NC0003468_1 DRSS CAP Part 2_Report_FINAL_20160210

F)l
Corrective Action Plan Part 2
Dan River Steam Station Ash Basin
Site Location:
NPDES Permit No.
Permittee and Current
Property Owner:
Consultant Information
Report Date:
Dan River Steam Station
900 South Edgewood Road
Eden, NC 27288
N C0003468
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
February 10, 2016

This page intentionally left blank

Corrective Action Plan Part 2
Dan River Steam Station Ash Basin

Contents
1 Introduction .......................................................................................................................................... 4
1.1 Regulatory Background ............................................................................................................. 4
1.2 Report Organization .................................................................................................................. 6
2 Summary of Previous and Current Studies ......................................................................................... 7
2.1 Comprehensive Site Assessment ............................................................................................. 7
2.1.1 Identification of COIs .................................................................................................... 8
2.1.2 Soil Delineation ............................................................................................................ 8
2.1.3 Groundwater Delineation ............................................................................................. 8
2.2 Corrective Action Plan Part 1 .................................................................................................... 9
2.2.1 Proposed Provisional Background Concentrations for Soil and Groundwater ............ 9
2.2.2 COI Occurrence and Distribution ................................................................................. 9
2.3 Round 2 Sampling ................................................................................................................... 10
2.3.1 Groundwater ............................................................................................................... 10
2.3.2 Round 1 and Round 2 Source Area and Groundwater Data Comparison ................. 11
2.3.3 Surface Water and Areas of Wetness ........................................................................ 14
2.4 Round 3 and Round 4 Background Well Sampling ................................................................. 16
2.5 Well Abandonment .................................................................................................................. 16
3 Site Conceptual Model ...................................................................................................................... 17
3.1 Identification of Potential Contaminants .................................................................................. 17
3.2 Identification and Characterization of Source Contaminants .................................................. 17
3.3 Delineation of Potential Migration Pathways through Environmental Media .......................... 18
3.3.1 Soil.............................................................................................................................. 18
3.3.2 Groundwater ............................................................................................................... 18
3.3.3 Surface Water and Sediment ..................................................................................... 19
3.4 Establishment of Background Areas ....................................................................................... 20
3.5 Environmental Receptor Identification and Discussion ........................................................... 20
3.6 Determination of System Boundaries ...................................................................................... 20
3.7 Site Geochemistry and Influence on COIs .............................................................................. 21
4 Updated Modeling ............................................................................................................................. 23
4.1 Groundwater Model Refinement ............................................................................................. 23
4.1.1 Flow Model Refinements ............................................................................................ 23
4.1.2 Fate and Transport Model Refinements..................................................................... 23
4.1.3 Summary of Modeled Scenarios ................................................................................ 24
4.1.4 Model Assumptions and Limitations ........................................................................... 25
4.1.5 Modeled Scenario Results ......................................................................................... 26
4.2 Surface Water Model Refinement ........................................................................................... 28
4.2.1 Methodology ............................................................................................................... 28
4.2.2 Results ....................................................................................................................... 28
4.3 Geochemical Modeling ............................................................................................................ 29
4.3.1 Objective .................................................................................................................... 29
4.3.2 Methodology ............................................................................................................... 30
4.3.3 Assumptions ............................................................................................................... 31
4.3.4 Results ....................................................................................................................... 31
4.4 Refined Site Conceptual Model ............................................................................................... 32
5 Risk Assessment ............................................................................................................................... 33
5.1 Step 1: Conceptual Site Model ................................................................................................ 33
i
Corrective Action Plan Part 2
Dan River Steam Station Ash Basin

5.2 Step 2: Risk-Based Screening ................................................................................................ 34
5.3 Step 3: Human Health Risk Assessment ................................................................................ 34
5.4 Step 4: Ecological Risk Assessment ....................................................................................... 35
6 Alternative Methods for Achieving Restoration ................................................................................. 37
6.1 Corrective Action Decision Process ........................................................................................ 37
6.1.1 Evaluation Criteria ...................................................................................................... 37
6.1.2 COIs Requiring Corrective Action .............................................................................. 38
6.1.3 Potential Exposure Routes and Receptors ................................................................ 38
6.2 Alternative Evaluation Criteria ................................................................................................. 39
6.2.1 Effectiveness .............................................................................................................. 39
6.2.2 Implementability/Feasibility ........................................................................................ 40
6.2.3 Environmental Sustainability ...................................................................................... 40
6.2.4 Cost ............................................................................................................................ 41
6.2.5 Stakeholder Acceptance ............................................................................................ 41
6.3 Remedial Alternatives to Achieve Regulatory Standards ....................................................... 41
6.3.1 Groundwater Remediation Alternatives ..................................................................... 41
6.3.2 Monitored Natural Attenuation Applicability to Site .................................................... 42
6.3.3 Site-Specific Alternatives Analysis ............................................................................. 43
6.3.4 Site-Specific Recommended Approach ..................................................................... 45
7 Selected Corrective Action(s) ............................................................................................................ 46
7.1 Selected Remedial Alternative for Corrective Action .............................................................. 46
7.2 Conceptual Design .................................................................................................................. 46
7.2.1 Source Removal – Excavation ................................................................................... 46
7.2.2 MNA............................................................................................................................ 47
8 Recommended Interim Activities ....................................................................................................... 49
8.1 Additional Information Needs .................................................................................................. 49
8.1.1 Additional Well Installation ......................................................................................... 49
8.1.2 Additional Groundwater Sampling and Analyses ....................................................... 49
9 Interim and Effectiveness Monitoring Plans ...................................................................................... 50
9.1 Interim Monitoring Plan ........................................................................................................... 50
9.1.1 Data Quality Objectives .............................................................................................. 50
9.1.2 Sampling Requirements ............................................................................................. 51
9.1.3 Reporting .................................................................................................................... 51
9.2 Effectiveness Monitoring Plan ................................................................................................. 52
9.2.1 Data Quality Objectives .............................................................................................. 52
9.2.2 Sampling Requirements ............................................................................................. 52
9.2.3 Reporting .................................................................................................................... 53
9.3 Sampling and Analysis ............................................................................................................ 53
9.3.1 Monitoring Well Measurements and Inspection ......................................................... 53
9.3.2 Surface Water and Area of Wetness Measurements ................................................. 54
9.3.3 Sample Collection ...................................................................................................... 54
9.3.4 Quality Assurance/Quality Control ............................................................................. 55
10 Implementation Cost and Schedule .................................................................................................. 57
10.1 Implementation Cost ............................................................................................................... 57
10.2 Implementation Schedule ........................................................................................................ 57
11 References ........................................................................................................................................ 59

ii
Corrective Action Plan Part 2
Dan River Steam Station Ash Basin

Tables
2-1 Summary of Horizontal Hydraulic Gradient Calculations
2-2 Comparison of 0.45 µm and 0.1 µm Filter Sample Results – Groundwater
2-3 Ash Porewater Analytical Results – Round 1 and Round 2
2-4 Ash Basin Water Analytical Results – Round 1 and Round 2
2-5 Background Groundwater Analytical Results – Round 1, 2, 3, and 4
2-6 Groundwater Results Upgradient of Ash Basin and Ash Storage Areas – Round 1 and
Round 2
2-7 Groundwater Results Beneath Ash Basin – Round 1 and Round 2
2-8 Groundwater Results Beneath Ash Storage Areas – Round 1 and Round 2
2-9 Groundwater Results Downgradient of the Ash Basin and Ash Storage Areas – Round 1
and Round 2
2-10 Constituents of Interest Evaluation
2-11 Surface Water Sample Analytical Results – Round 1 and Round 2
2-12 Areas of Wetness Sampling Analytical Results – Round 1, 2, and 3
2-13 NCDEQ Sample Location Analytical Results – Round 1 and Round 2
4-1 Summary of Modeled COI Results at the Compliance Boundary*
4-2 Eastern Unnamed Tributary Surface Water Concentrations*
4-3 Dan River Steam Station Surface Water Concentrations*
9-1 Interim Monitoring Plan Sample Locations
9-2 Sampling Parameters and Analytical Methods
10-1 Remedial Alternative Costs for MNA*
*Table is presented in the text of this CAP Part 2 Report; all other tables are attached
separately.

Figures
2-1 Site Sampling Locations
2-2 Potentiometric Surface Map – Shallow Flow Layer
2-3 Potentiometric Surface Map – Deep Flow Layer
2-4 Potentiometric Surface Map – Bedrock Flow Layer
2-5 Area of Exceedances of 2L Standards
3-1 Site Conceptual Model – 3D Representation
3-2 Site Conceptual Model Cross Sectional
3-3 Receptor Map
8-1 Additional Assessment Well Locations

iii
Corrective Action Plan Part 2
Dan River Steam Station Ash Basin

Appendices
A CSA Supplement 1
B Groundwater Flow and Transport Model
C Addendum to Soil Sorption Evaluation
D Surface Water Mixing Model Approach
E Geochemical Modeling Report
F Baseline Human Health and Ecological Risk Assessment
G Evaluation of Potential Groundwater Remedial Alternatives
H Monitored Natural Attenuation Technical Memorandum

Acronyms and Abbreviations
µg/L micrograms per liter
2B Standards North Carolina Surface Water Standards as Specified in T15 NCAC 02B .0211
and .0216 (Amended Effective January 2015)
2L Standards North Carolina groundwater Standards as Specified in T15A NCAC 02L
Standards
AOW area of wetness
ARAR applicable or relevant and appropriate requirements
ASTM ASTM International
BERA Baseline Ecological Risk Assessment
BG background
BR bedrock
CAMA North Carolina Coal Ash Management Act of 2014
CAP corrective action plan
CCR coal combustion residuals
COI constituent of interest
COPC constituent of potential concern
CSA comprehensive site assessment
CSM Conceptual Site Model
D deep
DO dissolved oxygen
DRCCS Dan River Combined Cycle Station
DRSS Dan River Steam Station
Duke Energy Duke Energy Carolinas, LLC
DWR NCDEQ Division of Water Resources
EPC exposure point concentration
HAO hydrous aluminum oxide
HFO hydrous ferric oxide
HQ hazard quotient
HSL health screening level
IMAC interim maximum allowable concentration
J Estimated concentration
J- Estimated concentration, biased low
J+ Estimated concentration, biased high
Kd linear sorption coefficient
iv
Corrective Action Plan Part 2
Dan River Steam Station Ash Basin

mg/kg milligrams per kilogram
mg/L milligrams per liter
MNA monitored natural attenuation
MW monitoring well
NCAC North Carolina Administrative Code
NCPSRGs North Carolina Preliminary Soil Remediation Goals
NCDENR North Carolina Department of Environment and Natural Resources
NCDEQ North Carolina Department of Environmental Quality
NCDHHS North Carolina Department of Health and Human Services
NPDES National Pollutant Discharge Elimination System
NTU Nephelometric Turbidity Unit
POG protection of groundwater
PPBC proposed provisional background concentrations
RMS root mean squared
S shallow
SCM site conceptual model
SU standard units
TDS total dissolved solids
USEPA U.S. Environmental Protection Agency
Work Plan Groundwater Assessment Work Plan

v

This page intentionally left blank

Corrective Action Plan Part 2
Dan River Steam Station Ash Basin

Acknowledgments
HDR would like to express its appreciation to Duke Energy for its comments and interim report
reviews, and to the parties listed below for their assistance with data analysis, report
preparation, quality reviews, and overall development of this corrective action plan:

• The University of North Carolina at Charlotte – Groundwater modeling and soil sorption
analysis
• Electric Power Research Institute – Groundwater flow and transport model third-party
peer review
• Geochemical, LLC – Monitored natural attenuation evaluation and soil sorption analysis
• CH2M Hill, Inc. – Remedial alternative analysis

Corrective Action Plan Part 2
Dan River Steam Station Ash Basin

Executive Summary
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 the Dan River Steam Station (DRSS) was submitted to the North Carolina
Department of Environment and Natural Resources (NCDENR) on September 25, 2014, and
was subsequently revised 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 evaluate the 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 2015a).
Subsequent to submittal of the CSA Report, CAMA requires submittal of a corrective action plan
(CAP) for each regulated facility no later than 180 days after submittal of the CSA. Duke Energy
Carolinas, LLC (Duke Energy) and North Carolina Department of Environmental Quality
(NCDEQ)1 mutually agreed to a two-part CAP submittal, with Part 1 being submitted within 90
days of submittal of the CSA Report and Part 2 being submitted no later than 180 days after
submittal of the CSA Report. The DRSS CAP Part 1 was submitted to NCDEQ on November
12, 2015.
In December 2015, NCDEQ released draft proposed impoundment risk classifications for Duke
Energy’s coal ash impoundments in North Carolina. The proposed risk classifications issued for
both cells of the DRSS ash basin is high. Risk classifications were based upon potential risk to
public health and the environment. A public meeting regarding the proposed risk classifications
is scheduled for March 1, 2016. NCDEQ will release final risk classifications after review of
public comments. However, regardless of the remaining public meetings, DRSS is considered
high priority by CAMA and therefore ash excavation of the ash basin is required.
Duke Energy owns and formerly operated the 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, CCR from DRSS’s coal combustion process was disposed in an ash basin located
northeast of the station and adjacent to the Dan River, since the basin was constructed.
Discharge from the ash basin is permitted by the NCDEQ Division of Water Resources (DWR)
under National Pollutant Discharge Elimination System (NPDES) Permit NC0003468.
Groundwater flows to the south/southeast from the ash storage areas and ash basin toward the
eastern unnamed tributary and the Dan River. The groundwater flow direction is away from the
direction of the nearest public or private water supply wells. The Dan River serves as the
primary hydrologic discharge feature for groundwater within the shallow, deep, and bedrock flow
layers at the site.
1 Prior to September 18, 2015, the NCDEQ was referred to as the NCDENR. Both naming conventions are used in
this report, as appropriate.
1

Corrective Action Plan Part 2
Dan River Steam Station Ash Basin

Based on results of the CSA, concentrations of constituents of interest (COIs)2 attributable to
the CCR source areas at DRSS are present beneath the ash basin, beneath ash storage areas,
and downgradient and east of the ash basin Secondary Cell. COI transport from the source
areas is generally in a southeasterly direction towards the Dan River and the eastern unnamed
tributary that flows to the Dan River. COIs directly attributable to ash handling at DRSS are
arsenic, boron, sulfate, and total dissolved solids (TDS). Cobalt, iron, manganese, and
vanadium were found to be naturally occurring constituents in site background soil. Further
sampling and analysis are necessary to determine if COI exceedances are the result of source-
related impacts (as discussed in Section 2).
The refined groundwater model predicts that several COIs will exceed regulatory standards at
the Compliance Boundary 3 as discussed in Section 4.1.5; however, based on results of the
groundwater to surface water modeling, all water quality model results are less than the 2B
Standards at the edge of the mixing zones in the Dan River.
A human health and ecological risk assessment was conducted as part of this CAP. Results of
the risk assessment indicate there are no unacceptable risks to human health, except for a
subsistence fisher. Consumption of fish caught off-site by a recreational fisher and subsistence
fisher was estimated to result in potential non-cancer risks. Evaluation of potential impacts to
ecological receptors indicates that exposure to the concentrations of vanadium in the eastern
unnamed tributary may adversely impact water-dependent birds and concentrations of
aluminum in the same location may impact water-dependent mammals.
Duke Energy plans to excavate ash contained in the ash storage areas and the ash basin
(Primary and Secondary cells). Excavated material will either be used beneficially off-site or will
be relocated to a new on-site lined landfill once constructed.
An evaluation of site conditions, consituents, and a review of alternative methods for restoring
groundwater quality found that, in conjunction with source removal at DRSS, Monitored Natural
Attenuation (MNA) is recommended as a corrective action for groundwater impacts beneath the
site. An interim monitoring plan has been developed to provide baseline seasonal analytical
data for DRSS and will be implemented with sampling activities planned for the First and
Second Quarters of 2016. Interim monitoring results will be used to evaluate compliance and
may be used, as needed, to refine the groundwater fate and transport, groundwater to surface
water interaction, and geochemical models. The monitoring results will also be used to confirm
that natural attenuation is continuing to occur and remains an effective remedial action for
DRSS. If MNA is deemed insufficient for restoration of groundwater quality, other alternatives
discussed in Section 6.3.3 should be evaluated and, if warranted, implemented to augment
2 If a constituent concentration exceeded the North Carolina Groundwater Quality Standards as specified in 15A
NCAC .0202L (2L Standards), Interim Maximum Allowable Concentration (IMAC), North Carolina Preliminary Soil
Remediation Goals for Protection of Groundwater (NCPSRG for POG), North Carolina Department of Health and
Human Services Health Screening Level (NCDHHS HSLs), North Carolina Surface Water Quality Standards (2B
Standards), or U.S. Environmental Protection Agency National Recommended Water Quality Criteria, it has been
designated as a “constituent of interest”. 3 Per 15A NCAC 02L .0102, “Compliance Boundary” means a boundary around a disposal system at and beyond
which groundwater quality standards may not be exceeded and only applies to facilities which have received a
permit issued under the authority of G.S. 143-215.1 or G.S. 130A.

2

Corrective Action Plan Part 2
Dan River Steam Station Ash Basin

MNA monitoring. The performance of these remedial alternatives will continue to be monitored
and evaluated to determine if modifications to the measures are required.
Per CAMA §130A-309.209.(b)(1), "The Groundwater Corrective Action Plan shall provide for the
restoration of groundwater in conformance with the requirements of Subchapter L of Chapter 2
of Title 15A of the North Carolina Administrative Code.” This CAP meets the requirements of
15A NCAC 02L .0106 and the requirements of the referenced section of CAMA.

3
Corrective Action Plan Part 2
Dan River Steam Station Ash Basin

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 combustion residuals
(CCR) from DRSS’s coal combustion process were disposed in an ash basin located northeast
of the station and adjacent to the Dan River, since the basin was constructed. Discharge from
the ash basin is currently permitted by the North Carolina Department of Environmental Quality
(NCDEQ)4 Division of Water Resources (DWR) under National Pollutant Discharge Elimination
System (NPDES) Permit NC0003468.
1.1 Regulatory Background
The North Carolina Coal Ash Management Act of 2014 (CAMA) directs owners of 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 the North Carolina Department of Environment and Natural Resources (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 the
horizontal and vertical extent of impacts to soil and groundwater attributable to CCR source
areas, identify potential receptors, and screen for potential risks to those receptors. The DRSS
CSA Report was submitted to NCDENR on August 14, 2015 (HDR 2015a).
CAMA Section §130A-309.209(b) requires implementation of corrective action for the
restoration of groundwater quality in accordance with Subchapter L of Chapter 2 of Title 15A of
the North Carolina Administrative Code (15A NCAC 02L) and requires the submittal of a
corrective action plan (CAP) for each regulated facility no later than 180 days after submittal of
the CSA. Duke Energy and NCDEQ mutually agreed to a two-part CAP submittal, with Part 1
being submitted within 90 days of submittal of the CSA and Part 2 being submitted no later than
180 days after submittal of the CSA.
The DRSS CAP Part 1 was submitted to NCDEQ on November 12, 2015 and consisted of the
following:
• background information
• brief summary of the CSA findings
• brief description of the site geology and hydrogeology
• summary of the previously completed receptor survey
• summary of constituent of interest (COI) exceedances and distribution
4 Prior to September 18, 2015, the NCDEQ was referred to as the NCDENR. Both naming conventions are used in
this report, as appropriate.
4

Corrective Action Plan Part 2
Dan River Steam Station Ash Basin

• development of proposed soil and groundwater provisional background concentrations
(PPBCs)
• detailed description of the site conceptual model (SCM)
• results of the groundwater flow and fate and transport model
• results of the groundwater to surface water interaction model
The purpose of this CAP Part 2 is to provide the following:
• a description of exceedances of groundwater quality standards, surface water quality
standards, and sample results greater than IMACs and North Carolina Department of
Health and Human Services (NCDHHS) Health Screening Levels (HSLs)
• a refined SCM
• refined groundwater flow and fate and transport model results
• refined groundwater to surface water model results
• site geochemical model results
• findings of the risk assessment
• evaluation of methods for achieving groundwater quality restoration
• conceptual plan(s) for recommended proposed corrective action(s)
• a schedule for implementation of the proposed corrective action
• a plan for monitoring and reporting on the effectiveness of the proposed corrective action
The information provided in the combined CAP Part 1 and CAP Part 2 meets the requirements
of regulation 15A NCAC 02L .0106 (f) for corrective action.
As required by CAMA, Duke Energy plans to excavate the primary source, which is the coal ash
contained in the ash storage areas and the ash basin (Primary and Secondary cells). Excavated
material will either be used beneficially off-site or will be relocated to a new on-site lined landfill.
Regulation 15A NCAC 02L .0106 (f)(4) requires that the secondary sources, which would be
potential continuing sources of possible pollutants to groundwater, be addressed in the CAP. At
DRSS, the soil located beneath the ash basin could be considered as a potential secondary
source. Preliminary information to date indicates that the thickness of soil impacted by ash
would generally be limited to the depth near the ash/soil interface. As discussed with NCDEQ,
after excavation, soils left on-site will be sampled and analyzed, and the analytical results will be
incorporated into the groundwater fate and transport model. If this evaluation indicates that
modification to the proposed CAP is required, Duke Energy will prepare and submit a revised
CAP.
In December 2015, NCDEQ released draft proposed risk classifications for Duke Energy’s coal
ash impoundments in North Carolina. The proposed risk classification issued for both cells of
the DRSS ash basin is high. Risk classifications were based upon potential risk to public health
and the environment. A public meeting regarding the proposed risk classification is scheduled
for March 1, 2016. NCDEQ will release final risk classifications after review of public comments;
however, regardless of the remaining public meetings, DRSS is considered “High Priority” by
CAMA and therefore ash excavation of the ash basin is required.
5
Corrective Action Plan Part 2
Dan River Steam Station Ash Basin

1.2 Report Organization
The information identified above has been organized in this CAP Part 2 Report as follows:
• Section 1 provides an introduction to the DRSS site and the intent of corrective action
under CAMA.
• Section 2 provides a summary of the CSA and CAP Part 1 reports; a comparison of
Round 1 and Round 2 groundwater, surface water, and areas of wetness (AOW)
sampling results; and a summary of Round 3 and Round 4 background well sampling
results.
• Section 3 discusses the SCM and site geochemical controls on contaminant mobility.
• Section 4 discusses the purpose, methodologies, and results of refined groundwater,
groundwater to surface water, and geochemical modeling. Refinement of the SCM
following evaluation of the model results is also discussed in this section.
• Section 5 provides details of the human health and ecological risk assessments.
• Section 6 presents an evaluation of remedial alternatives to achieve groundwater
restoration.
• Section 7 provides a concept-level discussion and plans for recommended corrective
action(s).
• Section 8 discusses recommended interim activities to be initiated in 2016.
• Section 9 provides a plan for groundwater monitoring subsequent to implementation of
the CAP.
• Section 10 provides a schedule and cost opinion for CAP implementation and post-CAP
monitoring.
Applicable tables, figures, and appendices with supporting documents are attached to this
report.

6
Corrective Action Plan Part 2
Dan River Steam Station Ash Basin

2 Summary of Previous and Current Studies
This section presents a summary of previous and current studies including the following:
• Summary of CSA;
• Summary of CAP Part 1;
• Presentation of Round 2 groundwater, surface water, and area of wetness (AOW)
sampling results and data comparison to Round 1 sampling; and
• Presentation of Round 3 and 4 background well sampling results.

Round 1 sampling data were previously provided in the CSA Report. Subsequent sampling
rounds occurred after the CSA submittal and are presented in this CAP Part 2 Report.
2.1 Comprehensive Site Assessment
The purpose of the DRSS CSA was to collect information necessary to characterize the extent
of contamination resulting from historical production and storage of coal ash, evaluate the
chemical and physical characteristics of the contaminants, investigate the geology and
hydrogeology of the site including factors relating to contaminant transport, and examine risk to
potential receptors and exposure pathways. The following assessment activities were included
as part of the CSA:
• Completion of soil borings and installation of groundwater monitoring wells to facilitate
collection and analysis of chemical, physical, and hydrogeological parameters of
subsurface materials encountered within and beyond the waste boundary and
Compliance Boundary.
• Evaluation of laboratory analytical data to supplement the SCM.
• Update of the receptor survey previously completed in September 2014 (and updated
November 2014).
• Completion of a screening-level risk assessment.
Note that subsequent to submittal of the CSA Report, additional evaluation of the Round 1
sampling results has been conducted. The results of this additional evaluation are presented in
the form of updated isoconcentration maps for each COI included in Appendix A. Also included
in Appendix A are responses to the NCDEQ site-specific comments, additional information in
response to the temporal data gaps identified in the CSA Report, and well abandonment
records for wells that have been removed due to ongoing site activities.

7
Corrective Action Plan Part 2
Dan River Steam Station Ash Basin

2.1.1 Identification of COIs
If a constituent concentration exceeded the North Carolina Groundwater Quality Standards, as
specified in 15A NCAC .0202L (2L Standards), the Interim Maximum Allowable Concentration
(IMAC)5, North Carolina Preliminary Soil Remediation Goals (NCPSRGs) for Protection of
Groundwater (POG), or North Carolina Surface Water Quality Standards (2B Standards)6 or
USEPA criteria, it was designated as a COI. The following constituents were reported as COIs
in the CSA:
• Soil: iron, manganese, arsenic, chromium,7 and selenium
• Groundwater: antimony, arsenic, boron, chromium, cobalt, iron, manganese, sulfate,
thallium, total dissolved solids (TDS), and vanadium
• Surface Water: aluminum, arsenic, copper, lead, and vanadium
In addition to COI identification, delineation of COIs in site media was also conducted during the
CSA.
2.1.2 Soil Delineation
The primary source areas at DRSS are defined as the ash basin (Primary and Secondary cells)
and the ash storage areas (Ash Storage 1 and Ash Storage 2). Horizontal and vertical
delineation of source-related soil contamination was presented in the CSA Report. Where soil
impacts were identified beneath the ash storage areas and ash basin, the vertical extent of
contamination beneath the ash/soil interface is generally limited to the uppermost soil sample
collected beneath the ash.
2.1.3 Groundwater Delineation
Groundwater contamination at the site attributable to ash handling and storage was delineated
during the CSA activities. The extent of groundwater impacts in the following areas warranted
refinement as noted below:
• 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
These areas of refinement were identified as additional assessment needs in the CSA Report.
Based on these additional assessment needs, and subsequent discussions with NCDEQ, a total
of 13 monitoring wells are scheduled to be installed at DRSS. Locations of the additional wells
have been identified in the field, and drilling is scheduled to begin in the first quarter of 2016.
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. 6 North Carolina surface water standards as specified in 15 NCAC 02B .0211 and .0216 (amended effective January
2015). 7 Unless otherwise noted, references to chromium in this document indicate total chromium.

8

Corrective Action Plan Part 2
Dan River Steam Station Ash Basin

Results from the installation and sampling of these additional wells will be submitted under
separate cover.
2.2 Corrective Action Plan Part 1
The purpose of CAP Part 1 was to summarize CSA findings, evaluate background conditions by
calculating PPBCs, evaluate exceedances per sample medium with regard to PPBCs, develop a
refined SCM, and present preliminary results of the groundwater flow and transport model and
groundwater to surface water model.
2.2.1 Proposed Provisional Background Concentrations for Soil and
Groundwater
HDR used background soil and groundwater concentrations to determine site-specific
background concentrations for each COI. Because COIs can be both naturally occurring and
related to the source areas, the selection of borings/monitoring wells used to establish
background concentrations is important in determining whether releases have occurred from the
source areas, and to define the concentrations of source-related COIs exceeding background
concentrations for corrective action. COIs were 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. A detailed analysis of DRSS background soil and
groundwater PPBCs is provided in the CAP Part 1 Report. Further refinement of the PPBCs is
anticipated following the completion of additional background monitoring well sampling in 2016.
2.2.2 COI Occurrence and Distribution
The following groundwater COIs for DRSS were evaluated in the CAP Part 1: arsenic, boron,
chromium, cobalt, iron, manganese, sulfate, TDS, and vanadium. Cobalt, iron, manganese, and
vanadium exceeded their respective 2L Standards or IMACs and PPBCs in source areas.
Antimony and selenium were observed as isolated occurrences and need further evaluation as
to source of these COIs. Arsenic, boron, chromium, sulfate, and TDS are attributable to ash
handling at the site.
Areas of 2L Standard exceedances of arsenic, boron, chromium, sulfate, and TDS are within or
immediately downgradient of the source areas indicating that physical and geochemical
processes beneath DRSS 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 northeast of the Secondary Cell where
COI concentrations in excess of 2B Standards were detected in the eastern unnamed tributary
that discharges to the Dan River. Groundwater model results for several COIs showed their
concentrations to be greater than applicable regulatory standards (2L Standard, IMAC,
NCDHHS HSL) at the Compliance Boundary for both current conditions and future scenarios
modeled; however, based on groundwater to surface water interaction model results, all water
quality standards are less than 2B Standards at the edge of the mixing zones in the Dan River.
Downward vertical migration of COIs has been measured in select well clusters 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.
9
Corrective Action Plan Part 2
Dan River Steam Station Ash Basin

2.3 Round 2 Sampling
Round 1 sampling was conducted in June 2015 as part of the CSA. A comprehensive
groundwater gauging event was conducted on September 2, 2015. Round 2 groundwater,
surface water, and AOW sampling activities were conducted between September 3 through 14,
2015. All water samples were collected in accordance with sampling procedures described in
the CSA Report. The following subsections provide a comparison of Round 1 and Round 2
groundwater elevations and groundwater, surface water, and AOW analytical results.
2.3.1 Groundwater
A total of 68 groundwater monitoring wells were sampled during the Round 2 event including 57
groundwater assessment wells, 8 compliance and voluntary wells, and 3 wells installed as part
of previous ash basin closure activities. Six locations (BG-1D, GWA-8S, GWA-10S, and GWA-
15S, MW -9, and MW -10) were dry at time of sampling. Monitoring well locations are depicted on
Figure 2-1.
2.3.1.1 Groundwater Water Levels
On September 2, 2015, all monitoring wells were manually gauged from the top of the PVC
casing using an electronic water level indicator accurate to 0.01 foot. Groundwater elevations
and contours for the shallow, deep and bedrock flow layer are depicted on Figures 2-2, 2-3,
and 2-4, respectively. Groundwater elevations measured during the Round 2 water level
gauging event were generally lower than those observed during the Round 1 event and are
most likely attributable to seasonal variations of the water table. Groundwater flow direction is
consistent with flow directions identified during Round 1 water level gauging event documented
in the CSA Report, generally in a south/southeast direction toward the Dan River and slightly
east toward an unnamed tributary on Duke Energy property that flows to the Dan River.
2.3.1.2 Horizontal and Vertical Gradients
Horizontal hydraulic gradients were updated using Round 2 groundwater elevations 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). Monitoring wells, groundwater elevations,
and length of flow paths utilized for horizontal hydraulic conductivity calculations are detailed in
Table 2-1. The average horizontal hydraulic gradients for Round 2 compared to Round 1 are
provided below:
• Shallow: Round 2 – 0.029 feet/foot; Round 1 – 0.030 feet/foot
• Deep: Round 2 – 0.029 feet/foot; Round 1 – 0.029 feet/foot
• Bedrock: Round 2 – 0.035 feet/foot; Round 1 – 0.037 feet/foot
Minor fluctuations in the shallow and bedrock flow layer may be attributable to seasonal
variations; however, horizontal hydraulic gradients were generally consistent with those
documented in the CSA Report.
10
Corrective Action Plan Part 2
Dan River Steam Station Ash Basin

Vertical hydraulic gradients were calculated for Round 2 and were presented in the CAP Part 1
Report. Based on review of the results, groundwater vertical gradients are generally downward
across the site. Sixteen of the 20 well pairs exhibited a downward gradient ranging from -0.002
feet/foot to -1.864 feet/foot. Four of the 20 well pairs exhibited an upward gradient ranging from
0.051 feet/foot to 0.333 feet/foot and each of these locations are in close proximity to and
influenced by the ash storage areas and ash basin. Details regarding the vertical gradients
across the site were presented in the CAP Part 1 Report.
2.3.1.3 Groundwater Sampling
Groundwater samples were collected using low-flow sampling techniques, as outlined in the
Low Flow Sampling Plan developed for the ash basin groundwater assessment program and
approved by NCDENR (CSA Report Appendix G). The samples were submitted to a laboratory
for analysis of total and dissolved inorganic parameters (USEPA Methods 200.7/200.8, 245.7,
and 218.7) and water quality parameters.
A 0.45-micron filter was used for collection of groundwater samples for dissolved concentration
analysis. During Round 2, additional sample volume was collected at select locations using a
0.1-micron filter for dissolved concentration analysis along two major flow paths at DRSS and at
locations with constituent concentrations that may be affected by turbidity. The following
monitoring wells were sampled using the 0.1-micron filter:

• AB-5D • AS-4D • GWA-3D • MW -11 • MW -315BR
• AB-10D • AS-8D • GWA-8D • MW -11D • MW -317BR
• AB-25D • AS-8BR • GWA-9D • MW -21S • MW -318D
• AB-30S • AS-12S • GWA-10D • MW -303BR • OW -308BR
• AB-30D • BG-5S • GWA-11S • MW -306BR • OW -310BR
• AB-30BR • BG-5D • GWA-11D • MW -306BR
• AB-35BR • GWA-1S • MW -9D • MW -310BR

Dissolved concentrations of COI samples using 0.45-micron and 0.1-micron filters were
generally similar. A comparison of the analytical results from 0.45-micron and 0.1-micron filtered
samples are presented in Table 2-2.
2.3.2 Round 1 and Round 2 Source Area and Groundwater Data Comparison
Round 1 and Round 2 ash porewater, ash basin water, and groundwater data are presented in
Tables 2-3 through 2-12. Ash porewater and groundwater concentrations in Round 2 have
generally remained similar to those observed in Round 1. Variation from Round 1 to Round 2
cannot be further interpreted at this time as the data set consists of only two comprehensive
sampling events and therefore does not fully consider seasonal fluctuations, or other temporal
changes.
11
Corrective Action Plan Part 2
Dan River Steam Station Ash Basin

2.3.2.1 Source Area Results
Ash Porewater
The ash basin is a permitted wastewater treatment facility and water in the basin is wastewater,
not groundwater. Ash porewater is compared to 2L Standards or IMACs and NCDHHS HSLs for
comparison purposes only.
Ash porewater samples were collected in Rounds 1 and 2 from locations within the source
areas associated with the ash basin and ash storage areas (Table 2-3). Fluctuations in the total
number of COIs reported at individual wells were noted when comparing Round 1 to Round 2.
Three additional COIs (barium, lead, and nickel) were identified at one location (AB-25S) in
Round 2.
Based on this data comparison, COIs in ash porewater are: antimony, arsenic, barium, boron,
chromium, cobalt, iron, lead, manganese, nickel, sulfate, thallium, TDS, and vanadium.
Ash Basin Water
The ash basin is a permitted wastewater treatment facility and water in the basin is wastewater
not surface water. Ash basin water is compared to both 2B Standards and 2L Standards or
IMACs and NCDHHS HSLs because ash basin water is the source of impacts to groundwater
and surface water and is for comparison purposes only.
Two ash basin water samples were collected (SW-1 and SW-9). For purposes of comparison,
ash basin water was compared to 2B Standards (Table 2-4). Ash basin water samples were
collected in Round 1 and Round 2. Fluctuations in the total number of COIs reported at
individual locations were noted when comparing Round 1 to Round 2.
Based on this data comparison, COIs in ash basin water are: aluminum, arsenic, copper,
manganese, and zinc.
2.3.2.2 Groundwater Results
Background Wells
Groundwater samples were collected in Round 1 and Round 2 from background locations
(Table 2-5). In general, concentrations from the background monitoring wells exhibited similar
results and constituents when comparing data from the Round 1 and Round 2 sampling events.
Background monitoring wells will continue to be sampled and PPBCs recalculated as the size of
the data set increases with additional sampling rounds.
Upgradient of the Ash Basin and Ash Storage Areas
Groundwater samples were collected in Round 1 and Round 2 from locations upgradient of the
ash basin and ash storage areas (Table 2-6). Fluctuations in the total number of COIs reported
at individual wells were noted when comparing Round 1 to Round 2. One additional COI
(beryllium) was identified in well GWA-7S in Round 2. Beryllium was detected in well GWA-7S
in Round 1, but did not exceed the IMAC value and, therefore, was not previously identified as a
COI.
12
Corrective Action Plan Part 2
Dan River Steam Station Ash Basin

Based on this data comparison, COIs in groundwater upgradient of source areas are: antimony,
beryllium, cobalt, iron, manganese, and vanadium.
Beneath the Ash Basin
Groundwater samples were collected in Round 1 and Round 2 from locations beneath the ash
basin (Table 2-7). Fluctuations in the total number of COIs reported at individual wells were
noted when comparing Round 1 to Round 2. No additional COIs were identified in Round 2.
Based on this data comparison, COIs in groundwater beneath the ash basin are: antimony,
arsenic, boron, cobalt, hexavalent chromium, iron, manganese, and vanadium.
Beneath the Ash Storage Areas
Groundwater samples were collected in Round 1 and Round 2 from locations beneath the ash
storage areas (Table 2-8). No additional COIs were identified in Round 2.
Based on this data comparison, COIs in groundwater beneath the ash basin are: antimony,
boron, cobalt, iron, manganese, selenium, sulfate, TDS, and vanadium.
Downgradient of the Ash Basin and Ash Storage Areas
Groundwater samples were collected in Round 1 and Round 2 from locations downgradient of
the ash basin and ash storage areas (Table 2-9). Fluctuations in the total number of COIs
reported at individual wells were noted when comparing Round 1 to Round 2. Chromium was
observed above the 2L Standard at GWA-14S during Round 2 (this well was dry during Round 1
and unable to be sampled).
Based on this data comparison, COIs in groundwater downgradient of the source areas are:
arsenic, cobalt, iron, manganese, sulfate, TDS, and vanadium.
Note that further evaluation of Round 1 and Round 2 data resulted in chromium being removed
as a COI for corrective action as part of this CAP. Although total chromium exceeded the 2L
Standard, dissolved concentrations were observed below the 2L Standard in all samples.
Additional data is needed to further assess whether chromium is a COI at the DRSS site.
2.3.2.3 Description of Groundwater Quality Standard Exceedances
Per CAMA, the CAP should include “A description of all exceedances of the groundwater quality
standards, including any exceedances that the owner asserts are the result of natural
background conditions”. To address this requirement, COIs identified during the Round 1 and
Round 2 sampling events were evaluated to assess if they are naturally occurring or attributable
to ash handling at the site.
Results of the COI evaluation are summarized in Table 2-10. Only analytical results which
exceeded their respective groundwater criteria are presented in this table.
• Where the COI concentration is less than the applicable PPBC, the cell is highlighted
green. The analytical results associated with the green highlighting are groundwater
criteria exceedances attributable to natural background conditions.
13
Corrective Action Plan Part 2
Dan River Steam Station Ash Basin

• Where the COI concentrations are greater than the applicable groundwater standard or
PPBC, the cell is highlighted orange. The analytical results associated with the orange
highlighted cells are exceedances associated with ash handling at the site.
It is important to note that this evaluation only includes two comprehensive sampling events and
additional sampling is needed to re-evaluate PPBCs and more appropriately assess COIs
compared to PPBCs at the site. Areas of exceedances of COIs attributable to ash handling at
DRSS are depicted on Figure 2-5.
As described in Section 2.3.2.2, based on further evaluation, chromium was eliminated as a
COI for corrective action as part of this CAP. Although total chromium exceeded the 2L
Standard, dissolved concentrations were observed below the 2L Standard in all samples.
2.3.3 Surface Water and Areas of Wetness
During the Round 2 sampling event, a total of 19 surface water and AOW samples were
collected at DRSS. AOW is defined as an area of ponded or flowing water that is not attributable
to stormwater runoff. Surface water and AOW sample locations are depicted on Figure 2-1.
• Surface Water (11 samples): SW-2 through SW -8 and SW-10 through SW-13
• AOWs (5 samples): S-1 through S-4 and S-6
• NCDEQ sampling locations (3 samples): CCSW001, CCSW002 (CCSW002OUT), and
DRRC001.
Surface Water
Surface water samples were collected in Round 1 and Round 2. Fluctuations in the total number
of COIs reported at individual locations were noted when comparing Round 1 to Round 2.
Based on groundwater results from Round 1 sampling, an additional data assessment need was
identified along the eastern unnamed tributary at the SW -3 location. Additional surface water
samples were collected upgradient (SW-11, SW-12 and SW-13) and downgradient (SW-10) of
the SW-3 location to resolve this additional data assessment need. Round 1 and Round 2
analytical results for the surface water samples are presented in Table 2-11.
Review of laboratory analysis of surface water samples collected from the eastern unnamed
tributary yields the following:
• Copper exceeded its 2B Standard in all surface water samples except SW-3, SW -6, and
SW -13.
• Lead exceeded its 2B Standard in samples SW-11, SW -12, SW-3, and SW-8.
• Aluminum exceeded its 2B Standard in all samples except SW-4, SW -12, and SW-13.
• Arsenic exceeded its 2B Standard in sample SW-10 during Round 2. Arsenic has
historically been detected above the 2B Standard at location SW-3, but was below the
2B Standard during Round 2 at this location.
Based on this data comparison, COIs in surface water are: aluminum, arsenic, copper and lead.
Note concentrations above 2B Standard of aluminum, copper and lead observed in samples
mentioned above were also observed in the upgradient surface water sample (SW -8).
14
Corrective Action Plan Part 2
Dan River Steam Station Ash Basin

Areas of Wetness
AOW samples were collected from locations S-1 through S-4 and S-6 near SW -3 to evaluate
potential sources of exceedances in the eastern unnamed tributary (Figure 2-1). Round 1 and
Round 2 analytical results for the AOW samples are presented in Table 2-12.
Review of laboratory analysis of AOW samples collected adjacent to the unnamed tributary
yields the following:
• Iron, manganese, and vanadium exceeded their respective 2L Standards or IMAC in
each of the AOW samples collected.
• Cobalt exceeded its IMAC in AOW samples S-1 and S-3.
• Arsenic, chromium, and lead exceeded their respective 2L Standards in AOW sample
S-1.
• Arsenic exceeded the 2L Standard in AOW sample S-6.
Based on this data comparison, COIs in AOWs are: arsenic, cobalt, iron, lead, manganese, and
vanadium. Although chromium exceeded its applicable 2L Standard, additional data is needed
to further assess whether chromium is a COI at the DRSS site.
March 2014 NCDEQ Sampling
NCDEQ performed a sampling event at DRSS in March 2014, which included nine sampling
locations (CSA Section 7.4.1). As part of the CSA, four of the nine locations (INFSW009,
CSSW001, CCSW002OUT, and DRRC001) were re-sampled in Round 1 to assess potential
influence of the ash basin at these locations. INFSW009 was dry at time of sampling.
Water samples were collected from NCDEQ sampling locations CCSW001, CCSW002
(CCSW002OUT), and DRRC001 in Round 2. INFSW009 was dry at time of sampling. NCDEQ
sample locations are compared to both 2L Standards, IMACs, NCDHHS HSLs, and 2B
Standards for comparative purposes only. Round 1 and Round 2 analytical results for the
NCDEQ water samples are presented in Table 2-13.
Review of laboratory analysis of water samples collected NCDEQ sampling locations yields the
following:
• Aluminum exceeded its 2B Standard in samples CCSW002 and DRRC001.
• Cobalt exceeded its 2B Standard in samples CCSW002 and DRRC001. Sample
CCSW002 also exceeded its 2L Standard.
• Copper and lead exceeded their 2B Standards in samples CCSW002 and DRRC001.
• Iron exceeded its 2L Standard in samples CCSW002 and DRRC001.
• Manganese exceeded its 2L Standard in all NCDEQ samples collected.
• Mercury exceeded its 2L Standard in sample DRRC001.
• Vanadium exceeded its IMAC in samples CCSW001, CCSW002, and DRRC001.
Based on this data comparison, COIs at NCDEQ sampling locations are: aluminum, cobalt,
copper, iron, lead, manganese, mercury, and vanadium.
15
Corrective Action Plan Part 2
Dan River Steam Station Ash Basin

In general, surface water and AOW concentrations in Round 2 have remained similar to those
observed in Round 1 with the exception of the AOW results detailed above. Note some AOW
samples were dry during Round 1, but were able to be sampled in Round 2, thus some
locations only have one data set. Variation from Round 1 to Round 2 cannot be further
interpreted at this time as the data set consists of only two comprehensive sampling events and
therefore does not fully consider seasonal fluctuations, or other temporal changes. Round 1 and
Round 2 analytical results for the surface water and AOW samples are presented in Tables 2-
11 and 2-13.
2.4 Round 3 and Round 4 Background Well Sampling
In response to a Duke Energy request for clarification of guidance, NCDEQ provided a table
titled “Clarification of Attachment 1 Groundwater Assessment Plan Conditional Letters of
Approval Items Related to Speciation – May 22, 2015” by electronic mail.
In the responses provided in this table, NCDEQ requested that Duke Energy “plan to sample the
existing and newly installed background wells two (2) additional times during 2015 as part of an
anticipated corrective action measure to support USEPA tiered site analysis and statistical
analysis”. The two additional sampling events referenced in this response correspond to
background sampling Round 3 and Round 4, performed in November and December 2015,
respectively.
The groundwater analytical parameters and methods are detailed in CSA Report. Groundwater
samples were collected in accordance with sampling procedures described in the CSA. The
results of the Round 3 and Round 4 background well sampling event are presented in Table 2-
5. Further evaluation of background sample results and PPBCs will be provided in subsequent
reports.
2.5 Well Abandonment
Due to source removal at DRSS, several monitoring wells within the source areas were
abandoned, including monitoring wells AS-4D, AS-6D, AS-10D, GWA-2S/D, GWA-3S/D, MW -
303BR, and MW-306BR. Well abandonment forms are provided in Appendix A, Attachment 5.
16
Corrective Action Plan Part 2
Dan River Steam Station Ash Basin

3 Site Conceptual Model
The SCM was initially presented in the CSA, and refined based on results from additional
sampling events and groundwater fate and transport and groundwater to surface water
modeling. The SCM was developed in general accordance with ASTM standard guidance
document E1689-95 (Reapproved 2014), “Standard Guide for Developing Conceptual Site
Models for Contaminated Sites” (ASTM 2014) to describe and integrate processes that
determine contaminant releases, contaminant migration, and environmental receptor exposure
to contaminants. The SCM is used to integrate site information and determine whether
additional information may be needed to further understand site hydrogeologic and potential
contaminant migration processes. The model was also used to facilitate selection of remedial
alternatives and effectiveness of remedial actions in reducing the exposure of environmental
receptors to contaminants (ASTM 2014). The SCM was developed using the six basic activities
outlined in ASTM E1689-95:
• Identification of potential contaminants;
• Identification and characterization of the source contaminants;
• Delineation of potential migration pathways through environmental media;
• Establishment of background areas;
• Environmental receptor identification and discussion; and
• Determination of system boundaries.
An expanded discussion of site geochemical controls on contaminant mobility and migration is
also provided in this section, as requested by the NCDEQ. A graphical representation of the
SCM is included as Figure 3-1.
3.1 Identification of Potential Contaminants
Potential contaminants (COIs) were identified in the CSA Report and are summarized in
Section 2.1 of this report.
3.2 Identification and Characterization of Source
Contaminants
The primary source areas at DRSS are defined as the ash basin (Primary and Secondary cells)
and the ash storage areas (Ash Storage 1 and Ash Storage 2) (see Figure 2-1). Source
characterization was performed through the completion of soil and rock borings, installation of
monitoring wells, and collection and analysis of associated solid- and aqueous-matrix samples
to identify physical and chemical properties of ash, ash basin water, ash porewater, and ash
basin AOWs. A geologic cross-section through the source areas is included as Figure 3-2.
Ash distribution and chemical and physical properties were evaluated through advancement and
sampling of 12 borings within the ash basin boundary, four borings within Ash Storage 1, and
four borings within Ash Storage 2. Ash within the ash basin was encountered at depths ranging
from the ground surface to approximately 17 to 45 feet below ground surface (bgs). Ash within
17
Corrective Action Plan Part 2
Dan River Steam Station Ash Basin

Ash Storage 1 was encountered from 10 to approximately 80 feet below ground surface. Ash
within Ash Storage 2 was encountered from 2 to approximately 27 feet below ground surface.
Ash porewater was evaluated through the sampling of four monitoring wells and two temporary
monitoring wells installed within the basin. Ash basin water was evaluated through sampling and
analysis of two ash basin water samples.
Based on the CSA results, concentrations of COIs at DRSS are present beneath the ash basin,
ash storage areas, and downgradient and east of the Secondary Cell. COIs that are attributable
to the source areas are limited to beneath the ash basin, ash storage areas and downgradient
and east of the Secondary Cell.
Review of laboratory analytical results of ash samples collected from the ash basin and ash
storage areas identified eight COIs: arsenic, barium, boron, cobalt, iron, manganese, selenium,
and vanadium. COIs identified in ash porewater samples were antimony, arsenic, boron,
chromium, cobalt, iron, manganese, sulfate, thallium, TDS, and vanadium. COIs identified in
ash basin water samples were aluminum, arsenic, copper, manganese, and zinc.
3.3 Delineation of Potential Migration Pathways through
Environmental Media
3.3.1 Soil
The approximate horizontal and vertical extent of soil contamination was delineated during the
CSA, with the exception of off-site areas north and east 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 ash. COIs identified
in soil were arsenic, boron, chromium, cobalt, iron, manganese, selenium, and vanadium. At
DRSS, the soil located below the ash basin could be considered as a potential secondary
source. As discussed with NCDEQ, after excavation, soils left on-site will be sampled and
analyzed, and the analytical results will be incorporated into the groundwater contaminant fate
and transport model. If this evaluation indicates that modification to the proposed CAP is
required, Duke Energy will prepare and submit a revised CAP.
Further assessment is underway, as recommended in the CSA and refined through
consideration of NCDEQ comments. Results of this assessment will be reported under separate
cover.
3.3.2 Groundwater
The approximate extent of groundwater impacts at DRSS is limited to beneath the ash basin
and ash storage areas with the following exceptions:
• Vertical extent in the vicinity of Ash Storage 1,
• Horizontal and vertical extent north of Ash Storage 1, and
• East of the Secondary Cell where groundwater to surface water interaction was
observed.
18
Corrective Action Plan Part 2
Dan River Steam Station Ash Basin

Based on exceedances of 2L Standards and IMACs the following groundwater COIs are being
evaluated for corrective action: antimony, arsenic, boron, cobalt, iron, manganese, selenium,
sulfate, thallium, TDS, and vanadium. As discussed in further detail in Section 2.3.1.6,
chromium was removed as a COI for corrective action as part of this CAP. Note that data
collected as part of the hydrogeologic study for proposed on-site landfill (AMECFW 2015) in the
vicinity of Ash Storage 1 was further evaluated. This data evaluation verified that groundwater
north of Ash Storage 1 flows to the north/northeast toward an unnamed tributary that discharges
to the Dan River.
Site hydrogeologic conditions were evaluated through sampling/testing conducted during
installation of three soil borings, 61 monitoring wells, and two temporary monitoring wells. Based
on the CSA investigation, the groundwater system in the natural materials (alluvium, soil,
soil/saprolite, and bedrock) is consistent with the regolith-fractured rock system and is an
unconfined, connected aquifer system without confining layers. The DRSS groundwater system
is divided into three layers referred to as the shallow, deep (transition zone), and bedrock flow
layers to distinguish the flow layers within the connected aquifer system.
In general, groundwater flows from the north and northwest extents of the DRSS property
boundary to the south and southeast toward the Dan River. As stated above, monitoring wells
installed outside the north/northeast boundaries of Ash Storage 1 indicate that groundwater
flows toward the eastern unnamed tributary that flows to the Dan River. Groundwater flow within
the bedrock flow layer is consistent with observed flow directions in the shallow and deep flow
layers, flowing from the northern portion of the site to the southeast toward the Dan River. The
Dan River serves as the lower hydrologic boundary for groundwater within the shallow, deep,
and bedrock flow layers at the site. Groundwater flow direction in the shallow, deep, and
bedrock flow layers is shown on Figures 2-2, 2-3, and 2-4, respectively.
3.3.3 Surface Water and Sediment
Two ash basin water samples were collected from the ash basin: one from the Primary Cell
(SW-9) and one from the Secondary Cell (SW-1). Two surface water samples (SW -3 and SW-4)
were obtained during the CSA from the eastern unnamed tributary that flows to the Dan River.
The location of surface water sample SW -3 was identified to be downgradient of the Secondary
Cell. In addition, one upgradient surface water sample (SW-5) was collected from the western
unnamed tributary that flows toward the Dan River, and three surface water samples were
collected from the upstream (SW -8) and downstream (SW -6 and SW -7) reaches of the Dan
River. Surface water flow directions in the Dan River and its tributaries adjacent to DRSS are
shown on Figure 2-1.
In general, COIs identified in surface water sample SW -3, collected from the eastern unnamed
tributary to the Dan River, are similar to groundwater COIs identified in monitoring wells located
between the eastern unnamed tributary and the Secondary Cell. Additional sampling of the
tributary was conducted during the Round 2 sampling event as summarized in Section 2.3.2.
Aluminum, copper, and lead were identified as COIs in the surface water samples collected
from the Dan River; however, copper did not exceed its 2L Standard in groundwater monitoring
wells located between the ash basin and the Dan River. Also, copper was not observed above
the 2L Standard in ash porewater samples collected from the ash basin. However, the dissolved
19
Corrective Action Plan Part 2
Dan River Steam Station Ash Basin

fraction of copper in ash basin water sample (SW-1) did exceed the 2B Standard. The dissolved
copper 2B standard was exceeded in the upgradient surface water sample (SW -8) in the Dan
River. The absence of copper exceedances in groundwater and ash porewater indicates that
there is likely an off-site source of copper to the Dan River.
3.4 Establishment of Background Areas
Background areas are located on the northern, northwestern, and western portions of DRSS.
Specifically for groundwater, the monitoring wells installed as background wells during the CSA
(BG-1D, BG-5S/D, and MW -23BR) and the existing NPDES ash basin background compliance
well (MW -23D) were installed in locations that represent upgradient, background groundwater
conditions. A detailed background monitoring well assessment is presented in Appendix B in the
CAP Part 1 Report.
3.5 Environmental Receptor Identification and Discussion
Duke Energy conducted a receptor survey of the area within 0.5 mile of the Compliance
Boundary in September 2014, and subsequently supplemented the receptor survey in
November 2014. Receptor locations identified during the surveys are shown on Figure 3-3.
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 and rural in nature. Residential properties are located north and northwest
of the ash basin Compliance Boundary within the 0.5-mile radius. One residence is located
south of DRSS across the Dan River within the 0.5-mile radius. Two industrial properties are
located northeast of DRSS. Farm land is located southeast of the site across the Dan River.
The receptor survey activities identified three private water supply wells and one water supply
spring, currently not in use, located within the 0.5-mile radius of the ash basin Compliance
Boundary. No wellhead protection areas were identified within a 0.5-mile radius of the ash basin
Compliance Boundary.
Several surface water bodies that flow from the topographic divide along South Edgewood Road
toward the Dan River were identified within a 0.5-mile radius of the ash basin Compliance
Boundary. No water supply wells (including irrigation wells and unused or abandoned wells)
were identified between the source areas and the Dan River.
There are no surface water features located at the site between the ash basin and the Dan
River. The groundwater flow direction is away from the direction of the nearest public or private
water supply wells. The Dan River serves as the lower hydrologic boundary for groundwater
within the shallow, deep, and bedrock flow layers at the site according to the LeGrand slope-
aquifer system model. Further, data from well pairs near the Dan River show upward gradients,
indicating discharge to the Dan River.
3.6 Determination of System Boundaries
The site, waste, and compliance boundaries for DRSS are shown on Figure 2-1. Spatially, the
SCM for DRSS is bounded by a hydrologic divide north of Ash Storage 1, the Dan River to the
20
Corrective Action Plan Part 2
Dan River Steam Station Ash Basin

south/southeast, an unnamed tributary to the east, and a hydrologic divide to the west that runs
approximately parallel to South Edgewood Road. The SCM extends vertically into bedrock,
which generally inhibits vertical migration of COIs at the site.
3.7 Site Geochemistry and Influence on COIs
Groundwater composition can be affected by an array of naturally occurring and anthropogenic
factors. Many of these factors can be causative agents for specific reduction-oxidation (redox)
processes or indicators of the implied redox state of groundwater as expressed by pH,
oxidation-reduction potential (ORP), and dissolved oxygen (DO). Groundwater pH is affected by
the composition of the bedrock and soil through which the water moves as well as other factors,
including exposure to carbonate rocks or lime-containing materials in well casings, exposure to
atmospheric carbon dioxide gas, and precipitation. In addition, metals and other elemental or
ionic constituents in groundwater, or the surrounding soil matrix, can act as electron donors or
acceptors as measured by ORP. The reactivity of different constituents can lead to oxidizing
(positive ORP) or reducing (negative ORP) environments in groundwater systems. DO in
groundwater can act as an oxidizing agent and is an indicator of redox state.
Based on site measurements, the primary redox categories at DRSS are oxic and mixed
(anoxic). At DRSS, DO levels exceeded the threshold of 0.5 mg/L in 43 of 83 samples (~52%)
and predominant redox processes are oxygen reduction with iron or manganese oxidation (i.e.,
controlled by O2 and Fe(III)/Fe(II) or Mn(IV)/Mn(II) couples). Under these conditions, more
oxidized species As(V), Se(VI), and Mn(IV) would be expected. There were five wells from
which porewater samples were collected and all five of those samples are classified as anoxic
or mixed (oxic-anoxic), which would indicate an increased potential for reduced forms of metals
to occur. However, it should be noted that 18 of the 76 (~72%) groundwater samples from wells
across the site are classified as suboxic or oxic categories where reduced species of metals
such as As(III) are less likely to be present.
Field measurements indicate that pH ranges from 4.77 to 11.44 Standard Units (SU) at the site.
Background well results indicate that pH ranges from 6.89 to 7.45 SU. Similarly, pH results for
upgradient wells beyond the waste boundary range from 5.39 to 7.81 SU. In contrast, pH values
within ash basin materials range from 6.46 to 8.61 SU. There is a wide range of ORP values,
spanning ranges that imply reduced (negative values) to highly oxidized (large positive values)
conditions. For ash porewater, both reducing and oxidizing conditions are indicated within the
ash basin. For groundwater, oxidizing conditions are generally present in all flow layers across
the site with the exception of reducing conditions observed at several locations beneath the ash
basin and ash storage areas including MW -22S, MW -308BR, MW -311BR, AS-2D, MW -315BR,
and MW-317BR. Reducing conditions were observed in the deep flow layer at three other
locations outside the waste boundary: GWA-1D, GWA-12D, and GWA-14D.
For the four background well samples (BG-1D, BG-5S/D, and MW-23BR), ORP values are
generally consistent with the inferred redox category of oxic. Measured ORP values from non-
background monitoring wells ranged from -81.4 millivolts (mV) (reducing) to +342.9 mV (strongly
oxidizing), whereas inferred redox conditions were generally oxic.
21
Corrective Action Plan Part 2
Dan River Steam Station Ash Basin

Speciation measurements were performed for samples collected from 12 groundwater and/or
porewater monitoring wells, depending on the analyte, and vary widely at the site. Speciation
measurements are summarized below:
• For the nine samples where speciated arsenic concentrations were above reporting
limits, the predominant species was in the reduced form [As(III)].
• In general, hexavalent chromium [Cr(VI)] was identified in upgradient and background
groundwater samples with similar concentrations to those collected from porewater wells
and groundwater monitoring wells beneath and downgradient of the source areas.
• The reduced form of iron [Fe(II)] was observed with highest concentrations downgradient
of the ash basin.
• The reduced form of manganese [Mn(II)] was observed with highest concentrations
downgradient of the source areas.
• The reduced form of selenium [Se(IV)] was present above detection limits in two
porewater samples (AB-10SL and AB-25S) and in one deep monitoring well GWA-10D.
The oxidized form of selenium [Se(VI)] was present above detection limits in the three
porewater samples (AB-10SL, AB-25S, and OW-308D).
Additional sampling will be needed to characterize the temporal and spatial characteristics of
groundwater composition for the site. Additional evaluations may also be beneficial to better
characterize the kinetics of redox reactions.
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. This chemical composition 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 cations and anions of groundwater
samples collected at a particular site.
Piper diagrams presented in the CSA Report provide evidence of mixing of ash porewater and
groundwater. In general, the ionic composition of groundwater and surface water at the site is
calcium, magnesium, and bicarbonate rich with the exception of downgradient groundwater,
which were observed to be trending closer to calcium, chloride, magnesium, and sulfate rich.

22
Corrective Action Plan Part 2
Dan River Steam Station Ash Basin

4 Updated Modeling
4.1 Groundwater Model Refinement
The groundwater flow and fate and transport models were refined to incorporate post-CSA data.
Model refinements are summarized in the following sections. The refined groundwater flow and
transport model report completed by HDR in conjunction with the University of North Carolina at
Charlotte (UNCC) is included as Appendix B.
4.1.1 Flow Model Refinements
Transient transport simulations for all COIs were calibrated and flow parameters were refined as
follows:
• Hydraulic conductivity measurements, obtained from slug test data collected during the
CSA, were utilized in the calibration of the flow model to better represent site-specific
conditions. This refinement led to reduction in the square root of the average square
error (also referred to as the root mean squared error, or RMS error) of the modeled
versus observed water levels for wells gauged in June 2015 to 5.5% compared to the
initial calibrated model of 9.95% in the CAP Part 1 model. The model calibration goal is
an RMS error less than 10% of the difference in head between the modeled and the
observed values across the model domain. The results are provided in Table 3 in
Appendix B.
• Recharge rates for the model were revised for both the ash basin and outside the ash
basin. Recharge applied to the areas outside the ash basin was assigned a value of 6.5
inches per year. Recharge within the ash basin was calculated using Darcy’s Law
considering the approximate area of the ash basin, the approximate depth of water or
saturated ash, and the range of measured hydraulic conductivity values within the ash
and fill. The calculation provided a range of recharge values, with a value of 13.14
inches per year. These refinements affect fate and transport of COIs at the site and are
more representative of current site conditions.
• Historic basin water levels were considered during calibration of the flow model.
However, the current flow model is calibrated to steady-state conditions. Continued
refinement of the model to consider transient flow may enhance integration of historic
water level data within the ash basin.
4.1.2 Fate and Transport Model Refinements
The groundwater fate and transport model was calibrated using the refined parameters from the
groundwater flow model as discussed above in Section 4.1.1, and presented below:
• The initial model used conservative (low) Kd (i.e., linear sorption coefficient) values to
achieve calibration of the transport models for each COI. Subsequent to submittal of the
CAP Part 1 Report, UNCC and Geochemical, LLC each provided recalculated Kd values
using linear and Freundlich isotherms (Appendix C). Both sets of recalculated Kd values
were considered during refinement of the transport models for each COI. Use of the
23
Corrective Action Plan Part 2
Dan River Steam Station Ash Basin

newly derived COI Kd values in the fate and transport model resulted in improved
calibration of source concentrations to measured concentrations in downgradient wells.
Note that final Kd values used to calibrate the fate and transport model may have fallen
outside the recalculated upper and lower limits; however, adjustment of Kd values within
the model to achieve calibration is considered acceptable practice.
• The initial model was not calibrated to background groundwater concentrations as
PPBCs were not developed in time for use in the model. The model has since been
refined to incorporate PPBCs for each COI. This refinement allows the model to account
for naturally occurring background concentrations and is particularly important for COIs
whose PPBC is greater than the 2L Standard, IMAC, or NCDHHS HSL. However, the
model is limited in that it applies the PPBC across the entire site, as shown on individual
COI concentration figures in Appendix B.
• The initial model used adjusted source area concentrations to achieve calibration at
downgradient monitoring wells. The flow model refinements discussed in Section 4.1.1
enabled refinement of the fate and transport model to better represent measured source
area porewater concentrations.
• Round 2 laboratory data were reviewed and no additional COIs were identified for
inclusion in the modeling.
• The background concentrations for the COIs were applied as initial concentrations.
Refinements to the groundwater models provide a more accurate representation of existing site
conditions and produce model results that more accurately depict closure scenarios at the site.
4.1.3 Summary of Modeled Scenarios
Two closure scenarios were modeled for DRSS: an Existing Conditions scenario with ash
sources left in place and an Excavation scenario with the accessible ash removed from the site.
These scenarios predict flow and transport results using the model parameters calibrated for
existing conditions. No modifications were made to the previously modeled Existing Conditions
scenario hydrogeologic parameters or structure.
4.1.3.1 Existing Conditions Scenario
The Existing Conditions scenario consists of using the calibrated model for steady-state
groundwater flow conditions and transient transport of COIs under existing conditions across the
site to predict when steady-state concentrations are reached at the Compliance Boundary. COI
concentrations remain the same or increase initially for this scenario with source concentrations
held at their constant value over time. Thereafter, the concentrations and discharge rates
remain constant. This scenario represents the most conservative case in terms of groundwater
concentrations on-site and off-site, with COIs discharging to surface water at steady-state.
The time to achieve a steady-state concentration plume depends on the source zone location
relative to the Compliance Boundary and its loading history. Areas close to the Compliance
Boundary will reach a steady-state concentration sooner. The time to steady-state concentration
is also dependent on the sorptive characteristics of each COI. Sorptive COIs will be transient for
a longer time period as their peak breakthrough concentration travels at a rate that is less than
24
Corrective Action Plan Part 2
Dan River Steam Station Ash Basin

groundwater pore velocity. Use of lower effective porosity values will result in shorter times to
achieve steady-state concentrations for both sorptive and non-sorptive COIs.
4.1.3.2 Excavation Model Scenario
In the Excavation scenario, the water in the basin is removed and ash from the ash basin and
ash storage areas is removed and transported off-site. The model did not account for backfilling
of excavation areas; however, the constant concentration sources of ash above and below the
water table are removed. This scenario assumes recharge rates become equal to rates
surrounding the ash basin (6.5 inches per year). Starting from the time that excavation is
complete, COIs already present in the groundwater continue to migrate downgradient as water
infiltrates from ground surface and recharges the aquifer at the water table. COIs move through
the saturated zone beneath the source areas at a rate dependent on aquifer properties and
geochemical interactions of the COI and groundwater. If the source areas become unsaturated,
COIs beneath the sources will decrease over time without a contributing constant source. COI
migration is retarded relative to the porewater velocity as sorptive COIs are attenuated by site
materials. The model uses the predicted concentrations from the 2015 calibration as the initial
COI concentrations.
4.1.4 Model Assumptions and Limitations
The model assumptions include the following:
• The steady-state flow model was calibrated to hydraulic heads measured at observation
wells in June 2015 and considered the ash basin water level. The model is not calibrated
to transient water levels over time, recharge, river flow, or river stage changes. A steady-
state calibration does not consider groundwater storage and does not calibrate the
groundwater flux into adjacent surface water bodies.
• A single domain MODFLOW modeling approach was used for simulating flow in the
primary porous groundwater flow layers.
• During model calibration, the constant source concentrations at the ash basins and ash
storage areas reasonably match 2015 groundwater COI concentrations.
• For the purposes of numerical modeling and comparing closure scenarios, it was
assumed that the selected closure scenario is implemented in 2015.
• Predictive simulations were performed and steady-state flow conditions were assumed
from the time the ash basins and ash storage areas were placed in service through the
current time until the end of the predictive simulations (Year 2265).
• COI source zone concentrations at the ash basin and ash storage areas were applied
uniformly within each source area and assumed to be constant with respect to time for
transport model calibration.
• Travel times are advective and do not account for sorption of COIs to site media , which may
cause the travel times to be reduced.
25
Corrective Action Plan Part 2
Dan River Steam Station Ash Basin

• The model does not account for varying geochemical conditions such as pH and redox
potential that could affect COI mobility; however, geochemical modeling was completed
and is further discussed in Section 4.3.
4.1.5 Modeled Scenario Results
Constituent concentrations were analyzed at downgradient monitoring wells (Figures 7 and 8 in
Appendix B) for all modeled COIs. These wells are downgradient from either the ash basin or
ash storage areas, and either upgradient of the Compliance Boundary or the Dan River.
Closure scenario results are presented as predicted concentration versus time curves in
downgradient monitoring wells and as groundwater concentration maps for each modeled COI
on Figures 17 through 134 in Appendix B, as discussed in the following subsections.
Concentration contours and concentration breakthrough curves are referenced to the date when
the ash basin came into service (i.e., 1957). Concentration maps are referenced to a time zero
that represents the time the closure action was implemented, which for the purposes of
modeling is assumed to be 2015.
A summary of the modeled COI results at the Compliance Boundary is provided in Table 4-1. A
“+” indicates that the concentration of a given COI has exceeded its applicable 2L Standard,
IMAC, or NCDHHS HSL. A “-” indicates that the concentration of a given COI has not exceeded
its applicable 2L Standard, IMAC, or NCDHHS HSL. Year 0 represents initial concentrations
observed in 2015 and Year 100 represents concentrations observed 100 years post-
implementation of each scenario.
Table 4-1 Summary of Modeled COI Results at the Compliance Boundary
Constituent
(Standard)
Appendix B
Figures Flow Layer Existing Conditions Excavation Scenario
Year 0 Year 100 Year 0 Year 100
Antimony
IMAC
(1 µg/L)
35 - 40
Shallow + + + +
Deep + + + +
Bedrock + + + +
Arsenic
2L
(10 µg/L)
23 - 28
Shallow - - - -
Deep - - - -
Bedrock - - - -
Boron
2L
(700 µg/L)
47 - 52
Shallow - + - -
Deep - + - -
Bedrock - + - -
Chromium
2L
(10 µg/L)
59 - 64
Shallow - - - -
Deep + - + -
Bedrock - - - -
Cobalt
IMAC
(1 µg/L)
71 - 76

Shallow + + + +
Deep + + + +
Bedrock + + + -
Hexavalent
Chromium
NCDHHS HSL
(0.07 µg/L)
81 - 86
Shallow + + + +
Deep + + + +
Bedrock + + + +
26
Corrective Action Plan Part 2
Dan River Steam Station Ash Basin

Constituent
(Standard)
Appendix B
Figures Flow Layer Existing Conditions Excavation Scenario
Year 0 Year 100 Year 0 Year 100
Selenium
2L
(20 µg/L)
93 - 98
Shallow - - - -
Deep - - - -
Bedrock - - - -
Sulfate
2L
(250,000 µg/L)
105 - 110
Shallow - + - -
Deep + + + -
Bedrock + + + -
Thallium
IMAC
(0.2 µg/L)
117 - 122
Shallow - + - +
Deep - + - +
Bedrock + + + +
Vanadium
IMAC
(0.3 µg/L)
129 - 134
Shallow + + + +
Deep + + + +
Bedrock + + + +

The model predictions are summarized as follows:
• In accordance with 15A NCAC 02L .0106(k), a CAP may be approved by NCDEQ
without requiring groundwater remediation to the 2L Standards if seven conditions are
met. Condition (4) specifies that 2L Standards must be met at a location no closer than
one year time of travel upgradient of an existing or foreseeable receptor. For DRSS, the
receptor is considered to be the Dan River. To evaluate this condition, HDR and UNCC
conducted particle tracking within the Existing Conditions scenario steady-state flow field
to identify the travel path and time to the model boundary. Monitoring wells used for this
evaluation and results of the particle tracking are presented in Appendix B, Table 6.
Four of seven locations in the shallow flow layer, six of seven locations in the deep flow
layer, and four of five locations in the bedrock flow layer do not meet the condition
described above.
• The model predicts that under the Existing Conditions and Excavation scenarios,
antimony, boron, cobalt, sulfate, thallium, and vanadium will exceed their respective 2L
Standards or IMACs at the Dan River model interface for all groundwater flow layers.
Also, hexavalent chromium is predicted to exceed the NCDHHS HSL at the Dan River.
For antimony, cobalt, thallium, and vanadium, the background concentrations used for
modeling also exceed the applicable groundwater standards at the Dan River.
• Refined model predictions do not show that COI concentrations will be effectively
reduced by ash removal under the Excavation scenario. The COIs that are predicted to
exceed their respective 2L Standard, IMAC, or NCDHHS HSL will remain above those
standards for the time period modeled (Years 2015−2115).
• The model predicts that under the Existing Conditions and Excavation scenarios, arsenic
and selenium will not exceed their respective 2L Standards at the Dan River model
interface.
• Among the COIs, boron and sulfate are similar in that both are considered conservative;
that is, neither of these COIs has a strong affinity to attenuate or adsorb to soil/rock
surfaces. As a result, the model predicts similar behavior for boron, sulfate, and other
27
Corrective Action Plan Part 2
Dan River Steam Station Ash Basin

COIs with low Kd values: rapid and nearly complete reduction to below the respective
standard or IMAC under the Excavation scenario.
4.2 Surface Water Model Refinement
4.2.1 Methodology
The methodology to complete the surface water model in CAP Part 2 (Appendix D) is
consistent with CAP Part 1 and incorporates new groundwater modeling results addressed in
Section 4.1, including:
• Revisions to the Kd values;
• Additional COIs based on review of Round 2 sampling data or as requested by NCDEQ;
• Updated groundwater flux data for input into the surface water model; and
• Round 2 sampling results that were used because additional sampling was conducted in
the eastern unnamed tributary to further evaluate arsenic exceedances in the tributary.
Groundwater to 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 DRSS.
4.2.2 Results
The calculated surface water COI concentrations in the eastern unnamed tributary and in the
Dan River downstream from DRSS are presented in Tables 4-2 and 4-3. The stream flows,
groundwater flows, and COI concentrations presented in Appendix B were used to complete
these calculations. In the eastern unnamed tributary, the average observed surface water
concentrations are presented for comparison to the flux-averaged discharge concentrations
calculated with the groundwater model.
Because there is negligible runoff flow in the eastern unnamed tributary that is available for
groundwater dilution, surface water mixing model calculations were not completed. Instead, the
groundwater model calculated flux-averaged concentrations entering the eastern unnamed
tributary were compared to the applicable 2B water quality standards. In addition, available
surface water samples in the eastern unnamed tributary were compared to 2B water quality
standards. Comparison of the groundwater model-calculated and surface water sample
concentrations to the 2B water quality standards indicated that the 2B water quality standards
are met for all COIs except for the human health 2B water quality standard for arsenic in the
eastern unnamed tributary. This conclusion is based on the average observed surface water
concentration at sample locations SW -3, SW -4, SW-10, SW-11, SW-12, and SW -13, which
equaled 11.3 µg/L. This appears to be a localized phenomenon, as model results for surface
water at other locations in the eastern unnamed tributary (i.e., SW-4, SW -11, SW-12, and SW -
13) were less than the 2B human health water quality standard for arsenic. In the Dan River, the
surface water mixing model calculated concentrations are all less than the 2B acute, chronic,
and human health water quality standards at the edge of the mixing zones.
28
Corrective Action Plan Part 2
Dan River Steam Station Ash Basin

Table 4-2 Eastern Unnamed Tributary Calculated Surface Water Concentrations
COI Observed
Conc. (µg/L)*
Average GW
Conc. (µg/L)
Water Quality Standard (µg/L)
Acute Chronic Human Health
Arsenic 11.3 0.956 340 150 10
Boron 97.1 48.2 NS NS NS
Chromium 4.62 4.73 NS NS NS
Hexavalent
Chromium 0.021 0.084 16 11 NS
Cobalt 0.618 0.483 NS NS 4
Sulfate 14,425 63,659 NS NS NS
Thallium 0.125 0.193 NS NS 0.47
Vanadium 0.856 0.948 NS NS NS
Notes:
1. All COIs are shown as dissolved, except chromium
2. NS – no water quality standard
3. * – average of data from site stations SW-3, SW -4, SW -10, SW-11, SW -12, and SW-13
4. Human Health / Water Supply (15A NCAC 02B .0211, 15A NCAC 02B .0216, effective January 1, 2015)
Table 4-3 Dan River Surface Water Concentrations
COI
Calculated Mixing Zone Conc. (µg/L) Water Quality Standard (µg/L)
Acute Chronic Human
Health Acute Chronic Human
Health
Arsenic 0.245 0.236 0.235 340 150 10
Boron 151.1 150.1 150.0* NS NS NS
Chromium 0.527 0.504 0.500* NS NS NS
Hexavalent
Chromium 0.496 0.499 0.500* 16 11 NS
Cobalt 0.464 0.465 0.465 NS NS 4
Sulfate 5,486 5,197 5,155* NS NS NS
Thallium 0.060 0.059 0.059 NS NS 0.47
Vanadium 1.64 1.64 1.65* NS NS NS
Notes:
1. All COIs are shown as dissolved except for total chromium
2. NS – no water quality standard
3. * – concentration calculated with annual mean river flow
4. Human Health / Water Supply (15A NCAC 02B .0211, 15A NCAC 02B .0216, effective January 1, 2015)

4.3 Geochemical Modeling
4.3.1 Objective
The objective of geochemical modeling is to describe the expected partitioning of COIs between
aqueous and solid phases (i.e., between groundwater and soil and between ash porewater and
ash) and anticipated changes in phase distributions given variations in DO, pH, and TDS.
Changes in DO affect the oxidation state of groundwater as measured by ORP, which is
generally expressed as Eh or electron activity (pE). Changes in pH affect the acidity of
groundwater and concurrently affect Eh. Changes in TDS affect ionic strength and ion
competition at sorption sites. Constituents evaluated for DRSS were: antimony, arsenic, boron,
chromium, cobalt, iron, manganese, pH, selenium, sulfate, TDS, thallium, and vanadium.
29
Corrective Action Plan Part 2
Dan River Steam Station Ash Basin

4.3.2 Methodology
Site-specific evaluations of COIs were performed for each of the monitoring wells using the
United States Geological Survey PHREEQC (v3.3.3) geochemical speciation code (Parkhurst
and Appelo 2013) and PhreePlot (Kinniburgh and Cooper 2011), a companion plotting package
that utilizes looping PHREEQC with a hunt and track approach to determine stability
boundaries. In using a single well approach, wells can be evaluated or grouped later based on
geochemical characteristics. The single well approach also allows fine resolution of
geochemical constituents and subtle differences between wells that have a significant bearing
on the overall geochemical characterization. Calculations were driven by measured
concentrations of COIs and other analytes such as ORP, alkalinity, sodium, and other ions in
groundwater for each of the 78 wells monitored at the DRSS.
PHREEQC calculations were performed to develop Pourbaix (Eh-pH) diagrams to display the
dominant geochemical forms (i.e., species) that would be expected in groundwater in the
absence of adsorption under equilibrium conditions and allowing for most probable mineral
precipitation where appropriate. Measured ORP and pH values for each well were plotted on the
Pourbaix diagram for each COI to evaluate the likely distribution of species at the DRSS.
Additional PHREEQC calculations were performed to simulate anticipated geochemical
speciation that would occur for each COI in the presence of adsorption to soils. Further
simulations were performed to evaluate model and COI response to changes in DO, pH, and
TDS in the presence of sediment adsorption. Adsorption to soils was represented using a
surface complexation theory approach with hydrous ferric oxides (HFO) and hydrous aluminum
oxides (HAO) representing weak and strong binding sites, respectively. Values for HFO and
HAO were determined from extractions from actual site sediment that were also the basis for
measured distribution coefficients (Kd values) for DRSS soils determined from adsorption
experiments conducted by UNCC.
To geochemically simulate changes to aquifers or test potential remediation strategies,
simulations in which DO, pH, redox, and TDS were varied were utilized. These geochemical
simulations are termed titrations for this report. Each set of titrations provides an estimate of the
percentage of each COI that would be adsorbed as a function of changing DO, pH, redox, or
TDS along with relevant changes to the dominant species across the gradient. For these
titrations, TDS was evaluated along with select cations and anions common in soils and
sediment at the site, including sodium, calcium, chloride, potassium, and sulfate. Changes to
DO, pH, and TDS were utilized for titrations due to the affinity for numerous COIs such as
metals to exist primarily as anionic or cationic species and their adsorption coefficient variations
to mineral surfaces, soils, sediment, rock, and ash. The titration method in geochemical
modeling can also account for mobility changes due to redox threshold changes and potential
mineral precipitation, indicated by saturation indices in outputs. Adsorption of anionic species is
typically greater at lower pH where anions are more strongly attracted to positively charged
surfaces (and vice versa regarding cationic species). Similarly, the solubility of mineral phases
is pH dependent and lower pH values tend to favor formation of more soluble cationic species
for most alkali elements, alkali earth elements, and transition metals. Methodologies are
discussed in further detail in Appendix E.
30
Corrective Action Plan Part 2
Dan River Steam Station Ash Basin

4.3.3 Assumptions
The following assumptions were incorporated in the PHREEQC modeling effort:
• Groundwater data were evaluated on a well-by-well basis.
• COI sorption in PHREEQC was represented by surface complexation. Surface
complexation models provide a molecular description of adsorption using an equilibrium
approach that defines surface species, chemical reactions, equilibrium constants, mass
and charge balances. A benefit of the surface complexation approach is that the charge
on both the adsorbing ion and the solid surface where sorption occurs.
• The surface complexation model was parameterized based on soil column tests and
extraction measurements reported by UNCC. The range of sorption properties was
parameterized as the minimum, mean, and maximum estimates for binding sites as
defined from soil extraction measurements. This range of sorption capacities was used
to develop pH, eH, DO, TDS, and COI titrations.
• The dominant attenuation process is adsorption to hydrous metal oxides, particularly
HFO and HAO. HFO and HAO are representative of clay minerals and similar facies that
are abundant in soils, the transition zone, and bedrock.
• COI concentrations used in PHREEQC model were as reported in the database.
Analytical results qualified as non-detects or estimated values (U- and J-flagged values)
were used as reported without modification.
• Nitrogen values are assumed to be primarily nitrate and alkalinity results are primarily
bicarbonate, not carbonate.
• TDS is evaluated as a summary of sodium, potassium, magnesium, calcium, sulfate and
chloride ions. These constituents account for approximately 60% of the TDS value.
Chloride does not have sorption constants, so this is addressed as a component of TDS.
• Pourbaix diagrams and/or predominance plots were completed in PhreePlot or
Geochemist’s Workbench for each COI to aid in demonstration of changes in Kd, pH,
and DO.
4.3.4 Results
The modeling effort described above provides both qualitative and quantitative estimations of
the chemical speciation and adsorption behavior of several key COIs. Relevant observations
from this modeling effort are as follows:

• The redox conditions vary widely at the site indicating that it has not reached equilibrium,
or data are not representative of the conditions sampled. Additional groundwater results
will assist in refining the model further and confirm these findings should sampled data
not be representative of actual groundwater conditions.
• The observed site condition of limited solubility of arsenic, chromium, cobalt, and
selenium in site groundwater is confirmed by the geochemical modeling.
31
Corrective Action Plan Part 2
Dan River Steam Station Ash Basin

• Each of the pH, Eh, and TDS figures can be further evaluated to address monitored
natural attenuation (MNA) or remediation. The addition of an engineered cap, which
would reduce infiltration and introduction of oxygen presumably creating a more anoxic
environment, the addition of acid or base to adjust the pH to conditions to prevent the
COIs from solubilizing, or impacts due to excavation and the release of TDS and other
metals.
• Methods such as capping could change groundwater flow, inhibit current microbial
reduction mechanisms, or cause secondary metal redox reductions that could be
modeled when options are chosen more specifically.
• Soil sorptive capacity for COIs such as boron is typically lower and for COIs such as
arsenic the sorptive capacity is higher.
4.4 Refined Site Conceptual Model
Groundwater and surface water models were revised to address comments from NCDEQ. An
updated SCM is provided on Figure 3-1. Based on review of refined models and the
geochemical model, no revisions are warranted for the SCM at this time.

32
Corrective Action Plan Part 2
Dan River Steam Station Ash Basin

5 Risk Assessment
The purpose of the human health and ecological risk assessment is to characterize potential
risks to humans and ecological receptors associated with exposure to the coal ash-derived
constituents that may be present in groundwater, surface water, sediments, soil, and air due to
release(s) from the coal ash basin at DRSS.
Results of the risk assessment and the information provided on background conditions and
groundwater flow (including fate and transport model results) provided in the CAP will aid in
focusing remedial actions which, when implemented, will provide future conditions that are
protective of human health and the environment, as required by CAMA.
The risk assessment was completed using methodology designed to be consistent with state
and federal guidance. This methodology represents a step-wise process whereby DRSS is
evaluated using the following methods:
• Step 1: Develop a conceptual site model (CSM), including receiving media, exposure
pathways, and human and ecological receptors.
• Step 2: Screen analytical data for applicable site media by comparing screening values
identified in the risk assessment work plan to identify constituents of potential concern
(COPCs).
• Step 3: Develop site-specific human health risk-based concentrations (RBCs) for the
COPCs, derive exposure point concentrations (EPCs), and compare EPCs to RBCs to
draw conclusions about the significance of potential human health risks.
• Step 4: Develop a site-specific baseline ecological risk assessment (BERA) for the
COPCs and, where appropriate, derive RBCs based on the risk assessment results.
5.1 Step 1: Conceptual Site Model
The CSM includes a site description, information on current and anticipated future land uses,
sources and potential migration pathways through which coal ash-derived COPCs may have
been transported to other environmental media (receiving media), and the human and
environmental receptors that may come in contact with the receiving media. The CSM is meant
to be a living model that can be updated and modified as additional data become available.
Initial CSMs were presented on Figure 12-1 (human health) and Figure 12-2 (ecological) of the
CSA Report. Updated CSMs are provided on Figures 2-3 and 2-4 in Appendix F. The CSMs
are intended to identify potential exposure pathways and receptors that may be applicable at the
site.
For DRSS, the following receptors and exposure scenarios are identified in the human health
CSM (Figure 2-3 in Appendix F):
• Current/future on-site trespasser with potential exposure to dust in outdoor air, soil
remaining post-excavation, AOW water and AOW soil, on-site surface water, and on-site
sediment;
33
Corrective Action Plan Part 2
Dan River Steam Station Ash Basin

• Current/future commercial/industrial worker with potential exposure to dust in outdoor
air, soil remaining post-excavation, AOW water and AOW soil, on-site surface water, and
on-site sediment;
• Current/future construction worker with potential exposure to dust in outdoor air, soil
remaining post-excavation, AOW soil, and groundwater;
• Current/future off-site resident with potential exposure to on-site groundwater and off-site
surface water as potential sources of potable water;
• Current/future off-site recreational swimmer, waders, and boaters with potential
exposure to off-site surface water and off-site sediment; and
• Current/future fishers with potential exposure to off-site surface water and off-site
sediment, and fish ingestion for recreational purposes and fish ingestion for subsistence.
The following ecological receptors and exposure scenarios are identified in the ecological CSM:
• Fish and benthic invertebrates with potential exposure to surface water and sediments;
• Aquatic birds with potential exposure to surface water, sediments, fish, and AOW water;
• Aquatic mammals with potential exposure to surface water, sediment, fish, and AOW
water; and
• Terrestrial birds and mammals with potential exposure to soil remaining post-excavation,
and AOW water and AOW soils.
5.2 Step 2: Risk-Based Screening
Groundwater, surface water, sediment, and soil data were evaluated during the CSA using risk-
based screening level concentrations for identified COPCs. Risk-based screening level
concentrations of COPCs were revised in the CAP Part 2 based upon additional groundwater,
surface water, sediment, and soil data collected in third and fourth quarter 2015. Screening
levels are concentrations of constituents in environmental media (e.g., soil) considered to be
protective under most circumstances; their use requires a detailed understanding of the
underlying assumptions in the CSM, including land use and the presence of sensitive
populations. The presence of a constituent in environmental media at concentrations below the
media and constituent-specific screening level is generally assumed not to pose a significant
threat to human health or the environment. If a constituent exceeds the screening level, it does
not necessarily indicate adverse effects on human health or the environment; rather, it only
indicates that additional evaluation may be warranted. Screening levels are used in this
assessment to help identify COPCs to be carried forward into the evaluation of risk at the site.
5.3 Step 3: Human Health Risk Assessment
COPCs were evaluated through a comparison of EPCs to calculated RBCs. The comparison
was made through calculation of risk ratios for cancer and non-cancer effects. The total risk
ratios among all compounds were then summed.
34
Corrective Action Plan Part 2
Dan River Steam Station Ash Basin

Risk ratios were calculated by first identifying whether COPC RBCs were based on cancer risk
or non-cancer hazard. For RBCs based on cancer risk, the risk ratio for each COPC was
calculated by dividing the EPC by the cancer-based RBC concentration. For RBCs based on
non-cancer risk, the risk ratio for each COPC was calculated by dividing the EPC by the non-
cancer-based RBC concentration.
A risk ratio less than 1 indicated that the EPC does not exceed the RBC, whereas a ratio
greater than 1 indicated that the EPC exceeds the RBC. Risk ratios were also used to evaluate
the cumulative receptor risk associated with each exposure point. Cumulative receptor risk was
calculated by summing the risk ratios among all COPCs on which the RBC was based.
In accordance with USEPA risk assessment guidance (USEPA 1991b), the cumulative cancer
risks and non-cancer hazard indices were evaluated against the USEPA target cancer risk
range of 1.0E-06 to 1.0E-04 for potential carcinogens and target non-cancer hazard index of 1
for non-carcinogens (that act on the same target organ by the same mechanism of action);
cumulative cancer risks and hazard indices that are above these limits indicate further
evaluation may be deemed necessary.
For DRSS, the results of the human health risk assessment indicate that on-site surface water,
AOW water, AOW soil, sediment, and groundwater pose no unacceptable risk for a trespasser,
commercial/industrial worker or construction worker under the exposure scenarios developed in
Step 1. Similarly, off-site surface water and sediment pose no unacceptable risk for a
recreational swimmer, wader, boater, or recreational fisher under the scenarios developed in
Step 1. A subsistence fisher results in a cumulative hazard index of 6.6E+00, which is above 1,
and indicates that exposure to COPCs in fish (using surface water as a surrogate) may pose
unacceptable hazard for this scenario.
5.4 Step 4: Ecological Risk Assessment
The purpose of a BERA is to: (1) determine whether unacceptable risks are posed to ecological
receptors from chemical stressors, (2) derive constituent concentrations that would not pose
unacceptable risks, and (3) provide the information necessary to make a risk management
decision concerning the practical need and extent of remedial action. The BERA was performed
according to the traditional ecological risk assessment paradigm: Problem Formulation, Analysis
(Exposure and Effects Characterization), and Risk Characterization. Because of the many
permutations of conservative parameters, an Uncertainty Evaluation section was also included.
The BERA generally adhered to the USEPA’s “Ecological Risk Assessment Guidance for
Superfund: Process for Designing and Conducting Ecological Risk Assessments” (USEPA
1997) and the “Supplemental Guidance to ERAGS: Region 4, Ecological Risk Assessment”
(USEPA 2015c), as well as NCDENR’s “Guidelines for Performing Screening Level Ecological
Risk Assessments” (NCDENR 2003).
Constituents of concern were selected based on factors including the type of source(s),
concentration, background levels, frequency of detection, persistence, bioaccumulation
potential, toxicity/potency, fate and transport (e.g., mobility to groundwater), and potential
biological effects. Screening consisted of comparing the maximum concentration of each
constituent in the applicable media to conservative environmental screening levels. Per Step 3
35
Corrective Action Plan Part 2
Dan River Steam Station Ash Basin

of the Region 4 ecological risk assessment guidance, COPCs that were retained using the risk-
based screening were evaluated using a multiple lines-of -evidence approach in the BERA
(USEPA 2015a, 2015b).
Risk characterization involved a quantitative estimation of risk followed by a description and/or
interpretation of the meaning of this risk. The purpose of the risk characterization was to
estimate potential hazards associated with exposures to COPCs and their significance. During
risk estimation, the exposure assessment and effects assessment were integrated to evaluate
the likelihood of adverse impacts to the wildlife receptors of interest (e.g., birds and mammals).
The risk estimate was calculated by dividing the dose estimate from the exposure assessment
by the applicable toxicity reference value (derived from the available literature) to obtain a
Hazard Quotient (HQ).
Receptors chosen for ecological risk assessment are often surrogates for the broad range of
potential ecological receptors in a given habitat. For DRSS, typical receptors were chosen for
their expected common presence in the habitats represented at the site, or because they are
common in the southeast and toxicity data are available. Aquatic receptors include fish, benthic
invertebrates, aquatic birds (represented by mallard duck and great blue heron), and aquatic
mammals (represented by muskrat and otter). Terrestrial receptors include birds, represented
by American robin and red-tailed hawk, and mammals, represented by meadow vole and red
fox.
At DRSS, four ecological exposure areas were defined (Figure 2-5 in Appendix F). These
include:
• Exposure Area 1, along the eastern unnamed tributary;
• Exposure Area 2, a portion of the Dan River along the length of the plant site;
• Exposure Area 3 between the DRSS plant and ash basin Primary cell; and
• Exposure Area 4 north of the service water settling pond.
Potentially affected areas on-site are classified as aquatic and terrestrial and evaluated for
exposure to site COPCs. Ecological habitats are presented on Figure 2-6 in Appendix F.
Evaluation of the surface water, AOW water, AOW soil, and sediment in Exposure Area 1
indicates a calculated HQ of 2 for a great blue heron’s exposure to vanadium and a HQ of 2 for
a muskrat’s exposure to aluminum using the No Observed Adverse Effects Level as the toxicity
reference value. Using the Lowest Observed Adverse Effects Level toxicity reference value,
vanadium’s HQ for a great blue heron decreases to 1 and aluminum’s HQ for a muskrat
decreases to 0.2. All other aquatic and terrestrial wildlife receptors have chemical HQs below 1.
The evaluation of off-site ecological exposures to surface water and sediment in Exposure Area
2 indicates that all aquatic receptors (i.e., mallard duck, great blue heron, muskrat and river
otter) have chemical HQs below 1.
Evaluation of AOW water in Exposure Area 3 indicates that all terrestrial receptors (i.e.,
American robin, red-tailed hawk, meadow vole and red fox) have chemical HQs below 1.
AOW water in Exposure Area 4 also presents no hazard to terrestrial receptors (i.e., American
robin, red-tailed hawk, meadow vole and red fox) with chemical HQs below 1.
36
Corrective Action Plan Part 2
Dan River Steam Station Ash Basin

6 Alternative Methods for Achieving
Restoration
This section discusses how remedial alternatives are evaluated and identifies the remedial
alternative selected to achieve restoration of groundwater quality at DRSS.
As described in Section 1, after removal of the ash, soils left on-site will be sampled and
analyzed, and the analytical results will be incorporated into the groundwater contaminant fate
and transport model. If this evaluation indicates that modification to the proposed CAP is
required, Duke Energy will prepare and submit a revised CAP. Therefore, remediation of soils is
not discussed in this document.
As noted in Section 2, exceedances of 2B or 2L Standards were measured at AOWs adjacent
to the unnamed tributary located to the east of the Secondary Cell. HDR and Duke Energy
consider that the water in the Secondary Cell is the likely primary source of the water supplying
these AOWs, and that as a result of dewatering of the basin, the flow at these AOWs will
decrease or possibly be eliminated. Physical characteristics of AOWs will be monitored
throughout excavation activities. Duke Energy proposes that remedial measures at these AOWs
be deferred until after basin dewatering and excavation of the ash. If, at that time, the
exceedances are still present, Duke Energy will evaluate those conditions and develop
corrective measures to address the exceedance(s). For DRSS, the Plan for Identification of New
Discharges was submitted to NCDEQ on May 19, 2015. This plan was developed to address
the requirements of North Carolina General Statute (GS)130A-309.210 (d) Identification and
assessment of discharges; correction of unpermitted discharges, as modified by North Carolina
Senate Bill 729. Identification of new discharges (AOWs) and any associated sampling of the
new discharge will be done in compliance with the document referenced above.
6.1 Corrective Action Decision Process
6.1.1 Evaluation Criteria
The goal of groundwater corrective action in accordance with 15A NCAC 2L.0106 is:
“…where groundwater quality has been degraded, the goal of any required corrective
action shall be restoration to the level of the standards, or as closely thereto as is
economically and technologically feasible”…using best available technology (j), or to
an alternate standard (k) or using natural attenuation mechanisms (l).
The evaluation of best available methods for groundwater remediation is based on the objective
of meeting groundwater standards at the Compliance Boundary with consideration of
implementability, time, and cost. The methods may include one or a combination of best
available technologies and natural attenuation processes. Source control measures (such as
cap-in-place or excavation) are being addressed separately, but are assumed to occur in
addition to the groundwater corrective action alternatives evaluated herein.
37
Corrective Action Plan Part 2
Dan River Steam Station Ash Basin

6.1.2 COIs Requiring Corrective Action
Data from the CSA were evaluated in CAP Part 1 to identify the following groundwater COIs that
are considered for current or potential future remedial action: antimony, arsenic, boron, cobalt,
iron, manganese, selenium, sulfate, TDS, and vanadium. These COIs are considered for
remedial action because they have been found to exceed their applicable regulatory standards
at the Compliance Boundary, in the Dan River, or may exceed their applicable regulatory
standards in the future due to fluctuations of COI concentrations as a result of closure activities.
There are some areas that show similar exceedances, while others do not demonstrate
similarities with non-detect and exceedance concentrations in consecutive sampling rounds. For
this reason, it is recommended that additional sampling be conducted to develop a conceptual
plan for remediating residual groundwater concentrations over time.
6.1.3 Potential Exposure Routes and Receptors
The Baseline Human Health and Ecological Risk Assessment (Appendix F) provides
information on current knowledge of DRSS and a conservative conditions assessment of the
potential risk associated with the COIs attributed to the currently defined sources at DRSS.
The primary source to groundwater receptor mechanism is leaching of ash porewater to
groundwater, which will migrate toward the Dan River. Therefore, the groundwater to surface
water migration is a potential route of exposure to the Dan River.
Secondary source to receptor mechanisms include:
• Wet and dry AOWs, which may contribute COIs to groundwater through infiltration; and
• Soil concentrations exceeding the soil NCPSRGs for POG standards that may contribute
to groundwater COIs.
Localized groundwater mounding associated with the current hydraulic head in the basin(s) will
be eliminated with the source control measures. The residual groundwater concentrations are
the focus of this CAP to prevent unacceptable risk to receptors such as water supply wells,
springs, surface water, and/or sediment.
It is likely that updated hydrogeologic and geochemical models will be required following basin
closure to determine if modifications to the SCM are warranted. The following assessment is
anticipated post-scenario selection to determine if exposure pathways or receptors may be
added or eliminated following closure:
• Groundwater elevation measurements. These will be completed to determine if there is
any change in groundwater flow direction post-scenario selection and implementation.
Groundwater flow direction is anticipated to continue toward the Dan River after
implementation; however, post-closure flow direction will be re-evaluated to update the
SCM.
• Re-evaluation of Geochemical Modeling – Geochemical modeling will be re-evaluated to
determine if potential fill material imported to the ash basins under the Excavation
scenario would change the groundwater chemistry in the ash basin upon
38
Corrective Action Plan Part 2
Dan River Steam Station Ash Basin

implementation. Subsequent evaluations will also be needed to determine if the
geochemical model would still support MNA of COIs in groundwater.
The primary routes of exposure are associated with COI concentrations exceeding the 2L
Standard, IMAC, or PPBC in groundwater and are the focus of this CAP with the intent to meet
these standards at the Dan River within a technically feasible timeframe.
6.2 Alternative Evaluation Criteria
Alternative Evaluation Criteria are chosen in general conformance with USEPA Office of Solid
Waste and Emergency Response Directive 9355.-27FS “A Guide to Selecting Superfund
Remedial Actions,” dated April 1990. This document provides threshold, balancing, and
modifying criteria for selection of a remedy.
Potential groundwater corrective action alternatives will be evaluated against the following
criteria:
• Effectiveness (Section 6.2.1)
• Implementability/Feasibility (Section 6.2.2)
• Environmental Sustainability (Section 6.2.3)
• Cost (Section 6.2.4)
• Stakeholder Acceptance (Section 6.2.5)
6.2.1 Effectiveness
Effectiveness is a comparison of the likely performance of technologies taking into consideration
the following:
1. The estimated area and volumes of media to be treated.
2. Demonstrated reliability to achieve constituent remedial goals under site conditions.
3. Demonstrated reliability to reduce potential risk to human health and the environment in
a timely manner.
Specific effectiveness criteria include:
• Has the potential remedial alternative been demonstrated to be effective at similar sites?
• Does the remedial technology involve treatment that will permanently destroy target
constituents?
• Does the remedial technology involve treatment that will permanently detoxify target
constituents?
• Does the remedial technology involve treatment that will permanently reduce the mobility
of target constituents?
• Will the remedial alternative permanently remove contaminants from site?
• Can the effectiveness of a potential remedial technology be monitored, measured, and
validated?
• Will a remedial technology reduce potential risk to human health when fully
implemented?
39
Corrective Action Plan Part 2
Dan River Steam Station Ash Basin

• Will a remedial technology reduce potential risk to the environment when fully
implemented?
• Will a remedial technology be protective of human health?
Technologies that are deemed to be less effective under site-specific conditions than otherwise
comparable technologies will be eliminated on the basis of effectiveness.
6.2.2 Implementability/Feasibility
The screening criteria of implementability evaluates whether implementation of a technology is
technically and administratively feasible. Specific implementability criteria include:
• Are the material resources and manpower readily available to fully implement the
remedial technology in a timely manner?
• Does the remedial technology require highly specialized resources and/or equipment?
• Is there sufficient on-site and off-site area to fully implement the remedy?
• Does the remedial technology require any permits, and can the permits be acquired in a
timely manner (e.g., wetlands permitting)?
• Can the remedial alternative be implemented safely?
• Can existing and future infrastructure support the remedial alternative?
• Will a remedial technology increase potential risk to public safety during implementation?
• Will a remedial technology increase potential risk to the environment during
implementation?
• Can a remedial technology meet all applicable or relevant and appropriate requirements
(ARARs)?
Technologies that are deemed impractical under site-specific conditions will be eliminated on
the basis of implementability/feasibility.
6.2.3 Environmental Sustainability
A remedy is environmentally sustainable when it maximizes short-term and long-term protection
of human health and the environment through the judicious use of limited resources. Metrics
used to measure environmental sustainability include:
• Will constituents be treated to reduce toxicity or mobility, or will treatment transfer the
constituent from one media to another (e.g., discharge constituents in extracted
groundwater to surface water)?
• Is the carbon footprint (energy consumption) of otherwise comparable remedial
alternatives significantly different?
• Will source materials used in the remediation process be recycled or reclaimed?
• Will waste materials generated during the remediation process be recycled or
reclaimed?
• Will renewable sources of energy be used during the remediation process?
• Will natural habitat restoration, enhancement, or replacement be integral to the remedy?
40
Corrective Action Plan Part 2
Dan River Steam Station Ash Basin

Duke Energy considers environmental sustainability in their alternative evaluation criteria and
where appropriate will incorporate “green” remedial strategies in their evaluation. Green
remedial strategies consider all environmental effects of remedy implementation and
incorporating options to maximize new environmental benefit of cleanup operations (USEPA
2008). Green remediation reduces the demand placed on the environment during remedial
operations to avoid collateral damage to the environment. Green remediation strategies
minimize adverse impacts to other environmental media, such as:
• Air pollution caused by emission of carbon dioxide, nitrous oxide, methane, and other
greenhouse gasses emitted during remediation
• Imbalance to the local and regional hydrologic regimes
• Soil erosion and nutrient depletion causing changes to soil geochemistry
• Ecological diversity and population reductions
6.2.4 Cost
The criterion of cost has been evaluated by evaluating the estimated capital cost and labor
required to implement technologies that will enhance future closure activities. This includes the
cost to implement remedial design technologies that can be used to enhance the effectiveness
of future closure activities. The cost evaluation considers design, construction, and operation
and maintenance over a 30-year period. Cost will not be the sole or primary basis for selecting a
technology or remedial alternative; however, cost will be considered when evaluating the
scenarios.
6.2.5 Stakeholder Acceptance
Stakeholders include state and federal regulatory agencies and the general public. Anticipated
stakeholder acceptance will be considered in the evaluation process; however, a re-evaluation
of remedial alternatives may be conducted if stakeholders oppose the recommended remedy.
Appropriate stakeholders will be notified pursuant to 15A NCAC 02L .0114.
6.3 Remedial Alternatives to Achieve Regulatory Standards
Source control is the primary remedial action for groundwater restoration at the site. As required
by CAMA, the ash basin at DRSS will be excavated due to the high priority designation by
CAMA. Source removal is already being implemented and includes the export of ash for landfill
or beneficial use. The following remedial alternatives were considered to enhance source
removal at DRSS and improve the effectiveness of the remedy.
6.3.1 Groundwater Remediation Alternatives
Remedial alternatives for restoration of groundwater in accordance with 15A NCAC 2L.0106
include the following:
41
Corrective Action Plan Part 2
Dan River Steam Station Ash Basin

• Source Control, which can include:
o Ash removal to prevent COIs from leaching into groundwater
o Placement of engineered cap to prevent COIs from leaching into groundwater
o Slurry walls or grout curtains to prevent groundwater interaction with source
material
o In-situ solidification/stabilization to reduce or eliminate leaching of COIs into
groundwater by mixing soil beneath source areas with pozzolanic materials (i.e.,
Portland cement or bentonite)
• Land Use Controls – State approval to restrict land use to prevent the use of surface
water and groundwater in the area.
• Monitored Natural Attenuation – MNA involves regular monitoring of select groundwater
monitoring wells for specific parameters to ensure COI concentrations in groundwater
are decreasing. Dilution from recharge to shallow groundwater, mineral precipitation,
and COI adsorption will occur over time, thus reducing COI concentrations through
attenuation.
• Enhanced Attenuation, which can include:
o Addition of materials with high adsorptive capacity to the saturated zone to
increase the reduction of COI levels in groundwater
o Air sparging and adjusting pH to enhance precipitation of iron and manganese
oxide/hydroxide minerals to increase the reduction of COI levels in groundwater
o Bioremediation for removal of COIs
• Permeable Reactive Barriers – Involves trenching and placement of selected material in
the trench that would chemically bond and remove COIs and reduce their levels in
groundwater.
• Water Treatment – Active in-situ groundwater remediation by injection of chemical
and/or air sparging, groundwater extraction and treatment, or passive groundwater
remediation through wetland construction.
A detailed description of available remedial alternatives is documented in Appendix G.
6.3.2 Monitored Natural Attenuation Applicability to Site
An MNA Tier I and Tier II evaluation was conducted for DRSS by Geochemical, LLC and is
included in Appendix H. The following is a summary of the Tier I and Tier II evaluation.
MNA is a strategy and set of procedures used to demonstrate that physiochemical and/or
biological processes in an aquifer will reduce concentrations of undesirable substances to levels
below regulatory standards. The mechanisms that regulate their release from solids and
movement through aquifers are, for the most part, the same processes that provide chemical
controls on movement of CCR leachate in an aquifer. These processes attenuate the
concentration of inorganics in groundwater by depositing inorganics on aquifer solids. MNA is
considered a viable remedial alternative for COIs in groundwater at DRSS.
The groundwater COIs for DRSS are arsenic, boron, iron, manganese, sulfate, TDS, and
vanadium with localized 2L Standard or IMAC exceedances for antimony, cobalt, selenium, and
thallium. Cobalt, iron, manganese, and vanadium occur naturally in regional groundwater;
however, these constituents are still considered COIs because concentrations exceeded their
42
Corrective Action Plan Part 2
Dan River Steam Station Ash Basin

respective 2L Standards or IMACs and were higher than observed background concentrations.
Sulfate and TDS are generally not attenuated by reactions with solids, but are reduced in
concentration by diffusion, mechanical mixing, or dilution.
Tier I analysis used two lines of evidence for attenuation: 1) Solid-water pair comparison of COI
concentrations were performed, a mutually rising relationship indicating attenuation (USEPA
2007b); and 2) laboratory determination of the solid-water partitioning coefficient or Kd value
(USEPA 1999), which was used as a measure of the susceptibility of COIs to adsorb to solids
and be attenuated. Tier I analysis indicates that antimony, arsenic, boron, cobalt, selenium, and
thallium should be advanced to Tier II analysis for determination of mechanisms. Although
related to background concentrations, vanadium should also be advanced to Tier II analysis to
improve the understanding of its site-specific occurrence and mobility.
Following successful completion of a Tier I demonstration that antimony, arsenic, boron, cobalt,
selenium, and thallium are attenuating in groundwater at DRSS, a conceptual model for COI
attenuation involving reversible and irreversible interaction with clay minerals, metal oxides, and
organic matter is proposed. A Tier II demonstration based on that conceptual model was
partially executed. The findings are as follows:
1. Representative site-specific samples were collected and tested.
2. Clay minerals and iron oxides were found in all samples. Due to lack of organic matter
(i.e., leaves, moss, wood, etc.) observed during the CSA, organic matter is not a
significant sink for COIs at DRSS.
3. Chemical extractions identified that COIs were concentrated in soil samples exposed to
groundwater containing higher concentrations of COIs, validating the attenuation.
4. Chemical extractions were used to determine a probable range of Kd values that suggest
attenuation is taking place for antimony, arsenic, boron, cobalt, selenium, thallium, and
vanadium.
As documented in Appendix E, titration results for DRSS wells can be used to support
evaluation of MNA or remediation impacts. For example, titration results can be used to
evaluate the following: 1) the expected impact that DO changes would have in response to
addition of an engineered cap (leading to reduced infiltration and lower recharge DO), 2) the
introduction of oxygen creating a more oxic environment, 3) the addition of acid or base to
adjust the pH to conditions that prevent COIs from being solubilized, and 4) the impact due to
excavation and the release of TDS and other metals. Changes in redox can occur also in
response to DO increases or decreases as well as the introduction of inorganic oxidants from
anthropogenic contamination or changes in groundwater flow vectors.
6.3.3 Site-Specific Alternatives Analysis
Additional data collection is necessary to complete the Tier II/III assessment with respect to
specific attenuation mechanisms for each COI, and quantification of the magnitude of that
attenuation by specific media to support numerical modeling. The Tier III assessment will be
performed in general accordance with USEPA’s “Monitored Natural Attenuation of Inorganic
Contaminants in Ground Water - Volume 2 Assessment for Non-Radionuclides Including
Arsenic, Cadmium, Chromium, Copper, Lead, Nickel, Nitrate, Perchlorate, and Selenium”
(USEPA 2007a).
43
Corrective Action Plan Part 2
Dan River Steam Station Ash Basin

Source control is the primary remedial action for groundwater restoration at DRSS. Source
control is actively being completed with the removal of ash from DRSS and is currently being
moved to an off-site lined permitted landfill. Removal of ash at DRSS is anticipated to be
completed no later than August 2019. Source removal is discussed in further detail in Section
7.2.1.The soil in the dike embankments will be removed and the non-impacted material will be
used in site re-grading. The depression left after ash removal has been completed will be filled
with on-site and/or imported fill material, re-graded, and appropriate vegetation planted to
establish a long-term stable, erosion resistant site condition.
A consideration in the remedial alternatives selection is the limited area available for use or
application of remedial alternatives until the ash basin and the ash basin dikes are removed and
the area is stabilized.
As detailed in Appendix G, the following remedial alternatives have been recommended for
consideration at DRSS to enhance or supplement the existing source removal activities.
The alternatives listed below were selected for further evaluation:
• No further action – this alternative is provided to establish a baseline for comparison to
other alternatives. There would be no remedial actions conducted at the site to remove
the source of COIs other than the removal of the ash, and no further remedial action
would be taken for groundwater. This measure does not include long-term monitoring or
institutional controls.
• MNA – Groundwater monitoring would be continued until remedial objectives are met
(that is, groundwater concentrations are at or below 2L Standards, IMACs or NCDHHS
HSL). At DRSS, ash will be removed from the source areas and the concentrations of
COIs in groundwater will continue to decrease over time. Attenuation will occur over time
due to natural processes, and its extent and progress will be monitored at appropriate
locations in the source area and between the source and the Compliance Boundary. The
modeling report results for the shallow flow layer show the following:
a) Antimony, cobalt, hexavalent chromium, thallium and vanadium currently exceed
their respective IMACs or NCDHHS HSL at the Compliance Boundary and are
projected to remain above standards into the future.
b) Boron and sulfate currently exceed their respective 2L Standards at the Compliance
Boundary, but are projected to decrease below 2L Standards in the future following
excavation.
c) Arsenic and selenium are currently below their respective 2L Standards at the
Compliance Boundary and are projected to remain below 2L Standards in the future.
d) Iron and manganese exceed their respective 2L Standards; however, these COIs
could not be properly evaluated as part of the groundwater modeling effort and were
evaluated in the geochemical model (Section 4.3).
• MNA and Enhanced Natural Adsorption by In-Situ Sorption or Chemical Fixation – Since
modeling has indicated MNA processes are occurring at the site, it is more cost-effective
to couple MNA with an active remedial alternative. Based on the COIs on the northeast
44
Corrective Action Plan Part 2
Dan River Steam Station Ash Basin

portion of DRSS, injecting an alkaline agent (e.g., sodium carbonate) and an oxidant or
air to enhance oxidation and precipitation of the iron already present in the water should
achieve the desired objectives (Evanko and Dzombak 1997; Ilavský 2008; Twidwell and
Williams-Beam 2002). The injection would occur downgradient of the Secondary Cell of
the ash basin between the basin and the eastern unnamed tributary. This may
accelerate attenuation sufficiently to reduce COI concentrations between the source
material and the eastern unnamed tributary/Compliance Boundary.
a) The quantity and type of additives required for promoting fixation through enhancing
adsorption and the approaches for delivering them effectively to the subsurface
would need to be determined as part of remedial action planning and design stages
of the project. Further evaluation of implementability along the eastern unnamed
tributary would be needed prior to design. Continued monitoring of groundwater
quality would be required to verify reduction in COIs as a result of the reagent
injection. Modeling and bench/pilot tests should be undertaken to confirm the
potential cost effectiveness.
b) The targeted area is approximately 400 feet by 700 feet at and upgradient of the
unnamed tributary on the eastern portion of DRSS. A barrier approach to injection,
consisting of three transects, one 400 feet long and two 500 feet long, is proposed to
be used.
Selection of site-specific remedial actions is based on results observed from the refined
groundwater model, as well as evaluation of effectiveness, implementability, feasibility,
environmental sustainability, and cost. Appendix G details evaluation of site specific remedial
alternatives considered.
6.3.4 Site-Specific Recommended Approach
At DRSS, source removal is the primary corrective action and is anticipated to decrease COI
concentrations and number of COIs at the site. Alternative 2 (MNA) is also recommended as a
proposed remedial action for DRSS. Based on results of the Tier I and Tier II evaluation, MNA is
an effective remedial action because COIs will attenuate over time to restore groundwater
quality at the site and is protective of both human health and the environment. This corrective
action will include installation of at least five additional wells along the Dan River to more
accurately monitor MNA parameters in this area once closure activities are complete. MNA is a
feasible remedial alternative and can easily be implemented at the site. Implementation of MNA
for a 30-year period is estimated to cost $6,490,000. Costs are discussed further in Section 10.
An Interim Monitoring Plan will be implemented at the site during sampling activities, which will
be conducted in the first half of 2016. The results of these sampling events will be evaluated to
determine which monitoring wells will be utilized for MNA and frequency of sampling, as
discussed in Section 9. Groundwater quality and effectiveness of MNA will need to be re-
evaluated after excavation of source material is complete. If results of MNA re-evaluation deem
MNA is not sufficient to reduce COI concentrations within an acceptable time period, remedial
alternatives should be re-evaluated and implemented to augment MNA.
45
Corrective Action Plan Part 2
Dan River Steam Station Ash Basin

7 Selected Corrective Action(s)
Remedial alternatives were evaluated for achieving restoration of groundwater at DRSS and are
detailed in Section 6. Given that excavation of ash is occurring, and based on the remedial
alternatives evaluation, MNA was determined to be the most appropriate corrective action for
DRSS; however, groundwater quality and effectiveness of MNA need to be re-evaluated after
excavation is complete. If results indicate MNA not be an effective corrective action, remedial
alternatives should be re-evaluated and implemented to augment MNA monitoring.
7.1 Selected Remedial Alternative for Corrective Action
COI transport in groundwater is primarily controlled by hydrogeologic and geochemical
conditions at the site (Section 3). COIs enter the groundwater system through the shallow flow
layer within the source areas. Evaluation of the geochemical modeling indicated COIs are
attenuated by a combination of sorption and/or precipitation. Sorption of antimony, sulfate,
thallium, and vanadium was not observed during geochemical modeling, namely due to low
concentrations in groundwater. TDS and sulfate generally are not attenuated, but
concentrations are reduced by diffusion, mechanical mixing, or dilution. Arsenic, cobalt, and
selenium were observed to have limited solubility, meaning these constituents more readily
adsorb. Groundwater fate and transport model predictions presented in Appendix B are
supported by findings of the geochemical modeling presented in Appendix E. Based on review
of the groundwater modeling, COIs with sorption coefficients that are similar to or higher than
arsenic are immobilized by sorption and/or precipitation. COIs with sorption coefficients that are
similar to or lower than boron do not readily adsorb and easily transported in groundwater.
7.2 Conceptual Design
7.2.1 Source Removal – Excavation
Excavation of ash at DRSS will remove the source areas, which are identified as the ash
storage areas and the ash basin (Primary and Secondary cells). Details regarding excavation
can be found in the Dan River Steam Station Coal Ash Excavation Plan submitted to NCDENR
by Duke Energy in November 2014 (http://portal.ncdenr.org/web/wq/ca-excavation-plans).
Initially, ash will be excavated from the ash storage areas in preparation for construction of the
on-site landfill, which is planned in the vicinity of Ash Storage 1. Material removed during Phase
I of excavation will be transported via rail car to the Maplewood (Amelia) Landfill in Jetersville,
Virginia. Once the construction of the on-site landfill and all applicable permits are in place, ash
will be removed from the ash basin and placed in the new on-site landfill. Excavation of the ash
will remove the primary source of groundwater contamination at the site, but will not eliminate or
reduce the COIs observed in the groundwater at the present. This CAP supplements the
existing excavation plan by providing post-excavation corrective actions that could remediate
residual contamination remaining after source removal and provides a monitoring plan for post-
excavation activities.
A stormwater permit was required prior to commencing excavation activities at DRSS. The
stormwater permit was issued on October 1, 2015. The first phase of excavation at DRSS
46
Corrective Action Plan Part 2
Dan River Steam Station Ash Basin

requires removal of two soil stockpiles on Ash Storage 1 prior to removal of ash. Removal of the
first soil stockpile began in November 2015 and has been completed. Removal of the second
soil stockpile began in December 2015 and this work is still underway. To date, 75,000 tons of
soil has been removed from Ash Storage 1 in preparation for excavation of ash. Once the soil
stockpiles are removed the second phase of work will require installation of a sedimentation
basin.
7.2.2 MNA
7.2.2.1 Demonstration of MNA
The use of MNA as a remedy involves the monitoring of select parameters to ensure COIs are
attenuating. Once the ash within the source areas is removed, groundwater quality will improve
over time due to dilution from the recharge to shallow groundwater, precipitation, and adsorption
of COIs.
Tier I and II analyses were conducted for DRSS (discussed in Section 6.3.2; Appendix H). A
geochemical site conceptual model for COI attenuation involving reversible and irreversible
interaction with site samples containing clay minerals, metal oxides, and organic matter was
completed. The sampling was determined to be representative of material into and through
which the COIs will migrate. The most significant finding was that precipitating iron and
manganese was removing other COIs through co-precipitation and adsorption, thus confirming
that attenuation is occurring. In support of this reaction, clay minerals and Fe-Mn-Al oxides were
observed in samples.
Groundwater modeling did not take into consideration the removal of COI via co-precipitation
with iron oxides, which likely resulted in an over-prediction of COI transport, causing some of
the COIs to exceed the 2L Standards or IMACs at the Compliance Boundary in the model
output. Surface water models have determined that even with over-prediction of COIs to the
Dan River, exceedance of the 2B Standards will not occur.
Based on these predictions, site conditions are favorable for MNA to be implemented at DRSS.
7.2.2.2 Verification of MNA
The MNA monitoring program and the data collection and evaluation to advance the Tier III
assessment should be implemented prior to initiating the excavation (removal) activities. The
monitoring will continue through the removal effort and be maintained until water quality meets
remedial objectives (i.e., 2L Standards, 2B Standards, or site-specific standards), as applicable.
The site monitoring requirements are discussed in Section 9.
The current groundwater monitoring network along with monitoring wells scheduled to be
installed to address additional assessment needs at the site are suitable for characterization of
COIs and monitoring effectiveness of MNA at DRSS. Monitoring wells within the ash storage
areas, ash basin, and associated dams will be abandoned as part of closure activities. To
adequately monitor downgradient of the basin along the Dan River, five additional wells are
proposed in this area once closure activities have been completed. These wells will be installed
to monitor the shallow flow layer downgradient of the ash basin. Wells GWA-10S/D and GWA-
47
Corrective Action Plan Part 2
Dan River Steam Station Ash Basin

11S/D are located downgradient of the ash storage areas and will monitor groundwater quality
between the ash storage areas and the basin. Monitoring wells along the perimeter of the
Compliance Boundary, downgradient, and background wells will be effective in monitoring MNA
parameters. Replacement monitoring wells within the source areas are not proposed as wells
chosen for MNA will adequately monitor groundwater quality within the Compliance Boundary.
As discussed in Section 9, results of the sampling activities conducted in 2016 will be evaluated
to determine monitoring wells to be utilized for MNA at DRSS.
If COIs or COI concentrations are observed to increase during the MNA monitoring,
effectiveness of MNA should be re-evaluated. If warranted, additional remedial alternatives
should be evaluated and implemented as necessary and as described above.

48
Corrective Action Plan Part 2
Dan River Steam Station Ash Basin

8 Recommended Interim Activities
As discussed in prior sections, several interim activities will occur at DRSS to address additional
information needs identified in the CSA and CAP Part 1 reports. Interim activities to be
completed include the installation of additional monitoring wells and monitoring of groundwater
during excavation activities.
8.1 Additional Information Needs
8.1.1 Additional Well Installation
Thirteen new monitoring wells are currently planned to be installed to address additional
information needs identified in the CSA Report and comments from NCDEQ. Proposed
locations of the additional wells are shown on Figure 8-1. The additional monitoring wells will be
installed to further refine the horizontal and vertical extent of groundwater impacts and refine the
understanding of groundwater flow direction.
8.1.2 Additional Groundwater Sampling and Analyses
In response to comments received from NCDEQ, dataloggers will be installed in wells GWA-
6S/D, MW-22S/D/BR, MW-10/10D, and AB-30S/D/BR to record groundwater elevation changes
and potential influence from changes in stage in the Dan River. A datalogger will also be
installed at one upgradient location (GWA-9S/D), which is not influenced by the ash basin, ash
storage areas, streams or river, to establish a baseline. In addition to monitoring groundwater
elevation changes, down-well water quality meters will be installed in select monitoring wells to
record potential changes in DO and evaluate redox conditions. Groundwater fluctuations will be
summarized in monitoring reports described in Section 9.

The additional monitoring wells and background wells will be incorporated into the groundwater
monitoring network in 2016 and sampled in conjunction with existing wells installed during the
CSA. Review of the data will be used to refine the understanding of natural background
concentrations of COIs and to refine the existing PPBCs.

49
Corrective Action Plan Part 2
Dan River Steam Station Ash Basin

9 Interim and Effectiveness Monitoring Plans
Interim and Effectiveness Monitoring Plans (Monitoring Plans) provide detailed information
regarding field activities to be performed during collection of groundwater, surface water, and
AOW samples at DRSS. The Monitoring Plans are intended to evaluate the effectiveness of the
proposed corrective actions; monitor the movement of COIs through groundwater during and
after excavation of the DRSS ash basin and ash storage areas; and evaluate baseline
conditions and seasonal variation in groundwater, surface water, and AOWs at DRSS. These
monitoring plans replace the monitoring plan provided in Section 16 of the CSA Report.
Protocols for groundwater, surface water, and AOW sample collection, analysis, and reporting
are consistent between the Monitoring Plans. The sampling and analysis will be completed in
accordance with the Monitoring Plans described below, the Work Plan, and the Low Flow
Sampling Plan (CSA Report Appendix G).
9.1 Interim Monitoring Plan
The Interim Monitoring Plan has been developed to provide baseline seasonal analytical data
associated with DRSS. The Interim Monitoring Plan will be implemented at DRSS during
sampling activities conducted in the first half of 2016. The Interim Monitoring Plan establishes
data quality objectives (DQOs) and sampling requirements associated with sampling frequency,
sampling locations, and analytical requirements. Upon completion of the Second Quarter 2016
sampling event, monitoring activities will be conducted in accordance with the Effectiveness
Monitoring Plan described in Section 9.2.
9.1.1 Data Quality Objectives
The following DQOs are associated with the Interim Monitoring Plan:
• Monitor the extent of groundwater contamination in and around the ash basin and ash
storage areas and evaluate seasonal trends associated with COIs.
• Monitor the movement of COIs within groundwater and the interaction with surface water
and AOWs.
• Determine seasonal groundwater flow direction and elevations throughout DRSS and
monitor potential changes to groundwater flow direction and elevation as the result of
closure activities.
The DQOs will be met through the following activities:
• Perform sampling of CSA, compliance, and voluntary monitoring wells, surface water,
and AOW locations twice during the first half of 2016. These sampling events, planned
for First and Second Quarters of 2016, will be combined with analytical data from CSA
Rounds 1 and 2 (collected in the Third and Fourth Quarters of 2015) to evaluate
seasonal water quality conditions at DRSS. Additional assessment wells are being
installed at DRSS in the First Quarter of 2016 and will be added to the interim monitoring
network of wells following installation. If monitoring indicates that dewatering and
50
Corrective Action Plan Part 2
Dan River Steam Station Ash Basin

excavation activities are mobilizing COIs towards off-site receptors, more frequent
sampling of select wells will be considered.
• Perform groundwater static water level measurements at CSA, compliance, and
voluntary monitoring wells concurrent with groundwater sampling described above.
• Perform water level gauging at stream and surface water locations, if available.
• Perform total depth measurements at CSA, compliance, and voluntary monitoring wells
on an annual basis.
• Refine groundwater models, if necessary, with additional analytical data upon
completion of the proposed seasonal monitoring.
• Prepare regular reports documenting sampling results and analysis for submittal to
NCDEQ, as specified in Section 9.1.3.
9.1.2 Sampling Requirements
9.1.2.1 Sample Frequency
To meet the DQOs and evaluate seasonal fluctuations in COI concentrations, sampling of CSA,
compliance, voluntary, and background groundwater monitoring wells and surface water and
AOW locations will be conducted at DRSS in the First and Second Quarters of 2016. Sampling
frequency will be revised as described in the Effectiveness Monitoring Plan (Section 9.2) upon
completion of the Second Quarter 2016 sampling event.
9.1.2.2 Sample Locations
Groundwater monitoring well, surface water, and AOW locations to be sampled under the
Interim Monitoring Plan are identified in Table 9-1 and depicted on Figure 2-1. Wells may be
added to the sampling program through the installation of additional wells or removed as
excavation of the ash basin and ash storage areas results in well abandonment. NCDEQ will be
notified and NCDEQ’s approval will be obtained prior to the abandonment of monitoring wells.
9.1.2.3 Analytical Requirements
Analytical parameters will be consistent with those specified in the approved Work Plan.
Analytes for monitoring wells, surface water, and AOWs include total and dissolved metals,
alkalinity, calcium, chloride, hexavalent chromium, potassium, magnesium, nitrate, sodium,
sulfate, total combined radium, total combined uranium, TDS, total organic carbon, and total
suspended solids. In addition, samples will be analyzed for ammonia for use in evaluation of
MNA as a corrective action. Analytical services will be provided by a North Carolina certified
laboratory. Chemical analytes, analytical methods, bottle requirements, and preservatives are
provided in Table 9-2.
9.1.3 Reporting
Validated analytical results from each sampling event is proposed to be transmitted to NCDEQ
within 90 days of completion of the sampling event. Within 120 days of completion of the
Second Quarter 2016 sampling event, Duke Energy will submit a groundwater monitoring report
to NCDEQ that summarizes the results from the two 2016 sampling events.
51
Corrective Action Plan Part 2
Dan River Steam Station Ash Basin

9.2 Effectiveness Monitoring Plan
The Effectiveness Monitoring Plan has been developed to monitor select wells for MNA
parameters to provide baseline MNA data before and/or during closure activities for use with
future MNA analysis. The Effectiveness Monitoring Plan is proposed to be implemented at
select wells following completion of the Second Quarter 2016 sampling event and will coincide
with NPDES monitoring sampling events for the remainder of effectiveness monitoring. The
Effectiveness Monitoring Plan will continue through 2020 (five years of CAP-related sampling)
and will be modified within the first five-year period if additional corrective actions are
implemented. Note that ash removal from the basin and ash storage areas will likely preclude
accurate analysis of MNA processes through August 2019.
9.2.1 Data Quality Objectives
The following DQOs are associated with the Effectiveness Monitoring Plan:
• Monitor the effectiveness of the corrective action (excavation and MNA).
• Monitor changes in groundwater, surface water, and AOW COI concentrations as
excavation of the ash basin and ash storage areas progress.
• Monitor the movement of COIs in groundwater and interaction with surface water and
AOWs.
• Monitor seasonal groundwater elevations and flow direction, and monitor potential
changes resulting from basin closure activities.
The DQOs will be met through the following activities:
• Perform regular groundwater, surface water, and AOW sampling, including MNA
parameters, for select monitoring wells, surface water, and AOW locations.
• Perform regular groundwater static water level measurements at CSA, compliance, and
voluntary monitoring wells concurrent with groundwater sampling described above.
• Perform regular water level gauging at stream and surface water locations, if available.
• Perform total depth measurements at CSA, compliance, and voluntary monitoring wells
on an annual basis.
• Prepare regular reports documenting sampling results and analysis for submittal to
NCDEQ.
9.2.2 Sampling Requirements
Following the Second Quarter 2016 monitoring event, four seasonal sampling events, including
CSA Rounds 1 and 2, will have occurred at DRSS. Results from the four seasonal sampling
events will be evaluated to establish an MNA sampling network of select monitoring well,
surface water, and AOW locations. Results will also be evaluated to determine the need for
increased frequency of sampling in the vicinity of the ash basin and ash storage areas to
monitor potential migration of COIs during excavation activities. Additional monitoring of the
MNA network will be proposed to confirm the effectiveness of MNA as a viable corrective action
52
Corrective Action Plan Part 2
Dan River Steam Station Ash Basin

and to determine if additional sampling is required at locations outside the MNA network to
monitor site conditions.
9.2.2.1 Sample Frequency
Following the establishment of the MNA network, one additional sampling event of the MNA
network will be conducted in 2016 in conjunction with the third (September) 2016 NPDES
compliance sampling event. Beginning in 2017, samples from the MNA network will be collected
three times per year, in conjunction with the NPDES compliance monitoring to correlate the
results from the MNA network with the NPDES results. Sampling frequency associated with
Effectiveness Monitoring Plan will be re-evaluated every five years. Upon completion of the first
5-Year sampling cycle, the potential for semi-annual sampling frequency will be evaluated.
Additional sampling beyond the MNA network, including excavation-specific monitoring, will be
evaluated after completion of the Second Quarter 2016 sampling event and will be proposed in
the subsequent sampling event.
9.2.2.2 Sample Locations
Sampling locations associated with the MNA network will be established after collection of the
Second Quarter 2016 sampling event, following evaluation of four seasonal sampling events.
Sample locations will be proposed to NCDEQ prior to implementation of the Effectiveness
Monitoring Plan.
9.2.2.3 Analytical Requirements
Groundwater samples collected from the MNA network will be analyzed for the parameters
described in the Interim Monitoring Plan in Section 9.1. Changes to the analytical requirements
may be proposed upon evaluation of the seasonal monitoring results obtained during the CSA
and interim monitoring.
9.2.3 Reporting
Monitoring reports summarizing the results from the each sampling event is proposed to be
submitted to NCDEQ within 120 days of completion of each sampling event.
9.3 Sampling and Analysis
9.3.1 Monitoring Well Measurements and Inspection
Groundwater sampling will be conducted at monitoring well locations associated with the CSA,
compliance, voluntary, and additional assessment wells as described in the monitoring plans
above.
During each sampling event, these wells will be measured for static water levels. The
measurements will be taken within one 24-hour period and prior to sampling to minimize
temporal variations. The depth to water measurements, along with date and time will be
recorded on a dedicated field form, a field notebook, and/or electronically via the iForms
software program or comparable system.
53
Corrective Action Plan Part 2
Dan River Steam Station Ash Basin

Monitoring wells will be measured for total depth during the Second Quarter 2016 sampling
event and during the first (March) sampling event in subsequent years. Measurements will be
recorded on a dedicated field form, a field notebook, and/or electronically via the iForms
software program or comparable system.
The thickness of sediment accumulated in each monitoring well will be calculated once each
year, during the second (June) sampling event of the year. Sediment thickness will be
calculated by comparing current total depth with historical data. If more than one foot of
sediment exists, wells will be redeveloped with a bailer or pump prior to the third (September)
sampling event. In addition, wells may be redeveloped if turbidity readings below 10 NTU
cannot be achieved during sample purging. Wells where turbidity less than 10 NTU cannot be
obtained may still be sampled in accordance with the Low Flow Sampling Plan.
Each monitoring well will be inspected while performing water level measurements for damage
to the casing, protective monuments, and bollards. Well caps and locks will be inspected to
determine whether they are in good working order and functioning properly. Flush-mounted
wells will be inspected for any damage by vehicular traffic and to ensure that the rubber seal is
functioning properly.
9.3.2 Surface Water and Area of Wetness Measurements
Stream stage measurements will be conducted at gauging locations along the Dan River and
eastern and western unnamed tributaries. Each stage location will have a designated datum
point from which the relative stream level will be measured.
9.3.3 Sample Collection
9.3.3.1 Monitoring Well Purging
All monitoring wells will utilize low-flow (minimal drawdown) groundwater purging and sampling
methods in accordance with the Low Flow Sampling Plan. The low-flow technique will be used
to determine when a well has been adequately purged and is ready to sample by monitoring the
pH, specific conductance, temperature, ORP, and turbidity. The volume of water that is removed
will also be observed and recorded. Wells that exhibit slow recharge, excessive drawdown, or
that require a higher pumping rate may be purged using the volume-averaging method.
An adequate purge is achieved when the pH, specific conductance, ORP, and temperature of
groundwater have stabilized and the turbidity is below 10 NTU.
9.3.3.2 Groundwater Sample Collection
After purging is complete, laboratory-supplied sample containers will be filled using the same
method utilized for purging. Appropriate sample containers, quantities, and preservatives for the
various analyses are listed in Table 9-2.
9.3.3.3 Surface Water and Area of Wetness Sample Collection
Grab samples will be collected from each surface water and AOW location. Water quality
parameters (pH, specific conductance, ORP, temperature, and turbidity) will be measured from
54
Corrective Action Plan Part 2
Dan River Steam Station Ash Basin

each location. After water quality parameters have been collected and recorded, AOW and
surface water samples will be collected by slightly submersing the lip of the sample container
under the water surface. Samples collected for dissolved target analyte list metals will be field-
filtered through a 0.45-micron filter on the day of collection using a peristaltic pump. Filled
laboratory-supplied sample containers will be labeled and placed in a cooler on ice (4°C) and
maintained at that temperature until delivery to the laboratory.
9.3.3.4 Sample Naming Convention
Samples will be identified by the following convention: site code, sample identification name,
sample matrix, and date code. The codes are further explained below.
• The two-digit Site Code for DRSS is DR.
• Sample Identification Name will be the sample location (i.e., BG-1D).
• Sample Matrix
o FD – Field Duplicate
o EB – Equipment Blank
o AMB – Ambient Blank
o FB – Filter Blank
o TB – Trip Blank
o NS – Normal Sample
• Date Code is a four-digit code indicating the quarter and year a sample was collected.
For example, a groundwater sample collected from monitoring well BG-1D in February 2016
would be designated DR-BG-1D-NS-1Q16. If a field duplicate was also collected from that
location it would be designated DR-BG-1D-FD-1Q16.
9.3.3.5 Waste Handling
Investigation-derived waste (IDW ) generated during this fieldwork will be handled in accordance
with applicable federal, state and local regulations. The IDW generated during the field activities
is expected to include purge water, personal protective equipment, trash, and decontamination
water. Waste minimization techniques will be employed where possible to reduce quantity of
IDW generated. IDW generated during the groundwater sampling will be characterized,
managed, and properly disposed following receipt of groundwater analytical results. Other
investigation-derived waste, including disposable tubing and gloves, will be bagged and
disposed as part of DRSS’s municipal solid waste.
9.3.3.6 Chain of Custody and Sample Delivery
All samples will be tracked using chain-of-custody procedures. A separate chain-of-custody will
be filled out by each sample team and accompany each cooler shipped. Samples will be hand-
delivered or shipped to the contract laboratory.
9.3.4 Quality Assurance/Quality Control
In addition to laboratory and other QA/QC procedures, field quality control measures are
implemented to ensure that data meets project requirements. The following field quality control
procedures will be utilized for this project:
55
Corrective Action Plan Part 2
Dan River Steam Station Ash Basin

• Field Duplicates – Field duplicate samples will be collected by filling two identical sets
of sample containers with water from the same sample location for each of the planned
analyses. Field duplicates will be given unique sample numbers.
• Equipment Blanks – Equipment blanks measure the cleanliness of field sampling
equipment. Equipment blanks will be created by pouring reagent-grade water over a
decontaminated pump and collecting the water in appropriate sample containers.
Equipment blanks will receive all the tests that are to be performed on the associated
samples.
• Ambient Blanks – Ambient blanks measure background contamination in the field.
Appropriate sample containers will be filled with reagent-grade water while other water
samples are collected at the site.
• Filter Blanks – Filter blanks evaluate the possible addition of chemicals from the filter to
the sample, thus measuring potential contamination in the filters. Filter blanks will be
created by filling the appropriate sample containers with reagent-grade water run
through a new filter.
• Trip Blanks – Trip blanks will be created by the laboratory and accompany low level
mercury samples through sample container shipment and delivery to the site, sample
collection, and delivery back to the laboratory for analysis.

56
Corrective Action Plan Part 2
Dan River Steam Station Ash Basin

10 Implementation Cost and Schedule
CAMA Section §130A-309.211(b)(1) requires implementation of corrective action within 30 days
of CAP approval. Surface ash impoundment closure planning and implementation is ongoing at
DRSS and compliance groundwater monitoring has been conducted since 2011.
10.1 Implementation Cost
The recommended corrective action at DRSS following source removal is MNA. A summary of
costs for the recommended corrective action are provided in Table 10-1. Additional cost details
are provided in Appendix G. Note that the actual costs will depend on actual conditions that
exist following excavation and completion of ash basin closure activities. Therefore, these
values represent an estimate for reference purposes when evaluating remedial alternatives.
Table 10-1 Estimated Capital and Annual Costs for Corrective Action - MNA
Proposed Activity Total
Capital Costs - Monitoring Well Installation
Monitoring Well Installation $60,000
Site Prep and Erosion and Sediment Control (ESC) $30,000
Field Management (15%) $16,500
Well Installation Reporting $5,000
Project Management (10%) $10,900
Contingency (20%) $23,900
Total Capital Costs $146,300
Annual Costs - Monitoring/Reporting
Lab Analysis $17,400
Data Validation $15,000
Equipment and Expendables $9,000
Sampling Labor $34,800
Reporting $60,000
Project Management (10%) $13,600
Escalation to Mid-Point (4%) $6,000
Annual Monitoring/Reporting Costs (3 events annually) $155,800
Total Capital/Annual Costs for Project Duration* $6,490,000
*Note: this total project cost includes the annual cost over the project duration of 30 years with a 4.25% discount factor per year.

10.2 Implementation Schedule
The Interim Monitoring Plan will be implemented at DRSS during sampling activities conducted
in the first half of 2016. Details of interim monitoring are discussed in Section 9. MNA will be re-
evaluated after closure activities are completed using results of the interim monitoring.
Beginning in September 2016, effectiveness monitoring events will coincide with compliance
monitoring events required by the NPDES permit, which are conducted in January, May, and
September. Groundwater sampling will be conducted in accordance with the Low Flow
57
Corrective Action Plan Part 2
Dan River Steam Station Ash Basin

Sampling Plan developed for the ash basin groundwater assessment program and approved by
NCDENR (CSA Report Appendix G).
MNA effectiveness and groundwater quality monitoring results at DRSS will be evaluated and
used to assess the dynamics of the contaminant plume. Based on the resulting monitoring data,
recommendations will be made regarding modifications in the monitoring program to ensure
representative data are being collected. Recommendations regarding changes in the
implementation of the selected remedy or other alternatives for consideration will also be made,
as needed.

58
Corrective Action Plan Part 2
Dan River Steam Station Ash Basin

11 References
ASTM. 2014. Standard Guide for Developing Conceptual Site Models for Contaminated Sites.
ASTM International.
Bethke, C. 2008. Geochemical and Biogeochemical Reaction Modeling, Cambridge University
Press.
Davis, J.A., R.O. James, and J.O. Leckie. 1978. Surface ionization and complexation at the
oxide-water interface.1. Computation of electrical double layer properties in simple
electrolytes. Journal of Colloid and Interface Science. 63(3): p. 480-499.
Dzombak, D.A. and F.M.M. Morel. 1990. Surface complexation modeling: hydrous ferric oxide.
New York: Wiley. xvii, 393.
Evanko, C.R., and D.A. Dzombak. 1997. Remediation of Metals-contaminated Soils and
Groundwater. Technology Evaluation Report 97-01. Ground-water Remediation
Technologies Evaluation Center, Pittsburgh, PA.
Goldberg, S., 1990. Use of Surface Complexation Models in Soil Chemical Systems, in
Advances in Agronomy, D.L. Sparks, Editor. 1990, Academic Press, Inc. p. 233-329
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-surveysHDR 2014a DRSS Receptor Survey
HDR, 2015a. Comprehensive Site Assessment Report. Dan River Steam Station Ash Basin.
August 14, 2015.
HDR, 2015b. Corrective Action Plan Part 1. Dan River Steam Station Ash Basin, November 12,
2015.
Hem, J.D. 1985. J.D., Study and Interpretation of the Chemical Characteristics of Natural Water,
United States Geological Survey Water Supply Paper 2254, 1985.
Hem, J.D. 1977. Reactions of metal ions at surfaces of hydrous iron oxide. Geochem.
Cosmochim. Acta., 1977. 41: p. 527-538.
Ilavský, J. 2008. “Removal of Antimony from Water by Sorption Materials.” Slovak Journal of
Civil Engineering. pp. 1–6. http://www.svf.stuba.sk/docs/sjce/2008/2008_2/file3.pdf.
Karamalidis, A. K. and Dzombak, D. A. 2011. Surface complexation modeling: gibbsite. John Wiley
& Sons,2011.
Karamalidis, A.K. and Dzombak, D.A. 2010. Surface Complexation Modeling: Gibbsite. 2010,
Hoboken, NJ: John Wiley and Sons, Inc.
59
Corrective Action Plan Part 2
Dan River Steam Station Ash Basin

Kinniburgh, D.G., and Cooper, D.M., 2011. PhreePlot – Creating graphical output with
PHREEQC. Available at http://www.phreeplot.org/, original date June 2011, last updated
December 31, 2015.
Martell, A.E. and R.K. Smith. 2001. Critical Stability Constants, Standard Reference Database
46, Version 6.30. 2001, National Institute of Standards, Gaithersburg, MD.
Miller, T. 2010. Conceptual model testing and development for neptunium and radium sorption
to SRS sediments, in Environmental Engineering and Earth Sciences. 2010, Clemson
University: Clemson
Parkhurst, D.L., and C.A.J. Appelo. 2013. Description of input and examples for PHREEQC
version 3—A computer program for speciation, batch-reaction, one-dimensional
transport, and inverse geochemical calculations: U.S. Geological Survey Techniques
and Methods, book 6, chap. A43, 497 p. [Online] URL: http://pubs.usgs.gov/tm/06/a43/
Pawlak, Z., P.S. Cartwright, A. Oloyede, and E. Bayraktar. 2002. “Removal of Toxic Arsenic and
Antimony from Groundwater Spiro Tunnel Bulkhead in Park City Utah Using Colloidal Iron
Hydroxide: Comparison with Reverse Osmosis.” Advanced Materials Research. Vols. 83–
86, pp. 553–562.
Sposito, G. 1989. The chemistry of soils. Oxford: Oxford University Press.
Stachowicz, M., T. Hiemstra, and W.H. van Riemsdijk. 2006. Surface speciation of As(III) and
As(V) in relation to charge distribution. Journal of Colloid and Interface Science, 2006.
302(1): p. 62-75.
Twidwell, L. G., and C. Williams-Beam. 2002. “Potential Technologies for Removing Thallium
from Mine and Process Wastewater: An Abbreviated Annotation of the Literature.” European
Journal of Mineral Processing and Environmental Protection. Vol. 2, no. 1. pp. 1–10.
USEPA (U.S. Environmental Protection Agency). 1991a. Risk Assessment Guidance for
Superfund, Part B
USEPA (U.S. Environmental Protection Agency). 1991b. Ecological Screening Levels.
USEPA (U.S. Environmental Protection Agency). 1999 laboratory determination of the solid-
water partitioning coefficient or Kd value
USEPA (U.S. Environmental Protection Agency). 2007a. Monitored Natural Attenuation of
Inorganic Contaminants in Ground Water - Volume 2 Assessment for Non-Radionuclides
Including Arsenic, Cadmium, Chromium, Copper, Lead, Nickel, Nitrate, Perchlorate, and
Selenium. EPA/600/R-07/140. Revised October 2007.
USEPA (U.S. Environmental Protection Agency). 2007b. Pair comparison of COI
concentrations, mutually rising relationship indicating attenuation
USEPA (U.S. Environmental Protection Agency). 2015a. Guidance for Conducting Screening
Level Ecological Risk Assessments.
60
Corrective Action Plan Part 2
Dan River Steam Station Ash Basin

USEPA (U.S. Environmental Protection Agency). 2015b. National Recommended Ambient
Water Quality Criteria.
http://water.epa.gov/scitech/swguidance/standards/criteria/current/index.cfm
USEPA (U.S. Environmental Protection Agency). 2015c. Supplemental Guidance to ERAGS:
Region 4, Ecological Risk Assessment. http://www2.epa.gov/sites/production/files/2015-
09/documents/r4_era_guidance_document_draft_final_8-25-2015.pdf
USEPA (U.S. Environmental Protection Agency). 2007. Monitored Natural Attenuation of
Inorganic Contaminants in Ground Water. Volume 2. Assessment for Non-Radionuclides
Including Arsenic, Cadmium, Chromium, Copper, Lead, Nickel, Nitrate, Perchlorate, and
Selenium.2007.
Zawislanski, P.T. and M. Zavarin. 1966. Nature and rates of selenium transformations: A
laboratory study of Kesterson Reservoir soils. Soil Science Society of America Journal,
1996. 60(3): p. 791- 800.

61