The URL can be used to link to this page

Home My WebLink About1 MSS CAP Part 2_Report_FINALF)R Corrective Action Plan Part 2 Marshall Steam Station Ash Basin Site Location: NPDES Permit No. Permittee and Current Property Owner: Consultant Information: Report Date: Marshall Steam Station 8320 NC Highway 150 E Terrell, NC 28682 NC0004987 Duke Energy Carolinas, LLC 526 South Church St Charlotte, NC 28202 704.382.3853 HDR Engineering, Inc. of the Carolinas 440 South Church St, Suite 900 Charlotte, NC 28202 704.338.6700 March 3, 2016 This page intentionally left blank Corrective Action Plan Part 2 F)� Marshall Steam Station Ash Basin Contents Acknowledgements ExecutiveSummary ................................................................................................................................ 1 Introduction.................................................................................................................................... 1.1 Regulatory Background....................................................................................................... 1.2 Report Organization............................................................................................................ 2 Summary of Previous and Current Studies................................................................................... 2.1 Comprehensive Site Assessment....................................................................................... 2.1.1 Identification of COls.............................................................................................. 2.1.2 Soil Delineation...................................................................................................... 2.1.3 Groundwater Delineation....................................................................................... 2.2 Corrective Action Plan Part 1.............................................................................................. 2.2.1 Proposed Provisional Background Concentrations for Soil and Groundwater ...... 2.2.2 COI Occurrence and Distribution........................................................................... 2.3 Round 2 Sampling............................................................................................................... 2.3.1 Groundwater........................................................................................................... 2.3.2 Round 1 and Round 2 Source Area and Groundwater Data Comparison ............. 2.3.3 Surface Water and Area of Wetness...................................................................... 2.4 Round 3 and Round 4 Background Well Sampling............................................................. 3 Site Conceptual Model.................................................................................................................. 3.1 Identification of Potential Contaminants.............................................................................. 3.2 Identification and Characterization of Source Contaminants .............................................. 3.3 Delineation of Potential Migration Pathways through Environmental Media ...................... 3.3.1 Soil.......................................................................................................................... 3.3.2 Groundwater........................................................................................................... 3.3.3 Surface Water and Sediment................................................................................. 3.4 Establishment of Background Areas................................................................................... 3.5 Environmental Receptor Identification and Discussion....................................................... 3.6 Determination of System Boundaries.................................................................................. 3.7 Site Geochemistry and Influence on COls.......................................................................... 4 Modeling........................................................................................................................................ 4.1 Groundwater Model Refinement......................................................................................... 4.1.1 Flow Model Refinements........................................................................................ 4.1.2 Contaminant Fate and Transport Model Refinements ........................................... 4.1.3 Summary of Modeled Results................................................................................ 4.1.4 Model Assumptions and Limitations....................................................................... 4.1.5 Modeled Scenario Results..................................................................................... 4.2 Surface Water Model Refinement....................................................................................... 4.2.1 Methodology........................................................................................................... 4.2.2 Results................................................................................................................... 4.3 Geochemical Modeling........................................................................................................ 4.3.1 Objective................................................................................................................ 4.3.2 Methodology........................................................................................................... 4.3.3 Assumptions........................................................................................................... 4.3.4 Geochemical Model Results.................................................................................. .5 .5 .7 .8 .8 .8 .9 .9 10 10 10 11 11 13 17 18 19 19 19 20 20 21 21 22 22 22 22 25 25 25 26 27 28 28 31 31 31 32 32 32 33 34 n Corrective Action Plan Part 2 F)� Marshall Steam Station Ash Basin 4.4 Refined Site Conceptual Model............................................................................................... 34 5 Risk Assessment............................................................................................................................... 35 5.1 Step 1: Conceptual Site Model................................................................................................ 35 5.2 Step 2: Risk -Based Screening................................................................................................ 36 5.3 Step 3: Human Health Risk Assessment................................................................................37 5.4 Step 4: Ecological Risk Assessment....................................................................................... 38 6 Alternative Methods for Achieving Restoration................................................................................. 40 6.1 Corrective Action Decision Process........................................................................................ 40 6.1.1 Evaluation Criteria......................................................................................................40 6.1.2 COIs Requiring Corrective Action.............................................................................. 41 6.1.3 Potential Exposure Routes and Receptors................................................................ 41 6.2 Alternative Evaluation Criteria.................................................................................................41 6.2.1 Effectiveness.............................................................................................................. 42 6.2.2 Implementability/Feasibility........................................................................................ 42 6.2.3 Environmental Sustainability......................................................................................43 6.2.4 Cost............................................................................................................................ 44 6.2.5 Stakeholder Input....................................................................................................... 44 6.3 Remedial Alternatives to Achieve Regulatory Standards....................................................... 44 6.3.1 Groundwater Remediation Alternatives..................................................................... 44 6.3.2 Monitored Natural Attenuation Applicability to Site....................................................45 6.3.3 Site -Specific Alternatives Analysis............................................................................. 46 6.3.4 Site -Specific Recommended Approach..................................................................... 49 7 Selected Corrective Action(s)............................................................................................................ 51 7.1 Selected Remedial Alternative for Corrective Action.............................................................. 51 7.2 Conceptual Design.................................................................................................................. 51 7.2.1 Source Control — Cap-in-Place.................................................................................. 51 7.2.2 MNA............................................................................................................................ 52 8 Recommended Interim Activities....................................................................................................... 53 8.1 Additional Monitoring Well Installation.................................................................................... 53 8.2 Additional Groundwater Sampling and Analyses.................................................................... 53 9 Interim and Effectiveness Monitoring Plans......................................................................................54 9.1 Interim Monitoring Plan........................................................................................................... 54 9.1.1 Data Quality Objectives.............................................................................................. 54 9.1.2 Sampling Requirements.............................................................................................55 9.1.3 Reporting....................................................................................................................55 9.2 Effectiveness Monitoring Plan.................................................................................................56 9.2.1 Data Quality Objectives.............................................................................................. 56 9.2.2 Sampling Requirements.............................................................................................56 9.2.3 Reporting....................................................................................................................57 9.3 Sampling and Analysis............................................................................................................ 57 9.3.1 Monitoring Well Measurements and Inspection......................................................... 57 9.3.2 Surface Water Measurements................................................................................... 58 9.3.3 Sample Collection...................................................................................................... 58 9.3.4 Quality Assurance/Quality Control............................................................................. 60 10 Implementation Cost and Schedule.................................................................................................. 61 10.1 Implementation Cost............................................................................................................... 61 Corrective Action Plan Part 2 F)� Marshall Steam Station Ash Basin 10.2 Implementation Schedule........................................................................................................ 62 11 References Tables 63 2-1 Summary of Horizontal Hydraulic Gradient Calculations 2-2 Exceptions to Vertical Hydraulic Gradients* 2-3 Comparison of COI Sample Results using 0.45 pm and 0.1 pm Filters 2-4 Ash Porewater Analytical Results — Round 1 and Round 2 2-5 Ash Basin Water Analytical Results — Round 1 and Round 2 2-6 Background Groundwater Analytical Results — Rounds 1 through 4 2-7 Groundwater Analytical Results Upgradient of Source Areas — Round 1 and Round 2 2-8 Groundwater Analytical Results Beneath and Adjacent to Ash Basin — Round 1 and Round 2 2-9 Groundwater Analytical Results Beneath the Dry Ash Landfill (Phase II) — Round 1 and Round 2 2-10 Groundwater Analytical Results Downgradient and East of Ash Basin and Dry Ash Landfill (Phase 1) — Round 1 and Round 2 2-11 Groundwater Analytical Results Downgradient and Southeast of Ash Basin — Round 1 and Round 2 2-12 Constituents of Interest Evaluation 2-13 Surface Water Sample Analytical Results — Round 1 and Round 2 2-14 Areas of Wetness Sampling Analytical Results — Round 1 and Round 2 4-1 Summary of Modeled COI Results at the Compliance Boundary* 4-2 Lake Norman Calculated Surface Water Concentrations 9-1 Interim Monitoring Plan Sample Locations 9-2 Sampling Parameters and Analytical Method 10-1 Estimated Capital and Annual Costs for Corrective Action - MNA* * Table is presented within the text of this CAP Part 2 Report. Figures 2-1 Site Sampling Locations 2-2 Water Table Surface Map — Shallow Wells (S) 2-3 Potentiometric Surface Map — Deep Wells (D) 2-4 Potentiometric Surface Map — Bedrock Wells (BR) 2-5 Areas of Exceedances 3-1 Site Conceptual Model — 3D Representation 3-2 Site Conceptual Model — Cross Sectional (4 Sheets) 3-3 Receptor Map 8-1 Additional Assessment Wells Appendices A CSA Addendum B Groundwater Flow and Transport Model C Addendum to Soil Sorption Evaluation D Surface Water Mixing Model Approach C Geochemical Modeling Report D Baseline Human Health and Ecological Risk Assessment E Evaluation of Potential Groundwater Remedial Alternatives F Monitored Natural Attenuation Technical Memorandum Corrective Action Plan Part 2 F)� Marshall Steam Station Ash Basin Acronyms and Abbreviations tag/L micrograms per liter 2B Standard North Carolina surface water standards as specified in T15 NCAC 02B .0211 and .0216 (amended effective January 2015) 2L Standard North Carolina groundwater standards as specified in T15A NCAC 02L 3-D three-dimensional AOW area of wetness ASTM ASTM International BERA Baseline Ecological Risk Assessment BG background BR bedrock CAMA North Carolina Coal Ash Management Act of 2014 CAMC North Carolina Coal Ash Management Commission CAP Corrective Action Plan CCR coal combustion residuals COI constituent of interest COPC contaminant of potential concern CSA Comprehensive Site Assessment D deep DO dissolved oxygen DQO data quality objectives Duke Energy Duke Energy Carolinas, LLC DWR NCDEQ Division of Water Resources Eh oxidation-reduction potential (see also ORP) EPC exposure point concentration EPRI Electric Power Research Institute FGD flue gas desulfurization ft bgs feet below ground surface ft/ft feet / foot HAO hydrous aluminum oxide HFO hydrous ferric oxide HQ hazard quotient HSL health screening level IMAC interim maximum allowable concentration Kd sorption coefficient mg/kg milligrams per kilogram MNA monitored natural attenuation MW monitoring well MSS Marshall Steam Station NC PSRGs North Carolina Preliminary Soil Remediation Goals NCAC North Carolina Administrative Code iv Corrective Action Plan Part 2 Marshall Steam Station Ash Basin NCDENR North Carolina Department of Environment and Natural Resources NCDEQ North Carolina Department of Environmental Quality NCDHHS North Carolina Department of Health and Human Services NPDES National Pollutant Discharge Elimination System NTU nephelometric turbidity units ORP oxidation-reduction potential POG protection of groundwater PPBC proposed provisional background concentration PRB permeable reactive barrier PV photovoltaic RBC risk -based concentrations SCM site conceptual model TDS total dissolved solids UNCC University of North Carolina at Charlotte USEPA U.S. Environmental Protection Agency USGS U.S. Geological Survey Work Plan Groundwater Assessment Work Plan Corrective Action Plan Part 2 Marshall Steam Station Ash Basin This page intentionally left blank Corrective Action Plan Part 2 F)� Marshall Steam Station Ash Basin Acknowledgements HDR would like to express its appreciation to Duke Energy for its comments and interim report reviews, and to the parties listed below for their assistance with data analysis, report preparation, quality reviews, and overall development of this corrective action plan. The University of North Carolina at Charlotte — Groundwater modeling and soil sorption analysis Electric Power Research Institute — Groundwater flow and transport model third -party peer review Geochemical, LLC — Monitored natural attenuation evaluation and soil sorption analysis CH2M Hill, Inc. — Remedial alternative analysis Leggette, Brashears & Graham, Inc — Groundwater modeling Haley & Aldrich, Inc. — Risk assessment third -party peer review Corrective Action Plan Part 2 Marshall Steam Station Ash Basin This page intentionally left blank Corrective Action Plan Part 2 F)� Marshall Steam Station Ash Basin Executive Summary 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 the Marshall Steam Station (MSS) was submitted to the North Carolina Department of Environment and Natural Resources (NCDENR) on September 24, 2014, and was subsequently revised on December 30, 2014. The revised Work Plan was conditionally approved by NCDENR on March 12, 2015. A Comprehensive Site Assessment (CSA) was performed to collect information necessary to evaluate the horizontal and vertical extent of impacts to soil and groundwater attributable to CCR source area(s), identify potential receptors, and screen for potential risks to those receptors. The MSS CSA Report was submitted to NCDENR on September 8, 2015 (HDR 2015a). Subsequent to submittal of the CSA Report, CAMA requires submittal of a Corrective Action Plan (CAP) for each regulated facility no later than 180 days after submittal of the CSA Report. Duke Energy Carolinas, LLC (Duke Energy) and the North Carolina Department of Environmental Quality (NCDEQ)' agreed to a two-part CAP submittal, with Part 1 being submitted within 90 days of submittal of the CSA Report and Part 2 being submitted no later than 180 days after submittal of the CSA Report. The MSS CAP Part 1 Report was submitted to NCDEQ on December 7, 2015 (HDR 2015b). On December 31, 2015, NCDEQ released draft proposed risk classifications for Duke Energy's coal ash impoundments in North Carolina. The proposed risk classification for the MSS ash basin was low -to -intermediate. Risk classifications were based upon potential risk to public health and the environment. A public meeting regarding the proposed risk classification for the MSS impoundments is scheduled for March 29, 2016. NCDEQ will release the final risk classifications, subject to approval by the North Carolina Coal Ash Management Commission (CAMC), after review of public comments. Duke Energy owns and operates MSS, located in Catawba County near the town of Terrell, North Carolina. MSS began operations in 1965 as a coal-fired generating station and currently operates four coal-fired units. CCR consisting of bottom and fly ash material from MSS has been disposed in the station's ash basin, located north of the station adjacent to Lake Norman, since the basin was constructed. Dry ash has been disposed in other areas at the site including the dry ash landfill units (Phases I and II) and Industrial Landfill No. 1. Flue gas desulfurization (FGD) residue (i.e., gypsum) and fly ash were disposed in the FGD residue landfill, which was placed in intermediate closure in October 2015. Fly ash utilized as structural fill was placed in the photovoltaic (PV) structural fill and was used as structural fill beneath portions of the Industrial Landfill No. 1. Discharge from the ash basin is permitted by the NCDEQ Division of Water Resources (DWR) under the National Pollutant Discharge Elimination System (NPDES) Permit NC0004987. Prior to September 18, 2015, the NCDEQ was referred to as the NCDENR. Both naming conventions are used in this report, as appropriate. 1 Corrective Action Plan Part 2 F)� Marshall Steam Station Ash Basin Groundwater at the MSS site generally flows from the north and northwest extents of the property boundary to the south and southeast beneath the source areas toward Lake Norman, which serves as the hydrologic boundary downgradient of the CCR source areas. Based on evaluation of data presented in the CSA Report, groundwater concentrations of constituents of interest (COIs)Z attributable to source areas at the MSS site were identified beneath the ash basin, beneath the dry ash landfill units, and downgradient of the ash basin. COI transport in groundwater is consistent with the direction of groundwater flow. COls in groundwater attributable to ash handling at the MSS site are: antimony, arsenic, barium, beryllium, boron, chromium3, cobalt, iron, manganese, selenium, sulfate, thallium, total dissolved solids (TDS), and vanadium. Note that antimony, barium, chromium, cobalt, hexavalent chromium, iron, manganese, thallium, and vanadium were also detected above their groundwater quality standard or criteria in site background groundwater. Further sampling and analyses are necessary to determine if COI exceedances are the result of source -related impacts or naturally occurring conditions (as discussed in Section 2). Groundwater fate and transport modeling was performed for two closure scenarios for the MSS ash basin during CAP Part 2: An Existing Conditions scenario with ash sources left in place and a Cap -in -Place scenario, which simulates the effects of covering the ash basin with an engineered cap at the beginning of the predictive simulation. The refined groundwater model predicts that certain COls will exceed regulatory standards or criteria at the Compliance Boundary4 in the Cap -in -Place closure scenario, as discussed in Section 4.1.5; however, based on results of the groundwater to surface water interaction modeling, no surface water quality standards or criteria are exceeded at the edge of the mixing zones in Lake Norman. A human health and ecological risk assessment was conducted as part of this CAP. Evaluation of potential impacts to humans indicates that risk estimates for several COPCs are above risk targets for the recreational fisher and subsistence fisher from ingestion of fish caught near the site. Additional data regarding site -specific conditions, delineation of source -related and background COls applicable to the risk assessment, and evaluation of the exposure parameters and fish ingestion models used in the risk assessment are needed to qualify these results. Evaluation of potential impacts to ecological receptors indicates that risk estimates for several COPCs are above risk targets for some water -dependent birds, if and where these species are present at the MSS site. Additional data and further refined assessment are needed to address uncertainties associated with the evaluation of these scenarios including the occurrence of these ecological receptors in the areas adjacent to the ash basin, delineation of source -related 2 If a constituent concentration exceeded the North Carolina Groundwater Quality Standards as specified in Title 15A NCAC .0202L (2L Standards), Interim Maximum Allowable Concentration (IMAC), North Carolina Preliminary Soil Remediation Goals for Protection of Groundwater (NC PSRGs for POG), North Carolina Department of Health and Human Services Health Screening Level (NCDHHS HSL), North Carolina Surface Water Quality Standards as specified in T15 NCAC 02B .0211 and .0216 (amended effective January 2015) (213 Standards), or U.S. Environmental Protection Agency National Recommended Water Quality Criteria, it has been designated as a "constituent of interest". 3 Unless otherwise noted, references to chromium in this document indicate total chromium. 4 Per 15A NCAC 02L .0102, "Compliance Boundary" means a boundary around a disposal system at and beyond which groundwater quality standards may not be exceeded and only applies to facilities which have received a permit issued under the authority of G.S. 143-215.1 or G.S. 130A. Corrective Action Plan Part 2 F)� Marshall Steam Station Ash Basin and background Cols applicable to the risk assessment, and refinement of the exposure and toxicity assumptions used in the ecological risk characterization. Based on evaluation of data collected to date, Duke Energy is proposing to utilize cap -in -place as a source control measure at the MSS ash basin. The cap -in -place option would consist of constructing an engineered cap system over the ash areas. The final closure option selected will be modified as required based on the final risk classification proposed by NCDEQ and approved by the North Carolina CAMC. An evaluation of site conditions, constituents, and a review of alternative methods for restoring groundwater quality found that, in conjunction with the planned source control, monitored natural attenuation (MNA) is recommended as a supplemental corrective action for groundwater impacts beneath the site. In addition, further investigation should be conducted to evalaute the effectiveness of a groundwater cutoff wall to intercept groundwater flowing from the ash basin to the unnamed tributary east of the basin. An Interim Monitoring Plan has been developed to provide baseline seasonal groundwater data for the MSS site and will be implemented with sampling activities planned for the first two quarters of 2016. Interim monitoring results will be used to evaluate compliance and may be used, as needed, to refine the groundwater contaminant fate and transport, groundwater to surface water interaction, and geochemical models. Although Duke Energy acknowledges that groundwater quality may be influenced by basin closure activities, an Effectiveness Monitoring Plan will be implemented prior to completion of closure to monitor these potential affects and establish a baseline for MNA effectiveness monitoring. The monitoring results will also be used to confirm that attenuation of COls is continuing to occur and remains an effective corrective action for groundwater at the MSS site. If MNA is deemed insufficient for restoration of groundwater quality, other alternatives discussed in Section 6.3.3 should be evaluated and, if warranted, implemented to augment the MNA processes. The performance of these corrective actions will continue to be monitored and evaluated to determine if modifications to the measures are warranted. Per CAMA §130A-309.209.(b)(1), "The Groundwater Corrective Action Plan shall provide for the restoration of groundwater in conformance with the requirements of Subchapter L of Chapter 2 of Title 15A of the North Carolina Administrative Code." This CAP meets the requirements of 15A NCAC 02L .0106 and the requirements of the referenced section of CAMA. Corrective Action Plan Part 2 Marshall Steam Station Ash Basin This page intentionally left blank Corrective Action Plan Part 2 F)� Marshall Steam Station Ash Basin 1 Introduction Duke Energy Carolinas, LLC (Duke Energy) owns and operates the Marshall Steam Station (MSS), located in Catawba County near the town of Terrell, North Carolina. MSS began operations in 1965 as a coal-fired generating station and currently operates four coal-fired units. Coal combustion residual (CCR) consisting of bottom and fly ash material from MSS has been disposed in the station's ash basin, located north of the station adjacent to Lake Norman, since construction of the ash basin. Dry ash has been disposed in other areas at the site including the dry ash landfill units (Phases I and II) and Industrial Landfill No. 1. Flue gas desulfurization (FGD) residue (i.e., gypsum) and fly ash were disposed in the FGD residue landfill, which was placed in intermediate closure in October 2015. Fly ash was used as structural fill in the photovoltaic (PV) structural fill and beneath portions of the Industrial Landfill No. 1. Discharge from the ash basin is permitted by the North Carolina Department of Environmental Quality (NCDEQ)5 Division of Water Resources (DWR) under the National Pollutant Discharge Elimination System (NPDES) Permit NC0004987. 1.1 Regulatory Background The North Carolina Coal Ash Management Act of 2014 (CAMA) directs owners of CCR surface impoundments in North Carolina to conduct groundwater monitoring, assessment, and remedial activities, if necessary. A Groundwater Assessment Work Plan (Work Plan) for MSS was submitted to North Carolina Department of Environment and Natural Resources (NCDENR) on September 24, 2014, followed by a revised Work Plan on December 30, 2014. The revised Work Plan was conditionally approved by NCDENR on March 12, 2015. A Comprehensive Site Assessment (CSA) was performed to collect information necessary to evaluate horizontal and vertical extent of impacts to soil and groundwater attributable to CCR source area(s), identify potential receptors, and screen for potential risks to those receptors. The MSS CSA Report was submitted to NCDENR on September 8, 2015 (HDR 2015a). CAMA Section §130A-309.209(b) requires implementation of corrective action for the restoration of groundwater quality in accordance with the North Carolina Administrative Code (T15A NCAC 02L) and requires the submittal of a Corrective Action Plan (CAP) for each regulated facility no later than 180 days after submittal of the CSA. Duke Energy and NCDEQ mutually agreed to a two-part CAP submittal, with Part 1 being submitted within 90 days of submittal of the CSA Report and Part 2 being submitted no later than 180 days after submittal of the CSA Report. The MSS CAP Part 1 Report (HDR 2015b) was submitted to NCDEQ on December 7, 2015 and consisted of the following: • background information • brief summary of the CSA findings • brief description of the site geology and hydrogeology 5 Prior to September 18, 2015, the NCDEQ was referred to as the North Carolina Department of Environment and Natural Resources (NCDENR). Both naming conventions are used in this report, as appropriate. Corrective Action Plan Part 2 F)� Marshall Steam Station Ash Basin • summary of the previously completed receptor survey • summary of constituent of interest (COI) exceedances and distribution • development of proposed provisional background concentrations (PPBCs) for soil and groundwater • detailed description of the site conceptual model (SCM) • results of the groundwater flow and fate and transport model • results of the groundwater to surface water interaction model The purpose of this CAP Part 2 is to provide the following: • a description of exceedances of groundwater quality standards, surface water quality standards, and sample results greater than the Interim Maximum Allowable Concentrations (IMAC) and North Carolina Department of Health and Human Services (NCDHHS) health screening levels (HSL) • present Round 3 and 4 background sampling results • a refined SCM • refined groundwater flow and fate and transport model results • refined groundwater to surface water model results • site geochemical model results • findings of the risk assessment • evaluation of methods for achieving groundwater quality restoration • conceptual plan(s) for recommended proposed corrective action(s) • a schedule for implementation of the proposed corrective action(s) • a plan for monitoring and reporting of the effectiveness of the proposed corrective action(s) The information provided in the combined CAP Part 1 and CAP Part 2 meets the requirements of regulation 15A NCAC 02L .0106 (f) for corrective action. Regulation 15A NCAC 02L .0106 (f)(4) requires that the secondary sources, which could be potential continuing sources of possible pollutants to groundwater, be addressed in the CAP. At the MSS site, the soil located below the ash basin, dry ash landfill units and PV structural fill could be considered as potential secondary sources. Information collected to date indicates that the thickness of soil impacted by ash is generally limited to the depth near the ash -soil interface. This proposed corrective action plan considers cap -in -place as the source control measure for the ash basin. An engineered cap will minimize infiltration through the covered area reducing possible impacts from potentially impacted soil. Therefore, remediation of soils is not discussed in this report. On December 31, 2015, NCDEQ released draft proposed risk classifications for Duke Energy's coal ash impoundments in North Carolina. The proposed risk classification issued for the MSS ash basin was low -to -intermediate. The risk classification was based upon potential risk to public health and the environment. A public meeting regarding the proposed risk classification for the MSS ash basin is scheduled for March 29, 2016. According to the CAMA, NCDEQ will submit its final risk classifications for review and approval by the North Carolina Coal Ash Management Commission (CAMC) after review of public comments. Corrective Action Plan Part 2 F)� Marshall Steam Station Ash Basin 1.2 Report Organization The information identified above has been organized in this CAP Part 2 report as follows: • Section 1 provides an introduction to the MSS site and the intent of corrective action under CAMA. • Section 2 provides a summary of the CSA and CAP Part 1 reports; a comparison of Round 1 and Round 2 groundwater, surface water, and area of wetness (AOW) sampling results; and a summary of Round 3 and Round 4 background well sampling results. • Section 3 discusses the SCM and site geochemical controls on contaminant mobility. • Section 4 discusses purpose, methodologies, and results of refined groundwater, groundwater to surface water interaction, and geochemical modeling. Refinement of the SCM following evaluation of model results is also discussed in this section. • Section 5 provides a summary of the human health and ecological risk assessments. • Section 6 presents an evaluation of remedial alternatives to achieve groundwater restoration. • Section 7 provides a concept -level discussion and plans for recommended corrective action(s). • Section 8 discusses recommended interim activities to be initiated in 2016. • Section 9 provides a plan for interim and effectiveness groundwater monitoring. • Section 10 provides a schedule and cost opinion for CAP implementation and post -CAP monitoring. Applicable tables, figures, and appendices with supporting documents are included with this report. Corrective Action Plan Part 2 F)� Marshall Steam Station Ash Basin 2 Summary of Previous and Current Studies This section presents a summary of previous and current studies including the following: • Summary of the CSA. • Summary of the CAP Part 1. • Presentation of Round 2 groundwater, surface water, and AOW sampling results including comparison to Round 1 sampling results. • Presentation of Round 3 and 4 background well sampling results. Round 1 sampling data were previously provided in the CSA Report. Subsequent sampling rounds occurred after the CSA Report submittal and are presented in this CAP Part 2 Report. 2.1 Comprehensive Site Assessment The purpose of the MSS CSA was to collect information necessary to characterize the extent of impacts resulting from historical production and storage of coal ash, evaluate the chemical and physical characteristics of the contaminants, investigate the geology and hydrogeology of the site including factors relating to contaminant transport, and examine risk to potential receptors and exposure pathways. The following assessment activities were performed as part of the CSA. • Completion of soil borings and installation of groundwater monitoring wells to facilitate collection and analysis of chemical, physical, and hydrogeological parameters of subsurface materials encountered within and beyond the waste and Compliance Boundary(s)s. • Evaluation of laboratory analytical data to support the SCM. • Update of the receptor survey previously completed in September 2014 (updated November 2014) (HDR 2014a, 2014b). • Completion of a screening -level risk assessment. Note that subsequent to submittal of the CSA Report, additional evaluation of Round 1 sampling results has been conducted. Responses to NCDEQ comments and additional information in response to the exceptions identified in the CSA Report are provided in Appendix A. 2.1.1 Identification of COIs If a constituent concentration exceeded the North Carolina Groundwater Quality Standards, as specified in T15A NCAC .0202L (2L Standards), IMACs', NCDHHS HSL (hexavalent chromium only), North Carolina Preliminary Soil Remediation Goals (NC PSRGs) for Protection of 6 Per 15A NCAC 02L .0102, "Compliance Boundary" means a boundary around a disposal system at and beyond which groundwater quality standards may not be exceeded and only applies to facilities which have received a permit issued under the authority of G.S. 143-215.1 or G.S. 130A. ' Appendix #1 of 15A NCAC Subchapter 02L Classifications and Water Quality Standards Applicable to The Groundwaters of North Carolina, lists Interim Maximum Allowable Concentrations (IMACs). The IMACs were issued in 2010 and 2011; however, NCDENR has not established a 2L Standard for these constituents as described in 15A NCAC 02L.0202(c). For this reason, IMACs noted in this report are for reference only. Corrective Action Plan Part 2 F)� Marshall Steam Station Ash Basin Groundwater (POG), North Carolina Surface Water Quality Standards as specified in 15 NCAC 02B .0211 and .0216 (amended effective January 2015) for Class WS-V waters (213 Standards), or U.S. Environmental Protection Agency (USEPA) National Water Quality Criteria (USEPA Criteria), it was designated as a COI. The following constituents were reported as COls in the CSA Report: • Soil: arsenic, barium, cobalt, iron, manganese, nickel, selenium, and vanadium • Groundwater: antimony, arsenic, barium, beryllium, boron, chromium$, cobalt, iron, manganese, selenium, sulfate, thallium, total dissolved solids (TDS), and vanadium. • Surface water: cobalt, iron, manganese, sulfate, and TDS. In addition to COI identification, delineation of COls in site media was also conducted during the CSA. Soil and groundwater delineation is further discussed below. 2.1.2 Soil Delineation The horizontal extent of soil impacts identified during the CSA was limited to the area beneath the ash basin and one location east and downgradient of the dry ash landfill (Phase 1). Where soil impacts were identified beneath the ash basin, the vertical extent of contamination beneath the ash/soil interface was generally limited to the uppermost soil sample collected beneath the ash. 2.1.3 Groundwater Delineation The approximate horizontal extent of groundwater impacts identified during the CSA was limited to within the ash basin Compliance Boundary, specifically beneath the ash basin and dry ash landfill (Phase II), east and downgradient of the ash basin and dry ash landfill (Phase 1), and southeast and downgradient of the ash basin, within the ash basin Compliance Boundary. The approximate vertical extent of groundwater impacts is generally limited to the shallow and deep flow layers, and vertical migration of COls is impeded by the underlying bedrock. Groundwater impacts at the MSS site attributable to ash handling and storage was delineated during the CSA activities with the following areas requiring refinement: • Horizontal and vertical extent of groundwater impacts downgradient and east of the ash basin and dry ash landfill (Phase 1). Based on the exceptions noted above and subsequent discussions with NCDEQ, a total of four additional monitoring wells are scheduled to be installed in this area of the site in March 2016 to further delineate groundwater impacts. In addition, one soil boring will be advanced east and downgradient of the dry ash landfill (Phase 1) to confirm the soil impacts identified during the CSA. Results of the additional monitoring well installation and groundwater sampling and soil sampling will be submitted under separate cover. 8 Unless otherwise noted, references to chromium in this document should be assumed to indicate total chromium. Corrective Action Plan Part 2 F)� Marshall Steam Station Ash Basin 2.2 Corrective Action Plan Part 1 The purpose of CAP Part 1 was to summarize CSA findings, evaluate background conditions by calculating PPBCs, evaluate exceedances per sample medium with regard to PPBCs, refine the SCM, and present the results of the groundwater flow and contaminant fate and transport model, and groundwater to surface water interaction model. 2.2.1 Proposed Provisional Background Concentrations for Soil and Groundwater HDR used background soil and groundwater concentrations to determine site -specific background concentrations for each COI. Because COls can be both naturally occurring and related to the source area, the selection of borings and monitoring wells used to establish background concentrations is important in determining whether releases have occurred from the source areas and to define the concentration of source -related compounds exceeding the background concentration for corrective action. A detailed analysis of background soil and groundwater PPBCs was provided in the CAP Part 1 Report. Further refinement of the PPBCs is anticipated following evaluation of data from Round 3 and Round 4 sampling events (completed in 2015), additional background monitoring planned in 2016, and the interim and effectiveness sampling planned in 2016 (Section 8 and 9). 2.2.2 COI Occurrence and Distribution Several COls exceeded both their NC PSRGs for POG and PPBCs in soil samples collected during the CSA and are listed below with respect to each area of the site. Beneath the Ash Basin: arsenic, iron, manganese, and vanadium. Beneath the Dry Ash Landfill (Phase II): arsenic, manganese, and vanadium. Beneath the PV Structural Fill: arsenic, manganese, and selenium. Outside Source Area Waste Boundaries: arsenic, iron, manganese, selenium, and vanadium. The vertical extent of soil impacts is generally limited to the shallow soil samples collected beneath the source areas. As a result of constituents leaching from the ash basin and dry ash landfill units (Phase I and II), and geochemical processes taking place in groundwater and soil beneath the site, several COls in groundwater exceeded their PPBCs and/or 2L Standard, IMAC or NCDHHS HSL (hexavalent chromium only), and are listed below with respect to each area of the site. Beneath the Ash Basin: antimony, boron, chloride, cobalt, iron, manganese, TDS, and vanadium. Beneath the Dry Ash Landfill (Phase II): barium, boron, chromium, cobalt, iron, manganese, selenium, sulfate, TDS, and vanadium. Downgradient and East of the Ash Basin and Dry Ash Landfill (Phase 1): beryllium, boron, chloride, chromium, hexavalent chromium, cobalt, manganese, thallium, TDS, and vanadium. 10 Corrective Action Plan Part 2 F)� Marshall Steam Station Ash Basin Downgradient of the Ash Basin Dam: arsenic, boron, cobalt, hexavalent chromium, iron, manganese, thallium, TDS, and vanadium. The areas of 2L Standard, IMAC, or NCDHHS HSL exceedances beneath and downgradient of the source areas indicate that physical and geochemical processes beneath the site inhibit the lateral migration of COls. Vertical migration of COls was observed in select well clusters (shallow, deep, and bedrock) 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. Groundwater from the shallow and deep flow layers at the site discharges to Lake Norman and the unnamed tributary east of the ash basin and dry ash landfill (Phase 1), as described in the SCM section of this report (Section 3). The extent of impacted groundwater is being refined through the installation of additional monitoring wells, as discussed in Section 2.1. In the CAP Part 1, groundwater model results for several Cols showed concentrations to be above applicable groundwater standards or criteria at the Compliance Boundary or Lake Norman for both current conditions and future scenarios modeled. However, based on groundwater to surface water interaction model results, all water quality standards are less than applicable criteria at the edge of the mixing zones in the Lake Norman. 2.3 Round 2 Sampling Round 2 groundwater, ash porewater, ash basin water, surface water, and AOW sampling activities were completed between September 28 and October 7, 2015. Groundwater analytical parameters and methods were consistent with those employed for Round 1, as detailed in CSA Report Table 7-3. The following subsections provide a comparison of Round 1 to Round 2 groundwater flow and analytical results. 2.3.1 Groundwater A total of 62 groundwater monitoring wells were sampled during the Round 2 event including, 12 compliance monitoring wells installed during the CSA, seven voluntary monitoring wells, and one FGD residue landfill monitoring well. Samples were collected for total and dissolved concentration analyses. Monitoring well locations are depicted on Figure 2-1. 2.3.1.1 Groundwater Water Levels On September 28, 2015, monitoring wells were manually gauged from the top of the PVC casing using an electronic water level indicator. Groundwater elevations and contours for the shallow, deep, and bedrock flow layers are depicted on Figures 2-2, 2-3, and 2-4, respectively. Groundwater elevations calculated during the Round 2 water level gauging event were generally similar to or slightly higher than those calculated from the Round 1 water level gauging data. The differences are likely attributable to seasonal variations of the water table. Depth to water measurements were not collected from all the monitoring wells gauged during the Round 1 sampling event and thus do not allow for groundwater contours to be depicted for the extent included in the CSA Report. 11 Corrective Action Plan Part 2 F)� Marshall Steam Station Ash Basin 2.3.1.2 Horizontal and Vertical Gradients Horizontal hydraulic gradients were updated using Round 2 groundwater elevations for the shallow, deep, and bedrock flow layers by calculating the difference in hydraulic heads over the length of the flow path between two wells with similar well construction (e.g., both wells having 15-foot screens within the same water -bearing unit). Monitoring wells, groundwater elevations, and length of flow paths utilized for horizontal hydraulic gradient calculations are detailed in Table 2-1. The average horizontal hydraulic gradients for Round 2 compared to Round 1 are provided below: • Shallow flow layer: Round 2 - 0.017 feet/foot; Round 1 - 0.018 feet/foot • Deep flow layer: Round 2 - 0.016 feet/foot; Round 1 - 0.017 feet/foot • Bedrock flow layer: Round 2 - 0.010 feet/foot; Round 1 - 0.010 feet/foot Horizontal hydraulic gradients were consistent with those documented in the CSA Report. From the Round 2 data, vertical hydraulic gradients were calculated for 39 shallow and deep well pairs and eight deep and bedrock well pairs by dividing the difference in groundwater elevation in each well pair by the difference in elevation of well screen midpoints. In general, the gradients showed potential vertical flow in the same direction in Rounds 1 and 2 with the exceptions listed below in Table 2-2. Table 2-2 Exceptions to Vertical Hydraulic Gradients Round 1 Vertical Round 2 Vertical Shallow Well Deep Well Gradient (ft/ft) Gradient (ft/ft) GWA-1 S GWA-1 D -0.001 0.016 GWA-4S GWA-4D -0.005 0.967 MW-4 MW-4D -0.011 0.400 Round 1 Vertical Round 2 Vertical Deep Well Bedrock Well Gradient (ft/ft) Gradient (ft/ft) MW-14D MW-14BR -0.233 0.057 2.3.1.3 Groundwater Sampling Procedures Groundwater samples were collected using low -flow sampling techniques, as outlined in the Low Flow Sampling Plan developed for the ash basin groundwater assessment program and approved by NCDENR (CSA Report Appendix G). The samples were submitted to a laboratory for analysis of total and dissolved inorganic parameters (USEPA Methods 200.7/200.8, 245.7, and 218.7) and water quality parameters. A 0.45-micron filter was used for collection of groundwater samples for dissolved concentration analysis. During Round 2, additional sample volume was collected at select locations using a 0.1-micron filter for dissolved concentration analysis along major flow paths and at locations with constituent concentrations that may be more affected by turbidity. The following monitoring wells were sampled using the 0.1-micron filter: 12 Corrective Action Plan Part 2 F)� Marshall Steam Station Ash Basin AB-2D AB-41D AB-5BR AB-8D AB-9BR AB-9D AB-15BR AB-15D AB-17D MW-8S AB-5D AB-6BR AB-6D AB-10D AB-11 D AB-12D AB-18D GWA-6D MW-81D Sample results from the 0.45-micron and 0.1-micron filters were generally similar. A comparison of the analytical results from filtered samples is presented in Table 2-3. 2.3.2 Round 1 and Round 2 Source Area and Groundwater Data Comparison Round 1 and Round 2 sample results are presented in Tables 2-4 through 2-8. Ash porewater and groundwater concentrations in Round 2 remained similar to those observed in Round 1. Variation from Round 1 to Round 2 cannot be further interpreted at this time as the data set consists of only two comprehensive sampling events and, therefore, does not fully consider seasonal fluctuations, or other temporal changes. 2.3.2.1 Source Area Results Ash Porewater The ash basin is a permitted wastewater treatment facility and ash porewater in the ash is wastewater, not groundwater. Ash porewater is compared to 2L Standards or IMACs and NCDHHS HSL for comparison purposes only. Ash porewater samples were collected in Rounds 1 and 2 from locations within the sources areas (Table 2-4). Ash porewater sample results from Round 2 were generally similar to Round 1 results except that zinc was identified as an additional COI in ash porewater (AB-5S) during Round 2. Although the concentrations of zinc observed at AB-5S were similar in Rounds 1 and 2 (890 and 1,200 fag/L), this was the only location where zinc exceeded its 2L Standard. Based on the Rounds 1 and 2 sampling results, COls in ash porewater are antimony, arsenic, barium, beryllium, boron, cadmium, chloride, chromium, cobalt, hexavalent chromium, iron, lead, manganese, nickel, pH, selenium, sulfate, thallium, TDS, vanadium, and zinc. Ash Basin Water The ash basin is a permitted wastewater treatment facility and water in the basin is wastewater, not surface water. Ash basin water is compared to both 2B and 2L Standards or IMACs and NCDHHS HSL for comparison purposes only, as ash basin water is a potential source of groundwater and surface water impacts. Ash basin water was collected from five locations: SW-1 through SW-5. Round 2 ash basin water sample results were generally similar to or less than Round 1 results with the exceptions noted below. • SW-1 through SW-5 — aluminum was not analyzed during Round 1, and exceeded its 2B Standard in each ash basin water sample during Round 2. • SW-3 — manganese exceeded its 2B Standard during Round 2. • SW-4 — arsenic exceeded its 2B Standard during Round 2. 13 Corrective Action Plan Part 2 F)� Marshall Steam Station Ash Basin • SW-4 and SW-5 — iron was not analyzed during Round 1, and exceeded its 2B Standard in samples SW-4 and SW-5 during Round 2. The following COls exceeded their regulatory standard or criteria in ash basin water samples collected in Rounds 1 and 2: aluminum, arsenic, beryllium, cadmium, chloride, cobalt, copper, iron, lead, manganese, mercury, nickel, selenium, sulfate, TDS, and zinc. Ash basin water sample results from Round 1 and 2 are presented in Table 2-5. 2.3.2.2 Groundwater Results Background Wells In general, the constituent concentrations from the background monitoring wells exhibited similar results when comparing data from the Round 1 and 2 sampling events. The following constituents had concentrations exceeding their regulatory standard or criteria in groundwater samples collected from background monitoring wells in Rounds 1 and 2: barium, chromium, cobalt, hexavalent chromium, iron, lead, manganese, pH, thallium, and vanadium. This list of COls is consistent with the COls reported in CAP Part 1. Note that hexavalent chromium was only analyzed during Round 1 sampling from monitoring wells BG-1 S/D, MW-4, and MW-4D. Background groundwater sample results from Round 1 and 2 are presented in Table 2-6. Background monitoring wells will continue to be sampled and PPBCs recalculated as the data set increases with additional sampling rounds. Upgradient of Source Areas Round 2 groundwater sample results for locations upgradient of the source areas were generally similar to or less than Round 1 results with the following exceptions. • GWA-3D — antimony and chromium did not exceed their 2L Standards during Round 2. • GWA-3S — manganese did not exceed its 2L Standards during Round 2. • GWA-4D — turbidity was less than 10 nephelometric turbidity units (NTU) during Round 2. Concentrations of iron and manganese decreased below their 2L Standards. Antimony increased slightly above its 2L Standard. • GWA-4S — turbidity was less than 10 NTU during Round 2. Although they still exceeded their 2L Standard or IMAC, concentrations of cobalt, iron, manganese, and vanadium decreased by approximately one order of magnitude. Chromium increased slightly above its 2L Standard. • GWA-5S — turbidity was less than 10 NTU during Round 2. Although they still exceeded their 2L Standard or IMAC, concentrations of iron, manganese, and vanadium decreased by approximately one order of magnitude. The concentration of cobalt decreased below its IMAC. • GWA-6D — antimony, iron, and manganese did not exceed their 2L Standards or IMAC during Round 2. Vanadium decreased by approximately one order of magnitude but still exceeded its IMAC. • GWA-7D — iron and manganese did not exceed their 2L Standards during Round 2. 14 Corrective Action Plan Part 2 F)� Marshall Steam Station Ash Basin • GWA-7S — chromium and iron did not exceed their 2L Standards during Round 2. Vanadium decreased by more than one order of magnitude but still exceeded its [MAC. • GWA-8S — manganese did not exceed its 2L Standard during Round 2. The following COls exceeded their regulatory standard or criteria in groundwater samples collected from upgradient wells in both Rounds 1 and 2: antimony, chromium, cobalt, hexavalent chromium, iron, manganese, pH, TDS, and vanadium. This list of COls is consistent with the COls reported in CAP Part 1. Groundwater sample results from monitoring wells located upgradient of the source areas for Round 1 and 2 are presented in Table 2-7. Beneath and Adjacent to the Ash Basin Round 2 groundwater sample results for locations beneath and adjacent to the ash basin were generally similar to or less than Round 1 results with the following exceptions. • AB-17D — turbidity was less than 10 NTU during Round 2. Cobalt and iron did not exceed their 2L Standard or IMAC during Round 2. • AB-11 S -turbidity was less than 10 NTU during Round 2. Cobalt, iron, and vanadium did not exceed their 2L Standard or IMAC during Round 2. The following COls exceeded their regulatory standard or criteria in groundwater samples collected from wells beneath and adjacent to the ash basin in both Rounds 1 and 2: antimony, boron, chloride, cobalt, iron, manganese, pH, TDS, and vanadium. This list of COls is consistent with the COls reported in CAP Part 1. Note that the only constituents that exceeded their regulatory standard or criteria in wells located adjacent to the ash basin included cobalt, iron, manganese, pH, and vanadium. Groundwater sample results from monitoring wells located beneath and adjacent to the ash basin for Round 1 and 2 are presented in Table 2-8 Beneath the Dry Ash Landfill (Phase 11) Round 2 groundwater sample results for locations beneath the dry ash landfill (Phase II) were generally similar to or less than Round 1 results with the following exceptions; • AL-213R — antimony concentrations were similar in Round 1 and 2, but increased above its IMAC in Round 2. Iron increased by more than one order of magnitude and exceeded the 2L Standard in Round 2. • AL-2D — selenium concentrations were similar in Round 1 and 2, but increased above the 2L Standard in Round 2. • AI4D — chromium concentrations were similar in Round 1 and 2, but increased above the 2L Standard in Round 2. The following COls exceeded their regulatory standard or criteria in groundwater samples collected from wells beneath the dry ash landfill (Phase II) in both Rounds 1 and 2: antimony, barium, boron, chromium, cobalt, hexavalent chromium, iron, manganese, pH, selenium, 15 Corrective Action Plan Part 2 F)� Marshall Steam Station Ash Basin sulfate, TDS, and vanadium. This list of COls is consistent with the COls reported in CAP Part 1. Groundwater sample results from monitoring wells located beneath the dry ash landfill (Phase 11) for Round 1 and 2 are presented in Table 2-9. Downgradient and East of Ash Basin and Dry Ash Landfill (Phase 1) Round 2 groundwater sample results for locations downgradient and east of the ash basin and dry ash landfill (Phase 1) were generally similar to or less than Round 1 results. The following COls exceeded their regulatory standard or criteria in groundwater samples collected from wells downgradient and east of the ash basin and dry ash landfill (Phase 1) in Rounds 1 and 2: barium, beryllium, boron, chloride, chromium, cobalt, hexavalent chromium, iron, manganese, pH, thallium, TDS, and vanadium. This list of COls is consistent with the COls reported in CAP Part 1. Groundwater sample results from monitoring wells located downgradient and east of the ash basin and dry ash landfill (Phase 1) for Round 1 and 2 are presented in Table 2-10. Downgradient and Southeast of Ash Basin Round 2 groundwater sample results for locations downgradient and southeast of the ash basin were generally similar to or less than Round 1 results with the exceptions noted below. • GWA-1 D —turbidity increased above 10 NTU during Round 2. Therefore, cobalt, iron, and manganese increased to concentrations that exceeded their 2L Standards or IMAC during Round 2. Voluntary monitoring wells MW-6D, MW-71D, MW-8S, MW-8D, MW-9S, and MW-91D were sampled during Round 2, but not Round 1. Sample results from these monitoring wells exhibited concentrations of COls exceeding regulatory standards similar to those observed in other monitoring wells sampled during Round 1 downgradient and southeast of the ash basin. The following COls exceeded their regulatory standard or criteria in groundwater samples collected from wells downgradient and southeast of the ash basin in Rounds 1 and 2: antimony, arsenic, boron, cobalt, hexavalent chromium, iron, manganese, pH, thallium, TDS, and vanadium. This list of COls is consistent with the COls reported in CAP Part 1. Groundwater sample results from monitoring wells located downgradient and southeast of the ash basin for Round 1 and 2 are presented in Table 2-11. 2.3.2.3 Description of Groundwater Quality Standard Exceedances Per CAMA, the CAP should include "A description of all exceedances of the groundwater quality criteria, including any exceedances that the owner asserts are the result of natural background conditions." To address this requirement, COls identified during the Round 1 and Round 2 sampling events were evaluated to assess if they are naturally occurring or attributable to ash handling at the site. 16 Corrective Action Plan Part 2 F)� Marshall Steam Station Ash Basin Results of the COI evaluations are summarized in Table 2-12. Only analytical results that exceeded their respective groundwater criteria are presented on this table. It is important to note that this evaluation only includes two sampling events and additional sampling is needed to re-evaluate PPBCs and more appropriately assess COls compared to PPBCs at the site. Areas with what appear to be source -related exceedances are shown on Figure 2-5. 2.3.3 Surface Water and Area of Wetness During the Round 2 sampling event, two AOWs and seven surface water locations were sampled at the MSS site. An AOW is defined as an area of ponded or flowing water that is not attributable to stormwater runoff. Surface water and AOW sample locations are depicted on Figure 2-1. Round 1 and Round 2 analytical results for the surface water and AOW samples are presented in Tables 2-13 and 2-14. Sampling locations are listed below. • AOWs: S-2 and MSSW001 • Surface water: SW-6, SW-7, SW-8, SW-9, SW-10, SW-11, and MSWW002 2.3.3.1 Surface Water Surface water samples were collected during Round 1 and Round 2. Surface water samples SW-7 through SW-11 were not sampled during the CSA (Round 1) sampling event. The Round 2 sample results for these surface water samples were evaluated and provided in CAP Part 1. Round 2 surface water sample results were generally similar to or less than Round 1 results except that sulfate exceeded its 2B Standard in SW-6 in Round 2. Surface water analytical results are provided in Table 2-13. The following COls exceeded their regulatory standard or criteria in surface water samples collected in Rounds 1 and 2: aluminum, cadmium, cobalt, copper, iron, lead, manganese, sulfate, and TDS. This list of COls is consistent with the COls reported in CAP Part 1. 2.3.3.2 Areas of Wetness Round 2 AOW sample results were generally similar to or less than Round 1 results with the exceptions noted below. • MSWW001 — iron and vanadium exceeded the 2L Standard and IMAC, respectively, in Round 2. Concentrations of boron, cobalt, hexavalent chromium, manganese, thallium, and TDS exceeding regulatory standards from Round 1 were not observed in Round 2. Turbidity was measured at 1.29 NTU during Round 1 compared to 38.5 NTU during Round 2. • S-2 — iron and chloride exceeded their 2L Standards in Round 2. Concentrations of arsenic, barium, beryllium, chromium, lead, selenium, and vanadium exceeding regulatory standards from Round 1 were not observed in Round 2. Turbidity was measured at 986 NTU during Round 1 compared to 9.02 NTU during Round 2. The following COls exceeded their regulatory standard or criteria in AOW samples collected in Rounds 1 and 2: arsenic, barium, beryllium, boron, chloride, chromium, hexavalent chromium, 17 Corrective Action Plan Part 2 F)� Marshall Steam Station Ash Basin cobalt, iron, lead, manganese, pH, selenium, thallium, TDS, and vanadium. This list of COls is consistent with the COls reported in CAP Part 1. Round 1 and Round 2 analytical results for the AOW samples are presented in Table 2-14. 2.4 Round 3 and Round 4 Background Well Sampling In response to a Duke Energy request for clarification of guidance, NCDEQ provided a table titled "Clarification of Attachment 1 Groundwater Assessment Plan Conditional Letters of Approval Items Related to Speciation — May 22, 2015" by electronic mail. In the responses provided in this table, NCDEQ requested that Duke Energy "plan to sample the existing and newly installed background wells two additional times during 2015 as part of an anticipated corrective action measure to support USEPA tiered site analysis and statistical analysis". The two additional sampling events referenced in this response correspond to background sampling Rounds 3 and Round 4, performed in November and December 2015, respectively. Groundwater sample collection and analysis were conducted in accordance with procedures described in the CSA Report. The results of the Round 3 and Round 4 background well sampling events are presented in Table 2-6. The locations of the background monitoring wells are presented on Figure 2-1. Further evaluation of background sample results and refinement of PPBCs will be provided in subsequent reports. 18 Corrective Action Plan Part 2 F)� Marshall Steam Station Ash Basin 3 Site Conceptual Model The SCM was initially presented in the CSA Report, and refined based on results from additional sampling events, groundwater contaminant fate and transport modeling, and groundwater to surface water interaction modeling. The SCM for the MSS site was developed in general accordance with the ASTM standard guidance document E1689-95 (Reapproved 2014), Standard Guide for Developing Conceptual Site Models for Contaminated Sites (ASTM 2014), to describe and integrate processes that determine contaminant releases, contaminant migration, and environmental receptor exposure to contaminants. The SCM is used to integrate site information and determine whether additional information may be needed to further understand site hydrogeologic and potential contaminant migration processes. The model is also used to support selection of remedial alternatives and effectiveness of corrective actions in reducing the potential exposure of environmental receptors to contaminants. The SCM was developed using the six basic activities outlined in ASTM E1689-95: • Identification of potential contaminants • Identification and characterization of the source(s) of contaminants • Delineation of potential migration pathways through environmental media • Establishment of background areas • Environmental receptor Information • Determination of system boundaries An expanded discussion of site geochemical controls on contaminant mobility and migration is also provided, as requested by NCDEQ. A graphical representation of the SCM is included as Figure 3-1. 3.1 Identification of Potential Contaminants Potential contaminants (COls) were identified in the CSA Report and are summarized in Section 2.1 of this report. 3.2 Identification and Characterization of Source Contaminants The source areas at MSS are defined as the ash basin, the dry ash landfill (Phases I and II), and the PV structural fill (Figure 2-1). Source characterization was performed through the completion of soil and rock borings, installation of monitoring wells, and collection and analysis of associated solid matrix and aqueous samples to identify physical and chemical properties of ash, ash basin water, ash porewater, and ash basin AOWs (Figure 3-1). A geologic cross- section through the source areas is shown on Figure 3-2 (4 sheets). A summary of source characterization analytical results per sample medium were provided in the CSA Report. Round 1 and Round 2 analytical results for ash porewater and ash basin water are provided in Tables 2-4 and 2-5, respectively. Ash distribution and chemical and physical properties were evaluated through advancement and sampling of 23 borings within the ash basin boundary, three borings in the dry ash landfill, and 19 Corrective Action Plan Part 2 F)� Marshall Steam Station Ash Basin eight borings within the PV structural fill boundary. Ash within the ash basin was encountered to depths ranging from the ground surface to approximately 85 feet below ground surface ( ft bgs). Ash in the dry ash landfill was encountered from approximately 2 to 111 ft bgs. Ash within the PV structural fill was encountered beneath the soil cap system to depths ranging from approximately 28 to 71 ft bgs. Ash porewater was evaluated through the sampling of 18 monitoring wells installed within the basin. Ash basin water was evaluated through sampling and analysis of five water samples. Based on the CSA results, concentrations of COls in groundwater that are attributable to the source areas are limited to beneath the ash basin, beneath the dry ash landfill (Phase 11), downgradient and east of the ash basin and dry ash landfill (Phase 1), and downgradient and southeast of the ash basin. Concentrations of COls in groundwater were higher beneath the dry ash landfill (Phase 11) and downgradient and east of the ash basin and dry ash landfill (Phase 1) compared to beneath the ash basin. COI transport is generally in a southeasterly direction towards Lake Norman and the unnamed tributary that flows to Lake Norman. Review of laboratory analytical results of ash samples collected from the ash basin, dry ash landfill (Phase 11), and PV structural fill identified nine COls: antimony, arsenic, barium, boron, cobalt, iron, manganese, selenium, and vanadium. COls identified in ash porewater samples include antimony, arsenic, barium, beryllium, boron, cadmium, chloride, chromium, cobalt, iron, lead, manganese, nickel, selenium, sulfate, thallium, TDS, and vanadium. COls identified in ash basin water samples include arsenic, beryllium, boron, cadmium, chloride, cobalt, copper, lead, manganese, nickel, selenium, sulfate, thallium, TDS, vanadium, and zinc. 3.3 Delineation of Potential Migration Pathways through Environmental Media 3.3.1 Soil The soil zone encountered beneath the ash basin ranged from approximately 5.5 to 76 feet. Generally, there is not an unsaturated soil zone beneath the ash basin to allow for sorption of COls to occur prior to reaching groundwater. The soil zone encountered beneath the dry ash landfill (Phase 11) ranged from approximately 34.5 to 78 feet in thickness. An unsaturated soil layer approximately 21 to 23 feet thick was present beneath portions of the ash landfill; however, the northeast portion of the ash landfill contained approximately two feet of saturated ash above saturated soil. The residual unsaturated soil/saprolite zone encountered beneath the PV structural fill was approximately one to 10 feet thick, which inhibits COls from leaching directly into groundwater. Based on depth to groundwater measurements directly east of the dry ash landfill (Phase 1), which was approximately 36 ft bgs during the CSA, there is a high likelihood of an unsaturated 20 Corrective Action Plan Part 2 F)� Marshall Steam Station Ash Basin soil buffer located beneath the dry ash landfill (Phase 1). Note that soil borings were not advanced within the footprint of the dry ash landfill (Phase 1). 3.3.2 Groundwater Site hydrogeologic conditions were evaluated through sampling/testing conducted during installation of 13 borings and 83 monitoring wells. Based on observations from the CSA investigation, the groundwater system in the natural materials (alluvium, soil, soil/saprolite, and bedrock) at MSS is consistent with the regolith-fractured rock system and is an unconfined, connected aquifer system. The MSS groundwater system is divided into three layers referred to as the shallow, deep (transition zone), and bedrock flow layers to distinguish the flow layers within the connected aquifer system. In general, groundwater within the shallow, deep, and bedrock layers flows from the north and northwest extents of the MSS site to the south and southeast toward Lake Norman. Monitoring wells installed outside the northwest, north, and northeast boundaries of the ash basin indicate that groundwater flows toward the ash basin along the west and north property boundaries. Shallow and deep groundwater at the site discharges to Lake Norman and an unnamed tributary that flows to Lake Norman east of the ash basin and dry ash landfill (Phase 1). Groundwater flows from the southeast portion of the ash basin to the east and beneath the dry ash landfill (Phase 1), and ultimately toward the unnamed tributary that flows to Lake Norman. Between the ash basin and Lake Norman (i.e., the southernmost portion of the ash basin), groundwater flows to the south/southeast toward Lake Norman. Groundwater flow direction in the shallow, deep and bedrock flow layers is shown on Figure 2-2, 2-3, and 2-4, respectively. 3.3.3 Surface Water and Sediment During the CSA, one surface water sample (SW-6) was obtained from the unnamed tributary east and downgradient of the dry ash landfill (Phase 1) and the ash basin. In addition, two upgradient surface water samples (SW-7 and SW-8) were collected from perennial streams that flow toward the ash basin and PV structural fill, and three surface water samples (SW-9, SW-10, and SW-1 1) were collected from Lake Norman downgradient of the ash basin and dry ash landfill (Phase 1). In general, COls identified in surface water sample SW-6, collected from the unnamed tributary to Lake Norman, are similar to groundwater COls identified in monitoring wells located between the unnamed tributary and the dry ash landfill (Phase 1) and ash basin. This supports the need for further evaluation of the COls identified in the SW-6 sample. Of the COls identified in the surface water samples collected from Lake Norman, manganese is the only COI that exceeded its 2L Standard or IMAC in groundwater monitoring wells located between the ash basin and Lake Norman. Cadmium, copper, and lead were identified as source -related COIs in the CSA and exceeded their 2B Standards in Lake Norman. However, these constituents were not detected above their 2L Standards in groundwater monitoring wells located between the ash basin and Lake Norman, indicating that there is not a complete pathway of exceedances of these constituents from the ash basin to Lake Norman. Note that 21 Corrective Action Plan Part 2 F)� Marshall Steam Station Ash Basin the 2B Standards for cadmium, copper, and lead are more than one order of magnitude less than their 2L Standards. 3.4 Establishment of Background Areas The background locations at MSS are located on the northeast side of the site. As described in the CAP Part 1, wells determined to represent background conditions at the site are compliance monitoring wells MW-4 and MW-4D, FGD Residue Landfill monitoring well MS-10, and CSA background monitoring wells BG-1 S/D, BG-2S/BR, and BG-3S/D. A detailed background monitoring well assessment is presented in CAP Part 1 Report, Appendix B. 3.5 Environmental Receptor Identification and Discussion Duke Energy conducted a receptor survey of the area within 0.5 miles of the ash basin Compliance Boundary in September 2014, and supplemented the receptor survey in November 2014. Locations of receptors identified during the surveys are shown on Figure 3-3. Properties located within a 0.5-mile radius of the MSS ash basin Compliance Boundary generally consist of undeveloped land and Lake Norman to the east, undeveloped land and residential properties to the north and west, portions of the MSS site outside the Compliance Boundary, undeveloped land and residences to the south, and commercial properties to the southeast along North Carolina Highway 150. The receptor survey activities identified four public water supply wells and 83 private water supply wells in use, along with six assumed private water supply wells, located within the 0.5- mile radius of the ash basin Compliance Boundary. No wellhead protection areas were identified within a 0.5-mile radius of the ash basin Compliance Boundary. Several surface water bodies that flow from the topographic divide along Sherrills Ford Road toward Lake Norman were identified within a 0.5-mile radius of the ash basin Compliance Boundary. No water supply wells (including irrigation wells and unused or abandoned wells) were identified between the source areas and Lake Norman. 3.6 Determination of System Boundaries The site boundary, waste boundaries, and Compliance Boundary at the MMS site are depicted on Figure 2-1. Spatially, the SCM for MSS is bounded by Lake Norman to the southeast, which is a hydrologic boundary, and topographically and hydrologically elevated areas west and north of the ash basin system. The SCM extends vertically into bedrock. 3.7 Site Geochemistry and Influence on COls Groundwater composition can be affected by an array of naturally -occurring and anthropogenic factors. Many of these factors can be causative agents for specific reduction -oxidation (redox) processes or indicators of the implied redox state of groundwater as expressed by pH, oxidation-reduction potential (ORP), and dissolved oxygen. Groundwater pH is affected by the composition of the bedrock and soil through which the water moves as well as other factors, including exposure to carbonate rocks or lime -containing materials in well casings, exposure to atmospheric carbon dioxide gas, and precipitation. In addition, metals and other elemental or 22 Corrective Action Plan Part 2 F)� Marshall Steam Station Ash Basin 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. Based on field measurements, the primary redox categories at the MSS site were determined to include oxic, suboxic, mixed (oxi-anoxic), mixed (anoxic), and anoxic conditions. At MSS, DO levels exceeded the threshold of 0.5 mg/L in 66 of 91 samples (72%) and predominant redox processes are oxygen reduction with iron or manganese oxidation (i.e., controlled by 02 and Fe(III)/Fe(II) or Mn(IV)/Mn(II) couples). Under these conditions, more oxidized species As(V), Se(VI), and Mn(IV) would be expected. There were 18 wells from which ash porewater samples were collected and 17 of those samples are classified as anoxic or mixed (oxi-anoxic); one sample was classified as suboxic. There is an increased potential for reduced forms of metals to occur under anoxic or mixed conditions. However, it should be noted that 33 of the 73 (-45%) groundwater samples from wells across the site are classified as suboxic or oxic categories where reduced species of metals such as As(III) are less likely to be present. Field measurements indicate that pH ranges from 4.5 to 11.9 SU in groundwater at the site. Background well results indicate that pH ranges from 5.7 to 10.9 SU. Similarly, pH results for upgradient wells beyond the waste boundary range from 5.4 to 11.9 SU. In contrast, pH values within ash porewater range from 2.8 to 8.5 SU. pH values in groundwater downgradient of the source areas range from 4.5 to 11.2 SU. There is a very wide range of ORP values, spanning ranges that imply reduced (negative values) to highly oxidized (large positive values) conditions. For ash porewater, reducing conditions are present throughout the ash basin except for the southwest portion of the ash basin. For groundwater, oxidizing conditions are generally present in the deep flow layer beneath the ash basin, dry ash landfill (Phase II), and PV structural fill. Reducing conditions are present in groundwater in several locations including AB-6D, AB-8D, AB-15D, AB-16D and all bedrock wells installed beneath the ash basin. For the three background well samples, ORP values are generally consistent with the inferred redox category of mixed (oxic-anoxic). In contrast, measured ORP values from non -background monitoring wells ranged from -148 mV (reducing) to +597 mV (strongly oxidizing), whereas inferred redox conditions were generally mixed (oxic-anoxic). Positive ORP values measured in samples where mixed (oxic-anoxic) conditions are inferred suggest that many samples may not be in redox equilibrium. Speciation measurements were performed for samples collected from 25 groundwater and/or ash porewater monitoring wells, depending on the analyte, and vary widely at the site. Speciation measurements are summarized below: For the five samples where speciated arsenic concentrations were above reporting limits, the predominant species was the reducing form As(III), representing 68% of the total arsenic. • In general, Cr(VI) was identified in upgradient and background groundwater samples with higher concentrations than those collected from ash porewater wells and groundwater monitoring wells beneath and downgradient of the on -site source areas. 23 Corrective Action Plan Part 2 F)� Marshall Steam Station Ash Basin • The reduced form of iron [Fe(II)] was mainly present downgradient and southeast of the ash basin. • The reduced form of manganese [Mn(II)] was mainly present beneath the central portion of the ash basin and downgradient and southeast of the ash basin. The reduced form of selenium [Se(IV)] was present above detection limits in just one sample. The oxidized form of selenium [Se(VI)] was present above detection limits in five samples. Se(VI) was present beneath the central portion of the ash basin (AB-12D) and in one deep background well (BG-1 D). The highest concentrations of Se(VI) were reported in two wells located downgradient and east of the ash basin and dry ash landfill (Phase 1) (MW-14S and MW-14D). Piper diagrams were generated as part of the CSA to compare geochemistry between ash basin porewater and groundwater. In general, the ionic composition of upgradient and background groundwater at the site is less chloride- and sulfate -rich than ash basin porewater, ash basin water, and downgradient groundwater, which were observed to be trending closer to calcium-, chloride-, magnesium-, and sulfate -rich. 24 Corrective Action Plan Part 2 F)� Marshall Steam Station Ash Basin 4 Modeling 4.1 Groundwater Model Refinement The groundwater flow and contaminant fate and transport models were refined to incorporate post-CSA data. Model refinements are summarized below. The refined groundwater flow and contaminant fate and transport model reports were completed by HDR in conjunction with the University of North Carolina at Charlotte (UNCC). An independent review of the refined MSS model was conducted by the Electric Power Research Institute (EPRI) and found that the model was set-up and meets its flow and transport calibration objectives sufficiently to meet its final objective of predicting effects of corrective action alternatives on groundwater quality. The refined groundwater flow and transport model report and the EPRI review of the calibrated MSS model are provided in Appendix B. 4.1.1 Flow Model Refinements Transient transport simulations for all modeled COls were calibrated and flow parameters were refined as follows: Hydraulic conductivity measurements, obtained from slug test data collected during the CSA, were utilized in the calibration of the flow model to better represent site -specific conditions. This refinement led to reduction in the square root of the average square error (also referred to as the root mean squared error) of the modeled versus observed water levels for wells gauged in July 2015 to 4.68% compared to the initial calibrated model root mean squared error of 5.75% in the difference in head between the modeled and the observed values across the model domain. These results are provided in Table 3 in Appendix B. • Recharge rates for the model were revised within and outside of the ash basin. Recharge applied to the inactive portion of the ash basin and areas outside the ash basin were assigned a value of 6.6 inches per year. The dry ash landfill (Phase 1), Industrial Landfill No. 1 and The FGD Residue Landfill were constructed with liners, and as a result the recharge was set to zero in these areas. The calibrated model recharge at the PV structural fill and dry ash landfill (Phase 11) were set at 6.6 and 4.0 inches per year, respectively. Recharge within the active portion of the ash basin was calculated using Darcy's Law and is based on the area of the ash basin, the approximate depth of water or saturated ash, and the range of measured hydraulic conductivity values within the ash. The recharge rate applied to the active portion of the ash basin area was set at 12.3 inches per year. Historic basin water levels were considered during calibration of the flow model. However, the current flow model is calibrated to steady-state conditions. Continued refinement of the model to consider transient flow may enhance integration of historic water level data within the ash basin. • The bedrock hydrostratigraphic layers were extended vertically in the refined model to correlate to the deepest reported off -site private water supply well, as obtained from the 25 Corrective Action Plan Part 2 F)� Marshall Steam Station Ash Basin completed questionnaires received from adjacent well owners during the receptor survey conducted in 2015 and/or the North Carolina State/Gaston or Mecklenburg County geographic information system (GIS) database. Private water supply wells were identified in the CSA Report. Residential wells were identified at four locations within the current model domain. Potential effects on the site - specific flow regime from pumping of these wells were simulated by applying a constant pumping rate of 400 gallons per day (gpd), which represents the average household usage per USEPA Water Sense Partnership Program (USEPA 2015a). 4.1.2 Contaminant Fate and Transport Model Refinements The groundwater contaminant fate and transport model was calibrated using the refined parameters from the groundwater flow model, as discussed in Section 4.1.1, and presented below. The initial model used conservative (low) sorption coefficient (Kd) values to achieve calibration of the contaminant fate and transport models for each COI. Subsequent to submittal of the CAP Part 1 Report, Kd values were recalculated using linear and Freundlich isotherms (Appendix C). Refinement of the flow model and use of the newly derived upper and lower Kd limits in the contaminant fate and transport model resulted in improved calibration of source concentrations to measured concentrations in downgradient wells. Note that final Kd values used to calibrate the fate and transport models may have fallen outside the recalculated upper and lower limits; however, adjustment of Kd values within the model to achieve calibration is considered acceptable practice. • The initial contaminant fate and transport model used adjusted source area concentrations to achieve calibration at downgradient monitoring wells. The flow model refinements discussed in Section 4.1.1 enabled refinement of the contaminant fate and transport model to better represent measured source area ash porewater concentrations. • The initial contaminant fate and transport model was not calibrated to background groundwater concentrations as PPBCs were not developed in time for use in the model. The model has since been refined to incorporate background concentrations (i.e., PPBCs) for each COI. This refinement allows the models to account for naturally occurring background concentrations and is particularly important for COls with a PPBC greater than the 2L Standard, IMAC, or NCDHHS HSL. However, the model is limited in that it applies the PPBC across the entire site, as shown on individual COI concentration figures in Appendix B. • Round 2 laboratory analytical data were reviewed and no additional COls were identified for including in the refined modeling efforts. • Potential COI impacts to private water supply wells located within the current model domain were evaluated through reverse particle tracking simulations by applying a constant pumping rate of 400 gallons per day at each well, as specified in Section 4.1.1. 26 Corrective Action Plan Part 2 F)� Marshall Steam Station Ash Basin Particle tracking simulations show advective travel time and do not account for reduced travel time associated with sorptive COls (i.e., arsenic). Refinements to the groundwater model in CAP Part 2 provide a more accurate representation of existing site conditions, which produces model results that more accurately simulate the proposed closure scenarios at the MSS site. 4.1.3 Summary of Modeled Results Two closure scenarios were modeled for MSS: an Existing Conditions scenario with ash sources left in place and a Cap -In -Place scenario, which simulates the effects of covering the ash basin at the beginning of the predictive simulation with an engineered cap. These simulations predict flow and transport results and are calibrated for existing conditions. Once the chosen scenario is decided on for corrective action, the model should be revised and recalibrated to improve its accuracy and reduce uncertainty. 4.1.3.1 Existing Conditions Scenario The Existing Conditions scenario consists of using the calibrated model for steady-state groundwater flow conditions and transient transport of COls under existing conditions across the site to predict when steady-state concentrations are reached at the Compliance Boundary. COI concentrations remain the same for this scenario with source concentrations being held at their constant value over time. This scenario represents the most conservative case in terms of groundwater concentrations on -site and off -site, with COls discharging to surface water at steady-state. The time to achieve a steady-state plume depends on location of the source areas relative to the Compliance Boundary and the COI loading history. Areas close to the Compliance Boundary will reach a steady-state concentration sooner. The time to steady-state concentration is also dependent on the sorptive characteristics of each COI. Sorptive COls are transient for a longer time period as their peak breakthrough concentration travels at a rate that is less than groundwater pore velocity. Use of lower effective porosity results in shorter times to achieve steady-state concentrations for both sorptive and non-sorptive COls. 4.1.3.2 Cap -in -Place Scenario The Cap -in -Place scenario simulates the effects of covering the ash basin at the beginning of the predictive simulation with an engineered cap. In the model, recharge at the ash basin is set to zero. In the model, non-sorptive COls move downgradient at the pore velocity of groundwater and are displaced by the passage of a single ash porewater volume, while sorptive COls migration in groundwater is retarded because of sorption with soil/rock. The model uses the predicted concentration from the 2015 calibration as the initial concentration at the start of the model scenario. COls move through the saturated zone beneath the source areas at a rate dependent on aquifer properties and geochemical interactions of the COI and groundwater. If the source areas become unsaturated, concentrations of COls beneath the source zones will decrease over time without a contributing constant source. 27 Corrective Action Plan Part 2 F)� Marshall Steam Station Ash Basin Groundwater flow is affected by this scenario as the water table is lowered, and groundwater velocities are reduced beneath the capped areas. The water table just upgradient of the ash basin dike in the ash basin was reduced by approximately 10 feet. In the ash basin between the PV structural fill and dry ash landfill (Phase II), the water table was reduced by approximately 1 foot. 4.1.4 Model Assumptions and Limitations The model assumptions include the following: • The steady-state flow model was calibrated to hydraulic heads measured at observation wells in July 2015 and reflects the ash basin water level. The model is not calibrated to transient water levels over time, recharge, or stage changes in Lake Norman. A steady- state calibration does not consider groundwater storage, and was not used to calibrate the groundwater flux into adjacent surface water bodies. • A single domain MODFLOW modeling approach was used for simulating flow in the primary porous groundwater flow layer and bedrock for contaminant transport at the MSS site. MODFLOW simulates flow through porous media and groundwater flow in the bedrock zone via fractures in the bedrock. • The model was calibrated by adjusting the constant source concentrations at the ash basin and ash storage areas to reasonably match 2015 COI concentrations in groundwater. • For the purposes of numerical modeling and comparing closure scenarios, it was assumed that the selected closure scenario was implemented in 2015. • Predictive simulations were performed and steady-state flow conditions were assumed from the time the ash basin and ash storage areas were placed in service (Year 1965) through the current time until the end of the predictive simulations (Year 2265). • COI source area concentrations were applied uniformly within each source area and assumed to be constant with respect to time for transport model calibration. • The background concentrations for the COls were applied as initial concentrations. • Travel times are advective and do not account for sorption of COls to site media, which may cause the travel times to be reduced. • Residential wells are assumed to have a constant pumping rate of 400 gpd and are completed in bedrock. • The model does not account for varying geochemical conditions such as pH and redox potential that could affect COI mobility; however, geochemical modeling was completed and is further discussed in Section 4.3. 4.1.5 Modeled Scenario Results Constituent concentrations were analyzed at downgradient monitoring wells (Figure 7 in Appendix B) for all modeled COls. These wells are downgradient from the ash basin and upgradient of the ash basin Compliance Boundary at Lake Norman. Wells AB-1 S and AB-2S 28 Corrective Action Plan Part 2 F)� Marshall Steam Station Ash Basin are located on the ash basin dam, while GWA-1 S is near the southeastern corner of the main coal storage area, and MW-6 is northeast of the ash basin dam and directly east of the dry ash landfill (Phase 1). Closure scenario results are presented as predicted concentration versus time curves in downgradient monitoring wells and as groundwater concentration maps for each of the 13 modeled COls on Figures 16 through 184 of Appendix B, as discussed in the following sub- sections. Concentration contours and concentration breakthrough curves are referenced to 1965, the year that the ash basin became active. Groundwater concentration maps are referenced to a time zero that represents the time the closure action planned to be implemented, which for the purposes of modeling is assumed to be 2015. A summary of the modeled COI results is provided below in Table 4-1. A "+" indicates that the predicted concentration of a given COI exceeds its applicable 2L Standard, IMAC, or NCDHHS HSL at the Compliance Boundary. A " ° indicates that the concentration of a given COI has not exceeded its applicable 2L Standard, IMAC, or NCDHHS HSL at the Compliance Boundary. Year 0 indicates the time the closure action was implemented (2015), and Year 100 indicates 100 years from the time the closure action was implemented (2115). Table 4-1 Summary of Modeled COI Results Constituent (Standard) Appendix B Figures Flow Layer Existing Conditions Cap -in -Place Scenario Year 0 Year 100 Year 0 Year 100 Antimony IMAC (1 pg/L) 16 - 28 Shallow + + + + Deep + + + + Bedrock + + + + Arsenic 2L (10 pg/L) 29 - 41 Shallow - - - - Deep - - - - Bedrock - - - - Barium 2L (700 pg/L) 42 - 54 Shallow - - - - Deep - - - - Bedrock - - - - Beryllium IMAC (4 pg/L) 55 - 67 Shallow - + - - Deep - + - - Bedrock - - - - Boron 2L (700 pg/L) 68 — 80 Shallow + + + + Dee + + + + Bedrock + + + + Chloride 2L (250,000 pg/L) 81 - 93 Shallow - - - - Deep - - - - Bedrock - - - - Chromium 2L (10 pg/L) 94 - 106 Shallow + + + + Deep + + + + Bedrock + + + + Cobalt IMAC (1 pg/L) 107-119 Shallow + + + + Deep + + + + Bedrock + + + + Hexavalent Chromium 120-132 Shallow + + + + Deep + + + + 29 Corrective Action Plan Part 2 Marshall Steam Station Ash Basin Constituent (Standard) NCDHHS HSL 0.07 /L Appendix B Figures Flow Layer Existing Conditions Cap -in -Place Scenario Year 0 Year 100 Year 0 Year 100 Bedrock + + + + Selenium 2L (20 pg/L) 133-145 Shallow - - - - Deep - - - - Bedrock - - - - Sulfate 2L (250,000 pg/L) 146-158 Shallow - - - - Deep - - - - Bedrock - - - - Thallium IMAC (0.2 pg/L) 159 - 171 Shallow + + + + Deep + + + + Bedrock + + + + Vanadium IMAC (0.3 pg/L) 172 - 184 Shallow + + + + Deep + + + + Bedrock + + + + The model predictions are summarized as follows: In accordance with 15A NCAC 02L.0106 (k), a CAP may be approved by NCDEQ without requiring groundwater remediation to the 2L Standards if certain conditions are met. Condition (4) specifies that 2L Standards must be met at a location no closer than one year time of travel upgradient of an existing or foreseeable receptor. For MSS, the receptor is considered to be the four nearby residential pumping wells north of the steam station that are included in the model domain. To evaluate this condition, HDR and UNCC conducted particle tracking within the Existing Conditions scenario steady-state flow field to identify the one-year travel time boundary. The residential pumping wells were set to pump at a rate of 400 gpd (representing the average USEPA daily household usage rate). The advective travel time one year from each well was performed using MODPATH (shown on Figure 15 in Appendix B). The one-year advective travel time pathlines do not intersect the Compliance Boundary. The refined model predicts that under the Existing Conditions and Cap -in -Place scenarios, antimony, beryllium, boron, chromium, cobalt, hexavalent chromium, , thallium and vanadium are predicted to exceed their respective 2L Standards or IMACs at the model interface (i.e., Lake Norman and the unnamed tributary) or the Compliance Boundary for all groundwater flow layers. Also, hexavalent chromium is predicted to exceed the NCDHHS HSL at Lake Norman for all groundwater flow layers. For these COls, the background concentrations used for modeling antimony, chromium, cobalt, hexavalent chromium, thallium, and vanadium also exceed the applicable groundwater standards, so the actual impact of the site sources on groundwater quality is unknown. The background concentration used to model boron was less than the 2L Standard and the modeling predicts that boron will exceed its 2L Standard at the model interface or Compliance Boundary. 30 Corrective Action Plan Part 2 F)� Marshall Steam Station Ash Basin • The model predicts that under the Existing Conditions and Cap -in -Place scenarios, arsenic, barium, chloride, selenium, and sulfate are not predicted to exceed the applicable groundwater standards at the model interface or Compliance Boundary. 4.2 Surface Water Model Refinement 4.2.1 Methodology The methodology to complete the surface water model in CAP Part 2 (Appendix D) is consistent with CAP Part 1 and incorporates new groundwater modeling results addressed in Section 4.1 including: • Refinement of Kd values. • Updated groundwater flux data for input into the surface water model. Groundwater to surface water interactions were evaluated 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 MSS site. 4.2.2 Results The calculated surface water COI concentrations in Lake Norman adjacent to and downstream of the MSS site are presented below in Table 4-2. The design flows, upstream surface water concentrations, groundwater flows, and groundwater COI concentrations presented in Appendix B were used to complete these calculations. The mixing model results indicate that no surface water quality criteria are exceeded for COls modeled at the edge of the mixing zone in Lake Norman. Descriptions of the mixing zones are provided in Appendix D. Table 4-2. Lake Norman Calculated Surface Water Concentrations Calculated Mixing Zone Conc. (Ng/L) Water Quality Standard (Ng/L) COI Acute Chronic HH/WS Acute Chronic HH / WS Arsenic 0.260 0.252 0.251 (c) 340 150 10 / 10 Barium 4.58 4.32 4.32 (nc) ns ns 200,000 / 1,000 Beryllium 0.103 0.101 0.100* 65 6.5 ns / ns Boron 26.9 25.4 25.2* ns ns ns / ns Chloride 560 432 432 (nc) ns ns ns / 250,000 Total Chromium 0.269 0.254 0.252* ns ns ns / ns Chromium VI 0.253 0.251 0.250* 16 11 ns / ns Cobalt 0.255 0.251 0.251 (nc) ns ns 4/3 Selenium 0.265 0.253 0.251* ns 5 ns / ns Sulfate 660 532 532 (nc) ns ns ns / 250,000 Thallium 0.051 0.050 0.050 (nc) ns ns 0.47 / 0.24 Notes: 1. All COls are shown as dissolved fraction except for total chromium, which is total recoverable metal 2. WS — water supply (15A NCAC 02B .0216, amended effective January 1, 2015) 3. HH — human health (15A NCAC 02B .0211, amended effective January 1, 2015) 4. c — carcinogen 5. nc — non -carcinogen 31 Corrective Action Plan Part 2 F)� Marshall Steam Station Ash Basin 6. ns — no water quality standard 7. *—calculated for 100% of induced flow 4.3 Geochemical Modeling 4.3.1 Objective Geochemical modeling was conducted to describe the speciation of COls and other groundwater constituents across the spectrum of groundwater conditions measured at the MSS site. The objective of geochemical modeling is to describe the expected partitioning of COls between aqueous and solid phases (i.e., between groundwater and soil and between ash porewater and ash) and anticipated changes in phase distributions given variations in DO, pH, and TDS. Changes in DO will affect the oxidation state of groundwater as measured by ORP, which is generally expressed as Eh or electron activity (pE). Changes in pH affect the acidity of groundwater and concurrently affect Eh. Changes in TDS affect ionic strength and ion competition at sorption sites. Constituents evaluated for MSS were: antimony, arsenic, barium, beryllium, boron, chloride, chromium, cobalt, iron, lead, manganese, nickel, selenium, sulfate, thallium, and vanadium. 4.3.2 Methodology Site -specific evaluations of COls were performed for each of the monitoring wells using the U.S. Geological Survey (USGS) PHREEQC (0.3.3) geochemical speciation code (Parkhurst and Appelo 2013) and PhreePlot (Kinniburgh and Cooper 2011), a companion plotting package that utilizes looping PHREEQC with a hunt and track approach to determine stability boundaries. By using a single well approach, wells can be evaluated or grouped later based on geochemical characteristics. The single well approach also allows fine resolution of geochemical constituents and subtle differences between wells that have a significant bearing on the overall geochemical characterization. Calculations were driven by measured concentrations of COls and other analytes such as ORP (also described as Eh), alkalinity, sodium, and other ions in groundwater for each of the 101 wells monitored at the site. PHREEQC calculations were performed to construct Pourbaix (Eh -pH) diagrams to display the dominant geochemical forms (i.e., species) that would be expected in groundwater in the absence of adsorption under equilibrium conditions and allowing for most probable mineral precipitation where appropriate. Measured ORP (presented as Eh) and pH values for each well were plotted on the Pourbaix diagram for each COI to evaluate the likely distribution of species at the MSSI site. Additional PHREEQC calculations were performed to simulate anticipated geochemical speciation that would occur for each COI in the presence of adsorption to soils. Further simulations were performed to evaluate model and COI response to changes in DO, pH, and TDS in the presence of sediment adsorption. Adsorption to soils was represented using a surface complexation theory approach with hydrous ferric oxides (HFO) and hydrous aluminum oxides (HAO) representing weak and strong binding sites, respectively. Values for HFO and HAO were determined from actual site sediment extractions that were also the basis for measured distribution coefficients (Kd values) for site soils determined from adsorption experiments conducted by UNCC. 32 Corrective Action Plan Part 2 F)� Marshall Steam Station Ash Basin In order to geochemically simulate changes to aquifers or test potential remediation strategies, DO, pH, redox, and TDS were varied in the simulations. These geochemical simulations are termed titrations for this report. Each set of titrations provides an estimate of the percentage of each COI that would be adsorbed as a function of changing DO, pH, reduction -oxidation (redox), or TDS along with relevant changes to the dominant species across the gradient. For these titrations, TDS was evaluated as the addition of a select set of cations and anions known to be common in soils and sediment at the MSS site to include sodium, calcium, chloride, potassium, and sulfate. Changes to DO, pH, and TDS were utilized for titrations due to the affinity for numerous COls such as metals to exist primarily as anionic or cationic species and their adsorption coefficient variations to mineral surfaces, soils, sediment, rock and ash. The titration method in geochemical modeling can also account for mobility changes due to redox threshold changes as well and potential mineral precipitation, indicated by saturation indices in outputs. Adsorption of anionic species is typically greater at lower pH where anions are more strongly attracted to positively charged surfaces (and vice versa regarding cationic species). Similarly, the solubility of mineral phases is pH dependent and lower pH levels tend to favor formation of more soluble cationic species for most alkali elements, alkali earth elements, and transition metals. Geochemical modeling methodologies are discussed in further detail in Appendix E. 4.3.3 Assumptions The following assumptions apply to PHREEQC modeling efforts for the MSS site: • Groundwater data evaluation based on an individual well basis. • COI sorption in PHREEQC was represented by surface complexation. Surface complexation models provide a molecular description of adsorption using an equilibrium approach that defines surface species, chemical reactions, equilibrium constants, mass and charge balances. A benefit of the surface complexation approach is that the charge on both the adsorbing ion and the solid surface where sorption occurs. • The surface complexation model was parameterized based on soil column tests and extraction measurements reported by UNCC. The range of sorption properties was parameterized as the minimum, mean, and maximum estimates for binding sites as defined from soil extraction measurements. This range of sorption capacities was used to develop pH, Eh, DO, TDS, and COI titrations. • The dominant attenuation process is adsorption to hydrous metal oxides, particularly HFO and HAO, which are representative of clay minerals and similar facies that are abundant in soil, transition zone, and bedrock. • COI concentrations used in PHREEQC model were as reported in the database. Analytical results qualified as non -detects or estimated values (U- and J-flagged values) were used as reported without modification. • Nitrogen values were assumed to be primarily nitrate and alkalinity results were assumed to be primarily bicarbonate, not carbonate. 33 Corrective Action Plan Part 2 F)� Marshall Steam Station Ash Basin • TDS was evaluated as a summary of calcium, sodium, potassium, magnesium, sulfate, and chloride ions. These constituents account for approximately 60% of the TDS value. Chloride does not have sorption constants, so this constituent was addressed as a component of TDS. • Eh/pH diagrams and/or predominance plots were developed in PhreePlot or Geochemist's Workbench for each COI to aid in demonstration of changes in Kd, pH, and DO. 4.3.4 Geochemical Model Results The modeling effort described above provides both qualitative and quantitative estimations of the chemical speciation and adsorption behavior of several key COls. Relevant observations from this modeling effort are as follows: • Redox conditions vary widely at the MSS site, indicating that it has not reached equilibrium, or data is not representative of the conditions sampled. Additional groundwater results will assist in refining the model further or confirm these findings should sampling data not be representative of actual groundwater conditions. • The observed site condition of limited solubility of arsenic, chromium, and cobalt in MSS site groundwater is confirmed by the modeling. • Each of the pH, Eh, and TDS figures can be further evaluated to potentially support MNA or remediation. The addition of an engineered cap would reduce infiltration and introduction of oxygen presumably creating a more anoxic environment. In addition, pH adjustment could be performed to make COls less soluble, thus limiting COI migration during excavation and the release of TDS and other metals. • Methods such as capping could change groundwater flow, inhibit current microbial reduction mechanisms, or cause secondary metal redox reductions that could be modeled when options are chosen more specifically. • Soil sorptive capacity for COls such as boron is typically small and relatively larger for COls such as arsenic or chromium. Refer to Appendix E for further details of MSS site geochemical model results. 4.4 Refined Site Conceptual Model Groundwater and surface water models were revised to address comments from NCDEQ. An updated SCM is provided on Figure 3-1. Based on review of refined models and the geochemical model, no revisions are warranted for the SCM at this time. 34 Corrective Action Plan Part 2 F)� Marshall Steam Station Ash Basin 5 Risk Assessment The purpose of the human health and ecological risk assessment is to characterize potential risks to humans and ecological receptors associated with exposure to the coal ash -derived constituents that may be present in groundwater, surface water, sediments, soil, and air due to release(s) from the coal ash basin at the MSS site. Results of the risk assessment and the information provided on background conditions and groundwater flow (including fate and transport model results) provided in the CAP reports will aid in focusing remedial actions which, when implemented, will provide future conditions that are protective of human health and the environment, as required by CAMA. The risk assessment was completed using a methodology designed to be consistent with state and federal guidance. This methodology represents a step -wise process whereby MSS was evaluated using the following methods: • Step 1: Develop a conceptual site model (CSM), including receiving media, exposure pathways, and human and ecological receptors. • Step 2: Screen analytical data for the applicable site media by comparing screening values identified in the risk assessment work plan to identify constituents of potential concern (COPCs). • Step 3: Develop site -specific human health risk -based concentrations (RBCs) for the COPCs, derive exposure point concentrations (EPCs), and compare EPCs to RBCs to draw conclusions about the significance of potential human health risks. • Step 4: Develop a site -specific baseline ecological risk assessment (BERA) for the COPCs and, where appropriate, derive RBCs based on the risk assessment results. 5.1 Step 1: Conceptual Site Model The CSM includes a site description, information on current and anticipated future land uses, sources, and potential migration pathways through which coal ash -derived COPCs may have been transported to other environmental media (receiving media), and the human and environmental receptors that may come in contact with the receiving media. The CSM is meant to be a living model that can be updated and modified as additional data become available. Initial CSMs were presented on CSA Report Figure 12-1 (human health) and Figure 12-2 (ecological). Updated CSMs are provided on Figures 2-3 and 2-4 in Appendix F. The CSMs are intended to identify potential exposure pathways and receptors that may be applicable at the site. For MSS, the following receptors and exposure scenarios are identified in the human health CSM: 35 Corrective Action Plan Part 2 F)� Marshall Steam Station Ash Basin • Current/future on -site trespasser with potential exposure to dust in outdoor air, soil remaining post- remediation9, AOW water and AOW soil, on -site surface water, and on - site sediment; • Current/future commercial/industrial worker with potential exposure to dust in outdoor air, soil remaining post-remediation, AOW water and AOW soil, on -site surface water, and on -site sediment; • Current/future construction worker with potential exposure to dust in outdoor air, soil remaining post-remediation, AOW soil and groundwater; • Current/future off -site resident with potential exposure to on -site groundwater and off -site surface water as potential sources of potable water; • Current/future off -site recreational swimmer and waders with potential exposure to off - site surface water and off -site sediment; and • Current/future recreational boaters and recreational and subsistence fishers with potential exposure to off -site surface water and off -site sediment, and fish ingestion for recreational and subsistence fishers. The following ecological receptors and exposure scenarios are identified in the ecological CSM: • Fish and benthic invertebrates with potential exposure to surface water and sediments. • Aquatic birds with potential exposure to surface water, sediments, fish, and AOW water. • Aquatic mammals with potential exposure to surface water, sediment, fish, and AOW water. Note the habitats where sampling was performed at MSS were considered primarily aquatic; therefore, the exposure pathways associated with terrestrial receptors were not evaluated for this site. 5.2 Step 2: Risk -Based Screening Groundwater, surface water, sediment, and soil data were evaluated during the CSA using risk - based screening level concentrations for identified COPCs. Risk -based screening level concentrations of COPCs were revised in the CAP Part 2 based upon additional groundwater, surface water, sediment, and soil data collected in the Third and Fourth Quarter 2015. Screening levels are concentrations of constituents in environmental media (e.g., soil) considered to be protective under most circumstances; their use requires a detailed understanding of the underlying assumptions in the CSM, including land use and the presence of sensitive populations. The presence of a constituent in environmental media at concentrations below the media and constituent -specific screening level is generally assumed not to pose a significant threat to human health or the environment. If a constituent exceeds the screening level, it does not necessarily indicate adverse effects on human health or the s The planned remedy for the ash basin pending NCDEQ risk classification is capping in place; therefore, it is not anticipated that soil will remain exposed after remedy implementation. It is included in this assessment as a receiving medium so that the risk -based methods for evaluating this medium in the future are in place. 36 Corrective Action Plan Part 2 F)� Marshall Steam Station Ash Basin environment; rather, it only indicates that additional evaluation may be warranted. Screening levels are used in this assessment to help in identifying COPCs to be carried forward into the evaluation of human health and ecological risk at the site. 5.3 Step 3: Human Health Risk Assessment COPCs were evaluated through a comparison of EPCs to calculated RBCs. The comparison was made through calculation of risk ratios for cancer and non -cancer effects. The total risk ratios among all compounds were then summed. Risk ratios were calculated by first identifying whether COPC RBCs were based on cancer risk or non -cancer hazard. For RBCs based on cancer risk, the risk ratio for each COPC was calculated by dividing the EPC by the cancer -based RBC concentration. For RBCs based on non -cancer risk, the risk ratio for each COPC was calculated by dividing the EPC by the non - cancer -based RBC concentration. A risk ratio less than 1 indicated that the EPC does not exceed the RBC, whereas a ratio greater than 1 indicated that the EPC exceeds the RBC. Risk ratios were also used to evaluate the cumulative receptor risk associated with each exposure point. Cumulative receptor risk was calculated by summing the risk ratios among all COPCs on which the RBC was based. In accordance with USEPA risk assessment guidance (USEPA 1991), the cumulative cancer risks and non -cancer hazard indices were evaluated against the USEPA target cancer risk range of 1.0E-06 to 1.0E-04 for potential carcinogens and target non -cancer hazard index of 1 for non -carcinogens (that act on the same target organ by the same mechanism of action); cumulative cancer risks and hazard indices that are above these limits indicate further evaluation may be deemed necessary. For MSS, the results of the human health risk assessment indicate that exposure to on -site surface water, AOW water, AOW soil, sediment, and groundwater pose no unacceptable risk for a trespasser, commercial/industrial worker, or construction worker under the exposure scenarios developed in Step 1. Similarly, off -site surface water and sediment pose no unacceptable risk for a recreational swimmer, wader, or boater, under the scenarios developed in Step 1. Consumption of fish (using on -site surface water data as a surrogate for fish tissue concentrations) by a recreational fisher and subsistence fisher resulted in hazard indices of 7.2E+00 and 2.1 E+02, respectively. Target endpoint analysis performed indicated hazard quotients above 1 for cobalt for both a recreational and subsistence fisher. The use of on -site surface water data to evaluate these potential exposure scenarios likely overestimates risk. This conservative approach was taken to provide an upper -bound estimate of potential surface water concentrations. As discussed in Section 4.2.2, modeled surface water concentrations for COPCs in Lake Norman are lower than their respective criteria (for COPCs that have applicable water quality criteria) at the edge of the modeled mixing zone. Thus, the potential risks calculated as part of this assessment need to be considered in this context, as well as considering the conservative exposure parameters used to evaluate these scenarios. 37 Corrective Action Plan Part 2 F)� Marshall Steam Station Ash Basin Additional data are needed regarding site -specific conditions including recreational and subsistence uses (if any) near the site, delineation of source -related and background COls applicable to the risk assessment, and the evaluation of the exposure parameters and fish ingestion models used in the risk assessment to qualify these results. 5.4 Step 4: Ecological Risk Assessment The purpose of a BERA is to: (1) determine whether unacceptable risks are posed to ecological receptors from chemical stressors, (2) derive constituent concentrations that would not pose unacceptable risks, and (3) provide the information necessary to make a risk management decision concerning the practical need and extent of remedial action. The BERA was performed according to the traditional ecological risk assessment paradigm: Problem Formulation, Analysis (Exposure and Effects Characterization), and Risk Characterization. Because of the many permutations of conservative parameters, an Uncertainty Evaluation section was also included. The BERA generally adhered to the USEPA's "Ecological Risk Assessment Guidance for Superfund: Process for Designing and Conducting Ecological Risk Assessments" (USEPA 1997) and the "Supplemental Guidance to ERAGS: Region 4, Ecological Risk Assessment" (USEPA 2015b), as well as NCDENR's "Guidelines for Performing Screening Level Ecological Risk Assessments" (NCDENR 2003). Constituents of concern were selected based on factors including the type of source(s), concentration, background levels, frequency of detection, persistence, bioaccumulation potential, toxicity/potency, fate and transport (e.g., mobility to groundwater), and potential biological effects. Screening consisted of comparing the maximum concentration of each constituent in the applicable media to conservative environmental screening levels. Per Step 3 of the USEPA Region 4 ecological risk assessment guidance, COPCs that were retained using the risk -based screening were evaluated using a multiple lines -of -evidence approach in the BERA (USEPA 2015c, 2015d). Risk characterization involved a quantitative estimation of risk followed by a description and/or interpretation of the meaning of this risk. The purpose of the risk characterization was to estimate potential hazards associated with potential exposures to COPCs and their significance. During risk estimation, the exposure assessment and effects assessment were integrated to evaluate the likelihood of adverse effects to wildlife receptors of interest (e.g., birds and mammals). The risk estimate was calculated by dividing the dose estimate from the exposure assessment by the applicable toxicity reference value (derived from the available literature) to obtain a hazard quotient. Receptors chosen for ecological risk assessment are often surrogates for the broad range of potential ecological receptors in a given habitat. For MSS, typical receptors were chosen for their expected common presence in the habitats represented at the site, or because they are common in the southeast and toxicity data are available. Aquatic receptors include fish, benthic invertebrates, aquatic birds (represented by mallard duck and great blue heron), and aquatic mammals (represented by muskrat and river otter). 38 Corrective Action Plan Part 2 F)� Marshall Steam Station Ash Basin At MSS, two ecological exposure areas were defined (Figure 2-5 in Appendix F). These include: • Ecological Exposure Area 1, located between the active ash basin and Lake Norman; and • Ecological Exposure Area 2, located in the northwest area of the MSS site and adjacent to the PV structural fill. Potentially affected areas on -site were classified as aquatic and were evaluated for exposure to site COPCs. Ecological habitats are presented on Figure 2-6 in Appendix F. Evaluation of surface water and sediment in the unnamed tributary, AOW water, and AOW sediment in Ecological Exposure Area 1 indicates a HQ of greater than 1 for a great blue heron's exposure to selenium and vanadium, at 3 and 22, respectively, using the NOAEL-based HQs; using the LOAEL-based HQs, the heron's exposure is reduced to 1 for selenium and 11 for vanadium. All other aquatic receptors have chemical HQs below 1. Evaluation of surface water in Ecological Exposure Area 2 indicates that all aquatic receptors have HQs below 1; the metal COPCs pose no unacceptable hazards to these receptors in these scenarios. A heron rookery and an active bald eagle nest are present on -site. Although bald eagles have been delisted at the federal and county level, they are still listed as a threatened species in North Carolina. Because the potential exists for this threatened species to inhabit/use the site, NOAEL-based HQs were used to evaluate potential hazard to this protected receptor. Since the eagle's diet is partially composed of fish, there may be a concern as the HQs for selenium and vanadium for the great blue heron, which subsists solely on fish, are 3 and 22, respectively. The body weight of the eagle is, however, two times greater than the heron. Additionally, the normalized fish ingestion rate is lower for the eagle than the heron (0.1 mg/kg/day vs. 0.18 mg/kg/day, respectively). Although the anticipated body burden of these COPCs would be at least two times lower than the great blue heron, additional evaluation is needed to determine what the potential risk from selenium and vanadium may be to the eagle, as sensitivity can be both species and constituent -specific. Additional data and further refined assessment are needed to address uncertainties associated with the evaluation of these scenarios including the occurrence of these ecological receptors in the areas adjacent to the ash basins, delineation of source -related and background COls applicable to the risk assessment, and refinement of the exposure and toxicity assumptions used in the ecological risk characterization. 39 Corrective Action Plan Part 2 F)� Marshall Steam Station Ash Basin 6 Alternative Methods for Achieving Restoration This section discusses how remedial alternatives were evaluated and identifies the remedial alternative selected to achieve restoration of groundwater quality at the MSS site. As described in Section 1, this CAP considers cap -in -place as the source control measure for the ash basin. An engineered cap will minimize infiltration through the covered area, thereby reducing the contact of infiltrating water with potentially impacted soil. Based on the foregoing, remediation of soils is not discussed in this document; rather, this CAP focuses on remediation of groundwater impacts associated with the MSS ash basin system. As noted in Section 2, exceedances of 2L Standards were measured in groundwater monitoring wells located between the ash basin and Lake Norman and an unnamed tributary that flows to the lake. In addition, exceedances of 2B Standards were measured in the surface water samples collected from the unnamed tributary located east of the ash basin and the dry ash landfill (Phase 1). It is likely that the primary source of water discharging to the unnamed tributary is water flowing from within and beneath the southeast portion of the ash basin. Dewatering of the ash basin will likely decrease the amount of discharge as well as the concentrations of COls that appear to be discharging to the unnamed tributary. Monitoring of the unnamed tributary will continue throughout source control activities and further assessment will be conducted to evaluate a potential groundwater cutoff wall in this specific area of the site, as described in the sections below. 6.1 Corrective Action Decision Process 6.1.1 Evaluation Criteria The goal of groundwater corrective action in accordance with 15A NCAC 2L .0106 is: "... where groundwater quality has been degraded, the goal of any required corrective action shall be restoration to the level of the standards, or as closely thereto as is economically and technologically feasible"...using best available technology 0), or to an alternate standard (k) or using natural attenuation mechanisms (1). The evaluation of best available methods for groundwater remediation is based on the objective of meeting groundwater standards at the Compliance Boundary with consideration of implementability, time, and cost. The methods may include one or a combination of best available technologies and natural attenuation processes. Source control measures (such as cap -in -place, excavation, or partial excavation/cap-in-place) are being addressed separately. The groundwater remedial alternatives herein are evaluated to supplement the selected source control measure. 40 Corrective Action Plan Part 2 F)� Marshall Steam Station Ash Basin 6.1.2 COls Requiring Corrective Action Data from the CSA were evaluated in CAP Part 1 to identify the following groundwater COls that are considered for current or potential corrective action: antimony, arsenic, barium, beryllium, boron, chloride, chromium, cobalt, hexavalent chromium, iron, manganese, selenium, sulfate, thallium, TDS, and vanadium. These COls are considered for corrective action because they have either exceeded their applicable 2L Standard, IMAC, or NCDHHS HSL, or may exceed them in the future (based on groundwater modeling predictive scenarios) due to fluctuations of COI concentrations as a result of closure activities. Variability in detections and concentrations of COls from Round 1 and Round 2 sampling events were previously discussed in Section 2.3.2. Based on this variability, additional sampling and data evaluation is recommended to confirm the magnitude and extent of COI exceedances and so that the most appropriate corrective action(s) for specific COls is selected. 6.1.3 Potential Exposure Routes and Receptors The Baseline Human Health and Ecological Risk Assessment (Appendix F) provides a conservative assessment of potential risk associated with the COls attributed to the source areas at the MSS site. The primary source -to -receptor exposure route is leaching of ash porewater to groundwater, which will discharge to Lake Norman and the unnamed tributary flowing to Lake Norman. A secondary source -to -receptor exposure route may be infiltration of COls in water and/or sediment from AOWs to groundwater. At the MSS site, there are no water supply wells downgradient between the source areas and Lake Norman. Lake Norman is a hydrologic boundary between the ash basin and properties to the southeast of Lake Norman and Duke Energy property; therefore, consideration of future water supply wells as receptors downgradient of the source areas does not warrant further assessment. Localized groundwater mounding associated with the current hydraulic head in the basin will be reduced as source control is implemented. After dewatering of the basin and source control, the groundwater model predicts that the water table elevation in the source areas will be lowered. These corrective actions have the potential to reduce or eliminate the flow in AOWs associated with the ash basin and the amount of discharge and concentrations of COls discharging to the unnamed tributary east of the ash basin, potentially eliminating the exposure pathway to surface water. 6.2 Alternative Evaluation Criteria Alternative evaluation criteria are chosen in general conformance with USEPA Office of Solid Waste and Emergency Response Directive 9355.-27FS, "A Guide to Selecting Superfund Remedial Actions" (USEPA 1990). This document provides threshold, balancing, and modifying criteria for selection of a remedy. 41 Corrective Action Plan Part 2 F)� Marshall Steam Station Ash Basin Potential groundwater corrective action alternatives will be evaluated against the following criteria: • Effectiveness (Section 6.2.1) • Implementability/Feasibility (Section 6.2.2) • Environmental Sustainability (Section 6.2.3) • Cost (Section 6.2.4) • Stakeholder Input (Section 6.2.5) 6.2.1 Effectiveness Effectiveness is a comparison of the likely performance of applicable technologies while taking into consideration the following: 1. The estimated area and volumes of media to be treated. 2. Demonstrated reliability to achieve constituent remedial goals under site conditions. 3. Demonstrated reliability to reduce potential risk to human health and the environment in a timely manner. Specific effectiveness criteria include: • Has the potential remedial alternative been demonstrated to be effective at similar sites? • Does the remedial technology involve treatment that will permanently destroy target constituents? • Does the remedial technology involve treatment that will permanently detoxify target constituents? • Does the remedial technology involve treatment that will permanently reduce the mobility of target constituents? • Will the remedial alternative permanently remove contaminants from the site? • Can the effectiveness of a potential remedial technology be monitored, measured, and validated? • Will a remedial technology reduce potential risk to human health when fully implemented? • Will a remedial technology reduce potential risk to the environment when fully implemented? • Will a remedial technology be protective of human health? Technologies that are deemed to be less effective under site -specific conditions than otherwise comparable technologies will be eliminated on the basis of effectiveness. 6.2.2 Implementability/Feasibility The screening criteria of implementability evaluates whether implementation of a technology is technically and administratively feasible. Specific implementability criteria include: • Are the material resources and manpower readily available to fully implement the remedial technology in a timely manner? • Does the remedial technology require highly specialized resources and/or equipment? 42 Corrective Action Plan Part 2 F)� Marshall Steam Station Ash Basin • Is there sufficient on -site and off -site area to fully implement the remedy? • Does the remedial technology require any permits, and can the permits be acquired in a timely manner (e.g., wetlands permitting)? • Can the remedial alternative be implemented safely? • Can existing and future infrastructure support the remedial alternative? • Will a remedial technology increase potential risk to public safety during implementation? • Will a remedial technology increase potential risk to the environment during implementation? • Can a remedial technology meet all applicable or relevant and appropriate requirements (ARARs)? Technologies that are deemed impractical under site -specific conditions will be eliminated on the basis of implementablity/feasibility. 6.2.3 Environmental Sustainability A remedy is environmentally sustainable when it maximizes short-term and long-term protection of human health and the environment through the judicious use of limited resources. Metrics used to measure environmental sustainability include: • Will constituents be treated to reduce toxicity or mobility, or will treatment transfer the constituent from one media to another (e.g., discharge constituents in extracted groundwater to surface water)? • Is the carbon footprint (energy consumption) of otherwise comparable remedial alternatives significantly different? • Will source materials used in the remediation process be recycled or reclaimed? • Will waste materials generated during the remediation process be recycled or reclaimed? • Will renewable sources of energy be used during the remediation process? • Will natural habitat restoration, enhancement, or replacement be integral to the remedy? Duke Energy considers environmental sustainability in its alternative evaluation criteria and where appropriate will incorporate "green" remedial strategies in their evaluation. Green remedial strategies consider all environmental effects of remedy implementation and incorporate options to maximize new environmental benefit of cleanup operations (USEPA 2008). Green remediation reduces the demand placed on the environment during remedial operations to avoid collateral damage to the environment. Green remediation strategies minimize adverse impacts to other environmental media such as: • Air pollution caused by emission of carbon dioxide, nitrous oxide, methane, and other greenhouse gases emitted during remediation. • Imbalance to the local and regional hydrologic regimes. • Soil erosion and nutrient depletion causing changes to soil geochemistry. • Ecological diversity and population reductions. 43 Corrective Action Plan Part 2 F)� Marshall Steam Station Ash Basin 6.2.4 Cost The criteria of cost will include estimated capital cost and labor that will be incurred during remedial design and implementation as well as operation and maintenance cost over a 30-year period. Cost will not be the sole or primary basis for selecting a technology or remedial alternative; however, cost will be considered when selecting between two or more remedial technologies or alternatives that are otherwise comparable. 6.2.5 Stakeholder Input Appropriate stakeholders will be notified pursuant to 15A NCAC 02L .0114. 6.3 Remedial Alternatives to Achieve Regulatory Standards Source control is the primary corrective action for groundwater restoration at the MSS site and may include capping in place. Source control options are being evaluated and designed outside of this CAP; however, source control and active/passive groundwater remediation are intertwined and should be considered together when evaluating remedial alternatives. Corrective action alternatives to restore groundwater to 2L Standards in conjunction with source control measures are discussed below. 6.3.1 Groundwater Remediation Alternatives Remedial alternatives for restoration of groundwater in accordance with T15A NCAC 2L .0106 include the following: Source Control, which can include: o Placement of engineered cap to prevent minimize infiltration and reduce subsequent COI leaching into groundwater; o Slurry walls or grout curtains to prevent groundwater interaction with source material; and/or, o In -situ solidification/stabilization to reduce or eliminate leaching of COls into groundwater by mixing soil beneath source areas with pozzolanic materials (i.e., Portland cement or bentonite). Monitored Natural Attenuation — Dilution from recharge to shallow groundwater, mineral precipitation, and COI sorption will occur over time, thus reducing COI concentrations through attenuation. Regular monitoring of select groundwater monitoring wells for specific parameters would be conducted to ensure COI concentrations in groundwater are decreasing. • Enhanced Recharge/Flushing — Short-term temporary methods or a permanent installation for COls that persist. Short-term approaches may include surface irrigation, infiltration galleries, or ponds/wetlands with permeable bottoms. For long-term applications, infiltration galleries are needed. • Enhanced Attenuation, which can include: o Addition of materials with high sorptive capacity to the saturated zone to reduce COI levels in groundwater; and 44 Corrective Action Plan Part 2 F)� Marshall Steam Station Ash Basin o Adjusting the pH and/or redox state to enhance precipitation of iron and manganese oxide/hydroxide minerals to reduce COI levels in groundwater. Permeable reactive barriers — Trenching and placement of selected material in the trench that would chemically bond with the COls and reduce their concentrations in groundwater. • Water Treatment — Active in -situ groundwater remediation by injection of chemical and/or air sparging, groundwater extraction and treatment, or passive groundwater remediation through wetland construction. A detailed description of remedial alternatives is included in Appendix G. 6.3.2 Monitored Natural Attenuation Applicability to Site MNA is a strong candidate for corrective action for groundwater impacts identified at the MSS site. MNA is a strategy and set of procedures used to demonstrate that physiochemical and/or biological processes in an aquifer will reduce concentrations of COls to levels below regulatory criteria. The mechanisms that regulate their release from solids and movement through an aquifer are, for the most part, the same processes that provide chemical controls on movement of CCR leachate in an aquifer. These processes attenuate the concentration of inorganics in groundwater by depositing inorganics on aquifer solids removing constituents from groundwater. An MNA Tier I and Tier II evaluation was conducted for the MSS site by Geochemical, LLC and is included in Appendix H. The following is a summary of the Tier I and Tier 11 evaluations. The Tier I analysis used two lines of evidence for attenuation, including: 1) solid -water pair comparison of COI concentrations was performed, and a mutually rising relationship indicated attenuation (USEPA 2007a); and 2) laboratory determination of the solid -water partitioning coefficient, or Kd value (USEPA 1999), was used as a measure of the susceptibility of COls to sorb to solids and be attenuated. The Tier I analysis indicated that arsenic, barium, beryllium, boron, chromium, cobalt, lead, thallium, and vanadium are attenuating in groundwater at the MSS site. The Tier II demonstration was then performed based on the conceptual model for COI attenuation involving reversible and irreversible interaction with clay minerals, metal oxides, and organic matter. Findings from the Tier II demonstration include the following: • The samples evaluated for Kd determinations were representative of site -specific conditions. • Clay minerals and Fe-Mn-AI oxides were found in all samples. Organic matter is likely not a significant sink for COls at the MSS site. • Chemical extractions indicated that COls were concentrated in soil samples exposed to groundwater containing higher concentrations of COls, which most likely indicates that precipitating iron and possibly manganese have removed other COls through coprecipitation and adsorption, thus validating attenuation is occurring at the site. • Chemical extractions were used to determine a probable range of Kd values that suggest attenuation is taking place for arsenic, barium, chromium, and thallium. 45 Corrective Action Plan Part 2 F)� Marshall Steam Station Ash Basin Additional data collection is necessary to complete the Tier II analysis and then conduct a Tier III analysis to evaluate specific attenuation mechanisms for each COI, and quantification of the magnitude of attenuation by specific media to support numerical modeling. The Tier III objective is to eliminate sites where site data and analysis show that there is insufficient capacity in the aquifer to attenuate the contaminant mass to groundwater concentrations that meet regulatory objectives or that the stability of the immobilized contaminant is insufficient to prevent remobilization due to future changes in ground -water chemistry. The Tier III assessment will be performed in general accordance with USEPA's "Monitored Natural Attenuation of Inorganic Contaminants in Ground Water - Volume 2 Assessment for Non -Radionuclides Including Arsenic, Cadmium, Chromium, Copper, Lead, Nickel, Nitrate, Perchlorate, and Selenium" (USEPA 2007b). 6.3.3 Site -Specific Alternatives Analysis Source control is the primary corrective action for groundwater restoration at the MSS site. Source control for the ash basin could include construction of an engineered cap system (i.e., cap -in -place), but will be modified as required based on the final risk ranking classification proposed by NCDEQ and approved by the CAMC. Additional remedial technologies were reviewed to develop site -specific alternatives for addressing groundwater impacts and are detailed in Appendix G. Six remedial alternatives were evaluated for the MSS site to enhance or supplement source control activities. Provided below are summaries of each technology's applicability to the site. 1. No further action — This alternative is provided to establish a baseline for comparison to other alternatives. There would be no remedial actions conducted at the site to remove the source of COls other than capping -in -place and no further remedial action would be taken for groundwater. This measure does not include long-term monitoring or institutional controls. 2. MNA — Attenuation will occur over time due to natural processes, and its extent and progress will be monitored at appropriate locations in the source area and between the source and the Compliance Boundary. Water quality will improve over time once the source area is controlled/capped and as recharge flushes non -impacted water through the aquifer. Groundwater monitoring would be continued until remedial objectives are met (i.e., groundwater concentrations are at or below applicable standards or criteria). Implementing land use controls downgradient of the ash basin will ensure future potential receptors will not be affected and controls would remain in place until remedial objectives are achieved. Adsorption to iron oxides and hydroxides is demonstrated for antimony, arsenic, boron, barium, cadmium, cobalt, chromium, iron(II), mercury, manganese, nickel, lead, selenium, sulfate, vanadium and zinc (Dzombak and Morel, 1990). Soil chemistry results at the MSS site show abundant Fe2O3 and other hydroxides in soils at the site (CSA Report Table 6-2), and a strong potential for adsorption. A Tier II demonstration based on that conceptual model was partially completed. Additional data collection is necessary to complete the Tier II assessment to determine the specific attenuation mechanisms for each COI and to determine the rate of attenuation such that it may be 46 Corrective Action Plan Part 2 F)� Marshall Steam Station Ash Basin included in groundwater modeling. The groundwater model did not allow for removal of COI via coprecipitation with iron oxides, which likely resulted in a conservative prediction of COI transport. Completion of the Tier II tests described in Appendix H will address this issue. If the completed Tier II analysis indicates a Tier III analysis is warranted, the additional analysis should be conducted to determine if natural processes are sufficient to prevent COI exceedances at the groundwater Compliance Boundary. Based on available assessment results, it is feasible that MNA can be used partially or entirely to remediate groundwater at the MSS site. It is noted that a groundwater cutoff wall may be an effective method for addressing impacted groundwater that appears to be flowing from the ash basin beneath the dry ash landfill (Phase 1) and discharging to the unnamed tributary that flows to Lake Norman. Further assessment should be conducted to evaluate the effectiveness of a groundwater cutoff wall in this area of the site. Two potential options include construction of a slurry wall or emplacement of a grout curtain. 3. Enhanced Recharge/Flushing with Groundwater Monitoring — It is possible to increase the rate of groundwater quality improvement by increasing infiltration of uncontaminated water into portions of a site, thereby flushing, diluting, and attenuating the remnant concentrations of COls. There are various ways to accomplish this, ranging from short- term or temporary methods (e.g., surface irrigation using mechanical sprayers) to the creation of groundwater infiltration galleries or ponds or wetlands with a permeable bottom, which could be temporary or permanent. Where a continuing source is present, permanent infiltration galleries may be needed. The best potential location for enhanced recharge is to the east of the ash basin, where the dry ash landfill units (Phases I and II) are located. The use of this technology is not considered appropriate for the MSS site due to the landfill units being located where enhanced recharge would be most effective. 4. In -situ Sorption or In -situ Chemical Fixation — Various measures can be taken to enhance attenuation of COls by blending materials that have a high sorptive capacity, such as clays, peat moss, and zeolites, into the impacted subsurface material. Contaminated groundwater can also be treated in -situ using chemical fixation to adjust the pH and/or redox state of the groundwater, which could result in enhancing precipitation of iron and manganese oxide and hydroxide minerals in the groundwater. Enhanced formation of these minerals removes iron and manganese from the groundwater and can effectively co -precipitate and attenuate other COls. Redox conditions can be adjusted either through addition of one or more reagents or through air sparging. Bench -scale treatability testing and/or pilot -scale tests are usually required to verify the effectiveness of this technology at a specific site prior to full-scale application, and to select the most appropriate reagent and dosage. Geochemical modeling suggests that iron and manganese are already precipitating beneath and downgradient of the source areas. Reagent addition (or air sparging) could encourage the precipitation of COls by changing the redox conditions over targeted areas and thereby enhancing natural attenuation. In -situ chemical fixation involves the injection of a chemical oxidant, such as potassium permanganate. Lifetime costs of 47 Corrective Action Plan Part 2 F)� Marshall Steam Station Ash Basin these technologies may be comparable. If these technologies are part of a selected alternative, it is recommended that both approaches be tested on -site with a pilot -scale study to evaluate effectiveness of each approach, since a myriad of variables affect their comparable performance. Due to the wide distribution of COls in the shallow and deep zones, the application of in - situ chemical fixation technology would require a very extensive well network to facilitate application of reagents. It is not feasible to use this technology to manage large land areas. It may be feasible to use chemical fixation for the treatment of groundwater east of the ash basin and dry ash landfill (Phase 1) to prevent the impacted groundwater from discharging to the unnamed tributary that flows to Lake Norman. This approach could reduce the flux of COls and therefore reduce loading for MNA treatment. However, boron is one of the COIs that exceeded groundwater standards at this location, and this approach has a limited ability to attenuate boron. Application of the technology for this purpose could be considered in the future following the re-evaluation of site conditions following completion of source control and potential hydraulic control such as a cutoff wall. 5. Permeable Reactive Barrier - A permeable reactive barrier (PRB) is a passive form of in - situ water treatment that removes COls by chemical bonding in the subsurface zone (ITRC 2005). PRBs are typically constructed by excavating a trench that fully penetrates the saturated zone of the unconsolidated aquifer and places material that would chemically bond with the selected COI in the trench to treat the groundwater. The media that are used for in -situ treatment are selected based on the contaminants required for removal. A funnel -and -gate system can also be used to channel the contaminant plume into a gate that contains the bonding material. The funnels are low -permeability, and the basic design consists of a single gate with walls extending from both sides. The advantage of the funnel -and -gate system is that a smaller region can be used to treat the plume, which can reduce costs. In addition, if the reactive media need to be replaced, it is much easier to do so because there is less material to replace. Site -specific media should be evaluated with a range of reactive adsorbents to best determine the type and blend ratio to effectively remove COls while maintaining hydraulic conductivity. The PRB lifespan is a function of the COI concentration and the media removal characteristics. PRBs may be placed as an interim or long-term measure. If it is anticipated that the COls will continue to persist in groundwater for multiple decades, long-term remediation may require the periodic replacement of the PRB's reactive media. There have been many successful PRB remedies at sites with a wide range of constituents, but only limited testing with water containing the constituents in ash leachates (EPRI 2006). Based on a review of data, it appears that the PRB could comprise a combination of limestone aggregate (to provide PRB stability, transmissivity, 48 Corrective Action Plan Part 2 F)� Marshall Steam Station Ash Basin and pH buffering) and organic materials (mulch, wood chips, etc.) to promote the reduction of sulfate to sulfide and precipitation of the inorganics, and potentially zero valent iron to help promote and sustain the reducing conditions. Laboratory tests would have to be conducted and then scaled to a pilot -scale level to confirm the effectiveness of PRB materials at the site and whether additives would be necessary to maintain the required level of permeability. Considering the size of the MSS site and the uncertainty of effectiveness, installation of a PRB is not expected to be cost effective or feasible. However, a PRB could potentially be installed near the unnamed stream east of the ash basin and dry ash landfill (Phase 1) where the depth to bedrock is relatively shallow, if warranted to treat impacted groundwater flowing toward the unnamed tributary. However, boron concentrations exceed the 2L Standard in this location, so pilot -scale tests of the adsorbents shown to remove boron in the laboratory would be necessary. Site media would need to be evaluated with a range of adsorbents to determine the optimal blend ratio for effectively removing COls and maintaining hydraulic conductivity. 6. Groundwater Treatment — As an alternative to in -situ groundwater treatment methods, groundwater can be removed and treated above ground. Impacted groundwater would be pumped to the surface (pump -and -treat) or captured at AOWs at the ground surface. Following treatment, the water may be discharged under an existing NPDES permit or re -injected underground. Water treatment can be active (requiring the continual addition of chemicals and typically, electrical power) or passive (systems that take advantage of reactions that occur in nature, such as constructed wetlands or limestone beds to provide neutralization). The use of passive systems is generally restricted to smaller flows because the approach typically requires much larger land area than active systems, but has the advantages of less maintenance and lower operating costs. Passive treatment systems, however, can be ineffective at removing certain COls, such as boron and TDS. Active treatment systems are generally costly to construct and operate, but can be designed to effectively lower the concentrations of all COls. While a zone of depression could be created, thereby minimizing off -site transfer of COls, it is anticipated that the system would have to operate a considerable time into the future until groundwater concentrations decrease below applicable standards or criteria. It may be feasible to evaluate applying this technology in localized portions of the site; however, the areal extent of the site makes centralized treatment more challenging. A detailed modeling and pilot study would be needed to predict drawdown and recovery for system design. 6.3.4 Site -Specific Recommended Approach As noted in Section 6.3.3, collection of additional groundwater data is needed to establish trends, better understand background conditions, and identify potential areas requiring remediation. Based on the limited data to date, observations/approaches are outlined below. 49 Corrective Action Plan Part 2 F)� Marshall Steam Station Ash Basin 6.3.4.1 Proposed Areas Evaluated for Remediation At the MSS site, there are no areas downgradient of the source areas where groundwater has been reported to exceed applicable 2L Standards, IMACs, or NCDHHS HSLs beyond the ash basin Compliance Boundary. However, based on the CSA results, it appears groundwater may be impacted beyond the Compliance Boundary east of the ash basin and dry ash landfill (Phase 1). Surface water samples collected from the unnamed tributary that flows to Lake Norman have exhibited exceedances of 2B Standards (specifically, ash -related COls sulfate and TDS). Additional assessment is actively occurring to delineate the extent of groundwater impacts in this area of the site, specifically to the east/northeast of the unnamed tributary. If active remediation is deemed necessary in this area of the site, controlling groundwater that flows from the ash basin beneath the dry ash landfill (Phase 1) toward the unnamed tributary should be evaluated to supplement MNA. If additional investigations confirm elevated COI concentrations that cannot be addressed by MNA, alternate corrective actions in the form of reagent additions or air sparging may be further evaluated in the future. 6.3.4.2 Recommended Corrective Action As noted in Section 6.3.3., available assessment results indicate it is feasible that MNA can be used partially or entirely to remediate groundwater at the MSS site in conjunction with source control by cap -in -place. It is noted that a Tier III MNA analysis is recommended to verify site - specific conditions are adequate for potential naturally occurring reactions. Existing data demonstrates MNA is a viable remedial approach, but additional data to assess system capacity and stability and evaluate the mechanisms of inorganic attenuation is needed to support this approach. In addition to MNA, a groundwater cutoff wall may be an effective method for remediating impacted groundwater that appears to be flowing from the ash basin beneath the dry ash landfill (Phase 1) and discharging to the unnamed tributary that flows to Lake Norman. Further assessment should be conducted to evaluate the effectiveness of a groundwater cutoff wall. Two potential options include construction of a slurry wall or emplacement of a grout curtain. The implementation cost of MNA for a 30-year period and additional assessment for evaluating groundwater diversion is estimated to cost $7,030,000. Monitoring to evaluate the effectiveness of MNA as a corrective action is summarized in Section 9.0. Costs are discussed further in Section 10. 50 Corrective Action Plan Part 2 F)� Marshall Steam Station Ash Basin 7 Selected Corrective Action(s) Corrective action(s) for the MSS site were selected based on the evaluation of site conditions as described in previous sections and the evaluation of alternatives described in Section 6. Based on the remedial alternatives evaluation, MNA was determined to be the most appropriate corrective action to support source control; however, groundwater quality should be monitored following implementation of the proposed closure activities to evaluate if MNA remains a viable corrective action after basin closure. The final closure option selected will be modified as required based on the final risk classification proposed by NCDEQ and approved by the CAMC. 7.1 Selected Remedial Alternative for Corrective Action If consistent with the final NCDEQ risk classification, and if ongoing monitoring data support it, MNA, in conjunction with an engineered cover system as source control, is recommended on the basis of environmental protection, technical effectiveness, and sustainability. Further assessment should be conducted to evaluate the effectiveness of a groundwater cutoff wall for remediating impacted groundwater that appears to be discharging to the unnamed tributary that flows to Lake Norman. Subsequent to closure, it is reasonable to assume that COls remaining in groundwater will decrease in concentration over time as upgradient non -impacted water moves through the aquifer. Groundwater flow and geochemical modeling indicates that attenuation by a combination of sorption, chemical precipitation, and dilution by surface water infiltration and fresh groundwater effectively dissipate COls in groundwater beneath and downgradient of the source areas. During and following ash basin dewatering and capping, groundwater hydrology and geochemistry will likely change and monitoring wells located within the cap footprint will be decommissioned and removed, as needed. The Tier III MNA analysis will be conducted in conjunction with or shortly after installation of the additional assessment wells described in Section 8. Groundwater monitoring will also be used to evaluate changes in hydrology and geochemistry and refine geochemical and groundwater modeling predictions presented in Section 4. A monitoring plan is discussed in detail in Section 9 of this report. 7.2 Conceptual Design 7.2.1 Source Control — Cap -in -Place This proposed CAP considers cap -in -place as the source control measure for the ash basin. An engineered cap will reduce infiltration through the covered area, thereby reducing the potential of leaching of COls into the groundwater underlying the closed basin. Note that the final closure option selected will be modified as required based on the final risk classification proposed by NCDEQ and approved by the CAMC. 51 Corrective Action Plan Part 2 Marshall Steam Station Ash Basin 7.2.2 MNA 7.2.2.1 Demonstration of MNA The use of MNA as a corrective action involves monitoring select parameters to determine if COls are attenuating as a result of the corrective action. Once the ash within the ash basin is capped, it is anticipated that groundwater quality will improve over time due to the attenuation of COls from the recharge of non -impacted groundwater, precipitation, and adsorption of Cols. Tier I and 11 analyses were conducted for the MSS site (discussed in Section 6.3.2 and presented in Appendix H). A geochemical site conceptual model for COI attenuation involving interaction with site samples containing clay minerals, metal oxides, and organic matter was completed. The most significant finding was that the precipitation of iron and manganese serves to remove other COls through co -precipitation and adsorption, thus confirming that attenuation is occurring. In support of this reaction, clay minerals and Fe-Mn-AI oxides were found in samples. Evaluation of geochemical modeling indicated COls are attenuated by a combination of sorption and/or precipitation. TDS and sulfate generally are not attenuated, but concentrations are reduced by diffusion, mechanical mixing, and/or dilution. Arsenic and chromium were calculated to have limited solubility; therefore, these constituents do not migrate readily in groundwater. Groundwater fate and transport model predictions presented in Appendix B are supported by the findings of the geochemical modeling presented in Appendix E. Based on review of the groundwater modeling results, COls with sorption coefficients similar to or greater than arsenic are immobilized by sorption and/or precipitation, but COls with sorption coefficients similar to or less than boron are not readily attenuated by sorption to solid site materials and are more readily transported by groundwater moving through impacted media. Groundwater modeling did not take into consideration the removal of COls via co -precipitation with iron oxides, which likely resulted in an over -prediction of COI transport, causing some of the COIs to exceed the 2L Standards at the Compliance Boundary in the model output. Surface water and groundwater interaction modeling has determined that, even with over -prediction of COls to Lake Norman, exceedances of 2B Standards are not predicted to occur. 7.2.2.2 Verification of MNA The MNA monitoring program, data collection, and evaluation to advance the Tier III assessment should be implemented prior to any cap -in -place activities. If the results of the Tier III assessment support the use of MNA, effectiveness monitoring should be continued until water quality meets remedial objectives (e.g., applicable regulatory standards or PPBCs, as applicable). The site monitoring requirements are discussed in Section 9. If the number of COls or COI concentrations are determined to be increasing over time during MNA monitoring, the effectiveness of MNA will be re-evaluated and additional remedial alternatives will be considered. If warranted, additional corrective actions will be implemented. Due to the changes in groundwater hydrology and geochemistry that are anticipated to occur as a result of proposed closure activities, interim monitoring activities are recommended along with the proposed corrective action, as outlined in Section 8. Monitoring plans to evaluate the effectiveness of MNA are detailed in Section 9, and the implementation schedule is presented in Section 10. 52 Corrective Action Plan Part 2 F)� Marshall Steam Station Ash Basin 8 Recommended Interim Activities Interim activities are proposed at the MSS site to address additional information needs identified in the CSA and CAP Part 1 reports and to refine geochemical and groundwater model predictions. These activities include the installation of additional monitoring wells and groundwater sampling during closure activities. 8.1 Additional Monitoring Well Installation Approximately 13 additional monitoring wells are currently being installed to address additional information needs identified in the CSA Report and during post-CSA review comment meetings with NCDEQ. The additional wells will be used to further refine the horizontal and vertical extent of groundwater impacts, refine the understanding of groundwater flow direction, refine geochemical and groundwater modeling predictions, and provide additional background monitoring locations. Locations of the proposed additional assessment monitoring wells are shown on Figure 8-1. The additional monitoring wells will be incorporated into the groundwater monitoring network in 2016 and sampled in conjunction with the existing monitoring wells, as described in Section 9. 8.2 Additional Groundwater Sampling and Analyses Additional sampling of select monitoring wells will be conducted at the MSS site during 2016. The number of qualifying sampling events and locations (i.e., where turbidity was less than 10 NTU) of background wells have been limited such that PPBCs presented in the CAP Part 1 report for select constituents could not be statistically derived. Groundwater sampling results from proposed First and Second Quarter 2016 sampling events, as described in Section 9, will be used to supplement the existing background well database and support statistical analysis for calculation of revised PPBCs. In response to NCDEQ comments to the CSA Report, additional radionuclide sampling will also be performed in wells along inferred groundwater flow pathways through the ash basin and in background wells. This sampling will be coordinated to coincide with either the First or Second Quarter 2016 sampling event. 53 Corrective Action Plan Part 2 F)� Marshall Steam Station Ash Basin 9 Interim and Effectiveness Monitoring Plans The Interim and Effectiveness Monitoring Plans (Monitoring Plans) provide detailed information regarding the purpose, schedule, and methods for collection of groundwater, surface water, and ash basin water samples at MSS. The Monitoring Plans are intended to evaluate the effectiveness of proposed corrective actions; monitor the movement of contaminants in groundwater during and after proposed future closure activities (i.e., capping) of the MSS ash basin system; and provide data for evaluation of baseline conditions and seasonal variation in groundwater, surface water, and ash basin water at the MSS site. These Monitoring Plans supersede the monitoring plan provided in the CSA (Section 16 — Interim Monitoring Plan). Protocols for groundwater, surface water, and ash basin water sample collection, analysis, and reporting are consistent between the Monitoring Plans. This sampling and analysis will be completed in general accordance with the Monitoring Plans presented below, the CSA Work Plan, and the Low Flow Sampling Plan (CSA Report Appendix G). 9.1 Interim Monitoring Plan The Interim Monitoring Plan has been developed to provide baseline seasonal analytical data associated with the MSS site. The Interim Monitoring Plan will be implemented through the first half of 2016. The Interim Monitoring Plan establishes data quality objectives (DQOs) and sampling requirements associated with sampling frequency, sampling locations, and analytical requirements. Upon completion of the Second Quarter 2016 sampling event, monitoring activities will be conducted in accordance with the Effectiveness Monitoring Plan described in Section 9.2. 9.1.1 Data Quality Objectives The following DQOs are associated with the Interim Monitoring Plan: • Monitor the extent of groundwater contamination in and around the MSS site and evaluate seasonal trends associated with COIs. • Monitor the movement of COls within groundwater and the interaction with surface water. • Determine seasonal groundwater flow direction and elevations, and monitor potential changes to groundwater flow direction and elevation resulting from seasonal variation. The DQOs will be met through the following activities: Perform groundwater, surface water, and ash basin water sampling at the locations depicted on Figure 2-1 and Table 9-1 through the first half of 2016. These monitoring events, planned for first and second quarters of 2016, will be combined with analytical data from CSA rounds 1 through 4 (collected in the second half of 2015) to evaluate seasonal water quality conditions and background groundwater concentrations at the MSS site. Additional assessment wells are being installed at the MSS site as of February 2016 and will be added to the Interim Monitoring network following installation. 54 Corrective Action Plan Part 2 F)� Marshall Steam Station Ash Basin • Perform groundwater static water level measurements at site -wide monitoring wells concurrent with groundwater sampling described above. • Perform water level gauging at previously established stream and surface water locations. • Perform total depth measurements at site -wide monitoring wells on an annual basis. • Prepare reports documenting sampling results and analysis for submittal to NCDEQ, as specified in Section 9.1.3 below. 9.1.2 Sampling Requirements 9.1.2.1 Sample Frequency To meet the DQOs and evaluate seasonal fluctuations in COI concentrations, sampling of CSA, compliance, voluntary, and background groundwater monitoring wells and surface water locations will be conducted at the MSS site in the First and Second Quarters of 2016. Sampling frequency will be revised as described in the Effectiveness Monitoring Plan (Section 9.2) upon completion of the Second Quarter 2016 monitoring event. 9.1.2.2 Sample Locations Groundwater monitoring well, surface water, and ash basin water locations to be sampled during the Interim Monitoring are identified in Table 9-1 and depicted on Figure 2-1. Monitoring wells may be added to the sampling program through the installation of additional monitoring wells (i.e., assessment wells shown on Figure 8-1) or removed as proposed closure activities lead to well abandonment. NCDEQ will be notified and NCDEQ's approval will be obtained prior to the abandonment of these monitoring wells. 9.1.2.3 Analytical Requirements Analytes for monitoring wells, surface water, and ash basin water include total and dissolved metals, alkalinity (total, bicarbonate, and carbonate), calcium, chloride, hexavalent chromium, potassium, magnesium, nitrate, sodium, sulfate, total combined radium, total combined uranium, total dissolved solids, total organic carbon, and total suspended solids. In addition, samples will be analyzed for ammonia for use in evaluation of MNA as a corrective action. Analytical services will be provided by a North Carolina certified laboratory. Chemical analytes, analytical methods, bottle requirements, and preservatives are provided in Table 9-2. 9.1.3 Reporting Validated analytical results from the First Quarter 2016 monitoring event are proposed to be transmitted to NCDEQ within 90 days of completion of the sampling event. Within 120 days of completion of the Second Quarter 2016 monitoring event, Duke Energy proposes to submit a groundwater monitoring report to NCDEQ that summarizes the results from the First and Second Quarter 2016 monitoring events. 55 Corrective Action Plan Part 2 F)� Marshall Steam Station Ash Basin 9.2 Effectiveness Monitoring Plan The Effectiveness Monitoring Plan has been developed to monitor select wells for MNA parameters to provide baseline MNA data prior to basin closure; to monitor potential influence of closure activities on COIs; and to monitor the effectiveness of MNA as a corrective action following basin closure. The Effectiveness Monitoring Plan will be implemented using select wells following completion of the Second Quarter 2016 sampling event and will continue through 2020 (five years of CAP -related sampling). The Effectiveness Monitoring Plan may be modified within the first five-year period if additional corrective actions are implemented. 9.2.1 Data Quality Objectives The following DQOs are associated with the Effectiveness Monitoring Plan: • Monitor the effectiveness of the approved remedies identified for the ash basin system. • Monitor changes in groundwater and surface water concentrations as the result of proposed closure activities (i.e., capping) associated with the ash basin system. • Monitor the movement of COls within groundwater and the interaction with surface water. • Collect additional analytical data from background wells to revise PPBCs. • Monitor seasonal groundwater flow direction and elevations, and monitor potential changes to groundwater flow direction and elevation resulting from proposed closure activities. The DQOs will be met through the following activities: • Perform groundwater and surface water sampling, including MNA parameters, at select locations. • Perform groundwater static water level measurements of select monitoring wells concurrent with groundwater sampling described above. • Perform water level gauging at previously established stream and surface water locations concurrent with sampling events. • Perform total depth measurements at monitoring wells on an annual basis. • Prepare reports documenting sampling results and analysis for submittal to NCDEQ. 9.2.2 Sampling Requirements Following the Second Quarter 2016 monitoring event, four seasonal monitoring events, including Rounds 1 and 2, will have occurred at the MSS site (Rounds 3 and 4 were background well only sampling events). Results from the four seasonal monitoring events will be evaluated to establish an MNA sampling network of select monitoring well and surface water locations. Additional monitoring of the MNA sampling network will be proposed to confirm the effectiveness of MNA as a proposed corrective action and to determine if additional sampling is warranted at locations outside the MNA network to monitor site conditions. 56 Corrective Action Plan Part 2 F)� Marshall Steam Station Ash Basin 9.2.2.1 Sample Frequency Following the establishment of the MNA network, one additional monitoring event of these locations will be conducted in 2016 in conjunction with the Third 2016 NPDES compliance sampling event in October. Beginning in 2017, samples from the MNA network will be collected three times per year, in conjunction with the NPDES compliance monitoring, in order to correlate the results from the MNA network results with the NPDES sample results. Sampling frequency associated with Effectiveness Monitoring will be re-evaluated every five years. Upon completion of the first five-year sampling cycle, the potential for semi-annual sampling frequency will be evaluated. In order to revise PPBCs using the additional background wells as described in Section 8, an additional monitoring event will be conducted in 2016 from all MSS background monitoring wells in order to obtain four rounds of analytical data in 2016. Based on the sampling schedule proposed in the Interim Monitoring Plan and the scheduled effectiveness monitoring to be conducted concurrent with the NPDES compliance monitoring (scheduled for February, June, and October 2016), it is anticipated this additional background sampling event would be conducted in the Second Quarter 2016 (i.e., June 2016). 9.2.2.2 Sample Locations Sampling locations associated with the Effectiveness Monitoring Plan will be established after collection of the Second Quarter 2016 sampling event, following evaluation of four seasonal monitoring events. Sample locations will be proposed to NCDEQ prior to implementation of the Effectiveness Monitoring Plan. 9.2.2.3 Analytical Requirements Samples collected from the MNA network will be analyzed for the parameters described in the Interim Monitoring Plan above (Section 9.1 and Table 9-2). Changes to the analytical requirements may be proposed upon evaluation of the seasonal monitoring results obtained during CSA and interim monitoring. 9.2.3 Reporting Monitoring reports analyzing the results from the each monitoring event are proposed to be submitted to NCDEQ within 120 days of completion of each sampling event. Results from the additional background monitoring event will be included in the next scheduled monitoring report. 9.3 Sampling and Analysis 9.3.1 Monitoring Well Measurements and Inspection Groundwater sampling will be conducted at monitoring well locations associated with the CSA, compliance, voluntary, and additional assessment wells as described in the Monitoring Plans above. During each monitoring event, monitoring wells will be measured for static water levels. These measurements will be taken within one 24-hour period and prior to sampling to minimize 57 Corrective Action Plan Part 2 F)� Marshall Steam Station Ash Basin temporal variations. The depth to water measurements, along with date and time, will be recorded on a dedicated field form, a field notebook, and/or electronically via the iForms software program or comparable system. Monitoring wells will be measured for total depth during the Second Quarter 2016 sampling event and during the first sampling event in subsequent years. Measurements will be recorded on a dedicated field form, a field notebook, and/or electronically via the iForms software program or comparable. The thickness of sediment accumulated in each monitoring well will be calculated once each year, during the second sampling event of the year. Sediment thickness will be calculated by comparing current total depth with historical data. If more than one foot of sediment exists, wells will be redeveloped with a pump or bailer prior to the third sampling event. In addition, wells may be redeveloped if turbidity readings below 10 NTU cannot be achieved during sample purging. Wells where turbidity less than 10 NTU cannot be obtained may still be sampled in accordance with the Low Flow Sampling Plan. Monitoring wells with turbidities greater than 10 NTU after redevelopment may be considered for replacement. Each monitoring well will be inspected while performing water level measurements for damage to the casing, protective casing, and bollards. Well caps and locks will be inspected to determine whether they are in good working order and functioning properly. Flush -mounted wells will be inspected for any damage by vehicular traffic and to ensure that the water -tight gasket is functioning properly. 9.3.2 Surface Water Measurements Stream stage measurements will be conducted at gauging locations along Lake Norman and the unnamed tributaries to Lake Norman. Each gauging location will have a designated datum point from which the relative stream level will be measured. 9.3.3 Sample Collection 9.3.3.1 Monitoring Well Purging Monitoring wells will be purged and sampled using low -flow (minimal drawdown) methods in accordance with the Low Flow Sampling Plan. The low -flow technique will be used to determine when a well has been adequately purged and is ready to sample by monitoring the pH, specific conductance, temperature, ORP, DO, and turbidity. The volume of water that is removed will also be observed and recorded. Wells with slow recharge rates, excessive drawdown, or that require a higher pumping rate may be purged using the volume -averaging method. An adequate purge is achieved when the pH, specific conductance, ORP, DO and temperature of groundwater have stabilized and the turbidity is below 10 NTU. 9.3.3.2 Groundwater Sample Collection After purging and stabilization are accomplished, laboratory -supplied sample containers will be carefully filled using the same method utilized for purging. Appropriate sample containers, quantities, and preservatives for the various analyses are listed in Table 9-2. 58 Corrective Action Plan Part 2 F)� Marshall Steam Station Ash Basin 9.3.3.3 Surface Water Sample Collection Grab samples will be collected from each surface water sample location shown on Figure 2-1. Water quality parameters (pH, specific conductance, ORP, temperature, DO and turbidity) will be measured at each location. After water quality parameters have been collected and recorded, surface water samples will be collected by slightly submerging the lip of the sample container under the water surface, or by using a dip cup sampler in areas with difficult access. Samples collected for dissolved metals analysis will be field -filtered through a 0.45-micron filter on the day of collection using a peristaltic pump. Filled laboratory -supplied sample containers will be labeled and placed in a cooler on ice (4°C) and maintained at that temperature until delivery to the laboratory. 9.3.3.4 Sample Naming Convention Samples will be identified by the following convention: site code, sample identification name, sample matrix, and date code. The codes are further explained below. • The two -digit Site Code for MSS is MA. • Sample Identification Name will be the sample location name (i.e., BG-1 D). • Sample Matrix o NS — Normal Sample o FD — Field Duplicate o EB — Equipment Blank o AMB — Ambient Blank o FB — Filter Blank o TB — Trip Blank • Date Code is a four -digit code indicating the quarter and year a sample was collected. For example, a groundwater sample collected from monitoring well BG-1 D in February 2016 would be designated MA-BG-1 D-NS-1 Q16. If a field duplicate was also collected from that location, it would be designated MA-BG-1 D-FD-1 Q16. 9.3.3.5 Waste Handling Purge water and decontamination water will be discharged to the ground surface at each individual sample location unless otherwise directed by Duke Energy. Other investigation - derived waste, including disposable tubing and gloves, will be handled in accordance with the MSS site solid waste protocols. 9.3.3.6 Chain of Custody and Sample Delivery All samples will be tracked using proper chain -of -custody procedures. A separate chain -of - custody will be filled out by each sample team and accompany each cooler shipped. Samples will be hand -delivered or shipped to the appropriate contract laboratory. 59 Corrective Action Plan Part 2 F)� Marshall Steam Station Ash Basin 9.3.4 Quality Assurance/Quality Control In addition to laboratory and other quality assurance/quality control procedures, field quality control measures will be implemented to ensure that data meets project requirements. The following field quality control procedures will be utilized for this project: • Field Duplicates — Field duplicate samples will be collected by filling two identical sets of sample containers with water from the same sample location for each of the planned analyses. Field duplicates will be given unique sample numbers. • Equipment Blanks — Equipment blanks measure the cleanliness of field sampling equipment. Equipment blanks will be created by pouring reagent -grade water over a decontaminated pump and collecting the water in appropriate sample containers. Equipment blanks will receive all the tests that are to be performed on the associated samples. • Ambient Blanks — Ambient blanks measure background contamination in the field. Appropriate sample containers will be filled with reagent -grade water while other water samples are collected at the site. • Filter Blanks — Filter blanks evaluate the possible addition of chemicals from the filter to the sample, thus measuring potential contamination in the filters. Filter blanks will be created by filling the appropriate sample containers with reagent -grade water run through a new filter. • Trip Blanks — Trip blanks will be created by the laboratory and accompany low level mercury samples through sample container shipment and delivery to the site, sample collection, and delivery back to the laboratory for analysis. 60 Corrective Action Plan Part 2 F)� Marshall Steam Station Ash Basin 10 Implementation Cost and Schedule CAMA Section §1 30A-309.21 1 (b)(1) requires implementation of corrective action within 30 days of CAP approval. Ash basin closure planning and implementation is ongoing at the MSS site and compliance groundwater monitoring has been conducted since 2011. 10.1 Implementation Cost The recommended corrective action to supplement source control at the MSS site (i.e., proposed capping based on pending risk classification) is MNA. In addition, further investigation should be conducted to evaluate the effectiveness of a potential groundwater cutoff wall east of the ash basin and dry ash landfill (Phase 1). A summary of costs associated with implementation of the MNA program and evaluation of a groundwater cutoff wall east of the ash basin is provided in Table 10-1. Note that the actual costs will be highly dependent on the actual conditions that exist following capping and completion of ash basin closure activities. Therefore, these values represent an estimate for reference purposes only. Table 10-1 Estimated Capital and Annual Costs for MNA and Evaluation of Groundwater Cutoff Wall Capital Costs — MNA Monitoring Well Installation Monitoring Well Installation (10 wells) $180,000 Site Preparation and Erosion and Sediment Control $30,000 Field Oversight (15%) $31,500 Well Installation Reporting $5,000 Project Management (10%) $24,700 Contingency (20%) $54,300 Subtotal Monitoring Well Installation $325,500 Capital Costs — Evaluation of Groundwater Cutoff Wall Planning Documents/Initial Preparation $35,000 Sediment and Erosion Control $15,000 Site Preparation $11,000 Field Investigation Activities $40,000 Subtotal Evaluation of Groundwater Cutoff Wall $101,000 Total Capital Costs $426,500 Annual Costs - Monitoring/Reporting Lab Analysis (32 samples) $19,200 Data Validation $15,000 Reporting $60,000 Equipment and Expendables $9,000 Sampling Labor $38,400 Project Management (10%) $14,160 Escalation to Mid -Point (4%) $6,230 Annual Monitoring/Reporting Costs $162,000 Total Capital/Annual Costs for Project Duration $7,030,000 61 Corrective Action Plan Part 2 F)� Marshall Steam Station Ash Basin 10.2 Implementation Schedule Interim activities, including advancement of the MNA Tier III assessment, will continue to be implemented at MSS until closure activities have been completed. Groundwater, surface water, ash basin water (pending capping status) and AOW sampling associated with the Interim Monitoring Plan will be implemented at the MSS site through the Second Quarter 2016, as detailed in Section 9.1. Following the Second Quarter 2016 sampling event, an MNA monitoring network of wells will be established and sampled concurrent with the NPDES compliance monitoring beginning in November 2016. Monitoring will be conducted three times per year in accordance with the Effectiveness Monitoring Plan described in Section 9.2 and coincide with NPDES compliance sampling events scheduled annually in February, June, and October. MNA will continue to be evaluated after closure activities are completed and subsurface conditions have stabilized. Should these results indicate that MNA is effectively reducing COI concentrations in groundwater, monitoring will continue per the Effectiveness Monitoring Plan. Based on the monitoring data, recommendations will be made regarding modifications in the monitoring program, changes in the implementation of the selected remedy, or if other alternatives need to be considered. 62 Corrective Action Plan Part 2 F)� Marshall Steam Station Ash Basin 11 References ASTM. 2014. E1689-95 (Reapproved 2014), Standard Guide for Developing Conceptual Site Models for Contaminated Sites. ASTM International. Dzombak, D. A. and F.M.M. Morel. 1990. Surface complexation modeling : hydrous ferric oxide. Wiley: New York; 1990, p xvii, 393 HDR. 2014a. Marshall Steam Station — Ash Basin Drinking Water Supply Well and Receptor Survey. [Online] URL: http://portal.ncdenr.org/web/wq/drinking-water-receptor-surveys HDR. 2014b. Marshall Steam Station — Ash Basin Supplement to Drinking Water Supply Well and Receptor Survey. [Online] URL: http://portal.ncdenr.org/web/wq/drinking-water- receptorsurveys HDR. 2015a. Comprehensive Site Assessment Report. Marshall Steam Station Ash Basin. September 8, 2015. HDR. 2015b. Corrective Action Plan Part 1. Marshall Steam Station Ash Basin, December 7, 2015. Kinniburgh, D. G., and Cooper, D. M., 2011. PhreePlot — Creating graphical output with PHREEQC. Available at http://www.phreeplot.org/, original date June 2011, last updated December 31, 2015. NCDENR (North Carolina Department of Environment and Natural Resources). 2003. "Guidelines for Performing Screening Level Ecological Risk Assessments". Parkhurst, D.L., and C.A.J. Appelo. 2013. Description of input and examples for PHREEQC version 3—A computer program for speciation, batch -reaction, one-dimensional transport, and inverse geochemical calculations: U.S. Geological Survey Techniques and Methods, book 6, chap. A43, 497 p. [Online] URL: http://pubs.usgs.gov/tm/06/a43/ USEPA (U.S. Environmental Protection Agency). 1990. "A Guide to Selecting Superfund Remedial Actions." Office of Solid Waste and Emergency Response Directive 9355.- 27FS April 1990 USEPA. 1991. Risk Assessment Guidance for Superfund: Volume I — Human Health Evaluation Manual: (Part B, Development of Risk -based Preliminary Remediation Goals). Interim, OSWER Directive 9285.6-03. December, 1991 USEPA. (U.S. Environmental Protection Agency). 1997. Ecological Risk Assessment Guidance for Superfund: Process for Designing and Conducting Ecological Risk Assessments EPA 540-R-97-006, OSWER 9285.7-25, PB97-963211, June 1997 USEPA (U.S. Environmental Protection Agency). 1999. Laboratory Determination of the Solid - Water Partitioning Coefficient or Kd Value. 63 Corrective Action Plan Part 2 F)� Marshall Steam Station Ash Basin USEPA (U.S. Environmental Protection Agency). 2007a. Pair Comparison of COI Concentrations, Mutually Rising Relationship Indicating Attenuation. USEPA (U.S. Environmental Protection Agency). 2007b. Monitored Natural Attenuation of Inorganic Contaminants in Ground Water - Volume 2 Assessment for Non -Radionuclides Including Arsenic, Cadmium, Chromium, Copper, Lead, Nickel, Nitrate, Perchlorate, and Selenium. EPA/600/R-07/140. Revised October 2007 USEPA (U.S. Environmental Protection Agency). 2008. Green Remediation: Incorporating Sustainable Environmental Practices in to the Remediation of Contaminated Sites. USEPA Office of Solid Water and Emergency Response. April 2008. USEPA. (U.S. Environmental Protection Agency). 2015a. Water Sense Partnership Program. http://www3.epa.gov/watersense/pubs/indoor.htmi USEPA U.S. Environmental Protection Agency). 2015b.Supplemental Guidance to ERAGS: Region 4, Ecological Risk Assessment USEPA. 2015c. USEPA Risk -Based Screening Levels. June 2015. Available at: http://www2.epa.gov/risk/risk-based-screening-table-generic-tables USEPA. 2015d. Final 2015 Updated National Recommended Human Health Water Quality Criteria. http://water.epa.gov/scitech/swguidance/standards/criteria/current/hhfinal.cfm 64