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Home My WebLink AboutNC0024406_1. BCSS CAP Part 1_Report_Final_20151208F)l Corrective Action Plan Part 1 Belews Creek Steam Station Ash Basin Site Location: NPDES Permit No. Permittee and Current Property Owner: Consultant Information Report Date: Belews Creek Steam Station 3195 Pine Hall Road Belews Creek, NC 27009 NC0024406 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 8, 2015 This page intentionally left blank Corrective Action Plan Part 1 Belews Creek 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 the Coal Ash Management Act of 2014, N.C. Gen. Stat. SS130A-309-200 et seq. \............ 1.6 Summary of Screening Level Risk Assessment.......................................................... 1.7 Geological/Hydrogeological Conditions....................................................................... 1.8 Results of the CSA Investigations................................................................................. 1.9 Regulatory Requirements.............................................................................................. 1.9.1 CAMA Requirements........................................................................................ 1.9.2 Standards for Site Media................................................................................. 2 Background Concentrations and Regulatory Exceedances................ 2.1 Introduction................................................................................ 2.2 Groundwater.............................................................................. 2.2.1 Background Wells and Concentrations ........................ 2.2.2 Groundwater Exceedances of 2L Standards or IMACs 2.2.3 Radionuclides in Groundwater ..................................... 2.3 Seeps........................................................................................ 2.3.1 CSA Seeps................................................................... 2.3.2 NCDENR Seeps........................................................... 2.4 Surface Water........................................................................... 2.5 Sediments................................................................................. 2.6 Soil............................................................................................. 2.6.1 Background Soil and Concentrations ........................... 2.6.2 Soil Exceedances of NC PSRGs for POG ................... 2.7 Ash............................................................................................ 2.8 Ash Porewater........................................................................... 2.9 Ash Basin Surface Water.......................................................... 2.10 PWR and Bedrock..................................................................... 2.11 COI Screening Evaluation Summary ........................................ 2.12 Interim Response Actions......................................................... 10 11 11 11 13 13 15 15 16 16 16 16 17 18 18 19 21 21 22 22 27 31 32 32 35 37 38 40 40 41 43 44 45 46 46 47 Corrective Action Plan Part 1 Belews Creek Steam Station Ash Basin 2.12.1 Source Control..................................................................................... 2.12.2 Groundwater Response Actions.......................................................... 3 Site Conceptual Model................................................................................................ 3.1 Site Hydrogeologic Conditions .......................................................................... 3.1.1 Hydrostratigraphic Units....................................................................... 3.1.2 Hydrostratigraphic Unit Properties....................................................... 3.1.3 Potentiometric Surface — Shallow Flow Layer ..................................... 3.1.4 Potentiometric Surface —Deep Flow Layer ........................................... 3.1.5 Potentiometric Surface — Bedrock Flow Layer ..................................... 3.1.6 Horizontal and Vertical Hydraulic Gradients ........................................ 3.2 Site Geochemical Conditions............................................................................ 3.2.1 COI Sources and Mobility in Groundwater ........................................... 3.2.2 Geochemical Characteristics............................................................... 3.2.3 Source Area Geochemical Conditions ................................................. 3.2.4 Mineralogical Characteristics............................................................... 3.3 Correlation of Hydrogeologic and Geochemical Conditions to COI Distribution 4 Modeling......................................................................................... 4.1 Groundwater Modeling......................................................... 4.1.1 Model Scenarios...................................................... 4.1.2 Calibration of Models ............................................... 4.1.3 Kd Terms.................................................................. 4.1.4 Flow and COI Transport Model Sensitivity Analysis 4.1.5 Fate and Transport Model ....................................... 4.1.6 Proposed Geochemical Modeling Plan ................... 4.2 Groundwater - Surface Water Interaction Modeling ............. 4.2.1 Mixing Model Approach ........................................... 4.2.2 Surface Water Model Results ................................. 4.3 Refinement of Models.......................................................... 5 Summary and Recommendations.................................................. 6 References..................................................................................... 47 48 49 49 49 50 50 51 51 51 53 54 59 66 66 67 Fit At F, Corrective Action Plan Part 1 Belews Creek Steam Station Ash Basin Tables 2-1 Initial COI Screening Evaluation 2-2 Background Concentrations for Groundwater COls Identified in the CSA: Ranges of Analytical Results with Sample Turbidity <10 NTU 2-3 Groundwater Results for COls Compared to 2L Standards, IMACs or NC DHHS HSL, Frequency of Exceedances and PPBCs 2-4 Radionuclide Concentrations 2-5A CSA Seep Results for COls Compared to 2L Standards, or IMACs and Frequency of Exceedances 2-513 NCDENR Seep Results Associated with Surface Water Discharges for COls Compared to 2B Standards or USEPA Criteria, and Frequency of Exceedances 2-6 Surface Water Results for COls Compared to Upgradient Surface Water Concentrations, 2B Standards or USEPA National Recommended Water Quality Criteria and Frequency of Exceedances 2-7 Sediment Results for COls Compared to NC PSRGs for POG, Upgradient Concentrations and Frequency of Exceedances 2-8 Proposed Provisional Background Soil Concentrations 2-9 Soil Results for COls Compared to NC PSRGs for POG, Frequency of Exceedances and PPBCs 2-10 Ash Exceedance Results for COls Compared to NC PSRGs for POG and Frequency of Exceedances 2-11 Ash Basin Porewater Results for COls Compared to 2L Standards, or IMACs, Frequency of Exceedances, and PPBCs 2-12 Ash Basin Surface Water Results for COls Compared to 2L Standards, IMACs, or NC DHHS HSL, 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 Well Pairs 3-3 Categories and Threshold Concentrations to Identify Redox Processes in Groundwater 3-4 Field Parameters from Belews Creek CSA 3-1 Vertical Gradients — Shallow and Deep Well Pairs 3-2 Vertical Gradients — Deep and Bedrock Pairs 4-1 Mixing Zone Sizes and Percentages of Upstream River Flows 4-2 Dan River 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 Seep and Surface Water Sample Locations 1-6 Receptor Survey Map 1-7 Site Vicinity Map 2-1 Porewater and Groundwater Analytical Results — Plan View (21- Standard and IMAC Exceedances) 2-2 Surface Water and Seep Analytical Results Corrective Action Plan Part 1 Belews Creek Steam Station Ash Basin 2-3 Soil Analytical Results — Plan View (NC PSRG Exceedances) 3-1 Site Conceptual Model — 3-D View 3-2 Site Conceptual Model — Cross -Sectional View 3-3 Water Table Surface — Shallow Wells 3-4 Potentiometric Surface — Deep Wells 3-5 Potentiometric Surface — Bedrock Wells 3-6 Vertical Gradient, Shallow to Deep Wells 3-7 Vertical Gradient, Deep to Bedrock Wells 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 BCSS Belews Creek Steam Station BG background bgs below ground surface CAMA North Carolina Coal Ash Management Act of 2014 CAP Corrective action plan CCR Coal combustion residuals COI Constituent of Interest COPC Contaminant of potential concern CSA Comprehensive site assessment cy cubic yards DHHS North Carolina Department of Health and Human Services DO dissolved oxygen DORS Distribution of residuals solids DWR NCDEQ Division of Water Resources EPRI Electric Power Research Institute FERC Federal Energy Regulatory Commission FGD flue gas desulfurization ft/ft feet / foot HDPE high -density polyethylene HSL health screening level IMAC Interim maximum allowable concentration Kd linear sorption coefficient mg/kg milligrams per kilogram MW megawatt iv Corrective Action Plan Part 1 Belews Creek Steam Station Ash Basin NAVD 88 North American Vertical Datum of 1988 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 NTU Nephelometric turbidity units NURE National Uranium Resource Evaluation ORP oxidation-reduction potential POG Protection of groundwater PPBC Proposed provisional background concentration PWR partially weathered rock 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 USGS U.S. Geological Survey USEPA U.S. Environmental Protection Agency UTL Upper tolerance limit Work Plan Groundwater assessment work plan WQC National recommended water quality criteria v This page intentionally left blank Executive Summary ES-1 Introduction ES-1.1 Regulatory Background Duke Energy Carolinas, LLC (Duke Energy) owns and operates the Belews Creek Steam Station (BCSS), located in Stokes County, North Carolina. BCSS began operation in 1974 as a coal-fired generating station and currently operates two coal-fired units. Historically, BCSS disposed of coal ash residue from the coal combustion process in the ash basin located across Pine Hall Road to the west-northwest of the station. In 1983, BCSS converted to dry handling of fly ash with disposal in on -site landfills. Discharge from the ash basin is permitted by the North Carolina Department of Environmental Quality (NCDEQ)' Division of Water Resources (DWR) under the National Pollutant Discharge Elimination System (NPDES) Permit NC0005088. 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 BCSS was submitted to NCDENR (now NCDEQ) on December 30, 2014. The Work Plan was conditionally approved by NCDENR on March 13, 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 BCSS CSA Report was submitted to NCDEQ on September 9, 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 BCSS 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 and Part 2 being submitted no later than 180 days after submittal of the CSA. The purpose of this CAP Part 1 is to provide a summary of site usage , 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 transport model, and results of the groundwater to surface water interaction model. The CAP Part 2 will include the remainder of the CAMA requirements, including proposed alternative methods for achieving groundwater quality restoration, conceptual plans for 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 Executive Summary, as appropriate. recommended corrective action, an estimated 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 BCSS began in March 2015 and was completed in September 2015. Sixty-four groundwater monitoring wells and eleven 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 BCSS site. Seep, surface water, and sediment samples were also collected. For the CSA, the source area was defined as the ash basin, the chemical pond located within the southern portion of the ash basin, and the closed Pine Hall Road Landfill. 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), 15A NCAC 213 (213 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 BCSS site and found that groundwater exceedances are a result of both naturally occurring conditions and CCR material contained in the ash basin. The approximate horizontal extent of groundwater impacts is limited to beneath the ash basin west and downgradient of the ash basin dam and Middleton Loop Road. Exceedances of 2L Standards or IMACs at seep location S-9 in the drainage south of Pine Hall Road and adjacent to the ash structural fill indicates potential groundwater impacts in that area The approximate vertical extent of groundwater impacts is generally limited to the shallow and deep flow layers. The horizontal extent of soil impacts is limited to the area beneath the ash basin. Where soil impacts were identified beneath the ash basin, the vertical extent of contamination beneath the ash/soil interface is generally limited to the upper soil samples beneath ash. The direction of groundwater transport is generally in a northerly direction towards the Dan River and not toward other off -site receptors (i.e., private drinking water wells). 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 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: NPDES compliance monitoring wells MW-202S and MW-202D, Pine Hall Road Landfill monitoring well MW-3, Craig Road Landfill monitoring well CRW10, FGD Landfill monitoring wells BC-23A and BC28, and CSA background monitoring wells 13G-1 D, BG-2S, BG-2D, BG-2BR and MW-202BR Groundwater PPBCs represent the statistically- derived prediction limit for constituents with sufficient sample size to allow the use of statistical methods and the highest reported value or laboratory reporting limit (for non -detects) for constituents that were not historically monitored at the site. At the request of NCDEQ, only samples with turbidity less than 10 NTU were included in the background calculations. PPBCs for some constituents exceed the 2L Standards, IMACs or NC DHHS HSLs, including antimony, hexavalent chromium, iron, manganese, pH, thallium and vanadium. These values were used for comparison purposes in the report, but not to establish which COls are evaluated for modeling purposes. Well development and sampling will continue to allow for additional analytical results to be incorporated into statistical background analysis once a sufficient data set has been obtained. The statistical evaluation methods used and results for the PPBCs are described in Section 2 and Appendix B of this report. Soil PPBCs (i.e., the 95% upper tolerance limit [UTL]) were calculated for those constituents analyzed in background soil borings. A detailed method review, statistical evaluation, and results for the PPBCs are included in Appendix B. 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. =S-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, IMACs, NC DHHS HSLs, and 213 Standards, 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 and transport modeling. Potential COI CSA COI Exceedances by Media COI to be Solid/ Aqueous Ash Pore- water2 Ash Basin Surface Water2 Ground- water Surface Water Seeps Sediment Soil Further Assessed in Groundwater Modeling Antimony Yes Arsenic Yes Beryllium Yes Boron Yes Cadmium Yes Chloride Yes Chromium Yes Hexavalent Chromium - _ _ _ _ Yes Cobalt Yes Copper3 No Iron Yes Lead No Manganese Yes pH Yes Selenium Yes Sulfate Yes TDS Yes Thallium Yes Vanadium Yes Notes: 1. Note that ash is not evaluated for remediation in CAP Part 1 because ash will be drained of water during remedial activities and excavated or capped. 2. Note that porewater and ash basin surface water are not evaluated for remediation in CAP Part 1 because both will be eliminated during ash basin closure activities. 3. Exceedance identified in dissolved concentration, but not total, for one surface water sample and not present in other media (copper) or one surface water sample and one ash basin surface water sample (lead). Site Conceptual Model The site conceptual model (SCM) is an interpretation of processes and characteristics associated with hydrogeological 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 BCSS site. The SCM was developed using data and analysis from the CSA Report. =S-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 BCSS is consistent with the regolith-fractured rock system and is an unconfined, connected aquifer system. The groundwater system is divided into three flow layers within the connected aquifer system: shallow, deep, and bedrock. In general, groundwater flow for all three flow layers is from the groundwater divide along Pine Hall Road, located south of the ash basin and the Pine Hall Road Landfill to the north toward the Dan River. Horizontal and vertical hydraulic gradients were calculated for each flow layer. Positive (upward) and negative (downward) vertical gradients varied across the site. Groundwater flow is generally downward or neutral in the ash basin and in most areas of the BCSS site. Negative (downward) vertical gradients in the ash basin increase the potential for migration of COls into the deep and bedrock layers. The neutral to low magnitude of the gradients in the ash basin limits the impact of vertical migration, which is supported by the generally lower COI concentrations in the deep layer and the limited number of COI exceedances of 2L Standards and IMACs in the bedrock layer. Positive (upward) gradients at the high points on the groundwater divides indicates flow from the deep and bedrock layers as groundwater in the shallow layer flows down the slope. Positive gradients below the ash basin indicate upward groundwater flow due to the sharp decrease in potentiometric head downgradient of the dam. 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 BCSS site was evaluated based on 73 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 (oxic-anoxic), mixed (anoxic) and anoxic conditions. Under these conditions, more oxidized species As(V), Se(VI), and Mn(IV) would be expected. Ash porewater samples included the entire range of redox categories found at the site: oxic, suboxic, mixed (oxic-anoxic), mixed(anoxic) and anoxic. There is an increased potential for reduced forms of metals to occur under anoxic or mixed conditions. Groundwater samples from wells elsewhere across the site are classified as suboxic or oxic categories where reduced species of metals such as As(III) are less likely to persist. 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 BCSS site. Key observations include: Cobalt, hexavalent chromium, iron, manganese, pH, and vanadium were the only COls with widespread exceedances in wells upgradient, beneath and downgradient of the ash basin, and in background locations. The concentrations of iron and manganese are highly pH dependent. Cobalt and iron were reported at higher concentrations at and downgradient of the ash basin, similar to other COls. Vanadium does not appear to represent impacts from ash handling because it was reported at similar concentrations upgradient and downgradient of the ash basin. Groundwater and geochemical conditions promote the mobility of vanadium across the site with contribution likely from naturally occurring vanadium and vanadium from source areas. • Antimony exceedances were reported in shallow, deep and bedrock flow layers at isolated locations around the ash basin and one background location. It was not reported above the IMAC as frequently as cobalt and vanadium. • Hexavalent chromium exceedances were reported in shallow, deep and bedrock flow layers at widespread locations but not in porewater samples within the ash. In each flow layer, exceedances were reported upgradient of the ash basin and/or downgradient of the ash basin dam, indicating that hexavalent chromium may be naturally occurring. • Arsenic, beryllium and cadmium exceedances were reported at a few locations at and downgradient of the ash basin dam, but not in upgradient or background locations. Arsenic and cadmium exceedances were also reported in downgradient wells at the Pine Hall Road Landfill. Arsenic has a relatively high Kd value at the site, which suggests that geochemical conditions favor low mobility of this COI. • Sulfate exceedances were reported in downgradient wells at the Pine Hall Road Landfill. Sulfate has a low Kd value and can be mobile in groundwater but exceedances were not reported in CSA groundwater samples beneath and around the ash basin. Boron, chloride, chromium, and TDS exceedances were detected frequently in wells at the ash basin and at downgradient locations. Boron has a low Kd value and can be mobile in groundwater. 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 -surface water modeling were conducted to evaluate COI migration and potential impacts following closure of the ash basin at BCSS. Under the direction of HDR, UNCC developed a site -specific, 3-D, steady- state groundwater flow and fate and transport model for the BCSS site using MODFLOW and MT3DMS. 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 modeling effort was to simulate steady-state groundwater flow conditions for the BCSS ash basin area, and simulate transient transport conditions in which COls enter groundwater via the ash basin over the period it was in service. 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 left in ash basin is covered by an engineered cap(s) • Excavation: assumes removal of ash from the ash basin 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. EPRI provided subsequent comments on November 20, 2015, which concluded the model was constructed and calibrated sufficiently to achieve its primary objective of comparing the effects of closure alternatives on nearby groundwater quality 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 10 site -specific samples of soil, or PWR from the transition zone. The results of these Kd tests were used as base inputs to the model and adjusted accordingly to achieve model calibration. ES-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 aquifer, transition zone, and fractured bedrock flow layers at the site flows toward the ash basin and then to the north and discharges to the Dan River. 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. The Cap -in -Place scenario simulated placement of an engineered cap by applying a recharge rate of zero to the source areas. The model assumption for this scenario is that the ash will remain in its current position and that there is no recharge through the cap. Groundwater flow is affected by this scenario as the water table is lowered and groundwater velocities may be reduced beneath the capped areas. In addition, the ash was assumed to be above the water table and the migration of COls from porewater to groundwater beneath the basin is stopped. The CAP Part 2 model assumptions will be revised such that COls in the saturated portion of the ash layer will be evaluated during the model simulation period. The Excavation scenario simulated the removal of all ash from the ash basin. The model assumption is that all ash above and below the water table is removed and the migration of COls from porewater to groundwater beneath the basin was stopped. The Excavation scenario also assumes recharge rates in the ash basin become equal to recharge rates in areas surrounding the basin. 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 to 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 arsenic, beryllium, boron, chloride, chromium, hexavalent chromium, cobalt, and thallium. Several COls were not advanced to modeling because of the following rationale: • Antimony was detected in isolated locations at BCSS, including background; although present in the three flow layers, it was not detected consistently with depth at the same location. As a result, antimony was not considered in the model simulations. • Cadmium was only reported above the 2L Standard in one location and there is no discernable plume. Due to cadmium's limited distribution and moderate sorptive capacity, model results from other COls at this location should bracket this constituent. • Iron, manganese, pH and TDS are naturally occurring in the groundwater system and require more complex modeling than the current MODFLOW/MT3DMS. The geochemical modeling will enhance the understanding of the processes taking place in the subsurface and ultimately aid in choosing the most appropriate remedial action for the site. Geochemical modeling will be completed and submitted in the CAP Part 2. • Vanadium concentrations were prevalent above the IMAC in wells throughout the BCSS site. However, vanadium was not present at higher concentration in downgradient areas; although present in the three flow layers, it was not detected consistently with depth at the same location and it was not detected consistently at adjacent wells. As a result, vanadium was not considered in the simulations. Under the Existing Conditions scenario, concentrations for all modeled COls, except beryllium, increase or reach steady-state conditions above 2L Standards, IMACs, or NC DHHS HSL at one or more of the selected well locations during the 250-year simulation period. Of the three model scenarios, the Existing Conditions scenario represents the most conservative conditions in terms of groundwater concentrations and COls reaching the compliance boundary. Under the Cap -In -Place scenario, concentrations of boron and chloride decrease below the 2L Standard within 15 years at the selected well locations; the other COls increase initially and then decrease during the 250-year simulation period but remain above their respective standards at the selected well locations. Under the Excavation model scenario, concentrations of beryllium, boron, chloride and thallium decrease below the 2L Standard and IMACs within 10 years at the selected well locations, while modeled concentrations of the other COls decrease slowly over the 250-year simulation period. The flow and fate and transport models will be updated during CAP Part 2 based on a review of additional sampling and water elevation data. =S-4.3 Groundwater -Surface Water Interaction Modeling Groundwater model output from the fate and transport modeling were used as inputs to the surface water assessment in the Dan River. A mixing model was used to assess potential downgradient surface water quality impacts. For each groundwater COI that discharges to surface waters at a concentration exceeding the 2B Standards or USEPA Criteria, 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 zones in the Dan River. 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 • 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. The updated results should be used to refine the areas requiring evaluation for remediation. • Additional sampling for radiological parameters along major groundwater flow paths is needed to perform a more comprehensive assessment of radionuclides from source areas. • Additional surface water and sediment sampling should be conducted in the Dan River and in the drainage channel between the ash basin and the Dan River to further evaluate constituent concentrations with regard to the ash basin discharge. • Hydrogeological and analytical data from data gap wells west of the ash basin dam should be reviewed to confirm the horizontal and vertical extent of groundwater impacts has been determined. • The groundwater flow and fate and transport model should be refined to consider site - specific conditions in CAP Part 2. Introduction Duke Energy Carolinas, LLC (Duke Energy) owns and operates the Belews Creek Steam Station (BCSS), which is located on Belews Lake in Stokes County, North Carolina. BCSS began operation in 1974 and operates two coal-fired units. BCSS disposed of coal ash residue from the coal combustion process in the ash basin until 1983. At that time, BCSS converted to dry handling of fly ash with disposal in on -site landfills with bottom ash continuing to be sluiced to the ash basin. Discharge from the ash basin is permitted by the North Carolina Department of Environmental Quality (NCDEQ)2 Division of Water Resources (DWR) under the National Pollutant Discharge Elimination System (NPDES) Permit NC0024406. 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 BCSS was submitted to NCDENR on September 25, 2014, followed by a revised Work Plan on December 30, 2014. The Work Plan was conditionally approved by NCDENR on March 13, 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 BCSS CSA Report was submitted to NCDENR on September 9, 2015 (HDR 2015). CAMA also requires the preparation of a Corrective Action Plan (CAP) for each regulated facility no later than 180 days after submittal of the CSA Report. 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 (Appendix A). The purpose of this CAP Part 1 is to provide background information, a brief summary of the CSA findings, an evaluation and refinement of "Constituents of Interest" (COls) for modeling purposes, a detailed description of the site conceptual model (SCM), results of the groundwater flow and transport model, and results of the groundwater to surface water interaction model. The CAP Part 2 will include the remainder of the CAMA requirements, including proposed alternative methods for achieving groundwater quality restoration, conceptual plans for recommended corrective actions, an estimated implementation schedule, and a plan for future monitoring and reporting. A risk assessment will also be submitted with the CAP Part 2 submittal. Z 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 1.1 Site History and Overview 1.1.1 Site Location, Acreage, and Ownership The BCSS site is located on the north side of Belews Lake on Pine Hall Road in Stokes County, North Carolina (Figure 1-1). The BCSS site, including the station and supporting facilities, is approximately 700 acres. The BCSS site lies within a 6,100-acre parcel owned by Duke Energy, of which Belews Lake comprises 3,800 acres. Site Description BCSS, one of Duke Energy's largest coal -burning power plants in the Carolinas, is a two -unit coal-fired electricity generating plant with a capacity of 2,220 megawatts (MVV). The station began commercial operations in 1974 with Unit 1 (1,110 MVV) followed by Unit 2 (1,110 MVV) in 1975. Cooling water for BCSS is provided by Belews Lake, a man-made lake formed when Duke Energy built the facility. The BCSS ash basin is located across Pine Hall Road to the northwest of the station and consists of a single cell impounded by an earthen dam located on the north end of the ash basin. The dam is approximately 2,000 feet long with a maximum height of approximately 140 feet. The top of the dam is at elevation 770 feet and the crest is 20 feet wide. The ash basin was constructed from 1970 to 1972 and has a surface area of approximately 283 acres. The full pond elevation of the BCSS ash basin is approximately 750 feet and the full pond capacity of the ash basin is estimated to be 17,656,000 cubic yards (cy). Ash basin water surface elevations typically ranged between 750.0 and 752.5 feet from January 2005 to April 2014. Beginning in May 2014, the ash basin water surface elevation was lowered, and from May 2015 through July 2015, generally ranged between 747.5 and 750 feet. Surface topography at the BCSS site ranges from an approximate high elevation of 878 feet (NAVD 88) southeast of the ash basin near the intersection of Pine Hall Road and Middleton Loop Road to an approximate low elevation of 646 feet at the toe of the earthen dike located at the north end of the ash basin. Middleton Loop Road and Pine Hall Road are located approximately along topographic divides. Topography to the west of Middleton Loop Road and north of the earthen dam and natural ridge generally slopes downward toward the Dan River, which is located approximately 2,000 feet north of the Ash Basin Compliance Boundary. Topography to the south and east of Pine Hall Road generally slopes downward toward Belews Lake. An unnamed stream channel extends from the base of the ash basin dam and flows approximately 4,400 feet from southeast to northwest where it enters the Dan River. The elevation at the discharge point of the tributary to the Dan River is approximately 578 feet. The elevation of Belews Lake is approximately 725 feet. Refer to Figure 1-2 for a map of the site layout. Coal ash residue from the coal combustion process was disposed of in the ash basin prior to 1983. In 1983, BCSS converted to dry handling of fly ash with disposal of fly ash in Pine Hall Road Landfill. Bottom ash has continued to be sluiced to the ash basin, and fly ash is sluiced to the ash basin during startup or maintenance activities. Disposal of coal ash residue continued at 11 the Pine Hall Road Landfill until construction of the Craig Road Landfill in 2008. Also in 2008, flue gas desulphurization (FGD) residue (gypsum) began to be generated as part of the air pollution control system and is disposed of in the FGD Residue Landfill. The landfill locations are shown on Figure 1-1. The Pine Hall Road Landfill was permitted in 1983 under NCDENR Solid Waste Permit No. 8503 to accept only fly ash from BCSS operations. The original landfill was unlined and was permitted with a soil cap 1-foot thick on the side slopes and 2-feet thick on flatter areas. The Phase 1 Expansion, permitted in 2003, was also unlined but with a synthetic cap system to be applied at closure. Ash disposal was halted after exceedances of 2L Standards were observed in groundwater monitoring wells near the landfill. Duke Energy installed an engineered cap as a corrective action measure, following NCDENR approval of the closure plan in December 2007. The engineered cap consisted of a 40-mil low -density polyethylene geomembrane, a geonet composite, 18 inches of compacted soil, and 6 inches of vegetative soil cover over a 37.9-acre area; an adjacent 14.5-acre area had additional soil cover applied and was graded to improve surface drainage. The cap was substantially completed in December 2008. The Craig Road Landfill was permitted in 2007 under NCDENR Solid Waste Permit No. 8504 to accept coal ash, wastewater treatment sludge, and off -spec FGD residue (gypsum) generated from BCSS operations. Waste disposal began in February 2008. The landfill was constructed with an engineered liner system, consisting of a leachate collection and removal system, a high - density polyethylene (HDPE) geomembrane, and a geosynthetic clay liner. The landfill began accepting waste in February 2008. The landfill covers an area of approximately 31 acres and is located on the south side of Belews Lake adjacent to the Belews Lake Canal (West Belews Creek). The FGD Residue Landfill was permitted for operation in 2008 under the NCDENR Solid Waste Permit No. 8505 to receive FGD residue (gypsum) and wastewater treatment clarifier sludge produced at the BCSS. Waste disposal began in April 2008. The landfill has an engineered liner system consisting of a leachate collection system, underlain by a HDPE geomembrane liner, underlain by a geo-synthetic clay liner. The landfill covers an area of approximately 24 acres and is located on the south side of Belews Lake, approximately one-half mile north of the Craig Road Landfill. An unlined structural fill comprised of compacted fly ash was constructed southeast of the ash basin. The ash structural fill is located south of the Pine Hall Road topographic divide, and therefore, groundwater flow beneath the fill should be predominantly away from the ash basin towards Belews Lake. This structural fill was constructed under the structural fill rules found in 15A NCAC 13B .1700. The Notification of the Beneficial Use Structural Fill was submitted by Duke Energy to NCDENR on May 7, 2003. Approximately 968,000 cy of ash were placed within the structural fill from February 2004 to the last ash placement in July 2009. An engineered cap similar to that previously described for the Pine Hall Road Landfill was constructed over the structural fill in 2012. The structural fill is currently used as an equipment/material staging area and for overflow parking. Per the approved Work Plan, ash used in the structural fill was not considered part of the source area and was not evaluated by the CSA. 12 Permitted Activities and Permitted Waste Duke Energy is authorized to discharge wastewater that has been adequately treated and managed from the BCSS ash basin to receiving waters of an unnamed stream, which is a tributary to the Dan River. This discharge is in accordance with NPDES Permit NC0024406, which was renewed on November 1, 2012 and expires February 28, 2017. Any other point source discharge to surface waters of the state is prohibited unless it is an allowable non- stormwater discharge or is covered by another permit, authorization, or approval. The NPDES permit authorizes discharges in accordance with effluent limitations monitoring requirements and other conditions set forth in the permit. A summary of NPDES and surface water sampling requirements, along with the associated NPDES site flow diagram, is provided in the CSA Report. There are four solid waste facilities associated with BCSS: • Craig Road Landfill, NCDENR Permit No. 8504-INDUS, active; • FGD Residue Landfill, NCDENR Permit No. 8505-INDUS, active; • Ash structural fill, closed; and • Pine Hall Road Landfill, NCDENR Permit No. 8503-INDUS, closed. The Craig Road Landfill, FGD Residue Landfill and the ash structural fill are located south of the ash basin and are not hydrogeologically connected to the ash basin (although leachate collected from the landfill facilities is routed to the ash basin). The Pine Hall Road Landfill is located on the south side of the ash basin north of Pine Hall Road and is hydrogeologically connected to the ash basin. History of Site Groundwater Monitoring Monitoring wells were installed by Duke Energy in 2006 as part of the voluntary monitoring system for groundwater for the ash basin. Eight voluntary groundwater monitoring wells were installed in 2006 and Duke Energy performed voluntary groundwater monitoring around the ash basin twice per year from November 2007 until May 2010 with the results submitted to NCDENR. Two of the voluntary monitoring wells (MW-102S and MW-102D) were recently abandoned as a result of reinforcement construction activities at the ash basin dam. The voluntary monitoring wells are not included in compliance monitoring and have not been sampled routinely since 2010. In accordance with the NPDES Permit, nine compliance wells were installed in December 2010. Compliance groundwater monitoring as required by the NPDES Permit, began in January 2011. The compliance monitoring wells have been sampled three times per year for a total of 14 times from January 2011 through July 2015. The compliance boundary for groundwater quality at the BCSS site is defined in accordance with Title 15A NCAC 02L .0107(a) as being established at either 500 feet from the waste boundary or at the property boundary, whichever is closer to the 13 waste boundary. A detailed description of NPDES and voluntary groundwater monitoring programs and results is provided in the CSA Report. Groundwater monitoring is conducted at the three permitted BCSS landfills (Pine Hall Road, Craig Road, and FGD Residue) in accordance with permit requirements. Monitoring is performed twice per year per an established scheduled at each landfill. Summary information for each landfill is provided below. • Pine Hall Road Landfill — the groundwater monitoring system currently consists of 13 monitoring wells and two surface water sample locations. Twelve wells are screened in the residual soil/saprolite layer and one well (MW-1 D) is screened in fractured bedrock. Groundwater monitoring wells MW-1, MW-2, MW-3, MW-4, and MW-5 were installed in 1989. Monitoring well MW-3 was determined to monitor background groundwater quality (Appendix B). The initial twice per year groundwater sampling was performed at these wells in October 1989. Monitoring wells MW-6, MW2-7, MW2-9, OB-4, OB-5, and OB-9 were installed, and monitoring initiated, as part of the site investigation for the Phase 1 Expansion and subsequent investigation of groundwater exceedances from 2000 to 2004. Monitoring wells MW-1 D and MW-7 were installed after installation of the engineered cap in 2008. Groundwater monitoring is performed in April and October. • Craig Road Landfill — the groundwater monitoring system currently consists of 17 monitoring wells, six surface water sample locations and three leachate sample locations. Monitoring well CRW-10 was determined to monitor background water quality (Appendix B). Monitoring wells were installed to monitor the transition zone (TZ) between the saprolite/partially weathered rock zone and bedrock. The initial twice per year groundwater sampling event was performed in January 2007 prior to initial placement of waste in February 2008. Groundwater monitoring is performed in January and July. FGD Residue Landfill — the groundwater monitoring system currently consists of 12 monitoring wells, one surface water sample location and one leachate sample location. Wells BC-23A and BC-28 were determined to monitor background groundwater quality (Appendix B).The monitoring wells were installed to monitor groundwater quality in the residual soil/saprolite layer. The initial twice per year groundwater sampling event was performed in November 2007 prior to initial waste placement in April 2008. Groundwater monitoring is performed in May and November. The location of the ash basin voluntary and compliance monitoring wells, the approximate ash basin waste boundary, the ash basin compliance boundary, the assessment wells at the Pine Hall Road Landfill and the landfill compliance boundary are shown on Figure 1-3. The Craig Road Landfill and the FGD Residue Landfill are located on the south side of Belews Lake, hydraulically isolated from the ash basin and Pine Hall Road Landfill; as such, those monitoring wells are not shown. 14 Summary of Comprehensive Site Assessment The CSA for the BCSS site began in March 2015 and was completed in September 2015. Sixty- four groundwater monitoring wells and 11 geotechnical/soil borings were installed/advanced as part of the assessment to characterize the ash, soil, rock, and groundwater at the BCSS site (Figure 1-4). Seep, surface water, and sediment samples were also collected (Figure 1-5). In addition, hydrogeological evaluation testing was performed on newly installed wells. 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 source areas. If a constituent3 concentration exceeded: (1) the North Carolina Groundwater Quality Standards, as specified in T15A NCAC .0202L (21- Standards) or Interim Maximum Allowable Concentration (IMAC)4, (2) North Carolina Surface Water Quality 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 the 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 surveys was to identify drinking water wells or other water sources within a 0.5- mile (2,640-foot) radius of the BCSS ash basin compliance boundary. The supplemental 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 and well details and usage. A detailed description of the receptor surveys is provided in the CSA Report. Results of the receptor survey are detailed on Figure 1-6. One public water supply well and 50 private water supply wells were identified within the 0.5- mile radius of the BCSS ash basin compliance boundary during the receptor survey (Figure 1-6). Of the 50 private water supply wells, 45 wells were confirmed by the survey and 20 had records at the Stokes County Division of Environmental Health; the presence of 5 private water supply wells were assumed based on the lack of public water supply in the area and proximity to other residences that have private wells. 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 Middleton Loop Road toward the Dan River were identified within a 0.5-mile radius of the ash basin. 3 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). a 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. 16 Surrounding Land Use Properties located within a 0.5-mile radius of the BCSS ash basin compliance boundary are located in Stokes County, North Carolina. The area surrounding BCSS generally consists of residential properties, farmland, undeveloped land, and Belews Lake, as shown on Figure 1-7. Residential properties are located to the southwest and residential farmland to the northeast, north, and west. Duke Energy property is located to the north, northwest, south, and east with Belews Lake beyond BCSS to the south and east. Findings of Drinking Water Supply Well Survey Conducted per the Coal Ash Management Act of 2014, N.C. Gen. Stat. SS130A-309-200 et seq. \ Section § 130A-309.209 (c) of the CAMA also indicates that NCDENR (now NCDEQ) will require sampling of public and private water supply wells to determine whether the wells may be adversely impacted by releases from CCR impoundments. Between February and April 2015, NCDENR arranged for independent analytical laboratories to collect and analyze water samples obtained from private wells identified during the Drinking Water Well Survey, if the owner agreed to have their well sampled. Seven wells were sampled by NCDENR between February 18 and April 9, 2015, and one well was re -sampled. The results of that testing were included in Appendix B of the BCSS CSA Report. Subsequent to the CSA submittal, NCDEQ sampled an additional 23 wells between August and October 2015. The results of that testing can be found on the NCDEQ website at the following link: http://www.ncwater.orq/?gape=603. 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 consistent with risk assessment protocols and will be presented in the CAP Part 2 report. Geological/Hydrogeological Conditions The BCSS site is located in the Milton terrane; the Dan River Triassic Basin is located approximately 3,000 feet north of the site. Geologic units mapped in the vicinity of the site include alluvium, terrace deposits, sedimentary rocks of the Dan River Basin, a diabase dike, and felsic gneisses and schists with interlayered hornblende gneiss and schist. Alluvial and 16 terrace deposits were not encountered in any of the boreholes in the area of the BCSS ash basin, but alluvial deposits were mapped along the unnamed stream downstream of the ash basin main dam and along the Dan River. The hydrogeologic regime at the BCSS is characterized by residual soil/saprolite and weathered rock overlying fractured crystalline rock separated by the TZ. Typically, the residual soil/saprolite is partially saturated and the water table fluctuates within it. Water movement is generally preferential through the weathered/fractured and fractured bedrock. Groundwater flow paths in the Piedmont are almost invariably restricted to the zone underlying the topographic slope extending from a topographic divide to an adjacent stream. Under natural conditions, the general direction of groundwater flow can be approximated from the surface topography (LeGrand 2004). Based on the site investigation completed for the CSA, the groundwater system in the natural materials (soil, soil/saprolite, and bedrock) at BCSS is consistent with the regolith-fractured rock system and is an unconfined, connected aquifer system without confining layers. The BCSS groundwater system is divided into three layers referred to as shallow, deep (TZ), and bedrock to distinguish the flow layers within the connected aquifer. Groundwater flow and transport at the BCSS site can be approximated from the surface topography. A topographic divide along Pine Hall Road separates the ash basin and Pine Hall Road landfill, both located north of the road, from the ash structural fill, coal pile, and power plant, located south of the road. Groundwater flow north of the road is to the north-northwest toward the Dan River, while groundwater flow south of the road is to the south-southeast towards Belews Lake. Additional topographic divides are located west and north of the ash basin approximately near Middleton Loop Road. These divides separate the surface drainage area containing the ash basin from adjacent drainage areas. While the topographic divides generally function as groundwater divides, groundwater flow across topographic divides may occur based on driving head conditions from the ash basin or preferential flow paths within the shallow and/or deep flow layers. Seeps located northwest of the ash basin within the Duke Energy property boundary indicate groundwater flow across the topographic divide of Middleton Loop Road based on elevated concentrations of source constituents. Results of the CSA Investigations Groundwater constituent exceedances were determined to be the result of source -related materials contained within the ash basin and naturally occurring conditions within the Duke Energy property boundary and surrounding vicinity. The CSA identified the source -related horizontal and vertical extent of groundwater contamination at the BCSS site and found it is limited to within the compliance boundary, except to the west of the ash basin dam. Where soil impacts were identified beneath the ash basin, the vertical extent of contamination beneath the ash/soil interface is generally limited to the upper soil samples collected beneath 17 the ash. Groundwater contamination at the site attributable to ash handling and storage was delineated during the CSA activities with the following exceptions: • Horizontal extent west and downgradient of the ash basin dam and Middleton Loop Road. • Horizontal and vertical extent in the area of seep location S-9 in the drainage south of Pine Hall Road and adjacent to the ash structural fill. Although some constituent concentrations were measured above NC PSRGs for POG in soil samples beneath the basin, concentrations in general were similar to those measured from soil samples collected at background well locations. Surface water samples collected from the Dan River during the CSA indicated chloride, manganese, thallium, and total dissolved solids (TDS) concentrations were higher in downstream samples (compared to upstream samples). Downstream sample concentrations for these constituents were higher than their respective 2B Standards or U.S. Environmental Protection Agency (USEPA) National Recommended Water Quality Criteria (WQC). Background monitoring well analytical results indicate the presence of naturally occurring metals and other constituents at concentrations that exceeded their respective regulatory standards or guidelines. These include antimony, iron, manganese, pH and vanadium. The CSA Report did not propose provisional background concentrations; however, they are proposed in Section 2 of this report. The geologic conditions present beneath the ash basin impede the vertical migration of contaminants. The direction of contaminant transport is generally to the north/northwest toward the Dan River and not toward off -site water supply wells. Additional details pertaining to the horizontal and vertical extent of soil and groundwater impacts at the BCSS site are detailed in the CSA Report. 1.9 Regulatory Requirements 1.9.1 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 18 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. 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 BCSS ash basin no later than August 1, 2029 or as otherwise dictated by NCDEQ risk classification. Closure for the BCSS ash basin was not defined in CAMA. Based on the results of soil and groundwater samples collected beneath the ash basin, some residual contamination may remain after closure; however, the degree of contamination and the persistence of this contamination over time cannot be determined at this time. CAMA requires that corrective action be implemented to restore groundwater quality where the CSA documents exceedances of groundwater quality standards. 1.9.2 Standards for Site Media Groundwater and seep sample analytical results were compared to 2L Standards or the 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 Level (HSL) developed for drinking water supply wells. 19 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 including applicable USEPA WQC. 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 (updated March 2015). Sediment sample analytical results were also compared to NC PSRGs for POG. 20 2 Background Concentrations and Regulatory Exceedances Introduction As part of the CSA, groundwater, seep, surface water, sediment, soil, partially weathered rock (PWR) and bedrock samples were collected between March 5 and July 27, 2015, from background locations, locations beneath the ash basin, and from locations outside of the waste boundaries. Groundwater samples were also collected from pre-existing voluntary and compliance wells and seep samples were collected from seeps previously identified by NCDENR. Data obtained from these sampling events were presented in the CSA Report. Groundwater samples were also collected in April 2015 from monitoring wells at the Pine Hall Road Landfill during semiannual sampling; these data were not included in the CSA Report but are summarized in Section 2.2.3 of this report. 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 exceedances with regard to PPBCs and applicable regulatory standards or guidelines (i.e., 2L Standards, IMACs, NC DHHS HSLs, 213 Standards, and NCPSRGs for POG); and determine which COls will be retained for further evaluation of corrective action. COls (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 or are naturally occurring. 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 further 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. 21 Table 2-1. Initial COI Screening Evaluation Potential COIs CSA COI Exceedance by Media COI to be Solid/ Aqueous Ash Pore- water Ash Basin Surface Water Ground- water Surface Water Seeps Sediment Soil PWR/ Bedrock Further Assessed in CAP I 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 Mercury - - - - - - - - - No Nickel - - - - - - - - - No Nitrate - - - - - - - - - No pH Yes Selenium Yes Sulfate Yes TDS Yes Thallium Yes Vanadium Yes Zinc - - - - - - - - - No Note: COI exceedance based on 2L Standard, IMAC, or 2B Standard for respective aqueous media and NC PSRGs for solid/soil-like media. Groundwater Background Wells and Concentrations Because COls can be both naturally occurring and related to the source areas, the choice of monitoring wells used to establish background concentrations is important in determining whether releases have occurred from the source areas. The determination of whether or not a monitoring well is a suitable background well is based on the following: 22 • 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 this time are compliance monitoring wells MW-202S and MW-202D, Pine Hall Road Landfill monitoring well MW-3, Craig Road Landfill monitoring well CRW10, FGD Landfill monitoring wells BC-23A and BC28, and CSA background monitoring wells BG-1 D, BG-2S, BG-2D, BG-2BR, BG-3S, BG-3D and MW-202BR (Figure 1-4). BG-1S was installed as a background well during the CSA; however, the well was dry and a groundwater sample could not be collected during the CSA. The Craig Road and FGD Landfills are on the south side of Belews Lake and those background wells are not shown on Figure 1-4 (but can be found in Appendix B on Figure B-1). Note that analytical results for samples collected with turbidity values greater than 10 Nephelometric Turbidity Units (NTU) were not included in the PPBC calculations. However, the evaluation of COls in CAP Part 1 does consider analytical data where turbidity was greater than 10 NTU. 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) that were collected during the CSA where turbidity was greater than 10 NTU were used in the contaminant fate and transport modeling discussed in Section 4. This should be taken into account when evaluating the results of the fate and transport model and considering the risk classification for the BCSS site. The range of background groundwater concentrations for the BCSS site, PPBCs, and regional background data are presented in Table 2-2. Background concentrations reported in Table 2-2 at BCSS are limited to samples collected from wells with turbidity less than 10 NTU. PPBCs were calculated as the Upper 95% Prediction Limit using the compliance and landfill monitoring wells, or where too few data were available to perform statistics (less than 8 samples), are the highest reported value (or highest laboratory reporting limit for non -detects) in the compliance, landfill, and newly installed background monitoring wells. Background concentrations identified for new background wells will be incorporated into statistical background analysis once a statistically valid data set has been obtained. Regional groundwater data in Table 2-2 were reviewed 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 was used for all constituents contained in the NURE database. NC DHHS county -level data were the secondary source for all constituents available. 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 BCSS CSA Report. Particularities of the NURE and NC DHHS data are as follows: 23 The NC DHHS collected data from private well owners whose water samples were analyzed at the North Carolina State Laboratory of Public Health from 1998-2010. Basic summary statistics of this data were subsequently calculated by the Superfund Research Translation Corps at the University of North Carolina at Chapel Hill, and posted for public use on the NC DHHS Epidemiology section website. These data are informative for illustrating broad spatial differences in groundwater quality across North Carolina counties, but have a number of limitations. Minimum, maximum, and average concentrations were calculated for each analyte by county; however, these statistics were calculated with no consideration of non -detected values, which causes a high bias to the results. Furthermore, certain concentration values provided by NC DHHS appear unusually high, such as the mean and maximum iron concentrations cited for Rowan County, suggesting that some issues may have been left unresolved in the data cleaning process. • Groundwater chemical concentration data in a 20-mile radius surrounding the BCSS site were collected from USGS NURE program between 1975 and 1980. The data have a high level of spatial resolution and consistent coverage over the state of North Carolina, making it a useful and appropriate resource for illustrating state-wide patterns (or variations) in groundwater quality. Data provided in the "2-10 Private Well Data" column identifies the range of values found for each constituent sampled in private wells owned by Duke Energy employees living within 2 and 10 miles from the BCSS waste boundary. The 2-10 private well results are provided for reference only due to the lack of well construction data, hydrostrati graphic data, and detailed geological context for these sample locations. An analysis of BCSS background groundwater concentrations is provided in Appendix B. 24 Table 2-2. Background Concentrations for Groundwater COls Identified in the CSA: Ranges of Analytical Results with Sample Turbidity <10 NTU Compliance Pine Hall Road Craig Road and Regional 2-10 Private Well Landfill Well FGD Landfill New Background Proposed Background Well Data (May Background Background Well Wells Provisional Constituent Groundwater 2015-August Concentrations Concentrations Background Groundwater Background Concentrations 2015) (pg/L) (2010 to 2015) (2000 to 2014) Concentrations Concentrations Concentrations (pg/L) (Ng/L) (Ng/L) (2008 to 2015) (June 2015) (pg/L) (pg/L) (Ng/L) Antimony <6 (North Carolina) <1 to 1.13 0.44J to 1.16 <1 to <5 Not reported 0.16J to 1.5 5 1.8 (mean) 0.5 to Arsenic 20 (range) (Stokes, <0.5 to 1.7 <0.5 to <1 <1 to <5 <1 to <5 0.2J to 1.9 5 Rockingham) Beryllium Not Determined <0.2 to <1 0.18J to <1 <1 to <5 Not reported 0.17J to 0.33 0.33 70 (mean) <10 to Boron 590 (EPRI - National near <5 to <50 37J to <50 <50 <50 to 12 0.37J to <50 50 power plants) 0.6 (mean) 0.5 to Cadmium 5 (range) (Stokes, <0.08 to <1 0.052J to <1 <1 <1 0.052J to 0.41 1 Rockingham) Below Detect to Chloride 55,700 (20 mile 1,400 to 11,000 1,200 to 3,300 7,170 to 9,810 <5,000 to 6,080 2,000 to 9,700 9,810 radius from site) 2.5 (mean) 0.5 to 80 (range) Chromium (Stokes, <0.5 to <5 0.87J to 15 <5 <5 to 3.7 0.77J to 4.8 8 Rockingham)Nor th Carolina) Hexavalent Chromium Not Determined <0.03 to 3 0.13 to 3.2 Not reported Not reported Not reported 3.2* Cobalt 1 to 2 (USGS) <0.5 to <1 <0.5 to <1 <1 to <5 Not reported 0.19J to 0.9 0.9* 654 (mean) 25 to Iron 33,970 (range) (Stokes, <10 to 389 17 to 7,280 59.9 to 1,280 33.3 to 1,820 <50 to 1,900 1,820 Rockingham) 26 Compliance Pine Hall Road Craig Road and Regional 2.10 Private Well Landfill Well FGD Landfill New Background Proposed Background Well Data (May Background Background Well Wells Provisional Constituent Groundwater 2015-August Concentrations Concentrations Background Groundwater Background Concentrations 2015) (pg/L) (2010 2015) (200t 2014) Concentrations Concentrations Concentrations (Ng/L) (Ng/L) (Ng/L) (2008 to 2015) (June 2015) (pg/L) (pg/L) (Ng/L) Below Detect to Manganese 785.8 (20 mile <0.5 to 195 2.9J to 413 <5 to 22.3 <5 to 96.2 2.7J to 93 96.2 radius from site) 4.5 to 8 SU (20 pH mile radius from 6.24 to 8.05 SU 5.3 to 6.3 SU 5.37 to 5.71 SU 5.35 to 6.14 SU 5.78 to 9.04 SU 4.9 to 8.5 SU site 2.7 (mean) 2.5 to Selenium 26 (range) <0.5 to 1.1 0.44J to <1 <1 to <10 <1 to <10 0.24J to 1.1 10 (Stokes, Rockingham) Sulfate Not Determined 1,400 to 8,500 120 to 9,600 100 to 1,590 <5,000 to 78,900 920J to 23,400 78,900 TDS Not Determined 150,000 to 30,000 to <20,000 to 63,000 44,000 to 57,000 to 173,000 169,000 170,000 133,000 169,000 Thallium <1 (Blue Ridge) <0.1 to <0.2 <0.1 <0.2 to <10 Not reported 0.024J to <0.1 10 <DL to 13.3 (20 Vanadium mile radius from 0.653 to 4.89 <0.1 to 2.3 <5 to 2.01 Not reported 0.31J to 7.4 7.4* site) Notes: 1. tag/L = micrograms per liter 2. SU = Standard Units 3. < indicates concentration less than laboratory reporting limit. 4. J = Estimated concentration 5. 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 is 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 BCSS CSA Report. 6. Reported compliance monitoring well (MW-202S/D) concentration ranges for beryllium, hexavalent chromium, cobalt, and vanadium are from an NPDES sampling event in April 2015 and the June 2015 CSA sampling event. Only June 2015 data were provided in the CSA Report. These constituents were historically not analyzed for as part of the NPDES sampling program. 7. PPBCs for constituents monitored during the CSA not considered COls are provided in Appendix B. 8. * = Sufficient data to statistically derive concentrations not available. PPBC presented is the highest reported value (or highest laboratory reporting limit for non -detects) in the compliance, landfill, and newly installed background monitoring wells. 26 Corrective Action Plan Part 1 Belews Creek Steam Station Ash Basin 2.2.2 Groundwater Exceedances of 2L Standards or IMACs Groundwater impacts at the BCSS site attributed to ash handling and storage were delineated during the CSA activities with the following exceptions: • Horizontal extent west and downgradient of the ash basin dam and Middleton Loop Road. • Horizontal and vertical extent in area of seep location S-9 in drainage south of Pine Hall Road and adjacent to the ash structural fill. Additional monitoring wells will be installed during the fourth quarter of 2015 and the first quarter of 2016 to address the above -referenced data gaps. Information gathered from additional assessment will be submitted under a separate cover. 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 in the table for reference purposes only. Groundwater sample locations and analytical results are depicted on Figure 2-1. Table 2-3. Groundwater Results for COls Compared to 2L Standards, IMACs or NC DHHS HSL, Frequency of Exceedances and PPBCs COI Proposed Provisional Background Concentrations (pg/L) NC 2L Standard IMAC or � NC DHHS HSL (Ng/L) Groundwater Concentrations Exceeding 2L Standards IMAC or NC DHHS HSL /L Number of Samples Exceeding 2L Standards or IMACs/Number of Samples Upgradient of Ash Basin Antimony* 5 1 1.3 to 2.5 4/25 Chromium 8 10 10.3 to 50.7 2/25 Hexavalent Chromium** 3.2 0.07 0.16 to 3.7 3/8 Cobalt* 0.9 1 1.3 to 16.5 10/25 Iron 1,820 300 610 to 2,200 9/25 Manganese 96.2 50 73 to 1,100 13/25 pH 4.9 to 8.5 SU 6.5 to 8.5 SU 4.36 to 11.52 SU 17/25 Thallium* 10 0.2 0.23 to 0.24 2/25 TDS 169,000 500,000 503,000 1 /25 Vanadium* 7.4 0.3 0.42J+ to 10.2 21/25 Pine Hall Road Landfill-Upgradient Wells pH 4.9 to 8.5 SU 6.5 to 8.5 SU 4.92 to 6.46 SU 3/3 Pine Hall Road Landfill-Downgradient Wells Antimony* 5 1 25.9 1/9 Arsenic 5 10 40.9 1 /9 Boron 50 700 2,720 to 28,700 4/9 27 Corrective Action Plan Part 1 Belews Creek Steam Station Ash Basin COI Proposed Provisional Background Concentrations (pg/L) NC 2L Standard IMAC or � NC DHHS HSL (Ng/L) Groundwater Concentrations Exceeding 2L Standards IMAC or NC DHHS HSL /L Number of Samples Exceeding 2L Standards or IMACs/Number of Samples Cadmium 1 2 2.01 and 3.02 2/9 Chromium 8 10 11.7 and 13.1 2/9 Cobalt* 0.9 1 1.3 and 2.13 2/9 Iron 1,820 300 365 to 2,300 5/9 Manganese 96.2 50 100 to 2,190 4/9 pH 4.9-8.5 SU 6.5-8.5 SU 5.13 to 6.12 SU 8/9 Selenium 10 20 27 to 332 4/9 Sulfate 78,900 250,000 1,210,000 and 1,580,000 3/9 Thallium* 10 0.2 8.74 1/9 TDS 169,000 500,000 1,590,000 to 2,630,000 3/9 Vanadium* 7.4 0.3 1.15 to 203 6/9 Beneath Ash Basin Antimony* 5 1 8.1 1 /15 Arsenic 5 10 39 1 /15 Boron 50 700 2,400 to 13,200 5/15 Chloride 9,810 250,000 307,000 to 541,000 5/15 Chromium 8 10 19.4 to 39.7 2/15 Hexavalent Chromium** 3.2 0.07 7.5 to 14 2/9 Cobalt* 0.9 1 1.1 to 108 8/15 Iron 1,820 300 320 to 12,800 7/15 Manganese 96.2 50 91 to 14,800 10/15 pH 4.9 to 8.5 SU 6.5 to 8.5 SU 4.47 to 11.18 SU 13/17 Thallium* 10 0.2 0.31 to 0.42 4/15 TDS 169,000 500,000 1,100,000 to 1,430,000 5/15 Vanadium* 7.4 0.3 0.33J to 47.2 12/15 Downgradient of Ash Basin Antimony* 5 1 1.6 1/16 Arsenic 5 10 79.1 1/16 Beryllium* 0.33 4 4.4 to 6.6 3/16 Boron 50 700 980 to 8,800 3/16 Cadmium 1 2 3.8 1/16 Chloride 9,810 250,000 280,000 to 407,000 2/16 Chromium 8 10 29.7 1/16 Hexavalent Chromium** 3.2 0.07 0.084 to 0.6 2/6 Cobalt* 0.9 1 1.1 to 413 11/16 Iron 1,820 300 320 to 92,200 9/16 28 Corrective Action Plan Part 1 Belews Creek Steam Station Ash Basin Proposed Groundwater Number of Samples Provisional NC 2L Standard � Concentrations Exceeding 2L COI Background IMAC or Exceeding 2L Standards or Concentrations NC DHHS HSL Standards IMAC or IMACs/Number of (pg/L) (Ng/L) NC DHHS HSL Samples /L Manganese 96.2 50 84 to 21,300 12/16 pH 4.9 to 8.5 SU 6.5 to 8.5 SU 4.14 to 11.98 SU 14/18 Thallium* 10 0.2 0.3 to 3.6 4/16 TDS 169,000 500,000 526,000 to 1,270,000 5/16 Vanadium* 7.4 0.3 0.32J to 7.9 7/16 Notes: 1. lag/L = micrograms per liter 2. SU = Standard Units 3. J = Laboratory estimated concentration 4. J+ = Estimated concentration, biased high 5. NC DHHS indicates the North Carolina Department of Health and Human Services 1. * =2L Standard not established for constituent; therefore, IMAC used for screening criteria 2. ** = 2L Standard not established for constituent; therefore, NC DHHS HSL for private water supply wells used Observations related to groundwater COls at BCSS are: • Arsenic was reported at concentrations greater than the 2L Standard in three groundwater samples: on the ash basin dam (AB-1 S), at the toe of the ash basin dam (MW-103S), and downgradient of Pine Hall Road Landfill (OB-4). The AB-1S sample concentration was 39 pg/L (1.2 pg/L dissolved fraction) with a turbidity of 14.5 NTU. The MW-103S sample concentration was 79.1 pg/L in the total and dissolved analyses with a turbidity of 4.7 NTU. The OB-4 sample concentration was 40.9 pg/L in the total analysis; dissolved analysis was not performed and the turbidity was 4.3 NTU. Although the exceedances are limited to three wells, this constituent cannot be ruled out as a COI as part of the CAP Part 1. Beryllium was reported exceeding its IMAC of 4 pg/L in three groundwater samples: two north of the ash basin dam (GWA-1S and MW-103D) and one northwest of the ash basin dam (GWA-11S); all three locations are downgradient of the ash basin. The GWA- 1S sample concentration was 6 pg/L (6.3 pg/L dissolved fraction) with a turbidity of 8.3 NTU. The MW-103D sample concentration was 4.4 pg/L (2.9 pg/L dissolved fraction) with a turbidity of 7.4 NTU. The GWA-11S sample concentration was 6.6 pg/L (6.9 pg/L dissolved fraction) with a turbidity of 3.1 NTU. Although the exceedances have limited spatial extent across the site, this constituent cannot be ruled out as a COI as part of the CAP Part 1. • Cadmium was reported at a concentration greater than the 2L Standard in two groundwater samples: one at the toe of the ash basin dam (MW-103D) and one downgradient of Pine Hall Road Landfill (OB-4). The MW-103D sample concentration was 3.8 pg/L (4.2 pg/L dissolved fraction) with a turbidity of 7.4 NTU. The OB-4 sample concentration was 2.01 pg/L (no dissolved analysis) with a turbidity of 4.3 NTU. Although the exceedances are limited to two wells across the site, exceedances of other COls in both wells indicate cadmium exceedances may be related to ash handling activities; as such, this constituent cannot be ruled out as a COI as part of the CAP Part 1. 29 Corrective Action Plan Part 1 Belews Creek Steam Station Ash Basin Chromium was reported at concentrations greater than the 2L Standards in seven groundwater samples: two on the ash basin dam (AB-1S and AB-2S), two upgradient of the ash basin (GWA-17S and MW-201D), one below the ash basin dam (GWA-1S) and two downgradient of Pine Hall Road Landfill (MW2-9 and MW-4). Chromium in the speciation sample for GWA-41D, an upgradient well southeast of the ash basin, also exceeded the 2L Standard. o Chromium exceedances in samples collected from the two upgradient wells were associated with turbidities greater than 10 NTU and dissolved fractions were less than the 2L Standard. The GWA-17S sample concentration was 50.7 pg/L (0.2 pg/L dissolved fraction) with a turbidity of 21.7 NTU. The MW-201 D sample concentration was 10.3 pg/L (2.3 pg/L dissolved fraction) with a turbidity of 17.4 NTU. o Chromium exceedances in samples collected from the wells located on or below the ash basin dam were associated with turbidities both above and below 10 NTU; however, dissolved fractions exceeded the 2L Standard. The AB-1 S sample concentration was 39.7 pg/L (12.8 pg/L dissolved fraction) with a turbidity of 14.5 NTU. The AB-2S sample concentration was 19.4 pg/L (14 pg/L dissolved fraction) with a turbidity of 8.2 NTU. The GWA-1S sample concentration was 29.7 pg/L (12.5 pg/L dissolved fraction) with a turbidity of 8.3 NTU. o No dissolved analyses were conducted on the two landfill samples. The MW2-9 sample concentration was 11.7 pg/L with a turbidity of 37.3 NTU. The MW-4 sample concentration was 13.1 pg/L with a turbidity of 4.9 NTU. o The GWA-41D speciation sample concentration was 11.8 pg/L with a turbidity of 8.3 NTU. o Based on the results described above, chromium exceedances do not appear to be associated with high turbidity. This constituent cannot be ruled out as a COI as part of CAP Part 1. • Hexavalent chromium was analyzed in select groundwater monitoring wells (primarily along presumed groundwater flow paths) at the site. Hexavalent chromium was reported above the NC DHHS HSL in the three background wells sampled (0.13 to 3.2 pg/L) and in seven of twenty-three wells located upgradient of the ash basin, beneath the ash basin and downgradient of the ash basin. Hexavalent chromium analysis will be performed on samples collected from additional wells during subsequent sampling events to further evaluate hexavalent chromium occurrence and distribution at the BCSS site. • Exceedances of pH beyond the 2L Standard range were observed throughout the BCSS site. Further evaluation is needed to determine if pH should remain as a COI. • The following groundwater COls are considered for further evaluation: • Antimony • Arsenic • Beryllium • Iron • Manganese • pH 30 Corrective Action Plan Part 1 Belews Creek Steam Station Ash Basin • Boron • Cadmium • Chloride • Chromium • Cobalt • Hexavalent chromium • Selenium • Sulfate • Thallium • TDS • Vanadium Of the COls identified at BCSS, boron, pH, sulfate, and TDS are considered to be detection monitoring constituents and are listed in 40 CFR 257 Appendix III of the USEPA's Hazardous and Solid Waste Management System; Disposal of Coal Combustion Residuals from Electric Utilities (CCR Rule). The USEPA considers these constituents to be potential indicators of groundwater contamination from CCR as they move rapidly through the surface layer, relative to other constituents, and thus provide an early detection of whether contaminants are migrating from the CCR unit. Additional details regarding the CCR Rule and applicable constituents can be found in the CSA Report (HDR 2015). PPBCs were determined to be greater than (or outside of the range of in the case of pH) the 2L Standards, IMACs, or NC DHHS HSL for the following constituents: • Antimony • Hexavalent Chromium • Iron • Manganese • pH • 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 HSLs during future sampling events. For PPBCs determined to be less than the 2L Standards, IMACs, or NC DHHS HSLs, the respective regulatory standard for that constituent will continue to be used for determining exceedances. 2.2.3 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 as part of the CSA: radium-226, radium-228, uranium, uranium-233, uranium-234, and uranium-236. Background monitoring wells BG-1 D, MW-202S, MW-202D, and MW-202BR and monitoring wells MW-200S, MW-200D, MW-200BR located below the ash basin dam were sampled for these analytes, and results of this analysis are presented in Table 2-4. 31 Corrective Action Plan Part 1 Belews Creek Steam Station Ash Basin Table 2-4. Radionuclide Concentrations Background Downgradient of Radionuclide USEPA MCL* Concentrations Source Area Concentrations Radium-226 5 pCi/L (combined) 6.55 to 8.74 pCi/L (combined) 4.74 to 12.84 pCi/L (combined) Radium-228 Uranium 30 lag/L 0.081J to 1.81 lag/L 0.0502J to 0.829 lag/L Uranium-233 30 lag/L <0.0015 lag/L < 0.0015 lag/L Uranium-234 (combined) (combined) (combined) U ran iu m-236 Notes: 1. pCi/L = Picocuries per liter 2. lag/L = micrograms per liter 3. J = Estimated concentration 4. < indicates concentration less than laboratory reporting limit 5. MCL = Maximum Contaminant Level 6. Bold indicates an exceedance of USEPA MCL 7. * USEPA MCL for uranium of 30 lag/L assumes combined concentration for all isotopes As shown in Table 2-4, the highest reported concentrations of radium-226 and radium-228 in background and downgradient monitoring wells exceeded the USEPA MCL. The downgradient monitoring wells had a greater range in concentrations than background monitoring wells although the mid -point concentrations were similar. Uranium was reported at greater concentrations in the background monitoring wells than in the downgradient monitoring wells; all monitoring well results were well below the USEPA MCL. Uranium-223, uranium-234, and uranium-236 were not reported above their laboratory reporting limits in any of the samples. 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 CSA Seeps Seep samples anticipated to be associated with the ash basin were collected at three locations during the CSA (S-6, S-10 and S-11). Seep S-6 is located downgradient of the original (closed) ash basin pond discharge to Belews Lake, and seeps S-10 and S-11 are located near the compliance boundary downgradient of the ash basin dam. Seep samples initially considered to be outside of expected impacts from the ash basin were collected at eight locations (S-1 through S-5 and S-7 through S-9) during the CSA. Seeps S-1 through S-5 are located west of the ash basin across Middleton Loop Road on forested property owned by Duke Energy; the locations are at least 1,000 feet from the western edge of the ash basin and flow north into drainage features toward the Dan River. Seep S-7 is located on the east side of Pine Hall Road near the entrance to BCSS and is also located on forested land owned by Duke Energy. Seep S-8 is located in a forested area south of 32 Corrective Action Plan Part 1 Belews Creek Steam Station Ash Basin Pine Hall Road and west of a plant access road. Seep S-9 is also located south of Pine Hall Road on the east side of the plant access road toward the ash structural fill. Seep samples are collected during required semiannual monitoring associated with the solid waste permit in drainage features on the eastern and western side of Pine Hall Road Landfill, which both drain to the ash basin. The sample locations (SW-1A and SW-2) are described as seeps, which are indicative of groundwater. CSA seep results for COls and comparison to 2L Standards or IMACs are provided in Table 2- 5A. Seep sample locations and analytical results are shown on Figure 2-2; analytical results of groundwater samples in close proximity to the seep samples are also shown for comparison. Table 2-5A. CSA Seep Results for COls Compared to 2L Standards, or IMACs and Frequency of Exceedances COI NC 2L Standard or IMAC (pg/L) Seep Concentrations Exceeding 2L Standards or IMAC (pg/L) Number of Samples Exceeding 2L Standards or IMACs/Number of Samples Associated with Ash Basin Boron 700 3,900 to 9,400 3/3 Chloride 250,000 414,000 to 434,000 2/3 Cobalt* 1 1.1 to 74.8 2/3 Iron 300 709 to 1,480 3/3 Manganese 50 440 to 8,500 3/3 Thallium* 0.2 0.22 to 0.41J+ 3/3 TDS 500,000 509,000 to 11,700,000 3/3 Vanadium* 0.3 9 1/3 Pine Hall Road Landfill Boron 700 1,630 to 12100 2/2 Cobalt* 1 1.59 1/2 Iron 300 511 1/2 Manganese 50 800 1/2 pH 6.5 to 8.5 SU 6.18 to 6.31 SU 2/2 Selenium 20 78.6 1/2 Sulfate 250,000 676,000 1/2 TDS 500,000 1,120,000 1/2 Vanadium* 0.3 1.12 1/2 Outside Ash Basin Impact Arsenic 10 16.9 1/8 Boron 700 2,800 1/8 Chromium 10 11.5 1/8 Cobalt* 1 1.1 to 6.7 3/8 Iron 300 502 to 1,730 4/8 Manganese 50 57 to 460 7/8 pH 6.5 to 8.5 SU 5.59 to 6.42 SU 6/8 33 Corrective Action Plan Part 1 Belews Creek Steam Station Ash Basin NC 2L Standard Seep Number of Samples COI or IMAC Concentrations Exceeding 2L Standards (pg/L) Exceeding 2L Standards or IMACs/Number of or IMAC /L Samples Sulfate 250,000 475,000 1 /8 TDS 500,000 791,000 1 /8 Vanadium* 0.3 0.45J to 2.6 7/8 Notes: 1. lag/L = micrograms per liter 2. SU = Standard Units 3. J = Laboratory estimated concentration 4. J+ = Estimated concentration, biased high 5. * = 2L Standard not established for constituent; therefore, IMAC used for screening criteria Observations related to seep COls at BCSS are: The COls reported at concentrations greater than the 2L Standards in seep samples S-1 to S-11 are generally present in multiple samples and are present in groundwater samples at BCSS, thus none can cannot be ruled out as a COI as part of the CAP Part 1. • Seep samples S-10 and S-11 were collected downgradient of the ash basin dam in the same area as the majority of the NCDENR seep samples, which are capturing water discharge from the embankment dam. The COls reported at concentrations greater than the 2L Standards at S-10 and S-11 (boron, chloride, cobalt, iron, manganese, thallium and total dissolved solids) were consistent with those reported in the NCDENR seep samples with variances due to the difference in the 2L and 2B Standards. The reported COI concentrations in the two sets of samples (seeps S-10/S-11 and the NCDENR seeps) were also similar, indicating a connection between the groundwater being discharged at natural seeps and the water collected by the toe drains. Arsenic was reported at a concentration greater than the 2L Standard in one seep sample collected south of Pine Hall Road (S-7). The sample concentration was 16.9 pg/L (2 pg/L dissolved fraction) with a turbidity of 18.2 NTU. Based on the sample location on the opposite side of the groundwater divide from the ash basin and the detection monitoring constituents boron, sulfate, and TDS not being detected above the laboratory method detection limits, the 2L Standard exceedances at S-7 are not considered to be related to the ash basin. • Boron, sulfate and TDS were reported at concentrations greater than the 2L Standards in seep sample S-9, but not in other seep samples located outside the expected area of ash basin impacts. S-9 is located in a small drainage on the west side of the structural fill south of Pine Hall Road and is not considered to be associated with the ash basin due to its location. Additional investigation in this area was recommended in the CSA data gap section to assess potential impacts from the structural fill. • Seep samples S-1 to S-5 were collected in separate, small drainage areas approximately 1,500 feet northwest of the ash basin. Iron, manganese, pH and vanadium were each reported in three to five of the samples at concentrations greater than the 2L Standards but consistent with background groundwater concentrations. 34 Corrective Action Plan Part 1 Belews Creek Steam Station Ash Basin Chromium and cobalt were reported at concentrations greater than the 2L Standards in S-3. • Seeps S-2 and S-4 are located on Duke Energy property west of Middleton Loop Road and the ash basin dam, and sampling reported elevated levels of TDS and chloride above background concentrations but less than their 2L Standards at these locations. This indicates potential groundwater flow and migration at the northwestern rim of the ash basin toward the Dan River. This flow direction is away from the direction of the nearest public or private water supply wells. • Based on a review of data from seeps identified by NCDENR and by Duke Energy during the CSA, the following seep COls will be considered for further evaluation: • Arsenic • pH • Boron • Selenium • Chloride • Sulfate • Chromium • Thallium • Cobalt • TDS • Iron • Vanadium • Manganese '.3.2 NCDENR Seeps Seep samples were also collected from 13 NCDENR-identified locations with nine samples collected below the ash basin dam and four samples collected outside the area of ash basin impacts. Samples collected below the ash basin dam consisted of eight samples at the base of the ash basin dam (primarily from toe drains installed within the structural fill of the dam (TF-1, TF-2, TF-3, HD-7A, HD-11A, HD-21, HD-26 and ABW)) and one sample from the ash basin discharge outlet (003). Samples collected outside the area of ash basin impacts consisted of one sample from the wastewater treatment plant effluent (BCWW--002); one sample adjacent to the FGD Landfill (BCSW-08); and two samples adjacent to railroad tracks near Belews Lake (BCSW-018A and BCSW-019). The NCDENR sample locations are associated with surface water discharges and are compared to 2B Standards or USEPA WQC. Surface water from the ash basin seeps through the ash basin embankment and is captured in a series of horizontal drains and engineered flumes before being routed through a Parshall flume for flow monitoring at the toe of the dam. This seepage is believed to be a result of preferential seepage paths between the seeps and the upstream ash basin and surface water. There is no background comparison currently available for seeps. The seep sample locations associated with the ash basin, Pine Hall Road Landfill and NCDENR locations are downgradient of potential source areas and do not represent background conditions. The seep sample locations outside of expected impacts from the ash basin are not yet confirmed to represent background conditions. NCDENR seep results associated with surface water discharges for COls, along with a comparison to 2B Standards or USEPA Criteria are provided in Table 2-51B. Seep sample 36 Corrective Action Plan Part 1 Belews Creek Steam Station Ash Basin locations and analytical results are shown on Figure 2-2; analytical results of groundwater samples in close proximity to the seep samples are also shown for comparison. Table 2-5B. NCDENR Seep Results Associated with Surface Water Discharges for COls Compared to 2B Standards or USEPA Criteria, and Frequency of Exceedances COI NC 2B Standard or USEPA Criteria (pg/L) Concentrations Exceeding 2B Standards or USEPA Criteria (pg/L) Number of Samples Exceeding 2B Standard/Number of Samples Below Ash Basin Dam Cadmium 0.15 0.18 to 2 8/9 Chloride 230,000 426,000 to 501,000 8/9 Cobalt 3 30.4 to 268 8/9 Dissolved Oxygen 5,000 minimum 3,620 to 4,300 2/9 Iron 1,000 1,170 to 4,240 3/9 Lead 0.54 0.56 to 1.2 2/9 Manganese 50 84 to 30,800 9/9 Mercury 0.012 0.0157 to 0.154 6/9 Nickel 16 36.7 to 46.7 2/9 pH 6 SU minimum 4.85 to 5.76 SU 8/9 Selenium 5 5.9 to 11.2 5/9 Thallium 0.24 0.25 to 1.2 9/9 TDS 250,000 1,140,000 to 14,000,000 8/9 Outside Ash Basin Impacts Chloride 230,000 4,490,000 1/4 Cobalt 3 4.4 1/4 Copper 2.7 3.7 1/4 Dissolved Oxygen 5,000 minimum 2,370 to 4,500 2/4 Iron 1,000 5,810 1/4 Manganese 50 78 to 6,010 3/4 Sulfate 250,000 854,000 1/4 TDS 250,000 13,800,000 1/4 Notes: 1. lag/L = micrograms per liter 2. SU = Standard Units Observations related to NCDENR seep COls at BCSS are: • Nine of the NCDENR seep samples were collected from a relatively small area at the base of the ash basin dam and cadmium, chloride, cobalt, manganese, pH, thallium and total dissolved solids were identified as exceedances of the 2B Standards in the majority of samples collected. Based on these results these constituents cannot be ruled out as a COI as part of the CAP Part 1. Lead was reported at concentrations greater than the 2B Standard in two seep samples collected below the ash basin dam (HD-21 and HD-26). Lead was not identified as a COI 36 Corrective Action Plan Part 1 Belews Creek Steam Station Ash Basin in the source characterization samples and, based on the limited exceedances in the seep samples, will not be carried forward for further assessment. Nickel was reported at concentrations greater than the 2B Standard in two seep samples collected below the ash basin dam (HD-21 and TF-3) at concentrations of 36.7 and 46.7 pg/L. Nickel was not identified as a COI in the source characterization samples, and based on the limited exceedances will not be carried forward for further assessment. Selenium was reported at concentrations greater than the 2B Standard in five seep samples collected below the ash basin dam at concentrations of 5.9 to 11.2 pg/L. The dissolved fraction concentration for four samples were below the 2B standard. The total and dissolved fraction concentrations in the fifth sample (003) were 5.9 and 5.6 pg/L, respectively. Based on these results. selenium cannot be ruled out as a COI as part of the CAP Part 1. Four of the NCDENR seep samples were collected from locations near Belews Lake and were not associated with the ash basin. Of the eight COls reported at concentrations greater than the 2B Standards, six were detected at only one location. The presence of COls at these locations will not be considered in selection of seep COls for further assessment. Surface Water Surface water samples were obtained during the CSA at four locations in the Dan River and Belews Lake. Samples SW-DR-U and SW-DR-D were collected upgradient and downgradient, respectively, from the NPDES outfall location of the unnamed water conveyance from the ash basin into the Dan River. The upgradient sample from Belews Lake (SW-BL-U) was collected on the south shore of the lake west (upstream) of the Craig Road Landfill. The downgradient sample from Belews Lake (SW-BL-D) was collected on the north shore of the lake near the boat ramp northeast of BCSS. Surface water sample 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 2B Standards for Class WS-IV waters. In the absence of a 2B Standard, constituent concentrations were compared to USEPA National Recommended Water Quality Criteria. Surface water exceedance results for COls, compared to upgradient surface water concentrations and applicable regulatory standards, are provided in Table 2-6. Surface water sample locations and analytical results are shown on Figure 2-2. 37 Corrective Action Plan Part 1 Belews Creek Steam Station Ash Basin Table 2-6. Surface Water Results for COls Compared to Upgradient Surface Water Concentrations, 2B Standards or USEPA National Recommended Water Quality Criteria and Frequency of Exceedances Concentrations NC 2B Number of Samples Exceeding 2B Standard or Upgradient Exceeding 2B COI Standards or USEPA Surface Water Standards or USEPA Criteria Criteria Concentrations USEPA (pg/L) (pg/L) (Ng/L) Criteria/Number of Samples Dan River Chloride 240,000 230,000 3,300 1/2 Manganese 60 and 240 50 60 2/2 pH 5.39 and 5.92 SU 6 SU minimum 5.92 SU 2/2 TDS 815,000 250,000 58,000 1/2 Thallium 0.55 0.24 <0.1 1/2 Belews Lake Dissolved Oxygen 4,300 <5,000 4,300 1/2 Notes: 1. lag/L = micrograms per liter 2. SU = Standard Units 3. <5,000 represents the minimum acceptable DO concentration for freshwater aquatic life. 4. * Indicates USEPA National Recommended Water Quality Criteria used for constituent. Observations related to surface water COls at BCSS are: • Dissolved oxygen was reported at concentration less than the 2B Minimum Standard in the Belews Lake upstream surface water sample (SW-BL-U). Since the downstream surface water sample (SW-BL-D) was within the standard, dissolved oxygen does not appear to be indicative of impacts from ash basin activities. As such, this parameter will not be carried forward for further assessment. • Exceedances of surface water quality standards were reported in the Dan River downstream surface water sample (SW-BL-D). Chloride and total dissolved solids were reported at concentrations greater than the 2B Standard; thallium was reported at concentrations greater than USEPA recommended criteria. Dissolved fraction concentrations were similar to the total concentrations although turbidity was 11.5 NTU. • The following surface water COls will be considered for further evaluation: • Chloride • Thallium • Manganese • TDS • pH i Sediments Sediment samples were collected at the same time as the surface water samples at upstream and downstream locations in Dan River (SD-DR-U and SD-DR-D) and Belews Lake (SD-BL-U and SD-BL-D). In the absence of NCDEQ sediment criteria, the sediment sample results were compared to the NCPSRGs for POG. However, it should be noted that NC PSRGs for POG 38 Corrective Action Plan Part 1 Belews Creek Steam Station Ash Basin were derived to be protective of groundwater. Application of these standards to sediment sample results is not consistent with the purpose of the standards and should not be used to evaluate the need for corrective action. Sediment samples were not collected during the CSA at seep locations near the ash basin (S-1 to S-11) or at NCDEQ-identified seeps (TF-1, TF-2, TF- 3, HD-7A, HD-11A, HD-21, HD-26, ABW, BCWW-002, BCSW-08, BCSW-018A and BCSW- 019). The sediment samples at these locations were collected during a second sampling event conducted at the end of September 2015 and will be addressed in the CAP Part 2 report. Sediment exceedance results for COls based on comparison to NC PSRGs for POG and upstream sediment concentrations are provided in Table 2-7. 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, Upgradient Concentrations and Frequency of Exceedances COI Concentrations Exceeding NC PSRGs for POG (mg/kg) NC PSRGs for POG (mg/kg) Upgradient Sediment Concentrations (mg/kg) Number of Samples Exceeding NC PSRGs for POG/Number of Samples Dan River Chromium 8.4 to 16.6 3.8* 8.4 2/2 Cobalt <6.7 to 9.3J- 0.9 <6.7 2/2 Iron 6,990 to 13,800 150 6,990 2/2 Manganese 90.4 to 217 65 90.4 2/2 Selenium 4.3J- to <6.7 2.1 <6.7 2/2 Vanadium 15 to 28.5J- 6 15 2/2 Belews Lake Chromium 5.2 3.8* 5.2 2/2 Iron 4,760 to 6,430 150 4,760 2/2 Manganese 68.3 65 62.2 2/2 Vanadium 1 11 to 14.6 1 6 11 1 2/2 Notes: 1. mg/kg = milligrams per kilogram 2. J- = Estimated concentration, biased low 3. < indicates concentration less than laboratory method detection limit 4. NC PSRG for POG is for hexavalent chromium, sediment analytical results are for total chromium Observations related to sediment COls at BCSS are: • Reported concentrations for iron and vanadium in all sediment samples, upstream and downstream, exceeded NC PSRG for POGs. • Chromium and manganese concentrations exceeded NC PSRGs for POG in both the upstream and downstream Dan River sediment samples and the downstream Belews Lake sediment sample. • The reported concentration in downstream sediment sample SD-DR-D exceeded the NC PSRGs for POG for cobalt and selenium. Reporting limits for the other samples were greater than the NC PSRGs for POG. 39 Corrective Action Plan Part 1 Belews Creek Steam Station Ash Basin • Reported concentrations for all COls were higher in the Dan River samples than in the Belews Lake samples, and concentrations in the downstream Dan River sample were approximately double those in the upstream sample. • All the COls detected above NC PSRGs for POG in the sediment samples exceed the standards in soil background samples and may not be indicative of impacts from ash handling. Based on comparison of sediment concentrations in upstream and downstream samples, impacts from ash handling were not observed in Belews Lake. For certain COls detected in the Dan River sediment sample, concentrations in the downstream sample were higher than in the upstream sample. However, this evaluation is based on a limited number of samples collected during a single sampling event. Additional sampling within the Dan River and along the ash basin discharge conveyance should be performed to refine the background concentrations and evaluate potential influence from the BCSS site. 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 releases have occurred from 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 are: BG-1 D, BG-2S/D, BG-3S and MW-202BR (Figure 1-4). Samples shallower than 5 feet below ground surface (bgs) were not included in the population of background samples to minimize possible impacts from surface contamination. 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. 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. 40 Corrective Action Plan Part 1 Belews Creek Steam Station Ash Basin Table 2-8. Proposed Provisional Background Soil Concentrations Constituent Number of Samples Number of Detections Range (mg/kg) Proposed Provisional Background Soil Concentrations (95 /o UTL) (mg/kg) Aluminum 18 18 2,050 to 20,000 18,700 Antimony 18 0 <2.8 to <8.4 8.4* Arsenic 18 9 1.6 to 30.7 21.4 Barium 18 18 26.8 to 153 168 Beryllium 18 18 0.29 to 11.8 9.87 Boron 18 9 <3.2 to 38.6 37 Cadmium 18 1 0.2J to <1.00 1* Calcium 18 7 55.2 to 3,480 2,030 Chloride 18 0 <278 to <405 405* Chromium 18 16 0.63J to 51.7 96.9 Cobalt 18 15 <3.2 to 37.9 50.9 Copper 18 18 0.86 to 40.7 83.9 Iron 18 18 1,130 to 98,200 198,000 Lead 18 18 3.9 to 77.3 62.4 Magnesium 18 18 283 to 9,400 9,750 Manganese 18 18 18.5 to 1,010 1,120 Mercury 18 3 0.0063 to 0.015 0.015* Molybdenum 18 1 <0.57 to 8.9 8.9* Nickel 18 13 0.67J to 32.8 28.9 Nitrate 18 0 <27.8 to <40.5 40.5* pH (field) 18 18 4.9 to 6.5 SU 4.9-6.5* SU Potassium 18 17 183 to 4,330 5,160 Selenium 18 4 2.2 to <8.4 3.47 Sodium 18 2 37.8 to <419 419* Strontium 18 8 1.5 to 18.3 13.4 Sulfate 18 0 <278 to <405 405* Thallium 18 0 <2.8 to <8.4 8.4 TOC 20 5 548 to 23,700 14,050 Vanadium 18 17 <3.2 to 293 587 Zinc 18 18 9.2 to 88 121 Notes: 1. mg/kg = milligrams per kilogram 2. SU = Standard Units 3. < indicates analytical result was less than the laboratory maximum reporting limit (MRL) 4. UTL = Upper tolerance limit (USEPA 2013) 5. * = Value shown is highest detection or highest ND. In these cases, there were too few detections to develop UTL. Ranges associated with zero detections indicate the range of detection limits 2.6.2 Soil Exceedances of NC PSRGs for POG The horizontal and vertical extent of soil contamination at the site attributed to ash handling and storage was delineated in the CSA Report. Soil exceedance results for COls, along with a 41 Corrective Action Plan Part 1 Belews Creek Steam Station Ash Basin comparison to NC PSRGs for POG, soil PPBCs, and background concentrations are provided in Table 2-9. Soil sample locations and analytical results are depicted on Figure 2-3. Table 2-9. Soil Results for COls Compared to NC PSRGs for POG, Frequency of Exceedances and PPBCs COI Background Soil Concentrations (mg/kg) NC PSRGs for POG (mg/kg) Soil mg/k s ( g) Concentrations Exceeding NC PSRGs for POG (mg/kg) Number of Samples Exceeding NC POG/Number of Samples Upgradient of Ash Basin Arsenic 1.6 to 30.7 5.8 21.4 8.5 to 50.2J 4/19 Barium 26.8-153 580 168 620J- 1/19 Chromium* 0.63J to 51.7 3.8 96.9 3.9 to 54.6J- 10/19 Cobalt <3.2-37.9 0.9 50.9 2.9 to 28.7J- 16/19 Iron 1,130 to 98,200 150 198,000 912 to 44,400 19/19 Manganese 18.5 to 1,010 65 1,120 103 to 700 19/19 Selenium 2.2 to <8.4 2.1 3.47 3.6J- to 5.6 4/19 Vanadium <3.2 to 293 6 587 7.9 to 95.7J- 17/19 Beneath Ash Basin Arsenic 1.6 to 30.7 5.8 21.4 6.1 to 128 11/19 Chromium* 0.63J to 51.7 3.8 96.9 4.6 to 34.3 17/19 Cobalt <3.2-37.9 0.9 50.9 3.5J to 120J- 16/19 Iron 1,130 to 98,200 150 198,000 9,450 to 40,600 19/19 Manganese 18.5 to 1,010 65 1,120 79.2 to 1,390 17/19 Selenium 2.2 to <8.4 2.1 3.47 3.8J to 11.3 3/19 Vanadium <3.2 to 293 6 587 12.4 to 117J 19/19 Downgradient of Ash Basin Arsenic 1.6 to 30.7 5.8 21.4 23.9 to 62.1 12/13 Chromium* 0.63J to 51.7 3.8 96.9 4.5 to 39.5 10/13 Cobalt <3.2-37.9 0.9 50.9 4.1J to 26.5 11/13 Iron 1,130 to 98,200 150 198,000 8,450 to 45,800 13/13 Manganese 18.5 to 1,010 65 11120 68.2 to 689 13/13 Vanadium <3.2 to 293 6 587 8 to 87.5 13/13 Notes: 1. mg/kg = milligrams per kilogram 2. J = Laboratory estimated concentration 3. J- = Estimated concentration, biased low 4. < indicates concentration less than laboratory method detection limit 5. NC PSRG for POG indicates the North Carolina Preliminary Soil Remediation Goal for Protection of Groundwater 6. *NC PSRG for POG is for hexavalent chromium, soil analytical results are for total chromium Observations related to soil COls at BCSS are: • Barium was reported at a concentration greater than the NCPSRG POG in one soil sample (GWA-9GTB, 40-41.5 feet bgs). The boring is located on Middleton Loop Road 42 Corrective Action Plan Part 1 Belews Creek Steam Station Ash Basin upgradient of the ash basin along the compliance boundary coincident with the property boundary. The sample was collected within the screened interval of monitoring well GWA-9S. The reported concentration of barium in GWA-9S (100 pg/L) was below the 2L Standard (700 pg/L). Because barium was not identified as a COI in source characterization samples, has not been detected above the NCPSRG for POG in other soil samples, and has not been detected above the 2L Standard in groundwater samples collected during the CSA, barium is not considered a COI for corrective action. • The following COls exceed the soil NC PSRGs for POG and will be considered COls for corrective action: • Arsenic • Manganese • Chromium • Selenium • Cobalt • Vanadium • Iron As seen in Section 2.6.1 above, PPBCs for these COls are greater than their respective NC PSRGs for POG. If the PPBCs are approved for the BCSS site, barium, chromium, iron, and vanadium would be eliminated from further evaluation. Ash Ash samples from the ash basin were collected and analyzed during the CSA. 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 it will be capped or excavated during ash basin closure activities. 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. 43 Corrective Action Plan Part 1 Belews Creek 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 Arsenic 5.8 8.5 to 66 10/12 Boron 45 54.1 to 105 4/12 Chromium 3.8 4.3 to 75.5 10/12 Cobalt 0.9 4.3J to 18.3 6/12 Iron 150 1,530 to 16,900 12/12 Manganese 65 79.5 to 82.6 2/12 Selenium 2.1 2.8J to 17.3J 8/12 Vanadium 6 6.8 to 129 8/12 Notes: 1. mg/kg = milligrams per kilogram 2. J = Laboratory estimated concentration Ash Porewater Porewater refers to water samples collected from monitoring wells installed in the ash basins and ash storage area that are screened within the ash layer. Porewater COls are representative of the source (CCR), but not representative of groundwater conditions. Porewater is not further evaluated for remediation in CAP Part 1 because porewater will be eliminated by dewatering and discharged following necessary treatment during ash basin closure activities. Porewater exceedance results for COls, along with a comparison to applicable regulatory standards or guidelines, are provided in Table 2-11. Note that until PPBCs are approved by NCDEQ, COI concentrations will be compared to their applicable 2L Standard or IMAC. At this time, PPBCs are shown in the table for reference purposes only. 44 Corrective Action Plan Part 1 Belews Creek Steam Station Ash Basin Table 2-11. Ash Basin Porewater Results for COls Compared to 2L Standards, or IMACs, Frequency of Exceedances, and PPBCs Proposed Porewater Number of Provisional NC 2L Concentrations Samples COI Background Standard or Exceeding 2L Exceeding 2L Concentrations IMAC Standards, IMAC, or NC Standards or (pg/L) (Ng/L) DHHS HSL IMACs/Number of (pg/L)Sam les Within the Active Ash Basin Antimony* 5 1 8.1 to 15.2 3/11 Arsenic 5 10 11.8 to 146 9/10 Boron 50 700 1,600 to 21,900J 9/10 Chloride 9,810 250,000 298,000 to 783,000 3/10 Cobalt* 0.9 1 1.1 to 243 4/10 Iron 1,820 300 310J+ to 77,800J- 8/10 Manganese 96.2 50 120 to 9,200 6/10 pH 4.9 to 8.5 SU 6.5 to 8.5 SU 5.7 to 10.6 SU 7/11 Selenium 10 20 38.4 1/10 Sulfate 78,900 250,000 395,000 to 439,000 2/10 Thallium* 0.2 0.2 0.22 to 1.4 4/10 TDS 169,000 500,000 595,000 to 2,940,000 6/10 Vanadium* 7.4 0.3 0.38J to 867 9/10 Notes: 1. lag/L = micrograms per liter 2. SU = Standard Units 3. J = Laboratory estimated concentration 4. J+ = Estimated concentration, biased high 5. J- = Estimated concentration, biased low 6. < indicates concentration less than laboratory method detection limit 7. NC DHHS indicates the North Carolina Department of Health and Human Services 8. Indicates 2L Standard not established for constituent; therefore, IMAC used for screening criteria 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. 46 Corrective Action Plan Part 1 Belews Creek Steam Station Ash Basin Table 2-12. Ash Basin Surface Water Results for COls Compared to 2L Standards, IMACs, or NC DHHS HSL, 2B or USEPA Standards, and Frequency of Exceedances Concentrations NC 2B Standard Number of Samples Exceeding 2B or USEPA NC 2L Standard, Exceeding 2L or 2B COI Standards or Criteria IMAC, or NC DHHS Standard/Number of USEPA Criteria (pg/L) HSL (pg/L) Samples (Ng/L) Antimony 1.2 to 1.3 5.6 1 3/9 Arsenic 10.4 150 10 1 /9 Boron 5,200 to 16,300J+ 45 700 5/9 Chloride 389,000 to 230,000 250,000 3/9 492,000 Iron 860 to 1420 1,000 300 2/9 Lead 0.72 0.54 15 1 /9 Manganese 58 to 330 50 50 6/9 Selenium 5.9 to 6.7 5 20 3/9 TDS 360,000 to 250,000 500,000 5/9 1,930,000 Thallium 0.34 to 1.4 0.24 0.2 4/9 Vanadium 0.52J to 9.2 6 0.3 6/9 Notes: 1. lag/L = micrograms per liter 2. J = Laboratory estimated concentration. 3. J+ = Estimated concentration, biased high 4. * 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, further evaluation of COls in solid matrix PWR or bedrock will not be conducted. 2.11 COI Screening Evaluation Summary Table 2-13 summarizes COls (by media) identified in Sections 2.1 through 2.7 and identifies those that require further evaluation to determine if they require possible corrective action. 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. 46 Corrective Action Plan Part 1 Belews Creek Steam Station Ash Basin Table 2-13. Updated COI Screening Evaluation Summary Potential COI CSA COI Exceedances by Media COI to be Solid/ Aqueous Ash Pore- water2 Ash Basin Surface Water2 Ground- water Surface Water Seeps Sediment Soil Further Assessed in Groundwater Modeling Antimony Yes Arsenic Yes Beryllium Yes Boron Yes Cadmium Yes Chloride Yes Chromium Yes Hexavalent Chromium - _ _ _ _ Yes Cobalt Yes Copper3 No Iron Yes Lead No Manganese Yes pH Yes Selenium Yes Sulfate Yes TDS Yes Thallium Yes Vanadium Yes Notes: 1. Note that ash is not evaluated for remediation in CAP Part 1 because ash will be drained of water during remedial activities and excavated or capped. 2. Note that porewater and ash basin surface water are not evaluated for remediation in CAP Part 1 because both will be eliminated during ash basin closure activities. 3. Exceedance identified in dissolved concentration, but not total, for one surface water sample and not present in other media (copper) or one surface water sample and one ash basin surface water sample (lead). 2.12 Interim Response Actions 2.12.1 Source Control No interim response actions are necessary at the BCSS site because there are no identified imminent hazards to human health or the environment. In accordance with CAMA, Duke Energy is required to implement closure and remediation of the BCSS ash basin no later than August 1, 2029 (or sooner if classified as intermediate or high risk). Closure for the BCSS ash basin was not defined in CAMA. 47 Corrective Action Plan Part 1 Belews Creek Steam Station Ash Basin Groundwater Response Actions Based on the results of CSA investigation, groundwater contamination is present beneath and downgradient of the ash basin. Duke Energy will pursue corrective action under 15A NCAC 02L .0106. The approaches to corrective action under rule .0106(k) or (1) will be evaluated along with other remedies depending on the results of groundwater modeling and evaluation of the site's suitability to use monitored natural attenuation or other industry -accepted methodologies. Impacted groundwater has apparently migrated outside the Duke Energy property boundary to the west of the BCSS ash basin dam. Groundwater sampling for the CSA found exceedances of 2L Standards or IMACs for antimony, beryllium, cobalt, iron, manganese, thallium, vanadium and total dissolved solids and elevated concentrations of chloride and boron in two monitoring wells (GWA-10S/D and GWA-11S/D) located northwest of the ash basin and west of Middleton Loop Road. These wells are located on Duke Energy property; however there is a 2.23 acre parcel (6982-00-18-5694) not owned by Duke Energy between these wells and the ash basin. In addition, samples collected at groundwater seeps downgradient of this area (S-2, S-3 and S-4) showed exceedances of 2L Standards or IMACs for cobalt, chromium, manganese, and vanadium, and elevated concentrations of boron, chloride, and total dissolved solids. Based on the groundwater and seep sampling results, groundwater beneath this parcel is likely impacted. The receptor survey (included in the CSA) indicates there are no public or private water supply wells located downgradient of the direction of groundwater flow from the ash basin and the area with isolated groundwater impacts mentioned above. The nearest receptor downgradient of the potential offsite groundwater impacts is the Dan River, which is located approximately 2,000 feet northwest of this area. The data gap section of the CSA recommended installation of additional groundwater monitoring wells and additional data assessment to fully delineate the horizontal extent of impacted groundwater west/northwest and downgradient of the ash basin and Middleton Loop Road. In accordance with the Settlement Agreement reached between the NCDEQ and Duke Energy on September 29, 2015, Duke Energy shall implement accelerated remediation at the BCSS site consistent with 15A NCAC 2L .0106 to address offsite groundwater impacts in isolated areas that are not impacting private wells. These accelerated remedial action(s) are currently being evaluated outside of this CAP Part 1, but will be considered during the remedial alternative analysis phase of CAP Part 2. 48 Corrective Action Plan Part 1 Belews Creek 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 purpose of the SCM is to evaluate areal distribution of COls with regard to site -specific geological/ hydrogeological and geochemical properties at the BCSS site. The SCM was developed using data and analysis from the CSA (HDR 2015). The sources and areas with 2L Standard, IMAC, or NC DHHS HSL 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 11 soil borings and 64 monitoring wells. The 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 BCSS site, as well as on-site/near-site geologic mapping, to further understand the site geology in support of the SCM. 3.1.1 Hydrostratigraph ic Units The following materials were encountered during the site assessment and are consistent with material descriptions from previous site exploration: • Ash (A) — Ash was encountered in borings advanced within the ash basin. Ash was generally described as gray to dark gray, non -plastic, loose to medium density, dry to wet, fine- to coarse -grained. The range of ash thickness measured at five locations on the BCSS site was 18 to 66 feet; ash was not observed in borings outside the ash basin. • Fill (F) — Fill material was used in the construction of the ash basin dikes and dam, and generally consisted of re -worked silts, clays, and sands that were borrowed from the site and re -distributed to other areas. Fill was generally classified in the boring logs as silty sand, clay with sand, clay, and sandy clay. The range of fill thickness observed at four locations on the ash basin dike and main dam at the BCSS site was 23 to 69 feet. • Residuum (M1) — Residuum is in -place soil that develops by weathering. At BCSS, it consisted primarily of silt with sand, clayey sand, sandy clay, clay with gravel, and clayey silts. The range of residuum thickness observed at the BCSS site was 5 to 68 feet. • Saprolite/Weathered Rock (M2) — Saprolite is soil developed by in -place weathering of rock that retains remnant bedrock structure. Saprolite consisted primarily of medium dense to very dense silty sand, sandy silt, sand, sand with gravel, sand with clay, clay with sand, and clay. Sand particle size ranged from fine to coarse -grained. Much of the saprolite was micaceous. The range of saprolite/weathered rock observed at the BCSS site was 0 to 49 feet. • Partially Weathered/Fractured Rock (Transition Zone) — Partially weathered (slight to moderate) and/or highly fractured rock was encountered below refusal (auger, casing 49 Corrective Action Plan Part 1 Belews Creek Steam Station Ash Basin advancer, etc.). The range of transition zone thickness observed at the BCSS site was 0 to 15 feet. • Bedrock (BR) — Bedrock is defined as sound rock in boreholes, generally slightly weathered to fresh and relatively unfractured. The maximum depth that borings extended into bedrock at the BCSS site was 62.5 feet (GWA-12BR). Based on the site investigation conducted as part of the CSA, the groundwater system in the natural materials (soil, soil/saprolite, and bedrock) is consistent with the regolith-fractured rock system and is characterized as an unconfined, connected aquifer system. The groundwater system beneath the BCSS site is divided into the following three layers to distinguish the connected aquifer system: the shallow flow layer, deep flow layer, and the bedrock flow layer. Hydrostratigraphic layers 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 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, M1, and M2 hydrostratigraphic layers 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 used to develop the groundwater flow and fate and transport model further discussed in Section 4. The effective porosity (primarily fracture porosity) and specific storage of the transition zone and bedrock were estimated from published data. Potentiometric Surface ')hallow Flow Layer The shallow flow layer was defined by data obtained from the shallow groundwater monitoring wells (S wells) and surface water elevations. In general, groundwater within the shallow flow layer flows to the north-northwest toward the Dan River and is influenced by the groundwater divide along Pine Hall Road, located south of the ash basin and the Pine Hall Road Landfill. The highest measured groundwater elevation during the CSA was approximately 812 feet (BG-3S) located near the intersection of Pine Hall Road and Middleton Loop Road on the western property boundary. To the north of Pine Hall Road, flow is to the north toward the Dan River; south of the road, flow is to the south toward Belews Lake. Groundwater flow in the shallow layer is generally toward the ash basin from the east and west; in the northwest portion of the basin, groundwater flow is influenced by the elevation of the ash basin surface water and appears to be to the west into the adjacent drainage basin. Groundwater flow below the ash basin dam is generally toward the small unnamed stream, except on the west side of the dam where flow in the shallow layer is to the northwest. The Dan River serves as a hydrologic boundary for groundwater, intercepting flow from the ash basin to shallow groundwater across the Dan River north of the BCSS site. The potentiometric surface of the shallow flow layer is shown on Figure 3-3. 60 Corrective Action Plan Part 1 Belews Creek Steam Station Ash Basin Potentiometric Surface —Deep Flow Layer The deep flow layer was defined by data obtained from the deep groundwater monitoring wells (D wells). In general, groundwater within the deep flow layer follows the same path as the shallow flow layer and flows in a north -northwesterly direction toward the Dan River from Pine Hall Road south of the Ash Basin. Groundwater elevations in the deep flow layer are generally within one foot of those in the shallow flow layer. The potentiometric surface of the deep flow layer is shown on Figure 3-4. 1.1.5 Potentiometric Surface — Bedrock Flow Layer The bedrock flow layer was defined by data obtained from the bedrock groundwater monitoring wells (BR wells). In general, groundwater within the bedrock flow layer flows to the north toward the Dan River from Pine Hall Road. The potentiometric surface of the bedrock flow layer is shown on Figure 3-5. 3.1.6 Horizontal and Vertical Hydraulic Gradients 3.1.6.1 Horizontal Hydraulic Gradient Horizontal hydraulic gradients were derived for the shallow and deep flow layers by calculating the difference in hydraulic heads over the length of the flow path between two wells with similar well construction (e.g., both wells having 15-foot screens within the same water -bearing unit). Insufficient data were available to calculate gradients for the bedrock flow layer. Applying this equation to wells installed during the CSA yields the following average horizontal hydraulic gradients (measured in feet/foot): • Shallow flow layer: 0.019 • Deep flow layer: 0.041 Vertical Hydraulic Gradients Vertical hydraulic gradients were calculated for 30 shallow (S) and deep (D) well pairs by dividing the difference in groundwater elevation in each well pair by the vertical difference between the well screen midpoints (Tables 3-1 and 3-2). Vertical hydraulic gradients were calculated for 5 deep (D) and bedrock (BR) well pairs. A positive value indicates potential upward flow (higher hydraulic head with depth) and a negative value indicates potential downward flow (lower hydraulic head with depth). Vertical hydraulic gradients between the shallow and deep flow layers are presented on Figure 3-6, and the vertical gradients between the deep and bedrock flow layers are presented on Figure 3-7. 61 Corrective Action Plan Part 1 Belews Creek Steam Station Ash Basin 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 0.057 GWA-17S GWA-17D -0.017 AB-2S AB-2D -0.104 GWA-1 S GWA-1 D -0.039 AB-3S AB-3D -0.118 GWA-2S GWA-2D -0.025 AB-4S AB-4D -0.006 GWA-3S GWA-3D -0.005 AB-5S AB-5D -0.004 GWA-5S GWA-5D -0.006 AB-6S AB-6D -0.024 GWA-6S GWA-6D -0.106 AB-7S AB-7D 0.015 GWA-7S GWA-7D -0.023 AB-8S AB-8D -0.004 GWA-8S GWA-8D -0.019 AB-9S AB-9D -0.060 GWA-9S GWA-9D -0.026 BG-2S BG-2D 0.074 MW-103S MW-103D -0.059 BG-3S BG-3D 0.036 MW-104S MW-104D 0.002 GWA-10S GWA-10D -0.026 MW-200S MW-200D 0.040 GWA-11 S GWA-11 D -0.074 MW-202S MW-202D 0.015 GWA-12S GWA-12D -0.052 MW-203S MW-203D 0.0002 GWA-16S GWA-16D -0.029 MW-204S MW-204D -0.006 Comparison of vertical gradients between shallow and deep flow layers: Neutral to negative gradients were observed at 23 of 30 well pairs throughout the BCSS site. The highest negative (downward flow) gradients were observed on the ash basin and chemical pond dams (AB-3S/D, AB-2S/D and AB-9S/D) and west of the ash basin dam (GWA-11S/D); the direction of groundwater flow in the shallow and deep flow layers at these locations is to the north. A higher negative gradient was also measured at GWA-6S/D located south of Pine Hall Road with groundwater flow to the east toward Belews Lake. • Positive gradients were observed at 7 of 30 well pairs. The highest positive (upward flow) gradients were observed at high points on the Pine Hall Road hydrologic divide (BG-2S/D and BG-3S/D) and below the ash basin dam (MW-200S/D). A higher positive gradient was also observed on the west side of the ash basin dam (AB-1 S/D) showing a change in vertical gradient from negative on the east side to positive on the west side of the dam. 62 Corrective Action Plan Part 1 Belews Creek Steam Station Ash Basin Table 3-2. Vertical Gradient Calculations for Deep/Bedrock Well Pairs Deep Well Bedrock Well Vertical Gradient (ft/ft) Deep Well Bedrock Well Vertical Gradient (ft/ft) AB-4D AB-4BR 0.002 GWA-5D GWA-5BR2 -0.086 AB-9D AB-9BR -0.014 MW-202D MW-202BR 0.006 BG-2D BG-2BR 0.056 MW-203D MW-203BR -0.081 GWA-12D GWA-12BR -0.206 Notes: 1. Vertical Gradients = AWE/ABS(AMSE), where A implies deep to shallow, WE is water elevation, and MSE is mid - screen elevation. 2. Positive gradient implies potential upward flow. 3. Depth to Water measurements taken on 7/7/15 Comparison of vertical gradients between deep and bedrock flow layers: • The highest negative (downward flow) vertical gradient was observed at GWA-12D/BR located south of Pine Hall Road where groundwater flow is toward Belews Lake. The vertical gradient at the MW-203 well cluster, on the west side of the ash basin near the upper end, changed from neutral in the S/D well pair to negative in the D/BR well pair. • The vertical gradients remained positive (upward flow) at BG-2D/BR located on Pine Hall Road at the groundwater high and at MW-202D/BR near Belews Lake. Although not calculated, a positive vertical gradient was observed at MW-200D/BR below the ash basin dam based on artesian flow from the bedrock well. Negative (downward) vertical gradients in the ash basin increase the potential for migration of COls into the deep and bedrock layers. The neutral to low magnitude of the gradients in the ash basin limits the impact of vertical migration, which is supported by the generally lower COI concentrations in the deep layer and the limited number of COI exceedances of 2L Standards and IMACs in the bedrock layer. Positive (upward) gradients at the high points on the groundwater divides indicates flow from the deep and bedrock layers as groundwater in the shallow layer flows down the slope. Positive gradients below the ash basin indicate upward groundwater flow due to the sharp decrease in potentiometric head downgradient of the dam. Site Geochemical Conditions The site geochemical conditions (specifically the Kd values) as described below were incorporated in the fate and transport modeling discussed further in Section 4. 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. The following site geochemical conditions were evaluated for site -specific COls as identified in Section 2.8. 63 Corrective Action Plan Part 1 Belews Creek Steam Station Ash Basin 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 elements 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 BCSS site consist of the ash basin including the chemical pond and the Pine Hall Road Landfill. These source areas are subject to different processes that generate Ieachate migrating into the underlying soil layers and into the groundwater. For example, the Pine Hall Road Landfill would leach as a result of infiltration of precipitation, while the ash basin would leach based on the contact with ponded water in the basin. Infiltration of precipitation into the ash landfill is limited by the engineered cap installed in 2008. Periodic inflows to the ash basin would likely affect the amount of Ieachate from constituents and their resulting concentrations over time. In addition, ash management practices can alter the concentration range of constituents in ash Ieachate, and certain groups of constituents are more prevalent in landfill versus pond management scenarios (EPRI 2004). The location of ash, precipitation, and process water in contact with ash are the most significant factors on geochemical conditions. Constituents would not be present in groundwater or soils at levels greater than background without ash -to -groundwater contact. Once leached by precipitation or process water, constituents can enter the soil -to -groundwater -to -rock system and their concentration and mobility are controlled by the principles of constituent transport in groundwater. Soil -to -groundwater -to -rock interaction and geochemical conditions present in the subsurface are also responsible for the natural occurrence of some constituents in background locations. These natural processes may also be responsible for a portion of constituents in groundwater. 3.2.1.2 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, 64 Corrective Action Plan Part 1 Belews Creek Steam Station Ash Basin 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. 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. 3.2.1.3 COI Distribution in Groundwater The spatial distribution of COls detected in groundwater samples collected at the BCSS site is described below. For the purposes of this discussion, the shallow flow layer includes the analytical results reported in the shallow (S) monitoring wells. The deep flow layer includes the analytical results reported in the deep (D) monitoring wells, and the bedrock flow layer includes the analytical results reported in the bedrock (BR) monitoring wells. • Antimony - Concentrations of antimony exceeded the IMAC in the shallow flow layer at AB-2S on the ash basin dam, at GWA-5S east of the ash basin and at OB-4 at Pine Hall Road Landfill. Antimony exceeded the IMAC in the deep flow layer at three locations around the ash basin (GWA-4D, GWA-10D and GWA-16D) and in a background well (BG-1 D). Antimony exceeded the IMAC in the bedrock flow layer at GWA-5BR2 east of the ash basin. Turbidity, total, and dissolved concentration results were available for the majority of these samples. Turbidity values were less than 10 NTU with the exception of GWA-5S. Dissolved and total concentration results were within an order of magnitude. Based on 65 Corrective Action Plan Part 1 Belews Creek Steam Station Ash Basin the available data, the antimony exceedances do not appear to be attributable to turbidity. • Arsenic - Arsenic concentrations exceeded the 2L Standard in the shallow flow layer at two locations near the ash basin dam (AB-1 S located on the dam and MW-103S at the toe of the ash basin dam), and one location at Pine Hall Road Landfill (OB-4). There were no arsenic exceedances of the 2L Standard in the deep or bedrock flow layers. The AB-1 S sample concentration was 39 pg/L (1.2 pg/L dissolved fraction) with a turbidity of 14.5 NTU. Turbidity values were less than 10 NTU at MW-103S and OB-4. The MW-103S sample concentration was the same in the total and dissolved concentration results (79.1 pg/L). Dissolved fraction analysis was not performed for OB- 4. Based on the available data, only the AB-1S exceedance for arsenic is potentially attributable to turbidity. Beryllium - IMAC exceedances of beryllium were reported in the shallow flow layer at two locations downgradient of the ash basin dam (GWA-1S and GWA-11S). Beryllium exceeded the IMAC in the deep flow layer at one location downgradient of the ash basin dam (MW-103D). There were no IMAC exceedances of beryllium in the bedrock flow layer. Turbidity was below 10 NTU for these three samples, and the dissolved and total concentration results were within an order of magnitude. Based on the limited data, the beryllium exceedances do not appear to be attributable to turbidity. Boron - Boron concentrations exceeding the 2L Standard in the shallow flow layer were reported at two locations on the ash basin dam (AB-1S and AB-3S), one location downgradient of the ash basin (MW-102S), and four Pine Hall Road Landfill downgradient wells. Boron concentrations exceeding the 2L Standard in the deep flow layer were reported at three locations on the ash basin dam (AB-1 D, AB-2D and AB-3D) and at two locations downgradient of the ash basin (MW-102D and MW-103D). There were no boron exceedances of the 2L Standard in the bedrock flow layer. Turbidity values were less than 10 NTU, with the exception of AB-1S (14.5 NTU), AB-1 D (11.0 NTU) and landfill well MW-7 (23.3 NTU). Dissolved analyses were not performed for the landfill wells. Dissolved and total concentration results for the CSA samples were within an order of magnitude. Based on the available data, the boron exceedances do not appear to be attributable to turbidity. Cadmium — Cadmium concentrations exceeding the 2L Standard were limited to two wells in the shallow flow layer at Pine Hall Road Landfill (OB-4 and OB-9) and one location in the deep flow layer downgradient of the ash basin (MW-103D). There were no 2L Standard exceedances of cadmium in the bedrock flow layer. Turbidity values were less than 10 NTU for these three samples. Dissolved analyses were not performed for the landfill wells. Dissolved and total concentration results for MW-103D were within an order of magnitude. Based on the available data, the cadmium exceedances do not appear to be attributable to turbidity. • Chloride - Chloride concentrations exceeding the 2L Standard in the shallow flow layer were reported at two locations on the ash basin dam (AB-1S and AB-3S). Chloride concentrations exceeding the 2L Standard in the deep flow layer were reported at three 66 Corrective Action Plan Part 1 Belews Creek Steam Station Ash Basin locations on the ash basin dam (AB-1 D, AB-2D and AB-3D) and at at two locations downgradient of the ash basin (MW-102D and MW-103D). No chloride exceedances of the 2L Standard were reported in groundwater samples collected from the bedrock flow layer. Turbidity values were less than 10 NTU, with the exception of AB-1S (14.5 NTU) and AB-1 D (11.0 NTU). Dissolved analyses for chloride were not performed. Based on the exceedances in samples with turbidity less than 10 NTU and the similarity with exceedances for boron, the chloride exceedances do not appear to be attributable to turbidity. Chromium - Chromium concentrations exceeding the 2L Standard in the shallow flow layer were reported in one upgradient well west of the ash basin (GWA-17S), two wells on the ash basin dam (AB-1S and AB-2S), one well downgradient of the ash basin (GWA-1S), and two Pine Hall Road Landfill downgradient wells (MW2-9 and MW-4). Chromium exceedances in the deep flow layer were limited to one upgradient well east of the ash basin (MW-201 D). There were no chromium exceedances of the 2L Standard reported in the bedrock flow layer. Turbidity values were less than 10 NTU, with the exception of MW-201 D (17.4 NTU), AB-1S (14.5 NTU), GWA-17S (21.7 NTU) and landfill well MW2-9 (37.3 NTU). Dissolved analyses were not performed for the landfill well. Dissolved and total concentration results were within an order of magnitude except for GWA-17S (total 50.7 pg/L and dissolved 0.23J+ pg/L ). Based on the available data, the chromium exceedances do not appear to be attributable to turbidity, with the possible exceptions of GWA-17S and MW2-9. • Cobalt - Cobalt concentrations exceeding the IMAC in the shallow flow layer were widespread across the site in the shallow flow layer; the highest cobalt concentration was located below the ash basin dam (MW-102S). Cobalt exceedances of the IMAC in the deep flow layer were also widespread but at lower concentrations than in the shallow flow layer; the highest cobalt concentration was detected in well AB-2D located on the ash basin dam. No cobalt exceedances were reported in groundwater samples collected from the bedrock flow layer. Turbidity values were less than 10 NTU for most of the 31 samples with IMAC exceedances. Turbidity values greater than 10 NTU were recorded for 9 samples (up to 130.5 NTU in AB-5D). However, dissolved and total concentration results were within an order of magnitude for all samples. Based on the available data, the cobalt exceedances do not appear to be attributable to turbidity. Hexavalent Chromium — Concentrations of hexavalent chromium exceeding the NC DHHS HSL were reported at 10 locations in the shallow (3), deep (4) and bedrock (3) flow layers. In each flow layer, exceedances were reported upgradient of the ash basin and/or downgradient of the ash basin dam, indicating that hexavalent chromium may be naturally occurring. Iron - Iron concentrations exceeding the 2L Standard in the shallow flow layer were widespread across the site, at and downgradient of the ash basin dam, upgradient of the 67 Corrective Action Plan Part 1 Belews Creek Steam Station Ash Basin ash basin, south of Pine Hall Road and in a background well. Iron exceedances of the 2L Standard in the deep flow layer were widespread, as in the shallow flow layer. No iron exceedances were reported in the bedrock flow layer. The highest iron concentration in both the shallow and deep flow layers was located below the ash basin dam at well MW- 103S/D. In both samples, turbidity was below 10 NTU and similar concentrations were reported in the total and dissolved fraction Note there are significant differences in total and dissolved concentrations. Twenty-eight samples had exceedances of the 2L Standard in the total concentration while only eight of those samples had exceedances in the dissolved concentrations. In fourteen samples with exceedances of the 2L standard in the total concentrations, the dissolved concentration was more than an order of magnitude less and below the 2L Standard. Turbidity values in those samples were 3.9 to 130.5 NTU and exceeded 10 NTU in ten samples. Based on the available data, the iron concentrations appear to be affected by turbidity. Manganese - Manganese concentrations exceeding the 2L Standard were reported throughout the site in the shallow, deep, and bedrock flow layer montoring wells. The dissolved phase maganese concentrations were similar to total concentrations. The distribution of the exceedances was similar to those for cobalt and iron. The highest manganese concentrations in both the shallow and deep flow layers were located on and below the ash basin dam at MW-102S, MW-103S/D, AB-21D, AB-3S and AB-1S. Thirty-nine samples had exceedances of the 2L Standard in the total concentration and in the dissolved concentrations, except for AB-9BR and BG-31D. AB-9BR had a total concentration of 91 pg/L (dissolved concentration 21 pg/L) with turbidity of 18.5 NTU. BG-3D had a total concentration of 70 pg/L (dissolved concentration 5.5 pg/L) with turbidity of 93 NTU; this was the only sample where the dissolved concentration was more than an order of magnitude less than the total concentration. Based on the available data, the manganese exceedances do not appear to be attributable to turbidity, except at BG-31D. pH - pH measurements outside of the 2L Standard range of 6.5-8.5 were encountered in shallow, deep, and bedrock flow layers, and were distributed across the site. The pH measurements outside of the 2L Standard range in shallow wells were primarily below the range, with acidic exceedances in 23 samples and basic exceedances in 1 sample (AB-2S). The pH measurements outside of the 2L Standard range in deep wells were mixed, with acidic exceedances in 16 samples and basic exceedances in 6 samples. The pH measurements outside of the 2L Standard range in bedrock wells were all above the range (basic). • Sulfate — Sulfate concentrations exceeding the 2L Standard in the shallow flow layer were reported in three Pine Hall Road Landfill downgradient wells (MW2-7, OB-4 and OB-9). No sulfate exceedances were reported in groundwater samples collected during the CSA from the shallow, deep or bedrock flow layers. • TDS - Concentrations of total dissolved solided (TDS) exceeding the 2L Standard in the shallow flow layer were reported at two locations on the ash basin dam (AB-1 S and AB- 3S), one location downgradient of the ash basin (MW-102S), and three Pine Hall Road 68 Corrective Action Plan Part 1 Belews Creek Steam Station Ash Basin Landfill downgradient wells (MW2-7, OB-4 and OB-9). TDS exceedances of the 2L Standard in the deep flow layer were reported at three locations on the ash basin dam (AB-1 D, AB-2D, and AB-3D) and three locations downgradient of the ash basin (GWA- 11 D, MW-102D and MW-103D). No TDS exceedances were reported in groundwater samples collected from the bedrock flow layer. • Thallium - Thallium concentrations exceeding the IMAC in the shallow flow layer were reported at two locations on the ash basin dam (AB-1S and AB- 3S) three locations downgradient of the ash basin (GWA-1S, GWA-11S, and MW-102S) and two locations upgradient of the ash basin (GWA-6S and GWA-9S). Thallium exceedances of the IMAC in the deep flow layer were were reported at two locations on the ash basin dam (AB-1 D and AB-2D) and one location downgradient of the ash basin (MW-103D). No thallium exceedances were reported in groundwater samples collected from the bedrock flow layer. Turbidity values ranged from 2.3 to 14.5 NTU and the total and dissolved concentrations for thallium were within one order of magnitude, except at GWA-9S. GWA-9S had a total concentration of 0.23 pg/L (dissolved concentration 0.094J pg/L) with turbidity of 42.2 NTU. Based on the available data, the thallium exceedances do not appear to be attributable to turbidity, except at GWA-9S. Vanadium - Vanadium concentrations exceeding the IMAC were reported throughout the site in the shallow, deep, and bedrock flow layer montoring wells. The vanadium method reporting limit provided by the analytical laboratory was 1.0 pg/L, which exceeds the IMAC for vanadium (0.3 pg/L). Vanadium concentrations exceeding the IMAC in the shallow flow layer were widespread. Vanadium concentrations exceeded the IMAC in most of the wells in the deep flow layer including the background wells; however, vanadium exceedances were not observed in one well on the ash basin dam and two wells downgradient of the ash basin MW-102D and MW-103D. Turbidity values were 1.8 to 130.5 NTU and the total and dissolved concentrations for vanadium were within one order of magnitude, except at GWA-9S. GWA-9S had a total concentration of 7.2 pg/L (dissolved concentration <1 pg/L) with turbidity of 42.2 NTU. Based on the available data, the vanadium exceedances do not appear to be attributable to turbidity, except at GWA-9S. 3.2.2 Geochemical Characteristics Groundwater composition can be affected by an array of naturally -occurring and anthropogenic (cultural) factors. Many of these factors can be causative agents for specific oxidation-reduction (redox) processes or indicators of the implied redox state of groundwater as expressed by pH, Oxidation -Reduction Potential (ORP), and dissolved oxygen (DO). The pH of a body of groundwater 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 69 Corrective Action Plan Part 1 Belews Creek Steam Station Ash Basin 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. 3.2.2.1 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 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. The relative concentrations and distribution of the cations and anions can be used to compare the relative ionic composition of different water quality samples through the use of piper diagrams. Piper diagrams were generated as part of the BCSS CSA to compare the cation and anion composition of groundwater, ash basin porewater, surface water, and seeps. Evidence of mixing of ash basin porewater and groundwater can be seen in the piper diagrams presented in the CSA Report. In general, the ionic composition of groundwater and surface water at the BCSS site is predominantly rich in calcium/magnesium cations; samples more indicative of a natural water source (background wells, deep wells, seeps with low ash basin impact potential) are biased toward carbonate/bicarbonate anions while samples indicative of ash impact (shallow ash basin wells, impacted deep wells on the ash basin dam, wells and seeps at the toe of the ash basin dam, and various other samples [S-9, SW-DR-D, and GWA-11 S/D]) are biased toward chloride/nitrate/sulfate anions. 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). 60 Corrective Action Plan Part 1 Belews Creek Steam Station Ash Basin 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. Moreover, groundwater is commonly not in redox equilibrium. 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 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 et al. 1995; Christensen et al. 2000; Paschke et al. 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 (NO34 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 an even 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 BCSS site. Speciation measurements were performed for arsenic, selenium, chromium, iron and manganese at select locations. Samples were collected using 61 Corrective Action Plan Part 1 Belews Creek Steam Station Ash Basin 0.45 micron (pm) 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 (11), iron (11), sulfate and sulfide measured at the BCSS site were used as inputs to the redox workbook for monitoring wells. Analytical results were reported as the sum of nitrate and nitrite. 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 BCSS site was evaluated based on 73 samples from the study area for which all six constituents were available, including porewater and groundwater. Based on site measurements, the primary redox categories were determined to include oxic, suboxic, mixed (oxic-anoxic), mixed (anoxic) and anoxic conditions. DO levels exceeded the threshold of 0.5 mg/L in 60 of 73 samples (82%) and predominant redox processes are oxygen reduction with iron or manganese oxidation (i.e., controlled by 02 and Fe(111)/Fe(II) or Mn(IV)/Mn(ll) couples). Under these conditions, more oxidized species As(V), Se(VI), and Mn(IV) would be expected. There were 10 wells from which porewater samples were collected and those include the entire range of redox categories found at the site: oxic, suboxic, mixed (oxic-anoxic), mixed(anoxic) and anoxic. Only two of these 10 porewater samples were considered to be anoxic, but another five were considered to be mixed. There is an increased potential for reduced forms of metals to occur under anoxic or mixed conditions. However, it should be noted that 27 of the 63 (-43%) groundwater samples from wells elsewhere across the site are classified as suboxic or oxic categories where reduced species of metals such as As(III) are less likely to persist. Table 3-3. Categories and Threshold Concentrations to Identify Redox Processes in Groundwater Dissolved Nitrate Iron/Sulfide Process Redox as Manganese Iron Sulfate mass Likely Category mg/L) Nitrogen (mg/L) (mg/L) (mg/L) ratio) Occurring (mg/L) at BCCS 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(ll 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 1 aand Yes Fe II I ) ( 4 Anoxic, <0.5 <0.5 - >_0.1 >_0.5 <0.3 No SO4 Anoxic, <0.5 <0.5 - >_0.1 <0.05 - No CH4 Notes: 1. Thresholds and concentrations from McMahon and Chapelle (2008) and Jurgen et al. (2009). 2. mg/L = milligrams per liter Ranges for a number of field measurements characterizing aspects of groundwater conditions outside and beneath the ash basins are presented in Table 3-4. Those measurements indicate 62 Corrective Action Plan Part 1 Belews Creek Steam Station Ash Basin that pH ranges from 4.1 to 12.0 SU. In contrast, background well results indicate that pH ranges from 5.3 to 9.0 SU, whereas pH within ash ranges from 5.7 to 10.2. There is a wide range of ORP values, spanning ranges that imply mildly reduced (negative values) to highly oxidized (large positive values) conditions. This both agrees and contrasts with the redox category assessment. In terms of redox categories, a number of wells and samples were considered to be oxic while others were anoxic with manganese or iron reduction with sulfur oxidation as a predominant process. Standard (equilibrium) electrode potentials for such reactions may be expected to be in the range of approximately -1,000 millivolts. In contrast, measured ORP values at the BCSS site were never less than -128.7 millivolts. 63 Corrective Action Plan Part 1 Belews Creek Steam Station Ash Basin Table 3-4. Field Parameters from Belews Creek CSA Range of Results for Groundwater Parameters No. of Spec. Diss.ORP/Redox Turbidity Well Locations Results pH (SU) Cond (pS) Oxy gen (mV) (NTU) Background Wells 9 5.29- 39 - 235.1 1.04 - 104 - 277.8 4.02 - 93 (includes NPDES points) 9.04 8.23 Downgradient Well 18 4.14 - 86 - 1,750 0.13 - 6.4 -48.3 - 443 2 - 115 11.98 Downgradient Well, Ash 5 5.64 - 773 - 0.22 - -4.6 - 206.3 1.63 - Basin Dam 11.06 1,779 2.57 14.54 Upgradient Wells 25 4.36- 23.4 - 314 0.19- -121.4- 1.77- 11.52 131 317.1 42.16 Within Ash Basin on 3 4.47 - 48 - 291 0.31 - -128.7 - 3.72 - Chemical Pond Dike 9.12 .1 1.66 188.8 23.2 Within Ash Basin, Below 8 6.05 - 98.6 - 870 -0.11 - -7.6 - 123.4 2.5 - Ash 11.18 1 73 130.5 Within Ash Basin, In Ash 11 5.7 - 140.8 - 0.24 - -52.2 - 0.26 - 10.18 2,570 0.74 166.1 26.24 Notes: 1. SU = Standard Units 2. NS = microsecond 3. mg/L = milligrams per liter 4. mV = millivolts 5. NTU = Nephelometric Turbidity Unit Differences between equilibrium (theoretical) and measured ORP values suggests that groundwater samples are not in full equilibrium with aquifer materials. Apparent disequilibrium is also suggested by the wide range of distribution (sorption) coefficients calculated from batch and column laboratory experiments (see Appendix D). In those experiments, batch tests were performed for a 24-hour period and column tests involved hydraulic residence times that were approximately 6-8 hours. Such differences in sorption test results suggest that reaction kinetics for redox processes very likely require longer periods of time for full equilibrium to occur. However, it is also worth noting that ORP values during sorption experiments differed widely from those measured in groundwater samples. �.2.2.3 Solute Speciation Groundwater samples were characterized in terms of solute speciation to evaluate the concentrations and ionic composition (oxidation states) of metal ions primary concern, including arsenic(III, V), chromium(III, VI), iron(II, III), manganese(II, IV), and selenium(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 BCSS site, speciation measurements were performed on samples from 37 groundwater and/or porewater monitoring wells, depending on the analyte. 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 each sample. For these calculations, analyte concentrations reported as below detection limits were assumed to equal the detection limit. Speciation measurements at the BCSS site vary widely, and are summarized below: 64 Corrective Action Plan Part 1 Belews Creek Steam Station Ash Basin • Arsenic: speciation was measured in 35 samples. In many cases, concentrations of As(III) and As(V) species were below detection limits and reported with U or UJ qualifiers. There were 18 samples where concentrations for both arsenic species were above reporting limits. In those samples As(III) and As(V) were present in equal proportions on average. Neither species was predominant, with As(III) representing 50% of the total arsenic and ranging from 5% to 90% of the total. • Chromium: speciation was measured in 31 samples. In 17 of those 31 cases (55%), hexavalent chromium [Cr(VI)] concentrations were below detection limits and reported with U qualifiers. In general, with the exception of one sample, Cr(VI) was a small component of total chromium, comprising approximately 2.5% of total chromium and ranging from 0% to 15% of the total. The one exception was within the ash basin on the chemical pond dike (AB-9S) where measured Cr(VI) was calculated as 15% of the total chromium. Iron: speciation was measured in 35 samples. Reduced iron [Fe(II)] was present above detection limits in 14 samples and for 12 of those samples total iron was also measured. For the 12 samples where Fe(II) and total iron were measured, Fe(II) comprised just 16% of the total but ranged from 2% to 94% of total iron. The sample where Fe(I I) comprised 94% of the total iron was a background compliance well (MW-202S). Manganese: speciation was measured in only three samples collected within or beneath the ash basin. Reduced manganese [Mn(II)] comprised 90% of total manganese and ranged from 71 % to 99% of the total. Selenium: speciation was measured in 37 samples. Reduced selenium [Se(IV)] was present above detection limits in just three samples. Oxidized selenium [Se(VI)] was present above detection limits in eight samples. For the two samples where both Se(IV) and Se(VI) were detected, Se(IV) ranged from 34% to 88% of the total. Se(IV) was present only in wells within the ash basin (AB-4SL, AB-5SL, and AB-8SL). Se(VI) was present in one background well (MW-202BR), one upgradient well (GWA-8S), one downgradient well (AB-2S), two wells beneath the ash basin (AB-4BR and AB-7D) and three wells within the ash basin (AB-4SL, AB-6S and AB-8SL). Given the range of conditions, next steps in the BCSS site evaluation process 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 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 As described in Section 3.2.1.2, a constituent may be removed from groundwater and onto mineral surfaces of the aquifer media through one of three types of sorption processes: • Adsorption — solutes are held at the water/solid interface as a hydrated species • Absorption — solutes are incorporated into the mineral structure at the surface 66 Corrective Action Plan Part 1 Belews Creek Steam Station Ash Basin • Ion Exchange — when an ion becomes sorbed to a surface by changing places with a similarly charged ion These processes result in decreased constituent concentrations and, therefore, the mass of the constituent as it is removed from groundwater and sorbed 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 10 site -specific samples of soil, or PWR from the transition zone. Solid samples were batch equilibrated to measure the sorption of COls at varying concentrations. For the BCSS site, column tests and batch tests were conducted. The methods used by the University of North Carolina at Charlotte (UNCC) and Kd results obtained from the testing are presented in Appendix D. The Kd data were used as an input parameter to evaluate contaminant fate and transport through the subsurface at the BCSS site, as described in greater detail in Section 4.1. Sorption coefficient (Kd) test results (see Appendix D) for column tests were highly variable. This suggests that the rates at which constituents sorb or desorb to or from particulate phases varies widely. For the same constituent, experimentally - determined Kd values generally vary by factors of 3-5 and sometimes by an order of magnitude. 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. Ash Basin Ash within the active ash basin was encountered from the ground surface to a maximum depth of 66 feet bgs, and ranged in thickness from 18 to 66 feet. Water levels ranged from approximately 3.4 to 7.6 feet bgs, causing ash to be saturated. Refusal was encountered between approximately 68 to 88 feet bgs. Generally no unsaturated soil zone exists beneath the ash to allow for sorption of COls to occur prior to reaching groundwater Constituents dissolve when ash is wet and those constituents leach into groundwater. Mobility of constituents is affected by sorprtion characteristics of each respective constituent. Pond level fluctuation also affects COI mobility due to increased dissolution of COls into groundwater thus increasing COI concentrations with increased pond levels. 1.2.4 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. 66 Corrective Action Plan Part 1 Belews Creek Steam Station Ash Basin The dominant minerals in soils at the BCSS site are quartz, feldspar (both alkali and plagioclase feldspars), kaolinite, illite, and muscovite/illite. Other minerals identified include vermiculite/illite, biotite, smectite, chlorite, and amorphous materials (that contain smectites, mica, and iron oxide/hydroxide). The major oxides in the soils are Si02 (47.20% - 74.97%), A1203 (12.18% - 26.40%), and Fe203 (2.78% - 12.00%). MnO ranges from 0.03% to 0.10%. The dominant minerals in the transition zone are quartz, feldspar (both alkali and plagioclase feldspars), illite, kaolinite, and biotite. The major oxides in the transition zone are Si02 (64.92% - 72.01 %), A1203 (13.17% - 16.65%), and Fe203 (2.96% - 6.39%). MnO ranges from 0.05% to 0.14%. The major oxides in the rock samples are Si02 (63.4% - 74.3%), A1203 (15.4% - 21.7%), and Fe203 (2.5% - 8.0%). 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 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. Increase in the concentrations of COls during the weathering and soil development is significant in a limited number of borings for arsenic (GWA-5Sa, GWA-10D, GWA-11 D, AB-3S, and AMID) and antimony (GWA-5Sa). The increasing abundance of clay during the natural weathering process can conceivably result in an increase in COI content with time. However, the values derived during the CSA suggest the concentrations are being introduced from the ash, especially in the borings noted above. Reported values for borings into the transition zone are also elevated for arsenic at GWA-21D (BCSS CSA Report, Table 6-6); reported values for borings into bedrock are elevated for arsenic and antimony at GWA-21D (BCSS CSA Report, Table 6-6). These reported values exceed average crustal abundances suggesting transport of these COI into the transition zone from the overlying ash. Other COls do not appear elevated in the transition zone over average crustal abundance except vanadium which is slightly elevated (BCSS CSA Report, Table 6-5). Correlation of Hydrogeologic and Geochemical Conditions to COI Distribution The COls found in both ash and porewater, as described in Section 2, include arsenic, boron, cobalt, iron, manganese, selenium and vanadium. Based on results of sampling and analysis performed during CSA activities, the following are groundwater COls at the BCSS site: antimony, arsenic, beryllium, boron, cadmium, chloride, chromium, cobalt, hexavalent chromium, iron, manganese, pH, selenium, sulfate, thallium, TDS, and vanadium. Some of these exceedances may be due to naturally occurring concentrations of the COls in groundwater at the site. The sources and areas with 2L Standard, IMAC, or NC DHHS HSL exceedances of these COls, as well as other BCSS site features, are illustrated the 3-D SCM presented on Figure 3-1 and in cross -sectional view on Figure 3-2. 67 Corrective Action Plan Part 1 Belews Creek Steam Station Ash Basin On Figure 3-1, the areas of 2L Standard or IMAC exceedances within or directly adjacent to the ash basin indicate that physical and geochemical processes beneath the BCSS site retard the lateral migration of the COls. Discharge of groundwater from shallow and deep flow layers into surficial water bodies, in accordance with LeGrand's slope -aquifer system characteristic of the Piedmont, is expected in the unnamed stream downgradient of the ash basin, the Dan River and Belews Lake. Vertical migration of COls observed in select well clusters (S, D, and BR) is likely influenced by infiltration of precipitation and/or ash basin water, where applicable, through the shallow and deep flow layers into underlying fractured bedrock. Exceedances of 2L Standard, IMAC, or NC DHHS HSL in the bedrock flow layer at BCSS was limited to antimony, hexavalent chromium, manganese, pH and vanadium, and the majority of exceedances were below the PPBCs listed in Table 2-2. Cobalt, hexavalent chromium, iron, manganese, pH, and vanadium were the only COls with widespread exceedances in wells upgradient, beneath and downgradient of the ash basin, and in background locations. The concentrations of iron and manganese are highly pH dependent. Cobalt and iron were reported at higher concentrations at and downgradient of the ash basin, similar to other COls. Vanadium does not appear to represent impacts from ash handling because it was reported at similar concentrations upgradient and downgradient of the ash basin. Groundwater and geochemical conditions promote the mobility of vanadium across the site with contribution likely from naturally occurring vanadium and vanadium from source areas. Antimony exceedances were reported in shallow, deep and bedrock flow layers at isolated locations around the ash basin and one background location. It was not reported above the IMAC as frequently as cobalt and vanadium. Hexavalent chromium exceedances were reported in shallow, deep and bedrock flow layers at widespread locations but not in porewater samples within the ash. The exceedances were generally not observed in multiple flow layers at the same location, except at MW-202 background wells where exceedances were reported in the shallow, deep and bedrock layers. The highest reported concentration was 14 pg/L at AB-413R, but exceedances were not observed in groundwater at AB-4D or porewater at AB-4S and AB-4SL. Arsenic, beryllium and cadmium exceedances were reported at a few locations at and downgradient of the ash basin dam, but not in upgradient or background locations. Arsenic and cadmium exceedances were also reported in downgradient wells at the Pine Hall Road Landfill. Arsenic has a relatively high Kd value at the site, which suggests that geochemical conditions favor low mobility of this COI. Sulfate exceedances were reported in downgradient wells at the Pine Hall Road Landfill. Sulfate has a low Kd value and can be mobile in groundwater but exceedances were not reported in CSA groundwater samples beneath and around the ash basin. Boron, chloride, chromium, and TDS exceedances were detected frequently in wells at the ash basin and at downgradient locations. Boron has a low Kd value and can be mobile in groundwater. 68 Corrective Action Plan Part 1 Belews Creek Steam Station Ash Basin Refinement of this SCM, as it pertains to groundwater fate and transport modeling, is discussed in Section 4.3. Furthermore, the SCM will continue to evolve as additional data become available during supplemental assessment activities. 69 Corrective Action Plan Part 1 Belews Creek 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 at the BCSS 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 BCSS 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 and subsequent data collection. The objective of the groundwater modeling effort was to simulate steady-state groundwater flow conditions for the BCSS site, and simulate transient transport conditions in which COls enter groundwater via the ash basin over the period it was 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 the Dan River The area, or domain, of the simulation included the BCSS ash basin and areas of the site that have been impacted by COls above 2L Standards, IMACs, or NC DHHS HSL. Groundwater flow and constituent loading to Belews Lake was not modeled because of a groundwater divide south and east of the ash basin along Pine Hall Road which prevents groundwater flow from the ash basin to Belews Lake. 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. 4.1.1 Model Scenarios The following ash basin closure scenarios were modeled for the BCSS site: • Existing Conditions: assumes current conditions with ash sources left in place • Cap -in -Place: assumes ash is left in the ash basin and covered by an engineered cap • Excavation: assumes removal of ash from the ash basin Model scenarios used steady-state groundwater flow conditions established during 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 70 Corrective Action Plan Part 1 Belews Creek Steam Station Ash Basin assess changing COI concentrations over time at the compliance boundary. The rate of natural attenuation is then described over the model period. 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 HSLs were used for model calibration purposes by introducing a constant source for each COI at the start of the 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 • 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 BCSS model was submitted to the Electric Power Research Institute (EPRI) on October 12, 2015, for independent review of the model. The 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 reviewers were provided the BCSS CSA Report, a BCSS 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 and UNCC submitted a revised model for EPRI review on November 16, 2015. EPRI provided subsequent comments on November 20, 2015, which concluded the model was constructed and calibrated sufficiently to achieve its primary objective of comparing the effects of closure alternatives on nearby groundwater quality. In addition, the reviewers identified limitations with the model, which are included in the discussion of model limitations in Appendix C. Kd Terms COls enter the ash basin in both dissolved and solid phases. In the ash basin, constituents may undergo phase changes including dissolution, precipitation, adsorption, and desorption. Dissolved phase constituents may undergo these phase changes as they are transported in 71 Corrective Action Plan Part 1 Belews Creek Steam Station Ash Basin groundwater flowing through the basin. Phase changes 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 is represented by a constant concentration in the saturated zone (i.e., porewater) of the basin, and is continually depleted by infiltrating recharge from above. As previously discussed in Section 3.2.2.4, laboratory Kd terms were developed by UNCC via column testing of 10 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 BCSS 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 at BCSS indicate that the COI Kds for background soils surrounding the ash basin are higher than the values used in modeling; COI Kds were lowered in the model to calibrate 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 at BCSS. 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. In the Cap -in -Place and Excavation scenarios, the assumed recharge was modified and the constant source concentration was set to zero. In the Cap -in -Place scenario, recharge within the ash basin was reduced from 5 in/year to 0 in/year to represent capping of the ash basin. In the Excavation scenario, the same recharge was applied to the ash basin area as the surrounding areas. Recharge rates for the Existing Conditions scenario are shown on Figure 5 in Appendix C. 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 beyond the ash basin pond, followed by horizontal hydraulic conductivity of the transition zone, then vertical hydraulic conductivity of the shallow flow layer. The model was least sensitive to vertical hydraulic conductivity of the transition zone. Sensitivity of the COI transport model was evaluated by varying key model assumptions for porosity, dispersivity, and Kd by a percentage above and below their respective calibration values. The transport model was most sensitive to porosity and Kd, as a decrease in these parameters increased the velocity of COls moving through the groundwater system. Dispersivity was less sensitive, as an increase in dispersivity increased the length of the COI plume initially, 72 Corrective Action Plan Part 1 Belews Creek Steam Station Ash Basin but did not result in an increase of COI concentrations at distance as quickly as a reduction in porosity or Kd. The transport model was calibrated primarily through modifications to the constant source concentrations and the linear sorption coefficient (Kd) for each COI. The parameters were adjusted to minimize residual concentrations in target wells. 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. Cap -in -Place Scenario The Cap -in -Place scenario results were used to estimate groundwater levels in the ash basin subsequent to placement of an engineered cap. The model assumption for this scenario is that the ash will remain in its current position and that there is no recharge through the cap. The model indicated that groundwater flow is affected by this scenario as the water table may be lowered and groundwater velocities may be reduced beneath the covered areas. For this scenario, the ash was assumed to be above the water table and the migration of COls from porewater to groundwater beneath the basin is stopped. However, the model results indicated the water table is lowered by approximately 15 feet near the center of the ash basin, compared to the Existing Condition scenario. As a result, a portion of the ash in the basin remains saturated in the Cap -in -Place scenario. To more accurately represent this scenario, the CAP Part 2 model assumptions will be revised to evaluate potential transport of COls from the saturated portion of the ash layer during the model simulation period. Excavation Scenario The Excavation scenario simulated removal of all ash from the ash basin. All ash above and below the water table was removed from the model scenario and, as in the Cap -in -Place scenario, the migration of COls from porewater to groundwater beneath the basin was stopped. Unlike the Cap -in -Place scenario, the Excavation scenario assumes recharge rates in the ash basin become equal to recharge rates in areas surrounding the basin. Upon completion of excavation, COls already present in the groundwater migrate downgradient as precipitation infiltrates and recharges the aquifer at the water table. Fate and Transport Model Each model scenario provides simulation of groundwater concentrations over time. The model does not account for changing background COI concentrations To better understand the movement and concentrations of COls, Figures 14 through 132 in the Groundwater Flow and Transport Model in Appendix C show predicted concentrations at selected wells during the 250-year simulation period and concentration isocontours 100 years into the simulation period for each COI. Predicted COI concentrations are shown at AB-2S and MW-102D for hexavalent chromium and AB-1S, AB-31D and MW-103S for all other COls. The 100-year mark was selected to provide a snap -shot of results showing increases or decreases in COI concentrations and movement of COI plumes over a significant period of time. 73 Corrective Action Plan Part 1 Belews Creek Steam Station Ash Basin COls from Section 2 evaluated in the fate and transport model are: arsenic, beryllium, boron, chloride, chromium, hexavalent chromium, cobalt, and thallium. Several COls were not advanced to modeling because of the following rationale: • Antimony was detected in isolated locations at BCSS, including background; although present in the three flow layers, it was not detected consistently with depth at the same location. As a result, antimony was not considered in the model simulations. • Cadmium was only reported above the 2L Standard in one location (MW-103D) and there is no discernable plume. Due to cadmium's limited distribution and moderate sorptive capacity, model results from other COls at this location should bracket this constituent. • Iron, manganese, pH and TDS are naturally occurring in the groundwater system and require more complex modeling than the current MODFLOW/MT3DMS. The geochemical modeling will enhance the understanding of the processes taking place in the subsurface and ultimately aid in choosing the most appropriate remedial action for the site. Geochemical modeling will be completed and submitted in CAP Part 2. • Vanadium concentrations were prevalent above the IMAC in wells throughout the BCSS site. However, vanadium was not present at higher concentrations in downgradient areas; although present in the three flow layers, it was not detected consistently with depth at the same location; and it was not detected consistently at adjacent wells. As a result, vanadium was not considered in the simulations. Existing Conditions The Existing Conditions scenario used the calibrated groundwater flow and transport model and extended the time period from the end of the calibration period (present day) to 250 years into the future. COI concentrations can only increase initially for this scenario with source concentrations being held at their constant value over the entire simulation period. Once steady- state conditions are reached, the concentrations and mass flux of dissolved constituents at the compliance boundary remain constant. Of the three model scenarios, the Existing Conditions scenario represents the most conservative conditions in terms of groundwater concentrations and COls reaching the compliance boundary. 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. Sorptive COls will be transient at a rate that is less than the groundwater pore velocity. Lower effective porosity increases the pore velocity of groundwater and results in shorter time periods to achieve steady-state concentrations for both sorptive and non-sorptive COIs. The results of the Existing Conditions scenario indicated that concentrations for all modeled COls, except beryllium, increase or reach steady-state conditions above 2L Standards, IMACs, or NC DHHS HSL at one or more of the selected well locations during the 250-year simulation period. 74 Corrective Action Plan Part 1 Belews Creek Steam Station Ash Basin At the end of 100 years in the Existing Conditions scenario, seven of eight constituents were estimated by the model to be above the 2L Standards or IMACs at the compliance boundary north of the ash basin dam; the exception was hexavalent chromium, which was below the NC DHHS HSL. Likewise, five of eight constituents were estimated by the model to be above the 2L Standards or IMACs at the compliance boundary northwest of the ash basin dam; the exceptions were arsenic, boron and hexavalent chromium, which were below their respective 2L Standards or NC DHHS HSL. Cap in Place The Cap -in -Place scenario simulates the effects of capping the ash basin at the beginning of the scenario (i.e., Year 2016). In the model, recharge and source area concentrations in the ash basin were set to zero. Under this scenario, groundwater flow rates are generally lower (compared to the Existing Conditions scenario) due to reduced groundwater velocities caused by the reduction in recharge and the lower groundwater table beneath the capped areas. In this scenario, non-sorptive COls move downgradient at the pore velocity of groundwater and are displaced by the passage of a single porewater volume, while migration of sorptive COls is retarded. Under the Cap -in -Place scenario, modeled concentrations of boron and chloride (i.e., non- sorptive COls) at the selected well locations decrease below their respective 2L Standards quickly (i.e., within 15 years). The other COI modeled concentrations increase initially and then decrease over the 250-year simulation period. The modeled concentrations for arsenic, chromium, hexavalent chromium, cobalt and thallium remained above their respective 2L Standards, IMACs, or NC DHHS HSL throughout the 250-year simulation period. The modeled concentrations for beryllium remain below its IMAC throughout the model simulation period. At the end of 100 years in the Cap -in -Place scenario, chromium, cobalt, and thallium were estimated by the model to be above the 2L Standard or IMACs at the compliance boundary north of the ash basin dam. Those three constituents and arsenic were estimated by the model to be above the 2L Standards or IMACs at the compliance boundary northwest of the ash basin dam at the 100-year mark t.1.5.3 Excavation The Excavation scenario simulates the effect of removing all ash from the ash basin at the beginning of the scenario (i.e., Year 2016). In the model, ash basin COI concentrations are set to zero while recharge to the excavation area is applied at the same rate as the surrounding area. Groundwater flow and COI transport beneath the ash basin is affected by this scenario as the basins are completely drained and clean water enters the area from upgradient and from infiltration of precipitation. As in the Cap -in -Place scenario, non-sorptive COls move downgradient at the pore velocity of groundwater and are displaced by the passage of a single porewater volume, while migration of sorptive COls is retarded. Groundwater flow through the area is greater in this scenario than in the Cap -in -Place scenario due to recharge from precipitation. Under the Excavation scenario, modeled concentrations at the selected well locations for boron and chloride (i.e., non-sorptive COls) as well as beryllium and thallium decrease below the 2L 76 Corrective Action Plan Part 1 Belews Creek Steam Station Ash Basin Standards and IMACs quickly (i.e., within 10 years). The other COI modeled concentrations decrease more slowly over the 250-year simulation period; chromium decreases below the 2L Standard at AB-1 S, while arsenic and cobalt decrease below their respective 2L Standard or IMAC near the end of the model simulation period. The modeled concentration of hexavalent chromium does not decrease below the NC DHHS HSL at AB-2S. However, the current model does not account for background concentrations of COls. Refinement of the groundwater flow and fate and transport model for this and other assumptions will be performed during the CAP Part 2. Further, the NC DHHS HSL for hexavalent chromium is conservatively low since it was devised to be protective under a human consumption scenario. As shown in Section 2.2, background concentrations for hexavalent chromium range from 0.13 pg/L to 3.2 pg/L. At the end of 100 years in the Excavation scenario, cobalt was the only COI estimated by the model to be above the 2L Standards, IMACs, or NC DHHS HSL at the compliance boundary north of the ash basin dam. No COls were estimated by the model to be above the 2L Standards, IMACs, or NC DHHS HSL at the compliance boundary northwest of the ash basin dam. 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, recharge, or river flow. A steady-state calibration does not consider groundwater storage and does not calibrate the groundwater flux into adjacent surface water bodies. • MODFLOW 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 at the BCSS site. • The model was calibrated by adjusting the constant source concentrations at the ash basins to reasonably match 2015 COI concentrations in groundwater. 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 2016. • Predictive simulations were performed and steady-state flow conditions were assumed from the time that the ash basin was placed in service through the current time until the end of the predictive simulations (Year 2265). • COI source area concentrations at the ash basin were assumed to be constant with respect to time for transport model calibration. 76 Corrective Action Plan Part 1 Belews Creek Steam Station Ash Basin • 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. • Model results along certain boundaries are impacted by the cell saturation level resulting in some numerical spikes in the calculations that do not represent concentrations in surrounding cells. These numerical spikes are localized calculations and do not impact the overall model results. • 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. t.1.6 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 site geochemical conceptual model to enhance planned geochemical modeling. Some of the model outputs: • Site -specific groundwater flow matches the original SCM; flow matches the regional flow processes in Piedmont Physiographic Province. • The dominant attenuation processes, as initially hypothesized, are adsorption to hydrous metal oxides (HFO, HMO, and HAO) and clay minerals. Hydrous metal oxides and clay minerals are abundant in the soil and transition zone, concentrations increasing with the degree of weathering of the bedrock. • Correlations exist between COI concentrations and HFO, HMO, and 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) Geochemical modeling will be used to perform the sensitivity analysis for pH, Eh, and TDS. 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. 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. 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 77 Corrective Action Plan Part 1 Belews Creek Steam Station Ash Basin 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. 4-.2 Groundwater - 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 surface water bodies adjacent to the BCSS site. 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 COI with exceedances of the 2L Standards, IMACS, or NC DHHS HSLs were used as inputs for the surface water assessment in the receiving waters adjacent to the BCSS site. The groundwater model showed that groundwater at the BCSS flows generally northward toward the Dan River and away from Belews Lake. The Dan River is classified by NCDEQ as Class C, WS-IV waters in the reach north of BCSS and the water quality is compared to the North Carolina Surface Water Pollutant Standards for Metals for freshwater aquatic life, water supply, or human health derived from 2B Standards for Class C and WS-IV waters. Given that river flows in the Dan River are unidirectional and groundwater discharge mixes with upstream flow, a mixing calculation was used to assess potential surface water quality impacts. 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 78 Corrective Action Plan Part 1 Belews Creek Steam Station Ash Basin 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 sizes and percentages of upstream river design flows used for assessing compliance with applicable water quality standards or criteria, as presented in Section 4.2.2, are provided in Table 4-1. 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 River Width or 12 feet 10% of 1 Q10 Chronic Aquatic Life 50% of River Width or 60 feet 50% of 7Q10 Human Health / Water Supply 50% of River Width or 60 feet 50% of 7Q10 (non -carcinogen) Human Health / Water Supply 100% of River Width or 120 feet 100% of Annual Mean (carcinogen) Notes: The 1Q10 flow is the lowest one -day average flow that occurs (on average) once every 10 years. The 7Q10 flow is the lowest seven-day average flow that occurs (on average) once every 10 years (USEPA 2013). Mean annual flow is the long-term average annual flow based on complete annual flow records. 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 79 Corrective Action Plan Part 1 Belews Creek Steam Station Ash Basin adjacent water body from the point of discharge. These surface water results were compared to applicable surface water quality standards or criteria to determine compliance at the edge of the mixing zone(s). The development of the mixing model inputs required additional data for upstream river flow and COI concentrations, which were obtained from readily available USGS data sources in addition to site -specific surface water quality data collected as part of the CSA. 4.2.2 Surface Water Model Results The calculated surface water COI concentrations in the Dan River downstream of the BCSS site are presented in Table 4-2. The river 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 all COls are below the water quality standard at the edge of the mixing zone in the Dan River. Table 4-2. Dan River Calculated Surface Water Concentrations Calculated Mixing Zone Conc. (Ng/L) Water Quality Standard (Ng/L) COI Acute Chronic HH / WS Acute Chronic HH / WS Arsenic 0.215 0.211 0.210 (c) 340 150 10 / 10 Beryllium 0.098 0.100 0.100* 65 6.5 ns / ns Boron 56.3 30.5 25.4* ns ns ns / ns Chloride 7,410 4,025 4,025 (nc) ns ns ns / 250,000 Total Chromium 0.879 0.872 0.870* ns ns ns / ns Chromium VI 0.845 0.866 0.870* 16 11 ns / ns Cobalt 0.309 0.260 0.260 (nc) ns ns 4/3 Thallium 0.049 0.050 0.050 (nc) ns ns 0.47 / 0.24 Notes: 1. All COls are shown as dissolved except for total chromium 2. WS — water supply 3. HH — human health 4. c — carcinogen 5. nc — non -carcinogen 6. ns — no water quality standard 7. * — concentration calculated with annual mean river flow Refinement of ModelF Groundwater and surface water models have been used to provide further information regarding the transport of COls toward the Dan River. All modeled COls are below the applicable 213 Standards or USEPA WQC at the edge of the mixing zone in the Dan River 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; 80 Corrective Action Plan Part 1 Belews Creek Steam Station Ash Basin • 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 may lead to 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. 81 Corrective Action Plan Part 1 Belews Creek 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. o Note that for the BCSS site, the groundwater PPBCs were calculated using historical groundwater quality data from the NPDES compliance wells, and the background monitoring wells installed for the Pine Hall Road, Craig Road and FGD Landfills. PPBCs were calculated as the Upper 95% Prediction Limit using the compliance and landfill monitoring wells where there was sufficient data for a statistical analysis. Where too few data were available to perform statistics, the PPBCs are the highest reported value (or highest laboratory reporting limit for non -detects). At the request of NCDEQ, groundwater analytical results that were 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. If the groundwater PPBCs are approved for the BCSS site, no COls would be eliminated from further evaluation, but the areas requiring evaluation for remediation of antimony, hexavalent chromium, iron, manganese, pH, thallium and vanadium would be reduced. o 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 reported concentration (or the highest method reporting limit for non - detect values). If the soil PPBCs are approved for the BCSS site, chromium, iron, and vanadium would be eliminated from further evaluation, and the locations requiring evaluation for remediation of arsenic, cobalt, manganese, and selenium would be reduced. COls were selected for groundwater fate and transport modeling, in part, based on comparison of constituent concentrations in monitoring wells located beneath and outside the ash basin to applicable regulatory standards or criteria. Data obtained from monitoring wells beneath and outside the ash basin 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 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 BCSS site will be completed and submitted in CAP Part 2. The geochemical model results, taken into consideration with the groundwater flow, fate and transport model and the surface water to groundwater model, will enhance the understanding of the processes taking place in the subsurface and ultimately aid in 82 Corrective Action Plan Part 1 Belews Creek Steam Station Ash Basin choosing the most appropriate remedial action 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. Three scenarios were developed for the groundwater model: Existing Conditions, Cap- in -Place, and Excavation. o The Existing Conditions scenario is used as a baseline for comparison to other scenarios. o The Cap -in -Place scenario assumes the ash will remain in its current position and that there is no recharge through the cap; the ash is also assumed to be above the water table with no direct migration of COls from porewater to groundwater beneath the basin. The CAP Part 2 model assumptions will be revised such that COls in the saturated portion of the ash layer are evaluated during the model simulation period. o The Excavation scenario assumes all ash above and below the water table is removed and, as in the Cap -in -Place scenario, the migration of COls from porewater to groundwater beneath the basin is stopped. Recharge rates in the ash basin are assumed to match recharge rates in areas surrounding the basin • Groundwater modeling was conducted for arsenic, beryllium, boron, chloride, chromium, hexavalent chromium, cobalt, and thallium. Figures in the modeling report (Appendix C) for each COI show predicted concentrations during the 250-year model simulation period at selected well locations upgradient of the compliance boundary and concentration isocontours 100 years into the simulation period. The results are summarized as follows: o Existing Conditions Scenario — Concentrations for seven of eight COls increase or reach a steady-state condition above the 2L Standards or IMACs at the compliance boundary after 100 years of the model simulation. The only exception is hexavalent chromium, which is not predicted to exceed the NC DHHS HSL at the compliance boundary after 100 years. o Cap -in -Place Scenario — Concentrations of boron and chloride decrease below the 2L Standard within 15 years at the selected well locations; the other COls increase initially and then decrease during the 250-year simulation period but remain above their respective standards at the selected well locations. In addition, concentrations of arsenic, chromium, cobalt, and thallium remain above the 2L Standards or IMACs at the compliance boundary after 100 years of the model simulation. o Excavation Scenario — Concentrations of beryllium, boron, chloride and thallium decrease below the 2L Standard and IMACs within 10 years at the selected well locations, while modeled concentrations of the other COls decrease slowly over the 250-year simulation period. Cobalt was the only COI estimated by the model to be above the 2L Standards, IMACs, or NC DHHS HSL at the compliance boundary after 100 years of the model simulation. • 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 in groundwater would result in exceedances of 2B surface water standards (or USEPA WQC) in the Dan River. Surface 83 Corrective Action Plan Part 1 Belews Creek Steam Station Ash Basin water modeling results show that water quality standards or criteria are not exceeded at the edge of the mixing zone in the Dan River. 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. • Ongoing monitoring of the Dan River for NPDES surface water quality indicates that the ash basin has not resulted in increased constituent concentrations above the 2B Standards downstream of the BCSS ash basin discharge for permitted constituents. Exceedances of 2B Standards for chloride and TDS and USEPA WQC for thallium were identified in the Dan River downstream water sample. Similar results were identified in Dan River sediment samples. • In accordance with the Settlement Agreement reached between the NCDEQ and Duke Energy on September 29, 2015, Duke Energy shall implement accelerated remediation at the BCSS site consistent with 15A NCAC 2L .0106 to address offsite groundwater impacts in isolated areas that are not impacting private wells. These accelerated remedial action(s) are currently being evaluated outside of this CAP Part 1, but will be considered during the remedial alternative analysis phase of CAP Part 2. The following recommendations are made to address areas needing further assessment: • 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. The updated results should be used to refine the areas requiring evaluation for remediation. • Additional sampling for radiological parameters along major groundwater flow paths is needed to perform a more comprehensive assessment of radionuclides from source areas. • Additional surface water and sediment sampling should be conducted in the Dan River and in the drainage channel between the ash basin and the Dan River to further evaluate constituent concentrations with regard to the ash basin discharge. • Hydrogeological and analytical data from data gap wells west of the ash basin dam should be reviewed to confirm the horizontal and vertical extent of groundwater impacts has been determined. • The groundwater flow and fate and transport model should be refined to consider site - specific conditions in CAP Part 2. 84 Corrective Action Plan Part 1 Belews Creek Steam Station Ash Basin 6 References Back, W., and I. Barnes. 1965. 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