NC0004987_1_FINAL_CSA Supplement 2_Marshall_Report_20160804

Comprehensive Site Assessment
Supplement 2
Marshall 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
Marshall Steam Station
8320 NC Highway 150 E
Terrell, NC 28682
Not Assigned
NC0004987
August 4, 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 35' 49" N, 800 57' 54 "W
This document has been reviewed for accuracy and quality
commensurate with the intended application.
cE N�ti��
Ir
Malcolm F. Schaeffer, L.G.
Senior Geologist

Duke Energy Carolinas, LLC I CSA Supplement 2
Marshall Steam Station Ash Basin
TABLE OF CONTENTS
Table of Contents
Page
ExecutiveSummary......................................................................................................................1
Section1 — 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.........................................................................................................6
1.3.3 Post-CSA Sampling.................................................................................................7
1.3.4 NCDEQ Water Supply Well Sampling.....................................................................7
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
3.1.1 Well Gauging and Sampling..................................................................................11
3.2 Additional Assessment Results....................................................................................11
3.2.1 Groundwater Flow Direction..................................................................................11
3.2.2 Sampling Results..................................................................................................12
Section 4 — Background Concentrations.....................................................................................17
4.1 Methodology.................................................................................................................17
4.2 Observation for Background Wells...............................................................................19
Section 5 — Anticipated Additional Assessment Activities...........................................................20
5.1 Proposed Additional Assessment Monitoring Wells.....................................................20
5.2 Implementation of the Effectiveness Monitoring Plan...................................................20
Section 6 — Conclusions and Recommendations.......................................................................22

Duke Energy Carolinas, LLC I CSA Supplement 2
Marshall Steam Station Ash Basin FN
TABLE OF CONTENTS
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 Barium Isoconcentration Contour Map - Shallow Wells (S)
3-4.5 Barium Isoconcentration Contour Map - Deep Wells (D and BRU)
3-4.6 Barium Isoconcentration Contour Map - Bedrock Wells (BR)
3-4.7 Boron Isoconcentration Contour Map - Shallow Wells (S)
3-4.8 Boron Isoconcentration Contour Map - Deep Wells (D and BRU)
3-4.9 Boron Isoconcentration Contour Map - Bedrock Wells (BR)
3-4.10 Chloride Isoconcentration Contour Map - Shallow Wells (S)
3-4.11 Chloride Isoconcentration Contour Map - Deep Wells (D and BRU)
3-4.12 Chloride 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 Mercury Isoconcentration Contour Map - Shallow Wells (S)
3-4.29 Mercury Isoconcentration Contour Map - Deep Wells (D and BRU)
3-4.30 Mercury Isoconcentration Contour Map - Bedrock Wells (BR)
3-4.31 Selenium Isoconcentration Contour Map - Shallow Wells (S)
3-4.32 Selenium Isoconcentration Contour Map - Deep Wells (D and BRU)
3-4.33 Selenium Isoconcentration Contour Map - Bedrock Wells (BR)
3-4.34 Sulfate Isoconcentration Contour Map - Shallow Wells (S)
3-4.35 Sulfate Isoconcentration Contour Map - Deep Wells (D and BRU)
3-4.36 Sulfate Isoconcentration Contour Map - Bedrock Wells (BR)
3-4.37 Total Dissolved Solids Isoconcentration Contour Map - Shallow Wells (S)
3-4.38 Total Dissolved Solids Isoconcentration Contour Map - Deep Wells (D and BRU)
3-4.39 Total Dissolved Solids Isoconcentration Contour Map - Bedrock Wells (BR)
3-4.40 Thallium Isoconcentration Contour Map - Shallow Wells (S)
3-4.41 Thallium Isoconcentration Contour Map - Deep Wells (D and BRU)
3-4.42 Thallium Isoconcentration Contour Map - Bedrock Wells (BR)
3-4.43 Vanadium Isoconcentration Contour Map - Shallow Wells (S)

Duke Energy Carolinas, LLC I CSA Supplement 2
Marshall Steam Station Ash Basin FN
TABLE OF CONTENTS
3-4.44 Vanadium Isoconcentration Contour Map — Deep Wells (D and BRU)
3-4.45 Vanadium Isoconcentration Contour Map — Bedrock Wells (BR)
3-5.1 Site Cross Section Locations
3-5.2 Cross Section A -A' (1 of 4)
3-5.3 Cross Section A -A' (2 of 4)
3-5.4 Cross Section A -A' (3 of 4)
3-5.5 Cross Section A -A' (4 of 4)
3-5.6 Cross Section B-B' (1 of 2)
3-5.7 Cross Section B-B' (2 of 2)
3-5.8 Cross Section C-C' (1 of 3)
3-5.9 Cross Section C-C' (2 of 3)
3-5.10 Cross Section C-C' (3 of 3)
3-6.1 Piper Diagram — Background Groundwater, Porewater, and Surface Water
3-6.2 Piper Diagram — Shallow Groundwater, Porewater, and Surface Water
3-6.3 Piper Diagram — Deep Groundwater, Porewater, and Surface Water
3-6.4 Piper Diagram — Bedrock Groundwater, Porewater, 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 Comment
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 Surface Water Locations
3-4 Summary of Groundwater Elevations
3-5 Summary of Cation -Anion Balance Differences
APPENDICES
A Monitoring Well Logs and Core Photos
B Field Sampling Forms and Slug Test Reports
C Laboratory Report and Chain -of -Custody Forms

Duke Energy Carolinas, LLC I CSA Supplement 2
Marshall Steam Station Ash Basin FN
EXECUTIVE SUMMARY
Executive Summary
Duke Energy Carolinas, LLC (Duke Energy) owns and operates the Marshall Steam Station
(MSS), located in Catawba County near the town of Terrell, North Carolina (Figure 1-1). MSS
began operations in 1965 as a coal-fired generating station and currently operates four coal-
fired units. Coal combustion residuals (CCR) consisting of bottom and fly ash material from
MSS have historically been disposed in the station's ash basin, located north of the station
adjacent to Lake Norman. Dry ash has been disposed in other areas at the site including the dry
ash landfill units (Phases I and II) and Industrial Landfill No. 1. Flue gas desulfurization (FGD)
residue (i.e., gypsum) and fly ash were disposed in the FGD Residue Landfill, which was placed
in intermediate closure in October 2015. Fly ash was also used as structural fill in the
photovoltaic (PV) structural fill and beneath portions of the Industrial Landfill No. 1. Discharge
from the ash basin is permitted by the North Carolina Department of Environmental Quality
(NCDEQ)' Division of Water Resources (DWR) under the National Pollutant Discharge
Elimination System (NPDES) Permit NC0004987.
This Comprehensive Site Assessment (CSA) Supplement 2 report addresses the following:
• Summary of groundwater, porewater, and ash basin surface water 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 data gaps 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) beneath and downgradient (south-southeast) of the ash
basin. Boron has not been detected in groundwater beyond the compliance boundary. The
hydrogeologic nature of the ash basin setting is the primary control mechanism on groundwater
flow and constituent transport.
Groundwater monitoring results from Round 5 of CSA well sampling and NPDES groundwater
monitoring data are presented herein. Updated summary tables, isoconcentration maps, cross
sections, and graphical representations of the data are included.
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
evaluation of whether the presence of constituents of interest (COls) downgradient of the source
areas is naturally occurring or potentially attributed to the source areas can be advanced in
more detail.
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 I CSA Supplement 2
Marshall Steam Station Ash Basin FYZ
EXECUTIVE SUMMARY
The following conclusions and recommendations are offered:
• 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).
• Additional monitoring wells (GWA-10S/D and GWA-11 S/D) were installed to refine the
understanding of constituent concentrations east of the ash basin and dry ash landfill
(Phase 1). Analytical results from these wells indicate boron, hexavalent chromium,
cobalt, iron, manganese, selenium, sulfate, and vanadium exceeded their applicable
criteria at these locations and will be further evaluated with additional analytical results
for evaluation of whether the horizontal extent of ash -related impacts is adequately
defined at the MSS site.
• Additional wells (GWA-12S/D/BR and GWA-13S/D) were installed to refine groundwater
flow and quality west of and upgradient of the site, however, these wells were not
installed in time for inclusion in the Round 5 sampling event. These monitoring wells
shall be reviewed with the Round 6 sampling event results, when available.
• The vertical extent of ash -related impacts shall be further refined east of the ash basin
and dry ash landfill (Phases I and II).
• Refinement of PPBCs should be conducted once the minimum number of viable
observations per background well are available.
• Additional monitoring wells should be installed east of the ash basin and dry ash landfill
(Phases I and 11) as well as the central portion of the ash basin to refine 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.

Duke Energy Carolinas, LLC I CSA Supplement 2
Marshall Steam Station Ash Basin FN
SECTION 1 — BACKGROUND
Section 1 — Background
Duke Energy Carolinas, LLC (Duke Energy) owns and operates the Marshall Steam Station
(MSS), located in Catawba County near the town of Terrell, North Carolina (Figure 1-1). MSS
began operations in 1965 as a coal-fired generating station and currently operates four coal-
fired units. Coal combustion residuals (CCR) consisting of bottom and fly ash material from
MSS have historically been disposed in the station's ash basin, located north of the station
adjacent to Lake Norman. Dry ash has been disposed in other areas at the site including the dry
ash landfill units (Phases I and 11) and Industrial Landfill No. 1. Flue gas desulfurization (FGD)
residue (i.e., gypsum) and fly ash were disposed in the FGD Residue Landfill, which was placed
in intermediate closure in October 2015. Fly ash was also used as structural fill in the
photovoltaic (PV) structural fill and beneath portions of the Industrial Landfill No. 1. Discharge
from the ash basin is permitted by the North Carolina Department of Environmental Quality
(NCDEQ)2 Division of Water Resources (DWR) under the National Pollutant Discharge
Elimination System (NPDES) Permit NC0004987.
The Comprehensive Site Assessment (CSA) report for MSS was submitted to NCDENR on
September 8, 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, NCDENR's
review of the CSA report led to requests for additional information. As such, CSA Supplement 1,
submitted to NCDEQ on March 3, 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 March and June 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;
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 I CSA Supplement 2
Marshall Steam Station Ash Basin FN
SECTION 1 — BACKGROUND
• An updated approach for the refinement of proposed provisional background
concentrations (PPBCs) for groundwater at the MSS site; and
• A description of additional planned assessment activities.
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 groundwater monitoring wells are shown on Figure 1-2 with green text labels.
1.2 Site Description
The MSS site is located in Catawba County near the town of Terrell, North Carolina. The MSS
site occupies 1,446 acres and is owned by Duke Energy. MSS is a four -unit, coal-fired electric
generating plant. The first two units (Units 1 and 2) began operation in 1965 and 1966,
generating 350 MW each. The remaining units (Units 3 and 4) began operation in 1969 and
1970, generating 648 MW each. Improvements to the plant since 1970 have increased the
electric generating capacity to 2,078 MW.
The ash basin system at the MSS site consists of a single cell impounded by an earthen dike
located on the southeast end of the ash basin, as further described in the CSA report. Inflows
from the station to the ash basin are discharged into the southern portion of the ash basin.
Discharge from the ash basin is through a concrete discharge tower located in the eastern
portion of the ash basin. The concrete discharge tower drains through a 30-inch-diameter,
slip -lined corrugated metal pipe that discharges to Lake Norman. The ash basin pond elevation
is controlled by the use of concrete stoplogs in the discharge tower.
The dry ash landfill consists of two units that are located adjacent to the east (Phase 1) and
northeast (Phase II) portions of the ash basin. The dry ash landfill units were constructed prior
to the requirement for lining industrial landfills and were closed with a soil and vegetative cover
system. The PV structural fill was constructed of fly ash in accordance with the structural fill
rules found in 15A NCAC 13B .1700 et seq. and is located adjacent to and partially on top of the
northwest portion of the ash basin. Portions of the subgrade beneath the Industrial Landfill No.
1, which is located over portions of the northernmost extent of the ash basin, were constructed
of fly ash under the structural fill rules found in 15A NCAC 13B .1700 et seq. The two lined
landfills at the site (FGD Residue Landfill and Industrial Landfill No. 1) also contain CCR
materials that were placed in each landfill in accordance with the landfill operating permit.
Topography at the MSS site generally slopes from the northwest to southeast, ranging from an
approximate high elevation of 900 feet elevation near the western (Sherrills Ford Road) and
northern (Island Point Road) boundaries of the site to an approximate low elevation of 760 feet
at the shoreline of Lake Norman. Ground surface elevation varies approximately 120 to 140 feet
over an approximate distance of 1.5 miles. Surface water drainage generally follows site
topography and flows from the northwest to the southeast across the MSS site except where
drainage patterns have been modified by the ash basins or other construction. A site layout map
is included as Figure 1-2.

Duke Energy Carolinas, LLC I CSA Supplement 2
Marshall Steam Station Ash Basin FN
SECTION 1 — BACKGROUND
The groundwater system in the natural materials (alluvium, soil, soil/saprolite, and bedrock) at
the MSS site is consistent with the Piedmont regolith-fractured rock system and is an
unconfined, connected aquifer system of flow layers. In general, groundwater within the shallow
and deep layers (S and D wells) and bedrock layer (BR wells) flows from the north and
northwest extents of the MSS site property boundary to the south and southeast toward Lake
Norman.
The source areas included in the CSA at the MSS site are defined as the ash basin, the dry ash
landfill (Phases I and 11), and the PV structural fill. Source characterization was performed
during and after the CSA to identify physical and chemical properties of ash, ash basin surface
water, ash porewater, and ash basin areas of wetness (AOW).
The compliance boundary for groundwater quality at the MSS 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 (21- 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 (NCDHHS)
Health Screening Level (HSL) (hexavalent chromium only) for the purpose of identifying COIs.
The IMACs were issued for certain consituents in 2010, 2011 and 2012; however, NCDEQ has
not established a 2L Standard for these 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, IMAC, or DHHS HSL exceedances beneath and downgradient of the
source areas indicate that physical and geochemical processes beneath the MSS site inhibit the
lateral migration of COIs. Vertical migration of COls was observed in select well clusters
(shallow, deep, and bedrock) and is likely influenced by infiltration of precipitation and/or ash
basin water, where applicable, through the shallow and deep flow layers into underlying
fractured bedrock. Groundwater from the shallow, deep, and bedrock flow layers at the MSS
site discharges to Lake Norman and the unnamed tributary east of the ash basin and dry ash
landfill (Phase 1).
History of Site Groundwater Monitoring
Monitoring wells were installed by Duke Energy in 2006 as part of the voluntary monitoring
system for groundwater near the ash basin. The voluntary groundwater monitoring wells were
sampled twice each year by Duke Energy and the analytical results were submitted to NCDENR
DWR. As required by the North Carolina Coal Ash Management Act of 2014 (CAMA), additional
monitoring wells were installed and sampled in 2015 and 2016. A review of voluntary monitoring
well sampling results obtained between 2006 and 2010 indicates the following:
• TDS exceeded the 2L Standard in wells MW-7S and MW-8S during consecutive
sampling events in 2009 and 2010
• Chloride exceeded the 2L Standard in wells MW-7S during consecutive sampling events
in 2009 and 2010

Duke Energy Carolinas, LLC I CSA Supplement 2
Marshall Steam Station Ash Basin FYZ
SECTION 1 - BACKGROUND
• Barium exceeded the 2L Standard in wells MW-6S during the sampling event in
February 2010
• Boron exceeded the 2L Standard in wells MW-6S and MW-7S during consecutive
sampling events in 2009 and 2010
• Iron exceeded the 2L Standard in wells MW-61D and MW-7S/D and MW-8S/D and MW-
9S/D during each of the voluntary sampling events
• Manganese exceeded the 2L Standard in wells MW-6S and MW-7S/D and MW-8S/D
and MW-9S/D during each of the voluntary sampling events
Construction details for monitoring wells installed during these activities are provided in Table 1-
1. The location of the ash basin voluntary and compliance monitoring wells, the approximate
ash basin waste boundary, and the compliance boundary are shown on Figure 1-2.
1.3.1 NPDES Sampling
NPDES compliance monitoring wells (compliance wells) were installed in July and August 2010
and the previously installed monitoring wells became part of the voluntary monitoring program.
Compliance groundwater monitoring, as required by the NPDES permit, began in February
2011. From February 2011 through July 2016, compliance groundwater monitoring wells at the
MSS site have been sampled three times each year, resulting in 17 total monitoring events.
A review of NPDES compliance well sampling results indicates the following:
• Boron, Sulfate, and TDS exceeded the 2L Standard in compliance wells MW-14S/D
• Iron exceeded the 2L Standard in MW-4, MW-41D, MW-10S/D, MW-11S/D, MW-12D,
MW-13S, and MW-14S
• Manganese exceeded the 2L Standard in compliance wells MW-10S/D, MW-12S, MW-
13S, and MW-14S/D
Historical analytical results and a summary of the range of exceedances within the NPDES
groundwater monitoring program results are provided in Tables 1-2 and 1-3, respectively.
1.3.2 CSA Sampling
The CSA for the MSS site began in March 2015 and was completed in September 2015. Eighty-
three groundwater monitoring wells and 13 soil borings were installed/advanced as part of this
assessment to characterize the ash, soil, rock and groundwater at the MSS site. One
comprehensive round of groundwater sampling and analysis was included in the CSA report
and included sampling of soil, groundwater, and surface water (Figure 1-2). In addition,
hydrogeological evaluation testing was performed on newly installed wells.
The following constituents were reported as COls in the CSA report:
• Soil: arsenic, barium, cobalt, iron, manganese, nickel, selenium, and vanadium
• Groundwater: antimony, arsenic, barium, beryllium, boron, chromium3, cobalt, iron,
manganese, selenium, sulfate, thallium, total dissolved solids (TDS), pH, and vanadium.
• Surface water: cobalt, iron, manganese, sulfate, and TDS
3 Unless otherwise noted, references to chromium in this document should be assumed to indicate total chromium.

Duke Energy Carolinas, LLC I CSA Supplement 2
Marshall Steam Station Ash Basin FYZ
SECTION 1 — BACKGROUND
Horizontal and vertical delineation of source -related soil impacts was presented in the CSA
report. Where soil impacts were identified beneath the ash basins (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 of groundwater impacts downgradient and east of the ash
basin and dry ash landfill (Phase 1).
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 December 7, 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 March 3,
2016). Round 5 of groundwater monitoring was conducted in March and April 2016, and is the
focus of the data evaluation in Section 3 of this CSA Supplement 2.
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 potential drinking water receptors within 1500-
feet of the MSS site compliance boundary in all directions, since the direction of groundwater
flow had not been determined at MSS at the time of the sampling. Between March and
September 2015, NCDEQ arranged for independent analytical laboratories to collect and
analyze water samples obtained from wells identified during the Drinking Water Well Survey4, 5 if
the owner agreed to have their well sampled. The NCDEQ-directed water supply well sampling
consisted of collection and analysis of the following:
• A total of 39 samples collected from 38 drinking water supply wells within 0.5 mile of the
MSS compliance boundary; and
• A total of 10 reference or background water supply wells in the vicinity of MSS.
In addition, Duke Energy collected samples from 29 background water supply wells located
within a 2- to 10-mile radius of the MSS site. The locations of the private water supply wells
identified within 0.5 mile of the MSS compliance boundary, including NCDEQ-directed sampling
locations with updated analytical results provided to Duke Energy, are shown on Figure 1-3.
The results of water supply well testing conducted by the NCDEQ in the vicinity of the MSS
facility indicated that boron was detected in 4 of the 39 NCDEQ-sampled water supply wells
4 HDR. 2014a. Marshall Steam Station Ash Basin Drinking Water Supply Well and Receptor Survey. NPDES Permit
NC0004987 September 30, 2014.
5 HDR. 2014b. Marshall Steam Station Ash Basin. Supplement to Drinking Water Supply Well and Receptor Survey.
NPDES Permit NC0004774. November 6, 2014

Duke Energy Carolinas, LLC I CSA Supplement 2
Marshall Steam Station Ash Basin FN
SECTION 1 — BACKGROUND
within 0.5 mile of the compliance boundary and in 5 of 39 background well samples collected by
NCDEQ and Duke Energy. In general, pH was below the state and federal drinking water
standard range in NCDEQ-sampled water supply wells and in NCDEQ- and Duke Energy -
sampled background wells. This result is not unexpected, based on a study published by the
United States Geological Survey6 and additional North Carolina -specific studies' showing that
groundwater pH in the state is commonly below the Maximum Contaminant Level (MCL) range
of 6.5 to 8.5 Standard Units. Lead was above the drinking water standard in 2 of the 39
NCDEQ-sampled water supply wells. None of the NCDEQ-sampled water supply well results
were above Federal primary drinking water standards (MCLs), with the exception of the pH and
lead results noted above.
"Do Not Drink" letters were issued by the DHHS for 38 water supply wells at MSS, with
hexavalent chromium and vanadium being the primary constituents listed in the letters. After
review of studies on how the federal government and other states manage these elements in
drinking water, state health officials withdrew the "Do Not Drink" warnings for these two
constituents. Letters were issued for other constituents as follows: iron (9 wells), lead (1 well),
manganese (3 wells), and sodium (1 well).
Based on data obtained during the NCDEQ water supply well sampling, Duke Energy used a
multiple -lines -of -evidence approach to evaluate whether the presence of constituents in water
supply wells near MSS are the result of migration of CCR-impacted groundwater. This approach
consisted of a detailed evaluation of groundwater flow and groundwater chemical signatures.
The results of the groundwater flow evaluation confirmed that groundwater flow is from the
higher topography located along the southern, western, and northern extent of the MSS
property to the southeast toward Lake Norman and slightly east toward an unnamed tributary
that empties into Lake Norman. Thus, groundwater flow from areas associated with the ash
basin and the ash storage area is away from the water supply wells. A review of topographic
and monitoring well groundwater elevation data at MSS found no evidence of mounding
associated with the ash basin.
The results of the groundwater chemical signature evaluation indicate that constituent
concentrations in the water supply wells are generally consistent with background levels,
including boron and sulfate. The conclusion from the groundwater chemical signature evaluation
is that water supply wells in the vicinity of the MSS facility are not impacted by CCR releases
from the ash basin.
6 Chapman, M.J.,Cravotta III, C.A., Szabo, Z. and Linsay, B.D. 2013 Naturally occurring contaminants in the
Piedmont and Blue Ridge crystalline -rock aquifers and Piedmont Early Mesozoic basin siliciclastic-rock aquifers,
eastern United States, 1994-2008 (Scientific Investigations Report No. 2013-5072). U.S. Geological Survey.
Briel, L.I. 1997. Water quality in the Appalachian Valley and Ridge, the Blue Ridge, and the Piedmont physiographic
provinces, eastern United States (Professional Paper No. 1422-D). U.S. Geological Survey.

Duke Energy Carolinas, LLC I CSA Supplement 2
Marshall Steam Station Ash Basin FN
SECTION 2 — CSA REVIEW COMMENTS
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 MSS.
NCDEQ General Comments and Responses
General comments applicable to the CSA reports, and responses to the comments, are
presented in Table 2-1.
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 19, 2016 as part of the CAP Part 2 report.
Errata
Editorial comments and corrections to the CSA report were included in the CSA Supplement 1,
which was submitted to NCDEQ on February 19, 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.

Duke Energy Carolinas, LLC I CSA Supplement 2
Marshall Steam Station Ash Basin FN
SECTION 3 — ADDITIONAL ASSESSMENT
Section 3 — Additional Assessment
Additional assessment activities identified in the CSA report were addressed and the findings
are discussed in the following sections.
Additional Assessment Activities
Additional assessment activities included monitoring well installation and sampling, as
discussed below.
3.1.1 Well Installation
The following locations at the MSS site required additional assessment to refine the extent of
groundwater impacts attributable to the ash basin:
• Bedrock wells BG-1 BR and BG-3BR, located adjacent to existing background wells BG-
1 S/D and BG-3S/D.
The purpose of these wells was to provide information regarding constituent concentrations to
assist in an understanding of site background concentrations. Subsequent discussions with
Duke Energy and NCDEQ resulted in the installation of additional monitoring wells beyond
those described above. A summary of those additional assessment wells and installation dates,
as well as the purpose for installation, is provided in the table below.
Boring/Well Installation
Identification Date
AL-1D-SB 3/1/2016
BG-1 BR
4/16/2016
BG-3BR
4/5/2016
GWA-10S
GWA-10D
GWA-11S
3/29/2016
3/24/2016
3/11/2016
GWA-11 D
3/9/2016
Purpose for Installation
Recollect soil sample to
confirm arsenic concentrations
in soil sample collected during
CSA.
Provide additional background
groundwater quality for
determination of PPBCs at the
site.
Further delineate groundwater
impacts identified east of the
ash basin and dry ash landfill
(Phase 1).
Results
Soil sample collected with an
arsenic concentration of 6.7 mg/kg
at 34 feet exceeded applicable
criteria
Concentrations of chromium,
hexavalent chromium, cobalt, iron,
manganese, and vanadium
exceeded applicable criteria
Concentrations of cobalt, iron,
manganese, and vanadium
exceeded applicable criteria
Concentrations of hexavalent
chromium, cobalt, manganese, and
vanadium exceeded applicable
criteria
Concentrations of cobalt, iron,
manganese, and vanadium
exceeded applicable criteria _
Concentrations of boron,
hexavalent chromium, iron,
selenium, sulfate, and vanadium
exceeded applicable criteria
Concentrations of boron,
hexavalent chromium, manganese,
and vanadium exceeded
applicable criteria
In

Duke Energy Carolinas, LLC I CSA Supplement 2
Marshall Steam Station Ash Basin
SECTION 3 — ADDITIONAL ASSESSMENT
Boring/Well
Installation
Purpose for Installation
Results
Identification
Date
GWA-12S
5/24/2016
Further assess groundwater
Not installed in time for sampling
flow and quality west of and
inclusion during the Round 5
upgradient of the site, per
sampling event
5/12/2016
GWA-12D
request of NCDEQ.
Not installed in time for sampling
inclusion during the Round 5
sampling event
Not installed in time for sampling
GWA-12BR
5/23/2016�
inclusion during the Round 5
sampling event
GWA-13S
5/25/2016
Not installed in time for sampling
inclusion during the Round 5
6/2/2016
sampling event
Not installed in time for sampling
GWA-13D
inclusion 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.
Additional assessment well GWA-14S/D was planned for installation to further assess
groundwater flow and quality west of and upgradient of the MSS site, but was not completed
due to off -site access issues.
3.1.1 Well Gauging and Sampling
Round 5 of groundwater, porewater, and ash basin surface water sampling activities was
completed between March and June 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 98 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
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, 3-2, and 3-3 for groundwater, porewater, and
ash basin surface water, respectively. A summary of groundwater elevations measured during
the Round 1 through 5 gauging events is presented in Table 3-4.
3.2.1 Groundwater Flow Direction
On June 22, 2015, 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 higher than those measured during Rounds 1 and 2; this is likely attributable to
seasonal variation of the water table. Groundwater flow direction was consistent with flow
directions identified in Rounds 1 and 2, and generally flows from the northern portion of the site
to the south, toward Lake Norman, and to unnamed tributaries that flow to Lake Norman.

Duke Energy Carolinas, LLC I CSA Supplement 2
Marshall Steam Station Ash Basin
SECTION 3 — ADDITIONAL ASSESSMENT
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. Water elevations of additional
assessment wells GWA-12S/D and GWA-13S/D were inadvertently excluded from the water
gauging event and not available for inclusion within this 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 COls were identified in groundwater as
a result of sampling conducted during the CSA: antimony, arsenic, barium, beryllium, boron,
cobalt, chromium, hexavalent chromium, iron, manganese, nickel, pH, selenium, sulfate,
thallium, TDS, and vanadium. The list of COls resulting from Round 5 of groundwater sampling
are consistent with the exception of arsenic, which did not have exceedances during the Round
5 sampling event, and the addition of chloride as a COI. In addition, there was one exceedance
of mercury in well AL-2S during the Round 5 sampling event, resulting in potential additional
COI for evaluation.
3.2.2.2 ROUND 5 POREWATER SAMPLING RESULTS
A total of 19 porewater wells were sampled in April 2016 as part of the Round 5 sampling event.
The analytical results for the porewater well samples are presented in Table 3-2.
Concentrations of antimony, arsenic, beryllium, boron, cadmium, chloride, chromium,
hexavalent chromium, cobalt, iron, manganese, nickel, selenium, sulfate, TDS, thallium,
vanadium and zinc that exceed the applicable 2L Standard or IMAC were detected in porewater
samples collected during the Round 5 sampling event. The range and number of exceedances
of each COI in porewater are listed below.
• Antimony: 1.2J pg/L to 25.5 pg/L; 4 exceedances/19 samples (4/19)
• Arsenic: 38.4 pg/L to 2,110 pg/L; 16/19
• Barium: no exceedances in porewater
• Beryllium: 22.5 pg/L; 1/19
• Boron: 760 pg/L to 71,400 pg/L; 14/19
• Cadmium: 3.5 pg/L to 4.2 pg/L; 2/19
• Chloride: 2,410,000 pg/L; 1/19
• Chromium: 13 pg/L to 46.9 pg/L; 2/19
• Hexavalent chromium: 0.15 pg/L to 0.19 pg/L; 2/19
• Cobalt: 2 pg/L to 170 pg/L; 10/19
• Iron: 2,010 pg/L to 2,140,000 pg/L; 14/19
• Manganese: 130 pg/L to 22,200 pg/L; 16/19
• Mercury: no exceedances in porewater
• Nickel: 202 pg/L to 364 pg/L; 2/19
• Selenium: 62.1 pg/L to 229 pg/L; 4/19
• Sulfate: 300,000 pg/L to 6,420,000 pg/L; 11/19
• TDS: 569,000 pg/L to 10,9000,000 pg/L; 13/19
• Thallium: 0.28 pg/L to 5.3 pg/L; 9/19
12

Duke Energy Carolinas, LLC I CSA Supplement 2
Marshall Steam Station Ash Basin FN
SECTION 3 — ADDITIONAL ASSESSMENT
• Vanadium: 0.49 pg/L to 413 pg/L; 14/19
• Zinc: 1,120 pg/L; 1/19
3.2.2.3 ROUND 5 GROUNDWATER SAMPLING RESULTS
A total of 79 groundwater monitoring wells were sampled during March and April 2016 as part of
the Round 5 sampling event. The analytical results for the groundwater monitoring well samples
are presented on Table 3-1. In general, the COls 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 pg/L to 2.9 pg/L; 7 exceedances/79 samples (7/79)
o Antimony exceeded the IMAC in three wells (AL-2S, GW-4S and MW-9S)
screened within the shallow flow layer and in three deep flow layer wells (AB-
20D, AL-3D and GWA2D). Only one bedrock flow layer well (AL-2BR) exeeded
the IMAC for antimony. The locations of the exceedances from flow layer to flow
layer were inconsistent with exception of wells AL-2S and AL-2BR, which are
located at the southern corner of the Phase II Dry Ash Landfill. In general, total
and dissolved concentrations were consistent in each sample.
• Arsenic: No exceedances
• Barium: 768 pg/L to 811 pg/L; 2/79
o Barium did not exceed the 2L Standard in wells screened within the shallow or
bedrock flow layers during the Round 5 sampling event. Two wells (BG-3D and
AL-1 D) screened in the deep flow layer exceeded the 2L Standard for barium. In
general, total and dissolved concentrations were consistent in each sample.
• Beryllium: 5.2 pg/L; 1/79
o Beryllium exceeded the 2L Standard in one well (AL-1 S) screened within the
shallow flow layer. There were no exceedances for beryllium in wells screened
within the deep and bedrock flow layer.
• Boron: 787 pg/L to 21,200 pg/L; 16/79
o Boron exceeded the 2L Standard in the shallow and deep flow layer in wells
located within the Phase II Dry Ash Landfill and adjacent to/downgradient of the
Phase I Dry Ash Landfill. Boron exceeded the 2L Standard in one well (AL-2BR)
in the the bedrock flow layer, which is located in the souther corner of the Phase
II Dry Ash Landfill. In general, total and dissolved concentrations were consistent
in each sample.
• Cadmium: no exceedances in groundwater
• Chloride: 276,000 pg/L to 354,000 pg/L; 2/79
o Chloride did not exceed the 2L Standard in wells screened within the shallow or
bedrock flow layers during the Round 5 sampling event. Two wells (AB-12D and
AL-1 D) screened in the deep flow layer exceeded the 2L Standard for chloride.
• Chromium: 10.1 pg/L to 114 pg/L; 10/79
o Chromium exceeded the 2L Standard in the shallow flow layer in a well located
within the Phase II Dry Ash Landfill and in wells downgradient of the Phase I Dry
Ash Landfill and in two wells in an unutilized northeastern portion of the MSS

Duke Energy Carolinas, LLC I CSA Supplement 2
Marshall Steam Station Ash Basin FN
SECTION 3 — ADDITIONAL ASSESSMENT
site. Chromium exceeded the 2L Standard in the deep flow layer in two wells
located in the southern portion of the MSS site. Chromium exceeded the 2L
Standard in the bedrock flow layer in one well (AL-213R) located in the southern
corner of the Phase II Dry Ash Landfill, in one well (AB-613R) located in the
central portion of the MSS site and in one other well (BG-1 BR) located in the
unutilized northern corner of the MSS site.
Hexavalent Chromium: 0.12 pg/L to 142 pg/L; 44/79
o Hexavalent chromium exceeded the DHHS HSL in the shallow flow layer in wells
located downgradient and sidegradient of the Phases I and II Dry Ash Landfills
and in several wells not adjacent to the Ash Landfills. Hexavalent chromium also
exceeded the DHHS HSL in the deep flow layer with the approximate same
frequency and concentrations with the exception of well GWA-2D, which
contained a hexavalent chromium concentration two to three time higher than the
majority of wells across the MSS site. Exceedances of hexavalent 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: 1.2 pg/L to 195 pg/L; 32/79
o Cobalt exceeded the IMAC in the shallow flow layer in wells across the MSS site.
Concentrations were generally the highest south and east of the Phase I Dry Ash
Landfill and ash basin. Cobalt exceedances in the deep flow layer were slightly
more frequent, but at lower concentrations than were observed in the shallow
flow layer. Cobalt exceedances in the bedrock flow layer were less frequent and
at concentrations generally consistent with the concentrations observed in the
deep flow layer. Total and dissolved concentrations were generally consistent
across the sample set.
• Iron: 311 pg/L to 9,160 pg/L; 34/79
o Iron exceeded the 2L Standard in the shallow flow layer in wells in multiple areas
of the MSS site including areas associated and not associated with ash storage.
Exceedances of iron in the deep flow layer were more frequent in the central
portion of the MSS site, which includes the Phases I and II Dry Ash Landfills and
the ash basin system. Exceedances of iron in the bedrock flow layer were limited
to four wells including two background wells. Total and dissolved concentrations
were generally consistent across the sample set.
• Manganese: 53.9 pg/L to 7,690 pg/L; 51/79
o Manganese exceeded the 2L Standard in the shallow deep and bedrock flow
layers in wells across the MSS site. However, the frequency of exceedances and
concentrations were substantially less in the bedrock flow layer. Total and
dissolved concentrations were generally consistent across the sample set.
• Mercury: 1.2 pg/L; 1/79
o Mercury exceeded the 2L Standard in one well (AL-2S) screened within the
shallow flow layer. There were no exceedances for mercury in wells screened
within the deep and bedrock flow layer.
• Nickel: no exceedances in groundwater

Duke Energy Carolinas, LLC I CSA Supplement 2
Marshall Steam Station Ash Basin FN
SECTION 3 — ADDITIONAL ASSESSMENT
Selenium: 22.5 pg/L to 119 pg/L; 6/79
o Selenium exceeded the 2L Standard in five wells screened within the shallow
flow layer and in one well screened within the deep flow layer. There were no
exceedances for selenium in wells screened within the bedrock flow layer. The
wells screened in the shallow flow layer were located in or adjacent to the
Phases I and II Dry Ash Landfills. Total and dissolved concentrations were
generally consistent across the sample set.
• Sulfate: 261,000 pg/L to 1,210,000 pg/L; 5/79
o Sulfate exceeded the 2L Standard in two wells (AL-2S and GWA-11 S) screened
within the shallow flow layer and three wells (AL-21D, AL-3D, and AL-4D) within
the deep flow layer. There were no exceedances for sulfate in wells screened
within the bedrock flow layer. These wells are located within or downgradient of
the Phases I and II Dry Ash Landfills.
• Thallium: 0.22J pg/L to 0.32 pg/L; 2/79
o Thallium exceeded the IMAC in two wells (AB-1 Sand AL-1 S) screened within the
shallow flow layer. There were no exceedances for thallium in wells screened
within the deep and bedrock flow layer.
• TDS: 560,000 pg/L to 1,640,000 pg/L; 10/79
o TDS exceeded the 2L Standard in four wells screened within the shallow flow
layer, five wells screened within the deep flow layer, and one well within the
bedrock flow layer. With the exception of one well (AB-6BR), these wells are
located within or downgradient of the Phases I and II Dry Ash Landfills.
• Vanadium: 0.39 pg/L to 23.6 pg/L; 59/79
o Vanadium exceeded the IMAC in the shallow and deep flow layers in wells
across the MSS site, including background wells. Total and dissolved
concentrations were generally consistent across the sample set.
• Zinc: no exceedances in groundwater
Note that although mercury was not previously identified as a COI, it exceeded the 2L Standard
in well AL-2S during the Round 5 sampling event. Also, pH was measured outside of the range
specified in 2L (6.5-8.5 SU) in the shallow, deep, and bedrock flow layers across the MSS site.
However, as discussed in Section 1.3.4, this can be expected in the Piedmont Province of
North Carolina.
The horizontal extent of exceedances is presented in the form of isoconcentration figures
(Figures 3-4.1 through 3-45). The vertical extent of boron is presented on applicable cross -
sections (Figures 3-5.1 through 3-5.10). 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. However, the majority of
groundwater COls that are likely attributable to the source areas at the MSS site were observed

Duke Energy Carolinas, LLC I CSA Supplement 2
Marshall Steam Station Ash Basin
SECTION 3 — ADDITIONAL ASSESSMENT
in four general areas: beneath the ash basin, downgradient and east of the ash basin and the
Phase I Dry Ash Landfill, beneath the Phase II Dry Ash Landfill, and downgradient and
southeast of the ash basin.
Additional assessment wells GWA-10S/D and GWA-11 S/D were installed for further horizontal
and vertical delineation of exceedances in the vicinity of the Phase I Dry Ash Landfill. Boron,
hexavalent chromium, cobalt, iron, manganese, selenium, and sulfate exceeded their 2L
Standards, IMACs, or DHHS HSLs at these locations. These analytical results are consistent
with exceedances observed within the ash basin area.
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 upgradient and background groundwater at the site is less
chloride and sulfate rich than ash basin porewater, ash basin surface water, and downgradient
groundwater, which were observed to be trending closer to calcium, chloride, magnesium, and
sulfate rich. Calcium, magnesium, and sulfate are generally elevated in ash basin porewater
compared to upgradient and background wells. 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 3-5.

Duke Energy Carolinas, LLC I CSA Supplement 2
Marshall Steam Station Ash Basin FN
SECTION 4 — BACKGROUND CONCENTRATIONS
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 NCDEQ.
4.1 Methodology
As stated in the USEPA Unified Guidance (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.8
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 Environmental Protection Agency's Hazardous
and Solid Waste Management System; Disposal of Coal Combustion Residuals from Electric
Utilities; Final Rule (EPA CCR).9 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.
a U.S. Environmental Protection Agency (USEPA) Unified Guidance (USEPA 2009), 5.2.1 Selecting Monitoring
Constituents and Adequate Sample Sizes
9 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.
17

Duke Energy Carolinas, LLC I CSA Supplement 2
Marshall Steam Station Ash Basin FN
SECTION 4 — BACKGROUND CONCENTRATIONS
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 MSS site. 10
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 approach.
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
10 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; to avoid
confusion, the UPL is generally chosen over the UTL.
m

Duke Energy Carolinas, LLC I CSA Supplement 2
Marshall Steam Station Ash Basin FN
SECTION 4 — BACKGROUND CONCENTRATIONS
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.
Observation for Background Wells
Currently, the MSS site has the following number of usable observations at background wells for
implementation of the background concentration methodology described in Section 4.1:
• MW-3 (Voluntary Monitoring Well) — 15 observations
• MW-41D (Voluntary Monitoring Well) — 19 observations
• BG-1 S (CSA Monitoring Well) — 5 observations
• BG-1 D (CSA Monitoring Well) — 6 observations
• BG-2S (CSA Monitoring Well) — 5 observations
• BG-2BR (CSA Monitoring Well) — 5 observations
• BG-31D (CSA Monitoring Well) — 6 observations
• BG-3S (CSA Monitoring Well) — 1 observations
• MS-10 (FGD Landfill Monitoring Well) — 3 observations
It is expected that with interim monitoring implementation, the MSS 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.

Duke Energy Carolinas, LLC I CSA Supplement 2
Marshall Steam Station Ash Basin
SECTION 5 — ANTICIPATED ADDITIONAL ASSESSMENT ACTIVITIES
Section 5 — Anticipated Additional Assessment
Activities
Anticipated additional assessment activities are summarized below.
r;; "i Proposed Additional Assessment Monitoring Wells
Based on review of site information and analytical data available at this time, there are several
locations at the MSS site where additional groundwater assessment is warranted to delineate
the vertical extent of groundwater impacts associated with potential coal ash -related
constituents based on feedback from NCDEQ. The following proposed locations are currently
under review:
Proposed Additional Location
Monitoring Wells 1
Purpose
AB-10BR
Within central portion
I Evaluate vertical extent of
of ash basin
boron (1,130 pg/L in AB-
_
10D). _
AB-12BR
Within central portion
Evaluate vertical extent of
of ash basin
boron (2,210 pg/L in AB-
12D), TDS (1,080,000 pg/L),
and chloride (354,000 pg/L)
beneath the central portion of
the ash basin.
AL-1 BR East of ash basin and
dry ash landfill (Phase
_ I)
AL-213RL On dry ash landfill
(Phase 11)
AL-3113111 On dry ash landfill
(Phase 11)
AL-4BR On dry ash landfill
(Phase 11)
Approximate
Monitoring Well
Depth(s) (ft)
125
125
Evaluate vertical extent of 140
TDS exceedance (330,000
pg/L) in AL-1 D.
Evaluate vertical extent of 260
boron exceedance (1,950
pg/L) in AL-213R.
Evaluate vertical extent of 235
boron (4,720 pg/L), TDS
(654,000 pg/L), and sulfate
(367,000 pg/L) exceedances
in AL-3D.
Evaluate vertical extent of 200
boron (21,200 pg/L),
chromium (9.8 pg/L), TDS
(906,000 pg/L), and sulfate
(587,000 pg/L) exceedances
in AL-4D.
The information obtained from the borings will be reviewed against the existing conceptual site
model to evaluate if modifications or refinement are required.
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, porewater, and ash

Duke Energy Carolinas, LLC I CSA Supplement 2
Marshall Steam Station Ash Basin
SECTION 5 — ANTICIPATED ADDITIONAL ASSESSMENT ACTIVITIES
basin surface water samples associated with the ash basin, the dry ash landfill (Phases I and II),
and the PV structural fill at the MSS 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, porewater, and ash basin surface water.
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 I CSA Supplement 2
Marshall Steam Station Ash Basin FN
SECTION 6 — CONCLUSIONS AND RECOMMENDATIONS
Section 6 — Conclusions and Recommendations
The following conclusions have been developed from the information presented in this CSA
Supplement 2 report:
• 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).
• Additional monitoring wells (GWA-10S/D and GWA-11 S/D) were installed to refine the
understanding of constituent concentrations east of the ash basin and dry ash landfill
(Phase 1). Analytical results from these wells indicate boron, hexavalent chromium,
cobalt, iron, manganese, selenium, sulfate, and vanadium exceeded their applicable
criteria at these locations and will be further evaluated with additional analytical results
for evaluation of whether the horizontal extent of ash -related impacts is adequately
defined at the MSS site.
• Additional wells (GWA-12S/D/BR and GWA-13S/D) were installed to refine groundwater
flow and quality west of and upgradient of the site, however, these wells were not
installed in time for inclusion in the Round 5 sampling event. These monitoring wells
shall be reviewed with the Round 6 sampling event results, when available.
• The vertical extent of ash -related impacts shall be further refined east of the ash basin
and dry ash landfill (Phases I and II).
Based on the conclusions presented above, the following recommendations are offered:
• Refinement of PPBCs should be conducted once the minimum number of viable
observations per background well are available.
• Additional monitoring wells should be installed east of the ash basin and dry ash landfill
(Phases I and II) as well as the central portion of the ash basin to refine 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.
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