NC0004961_Riverbend CSA Executive Summary_20150818

Duke Energy Carolinas, LLC I Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin FN
EXECUTIVE SUMMARY
Executive Summary
On August 20, 2014, the North Carolina General Assembly passed Senate Bill 729, the Coal
Ash Management Act of 2014 (CAMA). Section § 130A-309.209 of the bill requires the owner of
a coal combustion residuals surface impoundment to submit a Groundwater Assessment Plan
(Work Plan) to the North Carolina Department of Environment and Natural Resources
(NCDENR) no later December 31, 2014 and a Groundwater Assessment Report (herein
referred to as a Comprehensive Site Assessment (CSA)) no later than 180 days following
approval of the Work Plan. Duke Energy Carolinas, LLC (Duke Energy) submitted a Work Plan
to NCDENR for assessment and characterization of the Riverbend Steam Station (RSS) ash
basin, ash storage, and cinder storage areas on December 30, 2014. The Work Plan was
subsequently conditionally approved by the NCDENR in correspondence dated February 19,
2015. This CSA Report was prepared to comply with the CAMA and is submitted to NCDENR
within the allotted 180 day timeframe. Data generated during the CSA will be used in
development of the Corrective Action Plan (CAP), which is due 90 days after submittal of the
CSA.
The purpose of this CSA is to characterize the extent of contamination resulting from historical
production and storage of coal ash, evaluate the chemical and physical characteristics of the
contaminants, investigate the geology and hydrogeology of the site including factors relating to
contaminant transport, and examine risk to potential receptors and exposure pathways. This
CSA was prepared in general accordance with requirements outlined in the following regulations
and documents:
• Classifications and Water Quality Standards Applicable to the Groundwaters of North
Carolina in Title 15A NCAC 02L .0106(g),
• Coal Ash Management Act in G.S. 130A-309.209(a),
• Notice of Regulatory Requirements (NORR) issued by NCDENR on August 13, 2014,
• Conditional Approval of Revised Groundwater Assessment Work Plan issued by
NCDENR on February 16, 2015, and
• Subsequent meetings and correspondence between Duke Energy and NCDENR.
The assessment addresses the horizontal and vertical extent of contamination in soil,
groundwater, and surface water. If a constituent' concentration exceeded the North Carolina
Groundwater Quality Standards, as specified in T15A NCAC .0202L (21- Standards) or Interim
Maximum Allowable Concentration (IMAC)2, it has been designated as a "Constituent of
Interest" (COI). Some COls (e.g., iron and manganese) are also present in background
monitoring wells and thus require careful examination to determine whether their presence
' Constituents are elements, chemicals, or compounds that were identified in the approved Work Plan for sampling
and analysis, and include antimony, arsenic, boron, chromium, cobalt, iron, manganese, selenium, thallium,
vanadium, sulfate, and total dissolved solids (TDS).
2 Appendix #1 of 15A NCAC Subchapter 02L Classifications and Water Quality Standards Applicable to The
Groundwaters of North Carolina, lists Interim Maximum Allowable Concentrations (IMACs). The IMACs were issued
in 2010 and 2011; however, NCDENR has not established a 2L Standard for these constituents as described in 15A
NCAC 02L.0202(c). For this reason, IMACs noted in this report are for reference only.
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Duke Energy Carolinas, LLC I Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
EXECUTIVE SUMMARY
downgradient of the ash basin or ash storage areas is naturally occurring or a result of ash
handling and storage. In addition to evaluating the distribution of constituents across the RBSS
site, significant factors affecting constituent transport, and the geological and hydrogeological
features influencing the migration and chemical and physical character of the COls were also
evaluated.
The assessment consisted of the following activities:
• Completion of soil and rock borings and installation of groundwater monitoring wells to
facilitate collection and analysis of chemical, physical, and hydrogeological parameters
of subsurface materials encountered within and beyond the waste and compliance
boundaries;
• Evaluation of testing data to supplement the Conceptual Site Model (SCM);
• Revision to the Receptor Survey previously completed in 2014; and
• Completion of a Screening -level Risk Assessment.
Based on scientific evaluation of historical and new data obtained during completion of the
above -referenced activities, the following conclusions can be drawn:
• No imminent hazard to human health or the environment has been identified as a result
of groundwater migration from the ash basin or ash storage areas.
• Upgradient, background monitoring wells contain naturally occurring metals and other
constituents at concentrations that exceeded their respective 2L Standards or IMACs.
This information is used to evaluate whether concentrations in groundwater
downgradient of the ash basin and ash storage areas are also naturally occurring or
might be influenced by migration of constituents from the ash basin and ash storage
areas. Examples of naturally occuring metals and consituents include cobalt, iron,
manganese, and vanadium. These constituents were detected in background
groundwater samples at concentrations greater than 2L Standards or IMACs.
• Under the RBSS ash basin shallow groundwater flows to the north, east and west and
discharges to the Catawba River. Groundwater in the bedrock flows in a northern to
northeasterly direction from the southern (upgradient) area of the site to the Catawba
River. This flow direction is away from the direction of the nearest public or private water
supply wells. The Catawba River serves as a hydrologic boundary for shallow
groundwater flow layera, prohibiting the shallow groundwater flow from the ash basin to
properties north and east of the RBSS site.
• There are no water supply wells located between the ash basin and the Catawba River.
• Groundwater in the southwest portion of the site under the ash storage area flows to the
northwest, under the cinder storage area, to the Catawba River.
• The geological and hydrogeological features influencing the migration, chemical, and
physical characteristics of contaminants are related to the Piedmont hydrogeologic
system present at the site.
• Regional groundwater flow in the vicinity of the RBSS is east toward the Catawba River.
• Exceedances of the 2L Standards have been measured in samples from monitoring
wells south of the ash storage area between the ash storage area and the property
boundary; however, groundwater elevation measurements indicate that flow direction in
this area is to the northwest. Residences located south of the ash basins and ash
storage areas are served by municipal water.
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Duke Energy Carolinas, LLC I Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin FN
EXECUTIVE SUMMARY
• Ongoing NPDES surface water quality data for samples collected upstream and
downstream of the ash basin within the Catawba River do not indicate that the ash
storage onsite has resulted in increased constituent concentrations above the North
Carolina Surface Water Quality Standards (2B Standards).
• Boron and sulfate are the primary ash related constituents of interest identified in the
groundwater at concentrations that exceed the background concentrations and 2L
Standards. The constituents are detected above the 2L Standards beneath the ash
basin and ash storage areas in the groundwater.
• The horizontal migration of boron best represents the dominant flow and transport
system. Boron is highly soluble and was identified by the USEPA as one of the leading
indicators for releases of contaminants from ash. Because of these characteristics,
boron can be used to represent the general extent of the groundwater impacted by the
ash basin and ash storage areas.
• USEPA has identified constituents for groundwater detection monitoring programs that
can be used as indicators of groundwater contamination from CCR. Specifically boron
and sulfate would be expected to migrate rapidly in groundwater, and that would provide
early detection as to whether contaminants were migrating from the ash basin system.
Figure ES-1 indicates the impacted groundwater in the shallow monitoring wells onsite
as it relates to boron and sulfate.
• Cobalt, iron, manganese and vanadium were the primary constituents detected in
background wells and groundwater at concentrations that exceed 2L Standards. These
constituents were detected above the 2L Standards beneath the ash basins and ash
storage areas in the shallow aquifer.
Aluminum, lead, and zinc exceeded the 2B standards in the surface water sample (SW-
3) collected from the cinder storage area. Additionally, seep samples collected from
surface water features north and east of the ash basin (S-4, S-6, S-7, S-8 and S-13) had
aluminium, cobalt, copper, and lead in excess of the 2B standards, which can be
naturally occurring in the groundwaters of the Piedmont Carolinas.
The CSA serves to characterize the horizontal and vertical extent of ash -related
constituents and evaluate groundwater gradients which facilitate the development of the
Site Conceptual Model. This then facilitates developemnt of the Corrective Action Plan,
due 90 days after submittal of this CSA.
Note that as required to by CAMA, Duke Energy has agreed to remove the ash in the ash basin
and ash storage areas via excavation. Approximately 4.6 million tons of ash will be transported
to permitted lined landfills in Homer, Georgia and Marshall Steam Station. The majority of ash
at Riverbend is anticipated to be transported by rail to a lined clay mine reclamation project in
central North Carolina, pending permitting and approval. Final removal of ash is anticipated to
be completed no later than August 2019.
ESA Source Information
Duke Energy owns and operated the Riverbend Steam Station (RBSS), located near Mount
Holly in Gaston County, North Carolina. RBSS began operation as a coal-fired generating
station in 1929 and was subsequently retired in April 2013. Following initial station operation,
coal ash residue from RBSS's coal combustion process was deposited in a cinder storage area
and other areas near the cinder storage area and coal pile on site. Following installation of
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Duke Energy Carolinas, LLC I Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin FN
EXECUTIVE SUMMARY
precipitators and a wet sluicing system, coal ash residue was disposed of in the station's ash
basin system located adjacent to the station and Mountain Island Lake. Discharge from the
RBSS ash basin is currently permitted by the North Carolina Department of Environment and
Natural Resources (NCDENR) Division of Water Resources (DWR) under the National Pollutant
Discharge Elimination System (NPDES) Permit NC0004961.
The ash basin system at the plant was used to settle and retain ash generated from coal
combustion at RBSS. The ash basin system is located adjacent to the Catawba River (Mountain
Island Lake) and consists of a Primary Cell, a Secondary Cell, and associated embankments
and outlet works. The ash basin cells are unlined and are in the process of being closed. An
ash storage area also exists to the southwest and side -gradient to the Primary Cell and consists
of ash relocated from the Primary Cell. In addition, a cinder storage area is located west and is
down -gradient of the Primary Cell and consists of ash from station operations prior to the
construction of the ash basin in 1957. The ash storage areas are unlined and have a
vegetative soil cap.
The ash basin was operated as an integral part of the site's wastewater treatment system.
During operation of the coal-fired units, the ash basin received permitted variable inflows of fly
ash, bottom ash, pyrites, stormwater runoff (including runoff from the coal pile), cooling water,
powerhouse floor drains, sanitary waste effluent, station yard drainage sump, and boiler
chemical cleaning wastes. The coal ash was historically sluiced to the southwest corner of the
Primary Cell on a variable basis (i.e., dependent on RBSS operations) via sluice pipes.
The CSA found that exceedances of ash -related constituents in soil, groundwater, and surface
water are likely the result of leaching from the coal ash contained in the ash basins and ash
storage areas. However, some exceedances may be due in part to naturally occurring
conditions based on a review of background groundwater quality data.
ES.2 Initial Abatement and Emergency Response
The coal-fired units at the plant have been decommissioned. The ash management areas are
no longer in use. Duke is in the process of excavating the ash from the site. No imminent
hazard to human health or the environment has been identified
ES.3 Receptor Information
The purpose of the receptor survey was to identify the exposure locations that are critical to be
considered in the groundwater transport modeling and human health risk assessment. Duke
Energy completed and submitted a receptor survey to NCDENR (HDR 2014a) in September
2014, and subsequently submitted to NCDENR a supplement to the receptor survey (HDR
2014b) in November 2014. The supplementary information was obtained from responses to
water supply well survey questionnaires mailed to property owners within a 0.5-mile (2,640-foot)
radius of the RBSS ash basin compliance boundary requesting information on the presence of
water supply wells and well usage.
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Duke Energy Carolinas, LLC I Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin FN
EXECUTIVE SUMMARY
The survey activities included contacting and/or reviewing the following agencies/records to
identify public and private water supply sources, confirm the location of wells, and/or identify any
wellhead protection areas located within a 0.5-mile radius of the RBSS ash basin compliance
boundary:
• NCDENR Division of Water Resources Public Water Supply Section's (PWSS) most
current Public Water Supply Water Sources GIS point data set;
• NCDENR DWR Source Water Assessment Program (SWAP) online database for public
water supply sources;
• Environmental Data Resources (EDR) local/regional water agency records review;
• Mecklenburg County's Groundwater and Wastewater Services Well Information System
online database;
• Gaston County Environmental Health Department;
• Charlotte -Mecklenburg Utilities Department (CMUD);
• Mount Holly Public Utilities Department; and
• USGS National Hydrography Dataset.
The review of these records identified one private water supply well and several tributaries to
Mountain Island Lake within a 0.5-mile radius of the ash basin compliance boundary. The water
supply well is located across Mountain Island Lake from the ash basin system. No public water
supply wells or wellhead protection areas were identified within a 0.5-mile radius of the ash
basin compliance boundary. In addition, no water supply wells (including irrigation wells and
unused wells) were identified within the ash basin potential area of interest. No information
gathered as part of this assessment suggests that any water supply is impacted by the
Riverbend ash basin system.
As part of this CSA report, the previously completed Receptor Survey activities were updated
based on the CSA Guidelines. The update included contacting and/or reviewing the
agencies/records to identify public and private water supply sources identified in Section 4.1 and
reviewing any questionnaires that were received after the submittal of the November 2014
supplement to the September 2014 receptor survey (i.e. questionnaires received after October
31, 2014).
A summary of the receptor survey findings is provided below.
• One reported private water supply well is located at a residence located northeast of
RBSS within a 0.5-mile radius of the ash basin compliance boundary. This well is
located across Mountain Island Lake in Mecklenburg County (Well 1).
• No public water supply wells (including irrigation wells and unused wells) were identified
within a 0.5-mile radius of the RBSS ash basin compliance boundary.
According to Duke Energy, the two private water supply wells and one public water
supply well previously identified on the RBSS property were properly abandoned in June
2015.
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Duke Energy Carolinas, LLC I Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin FN
EXECUTIVE SUMMARY
• No wellhead protection areas were identified within a 0.5-mile radius of the ash basin
compliance boundary.
• Several surface water features that flow toward Mountain Island Lake were identified
within a 0.5-mile radius of the ash basin (Figure 4-5).
Based on the returned water supply well questionnaires since October 31, 2014, no additional
receptors were identified.
ESA Sampling / Investigation Results
ES.4.1 Background Findings
MW-7D and MW-7SR are designated as part of the RBSS compliance groundwater monitoring
program as background wells for comparison based on historical data. Analyses of groundwater
samples collected from wells MW-7D/SR indicated that the following naturally occurring metals
exceeded 2L Standards in background locations: chromium, iron, manganese and pH. As part
of the CSA, Duke Energy installed additional nested wells (three shallow, three deep, and one
bedrock monitoring well) to provide background soil and groundwater quality data. The
background locations selected at locations not expected to have been impacted by site activities
and would not be impacted by groundwater flow from areas potentially impacted by ash. The
additional background wells are designated BG-1 S/D, BG-2S/D/BR, BG-3S/D, and MW-7BR.
Analyses of groundwater samples collected from these wells indicated that the following
naturally occurring constituents exceeded 2L standards in background locations: antimony,
boron, chromium, cobalt, iron, manganese, and vanadium. The results for all other constituents
were reported below 2L Standards. A summary of COI, 2L Standard or IMAC values and
ranges of exceedances is provided as follows.
Constituent of Interest
Groundwater 2L
Standard or IMACs N /L
Background Wells
Range of Results
Antimony
1
<0.5 to 3.9
Chromium
10
0.25J+ to 27.5
Cobalt
1
<0.5 to 3
Iron
300
28J to 2,200
Manganese
50
<5 to 370
pH
6.5-8.5
5.0 to 5.9
Total Dissolved Solids
(TDS)
500,000
< 25,000 to 1,180,000
Vanadium
0.3
0.35J to 29.9
ES.4.2 Nature and Extent of Contamination
Soil and groundwater beneath the ash basin and ash storage areas (within the compliance
boundary) have been impacted by ash handling and storage at the RBSS site as described in
detail below. Concentrations of several constituents exceed their respective 2L Standards or
IMACs in groundwater across the site. The extent of the exceedances is as noted below:
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Duke Energy Carolinas, LLC I Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin
EXECUTIVE SUMMARY
• Antimony concentrations in shallow monitoring wells exceeded the IMAC in wells
associated with the ash basin, in GWA-22S located south of the ash storage area, and in
BG-3S, located east of the ash basin compliance boundary near Mountain Island Lake.
Antimony concentrations in the deep monitoring wells exceeded the IMAC in wells
associated with the ash storage area; the ash basins; GWA-1 D and GWA-1 OD, located
north of the ash basin Secondary Cell; MW-9D, located north of the cinder storage area;
and BG-1 D, located east of the ash basin compliance boundary. Antimony
concentrations in the bedrock monitoring wells exceeded the IMAC on the ash basin
Primary Dam (AB-6BRU), west of the ash basin Primary Cell and north of the cinder
storage area (MW-9BR), and GWA-9BR located immediately east of the ash basin
Secondary Cell.
• Arsenic was not reported above 2L Standards in the shallow or deep monitoring wells.
Arsenic concentrations exceeded the 2L Standards in bedrock monitoring well GWA-
9BR, located east of the ash basin Secondary Cell.
• Boron concentrations in shallow monitoring wells exceeded 2L Standards in monitoring
well AS-1 S, located within the ash storage area. No other exceedances of boron were
reported in the deep or bedrock monitoring wells.
• Chromium concentrations in shallow monitoring wells exceeded 2L Standards in
monitoring well AS-2S and GWA-20S, located in and adjacent to the ash storage area,
and in monitoring well GWA-1 S, located northwest of the ash basin Secondary Cell.
Chromium concentrations exceeding the 2L Standards were reported in deep monitoring
wells located beneath the ash basin, north of the cinder storage area, and in the newly
installed background well results. Chromium exceedances of the 2L Standards were
reported in the monitoring wells to the south of the ash storage area, northwest of the
ash basin Primary Cell, and in background well location MW-7BR.
• Cobalt concentrations in shallow monitoring wells exceeded the IMAC throughout the
majority of the site, including results at newly installed background well BG-1 S. Cobalt
concentrations in deep monitoring wells exceeded the IMAC in monitoring wells AB-1 D,
associated with ash basin Secondary Cell; GWA-3D, located northwest of the cinder
storage area; GWA-20D, GWA-22D, and MW-8D, located south of the ash storage area;
and newly installed background well BG-1 D, located east of the ash basin compliance
boundary. Cobalt concentrations in deep monitoring wells exceeded the IMAC in
monitoring wells AB-6BRU and AB-3BR, beneath the ash basin.
• Iron concentrations in shallow monitoring wells exceeded 2L Standards throughout the
majority of the site, including results from newly installed background well BG-1 S;
however, the dissolved phase iron result in BG-1 S was below the laboratory reporting
limit. Iron concentrations in the deep monitoring wells exceeded 2L Standards in
monitoring wells AB-1 D, associated with ash basin Secondary Cell; GWA-20D, GWA-
22D, and MW-8D, located south of the ash storage area; and GWA-8D and GWA-7D.
Iron concentrations also exceeded the 2L Standards in newly installed deep background
wells BG-1 D and BG-3D, located east of the ash basin compliance boundary; however,
the dissolved phase iron results in the background groundwater samples were reported
as less than the laboratory reporting limit. Iron concentrations in the bedrock monitoring
wells exceeded 2L Standards in monitoring wells AB-6BRU and AB-3BR, associated
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Duke Energy Carolinas, LLC I Comprehensive Site Assessment Report ���
Riverbend Steam Station Ash Basin
EXECUTIVE SUMMARY
with the ash basin; GWA-9BR located east of the ash basin Secondary Cell; and newly
installed background well BG-2BR; however, the dissolved phase iron results in the
background groundwater sample was reported as less than the laboratory reporting limit.
The dissolved iron concentrations varied significantly from the totals concentrations for
the majority of the groundwater samples collected. Dissolved iron concentrations in the
shallow monitoring wells exceeded the 2L Standards in AS-2S located in the ash
storage area; MW-13 located adjacent to Mountain Island Lake northeast of the ash
basin Secondary Cell; and MW-1S located at the western toe of the ash basin Primary
Cell. All other dissolved iron concentrations (where data is available) in the shallow,
deep, and bedrock wells were below the 2L Standards.
• Manganese concentrations in the shallow monitoring wells exceeded 2L Standards
throughout the majority of the site, including BG-1 S, BG-2S and BG-3S. Manganese
concentrations in deep monitoring wells exceeded 2L Standards in monitoring wells AB-
8D, associated with the ash basin Primary Cell; MW-1 D, and GWA-3D, located west of
ash basin Primary Cell; GWA-9D and GWA-8D located east of the ash basin Secondary
Cell, BG-3D, located east of the ash basin compliance boundary; and GWA-22D, located
south of the ash storage area. Manganese concentrations in the bedrock monitoring
wells exceeded 2L Standards in AB-313R and in GWA-2BRU located northwest of the
ash Primary Cell. The dissolved phase results for manganese in these wells were below
the laboratory reporting limits.
• Sulfate concentrations in the shallow monitoring wells exceeded 2L Standards in
monitoring well GWA-3SA located northwest of the cinder storage area. Sulfate
concentrations in the deep monitoring wells exceeded 2L Standards in monitoring wells
GWA-3D, located northwest of the cinder storage area, and GWA-20D, located south of
the ash storage area. Sulfate did not exceed 2L Standards in the bedrock monitoring
wells.
• Thallium was not reported above 2L Standards in shallow monitoring wells. Thallium
concentrations in the deep monitoring wells exceeded 2L Standards in monitoring well
GWA-20D, located south of the ash storage area. Thallium was not reported above 2L
Standards in the bedrock wells.
• TDS concentrations in the shallow monitoring wells exceeded 2L Standards in
monitoring wells AS-1 S, located in the western portion of the ash storage area; and
GWA-3SA, located northwest of the cinder storage area. TDS concentrations in the
deep monitoring wells exceeded 2L Standards in monitoring wells AB-3D, located in the
ash basin; GWA-20D, located south of the ash storage area; GWA-3D, located
northwest of the cinder storage area; and BG-1 D, located in the background location
east of the ash basin compliance boundary. TDS concentrations in the bedrock
monitoring wells exceeded 2L Standards in monitoring well GWA-213R, located west of
the ash basin Primary Cell; GWA-23BR, located south of the ash storage area; MW-7BR
located southeast of the ash basin; and GWA-4BR located southwest of the ash storage
area.
Vanadium concentrations exceeded 2L Standards in all of the shallow, deep, and
bedrock monitoring wells, including all background monitoring wells. The relative
concentrations of vanadium are generally higher in the deep and bedrock wells than in

Duke Energy Carolinas, LLC I Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin FN
EXECUTIVE SUMMARY
the shallow wells. The vanadium method reporting limit provided by the analytical
laboratory was 1.0 ug/L. The IMAC for vandium is 0.3 ug/L. The vanadium results
reported at concentrations less than the laboratory method reporting limit are estimated.
During subsequent monitoring events, a laboratory method reporting equal to or less
than the IMAC should be utilized.
ES.4.3 Maximum Contaminant Concentrations
The maximum contaminant concentrations reported in groundwater, ash porewater, seep water,
and ash basin surface water samples collected during the CSA are listed below.
Maximum Constituent of Interest (COI) Concentrations
COI
Groundwater
(pg/L)
Ash Porewater
(pJ/L)
Seep Water
(pg/L)
Ash Basin Surface Water
(p9/L)
Aluminum
7,120
6,370
330
310
Antimony
18.2
21.6
0.51J
0.2J
Arsenic
10.7
651
0.5U
2.4
Barium
370
2,400
69
100
Beryllium
1.9
0.4
0.21.1
0.11
Boron
2,200
2,400
490
530
Cadmium
0.21
0.1
0.081.1
0.32
Chromium
903
4.3
1.8
10.4
Cobalt
66.8
102
46.1
11.7
Copper
68 J+
8.2
2.2
35.9
Iron
30,800
175,000
4600
2,200
Lead
7.5
1.7
0.52
1.1
Manganese
12,700
3,200
3100
4,300
Nickel
61.3
30.8
7.1
10.5
Selenium
15.4
4
0.51-1
35.4
Sulfate
1,420,000
559,000
89300
119,000
TDS
23,000,000
1,110,000
195000
33,200
Thallium
3.2
0.48
0.1 U
0.22
Vanadium
1 40.5
1 231
1 3.2
1 2.8
Zinc
1 200 J
1 91
1 101.1
1 750
Notes:
1. N/A indicates that a constituent was not detected above the reporting detected limit.
2. J indicates an estimated concentration.
3. J+ indicates an estimated concentration, biased high.
ES.4.4 Source Characterization
Source characterization was performed through the completion of borings and installation of
groundwater monitoring wells within the footprint of the ash basin cells and ash and cinder
storage areas, and associated solid matrix (ash) and aqueous sample (ash pore water)
collection and analysis. Ash samples were collected for analysis of physical characteristics (e.g.,
grain size, porosity) to provide data for evaluation of retention/transport properties within and
beneath the ash basin and ash storage areas. Ash samples were collected for analysis of
chemical characteristics (e.g., total inorganics, leaching potential). The results of the
characterization will be used to refine the site conceptual model and to provide data for use in
the CAP. Ash porewater refers to water samples collected from wells installed within the ash
basins or ash storage area screened in the ash layer. HDR does not consider ash porewater
results to be representative of groundwater.
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Duke Energy Carolinas, LLC I Comprehensive Site Assessment Report
Riverbend Steam Station Ash Basin FN
EXECUTIVE SUMMARY
Review of laboratory analytical results of ash samples collected from the ash basin and ash
storage areas identified eight COls which include antimony, arsenic, boron, cobalt, iron,
manganese, selenium and vanadium within the ash basin boundary. COls identified in
porewater in the ash basin include antimony, arsenic, boron, cobalt, iron, manganese, thallium
vanadium and TDS. COls identified in surface water include aluminum, antimony, arsenic,
beryllium, cadmium, chromium, cobalt, copper, lead, nickel, thallium and zinc.
SPLP (Synthetic Precipitation Leaching Procedure) testing was conducted to evaluate the
leaching potential of Cols from ash. Although SPLP analytical results are being compared to
the 2L Standards and IMACs, these samples do not represent groundwater samples. The
results of SPLP analyses indicated that the following COls exceeded their 2L Standards:
antimony, arsenic, chromium, cobalt, iron, lead, manganese, nitrate, selenium, thallium, and
vanadium. However, many factors influence the transport of these Cols and any potential
impacts to groundwater over time will be investigated through modeling as part of the CAP.
Four seeps (S-2, S-5, S-9 and S-11) were identified to be associated with the ash basin at
RBSS. Constituents identified in seeps include aluminum, antimony, chromium, cobalt, copper,
iron, manganese, selenium and vanadium, but some of these COls are naturally occurring.
ES.4.6 Regional Geology and Hydrogeology
The RBSS site is located within the Charlotte terrane, one of a number of tectonostratigraphic
terranes that have been defined in the southern and central Appalachians and is in the western
portion of the larger Carolina superterrane (Figure 5-1; Horton et al. 1989; Hibbard et al. 2002;
Hatcher et al. 2007). On the northwest side, the Charlotte terrane is in contact with the Inner
Piedmont zone along the Central Piedmont suture along its northwest boundary and is
distinguished from the Carolina terrane to the southeast by its higher metamorphic grade and
portions of the boundary may be tectonic (Secor et al. 1998; Dennis et al. 2000).
The Charlotte terrane is dominated by complex sequence of plutonic rocks that intrude a suite of
metaigneous rocks (amphibolite metamorphic grade) including mafic gneisses, amphibolites,
metagabbros, and metavolcanic rocks with lesser amounts of granitic gneiss and ultramafic
rocks with minor metasedimentary rocks including phyllite, mica schist, biotite gneiss, and
quartzite with marble along its western portion (Butler and Secor 1991; Hibbard et al. 2002). The
general structure of the belt is primarily a function of plutonic contacts.
The fractured bedrock is overlain by a mantle of unconsolidated material known as regolith. The
regolith includes residual soil and saprolite zones and, where present, alluvial deposits.
Saprolite, the product of chemical weathering of the underlying bedrock, is typically composed
of clay and coarser granular material and reflects the texture and structure of the rock from
which it was formed.
The groundwater system is a two -medium system restricted to the local drainage basin. The
groundwater occurs in a system composed of two interconnected layers: residual soil/saprolite
and weathered rock overlying fractured crystalline rock separated by the transition zone.
Typically, the residual soil/saprolite is partially saturated and the water table fluctuates within it.
Water movement is generally preferential through the TZ (i.e., enhanced permeability zone).
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The near -surface fractured crystalline rocks can form extensive aquifers. The character of such
aquifers results from the combined effects of the rock type, fracture system, topography, and
weathering. Topography exerts an influence on both weathering and the opening of fractures,
while the weathering of the crystalline rock modifies both transmissive and storage
characteristics.
ES.4.7 Site Geology and Hydrogeology
The RBSS site and its associated ash basin, ash storage area, and cinder storage area are
located in the Charlotte terrane. The Charlotte terrane consists of an igneous complex of
Neoproterozoic to Paleozoic ages (Hibbard et al. 2002) that range from intermediate to mafic in
composition (Butler and Secor 1999). The Charlotte terrane is bordered on the east and
southeast by the Carolina terrane and to the west and northwest by the Inner Piedmont (Cat
Square and Tugaloo terranes) and the Kings Mountain terrane. The structural contact of the
Inner Piedmont and Charlotte terrane is the Central Piedmont Shear Zone. The most important
effects of structural geology on groundwater flow are the contacts of the meta-diabase and the
meta -quartz diorite, and the likely interconnected joint sets discussed in Sections 6.1.3 and
6.1.4. Since it is difficult to define joint sets based on dip angle alone, it is also difficult to define
which joints are most relevant with respect to groundwater flow.
Based on the CSA site investigation, the groundwater system in the natural materials (alluvium,
soil, soil/saprolite, and bedrock) at RBSS is consistent with the regolith-fractured rock system
and is an unconfined, connected system without confining layers. However, the hydraulic
conductivity data collected during the investigation and discussed in Section 11.2 indicates that
a distinct transition zone of higher permeability does not exist at the site. This is consistent with
Harned and Daniel's (1992) concept of the two types of rock structure (foliated/layered and
massive) in the Piedmont province discussed in Section 5.2. The RBSS is underlain by a
relatively massive meta -plutonic complex of the type that they believe may develop an indistinct
transition zone. The groundwater system at RBSS is a two -layer system: shallow (regolith) and
deep (bedrock).
In general, groundwater at the site flows to the north, east, and west and discharges to the
Catawba River. Groundwater in the southwest portion of the site under the ash storage area
flows to the northwest, under the cinder storage area to the Catawba River. Flow contours
developed from groundwater elevations measured in the shallow and deep wells in the
southeastern portion of the site depict groundwater flow generally to the northeast to the
Catawba River. Groundwater contours developed from the groundwater elevations in the
bedrock wells show groundwater moving in a northeasterly direction from the south side of the
site to the Catawba River.
ES.4.8 Existing Groundwater Monitoring Data
Groundwater monitoring prior to 2015 consisted of a voluntary groundwater monitoring program
in 2006 with the installation of an initial set of twelve monitoring wells adjacent to the RBSS
active ash basin. In December, 2008, Duke Energy implemented an expanded voluntary
groundwater monitoring around the RBSS active ash basin until June 2010. During this period,
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the voluntary groundwater monitoring wells were sampled two times per year and the analytical
results were submitted to NCDENR DWR.
Compliance groundwater monitoring as required by the NPDES Permit began in December
2010. From December 2010 through June 2015, The compliance monitoring wells are sampled
three times a year (February, June, and October) and 15 sampling events have been conducted
to date.
Background monitoring wells MW-7D and MW-7SR were installed in December 2006 and
November 2010, respectively, as a part of the compliance monitoring program to evaluate
background water quality at the site. New background monitoring well locations (BG) were
identified based on the SCM at the time the Work Plan was submitted. Background monitoring
wells include two existing compliance groundwater monitoring well (MW-7D and MW-7SR) and
eight newly installed groundwater monitoring wells (MW-7BR which is located near MW-7D and
MW-7SR, as well as BG-1 S/D, BG-2S/D/BR, and BG-3S/D which is located near the eastern
property boundary). Background groundwater monitoring wells are depicted in Section 10.
Groundwater flow in the vicinity of MW-7D and MW-7SR is to the northeast toward Mountain
Island Lake. Historical groundwater data dates back to December 2008 for MW-7D and
December 2010 for MW-7SR.
Newly installed background monitoring wells BG-1 S/D, BG-2S/D/BR, BG-3S/D, and MW-7BR
were installed to evaluate background water quality in the regolith and within the bedrock at the
site. Groundwater flow in the vicinity of these monitoring wells is generally to the north or
northeast towards Mountain Island Lake. Currently, insufficient data are available to qualify BG-
1 S/D, BG-2S/D/BR, and BG-3S/D as background monitoring wells and provide associated
statistical analysis.
ES.4.9 Screening Level Risk Assessments
The prescribed goal of the human health and ecological screening level risk assessments is to
evaluate the analytical results from the COI sampling and analysis effort and using the various
criteria taken from applicable guidance, determine which of the COls may present an
unacceptable risk, in what media, and therefore, should be carried through for further evaluation
in a baseline human health or ecological risk assessment or other analysis, if required.
Constituents of Probable Concern (COPCs) are those COls that have been identified as having
possible adverse effects on human or ecological receptors that may have exposure to the
COPCs at or near the site. The COPCs serve as the foundation for further evaluation of
potential risks to human and ecological receptors.
To support the CSA effort and inform corrective action decisions, a screening level evaluation of
potential risks to human health and the environment to identify preliminary, media -specific
COPCs has been performed in accordance with applicable federal and state guidance, including
the Guidelines for Performing Screening Level Ecological Risk Assessments within the North
Carolina Division of Waste Management (NCDENR, 2003). The criteria for identifying COPCs
vary by the type of receptor (human or ecological) and media, as shown in the comparison of

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contaminant concentrations in various media to corresponding risk -based screening levels
presented in Tables 12-1 through 12-9.
COls were not screened out as COPCs based on a comparison to background concentrations,
as NCDENR SLERA guidance does not allow for screening based on background. Site -specific
background concentrations, discussed above in Section 12.1.3 will be considered in the
uncertainty section of the baseline ecological risk assessment, if determined to be necessary.
This initial screening, does not specifically identify that health or environmental risks are
present, rather the results indicate constituents in the environmental media for further
investigation by a site -specific risk assessment. It should be noted that the observed levels of
certain Cols in the naturally occurring background at Riverbend would also warrant
consideration of a BERA.
ESAA0 Development of Site Conceptual Model
The human health and ecological risk assessment conceptual site models, illustrating potential
pathways of exposure from source to receptors, are provided in this report.
In the initial site conceptual hydrogeologic model presented in the Work Plan dated December
30, 2014, the geological and hydrogeological features influencing the migration, chemical, and
physical characteristics of contaminants were related to the Piedmont hydrogeologic system
present at the site.
A hydrogeological site conceptual model was developed from data generated during previous
assessments, existing groundwater monitoring data, and modified based on the results of the
2015 groundwater assessment activities. The CSA found the ash basin source areas discharge
porewater to the subsurface beneath the basins and via seeps through the embankments.
Groundwater flows in a generally northern, western, and easterly direction from the vicinity of
the primary and secondary cells to Mountain Island Lake.
ES.4.11 Identification of Data Gaps
HDR has identified data gaps that will require further evaluation to refine the CSM through
completion of additional groundwater assessment field activities and evaluation of data collected
during those activities. The data gaps have been separated into two groups: 1) data gaps
resulting from temporal constraints and 2) data gaps resulting from evaluation of data collected
during the CSA.
ES.4.11.1 Data Gaps Resulting from Temporal Constraints
Temporal data gaps identified in this category are generally present due to insufficient time to
collect, analyze, or evaluate data collected during the CSA activities. It is expected that the
majority of these data gaps will be remedied in supplemental information to the CSA report to be
submitted to NCDENR following completion of the second comprehensive groundwater
sampling event.
• Mineralogical characterization of soil and rock: a total of 17 soil, three TZ, and eight
bedrock samples were submitted to three third -party mineralogical testing laboratories
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for analysis of soil and rock composition. As of the date of this report, Duke Energy has
not received results of this testing; however, results should be available for inclusion in
the CSA supplement.
• Horizontal Delineation of Groundwater Contamination: as part of Work Plan
development prior to field mobilization, Duke Energy reviewed existing groundwater
quality data from compliance monitoring wells MW-8S/I/D to evaluate the potential for
off -site migration of COls and the potential need for addional on -site and off -site wells.
This evaluation prompted the installation of groundwater monitoring wells GWA-
20S/D/BR, GWA-22S/D/BR, and GWA-23S/D/BR on the RBSS property south of the ash
storage area, and GWA-21 S/D/BR on the adjacent property to the south of MW-8S/I/D
and south of Horseshoe Bend Beach Road, to better define groundwater flow in this
area and the distribution of COls . The sampling results from these wells was not
received in time for the evaluation of the results to be incorporated in this report. This
evaluation will be included in the submittal of the CSA supplement.
Additional Speciation Analyses: In order to meet the requirements of the NORR, Duke
Energy conducted speciation of samples for arsenic, chromium, iron, manganese, and
selenium along flow transects, at ash basin water sample locations, and at compliance
wells with historical exceedances of the 2L Standards for speciation constituents. Duke
Energy and NCDENR are currently conducting discussions concerning the specifics of
the requirements for sampling associated with additional speciation sampling.
• Groundwater analytical results from monitoring wells AS-3SA and GWA-4BR, and
bedrock analytical results from boring GWA-21 D, were not received with sufficient time
to include the results in this report. These groundwater analytical results will be included
in the CSA supplement.
ES.4.11.2 Data Gaps Resulting from Review of Data Obtained During CSA Activities
Additional refinement is needed for the horizontal and vertical extent of groundwater
impacts to the west of the ash and cinder storage areas near well GWA-3SA/D with
sulfate, manganese, and TDS concentrations exceeding the 2L Standards reported at
this location. Additional groundwater monitoring wells may be required to delineate
exceedances in this area.
• Additional refinement is needed for the horizontal and vertical extent of groundwater
impacts to to the south of the ash storage area. Sulfate and TDS 2L Standard
exceedances were reported in monitoring wells south of the ash storage area. However,
groundwater flow direction in this location is from the south to the north/northwest. To
address this data gap, monitoring wells GWA-21 S/D/BR are currently being installed
southeast of MW-8S/I/D and will refine the understanding of groundwater flow direction
in this area, and provide information regarding potential concentrations of sulfate and
TDS south of Horseshoe Bend Beach Road. The analytical results from GWA-
21 S/D/BR will be included in the CSA supplement and a determination will be made at
that time whether additional groundwater monitoring wells are needed south of the ash
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Groundwater samples were inadvertently not collected from compliance background
monitoring wells MW-7D and MW-7SR. Although historical analytical results are
available for these wells, groundwater from these wells was not analyzed for the full list
of parameters and constituents used during the assessment activities. Monitoring wells
MW-7D and MW-7SR should be sampled and analyzed along for the same parameter
and constitiuent list as the assessment wells onsite during any future monitoring events.
• The vanadium method reporting limit provided by the analytical laboratory was 1.0 ug/L.
The IMAC for vandium is 0.3 ug/L. The vanadium results reported at concentrations less
than the laboratory method reporting limit are estimated. During subsequent monitoring
events, a laboratory method reporting equal to or less than the IMAC should be utilized.
Newly installed monitoring wells GWA-9BR and C-1 BRU, and voluntary monitoring wells
MW-2S and MW-4S were noted as dry at the time of the sampling event. A groundwater
sample was not able to be collected from these well. An attempt should be made to
collect a groundwater sample from this well during subsequent monitoring events. If the
well remains dry NCDENR will be contacted regarding the potential replacement of the
well. Review of Non -Ash Contamination Information: Review of information regarding
areas of non -ash contamination (i.e., petroleum -contaminated areas) is needed to
evaluate potential interferences with possible future remedial actions, if applicable.
ES.5 Conclusions
The CSA identified the horizontal and vertical extent of groundwater contamination within the
compliance boundary at RBSS, and found that the source and cause of the contamination within
that boundary is the coal ash contained in the ash basin and ash storage areas. The cause of
contamination is leaching of constituents from the coal ash into the underlying soil and
groundwater.
Background monitoring wells contained naturally occurring metals and other constituents at
concentrations that exceeded their respective 2L Standards or IMAC. These included antimony,
chromium, cobalt, iron, manganese, and vanadium. The presence of these metals are used to
evaluate if concentrations detected downgradient of the ash basin and ash storage areas are
naturally occurring.
The CSA identified arsenic, cobalt, iron, manganese, selenium thallium and vanadium as soil
COls. Groundwater COls were identified as antimony, arsenic, boron, chromium, cobalt, iron,
lead, manganese, sulfate, thallium, TDS, and vanadium. Antimony, cobalt, chromium, iron,
manganese, and vanadium are constituents that may be naturally occurring in regional
groundwater. The CSA identified the horizontal and vertical extent of soil contamination, with
exception of off -site areas east and north of Ash Storage 1 (as described in Section 14.1.1).
Migration of each contaminant is related to the groundwater flow direction, the groundwater flow
velocity, and the rate at which a particular contaminant reacts with materials in the aquifer. The
data confirm that geologic conditions present beneath the ash basins impedes the vertical
migration of contaminants. The CSA found that the direction of mobile contaminant transport is
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generally in north, west, and east direction towards the Catawba River, as anticipated, and not
towards other off -site receptors.
The human health and ecological screening -level risk assessments did not specifically identify
the presence of health or environmental risks; however, the results indicate that constituents in
environmental media could be of concern and further investigation by a site -specific risk
assessment may be warranted. No imminent hazards to human health and the environment
were identified as a result of the assessment.
Duke Energy is required per CAMA and has committed to removing the ash in the ash basin
and ash storage areas via excavation. In conjunction with decommissioning activities and in
accordance with CAMA requirements, Duke Energy will permanently close the RBSS ash ponds
by August 1, 2019. Closure of the RBSS ash ponds was defined in CAMA as excavation of ash
from the site, and beneficial reuse of the material or relocation to a lined structural fill or landfill.
As part of the RBSS closure process, Duke Energy submitted a coal ash excavation plan to
NCDENR in November 2014. The excavation plan detailed a multiphase approach for removing
coal ash from the site with an emphasis on the first 12 to 18 months of activities.
Based on the results of soil and groundwater samples collected beneath the ash basins and the
ash storage areas, some residual contamination will remain after excavation (if required);
however, the degree of contamination and the persistence of this contamination over time
cannot be determined at this time. Duke Energy will pursue corrective action under 15A NCAC
02L .0106. The approaches to corrective action under rule .0106(k) or (1) will be evaluated along
with other remedies depending on the results of groundwater modeling and evaluation of the
site's suitability to use Monitored Natural Attenuation or other industry -accepted methodologies.
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