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CSS CAP Part 2_Report_FINAL_20160212F)l Corrective Action Plan Part 2 Cliffside Steam Station Ash Basin Site Location: NPDES Permit No. Permittee and Current Property Owner: Consultant Information Report Date: Cliffside Steam Station 573 Duke Power Rd Mooresboro, NC 28114 NC0005088 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 February 12, 2016 This page intentionally left blank Corrective Action Plan Part 2 Cliffside Steam Station Ash Basin Contents Acknowledgements ExecutiveSummary...................................................................................................................................... 1 1 Introduction.......................................................................................................................................... 5 1.1 Regulatory Background............................................................................................................. 5 1.2 Report Organization.................................................................................................................. 7 2 Summary of Previous and Current Studies......................................................................................... 8 2.1 Comprehensive Site Assessment............................................................................................. 8 2.1.1 Identification of COls.................................................................................................... 8 2.1.2 Soil Delineation............................................................................................................ 9 2.1.3 Groundwater Delineation............................................................................................. 9 2.2 Corrective Action Plan Part 1.................................................................................................... 9 2.2.1 Proposed Provisional Background Concentrations for Soil and Groundwater .......... 10 2.2.2 COI Occurrence and Distribution............................................................................... 10 2.3 Round 2 Sampling................................................................................................................... 10 2.3.1 Groundwater...............................................................................................................11 2.3.2 Round 1 and Round 2 Source Area and Groundwater Data Comparison ................. 12 2.3.3 Surface Water and Areas of Wetness........................................................................ 15 2.4 Round 3 and Round 4 Background Well Sampling................................................................. 17 2.5 Well Abandonment.................................................................................................................. 17 3 Site Conceptual Model...................................................................................................................... 18 3.1 Identification of Potential Contaminants.................................................................................. 18 3.2 Identification and Characterization of Source Contaminants.................................................. 18 3.3 Delineation of Potential Migration Pathways through Environmental Media .......................... 19 3.3.1 Soil.............................................................................................................................. 19 3.3.2 Groundwater...............................................................................................................20 3.3.3 Surface Water and Sediment..................................................................................... 20 3.4 Establishment of Background Areas....................................................................................... 21 3.5 Environmental Receptor Identification and Discussion........................................................... 21 3.6 Determination of System Boundaries...................................................................................... 22 3.7 Site Geochemistry and Influence on COls.............................................................................. 22 4 Updated Modeling............................................................................................................................. 25 4.1 Groundwater Model Refinement............................................................................................. 25 4.1.1 Flow Model Refinements............................................................................................ 25 4.1.2 Fate and Transport Model Refinements..................................................................... 26 4.1.3 Summary of Modeled Scenarios................................................................................ 27 4.1.4 Model Assumptions and Limitations........................................................................... 28 4.1.5 Modeled Scenario Results......................................................................................... 29 4.2 Surface Water Model Refinement........................................................................................... 31 4.2.1 Methodology...............................................................................................................31 4.2.2 Results....................................................................................................................... 32 4.3 Geochemical Modeling............................................................................................................ 34 4.3.1 Objective.................................................................................................................... 34 4.3.2 Methodology...............................................................................................................34 4.3.3 Assumptions...............................................................................................................35 4.3.4 Results....................................................................................................................... 36 Corrective Action Plan Part 2 Cliffside Steam Station Ash Basin 4.4 Refined Site Conceptual Model ........................................... 5 Risk Assessment.......................................................................... 5.1 Step 1: Conceptual Site Model ........................................... 5.2 Step 2: Risk -Based Screening ........................................... 5.3 Step 3: Human Health Risk Assessment ........................... 5.4 Step 4: Ecological Risk Assessment .................................. 6 Alternative Methods for Achieving Restoration ............................ 6.1 Corrective Action Decision Process ................................... 6.1.1 Evaluation Criteria ................................................. 6.1.2 COls Requiring Corrective Action ......................... 6.1.3 Potential Exposure Routes and Receptors ........... 6.2 Alternative Evaluation Criteria ............................................ 6.2.1 Effectiveness......................................................... 6.2.2 Implementability/Feasibility................................... 6.2.3 Environmental Sustainability................................. 6.2.4 Cost....................................................................... 6.2.5 Stakeholder Input .................................................. 6.3 Remedial Alternatives to Achieve Regulatory Standards ... 6.3.1 Groundwater Remediation Alternatives ................. 6.3.2 Monitored Natural Attenuation Applicability to Site 6.3.3 Site -Specific Alternatives Analysis ........................ 6.3.4 Site -Specific Recommended Approach ................ 7 Selected Corrective Action(s)....................................................... 7.1 Selected Remedial Alternative for Corrective Action ......... 7.2 Conceptual Design............................................................. 7.2.1 Source Removal — Excavation .............................. 7.2.2 Source Removal — Cap -In -Place ............................ 7.2.3 MNA........................................................................ 8 Recommended Interim Activities............................................................................ 8.1 Additional Well Installation........................................................................... 8.2 Additional Groundwater Sampling and Analyses ......................................... 8.2.1 Units 1-4 Inactive Ash Basin Groundwater Monitoring Plan During Excavation Activities........................................................................ 8.2.2 Groundwater Elevation Data Loggers ............................................. 9 Interim and Effectiveness Monitoring Plans ............................. 9.1 Interim Monitoring Plan .................................................. 9.1.1 Data Quality Objectives ..................................... 9.1.2 Sampling Requirements .................................... 9.1.3 Reporting........................................................... 9.2 Effectiveness Monitoring Plan ........................................ 9.2.1 Data Quality Objectives ..................................... 9.2.2 Sampling Requirements .................................... 9.2.3 Reporting........................................................... 9.3 Sampling and Analysis ................................................... 9.3.1 Monitoring Well Measurements and Inspection 9.3.2 Surface Water and AOW Measurements.......... 9.3.3 Sample Collection ............................................. 9.3.4 Quality Assurance/Quality Control .................... 37 38 38 39 39 40 43 43 43 44 44 45 45 46 46 47 47 47 47 48 49 53 55 55 55 55 55 56 57 57 57 57 58 59 59 59 60 61 61 61 62 62 62 62 63 63 65 Corrective Action Plan Part 2 Cliffside Steam Station Ash Basin 10 Implementation Cost and Schedule 10.1 Implementation Cost ............ 10.2 Implementation Schedule..... 11 References Tables 2-1 Summary of Horizontal Hydraulic Gradient Calculations 2-2 Comparison of COI Sample Results using 0.45 pm and 0.1 pm Filters — Ash Porewater 2-3 Comparison of COI Sample Results using 0.45 pm and 0.1 pm Filters - Groundwater 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 Inside the Waste Boundary — Round 1 and Round 2 2-8 Groundwater Analytical Results Outside the Waste Boundary — Round 1 and Round 2 2-9 Constituents of Interest Evaluation 2-10 Surface Water Sample Analytical Results — Round 1 and Round 2 2-11 Areas of Wetness Sampling Analytical Results — Round 1 and Round 2 2-12 NCDEQ Water Sampling Analytical Results— Round 1 and Round 2 4-1 Summary of Modeled COI Results at the Compliance Boundary* 4-2 Broad River Calculated Surface Water Concentrations* 4-3 Suck Creek Calculated Surface Water Concentrations* 9-1 Interim Monitoring Plan Sample Locations 9-2 Sampling Parameters and Analytical Methods 10-1 Remedial Alternative Costs for MNA* *Table is presented in the text of this CAP Part 2 Report; all other tables are attached separately. Figures 2-1 Site Sampling Locations 2-2 Potentiometric Surface Map — Shallow Flow Layer 2-3 Potentiometric Surface Map — Deep Flow Layer 2-4 Potentiometric Surface Map — Bedrock Flow Layer 2-5 Areas of Exceedances of 2L Standards 3-1 Site Conceptual Model — 3D Representation 3-2 Site Conceptual Model Cross Sectional 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 E Geochemical Modeling Report F Baseline Human Health and Ecological Risk Assessment i Corrective Action Plan Part 2 �� Cliffside Steam Station Ash Basin G Evaluation of Potential Groundwater Remedial Alternatives H Monitored Natural Attenuation Technical Memorandum Note: The hard copy of this CAP includes the report portion of appendices only. Complete appendices with all attachments are provided on the accompanying CAP Part 2 CD. Acronyms and Abbreviations pm micron pg/L microgram per liter 2B Standards North Carolina surface water standards as specified in T15 NCAC 02B .0211 and .0216 (amended effective January 2015) 2L Standards North Carolina groundwater standards as specified in T15A NCAC 02L AOW areas of wetness (referred to as seeps in previous reports) ARAR applicable or relevant and appropriate requirements BERA baseline ecological risk assessment bgs below ground surface CAMA North Carolina Coal Ash Management Act of 2014 CAP corrective action plan CCP coal combustion products CCR coal combustion residuals COPC constituents of potential concern COI constituent of interest CSA comprehensive site assessment CSM conceptual site model CSS Cliffside Steam Station 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 gpd gallons per day HAO hydrous aluminum oxide HFO hydrous ferric oxide HQ hazard quotient HSL health screening level IMAC interim maximum allowable concentration Kd sorption coefficient LOAEL lowest observed adverse effect level MNA monitored natural attenuation NC PSRGs North Carolina Preliminary Soil Remediation Goals NCAC North Carolina Administrative Code NCDENR North Carolina Department of Environment and Natural Resources NCDEQ North Carolina Department of Environmental Quality NCDHHS North Carolina Department of Health and Human Services NOAEL no observed adverse effects level NOD Notice of Deficiency NPDES National Pollutant Discharge Elimination System iv Corrective Action Plan Part 2 Cliffside Steam Station Ash Basin NTU nephelometric turbidity units ORP oxidation-reduction potential (see also Eh) POG protection of groundwater PPBC proposed provisional background concentration PRB permeable reactive barrier RBC risk -based concentrations RMS root mean squared error SCM site conceptual model SU standard unit 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 v This page intentionally left blank Corrective Action Plan Part 2 Cliffside 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 Corrective Action Plan Part 2 Cliffside Steam Station Ash Basin This page intentionally left blank Corrective Action Plan Part 2 Cliffside 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, remedial activities, if necessary. A Groundwater Assessment Work Plan (Work Plan) for the Cliffside Steam Station (CSS) was submitted to the North Carolina Department of Environment and Natural Resources (NCDENR) on September 25, 2014, and subsequently revised on December 30, 2014. The revised Work Plan was conditionally approved by NCDENR on February 19, 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 CSS CSA Report was submitted to NCDENR on August 18, 2015 (HDR 2015a). Subsequent to submittal of the CSA, CAMA requires submittal of a corrective action plan (CAP) for each regulated facility no later than 180 days after submittal of the CSA. Duke Energy Carolinas, LLC (Duke Energy) and North Carolina Department of Environmental Quality (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 CSS CAP Part 1 was submitted to NCDEQ on November 16, 2015 (HDR 2015b). In December 2015, NCDEQ released draft proposed impoundment risk classifications for Duke Energy's coal ash impoundments in North Carolina. The proposed risk classifications issued for the CSS Units 1-4 inactive ash basin and Unit 5 inactive ash basin is low. NCDEQ narrowed the proposed risk classification for the CSS active ash basin to low -to -intermediate. Risk classifications were based on potential risk to public health and the environment. Public meetings regarding the proposed risk classifications of the CSS impoundments are scheduled for March 14, 2016 in Cleveland and Rutherford Counties. NCDEQ will release final risk classifications after review of public comments. Note that Duke Energy received a Notice of Deficiency (NOD) from NCDEQ on March 5, 2014 for the dam impounding the Units 1-4 inactive ash basin after inspection noted deterioration and seepage in a corrugated metal spillway barrel and embankment drains. Based on the NOD, the basin's location adjacent to the Broad River, and the structural integrity analysis which indicated an insufficient factor of safety exists in the event of an earthquake, Duke Energy proposed to excavate the ash and close the basin in an Excavation Plan approved by NCDEQ. Therefore, Duke Energy is proceeding with complete excavation of the Units 1-4 inactive ash basin despite the NCDEQ draft proposed risk classification of low. Duke Energy owns and operates CSS, located in Mooresboro, in Rutherford and Cleveland Counties, North Carolina. CSS began operations in 1940 with Units 1 through 4. Unit 5 began operations in 1972, followed by Unit 6 in 2012. Units 1 through 4 were retired from service in 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 Cliffside Steam Station Ash Basin 2011 as part of Duke Energy's decommissioning and demolition program and were imploded in October 2015. Currently only Units 5 and 6 are in operation. The CCR and other liquid discharges from CSS's coal combustion process have been disposed in the station's ash basin system since the construction of the ash basin system, beginning in 1957. Discharge from the active ash basin is permitted by the NCDEQ Division of Water Resources (DWR) under the National Pollutant Discharge Elimination System (NPDES) Permit NC0005088. Groundwater at the site flows to the north from a topographic divide that runs approximately parallel to Duke Power (McCraw) Road, south of the site, toward the Broad River. The groundwater flow direction is away from the the nearest public or private water supply wells. The Broad River serves as the primary hydrologic discharge feature for groundwater within the shallow, deep, and bedrock layers at the site. Based on results of the CSA, concentrations of constituents of interest (COIs)2 attributable to the CCR source areas at the CSS site are present beneath and downgradient of the ash basins, ash storage area, east of Unit 6 and west of the active ash basin. COI transport from the source areas is generally in a northerly direction toward the Broad River and Suck Creek, which flows to the Broad River. COls in groundwater attributable to ash handling at the CSS site are arsenic, barium, boron, cobalt, chromium3, hexavalent chromium, iron, manganese, pH, sulfate, total dissolved solids (TDS), and vanadium. Cobalt, iron, manganese, and vanadium were found to be naturally occurring constituents in site background groundwater. Further sampling and analysis are necessary to determine if COI exceedances are the result of source -related impacts (as discussed in Section 2.0) or naturally occurring conditions. The refined groundwater model predicts that certain COls will exceed regulatory standards at the Compliance Boundary4, as discussed in Section 4.1.5; however, based on results of the groundwater to surface water interaction modeling, no water quality standards or criteria are exceeded at the edge of the mixing zones in the Broad River. Groundwater to surface water model results predict water quality standards will be exceeded in Suck Creek for lead (freshwater aquatic life chronic standard), and thallium (human health and water supply standards). These modeled exceedances are localized due to relatively low surface water flows into Suck Creek and do not result in exceedances of the 2B Standards at the edge of the mixing zone in the Broad River. Although the groundwater to surface water model predicts thallium in Suck Creek at concentrations greater than the human health and water supply water quality standards, actual surface water sampling results reported thallium concentrations below the water -quality standard. 2 If a constituent concentration exceeded the North Carolina Groundwater Quality Standards as specified in T15A 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. Note that the Unit 5 inactive ash basin Compliance Boundary is provisional, pending approval by NCDEQ. 2 Corrective Action Plan Part 2 Cliffside Steam Station Ash Basin A Human Health and Ecological Risk Assessment was conducted as part of this CAP. Results of the Human Health Risk Assessment indicated that there are no unacceptable risks to human health. Evaluation of potential impacts to ecological receptors indicates that risk estimates for several contaminants of potential concern (COPCs) are above risk targets for several ecological receptor species, if and where these species present at CSS. 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 to the risk assessment results, and refinement of the exposure and toxicity assumptions used in the ecological risk characterization. Duke Energy began excavating source material contained in the Units 1-4 inactive ash basin on October 26, 2015. As of February 8, 2016, approximately 98,500 tons of ash and cover soil have been excavated and transported to an on -site lined landfill. The active ash basin, ash storage area, and Unit 5 inactive basin are candidates for capping in place, pending the outcome of the final state risk classifications. An evaluation of site conditions, constituents, and a review of alternative methods for restoring groundwater quality found that, in conjunction with source removal and pending determination of closure options at the CSS site, monitored natural attenuation (MNA) is recommended as corrective action for groundwater impacts beneath the site. An Interim Monitoring Plan has been developed to provide baseline seasonal analytical data for the CSS 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 fate and transport, groundwater to surface water interaction, and geochemical models. The monitoring results will also be used to confirm natural attenuation is continuing to occur and remains an effective corrective action for the CSS 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 remedial alternatives will continue to be monitored and evaluated to determine if modifications to the measures are required. 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 Cliffside Steam Station Ash Basin This page intentionally left blank Corrective Action Plan Part 2 Cliffside Steam Station Ash Basin 1 Introduction Duke Energy Carolinas, LLC (Duke Energy) owns and operates the coal-fired Cliffside Steam Station (CSS), located in Mooresboro, in Rutherford and Cleveland Counties, North Carolina. CSS began operations in 1940 with Units 1 through 4. Unit 5 began operations in 1972, followed by Unit 6 in 2012. Units 1 through 4 were retired from service in 2011 as part of Duke Energy's decommissioning and demolition program and were imploded in October 2015. Currently only Units 5 and 6 are in operation. The coal combustion residuals (CCR) and other liquid discharges from CSS's coal combustion process have been disposed in the station's ash basin system since the basins were constructed. Discharge from the active 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 NC0005088. 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 CSS was submitted to the North Carolina Department of Environment and Natural Resources (NCDENR) on September 25, 2014, followed by a revised Work Plan on December 30, 2014. The revised Work Plan was conditionally approved by NCDENR on February 19, 2015. A Comprehensive Site Assessment (CSA) w .as 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 CSS CSA Report was submitted to NCDENR on August 18, 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 (15A 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 CSS CAP Part 1 was submitted to NCDEQ on November 16, 2015, and consisted of the following (HDR 2015b): • background information • brief summary of the CSA findings • brief description of the site geology and hydrogeology • summary of the previously completed receptor survey • summary of constituent of interest (COI) exceedances and distribution 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 Cliffside Steam Station Ash Basin • 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 (IMACs) and North Carolina Department of Health and Human Services (NCDHHS) health screening levels (HSLs) • 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 • a plan for monitoring and reporting 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 secondary sources, which would be potential continuing sources of possible pollutants to groundwater, be addressed in the CAP. At the CSS site, the soil located below the ash basins and ash storage area could be considered as a potential secondary source. Preliminary information to date indicates that the thickness of soil impacted by ash would generally be limited to the depth near the ash soil interface. As discussed with NCDEQ, after excavation of the Units 1-4 inactive ash basin, soils left on -site will be sampled and analyzed, and the analytical results will be incorporated into the groundwater contaminant fate and transport models. If this evaluation indicates that modification to the proposed CAP is required, Duke Energy will prepare and submit a revised CAP. The proposed corrective action plan calls for the Unit 5 inactive ash basin, ash storage area, and active ash basin to be covered with an engineered cap, which will minimize infiltration through the covered area reducing possible impacts from potentially impacted soil. Therefore, remediation of soils is not discussed in this document. In December 2015, NCDEQ released draft proposed impoundment risk classifications for Duke Energy's coal ash impoundments in North Carolina. The proposed risk classifications issued for the CSS Units 1-4 inactive ash basin and Unit 5 inactive ash basin is low. NCDEQ narrowed the proposed risk classification for the CSS active ash basin to low -to -intermediate. Risk classifications were based upon potential risk to public health and the environment. Public meetings regarding the proposed risk classifications of the CSS impoundments are scheduled for March 14, 2016 in Cleveland and Rutherford Counties. NCDEQ will release final risk classifications after review of public comments. Note that Duke Energy received a Notice of Corrective Action Plan Part 2 Cliffside Steam Station Ash Basin Deficiency (NOD) from NCDEQ on March 5, 2014 for the dam impounding the Units 1-4 inactive ash basin after inspection noted deterioration and seepage in a corrugated metal spillway barrel and embankment drains. Based on the NOD, the basin's location adjacent to the Broad River, and the structural integrity analysis which indicated an insufficient factor of safety existed in the event of an earthquake, Duke Energy proposed to excavate the ash and close the basin in an Excavation Plan approved by NCDEQ. Therefore, Duke Energy is proceeding with complete excavation of the Units 1-4 inactive ash basin despite the NCDEQ draft proposed risk classification of low. An industrial stormwater permit was required before ash excavation and transportation activities could be initiated for the Cliffside Unit 1-4 inactive ash basin Phase 1 excavation project. The industrial stormwater permit went into effect on October 1, 2015. Transportation of ash from the Units 1-4 inactive ash basin to the on -site landfill started on October 16, 2015. As of February 8, 2016, approximately 98,500 tons of ash and cover soil have been excavated from the Units 1-4 ash basin and transported to the Cliffside coal combustion products (CCP) landfill. 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 CSS 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 areas 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 the purpose, methodologies and results of refined groundwater and groundwater to surface water, and geochemical modeling. Refinement of the SCM following evaluation of the 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 Cliffside 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 sampling results and data comparison to Round • 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 submittal and are presented in this CAP Part 2 Report. 2.1 Comprehensive Site Assessment The purpose of the CSS 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 included 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 Boundaries6. • Evaluation of laboratory analytical data to supplement the SCM. • Update of the receptor survey previously completed in September 2014 (and updated in November 2014) (HDR 2014a, 2014b). • Completion of a screening -level risk assessment. Note that subsequent to submittal of the CSA, additional evaluation of the initial round of 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 15A NCAC .0202L (2L Standards), the IMACs', NCDHHS HSLs (hexavalent chromium only), North Carolina Preliminary Soil Remediation Goals (NC PSRGs) for Protection of Groundwater (POG), North Carolina Surface Water Quality Standards as specified in T15 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. Note that the Unit 5 inactive ash basin Compliance Boundary is provisional, pending approval by NCDEQ. ' 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 Cliffside Steam Station Ash Basin NCAC 02B .0211 and .0216 (amended effective January 2015) for Class WS-IV waters (2B Standards), or U.S. Environmental Protection Agency (USEPA) National Water Quality Criteria, it was designated as a COI. The following constituents were reported as COls in the CSA Report: • Soil: arsenic, cobalt, iron, manganese, selenium, thallium, and vanadium. • Groundwater: antimony, arsenic, barium, beryllium, boron, chromium$, cobalt, iron, lead, manganese, nickel, pH, sulfate, thallium, total dissolved solids (TDS), and vanadium. • Surface water: aluminum and pH. In addition to COI identification, delineation of COls in site media was also conducted during the CSA. 2.1.2 Soil Delineation Horizontal and vertical delineation of source -related soil impacts was presented in the CSA Report. Where soil impacts were identified beneath the active ash basin, ash storage area (eastern and western portions), Units 1-4 inactive ash basin, and Unit 5 inactive ash basin, the vertical extent of impacts beneath the ash/soil interface is generally limited to the upper soil samples collected beneath the ash. 2.1.3 Groundwater Delineation Groundwater impacts at the site attributable to ash handling and storage were delineated during the CSA activities with the following areas requiring refinement: • Horizontal and vertical extent west of the active ash basin in the vicinity of monitoring wells MW-23D and GWA-14D. • Horizontal and vertical extent east of the Unit 5 inactive ash basin to the east of monitoring wells MW-42D and GWA-41D. These exceptions were identified as areas needing additional assessment in the CSA Report. Based on the exceptions noted above and subsequent discussions with NCDEQ, a total of approximately 30 additional monitoring wells and four water level observation wells are scheduled to be installed at CSS. The locations of the additional wells have been identified in the field, and drilling began in the first quarter of 2016. Results of the additional assessment well installation and sampling will be submitted to NCDEQ under separate cover. 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, develop a refined SCM, and present results of the groundwater flow and transport model and groundwater to surface water model. 8 Unless otherwise noted, references to chromium in this document indicate total chromium. Corrective Action Plan Part 2 Cliffside Steam Station Ash Basin 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 areas, the selection of borings/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 CSS background soil and groundwater PPBCs are provided in the CAP Part 1 Report. Further refinement of the PPBCs is anticipated following the completion of additional sampling events of background monitoring wells in 2016. 2.2.2 COI Occurrence and Distribution The following soil COls were evaluated in CAP Part 1: antimony, arsenic, boron, chromium, cobalt, iron, manganese, selenium, thallium, and vanadium. If the soil PPBCs presented in the CAP Part 1 Report are approved for the CSS site, chromium would be eliminated from further evaluation, as there were no exceedances of the PPBCs reported for this constituent. The following groundwater COls for the CSS site were evaluated in CAP Part 1: antimony, arsenic, barium, beryllium, boron, chromium, cobalt, hexavalent chromium, iron, lead, manganese, nickel, pH, sulfate, thallium, TDS, and vanadium. Although mercury exceeded the 2L Standard in one location beneath the active ash basin, mercury was not considered a COI for further evaluation because the dissolved concentration was less than the 2L Standard. Additional sampling and analysis were identified as being necessary to determine if beryllium, lead, and nickel should remain as COls at the CSS site. If the groundwater PPBCs presented in the CAP Part 1 Report are approved for the CSS site, antimony and thallium would be eliminated from further evaluation as there were no exceedances of the PPBCs reported for these constituents. The areas of 2L Standard or IMAC exceedances within or directly adjacent to the sources indicate that physical and geochemical processes beneath the CSS site inhibit the lateral migration of COIs. In accordance with LeGrand's slope -aquifer system characteristics of the Piedmont, discharge of groundwater from shallow and deep flow layers into surficial waterbodies is expected in Suck Creek and the Broad River. Vertical migration of COls was observed in select well clusters and is likely influenced by infiltration of precipitation and/or ash basin water, where applicable, through the shallow and deep flow layers into underlying fractured bedrock. 2.3 Round 2 Sampling Round 2 groundwater, surface water, and AOW sampling activities were completed between September 3 and 17, 2015. Groundwater analytical parameters and methods for Round 2 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 and Round 2 groundwater flow and analytical results. 10 Corrective Action Plan Part 2 Cliffside Steam Station Ash Basin Groundwater A total of 140 groundwater and ash porewater monitoring wells were sampled during the Round 2 event including 125 wells installed as part of the groundwater assessment and 15 voluntary wells. Four locations (GWA-31 BR, MW-7D, MW-34S, and U5-3S) were dry at time of sampling. One well (U5-2S-SL) was obstructed or damaged. Groundwater samples could not be collected from the dry or damaged well locations. Monitoring well locations are depicted on Figure 2-1. 2.3.1.1 Groundwater Water Levels On August 27, 2015, monitoring wells were manually gauged from the top of the PVC casing using an electronic water level indicator accurate to 0.01 foot. Groundwater elevations and contours for the shallow, deep, and bedrock flow layer are depicted on Figures 2-2, 2-3, and 2- 4, respectively. Groundwater elevations measured in Round 2 were generally lower than those measured during Round 1; this is likely attributable to seasonal variations of the water table. Groundwater flow direction in August was consistent with flow directions identified in June and documented in the CSA Report. Groundwater flows generally from the southern portion of the site to the north, toward the Broad River, with groundwater in the central portion of the site flowing toward Suck Creek and then north to the Broad River. 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 within the same water -bearing unit. Monitoring wells, groundwater elevations, and length of flow paths utilized for horizontal hydraulic conductivity calculations are detailed in Table 2-1. The average horizontal gradients calculated for Rounds 1 and 2 are provided below. • Shallow: Round 2 - 0.049 feet/foot; Round 1 - 0.041 feet/foot • Deep: Round 2 - 0.049 feet/foot; Round 1 - 0.053 feet/foot • Bedrock: Round 2 - 0.020 feet/foot; Round 1 - 0.047 feet/foot Minor fluctuations in the horizontal gradient were observed in the flow layers and may be attributable to seasonal variations; however, horizontal hydraulic gradients were generally consistent with those documented in the CSA Report Vertical hydraulic gradients were calculated for Round 2 and were presented in the CAP Part 1 Report. Vertical hydraulic gradients were calculated for 52 shallow and deep/upper bedrock well pairs by taking the difference in groundwater elevation in each well pair over the vertical difference between the well screen midpoints. Vertical hydraulic gradients were also calculated for 16 deep/upper bedrock and bedrock well pairs. Details regarding the vertical gradients across the site were presented in the CAP Part 1 Report. In general, the gradients calculated for Round 2 were consistent with those observed during Round 1. 2.3.1.3 Groundwater Sampling 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 11 Corrective Action Plan Part 2 Cliffside Steam Station Ash Basin approved by NCDENR (CSA Report Appendix G). The groundwater samples collected from monitoring wells AS-4S and GWA-6S were obtained using disposable bailers because insufficient water was present in these wells to utilize the low flow sampling techniques. 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 volumes were 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: AB-1 S AB-1 D A13-2S AB-21D AB-31 A13-313RU AB-41D AB-61D AB-613R AS-1 SB AS-1 D AS-2S AS-2D AS-7D AS-7BR BG-1 S BG-1 D BG-1 BR CLMW-1 CLMW-6 GWA-11 S GWA-11 BRU GWA-14S GWA-14D GWA-20S GWA-20D GWA-21 S GWA-21 BRU GWA-21 BR GWA-25S GWA-25D GWA-2S GWA-2BRU GWA-6D IB-2AL IB-21 I13-213RU MW-30S MW-30D U54S U54D and U5-4BR Sample results from the 0.45-micron and 0.1-micron filters were generally similar. A comparison of the analytical results from 0.45-micron and 0.1-micron filtered samples are presented in Tables 2-2 and 2-3. 2.3.2 Round 1 and Round 2 Source Area and Groundwater Data Comparison Round 1 and Round 2 ash porewater, ash basin water, and groundwater data are presented in Tables 2-4 through 2-7. Variation from Round 1 to Round 2 cannot be 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 The source areas at CSS are defined as the active ash basin, ash storage area, Units 1-4 inactive ash basin, and Unit 5 inactive ash basin (Figure 2-1). Ash Porewater The ash basin is a permitted wastewater treatment facility and water in the basin is wastewater not groundwater. Ash porewater is compared to 2L Standards or IMACs and NCDHHS HSLs for comparison purposes only. Ash porewater samples were collected in Rounds 1 and 2 from locations within the source areas (Table 2-4). Fluctuations in the total number of COls reported at individual wells were noted when comparing Round 1 to Round 2. No additional COls were identified during Round 2. No strong correlation can be made between turbidity and the number of COls exceeding the applicable regulatory standard. In some cases turbidity increased and the number of COls in a particular well decreased. COls in ash porewater are antimony, arsenic, boron, cobalt, iron, lead, manganese, pH, sulfate, thallium, TDS, and vanadium. 12 Corrective Action Plan Part 2 Cliffside Steam Station Ash Basin Due to active excavation occurring within the Units 1-4 inactive ash basin, COI concentrations, including the number of COls identified in the wells, may vary in subsequent sampling rounds. With the movement of groundwater, mixing of ash material during the process of disposal, and changes to infiltration to the ash management area, the groundwater hydrology and geochemistry will change. Caution should be used in development of trends and any conclusions drawn from data collected in subsequent sampling events as they will only represent a snapshot of what is occurring on the site. 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, IMACs and NCDHHS HSLs for comparison purposes only as ash basin water is a source of groundwater and surface water impacts. Two active ash basin water samples (SW-5 and SW-7) and one water sample from the active ash basin discharge (CLFWW057) were collected during Round 1. An AOW sample collected at the toe of the active ash basin downstream dam during Round 1 was inadvertently identified as CLFWW057 during that sampling event. The results of this Round 1 sample are included with the NCDEQ water sampling results. During the Round 2 sampling event, CLFWW057 was correctly collected from the discharge of the active ash basin and the results are included on the ash basin water results table (Table 2-5). Fluctuations in the total number of COls reported at individual locations were noted when comparing Round 1 to Round 2. COls in the active ash basin water include: aluminum, antimony, arsenic, boron, cobalt, hexavalent chromium, iron, manganese, selenium, thallium, TDS, and vanadium. These COls are consistent with those reported in CAP Part 1 with the exception of antimony, boron, cobalt, and selenium, which were not reported in the ash basin water during the Round 1 sampling event. Speciation samples were not collected during Round 2; therefore, hexavalent chromium was not analyzed in the ash basin water during Round 2 sampling. 2.3.2.2 Groundwater Results Background Wells Groundwater samples were collected in Round 1 and Round 2 from background monitoring well locations within the shallow, deep, and bedrock flow layers (Table 2-6). Fluctuations in the total number of exceedances reported at individual wells were noted when comparing Round 1 to Round 2. Speciation samples were not collected during Round 2; therefore, hexavalent chromium was not analyzed in the background monitoring wells during Round 2 sampling. In general, the concentrations from the background monitoring wells exhibited similar results and constituents when comparing data from Round 1 and 2. Background monitoring wells were also sampled during Round 3 and Round 4. The results of these sampling events are also provided in Table 2-6. Background monitoring wells will continue to be sampled and PPBCs refined as the data set increases with additional sampling rounds. 13 Corrective Action Plan Part 2 Cliffside Steam Station Ash Basin Within and Beneath the Waste Boundaries HDR collected groundwater samples from monitoring wells located within and beneath the active ash basin, ash storage area, Units 1-4 inactive ash basin, and Unit 5 inactive ash basin waste boundaries during Round 2. The Round 1 and Round 2 groundwater sample results are presented in Table 2-7. The following COls exceeded their applicable regulatory standard in samples collected from groundwater monitoring wells in Rounds 1 and 2: antimony, arsenic, barium, beryllium, boron, chromium, cobalt, hexavalent chromium, iron, lead, manganese, mercury, nickel, pH, sulfate, thallium, TDS, and vanadium. This list of COls is consistent with the COls reported in CAP Part 1 with the exception of hexavalent chromium, which was not analyzed during Round 2. Fluctuations in the total number of COls reported at individual wells were noted when comparing Round 1 to Round 2. No additional COls were identified during Round 2. No strong correlation can be made between turbidity and the number of COls exceeding the 2L Standards, IMACs or NCDHHS HSLs. In some cases turbidity increased and the number of COls in a given well decreased. Outside the Waste Boundaries HDR collected groundwater samples from monitoring wells located outside the waste boundaries and within the Compliance Boundaries and at or beyond the Compliance Boundaries during Round 2. The wells were installed within the shallow, deep, and bedrock flow layers. The Round 1 and 2 groundwater sample results are presented in Table 2-8. The following COls exceeded their respective 2L Standards, IMACs, or NCDHHS HSLs in samples collected from groundwater monitoring wells in Rounds 1 and 2: antimony, barium, boron, chromium, cobalt, hexavalent chromium, iron, lead, manganese, nickel, pH, sulfate, thallium, TDS, and vanadium. This list of COls is consistent with the COls reported in CAP Part 1 with the exception of lead, which was only reported at a concentration greater than the 2L Standard in one well during Round 1 and no wells during Round 2, and hexavalent chromium, which was not analyzed for during Round 2. Fluctuations in the total number of COls reported at individual wells were noted when comparing Round 1 to Round 2. No additional COls were identified during Round 2. No strong correlation can be made between turbidity and the number of Cols exceeding the 2L Standards, IMACs, or NCDHHS HSLs. In some cases turbidity increased and the number of COls in a given well decreased. 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 standards, 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. Results of the COI evaluation are summarized in Table 2-9. Only analytical results which exceed their respective groundwater criteria are presented in this table. • Where the COI concentration is less than the applicable PPBC, the cell is highlighted green. The analytical results associated with the green highlighting are groundwater criteria exceedances considered attributable to natural background conditions. 14 Corrective Action Plan Part 2 Cliffside Steam Station Ash Basin • Where the COI concentrations are greater than the applicable groundwater standard or PPBC, the cell is highlighted orange. The analytical results associated with the orange highlighted cells are exceedances considered to be associated with ash handling at the site. Thallium concentrations did not exceed the PPBC in any of the groundwater samples collected. At the CSS site, thallium concentrations are consistent with natural conditions; therefore, it should not be considered a COI for CSS. It is important to note that this evaluation only includes two sampling events and additional sampling is needed to reevaluate PPBCs and more appropriately assess COls compared to PPBCs at the site. A depiction of the areas with exceedances associated with ash handling are presented on Figure 2-5. 2.3.3 Surface Water and Areas of Wetness During the Round 2 sampling event, a total of 26 surface water and AOW samples were collected at the CSS 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 2-10 through 2-12. Surface water (6 samples): SW-1, SW-2, SW-3, SW-4, S-1, and S-8 AOWs (15 samples): S-2, S-3, S-4, S-5, S-7, S-9, S-10, S-11, S-12, S-14, S-16, S-18, S-19, S-19A, and S-22 • NCDEQ water samples (5 samples): CLFTD004, CLFTD005, CLFSD063, CLFSTR064, and CLFSTR065 Surface Water Surface water samples were collected during Round 1 and Round 2. Fluctuations in the total number of COls reported at individual locations were noted when comparing Round 1 to Round 2. Review of Round 2 laboratory analysis of surface water samples yields the following variations from the Round 1 sampling event: • Lead and copper concentrations exceeded 213 Standards at SW-1 and SW-3. • The mercury concentration at S-1 exceeded the 213 Standard. Copper, lead, and mercury were not previously identified as surface water COls at CSS and should be included with the COls in surface water. The lead and copper total recoverable concentrations at SW-3 and the total recoverable copper concentration at SW-1 were less than the dissolved concentrations (and 213 Standards). The cause of this variation in results is unknown. These constituents will continue to be monitored during subsequent sampling events to see if they should remain as COls for surface water at the site. Aluminum was reported at a concentration greater than the 213 Standard at S-8 during Round 1 and an estimated concentration less than the 213 Standard during Round 2. Aluminum concentrations also decreased at S-1, SW-2, and SW-3 during Round 2 compared to Round 1, 15 Corrective Action Plan Part 2 Cliffside Steam Station Ash Basin with SW-2 and SW-3 results remaining greater than the 2B Standards and S-1 not reported at a concentration greater than the method detection limit (which was greater than the 2B Standard). The aluminum concentration at SW-4 increased from Round 1 to Round 2, remaining greater than the 2B Standards. Round 1 and Round 2 analytical results for the surface water samples are presented in Table 2-10. Areas of Wetness AOW samples were collected during Round 1 and Round 2. Variation from Round 1 to Round 2 cannot be 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. The following constituents exceeded their applicable 2L Standard, IMAC, or NCDHHS HSL in samples collected from AOWs in Rounds 1 and 2: arsenic, barium, beryllium, boron, chromium, cobalt, copper, hexavalent chromium, iron, lead, manganese, nickel, pH, selenium, sulfate, thallium, TDS, vanadium, and zinc. This list of COls is consistent with the COls reported in CAP Part 1 with the exception of hexavalent chromium, which was not analyzed for during Round 2, and the following exceptions which were reported at concentrations greater than their 2L Standard or IMAC during Round 1, but were less than their 2L Standard or IMAC during Round 2: • Barium at S-16 • Cadmium at S-14 and S-16 • Nickel at S-14, S-15, and S-16 • Selenium at S-14 • Thallium at S-19 • Zinc at S-20 Fluctuations in the total number of COls reported at individual AOW locations were noted when comparing Round 1 to Round 2. No additional COls were identified during Round 2. Round 1 and Round 2 analytical results for the AOW samples are presented in Table 2-11. NCDEQ Water Sample Locations Samples were collected from NCDEQ-identified water sampling locations during Round 1 and Round 2 and results are presented in Table 2-12. In general, concentrations in Round 2 sampling were 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. The following COls exceeded their applicable 2L Standard, IMAC, or NCDHHS HSL in samples collected from NCDEQ-identified water sample locations in Rounds 1 and 2: antimony, arsenic, beryllium, boron, chromium, cobalt, hexavalent chromium, iron, lead, manganese, pH, sulfate, thallium, TDS, and vanadium. This list of COls is consistent with the COls reported in CAP Part 1 with the exception of hexavalent chromium, which was not analyzed during Round 2, and the following exceptions: COls reported at concentrations greater than the 2L Standard or IMAC during Round 1, but were less than the 2L Standard or IMAC during Round 2: 16 Corrective Action Plan Part 2 Cliffside Steam Station Ash Basin • Beryllium and lead at CLFTD005 COls reported at concentrations less than their IMAC during Round 1, but were greater than their IMAC during Round 2: • Antimony at CLFTD005 Fluctuations in the total number of COls reported at individual NCDEQ-identified water sample locations were noted when comparing Round 1 to Round 2. 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 CAP Part 2 correspond to background sampling Round 3 and Round 4, performed in November and December 2015, respectively. In addition to the CSA background wells, during the October 20, 2015 meeting with NCDEQ at the Asheville Regional Office, NCDEQ requested that the Cliffside CCP Landfill (Permit No. 8106) background monitoring wells (CCP-MW1 S and CCP-MW1 D) be added to the background monitoring well network for the CSS site. Groundwater samples were collected from these background monitoring wells during Round 4 only. Groundwater samples were also collected from NPDES background monitoring wells MW-24D and MW-24DR during Round 4. Samples associated with the assessment activities at the site had been collected previously from these wells during the Round 1 sampling event. The groundwater analytical parameters and methods are detailed in CSA Report. Groundwater samples were collected in accordance with sampling procedures described in the CSA Report. The results of the Round 3 and Round 4 background well sampling event 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. Well Abandonment Due to source removal at the Units 1-4 inactive ash basin, several monitoring wells within the source area were abandoned. Monitoring wells IB-2S-SL, I13-21, IB-2AL, IB-2BRU, IB-4S-SL, I13- 4D, and IB-4BR have been abandoned. Well abandonment forms are provided in Appendix A, Attachment 5. 17 Corrective Action Plan Part 2 Cliffside Steam Station Ash Basin 3 Site Conceptual Model The SCM was initially presented in the CSA, and refined based on results from additional sampling events, groundwater fate and transport modeling, and groundwater to surface water modeling. The SCM for the CSS site was developed in general accordance with the ASTM standard guidance 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 was also used to support selection of remedial alternatives and effectiveness of remedial 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 contaminants • Delineation of potential migration pathways through environmental media • Establishment of background areas • Environmental receptor information and discussion • 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 CSS are defined as the active ash basin, ash storage area, Units 1-4 inactive ash basin, and Unit 5 inactive ash basin (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- and aqueous -matrix samples to identify physical and chemical properties of ash, ash basin water, ash porewater, and ash basin AOWs. A geologic cross-section through the active ash basin source area is shown on Figure 3-2. 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. Ash distribution and chemical and physical properties were evaluated through advancement and sampling of 19 borings within the active ash basin waste boundary, 13 borings in the ash 18 Corrective Action Plan Part 2 Cliffside Steam Station Ash Basin storage area, 11 borings in the Units 1-4 inactive ash basin, and 17 borings in the Unit 5 inactive ash basin waste boundary. Ash within the active ash basin was encountered at depths ranging from ground surface to approximately 73 feet below ground surface (bgs). Ash in the ash storage area was encountered from approximately 7 to 57 feet bgs. Ash within the Units 1-4 inactive ash basin was encountered from approximately 1 to 48.5 feet bgs. Ash within the Unit 5 inactive ash basin was encountered from approximately 23.5 to 67 feet bgs. Ash porewater was evaluated through the sampling of 10 monitoring wells installed within the active ash basin. Ash basin water was evaluated through sampling and analysis of two ash basin water samples. Based on the CSA results, groundwater impacts attributable to source areas were identified beneath and downgradient of the active ash basin, ash storage area, Units 1-4 inactive ash basin, Unit 5 inactive ash basin, east of the Unit 5 inactive ash basin, and west of the active ash basin, in the vicinity of Unit 6. COI transport is generally in a northerly direction towards the Broad River and in the central portion of the site toward Suck Creek, which flows to the north, discharging into the Broad River. Analytical results of samples collected from the source areas were reviewed to identify COls, as follows: Eight COls were identified in ash based on comparison to the NC PSRGs for POG: arsenic, barium, boron, cobalt, iron, manganese, selenium, and vanadium. • Eleven COls were identified in ash porewater samples based on comparison to 2L Standards and IMACs: antimony, arsenic, boron, cobalt, iron, manganese, pH, sulfate, thallium, TDS, and vanadium. • Six COls were identified in ash basin water samples based on comparison to both 2L Standards or IMACs for groundwater and 2B Standards/USEPA National Water Quality criteria for surface water: aluminum, arsenic, cadmium, cobalt, copper, and thallium. 3.3 Delineation of Potential Migration Pathways through Environmental Media 3.3.1 Soil The approximate horizontal extent of soil contamination was delineated during the CSA and is generally limited to the area beneath the ash basins and ash storage area. Where soil impacts were identified, the approximate vertical extent of contamination beneath the ash/soil interface is generally limited to the uppermost soil sample collected beneath the ash basins. COls identified in soil include antimony, arsenic, boron, chromium, cobalt, iron, manganese, selenium, thallium, and vanadium. At the CSS site, the soil located beneath the ash basins could be considered as a potential secondary source. As discussed with NCDEQ, after excavation of the Units 1-4 basin, soils left on -site will be sampled and analyzed, and the analytical results will be incorporated into the groundwater contaminant fate and transport models. If this evaluation indicates that modification to the proposed corrective action plan is required, Duke Energy will prepare and submit a revised corrective action plan. The proposed corrective action plan calls for the Unit 5 inactive ash basin, ash storage area, and active ash basin to be covered with an 19 Corrective Action Plan Part 2 Cliffside Steam Station Ash Basin engineered cap, which will eliminate infiltration through the covered area reducing possible impacts from potentially impacted soil. Therefore, remediation of soils is not discussed in this document. Further assessment is underway, as recommended in the CSA Report and refined through consideration of NCDEQ comments. Results of this assessment will be reported under separate cover. 3.3.2 Groundwater Site hydrogeologic conditions were evaluated through sampling/testing conducted during installation of 131 monitoring wells. Based on the site investigation, the groundwater system in natural materials (alluvium, soil, soil/saprolite, and bedrock) at CSS is consistent with the LeGrand slope -aquifer system and is an unconfined, connected aquifer system. The CSS groundwater system is divided into three layers referred to as shallow, deep (transition zone), and bedrock flow layers to distinguish the flow layers within the connected aquifer system. In general, groundwater flows from the southern extent of the CSS site property boundary northward toward the Broad River. Monitoring wells installed on the east side of the active ash basin indicate that groundwater flows toward the active ash basin along the eastern property boundary. Shallow and deep groundwater at the site discharge directly to the Broad River, while groundwater in the central portion of the site flows to Suck Creek, which then discharges to the Broad River. The Broad River serves as a hydrologic boundary for groundwater within the shallow, deep, and bedrock flow layers at the site. Groundwater flow direction in the shallow, deep, and bedrock flow layers based on water levels gauged during the Round 2 sampling event (August 2015) are shown on Figures 2-2, 2-3, and 2-4, respectively. The approximate horizontal extent of groundwater impacts is limited to areas beneath and downgradient of the ash basins and ash storage area, east of the Unit 5 inactive ash basin, and west of the active ash basin in the vicinity of Unit 6. The approximate vertical extent of groundwater impacts is generally limited to the shallow and deep flow layers, and vertical migration of COls is limited by the underlying bedrock. Additional groundwater monitoring wells are being installed to refine understanding the extent and sources of groundwater impacts and to address NCDEQ comments to the CSA Report. Based on exceedances of 2L Standards, IMACs, and NCDHHS HSLs, the following groundwater Cols are being evaluated for corrective action: antimony, arsenic, barium, beryllium, boron, chromium, cobalt, hexavalent chromium, iron, lead, manganese, nickel, pH, sulfate, thallium, TDS, and vanadium. 3.3.3 Surface Water and Sediment Ash basin water and groundwater are being removed as needed during ash removal activities at the Units 1-4 inactive ash basin. However, during the excavation, surface water and sediment COls outside of the managed areas will continue to migrate into groundwater through infiltration and contribute to the flux toward the Broad River. Sediment that exceeds soil NC PSRGs for POG can also contribute to groundwater concentrations and move toward the Broad River. During excavation, the processes that govern COI migration will fluctuate, and therefore, the influence of surface water and sediment COI migration to the Broad River cannot be fully ascertained until source removal is complete. 20 Corrective Action Plan Part 2 Cliffside Steam Station Ash Basin The site surface water flow direction(s) is shown on Figure 3-1 and generally flows from the basins and ash storage area to the Broad River. COls present in surface water and sediment from AOW samples are summarized in Section 2.3.2. Sediment samples were collected from AOW and surface water locations during the CSA. COls exceeding the NC PSRGs for POG in sediment samples include arsenic, boron, cobalt, iron, manganese, nickel, selenium, and vanadium. Cobalt, iron, manganese, and vanadium concentrations exceeded the NC PSRGs for POG, but are also considered naturally occurring constituents in background soil. Establishment of Background Areas Background areas are located on the south side of the site, north of Duke Power (McCraw) Road (Figure 2-1). Specifically for groundwater, the monitoring wells installed as background wells during the CSA (BG-1 S/D/BR, BG-2D, MW-30S/D, and MW-32S/D/BR) and the existing NPDES ash basin compliance monitoring wells (MW-24D/DR) are located in background areas of the site. The area south of Duke Power (McCraw) Road is south of a topographic divide that generally parallels the road. The monitoring wells were installed in locations that represent upgradient, background groundwater conditions relative to the landfill, and have also been included in the pool of background wells for the CSS site to assist with establishing background groundwater quality. A detailed background monitoring well assessment was presented in Appendix B in CAP Part 1. Environmental Receptor Identification and Discussion Duke Energy conducted a receptor survey of the area within 0.5 mile of the ash basins and ash storage area Compliance Boundaries in September 2014, and subsequently supplemented the receptor survey in November 2014. Receptor locations identified during the surveys are shown on Figure 3-3. Properties located within a 0.5-mile radius of the CSS site generally consist of rural residential properties, undeveloped land, and the Broad River. Properties in Cleveland County are primarily comprised of the CSS site, other land which is zoned heavy and light industrial, and residential properties to the south, east, and northeast of CSS. Properties located to the west along Highway 221A and northwest across the Broad River in Rutherford County are zoned rural residential, including portions of CSS property identified as average rural. A total of 71 private water supply wells located within a 0.5-mile radius of the ash basins and ash storage area Compliance Boundaries were identified during the receptor survey. Groundwater flow at the site is away from the nearest private water supply wells. Several surface water bodies that flow toward the Broad River were identified within a 0.5-mile radius of the ash basins and ash storage area Compliance Boundaries. No wellhead protection areas were identified within a 0.5-mile radius of the Compliance Boundaries and no water supply wells (including irrigation wells and unused or abandoned wells) were identified between the source areas and the Broad River. 21 Corrective Action Plan Part 2 Cliffside Steam Station Ash Basin 3.6 Determination of System Boundaries The site, waste, and Compliance Boundaries for the CSS facility are shown on Figure 2-1. Spatially, the SCM for CSS is bounded by the Broad River to the north, a topographic divide that runs approximately parallel to Duke Power (McCraw) Road to the south, and topographic divides to the east and west of the CSS site. The SCM extends vertically into bedrock, which generally inhibits vertical migration of COls at the site. 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 (DO). Groundwater pH is affected by the composition of the bedrock and soil through which the water moves as well as other factors, including possible exposure to lime -containing materials in well casings, exposure to atmospheric carbon dioxide gas, and precipitation. In addition, metals and other elemental or ionic constituents in groundwater, or the surrounding soil matrix, can act as electron donors or acceptors as measured by ORP. The reactivity of different constituents can lead to oxidizing (positive ORP) or reducing (negative ORP) environments in groundwater systems. DO in groundwater can act as an oxidizing agent and is an indicator of redox state. Based on field measurements at CSS, the predominant redox category is anoxic/mixed, and the predominant redox processes are ferrous iron/ferrous sulfate, so the reduced species As(III), Se(IV), and Mn(IV) would be expected. The redox conditions appear to be controlled at least partly by the SO4/S2 and Fe(I I I)/Fe(I I) redox couples, and these redox couples should be monitored to assess changing redox conditions. The U.S. Geological Survey (USGS) redox framework was applied to groundwater measurements from different environments across the CSS site. Speciation measurements were performed for arsenic, selenium, chromium, iron, and manganese at select locations. Samples were collected using 0.45-micron filters and analyzed for total and dissolved metals. Other field measurements included DO, ORP, temperature, pH, specific conductance, and turbidity. DO, nitrate as nitrogen, manganese (II), iron (II), sulfate, and sulfide measured at the CSS site were used as inputs to primary redox categories were determined to include oxic, suboxic, mixed (anoxic), and anoxic conditions. Field measurements indicate that pH ranges from 5.1 to 8.7 standard units (SU) at the site. Background wells had pH values from 5.09 to 5.4 SU, and pH values within the ash basin material were from 3.61 to 12.5 SU. There is a wide range of ORP values, spanning ranges that imply mildly reduced (negative values) to highly oxidized (large positive values) conditions. This both agrees and contrasts with the redox category assessment. In terms of redox categories, a number of wells and samples were considered to be oxic while others were anoxic with manganese or iron reduction with sulfur oxidation as a predominant process. Standard (equilibrium) electrode potentials for such reactions may be expected in the range of approximately -1,000 millivolts. In contrast, measured ORP values at the CSS site were never less than -100 millivolts. 22 Corrective Action Plan Part 2 Cliffside Steam Station Ash Basin Speciation measurements were performed for samples collected from 54 groundwater and/or ash porewater monitoring wells, depending on the analyte, and vary widely at the site. Speciation measurements are summarized below. • For the samples where speciated arsenic concentrations were above reporting limits, the predominant species was in the reduced form As(III). • In general, hexavalent chromium [Cr(VI)] was identified in upgradient and background groundwater samples with greater or similar concentrations to those collected from ash porewater wells and groundwater monitoring wells beneath and downgradient of the source areas. • The reduced form of iron [Fe(II)] was observed with highest concentrations within the ash porewater downgradient of the active ash basin. • The reduced form of manganese [Mn(II)] was observed with highest concentrations in the ash porewater and beneath and downgradient of the active ash basin. • The reduced form of selenium [Se(IV)] was present above detection limits in three of the nine ash porewater samples and in nine of the groundwater wells. The oxidized form of selenium [Se(VI)] was present above detection limits in two of the ash porewater samples and ten of the groundwater samples. Speciation measurements for the CSS site suggest that COls exist in a patchwork of different conditions (CSA Report Table 10-5). In many cases, COI concentrations were below laboratory detection or quantitation limits and flagged with a "U" qualifier (e.g., results for arsenic and iron at MW-21 D sampled on June 23, 2015). At other wells, some constituents were measured to be in a reduced state while others were in a oxidized state (e.g., more As(III) than As(V) and more Fe(II) than Fe(III) but less Mn(II) than Mn(IV) for MW-24D from the June 30, 2015 sampling event). These differences highlight potentially varying redox conditions across the CSS site. Additionally, sorption coefficient (Kd) test results for batch and column tests were highly variable (CAP Part 1, Appendix D). This suggests that the rates at which constituents sorb or desorb to or from particulate phases varies widely. For the same constituent, experimentally determined Kd values can vary by one to two orders of magnitude (CAP Part 1, Appendix D). Additional sampling will be needed to characterize the temporal and spatial characteristics of groundwater composition for the site. Additional evaluations may also be beneficial to better characterize the kinetics of redox reactions. Classification of the geochemical composition of groundwater aids in aquifer characterization and SCM development. As groundwater flows through the aquifer media, the resulting geochemical reactions produce a chemical composition which can be used to characterize groundwater that may differ in composition from groundwater from a different set of lithological and geochemical conditions. This depiction is typically performed using Piper diagrams to graphically depict the distribution of the major cations and anions of groundwater samples collected at a particular site. 23 Corrective Action Plan Part 2 Cliffside Steam Station Ash Basin Piper diagrams presented in the CSA Report provide evidence of mixing of ash basin porewater and groundwater. In general, the ionic composition of groundwater and surface water at the CSS site is predominantly rich in calcium and magnesium with the exception of downgradient groundwater monitoring wells, which trend closer to a calcium-, magnesium-, and sodium -rich geochemical composition. 24 Corrective Action Plan Part 2 Cliffside Steam Station Ash Basin 4 Updated Modeling Groundwater Model Refinement The groundwater flow and fate and transport model was refined to incorporate post-CSA data. Model refinements are summarized in the following sections. The refined groundwater flow and transport model report was completed by HDR in conjunction with the University of North Carolina at Charlotte (UNCC). An independent review of the refined CSS 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 CSS model are provided in Appendix B. 4.1.1 Flow Model Refinements Transient transport simulations for all 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, or RMS error) of the modeled versus observed water levels for wells gauged in June 2015 to 4.51 % compared to the initial calibrated model of 7.6% in the CAP Part 1 model. The model calibration goal is an RMS error less than 10% of the difference in head between the modeled and the observed values across the model domain. The results are provided in Table 3 in Appendix B. • Recharge rates for the model were revised within and outside the ash basins. Recharge applied to the area of the model outside the ash basins was assigned a value of 6.5 inches per year. The ash storage area was assigned a value of 7 inches per year. Recharge within the ash basins was calculated using Darcy's Law considering the approximate area of the ponds, the approximate depth of water or saturated ash, and the range of measured hydraulic conductivity values within the ash and fill. The calculation provided a range of recharge values, with a value of 7.5 inches per year applied to the Units 1-4 inactive ash basin and a value of 11 inches per year over the active ash basin (Appendix B). 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 basins. • The bedrock hydrostratigraphic layer was extended vertically in the refined model to correlate to the deepest reported off -site private water supply well, as obtained from the completed questionnaires received from adjacent well owners during the receptor survey conducted in 2014. 25 Corrective Action Plan Part 2 Cliffside Steam Station Ash Basin • Private water supply wells were identified in the CSA Report. Residential wells were identified at 13 locations within the current model domain. These wells have been included as active pumping wells within the bedrock layer for the calibration and predictive scenarios. Actual pumping rates are unknown, so a constant pumping rate of 400 gallons per day (gpd) was applied to each well, which is the average household usage per the USEPA Water Sense Partnership Program (USEPA 2015a), was used. 4.1.2 Fate and Transport Model Refinements The groundwater fate and transport model was calibrated using refined parameters from the groundwater flow model, as discussed in Section 4.1.1, and presented below: • The initial model used conservative (low) Kd values to achieve calibration of the transport models for each COI. Subsequent to submittal of the CAP Part 1 Report, UNCC and Geochemical, LLC each provided recalculated Kd values using linear and Freundlich isotherms (Appendix C). Both sets of recalculated Kd values were considered during refinement of the transport models for each COI. Use of the newly derived Kd values in the 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 an acceptable practice. • The initial 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 fate and transport model to better represent measured source area ash porewater concentrations. • The initial 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 PPBCs for each COI which were applied as initial concentrations. This refinement allows the model 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. • Review of laboratory analytical data from Round 2 sampling resulted in the addition of barium and beryllium as modeled COls. • Potential COI impacts to private water supply wells located within the current model domain were evaluated through particle tracking simulations by applying a constant pumping rate of 400 gpd at each well, as specified in Section 4.1.1 above. 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 models provide a more accurate representation of existing site conditions, which produces model results that more accurately depict closure scenarios at the CSS site. 26 Corrective Action Plan Part 2 Cliffside Steam Station Ash Basin 4.1.3 Summary of Modeled Scenarios Two closure scenarios were modeled for CSS: an Existing Conditions scenario with ash sources left in place, and the Excavation and Cap -in -Place scenario, which simulates the removal of the ash from the Units 1-4 inactive ash basin, while the active ash basin, ash storage area, and Unit 5 inactive ash basin are covered with an engineered cap. These simulations predict flow and transport results using the model parameters calibrated for existing conditions. Once the scenario for corrective action is selected, the model should be revised and recalibrated to improve its accuracy and reduce its uncertainty. No modifications were made to the previously modeled Existing Conditions scenario hydrogeologic parameters or structure. 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 or increase initially for this scenario with source concentrations being held at their constant value over time. Thereafter, the concentrations and discharge rates remain constant. This scenario represents the most conservative case in terms of groundwater concentrations on -site and off -site, with COls discharging to surface water at a steady state. The time to achieve a steady-state concentration plume depends on the source zone location relative to the Compliance Boundary and its loading history. Areas close to the Compliance Boundary reach a steady-state concentration sooner. The time to steady-state concentration is also dependent on the sorptive characteristics of each COI. Sorptive COls will be 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 values will result in shorter times to achieve steady-state concentrations for both sorptive and non-sorptive COls. 4.1.3.2 Excavation and Cap -in -Place Scenario The Excavation and Cap -In -Place scenario simulates the effects of fully removing water and ash from the Units 1-4 inactive ash basin, while the Unit 5 inactive ash basin, the active ash basin and the ash storage area are covered by an engineered cap at the beginning of the predictive simulation. In the model, the constant concentrations were not removed from the model for the capped areas where the material remained saturated, allowing the material (ash) that remains saturated to contribute source concentrations to the model domain. This scenario assumes recharge rates in the ash basins become equal to rates surrounding the ash basin (6.5 inches per year). The capped ash disposal areas were assigned a recharge rate of zero. Starting from the time that excavation is complete, COls already present in the groundwater continue to migrate downgradient as water infiltrates from ground surface and recharges the aquifer at the water table. 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 Cliffside Steam Station Ash Basin COI migration is retarded relative to the ash porewater velocity as sorptive Cols adsorb to the soil/rock surfaces. The model uses the predicted concentrations from the 2015 calibration as the initial COI concentrations. Groundwater flow is affected by this scenario as the water table is lowered and groundwater velocities may be reduced beneath the capped areas. Dewatering and capping are expected to substantially reduce the amount of ash contacting groundwater. Near the center of the Unit 5 inactive ash basin, the water table is lowered by approximately 11 feet relative to the level simulated under the Existing Conditions scenario. Near the center of the ash storage area, the water table is lowered by approximately 9 feet relative to the level simulated under the Existing Conditions scenario. In the active ash basin, the difference in water level near the center of the basin is approximately 10 feet. 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 monitoring wells in June 2015 and reflects ash basin water levels at that time. The model was not calibrated to transient water levels over time, recharge, river flow, or river stage changes. A steady-state calibration does not consider groundwater storage and is 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 zones and bedrock for contaminant transport at the CSS site. MODFLOW simulates flow through porous media and groundwater flow in the bedrock groundwater zone is via fractures in the bedrock. • During model calibration, the constant source concentrations at the active ash basin, inactive ash basins, and ash storage area reasonably match 2015 COI ash porewater concentrations. • For the purposes of numerical modeling and comparing closure scenarios, it was assumed that the selected closure scenario will be implemented in 2015. • Predictive simulations were performed and steady-state flow conditions were assumed from the time the ash basins and ash storage area were placed in service through the current time until the end of the predictive simulations (Year 2265). • COI source zone concentrations were applied non -uniformly within each source area and assumed to be constant with respect to time for transport model calibration. • 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. 28 Corrective Action Plan Part 2 Cliffside Steam Station Ash Basin 4.1.5 Modeled Scenario Results 4.1.5.1 Excavation and Cap -in -Place Scenario Constituent concentrations were analyzed at downgradient monitoring wells (Figures 6, 7 and 8 in Appendix B) for all modeled COls. These wells are downgradient from either the ash basins or ash storage area and upgradient of the Compliance Boundary, Suck Creek, or the Broad River. Closure scenario results are presented as predicted concentration versus time curves in downgradient monitoring wells and as groundwater concentration maps for each modeled COI on Figures 19 through 210 in Appendix B, as discussed in the following subsections. Concentration contours and concentration breakthrough curves are referenced to the date when the Units 1-4 ash basin came into service (i.e., 1957). Concentration maps are referenced to a time zero that represents the time the closure action was implemented, which for the purposes of modeling is assumed to be 2015. A summary of the modeled COI results at the Compliance Boundary is provided below in Table 4-1. A "+" indicates that the concentration of a given COI has exceeded 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 represents the time the closure action was implemented (2015), and year 100 indicates 100 years from the time the closure action was implemented (2115). 29 Corrective Action Plan Part 2 Cliffside Steam Station Ash Basin Table 4-1 Summary of Modeled COI Results at the Compliance Boundary —Excavation and Cap -In -Place Scenario Constituent Appendix B Figures Flow Layer Existing Conditions Excavation and Cap -in - Place Scenario Year 0 2015 Year 100 2115 Year 0 2015 Year 100 2115 Antimony IMAC (1 pg/L) 19 — 33 Shallow + + + + Deep + + + + Bedrock + + + + Arsenic 2L (10 pg/L) 34 — 48 Shallow + + + + Deep + + + + Bedrock + + + + Barium 2L (700 pg/L) 49 - 63 Shallow — — — — Deep — — — — Bedrock Beryllium IMAC (4 pg/L) 64 - 78 Shallow Deep + + + + Bedrock + + + + Boron 2L (700 pg/L) 79 — 93 Shallow Deep + + + + Bedrock — + — + Chromium 2L (10 pg/L) 94 — 108 Shallow + + + + Deep + + + + Bedrock + + + + Cobalt IMAC (1 pg/L) 109 — 123 Shallow + + + + Deep + + + + Bedrock + + + + Hexavalent Chromium NCDHHS HSL (0.07 pg/L) 124 — 135 Shallow + + + + Deep + + + + Bedrock + + + + Lead 2L (15 pg/L) 136 - 150 Shallow + + + + Deep + + + + Bedrock + + + + Nickel 2L (100 pg/L) 151 - 165 Shallow — — — — Deep — — — — Bedrock — — — — Sulfate 2L (250,000 pg/L) 166 — 180 Shallow + + + + Deep + + + + Bedrock + + + + Thallium IMAC (0.2 pg/L) 181 — 195 Shallow + + + + Deep + + + + Bedrock + + + + Vanadium IMAC (0.3 pg/L) 196 — 210 Shallow + + + + Deep + + + + Bedrock + + + + 30 Corrective Action Plan Part 2 Cliffside Steam Station Ash Basin 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 seven 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. To evaluate this condition, HDR and UNCC conducted particle tracking within the Existing Conditions scenario steady-state flow field to identify the travel path and time to the model boundary. Particles were placed at wells located near the Broad River and at the upgradient ends of the ash basins. The particle tracks are shown on Figure 17 in Appendix B and predicted advective travel times are provided in Table 7 in Appendix B. The one-year advective travel time pathlines of 13 monitoring wells intersect the Compliance Boundary or the Broad River at CSS indicating that the 2L Standard, IMAC, or NCDHHS HSL will be exceeded. As previously discussed, 13 residential pumping wells are included in the model domain, pumping at a rate of 400 gpd each within the bedrock layer. The particle tracks for these residential pumping wells are shown on Figure 18 in Appendix B. The one-year advective travel time from the residential pumping well pathlines do not intersect the Compliance Boundary at CSS. • Under the Existing Conditions and Excavation and Cap -in -Place scenario, antimony, arsenic, beryllium, boron (Existing Conditions scenario only), chromium, cobalt, hexavalent chromium, lead, sulfate, thallium, and vanadium are predicted to exceed their applicable 2L Standard or IMAC at the Broad River. Under the Existing Conditions and Excavation and Cap -in -Place scenarios, antimony, arsenic, barium, beryllium, boron, chromium, cobalt, nickel, sulfate, and vanadium are predicted exceed their respective PPBCs at the Broad River. The background concentrations used for modeling antimony, arsenic, chromium, hexavalent chromium, cobalt, thallium, and vanadium either exceed or equal their respective 2L Standard, IMAC, or NCDHHS HSL. • Under the Existing Conditions and Excavation and Cap -in -Place scenarios barium, boron (Excavation and Cap -in -Place scenario only), and nickel are not predicted to exceed their respective 2L Standards at the Broad River. Model predictions do not show that COI concentrations will be reduced by installing cap(s) over source areas. 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, and 31 Corrective Action Plan Part 2 Cliffside Steam Station Ash Basin • COls from Round 1 and Round 2 sampling events. Groundwater to surface water interactions were completed using groundwater model output and a surface water mixing model approach to evaluate potential surface water impacts of Cols in groundwater as they discharge to surface water bodies adjacent to the CSS site. 4.2.2 Results The calculated surface water COI concentrations in the Broad River adjacent to CSS and Suck Creek are presented in Tables 4-2 and 4-3. The stream design flows, groundwater flows, and groundwater COI concentrations presented in Appendix D were used to complete these calculations. The mixing model results indicate that water quality results for modeled Cols are less than the 2B Standards at the edge of the mixing zone in the Broad River. The descriptions of the mixing zones are provided in Appendix D. The mixing model results indicate that water quality standards are exceeded in Suck Creek for lead freshwater aquatic life chronic standard, and thallium human health and water supply standards. These calculated exceedances are localized due to relatively low surface water flows into Suck Creek and do not result in exceedances of the 2B Standard at the edge of the mixing zone in the Broad River. Three surface water sample locations (SW-2, SW-3, and SW-4) in Suck Creek, and one location (S-8) located in an unnamed tributary to Suck Creek (Figure 2-1) were sampled during the Round 1 and Round 2 sampling events in 2015. For the Cols included in the groundwater model (Table 4-3), results from the surface water samples were in general agreement with results from the mixing model approach that was applied. Surface water samples collected at these four locations were less than the water quality standards for antimony, arsenic, boron, chromium, cobalt, hexavalent chromium, nickel, sulfate and vanadium. Also in agreement with the mixing model, surface water sampling during Round 2 measured a lead concentration of 1.1 pg/L at SW-3, which exceeds the chronic water quality standard for freshwater aquatic life (0.54 pg/L) (Table 4-3). Although the mixing model indicates that thallium concentrations in Suck Creek were greater than the human health and water supply water quality standards, the surface water sampling results reported thallium concentrations below these water -quality standards (< 0.1 pg/L) at all four stations during Round 1 and Round 2. The surface water sampling in Suck Creek during Round 2 also identified two COls that were not identified until after completion of the groundwater modeling and mixing model calculations. Exceedances for aluminum (total recoverable) were measured at the three sampling stations in Suck Creek (including upgradient sample location SW-2) and the sample collected from the tributary to Suck Creek, with a mean concentration of 116 pg/L, which is greater than the freshwater chronic aquatic life water quality standard of 87 pg/L. At surface water sample location SW-3 (near the active ash basin upstream dam), the measured copper concentration of 10.4 pg/L exceeded both the acute (3.6 pg/L) and chronic (2.7 pg/L) water quality standards for freshwater aquatic life (assuming a hardness of 25 mg/L). The sampling results indicate that the surface water mixing model results are generally consistent with recent data collected in Suck Creek. Although the mixing model calculated 32 Corrective Action Plan Part 2 Cliffside Steam Station Ash Basin thallium water quality standard exceedances, the recent data did indicate that thallium is less than the human health and water supply water quality standards in Suck Creek. Copper and lead were not previously identified during the Round 1 sampling event as surface water Cols at the CSS site, and should be added to the list of COls for surface water. The lead and copper total recoverable concentrations at SW-3 and the total recoverable copper concentration at SW-1 were less than the dissolved concentrations and 2B Standards. Surface water sampling location (S-1) is a small tributary that discharges to the Broad River at the upstream boundary of CSS. During Round 2 sampling an aluminum concentration of 120 pg/L was reported, which exceeds the acute and chronic water quality standards. Additionally at this sample location, a mercury (total recoverable) concentration of 0.085 pg/L was measured in one sample, which exceeds the chronic water quality standard of 0.012 pg/L for freshwater aquatic life. It should be noted, however, that the measured value was a laboratory estimated concentration. Three newly identified COls (aluminum, copper, and mercury) were reported in localized surface waters with relatively low discharge, which includes Suck Creek and the small, unnamed tributary where sample S-1 is located when comparing Round 1 and Round 2 sampling events. The cause of this variation in results is unknown at this time. These constituents will continue to be monitored during subsequent sampling events to see if they should remain as COls for surface water at the site. Table 4-2 Broad River Calculated Surface Water Concentrations Calculated Mixing Zone Conc. (Ng/L) Water Quality Standard (Ng/L) COI Acute Chronic HH/WS Acute Chronic HH / WS Antimony 0.49 0.29 0.29 (nc) ns ns 640 / 5.6 Arsenic 0.75 0.33 0.26 (c) 340 150 10 / 10 Boron 28.4 25.5 25.1 * ns ns ns / ns Chromium 0.76 0.33 0.26* ns ns ns / ns Hexavalent Chromium 0.31 0.26 0.25* 16 11 ns / ns Cobalt 0.83 0.34 0.34 (nc) ns ns 4/3 Lead 0.56 0.13 0.06* 14 0.54 ns / ns Nickel 0.77 0.33 0.33 (nc) 140 16 ns / 25 Sulfate 2,422 790 790 (nc) ns ns ns / 250,000 Thallium 0.561 0.127 0.127 (nc) ns ns 0.47 / 0.24 Vanadium 1.02 0.58 0.51* ns ns ns / ns Notes: 1. All COls are shown as dissolved concentrations, except for chromium 2. HH - Human Health (15A NCAC 02B .0211, amended effective January 1, 2015) 3. WS - water supply (15A NCAC 02B .0216, amended effective January 1, 2015) 4. c - carcinogen 5. nc - non -carcinogen 6. ns - no water quality standard 7. * - concentration calculated with annual mean river flow 33 Corrective Action Plan Part 2 Cliffside Steam Station Ash Basin Table 4-3 Suck Creek Calculated Surface Water Concentrations Calculated Mixing Zone Conc. (Ng/L) Water Quality Standard (Ng/L) COI Acute Chronic HH/WS Acute Chronic HH / WS Antimony 1.66 0.52 0.52 (nc) ns ns 640 / 5.6 Arsenic 3.23 0.80 0.27 (c) 340 150 10 / 10 Boron 31.0 26.2 25.1 * ns ns ns / ns Chromium 3.44 1.09 0.58* ns ns ns / ns Hexavalent Chromium 0.80 0.57 0.53* 16 11 ns / ns Cobalt 3.33 0.78 0.78 (nc) ns ns 4/3 Lead 3.01 0.63 0.11 * 14 0.54 ns / ns Nickel 3.26 0.97 0.97 (nc) 140 16 ns / 25 Sulfate 3,073 1,485 1,485 (nc) ns ns ns / 250,000 Thallium 3.077 0.641 0.641 (nc) ns ns 0.47 / 0.24 Vanadium 1.92 0.83 0.59* ns ns ns / ns Notes: 1. All COls are shown as dissolved concentrations, except chromium 2. HH - Human Health (15A NCAC 02B .0211, amended effective January 1, 2015) 3. WS - water supply (15A NCAC 02B .0216, amended effective January 1, 2015) 4. c - carcinogen 5. nc - non -carcinogen 6. ns - no water quality standard 7. * - concentration calculated with annual mean creek 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 CSS 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 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 affects Eh. Changes in TDS affect ionic strength and ion competition at sorption sites. Constituents evaluated for CSS were: arsenic, antimony, boron, chromium, cobalt, iron, manganese, pH, selenium, sulfate, TDS, thallium, and vanadium. 4.3.2 Methodology Site -specific evaluations of COls were performed for each of the monitoring wells using the USGS PHREEQC (v3.3.3) 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 picture. 34 Corrective Action Plan Part 2 Cliffside Steam Station Ash Basin 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 78 wells monitored at the CSS 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 CSS. 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 extractions from actual site sediment that were also the basis for measured distribution coefficients (Kd values) for CSS soils determined from adsorption experiments conducted by collaborators at the UNCC. In order to geochemically simulate changes to aquifers or test potential remediation strategies, simulations in which DO, pH, redox, and TDS were varied were utilized. 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 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 values tend to favor formation of more soluble cationic species for most alkali elements, alkali earth elements, and transition metals. Methodologies are discussed in further detail in Appendix E. 4.3.3 Assumptions The following assumptions were incorporated in the PHREEQC modeling effort: • Groundwater data were evaluated 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. 35 Corrective Action Plan Part 2 Cliffside Steam Station Ash Basin • 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 soils, the 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 are assumed to be primarily nitrate and alkalinity results are primarily bicarbonate, not carbonate. • TDS is 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 is addressed as a component of TDS. • Eh/pH diagrams and/or predominance plots were completed in PhreePlot or Geochemist's Workbench for each COI to aid in demonstration of changes in Kd, pH, and DO. 4.3.4 Results • The modeling effort described above provides both qualitative and quantitative estimations of the chemical speciation and adsorption behavior of several key constituents of interest. Relevant observations from this modeling effort are as follows: • Redox conditions vary widely at the CSS site, indicating that the groundwater system has not reached equilibrium, or data is not representative of the conditions sampled. Additional groundwater results will assist in refining the model further and confirm these findings should sampled data not be representative of actual groundwater conditions. • The observed site condition of limited solubility of arsenic, chromium, and cobalt in the CSS site groundwater is confirmed by the geochemical modeling. • Each of the pH, Eh, and TDS figures can be further evaluated to support monitored natural attenuation (MNA) or remediation. The addition of an engineered cap over the Unit 5 inactive ash basin, active ash basin, and ash storage area would reduce infiltration and reduce 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 restricting the release of TDS and other metals. Methods such as the installation of an engineered cap over the Unit 5 inactive ash basin, active ash basin, and ash storage area 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. 36 Corrective Action Plan Part 2 Cliffside Steam Station Ash Basin • Soil sorptive capacities for COls varied from more sorptive (arsenic) to less sorptive (boron) at the CSS site. Refer to Appendix E for further details of the CSS site geochemical model results. 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. 37 Corrective Action Plan Part 2 Cliffside Steam Station Ash Basin 5 Risk Assessment The purpose of this 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 CSS. 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 will aid in focusing corrective 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 methodology designed to be consistent with state and federal guidance. This methodology represents a step -wise process whereby CSS is 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 Figure 12-1 (human health) and Figure 12-2 (ecological) of the CSA Report. Updated CSMs are provided in Appendix F. The CSMs are intended to identify potential exposure pathways and receptors that may be applicable at the site. For CSS, the following receptors and exposure scenarios were identified in the human health CSM (Figure 2-3 in Appendix F): • Current/future on -site trespasser with potential exposure to dust in outdoor air, soil remaining post -excavation, AOW water, AOW soil, on -site surface water, and on -site sediment; 38 Corrective Action Plan Part 2 Cliffside Steam Station Ash Basin • Current/future commercial/industrial worker with potential exposure to dust in outdoor air, soil remaining post -excavation, AOW water, 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 -excavation, 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, waders, and boaters with potential exposure to off -site surface water and off -site sediment. • Current/future anglers with potential exposure to off -site surface water and off -site sediment, fish ingestion for recreational fishers, and fish ingestion for 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. • Terrestrial birds and mammals with potential exposure to soil remaining post -excavation, and AOW water and AOW soils. 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 Round 1 and Round 2 sampling. 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 environment; rather, it only indicates that additional evaluation may be warranted. Screening levels are used in this assessment to help identify 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. 39 Corrective Action Plan Part 2 Cliffside Steam Station Ash Basin 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 1991a), 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 noncarcinogens (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 CSS, the results of the human health risk assessment indicate that exposure to on -site surface water, AOW water, AOW soil, sediment, and groundwater poses no carcinogenic risk exceeding 1.0E-04 and hazard index above 1 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 cancer risk and hazard index above 1 for a recreational swimmer, wader, boater, and recreational or subsistence angler under the scenarios developed in Step 1. Results of the Human Health Risk Assessment indicated that there are no unacceptable risks to human health. 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 "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 40 Corrective Action Plan Part 2 Cliffside Steam Station Ash Basin 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 exposures to COPCs and their significance. During risk estimation, the exposure assessment and effects assessment were integrated to evaluate the likelihood of adverse impacts to the 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 (HQ). Receptors chosen for ecological risk assessment are often surrogates for the broad range of potential ecological receptors in a given habitat. For CSS, 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). Terrestrial receptors include birds, represented by American robin and red-tailed hawk, and mammals, represented by meadow vole and red fox. At CSS, four ecological exposure areas were defined (Figure 2-4 in Appendix F). These include: • Exposure Area 1, located northeast of Units 1-4 inactive ash basin and the active ash basin along the Broad River; • Exposure Area 2, located south of Units 1-4 inactive ash basin and west of the active ash basin along Suck Creek; • Exposure Area 3, located south of the active ash basin along Suck Creek; and • Exposure Area 4, located north of Unit 5 inactive ash basin along the Broad River. Evaluation of the surface water, AOW water, AOW soil, AOW sediment and sediment in Exposure Area 1 indicates a calculated HQ of 6 for a muskrat's exposure to aluminum. The American robin has an HQ of 1 and meadow vole has an HQ of 8, also resulting from aluminum exposure, using the No Observed Adverse Effects Level (NOAEL) as the toxicity reference value. The American robin also has an HQ from selenium of 1. Using the Lowest Observed Adverse Effect Level (LOAEL) toxicity reference value, aluminum's HQ for a muskrat decreases to 0.6. Aluminum's HQ for the American robin decreases to 0.1 and 0.8 for the meadow vole. The American robin's HQ from selenium decreases to 0.5. All other aquatic and terrestrial wildlife receptors have chemical HQs below 1. The evaluation of ecological exposures to surface water, AOW water, AOW soil, AOW sediment and sediment in Ecological Exposure Area 2 indicates HQs of 2, 8, 3, 5, and 21 for a great blue heron's exposure to barium, copper, manganese, selenium and vanadium, respectively, using 41 Corrective Action Plan Part 2 Cliffside Steam Station Ash Basin the NOAEL. Under the LOAEL scenario, these HQs decrease to 1, 3, 2, 2, and 10, respectively for each COPC. Exposure to aluminum results in HQs of 42, 2, and 81 for a muskrat, American Robin and meadow vole, respectively, using the NOAEL; these HQs decrease to 4, 0.2, and 8, respectively, using the LOAEL. Evaluation of AOW water, AOW soil and AOW sediment in Exposure Area 3 indicates that all aquatic receptors (i.e., mallard, great blue heron, muskrat, and river otter) have chemical HQs below 1. The terrestrial meadow vole has a HQ at 1 from exposure to aluminum using the NOAEL and of 0.1 using the LOAEL. AOW water, AOW soil, and AOW sediment in Exposure Area 4 presents a hazard to aquatic receptors, with HQs of 1 from selenium and 2 for vanadium for the great blue heron using the NOAEL that decrease below 1 using the LOAEL; and 11 for aluminum, 2 for manganese, and 5 for selenium for the muskrat using the NOAEL that decrease 1, 1, and 3 using the LOAEL. The terrestrial species American robin has an HQ of 2 from aluminum, 1 from cobalt and manganese, and 7 from selenium using the NOAEL that decrease to 0.2, 0.7, and 4 for aluminum, manganese and selenium, respectively, while cobalt remains the same at 1. The meadow vole has HQs of 16 from aluminum, 2 from manganese, and 5 from selenium exposure using the NOAEL that decrease to 2, 1, and 3, respectively, using the LOAEL. All other aquatic and terrestrial wildlife receptors have chemical HQs below 1. Evaluation of potential impacts to ecological receptors indicates that risk estimates for several COPCs are above risk targets for several ecological receptor species if and where these species present at CSS. In the area along the Broad River adjacent to the active ash basin and Units 1-4 of the inactive ash basin, ecological receptor risks are below risk targets. In a small area along Suck Creek between the steam station and the active ash basin, risks are above risk targets for aluminum for the muskrat and meadow vole, and are above risk targets for copper, manganese, selenium, and vanadium for the heron. In the area immediately northwest of Unit 5 inactive ash basin along the river, potential risks are above risk targets for the selenium for the muskrat, robin and meadow vole, and for aluminum for the meadow vole. 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, and the conservative nature of the exposure and toxicity assumptions used in the ecological risk characterization. 42 Corrective Action Plan Part 2 Cliffside Steam Station Ash Basin 6 Alternative Methods for Achieving Restoration This section discusses how remedial alternatives are evaluated and identifies the remedial alternative selected to achieve restoration of groundwater quality at the CSS site. As described in Section 1, after removal of ash from the Units 1-4 inactive ash basin, soils left on -site will be sampled and analyzed, and the analytical results will be incorporated into the groundwater contaminant fate and transport models. If this evaluation indicates that modification to the proposed CAP is required, Duke Energy will prepare and submit a revised CAP. The proposed CAP calls for the Unit 5 inactive ash basin, ash storage area, and active ash basin to be covered with an engineered cap, which will minimize infiltration through the covered area, reducing possible impacts from potentially impacted soil. Therefore, remediation of soils is not discussed in this document. As noted in Section 2, exceedances of the 2L Standards or IMACs were measured at AOWs located north of the Unit 5 inactive ash basin, north of the Units 1-4 inactive ash basin, north of the ash storage area, and west and north of the active ash basin. HDR and Duke Energy consider that the water in the Units 1-4 inactive ash basin is the likely primary source of the water supplying the AOWs north of this basin along the Broad River. As a result of dewatering this basin, the flow at these AOWs will decrease or possibly be eliminated. The physical characteristics of the AOWs will be monitored during excavation activities. Based on the groundwater modeling results, the water table in the Unit 5 inactive ash basin, active ash basin, and ash storage area will be lowered by approximately 10 feet after an engineered cap is installed. The decreased head in these areas should result in a decrease in flow at the AOWs. Duke Energy proposes that remedial measures at the AOWs be deferred until after closure activities are completed in these areas. If at that time, exceedances are still present, Duke Energy will evaluate those conditions and develop corrective measures to address the exceedances. For CSS, the Plan for Identification of New Discharges was submitted to NCDEQ on May 19, 2015. This plan was developed to address the requirements of North Carolina General Statute (GS)l 30A-309.21 0 (d) Identification and assessment of discharges; correction of unpermitted discharges, as modified by North Carolina Senate Bill 729. Identification of new discharges (AOWs) and any associated sampling of the new discharge will be done in compliance with that document. 6.1 Corrective Action Decision Process 6.1.1 Evaluation Criteria The goal of groundwater corrective action in accordance with T15A 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). 43 Corrective Action Plan Part 2 Cliffside Steam Station Ash Basin 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 or excavation) are being addressed separately. The groundwater corrective action alternatives evaluated herein are evaluated to supplement source control. 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 future corrective action: antimony, arsenic, barium, beryllium, boron, chromium, cobalt, hexavalent chromium, iron, lead, manganese, nickel, pH, sulfate, thallium, TDS, and vanadium. These COls are considered for corrective action because they have been found to exceed their applicable 2L Standards, IMACs, or NCDHHS HSLs, or may exceed their applicable 2L Standards, IMACs, or NCDHHS HSLs in the future due to fluctuations of COI concentrations as a result of closure activities. There are some locations that have exceedances in both Round 1 and Round 2 results, while other locations do not present consistent exceedances when comparing Round 1 and Round 2 results. In some cases exceedances at locations in Round 1 were followed with non -detect results in Round 2 sampling. For this reason, it is recommended that additional groundwater sampling be conducted as recommended in Section 9 to confirm the effectiveness of proposed corrective action. 6.1.3 Potential Exposure Routes and Receptors The risk assessment provided in Appendix F provides a conservative assessment of potential ecological and human health risks associated with the COls attributed to the CSS source areas. The primary source -to -receptor exposure route from the ash disposal areas is the leaching of ash porewater to groundwater. Groundwater will then migrate to the Broad River, or to Suck Creek which discharges to the Broad River. A secondary source -to -receptor exposure route may be infiltration of COls in water and/or sediment at AOWs to groundwater. At the CSS site, there are no water supply wells downgradient of the ash disposal areas (Figure 3-3). The Broad River is a hydrologic boundary between the ash disposal areas and the properties to the north of the Broad River; therefore, consideration of future water supply wells downgradient of the ash basins and ash storage area as receptors do not warrant further assessment. Localized groundwater mounding associated with the current hydraulic head in the basin will be eliminated with the removal of ash from the Units 1-4 inactive ash basin. After capping the Unit 5 inactive ash basin, ash storage area, and the active ash basin, the groundwater model predicts that the water table in these areas will be lowered. These corrective actions have the potential to reduce or eliminate the flow in AOWs associated with these basins, potentially eliminating the exposure pathway to surface water. 44 Corrective Action Plan Part 2 Cliffside Steam Station Ash Basin 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. 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. 45 Corrective Action Plan Part 2 Cliffside Steam Station Ash Basin 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? • 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 implementability/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 their 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 incorporating 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: 46 Corrective Action Plan Part 2 Cliffside Steam Station Ash Basin • Air pollution caused by emission of carbon dioxide, nitrous oxide, methane, and other greenhouse gasses 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 6.2.4 Cost The criterion of cost has been evaluated by looking at the estimated capital cost and labor required to implement technologies that will enhance future closure activities. The cost evaluation considers design, construction, and operation and maintenance 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 evaluating the alternatives. 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 site, and may involve capping -in -place, excavating and landfilling on -site, or excavation and disposal or beneficial use off -site. The source control measures are being evaluated and designed separately. Source removal is already being implemented at the Units 1-4 inactive ash basin; ash is being excavated and relocated to the on -site lined landfill for disposal. Potentially applicable measures are summarized below. The remedial alternatives described in this section were considered to enhance source control measures at the CSS site and improve the effectiveness of the remedy. 6.3.1 Groundwater Remediation Alternatives Remedial alternatives for restoration of groundwater in accordance with T15A NCAC 2L.0106 include: Source Control, which can include: o Ash removal to prevent COls from leaching into groundwater; o Placement of an engineered cap(s) to prevent infiltration and 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). • Land Use Controls - State approval to restrict land use to prevent the use of surface water and groundwater in the area. 47 Corrective Action Plan Part 2 Cliffside Steam Station Ash Basin Monitored Natural Attenuation - Regular monitoring of select groundwater monitoring wells for specific parameters to ensure COI concentrations in groundwater are decreasing. Dilution from recharge to shallow groundwater, mineral precipitation, and COI adsorption will occur over time, thus reducing COI concentrations through attenuation. Enhanced Attenuation, which can include: o Addition of materials with high sorptive capacity to the saturated zone to reduce COI levels in groundwater; o Air sparging and adjusting pH to enhance precipitation of iron and manganese oxide/hydroxide minerals to reduce COI levels in groundwater; and o Bioremediation for removal of COls Permeable Reactive Barriers — Involves 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 available remedial alternatives is included in Appendix G. 6.3.2 Monitored Natural Attenuation Applicability to Site An MNA Tier I and Tier II evaluation was conducted for the CSS site by Geochemical, LLC and is included in Appendix H. The following is a summary of the Tier I and Tier II evaluation. 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 standards or criteria. The mechanisms that regulate their release from solids and movement through aquifers are, for the most part, the same processes that provide chemical controls on movement of CCR leachate in an aquifer. These processes attenuate the concentrations of inorganics in groundwater by depositing inorganics on aquifer solids, removing the constituent from the groundwater. MNA is a potential corrective action or COI exceedances in groundwater at the CSS site. The groundwater COls for CSS were identified as antimony, arsenic, barium, beryllium, boron, chromium, cobalt, hexavalent chromium, iron, lead, manganese, nickel, pH, sulfate, thallium, TDS, and vanadium. Antimony, chromium, cobalt, iron, manganese, and vanadium occur naturally in regional groundwater, but these constituents were detected at concentrations that exceeded the observed background conditions in locations associated with the ash disposal areas at the site. Sulfate and TDS are generally not strongly attenuated by reactions with solids, but are reduced in concentration by diffusion, mechanical mixing (dispersion), or dilution. Tier I analysis used two lines of evidence for attenuation: 1) Solid -water pair comparisons of COI concentrations are performed, and a mutually rising relationship indicates attenuation (USEPA 2007a) and 2) Laboratory determination of the solid -water partitioning coefficient, or Kd value (USEPA 1999), is used as a measure of the propensity of COls to adsorb to site -specific solids and be attenuated. Tier I analysis indicates that arsenic, barium, beryllium, boron, 48 Corrective Action Plan Part 2 Cliffside Steam Station Ash Basin chromium, cobalt, lead, thallium, and vanadium are COls that should be advanced to Tier II analysis. Following completion of the Tier I analysis, a conceptual model for COI attenuation involving reversible and irreversible interaction with clay minerals, metal oxides, and organic matter was created. A Tier II demonstration based on the conceptual model was completed, resulting in the following findings: 1. The samples evaluated for Kd determination were found to be representative of site - specific conditions under which Cols would migrate. 2. Clay minerals and iron oxides were found in all samples. Organic matter is not a significant sink for COls at CSS. 3. Chemical extractions identified that COls were concentrated in soil samples exposed to groundwater containing higher concentrations of COls, validating attenuation. 4. Chemical extractions were used to determine a probable range of Kd values. The resulting Kd values suggest attenuation is taking place for arsenic, barium, beryllium, boron, chromium, cobalt, lead, thallium and vanadium. As documented in Appendix E, titration results for CSS monitoring wells can be used to support evaluation of MNA or remediation impacts. For example, titration results can be used to help determine the expected impact that DO changes would have in response to addition of an engineered cap (leading to reduced infiltration and lower recharge DO), or the introduction of oxygen creating a more oxic environment, addition of acid or base to adjust the pH to conditions that prevent Cols from being solubilizing, or impact due to excavation and the release of TDS and other metals. Changes in redox can occur also in response to DO increases or decreases as well as the introduction of inorganic oxidants from anthropogenic contamination or changes in groundwater flow vectors. Results of the geochemical modeling support applicability of MNA as an effective remedial alternative for the site. Additional data collection is necessary to complete the Tier II/111 assessment with respect to specific attenuation mechanisms for each COI, and quantification of the magnitude of that 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 I II 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 CSS site. Source control is actively being completed with the removal of ash from the Units 1-4 inactive ash basin and placing the ash into an on -site lined permitted landfill. The initial phase of 49 Corrective Action Plan Part 2 Cliffside Steam Station Ash Basin excavation work began in October 2015. The active ash basin, ash storage area, and Unit 5 inactive ash basin are planned to be covered in place with an engineered cap. As detailed in Appendix G, the following remedial alternatives are being considered at the CSS site to enhance or supplement existing and planned source control activities. An overview of each technology's applicability to the site is provided: 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 or the excavation of the ash, and no further remedial action would be taken for groundwater. This measure does not include long-term monitoring or institutional controls. 2. MNA - Groundwater monitoring would be continued until remedial objectives are met (i.e., groundwater concentrations are at or below applicable standards or criteria). Given that ash will be removed from the Units 1-4 inactive ash basin, it is reasonable to assume that COls remaining in groundwater downgradient of Units 1-4 inactive ash basin would continually decrease in concentration over time as recharge flushes non - impacted water through the aquifer. 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. 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 CSS site show abundant Fe2O3 and MnO values in soils from the site (CSA Report Table 6-2) and a strong potential for adsorption. A Tier II demonstration based or 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 included in groundwater modeling. The groundwater model did not allow for removal of COI via coprecipitation with iron oxides, which likely resulted in an over -prediction of COI transport. Completion of the Tier 11 tests described in Appendix H will address this issue. It is feasible that MNA can be used partially or entirely to remediate the CSS site including areas downgradient of the Units 1-4 inactive ash basin that will be excavated as well as the ash storage area, Unit 5 inactive ash basin, and the active ash basin. It is noted that the Broad River is located immediately downgradient of the ash storage area and active ash basin, and as a result, evaluation of the groundwater impacts to surface water will be required. 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 50 Corrective Action Plan Part 2 Cliffside Steam Station Ash Basin bottom, which could be temporary or permanent. Where a continuing source is present, permanent infiltration galleries may be needed. A potential location for an infiltration basin could be located in the active ash basin if ash were consolidated on -site. The use of this technology may be applicable if areas of the existing active ash basin are excavated and become available. Directly behind the berm in the active ash basin there appears to be a limited amount of accumulated ash which facilitates use of this area for enhanced recharge/flushing. 4. In -situ Sorption or In -situ Chemical Fixation - Various measures can be taken to enhance adsorptive removal of COls by soil blending with materials that have a high sorptive capacity, such as clays, peat moss, and zeolites, into the contaminated material or affected groundwater. Contaminated groundwater can also be treated in -situ using chemical fixation by adjusting the pH and/or redox state of the groundwater, for example, by enhancing the 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. Reagent addition (or air sparging) could encourage the precipitation of COls by changing the redox conditions over targeted areas. Enhancing natural attenuation in this manner could be affected using in -situ chemical fixation or air sparging technologies. In -situ chemical fixation involves the injection of a chemical oxidant, such as potassium permanganate. Lifetime costs of these technologies may be comparable. If these technologies are part of a selected alternative, it is recommended that both approaches be tested on -site during a pilot study to see which works better, 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 application of wells and reagents and it may not be feasible to use this technology to manage large land areas. It may be feasible to use chemical fixation for the treatment of select areas with a greater mass of COls to reduce the flux of COls through aquifer to reduce loading for MNA treatment. 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. 5. Permeable Reactive Barrier - A permeable reactive barrier (PRB) is a passive form of in - situ water treatment that removes Cols in a 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. 51 Corrective Action Plan Part 2 Cliffside Steam Station Ash Basin 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 non -permeable, 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 the data, it appears that the PRB could comprise a combination of limestone aggregate (to provide PRB stability, transmissivity, 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. While impacted groundwater has moved beyond the Units 1-4 inactive ash basin berms and active ash basin dam, it would potentially be appropriate to install a PRB along the Broad River and base of the dam intercepting the impacted water but the feasibility of doing this may be limited along the dam by its structural integrity. Further, the COI concentrations may not be generating a 2B Standard exceedance in the Broad River. Beneath the Units 1-4 inactive ash basin, the grading of the ash berms may allow PRB installation access; however, once again the Compliance Boundary will be the surface water body in this vicinity, and there is limited shoreline for construction. Near the Unit 5 inactive ash basin there are multiple locations where the PRB could be constructed based on access; however, it is not clear from existing data if active remediation is required now or in the future. 6. Groundwater Treatment - As an alternative to in -situ groundwater treatment methods discussed above, groundwater can be removed and treated above grade. Impacted groundwater would be pumped to the surface (pump -and -treat) or captured at AOWs. Following treatment, the water may be discharged directly to a surface water body 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, 52 Corrective Action Plan Part 2 Cliffside Steam Station Ash Basin 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 over portions of the site; however, the areal extent of the site makes centralized treatment more challenging. 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: • Units 1-4 Inactive Ash Basin - Duke Energy has decided to remove the ash from the Units 1-4 inactive ash basin, due to repairs needed to the decant riser coupled with liquefaction results for the embankment. Based on the groundwater modeling results, the removal of ash should improve the water quality downgradient of the basin over time. Implementation of MNA is recommended to observe whether or not Cols decrease following completion of ash removal. Groundwater to surface water interaction modeling found that groundwater impacts did not result in exceedances of 2B Standards in the Broad River. Further monitoring will be performed to confirm the modeling predictions. Unit 5 Inactive Ash Basin - If ash at the Unit 5 inactive ash basin is capped in place, MNA alone may not be sufficient to achieve remedial goals; however, Tier III assessment activities should be completed to determine this. In the primary flow direction of the shallow and deep flow layers where impacts have been observed, continued evaluation is needed to determine whether MNA will be sufficient or if additional remedial alternatives need to be evaluated to meet groundwater standards or criteria at the provisional Compliance Boundary. Localized areas may be treated using in -situ chemical fixation or broader areas may require a more comprehensive approach in the direction of the primary flow path such as PRB. Active Ash Basin and Ash Storage Area - If the ash in the active ash basin and ash storage areas are capped in place, MNA alone may not be sufficient to remediate groundwater to meet applicable groundwater standards or criteria; however, the Broad River receives recharge from groundwater immediately downgradient of these areas. Groundwater to surface water modeling finds that after mixing with surface water, surface water 2B Standards will be met. As a contingency, focused source treatment could be considered. The selection of a source treatment technology will require further data collection, cost estimation, and bench -scale treatability testing. MNA is an effective corrective action because COls will attenuate over time to restore groundwater quality at the CSS site and is protective of both human health and the environment. This corrective action would include installation of additional monitoring wells downgradient of the ash basins and ash storage area between the ash management areas and 53 Corrective Action Plan Part 2 Cliffside Steam Station Ash Basin the Broad River to more accurately monitor MNA parameters in this area once closure activities are complete. MNA is a feasible corrective action and can be implemented at the site. The implementation cost for MNA for a 30-year period is estimated to cost $9.3 million. Monitoring to evaluate the effectiveness of MNA as a corrective action is summarized in Section 9.0. Costs are discussed further in Section 10. 54 Corrective Action Plan Part 2 Cliffside Steam Station Ash Basin 7 Selected Corrective Action(s) Remedial alternatives were evaluated for achieving restoration of groundwater at the CSS site, as detailed in Section 6. MNA was determined to be the most appropriate corrective action; however, groundwater quality should be monitored following excavation of the Units 1-4 inactive ash basin and closure of the Unit 5 inactive ash basin, the active ash basin, and the ash storage area to evaluate if MNA remains a viable corrective action after basin closure. 7.1 Selected Remedial Alternative for Corrective Action COI transport in groundwater is primarily controlled by hydrogeologic and geochemical conditions at the CSS site (Section 3). COls enter the groundwater system through the shallow flow layer within the source areas. Evaluation of the 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, chromium, and cobalt were observed to have limited solubility, meaning these constituents attenuate more readily. 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 site materials and are readily transported in groundwater. 7.2 Conceptual Design 7.2.1 Source Removal — Excavation Excavation of ash at the CSS site began at the Units 1-4 inactive ash basin in October 2015. Approximately 423,600 tons of ash is planned to be removed from this ash basin and relocated to an on -site lined landfill. Excavation of ash will remove the primary source of groundwater contamination at the site, but will not eliminate the COI concentrations presently observed in groundwater beneath the site. As discussed with NCDEQ, after excavation of the Units 1-4 inactive ash basin, soils left on -site will be sampled and analyzed, and the analytical results will be incorporated into the groundwater contaminant fate and transport models. If this evaluation indicates that modification to the proposed CAP is required, Duke Energy will prepare and submit a revised CAP. 7.2.2 Source Removal — Cap -In -Place A cap -in -place scenario was evaluated as the closure option for the active ash basin, ash storage area, and Unit 5 inactive ash basin. Capping in place involves covering the ash disposal areas with an engineered cap. This closure option will minimize infiltration occurring at the capped areas and is expected to reduce the concentrations of COls observed in groundwater over time. The proposed corrective action plan calls for the Unit 5 inactive ash basin, ash storage area, and active ash basin to be covered with an engineered cap, which will eliminate infiltration through the covered area reducing possible impacts from potentially impacted soil. Therefore, remediation of soils is not discussed in this document. This option is subject to the E1. Corrective Action Plan Part 2 Cliffside Steam Station Ash Basin closure requirements of CAMA for closure depending on the final risk classification assigned by NCDEQ. 7.2.3 MNA 7.2.3.1 Demonstration of MNA The use of MNA as a corrective action involves the monitoring of select parameters to determine if COls are attenuating as a result of the corrective action. Once the ash within the source areas is removed or capped, groundwater quality will improve over time due to dilution from the recharge to groundwater, precipitation, and adsorption of COls. Tier I and II analyses were conducted for CSS (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 sampling was determined to be representative of material into and through which the COls migrate. The most significant finding was that precipitating iron and manganese was removing additional 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 observed in samples. 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 COls to exceed the 2L Standards, IMACs, or NCDHHS HSL at the Compliance Boundary in the model output. Groundwater to surface water interaction models have determined that even with over -prediction of COls to the Broad River, exceedances of the 2B Standards should not occur. 7.2.3.2 Verification of MNA The MNA monitoring program and data collection and evaluation to advance the Tier III assessment should be implemented throughout ash removal activities at the Units 1-4 inactive ash basin and prior to any cap -in -place activities. If MNA is demonstrated to effectively reduce COI concentrations, it should be maintained until water quality meets remedial objectives (e.g., 2L Standards/IMACs/NCDHHS HSLs/2B Standards or site -specific standards, as applicable) are met. The site monitoring requirements are discussed in Section 9.0. If the number of COls or COI concentrations are observed to increase during MNA monitoring, the effectiveness of MNA will be re-evaluated and additional remedial alternatives will be considered. If warranted, additional remedial alternatives will be implemented, as necessary. 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. 56 Corrective Action Plan Part 2 Cliffside Steam Station Ash Basin 8 Recommended Interim Activities Several interim activities will occur at the CSS site to address additional information needs identified in the CSA and CAP Part 1 Reports. Interim activities to be completed include the installation of additional monitoring wells and monitoring of groundwater during excavation activities. 8.1 Additional Well Installation Approximately 30 monitoring wells and four groundwater level observation wells are currently being installed to address additional information needs identified in the CSA Report and during meetings with NCDEQ. Locations of the additional monitoring wells are shown on Figure 8-1. The additional wells will be used to further refine the horizontal and vertical extent of groundwater impacts, refine the understanding of groundwater flow direction, and replace monitoring wells that previously had insufficient water to collect groundwater samples. The additional monitoring wells will be incorporated into the groundwater monitoring network in 2016 and sampled in conjunction with the existing monitoring wells described in Section 9. Review of the data will be used to refine the understanding of COI distributions and groundwater flow directions. 8.2 Additional Groundwater Sampling and Analyses 8.2.1 Units 1-4 Inactive Ash Basin Groundwater Monitoring Plan During Excavation Activities Duke Energy previously submitted to NCDEQ a groundwater monitoring plan to be used during excavation activities at the Units 1-4 inactive ash basin. The sampling plan includes procedures and details for the following: • Sampling and analysis of monitoring wells surrounding the Units 1-4 inactive ash basin • Installation of data loggers to monitor groundwater fluctuations in monitoring wells surrounding the Units 1-4 inactive ash basin • Bi-weekly monitoring of field parameters in monitoring wells GWA-11 S/BRU • Submittal of analytical results and groundwater measurements to NCDEQ Monitoring wells IB-2S-SL, I13-21, IB-2AL, I13-213RU, IB-4S-SL, 113 4D, and I13-413R have already been abandoned in accordance with NCAC T15A 2C .0113 to support decommissioning of the Units 1-4 inactive ash basin. The abandonment records are included in Appendix A. Additional monitoring wells may need to be abandoned as part of decommissioning activities. NCDEQ will be notified and approval will be obtained prior to the abandonment of additional monitoring wells. The Interim and Effectiveness Monitoring Plans associated with the Unit 5 inactive ash basin, active ash basin, ash storage area, and the Units 1-4 inactive ash basin are detailed in Section 9. These monitoring plans will be conducted in conjunction with the Units 1-4 inactive ash basin excavation activities groundwater monitoring plan. 57 Corrective Action Plan Part 2 Cliffside Steam Station Ash Basin 8.2.2 Groundwater Elevation Data Loggers In response to comments from NCDEQ, Duke Energy will install data loggers in select monitoring wells at the site to evaluate the potential influence of changes in stage in the Broad River on nearby groundwater elevations. These data loggers will be installed in wells AS-2S/D, GWA-21 S/BRU/BR, and GWA-22S/BRU. A data logger will also be installed at one upgradient location (GWA-6S/D) not influenced by the ash basin, ash storage areas, Suck Creek, or the Broad River to establish a baseline. Groundwater fluctuations will be summarized in monitoring reports described in Section 9. 58 Corrective Action Plan Part 2 Cliffside Steam Station Ash Basin 9 Interim and Effectiveness Monitoring Plans The Interim and Effectiveness Monitoring Plans (Monitoring Plans) provide detailed information regarding field activities to be performed during collection of groundwater, surface water, and AOW samples associated with the Units 1-4 inactive ash basin, the Unit 5 inactive ash basin, the active ash basin, and ash storage area at CSS. The Monitoring Plans are intended to evaluate the effectiveness of proposed corrective actions; monitor the movement of contaminants in groundwater during and after excavation of the CSS Units 1-4 inactive ash basin; and address the need to evaluate baseline conditions and seasonal variation in groundwater, surface water, and AOWs at the CSS site. These Monitoring Plans replace the monitoring plan provided in Section 16 of the CSA Report. Protocols for groundwater, surface water, and AOW 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 Work Plan, and 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 CSS 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 CSS site and evaluate seasonal trends associated with COIs. Monitor the movement of Cols within groundwater and the interaction with surface water and AOWs. • Determine seasonal groundwater flow direction and elevations throughout the CSS site and monitor potential changes to groundwater flow direction and elevation as the result seasonal variations and closure activities. The DQOs will be met through the following activities: Perform groundwater, surface water, ash basin water, and AOW sampling at the locations depicted on Figure 2-1 and in Table 9-1 through the first half of 2016. These monitoring events, planned for the first and second quarters of 2016, will be combined with analytical data from Rounds 1 through 4 (collected in 2015) to evaluate seasonal water quality conditions and background groundwater concentrations at the CSS site. 59 Corrective Action Plan Part 2 Cliffside Steam Station Ash Basin Additional assessment wells are being installed at the CSS site in the First Quarter 2016 and will be added to the interim monitoring network following installation. If monitoring indicates that excavation activities are mobilizing COls towards off -site receptors, more frequent sampling of select wells associated with the Units 1-4 inactive ash basin will be considered. Additional surface water sample locations from the Broad River are proposed to further evaluate the groundwater to surface water interaction. • Perform groundwater static water level measurements at 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 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 and AOW locations will be conducted at the CSS 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, ash basin water, and AOW 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 wells or removed as excavation of the Units 1-4 inactive ash basin leads to well abandonment. NCDEQ will be notified and NCDEQ's approval will be obtained prior to the abandonment of monitoring wells. AOWs S-3, S-10, S-11, and S-13 were retained for sampling; however, excavation of the Units 1-4 inactive ash basin may cause previously identified AOWs to become dry or express in other locations. Periodic inspection of AOWs within the CSS site should be conducted to determine if new AOWs have been established as a result of closure activities. If new AOWs are encountered, they should be added to the inventory and the sampling program. The AOW inventory will be conducted per the CSS Plan for Identification of New Discharges. Identification of new discharges (AOWs) and any associated sampling of the new discharge will be done in compliance with that document. 9.1.2.3 Analytical Requirements Analytes for monitoring wells, surface water, and AOWs include total and dissolved metals, alkalinity, calcium, chloride, hexavalent chromium, potassium, magnesium, nitrate, sodium, sulfate, total combined radium, total combined uranium, TDS, total organic carbon, and total 60 Corrective Action Plan Part 2 Cliffside Steam Station Ash Basin 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. Effectiveness Monitoring Plan The Effectiveness Monitoring Plan has been developed to monitor select wells for MNA parameters to provide baseline MNA data before and/or during closure activities for use with future MNA analysis. The Effectiveness Monitoring Plan will be implemented on 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. Note that ash removal from the Units 1-4 inactive ash basin will likely preclude accurate analysis of MNA processes in this area through excavation activities and basin closure. 9.2.1 Data Quality Objectives The following DQOs are associated with the Effectiveness Monitoring Plan • Monitor the effectiveness of MNA for the active ash basin, ash storage areas, the Unit 5 inactive ash basin and the Units 1-4 inactive ash basin. • Monitor changes in groundwater, surface water, ash basin water, and AOW COI concentrations as the result of excavation of the Units 1-4 inactive ash basin. • Monitor the movement of COls within groundwater and interaction with surface water and AOWs. • Monitor seasonal groundwater flow direction and elevations, and monitor potential changes to groundwater flow direction and elevation resulting from seasonal variations or closure activities. The DQOs will be met through the following activities: • Perform groundwater, surface water, ash basin water, and AOW sampling, including MNA parameters at select monitoring well, surface water, and AOW locations. • Perform groundwater static water level measurements of the monitoring wells concurrent with groundwater sampling described above. • Perform water level gauging at stream and surface water locations, if available. • Perform total depth measurements at monitoring wells on an annual basis. 61 Corrective Action Plan Part 2 Cliffside Steam Station Ash Basin • 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 CSS 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, surface water, and AOW locations. Results will also be evaluated to determine the need for increased frequency of sampling in the vicinity of the Units 1-4 inactive ash basin to monitor potential migration of COls as the result of excavation activities. 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 required at locations outside the MNA network to monitor site conditions. 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. 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 with the NPDES 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. 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 in Section 9.1. Changes to the analytical requirements may be proposed upon evaluation of the seasonal monitoring results obtained during the 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. 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. 62 Corrective Action Plan Part 2 Cliffside Steam Station Ash Basin During each monitoring event, these wells will be measured for static water levels. These measurements will be taken within one 24-hour period and prior to sampling to minimize 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 system 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 bailer or pump prior to the third sampling event. In addition, wells may be redeveloped if turbidity readings below 10 nephelometric turbidity units (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 will be considered for replacement. Each monitoring well will be inspected while performing water level measurements for damage to the casing, protective metal casings, 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 rubber seal is functioning properly. 9.3.2 Surface Water and AOW Measurements Stream stage measurements will be conducted at gauging locations along the Broad River and along Suck Creek. Each stage 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 All monitoring wells will utilize low flow (minimal drawdown) groundwater purging and sampling 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, 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, and temperature of groundwater have stabilized and the turbidity is below 10 NTU. 63 Corrective Action Plan Part 2 Cliffside Steam Station Ash Basin 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. 9.3.3.3 Surface Water and AOW Sample Collection Grab samples will be collected from each surface water and AOW location. Water quality parameters (pH, specific conductance, ORP, temperature, and turbidity) will be measured from each location. After water quality parameters have been collected and recorded, AOW and surface water samples will be collected by slightly submerging the lip of the sample container under the water surface. Samples collected to be analyzed for dissolved target analyte list metals 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 CSS is CS. • 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 CS-BG-1 D-NS-1 Q16. If a field duplicate was also collected from that location, it would be designated CS-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 CSS. Other investigation -derived waste, including disposable tubing and gloves, will be bagged and disposed of as part of the CSS site's municipal solid waste. 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 contract laboratory. 64 Corrective Action Plan Part 2 Cliffside 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. 65 Corrective Action Plan Part 2 Cliffside 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. Surface ash impoundment closure planning and implementation is ongoing at the CSS site and compliance groundwater monitoring has been conducted since 2011. 10.1 Implementation Cost The recommended corrective action at the CSS site following source control is MNA. A summary of costs for the recommended corrective action are provided in Table 10-1. Additional cost details are provided in Attachment G. Note that the actual costs will be highly dependent on the actual conditions that exist following excavation and completion of ash basin closure activities. Therefore, these values represent an estimate for reference purposes. Table 10-1 Estimated Capital and Annual Costs for Corrective Action - MNA Proposed Activity Total Capital Costs - Monitoring Well Installation Monitoring Well Installation (13 wells) $195,000 Site Preparation and Erosion and Sediment Control $30,000 Field Management (15%) $34,000 Well Installation Reporting $5,000 Project Management (10%) $27,000 Contingency (20%) $58,000 Total Capital Costs $349,000 Annual Costs - Monitoring/Reporting Lab Analysis $36,000 Data Validation $15,000 Reporting $60,000 Equipment and Expendables $9,000 Sampling Labor $72,000 Project Management (10%) $19,000 Escalation to Mid -Point (4%) $8,000 Annual Monitoring/Reporting Costs $219,000 Total Capital/Annual Costs for Project Duration* $9,270,000 "Note: this total project cost includes the annual cost over the project duration of 30 years with a 4.25% discount factor per year. 10.2 Implementation Schedule Interim activities, including advancement of the MNA Tier III assessment, will continue to be implemented at the Units 1-4 inactive ash basin during the excavation activities until closure activities have been completed. Groundwater, surface water, and AOW sampling associated with the Interim Monitoring Plan will be implemented at the CSS site through the Second Quarter 2016, as detailed in Section 9.1. Following the Second Quarter 2016 monitoring event, an MNA monitoring network will be 66 Corrective Action Plan Part 2 Cliffside Steam Station Ash Basin established for the Units 1-4 inactive ash basin, Unit 5 inactive ash basin, active ash basin, and ash storage area. The MNA network will be sampled concurrent with the NPDES compliance monitoring beginning in December 2016 and sampling will be conducted three times per year in accordance with the Effectiveness Monitoring Plan established in Section 9.2. Compliance monitoring events are conducted in April, August, and December. MNA will continue to be evaluated after closure activities are completed and subsurface conditions have stabilized using the results of the Interim and Effectiveness Monitoring. Should these results indicate that MNA successfully addresses residual impacts in groundwater underlying the CSS site, MNA monitoring will continue per the Effectiveness Monitoring Plan. Note that ash removal from the Units 1-4 inactive ash basin will likely preclude accurate analysis of MNA processes in this area through excavation and closure activities. 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. 67 Corrective Action Plan Part 2 Cliffside 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 EPRI (Electric Power Research Institute). 2006. "Groundwater Remediation of Inorganic Constituents at Coal Combustion Product Management Sites: Overview of Technologies, Focusing on Permeable Reactive Barriers." EPRI, Palo Alto, CA: 2006. 1012584 HDR. 2014a. Cliffside 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. Cliffside Steam Station — Ash Basin Supplement to Drinking Water Supply Well and Receptor Survey. [Online] URL: http://portal.ncdenr.org/web/wq/drinking-water- receptor-surveys HDR. 2015a. Comprehensive Site Assessment Report. Cliffside Steam Station Ash Basin. August 18, 2015. HDR. 2015b. CAP Part 1 Report. Cliffside Steam Station Ash Basin. November 16, 2015. ITRC (Interstate Technology and Regulatory Council). 2005. Permeable Reactive Barriers: Lessons Learned/New Directions. Interstate Technology and Regulatory Council, Permeable Reactive Barriers Team, PRB-4, Washington, D.C. http://www.itrcweb.org 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. Division of Waste Management - Guidelines for Performing Screening Level Ecological Risk Assessments within North Carolina. [Online] URL: http://portal.ncdenr.org/web/wm/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. 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Supplemental Guidance to ERAGS: Region 4, Ecological Risk Assessment. http://www2.epa.gov/sites/production/files/2015- 09/documents/r4_era_guidance_document_draft_final_8-25-2015. pdf USEPA (U.S. Environmental Protection Agency). 2015a. Water Sense Partnership Program. http://www3.epa.gov/watersense/pubs/indoor.htmls USEPA (U.S. Environmental Protection Agency). 2015a. USEPA Risk -Based Screening Levels. June 2015. [Online] URL: http://www2.epa.gov/risk/risk-based-screening-table-generic- tables USEPA (U.S. Environmental Protection Agency). 2015b. Final 2015 Updated National Recommended Human Health Water Quality Criteria. [Online] URL: http://water.epa.gov/scitech/swguidance/standards/criteria/current/hhfinal.cfm 69 This page intentionally left blank