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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 This page intentionally left blank 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 1 2 5 7 9 10 11 11 11 12 13 13 14 14 14 15 15 15 16 16 17 18 19 19 23 26 27 28 30 30 30 32 34 36 37 38 38 40 41 41 41 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 v This page intentionally left blank 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. 10 Corrective Action Plan Part 1 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. 11 Corrective Action Plan Part 1 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 12 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. 13 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. 14 Corrective Action Plan Part 1 Marshall Steam Station Ash Basin 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 16 Corrective Action Plan Part 1 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. 16 Corrective Action Plan Part 1 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. 17 Corrective Action Plan Part 1 Marshall Steam Station Ash Basin 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. 18 Corrective Action Plan Part 1 Marshall Steam Station Ash Basin 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. 38 Corrective Action Plan Part 1 Marshall Steam Station Ash Basin 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. 39 Corrective Action Plan Part 1 Marshall Steam Station Ash Basin 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. 40 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. 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 41 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. 42 Corrective Action Plan Part 1 Marshall Steam Station Ash Basin 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 43 Corrective Action Plan Part 1 Marshall Steam Station Ash Basin 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). 44 Corrective Action Plan Part 1 Marshall Steam Station Ash Basin 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 46 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. 46 Corrective Action Plan Part 1 Marshall Steam Station Ash Basin 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. 47 Corrective Action Plan Part 1 Marshall Steam Station Ash Basin 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. 48 Corrective Action Plan Part 1 Marshall Steam Station Ash Basin 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 Marshall Steam Station Ash Basin 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 60 Corrective Action Plan Part 1 Marshall Steam Station Ash Basin (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). 61 Corrective Action Plan Part 1 Marshall Steam Station Ash Basin • 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 62 Corrective Action Plan Part 1 Marshall Steam Station Ash Basin 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 63 Corrective Action Plan Part 1 Marshall Steam Station Ash Basin 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 64 Corrective Action Plan Part 1 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 65 Corrective Action Plan Part 1 Marshall Steam Station Ash Basin 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 66 Corrective Action Plan Part 1 Marshall Steam Station Ash Basin 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). 67 Corrective Action Plan Part 1 Marshall Steam Station Ash Basin 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 68 Corrective Action Plan Part 1 Marshall Steam Station Ash Basin 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 69 Corrective Action Plan Part 1 Marshall Steam Station Ash Basin 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 60 Corrective Action Plan Part 1 Marshall Steam Station Ash Basin 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 61 Corrective Action Plan Part 1 Marshall Steam Station Ash Basin 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 62 Corrective Action Plan Part 1 Marshall Steam Station Ash Basin 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. 63 Corrective Action Plan Part 1 Marshall Steam Station Ash Basin 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. 64 Corrective Action Plan Part 1 Marshall Steam Station Ash Basin 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, 66 Corrective Action Plan Part 1 Marshall Steam Station Ash Basin 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. 66 Corrective Action Plan Part 1 Marshall Steam Station Ash Basin 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. 67 Corrective Action Plan Part 1 Marshall Steam Station Ash Basin 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. 68 Corrective Action Plan Part 1 Marshall Steam Station Ash Basin 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. 69 Corrective Action Plan Part 1 Marshall Steam Station Ash Basin 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. 70 Corrective Action Plan Part 1 Marshall Steam Station Ash Basin 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 71 Corrective Action Plan Part 1 Marshall Steam Station Ash Basin 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. 72 Corrective Action Plan Part 1 Marshall Steam Station Ash Basin 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. 73 Corrective Action Plan Part 1 Marshall Steam Station Ash Basin 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. 74 Corrective Action Plan Part 1 Marshall Steam Station Ash Basin 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. 76 Corrective Action Plan Part 1 Marshall Steam Station Ash Basin 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. 76 Corrective Action Plan Part 1 Marshall Steam Station Ash Basin 6 References AMEC. 2015. Natural Resources Technical Report, Marshall Steam Station, Gaston County, North Carolina. June 24. Back, W. and Barnes, I. 1965. Relation of electrochemical potentials and iron content to groundwater flow patterns. U.S. Geological Survey Professional Paper no. 498-C. Reston, Virginia: USGS. Baedecker, M. J. and Back, W. 1979. Hydrogeological processes and chemical reactions at a landfill. Ground Water 17, no. 5: 429-437. Bradley, P. M. 2003. History and ecology of chloroethene biodegradation: A review. 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