NC0004961_1_FINAL_CSA Sup 2_Report_20160826

Comprehensive Site Assessment
Supplement 2
Riverbend Steam Station Ash Basin
Site Name and Location
Groundwater Incident No.
NPDES Permit No.
Date of Report
Permittee and Current Property Owner
Consultant Information
Latitude and Longitude of Facility
Riverbend Steam Station
175 Steam Plant Rd
Mount Holly, NC 28120
Not Assigned
NC0004961
August 26, 2016
Duke Energy Carolinas, LLC
526 South Church St.
Charlotte, NC 28202-1803
704.382.3853
HDR Engineering, Inc. of the Carolinas
440 South Church St, Suite 900
Charlotte, NC 28202
704.338.6700
350 21' 38" N, 800 58' 26" W
This document has been reviewed for accuracy and quality
commensurate with the intended application.
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SEAL
of �.OL OG�;.• �Q •.•
FRANC IS S.•••
Malcolm F. Schaeffer, L.G.
Senior Geologist

Duke Energy Carolinas, LLC | CSA Supplement 2
Riverbend Steam Station Ash Basin
TABLE OF CONTENTS

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Table of Contents
Page
Executive Summary ................................................................................................................... 1
Section 1 – Background ............................................................................................................. 3
1.1 Purpose of CSA Supplement 2 .................................................................................... 3
1.2 Site Description ............................................................................................................ 4
1.3 History of Site Groundwater Monitoring ........................................................................ 5
1.3.1 NPDES Sampling ................................................................................................. 6
1.3.2 CSA Sampling ...................................................................................................... 7
1.3.3 Post-CSA Sampling .............................................................................................. 7
1.3.4 NCDEQ Water Supply Well Sampling ................................................................... 8
Section 2 – CSA Review Comments .......................................................................................... 9
2.1 NCDEQ General Comments and Responses ............................................................... 9
2.2 NCDEQ Site-Specific Comments and Responses ....................................................... 9
2.3 Errata ........................................................................................................................... 9
Section 3 – Additional Assessment ...........................................................................................10
3.1 Additional Assessment Activities ................................................................................10
3.1.1 Well Installation ....................................................................................................10
3.1.2 Well Gauging and Sampling .................................................................................11
3.2 Additional Assessment Results ...................................................................................11
3.2.1 Groundwater Flow Direction .................................................................................12
3.2.2 Sampling Results .................................................................................................12
Section 4 – Background Concentrations ...................................................................................16
4.1 Methodology ...............................................................................................................16
4.2 Observation for Background Wells ..............................................................................18
Section 5 – Anticipated Additional Assessment Activities ..........................................................19
5.1 Proposed Additional Assessment Monitoring Wells.....................................................19
5.2 Implementation of the Effectiveness Monitoring Plan ..................................................19
Section 6 – Conclusions and Recommendations ......................................................................20

Duke Energy Carolinas, LLC | CSA Supplement 2
Riverbend Steam Station Ash Basin
TABLE OF CONTENTS

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FIGURES
1-1 Site Location Map
1-2 Sample Location Map
1-3 NCDEQ Water Supply Well Sampling
3-1 Potentiometric Surface – Shallow Flow Layer
3-2 Potentiometric Surface – Deep Flow Layer
3-3 Potentiometric Surface – Bedrock Flow Layer
3-4.1 Antimony Isoconcentration Contour Map – Shallow Wells (S)
3-4.2 Antimony Isoconcentration Contour Map – Deep Wells (D and BRU)
3-4.3 Antimony Isoconcentration Contour Map – Bedrock Wells (BR)
3-4.4 Arsenic Isoconcentration Contour Map – Shallow Wells (S)
3-4.5 Arsenic Isoconcentration Contour Map – Deep Wells (D and BRU)
3-4.6 Arsenic Isoconcentration Contour Map – Bedrock Wells (BR)
3-4.7 Beryllium Isoconcentration Contour Map – Shallow Wells (S)
3-4.8 Beryllium Isoconcentration Contour Map – Deep Wells (D and BRU)
3-4.9 Beryllium Isoconcentration Contour Map – Bedrock Wells (BR)
3-4.10 Boron Isoconcentration Contour Map – Shallow Wells (S)
3-4.11 Boron Isoconcentration Contour Map – Deep Wells (D and BRU)
3-4.12 Boron Isoconcentration Contour Map – Bedrock Wells (BR)
3-4.13 Hexavalent Chromium Isoconcentration Contour Map – Shallow Wells (S)
3-4.14 Hexavalent Chromium Isoconcentration Contour Map – Deep Wells (D and BRU)
3-4.15 Hexavalent Chromium Isoconcentration Contour Map – Bedrock Wells (BR)
3-4.16 Chromium (Total) Isoconcentration Contour Map – Shallow Wells (S)
3-4.17 Chromium (Total) Isoconcentration Contour Map – Deep Wells (D and BRU)
3-4.18 Chromium (Total) Isoconcentration Contour Map – Bedrock Wells (BR)
3-4.19 Cobalt Isoconcentration Contour Map – Shallow Wells (S)
3-4.20 Cobalt Isoconcentration Contour Map – Deep Wells (D and BRU)
3-4.21 Cobalt Isoconcentration Contour Map – Bedrock Wells (BR)
3-4.22 Iron Isoconcentration Contour Map – Shallow Wells (S)
3-4.23 Iron Isoconcentration Contour Map – Deep Wells (D and BRU)
3-4.24 Iron Isoconcentration Contour Map – Bedrock Wells (BR)
3-4.25 Manganese Isoconcentration Contour Map – Shallow Wells (S)
3-4.26 Manganese Isoconcentration Contour Map – Deep Wells (D and BRU)
3-4.27 Manganese Isoconcentration Contour Map – Bedrock Wells (BR)
3-4.28 Sulfate Isoconcentration Contour Map – Shallow Wells (S)
3-4.29 Sulfate Isoconcentration Contour Map – Deep W ells (D and BRU)
3-4.30 Sulfate Isoconcentration Contour Map – Bedrock Wells (BR)
3-4.31 Total Dissolved Solids Isoconcentration Contour Map – Shallow Wells (S)
3-4.32 Total Dissolved Solids Isoconcentration Contour Map – Deep Wells (D and BRU)
3-4.33 Total Dissolved Solids Isoconcentration Contour Map – Bedrock Wells (BR)
3-4.34 Thallium Isoconcentration Contour Map – Shallow Wells (S)
3-4.35 Thallium Isoconcentration Contour Map – Deep Wells (D and BRU)
3-4.36 Thallium Isoconcentration Contour Map – Bedrock Wells (BR)
3-4.37 Vanadium Isoconcentration Contour Map – Shallow Wells (S)
3-4.38 Vanadium Isoconcentration Contour Map – Deep Wells (D and BRU)
3-4.39 Vanadium Isoconcentration Contour Map – Bedrock Wells (BR)
3-5.1 Site Cross Section Locations
3-5.2 Cross Section A-A’ (1 of 2)
3-5.3 Cross Section A-A’ (2 of 2)
3-5.4 Cross Section B-B’ (1 of 2)
Duke Energy Carolinas, LLC | CSA Supplement 2
Riverbend Steam Station Ash Basin
TABLE OF CONTENTS

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3-5.5 Cross Section B-B’ (2 of 2)
3-5.6 Cross Section C-C’ (1 of 1)
3-5.7 Cross Section D-D’ (1 of 1)
3-5.8 Cross Section E-E’ (1 of 1)
3-6.1 Piper Diagram – Background Groundwater, Porewater, Areas of Wetness, and Surface
Water
3-6.2 Piper Diagram – Shallow Groundwater, Porewater, Areas of Wetness, and Surface
Water
3-6.3 Piper Diagram – Deep Groundwater, Porewater, Areas of Wetness, and Surface Water
3-6.4 Piper Diagram – Bedrock Groundwater, Porewater, Areas of Wetness, and Surface
Water

TABLES
1-1 Well Construction Information
1-2 NPDES Historical Data
1-3 Range of 2L Groundwater Standard Exceedances from NPDES Sampling
2-1 Responses to General NCDEQ Comments
2-2 Total and Effective Porosity and Specific Storage by Flow Layer
3-1 Round 5 Analytical Results of Groundwater Monitoring
3-2 Round 5 Analytical Results of Porewater Monitoring
3-3 Round 5 Analytical Results of Ash Basin Surface Water Locations
3-4 Round 5 Analytical Results of Areas of Wetness
3-5 Summary of Groundwater Elevations
3-6 Summary of Cation-Anion Balance Differences

APPENDICES
A Monitoring Well Logs and Core Photos
B Field Sampling Forms and Slug Test Report
C Laboratory Report and Chain-of-Custody Forms and Validation Report

Duke Energy Carolinas, LLC | CSA Supplement 2
Riverbend Steam Station Ash Basin
EXECUTIVE SUMMARY

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Executive Summary
Duke Energy Carolinas, LLC (Duke Energy) owns and formerly operated the Riverbend Steam
Station (RBSS) located adjacent to the Mountain Island Lake portion of the Catawba River near
Mount Holly, Gaston County, North Carolina (Figure 1-1). RBSS began operation as a coal-
fired generating station in 1929 and was retired from service in April 2013. Decommissioning of
RBSS is ongoing. From 1929 to 1957, coal combustion residuals (CCR) from RBSS’s coal
combustion process were dredged from the primary basin to the ash storage area, where it then
decanted back to the primary pond area, leaving behind ash. Following installation of
precipitators and a wet sluicing system in 1957, CCR was disposed in the station’s ash basin
located adjacent to the station and Mountain Island Lake. Discharge from the ash basin is
currently permitted by the North Carolina Department of Environment Quality (NCDEQ)1
Division of Water Resources under the National Pollutant Discharge Elimination System
(NPDES) Permit NC0004961.
This Comprehensive Site Assessment (CSA) Supplement 2 report addresses the following:
• Summary of groundwater, porewater, ash basin surface water, and area of wetness
(AOW) monitoring data through April 2016;
• Responses to NCDEQ review comments pertaining to the CSA;
• Findings from assessment activities conducted since the submittal of the CSA report,
including additional assessments previously identified in the CSA;
• Update on the development of provisional background groundwater concentrations; and
• Description of planned additional source area assessment activities.
Boron, the primary site-derived constituent in groundwater, was detected at concentrations
greater than the 15A NCAC (North Carolina Administrative Code) 02L.0202 Groundwater
Quality Standards (2L Standards or 2L) within the ash basin porewater, but has not exceeded
the 2L Standard in groundwater. Exceedances of the 2L Standard for boron occurred during the
Round 1 sampling in AS-1 and during the Round 2 sampling in AS-3; however, these locations
are no longer being sampled due to having been abandoned as part of the ongoing ash
excavation activities.
Groundwater monitoring results from Round 5 of CSA well sampling (completed between
February and April 2016) and NPDES groundwater monitoring data are presented herein.
Updated summary tables, isoconcentration maps, cross sections, and other figures are
included. In general, groundwater data reported from previous rounds of monitoring are
consistent with current site conditions, specifically the extent of impact to groundwater from ash
basin-related constituents (e.g., sulfate).
Presentation of site-specific proposed provisional background concentrations (PPBCs) was
included in the Corrective Action Plan (CAP) Part 1 report and should be refined as more data
becomes available and pending input from NCDEQ. With refinement of the PPBCs, the
1 Prior to September 18, 2015, the NCDEQ was referred to as the North Carolina Department of Environment and
Natural Resources (NCDENR). Both naming conventions are used in this report, as appropriate.
Duke Energy Carolinas, LLC | CSA Supplement 2
Riverbend Steam Station Ash Basin
EXECUTIVE SUMMARY

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evaluation of whether the presence of constituents of interest (COIs) downgradient of the source
area is naturally occurring or potentially attributed to the source area can be advanced in more
detail when additional sampling data is available.
The following conclusions and recommendations are offered:
• Based on its location across Mountain Island Lake from the ash basin and being
hydraulically isolated, the water supply well (Well 1) in the vicinity of the RBSS facility is
not impacted by CCR releases from the ash basin.
• Groundwater monitoring results from Round 5 of sampling, including data from additional
assessment groundwater monitoring wells, indicate consistency with previous sampling
results, specifically the extent of impact to groundwater from ash basin-related
constituents (e.g., boron and sulfate).
• Additional monitoring wells (GWA-11S/D and GWA-12S/D) within the vicinity of GWA-
2BR/BRU were installed to refine understanding of constituent concentrations west of
the ash and cinder storage areas near groundwater monitoring well GWA-3SA/D. Cobalt
concentrations in the additional assessment wells were approximately one order of
magnitude higher than in well GWA-3SA/D during the Round 5 sampling event. Iron
concentrations in the additional assessment wells were lower in the shallow flow layer
than in well GWA-3SA, and similar or higher in the deep flow layer than in well GWA-3D.
Manganese concentrations in the additional assessment wells were lower in both the
shallow and deep flow layers than in GWA-3SA/D. Groundwater monitoring results from
Round 6 of sampling shall be evaluated for the additional assessment wells as several
were not installed for inclusion within the Round 5 sampling event.
• Refinement of PPBCs should be conducted once the minimum number of viable
observations (e.g., eight) per background well is available.
• Additional monitoring wells (GWA-3BR, GWA-15S, and GWA-15D) should be installed
northwest and north of the cinder storage area to refine horizontal and vertical
delineation of groundwater impacts from these areas.
• Duke Energy will implement the effectiveness monitoring plan in accordance with
recommendations provided in the CAP Part 2 report as well as subsequent discussions
with NCDEQ.
Duke Energy Carolinas, LLC | CSA Supplement 2
Riverbend Steam Station Ash Basin
SECTION 1 – BACKGROUND

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Section 1 – Background
Duke Energy Carolinas, LLC (Duke Energy) owns and formerly operated the Riverbend Steam
Station (RBSS) located adjacent to the Mountain Island Lake portion of the Catawba River near
Mount Holly, Gaston County, North Carolina (Figure 1-1). RBSS began operation as a coal-
fired generating station in 1929 and was retired from service in April 2013. Decommissioning of
RBSS is ongoing. From 1929 to 1957, coal combustion residuals (CCR) from RBSS’s coal
combustion process were dredged from the primary basin to the ash storage area, where it then
decanted back to the primary pond area, leaving behind ash. Following installation of
precipitators and a wet sluicing system in 1957, CCR was disposed in the station’s ash basin
located adjacent to the station and Mountain Island Lake. Discharge from the ash basin is
currently permitted by the North Carolina Department of Environment Quality (NCDEQ)2
Division of Water Resources under the National Pollutant Discharge Elimination System
(NPDES) Permit NC0004961.
The Comprehensive Site Assessment (CSA) for the RBSS was submitted to NCDEQ on August
18, 2015. Given the compressed timeframe for submittal, certain information was not included in
the CSA report because the data was not yet available. Thus, Duke Energy committed to
providing this information after submittal of the CSA report. In addition, NCDEQ’s review of the
CSA report led to requests for additional information. As such, CSA Supplement 1, submitted to
NCDEQ on February 12, 2016, as an appendix to the Corrective Action Plan (CAP) Part 2,
provided information to address the temporal constraints, information requested by NCDEQ
subsequent to submittal of the CSA report, additional data validation reporting, and a response
to site-specific NCDEQ comments obtained during in-person meetings.
1.1 Purpose of CSA Supplement 2
The purpose of this CSA Supplement 2 is to provide data obtained during additional well
installation and sampling conducted between January and April 2016. These activities were
conducted to refine the understanding of subsurface geologic/hydrogeologic conditions and the
extent of impacts from historical production and storage of coal ash. This CSA Supplement 2
was prepared in coordination with Duke Energy and NCDEQ as a result of requests for
additional information and areas identified for additional assessment. It includes the following
information:
• A brief summary and update of groundwater sampling data from the NPDES, CSA, and
post-CSA monitoring well sampling events;
• A brief summary of results of NCDEQ water supply well sampling activities;
• A summary of NCDEQ comments on the CSA report and responses to those comments;
• A description of additional assessment activities conducted since submittal of the CSA
report and the findings of those assessment activities;
• An updated approach for the refinement of proposed provisional background
concentrations (PPBCs) for groundwater at the RBSS site; and
• A description of additional planned assessment activities.
2 Prior to September 18, 2015, the NCDEQ was referred to as the North Carolina Department of Environment and
Natural Resources (NCDENR). Both naming conventions are used in this report, as appropriate.
Duke Energy Carolinas, LLC | CSA Supplement 2
Riverbend Steam Station Ash Basin
SECTION 1 – BACKGROUND

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As a complement to the CSA report and the CSA Supplement 1, this CSA Supplement 2
provides an updated evaluation of the extent of impacts from the ash basin and related ash
storage facilities based on existing CSA groundwater monitoring wells and monitoring wells
installed subsequent to submittal of the CSA report (herein referred to as additional assessment
wells). Additional assessment groundwater monitoring wells are shown on Figure 1-2 with
green text labels.
1.2 Site Description
The RBSS site was named for a bend in Mountain Island Lake on which it is located. The site is
north of Horseshoe Bend Beach Road near the town of Mount Holly in Gaston County, North
Carolina, and occupies approximately 340.7 acres of land. RBSS was a seven-unit, 454 MW,
coal-fired, electricity-generating facility. The station began commercial operations in 1929 with
Units 1–4. Units 5–7 began commercial operations sequentially from 1952 through 1954. 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 turbine (CT) units at the site from 1969 until October
2012.
The ash basin system at RBSS consists of a Primary Cell, a Secondary Cell, and associated
embankments and outlet works. An ash storage area is located southwest and side gradient of
the Primary Cell and a cinder storage area is located west and downgradient of the Primary
Cell. The ash basin system is located approximately 2,400 feet to the northeast of the power
plant, adjacent to Mountain Island Lake, as shown on Figure 1-2. The Primary Cell is
impounded by an earthen dike located on the west side of the Primary Cell. The surface area of
the Primary Cell is approximately 41 acres with an approximate maximum pond elevation of 724
feet.3 The Secondary Cell is impounded by an earthen dike located along the northeast side of
the Secondary Cell close to Mountain Island Lake. The surface area of the Secondary Cell is
approximately 28 acres with an approximate maximum pond elevation of 714 feet. The full pond
elevation of Mountain Island Lake is approximately 646.8 feet.
The RBSS site is generally forested along Mountain Island Lake. Buildings and other structures
associated with the retired 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. Topography at the RBSS site generally slopes from this divide to
Mountain Island Lake (i.e., south to north).
The RBSS site contains two switchyards and associated transmission lines. While power
production no longer exists at the site, these facilities continue to support power transmission on
the Duke Energy system.
The Lark Maintenance Center is also located at the RBSS site west of the retired coal-fired
units. This facility is an advanced machining and welding shop that supports various Duke
Energy power plants in the region.
3 The datum for all elevation information presented in this report is NAVD88. Reported pond elevations are indicative
of conditions prior to ash basin excavation and/or dewatering activities.
Duke Energy Carolinas, LLC | CSA Supplement 2
Riverbend Steam Station Ash Basin
SECTION 1 – BACKGROUND

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The groundwater system in the natural materials (alluvium, soil, soil/saprolite, and bedrock) at
RBSS is consistent with the Piedmont regolith-fractured rock system and is an unconfined,
connected system without confining layers. The groundwater system at RBSS is a two-layer
system: shallow (regolith) and deep (bedrock). In general, groundwater at the site flows to the
north, east, and west and discharges to the Mountain Island Lake portion of the Catawba River.
Groundwater beneath the southwest portion of the waste boundary flows to the northwest to
Mountain Island Lake. Groundwater in the southwest portion of the site under the ash storage
area flows to the northwest, under the cinder storage area to the Mountain Island Lake. 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.
The source area at RBSS is defined as the ash basin (Primary and Secondary cells), ash
storage area, and the cinder storage area. Source characterization was performed to identify
physical and chemical properties of ash, ash basin surface water, porewater, and areas of
wetness (AOW) outside the ash basin.
The compliance boundary for groundwater quality at the RBSS site is defined in accordance
with Title 15A NCAC 02L .0107(a) as being established at either 500 feet from the waste
boundary or at the property boundary, whichever is closer to the waste boundary. As described
in the CSA report, analytical results for groundwater samples were compared to the North
Carolina Groundwater Quality Standards, as specified in 15A NCAC 2L.0202 (2L Standards or
2L) or Interim Maximum Allowable Concentration (IMAC) established by NCDEQ pursuant to
15A NCAC 2L.0202(c), or North Carolina Department of Health and Human Services (DHHS)
Health Screening Level (HSL) (hexavalent chromium only) for the purpose of identifying
constituents of interest (COIs). The IMACs were issued for certain constituents in 2010, 2011,
and 2012; however, NCDEQ has not established a 2L Standard for those constituents as
described in 15A NCAC 2L.0202(c). For this reason, the IMACs noted in this document are for
reference purposes only.
Areas of 2L Standard or IMAC exceedances of antimony, arsenic, boron, chromium 4, cobalt,
iron, manganese, sulfate, thallium, total dissolved solids (TDS), and vanadium are within and
beneath or immediately adjacent to the RBSS source area.
1.3 History of Site Groundwater Monitoring
Fourteen monitoring wells (MW -1S/D, MW -2S/D, MW -3S/D, MW -4S/D, MW-5S/D, MW-6S/D,
and MW-7S/D) were installed by Duke Energy in 2006 as part of the voluntary monitoring
system for groundwater adjacent to the ash basin Primary and Secondary cells. 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/D, MW -5S/D, MW -6S/D and MW -
4 Unless otherwise noted, references to chromium in this document indicate total chromium.
Duke Energy Carolinas, LLC | CSA Supplement 2
Riverbend Steam Station Ash Basin
SECTION 1 – BACKGROUND

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7S/D since February 2013 as part of groundwater assessment efforts. A review of voluntary
monitoring well sampling results obtained between 2006 and 2016 indicates the following:
• Iron and manganese have intermittently exceeded the 2L Standards in voluntary wells
screened in the shallow and deep flow layers located across the site. However, these
exceedances may be attributable to naturally occurring conditions and require additional
evaluation as site-specific PPBCs are refined.
• Lead exceeded the 2L Standard in the shallow flow layer in well MW -4S during the
February 2013 and October 2015 voluntary sampling events. No lead exceedances were
exhibited in the deep flow layer during any of the voluntary sampling events.
• Antimony concentrations have intermittently exceeded the IMAC in the deep flow layer
across the site. No exceedances of antimony were exhibited in the shallow flow layer.
• TDS exceeded the 2L Standard in the shallow flow layer in well MW -4S during the
February 2013 sampling event. No TDS exceedances were exhibited in the deep flow
layer during any of the voluntary sampling events.
Construction details for voluntary monitoring wells are provided in Table 1-1. The location of the
ash basin voluntary monitoring wells, the approximate ash basin waste boundary, and the
compliance boundary are shown on Figure 1-2.
As described in Section 1.2, the compliance boundaries for groundwater quality at the RBSS
site are defined in accordance with Title 15A NCAC 02L .0107(a) as being established at either
500 feet from the waste boundaries or at the property boundary, whichever is closer to the
waste boundary. After additional review, Duke Energy is aware that the compliance boundaries
currently depicted on CSA and CAP report figures need to be revised to accurately reflect Duke
Energy property ownership. The current property depiction was obtained from county GIS
records and shows certain areas consisting of rights-of-ways as not being owned by Duke
Energy. Duke Energy is in the process of revising these areas and will submit revised
compliance boundary figures to NCDEQ for approval.
1.3.1 NPDES Sampling
NPDES compliance monitoring wells (compliance wells) were installed in December 2010.
Compliance groundwater monitoring, as required by the NPDES permit, began in December
2010. From December 2010 through June 2016, compliance groundwater monitoring wells at
the RBSS site have been sampled three times each year, resulting in 17 total monitoring events
during that time.
One or more groundwater quality standards (2L Standards) have been exceeded in
groundwater samples collected from the compliance monitoring wells. Exceedances have
occurred in one or more wells during one or more sampling events for chromium, iron,
manganese, antimony. A review of NPDES compliance well sampling results indicates the
following:
• During one sampling event (June 1, 2015), chromium exhibited a groundwater
concentration that exceeded the 2L Standard in compliance well MW-7SR screened
within the shallow flow layer along the southern waste boundary of the ash basin
Primary Cell.
Duke Energy Carolinas, LLC | CSA Supplement 2
Riverbend Steam Station Ash Basin
SECTION 1 – BACKGROUND

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• Iron and manganese have intermittently exceeded the 2L Standards in compliance wells
screened in the shallow and deep flow layers located across the site during the NPDES
sampling period. However, these exceedances may be attributable to naturally occurring
conditions and require additional evaluation as site-specific PPBCs are refined.
• Antimony concentrations have intermittently exceeded the IMAC in the shallow and deep
flow layers across the site during the 2013 and 2014 sampling events. Several
exceedances were detected in wells during the February 2016 sampling event.
Historical analytical results and a summary of the range of exceedances within the NPDES
groundwater monitoring program are provided in Tables 1-2 and 1-3, respectively.
1.3.2 CSA Sampling
The CSA for the RBSS site began in February 2015 and was completed in August 2015.
Seventy-eight groundwater monitoring wells and six soil borings were installed/advanced as part
of this assessment to characterize the ash, soil, rock, and groundwater at the RBSS site. One
comprehensive round of sampling and analysis was included in the CSA report and included
sampling of ash, ash basin surface water, porewater, groundwater, and Areas of Wetness
(AOW) (Figure 1-2). In addition, hydrogeological evaluation testing was performed on newly
installed wells.
The following constituents were reported as COIs in the CSA report:
• Soil: arsenic, boron, cobalt, iron, manganese, nickel, selenium, and vanadium.
• Groundwater: antimony, arsenic, boron, chromium, cobalt, iron, manganese, sulfate,
TDS, thallium, and vanadium.
• Surface water: aluminum, cadmium, chromium, cobalt, copper, iron, lead, manganese,
selenium, thallium, vanadium, and zinc.
Horizontal and vertical delineation of source-related soil impacts was presented in the CSA
report. Where soil impacts were identified beneath the ash basin (beneath the ash/soil
interface), the vertical extent of contamination was generally limited to the soil samples collected
just beneath the ash.
Groundwater impacts at the site attributable to ash handling and storage were delineated during
the CSA activities with the following areas requiring refinement:
• Horizontal and vertical extent north of the coal pile and west of the cinder storage area.
• Horizontal and vertical extent outside the waste boundary northeast of the ash basins.
1.3.3 Post-CSA Sampling
Four additional rounds of groundwater sampling of the CSA wells have occurred since submittal
of the CSA report. Round 2 of groundwater monitoring occurred in September 2015 and was
reported in the Corrective Action Plan (CAP) Part 1 (submitted on November 16, 2015). Rounds
3 and 4 of groundwater monitoring occurred in November and December 2015, and results
were reported in the CSA Supplement 1 as part of the CAP Part 2 report (submitted on
February 12, 2016). Round 5 of groundwater monitoring was conducted in March and April 2016
and is the focus of the data evaluation presented in Section 3 of this CSA Supplement 2.
Duke Energy Carolinas, LLC | CSA Supplement 2
Riverbend Steam Station Ash Basin
SECTION 1 – BACKGROUND

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1.3.4 NCDEQ Water Supply Well Sampling
Section § 130A-309.209 (c) of CAMA indicates that NCDEQ requires sampling of water supply
wells to evaluate whether the wells may be adversely impacted by releases from CCR
impoundments. NCDEQ required sampling of all drinking water receptors within 0.5 mile of the
RBSS ash basin compliance boundary in all directions, since the direction of groundwater flow
had not been determined at RBSS at the time of sampling. However, no private wells within 0.5
mile of the compliance boundary were sampled at the RBSS sites, and “Do Not Drink” letters
were not issued by the DHHS.
One reported private water supply well is located at a residence northeast of RBSS within a 0.5-
mile radius of the ash basin compliance boundary. This well is located on the northern side of
Mountain Island Lake in Mecklenburg County (Well 1 shown on Figure 1-3). Information
evaluated as part of the CSA and the additional data in this report indicates that the identified
water supply well would not be impacted as it is hydraulically isolated and across Mountain
Island Lake from the ash basin. As such, this water supply well in the vicinity of the RBSS
facility is not impacted by CCR releases from the ash basin.

Duke Energy Carolinas, LLC | CSA Supplement 2
Riverbend Steam Station Ash Basin
SECTION 2 – CSA REVIEW COMMENTS

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Section 2 – CSA Review Comments
Representatives of NCDEQ’s Central Office and Mooresville Regional Office (MRO) met with
Duke Energy and HDR on October 15, 2015 to present their review comments to the CSA
report. Comments were organized in two categories: general comments applicable to all Duke
Energy Carolinas CSA reports regardless of site and site-specific comments applicable to
RBSS.
2.1 NCDEQ General Comments and Responses
General comments applicable to the CSA reports, and responses to the comments, are
presented in Table 2-1.
2.2 NCDEQ Site-Specific Comments and Responses
Site-specific comments and responses were included in the CSA Supplement 1, which was
submitted to NCDEQ on February 12, 2016 as part of the CAP Part 2 report.
2.3 Errata
Editorial comments and corrections to the CSA report were included in the CSA Supplement 1,
which was submitted to NCDEQ on February 12, 2016 as part of the CAP Part 2 report. Since
the issuance of the CAP Part 2 report, additional evaluation of site data has occurred, resulting
in refinement by flow layer of total porosity, secondary (effective) porosity, and specific storage
for the lower hydrostratigraphic units (deep transition zone) and bedrock), as provided in Table
2-2.

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SECTION 3 – ADDITIONAL ASSESSMENT

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Section 3 – Additional Assessment
Additional assessment activities identified in the CSA report were addressed and the findings
are discussed in the following sections.
3.1 Additional Assessment Activities
Additional assessment activities included monitoring well installation and sampling, as
discussed below.
3.1.1 Well Installation
Additional monitoring wells were installed to refine understanding of groundwater flow direction
and extent of exceedances at RBSS. Refinement of exceedances focused primarily on the
extent of groundwater impacts to the west of the ash and cinder storage areas near
groundwater monitoring well GWA-3SA/D, and additional monitoring wells GWA-11S/D and
GWA-12S/D were installed. In addition, monitoring well MW -2S has been repeatedly found dry
during previous rounds (Rounds 1 and 2) of sampling and analytical results are presented
herein from the Round 5 sampling event.
Subsequent discussions with Duke Energy and NCDEQ resulted in the installation of additional
monitoring wells beyond those described above. A summary of these additional assessment
wells and installation dates, as well as the purpose for installation, is provided in the table
below.
Boring/Well
Identification
Installation
Date
Purpose for Installation Result
BG-4S 3/4/2016 Background well locations
evaluated during CAP Part 1
determined that the background
well locations may not be truly
background. Additional
background well locations
necessary.
Not installed and developed in
time for sampling inclusion
during the Round 5 sampling
event.
BG-4D 3/4/2016 Not installed and developed in
time for sampling inclusion
during the Round 5 sampling
event.
BG-5D 3/17/2016 Background well locations
evaluated during CAP Part 1
determined that the background
well locations may not be truly
background. Additional
background well locations
necessary.
Not installed and developed in
time for sampling inclusion
during the Round 5 sampling
event.
BG-5BR 4/4/2016 Concentrations of antimony,
hexavalent chromium, and
vanadium exceeded applicable
criteria.
GWA-11S 2/4/2016 Wells to be installed to determine
horizontal extent of exceedances
reported at GWA-3 and to better
define groundwater flow direction.
Concentrations of cobalt, iron,
manganese, and vanadium
exceeded applicable criteria.
GWA-11D 2/21/2016 Concentrations of hexavalent
chromium, cobalt, manganese,
sulfate, thallium, and TDS
exceeded applicable criteria.
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Boring/Well
Identification
Installation
Date
Purpose for Installation Result
GWA-12S 1/30/2016 Wells to be installed to determine
horizontal extent of exceedances
reported at GWA-4 and AS-1 and
to better define groundwater flow
direction.
Concentrations of beryllium,
hexavalent chromium, cobalt,
iron, manganese, and vanadium
exceeded applicable criteria.
GWA-12D 1/31/2016 Concentrations of cobalt, iron,
manganese, sulfate, TDS, and
vanadium exceeded applicable
criteria.
GWA-13S 4/28/2016 Wells to be installed to determine
horizontal extent of exceedances
reported at GWA-20 and to better
define groundwater flow direction.
Not installed in time for
sampling inclusion during the
Round 5 sampling event.
GWA-13D 4/29/2016 Not installed in time for
sampling inclusion during the
Round 5 sampling event.
GWA-14S 2/15/2016 NCDEQ has requested a well
cluster at this location to determine
horizontal extent of exceedances
reported at GWA-6 and to better
define groundwater flow direction.
Concentrations of cobalt and
manganese exceeded
applicable criteria.
GWA-14D 4/28/2016 Not installed in time for
sampling inclusion during the
Round 5 sampling event.
MW-2S-A 2/6/2016 Well requested by NCDEQ as a
replacement well for dry well MW -
2S.
MW -2S was not dry at the time
of the Round 5 sampling event
included in the sampling. MW -
2S-A was not sampled during
the Round 5 sampling event.

Monitoring well logs and core photos, field sampling and slug test records, and analytical
laboratory reports are included in Appendices A, B, and C, respectively.
Several monitoring wells exhibit pH results of 9 or above, which is indicative of cement leakage
beyond the borehole seal. In addition, several monitoring wells were sampled with turbidity
greater than 10 NTU. The evaluation of groundwater quality data obtained from these wells
during Round 5 sampling must be qualified and further evaluated. The additional assessment
wells were installed by North Carolina-licensed drillers according to construction standards
described in 15A NCAC 2C.0107.
3.1.2 Well Gauging and Sampling
Round 5 of groundwater, porewater, ash basin surface water, AOW, and ash basin water
sampling activities was completed between February 19 and April 22, 2016. Groundwater
analytical parameters and methods for Round 5 were consistent with those used during
previous sampling events, as presented in previous reports. A total of 87 groundwater and
porewater monitoring wells were sampled during the Round 5 event. Sample locations are
depicted on Figure 1-2.
3.2 Additional Assessment Results
Findings and results from Round 5 of sampling and analysis and additional assessment
activities are presented below. Note that the Round 3 and 4 monitoring events focused on
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sampling of background wells only. Therefore, groundwater elevation and analysis results are
compared to data previously obtained during Round 1 and 2 monitoring events. A summary of
the analytical results is presented in Tables 3-1 through 3-4 for groundwater, porewater, ash
basin surface water, and AOW s, respectively. A summary of groundwater elevations measured
during the Round 1 through 5 gauging events is presented in Table 3-5.
3.2.1 Groundwater Flow Direction
On June 15, 2016, monitoring wells were manually gauged from the top of the PVC casing
using an electronic water level indicator accurate to 0.01 foot. Groundwater elevations were
generally lower, with a few exceptions, than those measured during Rounds 1 and 2 which is
likely attributable to seasonal variation of the water table as well as the activities associated with
ash basin dewatering and excavation. Groundwater flow direction was consistent with flow
directions identified in Rounds 1 and 2, and generally flows to the north/northwest from the ash
basin Primary and Secondary cells, ash storage area, and cinder storage area toward Mountain
Island Lake. Groundwater elevations and inferred contours for the shallow, deep, and bedrock
flow layers are depicted on Figures 3-1, 3-2, and 3-3, respectively. These groundwater flow
directions are consistent with interpretations made in the CSA report.
3.2.2 Sampling Results
3.2.2.1 SUMMARY OF ROUND 1 AND 2 GROUNDWATER SAMPLING RESULTS
As previously mentioned in Section 1.3.2, the following COIs were identified in groundwater as
a result of sampling conducted during the CSA: antimony, arsenic, boron, chromium, cobalt,
iron, manganese, sulfate, TDS, thallium, and vanadium. The list of COIs resulting from Round 5
of groundwater sampling is consistent with the exception of boron, which did not have an
exceedance during the Round 5 sampling event. In addition, beryllium and hexavalent
chromium exceeded applicable criteria during the Round 5 sampling event and may be
considered additional COIs.
3.2.2.2 ROUND 5 POREWATER SAMPLING RESULTS
A total of seven porewater samples were collected from monitoring wells (AB-3S, AB-4S, AB-
5S/SL, AB-7S, and C-1S) screened within the ash basin Primary and Secondary cells and the
cinder storage area. A Field Duplicate sample was collected from well C-1S for seven total
samples. Antimony, arsenic, boron, hexavalent chromium, cobalt, iron, manganese, sulfate,
TDS, thallium, and vanadium exceeded applicable criteria in porewater samples collected
during the Round 5 sampling event. The range and number of exceedances of each COI in
porewater is listed below.
• Antimony: 2.7J µg/L to 18 µg/L; 2 exceedances/7 samples
• Arsenic: 10.9 µg/L to 497 µg/L; 7/7
• Boron: 1,540 µg/L to 2,160 µg/L; 2/7
• Hexavalent Chromium: 0.21 µg/L; 1/7
• Cobalt: 1.2 µg/L to 132 µg/L; 2/7
• Iron: 3,280 µg/L to 68,500 µg/L; 4/7
• Manganese: 90.3 µg/L to 3,100 µg/L; 6/7
• Sulfate: 410,000J µg/L to 415,000J µg/L; 2/7
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• TDS: 515,000 µg/L to 691,000 µg/L; 3/7
• Thallium: 0.22 µg/L to1 µg/L ; 5/7
• Vanadium: 0.57 µg/L to 183 µg/L ; 7/7
3.2.2.3 ROUND 5 GROUNDWATER SAMPLING RESULTS
A total of 81 groundwater monitoring wells were sampled during February, March, and April
2016 as part of the Round 5 sampling event. In addition, a total of six Field Duplicate samples
were collected from the groundwater wells. The analytical results for the groundwater monitoring
well samples are presented on Table 3-1. In general, the COIs identified during Round 5
groundwater sampling are consistent with the results obtained during Rounds 1 and 2. A
summary of Round 5 sampling results per COI identified during the CSA is as follows:
• Antimony: 1.1 µg/L to 15 µg/L; 20 exceedances/87 samples (20/87)
o Antimony exceeded the IMAC in one well (AB-2S) screened within the shallow
flow layer located downgradient of the ash basin Primary Cell. Antimony
exceedances were detected in the deep flow layer in wells located within and
downgradient of the ash basin Primary and Secondary cells. Antimony
exceedances in the bedrock flow layer were detected more frequently and at
concentrations that were generally higher than the deep flow layer. Antimony was
detected above the IMAC in background well BG-5BR, indicating that antimony
may be present as a naturally occurring constituent. In general, total and
dissolved concentrations were consistent in each sample.
• Arsenic: No exceedances in groundwater
• Beryllium: 5.5 µg/L; 1/87
o Beryllium exceeded the 2L Standard in one well (GWA-12S) screened within the
shallow flow layer. There were no exceedances for beryllium in wells screened
within the deep and bedrock flow layer.
• Boron: No exceedances in groundwater
• Chromium: 11.1 µg/L to 160 µg/L; 19/87
o Chromium exceeded the 2L Standard in the deep flow layer in wells located
within the ash basin Primary Cell, in wells sidegradient and upgradient of the ash
storage area, and in wells downgradient of the ash basin Primary and Secondary
cells. Chromium exceeded the 2L Standard in the bedrock flow layer in five wells
located upgradient and sidegradient of the ash storage area and the ash basin
Primary Cell. No exceedances of chromium were detected in the shallow flow
layer. In general, total and dissolved concentrations were consistent in each
sample.
• Hexavalent Chromium: 0.072J µg/L to 56.1J µg/L; 54/87
o Hexavalent chromium exceeded the DHHS HSL in the shallow flow layer in wells
located across the RBSS site. Hexavalent chromium also exceeded the DHHS
HSL in the deep flow layer with more frequency and higher concentrations than
the shallow flow layer wells. Several deep flow layer wells exhibited exceedances
within and adjacent to the waste boundary of the ash basin Primary and
Secondary cells, and the ash storage area. Exceedances of hexavalent
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chromium in the bedrock flow layer wells were less frequent and sporadically
located across the site. Note that dissolved phase analyses were not performed
on groundwater samples for hexavalent chromium.
• Cobalt: 1.3 µg/L to 78.9 µg/L; 31/87
o Cobalt exceeded the IMAC in the shallow flow layer in wells across the RBSS
site. The concentrations were generally the highest in wells located outside the
waste boundary of the cinder storage area, the ash storage area, and the ash
basin Primary and Secondary cells. Cobalt exceedances in the deep flow layer
were significantly less frequent and at lower concentrations than were observed
in the shallow flow layer. Cobalt exceedances in the bedrock flow layer were
exhibited in one bedrock well (MW -15BR), which is located downgradient of the
ash basin Primary Cell. Total and dissolved concentrations were generally
consistent across the sample set.
• Iron: 305 µg/L to 29,500 µg/L; 20/87
o Iron exceeded the 2L Standard in the shallow flow layer in wells in multiple areas
of the RBSS site. Exceedances of iron in the deep flow layer were less frequent
and located in wells within the ash basin Primary Cell, upgradient of the ash
storage area, and side gradient of the cinder storage area. Exceedances of iron
in the bedrock flow layer were limited to one well (GWA-20BR), which was
detected in the Field Duplicate sample. Total and dissolved concentrations were
generally inconsistent across the sample set.
• Manganese: 53.2 µg/L to 12,700 µg/L; 35/87
o Manganese exceeded the 2L Standard in the shallow, deep, and bedrock flow
layers in wells across the RBSS site. However, the frequency of exceedances
and concentrations were substantially less in the bedrock flow layer, with only
one bedrock well (MW -15BR) exhibiting manganese exceedances. Total and
dissolved concentrations were generally consistent across the sample set.
• Sulfate: 394,000 µg/L to 1,580,000J µg/L; 7/87
o Sulfate exceeded the 2L Standard in three wells screened within the shallow flow
layer and four wells within the deep flow layer. There were no exceedances for
sulfate in wells screened within the bedrock flow layer. The shallow and deep
screened wells are located sidegradient and downgradient of the ash storage
area and the cinder storage area.
• Thallium: 1J µg/L; 1/87
o Thallium exceeded the IMAC in one well (GWA-11D) screened within the deep
flow layer. There were no exceedances for thallium in wells screened within the
shallow and bedrock flow layers.
• TDS: 588,000 µg/L to 3,240,000 µg/L; 11/87
o TDS exceeded the 2L Standard in three wells screened within the shallow flow
layer, six wells screened within the deep flow layer, and two wells screened
within the bedrock flow layer. With the exception of one well (AB-3D), these wells
are located outside of the ash basin Primary and Secondary cells. The bedrock
flow layer wells with TDS exceedances are located upgradient of the Primary and
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Secondary cells. The shallow and deep wells with TDS exceedances are located
sidegradient and downgradient of the Primary Cell.
• Vanadium: 0.35 µg/L to 38.2 µg/L; 58/87
o Vanadium exceeded the IMAC in the shallow, deep, and bedrock flow layers in
wells across the RBSS site, including background wells. Total and dissolved
concentrations were generally consistent across the sample set.
The horizontal extent of exceedances is presented in the form of isoconcentration figures
(Figures 3-4.1 through 3-4.39). The vertical extent of sulfate is presented on applicable cross
sections (Figures 3-5.1 through 3-5.8). In addition, cross sections show hydrostratigraphic
layers, rock lithology, rock core recovery (REC) and rock quality designation (RQD; a measure
of rock mass discontinuities/fracturing) in response to comments received from NCDEQ.
3.2.2.4 COMPARISON OF POREWATER AND GROUNDWATER RESULTS
Based upon review of data collected during Round 5 sampling, there was no discernable trend
in comparison of the COI concentrations in the porewater versus COI concentrations for
groundwater in wells screened within the shallow flow layer. The concentrations of COIs
identified in porewater were generally consistent with groundwater concentrations with the
possible exceptions of iron and sulfate. Exceedances of beryllium and chromium were exhibited
in shallow groundwater wells, but not in porewater wells, and exceedances of arsenic and boron
were exhibited in porewater wells, but not in shallow groundwater wells.
Piper diagrams presented in the CSA report provided evidence of mixing ash basin porewater
and groundwater, and Round 5 analytical results are consistent with previous presentations. In
general, the ionic composition of groundwater and surface water at the RBSS site is
predominantly rich in calcium/magnesium. Piper diagrams with cation-anion balance differences
< 10% are presented in Figures 3-6.1 through 3-6.4. In addition, overall cation-anion balance
differences are summarized in Table 3-6.
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Riverbend Steam Station Ash Basin
SECTION 4 – BACKGROUND CONCENTRATIONS

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Section 4 – Background Concentrations
Presentation of site-specific PPBCs was included in the CAP Part 1 report and is pending
refinement as the required minimum number of additional sampling results become available.
Regulations providing North Carolina groundwater quality standards are provided in T15A
NCAC 02L .0202. Section (b)(3) of the regulation provides that:
Where naturally occurring substances exceed the established standard, the standard shall
be the naturally occurring concentration as determined by the Director.
The reference background concentrations determined by the methodology described below will
be submitted to the NCDEQ DWR as the proposed naturally occurring site background
concentrations for the specific constituents. A site-specific report documenting the procedures,
evaluations, and calculation will be prepared and submitted to the NCDEQ.
4.1 Methodology
As stated in the USEPA (USEPA 2009) (Unified Guidance):
The Unified Guidance recommends that a minimum of at least 8 to 10
independent background observations be collected before running most
statistical tests. Although still a small sample size by statistical standards, these
levels allow for minimally acceptable estimates of variability and evaluation of
trend and goodness-of fit. However, this recommendation should be considered
a temporary minimum until additional background sampling can be conducted
and the background sample size enlarged.5
Once the required minimum number of samples is available, HDR will calculate PPBCs utilizing
the appropriate methods in the Unified Guidance, the USEPA ProUCL software, and guidance
found in the North Carolina Division of Water Quality (NCDWQ) technical assistance document
Evaluating Metals in Groundwater at DWQ Permitted Facilities.
This process will also follow HDR’s proposed method to establish reference background
concentrations for constituents according to the USEPA’s Hazardous and Solid Waste
Management System; Disposal of Coal Combustion Residuals from Electric Utilities; Final Rule
(EPA CCR).6 The proposed method will be developed in consultation with Synterra, Duke
Energy’s groundwater assessment consultant for Duke Energy Progress sites, to ensure
consistency in approach.
5 U.S. Environmental Protection Agency (USEPA) Unified Guidance (USEPA 2009), 5.2.1 Selecting Monitoring
Constituents and Adequate Sample Sizes 6 HDR modified its earlier methods to establish reference background concentration so that both state and federal
regulations are comparable. Having similar processes to address the two sets of regulations will minimize confusion.
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As recommended by the USEPA Unified Guidance, HDR will calculate the 95 percent upper
prediction limits (UPL95) as the proposed reference background concentration value for each
constituent at the RBSS site.7
HDR will calculate UPL95 values for each of the constituents using their respective
concentrations observed in the samples taken from the set of site-specific background wells
once a minimum of eight observations per constituent are available. Samples will not be used to
develop reference background concentrations whenever turbidity is 10 NTU or greater. Only
non-filtered results will be utilized. HDR will review and evaluate the corresponding filtered
results; however, they will not be used for compliance purposes at this time.
The data across the background wells will be pooled prior to estimating the reference
background concentration using the UPL95.
When implementing this approach, HDR will consider that the background wells are screened in
different hydrostratigraphic units (shallow, transition zone, or bedrock). While there are
differences as described, the fundamental assumption will be that the constituent concentrations
sampled at these background wells, when pooled, will serve as an estimate of overall well field
conditions for a given constituent. HDR will test this assumption using statistical methods and if
distinct sub-groups exist, separate background concentrations for each distinct sub-group of
wells by hydrostratigraphic unit (shallow, transition zone, and bedrock) will be calculated.
The methodology used to calculate upper prediction limits (UPLs) for the constituents will be
generally completed in three parts as follows:
1. Analyze Preliminary Data
2. Determine Differences Across Sub-Groups
3. Develop Background Threshold Values (UPLs)
Part 1 of the process includes the preliminary data analyses used to assess and transform the
data where necessary such that the data can be used to calculate UPLs. Statistical methods will
be used to evaluate outliers, serial correlation, seasonality, spatial variability, trends, and
appropriateness of the period of record (sampling period).
Part 2 of the process includes describing the approach to test for sub-group differences. Types
of sub-groups to test include seasonal sub-groups (winter, spring, summer, and fall) and well
class sub-groups (bedrock, shallow, or deep). If the groups are statistically different after testing,
the same steps described in Part 1 can be applied to the partitioned data to better understand
the distribution of the samples within a sub-group for each constituent. The reference
7 There are different methods that can be used to estimate the reference background concentrations such as the UPL
and the upper tolerance limit (UTL). HDR selected the UPL as it is the statistic recommended by the USEPA Unified
Guidance (page 2-15). The Unified Guidance recommends the UPL over the UTL for the following reasons, (1). The
ability to estimate a UTL which can control for Type I error rates when simultaneously testing an exact number of
multiple future or independent observations is not as precise as when estimating the appropriate UPL. (2) The
mathematical underpinnings of UPLs under re-testing strategies are well established, while those for re-testing with
tolerance limits are not. Re-testing strategies are now encouraged and sometimes required under assessment
monitoring situations. (3) Statistically, the two limits are similar, especially under normal assumptions.
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SECTION 4 – BACKGROUND CONCENTRATIONS

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background concentration values using the UPL95 for a constituent will be produced for each
sub-group of samples, provided the sub-groups represent distinct populations.
Part 3 of the process involves describing the statistical analyses and presenting the resulting
background threshold values (UPLs) for each constituent.
4.2 Observation for Background Wells
Currently, the RBSS site has the following number of usable observations at background wells
for implementation of the background concentration methodology described in Section 4.1:
• BG-1S (CSA Monitoring Well) – 3 observations
• BG-1D (CSA Monitoring Well) – 5 observations
• BG-2S (CSA Monitoring Well) – 5 observations
• BG-2D (CSA Monitoring Well) – 4 observations
• BG-2BR (CSA Monitoring Well) – 5 observations
• BG-3S (CSA Monitoring Well) – 5 observations
• BG-3D (CSA Monitoring Well) – 4 observations
• BG-4S (CSA Monitoring Well) – 0 observations
• BG-4D (CSA Monitoring Well) – 0 observations
• BG-4BRU (CSA Monitoring Well) – 0 observations
• BG-5D (CSA Monitoring Well) – 0 observations
• BG-5BR (CSA Monitoring Well) – 0 observations
• MW -7SR (Compliance Monitoring Well) – 16 observations
• MW -7D (Voluntary Monitoring Well) – 18 observations
• MW -7BR (CSA Monitoring Well) – 3 observations

It is expected that with interim monitoring implementation, the RBSS site will have the
appropriate number of data points to perform the calculation of UPL95s. HDR is considering
alternatives that will provide the required number of sample events and will provide an update
on that evaluation at a later date.
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Riverbend Steam Station Ash Basin
SECTION 5 – ANTICIPATED ADDITIONAL ASSESSMENT ACTIVITIES

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Section 5 – Anticipated Additional Assessment
Activities
Anticipated additional assessment activities are summarized below.
5.1 Proposed Additional Assessment Monitoring Wells
Based on review of site information and analytical data available at this time, there are several
locations at the site where additional groundwater assessment is warranted to refine delineation
of the vertical extent of groundwater impacts associated with potential coal ash-related
constituents. The following wells are currently planned for installation during Fall of 2016:
Proposed Additional
Monitoring Wells
Location Purpose Approximate
Monitoring Well
Depth(s) (ft)
GWA-3BR Northwest of cinder
storage area
Evaluate vertical extent of
exceedances northwest of
cinder storage area and
those reported in well GWA-
3D (TDS, sulfate, Mn).
90
GWA-15S North of cinder
storage area
Evaluate extent of
exceedances north of cinder
Storage area and those
reported in wells C-1S (As,
Co, Fe, Mn, Sulfate, and
TDS) and C-1BRU (Sb and
TDS).
30
GWA-15D North of cinder
storage area
Evaluate extent of
exceedances north of cinder
storage area and those
reported in wells C-1S (As,
Co, Fe, Mn, Sulfate, and
TDS) and C-1BRU (Sb and
TDS).
125

5.2 Implementation of the Effectiveness Monitoring Plan
The effectiveness monitoring plan proposed in CAP Part 2 provided detailed information
regarding field activities to be performed during collection of groundwater, ash basin surface
water, and AOW samples associated with the ash basin (Primary and Secondary cells), ash
storage area, and the cinder storage area at the RBSS site. The monitoring plan is intended to
evaluate the effectiveness of proposed corrective actions and address the need to evaluate
baseline conditions and seasonal variation in groundwater, ash basin surface water, and AOWs.
Duke Energy will implement the effectiveness monitoring plan in accordance with
recommendations provided in the CAP Part 2 report as well as subsequent discussions with
NCDEQ.
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Riverbend Steam Station Ash Basin
SECTION 6 – CONCLUSIONS AND RECOMMENDATIONS

20
Section 6 – Conclusions and Recommendations
The following conclusions have been developed from the information presented in this CSA
Supplement 2 report:
• Based on its location across Mountain Island Lake from the ash basin and being
hydraulically isolated, the water supply well (Well 1) in the vicinity of the RBSS facility is
not impacted by CCR releases from the ash basin.
• Groundwater monitoring results from Round 5 of sampling, including data from additional
assessment groundwater monitoring wells; indicate consistency with previous sampling
results, specifically the extent of impact to groundwater from ash basin-related
constituents (e.g., boron and sulfate).
• Additional monitoring wells (GWA-11S/D and GWA-12S/D) within the vicinity of GWA-
2BR/BRU were installed to refine understanding of constituent concentrations west of
the ash and cinder storage areas near groundwater monitoring well GWA-3SA/D. Cobalt
concentrations in the additional assessment wells were approximately one order of
magnitude higher than in well GWA-3SA/D during the Round 5 sampling event. Iron
concentrations in the additional assessment wells were lower in the shallow flow layer
than in well GWA-3SA, and similar or higher in the deep flow layer than in well GWA-3D.
Manganese concentrations in the additional assessment wells were lower in both the
shallow and deep flow layers than in GWA-3SA/D. Iron concentrations in the additional
assessment wells were lower in the shallow flow layer than in well GWA-3SA, and
similar or higher in the deep flow layer than in well GWA-3D. Manganese concentrations
in the additional assessment wells were lower in both the shallow and deep flow layers
than in GWA-3SA/D.
Based on the conclusions presented above, the following recommendations are offered:
• Groundwater monitoring results from Round 6 of sampling shall be evaluated for the
additional assessment wells as several were not installed for inclusion within the Round
5 sampling event.
• Refinement of PPBCs should be conducted once the minimum number of viable
observations (e.g. eight) per background well is available.
• Additional monitoring wells (GWA-3BR, GWA-15S, and GWA-15D) should be installed
northwest and north of the cinder storage area to refine horizontal and vertical
delineation of groundwater impacts in these areas.
• Duke Energy will implement the effectiveness monitoring plan in accordance with
recommendations provided in the CAP Part 2 report as well as subsequent discussions
with NCDEQ.

Figures

Tables

A
Appendix A
Monitoring Well Boring Logs
Core Photos

B
Appendix B
Field Sampling Forms
Slug Test Report

C
Appendix C
Laboratory Reports and
Chain-of-Custody Forms
Validation Report