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Home My WebLink AboutNC0004961_RBSS CAP I_Report_Final_20151116F)R Corrective Action Plan Part 1 Riverbend Steam Station Ash Basin Site Location: NPDES Permit No. Permittee and Current Property Owner: Consultant Information Report Date: Mark Filardi, L.G. Senior Geologist NC LG #1886 Riverbend Steam Station 175 Steam Plant Rd Mount Holly, NC 28120 NC0004961 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 November 16, 2015 Vincent Carbone, P.G. Geologist This page intentionally left blank Corrective Action Plan Part 1 Riverbend 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........................................................................................ 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 Report ................................................. 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........................................................................................ 2.4 Surface Water........................................................................... 2.5 Sediment................................................................................... 2.6 Soil............................................................................................. 2.6.1 Background Soil Concentrations .................................. 2.6.2 Soil Exceedances of NC PSRGs for POGs ................. 2.7 Ash............................................................................................ 2.8 Porewater.................................................................................. 2.9 PWR and Bedrock..................................................................... 2.10 COI Screening Evaluation Summary ........................................ 2.11 Interim Response Actions......................................................... 2.11.1 Source Control............................................................. 2.11.2 Groundwater Response Actions .................................. 3 Site Conceptual Model........................................................................ 3.1 Site Hydrogeologic Conditions .................................................. 1 1 2 5 7 11 12 12 12 13 13 14 14 14 15 15 16 16 17 17 19 20 20 21 21 24 26 27 28 29 30 30 32 33 33 33 33 34 34 35 36 36 Corrective Action Plan Part 1 Riverbend Steam Station Ash Basin 3.1.1 Hydrostratigraphic Units............................................................................................. 36 3.1.2 Hydrostratigraphic Unit Properties............................................................................. 37 3.1.3 Potentiometric Surface — Shallow and Deep Flow Layers ......................................... 37 3.1.4 Potentiometric Surface — Bedrock Flow Layer........................................................... 38 3.1.5 Horizontal and Vertical Hydraulic Gradients.............................................................. 38 3.2 Site Geochemical Conditions.................................................................................................. 40 3.2.1 COI Sources and Mobility in Groundwater.................................................................40 3.2.2 Geochemical Characteristics..................................................................................... 43 3.2.3 Source Area Geochemical Conditions....................................................................... 49 3.2.4 Mineralogical Characteristics..................................................................................... 50 3.3 Correlation of Hydrogeologic and Geochemical Conditions to COI Distribution ..................... 50 4 Modeling............................................................................................................................................ 52 4.1 Groundwater Modeling............................................................................................................52 4.1.1 Model Scenarios......................................................................................................... 52 4.1.2 Calibration of Models.................................................................................................. 53 4.1.3 Kd Terms..................................................................................................................... 53 4.1.4 Flow Model................................................................................................................. 54 4.1.5 Fate and Transport Model.......................................................................................... 55 4.2 Groundwater - Surface Water Interaction Modeling................................................................ 58 4.2.1 Mixing Model Approach.............................................................................................. 58 4.2.2 Surface Water Model Results.................................................................................... 60 4.2.3 Surface Water Quality Assessment Results.............................................................. 62 4.2.4 Results.......................................................................................................................62 4.3 Refinement of SCM................................................................................................................. 63 5 Summary and Recommendations.....................................................................................................64 6 References........................................................................................................................................ 67 Corrective Action Plan Part 1 Riverbend Steam Station Ash Basin Tables 2-1 Initial COI Screening Evaluation 2-2 Background Groundwater Concentrations for the RBSS Site: Ranges of Samples with Turbidity <10 NTU 2-3 Groundwater Results for COls Compared to PPBCs, 2L Standards, IMACs, or DHHS HSLs, and Frequency of Exceedances 2-4 Radionuclide Concentrations 2-5 Seep Results for COls Compared to 2L Standards or IMACs and Frequency of Exceedances 2-6A Surface Water Results for COls Compared to Upgradient Surface Water Concentrations, 2B Standards or USEPA Criteria, and Frequency of Exceedances 2-7 Sediment COI Exceedances Compared to Upgradient Sediment Concentration, NC PSRGs for POG and Frequency of Exceedances 2-8 Proposed Provisional Soil Background Soil Concentrations 2-9 Soil Exceedance Results for COls Compared to NC PSRGs for POG, Background Concentrations, and Frequency of Exceedances 2-10 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 Range of Results for Groundwater Parameters 4-1 Mixing Zone Sizes and Percentages of Upstream River Flows 4-2 East Basin Calculated Surface Water Concentrations 4-3 Mountain Island Lake (main river channel) Calculated Surface Water Concentrations 4-4 Mountain Island Lake (entire site) Calculated Surface Water Concentrations Figures ES-1 Site Conceptual Model — Plan View ES-2 Existing Conditions Cross -Section Conceptual Site Model ES-3 Source Removal Cross -Section Conceptual Site Model 1-1 Site Location Map 1-2 Site Layout Map 1-3 Compliance and Voluntary Monitoring Wells 1-4 Receptor Map 1-5 Site Vicinity 1-6 Municipal Water Intakes Map 1-7 Surface Water Features Map 1-8 Monitoring Well and Sample Locations 2-1 Groundwater Analytical Results Map 2-2 Surface Water and Seep Analytical Results 2-3 Soil Analytical Results — Plan View 3-1 Site Conceptual Model — 3-D View 3-2 Site Conceptual Model — Cross Section A -A' 3-3 Potentiometric Surface Map — "S" Wells 3-4 Potentiometric Surface Map — "D" Wells 3-5 Potentiometric Surface Map — "BR" Wells 3-6 Potentiometric Surface Map — "D and BRU" Wells 3-7 Vertical Gradient Map — "D to BR" Wells Corrective Action Plan Part 1 Riverbend Steam Station Ash Basin 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 BG background bgs below ground surface CAMA North Carolina Coal Ash Management Act of 2014 CAP Corrective Action Plan CCR Coal Combustion Residuals cfs cubic feet per second COI Constituent of Interest COPC Contaminant of Potential Concern CSA Comprehensive Site Assessment DHHS Department of Health and Human Services DWR NCDEQ Division of Water Resources ft feet / foot HSL health screening level IMAC Interim Maximum Allowable Concentration mg/kg milligrams per kilogram MW megawatt NC PSRGs North Carolina Preliminary Soil Remediation Goals NCAC North Carolina Administrative Code NCDENR North Carolina Department of Environment and Natural Resources NCDEQ North Carolina Department of Environmental Quality NPDES National Pollutant Discharge Elimination System NTU Nephelometric Turbidity Unit NURE National Uranium Resource Evaluation POG Protection of Groundwater PPBC Proposed Provisional Background Concentration PWR partially weathered rock RBSS Riverbend Steam Station SCM Site Conceptual Model SU Standard Unit TDS total dissolved solids TZ transition zone UNCC University of North Carolina at Charlotte USEPA U.S. Environmental Protection Agency USGS U.S. Geological Survey Work Plan Groundwater Assessment Work Plan iv Corrective Action Plan Part 1 Riverbend Steam Station Ash Basin Executive Summary ES-1 Introduction ES-1.1 Regulatory Background Duke Energy Carolinas, LLC (Duke Energy) owns and formerly operated Riverbend Steam Station (RBSS) located adjacent to the Mountain Island Lake portion of the Catawba River (Mountain Island Lake) near Mount Holly, Gaston County, North Carolina. RBSS began operation as a coal-fired generating station in 1929 and was retired from service in April 2013. Decommissioning of RBSS is ongoing. From 1929 to 1957, coal ash residue from RBSS's coal combustion process was deposited in the ash storage area and cinder storage area. Following installation of precipitators and a wet sluicing system in 1957, coal ash residue was disposed of in the station's ash basin located adjacent to the station and Mountain Island Lake. Discharge from the ash basin is currently permitted under North Carolina Department of Environment Quality (NCDEQ)1 Division of Water Resources (DWR) under the National Pollutant Discharge Elimination System (NPDES) Permit NC0004961. 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 RBSS was submitted to NCDENR (now NCDEQ) on September 25, 2014, followed by a revised Work Plan on December 30, 2014. The Work Plan was conditionally approved by NCDENR on February 16, 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 RBSS CSA Report was submitted to NCDENR on August 18, 2015 (HDR 2015). North Carolina 15A NCAC 02L .0106(g)(2) requires a site assessment to identify any imminent hazards to public health and safety and the actions taken to mitigate them in accordance with Paragraph (f) of .0106(g). The CSA found no imminent hazards to public health and safety; therefore, no actions to mitigate imminent hazards are required. However, corrective action at the RBSS site is required to address soil and groundwater contamination resulting from source areas. In addition, a plan for continued monitoring of select monitoring wells and parameters/constituents will be implemented following NCDEQ's approval. CAMA also requires the submittal of a Corrective Action Plan (CAP) for each regulated facility no later than 180 days after submittal of the CSA. Duke Energy and NCDEQ mutually agreed to a two-part CAP submittal, with Part 1 being submitted within 90 days of submittal of the CSA Report and Part 2 being submitted no later than 180 days after submittal of the CSA Report. Prior to September 18, 2015, the NCDEQ was referred to as the North Carolina Department of Environment and Natural Resources (NCDENR). Both naming conventions are used in this report, as appropriate. Corrective Action Plan Part 1 Riverbend Steam Station Ash Basin The purpose of this CAP Part 1 is to provide a summary of site usage, brief summary of the CSA findings, an evaluation and refinement of COls for modeling purposes, a detailed description of the Site Conceptual Model (SCM), and results of the groundwater flow and transport model, and results of the groundwater to surface water interaction model. CAP Part 2 will include the remainder of the CAMA requirements, including alternative methods for achieving groundwater quality restoration, conceptual plans for recommended corrective actions, implementation schedule and a plan for future monitoring and reporting. The risk assessment will be submitted under a separate cover with the CAP Part 2 submittal. ES-1.2 Summary of CSA Based on the CSA findings, the source and cause of certain constituent exceedances in areas of the RBSS site is the coal ash contained in the ash basin, ash storage area, and cinder storage area. The cause of these exceedances is leaching of constituents from the coal ash into the underlying soil and groundwater. However, some groundwater, surface water, and soil standards were also exceeded due to naturally occurring elements found in the subsurface as identified in section 2.0. If a constituent concentration exceeded the North Carolina Groundwater Quality Standards, as specified in T15A NCAC .0202L (21- Standards) or Interim Maximum Allowable Concentration (IMAC), the North Carolina Department of Health and Human Services (DHHS) Health Screening Levels (HSL), North Carolina Preliminary Soil Remediation Goals (NC PSRGs) for Protection of Groundwater (POG), or T15 NCAC 02B .0211 and .0216 (213 Standards) or United States Environmental Protection Agency (USEPA) Criteria, the constituent was designated as a "Constituent of Interest" (COI). The CSA found the exceedances in the groundwater for the following constituents: antimony, arsenic, boron, cobalt, iron, manganese, TDS, thallium, and vanadium. The CSA found the exceedances in the soil for the following constituents: antimony, arsenic, boron, chromium, cobalt, iron, manganese, sulfate, thallium, total dissolved solid (TDS), and vanadium. Additional details pertaining to the horizontal and vertical extent of soil and groundwater impacts at the RBSS site are detailed in the CSA Report. ES-2 Background Concentrations and COI Screening Level Summary During the CSA, the source areas were defined as the ash basin (Primary and Secondary Cells), the ash storage area and cinder storage area. Source characterization was performed to identify physical and chemical properties of ash, ash basin surface water, porewater, and ash basin seeps. The analytical results for source characterization samples were compared to 2L Standards or IMACs, NC PSRGs for POG, and 2B Standards or USEPA criteria for the purpose of identifying Corrective Action Plan Part 1 Riverbend Steam Station Ash Basin COls that may be associated with potential impacts to soil, groundwater, sediment, and surface water from the source areas. COls were also found to be naturally occurring in groundwater samples collected at background and upgradient monitoring wells and in soil samples collected from locations that were not impacted by ash. Examples of naturally occurring COls include antimony, chromium, cobalt, iron, manganese, and vanadium. The occurrence of these COls in areas potentially impacted by ash require examination to determine whether their presence in source areas is naturally occurring or can be attributed to ash handling and storage activities at the RBSS site. ES-2.1 Proposed Provisional Background 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: • 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 Proposed Provisional Background Concentrations (PPBCs) were calculated for those groundwater constituents analyzed in background compliance wells included in the NPDES monitoring program. Only samples with turbidity values less than 10 NTU were included in the background calculations. The methodology followed the Statistical Analysis of Groundwater Monitoring Data at RCRA Facilities — Unified Guidance (USEPA 2009). A detailed method review, statistical evaluation, and results for the PPBCs are included in Section 2 and Appendix B of this report. PPBCs for some constituents exceed the 2L Standards, IMACs or DHHS HSLs, including: antimony, cobalt, chromium, hexavalent chromium, iron, manganese, pH, thallium, and vanadium. These values have been used for comparison purposes in the report, but have not been used to establish which COls are moved forward for modeling purposes. Well development and sampling will continue and these concentrations will be incorporated into statistical background analysis once a sufficient data set has been obtained. Soil PPBCs were calculated for those constituents analyzed in background soil borings. The methodology followed ProUCL Technical Guidance, Statistical Software for Environmental Applications for Data Sets with and without Nondetect Observations (USEPA 2013). A detailed method review, statistical evaluation, and results for the PPBCs are included in Appendix B. The soil PPBCs were compared to the NCDEQ PSRGs for POG and, for most COls, the PPBC is higher than the PSRG for POG. Therefore, site -specific soil remediation goals may need to be established. During development of PPBC concentrations, the review of the analytical results and groundwater contour maps generated from the June/July 2015 sampling event suggest that certain background wells identified in the CSA may not truly represent a background condition. Corrective Action Plan Part 1 Riverbend Steam Station Ash Basin To obtain a representative sample of background groundwater additional background wells may be required. Based on horizontal and vertical differences from source areas to well locations, groundwater quality data, and regional background of site -specific COls, monitoring wells MW-7SR, MW-7D, MW-7BR and BG-1 S, BG-1 D are considered to confidently reflect background water quality. Updated COI Screening Evaluation Summary The table below summarizes COls (by medium) and identifies those that require further evaluation to determine if they require possible corrective action. The COls were based on findings from the CSA and comparison to 2L Standards, IMAC, DHHS HSLs, and 2B Standards for respective aqueous media and PSRGs for POG for solid media. These COls then underwent further evaluation as an element of this CAP Part 1, as applicable, in the groundwater flow and fate and transport modeling. Ash and porewater were assessed for COls during the CSA phase, but were not evaluated for remediation in this CAP because the source areas will be drained of water during remedial activities and excavated. Potential COI COI Exceedance by Media COI To Be Further Assessed in Section 3.0 Solid/ Aqueous Ash' Pore Water Ground- water Surface Water Ash Basin Surface Water Seeps Sediment Soil PWR/ 3 Bedrock Aluminum - - - - - - - - - - Antimony - - - - - - - - Arsenic - - - - - - Barium - - - - - - - - Beryllium - - - - - - - - - - Boron - - - - - - - Chromium - - - - - - - - Cobalt - - - - - Copper - - - - - - - - - - Hexavalent Chromium - - - - - - Iron - - - - - Lead - - - - - - - - - - Manganese - - - - - pH- - - - - - - - - - Selenium - - - - - - - - Sulfate - - - - - - - - - - Thallium - - - - - - - - - - TDS - - - - - - - - - - Vanadium - - - - - Notes: 1. Note that ash is not evaluated for remediation in this CAP because ash will be drained of water during remedial activities and excavated. 2. Note that porewater is not evaluated for remediation in this CAP because porewater will be eliminated during ash basin closure activities. 3. Note that PWR and bedrock are not evaluated for remediation in this CAP because there are not regulatory cleanup standards associated with PWR or bedrock. Corrective Action Plan Part 1 Riverbend Steam Station Ash Basin Site Conceptual Model The purpose of the SCM is to evaluate areal distribution of COls with regard to site -specific geological/ hydrogeological and geochemical properties at the RBSS site. The SCM was developed using data and analysis from the CSA Report. Geological/Hydrogeological Properties Seven hydrostratigraphic units were identified as part of the CSA. These units are part of the regolith-fractured rock system, which is characterized as 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 to the north and northeast across the RBSS property boundary to the Mountain Island Lake. Horizontal and vertical hydraulic gradients were calculated for each flow layer. Downward gradient is exhibited across the site within the shallow and deep flow layers. Upward gradient was identified along the northern perimeter of the waste boundary. This gradient is produced because the elevation of the waste is 52 feet higher than the ground surface at the base of the dike. This creates a pressure at the wells downgradient of the dikes Following excavation, this pressure will be reduced and likely eliminate the upward gradient. 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 observed groundwater conditions at the RBSS site span oxidizing to moderately reducing conditions. A review of equilibrium chemistry shows some oxidized species (e.g., chromium (VI)) present in reduced conditions and some reduced species (e.g., selenium (IV)) in oxic conditions. The observed groundwater conditions, showing a mixture of redox conditions and variability in species, taken broadly, indicate a dynamic redox environment at the RBSS site that is not in equilibrium for some COls. Given this diverse range of conditions, logical next steps in the RBSS site evaluation process may include: equilibrium geochemical speciation evaluation using modeling tools such as PHREEQC (USGS, 2013) and groundwater transport and chemical transport modeling. Additional sampling will be needed to characterize the temporal and spatial characteristics of groundwater composition for the site. Additional evaluations may also be beneficial to better characterize the kinetics of redox reactions. Constituents may be removed from groundwater and onto mineral surfaces of the aquifer media through one of the three types of sorption processes: adsorption, absorption, and ion exchange. These sorption processes result in a change of the constituent concentration, and therefore, the mass of the constituent as it is removed from the groundwater onto the solid material. The effect of these processes for a particular constituent can be expressed by the sorption coefficient (Kd). Kd relates the quantity of the adsorbed constituent per unit mass of solid to the quantity of the constituent remaining in solution. Corrective Action Plan Part 1 Riverbend Steam Station Ash Basin Laboratory determination of Kd was performed by the University of North Carolina at Charlotte (UNCC) on 14 site -specific samples of soil. Solid samples were tested in flow through columns to measure the adsorption of COls at varying concentrations. For the RBSS site, 12 column tests and 14 batch tests were conducted. The methods used by 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 RBSS site, as described in greater detail in Section 4.1. ES-3.3 Correlation of Hydrogeologic and Geochemical Conditions of COI Distribution Based on results of sampling and analysis performed during CSA activities, the following are groundwater COls at the RBSS site: Ash and porewater 2L Standard or IMAC exceedances identified in Section 2 include antimony, arsenic, boron, cobalt iron, manganese, selenium, sulfate, TDS, thallium, and vanadium. Antimony concentrations in the shallow flow layer exceeded the IMAC in wells within the ash basin, south of the ash storage area, and in BG-3S. Antimony concentrations in the deep flow layer exceeded the IMAC in wells associated with the ash storage area, the ash basins, MW-91D located north of the cinder storage area, and in background well BG-1 D. Antimony concentrations in the bedrock flow layer exceeded the IMAC in wells within the ash basin, west of the ash basin Primary Cell, and north of the cinder storage area Boron, selenium, sulfate and thallium were identified above their respective 2L Standards or IMAC in three or fewer sample locations. Each of these COls has a low Kd value and can be highly mobile in groundwater. The absence of these constituents in groundwater suggests that geochemical conditions are such that they can attenuate the COI, or that the original composition of source material did not contain large concentrations of these COls. Arsenic has a relatively high Kd value, which suggests that geochemical conditions favor low mobility of this COI. Combined with a lack of detections above the 2L Standard in groundwater, arsenic appears to have a limited distribution. Iron and manganese exceedances of the 2L Standards are widely distributed across the site including both background and source area samples and are source related constituents. The concentration of these COls are variable and very dependent on pH. It is likely that slightly acidic soils located at the RBSS contribute to the mobility and concentration of these COls. Vanadium is above the IMAC in 71 of 79 samples. Groundwater and geochemical conditions promote the mobility of this COI across the site with contributions from naturally occurring vanadium and vanadium from source areas. TDS exceedances are primarily downgradient of the coal pile area. The absence of TDS in other areas suggests that geochemical conditions are not conducive to elevate this COI above its 2L Standard. TDS west of the cinder storage area will require further assessment. Corrective Action Plan Part 1 Riverbend Steam Station Ash Basin Refinement of the RBSS 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 becomes available during supplemental site investigation activities. ES-4 Modeling Groundwater flow, groundwater fate and transport, and groundwater to surface water modeling was conducted to evaluate COI migration and potential impacts following closure of the primary and secondary cells of the ash basin system and ash storage area and cinder storage area at the RBSS site. Under the direction of HDR, The University of North Carolina - Charlotte (UNCC) developed a site -specific, 3-D, steady-state groundwater flow and fate and transport model for the RBSS 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 RBSS ash basin areas, and simulate transient transport conditions in which COls enter groundwater via the ash basin system over the period it was in service. Model Scenarios The following ash basin closure scenarios were modeled for the RBSS site: Existing Conditions (EC): assumes current site conditions with ash sources left in place Cap -in -Place (CIP): assumes ash left in ash basin, cinder storage area, and ash storage area covered by engineered caps Excavation (EX): assumes accessible ash removed Each model scenario utilized steady-state flow conditions established during flow model calibration and transient transport of COIs. __ �... Model Output ES-4.3.1 Groundwater Flow Model The 3-D groundwater flow model results indicate that groundwater in the shallow, deep and transition zone flow layers, and the fractured bedrock flows radially from the southern extent of the property to drainage features north and northeast of the site toward Mountain Island Lake, which is consistent with the groundwater potentiometric surface maps and interpreted groundwater flow directions presented in the RBSS CSA Report. Moreover, the model indicates that groundwater flow originating from the ash basin starts vertically downward then moves horizontally at depth before discharging as baseflow to Mountain Island Lake. The maximum modeled groundwater travel times from southern boundary of the model domain is 662 years in the deep groundwater zone to Mountain Island Lake. Corrective Action Plan Part 1 Riverbend Steam Station Ash Basin The EC Scenario models the condition of source areas assuming that no active remediation has been performed and the site remains "As Is." The CIP scenario results were used to estimate groundwater levels in the ash management area subsequent to placement of an engineered geosynthetic soil cap (assuming the ash will remain in its current position). The model results indicate that groundwater levels decreased by approximately 28 to 40 feet in the center of the Ash Basin Primary Cell and 44 feet in the center of the Ash Basin Secondary Cell. The placement of an engineered cap is a viable option and for the source areas; however, excavation of the source areas has been initiated. In the EX scenario, complete removal of the ash layers was simulated in the model. Flow conditions and groundwater levels post -excavation cannot be accurately estimated until the depth of excavation and the hydraulic parameters of the replacement fill material are known. ES-4.3.2 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 to predict the fate of these COls over time. Selected COls were modeled individually under the EC, CIP, and EX scenarios. COls evaluated in the fate and transport model include: Antimony, arsenic, boron, chromium, cobalt, iron, manganese, pH, selenium, thallium, sulfate, TDS, and vanadium. Several COls were not advanced to modeling because of the following rationale: • Arsenic, boron, selenium, and thallium were found above the 2L Standards or IMAC in only 1 location within the waste boundary. Provided that concentrations above the 2L Standards or IMAC are limited and the source areas are being actively removed, they were eliminated from modeling. • Geochemical modeling of the RBSS site will be completed and submitted under cover of the CAP Part 2. The geochemical model results coupled with the groundwater flow, fate and transport and surface water -groundwater models 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. The geochemical model is key to understanding mobility of iron, manganese, pH and TDS since it cannot adequately be modeled using MODFLOW/MT3DMS. • Cobalt and vanadium concentrations were prevalent above the IMAC in wells hydraulically upgradient of the source areas and throughout the RBSS site. Because of the elevated concentrations in background wells, source area concentrations could not be modeled and would not be representative of source area concentrations. As a result, cobalt and vanadium were not modeled. • Hexavalent chromium was modeled as a flux of total chromium to Mountain Island Lake. Surface water modeling will compare this flux to hexavalent and total chromium. COls evaluated in the fate and transport model were antimony, total chromium, and sulfate. These COls represent the remaining source related groundwater constituents that can be Corrective Action Plan Part 1 Riverbend Steam Station Ash Basin utilized in the fate and transport model as they do not require further evaluation as background or in the geochemical model. Under the EC scenario, the COls modeled included antimony, chromium and sulfate. Results of the EC scenario indicate that concentrations for all modeled COls increase or reach steady- state conditions above 2L Standards or IMACs during the modeled period. The CIP scenario simulated the effects of capping the ash management area at the beginning of the scenario. Under the CIP scenario, the COls simulated by the model are provided below. Concentrations will intercept Mountain Island Lake prior to reaching the compliance boundary: • Antimony: Predicted concentrations decreased below the IMAC in 2040 for the bedrock flow layer and 2070 in the shallow and deep flow layers. Chromium: Concentrations will increase in shallow and deep flow layers to above the 2L Standard in GWA-2 (west side of site) between 2030 and 2090 and in MW-6 (east side of site) between 2095 and 2135. Chromium will not impact MW-15 on the north side of the site adjacent to Mountain Island Lake. Sulfate: Predicted concentrations decreased in each flow layer in downgradient wells and were below the 2L Standard within 15 years of CIP implementation. The EX scenario simulates the effects of removing the ash basin, cinder storage area, and ash storage area at the beginning of this scenario. Under the EX scenario, the COls simulated by the model are provided below. Concentrations will intercept Mountain Island Lake prior to reaching the compliance boundary. • Antimony at well GWA-2 is predicted to decrease and concentrations continue to fall below the IMAC beginning in 2055. At MW-15, antimony concentrations in the shallow and deep flow layers will remain below the IMAC and antimony in the bedrock flow layer will decrease to the IMAC by 2055. At MW-6, antimony will continue to decrease in all flow layers. By 2070, antimony will be below the IMAC across the site. • Chromium concentrations in GWA-2 will increase and exceed the 2L Standard in the shallow flow layer by 2020 and in the deep flow layer by 2060. Chromium will remain above the 2L Standard through 2260. Chromium will not be detectable in MW-15 in any flow layers. In MW-6, chromium concentrations will increase to levels above the 2L Standard in 2060 in the shallow flow layer and 2080 in the deep flow layer and remain above the 2L Standard throughout the modeled time period. • Sulfate concentrations will remain in the groundwater at all three flow layers but below the 2L Standard. The sulfate concentrations are less than the 2L Standard by 2043. Groundwater -Surface Water Interaction Modeling Groundwater model outputs from the fate and transport model were used as inputs to the surface water model to assess water quality conditions in the adjacent Mountain Island Lake receiving water. A mixing model was used to assess potential surface water quality impacts. Groundwater flow rates and concentrations of COls from the groundwater model were used as input to a groundwater -surface water interaction model to determine if 2L Standard (or IMAC) Corrective Action Plan Part 1 Riverbend Steam Station Ash Basin exceedances would result in exceedances of 2B surface water standards or USEPA water quality criteria in Mountain Island Lake. Surface water quality modeling results indicate antimony, chromium, and sulfate concentrations in Mountain Island Lake are all less than the applicable water quality standards or criteria. ES-5 Recommendations The following recommendations have been made based on review of data in CAP Part 1: Groundwater flow, fate and transport and surface water models should be updated with results from second -round sampling at the RBSS site and should be included in CAP Part 2. Further assessment of the area downgradient to the west of cinder storage area is recommended. TDS, sulfate, arsenic, and magnesium COls in this area outside the waste boundary are at concentrations exceeding the applicable regulatory standards and are higher than source area concentrations. CAP Part 1 reviewed groundwater analytical data and groundwater elevation data collected between June/July 2015. Background wells identified in the CSA (BG wells and MW 7-SR and MW-7D) are not situated in an upgradient location to the RBSS source areas and impact to these wells from source could not be ruled out. Additional upgradient wells are recommended to provide background groundwater chemistry for the site south of the source area on Duke Energy property. 10 Corrective Action Plan Part 1 Riverbend Steam Station Ash Basin Introduction Duke Energy Carolinas, LLC (Duke Energy) owns and formerly operated Riverbend Steam Station (RBSS) located adjacent to the Mountain Island Lake portion of the Catawba River (Mountain Island Lake) near Mount Holly, Gaston County, North Carolina. RBSS began operation as a coal-fired generating station in 1929 and was retired from service in April 2013. Decommissioning of RBSS is ongoing. From 1929 to 1957, coal ash residue from RBSS's coal combustion process was deposited in an on -site cinder storage area. Following installation of precipitators and a wet sluicing system in 1957, coal ash residue was disposed of in the station's ash basin located adjacent to the station and Mountain Island Lake. Discharge from the ash basin is currently permitted under North Carolina Department of Environment Quality (NCDEQ)2 Division of Water Resources (DWR) under the National Pollutant Discharge Elimination System (NPDES) Permit NC0004961. 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 RBSS was submitted to NCDENR on September 25, 2014, followed by a revised Work Plan on December 30, 2014. 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 RBSS CSA Report was submitted to NCDENR on August 18, 2015 (HDR 2015). CAMA also requires the submittal of a Corrective Action Plan (CAP) for each regulated facility no later than 180 days after submittal of the CSA 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. The purpose of this CAP Part 1 is to provide background information, a brief summary of the CSA findings, an evaluation and refinement of COls for modeling purposes, a detailed description of the site conceptual model, and results of the groundwater flow and transport model and groundwater to surface water model. CAP Part 2 will include the remainder of the CAP, alternative methods for achieving groundwater quality restoration, conceptual plans for recommended corrective actions, implementation schedule, and a plan for future monitoring and reporting. The risk assessment will be submitted under a separate cover 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. 11 Corrective Action Plan Part 1 Riverbend Steam Station Ash Basin 1.1 Site History and Overview 1.1.1 Site Location, Acreage, and Ownership The RBSS site was named for a bend in Mountain Island Lake on which it is located. The site is north of Horseshoe Bend Beach Road near the town of Mount Holly in Gaston County, North Carolina, and occupies approximately 340.7 acres of land (Figure 1-1). In addition to the RBSS site, Duke Energy owns and operates Mountain Island Lake as part of the Catawba-Wateree Hydroelectric Project (FERC Project No. 2232). Mountain Island Lake borders the RBSS site to the north and is used for recreation and water supply for the Charlotte municipal area. Duke Energy performed a review of property ownership within the ash basin compliance boundary (defined in accordance with TitIe15A NCAC 02L .0107(a) as being established at either 500 feet from the waste boundary or at the property boundary, whichever is closer to the waste). The review indicated that Duke Energy owns the bottom of Mountain Island Lake within the compliance boundary. A site layout map is shown on Figure 1-2. Site Description RBSS was a seven -unit, 454 MW, coal-fired, electricity -generating facility. The station began commercial operations in 1929 with Units 1-4. Units 5-7 began commercial operations sequentially from 1952 through 1954. Units 1-3 were retired from service in the 1970s and Units 4-7 were retired from service on April 1, 2013. During its final years of operation, the plant was considered a cycling station and was brought online to supplement energy supply when electricity demand was at its highest. Duke Energy also operated four combustion turbine (CT) units at the site from 1969 until October 2012. The ash basin system consists of a Primary Cell, a Secondary Cell, and associated embankments and outlet works. An ash storage area is located southwest and side gradient of the Primary Cell and a cinder storage area is located west and downgradient of the Primary Cell. The ash basin system is located approximately 2,400 feet to the northeast of the power plant, adjacent to Mountain Island Lake, as shown on Figure 1-2. The Primary Cell is impounded by an earthen dike located on the west side of the Primary Cell. The surface area of the Primary Cell is approximately 41 acres with an approximate maximum pond elevation of 724 feet.3 The Secondary Cell is impounded by an earthen dike located along the northeast side of the Secondary Cell. The surface area of the Secondary Cell is approximately 28 acres with an approximate maximum pond elevation of 714 feet. The full pond elevation of Mountain Island Lake is approximately 646.8 feet. The RBSS site is generally forested along Mountain Island Lake. Buildings and other structures associated with the retired power production facilities are located on the north side of Horseshoe Bend Beach Road, which extends from west to east and is generally located along a local topographic divide. Topography at the RBSS site generally slopes from this divide to Mountain Island Lake (i.e., south to north). 3 The datum for all elevation information presented in this report is NAVD88. 12 Corrective Action Plan Part 1 Riverbend Steam Station Ash Basin The RBSS site contains two switchyards and associated transmission lines. While power production no longer exists at the site, these facilities continue to support power transmission on the Duke Energy system. The Lark Maintenance Center is also located at the RBSS site west of the retired coal-fired units. This facility is an advanced machining and welding shop that supports various Duke Energy power plants in the region. Permitted Activities and Permitted Waste Duke Energy is authorized to discharge wastewater that has been adequately treated and managed to the surface waters of North Carolina or to a separate storm sewer system in accordance with NPDES Permit NC0004961, which most recently became effective March 1, 2011. This permit expired in March 2015. NCDENR DWR issued a draft of the new NPDES Permit NC0004961 in March 2015. Duke Energy provided comments on the draft permit to NCDENR DWR on May 4, 2015. As of the issuance of this CAP Part 1 report, the draft NPDES permit has not become effective and the site is operating under the March 1, 2011 permit requirements. No active or inactive permitted solid waste facilities (landfills) are located at the site and the property watershed classification prohibits construction of future landfills. As station decommissioning and ash removal activities continue at the site, Duke Energy is working with NCDEQ and other state and federal agencies to ensure all necessary permits and approvals are received. Duke Energy has commenced with removing the ash in the ash basin, ash storage area, and cinder storage area via excavation in May 2015. Approximately 4.6 million tons of ash will be transported to permitted lined landfills and/or structural fills during the excavation project. This initial phase has begun in the northeast corner of the ash storage area and has involved hauling ash by truck to a permitted lined landfill in Homer, Georgia and to a permitted lined landfill at the Duke Energy Marshall Steam Station in Mooresville, North Carolina. Following the initial ash removal phase, the majority of ash at RBSS is anticipated to be transported by rail to a lined clay mine reclamation project in central North Carolina, pending permitting and approvals. Final removal of ash at RBSS is anticipated to be completed no later than August 2019. History of Site Groundwater Monitoring Duke Energy implemented compliance groundwater monitoring at the RBSS in 2010 in accordance with NPDES Permit NC0004961 requirements. From December 2008 to June 2010, voluntary groundwater monitoring was performed twice annually around the RBSS ash basin with analytical results submitted to NCDENR DWR. From December 2010 through June 2015, the compliance groundwater monitoring wells at the RBSS site have been sampled a total of 15 times. The locations of the voluntary and compliance wells are identified on Figure 1-3. The compliance boundary for groundwater quality at the RBSS site is defined in accordance with Title 15A NCAC 02L .0107(a) as being established either 500 feet from the waste boundary or at the property boundary, whichever is 13 Corrective Action Plan Part 1 Riverbend Steam Station Ash Basin closer to the waste boundary. A detailed description of NPDES and voluntary groundwater monitoring programs and results is provided in the CSA Report. Summary of Comprehensive Site Assessment Report The CSA for the RBSS site began in February 2015 and was completed in August 2015. Seventy-eight groundwater monitoring wells and 6 soil borings were installed/advanced as part of the assessment to characterize the ash, soil, rock, and groundwater at the RBSS site (Figure 1-8). Seep, surface water, and sediment samples were also collected (Figure 1-7). In addition, hydrogeological evaluation testing was performed on newly installed wells. Information obtained during the CSA was utilized to determine existing background constituent concentrations, source related constituents concentrations and to evaluate the horizontal and vertical extent of impacts to soil and groundwater at the site. If a constituent4 concentration exceeded the North Carolina Groundwater Quality Standards, as specified in T15A NCAC .0202L (2L Standards) or Interim Maximum Allowable Concentration (IMAC)5, it was designated in the CSA as a "Constituent of Interest' (COI). In addition, the CSA Report presented information from a receptor survey completed in 2014 and a screening level human health and ecological risk assessment. Additional details of the CSA Report 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 within a 0.5-mile (2,640-foot) radius of the RBSS 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 survey results is provided in the CSA Report. Results of the receptor survey are detailed on Figure 1-4. Surrounding Land Use The area surrounding RBSS generally consists of residential properties, undeveloped land, and Mountain Island Lake (Figure 1-5). Properties on the northern side of Mountain Island Lake are located in the Town of Huntersville, Mecklenburg County, North Carolina. The Town of Huntersville identifies most of these properties as a park, nature preserve, or wildlife refuge. A residential property is located to the northeast of the ash basin on the northern side of Mountain Island Lake. 4 Constituents are elements, chemicals, or compounds that were identified in the approved Work Plan for sampling and analysis, and include antimony, arsenic, boron, chromium, cobalt, iron, manganese, selenium, thallium, vanadium, sulfate, and total dissolved solids (TDS). s Appendix #1 of 15A NCAC Subchapter 02L Classifications and Water Quality Standards Applicable to The Groundwaters of North Carolina, lists Interim Maximum Allowable Concentrations (IMACs). The IMACs were issued in 2010 and 2011; however, NCDENR has not established a 2L Standard for these constituents as described in 15A NCAC 021_.0202(c). For this reason, IMACs noted in this report are for reference only. 14 Corrective Action Plan Part 1 Riverbend Steam Station Ash Basin Properties on the southern side of Mountain Island Lake are located in Mount Holly, Gaston County, North Carolina. The majority of the property in this area is owned by Duke Energy and associated with RBSS. Residential properties are located to the south and southeast of the RBSS site (south of Horseshoe Bend Beach Road). With the exception of RBSS decommissioning activities, future surrounding land uses are assumed to remain similar to their current uses (undeveloped land, wildlife refuge, nature preserve, and residential). Mountain Island Lake supplies water to the Charlotte municipal area, as well as the towns of Gastonia and Mount Holly, North Carolina. The Charlotte intake is located 3.4 miles downstream from the RBSS site and the Gastonia and Mount Holly intakes are located approximately 6.9 miles downstream from the RBSS site. Water supply intake locations are shown on Figure 1-6. 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. Based on the receptor surveys, no public water supply wells (including irrigation wells or unused wells) were identified within a 0.5-mile radius of the RBSS ash basin compliance boundary. Wells historically used by Duke Energy at the RBSS facility are no longer in operation and have been properly abandoned. No wellhead protection areas were identified within a 0.5-mile radius of the ash basin compliance boundary. Receptors identified within the RBSS compliance boundary include surface water features that direct flow towards Mountain Island Lake (Figure 1-4) . One reported private water supply well is located at a residence northeast of RBSS within a 0.5- mile radius of the ash basin compliance boundary. This well is located on the northern side of Mountain Island Lake in Mecklenburg County (Well 1 shown on Figure 1-4). Information evaluated as part of the CSA indicated that the identified water supply well would not be impacted as it is hydraulically isolated and across Mountain Island Lake from the ash basin. Summary of Screening Level Risk Assessment A screening level human health and ecological risk assessment was performed as a component of the CSA Report (HDR, 2015). Each screening level risk assessment identified the exposure media for human and ecological receptors. Human health and ecological exposure media includes potentially impacted groundwater, soil, surface water, and sediments. The human health exposure routes associated with the evaluated pathways for the site include ingestion, inhalation, and dermal contact of environmental media. Potential human receptors under a current or hypothetical future use include construction/outdoor workers, off -site residents, recreational users and trespassers. The ecological exposure routes associated with the evaluated pathways for the site include dermal contact/root absorption/gill uptake and ingestion of environmental media. Potential ecological receptors under a current or hypothetical future use include aquatic, riparian, and terrestrial biota. 16 Corrective Action Plan Part 1 Riverbend Steam Station Ash Basin The screening level risk assessment will continue to be refined consistent with risk assessment protocol in parallel with the CAP Part 2 schedule. Geological/Hydrogeological Conditions The RBSS site is within the Charlotte terrane, one of a number of tectonostratigraphic terranes that have been defined in the southern and central Appalachians and is in the western portion of the larger Carolina superterrane (Horton et al. 1989; Hibbard et al. 2002; Hatcher et al. 2007). The Charlotte terrane is dominated by complex sequence of plutonic rocks that intrude a suite of metaigneous rocks (amphibolite metamorphic grade) including mafic gneisses, amphibolites, metagabbros, and metavolcanic rocks with lesser amounts of granitic gneiss and ultramafic rocks with minor metasedimentary rocks including phyllite, mica schist, biotite gneiss, and quartzite with marble along its western portion (Butler and Secor 1991; Hibbard et al. 2002). The general structure of the belt is primarily a function of plutonic contacts. The geologic and hydrogeologic system in natural materials (alluvium, soil, soil/saprolite, and bedrock) at RBSS is consistent with the regolith-fractured rock system and is an unconfined, connected system without confining layers. On the northwest side, the Charlotte terrane is in contact with the Inner Piedmont zone along the Central Piedmont suture along its northwest boundary and is distinguished from the Carolina terrane to the southeast by its higher metamorphic grade. Bedrock at the RBSS site consists of meta -quartz diorite and meta-diabase. Based on rock core descriptions, the meta -quartz diorite color typically is a white to light gray matrix with dark greenish gray, dark gray and black phenocrysts. In general, groundwater at the site flows to the north, east, and west and discharges to Mountain Island Lake (Figure 3-1). Groundwater beneath the southwest portion of the waste boundary flows to the northwest to Mountain Island Lake. Flow contours developed from groundwater elevations measured in the shallow and deep wells in the southeastern portion of the site depict groundwater flow generally to the northeast to Mountain Island Lake. Groundwater contours developed from the groundwater elevations in the bedrock wells show groundwater moving generally in a north/northwesterly direction from the south side of the site to Mountain Island Lake. Results of the CSA Investigations Groundwater exceedances were identified at the RBSS site during the CSA investigation. These groundwater impacts are a result of both naturally occurring conditions and from CCR material contained in the ash basin and the ash storage area and cinder storage areas. The horizontal and vertical extent of source -related soil contamination was also identified during the CSA with the exception of the area outside the cinder storage area southwest of the ash basins and north of the coal pile area. The source of the groundwater contamination at the site was found to be the coal ash contained within the ash basin, ash storage areas, and cinder storage area. Groundwater contamination at the site attributable to ash handling and storage was delineated during the CSA activities with the following exceptions: 16 Corrective Action Plan Part 1 Riverbend Steam Station Ash Basin • Horizontal and vertical extent north of the coal pile and west of the cinder storage area. • Horizontal and vertical extent outside the waste boundary northeast of the ash basins. The data confirm that geologic conditions present beneath the ash basins impede the vertical migration of contaminants. The contaminant transport is generally to the north, west, and east toward the Mountain Island Lake and not towards other off -site receptors. Additional details pertaining to the horizontal and vertical extent of soil and groundwater impacts at the RBSS site are detailed in the CSA Report. Background monitoring wells contained naturally occurring metals and other constituents at concentrations that exceeded their respective regulatory standards or guidelines. These included antimony, chromium, cobalt, iron, manganese, and vanadium. The CSA Report did not propose provisional background concentrations for soil, groundwater, and surface water COls identified in the CSA; however, these concentrations are discussed in Section 2 of this this CAP Part 1 report. Duke Energy is required per the CAMA and has committed to removing the ash in the ash basin and ash storage areas via excavation to a lined facility and/or beneficial reuse. As part of the RBSS closure process, Duke Energy submitted a coal ash excavation plan to the NCDENR in November 2014. The excavation plan has been approved, and removal actions have has been commenced and excavation activities are being completed concurrently with development of this CAP. In conjunction with decommissioning activities and in accordance with CAMA requirements, Duke Energy is anticipated required to complete closure of the RBSS ash ponds by August 1, 2019 per NC CAMA. Based on the results of soil and groundwater samples collected beneath the ash basins and the ash management areas, some residual contamination will remain after excavation; however, the degree of contamination and the persistence of this contamination over time cannot be determined at this time. The CAMA requires that corrective action be implemented to restore groundwater quality where the CSA documents exceedances of groundwater quality standards. Additional information concerning groundwater corrective action will be provided as part of the subsequent CAP Part 2 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. 17 Corrective Action Plan Part 1 Riverbend Steam Station Ash Basin (1) No later than 90 days from submission of the Groundwater Assessment Report required by subsection (a) of this section, or a time frame otherwise approved by the Department not to exceed 180 days from submission of the Groundwater Assessment Report, the owner of the coal combustion residuals surface impoundment shall submit a proposed Groundwater Corrective Action Plan to the Department for its review and approval. The Groundwater Corrective Action Plan shall provide restoration of groundwater in conformance with the requirements of Subchapter L of Chapter 2 of Title 15A of the North Carolina Administrative Code. The Groundwater Corrective Action Plan shall include, at a minimum, all of the following: a. A description of all exceedances of the groundwater quality standards, including any exceedances that the owner asserts are the result of natural background conditions. 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. As required under the CAMA, Duke Energy is currently closing the RBSS ash basin, ash storage area, and cinder storage area in accordance with the RBSS Coal Ash Excavation Plan, which was submitted by Duke Energy to the NCDENR on November 13, 2014 (Duke Energy 2014). All necessary permits were received in May 2015 and ash is actively being removed from the facility. Duke Energy is anticipated to complete closure of the RBSS ash basin and cinder storage area by August 1, 2019. Based on the results of soil and groundwater samples collected beneath the ash basin and cinder storage area, some residual contamination will remain after excavation; however, the degree of contamination and the persistence of this contamination over time cannot be 18 Corrective Action Plan Part 1 Riverbend Steam Station Ash Basin determined at this time. The 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 North Carolina Groundwater Quality Standards found in the North Carolina Administrative Code Title 15A, Subchapter 2L.0202 (2L Standards) or the Interim Maximum Allowable Concentrations (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 has also requested that hexavalent chromium be compared to the North Carolina Department of Health and Human Services (NCDHHS) Health Screening Level (HSL). Surface water sample analytical results were compared to the appropriate North Carolina Surface Water Quality Standards (213 Standards), selected from a list of standards published by NCDENR dated April 22, 2015 and applicable U.S. Environmental Protection Agency (USEPA) National Recommended Water Quality Criteria. A two-step process was employed in this assessment. First, if surface water bodies receiving surface water discharge from the ash basin are classified for drinking water use, the standards designated as'water supply' were used. Next, this value was compared to the Iowest'aquatic life' value and the lower of these two values was used in comparison tables included herein addressing surface water quality (http://portal.ncdenr.org/web/wq/ps/csu), accessed on October 17, 2015). Soil sample analytical results were compared to North Carolina Preliminary Soil Remediation Goals (NC PSRGs) 'new format' tables for Protection of Groundwater (POG) exposures (updated March 2015). Sediment sample analytical results were also compared to NC PSRGs for POG. 19 Corrective Action Plan Part 1 Riverbend Steam Station Ash Basin 2 Background Concentrations and Regulatory Exceedances Introduction As part of this CSA, groundwater, seep, surface water, sediment, and soil samples were collected between June 5, and July 27, 2015, from background locations, within the ash management areas, and from locations outside the RBSS waste boundary. Groundwater samples were also collected from previously installed voluntary and compliance monitoring wells on the site and seep samples were collected from seeps previously identified by NCDENR. Data obtained from this sampling event were presented in the CSA report and are summarized in Section 1.5.2 of this CAP Part 1. The purpose of this section is to present proposed provisional background concentrations (PPBCs) for COls per affected medium; discuss the nature and extent of COI impacts to media with regard to PPBCs and applicable regulatory standards or guidelines (i.e., 2L Standards, IMACs, 213 Standards, PSRGs for POG); and determine which COls will be retained for further evaluation. This section also compares background, downgradient, and source area constituent concentrations to applicable regulatory standards or guidelines to determine if constituent exceedances are attributable to the source area. Some COls identified in the CSA are present in background and upgradient monitoring wells, soil borings, and surface water locations and may be naturally occurring, and thus require consideration to determine whether their presence downgradient of the source area is naturally occurring or potentially attributed to the source area. 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 becomes available with input from NCDEQ. The COls identified in the CSA report are organized by media and presented in Table 2-1. 20 Corrective Action Plan Part 1 Riverbend Steam Station Ash Basin Table 2-1. Initial COI Screening Evaluation Potential COI CSA COI Exceedance by Media COI To Be Solid/ Aqueous Ash Pore- water Ground- water Surface Water Ash Basin Surface Water Seeps Sediment Soil PWR/ Bedrock Further Assessed in CAP Part 1 Aluminum - - - - - - - Yes Antimony - - - - - Yes Arsenic - - - Yes Barium - - - - - - Yes Beryllium - - - - - - - - No Boron - - - - Yes Cadmium - - - - - - - Yes Chloride - - - - - - - - - No Chromium - - - - - - Yes Cobalt Yes Copper - - - - - - - Yes Iron Yes Lead - - - - - - - Yes Manganese Yes Mercury - - - - - - - - - No Nickel - - - - - - - Yes Nitrate - - - - - - - - - No H - - - - - - - Yes Selenium - - - - - - Yes Sulfate - - - - - - - Yes Thallium - - - - Yes Vanadium - Yes Zinc - - - - - - Yes TDS - - - - - - Yes Note: COI Exceedance based on 2L, IMAC, 213 for respective Aqueous media and PSRGs for solid/soil like media COls resulting from ash and porewater exceedances at the RBSS site are representative of source characterization data with respect to groundwater. Since there is active removal of the source areas, ash and porewater are not considered in this CAP Groundwater �.2.1 Background Wells and Concentrations Because COls can be both naturally occurring and related to the source areas, the groundwater monitoring wells used to establish background concentrations are 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: 21 Corrective Action Plan Part 1 Riverbend Steam Station Ash Basin • 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 in the CSA are compliance monitoring wells MW-7SR and MW-7D and newly installed background monitoring wells BG-1S, BG-1D, BG-2S, BG-2D, BG-2BR, BG-3S and BG-3D, and MW-7BR (Figure 2-1). During development of PPBC concentrations, analytical results and groundwater contour maps generated from the June/July sampling event suggest that the compliance and background wells identified in the CSA may not truly represent a background condition. To obtain a representative sample of background groundwater additional background wells may be required. 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 evaluations of COls in this CAP Part 1 does consider analytical data where turbidity was greater than 10 NTU. Duke Energy acknowledges that eliminating data greater than 10 NTU in comparison to PPBCs represents a conservative approach. Additional evaluation on a well -by - well and constituent -by -constituent basis may be required and will be addressed as part of a post remedial monitoring plan included in CAP Part 2. That level of evaluation was not possible using the limited data set acquired under the time constraints specified in CAMA. In addition, porewater and groundwater sample results (other than background) which were collected during the CSA 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 fate and transport model considering the risk classification for the RBSS site. Background groundwater concentrations for the RBSS site, PPBCs, and regional background data are presented in Table 2-2. Background concentrations reported for RBSS are limited to samples collected from wells where turbidity was less than 10 NTU. PPBCs represent either the prediction limit for statistically derived concentrations utilizing the compliance monitoring wells, or the highest detected value or the highest laboratory reporting limit for non -detect values as observed in the compliance or newly installed background monitoring wells. Background concentrations will be incorporated into statistical background analysis once sufficient data set has been obtained. Regional Background Groundwater Concentrations for COls listed in Table 2-2 come from publicly available groundwater data sources, including National Uranium Resource Evaluation (NURE) data collected from within 20 miles of the RBSS site, county -wide background data collected from Gaston County by Department of Health and Human Services (DHHS) and state- wide data collected by USGS. 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 employees living within 2 and 10 miles from the RBSS waste boundary. The 2-10 Private Well results are provided for reference only due to the lack of well construction data, 22 Corrective Action Plan Part 1 Riverbend Steam Station Ash Basin hydrostratigraphic data, and detailed geological context for these sample locations. A detailed analysis of RBSS background groundwater concentrations is provided in Appendix B. Table 2-2. Background Groundwater Concentrations for the RBSS Site: Ranges of Analytical Results with Turbidity <10 NTU. MW-7SR and MW- New Proposed Regional 2-10 Private 7D Compliance Background Provisional Background Well Data Well Background Well Background Constituent Groundwater (February- Groundwater Groundwater Groundwater Concentrations July 2015) Concentrations Concentrations Concentrations (pg/L) (pg/L) (2010 to 2015) (June 2015) (pg/L) (pg/L)*** /L Antimony* North Carolina <1 to 1.5 <1 to 1.04 0.33J to 3.3 1 0.5 to 232; 0.5 to 15 Arsenic (Gaston; <0.5 to <1 <1 to 2 0.12J to 2 1 Mecklenburg) Boron Not reported <5 to <50 <100 27J to <50 50 5 to 30; 0.5 to 80 Chromium (Gaston; <0.5 to <5 <1 to 14 0.25J+ to 57.5 5 Mecklenburg) Cobalt* Not reported <0.5 to 7.8 <1 <0.5 to 3 3 Below detect to Iron 98,000; 25 to 58,560 <0.01 to <10 to 790 28J to 1,500 790 (Gaston; 0.377 Mecklenburg) Below detect to 434 Manganese (20-mile radius from 1.7 to 65 <5 to 413 <5 to 370 413 site 5.4to8.2SU 6.52to7.38 pH (20-mile radius from SU 4.96 to 5.92 SU 6.15 to 8.2 SU 4.56-8.5 SU site Sulfate Not reported 800 to 8,300 120 to <1,000 2,400 to 48,400J 970 <1 Thallium* (Blue Ridge <0.1 to <0.2 <0.2 0.033J+ to <0.1J 0.2 Mountain and Piedmont aquifers) TDS Not reported 79,000 to 10,000 to 120,000 57,000 to 120,000 130,000 1,180,000 <0.1 to 14.8 0.365 to Vanadium* (20-mile radius from 9.83 No Data Available** 0.35J to 29.9 29.9 site Notes: 1. Ng/L = micrograms per liter; SU = Standard Units 2. < indicates concentration less than laboratory reporting limit. 3. J = Estimated concentration; J+ = Estimated concentration, biased high 4. * Indicates 2L Standard not established for constituent; therefore, IMAC used for screening criteria. 5. **Vanadium has not analyzed in compliance monitoring wells MW-7SR and MW-7D during prior events 6. Regional groundwater concentration data are from NURE data in a 20-mile radius from the site for all constituents contained in the NURE database. DHHS county -level data were subsequently used for all constituents available. Remaining constituents for which there are no NURE or DHHS data were pulled from the most spatially relevant, publicly available sources. Further source information is found in Section 10.1 of the RBSS CSA report. 7. *** As stated in the prior sections background concentrations may require the installation of additional background monitoring wells to develop the data set. 8. Reported range for COls cobalt and vanadium is from an NPDES sampling event in May 2015 and a CSA sampling event in June 2015. Only June 2015 data were provided in the CSA report. 23 Corrective Action Plan Part 1 Riverbend Steam Station Ash Basin Notes (cont'd): 9. Reported compliance monitoring well (MW-7SR and MW-7D) concentration ranges for hexavalent chromium, and cobalt, are from an NPDES sampling event in April 2015. These constituents were historically not analyzed for as part of the NPDES sampling program. 10. PPBCs were statistically derived using the historical data from the compliance monitoring wells (MW-24D and MW-24DR) and the CCP Landfill monitoring wells (CCP-MW1S and CCPMW-1 D). See Appendix B for additional information regarding the data set and the determination of the PPBCs. During development of PPBC concentrations, analytical results and groundwater contour maps generated from the June/July sampling event suggest that the compliance and background wells identified in the CSA may not truly represent a background condition. To obtain a representative sample of background groundwater additional background wells may be required. 2.2.2 Groundwater Exceedances of 2L Standards or IMACs Groundwater contamination at the RBSS site attributed to ash management areas was delineated during the CSA activities with the following exceptions: • Horizontal and vertical extent downgradient of the coal pile and cinder storage area. • Horizontal and vertical extent outside the northeast boundary of the ash basin. These exceptions were identified as data gaps in the CSA. Additional assessment is planned to address the exceptions noted above and will likely include additional soil sampling and monitoring well installation. A specific scope of work will be determined as part of a future activities. Information gathered from additional assessment will be submitted under a separate cover. Groundwater results for COls, along with a comparison to applicable regulatory standards or guidelines, are provided in Table 2-3. There is no 2L Standard or IMAC for hexavalent chromium. NCDEQ has requested that hexavalent chromium results be compared to the DHHS Health Screening Levels (HSLs) for private water supply wells (0.07 pg/L). Groundwater sample locations and analytical results are depicted on Figure 2-1. Table 2-3. Groundwater Results for COls Compared to PPBCs, 2L Standards, IMACs, or DHHS HSLs, and Frequency of Exceedances COI Proposed Provisional Background Concentrations (pg/L)DHHS NC 2L Standard, IMAC, or DHHS HSL (pg/L) Groundwater Concentrations Exceeding 2L Standards, IMACs, or HSLs /L Number of Samples Exceeding 2L Standard, IMACs, or DHHS HSLs Number of Samples Antimony* 1 1 1.1 to 18.2 19/79 Arsenic 1 10 10.7 1/79 Boron 50 700 2,200 1/79 Chromium 5 10 11.6 to 903 22/79 Cobalt* 3 1 1.1 to 66.8 28/79 Hexavalent Chromium' NS 0.07 .014J to 810 28/79 I ron 790 300 370 to 30,800 27/79 Manganese 413 50 57 to 12,700 30/79 pH 4.56 to 8.5 SU 6.5 to 8.5 SU 3.56 to 12.5 SU 82/90 24 Corrective Action Plan Part 1 Riverbend Steam Station Ash Basin Proposed Groundwater Number of Samples Provisional NC 2L Standard, Concentrations Exceeding 2L COI Background IMAC, or DHHS Exceeding 2L Standard, IMACs, or Concentrations HSL (pg/L) Standards, IMACs, or DHHS HSLs Number (pg/L)DHHS HSLs /L of Samples Sulfate 970 250,000 376, OOOJ+ to 1,420,000 3/78 Thallium* 0.2 0.2 3.2 1/73 TDS 120,000 500,000 750,000J- to 23,000,000 8/78 Vanadium* 29.9 0.3 0.32J+ to 67.5 71/79 Notes: 1. Ng/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. * Indicates 2L Standard not established for constituent; therefore, IMAC used for screening criteria. 8. NS = No standard (PPBC) available Observations related to groundwater COls at RBSS are: • Arsenic, boron, and thallium concentrations each exceed the 2L Standard or IMAC in one groundwater sample. Boron is considered to be a detection monitoring constituent and is 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 boron to be a potential indicator of groundwater contamination from CCR as it moves rapidly through the groundwater relative to other constituents, and thus provide an early detection of whether contaminants are migrating from the CCR unit. The isolated presence of these constituents suggests that migration of contaminants from the source areas is minimal. Arsenic, boron and thallium will not be evaluated further based on the isolated nature of the exceedances and based on the ongoing removal of source area media as part of the ash removal activities. • Sulfate concentrations exceed the 2L Standard in three groundwater samples collected from RBSS. The greatest sulfate concentrations are identified downgradient of the coal pile and cinder storage area and not present above 2L Standards downgradient of the ash basins. Sulfate is also considered to be a detection monitoring constituent listed in the CCR Rule. Sulfate is further assessed in Section 4 to determine if there are surface water impacts to Mountain Island Lake. Further assessment of the cinder storage area was recommended in the CSA. • Antimony, chromium, cobalt, iron, manganese, vanadium, and TDS all exceed their respective 2L Standards or IMACs in at least one background well. It is likely that these COls are in part related to natural background conditions and will be carried further in the evaluation. • Removal of the ash basins will minimize further contribution of COls from source areas. A post -excavation monitoring plan will be evaluated as part of the CAP Part 2 submission to monitor trends in COI concentrations. Evaluation of the post excavation data will provide information on whether these COls are more related to background conditions upon removal of the source area and to the effectiveness of the remedy. 26 Corrective Action Plan Part 1 Riverbend Steam Station Ash Basin • Cobalt and vanadium are identified above the 2L Standard or IMAC in 28 and 71 of 79 monitoring wells respectively. These exceedances were identified in background, source area, and downgradient wells, indicating naturally occurring conditions contribute to regulatory exceedances. Contribution of these COls from upgradient groundwater exists and varies across the site. Considering the active excavation occurring at the RBSS site, and that groundwater models presented in Section 4 are unable to incorporate background conditions into the models, performing models on these COls would not provide a true representation of source contribution. As such, these COls have not been advanced into modeling. • Antimony and chromium exceed the 2L Standard or IMAC downgradient of the ash management area. Because these COls exceed the 2L Standard or IMAC immediately downgradient of the ash management areas, these COls will be modeled in Section 4.0. The PPBCs were determined to be greater than (or outside of the range of in the case of pH) the 2L Standards or IMACs for the following constituents: • Cobalt • Iron • Manganese • pH • Vanadium Pending approval of the PPBC concentrations for these constituents by NCDEQ, PPBCs for the constituents listed above will be used for identifying groundwater quality exceedances of COls instead of the 2L Standards or IMACs during future sampling events. For PPBCs determined to be less than the 2L Standards or IMAC 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. Monitoring well MW-13 located downgradient of the ash basins and background monitoring wells BG-1S/D were sampled for these analytes, and the results of this analysis are presented in Table 2-4. 26 Corrective Action Plan Part 1 Riverbend Steam Station Ash Basin Table 2-4 Radionuclide Concentrations Radionuclide USEPA MCL* Background Concentrations MW-13 Radium-226 5 pCi/L (combined) 3.832J+ to <4 pCi/L (combined) <4 (combined) Radium-228 Uranium 30 lag/L 0. 112J to 0. 331 lag/L <0.2 Uranium-233 30 lag/L (combined) <0.15 (combined) <0.15 (combined) Uranium-234 Uranium-236 Notes: 1. pCi/L = Picocuries per liter 2. lag/L = micrograms per liter 3. J = Estimated concentration 4. J+ = Estimated concentration, biased high 5. ND= Not detected above laboratory detection limits. 6. MCL = Maximum Contaminant Level 7. * USEPA MCL for uranium of 30 lag/L assumes combined concentration for all isotopes. Radium isotopes and natural uranium were detected below USEPA Maximum Contaminant Levels (MCLs) in background wells; these radionuclides were not detected above laboratory detection limits in downgradient monitoring well MW-13. Uranium isotopes were not detected above laboratory reporting limits in either background wells or MW-13. Seeps Duke Energy, at the request of NCDEQ, identified 12 areas of wetness (AOWs) at the RBSS site. These AOWs were located in areas associated with both surface water and groundwater expression and were compared to either surface water or groundwater standards as applicable. A thirteenth area was selected as background for all AOWs from an unnamed tributary east of the ash management areas (S-13).Due to their proximity to Mountain Island Lake, AOWs S-4, S-6, S-7, and S-8 were considered surface water locations and are discussed in Section 2-4 below. AOWs S-2, S-5, S-9, and S-11 were located at the toe of the basins and were considered seeps due to the assumed connectivity to groundwater. COI results for the four identified seeps compared to 2L Standards or IMACs are provided in Table 2-5. Seep sample locations and analytical results are shown on Figure 2-2. Table 2-5. Seep Results for COls Compared to 2L Standards or IMACs and Frequency of Exceedances Seep Concentrations Number of Samples COI NC 2L Standard or IMAC Exceeding 2L Standards Exceeding 2L Standards (pg/L) or IMACs or IMACs/ Number of (pg/L) Samples Cobalt* 1 1.4 to 46.1 4/4 Iron 300 4,600 1/2 Manganese 50 210 to 3,100 4/4 Vanadium* 0.3 0.33J to 3.2 4/4 Notes: 1. lag/L = micrograms per liter 27 Corrective Action Plan Part 1 Riverbend Steam Station Ash Basin Notes (cont'd): 2. J = Laboratory estimated concentration 3. * Indicates 2L Standard not established for constituent; therefore, IMAC used for screening criteria. With the exception of iron, manganese, and vanadium, the results for all other constituents reported at background seep sample location S-13 were less than the 2L Standards or IMACs. Following removal of the source areas from RBSS and final grading as part of site restoration, the current seeps may be eliminated, modified, or remain as surface expressions of groundwater in their current locations. Evaluation of seep COls have not been carried forward in this CAP Part 1 based on the uncertainty surrounding the seeps as a result the ongoing ash excavation. A post -monitoring excavation plan will be recommended as part of CAP Part 2 that will provide for further identification, assessment and monitoring, if necessary, of seeps 2.4 Surface Water Two surface water samples (RBSW001 and RBSW002) were collected from Mountain Island Lake at the cooling water intake canal located immediately northwest of the station. One surface water sample was collected from ponded water in the excavated area of the cinder storage area (SW-3). Four surface water samples were collected from seep locations beyond the ash basin waste boundary (AOWs S-4, S-6, S-7, and S-8). During the CSA, these locations were considered surface water tributaries outside the ash basin and were compared to surface water standards. The background surface water sample was identified as AOW S-13 and was collected on an unnamed tributary leading to Mountain Island Lake Surface water concentrations were compared to the more stringent of the North Carolina Surface Water Pollutant Standards for Metals for freshwater aquatic life, water supply, or human health derived from 213 Standards for Class B6, WS-IV' waters. In the absence of a 213 Standard, constituent concentrations were compared to USEPA National Recommended Water Quality Criteria. Surface water results for COls, compared to upgradient surface water concentrations (S-13) and applicable regulatory standards or criteria, are provided in Table 2-6. Surface water sample locations and analytical results are depicted on Figure 2-2. Table 2-6. Surface Water Results for COls Compared to Upgradient Surface Water Concentrations, 2B Standards or USEPA Criteria, and Frequency of Exceedances 2B Concentrations SW-13 Number of Samples Standard or Exceeding 2B Upgradient Exceeding 2B COI USEPA Standards or USEPA Surface Water Standard or USEPA Criteria Criteria Concentrations Criteria/Number of (pg/L)/L /L Samples Within Ash Basin Waste Boundary Aluminum* 87 310 130 1/1 Lead 0.54 0.58 0.13 1/1 Zinc 36 220 3J 1/1 6 Class B refers to surface waters protected for recreation (e.g., swimming, fishing, wading, boating, etc.), wildlife, fish consumption, aquatic life, agriculture, and other similar uses. Water Supply WS-IV classification is for surface waters used as sources of water supply for drinking, culinary or food processing purposes where other water supply classifications are not feasible. WS-IV waters are generally in moderately to highly developed watersheds. 28 Corrective Action Plan Part 1 Riverbend Steam Station Ash Basin COI 2B Standard or USEPA Criteria (pg/L)/L Concentrations Exceeding 2B Standards or USEPA Criteria SW-13 Upgradient Surface Water Concentrations /L Number of Samples Exceeding 2B Standard or USEPA Criteria/Number of Samples Intake Canal Aluminum* 87 230 130 1/2 Cadmium 0.15 0.32 to 2 <0.08 2/2 Copper 2.7 9.8 to 35.9 0.92J 2/2 Lead 0.54 0.89 to 1.5 0.13 2/2 Selenium 5 6 to 35.4 <0.5 2/2 Zinc 36 140 to 750 3J 2/2 Downgradient Aluminum* 87 110 130 1/1 Cobalt* 3 9.6 to 11.7 0.24J 2/4 Copper 2.7 2.9 0.92J 1/4 Lead 0.54 1.1 0.13 1 /4 Notes: 1. pg/L = micrograms per liter 2. J = Laboratory estimated concentration 3. * Indicates 2L Standard not established for constituent; therefore, IMAC used for screening criteria. Since ash is being removed from the ash basin, surface water results from sampling location SW-3 (inside the ash basin) will not be further evaluated as part of this CAP Part 1 report None of the COls listed in Table 2-6 were identified as porewater or groundwater COls, therefore, they are not attributable to source areas and are not considered for further CAP review in this media. Surface water sample analytical results collected as part the NPDES permit requirements were reviewed for one upstream (River Mile 278.0) and one downstream (River Mile 277.5) location in Mountain Island Lake. Twice -yearly surface water sample results from 2011 to 2015 were reviewed. Constituents analyzed consisted of arsenic, cadmium, chromium, copper, mercury, lead, selenium, and zinc. No exceedances of 2B Standards were detected for the constituents analyzed. Sediment Sediment samples were collected concurrently with each of the surface water and seep samples with the exception of samples collected within ash management areas. Seeps S-1, S- 3, S-10, and S-12 were noted to be dry at the time of sample collection; however, sediment samples were collected. In the absence of NCDEQ sediment criteria, the sediment sample results were compared to North Carolina Preliminary Soil Remediation Goals (NC PSRGs) for Protection of Groundwater (POG). Sediment sample locations and analytical results are depicted on Figure 2-3. Table 2-7 provides a summary of the sediment analytical results compared to NC PSRG POGs. 29 Corrective Action Plan Part 1 Riverbend Steam Station Ash Basin Table 2-7. Sediment COI Exceedances Compared to Upgradient Sediment Concentration, 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 Arsenic 5.8 11.2J to 61.8 2/12 Barium 580 1,960 1/12 Boron 45 45.1 J 1/12 Cobalt 0.9 4.6J to 355 12/12 1 ron 150 8,100 to 92,500 12/12 Manganese 65 92.8 to 64,400 12/12 Vanadium 6 23.2J to 126 12/12 1. mg/kg = milligrams per kilogram 2. J = Laboratory estimated concentration. 3. < indicates concentration less than laboratory method detection limit. Arsenic exceeded the NC PSRG for POG in sediment samples collected at S-2 and S-12. Barium and boron exceeded the NC PSRGs for POG in sediment sample S-6. Cobalt, iron, manganese, and vanadium concentrations exceeded the NC PSRG for POGs in all sediment samples. As described above, it is unclear if the current seeps will be eliminated, modified, or remain as surface expressions of groundwater in their current locations. As a result, COls attributable to soils impacted by seeps will not be further evaluated as part of the CAP Part 1 report based on the uncertainty surrounding the seeps as a result the ongoing ash excavation. A post -monitoring excavation plan will be recommended as part of CAP Part 2 that will provide for further identification, assessment and monitoring, if necessary, of seeps. 2.6 Soil 2.6.1 Background Soil Concentrations Because some COls 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 2.2.1) from which background soil samples were collected from include: compliance wells MW-7SR and MW-7D, CSA background monitoring wells BG- 1S, BG-1D, BG-2S, BG-21D, BG-2BR, BG-3S, BG-31D, and MW-7BR (Figure 2-3). Additional samples may be required if additional background wells are installed. 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 that 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 30 Corrective Action Plan Part 1 Riverbend Steam Station Ash Basin 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 highest reported concentration or the highest non -detect value. Table 2-8. Proposed Provisional Soil Background Soil Concentrations Constituent No. of Samples No. of Detections Range (mg/kg) 95% UTL (mg/kg) Aluminum 17 17 5,650-26,700 28,400 Antimony 17 0 <5.7-<8.3 8.3* Arsenic 17 0 <5.7-<8.3 8.3* Barium 17 17 20.4-282 361 Beryllium 17 14 0.23-1.4 1.38 Boron 17 2 12.0-<20.8 20.8* Cadmium 17 0 <0.69-<1.0 1.0* Calcium 17 11 96-2,480 2,580 Chloride 17 0 <284-<427 427* Chromium 17 17 3.1-20.6 20.6 Cobalt 17 15 5.0-22.8 22.1 Copper 17 17 4.3-93.3 116 Iron 17 17 12, 300-32,200 22,800 Lead 17 12 3.3-15.2 14.6 Magnesium 17 16 <162-10,200 12,500 Manganese 17 17 29.1-1,440 1,550 Mercury 17 2 <0.0093-0.048 0.048* Molybdenum 17 0 <2.9-<4.2 4.2* Nickel 17 17 1.3-8.5 10.2 Nitrate 17 0 <28.4-<42.7 42.7* pH (field) 17 17 4.5-6.5 4.5-6.5* Potassium 17 16 216-8,700 10,700 Selenium 16 1 4.7-<8.3 8.3* Sodium 17 0 286-417 417* Strontium 17 13 2.1-22.2 27.2 Sulfate 17 0 <284-<427 427* Thallium 17 0 <5.7-<8.3 8.3* TOC 17 1 448-<1,020 1,020* Vanadium 17 17 29.2-89.2 92.7 Zinc 17 16 <6.5-59.8 71.4 Notes: 1. mg/kg = milligrams per kilogram 2. UTL = Upper tolerance limit (USEPA 2013) 3. * = Value is highest detection or highest ND. Too few detections to develop UTL. 31 Corrective Action Plan Part 1 Riverbend Steam Station Ash Basin 2.6.2 Soil Exceedances of NC PSRGs for POGs The horizontal and vertical extent of soil contamination at the site attributed to ash management area activities was delineated in the CSA report, with the exception of the horizontal and vertical extent of source -related impacts north and west of the coal pile and cinder storage area. Additional assessment is planned to address the exceptions noted above. These exceptions were identified as data gaps in the CSA and will be addressed under separate cover. Soil exceedance results for COls, along with a 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 ash storage was delineated during the CSA activities with the following exception: • Horizontal and vertical extent downgradient of the coal pile and cinder storage area. • Horizontal and vertical extent outside the northeast boundary of the ash basin. Table 2-9. Soil Exceedance Results for COls Compared to NC PSRGs for POG, Background Concentrations, and Frequency of Exceedances COI Concentrations Exceeding NC PSRGs for POG (mg/kg) NC PSRGs for POG (mg/kg) Background Soil Concentrations ( g g) m /k Soil PPBCs (mg/kg) Number of Samples Exceeding NC PSRGs for POG/Number of Samples Within Ash Basin Waste Boundary Arsenic 3.5J to 33.8 5.8 <5.7 to <8.3 8.3 8/34 Boron 50 to 54.3 45 12 to <20.8 20.8 3/34 Cobalt 3.1 J to 70.1 0.9 5J to 22.8 22.1 27/34 Iron 5,230 to 71,900 150 12,300 to 32,200 22,800 34/34 Manganese 68.9 to 3,320 65 29.1 to 1,440 1,550 25/34 Nickel 167 130 1.3J to 8.5 10.2 1/34 Vanadium 9.9 to 161J+ 6 29.2 to 89.2 92.7 34/34 Within Ash Basin Waste Boundary Arsenic 3.5J to 12.2 5.8 <5.7 to <8.3 8/3 3/40 Cobalt 3.1 J to 102 0.9 5J to 22.8 22.1 35/40 Iron 4,950 to 109,000 150 12,300 to 32,200 22,800 40/40 Manganese 68.4 to 2,940 65 29.1 to 1,440 1,550 40/40 Nickel 151 130 1.3J to 8.5 10.2 1/40 Selenium 5.6J 2.1 4.7J to <8.3 8.3 1/39 Vanadium 7J to 209 6 29.2 to 89.2 92.7 34/35 Notes: 1. mg/kg = milligrams per kilogram 2. J = Laboratory estimated concentration. 3. < indicates concentration less than laboratory method detection limit. 4. NC PSRG for POG indicates the North Carolina Preliminary Soil Remediation Goal for Protection of Groundwater 32 Corrective Action Plan Part 1 Riverbend Steam Station Ash Basin Ash Ash samples were collected and analyzed from the ash disposal areas during the CSA. COls identified in ash characterize the source material from which COls were evaluated with respect to releases from the ash disposal areas. Ash is not evaluated as a separate medium for remediation in this CAP because it will be capped or excavated during ash basin closure activities. 1.8 Porewater Porewater refers to water samples collected from monitoring wells installed in the ash management areas and screened within the ash layer. HDR does not consider porewater results to represent groundwater, but the results are compared to the 2L Standards or IMACs for purposes of discussion. Porewater results are representative of the source characterization data with respect to contamination at the site that is attributed to ash handling and storage. Note that porewater is not further evaluated for remediation in this CAP Part 1 because porewater will be eliminated during ash basin closure activities. 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. COI Screening Evaluation Summary Table 2-10 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. Table 2-10. Updated COI Screening Evaluation Summary Potential COI CSA COI Exceedance by Media COI To Be Further Assessed in Section 3.0 Solid/ Aqueous Ash Pore Water Ground- water Surface Water Ash Basin Surface Water Seeps Sediment Soil PWR/ Bedrock Aluminum - - - - - - - - - - Antimony - - - - - - - - Arsenic - - - - - - Barium - - - - - - -� - - Beryllium - - - - - - - - - - Boron - - - - - -� - - Chromium - - - - - - - - Cobalt - - - - - -� - Copper - - - - - - - - - Iron - - - - - Lead - - - - - - - - - - 33 Corrective Action Plan Part 1 Riverbend Steam Station Ash Basin Potential COI CSA COI Exceedance by Media COI To Be Further Assessed in Section 3.0 Solid/ Aqueous Ash Pore Water Ground- water Surface Water Ash Basin Surface Water Seeps Sediment Soil PWR/ Bedrock Manganese -- pH- - - - - - - - - - Selenium - - - - - - - - - Sulfate - - - - - - - - - - Thallium - - - - - - - - - - TDS - - - - - - - - - - Vanadium - - - - -� - 2.11 Interim Response Actions 2.11.1 Source Control No interim response actions are necessary at the RBSS site because there are no identified imminent hazards to human health or the environment. In conjunction with decommissioning activities and in accordance with CAMA requirements, Duke Energy plans to permanently close the RBSS ash basin by August 2019. Closure of the RBSS ash basin was defined in the CAMA as excavation of ash from the site and beneficial reuse of the material or relocation to a lined structural fill or landfill. As part of the RBSS closure process, Duke Energy submitted a coal ash excavation plan to state regulators in November 2014. The excavation plan details a multi -phase approach for removing coal ash from the site with an emphasis on the first 12 to 18 months of activities. Duke Energy has commenced with removing the ash in the ash basin, ash storage area, and cinder storage area via excavation in May 2015. Approximately 4.6 million tons of ash from the ash basin, ash storage area, and cinder storage area will be transported to permitted lined landfills and/or structural fills during the excavation project. This initial phase has begun in the northeast corner of the ash storage area and has involved hauling ash by truck to a permitted lined landfill in Homer, Georgia and to a permitted lined landfill at the Duke Energy Marshall Steam Station in Mooresville, North Carolina. The majority of ash at Riverbend is anticipated to be transported by rail to a lined clay mine reclamation project in central North Carolina, pending permitting and approvals. Final removal of ash at Riverbend and is anticipated to be completed no later than August 2019. The soil dams will be removed and the unimpacted material will be used in site re -grading. The depression left after ash removal is planned to be filled with on -site and imported fill material, re -graded, and appropriate vegetation planted to establish a long-term stable, erosion resistant site condition Ash impoundments are anticipated to be closed by August 1, 2019. The coal-fired units at the RBSS plant have been decommissioned. The ash management areas are no longer in use and Duke Energy is in the process of removing the ash from the site. No imminent hazard to human health or the environment has been identified. 34 Corrective Action Plan Part 1 Riverbend Steam Station Ash Basin Groundwater Response Actions Based on the results of CSA investigation activities, impacted groundwater has not migrated to off -site public or private water supply wells. The CSA found the ash basin source areas discharge porewater to the subsurface beneath the basins and via seeps through the embankments. Groundwater flows in a generally northern, western, and easterly direction from the ash management areas to Mountain Island Lake. No information assessed or reviewed as part of the CSA has shown surface water exceedances of the 2B Standard for source area COls. Further assessment of groundwater and soil will be required of the cinder storage area. 36 Corrective Action Plan Part 1 Riverbend 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 RBSS site. The SCM was developed utilizing data and analysis from the CSA Report (HDR 2015). The sources and areas with 2L Standards or IMAC exceedances of constituents attributable to ash handling are illustrated in the 3-D SCM presented on Figure 3-1 and in the CSA cross -sectional view presented on Figure 3-2. Site Hydrogeologic Conditions Site hydrogeologic conditions were evaluated through the installation and sampling of 78 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 RBSS site, as well as on-site/near-site geologic mapping, to further understand the site geology in support of the SCM. Hydrostratigraph ic Units The following materials were encountered during the CSA investigation and are consistent with material descriptions from previous site exploration: • Ash (A) — Ash was encountered in borings advanced within the ash basin, ash storage area, and cinder storage area, as well as in some borings advanced through the pond perimeter and dikes. Ash was generally described as gray to dark bluish gray, highly plastic to non -plastic, loose to medium dense and very soft (wet) to very stiff (dry), dry to wet, fine to medium grained. The range of ash thickness observed at the RBSS site was 0 to 76 feet. • Fill (F) — Fill material generally consisted of re -worked silts, clays, and sands that were borrowed from one area of the site and re -distributed to other areas. Fill was generally classified as silty sand, clay with sand, clay, and sandy clay. Fill was used in the construction of dikes, as cover for the ash and cinder storage areas, and as bottom liner for the ash storage area. The range of fill thickness observed at the RBSS site was 0 to 78.5 feet. • Alluvium (S) —Alluvium encountered in borings during the project subsurface exploration activities was classified as gravel with clay and sand, sand with gravel, and silt. In some cases, alluvium was logged beneath ash. The range of alluvium thickness observed at the RBSS site was 0 to 28.5 feet. Residuum (M1) — Residuum is the in -place weathered soil that consists primarily of silt with sand, clayey sand, sandy clay, clay with gravel, and clayey silts. Residuum varied in thickness and was relatively thin compared to the thickness of saprolite. The range of residuum thickness observed at the RBSS site was 0 to 79 feet. Saprolite/Weathered Rock (M1/M2) — Saprolite is soil developed by in -place weathering of rock that retains remnant bedrock structure. Saprolite consists primarily of 36 Corrective Action Plan Part 1 Riverbend Steam Station Ash Basin medium dense to very dense silty sand, sandy silt, sand, sand with gravel, sand with clay, clay with sand, and clay. Sand particle size ranges from fine to coarse grained. Much of the saprolite is micaceous. The range of saprolite/weathered rock thickness observed at the RBSS site was 0 to 246 feet. • Partially Weathered/Fractured Rock (TZ wells labeled BRU) — Partially weathered (slight to moderate) and/or highly fractured rock encountered below refusal (auger, casing advancer, etc.). The range of TZ thickness observed at the RBSS site was 0 to 51.7 feet. • Bedrock (BR) — Sound rock in boreholes, generally fresh to slightly weathered and relatively unfractured. The maximum depth that borings extended into bedrock was 44 feet. Based on the site investigation conducted as part of the CSA, the groundwater system in the natural materials (alluvium, soil, soil/saprolite, and bedrock) is consistent with the regolith- fractured rock system and is characterized as an unconfined, connected aquifer system. Evaluation of geologic and hydrogeologic data from the CSA has shown that the TZ was absent at the RBSS site. Therefore, from a geologic and hydrogeologic perspective, this site only has two flow layers. For the purpose of this CAP Part 1 report, and for consistency with the CSA Report, shallow (S), deep (D), and bedrock (BR and BRU) wells are discussed. The SCM can be simplified to a two -layer system. The absence of a TZ layer may require revisions to the 3-D groundwater models (and potentially the surface water model) and will be addressed, if necessary, in CAP Part 2. Hydrostratigraph ic Unit Properties Material properties required for the groundwater flow and transport model are total porosity, effective porosity, specific yield, and specific storage. These properties were developed from laboratory testing of ash, fill, alluvium, and soil/saprolite and are presented in the CSA Report. Specific yield/effective porosity was determined for a number of samples of the A, F, S, M1, and M2 hydrostratigraphic layers to provide an average and range of expected 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, and soil/saprolite). Results from these tests were utilized to develop the groundwater flow and fate and transport model further discussed in Section 4. Refinements to the SCM will also be made in CAP Part 2. Potentiometric Surface — Shallow and Deep Flow Layers The shallow and deep flow layers are defined by data obtained from the shallow and deep groundwater monitoring wells (S and D wells, respectively). In general, shallow and deep groundwater at the site flows to the north, east, and west and discharges to Mountain Island Lake. Groundwater in the southwest portion of the site under the cinder storage area flows to the northwest, to Mountain Island Lake. Flow contours developed from groundwater elevations measured in the shallow and deep wells in the southeastern portion of the site depict 37 Corrective Action Plan Part 1 Riverbend Steam Station Ash Basin groundwater flow generally to the northeast to Mountain Island Lake. Figures 3-3 and 3-4 show the potentiometric surfaces of the shallow and deep flow layers. Potentiometric Surface - ;edrock Flow Layer The bedrock flow layer is defined by data obtained from the bedrock groundwater monitoring wells (BR or BRU wells). Groundwater contours developed from the groundwater elevations in the bedrock wells show groundwater moving generally in a north/northeasterly direction from the south side of the site to Mountain Island Lake. Groundwater flow within the bedrock layer does not express the west to northwest flow direction observed in shallow and deep flow layers. Instead, flow in the bedrock layer is generally northward. The potentiometric surface for the bedrock flow layer is illustrated on Figure 3-5. 3.1.5 Horizontal and Vertical Hydraulic GradientE 3.1.5.1 Horizontal Hydraulic Gradients Horizontal hydraulic gradients were derived for the shallow, deep, and bedrock flow layers by calculating the difference in hydraulic head 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). Applying this equation to wells installed during the CSA yields the following average horizontal hydraulic gradients (measured in feet / foot): • Shallow flow layer: 0.032 • Deep flow layer: 0.028 • Bedrock flow layer: 0.032 Vertical Hydraulic Gradients Vertical hydraulic gradients were calculated (Table 3-1 and 3-2) for 27 shallow (S) and deep (D) wells pairs by taking the difference in groundwater elevation in each well pair over the difference in mid well screen of each well pair. Vertical hydraulic gradients were calculated for three deep (D) and bedrock (BR and BRU) well pairs. The vertical gradients are presented below. 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 deep and bedrock well pairs are presented on Figure 3-7. Table 3-1. Vertical Gradient Calculations for Shallow/Deep Well Pairs Shallow Well Deep Well Vertical Gradient (ft/ft) AB-1 D AB-1 S -0.123 AB-2D AB-2S 0.074 AB-3D AB-3S -0.039 AB-4D AB-4S -0.017 AB-5D AB-5S -0.217 AB-7D AB-7S -0.007 AB-8D AB-8S 0.028 38 Corrective Action Plan Part 1 Riverbend Steam Station Ash Basin Shallow Well Deep Well Vertical Gradient (ft/ft) AS-1 D AS-1 S -0.033 AS-2D AS-2S 0.013 BG-1 D BG-1 S 0.022 BG-3D BG-3S -0.002 C-1 BRU C-1 S -0.960 GWA-10BRU GWA-10S 0.001 GWA-1 BRU GWA-1 S 0.024 GWA-20D GWA-20S -0.008 GWA-22D GWA-22S -0.024 GWA-3D GWA-3S -0.033 GWA-4D GWA-4S -0.003 GWA-6D GWA-6S -0.008 GWA-7D GWA-7S 0.009 GWA-8D GWA-8S -0.002 MW-11 DR MW-11 SR -0.006 MW-15D MW-15S 0.003 MW-3D MW-3S 0.321 MW-6D MW-6S 0.013 MW-8D MW-8S -0.037 MW-9D MW-9 0.002 Notes: 1. Vertical Gradients = OWE/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 7/9/15 Table 3-2. Vertical Gradient Calculations for Deep/Bedrock Well Pairs Deep Well Bedrock Well Vertical Gradient (ft/ft) AB-3BR AB-3D -3.541 GWA-7BR GWA-7D 0.001 Notes: 1. Vertical Gradients = OWE/ABS(AMSE), where A implies bedrock to deep, WE is water elevation, and MSE is mid -screen elevation 2. N/A implies a lack of sufficient information to satisfy the above formula; a note is included to explain why 3. Positive gradient implies potential upward flow 4. Depth to Water measurements taken on 7/9/15 Comparison of vertical gradients between shallow and deep flow layers: Downward gradient is exhibited across the site within the shallow and deep flow layers. Upward gradient was identified along the northern perimeter of the waste boundary. This gradient is produced because the elevation of the waste is 52 feet higher than the ground surface at the base of the dike. This creates a pressure at the wells downgradient of the dikes. Following excavation, this pressure will be reduced and likely 39 Corrective Action Plan Part 1 Riverbend Steam Station Ash Basin eliminate the upward gradient. It is also likely that the direction of groundwater flow will change. Comparison of vertical gradients deep and bedrock flow layers: • Potential downward flow was identified within the ash basin from the A13-3 well pair. • A slight upward gradient was identified with A13-313R. 3.2 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. 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, and titanium) make up an additional 8%, while trace constituents account for less than 1 %. The following constituents are considered to be trace elements: arsenic, barium, cadmium, chromium, lead, mercury, selenium, copper, manganese, nickel, lead, vanadium, and zinc (EPRI 2010). COI sources at the RBSS site consist of the following areas collectively identified as the ash management area: Ash Basin — The Primary Cell is impounded by an earthen embankment dam, referred to as Dam #1 (Primary), located on the west side of the Primary Cell. The Secondary Cell is impounded by an earthen embankment dam, referred to as Dam #2 (Secondary), located along the northeast side of the Secondary Cell. Ash Storage Area — An unlined ash storage area is located topographically cross- gradient/upgradient and adjacent to the southwest side of the Primary Cell. The footprint is approximately 29 acres and is estimated to contain approximately 1.5 million tons of ash. The ash storage area was constructed during two ash basin clean -out projects: one which occurred around 2000-2001 and another which occurred from late 2006 to early 2008. • Cinder Storage Area — The unlined cinder storage area is located topographically cross -gradient and immediately west/southwest of the Primary Cell, and northwest of the ash storage area. The footprint is approximately 13 acres and is located in a triangular area northeast of the coal pile and northwest of the rail spur. 40 Corrective Action Plan Part 1 Riverbend Steam Station Ash Basin These source areas are subject to different processes that result in constituents leaching into the underlying soil layers and into the groundwater. For example, constituents in the ash storage area and cinder storage area would leach as a result of infiltration of precipitation, while constituents in the ash basin would leach based on the pond elevation in the basin. Periodic inflows to the ash basin would likely affect the amount of leachate 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 and USDOE 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 above 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. 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. The two processes mainly involved in transport the dissolved concentrations in groundwater flow are advection and dispersion. Advection is the movement of dissolved and colloidal COls by groundwater flow and is the primary mechanism for movement of a dissolved concentration. The rate of advection can be described by Darcy flow. The second process that affecting the location and concentration of inorganic COls in groundwater flow is mechanical dispersion. This mixing process happens as groundwater undergoes tortuous paths of various lengths to arrive at the same location; some water moves faster than other water, which causes longitudinal and lateral spreading of plumes. Dispersion is scale dependent and increases with plume length and groundwater flow velocity. The third process involved in the transport of a dissolved concentration is molecular diffusion. Molecular diffusion 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 cause the movement of concentrations of a COI to over a site and can also cause decreases in concentrations over distances and time, without consideration of other geochemical processes. 41 Corrective Action Plan Part 1 Riverbend Steam Station Ash Basin Retardation of constituent concentrations relative to an initial concentration can occur due to adsorption, absorption, or ion exchange. Which of these three processes occur and the degree to which they occur depends on factors such as the properties of the solute, the properties of the soil/rock media, and geochemical conditions. Inorganic constituents have a varying propensity to interact with the mineral and organic matter contained in aquifer media. Depending on the constituent and the mechanism of interaction, the retention of a constituent to the soil or aquifer material, and removal of the constituent from groundwater, may be a non -reversible or a reversible condition. In some cases, the degree of retardation or attenuation of a constituent to the aquifer media may be so great that the constituent will not be mobile and will not transport. In these cases, attenuation may result in reduction of constituent concentrations to acceptable levels before reaching the point of compliance or receptors. In other cases, the degree of retardation or attenuation of a constituent may be weaker resulting in greater mobility through the aquifer media. 3.2.1.3 COI Distribution in Groundwater The spatial distribution of COls detected in groundwater samples collected at the RBSS site is described below. For the purposes of this discussion, the shallow flow layer includes the analytical results reported in the shallow (S) and intermediate (1) wells, the deep flow layer includes the analytical results reported in the deep (D) wells, and the bedrock flow layer includes the analytical results reported in the bedrock (BR and BRU) wells. Shallow and Deep Distribution in Groundwater Antimony — Antimony concentrations in the shallow flow layer exceeded the IMAC in wells within the ash basin, south of the ash storage area, and in BG-3S. Antimony concentrations in the deep flow layer exceeded the IMAC in wells associated with the ash storage area, the ash basins, MW-9D located north of the cinder storage area, and in background well BG-1 D. Antimony concentrations in the bedrock flow layer exceeded the IMAC in wells within the ash basin, west of the ash basin Primary Cell, and north of the cinder storage area. • Boron —Boron was identified atone location within the Primary Cell (AS-1S) above the 2L Standard. This location is scheduled for ash removal. • Chromium — Chromium concentrations in the shallow flow layer exceeded the 2L Standard in wells adjacent to the ash management areas and northwest of the ash basin. Chromium concentrations exceeded the 2L Standard in the deep flow layer located within the ash basin, north of the cinder storage area, and in the background wells. Chromium exceedances of the 2L Standard in the bedrock flow layer were reported in the monitoring wells south of the ash storage area, northwest of the ash basin Primary Cell, and in background well MW-7BR. • Cobalt — Concentrations of cobalt above the IMAC varied in the shallow and deep flow layers and were widely distributed across the site above the 2L Standard in both background and downgradient monitoring well locations. This highest concentration of cobalt was identified downgradient of the coal pile in C-1S. 42 Corrective Action Plan Part 1 Riverbend Steam Station Ash Basin • Hexavalent Chromium — Hexavalent Chromium was identified primarily in the shallow and deep groundwater layers within and the waste boundary. • Iron — Concentrations of iron above the 2L Standard varied in both the shallow, deep, and bedrock flow layers across the site. Iron concentrations are attributable to naturally occurring conditions at the RBSS site, with the exception of elevated concentrations above both 2L and PPBCs below the ash basin and ash storage areas. • Manganese — Concentrations of manganese exceeded the 2L Standard in shallow and deep flow layers across the site. Manganese concentrations are attributable to naturally occurring conditions at the RBSS site, with the exception of elevated concentrations beneath and downgradient of the ash management areas. • pH — pH measurements outside of the 2L Standard of 6.5-8.5 were encountered in shallow, deep, and bedrock flow layers. Many of the shallow monitoring wells were acidic while deep and bedrock monitoring wells were generally basic. • Sulfate — Sulfate concentrations in the shallow flow layer exceeded the 2L Standard in monitoring well GWA-3SA located northwest of the cinder storage area. Sulfate concentrations in the deep flow layer exceeded the 2L Standard in monitoring wells GWA-31D located northwest of the cinder storage area and GWA-20D located south of the ash storage area. Sulfate exceedances were limited to three locations; the highest concentrations of sulfate are located downgradient of the coal pile and cinder storage area. Further assessment is recommended. • Thallium —Thal lium exceeded the IMAC atone location (GWA-20D) located south of the ash storage area in the deep flow layer. This exceedance may be the result of ash management. This location is on the southern border of the waste boundary. • TDS — Concentrations of TDS exceeded the 2L Standard in the shallow flow layer at AS- 1 S and GWA-3SA; the deep flow layer at AB-31D, BG-1 D, GWA-31D, and GWA-20D; and the bedrock flow layer at MW-7BR, GWA-2313R, and GWA-213RU. It is likely that TDS impacts are attributable to ash handling at the RBSS site, however further analysis may be required in a post excavation monitoring plan. • Vanadium — Concentrations of vanadium exceeded the IMAC in shallow, deep and bedrock layers across the site. Vanadium concentrations are in part naturally occurring in the area and found in the background wells. 3.2.2 Geochemical Characteristics 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. In general, the ionic composition of groundwater and surface water at the site is predominantly calcium, magnesium, and bicarbonate rich with the exception of ash basin water, ash basin 43 Corrective Action Plan Part 1 Riverbend Steam Station Ash Basin porewater, and downgradient groundwater monitoring wells which were observed to be trending closer to calcium, magnesium and sulfate rich geochemical makeup. Seep data indicates similar geochemistry to ash basin water, ash basin porewater, and shallow wells in the ash basin. Redox Potential When elements dissolve in water, they are often ionized or charged. Sometimes this is because the compounds are ionic solids like salt (NaCI), and when dissolved will form negatively charged (anions - CI-) and positively charged (cations - Na+) ions. Other times elements change charge because they gain or lose electrons from their outer shells. These are reduction -oxidation or "redox" reactions. An example of that is oxidized ferric iron (+3 charge) transforming to ferrous iron (+2 charge) by accepting an electron. Redox reactions are balanced —every electron donor has to have an electron receptor. Redox reactions such as the iron example above can greatly influence the presence of contaminants in water. Mobility and transport of iron and manganese is controlled by their oxidation state. As seen from the speciation sampling discussed below and the redox measurement activities, reduced zones are expected to have higher mobility for most species. In highly reducing conditions, the formation of sulfides such as pyrite can also control the mobility of iron and manganese. The iron -manganese redox system deserves attention in this hydrogeologic setting because of its prevalence and the indications of variable redox conditions. As iron and manganese precipitate due to redox reactions (forming rust), these solids adsorb and attenuate many constituents. Reductive dissolution of iron oxyhydroxides release adsorbed material. 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 contaminants. It is problematic that field measurement of redox conditions using probes or single redox couples are very difficult to conduct accurately. At the RBSS site, the approach taken was to measure multiple redox couples and dissolved gasses in addition to probe measurements. Just as pH reactions are limited by compounds that buffer changes in pH, the presence of high concentrations of redox couples causes the solution to be poised near a certain redox potential. At RBSS, the predominant redox category is anoxic/mixed and the predominant redox processes are ferrous iron/ferrous sulfate, so the reduced species As(III), Se(IV), and Mn(IV) would be expected. The redox conditions appear to be controlled at least partly by the SO4/S2 and Fe(I I I)/Fe(I I) redox couples, and these redox couples should be monitored to assess changing redox conditions. Solute Speciation Background As described by McMahon and Chapelle (2008), reduction/oxidation (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 44 Corrective Action Plan Part 1 Riverbend Steam Station Ash Basin byproducts such as dissolved manganese (Mn 2+), ferrous iron (Fe 2+), hydrogen sulfide (112S), 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 U.S. Geological Survey (USGS) developed a framework to assess redox processes based on commonly measured water quality parameters (McMahon and Chapelle, 2008; Jurgens, et al., 2009). The redox framework allows the state of a groundwater sample and dominant type of redox reaction or process occurring to be inferred from water quality data. An implementation of this framework is provided in the USGS "Excel® Workbook for Identifying Redox Processes in Ground Water' (Jurgens, et al., 2009), which is detailed in USGS Open File Report 2009-1004. The primary aquifer system in western North Carolina is considered to be of the New England, Piedmont and Blue Ridge type and is representative of crystalline -rock aquifers (McMahon and Chapelle, 2008). Precise identification of redox conditions in groundwater can be difficult to determine because groundwater is commonly not in redox equilibrium and multiple redox conditions may exist simultaneously as groundwater progresses from more oxygenated (i.e., oxic) states to more reduced states (i.e., anoxic). For example, decreases in nitrate concentrations during denitrification can occur concomitant with increases in manganese concentrations as a result of solid -phase manganese (IV) reduction. 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 and others, 1995; Christensen and others, 2000; Paschke and others, 2007; McMahon and Chapelle, 2008). Redox conditions are generally facilitated by microorganisms, which gain energy by transferring electrons from donors (usually organic carbon) to acceptors (usually inorganic species) (McMahon and Chapelle, 2008). Because some electron acceptors provide more energy than others, electron acceptors that yield the most energy are utilized first and species that yield less energy are utilized in order of decreasing energy gain. This process continues until all available donors or acceptors have been used. If carbon sources are not a limiting factor, the predominant electron acceptor in water will usually follow an ecological succession from dissolved oxygen (02), to nitrate (NO3 ), to manganese (IV), to iron (III), to sulfate (S042-), and finally to carbon dioxide (CO2(g)) (Table 3-3). Although some redox processes overlap as groundwater becomes progressively more reduced, there is usually one TEAP that dominates the chemical signature. Consequently, concentrations of soluble electron acceptors (02, NO3 , S042-) and TEAP end products (Mn 2+, Fe2+, 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 broadly applicable to site -specific 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 (:50.45pm) and acidified, total dissolved concentrations are generally accurate estimates of Mn2+ and Fe2+ above the threshold concentrations (50 and 100 pg/L, respectively) for pH ranges normally found in ground water 46 Corrective Action Plan Part 1 Riverbend Steam Station Ash Basin (6.5-8.5) (Kennedy and others, 1974; Hem, 1989). At lower pH values there is an even greater likelihood that dissolved concentrations (total filtered) are equal to the primary species of interest: Mn2+ and Fee+ The USGS redox framework was applied to groundwater measurements from different environments across the RBSS site. Speciation measurements were performed for arsenic, selenium, chromium, iron and manganese at select locations. Samples were collected using 0.45 micron (pm) filters and analyzed for total and dissolved metals. Other field measurements include dissolved oxygen (DO), Oxidation -Reduction Potential (ORP), temperature, pH, specific conductance, and turbidity. Based on site measurements, the primary redox categories were determined to include oxic, mixed (anoxic), and anoxic conditions. Table 3-3. Range of Results for Groundwater Parameters Well Locations No. of pH (std. Spec. Diss. ORP/Redox Turbidity Results unit) Cond Oxygen (mV) (NTU) (PS) (mg/L) Beneath Ash 5.10 - 26.8 - 18 0.20 - 8.21 -265.9 - 257.6 1.86- 416.5 Basin 11.91 3092 Ash Basin - Beyond Waste Boundary and 4.31 - 0.95 - 30 61 - 5030 0.03 - 136 -169 - 311 Within 12.11 137.3 Compliance Boundary Ash Basin 8.00 - 40.3 - 15.95 - 2 5.4 - 6.0 143.4 - 209.5 Surface Water 9.10 128.3 25.47 Seeps Beyond Waste Boundary and 6.93 - 102.3 - 4 3.55 - 5.27 81.5 - 200.7 0.8 - 9.05 Within 8.05 394.3 Compliance Boundary Seeps at or Beyond Ash 6.81 - 72.7- 5.63- Basin 3 5.17 - 5.19 227.4 - 240.6 7.62 96.5 14.63 Compliance Boundary Beneath Ash 5.3 - 100 - 6 0.50 - 6.60 -88.8 - 172 6.2 - 48.37 Storage Area 11.57 1592 Ash Storage Area - Beyond Waste 3.56 - 22 - -89.4 - 239.9 Boundary 15 3.1 - 8.6 1.04 - 84.3 12.50 12,552 Within Compliance Boundary Beneath Cinder 4.16 - 3 220 - 801 0.35 - 4.35 -25.2 - 479.9 3.14 - 7.62 Storage Area 9.36 Cinder Storage 5 5.94 - 128.3 - 0.25 - 4.6 -43.4 - 207.4 1.55 - 8.43 46 Corrective Action Plan Part 1 Riverbend Steam Station Ash Basin Well Locations No. of Results pH (std. unit) Spec. Cond S Diss. Oxygen m /L ORP/Redox (mV) Turbidity (NTU) Area - Beyond 11.49 2263 Storage Waste Boundary Within Compliance Boundary Background 5.75 — 33.3 — 1.93 — 4 2... 1 — 64 1613 — 288 Wells 8.20 180.2 123.6 Notes: 1. Thresholds and concentrations from McMahon and Chapelle (2008) and Jurgen et al. (2009). Ranges for a number of field measurements characterizing aspects of groundwater conditions outside and beneath the ash basins are presented in Table 3-3. Those measurements indicate that pH ranges from 3.56 to 12.50 standard units. In contrast, background well results indicate that pH ranges from 5.75 to 8.20, whereas pH within the ash basin materials range from 5.10 to 11.91. Importantly, there is a very wide range of ORP values. In most cases shown, ORP values span ranges that imply highly reduced (large negative values) to highly oxidized (large positive values) environments. 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 RBSS site were never less than -270 millivolts. Discussion 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 redox processes or indicators of the implied redox state of groundwater as expressed by pH, ORP, and 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 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. Although redox processes affect the mobility of constituents associated with aquifer materials, precise identification of redox conditions in groundwater can be difficult to determine. 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. Multiple redox conditions may exist simultaneously as water progresses from more oxygenated states to more reduced states (McMahon and Chapelle, 2008). 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 47 Corrective Action Plan Part 1 Riverbend Steam Station Ash Basin 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. During the RBSS CSA, select wells were sampled for chemical speciation. Laboratory results were used to calculate speciation for those species not determined in the lab. Totals and speciation were measured in the lab or calculated for: • Total arsenic, arsenic (III), and arsenic (V) • Total chromium, chromium (III) and chromium (VI) • Total iron, iron (11), and iron (III) • Total manganese, manganese (11), and manganese (IV) • Total selenium, selenium (IV), and selenium (VI) Results for chemical speciation of groundwater at the RBSS site are presented in the CSA Report. Chemical speciation was determined for 37 samples and redox calculations were performed for each of those samples. The goal was to create a representative data set for evaluation of trends in speciation, and the probable redox controls on site observations. Of the 37 speciation samples, 14 were oxic and 19 were anoxic or mixed samples. Evaluation of the redox chemistry reported in the CSA indicates that redox conditions in RBSS site groundwater range from anoxic to oxic, with anoxic or mixed samples slightly outnumbering the oxic samples. Dissolved oxygen is the primary driver for oxic conditions in site groundwater. Anoxic (or reducing) conditions are evidenced by Fe(III) and sulfate concentrations in site groundwater. Reduced arsenic (III) was detected much less frequently than the oxidized arsenic (V). Evaluation of all samples together shows arsenic (111) concentrations are about half of the arsenic (V) concentrations. And, arsenic (111) concentrations decrease in reducing conditions; which is counterintuitive. This may be due in part to the limited number of arsenic (111) samples available for comparison (2 oxic and 4 anoxic). Total chromium was detected in all samples but AB-613RU. Chromium (VI) was detected in 32 of 37 speciation samples, including samples from anoxic groundwater conditions. The average concentration of chromium (VI) is higher in anoxic versus oxic groundwater conditions. There is also a corresponding increase in total chromium concentrations in anoxic versus oxic groundwater conditions, which may in part explain why chromium (VI) concentrations increase under these same conditions. Chromium (VI) concentrations are partly proportional to total chromium concentrations. Other factors, such as redox influence on solid media phases that adsorb chromium (VI) also likely influence chromium (VI) concentrations at the RBSS site. Manganese speciation results did not exhibit the high manganese (11) concentrations that would be expected in an anoxic (reducing) environment. Iron speciation results show similar trends to manganese. The occurrence of reduced selenium(selenium (IV)) concentrations dominated in both oxic and anoxic environments. Total selenium concentrations (selenium (IV) + selenium (VI)) are higher in oxic groundwater conditions compared to anoxic conditions. 48 Corrective Action Plan Part 1 Riverbend Steam Station Ash Basin In summary, the observed groundwater conditions at the RBSS site span oxidizing to moderately reducing conditions. A review of equilibrium chemistry shows some oxidized species (e.g., chromium (VI)) present in reduced conditions and some reduced species (e.g., selenium (IV)) in oxic conditions. The observed groundwater conditions, showing a mixture of redox conditions and variability in species, taken broadly, indicate a dynamic redox environment at the RBSS site that is not in equilibrium for some COls. Given this diverse range of conditions, logical next steps in the RBSS site evaluation process may include: equilibrium geochemical speciation evaluation using modeling tools such as PHREEQC (USGS, 2013) and groundwater transport and chemical transport modeling. Additional sampling will be needed to characterize the temporal and spatial characteristics of groundwater composition for the site. Additional evaluations may 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 the three types of sorption processes: • Adsorption — solutes are held at the water/solid as a hydrated species, • Absorption — solutes are incorporated into the mineral structure at the surface, • Ion Exchange — when an ion becomes sorbed to a surface by changing places with a similarly charged ion. These processes result in a decrease of the concentration and therefore the mass of the constituent as it is removed from the groundwater onto the solid material. The effect of these processes for a particular constituent can be expressed by the distribution coefficient (or partition coefficient) Kd. Kd relates the quantity of the adsorbed constituent per unit mass of solid to the quantity of the constituent remaining in solution. Laboratory determination of Kd was performed by UNCC on 14 site -specific samples of soil. Solid samples were tested in flow through columns to measure the adsorption of COls at varying concentrations. For the RBSS site, 12 column tests and 14 batch tests were conducted. The methods used by UNCC and Kd results obtained from the testing are presented in Appendix E. The Kd data were used as an input parameter to evaluate contaminant fate and transport through the subsurface at the RBSS site, as described in greater detail in Section 4.1. Source Area Geochemical Conditions Geochemical conditions will vary in the source area based on the COI, groundwater, and soil sorption Kd properties. The variability in Kd varies from thousands of ml/g to single digits. Based on the Kd calculated for the source area, COls arsenic and vanadium will be the most sorbed of the COls analyzed while boron is the least sorbed. At the RBSS facility, excavation of the source areas will change the geochemical conditions of the source areas. Geochemical conditions will change based on the type of fill material to be placed into the source areas following ash removal. 49 Corrective Action Plan Part 1 Riverbend Steam Station Ash Basin Mineralogical Characteristics Soil and rock mineralogy and chemical analyses completed to date are sufficient to support evaluation of geochemical conditions. Soil mineralogy and chemistry results through July 31, 2015, were presented in the CSA Report. The dominant mineral constitutes in the soils are quartz, feldspar (both alkali and plagioclase feldspars), kaolinite, and illite. Soils exhibiting a higher degree of weathering show an increase in kaolinite and illite. Other minerals identified include chlorite, biotite, muscovite, and amphibole. One sample (GWA-2S: RB-05) had a relatively high amount of smectites (16.0%). The major oxides in the soils are Si02 (45.76%-77.80%), A1203 (2.77%-28.07%), and Fe203 (3.53%-14.60%). MnO ranges from 0.03% to 0.12%. The major oxides in the rock samples are Si02 (49.55%-72.14%), A1203 (15.24%-21.62%), and Fe203 (2.63%-10.31 %). Bedrock at the RBSS site consists of meta -quartz diorite and meta-diabase. Based on rock core descriptions, the meta -quartz diorite color typically is a white to light gray matrix with dark greenish gray, dark gray, and black phenocrysts. The texture is described as phaneritic, fine to coarse grained, non -foliated and massive. Foliation is rarely noted. The meta -quartz diorite is composed dominantly of plagioclase, quartz, biotite, hornblende, and epidote. 3.3 Correlation of Hydrogeologic and Geochemical Conditions to COI Distribution Ash and porewater 2L exceedances identified in Section 2 include antimony, arsenic, boron, cobalt iron, manganese, selenium, sulfate, thallium, vanadium and TDS. The sources and areas where these COls exceed 2L Standard as well as other RBSS site features are illustrated the 3- D SCM presented on Figure 3-1 and in cross -sectional view on Figure 3-2. Antimony concentrations in the shallow flow layer exceeded the IMAC in wells within the ash basin, south of the ash storage area, and in BG-3S. Antimony concentrations in the deep flow layer exceeded the IMAC in wells associated with the ash storage area, the ash basins, MW-9D located north of the cinder storage area, and in background well BG-1 D. Antimony concentrations in the bedrock flow layer exceeded the IMAC in wells within the ash basin, west of the ash basin Primary Cell, and north of the cinder storage area. Boron, sulfate, selenium and thallium were identified above the 2L Standard in three or fewer sample locations. Each of these COls has a low Kd value and can be mobile in groundwater. The absence of these constituents in groundwater suggests that geochemical conditions are such that they can attenuate the COI, or that the original composition of source material did not contain large concentrations of these COls. Arsenic has a relatively high Kd value on the site, which suggests that geochemical conditions favor low mobility of this COI. Combined with a lack of detections above the 2L in groundwater, arsenic appears to have a limited distribution. Iron and manganese are widely distributed above 2L Standard in monitoring wells on site being found above 2L regulatory standards in both background and source area samples. The 60 Corrective Action Plan Part 1 Riverbend Steam Station Ash Basin concentration of these COls are variable and very dependent on pH. It is likely that slightly acidic soils located at the RBSS contribute to the mobility and concentration of these COls. Vanadium is above 2L Standard in 71 of 79 samples. Groundwater and geochemical conditions promote the mobility of this COI across the site with contribution from naturally vanadium and vanadium in source areas. TDS exceedances are primarily downgradient of the coal pile. The absence of TDS in other areas suggests that geochemical conditions are not present to elevate this COI. TDS downgradient of the coal pile will require further assessment. 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 becomes available during supplemental site investigation activities. 61 Corrective Action Plan Part 1 Riverbend Steam Station Ash Basin Modeling Groundwater flow and fate and transport, and groundwater to surface water models were developed to evaluate COI migration and potential impacts to groundwater and surface water resulting from closure of the ash basin (Primary and Secondary cells), cinder storage area, and ash storage area at the RBSS site. Groundwater Modeling UNCC and HDR developed a site -specific, 3-D, steady-state groundwater flow and fate and transport model for the RBSS site using MODFLOW and MT3DMS. The groundwater model is based on the SCM presented in Section 3 and incorporates site -specific data obtained during the CSA and subsequent data collection. Modeling objectives included simulation of steady- state groundwater flow conditions at the RBSS ash basin area and simulation of transport conditions in which COls enter groundwater via the ash management areas over the period it was in service. 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 adjacent downgradient unnamed tributaries and Mountain Island Lake The area, or domain, of the simulation included the RBSS ash management areas that have been impacted by COls above 2L Standards or IMACs. For model output, three monitoring wells near the compliance boundary (GWA-2 on the west side, MW-15 on the north side and MW-6 on the east side) were selected to demonstrate COI behavior. The model was developed in general accordance with NCDENR DWQ's Groundwater Modeling Policy dated May 31, 2007. Details of the groundwater modeling conducted by UNCC and HDR are presented in Appendix C. 4.1.1 Model Scenarios The following ash basin closure scenarios were modeled for the RBSS site: • Existing Conditions (EC): assumes current site conditions with ash sources left in place • Cap -in -Place (CIP): assumes ash left in ash basin, cinder storage area, and ash storage area covered by engineered caps • Excavation (EX): assumes accessible ash removed Model scenarios used COls identified in Sections 2 and 3 for further analysis. Each COI was modeled individually using the transient transport model for a period of 250 years. This time period was selected as the model duration boundary condition to assess changing COI concentrations over time at the compliance boundary. The rate of natural attenuation is then described over the model period. 62 Corrective Action Plan Part 1 Riverbend Steam Station Ash Basin Calibration of Models The groundwater flow model was calibrated to steady-state flow conditions using water level observations taken at the site during June/July 2015 in shallow, deep, and bedrock wells. The transient transport of COls was calibrated to COI concentrations from June/July 2015 for each constituent. A detailed account of the flow and transient transport model calibrations are included in Appendix C. Ranges of measured hydrogeologic properties from the CSA were used as a guide for selecting model input parameters during the calibration process. 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 within each flow layer within the basin (e.g., ash, unconsolidated soils, and fractured bedrock zone); • The infiltration rate applied to the water table of the ash basin system; • The net infiltration due to precipitation applied to other areas of the site; • The variation of measured porewater COI concentrations; • The effective porosity of each model layer; and • The Kd value of each COI. Calibration results indicate the model adequately represents steady-state groundwater flow conditions at the site and meets transport calibration objectives. An independent review of the calibrated RBSS model was conducted by the Electric Power Research Institute (EPRI) and found that the model was sufficient to meet the objective of predicting effects of corrective action alternatives on groundwater quality. The EPRI review of the calibrated RBSS model is provided in Appendix C. Kd Terms COls enter the ash basin system in both dissolved and solid phases. In the ash management area, constituents may undergo phase changes including dissolution, precipitation, adsorption, and desorption. Dissolved phase constituents may undergo these phase changes as they are transported in groundwater flowing through the basin. Phase changes (dissolution, precipitation, adsorption, and desorption) are collectively taken into account by specifying a linear soil - groundwater partitioning coefficient (sorption coefficient [Kd]). In the fate and transport model, the entry of constituents into the ash basin and ash storage area is represented by a constant concentration in the saturated zone (porewater) of the basin, and is continually replaced 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 14 site -specific samples of soil. The methods used by UNCC and Kd results obtained from the testing are presented in Appendix D. The Kd data were used as an input parameter to evaluate COI fate and transport through the subsurface at the RBSS site. Sorption studies on soil samples obtained during the CSA at RBSS indicate that COI Kds for native soils surrounding the ash basin and ash storage area are higher than the values used in modeling. 63 Corrective Action Plan Part 1 Riverbend Steam Station Ash Basin Flow Model The groundwater model, calibrated for flow and constituent fate and transport under existing conditions, was applied to evaluate closure scenarios at RBSS. 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 becomes available in order to improve model accuracy and reduce uncertainty. The model domain developed for existing conditions was applied without modification for the EC and CIP scenarios. The EC scenario is used as a baseline for comparison to other scenarios. The assumed recharge was modified and the constant source concentration was removed in the other two scenarios. In the CIP scenario, recharge within the ash basin was reduced from 6 in/year to 0 in/year to represent capping of the ash management area For the EX scenario, the ash management areas including the primary and secondary cells of the ash basin, the ash storage area, and the cinder storage area were removed. The flow parameters for this model were identical to the EC scenario except for the removal of ash related layers, and the same recharge was applied over the remainder of the model domain (i.e.; 4 in/year). 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 horizontal hydraulic conductivity of the shallow flow layer, followed by recharge to areas beyond the ash management areas, and hydraulic conductivity of the regolith. The model was less sensitive to vertical hydraulic conductivity beyond the ash management area, where the dominant component of groundwater flow is lateral. The elevation of the water table within the ash basin system is particularly sensitive to recharge, although the effect on site -wide NRMSE is limited. 4.1.4.1 Existing Conditions Scenario This scenario models the condition of source areas assuming that no active remediation has been performed and the site remains "As Is." Cap -in -Place Scenario The CIP scenario results were used to estimate groundwater levels in the ash management area subsequent to placement of an engineered geosynthetic soil cap (assuming the ash will remain in its current position). The model results indicate that groundwater levels decreased by approximately 28 to 40 feet in the center of the Ash Basin Primary Cell and 44 feet in the center of the Ash Basin Secondary Cell. The placement of an engineered cap is a viable option and for the source areas; however, excavation of the source areas has been initiated. 64 Bad image. Missing page 55 Corrective Action Plan Part 1 Riverbend Steam Station Ash Basin 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 its sorptive characteristics. Source areas close to the compliance boundary will reach steady-state concentrations sooner than those farther away. Sorptive COls will be transient at a rate that is less than the groundwater pore velocity. Lower effective porosity will result in shorter time periods to achieve steady-state concentrations for both sorptive and non-sorptive COls. Results of the EC scenario indicate that concentrations for all modeled COls increase or reach steady-state conditions above 2L Standards or IMACs during the modeled period. t.1.5.2 Cap -In -Place The CIP scenario simulated the effects of capping the ash management area at the beginning of the scenario. In the model, recharge and source area concentrations were set to zero at the time the cap was installed. Under this scenario, groundwater flow rates are lower (compared to the EC scenario) due to the reduction in recharge and decrease in groundwater table elevation beneath the capped areas. Under the CIP scenario, the COls simulated by the model are provided below. Concentrations will intercept Mountain Island Lake prior to reaching the compliance boundary: • Antimony: Predicted concentrations decreased below the IMAC in 2040 for the bedrock flow layer and 2070 in the shallow and deep flow layers. Chromium: Concentrations will increase in shallow and deep flow layers to above the 2L Standard in GWA-2 (west side of site) between 2030 and 2090 and in MW-6 (east side of site) between 2095 and 2135. Chromium will not impact MW-15 on the north side of the site adjacent to Mountain Island Lake. Sulfate: Predicted concentrations decreased in each flow layer in downgradient wells and were below the 2L Standard within 15 years of CIP implementation. Excavation The EX scenario simulates the effects of removing the ash basin, cinder storage area, and ash storage area at the beginning of this scenario. In the model, source area concentrations from the ash basin, cinder storage area, and ash storage area are set to zero while recharge is applied at the same rate as the surrounding area. Groundwater flow beneath the ash basin is changed by this scenario because the basin is completely drained. Under the EX scenario, the COls simulated by the model are provided below. Concentrations will intercept Mountain Island Lake prior to reaching the compliance boundary. • Antimony at well GWA-2 is predicted to decrease and concentrations continue to fall below the IMAC beginning in 2055. At MW-15, antimony concentrations in the shallow and deep flow layers will remain below the IMAC and antimony in the bedrock flow layer will decrease to the IMAC by 2055. At MW-6, antimony will continue to decrease in all flow layers. By 2070, antimony will be below the IMAC across the site. • Chromium concentrations in GWA-2 will increase and exceed the 2L Standard in the shallow flow layer by 2020 and in the deep flow layer by 2060. Chromium will remain 66 Corrective Action Plan Part 1 Riverbend Steam Station Ash Basin above the 2L Standard through 2260. Chromium will not be detectable in MW-15 in any flow layers. In MW-6, chromium concentrations will increase to levels above the 2L Standard in 2060 in the shallow flow layer and 2080 in the deep flow layer and remain above the 2L Standard throughout the modeled time period. Sulfate concentrations will remain in the groundwater at all three flow layers but below the 2L Standard. The sulfate concentrations are less than the 2L Standard by 2043. Key Model Assumptions The key model assumptions and limitations include, but are not limited to, the following: • The steady-state groundwater flow model is calibrated to a single water level gauging event in June/July 2015 and is not calibrated to transient water levels, recharge, or river flow. A steady-state flow calibration does not calculate storage within the groundwater system and but estimates the groundwater flux into adjacent water bodies. • Steady-state groundwater flow conditions are assumed from the time the ash basin was placed in service through the simulation period. • COI source concentrations and recharge in the ash management area is assumed to be constant with respect to time. • The model is configured to simulate groundwater flow and transport conditions within the RBSS site (i.e., domain) and is not able to simulate off -site water level or COI transport conditions. • The river is represented by a constant head, which does not allow the stage of the river to change. • Sorption coefficients were applied in the model to allow for transport model calibration consistent with the conceptual transport model and measured COI concentrations. • The model does not account for varying geochemical conditions such as pH and redox potential that could affect COI mobility. Refer to Appendix C for additional details regarding model assumptions. Proposed Geochemical Modeling Plan Data obtained during the CSA and subsequent interpretation, determination of groundwater flow, and fate and transport modeling have resulted in improvements to the site geochemical conceptual model to enhance planned geochemical modeling. Some of the conclusions: • Site -specific groundwater flow matches the original SCM; flow matches the regional flow processes in Piedmont Terrain. • The dominant attenuation processes, as initially hypothesized, are adsorption to hydrous metal oxides (HFO, HMO, HAO) and clay minerals. Hydrous metal oxides and clay minerals are abundant in the soil and TZ, concentrations increasing with the degree of weathering of the bedrock. • Correlations exist between the concentrations of COls and HFO, HMO, and HAO and clay minerals. • Variability in pH and redox conditions across the site is high; the significant variation indicates pH and redox influence on COI attenuation should be evaluated. The binding 67 Corrective Action Plan Part 1 Riverbend Steam Station Ash Basin 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 are stable, may dissolve, or may actively precipitate. Clay mineral sorption is sensitive to pH and ionic strength (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 GIS and the retardation equation (see Appendix C). Retardation represents the combined attenuation effect of reactive area (porosity and bulk density) and Kd. Use of GIS allows overlay of a retardation function at multiple depths and evaluation of correlation and sensitivity to other measured parameters such as HFO or clay mineral content. COI Adsorption under Variable pH and Redox Conditions: The dominant attenuation processes are highly sensitive to pH and redox values and variability. Sensitivity will be evaluated by: • Using PHREEQC (USGS 2013) to determine the redox and pH changes that take place under source term conditions of capping (cessation of oxygen delivery by recharge and adjustment to a new dynamic equilibrium, draining and change in water/rock ratio). These results will be used to determine if there are changes in leachate chemistry, and if so, if the changes in leachate chemistry affect mobility outside the ash. • Under the observed variability in pH and redox, and postulated changes in pH and redox, evaluate the sensitivity of Kd to these conditions. With quantitative mineralogy and reactive surface area inputs site -specific sample attenuation can be simulated in PHREEQC using surface complexation subroutines. Surface complexation is reflected 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 " " 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 exceedances of the 2L Standards or IMACs) were used as inputs for the surface water assessment in the adjacent Mountain Island Lake receiving water. References to figures and tables relate to the surface water modeling report in Attachment D. Given that Mountain Island Lake is unidirectional and groundwater discharge 68 Corrective Action Plan Part 1 Riverbend Steam Station Ash Basin mixes with upstream river 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 along with analytical solutions to estimate the lateral spreading and mixing of the groundwater plume in the adjacent water body. This approach will be similar to that presented in USEPA's Technical Support Document for Water Quality based Toxics Control (USEPA/505/2-90-001) for ambient induced mixing that considers a lateral dispersion coefficient, upstream flow and shear velocity. The results from this analysis provide information on constituent concentration as a function of the spatial 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 2B .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 the surface water assessment. Mixing zone sizes and percentages of upstream river flows used for assessing compliance with applicable water quality standards or criteria as presented in Appendix E Figure E-1 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 Average River Width o 10 /o of 1 Q10 or 80 feet 69 Corrective Action Plan Part 1 Riverbend Steam Station Ash Basin Criteria Mixing Zone Size Percent of Design� River Flow Chronic Aquatic Life 50% of Average River Width 50% of 7Q10 or 400 feet Water Supply 50% of Average River Width 50% of 7Q10 or 400 feet Human Health 100% of Average River Width 100% of Annual Mean or 800 feet Notes: 1. The 1 Q10 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 (LISEPA 2013). Mean annual flow is the long-term average annual flow based on complete annual flow records. Using the mixing model approach, output from the groundwater model (e.g., flow and COI concentrations) was used in the mixing calculation to determine the resulting constituent concentrations in the adjacent water body from the point of discharge. These surface water results were used for comparison to the 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 U.S. Geological Survey (USGS) data sources in addition to site -specific surface water quality data collected by NCDEQ as part of their NPDES monitoring program. 4.2.2 Surface Water Model Results " ^ IN I Water Quality Assessment for the East Basin For the purpose of this surface water model, a small, semi -enclosed basin (hereafter, "East Basin") is located on the downstream (east) side of the RBSS site (Figure 1-2). The East Basin receives limited upstream inflow from the Mountain Island Lake through a narrow, shallow channel that connects back to Mountain Island Lake downstream through two larger openings. Mountain Island Lake upstream of the East Basin inflow channel is potentially influenced by COls from local groundwater inflow and COls in the upstream Mountain Island Lake that will flow into the East Basin. The East Basin inflow channel and the basin itself are potentially influenced further by COls from local groundwater inflow and from the discharge of a small, unnamed tributary (Figure 1-2). The calculated surface water COI concentrations for the East Basin are shown in Table 4-2. The stream flows, groundwater flows, and COI concentrations presented in Appendix E were used to complete these calculations. For the purpose of this surface water model, a small, semi -enclosed basin (hereafter, "East Basin") is located on the downstream (east) side of the RBSS site (Figure 1-2). The East Basin receives limited upstream inflow from the Mountain Island Lake through a narrow, shallow channel that connects back to the Mountain Island Lake downstream through two larger openings. Mountain Island Lake upstream of the East Basin inflow channel is potentially 60 Corrective Action Plan Part 1 Riverbend Steam Station Ash Basin influenced by COls from local groundwater inflow and COls in the upstream Mountain Island Lake that will flow into the East Basin. The East Basin inflow channel and the basin itself are potentially influenced further by COls from local groundwater inflow and from the discharge of a small, unnamed tributary (Figure 1-2). Thus, to assess water quality in the East Basin, mixing zone dilution calculations were first performed for COls in groundwater inflow to the Mountain Island Lake upstream of the East Basin inflow channel. The results of those calculations served as inflow values to the East Basin, with reduced inflow rates specified as a small fraction of the Mountain Island Lake design flows (i.e., 7Q10, 1Q10, annual mean). COI loadings specific to the East Basin from groundwater and from the unnamed tributary were applied. The calculations were re -run specific to the East Basin for mixing zone dilution calculations using local groundwater fluxes and the reduced Mountain Island Lake inflow rates to the basin. Table 4-2. East Basin Calculated Surface Water Concentrations Observed or Calculated Concentrations WQS /L (pg ) Estimated (pg/L) COI Upstream Cons. /L Acute Chronic HH/WS Acute Chronic HH/WS Antimony** 0.25 NA NA 0.26 NA NA 5.6** Chromium 0.5 15.5 2.6 n.a. 16 11 NA Sulfate 500 NA NA 3,021 NA NA 250,000 Notes: 1. pg/L — micrograms per liter 2. WQS — Water Quality Standards 3. * Total recoverable GW and SW concentrations; chromium VI dissolved WQS. 4. ** USEPA Human health WQS 5. NA — not applicable 6. HH/WS — human health / water supply The calculated surface water concentrations in Mountain Island Lake downstream from the RBSS site (main river channel and downstream from the entire RBSS site) are presented in Table 4-3 and 4-4. 61 Corrective Action Plan Part 1 Riverbend Steam Station Ash Basin Table 4-3. Mountain Island Lake (main river channel) Calculated Surface Water Concentrations Observed or Downstream Calculated WQS /L (pg ) Estimated Concentrations (pg/L) COI Upstream Cons. /L Acute Chronic HH/WS Acute Chronic HH/WS Antimony** 0.25 NA NA 0.25 NA NA 5.6** Chromium 0.5 0.78 0.53 NA 16 11 NA Sulfate 500 NA NA 1,271 NA NA 250,000 Notes: 1. pg/L — micrograms per liter 2. WQS — Water Quality Standards 3. ** USEPA Human health WQS 4. NA — not applicable 5. HH/WS — human health / water supply Table 4-4. Mountain Island Lake (entire site) Calculated Surface Water Concentrations Observed or Downstream Calculated WQS /L (pg ) Estimated Concentrations (pg/L) COI Upstream Cons. /L Acute Chronic HH/WS Acute Chronic HH/WS Antimony** 0.25 NA NA 0.25 NA NA 5.6** Chromium 0.5 1.00 0.55 NA 16 11 NA Sulfate 500 NA NA 1,872 NA NA 250,000 Notes: 1. pg/L — micrograms per liter 2. WQS — Water Quality Standards 3. * Total recoverable GW and SW concentrations; chromium VI dissolved WQS. 4. ** USPEA Human health WQS 5. NA — not applicable 6. HH/WS — human health / water supply 4.2.3 Surface Water Quality Assessment Results Based on the unnamed stream calculations, antimony, chromium (VI) and sulfate concentrations are below the 2B Standards and/or USEPA criteria. 4.2.4 Results The calculated surface water concentrations for antimony, chromium and sulfate concentrations in the main channel of Mountain Island Lake, East Basin and downstream Mountain Island Lake (entire site) are all less than the applicable 2B surface water standards and USEPA criteria for acute, chronic, water supply or human health water quality standards. The fraction of Mountain Island Lake design flows specified as inflow to the East Basin was adjusted until the COI concentrations within the East Basin met the 2B Standards or USEPA criteria. Through this analysis, it was determined that if only 1.1 % of the Mountain Island Lake design flows entered the East Basin through the narrow inflow channel, COI concentrations 62 Corrective Action Plan Part 1 Riverbend Steam Station Ash Basin within the basin mixing zone would meet the 2B Standards and/or USEPA criteria. This small fraction of the Mountain Island Lake flow entering the East Basin appears realistic (and probably conservative) since the East Basin inflow channel width is about 10% of the river width at this location. For the unnamed tributary to the east of the RBSS site, one adjacent upgradient groundwater well had a total chromium concentration of 55.9 pg/L, which is greater than the USEPA chromium (VI) acute and chronic water quality criteria. However, the total chromium concentration for seep location S-8 (see Appendix E, Figure E-1) in the unnamed tributary was 0.63 pg/L and less than the chromium (VI) acute and chronic water quality USEPA criteria. In addition, seep location S-8 had an antimony concentration of 0.5 pg/L and sulfate concentration of 1,000 pg/L. Both of these concentrations are less than the applicable water quality criteria. 4.3 Refinement of SCM The SCM in Section 3 identifies Mountain Island Lake as a potential receptor to Source area COls through the groundwater to surface water exposure pathway. Groundwater and surface water models have been used to provide further information regarding the transport of COls toward Mountain Island Lake. COls addressed in the models do not identify impacts to the surface waters of Mountain Island Lake, the East Basin, or the unnamed tributary for the exposure pathway. 63 Corrective Action Plan Part 1 Riverbend 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. At the request of NCDEQ, groundwater results that were obtained with turbidity greater than 10 NTU were removed prior to performing statistical analyses. For groundwater constituents that were not historically analyzed in the compliance wells, the groundwater PPBCs were developed by selecting the highest concentration (or highest method reporting limit for non -detects) of a given constituent across the range of concentrations observed for that constituent in the qualifying events. PPBCs will be refined as additional data sets are obtained from subsequent sampling events collected from the background monitoring wells. Soil PPBCs were calculated as the 95% upper tolerance limit for soil constituents. For constituents where too few detections were reported to use the statistical methodology, the PPBCs were established by setting the value equal to the highest concentration or the highest method reporting limit for non - detect values. As more data become available for groundwater from additional sampling events in 2015, PPBCs will be refined and reported. • COls were selected for groundwater fate and transport modeling, in part, based on comparison of constituent concentrations in background wells to source and downgradient wells. Data obtained from source and downgradient wells were not eliminated using the 10 NTU turbidity limit applied to PPBCs, even though the analytical results for selected constituents can be biased upward due to the effects of turbidity. Groundwater samples collected during the CSA, and subsequent monitoring events, were analyzed for total and dissolved phase constituents to evaluate potential effects of turbidity. The list of COls to be carried forward in CAP Part 2 will be modified, if warranted, as additional groundwater quality data are obtained and the possible effects of turbidity on the analytical results are evaluated. Source removal is actively being conducted on the RBSS site. Source removal is being completed pursuant to a NCDEQ approved Excavation Plan. The Excavation Plan removes source areas identified in the CSA ash basins 1 and 2, the ash storage area, and the cinder storage area. The CAP Part 1 Report does not carry forward COls identified within ash because they are being removed. • Due to the continuing source removal action, a post -excavation monitoring plan will be presented in CAP Part 2 that will address remaining COls outside the excavation area for reported media. The post excavation monitoring plan will provide a sampling and analysis plan for groundwater, and modify the existing seep inventory for sampling to reflect changes in seeps following excavation. • Geochemical modeling of the RBSS site will be completed and submitted under cover of the CAP Part 2. The geochemical model results taken into consideration with the groundwater flow, fate, and transport and surface water -groundwater models will enhance the understanding of the processes taking place in the subsurface and ultimately aid in choosing the most appropriate remedial actions for the site. The 64 Corrective Action Plan Part 1 Riverbend Steam Station Ash Basin geochemical model is key to understanding mobility of iron, manganese, and TDS since these cannot adequately be modeled using MODFLOW/MT3DMS. The groundwater modeling results for the flow model indicate that under the CIP scenario groundwater levels decrease by approximately 28 to 40 feet in the center of the Ash Basin Primary Cell and 44 feet in the center of the Ash Basin Secondary Cell. The placement of an engineered cap reduces the groundwater elevation to an extent where groundwater flow direction will be changed. In the excavation scenario, groundwater levels post -excavation cannot be accurately estimated until the depth of excavation and the hydraulic parameters of the replacement fill material are known. Model simulations are summarized as follows: • Existing Conditions Scenario — For the fate and transport modeling, the results of the EC scenario indicate that concentrations for all modeled COls increase and reach steady-state above 2L Standards or IMACs at Mountain Island Lake through the 250 year modeling period. • Cap -In -Place Scenario — o Predicted concentrations of antimony decrease below the IMAC in 2040 for the bedrock flow layer and 2070 in the shallow and deep flow layers. Predicted concentrations of sulfate decrease in each flow layer in downgradient wells and fall below the 2L Standard within 15 years of CIP implementation. o Chromium concentrations increase in shallow and deep flow layers to above the 2L Standard in GWA-2 (west side of site) between 2030 and 2090 and in MW-6 (east side of site) between 2095 and 2135. Chromium will not impact MW-15 on the north side of the site adjacent to Mountain Island Lake. o Sulfate: Predicted concentrations decreased in each flow layer in downgradient wells and fall below the 2L Standard within 15 years of CIP implementation. • Excavation Scenario — o In the EX scenario, antimony will decrease below the IMAC across the site by 2070. o Chromium concentrations in GWA-2 will increase and exceed the 2L Standard in the shallow flow layer by 2020 and in the deep flow layer by 2060. Chromium will remain above the 2L Standard through 2260. Chromium will not be detectable in MW-15 in any flow layers. In MW-6, chromium concentrations will increase to levels above the 2L Standard in 2060 in the shallow flow layer and 2080 in the deep flow layer and remain above the 2L Standard to the end of the model run. o Sulfate concentrations will remain in the groundwater in all three flow layers below the 2L Standard. The sulfate concentrations are less than the 2L Standard by 2043. The surface water modeling results: • Groundwater flow rates and concentrations of COls from the groundwater model were used as input to a groundwater -surface water interaction model to determine if 2L 66 Corrective Action Plan Part 1 Riverbend Steam Station Ash Basin Standard (or IMAC) exceedances would result in exceedances of 2B surface water standards or USEPA water quality criteria in Mountain Island Lake. • Antimony, chromium and sulfate concentrations in Mountain Island Lake are all less than the applicable water quality standards or criteria. The following recommendations are made to address areas needing further assessment: • Groundwater flow, fate and transport and surface water models should be updated with results from second -round sampling at the RBSS site and should be included in CAP Part 2. • Further assessment of the area downgradient to the west of cinder storage area is recommended. TDS, sulfate, arsenic, and magnesium COls in this area outside the waste boundary are at concentrations exceeding the applicable regulatory standards or criteria and are higher than source area concentrations. Cap Part 1 reviewed groundwater analytical data and groundwater elevation data collected between June/July 2015. Background wells identified in the CSA (BG wells and MW 7-SR and MW-7D) are not situated in an upgradient location to the RBSS source areas and impact to these wells from source could not be ruled out. Additional upgradient wells are recommended to provide background groundwater chemistry for the site south of the source area on Duke Energy property. 66 Corrective Action Plan Part 1 Riverbend Steam Station Ash Basin 6 References Back and Barnes 1965. Relation of electrochemical potentials and iron content to groundwater flow patterns. U.S. Geological Survey Professional Paper no. 498-C. Reston, Virginia: USGS. Baedecker, M.J., and W. Back. 1979. 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