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RBSS CAP Part 2_Report_FINAL_20160212F)l Corrective Action Plan Part 2 Riverbend Steam Station Ash Basin Site Location: NPDES Permit No. Permittee and Current Property Owner: Consultant Information Report Date: Riverbend Steam Station 175 Steam Plant Rd Mount Holly, NC 28120 NC0004961 Duke Energy Carolinas, LLC 526 South Church St Charlotte, NC 28202 704.382.3853 HDR Engineering, Inc. of the Carolinas 440 South Church St, Suite 900 Charlotte, NC 28202 704.338.6700 February 12, 2016 This page intentionally left blank Corrective Action Plan Part 2 Riverbend Steam Station Ash Basin i Contents Executive Summary ...................................................................................................................................... 3 1 Introduction .......................................................................................................................................... 7 1.1 Regulatory Background ............................................................................................................. 7 1.2 Report Organization .................................................................................................................. 9 2 Summary of Previous and Current Studies ....................................................................................... 10 2.1 Comprehensive Site Assessment ........................................................................................... 10 2.1.1 Identification of COIs .................................................................................................. 10 2.1.2 Soil Delineation .......................................................................................................... 11 2.1.3 Groundwater Delineation ........................................................................................... 11 2.2 Corrective Action Plan Part 1 .................................................................................................. 11 2.2.1 Proposed Provisional Background Concentrations for Soil and Groundwater .......... 12 2.2.2 COI Occurrence and Distribution ............................................................................... 12 2.3 Round 2 Sampling ................................................................................................................... 13 2.3.1 Groundwater ............................................................................................................... 13 2.3.2 Round 1 and Round 2 Source Area and Groundwater Data Comparison ................. 14 2.3.3 Surface Water and Areas of Wetness ........................................................................ 17 2.4 Round 3 and Round 4 Background Well Sampling ................................................................. 18 2.5 Well Abandonment .................................................................................................................. 18 3 Site Conceptual Model ...................................................................................................................... 19 3.1 Identification of Potential Contaminants .................................................................................. 19 3.2 Identification and Characterization of Source Contaminants .................................................. 19 3.3 Delineation of Potential Migration Pathways through Environmental Media .......................... 20 3.3.1 Soil.............................................................................................................................. 20 3.3.2 Groundwater ............................................................................................................... 21 3.3.3 Surface Water and Sediment ..................................................................................... 21 3.4 Establishment of Background Areas ....................................................................................... 22 3.5 Environmental Receptor Identification and Discussion ........................................................... 22 3.6 Determination of System Boundaries ...................................................................................... 23 3.7 Site Geochemistry and Influence on COIs .............................................................................. 23 4 Modeling ............................................................................................................................................ 26 4.1 Groundwater Model Refinement ............................................................................................. 26 4.1.1 Flow Model Refinements ............................................................................................ 26 4.1.2 Fate and Transport Model Refinements..................................................................... 27 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 ........................................................................................... 32 4.2.1 Methodology ............................................................................................................... 32 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 4.4 Refined Site Conceptual Model ............................................................................................... 36 Corrective Action Plan Part 2 Riverbend Steam Station Ash Basin ii 5 Risk Assessment ............................................................................................................................... 37 5.1 Step 1: Conceptual Site Model ................................................................................................ 37 5.2 Step 2: Risk-Based Screening ................................................................................................ 38 5.3 Step 3: Human Health Risk Assessment ................................................................................ 38 5.4 Step 4: Ecological Risk Assessment ....................................................................................... 39 6 Alternative Methods for Achieving Restoration ................................................................................. 42 6.1 Corrective Action Decision Process ........................................................................................ 42 6.1.1 Evaluation Criteria ...................................................................................................... 42 6.1.2 COIs Requiring Corrective Action .............................................................................. 43 6.1.3 Potential Exposure Routes and Receptors ................................................................ 43 6.2 Alternative Evaluation Criteria ................................................................................................. 43 6.2.1 Effectiveness .............................................................................................................. 44 6.2.2 Implementability/Feasibility ........................................................................................ 44 6.2.3 Environmental Sustainability ...................................................................................... 45 6.2.4 Cost ............................................................................................................................ 45 6.2.5 Stakeholder Input ....................................................................................................... 45 6.3 Remedial Alternatives to Achieve Regulatory Standards ....................................................... 46 6.3.1 Groundwater Remediation Alternatives ..................................................................... 46 6.3.2 Monitored Natural Attenuation Applicability to Site .................................................... 47 6.3.3 Site-Specific Alternatives Analysis ............................................................................. 48 6.3.4 Site-Specific Recommended Approach ..................................................................... 49 7 Selected Corrective Action(s) ............................................................................................................ 50 7.1 Selected Remedial Alternative for Corrective Action .............................................................. 50 7.2 Conceptual Design .................................................................................................................. 50 7.2.1 Source Removal – Excavation ................................................................................... 50 7.2.2 Monitored Natural Attenuation ................................................................................... 51 8 Recommended Interim Activities ....................................................................................................... 52 8.1 Well Installation ....................................................................................................................... 52 8.2 Additional Groundwater Sampling and Analyses .................................................................... 52 9 Interim and Effectiveness Monitoring Plans ...................................................................................... 53 9.1 Interim Monitoring Plan ........................................................................................................... 53 9.1.1 Data Quality Objectives .............................................................................................. 53 9.1.2 Sampling Requirements ............................................................................................. 54 9.1.3 Reporting .................................................................................................................... 54 9.2 Effectiveness Monitoring Plan ................................................................................................. 55 9.2.1 Data Quality Objectives .............................................................................................. 55 9.2.2 Sampling Requirements ............................................................................................. 55 9.2.3 Reporting .................................................................................................................... 56 9.3 Sampling and Analysis ............................................................................................................ 56 9.3.1 Monitoring Well Measurements and Inspection ......................................................... 56 9.3.2 Sample Collection ...................................................................................................... 57 9.3.3 Quality Assurance/Quality Control ............................................................................. 58 10 Implementation Cost and Schedule .................................................................................................. 60 10.1 Implementation Cost ............................................................................................................... 60 10.2 Implementation Schedule ........................................................................................................ 60 11 References ........................................................................................................................................ 62 Corrective Action Plan Part 2 Riverbend Steam Station Ash Basin iii Tables 2-1 Summary of Horizontal Hydraulic Gradient Calculations 2-2 Comparison of 0.45 Micron and 0.1 Micron Filter Sample Results - Groundwater 2-3 Ash Porewater Analytical Results – Round 1 and Round 3 2-4 Ash Basin Water Analytical Results – Round 1 and Round 3 2-5 Groundwater Results within Waste Boundary 2-6 Background Groundwater Analytical Results – Rounds 1, 2, 3, and 4 2-7 Groundwater Outside the Waste Boundary Analytical Results – Round 1 and Round 2 2-8 Constituents of Interest Evaluation 2-9 Surface Water and Areas of Wetness Sample Analytical Results – Round 1 and Round 2 4-1 Summary of Modeled COI Results at the Compliance Boundary* 4-2 Mountain Island Lake Surface Water Concentrations* 4-3 East Basin Surface Water Concentrations* 9-1 Interim Monitoring Plan Sample Locations 9-2 Sampling Parameters and Analytical Methods 10-1 Estimated Capital and Annual Costs for Corrective Action - MNA * * Table is presented in the text of this CAP Part 2 Report; all other tables are attached separately Figures 2-1 Groundwater, Surface Water, and Area of Wetness Sampling Locations Map 2-2 Potentiometric Surface Map – Shallow Flow Layer 2-3 Potentiometric Surface Map – Deep Flow Layer 2-4 Potentiometric Surface Map – Bedrock Flow Layer 3-1 Site Conceptual Model – 3D Representation 3-2 Site Conceptual Model Cross Sectional 3-3 Receptor Map 3-4 Site Vicinity Map 3-5 Water Supply Intake Locations 8-1 Additional Assessment Wells Appendices A CSA Supplement 1 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 G Evaluation of Potential Groundwater Remedial Alternatives H Monitored Natural Attenuation Technical Memorandum Note this hard copy includes the report portion of appendices only. Complete appendices with all attachments are provided on the accompanying CAP Part 2 CD. Corrective Action Plan Part 2 Riverbend Steam Station Ash Basin iv Acronyms and Abbreviations µg/L micrograms 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 Standards AOW area of wetness BERA baseline ecological risk assessment BG background BR bedrock CAMA North Carolina Coal Ash Management Act of 2014 CAP corrective action plan CCR coal combustion residuals COI constituent of interest COPC constituent of potential concern CSA comprehensive site assessment CSM Conceptual Site Model D deep DO dissolved oxygen DQO data quality objective Duke Energy Duke Energy Carolinas, LLC EPC exposure point concentration HAO hydrous aluminum oxide HFO hydrous ferric oxide HQ hazard quotient HSL health screening level IMAC interim maximum allowable concentration Kd sorption coefficient MNA monitored natural attenuation MW monitoring well NCAC North Carolina Administrative Code NC PSRGs North Carolina Preliminary Soil Remediation Goals NCDENR North Carolina Department of Environment and Natural Resources NCDEQ North Carolina Department of Environmental Quality NCDHHS North Carolina Department of Health and Human Services NPDES National Pollutant Discharge Elimination System NTU Nephelometric Turbidity Unit POG protection of groundwater PPBC proposed provisional background concentrations RBC risk-based concentrations RBSS Riverbend Steam Station RMS root mean squared S shallow SCM site conceptual model SU standard units TDS total dissolved solids USEPA U.S. Environmental Protection Agency Work Plan Groundwater Assessment Work Plan Corrective Action Plan Part 2 Riverbend Steam Station Ash Basin 1 Acknowledgments HDR Engineering, Inc. of the Carolinas would like to express its appreciation to Duke Energy Carolinas, LLC for its guidance 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 Riverbend Steam Station Ash Basin 2 This page intentionally left blank Corrective Action Plan Part 2 Riverbend Steam Station Ash Basin 3 Executive Summary The North Carolina Coal Ash Management Act of 2014 (CAMA) directs owners of coal combustion residuals (CCR) surface impoundments in North Carolina to conduct groundwater monitoring, assessment, and remedial activities, if necessary. A Groundwater Assessment Work Plan (Work Plan) for the Riverbend Steam Station (RBSS) was submitted to the North Carolina Department of Environment and Natural Resources (NCDENR 1) 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 RBSS 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 the 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 and Part 2 being submitted no later than 180 days after submittal of the CSA. The RBSS CAP Part 1 was submitted to NCDEQ on November 16, 2015. Based on the “Coal Combustion Residual Impoundment Risk Classifications” report published by NCDEQ in January 2016, CAMA has identified RBSS as a high priority site. A public meeting regarding the proposed risk classifications is scheduled for March 1, 2016. NCDEQ will release final risk classifications after review of public comments. Duke Energy owns and formerly operated the RBSS, located on the Catawba River/Mountain Island Lake in Gaston County near Mount Holly, North Carolina. RBSS began operation as a coal-fired generating station in 1929 and was retired from service in April 2013. Decommissioning of RBSS is ongoing. After 1957, following installation of precipitators and a wet sluicing system, CCR generated at the site was disposed in the station’s ash basin located adjacent to the station and Mountain Island Lake. Discharge from the ash basin is permitted under the National Pollutant Discharge Elimination System (NPDES) Permit NC0004961. Groundwater at this site flows to the north/northwest from the Primary and Secondary Cells of the ash basin, ash storage area, and cinder storage area toward Mountain Island Lake. The groundwater flow direction is away from the direction of the nearest public or private water supply wells. The Catawba River/Mountain Island Lake serves as the primary hydrologic discharge feature for groundwater within the shallow, deep, and bedrock layers at the site. No drinking water wells are located within the downgradient Compliance Boundary of RBSS. 1 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 Riverbend Steam Station Ash Basin 4 Based on results of the CSA, concentrations of constituents of interest (COIs)2 attributable to the CCR source areas at the RBSS site are beneath the ash basin, ash storage area, and west of the cinder storage area. COI transport from the source areas is generally in a northerly direction towards the Catawba River/Mountain Island Lake. COIs in groundwater that are attributable to ash handling at the RBSS site are antimony, arsenic, boron, chromium, cobalt, hexavalent chromium, iron, manganese, sulfate, total dissolved solids (TDS), thallium, and vanadium. Chromium, cobalt, iron, manganese, and vanadium were found to be naturally occurring constituents in groundwater across the site. Further sampling and analysis are necessary to determine if COI exceedances are the result of source-related impacts or are from naturally occurring conditions (as discussed in Section 2). The refined groundwater model predicts that several COIs exceed regulatory standards at the Compliance Boundary as discussed in Section 4.1.5; however, based on results of the groundwater to surface water modeling, no water quality standards or criteria are exceeded at the edge of the mixing zones in Mountain Island Lake and the East Basin. A human health and ecological risk assessment was conducted as part of this CAP. The ecological risk assessment indicates that potential risks are above risk targets for several constituent of potential concern for some water-dependent mammals and birds. 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 COIs, and refinement of the exposure and toxicity assumptions used in the ecological risk characterization. The human health risk assessment indicates that potential risks are above risk targets for the recreational fisher and subsistence fisher from ingestion of fish caught near the site. Similar to the ecological risk assessment, additional data regarding site-specific conditions, delineation of source-related and background COIs to the risk assessment results, and the evaluation of the exposure parameters and fish ingestion models used in the risk assessment are needed to address these results. Duke Energy is actively excavating ash at the RBSS site. Excavated ash will be used beneficially off-site or will be relocated to a new off-site lined landfill. An evaluation of site conditions, consituents, and a review of alternative methods for restoring groundwater quality found that, in conjunction with source removal at the RBSS 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 RBSS 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 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) (2B Standards), or U.S. Environmental Protection Agency National Recommended Water Quality Criteria, it has been designated as a “constituent of interest”. Corrective Action Plan Part 2 Riverbend Steam Station Ash Basin 5 water interaction, and geochemical models. The monitoring results will also be used to confirm natural attenuation continues to occur and remains an effective corrective action for the RBSS site. If MNA is deemed insufficient for restoration of groundwater quality, other alternatives discussed in Section 6.3.3 will be evaluated and, if warranted, implemented to augment MNA. The performance of these remedial alternatives will continue to be monitored and evaluated to determine if modifications to the measures are required. Per CAMA, "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 Riverbend Steam Station Ash Basin 6 This page intentionally left blank Corrective Action Plan Part 2 Riverbend Steam Station Ash Basin 7 1 Introduction Duke Energy Carolinas, LLC (Duke Energy) owns and formerly operated the Riverbend Steam Station (RBSS) located adjacent to the Mountain Island Lake portion of the Catawba River (Mountain Island Lake) near Mount Holly, Gaston County, North Carolina. RBSS began operation as a coal-fired generating station in 1929 and was retired from service in April 2013. Decommissioning of RBSS is ongoing. From 1929 to 1957, coal combustion residuals (CCR) from RBSS’s coal combustion process were dredged from the primary basin to the ash storage area, where it then decanted back to the primary pond area, leaving behind ash. Following installation of precipitators and a wet sluicing system in 1957, CCR was disposed in the station’s ash basin located adjacent to the station and Mountain Island Lake. Discharge from the ash basin is currently permitted under North Carolina Department of Environment Quality (NCDEQ)3 Division of Water Resources under the National Pollutant Discharge Elimination System (NPDES) Permit NC0004961. 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 RBSS was submitted to 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) 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 RBSS 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 Subchapter L of Chapter 2 of Title 15A of the North Carolina Administrative Code (T15A NCAC 02L) and requires the submittal of a corrective action plan (CAP) for each regulated facility no later than 180 days after submittal of the CSA. Duke Energy and NCDEQ mutually agreed to a two-part CAP submittal, with Part 1 being submitted within 90 days of submittal of the CSA and Part 2 being submitted no later than 180 days after submittal of the CSA. The RBSS CAP Part 1 Report (HDR 2015b) was submitted to NCDEQ on November 16, 2015 and consisted of the following: • background information • a brief summary of the CSA findings • a brief description of the site geology and hydrogeology • a summary of the previously completed receptor survey • a summary of constituent of interest (COI) exceedance and distribution 3 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 Riverbend Steam Station Ash Basin 8 • development of proposed provision background concentrations (PPBCs) for soil and groundwater • a 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, of surface water quality standards, and of sample results greater than interim maximum allowable concentrations (IMACs) and North Carolina Department of Health and Human Services (NCDHHS) health screening levels (HSLs) • a refined SCM • revised groundwater flow and fate and transport model results • revised groundwater to surface water model results • site geochemical model results • findings of the risk assessment • evaluation of methods for achieving groundwater quality restoration • conceptual plan(s) for recommended proposed corrective action(s) • a schedule for implementation of the proposed corrective action(s) • a plan for monitoring and reporting on the effectiveness of the proposed corrective action The inf ormation provided in the combined CAP Part 1 and CAP Part 2 meets the requirements of regulation 15A NCAC 02L .0106 (f) for corrective action. As required by CAMA, Duke Energy plans to excavate the primary source, which is the coal ash contained in the ash basin (Primary and Secondary cells). In addition, Duke Energy is removing the ash stored in the ash storage area and cinder storage area. Excavated and removed material will either be used beneficially off-site or will be relocated to a new off-site lined landfill. Regulation 15A NCAC 02L .0106 (f)(4) requires that the secondary sources, which would be potential continuing sources of possible pollutants to groundwater, be addressed in the CAP. At the RBSS site, the soil located below the ash basin 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, 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. Based on the NCDEQ January 2016 Report, “Coal Combustion Residual Impoundment Risk Classifications, CAMA has identified RBSS as a high priority site. A public meeting regarding the proposed risk classifications is scheduled for March 1, 2016. NCDEQ will release final risk classifications after review of public comments. However, regardless of the remaining public meetings, RBSS is considered “High Priority” by CAMA and therefore excavation of the ash management area is required. Corrective Action Plan Part 2 Riverbend Steam Station Ash Basin 9 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 RBSS site and the intent of corrective action under CAMA. • Section 2 provides a summary of the CSA and CAP Part 1 reports and comparison of Round 1 and Round 2 groundwater, surface water, and area of wetness (AOW) sampling results and a summary of Round 3 and Round 4 background well sampling results. • Section 3 discusses the SCM and site geochemical controls on contaminant mobility. • Section 4 discusses the purpose, methodologies, and results of refined groundwater, groundwater to surface water interaction, and geochemical modeling. Refinement of the SCM following evaluation of the model results is also discussed in this section. • Section 5 summarizes 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 Riverbend Steam Station Ash Basin 10 2 Summary of Previous and Current Studies This section presents a summary of previous and current studies including the following: • Summary of CSA; • Summary of CAP Part 1; • Presentation of Round 2 groundwater sampling results and data comparison to Round 1 sampling; and • 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 RBSS 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 boundary and Compliance Boundary4. • Evaluation of laboratory analytical data to supplement the SCM. • Update of the receptor survey previously completed in September 2014 (updated November 2014). • Completion of a screening-level risk assessment. Note that subsequent to submittal of the CSA Report, 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 5, NCDHHS HSLs (hexavalent 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. 5 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 Riverbend Steam Station Ash Basin 11 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 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 COIs in the RBSS site CSA: • Soil: arsenic, boron, cobalt, iron, manganese, nickel, selenium, and vanadium (CSA Table 8-4). • Groundwater: antimony, arsenic, boron, chromium 6, cobalt, iron, manganese, sulfate, total dissolved solids (TDS), thallium, and vanadium (CSA Table 10-8). • Surface water: aluminum, cadmium, chromium, cobalt, copper, iron, lead, manganese, selenium, thallium, vanadium, and zinc (CSA Table 9-1). 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 primary and secondary ash basins, the ash storage area, and the cinder storage area 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 was delineated during the CSA activities with the following areas requiring refinement: • Horizontal and vertical extent to the west of the ash and cinder storage areas • Horizontal and vertical extent outside the northeast boundary of the ash basin Additional groundwater monitoring wells are in the process of being installed to delineate these areas. Based on the results of the CSA, eight additional assessment monitoring wells, one replacement well, and six new background monitoring wells are currently being installed. The additional background wells are located hydraulically upgradient of the ash management area. Data obtained from these wells will be used to increase the understanding of background conditions at RBSS and determine naturally occurring concentrations of COIs. 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 the CSA findings, evaluate background conditions by calculating PPBCs, evaluate exceedences per sample with regard to PPBCs, develop a refined SCM, and present preliminary results of results of the groundwater flow and transport model and groundwater to surface water model. 6 Unless otherwise noted, references to chromium in this document indicate total chromium. Corrective Action Plan Part 2 Riverbend Steam Station Ash Basin 12 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 COIs 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 concentrations for corrective action. Findings of the RBSS CSA indicate that some of the previously installed wells thought to represent background data were determined to be in locations downgradient or side gradient of the source areas and may not represent background conditions for designation of naturally occurring COIs. During CAP Part 1, additional background wells were recommended to develop a more complete understanding of the naturally occurring concentrations of COIs in groundwater. For the purpose of the CAP and for consistency with the CSA, only the background wells identified in the CSA were used to develop RBSS background soil and groundwater concentrations. Further refinement of the PPBCs is anticipated following the completion of additional background well sampling events in 2016. 2.2.2 COI Occurrence and Distribution The following soil COIs for the RBSS site were identified in the CSA: arsenic, boron, cobalt, iron, manganese, nickel, selenium, and vanadium as referenced in the CSA Table 8-4. Outside the waste boundary, the primary soil COIs are cobalt, iron, vanadium, and manganese. Selenium occurs in localized areas outside the waste boundary. Arsenic is located only within the waste boundary. Due to their widespread distribution, cobalt, iron, manganese, and vanadium have naturally occurring concentrations that may exceed the soil (NC PSRGs for POG) regulatory criteria. The following groundwater COIs for the RBSS site were identified in the CSA: antimony, arsenic, boron, chromium, cobalt, iron, manganese, sulfate, thallium, TDS, and vanadium as referenced in the CSA Table 10-6 and Table 10-8. Hexavalent chromium was added as a groundwater COI in CAP Part 1. Arsenic, boron, and thallium were identified within the waste boundary and planned ash basin excavation area that will be dewatered. Arsenic and boron exceeded the 2L Standard and thallium exceeded the IMAC in only one groundwater sampling location each during Round 1 sampling. Each of the locations is within the planned excavation area. The absence of these COIs in remaining areas at the RBSS site suggests that either their presence in ash is limited, or that these COIs are relatively immobile due to geochemical properties and processes. • Hexavalent chromium was identified within the waste boundary and primarily west and northwest of the waste boundary. The highest concentration was located in the ash storage area. CSA background well speciation sampling for hexavalent chromium was not performed during Round 1 sampling. • Sulfate and TDS concentrations support the conclusions made in the CSA and CAP Part 1 that further evaluation in the vicinity of the cinder storage area is recommended. Corrective Action Plan Part 2 Riverbend Steam Station Ash Basin 13 • Antimony, chromium, cobalt, iron, manganese, vanadium, and TDS exceeded their respective 2L Standards or IMACs in at least one existing or CSA-identified background monitoring well (MW -7D, MW -7SR, MW -7BR, BG-1S, and BG-1D). With the exception of TDS, it is likely that these COIs are in part related to natural background conditions and will be evaluated further after the installation of additional background wells. 2.3 Round 2 Sampling Round 2 groundwater, surface water, and AOW sampling activities were completed between September 4 and 19, 2015. Groundwater analytical parameters and methods for Round 2 were consistent with those employed for Round 1 in accordance with low flow sampling procedures described in the CSA Report. The following subsections provide a comparison of Round 1 and Round 2 groundwater flow and analytical results. 2.3.1 Groundwater A total of 83 monitoring wells were sampled during the Round 2 sampling event including 49 groundwater assessment wells, 24 source area wells, and 10 CSA identified background wells. Monitoring well locations are depicted on Figure 2-1. 2.3.1.1 Groundwater Water Levels On September 15, 2015, all 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 based on Round 2 data for the shallow, deep, and bedrock flow layers are depicted on Figures 2-2 through 2-4, respectively. Groundwater elevations measured during the Round 2 water level gauging event were generally lower than those measured during the Round 1 event; this is likely attributable to seasonal variations of the water table. Groundwater flow directions based on Round 2 data are consistent with flow directions identified during Round 1 water level gauging event documented in the CSA Report. 2.3.1.2 Horizontal and Vertical Gradients Horizontal hydraulic gradients were derived using the Round 2 groundwater elevations for the shallow, deep, and bedrock flow layers by calculating the difference in hydraulic heads over the length of the flow path between two wells with similar well construction (e.g., both wells having 15-foot screens within the same water-bearing unit). Monitoring wells, groundwater elevations, and length of flow paths used for horizontal hydraulic conductivity calculations are detailed in Table 2-1. The average horizontal hydraulic gradients for Round 2 compared to Round 1 are provided below. • Shallow: Round 2 – 0.043 feet/foot; Round 1 – 0.032 feet/foot • Deep: Round 2 – 0.031 feet/foot Round 1 – 0.028 feet/foot • Bedrock: Round 2 – 0.021 feet/foot Round 1 – 0.032 feet/foot Minor fluctuations were observed in the shallow and bedrock flow layers and may be attributable to seasonal variations, but in general, horizontal hydraulic gradients were consistent with those documented in the CSA Report. Corrective Action Plan Part 2 Riverbend Steam Station Ash Basin 14 Vertical hydraulic gradients were calculated in Round 2 for 11 shallow and deep well pairs and two deep and bedrock well pairs by taking the difference in groundwater elevation in each well pair over the difference in mid-well screen of each well pair. 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 and are summarized below. A downward gradient between the shallow and deep zones generally exists across the Site. An upward gradient was identified along the northern and eastern perimeter of the waste boundary. This gradient is produced because the elevation of the water in the basin is 52 feet higher than the ground surface at the base of the dike. This creates a pressure head resulting in an upward vertical gradient at the wells downgradient of the dikes. Following excavation, this pressure head will be reduced with removal of the water in the basin and it is anticipated that the vertical gradient will be reduced. The vertical gradients between the deep and bedrock zones exhibited a downward trend within the ash basin from the AB-3 monitoring well pair and a slight upward trend was exhibited east of the ash basin from the GWA-7 well pair. 2.3.1.3 Groundwater Sampling Groundwater samples were collected using low flow sample collection techniques, as outlined in the Low Flow Sampling Plan developed for the Ash Basin Groundwater Assessment Program and approved by NCDENR (CSA Report Appendix G). Field water quality parameters were measured at sampling. 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). A 0.45-micron filter was used for collection of groundwater samples for dissolved concentration analysis. During Round 2, additional sample volume was collected at select locations using a 0.1-micron filter and analyzed for dissolved constituent concentration along major flow paths and at locations with constituent concentrations that may be affected by turbidity. The following monitoring wells were sampled using the 0.1-micron filter. • AB-2D • AB-7D • C-2S • GWA-21S • MW -9D • AB-2S • AS-7I • C-2D • GWA-23BR • MW -9BR • AB-3BR • AS-2D • GWA-6S • GWA-23D • GWA-21D • AB-3D • AS-2S • GWA-6D • GWA-23S • GWA-21BR • AB-5D • AS-3D • GWA-9S • MW -1D • AB-6S • AB-6BRU • AS-3SA • GWA-9D • MW -1S Based on review of Round 2 analytical results, minimal to no differences in concentrations were observed in the samples collected with the 0.45-micron and 0.1-micron filters. The analytical results comparing the 0.45-micron and 0.1-micron filters are presented in Table 2-2. 2.3.2 Round 1 and Round 2 Source Area and Groundwater Data Comparison Round 1 and Round 2 sample results are presented in Tables 2-3 through 2-7. 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. Corrective Action Plan Part 2 Riverbend Steam Station Ash Basin 15 2.3.2.1 Source Area Results Ash Porewater The ash basin is a permitted wastewater treatment facility and water in the basin is wastewater not groundwater. The comparison to 2L Standards, IMACs, and NCDHHS HSLs is for comparison purposes only. Ash porewater samples were collected in Round 1 and Round 2 from locations within the source area. Fluctuations in the total number of COIs reported at individual wells were noted when comparing Round 1 and Round 2. No strong correlation can be made between turbidity and the number of COIs exceeding the 2L Standard, IMACs, and NCDHHS HSLs. In some cases turbidity increased and the number of COIs decreased. Ash porewater sample results from Round 1 and 2 are presented in Table 2-3. Ash Basin Water The ash basin is a permitted wastewater treatment facility and water in the basin is classified in the NPDES permit as wastewater, not groundwater. 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 water samples (SW-1 and SW -2) were collected from within the ash basin Secondary Cell in Round 1. These two locations were not sampled during Round 2, but were sampled in November 2015 during Round 3. Aluminum, antimony, arsenic, chromium, cobalt, iron, lead, manganese, nickel, thallium, and zinc concentrations exceeded their respective 2B Standards, 2L Standards, or IMACs in at least one of the two water samples collected from the ash basin Secondary Cell. Ash basin water sample results from Round 1 and 3 are presented in Table 2-4. Groundwater within the Waste Boundary A total of 24 monitoring wells were sampled within the waste boundary during the Round 2 sampling event. The following six wells were sampled during the Round 1 event but were not sampled during Round 2 due to active excavation activities: AB-3S, AB-7S, AB-4S, AB-5S, AB- 5SL, and C-1S. In general, the Round 2 groundwater concentrations within the waste boundary are stable when compared to the Round 1 data with some COIs exhibiting increasing and decreasing concentrations. The TDS concentrations in Round 2 decreased from Round 1 concentrations in most of the monitoring wells installed in the ash storage area and the ash basin area. The most significant decrease in TDS was exhibited in monitoring well AS-3SA, which decreased from 519,000 µg/L in Round 1 to 92,000 µg/L in Round 2. However, the Round 2 TDS concentrations in ash basin well AB-2S increased from 30,000 µg/L to 197,000 µg/L but did not exceed the 2L standard. Other COI concentrations that increased in Round 2 include manganese in ash basin area wells AB-2S and AB-6S; and chromium and iron in cinder storage area well C-2S. The Round 2 concentrations of these COI exceeded their respective 2L standards (Table 2-5). Corrective Action Plan Part 2 Riverbend Steam Station Ash Basin 16 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). Differences in the total number of exceedances and COIs reported at individual wells were noted when comparing Round 1 to Round 2. Additional background wells are being installed and will be sampled starting in spring 2016. In general, background monitoring wells exhibited similar concentrations and constituents when comparing data from the Round 1 and 2 sampling events. Background monitoring wells were also sampled during Round 3 and Round 4. The results of these sampling events are discussed in Section 2.4. Background monitoring wells will continue to be sampled and PPBCs will be refined as the data set increases with additional sampling rounds. Areas Outside of the Waste Boundary This section provides a brief summary of groundwater analytical results outside the waste boundary. The waste boundary is designated by the blue line that surrounds the ash management area as presented on Figure 2-1. HDR collected groundwater samples from 49 monitoring wells located outside the waste boundary during Round 2 sampling (Table 2-7). The wells are installed either within the shallow, deep or bedrock flow layers. The following COIs exceeded the 2L standards or IMACs in groundwater samples collected from wells outside the waste boundary in Rounds 1 and 2: antimony, chromium, cobalt, iron, manganese, vanadium, thallium and TDS. This list of COIs is consistent with the COIs that were reported in CAP Part 1. Most of the shallow wells exhibited detections of cobalt, manganese, and vanadium in Round 1 and 2 sampling along with detections of iron in Round 2. Iron and manganese were not detected in shallow wells MW -5S and MW -6, which are located to the northeast of the waste management area. Vanadium concentrations exceeded the IMAC in all shallow, deep and bedrock wells except shallow wells to the north, east, and south of the waste management area (MW -1S, GWA-8S, GWA-10S, GWA-21S, GWA-22S, and GWA-23S). 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, COIs identified during the Round 1 and Round 2 sampling events were evaluated to determine if they are naturally occurring or attributable to ash handling at the site. Results of the COI evaluation are provided in Table 2-8. Only analytical results which exceed their respective groundwater criteria are presented in this table. Corrective Action Plan Part 2 Riverbend Steam Station Ash Basin 17 • Where the exceedance is less than the applicable PPBC, the cell is highlighted green. The analytical results associated with the green highlighting are likely exceedances considered attributable to natural background conditions. • Where the COI concentrations are greater than the applicable groundwater standard and/or PPBC and the cell is highlighted orange. The analytical results associated with the orange highlighted cells are considered to be exceedances associated with ash handling at the site. It is important to note that this evaluation only includes two sampling events and additional sampling is needed to re-evaluate PPBCs and more appropriately assess COIs compared to PPBCs at the site. Areas of exceedances of COIs attributable to ash handling at the RBSS site are depicted on Figure 3-1. 2.3.3 Surface Water and Areas of Wetness As part of the CSA activities AOW and surface water samples were collected at RBSS. Three AOW locations (S-2, S-9, and S-11) were sampled during both Round 1 and 2. As reported in CAP Part 1, during the CSA locations S-4, S-6, S-7, and S-8 were originally identified as AOW samples. As discussed in CAP Part 1 Report, Section 2.4, due to the proximity of these locations relative to Mountain Island Lake, these four AOW locations were compared to the North Carolina surface water standards as specified in the 2B Standards and as amended effective January 2015. These four locations are discussed as surface water samples in this report. The results from the remaining AOW sample locations were compared to 2L Standards. AOW s reported in CAP Part 1, cobalt, hexavalent chromium, iron, manganese, and vanadium were identified as COIs in AOW s during Round 1 sampling. These COIs, except hexavalent chromium, also exceeded their respective 2L Standards or IMACs in most sample locations during Round 2. In addition, TDS exceeded the 2L Standard in AOW sample S-2. The TDS concentration in sample S-2 increased from 195,000 µg/L in Round 1 to 51,600,000 µg/L in Round 2. Water flows discharging from AOW s were measured by field personnel at several AOW locations during April 2014 and November 2015. In general, the AOW flows in November 2015 were lower than the flows measured in April 2014, although the flow at S-2 increased slightly. AOW S-7 exhibited the greatest decrease in flow from 23 gallons per minute in April 2014 to 7.6 gallons per minute in November 2015. In general, the COIs exhibited higher concentrations during the lower water flow rates, and lower concentrations during greater water flow rates. AOW locations S-3, S-5 and S-12 exhibited no flow during the November 2015 monitoring event. The fluctuating flow rates are likely attributed to seasonal trends in precipitation and water surface runoff infiltrating into the subsurface. Four AOW samples (S-4, S-6, S-7, and S-8) and surface water sample SW-3 were collected during the Round 1 and Round 2 sampling events. Surface water locations RBSW001 and RBSW002 were only sampled during the Round 1 event. The surface water samples in Round 1 exhibited exceedances of 2B Standards for aluminum, chromium, cobalt, cadmium, copper, iron, lead, manganese, selenium, thallium, vanadium and zinc. Concentrations of COIs in Corrective Action Plan Part 2 Riverbend Steam Station Ash Basin 18 Round 2 samples were higher than Round 1. Additional COIs exceeded their respective 2B standards during Round 2, including arsenic, barium, and nickel. The majority of these COIs were from sample locations S-6 and S-7 located near Mountain Island Lake northeast of the primary ash basin. Round 1 and Round 2 analytical results for the surface water and AOW samples are presented in Table 2-9. Following the removal of the source areas from RBSS and final grading as part of site restoration, the current AOWs may be eliminated, modified, or remain as surface expressions of groundwater in their current locations. Interim and Effectiveness Monitoring outlined in Section 9.0 recommends further inventory, identification and monitoring, if necessary, of AOWs. 2.4 Round 3 and Round 4 Background Well Sampling In response to a Duke Energy request for clarification of guidance, NCDEQ provided a table titled “Clarification of Attachment 1 Groundwater Assessment Plan Conditional Letters of Approval Items Related to Speciation – May 22, 2015” by electronic mail. In the responses provided in this table, NCDEQ requested that Duke Energy “plan to sample the existing and newly installed background wells two (2) additional times during 2015 as part of an anticipated corrective action measure to support USEPA tiered site analysis and statistical analysis”. The two additional sampling events referenced in this response correspond to background sampling Round 3 and Round 4, performed in November and December 2015, respectively. The groundwater analytical parameters and methods are detailed in CSA Report Table 7-3. Groundwater samples were collected in accordance with sampling procedures described in the CSA. The results of the Round 3 and Round 4 background well sampling event are presented in Table 2-5. Further evaluation of background sample results and PPBCs will be provided in subsequent reports. 2.5 Well Abandonment Due to source removal several monitoring wells within the source area were abandoned. Monitoring wells AS-1S, AS-1D, AS-2D, AS-3D and AS-3SA have been abandoned. Well abandonment forms can be found in Appendix A, Attachment 4. Corrective Action Plan Part 2 Riverbend Steam Station Ash Basin 19 3 Site Conceptual Model The SCM was initially presented in the CSA, and refined based on results from additional sampling events and refined groundwater modeling. The SCM for RBSS was developed in general accordance with ASTM International standard guidance document E1689-95 (Reapproved 2014), Standard Guide for Developing Conceptual Site Models for Contaminated Sites (ASTM 2014), to describe and integrate processes that determine contaminant releases, contaminant migration, and environmental receptor exposure to contaminants. The SCM is used to integrate site information and determine whether additional information may be needed to further understand site hydrogeologic and potential contaminant migration processes. The model is also used to support selection of remedial alternatives and effectiveness of remedial actions in reducing the exposure of environmental receptors to contaminants. The SCM was developed using the six basic activities outlined in ASTM E1689-95: • Identification of potential contaminants; • Identification and characterization of the source(s) of contaminants; • Delineation of potential migration pathways through environmental media; • Establishment of background areas; • Environmental receptor information; and • Determination of system boundaries. An expanded discussion of site geochemical controls on contaminant mobility and migration is also provided in this section, as requested by the NCDEQ. A graphical representation of the SCM is included as Figure 3-1. 3.1 Identification of Potential Contaminants Potential contaminants (COIs) 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 RBSS are defined as the ash basin (Primary and Secondary cells), ash storage area, and the cinder storage area (see 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 AOWs outside the ash basins (Figure 3-1). A geologic cross-section through the source areas is included as Figure 3-2. Round 1 and Round 2 analytical results for ash porewater and ash basin water are provided in Table 2-3 and 2-4. Ash distribution and chemical and physical properties were evaluated through advancement and sampling of 17 borings within the ash basin, 6 borings in the ash storage area, and 4 borings within the cinder storage area. Ash within the ash basin was encountered to depths ranging from the surface to approximately 76 feet below ground surface (bgs). Ash within the ash Corrective Action Plan Part 2 Riverbend Steam Station Ash Basin 20 storage area was encountered from the surface to a maximum depth of 78 feet bgs. Within the cinder storage area ash was encountered from the surface to a maximum depth of 14.5 feet bgs. Ash porewater was evaluated through the sampling of 7 monitoring wells installed within the 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 the source areas were identified beneath the ash basin, ash storage area, and cinder storage area. The need for further refinement of exceedences was identified and is being conducted in the area downgradient of the cinder storage area. COI transport is generally in a northern, northwestern, and northeastern direction toward Mountain Island Lake. Analytical results of samples collected from the source areas were reviewed to identify COIs, as follows: • Seven COIs were identified in ash based on comparison to the NC PSRGs for POG: antimony, arsenic, cobalt, iron, manganese, selenium, and vanadium. • Eleven COIs 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. • Sixteen COIs 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: aluminum, antimony, arsenic, barium, beryllium, cadmium, chromium, cobalt, copper, iron, lead, manganese, nickel, thallium, vanadium, and zinc. 3.3 Delineation of Potential Migration Pathways through Environmental Media 3.3.1 Soil The approximate horizontal extent of soil impacts was delineated during the CSA and is generally limited to the area beneath the ash basin and one location along the waste boundary south of the 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 ash. COIs identified in soil include arsenic, boron, cobalt, iron, manganese, selenium, and vanadium. At the RBSS site, the soil located beneath the ash basin could be considered as a potential secondary source. As discussed with NCDEQ, after excavation, soils left on-site will be sampled and analyzed, and the analytical results will be incorporated into the groundwater contaminant fate and transport model. If this evaluation indicates that modification to the proposed CAP is required, Duke Energy will prepare and submit a revised CAP. Further assessment west of the cinder storage area is underway as recommended in the CSA Report. Results of this assessment will be reported under separate cover. Corrective Action Plan Part 2 Riverbend Steam Station Ash Basin 21 3.3.2 Groundwater Site hydrogeologic conditions were evaluated through the sampling of 78 monitoring wells during Round 1 sampling and 81 monitoring wells during Round 2 sampling. The wells were screened within the shallow, deep, and bedrock flow layers beneath the site. Based on the site investigation, the groundwater system in the natural materials (alluvium, soil, soil/saprolite, and bedrock) at RBSS is consistent with the LeGrand slope-aquifer system and is an unconfined, connected aquifer system. In general, groundwater within the shallow, deep, and bedrock flow layers flows from the southern extent of the RBSS site property boundary to the north, northeast, and northwest and discharges into Mountain Island Lake. Flow contours developed from groundwater elevations measured in the shallow and deep wells in the southeastern portion of the site depict groundwater flow generally to the northeast discharging to Mountain Island Lake. Groundwater flow direction in the shallow, deep flow layers based on water levels gauged during the Round 2 sampling event (August 2015) are shown on Figures 2-2 and 2-3, respectively. The approximate horizontal extent of groundwater impacts is limited to beneath the waste boundary and northeast of the ash basin. The extent of groundwater impacts in the following areas requires refinement as noted below: • horizontal and vertical extent west of the ash and cinder storage areas near well GWA- 3SA/D; and • horizontal and vertical extent to the south of the ash storage area. The approximate vertical extent of groundwater impacts is generally limited to the shallow and deep zones (although a transition zone as defined in the CSA is absent at RBSS), and vertical migration of COIs is impeded by the underlying bedrock. The bedrock flow layer is defined by data obtained from the bedrock groundwater monitoring wells (BR or BRU wells). Groundwater contours developed from the groundwater elevations in the bedrock wells show groundwater flowing generally in a north/northwest direction from the south side of the RBSS site discharging to Mountain Island Lake. Groundwater flow direction in the bedrock flow layer is illustrated on Figure 2-4. Following excavation of the ash basin, stabilization of groundwater elevations, and evaluation of analytical results re-evaluation of the SCM will be required. Groundwater elevation measurements, AOW inventory, and groundwater contours mapping will likely be warr anted to determine if changes that have occurred to groundwater or exposure routes described in the SCM. 3.3.3 Surface Water and Sediment Two surface water samples were collected from the intake channel: RBSW001 and RBSW002. COIs exceeding their respective 2B Standards include aluminum, cadmium, copper, lead, and zinc. Surface water generally flows from the basins to Mountain Island Lake as shown on Figure 3-1. Corrective Action Plan Part 2 Riverbend Steam Station Ash Basin 22 Sediment samples were collected from 11 locations, including dry AOWs (S-1, S-3, S-10, and S-12), during the CSA. COIs exceeding the NC PSRGs for POG in sediment samples include arsenic, barium, boron, cobalt, iron, manganese, and vanadium. Cobalt, iron, manganese, and vanadium concentrations exceeded the NC PSRGs for POG, but are also naturally occurring constituents in background soil. A summary of COIs related to surface water and sediment from AOW samples is provided in Section 2.3.2. Ash basin water from within the waste boundary and groundwater will be removed as needed during ash excavation activities. During excavation, new AOWs may arise and existing AOW may no longer be present. Surface water for existing AOWs and sediment at dry AOWs identified in the CSA may continue to contribute to groundwater through infiltration, reaching groundwater and ultimately moving toward Mountain Island Lake. It is anticipated that the decrease in ash basin water levels will cause a decrease in flows at these AOWs. Sediment that exceeds the NC PSRGs for POG can also contribute to groundwater concentrations. The effect of sediment COIs on receptors is evaluated in the risk assessment (see Section 5). During excavation, the processes that govern COI migration will fluctuate, and therefore, the influence of surface water and sediment COI migration to Mountain Island Lake cannot be fully ascertained until source removal is complete. 3.4 Establishment of Background Areas Background areas at the RBSS site are located south and beyond the immediate boundary of the ash storage area and south of Horseshoe Bend Beach Road (Figure 2-1). In addition to the existing NPDES ash basin background compliance well, monitoring wells installed as background wells during the CSA are not in locations that represent true upgradient, background groundwater conditions (See Figures 2-2, 2-3, and 2-4). A detailed background monitoring well assessment is presented in Appendix B in the CAP Part 1 Report. As a result of this assessment, Duke Energy commenced installation of additional monitoring wells in January 2016 to aid in the evaluation of naturally occurring COIs upgradient of the facility. Once the well installation is complete and groundwater sampling results are available, refinement of groundwater flow direction and distribution and influence of naturally occurring COIs will be re- evaluated. 3.5 Environmental Receptor Identification and Discussion Duke Energy conducted a receptor survey of the area within 0.5 mile of the Compliance Boundary 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 RBSS site generally consists of residential properties, undeveloped land, and Mountain Island Lake (Figure 3-4). Properties north of Mountain Island Lake are located in the Town of Huntersville, Mecklenburg County, North Carolina. The Town of Huntersville identifies most of these properties as a park, nature preserve, or wildlife refuge. A residential property is located to the northeast of the ash basin on the northern side of Mountain Island Lake. Corrective Action Plan Part 2 Riverbend Steam Station Ash Basin 23 Properties south of Mountain Island Lake are located in Mount Holly, Gaston County, North Carolina. The majority of properties in this area are owned by Duke Energy and are associated with RBSS. Residential properties are located south and southeast of the RBSS site (south of Horseshoe Bend Beach Road). One well was identified north of Mountain Island Lake. Based on the topography on the north shore, groundwater flow is toward Mountain Island Lake. This observation, combined with the distance away from the RBSS site, suggests that this well is not hydraulically connected with groundwater at RBSS. No water supply wells (including irrigation wells and unused or abandoned wells) were identified between the source area and Mountain Island Lake. Mountain Island Lake supplies water to the Charlotte municipal area, as well as the towns of Gastonia and Mount Holly, North Carolina. The Charlotte intake is located 3.4 miles downstream of the RBSS site and the Gastonia and Mount Holly intakes are located approximately 6.9 miles downstream of the RBSS site. Water supply intake locations are shown on Figure 3-5. 3.6 Determination of System Boundaries The site, waste, and Compliance Boundaries for the RBSS site are shown on Figure 2-1. Spatially, the SCM for RBSS is bounded by Mountain Island Lake to the north and west and topographic divides to the east and south of the site. The SCM extends into bedrock, which inhibits vertical migration of COIs at the site. 3.7 Site Geochemistry and Influence on COIs As excavation activities continue at RBSS, geochemistry in the ash management areas and the excavation areas will change. Geochemistry described within this CAP Part 2 Report represents a snapshot of what is occurring at RBSS. Groundwater composition can be affected by an array of naturally occurring and anthropogenic factors. Many of these factors can be causative agents for specific oxidation- reduction (redox) processes or indicators of the implied redox state of groundwater as expressed by pH, oxidation-reduction potential (ORP; expressed as Eh), 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 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 a redox state. Based on field measurements at RBSS, the predominant redox category is anoxic/mixed and the predominant redox processes are ferrous iron/ferrous sulfate, so the reduced species As(III), Se(IV), and Mn(IV) would be expected. The redox conditions appear to be controlled at least partly by the SO4/S2 and Fe(III)/Fe(II) redox couples, and these redox couples should be monitored to assess changing redox conditions. Corrective Action Plan Part 2 Riverbend Steam Station Ash Basin 24 Chemical speciation measurements were determined for 37 groundwater and/or ash porewater monitoring wells samples, and redox calculations were performed for each of those samples. Of the 36 speciation samples, 14 were oxic and 19 were anoxic or mixed samples. Those measurements indicate that pH ranges from 3.56 to 12.50 standard units (SU). It is anticipated that high pH ranges are related to grout from the well installation and are not representative of groundwater. In contrast, background well results indicate that pH ranges from 5.75 to 8.20 SU, whereas pH within the ash basin materials ranges from 5.10 to 11.91 SU. A wide range of ORP values was measured; in most cases, the ORP value ranges implied highly reduced (large negative values) to highly oxidized (large positive values) environments. Standard (equilibrium) electrode potentials for such reactions may be expected to be approximately -1,000 millivolts. In contrast, measured ORP values at the RBSS site were never less than -270 millivolts. Totals and speciation were measured in the lab or calculated for: • Total arsenic, As(III), and As(V) • Total chromium, Cr (III) and Cr (VI) • Total iron, Fe (II), and Fe (III) • Total manganese, Mn (II), and Mn (IV) • Total selenium, Se (IV), and Se (VI) Speciation analytical results are summarized below: • Reduced arsenic [As (III)] was detected less frequently than oxidized arsenic [AS (V)]. As (III) concentrations were about half of the As (V) concentrations. Also, As (III) concentrations decreased in reducing conditions, which is counterintuitive, and may be due in part to the limited number of As (III) samples available for comparison (two oxic and four anoxic). • Total chromium was detected in all samples except AB-6BRU. Hexavalent chromium [Cr (VI)] was detected in 32 of 37 speciation samples, including samples from anoxic groundwater conditions. The average concentration of Cr (VI) was higher in anoxic versus oxic groundwater conditions. There was also a corresponding increase in total chromium concentrations in anoxic versus oxic groundwater conditions, which may in part explain why Cr (VI) concentrations increased under these same conditions. Cr (VI) concentrations were partly proportional to total chromium concentrations. Other factors, such as redox influence on solid media phases that adsorb Cr (VI), also likely influenced Cr (VI) concentrations at the RBSS site. • Iron speciation results showed similar trends as manganese. The reduced form of iron [Fe(II)] was mainly present downgradient and southeast of the ash basin. • Manganese speciation results did not exhibit the high reduced form of manganese [Ms (II)] concentrations that would be expected in an anoxic (reducing) environment. • Ash porewater speciation results indicate a favorability of the reduced form of selenium [Se (IV)] over the oxidized form of selenium [Se (VI)]. In summary, the observed groundwater conditions at the RBSS site span oxidizing to moderately reducing conditions. A review of equilibrium chemistry shows some oxidized species Corrective Action Plan Part 2 Riverbend Steam Station Ash Basin 25 (e.g., Cr (VI)) present in reduced conditions and some reduced species (e.g., Se (IV)) in oxic conditions. In general, the observed groundwater conditions, showing a mixture of redox conditions and variability in species, indicate a dynamic redox environment at the RBSS site and that conditions are not in equilibrium for some COIs. Given this diverse range of conditions at the RBSS site, further evaluation and modeling of geochemistry will be completed using the PHREEQC (Parkhurst and Appelo 2013) modeling tool and groundwater transport and chemical transport modeling. The results of geochemical and groundwater modeling and their relevance to remedial alternatives are presented in Section 4. Corrective Action Plan Part 2 Riverbend Steam Station Ash Basin 26 4 Modeling 4.1 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 RBSS model was conducted by the Electric Power Research Institute (EPRI) and found that the model was sufficient to meet the objective of predicting effects of corrective action alternatives on groundwater quality. The refined groundwater flow and transport model report and the EPRI review of the calibrated RBSS model are provided in Appendix B. 4.1.1 Flow Model Refinements Transient transport simulations for all COIs 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 5.5% compared to the initial calibrated model of 9.95% 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 head across the model domain. The results are provided in Table 3 in Appendix B. • Recharge rates for the model were also refined within the ash basin footprint and also for the area beyond the ash basin waste boundary. Recharge within the ash basin footprint was calculated using Darcy’s Law considering the approximate area of the Primary and Secondary Cells, the approximate depth of water or saturated ash, and the range of measured hydraulic conductivity values within the ash and fill. The mean annual recharge in the Piedmont ranges from 4.0 to 9.7 inches per year (Daniel 2001). The recharge rate applied in the groundwater model was 21.5 inches/year within the ash basin footprint and 6.5 inches/year across the rest of the model domain. These model refinements affect fate and transport of COIs at the site and are representative of current site conditions. • Historic basin water levels were considered during calibration of the flow model. However, the current flow model is calibrated to steady-state conditions. Continued refinement of the model to consider transient flow may enhance integration of historic water level data within the ash basin. Corrective Action Plan Part 2 Riverbend Steam Station Ash Basin 27 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) sorption coefficient (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 COI Kd values in the fate and transport models resulted in improved calibration of source concentrations to measured concentrations in downgradient wells. Note that final Kd values used to calibrate the fate and transport models may have fallen outside the recalculated upper and lower limits; however, adjustment of Kd values within the model to achieve calibration is considered acceptable practice. • The initial 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 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. This refinement allows the model to account for naturally occurring background concentrations and is particularly important for COIs whose PPBC is 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. • Additional COIs were added to the modeling scenario for COIs identified within the waste boundary including: Arsenic, boron, hexavalent chromium, sulfate, thallium, and vanadium. • The background concentrations for the COIs were applied as initial concentrations. Refinements to the groundwater model provide a more accurate representation of existing site conditions and produce model results that more accurately depict closure scenarios at the site. 4.1.3 Summary of Modeled Scenarios Two closure scenarios were modeled for RBSS: an Existing Conditions scenario with ash sources left in place and an Excavation scenario with the accessible ash removed from the site. 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. Corrective Action Plan Part 2 Riverbend Steam Station Ash Basin 28 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 COIs 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 a 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- and off-site, with COIs 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 will reach a steady-state concentration sooner. The time to steady-state concentration is also dependent on the sorptive characteristics of each COI. Sorptive COIs 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 COIs. 4.1.3.2 Excavation Scenario In the Excavation scenario, the water in the ash basin is removed and the ash from the ash basin, cinder storage area, and ash storage area is removed and transported off-site. In the model, the constant concentration sources of ash above and below the water table are removed. The flow parameters for this model scenario are identical to the Existing Conditions scenario, except for removal of ash layers. This scenario assumes in-basin recharge rates become equal to rates surrounding the ash basin (6.5 inches per year). Starting from the time that excavation is complete, COIs already present in the groundwater and not removed by geochemical processes continue to migrate downgradient as clean water infiltrates from ground surface and recharges the aquifer at the water table. The COIs are removed from the saturated zone beneath the source areas. COIs with high Kd values will have migration retarded relative to the ash porewater velocity as sorptive COIs are attenuated by site materials. The model uses the predicted concentration from the 2015 calibration as the initial COI concentrations. 4.1.4 Model Assumptions and Limitations The model assumptions include the following: • The steady-state flow model was calibrated to hydraulic heads measured at observation wells in June 2015 and considered the ash basin water level. The model is not calibrated to transient water levels over time, recharge, river flow, or river stage (i.e., Mountain Island Lake) changes. A steady-state calibration does not consider groundwater storage and does not calibrate the groundwater flux into adjacent surface water bodies. • A single domain MODFLOW modeling approach was used for simulating flow in the primary porous groundwater flow layers. • During model calibration, the constant source concentrations at the ash basins and ash storage area and cinder storage area reasonably match 2015 COI groundwater concentrations. Corrective Action Plan Part 2 Riverbend Steam Station Ash Basin 29 • For the purposes of numerical modeling and comparing closure scenarios, it is assumed that the selected closure scenario is implemented in 2015. • Predictive simulations were performed and steady-state flow conditions were assumed from the time the ash basins and ash storage areas were placed in service through the current time until the end of the predictive simulations (Year 2265). • COI source zone concentrations within the waste boundary were applied uniformly within each source area and assumed to be constant with respect to time for transport model calibration. • Since Mountain Island Lake is modeled as a constant head boundary in the numerical model, it will not be possible to assess the effects of pumping wells or other groundwater sinks that are near the river. • Travel times are advective and do not account for sorption of COIs to host rock, which may cause the travel times to be reduced. • The model does not predict co-precipitation of COIs with iron and manganese. Therefore, COI concentrations generated by the model may be over-estimated (Section 4.3); however, geochemical modeling was conducted to supplement the groundwater modeling and is discussed further in Section 4.3. • The model does not account for varying geochemical conditions such as pH and redox potential that could affect COI mobility; however, geochemical modeling was completed and is further discussed in Section 4.3. 4.1.5 Modeled Scenario Results Constituent concentrations were analyzed at three downgradient monitoring wells (MW -3S, MW -5S, and MW -6S) for all COIs except hexavalent chromium (Appendix B, Figure 6). Hexavalent chromium analysis was limited to downgradient monitoring well MW-9D (Appendix B, Figure 7). Closure scenario results are presented as predicted concentration versus time curves in downgradient monitoring wells and as groundwater concentration maps for each of the seven modeled COIs on Figures 15 through 121 in Appendix B, as discussed in the following subsections. Concentration contours and concentration breakthrough curves are referenced to 1957, the year that the ash basin became effective. Concentration contours and concentration breakthrough curves 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 in Table 4-1. A “+”indicates that the concentration of a given COI has exceeded its applicable 2L Standard, IMAC, or NCDHHS HSL. A “-” indicates that the concentration of a given COI has not exceeded its applicable 2L Standard, IMAC, or NCDHHS HSL. Corrective Action Plan Part 2 Riverbend Steam Station Ash Basin 30 Table 4-1 Summary of Modeled COI Results at the Compliance Boundary Constituent Appendix B Figures Flow Layer Existing Conditions Scenario Excavation Scenario Year 0 (2015) Year 100 (2115) Year 0 (2015) Year 100 (2115) Antimony IMAC (1 µg/L) 15 – 26 Shallow + + + + Deep + + + + Bedrock + + + + Arsenic 2L (10 µg/L) 27 – 38 Shallow     Deep     Bedrock     Boron 2L (700 µg/L) 39 – 50 Shallow     Deep     Bedrock     Chromium 2L (10 µg/L) 51 – 62 Shallow     Deep     Bedrock     Cobalt IMAC (1 µg/L) 63 – 74 Shallow + + + + Deep + + + + Bedrock + + + + Hexavalent Chromium NCDHHS HSL (0.07 µg/L) 75 – 84 Shallow + + + + Deep + + + + Bedrock + + + + Sulfate 2L (250,000 µg/L) 85 – 96 Shallow     Deep     Bedrock     Thallium IMAC (0.2 µg/L) 97 – 108 Shallow + + + + Deep + + + + Bedrock + + + + Vanadium IMAC (0.3 µg/L) 109 – 120 Shallow + + + + Deep + + + + Bedrock + + + + The model predictions are summarized as follows: • In accordance with 15A NCAC 02L .0106 (k), a CAP may be approved by NCDEQ without requiring groundwater remediation to the 2L Standards if 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. For RBSS, the receptor is considered to be Mountain Island Lake. To evaluate this condition, HDR and UNCC conducted particle tracking using the excavation steady-state flow field to identify the one-year travel time boundary. Particles were placed at select wells located near Mountain Island Lake and also at the side-gradient ends of the ash basin. The advective travel time one year from each well was performed using MODPATH and is shown on Corrective Action Plan Part 2 Riverbend Steam Station Ash Basin 31 Figure 14 in Appendix B. Based on the results of particle tracking, boron exceeds its 2L Standard beyond the one-year travel time boundary, and thus, remediation would be required to meet the 2L Standard. Thus, potential groundwater extraction wells were added to the model to evaluate the degree of pumping required to provide hydraulic control of the boron plume such that the 2L Standard is met at one year’s advective travel time upgradient of Mountain Island Lake. The monitoring locations are shown on Figure 121 in Appendix B and are labeled as extraction wells for the purpose of the simulation. • The simulation was performed using six wells pumping at a rate of 3 gallons per minute. Results of the simulation show that the modeled well configuration and pumping rate would not adequately capture groundwater in the shallow zone that has been impacted by the ash basin and other source areas. If Duke Energy were to pursue remediation under 15A NCAC 02L .0106 (k), a more detailed modeling analysis would be needed to predict recovery rates and design an efficient pumping recovery system. • The model predicts that under the Existing Conditions and Excavation scenarios, antimony, cobalt, thallium, and vanadium exceed their respective IMACs at Mountain Island Lake. Also, hexavalent chromium is predicted to exceed the NCDHHS HSL at Mountain Island Lake. For these COIs, the background concentrations used for modeling exceed the applicable groundwater standards, so the actual impact of the site sources on groundwater quality is in part related to background conditions. Further sampling of background wells, statistical evaluation, and geochemical modeling will provide further insight on contributions from the source area. • Model predictions do not show that COI concentrations will be effectively reduced by ash removal under the Excavation scenario. The COIs that are predicted to exceed their respective 2L Standard, IMAC, or NCDHHS HSL will not achieve compliance within the time period modeled (2015-2265). • The model predicts that under the Existing Conditions and Excavation scenarios, arsenic, boron, chromium, and sulfate will not exceed their respective 2L Standards at Mountain Island Lake. • Among the COIs, sulfate and boron are similar in that both are considered conservative; that is, neither of these COIs has a strong affinity to attenuate or adsorb to soil/rock surfaces. As a result, the model predicts similar behavior for sulfate, boron, and other COIs with low Kd values: rapid and nearly complete reduction to below the respective standard or IMAC under the Excavation scenario. Corrective Action Plan Part 2 Riverbend Steam Station Ash Basin 32 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: • Revisions to the Kd values; • Updated groundwater flux data for input into the surface water model; and • Additional COIs based on review of Round 2 sampling data or as requested by NCDEQ. New data were used to evaluate potential surface water impacts of COIs in groundwater as they discharge to surface water bodies adjacent to the RBSS site. 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 COIs in groundwater as they discharge to surface water bodies adjacent to the RBSS site and within the Compliance Boundary. Assessment of surface water quality was performed for concentrations and mass flux of COIs to Mountain Island Lake and separately for local groundwater loads to a small, semi-enclosed basin (hereafter, “East Basin”) located on the downstream (east) side of the RBSS site (Figure 2-1). The East Basin receives limited upstream inflow through a narrow, shallow channel and connects back to Mountain Island Lake downstream. Mountain Island Lake, upstream of the East Basin inflow channel, is influenced by COIs from local groundwater inflow, and COIs in the upstream section will flow into the East Basin. The East Basin inflow channel and the basin itself are influenced further by COIs from local groundwater inflow and from the discharge of a small, unnamed tributary (Figure 2-1). Groundwater loading of COIs to Mountain Island Lake and the East Basin were calculated as the product of volumetric groundwater fluxes and corresponding COI concentrations calculated with the groundwater model. 4.2.2 Results The mixing model results indicate that impacts from groundwater exceedances do not cause violation of 2B surface water quality standards at the edge of the mixing zones. The calculated surface water COI concentrations in Mountain Island Lake downstream of the RBSS site and separately for the East Basin are presented in Table 4-2 and Table 4-3. The river flows, upstream surface water concentrations, groundwater flows, and groundwater COI concentrations presented in Appendix B and Appendix D were used to complete these calculations. Corrective Action Plan Part 2 Riverbend Steam Station Ash Basin 33 Table 4-2 Mountain Island Lake Surface Water Concentrations COI Calculated Mixing Zone Conc. (µg/L) Water Quality Standard (µg/L) Acute Chronic HH / WS Acute Chronic HH / WS Antimony 0.273 0.253 0.253 (nc) NS NS 640 / 5.6 Arsenic 0.272 0.252 0.250 (c) 340 150 10 / 10 Boron 27.5 25.3 25.0* NS NS ns / ns Total Chromium 0.633 0.514 0.501* NS NS ns / ns Hexavalent Chromium 0.573 0.508 0.500* 16 11 ns / ns Cobalt 0.423 0.269 0.269 (nc) NS NS 4 / 3 Sulfate 1,233 580 580 (nc) NS NS ns / 250,000 Thallium 0.055 0.051 0.051 (nc) NS NS 0.47 / 0.24 Vanadium 0.518 0.502 0.500* NS NS ns / ns Notes: 1. All COIs are shown as dissolved except for total chromium 2. WS – water supply (15A NCAC 02B .0216, amended effective January 1, 2015) 3. HH – human health (15A NCAC 02B .0211, amended effective January 1, 2015) 4. c – carcinogen 5. nc – non-carcinogen 6. ns – no water quality standard 7. * – concentration calculated with annual mean river flow Table 4-3 East Basin Surface Water Concentrations COI Calculated Mixing Zone Conc. (µg/L) Water Quality Standard (µg/L) Acute Chronic HH / WS Acute Chronic HH / WS Antimony 0.369 0.264 0.264 (nc) NS NS 640 / 5.6 Arsenic 0.360 0.263 0.251 (c) 340 150 10 / 10 Boron 40.1 26.8 25.1* NS NS ns / ns Total Chromium 1.157 0.579 0.504* NS NS ns / ns Hexavalent Chromium 0.512 0.502 0.500* 16 11 ns / ns Cobalt 1.206 0.365 0.365 (nc) NS NS 4 / 3 Sulfate 4,132 937 937 (nc) NS NS ns / 250,000 Thallium 0.072 0.053 0.053 (nc) NS NS 0.47 / 0.24 Vanadium 0.601 0.512 0.501* NS NS ns / ns Notes: 1. Mixing zone concentrations assume that 10% of upstream Catawba River flow enters the East Basin through the inflow channel. 2. All COIs are shown as dissolved except for total chromium 3. WS – water supply (15A NCAC 02B .0216, amended effective January 1, 2015) 4. HH – human health (15A NCAC 02B .0211, amended effective January 1, 2015) 5. c – carcinogen 6. nc – non-carcinogen 7. ns – no water quality standard 8. * – concentration calculated with annual mean river flow Corrective Action Plan Part 2 Riverbend Steam Station Ash Basin 34 4.3 Geochemical Modeling 4.3.1 Objective The objective of geochemical modeling is to describe the expected partitioning of COIs 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 dissolved oxygen (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. COIs evaluated for RBSS were: arsenic, antimony, boron, chromium, cobalt, iron, manganese, pH, sulfate, TDS, thallium, and vanadium. 4.3.2 Methodology Site-specific evaluations were performed for each of the monitoring wells using the United States Geological Survey (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. In using a single well approach, wells can be evaluated or grouped later based on geochemical characteristics. The single well approach also allows fine resolution of geochemical constituents and subtle differences between wells that have a significant bearing on the overall geochemical characterization. Calculations were driven by measured concentrations of COIs and other analytes such as ORP alkalinity, and sodium, and other ions in groundwater for each of the 103 wells monitored at the RBSS. 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 and pH values for each well were plotted on the Pourbaix diagram for each COI to evaluate the likely distribution of species at the RBSS. 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 Kd values for RBSS soils determined from adsorption experiments conducted by UNCC. To geochemically simulate changes to aquifers or test potential remediation strategies, simulations in which DO, pH, redox, and TDS 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, redox, or TDS along with relevant changes to the dominant species across the gradient. For these titrations, TDS was evaluated along with select cations and anions known to be common in soils Corrective Action Plan Part 2 Riverbend Steam Station Ash Basin 35 and sediment at the site including sodium, calcium, chloride, potassium, and sulfate. Changes to DO, pH, and TDS were utilized for titrations due to the affinity for numerous COIs 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 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 surf aces (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 individually on a well-by-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 electric charge balances. A benefit of the surface complexation approach is that the charge on both the adsorbing ion and the solid surface where sorption occurs. • The surface complexation model was parameterized based on soil column tests and extraction measurements reported by UNCC. The range of sorption properties was parameterized as the minimum, mean, and maximum estimates for binding sites as defined from soil extraction measurements. This range of sorption capacities was used to develop pH, Eh, DO, TDS, and COI titrations. • The dominant attenuation process is adsorption to hydrous metal oxides, particularly HFO and HAO. HFO and HAO 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 sodium, potassium, magnesium, calcium, 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. • Pourbaix 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. Corrective Action Plan Part 2 Riverbend Steam Station Ash Basin 36 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 COIs. Relevant observations from this modeling effort are as follows: • The redox conditions vary widely at the site indicating that it has not reached equilibrium or data is not representative of the conditions sampled. Additional groundwater results will assist in refining the model further and confirm these findings should sampled data not be representative of actual groundwater conditions • Sorption of all of the aqueous groundwater species identified by the CSA would consume only a fraction of the HFO and HAO sorption sites available in site soils. This will be evaluated further under the Tier III MNA evaluation to be completed after this CAP. • The observed site condition of limited solubility of arsenic, chromium, cobalt, and selenium in site groundwater is confirmed by the modeling. • Each of the pH, Eh, and TDS figures can be further evaluated to support monitored natural attenuation (MNA) or remediation. In addition, pH adjustment could be performed to make COIs less soluble, thus limiting COI migration during excavation and restricting the release of TDS and other metals. • Soil sorptive capacities for COIs such as boron are typically lower than COIs such as arsenic which have higher sorptive capacities. 4.4 Refined Site Conceptual Model Groundwater and surface water models were revised to address comments from NCDEQ. An updated SCM is provided on Figure 3-1. Based on updated groundwater and geochemical model, the SCM has not changed significantly with the exception of the following: COIs represented in the groundwater model may be over-estimated since they did not allow for co-precipitation of COIs in the geochemical model. In terms of the SCM, groundwater fluxes derived from the groundwater models may be conservatively high. Even with potentially over- estimated modeling results, the surface water mixing models do not show exceedances of the 2B Standards. It is likely that, upon source removal, recreational receptors at Mountain Island Lake will not be impacted. Corrective Action Plan Part 2 Riverbend Steam Station Ash Basin 37 5 Risk Assessment The purpose of the human health and ecological risk assessment is to characterize potential risks to humans and ecological receptors associated with exposure to the coal ash-derived constituents that may be present in groundwater, surface water, sediments, soil, and air due to release(s) from the coal ash basin at RBSS. 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 remedial actions which, when implemented, will provide future conditions that are protective of human health and the environment, as required by CAMA. The risk assessment was completed using a methodology designed to be consistent with state and federal guidance. This methodology represents a step-wise process whereby RBSS 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 into 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 RBSS, the following receptors and exposure scenarios are 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 and AOW soil, on-site surface water, and on-site sediment Corrective Action Plan Part 2 Riverbend Steam Station Ash Basin 38 • 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 swimmers, waders and boaters with potential exposure to off-site surface water and off-site sediment • Current/future recreational and subsistence fishers with potential exposure to off-site surface water and off-site sediment, and fish ingestion for recreational purposes and ingestion for subsistence fishers The following ecological receptors and exposure scenarios are identified in the ecological CSM: • Fish with potential exposure to surface water 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 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. Corrective Action Plan Part 2 Riverbend Steam Station Ash Basin 39 Risk ratios were calculated by first identifying whether COPC RBCs were based on cancer risk or non-cancer hazard. For RBCs based on cancer risk, the risk ratio for each COPC was calculated by dividing the EPC by the cancer-based RBC concentration. For RBCs based on non-cancer risk, the risk ratio for each COPC was calculated by dividing the EPC by the non- cancer-based RBC concentration. A risk ratio less than 1 indicated that the EPC does not exceed the RBC, whereas a ratio greater than 1 indicated that the EPC exceeds the RBC. Risk ratios were also used to evaluate the cumulative receptor risk associated with each exposure point. Cumulative receptor risk was calculated by summing the risk ratios among all COPCs on which the RBC was based. In accordance with USEPA risk assessment guidance (USEPA 1991), the cumulative cancer risks and non-cancer hazard indices were evaluated against the USEPA target cancer risk range of 1.0E-06 to 1.0E-04 for potential carcinogens and target non-cancer hazard index of 1 for 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 RBSS, the results of the human health risk assessment indicate that exposure to on-site surface water, AOW water, AOW soil, sediment, and groundwater pose no unacceptable risk or hazard for a trespasser and construction worker under the exposure scenarios developed in Step 1. Exposure to on-site surface water, AOW water, AOW soil, and sediment for a commercial/industrial worker results in a hazard index of 1.2E+00, but a target endpoint analysis indicates target organ-specific hazard quotients (HQs) below 1; thus, on-site media pose no unacceptable human health hazard under this scenario. No carcinogenic risk is above 1.0E-04 for the commercial/industrial worker. Similarly, off-site surface water and sediment pose no unacceptable cancer risk or hazard for a recreational swimmer, wader, or boater under the scenarios developed in Step 1. Consumption of fish (using on-site surface water concentrations as surrogate data) by a recreational fisher and subsistence fisher results in hazard indices of 4.0E+00 and 1.2E+02, respectively; target endpoint analysis performed indicates HQs above 1 for selenium, thallium, and zinc for a recreational fisher and HQs above 1 for cadmium, selenium, thallium, and zinc for a subsistence fisher. No carcinogenic risk is above 1.0E-04 for either fisher. Thus, the human health risk assessment indicates that potential risks are above risk targets for the recreational fisher and subsistence fisher from ingestion of fish caught near the site. These scenarios were evaluated using data for on-site surface water as a conservative surrogate in the absence of off-site surface water data. Additional data regarding site-specific conditions as well as evaluation of the very conservative nature of the exposure parameters and fish ingestion models used in the risk assessment are needed to address these results. 5.4 Step 4: Ecological Risk Assessment The purpose of a BERA is to: (1) determine whether unacceptable risks are posed to ecological receptors from chemical stressors, (2) derive constituent concentrations that would not pose unacceptable risks, and (3) provide the information necessary to make a risk management decision concerning the practical need and extent of remedial action. The BERA was performed Corrective Action Plan Part 2 Riverbend Steam Station Ash Basin 40 according to the traditional ecological risk assessment paradigm: Problem Formulation, Analysis (Exposure and Effects Characterization), and Risk Characterization. Because of the many combinations of conservative assumptions, 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 2015a), as well as NCDENR’s “Guidelines for Performing Screening Level Ecological Risk Assessments” (NCDENR 2003). Constituents of concern were selected based on factors including the type of source(s), concentration, background levels, frequency of detection, persistence, bioaccumulation potential, toxicity/potency, fate and transport (e.g., mobility to groundwater), and potential biological effects. Screening consisted of comparing the maximum concentration of each constituent in the applicable media to conservative environmental screening levels. Per Step 3 of the USEPA Region 4 ecological risk assessment guidance, COPCs that were retained using the risk-based screening were evaluated using a multiple lines-of-evidence approach in the BERA (USEPA 2015a, 2015b). 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 an HQ. Receptors chosen for ecological risk assessment are often surrogates for the broad range of potential ecological receptors in a given habitat. For RBSS, 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 or water-dependent receptors include: fish, benthic invertebrates, aquatic birds (represented by mallard duck and great blue heron), and aquatic mammals (represented by muskrat and river otter). At RBSS, three ecological exposure areas were defined (Figure 2-5 in Appendix F). These include: • Ecological Exposure Area 1, located near an inlet in the area west of the steam station • Ecological Exposure Area 2, located west of the ash basin Primary Cell and along Mountain Island Lake • Ecological Exposure Area 3, located west, north and east of the ash basin Secondary Cell and along the Catawba River/Mountain Island Lake Potentially affected areas on-site are classified as aquatic and evaluated for exposure to site COPCs. Ecological habitats are presented on Figure 2-6 in Appendix F. Evaluation of surface water in Exposure Area 1 indicates a calculated HQ of 3 for a great blue heron’s exposure to selenium; using the No Observed Adverse Effects Level (NOAEL) as the Corrective Action Plan Part 2 Riverbend Steam Station Ash Basin 41 toxicity reference value. Using the Lowest Observed Adverse Effect Level (LOAEL) toxicity reference value, the great blue heron’s selenium-based HQ decreases to 2. All other aquatic wildlife receptors have chemical HQs below 1. The evaluation of ecological exposures to surface water, AOW water and AOW sediment in Exposure Area 2 indicates that no aquatic receptors have HQs above 1. Evaluation of AOW water and AOW sediment in Exposure Area 3 indicates that the great blue heron and muskrat have chemical HQs above 1. These include, for the great blue heron, a HQ of 1 from barium, 2 from cobalt and 6 from vanadium using the NOAEL; the HQs decrease to 0.6 for barium, 3 for vanadium, but remains the same at 2 for cobalt under the LOAEL scenario. For the muskrat, HQs include a value of 2 for aluminum, 3 for barium and 11 for manganese exposure; the HQs decrease to 0.2, 2 and 8, respectively, using the LOAEL. Other aquatic receptors have chemical HQs below 1. Thus, the ecological risk assessment indicates that potential risks are above risk targets for barium, cobalt, manganese, selenium and vanadium for some water-dependent mammals and birds. 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. However, since the remediation of the ash basin area is planned, any future exposure would be eliminated and, therefore, any subsequent ecological risks would be mitigated. Corrective Action Plan Part 2 Riverbend Steam Station Ash Basin 42 6 Alternative Methods for Achieving Restoration 6.1 Corrective Action Decision Process This section discusses how remedial alternatives are evaluated and identifies the remedial alternative selected to achieve restoration of groundwater quality at the RBSS site. As described in Section 1, after removal of ash, soils left on-site will be sampled and analyzed, and the analytical results will be incorporated into the groundwater contaminant fate and transport model. If this evaluation indicates that modification to the proposed CAP is required, Duke Energy will prepare and submit a revised CAP. Therefore, remediation of soils is not discussed in this document. As noted in Section 2, exceedances of 2B or 2L Standards or IMACs were measured at AOWs adjacent to Mountain Island Lake. HDR and Duke Energy consider that the water in the Primary and Secondary cells is the likely source of the water supplying these AOWs, and that dewatering the ash basin will result in reduced or no flow at these AOWs. Duke Energy proposes that remedial measures at these AOWs be deferred until after basin dewatering and excavation of the ash. If at that time, the exceedances are still present, Duke Energy will evaluate those conditions and develop corrective measures to address the exceedance(s). For RBSS, the Plan for Identification of New Discharges was submitted to NCDEQ on May 8, 2015. This plan was developed to address the requirements of North Carolina General Statute (GS)130A-309.210 (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 the document referenced above.” 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 (j), or to an alternate standard (k) or using natural attenuation mechanisms (l). 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 implementation, time, and cost. The methods may include one or a combination of best available technologies and natural attenuation processes. For RBSS, the primary corrective action is removal of source material by excavation and off-site disposal. The groundwater corrective action alternatives discussed herein are evaluated to supplement source control. Corrective Action Plan Part 2 Riverbend Steam Station Ash Basin 43 6.1.2 COIs Requiring Corrective Action Data from the CSA were evaluated in CAP Part 1 to identify the following groundwater COIs that are considered for current or potential future corrective action: antimony, arsenic, boron, chromium, cobalt, hexavalent chromium, iron, manganese, sulfate, thallium, TDS, and vanadium. These COIs 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. 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 Baseline Human Health and Ecological Risk Assessment (Appendix F) provides information on the current knowledge of the RBSS site and conservative conditions assessment of the potential risk associated with COIs attributed to the currently defined sources at RBSS. The primary source-to-receptor exposure route is leaching of ash porewater to groundwater. Groundwater then migrates and reaches Mountain Island Lake prior to the Compliance Boundary. There are no receptors to groundwater through public and private wells within 0.5 mile of the RBSS Compliance Boundary. The groundwater to surface water route is the primary route of exposure of groundwater to receptors. A secondary source-to-receptor exposure route may be infiltration of COIs in water and/or sediment at AOWs to groundwater. Localized groundwater mounding associated with the current hydraulic head in the ash basin will be eliminated with the source control measures (i.e., excavation). The residual groundwater concentrations are the focus of this CAP. 6.2 Alternative Evaluation Criteria Alternative evaluation criteria are selected in general conformance with USEPA Office of Solid Waste and Emergency Response Directive 9355.-27FS, “A Guide to Selecting Superfund Remedial Actions” dated April 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) Corrective Action Plan Part 2 Riverbend Steam Station Ash Basin 44 6.2.1 Effectiveness Effectiveness is a comparison of the likely performance of applicable technologies taking into consideration the following: 1. The estimated area and volumes of media to be treated. 2. Demonstrated reliability to achieve constituent remedial goals under site conditions. 3. Demonstrated reliability to reduce potential risk to human health and the environment in a timely manner. Specific effectiveness criteria include: • Has the potential remedial alternative been demonstrated to be effective at similar sites? • Does the remedial technology involve treatment that will permanently destroy target constituents? • Does the remedial technology involve treatment that will permanently detoxify target constituents? • Does the remedial technology involve treatment that will permanently reduce the mobility of target constituents? • Will the remedial alternative permanently remove contaminants from the site? • Can the effectiveness of a potential remedial technology be monitored, measured, and validated? • Will a remedial technology reduce potential risk to human health when fully implemented? • Will a remedial technology reduce potential risk to the environment when fully implemented? • Will a remedial technology be protective of human health? Technologies that are deemed to be less effective under site-specific conditions than otherwise comparable technologies will be eliminated on the basis of effectiveness. 6.2.2 Implementability/Feasibility The screening criteria of implementability evaluates whether implementation of a technology is technically and administratively feasible. Specific implementability criteria include: • Are the material resources and manpower readily available to fully implement the remedial technology in a timely manner? • Does the remedial technology require highly specialized resources and/or equipment? • 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 the environment during implementation? Corrective Action Plan Part 2 Riverbend Steam Station Ash Basin 45 • Can a remedial technology meet all applicable or relevant and appropriate requirements (USEPA 1997)? 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: • 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. Corrective Action Plan Part 2 Riverbend Steam Station Ash Basin 46 6.3 Remedial Alternatives to Achieve Regulatory Standards Source control is the primary corrective action for groundwater restoration at the site. As required by CAMA, the ash basin at RBSS is being excavated due to the high priority designation established by CAMA. Source removal at RBSS includes the export of ash for landfill or beneficial use. The remedial alternatives described in this section were considered to enhance source control measures at the RBSS 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 the following: • Source Control, which can include: o Ash removal to prevent COIs from leaching into groundwater o Placement of engineered cap to minimize infiltration and prevent COIs from leaching into groundwater o Slurry walls or grout curtains to prevent groundwater interaction with source material o In-situ solidification/stabilization to reduce or eliminate leaching of COIs 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. • Monitored Natural Attenuation – MNA involves 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 adsorptive capacity to the saturated zone to increase the reduction of COI levels in groundwater o Air sparging and adjusting pH to enhance precipitation of iron and manganese oxide/hydroxide minerals to increase the reduction of COI levels in groundwater o Bioremediation for removal of COIs • Permeable Reactive Barriers – Involves trenching and placement of selected material in the trench that would chemically bond and remove COIs and reduce their levels 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. Corrective Action Plan Part 2 Riverbend Steam Station Ash Basin 47 A detailed description of available remedial alternatives is documented in Appendix G. 6.3.2 Monitored Natural Attenuation Applicability to Site A MNA Tier I and Tier II evaluation was conducted for the RBSS 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 COIs 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 concentration of inorganics in groundwater by depositing inorganics on aquifer solids removing and the constituent from the groundwater. MNA is considered a viable remedial alternative for COIs in groundwater at the RBSS site. The following groundwater COIs were identified at RBSS: antimony, arsenic, boron, chromium, cobalt, hexavalent chromium, iron, manganese, sulfate, thallium, TDS, and vanadium. Cobalt, iron, manganese, and vanadium occur naturally in regional groundwater; however, these constituents are still considered COIs because at certain locations, concentrations exceed their respective 2L Standards or IMACs and were higher than observed background concentrations. Sulfate and TDS are generally not attenuated by reactions with solids, but are reduced in concentration by diffusion, mechanical mixing, or dilution. Tier I analysis used two lines of evidence for attenuation:1) Solid-water pair comparison of COI concentrations were performed, with a mutually rising relationship indicating attenuation (USEPA 2007a); and 2) laboratory determination of the solid-water partitioning coefficient or Kd value (USEPA 1999) was used as a measure of the susceptibility of COIs to adsorb to solids and be attenuated. Tier I analysis indicates that arsenic, boron, chromium, selenium, and thallium should be carried through to Tier II analysis. Following completion of a Tier I analysis, a conceptual model for COI attenuation involving reversible and irreversible interaction with clay minerals, metal oxides, and organic matter is proposed. A Tier II demonstration based on that conceptual model was partially executed. The findings are as follows: 1. The samples evaluated for Kd determination were found to be representative of site- specific conditions under which COIs would migrate. 2. Clay minerals and Fe-Mn-Al oxides were found in all samples. Organic matter is not a significant sink for COIs at RBSS. 3. Chemical extractions identified that COIs were concentrated in soil samples exposed to groundwater containing higher concentrations of COIs, validating attenuation. 4. Chemical extractions were used to determine a probable range of Kd values that suggest attenuation is taking place for arsenic, chromium, selenium and thallium. 5. Geochemical modeling performed titration tests for the COIs. Arsenic is completely adsorbed. Antimony and boron are the least adsorbed. If groundwater is maintained at a pH range between 6-8 chromium, cobalt, manganese, selenium, and vanadium will have the most effective adsorptive capacity. Corrective Action Plan Part 2 Riverbend Steam Station Ash Basin 48 As documented in Appendix E, titration results for RBSS 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 a 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 COIs from being solubilizing, or impact due to excavation and the release of TDS and other metals. Changes in redox can also occur 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/III 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 III assessment will be performed in general accordance with USEPA’s “Monitored Natural Attenuation of Inorganic Contaminants in Ground Water - Volume 2 Assessment for Non-Radionuclides Including Arsenic, Cadmium, Chromium, Copper, Lead, Nickel, Nitrate, Perchlorate, and Selenium” (USEPA 2007b). 6.3.3 Site-Specific Alternatives Analysis Source control (i.e., removal of ash) is currently underway at the RBSS site following NCDEQ approval of the excavation plan. Approximately 4.6 million tons of ash will be transported to permitted lined landfills and/or structural fills, or designated for beneficial reuse. The initial phase of excavation work began in May 2015 and includes removal of materials in the northeast corner of the ash storage area. The majority of ash at the RBSS site is anticipated to be transported by rail to a lined clay mine reclamation project in central North Carolina. This activity began in January 2016. Removal of ash at the RBSS site is scheduled to be complete no later than August 2019. The earthen dams will be removed and the non-impacted material will be used in site re-grading. Fill material (from on-site and/or imported sources) will be used to fill the void left after ash removal and the area will be re-graded and vegetated to establish a long-term, stable, erosion-resistant site condition. As detailed in Appendix G, the following three remedial alternatives have been recommended for consideration at the RBSS site to enhance or supplement the existing source removal activities. 1. No Further Action – This alternative is provided to establish a baseline for comparison to other alternatives. Under this alternative, there would be no corrective actions conducted at the site to remove the source of COIs other than the removal of the ash, and no further corrective action would be taken for groundwater. This measure does not include long-term monitoring or institutional controls. Corrective Action Plan Part 2 Riverbend Steam Station Ash Basin 49 2. MNA – Groundwater monitoring would be continued until remedial objectives are met (that is, groundwater concentrations are at or below applicable standards at the Compliance Boundary). At RBSS, ash will be removed from the source areas. COIs may remain in groundwater and concentrations continually decrease over time. Attenuation will occur over time due to natural processes, and its extent can be monitored between the source and the Compliance Boundary. 3. MNA and Permeable Reactive Barrier at Key Locations – It is anticipated that removal of the ash, the presumed source of most of the COIs at the site, will resolve most of the groundwater contamination at the RBSS site, although it will take time for groundwater concentrations in downgradient areas to see decreasing concentrations. While COIs from the RBSS site do not currently result in 2B Standard exceedances in Mountain Island Lake, if ash removal activities result in 2B exceedances of COIs in the lake, a permeable reactive barrier(s) could be constructed between the source area and the lake. The barrier(s) would be designed to reduce contaminant loading for certain sorptive COIs. Pilot studies would be required, both for optimal placement and selection of appropriate reactive or adsorptive media to incorporate into the permeable reactive barriers. Laboratory tests would be required to confirm the effectiveness of the media on the site-specific COIs. Depth to bedrock at the RBSS site would need to be considered as the relative deep bedrock layer may make installation of a permeable reactive barrier technically infeasible at this site. Selection of site-specific corrective actions will be based on results observed from the refined groundwater model and modeling of remedial alternative actions for the site, as well as evaluation of effectiveness, implementability, feasibility, environmental sustainability, and cost. 6.3.4 Site-Specific Recommended Approach At the RBSS site, source removal is the primary corrective action and is anticipated to decrease COI concentrations and number of COIs at the site. Alternative 2 (MNA) is recommended as a supplemental corrective action for the RBSS site. Based on results of the Tier I and Tier II evaluation, MNA is an effective corrective action because COIs will attenuate over time to restore groundwater quality at the site and is protective of both human health and the environment. MNA is a feasible corrective action and can be implemented at the site. Implementation of MNA for a 30-year period is estimated to cost $6.7 million. Costs are discussed further in Section 10. Select monitoring wells, surface water locations, and AOWs will be used for MNA and will be monitored in accordance with the Interim Monitoring Plan and the Effectiveness Monitoring Plan discussed further in Section 9. Groundwater quality and effectiveness of MNA will need to be re-evaluated after excavation of source material is complete. If results of MNA re-evaluation deem MNA is not a sufficient to reduce COI concentrations within an acceptable time period, remedial alternatives should be reconsidered and implemented to augment MNA. Corrective Action Plan Part 2 Riverbend Steam Station Ash Basin 50 7 Selected Corrective Action(s) Remedial alternatives were evaluated for achieving restoration of groundwater at the RBSS site and are detailed in Section 6 of this report. Source control via excavation of ash from the ash basin, ash storage area, and cinder storage area is the primary corrective action for the RBSS site. MNA was determined to be the most appropriate corrective action to supplement source control; however, groundwater quality will need to be monitored during excavation, and the effectiveness of MNA should be re-evaluated after basin closure is complete. Post-excavation MNA requires re-evaluation of the SCM, geochemical, hydrogeological, and surface water models. If MNA is determined to not be an effective corrective action, then re-evaluation of alternatives will be performed to determine the appropriate technology. Selection of MNA for corrective action and conceptual design is discussed further in this section. 7.1 Selected Remedial Alternative for Corrective Action COI transport in groundwater is primarily controlled by hydrogeologic and geochemical conditions at the site (Section 3). COIs enter the groundwater system through the shallow flow layer within the source areas. Evaluation of the geochemical modeling indicated COIs are attenuated by a combination of sorption and/or precipitation and at a pH range between 6 and 8 SU. TDS and sulfate generally are not attenuated, but concentrations are reduced by diffusion, mechanical mixing, and/or dilution. Arsenic, cobalt, and selenium 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 findings of the geochemical modeling presented in Appendix E. Based on review of the modeling, COIs with sorption coefficients similar to or greater than arsenic are immobilized by sorption and/or precipitation. COIs with sorption coefficients similar to or less than boron do not readily attenuate and easily transport in groundwater. 7.2 Conceptual Design 7.2.1 Source Removal – Excavation Excavation of ash at the RBSS site will remove the source areas which are identified as the ash basin, ash storage area, and cinder storage area. Details regarding excavation can be found in the Riverbend Steam Station Coal Ash Excavation Plan submitted to NCDENR by Duke Energy in November 2014 (http://portal.ncdenr.org/web/wq/ca-excavation-plans). The excavation plan covers the first 12 to 18 months of ash basin excavation activities, including the initiation of basin dewatering, ash storage removal, and other permitted ash activities within the waste boundary. The excavation plan is a living document and can be modified based on changing site conditions. Currently, ash has been transported to three locations: Waste Management’s R&B Landfill in Homer, GA; Duke Energy’s Marshall FGD Landfill in Mooresville, NC; and Charah’s Brickhaven Mine. The plan will be updated and submitted to NCDEQ annually or as required during the excavation process. This CAP supplements the existing excavation plan by proposing corrective action and a post-closure monitoring plan. Corrective Action Plan Part 2 Riverbend Steam Station Ash Basin 51 7.2.2 Monitored Natural Attenuation 7.2.2.1 Demonstration of MNA The use of MNA as a remedy involves the monitoring of select parameters to ensure COIs are attenuating. Once the ash within the source areas is removed, groundwater quality will improve over time due to dilution from the recharge to shallow groundwater, precipitation, and adsorption of COIs. Tier I and II analyses were conducted for RBSS (discussed in Section 6.3.2 and Appendix H). A geochemical site conceptual model for COI attenuation involving reversible and irreversible 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 COIs will migrate. The most significant finding was that precipitating iron and manganese was removing other COIs through co-precipitation and adsorption, thus confirming that attenuation is occurring. In support of this reaction, clay minerals and Fe-Mn-Al oxides were observed in samples. Groundwater modeling did not take into consideration the removal of COIs via co-precipitation with iron oxides, which likely resulted in an over-prediction of COI transport, causing some of the COIs to exceed the 2L Standards or IMACs at the compliance boundary in the model output. Surface water models have determined that even with over-prediction of COIs to Mountain Island Lake, exceedance of the 2B Standards will not occur. Based on these predictions, site conditions are favorable for MNA to be implemented at the RBSS site. 7.2.2.2 Verification of MNA The MNA monitoring program and the data collection and evaluation to advance the Tier III assessment should be implemented during excavation (removal) activities. The monitoring will continue through the removal effort and be maintained until water quality meets remedial objectives whether that be 2L/2B Standards or a site-specific standards based on background conditions, as applicable. The site monitoring requirements are discussed in Section 9. The current groundwater monitoring network along with monitoring wells scheduled to be installed to address the need for additional assessment at the site are suitable for characterization of COIs and monitoring effectiveness of MNA at RBSS. Monitoring wells within the waste boundary will be abandoned as part of closure activities. Well abandonment reports are located in Appendix A. With the exception of background wells no new wells are proposed at this time. If COIs or COI concentrations are observed to increase during the MNA monitoring, effectiveness of MNA should be re-evaluated. If warranted, additional remedial alternatives should be evaluated and implemented as necessary and as described above. Corrective Action Plan Part 2 Riverbend Steam Station Ash Basin 52 8 Recommended Interim Activities Several interim activities will occur at the RBSS site to address additional assessment 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 Well Installation Eight additional groundwater assessment wells, one replacement well, and six background wells are currently being installed to address data needs identified in the CSA. These wells are being installed to better assess background conditions upgradient of the ash storage areas at the RBSS site and to further delineate COIs. Additional well locations are presented on Figure 8-1. Assessment wells are being placed in shallow and deep locations in the vicinity of the cinder storage area. The purpose of these wells is to better delineate COIs in the vicinity of the cinder storage area. The wells will also better define groundwater flow direction in the vicinity of the cinder storage, and Mountain Island Lake. Concurrent with the additional assessment wells, six new background wells are being installed on Duke Energy property hydrogeologically upgradient of the source area. These wells will increase the understanding of naturally occurring COI concentrations at RBSS. The new background wells are designated BG-4S, BG-4D, and BG-4BR; and BG-5S, BG-5D, and BG-5BR. The new assessment wells are designated GWA-11S, GWA-11D, GWA-12S, GWA-12D, GWA-13S, GWA-13D, GWA 14S, GWA 14D, and replacement well MW -2S. 8.2 Additional Groundwater Sampling and Analyses The additional assessment wells and background wells will be incorporated into the groundwater monitoring network in 2016 and sampled concurrently and in accordance with the Interim and Effectiveness Monitoring Plans. Review of the data will be used to refine the understanding of natural background concentrations of COIs, refine the existing PPBCs, and to delineate the vertical and horizontal extent of COI impacts. The Interim and Effectiveness Monitoring Plans associated with RBSS are described in Section 9. Corrective Action Plan Part 2 Riverbend Steam Station Ash Basin 53 9 Interim and Effectiveness Monitoring Plans Interim and Effectiveness Monitoring Plans (Monitoring Plans) provide detailed information on field activities to be performed during collection of groundwater and AOW samples at the RBSS site. The Monitoring Plans are intended to evaluate the effectiveness of the proposed corrective actions; monitor the movement of contaminants in groundwater during and after excavation of the RBSS site’s ash basin Primary and Secondary Cells, the ash storage area, and the cinder storage area; and address the need to evaluate baseline conditions and seasonal variation in groundwater and AOWs at RBSS. These Monitoring Plans replace the monitoring plan provided in the CSA Report (Section 16 - Interim Monitoring Plan). Protocols for groundwater and AOW sample collection, analysis, and reporting are consistent between the Monitoring Plans. This sampling and analysis will be completed in accordance with the Monitoring Plans presented below, the CSA Work Plan, and the Low Flow Sampling Plan (CSA Report Appendix G). 9.1 Interim Monitoring Plan The Interim Monitoring Plan has been developed to provide baseline seasonal analytical data associated with RBSS. The Interim Monitoring Plan will be implemented at RBSS 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 ash basin, ash storage area, and cinder storage area and evaluate seasonal trends associated with COIs. • Monitor the movement of COIs within groundwater and the interaction of groundwater with AOW s. • Determine seasonal groundwater flow direction and elevations throughout RBSS and monitor potential changes to groundwater flow direction and elevation as the result of closure activities. The DQOs will be met through the following activities: • Perform groundwater and AOW sampling at the locations depicted on Figure 2-1 and identified in Table 9-1 through the first half of 2016. These monitoring events, planned for the First and Second Quarters 2016, will be combined with analytical data from Rounds 1 through 4 (collected in 2015) to evaluate seasonal water quality conditions at the RBSS site. Additional assessment wells are being installed at the RBSS site in First Corrective Action Plan Part 2 Riverbend Steam Station Ash Basin 54 Quarter 2016 and may be added to the interim monitoring network following installation. If monitoring indicates that excavation activities are mobilizing COIs towards off-site receptors, more frequent sampling of select wells will be considered. • Perform groundwater static water level measurements at monitoring wells on an annual basis. • 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 AOW locations will be conducted at the RBSS 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 sampling event. 9.1.2.2 Sample Locations Groundwater monitoring well and AOW locations to be sampled during the interim monitoring are identified in Table 9-1 and depicted on Figure 2-1. Wells may be added to the sampling program through the installation of additional monitoring wells or removed as excavation of the ash basin, ash storage area, and cinder storage area leads to well abandonment. NCDEQ will be notified and NCDEQ’s approval will be obtained prior to the abandonment of these monitoring wells. 9.1.2.3 Analytical Requirements Analytical parameters will be consistent with those specified in the approved Work Plan. Analytes for monitoring wells and AOWs include total and dissolved metals, alkalinity, calcium, chloride, hexavalent chromium, potassium, magnesium, nitrate, sodium, sulf ate, total combined radium, total combined uranium, total dissolved solids, total organic carbon, and total suspended solids. In addition, samples will be analyzed for ammonia for use in evaluation of MNA as a corrective action. Analytical services will be provided by a North Carolina certified laboratory. Chemical analytes, analytical methods, bottle requirements, and preservatives are provided in Table 9-2. 9.1.3 Reporting Validated analytical results from the First Quarter 2016 sampling 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 sampling event, Duke Energy proposes to submit a groundwater monitoring report to NCDEQ that summarizes the results from the First and Second Quarter 2016 sampling events. Corrective Action Plan Part 2 Riverbend Steam Station Ash Basin 55 9.2 Effectiveness Monitoring Plan The Effectiveness Monitoring Plan has been developed to monitor select wells during excavation to develop baseline analytical data for use with future MNA analysis. The Effectiveness Monitoring Plan will be implemented at select locations 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 within the waste boundary will likely preclude accurate analysis of MNA processes through August 2019. 9.2.1 Data Quality Objectives The following DQOs are associated with the Effectiveness Monitoring Plan: • Monitor the effectiveness of the approved remedy (excavation and MNA) • Monitor changes in groundwater and AOW COI concentrations as the result of excavation within the waste boundary. • Monitor the potential migration of COIs within groundwater and the expression of groundwater as AOW s. • Collect additional analytical data from new background wells to establish new PPBCs. • Monitor seasonal groundwater flow direction and elevations, and monitor potential changes to groundwater flow direction and elevation resulting from closure activities. The DQOs will be met through the following activities: • Perform groundwater and AOW sampling, including MNA parameters at select monitoring well and AOW locations. • Perform groundwater static water level measurements at CSA, compliance, and voluntary monitoring wells concurrent with groundwater sampling described above. • Perform total depth measurements at CSA, compliance, and voluntary monitoring wells on an annual basis. • Prepare reports documenting sampling results and analysis for submittal to NCDEQ. 9.2.2 Sampling Requirements Following the Second Quarter 2016 sampling event, four seasonal sampling events, including CSA Rounds 1 and 2, will have occurred at the RBSS site. Results from the four seasonal sampling events will be evaluated to establish an MNA sampling network of select monitoring well and AOW locations. Results will also be evaluated to determine the need for increased frequency of sampling in beyond the waste boundary to monitor potential migration of COIs as the result of excavation activities. Additional monitoring of the MNA 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 sampling event of these locations will be conducted in 2016 in conjunction with the Third Quarter 2016 NPDES Corrective Action Plan Part 2 Riverbend Steam Station Ash Basin 56 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 to correlate the results from the MNA network sampling with the NPDES results. Sampling frequency associated with the Effectiveness Monitoring Plan 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. Additional sampling beyond the MNA network, including excavation-specific monitoring associated with the ash management area, will be evaluated after completion of the Second Quarter 2016 sampling event and will be proposed prior to the subsequent event sampling event. In order to establish PPBCs using the additional background wells described in Section 8, an additional monitoring event will be collected in 2016 from all RBSS background monitoring wells in order to obtain four rounds of analytical data in 2016. Based on the sampling schedule proposed in the Interim Monitoring Plan and the scheduled effectiveness monitoring to be conducted concurrent with the compliance monitoring (scheduled for October 2016), it is anticipated this additional background sampling event would be conducted in the Third Quarter 2016. 9.2.2.2 Sample Locations Sampling locations associated with the Effectiveness Monitoring Plan will be established after collection of the Second Quarter 2016 sampling event, following evaluation of four seasonal sampling 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 each sampling event will be submitted to NCDEQ within 120 days of completion of each sampling event. Results from the additional background monitoring event will be included in the next scheduled monitoring report. 9.3 Sampling and Analysis 9.3.1 Monitoring Well Measurements and Inspection Groundwater sampling will be conducted at monitoring well locations associated with the CSA and additional assessment wells, as described in the Monitoring Plans above. During each sampling 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 Corrective Action Plan Part 2 Riverbend Steam Station Ash Basin 57 recorded on a dedicated field form, a field notebook, and/or electronically via the iForms 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 a 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 1 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. Each monitoring well will be inspected while performing water level measurements for damage to the casing, protective monuments, 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 Sample Collection 9.3.2.1 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 draw down, 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. 9.3.2.2 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. 9.3.2.3 AOW Sample Collection Grab samples will be collected from each 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 samples will be collected by slightly submersing 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 Corrective Action Plan Part 2 Riverbend Steam Station Ash Basin 58 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. 9.3.2.4 Sample Naming Convention Samples are 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 RBSS is RV. • Sample Identification Name will be the sample location name (i.e., BG-1D). • Sample Matrix o NS – Normal Sample o FD – Field Duplicate o AMB – Ambient Blank o FB – Filter Blank o EB – Equipment 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-1D in February 2016 would be designated RV-BG-1D-NS-1Q16. If a field duplicate was also collected from that location it would be designated RV-BG-1D-FD-1Q16. 9.3.2.5 Waste Handling Purge water and decontamination water will be discharged to the ground surface at the RBSS site. Other investigation derived waste, including disposable tubing and gloves, will be bagged and disposed of as part of the RBSS site’s municipal solid waste. 9.3.2.6 Chain of Custody and Sample Delivery All samples will be tracked using chain-of-custody procedures. A separate chain-of-custody form will be filled out by each sample team and accompany each cooler shipped. Samples will be hand delivered or shipped to the contract laboratory. 9.3.3 Quality Assurance/Quality Control In addition to laboratory and other quality assurance/quality control procedures, field quality control measures are implemented to ensure that data meet project requirements. The following field quality control procedures are 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. Corrective Action Plan Part 2 Riverbend Steam Station Ash Basin 59 Equipment blanks receive all the tests which are to be performed on the associated samples. • Ambient Blanks – Field blanks are also filled with reagent grade water. Field blanks are created in the field and are intended to measure background contamination in the field. Field blanks will be created by filling the appropriate sample containers while at the site collecting other water samples. • Filter Blanks – Filter blanks evaluate the possible addition of chemicals from the filter to the sample. Filter blanks are created in the field and are intended to measure 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 are created by the laboratory and accompany low level mercury samples the entire time they are being shipped and are out in the field. Corrective Action Plan Part 2 Riverbend Steam Station Ash Basin 60 10 Implementation Cost and Schedule CAMA Section §130A-309.211(b)(1) requires implementation of corrective action within 30 days of CAP approval. 10.1 Implementation Cost The recommended corrective action at RBSS following source removal is MNA. A summary of costs for the selected remedy is provided in Table 10-1. Note that the actual cost will be dependent on the actual conditions that exist following excavation and completion of 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 – 15 new wells $126,000 Site Prep and Erosion Sediment Control $30,000 Field Management (15%) $23,000 Well Install Reporting (GW-1s/construction records) $5,000 Project Management (10%) $18,000 Contingency (20%) $41,000 Total Capital Costs $243,000 Annual Costs - Monitoring/Reporting Lab Analysis $19,000 Data Validation $15,000 Reporting $60,000 Equipment and Expendables $9,000 Sampling Labor $37,000 Project Management (10%) $14,000 Escalation to Mid-Point (4%) $6,000 Annual Monitoring/Reporting Costs $160,000 Total Capital/Annual Costs for Project Duration* $6,700,000 *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 The Interim Monitoring Plan will be implemented at the RBSS site during sampling activities conducted in the first half of 2016. Details of interim monitoring are discussed in Section 9. MNA will be re-evaluated after closure activities are completed using results of the interim monitoring. Beginning in September 2016, effectiveness monitoring events will coincide with compliance monitoring events required by the NPDES permit, which are conducted in January, May, and September. MNA effectiveness and groundwater quality monitoring results at the RBSS site will be evaluated and used to assess the effectiveness of MNA as a remedial alternative. Based on the Corrective Action Plan Part 2 Riverbend Steam Station Ash Basin 61 resulting monitoring data, recommendations will be made regarding modifications in the monitoring program to ensure representative data are being collected, changes in the implementation of the selected remedy, or if other alternatives need to be considered. Corrective Action Plan Part 2 Riverbend Steam Station Ash Basin 62 11 References ASTM. 2014. 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