented
in Appendix E were used to complete these calculations. In the east tributary, the observed
surface water concentrations at station SW-3 are presented in addition to the mixing model
calculations for comparison purposes. The mixing model results indicate that all water quality
standards are attained at the edge of the mixing zones except for arsenic in the unnamed east
tributary for the human health standard based on the observed concentration at station SW-3.
Corrective Action Plan Part I
Dan River Steam Station Ash Basin

65
Table 4-2. Unnamed East Tributary Calculated Surface Water Concentrations
COI
Observed
SW-3 Conc.
(µg/L)
Average GW
Conc. (µg/L)
Water Quality Standard (µg/L)
Acute Chronic Human
Health
Arsenic 42.7 4.66 340 150 10
Boron 128.0 249.3 ns ns ns
Total Chromium 33.6 17.0 ns ns ns
Chromium VI no data 0.009 16 11 ns
Cobalt <1 0.040 ns ns 4
Sulfate 23,000 153,869 ns ns ns
Thallium <0.2 0.004 ns ns 0.47
Vanadium 1.25 0.545 ns ns ns
Notes:
1. µg/L – micrograms per liter
2. All COIs are shown as dissolved except for total chromium
3. ns – no water quality standard
4. * – concentration calculated with annual mean river flow

Table 4-3. Dan River Calculated Surface Water Concentrations
COI
Calculated Mixing Zone Conc. (µg/L) Water Quality Standard (µg/L)
Acute Chronic Human
Health Acute Chronic Human
Health
Arsenic 0.473 0.186 0.145 340 150 10
Boron 171.1 170.2 170.0* ns ns ns
Total
Chromium 0.675 0.602 0.591* ns ns ns
Chromium VI 0.592 0.590 0.590* 16 11 ns
Cobalt 0.326 0.321 0.321 ns ns 4
Sulfate 5,347 5,048 5,005* ns ns ns
Thallium 0.051 0.050 0.050 ns ns 0.47
Vanadium 0.702 0.692 0.690* ns ns ns
Notes:
1. µg/L – micrograms per liter
2. All COIs are shown as dissolved except for total chromium
3. ns – no water quality standard
4. * – concentration calculated with annual mean river flow

4.3 Refinement of SCM
The SCM developed and presented in Section 3 (Figures 3-1 and 3-2) adequately illustrates
hydrogeologic processes and characteristics as well as source areas and areas with 2L
exceedances of COIs attributable to ash handling at the DRSS site. Based on review of the
groundwater flow, fate and transport, and groundwater-surface water models the SCM does not
require further refinement.
Corrective Action Plan Part I
Dan River Steam Station Ash Basin

66
5 Summary and Recommendations
Based on the data presented herein, and the analysis of these data, Duke Energy provides the
following conclusions and recommendations:
 Duke Energy submitted a receptor survey to the NCDENR in September 2014, and
subsequently submitted a supplement to the receptor survey in November 2014. Three
private water supply wells, one private water supply spring, not currently in use, and
several tributaries to the Dan River were identified within a 0.5-mile radius of the ash
basin compliance boundary. All three water supply wells are located more than 2,000
feet away from the Dan River ash basin compliance boundary and are either upgradient
or across the Dan River from the ash basin system. No information gathered as part of
this assessment suggests that water supply wells or the spring located within the 0.5-
mile radius of the compliance boundary are impacted, or have the potential to be
impacted, by the Dan River ash basin system.
 PPBCs were calculated for soil and groundwater at the site and are presented in
Section 2. Note that for the DRSS site, the PPBCs were calculated using historical
groundwater quality data from NPDES compliance wells. At the request of NCDEQ,
groundwater results that were obtained with turbidity greater than 10 NTU were removed
from the statistical population. Given that the DRSS site has only one background
compliance well, the removal of these data resulted in an insufficient amount of
qualifying data (less than 8 data points) for statistical evaluation. Therefore, PPBCs
were developed by selecting the highest concentration of a given constituent across the
range of concentrations observed for that constituent in the qualifying events. PPBCs
will be refined as additional data sets are obtained from subsequent sampling events
collected from the background wells.
 COIs were selected for groundwater fate and transport modeling, in part, based on
comparison of constituent concentrations in background wells versus non-background
wells. As mentioned above, PPBCs were developed in background wells exclusive of
data collected with turbidity greater than 10 NTU. Note that data obtained from non-
background (e.g., cross- or down-gradient) wells was not eliminated to the same turbidity
criteria, even though the analytical results for selected constituents can be biased
upward due to the effects of turbidity. As such, the list of COIs to be carried forward in
CAP Part II will be modified, if warranted, as additional groundwater quality data is
obtained and the possible effects of turbidity on the analytical results are evaluated.
 Groundwater samples collected during the CSA and subsequent monitoring events were
analyzed for total and dissolved phase constituents to evaluate potential effects of
turbidity. While dissolved concentrations are not approved by the NCDEQ for use in
compliance well evaluations, they do provide meaningful data in areas of the DRSS site
where elevated turbidity is driven by site-specific geologic conditions. A more detailed
constituent-specific and location-specific analysis of turbidity and its effect on
groundwater quality results will be completed in CAP Part II to refine the list of COIs to
be remediated (if necessary).
Corrective Action Plan Part I
Dan River Steam Station Ash Basin

67
 Review of the groundwater modeling yields the following:
o Existing Conditions Scenario - The results of the EC scenario indicated that
concentrations for all modeled COIs increase or reach steady state conditions
above 2L Standards or IMACs at the compliance boundary during the modeled
period.
o Cap-in-Place Scenario – On-site model concentrations of arsenic, cobalt,
thallium and vanadium decrease, but remain above the 2L Standards or IMACs
at the compliance boundary. Boron, chromium, hexavalent chromium, and
sulfate are removed from the model domain under this scenario.
o Excavation Scenario - The simulation fails to reach steady state concentrations
after 250 years. The results of the EX scenario indicated that concentrations of
arsenic, boron, chromium, hexavalent chromium, cobalt, sulfate, thallium, and
vanadium are removed from the model domain under this scenario.
Concentrations of the above constituents will be below 2L Standards at the
compliance boundary.
 Groundwater flow rates and concentrations of COIs from the groundwater model were
used to determine if 2L exceedances would result in exceedances of 2B surface water
standards in the Dan River and in the tributary stream located east of the ash basin. The
groundwater-surface water interaction was performed using maximum concentrations of
COIs and those results show arsenic exceeds 2B Standards in the eastern unnamed
tributary. The mixing model results indicate that all water quality standards are attained
at the edge of the mixing zones except for arsenic in the unnamed east tributary for the
human health standard based on the observed concentration at station SW-3.
 Data gaps identified as part of the CSA will be assessed and information collected as
part of that assessment will be included in the CSA supplement to be provided after CAP
Part II.

The following recommendations have been made to address areas needing further assessment:
 Geochemical modeling of the DRSS site will be completed and submitted under cover of
the CAP Part II. The geochemical model results coupled with the groundwater flow, fate
and transport and surface water-groundwater models will enhance the understanding of
the processes taking place in the subsurface and ultimately aid in choosing the most
appropriate remedial action for the site. The geochemical model is key to understanding
mobility of iron, manganese, and TDS since it cannot adequately be modeled using
MODFLOW/MT3DMS.
 Additional sampling for radiological parameters on major flow paths is needed to perform
a more comprehensive assessment of radionuclides from source areas and
downgradient wells along major flow paths.
 Models should be updated with results from second round sampling at the DRSS site
and will be included in CAP Part II.
 Two additional background well sampling events are scheduled before the end of 2015.
These additional results will be utilized for statistical analysis of background
concentrations at the DRSS site and will be included in CAP Part II.
Corrective Action Plan Part I
Dan River Steam Station Ash Basin

68
6 References
Griffith, G. E., Omernik, J. M., Comstock, J. A., Shafale, M. P., McNab, W. H., Lenat, D. R.,
Glover, J. B., and Shelburne, V. B. 2002. Ecoregions of North Carolina and South
Carolina, (color poster with map, descriptive text, summary tables, and photographs):
Reston, Virginia, U.S. Geological Survey (map scale 1:1,500,000).
Duke Energy. 2014. Dan River Steam Station Coal Ash Excavation Plan. [Online] URL:
http://portal.ncdenr.org/c/document_library/get_file?p_l_id=1169848&folderId=23390982
&name=DLFE-106220.pdf
HDR. 2015. Comprehensive Site Assessment Report. Dan River Steam Station Ash Basin.
August 14, 2015.
HDR. 2014a. Dan River Steam Station – Ash Basin Drinking Water Supply Well and Receptor
Survey. [Online] URL: http://portal.ncdenr.org/web/wq/drinking-water-receptor-surveys
HDR. 2014b. Dan River Steam Station – Ash Basin Supplement to Drinking Water Supply Well
and Receptor Survey. [Online] URL: http://portal.ncdenr.org/web/wq/drinking-
waterreceptor-surveys
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.
North Carolina Department of Environment and Natural Resources. 2015. NPDES Permit for
Dan River Steam Station. [Online] URL:
http://portal.ncdenr.org/c/document_library/get_file?uuid=38c490a7-5b93-4540-a593-
d8ae3014d724&groupId=38364
Pollock, D. 2012. MODPATH: A Particle-Tracking Model for MODFLOW. U.S. Geological
Survey Office of Groundwater.
U.S. Environmental Protection Agency. 2013. Flow 101. [Online] URL:
http://water.epa.gov/scitech/datait/models/dflow/flow101.cfm
U.S. Environmental Protection Agency. 2009. Statistical Analysis of Groundwater Monitoring
Data at RCRA Facilities Unified Guidance.
Venkatakrishnan, R. and Gheorghiu, F. 2003. Conceptual groundwater flow models identified in
Triassic Basins, eastern United States: EGS-AGU-EUG Joint Assembly, Abstract from
Meeting, Held in Nice, France, 6-11 April 2003, Abstract #8569.
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.