NC0004961_RBSS CSA Report_NCDENR Submittal_20150818

Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
Site Name and Location Riverbend Steam Station
175 Steam Plant Rd
Mount Holly, NC 28120
Groundwater Incident No. Not Assigned
NPDES Permit No. NC0004961
Date of Report August 18, 2015
Permittee and Current Property Owner Duke Energy Carolinas, LLC
526 South Church St
Charlotte, NC 28202
800.559.3853
Consultant Information HDR Engineering, Inc. of the Carolinas
440 South Church St, Suite 900
Charlotte, NC 28202
704.338.6700
Latitude and Longitude of Facility 35° 21’ 40” N, 80° 58’ 32” W

This document has been reviewed for accuracy and quality
commensurate with the intended application.

Scott Spinner, L.G. Malcolm Schaeffer, L.G.
Environmental Geologist Senior Geologist

This page is intentionally left blank.
Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
EXECUTIVE SUMMARY

Executive Summary
.
ES-i

This page is intentionally left blank.
Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
TABLE OF CONTENTS

Table of Contents
Section Page No.
1.0 Introduction ........................................................................................................................ 1
1.1 Purpose of Comprehensive Site Assessment ................................................................ 1
1.2 Regulatory Background .................................................................................................. 2
1.2.1 NCDENR Requirements .......................................................................................... 2
1.2.2 Notice of Regulatory Requirements ........................................................................ 3
1.2.3 Coal Ash Management Act Requirements .............................................................. 3
1.3 NCDENR-Duke Energy Correspondence ....................................................................... 4
1.4 Approach to Comprehensive Site Assessment .............................................................. 4
1.4.1 NORR Guidance ..................................................................................................... 5
1.4.2 EPA Monitored Natural Attenuation Approach ........................................................ 5
1.4.3 ASTM Conceptual Site Model Guidance ................................................................. 5
1.5 Limitations and Assumptions .......................................................................................... 6
2.0 Site History and Description ............................................................................................... 8
2.1 Site Location, Acreage, and Ownership ......................................................................... 8
2.2 Site Description .............................................................................................................. 8
2.3 Adjacent Property, Zoning, and Surrounding Land Uses ............................................... 9
2.4 Adjacent Surface Water Bodies and Classifications ....................................................... 9
2.5 Meteorological Setting .................................................................................................... 9
2.6 Hydrologic Setting ........................................................................................................ 10
2.7 Permitted Activities and Permitted Waste .................................................................... 11
2.8 NPDES and Surface Water Monitoring ........................................................................ 11
2.9 NPDES Flow Diagram .................................................................................................. 12
2.10 History of Site Groundwater Monitoring ........................................................................ 12
2.10.1 Voluntary Groundwater Monitoring Wells .............................................................. 13
2.10.2 Compliance Groundwater Monitoring Wells .......................................................... 13
2.11 Assessment Activities or Previous Site Investigations.................................................. 14
2.12 Decommissioning Status .............................................................................................. 15
3.0 Source Characteristics ..................................................................................................... 16
3.1 Coal Combustion and Ash Handling System ................................................................ 16
3.2 Description of Ash Basin and Other Ash Storage Areas .............................................. 16
i

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
TABLE OF CONTENTS

3.2.1 Ash Basin .............................................................................................................. 16
3.2.2 Ash Storage Area .................................................................................................. 17
3.2.3 Cinder Storage Area .............................................................................................. 18
3.3 Physical Properties of Ash ............................................................................................ 18
3.4 Chemical Properties of Ash .......................................................................................... 19
4.0 Receptor Information ........................................................................................................ 21
4.1 Summary of Previous Receptor Survey Activities ........................................................ 21
4.2 Summary of CSA Receptor Survey Activities and Findings ......................................... 22
4.3 NCDENR Well Water Testing Program ........................................................................ 23
5.0 Regional Geology and Hydrogeology .............................................................................. 24
5.1 Regional Geology ......................................................................................................... 24
5.2 Regional Hydrogeology ................................................................................................ 24
6.0 Site Geology and Hydrogeology ...................................................................................... 27
6.1 Site Geology ................................................................................................................. 27
6.1.1 Soil Classification .................................................................................................. 27
6.1.2 Rock Lithology ....................................................................................................... 28
6.1.3 Structural Geology ................................................................................................. 28
6.1.4 Geologic Mapping ................................................................................................. 29
6.1.5 Fracture Trace Analysis ........................................................................................ 29
6.1.6 Effects of Structure on Groundwater Flow ............................................................ 31
6.1.7 Soil and Rock Mineralogy and Chemistry ............................................................. 31
6.2.1 Groundwater Flow Direction .................................................................................. 32
6.2.2 Hydraulic Gradient ................................................................................................. 33
6.2.3 Effects of Geologic/Hydrogeologic Characteristics on Contaminants ................... 33
6.2.4 Hydrogeologic Site Conceptual Site Model ........................................................... 33
7.0 Source Characterization ................................................................................................... 35
7.1 Ash Basin Primary and Secondary Cells ...................................................................... 36
7.1.1 Ash (Sampling and Chemical Characteristics) ...................................................... 36
7.1.2 Ash Basin Water (Sampling and Chemical Characteristics) .................................. 36
7.1.3 Porewater (Sampling and Chemical Characteristics) ............................................ 36
7.1.4 Ash Porewater Speciation ..................................................................................... 36
7.2 Ash Storage Area ......................................................................................................... 37
7.2.1 Ash (Sampling and Chemical Characteristics) ...................................................... 37
ii

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
TABLE OF CONTENTS

7.2.2 Porewater (Sampling and Chemical Characteristics) ............................................ 37
7.3 Cinder Storage Area ..................................................................................................... 37
7.3.1 Ash (Sampling and Chemical Characteristics) ...................................................... 37
7.3.2 Porewater (Sampling and Chemical Characteristics) ............................................ 37
7.4 Leaching Potential of Ash ............................................................................................. 37
7.5 Seeps ........................................................................................................................... 38
7.5.1 Review of NCDENR March 2014 Sampling Results ............................................. 38
7.5.2 Seep Sampling Results – CSA Activities .............................................................. 39
7.6 Constituents of Interest ................................................................................................. 40
7.6.1 COIs in Ash ........................................................................................................... 40
7.6.2 COIs in Ash Basin Surface Water ......................................................................... 40
7.6.3 COIs in Porewater ................................................................................................. 41
7.6.4 COIs in Seeps ....................................................................................................... 41
8.0 Soil and Rock Characterization ........................................................................................ 42
8.1 Background Sample Locations ..................................................................................... 42
8.2 Analytical Methods and Results ................................................................................... 42
8.3 Comparison of Soil Results to Applicable Levels ......................................................... 43
8.4 Comparison of Soil Results to Background .................................................................. 43
8.4.1 Background Soil .................................................................................................... 43
8.4.2 Soil Beneath Ash Basin and Within Waste Boundary ........................................... 43
8.4.3 Soil Beneath Ash Storage Area ............................................................................. 44
8.4.4 Soil Beneath Cinder Storage Area ........................................................................ 44
8.4.5 Soil Outside the Waste Boundary and Within Compliance Boundary ................... 44
8.5 Comparison of PWR and Bedrock Results to Background........................................... 44
8.5.1 Background PWR and Bedrock ............................................................................. 44
8.5.2 PWR and Bedrock Beneath Ash Basin and Within Ash Basin Waste Boundary... 45
8.5.3 PWR and Bedrock Beneath Ash Storage Area ..................................................... 45
8.5.4 PWR and Bedrock Outside the Waste Boundary and within Compliance
Boundary ............................................................................................................... 45
9.0 Surface Water and Sediment Characterization ................................................................ 46
9.1 Surface Water ............................................................................................................... 46
9.1.1 Comparison of Exceedances to 2B Standards ...................................................... 46
9.1.2 Results for Constituents without 2B Standards ..................................................... 47
iii

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
TABLE OF CONTENTS

9.1.3 Results for Select Constituents in Mountain Island Lake ...................................... 48
9.2 Surface Water Speciation ............................................................................................. 48
9.3 Sediment ...................................................................................................................... 48
10.0 Groundwater Characterization ......................................................................................... 49
10.1 Regional Groundwater Data for Constituents of Interest .............................................. 49
10.1.1 Antimony ............................................................................................................... 50
10.1.2 Arsenic .................................................................................................................. 50
10.1.3 Barium ................................................................................................................... 50
10.1.4 Boron ..................................................................................................................... 51
10.1.5 Chromium .............................................................................................................. 51
10.1.6 Cobalt .................................................................................................................... 52
10.1.7 Iron ........................................................................................................................ 52
10.1.8 Manganese ............................................................................................................ 52
10.1.9 Sulfate ................................................................................................................... 53
10.1.10 Thallium ............................................................................................................. 54
10.1.11 Vanadium ........................................................................................................... 54
10.1.12 pH ...................................................................................................................... 54
10.2 Background Wells ......................................................................................................... 55
10.3 Discussion of Redox Conditions ................................................................................... 56
10.4 Groundwater Analytical Results ................................................................................... 56
10.4.1 Beneath Ash Basin and Within Waste Boundary .................................................. 58
10.4.2 Beneath Ash Storage Areas .................................................................................. 58
10.4.3 Beneath Cinder Storage Area ............................................................................... 58
10.4.4 Outside the Waste Boundary and Within Compliance Boundary .......................... 58
10.5 Comparison of Results to 2L Standards ....................................................................... 59
10.6 Comparison of Results to Background ......................................................................... 59
10.6.1 Background Wells MW-7D and MW-7SR .............................................................. 59
10.6.2 Newly Installed Background Wells ........................................................................ 60
10.6.3 Regional Groundwater Data .................................................................................. 60
10.6.4 Groundwater Beneath Ash Basin and Within Waste Boundary ............................ 61
10.6.5 Groundwater Beneath Ash Storage Area .............................................................. 61
10.6.6 Groundwater Beneath Cinder Storage Area ......................................................... 61
iv

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
TABLE OF CONTENTS

10.6.7 Groundwater Beyond Ash Basin Waste Boundary and within Compliance
Boundary ............................................................................................................................. 61
10.7 Cation and Anion Water Quality Data ........................................................................... 62
10.8 Groundwater Speciation ............................................................................................... 62
10.9 Radiological Laboratory Testing ................................................................................... 63
10.10 CCR Rule Groundwater Detection and Assessment Monitoring Parameters ........... 63
11.0 Hydrogeological Investigation .......................................................................................... 65
11.1 Hydrostratigraphic Layer Development ........................................................................ 65
11.2 Hydrostratigraphic Layer Properties ............................................................................. 66
11.2.1 Borehole In-Situ Tests ........................................................................................... 67
11.2.2 Monitoring Well and Observation Well Slug Tests ................................................ 67
11.2.3 Laboratory Permeability Tests ............................................................................... 68
11.2.4 Hydrostratigraphic Layer Parameters .................................................................... 68
11.3 Vertical Hydraulic Gradients ......................................................................................... 69
11.4 Groundwater Velocity ................................................................................................... 69
11.5 Contaminant Velocity .................................................................................................... 69
11.6 Plume's Physical and Chemical Characterization ........................................................ 70
11.7 Groundwater / Surface Water Interaction ..................................................................... 72
11.8 Estimated Seasonal High Groundwater Elevations – Compliance Wells ..................... 73
12.0 Screening-Level Risk Assessment ................................................................................... 74
12.1 Human Health Screening ............................................................................................. 74
12.1.1 Introduction ............................................................................................................ 74
12.1.2 Conceptual Site Model .......................................................................................... 75
12.1.3 Human Health Risk-Based Screening Levels ....................................................... 77
12.1.4 Site-Specific Risk Based Remediation Standards ................................................. 78
12.1.5 NCDENR Receptor Well Investigation .................................................................. 78
12.2 Ecological Screening .................................................................................................... 79
12.2.1 Introduction ............................................................................................................ 79
12.2.2 Ecological Setting .................................................................................................. 79
12.2.3 Fate and Transport Mechanisms ........................................................................... 84
12.2.4 Comparison to Ecological Screening Levels ......................................................... 84
12.2.5 Uncertainty and Data Gaps ................................................................................... 85
12.2.6 Scientific/Management Decision Point .................................................................. 86
v

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
TABLE OF CONTENTS

12.2.7 Ecological Risk Screening Summary .................................................................... 86
13.0 Groundwater Modeling ..................................................................................................... 87
13.1 Fate and Transport Groundwater Modeling .................................................................. 87
13.2 Batch Geochemical Modeling ....................................................................................... 88
13.3 Geochemical Site Conceptual Model ........................................................................... 88
14.0 Data Gaps – Conceptual Site Model Uncertainties .......................................................... 92
14.1 Data Gaps .................................................................................................................... 92
14.1.1 Data Gaps Resulting from Temporal Constraints .................................................. 92
14.1.2 Data Gaps Resulting from Review of Data Obtained During CSA Activities ......... 93
14.2 Site Heterogeneities ..................................................................................................... 93
14.3 Impact of Data Gaps and Site Heterogeneities ............................................................ 94
15.0 Planned Sampling for CSA Supplement .......................................................................... 95
15.1 Sampling Plan for Inorganic Constituents .................................................................... 95
15.2 Sampling Plan for Speciation Constituents .................................................................. 95
16.0 Interim Groundwater Monitoring Plan .............................................................................. 96
16.1 Sampling Frequency ..................................................................................................... 96
16.2 Constituent and Parameter List .................................................................................... 96
16.3 Proposed Sampling Locations ...................................................................................... 96
16.4 Proposed Background Wells ........................................................................................ 96
17.0 Discussion ........................................................................................................................ 97
17.1 Summary of Completed and Ongoing Work ................................................................. 97
17.2 Nature and Extent of Contamination ............................................................................ 98
17.3 Maximum Contaminant Concentrations ....................................................................... 99
17.4 Contaminant Migration and Potentially Affected Receptors ......................................... 99
18.0 Conclusions .................................................................................................................... 101
18.1 Source and Cause of Contamination .......................................................................... 101
18.2 Imminent Hazards to Public Health and Safety and Actions Taken to Mitigate them in
Accordance to 15A NCAC 02L .0106(f) ................................................................................ 101
18.3 Receptors and Significant Exposure Pathways .......................................................... 101
18.4 Horizontal and Vertical Extent of Soil and Groundwater Contamination and Significant
Factors Affecting Contaminant Transport .............................................................................. 101
18.5 Geological and Hydrogeological Features influencing the Migration, Chemical, and
Physical Character of the Contaminants ............................................................................... 103
18.6 Proposed Continued Monitoring ................................................................................. 104
vi

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
TABLE OF CONTENTS

18.7 Preliminary Evaluation of Corrective Action Alternatives ............................................ 104
19.0 References ..................................................................................................................... 105

vii

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
LIST OF FIGURES

List of Figures (organized by CSA report section)
Executive Summary
• ES-1: Site Conceptual Model – Plan View Map

1.0 Introduction
<No Figures>

2.0 Site History and Description
• Figure 2-1: Site Location Map
• Figure 2-2: Site Layout Map
• Figure 2-3: Pre-Ash Basin 1948 USGS Map
• Figure 2-4: Site Features Map
• Figure 2-5: Site Vicinity Map
• Figure 2-6: Riverbend Steam Station Water Schematic Flow Diagram
• Figure 2-7: Compliance and Voluntary Monitoring Wells

3.0 Source Characteristics
• Figure 3-1: Photo of Fly Ash and Bottom Ash
• Figure 3-2: Elemental Composition for Bottom Ash, Fly Ash, Shale, and Volcanic
Ash
• Figure 3-3: Coal Ash TCLP Leachate Concentration vs. Regulatory Limits
• Figure 3-4: Trace Elements in Fly Ash vs Soil
• Figure 3-5: Trace Elements in Bottom Ash vs Soil

4.0 Receptor Information
• Figure 4-1: Receptor Map – USGS Base
• Figure 4-2: Receptor Map – Aerial Base
• Figure 4-3: Ash Basin Underground Features Map
• Figure 4-4: Ash Storage Area Underground Features Map
• Figure 4-5: Surface Water Bodies
• Figure 4-6: Surrounding Property Owners

5.0 Regional Geology and Hydrogeology
• Figure 5-1: Tectonostratigraphic Map of the Southern and Central Appalachians
• Figure 5-2: Regional Geologic Map
• Figure 5-3: Interconnected, Two-Medium Piedmont Groundwater System
• Figure 5-4: Conceptual Variations of the Transition Zone due to Rock Type /
Structure
viii

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
LIST OF FIGURES

• Figure 5-5: Piedmont Slope-Aquifer System

6.0 Site Geology and Hydrogeology
• Figure 6-1: Site Geologic Map
• Figure 6-2: Monitoring Well and Sampling Location Map
• Figure 6-3: Topographic Lineaments and Rose Diagram
• Figure 6-4: Aerial Photography Lineaments and Rose Diagram
• Figure 6-5: Potentiometric Surface Map – S Wells
• Figure 6-6: Potentiometric Surface Map – D Wells
• Figure 6-7: Potentiometric Surface Map – BR Wells

7.0 Source Characterization
• Figure 7-1: Source Characterization Sample Location Map

8.0 Soil and Rock Characterization
• Figure 8-1: Soil Analytical Results – Plan View (PSRG Exceedances)
• Figure 8-2: Cross Section A-A’ with Soil Analytical Results
• Figure 8-3: Cross Section B-B’ with Soil Analytical Results
• Figure 8-4: Cross Section C-C’ with Soil Analytical Results
• Figure 8-5: Cross Section D-D’ with Soil Analytical Results
• Figure 8-6: Cross Section E-E’ with Soil Analytical Results
• Figure 8-7: Cross Section F-F’ with Soil Analytical Results
• Figure 8-8: Cross Section G-G’ with Soil Analytical Results
• Figure 8-9: Cross Section H-H’ with Soil Analytical Results
• Figure 8-10: Cross Section I-I’ with Soil Analytical Results
• Figure 8-11: Cross Section J-J’ with Soil Analytical Results
• Figure 8-12: Cross Section K-K’ with Soil Analytical Results
• Figure 8-13: Cross Section L-L’ with Soil Analytical Results

9.0 Surface Water and Sediment Characterization
• Figure 9-1: Seep and Surface Water Sample Locations
• Figure 9-2: NCDENR March 2014 Sample Locations

10.0 Groundwater Characterization
• Figure 10-1: Mean Arsenic Groundwater Concentrations by County
• Figure 10-2: Mean Iron Groundwater Concentrations by County
• Figure 10-3: Manganese Concentrations in Well Water Compared to Soil Systems
• Figure 10-4: Regional Groundwater Quality – Manganese
• Figure 10-5: Thallium Concentrations in Soil
ix

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
LIST OF FIGURES

• Figure 10-6: Regional Groundwater Quality - Vanadium
• Figure 10-7: Regional Groundwater Quality – pH
• Figure 10-8: Monitoring Well and Sample Location Map
• Figure 10-9: Typical Well Construction Details
• Figure 10-10: Time Series Plot: Compliance Well MW-7SR Chromium and Turbidity
• Figure 10-11: Time Series Plot: Compliance Well MW-10 Iron and Turbidity
• Figure 10-12: Time Series Plot: Compliance Well MW-11SR Iron and Turbidity
• Figure 10-13: Time Series Plot: Compliance Well MW-13 Iron and Turbidity
• Figure 10-14: Time Series Plot: Compliance Well MW-14 Iron and Turbidity
• Figure 10-15: Time Series Plot: Compliance Well MW-15 Iron and Turbidity
• Figure 10-16: Time Series Plot: Compliance Well MW-7SR Iron and Turbidity
• Figure 10-17: Time Series Plot: Compliance Well MW-8D Iron and Turbidity
• Figure 10-18: Time Series Plot: Compliance Well MW-8I Iron and Turbidity
• Figure 10-19: Time Series Plot: Compliance Well MW-9 Iron and Turbidity
• Figure 10-20: Time Series Plot: Compliance Well MW-10 Manganese and Turbidity
• Figure 10-21: Time Series Plot: Compliance Well MW-11SR Manganese and
Turbidity
• Figure 10-22: Time Series Plot: Compliance Well MW-13 Manganese and Turbidity
• Figure 10-23: Time Series Plot: Compliance Well MW-14 Manganese and Turbidity
• Figure 10-24: Time Series Plot: Compliance Well MW-15 Manganese and Turbidity
• Figure 10-25: Time Series Plot: Compliance Well MW-7SR Manganese and
Turbidity
• Figure 10-26: Time Series Plot: Compliance Well MW-8D Manganese and Turbidity
• Figure 10-27: Time Series Plot: Compliance Well MW-8I Manganese and Turbidity
• Figure 10-28: Time Series Plot: Compliance Well MW-9 Manganese and Turbidity
• Figure 10-29: Time Series Plot: Compliance Well MW-10 pH and Turbidity
• Figure 10-30: Time Series Plot: Compliance Well MW-11DR pH and Turbidity
• Figure 10-31: Time Series Plot: Compliance Well MW-11SR pH and Turbidity
• Figure 10-32: Time Series Plot: Compliance Well MW-13 pH and Turbidity
• Figure 10-33: Time Series Plot: Compliance Well MW-15 pH and Turbidity
• Figure 10-34: Time Series Plot: Compliance Well MW-7D pH and Turbidity
• Figure 10-35: Time Series Plot: Compliance Well MW-7SR pH and Turbidity
• Figure 10-36: Time Series Plot: Compliance Well MW-8D pH and Turbidity
• Figure 10-37: Time Series Plot: Compliance Well MW-8I pH and Turbidity
• Figure 10-38: Time Series Plot: Compliance Well MW-8S pH and Turbidity
• Figure 10-39: Time Series Plot: Compliance Well MW-9 pH and Turbidity
• Figure 10-40: Time Series Plot: Compliance Well MW-7D Antimony and Turbidity
• Figure 10-41: Time Series Plot: Compliance Well MW-7D vs Deep Wells –
Chromium (Total)
x

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
LIST OF FIGURES

• Figure 10-42: Time Series Plot: Compliance Well MW-7SR vs Shallow Wells –
Chromium (Total)
• Figure 10-43: Time Series Plot: Compliance Well MW-7D vs Deep Wells – Iron
(Total)
• Figure 10-44: Time Series Plot: Compliance Well MW-7SR vs Shallow Wells – Iron
(Total)
• Figure 10-45: Time Series Plot: Compliance Well MW-7D vs Deep Wells –
Manganese (Total)
• Figure 10-46: Time Series Plot: Compliance Well MW-7SSR vs Deep Wells –
Manganese (Total)
• Figure 10-47: Time Series Plot: Compliance Well MW-7D vs Deep Wells – pH, Field
• Figure 10-48: Time Series Plot: Compliance Well MW-7SR vs Shallow Wells – pH,
Field
• Figure 10-49: Time Series Plot: Compliance Well MW-7D vs Deep Wells –
Antimony (Total)
• Figure 10-50: Time Series Plot: Compliance Well MW-7SR vs Shallow Wells –
Antimony (Total)
• Figure 10-51: Stacked Time Series Plots – Compliance Wells – Antimony
• Figure 10-52: Stacked Time Series Plots – Compliance Wells – Iron
• Figure 10-53: Stacked Time Series Plots – Compliance Wells – Manganese
• Figure 10-54: Stacked Time Series Plots – Compliance Wells – Chromium
• Figure 10-55: Stacked Time Series Plots – Compliance Wells – pH
• Figure 10-56: Correlation Plot – Compliance Wells vs Background – Iron (Total)
• Figure 10-57: Correlation Plot – Compliance Wells vs Background – Manganese
(Total)
• Figure 10-58: Correlation Plot – Compliance Wells vs Background – Chromium
(Total)
• Figure 10-59: Correlation Plot – Compliance Wells vs Background – pH (Total)
• Figure 10-60: Correlation Plot – Compliance Wells vs Background – Antimony
(Total)
• Figures 10-61 through 10-117 are not used
• Figure 10-118: Groundwater Analytical Results – Plan View (2L Exceedances)
• Figure 10-119: Antimony Isoconcentration Contour Map – Shallow Wells (S)
• Figure 10-120: Antimony Isoconcentration Contour Map – Deep Wells (D)
• Figure 10-121: Antimony Isoconcentration Contour Map – Bedrock Wells (BRU and
BR)
• Figure 10-122: Arsenic Isoconcentration Contour Map – Shallow Wells (S)
• Figure 10-123: Arsenic Isoconcentration Contour Map – Deep Wells (D)
• Figure 10-124: Arsenic Isoconcentration Contour Map – Bedrock Wells (BRU and
BR)
• Figure 10-125: Boron Isoconcentration Contour Map – Shallow Wells (S)
• Figure 10-126: Boron Isoconcentration Contour Map – Deep Wells (D)
xi

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
LIST OF FIGURES

• Figure 10-127: Boron Isoconcentration Contour Map – Bedrock Wells (BRU and
BR)
• Figure 10-128: Chromium Isoconcentration Contour Map – Shallow Wells (S)
• Figure 10-129: Chromium Isoconcentration Contour Map – Deep Wells (D)
• Figure 10-130: Chromium Isoconcentration Contour Map – Bedrock Wells (BRU
and BR)
• Figure 10-131: Cobalt Isoconcentration Contour Map – Shallow Wells (S)
• Figure 10-132: Cobalt Isoconcentration Contour Map – Deep Wells (D)
• Figure 10-133: Cobalt Isoconcentration Contour Map – Bedrock Wells (BRU and
BR)
• Figure 10-134: Iron Isoconcentration Contour Map – Shallow Wells (S)
• Figure 10-135: Iron Isoconcentration Contour Map – Deep Wells (D)
• Figure 10-136: Iron Isoconcentration Contour Map – Bedrock Wells (BRU and BR)
• Figure 10-137: Manganese Isoconcentration Contour Map – Shallow Wells (S)
• Figure 10-138: Manganese Isoconcentration Contour Map – Deep Wells (D)
• Figure 10-139: Manganese Isoconcentration Contour Map – Bedrock Wells (BRU
and BR)
• Figure 10-140: Sulfate Isoconcentration Contour Map – Shallow Wells (S)
• Figure 10-141: Sulfate Isoconcentration Contour Map – Deep Wells (D)
• Figure 10-142: Sulfate Isoconcentration Contour Map – Bedrock Wells (BRU and
BR)
• Figure 10-143: TDS Isoconcentration Contour Map – Shallow Wells (S)
• Figure 10-144: TDS Isoconcentration Contour Map – Deep Wells (D)
• Figure 10-145: TDS Isoconcentration Contour Map – Bedrock Wells (BRU and BR)
• Figure 10-146: Thallium Isoconcentration Contour Map – Shallow Wells (S)
• Figure 10-147: Thallium Isoconcentration Contour Map – Deep Wells (D)
• Figure 10-148: Thallium Isoconcentration Contour Map – Bedrock Wells (BRU and
BR)
• Figure 10-149: Vanadium Isoconcentration Contour Map – Shallow Wells (S)
• Figure 10-150: Vanadium Isoconcentration Contour Map – Deep Wells (D)
• Figure 10-151: Vanadium Isoconcentration Contour Map – Bedrock Wells (BRU and
BR)
• Figure 10-152: Cross Section A-A’ with Groundwater Analytical Results
• Figure 10-153: Cross Section A-A’ with Groundwater Analytical Results
• Figure 10-154: Cross Section B-B’ with Groundwater Analytical Results
• Figure 10-155 Cross Section B-B’ with Groundwater Analytical Results
• Figure 10-156: Cross Section C-C’ with Groundwater Analytical Results
• Figure 10-157: Cross Section D-D’ with Groundwater Analytical Results
• Figure 10-158: Cross Section E-E’ with Groundwater Analytical Results
• Figure 10-159: Cation/Anion Concentrations in Ash Basin Porewater Samples
xii

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
LIST OF FIGURES

• Figure 10-160: Cation/Anion Concentrations in Surface Water and Ash Basin
Samples
• Figure 10-161: Cation/Anion Concentrations in Seep Samples
• Figure 10-162: Cation/Anion Concentrations in Background Wells
• Figure 10-163: Cation/Anion Concentrations in Shallow Wells
• Figure 10-164: Cation/Anion Concentrations in Shallow Wells
• Figure 10-165: Cation/Anion Concentrations in Deep Wells
• Figure 10-166: Cation/Anion Concentrations in Bedrock Wells
• Figure 10-167: Sulfate/Chloride Ratios in Porewater
• Figure 10-168: Sulfate/Chloride Ratios in Surface Water and Ash Basin Water
• Figure 10-169: Sulfate/Chloride Ratios in Seep Samples
• Figure 10-170: Sulfate/Chloride Ratios in Background Wells
• Figure 10-171: Sulfate/Chloride Ratios in Shallow Wells
• Figure 10-172: Sulfate/Chloride Ratios in Deep Wells
• Figure 10-173: Sulfate/Chloride Ratios in Bedrock Wells
• Figure 10-174: Piper Diagram – Ash Basin Porewater, Water, and Background
Monitoring Wells
• Figure 10-175: Piper Diagram – Ash Basin Porewater, Water, and Seeps
• Figure 10-176: Piper Diagram – Ash Basin Porewater, Water, and Downgradient
“S”, “D”, and “BR” Monitoring Wells
• Figure 10-177: Piper Diagram – Ash Basin Porewater, Water, and Upgradient “S”,
“D”, and “BR” Monitoring Wells
• Figure 10-178: Piper Diagram – Ash Basin Porewater, “S”,”D”, and “BR” Monitoring
Wells
• Figure 10-179: Piper Diagram – Ash Basin “S” Wells and Seeps
• Figure 10-180: Detection Monitoring Constituents Detected in Shallow Wells
• Figure 10-181: Detection Monitoring Constituents Detected in Deep Wells
• Figure 10-182: Detection Monitoring Constituents Detected in Bedrock Wells
• Figure 10-183: Assessment Monitoring Constituents Detected in Shallow Wells
• Figure 10-184: Assessment Monitoring Constituents Detected in Deep Wells
• Figure 10-185: Assessment Monitoring Constituents Detected in Bedrock Wells

11.0 Hydrogeological Investigation
• Figure 11-1: Major Transects
• Figure 11-2: Hydrostratigraphic Cross Section A-A’
• Figure 11-3: Hydrostratigraphic Cross Section B-B’
• Figure 11-4: Hydrostratigraphic Cross Section C-C’
• Figure 11-5: Hydrostratigraphic Cross Section D-D’
• Figure 11-14: Groundwater Analytical Results – Plan View (2L Exceedances)
• Figure 11-15: Groundwater Velocities – S Wells
xiii

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
LIST OF FIGURES

• Figure 11-16: Groundwater Velocities – D Wells
• Figure 11-17: Groundwater Velocities – BR Wells
12.0 Screening Level Risk Assessment
• Figure 12-1 Human Health Screening Conceptual Site Model
• Figure 12-2: Ecological Screening Conceptual Site Model
13.0 Groundwater Modeling
<No Figures>
14.0 Data Gaps – Conceptual Site Model Uncertainties
<No Figures>
15.0 Planned Sampling for CSA Supplement
<No Figures>
16.0 Interim Groundwater Monitoring Plan
<No Figures>
17.0 Discussion
<No Figures>
18.0 Conclusions and Recommendations
<No Figures>
19.0 References
<No Figures>

xiv

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
LIST OF TABLES

List of Tables (organized by CSA report section)
Executive Summary
<No Tables>

1.0 Introduction
• Table 1-1: Comparison of Sampling Data to Federal and State Regulatory
Standards

2.0 Site History and Description
• Table 2-1: NPDES Groundwater Monitoring Requirements
• Table 2-2: Exceedances of 2L Standards at Compliance Wells (March 2011 – June
2015)
• Table 2-3: Summary of Onsite Environmental Incidents

3.0 Source Characteristics
• Table 3-1: Range (10th percentile – 90th percentile) in Bulk Composition of Fly Ash,
Bottom Ash, Rock, and Soil

4.0 Receptor Information
• Table 4-1: Public and Private Water Supply Wells within 0.5-mile Radius of Ash
Basin Compliance Boundary
• Table 4-2: Property Owner Addresses Contiguous to the Ash Basin Waste
Boundary

5.0 Regional Geology and Hydrogeology
<No Tables>

6.0 Site Geology and Hydrogeology
• Table 6-1: Soil Mineralogy Results
• Table 6-2: Soil Chemistry Results – Oxides
• Table 6-3: Soil Chemistry Results – Elemental
• Table 6-4: Transition Zone Mineralogy Results
• Table 6-5: Chemical Composition of Transition Zone Samples
• Table 6-6: Whole Rock Chemistry Results - Oxides
• Table 6-7: Whole Rock Chemistry Results - Elemental
• Table 6-8: Compliance and Voluntary Monitoring Well Construction Information
• Table 6-9: 2015 Groundwater Assessment Monitoring Well Construction Information
• Table 6-10: Summary of Horizontal Hydraulic Gradient Calculations

xv

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
LIST OF TABLES

7.0 Source Characterization
• Table 7-1: Solid Matrix Parameters and Analytical Methods
• Table 7-2: Ash Sample Results
• Table 7-3: Aqueous Matrix Parameters and Analytical Methods
• Table 7-4: Ash Basin Surface Water Sample Results
• Table 7-5: Ash Basin Porewater Sample Results
• Table 7-6: Porewater Speciation Results
• Table 7-7: Ash Sample SPLP Results
• Table 7-8: Seep Sample Results
• Table 7-9: Field Parameters for Seep Sampling
• Table 7-10: NCDENR March 2014 Sampling Results

8.0 Soil and Rock Characterization
• Table 8-1: Solid Matrix Parameters and Analytical Methods or Soil, Ash, and Rock
Parameters and Constituent Analysis – Analytical Methods
• Table 8-2: Background Soil Sample Results
• Table 8-3: Background PWR and Bedrock Sample Results
• Table 8-4: Soil Total Inorganic Results
• Table 8-5: PWR and Bedrock Totals Inorganic Results
• Table 8-6: Background Soil SPLP Results
• Table 8-7: Soil SPLP Results
• Table 8-8: Frequency and Concentration Ranges in Soil for COI Exceedances of
North Carolina PSRGs
• Table 8-9: Frequency and Concentration Ranges in PWR and Bedrock for COI
Exceedances of North Carolina PSRGs
• Table 8-10: Constituents in Soil Beneath Ash Basin and Within Waste Boundary
• Table 8-11: Constituents in Soil Beneath Ash Storage Area
• Table 8-12: Constituents in Soil Beneath Cinder Storage Area
• Table 8-13: Constituents in Soil Outside the Waste Boundary and Within
Compliance Boundary
• Table 8-14: Constituents in PWR and Bedrock Beneath Ash Basin and Within
Waste Boundary
• Table 8-15: Constituents in PWR and Bedrock Beneath Ash Storage Area
• Table 8-16: Constituents in PWR and Bedrock Outside the Waste Boundary and
Within the Compliance Boundary

9.0 Surface Water and Sediment Characterization
• Table 9-1: Surface Water Sample Results
• Table 9-2: Range of Constituent Concentrations for 2B Standards in Background
and NCDENR Surface Water Samples
xvi

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
LIST OF TABLES

• Table 9-3: Range of Constituent Concentrations for 2B Standards in Background
and Cinder Storage Area Samples
• Table 9-4: Range of Constituent Concentrations for 2B Standards in Background
and Stream Surface Water Samples
• Table 9-5: Range of Concentrations for Constituents without 2B Standards in
Background and NCDENR Surface Water Samples
• Table 9-6: Range of Concentrations for Constituents without 2B Standards in
Background and Cinder Storage Area Surface Water Samples
• Table 9-7: Range of Concentrations for Constituents without 2B Standards in
Background and Stream Surface Water Samples
• Table 9-8: Surface Water Speciation Results
• Table 9-9: Sediment Sample Results - Totals

10.0 Groundwater Characterization
• Table 10-1: State and Federal Standards for COIs
• Table 10-2: Gaston and Mecklenburg County Private Well Statistics for COIs
• Table 10-3: Regional Average Concentrations of Iron and Manganese in
Groundwater
• Table 10-4: Background Groundwater Sample Results
• Table 10-5: Redox Potential
• Table 10-6: Groundwater Sampling Parameters and Constituent Analytical Methods
• Table 10-7: Background Groundwater Sample Results – Totals and Dissolved
• Table 10-8: Background Groundwater Speciation Results
• Table 10-9: Groundwater Sample Results – Totals and Dissolved
• Table 10-10: Groundwater Speciation Results
• Table 10-11: Comparison of Groundwater Results to 2L Standards and IMACs
• Table 10-12: Constituents in Groundwater Beneath Ash Basin and Within Waste
Boundary
• Table 10-13: Constituents in Groundwater Beneath Ash Storage Area
• Table 10-14: Constituents Beneath Cinder Storage Area
• Table 10-15: Constituents in Groundwater Beyond Ash Basin Waste Boundary and
within Compliance Boundary
• Table 10-16: Groundwater Radiological Testing Results

11.0 Hydrogeological Investigation
• Table 11-1: Soil/Material Properties for Ash, Fill, Alluvium, Soil/Saprolite
• Table 11-2: Field Permeability Test Results
• Table 11-3: Slug Test Permeability Results
• Table 11-4: Historic Slug Test Permeability Results
• Table 11-5: Laboratory Permeability Test Results
• Table 11-6: Historic Laboratory Permeability Test Results
xvii

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
LIST OF TABLES

• Table 11-7: Estimated Total Porosity/Specific Yield and Specific Storage for Upper
Hydrostratigraphic Units (A, F, S, M1, and M2)
• Table 11-8: Estimated Effective Porosity/Specific Yield and Specific Storage for
Upper Hydrostratigraphic Units (A, F, S, M1, and M2)
• Table 11-9: Hydrostratigraphic Layer Properties – Horizontal Hydraulic Conductivity
• Table 11-10: Hydrostratigraphic Layer Properties – Vertical Hydraulic Conductivity
• Table 11-11: Total Porosity, Secondary (Effective) Porosity/Specific Yield, and
Specific Storage for Lower Hydrostratigraphic Units (TZ and BR)
• Table 11-12: Table 11-13: Hydraulic Gradients – Vertical
• Table 11-13: Groundwater Velocities

12.0 Screening Level Risk Assessment
• Table 12-1: Selection of Human Health COPCs – Groundwater
• Table 12-2: Selection of Human Health COPCs – Soil
• Table 12-3: Selection of Human Health COPCs – Surface Water
• Table 12-4: Selection of Human Health COPCs – Sediment
• Table 12-5: Contaminants of Potential Human Health Concern
• Table 12-6: Selection of Ecological COPCs – Soil
• Table 12-7: Selection of Ecological COPCs – Freshwater
• Table 12-8: Selection of Ecological COPCs – Sediment
• Table 12-9: Contaminants of Potential Ecological Concern
• Table 12-10: Threatened and Endangered Species in Cleveland and Rutherford
Counties

13.0 Groundwater Modeling
<No Tables>

14.0 Data Gaps – Conceptual Site Model Uncertainties
<No Tables>

15.0 Planned Sampling for CSA Supplement
• Table 15-1: Wells with 2L Standard Exceedances – Constituents to be Speciated

16.0 Interim Groundwater Monitoring Plan
• Table 16-1: Recommended Parameters and Constituents
• Table 16-2: Sample Locations in Interim Groundwater Monitoring Plan

17.0 Discussion
<No Tables>

xviii

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
LIST OF TABLES

18.0 Conclusions and Recommendations
<No Tables>

19.0 References
<No Tables>
xix

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
LIST OF APPENDICES

List of Appendices (provided electronically)

Appendix A: Introduction
• NORR Letter
• Summary of Work Plan Submittals and NCDENR-Duke Energy Correspondence
• Revised Groundwater Assessment Work Plan

Appendix B: Receptor Information
• Updated Receptor Survey Report

Appendix C: Source Characterization
• Drilling Procedures
• Drilling and Installation Variances

Appendix D: Soil and Rock Characterization
• Sampling Procedures
• Sampling Variances

Appendix E: Field, Sampling, and Data Analysis Quality Assurance / Quality
Control
• Field and Sampling Quality Assurance / Quality Control Procedures
• Data Analysis Quality Assurance / Quality Control Procedures

Appendix F: Surface Water and Sediment Characterization
• Sampling Procedures
• Sampling Variances

Appendix G: Groundwater Characterization
• Well Development Procedure
• Well Development Forms
• Sampling Procedures
• Sampling Forms
• Sampling Variances
• Evaluation of Turbidity in Existing Voluntary and Compliance Wells
• Evaluation of Need for Off-site Monitoring Wells
xx

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
LIST OF APPENDICES

• Statistical Analysis of Groundwater Results

Appendix H: Hydrogeological Investigation
• Boring Logs
• Well Construction Records
• Historical Boring Logs and Well Construction Records
• Soil Physical Lab Reports
• Mineralogy Lab Reports
• Slug Test Reports
• Field Permeability Data
• Historic Permeability Data
• Fetter-Bear Diagrams – Porosity
• Estimated Seasonal High and Low Groundwater Elevations Calculation

Appendix I: Screening Level Risk Assessment Supporting Data
• Trustee Letters and Responses
• Checklist for Ecological Assessments/Sampling

Appendix J: Analytical Results Table

Appendix K: Laboratory Reports

Appendix L: Soil Sample and Rock Core Photographs

Appendix M: Certification Form for CSA

xxi

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
LIST OF ACRONYMS AND ABBREVIATIONS

List of Acronyms and Abbreviations
µg/L micrograms per liter
2L Standards 15A NCAC 02L .0202 Groundwater Quality Standards
AMEC AMEC Environment & Infrastructure
APS NCDENR DWR Aquifer Protection Section
AST Aboveground Storage Tank
ASTM American Society for Testing and Materials
BG Background
bgs Below ground surface
BR Bedrock
CAMA Coal Ash Management Act
CAP Corrective Action Plan
CCP Coal Combustion Products
CCR Coal Combustion Residuals
CNS Catawba Nuclear Station
COI Constituent of Interest
COPC Contaminant of Potential Concern
CSA Comprehensive Site Assessment
CSM Conceptual Site Model
DO Dissolved oxygen
DTW Depth to Water
Duke Energy Duke Energy Carolinas, LLC
DWR NCDENR Division of Water Resources
EDR Environmental Data Resources
EPD Georgia Environmental Protection Division
EPRI Electric Power Research Institute
ESH Estimated Seasonal High
ESL Estimated Seasonal Low
GSCM Geochemical Site conceptual model
GIS Geographic Information Systems
HFO Hydrous ferric oxide
HQ Hazard Quotient
IMAC Interim Maximum Allowable Concentration
Kd Sorption Coefficient
mD millidarcies
MDL Method detection limit
mg/kg Milligrams per kilogram
MGD Million gallons per day
xxii

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
LIST OF ACRONYMS AND ABBREVIATIONS

mm milligrams
MNA Monitored Natural Attenuation
MRL Method reporting limit
MSL Mean sea level
MW Megawatt
N Standard Penetration Testing Values
NRCS Natural Resources Conservation Service
NCAC North Carolina Administrative Code
NCDENR North Carolina Department of Environment and Natural Resources
NCDHHS North Carolina Department of Health and Human Services
NCNHP North Carolina Natural Heritage Program
NCWRC North Carolina Wildlife Resources Commission
ng/L Nanograms per liter
NHD USGS National Hydrography Dataset
NORR Notice of Regulatory Requirements
NPDES National Pollutant Discharge Elimination System
NTU Nephelometric Turbidity Unit
NURE National Uranium Resource Evaluation
PL Prediction Limit
PMCL Primary Maximum Contaminant Level
ppb parts per billion
ppm parts per million
PSRG Preliminary Soil Remediation Goal
PWR Partially Weathered Rock
PWSS NCDENR Division of Water Resources Public Water Supply Section
RBSS Riverbend Steam Station
RCRA Resource Conservation and Recovery Act
REC Recovery
RL Reporting Limit
RQD Rock Quality Designation
RSL USEPA Regional Screening Level
SCM Site Conceptual Model
SCS U.S. Department of Agriculture Soil Conservation Service
SLERA Screening Level Ecological Risk Assessment
SMCL Secondary Maximum Contaminant Level
SMDP Scientific/Management Decision Point
SPLP Synthetic Precipitation Leaching Procedure
SQL Sample Quantitation Limit
SWAP NCDENR DWR Source Water Assessment Program
xxiii

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
LIST OF ACRONYMS AND ABBREVIATIONS

TCLP Toxicity Characteristic Leaching Procedure
TDS Total Dissolved Solids
TZ Transition Zone
UNC University of North Carolina
UNCC University of North Carolina at Charlotte
USCS Unified Soil Classification System
USDA U.S. Department of Agriculture
USEPA U.S. Environmental Protection Agency
USGS U.S. Geological Survey
UST Underground Storage Tank

xxiv

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
1.0 INTRODUCTION

1.0 Introduction
Duke Energy Carolinas, LLC (Duke Energy) owns and operates the Riverbend Steam Station
(RBSS), located near Mount Holly, in Gaston County, North Carolina. RBSS began operation in
1929 as a coal-fired generating station and was subsequently retired in April 2013. Following
initial station operation in 1929, coal ash residue from RBSS’s coal combustion process was
deposited in a cinder storage area on site. Following installation of precipitators and a wet
sluicing system around 1957, coal ash residue was disposed of in the station’s ash basin
located adjacent to the station and Mountain Island Lake. Discharge from the ash basin is
currently permitted under North Carolina Department of Environment and Natural Resources
(NCDENR) Division of Water Resources (DWR) under the National Pollutant Discharge
Elimination System (NPDES) Permit NC0004961.
Since 2008, Duke Energy has performed voluntary and NPDES permit-required groundwater
monitoring at RBSS. Voluntary groundwater monitoring around the RBSS ash basin was
performed twice each year from December 2008 until June 2010, with analytical results
submitted to NCDENR DWR. Compliance groundwater monitoring as required by the NPDES
permit began in March 2011. From December 2010 to June 2015, the compliance groundwater
monitoring wells at the RBSS site have been sampled three times per year for a total of 15
times.
Recent monitoring events have indicated exceedances of 15A NCAC 02L .0200 Groundwater
Quality Standards (2L Standards; refer to North Carolina Administrative Code Title 15A
Department of Environmental and Natural Resources Division of Water Quality Subchapter 2L
Section .0100, .0200, .0300 Classifications and Water Quality Standards Applicable to the
Groundwaters of North Carolina) at RBSS, prompting NCDENR’s requirement for Duke Energy
to perform a groundwater assessment at the site and prepare a Comprehensive Site
Assessment (CSA) report. The Coal Ash Management Act of 2014 (CAMA) also directed
owners of coal combustion residuals (CCR) surface impoundments to conduct groundwater
monitoring and assessment and submit a Groundwater Assessment Report. This CSA is
submitted to meet the requirements of both NCDENR and the CAMA.
1.1 Purpose of Comprehensive Site Assessment
The purpose of this Comprehensive Site Assessment (CSA) is 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. This CSA was prepared in general accordance with
requirements outlined in the following regulations and documents:
• Classifications and Water Quality Standards Applicable to the Groundwaters of North
Carolina in Title 15A NCAC 02L .0106(g);
• Coal Ash Management Act in G.S. 130A-309.209(a);
1

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
1.0 INTRODUCTION

• Notice of Regulatory Requirements (NORR) issued by NCDENR on August 13, 2014;
• Conditional Approval of Revised Groundwater Assessment Work Plan issued by
NCDENR on February 16, 2015; and
• Subsequent meetings and correspondence between Duke Energy and NCDENR.
This assessment includes evaluation of possible impacts from the ash basins and related ash
storage facilities, and consisted of the following activities:
• Completion of soil borings and installation of groundwater monitoring wells to faciliatate
collection and analysis of chemical, physical, and hydrogeological parameters of
subsurface materials encountered within and beyond the waste and compliance
boundaries;
• Evaluation of testing data to supplement the site conceptual model (SCM);
• Update of the receptor survey previously completed in 2014; and
• Completion of a screening-level risk assessment.

In this report, constituents are those chemicals or compounds that were identified in the
approved Work Plan for sampling and analysis. For RBSS, these include antimony, arsenic,
boron, chromium, cobalt, iron, manganese, selenium, thallium, vanadium, sulfate, and total
dissolved solids (TDS). If a constituent exceeded its respective regulatory standard or screening
level in the medium in which it was found, the constituent was then termed a Constituent of
Interest (COI) and evaluated in the human health and ecological screening-level risk
assessment (Section 12.0).
1.2 Regulatory Background
1.2.1 NCDENR Requirements
NCDENR DWR regulates wastewater discharges from coal ash ponds to state waters, streams,
and lakes, and requires groundwater monitoring and stormwater management at these facilities.
Duke Energy’s coal-fired power facilities are regulated through federal NPDES wastewater
permits. As part of these permits, the facilities must comply with the state water quality
standards and U.S. Environmental Protection Agency (USEPA) water quality criteria.
Groundwater monitoring is performed at Duke Energy facilities in accordance with approved
monitoring plans and NPDES permits for each site. Included in these monitoring evaluations is a
determination if site-specific background concentrations (i.e., naturally occurring constituents in
the soil profile and groundwater) for various constituents (e.g., iron and manganese) contribute
to reported concentrations. For each facility, if it is determined that activities on the property are
causing noncompliance with NCDENR DWR regulatory requirements, the agency will
coordinate with the permittee to develop and implement a Corrective Action Plan (CAP) in
accordance with state regulation.
2

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
1.0 INTRODUCTION

1.2.2 Notice of Regulatory Requirements
Chapter 143, North Carolina General Statutes, authorizes and directs the Environmental
Management Commission of the Department of Environment and Natural Resources to protect
and preserve the water and air resources of the State. The NCDENR DWR has the delegated
authority to enforce adopted pollution control rules. NCDENR DWR Rule 15A NCAC 02L
.0103(d) states that “no person shall conduct or cause to be conducted any activity which
causes the concentration of any substance to exceed that specified in” 15A NCAC 02L .0202,
Groundwater Quality Standards.
On August 13, 2014, NCDENR issued a Notice of Regulatory Requirements (NORR) letter
notifying Duke Energy that exceedances of the groundwater quality standards 15A NCAC 02L
.0200 Classifications and Water Quality Standards Applicable to the Groundwaters of North
Carolina were reported at 14 coal ash facilities owned and operated by Duke Energy, including
RBSS. The NORR stipulated that for each coal ash facility , Duke Energy shall conduct a CSA
following submittal of a Groundwater Assessment Work Plan (Work Plan) and receptor survey.
In accordance with the NORR requirements, a receptor survey was performed to identify all
receptors within a 0.5-mile radius (2,640 feet) of the RBSS ash basin compliance boundary, and
a CSA was conducted for each facility. The NORR letter is included as Appendix A.
1.2.3 Coal Ash Management Act Requirements
The Coal Ash Management Act (CAMA) of 2014 – General Assembly of North Carolina Senate
Bill 729 Ratified Bill (Session 2013) (SB 729) requires ash from Duke Energy coal plant sites
located in the State either (1) be excavated and relocated to fully lined storage facilities or (2) go
through a classification process to determine closure options and schedule. Closure options can
include a combination of excavating and relocating ash to a fully lined structural fill, excavating
and relocating the ash to a lined landfill (on-site or off-site), and/or capping the ash with an
engineered synthetic barrier system, either in place or after being consolidated to a smaller area
on-site.
As a component of implementing this objective, CAMA provides instructions for owners of coal
combustion residuals surface impoundments to perform various groundwater monitoring and
assessment activities. Section §130A-309.209 of the CAMA ruling specifies groundwater
assessment and corrective actions, drinking water supply well surveys and provisions of
alternate water supply, and reporting requirements as follows:
(a) Groundwater Assessment of Coal Combustion Residuals Surface Impoundments.
– The owner of a coal combustion residuals surface impoundment shall conduct
groundwater monitoring and assessment as provided in this subsection. The
requirements for groundwater monitoring and assessment set out in this
subsection are in addition to any other groundwater monitoring and assessment
requirements applicable to the owners of coal combustion residuals surface
impoundments.
3

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
1.0 INTRODUCTION

(1) No later than December 31, 2014, the owner of a coal combustion residuals
surface impoundment shall submit a proposed Groundwater Assessment
Plan for the impoundment to the Department for its review and approval. The
Groundwater Assessment Plan shall, at a minimum, provide for all of the
following:
a. A description of all receptors and significant exposure pathways.
b. An assessment of the horizontal and vertical extent of soil and
groundwater contamination for all contaminants confirmed to be present
in groundwater in exceedance of groundwater quality standards.
c. A description of all significant factors affecting movement and transport
of contaminants.
d. A description of the geological and hydrogeological features influencing
the chemical and physical character of the contaminants.
2) The Department shall approve the Groundwater Assessment Plan if it
determines that the Plan complies with the requirements of this subsection
and will be sufficient to protect public health, safety, and welfare; the
environment; and natural resources.
(3) No later than 10 days from approval of the Groundwater Assessment Plan,
the owner shall begin implementation of the Plan.
(4) No later than 180 days from approval of the Groundwater Assessment Plan,
the owner shall submit a Groundwater Assessment Report to the Department.
The Report shall describe all exceedances of groundwater quality standards
associated with the impoundment.
1.3 NCDENR-Duke Energy Correspondence
In response to both the NORR letter and CAMA requirements, Duke Energy submitted a Work
Plan to NCDENR DWR on September 25, 2014, establishing proposed site assessment
activities and schedules for the implementation, completion, and submission of a CSA report in
accordance with 15A NCAC 02L .0106(g). NCDENR DWR reviewed the Work Plan and
provided Duke Energy with initial comments on November 4, 2014. A revised Work Plan was
subsequently submitted to NCDENR DWR on December 30, 2014, and NCDENR DWR
provided final comments and conditional approval of the revised Work Plan on February 19,
2015. In addition, Duke Energy submitted proposed adjustments to the CSA guideline and
requested clarifications regarding groundwater sampling and speciation of selected constituents
to NCDENR on May 14 and May 22, 2015. NCDENR provided responses to these proposed
revisions and clarifications in June 2015. Copies of this correspondence including Work Plan
submittals are included in Appendix A.
1.4 Approach to Comprehensive Site Assessment
The CSA approach was developed based on the NORR guidelines and CAMA requirements.
Development of the SCM is based on several documents including but not limited to USEPA’s
4

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
1.0 INTRODUCTION

Monitored Natural Attenuation tiered approach, ASTM 1689-95 (2014) Standard Guide for
Developing Site Conceptual Models for Contaminated Sites, and comments received by
NCDENR.
1.4.1 NORR Guidance
The NORR letter (Appendix A) outlined general guidelines for the CSA report, including
guidance from 15A NCAC 02L .0106(g) as described in Section 1.1. The NORR letter also
included Guidelines for Comprehensive Site Assessment for those involved in the investigation
of contaminated soil and/or groundwater. The components included in the NORR guidelines
were used in developing the site Work Plan and this CSA report.
1.4.2 EPA Monitored Natural Attenuation Approach
In accordance with NCDENR requirements and the February 16, 2015 Conditional Approval
letter (Appendix A), the elements of the USEPA’s Monitored Natural Attenuation (MNA) tiered
approach has been utilized as part of the investigation associated with the CSA.
MNA may be used as a component to meet corrective action requirements if site conditions
meet the requirements associated with use of MNA. The approach involves a detailed analysis
of site characteristics controlling and sustaining attenuation to support evaluation and selection
of MNA as part of a cleanup action for inorganic contaminant plumes in groundwater (USEPA
2007). The site characterization is conducted in a step-wise manner to facilitate collection of
data necessary to progressively evaluate the effectiveness of natural attenuation processes
within the site aquifer(s). Four general elements are included in the tiered site analysis
approach:
• Demonstration of active contaminant removal from groundwater and dissolved plume
stability;
• Determination of the mechanism and rate of attenuation;
• Determination of the long-term capacity for attenuation and stability of immobilized
contaminants, before, during, and after any proposed remedial activities; and
• Design of performance monitoring program, including defining triggers for assessing the
remedial action strategy failure, and establishing a contingency plan.

Duke Energy will evaluate the USEPA MNA approach further during preparation of the CAP.
1.4.3 ASTM Conceptual Site Model Guidance
ASTM standard guidance document E1689-95 “Developing Conceptual Site Models for
Contaminated Sites” (2014) was used as a general guide for developing the Site Conceptual
Model (SCM). The guidance document provides direction in developing SCMs used for the
integration of technical information from multiple sources, selection of sampling locations to
establish background concentrations of substances, identification of data needs and guidance of
data collection activities, and evaluation of risks to human and environmental health posed by a
5

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
1.0 INTRODUCTION

contaminated site. According to ASTM E1689-95, six basic activities are associated with
developing a SCM:
• Identification of potential contaminants;
• Identification and characterization of the source(s) of contaminants;
• Delineation of potential migration pathways through environmental media, such as
groundwater, surface water, soils, sediment, biota, and air;
• Establishment of background areas of contaminants for each contaminated medium;
• Identification and characterization of potential environmental receptors (human and
ecological); and
• Determination of the limits of the study area or system boundaries.

Development of a SCM is typically iterative and the complexity of the model should be
consistent with the complexity of the site and available data. Information gained through site
investigation activities is used to characterize the existing physical, biological, and chemical
systems at a site. The SCM describes and integrates processes that determine contaminant
releases, contaminant migration, and environmental receptor exposure to contaminants.
Development of the model is essential to determine potential exposure routes and identifying
possible impacts to human health and the environment (ASTM 2014).
The SCM is used to integrate site information, identify data gaps, and determine whether
additional information is needed at the site. The model is also used to facilitate selection of
remedial alternatives and effectiveness of remedial actions in reducing the exposure of
environmental receptors to contaminants (ASTM 2014).
This CSA was conducted in accordance with the conditionally approved Work Plan to meet the
NCDENR, NORR, and CAMA regulatory requirements described in Section 1.2, and using the
NORR, USEPA, and ASTM approaches described above. This assessment information will be
used to develop a CAP, to be submitted separately, for the RBSS site that will provide a
demonstration of these criteria in support of the recommended site remedy.
Data obtained from sampling during this CSA are compared to federal and state regulatory
standards shown in Table 1-1. Beginning in Section 7.0, laboratory results are compared to the
above-referenced regulatory standards and discussed as either “exceeding” or “not exceeding”
those standards. The evaluation of exceedances of these standards forms the basis for
determining the need for additional work later in this document
1.5 Limitations and Assumptions
Development of this CSA is based on information provided to HDR by both public and private
entities including universities, federal, state and local governments, and information and
analytical reports generated by Duke Energy. HDR assumes the information in these
documents to be accurate and reliable. This information was used to estimate exposure routes
and migration pathways in the subsurface. This CSA was developed using a standard of care
ordinarily used by engineering practice under the same or similar circumstances, but may
6

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
1.0 INTRODUCTION

include assumptions based on the accuracy and reliability of data from various entities. CAMA
Section §130A-309.209 requires that “No later than 180 days from approval of the Groundwater
Assessment Plan, the owner shall submit a Groundwater Assessment Report to the
Department”. The schedule dictated by CAMA is compressed; therefore, data interpretation is
limited and subject to change upon receipt of additional data in subsequent rounds of sampling
and additional data collected to resolve data gaps identified in Section 14.0. The additional data
will be used to inform the corrective actions identified in the CAP.

7

This page is intentionally left blank.
Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
2.0 SITE HISTORY AND DESCRIPTION

2.0 Site History and Description
This section provides a description of the RBSS site based on relevant historical data and
representative information. The purpose of this characterization is to familiarize readers with the
site and use the general information as part of the overall ASTM CSM development approach.
2.1 Site Location, Acreage, and Ownership
The RBSS site is located on a peninsula in the Catawba River on the north side of Horseshoe
Bend Beach Road, near the town of Mount Holly in Gaston County, North Carolina (Figure 2-1).
The entire RBSS site is approximately 340.7 acres in area and is owned by Duke Energy
(Figure 2-2). As of the date of this report, site ownership and land use prior to Duke Energy
could not be determined from available records.
In addition to the RBSS power plant property, Duke Energy owns and operates Mountain Island
Lake as part of the Catawba-Wateree Hydroelectric Project (FERC Project No. 2232). Mountain
Island Lake surrounds the RBSS site and is used for municipal water supply and recreation.
Duke Energy performed a review of property ownership of the FERC project boundary property
within the ash basin compliance boundary (defined in accordance with Title15A NCAC 02L
.0107(a) as being established at either 500 feet from the waste boundary or at the property
boundary, whichever is closer to the waste). The review indicated that Duke Energy owns the
lake bottom of Mountain Island within the compliance boundary, as shown on Figure 2-2.
2.2 Site Description
RBSS is a former seven-unit coal-fired electricity generating facility with a capacity of 454 MW.
Coal was delivered to the station by rail. The station began commercial operation in 1929 with
operation of coal-fired Units 1-4. Coal-fired Units 5-7 began operations in 1952 through 1954. All
of the coal-fired units were located in a single power plant. Units 1-3 were retired from service in
the 1970s, and Units 4-7 were retired from service on April 1, 2013. During its final years of
operation, the plant was considered a cycling station and was brought online to supplement
energy supply when electricity demand was at its highest. Duke Energy also operated four
combustion turbines (CT) at the site from 1969 until October 2012. The CT units could be fired
by natural gas or oil, and are located to the west of the coal-fired units. Refer to Figure 2-4 for a
map of site features.
The RBSS site is generally forested along the Catawba River. The buildings and other
structures associated with the power production facilities are located on the north side of
Horseshoe Bend Beach Road, which extends from west to east and is generally located along a
local topographic divide. The topography at the site generally slopes downward from this divide
on the south to Mountain Island Lake on the north. A 1948 USGS topographic map depicting the
site prior to construction of the ash basin features is shown on Figure 2-3. Refer to Section 3.2
for a detailed description of the RBSS ash basin and other ash storage facilities.
8

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
2.0 SITE HISTORY AND DESCRIPTION

Other areas of the site are occupied by facilities supporting the production or transmission of
power. The site contains two switchyards and associated transmission lines. The Lark
Maintenance Center is located at the RBSS site to the west of the coal-fired units. The Lark
facility is an advanced machining and welding shop that supports various Duke Energy power
plants.
2.3 Adjacent Property, Zoning, and Surrounding Land Uses
The area surrounding RBSS generally consists of residential properties, undeveloped land, and
Mountain Island Lake (Figure 2-5). Properties north of Mountain Island Lake are located in the
Town of Huntersville, Mecklenburg County, North Carolina. These properties are primarily
comprised of a wildlife refuge located to the northwest and a nature preserve identified by the
Town of Huntersville as a Park or Nature Preserve and a residential property located to the
northeast of the ash basin and Mountain Island Lake, which is zoned as Rural (R). According to
the Town of Huntersville zoning descriptions, the Rural District is intended to encourage the
development of neighborhoods and rural compounds that set aside natural vistas and
landscape features for permanent conservation.
Properties south of Mountain Island Lake are located in Mount Holly, Gaston County, North
Carolina. The majority of the property in this area is owned by Duke Energy and associated with
the RBSS, and is zoned by the Town of Mount Holly as Heavy Industrial (H-1) to be used for
industrial and commercial uses, and Light Industrial (L-1) to be used for certain kind of
commercial uses that are located as neighbors of industrial use. Residential properties are
located south and southeast of RBSS to the south of Horseshoe Bend Beach Road, and are
zoned Single Family Residential (R-12) to be used for single-family residential purposes.
With the exception of the decommissioning of RBSS, future surrounding land uses are assumed
to remain similar to their current uses (undeveloped land, wildlife refuge, nature preserve, heavy
and light industrial, and residential).
2.4 Adjacent Surface Water Bodies and Classifications
The site is located in the Mountain Island Lake/Catawba River watershed and the ash basin is
adjacent to Mountain Island Lake. Surface water classifications in North Carolina are defined in
15A NCAC 02B. 0101 (c). The surface water classification for Mountain Island Lake is Class
WS-IV, Class B, and Class C. Class WS-IV waters are protected as water supplies which are
generally in moderately to highly developed watersheds. Class C are waters protected for uses
such as secondary recreation, fishing, wildlife, fish consumption, aquatic life including
propagation, survival and maintenance of biological integrity, and agriculture. Class B waters
are protected for all Class C uses in addition to primary recreation (swimming). Surface water
features located on the site are shown on Figure 2-2 and Figure 4-5.
2.5 Meteorological Setting
In winter, the average temperature for Gaston County is 43°F and the average daily minimum
temperature is 32°F. In summer, the average temperature is 77°F and the average daily
9

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
2.0 SITE HISTORY AND DESCRIPTION

maximum temperature is 88°F (USDA-SCS 1989). The average annual precipitation in Mount
Holly is 41.63 inches (over the 30-year period of record). Severe local storms occasionally occur
in or near Gaston County. These events, such as tropical depressions or remnants of
hurricanes moving inland can cause isolated heavy rain fall. The average annual precipitation in
the piedmont, where this site is located, ranges from approximately 42 inches to 46 inches
(USDA-SCS 1989). The average relative humidity in midafternoon is approximately 70 percent,
with humidity reaching higher levels at night. The prevailing wind is from the southwest, and
average wind speed is highest (9 miles per hour) in spring (USDA-SCS 1989).
2.6 Hydrologic Setting
RBSS is located on a peninsula in the Catawba River (Mountain Island Lake) on the north side
of Horseshoe Bend Beach Road. This road runs generally west to east and is located along a
local topographic divide. Based on the slope-aquifer system, groundwater at the site is expected
to flow downward from this topographic divide to the south to the ash basins and discharge into
Mountain Island Lake (Catawba River) to the north .
Cowans Ford Hydroelectric Station (Cowans Ford) is part of the cascade of hydropower stations
constructed on the Catawba River. The Cowans Ford Project forms Lake Norman and is located
8 river miles upstream of Riverbend ash basin. Cowans Ford discharges water directly into
Mountain Island Lake. Mountain Island Hydroelectric Station is part of the dam that creates
Mountain Island Lake and is located 8.5 river miles downstream of Riverbend ash basin.
The water surface elevation of Mountain Island Lake is measured on the upstream side of
Mountain Island Dam and typically varies from 643.5 feet to 647.0 feet above mean sea level
(MSL). Operation of the hydroelectric station causes changes in the water levels in Mountain
Island Lake on a frequent basis (i.e., over the course of a year, and with significant weekly
variation) based on information provided by Duke Energy’s Hydro Central Operations Center via
electronic mail dated April 10, 2013.
The ash basins affect local groundwater elevations . Select monitoring wells located
downgradient of the primary and secondary ash ponds have periodically exhibited localized
groundwater mounding in the vicinity of the basins.
Water levels within the primary and secondary cells of the ash basin have historically fluctuated
approximately within a range of 4 feet from June 1982 until late 2013. The water elevation in the
primary cell ranged from approximately 721 to 725 feet and the water elevation in the secondary
cell ranged from approximately 711 to 715 feet. Since the station was retired in April 2013, the
water levels in both cells have dropped four feet from the typical operation range. The primary
cell no longer has standing water and the secondary cell water elevation is approximately 704.6
feet.
Groundwater recharge in the area is derived from infiltration of local precipitation and from
inflow and subsequent infiltration to the ash basins. Groundwater recharge occurs in areas of
higher topography (i.e., hilltops) and groundwater discharge occurs in lowland areas bordering
surface water bodies, marshes, and floodplains (LeGrand 2004). Of the average annual
10

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
2.0 SITE HISTORY AND DESCRIPTION

precipitation in the area of 42 to 46 inches, mean annual recharge ranges from 4.0 to 9.7 inches
per year (Daniel 2001).
The hydrologic setting is described in further detail in Section 11.0.
2.7 Permitted Activities and Permitted Waste
Duke Energy is authorized to discharge wastewater to the surface waters of North Carolina or
separate storm sewer system that has been adequately treated and managed in accordance
with NPDES Permit NC0004961, which most recently became effective March 1, 2011. Any
other point source discharge to surface waters of the state is prohibited unless it is an allowable
non-stormwater discharge or is covered by another permit, authorization, or approval.
The NPDES permit authorizes the following discharges in accordance with effluent limitations,
monitoring requirements, and other conditions set forth in the permit:
• Once through cooling water (Outfall 001) consisting of intake screen backwash and
water from the plant chiller system, turbine lube oil coolers, condensate coolers, main
turbine steam condensers and the intake tunnel dewatering sump;
• Ash basin discharge (Outfall 002) consisting of induced draft fan and preheater bearing
cooling water, stormwater from roof drains and paving, treated groundwater, track
hopper sump (groundwater), coal pile runoff, laboratory drain and chemical makeup
tanks and drums rinsate wastes, general plant/trailer sanitary wastewater, turbine and
boiler room sumps, vehicle rinse water, and stormwater from pond areas, upgradient
watershed, and miscellaneous stormwater flows; and
• Yard sump overflow (Outfall 002A).

No active or inactive permitted solid waste facilities (landfills) are located at the site, and the
property watershed classification prohibits construction of future landfills.
Duke Energy is permitted to discharge stormwater to the surface waters of North Carolina or
separate storm sewer system that has been adequately treated and managed in accordance
with the RBSS NPDES permit. Any other point source discharge to surface waters of the state is
prohibited unless it is an allowable non-stormwater discharge or is covered by another permit,
authorization, or approval.
The Lark Maintenance Center operates under North Carolina Division of Air Quality permit
07248R054, with effective dates from June 9, 2015 through May 31, 2023.
2.8 NPDES and Surface Water Monitoring
The NPDES program regulates wastewater discharges to surface waters to ensure that surface
water quality standards are maintained. The NPDES permitting program requires that permits
be renewed every five years.
11

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
2.0 SITE HISTORY AND DESCRIPTION

RBSS received its first NPDES wastewater permit in the early to mid-1970s. The most recent
NPDES permit became effective March 1, 2011 and expired on February 28, 2015. Duke
Energy submitted a permit renewal application to NCDENR DWR on May 15, 2014. The
NCDENR DWR issued a draft of the new NPDES Permit NC0004961 on March 6, 2015, based
on the permit renewal application submitted by Duke Energy on May 15, 2014. The draft permit
requires additional surface water monitoring of 12 potentially contaminated groundwater seeps.
The seeps (identified in the permit as S-1 to S-12) are collectively classified as Outfall 010.
Location information for each seep is identified in the permit. The draft permit also identifies
Outfall 011 as permitted for wastewater, stormwater, and groundwater. Duke Energy provided
comments on the draft permit to NCDENR DWR on May 4, 2015. As of the issuance of this
report, the draft NPDES permit has not become effective, and the site is operating under the
March 1, 2011 permit requirements.
The permit requires surface water monitoring as part of the permit conditions. Surface water
samples are required to be collected associated with Outfall 001, Outfall 002, and Outfall 002A
(reference Section 2.10). The sample locations, parameters, and constituents to be measured
and analyzed, and the requirements for sampling frequency and reporting results are outlined in
the permit.
2.9 NPDES Flow Diagram
The current NPDES flow diagram from the submitted NPDES permit application for RBSS is
provided in Figure 2-6. Current approximate quantities of inflows into the ash basin include
0.139 million gallons per day (MGD) from the yard drain sump, 0.021 MGD from stormwater,
and 0.025 MGD from plant sumps. The contributing sources to these inflows are depicted on
Figure 2-6.
The NPDES flow diagram provided in Figure 2-6 depicts former flow estimates when the station
was operational. Former inflows and approximate quantities into the ash basin included 3 MGD
from the ash removal system, 1.4 MGD from the yard drain sump, 1.3 MGD from the boiler
room sumps, and 0.31 MGD from stormwater..
2.10 History of Site Groundwater Monitoring
The location of the ash basin voluntary and compliance monitoring wells, the approximate ash
basin waste boundary, and the compliance boundary are shown in Figure 2-7. The compliance
boundary for groundwater quality at the RBSS ash basin site is defined in accordance with Title
15A NCAC 02L .0107(a) as being established at either 500 feet from the waste boundary or at
the property boundary, whichever is closer to the waste.
12

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
2.0 SITE HISTORY AND DESCRIPTION

2.10.1 Voluntary Groundwater Monitoring Wells
Monitoring wells MW-1S, MW-1D, MW-2S, MW-2D, MW-3S, MW-3D, MW-4S, MW-4D, MW-5S,
MW-5D, MW-6S, and MW-6D were installed by Duke Energy in 2006 as part of a voluntary
monitoring system. Duke Energy implemented enhanced voluntary groundwater monitoring
around the RBSS ash basin from December 2008 until June 2010. During this period, the
voluntary groundwater monitoring wells were sampled two times per year and the analytical
results were submitted to NCDENR DWR. Samples have been collected from monitoring wells
MW-4S, MW-4D, MW-5S, and MW-5D since February 2013 as part of groundwater assessment
efforts. No samples are currently being collected from the other voluntary wells. The voluntary
wells are shown on Figure 2-7.
2.10.2 Compliance Groundwater Monitoring Wells
Groundwater monitoring as required by the RBSS NPDES Permit NC0004961 began in March
2011. NPDES Permit Condition A (11), Version 1.1, dated June 15, 2011, lists the groundwater
monitoring wells to be sampled, the parameters and constituents to be measured and analyzed,
and the requirements for sampling frequency and reporting results (provided in Table 2-1).
Locations for the compliance groundwater monitoring wells were approved by the NCDENR
DWR Aquifer Protection Section (APS).
The compliance groundwater monitoring system for the ash basin consists of the following
monitoring wells: MW-7SR, MW-7D, MW-8S, MW-8I, MW-8D, MW-9, MW-10, MW-11SR, MW-
11DR, MW-13, MW-14, and MW-15. The compliance wells were installed in October,
November, and December 2010, and February 2011. All compliance monitoring wells listed in
Table 2-1 are sampled three times per year (in February, June, and October). Analytical results
for the constituents listed in Table 2-1 are submitted to the NCDENR DWR before the last day
of the month following the month of sampling for all compliance monitoring wells. The
compliance groundwater monitoring is performed in addition to the current NPDES monitoring of
the discharge flows from the ash basin.
Due to the proximity of Mountain Island Lake and associated wetland areas, monitoring wells
MW-9, MW-10, and MW-13 were installed inside the 500-foot compliance boundary. These
monitoring wells are sampled three times per year, compliance with 2L Standards had been
determined using predictive calculations or a groundwater model to demonstrate compliance.
Per the electronic correspondence from Eric Smith of NCDENR to Duke Energy on March 9,
2015, these wells have been removed from the groundwater assessment plan. However, Duke
Energy is still required to submit groundwater quality data for these wells.
The Supplemental Groundwater Monitoring Report, Riverbend Steam Station Ash Basin,
NPDES Permit NC0004961 (Altamont Environmental, Inc. 2012) and the 2012 Supplemental
Groundwater Monitoring Report, Riverbend Steam Station Ash Basin, NPDES Permit
NC0004961 (HDR 2013) were submitted to NCDENR with results of the groundwater modeling
for monitoring wells MW-9, MW-10, and MW-13.
13

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
2.0 SITE HISTORY AND DESCRIPTION

Monitoring wells MW-7SR and MW-7D are considered by Duke Energy to represent background
water quality. Monitoring wells MW-8S, MW-8I, and MW-8D are located to the south of an ash
storage area and to the north of Horseshoe Bend Beach Road. Monitoring well MW-9 is located
to the north of a cinder storage area. MW-10 is located downgradient of the Primary Cell.
Monitoring wells MW-11SR and MW-11DR are located northwest of the dam dividing the
Primary Cell and the Secondary Cell. Monitoring wells MW-13, MW-14, and MW-15 are located
downgradient of the Secondary Cell. With the exception of monitoring wells MW-9, MW-10, and
MW-13, the ash basin compliance monitoring wells were installed at or near the compliance
boundary.
From December 2010 through June 2015, the compliance groundwater monitoring wells at the
RBSS site have been sampled a total of 15 times. During this period, these monitoring wells
were sampled in:
• December 2010
• February, June, and October 2011
• February, June, and October 2012
• February, June, and October 2013
• February, June, and October 2014
• February, June 2015

One or more groundwater quality standards (2L Standards) have been exceeded in
groundwater samples collected at every compliance monitoring well. Exceedances have
occurred in one or more wells during one or more sampling events for chromium, iron,
manganese, and pH. An exceedance of the interim maximum allowable concentration (IMAC)
groundwater quality standard for antimony has also been measured at MW-7D. Table 2-2
presents exceedances measured from March 2011 through June 2015.
2.11 Assessment Activities or Previous Site Investigations
Several historical site investigations have been conducted onsite due to fuel oil releases
associated with the piping and aboveground storage tank (AST) associated with the former
combustion turbine system at the site. A summary of historical environmental incidents are
provided in Table 2-3.
In a letter dated March 16, 2012, the NCDENR DWQ Aquifer Protection Section (APS)
requested that Duke Energy begin additional assessment activities at stations where measured
and modeled concentrations of groundwater constituents exceed the 2L Standards at the
compliance boundary. Duke Energy submitted the report Groundwater Assessment, Duke
Energy Carolinas, LLC, Riverbend Steam Station Ash Basin, NPDES Permit NC0004961 (HDR
2013) to address this request by NCDENR for RBSS. Duke Energy received comments on the
May 31, 2013 report from NCDENR in a letter dated September 8, 2014. The NCDENR
recommendations and requests were generally incorporated during the development of the CSA
Work Plan.
14

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
2.0 SITE HISTORY AND DESCRIPTION

2.12 Decommissioning Status
Initial decommissioning activities at RBSS began in the fall of 2013 and involved demolition of
gas-fired combustion turbine generation units, coal-fired turbine units, water tank, precipitators,
and coal handling equipment, as well as initiation of asbestos removal. By mid-2014, asbestos
removal activities were completed, relocation of electrical equipment began, and auxiliary
buildings and structures were demolished. Ongoing decommissioning activities include removal
of remaining power plant equipment and material for salvage (planned to occur in late 2016),
demolition of the powerhouse and chimneys (planned to occur in late 2016), and restoration of
the RBSS plant site (planned to begin in late 2017).
In conjunction with decommissioning activities and in accordance with CAMA requirements,
Duke Energy will permanently close the RBSS ash ponds per the North Carolina CAMA-
required date associated with the risk ranking that the ash basins receive from the Coal Ash
Management Commission.
All necessary permits were received in May 2015 to commence removing ash by truck from the
ash storage area. Duke Energy is awaiting permits to commence dewatering and ash removal
from the primary and secondary ash basin cells.

15

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
3.0 SOURCE CHARACTERISTICS

3.0 Source Characteristics
This section provides a general description of the RBSS coal combustion and ash handling
system, a description of the ash basin and other ash storage areas, and provides a discussion
on the general physical and chemical properties of ash.
3.1 Coal Combustion and Ash Handling System
Coal ash is produced from the combustion of coal. The coal is dried, pulverized, and conveyed
to the burner area of a boiler. The smaller particles produced by coal combustion, referred to as
fly ash, are carried upward in the flue gas and are captured by an air pollution control device,
such as an electrostatic precipitator. The larger particles of ash that fall to the bottom of the
boiler are referred to as bottom ash.
Coal ash residue from the coal combustion process was sluiced to the RBSS ash basin from
approximately 1957 until the last coal-fired generating units were retired in April 2013. After
collection, both fly ash and bottom ash/boiler slag were sluiced to the ash basin using
conveyance water withdrawn from the Catawba River. During operation, RBSS produced
approximately 100,000 tons of ash per year. The sluice lines convey the water/ash slurry and
other flows to the southwest corner of the Primary Cell. Refer to Figure 2-4 for a depiction of
these features.
3.2 Description of Ash Basin and Other Ash Storage Areas
The ash at RBSS was originally stored in an area known as the cinder storage area. In 1957, a
single-cell ash basin was commissioned to serve as an effective treatment system for
wastewater containing coal ash. The ash basin was expanded in 1979 and divided by
constructing an intermediate (divider) dike to form two separate cells. These cells, known as the
Primary and Secondary Cells, were previously used to retain and settle ash generated from coal
combustion at RBSS. In addition, a cinder storage area and an ash storage area on the property
store ash from station operations prior to the construction of the ash basin as well as ash basin
clean-out projects respectively. Descriptions of these ash storage areas are provided in the
following sections. The ash basin and ash storage areas are shown on Figure 2-2. A 1948
USGS topographic map showing the site prior to the construction of the ash basin, cinder
storage area, and ash storage area is presented as Figure 2-3.
3.2.1 Ash Basin
The unlined ash basin is located approximately 2,400 feet to the northeast of the power plant,
adjacent to Mountain Island Lake, as shown on Figure 2-2. The Primary Cell is impounded by
an earthen embankment dam, referred to as Dam #1 (Primary), located on the west side of the
Primary Cell. The Secondary Cell is impounded by an earthen embankment dam, referred to as
Dam #2 (Secondary), located along the northeast side of the Secondary Cell. The toe areas for
both dams are in close proximity to Mountain Island Lake.
16

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
3.0 SOURCE CHARACTERISTICS

The intermediate (divider dike) constructed in 1979 was constructed of soil on top of the existing
ash in the basin. This construction provides hydraulic connection between the Primary Cell and
Secondary Cell. Borrow areas within the ash basin are depicted in some RBSS site drawings,
but are omitted from others. Based on information provided by Duke Energy, a dredge pond
was previously located south of the primary cell in the approximate location of the current ash
storage area, as described below in Section 3.2.2. Between 1993 and 2000, the dredged ash
was allowed to dry and was hauled offsite for reuse at least once.
The surface area of the Primary Cell is approximately 41 acres with an approximate maximum
pond elevation of 724 feet. The Primary Cell contains approximately 1.9 million cubic yards of
ash. The surface area of the Secondary Cell is approximately 28 acres with an approximate
maximum pond elevation of 714 feet. The Secondary Cell contains approximately 700,000 cubic
yards of ash. The full pond elevation of Mountain Island Lake is approximately 646.8 feet.
During operation of the coal-fired units, the ash basin system was operated as an integral part
of the site’s wastewater treatment system. This system predominantly received inflows from the
ash removal system, station yard drain sump, and stormwater flows. During station operations,
inflows to the ash basin were highly variable due to the cyclical nature of station operations. The
inflows from the ash removal system and the station yard drain sump are conveyed through
sluice lines into the Primary Cell. Discharge from the Primary Cell to the Secondary Cell is
conveyed through a concrete discharge tower located near the divider dike.
Although the RBSS station is retired, wastewater effluent from other non-ash-related station
flows to the ash basin and may be conveyed from the Secondary Cell, through a concrete
discharge tower, to Mountain Island Lake. The concrete discharge tower drains through a 30-
inch-diameter corrugated metal pipe into a concrete-lined channel that conveys to Mountain
Island Lake. The ash basin pond elevation is controlled by the use of concrete stop logs,
although no discharge has occurred from the basin in over one year.
Duke Energy is in the process of evaluating alternatives for removing these flows to the ash
basin to allow total decommissioning of the ash basin.
3.2.2 Ash Storage Area
An unlined ash storage area is located topographically cross-gradient/upgradient and adjacent
to the southwest side of the Primary Cell (Figure 2-2). The footprint is approximately 29 acres
and is estimated to contain approximately 1.5 million tons of ash. The ash storage area was
constructed during two ash basin clean-out projects: one which occurred around 2000-2001 and
another which occurred from late 2006 to early 2008. The clean-out projects were performed to
provide additional capacity in the ash basins for future sluiced ash.
The storage area currently has a 1.5- to 2-foot soil cover and vegetation cover that has been
maintained following the completion of ash placement. For the purpose of water management,
the stormwater runoff from the ash storage area is routed to the cinder storage area.
As required by CAMA and per Duke Energy’s November 13, 2014 proposed Coal Ash
Excavation Plan for RBSS, Duke Energy will permanently close the RBSS ash ponds by August
17

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
3.0 SOURCE CHARACTERISTICS

1, 2019. The first phase this plan will include the excavation and removal of approximately 1.0
million tons of ash from the storage area. Subsequent phase(s) of excavation will remove the
remaining ash in the ash storage area of the site. Ash removed from the site will be transported
by the contractor to facilities permitted for this type of waste.
3.2.3 Cinder Storage Area
The unlined cinder storage area is located topographically cross-gradient and immediately
west/southwest of the Primary Cell, and northwest of the ash storage area (Figure 2-2). The
footprint is approximately 13 acres and is located in a triangular area northeast of the coal pile
and northwest of the rail spur. Following initial station operation in 1929 and prior to initial ash
basin operation in 1957, bottom ash (cinders) generated as part of the coal combustion process
was deposited in the cinder storage area and other areas near the cinder storage area and coal
pile. This area was also utilized for storage of ash material at the station prior to the installation
of precipitators and a wet sluicing system around 1958. The storage area is estimated to contain
approximately 300,000 tons of ash. Per Duke Energy’s November 13, 2014 proposed Coal Ash
Excavation Plan for RBSS, the ash contained within the cinder storage area will be removed.
3.3 Physical Properties of Ash
Ash in the RBSS ash basin consists of fly ash and bottom ash produced from the combustion of
coal. The physical and chemical properties of coal ash result from reactions that occur during
the combustion of the coal and subsequent cooling of the flue gas. In general, coal is dried,
pulverized, and conveyed to the burner area of a boiler for combustion. As described in Section
3.1, material that forms larger particles of ash and falls to the bottom of the boiler is referred to
as bottom ash. Smaller particles of ash, known as fly ash, are carried upward in the flue gas and
are captured by an air pollution control device.
Approximately 70 to 80 percent of the ash produced during coal combustion is fly ash (EPRI
1993). Typically 65 to 90 percent of fly ash has particle sizes that are less than 0.010 millimeter
(mm). In general, fly ash has a grain size distribution similar to that of silt. The remaining 20 to
30 percent of ash produced is considered to be bottom ash. Bottom ash consists of angular
particles with a porous surface and is normally gray to black in color. Bottom ash particle
diameters can vary from approximately 0.05 to 38 to mm. In general, bottom ash has a grain
size distribution similar to that of fine gravel to medium sand (EPRI 1995).
Based on published literature not specific to the site, the specific gravities of fly ash typically
range from 2.1 to 2.9 and the specific gravity of bottom ash typically ranges from 2.3 to 3.0. The
permeability of fly ash and bottom ash vary based on material density, but would be within the
range of a sand-gravel with a similar gradation, grain-size distribution and density (EPRI 1995).
Hydraulic and physical properties of ash within the RBSS ash basin are presented later in this
report.
18

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
3.0 SOURCE CHARACTERISTICS

3.4 Chemical Properties of Ash
In general, the specific mineralogy of coal ash varies based on many factors including the
chemical composition of the coal, which is directly related to the geographic region where the
coal was mined, the type of boiler where the combustion occurs (i.e., thermodynamics of the
boiler), and air pollution control technologies employed.
The overall chemical composition of coal ash resembles that of siliceous rocks from which it
was derived, particularly shale. Oxides of silicon, aluminum, iron, and calcium make up more
than 90 percent of most siliceous rocks, soils, fly ash, and bottom ash. Other major and minor
elements (sulfur, sodium, potassium, magnesium, titanium) make up an additional 8 percent,
while trace constituents account for less than 1 percent. The following constituents are
considered to be trace elements: arsenic, barium, cadmium, chromium, lead, mercury,
selenium, copper, manganese, nickel, lead, vanadium, and zinc (EPRI 2010).
According to Duke Energy the primary source of coal burned at RBSS was bituminous coal from
Eastern Kentucky.
The majority of fly ash particles are glassy spheres mainly composed of amorphous or glassy
aluminosilicates, crystalline matter, and carbon. Figure 3-1 presents a photograph of ash
collected from the ash basin at Duke Energy’s Cliffside Steam Station showing a mix of fly ash
and bottom ash at 10 µm and 20 µm magnifications. The glassy spheres can be observed in the
photograph. The glassy spheres themselves are generally resistant to dissolution. During the
later stages of the combustion process and as the combustion gases are cooling after exiting
the boiler, molecules from the combustion process condense on the surface of the glassy
spheres. These surface condensates consist of soluble salts (e.g. calcium (Ca2+), sulfate (SO42-
), metals (copper (Cu), zinc (Zn), and other minor elements (e.g. boron (B), selenium (Se), and
arsenic (As)) (EPRI 1994).
The major elemental composition of fly ash (approximately 95 percent by weight) is composed
of mineral oxides of silicon, aluminum, iron, calcium. Oxides of magnesium, potassium, titanium
and sulfur comprise approximately 4 percent by weight (EPRI 1995). Trace elemental
composition typically is approximately 1 percent by weight and may include arsenic, antimony,
barium, boron, cadmium, chromium, copper, manganese, mercury, nickel, lead, selenium,
silver, thallium, zinc, and other elements. For comparison, Figure 3-2 shows the elemental
composition of fly ash and bottom ash compared with typical values for shale and volcanic ash.
Table 3-1 shows the bulk composition of fly ash and bottom ash compared with typical values
for soil and rock. In addition to these constituents, fly ash may contain unburned carbon.
Bituminous coal ash typically yields slightly acidic to alkaline solutions with pH levels ranging
from approximately 5 to 10 on contact with water. As noted in Table 3-1, aluminum, silicon,
calcium, and iron represent the larger fractions of fly ash by weight.
The geochemical factors controlling the reactions associated with leaching of ash are complex.
Factors such as the chemical speciation of the constituent, solution pH, solution-to-solid ratio,
and other factors control the chemical concentration of the resultant solution. Constituents that
are held on the glassy surfaces of fly ash such as boron, arsenic, and selenium may initially
19

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
3.0 SOURCE CHARACTERISTICS

leach more readily than other constituents. As noted in Table 3-1, aluminum, silicon, calcium,
and iron represent the larger fractions of fly ash and bottom ash by weight. The presence of
calcium may limit the release of arsenic by forming calcium-arsenic precipitates. Formation of
iron hydroxide compounds may also sequester arsenic and retard or prevent release of arsenic
to the environment. Similar processes and reactions may affect other constituents of concern;
however, certain constituents such as boron and sulfate will likely remain highly mobile.
In addition to the variability that might be seen in the mineralogical composition of the ash,
which is based on different coal types, different age of ash in the basin, and other factors, it is
anticipated that the chemical environment of the ash basin varies over time, distance, and
depth.
EPRI (2010) reported that 64 samples of coal combustion residuals (including fly ash, bottom
ash, and flue gas desulfurization residue) from 50 different power plants were subjected to EPA
Method 1311 Toxicity Characteristic Leaching Procedure (TCLP) leaching and no TCLP result
exceeded the TCLP hazardous waste limit. Figure 3-3 provides the results of that testing. The
report also presents the trace element concentrations for fly ash and bottom ash compared to
EPA Residential Soil Screening Levels (RSLs) for ingestion and dermal exposure. Figure 3-4
shows the 10th to 90th percentile range for trace element concentrations (mg/kg) in fly ash and
the associated EPA RSLs. The trace element concentrations for arsenic were greater than the
RSL for arsenic. The RSLs of the remaining constituents were greater than or within the 10th to
90th percentile range for their trace element concentrations.
Figure 3-5 shows similar data for bottom ash. As with fly ash, the trace element concentrations
for arsenic in bottom ash were greater than the RSL for arsenic. The RSL for chromium was
within the 10th to 90th percentile range of concentrations for chromium in bottom ash. The 10th
and 90th percentile range for the remaining constituents were below their respective RSLs.
Site-specific ash data are discussed in Section 7.0 of this report.

20

This page is intentionally left blank.
Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
4.0 RECEPTOR INFORMATION

4.0 Receptor Information
Section §130A-309.201(13) of the CAMA defines receptor as “any human, plant, animal, or
structure which is, or has the potential to be, affected by the release or migration of
contaminants. Any well constructed for the purpose of monitoring groundwater and contaminant
concentrations shall not be considered a receptor.” In accordance with the NORR CSA
guidance, receptors cited in this section refer to public and private water supply wells (including
irrigation wells and unused wells) and surface water features. Refer to Section 12.0 for a
discussion of receptors that were evaluated as part of this CSA effort.
The NORR CSA receptor survey guidance requirements include listing and depicting all water
supply wells, public or private, including irrigation wells and unused wells (other than those that
have been properly abandoned in accordance with 15A NCAC 2C .0100) within a minimum of
1,500 feet of the known extent of contamination. In NCDENR’s June 2015 response to Duke
Energy’s proposed adjustments to the CSA guidelines, NCDENR DWR acknowledged the
difficulty with determining the known extent of contamination at this time and stated that they
expected all drinking water wells located 2,640 feet (0.5-miles) downgradient from the
established compliance boundary to be documented in the CSA reports as specified in the
CAMA requirements. The approach to the receptor survey in this CSA includes listing and
depicting all water supply wells (public or private, including irrigation wells and unused wells)
within a 0.5-mile radius of the ash basin compliance boundary.
Note that the NORR CSA guidance requires that subsurface utilities be mapped within 1,500
feet of the known extent of contamination in order to evaluate the potential for preferential
pathways. Drawings of underground utilities were not readily available for review. The flow of
groundwater from the ash basin, ash storage area, and cinder storage area is to Mountain
Island Lake. Therefore, the mapping of underground features that serve as potential preferential
pathways was limited to underground piping and drains located between the ash basin waste
boundary and Mountain Island Lake depicted in Figures 4-3 and 4-4.
4.1 Summary of Previous Receptor Survey Activities
Duke Energy completed and submitted a receptor survey to NCDENR (HDR 2014a) in
September 2014, and subsequently submitted to NCDENR a supplement to the receptor survey
(HDR 2014b) in November 2014. The purpose of the receptor survey was to identify the
potential exposure locations that are critical to be considered in the groundwater transport
modeling and human health risk assessment. The supplementary information was obtained
from responses to water supply well survey questionnaires mailed to property owners within a
0.5-mile (2,640-foot) radius of the RBSS ash basin compliance boundary requesting information
on the presence of water supply wells and well usage.
The survey activities included contacting and/or reviewing the following agencies/records to
identify public and private water supply sources, confirm the location of wells, and/or identify any
wellhead protection areas located within a 0.5-mile radius of the RBSS ash basin compliance
boundary:
21

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
4.0 RECEPTOR INFORMATION

• NCDENR Division of Water Resources (DWR) Public Water Supply Section’s (PWSS)
most current Public Water Supply Water Sources GIS point data set;
• NCDENR DWR Source Water Assessment Program (SWAP) online database for public
water supply sources;
• Environmental Data Resources (EDR) local/regional water agency records review;
• Mecklenburg County’s Groundwater and Wastewater Services Well Information System
online database;
• Gaston County Environmental Health Department;
• Charlotte-Mecklenburg Utilities Department (CMUD);
• Mount Holly Public Utilities Department; and
• USGS National Hydrography Dataset.

In addition, a field reconnaissance was performed on January 27, 2014, to identify public and
private water supply wells (including irrigation wells and unused or abandoned wells) and
surface water features located within a 0.5-mile radius of the RBSS ash basin compliance
boundary. A windshield survey was conducted from public roadways to identify water meters,
fire hydrants, valves, and any potential well heads/well houses. Duke Energy site personnel
provided information regarding water supply wells located on Duke Energy property.
During the week of October 8, 2014, 328 water supply well survey questionnaires were mailed
to property owners within a 0.5-mile radius of the RBSS ash basin compliance boundary
requesting information on the presence of water supply wells and well usage information for
each property. The mailing list was compiled from a query of the parcel addresses included in
the Gaston and Mecklenburg counties’ GIS databases utilizing the 0.5-mile offset.
Between July 8 and July 23, 2015, the agencies/records listed above were contacted to provide
additional update information. Updated information is provided in Appendix B.
4.2 Summary of CSA Receptor Survey Activities and Findings
As part of this CSA report, the previously completed Receptor Survey activities were updated
based on the CSA Guidelines. The update included contacting and/or reviewing the
agencies/records to identify public and private water supply sources identified in Section 4.1 and
reviewing any questionnaires that were received after the submittal of the November 2014
supplement to the September 2014 receptor survey (i.e. questionnaires received after October
31, 2014).
A summary of the receptor survey findings is provided below. The identified water supply wells
are shown on the USGS receptor map on Figure 4-1 and on an aerial photograph on Figure 4-2.
Available property and well information for the identified water supply wells is provided in Table
4-1.
Underground features including underground piping and drains identified in the vicinity of the
Ash Basin are shown in Figure 4-3. Underground features including underground piping and
drains identified in the vicinity of the Cinder Storage and Ash Storage Areas are shown in figure
22

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
4.0 RECEPTOR INFORMATION

4-4. The dams associated with the ash basin contain engineered drainage features associated
with dam drainage and stability. These features are internal or adjacent to the dams and are not
included in the underground features mapping. The underground piping and drains on the site
appear to convey water from various portions of the RBSS site toward Mountain Island Lake.
The underground piping or drains do not act as preferential pathways between the impacted
portions of the RBSS site to the identified water supply well on the north side of the lake.
Table 4-2 presents names and addresses of property owners contiguous to the ash basin waste
boundary which corresponds to the parcels depicted on Figure 4-6.
• One reported private water supply well is located at a residence located northeast of
RBSS within a 0.5-mile radius of the ash basin compliance boundary. This well is
located across Mountain Island Lake in Mecklenburg County (Well 1).
• No public water supply wells (including irrigation wells and unused wells) were identified
within a 0.5-mile radius of the RBSS ash basin compliance boundary.
• According to Duke Energy, the two private water supply wells and one public water
supply well previously identified on the RBSS property were properly abandoned in June
2015.
• No wellhead protection areas were identified within a 0.5-mile radius of the ash basin
compliance boundary.
• Several surface water features that flow toward Mountain Island Lake were identified
within a 0.5-mile radius of the ash basin (Figure 4-5).

Based on the returned water supply well questionnaires since October 31, 2014, no additional
receptors were identified. Further details of HDR’s receptor survey activities and findings are
presented in Appendix B.
4.3 NCDENR Well Water Testing Program
Section § 130A-309.209 (c) of the CAMA requires the owner of a coal combustion residuals
surface impoundment to conduct a Drinking Water Supply Well Survey that identifies all drinking
water supply wells within one-half mile down-gradient from the established compliance
boundary of the impoundment and submit the Survey to the Department. Since the direction of
groundwater flow had not been fully established at the sites, NCDENR required the sampling to
include all potential drinking water receptors within a half mile of the compliance boundaries in
all directions. Between February and July 2015, NCDENR arranged for independent analytical
laboratories to collect and analyze water samples obtained from private wells identified during
the Well Survey, if the owner agreed to have their well sampled. At the RBSS site, no private
wells within the half mile of the compliance boundary were sampled.

23

This page is intentionally left blank.
Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
5.0 REGIONAL GEOLOGY AND HYDROGEOLOGY

5.0 Regional Geology and Hydrogeology
5.1 Regional Geology
North Carolina is divided into three physiographic provinces: the Atlantic Coastal Plain,
Piedmont, and Blue Ridge (Fenneman 1938). The RBSS site is located in in the Piedmont
province. The Piedmont province is bounded to the east and southeast by the Atlantic Coastal
Plain and to the west by the escarpment of the Blue Ridge Mountains, with a width ranging from
150 miles to 225 miles in the Carolinas (LeGrand 2004).
The topography of the Piedmont region is characterized by low, rounded hills and long, rolling,
northeast-southwest trending ridges (Heath 1984). Stream valley to ridge relief in most areas
ranges from 75 feet to 200 feet. Along the Coastal Plain boundary, the Piedmont region rises
from an elevation of 300 feet above mean sea level, to the base of the Blue Ridge Mountains at
an elevation of 1,500 feet (LeGrand 2004).
The RBSS site is within the Charlotte terrane, one of a number of tectonostratigraphic terranes
that have been defined in the southern and central Appalachians and is in the western portion of
the larger Carolina superterrane (Figure 5-1; Horton et al. 1989; Hibbard et al. 2002; Hatcher et
al. 2007). On the northwest side, the Charlotte terrane is in contact with the Inner Piedmont
zone along the Central Piedmont suture along its northwest boundary and is distinguished from
the Carolina terrane to the southeast by its higher metamorphic grade and portions of the
boundary may be tectonic (Secor et al. 1998; Dennis et al. 2000).
The Charlotte terrane is dominated by complex sequence of plutonic rocks that intrude a suite of
metaigneous rocks (amphibolite metamorphic grade) including mafic gneisses, amphibolites,
metagabbros, and metavolcanic rocks with lesser amounts of granitic gneiss and ultramafic
rocks with minor metasedimentary rocks including phyllite, mica schist, biotite gneiss, and
quartzite with marble along its western portion (Butler and Secor 1991; Hibbard et al. 2002). The
general structure of the belt is primarily a function of plutonic contacts. A geologic map of the
area around the Riverbend Steam Station is shown in Figure 5-2.
The fractured bedrock is overlain by a mantle of unconsolidated material known as regolith. The
regolith includes residual soil and saprolite zones and, where present, alluvial deposits.
Saprolite, the product of chemical weathering of the underlying bedrock, is typically composed
of clay and coarser granular material and reflects the texture and structure of the rock from
which it was formed. The weathering products of granitic rocks are quartz-rich and sandy
textured. Rocks poor in quartz and rich in feldspar and ferro-magnesium minerals form a more
clayey saprolite.
5.2 Regional Hydrogeology
The groundwater system in the Piedmont Province, in most cases, is described as being
comprised of two interconnected layers, or two-medium system 1) residual soil/saprolite and
weathered fractured rock (regolith) overlying 2) fractured crystalline bedrock (Heath 1980;
24

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
5.0 REGIONAL GEOLOGY AND HYDROGEOLOGY

Harned and Daniel 1992; Figure 5-3). The regolith layer is a thoroughly weathered and
structureless residual soil that occurs near the ground surface with the degree of weathering
decreasing with depth. The residual soil grades into saprolite, a coarser grained material that
retains the structure of the parent bedrock. Beneath the saprolite, partially weathered/fractured
bedrock occurs with depth until sound bedrock is encountered. This mantle of residual soil,
saprolite, and weathered/fractured rock is a hydrogeologic unit that covers and crosses various
types of rock (LeGrand 1988). This regolith layer serves as the shallow unconfined groundwater
system and provides an intergranular medium through which the recharge and discharge of
water from the underlying fractured rock occurs. Within the fractured crystalline bedrock layer,
the fractures control both the hydraulic conductivity and storage capacity of the rock mass. A
transition zone (TZ) at the base of the regolith has been interpreted to be present in many areas
of the Piedmont. Harned and Daniel (1992) describe the TZ as consisting of partially
weathered/fractured bedrock and lesser amounts of saprolite that grades into bedrock and they
described the TZ as “being the most permeable part of the system, even slightly more
permeable than the soil zone”. Harned and Daniel (1992) suggested the TZ may serve as a
conduit of rapid flow and transmission of contaminated water.
Most of the information supporting the existence of the TZ, until recently, was qualitatively
based on observations made during the drilling of boreholes and water-wells, although some
quantitative data is available for the Piedmont region (Stewart 1964; Stewart and others 1964;
Nutter and Otton 1969; Harned and Daniel 1992). Schaeffer (2009; 2014a) using a database of
669 horizontal hydraulic conductivity measurements in boreholes at six locations in the Carolina
Piedmont statistically showed that a transition zone of higher hydraulic conductivity exists in the
Piedmont groundwater system when considered within Harned and Daniel’s (1992) two types of
bedrock conceptual framework.
The transition zone is described as being comprised of partially weathered rock, open, steeply
dipping fractures, and low angle stress relief fractures, either singly or in various combinations
below refusal (auger, roller cone, or casing advancer; Schaeffer 2011; 2014b). The TZ has less
advanced weathering relative to the regolith and generally the weathering has not progressed to
the development of clay minerals that would decrease the permeability of secondary porosity
developed during weathering, and new fractures develop during the weathering process, and /or
existing fractures are opened. The characteristics of the transition zone can vary widely based
on the interaction of rock type, degree of weathering, degree of systematic fracturing, presence
of stress-relief fracturing, and the general characteristics of the bedrock (foliated/layered,
massive, or in between). The transition zone is not a continuous layer between the regolith and
bedrock; it thins and thickens within short distances and is absent in places (Schaeffer 2011;
2014b). The absence, thinning, and thickening of the TZ is related to the characteristics of the
underlying bedrock (Schaeffer 2014b).
As previously mentioned, the TZ may vary due to different rock types and associated rock
structure. Harned and Daniel (1992) further divided the bedrock into two conceptual models: 1)
foliated/layered (metasedimentary and metavolcanic sequences); and 2) massive/plutonic
(plutonic and metaplutonic complexes) structures (Figure 5-4). Strongly foliated/layered rocks
are thought to have a well-developed transition zone due to the breakup and weathering along
25

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
5.0 REGIONAL GEOLOGY AND HYDROGEOLOGY

the foliation planes or layering, resulting in numerous rock fragments (Harned and Daniel 1992).
More massive rocks are thought to develop an indistinct transition zone because they do not
contain foliation/layering and tend to weather along relatively widely spaced fractures (Harned
and Daniel 1992). Schaeffer (2014a) proved Harned and Daniel’s (1992) hypothesis that
foliated/layered bedrock would have a better developed transition zone than plutonic/massive
bedrock. The foliated/layered bedrock transition zone has a statistically significant higher
hydraulic conductivity than the massive/plutonic bedrock transition zone (Schaeffer 2014a).
LeGrand’s (1988; 1989) conceptual model of the groundwater setting in the Piedmont
incorporates Daniel and Harned’s (1989) above two-medium system into an entity that is useful
for the description of groundwater conditions. That entity is the surface drainage basin that
contains a perennial stream (LeGrand 1988). Each basin is similar to adjacent basins and the
conditions are generally repetitive from basin to basin. Within a basin, movement of
groundwater is generally restricted to the area extending from the drainage divides to a
perennial stream (Slope-Aquifer System; Figure 5-5; LeGrand 1988; 1989; 2004). Rarely does
groundwater move beneath a perennial stream to another more distant stream or across
drainage divides (LeGrand 1989). The crests of the water table underneath topographic
drainage divides represent natural groundwater divides within the slope-aquifer system and may
limit the area of influence of wells or contaminant plumes located within their boundaries. The
concave topographic areas between the topographic divides may be considered as flow
compartments that are open-ended down slope.
Therefore, the groundwater system is a two-medium system restricted to the local drainage
basin. The groundwater occurs in a system composed of two interconnected layers: residual
soil/saprolite and weathered rock overlying fractured crystalline rock separated by the transition
zone. Typically, the residual soil/saprolite is partially saturated and the water table fluctuates
within it. Water movement is generally preferential through the weathered/fractured and
fractured bedrock of the TZ (i.e., enhanced permeability zone). The near-surface fractured
crystalline rocks can form extensive aquifers. The character of such aquifers results from the
combined effects of the rock type, fracture system, topography, and weathering. Topography
exerts an influence on both weathering and the opening of fractures, while the weathering of the
crystalline rock modifies both transmissive and storage characteristics.
The igneous and metamorphic bedrock in the Piedmont consist of interlocking crystals and
primary porosity is very low, generally less than 3 percent. Secondary porosity of crystalline
bedrock due to weathering and fractures ranges from 1 to 10 percent (Freeze and Cherry 1979);
but, porosity values of 1 to 3 percent are more typical (Daniel and Sharpless 1983). Daniel
(1990) reported that the porosity of the regolith ranges from 35 to 55 percent near land surface
but decreases with depth as the degree of weathering decreases.
Groundwater flow paths in the Piedmont are almost invariably restricted to the zone underlying
the topographic slope extending from a topographic divide to an adjacent stream. Under natural
conditions, the general direction of groundwater flow can be approximated from the surface
topography (LeGrand 2004).
26

This page is intentionally left blank.
Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
6.0 SITE GEOLOGY AND HYDROGEOLOGY

6.0 Site Geology and Hydrogeology
6.1 Site Geology
The RBSS site and its associated ash basin, ash storage area, and cinder storage area are
located in the Charlotte terrane. The Charlotte terrane consists of an igneous complex of
Neoproterozoic to Paleozoic ages (Hibbard et al. 2002) that range from intermediate to mafic in
composition (Butler and Secor 1999). The Charlotte terrane is bordered on the east and
southeast by the Carolina terrane and to the west and northwest by the Inner Piedmont (Cat
Square and Tugaloo terranes) and the Kings Mountain terrane. The structural contact of the
Inner Piedmont and Charlotte terrane is the Central Piedmont Shear Zone. A bedrock geologic
map displaying the site and boring locations is presented in Figure 6-1. The installed well and
sample locations are shown on Figure 6-2.
6.1.1 Soil Classification
The following soils/materials were encountered in the boreholes:
• Ash – Ash was encountered in borings advanced within the ash basin, ash storage area,
and cinder storage area, as well as in one boring at the ash storage area perimeter. Ash
was generally described as gray to dark bluish gray with a silty to sandy texture,
consistent with fly ash and bottom ash.
• Fill – Fill material generally consisted of re-worked sandy silts, clays, and sands that
were borrowed from one area of the site and re-distributed to other areas. Fill was
classified in the boring logs as sandy silt, clay with sand, clay, sandy clay, and clay with
gravel. Fill was used in the construction of dikes and as cover for the ash storage area.
• Alluvium – Alluvium is unconsolidated soil and sediment that has been eroded and
redeposited by streams and rivers. Alluvium may consist of a variety of materials ranging
from silts and clays to sands and gravels. During site construction and plant operation,
alluvial deposits have been removed or covered. Alluvium was encountered in borings
along Mountain Island Lake during the project subsurface exploration activities. Alluvium
was logged in GWA-5D and MW-15D. Designations between alluvium and fill are
approximate and were challenging to distinguish due to the similarities in material.
• Residuum (Residual soils) – Residuum is the in-place weathered soil that consists
primarily of sandy silt, clay, silt, clayey sand, sand with silt and gravel, and clay with
sand and gravel at the RBSS site. Residuum varied in thickness and was relatively thin
compared to the thickness of saprolite. Designations between residuum and fill are
approximate and were challenging to distinguish due to the similarities in material.
• Saprolite – Saprolite is soil developed by in-place weathering of rock that retains
remnant bedrock structure. The primary distinction from residuum is that saprolite
typically retains some structure (e.g., mineral banding) from the parent rock. Saprolite at
the RBSS site consists primarily of lean clay, sand with clay, silt with sand, sand, sandy
silt (variably micaceous), silty sand, silty sand with gravel, and silt with gravel. Sand
27

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
6.0 SITE GEOLOGY AND HYDROGEOLOGY

particle size ranges from fine to coarse grained. Saprolite in typically logged as
micaceous.

Undisturbed and split-spoon samples collected during the field investigation were tested for
various geotechnical parameters including natural moisture content, Atterberg Limits
(undisturbed samples only), grain size with hydrometer, total porosity (undisturbed samples
only), and vertical hydraulic conductivity (undisturbed samples only), and were classified using
the Unified Soil Classification System (USCS). Geotechnical index property testing was
completed for disturbed and undisturbed samples in accordance with ASTM standards. Thirty-
five undisturbed ('Shelby Tube') samples were submitted for geotechnical index testing. Index
property testing for undisturbed samples comprised USCS (ASTM D 2487), natural moisture
content (ASTM D 2216), Atterberg Limits (ASTM D 4318), grain size distribution, including sieve
analysis and hydrometer (ASTM D 422), total porosity calculated from Specific Gravity (ASTM D
854), and hydraulic conductivity (ASTM D 5084). Four undisturbed samples were not subjected
to the full suite of index property tests due to low recovery, wax and gravel mixed in the tube,
loose material, or damaged tubes. Twenty disturbed ('Split Spoon,' or 'Jar') samples were
analyzed for grain size distribution with hydrometer (ASTM D 422) and natural moisture content
(ASTM D 2216). The results are presented in Section 11.0.
6.1.2 Rock Lithology
Bedrock at the RBSS site consists of meta-quartz diorite and meta-diabase. Based on rock core
descriptions, the meta-quartz diorite color typically is a white to light gray matrix with dark
greenish gray, dark gray, and black phenocrysts. The texture is described as phaneritic, fine to
coarse grained, non-foliated and massive. Foliation is rarely noted. The meta-quartz diorite is
composed dominantly of plagioclase, quartz, biotite, hornblende, and epidote.
The meta-diabase is very dark to dark greenish gray black to very dark greenish gray, is mostly
non-foliated, and is mostly noted as aphanitic but a porphyritic texture with phenocrysts is noted
on some of the boring logs. Banding and foliation is noted in the meta-diabase and generally
parallel the dike contacts. Vugs and fractures with calcite mineralization are occasionally noted
in the meta-diabase. Meta-quartz diorite is the primary rock type underlying the site with the
meta-diabase occurring during a late syn-plutonic stage similar the relationships noted at the
Catawba Nuclear Station (CNS) located approximately 24 miles south of RBSS (Gilbert et al.
1982). Diorite xenoliths are noted in GWA 2BR and GWA 8D. Greenstone (meta-basalt)
xenoliths are present within the meta-quartz diorite suggest it was intruded into a mafic volcanic
sequence that is widespread in the Charlotte terrane (Goldsmith et al. 1988; Butler and Secor
1991). Figure 6-1 shows the extent of each unit in the near-site vicinity.
6.1.3 Structural Geology
The Charlotte terrane is a metaigneous terrane consisting of volcanic and plutonic rocks that
have been subjected to deformation and high grade metamorphism due to tectonic stress during
and after intrusion of the igneous units. Foliation is noted only in some of the rock core and is
not dominant with respect to the structure of the rock mass.
28

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
6.0 SITE GEOLOGY AND HYDROGEOLOGY

Data from the rock core also shows a number of joint dip angles that cannot be properly defined
as joint sets since there is no strike orientation information. For the purpose of this discussion,
the joints have been assessed based on dip angle alone. The most prevalent dip angle is from
10 to 20 degrees and from 30 to 40 degrees. These two sets are predominant based on the
number of joints noted on the boring logs. A steeply dipping set ranging from 70 to 80 degrees
is not noted as frequently the boring logs since a vertical borehole is less likely to intercept sub-
vertical joints. It is possible that the 70-80 degree dipping set is at least as prevalent as the
aforementioned sets. Less predominant joint sets dipping 50-60 degrees and 0-10 are also
noted on the boring logs. Iron and manganese staining is noted on joints of all the above sets
indicating that the joints are pathways for groundwater flow. The degree of openness (aperture)
of any of the joints is difficult to assess from rock core since the core is often broken at a joint
and no longer retains its actual in-place aperture. However, some of the logs describe some
joints as open. Geologic mapping was not successful in defining any joint sets in outcrop that
could help define the dip strike of any of the joint sets.
The orientation of the dikes at RBSS are probably similar to that noted at CNS (Gilbert et al.
1982), trending both northeast and northwest and with minor shearing along the contacts and a
secondary foliation developing within the dikes from the shearing.
6.1.4 Geologic Mapping
Geologic mapping was conducted in April 2015 to map outcrops at the site and within a 2-mile
radius of the site to identify rock types and utilizing a Brunton compass to attempt to
characterize the orientation (strike and dip) of structure such as foliation, joint sets, folds, and
shear zones. Due to limited outcrop locations, geologic mapping was not successful in defining
major rock structure. Figure 6-1 shows the locations where outcrop were mapped. The site
location and well locations are overlaid on the Geologic Map of the Charlotte 1° x 2°
Quadrangle, North Carolina and South Carolina (Goldsmith et al. 1988). Field mapping and use
of all of the borehole data confirm the geologic units and location of contacts of the units with no
evidence for differing geologic units or contact locations.
6.1.5 Fracture Trace Analysis
6.1.5.1 Introduction
Fracture trace analysis is a remote sensing technique used to identify lineaments on
topographic maps and aerial photography that may correlate to locations of bedrock fractures
exposed at the earth’s surface. Although fracture trace analysis is a useful tool for identifying
potential fracture locations, and hence potential preferential pathways for infiltration and flow of
groundwater near a site, results are not definitive. Lineaments identified as part of fracture trace
analysis may or may not correspond to actual locations of fractures exposed at the surface, and
if fractures are present, it cannot be determined from fracture trace analysis whether these are
open or healed. Healed fractures intruded by diabase are common in the vicinity of the site.
Strongly linear features at the earth’s surface are commonly formed by weathering along steeply
dipping to vertical fractures in bedrock. Morphological features such as narrow, sharp-crested
29

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
6.0 SITE GEOLOGY AND HYDROGEOLOGY

ridges, narrow linear valleys, linear escarpments, and linear segments of streams otherwise
characterized by dendritic patterns are examples. Linear variations in vegetative cover are also
sometimes indicative of the presence of exposed fractures, though in many cases these result
from unrelated human activity or other geological considerations (e.g., change in lithology).
Straight (as opposed to curvilinear) features are commonly associated with the presence of
steeply dipping fractures. Curvilinear features in some cases are associated with exposed,
moderately dipping fractures, but these also can be a result of preferential weathering along
lithologic contacts, or along foliation planes or other geologic structure. As part of this study,
only strongly linear features were considered, as these are far more commonly indicative of
steeply dipping or vertical fractures.
The effectiveness of fracture trace analysis in the eastern United States, including in the
Piedmont, is commonly hampered by the presence of dense vegetative cover, and oftentimes
extensive land-surface modification owing to present and past human activity. Aerial-
photography interpretation is most affected, as identification of small-scale features can be
rendered difficult or impossible in developed areas. Substantial surface alteration occurs over
an estimated 30 percent of the aerial-photography study area for the RBSS site.
6.1.5.2 Methods
Available geologic maps for the area were consulted prior to performance of aerial photography
and topographic map interpretation to identify lithologies and structures in the area, and likely
fracture orientations. Both low-altitude aerial photography provided by Duke Energy (from WSP
Global, Inc,) covering approximately 4 square miles, and USGS 1:24000 scale topographic
maps covering an area of approximately 20 square miles were examined.
Maps examined included portions of the Mountain Island Lake, N.C. and Lake Norman South
N.C. USGS 7.5’ (1:24,000 Scale) topographic quadrangles. Digital copies of the quadrangles
were obtained and viewed on a monitor at up to 7x magnification. Lineaments identified were
plotted directly on the digital images. Lineaments identified from topographic maps and
lineament trends indicated by a rose diagram are included on Figure 6-3.
Photography provided for review included 1”=600’ scale, 9 x 9 inch black-and-white (grayscale)
contact prints dated April 17, 2014. Stereo coverage was complete across the area shown on
Figure 6-4. The photography was examined using a Lietz Sokkia MS-27 mirror stereoscope with
magnifying binocular eyepiece. Lineaments identified on the photographs were marked on hard
copies of scanned images (600dpi resolution), and subsequently compiled onto a photomosaic
base.
Rose diagrams were prepared for lineament trends identified from both aerial photography and
topographic map interpretation, and are included as inserts on the respective figures.
6.1.5.3 Results
A total of 17 well-defined lineaments were identified. Trends are predominantly toward the
northeast in accordance with the predominant structural trend in polydeformed rocks beneath
30

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
6.0 SITE GEOLOGY AND HYDROGEOLOGY

the study area. Less pervasive northwest-trending lineaments are also relatively common.
Fractures in the area may have formed as a result of non-ductile deformation postdating the
peak of metamorphism, as lineaments suggestive of fracture traces are present in an
Ordovician to Devonian granitoid body that intrudes older kyanite to sillimanite grade rocks in
the southwest part of the study area.
Lineaments identified from aerial photography are shown and lineament trends indicated by a
rose diagram are included on Figure 6-4. A total of nine lineaments were identified from aerial
photography interpretation. Three of these correspond to lineaments identified from topographic
map interpretation as shown on Figure 6-3. The remainder are small-scale features not visible
at the scale of the topographic maps. Similar to the lineaments identified on topographic maps,
the dominant trend is toward the northeast, and to a lesser extent toward the northwest.
6.1.6 Effects of Structure on Groundwater Flow
The most important effects of structural geology on groundwater flow are the contacts of the
meta-diabase and the meta-quartz diorite, and the likely interconnected joint sets discussed in
Sections 6.1.3 and 6.1.4. Since it is difficult to define joint sets based on dip angle alone, it is
also difficult to define which joints are most relevant with respect to groundwater flow.
6.1.7 Soil and Rock Mineralogy and Chemistry
Soil and rock mineralogy and chemistry analyses are incomplete as of the date of this report.
Soil and mineralogy and chemistry results through July 31, 2015 are shown in Table 6-1
(mineralogy), Table 6-2 (chemistry, % oxides), and Table 6-3 (chemistry, elemental
composition). The mineralogy and chemical composition of TZ materials are presented in Table
6-4 (mineralogy) and Table 6-5 (chemistry). Whole rock chemistry results (% oxides and
elemental composition) are shown in Tables 6-6 and 6-7, respectively. Petrographic analysis of
rock (thin-sections) and the remaining soil mineralogy and chemistry analyses will be included in
the CSA supplement.
The dominant mineral constitutes in the soils are quartz, feldspar (both alkali and plagioclase
feldspars), kaolinite, and illite. Soils exhibiting a higher degree of weathering show an increase
in kaolinite and illite. Other minerals identified include chlorite, biotite, muscovite, and
amphibole. One sample (GWA-2S: RB-05) had a relatively high amount of smectites (16.0%).
The major oxides in the soils are SiO2 (45.76% - 77.80%), Al2O3 (2.77% - 28.07%), and Fe2O3
(3.53% - 14.60%). MnO range from 0.03% to 0.12%). Major TZ minerals are quartz, feldspar,
muscovite/vermiculite/illite, kaolinite, chlorite, and smectite. The major oxides are SiO2 (49.8% -
68.5%), Al2O3 (17.2% - 21.8%), and Fe2O3 (3.4% - 13.2%). The major oxides in the rock
samples are SiO2 (49.55% - 72.14%), Al2O3 (15.24% - 21.62%), and Fe2O3 (2.63% - 10.31%).
31

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
6.0 SITE GEOLOGY AND HYDROGEOLOGY

6.2 Site Hydrogeology
6.2.1 Groundwater Flow Direction
Based on the CSA site investigation, the groundwater system in the natural materials (alluvium,
soil, soil/saprolite, and bedrock) at RBSS is consistent with the regolith-fractured rock system
and is an unconfined, connected system without confining layers. However, the hydraulic
conductivity data collected during the investigation and discussed in Section 11.2 indicates that
a distinct transition zone of higher permeability does not exist at the site. This is consistent with
Harned and Daniel’s (1992) concept of the two types of rock structure (foliated/layered and
massive) in the Piedmont province discussed in Section 5.2. The RBSS is underlain by a
relatively massive meta-plutonic complex of the type that they believe may develop an indistinct
transition zone. The groundwater system at RBSS is a two-layer system: shallow (regolith) and
bedrock.
Accessible voluntary, compliance, and ash basin assessment monitoring wells were gauged for
depth to water and total well depth during a groundwater elevation measurement event on July
8, 9, and 10, 2015. Depth to water measurements were subtracted from surveyed top of well
casing elevations to produce groundwater elevations in shallow, deep, and bedrock monitoring
wells. Groundwater flow direction was estimated by contouring these groundwater elevations.
Two observation wells (OB-1 and OB-2) were installed across the water table and were used for
measuring water levels only (no water quality samples) in conjunction with monitoring well
GWA-7S to characterize groundwater flow in the region between the ash basin and the
background monitoring wells MW-7SR/D. Note that monitoring well GWA-7S was installed to
replace the proposed observation well OB-3, which was included in Duke Energy’s NPDES
permit renewal application dated May 15, 2014.
In general, groundwater at the site flows to the north, east, and west and discharges to the
Catawba River. Groundwater in the southwest portion of the site under the ash storage area
flows to the northwest, under the cinder storage area to the Catawba River. Flow contours
developed from groundwater elevations measured in the shallow and deep wells in the
southeastern portion of the site depict groundwater flow generally to the northeast to the
Catawba River. Groundwater contours developed from the groundwater elevations in the
bedrock wells show groundwater moving generally in a north/northwesterly direction from the
south side of the site to the Catawba River.
Shallow groundwater flow direction is shown on Figure 6-5. Groundwater flow within the deep
wells is shown on Figure 6-6. Groundwater flow within fractured bedrock is shown on Figure 6-
7. The relative position of the groundwater monitoring wells (upgradient / downgradient) in
relation to the ash basins or storage area are presented in Table 6-8 for voluntary and
compliance wells and Table 6-9 for the newly installed groundwater assessment wells.
32

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
6.0 SITE GEOLOGY AND HYDROGEOLOGY

6.2.2 Hydraulic Gradient
Horizontal hydraulic gradients were derived for the shallow, TZ, and fractured bedrock wells 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). The following equation was used to calculate horizontal hydraulic gradient:
i = dh / dl
where i is the hydraulic gradient; dh is the difference between two
hydraulic heads; and dl is the flow path length between the two
wells.

Applying this equation to wells installed during the CSA investigation yields the following
average horizontal hydraulic gradients (measured in feet/foot):
• S wells: 0.032
• D wells: 0.028
• BR wells: 0.032

A summary of hydraulic gradient calculations is presented in Table 6-10. Note that vertical
hydraulic gradients are discussed in Section 11.3.
6.2.3 Effects of Geologic/Hydrogeologic Characteristics on Contaminants
Migration, retardation, and attenuation of COIs in the subsurface is a factor of both physical and
chemical properties of the media in which the groundwater passes. Soil samples were collected
and analyzed for grain size, total porosity, soil sorption (Kd), and anions / cations to provide
data necessary for completion of the three-dimensional groundwater model discussed in
Section 13.0. As previously agreed upon with NCDENR, the results of the groundwater model
will be presented in the CAP.
6.2.4 Hydrogeologic Site Conceptual Site Model
The hydrogeologic site conceptual model (referred to as the site conceptual model, or SCM) is a
conceptual interpretation of the processes and characteristics of a site with respect to the
groundwater flow and other hydrologic processes at the site. The May 31, 2007 NCDENR
document, “Hydrogeologic Investigation and Reporting Policy Memorandum” was used as
general guidance to developing the model. General components of the SCM consist of
developing and describing the following aspects of the site: geologic/soil framework, hydrologic
framework, and the hydraulic properties of site materials. More specifically, the SCM describes
how these aspects of the site affect the groundwater flow at the site. In addition to these site-
specific aspects, the SCM:
33

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
6.0 SITE GEOLOGY AND HYDROGEOLOGY

• Describes the regional and site geology and hydrogeology (Sections 5.0, 5.1, 6.1 and
6.2);
• Presents longitudinal and transverse cross-sections showing the hydrostratigraphic
layers (Section 8.2);
• Develops the hydrostratigraphic layer properties required for the groundwater model
(Section 11.2);
• Presents a groundwater contour map showing the potentiometric surface of the shallow
aquifer, (Section 6.2.1); and
• Presents information on horizontal (Section 6.2.2) and vertical groundwater gradients
(Section 11.3).

The SCM serves as the basis for developing and understanding the hydrogeologic
characteristics of the site and for developing a groundwater flow and transport model. Historic
site groundwater elevations and ash basin water elevations were used to develop a historic
estimated seasonal high groundwater contour map for the site. A fracture trace analysis
(described previously in Section 6.1.5) was also performed for the RBSS site, as well as on-
site/near-site geologic mapping, to better understand site geology and to confirm and support
the SCM.
As anticipated in the initial site conceptual hydrogeologic model presented in the Work Plan, the
geological and hydrogeological features influencing the movement, chemical, and physical
characteristics of contaminants are related to the Piedmont hydrogeologic system present at the
site. The CSA found that the direction of the movement of the contaminants is toward the
Catawba River, as anticipated.
Based on the CSA site investigation, the groundwater system in the natural materials (alluvium,
soil, soil/saprolite, and bedrock) at RBSS is consistent with the regolith-fractured rock system
and is an unconfined, connected system without confining layers. However, the hydraulic
conductivity data collected during this study and classified as above into the various
hydrostratigraphic units indicates that a distinct TZ of higher permeability does not exist at the
site. The horizontal hydraulic conductivity of the TZ is not higher than that of the overlying M2
unit and the horizontal hydraulic conductivity at RBSS increases with depth; a TZ as defined
does not exist.
Further development of the hydrostratigraphic units taking into account the lack of a TZ, the
hydrostratigraphic layer parameters, and other parameters required for the flow and
contaminant transport model will be provided in the CAP. An updated SCM, based on data
obtained during the CSA activities and refined through completion of groundwater modeling, will
be presented in the CAP.

34

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
7.0 SOURCE CHARACTERIZATION

7.0 Source Characterization
For purposes of this assessment the source area is defined by the ash waste boundary as
depicted on Figure 2-2. For the RBSS site, sources include the ash basin, ash storage area,
and cinder storage area. Source characterization was performed to identify the physical and
chemical properties of the ash in the source area. The source characterization involved
developing selected physical properties of ash, identifying the constituents found in ash,
measuring concentrations of constituents present in the ash porewater, and performing
laboratory analyses to estimate constituent concentrations resulting from the leaching process.
The physical and chemical properties evaluated as part of this characterization will be used to
better understand impacts to soil and groundwater from the source area and will also be utilized
as part of groundwater model development in the CAP.
At the RBSS site, source characterization was performed through the completion of soil borings,
installation of monitoring wells, and collection and analysis of associated solid matrix and
aqueous samples. Ash samples were collected for analysis of physical characteristics (e.g.,
grain size, porosity, etc.) to provide data for evaluation of retention/transport properties within
and beneath the ash basin Primary and Secondary Cells, ash storage area and cinder storage
area. Ash samples were collected for analysis of chemical characteristics (e.g., total inorganics,
leaching potential, etc.) to provide data for evaluation of constituent concentrations and
distribution. Samples were collected in general accordance with the Work Plan. Sampling
variances are documented in Appendix F. For the purpose of this CSA report and for use
throughout this section, the term COI is used to refer to any constituent or parameter that
exceeded its applicable regulatory standard or criteria.
Ash, ash basin water, porewater, and seep sample locations used for source characterization
are shown on Figure 7-1. Porewater refers to water samples collected from wells installed within
the ash basin Primary Cell and Secondary Cell, ash storage area, or cinder storage area and
screened in the ash layer. HDR does not consider these results to be representative of
groundwater.
A summary of constituents and laboratory methods used for analysis of solid matrix (soil, rock,
and ash) samples is presented in Table 7-1. Laboratory results of total inorganic and
anion/cation analyses of ash samples are presented in Table 7-2.
A summary of laboratory results for aqueous matrix (groundwater, surface water, and seeps)
parameters and analytical methods is presented in Table 7-3. Laboratory results of ash basin
surface water samples are presented in Table 7-4. Ash basin porewater sample results are
presented in Table 7-5. Speciation results for ash basin porewater are presented in Table 7-6.
Results from Synthetic Potential Leaching Procedure (SPLP) analyses of ash samples are
presented in Table 7-7. Seep sample results are presented in Table 7-8 for the background
location and downgradient locations. Field parameters for samples collected as part of the
GWAP are presented in Table 7-9. Analytical results for water samples collected by NCDENR in
March 2014 are presented in Table 7-10.
35

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
7.0 SOURCE CHARACTERIZATION

As described in the approved Work Plan, both unfiltered and filtered (0.45 um filter) porewater
and surface water samples were collected for analyses of constituents whose results may be
biased by the presence of turbidity.1 Unless otherwise noted, concentration results discussed
are for the unfiltered samples and would represent total concentrations.
7.1 Ash Basin Primary and Secondary Cells
7.1.1 Ash (Sampling and Chemical Characteristics)
Four borings (AB-3D, AB-4D, AB-5D, and AB-7D) were advanced within the ash basin waste
boundary to obtain ash samples for chemical analyses.
Six COIs (antimony, arsenic, cobalt, iron, manganese, and vanadium) were reported above the
North Carolina PSRGs for Industrial Health and/or Protection of Groundwater for ash samples
collected within the ash basin waste boundary (see Table 7-2).
7.1.2 Ash Basin Water (Sampling and Chemical Characteristics)
Two water samples (SW-1 and SW-2) were collected from the water within the ash basin
Secondary Cell. Aluminum, antimony, arsenic, barium, beryllium, cadmium, chromium, cobalt,
copper, iron, lead, manganese, nickel, thallium, vanadium, and zinc concentrations exceeded
the North Carolina 2B Standards, 2L Standards or IMAC in at least one of the two water
samples collected from the ash basin Secondary Cell. The ash basin water is compared to the
2B and 2L Standards for comparative purposes and is not considered surface water or
groundwater. Ash basin water sample locations are shown on Figure 7.1 and are listed in Table
7-4. There were only two parameters for which dissolved (filtered) concentrations exceeded
their respective North Carolina 2B Standards in at least one of the two samples: arsenic and
thallium.
7.1.3 Porewater (Sampling and Chemical Characteristics)
Five porewater monitoring wells (AB-3S, AB-4S, AB-5S, AB-5SL, and AB-7S) were installed
within the waste boundary of the ash basin Primary and Secondary Cells.
Nine COIs (antimony, arsenic, boron, cobalt, iron, manganese, pH, thallium, vanadium, and
TDS) have been reported above the 2L Standards or IMAC in porewater samples collected from
wells within the waste boundary of the ash basin Primary and Secondary Cells. Porewater
sample locations are shown on Figure 7.1 and are listed in Table 7-5. See Section 17.3 for
maximum contaminant concentrations for porewater.
7.1.4 Ash Porewater Speciation
Four locations, AB-3S, AB-5S, AB-5SL, and AB-7S, were sampled for speciation analyses of
arsenic (III), arsenic (V), chromium (VI), iron (II), iron (III), manganese (II), manganese (IV),
1 The USEPA (EPA 2002) recommends that when possible, especially when sampling for contaminants
that may be biased by the presence of turbidity, the turbidity reading is desired to stabilize at a value
below 10 Nephelometric Turbidity Units (NTUs)
36

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
7.0 SOURCE CHARACTERIZATION

selenium (IV), and selenium (VI). Results for chemical speciation of porewater samples are
presented in Table 7-6. Further evaluation of speciation results will be included in the CAP.
7.2 Ash Storage Area
7.2.1 Ash (Sampling and Chemical Characteristics)
Three borings (AS-1D, AS-2D, AS-2S, and AS-3D) were advanced within the ash storage area
waste boundary to obtain ash samples for chemical analyses.
Seven COIs (antimony, arsenic, cobalt, iron, manganese, selenium, and vanadium) were
reported above the North Carolina PSRGs for Industrial Health and/or Protection of
Groundwater Standards within the ash storage area waste boundary (see Table 7-2).
7.2.2 Porewater (Sampling and Chemical Characteristics)
There were no porewater samples collected from the ash storage area due to groundwater in
the ash storage area being below the bottom of the ash layer.
7.3 Cinder Storage Area
7.3.1 Ash (Sampling and Chemical Characteristics)
Two borings (C-1D and C-2D) were advanced within the cinder storage area waste boundary to
obtain ash samples for chemical analyses.
Six COIs (arsenic, cobalt, iron, manganese, selenium, and vanadium) were reported above the
North Carolina PSRGs for Industrial Health and/or Protection of Groundwater Standards within
the cinder storage area waste boundary (see Table 7-2).
7.3.2 Porewater (Sampling and Chemical Characteristics)
One porewater monitoring well (C-1S) was installed within the cinder storage area waste
boundary.
Seven COIs (arsenic, cobalt, iron, manganese, pH, TDS, vanadium, sulfate, and TDS) have
been reported above the 2L Standards or IMAC in porewater samples collected from well within
the waste boundary of the cinder storage area. The porewater sample location is shown on
Figure 7.1 and are listed in Table 7-5. See Section 17.3 for maximum contaminant
concentrations for porewater.
7.4 Leaching Potential of Ash
In addition to total inorganic testing of ash samples, 8 ash samples collected from borings
completed within the ash basin Primary and Secondary Cells, ash storage area, and cinder
storage area were analyzed for leachable inorganics using SPLP (see Table 7-7). The purpose
of the SPLP testing is to evaluate the leaching potential of COIs that may result in impacts to
groundwater above the 2L Standards or IMAC. The results of the SPLP analyses indicated that
37

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
7.0 SOURCE CHARACTERIZATION

the following COIs exceeded their respective 2L Standards or IMAC in at least one sample
results: antimony, arsenic, chromium, cobalt, iron, lead, manganese, selenium, thallium,
vanadium, and nitrate.
Leaching of constituents from ash stored in the ash storage or cider storage area will be likely
be different from the leaching that occurs when ash is stored in a saturated condition. The ash
in these two different storage environments would experience differences in the time of
exposure to the leaching solution, the liquid to solid ratio, and the chemical properties of
leaching liquid. This would likely lead to differences in the constituents leached in the two
differing environments and in the concentrations of the leached constituents.
In general the infiltration for the ash storage area and the cinder storage area will be variable
and intermittent, as infiltration is precipitation induced. The infiltration rate is dependent on a
number of factors with the primary factors being climate, vegetation, and soil properties. The
precipitation and air temperature are the two aspects of climate that most directly affect
groundwater infiltration. Vegetation affects the infiltration rate through interception and by
means of transpiration. The primary soil properties that affect infiltration are represented by the
hydraulic conductivity of the material.
For areas where saturated conditions exist, the infiltration and subsequent groundwater
recharge would be represented by Darcy’s law. However, in the case of an ash basin the
recharge flow rate calculation is complicated by the flow through the earthen dike, and through
the material underlying the ash basin. An area where the surface is saturated or where water is
present in the ash basin will receive constant infiltration with the rate being controlled by the
factors described above.
The potential migration of contaminants from the ash basin, ash storage area, and the cinder
storage area will occur by the movement of ash leachate into the underlying soil layers and
groundwater through infiltration. The infiltration of precipitation for the ash storage area and the
cinder storage area and the infiltration of the ash basin water into the underlying soil material
will be modeled in the groundwater model being prepared for the CAP.
7.5 Seeps
7.5.1 Review of NCDENR March 2014 Sampling Results
In March 2014, NCDENR sampled the following seeps at RBSS (see Figure 7-2):
• RBWW002 (east of Ash Basin Secondary Cell tower)
• RBSP001 (west of Ash Basin Primary Cell)
• RBSP002 (north of Ash Basin Secondary Cell)
• RBSP003 (east of Ash Basin Secondary Cell)
• RBSW001 (southwest of RBSS intake canal)
• RBSW002 (southwest of RBSS intake canal)
• RBSW003 (southwest of RBSS intake canal)
• RBSW010 (west of RBSS at property boundary)
• RBSW011 (west of RBSS and outside of property boundary)
38

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
7.0 SOURCE CHARACTERIZATION

Based on review of the March 2014 sampling results and measured field parameters, the
following constituents exceeded the 2B Standards (class WS): aluminum, antimony, chromium,
copper, DO, pH, selenium, and zinc (see Table 7-10). The March 2014 sampling results and
measured field parameters also exceeded the 2L Standards or IMAC for antimony, chromium,
pH, and selenium (see Table 7-10).
Only NCDENR seep locations that correspond to the seeps identified in Section 7.5.2.2 or the
surface water samples identified in Section 9.1 were re-sampled as part of the CSA assessment
activities.
7.5.2 Seep Sampling Results – CSA Activities
7.5.2.1 Seeps: Background
The background seep location (S-13) was identified based on the SCM at the time the Work
Plan was submitted. The background seep location was chosen in an area assumed not to be
impacted by the ash basin, ash storage area, and cinder storage area. Based on the developed
groundwater surface water contours (Figures 6-5 through 6-7) and the updated SCM, the
background seep location is considered to be hydraulically side-gradient from the ash basin and
is likely representative of background groundwater quality conditions at the site. Seep sample
location is shown on Figure 7.1 and are listed in Table 7-10.
With the exception of iron, manganese, pH, and vanadium the results for all other constituents
reported at seep sample location S-13 were less than the 2L Standards or IMAC. The
background concentrations reported for constituents that are considered COIs in the seeps at
the RBSS site are provided below. Results with a J qualifier indicate an estimated concentration
less than the laboratory method reporting limits.
• Cobalt 0.24J µg/L
• Iron 1,600 µg/L
• Manganese 110 µg/L
• pH 6.8 SU
• Vanadium 1.4 µg/L
7.5.2.2 Seeps: Ash Basin
Four seeps (S-2, S-5, S-9, and S-11) are associated with the ash basin at RBSS. The seeps are
located between the ash basin Primary and Secondary Cells and the Catawba River. Seep S-2
is downgradient of the ash basin Primary Cell. Seeps S-5, S-9, and S-11 are located
downgradient of the ash basin Secondary Cell. Seep locations are shown on Figure 7-1.
Five COIs (cobalt, iron, manganese, and vanadium) were reported in the seep samples at
concentrations exceeding the 2L Standards or IMAC (see Table 7-8).
39

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
7.0 SOURCE CHARACTERIZATION

7.5.2.3 Seeps: Speciation
One location (S-5) were sampled for speciation analyses of arsenic (III), arsenic (V), chromium
(VI), iron (II), iron (III), manganese (II), manganese (IV), selenium (IV), and selenium (VI).
Results for speciation of surface water are presented in Table 9-2. Further evaluation of
speciation results will be included in the CAP.
7.6 Constituents of Interest
Based on evaluation of the ash, ash basin, surface water, ash porewater, and seep samples
collected in the source area, the constituents listed below were identified as COIs. As noted
above, these constituents were identified as COIs based on comparison to the following
standards or criteria:
• Ash – compared to North Carolina PSRGs for Industrial Health and/or Protection of
Groundwater Standards
• Ash Basin water samples – compared to 2B, and to respective 2L standard or IMAC
• Ash basin porewater – compared to respective 2L standard or IMAC

The comparison of these samples to the standards or criteria was performed only for discussion
purposes. These comparisons are useful in understanding potential impacts to soil, groundwater
and surface water. However, the fact that exceedances of these standards or criteria are
identified in this comparison does not necessarily indicate that exceedances of groundwater,
surface water, or soil standards are present.
7.6.1 COIs in Ash (based on total inorganics analysis, as shown in Table 7-2)
• Antimony
• Arsenic
• Cobalt
• Iron
• Manganese
• Selenium
• Vanadium
7.6.2 COIs in Ash Basin Surface Water (based on water quality analysis, as shown in
Table 7-4)
• Aluminum
• Antimony
• Arsenic
• Barium
• Beryllium
• Cadmium
• Chromium
40

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
7.0 SOURCE CHARACTERIZATION

• Cobalt
• Copper
• Lead
• Manganese
• Nickel
• Thallium
• Vanadium
• Zinc
7.6.3 COIs in Porewater (based on water quality analysis, as shown in Table 7-5)
• Antimony
• Arsenic
• Boron
• Cobalt
• Iron
• Manganese
• pH
• Thallium
• Vanadium
• Sulfate
• TDS
7.6.4 COIs in Seeps (based on water quality analysis, as shown in Table 7-8)
• Cobalt
• Iron
• Manganese
• Vanadium

41

This page is intentionally left blank.
Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
8.0 SOIL AND ROCK CHARACTERIZATION

8.0 Soil and Rock Characterization
The purpose of soil and rock characterization is to evaluate the physical and geochemical
properties in the subsurface with regard to COI presence, retardation, and migration. Soil and
rock sampling was performed in general accordance with the procedures described in the Work
Plan. Refer to Appendix D for a detailed description of these methods and Appendix E for field
and sampling quality control / quality assurance protocols.
Soil, PWR, and bedrock samples were collected from background locations, beneath the ash
basin, ash storage area, and cinder storage areas, and from locations beyond the waste
boundary. As of the date of this report the bedrock sample collected from boring locations GWA-
21D has not been received. The analytical results from these locations will be incorporated into
the CSA supplement.
Appendix D summarizes the soil and rock sampling plan utilized for groundwater assessment
activities. Variances from the proposed Work Plan are also presented in Appendix D. The boring
locations are shown on Figure 6-1.
8.1 Background Sample Locations
Background (BG) boring locations were identified based on the SCM at the time the Work Plan
was submitted. The BG locations were chosen in areas assumed not to be impacted by and
topographically cross-gradient from the ash basin, ash storage, and cinder storage areas.
Based on the developed groundwater surface water contours shown on Figures 6-5 through 6-
7, and the updated SCM, the BG locations are considered to be hydraulically cross-gradient
from the ash basin, ash storage, and cinder storage areas. As a result, the BG boring locations
are considered to be representative of background soil conditions at the site.
8.2 Analytical Methods and Results
Table 8-1 summarizes the parameters and constituent analytical methods for soil, PWR, and
bedrock samples collected.
Total inorganic results for background soil samples are presented in Table 8-2. Total inorganic
results for background PWR and bedrock soil samples are presented in Table 8-3. Total
inorganic results for soil samples are presented in Table 8-4. Total inorganic results for PWR
and bedrock samples are presented in Table 8-5.
Figure 8-1 depicts the total inorganic results for soil, PWR and bedrock analysis. Cross-section
transects are presented in plan view on Figure 11-1. Cross-sections presenting the vertical
distribution COIs along each transect are depicted on Figures 8-2.1 through 8-6.
SPLP results for background soil samples are presented in Table 8-6. SPLP results for soil
samples are presented on Table 8-7. Although SPLP analytical results are being compared to
the 2L Standards or IMAC, these samples do not represent groundwater samples.
42

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
7.0 SOURCE CHARACTERIZATION

8.3 Comparison of Soil Results to Applicable Levels
The soil analytical results are compared to the North Carolina Preliminary Soil Remediation
Goals (PSRGs) for Industrial Health and Protection of Groundwater Standards presented in
Tables 8-2 through 8-5. Frequency and concentration ranges for soil, PWR and bedrock COI
exceedances of North Carolina PSRGs are presented in Tables 8-8 and 8-9. The subsections
below provide a summary of COIs with PSRG exceedances in at least one of the samples
analyzed. Parameters not listed below were not reported at concentrations exceeding the North
Carolina PSRGs in the collected soil samples.
8.4 Comparison of Soil Results to Background
In addition to comparison of results to regulatory criteria, soil sample results have also been
compared to background concentrations as discussed below. Please refer to Figure 8-1 for soil
boring locations.
8.4.1 Background Soil
Background soil locations are identified as BG-1D, BG-2D, BG-3D, and MW-7BR. Background
soil concentration ranges are listed below for constituents that exceeded the North Carolina
PSRGs at least one soil sampling location at the RBSS site. A summary of results is presented
is in Table 8-2. Results with a J qualifier indicate an estimated concentration reported between
the laboratory method detection limit and the method reporting limit.
• Arsenic 5.5J milligrams per kilogram (mg/kg) to <8.3 mg/kg
• Barium 17.7J mg/kg to 282 mg/kg
• Boron 12J mg/kg to <20.8 mg/kg
• Cobalt 5J mg/kg to 22.8 mg/kg
• Iron 12,300J mg/kg to 36,500 mg/kg
• Manganese 11.4 mg/kg to 1,440 mg/kg
• Nickel 1.3J mg/kg to 8.5 mg/kg
• Selenium 4.7J mg/kg to <8.3 mg/kg
• Vanadium 29.2 mg/kg to 89.2 mg/kg
8.4.2 Soil Beneath Ash Basin and Within Waste Boundary
Soil samples beneath the ash basin were obtained from AB-1D, AB-2D, AB-3D, AB-4D, AB-4S,
AB-5D, AB-6D, AB-7D, AB-7S, and AB-8S. The range of constituent concentrations along with a
comparison to the range of reported background soil concentrations is provided in Table 8-10.
Constituent concentrations of arsenic, boron, cobalt, iron, manganese, nickel, and vanadium in
soils beneath the ash basins tend to be generally higher compared to background soil
concentrations. Selenium was detected only once in in these borings (2.4J mg/kg). Method
reporting limits for selenium are similar to background soil concentrations.
43

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
7.0 SOURCE CHARACTERIZATION

8.4.3 Soil Beneath Ash Storage Area
Soil samples beneath the ash storage area were obtained from AS-1D, AS-2D, AS-2S, AS-3D,
and GWA-23D. The range of constituent concentrations along with a comparison to the range of
reported background soil concentrations is provided in Table 8-11.
Constituent concentrations of arsenic, cobalt, iron, manganese, and vanadium in soils beneath
the ash storage were at or below background concentrations, other than a single exceedance
for arsenic in AS-2D.
8.4.4 Soil Beneath Cinder Storage Area
Soil samples beneath the cinder storage area were obtained from C-1D, C-2S, and C-2D. The
range of constituent concentrations along with a comparison to the range of reported
background soil concentrations is provided in Table 8-12.
Constituent concentrations of cobalt, iron, manganese and vanadium in soil beneath the cinder
storage area are similar to background soil concentrations.
8.4.5 Soil Outside the Waste Boundary and Within Compliance Boundary
Soil samples outside the waste boundary and within compliance boundary were obtained from
MW-9D, MW-15D, GWA-1D, GWA-1S, GWA-2D, GWA-3D, GWA-4S, GWA-5D, GWA-6D,
GWA-7D, GWA-7S, GWA-8D, GWA-9D, GWA-10S, GWA-20D, GWA-21D, GWA-22D, OB-1,
and OB-2. The range of constituent concentrations along with a comparison to the range of
reported background soil concentrations is provided in Table 8-13.
Constituent concentrations of arsenic, barium, cobalt, iron, manganese, nickel, selenium, and
vanadium of soils outside the waste boundary and within compliance boundary tend to be
generally higher than background soil concentrations.
8.5 Comparison of PWR and Bedrock Results to Background
In addition to comparison of results to regulatory criteria, PWR and bedrock sample results have
also been compared to background concentrations as discussed below.
8.5.1 Background PWR and Bedrock
Background PWR and bedrock sample locations were obtained from BG-2D, BG-2BR, and MW-
7BR. Background PWR and bedrock sample concentration ranges are listed below for
constituents that exceeded the North Carolina PSRGs at least one soil sampling location at the
RBSS site. Results with a J qualifier are estimated concentrations less than the laboratory
method reporting limit.
• Cobalt 2.6J mg/kg to 10.4 mg/kg
• Iron 4,100 mg/kg to 18,800 mg/kg
• Manganese 80.1 mg/kg to 754 mg/kg
• Vanadium 8.5 mg/kg to 30.4 mg/kg
44

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
7.0 SOURCE CHARACTERIZATION

8.5.2 PWR and Bedrock Beneath Ash Basin and Within Ash Basin Waste Boundary
One PWR sample within the waste boundary was obtained from AB-8D. Additional PWR and
bedrock samples were not collected within the ash basin. The constituent concentrations along
with a comparison to the range of reported background PWR and bedrock concentrations is
provided in Table 8-14.
PWR concentrations of cobalt, iron, manganese, and vanadium within the waste boundary are
within the range of background PWR concentrations.
8.5.3 PWR and Bedrock Beneath Ash Storage Area
PWR and bedrock samples collected beneath the ash storage area were obtained from GWA-
23BR. The range of constituent concentrations along with a comparison to the range of reported
background PWR and bedrock concentrations is provided in Table 8-15.
Cobalt concentrations in PWR and bedrock beneath ash storage area tend to be generally
higher than the background concentrations. Iron, manganese, and vanadium concentrations are
similar to the background concentrations.
8.5.4 PWR and Bedrock Outside the Waste Boundary and within Compliance
Boundary
PWR and bedrock samples collected outside the waste boundary and within compliance
boundary were obtained from MW-9D, MW-9BR, MW-15D, MW-15BR, GWA-1BRU, GWA-2D,
GWA-2BR, GWA-3D, GWA-4D, GWA-4BR, GWA-5D, GWA-6D, GWA-7D, GWA-9D, GWA-
9BR, GWA-10D, GWA-20BR, and GWA-22BR. The range of constituent concentrations along
with a comparison to the range of reported background PWR and bedrock concentrations is
provided in Table 8-16.
Cobalt, iron, manganese, and vanadium concentrations in PWR and bedrock outside the waste
boundary and within compliance boundary tend to be greater than the background
concentrations.

45

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
9.0 SURFACE WATER AND SEDIMENT CHARACTERIZATION

9.0 Surface Water and Sediment
Characterization
The purpose of surface water and sediment characterization is to evaluate whether storage of
ash has resulted in impacts to surface waters in the vicinity of the ash basin, ash storage areas,
and cinder storage area. The surface water and sediment characterization was performed in
general accordance with the procedures described in the Work Plan. Sampling methodology
and variances to that methodology are described in Appendix F. Surface water and sediment
sample locations are shown on Figure 9-1.
As described in the approved Work Plan, both unfiltered and filtered (0.45 um filter) samples
were collected for analyses of constituents whose results may be biased by the presence of
turbidity.[1] Unless otherwise noted, concentration results discussed are for the unfiltered
samples and would represent total concentrations.
9.1 Surface Water
Two surface water samples were obtained from Mountain Island Lake from the plant surface
water intake canal (RBSW001 and RBSW002) located just northwest of the station. One surface
water sample was collected from the ponded water in the excavated area in the cinder storage
area (SW-3). Four additional surface water samples were collected from surface waters
generally located beyond the ash basin waste boundary (S-4, S-6, S-7, and S-8). Surface water
parameters and laboratory methods used for analysis are presented in Table 7-1. Surface water
sample results for total and dissolved fractions of constituents are presented in Table 9-1.
9.1.1 Comparison of Exceedances to 2B Standards
Surface water analytical results are compared to the 2B Standards. Exceedances of the 2B
Standards were reported for aluminum, cadmium, copper, lead, and zinc in surface water
samples collected from RBSW001 and RBSW002. Exceedances of the 2B Standards were
reported for aluminum, lead, and zinc in surface water samples collected from the cinder
storage area (SW-3). Aluminum, cobalt, copper, and lead exceeded the 2B Standards in surface
water samples collected from streams beyond the ash basin waste boundary. Beryllium and
sulfide was not detected in surface water.
9.1.1.1 Background Surface Water
The background surface water location is identified as S-13 and is located on an unnamed draw
leading to Mountain Island Lake beside monitoring well location BG-3, sidegradient of the ash
basins and ash storage areas. Aluminum was the only constituent exceeding the 2B Standard in
[1] The USEPA (EPA 2002) recommends that when possible, especially when sampling for contaminants
that may be biased by the presence of turbidity, the turbidity reading is desired to stabilize at a value
below 10 Nephelometric Turbidity Units (NTUs)
46

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
9.0 SURFACE WATER AND SEDIMENT CHARACTERIZATION

the background surface water sample. The dissolved phase concentration of aluminum reported
in S-13 was less than the 2B Standards (Table 9-2).
9.1.1.2 NCDENR Surface Water Sample Locations
NCDENR collected two surface water sample locations from the west side of the intake canal.
The surface water samples collected from RBSW001 and RBSW002 exceeded 2B Standards
for aluminum, cadmium, copper, lead, and zinc (Table 9-2). The dissolved phase concentration
of lead reported in RBSW001 was less than the 2B Standards.
9.1.1.3 Cinder Storage Surface Water
One surface water sample location was collected for the surface water feature in the cinder
storage area. Samples collected from SW-3 exceeded 2B Standards for aluminum, lead, and
zinc (Table 9-3). The dissolved phase concentration of aluminum and lead were less than the
detection limit.
9.1.1.4 Stream Surface Water
Four surface water sample locations were collected from surface water features outside of the
waste boundary, S-4, S-6, S-7, and S-8. Samples collected from stream surface water locations
exceeded 2B Standards for aluminum, cobalt, copper, and lead (Table 9-4). The dissolved
phase concentration of aluminum, copper, and lead were less than the 2B Standards. The
dissolved concentrations for cobalt exceedances showed a reduction from the total values.
9.1.2 Results for Constituents without 2B Standards
Surface water samples were also analyzed for the following constituents that do not have 2B
Standards: boron, calcium, iron, manganese, mercury, selenium, and vanadium. Each of these
constituents was detected in at least one surface water sample. Samples for each area will be
compared to the background.
9.1.2.1 NCDENR Surface Water Sample Locations
NCDENR surface water samples collected from RBSW001 and RBSW002 exceeded the
background for calcium, selenium, and vanadium (Table 9-5).
9.1.2.2 Cinder Storage Surface Water
The cinder storage area sample collected from SW-3 exceeded background for boron, calcium,
and manganese (Table 9-6).
9.1.2.3 Stream Surface Water
Stream surface water collected exceeded the background for boron, iron, manganese, and
vanadium (Table 9-7).
47

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
9.0 SURFACE WATER AND SEDIMENT CHARACTERIZATION

9.1.3 Results for Select Constituents in Mountain Island Lake
Surface water sample analytical results collected as part the NPDES permit requirements, were
reviewed for an upstream (278.0) and one downstream (277.5) location in Mountain Island
Lake. Surface water sampling results from the two samples per year were reviewed for data
from 2011 to 2015. No exceedances were detected for the select constituents analyzed.
Surface water sample results from Mountain Island Lake are presented in Table 9-8.
9.2 Surface Water Speciation
Speciation is the analysis of the composition of a particular analyte in a system. Speciation is
important for understanding the fate and transport of COIs. Two locations, SW-3 and S-6, were
sampled for chemical speciation analyses of arsenic (III), arsenic (V), chromium (VI), iron (II),
iron (III), manganese (II), manganese (IV), selenium (IV), and selenium (VI). Results for
chemical speciation of surface water are presented in Table 9-9. Further evaluation of chemical
speciation results will be included in the CAP.
9.3 Sediment
Sediment samples were attempted to be collected coincidentally with each of the surface water
and seep samples with the exception of the surface water samples collected from the ash basin
Secondary Cell and the pond in the cinder storage area. Seeps S-1, S-3, S-10, and S-12 were
noted to be dry at the time of sample collection; however, sediment samples were collected
from the dry seeps. Sediment samples were analyzed for the constituent and parameter list
used for solid matrix characterization (see Table 8-1). In the absence of NCDENR sediment
criteria, the sediment sample results were compared to North Carolina soil PSRGs for
Protection of Groundwater and Industrial Soil and are presented in Table 9-10. Sediment
sample locations are shown on Figure 7-1.
Sediment sample results for arsenic, barium, boron, cobalt, iron, manganese and vanadium
exceeded one or both of the North Carolina PSRGs in all sediment samples. Cobalt, iron,
manganese, and vanadium concentrations exceeded the North Carolina PSRGs for Protection
of Groundwater in all sediment samples. Arsenic exceeded the North Carolina PSRG for
Protection of Groundwater and Industrial Soil in sediment samples collected at S-2 and S-12.
Boron and barium exceeded the North Carolina PSRG for Protection of Groundwater in
sediment sample S-6. Antimony, selenium, and thallium were not detected in sediment samples
collected at the RBSS site.
48

This page is intentionally left blank.
Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
10.0 GROUNDWATER CHARACTERIZATION

10.0 Groundwater Characterization
The purpose of groundwater characterization is to characterize the groundwater on the site for
comparison to the 2L Standards or interim maximum allowable concentrations (IMACs).
Groundwater sampling methods and the rationale for sampling locations were in general
accordance with the procedures described in the Work Plan. Refer to Appendix G for a detailed
description of these methods. Variances from the proposed well installation locations, methods,
quantities, and well designations are presented in Appendix D. The groundwater monitoring well
installation plan utilized is included in Appendix A.
As described in the approved Work Plan, both unfiltered and filtered (0.45 um filter) samples
were collected for analyses of constituents whose results may be biased by the presence of
turbidity.2 Unless otherwise noted, concentration results discussed are for the unfiltered
samples and would represent total concentrations.
10.1 Regional Groundwater Data for Constituents of Interest
Individual sampling events serve to characterize the hydrogeologic and chemical conditions at a
particular monitoring location at a particular time. When interpreting the results from a sampling
event, a number of factors that affect the sample results should be taken into consideration.
Among these are the geologic and hydrogeologic setting, the location of the sample points in
the regional groundwater flow system, potential interactions between suspected contaminants
and the geological and biological constituents present in the formation (Barcelona 1985).
As a result of these factors it may be possible that the analytical results of a given constituent
are influenced by naturally occurring conditions as opposed to conditions caused by releases
from the ash basin. This section presents an overview of the regional and statewide
groundwater conditions for COIs found at the RBSS site that have promulgated state or federal
standards.
The 2L Standards recognize that the concentrations of naturally occurring substances in
groundwater may exceed the standard established in .0202(g). Rule .0202(b)(3) states that
when this occurs, the Director of the DWR will determine the standard.
Table 10-1 lists the COIs at the RBSS site along with their associated North Carolina 2L
Groundwater Standards, IMACs, and federal drinking water standards (Primary Maximum
Contaminant Levels [MCLs] and Secondary Maximum Contaminant Levels [SMCLs]). North
Carolina 2L Standard are established by NCDENR, whereas federal MCLs and SMCLs are
established by the USEPA. Primary MCLs are legally enforceable standards for public water
supply systems set to protect human health in drinking water. Secondary MCLs are non-
enforceable guidelines set to account for aesthetic considerations, such as taste, color, and
odor (USEPA 2014).
2 The USEPA (EPA 2002) recommends that when possible, especially when sampling for contaminants
that may be biased by the presence of turbidity, the turbidity reading is desired to stabilize at a value
below 10 Nephelometric Turbidity Units (NTUs)
49

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
10.0 GROUNDWATER CHARACTERIZATION

Regional background information on COIs at the RBSS site are provided (in alphabetical order)
below in Sections 10.1.1 through 10.1.10. In addition, regional background information on pH is
also provided in Section 10.1.11 as pH levels can affect the leachability of metal ions in
groundwater.
10.1.1 Antimony
Antimony is a silvery-white, brittle metal. In nature, antimony combines with other elements to
form antimony compounds. Small amounts of antimony are naturally present in rocks, soils,
water, and underwater sediments.
Only a few ores of antimony have been encountered in North Carolina. Antimony has been
found in combination with other metals; and is found most commonly in Cabarrus County and
other areas of the Carolina Slate Belt (Chapman 2013).
In a USGS study of naturally occurring trace minerals in North Carolina, 57 private water supply
wells were sampled to obtain trace mineral data. Of the wells sampled, no wells contained
antimony above the USEPA Primary MCL (Chapman 2013). Antimony is compared to IMAC
since no 2L Standard has been established for this constituent by NCDENR.
10.1.2 Arsenic
Natural arsenic occurs commonly and comes mainly from the soil. The EPA estimates that the
amount of natural arsenic released into the air as dust from the soil is approximately equal to
the amount of arsenic released by all human activities (EPRI 2008).
RBSS is located in Gaston County, North Carolina. Since it is near the border of Mecklenburg
County, statistics for both counties are included here. Data collected from 3,756 private wells
across Gaston and Mecklenburg Counties from 1998 to 2010 indicated that 56 samples had
arsenic concentrations exceeding the PMCL. The summary statistics for both counties are
provided in Table 10-2.
In a state-wide investigation into arsenic concentrations in private wells, Sanders et al. (2011)
found strong geological patterns in groundwater arsenic concentrations across the state of
North Carolina (see Figure 10-1). The RBSS site is located in an area where the average
concentrations of naturally occurring arsenic in groundwater is between 1.1 – 2.5 µg/L.
10.1.3 Barium
Two forms of barium, barium sulfate and barium carbonate, are often found in nature as
underground ore deposits. Barium is sometimes found naturally in drinking water and food.
However, since certain barium compounds (barium sulfate and barium carbonate) do not mix
well with water, the amount of barium found in drinking water is typically small.
Barium compounds such as barium acetate, barium chloride, barium hydroxide, barium nitrate,
and barium sulfide dissolve more easily in water than barium sulfate and barium carbonate, but
because they are not commonly found in nature, they do not usually occur in drinking water
50

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
10.0 GROUNDWATER CHARACTERIZATION

unless the water is contaminated by barium compounds that are released from waste sites
(EPRI 2008).
Barium is naturally released into the air by soils as they erode in wind and rain, and is released
into the soil and water by eroding rocks. Barium released into the air by human activities comes
mainly from barium mines, metal production facilities, and industrial boilers that burn coal and
oil. Anthropogenic sources of barium in soil and water include copper smelters and oil drilling
waste disposal sites. Industries reporting to the USEPA released 119,646 tons of barium and
barium compounds into the environment in 2005 (EPRI 2008).
Regional metamorphic grade greenschist to upper amphibolite in the Piedmont’s King’s
Mountain Belt contains deposits of barium sulfate (barite). Barium is especially common as
concretions and vein fillings in limestone and dolostone, which are not common geologic facies
in North Carolina; however, at various times in the past century and a half, the Carolinas have
been major producers of barite (USEPA 2014).
In a statistical summary of groundwater quality in North Carolina, the Superfund Research
Program at the University of North Carolina (UNC) analyzed 1,898 private well water samples in
Gaston and Mecklenburg Counties. The samples were tested by the North Carolina State
Laboratory of Public Health from 1998 - 2012. This study found an average barium
concentration of 50 µg/L. No samples exceeded the 2,000 µg/L PMCL for barium (NC DHHS
2010).
10.1.4 Boron
While boron is relatively abundant on the earth’s surface, boron and boron compounds are
relatively rare in all geological provinces of North Carolina. Natural sources of boron in the
environment include volatilization from seawater, geothermal vents, and weathering of clay-rich
sedimentary rocks. Total contributions from anthropogenic sources are less than contributions
from natural sources. Anthropogenic sources of boron include agriculture, refuse, coal and oil
burning power plants, by-products of glass manufacturing, and sewage and sludge disposal
(EPRI 2005).
Boron is usually present in water at low concentrations. Surface waters typically have
concentrations of 0.001 to 5 mg/L, with an average concentration of about 0.1 mg/L.
Background boron concentrations in groundwater near power plants were compiled from data
presented in EPRI technical reports, and ranged from <0.01 to 0.59 mg/L with a median
concentration of 0.07 mg/L (EPRI 2005).
10.1.5 Chromium
Chromium is a blue-white metal found naturally only in combination with other substances. It
occurs in rocks, soil, plants, and volcanic dust and gases (EPRI 2008). Background
concentrations of chromium in groundwater generally vary according to the media in which they
occur. Most chromium concentrations in groundwater are low, averaging less than 1.0 µg/L
worldwide. Chromium tends to occur in higher concentrations in felsic igneous rocks (such as
granite and metagranite) and ultramafic igneous rocks; however, it is not a major component of
51

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
10.0 GROUNDWATER CHARACTERIZATION

the igneous or metamorphic rocks found in the North Carolina Piedmont or the Blue Ridge
(Chapman 2013).
In a statistical summary of groundwater quality in North Carolina, the Superfund Research
Program at UNC analyzed 1,898private well water samples in Gaston and Mecklenburg
Counties. The samples were tested by the North Carolina State Laboratory of Public Health
from 1998 to 2012. Summary statistics for both counties are included in Table 10-2.The average
chromium concentrations were 5.1 µg/L and 5.2 µg/L in Gaston and Mecklenburg Counties,
respectively.
10.1.6 Cobalt
The concentration of cobalt in surface and groundwater in the United States is generally low—
between 1 and 10 parts of cobalt in 1 billion parts of water (ppb) in populated areas. The
concentration may be hundreds or thousands times higher in areas that are rich in cobalt-
containing minerals or in areas near mining or smelting operations. In most drinking water,
cobalt levels are less than 1 to 2 ppb (USGS 1973). Cobalt is compared to IMAC since no 2L
Standard has been established for this constituent by NCDENR.
10.1.7 Iron
Iron is a naturally occurring element that may be present in groundwater from the erosion of
natural deposits (NC DHHS 2010). According to Harden (2009), iron commonly exceeds state
and federal regulatory standards in North Carolina groundwater. Iron exceedances occurred in
over half of the state’s 10 geozones. The average concentration of iron detected in North
Carolina private well water from sampling conducted in 2010 (NC DHHS 2010) is shown on
Figure 10-2. A study by the Superfund Research program at UNC found that only 15 of the 100
counties in North Carolina had average concentrations below the SMCL of 300 µg/L. The
average iron concentrations were 781.2 µg/L and 506 µg/L in Gaston and Mecklenburg
Counties, respectively.
A 2015 study by DENR (Summary of North Carolina Surface Water Quality Standards 2007-
2014) found that while concentrations vary regionally, “iron occurs naturally at significant
concentrations in the groundwaters of NC,” with a statewide average concentration of 1320
µg/L. Regional variations from the study are summarized in Table 10-3.
10.1.8 Manganese
Manganese is a naturally occurring silvery-gray transition metal that resembles iron, but is more
brittle and is not magnetic. It is found in combination with iron, oxygen, sulfur, or chlorine to form
manganese compounds. Manganese occurs naturally in soils, saprolite, and bedrock and is
thus a natural component of groundwater (EPRI 2008).
Manganese concentrations tend to cluster by soil system and geozone throughout North
Carolina, as shown on Figure 10-3. The Carolina Slate and Milton geozones have the highest
proportions of manganese exceedances, although six other geozones exceeded the state
standard as well (Gillespie 2013). Geozones with magmatic-arc rocks and low-grade
52

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
10.0 GROUNDWATER CHARACTERIZATION

metamorphic rocks, seen on Figure 10-3 tend to include abundant manganese-bearing mafic
minerals are likely to contribute manganese for subsurface water cycling (Gillespie 2013).
These rock types are distributed throughout North Carolina and contribute to spatial variations
of manganese concentrations throughout the state. High manganese concentrations are
associated with silty soils, and sedimentary, unconsolidated, or weathered lithologic units. Low
concentrations are associated with non-weathered igneous bedrock and soils with high
hydraulic conductivity (Gillespie 2013, Polizzoto 2014).
Manganese is most readily released to the groundwater through the weathering of mafic or
siliceous rocks (Gillespie 2013). When manganese-bearing minerals in saprolite, such as biotite,
are exposed to acidic weathering, the metal can be liberated from the host-mineral and released
to groundwater. It can then migrate through pre-existing fractures during the movement of
groundwater through bedrock. If this aqueous-phase manganese is exposed to higher pH in the
groundwater system, it will precipitate out of solution. This results in preferential pathways
becoming “coated” in manganese oxides and introduces a concentrated source of manganese
into groundwater (Gillespie 2013).
Manganese(II) in suspension of silt or clay is commonly oxidized by microorganisms present in
soil, leading to the precipitation of manganese minerals (ATSDR 2012).
Roughly 40-50% of North Carolina wells have manganese concentrations higher than the state
drinking water standard (Gillespie 2013). Concentrations are spatially variable throughout the
state, ranging from less than 0.01 mg/L to more than 2 mg/L. This range of values reflects
naturally derived concentrations of the constituent and is largely dependent on the bedrock’s
mineralogy and extent of weathering (Gillespie 2013).
In a 2015 study by DENR (Summary of North Carolina Surface Water Quality Standards 2007-
2014) it was found that manganese concentrations vary regionally; however “manganese occurs
naturally at significant concentrations in the groundwater of NC,” with a statewide average
concentration of 102 µg/L. The study found the regional variations summarized in Table 10-3.
Using the USGS National Uranium Resource Evaluation (NURE) database, all manganese
groundwater test results from water supply wells within a 20-mile radius of the RBSS site are
shown on (Figure 10-3).
10.1.9 Sulfate
Sulfate is a naturally occurring substance found in minerals, soil, and rocks. It is present in
ambient air, groundwater, plants, and food. The principal commercial use of sulfate is in the
chemical industry. Sulfate is discharged into water in industrial wastes and through atmospheric
deposition (USEPA 2003).
While sulfate has an SMCL, and no enforceable maximum concentration set by the USEPA,
ingestion of water with high concentrations of sulfate may be associated with diarrhea,
particularly in susceptible populations, such as infants and transients (USEPA 2012).
53

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
10.0 GROUNDWATER CHARACTERIZATION

In the Piedmont and Blue Ridge Aquifers chapter of the USGS Ground Water Atlas of the
United States, the groundwater of this region as a whole is described as “generally suitable for
drinking… but iron, manganese, and sulfate locally occur in objectionable concentrations,”
(USGS 1997).
10.1.10 Thallium
Pure thallium is a soft, bluish white metal that is widely distributed in trace amounts in the
earth's crust. In its pure form, it is odorless and tasteless. It can be found in pure form or mixed
with other metals in the form of alloys. It can also be found combined with other substances
such as bromine, chlorine, fluorine, and iodine to form salts (EPRI 2008).
Traces of thallium naturally exist in rock and soil. As rock and soil erode, small amounts of
thallium can occur in groundwater. In a USGS study of trace metals in soils, the variation in
thallium concentrations in A (i.e., surface) and C (i.e., substratum) soil horizons was estimated
across the United States. The overall thallium concentrations range from <0.1 mg/kg to 8.8
mg/kg. North Carolina concentrations from this study are depicted on Figure 10-5. Thallium is
compared to IMAC since no 2L Standard was established for this constituent by NCDENR.
In a study by the Georgia Environmental Protection Division (EPD) of the Blue Ridge Mountain
and Piedmont aquifers, 120 testing sites were sampled for various constituents. Thallium was
not detected at any of these sites (MRL = 1 µg/L) (Donahue 2007).
10.1.11 Vanadium
Vanadium is widely distributed in the earth’s crust at an average concentration of 100 ppm
(approximately 100 mg/kg), similar to that of zinc and nickel. Vanadium is the 22nd most
abundant element in the earth’s crust (EPRI 2008). V(V) and V(IV) are the most important
species in natural water, with V(V) likely the most abundant under environmental conditions
(Wright and Belitz 2010). Vanadium is compared to IMAC since no 2L Standard has been
established for this constituent by NCDENR.
A study by the Georgia EPD, 120 sites in the Blue Ridge and Piedmont physiographic regions
(regions shared with North Carolina) were sampled and detectible traces of vanadium were
found in six samples (with a reporting limit of 10 µg/L). Only two of these samples were in basic
pH groundwater while the rest were sampled in more acidic waters.
Using the USGS NURE database, all vanadium groundwater test results within a 20-mile radius
of RBSS are shown on Figure 10-6. Concentrations in the region surrounding the plant range
from 0.10 to 14.8 µg/L.
10.1.12 pH
The pH scale is used to measure acidity or alkalinity. A pH value of 7 indicates neutral water. A
value lower than the USEPA-established SMCL range (<6.5 Standard Units) is associated with
bitter, metallic tasting water, and corrosion. A value higher than the SMCL range (>8.5 Standard
Units) is associated with a slippery feel, soda taste, and deposits in the water (USEPA 2013).
54

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
10.0 GROUNDWATER CHARACTERIZATION

In a statistical summary of groundwater quality in North Carolina, the Superfund Research
Program at UNC analyzed 3,759 private well water samples for pH in Gaston and Mecklenburg
Counties. The samples were analyzed by the North Carolina State Laboratory of Public Health
from 1998 to 2012. This study found that 19.85% of wells in Gaston County and 6.44% of wells
in Mecklenburg County had a pH result outside of the EPA’s SMCL range (Table 10-2).
Using the USGS NURE database, all pH groundwater test results within a 20-mile radius of the
RBSS site are shown on Figure 10-7.
10.2 Background Wells
New background monitoring well locations (BG) were identified based on the SCM at the time
the Work Plan was submitted. The BG locations were chosen in areas assumed not to be
impacted by the ash basin, ash storage area, or cinder storage area. Based on the developed
groundwater contours (Figures 6-5 through 6-7) and the updated SCM, the BG locations are
considered to be hydraulically side-gradient from the ash basin. Groundwater flow beneath the
ash storage area and cinder storage area is to the north and northwest, away from the BG
locations. Based on this information the BG monitoring well locations are representative of
background groundwater quality conditions at the site.
Background monitoring wells include two existing compliance groundwater monitoring well
(MW-7D and MW-7SR) and eight newly installed groundwater monitoring wells (MW-7BR which
is located near MW-7D and MW-7SR, as well as BG-1S/D, BG-2S/D/BR, and BG-3S/D which is
located near the eastern property boundary). Background groundwater monitoring wells are
depicted on Figure 10-8 and the data provided in Table 10-4. Well construction details are
summarized in Tables 6-8 and 6-9. A generalized well construction diagram for newly installed
wells is shown on Figure 10-9. Well installation procedures are documented in Appendix G,
along with variances from the Work Plan. Boring logs are provided in Appendix H.
Background monitoring wells MW-7D and MW-7SR were installed in December 2006 and
November 2010, respectively, as a part of the compliance monitoring program to evaluate
background water quality at the site. Monitoring well MW-7D was installed to a depth of 100.5
feet bgs and screened from 95.5 to 100.5 feet bgs. Monitoring well MW-7SR was installed to a
depth 63.9 feet bgs and screened from 43.9 to 63.9 feet bgs. Groundwater flow in the vicinity of
MW-7D and MW-7SR is to the northeast toward Mountain Island Lake. Historical groundwater
data dates back to December 2008 for MW-7D and December 2010 for MW-7SR. The
compliance monitoring wells are sampled three times a year (February, June, and October) and
15 sampling events have been conducted to date. This is considered sufficient data to
adequately perform statistical analysis of the background concentrations in wells MW-7D and
MW-7SR (see Appendix G). Duke Energy recognizes that the NCDENR DWR Director is
responsible for establishing site-specific background levels for groundwater as stated in 15A
NCAC 02L .0202(a)(3). The concentrations in the statistical report are provided as information
to aid in this determination, and for comparative purposes for groundwater at the site.
Newly installed background monitoring wells BG-1S/D, BG-2S/D/BR, BG-3S/D, and MW-7BR
were installed to evaluate background water quality in the regolith, TZ, and within the bedrock at
55

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
10.0 GROUNDWATER CHARACTERIZATION

the site. Groundwater flow in the vicinity of these monitoring wells is generally to the north or
northeast towards Mountain Island Lake. BG-1S was installed to 44.7 feet bgs and screened
from 29.7 to 44.7 feet bgs, BG-1D was installed to 199.2 feet bgs and screened from 194.2 to
199.2 feet bgs, BG-2S was installed to 65.0 feet bgs and screened from 49.0 to 65.0 feet bgs,
BG-2D was installed to 160.0 feet bgs and screened from 155.0 to 160.0 feet bgs, BG-2BR was
installed to 208 feet bgs and screened from 208 to 208 feet bgs, BG-3S was installed to 37.2
feet bgs and screened from 21.2 to 37.2 feet bgs, and BG-3D was installed to 105.0 feet bgs
and screened from 100.0 to 105.0 feet bgs. MW-7BR was installed to 201 feet bgs and
screened from 196 to 201 feet bgs. Currently, insufficient data are available to qualify BG-1S/D,
BG-2S/D/BR, and BG-3S/D as background monitoring wells and provide associated statistical
analysis. As data become available, statistical analysis will be performed and determination
made as to whether these wells qualify as background monitoring wells.
Based on review of available information, the number of background wells located within the
property boundary of the site is adequate for monitoring background groundwater quality. The
background wells are located hydrologically cross-gradient and were strategically placed to
maximize physical separation from the ash basin, ash storage area, and cinder storage area.
Time series plots, time history plots, stacked time series plots, and correlation plots for
compliance wells are depicted in Figures 10-10 – 10-86.
10.3 Discussion of Redox Conditions
Determination of the reduction/oxidation (redox) condition of groundwater is an important
component of groundwater assessments, and helps to understand the mobility, degradation,
and solubility of contaminants. By applying the framework of the Excel Workbook for Identifying
Redox Processes in Ground Water (Jurgens, McMahon, Chapelle, and Eberts 2009) to the
analytical results in the following sections, the predominant redox process, or category, to
samples collected during the groundwater assessment was assigned. Categories include oxic,
suboxic, anoxic, and mixed. Assignment of redox category was based upon concentrations of
DO, nitrate as nitrogen, manganese (II), iron (II), sulfate, and sulfide as inputs. Constituent
criteria appropriate for inputs to the Excel Workbook, as well as an explanation of the redox
assignments, can be found in Tables 1 and 2, respectively, of the USGS Open File Report
2009-1004 (Jurgens, McMahon, Chapelle, and Eberts 2009).
Redox assignment results are presented in Table 10-5.
10.4 Groundwater Analytical Results
A total of 78 groundwater monitoring wells were installed at RBSS between February and
August 2015 as part of the groundwater assessment program. Groundwater monitoring well
locations are shown on Figure 6-2. Monitoring well information is provided in Tables 6-8 and 6-
9. Monitoring wells were installed in general accordance with procedures described in the Work
Plan and a detailed description is provided in Appendix H. Boring logs are also provided in
Appendix H.
56

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
10.0 GROUNDWATER CHARACTERIZATION

Table 7-9 summarizes parameters and constituent analytical methods for the groundwater
samples collected. Groundwater sample results are compared to the North Carolina 2L
Standards and IMACs. Background groundwater sample laboratory results for totals and
dissolved inorganic parameters are summarized in Table 10-4. Background speciation
groundwater results are presented in Table 10-7. Groundwater sample field parameters and
laboratory results for totals and dissolved are summarized in Table 10-8 and groundwater
speciation results are summarized in Table 10-7. Field parameters are summarized in Tables 7-
11 and 7-12. Groundwater sampling results are depicted on Figure 10-118. Variances from the
proposed sampling plans are presented in Appendix H. Field and sampling quality control /
quality assurance protocols are provided in Appendix E.
Duke Energy conducted speciation of groundwater samples for arsenic, chromium, iron,
manganese, and selenium from selected wells along inferred groundwater flow transects.
Speciation sampling was performed along flow transects, at ash basin water sample locations,
and at compliance wells with historical exceedances of the 2L Standards for speciation
constituents. Available speciation results for background groundwater and groundwater samples
are provided in the above referenced tables and the remaining results will be included in the
CSA Supplement.
Well designations and descriptions for the installed assessment monitoring wells include:
• S – Shallow monitoring wells installed in regolith or ash that were screened to bracket
the water table surface at the time of installation.
• SL – Monitoring wells that were installed with the bottom of the well screen set above the
ash-regolith interface
• I – Intermediate monitoring wells installed in regolith that were screened wholly within
the regolith zone, below the ash in the ash basin, and beneath the water table.
• D – Deep monitoring wells were installed with the screened interval within the partially
weathered/fractured bedrock transistion zone at the base of the regolith.
• BRU – Bedrock Upper monitoring wells are wells that were originally proposed to be “D”
wells; however, a partially weathered/fractured bedrock transition zone was not
encountered in the boring. These wells were screened within the first 15 feet of fresh,
competent bedrock encoountered below the regolith.
• BR – Bedrock monitoring wells were screened across water-bearing fractures within
fresh competent bedrock after continuous coring of at least 50 feet into competent
bedrock.

Groundwater monitoring wells were developed prior to sampling activities in general accordance
with well development procedures detailed in Appendix G. The well development forms are also
included in Appendix G. Groundwater samples were collected and analyzed in general
accordance with the procedures and methods described in the Work Plan and in Duke Energy’s
Low Flow Sampling Plan, Duke Energy Facilities, Ash Basin Groundwater Assessment
Program, dated May 22, 2015. Refer to Appendix D for a detailed description of these methods.
Appendix G includes a summary of variances from the well development and sampling plans.
Appendix E includes the field and sampling quality control / quality assurance protocols.
57

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
10.0 GROUNDWATER CHARACTERIZATION

Groundwater samples were collected from background locations, beneath the ash basin,
beneath the ash storage area, beneath the cinder storage area, and from locations beyond the
waste boundary (note the waste boundary encompasses the ash basin, ash storage area, and
cinder storage area). Groundwater samples were also collected from pre-existing voluntary and
compliance wells on the site.
10.4.1 Beneath Ash Basin and Within Waste Boundary
A total of 19 groundwater monitoring wells (10 shallow, eight deep and one bedrock) were
installed within the footprint of the ash basin Primary and Secondary Cells and associated
dams. These borings include the following: AB-1S/D, AB-2S/D, AB-3S/D/BR, AB-4S/D, AB-
5S/SL/D, AB-6S/BRU, AB-7S/I/D, and AB-8S/D. These groundwater monitoring wells were
installed to evaluate groundwater quality beneath the ash basin and within the waste boundary.
10.4.2 Beneath Ash Storage Areas
A total of nine groundwater monitoring wells (four shallow, four deep, and one bedrock) were
installed within the footprint of the ash storage area waste boundary to evaluate the impact of
the ash storage area on groundwater quality. These monitoring wells include the following: AS-
1S/D, AS-2S/D, AS-3SA/D, and GWA-23S/D/BR.
Groundwater sample analytical results from monitoring well GWA-3SA were not received in
sufficient time to incorporate the results into this CSA report. The analytical results from this well
with be provided in the CSA supplement.
10.4.3 Beneath Cinder Storage Area
A total of three groundwater monitoring wells (one shallow, one deep, and one bedrock) were
installed within the footprint of the cinder storage area waste boundary to evaluate the impact of
the cinder storage area on groundwater quality. These monitoring wells are identified as C-
1BRU and C-2S/D.
Groundwater sample analytical results from monitoring well C-1BRU were not received in
sufficient time to incorporate the results into this CSA report. The analytical results from this
sample with be provided in the CSA supplement.
10.4.4 Outside the Waste Boundary and Within Compliance Boundary
A total of 40 groundwater monitoring wells (14 shallow, 16 deep, and 10 bedrock) were installed
outside the waste boundary of the ash basin, ash storage area, and cinder storage area and
within the compliance boundary to evaluate the impact these areas have on groundwater
quality. These monitoring wells include the following: GWA-1S/BRU, GWA-2S/BRU/BR, GWA-
3SA/D, GWA-4S/D/BR, GWA-5S/D, GWA-6S/D, GWA-7S/D/BR, GWA-8S/D, GWA-9S/D/BR,
GWA-10S/BRU, GWA-20S/D/BR, GWA-21S/D/BR, GWA-22S/D/BR-A, GWA-23S/D/BR, MW-
9D/BR, and MW-15D/BR.
Monitoring wells GWA-9BR, MW-2S, and MW-4S were dry at the time of sample collection and
groundwater samples from these wells could not be obtained. Groundwater sample analytical
58

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
10.0 GROUNDWATER CHARACTERIZATION

results from monitoring wells AS-3SA and GWA-4BR were not received in sufficient time to
incorporate the results into this CSA report. The analytical results from these wells with be
provided in the CSA supplement.
A total of 22 existing voluntary and compliance groundwater monitoring wells were also sampled
to supplement groundwater quality data for this groundwater assessment. These wells include
the following: MW-1S/D, MW-2S/BRU, MW-3S/D, MW-4S/D, MW-5S/D, MW-6S/D, MW-8S/I/D,
MW-9, MW-10, MW-11SR/DR, MW-13, MW-14, and MW-15.
10.5 Comparison of Results to 2L Standards
Groundwater results were compared to 2L Standards and IMACs and exceedances are
summarized below. Table 10-8 presents groundwater results with exceedances of 2L Standards
and IMACs and Figure 10-87 depicts groundwater sample exceedances of 2L Standards and
IMACs. See Section 17.3 for maximum contaminant concentrations for groundwater.
10.6 Comparison of Results to Background
10.6.1 Background Wells MW-7D and MW-7SR
Background wells MW-7D and MW-7SR were selected based on the amount of historical data
available of the ash basin, ash storage area, and cinder storage area. These background wells
selected are located side-gradient of the ash basin, ash storage area, and upgradient of the
cinder storage area. These two wells have been part of the compliance monitoring program
initiated in 2010. With the exception of antimony (one exceedance), chromium (one
exceedances), iron, and manganese, the results for all other constituents were reported at less
than the respective 2L Standards or IMACs throughout their monitoring history. The background
concentration range for constituents that are considered COIs in groundwater at the RBSS site
are provided below:
• Antimony <1 µg/L to 1.04 µg/L
• Arsenic <1 µg/L to <2 µg/L
• Boron <50 µg/L to <100 µg/L
• Chromium <1 µg/L to 14 µg/L
• Cobalt <1 µg/L
• Iron <10 µg/L to 790 µg/L
• Manganese <5 µg/L to 413 µg/L
• pH 4.96 SU to 5.92 SU
• Sulfate 120 µg/L to <1,000 µg/L
• Thallium <0.2 µg/L
• TDS 10,000 µg/L to 120,000 µg/L
• Vanadium NA (NA indicates no data available)
59

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
10.0 GROUNDWATER CHARACTERIZATION

10.6.2 Newly Installed Background Wells
Recently installed background wells are designated BG-1S/D, BG-2S/D/BR, BG-3S/D, and MW-
7BR. Newly installed background wells will be compared to the RBSS well network in the future
after additional groundwater sampling events are evaluated. With the exception of antimony,
chromium, cobalt, iron, manganese, vanadium, and TDS, the results for all other constituents
were reported at less than the 2L Standards or IMACs. Results from the newly installed
background wells report concentrations for antimony, chromium, cobalt, iron, sulfate, vanadium
and TDS were higher than historically reported in MW-7D and MW-7SR. The results for the
remaining constituents are similar between the newly installed background wells and the
compliance background wells. The concentration range for COIs in newly installed background
wells in groundwater at the RBSS site are provided below. Results with a J qualifier indicate an
estimated concentration.
• Antimony 0.16J µg/L to 3.3 µg/L
• Arsenic 0.12J µg/L to 2 µg/L
• Boron 27J µg/L to <50.0 µg/L
• Chromium 0.25J+ µg/L to 57.5 µg/L
• Cobalt <0.5 µg/L to 3 µg/L
• Iron 28J µg/L to 2,200 µg/L
• Manganese <5 µg/L to 370 µg/L
• pH 5.75 to 8.2 SU
• Sulfate 580J µg/L to 48,400J+ µg/L
• Thallium 0.032J µg/L to <0.1 µg/L
• TDS <25,000 µg/L to 1,180,000 µg/L
• Vanadium 0.35J µg/L to 29.9 µg/L
10.6.3 Regional Groundwater Data
Information regarding the regional groundwater quality data is presented in Section 10.1. The
concentration range for constituents for county and/or state ranges for COIs in groundwater at
the RBSS site are provided below.
• Arsenic 0.5 µg/L to 232 µg/L (Gaston and Mecklenburg Counties)
• Boron <0.01 µg/L to 0.59 µg/L
• Chromium 0.5 µg/L(non-detect) to 80 µg/L (Gaston and Mecklenburg Counties)
• Iron 0 µg/L (non-detect) to 98,000 µg/L (Gaston and Mecklenburg Counties)
• Manganese 102 µg/L (NC state average)
• pH 4.6 SU to 10.3 SU
• Thallium <1 µg/L (Blue Ridge Mountain and Piedmont aquifers)
• Vanadium 0.1 µg/L to 14.8 µg/L (20-mile radius from site)
60

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
10.0 GROUNDWATER CHARACTERIZATION

10.6.4 Groundwater Beneath Ash Basin and Within Waste Boundary
Groundwater monitoring locations beneath the ash basin are identified as AB-1S/D, AB-2S/D,
AB-3D/BR, AB-4D, AB-5D, AB-6S/BRU, AB-7I/D, and AB-8S/D. The range of COI
concentrations along with a comparison to the range of reported background groundwater
concentrations in monitoring wells MW-7D and MW-7SR and the regional groundwater data is
provided in Table 10-9.
Concentrations of several constituents in groundwater beneath the ash basin and within the
waste boundary are higher than background and regional concentrations, including: antimony,
chromium, cobalt, iron, manganese, pH, vanadium, and TDS.
10.6.5 Groundwater Beneath Ash Storage Area
Groundwater monitoring locations outside the ash storage area and within the compliance
boundary are identified as AS-1S/D, AS-2S/D, AS-3D, and GWA-23S/D/BR. The range of COI
concentrations along with a comparison to the range of reported background groundwater
concentrations in monitoring wells MW-7D and MW-7SR and the regional groundwater data is
provided in Table 10-10.
Concentrations of several constituents in groundwater beneath the ash storage area are higher
than background and regional concentrations, including: antimony, boron, chromium, cobalt,
manganese, pH, and vanadium.
10.6.6 Groundwater Beneath Cinder Storage Area
Groundwater monitoring locations beneath the cinder storage area are identified as C-2S/D.
The range of COI concentrations along with a comparison to the range of reported background
groundwater concentrations in monitoring wells MW-7D and MW-7SR and the regional
groundwater data is provided in Table 10-11.
As expected, concentrations of several constituents in groundwater beneath the cinder storage
area are higher than background and regional concentrations, including: cobalt, iron,
manganese, pH, and vanadium.
10.6.7 Groundwater Beyond Ash Basin Waste Boundary and within Compliance
Boundary
Groundwater samples collected from outside the ash basin waste boundary and within the
compliance boundary are identified as GWA-1S/BRU, GWA-2S/BRU/BR, GWA-3SA/D, GWA-
4S/D, GWA-5S/D, GWA-6S/D, GWA-7S/D/BR, GWA-8S/D, GWA-9S/D/BR, GWA-10S/BRU,
GWA-20S/D, GWA-22S/D/BR, MW-1S/D, MW-2S/D, MW-3S/D, MW-4D, MW-5S/D, MW-6S/D,
MW-7BR, MW-8S/I/D, MW-9/D/BR, MW-10, MW-11SR/DR, MW-13, MW-14, and MW-15/D/BR.
The range of COI concentrations along with a comparison to the range of reported background
groundwater concentrations in monitoring wells MW-7D and MW-7SR and the regional
groundwater data is provided in Table 10-12.
61

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
10.0 GROUNDWATER CHARACTERIZATION

Concentrations of several constituents in groundwater beyond the ash basin waste boundary
are higher than background and regional concentrations, including: antimony, arsenic,
chromium, cobalt, manganese, sulfate, TDS, thallium, vanadium and TDS.
The USEPA recommends that when possible, especially when sampling for constituents that
may be biased by the presence of turbidity, that turbidity values in the stabilized well should be
less than 10 Nephelometric turbidity units (NTUs) (USEPA 2002). Compliance monitoring wells
with analytical results exceeding the 2L Standards for iron and/or manganese have been
individually plotted with the associated turbidity values (Figures 10-10 through 10-60). Maximum
contaminant concentrations for groundwater can be found in Section 17.3.
Groundwater isoconcentration contours with respect to each COI are depicted in Figures 10-119
through 10-151.
10.7 Cation and Anion Water Quality Data
Cation and anion concentrations can be used to describe the chemical composition of
groundwater in an aquifer. In natural waters, the cations calcium, magnesium, sodium and
potassium and the anions, chloride, sulfate, carbonate, and bicarbonate will make up 95% to
100% of the ions in solution.
Cation and anion concentrations at the RBSS site from upgradient groundwater monitoring wells
and ash basin groundwater monitoring wells are compared on Figures 10-107 and 10-108. In
general, calcium, chloride, magnesium, sodium, and sulfate are elevated, but calcium and
sulfate are higher in ash basin groundwater monitoring wells compared to the upgradient
monitoring wells.
The relative concentrations and distribution of the cations and anions can be used to compare
the relative ionic composition of different water quality samples through the use of Piper
diagrams.
Piper diagrams were generated for the RBSS site to show comparison of geochemistry between
ash basin porewater, ash basin water, seeps, upgradient and downgradient groundwater
monitoring wells and background groundwater monitoring wells. In general, the ionic
composition of groundwater and surface water at the site is predominantly calcium, magnesium,
and bicarbonate rich with the exception of ash basin water, ash basin porewater, and
downgradient groundwater monitoring wells which were observed to be trending closer to
calcium, magnesium and sulfate rich geochemical makeup. Seep data indicates similar
geochemistry to ash basin water, ash basin porewater, and shallow wells in the ash basin. Piper
diagrams are included as Figures 10-186 through 10-191.
10.8 Groundwater Speciation
Thirty-eight locations, were sampled for chemical speciation analyses of arsenic (III), arsenic
(V), chromium (VI), iron (II), iron (III), manganese (II), manganese (IV), selenium (IV), and
62

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
10.0 GROUNDWATER CHARACTERIZATION

selenium (VI). Results for chemical speciation of surface water are presented in Table 10-7.
Further evaluation of chemical speciation results will be included in the CAP.
10.9 Radiological Laboratory Testing
Radionuclides may exist dissolved in water from natural sources (e.g. soil or rock). The USEPA
regulates various radionuclides in drinking water. For purposes of this assessment, radium-226,
radium-228, natural uranium, uranium-233, uranium-234, and uranium-236 were analyzed. Four
locations, BG-1D, BG-1S and MW-13, were sampled for the analytes listed above. Results for
radiological laboratory testing are presented in Table 10-6. Further evaluation of radiological
laboratory testing results will be included in the CAP.
10.10 CCR Rule Groundwater Detection and Assessment
Monitoring Parameters
Appendix III to Part 257 Constituents for Detection Monitoring and Appendix IV to Part 257
Constituents for Assessment Monitoring
On April 17, 2015, the USEPA published its final rule “Disposal of Coal Combustion Residuals
from Electric Utilities” (Final Rule) to regulate the disposal of CCR as solid waste under Subtitle
D of the Resource Conservation and Recovery Act (RCRA). Among other requirements, the
Final Rule establishes requirements for a groundwater monitoring program to be implemented
for CCR surface impoundments consisting of groundwater detection monitoring and, if
necessary, assessment groundwater monitoring and corrective action.
The USEPA selected constituents to be used in the groundwater detection monitoring program
as indicators of groundwater contamination from CCR. USEPA selected constituents for
detection monitoring that are present in CCR, would be expected to migrate rapidly, and that
would provide early detection as to whether contaminants were migrating from the disposal unit.
(80 FR 74: 21397).
As stated in the FR (80 FR 74: 21342):
These detection monitoring constituents or inorganic indicator parameters are: boron,
calcium, chloride, fluoride, pH, sulfate and total dissolved solids (TDS). These inorganic
indicator parameters are known to be leading indicators of releases of contaminants
associated with CCR and the Agency strongly recommends that State Directors add
these constituents to the list of indicator parameters to be monitored during detection
monitoring of groundwater if and when a MSWLF decides to accept CCR. (Emphasis
added)
NCDENR requested that figures be included in the CSA report that depict groundwater analytical results
for the constituents in 40 CFR 257, Appendix III detection monitoring and 40 CFR 257, Appendix IV
assessment monitoring.
63

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
10.0 GROUNDWATER CHARACTERIZATION

Constituents for detection monitoring listed in 40 CFR 257 Appendix III are:
• Boron
• Calcium
• Chloride
• Fluoride (this constituent was not analyzed for in the CSA)
• pH
• Sulfate
• TDS

The analytical results for the detection monitoring constituents are found on Figure 10- 192 through 10-
194.
Constituents for assessment monitoring listed in 40 CFR 257 Appendix IV include:
• Antimony
• Arsenic
• Barium
• Beryllium
• Cadmium
• Chromium
• Cobalt
• Fluoride (not analyzed for the CSA)
• Lead
• Lithium (not analyzed for the CSA)
• Mercury
• Molybdenum
• Selenium
• Thallium
• Radium 226 and 228 combined

The analytical results for the assessment monitoring constituents are found on Figures 10-195 through
10-197.
Aluminum, copper, iron, manganese, and sulfide were included in the Appendix IV constituents in the
draft rule; USEPA removed these constituents in the Final Rule. These constituents were removed from
the Appendix IV list because they lack MCLs and were not shown to be constituents of concern based on
either the risk assessment conducted for the CCR Rule or the damage cases referenced in the CCR
Rule. Therefore, these constituents are not included on the above-referenced figures. In addition,
NCDENR requested that vanadium be included on these figures. Figures 10-118 shows vanadium as well
as the other constituents where they exceeded the relevant regulatory standards.

64

This page is intentionally left blank.
This page is intentionally left blank.
Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
11.0 HYDROGEOLOGICAL INVESTIGATION

11.0 Hydrogeological Investigation
The purpose of the hydrogeological investigation is to characterize site hydrogeological
conditions including groundwater flow direction, hydraulic gradient and conductivity,
groundwater and contaminant velocity, and slug and aquifer test results. The hydrogeological
investigation was performed in general accordance with the procedures described in the Work
Plan. Refer to Appendix H for a description of these methods.
11.1 Hydrostratigraphic Layer Development
The following materials were encountered during the site exploration and are consistent with
material descriptions from previous site exploration studies:
• Ash – Ash was encountered in borings advanced within the ash basin, ash storage area,
and cinder storage area, as well as in some borings advanced through the pond
perimeter and dikes. Ash was generally described as gray to dark bluish gray, highly
plastic to non-plastic, loose to medium dense and very soft (wet) to very stiff (dry), dry to
wet, fine to medium grained.
• Fill – Fill material generally consisted of re-worked silts, clays, and sands that were
borrowed from one area of the site and re-distributed to other areas. Fill was generally
classified as silty sand, clay with sand, clay, and sandy clay on the boring logs. Fill was
used in the construction of dikes, as cover for the ash and cinder storage areas, and as
bottom liner for the ash storage area.
• Alluvium –Alluvium encountered in borings during the project subsurface exploration
activities was classified as gravel with clay and sand, sand with gravel, and silt. In some
cases alluvium was logged beneath ash.
• Residuum (Residual soils) – Residuum is the in-place weathered soil that consists
primarily of silt with sand, clayey sand, sandy clay, clay with gravel, and clayey silts.
Residuum varied in thickness and was relatively thin compared to the thickness of
saprolite.
• Saprolite/Weathered Rock – Saprolite is soil developed by in-place weathering of rock
that retains remnant bedrock structure. Saprolite consists primarily of medium dense to
very dense silty sand, sand silt, sand, sand with gravel, sand with clay, clay with sand,
and clay. Sand particle size ranges from fine to coarse grained. Much of the saprolite is
micaceous.
• Partially Weathered/Fractured Rock – Partially weathered (slight to moderate) and/or
highly fractured rock encountered below refusal (auger, casing advancer, etc.).
• Bedrock – Sound rock in boreholes, generally slightly weathered to fresh and relatively
unfractured.

Based on the CSA site investigation, the groundwater system is consistent with the regolith-
fractured bedrock system discussed in Section 5.2. To define the hydrostratigraphic units, the
classification system of Schaeffer (2014a) used to show that the TZ is present in the Piedmont
groundwater system (discussed in Section 5-2) was modified to define the hydrostratigraphic
65

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
11.0 HYDROGEOLOGICAL INVESTIGATION

layers of the natural groundwater system. The classification system is based on Standard
Penetration Testing values (N) and the Recovery (REC) and Rock Quality Designation (RQD)
collected during the drilling and logging of the boreholes (Borehole/Well logs in Appendix H).
The ash, fill, and alluvial layers are as encountered at the site. The natural system (except
alluvium) includes the following layers:
• M1 – Soil/Saprolite: N<50
• M2 – Saprolite/Weathered Rock: N>50 or REC<50%
• TZ – Transition Zone: REC>50% and RQD<50%
• BR – Bedrock: REC>85% and RQD>50%.

Rock core runs that fell between the values for TZ and BR (REC<85% and RQD>50% or
REC>85% and RQD<50%) were assigned a hydrostratigraphic layer based on a review of the
borehole logs, rock core photographs, and geologic judgment. The same review was performed
in making the final determination of the thickness of the TZ as it could extend into the next core
run that meets the BR criterion because of potential core loss or fractured/jointed rock with
indications of water movement (iron/manganese staining).
The above layer designations (M1, M2, TZ, and BR) are used on the geologic cross-sections
with locations shown on Figure 11-1. The ash, fill, and alluvial layers are represented by A, F,
and S, respectively on the cross-sections and tables. Groundwater analytical results are
presented on Figure 11-X.
11.2 Hydrostratigraphic Layer Properties
The material properties required for the groundwater flow and transport model (total porosity,
effective porosity, specific yield and specific storage) for ash, fill, alluvium, and soil/saprolite
were developed from laboratory testing (Table 11-1; test data in Appendix H) and published
data (Domenico and Mifflin 1965). Table 11-1 has a column labeled ‘Estimated Specific
Yield/Effective Porosity’ and the values are estimated from laboratory soil data (grain size
analysis) utilizing Fetter-Bear diagrams (worksheets in Appendix H), as described by Johnson
(1967). This technique provides a simple method to estimate specific yield; however, there are
limitations to the method that may not provide an accurate determination of the specific yield of
a single sample (Robson 1993). Specific yield/effective porosity were determined for a number
of samples of the A, F, S, M1, and M2 layers to provide an average and range of expected
values. The effective porosity (primarily fracture porosity) and specific storage of the TZ and
bedrock were estimated from published data (Sanders 1998; Domenico and Mifflin
1965).Hydraulic conductivity (horizontal and vertical) of all layers, except the TZ and bedrock
(BR), was developed utilizing existing site data and historic data, in-situ permeability testing
(falling head, constant head, and packer testing where appropriate), slug tests in completed
monitoring wells, and laboratory testing of undisturbed samples (ash, fill, soil/saprolite: test
results in Appendix H) and, as necessary, from a database of Piedmont soil and rock properties
developed by HDR.

66

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
11.0 HYDROGEOLOGICAL INVESTIGATION

11.2.1 Borehole In-Situ Tests
In-situ horizontal (open hole) and vertical (flush bottom) permeability tests, either falling or
constant head as appropriate for field conditions, were performed in each of the
hydrostratigraphic units above refusal, ash, fill, alluvium, and soil/saprolite. In-situ borehole
horizontal permeability tests, either falling or constant head tests as appropriate for field
conditions, were performed just below refusal in the first 5 feet of a rock cored borehole (TZ if
present).
The flush bottom test involves advancing the borehole through the overburden with a casing
advancer until the test interval is reached. The cutting tool is removed from the casing and the
casing is filled with water to the top and the drop of the water level in the casing is measured
over a period of 60 minutes. In the open hole test, after the top of the test interval is reached,
the cutting tool (but not the casing) is advanced an additional number of feet (5 feet in the
majority of tests) and drop of the water level in the casing is measured over a time period of 60
minutes. The constant head test is similar except the water level is kept at a constant level in
the casing and the water flow-in is measured over a period of 60 minutes. The constant head
test was only used when the water level in the borehole was dropping too quickly back to the
static water level such that the time interval was insufficient to calculate the hydraulic
conductivity. The results from the field permeability testing are summarized in Table 11-2 and
the worksheets are provided in Appendix H.
Packer tests (shut-in and pressure tests) were conducted in a minimum of five boreholes. The
shut-in test is performed by isolating the zone between the packers (in effect, a piezometer) and
measuring the resulting water level over time until the water level is stable. The shut-in test
provides an estimate of the vertical gradient during the test interval. The pressure test involves
forcing water under pressure into rock through the walls of the borehole providing a means of
determining the apparent horizontal hydraulic conductivity of the bedrock. Each interval is tested
at three pressures with three steps of 20 minutes up and two steps of 5 minutes back down. The
pressure test results are summarized in Table 11-2 and the shut-in and packer tests worksheets
are provided in Appendix H.
Where possible, tests were conducted at borehole locations specified in the Work Plan and at
test intervals based on site-specific conditions at the time of the groundwater assessment work.
The U.S. Bureau of Reclamation (1995) test method and calculation procedures, as described
in Chapter 10 of their Ground Water Manual (2nd Edition), were used for the field permeability
and packer tests.
11.2.2 Monitoring Well and Observation Well Slug Tests
Hydraulic conductivity (slug) tests were completed in monitoring wells and observation wells
under the direction of the Lead Geologist/Engineer. Slug tests were performed to meet the
requirements of the May 31, 2007 NCDENR Memorandum titled, Performance and Analysis of
Aquifer Slug Tests and Pumping Tests Policy. Water level change during the slug tests was
recorded by a data logger. The slug test was performed for no less than 10 minutes, or until
such time as the water level in the test well recovered 95 percent of its original pre-tests level,
67

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
11.0 HYDROGEOLOGICAL INVESTIGATION

whichever occurred first. Slug tests were terminated after 60 minutes, even if the 95 percent
pre-test level was not achieved. Slug test field data was analyzed using the Aqtesolv (or similar)
software and the Bouwer and Rice method.
The slug test results are presented in Table 11-3 and the Slug Test Report is provided in
Appendix H. Historic slug test data is presented in Table 11-4.
11.2.3 Laboratory Permeability Tests
Laboratory permeability tests were conducted on undisturbed samples (Shelby Tubes) of ash,
fill, soil, and saprolite collected during the field investigation. The tests were performed in
accordance with ASTM D 5084 (ASTM 2010). Results of the laboratory permeability tests are
presented in Table 11-5 and historic laboratory permeability tests are presented in Table 11-6.
11.2.4 Hydrostratigraphic Layer Parameters
The soil material parameters for the A (ash), F (fill), S (alluvium), M1 (soil/saprolite), and M2
(saprolite/weathered rock) were developed by grouping the data into their respective
hydrostratigraphic units and calculating the mean, median, and standard deviation of the
different parameters. Estimated values for total porosity for the hydrostratigraphic layers A, F, S,
M1, and M2 are presented in Table 11-7. Values for estimated specific yield/effective porosity
are presented in Table 11-8 as well as estimates for specific storage based on published data
(Domenico and Mifflin 1965).
The hydraulic conductivity parameters were developed by grouping the data into their respective
hydrostratigraphic units and calculating the geometric mean, median, and standard deviation of
the different parameters. Vertical hydraulic conductivity values are not available for the TZ and
BR units, but are unlikely to be equal. As an initial assumption, vertical hydraulic conductivity for
these units can be considered to be equal to the horizontal hydraulic conductivity and adjusted
as necessary during the groundwater flow model calibration. Horizontal and vertical hydraulic
conductivity parameters for all hydrostratigraphic units are presented Tables 11-9 and 11-10,
respectively. The values of secondary (effective) porosity and specific storage for the TZ and
BR units are based on published values (Sanders 1998; Domenico and Mifflin 1965) and are
presented in Table 11-11.
Based on the CSA site investigation, the groundwater system in the natural materials (alluvium,
soil, soil/saprolite, and bedrock) at RBSS is consistent with the regolith-fractured rock system
and is an unconfined, connected system without confining layers. However, the hydraulic
conductivity data collected during this study and classified as above into the various
hydrostratigraphic units indicates that a distinct TZ of higher permeability does not exist at the
site. The horizontal hydraulic conductivity of the TZ is not higher than that of the overlying M2
unit and the horizontal hydraulic conductivity at RBSS increases with depth; a TZ as defined
does not exist.
Daniel and Harned (1992) noted the TZ that develops over massive/plutonic rock complexes
may not have a well-delineated TZ. Schaeffer (2009) found that a TZ as defined by Harned and
Daniel (1992) is not present in the bedrock at CNS, located about 24 miles south of RBSS. The
68

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
11.0 HYDROGEOLOGICAL INVESTIGATION

CNS site is underlain by a similar suite of meta-plutonic rocks as RBSS with a similar overall
thickness of regolith.
Further development of the hydrostratigraphic units taking into account the lack of a TZ, the
hydrostratigraphic layer parameters, and other parameters required for the flow and
contaminant transport model will be provided in the CAP.
11.3 Vertical Hydraulic Gradients
Horizontal hydraulic gradient is calculated by taking the difference in hydraulic head over the
distance between two wells with similar well construction. Section 6.2.2 provides additional
details for horizontal hydraulic gradients calculated for the site.
Vertical hydraulic gradient was calculated by taking the difference in groundwater elevation in a
deep and shallow well pair over the difference in total well depth of the deep and shallow well
pair. A positive output indicates upward flow and a negative output indicates downward flow.
Nine well pair locations, each consisting of a shallow and deep groundwater monitoring well,
were used to calculate vertical hydraulic gradient across the site. Based on review of the results,
vertical gradient of groundwater is generally downward across the site. Vertical gradient
calculations are summarized in Table 11-12.
11.4 Groundwater Velocity
Darcy’s Law is an equation that describes the flow rate or flux of fluid through a porous media.
To calculate the velocity that water moves through a porous medium, the specific discharge, or
Darcy flux, is divided by the effective porosity, ne. The result is the average linear velocity or
seepage velocity of groundwater between two points.
The following equation was used to calculate seepage velocities through each
hydrostratigraphic unit present at the site:
v = Ki
ne
where v is velocity; K is horizontal hydraulic conductivity; i is
horizontal hydraulic gradient; and ne is the effective porosity

Seepage velocities for groundwater were calculated using horizontal hydraulic gradients
established in Section 6.2.2, horizontal hydraulic conductivity values for each hydrostratigraphic
unit established in Table 11-9, and effective porosity values established in Table 11-11.
Hydrostratigraphic layers are defined in Section 11.1. Average groundwater velocity results are
summarized in Table 11-12.
11.5 Contaminant Velocity
Contaminant velocity depends on factors such as; the rate of groundwater flow, the effective
porosity of the aquifer material, and the soil-water partition coefficient, or Kd term. Site specific
69

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
11.0 HYDROGEOLOGICAL INVESTIGATION

Kd terms will be developed using samples collected during the site investigation. The testing to
develop the Kd terms is still underway and the results will be presented in the CAP. The
groundwater modeling to be performed in the CAP will present the velocities for the modeled
contaminants.
11.6 Plume's Physical and Chemical Characterization
Plume physical and chemical characterization is detailed below for each constituent detected in
porewater and groundwater samples, and is based on the extent presented on the
isoconcentration maps and cross sections. These descriptions are based on a single
groundwater sampling event.
As described in the approved Work Plan, both unfiltered and filtered (0.45 µm filter) samples
were collected for analyses of constituents whose results may be biased by the presence of
turbidity.3 Unless otherwise noted, concentration results discussed are for the unfiltered
samples and represent total concentrations.
• Antimony concentrations in shallow monitoring wells exceeded the IMAC in wells
associated with the ash basin, in GWA-22S located south of the ash storage area, and in
BG-3S, located east of the ash basin compliance boundary near Mountain Island Lake.
Antimony concentrations in the deep monitoring wells exceeded the IMAC in wells
associated with the ash storage area; the ash basins; GWA-1D and GWA-10D, located
north of the ash basin Secondary Cell; MW-9D, located north of the cinder storage area;
and BG-1D, located east of the ash basin compliance boundary. Antimony
concentrations in the bedrock monitoring wells exceeded the IMAC on the ash basin
Primary Dam (AB-6BRU), west of the ash basin Primary Cell and north of the cinder
storage area (MW-9BR), and GWA-9BR located immediately east of the ash basin
Secondary Cell.
• Arsenic was not reported above 2L Standards in the shallow or deep monitoring wells.
Arsenic concentrations exceeded the 2L Standards in bedrock monitoring well GWA-
9BR, located east of the ash basin Secondary Cell.
• Boron concentrations in shallow monitoring wells exceeded 2L Standards in monitoring
well AS-1S, located within the ash storage area. No other exceedances of boron were
reported in the deep or bedrock monitoring wells.
• Chromium concentrations in shallow monitoring wells exceeded 2L Standards in
monitoring well AS-2S and GWA-20S, located in and adjacent to the ash storage area,
and in monitoring well GWA-1S, located northwest of the ash basin Secondary Cell.
Chromium concentrations exceeding the 2L Standards were reported in deepmonitoring
wells located beneath the ash basin, north of the cinder storage area, and in the newly
installed background well results. Chromium exceedances of the 2L Standards were
reported in the monitoring wells to the south of the ash storage area, northwest of the
ash basin Primary Cell, and in background well location MW-7BR.
3 The USEPA (EPA 2002) recommends that when possible, especially when sampling for contaminants
that may be biased by the presence of turbidity, the turbidity reading is desired to stabilize at a value
below 10 Nephelometric Turbidity Units (NTUs)
70

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
11.0 HYDROGEOLOGICAL INVESTIGATION

• Cobalt concentrations in shallow monitoring wells exceeded the IMAC throughout the
majority of the site, including results at newly installed background well BG-1S. Cobalt
concentrations in deep monitoring wells exceeded the IMAC in monitoring wells AB-1D,
associated with ash basin Secondary Cell; GWA-3D, located northwest of the cinder
storage area; GWA-20D, GWA-22D, and MW-8D, located south of the ash storage area;
and newly installed background well BG-1D, located east of the ash basin compliance
boundary. Cobalt concentrations in deep monitoring wells exceeded the IMAC in
monitoring wells AB-6BRU and AB-3BR, beneath the ash basin.
• Iron concentrations in shallow monitoring wells exceeded 2L Standards throughout the
majority of the site, including results from newly installed background well BG-1S;
however, the dissolved phase iron result in BG-1S was below the laboratory reporting
limit. Iron concentrations in the deep monitoring wells exceeded 2L Standards in
monitoring wells AB-1D, associated with ash basin Secondary Cell; GWA-20D, GWA-
22D, and MW-8D, located south of the ash storage area; and GWA-8Dand GWA-7D.
Iron concentration also exceeded the 2L Standards in newly installed deep background
wells BG-1D and BG-3D, located east of the ash basin compliance boundary; however,
the dissolved phase iron results in the background groundwater samples were reported
as less than the laboratory reporting limit. Iron concentrations in the bedrock monitoring
wells exceeded 2L Standards in monitoring wells AB-6BRU and AB-3BR, associated
with the ash basin; GWA-9BR located east of the ash basin Secondary Cell; and newly
installed background well BG-2BR; however, the dissolved phase iron results in the
background groundwater sample was reported as less than the laboratory reporting limit.
The dissolved iron concentrations varied significantly from the totals concentrations for
the majority of the groundwater samples collected. Dissolved iron concentrations in the
shallow monitoring wells exceeded the 2L Standards in AS-2S located in the ash storage
area; MW-13 located adjacent to Mountain Island Lake northeast of the ash basin
Secondary Cell; and MW-1S located at the western toe of the ash basin Primary Cell. All
other dissolved iron concentrations (where data is available) in the shallow, deep, and
bedrock wells were below the 2L Standards.
• Manganese concentrations in the shallow monitoring wells exceeded 2L Standards
throughout the majority of the site, including BG-1S, BG-2S and BG-3S. Manganese
concentrations in deep monitoring wells exceeded 2L Standards in monitoring wells AB-
8D, associated with the ash basin Primary Cell; MW-1D, and GWA-3D, located west of
ash basin Primary Cell; GWA-9D and GWA-8D located east of the ash basin Secondary
Cell, BG-3D, located east of the ash basin compliance boundary; and GWA-22D, located
south of the ash storage area. Manganese concentrations in the bedrock monitoring
wells exceeded 2L Standards in AB-3BR and in GWA-2BRU located northwest of the
ash Primary Cell. The dissolved phase results for manganese in these wells were below
the laboratory reporting limits.
• Sulfate concentrations in the shallow monitoring wells exceeded 2L Standards in
monitoring well GWA-3SA located northwest of the cinder storage area. Sulfate
concentrations in the deep monitoring wells exceeded 2L Standards in monitoring wells
GWA-3D, located northwest of the cinder storage area, and GWA-20D, located south of
71

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
11.0 HYDROGEOLOGICAL INVESTIGATION

the ash storage area. Sulfate did not exceed 2L Standards in the bedrock monitoring
wells.
• Thallium was not reported above 2L Standards in shallow monitoring wells. Thallium
concentrations in the deep monitoring wells exceeded 2L Standards in monitoring well
GWA-20D, located south of the ash storage area. Thallium was not reported above 2L
Standards in the bedrock wells.
• TDS concentrations in the shallow monitoring wells exceeded 2L Standards in
monitoring wells AS-1S, located in the western portion of the ash storage area; and
GWA-3SA, located northwest of the cinder storage area. TDS concentrations in the deep
monitoring wells exceeded 2L Standards in monitoring wells AB-3D, located in the ash
basin; GWA-20D, located south of the ash storage area; GWA-3D, located northwest of
the cinder storage area; and BG-1D, located in the background location east of the ash
basin compliance boundary. TDS concentrations in the bedrock monitoring wells
exceeded 2L Standards in monitoring well GWA-2BR, located west of the ash basin
Primary Cell; GWA-23BR, located south of the ash storage area; MW-7BR located
southeast of the ash basin; and GWA-4BR located southwest of the ash storage area.
• Vanadium concentrations exceeded 2L Standards in all of the shallow, deep, and
bedrock monitoring wells, including all background monitoring wells. The relative
concentrations of vanadium are generally higher in the deep and bedrock wells than in
the shallow wells. The vanadium method reporting limit provided by the analytical
laboratory was 1.0 ug/L. The IMAC for vandium is 0.3 ug/L. The vanadium results
reported at concentrations less than the laboratory method reporting limit are estimated.
During subsequent monitoring events, a laboratory method reporting equal to or less
than the IMAC should be utilized.

Boron is mobile when released to groundwater; it does not readily precipitate, and has a
relatively low affinity for sorption. Boron was identified by the USEPA as one of the leading
indicators for releases of contaminants associated with ash. Because of these characteristics,
boron can be used to represent the general extent of groundwater impacted by ash.
11.7 Groundwater / Surface Water Interaction
As discussed in Section 5.2, shallow and deep groundwater flow typically follows the
topographic gradient and shallow groundwater generally discharges to nearby surface water
bodies (i.e. streams).
Groundwater/surface water interaction is evident at the site based on review of parameters
present in groundwater and seep sample results. Piper diagrams were generated for the RBSS
site to show comparison of geochemistry between ash basin porewater, surface water, seeps,
upgradient and downgradient groundwater monitoring wells and background groundwater
monitoring wells. In general, geochemistry of groundwater and surface water at the site is
predominantly calcium, magnesium, and bicarbonate rich with the exception of ash basin water,
ash basin porewater, and downgradient groundwater monitoring wells which were observed to
be trending closer to calcium, magnesium and sulfate rich geochemical makeup. Seep data
72

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
11.0 HYDROGEOLOGICAL INVESTIGATION

indicates similar geochemistry to ash basin water, ash basin porewater, and shallow wells in the
ash basin.
The hydroelectric generating station on Mountain Island Lake is operated in conjunction with the
upstream reservoir, Lake Norman. The operation the hydroelectric stations on Lake Norman
and Mountain Island Lake cause changes in the water levels in Mountain Island Lake on a
frequent basis. Monitoring well MW-13 is located downgradient from the ash basin Secondary
Cell and is approximately 100 feet from the bank of Mountain Island Lake (based on field
measurements made on April 15, 2013 by HDR). During the preliminary groundwater
assessment performed at RBSS in 2013, Duke Energy installed a pressure transducer in MW-
13 to record data every 15 minutes for the period of February 14, 2013 to March 22, 2013. The
results of the data evaluation in the 2013 report, comparing the water levels in the well to the
water levels in the lake, concluded that the water elevation in MW-13 appears to experience
direct and rapid changes in elevation due to changes in elevations of Mountain Island Lake.
Duke Energy is in the process of placing pressure transducers in several of the compliance
monitoring wells located adjacent to Mountain Island Lake to record groundwater elevation
measurements in these wells. An evaluation of the water level measurements from the pressure
transducers in these wells, compared to lake elevation data, will be provided in the CSA
Supplement.
11.8 Estimated Seasonal High Groundwater Elevations –
Compliance Wells
Estimated Seasonal Low (ESL) and Estimated Seasonal High (ESH) groundwater elevations
were calculated using historical groundwater elevations for select compliance and voluntary
wells at the site. The calculated ESL and ESH depth to water (DTW) was performed statistically
by multiplying the standard deviation of the historical DTW measurements by a factor of 1.2
then adding to the mean DTW measurement. To obtain the site modification factors for ESL and
ESH conditions, the calculated ESL and ESH DTW in the historical site wells were compared to
the current groundwater levels on site and the difference was calculated. The difference
between ESH and ESL DTW and current conditions was then averaged for the representative
site wells to create a modification factor to add to current DTW. Monitoring wells MW-10, MW-
13, MW-15, MW-4S, MW-5S, MW-9, MW-7SR, and MW-8S were selected as the most
representative shallow wells for natural seasonal fluctuations at the site, as they are located
outside of the ash basin embankments and are, therefore, less likely to be influenced by the
water level in the ash basin. Appendix H summarizes calculated ESH and ESL groundwater
elevations for newly installed groundwater monitoring wells.

73

This page is intentionally left blank.
Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
12.0 SCREENING-LEVEL RISK ASSESSMENT

12.0 Screening-Level Risk Assessment
The prescribed goal of the human health and ecological screening level risk assessments is to
evaluate the analytical results from the COI sampling and analysis effort and using the various
criteria taken from applicable guidance, determine which of the COIs may present an
unacceptable risk, in what media, and therefore, should be carried through for further evaluation
in a baseline human health or ecological risk assessment or other analysis, if required.
Constituents of Probable Concern (COPCs) are those COIs that have been identified as having
possible adverse effects on human or ecological receptors that may have exposure to the
COPCs at or near the site. The COPCs serve as the foundation for further evaluation of
potential risks to human and ecological receptors.
To support the CSA effort and inform corrective action decisions, a screening level evaluation of
potential risks to human health and the environment to identify preliminary, media-specific
COPCs has been performed in accordance with applicable federal and state guidance, including
the Guidelines for Performing Screening Level Ecological Risk Assessments within the North
Carolina Division of Waste Management (NCDENR 2003). The criteria for identifying COPCs
vary by the type of receptor (human or ecological) and media, as shown in the comparison of
contaminant concentrations in various media to corresponding risk-based screening levels
presented in Tables 12-1 through 12-9.
In the human health and ecological screening level risk assessments, the maximum
concentrations detected for all COIs, [or other appropriate data point (i.e., the analytical
reporting limit [RL]) in the 2015 sampling and analyses for coal ash detection and assessment
monitoring analytes were compared against established and conservative human health and
ecological screening toxicity reference values, likely to be protective for even the most sensitive
types of receptors.
These comparisons are used to determine which COIs present a potentially unacceptable risk to
human and/or ecological receptors and may warrant further evaluation. Those COIs determined
to pose a potential for adverse impacts are identified as preliminary COPCs.
Other factors that will be considered qualitatively in the evaluation of final COPCs that would be
incorporated into a baseline risk assessment include frequency of detection and a comparison
to background. Site- and media-specific risk-based remediation standards may be calculated,
pending additional sample collection, if and where additional sampling and site-specific
standards are deemed necessary.
12.1 Human Health Screening
12.1.1 Introduction
This screening level human health risk assessment has been prepared in accordance with
applicable NCDENR and USEPA guidance and the approved Work Plan.
74

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
12.0 SCREENING-LEVEL RISK ASSESSMENT

12.1.2 Conceptual Site Model
The Conceptual Site Model (CSM) is a dynamic tool for understanding site conditions and
potential exposure scenarios for human receptors that may be exposed to site-related
contamination. The CSM provides graphical representation of the following:
• A source and mechanism of chemical release;
• A retention or transport medium for COPCs;
• A point of contact between the human receptor and the medium; and
• A route of exposure to constituents for the potential human receptor at the contact point.

An exposure pathway is considered complete only if all four “source to receptor” components
are present. A CSM has been prepared illustrating potential exposure pathways from the source
area to possible receptors (see Figure 12-1). The information in the CSM has been used in
conjunction with the analytical data collected as part of the CSA to determine COPCs for the
site.
Potential receptors are defined as human populations that may be subject to contaminant
exposure. Both current and future land and water use conditions were considered when
determining exposure scenarios. Current and future land use of the RBSS site and associated
ash basin and ash storage area is expected to remain predominantly industrial while
decommissioning and restoration of the site is in progress (Duke Energy 2013b). The
hydroelectric stations on either end of Mountain Island Lake (i.e., the upstream Cowans Ford
Hydroelectric Station and the downstream Mountain Island Hydroelectric Station) will also
remain in active use for the foreseeable future. Lands surrounding the site include residential
and public recreation areas, as well as Mountain Island Lake, which supplies water to various
municipalities (Charlotte-Mecklenburg Storm Water Services 2011).
The following potential receptors are identified in the CSM:
• Current/future on-site construction worker with potential exposure to groundwater in
trenches, surface and subsurface soil and surface water;
• Current/future on-site outdoor worker with potential exposure to surface soil and surface
water;
• Current/future adult and child off-site resident with potential exposure to surface soil and
groundwater; and
• Current/future on-site trespasser with potential exposure to surface soil, surface water
and sediment.

Other exposure pathways for all potential receptors were evaluated and it was determined that
they would not have a significant impact on the risk assessment (e.g., outdoor worker inhalation
of inorganics in surface water in open air). Other exposure scenarios will also serve as
surrogates that will provide information about the magnitude of these potential risks.
The following presents a description of each receptor and potentially complete exposure
pathway:
75

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
12.0 SCREENING-LEVEL RISK ASSESSMENT

12.1.2.1 Current/Future Construction Worker
It was assumed that construction activities during decommissioning and restoration of RBSS
could take place on-site and that construction workers would potentially be exposed to COPCs
in various media during this timeframe. The potentially complete exposure pathways include
incidental ingestion, dermal contact and particulate inhalation exposure to surface and
subsurface soil. Construction workers in a trench with contact to groundwater may have
inhalation of metal COPCs with inhalation toxicity criteria and incidental ingestion of and dermal
contact (over limited parts of the body) with groundwater. Given the presence of ash basin and
ash storage area, dermal contact and incidental ingestion exposure to surface water would also
be considered a potentially complete exposure pathway for this receptor.
12.1.2.2 Current/Future Outdoor Worker
Outdoor workers are assumed to be involved with non-intrusive activities (e.g., landscapers that
will maintain the site). This receptor reflects a longer timeframe and different exposure pathways
than that of construction workers. Outdoor workers are assumed to have incidental ingestion,
dermal contact and particulate inhalation exposure to surface soil as well as dermal contact and
incidental ingestion exposure to surface water (e.g., ash basins).
Exposure to COPCs in groundwater is not identified in the CSM because outdoor workers are
assumed not to ingest untreated water; any COPCs aerosols or fumes will dissipate in open air,
and there is limited opportunity for dermal contact. Construction worker exposure scenarios are
considered a conservative surrogate to estimate the potential risk from groundwater to outdoor
workers.
12.1.2.3 Current/Future Off-Site Resident (Adult/Child)
The potential for off-site residents to be exposed to COPCs in untreated groundwater is
included in the CSM as one private water supply well was identified in Mecklenburg County
within a 0.5-mile radius of the ash basin compliance boundary northeast of RBSS, as described
in Section 4.0 above and the 2014 Drinking Water Supply Well and Receptor Survey and its
Supplement (HDR 2014a and b) (Figure 4-1). These exposures will consider all on and off-site
monitoring well data, excluding the receptor survey data, which is being handled independent of
this risk analysis.
Exposure routes are to include ingestion of groundwater (not incidental, but potable use) as well
as dermal contact during bathing/showering and inhalation during bathing/showering for those
metals in groundwater with available inhalation-based toxicity criteria.
Residents are assumed to be exposed to contaminants in surface soil during non-intrusive
outdoor activities (e.g., gardening); the potential exposure pathways include ingestion, dermal
and inhalation of soil particulates.
The Catawba River/Mountain Island Lake is a public drinking water supply that is treated before
consumption. Therefore, residential exposure to untreated surface water has not been
evaluated.
76

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
12.0 SCREENING-LEVEL RISK ASSESSMENT

12.1.2.4 Current/Future Trespasser (Adolescent/Adult)
Trespassers may come into direct contact with or incidentally ingest surface water and sediment
while on-site and near the Catawba River/Mountain Island Lake during what is assumed to be
predominantly recreational activity. This will occur at different rates depending on the specific
activity and setting. The exposure parameters for this scenario will be determined and will
incorporate all on- and off-site data for these media.
Exposure routes are to include incidental ingestion, dermal contact and particulate inhalation of
surface soil, as well as incidental ingestion and dermal contact with surface water and sediment.
This receptor reflects greater exposure to surface water, sediment and soil COPCs compared to
potential exposures of similar potential receptors (e.g., off-site recreator).
12.1.3 Human Health Risk-Based Screening Levels
A comparison of contaminant concentrations in various media to corresponding risk-based
screening levels has been made and is presented in Tables 12-1 through 12-5. These include:
• Soil: USEPA industrial soil Regional Screening Levels (RSLs) at a target cancer risk of
1E-06 and noncancer Hazard Quotient of 0.1
• Groundwater: USEPA tap water RSLs and NCDENR 2L Standards
• Surface water: USEPA National Recommended Water Quality Criteria and NCDENR 2B
Standards, considering the surface water classification for local water bodies
• Sediment: USEPA residential soil RSLs

Tables 12-1 through 12-5 present the constituents that were detected at concentrations
exceeding their relevant human health or other applicable criteria on a media-specific basis, in
ground and surface water, sediment, and soil. A summary of the COPCs is provided in Table
12-5.
Those COIs exceeding relevant screening criteria are identified as COPCs for purposes of the
human health risk assessment.
• In groundwater: beryllium, copper, lead, titanium, and zinc were eliminated as COPCs.
See Table 12-1 for maximum concentrations detected, the detailed screening results,
identification of COPCs, and contaminant categories.
• In soil: arsenic, cobalt, iron, manganese and thallium were detected at concentrations
exceeding the industrial soil screening levels and are determined to be COPCs. Sodium
was retained by default due to a lack of screening criterion for comparison. See Table
12-2 for the soil maximum concentrations, COPC and contaminant category data.
• Aluminum, antimony, arsenic, barium, cobalt, molybdenum, nickel thallium and strontium
have been excluded as COPCs in surface water, as shown in Table 12-3. No COIs
exceeded their respective screening values; all other constituents were identified as
COPCs based on a lack of screening values for comparison.
• Sediment COPCs and contaminant categories are presented in Table 12-4, which shows
that aluminum, antimony, arsenic, barium, cobalt, iron, manganese, thallium, and
77

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
12.0 SCREENING-LEVEL RISK ASSESSMENT

vanadium were determined to be COPCs based on their maximum concentrations
exceeding screening values. All other COIs have been excluded.

Constituents were not screened out as COPCs based on a comparison to background
concentrations, as USEPA recommends all COIs exceeding risk-based screening levels be
considered in a baseline risk assessment (USEPA 2002). Statistical background concentrations
have been developed as Prediction Limits (PLs), calculated for each constituent using
groundwater data in site background wells. PLs are a calculation of the upper limit of possible
future values based on the Statistical Analysis of Groundwater Monitoring Data at RCRA
Facilities Unified Guidance (USEPA, March 2009). If concentrations of COIs detected exceed
the PL, then the groundwater concentrations are assumed to have increased above background
levels. Site-specific background concentrations will be considered in the uncertainty section of
this baseline risk assessment, if determined to be required.
12.1.4 Site-Specific Risk Based Remediation Standards
Based on the results of the comparison to risk-based screening levels, media-specific
remediation standards will be calculated in accordance with the Eligibility Requirements and
Procedures for Risk-Based Remediation of Industrial Sites Pursuant to North Carolina General
Statute 130A-310.65 to 310.77, should additional sample collection and site-specific standards
be deemed necessary.
12.1.5 NCDENR Receptor Well Investigation
Numerous off-site private water supply wells were sampled and analyzed for constituents as
part of NCDENR’s well testing program of receptors within 1,500 feet of the RBSS compliance
boundary, as described in Section 4.0. No information on sampling results or any
recommendations from NCDENR on well water consumption for RBSS is available at this time.
12.1.6 Human Health Risk Screening Summary
A CSM was developed to identify potential pathways of exposure from COPC source to receptor
populations; including several possible exposure scenarios. Maximum concentrations of
constituents were compared to media-specific screening levels; constituents exceeding
screening levels and those having no screening levels or issues with RLs were retained as
COPCs, in accordance with guidance. As a result of the screening, the majority of constituents
were determined to be COPCs in groundwater; beryllium, copper, lead, titanium and zinc were
excluded. Only five constituents exceeded their soil screening values, these include arsenic,
cobalt, iron, manganese and thallium. No constituents exceeded screening values for surface
water, any retained as COPCs are by default due to a lack of criteria being available for
comparison. Nine constituents were determined to be COPCs based on exceedances of their
screening values in sediment.

78

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
12.0 SCREENING-LEVEL RISK ASSESSMENT

12.2 Ecological Screening
12.2.1 Introduction
This screening-level ecological risk assessment (SLERA) has been prepared in accordance with
the Guidelines for Conducting a Screening Level Ecological Risk Assessments within the North
Carolina Division of Waste Management (NCDENR 2003). An ecological CSM has been
developed for the RBSS site and is provided as Figure 12-2.
12.2.2 Ecological Setting
12.2.2.1 Site Summary
Refer to Section 2.0 for a description of the RBSS site.
12.2.2.2 Regional Ecological Setting
The site is located in the Southern Outer Piedmont eco-region of North Carolina adjacent to
Mountain Island Lake, which is part of Catawba River and is bordered by the Northern Inner
Piedmont and Carolina Slate Belt eco-regions (Griffith et al. 2002).
12.2.2.3 Description of the Eco-Region and Expected Habitats
The region consists of irregular plains and low to moderate gradient streams with less
precipitation and elevation than the Inner Piedmont. The common rock types include gneiss,
schist, and granite covered by deep saprolite and mostly red, clayey subsoil. Land cover
consists of mixed white oak forests, croplands, and pastures as well as pine plantations (Griffith
et al. 2002).
12.2.2.4 Watershed in which the Site is Located
The site is located in the Catawba River Basin watershed. The North Carolina portion of the
river basin encompasses approximately 3,300 miles in all or in part of 11 counties. It straddles
the southeastern corner of the Blue Ridge eco-region and the southwestern portion of the
Piedmont eco-region.
12.2.2.5 Average Rainfall
The average annual precipitation for Mt. Holly has been 41.63 inches over the past 30 years.
The average for the State of North Carolina is 48.87 inches. (Weather DB, 2015).
12.2.2.6 Average Temperature
The average temperature for Mt. Holly is 61.55° F. The average winter temperature is 49.1° F.
The average spring temperature is 57.1° F. The average summer temperature is 76.6° F and
average fall temperature is 63.3° F. (Weather DB, 2015).
79

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
12.0 SCREENING-LEVEL RISK ASSESSMENT

12.2.2.7 Length of Growing Season
According to the North Carolina State University Cooperative Extension, the average growing
season for Gaston County is 208 days, with a standard deviation of 22 days.
12.2.2.8 Threatened and Endangered Species that use Habitats in the Eco-Region
A list of threatened and endangered species for Gaston and Mecklenburg Counties is provided
in Table 12-10.
12.2.2.9 Site-Specific Ecological Setting
An ecological checklist and habitat figure has been completed for this site and is provided in
Appendix I.
Several streams are located on-site and may be intermittent during dry periods. Evidence of
flooding was also noted during HDR’s June 3, 2015 site visit.
Requests for information were submitted to several federal and state agencies, in accordance
with the North Carolina Guidelines for Performing Screening Level Ecological Risk Assessments
(NCDENR, 2003). A copy of the requests and responses are provided in Appendix I and a
summary of the information is provided, as follows.
North Carolina Department of Cultural Resources
In a letter dated June 23, 2015, the North Carolina Department of Cultural Resources indicated
that there are “no historic resources which would be affected by the project”.
North Carolina Natural Heritage Program
In a letter dated June 9, 2015, the North Carolina Natural Heritage Program (NCNHP) provided
information obtained from their database, both for the RBSS site and within a one-mile radius of
the site (see Appendix I). According to the NCNHP, their database did not identify records for
rare species or important natural areas located within the area evaluated. There are a total of
four Managed Areas (North Carolina Clean Water Management Trust Fund Easement)
documented within the site.
According to the NCNHP database, there are two Natural Areas, Mount Olive Church Basic
Forest and Mountain Island Lake Forest, located within a one-mile radius of the site. One
Managed Area, Cowan’s Ford Wildlife Refuge, is located within a one-mile radius of the site.
North Carolina Wildlife Resources Commission
In a letter dated June 19, 2015, the North Carolina Wildlife Resources Commission (NCWRC)
reported the following:
• The site drains to Mountain Island Reservoir in the Catawba River basin.
• There are records for the state threatened bald eagle (Haliaeetus leucocephalus) and
Northern cup-plant (Silphium perfoliatum), the state special concern oldfield deermouse
80

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
12.0 SCREENING-LEVEL RISK ASSESSMENT

(Peromyscus polionotus), and the state significantly rare-throughout glade wild quinine
(Parthenium auriculatum) near the site. There are historical records for the federal
species of concern and state endangered tall larkspur (Delphinium exaltatum) and the
state significantly rare helicta satyr (Neonympha helicta) near the site.
• Bald eagles forage in the area.
• There is recreational fishing in Mountain Island Reservoir. Recreational species include:
striped bass, largemouth bass, catfish, crappie, sunfish, white bass, and white perch.

United States Department of Agriculture, National Forests in North Carolina
In an email dated May 28, 2015, it was reported that there are no Designated and Proposed
Federal Wilderness and Natural Areas, National Preserves and Forests, or Federal Land
Designated for the Protection of Natural Ecosystems with a half-mile of the RBSS site.
United States Department of the Interior, National Park Service
In an email dated June 3, 2015, the United States Department of the Interior, National Park
Service indicated that “the NPS has not identified any resource concerns at this time”.
12.2.2.10 On-site and Off-site Land Use
On-site land use is approximately 30% heavy industrial, 5% light industrial, 55% undisturbed,
and 10% other (e.g., water bodies). Land use within a one-mile radius of the site is 30%
residential, 20% undisturbed, 20% rural, and 30% other (including Mountain Lake and the
Catawba River).
12.2.2.11 Habitats within the Site Boundary
Based on HDR’s June 3, 2015 site visit, the following habitats are present on site.
• 152 acres of Mixed Hardwoods
• 4.5 acres of Pine Plantation
• 24 acres of Bottomland Hardwoods
• 6.8 acres of Shrub/Scrub
• 30 acres of Open Fields
• Aquatic features including ash basins, streams, and wetlands

Forested areas with trees having exfoliating bark (i.e., hickories and white oaks) may be
potential roosting trees for the federally threatened northern long-eared bat. Power line
clearings, woodland edges, and openings are potential habitat for federally endangered
Schweinitz’s sunflower, Michaux’s Sumac, and smooth coneflower. Large pines along the
Catawba River and Mountain Lake may serve as potential nest trees for the bald eagle which
likely forage in the area. For a detailed description of habitats, refer to the Checklist for
Ecological Assessments located in Appendix I.
81

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
12.0 SCREENING-LEVEL RISK ASSESSMENT

12.2.2.12 Description of Man-made Units that may Act as Habitat
A 69-acre ash basin (Primary and Secondary Cells) is present on site and may act as man-
made habitat.
12.2.2.13 Site Layout and Topography
Topography at the RBSS site ranges from an approximate elevation of 786 feet near the south
edge of the property near Horseshoe Bend Beach Road to an elevation of 646 feet at the
boundary with Mountain Island Lake on the northern extent of the site.
The site generally slopes from south to north with an elevation loss of approximately 140 feet
over 3,500 feet. Surface water drainage generally follows site topography and flows from the
south to the north across the site, except where natural drainage patterns have been modified
by the ash basin, ash storage area, or other construction.
12.2.2.14 Surface Water Runoff Pathways
Swales and depressions were observed during HDR’s June 3, 2015 site visit. Unnamed
drainage features are located on the eastern and northwestern portions of the site and generally
flow north toward Mountain Island Lake.
12.2.2.15 Soil Types
Based on lithological data included in soil boring and monitoring well installation logs provided
by Duke Energy (ARCADIS G&M of North Carolina, Inc., 2007 and MACTEC, 2011), subsurface
stratigraphy consists of the following material types: fill, ash, residual soil, saprolite, alluvium,
PWR, and bedrock.
In general, residual soil, saprolite, and PWR were encountered on most areas of the site.
Alluvium was encountered in borings advanced along the northeastern extent of the Secondary
Cell, within close proximity to Mountain Island Lake. Bedrock was encountered sporadically
across the site ranging in depth from 34 feet on the northern extent of the site to greater than
200 feet on the southern extent of the site near Horseshoe Bend Beach Road.
12.2.2.16 Species Normally Expected to Use Site under Relatively Unaffected Conditions
Terrestrial communities occur in both natural and disturbed habitats in the study area; these
may support a diversity of wildlife species. Information on the species that would normally be
expected to use this and similar sites in the Piedmont eco-region under relatively unaffected
conditions was obtained from relevant literature, mainly the Biodiversity of the Southeastern
United States, Upland Terrestrial Communities (Wiley and Sons 1993) and Biodiversity of the
Southeastern United States, Aquatic Communities (Wiley and Sons 1993).
Mammal species that may be present include eastern cottontail (Sylvilagus floridanus), gray
squirrel (Sciurus carolinensis), various vole, rat and mice species, red (Vulpes vulpes) and gray
fox (Urocyon cinereoargenteus), raccoon (Procyon lotor), Virginia opossum (Didelphis
virginiana), and white-tailed deer (Odocoileus virginiana).
82

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
12.0 SCREENING-LEVEL RISK ASSESSMENT

Avian species are the most diverse. Canopy dwellers include the great crested flycatcher
(Myiarchus crinitus), Carolina chickadee (Parus carolinensis), tufted titmouse (P. bicolor), white-
breasted nuthatch (Sitta carolinensis), blue-gray gnatcatcher (Polioptila caerulea), red-eyed
vireo (Vireo olivaceus), yellow-throated vireo (V. flavifrons), various warblers and tanagers, and
American redstart (Setophaga ruticilla).
Subcanopy species include a variety of woodpeckers, eastern pewee (Contopus virens),
Acadian flycatcher (Empidonax virescens), American crow (Corvus brachyrhynchos), blue jay
(Cyanocitta cristata), and Carolina wren (Thryothorus ludovicianus).
Catbirds (Dumetella carolinensis), brown thrashers (Toxostoma rufum), and mockingbirds
(Mimus polyglottos) are found along adjacent brushy edges, fields, and thickets.
Understory species include wood thrush (Hylocichla mustelina), American robin (Turdus
migratorius), white-eyed vireo (Virea griseus), Kentucky warbler (Oporornis formosus), common
yellow-throat (Geothlypis trichas), and yellow breasted chat (Icteria virens). Predatory birds
include several hawk and owl species and the turkey vulture (Cathartes aura).
Amphibians and reptiles that tend to be associated with the terrestrial-aquatic interface in
streams, rivers, and open waters may include certain turtles (e.g., the Striped Mud and Gulf
Coast Spiny Softshell turtles); and frogs, snakes, and amphibians such as the Three‐lined
salamander. For a more detailed description, see Appendix I.
Streams of the southeastern piedmont support a range of aquatic benthic macroinvertebrate
groups including mayflies (Ephemeroptera), stoneflies (Plecoptera), caddisflies (Trichoptera),
water beetles (Coleoptera), dragonflies and damselflies (Odonata), dobsonflies and alderflies
(Megaloptera), true flies (Diptera), worms (Oligochaeta), crayfish (Crustacea), and clams and
snails (Mollusca).
Streams, rivers, ponds, and reservoirs support populations of game fish that may include
redbreast sunfish (Lepomis auritus), bluegill (Lepomis macrochirus), warmouth (Lepomis
gulosus), and largemouth bass (Micropterus salmoides). The most widespread non-game fish
species are American eel (Anguilla rostrata), eastern silvery minnow (Hybognathus regius),
bluehead chub (Nocomis leptocephalus), golden shiner (Notemigonus crysoleucas), spottail
shiner (Notropis hudsonius), whitefin shiner (N. niveus), swallowtail shiner (N. procne), creek
chub (Semotilus atromaculatus), creek chubsucker (Erimyzon oblongus), silver redhourse
(Moxostoma anisurum), yellow bullhead (Ictalurus natalis), flat bullhead (I. platycephalus),
margined madtom (Noturus insignis), and tessellated darter (Etheostoma olmstedi).
12.2.2.17 Species of Special Concern
For a detailed list of species of special concern that may be present, see Table 12-10.
12.2.2.18 Nearby Critical and/or Sensitive Habitats
For a detailed description, see Section III.D of the Ecological Checklist provided in Appendix I.
83

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
12.0 SCREENING-LEVEL RISK ASSESSMENT

12.2.3 Fate and Transport Mechanisms
Potential fate and transport mechanisms at/near the RBSS site include erosion, seeps,
stormwater runoff and flow of surface water bodies. An Ecological CSM (Figure 12-2) has been
prepared illustrating potential exposure pathways from the source area to possible ecological
receptors. The information in the ecological CSM has been used in conjunction with the
analytical data collected as part of the CSA to develop an understanding of the sources,
pathways and media of exposure as well as the receptors potentially impacted by site-related
COIs.
12.2.4 Comparison to Ecological Screening Levels
The sampling and analysis program completed as part of the RBSS CSA investigation is
described earlier in this report. Media of primary concern for ecological receptors (i.e., surface
water, sediment, and soil) have been sampled extensively, in accordance with the NCDENR
approved Work Plan.
The results of the comparison of COI concentrations in various media to risk-based screening
levels are presented in Tables 12-6 through 12-9, and include:
• USEPA Region IV Recommended Ecological Screening Values for soil, surface water
and sediment
• USEPA National Recommended Water Quality Criteria and North Carolina Freshwater
Aquatic Life Standards

The potential for ecological risk was also estimated by calculating screening hazard quotients
(HQ) using the appropriate screening value of each contaminant and comparing that value to
the USEPA Region IV Ecological Screening Values. Constituents having an HQ greater than or
equal to 1 are identified as COPCs.
NCDENR guidance requires a determination of which contaminant category the COPCs fall into
as a result of the data comparison to screening levels is also presented in the ecological COPC
tables (Tables 12-6 through 12-9). These include:
• Category 1 – Contaminants whose maximum detection exceeds the media specific
ecological screening value included in the COPC tables.
• Category 2 – Contaminants that generated a laboratory SQL that exceeds the USEPA
Region IV media-specific ecological screening value for that contaminant.
• Category 3 – Contaminants that have no USEPA Region IV ecological screening value,
but were detected above the laboratory SQLs.
• Category 4 – Contaminants that were not detected above the laboratory SQLs and have
no USEPA Region IV ecological screening value.
• Category 5 – Contaminants whose SQL or maximum detection exceeds the North
Carolina 2B Standards.
84

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
12.0 SCREENING-LEVEL RISK ASSESSMENT

Table 12-9 presents a summary of the COIs that were detected at concentrations exceeding
their relevant ecological screening media-specific criteria. Those constituents exceeding the
relevant criteria are identified as ecological COPCs for purposes of the SLERA.
Note that NCDENR SLERA guidance does not allow for exclusion of COIs as COPCs based on
a comparison to background concentrations.
In soil, the majority of COIs were detected at concentrations exceeding the ecological soil
screening levels. Cadmium and lead were excluded as COPCs, as they were detected at levels
below their screening values. Sodium and strontium are retained by default, as having no
screening value available for comparison. See Table 12-6 for detailed information, including the
maximum concentrations detected and characterization as to the category into which each
COPC falls. Six COPCs were detected at concentrations an order of magnitude or more than
their respective screening levels.
Based on the comparison of maximum detected concentrations to screening criteria, aluminum,
mercury and zinc are identified as ecological COPCs in surface water. Barium, cobalt,
manganese, molybdenum, strontium, and vanadium are retained by default due to the fact that
there are no ecological criteria available. Further information on the screening performed and
characterization as to the contaminant category each COPC falls into is provided in Table 12-7.
Only lead was excluded as a COPC in sediment. Many COIs are retained by default due to a
lack of available screening values. Antimony, arsenic, total chromium, copper, mercury, nickel,
and zinc were retained for exceeding their respective screening concentrations. Details on the
COPC screening, maximum concentrations detected, and contaminant category are provided in
Table 12-8.
COIs were not screened out as COPCs based on a comparison to background concentrations,
as NCDENR SLERA guidance does not allow for screening based on background. Site-specific
background concentrations, discussed above in Section 12.1.3 will be considered in the
uncertainty section of the baseline ecological risk assessment, if determined to be necessary.
12.2.5 Uncertainty and Data Gaps
There are uncertainties inherent in any environmental investigation and risk evaluation that
involve natural heterogeneity of the media, nature, and extent of constituents in the
environment, due to their individual fate and transport characteristics, and varied, site-specific
conditions. These uncertainties are considered in developing the sampling and analysis plan,
data quality assurance processes and understanding of the site.
These screening level assessments are designed to be very conservative in identifying potential
COPCs that would be carried forward into a baseline human health and/or ecological risk
assessment. They include all on- and off-site analytical data, and use the maximum
concentration detected as the comparison point to applicable screening criteria. Also, no
constituents were eliminated as COPCs based on background levels; this will be evaluated in
the baseline risk assessment, if they are required to be performed. These are highly unlikely to
85

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
12.0 SCREENING-LEVEL RISK ASSESSMENT

be the actual exposure concentrations, given the natural attenuation, dilution and distances to
potential receptors.
There is a high level of confidence that any constituent with potential to impact human health or
ecological receptors has been identified as a result of these assessments.
12.2.6 Scientific/Management Decision Point
If through the HQ analysis it is determined that constituents have been detected at maximum
concentrations that exceed applicable screening criteria that is an indication that additional
assessment of potential risks is warranted. This does not mean that impacts are in fact,
occurring; only that further data collection or evaluation should be considered.
This determination is known as the Scientific/Management Decision Point (SMDP) and the
conclusion reached must be one of the following:
• There is adequate information to conclude that the ecological risks are neglibible; or
• Site has inadequate data to complete the risk characterization. Data gaps need to be
filled prior to completion of the screening process; or
• The information indicates a potential for adverse ecological effects and a more thorough
assessment is warranted.

Given that several COPCs have been identified as having an HQ of greater than 1 in soil,
surface water, and sediment, there is adequate information indicating a potential for adverse
effects to occur and a baseline ecological risk assessment may be warranted. The need for a
separate baseline ecological risk assessment is being considered in light of the other ongoing or
planned environmental impact studies for this site.
12.2.7 Ecological Risk Screening Summary
The SLERA has identified that the potential exists for adverse ecological impacts due to
exposure to COPCs in soil, surface water, and/or sediment. Cadmium and lead are the only
constituents that has been excluded as COPCs in soil and numerous COPCs exceed their
respective screening criteria by one or two orders of magnitude. Notably fewer COIs have been
identified in surface water and sediment and several of those are retained by default for having
no criteria or due to RL issues, not due to maximum concentrations actually exceeding
screening criteria. Impacts from limited ecological receptor groundwater exposure are minimal
and have not been evaluated. For RBSS, identification of potential data gaps and overall
coordination of further ecological risk assessment efforts, specifically for surface water and
sediment impacts should consider any other activities that are ongoing related to ash basin
closure activities to avoid duplication of effort.

86

This page is intentionally left blank.
Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
13.0 GROUNDWATER MODELING

13.0 Groundwater Modeling
Groundwater modeling will be performed and submitted in the CAP in accordance with
NCDENR’s Conditional Approval letter. The groundwater modeling will consist of groundwater
flow and fate and transport modeling, performed with MODLFOW and MT3DMS, and batch
geochemical modeling, performed with PHREEQC. The following section presents an overview
of the fate and transport modeling, the batch geochemical modeling and the site geochemical
conceptual model.
The CAP will also present a discussion of the geochemical properties of the COIs and how
these properties relate to the retention and mobility of these constituents.
13.1 Fate and Transport Groundwater Modeling
A three-dimensional groundwater flow and contaminant fate and transport model
(MODFLOW/MT3DMS Model) will be developed for the ash basin site. The objective of the
modeling will be to predict the following in support of the CAP:
• Predict concentrations of the COIs at the compliance boundary or other locations of
interest over time,
• Estimate the groundwater flow and constituent loading to surface water discharge areas,
and
• Predict approximate groundwater elevations in the ash for the proposed corrective action

The model and model report will be developed in general accordance with the guidelines found
in the memorandum Groundwater Modeling Policy, NCDENR DWQ, May 31, 2007 (DENR
modeling guidelines).
The groundwater model will be developed from the hydrogeologic conceptual model presented
in the CSA, from existing wells and boring information provided by Duke Energy, and from
information developed during the site investigation. The model will also be supplemented with
additional information developed by HDR from other Piedmont sites, as applicable. The site
conceptual Model SCM is a conceptual interpretation of the processes and characteristics of a
site with respect to the groundwater flow, boundary conditions, and other hydrologic processes
at the site.
Although the site is anticipated in general to conform to the LeGrand conceptual groundwater
model, due to the configuration of the ash basin, the additional possible sources (ash storage
area and cinder storage area), and the boundary conditions present at the site, a three-
dimensional groundwater model is warranted.
The groundwater modeling will be performed under the direction of Dr. William Langley, P.E.,
Department of Civil and Environmental Engineering, University of North Carolina Charlotte
(UNCC). Groundwater flow and constituent fate and transport will be modeled using Visual
MODFLOW 2011.1 (flow engine USGS MODFLOW 2005) and MT3DMS.
87

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
13.0 GROUNDWATER MODELING

The modeling process, the development of the model, the development of the hydrostratigraphic
layers, the model extent (or domain), and the proposed model boundary conditions were
described in Section 7.0 of the work plan. To date, no changes to the proposed model
development are warranted based on data collected during the site investigation.
The MT3DMS model will use site specific Kd values developed from samples collected along
the major flow transects. The testing to develop the Kd terms is underway, but is not complete
at this time, therefore the results of that testing will be presented in the CAP. The methods used
to develop the Kd terms was presented in Section 7.7.2 of the work plan.
13.2 Batch Geochemical Modeling
As described in the Work Plan, batch geochemical simulations using PHREEQC will be used to
estimate sensitivity of the proposed sorption constants used with MT3DMS and to assist in
understanding the mechanisms involved in attenuation of selected constituents. Geochemical
modeling using PHREEQC can be used to indicate the extent to which a COI is subject to
solubility constraints a variable Kd, or other processes. PHREEQC can also identify postulated
solid phases calculation of their respective saturation indices. The specific locations where the
batch geochemical modeling will be performed will be determined after the development of the
Kd terms and a review of the site data.
13.3 Geochemical Site Conceptual Model
SCMs are developed to be a representation of what is known or suspected about contamination
sources, release mechanisms, transport, and fate of those contaminants.4 An SCM can be a
written and/or graphic presentation of site conditions to reflect the current understanding of the
site, identify data gaps, and be updated as new information is collected throughout the project.
SCMs can be utilized to develop understanding of the different aspects of site conditions, such
as a hydrogeologic conceptual site model, to help understand the site hydrogeologic condition
affecting groundwater. SCMs can also be used in a risk assessment to understand contaminant
migration and pathways to receptors.
On June 25, 2015, NCDENR made the following request:
Since speciation of groundwater and surface water samples is a critical
component of both the site assessments and corrective action, the Division
expects a geochemical site conceptual site model (CSM) developed as a
subsection in the Comprehensive Site Assessment (CSA) Reports. The
geochemical CSM should provide a summary of the geochemical interactions
between the solution and solid phases along the groundwater flowpath that
impact the mobility of metal constituents. At a minimum, the geochemical CSM
will describe the adsorption/desorption and mineral precipitation/dissolution
processes that are believed to impact dissolved concentrations along the aquifer
flowpaths away from the ash basin sources. The model descriptions should
4 EPA MNA Volume 1
88

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
13.0 GROUNDWATER MODELING

include the data upon which the conceptual model is based and any calculations
(such as mineral saturation indices) that are made to develop the site-specific
model.
Metal speciation analyses cover a broad aspect of metals’ geochemistry,
including solution complexation with other dissolved species and specific
association with aquifer solids, such as a metal adsorbed onto HFO or
precipitated as a sulfate mineral. A comprehensive speciation analysis that
requires a relatively complete groundwater analysis is expected that includes use
of an ion speciation computer code (such as PHREEQC) capable of calculating
solution complexes, surface complexation onto HFO, and mineral saturation
indices. This type of speciation calculation is necessary for the development of a
geochemical SCM and understanding metal mobility in an aquifer.
In previous correspondence, NCDENR agreed that the proposed geochemical modeling
described in the Work Plan, to be performed using PHREEQC, will be included in the CAP.
Specifically, the model descriptions and calculations, such as mineral saturation indices, will be
provided in the CAP. This approach will allow completion of the testing to develop the site-
specific Kd terms and site mineralogy, and will allow the geochemical modeling to be
coordinated with the groundwater flow and transport model.
Elements of the geochemical site conceptual site model (GSCM) described below will be
incorporated into the fate and transport and the geochemical modeling performed for the CAP.
The GSCM will be updated as additional data and information associated with contaminants,
site conditions, or processes such as migration of contaminants is developed. The GSCM will
be useful in understanding the transport and attenuation factors that affect the mobility of
contaminants at the site and the long-term capacity of the site for attenuation and stability of
immobilized contaminants.
The GSCM will describe the geochemical aspects of the site sources that influence contaminant
transport. Site sources at RBSS consist of the ash basin, ash storage area, and the cinder
storage area; these source areas are subject to different processes that generate leachate
migrating into the underlying soil layers and into the groundwater. For example, the dry ash
storage area would generate leachate as a result of infiltration of precipitation, while the ash
basin would generate leachate based on the pond elevation in the basin. General factors
affecting the geochemistry of the site are as follows:
Factors Affecting Ash Formation (Primary Source):
• Chemical and mineralogical composition of coal
• Thermodynamics of coal combustion process
• Factors Affecting Leaching in Ash Basin (Primary Source Release Mechanism):
• Chemical composition of ash
• Mineral phase of ash
• Physical characteristics of ash
• Inflow of water into/out of basin
89

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
13.0 GROUNDWATER MODELING

• Period of time ash has been in basin
• Geochemical conditions in ash basin
• Precipitation-dissolution reactions
• Sorptive properties of materials in ash

Factors Affecting Leaching in Ash Basin (Primary Source Release Mechanism):
• Chemical composition of ash in storage area
• Mineral phase of ash in storage area
• Physical characteristics of ash in storage area
• Inflow of precipitation in to ash storage area
• Period of time ash has been in storage
• Geochemical conditions in ash storage area
• Precipitation-dissolution reactions
• Sorptive properties of materials in ash

Factors Affecting Leaching in the Ash Storage Area (Primary Source Release
Mechanism):
• Chemical composition of ash in storage area
• Mineral phase of ash in storage area
• Physical characteristics of ash in storage area
• Inflow of precipitation in to ash storage area
• Period of time ash has been in storage
• Geochemical conditions in ash storage area
• Precipitation-dissolution reactions
• Sorptive properties of materials in ash
•
Factors Affecting Leaching in the Cinder Storage Area (Primary Source Release
Mechanism):
• Chemical composition of ash in storage area
• Mineral phase of ash in storage area
• Physical characteristics of ash in storage area
• Inflow of precipitation in to ash storage area
• Period of time ash has been in storage
• Geochemical conditions in ash storage area
• Precipitation-dissolution reactions
• Sorptive properties of materials in ash

90

Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
13.0 GROUNDWATER MODELING

Factors Affecting Sorption and Precipitation of Constituents onto Soil/Aquifer Materials
Beneath Ash (Secondary Source Release Mechanism):
• Chemical composition of soil
• Physical composition of soil
• Rate of infiltration/percolation of porewater
• Chemical composition of leachate infiltrating into soil
• Sorption capacity of soil
• Geochemistry of groundwater flowing beneath unit

Factors Affecting Desorption and Dissolution of Constituents From Soil/Aquifer Materials
Beneath Ash (Secondary Source Release Mechanism):
• Chemical composition of soil
• Physical composition of soil
• Rate of infiltration/percolation of porewater
• Attenuation capacity of soil
• Chemical composition of leachate or precipitation infiltrating into soil
• Geochemistry of groundwater flowing beneath unit

The results of the Kd testing, the results from the site mineralogy testing, and the geochemical
modeling developed in the CAP will be used to refine the GSCM.

91

This page is intentionally left blank.
Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
14.0 DATA GAPS – CONCEPTUAL SITE MODEL UNCERTAINTIES

14.0 Data Gaps – Conceptual Site Model
Uncertainties
14.1 Data Gaps
Through completion of groundwater assessment field activities and evaluation of data collected
during those activities, Duke Energy has identified data gaps that will require further evaluation
to refine the SCM. The data gaps have been separated into two groups: 1) data gaps resulting
from temporal constraints and 2) data gaps resulting from evaluation of data collected during the
CSA.
14.1.1 Data Gaps Resulting from Temporal Constraints
Data gaps identified in this category are generally present due to insufficient time to collect,
analyze, or evaluate data collected during the CSA activities. It is expected that the majority of
these data gaps will be remedied in a CSA supplement to be submitted in consultation with
NCDENR.
• Mineralogical characterization of soil and rock: a total of 17 soil, three TZ, and eight
bedrock samples were submitted to three third-party mineralogical testing laboratories
for analysis of soil and rock composition. As of the date of this report, Duke Energy has
not received results of this testing; however, results should be available for inclusion in
the CSA supplement.
• Horizontal Delineation of Groundwater Contamination: as part of Work Plan
development prior to field mobilization, Duke Energy reviewed existing groundwater
quality data from compliance monitoring wells MW-8S/I/D to evaluate the potential for
off-site migration of COIs and the potential need for addional on-site and off-site wells.
This evaluation prompted the installation of groundwater monitoring wells GWA-
20S/D/BR, GWA-22S/D/BR, and GWA-23S/D/BR on the RBSS property south of the ash
storage area, and GWA-21S/D/BR on the adjacent property to the south of MW-8S/I/D
and south of Horseshoe Bend Beach Road, to better define groundwater flow in this
area and the distribution of COIs . The sampling results from these wells was not
received in time for the evaluation of the results to be incorporated in this report. This
evaluation will be included in the submittal of the CSA supplement.
• Additional Speciation Analyses: In order to meet the requirements of the NORR, Duke
Energy conducted speciation of samples for arsenic, chromium, iron, manganese, and
selenium along flow transects, at ash basin water sample locations, and at compliance
wells with historical exceedances of the 2L Standards for speciation constituents. Duke
Energy and NCDENR are currently conducting discussions concerning the specifics of
the requirements for sampling associated with additional speciation sampling.
• Groundwater analytical results from monitoring wells AS-3SA and GWA-4BR, and
bedrock analytical results from boring GWA-21D, were not received with sufficient time
92
Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
14.0 DATA GAPS – CONCEPTUAL SITE MODEL UNCERTAINTIES

to include the results in this report. These groundwater analytical results will be included
in the CSA supplement.
14.1.2 Data Gaps Resulting from Review of Data Obtained During CSA Activities
• Additional refinement is needed for the horizontal and vertical extent of groundwater
impacts to the west of the ash and cinder storage areas near well GWA-3SA/D with
sulfate, manganese, and TDS concentrations exceeding the 2L Standards reported at
this location. Additional groundwater monitoring wells may be required to delineate
exceedances in this area.
• Additional refinement is needed for the horizontal and vertical extent of groundwater
impacts to to the south of the ash storage area. Sulfate and TDS 2L Standard
exceedances were reported in monitoring wells south of the ash storage area. However,
groundwater flow direction in this location is from the south to the north/northwest. To
address this data gap, monitoring wells GWA-21S/D/BR are currently being installed
southeast of MW-8S/I/D and will refine the understanding of groundwater flow direction
in this area, and provide information regarding potential concentrations of sulfate and
TDS south of Horseshoe Bend Beach Road. The analytical results from GWA-21S/D/BR
will be included in the CSA supplement and a determination will be made at that time
whether additional groundwater monitoring wells are needed south of the ash storage
area.
• Groundwater samples were inadvertently not collected from compliance background
monitoring wells MW-7D and MW-7SR. Although historical analytical results are
available for these wells, groundwater from these wells was not analyzed for the full list
of parameters and constituents used during the assessment activities. Monitoring wells
MW-7D and MW-7SR should be sampled and analyzed along for the same parameter
and constitiuent list as the assessment wells onsite during any future monitoring events.
• The vanadium method reporting limit provided by the analytical laboratory was 1.0 ug/L.
The IMAC for vanadium is 0.3 ug/L. The vanadium results reported at concentrations
less than the laboratory method reporting limit are estimated. During subsequent
monitoring events, a laboratory method reporting equal to or less than the IMAC should
be utilized.
• Newly installed monitoring wells GWA-9BR and C-1BRU, and voluntary monitoring wells
MW-2S and MW-4S were noted as dry at the time of the sampling event. A groundwater
sample was not able to be collected from these well. An attempt should be made to
collect a groundwater sample from this well during subsequent monitoring events. If the
well remains dry NCDENR will be contacted regarding the potential replacement of the
well. Review of Non-Ash Contamination Information: Review of information regarding
areas of non-ash contamination (i.e., petroleum-contaminated areas) is needed to
evaluate potential interferences with possible future remedial actions, if applicable.
14.2 Site Heterogeneities
In general, the groundwater flow direction within the shallow wells and deep wells flows radially
north, east, and west towards Mountain Island Lake. Groundwater in the southwest portion of
93
Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
14.0 DATA GAPS – CONCEPTUAL SITE MODEL UNCERTAINTIES

the site under the ash storage area flows to the northwest, under the cinder storage area to the
Catawba River. Flow contours developed from groundwater elevations measured in the shallow
and deep wells in the southeastern portion of the site depict groundwater flow generally to the
northeast to the Catawba River. Groundwater contours developed from the groundwater
elevations in the bedrock wells show groundwater moving in a northeasterly direction from the
south side of the site to the Catawba River.
Heterogeneities, with regard to COI concentrations, were not identified during completion of this
CSA. However, heterogeneities may be identified following completion of the groundwater
model for the RBSS site.
14.3 Impact of Data Gaps and Site Heterogeneities
Certain data gaps can be addressed with additional groundwater sampling at existing wells and
the collection of additional groundwater samples on the western portion and south of the site. As
discussed in Section 15, the second comprehensive groundwater sampling event is planned for
August/September 2015. A plan for interim groundwater sampling between submittal of the CSA
and implementation of the anticipated CAP is proposed in Section 16 and will further
supplement the existing data.

94
This page is intentionally left blank.
Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
15.0 PLANNED SAMPLING FOR CSA SUPPLEMENT

15.0 Planned Sampling for CSA Supplement
In accordance with CAMA, a second comprehensive groundwater sampling event at the RBSS
site is currently under discussion in consultation with NCDENR and Duke Energy. The second
sampling event will be conducted to:
• Supplement data obtained during the initial sampling event;
• Evaluate seasonal variation in groundwater results; and
• Collect additional samples for chemical speciation of arsenic, chromium, iron,
manganese, and selenium constituents should be further analyzed, if discussions with
NCDENR determine that this speciation sampling is required.
15.1 Sampling Plan for Inorganic Constituents
The second sampling event for inorganic constituents will consist of the following: sampling of
all locations (monitoring wells, seeps, surface water, and sediment) that were sampled during
the initial sample event. Locations that were previously dry will be re-evaluated and sampled if
sufficient water is present to do so. All samples collected will be analyzed for total inorganic
compounds. Groundwater samples with exceedances of 2L Standards during the initial
sampling event will also be analyzed for dissolved-fraction inorganics.
• Collection of second set of data for all new site assessment wells, seeps and surface
water for CSA work plan parameters (including total and dissolved metals using 0.45 µm
filters);
• Locations that were previously dry will be re-evaluated and sampled if sufficient water is
present to do so; and
15.2 Sampling Plan for Speciation Constituents
In consultation with NCDENR and Duke Energy, speciation sampling is anticipated to be
performed as follows:
• Collection of confirmation speciation data where initial data set provided apparent
anomalies as identified during the preparation of the CSAs (for hexavalent Cr in
particular);
• Collection of speciation data gaps that may have been identified during the preparation
of the CSA; and
• Collection of data gap information identified in the CSA’s for existing wells/surface
water/seep locations.

A summary of the proposed sampling program for the second comprehensive sampling event is
included in Table 15-1 and anticipated sampling locations are shown on Figure 6-2.

95
This page is intentionally left blank.
Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
16.0 INTERIM GROUNDWATER MONITORING PLAN

16.0 Interim Groundwater Monitoring Plan
CAMA requires a schedule for continued / interim groundwater monitoring. Given that Duke
Energy has recommended excavation of the existing ash basin, ash storage area, and cinder
storage area and removing this material to a lined landfill or other allowed beneficial use, certain
groundwater monitoring wells in these areas will be abandoned. As such, Duke Energy plans to
conduct interim groundwater monitoring of select wells, as identified in Section 16.3, to bridge
the gap between completion of CSA activities and implementation of the proposed CAP.
16.1 Sampling Frequency
One additional interim groundwater sampling is planned to occur during 2015 and the results
will be submitted in coordination with NCDENR. Interim groundwater sampling on a quarterly
basis is proposed until the CAP is approved by NCDENR. This sampling frequency will allow for
evaluation of seasonal fluctuations in COI concentrations, as well as provide additional data for
statistical analysis of site-specific background concentrations.
16.2 Constituent and Parameter List
The proposed constituents and parameters for analysis are presented in Table 16-1.
16.3 Proposed Sampling Locations
The proposed sampling locations are presented in Table 16-2.
16.4 Proposed Background Wells
The proposed background wells are MW-7BR, BG-1S, BG-1D, BG-2S, BG-2D, BG-2BR, BG-
3S, and BG-3D. Existing compliance background wells are MW-7SR and MW-7D. Interim
Groundwater Monitoring Plan, background wells are planned to be sampled during the interim
groundwater sampling event in 2015.

96
This page is intentionally left blank.
Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
17.0 DISCUSSION

17.0 Discussion
17.1 Summary of Completed and Ongoing Work
To date, the following activities have been completed in support of this CSA:
• Installation of 78 groundwater monitoring wells within the ash basin, ash storage area,
and cinder storage area, beyond the waste boundary, in background locations, and on
an adjacent off-site property;
• Completion of topographic and well/boring location surveys;
• Collection of ash samples from borings completed within the waste boundary and
analysis for total inorganics, TOC, anions/cations, SPLP, and physical properties;
• Collection of soil samples from borings completed within the waste boundary, beyond
the waste boundary, and background locations and analysis for total inorganics, TOC,
anions/cations, and physical properties;
• Collection of PWR and bedrock samples from borings completed within the waste
boundary, beyond the waste boundary, and background locations and analysis for total
inorganics, TOC, and anions/cations;
• Collection of soil samples for analysis of chemistry and mineralogy;
• Collection of rock samples for chemical analysis;
• Collection of rock samples for petrographic analysis (thin-sections);
• Performance of in-situ horizontal (open hole) and vertical (flush bottom) permeability
tests;
• Completion of packer tests in 17 bedrock borings;
• Completion of rising- and falling-head slug tests in 67 newly installed monitoring wells;
• Collection of groundwater samples from 104 newly installed, compliance, voluntary, and
previously installed monitoring wells, and analysis of samples for total and dissolved
inorganics and anions/cations;
• Speciation of groundwater samples for arsenic, chromium, iron, manganese, and
selenium in groundwater samples collected from 38 monitoring wells installed along
anticipated groundwater flow transects;
• Collection of 8 surface water samples, 4 groundwater seep samples, and 11 sediment
samples, and analysis for total inorganics and anions/cations;
• Speciation of 3 surface water samples and groundwater seep samples for arsenic,
chromium, iron, manganese, and selenium;
• Evaluation of solid and aqueous matrix laboratory data;
• Completion of an updated Receptor Survey;
• Completion of fracture trace analysis; and
• Preparation of this CSA Report.
The following activities are on-going (as described in more detail in Section 14.1.1) and will be
provided to NCDENR in the CSA supplement:
97
Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
17.0 DISCUSSION

• Analysis of soil samples for chemistry and mineralogy and rock samples for chemistry
and petrography; and
• Additional speciation of constituents found to be in excess of their respective 2L
Standards or IMACs, depending on the results of discussions between NCDENR and
Duke Energy.
• Installation and sampling of groundwater monitoring wells GWA-21D/BR.
17.2 Nature and Extent of Contamination
Soil and groundwater beneath the ash basins and ash storage area (within the compliance
boundary) have been impacted by ash handling and storage at the RBSS Site. However, the
presence and magnitude of exceedances for certain constituents may be attributed to naturally
occurring conditions and not necessarily attributed to ash handling at the RBSS site. The extent
of the contamination is noted in the following sections.
As noted on Figure 10-118, exceedances of 2L standards or IMACs were observed in nearly all
monitoring wells across the site, including in monitoring wells located at the outermost extent of
the monitoring well system. Preliminary review of these exceedances indicates, in most cases,
the exceedances observed in the outermost extent of the monitoring wells appear to be related
to background water quality, naturally occurring conditions and/or sampling conditions. A
second round of sampling will be performed at all locations sampled during the CSA. The
results from the CSA sampling, the second round of sampling, and the site specific background
concentrations will be used to confirm that these observed exceedances do not represent
groundwater or surface water impacts related to ash basins or ash storage at the site. The
results of this evaluation will be presented in the CSA supplement.
Boron and sulfate are identified by the USEPA as selected constituents that would be expected
to migrate rapidly, and that would provide early detections as to whether contaminants were
migrating from the waste boundary. Boron and sulfate rarely exceed their respective 2L
Standards across the site. The single boron exceedance is located within the shallow monitoring
well AS-1S within the ash storage area. The porewater results for boron are generally higher
than the exceedances reported within the ash storage area and are potentially impacting these
locations. Soil and groundwater beneath the ash basin, ash storage area, and cinder storage
area have been impacted by ash handling and storage at the RBSS site. Concentrations of
antimony, chromium, cobalt, iron, manganese, TDS, and vanadium exceeded their respective
2L Standards or IMACs in groundwater samples collected from the background wells installed
as part of the groundwater assessment activities.
Ten COIs exceeded the 2L Standards or IMACs in one or more wells in the groundwater
samples collected from the shallow monitoring wells: antimony, boron, chromium, cobalt, iron,
manganese, pH, sulfate, TDS, and vanadium.
Ten COIs exceeded the 2L Standards or IMACs in one or more wells in the groundwater
samples collected from the deep monitoring wells: antimony, chromium, cobalt, iron,
manganese, pH, sulfate, TDS, thallium, and vanadium.
98
Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
17.0 DISCUSSION

Nine COIs exceeded the 2L Standards or IMACs in one or more wells in the groundwater
samples collected from the bedrock wells: antimony, arsenic, chromium, cobalt, iron,
manganese, pH, TDS, and vanadium.
Sixteen COIs exceeded the 2B Standards in surface water samples: aluminum, antimony,
arsenic, barium, beryllium, cadmium, chromium, cobalt, copper, iron, lead, manganese, nickel,
thallium, vanadium, and zinc.
17.3 Maximum Contaminant Concentrations
Maximum contaminant concentrations were determined for ash, soil, groundwater, and surface
water based on the results of sample analyses for each medium. These concentrations were
used to perform screening-level ecological risk assessments based on the North Carolina
Division of Waste Management guidelines (NCDENR 2003).
COIs evaluated for maximum contaminant concentrations for groundwater included antimony,
arsenic, boron, chromium, cobalt, iron, manganese, pH, sulfate, thallium, TDS, and vanadium.
COIs evaluated for maximum contaminant concentrations for porewater included antimony,
arsenic, boron, cobalt, iron, manganese, pH, sulfate, thallium, TDS, and vanadium. Maximum
constituent concentrations are shown on Figure 10-118 and Table 10-8.

For the COIs identified on the basis of ash basin porewater concentrations, boron is the most
prevalent in groundwater with the highest concentration being detected in the shallow
monitoring wells and is limited to the area within and around the primary and secondary ash
basins and within the ash storage and cinder storage areas (reference Figures 10-122 through
10-124 and Table10-10). Groundwater affected by boron discharges to the Catawba River via
groundwater flow. The maximum concentration of boron in soil was detected in a sample
collected at AB-6BRU, at 28.5 to 30 feet below ground surface in the primary cell ash basin.
The highest concentration of arsenic in groundwater occurs outside the ash basin secondary
cell at GWA-9BR (reference Figures 10-125 through 10-127 and Table 10-12).
The highest concentration of cobalt in groundwater was detected in a shallow well (GWA-8S)
downgradient of the ash basin secondary cell. Cobalt was also detected in samples from
multiple background well locations (reference Figures 10-131 through 10-133 and Table 10-10).
17.4 Contaminant Migration and Potentially Affected Receptors
In general, groundwater flows radially from the ash basin and the adjacent ash storage area to
the north, east, and west and discharges to the Catawba River. Piper diagrams were generated
and reviewed for the RBSS site to show relative comparison of ionic composition between ash
basin porewater, surface water, seeps, upgradient and downgradient groundwater monitoring
wells relative to the waste boundary, and background groundwater monitoring wells. Seep data
indicates similar ionic composition to ash basin water, ash basin porewater, and shallow wells in
the ash basin. Piper diagrams are included as Figures 10-186 through 10-191.
99
Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
17.0 DISCUSSION

The human health and ecological CSMs, provided as Figures 12-1 and 12-2 illustrate the
potentially affected receptors; these will be reviewed and revised as necessary based on
information indicated above.

100
Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
18.0 CONCLUSIONS

18.0 Conclusions
18.1 Source and Cause of Contamination
The CSA found that the source and cause of the contamination for certain parameters in some
areas of the RBSS site is the coal ash contained in the ash basin, ash storage area, and cinder
storage area. The cause of this contamination is leaching of constituents from the coal ash into
the underlying soil and groundwater. However, some groundwater, surface water and soil
standards were also exceeded due to naturally occurring constituents found in the subsurface.
18.2 Imminent Hazards to Public Health and Safety and Actions
Taken to Mitigate them in Accordance to 15A NCAC 02L
.0106(f)
15A NCAC 02L .0106(g)(2) requires the site assessment to identify any imminent hazards to
public health and safety and the actions taken to mitigate them in accordance with Paragraph (f)
of .0106(g). The CSA found no imminent hazards to public health and safety; therefore, no
actions to mitigate imminent hazards are required. However, corrective action at the RBSS site
is required to address soil and groundwater contamination.
18.3 Receptors and Significant Exposure Pathways
The requirement contained in the NORR and the CAMA concerning receptors was completed
with the results provided in Section 4.0. A screening-level human health risk assessment and
screening-level ecological risk assessment was performed with the results provided in Section
12.0. The identified receptors and significant exposure pathways are identified in the human
health and ecological CSMs (Figures 12-1 and 12-2).
18.4 Horizontal and Vertical Extent of Soil and Groundwater
Contamination and Significant Factors Affecting
Contaminant Transport
The CSA identified the horizontal and vertical extent of groundwater contamination within the
compliance boundary, and found that the source and cause of the groundwater exceedances
within that boundary is the coal ash contained in the ash basins and ash storage area. The
cause of contamination is leaching of constituents from the coal ash into the underlying soil and
groundwater.
The approximate horizontal and vertical extent of soil contamination is shown with the exception
of the areas associated with the data gaps identified in Section 14.1.1 on Figures 10-67 through
10-103.
101
Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
18.0 CONCLUSIONS

The approximate horizontal and vertical extent of groundwater contamination is shown with
exception of the areas associated with the data gaps identified in Section 14.1.1 on Figures 10-
47 through 10-94. Groundwater contamination is considered to be present where the analytical
results were in excess of the site background concentrations and in excess of the 2L Standards.
The approximate extent of groundwater contamination is shown on these figures and is limited
to the ash basins, ash storage area, and within the compliance boundary associated with the
waste boundary at these locations, with exception of the areas associated with the data gaps
identified in Section 14.1.1. The assessment found the groundwater COIs to be antimony,
arsenic, boron, chromium, cobalt, iron, manganese, sulfate, thallium, TDS, and vanadium. Iron
and manganese are constituents that may be naturally occurring in regional groundwater as
previously discussed in Section 10.1.3 and 10.1.4 respectively.
Background monitoring wells contain naturally occurring metals and other constituents at
concentrations that exceeded their respective 2L Standards or IMACs. Examples of naturally
occurring metals and constituents include antimony, cobalt, iron, manganese and vanadium.
Some of these naturally occurring metals and constituents were also detected in newly installed
background monitoring well groundwater samples at concentrations greater than 2L Standards
or IMACs.
The approximate horizontal and vertical extent of soil contamination is shown on Figures 8-1
through 8-13. Soil contamination is considered to be present where soil analytical results were
in excess of the site soil background concentrations or in excess of the soil screening levels
protective of groundwater. The assessment found the soil COIs to be arsenic, barium, boron,
cobalt, iron, manganese, nickel, selenium, and vanadium.
The significant factors affecting contaminant transport are those factors that determine how the
contaminant reacts with the soil/rock matrix, resulting in retention by the soil/rock matrix and
removal of the contaminant from groundwater. The interaction between the contaminant and the
retention by soils are affected by the chemical and physical characteristics of the soil, the
geochemical conditions present in the matrix (if present), the matrix materials, and the chemical
characteristics of the contaminant. Migration of each contaminant is related to the groundwater
flow direction, the groundwater flow velocity, and the rate at which a particular contaminant
reacts with materials in the respective soil/rock matrix. The data indicates that geologic
conditions present beneath the ash basins impedes the vertical migration of contaminants. The
CSA found that the direction of mobile contaminant transport is generally to the west, north, and
east, towards the Catawba River, and not towards other off-site receptors. No information
gathered as part of this CSA suggests that water supply wells or springs within the 0.5-mile
radius of the compliance boundary are impacted by the RBSS ash basin system.
The two primary mechanisms that immobilize metals (iron and manganese) and semi-metals
(arsenic, boron, and selenium) and prevent their migration in groundwater are sorption and
precipitation (Ref NCDENR). The major attenuation mechanism for sulfate, a non-metal, is
sorption (EPRI). In these processes, the contaminant is in effect removed from groundwater and
partitions onto the surface of the soil/rock matrix (adsorption) or precipitates into a solid phase,
in both cases, removing the contaminant from groundwater.
102
Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
18.0 CONCLUSIONS

A number of factors specific to the constituent and to site conditions are involved in determining
which of these mechanisms occur and how much of the contaminant partitions out of the
groundwater.
Sections 7.0, 8.0, 9.0, and 10.0 present the results of testing performed to determine the
chemical, physical, and mineralogical characteristics of the soil and rock materials and the site
groundwater. As described above, the determination of the mechanism and the amount of the
contaminant removed from the groundwater depends on a number of site specific factors. The
adsorptive capacity of the site soil/rock materials to the specific groundwater contaminants by
development of site specific partition coefficient Kd terms, as described in Section 13.0. The Kd
testing will provide site specific values for the ability and capacity of site soils to remove
contaminants from groundwater and will assist in understanding the mechanisms affecting
contaminant transport at the site. The Kd tests and the associated groundwater modeling will
also allow for evaluation of the long-term contaminant loading and the capacity of the site soil
and rock material to attenuate this loading. The results of this testing, the groundwater
modeling, and the evaluation of the long term groundwater conditions at the site will be
presented in the CAP.
18.5 Geological and Hydrogeological Features influencing the
Migration, Chemical, and Physical Character of the
Contaminants
The initial site conceptual hydrogeologic model presented in the Work Plan dated December 30,
2014, the geological and hydrogeological features influencing the migration, chemical, and
physical characteristics of contaminants are related to the Piedmont hydrogeologic system
present at the site. The migration of the contaminants is related to the groundwater flow
direction, the groundwater flow velocity, and the rate at which a particular contaminant reacts
with soil/rock materials in the aquifer. The CSA found that the direction of the contaminant
migration is towards the Catawba River. The rate of groundwater migration varies with the
hydraulic conductivity and porosity of the site soil and rock materials and ranged from 39.1 ft/yr
to 288.1 ft/yr in soils, and 9.9x104 ft/yr to 1.1x108 ft/yr in rock.
The geological and hydrogeological features of the site influence the migration of the
contaminants by removal of constituents through sorption or precipitation of contaminants. The
degree and the rate at which these actions occur depend on many factors associated with the
solution containing the contaminant and the potentially sorbing soil or aquifer material.
These factors include redox conditions, the concentration of the solution, the chemical
composition of the solution and the contaminant, and the mineralogy of the soil or rock matrix.
The influence of these factors as determined by the chemical, physical, hydrologic, and
mineralogical characterization of the ash, ash basin porewater, the groundwater, and the site
soil and rock will be incorporated into the groundwater modeling discussed in Section 13.0.
Geological and hydrogeological features at the site do not influence the physical character of
the constituents other than through the process of sorption and precipitation. The Kd term
103
Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
18.0 CONCLUSIONS

development and the leaching tests results, that will be presented in the CAP, will be key to
understanding the influences of the site soils and rock on the migration of the COIs.
The groundwater model will provide information to allow evaluation of the capacity of the site
soil and aquifer material to attenuate the loading imposed by the conditions modeled for the
proposed corrective action.
18.6 Proposed Continued Monitoring
A plan for continued monitoring of select monitoring wells and parameters/constituents is
presented in Section 16.0 and will be implemented following approval of this CSA report.
18.7 Preliminary Evaluation of Corrective Action Alternatives
Duke Energy recommends removing the ash in the ash basin, ash storage area, and cinder
storage area via excavation. Approximately 4.6 million tons of ash will be transported to
permitted lined landfills and/or structural fills during the excavation project. The initial phase of
ash removal commenced at RBSS in May 2015, pending permitting. This initial phase has
begun in the northeast corner of the Ash Storage Area and has involved hauling ash by truck to
a permitted lined landfill in Homer, Georgia and to a permitted lined landfill at the Duke Energy
Marshall Steam Station in Mooresville, North Carolina. The majority of ash at Riverbend is
anticipated to be transported by rail to a lined clay mine reclamation project in central North
Carolina, pending permitting and approvals. Final removal of ash at Riverbend and is
anticipated to be completed no later than August 2019. The soil dams will be removed and the
unimpacted material will be used in site re-grading. The depression left after ash removal will be
filled with on-site and imported fill material, re-graded, and appropriate vegetation planted to
establish a long-term stable, erosion resistant site condition.
Based on the results of soil samples and groundwater samples collected beneath the ash basin
and the ash storage areas, after excavation residual contamination will remain. However the
degree of contamination and the persistence of this contamination over time cannot be
determined at this time. In the subsequent CAP, Duke Energy will pursue corrective action
under 15A NCAC 02L .0106 (k) or (l) depending on the results of the groundwater modeling and
the evaluation of the site’s suitability to use MNA. This would potentially require evaluation of
MNA using the approach found in Monitored Natural Attenuation of Inorganic Contaminants in
Groundwater, Volumes 1 and 2 (EPA 2007) and the potential modeling of groundwater surface
water interaction. If these approaches are found to not be satisfactory, additional measures
such as active remediation by hydraulic capture and treatment, among others, would be
evaluated. When properly applied, alternatives such as these can provide effective long term
management of sites requiring corrective action.
104
Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
19.0 REFERENCES

19.0 References
Altamont Environmental, Inc. 2012. Supplemental Groundwater Monitoring Report, Riverbend
Steam Station Ash Basin, NPDES Permit NC000496.
American Society of Testing and Materials. 2014. E1689-95, “Developing Site conceptual
models for Contaminated Sites,” ASTM International, West Conshohocken, PA, DOI:
10.1520/E1689.
American Society of Testing and Materials. 2010a. D2216, "Standard Test Methods for
Laboratory Determination of Water (Moisture) Content of Soil and Rock by Mass," ASTM
International, West Conshohocken, PA, DOI: 10.1520/ D2216-10.
American Society of Testing and Materials. 2010b. D4318, "Standard Test Methods for Liquid
Limit, Plastic Limit, and Plasticity Index of Soils," ASTM International, West
Conshohocken, PA, DOI: 10.1520/D4318-10.
American Society of Testing and Materials. 2010c. D854, "Standard Test Methods for Specific
Gravity of Soil Solids by Water Pycnometer," ASTM International, West Conshohocken,
PA, DOI: 10.1520/D0854-1.
American Society of Testing and Materials. 2010d. D5084, "Standard Test Methods for
Measurement of Hydraulic Conductivity of Saturated Porous Materials Using a Flexible
Wall Permeameter," ASTM International, West Conshohocken, PA, DOI:
10.1520/D5084-1.
American Society of Testing and Materials. 2007. D422, "Standard Test Method for Particle-
Size Analysis of Soils," ASTM International, West Conshohocken, PA, DOI:
10.1520/D0422-63R07.
American Society of Testing and Materials. 2001. D2487, “Standard Practice for Classification
of Soils for Engineering Purposes (Unified Soil Classification System),” ASTM
International, West Conshohocken, PA, DOI: 10.1520/D2487-11.
ARCADIS G&M of North Carolina, Inc. 2007. Ash Basin Drilling Services for: Riverbend Steam
Station, Mount Holly, North Carolina, January 7, 2007.
Butler, J. R. and Secor, D. T., Jr. 1991. The Central Piedmont, in, Horton, J. W., Jr. and Zullo, V.
A., eds., The Geology of the Carolinas: The University of Tennessee Press, Knoxville, p.
59-78.
Charlotte-Mecklenburg Storm Water Services. Undated. Catawba River Basin. [Online] URL:
http://charmeck.org/stormwater/whatsmywatershed/Documents/CatawbaWatershed.pdf
Cunningham, W. L. and Daniels, C. C, III. 2001. Investigation of ground-water availability and
quality in Orange County, North Carolina: U. S. Geological Survey, Water-Resources
Investigations Report 00-4286, 59 p.
105
Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
19.0 REFERENCES

Daniel, C. C., III, and Sharpless, N. B. 1983. Ground-water supply potential and procedures for
well-site selection upper Cape Fear basin, Cape Fear basin study, 1981-1983: North
Carolina Department of Natural Resources and Community Development and U.S.
Water Resources Council in cooperation with the U.S. Geological Survey, 73 p.
Daniels, J. L. and Das, G. P. 2014. Practical Leachability and Sorption Considerations for Ash
Management, Geo-Congress 2014 Technical Papers: Geocharacterization and Modeling
for Sustainability. Wentworth Institute of Technology, Boston, MA.
Deer, W. A., R. A. Howie, and J. Zussman. 1966, An Introduction to the Rock-Forming Minerals:
John Wiley & Sons Inc., New York, N. Y., 528p.
Dennis, A. J, Secor, D. T., and Shervais, J. W. 2000. Faults bounding eclogite-bearing
gneisses: Geological Society of America Abstracts with Program, v. 32 (2), p. A-14.
Duke Energy. 2014a. Coal Ash Excavation and Removal - Riverbend Steam Station [Online]
URL: https://www.duke-energy.com/pdfs/am-riverbend-excavation-fact-sheet.pdf
Duke Energy. 2014b. 2014 News Release [Online] URL: https://www.duke-
energy.com/news/releases/2014111301.asp
Duke Energy. 2014c. Technical Memorandum, “Cliffside Seepage Sample Metallurgy File
#5153 Revision 1”.
Duke Energy Corporation. 2013a. Mountain Island Lake. [Online] URL: http://www.duke-
energy.com/lakes/facts-and-maps/mountain-island.asp
Duke Energy. 2013b. Riverbend Decommissioning Project. [Online] URL: https://www.duke-
energy.com/pdfs/am-riverbend-decommissioning-project.pdf
Electric Power Research Institute. 2010. Comparison of Coal Combustion Products to Other
Common Materials, 1020556, Final Report September 2010.
Electric Power Research Institute. 2004. Electric Power Research Institute, “Chemical
Attenuation Coefficients for Arsenic Species Using Soil Samples Collected from
Selected Power Plant Sites: Laboratory Studies”, Product ID: 1005505, December 2004.
Electric Power Research Institute. 1995. Coal Ash Disposal Manual: Third Edition, EPRI TR-
104137, January 1995.
Electric Power Research Institute. 1994. “A Field and Laboratory Study of Solute Release from
Sluices Fly Ash”, EPRI TR-104585, December 1994.
Electric Power Research Institute. 1993. Electric Power Research Institute, Physical and
Hydraulic Properties of Fly Ash and Other By-Products from Coal Combustion, EPRI TR-
101999. February 1993.
Fenneman, N. M. 1938. Physiography of eastern United States. McGraw-Hill.
106
Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
19.0 REFERENCES

Find Average Temperature for US Cities. 2015. [Online] URL:
http://temperature.weatherdb.com/
Freeze, R. A. and Cherry, J. A. 1979. Ground Water, Englewood Cliffs, NJ, Prentice-Hall.
Gilbert, N. J., Brown, H. S., and Schaeffer, M. F. 1982. Structure and geologic history of a part
of the Charlotte belt, South Carolina Piedmont: Southeastern Geology, v. 12, no. 3, p.
129-145.
Gillispie, E. C., Austin, R., Abraham, J., Wang, S., Bolich, R., Bradley, P., Amoozegar, A.,
Duckworth, O., Hesterberg, D., and Polizzotto, M. L. 2014. Sources and variability of
manganese in well water of the North Carolina Piedmont. Water Resources Research
Institute of the University of North Carolina System Annual 2014 Conference, Raleigh,
NC, March 2014. Poster Presentation.
Goldsmith, R., Milton, D. J. and Horton, J. W., Jr. 1988. Geologic map of the Charlotte 1o x 2o
quadrangle, North Carolina and South Carolina: United States Geological Survey,
Miscellaneous Investigations Series, Map I-1251-E, scale 1:250,000.
Harned, D. A. and Daniel III, C. C. 1992. The transition zone between bedrock and regolith:
Conduit for contamination?, p. 336-348, in Daniel, C. C., III, White, R. K., and Stone, P.
A., eds., Groundwater in the Piedmont: Proceedings of a Conference on Ground Water
in the Piedmont of the Eastern United States, October 16-18, 1989, Clemson University,
693p.
Hatcher, R. D. Jr., Bream, B. R., and Merschat, A. J. 2007. Tectonic map of the southern and
central Appalachians: A tale of three orogens and a complete Wilson cycle, in Hatcher,
R. D., Jr, Carlson, M. P., McBride, J. H., and Martinez Catalan, J. R., eds., 4-D
Framework of Continental Crust: Geological Society of America Memoir 200, p. 595-632.
HDR. 2015. Riverbend Steam Station – NC Ash Basin Groundwater Assessments, Natural
Resources Avoidance Memo. April 14.
HDR. 2014a. Riverbend Steam Station – Ash Basin Drinking Water Supply Well and Receptor
Survey, NPDES Permit NC0004961. [Online] URL:
http://portal.ncdenr.org/web/wq/drinking-water-receptor-surveys
HDR. 2014b. Riverbend Steam Station – Ash Basin Supplement to Drinking Water Supply Well
and Receptor Survey. [Online] URL: http://portal.ncdenr.org/web/wq/drinking-water-
receptor-surveys
HDR. 2014c. Riverbend Steam Station – Ash Basin. Proposed Groundwater Assessment Work
Plan (Rev. 1), NPDES Permit NC0004961.
HDR. 2013a. Groundwater Assessment, Duke Energy Carolinas, LLC, Riverbend Steam Station
Ash Basin, NPDES Permit NC0004961 (HDR 2013a)
107
Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
19.0 REFERENCES

HDR. 2013b. 2012 Supplemental Groundwater Monitoring Report, Riverbend Steam Station
Ash Basin, NPDES Permit NC0004961 (HDR 2013b)
Heath, R. C. 1984. “Ground-water regions of the United States.” U.S. Geological Survey Water-
Supply Paper 2242, 78 p.
Heath, R. C. 1980. Basic elements of groundwater hydrology with reference to conditions in
North Carolina: U.S. Geo-logical Survey Open-File Report 80–44, 86 p.
Hibbard, J. P., Stoddard, E. F., Secor, D. T., and Dennis, A. J. 2002. The Carolina Zone:
overview of Neoproterozoic to Early Paleozoic peri-Gondwanan terranes along the
eastern flank of the southern Appalachians: Earth-Science Reviews 57, p. 299-339.
Horton, J. W., Jr., Drake, A. A., Jr., and Rankin, D. W. 1989. Tectonostratigraphic terranes and
their Paleozoic boundaries in the central and southern Appalachians, in, Dallmeyer, R;
D., ed., Terranes in the circum-Atlantic Paleozoic orogens: Geological Society of
America Special Paper 230, p. 213-245.
Krauskopf, K. B. 1972. Geochemistry of micronutrients: in Micronutrients in Agriculture, J.J.
Mortvedt, F.R. Cox, L.M. Shuman, and R.M. Walsh, eds., Soil Science Society of
America, Madison, Wisconsin, p. 7-36.
LeGrand, H. E. 2004. A Master Conceptual Model for Hydrogeological Site Characterization in
the Piedmont and Mountain Region of North Carolina, A Guidance Manual, North
Carolina Department of Environment and Natural Resources Division of Water Quality,
Groundwater Section, 50p.
LeGrand, H. E. 1992. A conceptual model of ground water settings in the Piedmont region, p.
317-327, in Daniel, C. C., III, White, R. K., and Stone, P. A., eds., Groundwater in the
Piedmont: Proceedings of a Conference on Ground Water in the Piedmont of the
Eastern United States, October 16-18, 1989, Clemson University, 693p.
LeGrand, H. E. 1989. A conceptual model of ground water settings in the Piedmont region. In
Ground Water in the Piedmont , ed. C.C. Daniel III, R.K. White, and P.A. Stone, 693.
Proceedings of a Conference on Ground Water in the Piedmont of the Eastern United
States, Clemson University, Clemson, South Carolina. Charlotte, NC: U.S. Geological
Survey.
LeGrand, H. E. 1988. Region 21, Piedmont and Blue Ridge, p.201-208, in Black, W.,
Rosenhein, J. S., and Seaber, P. R., eds., Hydrogeology: Geological Society of America,
The Geology of North America, v. O-2, Boulder, Colorado, 524p.
MACTEC. 2011. Amended Ash Basin Monitoring Well Installation Report, Riverbend Steam
Station, MACTEC Project No. 6228-10-5284, March 30, 2011.
Mountain Island Lake Work Group. 2005. Mountain Island Lake Watershed Protection
Guidelines. [Online] URL:
108
Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
19.0 REFERENCES

http://charmeck.org/stormwater/creekslakesponds/Lakes/Documents/MOUGuidelines.pd
f
North Carolina Department of Environment and Natural Resources. 2015. NPDES Permit for
Riverbend Steam Station. [Online] URL:
http://portal.ncdenr.org/c/document_library/get_file?p_l_id=1169848&folderId=25061489
&name=DLFE-113318.pdf
North Carolina Department of Environment and Natural Resources. 2013a. 15A NCAC 2B
.0200s. Classifications and Water Quality Standards Applicable to the Surface Waters
and Wetlands of North Carolina. NC and EPA Combined Surface Water Quality
Standards and Criteria Table. May 15. [Online] URL:
http://portal.ncdenr.org/web/wq/ps/csu/swstandards
North Carolina Department of Environment and Natural Resources. 2013b. 15A NCAC 02L.
Groundwater Rules. Groundwater Standards Table. April 1. [Online] URL:
http://portal.ncdenr.org/web/wq/ps/csu/gwstandards#4
North Carolina Department of Environment and Natural Resources. 2013c. 15A NCAC 02L.
Groundwater Rules. Interim Maximum Allowable Concentrations (IMACs) Table. April 1.
[Online] URL: http://portal.ncdenr.org/web/wq/ps/csu/gwstandards#4
North Carolina Department of Environment and Natural Resources. 2007a. “Hydrogeologic
Investigation and Reporting Policy Memorandum”, dated May 31, 2007.
North Carolina Department of Environment and Natural Resources. 2007b. “Performance and
Analysis of Aquifer Slug Tests and Pumping Tests Policy,” May 31, 2007.
North Carolina Department of Environment and Natural Resources. 2004. A Master Conceptual
Model for Hydrogeological Site Characterization in the Piedmont and Mountain Region
of North Carolina, A Guidance Manual. Division of Water Quality, Groundwater Section.
Prepared for the Groundwater Section by Harry E. LeGrand Sr., Independent
Hydrogeologist. 2004.
North Carolina Department of Environment and Natural Resources. 2003. Guidelines for
Performing Screening Level Ecological Risk Assessments with the North Carolina
Division of Waste Management. North Carolina Department of Environment and Natural
Resources, Division of Waste Management. October 2003. [Online] URL:
http://portal.ncdenr.org/web/wm/guidelines-for-performing-screening-level-ecological-
risk-assessments
North Carolina Department of Environment and Natural Resources – Division of Water Quality.
2012. Evaluating Metals in Groundwater at DWQ Permitted Facilities: A Technical
Assistance Document for DWQ Staff.
North Carolina Department of Environment and Natural Resources – Division of Water Quality.
2007. North Carolina Water Supply Watersheds, Division of Water Quality. [Online]
109
Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
19.0 REFERENCES

URL:http://portal.ncdenr.org/c/document_library/get_file?uuid=df30416f-cd0e-46bd-
9ce4-fcc470aa324d&groupId=38364
North Carolina Department of Environment and Natural Resources – Division of Water
Resources. 2015. NC Surface Water Classifications, NC Classifications Website [Online]
URL:
http://ncdenr.maps.arcgis.com/apps/webappviewer/index.html?id=6e125ad7628f494694
e259c80dd64265
North Carolina Department of Public Health. 2015. Fish Consumption Advisories. Division of
Public Health. March 27. [Online] URL:
http://epi.publichealth.nc.gov/oee/fish/advisories.html
North Carolina State University, A&T State University, NC Cooperative Extension Resources.
1996. Table 1. Average length of growing season for selected North Carolina Counties
(1951-1980) [Online] URL:http://content.ces.ncsu.edu/average-growing-season-for-
selected-north-carolina-locations/
North Carolina Geological Survey. 1985. Geologic map of North Carolina: North Carolina
Geological Survey, General Geologic Map, scale 1:500000.
Nutter, L. J. and Otton, E. G. 1969. Groundwater occurrence in the Maryland Piedmont:
Maryland Geological Survey Report of Investigations, No. 10, 42p.
Parkhurst, D. L., and Appelo, C. A. J. 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.
Pippin, C. G., Chapman, M. J., Huffman, B. A., Heller, M. J., and Schelgel, M. E. 2008.
Hydrogeologic setting, ground-water flow, and ground-water quality at the Lantree
Peninsula research station, Iredell County, North Carolina, 2000–2005: U.S. Geological
Survey Scientific Investigations Report 2008–5055, 89 p. [Online] URL:
http://pubs.water.usgs.gov/sir2008–5055
Schaeffer, M. F. 2014a. Piedmont groundwater system, Part 1 – The transition zone between
regolith and bedrock: Existence: Geological Society of America, Abstracts with Program,
April 2014, v. 46, no. 3, p. 26-27.
Schaeffer, M. F. 2014b. Piedmont groundwater system, Part 2 – The transition zone between
regolith and bedrock: Characteristics: Geological Society of America, Abstracts with
Program, April 2014. v. 46, no. 3, p. 27.
Schaeffer, M. F. 2011. Carolina Piedmont groundwater system: What does the transition zone
look like?: 19th Annual David S. Snipes/Clemson Hydrogeology Symposium, April 7,
2011, p. 43-44.
110
Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
19.0 REFERENCES

Schaeffer, M. F. 2009. Hydraulic conductivity of Carolina Piedmont soil and bedrock: Is a
transition zone present between the regolith and bedrock?: 17th Annual David S.
Snipes/Clemson Hydrogeology Symposium, April 2, 2009, p. 32-36.
Secor, D. T, Barker, C., Balinsky, M., and Colquhoun, D. 1998. The Carolina terrane in
northeastern South Carolina: history of an exotic volcanic arc: South Carolina Geology,
v. 40, p. 1-17.
Stewart, J. W. 1964. Infiltration and permeability of weathered crystalline rocks, Georgia Nuclear
Laboratory, Dawson County, Georgia: United States Geological Survey, Bulletin 1133-D,
p. D1-D57.
Stewart, J. W., Callahan, J. T., Carter, R. F., et al. 1964. Geologic and hydrologic investigation
at the site of the Georgia Nuclear Laboratory, Dawson County, Georgia: United States
Geological Survey Bulletin 1133-F, p. F1-F90.
Tang, G., Mayes, M. A., Parker, J. C., and Jardine, P. M. 2010. CXTFIT/Excel–A modular
adaptable code for parameter estimation, sensitivity analysis and uncertainty analysis for
laboratory or field tracer experiments. Computers & Geosciences, 36(9), 1200-1209.
U.S. Department of Agriculture – Soil Conservation Service. 1989. Soil Survey of Gaston
County, North Carolina.
U.S. Environmental Protection Agency. Undated. RCRA Corrective Action Workshop on
Results-Based Project Management.
U.S. Environmental Protection Agency. 2015. Regional Screening Levels Generic Tables.
January. [Online] URL: http://www.epa.gov/reg3hwmd/risk/human/rb-
concentration_table/Generic_Tables/index.htm
U.S. Environmental Protection Agency. 2014. National Recommended Water Quality Criteria.
Aquatic and Human Health Criteria Tables. December 3. [Online] URL:
http://water.epa.gov/scitech/swguidance/standards/criteria/current/index.cfm
U.S. Environmental Protection Agency. 2013. Guidance on the Management of Contaminated
Land And Groundwater At EPA Licensed Sites.
U.S. Environmental Protection Agency. 2007. Monitored Natural Attenuation of Inorganic
Contaminants in Ground Water. Volume 1, Technical Basis for Assessment.
U.S. Environmental Protection Agency Region 4. 2001. Ecological Risk Assessment Bulletins--
Supplement to RAGS. Tables 1 - 4. November 30. [Online] URL:
http://www.epa.gov/region4/superfund/programs/riskassess/ecolbul.html#tbl1
U.S. Environmental Protection Agency. 1998. Guidelines for Ecological Risk Assessment. April.
[Online] URL http://www2.epa.gov/sites/production/files/2014-
11/documents/eco_risk_assessment1998.pdf
111
Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
19.0 REFERENCES

U.S. Environmental Protection Agency. 1997. Ecological Risk Assessment Guidance for
Superfund: Process for Designing and Conducting Ecological Risk Assessments, Interim
Final. June. [Online] URL: http://www.epa.gov/oswer/riskassessment/ecorisk/ecorisk.htm
U.S. Environmental Protection Agency. 1992. Batch-type procedures for estimating soil
adsorption of chemicals. TechnicalResource Document 530/SW-87/006-F.
U.S. Fish and Wildlife Service. 2015. Endangered Species, Threatened Species, Federal
Species of Concern, and Candidate Species, Gaston County, North Carolina. United
States Fish & Wildlife Service. April 2. [Online] URL:
http://www.fws.gov/raleigh/species/cntylist/nc_counties.html
Weather DB. 2015. Mount Holly, North Carolina Average Rainfall. [Online] URL:
http://rainfall.weatherdb.com/l/17738/Mount-Holly-North-Carolina

112