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Home My WebLink AboutNC0004979_Allen CAP I_Report_Final_20151120F)l Corrective Action Plan Part 1 Allen Steam Station Ash Basin Site Location: NPDES Permit No.: Permittee and Current Property Owner: Consultant Information Report Date: Allen Steam Station 253 Plant Allen Road Belmont, NC 28012 NC0004979 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 20, 2015 This page intentionally left blank Corrective Action Plan Part 1 �� Allen Steam Station Ash Basin Contents ExecutiveSummary......................................................................................................................... ES-1 Introduction............................................................................................................................. ES-1.1 Regulatory Background.......................................................................................... ES-1.2 Summary of CSA.................................................................................................... ES-2 Background Concentrations and COI Screening Level Summary .......................................... ES-2.1 Proposed Provisional Background Concentrations ............................................... ES-2.2 Updated COI Screening Evaluation Summary ...................................................... ES-3 Site Conceptual Model........................................................................................................... ES-3.1 Geological/Hydrogeological Properties.................................................................. ES-3.2 Site Geochemical Conditions................................................................................. ES-3.3 Correlation of Hydrogeologic and Geochemical Conditions of COI Distribution ... ES-4 Modeling ES-4.1 Model Scenarios.................................................................................................... ES-4.2 Groundwater Modeling Conclusions...................................................................... ES-4.2.1 Flow Model................................................................................................. ES-4.2.2 Fate and Transport Model.......................................................................... ES-4.3 Groundwater -Surface Water Interaction Modeling .......................................... ES-5 Recommendations................................................................................................................. 1 Introduction............................................................................................................................. 1.1 Site History and Overview............................................................................................ 1.1.1 Site Location, Acreage, and Ownership.......................................................... 1.1.2 Site Description............................................................................................... 1.2 Permitted Activities and Permitted Waste.................................................................... 1.3 History of Site Groundwater Monitoring....................................................................... 1.4 Summary of Comprehensive Site Assessment............................................................ 1.5 Receptor Survey........................................................................................................... 1.5.1 Surrounding Land Use.................................................................................... 1.5.2 Findings of Drinking Water Supply Well Survey Conducted per the Coal Ash Management Act of 2014, N.C. Gen. Stat. SS 1 30A-309-200 et seq.............. 1.6 Summary of the 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........................................................................ .1 .1 .1 .2 .2 .3 .4 .5 .5 .5 .6 .7 .7 .7 .7 .8 .9 .9 11 12 12 12 14 14 15 15 16 16 16 17 17 18 18 19 21 21 23 23 26 29 Corrective Action Plan Part 1 �� Allen Steam Station Ash Basin 3 4 5 6 2.3 Seeps.................................................................................................................... 2.4 Surface Water....................................................................................................... 2.5 Sediments............................................................................................................. 2.6 Soils....................................................................................................................... 2.6.1 Background Soil and 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......................................................................................... SiteConceptual Model.................................................................................................... 3.1 Site Hydrogeologic Conditions.............................................................................. 3.1.1 Hydrostratigraphic Units........................................................................... 3.1.2 Hydrostratigraphic Unit Properties........................................................... 3.1.3 Potentiometric Surface — Shallow Flow Layers ........................................ 3.1.4 Potentiometric Surface — Deep Flow Layer ............................................. 3.1.5 Potentiometric Surface — Bedrock Flow Layer ......................................... 3.1.6 Horizontal and Vertical Hydraulic Gradients ............................................ 3.2 Site Geochemical Conditions................................................................................ 3.2.1 COI Sources and Mobility in Groundwater ............................................... 3.2.2 Geochemical Characteristics................................................................... 3.2.3 Source Area Geochemical Conditions..................................................... 3.2.4 Mineralogical Characteristics................................................................... 3.3 Correlation of Hydrogeologic and Geochemical Conditions to COI Distribution... Modeling.......................................................................................................................... 4.1 Groundwater Modeling.......................................................................................... 4.1.1 Model Scenarios....................................................................................... 4.1.2 Calibration of Models................................................................................ 4.1.3 Kd Terms................................................................................................... 4.1.4 Flow Model............................................................................................... 4.1.5 Fate and Transport Model........................................................................ 4.2 Groundwater - Surface Water Interaction Modeling .............................................. 4.2.1 Mixing Model Approach............................................................................ 4.2.2 Surface Water Model Results.................................................................. 4.3 Refinement of SCM............................................................................................... Summary and Recommendations................................................................................... References...................................................................................................................... .. 30 :1 r!- 70 71 K*6 Corrective Action Plan Part 1 Allen Steam Station Ash Basin Tables 2-1 Initial COI Screening Evaluation 2-2 Background Groundwater Concentrations for the Allen Site: Ranges of Analytical Results with Sample Turbidity <10 NTU 2-3 Groundwater Exceedance Results for COls Compared to PPBCs, 2L Standards, IMACs, or DHHS HSLs, and Frequency of Exceedances 2-4 Radionuclide Concentrations 2-5.1 Seep Exceedance Results for COls Compared to 2L Standards, IMACs or DHHS HSLs and Frequency of Exceedances 2-5.2 Seep Exceedance Results for COls Compared to 2B Standards or USEPA Criteria and Frequency of Exceedances 2-6.1 Surface Water Exceedance Results for COls Compared 2B or USEPA Standards, and Frequency of Exceedances 2-6.2 Surface Water Exceedance Results for COls Compared 2L or IMAC, and Frequency of Exceedances 2-7 Sediment COls Exceedances Compared to Upstream Sediment Concentrations, NC PSRGs for POG and Frequency of Exceedances 2-8 Proposed Provisional Background Soil Concentrations 2-9 Soil Exceedance Results for COls Compared to NC PSRGs for POG, PPBCs, and Frequency of Exceedances 2-10 Ash Exceedance Results for COls Compared to NC PSRGs for POG, PPBCs, and Frequency of Exceedances 2-11 Porewater Exceedance Results for COls Compared to 2L Standards, IMACs, or DHHS HSLs, and Frequency of Exceedances 2-12 Updated COI Screening Evaluation Summary 3-1 Vertical Gradient Calculations for Shallow/Deep Well Pairs 3-2 Vertical Gradient Calculations for Deep/Bedrock Well Pairs 3-3 Categories and Threshold Concentrations to Identify Redox Processes in Groundwater 3-4 Field Parameters from CSA 4-1 Mixing Zone Sizes and Percentages of Upstream River Flows 4-2 Catawba River (Lake Wylie) Calculated Surface Water Concentrations Corrective Action Plan Part 1 Allen Steam Station Ash Basin Figures 1-1 Site Location Map 1-2 Site Layout Map 1-3 Compliance and Voluntary Monitoring Wells 1-4 Monitoring Well and Sample Locations 1-5 Seep and Surface Water Sample Locations 1-6 Receptor Map 1-7 Site Vicinity Map 2-1 Groundwater Analytical Results Map 2-2 Surface Water and Seep Analytical Results 2-3 Soil Analytical Results 3-1 Site Conceptual Model — 3-D View 3-2.1 Site Conceptual Model — Cross Sectional 3-2.2 Site Conceptual Model — Cross Sectional 3-3 Water Table Surface Map — Shallow Wells 3-4 Potentiometric Surface Map — Deep Wells 3-5 Potentiometric Surface Map — Bedrock Wells 3-6 Vertical Gradient — Shallow to Deep Wells 3-7 Vertical Gradient — Deep to Bedrock Appendices A Regulatory Correspondence B Background Well Analysis C UNCC Groundwater Flow and Transport Model D UNCC Soil Sorption Evaluation E Surface Water Modeling Methods iv Corrective Action Plan Part 1 Allen Steam Station Ash Basin 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 COI Constituent of Interest COPC Contaminant of Potential Concern CSA Comprehensive Site Assessment DHHS Department of Health and Human Services DORS Distribution of Residuals Solids DWR NCDEQ Division of Water Resources FGD flue gas desulfurization ft/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 Units POG Protection of Groundwater PPBC Proposed Provisional Background Concentration RAB Retired Ash Basin SCM Site Conceptual Model SU Standard Unit TDS total dissolved solids UNCC University of North Carolina at Charlotte USGS U.S. Geological Survey USEPA U.S. Environmental Protection Agency Work Plan Groundwater Assessment Work Plan v Corrective Action Plan Part 1 Allen Steam Station Ash Basin Executive Summary ES-1 Introduction ES-1.1 Regulatory Background Duke Energy Carolinas, LLC (Duke Energy) owns and operates the Allen Steam Station (Allen), located on the Catawba River (specifically Lake Wylie) in Gaston County near Belmont, North Carolina. Allen began operation as a coal-fired generating station in 1957 and is still in service today. The coal ash residue and other liquid discharges from Allen's coal combustion process has been disposed of in station's two ash basins (active and inactive) since their respective construction. The ash basins are located south/southwest of the station and adjacent to the Catawba River.. Discharge from the active ash basin is permitted by the North Carolina Department of Environmental Quality (NCDEQ)' Division of Water Resources (DWR) under the National Pollutant Discharge Elimination System (NPDES) Permit NC0004979. 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 Allen was submitted to NCDENR (now NCDEQ) on September 24, 2014, followed by a revised Work Plan on December 30, 2014. The Work Plan was conditionally approved by NCDENR on February 24, 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 Allen CSA Report was submitted to NCDENR on August 23, 2015 (HDR 2015). The CSA found no imminent hazards to public health and safety; therefore, no actions to mitigate imminent hazards were required. However, corrective action at the Allen site is required to address soil and groundwater contamination resulting from the source areas. In addition, a plan for continued groundwater monitoring will be implemented following NCDEQ's approval. CAMA also requires the submittal of a Corrective Action Plan (CAP) for each regulated facility no later than 180 days after submittal of the CSA. Duke Energy and NCDEQ mutually agreed to a two-part CAP submittal, with Part 1 being submitted within 90 days of submittal of the CSA and Part 2 being submitted no later than 180 days after submittal of the CSA. The purpose of this CAP Part 1 is to provide a summary of site usage, a brief summary of the CSA findings, an evaluation and refinement of COls for modeling purposes, a detailed description of the site conceptual model (SCM), results of the groundwater flow and transport model, and results of the groundwater to surface water interaction model. Prior to September 18, 2015, the NCDEQ was referred to as the North Carolina Department of Environment and Natural Resources (NCDENR). Both naming conventions are used in this Executive Summary, as appropriate. Corrective Action Plan Part 1 Allen Steam Station Ash Basin The CAP Part 2 will include the remainder of the CAMA requirements, including proposed alternative methods for achieving groundwater quality restoration, conceptual plans for recommended corrective action, an estimated implementation schedule, and a plan for future monitoring and reporting. A risk assessment will be submitted under a separate cover with the CAP Part 2 submittal. ES-1.2 Summary of CSA The CSA for Allen began in February 2015 and was completed in August 2015. Eighty groundwater monitoring wells and nine soil borings were installed/advanced as part of the assessment to characterize media (soil, rock, and groundwater) potentially impacted by the source areas at the Allen site. Seep, surface water, and sediment samples were also collected. For the CSA, the source area was defined as the ash basin system, which comprises the inactive ash basin and the active ash basin. A lined Retired Ash Basin (RAB) dry ash landfill, two unlined Distribution of Residuals Solids (DORS) structural fill units, and two unlined dry ash storage areas are also located on top of the inactive ash basin. Source characterization was performed to identify physical and chemical properties of ash, ash basin surface water, ash porewater, and ash basin seeps. The analytical results for source characterization samples were compared to North Carolina Groundwater Quality Standards, as specified in 15A NCAC 2L.0202 (21L Standards), or Interim Maximum Allowable Concentrations (IMACs), and other regulatory screening levels for the purpose of identifying constituents of interest (COls) that may be associated with source -related impacts to soil, groundwater, and surface water. In addition, hydrogeological evaluation testing was performed on newly installed and existing monitoring wells at the site. The CSA identified groundwater impacts at the Allen site and found that groundwater exceedances are a result of both naturally occurring conditions and CCR material contained in the ash basin system. The approximate horizontal extent of groundwater exceedances is limited to beneath the ash basin system within the ash basin compliance boundary. The approximate vertical extent of groundwater impacts is generally limited to the shallow and deep flow layers. The horizontal extent of soil impacts is limited to the area beneath the ash basin and the vertical extent of exceedances beneath the ash/soil interface is generally limited to the uppermost soil sample collected beneath ash. Additional details pertaining to the horizontal and vertical extent of soil and groundwater exceedances and the additional delineation identified as data gaps at the site are detailed in the CSA report. ES-2 Background Concentrations and COI Screening Level Summary Some COls identified in the CSA are present in background and upgradient monitoring wells and may be naturally occurring, and thus require examination to determine whether their presence downgradient of the source areas is naturally occurring or potentially attributed to the source areas. Therefore, proposed provisional background concentrations (PPBCs) were Corrective Action Plan Part 1 Allen Steam Station Ash Basin calculated for groundwater and soil to aid in evaluating whether or not COI impacts identified in the CSA are attributable to the source areas and which COls will be further evaluated for corrective action. ES-2.1 Proposed (Provisional Background Concentrations Because COls can be both naturally occurring and related to the source areas, background groundwater monitoring wells are important for establishing background concentrations to evaluate whether releases have occurred from the source areas at the site. To determine whether or not a monitoring well is suitable for developing site -specific background concentrations, the following criteria were evaluated: • The topographic location of the well with respect to the source areas (distance from source areas and located hydraulically upgradient of source areas) • Stratigraphic unit being monitored • Screened intervals of well relative to source water elevation • Direction of groundwater flow in the region of the well relative to source areas Wells that have been determined to represent background conditions are BG-1 S/D and BG-3D. Existing ash basin compliance monitoring well AB-1 R has historically been considered by Duke Energy to represent background water quality at the site since being installed in 2010. AB-1 R is located northwest of the inactive ash basin at the compliance boundary and has not previously exhibited 2L Standard exceedances. However, recent increasing trends of detection monitoring constituents in samples collected from AB-1 R indicate groundwater in the vicinity of AB-1 R may be influenced by the northwest portion of the inactive ash basin. Therefore, only those CSA background (BG) monitoring wells that meet the detailed analysis provided in Appendix B were included. As a result, installation of additional background well(s) are recommended. Samples with turbidity less than 10 NTU collected from background wells were utilized to develop Proposed Provisional Background Concentrations (PPBCs). PPBCs represent the highest reported value (or highest laboratory reporting limit for non -detects) in the newly installed background monitoring wells based upon the single sample event data available. Additional constituent concentrations reported for new background wells will be incorporated into a statistical background analysis once a statistically valid 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. Corrective Action Plan Part 1 �� Allen Steam Station Ash Basin ES-2.2 Updated COI Screening Evaluation Summary The table below summarizes COls (by media) that are potentially attributable to the source areas that will be utilized in the evaluations to determine if corrective action is warranted. In addition to comparing COI concentrations to PPBCs, aqueous media concentrations were compared to 2L Standards, IMACs, DHHS HSL, and 213 Standards, and solid media were compared to NC PSRGs for POG. Following the comparison of COls to PPBCs and the regulatory standards listed above, the COls that are potentially attributable to the source areas underwent further evaluation in the groundwater flow and fate and transport modeling. Potential COI CSA COI Exceedance by Media COI To Be Further Pore- Surface Assessed Ash Ground- PWR/ 1 Water Water Water Seeps Sediment Soil Bedrock in 3 Section 3.0 - - - - Yes Yes Solid/ Aqueous Aluminum Antimony Arsenic Yes Barium - - - - Yes Beryllium - Yes Boron - Yes Cadmium - - Yes Chromium - - - Yes Hexavalent Chromium - - - - - Yes Cobalt Yes Copper Yes Iron 1I - - Yes Lead - - - - - - - Yes Manganese Yes Mercury - - Yes Nickel - Yes pH 1! - Yes Selenium Yes Sulfate 1f - - Yes Thallium - - Yes TDS - - - Yes Vanadium Yes Zinc - - - - Yes Notes: 1. Ash is not evaluated for remediation in this CAP because ash will be drained of water during remedial activities and capped. 2. Porewater is not evaluated for remediation in this CAP because porewater will be eliminated during ash basin closure activities due to the lack of saturated ash in the final post closure scenario. 3. The geochemical model is key to understanding mobility of iron, manganese, pH and TDS since it cannot adequately be modeled using MODFLOW/MT3DMS. Corrective Action Plan Part 1 �� Allen Steam Station Ash Basin ES-3 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 Allen site. The SCM was developed using data and analysis from the CSA Report. ES-3.1 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 from the western and southern extent of the Allen property boundary to the east and southeast toward the Catawba River, as well as north and northeast toward the station discharge canal. Horizontal and vertical hydraulic gradients were calculated for each flow layer. Positive and negative vertical gradients varied across the site. From the data available, groundwater flow is generally downward on the western side of the site and below the ash basins. Below the ash basin dams adjacent to the Catawba River, positive vertical gradients appear to be the result of hydraulic pressure from the elevation difference between the basin and the downgradient wells. Further evaluation will be undertaken as additional water elevation data are collected. ES-3.2 Site Geochemical Conditions Determination of the reduction/oxidation (redox) condition of groundwater is an important component of groundwater assessments, and helps to understand the mobility, degradation, and solubility of constituents. The redox state of the Allen site was evaluated based on 38 samples from the study area for which all six constituents were available, including porewater and groundwater. Based on site measurements, the primary redox categories were determined to include oxic, suboxic, mixed (anoxic), and anoxic conditions. At Allen, predominant redox processes involved ferrous iron/ferrous sulfate, so reduced species As(III), Se(IV), and Mn(IV) would be expected. Other redox processes involving manganese reduction may also occur. Redox conditions appear to be controlled at least partly by the SO4/S2 and Fe(III)/Fe(II) redox couples. However, it should be noted that 12 of the 38 (--32%) wells were in Suboxic or Oxic categories. Further, 6 of 10 wells from which porewater samples were collected are classified as Suboxic. There is a decreased potential for reduced forms of metals to persist under suboxic or oxic conditions. Groundwater samples were characterized in terms of solute speciation to evaluate the concentrations and ionic composition (oxidation states) of metal ions primary concern, including arsenic(III, V), chromium(III, VI), iron(II, III), manganese(II, IV), and selenium(IV, VI). In general, reduced forms of metals (i.e., species in lower oxidation states) are more readily transported in groundwater than those that are more oxidized. At the Allen site, speciation measurements were performed on 35 to 67 samples, depending on the analyte. To provide a general indication of sample composition, the relative percentage of the reduced specie concentration to the sum Corrective Action Plan Part 1 Allen Steam Station Ash Basin of the reduced and oxidized specie concentration were calculated. These relative percentages express the proportion of the reduced form metal present each sample. Speciation measurements at the Allen site show very high variations. For arsenic, approximately 50% is present as arsenic(III), with a range of 9% to 93%. Similarly, for chromium approximately 68% is present as chromium(III), with a range of 6% to 99%. For selenium, the variation is somewhat smaller but still relatively large, with 53% present as Se(IV) and a range of 40% to 87%. This highly variable composition is consistent with the wide range of redox categories that may exist across the site. Given this diverse range of conditions, next steps in the Allen site evaluation process will include: equilibrium geochemical speciation evaluation using modeling tools such as PHREEQC (USGS 2013) and groundwater and chemical transport modeling. Additional sampling will be needed to further characterize the temporal and spatial characteristics (i.e., redox reactions) of groundwater composition for the site. Constituents may be removed from groundwater and onto mineral surfaces of the aquifer media through sorption processes that may 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 (KA Soils from the Allen site were analyzed in the laboratory by the University of North Carolina at Charlotte (UNCC) to measure the adsorption of COls at varying concentrations. 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 Allen site, as described in greater detail in Section 4.1. ES-3.3 Correlation of Hydrogeologic and Geochemical Conditions of COI Distribution The SCM was used to evaluate areal distribution of COls with regard to site -specific geological/ hydrogeological and geochemical properties at the Allen site. Key observations include: Beryllium, cadmium, nickel, and zinc exceeded their respective 2L Standards or IMAC in one groundwater sample and selenium and thallium exceeded their respective 2L Standard and IMAC in a limited number of groundwater samples. The relative absence of these constituents in groundwater could suggest that the subsurface geochemical conditions may attenuate these COls or that the original composition of source material did not contain large concentrations of these COls to begin with. Additional sample data and refinement of the groundwater models is being performed and will be refined in CAP Part 2 to provide a better understanding of these COIs. Horizontal migration of COls was evident in groundwater flow to the east towards the Catawba River, with decreasing concentrations moving farther away from the source areas. Vertical migration of Cols was observed in select well clusters and is likely influenced by infiltration of precipitation and/or ash basin water, where applicable, through the shallow and deep flow layers into underlying fractured bedrock. Corrective Action Plan Part 1 �� Allen Steam Station Ash Basin Speciation analysis revealed that Allen groundwater is predominantly anoxic/mixed (oxic to suboxic in certain portions of the site) with detectable dissolved oxygen in nearly every well. Dissolved oxygen is the primary control on redox conditions in Allen groundwater. Mixed anoxic conditions are reflected in the speciation of redox-sensitive species (e.g., ferrous iron/ferrous sulfate are the predominant redox processes). Redox condition is a reasonable predictor of the presence and relative concentration of oxidized and reduced species of arsenic, chromium, iron, selenium and sulfate. Speciation of constituents at Allen is an important consideration in developing corrective actions for the site. 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 were conducted to evaluate COI migration and potential impacts following closure of the ash basin system at Allen. In conjunction with HDR, UNCC developed a site -specific, 3-D, steady-state groundwater flow and fate and transport model for the Allen site using MODFLOW and MT3DMS. UNCC provided the calibrated flow and fate and transport models and HDR performed the predictive simulations. 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 Allen 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. ES-4.1 Model Scenarios The following groundwater model scenarios were simulated for the purpose of this CAP Part 1: • Existing Conditions: assumes current site conditions with ash sources left in place • Cap -in -Place (CIP): assumes ash left in ash basins and ash storage area is above the water table and is covered by an engineered cap • Excavation (EX): assumes removal of ash from source areas Each model scenario utilized steady-state flow conditions established during flow model calibration and transient transport of COls. ES-4.2 Groundwater Modeling Conclusions ES-4.2.1 Flow Model The 3-D groundwater flow model results indicate that under existing conditions, groundwater in the shallow aquifer, transition zone, and fractured bedrock flow layers at the site transitions from vertical to primarily horizontal flow directly beneath the ash basins due to the absence of a dike or fill layer beneath the ash and above the shallow zone. Groundwater within the shallow, deep, and bedrock zones flows from the west and southwest to the east and discharges to the Corrective Action Plan Part 1 �� Allen Steam Station Ash Basin Catawba River. These findings are consistent with groundwater potentiometric surface maps and interpreted groundwater flow directions presented in the CSA Report. The Cap -In -Place scenario results were used to estimate groundwater levels in the ash management area subsequent to placement of an engineered cap. The model results indicate that near the center of the inactive ash basin, the water table is lowered by approximately 26 feet relative to the level simulated under the Existing Conditions scenario. In the active ash basins, the difference in water level is approximately 36 feet as compared to the water level in the Existing Condition scenario. In the Excavation 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-4.2.2 Fate and Transport Model The fate and transport modeling was used to assess the transport of selected Cols through shallow, deep, and bedrock flow layers and predict the fate of these COls over time. Each selected COI was modeled individually under the Existing Condition, Cap -In -Place, and Excavation scenarios. COls evaluated in the fate and transport model include: aluminum, antimony, arsenic, barium, beryllium, boron, cadmium, chloride, chromium, hexavalent chromium, cobalt, copper, iron, lead, manganese, mercury, molybdenum, nickel, nitrate, pH, selenium, strontium, sulfate, thallium, TDS, vanadium, and zinc. Several COls were not advanced to modeling based on the following rationale: • Beryllium, cadmium, nickel, thallium, and zinc were found above the 2L Standards or IMACs in only one location within the waste boundary. Provided that concentrations above the 2L Standards or IMACs are limited to the source areas and there is no discernable plume, they could be eliminated from modeling. Geochemical modeling of the Allen site will be completed and submitted under cover of the CAP Part 2. The geochemical model results 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. • Aluminum, chloride, copper, lead, mercury, molybdenum, and nitrate concentrations were not reported above their respective 21-Standards in groundwater in background wells, source areas, or downgradient areas. COls evaluated in the fate and transport model were antimony, hexavalent chromium, arsenic, barium, boron, cobalt, lead, mercury, selenium, total chromium, sulfate and vanadium. These COls represent the remaining source related groundwater constituents that can be utilized in the fate and transport model as they do not require further evaluation as background or in the geochemical model. Corrective Action Plan Part 1 Allen Steam Station Ash Basin Under the Existing Condition scenario, steady-state concentrations for seven of the ten COls modeled increase, or reach a steady-state condition greater than the 2L Standards or IMACs at the eastern compliance boundary during the modeled period. Under the Cap -In -Place scenario, concentrations for the COls modeled decrease at the eastern compliance boundary over the model simulation period with the exception of chromium, hexavalent chromium and vanadium. The majority of the COls still present concentrations greater than their respective 2L Standards, IMACs, or DHHS HSL, with the exceptions of barium, boron and sulfate. Modeled concentrations of boron and sulfate (non-sorptive COls) are depleted in the model simulation within 40 years of completion of closure. Under the Excavation scenario, model outputs are highly variable for each COI. As in the Cap - In -Place scenario, boron and sulfate are depleted from the model domain within 20 years of excavation. Model outputs for arsenic, cobalt, and vanadium exhibit variable concentrations and exceedances of the 2L Standards or IMACs at the eastern compliance boundary with the Catawba River through the end of the modeling period. Antimony, barium, concentrations decrease and fall below the 2L Standards of IMACs at the end of the modelling period. The flow and fate and transport models will be updated, if necessary, based on a review of additional sampling and water elevation data as it becomes available. ES-4.3 Groundwater -Surface Water Interaction Modeling Groundwater model output from the fate and transport modeling was used as input to the surface water assessment in the Catawba River adjacent to the Allen site. A mixing model was used to assess potential downgradient surface water quality impacts. For each groundwater COI that discharges to surface waters at a concentration exceeding the 213 Standards or USEPA's National Recommended Water Quality Criteria, the appropriate dilution factor and upstream (background) concentration were applied to determine the surface water concentrations at the edge of the mixing zone. This concentration was then compared to the applicable water quality standard or criteria to determine compliance. The surface water model results indicate that no water quality standards are exceeded for COls modeled at the edge of the mixing zones in the Catawba River. As additional data are obtained during subsequent sampling events, the surface water modeling will be refined and re- assessed, if necessary. ES-5 Recommendations The following recommendations have been made to address areas needing further assessment: • PPBCs should be refined based on additional sampling of background wells (four sampling rounds in 2015). Additional upgradient wells are recommended to provide background groundwater chemistry for the site west and north of the source area. Corrective Action Plan Part 1 �� Allen Steam Station Ash Basin • Additional groundwater elevation data are needed in the vicinity of BG-2 and along South Point Road to better define a likely groundwater flow direction and groundwater quality. These data will be collected as part of the data gap well installations. The well cluster BG-1 had groundwater elevation at the same or lower elevation than porewater levels in the ash basin. Additional wells are recommended for installation in the vicinity of the BG-1 cluster to further define groundwater flow direction and groundwater quality. Once this is accomplished, the use of this well cluster for background water quality will be re-evaluated. • Groundwater and seep COls should be updated with results from the second -round sampling performed at the Allen site and will be included in the CAP Part 2 report. • On -site background surface water and seep sample location(s) (if applicable) should be identified and sampled. • Additional sampling for radiological parameters along major groundwater flow paths is needed to perform a more comprehensive assessment of radionuclides from source areas and downgradient wells along major flow paths. • The SCM and groundwater models will be updated with groundwater elevations and analytical results from second -round sampling at the Allen site and will be included in the CAP Part 2 report. • The model will be revised during CAP Part 2 to accomplish the following tasks: o The model will be further refined to more rigorously reflect compliance, voluntary and newly installed CSA wells with non -detectable COI concentrations. o The model domain will be expanded to include nearby water supply wells and PBBCs. o The model results will be further assessed to identify data gaps that would improve the conceptual site model. o Antimony will be assessed. o Additional results from monitor well sampling will be included in further refinement of the calibration. o The Kd value used for non -conservative COls will be further assessed during refinement and recalibration of the model. o Remedial alternatives will be simulated to evaluate potential source control or removal options. • The groundwater to surface water model will refined based on results from second -round sampling activities at the Allen site. 10 Corrective Action Plan Part 1 �� Allen Steam Station Ash Basin 1 Introduction Duke Energy Carolinas, LLC (Duke Energy) owns and operates the Allen Steam Station (Allen), located on the Catawba River (specifically Lake Wylie) in Gaston County near Belmont, North Carolina. Allen began operation as a coal-fired generating station in 1957 and is still in service today. The coal ash residue and other liquid discharges from Allen's coal combustion process have been disposed of in station's two ash basins (active and inactive) since their respective construction. The ash basins are located south/southwest of the station and adjacent to the Catawba River. Discharge from the ash basins is permitted by the North Carolina Department of Environmental Quality (NCDEQ)2 Division of Water Resources (DWR) under the National Pollutant Discharge Elimination System (NPDES) Permit NC0004979. 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 Allen was submitted to NCDENR (now NCDEQ) on September 24, 2014, followed by a revised Work Plan on December 30, 2014 and approved on February 24, 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 Allen CSA Report was submitted to NCDENR on August 23, 2015 (HDR 2015). CAMA also requires the preparation of a Corrective Action Plan (CAP) for each regulated facility no later than 180 days after submittal of the CSA. 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 (Appendix A). The purpose of CAP Part 1 is to provide a summary of site usage, a brief summary of the CSA findings, an evaluation and refinement of the constituents of interest (COls) for modeling purposes, a detailed description of the site conceptual model (SCM), and results of the groundwater flow and transport model and groundwater to surface water interaction model. CAP Part 2 will include the remainder of the CAMA requirements including an evaluation of 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. 2 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 _ Allen Steam Station Ash Basin 1.1 Site History and Overview 1.1.1 Site Location, Acreage, and Ownership The Allen site is located on the west bank of the Catawba River on Lake Wylie in Belmont, Gaston County, North Carolina. The entire Allen site is approximately 1,009 acres in area and is owned by Duke Energy. Duke Energy also owns property along the Station Discharge Canal to the east and west of South Point Road (NC 273), as shown on Figure 1-1. In addition to the power plant property, Duke Energy owns and operates the Catawba-Wateree Project (Federal Energy Regulatory Commission [FERC] Project No. 2232). Lake Wylie reservoir is part of the Catawba-Wateree Project and is used for hydroelectric generation, municipal water supply, and recreation. Duke Energy performed a review of property ownership of the FERC project boundary property 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 all of the property within the compliance boundary with the exception of one parcel located east of the active ash basin within the FERC boundary (Catawba River) and bisected by the ash basin compliance boundary. Duke Energy has water rights for the parcel as the creation of Lake Wylie inundated the parcel. The Duke Energy property boundary and the ash basin compliance boundary are shown on Figure 1-1. 1.1.2 Site Description Allen is a coal-fired electricity generating facility with a capacity of 1,155 megawatts (MW) along the Catawba River. The five -unit station began commercial operation in 1957 with Units 1 and 2 (330 MW total). Unit 3 (275 MW) was placed into commercial operation in 1959, followed by Unit 4 (275 MW) in 1960, and Unit 5 (275 MW) in 1961. The Allen ash basin is situated between the Allen powerhouse to the north and topographic divides to the west (along South Point Road) and south (along Reese Wilson Road) (Figure 1-1). Coal ash residue from the coal combustion process has historically been disposed of in the Allen ash basin system, which is comprised of the inactive ash basin and the active ash basin. A lined Retired Ash Basin (RAB) dry ash landfill, two unlined Distribution of Residuals Solids (DORS) structural fill units, and two unlined dry ash storage areas are also located on top of the inactive ash basin. The ash basin system waste boundary encompasses approximately 322 acres. The active ash basin, located on the southern portion of the property, is approximately 169 acres in area and contains an estimated 7,700,000 tons of ash. The inactive ash basin, located between the generating units and the active ash basin, is approximately 132 acres in area and contains approximately 3,900,000 tons of ash. The inactive ash basin was commissioned in 1957 and is located adjacent to and north of the active ash basin. The inactive ash basin was formed by constructing the earthen North Dike (located along the west bank of the Catawba River), and the northern portion of the East Dike (located between active and inactive ash basins) across drainage features along the shore of the Catawba River. The active ash basin was constructed in 1973 and was formed by constructing the southern portion of the East Dike (Figure 1-2). 12 Corrective Action Plan Part 1 �� Allen Steam Station Ash Basin Fly ash precipitated from flue gas and bottom ash collected in the bottom of the boilers were sluiced to the ash basin using conveyance water withdrawn from the Catawba River. Since 2009, fly ash has been dry -handled and disposed of in the on -site ash landfill, and bottom ash has continued to be sluiced to the active ash basin. Coal ash was sluiced to the inactive ash basin until the active ash basin was constructed in 1973. During operations, the sluice lines discharge the water/ash slurry (and other permitted flows) into the three Primary Ponds on the northern portion of the active ash basin. Primary Ponds 1, 2, and 3 were constructed in approximately 2004. Currently, Primary Ponds 2 and 3 are utilized for settling purposes. The ash basin is operated as an integral part of the station's wastewater treatment system, which receives flows from the ash removal system, coal pile runoff, landfill leachate, flue gas desulfurization (FGD) wastewater, the station yard drain sump, and site stormwater. Due to variability in station operations and weather, the inflows to the ash basin are highly variable. Effluent from the ash basin is discharged from the discharge tower to the Catawba River via a 42-inch-diameter reinforced concrete pipe located in the southeastern portion of the ash basin (Outfall 002). The water surface elevation in the ash basin is controlled by the use of stop logs in the discharge tower. RAB Ash Landfill unit (NCDENR Division of Waste Management [DWM] Solid Waste Section Permit No. 3612-INDUS), is located on the eastern portion of the Allen site, on top of the inactive ash basin. The landfill was constructed in 2009 and is bound to the north, east, and south by earthen dikes. The retired ash basin dam comprises the northern and eastern boundaries of the landfill. The lined landfill is permitted to receive CCR materials including fly ash, bottom ash, boiler slag, mill rejects, and FGD waste generated by Duke Energy. In addition to these CCR materials, the landfill is permitted to receive non -hazardous sandblast material, limestone, coal, carbon, sulfur pellets, cation and anion resins, sediment from sumps, and cooling tower sludge. The two unlined DORS structural ash fills are located on top of the western portion of the inactive ash basin, adjacent to and west of the RAB Ash Landfill. These fills were constructed of ponded ash removed from the active ash basin per Duke Energy's DORS Permit issued by NCDENR DWQ. Placement of dry ash in the structural fills began in 2003 and was completed in 2009. During and following the completion of filling, the structural fill areas were graded to drain, soil cover was placed on the top slopes and side slopes, and vegetation was established. The eastern structural fill covers approximately 17 acres and contains approximately 500,000 tons of ash. The western structural fill covers approximately 17 acres and contains approximately 328,000 tons of ash. The two unlined ash storage areas are located on top of the western portion of the inactive ash basin, adjacent to and west of the two DORS structural fills. Similar to the two DORS structural fills, the ash storage areas were constructed in 1996 by excavating ash from the northern portion of the active ash basin in order to provide capacity for sluiced ash in the active ash basin and the future construction of Primary Ponds 1, 2, and 3. Following the completion of stockpiling, the ash storage areas were graded to drain, and a minimum of 18 and 24 inches of 13 Corrective Action Plan Part 1 Allen Steam Station Ash Basin soil cover were placed on the top slopes and side slopes, respectively, and vegetation was established. Approximately 300,000 cubic yards of ash is stored in the ash storage areas, which encompass an area of approximately 15 to 20 acres of the western portion of the inactive ash basin. The air pollution control system for the coal-fired units at Allen includes an FGD system that began operations in 2009. Coal is delivered to the station by rail. Other areas of the site are occupied by facilities supporting power production and transmission (two switchyards and associated transmission lines), the FGD wastewater treatment system, and the gypsum handling station (associated with the FGD system). The Allen site is bounded by the west bank of the Catawba River to the east, Reese Wilson Road and Nutall Oak Lane to the south, South Point Road to the west, and a local topographic divide to the north of the Station Discharge Canal. The Station Discharge Canal is located northwest of the ash basin and Plant Allen Road. South Point Road runs north to south and is generally located along a local topographic divide. Reese Wilson Road and Nutall Oak Lane run from west to east and are generally located along a local topographic divide. Topography at the Allen site ranges from an approximate high elevation of 650 feet to 680 feet near the west and southwest boundaries of the site to an approximate low elevation of 570 feet at the shoreline of the Catawba River. Permitted Activities and Permitted Waste Duke Energy is authorized to discharge wastewater that has been adequately treated and managed to the Catawba River and South Fork Catawba River in accordance with NPDES Draft Permit NC00004979 (dated May 15, 2015). Issuance of the final permit is pending. Any other point source discharge to surface waters of the state is prohibited unless it is an allowable non- stormwater discharge or is covered by another permit, authorization, or approval. Duke Energy is permitted to discharge stormwater to the Catawba River and South Fork Catawba River in accordance with the NPDES Draft Permit NCS000546 dated June 1, 2015. The NPDES permits authorize discharges in accordance with effluent limitations monitoring requirements and other conditions set forth in the permit. A detailed description of NPDES and surface water sampling requirements, along with the associated NPDES site flow diagram, is provided in the CSA Report. One active lined landfill (RAB Ash Landfill), two unlined structural fill units, and two unlined ash storage units are located at the site (as described previously above in Section 1.1.2). History of Site Groundwater Monitoring Monitoring wells were installed by Duke Energy in 2010 as part of the voluntary monitoring system for groundwater near the ash basin and ash storage areas. Duke Energy performed voluntary groundwater monitoring around the site from the ash basin and ash storage areas. The voluntary groundwater monitoring wells were sampled twice each year and the analytical results were submitted to NCDENR DWR. 14 Corrective Action Plan Part 1 �� Allen Steam Station Ash Basin Compliance groundwater monitoring, as required by the NPDES permit, began in May 2004. From May 2004 to November 2010, voluntary groundwater monitoring was performed twice annually around the Allen ash basins with analytical results submitted to NCDENR DWR. In late 2010, new compliance monitoring wells were installed with an increase in the frequency of sampling (three times per a year) and analytical parameters outlined in the NPDES permit. From March 2011 through July 2015, the compliance groundwater monitoring wells at the Allen site have been sampled three times per year for a total of 14 events, as required by the NPDES permit. The location of the ash basin voluntary and compliance monitoring wells, the approximate ash basin waste boundary, and the compliance boundary are shown on Figure 1-3. The compliance boundary for groundwater quality at the Allen site is defined in accordance with Title 15A NCAC 02L .0107(a) as being established at either 500 feet from the waste boundary or at the property boundary, whichever is closer to the waste boundary. A detailed description of NPDES and voluntary groundwater monitoring programs and results is provided in the CSA Report. 1.4 Summary of Comprehensive Site Assessment The CSA for the Allen site began in February 2015 and was completed in August 2015. Eighty groundwater monitoring wells and nine soil borings were advanced as part of the assessment to characterize the ash, soil, rock, and groundwater at the Allen site (Figure 1-4). Seep, surface water, and sediment samples were also collected (Figure 1-5). In addition, hydrogeological evaluation testing was performed on newly installed wells. Information obtained during the CSA was used to determine existing background and source - concentrations, as well as to evaluate the horizontal and vertical extent of impacts to soil and groundwater at the site related to source areas. If a constituent3 concentration exceeded 1) the North Carolina Groundwater Quality Standards, as specified in T15A NCAC .0202L (21L Standards) or Interim Maximum Allowable Concentration (IMAC)4, 2) the North Carolina Surface Water Quality Standard (213 Standard), or 3) the North Carolina Preliminary Soil Remediation Goals (NCPSRGs) for Protection of Groundwater (POG) it was designated in the CSA as a "Constituent of Interest" (COI). In addition, the CSA presented information from a receptor survey completed in 2014 and a screening level human health and ecological risk assessment. Additional details of the CSA findings are discussed in following sections. 1.5 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 Allen ash basin compliance boundary. The supplemental information was obtained from 3 Constituents are elements, chemicals, or compounds that were identified in the approved Work Plan for sampling and analysis, and include antimony, arsenic, boron, chromium, cobalt, iron, manganese, selenium, thallium, vanadium, sulfate, and total dissolved solids (TDS). a Appendix #1 of 15A NCAC Subchapter 02L Classifications and Water Quality Standards Applicable to The Groundwaters of North Carolina, lists Interim Maximum Allowable Concentrations (IMACs). The IMACs were issued in 2010, 2011 and 2012; however, NCDENR has not established a 2L Standard for these constituents as described in 15A NCAC 02L.0202(c). For this reason, IMACs noted in this report are for reference only. 15 Corrective Action Plan Part 1 �� Allen Steam Station Ash Basin responses to water supply well survey questionnaires mailed to property owners within the required distance requesting information on the presence of water supply wells and well details and usage. A detailed description of the receptor surveys is provided in the CSA Report. Further findings of the receptor survey are detailed on Figure 1-6. The City of Belmont provides potable water to the Allen site and some of the surrounding area within the City of Belmont, North Carolina. A combination of private drinking water wells and the Catawba River supply drinking water for the surrounding area. 1.5.1 Surrounding Land Use Properties located within a 0.5-mile radius of the Allen ash basin compliance boundary are predominately located in and south of Belmont, Gaston County, North Carolina. The majority of the land is residential or undeveloped property as shown on Figure 1-7 and land use is typical of rural areas. Residential properties are located primarily west and south of the ash basin compliance boundary within the 0.5-mile radius. In addition, residential properties are present across the Catawba River to the east in Charlotte, Mecklenburg County, North Carolina. 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. As part of the CSA report, Duke Energy updated the previously completed Receptor Survey activities based on the CSA Guidelines provided in the Notice of Regulatory Requirement (NORR) issued by NCDENR. Four (4) public water supply wells and 219 private water supply wells were identified within a 0.5-mile radius of the ash basin compliance boundary. Section § 130A-309.209 (c) of CAMA also indicates that NCDENR (now NCDEQ) will require sampling of wells to predict whether the wells may be adversely impacted by releases from CCR impoundments. NCDENR required sampling to include all potential drinking water receptors within 0.5-mile of the compliance boundary in all directions, since the direction of groundwater flow had not been determined at Allen at the time of the sampling. Between February and August 2015, NCDENR arranged for independent analytical laboratories to collect and analyze water samples obtained from private wells identified during the Drinking Water Well Survey, if the owner agreed to have their well sampled. The CSA included a water supply well tracking spreadsheet provided by NCDENR related to NCDENR-conducted well sampling. Approximately 150 private drinking water supply wells within 0.5 mile of the Allen compliance boundary were sampled by NCDENR. The analytical results are tabulated in Appendix B of the CSA Report. NCDENR also provided analytical results for several drinking water wells located outside of the 0.5-mile radius that would be considered to representative of background water quality. Those results are also included in Appendix B of the CSA Report. 1.6 Summary of the Screening Level Risk Assessment A screening level human health and ecological risk assessment was performed as a component of the CSA Report. 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. 16 Corrective Action Plan Part 1 �� Allen Steam Station Ash Basin The human health exposure routes associated with the evaluated pathways for the site include ingestion, inhalation, and dermal contact of environmental media. Potential human receptors under a current or hypothetical future use include construction/outdoor workers, off -site residents, recreational users and trespassers. The ecological exposure routes associated with the evaluated pathways for the site include dermal contact/root absorption/gill uptake and ingestion of environmental media. Potential ecological receptors under a current or hypothetical future use include aquatic, riparian, and terrestrial biota. The screening level risk assessment will continue to be refined consistent with risk assessment protocol in parallel with the CAP Part 22 schedule. 1.7 Geological/Hydrogeological Conditions The Allen site is located in the Charlotte terrane. The Charlotte terrane consists of an igneous complex of Neoproterozoic to Paleozoic ages (Hibbard et al. 2002) that range from intermediate to mafic in composition (Butler and Secor 1999). The Charlotte terrane is bordered on the east and southeast by the Carolina terrane and to the west and northwest by the Inner Piedmont (Cat Square and Tugaloo terranes) and the Kings Mountain terrane. The structural contact of the Inner Piedmont and Charlotte terrane is the Central Piedmont Shear Zone. The Charlotte terrane is a meta -igneous terrane consisting of volcanic and plutonic rocks that have been subjected to deformation and high grade metamorphism due to tectonic stress during and after intrusion of the igneous units. Foliation is noted in many of the boring logs but is not intense and is not dominant with respect to structure of the rock mass. Bedrock at the site consists of meta -quartz diorite and meta-diabase. The meta -quartz diorite is very light gray to dark gray, fine to coarse grained, non -foliated and massive to foliated, composed dominantly of plagioclase, quartz, biotite, and hornblende. Epidote was not noted in the cores. The meta-diabase is greenish black to very dark greenish gray, is mostly non -foliated and is noted as aphanitic to fine grained although it is described as fine to coarse grained in some logs. Meta -quartz diorite is the primary rock type underlying the site with the meta-diabase occurring as dikes within the meta -quartz diorite during a late syn-plutonic stage. Based on the CSA investigation, the groundwater system in the natural materials (alluvium, soil, soil/saprolite, and bedrock) at the Allen site is consistent with the Piedmont regolith-fractured rock system and is an unconfined, connected system of flow layers. In general, groundwater within the shallow and deep layers (S and D wells) and bedrock layer (BR wells) flows from west and southwest to the east toward the Catawba River and to the north toward Duke Energy property and the Station Discharge Canal. 1.8 Results of the CSA Investigations Groundwater constituent exceedances were determined to be the result of both source related materials contained within the ash basins and ash storage area as well as naturally occurring conditions within the Duke Energy property boundary and surrounding vicinity. The CSA identified the source related horizontal and vertical extent of groundwater contamination at the 17 Corrective Action Plan Part 1 Allen Steam Station Ash Basin Allen site and found it is limited to within the compliance boundary. Additional assessment is recommended offsite and west of the ash basin to further evaluate groundwater quality and groundwater flow west of the ash basin. The horizontal and vertical extent of source related soil contamination was identified during the CSA. Where soil impacts were identified beneath the inactive ash basin and the active ash basin, the vertical extent of contamination beneath the ash/soil interface is generally limited to the uppermost soil sample collected beneath the ash. Groundwater contamination at the site attributable to ash handling and storage was delineated during the CSA activities. Background monitoring wells analytical results contained naturally occurring metals and other constituents at concentrations that exceeded their respective regulatory standards or guidelines. These included: antimony, barium, chromium, cobalt, iron, manganese, pH, total dissolved solids (TDS), 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 CAP Part 1. The geologic conditions present beneath the inactive ash basin and active basin generally impede the vertical migration of contaminants. The direction of contaminant transport is generally in a northeast and east direction towards the Catawba River, as anticipated, and not toward other off -site receptors (i.e., private drinking water wells). Additional details pertaining to the horizontal and vertical extent of soil and groundwater impacts at the Allen site are detailed in the CSA Report. 1.9 Regulatory Requirements 1.9.1 CAMA Requirements CAMA Section §130A-309.209 requires implementation of corrective actions for the restoration of groundwater quality. Analysis and reporting requirements are as follows: (b) Corrective Action for the Restoration of Groundwater Quality. - The owner of a coal combustion residuals surface impoundment shall implement corrective action for the restoration of groundwater quality as provided in this subsection. The requirements for corrective action for the restoration of groundwater quality set out in the subsection are in addition to any other corrective action for the restoration of groundwater quality requirements applicable to the owners of coal combustion residuals surface impoundments. (1) No later than 90 days from submission of the Groundwater Assessment Report required by subsection (a) of this section, or a time frame otherwise approved by the Department not to exceed 180 days from submission of the Groundwater Assessment 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: 18 Corrective Action Plan Part 1 �� Allen Steam Station Ash Basin a. A description of all exceedances of the groundwater quality standards, including any exceedances that the owner asserts are the result of natural background conditions. b. A description of the methods for restoring groundwater in conformance with requirements of Subchapter L of Chapter 2 of Title 15A of the North Carolina Administrative Code and a detailed explanation of the reasons for selecting these methods. c. Specific plans, including engineering details, for restoring groundwater quality. d. A schedule for implementation of the Plan. e. A monitoring plan for evaluating effectiveness of the proposed corrective action and detecting movement of any contaminant plumes. f. Any other information related to groundwater assessment required by the Department. (2) The Department shall approve the Groundwater Corrective Action Plan if it determines that the Plan complies with the requirements of this subsection and will be sufficient to protect public health, safety, and welfare; the environment, and natural resources. (3) No later than 30 days from the approval of the Groundwater Corrective Action Plan, the owner shall begin implementation of the Plan in accordance with the Plan's schedule. Duke Energy is required by CAMA to close the Allen ash basin no later than August 1, 2029 or as otherwise dictated by NCDEQ risk classification. Closure for the Allen ash basin was not defined in CAMA. CAMA requires that corrective action be implemented to restore groundwater quality where the CSA documents exceedances of groundwater quality standards. 1.9.2 Standards for Site Media Groundwater and seep sample analytical results were compared to 2L or 2L Standards or IMACs established by NCDEQ pursuant to 15A NCAC 02L.0202(c). The IMACs were issued in 2010, 2011, and 2012, however NCDEQ has not established 2L Standards for these constituents as described in 15A NCAC 02L.0202(c). For this reason, IMACs noted in this report are for reference only. NCDEQ has also requested that hexavalent chromium be compared to the North Carolina Department of Health and Human Services (DHHS) Health Screening Level (HSL). Surface water sample analytical results were compared to the appropriate North Carolina Surface Water Quality Standards (213 Standards), selected from the list of standards published by NCDENR dated April 22, 2015 and including applicable U.S. Environmental Protection Agency (USEPA) National Recommended Water Quality Criteria. Ash pond surface water sample analytical results were also compared to 2L Standards or IMACs previously referenced due to potential surface water infiltration into groundwater. The water quality standards were published by NCDEQ in North Carolina Administrative Code 15A NCAC 2B, amended effective January 1, 2015. The most stringent of the values from the following three criteria (as 19 Corrective Action Plan Part 1 �� Allen Steam Station Ash Basin applicable) was selected for comparison of the surface water analytical results: Freshwater Aquatic Life, Water Supply, Human Health (NCDEQ DWR 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. 20 Corrective Action Plan Part 1 �� Allen Steam Station Ash Basin 2 Background Concentrations and Regulatory Exceedances Introduction As part of the CSA, groundwater, seep, surface water, sediment, and soil samples were collected between June 13 and July 14, 2015, from background locations, beneath the ash basins (including active and inactive), and from locations outside the Allen waste boundary. Groundwater samples were also collected from previously installed voluntary and compliance wells on the site. 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 media; 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); 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 examination to determine whether their presence downgradient of the source area is naturally occurring or potentially attributed to the source areas. Note that COls identified in the CSA were based on one sampling event and that the PPBCs presented in the subsections below are provisional values. The PPBCs will be updated as more data become available with input from NCDEQ. The COls identified in the CSA Report are organized by media and presented in Table 2-1. 21 Corrective Action Plan Part1 FN Allen 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 Seeps Sediment Soil PWR/ Bedrock Further Assessed in CAP I Aluminum _ _ `� _ - Yes Antimony - `� - - - - Yes Arsenic - - Yes Barium - - - - Yes Beryllium - - - - Yes Boron `� Yes Cadmium - Yes Chloride - - - No Chromium - - - - Yes Hexavalent Chromium - _ _ - Yes Cobalt - Yes Copper - - Yes Iron - Yes Lead - - - - - - - Yes Manganese Yes Molybdenum - - - No Mercury - - Yes Nickel - - - - Yes Nitrate - - - No H - - - - - Yes Selenium - Yes Strontium - - - - - No Sulfate - - - Yes Thallium - `� - - - - - Yes TDS - - - - Yes Vanadium `� Yes Zinc - - - Yes Note: COI Exceedances based on 2L Standard, [MAC, and 2B Standards for respective aqueous media and PSRGs for solid/soil media. COls resulting from ash and porewater exceedances at the Allen site are representative of source characterization data with respect to groundwater and soil impacts at the site attributable to ash handling and storage. Note that ash is not evaluated for remediation in this CAP Part 1 because ash will be addressed as part of ash basin closure activities. 22 Corrective Action Plan Part 1 Allen Steam Station Ash Basin j 2.2 Groundwater 2.2.1 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: • 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 identified to represent background conditions are BG-1 S/D and BG-3D (Figure 2-1). Existing ash basin compliance monitoring well AB-1 R has been considered by Duke Energy to represent background water quality at the site since being installed in 2010. AB-1 R is located northwest of the inactive ash basin at the compliance boundary and has not previously exhibited exceedances of 2L Standards. However, recent increasing trends of detection monitoring constituentss(See Section 2.2.2 for more detail). in samples collected from AB-1 R indicate groundwater in the vicinity of AB-1 R may be influenced by the northwest portion of the inactive ash basin. Therefore, only those CSA background (BG) monitoring wells that meet the detailed analysis provided in Appendix B were included. Note that NCDEQ requested that analytical results for samples collected with turbidity values greater than 10 Nephelometric Turbidity Units (NTU) should not be included in the PPBC calculations. However, the evaluation of COls in this CAP Part 1 does consider analytical data where turbidity was greater than 10 NTU, but these results are not utilized for the determination of PPBCs. 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 as part of a post -remedial monitoring plan to be completed in CAP Part 2. That level of evaluation was not possible using the limited data set acquired under the time constraints specified in CAMA. In addition, porewater and groundwater sample results (other than background) which were collected during the CSA where turbidity was greater than 10 NTU were used in the contaminant fate and transport modeling discussed in Section 4. This should be taken into account when evaluating the results of the fate and transport model and considering the risk classification for the Allen site. Background groundwater concentrations for the Allen site, PPBCs, and regional background data are presented in Table 2-2. Background concentrations reported at Allen are limited to samples collected from wells with turbidity less than 10 NTU. PPBCs represent the highest reported value (or highest laboratory reporting limit for non -detects) in the newly installed 5 Constituents 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) 23 Corrective Action Plan Part 1 Allen Steam Station Ash Basin background monitoring wells. Background concentrations identified for new background wells will be incorporated into statistical background analysis once a statistically valid data set has been obtained. Regional groundwater data in Table 2-2 were reviewed from publicly available data with the most relevant spatial resolution. U.S. Geological Survey (USGS) National Uranium Resource Evaluation (NURE) data in a 20-mile radius from the site were used for all constituents contained in the NURE database. North Carolina DHHS county -level data were subsequently used for all constituents available. Remaining constituents for which there are no NURE or DHHS data were acquired from the most spatially relevant, publicly available sources, which are cited in the Allen CSA. Particularities of the NURE and DHHS data are as follows: The DHHS collected data from private well owners whose water samples were analyzed at the North Carolina State Laboratory of Public Health from 1998-2010. Basic summary statistics of these data were subsequently calculated by the Superfund Research Translation Corps at the University of North Carolina at Chapel Hill, and posted for public use on the DHHS Epidemiology section website. These data are informative for illustrating broad spatial differences in groundwater quality across North Carolina counties, but have a number of limitations. Minimum, maximum, and average concentrations were calculated for each analyte by county; however, these statistics were calculated with no consideration of non -detected values, which causes a high bias to the results. • Groundwater chemical concentration data in a 20-mile radius surrounding the Allen site were collected from USGS NURE program between 1975 and 1980. The data have a high level of spatial resolution and consistent coverage over the state of North Carolina, making the date set a useful and appropriate resource for illustrating state-wide patterns (or variations) in groundwater quality. Data provided in the "2-10 Private Well Data" column identifies the range of values found for each constituent sampled in private wells owned by Duke Energy employees living within 2 and 10 miles from the Allen waste boundary. Data provided in the "NCDEQ Sampled Private Water Well Data" column identifies the range of values found for each constituent sampled in private wells owned by area residents. The 2-10 private well results and NCDEQ well results are provided for reference only due to the lack of well construction data, hydrostratigraphic data, and detailed geological context for these sample locations. An analysis of Allen background groundwater concentrations is provided in Appendix B. 24 Corrective Action Plan Part 1 �� Allen Steam Station Ash Basin Table 2-2. Background Groundwater Concentrations for the Allen Site: Ranges of Analytical Results with Sample Turbidity <10 NTU 2Private NCDEQ New Proposed Regional Well Data We Sampled Background Provisional Background (February private Well Background COI Groundwater 2015 to May Drinking Water Groundwater Groundwater Concentrations 2015) Well Data Concentrations Concentrations (pg/L) (pg/L) (August 2015) (June 2015) (Ng/L) (Ng/L) (Ng/L) Aluminum 10.7 to 241.7 <10 to 606 <10 to 93.4 130 to 860 860 Antimony North Carolina 0.53 to 1.22 <0.5 0.4J to <0.5 0.5 0.5 to 232; 0.5 Arsenic to 15 (Gaston; <0.5 to 1.14 <0.5 0.21J to 2.3 2.3 Mecklenburg) Barium 50; 50 (Gaston; 3.4 to 126 8.5 to 77.3 19 to 99 99 Mecklenburg) Beryllium Not Reported <0.2 to <1 <0.2 <0.2 0.2 Boron Not Reported <5 to 59 <5 to 12.4 <50 50 0.5 to 2.5; 0.5 to Cadmium 5 (Gaston; <0.8 to <1 <0.08 0.028J to <0.08 0.08 Mecklenburg) 5 to 30; 0.5 to Chromium 80 (Gaston; <0.5 to 6.1 <0.5 to 1.2 1.1 to 16 16 Mecklenburg) Hexavalent Chromium Not Reported 0.064 to 5.8 <0.6 to 4.5 Not Analyzed Not Determined Cr VI Cobalt Not Reported <0.5 to 4.99 <0.5 0.32J to 0.74 0.74 25 to 6,630; 18 Copper to 7,780 (Gaston; <5 to 992 <1 to 840 1.4 to 3 3 Mecklenburg) Below detect to Iron 98,000; 25 to <10 to 4,200 <50 to 129 110 to 960 960 58,560 (Gaston; Mecklenburg) 2.5to291;2.5 Lead to 703 (Gaston; <0.1 to 33 <0.1 to 0.72 <0.1 to 0.85 0.85 Mecklenburg) Below detect to Manganese 352.6 (20-mile <0.5 to 4,820 <0.5 to 8.3 29 to 38 38 radius from site Molybdenum Not Determined 1.25 to 2.73 <0.5 to 0.96 0.38J to 5.6 5.6 0.3 to 0.6; 0.3 Mercury (Gaston; <0.05 <0.2 <0.2 0.2 Mecklenburg) Nickel Not Reported <0.0005 to <0.5 to 1 0.76 to 7.3 7.3 0.012 5.4 to 8.4 pH (SU) (20-mile radius 6.95 to 7.86 6.28 to 7.85 5.89 to 10.2 5.89 to 10.2 from site 0.3to41; 0.3 to <0.5 pgg/ to 0.7 Selenium 24.4 (Gaston; <0.5 to 1.1 <0.5 0.7 Mecklenburg) Strontium Not Determined 43 to 185 84.4 to 760 30 to 210 210 Sulfate Not Determined Not <2,000 to 39,400 760J to 30,300 30,300 Determined 25 Corrective Action Plan Part 1 �� Allen Steam Station Ash Basin 2-10 Private NCDEQ Proposed Regional Well Data Sampledprivate Backew ground ound Provisional Background (February Well Background COI Groundwater 2015 to May Drinking Water Groundwater Groundwater Concentrations ) Well Data Concentrations Concentrations (pg/L) (pg/N9/L) (August 2015) (June 2015) (N9/L) (pg/L) (Ng/L) <1 (Blue Ridge Thallium Mountain and <0.2 <0.1 <0.1 0.1 Piedmont Aquifers) TDS Not Reported 93,000 to 8,200 to 158,000 33,000 to 198,000 320,000 198,000 Below detect to Vanadium 26.6 <0.3 to 19.8 <1 to 23.7 0.63J to 22.5 22.5 (20-mile radius from site 8 to 26,000; 25 Zinc to 26,000 <0.005 to <5 to 25.9 5.6 J to 12 12 (Gaston; 0.844 Mecklenburg) Notes: 1. pg/L = micrograms per liter 2. SU = Standard Units 3. < indicates concentration less than laboratory method detection limit 4. J = Estimated concentration 5. Regional groundwater concentration data is from NURE data in a 20-mile radius from the site for all constituents contained in the NURE database. DHHS county -level data was 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 Allen CSA report. 6. PPBCs for constituents monitored during the CSA not considered COls are provided in Appendix B. 7. Sufficient data set to statistically derive concentrations not available. PPBC presented is the highest reported value (or highest laboratory reporting limit for non -detects) in the newly installed background monitoring wells. 8. Reported NCDEQ sampled private drinking water well concentration ranges are from various NPDEQ sampling events reported on the "Groundwater Reconnaissance Well Water Sampling Study Results Posted" blog, posted August 28, 2015. 2.2.2 Groundwater Exceedances of 2L Standards or IMACs Groundwater impacts at the Allen site attributed to ash handling and storage was delineated during the CSA activities with the following exceptions identified as data gaps in the CSA Report: • Horizontal and vertical extent under the RAB Ash landfill • Horizontal and vertical extent north/northeast of the RAB Ash landfill • Horizontal extent west and southwest of the active ash basin beyond the waste boundary Additional monitoring wells will be installed during the first quarter of 2016 to address the above - referenced data gaps. Information gathered from additional assessment will be submitted under a separate cover. Groundwater exceedance results for COls, along with a comparison to applicable regulatory standards or guidelines, are provided in Table 2-3. Note that until PPBCs are approved by NCDEQ, COI concentrations will be compared to their applicable 2L Standard or IMAC. In the absence of a 2L Standard or IMAC for hexavalent chromium, NCDEQ has requested that 26 Corrective Action Plan Part 1 �� Allen Steam Station Ash Basin hexavalent chromium results be compared to the DHHS HSL for private water supply wells (0.07 lag/L). At this time, PPBCs are shown on the table for reference purposes only. Groundwater sample locations and analytical results are depicted on Figure 2-1. Table 2-3. Groundwater Exceedance Results for COls Compared to PPBCs, 2L Standards, IMACs, or DHHS HSLs, and Frequency of Exceedances COI Proposed Provisional Background Concentrations (lag/L) NC 2L Standard, IMAC, or DHHS HSL (lag/L) Groundwater Concentrations Exceeding 2L Standards, IMACs, or DHHS HSL (Range, lag/L) Number of Samples Exceeding 2L Standards, IMACs or DHHS HSL/Number of Samples Beneath the Active Ash Basin Antimony* 0.5 1 1.1 to 1.3 2/9 Boron 50 700 1,500 1/9 Chromium 16 10 13.3 to 65.6 4/9 Hexavalent Chromium** Not Determined 0.07 0.83 to 20 6/6 Iron 960 300 449 1 /9 pH (SU) 5.89 to 10.22 6.5 to 8.5 SU 8.71 to 11.84 8/9 TDS 198 500,000 590,000 to 1,280,000 2/9 Vanadium* 22.5 0.3 6.3 to 36.9 9/9 Beneath the Inactive Ash Basin (including Ash Storage, Structural Fill and Retired Ash Basin) Arsenic 2.3 10 369 1/13 Barium 99 700 990 1/13 Chromium 16 10 24.7 1/13 Hexavalent Chromium** Not Determined 0.07 0.34 to 6.1 5/10 Cobalt* 0.74 1 1.4 to 40 5/13 Iron 960 300 420 to 95,900 6/13 Manganese 38 50 66 to 7,000 7/13 pH (SU) 5.89 to 10.22 6.5 to 8.5 SU 6.05 to 10.27 8/10 Sulfate 30.3 250,000 337,000 1/13 TDS 198 500,000 761,000 1/13 Vanadium* 22.5 0.3 0.39J to 21.1 11/13 Downgradient of Ash Basin Antimony* 0.5 1 2.7 to 6 2/26 Arsenic 2.3 10 193 1/26 Boron 50 700 720 to 1,800 5/26 Beryllium* 0.2 4 48.6 1/26 Cadmium 0.08 2 10.1 1/26 Chromium 16 10 29.8 1/26 Hexavalent Chromium** Not Determined 0.07 0.16 to 1.4 3/8 Cobalt* 0.74 1 2 to 4,160 12/26 Iron 960 300 340 to 23,000 13/26 27 Corrective Action Plan Part 1 �� Allen Steam Station Ash Basin COI Proposed Provisional Background Concentrations (pg/L) NC 2L Standard, IMAC, or DHHS HSL (pg/L) Groundwater Concentrations Exceeding 2L Standards, IMACs, or DHHS Number of Samples Exceeding 2L Standards, IMACs or DHHS Manganese 38 50 52 to 207,000 15/26 Nickel 7.3 100 1,010 1/26 pH 5.89 to 10.22 6.5 to 8.5 SU 3.68 to 11.52 15/19 Selenium 0.7 20 73.5 1/26 Sulfate 30.3 250,000 1,830,000 1/26 Thallium* 0.1 0.2 1.3 1/26 TDS 198 500,000 3,920,000 1/26 Vanadium* 22.5 0.3 0.39J to 22.5 23/26 Zinc 10 1,000 2,100 1/26 Notes: 1. pg/L = micrograms per liter 2. SU = Standard Units 3. J = Laboratory estimated concentration 4. < indicates concentration less than laboratory method detection limit. * Indicates 2L Standard not established for constituent; therefore, IMAC used for screening criteria. Observations related to Allen groundwater COls are listed below: Hexavalent chromium was analyzed in multiple groundwater monitoring wells across the site and will need further evaluation as to prevalence at the Allen site. Additional sampling and analysis is required to determine if hexavalent chromium should remain as a COI, and therefore this constituent cannot be ruled out as a COI as part of the CAP Part 1. Iron, manganese, and TDS were reported at concentrations greater than their respective 2L Standards. Iron and manganese were reported in multiple groundwater samples while TDS was only reported in three source area locations and one downgradient location. The geochemical model is key to understanding mobility of iron, manganese, and TDS since it cannot adequately be modeled using MODFLOW/MT3DMS. Additional analysis is required to determine if lead and/or nickel should remain on the COI list; therefore, these constituents cannot be ruled out as COls as part of this CAP Part 1. • Beryllium, cadmium, nickel, thallium, and zinc exceeded their respective 2L Standards or IMAC in one source area location (GWA-6S). Reported dissolved fractions were similar to total concentrations for all constituents. The GWA-6S sample had a turbidity of 7.21 NTU. Additional sampling and analysis is required to determine if these constituents should remain on the COI list; therefore, these constituents cannot be ruled out as COls as part of the CAP Part 1. 28 Corrective Action Plan Part 1 Allen Steam Station Ash Basin The following groundwater COls will be considered for further evaluation: • Antimony • Iron • Arsenic • Manganese • Barium • Nickel • Boron • pH • Beryllium • Selenium • Cadmium • Sulfate • Chromium • Thallium • Hexavalent chromium • TDS • Cobalt • Vanadium • Zinc Of the COls identified at Allen, boron, pH, sulfate, and TDS are detection monitoring constituents listed in 40 CFR 257 Appendix III of the USEPA's Hazardous and Solid Waste Management System; Disposal of Coal Combustion Residuals from Electric Utilities (CCR Rule). The USEPA considers these inorganic parameters to be leading indicators of releases of contaminants associated with CCR as they move rapidly through the surface layer, relative to other constituents, and thus provide an early detection of whether contaminants are migrating from the CCR unit. Additional details regarding the CCR Rule and applicable constituents can be found in the CSA Report. The PPBCs were determined to be greater than (or outside of the range of, in the case of pH) the 2L Standards, IMACs, or DHHS HSL for the following constituents and parameter: • Chromium • Iron • 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, IMACs, or DHHS HSLs during future sampling events. For PPBCs determined to be less than the 2L Standards, IMAC, or DHHS HSL, 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. Three monitoring wells, AB-9S, BG-3D, and BG-3S, were sampled for these analytes, and the results of this analysis are presented in Table 2-4. 29 Corrective Action Plan Part 1 Allen Steam Station Ash Basin Ji Table 2-4. Radionuclide Concentrations Background Downgradient of Radionuclide USEPA MCL* Concentrations Source Area Concentrations (BG-3S/D) (AB-9S) Radium-226 5 pCi/L (combined) 3.778J to 5.47 pCi/L (combined) 6.21J pCi/L combined (combined) Radium-228 Uranium 30 pg/L < 0.2 to 0.449 < 0.2 pg/L pg/L Uranium-233 30 pg/L < 0.0015 pg/L < 0.0015 pg/L Uranium-234 (combined) (combined) (combined) Uranium-236 Notes: 1. pCi/L = Picocuries per liter 2. pg/L = micrograms per liter 3. < indicates concentration less than laboratory reporting limit. 4. J = Estimated concentration 5. MCL = Maximum Contaminant Level 6. * USEPA MCL for uranium of 30 pg/L assumes combined concentration for all isotopes. As shown in Table 2-4, concentrations of radium-226 and radium-228 are lower in background monitoring wells compared to the concentrations reported in the downgradient monitoring well. Uranium concentrations are higher in background monitoring wells compared to concentrations reported downgradient of the source area. Uranium-223, uranium-234, and uranium-236 were not reported at concentrations greater than the laboratory reporting limits at any of the locations sampled. Based on a review of available radiological data, additional data for radionuclides at the site are needed for a more comprehensive assessment and may be required as part of a post remedial monitoring plan to be completed in CAP Part 2. Seeps Seep samples associated with the active ash basin were identified from locations at the toe of the active ash basin downstream dam adjacent to the Catawba River (seeps S-2 through S-8). Seep sample S-9 was collected from a location at the toe of the inactive basin downstream dam adjacent to the Catawba River. Seep S-1 is located near the south property boundary, southeast of existing compliance monitoring well AB-11 D. Water sufficient for sampling was only present in seeps S-3 and S-4. Seeps S-1, S-2, and S-5 through S-9 were dry during multiple sampling attempts. In March 2014, NCDENR conducted sampling of 15 locations associated with seeps, surface water, stormwater outfalls, and NPDES permitted outfalls. Nine of the 15 NCDENR March 2014 sample locations (ANSP001, ANSP002, ANSP003, ANSP005, ANSW015, ANTD001, ANWW002, ANWW004, and ANFD001) are seeps or surface water sample points potentially influenced by the ash basin system. 30 Corrective Action Plan Part 1 Allen Steam Station Ash Basin Five of the NCDENR March 2014 sample locations were resampled during the CSA activities: ANSW015, ANWW001, ANWW002, ANWW003, and ANWW004. The remaining sample locations were dry during the sampling activities. Seep sample locations and analytical results are depicted on Figure 2-2. With the exception of ANSW015, the remaining NCDENR March 2014 sample locations are discharges associated with the Allen site NPDES permit. This includes ANWWO01 (NPDES 001), ANWWO02 (NPDES 002), ANWWO03 (NPDES 003), and ANWWO04 (NPDES 004). Background comparison for seeps is not appropriate since they are hydraulically and topographically downgradient of the ash basin and ash storage area. Seep exceedance results for COls, along with a comparison to 2L Standards, IMACs, and DHHS HSLs, are provided in Table 2-5.1. Seep sample results (S-3, S-4, ANSW015) were compared to 2L Standards or IMACs (as appropriate) because the seeps were emanating from the ground and samples were representative of groundwater conditions. Table 2-5.1. Seep Exceedance Results for Cols Compared to 2L Standards, IMACs or DHHS HSLs and Frequency of Exceedances NC 2L Standard Concentrations Number of Samples COI or IMAC Exceeding 2L Standards Exceeding 2L Standards (pg/L) or IMACs or IMACs/Number of (Range, pg/L) Samples Allen Steam Station Seeps Boron 700 800 1 /3 Cobalt* 1 1.1 to 1.2 2/3 Iron 300 1,900 to 2,200 2/3 Manganese 50 160 to 890 3/3 Vanadium* 0.3 0.94J to 1.3 3/3 Notes: 1. pg/L = micrograms per liter 2. SU = Standard Units 3. J = Laboratory estimated concentration 4. DHHS indicates the North Carolina Department of Health and Human Services. * Indicates 2L Standard not established for constituent; therefore, IMAC used for screening criteria. Observations related to seep COls observed in samples collected from the toe of the respective ash basin dams at Allen are: Cobalt was reported in two seep samples (S-3 and S-4) at concentrations of 1.2 pg/L and 1.1 pg/L, respectively. The corresponding turbidity measurements of these two samples were 8.46 NTU and 5.33 NTU, respectively. The IMAC for cobalt is 1.0 pg/L. The dissolved fraction result for both these samples was not detected at a concentration greater than the laboratory method detection limit (<0.5 pg/L). Based on the dissolved cobalt concentration, cobalt is not considered a COI for further evaluation. Iron was reported in two seep samples (S-3 and S-4) at concentrations of 2,200 pg/L and 1,900 pg/L, respectively. The corresponding turbidity measurements of these two samples were 8.46 NTU and 5.33 NTU, respectively. The 2L Standard for iron is 300 31 Corrective Action Plan Part 1 Allen Steam Station Ash Basin j pg/L. The dissolved fraction result for both these samples was not detected at a concentration greater than the laboratory method detection limit (<50 pg/L). Based on the dissolved iron concentration, iron is not considered a COI for further evaluation. The following seep -related COls will be considered for further evaluation: • Boron • Manganese • Vanadium In instances where a seep was observed flowing from a stormwater outfall (ANWW001, ANWW002, ANWW003, and ANWW004), sample results were compared to 2B Standards. (Table 2-5.2). Some stormwater outfalls (i.e. ANWW002) may be routed to the ash basin for treatment. An attempt to identify an on -site background surface water sample location will be made to establish background surface water concentrations relative to the seeps. Table 2-5.2. Seep Exceedance Results for Cols Compared to 2113 Standards or USEPA Criteria and Frequency of Exceedances COI NC 26 Standard or USEPA Criteria* (pg/L) Concentrations Exceeding 213 Standards or USEPA Criteria* (Range, pg/L) Number of Samples Exceeding 213 Standards or USEPA Criteria/Number of Samples Allen Steam Station Seeps Aluminum 87 98J 1/4 Copper 2.7 3.6J+ to 14.5 4/4 Lead 0.54 0.67 1/4 Manganese* 50 50 to 100 2/4 Mercury 0.012 0.16J+ 1/4 TDS 250,000 582,000 1/4 Notes: 1. tag/L = micrograms per liter 2. SU = Standard Units 3. J = Laboratory estimated concentration 4. J+ = Estimated concentration, biased high * Indicates 2B Standard not established for constituent; therefore, USEPA National Recommended Water Quality Criteria used for screening criteria. Although exceedances of the 2B Standards were noted in seep samples where the seep was observed flowing from a stormwater outfall (ANWW001 [NPDES 001], ANWWO02 [NPDES 002], ANWWO03 [NPDES 003], and ANWWO04 [NPDES 004]), HDR does not consider the stormwater outfall related to the plant as seep results to represent surface water. 32 Corrective Action Plan Part 1 Allen Steam Station Ash Basin The following ANWW002 (NPDES 002) seep -related COls will be considered for further evaluation: • Copper • Manganese • TDS Surface Water Surface water samples were obtained during the CSA from four locations within the onsite ash basin ponds. These locations include: one location from Primary Pond 2, one location from Primary Pond 3, and two locations from the active discharge pond. 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 for Class C and WS-V waters. In the absence of a 2B Standard, constituent concentrations were compared to USEPA National Recommended Water Quality Criteria. In addition, due to the nature of the surface water locations at the Allen site (i.e. directly from onsite ash basin ponds) the samples were also compared to applicable 2L Standards or IMAC guidelines (due to potential groundwater infiltration). No surface water samples representative of background conditions were collected during the first round sampling event. Attempts will be made to collect on -site background surface water samples and additional downgradient surface water locations during the second round sampling event. Surface water exceedance results for COls compared to applicable regulatory standards, are provided in Table 2-6.1 and Table 2-6.2. Surface water sample locations and analytical results are depicted on Figure 2-2. 33 Corrective Action Plan Part 1 �� Allen Steam Station Ash Basin Table 2-6.1. Surface Water Exceedance Results for COls Compared 213 or USEPA Standards, and Frequency of Exceedances NC 2B Standard or Concentrations Number of Samples COI EPA Criteria Exceeding 2B Exceeding 2B (pg/L) Standards Standard/Number of (Range, lag/L) Samples Ash Basin Surface Water Aluminum* 87 380 to 470 2/4 Copper 2.7 4.1 J+ to 5.7 3/4 Lead 0.54 0.74 1 /4 Manganese 50 96 to 150 3/4 Mercury 0.012 0.012J+ to 0.13J+ 4/4 TDS 250,000 410,000 to 2/4 418,000 Notes: 1. pg/L = micrograms per liter 2. mg/L = milligrams per liter 3. SU = Standard Units 4. J = Laboratory estimated concentration. 5. >_5 represents the minimum acceptable DO concentration for freshwater aquatic life. * Indicates USEPA National Recommended Water Quality Criteria used for constituent Table 2-6.2. Surface Water Exceedance Results for COls Compared 2L or IMAC, and Frequency of Exceedances COI NC 2L Standard or IMAC (lag/L) Concentrations Exceeding 2L Standards or IMACs (Range, lag/L) Number of Samples Exceeding 2L Standards or IMACs/Number of Samples Ash Basin Surface Water Boron 700 1,800 to 1,900 2/4 Cobalt 1 2.2 1 /4 Hexavalent Chromium 0.07 0.082J- to 0.089J- 2/4 Iron 300 820 1 /4 Manganese 50 96 to 150 3/4 Vanadium* 0.3 0.7J to 8.2 4/4 Notes: 1. Ng/L = micrograms per liter 2. mg/L = milligrams per liter 3. SU = Standard Units 4. J = Laboratory estimated concentration. Indicates 2L Standard not established for constituent; therefore, IMAC used for screening criteria. Observations related to surface water COls listed at Allen are: • Aluminum, boron, cobalt, copper, iron, lead, manganese, mercury, TDS, and vanadium require additional sampling and analysis to determine if surface water COls should remain under consideration as a COI, and therefore these constituent cannot be ruled out as a COI as part of the CAP Part I. Further lack of comparison to background or upstream surface water concentrations of constituents limits interpretation. 34 Corrective Action Plan Part 1 �� Allen Steam Station Ash Basin 2.5 Sediments Sediment samples were collected at the same time as the seep samples and NCDENR- identified seep samples. These locations include seep/sediment locations S-1 through S-7. A sediment sample was not collected at S-8 because this location is at the end of a concrete NPDES discharge culvert. Location S-9 was not sampled due to safety (i.e. Catawba River water's edge and surge rip rap) and accessibility concerns (i.e. barbed wire fencing). In the absence of NCDEQ sediment criteria, the sediment sample results were compared to NC PSRGs for Protection of Groundwater (POG). Sediment exceedances results for Cols, along with a comparison to NC PSRGs for POG are provided in Table 2-7. Sediment sample locations and analytical results are depicted on Figure 2-3. No background comparisons are available for sediment sample locations because they are hydraulically and topographically downgradient of the ash basin and ash storage areas. Table 2-7. Sediment COls Exceedances Compared to Upstream Sediment Concentrations, NC PSRGs for POG and Frequency of Exceedances Concentrations Number of Samples COI NC PSRGs for POG Exceeding NC PSRGs Exceeding NC (mg/kg) (Range, mg/kg) PSRGs/Number of Samples Sediment Cobalt 0.9 5AJ to 30.1 7/7 Iron 150 4,520 to 26,500 7/7 Manganese 65 119 to 1,510 7/7 Vanadium 6 11.1 to 65.3 7/7 Notes: 1. mg/kg = milligrams per kilogram 2. J = Laboratory estimated concentration. 3. < indicates concentration less than laboratory method detection limit. Observations related to sediment COls at Allen are: • There are not enough data to establish a statistically viable PPBC for these constituents in sediment. • In addition, there are no upgradient or background sediment sample locations to further refine the COls. Based on comparison to NC PSRGs for POG, the following sediment COls will be considered for further evaluation: • Cobalt • Iron • Manganese • Vanadium 35 Corrective Action Plan Part 1 �� Allen Steam Station Ash Basin 2.6 Soils 2.6.1 Background Soil and Concentrations Because some constituents are naturally occurring in soil and are also present in the source areas, establishing background concentrations is important for determining whether releases have occurred from the source areas. Boring locations that have been determined to represent background conditions (see Section 2.1.1) from which background soil samples were collected include: BG-1 S, BG-1 D, and BG-3D (Figure 1-4). Samples shallower than 5 feet below ground surface (bgs) were not included in the population of background samples to minimize possible surface impacts. A review of site geology determined that the soils are from the same geologic formations and thus could be pooled as a single population. Partially weathered rock (PWR) and bedrock samples were not included in the calculations for soil background statistics, because the mineralogy may be different. Soil PPBCs (i.e., the 95% upper tolerance limit [UTL]) were calculated for those constituents analyzed in background soil borings, as shown in Table 2-8. The methodology followed ProUCL Technical Guidance, Statistical Software for Environmental Applications for Data Sets with and without Nondetect Observations (USEPA 2013a). A detailed method review, statistical evaluation, and results for the PPBCs are included in Appendix B. For COls where there were too few detections reported to use the statistical methodology, the PPBCs were established by setting the value equal to the greatest reported concentration or the greatest non -detect value. 36 Corrective Action Plan Part 1 �� Allen Steam Station Ash Basin Table 2-8. Proposed Provisional Background Soil Concentrations Constituent Number of Samples Number of Detections Range (mg/kg) Proposed Provisional Background Soil Concentrations (95% UTL) (mg/kg) Aluminum 16 16 2,500-21,700 27,000 Antimony 16 0 <1.2-<7.6 7.6* Arsenic 16 1 0.68J-<7.6 7.6* Barium 16 16 26.3-269 324 Beryllium 16 15 0.071-1.6 1.54 Boron 16 4 <2.9-48.2 40.1 Cadmium 16 1 <0.14-<0.92 0.92* Calcium 16 14 59.8-11,100 10,500 Chloride 16 0 <291-<363 363* Chromium 16 15 1.0-58.1 106 Cobalt 16 15 3.9-28.7 33.2 Copper 16 16 3.2-55.7 62.3 Iron 16 16 6,400-53,600 59,800 Lead 16 11 1.6-31.6 25.5 Magnesium 16 16 433-14,400 17,200 Manganese 16 16 172-1340 1,360 Mercury 16 3 0.0071-0.016 0.016* Molybdenum 16 1 <0.59-<3.8 3.8* Nickel 16 13 0.78-29.2 69.5 Nitrate 16 0 <29.1-<36.3 36.3* pH (field) 16 16 5.3-7.5 5.3-7.5* Potassium 16 15 <339-10,100 5,950 Selenium 16 1 <1.2-<7.6 7.6* Sodium 16 5 33.3-<384 284 Strontium 16 14 1.3-62.8 63.9 Sulfate 16 0 <291-<363 363* Thallium 16 0 <1.2-<7.6 7.6* TOC 15 6 415-3,470 2,090 Vanadium 16 16 15.3-143 142 Zinc 16 16 13.2-88.2 105 Notes: 1. mg/kg = milligrams per kilogram 2. UTL — Upper Tolerance Limit 3. < indicates less than the laboratory method detection limit Value shown is the greatest detection or greatest non -detect (<) value. In these cases, there were too few detections to develop UTL. Ranges associated with zero detections indicate the range of detection limits. 37 Corrective Action Plan Part 1 �� Allen 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 handling and storage was delineated in the CSA Report. Soil exceedance results for COIs, 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. Table 2-9. Soil Exceedance Results for COls Compared to NC PSRGs for POG, PPBCs, and Frequency of Exceedances Constituent NC PSRGs for POG (mg/kg) Background Soil Concentrations (Range, mg/kg) Proposed Provisional Background Concentrations (mg/kg) Concentrations Exceeding NC PSRGs for POG (Range, mg/kg) Number of Samples Exceeding NC PSRGs for POG/Number of Samples Beneath the Active Ash Basin Arsenic 5.8 0.68J to <8.7 7.6 5.8 to 36 3/20 Barium 580 20.5 to 269 324 1,420 1/20 Cobalt 0.9 3.9 to 28.7J- 33.2 2.7J to 931 20/20 Iron 150 6,400 to 53,600 59,800 15,400 to 43,900 20/20 Manganese 65 172 to 1,340 1,360 157 to 19,500 19/20 Selenium 2.1 <1.2 to 7.6 7.6 2AJ to 5 4/20 Vanadium 6 15.3 to 143 L 142 42.7 to 104 20/20 Beneath the Inactive Ash Basin (including Ash Storage, Structural Fill and Retired Ash Basin) Arsenic 5.8 0.68J to <8.7 7.6 11.3 to 39.8 2/33 Barium 580 20.5 to 269 324 660 1/33 Boron 45 <2.9 to 48.2J 40.1 45.7 to 78.5 J 4/33 Cobalt 0.9 3.9 to 28.7J- 33.2 2 to 82.3 31/33 Iron 150 6,400 to 53,600 59,800 4820 to 73,400 33/33 Manganese 65 172 to 1,340 1,360 121 to 1,710 30/33 Selenium 2.1 <1.2 to 7.6 7.6 3.71J to 7 3/33 Vanadium 6 15.3 to 143 142 13.2 to 101 33/33 Outside of Ash Basin Cobalt 0.9 3.9 to 28.7J 33.2 4.7J to 39.8 39/41 Iron 150 6,400 to 53,600 59,800 13,500 to 47,600 41/41 Manganese 65 1,360 1,360 131 to 1,990 41/41 Selenium 2.1 <1.2 to 7.6 7.6 2.5J to 3.5J 4/41 Vanadium 6 15.3 to 143 142 30.5 to 136 41/41 Notes: 1. mg/kg = milligrams per kilogram 2. J = Laboratory estimated concentration. 3. J- = Estimated concentration, biased low. 4. < indicates concentration less than laboratory method detection limit. 5. NC PSRG for POG indicates the North Carolina Preliminary Soil Remediation Goal for Protection of Groundwater " NC PSRG for POG is for hexavalent chromium, soil analytical results are for total chromium. 38 Corrective Action Plan Part 1 �� Allen Steam Station Ash Basin Observations related to soil COls at Allen are: • Arsenic, boron, cobalt, iron, manganese, selenium and vanadium appear in at least one or more of the background soil boring locations at concentrations exceeding the respective NC PSRGs for POG. Barium is the only exception of these listed COls. Where reported at concentrations greater than the laboratory method detection limit, arsenic, barium, boron, cobalt, iron, manganese, selenium, and vanadium exceed their respective NC PSRGs for POG and are potentially attributable to ash handling and storage at the site. • As seen above, PPBCs for arsenic, cobalt, iron, manganese, selenium and vanadium are greater then their respective PSRGs for POG. If the PPBCs are approved for the Allen site, the following COls in soil would no longer be considered for further evaluation: o Selenium o Vanadium The following COls exceed the soil PSRGs for POG and will be considered COls for further evaluation: • Arsenic • Barium • Boron • Cobalt • Iron • Manganese • Selenium • Vanadium PPBCs were determined to be greater than the PSRGs for POG for the following constituents: • Arsenic • Cobalt • Iron • Manganese • Selenium • Vanadium Pending approval of the PPBC concentrations for these constituents by NCDEQ, PPBCs for the constituents listed above will be used for identifying soil exceedances of COls instead of the PSRGs for POG during future sampling events. For PPBCs determined to be less than the PSRGs for POG, the respective regulatory standard for that constituent will continue to be used for determining exceedances. Further evaluation may result in revisions to the list above and will be addressed in the CAP Part 2 report. 39 Corrective Action Plan Part 1 �� Allen Steam Station Ash Basin 2.7 Ash Ash samples were collected and analyzed from the ash management areas during the CSA. COls identified in ash characterize the source material from which COls were evaluated with respect to releases from the ash management areas. Ash exceedance results for COls, along with a comparison to NC PSRGs for POG, soil PPBCs, and background concentrations are provided in Table 2-10. Ash sample locations and analytical results are depicted on Figure 2-3. Table 2-10. Ash Exceedance Results for COls Compared to NC PSRGs for POG, PPBCs, and Frequency of Exceedances Ash Number of Proposed NC PSRG Concentrations Samples Provisional Protection of Exceeding NC Exceeding 2L COI Background Groundwater PSRG Protection Standards, IMACS Concentrations (mg/kg) of Groundwater or DHHS (mg/kg) (Range, mg/kg) HSL/Number of Samples Beneath the Active Ash Basin Arsenic 7.6 5.8 19.6 to 58.3 15/15 Boron 40.1 45 46.9 1 /15 Cobalt 33.2 0.9 2.1 to 13.4 15/15 Iron 59,800 150 3,580 to 15,900 15/5 Manganese 1,360 65 72.3 to 202 3/15 Selenium 7.6 2.1 2.3 to 19.2 11/15 Vanadium 142 6 9.8 to 124 15/15 Beneath the Inactive Ash Basin (including Ash Storage, Structural Fill and Retired Ash Basin) Arsenic 7.6 5.8 10.3 to 105 26/29 Barium 324 580 784 1/29 Boron 40.1 45 46.3 1 /29 Cobalt 33.2 0.9 2.1 to 16.9 23/29 Iron 59,800 150 4,010 to 31,400 29/29 Manganese 1,360 65 71.3 to 360 15/29 Selenium 7.6 2.1 2.3 to 19.2 18/29 Vanadium 142 6 9.4 to 101 28/29 Notes: 1. mg/kg = milligrams per kilogram 2. NC PSRG for POG indicates the North Carolina Preliminary Soil Remediation Goal for Protection of Groundwater Observations related to ash COIs at Allen are: Arsenic, barium, boron, cobalt, iron, manganese, selenium, and vanadium appear in at least one or more of the ash sample locations and/or depths at concentrations exceeding the respective NC PSRGs for POG. As seen above, PPBCs for arsenic, cobalt, iron, manganese, selenium and vanadium are greater then their respective PSRGs for POG. The following COls were measured at concentrations ranging in exceedance of their respective PSRG for POG and PPBCs in one or more sample location: 40 Corrective Action Plan Part 1 �� Allen Steam Station Ash Basin o Arsenic o Barium o Boron o Selenium For PPBCs determined to be less than the PSRGs for POG, the respective regulatory standard for that constituent will continue to be used for determining exceedances. Further evaluation may result in revisions to the list above and will be addressed in the CAP Part 2 report. 2.8 Porewater Porewater refers to water samples collected from monitoring wells installed in the ash basin and ash storage areas that are screened within the ash layer. Porewater COls are representative of the source (CCR), but not representative of groundwater conditions. Porewater exceedance results for COls, along with a comparison to applicable regulatory standards or guidelines, are provided in Table 2-11. Note that until PPBCs are approved by NCDEQ, COI concentrations will be compared to their applicable 2L Standard or IMAC. At this time, PPBCs are shown on the table for reference purposes only. Porewater sample locations and analytical results are depicted on Figure 2-1. 41 Corrective Action Plan Part 1 FN Allen Steam Station Ash Basin Table 2-11. Porewater Exceedance Results for COls Compared to 2L Standards, IMACs, or DHHS HSLs, and Frequency of Exceedances Constituent Proposed Provisional Background Concentrations (pg/L) NC 2L Standard, IMAC, or DHHS HSL (lag/L) Porewater Concentrations Exceeding 2L Standards, IMACs, or DHHS HSL (Range, pg/L) Number of Samples Exceeding 2L Standards, IMACs or DHHS HSL/Number of Samples Beneath the Active Ash Basin Antimony* 0.5 1 1.3 to 9.9 3/10 Arsenic 2.3 10 20.9 to 1,370 10/10 Boron 50 700 960 to 7,400 6/10 Cobalt* 0.74 1 2.1 to 42.3 5/10 Iron 960 300 620 to 25,200 5/10 Manganese 38 50 59 to 4,400 9/10 pH (SU) 5.89 to 10.2 SU 6.5-8.5 SU 5.95 to 8.8 SU 4/9 Sulfate 30,300 250,000 390,000 1/10 TDS 198,00 500,000 527,000 to 1,120,000 3/10 Thallium* 0.1 0.2 0.44 J+ to 0.71 2/10 Vanadium* 22.5 0.3 0.99 J to 47.4 9/10 Beneath the Inactive Ash Basin (including Ash Storage, Structural Fill and Retired Ash Basin) Antimony* 0.5 1 1.2 to 10.3 2/8 Arsenic 2.3 10 10.9 to 636 6/8 Boron 50 700 830 to 2,400 4/8 Hexavalent Chromium** Not determined 0.07 0.66 1/7 Cobalt* 0.74 1 1.1 to 17.6 4/8 Iron 960 300 370 to 60,700 6/8 Manganese 38 50 970 to 10,900 5/8 pH (SU) 5.89 to 10.2 SU 6.5-8.5 SU 5.9 to 10.32 SU 2/6 Sulfate 30,300 250,000 313,000 1/8 TDS 198,000 500,000 758,000 1/8 Vanadium* 22.5 0.3 1.1 to 70.8 7/8 42 Corrective Action Plan Part 1 Allen Steam Station Ash Basin Observations related to Allen porewater COls are listed below: Hexavalent chromium was analyzed in multiple monitoring wells across the site and will need further evaluation as to prevalence at the Allen site. Additional sampling and analysis is required to determine if hexavalent chromium should remain as a COI, and therefore this constituent cannot be ruled out as a COI as part of the CAP Part 1. • Iron, manganese, and TDS were reported at concentrations greater than their respective 2L Standards. Iron and manganese were reported in multiple porewater samples while TDS was only reported in three source area locations. The geochemical model is key to understanding mobility of iron, manganese, and TDS since it cannot adequately be modeled using MODFLOW/MT3DMS. • Remaining identified COls (antimony, arsenic, boron, cobalt, pH, sulfate, thallium, and vanadium) exceeded their respective 2L Standards or IMAC within the source area location. Additional sampling and analysis is required to determine if these constituents should remain on the COI list; therefore, these constituents cannot be ruled out as COls as part of the CAP Part 1. The following porewater COls will be considered for further evaluation: • Antimony • Arsenic • Boron • Cobalt • Hexavalent chromium • Iron • Manganese • pH • Sulfate • Thallium • TDS • Vanadium Pending approval of the PPBC concentrations for these constituents by NCDEQ, PPBCs for the constituents listed above will be used for identifying porewater quality exceedances of COls instead of the 2L Standards, IMACs, or DHHS HSLs during future sampling events. For PPBCs determined to be less than the 2L Standards, IMAC, or DHHS HSL, the respective regulatory standard for that constituent will continue to be used for determining exceedances. 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. 43 Corrective Action Plan Part 1 �� Allen Steam Station Ash Basin 2.10 COI Screening Evaluation Summary Table 2-12 summarizes COls (by media) identified in Sections 2.1 through 2.9 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 2-12. Updated COI Screening Evaluation Summary Potential COI CSA Ground- water COI Exceedance by Media COI To Be Further PWR/ Assessed Bedrock in Section 3.0 Yes _ Yes Yes - Yes - Yes Solid/ Aqueous Ash Pore- Water 2 Surface Water ti'' Seeps - - - Sediment Soil Aluminum - Antimony _ _ Arsenic Barium - - Beryllium - Boron Yes Cadmium - - Yes Chromium - - - Yes Hexavalent Chromium _ _ _ _ _ Yes Cobalt Yes Copper Yes Iron (3) Yes Lead - - - - - - - Yes Manganese Yes Mercury - - Yes Nickel - - Yes H 3 - - Yes Selenium - - - Yes Sulfate - - - - - - Yes Thallium - - - - Yes TDS 3 - - - - - Yes Vanadium Yes Zinc - - - - - - Yes Notes: ' Ash is not evaluated for remediation in this CAP because ash will be drained of water during remedial activities and capping. 2. Porewater is not evaluated for remediation in this CAP because porewater will be eliminated during ash basin closure activities. 3. The geochemical model is key to understanding mobility of iron, manganese, pH and TDS since it cannot adequately be modeled using MODFLOW/MT31DMS. 44 Corrective Action Plan Part 1 _ Allen Steam Station Ash Basin 2.11 Interim Response Actions 2.11.1 Source Control Duke Energy is required by CAMA to close the Allen ash basin no later than August 1, 2029 or as otherwise dictated by NCDEQ risk classification. Closure for the Allen ash basin was not defined in CAMA. CAMA requires that corrective action be implemented to restore groundwater quality where the CSA documents exceedances of groundwater quality standards. No interim response actions are necessary at the Allen site because there are no identified imminent hazards to human health or the environment. 2.11.1.1 Groundwater Response Actions Based on the results of CSA activities, impacted groundwater has not migrated beyond the Duke Energy property boundary north, south or west of the Allen site. Potential impacts east, towards the Catawba River, will be further assessed pending results of the second round of surface water sampling to determine if there are potential impacts to the Catawba River resulting from groundwater inflow to the river. Results of groundwater to surface water modeling are presented in Section 4.2. As recommended in the CSA report, additional on- and off -site groundwater monitoring wells will be installed and sampled to further evaluate background groundwater, groundwater flow, and groundwater impacts from source areas located at the Allen site. These monitoring wells are planed to be installed on Duke Energy property or Department of Transportation public right-of- ways. 45 Corrective Action Plan Part 1 �� Allen Steam Station Ash Basin 3 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 Allen site. The SCM was developed utilizing data and analysis from the CSA Report. The sources and areas with 2L Standard, IMAC or DHHS HSL exceedances of COls attributable to ash handling are illustrated in the 3-D SCM presented on Figure 3-1 and in cross -sectional view on Figure 3-2.1 and 3-2.2. 3.1 Site Hydrogeologic Conditions Site hydrogeologic conditions were evaluated through the installation and sampling of 80 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 Allen site, as well as on-site/near-site geologic mapping, to further understand the site geology in support of the SCM. 3.1.1 Hydrostratigraphic 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 ponds and ash storage areas, as well as in some borings advanced through the pond perimeter and dikes. Ash was generally described as gray to dark gray, highly plastic with non -plastic dry to wet silty to sandy texture, consistent with fly ash and bottom ash. The range of ash thickness observed at the Allen site was 0 to 54.9 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, clayey sand, and sand with clay and gravel on the boring logs. Fill was used in the construction of dikes, and as cover for ash storage areas. The range of fill thickness observed at the Allen site was 0 to 84.5 feet. • Alluvium (S) — Alluvium was encountered in borings located along the Catawba River in historic stream tributaries during the project subsurface exploration. Alluvium was classified as a well -sorted medium fine-grained sand, sand with silt and gravel, and silty gravel. The range of alluvium thickness observed at the Allen site was 0 to 34.5 feet. • Residuum (Residual soils) (M1) — Residuum is in -place weathered soil that consists primarily of micaceous silty sand, micaceous silt, and clayey sand. Residuum thickness varies across the site from a very thin layer where weathered soil is near the surface to as much as 82 feet in other areas. The range of residuum thickness observed at the Allen site was 0 to 82 feet. 46 Corrective Action Plan Part 1 �� Allen Steam Station Ash Basin Saprolite/Weathered Rock (M2) — Saprolite is soil developed by in -place weathering of rock that retains remnant bedrock structure. Saprolite consists primarily of dense to very dense silty sand, sand, silty sand, sand with gravel, noted as micaceous in some boring logs and not in others. Saprolite thickness varies across the site from a very thin mantle where bedrock is near the surface to as much as 75 feet in other areas. Partially Weathered/Fractured Rock (Transition Zone) — Partially weathered (slight to moderate) and/or highly fractured rock encountered below refusal (auger, casing advancer, etc.) is defined as the transition zone. The range of transition zone thickness observed at the Allen site was 0 to 33 feet. • Bedrock (BR) — Sound rock in boreholes, generally fresh to slightly weathered and relatively unfractured was defined as bedrock. The maximum depth that borings extended into bedrock was 73 feet. Based on the site investigation conducted as part of the CSA, the groundwater system in the natural materials (alluvium, soil, soil/saprolite, PWR, and bedrock) is consistent with the regolith- fractured rock system and is characterized as an unconfined, connected aquifer system. The groundwater system beneath the Allen site is divided into the following three layers to distinguish the connected aquifer system: the shallow flow layer, deep flow layer, and the bedrock flow layer. Hydrostratigraphic layers are shown on cross -sections presented in the CSA Report. 3.1.2 Hydrostratigraphic 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 values. These properties were obtained through in -situ permeability testing (falling head, constant head, and packer testing where appropriate); slug tests in completed monitoring wells; and laboratory testing of undisturbed samples (ash, fill, soil/saprolite). Results from these tests were used to develop the groundwater flow and fate and transport model further discussed in Section 4. 3.1.3 Potentiometric Surface — Shallow Flow Layers The shallow flow layer was defined by data obtained from the shallow groundwater monitoring wells (S wells) and surface water elevations. In general, groundwater within the shallow flow layer flows from the western and southern extent of the Allen site property boundary east and southeast toward the Catawba River, as well as north and northeast along Plant Allen Road (Duke Energy plant access) towards the Station Discharge Canal and administration buildings. However, in the area northwest of AB-1 R and south of the Station Discharge Canal, groundwater elevation data indicate the likely presence of a groundwater divide extending from BG-2 south along South Point Road. The Station Discharge Canal forms a groundwater divide, 47 Corrective Action Plan Part 1 Allen Steam Station Ash Basin and groundwater flow in the vicinity of BG-3S on its north side, flows southeast. Additional groundwater elevation data are needed to confirm flow direction in this area. Figure 3-3 shows the potentiometric surface of the shallow flow layer. 3.1.4 Potentiometric Surface — Deep Flow Layer The deep flow layer was defined by data obtained from the deep groundwater monitoring wells (D wells) and the upper bedrock (BRU wells) groundwater monitoring wells. In general, groundwater within the deep flow layer (similar to surface flow layer) flows from the western and southern extent of the Allen site property boundary east and southeast toward the Catawba River, as well as north and northeast to the Duke Energy property and the Station Discharge Canal. Additional groundwater elevation data are needed to confirm flow direction in this area and along South Point Road. Figure 3-4 shows the potentiometric surface of the deep flow layer. 3.1.5 Potentiometric Surface — Bedrock Flow Layer The bedrock flow layer is defined by data obtained from the bedrock groundwater monitoring wells (BR wells). In general, groundwater within the bedrock flow layer is consistent with observed flow directions in the shallow and deep flow layers, predominately eastward toward the Catawba River. Only one bedrock well is located in the northwest portion of the site. Additional groundwater elevation data are needed to confirm groundwater flow direction in this area and along South Point Road. Figure 3-5 shows the potentiometric surface of the bedrock flow layer. 3.1.6 Horizontal and Vertical Hydraulic Gradients 3.1.6.1 Horizontal Hydraulic Gradient 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: 0.020 • Deep flow layer: 0.033 • Bedrock flow layer: 0.030 3.1.6.2 Vertical Hydraulic Gradients Vertical hydraulic gradients were calculated for 40 shallow (S) and deep (D and BRU) well pairs and 8 deep and bedrock (D and BR) by taking the difference in groundwater elevation in each well pair over the vertical difference between the well screen midpoints (Table 3-1 and 3-2). A positive value indicates potential upward flow (higher hydraulic head with depth) and a negative value indicates potential downward flow (lower hydraulic head with depth). Vertical hydraulic gradients between the shallow and deep flow layers are presented on Figure 3-6, and the vertical gradients between the deep and bedrock flow layers are presented on Figure 3-7. 48 Corrective Action Plan Part 1 �� Allen Steam Station Ash Basin Table 3-1. Vertical Gradient Calculations for Shallow/Deep Well Pairs Shallow Well Deep Well Vertical Gradient (ft./ft.) Shallow Well Deep Well Vertical Gradient (ft./ft.) AB-10S AB-10D -0.0004 AB-36S AB-36D 0.002 AB-12S AB-12D 0.019 AB-37S AB-37D 0.002 AB-13S AB-13D 0.004 AB-38S AB-38D -0.009 AB-20S AB-20D -0.015 AB-39S AB-39D 0.003 AB-21 S AB-21 D 0.001 A134S A134D 0.005 AB-22S AB-22D -0.113 AB-9S AB-9D 0.032 AB-23S AB-23BRU -0.340 BG-1 S BG-1 D 0.002 AB-24S AB-24D -0.023 BG-2S BG-2D -0.051 AB-25S AB-25BRU -0.038 BG-3S BG-3D -0.005 AB-26S AB-26D 0.017 GWA-14S GWA-14D -0.002 AB-27S AB-27D -0.221 GWA-15S GWA-15D -0.032 AB-28S AB-28D -0.079 GWA-1 S GWA-1 D -0.020 AB-29S AB-29D 0.014 GWA-2S GWA-2D 0.035 AB-2S A13-2D -0.010 GWA-3S GWA-3D 0.101 AB-30S AB-30D 0.022 GWA-4S GWA-4D -0.010 AB-31 S AB-31 D 0.016 GWA-5S GWA-5D 0.040 AB-32S AB-32D -0.104 GWA-6S GWA-6D -0.133 AB-33S AB-33D -0.057 GWA-7S GWA-7D -0.052 AB-34S AB-34D -0.007 GWA-8S GWA-8D 0.031 AB-35S AB-35D 0.001 GWA-9S GWA-9D -0.0002 Table 3-2. Vertical Gradient Calculations for Deep/Bedrock Well Pairs Deep Well Bedrock Well Vertical Gradient (ft./ft.) AB-21 D AB-21 BR -0.002 AB-25BRU AB-25BR 0.042 AB-35D AB-35BR 0.002 BG-2D BG-2BR -0.053 GWA-1 D GWA-1 BR -0.0003 GWA-3D GWA-3BR -0.001 GWA-5D GWA-513R -0.003 GWA-6D GWA-6BR -0.085 Notes: 1. Vertical Gradients = AWE/ABS (AMSE), where A implies bedrock to deep, WE is water elevation, and MSE is mid -screen. 2. Positive gradient implies potential upward flow. 3. Depth to Water measurements gauged on June 2015. Comparison of vertical gradients between the shallow and deep flow layers: Active ash basin - Negative vertical gradients were predominately noted in well pairs in the active basin (i.e. western, northern and eastern portions), with the exception of the AB-21 well pair in the south portion, which has a neutral to positive vertical gradient. 49 Corrective Action Plan Part 1 �� Allen Steam Station Ash Basin Along the northern dike (AB-27 and AB-28), negative vertical gradients were reported while positive and negative gradients were reported along the eastern dike (AB-26 and AB-22, respectively). Well pairs downgradient of the active ash basin downstream dam show predominantly upward gradients near the Catawba River. This would be expected in this location because the ash basin is higher than the ground surface at the base of the dike which creates hydraulic pressure at the wells downgradient of the dikes. • Inactive ash basin — Negative vertical gradients were noted throughout the majority of the northern extent of the inactive basin indicating potential downward groundwater flow while positive gradients were reported in the eastern and southern portions of the inactive basin. The RAB Landfill is capped and should not present a recharge point to the inactive ash basin. o Ash storage area — Negative vertical gradients were noted in the northern portion of the ash storage area while gradients in the southern portion were neutral to positive (wetlands were present near AB-36 and AB-37). o Structural fill — Neutral to positive vertical gradients were noted in the structural fill portions of the ash storage area. Groundwater under the structural fill flows from the west side of the inactive ash basin. No dams are located in this vicinity that would cause an increase in hydraulic pressure at these wells. In general the ash storage area shows upward groundwater movement within the structural fill area. • Background monitoring wells — Negative vertical gradients were noted in the background wells northwest of the inactive ash basin near the Station Discharge Canal. The background wells located southwest of the active ash basin exhibited a neural to positive vertical gradient. Comparison of vertical gradients between the deep and bedrock flow layers: • Active ash basin —The well pair (AB-25) near Primary Ponds 2 and 3 had a positive vertical gradient while the well pair (AB-21) (central portion of the active ash basin) had a neutral to positive gradient. Both downgradient well pairs, GWA-3 (active ash basin downstream dam) and GWA-1 (active ash basin side gradient) show a neutral to negative vertical gradient. • Inactive ash basin — A neutral to positive vertical gradient was noted in the only active ash basin/structural fill well pair (AB-35). o Ash storage area — No bedrock well pairs were available for interpretation. o Structural fill — As stated above, AB-35 reported a neural to positive vertical gradient. Background monitoring wells — The only background bedrock well pair, BG-2, reported a negative vertical gradient. This well pair is located northwest of the inactive ash basin near the Station Discharge Canal. 50 Corrective Action Plan Part 1 �� Allen Steam Station Ash Basin The vertical gradients will be refined during future sampling events. 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, 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, vanadium, and zinc (EPRI 2010). COI sources at the Allen site consist of the active and inactive ash basins (including ponded ash beneath the RAB Ash Landfill Area) and the ash storage areas (including the structural fills). These source areas are subject to different processes that result in constituents leaching into the underlying soil layers and into the groundwater. For example, the ash storage area would leach as a result of infiltration of precipitation, while the active ash basin would leach based on the contact with ponded water in the basin. Periodic inflows to the ash basin would likely affect the amount of Ieachate from constituents and their resulting concentrations over time. In addition, ash management practices can alter the concentration range of constituents in ash Ieachate, and certain groups of constituents are more prevalent in landfill versus pond management scenarios (EPRI and USDOE 2004). The location of ash, precipitation, and process water in contact with ash are the most significant factors on geochemical conditions. COls in areas downgradient of the source would not be present in groundwater or soils at levels greater than background without ash -to -groundwater contact. Once leached by precipitation or process water, constituents can enter the soil -to -groundwater - to -rock system and their concentration and mobility are controlled by the principles of constituent transport in groundwater. Soil -to -groundwater -to -rock interaction and geochemical conditions present in the subsurface are also responsible for the natural occurrence of some constituents in background locations. These natural processes may also be responsible for a portion of constituents in groundwater. 51 Corrective Action Plan Part 1 �� Allen Steam Station Ash Basin 3.2.1.2 COI Mobility in Groundwater After leaching has occurred, the distribution and concentrations of constituents in groundwater depends upon factors such as how the dissolved concentrations are transported through the soil/rock media, the composition of the soil/rock media in the flow path, and the geochemical conditions present along those flow paths. There are three main processes involved in the transport of dissolved concentrations in groundwater flow: advection, dispersion, and diffusion. Advection is the movement of dissolved and colloidal constituents by groundwater flow and is the primary mechanism for movement of a dissolved concentration. The rate of advection is based on Darcy's Law, which describes the flow of a fluid (i.e., groundwater) through a porous medium (i.e., soil, rock, etc.). The second process affecting the location and concentration of inorganic constituents in groundwater flow is mechanical dispersion. This mixing process happens as groundwater undergoes tortuous paths of various lengths to arrive at the same location; some water moves faster than other water, which causes longitudinal and lateral spreading of plumes. Dispersion is scale dependent and increases with plume length and groundwater flow velocity. The third process involved in the transport of a dissolved concentration is molecular diffusion which occurs when particles spread due to molecular motion, as in stagnant water. When mechanical dispersion and molecular diffusion processes are combined, the resultant mixing factor is called hydrodynamic dispersion. Hydrodynamic dispersion is a scale -dependent phenomenon. There is greater mixing opportunity over long distances than over short distances, so the hydrodynamic dispersion is greater for long distances. Advection, dispersion, and diffusion can result in changes to constituent concentrations across a site, and can also result in decreases in constituent concentrations over distance and time, without consideration of other geochemical processes. Retardation of constituent concentrations relative to an initial concentration can occur due to adsorption, absorption, or ion exchange. Which of these three processes occur, and the degree to which they occur, depends on factors such as the properties of the solute, the properties of the soil/rock media, and geochemical conditions. Inorganic constituents have varying propensities 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. 52 Corrective Action Plan Part 1 Allen Steam Station Ash Basin 3.2.1.3 COI Distribution in Groundwater The spatial distribution for COls detected in groundwater samples collected at the Allen site is described below. For the purposes of this discussion, the shallow flow layer includes the analytical results reported in the shallow (S) monitoring wells. The deep flow layer includes the analytical results reported in the deep (D) and upper bedrock (BRU) monitoring wells, and the bedrock flow layer includes the analytical results reported in the bedrock (BR) monitoring wells. Antimony — Antimony concentrations that exceed the IMAC are mainly limited to the deep and bedrock flow layers, including in background samples. One exceedance was reported in the shallow background well BG-3S (4.8 pg/L ). In the deep flow layer, IMAC exceedances were reported beneath the north portion of the active ash basin (AB-24D; 1.3 pg/L), immediately downgradient of the eastern portion of the active ash basin (AB- 23BRU; 1.1 fag/L; AB-26D; 2.6 fag/L), immediately downgradient and east of the inactive ash basin (AB-31 D; 6 fag/L), and upgradient and offsite to the west (GWA-14D; 1.6 pg/L). In the bedrock flow layer, exceedances were reported in the bedrock background monitoring well BG-2BR (1.8 fag/L). Turbidity, total, and dissolved concentration results were available for the majority of these samples. Turbidity values were typically less than 10 NTU with the exception of BG-3S and AB-26D. Dissolved and total concentration results were within an order of magnitude. Based on these limited data, the antimony exceedances do not appear to be attributable to turbidity. • Arsenic — Concentrations of arsenic exceeding the 2L Standard are limited to the shallow flow layer beneath the westernmost extent of the inactive ash basin (AB-36S; 369 pg/L) and immediately downgradient and north of the inactive ash basin (GWA-6S; 193 pg/L). No arsenic exceedances were reported in deep or bedrock wells. Turbidity, total, and dissolved concentration results were available for the both of these samples. Turbidity was typically less than 10 NTU. Dissolved and total concentration results were similar. Based on these limited data, the arsenic exceedances do not appear to be attributable to turbidity. • Barium — Barium concentrations that exceeded the 2L Standard are limited to shallow well AB-36S (990 fag/L) located in beneath the western portion of the inactive ash basin and the background bedrock well BG-2BR (1,400 pg/L). No groundwater samples exceeded background barium concentrations, and no other barium exceedances were reported in shallow, deep, and bedrock wells. Turbidity, total, and dissolved concentration results were available for the both of these samples. Turbidity was typically less than 10 NTU. Dissolved and total concentration results were similar. Based on these limited data, the barium exceedances do not appear to be attributable to turbidity. 53 Corrective Action Plan Part 1 Allen Steam Station Ash Basin Beryllium — Beryllium concentrations that exceed the IMAC are limited to one shallow monitoring well, GWA-6S (48.6 pg/L). No other beryllium exceedances were reported in shallow, deep, and bedrock wells. The GWA-6S turbidity value, as well as the total and dissolved concentration results, were available for this sample. Turbidity was 7.21 NTU. Dissolved and total concentration results were similar. Based on these limited data, the beryllium exceedance does not appear to be attributable to turbidity. • Boron — concentrations that exceeded the 2L Standard are present in the shallow, deep, and bedrock flow layers. In the shallow flow layer, boron exceedances were reported immediately downgradient and east of the active ash basin (AB-26S; 730 pg/L), beneath the inactive ash basin dam (RAB Ash Landfill Area) (AB-31 S; 1,800 pg/L), and east toward the Catawba River (GWA-4S; 1,700 pg/L). In the deep flow layer, exceedances were reported beneath the northeast portion of the active ash basin (AB-27D; 1,500 pg/L) and immediately downgradient of the southeast portion of the active ash basin (AB-22D; 1,500 pg/L), which is screened in the transition zone below the active ash basin dam. In the bedrock flow layer, one boron exceedance was reported downgradient and direclty east of the inactive ash basin (GWA-5BR; 720 pg/L). No other boron exceedances were reported in shallow, deep, or bedrock wells. Turbidity, total, and dissolved concentration results were available for all of these samples. Turbidity was typically less than 10 NTU. Dissolved and total concentration results were similar. Based on these limited data, the boron exceedances do not appear to be attributable to turbidity. • Cadmium — Cadmium concentrations that exceed the 2L Standard are limited to one shallow monitoring well, GWA-6S (10.1 pg/L). No other cadmium exceedances were reported in shallow, deep, and bedrock wells. The GWA-6S turbidity value, as well as the total and dissolved concentration results, were available for this sample. Turbidity was 7.21 NTU. Dissolved and total concentration results were similar. Based on these limited data, the cadmium exceedance does not appear to be attributable to turbidity. Chromium — Chromium concentrations that exceed the 2L Standard are present in the shallow, deep and bedrock flow layers including background samples. In the shallow flow layer, exceedances are limited to voluntary monitoring wells located south of the ash basin (AB-5; 12.5 pg/L) and downgradient to the east of the active ash basin (AB-6R; 15.9 pg/L). In the deep flow layer, exceedances are limited to beneath the central portion of the active basin (AB-21 D; 13.9 pg/L and AB-24D; 13.3 pg/L), immediately downgradient and east of the active ash basin (AB-23BRU; 65.6 pg/L; AB-26D; 29.8 pg/L), upgradient and west of the inactive ash basin (GWA-14D; 24.5 pg/L), and the background well BG-1 D (16 pg/L). In the bedrock flow layer, exceedances are limited to beneath the south-central portion of the active ash basin (AB-21 BR; 18.9 pg/L) and the background monitoring well BG-2BR (25.6 pg/L). 54 Corrective Action Plan Part 1 �� Allen Steam Station Ash Basin Turbidity, total, and dissolved concentration results were available for each of these samples. Turbidity was typically less than 10 NTU with the exception of AB-5 and AB- 26D. Dissolved and total concentration results were similar. Based on these limited data, the chromium exceedances do not appear to be attributable to turbidity. Hexavalent chromium (Cr(VI)) — Hexavalent chromium concentrations that exceeded the NC Department of Health and Human Services Health Screening Level (DHHS HSL) hexavalent chromium health screening level of 0.07 pg/L are present across the site (active and inactive ash basin) in the shallow, deep and bedrock flow layers including background samples. In addition, exceedances were limited to one inactive ash basin well in porewater. Turbidity, total, and speciation concentration results were available for each of these samples. Turbidity was typically less than 10 NTU. Speciation and total concentration results indicate the Cr(VI) exceedances do not appear to be attributable to turbidity. Cobalt — concentrations that exceeded the IMAC in the shallow and deep flow layers are generally spread across the site, including in background samples. In the shallow layer, exceedances are concentrated downgradient and east of the ash basin from GWA-1 S (11.4 pg/L) to the north toward GWA-7S (51.6 pg/L), west of the ash basin at monitoring wells AB-2 (2.0 pg/L), AB-13S (1.1 pg/L) and GWA-14S (8.3 pg/L), and one isolated exceedance at GWA-9S (6.8 pg/L). The highest concentration of cobalt was reported in shallow well GWA-6S (4,160 pg/L). In the deep flow layer, cobalt concentrations that exceeded the IMAC are also generally across the site. Cobalt was reported above the IMAC in shallow background well BG-3S (1.6 pg/L) and deep background well BG-2D (2.4 pg/L). No cobalt exceedances were reported in bedrock wells. Turbidity, total, and dissolved concentration results were available for each of these samples. Turbidity was typically less than 10 NTU. Dissolved and total concentration results were similar. Based on these limited data, the cobalt exceedances do not appear to be attributable to turbidity. • Iron — Iron concentrations that exceeded the 2L Standard vary in the shallow and deep wells and are generally spread across the site, including background wells. In the bedrock wells, iron exceedances were only reported in monitoring wells GWA-1 BR (410 pg/L) and GWA-6BR (520 pg/L). Turbidity, total, and dissolved concentration results were available for each of these samples. Turbidity was typically less than 10 NTU. Dissolved and total concentration results vary at certain locations. Some of the iron exceedances on the site appear to be attributable to turbidity. • Managanese — Manganese concentrations that exceeded the 2L Standard vary in the shallow and deep wells and are generally spread across the site, including background 55 Corrective Action Plan Part 1 Allen Steam Station Ash Basin wells. In the bedrock wells, only one exceedance of manganese was reported in GWA- 6BR (210 pg/L). Turbidity, total, and dissolved concentration results were collected for all groundwater samples. Most of the turbidity readings were less than 10 NTU, and the total and dissolved concentrations were similar. Based on these limited data, manganese exceedances do not appear to be attributable to turbidity. • Nickel — Nickel concentrations that exceed the 2L Standard are limited to one shallow monitoring well, GWA-6S (1,010 pg/L). No other nickel exceedances were reported in shallow, deep, and bedrock wells. The GWA-6S turbidity value, as well as the total and dissolved concentration results, were available for this sample. Turbidity was 7.21 NTU. Dissolved and total concentration results were similar. Based on these limited data, the nickel exceedance does not appear to be attributable to turbidity. • pH — pH measurements outside of the 2L Standard range of 6.5-8.5 were encountered in shallow, deep, and bedrock flow layers, distributed across the site in no discernable pattern. pH excursions from the standard were both acidic and basic. Some of the basic pH exceedance results may be the result of monitoring well construction. • Selenium — Selenium concentrations that exceeded the 2L Standard are limited to one shallow monitoring well, GWA-6S (73.5 pg/L). No other selenium exceedances were reported in shallow, deep, and bedrock wells. The GWA-6S turbidity value, as well as the total and dissolved concentration results, were available for this sample. Turbidity was 7.21 NTU. Dissolved and total concentration results were similar. Based on these limited data, the selenium exceedance does not appear to be attributable to turbidity. • Sulfate — concentrations that exceeded the 2L Standard are limited to the shallow flow layer at the north boundary of the inactive ash basin (RAB Ash Landfill Area) (AB-33S; 337,000 pg/L) and immediately north and downgradient of the inactive ash basin (GWA- 6S; 1,830,000 pg/L). No sulfate exceedances were reported in deep and bedrock wells. The AB-33S and GWA-6S turbidity values were less than 10 NTU. Based on these limited data, the sulfate exceedances do not appear to be attributable to turbidity. Thallium — Thallium concentrations that exceeded the IMAC are limited to one shallow monitoring well, GWA-6S (1.3 pg/L). No other thallium exceedances were reported in shallow, deep, and bedrock wells. The GWA-6S turbidity value, as well as the total and dissolved concentration results, were available for this sample. Turbidity was 7.21 NTU. Dissolved and total 56 Corrective Action Plan Part 1 �� Allen Steam Station Ash Basin concentration results were similar. Based on these limited data, the thallium exceedance does not appear to be attributable to turbidity. TDS — concentrations that exceeded the 2L Standard are present in the shallow, deep, and bedrock flow layers, including background bedrock well BG-2BR. In the shallow flow layer, exceedances are limited to the north boundary of the inactive ash basin (RAB Ash Landfill Area) (AB-33S; 761,000 tag/L) and immediately north and downgradient of the inactive ash basin (GWA-6S; 3,920,000 pg/L). In the deep flow layer, exceedances are limited to beneath the north portion of the active ash basin (AB-24D; 1,280,000 pg/L) and upgradient and offsite to the west of the inactive ash basin (GWA-14D; 750,000 pg/L). In the bedrock flow layer, exceedances are limited to beneath the west portion of the active ash basin (AB-23BRU; 530,000 pg/L) and the background bedrock monitoring well located approximately 1,700 feet west of the inactive ash basin (BG-2BR; 2,040,000 tag/L). The 2L exceedances of TDS at GWA-14D and BG-2BR are not likely attributable to the ash basin, because they are hydraulically isolated from the basins. Vanadium — concentrations that exceeded the IMAC vary in the shallow, deep, and bedrock wells and are generally distributed across the site, including in background wells. The concentrations of vanadium are generally higher in the deep and bedrock wells than in the shallow wells. Turbidity, total, and dissolved concentration results were available for each of these samples. Turbidity was typically less than 10 NTU. Total and dissolved concentrations were similar. Based on these limited data, the vanadium exceedances do not appear to be attributable to turbidity. • Zinc — Zinc concentrations that exceed the 2L Standard are limited to one shallow monitoring well, GWA-6S (2,100 pg/L). No other zinc exceedances were reported in shallow, deep, and bedrock wells. The GWA-6S turbidity value, as well as the total and dissolved concentration results, were available for this sample. Turbidity was 7.21 NTU. Dissolved and total concentration results were similar. Based on these limited data, the zinc exceedance does not appear to be attributable to turbidity. Boron, cobalt, hexavalent chromium, iron, manganese, pH, TDS, and vanadium were the COls with the most widespread exceedances beneath the ash basins and inactive basins (including ash storage area), downgradient of the source areas, and in background monitoring well locations. Although the concentrations of these COls exceeded their respective 2L Standards, IMACs, or health screening level in many background and upgradient locations, concentrations of these MIS were generally higher in the ash basins, ash storage area, and areas downgradient from the source areas. Antimony, boron, chromium, hexavalent chromium, iron, pH, TDS and vanadium exceedances were detected in wells within the footprint of the active ash basins and in wells within the compliance boundary downgradient of these ash management areas. Arsenic, barium, chromium, hexavalent chromium, cobalt, iron, manganese, pH, sulfate, TDS, and vanadium 57 Corrective Action Plan Part 1 �� Allen Steam Station Ash Basin exceeded their respective 2L Standards within and near the inactive ash basin including the ash storage area. Concentrations of these constituents were greater than the PPBCs and concentrations observed in other areas of the site. Beryllium, cadmium, nickel, selenium, thallium, and zinc exceedances were reported beneath and/or adjacent to the inactive ash basins (northeast portion of the basin, adjacent to the Allen coal pile) and not in upgradient or background locations. These COls may be attributable to ash handling at the site. Note that the frequency of exceedances of COls were only detected above their respective standard or guideline value in one well (GWA-6S) sampled at the Allen site. Antimony, arsenic, barium, boron, cobalt, chromium, hexavalent chromium, selenium, sulfate, TDS, and vanadium exceedances were detected in wells within the footprint of ash basin and/or in wells within the compliance boundary downgradient of these ash management areas and could be attributed to ash handling practices. Subsequent sampling and data interpretation will further refine this summary of findings. Groundwater flows through the majority of the ash basin system to the east toward the Catawba River and the northern extent of the basin to the north/northeast toward the Station Discharge Canal and ultimately the Catawba River. Groundwater sampling results indicate elevated concentrations of COls attributable to the ash basins are mainly in the shallow flow layer immediately downgradient of the ash basins to the east and north in the direction of groundwater flow, as wells as in the deep flow layer beneath and downgradient of the eastern portion of the ash basins. 3.2.2 Geochemical Characteristics Groundwater composition can be affected by an array of naturally -occurring and anthropogenic (cultural) factors. Many of these factors can be causative agents for specific oxidation-reduction (redox) processes or indicators of the implied redox state of groundwater as expressed by pH, Oxidation -Reduction Potential (ORP), and dissolved oxygen (DO). The pH of a body of groundwater is affected by the composition of the bedrock and soil through which the water moves. Exposure to carbonate rocks (or lime -containing materials in well casings) can increase pH. Exposure to atmospheric carbon dioxide gas will lead to formation of carbonic acid and can lower pH. The pH of precipitation that falls on the watershed of an aquifer can also impact groundwater pH. In addition, metals and other elemental or ionic constituents in groundwater, or the surrounding soil matrix, can act as electron donors or acceptors as measured by ORP. The reactivity of different constituents can lead to oxidizing (positive ORP) or reducing (negative ORP) environments in groundwater systems. DO in groundwater can act as an oxidizing agent and is an indicator of redox state. 3.2.2.1 Cations/Anions Classification of the geochemical composition of groundwater aids in aquifer characterization and SCM development. As groundwater flows through the aquifer media, the resulting geochemical reactions produce a chemical composition that can be used to characterize groundwater that may differ in composition from groundwater from a different set of lithological and geochemical conditions. This depiction is typically performed using Piper diagrams to 58 Corrective Action Plan Part 1 �� Allen Steam Station Ash Basin graphically depict the distribution of the major cations and anions of groundwater samples collected at a particular site. The relative concentrations and distribution of the cations and anions can be used to compare the relative ionic composition of different water quality samples through the use of Piper diagrams. Piper diagrams were generated as part of the Allen CSA to compare the geochemistry between groundwater, ash basin porewater, surface water, and seeps. Evidence of mixing of ash basin porewater and groundwater can be seen in the piper diagrams presented in the CSA Report. 3.2.2.2 Redox Conditions As described by McMahon and Chapelle (2008), redox processes affect the chemical quality of groundwater in all aquifer systems. The descriptions that follow were adapted in whole or in part from McMahon and Chapelle (2008) and Jurgens et al. (2009). Redox processes can alternately mobilize or immobilize constituents associated with aquifer materials (Lovley et al. 1991; Smedley and Kinniburgh 2002), contribute to degradation of anthropogenic contaminants (Korom 1992; Bradley 2000, 2003), and can generate undesirable byproducts such as dissolved manganese (Mn2+), ferrous iron (Fe 21), hydrogen sulfide (H2S), and methane (CH4) (Back and Barnes 1965; Baedecker and Back 1979; Chapelle and Lovley 1992). Using data from the National Water -Quality Assessment (NAWQA) Program, researchers from the USGS developed a framework to assess redox processes based on commonly measured water quality parameters (McMahon and Chapelle 2008; Jurgens, et al. 2009). The redox framework allows the state of a groundwater sample and dominant type of redox reaction or process occurring to be inferred from water quality data. An implementation of this framework is provided in the USGS "Excel® Workbook for Identifying Redox Processes in Ground Water" (Jurgens et al. 2009), which is detailed in USGS Open File Report 2009-1004. The primary aquifer system in western North Carolina is considered to be of the New England, Piedmont and Blue Ridge type and is representative of crystalline -rock aquifers (McMahon and Chapelle 2008). Precise identification of redox conditions in groundwater can be difficult to determine because groundwater is commonly not in redox equilibrium and multiple redox conditions may exist simultaneously as groundwater progresses from more oxygenated (i.e., oxic) states to more reduced states (i.e., anoxic). Redox reactions express thermodynamic equilibrium conditions (i.e., an ultimate state). However, the time required for reactions to reach equilibrium (i.e., reaction rate kinetics) cannot be determined from the equilibrium state. Moreover, groundwater is commonly not in redox equilibrium. Thus, it is not unusual for differences in implied or measured redox conditions to exist between different wells (spatial differences) or over time at an individual well (temporal differences). Transient disturbances attributable to well construction may also alter groundwater composition. For example, pH and other compositional properties of groundwater samples may vary widely immediately following well construction and gradually become more consistent over time. In addition, groundwater samples are often mixtures of water from multiple flow layers that may have different redox conditions. Consequently, mixing within the well bore can produce chemistry results that suggest multiple redox conditions. Recognizing these limitations, researchers have classified groundwater on the basis of a predominant redox process or 59 Corrective Action Plan Part 1 �� Allen Steam Station Ash Basin terminal electron accepting process (TEAP) using concentrations of redox sensitive species (Chapelle and others, 1995; Christensen and others, 2000; Paschke and others, 2007; McMahon and Chapelle 2008). Redox conditions are generally facilitated by microorganisms, which gain energy by transferring electrons from donors (usually organic carbon) to acceptors (usually inorganic species) (McMahon and Chapelle, 2008). Because some electron acceptors provide more energy than others, electron acceptors that yield the most energy are utilized first and species that yield less energy are utilized in order of decreasing energy gain. This process continues until all available donors or acceptors have been used. If carbon sources are not a limiting factor, the predominant electron acceptor in water will usually follow an ecological succession from dissolved oxygen (02), to nitrate (NO3), to manganese (IV), to iron (III), to sulfate (S042-), and finally to carbon dioxide (CO2(g)) (Table 3-3). Although some redox processes overlap as groundwater becomes progressively more reduced, there is usually one TEAP that dominates the chemical signature. Consequently, concentrations of soluble electron acceptors (02, NO3 , S042-) and TEAP end products (Mn(II), Fe(II), H2S(g), CH4(g)) can be used to distinguish between redox processes. The redox evaluation approach uses these commonly measured constituents in conjunction with concentration thresholds applicable to groundwater quality investigations. Although most water quality studies analyze for total dissolved manganese and iron rather than the speciated forms of these elements, in samples that have been filtered (:50.45 micron and acidified, total dissolved concentrations are generally accurate estimates of Mn(II), Fe(II) above the threshold concentrations (50 and 100 pg/L, respectively) for pH ranges normally found in ground water (6.5-8.5) (Kennedy and others, 1974; Hem, 1989). At lower pH values there is an even greater likelihood that dissolved concentrations are equal to the primary species of interest: Mn(II), Fe(II). The USGS redox framework was applied to groundwater measurements from different environments across the Allen 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 including DO, ORP, temperature, pH, specific conductance, and turbidity. DO, nitrate/nitrite as nitrogen, manganese (II) (dissolved manganese), iron (II), sulfate and sulfide measured at the Allen site these analytical results for these six constituents were used as inputs to the redox workbook for monitoring wells. Analytical results were reported as the sum of nitrate and nitrite was reported. For the purpose of this redox assessment, nitrate was assumed to be equal to the reported nitrate/nitrite concentration (i.e., 100% nitrate). Similarly, manganese(II) was assumed to equal the reported dissolved manganese concentration. The redox state of the Allen site was evaluated based on 38 samples from the study area for which all six constituents were available, including porewater and groundwater. Based on site measurements, the primary redox categories were determined to include oxic, suboxic, mixed (anoxic), and anoxic conditions. At Allen, predominant redox processes involved ferrous iron/ferrous sulfate, so reduced species As(III), Se(IV), and Mn(IV) would be expected. Other redox processes involving manganese reduction may also occur. Redox conditions appear to be controlled at least partly by the SO4/S2 and Fe(lll)/Fe(II) redox couples. However, it should be noted that 12 of the 38 (-32%) wells were in Suboxic or Oxic categories. Further, 6 of 10 wells from which 60 Corrective Action Plan Part 1 �� Allen Steam Station Ash Basin porewater samples were collected are classified as Suboxic. There is a decreased potential for reduced forms of metals to persist under suboxic or oxic conditions. Table 3-3. Categories and Threshold Concentrations to Identify Redox Processes in Groundwater Redox Dissolved Nitrate as Manganese Iron Sulfate Iron/Sulfide Process Likely Category Oxygen Nitrogen (mg/L) (mg/L) (mg/L) (mass ratio) Occurring at (mg/L) (mg/L) Allen Oxic (02) >_0.5 - <0.05 <0.1 - - Yes Suboxic <0.5 <0.5 <0.05 <0.1 - - Yes (Low 02) Anoxic, <0.5 >_0.5 <0.5 <0.1 - - Yes NO3 Anoxic, <0.5 <0.5 >_0.05 <0.1 - - Yes Mn(IV) Anoxic, <0.5 <0.5 - >_0.1 >_0.5 no data Yes Fe(III)/SO4 Anoxic, <0.5 <0.5 - >_0.1 >_0.5 >10 Yes Fe(III) Mixed, <0.5 <0.5 - >_0.1 >_0.5 >-0.3 and Yes Fe(l1l)-SO4 :510 Anoxic, <0.5 <0.5 - >_0.1 >_0.5 <0.3 No SO4 Anoxic, <0.5 <0.5 - >_0.1 <0.05 - No CH4 Notes: Thresholds and concentrations from McMahon and Chapelle (2008) and Jurgens et al. (2009). 61 Corrective Action Plan Part 1 �� Allen Steam Station Ash Basin Table 3-4. Field Parameters from CSA Range of Results for Groundwater Parameters Well No. of pH Spec. Diss.ORP/Redox Turbidity Locations Results (std. unit) Cond (pS) Oxygen (mV) (NTU) (mg/L) Background 7 5.89 - 12.66 1.05 - 8.89 -50.2 - 165 5.3 - 56.5 102- 0.74-9.92 Downgradient 20 3.68- 11.52 50 - 2940 0.07 - 9.02 -177 - 544 AB-26D (815.4) Downgradient, 8 5.4 - 6.73 31 - 379 0.17 - 6.56 -37. 8 - 199.5 1.19 - 38.8 Compliance Upgradient 4 4.7 - 6.96 52 - 300 0.56 - 6.42 137.5 - 277.2 6.4 - 16.8 Upgradient, 2 4.97 - 6.03 132 - 133 00 - 2.82 145.6 - 162-4 6.92 - 9.37 Compliance Upgradient, 2 4.53 - 6.32 21 - 88 2.96 - 5.56 147.2 - 330.2 2.43 - 6.22 Voluntary Unknown 2 5.36 - 6.9 101 - 1.09 - 3.1 -3 - 191.6 6.99 - 83.8 (GWA-9D/9S) 131.2 Porewater 42 5.9 - 11.84 102 0.04 - 62.1 -188.3 - 125 1.42 - 34 (Ash Basin) 2022 Ranges for a number of field measurements characterizing aspects of groundwater conditions outside downgradient of and beneath ash basins are presented in Table 3-4 above. Those measurements indicate that pH ranges from 3.68 to 11.52 standard units. In contrast, background well results indicate that pH ranges from 5.89 to 12.66, whereas pH within the ash basin materials range from 5.9 to 11.84. 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). This both agrees and contrasts with the redox category assessment. In terms of redox categories, a number of wells and samples were considered to be oxic while others were anoxic with manganese or iron reduction with sulfur oxidation as a predominant process. Standard (equilibrium) electrode potentials for such reactions may be expected to be in the range of approximately -1,000 millivolts. In contrast, measured ORP values were never less than -200 millivolts. Differences between equilibrium (theoretical) and measured ORP values suggests that groundwater samples are not in full equilibrium with aquifer materials. Apparent disequilibrium is also suggested by the wide range of distribution (sorption) coefficients for determent from batch and column laboratory experiments (see Appendix D). In those experiments, batch tests were performed for a 24 hour period and column tests involved hydraulic residence times that were approximately 6-8 hours. Such differences in sorption test results suggest that reaction kinetics for redox processes very likely require longer periods of time for full equilibrium to occur. However, it is also worth noting that ORP values during sorption experiments differed widely from those measured in groundwater samples. 62 Corrective Action Plan Part 1 �� Allen Steam Station Ash Basin 3.2.2.3 Solute Speciation Groundwater samples were characterized in terms of solute speciation to evaluate the concentrations and ionic composition (oxidation states) of metal ions primary concern, including arsenic(III, V), chromium(III, VI), iron(II, III), manganese(II, IV), and selenium(IV, VI). In general, reduced forms of metals (i.e., species in lower oxidation states) are more readily transported in groundwater than those that are more oxidized. At the Allen site, speciation measurements were performed on 35 to 67 samples, depending on the analyte. To provide a general indication of sample composition, the relative percentage of the reduced specie concentration to the sum of the reduced and oxidized specie concentration were calculated. These relative percentages express the proportion of the reduced form metal present each sample. Speciation measurements at the Allen site show very high variations. For arsenic, approximately 50% is present as arsenic(III), with a range of 9% to 93%. Similarly, for chromium approximately 68% is present as chromium(III), with a range of 6% to 99%. For selenium, the variation is somewhat smaller but still relatively large, with 53% present as Se(IV) and a range of 40% to 87%. This highly variable composition is consistent with the wide range of redox categories that may exist across the site. 3.2.2.4 Kd (Sorption) Testing and Analysis As described in Section 3.2.1.2, a constituent may be removed from groundwater and onto mineral surfaces of the aquifer media through one of three types of sorption processes: • Adsorption — solutes are held at the water/solid interface as a hydrated species • Absorption — solutes are incorporated into the mineral structure at the surface • Ion Exchange — when an ion becomes sorbed to a surface by changing places with a similarly charged ion These processes result in decreased constituent concentrations and, therefore, the mass of the constituent as it is removed from groundwater onto the solid material. The effect of these processes for a particular constituent can be expressed by the sorption coefficient (or partition coefficient) Kd. Kd relates the quantity of the sorbed constituent per unit mass of solid to the quantity of the constituent remaining in solution. Laboratory determination of sorption values (Kd) was performed on 12 site -specific samples of soil, or PWR from the transition zone. Solid samples were batch equilibrated and/or tested in flow through columns to measure the sorption of COls at varying concentrations. For the Allen site, 24 batch tests were conducted. The methods used by the University of North Carolina at Charlotte (UNCC) and Kd results obtained from the testing are presented in Appendix E. The Kd data were used as an input parameter to evaluate contaminant fate and transport through the subsurface at the Allen site, as described in greater detail in Section 4.1. 63 Corrective Action Plan Part 1 �� Allen Steam Station Ash Basin 3.2.3 Source Area Geochemical Conditions 3.2.3.1 Active Ash Basin Ash within the active ash basin was encountered in all boring locations and extended below gauged water levels in several instances. Ash was detected from the surface to a maximum depth of 55 feet bgs. Refusal depths were generally deep relative to ash and ranged from approximately 101 to 128 feet bgs. Water levels tended to be shallow relative to ash, with a median water level of approximately 9 feet bgs and a range of 3 to 117 feet bgs. Ash extended below gauged water levels at AB-20, AB-21, AB-24, and AB-25 locations. The absence of an unsaturated soil buffer below the ash encountered in these locations could cause COls to leach directly to groundwater. Sorption cannot take place under such circumstances, thereby allowing COls to readily dissolve into groundwater and become mobile. Ash remained situated above gauged water levels at A13-23, AB-27, and AB-28 locations. AB-23 is underlain by a thick, unsaturated native soil/saprolite buffer and competent bedrock (approximately 87 feet soil/saprolite and four feet of rock above gauged water level), thereby inhibiting COls from directly leaching into groundwater at this particular location. Ash in AB-27 and AB-28 are underlain by thin layers of unsaturated, clayey fill and thicker layers of saturated fill and native soil/saprolite. There is minimal unsaturated buffer at AB-27 and A13-28. Mobility of constituents is affected by sorption characteristics of each respective constituent. Pond -level fluctuation also affects constituent movement by increasing the driving head on the water column. Higher pond levels increase dissolution (and thus mobility) of constituents into the groundwater, and vice versa. 3.2.3.2 Inactive Ash Basin During drilling activities within the inactive ash basin (which includes the RAB Ash Landfill Area, ash storage and structural fill areas), ash was encountered in all but one boring location and extended below gauged water levels in most instances. Ash was detected from the surface to a maximum depth of approximately 57 feet bgs. Refusal depths were moderately deep to deep relative to ash and ranged from 46.5 to 144.5 feet bgs. Similar to the active ash basin, water levels tended to be shallow relative to ash and ranged from 2 to 41 feet bgs. Ash was not encountered in A13-36. Ash extended below gauged water levels at AB-29, A13-30, AB-35, AB-37, A13-38, and AB-39 locations. The absence of an unsaturated soil buffer below the ash encountered at these locations could cause COls to leach directly to groundwater. Sorption cannot take place under such circumstances, thereby allowing COls to readily dissolve into groundwater and become mobile. Ash remained situated above gauged water levels at AB-33 and AB-34; these locations are underlain by thin layers of unsaturated clayey soil. Though there is minimal unsaturated buffer at AB-33 and A13-34, opportunity exists for sorption to take place prior to COls reaching the groundwater, thus reducing the quantity of dissolved COls and mobility of those COls. 64 Corrective Action Plan Part 1 �� Allen Steam Station Ash Basin 3.2.4 Mineralogical Characteristics Soil and rock mineralogy and chemical analyses completed to date are sufficient to support evaluation of geochemical conditions. Soil mineralogy and chemistry results through July 31, 2015 were presented in the CSA Report. The dominant minerals in soils at the Allen site 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 biotite, dolomite, amphibole, and zeolite. The major oxides in the soils are Si02 (44.78% - 65.69%), AI203 (12.91 % - 26.73%), and Fe203 (2.83% - 10.59%). MnO ranges from 0.05% to 0.15%. Major transition zone minerals are quartz, feldspar, illite, kaolinite, biotite and amphibole. The major oxides are Si02 (65.2% - 68.15%), AI203 (12.87% - 14.56%), and Fe203 (6.91 % - 8.56%). The major oxides in the rock samples are Si02 (49.29% - 66.75%), A1203 (15.61 % - 20.60%), and Fe203 (2.02% - 12.28%). The high Si02 in the bedrock samples is consistent with their sedimentary origin. These highly weathered Piedmont soils, saprolite, and rock contain high percentages of clay minerals and hydrous metal oxides and oxyhydroxides. These geologic materials are very fine- grained and have a large surface area compared to their volume. They are also chemically reactive, and the attenuation of inorganic compounds by clays and oxides has been a subject of intense study for over 100 years. The abundant clay content of the soils and host rock lithologies suggests much of the COI concentrations in the ash basins and ash storage area may be attenuated by these materials. Soil formation typically results in the loss of common soluble cations and the accumulation of quartz and clay. Feldspars are hydrolyzed to clays. Concentrations of COls during the weathering and soil development on the lithologies noted above are negligible, with the exception of a potential increase in vanadium and cobalt from diabase weathering. Soil chemistry results do not show marked deviation from normal crustal abundances at the Allen site. Accordingly, the residual soils do not appear to contribute significantly to the COI exceedances in the soils. 3.3 Correlation of Hydrogeologic and Geochemical Conditions to COI Distribution During the CSA, COls identified in ash and porewater were antimony, arsenic, barium, boron, cobalt, iron, manganese, pH, selenium, sulfate, thallium, TDS, and vanadium. Although ash and porewater are not evaluated for remediation during this CAP, the presence of these COls in ash and porewater could be related to similar constituents in other media, such as underlying soil, downgradient groundwater, seeps, and surface water. The following constituents were detected in downgradient wells evaluated as part of the CSA at concentrations exceeding their respective 2L Standards, IMACs or DHHS HSLs: antimony, arsenic, boron, cobalt, iron, manganese, selenium, sulfate, thallium, TDS, and vanadium. The source areas with exceedances of these COls, as well as other Allen site features, are illustrated on the 3-D SCM presented on Figure 3-1 and in cross -sectional view on Figure 3-2. 65 Corrective Action Plan Part 1 �� Allen Steam Station Ash Basin On Figure 3-1, the areas of 2L Standard or IMAC exceedances within or directly adjacent to the sources indicate that physical and geochemical processes beneath the Allen site retard the lateral migration of the Cols. Discharge of groundwater from shallow and deep flow layers into surficial water bodies, in accordance with LeGrand's slope -aquifer system characteristic of the Piedmont, is expected in the Catawba River. Vertical migration of COls were observed in select well clusters (S, D, and BR) and is likely influenced by infiltration of precipitation and/or ash basin water, where applicable, through the shallow and deep flow layers into underlying fractured bedrock. Beryllium, cadmium, nickel, and zinc exceeded their respective 2L Standards or IMAC in one sample and selenium and thallium exceeded their respective 2L Standard and IMAC in a limited number of samples. The relative absence of these constituents in groundwater could suggest that the subsurface geochemical conditions may attenuate these COls or that the original composition of source material did not contain large concentrations of these COls to begin with. Additional sample data and refinement of the groundwater models, if necessary, may provide a better understanding of these COls. Horizontal migration of COls was evident in groundwater flow to the east towards the Catawba River, with decreasing concentrations moving farther away from the source areas. Discharge of groundwater from shallow and deep flow layers into surficial water bodies is not currently known in the absence of surface water bodies (i.e., tributaries) where flow measurements can be made. Vertical migration of COls was observed in select well clusters (S, D (BRU), and BR) and is likely influenced by infiltration of precipitation and/or ash basin water, where applicable, through the shallow and deep flow layers into underlying fractured bedrock. Advection, dispersion, and diffusion can result in changes to constituent concentrations across the site and can also result in decreases in constituent concentrations over distances and time, without consideration of other geochemical processes. Redox reactions such as the iron example above can greatly influence the presence of contaminants in water. 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. Speciation is important for understanding the fate and transport of constituents as species react differently. Speciation analysis revealed that Allen groundwater is predominantly anoxic/mixed (oxic to suboxic in certain portions of the site) with detectable dissolved oxygen in nearly every well. Dissolved oxygen is the primary control on redox conditions in Allen groundwater. Mixed anoxic conditions are reflected in the speciation of redox-sensitive species (e.g., ferrous iron/ferrous sulfate are the predominant redox processes). At Allen, predominant redox processes involved ferrous iron/ferrous sulfate, so reduced species As(III), Se(IV), and Mn(IV) would be expected. Other redox processes involving manganese reduction may also occur. Redox conditions appear to be controlled at least partly by the SO4/S2 and Fe(III)/Fe(II) redox couples. Redox condition is a reasonable predictor of the presence and relative concentration of oxidized and reduced species of arsenic, chromium, iron, selenium, and sulfate. Speciation of constituents at Allen is an important consideration in developing corrective actions for the site. 66 Corrective Action Plan Part 1 �� Allen Steam Station Ash Basin Piper diagrams prepared during the CSA showed source area groundwater less calcium- and chloride -rich than ash basin porewater, ash basin surface water, and downgradient groundwater which were observed to be trending closer to calcium, magnesium, and sulfate rich. Seep data indicate similar geochemistry to ash basin porewater, and downgradient monitoring wells. Evidence of mixing of ash basin porewater and groundwater can be seen in the piper diagrams presented in the CSA Report. Mineralogical characteristics of the highly weathered Piedmont soils, saprolite, and rock contain high percentages of clay minerals and hydrous metal oxides and oxyhydroxides. These geologic materials are very fine-grained and have a large surface area compared to their volume. These materials are also chemically reactive, and the abundant clay content of the soils and host rock lithologies suggests much of the COI concentrations in the ash basins and ash storage area may be attenuated by these materials. Additional mineralogical characterization was identified as a data gap in the CSA. Refinement of this SCM, as it pertains to groundwater fate and transport modeling will continue to evolve as additional data become available during supplemental site investigation activities. 67 Corrective Action Plan Part 1 Allen Steam Station Ash Basin Modeling Groundwater flow and fate and transport, and groundwater to surface water models were conducted to evaluate COI migration and potential impacts following closure of the ash basin system and ash storage area at the Allen site. Groundwater Modeling Under the direction of HDR, UNCC developed a site -specific, 3-D, steady-state groundwater flow and fate and transport model for the Allen site using MODFLOW and MT3DMS. UNCC provided the calibrated flow and fate and transport models and HDR performed the predictive simulations. The model was developed in accordance with NCDENR DWQ's Groundwater Modeling Policy dated May 31, 2007. The groundwater flow and fate and transport model is based on the SCM presented in Section 3 and incorporates site -specific data obtained during the CSA and subsequent data collection. The objective of the groundwater modeling effort was to simulate steady-state groundwater flow conditions for the Allen site, and simulate transient transport conditions in which COls enter groundwater via the ash basin system over the period it was in service. These model simulations serve to: • Predict groundwater elevations in the ash and underlying groundwater flow layers for the proposed closure scenarios • Predict concentrations of the COls at the compliance boundary or other downgradient locations of interest over time • Estimate the groundwater flow and constituent loading to the Catawba River The area, or domain, of the simulation included the Allen ash basin system and areas of the surrounding site that have been impacted by COls above 2L Standards, IMACs, or DHHS HSLs. Note that modeling took a conservative approach by not incorporating wells in which a given constituent was reported below the 2L Standard, IMAC, or DHHS HSL. The UNCC Groundwater Flow and Transport Model report is included in Appendix C. 4.1.1 Model Scenarios The following ash basin closure scenarios were modeled for the Allen site: • Existing Conditions: assumes current site conditions with ash sources left in place • Cap -in -Place (CIP): assumes ash left in ash basins and ash storage area is above the water table and is covered by an engineered cap • Excavation: assumes removal of ash from source areas Model scenarios used steady-state groundwater flow conditions established during model calibration and transient transport of COls identified in Sections 2 and 3 for further analysis. Each COI was modeled individually using the transient transport model over a 250-year simulation period. This time period was selected as the model duration boundary condition to assess changing COI concentrations over time at the compliance boundary. The rate of natural attenuation is then described over the model simulation period. 68 Corrective Action Plan Part 1 �� Allen Steam Station Ash Basin 4.1.2 Calibration of Models The groundwater flow model was calibrated to steady-state flow conditions using water level measurements taken at the site during June 2015 in shallow, deep, and bedrock wells. Transient transport of each COI was calibrated to groundwater water quality samples collected in June 2015. Only COI concentrations above the 2L Standards, IMACs, or DHHS HSLs were used for model calibration purposes by introducing a constant source for each COI at the start of the ash basin operations and running the model until June 2015. A detailed account of the flow and transient transport model calibration process is included in Appendix C. Ranges of measured hydrogeologic properties from the CSA were used as a guide for selecting model input parameters during calibration. The groundwater flow model was calibrated by adjusting model parameters within the upper and lower bounds of measured hydrogeologic parameters at the site, including: • The hydraulic conductivity distribution of each flow layer within the basin (e.g., ash, dike, upper unconsolidated zone, transition zone, and fractured bedrock zone) • The infiltration rate applied to the water table within the ash basin system • The net infiltration due to precipitation applied to other areas of the site • The variation of measured porewater COI concentrations • The effective porosity of each model layer • The Kd value for each COI Calibration results indicate the model adequately represents steady-state groundwater flow conditions at the site and meets transport calibration objectives. The calibrated Allen model was submitted to the Electric Power Research Institute (EPRI) on September 27, 2015, for independent review of the model. The model third -party peer review team was coordinated by EPRI which included Dr. Chunmiao Zheng from the University of Alabama, James Rumbaugh from Environmental Simulations, Inc., and experienced modelers from Intera, Inc. The reviewers were provided the Allen CSA Report, an Allen draft model report, and digital model input and output files, allowing them to reconstruct the model for independent review. During the course of the review, the reviewers communicated with the modelers in order to better understand how the model was developed and calibrated. As a result of these communications, the model was modified and recalibrated, which allowed the reviewers to conclude that the model was constructed and calibrated sufficiently to achieve its primary objective of comparing the effects of closure alternatives on nearby groundwater quality. In addition, the reviewers identified limitations with the model, which are included in the discussion of model limitations in Appendix C. After EPRI acceptance of the Allen Groundwater model, the model was improved using measured aquifer test and water level results from monitor wells at the Allen site. In addition, a drainage feature was removed from the model near the northwest model boundary after further review of the CSA data. These changes did not change the model structure or boundaries and did not deviate from EPRI guidelines. 69 Corrective Action Plan Part 1 Allen Steam Station Ash Basin 4.1.3 Kd Terms COls enter the ash basin system in both dissolved and solid phases. In the ash basin system, constituents may undergo phase changes including dissolution, precipitation, adsorption, and desorption. Dissolved phase constituents may undergo these phase changes as they are transported in groundwater flowing through the basin. Phase changes (dissolution, precipitation, adsorption, and desorption) are collectively taken into account by specifying a linear soil -groundwater sorption coefficient (Kd). In the fate and transport model, the entry of constituents into the active ash basin and inactive ash basin (including ash storage, and structural fill areas) is represented by a constant concentration in the saturated zone (i.e., porewater) of the basin, and is continually depleted by infiltrating recharge from the surface. As previously discussed in Section 3.2.2.4, laboratory Kd terms were developed by UNCC via column testing of 14 site -specific samples of soil, or PWR (from the transition zone). The methods used by UNCC and Kd results obtained from the testing are provided in UNCC's Soil Sorption Evaluation report (Appendix D). The Kd data were used as an input parameter to evaluate COI fate and transport through the subsurface at the Allen site. Note that Kd characteristics were each represented by an isotherm from which the sorption coefficient Kd, with units of ml/gram, is calculated. Sorption studies on soil samples obtained during the CSA at Allen indicate that the COI Kds for background soils surrounding the ash basins and ash storage areas are higher than the values used in modeling; COI Kds were adjusted (typically lowered) in the model to facilitate movement of COls within the model. 4.1.4 Flow Model The groundwater model, calibrated for flow and constituent fate and transport under existing conditions, was applied to evaluate closure scenarios at Allen. Being predictive; these simulations produce flow and transport results for conditions that are beyond the range of those considered during the calibration. Thus, the model should be recalibrated and verified over time as new data become available in order to improve model accuracy and reduce uncertainty. The model domain developed for existing conditions was applied without modification for the Existing Conditions and Cap -In -Place scenarios. The Existing Conditions scenario is used as a baseline for comparison to other scenarios. The assumed recharge was modified and the constant source concentration was depleted in the Cap -in -Place and Excavation scenarios. In the Cap -in -Place scenario, recharge within the ash basin was reduced from 6 in/year to 0 in/year to represent capping of the active ash basin and inactive ash basin. For the Excavation scenario, the active ash basin and inactive ash basin were depleted. The flow parameters for this model were identical to the Existing Conditions model except for the removal of ash -related layers, with the same recharge applied to the ash basin as the remainder of the site. Sensitivity of the groundwater flow model was evaluated by varying key model assumptions by a percentage above and below their respective calibration values and calculating the normalized root mean square error (NRMSE) for comparison with the calibration value (Appendix C). Based on this approach, the groundwater flow model was most sensitive to hydraulic conductivity of the shallow flow layer, followed by recharge to areas beyond the ash basin 70 Corrective Action Plan Part 1 �� Allen Steam Station Ash Basin system. The model was less sensitive to vertical hydraulic conductivity beyond the ash basin system, where the dominant groundwater flow direction 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. Groundwater flow transitions from vertical to primarily horizontal flow directly beneath the ash basins due to the absence of a dike or fill layer beneath the ash and above the shallow zone. Groundwater within the shallow, deep, and bedrock zones flows from the west and southwest to the east and discharges to the Catawba River. Locally, the water table gradient is reduced beneath Primary Ponds 1 through 3, and increased beneath the RAB Ash Landfill and the southeastern portion of the active ash basin. Relatively steep hydraulic gradients are associated with the North, East, and RAB Dikes. 4.1.4.1 Existing Conditions Scenario The Existing Conditions scenario served as the basis of comparison to the Cap -in -Place and Excavation scenarios. No changes in flow are seen in this scenario in the flow model results. This scenario represents the most conservative conditions in terms of groundwater concentrations on- and off -site, and COls discharging to the Catawba River. 4.1.4.2 Cap -in -Place Scenario The Cap -in -Place scenario simulated placement of an engineered cap by applying a recharge rate of zero to the inactive and active ash basins. Groundwater flow is affected by this scenario as the water table is lowered and groundwater velocities may be reduced beneath the capped areas. Near the center of the inactive ash basin, the water table is lowered by approximately 26 feet relative to the level simulated under the Existing Conditions scenario. In the active ash basins, the difference in water level is approximately 36 feet. In the model, non-sorptive COls move downgradient at the pore velocity of groundwater and are displaced by the passage of a single porewater volume, while migration of sorptive COls in groundwater is retarded due to sorption with soils/rocks. 4.1.4.3 Excavation Scenario In the Excavation scenario, all ash from the inactive ash basin and active ash basin is removed and transported offsite. The Excavation scenario simulated complete removal of the ash layers in the model and, therefore, this scenario is incapable of estimating resulting groundwater levels in the ash basin system. In the model, the constant concentration sources and all ash above and below the water table are removed. Unlike the Cap -in -Place scenario, this scenario assumes recharge rates become equal to rates surrounding the ash basins, except for the RAB Ash Landfill where the recharge rate remains zero inches per year in all scenarios. 4.1.5 Fate and Transport Model Each model scenario provides simulation of groundwater concentrations over time. The model does not account for changing background COI concentrations. Ultimately, COI concentrations will reach equilibrium in groundwater across the site. 71 Corrective Action Plan Part 1 �� Allen Steam Station Ash Basin Groundwater COls from Section 2 include: aluminum, antimony, arsenic, barium, beryllium, boron, cadmium, chloride, chromium, hexavalent chromium, cobalt, copper, iron, lead, manganese, mercury, molybdenum, nickel, nitrate, pH, selenium, strontium, sulfate, thallium, TDS, vanadium, and zinc. The Cols modeled included: antimony, arsenic, barium, boron, chromium, hexavalent chromium, cobalt, selenium, sulfate, and vanadium. Several COls were not advanced to modeling based on the following rationale: • Beryllium, cadmium, nickel, thallium, and zinc were found above the 2L Standards or IMACs in only one location within the waste boundary (GWA-6S). Provided that concentrations above the 2L Standards or IMACs are limited to the source areas and there is no discernable plume, they could be eliminated from modeling. Geochemical modeling of the Allen site will be completed and submitted under cover of the CAP Part 2. The geochemical model results 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. • Aluminum, chloride, copper, lead, mercury, molybdenum, and nitrate concentrations were not reported above their respective 2LStandards in groundwater in background wells, source areas, or downgradient areas. COls evaluated in the fate and transport model were antimony, hexavalent chromium, arsenic, barium, boron, cobalt, lead, mercury, selenium, total chromium, sulfate and vanadium. These COls represent the remaining source related groundwater constituents that can be utilized in the fate and transport model as they do not require further evaluation as background or in the geochemical model. Sensitivity of the COI transport model was evaluated by varying key model assumptions for porosity, dispersivity, and Kd by a percentage above and below their respective calibration values. The transport model was most sensitive to increased and decreased horizontal hydraulic conductivity of the shallow aquifer, followed by decreased recharge outside the inactive and active ash basins, and then decreased recharge within the ash basins. The model was less sensitive to changing vertical hydraulic conductivity in the shallow and transition zones (S/M 1 /M2/TZ), as groundwater flow is dominantly horizontal in these hydrostratigraphic layers. In addition to the flow model sensitivity analysis, the calibrated transport model parameters, including effective porosity and Kd, were varied by 20% above and below their respective calibration values to determine the affect on COI transport. The average modeled concentration change relative to the calibrated concentrations of arsenic and boron at monitoring wells used during transport model calibration, were calculated to determine the transport model sensitivity. 72 Corrective Action Plan Part 1 �� Allen Steam Station Ash Basin 4.1.5.1 Existing Conditions The Existing Condition scenario used the calibrated groundwater flow and transport model and extended the time period from the end of the calibration period (present day) to 250 years into the future. The COls modeled included: antimony, arsenic, barium, boron, chromium, hexavalent chromium, cobalt, selenium, sulfate, and vanadium. No changes were made to the model assumptions for this scenario and it was used as a baseline for comparison to the other model scenarios discussed below. The time to achieve a steady-state COI concentration depends on where the particular COI plume is located relative to the compliance boundary, its loading history, and if it is sorptive or non-sorptive. Source areas close to the compliance boundary reach steady-state concentrations sooner than those farther away. The time to achieve steady-state concentrations is also dependent on the sorptive characteristic of each COI. Sorptive COls are transient at a rate that is less than the groundwater pore velocity. Lower effective porosity results in shorter time periods to achieve steady-state concentrations for both sorptive and non-sorptive COls. The results of the Existing Condition scenario indicated that concentrations for modeled COls predominately increase or reach steady-state conditions above 2L Standards, IMACs, or DHHS HSL during the modeled period. The fate and transport modeling was performed for a 250-year simulation period. At the end of the 250-year simulation period, 7 of 10 constituents were estimated by the model to be above the 2L Standards, IMACs, or DHHS HSL at the Catawba River (i.e., eastern boundary of the site). Additional modeling to determine the timeframe within which COI concentrations would drop below the 2L Standards, IMACs, or DHHS HSL was not performed due to CAP Part 1 time constraints. To better understand the movement and concentrations of COls, Figures 16 through 163 in the Groundwater Flow and Transport Model in Appendix C are provided to show concentration isocontours for all COls 100 years into the simulation period. The 100-year mark was selected to provide the reader a snap -shot of results showing increases or decreases in COI concentrations and movement of COI plumes over a significant period of time. Note that there is no specific relationship to 100-year or 250-year analysis periods. For many COls, the peak modeled concentration occurred within either the 100-year or 250-year simulation periods. 4.1.5.2 Cap -in -Place The Cap -In -Place scenario simulates the effects of capping the inactive ash basin and active ash basin (including the RAB landfill, ash storage, and structural fill areas) at the beginning of the scenario (i.e., Year 2015). In the model, recharge and source area concentrations from the inactive ash basin and active ash basin were set to zero. Under this scenario, groundwater flow rates are lower (compared to the Existing Conditions scenario) due to reduced groundwater velocities caused by the reduction in recharge and the reduction of the groundwater table beneath the capped areas. Groundwater flow is affected by this scenario as the water table is lowered and groundwater velocities may be reduced beneath the capped areas. Near the center of the inactive ash basin, the water table is lowered by approximately 26 feet relative to the level simulated under the Existing Conditions scenario. In the active ash basins, the difference in water level is approximately 36 feet. In the model, non-sorptive COls move downgradient at the pore velocity of groundwater and are displaced by the passage of a single porewater volume, 73 Corrective Action Plan Part 1 �� Allen Steam Station Ash Basin while sorptive Cols' migration in groundwater is retarded because of sorption with soils/rocks. This water table rebound effect and its influence on constituent concentrations in the model assumes ash is above these elevations in the Cap -In -Place scenario. Under the Cap -In -Place scenario, all COI modeled concentrations decrease at the compliance boundary over the 250-year simulation period with the exception of chromium, hexavalent chromium, and vanadium. The majority of the COls still remain greater than their respective 2L Standards, IMACs, or DHHS HSL, with the exceptions of barium, boron, and sulfate. Modeled concentrations of boron and sulfate (non-sorptive COls) are depleted in the model simulation within 40 years or go dry as a result of the cap. Antimony: concentrations decrease in each flow layer, but remain above the IMAC beneath the inactive and active ash basins at 100 years. Antimony concentrations east of the inactive ash basin near the northeast corner reach a maximum concentration at 40 years and then decline, but remain higher than the IMAC throughout the 250-year simulation period. Antimony concentrations east of the active ash basin (east dike) near the Catawba River decrease at 50 years and continue to decline approaching 250 years but remain above the IMAC. Arsenic: arsenic concentrations begin to decrease upon cap implementation, but remain above the 2L Standard through the modeling period at monitoring well AB-22S. For monitoring well GWA-6S, this area of the model becomes dry due to lack of recharge. Arsenic concentrations remain above the 2L Standard in the inactive ash basin, mostly in the eastern portion of the basin and downgradient of the basin towards the Catawba River in all three flow layers. Concentrations remain above the 2L Standard at 100 years in this portion of the site. Barium: barium concentrations decrease upon cap implementation and decrease slowly throughout the modeling period remaining below the 2L Standard in the downgradient monitoring wells throughout the 250-year modeling period. Barium concentrations remain below the 2L Standard at the compliance boundary at the Catawba River downgradient of the inactive ash basin and active ash basin. Barium is predominantly reported at higher concentrations in the shallow flow layers, across the site, but more concentrated along the eastern portions of the ash basins. • Boron: boron concentrations in all wells decrease sharply immediately after cap implementation. At the start of the model simulation period, boron concentrations decrease in each flow layer and are below 2L Standards within 40 years. Model results predict boron will be depleted from the model domain within the 250-year simulation period. Boron is below the 2L Standard at the compliance boundary at the Catawba River downgradient of the inactive ash basin and active ash basin. Chromium: chromium concentrations increase upon cap implementation in the majority of the wells, but remain below the 2L Standard throughout the modeling period. At well AB-6R, chromium concentrations decrease, but remain above the 2L Standard until 210- years after cap implementation. Chromium is above the 2L Standard at the compliance 74 Corrective Action Plan Part 1 �� Allen Steam Station Ash Basin boundary at the Catawba River downgradient of the inactive ash basin and active ash basin in all three groundwater flow layers. Hexavalent Chromium: hexavalent chromium concentrations increase after cap implementation and continue to increase (AB-9D) or remain steady (AB-31 S & GWA-3D) during the model period. Hexavalent chromium remains above the DHHS HSL, which is 0.07 pg/I, at the compliance boundary at the Catawba River. • Cobalt: cobalt concentrations show an overall decrease over time, but remain above the IMAC. The cobalt concentrations under this scenario remain above the IMAC in the 100- year period after cap implementation. Under the Cap -in -Place scenario, cobalt remains above the IMAC at the compliance boundary at the Catawba River. • Selenium: selenium concentrations decrease to below the 2L Standard within 10 years following cap implementation. Selenium is associated with the inactive ash storage basin and is highest in the shallow layer. Concentrations are greatest in the northeast corner of the inactive basin. Under the Cap -in -Place scenario, selenium remains above the 2L Standard at the compliance boundary at the Catawba River in the deep and bedrock flow layers, but is above the 2L Standard within the shallow flow layer. • Sulfate: under the Cap -in -Place scenario, the monitoring wells downgradient of the active ash basin become dry due to reduced recharge to the groundwater system. Sulfate concentrations decrease to zero shortly after capping. This is depicted in the all three groundwater flow layers under this scenario 100 years after cap implementation. Sulfate concentrations under the Cap -in -Place scenario fall below the 2L Standard at the compliance boundary at the Catawba River downgradient of the inactive ash basin and active ash basin. • Vanadium: vanadium concentrations increase over time and then remain fairly constant on the downgradient side (AB-6R, GWA-4S, and GWA-513R) of the active ash basin. Vanadium concentrations under the Cap -in -Place scenario remain above the IMAC at the compliance boundary at the Catawba River downgradient of the inactive ash basin and active ash basin. 4.1.5.3 Excavation The Excavation scenario simulates the effect of removing the inactive ash basin and active ash basin (including the RAB landfill, ash storage, and structural fill areas) at the beginning of the scenario (i.e., Year 2015). In the model, source area concentrations from the ash basins 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 basins is affected by this scenario as the basins are completely drained. Under the Excavation model scenario, non-sorptive COls, boron, and sulfate, are depleted from the model within 20 years and nearly at the start of the model simulation period, respectively. Results from the Excavation model scenario indicate the following: 75 Corrective Action Plan Part 1 �� Allen Steam Station Ash Basin • Antimony: antimony concentrations decrease and fall below the IMAC toward the end of the modeling period. Antimony concentrations decrease under this scenario in the shallow, deep, and fractured bedrock flow layers within 100 years post -excavation. Arsenic: arsenic concentrations decrease upon excavation and fall below the 2L Standard at monitoring well GWA-6S near the end of the modeling period, approximately 175 years out. Arsenic remains above the 2L Standard at the compliance boundary at the Catawba River. • Barium: barium concentrations decrease upon excavation and continue to gradually decrease throughout the modeling period and remain below the 2L Standard in the downgradient monitoring wells. Barium remains below the 2L Standard at the compliance boundary at the Catawba River downgradient of the inactive ash basin and active ash basin. Boron: boron concentrations in all wells decrease sharply immediately after excavation. Under this scenario, boron drops below the IMAC within 20 years and is depleted within 80 years. Both AB-22D and AB-26S show similar sharp decreases within the first 100 years of the simulation period. Boron is below the 2L Standard at the compliance boundary at the Catawba River downgradient of the inactive ash basin and active ash basin. • Chromium: chromium concentrations increase upon excavation, but remain below the 2L Standard throughout the modeling period. At well AB-6R, chromium concentrations decrease, but remain above the 2L Standard for 60 years post -excavation. Chromium is below the 2L Standard at the compliance boundary at the Catawba River downgradient of the inactive ash basin and active ash basin in the deep and bedrock groundwater flow layers and above the 2L Standard in the shallow groundwater flow layer. • Hexavalent Chromium: hexavalent chromium concentrations increase after excavation and slowly decrease during the model simulation period. Hexavalent chromium remains above the DHHS HSL at the compliance boundary at the Catawba River. • Cobalt: cobalt concentrations show an overall decrease over time and fall below the IMAC in three out of four monitoring wells. Concentrations in AB-10S, AB-26D, and AB-32D uniformly drop for the entire duration of the modeling period. Cobalt remains above the IMAC at the compliance boundary at the Catawba River. • Selenium: selenium concentrations decrease below the 2L Standard following excavation. The shallow, deep, and fractured bedrock flow layers under this scenario depict similar active ash basin influences with the greatest response in the bedrock groundwater flow layer. Under the Excavation scenario, selenium is below the 2L Standard at the compliance boundary at the Catawba River in all groundwater flow layers. 76 Corrective Action Plan Part 1 �� Allen Steam Station Ash Basin • Sulfate: sulfate concentrations decrease to zero shortly after excavation. Sulfate concentrations under the Excavation scenario fall below the 2L Standard at the compliance boundary at the Catawba River downgradient of the inactive ash basin and active ash basin. From the start of the model simulation period, sulfate concentrations decrease in each flow layer and were depleted from the model domain within only a few years after implementation. • Vanadium: vanadium concentrations increase during the first approximately 60 years of the model simulation period, level out, and then begin to decline towards the end of the modeling period. Three of four wells have initial increases and then begin to decrease between 40 to 100 years after excavation and continue to gradually decrease over the remainder of modeling period. Vanadium concentrations under the Excavation scenario remain above the IMAC at the compliance boundary at the Catawba River downgradient of the inactive ash basin and active ash basin. 4.1.5.4 Key Model Assumptions and Limitations The key model assumptions and limitations of the fate and transport model include, but are not limited to, the following: • The steady-state flow model was calibrated to hydraulic heads measured at monitoring wells in June 2015 and considered the active ash basin surface water level. The model is not calibrated to transient water levels over time, recharge, or river flow. A steady- state calibration does not consider groundwater storage and does not calibrate the groundwater flux into adjacent surface water bodies. • MOFLOW simulates flow through porous media, and groundwater flows in the bedrock flow layer via fractures in the bedrock. A single domain MODFLOW modeling approach for simulating flow within the groundwater flow layers and bedrock was used for contaminant transport at the Allen site. The model was calibrated by adjusting the constant source concentrations at the ash basins and ash storage area to reasonably match new installed CSA wells with detectable COI concentrations in groundwater. This model assumption was utilized as it reflected a simplified and more conservative approach to meet initial modeling requirements of CAMA. • For the purposes of numerical modeling and comparing closure scenarios, it is assumed that the selected closure scenario will be completed in Year 2015. Predictive simulations were performed and steady-state flow conditions were assumed from the time that the inactive ash basin and active ash basin were placed in service through the current time until the end of the predictive simulations (Year 2265). • COI source area concentrations at the inactive ash basin and active ash basin were assumed to be constant with respect to time for transport model calibration. 77 Corrective Action Plan Part 1 Allen Steam Station Ash Basin • The uncertainty in model parameters and predictions has not been quantified, and therefore the error in the model predictions is not known. It is assumed the model results are suitable for a relative comparison of closure scenarios. Model results along certain boundaries are impacted by the cell saturation level resulting in some numerical spikes in the calculations that do not represent concentrations in surrounding cells. These numerical spikes are localized calculations and do not impact the overall model results. • Since the Catawba River is modeled as a constant head boundary in the numerical model, it is not be possible to assess the effects of pumping wells or other groundwater sinks near the river. The model will be revised during CAP2 to accomplish the following tasks: • The model will be further refined to more rigorously reflect compliance, voluntary and newly installed CSA wells with non -detectable COI concentrations, • The model domain will expanded to include nearby water supply wells and PBBCs, • The model results will be further assessed to identify data gaps that would improve the conceptual site model, • Antimony will be assessed • Additional results from monitor well sampling will be included in further refinement of the calibration • The Kd value used for non -conservative COls will be further assessed during refinement and recalibration of the model. • Remedial alternatives will be simulated to evaluate potential source control or removal options. Refer to Appendix C for additional details regarding model assumptions. 4.1.5.5 Proposed Geochemical Modeling Data obtained during the CSA and subsequent interpretation, determination of groundwater flow, and fate and transport modeling have resulted in improvements to the SCM to enhance planned geochemical modeling. It has been determined that: • Site -specific groundwater flow and groundwater fate and transport is as hypothesized; it is operating within the regional flow and transport process in Piedmont Terrain. • Kd values can be calculated from batch adsorption isotherms for all COls at the Allen site. • The groundwater model can be constrained by site -specific Kd values. • Kd values within the observed site -specific range (determined in the laboratory) were used successfully to improve fate and transport model calibration. • The dominant attenuation processes, as initially hypothesized, are adsorption to hydrous metal oxides (HFO, HMO, HAO) and clay minerals. Hydrous metal oxides and clay minerals are abundant in the soil and transition zone, and it is assumed that 78 Corrective Action Plan Part 1 �� Allen Steam Station Ash Basin concentrations increase with the degree of bedrock weathering. • Correlations exist between COI concentrations and HFO, HMO, HAO, and clay minerals. • There is variability in pH and redox conditions across the Allen site; significant enough that pH and redox influences on COI attenuation should be evaluated. The binding of COls to HFO, HMO, HAO and clay minerals is known to be pH and redox sensitive. Under certain redox and pH conditions HFO, HMO, and HAO may be stable, may dissolve, or may actively precipitate. Clay mineral sorption is sensitive to pH and ionic strength (for example TDS). Sensitivity analysis of COI attenuation under variable conditions is warranted. As previously discussed in the CSA, the appropriate manner to conduct the sensitivity analysis for pH, Eh, and TDS is the use of geochemical models. Other sensitivity analysis can use more standard Geographical Information System (GIS) tools. 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. 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. Given this diverse range of conditions, next steps in the Allen site evaluation process may include: equilibrium geochemical speciation evaluation using modeling tools such as PHREEQC (USGS 2013) and groundwater and chemical transport modeling. Additional sampling will be needed to further characterize the temporal and spatial characteristics (i.e., redox reactions) of groundwater composition for the site. Mineral Stability: Pourbaix plots for HFO, HMO, and HAO and Observed Clay Minerals (OCM) will be created using site -specific chemistry to determine if minerals are stable under observed and postulated conditions. Spatial Variability in Retardation: Spatial variability in attenuation capacity and strength will be evaluated using geographic information systems (GIS) and the retardation equation (see Appendix C). Retardation represents the combined attenuation effect of reactive area (porosity and bulk density) and Kd. Use of GIS allows overlay of a retardation function at multiple depths and evaluation of correlation and sensitivity to other measured parameters such as HFO or clay mineral content. COI Adsorption under Variable pH and Redox Conditions: The dominant attenuation processes are highly sensitive to pH and redox values and variability. Sensitivity will be evaluated by: Using PHREEQC (USGS 2013) to determine the redox and pH changes that take place under source term conditions of capping (cessation of oxygen delivery by recharge and adjustment to a new dynamic equilibrium, draining and change in water/rock ratio). These results will be used to determine if there are changes in leachate chemistry, and if so, if the changes in leachate chemistry affect mobility outside the ash. 79 Corrective Action Plan Part 1 �� Allen Steam Station Ash Basin Under the observed variability in pH and redox, and postulated changes in pH and redox, evaluate the sensitivity of Kd to these conditions. With quantitative mineralogy and reactive surface area inputs site -specific sample attenuation can be simulated in PHREEQC using surface complexation subroutines. Surface complexation is analogous to Kd, but allows the variability of pH and TDS on adsorption to be modeled. PHREEQC will also be used to calculate redox conditions and speciation. The output of PHREEQC simulations on the effect of change on surface sorption properties will be used to determine what the expected distribution of species (e.g., As(II I)/As(V)) would be under those same changed conditions. 4.2 Groundwater - Surface Water Interaction Modeling Groundwater -surface water interactions were completed using groundwater model output and a surface water mixing model approach to evaluate potential surface water impacts of COls in groundwater as they discharge to surface water bodies adjacent to the Allen site. 4.2.1 Mixing Model Approach Groundwater model output from the fate and transport modeling discussed in Section 4.1 (i.e., groundwater volume flux and concentrations of COI with exceedances of the 2L Standards, IMACS, or DHHS HSLs were used as inputs for the surface water assessment in the Catawba River receiving waters adjacent to the Allen site. The Catawba River near the Allen site is classified as by NCDEQ as Class C and WS-V waters, and water quality is compared to the North Carolina Surface Water Pollutant Standards for Metals for freshwater aquatic life, water supply, or human health derived from the 2B Standards. Given that river flow in the Catawba River is unidirectional and groundwater discharge mixes with upstream flow, a mixing calculation was used to assess potential surface water quality impacts. A summary of this approach and NCDEQ's mixing zone regulations is presented below. Mixing Model Approach — This approach includes the effects of upstream flow on mixing and dilution of the groundwater plume within an allowable mixing zone. The results from this analysis provide information on constituent concentration as a function of the mixing zone distance from the groundwater input to the adjacent water body. Mixing Zone Regulations — A mixing zone is defined in the NCDEQ water quality standards (Subchapter 2B, 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 80 Corrective Action Plan Part 1 �� Allen Steam Station Ash Basin 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 [as defined by Rule .0202(1)] or prevent free passage of aquatic organisms around the mixing zone; o Result in offensive conditions; o Produce undesirable aquatic life habitat or result in a dominance of nuisance species outside of the assigned mixing zone; or o Endanger the public health or welfare. Although the NCDEQ mixing zone regulations are typically applied to point source discharges, the "free zone of passage" provision in the regulation was used in this surface water assessment. Mixing zone sizes and percentages of upstream river design flows used for assessing compliance with applicable water quality standards or criteria as presented in Section 4.2.2 and are provided in Table 4-1. Table 4-1. Mixing Zone Sizes and Percentages of Upstream River Flows Criteria Mixing Zone Size Percent of Design River Flow' Acute Aquatic Life 10% of River Width or 150 feet 10% of 1 Q10 Chronic Aquatic Life 50% of River Width or 750 feet 50% of 7Q10 Human Health/Water 50% of River Width or 750 feet 50% of 7Q10 Supply(non-carcinogen) Human Health/Water 100% of River Width or 1500 feet 100% of Annual Mean Supply (carcinogen) 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 (USEPA 2013b). Mean annual flow is the long-term average annual flow based on complete annual flow records. Using the mixing zone approach, output from the groundwater model (e.g., flows and COI concentrations) were used in the mixing calculation to determine COI concentrations in the adjacent water body from the point of discharge. These surface water results were compared to applicable surface water quality standards or criteria to evaluate compliance at the edge of the mixing zone(s). The development of the mixing model inputs required additional data for upstream river flow and COI concentrations, which were obtained from readily available USGS data sources in addition to site -specific surface water quality data collected as part of the CSA. 4.2.2 Surface Water Model Results The calculated surface water COI concentrations in the Catawba River downstream of the Allen site are presented in Table 4-2. The river design flows, upstream surface water concentrations, groundwater flows, and groundwater COI concentrations presented in Appendix E were used to complete these calculations. The surface water quality modeling results indicate that no water 81 Corrective Action Plan Part 1 Allen Steam Station Ash Basin quality standards or criteria are exceeded for COls modeled at the edge of the mixing zones in the Catawba River. Table 4-2. Catawba River (Lake Wylie) Calculated Surface Water Concentrations COI Calculated Mixing Zone Conc. (tag/L) Water Quality Criteria (tag/L) Acute Chronic HH / WS Acute Chronic HH/WS Antimony 0.413 0.268 0.268 (nc) ns ns 640 / 5.6 Arsenic 0.627 0.291 0.252 (c) 340 150 10 / 10 Barium 5.36 4.59 4.59 (nc) ns ns 200,000 / 1,000 Boron 51.4 27.9 25.1 * ns ns ns / ns Chromium VI 0.328 0.259 0.259* 16 11 ns / ns Cobalt 0.646 0.293 0.293 (nc) ns ns 4/3 Selenium 0.479 0.275 0.251 * ns 5 ns / ns Sulfate 3,689 847 847 (nc) ns ns ns / 250,000 Vanadium 0.739 0.526 0.501 * ns ns ns / ns Notes: 1. All Cols are shown as dissolved fraction except for total chromium, which is total recoverable metal 2. HH / WS — Human Health / Water Supply 3. Water Quality Criteria refers to USEPA National Recommended Water Quality Criteria 4. c — carcinogen 5. nc — non -carcinogen 6. ns — no water quality standard 7. * — concentration calculated with annual mean river flow Refinement of SCM Groundwater and surface water models have been used to provide further information regarding the transport of COls toward the Catawba River. The groundwater model will be further refined (i.e. recalibrated) to include nearby public and private drinking water wells and additional groundwater water quality samples. The refined model will be evaluated to assess the potential influence to groundwater flow and fate and transport of COls. If determined necessary, remedial alternatives will be simulated to evaluate potential source control or removal options to ensure no impacts to public and private drinking water wells. Although, the groundwater - surface water interaction model did not identify potential impacts to surface waters of the Catawba River further refinement of COls will be addressed with additional on -site and downgradient sample locations. 82 Corrective Action Plan Part 1 �� Allen Steam Station Ash Basin 5 Summary and Recommendations Based on the analysis of the data presented in this report, Duke Energy provides the following summary and recommendations: • PPBCs were calculated for soil at the site and are presented in Section 2. Soil PPBCs were calculated as the 95% upper tolerance limit for soil constituents. For constituents where there were too few laboratory detections reported to use the statistical methodology, the PPBCs were established by setting the value equal to the highest concentration (or the highest method reporting limit for non -detect values). HDR reviewed groundwater sampling data from the newly installed CSA background monitoring wells in conjunction with sampling data from the existing background compliance wells to evaluate their suitability for use as background conditions. Based on this review, compliance monitoring background well AB-1 R was determined to be unsuitable for use as a background well. Also at the request of NCDEQ, groundwater analytical results with turbidity greater than 10 NTU were removed from the data set prior to establishing PPBCs. As a result, too few samples were available to complete statistical analysis to determine background concentrations. Therefore, the highest concentration or highest method reporting limit for non -detects were selected as the PPBC for each constituent. PPBCs will be refined as additional data are obtained from newly installed background monitoring wells during subsequent sampling events. • 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 high due to the effects of turbidity. Groundwater samples collected during the CSA, and subsequent monitoring events, were analyzed for total and dissolved phase constituents to evaluate potential effects of turbidity. The list of COls to be carried forward in CAP Part 2 will be modified, if warranted, as additional groundwater quality data is obtained and the possible effects of turbidity on the analytical results are evaluated. Geochemical modeling of the Allen site will be completed and submitted in the CAP Part 2 report. The geochemical model results, taken into consideration with the groundwater flow, fate, and transport and surface water -groundwater models enhance the understanding of the processes taking place in the subsurface and will ultimately aid in choosing the most appropriate remedial action for the site. The geochemical model is key to understanding mobility of iron, manganese, and TDS since it cannot adequately be modeled using MODFLOW/MT3DMS. 83 Corrective Action Plan Part 1 �� Allen Steam Station Ash Basin • The groundwater modeling results are summarized as follows: o Existing Conditions Scenario — The results of the Existing Conditions scenario indicated that concentrations for all modeled COls predominately increase or reach steady-state conditions above 2L Standards, IMACs, or DHHS HSL during the modeled period. The fate and transport modeling was performed for a 250-year simulation period. At the end of the 250-year simulation period, 7 of 10 constituents were estimated by the model to be above the 2L Standards, IMACs, or DHHS HSL at the compliance boundary adjacent to the Catawba River (i.e., eastern boundary of the site). o Cap -in -Place Scenario — The COI modeled concentrations predominately decrease over the 250-year simulation period with the exception of chromium, hexavalent chromium, and vanadium. Modeled concentrations of the majority of the COls still remain higher than their respective 2L Standards, IMACs, or DHHS HSL (the exceptions being barium, boron, and sulfate). Modeled concentrations of boron and sulfate (non-sorptive COls) are depleted in the model simulation within 40 years as a result of the cap. o Excavation Scenario — Model results for COI concentrations are highly variable overtime. Under the Excavation model scenario, non-sorptive COls, boron and sulfate, are depleted from the model in 20 years or less. Model results indicate that concentrations of chromium, hexavalent chromium, and vanadium continue to increase in over the model simulation period. Antimony, arsenic, hexavalent chromium, cobalt, and vanadium exceed their respective 2L Standards or IMACs at the end of the model simulation period (i.e., 250 years). Model results for other constituents are below the 2L Standards, IMACs, or DHHS HSL in time periods ranging from 50 to 250 years. Regarding persistence of COls, sulfate and boron are similar in that both are considered conservative; that is, neither has a strong affinity to attenuate nor adsorb to soil/rock surfaces. As a result, similar behavior is predicted for both of these COls, and other COls with low sorption coefficients (Kd) - rapid and nearly complete reduction predicted under all closure scenarios, with the Excavation scenario proving to be the most effective. Groundwater flow rates and concentrations of COls from the groundwater model were used as inputs to a groundwater -surface water interaction model to determine if 2L Standard, IMAC, or DHHS HSL exceedances would result in exceedances of 2B surface water standards (or USEPA National Recommended Water Quality Criteria) in the Catawba River. Surface water modeling results indicate that water quality standards or criteria are not exceeded at the edge of the mixing zones in the Catawba River adjacent to the Allen site. 84 Corrective Action Plan Part 1 Allen Steam Station Ash Basin The following recommendations are made to address areas needing further assessment: • PPBCs should be refined based on additional sampling of background wells (four sampling rounds in 2015). Additional upgradient wells are recommended to provide background groundwater chemistry for the site west and north of the source area. • Groundwater and seep COls should be updated with results from the second -round sampling performed at the Allen site and should be included in the CAP Part 2 report. • An on -site background surface water and seep sample location (if applicable) should be identified and sampled. • Additional sampling for radiological parameters along major groundwater flow paths is needed to perform a more comprehensive assessment of radionuclides from source areas and downgradient wells along major flow paths. • The SCM and groundwater models should be updated with groundwater elevations and analytical results from second -round sampling at the Allen site and should be included in the CAP Part 2 report. • The model will be revised during CAP Part 2 to accomplish the following tasks: o The model will be further refined to more rigorously reflect compliance, voluntary and newly installed CSA wells with non -detectable COI concentrations. o The model domain will be expanded to include nearby water supply wells and PBBCs. o The model results will be further assessed to identify data gaps that would improve the conceptual site model. o Antimony will be assessed. o Additional results from monitor well sampling will be included in further refinement of the calibration. o The Kd value used for non -conservative COls will be further assessed during refinement and recalibration of the model. o Remedial alternatives will be simulated to evaluate potential source control or removal options. • The groundwater to surface water model should refined based on results from second round sampling activities at the Allen site. • Additional groundwater elevation data are needed in the vicinity of 13G-2 and along South Point Road to better define a likely groundwater flow direction and groundwater quality. These data will be collected as part of the data gap well installations. The well cluster BG-1 had groundwater elevation at the same or lower elevation than porewater levels in the ash basin. Additional wells are recommended for installation in 85 Corrective Action Plan Part 1 �� Allen Steam Station Ash Basin the vicinity of the BG-1 cluster to further define groundwater flow direction and groundwater quality. Once this is accomplished, the use of this well cluster for background water quality will be re-evaluated. 86 Corrective Action Plan Part 1 �� Allen Steam Station Ash Basin 6 References AMEC. 2015. Natural Resources Technical Report, Allen Steam Station, Gaston County, North Carolina. June 24. Back, W., and I. 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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