HomeMy WebLinkAboutNC0004987_1. MSS CAP Part 1_Report_Final_20151207Corrective Action Plan Part 1
Marshall Steam Station Ash Basin
Site Location:
NPDES Permit No.:
Permittee and Current
Property Owner:
Consultant Information
Report Date:
Marshall Steam Station
8320 NC Highway 150 E
Terrell, NC 28682
NC0004987
Duke Energy Carolinas, LLC
526 South Church St
Charlotte, NC 28202
704.382.3853
HDR Engineering, Inc. of the Carolinas
440 South Church St, Suite 900
Charlotte, NC 28202
704.338,6700
December 7, 2015
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Corrective Action Plan Part 1
Marshall Steam Station Ash Basin
Contents
Executive Summary ..........................................................................................
ES-1 Introduction....................................................................................
ES-2 Background Concentrations and COI Screening Level Summary
ES-3 Site Conceptual Model..................................................................
ES-4 Modeling........................................................................................
ES-5 Recommendations........................................................................
1 Introduction.....................................................................................................................
1.1 Site History and Overview....................................................................................
1.1.1 Site Location, Acreage, and Ownership ..................................................
1.1.2 Site Description.......................................................................................
1.2 Permitted Activities and Permitted Waste............................................................
1.3 History of Site Groundwater Monitoring...............................................................
1.4 Summary of Comprehensive Site Assessment....................................................
1.5 Receptor Survey...................................................................................................
1.5.1 Surrounding Land Use............................................................................
1.5.2 Findings of Drinking Water Supply Well Survey Conducted per Section
§1 30A-309.21 1 (c) of CAMA....................................................................
1.6 Summary of Screening Level Risk Assessment ..................................................
1.7 Geological/Hydrogeological Site Description.......................................................
1.8 Results of the CSA Investigation..........................................................................
1.9 Regulatory Background........................................................................................
1.9.1 CAMA Requirements...............................................................................
1.9.2 Standards for Site Media.........................................................................
2 Background Concentrations and Regulatory Exceedances................
2.1 Groundwater..............................................................................
2.1.1 Background Wells and Concentrations ........................
2.1.2 Groundwater Exceedances of 2L Standards or IMACs
2.1.3 Radionuclides in Groundwater .....................................
2.2 Seeps........................................................................................
2.3 Surface Water...........................................................................
2.4 Sediment...................................................................................
2.5 Soil.............................................................................................
2.5.1 Background Soil and Concentrations ...........................
2.5.2 Soil Exceedances of NC PSRGs for POG ...................
2.6 Ash............................................................................................
2.7 Ash Porewater...........................................................................
2.8 Ash Basin Surface Water..........................................................
2.9 PWR and Bedrock.....................................................................
2.10 COI Screening Evaluation Summary ........................................
2.11 Interim Response Actions.........................................................
3 Site Conceptual Model........................................................................
3.1 Site Hydrogeologic Conditions ..................................................
3.1.1 Hydrostratigraphic Units ...............................................
1
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2
5
7
9
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11
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12
13
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14
14
14
15
15
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36
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Corrective Action Plan Part 1
Marshall Steam Station Ash Basin
3.1.2 Hydrostratigraphic Unit Properties.............................................................................
42
3.1.3 Potentiometric Surface — Shallow and Deep Flow Layers .........................................
42
3.1.4 Potentiometric Surface — Bedrock Flow Layer...........................................................
43
3.1.5 Horizontal and Vertical Hydraulic Gradients..............................................................
43
3.2 Site Geochemical Conditions..................................................................................................
45
3.2.1 COI Sources and Mobility in Groundwater.................................................................45
3.2.2 Geochemical Characteristics.....................................................................................
54
3.2.3 Source Area Geochemical Conditions.......................................................................
60
3.2.4 Mineralogical Characteristics.....................................................................................
62
3.3 Correlation of Hydrogeologic and Geochemical Conditions to COI Distribution .....................
62
4 Modeling............................................................................................................................................
64
4.1 Groundwater Modeling............................................................................................................64
4.1.1 Model Scenarios.........................................................................................................
64
4.1.2 Calibration of Models..................................................................................................65
4.1.3 Kd Terms.....................................................................................................................
65
4.1.4 Flow and COI Transport Model Sensitivity Analysis ..................................................
66
4.1.5 Fate and Transport Model..........................................................................................67
4.1.6 Proposed Geochemical Modeling Plan......................................................................
70
4.2 Groundwater - Surface Water Interaction Modeling................................................................
71
4.2.1 Mixing Model Approach..............................................................................................
71
4.2.2 Surface Water Model Results....................................................................................
73
4.3 Refinement of Models.............................................................................................................
74
5 Summary and Recommendations.....................................................................................................
75
6 References........................................................................................................................................
77
Corrective Action Plan Part 1
Marshall Steam Station Ash Basin
Tables
2-1 Initial COI Screening Evaluation
2-2 Background Groundwater Concentrations for the MSS Site: Ranges of Analytical Results
with Sample Turbidity <10 NTU
2-3 Groundwater Results for COls Compared to PPBCs, 2L Standards, IMACs, or NC
DHHS HSL, and Frequency of Exceedances
2-4 Radionuclide Concentrations
2-5 Seep Results for COls Compared to 2L Standards or IMACs and Frequency of
Exceedances
2-6 Surface Water Results for COls Compared to Upgradient Surface Water Concentrations,
2B or USEPA Standards, and Frequency of Exceedances
2-7 Sediment COls Compared to NC PSRGs for POG and Frequency of Exceedances
2-8 Proposed Provisional Background Soil Concentrations
2-9 Soil Results for COls Compared to PPBCs, Background Concentrations, and NC
PSRGs for POG, and Frequency of Exceedances
2-10 Ash Exceedance Results for COls Compared to NC PSRGs for POG and Frequency of
Exceedances
2-11 Porewater Exceedance Results for COls Compared to 2L Standards, IMACs, or NC
DHHS HSLs, and Frequency of Exceedances
2-12 Ash Basin Surface Water Results for COls Compared to 2B or USEPA Standards, and
Frequency of Exceedances
2-13 Updated COI Screening Evaluation Summary
3-1 Vertical Gradient Calculations for Shallow/Deep Well Pairs
3-2 Vertical Gradient Calculations for Deep/Bedrock Pairs
3-3 Categories and Threshold Concentrations to Identify Redox Processes in Groundwater
3-4 Field Parameters from CSA
4-1 Mixing Zone Sizes and Percentages of Upstream River Flows
4-2 Lake Norman Calculated Surface Water Concentrations
Figures
1-1 Site Location Map
1-2 Site Layout Map
1-3 Compliance and Voluntary Monitoring Wells
1-4 Monitoring Well and Sample Locations
1-5 Receptor Survey Map
1-6 Site Vicinity Map
2-1 Groundwater Analytical Results — Plan View (2L Standard, IMAC or NC DHHS
Exceedances)
2-2 Surface Water and Seep Analytical Results
2-3 Soil Analytical Results — Plan View (NC PSRG for POG Exceedances)
3-1 Site Conceptual Model — 3-D Representation
3-2.1 Site Conceptual Model — Cross Section A -A'
3-2.2 Site Conceptual Model — Cross Section A -A'
3-2.3 Site Conceptual Model — Cross Section A -A'
3-2.4 Site Conceptual Model — Cross Section A -A'
Corrective Action Plan Part 1
Marshall Steam Station Ash Basin
3-3 Water Table Surface Map — Shallow Wells
3-4 Potentiometric Surface Map — Deep Wells
3-5 Potentiometric Surface Map — Bedrock Wells
3-6 Vertical Gradient — Shallow to Deep Wells
3-7 Vertical Gradient — Deep to Bedrock
Appendices
A Regulatory Correspondence
B Background Well Analysis
C UNCC Groundwater Flow and Transport Model
D UNCC Soil Sorption Evaluation
E Surface Water Modeling Methods
Acronyms and Abbreviations
pg/L micrograms per liter
2B Standards North Carolina Surface Water Quality Standards
2L Standards NCAC Title 15A, Subchapter 2L.0202
3-D
Three-dimensional
BG
background
BR
Bedrock
bgs
below ground surface
CAMA
North Carolina Coal Ash Management Act of 2014
CAP
Corrective Action Plan
CCR
Coal Combustion Residuals
cfs
cubic feet per second
COI
Constituent of Interest
COPC
Contaminant of Potential Concern
CSA
Comprehensive Site Assessment
D
Deep
DHHS
North Carolina Department of Health and Human Services
DO
Dissolved oxygen
DORS
Distribution of Residuals Solids
Duke Energy
Duke Energy Carolinas, LLC
DWR
NCDEQ Division of Water Resources
EPRI
Electric Power Research Institute
FERC
Federal Energy Regulatory Commission
FGD
flue gas desulfurization
ft/ft
feet / foot
GIS
Geographic Information Systems
HSL
health screening level
IMAC
Interim Maximum Allowable Concentration
J
Estimated concentration
J-
Estimated concentration, biased low
J+
Estimated concentration, biased high
MCL
maximum contaminant level
iv
Corrective Action Plan Part 1
Marshall Steam Station Ash Basin
mg/kg
milligrams per kilogram
MGD
million gallons per day
MSS
Marshall Steam Station
MW
megawatt
NC PSRGs
North Carolina Preliminary Soil Remediation Goals
NCAC
North Carolina Administrative Code
NCDENR
North Carolina Department of Environment and Natural Resources
NCDEQ
North Carolina Department of Environmental Quality
NPDES
National Pollutant Discharge Elimination System
NRMSE
Normalized Root Mean Square Error
NTU
Nephelometric Turbidity Units
NURE
National Uranium Resource Evaluation
OCM
Observed Clay Minerals
ORP
Oxidation -Reduction Potential
pCi/L
picocuries per liter
POG
Protection of Groundwater
PPBC
Proposed Provisional Background Concentration
PV
Photovoltaic
RAB
Retired Ash Basin
Redox
Reduction -Oxidation
S
Shallow
SCM
Site Conceptual Model
SU
Standard Unit
TDS
total dissolved solids
TEAP
Terminal Electron Accepting Process
TZ
transition zone
UNCC
University of North Carolina at Charlotte
USDOE
U.S. Department of Energy
USEPA
U.S. Environmental Protection Agency
USGS
U.S. Geological Survey
UTL
Upper Tolerance Limit
Work Plan
Groundwater Assessment Work Plan
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Corrective Action Plan Part 1
Marshall Steam Station Ash Basin
Executive Summary
ES-1 Introduction
Duke Energy Carolinas, LLC (Duke Energy) owns and operates Marshall Steam Station (MSS),
which is located on Lake Norman in Catawba County near the town of Terrell, North Carolina.
MSS began operations in 1965 as a coal-fired generating station and currently operates four
coal-fired units. Coal combustion residual (CCR) consisting of bottom and fly ash material from
MSS is disposed in the station's ash basin, located to the north of the station. Dry ash has been
disposed in six 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, gypsum, is disposed in the
FGD residue landfill. Fly ash utilized as structural fill was placed in the photovoltaic (PV)
structural fill and was used as structural fill beneath portions of the Industrial Landfill No. 1.
Discharge from the ash basin is permitted by the North Carolina Department of Environmental
Quality (NCDEQ, formerly referred to as NCDENR)' Division of Water Resources (DWR) under
the National Pollutant Discharge Elimination System (NPDES) Permit NC0004987.
The North Carolina Coal Ash Management Act of 2014 (CAMA) directs owners of CCR surface
impoundments in North Carolina to conduct groundwater monitoring, assessment, and remedial
activities, if necessary. A groundwater assessment work plan (Work Plan) for MSS was
submitted to NCDENR on November 4, 2014, followed by a revised Work Plan on December
30, 2014. The revised Work Plan was conditionally approved by NCDENR on March 12, 2015.
A Comprehensive Site Assessment (CSA) was performed to collect information necessary to
determine horizontal and vertical extent of impacts to soil and groundwater attributable to CCR
source area(s), identify potential receptors, and screen for potential risks to those receptors.
The MSS was submitted to NCDENR on September 8, 2015 (HDR 2015).
The CSA found no imminent hazards to public health and safety; therefore, no actions to
mitigate imminent hazards were required. However, corrective action at the MSS site is required
to address soil and groundwater contamination resulting from the source areas. In addition, a
plan for continued groundwater monitoring will be implemented following NCDEQ's approval.
CAMA also requires the submittal of a Corrective Action Plan (CAP) for each regulated facility
no later than 180 days after submittal of the CSA. Duke Energy and NCDEQ mutually agreed to
a two-part CAP submittal, with Part 1 being submitted within 90 days of submittal of the CSA
Report and Part 2 being submitted no later than 180 days after submittal of the CSA Report.
The purpose of this CAP Part 1 is to provide background information, a brief summary of the
CSA findings, an evaluation and refinement of COls for modeling purposes, a detailed
description of the site conceptual model (SCM), results of the groundwater flow and fate and
transport model, and results of the groundwater to surface water interaction model.
CAP Part 2 will include the remainder of the CAMA requirements, including an evaluation of
proposed alternative methods for achieving groundwater quality restoration, conceptual plans
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.
Corrective Action Plan Part 1
Marshall Steam Station Ash Basin
for recommended corrective actions, implementation schedule, and a plan for future monitoring
and reporting. A risk assessment will be submitted under a separate cover with the CAP Part 2
submittal.
Summary of CSA
The CSA for MSS 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 the
assessment to characterize media (soil, rock, and groundwater) potentially impacted by the
source areas at the site. Seep, surface water, and sediment samples were also collected. For
the CSA, the source area was defined as the ash basin, dry ash landfill (Phases I and 11), and
PV structural fill. Source characterization was performed to identify physical and chemical
properties of ash, ash basin surface water, ash porewater, and ash basin seeps. The analytical
results for source characterization samples were compared to North Carolina Groundwater
Quality Standards, as specified in 15A NCAC 2L.0202 (2L Standards), or Interim Maximum
Allowable Concentrations (IMACs), and other regulatory screening levels for the purpose of
identifying constituents of interest (COls) that may be associated with source -related impacts to
soil, groundwater, and surface water. In addition, hydrogeological evaluation testing was
performed on newly installed and existing monitoring wells at the site.
The CSA identified groundwater impacts at the MSS site and found that exceedances are a
result of both naturally -occurring conditions and CCR material contained in the ash basin, dry
ash landfill (Phases I and 11), and the PV structural fill. The approximate horizontal extent of
groundwater impacts is limited to beneath the ash basin and dry ash landfill (Phase 11), east and
downgradient of the ash basin and dry ash landfill (Phase 1), and southeast and downgradient of
the ash basin, within the ash basin compliance boundary. The approximate vertical extent of
groundwater impacts is generally limited to the shallow and deep flow layers.
Surface water impacts were identified in the unnamed tributary that flows to Lake Norman
located downgradient of the dry ash landfill (Phase 1).
The horizontal extent of soil impacts is limited to the area beneath the ash basin and one
location east and downgradient of the dry ash landfill (Phase 1). Where soil impacts were
identified beneath the ash basin, the vertical extent of contamination beneath the ash/soil
interface is generally limited to the uppermost soil sample collected beneath ash.
Additional details pertaining to the horizontal and vertical extent of soil and groundwater impacts
at the site are detailed in the CSA Report.
ES-2 Background Concentrations and COI Screening Level
Summary
Some COls identified in the CSA are present in background and upgradient monitoring wells
and may be naturally occurring, and thus require examination to determine whether their
presence downgradient of the source areas is naturally occurring or potentially attributed to the
source areas. Therefore, proposed provisional background concentrations (PPBCs) were
calculated for groundwater and soil to aid in evaluating whether or not COI impacts identified in
Corrective Action Plan Part 1
Marshall Steam Station Ash Basin
the CSA are attributable to the source areas and which COls will be further evaluated for
corrective action.
Proposed Provisional Background Concentrations
To determine if a monitoring well is suitable for developing site -specific background
concentrations, the following criterion was evaluated:
• The topographic location of the well with respect to the source areas (distance from
source areas and located hydraulically upgradient of source areas)
• Stratigraphic unit being monitored
• Screened intervals of well relative to source water elevation
• Direction of groundwater flow in the region of the well relative to source areas
Wells that have been determined to represent background conditions at the site are compliance
monitoring wells MW-4 and MW-4D, FGD Residue Landfill monitoring well MS-10, and CSA
background monitoring wells BG-1S/D, BG-2S/BR, and BG-3S/D.
Groundwater PPBCs represent either the statistically derived prediction limit concentration, for
constituents with sufficient data to use statistical methods (from compliance wells MW-4 and
MW-4D and FGD Residue Landfill monitoring well MS-10), or the highest reported value or
laboratory reporting limit (for non -detects) for constituents that were not historically monitored at
the site (from the compliance, FGD Residue Landfill, and newly installed background monitoring
wells). As additional data are collected from newly installed background wells, the results will be
incorporated into statistical background analysis used in the determination of site -specific
PPBCs.
Soil PPBCs were calculated for those constituents analyzed in background soil borings. The
methodology followed ProUCL Technical Guidance, Statistical Software for Environmental
Applications for Data Sets with and without Nondetect Observations (USEPA 2013). The soil
PPBCs were compared to the NC PSRGs for POG and, for most COls, the PPBC is higher than
the NC PSRG for POG. Therefore, site -specific soil remediation goals may need to be
established.
ES-2.2 Updated COI Screening Evaluation Summary
The table below summarizes COls (by media) that are potentially attributable to the source
areas that require further evaluation to determine if corrective action is warranted. In addition to
comparing COI concentrations to PPBCs, aqueous media concentrations were compared to 2L
Standards, IMACS2, North Carolina Department of Health and Human Services (NC DHHS)
Health Screening Levels (HSLs), and North Carolina Surface Water Quality Standard (213
Standard), and solid media were compared to NC PSRGs for POG. Following the comparison of
COls to PPBCs and the regulatory standards listed above, the COls that are potentially
attributable to the source areas underwent further evaluation in the groundwater flow and fate
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, 2011, and 2012; 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.
Corrective Action Plan Part 1
Marshall Steam Station Ash Basin
and transport modeling. Concentrations of COls from the source areas were incorporated in the
groundwater flow and contaminant transport model.
CSA COI Exceedance by Media
COI To Be
Constituent
Ash
Ash
Pore-
water
Ash Basin
Surface
Water
Seeps and
NCDENR
Resamples
Ground-
water
Surface
Water
Sediment
Soil
Further
Assessed in
Groundwater
Modeling
Aluminum
-
-
-
-
-
-
-
-
No
Antimony
-
-
-
Yes
Arsenic
-
-
-
Yes
Barium
-
-
-
Yes
Beryllium
-
-
-
Yes
Boron
-
-
-
Yes
Cadmium
-
-
-
No
Chloride
-
-
-
Yes
Chromium
-
-
-
Yes
Hexavalent
Chromium
-
-
-
_
_
_
Yes
Cobalt
-
-
-
Yes
Copper
-
-
-
No
Iron
-
-
-
Yes
Lead
-
-
-
No
Manganese
-
-
-
Yes
Molybdenum
-
-
-
-
-
-
-
-
No
Mercury
-
-
-
-
-
-
-
-
No
Nickel
-
-
-
-
-
-
-
-
No
Nitrate
-
-
-
-
-
-
-
-
No
pH
-
-
-
Yes
Selenium
-
-
-
Yes
Strontium
-
-
-
-
-
-
-
-
No
Sulfate
-
-
-
Yes
Thallium
-
-
-
Yes
TDS4
-
-
-
Yes
Vanadium
-
-
-
Yes
Zinc
-
-
-
-
-
-
-
-
No
Notes:
1. Note that ash is not evaluated for remediation in CAP Part 1 because ash will be addressed as part of corrective
action(s) to be evaluated in CAP Part 2.
2. Note that ash porewater is not evaluated for remediation in CAP Part 1 because porewater will be addressed as part of
corrective action(s) to be evaluated in CAP Part 2.
3. Note that ash basin surface water is not evaluated for remediation in CAP Part 1 because ash basin surface water will
be addressed as part of corrective action(s) to be evaluated in CAP Part 2.
4. Geochemical modeling will be performed in CAP Part 2 to evaluate impacts of iron, manganese, pH and TDS since
these constituents cannot adequately be modeled using MODFLOW/MT3DMS.
Corrective Action Plan Part 1
Marshall Steam Station Ash Basin
Site Conceptual Model
The site conceptual model (SCM) is an interpretation of processes and characteristics
associated with hydrogeologic conditions and COI interactions at the site. The SCM is used to
evaluate areal distribution of COls with regard to site -specific geological/hydrogeological and
geochemical properties at the MSS site. The SCM was developed using data and analysis from
the CSA Report.
ES-3.1 Geological/Hydrogeological Properties
Based on the CSA site investigation, the groundwater system in the natural materials (alluvium,
soil, soil/saprolite, and bedrock) at MSS is consistent with the regolith-fractured rock system and
is an unconfined, connected aquifer system. The groundwater system is divided into three
layers to distinguish flow layers within the connected aquifer system: the shallow, deep, and
bedrock flow layers. In general, groundwater flows from the north and northwest extents of the
MSS site property boundary to the south and southeast toward Lake Norman.
Horizontal and vertical hydraulic gradients were calculated for each flow layer. Negative vertical
gradients, which indicate potential for downward flow, were calculated for the majority of well
pairs located upgradient of the ash basin that are not in close proximity to surface waters and
for well pairs located in the southwest, west -central, and north portions of the ash basin.
Positive vertical gradients, which indicate potential for upward flow, were calculated for well
pairs located in close proximity to surface waters upgradient and downgradient of the ash basin
and for well pairs in close proximity to portions of the ash basin where there is ponded water or
where intermediate dikes/haul roads are preventing flow through the ash basin. In general,
these positive gradients indicate potential for groundwater to surface water interaction. Despite
water elevation differences, gradients are not well defined. Further evaluation will be undertaken
as additional water elevation data are collected.
ES-3.2 Site Geochemical Conditions
Determination of the reduction/oxidation (redox) condition of groundwater is an important
component of groundwater assessments, and helps to understand the mobility, degradation,
and solubility of constituents. The redox state of the MSS site was evaluated based on 91
samples from the study area for which all six constituents (DO, nitrate as nitrogen, manganese,
iron, sulfate, and sulfide) were available, including porewater and groundwater. Based on site
measurements, the primary redox categories were determined to include oxic, suboxic, mixed
(oxi-anoxic), mixed (anoxic), and anoxic conditions. The predominant redox processes are
oxygen reduction with iron or manganese oxidation. Under these conditions, more oxidized
species As(V), Se(VI), and Mn(IV) would be expected.
Ash porewater samples were predominantly classified as anoxic or mixed (oxi-anoxic)
conditions, which indicates increased potential for reduced forms of metals. Approximately 45%
of the groundwater samples collected across the site were classified as suboxic or oxic
categories, where reduced species of metals such as As(III) are less likely to be present.
Groundwater samples were characterized in terms of solute speciation to evaluate the
concentrations and ionic composition (oxidation states) of metal ions of primary concern,
Corrective Action Plan Part 1
Marshall Steam Station Ash Basin
including As(III, V), Cr(III, VI), Fe(II, III), Mn(II, IV), and Se(IV, VI). In general, reduced forms of
metals (i.e., species in lower oxidation states) are more readily transported in groundwater than
those that are more oxidized. At the MSS site, speciation measurements were performed for 25
samples. To provide a general indication of sample composition, the relative percentage of the
reduced specie concentration to the sum of the reduced and oxidized specie concentration were
calculated. These relative percentages express the proportion of the reduced form metal
present in each sample. Speciation measurements at the MSS site vary widely. In general,
reduced forms of arsenic, iron, and manganese were observed in groundwater samples
collected along flow transects at the MSS site.
ES-3.3 Correlation of Hydrogeologic and Geochemical Conditions of COI
Distribution
The SCM was used to evaluate areal distribution of COls with regard to site -specific geological/
hydrogeological and geochemical properties at the MSS site. Key observations include:
• Horizontal migration of COls is evident with groundwater flow direction at the site. From
beneath the dry ash landfill (Phase 11), concentrations decrease as COls migrate toward
the central portion of the ash basin. COls that are present in groundwater east and
downgradient of the ash basin and dry ash landfill (Phase 1) are present in the surface
water sample (SW-6) collected from the downgradient unnamed tributary that flows to
Lake Norman. Also, several COls were reported in groundwater between the ash basin
dam and Lake Norman. Vertical migration of COls observed in select well clusters (S, D,
and BR) indicates groundwater impacted by the on -site areas is primarily limited to the
shallow and deep flow layers with the exception of the underlying fractured bedrock
beneath the dry ash landfill (Phase 11).
• Cobalt, iron, manganese, pH, and vanadium were the COls with the most widespread
exceedances beneath the on -site source areas, downgradient of the source areas,
upgradient of the source areas, and in background monitoring well locations. These
constituents are naturally occurring in soil and groundwater. Concentrations of iron and
manganese are highly pH dependent. Groundwater and geochemical conditions
promote the mobility of vanadium across the site with contribution likely from naturally
occurring vanadium and vanadium from source areas.
• As a result of constituents leaching from the ash basin and dry ash landfill units (Phase I
and 11), and geochemical processes taking place in groundwater and soil beneath the
site, several COls exceeded their PPBC and/or 2L Standard, IMAC or NC DHHS HSL,
and are listed below for each source area at the site.
o Beneath the Ash Basin: antimony, boron, chloride, cobalt, iron, manganese,
TDS, and vanadium.
o Beneath the Dry Ash Landfill (Phase 11): barium, boron, chromium, cobalt, iron,
manganese, selenium, sulfate, TDS, and vanadium.
Corrective Action Plan Part 1
Marshall Steam Station Ash Basin
o Downgradient and East of the Ash Basin and Dry Ash Landfill (Phase 1):
beryllium, boron, chloride, chromium, hexavalent chromium, cobalt, manganese,
thallium, TDS, and vanadium.
o Downgradient of the Ash Basin Dam: arsenic, boron, hexavalent chromium,
cobalt, iron, manganese, thallium, TDS, and vanadium.
The SCM will continue to evolve as additional data become available during supplemental site
investigation activities.
ES-4 Modeling
Groundwater flow, fate and transport, and groundwater to surface water modeling were
conducted to evaluate COI migration and potential impacts following closure of the ash basin
and ash storage areas at the MSS site.
Under the direction of HDR, UNCC developed a site -specific, 3-D, steady-state groundwater
flow and fate and transport model for the MSS site using MODFLOW and MT3DMS. The
groundwater flow and fate and transport model was based on the SCM presented in Section 3
and incorporates site -specific data obtained during the CSA. The objective of the groundwater
modeling effort was to simulate steady-state groundwater flow conditions for the MSS site, and
simulate transient transport conditions in which COls enter groundwater via the source areas
over the period they have been in service.
ES-4.1 Model Scenarios
The following groundwater model scenarios were simulated for the purpose of this CAP Part 1:
• Existing Conditions: assumes current site conditions with ash sources left in place.
• Cap -in -Place: assumes ash remaining in source areas is covered by an engineered cap.
• Excavation: assumes removal of ash from ash basin (outside the footprints of the dry
ash landfill units and PV structural fill).
Each model scenario utilized steady-state flow conditions established during flow model
calibration and transient transport of COls. Only COI concentrations above the 2L Standards,
IMACs, or NC DHHS HSL were used for model calibration purposes by introducing a constant
source for each COI at the start of ash basin operations and running the model until July 2015.
The calibrated flow and transport model was reviewed by a third -party peer review team was
coordinated by EPRI. The EPRI review included the arsenic and boron transport calibrations,
which represent a sorptive and non-sorptive COI, respectively. Subsequent to a revision, EPRI
issued a memorandum to Duke Energy on December 2, 2015 deeming the model sufficient to
achieve its primary objective.
As a primary input to the transport model, Duke Energy, through UNCC, generated site -specific
sorption coefficients (or partition coefficient (Kd)) for COls identified during the CSA. Kd relates
the quantity of the sorbed constituent per unit mass of solid to the quantity of the constituent
remaining in solution. Laboratory determination of Kd was performed on 12 site -specific
samples of soil, or PWR from the transition zone. For the MSS site, 12 column tests and 18
Corrective Action Plan Part 1
Marshall Steam Station Ash Basin
batch tests were conducted by UNCC. The results of these Kd tests were used as base inputs
to the model and adjusted accordingly to achieve model calibration.
=S-4.2 Groundwater Modeling Conclusions
ES-4.2.1 Flow Model
The 3-D groundwater flow model results indicate that under existing conditions, groundwater in
the shallow, deep, and fractured bedrock flow layers at the site flows toward the ash basin and
then south-southeast beneath the ash basin before discharging to Lake Norman.
The Existing Conditions scenario served as the basis of comparison to the Cap -in -Place and
Excavation scenarios. This scenario represents the most conservative conditions in terms of
groundwater concentrations on- and off -site, and COls reaching the compliance boundary, and
discharging to Lake Norman.
The Cap -in -Place scenario simulated placement of an engineered geosynthetic cap by applying
a recharge rate of zero to the source areas. Groundwater flow is affected by this scenario as the
water table is lowered and groundwater velocities may be reduced beneath the capped areas.
The Excavation scenario simulated the removal of all ash from the ash basin outside the
footprints of the dry ash landfill units and the PV structural fill. Flow conditions and groundwater
levels post -excavation cannot be accurately estimated until the depth of excavation and the
hydraulic parameters of the replacement fill material are known.
ES-4.2.2 Fate and Transport Model
The fate and transport modeling was used to assess the transport of selected COls through
shallow, deep, and bedrock flow layers and predict the fate of these COls over time. Each
selected COI was modeled individually under the Existing Conditions, Cap -in -Place, and
Excavation scenarios. COls evaluated in the fate and transport model include: antimony,
arsenic, barium, beryllium, boron, chloride, chromium, hexavalent chromium, cobalt, selenium,
sulfate, thallium, and vanadium. Note that iron, manganese, pH, and TDS were not included
because they cannot be adequately modeled using MDFLOW/MT3DMS. These constituents will
be modeled using geochemical modeling performed in CAP Part 2.
The Existing Conditions and Cap -In -Place scenarios were modeled over a 250-year time period
and the Excavation scenario was modeled over a 100-year time period.
For each scenario, the fate and transport model indicated the following:
• Existing Conditions Scenario — Concentrations of modeled COls predominately
increase or reach steady-state conditions above 2L Standards, IMACs, or NC DHHS
HSL during the 250-year model simulation period.
• Cap -in -Place Scenario — Concentrations of all modeled COls exceeded their respective
2L Standards or IMACs at the compliance boundary within 100 years after the start of
the simulation period, with the exception of chloride, hexavalent chromium, and
selenium. Chloride, hexavalent chromium, and selenium concentrations decrease to
Corrective Action Plan Part 1
Marshall Steam Station Ash Basin
below their 2L Standard or NC DHHS HSL (for hexavalent chromium only) within 100
years.
• Excavation Scenario — Concentrations of non-sorptive COls chloride and sulfate, as
well as sorptive COls antimony, barium, beryllium and selenium, decrease to below their
2L Standard or IMAC at the compliance boundary at the end of the 100-year model
period. The remaining COls exceeded their respective 2L Standard, IMAC, or NC DHHS
HSL at the end of the 100-year model period.
The flow and fate and transport models will be updated, if necessary, based on a review of
additional sampling and water elevation data as they become available.
ES-4.3 Groundwater -Surface Water Interaction Modeling
Groundwater model output from the fate and transport modeling was used to generate a mixing
model to assess potential impacts to Lake Norman. For each groundwater COI that discharges
to surface waters, the appropriate dilution factor and upstream (background) concentration were
applied to determine the surface water concentrations at the edge of the mixing zone. This
concentration was then compared to the applicable water quality standard or criteria to
determine compliance.
The surface water model results indicate that no water quality standards are exceeded for COls
modeled at the edge of the mixing zone in Lake Norman. As additional data are obtained during
subsequent sampling events, the surface water modeling will be refined and re -assessed, if
necessary.
ES-5 Recommendations
The following recommendations have been made to address areas needing further assessment:
• Additional sampling for radiological parameters along major groundwater flow paths is
needed to perform a more comprehensive assessment of radionuclides from source
areas.
• The SCM and groundwater models should be updated with results from second -round
sampling at the MSS site and should be included in the CAP Part 2 report.
• Background monitoring well development and sampling should continue and new data
obtained from the sampling events should be incorporated into statistical background
analysis once a sufficient data set has been obtained.
Corrective Action Plan Part 1
Marshall Steam Station Ash Basin
Introduction
Duke Energy Carolinas, LLC (Duke Energy) owns and operates Marshall Steam Station (MSS),
which is located on Lake Norman in Catawba County near the town of Terrell, North Carolina.
MSS began operations in 1965 as a coal-fired generating station and currently operates four
coal-fired units. Coal combustion residual (CCR) consisting of bottom and fly ash material from
MSS is disposed in the station's ash basin, located to the north of the station. Dry ash has been
disposed in six 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, gypsum, is disposed in the
FGD residue landfill. Fly ash utilized as structural fill was placed in the photovoltaic (PV)
structural fill and was used as structural fill beneath portions of the Industrial Landfill No. 1.
Discharge from the ash basin is permitted by the North Carolina Department of Environmental
Quality (NCDEQ, formerly referred to as NCDENR)3 Division of Water Resources (DWR) under
the National Pollutant Discharge Elimination System (NPDES) Permit NC0004987.
The North Carolina Coal Ash Management Act of 2014 (CAMA) directs owners of coal
combustion residuals (CCR) surface impoundments in North Carolina to conduct groundwater
monitoring, assessment, and remedial activities, if necessary. A groundwater assessment work
plan (Work Plan) for MSS was submitted to NCDENR on November 4, 2014, followed by a
revised Work Plan on December 30, 2014. The revised Work Plan was conditionally approved
by NCDENR on March 12, 2015. A Comprehensive Site Assessment (CSA) was performed to
collect information necessary to determine horizontal and vertical extent of impacts to soil and
groundwater attributable to CCR source area(s), identify potential receptors, and screen for
potential risks to those receptors. The MSS CSA Report was submitted to NCDENR on
September 8, 2015 (HDR 2015).
CAMA also requires the submittal of a Corrective Action Plan (CAP) for each regulated facility
no later than 180 days after submittal of the CSA. Duke Energy and NCDEQ mutually agreed to
a two-part CAP submittal, with Part 1 being submitted within 90 days of submittal of the CSA
Report and Part 2 being submitted no later than 180 days after submittal of the CSA Report.
The purpose of this CAP Part 1 is to provide background information, a brief summary of the
CSA findings, an evaluation and refinement of COls for modeling purposes, a detailed
description of the site conceptual model (SCM), results of the groundwater flow and
contaminant transport model, and results of the groundwater to surface water interaction model.
Information included in CAP Part 1 is intended to support the NCDEQ's risk ranking
classification process for the MSS site, as required by CAMA.
CAP Part 2 will include the remainder of the CAMA requirements, including an evaluation of
proposed alternative methods for achieving groundwater quality restoration, conceptual plans
for recommended corrective actions, implementation schedule, and a plan for future monitoring
and reporting. A risk assessment will be submitted under a separate cover with the CAP Part 2
submittal.
3 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.
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Marshall Steam Station Ash Basin
1.1 Site History and Overview
1.1.1 Site Location, Acreage, and Ownership
The MSS site is located on the west bank of Lake Norman near the town of Terrell, Catawba
County, North Carolina (Figure 1-1). The entire MSS site is approximately 1,446 acres in area
and is owned by Duke Energy.
In addition to the power plant property, Duke Energy owns and operates the Catawba-Wateree
Hydroelectric Project (Federal Energy Regulatory Commission [FERC] Project No. 2232). Lake
Norman reservoir is part of the Catawba-Wateree project and is used for hydroelectric
generation, a source of cooling water for MSS and Duke Energy's McGuire Nuclear Station,
municipal water supply, and recreation. Duke Energy performed a review of property ownership
within the FERC project boundary property surrounding the ash basin compliance boundary.
The review indicated that Duke Energy owns all of the property within the ash basin compliance
boundary, which is also located within the FERC project boundary. The Duke Energy property
boundary and ash basin compliance boundary are shown on Figure 1-2.
Site Description
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.
Natural topography at the 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 site except where
drainage patterns have been modified by the ash basins or other construction. A site layout map
is provided on Figure 1-2.
The ash basin system at MSS 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 (HDR 2015). The
ash basin system is located north of the power plant. Inflows from the station to the ash basin
are discharged into the southwest 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 into 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 which are located adjacent to the east (Phase 1) and
northeast (Phase 11) 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.
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Marshall Steam Station Ash Basin
The PV structural fill was constructed of fly ash under 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.
Approximate volumes of CCR materials are provided below for each area where ash is stored at
the site.
• Ash Basin — 13,351,000 cubic yards
• Dry Ash Landfill (Phase 1) — 522,000 cubic yards
• Dry Ash Landfill (Phase 11) — 4,064,000 cubic yards
• PV Structural Fill — 5,410,000 cubic yards
• Subgrade Fill Beneath Industrial Landfill No. 1 — 726,000 cubic yards
• Industrial Landfill No. 1 (lined) — 390,000 cubic yards (through April 2014)
• FGD Residue Landfill (lined) — 569,000 cubic yards (through April 2014)
Based on the estimated volumes provided above, a total of approximately 25,032,000 cubic
yards of CCR material is stored at the MSS site. The relationship between the volumes and the
dry and moist unit weights of CCR material can vary based on the placement and storage
methods used at the site. For example, CCR material within the portion of the ash basin that
receives sluiced material will contain more water weight than ash placed in the PV structural fill
which was compacted in lifts during construction and likely maintains more consistent dry and
moist unit weights. Based on estimated dry unit weights and estimated moist unit weights for
each CCR storage area, the moist weight of CCR material at the MSS site is estimated to be
greater than 30,000,000 tons.
Permitted Activities and Permitted Waste
Duke Energy is authorized to discharge wastewater that has been adequately treated and
managed from MSS to receiving waters designated as the Catawba River in accordance with
NPDES Permit NC0004987.
The NPDES permit authorizes discharges in accordance with effluent limitations, monitoring
requirements, and other conditions set forth in the permit. A detailed description of the NPDES
and surface water sampling requirements, along with the associated NPDES site flow diagram,
is provided in the CSA Report.
Two active permitted landfills (the FGD Residue Landfill and Industrial Landfill No. 1), one
closed dry ash landfill consisting of two units (dry ash landfill Phases I and 11), one closed
demolition and construction debris landfill (no CCR), one closed asbestos landfill (no CCR), and
one fly ash structural fill unit (PV structural fill) are located partially or wholly outside the ash
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Corrective Action Plan Part 1
Marshall Steam Station Ash Basin
basin footprint. Further details regarding the waste management units than contain CCR
material are provided above and in the CSA Report.
History of Site Groundwater Monitoring
Groundwater monitoring, as required by the MSS NPDES Permit NC0004987, began in
February 2011. The compliance groundwater monitoring wells have been sampled three times a
year for a total of 14 times from February 2011 through June 2015. Prior to the requirement for
compliance groundwater monitoring, Duke Energy implemented voluntary groundwater
monitoring around the ash basin from November 2007 until October 2011. During this period,
the voluntary groundwater monitoring wells were sampled a total of nine times and the analytical
results were submitted to NCDENR DWR.
The location of the ash basin voluntary and compliance monitoring wells, the ash basin waste
boundary, and the compliance boundary are shown on Figure 1-3. The compliance boundary
for groundwater quality at the MSS ash basin 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. A detailed description of NPDES and
voluntary groundwater monitoring programs and results is provided in the CSA Report.
Summary of Comprehensive Site Assessment
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 the
assessment to characterize media (soil, rock, and groundwater) potentially impacted by the
source areas at the MSS site (Figure 1-4). Seep, surface water, and sediment samples were
also collected. For the CSA, the source areas were defined as the ash basin, dry ash landfill
(Phases I and II), and PV structural fill. Source characterization was performed to identify
physical and chemical properties of ash, ash basin surface water, ash porewater, and ash basin
seeps. The analytical results for source characterization samples were compared to North
Carolina Groundwater Quality Standards, as specified in 15A NCAC 2L.0202 (21- Standards), or
Interim Maximum Allowable Concentrations (IMACs), and other regulatory screening levels for
the purpose of identifying constituents of interest (COls) that may be associated with potential
impacts to soil, groundwater, and surface water from the source areas. In addition,
hydrogeological evaluation testing was performed on newly installed and existing monitoring
wells at the site.
Information obtained during the CSA was used to determine existing background and source -
related constituent concentrations, as well as to evaluate the horizontal and vertical extent of
impacts to soil and groundwater at the site related to the source areas. If a constituent4
concentration exceeded: (1) 2L Standard or IMAC5, (2) North Carolina Surface Water Quality
4 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).
s 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, 2011, and 2012; 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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Corrective Action Plan Part 1
Marshall Steam Station Ash Basin
Standard (213 Standard), or (3) North Carolina Preliminary Soil Remediation Goals (NC PSRGs)
for Protection of Groundwater (POG) it was designated in the CSA as a COI. In addition, the
CSA presented information from a receptor survey completed in 2014 and a screening -level
human health and ecological risk assessment. Additional details of the CSA findings are
discussed in following sections.
Receptor Survey
Duke Energy submitted a receptor survey to NCDENR (HDR 2014a) in September 2014,
followed by a supplement to the receptor survey (HDR 2014b) in November 2014. The purpose
of the receptor survey was to identify drinking water wells within a 0.5-mile (2,640-foot) radius of
the MSS ash basin compliance boundary. The supplemental receptor survey information was
obtained from responses to water supply well survey questionnaires mailed to property owners
within the required distance requesting information on the presence of water supply wells, well
details, and well usage. A detailed description of the receptor surveys is provided in the CSA
Report. Results of the receptor survey are detailed on Figure 1-5.
1.5.1 Surrounding Land Use
The area surrounding MSS generally consists of residential properties, undeveloped land, and
Lake Norman. Properties located within a 0.5-mile radius of the MSS ash basin compliance
boundary generally consist of undeveloped land and Lake Norman to the east, undeveloped
land and residential properties located to the north and west, portions of the MSS site (outside
the compliance boundary), undeveloped land, and residences to the south, and commercial
properties to the southeast along North Carolina Highway 150. Properties surrounding the MSS
site are shown on Figure 1-6.
Findings of Drinking Water Supply Well Survey Conducted per Section
§130A-309.211(c) of CAMA
The receptor survey activities identified four public water supply wells and 83 private water
supply wells in use, along with six assumed private water supply wells, located within the 0.5-
mile radius of the ash basin compliance boundary (Figure 1-5). No wellhead protection areas
were identified within a 0.5-mile radius of the ash basin compliance boundary. Several surface
water bodies that flow from the topographic divide along Sherrills Ford Road toward Lake
Norman were identified within a 0.5-mile radius of the ash basin compliance boundary. No water
supply wells were identified between the source areas and Lake Norman.
The direction of groundwater flow was not fully established at the site prior to the CSA.
Therefore, NCDEQ required sampling of potential drinking water well receptors within 1,500 feet
of the compliance boundary in all directions. Between February and May 2015, NCDENR
arranged for independent analytical laboratories to collect and analyze water samples obtained
from private wells identified during the Drinking Water Supply Well Survey, if the owner agreed
to have their well sampled. Approximately 35 samples were collected from water supply wells
sampled within the 0.5-mile radius of the MSS ash basin compliance boundary. The NCDENR
sample results were included in Appendix B of the CSA Report.
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Summary of Screening Level Risk Assessment
A screening level human health and ecological risk assessment was performed as a component
of the CSA Report (HDR 2015). Each screening level risk assessment identified the exposure
media for human and ecological receptors. Human health and ecological exposure media
includes potentially impacted groundwater, soil, surface water, and sediments.
The human health exposure routes associated with the evaluated pathways for the site include
ingestion, inhalation, and dermal contact of environmental media. Potential human receptors
under a current or hypothetical future use include construction/outdoor workers, off -site
residents, recreational users and trespassers. The ecological exposure routes associated with
the evaluated pathways for the site include dermal contact/root absorption/gill uptake and
ingestion of environmental media. Potential ecological receptors under a current or hypothetical
future use include aquatic, riparian, and terrestrial biota.
The screening level risk assessment will continue to be refined and will be included in the CAP
Part 2 report.
Geological/Hydrogeological Site Description
The MSS site and its associated ash basin system is underlain by the Charlotte and Kings
Mountain terranes, two of a number of tectonostratigraphic terranes that have been defined in
the southern and central Appalachians and is located within the western portion of the larger
Carolina superterrane (Horton et al. 1989; Hibbard et al. 2002; Hatcher et al. 2007). The
Charlotte terrane is dominated by a 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. The Kings Mountain terrane has a distinctive
metasedimentary sequence with interlayered quartzite, metaconglomerate, marble, and schists
derived from both sedimentary and volcanic protoliths (Keith and Sterrett 1931; Kesler 1944;
King 1955; Horton and Butler 1977).
Based on the site investigation, the groundwater system in the natural materials (alluvium, soil,
soil/saprolite, and bedrock) at MSS is consistent with the regolith-fractured rock system and is
an unconfined, connected aquifer system without confining layers, as discussed in the CSA
Report. The MSS groundwater system is divided into three layers within the connected aquifer
system: the shallow, deep (TZ), and bedrock flow layers. All three layers generally flow in a
direction similar to the topographic gradient. In general, groundwater within the shallow, deep,
and bedrock flow layers beneath the source areas flows to the southeast toward Lake Norman
and slightly east toward an unnamed tributary on Duke Energy property that flows to Lake
Norman.
Results of the CSA Investigation
The CSA identified groundwater impacts at the MSS site and found that exceedances are a
result of both naturally -occurring conditions and CCR material contained in the ash basin, dry
ash landfill (Phases I and II), and the PV structural fill. The approximate horizontal extent of
groundwater impacts is limited to beneath the ash basin and dry ash landfill (Phase II), east and
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Marshall Steam Station Ash Basin
downgradient of the ash basin and dry ash landfill (Phase 1), and southeast and downgradient of
the ash basin, within the ash basin compliance boundary. The approximate vertical extent of
groundwater impacts is generally limited to the shallow and deep flow layers.
Surface water impacts were identified in the unnamed tributary that flows to Lake Norman
located downgradient of the dry ash landfill (Phase 1).
The horizontal extent of soil impacts is limited to the area beneath the ash basin and one
location east and downgradient of the dry ash landfill (Phase 1). Where soil impacts were
identified beneath the ash basin, the vertical extent of contamination beneath the ash/soil
interface is generally limited to the uppermost soil sample collected beneath ash.
Additional details pertaining to the horizontal and vertical extent of soil and groundwater impacts
at the site are detailed in the CSA Report.
Groundwater samples collected from background monitoring wells contained naturally -occurring
metals and other constituents at concentrations that exceeded their respective regulatory
standards or guidelines. These included barium, chromium, cobalt, iron, lead, manganese,
thallium, and vanadium The CSA Report did not propose provisional background
concentrations; however, they are proposed in Section 2 of this CAP Part 1.
Regulatory Background
CAMA Requirements
CAMA Section §130A-309.209 requires implementation of corrective actions for the restoration
of groundwater quality. Analysis and reporting requirements are as follows:
(b) Corrective Action for the Restoration of Groundwater Quality. - The owner of a coal
combustion residuals surface impoundment shall implement corrective action for the restoration
of groundwater quality as provided in this subsection. The requirements for corrective action for
the restoration of groundwater quality set out in the subsection are in addition to any other
corrective action for the restoration of groundwater quality requirements applicable to the
owners of coal combustion residuals surface impoundments.
(1) No later than 90 days from submission of the Groundwater Assessment Report
required by subsection (a) of this section, or a time frame otherwise approved by the
Department not to exceed 180 days from submission of the Groundwater Assessment
Report, the owner of the coal combustion residuals surface impoundment shall submit
a proposed Groundwater Corrective Action Plan to the Department for its review and
approval. The Groundwater Corrective Action Plan shall provide restoration of
groundwater in conformance with the requirements of Subchapter L of Chapter 2 of
Title 15A of the North Carolina Administrative Code. The Groundwater Corrective
Action Plan shall include, at a minimum, all of the following:
a. A description of all exceedances of the groundwater quality standards,
including any exceedances that the owner asserts are the result of natural
background conditions.
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Marshall Steam Station Ash Basin
b. A description of the methods for restoring groundwater in conformance with
requirements of Subchapter L of Chapter 2 of Title 15A of the North Carolina
Administrative Code and a detailed explanation of the reasons for selecting
these methods.
c. Specific plans, including engineering details, for restoring groundwater quality.
d. A schedule for implementation of the Plan.
e. A monitoring plan for evaluating effectiveness of the proposed corrective
action and detecting movement of any contaminant plumes.
f. Any other information related to groundwater assessment required by the
Department.
(2) The Department shall approve the Groundwater Corrective Action Plan if it determines
that the Plan complies with the requirements of this subsection and will be sufficient to
protect public health, safety, and welfare, the environment, and natural resources.
(3) No later than 30 days from the approval of the Groundwater Corrective Action Plan, the
owner shall begin implementation of the Plan in accordance with the Plan's schedule.
Duke Energy is required by CAMA to close the MSS ash basin system no later than August 1,
2029 or as otherwise dictated by NCDEQ risk ranking classification.
CAMA requires that corrective action be implemented to restore groundwater quality where the
CSA documents exceedances of groundwater quality standards.
Standards for Site Media
Groundwater and seep sample analytical results were compared to 2L Standards or IMACs
established by NCDEQ pursuant to 15A NCAC 02L.0202(c). The IMACs were issued in 2010,
2011, and 2012; however, NCDEQ has not established 2L Standards for these constituents as
described in 15A NCAC 02L.0202(c). For this reason, IMACs noted in this report are for
reference only. NCDEQ also requested that hexavalent chromium be compared to the North
Carolina Department of Health and Human Services (NC DHHS) Health Screening Levels
(HSLs) developed for drinking water supply wells.
Surface water sample analytical results were compared to the appropriate 2B Standards,
selected from a list of standards published by NCDENR dated April 22, 2015, and applicable
U.S. Environmental Protection Agency (USEPA) National Recommended Water Quality Criteria.
The water quality standards were published by NCDEQ in North Carolina Administrative Code
15A NCAC 213, amended effective January 1, 2015. The most stringent of the values from the
following three criteria (as applicable) was selected for comparison of the surface water
analytical results; Freshwater Aquatic Life, Water Supply, and Human Health (NCDEQ DWR
2015).
Soil sample analytical results were compared to NC PSRGs for POG exposures (updated
March 2015). Sediment sample analytical results were also compared to NC PSRGs for POG.
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Background Concentrations and Regulatory
Exceedances
As part of the CSA, groundwater, seep, surface water, sediment, and soil samples were
collected from background locations, beneath the source areas, and from locations upgradient
and downgradient of the source areas. Groundwater samples were also collected from
previously installed voluntary and compliance wells at the site. Data obtained from this sampling
event were presented in the CSA Report and the extent of impacts is summarized in Section
1.7 of CAP Part 1.
Some COls identified in the CSA are present in background and upgradient monitoring wells,
soil borings, and surface water locations and may be naturally -occurring, and thus require
consideration to determine whether their presence downgradient of the source areas is
naturally -occurring or potentially attributed to the source areas. The purpose of this section is to
present proposed provisional background concentrations (PPBCs) for groundwater, surface
water, sediment, and soil and discuss the nature and extent of COI impacts with regard to
PPBCs and applicable regulatory standards or guidelines (i.e., 2L Standards, IMACs, NC DHHS
HSLs, 213 Standards, and NC PSRGs for POG); and determine which COls will be retained for
further evaluation of corrective action.
COls resulting from ash, ash porewater, ash basin surface water, and seep sample results (as
identified in the CSA) were evaluated to determine if groundwater, surface water, sediment, and
soil impacts at the site are attributable to ash handling and storage activities. These COls are
provided in Table 2-1 (organized by media) for reference purposes. Details regarding source
characterization COls, including sample locations and resulting concentrations, are provided in
the CSA Report. Source characterization media (i.e., ash, ash porewater, and ash basin surface
water) are not evaluated for remediation in CAP Part 1 because they will be addressed as part
of corrective action(s) to be evaluated in CAP Part 2. However, concentrations of COls from the
source areas were considered when evaluating COls in media downgradient of the source
area(s) and were incorporated in the groundwater flow and contaminant transport model as
discussed in Section 4 and the UNCC Groundwater Modeling Report provided in Appendix D.
Note that COls identified in the CSA were based on one sampling event and that the PPBCs
presented in the subsections below are provisional values. The PPBCs will be updated as more
data become available with input from NCDEQ.
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Table 2-1. Initial COI Screening Evaluation
CSA COI Exceedance by Media
COI To Be
Constituent
Ash
Ash
Pore-
water
Ash Basin
Surface
Water
Seeps and
NCDENR
Resamples
Ground-
water
Surface
Water
Sediment
Soil
Further
Assessed
in CAP 1
Aluminum
-
-
-
-
-
-
-
-
No
Antimony
Yes
Arsenic
Yes
Barium
Yes
Beryllium
Yes
Boron
Yes
Cadmium
-
-
-
-
-
-
Yes
Chloride
Yes
Chromium
Yes
Hexavalent
Chromium
-
_
Yes
Cobalt
Yes
Copper
-
-
-
-
-
-
-
Yes
Iron
Yes
Lead
-
-
-
-
-
Yes
Manganese
Yes
Molybdenum
-
-
-
-
-
-
-
-
No
Mercury
-
-
-
-
-
-
-
-
No
Nickel
Yes
Nitrate
-
-
-
-
-
-
-
-
No
pH
Yes
Selenium
Yes
Strontium
-
-
-
-
-
-
-
-
No
Sulfate
Yes
Thallium
Yes
TDS
Yes
Vanadium
Yes
Zinc
-
-
-
-
-
-
-
No
Note: COI Exceedances based on 2L Standard, IMAC, and 213 Standards for respective aqueous media and NC
PSRGs for POG for solid/soil media.
Groundwater
Background Wells and Concentrations
To determine if a monitoring well is suitable for developing site -specific background
concentrations, the following criterion was evaluated:
• The topographic location of the well with respect to the source areas (distance from
source areas and located hydraulically upgradient of source areas)
• Stratigraphic unit being monitored
19
Corrective Action Plan Part 1
Marshall Steam Station Ash Basin
• Screened intervals of well relative to source water elevation
• Direction of groundwater flow in the region of the well relative to source areas
Wells that have been determined to represent background conditions at the site are compliance
monitoring wells MW-4 and MW-4D, FGD Residue Landfill monitoring well MS-10, and CSA
background monitoring wells BG-1S/D, BG-2S/BR, and BG-3S/D. A detailed analysis for each
background monitoring well determination is provided in Appendix B.
Site -specific PPBCs and regional background data are presented below in Table 2-2 for all
groundwater COls identified in the CSA. Groundwater PPBCs represent either the statistically
derived prediction limit concentration for constituents with sufficient data to use statistical
methods (from compliance wells MW-4 and MW-4D and FGD Residue Landfill monitoring well
MS-10) or the highest reported value or laboratory reporting limit (for non -detects) for
constituents that were not historically monitored at the site (from the compliance, FGD Residue
Landfill, and newly installed background monitoring wells). As additional data are collected from
newly installed background wells, the results will be incorporated into statistical background
analysis used in the determination of site -specific PPBCs. The statistical evaluation methods
used for determination of PPBCs for constituents that have been historically monitored at the
site are included in Appendix B.
Note that NCDEQ requested that analytical results for samples collected with turbidity readings
greater than 10 Nephelometric Turbidity Units (NTU) should not be included in the PPBC
calculations. However, the evaluation of COls in CAP Part 1 does consider analytical data from
wells other than background with measured turbidity greater than 10 NTU. Duke Energy
acknowledges that eliminating data greater than 10 NTU in comparison to PPBCs represents a
conservative approach. Additional evaluation on a well -by -well and constituent -by -constituent
basis may be warranted as part of a post -remedial monitoring plan to be completed in CAP Part
2. That level of evaluation was not possible using the limited data set acquired under the time
constraints specified in CAMA. In addition, porewater and groundwater sample results (other
than background) which were collected during the CSA with turbidity readings greater than 10
NTU were utilized in the contaminant fate and transport modeling discussed in Section 4. This
should be taken into consideration when evaluating the results of the fate and transport model
and considering the risk classification for the MSS site.
Regional groundwater data provided in Table 2-2 were obtained from publicly available data
with the most relevant spatial resolution. U.S. Geological Survey (USGS) National Uranium
Resource Evaluation (NURE) data in a 20-mile radius from the site were used for all
constituents contained in the NURE database. NC DHHS county -level data were subsequently
used for all constituents where available. Data for the remaining constituents for which there is
no NURE or NC DHHS data were acquired from the most spatially relevant, publicly available
sources, which are cited in the MSS CSA Report.
The "2-10 Private Well Data" provided in Table 2-2 identifies the range of values found for each
constituent sampled in private wells owned by Duke Energy employees living between
approximately 2 and 10 miles from the MSS ash basin waste boundary. The 2-10 private well
results are provided for reference only due to the lack of well construction data,
hydrostratigraphic data, and detailed geological context for these sample locations.
20
Corrective Action Plan Part 1
Marshall Steam Station Ash Basin
Table 2-2. Background Concentrations for Groundwater COls Identified in the CSA: Ranges of
Analytical Results with Sample Turbidity <10 NTU
2-10 Private
Compliance
FGD Landfill
New
Regional
Well Data
Wells MW-4 and
Well MS-10
Background
Constituent
Groundwater
(Feb 2015 to
MW-4D
Groundwater
Wells
PPBCs
Concentrations
August
Groundwater
Concentrations
Groundwater
(pg/L)
(pg/L)
2015)
Concentrations
(g/L)
N
Concentrations
(Ng/L)
(Ng/L)
(Ng/L)
Antimony
0.64 to 1.46
0.33J to <2.5
NR
<0.5
2.5
(North Carolina)
0.5 to 133;
0.5 to 9
(Catawba;
Arsenic
Iredell)
<0.5 to 6.81
0.33J to <2.5
<1 to <5
0.17J to 0.77
5
1.1 to 2.5
(Average in
Iredell)
50; 50
Barium
(Catawba;
<5 to 486
40 to 48
131 to 160
28 to 760
157.3
Iredell)
Beryllium
Not Determined
<0.2 to <1
<0.2 to <1*
NR
<0.2
1
Boron
Not Determined
<5 to 107
<50
<50
26J+ to <50
100
Chloride
<38.4
4,100 to
11200 to 1,900
1,096 to <5,000
2,700 to 4,800
3,500
19,000
713.6; 5.3
Chromium
(Average in
<0.5 to 9
1.2J to 2.5J
<1 to <5
3.1 to 9
11.3
Catawba and
Iredell)
Hexavalent
Not Determined
<0.03 to 3.7
0.32 to 1 *
NR
0.78J to 2.8
2.8
Chromium
Cobalt
Not Determined
<0.5 to 1.33
<0.5 to <2.5*
NR
0.38J to 3.3
2.5
25 to 3,733,000;
0 to 68,200
(Catawba;
Iredell)
1,606; 412
Iron
(Average in
<10 to 3,920
77 to 510
28 to 382
140 to 480
467.1
Catawba and
Iredell)
624
(Average in
Piedmont)
2.5 to 1,071;
2.5 to 602
(Catawba;
Lead
Iredell)
0.19 to 6.3
0.078J to 0.78
<1 to <5
0.19 to 0.3
35.9
7.7; 4.3
(Average in
Catawba and
Iredell)
<DL to 271.6
Manganese
(20-mile radius
<5 to 988
4.1J to 19
25.2 to 35.9
20 to 160
48
from site)
21
Corrective Action Plan Part 1
Marshall Steam Station Ash Basin
2-10 Private
Compliance
FGD Landfill
New
Regional
Well Data
Wells MW-4 and
Well MS-10
Background
Constituent
Groundwater
(Feb 2015 to
MW-4D
Groundwater
Wells
PPBCs
Concentrations
August
Groundwater
Concentrations
Groundwater
(pg/L)
(pg/L)
2015)
Concentrations
(Ng/L)
Concentrations
(Ng/L)
(Ng/L)
(Ng/L)
pH
5.4 to 7.7 (20-
68.50
(SU)
mile radius from
7.31 to 8.06
5.52 to 6.46
4.77 to 5.14
6.1 to 10.9
site)
2.5; 2.5 to 19.5
Selenium
(Catawba;
<0.5 to 1.7
<2.5
<1 to <10
0.37J to 0.65
10
I rede ll)
Sulfate
Not Determined
360 to
170,000
<1,000
<100 to <5,000
1,100 to 16,000
1,460
Thallium
(Piedmont)
<0.1 to 0.354
<0.2
NR
0.018J to 0.023J
0.5
TDS
Not Determined
140,000 to
42,000J+ to
21,000 to 48,000
114,000 to
85,400
370,000
821000J+
183,000
Vanadium
<DL to 19.2
<0.3 to 22.9
2.2J to 2.8*
NR
3.5 to 44
3.9
Notes:
1. lag/L = micrograms per liter
2. SU = Standard Units
3. J = Estimated concentration
4. NR = No results
5. DL = detection limit
6. < indicates concentration less than laboratory reporting limit.
7. Regional groundwater concentration data are from NURE data in a 20-mile radius from the site for all
constituents contained in the NURE database. NC DHHS county -level data were subsequently used for all
constituents available. Remaining constituents for which there are no NURE or NC DHHS data were pulled from
the most spatially relevant, publicly available sources. Further source information is found in Section 10.1 of the
MSS CSA Report.
8. PPBCs for constituents monitored during the CSA not considered COls are provided in Appendix B.
9. * indicates background concentrations provided for compliance wells MW-4 and MW-41D are from the July 2015
CSA sampling event only (for beryllium, cobalt, hexavalent chromium, and vanadium).
PPBCs were determined to be greater than the 2L Standards, IMACs, or NC DHHS HSL (for
hexavalent chromium only) for the following constituents:
• Cobalt
• Chromium
• Hexavalent chromium
• Iron
• Lead (no exceedances reported in groundwater samples at the site)
• Thallium
• Vanadium
Pending approval of the PPBC concentrations for these constituents by NCDEQ, PPBCs for the
constituents listed above will be used for identifying groundwater exceedances of COls instead
of the 2L Standards, IMACs or NC DHHS HSL during future sampling events.
For PPBCs determined to be less than the 2L Standards or IMACs, the respective regulatory
standard for that constituent will continue to be used for determining exceedances.
22
Corrective Action Plan Part 1
Marshall Steam Station Ash Basin
Groundwater Exceedances of 2L Standards or IMACs
Groundwater impacts attributed to the source areas at the site were delineated during the CSA
activities with the exception of further refinement of the horizontal and vertical extent of
exceedances east and downgradient of dry ash landfill (Phase 1). The need for additional
assessment in this area was identified as a data gap in the CSA and additional data collection in
this area is planned to commence during the first quarter of 2016.
To better understand groundwater COls relative to the source areas, groundwater exceedances
were compared to PPBCs and regulatory standards or criteria, and are summarized and
organized by area in Table 2-3. In addition, frequency of exceedances are provided for each
COI in each area. In the absence of a 2L Standard or IMAC for hexavalent chromium, NCDEQ
has requested that hexavalent chromium results be compared to the NC DHHS HSL for private
water supply wells (0.07 pg/L). At this time, PPBCs are shown on the table for reference
purposes only. Groundwater sample locations and analytical results are depicted on Figure 2-1.
Further details pertaining to the concentrations observed and specific locations of COls in
groundwater is included in Section 3.2.1.3 of CAP Part 1.
Table 2-3. Groundwater Results for COls Compared to PPBCs, 2L Standards, IMACs, or NC DHHS
HSL and Frequency of Exceedances
COI
Proposed
Provisional
Background
Concentrations
(pg/L)
NC 2L
Standard,
IMAC, or
NC DHHS HSL
(pg/L)
Groundwater
Concentrations
Exceeding 2L
Standards, IMAC, or
NC DHHS HSL
(pg/L)
Number of Samples
Exceeding 2L
Standards, IMACs,
or NC DHHS
HSL/Number of
Samples
Beneath the Ash Basin
Antimony*
2.5
1
1.3
1/25
Boron
100
700
1,200 to 5,500
3/25
Chloride
3,500
250,000
464,000
1/25
Cobalt*
2.5
1
1.4 to 28.1
13/25
Iron
467.1
300
370 to 10,900
20/25
Manganese
48
50
80 to 2,300
19/25
pH
6.5 to 8.5
6.5 to 8.5 SU
5.5 to 10.7 SU
16/25
TDS
85,400
500,000
1,530,000
1/25
Vanadium*
3.9
0.3
0.52J to 57.5
23/25
Beneath the Dry Ash Landfill (Phase II)
Barium
157.3
700
960
1/5
Boron
100
700
2,100 to 15,200
5/5
Chromium
11.3
10
15.5 to 17.5
2/5
Cobalt*
2.5
1
1.3 to 15.8
4/5
Iron
467.1
300
1,900 to 54,000
4/5
Manganese
48
50
530 to 8,400
4/5
pH
6.5 to 8.5
6.5 to 8.5 SU
5.4 to 8.78 SU
4/5
Selenium
10
20
24 to 108
2/5
Sulfate
1,460
250,000
308,000 to 979,000
4/5
23
Corrective Action Plan Part 1
Marshall Steam Station Ash Basin
COI
Proposed
Provisional
Background
Concentrations
(pg/L)
NC 2L
Standard,
IMAC, or
NC DHHS HSL
(pg/L)
Groundwater
Concentrations
Exceeding 2L
Standards, IMAC, or
NC DHHS HSL
(pg/L)
Number of Samples
Exceeding 2L
Standards, IMACs,
or NC DHHS
HSL/Number of
Samples
TDS
85,400
500,000
582,000 to 1,610,000
4/5
Vanadium*
3.9
0.3
0.65J to 6.5
5/5
Downgradient and East of the Ash
Basin and Dry Ash Landfill (Phase 1)
Beryllium*
1
4
9.9
1/5
Boron
100
700
1,300 to 4,600
4/5
Chloride
3,500
250,000
260,000
1/5
Chromium
11.3
10
30.9
1/5
Hexavalent
Chromium**
2.8
0.07
0.11 to 0.32
2/2
Cobalt*
2.5
1
1.3 to 11.8
4/5
Iron
467.1
300
320 to 1,300
3/5
Manganese
48
50
54 to 3,600
2/5
pH
6.5 to 8.5
6.5 to 8.5 SU
4.48 to 6.24
4/5
Thallium*
0.5
0.2
0.33
1/5
TDS
85,400
500,000
552,000 to 831,000
3/5
Vanadium*
3.9
0.3
0.35J to 8.5
5/5
Downgradient of
the Ash Basin Dam and Within the Compliance Boundary
Arsenic
5
10
10.4
1 /11
Boron
100
700
5,200 to 5,300
2/11
Hexavalent
Chromium**
2.8
0.07
0.14 to 0.42
2/5
Cobalt*
2.5
1
1.7 to 57.6
6/11
Iron
467.1
300
470 to 3,900
8/11
Manganese
48
50
54 to 8,000
8/11
pH
6.5 to 8.5
6.5 to 8.5 SU
4.3 to 11.24 SU
10/11
Thallium*
0.5
0.2
0.28 to 0.37
2/11
TDS
85,400
500,000
541,000 to 800,000
3/11
Vanadium*
3.9
0.3
0.43J to 9.3
9/11
Upgradient of the Ash Basin and Within the Compliance Boundary
Antimony*
2.5
1
4.1 to 11.4
3/22
Chromium
11.3
10
17.2 to 182
4/22
Hexavalent
Chromium**
2.8
0.07
0.25 to 4.6
5/5
Cobalt*
2.5
1
2 to 8.4
6/22
Iron
467.1
300
420 to 20,500 J+
11/22
Manganese
48
50
67 to 1,700
10/22
pH
6.5 to 8.5
6.5 to 8.5 SU
5.4 to 11.9 SU
19/22
TDS
85,400
500,000
650,000
1/22
Vanadium*
3.9
0.3
0.33J to 24.6
21/22
24
Corrective Action Plan Part 1
Marshall Steam Station Ash Basin
Notes:
1. Ng/L = micrograms per liter
2. SU = Standard Units
3. J = Estimated concentration
4. J+ = Estimated concentration, biased high
5. J- = Estimated concentration, biased low
6. < indicates concentration less than laboratory reporting limit.
7. * Indicates 2L Standard not established for constituent; therefore, IMAC used.
8. ** Indicates 2L Standard not established for constituent; therefore, NC DHHS HSL for private water supply wells
used.
Observations related to groundwater COls at MSS include:
The majority of groundwater COls that are likely attributable to the source areas at the
site were observed in four general areas: beneath the ash basin, beneath the dry ash
landfill (Phase 11), downgradient and east of the ash basin and dry ash landfill (Phase 1),
and downgradient and southeast of the ash basin.
• Locations with 2L Standard or IMAC exceedances of cobalt, iron, and manganese are
generally widespread beneath the ash basin.
• Concentrations of boron, chloride, manganese, and TDS exceeded their 2L Standards in
the deep flow layer beneath the central portion of the ash basin (near AB-12D). Boron
was also detected at concentrations exceeding its 2L Standard in the deep flow layer
beneath the west -central portion of the ash basin (AB-61D) and the east portion of the
ash basin south of the dry ash landfill (Phase 11) (AB-10D). Antimony exceeded its IMAC
in one groundwater sample beneath the ash basin (AB-61D).
• Vanadium exceeded its IMAC in all groundwater samples collected from beneath ash
basin with the exception of bedrock monitoring well AB-15BR and shallow monitoring
well AB-16S, which is located immediately adjacent to the ash basin.
• Concentrations of barium, boron, chromium, cobalt, iron, manganese, selenium, sulfate,
TDS, and vanadium exceeded their 2L Standards or IMACs in one or more samples
collected beneath the dry ash landfill (Phase 11).
• Concentrations of beryllium, boron, chloride, chromium, cobalt, iron, manganese,
thallium, TDS, and vanadium exceeded their 2L Standards or IMACs in one or more
samples collected east and downgradient of the ash basin and dry ash landfill (Phase 1).
• Concentrations of arsenic, boron, cobalt, iron, manganese, thallium, TDS, and vanadium
exceeded their 2L Standards or IMACs in one or more samples collected southeast and
downgradient of the ash basin dam, specifically from monitoring wells MW-7S and AB-
1S/D/BR. Cobalt, iron, manganese, and vanadium were the only COls detected above
their 2L Standards or IMAC in samples collected from monitoring wells GWA-1S/D/BR,
which are located downgradient of the ash basin and the MSS coal pile.
• Concentrations of hexavalent chromium exceeded the NC DHHS HSL in groundwater
samples collected east and downgradient of the ash basin and dry ash landfill (Phase 1),
and southeast and downgradient of the ash basin dam.
Several COls exceeded their regulatory standards or criteria in upgradient wells
(separate from background wells). In general, hexavalent chromium concentrations were
26
Corrective Action Plan Part 1
Marshall Steam Station Ash Basin
higher in upgradient and background samples when compared to samples collected
beneath and downgradient of the on -site source areas. Concentrations of antimony,
chromium, TDS, and vanadium exceeded their 2L Standard or IMAC in one upgradient
deep monitoring well (GWA-2D) located east-northeast of the gypsum storage area at
the site. Cobalt, iron, and manganese exceeded their 2L Standard or IMAC in several
wells located upgradient of the on -site source areas, some of which are located along
the west and northwest property boundary.
• pH was measured at values outside the 2L Standard range in several monitoring wells
located upgradient, beneath the on -site source areas, and downgradient of the on -site
source areas. pH will be further evaluated in the geochemical modeling portion of CAP
Part 2.
Boron, chloride, sulfate, and TDS, all of which exceeded their 2L Standards and PPCBs either
beneath or downgradient of the source areas, are considered to be detection monitoring
constituents in 40 CFR 257 Appendix I I I of the USEPA's Hazardous and Solid Waste
Management System; Disposal of Coal Combustion Residuals from Electric Utilities CCR Rule).
The USEPA detection monitoring constituents are potential indicators of groundwater
contamination from CCR as these constituents are associated with CCR and move rapidly
through the groundwater relative to other constituents that may migrate from a CCR unit. The
isolated presence of these constituents suggests that migration of contaminants are primarily
beneath the central portion of the ash basin, beneath the dry ash landfill (Phase 11), east and
downgradient of the ash basin and dry ash landfill (Phase 1), and southeast and downgradient of
the ash basin dam. Additional details regarding the CCR Rule and applicable constituents can
be found in the CSA Report.
Based on exceedances of 2L Standards, IMACs, and NC DHHS HSL screening levels, the
following groundwater COls will be further evaluated for corrective action:
• Antimony, arsenic, barium, beryllium, boron, chloride, chromium, hexavalent chromium,
cobalt, iron, manganese, pH, selenium, sulfate, thallium, TDS, and vanadium.
Radionuclides in Groundwater
Radionuclides may be present in groundwater from natural sources (e.g., soil or rock). The
USEPA regulates various radionuclides in drinking water. The following radionuclides were
analyzed in samples collected from background monitoring wells (BG-1 S/D) and groundwater
monitoring wells located downgradient of the ash basin and dry ash landfill (Phase 1) (MW-
14S/D) and the ash basin (MW-7S) as part of the CSA: radium-226, radium-228, uranium-238,
uranium-233, uranium-234, and uranium-236. The results are presented in Table 2-4.
26
Corrective Action Plan Part 1
Marshall Steam Station Ash Basin
Table 2-4. Radionuclide Concentrations
Background
Downgradient of Dry
Downgradient of
Radionuclide
USEPA MCL
Concentrations
Ash Landfill (Phase 1)
Ash Basin
(BG-1S and BG-1D)
Concentrations
Concentrations
(MW-14S and MW-14D)
(MW-7S)
Radium-226
5 pCi/L
(combined)
4.76 to 9.74 pCi/L
(combined)
5.96 to 96.1J pCi/L
(combined)
7.71J pCi/L
(combined)
Radium-228
Uranium-238
30 pg/L
(combined)
0.447J+ to 3.46 lag/L
(combined)
0.0838J to 0.107J lag/L
(combined)
1.67 lag/L
(combined)
Uranium-233
Uranium-234
U ran iu m-236
Notes:
1. pCi/L = picocuries per liter
2. Ng/L = micrograms per liter
3. J = estimated concentration
4. J+ = estimated concentration, biased high
5. MCL = maximum contaminant level
6. Bold indicates an exceedance of USEPA MCL
7. USEPA MCL for uranium of 30 Ng/L assumes combined concentration for all isotopes.
Concentrations of radium isotopes were higher in deep wells BG-1 D and MW-14D compared to
shallow wells BG-1S and MW-14S. The highest radium concentration was detected in deep
downgradient monitoring well MW-14D. Radium concentrations detected in shallow
downgradient monitoring wells MW-7S and MW-14S were less than the highest background
radium concentration. The highest concentration of uranium-238 was detected in shallow
background monitoring well BG-1S. Uranium-233, uranium-234 and uranium-236 were not
reported above their laboratory reporting limits in any of the samples. The combined uranium
isotope concentrations for each sample were less than the USEPA MCL for uranium.
Based on a review of available radiological data, additional data for radionuclides at the site are
needed for a more comprehensive assessment and may be warranted as part of a post
remedial monitoring plan to be completed in CAP Part 2.
Seeps
Two seeps potentially associated with the MSS ash basin (S-2 and the NCDENR-identified seep
MSSW001 S001) were sampled during the CSA activities. The S-2 sample location was
downgradient of the ash basin between the toe of the ash basin dam and topographically
downgradient of MSSW001 S001. Sample MSSW001 S001 was collected from the discharge
location (at the end of the rip rap dissipating structure) where seepage from the toe of the ash
basin dam daylights before reaching Lake Norman.
Following CSA sampling, it was determined that seep S-2 is not a seep from the ash basin. A
tree in the area of S-2 was uprooted and caused pooling of seepage water that was flowing from
the location of MSSW001 S001 toward Lake Norman, which is where the S-2 sample was
collected. The sample collected at MSSW001 S001 is representative of seepage flowing from
the toe of the ash basin dam and is the only seep sample considered for evaluating COls in
CAP Part 1.
27
Corrective Action Plan Part 1
Marshall Steam Station Ash Basin
Seep results for COls that require further evaluation for corrective action, along with a
comparison to applicable regulatory standards or criteria, are provided in Table 2-5. Seep
sample locations and analytical results are shown on Figure 2-2.
Table 2-5. Seep Results for COls Compared to 2L Standards or IMACs and Frequency of
Exceedances
NC 2L Standard,
Seep
Number of Samples
IMAC or
Concentrations
Exceeding 2L
COI
NC DHHS HSL
Exceeding 2L Standards,
Standards or
(pg/L)
IMAC, or NC DHHS HSL
IMACs/Number of
(pg/L)Samples
Boron
700
6,000
1/1
Hexavalent
0.07
0.12
1/1
Chromium**
Cobalt*
1
92
1/1
Manganese
50
8,500
1/1
Thallium*
0.2
0.6
1/1
TDS
500,000
989,000
1/1
Notes:
1. lag/L = micrograms per liter
2. NC DHHS = North Carolina Department of Health and Human Services.
3. * Indicates 2L Standard not established for constituent; therefore, IMAC used for screening criteria.
4. ** Indicates 2L Standard not established for constituent, therefore, NC DHHS HSL for private water supply wells
used.
In general, COls identified in seep sample MSSWO01 S001 are similar to groundwater COls
identified in shallow monitoring wells that were sampled in close proximity to this seep (MW-7S
and A13-1S). This supports the need to evaluate COls listed in Table 2-5 with the exception of
hexavalent chromium, which was reported at a much lower concentration in the MSSWO01
S001 seep sample compared to the groundwater PPBC for hexavalent chromium provided in
Table 2-4.
Surface Water
One surface water sample (SW-6) was obtained during the CSA from an unnamed tributary that
flows to Lake Norman. The location of surface water sample SW-6 was identified to be
downgradient of the dry ash landfill (Phase 1) and the ash basin. No surface water samples were
collected from upgradient locations or Lake Norman during the CSA and this was identified as a
data gap in the CSA Report. To address this data gap, two upgradient surface water samples
(SW-7 and SW-8) were collected from perennial streams that flow toward the ash basin and PV
structural fill, and three surface water samples (SW-9, SW-10, and SW-1 1) were collected from
Lake Norman downgradient of the ash basin and dry ash landfill (Phase 1) in October 2015. The
results from these surface water sample locations are included in CAP Part 1. SW-6 was also
sampled in October 2015.
Surface water concentrations were compared to the more stringent of the North Carolina
Surface Water Pollutant Standards for Metals for freshwater aquatic life, water supply, or human
health derived from 213 Standards for Class B, C and WS-IV waters. The water quality standards
were published by NCDEQ in North Carolina Administrative Code 15A NCAC 213, amended
effective January 1, 2015. In the absence of a 213 Standard, constituent concentrations were
28
Corrective Action Plan Part 1
Marshall Steam Station Ash Basin
compared to USEPA National Recommended Water Quality Criteria. Surface water results for
COls that require further evaluation downgradient of the source areas, along with a comparison
to upgradient surface water concentrations (SW-7 and SW-8) and applicable regulatory
standards or criteria, are provided in Table 2-6. Surface water sample locations and analytical
results from July 2015 (SW-6 only) and October 2015 are shown on Figure 2-2.
Table 2-6. Surface Water Results for COls Compared to Upgradient Surface Water Concentrations
and 2B Standards or USEPA National Recommended Water Quality Criteria, and Frequency of
Exceedances
COI
NC 2B
Standard or
USEPA
Criteria
(pg/L)
Upgradient
Surface Water
(SW-7 and SW-8)
Concentrations
(pg/L)
Concentrations
Exceeding 2B
Standards or USEPA
Criteria
(pg/L)
Number of
Samples
Exceeding 2B
Standards or
USEPA
Criteria/Number of
Samples
Unnamed Tributary to Lake Norman (SW-6 July and October 2015)
Cobalt
3
0.35J to 3.5
4.5 to 24.6
2/2
Iron
1,000
1,100 to 4,100
2,300 to 6,420
2/2
Manganese
50
57 to 270
709 to 1,600
2/2
Sulfate
250,000
7,500 to 31,700
376,000J+
1/2
TDS
250,000
84,000 to 119,000
1 466,000 to 674,000
2/2
Lake Norman (SW-9, SWA 0, and SWA 1 October 2015)
Cadmium
0.15
<0.08
0.16
1 /3
Copper
2.7
0.8J+ to 0.94J+
2.9J+ to 9.2
2/3
Lead
0.54
0.36 to 0.58
0.8
1/3
Manganese
50
57 to 270
93 to 100
3/3
Notes:
1. lag/L = micrograms per liter
2. J = Laboratory estimated concentration.
3. J+ = Laboratory estimated concentration, biased high.
4. < indicates concentration less than laboratory method detection limit.
5. Indicates USEPA National Recommended Water Quality Criteria used for constituent.
In general, COls identified in surface water sample SW-6, collected from the unnamed tributary
to Lake Norman, are similar to groundwater COls identified in monitoring wells located between
the unnamed tributary and the dry ash landfill (Phase I) and ash basin. This supports the need
for further evaluation of the COls identified in the SW-6 sample.
Of the COls identified in the surface water samples collected from Lake Norman, manganese is
the only COI that exceeded its 2L Standard or IMAC in groundwater monitoring wells located
between the ash basin and Lake Norman. Cadmium, copper, and lead were identified as
source -related COls in the CSA. Although these three constituents were detected above their
respective 2B Standards in surface water samples collected from Lake Norman (downgradient
of the ash basin), they were not detected above their 2L Standards in groundwater monitoring
wells located between the ash basin and Lake Norman, indicating that there is not a complete
pathway of exceeding concentrations of these constituents from the ash basin to Lake Norman.
Although not considered COls at this time, these three constituents will continue to be
29
Corrective Action Plan Part 1
Marshall Steam Station Ash Basin
monitored in surface water samples collected during future sampling events to monitor surface
water quality downgradient of the ash basin.
Sediment
Sediment samples were collected coincidentally with surface water sample SW-6 and seep
sample S-2 during the CSA. In the absence of NCDEQ sediment criteria, the sediment sample
results were compared to NC PSRGs for POG. Sediment results for COls that require further
evaluation, along with a comparison to NC PSRGs for POG are provided in Table 2-7. The
sediment sample locations and analytical results are depicted on Figure 2-3.
Table 2-7. Sediment Results for COls Compared to NC PSRGs for POG and Frequency of
Exceedances
COI
NC PSRGs for POG
(mg/kg)
Concentrations
Exceeding
NC PSRGs for POG
(mg/kg)
Number of Samples
Exceeding NC PSRGs for
POG/Number of Samples
Seep Sediment (S-2)
Cobalt
0.9
17.3
1/1
Iron
150
8,040J-
1/1
Manganese
65
153J-
1/1
Selenium
2.1
4.8J
1/1
Vanadium
6
27
1/1
Tributary Sediment (SW-6)
Cobalt
0.9
11.3
1/1
Iron
150
16,600
1/1
Manganese
65
115
1/1
Selenium
2.1
4.1J
1/1
Vanadium
6
59.7
1/1
Notes:
1. mg/kg = milligrams per kilogram
2. J = Laboratory estimated concentration.
3. J- = Estimated concentration, biased low.
Cobalt, iron, manganese, and vanadium concentrations exceeded the NC PSRGs for POG in all
sediment samples. During the CSA, background sediment samples were not collected for
comparison. Background sediment samples will be collected and analyzed during CSA
supplemental sampling to better understand if the sediment exceedances are attributable to the
source areas and should be evaluated as sediment COls.
Soil
Background Soil and Concentrations
Because some constituents are naturally occurring in soil and are present in the source areas,
establishing background concentrations is important for determining whether soil has been
impacted by the source areas. Boring locations that have been determined to represent
background conditions (see Section 2.1.1) from which background soil samples were collected
30
Corrective Action Plan Part 1
Marshall Steam Station Ash Basin
include: BG-1S/D, BG-2BR and BG-3D. Samples shallower than 5 feet below ground surface
(bgs) were not included in the population of background samples to minimize possible surface
impacts. Site geology was reviewed to determine if the soils were from the same geologic
formations and thus could be pooled as a single population. PWR and bedrock samples were
not included in the calculations for soil background statistics, because the mineralogy may be
different. The number of samples collected in PWR and bedrock was not enough to support the
development of separate background statistics for these solid matrices.
Soil PPBCs (i.e., the 95% upper tolerance limit [UTL]) were calculated for those constituents
analyzed in background soil borings, as shown in Table 2-8. The methodology followed ProUCL
Technical Guidance, Statistical Software for Environmental Applications for Data Sets with and
without Nondetect Observations (USEPA 2013). A detailed method review, statistical
evaluation, and results for the PPBCs are included in Appendix B. For COls where there were
too few detections reported to use the statistical methodology, the PPBCs were established by
setting the value equal to the greatest reported concentration or the greatest non -detect value.
Table 2-8. Proposed Provisional Background Soil Concentrations
Constituent
Number of
Samples
Number of
Detections
Range
(mg/kg)
Proposed Provisional
Background Soil
Concentrations (95% UTL)
(mg/kg)
Aluminum
8
8
10,400 to 29,900
43,450
Antimony
8
0
<5.2 to <7.2
7.2*
Arsenic
8
5
3.4 to <7.2
6.47
Barium
8
8
86.7 to 1,670
2,740
Beryllium
8
8
0.32 to 1.2
1.78
Boron
8
0
<12.9 to <68.6
68.6*
Cadmium
8
0
<0.62 to <0.87
0.87*
Calcium
8
8
276 to 10,300
17,380
Chloride
8
0
<255 to <351
351*
Chromium
8
8
13.5 to 660
1,244
Cobalt
8
8
13 to 41.8
55.6
Copper
8
8
6.6 to 82.7
220
Iron
8
8
20,400 to 45,400
53,510
Lead
8
7
3.2 to 10.2
12.4
Magnesium
8
8
6,230 to 44,000
76,790
Manganese
8
8
243 to 799
947
Mercury
7
1
<0.0082 to 0.079
0.079*
Molybdenum
8
0
<2.6 to <3.6
3.6*
Nickel
8
8
10.1 to 380
684
Nitrate
8
0
<25.5 to <35.1
35.1
pH (field)
8
8
5.9 to 7.9
5.9 to 7.9*
Potassium
8
8
1,450 to 25,500
42,880
Selenium
8
1
3.7 to <7.2
7.2*
Sodium
8
5
204 to <361
322
31
Corrective Action Plan Part 1
Marshall Steam Station Ash Basin
Constituent
Number of
Samples
Number of
Detections
Range
(mg/kg)
Proposed Provisional
Background Soil
Concentrations (95% UTL)
(mg/kg)
Strontium
8
8
36.9 to 200
265
Sulfate
8
0
<255 to <351
351 *
Thallium
8
0
<5.2 to <7.2
7.2*
TOC
8
2
363 to 1,140
1,140*
Vanadium
8
8
53 to 83.3
105
Zinc
8
8
42.5 to 78.2
101
Notes:
1. mg/kg = milligrams per kilogram
2. "<" indicates analytical result was less than the laboratory maximum reporting limit (MRL).
3. UTL = Upper Tolerance Limit (USEPA 2013)
4. * Value shown is the greatest detection or greatest non -detect (<) value. In these cases, there were too few
detections to develop UTL. Ranges associated with zero detections indicate the range of detection limits.
PPBCs were determined to be greater than the NC PSRGs for POG for the following
constituents:
• Arsenic
• Manganese
• Barium
• Nickel
• Cobalt
• Selenium
• Iron
• Vanadium
Pending approval of the PPBC concentrations for these constituents by NCDEQ, PPBCs for the
constituents listed above will be used for identifying soil exceedances of COls instead of the NC
PSRGs for POG during future sampling events.
For PPBCs determined to be less than the NC PSRGs for POG, the respective regulatory
standard for that constituent will continue to be used for determining exceedances.
Further evaluation may result in revisions to the list above and will be addressed in the CAP
Part 2 report.
2.5.2 Soil Exceedances of NC PSRGs for POG
Soil results for COls, along with a comparison to soil PPBCs, background concentrations (for
COls with no PPBC), and NC PSRGs for POG, are provided in Table 2-9. Soil sample locations
and analytical results are depicted on Figure 2-3.
32
Corrective Action Plan Part 1
Marshall Steam Station Ash Basin
Table 2-9. Soil Results for COls Compared to PPBCs, Background Concentrations and NC PSRGs
for POG, and Frequency of Exceedances
COI
Soil
PPBCs
(mg/kg)
Background Soil
Concentrations
(mg/kg)
NC PSRGs
for POG
(mg/kg)
Concentrations
Exceeding NC
PSRGs for POG
(mg/kg)
Number of Samples
Exceeding NC
PSRGs for
POG/Number of
Samples
Beneath the Ash Basin
Arsenic
6.47
3.4 to <7.2
5.8
6.2J to 18.9
6/43
Barium
2,740
86.7 to 1,670
580
649 to 1,360
5/43
Cobalt
55.6
13 to 41.8
0.9
4.2J to 33.6
37/43
Iron
53,510
20,400 to 45,400
150
4,370 to 90,500
43/43
Manganese
947
243 to 799
65
86 to 1,020
43/43
Nickel
684
10.1 to 380
130
141 to 203
4/43
Selenium
7.2
3.7 to <7.2
2.1
2.6J to 5.9J
5/43
Vanadium
105
53 to 83.3
6
14.5 to 215
41/43
Beneath the
Dry Ash Landfill (Phase II)
Arsenic
6.47
3.4 to <7.2
5.8
6J to 6.3J
2/6
Barium
2,740
86.7 to 1,670
580
614 to 1,530
4/6
Cobalt
55.6
13 to 41.8
0.9
6.2J to 37.8
6/6
Iron
53,510
20,400 to 45,400
150
11,600 to 51,600
6/6
Manganese
947
243 to 799
65
162 to 1,150
6/6
Nickel
684
10.1 to 380
130
138 to 281
2/6
Selenium
7.2
3.7 to <7.2
2.1
4.5J to 6J
2/6
Vanadium
105
53 to 83.3
6
20.4 to 132
6/6
Beneath the
PV Structural
Fill
Arsenic
6.47
3.4 to <7.2
5.8
60.4
1/6
Cobalt
55.6
13 to 41.8
0.9
7.4 to 38.8
6/6
Iron
53,510
20,400 to 45,400
150
7,790 to 40,100
6/6
Manganese
947
243 to 799
65
342 to 1,560
5/6
Selenium
7.2
3.7 to <7.2
2.1
30
1/6
Vanadium
105
53 to 83.3
6
31.7 to 60.7
6/6
Outside Source Areas Waste Boundaries
Arsenic
6.47
3.4 to <7.2
5.8
7.8 to 15.3
4/29
Barium
2,740
86.7 to 1,670
580
828
1/29
Cobalt
55.6
13 to 41.8
0.9
3.1 J to 28.2
23/29
Iron
53,510
20,400 to 45,400
150
2,020 to 57,800
29/29
Manganese
947
243 to 799
65
78.1 to 1,160J-
28/29
Selenium
7.2
3.7 to <7.2
2.1
4AJ to 9.6
4/29
Vanadium
105
53 to 83.3
6
11.2 to 231
28/29
Notes:
1. mg/kg = milligrams per kilogram.
2. J = Laboratory estimated concentration.
3. J+ = Estimated concentration, biased high.
4. J- = Estimated concentration, biased low.
33
Corrective Action Plan Part 1
Marshall Steam Station Ash Basin
< indicates concentration less than laboratory method detection limit.
NC PSRG for POG indicates the North Carolina Preliminary Soil Remediation Goal for Protection of
Groundwater.
Observations related to soil COls at MSS include:
• Arsenic, barium, cobalt, iron, manganese, nickel, selenium, and vanadium were detected
above their respective NC PSRGs for POG in soil samples collected from beneath the
ash basin and dry ash landfill (Phase II).
• Arsenic, cobalt, iron, manganese, selenium, and vanadium were detected above their
respective NC PSRGs for POG in soil samples collected from beneath the PV structural
fill.
• All the COls listed above that exceeded NC PSRGs for POG in soil samples beneath the
source areas, except for barium, were detected above NC PSRGs for POG in soil
samples collected outside the source areas waste boundaries. The concentrations
detected in the soil samples outside the source area waste boundaries were generally
similar to those collected beneath the source areas.
• The vertical extent of soil impacts is generally limited to the shallow soil samples
collected beneath the source areas.
• Arsenic, iron, manganese, selenium, and vanadium are the only soil COls that exceeded
their PPBCs and the NC PSRGs for POG in one or more soil samples.
• Barium, cobalt, and nickel did not exceed their NC PSRGs for POG and PPBCs, and
therefore should not be considered soil COls for corrective action.
Ash
Ash samples were collected and analyzed from the source areas as described in the CSA
Report. COls identified in ash characterize the source material from which COls were evaluated
with respect to releases from the ash management areas. Ash is not evaluated as a separate
medium for remediation in CAP Part 1 because ash will be addressed as part of corrective
action(s) to be evaluated in CAP Part 2. Ash exceedance results for COls are provided in Table
2-10 for reference. Ash sample locations are provided in the CSA Report.
34
Corrective Action Plan Part 1
Marshall Steam Station Ash Basin
Table 2-10. Ash Exceedance Results for COls Compared to NC PSRGs for POG and
Frequency of Exceedances
COI
NC PSRGs for POG
(mg/kg)
Concentrations
Exceeding NC
PSRGs for POG
(mg/kg)
Number of Samples Exceeding
NC PSRGs for POG/Number of
Samples
Within the Ash Basin
Antimony
0.9
2.3J
1/38
Arsenic
5.8
7.9 to 127
33/38
Barium
580
612
1/38
Boron
45
51.9 to 56.9
2/38
Cobalt
0.9
4.4J to 22.4
29/38
Iron
150
3,220 to 36,900
38/38
Manganese
65
69.4 to 782
16/38
Selenium
2.1
3.5J to 25.6
26/38
Vanadium
6
6.7J to 113
36/38
Within the Dry Ash Landfill (Phase II)
Arsenic
5.8
35.2 to 113
6/6
Boron
45
50.7 to 174
3/6
Cobalt
0.9
10.3 to 20.6
6/6
Iron
150
7,090 to 11,600
6/6
Manganese
65
73.6 to 110
3/6
Selenium
2.1
13.6 to 26.9
6/6
Vanadium
6
64.2 to 139
6/6
Within the PV Structural Fill
Arsenic
5.8
35.5 to 70.1
7/8
Boron
45
48.6 to 75.6
4/8
Cobalt
0.9
6.8 to 13.6
8/8
Iron
150
7,110 to 17,500
8/8
Manganese
65
81 to 118J+
3/8
Selenium
2.1
17.3 to 24.8
7/8
Vanadium
6
45.8 to 78.1
8/8
Notes:
1. mg/kg = milligrams per kilogram
2. NC PSRG for POG indicates the North Carolina Preliminary Soil Remediation Goal for Protection of Groundwater
36
Corrective Action Plan Part 1
Marshall Steam Station Ash Basin
Ash Porewater
Porewater refers to water samples collected from monitoring wells installed in the ash basin, dry
ash landfill (Phase II), and PV structural fill that are screened within the ash layer. Porewater
COls are representative of the source (CCR), but not representative of groundwater conditions.
Porewater is not evaluated for remediation in CAP Part 1 because porewater will be addressed
as part of corrective action(s) to be evaluated in CAP Part 2. Porewater exceedance results for
COls are provided in Table 2-11 for reference. Porewater sample locations are provided in the
CSA Report.
Table 2-11. Porewater Exceedance Results for COls Compared to 2L Standards, IMACs,
or NC DHHS HSLs, and Frequency of Exceedances
COI
NC 2L
Standard,
IMAC, or
NC DHHS HSL
(pg/L)
Porewater
Concentrations
Exceeding 2LStandards IMAC or NC
DHHS HSL
(HS IHI
Number of Samples
Exceeding 2L
Standards, IMACs, or
NC DHHS HSL/Number
of Samples
Within the Ash Basin
Antimony*
1
1.3 to 21.8
2/17
Arsenic
10
145 to 6,380
14/17
Barium
700
780
1/17
Beryllium*
4
23.5
1/17
Boron
700
790 to 66,600
15/17
Cadmium
2
3.4
1/17
Chloride
250,000
3,650,000
1/17
Chromium (total)
10
53.9
1/17
Hexavalent
Chromium**
0.07
0.082
1/17
Cobalt*
1
2.7 to 134
9/17
Iron
300
2,100 to 2,300,000
15/17
Manganese
50
320 to 19,400
15/17
Nickel
100
333
1/17
pH
6.5 to 8.5 SU
2.84 to 6.4
6/17
Sulfate
250,000
353,000 to 8,850,000
9/17
Thallium*
0.2
0.24 to 2
8/17
TDS
500,000
620,000 to 11,600,000
11/17
Vanadium*
0.3
0.84J to 24.1
16/17
Within the Dry Ash Landfill (Phase II)
Antimony*
1
12.2
1/17
Arsenic
10
189
1/17
Boron
700
73,400
1/17
Cadmium
2
6.3
1/17
Cobalt*
1
4.1
1/17
Iron
300
5,600
1/17
Manganese
50
240
1/17
Selenium
20
28.5
1/17
36
Corrective Action Plan Part 1
Marshall Steam Station Ash Basin
COI
NC 21L
Porewater
Number of Samples
Sulfate
250,000
3,160,000
1/17
Thallium*
0.2
1.8
1/17
TDS
500,000
4,810,000
1/17
Vanadium*
0.3
163
1/17
Within the PV Structural Fill
Antimony*
1
26.6
1/17
Arsenic
10
95.4
1/17
Beryllium*
4
5.4
1/17
Boron
700
41,700
1/17
Cadmium
2
5.3
1/17
Chromium (total)
10
71.6
1/17
Cobalt*
1
423
1/17
Iron
300
12,800
1/17
Lead
15
28.7
1/17
Manganese
50
8,400
1/17
Nickel
100
310
1/17
Selenium
20
454
1/17
Sulfate
250,000
3,560,000
1/17
Thallium*
0.2
14.8
1/17
TDS
500,000
5,170,000
1/17
Vanadium*
0.3
157
1/17
Notes:
1. lag/L = micrograms per liter
2. SU = Standard Units
3. * Indicates 2L Standard not established for constituent; therefore, IMAC used.
4. ** Indicates 2L Standard not established for constituent, therefore, NC DHHS HSL for private water supply wells
used.
Ash Basin Surface Water
Ash basin surface water will be addressed as part of corrective action(s) to be evaluated in CAP
Part 2. Ash basin surface water results for COls are provided in Table 2-12 for reference. Ash
basin surface water sample locations are provided in the CSA Report.
Table 2-12. Ash Basin Surface Water Results for COls Compared to 2B or USEPA Standards, and
Frequency of Exceedances
Concentrations
Number of Samples
NC 2B Standard
Exceeding 2B Standards or
Exceeding 2B
COI
or USEPA Criteria
USEPA Criteria
Standards or USEPA
(pg/L)
(pg/L)
Criteria/Number of
Samples
Arsenic
10
11.3 to 24.4
3/5
Beryllium
6.5
14.4
1/5
Cadmium
0.15
0.42 to 1.8
3/5
Chloride
230,000
231,000
1/5
Cobalt
3
33.9 to 291
3/5
37
Corrective Action Plan Part 1
Marshall Steam Station Ash Basin
Concentrations
Number of Samples
NC 213 Standard
Exceeding 213 Standards or
Exceeding 213
COI
or USEPA Criteria
USEPA Criteria
Standards or USEPA
(pg/L)
(pg/L)
Criteria/Number of
Samples
Copper
2.7
3.5 to 17.7
3/5
Lead
0.54
1.4 to 2.6
3/5
Manganese
50
470 to 3,900
4/5
Nickel
16
58.2 to 115
3/5
Selenium
5
6.6 to 33.2
3/5
Sulfate
250,000
694,000 to 1,210,000
3/5
Thallium
0.24
1.5 to 2.3
2/5
TDS
250,000
939,000 to 1,7101000
5/5
Zinc
36
150 to 160
2/5
Notes:
6. lag/L = micrograms per liter
7. J = Laboratory estimated concentration.
8. J+ = Laboratory estimated concentration, biased high.
9. < indicates concentration less than laboratory method detection limit.
10. Indicates USEPA National Recommended Water Quality Criteria used for constituent.
PWR and Bedrock
As requested by NCDEQ, samples of PWR and bedrock were obtained from rock cores during
the CSA and analyzed. NCDEQ does not have regulatory standards applicable to PWR and
bedrock. For this reason, evaluation of COls in solid matrix PWR or bedrock will not be
conducted.
2.10 COI Screening Evaluation Summary
Table 2-13 summarizes COls (by media) identified in Sections 2.1 through 2.5 that are
believed to be attributable to the source areas and that require further evaluation to determine if
corrective action(s) is warranted. The SCM is presented in Section 3 and 3-D groundwater fate
and transport modeling was performed, as applicable (Section 4), to further evaluate these
cols.
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Table 2-13. Updated COI Screening Evaluation Summary
CSA COI Exceedance by Media
COI To Be
Constituent
Ash
Ash
Pore-
water
Ash Basin
Surface
Water
Seeps and
NCDENR
Resamples
Ground-
water
Surface
Water
Sediment
Soil
Further
Assessed in
Groundwater
Modeling
Aluminum
-
-
-
-
-
-
-
-
No
Antimony
-
-
-
Yes
Arsenic
-
-
-
Yes
Barium
-
-
-
Yes
Beryllium
-
Yes
Boron
-
Yes
Cadmium
-
No
Chloride
-
Yes
Chromium
-
Yes
Hexavalent
Chromium
-
_
_
_
_
_
Yes
Cobalt
-
Yes
Copper
-
No
Iron
-
Yes
Lead
-
No
Manganese
-
Yes
Molybdenum
-
-
-
-
-
-
-
-
No
Mercury
-
-
-
-
-
-
-
-
No
Nickel
-
-
-
-
-
-
-
-
No
Nitrate
-
-
-
-
-
-
-
-
No
pH
-
Yes
Selenium
-
Yes
Strontium
-
-
-
-
-
-
-
-
No
Sulfate
-
-
Yes
Thallium
-
-
Yes
TDS4
-
Yes
Vanadium
Yes
Zinc
-
-
-
-
-
-
-
-
No
Notes:
1. Note that ash is not evaluated for remediation in CAP Part 1 because ash will be addressed as part of corrective
action(s) to be evaluated in CAP Part 2.
2. Note that ash porewater is not evaluated for remediation in CAP Part 1 because porewater will be addressed as
part of corrective action(s) to be evaluated in CAP Part 2.
3. Note that ash basin surface water is not evaluated for remediation in CAP Part 1 because ash basin surface
water will be addressed as part of corrective action(s) to be evaluated in CAP Part 2.
4. Geochemical modeling will be performed in CAP Part 2 to evaluate impacts of iron, manganese, pH and TDS
since these constituents cannot adequately be modeled using MODFLOW/MT3DMS.
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Corrective Action Plan Part 1
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2.11 Interim Response Actions
Duke Energy is required by CAMA to close the MSS ash basin no later than August 1, 2029 or
as otherwise dictated by NCDEQ risk classification. Closure for the MSS ash basin was not
defined in CAMA.
CAMA requires that corrective action be implemented to restore groundwater quality where the
CSA documents exceedances of groundwater quality standards.
No interim response actions are necessary at the MSS site because there are no identified
imminent hazards to human health or the environment.
Groundwater Response Actions
COls listed as a "COI to be Further Assessed in Groundwater Modeling" in Table 2-13 will be
further evaluated in Section 3 (Site Conceptual Model) and/or Section 4 (Modeling) to continue
refining the list of COls that will be addressed in CAP Part 2. As part of CAP Part 2, additional
model refinement will be conducted and remedial alternatives will be evaluated for potential
corrective action(s) at the site.
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Corrective Action Plan Part 1
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Site Conceptual Model
The site conceptual model (SCM) is an interpretation of processes and characteristics
associated with hydrogeologic conditions and COI interactions at the site. The SCM is used to
evaluate areal distribution of COls with regard to site -specific geological/hydrogeological and
geochemical properties at the MSS site. The SCM was developed using data and analysis from
the CSA Report. The sources and areas with 2L Standard or IMAC exceedances of COls
attributable to ash handling are illustrated in the 3-dimensional (3-D) SCM presented on Figure
3-1 and in cross -sectional view on Figure 3-2.
Site Hydrogeologic Conditions
Site hydrogeologic conditions were evaluated in the CSA through sampling/testing conducted
during installation of 13 borings and 83 monitoring wells. Groundwater monitoring wells were
screened within the shallow, deep, and bedrock flow layers beneath the site. Additional
information obtained during in -situ testing (packer testing) and slug testing was also utilized to
evaluate site conditions. A fracture trace analysis was performed for the MSS site, as well as
on-site/near-site geologic mapping, to further understand the site geology in support of the
SCM.
�.1.1 Hydrostratigraph ic Units
The following materials were encountered during the CSA investigation and are consistent with
material descriptions from previous site exploration:
• Ash (A) — Ash was encountered in borings advanced within the ash basin, dry ash
landfill (unit 2), and structural fill, as well as in some borings advanced through the ash
basin perimeter and dikes. Ash was generally described as dark yellow brown to very
dark gray, non -plastic, loose to very loose, and dry to wet.
• Fill (F) — Fill material generally consisted of re -worked sandy silts, clays, and sands that
were borrowed from areas of the site and re -distributed to other areas. Fill was classified
in the boring logs as silty sand, gravel with clay and sand, sand with silt and gravel, and
silt. Fill was primarily used in the construction of dikes, as cover for ash storage areas,
and as bottom liner for ash storage areas.
• Alluvium (S) — Alluvium encountered in borings AB-91D, AB-11 D, GWA-31D and AB-20D
was classified as wet and medium dense sand, sand with silt, and gravel with sand.
• Residuum (M1) — Residuum is the in -place weathered soil that consists primarily of silt,
sand with silt, clay with sand, sandy silt with gravel, clay, sandy clay, and sandy clay with
gravel at the MSS site. Residuum varied in thickness and was relatively thin compared
to the thickness of saprolite.
• Saprolite (M1/M2) — Saprolite is soil developed by in -place weathering of rock that
retains remnant bedrock structure. Saprolite at the MSS site is classified primarily as
sand with silt, silty sand, sand with silt and gravel, sand, clayey sand, clayey sand with
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Corrective Action Plan Part 1
Marshall Steam Station Ash Basin
gravel, and sand with gravel. Saprolite is typically tens of feet thick and in some cases
over 80 feet thick.
• Partially Weathered/Fractured Rock (Transition Zone) — Partially weathered (slight to
moderate) and/or highly fractured rock was encountered below refusal (auger, casing
advancer, etc.). The range of transition zone thickness observed at the MSS site was 0
to 26 feet.
Bedrock (BR) — Bedrock is defined as sound rock encountered in boreholes, generally
slightly weathered to fresh and relatively unfractured. The maximum depth that borings
extended into bedrock was approximately 70 feet.
Based on the site investigation conducted as part of the CSA, the groundwater system in the
natural materials (alluvium, soil, soil/saprolite, and bedrock) at MSS is consistent with the
regolith-fractured rock system and is an unconfined, connected aquifer system. The
groundwater system beneath the MSS site is divided into three layers to distinguish flow layers
within the connected aquifer system: the shallow, deep (transition zone), and bedrock flow
layers. Hydrostrati graphic units are shown on cross -sections presented in the CSA Report.
Hydrostratigraph ic Unit Properties
Material properties used in the groundwater flow and transport model are total porosity, effective
porosity, specific yield, and specific storage. These properties were developed from laboratory
testing of ash, fill, alluvium, and soil/saprolite and are presented in the CSA Report. Specific
yield/effective porosity was determined for a number of samples of the A, F, S, M1, and M2
hydrostratigraphic units to provide an average and range of values.
These properties were obtained through in -situ permeability testing (falling head, constant head,
and packer testing where appropriate); slug tests in completed monitoring wells; and laboratory
testing of undisturbed samples (ash, fill, soil/saprolite). Results from these tests were utilized to
develop the groundwater flow and fate and transport model further discussed in Section 4.
Potentiometric Surface — Shallow and Deep Flow Layers
The shallow and deep flow layers were defined by data obtained from the shallow and deep
groundwater monitoring wells (S and D wells, respectively) and surface water elevations. In
general, groundwater within the shallow and deep flow layers is consistent with the LeGrand
slope -aquifer system and flows from the north and northwest extents of the MSS site property
boundary to the south and southeast toward Lake Norman. Monitoring wells installed outside
the northwest, north, and northeast boundaries of the ash basin indicate that groundwater flows
toward the ash basin along the west and north property boundaries. Shallow and deep
groundwater at the site discharges to Lake Norman and an unnamed tributary that flows to Lake
Norman east of the ash basin and dry ash landfill (Phase I). Groundwater flows from the
southeast portion of the ash basin to the east and beneath the dry ash landfill (Phase I), and
ultimately toward the unnamed tributary that flows to Lake Norman. Between the ash basin and
Lake Norman (i.e., the southernmost portion of the ash basin), groundwater flows to the
south/southeast toward Lake Norman. Potentiometric surfaces for the shallow and deep flow
layers are based on groundwater level measurements collected during the CSA activities and
are illustrated on Figures 3-3 and 3-4.
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Potentiometric Surface - 3edrock Flow Layer
The bedrock flow layer was defined by data obtained from bedrock groundwater monitoring
wells (BR wells). In general, groundwater flow within the bedrock flow layer is consistent with
observed flow directions in the shallow and deep flow layers, flowing from the northern portion
of the site to the southeast toward Lake Norman. The potentiometric surface for the bedrock
flow layer is based on groundwater level measurements collected during the CSA activities and
is illustrated on Figure 3-5.
3.1.5 Horizontal and Vertical Hydraulic Gradients
3.1.5.1 Horizontal Hydraulic Gradient
Horizontal hydraulic gradients were derived for the shallow, deep, and bedrock flow layers by
calculating the difference in hydraulic heads over the length of the flow path between two wells
with similar well construction. Applying the horizontal hydraulic equation [i = dh/dl to wells
installed during the CSA yields the following average horizontal hydraulic gradients (measured
in feet/foot):
• Shallow flow layer: 0.018
• Deep flow layer: 0.017
• Bedrock flow layer: 0.010
Vertical Hydraulic Gradients
Vertical hydraulic gradients were calculated for 42 shallow (S) and deep (D) well pairs and nine
deep (D) and bedrock (BR) well pairs by dividing the difference in groundwater elevation in each
well pair by the difference in elevation of well screen midpoints (Tables 3-1 and 3-2). A positive
value indicates potential upward flow and a negative value indicates potential downward flow.
Vertical hydraulic gradients between the shallow and deep flow layers are depicted on Figure 3-
6 and the vertical gradients between the deep and bedrock flow layers are depicted on Figure
3-7.
Table 3-1. Vertical Gradient Calculations for Shallow/Deep Well Pairs
Shallow
Well
Deep
Well
Vertical Gradient
ft/ft
Shallow
Well
Deep Well
Vertical Gradient
ft/ft
AB-1 S
AB-1 D
0.038
AL-3S
AL-3D
-0.009
AB-2S
AB-2D
0.085
BG-1 S
BG-1 D
-0.091
AB-3S
AB-3D
-0.010
BG-3S
BG-3D
0.084
AB-4S
AB-4D
-0.00002
GWA-1 S
GWA-1 D
-0.001
AB-5S
AB-5D
-0.002
GWA-2S
GWA-2D
-0.007
AB-6S
AB-6D
-0.038
GWA-3S
GWA-3D
0.089
AB-7S
AB-7D
-0.012
GWA-4S
GWA-4D
-0.005
AB-8S
AB-8D
-0.020
GWA-5S
GWA-5D
0.013
AB-9S
AB-9D
-0.010
GWA-6S
GWA-6D
-0.004
AB-1 OS
AB-10D
-0.002
GWA-7S
GWA-7D
0.258
AB-11 S
AB-11 D
0.028
GWA-8S
GWA-8D
-0.040
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Shallow
Well
Deep
Well
Vertical Gradient
ft/ft
Shallow
Well
Deep Well
Vertical Gradient
ft/ft
AB-12S
AB-12D
0.003
MW-4
MW-4D
-0.011
AB-13S
AB-13D
0.006
MW-6S
MW-6D
0.042
AB-14S
AB-14D
0.012
MW-7S
MW-7D
0.151
AB-15S
AB-15D
-0.032
MW-8S
MW-8D
0.028
AB-16S
AB-16D
-0.009
MW-9S
MW-9D
-0.019
AB-17S
AB-17D
0.006
MW-10S
MW-10D
0.008
AB-18S
AB-18D
0.003
MW-11 S
MW-11 D
0.0002
AB-20S
AB-20D
-0.023
MW-12S
MW-12D
-0.003
AB-21 S
AB-21 D
0.028
MW-13S
MW-13D
0.052
AL-1 S
AL-1 D
-0.117
MW-14S
MW-14D
-0.022
Notes:
1. Depth to water measurements gauged from July 22, 2015 through July 24, 2015.
Observations of vertical gradients between shallow (S) and deep (D) flow layers:
• In general, positive vertical gradients were found for well pairs located in close proximity
to surface waters upgradient and downgradient of the ash basin and for well pairs in
close proximity to portions of the ash basin where there is ponded water or where
intermediate dikes/haul roads are preventing flow through the ash basin. These positive
gradients indicate potential for groundwater to surface water interaction.
• Positive vertical gradients were calculated for well pairs located within the footprint of the
dry ash landfill (Phase 11).
• Negative vertical gradients were calculated for the majority of well pairs located
upgradient of the ash basin that are not in close proximity to surface waters and for well
pairs located in the southwest, west -central, and northern portions of the ash basin.
• Negative vertical gradients were calculated for the well pair located within the footprint of
the PV structural fill and for compliance well pair MW-12S/D located at the Duke Energy
property boundary west of the PV structural fill.
• Negative vertical gradients were calculated for well pairs AL-1 S/D and MW-14S/D which
are located downgradient and east of the ash basin and dry ash landfill (Phase 1), as well
as one well pair located downgradient of the ash basin dam (MW-9S/D) and one well
pair located on the southwest boundary of the ash basin dam (GWA-1 S/D).
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Table 3-2. Vertical Gradient Calculations for Deep/Bedrock Well Pairs
Deep Well
Bedrock Well
Vertical Gradient (ft/ft)
AB-1 D
AB-1 BR
0.038
AB-5D
AB-5BR
-0.003
AB-6D
AB-6BR
-0.018
AB-9D
AB-9BR
0.043
AB-15D
AB-15BR
0.030
AL-2D
AL-2BR
0.036
GWA-1 D
GWA-1 BR
0.102
MW-13D
GWA-9BR
0.079
MW-14D
MW-14BR
-0.229
Notes:
1. Depth to water measurements taken from July 22, 2015 through July 24, 2015.
Observations of vertical gradients between deep (D) and bedrock (BR) flow layers:
Positive vertical gradients were calculated for well pairs located northwest of the ash
basin and PV structural fill (GW-9D/BR), within the north -central portion of the ash basin
(AB-15D/BR), within the footprint of the dry ash landfill (Phase 11) (AL-21D/13R), adjacent
to the central portion of the ash basin (AB-9D/BR), and at two locations between the ash
basin and Lake Norman (GWA-1 D/BR and A13-1 D/BR).
• Negative vertical gradients were calculated for well pairs located in the southwest (AB-
5D/BR) and west -central (AB-6D/BR) portions of the ash basin, and at one well pair
located east and downgradient of the ash basin and dry ash landfill (Phase 1) (MW-
14D/BR).
Site Geochemical Conditions
The following site geochemical conditions were evaluated for site -specific COls identified in
Section 2.10. Further geochemical analysis will be performed as part of the CAP Part 2. The
SCM will be updated as additional data and information associated with COls and site
conditions are developed.
3.2.1 COI Sources and Mobility in Groundwater
3.2.1.1 COI Sources
The overall chemical composition of coal ash resembles that of siliceous rocks from which it
was derived, particularly shale. Oxides of silicon, aluminum, iron, and calcium make up more
than 90% of most siliceous rocks, soils, fly ash, and bottom ash. Other major and minor
elements (sulfur, sodium, potassium, magnesium, titanium) make up an additional 8%, while
trace constituents account for less than 1%. The following constituents are considered to be
trace elements: arsenic, barium, cadmium, chromium, lead, mercury, selenium, copper,
manganese, nickel, lead, vanadium, and zinc (EPRI 2010).
COI sources at the MSS site consist of the ash basin, the dry ash landfill (Phases I and 11), and
the PV structural fill. These source areas are subject to different processes that generate
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Corrective Action Plan Part 1
Marshall Steam Station Ash Basin
leachate migrating into the underlying soil layers and into the groundwater. For example, the dry
ash landfill units and PV structural fill would generate leachate as a result of infiltration of
precipitation, while the ash basin would generate leachate based on the pond elevation in the
basin. In addition, ash management practices can alter the concentration range of constituents
in ash leachate, and certain groups of constituents are more prevalent in landfill versus pond
management scenarios (EPRI and USDOE 2004). Based on the CSA results, it is evident that
concentrations of COls at the MSS site are more prevalent in groundwater beneath the dry ash
landfill (Phase II) and downgradient and east of the ash basin and dry ash landfill (Phase I) than
in groundwater beneath the ash basin. The extent of groundwater COls relative to the source
areas is described in Section 3.2.1.5.
The location of ash, precipitation, and process water in contact with ash is the most significant
factor on geochemical conditions. COls would not be present in groundwater or soils at levels
greater than background without ash -to -water contact. Once leached by precipitation or process
water, COls can enter the soil -to -groundwater -to -rock system and their concentration and
mobility are controlled by the principles of COI transport in groundwater. Soil -to -groundwater -to -
rock interaction and geochemical conditions present in the subsurface are also responsible for
the natural occurrence of selected constituents in background locations. These natural
processes may also be responsible for a portion of selected constituents in groundwater.
COI Mobility in Groundwater
After leaching has occurred, the distribution and concentrations of constituents in groundwater
depends upon factors such as how the dissolved concentrations are transported through the
soil/rock media, the composition of the soil/rock media in the flow path, and the geochemical
conditions present along those flow paths.
There are three main processes involved in the transport of dissolved concentrations in
groundwater flow: advection, dispersion, and diffusion. Advection is the movement of dissolved
and colloidal constituents by groundwater flow and is the primary mechanism for movement of a
dissolved concentration. The rate of advection is based on Darcy's law which describes the flow
of a fluid (i.e., groundwater) through a porous medium (i.e., soil, rock, etc.). The second process
affecting the location and concentration of inorganic constituents in groundwater flow is
mechanical dispersion. This mixing process happens as groundwater undergoes tortuous paths
of various lengths to arrive at the same location; some water moves faster than other water,
which causes longitudinal and lateral spreading of plumes. Dispersion is scale dependent and
increases with plume length and groundwater flow velocity. The third process involved in the
transport of a dissolved concentration is molecular diffusion which occurs when particles spread
due to molecular motion, as in stagnant water. When mechanical dispersion and molecular
diffusion processes are combined, the resultant mixing factor is called hydrodynamic dispersion.
Hydrodynamic dispersion is a scale -dependent phenomenon. There is greater mixing
opportunity over long distances than over short distances, so the hydrodynamic dispersion is
greater for long distances.
Advection, dispersion, and diffusion can result in changes to constituent concentrations across a
site, and can also result in decreases in constituent concentrations over distance and time,
without consideration of other geochemical processes.
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Corrective Action Plan Part 1
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Retardation of constituent concentrations relative to an initial concentration can occur due to
adsorption, absorption, or ion exchange. Which of these three processes occur, and the degree
to which they occur, depends on factors such as the properties of the solute, the properties of
the soil/rock media, and geochemical conditions.
Inorganic constituents have a varying propensity to interact with the mineral and organic matter
contained in aquifer media. Depending on the constituent and the mechanism of interaction, the
retention of a constituent to the soil or aquifer material, and removal of the constituent from
groundwater, may be a non -reversible or a reversible condition.
In some cases, the degree of retardation or attenuation of a constituent to the aquifer media
may be so great that the constituent will not be mobile and will not transport. In these cases,
attenuation may result in reduction of constituent concentrations to acceptable levels before
reaching the point of compliance or receptors. In other cases, the degree of retardation or
attenuation of a constituent may be weaker resulting in greater mobility through the aquifer
media.
As discussed in the CSA Report, groundwater COls that are attributable to the source areas are
limited to the shallow and deep flow layers, and the direction of COI transport is generally in a
southeasterly direction towards Lake Norman and the unnamed tributary that flows to Lake
Norman.
3.2.1.3 COI Distribution in Groundwater
The spatial distribution and reported concentrations for each groundwater COI that exceeded
applicable regulatory standards or criteria at the MSS site is detailed below relative to each
source area. Note that comparison to PPBCs was conducted for those COls where PPBCs are
greater than the applicable regulatory standard or criteria. For the purposes of this discussion,
the shallow flow layer includes the analytical results reported in the shallow (S) wells, the deep
flow layer includes the analytical results reported in the deep (D) wells, and the bedrock flow
layer includes the analytical results reported in the bedrock (BR) wells.
Due to the COls discussed below (and included throughout CAP Part 1) being determined from
only one sampling event, dissolved concentrations are compared to total concentrations and
field parameter readings of turbidity and pH are discussed for each COI and sample, as
appropriate.
Beneath the Ash Basin
• Antimony was reported at a concentration that exceeded its IMAC (1 pg/L) in one
bedrock monitoring well, AB-6BR (1.3 pg/L), which is located beneath the west -central
portion of the ash basin. The PPBC for antimony is 2.5 pg/L.
The reported dissolved concentration (1.2 pg/L) was similar to (i.e., within one order of
magnitude of) the total concentration. Turbidity was measured at less than 10 NTU when
the sample was collected. pH was measured at a relatively elevated reading of 10.4
standard units (SU), and may be influenced by grout used for constructing the
monitoring well. Antimony was not detected above the laboratory reporting limit in deep
monitoring well AB-6D.
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Corrective Action Plan Part 1
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It does not appear that the antimony exceedance in bedrock well A13-6I3R is source -
related since antimony did not exceed the IMAC in the adjacent (and shallower) deep
well, A13-6D.
• Boron was reported at concentrations that exceeded its 2L Standard (700 pg/L) in deep
monitoring wells AB-10D (1,200 pg/L), AB-12D (3,500 pg/L), and A13-6D (5,500 pg/L).
AB-10D and AB-12D are located in the central portion of the ash basin south of the dry
ash landfill (Phase II). A13-6D is located in the western portion of the ash basin. No other
2L exceedances for boron were detected in deep and bedrock wells located beneath the
ash basin, indicating boron exceedances are limited to the deep flow layer beneath the
central and west -central portions of the ash basin.
Chloride was reported at a concentrations that exceeded its 2L Standard (250,000 pg/L)
in deep monitoring well AB-12D (464,000 pg/L). AB-12D is located in the central portion
of the ash basin. No other 2L exceedances for chloride were detected in deep or
bedrock wells located beneath the ash basin, indicating chloride exceedances are
limited to the deep flow layer beneath the central portion of the ash basin
Cobalt was reported at concentrations that exceeded its PPBC (2.5 pg/L) and IMAC (1
pg/L) in deep monitoring wells A13-5D, AB-1 OD and AB-21 D, and bedrock monitoring
well A13-513R, all located beneath the footprint of the ash basin. Cobalt was also detected
above its PPBC and IMAC in three monitoring wells located adjacent to the ash basin,
including shallow monitoring wells A13-11S and A13-16S, and deep monitoring well AB-
9D. The highest concentrations of cobalt were observed in A13-9D (28.1 pg/L) and AB-
16S (22.6 pg/L).
The dissolved concentrations were similar to total concentrations for these samples
although turbidity was measured slightly above 10 NTU for each of the samples. pH
values were recorded at 6.4 SU for sample A13-9D and 5.9 SU for sample A13-16S.
The cobalt exceedances are generally limited to the deep flow layer beneath the
southwest and east -central portion of the ash basin, as well as the shallow flow layer
adjacent to the northeast and east portions of the ash basin.
Iron was reported at concentrations that exceeded its PPBC (467.1 pg/L) and 2L
Standard (300 pg/L) in several monitoring wells beneath the ash basin, including A13-4D,
A13-513R, A13-5D, A13-6D, AB-7D, A13-8D, A13-10D, A13-11 D, A13-13D, A13-14D, A13-1513R,
A13-15D, A13-17D, and AB-21 D. The highest concentrations of iron were reported in
monitoring wells A13-5D (10,900 pg/L), A13-6D (5,800 pg/L), A13-7D (2,400 pg/L), AB-1OD
(4,400 pg/L), and AB-15D (2,700 pg/L). Iron also exceeded its PPBC and 2L Standard in
monitoring wells located adjacent to the ash basin, including shallow monitoring wells
A13-11S (5,900 pg/L) and AB-16S (5,600 pg/L), and deep monitoring well AB-16D (2,100
pg/L).
Dissolved concentrations were similar to total concentrations reported for each sample
except for AB-7D, A13-8D, A13-11 D, A13-11S, A13-13D, and A13-17D. Elevated turbidity
values (i.e., greater than 10 NTU) were measured for samples A13-5D, A13-8D, A13-11S,
A13-13D, A13-16S, A13-17D, and A13-18D. pH was measured outside of the PPBC and 2L
Standard ranges of 6.5 to 8.5 SU for samples A13-4D, A13-5D, A13-6D, AB-7D, A13-10D,
A13-14D, and AB-21 D.
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Iron exceedances are widespread in the deep and bedrock flow layers beneath the ash
basin and the shallow flow layer adjacent to the east and northeast portions of the ash
basin.
Manganese was reported at concentrations that exceeded its 2L Standard (50 pg/L) in
nearly all montioring wells located beneath and adjacent to the ash basin. Elevated
concentrations were reported in deep monitoring wells AB-5D (1,800 pg/L) and AB-10D
(2,300 pg/L), and shallow monitoring well AB-16S (1,400 pg/L).
Dissolved concentrations were similar to total concentrations reported for all samples
collected beneath and adjacent to the ash basin that exceeded its 2L Standard.
Manganese exceedances are widespread in the deep and bedrock flow layers beneath
the ash basin and the shallow flow layer adjacent to the east and northeast portions of
the ash basin.
• TDS was reported at a concentration of 1,530,000 pg/L in deep monitoring well A13-12D,
which is higher than its 2L Standard (500,000 pg/L). TDS exceedances are limited to the
deep flow layer beneath the central portion of the ash basin.
Vanadium was reported at concentrations that exceeded its PPBC (3.9 pg/L) and IMAC
(0.3 pg/L) in several monitoring wells beneath the ash basin, including A13-5D, A13-613R,
AB-7D, A13-8D, A13-10D, A13-17D, and A13-18D. The highest concentration was reported
in monitoring well A13-7D (57.5 pg/L). Vanadium was also reported above the PPBC and
2L Standard in shallow monitoring well A13-11S (20.8 pg/L), which is located adjacent to
ash basin.
Dissolved concentrations were similar to the total concentrations reported except for AB-
8D, AB-1OD, and A13-11S. Turbidity was measured above 10 NTU for samples A13-5D,
A13-8D, A13-11S, A13-17D, and A13-18D. pH was measured outside the PPBC and 2L
Standard ranges of 6.5 to 8.5 SU for samples A13-5D, A13-613R, AB-7D, A13-10D, and AB-
11 D.
Vanadium exceedances are widespread in the deep flow layer beneath the southwest,
west -central, and north portions of the ash basin, as well as in the shallow flow layer
adjacent to the east portion of the ash basin.
Beneath the Dry Ash Landfill (Phase II)
Barium was reported at a concentration of 960 pg/L in the shallow well AL-2S, which is
higher than its 2L Standard (700 pg/L). Turbidity was measured greater that 10 NTU and
the dissolved concentration was reported at a concentration of 38 pg/L, indicating that
turbidity may have influenced the reported total concentration that exceeded the 2L
Standard for barium.
• Boron was reported at concentrations that exceeded its 2L Standard (700 pg/L) in
shallow monitoring well AL-2S (6,500 pg/L) and deep monitoring wells AL-2D (6,500
pg/L), AL-3D (4,000 pg/L), and AL-4D (15,200 pg/L).
• Chromium was reported at concentrations that exceeded its PPBC (11.3 pg/L) and 2L
Standard (10 pg/L) in shallow monitoring well AL-2S (15.5 pg/L) and bedrock monitoring
well AL-213R (17.5 pg/L). The dissolved concentration reported for the AL-213R sample
49
Corrective Action Plan Part 1
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was similar to the total concentration listed above and turbidity was measured at less
than 10 NTU. pH was measured outside the PPBC and 2L Standard ranges of 6.5 to 8.5
SU for samples AL-2S and AL-213R.
As mentioned above, turbidity was measured greater than 10 NTU for the AL-2S
sample. Similar to the dissolved versus total concentrations for barium, the dissolved
concentration for chromium was reported as an estimated value of 0.97J+ pg/L, which is
much less than the total concentration, indicating turbidity may have influenced the
reported total concentration that exceeded the PPBC and 2L Standard for chromium.
The chromium exceedances in bedrock well AL-2BR do not appear to be source related
since there were no exceedances in the deep flow layer beneath the dry ash landfill
(Phase II).
• Cobalt was reported at concentrations that exceeded its PPBC (2.5 pg/L) and IMAC (1
pg/L) in shallow monitoring well AL-2S (4.4 pg/L), and deep monitoring wells AL-2D
(15.8 pg/L) and AL-4D (7.4 pg/L). Dissolved concentrations were similar to the reported
total concentrations listed above. Note that turbidity was measured at greater than 10
NTU for the AL-4D sample.
Iron was reported at concentrations that exceeded its PPBC (467.1 pg/L) and 2L
Standard (300 pg/L) in shallow monitoring well AL-2S (54,000 pg/L), and deep
monitoring wells AL-2D (3,000 pg/L), AL-3D (1,900 pg/L), and AL-4D (3,100 pg/L).
Dissolved concentrations were similar to the reported total concentrations listed above
for the deep monitoring well samples. The dissolved concentration for the AL-2S sample
was significantly less (440 pg/L) than the total concentration listed above.
Similar to the barium and chromium results, the difference in dissolved and total
concentrations, as well as the elevated turbidity reading for the AL-2S sample, indicate
that turbidity may have influenced the reported total concentration that exceeded the
PPBC and 2L Standard for iron. However, iron exceedances are present in the deep
flow layer beneath the dry ash landfill (Phase II).
Manganese was reported at concentrations that exceeded its 2L Standard of 50 pg/L in
shallow monitoring well AL-2S (1,200 pg/L), and deep monitoring wells AL-2D (8,400
pg/L), AL-3D (590 pg/L), and AL-4D (530 pg/L). Dissolved concentrations were similar to
the reported total concentrations listed above for the deep monitoring well samples. The
dissolved concentration for the AL-2S sample was less (380 pg/L) than the total
concentration listed above, but still within one order of magnitude and exceeded the 2L
Standard.
Similar to barium, chromium, and iron, the manganese exceedance in shallow well AL-
2S may have been influenced by turbidity. However, manganese exceedances are
present in the deep flow layer beneath the dry ash landfill (Phase II).
• Selenium was reported at concentrations that exceeded its 2L Standard (20 pg/L) in
shallow monitoring well AL-2S (108 pg/L) and bedrock monitoring well AL-2BR (24
pg/L). Dissolved concentrations were similar to total concentrations reported in each of
these samples. The dissolved concentration reported in deep monitoring well AL-3D
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Corrective Action Plan Part 1
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(22.5 pg/L) exceeded the 2L Standard. The total concentration reported for AL-31D (16.7
pg/L) was similar but less than the 2L Standard.
Selenium exceedances are present in the shallow, deep, and bedrock flow layers
beneath the dry ash landfill (Phase 11).
Sulfate was reported at concentrations that exceeded its 2L Standard (250,000 pg/L) in
shallow monitoring well AL-2S (979,000 pg/L), and deep monitoring wells AL-21D
(386,000 pg/L), AL-31D (402,000 pg/L), and AL-41D (308,000 pg/L), indicating sulfate
exceedances are limited to the shallow and deep flow layers beneath the dry ash landfill
(Phase 11).
• TDS was reported at concentrations that exceeded its 2L Standard (500,000 pg/L) in
shallow monitoring well AL-2S (1,610,000 pg/L), and deep monitoring wells AL-21D
(761,000 pg/L), AL-31D (692,000 pg/L), and AL-41D (582,000 pg/L).
Similar to the differences in dissolved versus total concentrations for barium, chromium,
iron, and manganese, the elevated TDS concentration reported in AL-2S may be
influenced by turbidity. However, TDS exceedances are present in the deep flow layer
beneath the dry ash landfill (Phase 11).
Vanadium was reported at concentrations that exceeded its PPBC (3.9 pg/L) and IMAC
(0.3 pg/L) in monitoring wells AL-2S (6.5 pg/L) and AL-2BR (4.5 pg/L). The dissolved
concentration was similar to the total concentration reported for sample AL-2BR, but was
more than one order of magnitude different for sample AL-2S. Turbidity was measured
at 30.8 NTU for sample AL-2S, which indicates the total concentration results may be
influenced by turbidity. pH was measured outside the PPBC and 2L Standard ranges of
6.5 to 8.5 SU for samples AL-2BR and AL-2S.
Similar to barium, chromium, iron, and manganese, the vanadium exceedance in
shallow well AL-2S may have been influenced by turbidity. The vanadium exceedances
in bedrock well AL-2BR do not appear to be source -related since there were no
exceedances in the deep flow layer.
Downgradient and East of the Ash Basin and Dry Ash Landfill (Phase 1)
Beryllium was reported at a concentration of 9.9 pg/L in the shallow well AL-1S, which
exceeded its IMAC (4 pg/L). The dissolved concentration was similar to the total
concentration. Therefore, it appears beryllium exceedances are limited to the shallow
flow layer immediately downgradient and east of the ash basin and dry ash landfill
(Phase 1).
• Boron was reported at concentrations that exceeded its 2L Standard (700 pg/L) in
shallow monitoring wells AL-1S (4,600 pg/L) and MW-14S (2,700 pg/L), and deep
monitoring wells AL-11D (1,300 pg/L) and MW-14D (2,600 pg/L). Boron was reported as
an estimated concentration much lower than its 2L Standard in bedrock monitoring well
MW-14BR.
Boron exceedances are limited to the shallow and deep flow layers downgradient and
east of the ash basin and dry ash landfill (Phase 1).
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• Chloride was reported at a concentration of 260,000 pg/L in shallow well AL-1 S, which
exceeded its 2L Standard (250,000 pg/L), indicating chloride exceedances are limited to
the shallow flow layer immediately downgradient and east of the ash basin and dry ash
landfill (Phase 1).
Chromium was reported at a concentration of 30.9 pg/L in shallow well AL-1 S, which
exceeded its PPBC (11.3 pg/L) and 2L Standard (10 pg/L). The dissolved concentration
for the AL-1 S sample was less (13.8 pg/L) than the total concentration, but still within
one order of magnitude and exceeded the PPBC and 2L Standard. pH was measured at
4.48 SU for the AL-1 S sample, which is outside the PPBC and 2L Standard ranges of
6.5 to 8.5 SU. Chromium exceedances appear to be limited to the shallow flow layer
immediately downgradient and east of the ash basin and dry ash landfill (Phase 1).
Hexavalent Chromium was reported at concentrations or 0.32 pg/L and 0.11 pg/L in
monitoring wells MW-14S and MW-14D, respectively, which exceeded the NC DHHS
HSL (0.07 pg/L), but was less than the PPBC (2.8 pg/L). Note that MW-14S and MW-
14D were the only monitoring wells sampled downgradient and east of the ash basin and
dry ash landfill (Phase 1).
• Cobalt was reported at concentrations that exceeded its PPBC (2.5 pg/L) and IMAC (1
pg/L) in shallow monitoring wells AL- 1S (11.8 pg/L) and MW-14S (8.2 pg/L). Dissolved
concentrations were similar to the total concentrations listed above.
Cobalt exceedances are limited to the shallow flow layer downgradient and east of the
ash basin and dry ash landfill (Phase 1).
• Manganese was reported at concentrations that exceeded its 2L Standard (50 pg/L) in
shallow monitoring wells AL-1S (3,600 pg/L) and MW-14S (54 pg/L). Dissolved
concentrations were similar to the total concentrations listed above.
Manganese exceedances are limited to the shallow flow layer downgradient and east of
the ash basin and dry ash landfill (Phase 1).
Thallium was reported at a concentration of 0.33 pg/L in shallow monitoring well AL-1S,
which exceeded its IMAC (0.2 pg/L), but is less than the PPBC (0.5 pg/L). The dissolved
concentration for the AL-1S sample was reported as an estimated value of 0.16J+ pg/L,
which is less than PPBC and 2L Standard, but still within one order of magnitude of the
total concentration. As noted above, pH was measured at 4.48 SU for the AL-1 S sample,
which is below the PPBC and 2L Standard ranges of 6.5 to 8.5 SU.
TDS was reported at concentrations that exceeded its 2L Standard (500,000 pg/L) in
shallow monitoring wells AL-1S (831,000 pg/L) and MW-14S (552,000 pg/L), and deep
monitoring well AL-1 D (744,000 pg/L).
TDS exceedances are present in the shallow and deep flow layers downgradient and
east of the ash basin and dry ash landfill (Phase 1).
Vanadium was reported at concentrations that exceeded its PPBC (3.9 pg/L) and IMAC
(0.3 pg/L) in monitoring wells AL-1S (8.5 pg/L) and MW-14BR (4.3 pg/L). The dissolved
concentration was similar to the total concentration reported for sample MW-14BR, but
was less than the laboratory reporting limits for sample AL-1S. Turbidity was measured
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below 10 NTU for each sample. pH was measured outside the PPBC and 2L Standard
ranges of 6.5 to 8.5 SU for sample AL-1S.
The vanadium exceedances appear to be limited to the bedrock flow layer downgradient
and east of the ash basin and dry ash landfill (Phase 1) and do not appear to be source -
related since there were no exceedances in the deep flow layer.
Downgradient and Southeast of the Ash Basin
Arsenic was reported at a concentration of 10.4 pg/L in shallow well MW-7S, which
exceeded its 2L Standard (10 pg/L). The dissolved concentration was similar to the total
concentration. pH was measured at 4.3 SU for the MW-7S sample, which is outside the
PPBC and 2L Standard ranges of 6.5 to 8.5 SU.
Arsenic exceedances are limited to the shallow flow layer downgradient and southeast of
the ash basin in the vicinity of MW-7S.
• Boron was reported at concentrations that exceeded its 2L Standard (700 pg/L) in
shallow monitoring wells AB-1S (5,200 pg/L) and MW-7S (5,300 pg/L).
• Cobalt was reported at concentrations that exceeded its PPBC (2.5 pg/L) and IMAC (1
pg/L) in shallow monitoring wells AB-1S (27.1 pg/L), AB-2S (3.1 pg/L), MW-7S (57.6
pg/L) and GWA-1S (5.5 pg/L), and deep monitoring well AB-1D (4.3 pg/L). Dissolved
concentrations were similar to the total concentrations listed above.
Cobalt are exceedances are generally limited to the shallow flow layer downgradient and
southeast of the ash basin, with the exception of one exceedance in the deep flow layer
in the vicinity of AB-1 D.
Hexavalent Chromium was reported at concentrations or 0.14 pg/L and 0.42 pg/L in
monitoring wells MW-10S and MW-10D, respectively, which exceeded the NC DHHS
HSL (0.07 pg/L), but was less than the PPBC (2.8 pg/L). Note that hexavalent chromium
was not detected above the laboratory reporting limit in monitoring wells AB-1S/D/BR,
which are located immediately downgradient and southeast of the ash basin dam, and
between the ash basin and MW-10S/D.
Based on no exceedances of hexavalent chromium in monitoring wells AB-1S/D/BR, the
hexavalent chromium exceedances in MW-10S and MW-1 OD do not appear to be a
result of the ash basin.
Iron was reported at concentrations that exceeded its PPBC (467.1 pg/L) and 2L
Standard (300 pg/L) in shallow monitoring well AB-1S (2,200 pg/L), AB-2S (470 pg/L),
GWA-1S (3,900 pg/L) and MW-7S (2,200J+ pg/L); deep monitoring well AB-2D (1,200
pg/L); and bedrock monitoring wells AB-1 BR (2,800 pg/L) and GWA-1 BR (780 pg/L).
Dissolved concentrations were similar to the reported total concentrations listed above,
except for AB-2S, GWA-1S and MW-7S.
pH was measured outside the PPBC and 2L Standard ranges of 6.5 to 8.5 SU for
samples AB-1S, AB-2S, GWA-1S, and MW-7S, which may have influenced the results of
the elevated total concentration reported for iron in these samples. Turbidity was
measured greater than 10 NTU for samples GWA-1 S. Based on the difference in
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dissolved and total concentrations and the elevated turbidity reading, turbidity may have
influenced the validitiy of the reported total concentration that exceeded the PPBC and
2L Standard for iron in GWA-1 S. However, iron exceedances are present in the shallow,
deep, and bedrock flow layers downgradient and southeast of the ash basin.
• Manganese was reported at concentrations that exceeded its 2L Standard (50 pg/L) in
monitoring wells AB-1 S/D/BR, AB-2S/D, GWA-1 S/BR, and MW-7S. The highest
concentrations of mangenese were reported in shallow monitoring wells AB-1S (8,000 pg/L)
and MW-7S (6,000 pg/L). Dissolved concentrations were similar to total concentrations.
Manganese exceedances are widespread immediately downgradient and southeast of
the ash basin.
• Thallium was reported at concentrations that exceeded its IMAC (0.2 pg/L) in shallow
monitoring wells AB-1S (0.28 pg/L) and MW-7S (0.37 pg/L). These reported
concentrations were less than the PPBC (0.5 pg/L). Dissolved concentrations were
similar to the total concentrations listed above.
• TDS was reported at concentrations that exceeded its 2L Standard (500,000 pg/L) in
shallow monitoring wells AB-1S (781,000 pg/L) and MW-7S (800,000) pg/L), and deep
monitoring well AB-1 D (541,000 pg/L).
Vanadium was reported at concentrations that exceeded its PPBC (3.9 pg/L) and IMAC
(0.3 pg/L) in monitoring wells GWA-1 D (9.3 pg/L) and MW-7S (4.6 pg/L). Turbidity was
measured less than 10 NTU for samples GWA-1 D and MW-7S.
Vanadium exceedances are limited to the shallow and deep flow layers downgradient
and southeast of the ash basin.
t Geochemical Characteristics
Groundwater composition can be affected by an array of naturally -occurring and anthropogenic
factors. Many of these factors can be causative agents for specific reduction -oxidation (redox)
processes or indicators of the implied redox state of groundwater as expressed by pH,
oxidation-reduction potential (ORP), and dissolved oxygen (DO). Groundwater pH is affected by
the composition of the bedrock and soil through which the water moves. Exposure to carbonate
rocks (or lime -containing materials in well casings) can increase pH. Exposure to atmospheric
carbon dioxide gas will lead to formation of carbonic acid and can lower pH. The pH of
precipitation that falls on the watershed of an aquifer can also impact groundwater pH. In
addition, metals and other elemental or ionic constituents in groundwater, or the surrounding
soil matrix, can act as electron donors or acceptors as measured by ORP. The reactivity of
different constituents can lead to oxidizing (positive ORP) or reducing (negative ORP)
environments in groundwater systems. DO in groundwater can act as an oxidizing agent and is
an indicator of redox state.
Cations/Anions
Classification of the geochemical composition of groundwater aids in aquifer characterization
and SCM development. As groundwater flows through the aquifer media, the resulting
geochemical reactions produce a chemical composition that can be used to characterize
groundwater that may differ in composition from groundwater from a different set of lithological
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Marshall Steam Station Ash Basin
and geochemical conditions. This depiction is typically performed using piper diagrams to
graphically depict the distribution of the major cations and anions of groundwater samples
collected at a particular site.
Piper diagrams were generated as part of the CSA to compare geochemistry between ash basin
porewater and ash basin surface water to background monitoring wells, upgradient monitoring
wells, downgradient monitoring wells, seeps, and surface water sample SW-6. 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
3.2.2.2 Redox Potential
As described by McMahon and Chapelle (2008), redox processes affect the chemical quality of
groundwater in all aquifer systems. The descriptions that follow were adapted in whole or in part
from McMahon and Chapelle (2008) and Jurgens et al. (2009). Redox processes can alternately
mobilize or immobilize constituents associated with aquifer materials (Lovley et al. 1991;
Smedley and Kinniburgh 2002), contribute to degradation of anthropogenic contaminants
(Korom 1992; Bradley 2000, 2003), and can generate undesirable byproducts such as dissolved
manganese (Mn2+), ferrous iron (Fe 2+), hydrogen sulfide (1-12S), and methane (CH4) (Back and
Barnes 1965; Baedecker and Back 1979; Chapelle and Lovley 1992). Using data from the
National Water -Quality Assessment (NAWQA) Program, researchers from the USGS developed
a framework to assess redox processes based on commonly measured water quality
parameters (McMahon and Chapelle 2008; Jurgens et al. 2009). The redox framework allows
the state of a groundwater sample and dominant type of redox reaction or process occurring to
be inferred from water quality data. An implementation of this framework is provided in the
USGS "Excel® Workbook for Identifying Redox Processes in Ground Water' (Jurgens et al.
2009), which is detailed in USGS Open File Report 2009-1004. The primary aquifer system in
western North Carolina is considered to be of the New England, Piedmont and Blue Ridge type
and is representative of crystalline -rock aquifers (McMahon and Chapelle 2008).
Precise identification of redox conditions in groundwater can be difficult to determine because
groundwater is commonly not in redox equilibrium and multiple redox conditions may exist
simultaneously as groundwater progresses from more oxygenated (i.e., oxic) states to more
reduced states (i.e., anoxic). Redox reactions express thermodynamic equilibrium conditions
(i.e., an ultimate state). However, the time required for reactions to reach equilibrium (i.e.,
reaction rate kinetics) cannot be determined from the equilibrium state. Thus, it is not unusual
for differences in implied or measured redox conditions to exist between different wells (spatial
differences) or over time at an individual well (temporal differences). Transient disturbances
attributable to well construction may also alter groundwater composition. For example, pH and
other compositional properties of groundwater samples may vary widely immediately following
well construction and gradually become more consistent over time.
In addition, groundwater samples are often mixtures of water from multiple flow layers that may
have different redox conditions. Consequently, mixing within the well bore can produce
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chemistry results that suggest multiple redox conditions. Recognizing these limitations,
researchers have classified groundwater on the basis of a predominant redox process or
terminal electron accepting process (TEAP) using concentrations of redox sensitive species
(Chapelle and others, 1995; Christensen and others, 2000; Paschke and others, 2007;
McMahon and Chapelle 2008).
Redox conditions are generally facilitated by microorganisms, which gain energy by transferring
electrons from donors (usually organic carbon) to acceptors (usually inorganic species)
(McMahon and Chapelle, 2008). Because some electron acceptors provide more energy than
others, electron acceptors that yield the most energy are utilized first and species that yield less
energy are utilized in order of decreasing energy gain. This process continues until all available
donors or acceptors have been used. If carbon sources are not a limiting factor, the
predominant electron acceptor in water will usually follow an ecological succession from
dissolved oxygen (02), to nitrate (NO3 ), to manganese (IV), to iron (III), to sulfate (S042-), and
finally to carbon dioxide (CO2(g)) (Table 3-3).
Although some redox processes overlap as groundwater becomes progressively more reduced,
there is usually one TEAP that dominates the chemical signature. Consequently, concentrations
of soluble electron acceptors (02, NO3 , S042-) and TEAP end products (Mn(II), Fe(II), H2S(g),
CH4(g)) can be used to distinguish between redox processes. The redox evaluation approach
uses these commonly measured constituents in conjunction with concentration thresholds
applicable to groundwater quality investigations. Although most water quality studies analyze for
total dissolved manganese and iron rather than the speciated forms of these elements, in
samples that have been filtered (:50.45 micron and acidified), total dissolved concentrations are
generally accurate estimates of Mn(II), Fe(II) above the threshold concentrations (50 and 100
pg/L , respectively) for pH ranges normally found in ground water (6.5-8.5 SU) (Kennedy et al.
1974; Hem, 1989). At lower pH values there is a greater likelihood that dissolved concentrations
are equal to the primary species of interest: Mn(II), Fe(II).
The USGS redox framework was applied to groundwater measurements from different
environments across the MSS site. Speciation measurements were performed for arsenic,
selenium, chromium, iron, and manganese at select locations. Samples were collected using
0.45 micron filters and analyzed for total and dissolved metals. Other field measurements were
recorded including DO, ORP, temperature, pH, specific conductance, and turbidity. DO, nitrate
as nitrogen, manganese(II), iron(II), sulfate, and sulfide measured at the site were used as
inputs to the redox workbook for monitoring wells. For the purpose of this redox assessment,
nitrate was assumed to be equal to the reported nitrate/nitrite concentration (i.e., 100% nitrate).
Similarly, manganese(II) was assumed to equal the reported dissolved manganese
concentration.
The redox state of the MSS site was evaluated based on 91 samples from the study area for
which all six constituents (DO, nitrate as nitrogen, manganese, iron, sulfate, and sulfide) were
available, including porewater and groundwater. Based on site measurements, the primary
redox categories were determined to include oxic, suboxic, mixed (oxi-anoxic), mixed (anoxic),
and anoxic conditions. Conditions across samples were heterogeneous (i.e., there were wide
ranges of DO and other parameters). At MSS, DO levels exceeded the threshold of 0.5 mg/L in
66 of 91 samples (72%) and predominant redox processes are oxygen reduction with iron or
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manganese oxidation (i.e., controlled by 02 and Fe(III)/Fe(II) or Mn(IV)/Mn(II) couples). Under
these conditions, more oxidized species As(V), Se(VI), and Mn(IV) would be expected. There
were 18 wells from which porewater samples were collected and 17 of those samples are
classified as anoxic or mixed (oxi-anoxic) and one sample classified as suboxic. There is an
increased potential for reduced forms of metals to occur under anoxic or mixed conditions.
However, it should be noted that 33 of the 73 (-45%) groundwater samples from wells across
the site are classified as suboxic or oxic categories where reduced species of metals such as
As(III) are less likely to be present.
Table 3-3. Categories and Threshold Concentrations to Identify Redox Processes in Groundwater
Dissolved
Nitrate
Process
Redox
Oxygen
as
Manganese
Iron
Sulfate
Iron/Sulfide
Likely
Category
(mg/L)
Nitrogen
(mg/L)
(mg/L)
(mg/L)
(mass ratio)
Occurring at
(mg/L)
MSS
Oxic (02)
>_0.5
-
<0.05
<0.1
-
Yes
Suboxic
<0.5
<0.5
<0.05
<0.1
-
Yes
(Low 02)
Anoxic,
<0.5
>_0.5
<0.5
<0.1
-
No
NO3
Anoxic,
<0.5
<0.5
>_0.05
<0.1
-
Yes
Mn(IV)
Anoxic,
<0.5
<0.5
-
>_0.1
>_0.5
no data
No
Fe(l I I)/SO4
Anoxic,
<0.5
<0.5
-
>_0.1
>_0.5
>10
Yes
Fe(III)
Mixed,
<0.5
<0.5
-
>_0.1
>_0.5
>_0.3 and
Yes
Fe(Ill)-SO4
:510
Anoxic,
<0.5
<0.5
-
>_0.1
>_0.5
<0.3
No
SO4
Anoxic,
<0.5
<0.5
-
>_0.1
<0.05
No
CH4
Note: Thresholds and concentrations from McMahon and Chapelle (2008) and Jurgen et al. (2009).
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Table 3-4. Field Parameters from MSS CSA
Range of Results for Groundwater Parameters
No. of
pH (std.
Spec.
Diss.
ORP/Redox
Turbidity
Well Locations
Results
unit)
Cond
Oxygen
(mV)
(NTU)
S
m /L
Ash Porewater
19
2.84 -
140 -
0.1 - 2.57
-195.4 -
1.4 -
8.5
12002
405.8
358.2
Ash Porewater (Dry Ash
1
7.88
8308
0.26
-183.6
39.51
Landfill (Phase II))
Background
8
5.7 -
70.9 -
1'9 - 4'9
-45.1 -
6.2 -
10.9
388.3
183.8
413.7
Beneath Ash Basin
18
5.9 -
149.E -
0.1 - 2.8
-148.1 -
3.5 - 45.6
10.7
2120
233
Beneath Ash Basin
7
5.5 - 9.6
46.7 -
0.2 - 2.3
-106.4 -
2.9 - 13.8
(Adjacent to Ash Basin)
380.9
235.4
Beneath Dry Ash Landfill
5
5.4 -
648 -
1..72
14 - 3
-10.2 -
6.03 -
(Phase II)
8.78
1677
189.5
30.8
Downgradient and East of
4.48 -
219.6 -
Ash Basin and Dry Ash
5
6.24
1749
0.5 - 2.9
-5.5 - 597
4.5 - 9.06
Landfill Phase I
Downgradient and
11
5.2 -
16.8 -
0.66 - 5.6
-95.7
3.2 - 29.8
Southeast of Ash Basin
11.24
1295
312.6
Downgradient of Ash Basin
1
5.6
26.8
6.9
242.7
8.8
Upgradient (Compliance
6
5.56 -
20.4 -
3....3
85 - 64
-42 - 325
7.53 -
Well)
7.1
95.2
50.08
Upgradient and Beyond
5.4 -
59.1 -
-97.9 -
2.4 -
Waste Boundary
15
11.9
2593
1.6 - 6.6
286.7
152.4
Ranges for a number of field measurements characterizing aspects of groundwater conditions
outside, downgradient of, and beneath source areas are presented in Table 3-4 above. Those
measurements indicate that pH ranges from 2.8 to 11.9 SU. Background well results indicate
that pH ranges from 5.7 to 10.9 SU. Similarly, pH results for upgradient wells beyond the waste
boundary range from 5.4 to 11.9 SU. In contrast, pH values within ash basin materials range
from 4.9 to 11.9 SU. There is a very wide range of ORP values, spanning ranges that imply
reduced (negative values) to highly oxidized (large positive values) conditions. This both agrees
and contrasts with the redox category assessment. For porewater samples, 15 of 18 samples
have negative ORP values (reducing conditions) and in agreement with the redox assessment
category. ORP values for the remaining three porewater samples are positive (+100 to +450
mV) and indicate oxidizing conditions. For groundwater samples, measured ORP values and
inferred redox conditions are more variable. For the three background well samples, ORP
values (-30 to +4.4 mV) are generally consistent with the inferred redox category of mixed (oxic-
anoxic). In contrast, measured ORP values from non -background monitoring wells ranged from
-148 mV (reducing) to +597 mV (strongly oxidizing), whereas inferred redox conditions were
generally mixed (oxic-anoxic). Positive ORP values measured in samples where mixed (oxic-
anoxic) conditions are inferred suggest that many samples may not be in redox equilibrium.
Solute Speciation
Groundwater samples were characterized in terms of solute speciation to evaluate the
concentrations and ionic composition (oxidation states) of metal ions of primary concern,
including As(III, V), Cr(III, VI), Fe(II, III), Mn(II, IV), and Se(IV, VI). In general, reduced forms of
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metals (i.e., species in lower oxidation states) are more readily transported in groundwater than
those that are more oxidized. At the MSS site, speciation measurements were performed for
samples collected from 25 groundwater and/or porewater monitoring wells, depending on the
analyte. Results for chemical speciation of groundwater are presented in Table 10-16 of the
MSS CSA Report. To provide a general indication of sample composition, the relative
percentage of the reduced specie concentration to the sum of the reduced and oxidized specie
concentration were calculated. These relative percentages express the proportion of the
reduced form metal present in each sample. For these calculations, analyte concentrations
reported as below detection limits were assumed to equal the detection limit. Speciation
measurements at the MSS site vary widely, and are summarized below:
Arsenic speciation was measured in 25 samples. In many cases concentrations of As(III)
and As(V) species were below detection limits and reported with UJ qualifiers, which
indicates the reported concentration is estimated as the laboratory reporting limit. There
were five samples where speciated arsenic concentrations were above reporting limits
and in those samples As(III) was the predominant species, representing 68% of the total
arsenic.
• Chromium speciation was measured in 22 samples. In nine cases, hexavalent chromium
[Cr(VI)] concentrations were below detection limits and reported with U or UJ qualifiers.
In general, with the exception of one sample, Cr(VI) was a small component of total
chromium, comprising approximately 13% of total chromium (ranged from <1 % to 48%
of the total). The one exception was a background well (BG-1 S) were measured Cr(VI)
was calculated as 96% of the total chromium.
In general, Cr(VI) was identified in upgradient and background groundwater samples
with higher concentrations than those collected from porewater wells and groundwater
monitoring wells beneath and downgradient of the on -site source areas.
Iron speciation was measured in 24 samples. Fe(II) was present above detection limits
in 10 samples. For those 10 samples, Fe(I1) comprised 33% of the total but ranged from
6% to 90%. In groundwater, Fe(II) was mainly present downgradient and southeast of
the ash basin.
• Manganese speciation was measured in 25 samples. Mn(II) comprised 81% of total
manganese and ranged from 18% to 100% of the total. In groundwater, elevated
concentrations of Mn(II) were mainly present beneath the central portion of the ash basin
and downgradient and southeast of the ash basin.
Selenium speciation was measured in 25 samples. Se(IV) was present above detection
limits in just one sample. Se(VI) was present above detection limits in five samples. For
the one sample where both Se(IV) and Se(VI) were detected, Se(IV) comprised 55% of
the total. Se(VI) was present beneath the central portion of the ash basin (AB-12D) and
in one deep background well (BG-1 D). The highest concentrations of Se(VI) were
reported in two wells located downgradient and east of the ash basin and dry ash landfill
(Phase 1) (MW-14S and MW-14D).
Given the range of conditions, next steps in the MSS site evaluation process include equilibrium
geochemical speciation evaluation using modeling tools such as PHREEQC (USGS 2013) and
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groundwater and chemical transport modeling. Additional sampling will be needed to
characterize the temporal and spatial characteristics of groundwater composition for the site.
Additional evaluations will also be beneficial to better characterize the kinetics of redox
reactions.
Kd (Sorption) Testing and Analysis
Sorption (Kd) is the removal of solutes from solution onto mineral surfaces (also referred to as
binding). There are three types:
• Adsorption — solutes are held at the mineral surface as a hydrated species.
• Absorption — solutes are incorporated into the mineral structure at the surface.
• Ion Exchange — when an ion becomes sorbed to a surface by changing places with a
similarly changed ion.
These processes result in decreased constituent concentrations and, therefore, the mass of the
constituent as it is removed from groundwater onto the solid material. The effect of these
processes for a particular constituent can be expressed by the sorption coefficient (or partition
coefficient) Kd. Kd relates the quantity of the sorbed constituent per unit mass of solid to the
quantity of the constituent remaining in solution.
Laboratory determination of Kd was performed on 12 site -specific samples of soil, or PWR from
the transition zone. Solid samples were tested in flow through columns to measure the
adsorption of COls at varying concentrations. For the MSS site, 12 column tests and 18 batch
tests were conducted. The methods used by UNCC and Kd results obtained from the testing are
presented in Appendix E. The Kd data were used as an input parameter to evaluate
contaminant fate and transport through the subsurface at the site, as described in greater detail
in Section 4.1.
3.2.3 Source Area Geochemical Conditions
COls will predominantly be attenuated in the groundwater by adsorption and precipitation.
Constituents dissolve while ash receives precipitation and those constituents leach into
groundwater. Mobility of constituents is affected by sorption characteristics of each respective
constituent. Geochemical modeling of COls will provide a better understanding of geochemical
conditions/processes and their effect on COI mobility in groundwater. Geochemical modeling
was not completed as part of this CAP Part 1, but plan for geochemical modeling is discussed in
further detail in Section 4.
3.2.3.1 Ash Basin
Ash within the ash basin was encountered to depths ranging from the ground surface to
approximately 85 feet bgs. Water levels ranged from approximately 2 ft bgs to 24 ft bgs. As a
result, much of the ash within the ash basin is saturated. The soil zone encountered beneath the
ash basin ranged from approximately 5.5 to 76 feet. Generally there is not an unsaturated soil
zone beneath the ash basin to allow for sorption of COls to occur prior to reaching groundwater.
Due to the lack of unsaturated soil beneath the ash basin, COls are likely to leach directly into
groundwater beneath the ash basin. Therefore, sorption may be less, immediately beneath the
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ash basin, allowing COls to readily dissolve in groundwater and become mobile. Pond level
fluctuation also affects COI mobility due to increased dissolution of COls into groundwater thus
increasing COI concentrations with increased pond levels.
Dry Ash Landfill (Phase 11)
Ash in the dry ash landfill was encountered from approximately 2 to 111 feet bgs in borings
advanced during the CSA. The bottom of the dry ash landfill (Phase 11) is between 77 and 111
feet bgs. The soil zone encountered beneath the dry ash landfill (Phase 11) ranged from
approximately 34.5 to 78 feet. An unsaturated soil layer approximately 21 to 23 feet thick was
present beneath portions of the ash landfill. The northeast portion of the ash landfill contained
approximately 2 feet of saturated ash above saturated soil.
In the portion of the ash landfill with saturated ash and no unsaturated soil buffer, COls are
likely to leach directly into groundwater and sorption may be less, immediately beneath the ash,
allowing COls to readily dissolve in groundwater and become mobile. In the portion of the ash
landfill with unsaturated soil beneath ash, it is less likely for COls to leach directly into the
groundwater. Therefore, sorption is more likely to occur, which reduces the ability of COls to
readily dissolve in groundwater and become mobile. However, the highest concentrations for
several COls at the site were identified in groundwater beneath the ash landfill where there is
unsaturated soil beneath the ash. There were also elevated concentrations of COls in the
groundwater beneath the portion of the ash landfill with no unsaturated soil. The elevated COI
concentrations beneath the ash landfill may be a result of COls leaching into groundwater
during construction of the dry ash landfill (Phase 11), which occurred between 1986 and 1999.
Boring logs indicate the landfill is an ash monofill and no interim soil cover was used when
constructing the landfill. An open ash monofill exposed to precipitation is effectively a
mechanism for continuous recharge and leaching of COls from the ash landfill. The elevated
COI concentrations beneath the ash landfill may be in part due to leaching of COls from the ash
placed in the ash landfill during construction.
3.2.3.3 PV Structural Fill
Ash within the PV structural fill was encountered beneath the soil cap system to depths ranging
from approximately 28 to 71 ft bgs. The borings advanced through the PV structural fill within
the ash basin waste boundary indicated a soil fill layer approximately 2 to 5 feet thick was
beneath the structural fill and on top of the ash basin in these areas. These borings also
indicated there was saturated soil beneath the ash layer of the ash basin. The borings advanced
through the portions of the structural fill that were constructed above residual soil (SB-4, SB-5,
SB-8) indicated there was an approximately 1 to 10 foot thick layer of unsaturated soil/saprolite
beneath structural fill ash, which inhibits COls from leaching directly into groundwater. However,
the locations (similar to the ash basin) with no unsaturated soil beneath ash allow for less
sorption and a higher likelihood of COls leaching directly to groundwater.
Dry Ash Landfill (Phase 1)
No borings were advanced within the footprint of the dry ash landfill (Phase 1) during the CSA.
However, one shallow (AL-1S) and one deep monitoring well (AL-1D) were installed
immediately east and downgradient of the dry ash landfill (Phase 1). The depth to groundwater
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measured in the shallow well during the CSA was approximately 36 ft bgs. This indicates a high
likelihood of an unsaturated soil buffer located beneath the dry ash landfill (Phase 1). Similar to
COI concentrations observed in groundwater beneath the dry ash landfill (Phase 11), elevated
COI concentrations were reported in shallow and deep monitoring wells located immediately
east and downgradient of the dry ash landfill (Phase 1). The ash basin may be contributing to the
elevated COI concentrations in this area of the site.
3.2.4 Mineralogical Characteristics
Soil and rock mineralogy and chemical analyses completed to date are sufficient to support
evaluation of geochemical conditions. Soil mineralogy and chemistry results through July 31,
2015 were presented in the CSA Report.
The dominant minerals in the soils are quartz, feldspar (both alkali and plagioclase feldspars),
kaolinite, muscovite/illite, and biotite. Three samples (MS-02, MS-06, and AB-613R) reported
tremolite from 15.3 to 32.2 (wt %). Other minerals identified include vermiculite, hydroxyapatite,
hematite, ilmenite, magnetite, mullite, and amorphous materials (that contain smectites,
amorphous, mica, and/or amorphous iron oxide/hydroxide). The major oxides in the soils are
Si02 (51.15% - 68.78), A1203 (11.49% - 25.51 %), and Fe203 (2,67% - 10.04%). MnO ranges
from 0.02% to 0.11%. The dominant minerals in the transition zone are quartz, feldspar (both
alkali and plagioclase feldspars), biotite, and amphibolite. The major oxides in the transition
zone are Si02 (52.92% - 57.81 %), A1203 (16.53%-19.15%), and Fe203 (6.51 % - 10.51 %). MnO
ranges from 0.09% to 0.15%. The major oxides in the rock samples are Si02 (50.83% -
62.09%), A1203 (10.93% - 20.79%), and Fe203 (4.36% - 8.63 %). MnO ranges from 0.06% to
0.11% in the rock samples.
These highly weathered Piedmont soils, saprolite, and rock contain high percentages of clay
minerals and hydrous metal oxides and oxyhydroxides. These geologic materials are very fine-
grained and have a large surface area compared to their volume. They are also chemically
reactive, and the attenuation of inorganic compounds by clays and oxides has been a subject of
intense study for over 100 years. The abundant clay content of the soils and host rock
lithologies suggests much of the COI concentrations in the ash basin and ash storage areas
may be attenuated by these materials.
Soil formation typically results in the loss of common soluble cations and the accumulation of
quartz and clay. Feldspars are hydrolyzed to clays. The natural concentration of COls by
weathering and soil development on the lithologies noted above are negligible other than for a
potential increase in vanadium and cobalt from diabase weathering. Soil chemistry results do
not show marked deviation from normal crustal abundances at MSS. Accordingly, the residual
soils do not appear to contribute significantly to COI exceedances in soils at the site.
Correlation of Hydrogeologic and Geochemical
Conditions to COI Distribution
Source characterization COls identified in ash, ash porewater and ash basin surface water
during the CSA serve as indicators of constituents that could impact other media, such as
underlying soil, groundwater, seeps and surface water. A review of the groundwater sampling
results from the CSA show the following constituents were detected at concentrations
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exceeding their PPBCs (Section 2.1.2) or their respective 2L Standards, IMACs or NC DHHS
HSL in monitoring wells located beneath or downgradient of the source areas: antimony,
arsenic, barium, beryllium, boron, chloride, chromium, hexavalent chromium, cobalt, iron,
manganese, selenium, sulfate, thallium, TDS, and vanadium. The source areas, other MSS site
features, and areas where COls exceeded their PPBCs and applicable regulatory standards or
criteria in groundwater beneath and downgradient of the source areas, are illustrated on Figure
3-1 and in cross -sectional view on Figures 3-2.1 and 3.2.4.
Horizontal migration of COls is evident with groundwater flow direction at the site. From beneath
the dry ash landfill (Phase 11), concentrations decrease as COls migrate toward the central
portion of the ash basin. COls that are present in groundwater east and downgradient of the ash
basin and dry ash landfill (Phase I) are present in the surface water sample (SW-6) collected
from the downgradient unnamed tributary that flows to Lake Norman. Also, several COls were
reported in groundwater between the ash basin dam and Lake Norman. Vertical migration of
COls observed in select well clusters (S, D, and BR) indicates groundwater impacted by the on -
site areas is primarily limited to the shallow and deep flow layers with the exception of the
underlying fractured bedrock beneath the dry ash landfill (Phase 11).
Cobalt, iron, manganese, pH, and vanadium were the COls with the most widespread
exceedances beneath source areas, downgradient of the source areas, upgradient of the
source areas, and in background monitoring well locations. These constituents are naturally
occurring in soil and groundwater. Concentrations of iron and manganese are highly pH
dependent. Groundwater and geochemical conditions promote the mobility of vanadium across
the site with contribution likely from naturally occurring vanadium.
As a result of constituents leaching from the ash basin and dry ash landfill units (Phase I and 11),
and geochemical processes taking place in groundwater and soil beneath the site, several COls
exceeded their PPBCs and/or 2L Standards, IMACs or NC DHHS HSL, and are listed below for
each area at the site.
Beneath the Ash Basin: antimony, boron, chloride, cobalt, iron, manganese, TDS, and
vanadium.
Beneath the Dry Ash Landfill (Phase 11): barium, boron, chromium, cobalt, iron, manganese,
selenium, sulfate, TDS, and vanadium.
Downgradient and East of the Ash Basin and Dry Ash Landfill (Phase 1): beryllium, boron,
chloride, chromium, hexavalent chromium, cobalt, manganese, thallium, TDS, and vanadium.
Downgradient of the Ash Basin Dam: arsenic, boron, hexavalent chromium, cobalt, iron,
manganese, thallium, TDS, and vanadium.
Refinement of the SCM will continue to evolve as additional data become available during
supplemental site investigation activities.
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Modeling
Groundwater flow and fate and transport, and groundwater to surface water models were
conducted to evaluate COI migration and potential impacts following closure of the ash basin
and ash storage areas at the MSS site.
Groundwater Modeling
Under the direction of HDR, UNCC developed a site -specific, 3-D, steady-state groundwater
flow and fate and transport model for the MSS site using MODFLOW and MT3DMS. The model
was developed in accordance with NCDENR DWQ's Groundwater Modeling Policy dated May
31, 2007. The groundwater flow and fate and transport model is based on the SCM presented in
Section 3 and incorporates site -specific data obtained during the CSA. The objective of the
groundwater modeling effort was to simulate steady-state groundwater flow conditions for the
MSS site, and simulate transient transport conditions in which COls enter groundwater via the
source areas over the period they have been in service. These model simulations serve to:
• Predict groundwater elevations in the ash and underlying groundwater flow layers for the
proposed closure scenarios
• Predict concentrations of the COls at the compliance boundary or other downgradient
locations of interest over time
• Estimate the groundwater flow and constituent loading to Lake Norman and the
unnamed tributary that flows to Lake Norman
The area, or domain, of the simulation included the MSS site, source areas, and areas of the
site that have been impacted by COls above 2L Standards, IMACs, or NC DHHS HSL. Note
that modeling took a conservative approach by not incorporating wells in which a given
constituent was reported below the 2L Standard, IMAC, or NC DHHS HSL. The UNCC
Groundwater Flow and Transport Model report is included in Appendix C.
Model Scenarios
The following groundwater model scenarios were simulated for the purpose of this CAP Part 1:
• Existing Conditions: assumes current site conditions with ash sources left in place.
• Cap -in -Place: assumes ash remaining in source areas is covered by an engineered cap.
• Excavation: assumes removal of ash from the ash basin (outside the footprints of the PV
structural fill and dry ash landfill units).
Model scenarios used steady-state groundwater flow conditions established during flow model
calibration and transient transport of COls identified in Sections 2 and 3 for further analysis.
Each COI was modeled individually using the transient transport model over a 250-year
simulation period. This time period was selected as the model duration boundary condition to
assess changing COI concentrations over time at the compliance boundary. The rate of natural
attenuation is then described over the model simulation period.
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Calibration of Models
The groundwater flow model was calibrated to steady-state flow conditions using water level
measurements taken at the site during July 2015 in shallow, deep, and bedrock wells. Transient
transport of each COI was calibrated to groundwater water quality samples collected in July
2015. Only COI concentrations above the 2L Standards, IMACs, or NC DHHS HSL were used
for model calibration purposes by introducing a constant source for each COI at the start of ash
basin operations and running the model until July 2015. A detailed account of the flow and
transient transport model calibration process is included in Appendix C. Ranges of measured
hydrogeologic properties from the CSA were used as a guide for selecting model input
parameters during calibration. The groundwater flow model was calibrated by adjusting model
parameters within the upper and lower bounds of measured hydrogeologic parameters at the
site, including:
• The hydraulic conductivity distribution of each flow layer within the basin (e.g., ash, dike,
upper unconsolidated zone, transition zone, and fractured bedrock zone)
• The infiltration rate applied to the water table within the ash basin system
• The net infiltration due to precipitation applied to other areas of the site
• The variation of measured porewater COI concentrations
• The effective porosity of each model layer
• The Kd value for each COI
Calibration results indicate the model adequately represents steady-state groundwater flow
conditions at the site and meets transport calibration objectives. The calibrated flow and
transport model third -party peer review team was coordinated by EPRI and included Dr.
Chunmiao Zheng from the University of Alabama, James Rumbaugh from Environmental
Simulations, Inc, and experienced modelers from Intera, Inc. The EPRI review included the
arsenic and boron transport calibrations, which represent a sorptive and non-sorptive COI,
respectively. The reviewers were provided the MSS CSA Report, a MSS draft model report, and
digital model input and output files, allowing them to reconstruct the model for independent
review. During the course of the review, the reviewers communicated with the modelers in order
to better understand how the model was developed and calibrated. As a result of these
communications, the model was modified and recalibrated, which allowed the reviewers to
conclude that the model was constructed and calibrated sufficiently to achieve its primary
objective of comparing the effects of closure alternatives to nearby groundwater quality. The
initial model that was reviewed and deemed sufficient by EPRI as discussed in the November
11, 2015 model review memorandum (Appendix C) was revised on November 30, 2015. The
revised model was reviewed by EPRI and an additional review memorandum was issued to
Duke Energy on December 2, 2015 deeming the model sufficient to achieve its primary
objective with the limitations discussed (Appendix C).
4.1.3 Kd Terms
COls enter the ash basin system in both dissolved and solid phases. In the ash basin system,
constituents may undergo phase changes including dissolution, precipitation, adsorption, and
desorption. Dissolved phase constituents may undergo these phase changes as they are
transported in groundwater flowing through the basin. Phase changes (dissolution, precipitation,
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adsorption, and desorption) are collectively taken into account by specifying a linear
soil -groundwater sorption coefficient (Kd). In the fate and transport model, the entry of
constituents into the ash basin, dry ash landfill units, and PV structural fill is represented by a
constant concentration in the saturated zone (i.e., porewater) of the basin, and is continually
depleted by infiltrating recharge from the surface.
As previously discussed in Section 3.2.2.4, laboratory Kd terms were developed by UNCC via
column testing of 12 site -specific samples of soil, or PWR (from the transition zone). The
methods used by UNCC and Kd results obtained from the testing are provided in UNCC's Soil
Sorption Evaluation report (Appendix D). The Kd data were used as an input parameter to
evaluate COI fate and transport through the subsurface at the site. Note that Kd characteristics
were each represented by an isotherm from which the sorption coefficient Kd, with units of
ml/gram, is calculated. Sorption studies on soil samples obtained during the CSA indicate that
the COI Kds for background soils surrounding the ash basin and ash storage areas are higher
than the values used in modeling; COI Kds were adjusted (typically lowered) in the model to
facilitate movement of COls within the model.
Flow and COI Transport Model Sensitivity Analysis
The groundwater model, calibrated for flow and constituent fate and transport under existing
conditions, was applied to evaluate closure scenarios. Being predictive; these simulations
produce flow and transport results for conditions that are beyond the range of those considered
during the calibration. Thus, the model should be recalibrated and verified over time as new
data become available in order to improve model accuracy and reduce uncertainty.
The model domain developed for existing conditions was applied without modification for the
Existing Conditions and Cap -In -Place scenarios. The Existing Conditions scenario is used as a
baseline for comparison to other scenarios. The assumed recharge was modified and the
constant source concentration was depleted in the Cap -in -Place and Excavation scenarios. In
the Cap -in -Place scenario, recharge within the ash basin was reduced from 4.5 in/year to 0
in/year to represent an impermeable cap system.
For the Excavation scenario, all ash was removed from the ash basin outside the footprints of
the dry ash landfill units and the PV structural fill. The flow parameters for this model were
identical to the Existing Conditions model except for the removal of ash -related layers, with the
same recharge applied to the ash basin as the remainder of the site.
Sensitivity of the groundwater flow model was evaluated by varying key model assumptions by
a percentage above and below their respective calibration values and calculating the normalized
root mean square error (NRMSE) for comparison with the calibration value (Appendix C).
Based on this approach, the groundwater flow model was most sensitive to varying recharge
outside the ash basin, followed by recharge within the ash basin, vertical hydraulic conductivity
of the shallow zone, then horizontal hydraulic conductivity of the shallow zone. The model was
least sensitive to vertical hydraulic conductivity within the transition zone.
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4.1.4.1 Existing Conditions Scenario.
The Existing Conditions scenario served as the basis of comparison to the Cap -in -Place and
Excavation scenarios. No changes in flow are seen in this scenario in the flow model results.
This scenario represents the most conservative conditions in terms of groundwater
concentrations on- and off -site, and COls reaching the compliance boundary, and discharging to
Lake Norman.
Cap -in -Place Scenario
The Cap -in -Place scenario simulated placement of an engineered geosynthetic cap by applying
a recharge rate of zero to the source areas. Groundwater flow is affected by this scenario as the
water table is lowered and groundwater velocities may be reduced beneath the capped areas.
4.1.4.3 Excavation Scenario
The Excavation scenario simulated the removal of all ash from the ash basin outside the
footprints of the dry ash landfill units and the PV structural fill. In the model, the constant
concentration sources and all ash above and below the water table were made inactive. This
scenario assumes recharge rates in the ash basin become equal to rates surrounding the basin.
The recharge rates for the dry ash landfill units and PV structural fill remain the same as the
Existing Conditions scenario.
Fate and Transport Model
Each model scenario provides simulation of groundwater concentrations over time. The model
does not account for background COI concentrations.
The Existing Conditions and Cap -In -Place scenarios were modeled over a 250-year time period
and the Excavation scenario was modeled over a 100-year time period.
Additional modeling to determine the timeframe within which COI concentrations would drop
below the 2L Standards, IMACs, or NC DHHS HSL was not performed due to CAP Part 1 time
constraints. To better understand the movement and concentrations of COls, Figures 13
through 201 in the Groundwater Flow and Transport Model in Appendix C are provided to show
concentration isocontours for all COls 100 years into the simulation periods. The 100-year mark
was selected to provide the reader a snap -shot of results showing increases or decreases in
COI concentrations and movement of COI plumes over a significant period of time. Note that
there is no specific relationship to 100-year or 250-year analysis periods.
COls from Section 2 that were evaluated in the fate and transport model include: antimony,
arsenic, barium, beryllium, boron, chloride, chromium, hexavalent chromium, cobalt, selenium,
sulfate, thallium, and vanadium. Note that iron, manganese, pH, and TDS were not included
because they cannot be adequately modeled using MODFLOW/MT3DMS.
Iron, manganese, pH, and TDS will be evaluated with the geochemical modeling to be
completed and submitted in the CAP Part 2 report. The geochemical model results, taken into
consideration with the groundwater flow, fate, and transport and surface water -groundwater
models enhance the understanding of the processes taking place in the subsurface and will
ultimately aid in choosing an appropriate corrective action for the site.
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The following sections provide a high-level overview of the modeling results for the respective
scenario. Refer to Appendix C for detailed discussion on the model results.
Existing Conditions
The Existing Condition scenario used the calibrated groundwater flow and transport model. The
time to achieve a steady-state COI concentration depends on where the particular COI plume is
located relative to the compliance boundary, its loading history, and if it is sorptive or non-
sorptive. Source areas close to the compliance boundary reach steady-state concentrations
sooner than those farther away. Sorptive COls are transient at a rate that is less than the
groundwater pore velocity. Lower effective porosity results in shorter time periods to achieve
steady-state concentrations for both sorptive and non-sorptive COls.
The results of the Existing Condition scenario indicated that concentrations of modeled COls
predominately increase or reach steady-state conditions above 2L Standards, IMACs, or NC
DHHS HSL during the 250-year model simulation period.
In 250 years, 9 of 13 constituents were estimated by the model to be above the 2L Standards,
IMACs, or NC DHHS HSL at the compliance boundary or Lake Norman.
Cap -In -Place
The Cap -In -Place scenario simulates the effects of capping the source areas at the beginning of
the scenario (i.e., Year 2015) and has a 250-year model period. In the model, recharge and
source area concentrations were set to zero. Under this scenario, groundwater flow rates are
lower (compared to the Existing Conditions scenario) due to reduced groundwater velocities
caused by the reduction in recharge and the reduction of the groundwater table beneath the
capped areas. The model indicates that the water table within the downgradient portion of the
ash basin is lowered by approximately 14 feet. The water table is lowered by approximately 2
feet, 3 feet, and 7 feet beneath the dry ash landfill (Phase 1), dry ash landfill (Phase 11), and the
PV structural fill, respectively.
Under the Cap -In -Place scenario, concentrations of all modeled COls exceeded their respective
2L Standards or IMACs at the compliance boundary within 100 years after the start of the
simulation period, with the exception of chloride, hexavalent chromium, and selenium. Chloride,
hexavalent chromium, and selenium concentrations decrease to below their 2L Standard or NC
DHHS HSL (for hexavalent chromium only) within 100 years.
Excavation
The Excavation scenario simulates the effect of removing ash from the ash basin outside the
footprints of the dry ash landfill units and the PV structural fill at the beginning of the scenario
(i.e., Year 2015). In the model, ash basin concentrations are set to zero while recharge to the
excavation area is applied at the same rate as the surrounding area. Source area
concentrations remain the same as the Existing Conditions scenario for the dry ash landfill units
and PV structural fill, as well as the portion of the ash basin that is located beneath the dry ash
landfill (Phase 11) and PV structural fill. Groundwater flow beneath the site is affected by this
scenario as saturated ash located within the ash basin is drained.
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Under the Excavation scenario, concentrations of non-sorptive COls chloride and sulfate, as
well as sorptive COls antimony, barium, beryllium and selenium, decrease to below their 2L
Standard or IMAC at the compliance boundary at the end of the 100-year model period. The
remaining COls exceeded their respective 2L Standard, IMAC, or NC DHHS HSL at the end of
the 100-year model period.
Key Model Assumptions
The key model assumptions and limitations of the fate and transport model include, but are not
limited to, the following:
• The steady-state flow model was calibrated to hydraulic heads measured at monitoring
wells in July 2015 and considered the ash basin surface water level. The model is not
calibrated to transient water levels over time, recharge, or river flow. A steady-state
calibration does not consider changes in groundwater storage and groundwater flux to
surface water with time.
• MOFLOW simulates flow through porous media, and groundwater flows in the bedrock
flow layer via fractures in the bedrock. A single domain MODFLOW modeling approach
for simulating flow within the groundwater flow layers and bedrock was used for
contaminant transport.
• The model was calibrated by adjusting the constant source concentrations at the ash
basin and ash storage areas to reasonably match COI concentrations from the CSA
Report. This model assumption was utilized as it reflected a simplified and more
conservative approach to meet initial modeling requirements of CAMA.
• For the purposes of numerical modeling and comparing closure scenarios, it is assumed
that the selected closure scenario will be completed in Year 2015.
• Predictive simulations were performed and steady-state flow conditions were assumed
from the time that the ash basin and ash storage areas were placed in service through
the current time until the end of the predictive simulations (Year 2265).
• COI source area concentrations at the ash basin and ash storage areas were applied
uniformly within each source area and assumed to be constant with respect to time for
transport model calibration.
• The uncertainty in model parameters and predictions has not been quantified, and
therefore the error in the model predictions is not known. It is assumed the model results
are suitable for a relative comparison of closure scenarios.
• Since Lake Norman is modeled as a constant head boundary in the numerical model, it
is not possible to assess the effects of pumping wells or other groundwater sinks near
this boundary.
• The model does not account for varying geochemical conditions such as pH and redox
potential that could affect COI mobility and change modeling results. As mentioned
above, site -specific geochemistry and geochemical modeling will be considered in CAP
Part 2.
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Note that refinements to the groundwater model are currently being conducted and will be
provided in the CAP Part 2 report. Refer to Appendix C for additional details regarding model
assumptions.
Proposed Geochemical Modeling Plan
Data obtained during the CSA and subsequent interpretation, determination of groundwater
flow, and fate and transport modeling have resulted in improvements to the SCM to enhance
planned geochemical modeling. It has been determined that:
• Site -specific groundwater flow and groundwater fate and transport is as hypothesized; it
is operating within the regional flow and transport process in Piedmont Physiographic
Provence.
• Kd values can be calculated from batch adsorption isotherms for all COls at the site.
• The groundwater model can be constrained by site -specific Kd values.
• Kd values within the observed site -specific range (determined in the laboratory) were
used successfully to improve fate and transport model calibration.
• The dominant attenuation processes, as initially hypothesized, are adsorption to hydrous
metal oxides (HFO, HMO, HAO) and clay minerals. Hydrous metal oxides and clay
minerals are abundant in the soil and transition zone, and it is assumed that
concentrations increase with the degree of bedrock weathering.
• Correlations exist between COI concentrations and HFO, HMO, HAO, and clay minerals.
• There is variability in pH and redox conditions across the site; significant enough that pH
and redox influences on COI attenuation should be evaluated. The binding of COls to
HFO, HMO, HAO and clay minerals is known to be pH and redox sensitive. Under
certain redox and pH conditions HFO, HMO, and HAO may be stable, may dissolve, or
may actively precipitate. Clay mineral sorption is sensitive to pH and ionic strength (for
example TDS).
Sensitivity analysis of COI attenuation under variable conditions is warranted. As previously
discussed in the CSA, the appropriate manner to conduct the sensitivity analysis for pH, Eh, and
TDS is the use of geochemical models. The following sensitivity analysis will be conducted to
support the Kd values used in fate and transport modeling: Mineralogical Stability, Spatial
Variability in Retardation, and COI Adsorption under Variable pH and Redox Conditions.
Given this range of conditions, next steps in the evaluation process may include: equilibrium
geochemical speciation evaluation using modeling tools such as PHREEQC (USGS 2013) and
groundwater and chemical transport modeling. Additional sampling will be needed to further
characterize the temporal and spatial characteristics (i.e., redox reactions) of groundwater
composition for the site.
Mineral Stability: Pourbaix plots for HFO, HMO, and HAO and Observed Clay Minerals (OCM)
will be created using site -specific chemistry to determine if minerals are stable under observed
and postulated conditions.
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Spatial Variability in Retardation: Spatial variability in attenuation capacity and strength will
be evaluated using geographic information systems (GIS) and the retardation equation (see
Appendix C). Retardation represents the combined attenuation effect of reactive area (porosity
and bulk density) and Kd. Use of GIS allows overlay of a retardation function at multiple depths
and evaluation of correlation and sensitivity to other measured parameters such as HFO or clay
mineral content.
COI Adsorption under Variable pH and Redox Conditions: The dominant attenuation
processes are highly sensitive to pH and redox values and variability. Sensitivity will be
evaluated by:
• Using PHREEQC (USGS 2013) to determine the redox and pH changes that take place
under source term conditions of capping (cessation of oxygen delivery by recharge and
adjustment to a new dynamic equilibrium, draining and change in water/rock ratio).
These results will be used to determine if there are changes in leachate chemistry, and if
so, if the changes in leachate chemistry affect mobility outside the ash.
• Under the observed variability in pH and redox, and postulated changes in pH and
redox, evaluate the sensitivity of Kd to these conditions. With quantitative mineralogy and
reactive surface area inputs site -specific sample attenuation can be simulated in
PHREEQC using surface complexation subroutines. Surface complexation is analogous
to Kd, but allows the variability of pH and TDS on adsorption to be modeled.
PHREEQC will also be used to calculate redox conditions and speciation. The output of
PHREEQC simulations on the effect of change on surface sorption properties will be
used to determine what the expected distribution of species (e.g., As(III)/As(V)) would be
under those same changed conditions.
Groundwatei Surface Water Interaction Modeling
Groundwater -surface water interactions were completed using groundwater model output and a
surface water mixing model approach to evaluate potential surface water impacts of COls in
groundwater as they discharge to adjacent surface water bodies.
Mixing Model Approach
Groundwater model output from the fate and transport modeling discussed in Section 4.1 (i.e.,
groundwater volume flux and concentrations of groundwater COls were used as inputs for the
surface water assessment in the receiving surface water bodies adjacent to the MSS site. The
MSS is located on a semi -enclosed arm of Lake Norman. Although several small streams
discharge to the lake arm, natural surface -water inflow is likely minor and intermittent. However,
the MSS plant withdraws condenser non -contact cooling water from the head -end of the lake
arm and discharges via a canal to a different arm of Lake Norman, which will induce a relatively
uni-directional flow of approximately 1,093 MGD (1,691 cfs) past the MSS groundwater
discharge. Figure 1 (of Appendix E) presents the MSS study area and intake and discharge
flow locations from and to Lake Norman. A mixing -zone calculation was used to assess
potential surface water quality impacts when groundwater migrating from the MSS merges with
this induced flow in the lake arm. Wind -driven circulation will also induce groundwater
dispersion in surface waters adjacent to the MSS, but the lake arm steady flow induced by
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intake of non -contact condenser cooling water was used in the mixing calculations. A summary
of this approach and NCDEQ's mixing zone regulations is presented below.
• Mixing Model Approach — This approach includes the effects of upstream flow on mixing
and dilution of the groundwater plume within an allowable mixing zone. The results from
this analysis provide information on constituent concentration as a function of the mixing
zone distance from the groundwater input to the adjacent water body.
• Mixing Zone Regulations —A mixing zone is defined in the NCDEQ water quality
standards (Subchapter 213, Section .0100) as "a region of the receiving water in the
vicinity of a discharge within which dispersion and dilution of constituents in the
discharge occurs and such zones shall be subject to conditions established in
accordance with 15A NCAC 213.0204(b)".
• Additional details on mixing zones provided in 15A NCAC 2B .0204(b) are as follows:
A mixing zone may be established in the area of a discharge in order to provide
reasonable opportunity for the mixture of the wastewater with the receiving
waters. Water quality standards shall not apply within regions defined as mixing
zones, except that such zones shall be subject to the conditions established in
accordance with this Rule. The limits of such mixing zones shall be defined by
the division on a case -by -case basis after consideration of the magnitude and
character of the waste discharge and the size and character of the receiving
waters. Mixing zones shall be determined such that discharges shall not:
o Result in acute toxicity to aquatic life has defined by Rule .0202(1)] or prevent
free passage of aquatic organisms around the mixing zone;
o Result in offensive conditions;
o Produce undesirable aquatic life habitat or result in a dominance of nuisance
species outside of the assigned mixing zone; or
o Endanger the public health or welfare.
Although the NCDEQ mixing zone regulations are typically applied to point source discharges,
the "free zone of passage" provision in the regulation was used in this surface water
assessment. Mixing zone regulations apply more typically to time -variable naturally flowing
systems such as rivers and streams, and the regulations incorporate statistically derived design
flows (e.g., 1Q10, 7Q10) to characterize the irregularity of natural flows. These fundamental
concepts can be applied to a regulated, quasi -steady flow. Mixing zone sizes and percentages
of upstream river flows used for assessing compliance with applicable water quality standards
or criteria as presented in Section 4.2.2 are provided below in Table 4-1.
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Table 4-1. Mixing Zone Sizes and Percentages of Upstream River Flows
Criteria
Mixing Zone Size
Percent of Design River Flow'
Acute Aquatic Life
10% of Average Lake Arm Width
10% of flow
or 50 feet
Chronic Aquatic Life
50% of Average Lake Arm Width
50% of flow
or 250 feet
Human Health/Water
50% of Average Lake Arm Width
50% of flow
Supply (non -carcinogen)
or 250 feet
Human Health/Water
100% of Average Lake Arm Width
100% of flow
Supply (carcinogen)
500 feet
Notes:
1. Lake arm flow past the MSS site induced by intake of non -contact condenser cooling water.
Using the mixing zone approach, output from the groundwater model (e.g., flows and COI
concentrations) was used in the mixing calculation to determine COI concentrations in the
adjacent water body from the point of discharge. These surface water results were compared to
applicable surface water quality standards or criteria to evaluate compliance at the edge of the
mixing zone(s).
1.2.2 Surface Water Model Results
The calculated surface water COI concentrations in Lake Norman adjacent to and downstream
of the MSS site are presented below in Table 4-2. The design flows, upstream surface water
concentrations, groundwater flows, and groundwater COI concentrations presented in
Appendix E were used to complete these calculations.
The mixing model results indicate that COI concentrations do not exceed surface water quality
standards at the edge of the mixing zones in Lake Norman. In addition, this water quality
assessment can be considered conservative because the groundwater that mixes with the non -
contact cooling water withdrawn into the MSS and discharged to the downstream lake arm
discharge canal will be mixed with the entire non -contact cooling water flow not just the fraction
used in the mixing model calculations.
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Table 4-2. Lake Norman Calculated Surface Water Concentrations
Calculated Mixing Zone Conc. (pg/L)
Water Quality Standard (pg/L)
COI
Acute
Chronic
HH / WS
Acute
Chronic
HH / WS
Arsenic
0.289
0.258
0.254 (c)
340
150
10 / 10
Barium
4.50
4.30
4.30 (nc)
ns
ns
200,000 / 1,000
Beryllium
0.102
0.100
0.100*
65
6.5
ns / ns
Boron
26.8
25.4
25.2*
ns
ns
ns / ns
Chloride
447
409
409 (nc)
ns
ns
ns / 250,000
Total Chromium
0.255
0.251
0.251*
ns
ns
ns / ns
Chromium VI
0.250
0.250
0.250*
16
11
ns / ns
Cobalt
0.262
0.252
0.252 (nc)
ns
ns
4/3
Selenium
0.255
0.251
0.251 *
ns
5
ns / ns
Sulfate
803
561
561 (nc)
ns
ns
ns / 250,000
Thallium
0.050
0.050
0.050 (nc)
ns
ns
0.47 / 0.24
Notes:
1. All COls are shown as dissolved fraction except for total chromium, which is total recoverable metal
2. WS — water supply
3. HH — human health
4. c — carcinogen
5. nc — non -carcinogen
6. ns — no water quality standard
7. * — concentration calculated for 100% of induced flow
` Refinement of Models
Groundwater and surface water models have been used to provide further information regarding
the transport of COls toward Lake Norman.
The groundwater model will be further refined in CAP Part 2 to accomplish the following tasks:
• Geochemical modeling will be performed as discussed in Section 4.1.6;
• The groundwater model will be further refined to more rigorously reflect all detectable
and non -detectable COI concentrations from compliance, voluntary, and CSA wells;
• The groundwater model results will be further assessed to identify data gaps that would
improve the conceptual site model;
• The Kd value used for non -conservative COls will be further assessed during refinement
and recalibration of the groundwater model; and
• If necessary, remedial alternatives will be simulated in the groundwater model to
evaluate corrective action(s) at the site.
The groundwater to surface water interaction model will be refined as necessary following
refinement of the groundwater model.
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5 Summary and Recommendations
Based on the data presented herein, and the analysis of these data, Duke Energy provides the
following summary and recommendations:
PPBCs were calculated for soil and groundwater at the site and are presented in
Section 2. Note that for the MSS site, groundwater PPBCs were calculated using
historical groundwater quality data from the NPDES compliance wells MW-4 and MW-
4D, and at the request of NCDEQ, the MSS FGD Residue Landfill monitoring well MS-
10. For groundwater constituents that were not historically analyzed for in the
compliance or landfill wells, groundwater PPBCs were developed from the compliance,
landfill, and CSA background monitoring well results by selecting the highest
concentration (or highest method reporting limit for non -detects) across the range of
concentrations observed for that constituent. Also, at the request of NCDEQ,
groundwater analytical results for samples obtained with turbidity greater than 10 NTU
were removed from the data set prior to establishing PPBCs. PPBCs will be refined as
additional data are obtained from background monitoring wells during subsequent
sampling events. Soil PPBCs were calculated as the 95% upper tolerance limit for soil
constituents. For constituents where there were too few laboratory detections reported to
use the statistical methodology, the PPBCs were established by setting the value equal
to the highest concentration (or the highest method reporting limit for non -detect values).
COls were selected for groundwater fate and transport modeling, in part, based on
constituent concentrations in monitoring wells located beneath and downgradient of the
source areas compared to regulatory standards or criteria and background
concentrations. Data obtained from source and downgradient wells were not eliminated
using the 10 NTU turbidity limit applied to PPBCs, even though the analytical results for
selected constituents can be biased high due to the effects of turbidity. Groundwater
samples collected during the CSA and subsequent monitoring events were analyzed for
total and dissolved phase constituents to evaluate potential effects of turbidity. The list of
COls to be carried forward in CAP Part 2 will be modified, if warranted, as additional
groundwater quality data are obtained and the possible effects of turbidity on the
analytical results are evaluated.
• Geochemical modeling of the MSS site will be completed and submitted in CAP
Part 2. The geochemical model results, taking in to consideration with the groundwater
flow and fate and transport model, as well as the surface water to groundwater
interaction model enhance the understanding of the processes taking place in the
subsurface and will ultimately aid in choosing appropriate corrective action(s) for the site.
The geochemical model is key to understanding mobility of iron, manganese, and TDS
since they cannot adequately be modeled using MODFLOW/MT3DMS.
• The groundwater modeling results are summarized as follows:
o Existing Conditions Scenario — Concentrations of modeled COls
predominately increase or reach steady-state conditions above 2L Standards,
IMACs, or NC DHHS HSL during the 250-year model simulation period.
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o Cap -in -Place Scenario — Concentrations of all modeled COls exceeded their
respective 2L Standards or IMACs at the compliance boundary within 100 years
after the start of the simulation period, with the exception of chloride, hexavalent
chromium, and selenium. Chloride, hexavalent chromium, and selenium
concentrations decrease to below their 2L Standard or NC DHHS HSL within 100
years.
o Excavation Scenario — Concentrations of non-sorptive COls chloride and
sulfate, as well as sorptive COls antimony, barium, beryllium and selenium,
decrease to below their 2L Standard or IMAC at the compliance boundary at the
end of the 100-year model period. The remaining COls exceeded their respective
2L Standard, IMAC, or NC DHHS HSL at the end of the 100-year model period.
• Groundwater flow rates and concentrations of COls from the groundwater model were
used as inputs to a groundwater -surface water interaction model to determine if 2L
Standard, IMAC, or NC DHHS HSL exceedances would result in exceedances of 2B
surface water standards (or USEPA National Recommended Water Quality Criteria) in
Lake Norman and the unnamed tributary that flows to Lake Norman. Surface water
modeling results show that water quality standards are not exceeded at the edge of the
mixing zone in Lake Norman.
• Data gaps identified as part of the CSA will be assessed, and information collected as
part of that assessment will be included in the CSA supplement to be submitted in
conjunction with the CAP Part 2 submittal to NCDEQ.
The following recommendations are made to address areas needing further assessment:
• Additional sampling for radiological parameters along major groundwater flow paths is
needed to perform a more comprehensive assessment of radionuclides from source
areas.
The SCM and groundwater models should be updated with results from second -round
sampling at the MSS site and should be included in the CAP Part 2 report.
• Background monitoring well development and sampling should continue and new data
obtained from the sampling events should be incorporated into statistical background
analysis once a sufficient data set has been obtained.
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