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HomeMy WebLinkAboutNC0004961_RBSS_SARP_Rev0_Narrative_20161219SITE ANALYSIS AND REMOVAL PLAN RIVERBEND STEAM STATION REVISION 0 Prepared for DUKE ENERGY,. Duke Energy 550 South Tryon Street Charlotte, North Carolina 28202 December 2016 Prepared by IT amec aC foster wheeler Amec Foster Wheeler Environment & Infrastructure, Inc. t�,e t;)���de yR`t1� �M�o�ao C�Oo �f SEAL 0321 '1 Ni v 9:2S O� Amec Foster Wheeler Environment & Infrastructure, Inc. December 2016 Duke Energy Coal Combustion Residuals Management Program Riverbend Steam Station Site Analysis and Removal Plan Revision 0 i EXECUTIVE SUMMARY Amec Foster Wheeler Environment & Infrastructure, Inc. (Amec Foster Wheeler) has prepared this Site Analysis and Removal Plan (Removal Plan) in support of the proposed closure of the Coal Combustion Residuals (CCR) Facilities (CCR Management Facilities) at the Riverbend Steam Station (Riverbend) located on the Catawba River near Mt. Holly in Gaston County, North Carolina. The purpose of this Removal Plan is to seek the North Carolina Department of Environmental Quality’s (NCDEQ) concurrence with the Duke Energy Carolinas, LLC (Duke) plan for closure of the CCR Management Facilities located at Riverbend. This Removal Plan is submitted to NCDEQ on behalf of Duke. The work to be performed in support of the closure of the basins is summarized in this document, which is consistent with the requirements of the North Carolina Coal Ash Management Act (CAMA). This Removal Plan incorporates consideration of impacts to communities and managing cost. The drawings presented herein are subject to change in response to actual site conditions encountered as work progresses. The closure method entails excavation of CCR within the CCR Management Facilities, establishing final grades to promote drainage, and breaching the existing dams that form the ash basins. Duke has retired the coal-fired generating facility at Riverbend. CCR Management Facilities closure is being undertaken as part of the overall Riverbend decommissioning efforts. Specifically, Riverbend CCR Management Facilities include two ash basins known as the Primary and the Secondary Ash Basin and two dry ash storage areas known as Cinder Pit Storage Area and Dry Ash Stack. Results of evaluations reported in this Removal Plan indicate that the total volume of CCR contained at the Primary Ash Basin is estimated to contain approximately 2,183,000 cubic yards (cy) (approximately 2.6 million tons-assuming a moist density of approximately 1.2 tons/cy), and the Secondary Ash Basin is estimated to contain approximately 829,000 cy (approximately 1.0 million tons). In addition to the volumes impounded in the Ash Basins, there is an estimated 1,135,000 cy (1.4 million tons) and 169,000 cy (approximately 203,000 tons) of CCR in the Dry Ash Stack and Cinder Pit Storage Area, respectively. In summary, the estimated quantity of CCR in the existing CCR management Facilities at the Riverbend is approximately 4,316,000 cy (5,179,000 tons). Note that removal of CCR in the Dry Ash Stack commenced on May 21, 2015 and is ongoing. Assessment activities for the Riverbend facility were performed by HDR Engineering Inc. of the Carolinas (HDR) and were reported in a Comprehensive Site Assessment (CSA) Report dated August 18, 2015, a Corrective Action Plan (CAP) Part 1 dated November 16, 2015, and a CAP Part 2 dated February 12, 2016. Assessment work included a source area assessment in the ash basins, Dry Ash Stack, and Cinder Pit Storage Area. Source area impact delineation included the collection of samples in surrounding soil, partially weathered rock (PWR), bedrock, surface water, sediment and groundwater. Results of assessment identified the following constituents of interest (COIs) in soil: arsenic, boron, cobalt, iron, manganese, nickel, selenium, and vanadium. The approximate horizontal extent of soil impacts was delineated during the CSA and found to be limited to the area beneath the ash basin and one location along the waste boundary south of the Dry Ash Stack. Where soil impacts were identified, the approximate vertical extent of contamination beneath the ash/soil interface was generally limited to the uppermost soil sample collected beneath ash. COIs identified in groundwater included: Amec Foster Wheeler Environment & Infrastructure, Inc. December 2016 Duke Energy Coal Combustion Residuals Management Program Riverbend Steam Station Site Analysis and Removal Plan Revision 0 ii antimony, arsenic, boron, chromium (total), cobalt, iron, manganese, sulfate, TDS, thallium, and vanadium. The approximate horizontal extent of groundwater impacts was found to be limited to beneath the waste boundary and northeast of the ash basin, however, additional delineation was recommended. The approximate vertical extent of groundwater impacts was found to be limited to the shallow and deep zones, with vertical migration of COIs being impeded by geologic conditions present beneath the source area. Surface water COIs included: aluminum, cadmium, chromium, cobalt, copper, iron, lead, manganese, selenium, thallium, vanadium, and zinc. Surface water was found to generally flow from the south side of the site to Mountain Island Lake. Sediment COIs (arsenic, barium, boron, cobalt, iron, manganese, and vanadium). Cobalt, iron, manganese, and vanadium were also detected naturally occurring constituents in background soil. A preliminary geotechnical evaluation was performed and is presented in this Removal Plan. The results of the investigations indicate that the subsurface materials primarily consist of, from top to bottom, CCR (within the CCR management Facilities) or Dike Fill (at the perimeters of the basins), and Foundation Soils (consisting primarily of alluvium overlying residual soils). A partially weathered rock zone was encountered at the transition between the residual soils and the competent bedrock. The Primary and Secondary Ash Basins were operated as an integral part of Riverbend's wastewater and stormwater management system. Description of the existing stormwater and wastewater management facilities, as well as provisions for stormwater and wastewater management during and after the ash basins closure, are provided in this Removal Plan. This Removal Plan also presents a summary of the engineering evaluation and analyses performed, as well as a Construction Quality Assurance (CQA) Plan. Applicable permits required for closure of the CCR Management Facilities are summarized in this Removal Plan. A Post-Closure Operations Maintenance and Monitoring Plan is provided, including the interim groundwater monitoring program currently under evaluation by NCDEQ. This Removal Plan presents estimated schedule milestones related to basin closure and post-closure activities. Amec Foster Wheeler Environment & Infrastructure, Inc. December 2016 Duke Energy Coal Combustion Residuals Management Program Riverbend Steam Station Site Analysis and Removal Plan Revision 0 iii LIST OF ACRONYMS AND ABBREVIATIONS Acronym/ Abbreviation Definition 2B NCAC Title 15A, Subchapter 2B Surface Water and Wetland Standards 2L NCAC Title 15A, Subchapter 2L Groundwater Classification and Standards ASTM American Society for Testing Materials CAMA Coal Ash Management Act CAP Corrective Action Plan CCP Coal Combustion Products CCR Coal Combustion Residuals COI Constituent of Interest CMP Corrugated Metal Pipe CQA Construction Quality Assurance CSA Comprehensive Site Assessment DO Dissolved Oxygen EPA United States Environmental Protection Agency FGD Flue Gas Desulfurization HDPE High Density Polyethylene H&H Hydrology and Hydraulic IMAC Interim Maximum Allowable Concentrations Kd Partition Coefficient LLDPE Linear Low Density Polyethylene MSW Municipal Solid Waste NCAC North Carolina Administrative Code NCDEQ North Carolina Department of Environmental Quality NCGS North Carolina General Statutes NPDES National Pollutant Discharge Elimination System OM&M Operations Maintenance and Monitoring PMP Probable Maximum Precipitation PTI Permit to Install PWR Partially Weathered Rock RCP Reinforced Concrete Pipe RCRA Resource Conservation and Recovery Act RSL USEPA Regional Screening Level SPLP Synthetic Precipitation Leaching TBD To be determined TCLP Toxicity Characteristic Leaching Procedure USS Unified Soil Classification System Amec Foster Wheeler Environment & Infrastructure, Inc. December 2016 Duke Energy Coal Combustion Residuals Management Program Riverbend Steam Station Site Analysis and Removal Plan Revision 0 iv RECORD OF REVISION Revision Number Revision Date Section Revised Reason for Revision Description of Revision 0 12/2016 N/A N/A Initial Issue 1 2 Amec Foster Wheeler Environment & Infrastructure, Inc. December 2016 Duke Energy Coal Combustion Residuals Management Program Riverbend Steam Station Site Analysis and Removal Plan Revision 0 v TABLE OF CONTENTS EXECUTIVE SUMMARY ............................................................................................................ i LIST OF ACRONYMS AND ABBREVIATIONS ......................................................................... iii RECORD OF REVISIONS ........................................................................................................ iv 1. INTRODUCTION ............................................................................................................. 1 1.1 Site Analysis and Removal Plan Objectives ....................................................................... 1 1.2 Report Organization ............................................................................................................ 1 2. GOVERNING REQUIREMENTS ..................................................................................... 2 3. FACILITY DESCRIPTION AND EXISTING SITE FEATURES ........................................ 4 3.1 Surface Impoundment Description ...................................................................................... 4 3.1.1 Site History and Operations ................................................................................... 4 3.1.2 Estimated Volume of CCR Materials in Impoundments ........................................ 7 3.1.3 Description of Surface Impoundment Structural Integrity ...................................... 8 3.1.4 Sources of Discharges into Surface Impoundments............................................ 10 3.1.5 Existing Liner System .......................................................................................... 11 3.1.6 Inspection and Monitoring Summary ................................................................... 11 3.2 Site Maps .......................................................................................................................... 13 3.2.1 Summary of Existing CCR Impoundment Related Structures ............................. 13 3.2.2 Receptor Survey .................................................................................................. 15 3.2.3 Existing On-Site Landfills ..................................................................................... 16 3.3 Monitoring and Sampling Location Plan ........................................................................... 16 4. RESULTS OF HYDROGEOLOGIC, GEOLOGIC, AND GEOTECHNICAL INVESTIGATIONS ........................................................................................................ 18 4.1 Hydrogeology and Geologic Descriptions ......................................................................... 18 4.2 Stratigraphy of the Geologic Units Underlying Surface Impoundments ........................... 19 4.3 Hydraulic Conductivity Information ................................................................................... 19 4.4 Geotechnical Properties .................................................................................................... 21 4.4.1 Primary Ash Basin................................................................................................ 21 4.4.2 Secondary Ash Basin ........................................................................................... 23 4.4.3 Intermediate Dam................................................................................................. 24 4.5 Chemical Analysis of Impoundment Water, CCR Materials and CCR Affected Soil ........ 25 4.5.1 Source Area Characterization .............................................................................. 26 4.5.2 Soil, Partially Weathered Rock and Bedrock Assessment .................................. 27 4.5.3 Surface Water and Sediment Assessment .......................................................... 28 4.6 Historical Groundwater Sampling Results ........................................................................ 29 4.7 Groundwater Potentiometric Contour Maps ..................................................................... 31 4.8 Figures: Cross Sections Vertical and Horizontal Extent of CCR within the Impoundments .......................................................................................................................................... 31 5. GROUNDWATER MODELING ANALYSIS................................................................... 32 Amec Foster Wheeler Environment & Infrastructure, Inc. December 2016 Duke Energy Coal Combustion Residuals Management Program Riverbend Steam Station Site Analysis and Removal Plan Revision 0 vi 5.1 Site Conceptual Model Predictions ................................................................................... 32 5.2 Groundwater Chemistry Effects ........................................................................................ 34 5.3 Groundwater Trend Analysis Methods .............................................................................. 35 6. BENEFICIAL USE AND FUTURE USE ........................................................................ 41 6.1 CCR Material Use ............................................................................................................. 41 6.2 Site Future Use ................................................................................................................. 41 7. CLOSURE DESIGN DOCUMENTS .............................................................................. 42 7.1 Engineering Evaluations and Analyses ............................................................................ 42 7.1.1 Freeboard During Dam Decommissioning ........................................................... 42 7.1.2 Stormwater Management During Interim Conditions ........................................... 42 7.1.3 Stormwater Management During Final Conditions .............................................. 43 7.2 Removal Plan Drawings .................................................................................................... 44 7.3 Construction Quality Assurance Plan ............................................................................... 44 8. MANAGEMENT OF WASTEWATER AND STORMWATER ........................................ 45 8.1 Stormwater Management .................................................................................................. 45 8.2 Wastewater Management ................................................................................................. 45 9. DESCRIPTION OF FINAL DISPOSITION OF CCR MATERIALS ................................. 46 10. APPLICABLE PERMITS FOR CLOSURE .................................................................... 47 10.1 Decommissioning Request and Approval ......................................................................... 47 11. POST-CLOSURE MONITORING AND CARE .............................................................. 49 11.1 Groundwater Monitoring Program ..................................................................................... 49 12. PROJECT MILESTONES AND COST ESTIMATES ..................................................... 50 12.1 Project Schedule ............................................................................................................... 50 12.2 Closure and Post-Closure Cost Estimate ......................................................................... 50 13. REFERENCED DOCUMENTS ...................................................................................... 51 Amec Foster Wheeler Environment & Infrastructure, Inc. December 2016 Duke Energy Coal Combustion Residuals Management Program Riverbend Steam Station Site Analysis and Removal Plan Revision 0 vii Tables Table 2-1 North Carolina CAMA Closure Plan Requirements, Summary and Cross Reference Table Table 3-1 Estimated Volume and Weight of CCR Materials in Impoundments Table 4-1 Hydrostratigraphic Unit Properties - Horizontal Hydraulic Conductivity Table 4-2 Hydrostratigraphic Unit Properties - Vertical Hydraulic Conductivity Table 4-3 Exceedances of 2L Standards within Compliance Wells Table 5-1 Summary of Modeled COI Results at the Compliance Boundary Table 9-1 List of Approved Lined Landfills and Structural Fills for Riverbend CCR Materials Figures Figure 1 Site Vicinity Map Figure 2 Site Aerial Map – CCR Units Figure 3 Compliance Boundary Appendices Appendix A Riverbend Steam Station Ash Inventory Appendix B Tables and Select Figures from Comprehensive Site Assessment Report (HDR, 2015a) and Corrective Action Plan – Part 1 & Part 2 (HDR, 2015b and HDR, 2016) Appendix C Tables and Select Figures from Phase 2 Reconstitution of Ash Pond Designs Report, URS, 2014 Appendix D Riverbend Steam Station Ash Pond CCR Removal Grading Plan Appendix E Engineering Evaluations and Analyses of Riverbend Closure Design Grading Plans Appendix F Riverbend Construction Quality Assurance Plan Appendix G Riverbend Post-Closure Operations Maintenance and Monitoring Plan Appendix H Riverbend Closure and Post-Closure Cost Estimates (to be added at a later date) Amec Foster Wheeler Environment & Infrastructure, Inc. December 2016 Duke Energy Coal Combustion Residuals Management Program Riverbend Steam Station Site Analysis and Removal Plan Revision 0 1 1. INTRODUCTION Amec Foster Wheeler has prepared the following Site Analysis and Removal Plan (Removal Plan) for the Duke Energy (Duke) Riverbend Steam Station. The Riverbend Steam Station is located at 175 Steam Plant Road, Mt. Holly, Gaston County, North Carolina on the western bank of the Catawba River near Horseshoe Bend Beach Road. The site is located approximately 13 miles northwest of Charlotte, North Carolina. The project location from a regional context is illustrated on Figure 1. Duke has retired the coal-fired generating facility at the Riverbend Steam Station property. Ash management facility closure is being undertaken as part of the overall station decommissioning efforts. Specifically, the Riverbend Steam Station ash management facilities include two (2) ash basins known as the Primary and the Secondary Ash Basins and two (2) dry ash storage areas known as Cinder Pit Storage Area and Dry Ash Stack. The ash management facilities are shown on Figure 2. Duke intends to close the Primary and Secondary Ash Basins as well as Cinder Pit Storage Area and Dry Ash Stack. Both basins will be closed by removal of the coal ash for transport to an off-site landfill or structural fill. The purpose of this document is to present the plan and objectives to achieve closure for the Riverbend Steam Station ash management facilities and meet the requirements of applicable State rules. 1.1 Site Analysis and Removal Plan Objectives This Removal Plan has been prepared to address closure through removal of coal combustion residuals (CCRs) from the Riverbend Steam Station and to comply with the statutory requirements of the North Carolina Coal Ash Management Act (CAMA). 1.2 Report Organization Although closure of the CCR surface impoundments at Riverbend is controlled by Part II, Sections 3.(b) and 3.(c) of CAMA (and not N.C.G.S. § 130A-309.214), for purposes of consistency with the closure plans for those non-high-priority Duke facilities to which N.C.G.S. § 130A-309.214 applies, this Removal Plan is structured to follow generally the closure plan elements set forth in N.C.G.S. § 130A-309.214(a)(4). Amec Foster Wheeler Environment & Infrastructure, Inc. December 2016 Duke Energy Coal Combustion Residuals Management Program Riverbend Steam Station Site Analysis and Removal Plan Revision 0 2 2. GOVERNING REQUIREMENTS In August of 2014, the North Carolina General Assembly passed Senate Bill (S.B.) 729 known as CAMA, which lists specific requirements for CCR surface impoundment closure. For the Riverbend Steam Station, “surface impoundment” as defined in NCGS §130A-309.201(6) is interpreted to include the Primary Ash Basin and Secondary Ash Basin. However, closure of the Dry Ash Stack and Cinder Pit Storage Area will be implemented in conjunction with ash basin closure. The CAMA closure plan requirements are summarized in Table 2-1 for reference. CAMA deems the Riverbend Steam Station a “high-priority” site and specifically requires closure by August 1, 2019, which entails dewatering the ash basins to the maximum extent practicable and removing and transferring CCR from basins to a lined landfill or structural fill. (Note that ash removal is required to be complete by August 1, 2019; however, dam decommissioning and final grading of the former ash basin areas and completion of corrective action to restore groundwater quality, if needed, as provided in N.C.G.S. § 130A-309.204, may extend beyond this date.) Note that ash removal is required to be complete by August 1, 2019, however, dam decommissioning and final grading of the former ash basin areas and completion of corrective action may extend beyond this date. The closure plan requirements are set out for non-high-priority sites in NCGS § 130A- 309.214(a)(4). Although not specifically applicable to Riverbend Steam Station, which is a high- priority site required to close pursuant to Part II, Sections 3.(b) and 3.(c) of CAMA, this Removal Plan relies on NCGS § 130A-309.214(a)(4) solely to inform its organization. The Riverbend Steam Station Removal Plan includes the following:  Facility description  Site maps  Hydrogeologic, geologic, geotechnical characterization results  Groundwater potentiometric maps and extent of contaminants of concern  Groundwater modeling  Description of beneficial reuse plans  Closure plan drawings, design documents, and specifications  Description of the construction quality assurance and quality control program  Description of waste water disposal and stormwater management provisions  Description of how the final disposition of CCRs will be provided  List of applicable permits to complete closure  Description of post-closure monitoring and care plans  Estimated closure and post-closure milestone dates  Estimated costs of assessment, corrective action, closure and post closure care Amec Foster Wheeler Environment & Infrastructure, Inc. December 2016 Duke Energy Coal Combustion Residuals Management Program Riverbend Steam Station Site Analysis and Removal Plan Revision 0 3  Future site use description In addition to the closure pathway and closure plan requirements, CAMA outlines groundwater assessment and corrective action requirements summarized as follows:  Submit Groundwater Assessment Plans by December 31, 2014;  Within 180 days of Groundwater Assessment Plan approval, complete groundwater assessment and submit a Groundwater Assessment Report; and  Provide a Corrective Action Plan (if required) within 90 days (and no later than 180 days) of Groundwater Assessment Report completion. The groundwater assessment for the Riverbend Steam Station was reported in the Comprehensive Site Assessment Report (CSA Report (HDR, 2015a)) prepared by HDR in August 2015. Corrective action(s) including removal of the CCR materials at the station are being developed in parallel with Removal Plan development. Information from the CSA Report (HDR, 2015a) has been incorporated into this Removal Plan. Information from the CSA Supplements has not been incorporated into this Removal Plan. Duke received permission from NCDEQ to submit a Corrective Action Plan (CAP) in two phases. The first phase, herein referenced as the CAP Part 1 was submitted on November 16, 2015 and includes background information, a brief summary of the CSA findings, a description of site geology and hydrogeology, a summary of the previously completed receptor survey, a description of NCAC Subchapter 2L Groundwater Standards (2L Standards) and NCAC Subchapter 2B Surface Water Standards (2B Standards) exceedances, proposed site-specific groundwater background concentrations, a detailed description of the site conceptual model, and groundwater flow and transport modeling. The second phase, herein referenced as the CAP Part 2, was submitted on February 12, 2016 and includes groundwater and surface water model refinement, risk assessment, alternative methods for achieving restoration, conceptual plans for recommended corrective actions, implementation schedule, and a plan for future monitoring and reporting. Amec Foster Wheeler Environment & Infrastructure, Inc. December 2016 Duke Energy Coal Combustion Residuals Management Program Riverbend Steam Station Site Analysis and Removal Plan Revision 0 4 3. FACILITY DESCRIPTION AND EXISTING SITE FEATURES 3.1 Surface Impoundment Description The following section provides a summary of the history and operations of the CCR facilities at the Riverbend Steam Station. 3.1.1 Site History and Operations The Riverbend Steam Station is located at 175 Steam Plant Road, Mt. Holly, Gaston County, North Carolina on the western bank of the Catawba River near Horseshoe Bend Beach Road. The site is located approximately 13 miles northwest of Charlotte, North Carolina. Commercial operations of the station began in 1929 with two units, expanding to seven units in 1954 for a total combined peak generating capacity of 454 megawatts (MW). After expansion, the station continued operation until all units were retired in April 2013. The CCR storage areas at the Riverbend site are identified as the Cinder Pit Storage Area, the Dry Ash Stack, and the ash basins consisting of the Primary and Secondary Ash Basins. CCR storage at the Riverbend site was initially contained within the Cinder Pit Storage Area from startup in 1929 until a single cell ash basin was constructed in 1957. During operation of the Cinder Pit Storage Area, CCR materials were transported from the plant by rail to the storage area. Upon completion of the single cell ash basin in 1957, sluicing of the CCR materials began. The single cell ash basin was expanded in 1979 to the existing configuration by construction of the Intermediate Dam to effectively create two cells and raising the crest of the Primary Ash Basin Dam by approximately ten feet. The location of the Riverbend Steam Station and associated CCR storage areas is presented in Figure 2. Prior to eliminating sluicing of CCR materials to the ash basins, the Primary Ash Basin was generally used for initial treatment, with secondary treatment occurring in the Secondary Ash Basin before discharge to the Catawba River. Discharge from the Primary Ash Basin to the Secondary Ash Basin occurred via a discharge tower in the northernmost corner of the Primary Ash Basin near the Intermediate Dam. Discharge from the Secondary Ash Basin formerly occurred via a similar concrete discharge tower and a 30-inch corrugated metal pipe (CMP) into a concrete lined channel that eventually flowed into Mountain Island Lake (Catawba River). The outlet pipe has been grouted and discharge is via the dewatering pump system and wastewater treatment system only. The site has three regulated impoundment structures, the Primary Ash Basin Dam (State ID GASTO-097), the Secondary Ash Basin Dam (State ID GASTO-098), and the Intermediate Dam (State ID GASTO-099). The following is a summary description of each impoundment structure: 3.1.1.1 Primary Ash Basin Dam (GASTO-097) The Primary Ash Basin Dam (GASTO-097) is classified by North Carolina Department of Environmental Quality (NCDEQ) as a high hazard dam. This classification is driven by the potential environmental effects if a failure of the Primary Ash Basin Dam were to occur. The Amec Foster Wheeler Environment & Infrastructure, Inc. December 2016 Duke Energy Coal Combustion Residuals Management Program Riverbend Steam Station Site Analysis and Removal Plan Revision 0 5 Primary Ash Basin Dam is located along the western edge of the Primary Ash Basin and was constructed during the original commissioning in 1957, and was raised 10 feet in 1979 (the vertical extension). It impounds a surface area of approximately 41 acres. Mountain Island Lake is directly downstream of the dam, northeast of the CCR facilities. The main characteristics of the dam, which is depicted in Appendix C Figure 2 (AECOM 2015), are:  Dam Length: 1,550 feet  Maximum Dam Height: 80 feet  Crest Elevation: minimum of 727.3 feet on 2014 topographic mapping  Crest Width: 15 feet  Principal Spillway: Concrete vertical riser with stop log level control and 36-inch diameter reinforced concrete outlet pipe  Normal Pool Elevation: 722 feet above mean sea level (AMSL) (as designed)  Maximum Basin Elevation: approximately 724 feet AMSL  Ash levels varied from approximately elevations 712 to 718 feet prior to commencement of removal The Primary Ash Basin Dam is constructed of a central compacted soil embankment bearing on a foundation of residuum consisting of silty sands, underlain by partially weathered rock. Interior and exterior slopes along the dam are inclined at 2H:1V to 2.5H:1V (horizontal to vertical), except for the upstream slope of the vertical extension, which is 3.5H:1V. There are two benches on the downstream slope. 3.1.1.2 Secondary Ash Basin Dam (GASTO-098) The Secondary Ash Basin Dam (State ID GASTO-098) is classified by NCDEQ as a high- hazard dam. This classification is driven by the potential environmental effects if a failure of the Secondary Ash Basin Dam were to occur. The Secondary Ash Basin Dam was constructed in 1957 during the original power plant commissioning, and raised 10 feet (by the downstream method) in 1979. The Secondary Ash Basin impounds a surface area of 28 acres, and Mountain Island Lake is directly downstream of the dam. The main characteristics of the dam, which is depicted in Appendix C Figure 2 (AECOM 2015), are:  Dam Length: 3,070 feet  Maximum Dam Height: 70 feet  Crest Elevation: minimum of 717.7 feet on 2014 topographic mapping  Crest Width: 15 – 20 feet  Principal Spillway: Concrete vertical riser with stop log level control (out of service)  Spillway Outlet: 30-in diameter CMP (out of service) Amec Foster Wheeler Environment & Infrastructure, Inc. December 2016 Duke Energy Coal Combustion Residuals Management Program Riverbend Steam Station Site Analysis and Removal Plan Revision 0 6  Normal Pool Elevation: 712 feet AMSL (as designed)  Maximum Basin Elevation: approximately 714 feet AMSLAsh levels vary from approximately elevations 682 to 712 feet AMSL The principal spillway and discharge outlet pipe were taken out of service in January 2016. The 30-in diameter CMP was plugged and grouted, and the metal weir box at the pipe outlet was removed. Dewatering of the Secondary Ash Basin via a pump system has begun. The pump currently discharges to an on-site wastewater treatment system. The Secondary Ash Basin Dam is constructed of a compacted embankment bearing on a foundation of alluvium overlying residuum consisting of silty sand. Interior and exterior slopes along the dam are inclined at 2H:1V to 2.5H:1V. A bench approximately 15 feet wide has been constructed at approximately the mid-elevation point of the downstream slope along the southern two thirds of the dam. 3.1.1.3 Intermediate Dam (GASTO-099) The Intermediate Dam (State ID GASTO-099, high hazard) is a divider dike that separates the Primary Ash Basin from the Secondary Ash Basin. Therefore, the Intermediate Dam is located downstream of the Primary Ash Basin and upstream of the Secondary Ash Basin. The principal spillway of the Primary Ash Basin is functional and capable of allowing flow through the Intermediate Dam and into the Secondary Ash Basin. The Intermediate Dam was constructed on top of pond ash using compacted ash and possibly soil in 1979. The main characteristics of the dam, which is depicted in Appendix C Figure 2 (AECOM 2015), are:  Dam Length: 945 feet  Maximum Dam Height: 20 feet  Crest Elevation: 728 to 729 feet AMSL  Crest Width: 15 feet  Principal Spillway: None  Normal Pool Elevation: Upstream (Primary Ash Basin) 722 feet AMSL (as designed)  Downstream (Secondary Ash Basin) 712 feet AMSL (as designed)  Maximum Basin Elevation: 724 feet AMSL The Intermediate Dam is constructed of an upper interval of fill consisting of sandy silts and clayey silty sands. The foundation of the Intermediate Dam consists of sluiced ash. The exterior slopes along the upstream and downstream sides of the Intermediate Dam are inclined at an approximate 3H:1V ratio. In addition to the three impoundment structures, the Cinder Pit Storage Area and the Dry Ash Stack have been used as storage areas for CCR materials. The following is a summary of basic details relevant to each storage area: Amec Foster Wheeler Environment & Infrastructure, Inc. December 2016 Duke Energy Coal Combustion Residuals Management Program Riverbend Steam Station Site Analysis and Removal Plan Revision 0 7 3.1.1.4 Cinder Pit Storage Area Prior to construction of the ash basin system, bottom ash (“cinders”) was deposited in the “Cinder Pit” (primarily dry condition). The Cinder Pit Storage Area is approximately 13 acres and is located in a triangular area northeast of the coal pile and northwest of the rail spur. This area was used for storage of ash material at the station prior to the installation of precipitators and a wet sluicing system. The Cinder Pit Storage Area contains predominantly dry bottom ash and a significant portion of the area is currently covered with moderate to dense vegetation. 3.1.1.5 Dry Ash Stack The Dry Ash Stack was constructed of ash removed from the Primary and Secondary Ash Basins in order to prolong the life of the basins. The Dry Ash Stack is approximately 29 acres and is located south of the Primary Ash Basin adjacent to the existing rail spur. The Dry Ash Stack consists of material removed from the ash basins during two stages, the most recent of which occurred in 2007. The CCR materials were covered with at least eighteen inches of soil cover and seeded following construction of each additional level. 3.1.2 Estimated Volume of CCR Materials in Impoundments The principal ash storage areas at the Riverbend Steam Station are the Primary and Secondary Ash Basins formed by the three impoundment structures described in Section 3.1. Although hydraulically separated by the Intermediate Dam which is founded on stored CCR material, the Primary and Secondary Ash Basins effectively form a single storage area. The total volume of CCR materials contained at the Primary Ash Basin is estimated to be approximately 2,182,800 cubic yards and the Secondary Ash Basin is estimated to be approximately 829,500 cubic yards. Factoring for moisture content, this volume represents approximately 2,619,400 tons of CCR material in the Primary Ash Basin and 995,400 tons in the Secondary Ash Basin. In addition to the volumes impounded in the ash basins, there is an estimated 1,134,500 and 168,900 cubic yards of CCR material in the Dry Ash Stack and Cinder Pit Storage Area, respectively. Factoring for moisture content, this volume represents approximately 1,361,400 tons and 202,700 tons of CCR material in the Dry Ash Stack and Cinder Pit Storage Area, respectively. In summary, the estimated quantity of CCR materials in the existing storage areas at the Riverbend Steam Station is approximately 4,316,000 cubic yards or approximately 5.2 million tons (accounting for moisture). Table 3-1, presented below, summarizes the estimated weight and volume of CCR materials and Appendix A of this Removal Plan provides a detailed ash inventory for the Riverbend Steam Station based on bathymetric and topographic surveys of the CCR facilities. Note that these estimates may be updated as new information becomes available. Also note that soil shall be removed at least to a depth that no longer visually exhibits ash intermingled with soil during the excavation of the CCR facilities. Amec Foster Wheeler Environment & Infrastructure, Inc. December 2016 Duke Energy Coal Combustion Residuals Management Program Riverbend Steam Station Site Analysis and Removal Plan Revision 0 8 Table 3-1 Estimated Volume and Weight of CCR Materials in Impoundments Location Estimated CCR Volume (cy) Estimated Moist Weight* (tons) Primary Ash Basin 2,183,000 2.6 million Secondary Ash Basin 829,000 1.0 million Ash Stack 1,135,000 1.4 million Cinder Pit 169,000 203,000 Totals 4,316,000 5.2 million Notes: 1. Values in Table 3-1 are rounded from Appendix A. * – Estimated Moist Weight is calculated using a factor of 1.2 tons/cubic yard to account for moisture content in ash. 3.1.3 Description of Surface Impoundment Structural Integrity Tables and select figures from the Phase 2 Reconstitution of Ash Pond Designs Report (Phase 2 Report) prepared by AECOM (formerly URS) in 2015 are included in Appendix C of this Closure Report. The key findings from the Phase 2 Report are summarized below: 3.1.3.1 Primary Ash Basin Dam  No seeps were observed within the embankments of the Primary Ash Basin Dam. Seepage has historically been observed beyond the downstream toe of Primary Ash Basin dam. This seepage was observed during the site reconnaissance for the Phase 2 Report work and has been reported in subsequent weekly inspection reports.  Artesian conditions have been noted in groundwater monitoring wells located downstream of the toe of Primary and Secondary Ash Basin Dams. These conditions were observed only in select wells located a limited distance beyond the toe of the embankment. Overall, artesian conditions appear to result from a substantial difference in hydraulic head between the ash basin and the groundwater system.  Based upon site reconnaissance conducted between May and September 2014, the subsurface evaluations and observation of groundwater levels in existing and newly installed piezometers, no conditions were observed or identified that represent a dam safety condition requiring immediate attention.  Slope stability analyses completed for the two identified critical cross sections for the Primary Ash Basin Dam (Station 6+50, Station 10+00) indicate that the minimum factors of safety meet programmatic criteria under all static and pseudo-static conditions evaluated. Amec Foster Wheeler Environment & Infrastructure, Inc. December 2016 Duke Energy Coal Combustion Residuals Management Program Riverbend Steam Station Site Analysis and Removal Plan Revision 0 9  Rapid Drawdown analyses indicate that the programmatic criteria for a minimum factor of safety of 1.25 were met.  Post-earthquake stability analyses resulted in a minimum factor of safety of 1.66, which is greater than the minimum requirement of 1.1.  Embankment and foundation soils associated with the Primary Ash Basin Dam have low susceptibility to liquefaction, and risk of excessive deformation or settlement of the embankment is considered negligible during the Maximum Design Earthquake. 3.1.3.2 Secondary Ash Basin Dam  No seeps were observed within the embankments of the Secondary Ash Basin Dam. Seepage has historically been observed beyond the downstream toe of the Secondary Ash Basin dam. This seepage was observed during the site reconnaissance for the Phase 2 work and has been reported in subsequent weekly inspection reports.  Seepage has historically been observed beyond the downstream toe of Secondary Ash Basin dam. The likely sources of this seepage are discharges from the blanket drain and locally poor grading that may trap surface water runoff from the slope.  Based upon site reconnaissance conducted between May and September 2014, the subsurface evaluations and observation of groundwater levels in existing and newly installed piezometers, no conditions were observed or identified that represent a dam safety condition requiring immediate attention.  Slope stability analyses completed for the three identified critical cross section for the Secondary Ash Basin Dam (Station 26+38, Station 39+20 and Station 40+21) indicate that the minimum factors of safety meet programmatic criteria under all static and pseudo-static conditions evaluated.  Rapid Drawdown analyses indicate that the programmatic criteria for a minimum factor of safety of 1.25 were met.  Embankment and foundation soils associated with the Secondary Ash Basin Dam have low susceptibility to liquefaction, and risk of excessive deformation or settlement of the embankment is considered negligible during the Maximum Design Earthquake.  The principal spillway riser structure located in the Secondary Ash Basin is subject to excessive rocking deformation that could potentially result in separation between the riser and outlet barrel and breach of the dam when subject to seismic conditions using the Maximum Design Earthquake. Mitigation of this condition is required to meet program performance criteria. Note that the riser structure has subsequently been taken out of service and a portable pump system has been installed to serve as the spillway. Dewatering of the Secondary Ash Basin has begun in anticipation of basin closure. Amec Foster Wheeler Environment & Infrastructure, Inc. December 2016 Duke Energy Coal Combustion Residuals Management Program Riverbend Steam Station Site Analysis and Removal Plan Revision 0 10  The riser tower in the Secondary Ash Basin exhibits inadequate stability due to wind load while the basin is being dewatered. The design wind loading on the exposed riser structure could result in excessive deformation at the joint between the riser and the outlet barrel. However, the outlet barrel has been grouted to prevent loss of ash and liquid from the Secondary Ash Basin.  The outlet pipe barrel for the Secondary Ash Basin spillway was constructed using CMP, a pipe material known to be subject to deterioration and failure with age. Pipe inspections completed in 2014 and 2015 did not reveal any immediate concerns or issues. Note that the CMP was subsequently plugged and grouted in January 2016 and a portable pump system has been installed to serve as the spillway for the basin. 3.1.3.3 Intermediate Dam  Ash comprising the foundation of the Intermediate Dam is susceptible to liquefaction during the Maximum Design Earthquake and will be unstable immediately following such an event, which will result in large scale deformations in excess of the criteria provided in the Programmatic Document. If the basins are at or near design normal pool elevations, portions of the Intermediate Dam could breach. Under these conditions, however, breach of the Intermediate Dam will not result in breach or overtopping of the Secondary Ash Basin Dam. 3.1.4 Sources of Discharges into Surface Impoundments While the combustion units at the Riverbend Steam Station were operational, the majority of the influent discharged into the Primary and Secondary Ash Basins consisted of wastewater and sluiced CCR materials produced by steam generation systems. These discharges included ash transport water, combustion turbine cooling water from the turbine and boiler room sumps. Additional permitted discharges into the ash basins consisted of induced draft fan and preheater bearing cooling water, stormwater from roof drains and paving, treated groundwater, track hopper sump (groundwater), coal pile runoff, laboratory drain and chemical makeup tanks and drums, rinsate wastes, general plant/trailer sanitary wastewater, chemical metal cleaning waste, vehicle rinse water, and stormwater from pond areas and upgradient watershed. The combustion units at the Riverbend Steam Station were retired in April 2013. Following retirement of the steam generation operations, discharges into the ash basins were limited to stormwater from roof drains, paving, pond areas and upgradient watershed, treated groundwater and general plant/trailer sanitary wastewater. Figure 2-6 of the CSA Report (HDR, 2015a) provides a flow schematic for the station and is included in Appendix B of this Removal Plan. Wastewater collects in the Secondary Ash Basin and is discharged via a pump dewatering system to an on-site wastewater treatment system. The total average influent from the current sources combined was approximately 0.185 million gallons per day (MGD) in 2014. This is a significant decrease from an average of 5 MGD in 2009. As the decommissioning activities continue at the station, the total average influent rates will continue to decrease. Amec Foster Wheeler Environment & Infrastructure, Inc. December 2016 Duke Energy Coal Combustion Residuals Management Program Riverbend Steam Station Site Analysis and Removal Plan Revision 0 11 Other permitted discharges occur or have occurred at the Riverbend Steam Station, however, these discharges are not directed into the Primary and Secondary Ash Basins. These other permitted discharges include once through cooling water (Outfall 001) consisting of intake screen backwash and water from the plant chiller system, turbine lube oil coolers, condensate coolers, main turbine steam condensers and the intake tunnel dewatering sump; yard sump overflow from the former coal yard area (Outfall 002A); twelve potentially contaminated groundwater seeps (Outfalls 101 - 112); and, wastewater, stormwater and groundwater (Outfall 011). Discharge requirements for the Riverbend Steam Station are specified in NPDES Permit No. NC0004961, which was issued February 12, 2016. Duke’s Riverbend Steam Station Wet Ash Basins Facility O&M Plan (O&M Plan) provides guidance for managing the effluent discharge from the ash basins. The locations of the permitted discharges Outfalls 001, 002 and 002A and a flow diagram of the process discharges into the ash basins are presented in Figure 1 and Figure 2, respectively, in the O&M Plan. 3.1.5 Existing Liner System Based on historical information, no liner system was installed under the Riverbend Steam Station Primary and Secondary Ash Basins, the Cinder Pit Storage Area or the Dry Ash Stack. 3.1.6 Inspection and Monitoring Summary Weekly, monthly and annual inspections of the ash management facilities at the Riverbend Steam Station have been conducted consistent with CAMA and in accordance with O&M Plan. Independent third-party inspections are performed of the Riverbend Steam Station ash basins once every year. This was previously required every 5 years; however, in a letter dated August 13, 2014, NCDEQ required these inspections to be increased to annually at all of Duke’s fourteen coal ash impoundment facilities in North Carolina. These inspections are to promote structural integrity and the design, operation, and maintenance of the surface impoundment in accordance with generally accepted engineering standards. Inspection reports are to be submitted to NCDEQ within 30 days of the completion of the inspection. Annual inspections are performed to gather information on the current condition of the dams and appurtenant works. This information is then used to establish needed repairs and repair schedules, to assess the safety and operational adequacy of the dam, and to assess compliance activities with respect to applicable permits, environmental and dam regulations. Annual inspections are also performed to evaluate previous repairs. Annual inspections of the Riverbend Steam Station ash basin dams were conducted in October 2014, October 2015 and May 2016, and the subsequent inspection reports were issued in December 2014 (AMEC, 2014), February 2016 (Amec Foster Wheeler, 2016a), and June 2016 (Amec Foster Wheeler 2016b). The annual inspections included observations of the ash basin dams, discharge towers and drainage pipes. In addition to the field observations of the physical features of the impoundments, the annual inspections included a review of available design documents and inspection records. Amec Foster Wheeler Environment & Infrastructure, Inc. December 2016 Duke Energy Coal Combustion Residuals Management Program Riverbend Steam Station Site Analysis and Removal Plan Revision 0 12 The annual inspections did not identify any features or conditions in the ash basin dams, their outlet structures or spillways that indicate an imminent threat of impending failure hazard. Review of critical analyses indicated the design conforms to current engineering state of practice to a degree that no immediate actions are required other than the recent and ongoing surveillance and monitoring activities already being practiced. A general summary of the recommendations from the annual inspections include:  Continue weekly inspections on all dams.  Monitor seepage flow for change in flow volume or presence of turbidity.  Monitor and document wetness on berms and at the toe of the slopes.  Maintain slopes, reseed and mow to maintain good grass cover, as necessary. The 2014 annual inspection also included a few specific recommendations for each dam as follows:  Primary Ash Basin Dam o Monitor and clean out accumulated debris in a drainage pipe under a roadway. o Monitor and clean out accumulated sediment in rip rap swales at the toe and berm.  Secondary Ash Basin Dam o Monitor for seepage and erosion in swales. Repair/replace rip rap, where necessary. o Monitor and clean out accumulated debris or vegetation in a concrete drainage ditch. o Monitor and clean out accumulated sediment in rip rap swales at the toe and berm.  Intermediate Dam o Monitor and stabilize sloughed areas of the upstream slope, as required. o Monitor the downstream slope for increased wave erosion. o Monitor and repair animal burrow holes. Note that these recommendations have been subsequently addressed or continue to be monitored through the routine inspections of the dams described below. Weekly ash basin inspections include observation of downstream slopes, toes, abutment contacts and adjacent drainageway(s); spillway(s) and associated structure(s); upstream slopes and shorelines; and, other structures and features of the dams. Monthly inspections of the ash basins include the weekly monitoring elements with the addition of piezometer and observation well readings; water level gauges/sensors; and, visual observations and documentation of slopes and benches of the Dry Ash Stack. Amec Foster Wheeler Environment & Infrastructure, Inc. December 2016 Duke Energy Coal Combustion Residuals Management Program Riverbend Steam Station Site Analysis and Removal Plan Revision 0 13 Daily inspections of the ash management facilities at the Riverbend Steam Station are not routinely required. However, on a case-by-case basis, the ash basin facilities may be inspected daily beginning at such times, and continued for the duration as specified by Plant Management. Such daily inspections might be initiated, for example, during a repair activity on the dam or in response to a specific imposed regulatory agency requirement. Special inspections of the Riverbend Steam Station ash basins may be performed during episodes of high-flow, earthquake, emergency, or other special events. Visual inspections are performed after a heavy precipitation event when accumulation of 4 inches of rainfall or greater occurs within a 24-hour or lesser period. An internal inspection will be performed after an earthquake event if the seismic event was felt at the station or measured by the U.S. Geological Survey was greater than a Magnitude 3 and with an epicenter within 50 miles of the dam. A special inspection would also be performed during an emergency, such as when a potential dam breach condition might be identified or when construction activities (e.g., basin clean-out) are planned on or near the dam. They are also made when the ongoing surveillance program identifies a condition or a trend that appears to warrant special evaluation. 3.2 Site Maps 3.2.1 Summary of Existing CCR Impoundment Related Structures This section provides descriptions of the structures associated with the operation of the Primary and Secondary Ash Basins. A more detailed discussion of the stability and strength of the outlet structures for the ash basins at Riverbend Steam Station can be found in the Phase 2 Report (AECOM, 2015). Figure 4-3 of the CSA Report (HDR, 2015a) (see Appendix B) provides an aerial photographic map depicting the ash basins and location of structures associated with ash management at the Riverbend Steam Station. 3.2.1.1 Primary Ash Basin Structures The Primary Ash Basin Dam and impoundment were constructed on natural ground in 1957. According to the 1957 Design Drawings, soils used to construct the earthen embankments were excavated from the impoundment area, including areas where ash was previously placed. The Primary Ash Basin Dam is constructed of a central compacted embankment bearing on a foundation of residual soils consisting of clayey or sandy silts to silty sands underlain by partially weathered rock (PWR). The dam was raised by 10 feet and related improvements were completed in 1979. No documentation of dam related deficiencies are available, but the improvements were evidently intended to improve dam performance as well as to increase basin capacity. Some sluiced ash is shown as being present within the upper intervals of the embankment on the upstream side of the embankment. Design drawings indicate that the ash that formed the subgrade for the raise was allowed to consolidate prior to commencement of the dam improvements. Interior and exterior design slopes along the dam are inclined at 2H:1V to 2.5H:1V, respectively. As part of the 1979 expansion, an embankment berm was constructed on the downstream side Amec Foster Wheeler Environment & Infrastructure, Inc. December 2016 Duke Energy Coal Combustion Residuals Management Program Riverbend Steam Station Site Analysis and Removal Plan Revision 0 14 of the Primary Ash Basin. Benches, approximately 35 feet wide, were constructed at approximately the mid-elevation point of the downstream slope along and near the downstream toe along the southernmost two thirds of the dam. A graded blanket drain was constructed using sand and crushed stone aggregate beneath the newly placed embankment. A rip rap revetment was placed on the slope at the discharge of the blanket drain. There are no records of modifications to the Primary Ash Basin Dam since the 1979 expansion. Field reconnaissance conducted in 2014 indicates that the visible portions of the dam are generally consistent with the design drawings provided. A cross section of the Primary Ash Basin Dam that includes the 1979 downstream expansion is provided in Figure 11 of the Phase 2 Report (AECOM, 2015). The principal spillway of the Primary Ash Basin is reinforced concrete pipe (RCP) vertical riser structure with stop log level control and 30-inch RCP barrel to an earthen discharge channel. The spillway riser structure, which is pile supported, meets programmatic criteria for structural integrity, overturning and buoyancy under static and seismic conditions for the full range of operating levels in the basin. The principal spillway of the Primary Ash Basin is functional and capable of allowing water to flow through the Intermediate Dam into the Secondary Ash Basin. It will remain functional only until the Intermediate Dam is decommissioned in 2017. The outlet pipe or barrel for the Primary Ash Basin is RCP and has been in service since the Intermediate Dam was constructed in 1979. According to recent video inspections, the pipe is in good condition and exhibits a full cross section and is under relatively low vertical stresses. Crushing is not an anticipated failure mode within the remaining life of this structure. 3.2.1.2 Secondary Ash Basin Structures Like the Primary Ash Basin, the Secondary Ash Basin was constructed on natural ground in 1957 using soils excavated from the impoundment area. No seepage control or cutoff was constructed. The Secondary Ash Basin Dam is constructed of a central compacted embankment bearing on a foundation of residual clayey to sandy silts to silty sands underlain by PWR. No documentation of the original construction, including photographs, is available. Design drawings for the 1979 expansion indicate that cracking of the original downstream slope was to be repaired by removal and replacement with compacted soil. It can be concluded that the dam expansion also served to improve performance of Secondary Ash Basin by constructing the drain and toe berm described below. As part of the 1979 dam expansion, an approximately 35 feet wide bench was constructed at approximately the mid-elevation point of the downstream slope. A second bench (not shown on the typical cross section from the 1979 design drawings) was constructed near the downstream toe from approximate station 33+00 to the south abutment of the dam. A toe drain and filter – similar in design to the filter described for the Primary Ash Basin above – was constructed using sand and crushed stone aggregate beneath the newly placed embankment. A rip rap revetment was placed at the discharge of the blanket drain. A cross section of the Secondary Ash Basin Dam that includes the 1979 expansion is provided in Figure 12 of the Phase 2 Report (AECOM, 2015). Amec Foster Wheeler Environment & Infrastructure, Inc. December 2016 Duke Energy Coal Combustion Residuals Management Program Riverbend Steam Station Site Analysis and Removal Plan Revision 0 15 The principal spillway of the Secondary Ash Basin is a reinforced concrete vertical riser structure with stop log level control. The spillway riser structure for the Secondary Ash Basin, which is founded on a concrete mat, meets programmatic criteria for structural integrity. However, the riser could experience excessive overturning deformation during seismic conditions, as well as from wind loading when basin levels are reduced during dewatering. However, the outlet pipe is plugged and grouted so release of liquid and ash would not be anticipated. Lowering of the water level within the Secondary Ash Basin began early in 2016 in anticipation of basin closure. The former CMP outlet pipe or barrel for the Secondary Ash Basin was placed in service when the dams at Riverbend Steam Station were constructed in 1957. According to recent video inspections, the pipe was in good condition and exhibited a full cross section. However, CMP is considered to be prone to deterioration over time. Duke was granted approval to grout and abandon the horizontal outlet pipe in a letter from NCDEQ dated August 6, 2015. The CMP was plugged and grouted, and the metal weir box at the pipe outlet was removed in January 2016. 3.2.1.3 Intermediate Dam Structures The single cell ash basin was expanded in 1979 to the existing configuration by construction of the Intermediate Dam to effectively create two cells and raising the crest of the Primary Ash Basin Dam by approximately ten feet. The Intermediate Dam is constructed of an upper interval of fill overlying sluiced ash. The exterior slopes along the upstream and downstream sides of the Intermediate Dam are inclined at an approximate 3.5H:1V with an approximately 10-ft wide bench constructed on the downstream slope (Secondary Ash Basin side) of the embankment. A cross section of the Intermediate Dam is provided in Figure 14 of the Phase 2 Report (AECOM, 2015). There are no principal or auxiliary spillway or outlet structures associated with the Intermediate Dam at the Riverbend Steam Station. 3.2.1.4 Dry Ash Stack and Cinder Pit Storage Area Structures The locations of the Dry Ash Stack and Cinder Pit Storage Area are illustrated in Figures 1 and 2 of this Removal Plan. There are no principal or auxiliary spillway or outlet structures associated with the Dry Ash Stack or Cinder Pit Storage Area at the Riverbend Steam Station. 3.2.2 Receptor Survey The following information has been adopted from the CSA which included data obtained during receptor surveys conducted in 2014. The receptor survey update is included in Appendix B of the CSA Report (HDR, 2015a). Receptor surveys completed to date are based on responses to water supply well survey questionnaires mailed to property owners within 0.5-mile (2,640-foot) of the Riverbend Steam Station Ash Basin compliance boundary and the review of available records to identify public and private water supply sources, confirm the location of wells, and/or identify any wellhead protection areas located within a 0.5-mile radius of the compliance Amec Foster Wheeler Environment & Infrastructure, Inc. December 2016 Duke Energy Coal Combustion Residuals Management Program Riverbend Steam Station Site Analysis and Removal Plan Revision 0 16 boundary. The compliance boundary is depicted in Figure 3 of this Closure Report. The following is a summary of the receptor survey findings:  One reported private water supply well is located at a residence located northeast of RBSS within a 0.5-mile radius of the ash basin compliance boundary. This well is located across Mountain Island Lake in Mecklenburg County.  No public water supply wells (including irrigation wells and unused wells) were identified within a 0.5-mile radius of the Riverbend Steam Station Ash Basin compliance boundary. According to Duke, the two private water supply wells and one public water supply well previously identified on the Riverbend Steam Station property were properly abandoned in June 2015.  No wellhead protection areas were identified within a 0.5-mile radius of the compliance boundary.  Several surface water features that flow toward Mountain Island Lake were identified within a 0.5-mile radius of the ash basin. 3.2.3 Existing On-Site Landfills There are no permitted landfills (active or closed) at the Riverbend Steam Station. 3.3 Monitoring and Sampling Location Plan Figure 10-8 of the CSA Report (HDR, 2015a) (see Appendix B) shows existing monitoring locations and related information for the Riverbend Steam Station ash basins including groundwater monitoring wells, surface water sample locations, the property boundary and impoundment compliance boundaries, and existing site topography. Note that the sampling locations presented in the figures are current as of the date of the CSA Report (HDR, 2015a) and some wells including AS-3 series wells have been or will be abandoned during the closure activities. The Revised Groundwater Assessment Work Plan (GWA Work Plan) was submitted by HDR in December, 2014 and was subsequently granted conditional approval by NCDEQ in February 2015. The results of the groundwater assessment at Riverbend Steam Station are presented in the CSA Report (HDR, 2015a) prepared by HDR in August 2015. The purpose of the report is to characterize the extent of contamination 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 report was prepared in general accordance with requirements outlined in the following regulations and documents:  Classifications and Water Quality Standards Applicable to the Groundwaters of North Carolina in Title 15A NCAC 02L .0106(g),  Coal Ash Management Act in G.S. 130A-309.209(a), Amec Foster Wheeler Environment & Infrastructure, Inc. December 2016 Duke Energy Coal Combustion Residuals Management Program Riverbend Steam Station Site Analysis and Removal Plan Revision 0 17  Notice of Regulatory Requirements (NORR) issued by NCDEQ on August 13, 2014,  Conditional Approval of Revised Groundwater Assessment Work Plan issued by NCDEQ on February 16, 2015, and  Subsequent meetings and correspondence between Duke Energy and NCDEQ. In accordance with 15A NCAC 02L Groundwater Rules, the results of the groundwater monitoring are to be compared to the 2L Standards, Interim Maximum Allowable Concentrations (IMACs) and, where North Carolina standards do not exist, the U.S. EPA Maximum Contaminant Levels (MCLs). Assessment monitoring with potential implementation of corrective action measures may be required for Constituents of Interest (COIs) with a Statistically Significant Increase (SSI) over background. If a SSI over background is not found, monitoring is to continue for the active life of the CCR units and post-closure period. Remedy completion is achieved once COI concentrations are at or below the associated standards at all compliance points for a period of three years. Additional monitoring wells may have been and may be installed on the site as part of supplemental assessments of the CCR units. As a result, sampling locations may be modified following analysis and interpretation of additional data. Sampling will be in accordance with the groundwater monitoring plan in effect at the time of sampling. Amec Foster Wheeler Environment & Infrastructure, Inc. December 2016 Duke Energy Coal Combustion Residuals Management Program Riverbend Steam Station Site Analysis and Removal Plan Revision 0 18 4. RESULTS OF HYDROGEOLOGIC, GEOLOGIC, AND GEOTECHNICAL INVESTIGATIONS 4.1 Hydrogeology and Geologic Descriptions North Carolina is divided into distinct regions which are portions of three physiographic provinces: the Atlantic Coastal Plain, Piedmont, and Blue Ridge, as illustrated below. Riverbend Steam Station is situated on the bank of the Catawba River (also known as Mountain Island Lake) approximately 5 miles east of the Boogertown Shear Zone within the Charlotte lithotectonic belt of the Piedmont physiographic province. The Charlotte belt primarily consists of metavolcanic and metaplutonic rocks deformed pre-, syn-, and post-major North American tectonic events. Bedrock at the site has been described as late-Proterozoic era to early- Cambrian period metamorphosed quartz diorite and tonalite. Younger (Paleozoic Era) plutons consisting of gabbro and metagranite are situated less than one mile east and west of the site (Goldsmith et al., 1988; LeGrand and Mundorff, 1952). Topography in the area is characterized as low uplands and streams with relatively narrow floodplains. The Mountain Island highland area about the lake dominates the largest local topographic relief. At the Riverbend Steam Station ash basin area, topographic relief is approximately 150 feet. Native soils above the bedrock consist of completely weathered rock (saprolite) and Quaternary period alluvial sediments deposited in the floodplains of streams dissecting the area. Generally, soils in the area consist of well-drained sandy loams with a clayey subsoil (McCachren, 1980, as reported in Bales et al., 2001). Groundwater in the Piedmont physiographic province typically occurs in the overburden under unconfined (i.e., water table) conditions, and in the underlying bedrock under both unconfined and confined conditions. Groundwater in the overburden occurs within pore spaces of the unconsolidated medium. Due to the typical fine-grained nature of saprolite, the formation Amec Foster Wheeler Environment & Infrastructure, Inc. December 2016 Duke Energy Coal Combustion Residuals Management Program Riverbend Steam Station Site Analysis and Removal Plan Revision 0 19 normally possesses a relatively low permeability and is not usually utilized for groundwater production. The overburden is recharged by the infiltration of precipitation where the formation is exposed and acts as a storage medium for groundwater that is slowly released to surface water bodies and the underlying bedrock. Groundwater in the underlying bedrock occurs along zones of secondary porosity, such as fractures, bedding planes, foliations, and solution voids (Horton and Zullo, 1991). 4.2 Stratigraphy of the Geologic Units Underlying Surface Impoundments Stratigraphy at the site was interpreted based on a review of historical boring logs, dam construction methods, and borings completed during recent site characterization activities. The primary units identified at the site are: undifferentiated fill material (e.g., material imported or excavated on site to construct the embankment), ash, alluvial sands/silts/clays, saprolite, partially weathered bedrock, and competent bedrock. Outside of the areas disturbed by construction and beneath the fill material, the surficial unit consists of alluvial deposits dominated by clay with varying amounts of silt and sand. Thickness of the alluvial material ranges from approximately 6 to 40 feet. Beneath the alluvium, saprolite, resulting from the complete in situ degradation of rock, is present. Saprolite thickness ranges from approximately 25 to 85 feet. The saprolite transitions to bedrock through a zone of partially weathered rock, interpreted to range in thickness from 5 to 40 feet. Bedrock in this area has been described by others in the field as gneiss due to apparent compositional banding in rock cores; similarly, the rock has been classified on regional geologic maps as metamorphosed quartz diorite (Arcadis, 2007). The contact between the saprolite and the bedrock/foundation unit may be characterized as rolling across the site, occurring at a range of elevations and depths from ground surface. This is typical for the region, where extensive and differential weathering and alteration have occurred over time. 4.3 Hydraulic Conductivity Information According to the CSA Report (HDR, 2015a), the groundwater system in the natural materials (alluvium, soil, soil/saprolite, and bedrock) at Riverbend Steam Station is consistent with the regolith-fractured rock system and is an unconfined, connected system without confining layers. However, the hydraulic conductivity data collected during the groundwater assessment indicates that a distinct transition zone of higher permeability does not exist at the site. This is consistent with Harned and Daniel’s (1992) concept of the two types of rock structure (foliated/layered and massive) in the Piedmont province. The Riverbend Steam Station is underlain by a relatively massive meta-plutonic complex of the type that may develop an indistinct transition zone. The groundwater system at the site is a two-layer system: shallow (regolith) and bedrock. Hydraulic conductivity and groundwater quality were assessed at the Riverbend Steam Station for the CSA Report (HDR, 2015a) using data collected during field activities conducted in 2015 as well as from subsurface investigations previously conducted at the site. Measurements, sampling and testing methods used in determining hydraulic conductivity included boring logs and construction records for new and historic monitoring wells; slug tests and field permeability Amec Foster Wheeler Environment & Infrastructure, Inc. December 2016 Duke Energy Coal Combustion Residuals Management Program Riverbend Steam Station Site Analysis and Removal Plan Revision 0 20 data; particle size analysis and porosity results; as well as laboratory analysis of physical, chemical and mineralogical properties of site soils. A detailed review of the methods used to determine the hydraulic conductivities of the hydrostratigraphic units (ash, fill, alluvium, soil/saprolite, saprolite/weathered rock, transition zone and bedrock) at the Riverbend Steam Station can be found in Section 11 and Appendix B of the CSA Report (HDR, 2015a). Tables 4.1 and 4.2 below, provide a summary of the horizontal and vertical hydraulic conductivities adopted from the CSA. Table 4-1 Hydrostratigraphic Unit Properties - Horizontal Hydraulic Conductivity Hydrostratigraphic Unit N Geometric Mean (cm/sec) Geometric Mean + 1SD (cm/sec) Geometric Mean - 1SD (cm/sec) Geometric Median (cm/sec) Minimum (cm/sec) Maximum (cm/sec) Ash 44 1.1E-03 6.0E-03 2.1E-04 1.3E-03 3.0E-05 1.7E-02 Fill 14 4.3E-05 1.5E-04 1.3E-05 5.4E-05 5.4E-06 2.4E-04 Alluvium 11 4.3E-04 4.5E-03 4.0E-05 9.2E-04 7.1E-06 2.6E-02 Soil/Saprolite 32 3.8E-04 2.6E-03 5.4E-05 9.2E-04 8.6E-06 1.7E-02 Saprolite/Weathered Rock 19 2.7E-04 2.3E-03 3.2E-05 2.1E-04 5.4E-06 1.5E-02 Transition Zone 18 7.6E-05 3.8E-04 1.5E-05 9.8E-05 5.9E-06 1.2E-03 Bedrock 34 1.7E-05 1.6E-04 1.8E-06 1.2E-05 2.2E-07 1.5E-03 Table 4-2 Hydrostratigraphic Unit Properties - Vertical Hydraulic Conductivity Hydrostratigraphic Unit N Geometric Mean (cm/sec) Geometric Mean + 1SD (cm/sec) Geometric Mean - 1SD (cm/sec) Geometric Median (cm/sec) Minimum (cm/sec) Maximum (cm/sec) Ash 45 1.1E-04 5.8E-04 2.1E-05 1.2E-04 3.0E-06 3.1E-03 Fill 24 8.4E-06 5.3E-05 1.3E-06 9.99E-06 1.1E-04 1.1E-04 Alluvium 10 2.7E-06 6.0E-05 1.2E-07 1.6E-06 4.6E-08 6.2E-04 Soil/Saprolite 26 8.2E-06 7.4E-05 9.1E-07 7.2E-06 7.3E-08 2.4E-04 Amec Foster Wheeler Environment & Infrastructure, Inc. December 2016 Duke Energy Coal Combustion Residuals Management Program Riverbend Steam Station Site Analysis and Removal Plan Revision 0 21 Hydrostratigraphic Unit N Geometric Mean (cm/sec) Geometric Mean + 1SD (cm/sec) Geometric Mean - 1SD (cm/sec) Geometric Median (cm/sec) Minimum (cm/sec) Maximum (cm/sec) Saprolite/Weathered Rock 3 5.2E-05 3.2E-04 8.3E-06 2.0E-05 1.6E-05 4.3E-04 Transition Zone 0 - - - - - - Bedrock 0 - - - - - - In general, the infiltration for the CCR material at the Riverbend Steam Station will be variable and intermittent, as infiltration is precipitation induced. The infiltration rate is dependent on a number of factors with the primary factors being climate, vegetation, and soil properties. The precipitation and air temperature are the two aspects of climate that most directly affect groundwater infiltration. Vegetation affects the infiltration rate through interception and by means of transpiration. The primary soil properties that affect infiltration are represented by the hydraulic conductivity of the material. 4.4 Geotechnical Properties Geotechnical properties of embankment soils, ash, and residual soils are summarized in this section. The geotechnical properties summarized are from the following borings (and associated laboratory tests) by Amec Foster Wheeler: borings located along the crest of the dams (i.e., B-104 and B-105 for the primary dam and B-112 through B-115 for the secondary dam, and B-108 through B-111 for the intermediate dam), in residual soil adjacent to the ash basins (i.e., B-103), and in the primary ash basin (i.e., B-106). The material descriptions in the following sections are supplemented by data provided by Duke Energy, specifically the Phase 2 Reconstitution of Ash Pond Designs Report (AECOM 2015), referred to as the Phase 2 Report. The data from the Phase 2 Report are generally consistent with the Amec Foster Wheeler logs and test results. 4.4.1 Primary Ash Basin The Primary Ash Basin Dam and impoundment were constructed on natural ground in 1957. According to the 1957 design drawings, soils used to construct the earthen embankments were excavated from the impoundment area, including areas where ash was previously placed. The Primary Ash Basin Dam is an earthen embankment constructed of controlled, compacted soils bearing on a foundation of residual soils consisting of clayey or sandy silts to silty sands underlain by PWR. No seepage control, filters, or cutoff were constructed with the original embankment. No construction documentation or photos of the original construction are available. In 1979, a vertical raise extension was constructed and additional soil was placed on the downstream side of the dam, forming two benches. Amec Foster Wheeler Environment & Infrastructure, Inc. December 2016 Duke Energy Coal Combustion Residuals Management Program Riverbend Steam Station Site Analysis and Removal Plan Revision 0 22 Embankment Based on Borings B-104 and B-105, the embankment materials generally consist of sandy clay (SC) and elastic silt (MH), with a natural moisture content average of 30%, liquid limit average of 57, plasticity index average of 13, and fines content (silt and clay sized particles) average of 77%. Based on information presented by AECOM (2015): The embankment fill materials at the Primary Ash Basin Dam generally consist of stiff consistency, moist, elastic silt (MH) varying to sandy silt (ML) with pockets of localized fine to medium grained sand. At isolated intervals of limited thickness, the fill classified as sandy clay (SC). Overall, the embankment fill extends from the crest elevation (approximately 730 MSL) to elevations of approximately 670 MSL to 660 MSL, resulting in an embankment varying from 60 to 70 feet in thickness at the crest. The embankment soils exhibit the following index soil characteristics on average: N-value of 12, natural moisture content of 27%, liquid limit of 51, plasticity index of 20, fines content of 65%, and wet unit weight of 121 pound per cubic foot (pcf). Ash Based on two samples of primary basin ash obtained from Boring B-106 (located in the basin interior), the ash has an average fines content of about 66%. The boring log shows ash gradations varying from silty fine to coarse sand (SM) to sandy silt (ML). For the borings located on the Primary Dam, no ash was encountered in B-104, and a 1-foot thickness of ash was encountered in B-105 at approximately elevation 710 MSL. Based on information presented by AECOM (2015): Ash is present within the upper intervals of the Primary Ash Basin Dam embankment and along the upstream side on the embankment. The ash was generally encountered from approximately elevation 720 MSL to 715 MSL, and is indicative of the sluiced ash elevation in place when the embankment was raised to elevation 730-ft in 1979. Therefore, the thickness of the ash interval is expected to be approximately 4-5 feet, at most, directly beneath the crest and increasing with thickness upstream toward the primary basin. Sampling was limited along the Primary Dam, but where sampled, the ash consisted of soft consistency, wet to saturated, gray silt (ML) based on field classification. Foundation Based on field classification from Borings B-104 and B-105, the residual soil under the embankment fill consists of lean clay (CL) and fat clay (CH) underlain by silt and sandy silt (ML). No laboratory testing was performed on residual soil samples from these borings. Based on information presented by AECOM (2015): The foundation materials of the Primary Ash Basin Dam consist of alluvial deposits from the Catawba River overlying residual soils, or, in the absence of alluvium, residuum only. Overall, the foundation soils consist of stiff consistency, moist, sandy silt (ML) to elastic silt (MH) with some fine to medium grained sand. At isolated intervals of limited thickness, the residual soils are classified as silty sand (SM). Overall, the foundation soils extend to elevations of approximately 610 MSL to 600 MSL, generally averaging 60 to 70 feet in thickness. With depth, the foundation soils transition to PWR. PWR is underlain by bedrock, which was not characterized or sampled as part of the Phase 2 Amec Foster Wheeler Environment & Infrastructure, Inc. December 2016 Duke Energy Coal Combustion Residuals Management Program Riverbend Steam Station Site Analysis and Removal Plan Revision 0 23 investigation. The foundation soils exhibit the following index characteristics on average: N- value of 14, natural moisture content of 34%, liquid limit of 43, plasticity index of 9, fines content of 63%, and wet unit weight of 117 pcf. 4.4.2 Secondary Ash Basin The Secondary Ash Basin Dam and impoundment were constructed on natural ground in 1957. The Secondary Ash Basin Dam is an earthen embankment constructed of controlled, compacted soils bearing on a foundation of residual soils consisting of clayey or sandy silts to silty sands underlain by PWR. No seepage control, filters, or cutoff were constructed with the original embankment. No construction documentation or photos of the original construction are available. In 1979, a vertical raise extension was constructed and additional soil was placed on the downstream side of the dam, forming two benches. Embankment Based on Borings B-112 through B-115, the embankment materials generally consist of silt (ML) with some zones of elastic silt (MH), silty sand (SM), and fat clay (CH). Results of laboratory testing performed on samples of elastic silt (MH) indicate a natural moisture content average of 29%, liquid limit average of 63, plasticity index average of 18, and fines content average of 72%. Based on information presented by AECOM (2015): The embankment fill materials generally consists of stiff consistency, moist, elastic silt (MH) varying to silty clay (CL) and clayey silt (ML) with some fine to medium grained sand. Overall, the embankment fill extends to elevations of approximately 654 MSL to 631 MSL, resulting in an embankment varying from 60 to 70 feet in thickness at the crest. The embankment soils exhibit the following index soil characteristics on average: N-value of 11, natural moisture content of 36%, liquid limit of 51, plasticity index of 14, fines content of 70%, and wet unit weight of 117 pcf. Ash Ash was not present in Borings B-112 through B-115, which were drilled along the Secondary Dam. Based on information presented by AECOM (2015): Ash was not present at any boring locations conducted along the Secondary Dam. Foundation Based on field classification from Borings B-112 and B-115, the residual soil under the embankment fill generally consists of silt (ML). In B-113, the silt was overlain by a 5-foot thickness of fat clay with organic debris. No laboratory testing was performed on residual soil samples from these borings Based on information presented by AECOM (2015): The foundation materials consist of alluvial deposits from the Catawba River overlying residual soils, or, in the absence of alluvium, residual soil. Overall, the foundation soils generally consist of stiff consistency, moist, sandy silt (ML) with some fine to medium grained sand. At isolated intervals of limited thickness, the residual soils classified as silty sand (SM). Overall, the foundation soils extend to elevations of approximately Amec Foster Wheeler Environment & Infrastructure, Inc. December 2016 Duke Energy Coal Combustion Residuals Management Program Riverbend Steam Station Site Analysis and Removal Plan Revision 0 24 620 MSL, generally averaging about 35 to 45 feet in thickness. With depth, the foundation soils transition to PWR. PWR is underlain by bedrock, which was not characterized or sampled as part of the Phase 2 investigation. The foundation soils exhibit the following characteristics on average: N-value of 15, liquid limit of 51, plasticity index of 13, fines content of approximately 57%, and wet unit weight of 117 pcf. 4.4.3 Intermediate Dam The Intermediate Dike is constructed of fill. The Intermediate Dike foundation consists of sluiced ash. Residual soils underlie the sluiced ash.The exterior slopes along the upstream and downstream sides of the Intermediate Dike are inclined at approximately 3.5 Horizontal:1 Vertical with a 10-ft wide bench constructed on the Secondary Ash Basin side. Embankment Intermediate dike fill ranges from sandy silt (ML) and lean clay (CL) to clayey sand (SC) and ranges in thickness from 15 to 26 feet below the ground surface. AECOM (2015) reports that fill consists of medium stiff to stiff consistency, moist, elastic silt (MH) varying to sandy silt (ML) with isolated intervals of sandy clay (SM). The following average index soil characteristics were reported: • N-value of 7 • Natural Moisture Content of 23% • Liquid Limit of 47 • Plasticity Index of 13 • Approximately 63% consists of fine (silt and clay sized) particles • Wet unit weight of 122 pcf Ash Ash is present under the intermediate dike fill and is classified as sandy silts (ML) and silty sands (SM) by grain size. AECOM (2015) reports that, ash consists of very soft to soft consistency, saturated gray silt (ML). AECOM (2015) also reports that based upon historical laboratory testing the ash exhibits the following average index characteristics: • N-value of 2 • Natural Moisture Content of 51% • Non-Plastic • Approximately 67% consists of fine (silt and clay sized) particles • Wet unit weight of 90 pcf Foundation Amec Foster Wheeler Environment & Infrastructure, Inc. December 2016 Duke Energy Coal Combustion Residuals Management Program Riverbend Steam Station Site Analysis and Removal Plan Revision 0 25 Soils below the ash consist of residual soils classifying as sandy silt (ML) and lean clay (CL). Some sands (SP and SC) were reported in boring logs. AECOM (2015) reported that residual soils generally consist of stiff to very stiff consistency, moist, sandy silt (ML) varying to elastic silt (MH) with some fine to medium grained sand. Residual soils are reported to transition to PWR underlain by bedrock. Residuum is reported to exhibit the following average index characteristics: • N-value of 19 • Liquid Limit of 54 • Plasticity Index of 19 • Approximately 73% consists of fine (silt and clay sized) particles • Wet unit weight of 116 pcf 4.5 Chemical Analysis of Impoundment Water, CCR Materials and CCR Affected Soil According to the CSA Report (HDR, 2015a), source characterization was performed to identify the physical and chemical properties of the ash in source areas at the Riverbend Steam Station. Source areas identified in the CSA include the Primary and Secondary Ash Basins, Dry Ash Stack and the Cinder Pit Storage Area and seeps associated with the ash basins. Source characterization was performed through the completion of soil borings, installation of monitoring wells, and collection and analysis of associated solid matrix and aqueous samples. Specifically, ash, ash basin water, porewater (interstitial or pore-space water), water from seeps and soils beneath the ash basins were sampled for source characterization. Porewater refers to water samples collected from wells installed and screened within the ash layer of the Primary and Secondary Ash Basins, Dry Ash Stack or Cinder Pit Storage Area. Note that the CSA does not consider porewater results to be representative of groundwater. The source characterization involved developing selected physical properties of ash, identifying the constituents found in ash, measuring concentrations of constituents present in the ash porewater, and performing laboratory analyses to estimate constituent concentrations resulting from the leaching process. The analysis of solid matrix (soil, rock, and ash) samples included total inorganics (metals), pH, and total organic carbon. Select ash samples were also analyzed for leaching potential using SPLP extraction in conjunction total inorganics. The analysis of aqueous matrix (groundwater, ash basin water, porewater, surface water and seeps) samples included field parameters, total inorganics, and anions/cations. A summary of the constituents and laboratory methods used for analysis of samples is presented in Tables 7-1 through 10-13 of the CSA Report (HDR, 2015a) (see Appendix B). Because impacts to groundwater were identified in the CSA, NCGS Section §130A-309.209(b) requires the implementation of corrective action for the restoration of groundwater quality in accordance with 2L Standards and required Duke to submit a CAP. Duke and NCDEQ mutually agreed to a two-part submittal identified as CAP Part 1 (HDR, 2015b) and CAP Part 2 (HDR, Amec Foster Wheeler Environment & Infrastructure, Inc. December 2016 Duke Energy Coal Combustion Residuals Management Program Riverbend Steam Station Site Analysis and Removal Plan Revision 0 26 2016). The following discussion summarizes the results of assessment work conducted by HDR on behalf of Duke and describes the COIs identified at the Riverbend Steam Station. 4.5.1 Source Area Characterization Ash Chemical Characteristics Four borings were advanced within the Primary and Secondary Ash Basin waste boundary to obtain ash samples for chemical analyses. Inorganics including antimony, arsenic, cobalt, iron, manganese, and vanadium were reported above the North Carolina Preliminary Remediation Goal (PSRGs) for Industrial Soil and/or Protection of Groundwater for ash samples. These inorganics were therefore identified as COIs within the ash basin waste boundary of the ash basins. Three borings were advanced within the Dry Ash Stack waste boundary to obtain ash samples for chemical analyses. Seven COIs including antimony, arsenic, cobalt, iron, manganese, selenium, and vanadium were reported above the North Carolina PSRGs for Industrial Soil and/or Protection of Groundwater Standards within the waste boundary of the Dry Ash Stack. Two borings were advanced within the Cinder Pit Storage Area waste boundary to obtain ash samples for chemical analyses. COIs including arsenic, cobalt, iron, manganese, selenium, and vanadium were reported above the North Carolina PSRGs for Industrial Soil and/or Protection of Groundwater Standards within the waste boundary of the Cinder Pit Storage Area. Ash Basin Water Chemical Characteristics Two water samples were collected from within the Secondary Ash Basin. COIs including aluminum, antimony, arsenic, barium, beryllium, cadmium, chromium, cobalt, copper, iron, lead, manganese, nickel, thallium, vanadium, and zinc concentrations were identified in the ash basin water. These inorganics exceeded the North Carolina 2B Standards, 2L Standards or Interim Maximum Allowable Concentration (IMAC), in at least one of the two water samples collected from the Secondary Ash Basin. According to the CSA Report (HDR, 2015a), the ash basin water is compared to the 2B and 2L Standards for comparative purposes and is not considered surface water or groundwater. Dissolved (filtered) concentrations of arsenic and thallium exceeded their respective North Carolina 2B Standards in at least one of the two samples. Porewater Chemical Characteristics Five porewater monitoring wells were installed within the waste boundary of Primary and Secondary Ash Basins. COIs including antimony, arsenic, boron, cobalt, iron, manganese, pH, thallium, vanadium, and total dissolved solids (TDS) were reported above the 2L Standards or IMAC in the porewater samples from the ash basins. One porewater monitoring well was installed within the Cinder Pit Storage Area waste boundary. COIs including arsenic, cobalt, iron, manganese, pH, vanadium, sulfate, and TDS were reported above the 2L Standards or IMAC in porewater samples collected within the waste boundary of the Cinder Pit Storage Area. Amec Foster Wheeler Environment & Infrastructure, Inc. December 2016 Duke Energy Coal Combustion Residuals Management Program Riverbend Steam Station Site Analysis and Removal Plan Revision 0 27 There were no porewater samples collected from the Dry Ash Stack. Groundwater within the Dry Ash Stack waste boundary was below the bottom of the ash layer. Leaching Potential of Ash Eight ash samples were collected from borings completed within the Primary and Secondary Ash Basins, Dry Ash Stack, and Cinder Pit Storage Area and analyzed for leachable inorganics using SPLP. The results of the SPLP analyses indicated that COIs including antimony, arsenic, chromium, cobalt, iron, lead, manganese, selenium, thallium, vanadium, and nitrate exceeded their respective 2L Standards or IMAC in at least one sample. According to the CSA Report (HDR, 2015a), leaching of constituents from ash stored in the Dry Ash Stack or Cinder Pit Storage Area will be likely be different from the leaching that occurs when ash is stored in a saturated condition such as in the ash basins at the Riverbend Steam Station. The ash in these two different storage environments would experience differences in the time of exposure to the leaching solution, the liquid to solid ratio, and the chemical properties of leaching liquid. This would likely lead to differences in the constituents and in the concentrations leached in the two differing environments. 4.5.2 Soil, Partially Weathered Rock and Bedrock Assessment Soil Chemical Characteristics Soil samples were collected from borings within the waste boundary beneath the Primary and Secondary Ash Basins. Constituent concentrations of arsenic, boron, cobalt, iron, manganese, nickel, and vanadium in soils beneath the ash basins tend to be generally higher compared to background soil concentrations. Selenium was detected only once in these borings (2.4J mg/kg). Method reporting limits for selenium are similar to background soil concentrations. Soil samples were collected from borings within the waste boundary beneath the Dry Ash Stack. Constituent concentrations of arsenic, cobalt, iron, manganese, and vanadium in these soils were at or below background concentrations. A single exceedance for arsenic in one boring (AS-2D) was reported above background. Soil samples were collected from borings within the waste boundary beneath the Cinder Pit Storage Area. Constituent concentrations of cobalt, iron, manganese and vanadium in these soils were similar to background soil concentrations. Soil samples collected outside the waste boundary and within compliance boundary were obtained from nineteen boring locations at the Riverbend Steam Station. Constituent concentrations of arsenic, barium, cobalt, iron, manganese, nickel, selenium, and vanadium in these soils tend to be generally higher than background soil concentrations. Partially Weathered Rock (PWR) and Bedrock Chemical Characteristics One PWR sample was collected within the waste boundary of the ash basins at Riverbend Steam Station. Constituent concentrations of cobalt, iron, manganese, and vanadium in the PWR within the waste boundary were within the range of background PWR concentrations. Amec Foster Wheeler Environment & Infrastructure, Inc. December 2016 Duke Energy Coal Combustion Residuals Management Program Riverbend Steam Station Site Analysis and Removal Plan Revision 0 28 PWR and bedrock samples were collected beneath the Dry Ash Stack. Cobalt concentrations in these samples tend to be generally higher than background concentrations. Iron, manganese, and vanadium concentrations are similar to background concentrations. PWR and bedrock samples collected outside the waste boundary and within compliance boundary were obtained from eighteen boring locations at the Riverbend Steam Station. Constituent concentrations of cobalt, iron, manganese and vanadium in these samples tend to be greater than background concentrations. 4.5.3 Surface Water and Sediment Assessment Seep Water Chemical Characteristics Several seeps are located within the compliance boundary at Riverbend Steam Station and are associated with the Primary and Secondary Ash Basins. The seeps are located between the ash basins and the Catawba River. COIs including cobalt, iron, manganese, and vanadium were reported in four seep samples at concentrations exceeding the 2L Standards or IMAC. Specifically, the COIs were reported in one seep (S-2) located downgradient of the Primary Ash Basin, and three seeps (S-5, S-9 and S-11) located downgradient of the Secondary Ash Basin. Surface Water Chemical Characteristics Both unfiltered and filtered surface water samples were collected for analyses of constituents whose results may be biased by the presence of turbidity. Unless otherwise noted, concentration results discussed below are for the unfiltered samples and represent total concentrations. Surface water analytical results were compared to the 2B Standards. Surface water samples were also analyzed for the constituents that do not have 2B Standards and were therefore compared to background concentrations. Note that boron, calcium, iron, manganese, mercury, selenium, and vanadium do not have corresponding 2B Standards. Each of these constituents was detected in at least one surface water sample. A background surface water sample was collected from an unnamed draw leading to Mountain Island Lake near monitoring well location BG-3, side-gradient of the ash basins and ash storage areas. Aluminum was the only constituent exceeding the 2B Standard in the background surface water sample. The dissolved phase concentration of aluminum in the background sample was less than the 2B Standards. In 2014, NCDEQ collected two surface water samples from Mountain Island Lake in the plant surface water intake canal located just northwest of the station. These surface water samples exceeded 2B Standards for aluminum, cadmium, copper, lead, and zinc. The dissolved phase concentration of lead reported in one of the samples was less than the 2B Standards. These samples also exceeded the background concentration for calcium, selenium, and vanadium. One surface water sample was collected from ponded water in the excavated area within the Cinder Pit Storage Area. This sample exceeded 2B Standards for aluminum, lead, and zinc. The dissolved phase concentration of aluminum and lead were less than the detection limit for this sample. This sample also exceeded the background concentration for boron, calcium, and manganese. Amec Foster Wheeler Environment & Infrastructure, Inc. December 2016 Duke Energy Coal Combustion Residuals Management Program Riverbend Steam Station Site Analysis and Removal Plan Revision 0 29 Four additional surface water samples were collected from surface waters located outside of the ash basin waste boundary. These samples exceeded 2B Standards for aluminum, cobalt, copper, and lead. The dissolved phase concentration of aluminum, copper, and lead were less than the 2B Standards. The dissolved concentrations for cobalt exceedances showed a reduction from the total values. These samples also exceeded the background concentration for boron, iron, manganese, and vanadium. Surface water sample analytical results collected as part the NPDES permit requirements, were reviewed for an upstream and one downstream location in Mountain Island Lake. Surface water sampling results from the two sample locations were reviewed for data from 2011 to 2015. No exceedances were detected for the select constituents analyzed. Sediment Chemical Characteristics Sediment samples were collected at the twelve seeps (S-1 through S-12) identified at Riverbend Steam Station. Note that four seeps (S-1, S-3, S-10, and S-12) were dry at the time of sample collection; however, sediment samples were collected from these seeps. Sediment samples were analyzed for the constituent and parameter list used for solid matrix characterization (soils). In the absence of NCDEQ sediment criteria, the sediment sample results were compared to North Carolina PSRGs for Industrial Soil and Protection of Groundwater. Sediment sample results for arsenic, barium, boron, cobalt, iron, manganese and vanadium exceeded one or both of the North Carolina PSRGs in all sediment samples. Cobalt, iron, manganese, and vanadium concentrations exceeded the North Carolina PSRGs for Protection of Groundwater in all sediment samples. Arsenic exceeded the North Carolina PSRG for Industrial Soil and Protection of Groundwater in sediment samples collected at S-2 and S-12. Boron and barium exceeded the North Carolina PSRG for Protection of Groundwater in sediment sample S-6. Antimony, selenium, and thallium were not detected in sediment samples collected at Riverbend Steam Station. 4.6 Historical Groundwater Sampling Results In 2006, as part of a voluntary monitoring program at the Riverbend Steam Station, Duke installed a series of shallow and deep monitoring wells including MW -1S, MW-1D, MW-2S, MW- 2D, MW-3S, MW-3D, MW-4S, MW-4D, MW-5S, MW-5D, MW-6S and MW-6D. Duke implemented an enhanced voluntary groundwater monitoring around the ash basins from December 2008 until June 2010. During this time, the voluntary groundwater monitoring wells were sampled two times per year and the analytical results were submitted to NCDEQ Division of Water Resources. Samples have been collected from monitoring wells MW -4S, MW-4D, MW- 5S and MW-5D since February 2013 as part of groundwater assessment efforts. No samples are currently being collected from the other voluntary wells. Groundwater compliance monitoring was required per wastewater NPDES Permit NC0004961, issued in March 2011. In 2010 and 2011, as part of the compliance monitoring program, the following monitoring wells were installed: MW-7SR, MW-7D, MW-8S, MW-8I, MW-8D, MW-9, MW -10, MW-11SR, MW-11DR, MW-13, MW-14, and MW-15. The compliance monitoring wells are sampled three times per year (in February, June, and October) for the following parameters: Amec Foster Wheeler Environment & Infrastructure, Inc. December 2016 Duke Energy Coal Combustion Residuals Management Program Riverbend Steam Station Site Analysis and Removal Plan Revision 0 30 antimony, arsenic, barium, boron, cadmium, chromium, copper, iron, lead, manganese, mercury, nickel, selenium, thallium, zinc, chloride, nitrate, sulfate, pH, TDS and water levels. Table 4-3 lists the exceedances of 2L Standards within the compliance wells between March 2011 and June 2015. Table 4-3 Exceedances of 2L Standards within Compliance Wells (March 2011 - June 2015) Parameter Chromium Iron Manganese pH Antimony Units μg/L μg/L μg/L SU μg/L 2L Standard 10 300 50 6.5 - 8.5 1** Well ID Range of Exceedances MW -7SR 14 445 – 532 54 – 304 5.0 – 5.4 N/E MW -7D N/E N/E N/E 5.5 – 5.8 1.04 MW -8S N/E N/E 55 – 144 4.3 – 5.2 N/E MW -8I N/E 436 – 2,510 52 – 168 5.7 – 6.4 N/E MW -8D N/E 658 – 4,160 74 – 622 6.3 – 6.5 N/E MW -9* N/E 341 – 1,950 62 – 87 5.8 – 6.4 N/E MW -10* N/E 301 – 921 67 – 355 4.8 – 5.4 N/E MW -11SR N/E N/E 59 5.6 – 5.8 N/E MW -11DR N/E N/E 51 – 103 5.6 - 5.9 N/E MW -13* N/E 7,690 – 37,700 8,070 – 10,500 5.8 – 6.3 N/E MW -14 N/E 369 – 935 56 – 353 N/E N/E MW -15 N/E 399 - 465 52 – 86 5.1 – 5.2 N/E Notes: * - Monitoring wells located inside of the compliance boundary. ** - Interim Maximum Allowable Concentration (IMAC) for Antimony, as listed in 15A NCAC 02L .0200. N/E - No Exceedances. The compliance boundary for groundwater quality for the ash basins at the Riverbend Steam Station is defined in accordance with Title 15A NCAC 02L .0107(a) as being established at either 500 feet from the waste boundary or at the property boundary, whichever is closer to the Amec Foster Wheeler Environment & Infrastructure, Inc. December 2016 Duke Energy Coal Combustion Residuals Management Program Riverbend Steam Station Site Analysis and Removal Plan Revision 0 31 waste. The location of the approximate ash basin waste boundary and the compliance boundary are shown in Figure 3 of this Removal Plan. The location of the historical groundwater monitoring wells (voluntary and compliance monitoring wells) and wells installed as part of the CSA activities are provided in Figure 10-8 of the CSA Report (HDR, 2015a) (see Appendix B). 4.7 Groundwater Potentiometric Contour Maps As anticipated in the GWA Work Plan, the geological and hydrogeological features influencing the movement, chemical, and physical characteristics of contaminants are related to the Piedmont hydrogeologic system present at the site. The CSA concluded that the direction of the movement of the contaminants is toward the Catawba River, as anticipated. Figures 6-5, 6-6 and 6-7 of the CSA Report (HDR, 2015a) (see Appendix B) provide potentiometric surface maps for shallow, deep and bedrocks based on data collected in July 2015. The groundwater flow model, presented in the CAP Part 1 (HDR, 2015b), indicates that groundwater flow originating from the ash basin starts vertically downward then moves horizontally at depth before discharging as baseflow to Mountain Island Lake. The maximum modeled groundwater travel time from the southern boundary of the model domain is 662 years in the deep groundwater zone to Mountain Island Lake. 4.8 Figures: Cross Sections Vertical and Horizontal Extent of CCR within the Impoundments The figures and accompanying calculations in the Ash Inventory, Appendix A of this Removal Plan, are based on bathymetric and topographic surveys of the CCR facilities at the Riverbend Steam Station compared to historical topographic data and boring data of limited quality and quantity. Appendix A includes isopachs illustrating thickness variations of the CCR materials in the ash basins, Dry Ash Stack and Cinder Pit Storage Area on Figure 1.4, Figure 2.4 and Figure 3.4, respectively. Appendix A also includes cross sections illustrating vertical and horizontal variations of the CCR materials in the ash basins, Dry Ash Stack and Cinder Pit Storage Area on Figure 1.5, Figure 2.5 and Figure 3.5, respectively. Figures 10-152 through 10-158 of the CSA Report (HDR, 2015a) (see Appendix B) provide cross sections the CCR facilities at the Riverbend Steam Station illustrating the strategic units and concentrations of COIs that exceed 2L Standards or IMAC. Amec Foster Wheeler Environment & Infrastructure, Inc. December 2016 Duke Energy Coal Combustion Residuals Management Program Riverbend Steam Station Site Analysis and Removal Plan Revision 0 32 5. GROUNDWATER MODELING ANALYSIS 5.1 Site Conceptual Model Predictions From the initial site conceptual hydrogeologic model (SCM) presented in the GWA Work Plan, the geological and hydrogeological features influencing the migration, chemical, and physical characteristics of contaminants were related to the Piedmont hydrogeologic system present at the site, and described in Section 4 above. The SCM was developed from data generated during previous assessments, existing groundwater monitoring data, and modified based on the results of the 2015 groundwater assessment activities. The CSA found the ash basin source areas discharge to the subsurface beneath the basins and via seeps through the embankments. Groundwater flows in a generally northerly, westerly, and easterly direction from the vicinity of the ash basins to Mountain Island Lake. HDR developed a SCM in accordance with ASTM standard guidance document E1689-95 “Developing Conceptual Site Models for Contaminated Sites” (2014) using the following criteria:  Identification of potential contaminants  Identification and characterization of the source contaminants  Delineation of potential migration pathways through environmental media  Establishment of background areas  Environmental receptor identification and discussion  Determination of system boundaries Below is a summary of the SCM results for each of the criterion. Potential Contaminants Sections 4.5 and 4.6 of this Removal Plan describe the results of CSA activities for the Riverbend Steam Station and present the results of the potential contaminants for the site. To summarize, the following constituents were reported as COIs in the CSA:  Soil: arsenic, boron, cobalt, iron, manganese, nickel, selenium, and vanadium  Groundwater: antimony, arsenic, boron, chromium (total), cobalt, iron, manganese, sulfate, TDS, thallium, and vanadium  Surface water: aluminum, cadmium, chromium, cobalt, copper, iron, lead, manganese, selenium, thallium, vanadium, and zinc  Sediment: 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 Amec Foster Wheeler Environment & Infrastructure, Inc. December 2016 Duke Energy Coal Combustion Residuals Management Program Riverbend Steam Station Site Analysis and Removal Plan Revision 0 33 Source Area Characterization and Contaminants The source areas at the Riverbend Steam Station are defined as the Primary and Secondary Ash Basins, the Dry Ash Stack and the Cinder Pit Storage Area. Source area contaminants from ash, ash basin water, porewater and seeps are summarized in Section 4.5.1 above. In the CAP Part 2, HDR reported that ash within the basins was encountered at depths ranging from the surface to 76 feet below ground surface (bgs). Ash within the Dry Ash Stack was encountered from the surface to 78 feet bgs. Ash within the Cinder Pit Storage Area was encountered from the surface to 14.5 feet bgs. The 3-D representation and the vertical and horizontal cross-section of the CSM are illustrated in Figures 3-1 and 3-2 of the CAP Part 2 (HDR, 2016) (Appendix B). Delineation of Potential Migration Pathways  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 Dry Ash Stack. 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.  Groundwater: In general, groundwater within the shallow, deep, and bedrock flow layers flows from the southern extent of the station 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 indicate that groundwater generally flows to the northeast, discharging to Mountain Island Lake. The approximate horizontal extent of groundwater impacts is limited to beneath the waste boundary and northeast of the ash basin, however, additional delineation is likely to be needed. The approximate vertical extent of groundwater impacts is generally limited to the shallow and deep zones and vertical migration of COIs is impeded by the geologic conditions present beneath the source area. 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 site and discharging to Mountain Island Lake.  Surface Water: Surface water generally flows from the south side of the site to Mountain Island Lake. Background Areas Background areas at the Riverbend Steam Station are located south and beyond the immediate boundary of the Dry Ash Stack and south of Horseshoe Bend Beach Road. Existing and recently installed background monitoring wells will be used to refine groundwater flow direction and distribution, and further assess the influence of naturally occurring COIs. Amec Foster Wheeler Environment & Infrastructure, Inc. December 2016 Duke Energy Coal Combustion Residuals Management Program Riverbend Steam Station Site Analysis and Removal Plan Revision 0 34 Receptor Identification In September and November 2014, Duke conducted and updated a receptor survey of the area within a 0.5-mile radius of the compliance boundary. The results of the survey are summarized in Section 3.2.2 of this Removal Plan. Potential receptors are depicted on CAP Part 1 Figure 1-4 (HDR, 2015b) (Appendix B). 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 Riverbend Steam Station, and the Gastonia and Mount Holly intakes are located approximately 6.9 miles downstream of the station. Water supply intake locations are shown on CAP Part 1 Figure 1-6 (HDR, 2015b) (Appendix B). System Boundaries The site, waste, and compliance boundaries for the Riverbend Steam Station are shown on CAP Part 2 Figure 2-1 (HDR, 2016) (Appendix B). Spatially, the SCM for the station 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. 5.2 Groundwater Chemistry Effects As part of the CSA Report (HDR, 2015a) investigation, HDR completed a cation and anion geochemical evaluation of groundwater from upgradient monitoring wells and ash basin groundwater monitoring wells. In general, HDR concluded that calcium and sulfate are higher in ash basin groundwater monitoring wells compared to the upgradient monitoring wells. HDR generated piper diagrams for site data to compare the geochemistry between ash basin porewater, surface water, seeps, upgradient and downgradient groundwater monitoring wells, and background groundwater monitoring wells. In general, based on the piper diagrams, the ionic composition of groundwater and surface water at the site is predominantly calcium, magnesium, and bicarbonate rich with the exception of ash basin water, ash basin porewater, and downgradient groundwater monitoring wells which were observed to be trending closer to calcium, magnesium and sulfate rich geochemical makeup. Seep data indicate similar geochemistry to ash basin water, ash basin porewater, and shallow wells in the ash basin. Piper diagrams are included as Figures 10-186 through 10-191 of the CSA Report (HDR, 2015a) (see Appendix B). Thirty-eight locations, were sampled for chemical speciation analyses of arsenic (III), arsenic (V), chromium (VI), iron (II), iron (III), manganese (II), manganese (IV), selenium (IV), and selenium (VI). Results for chemical speciation of surface water are presented in Table 10-7 of the CSA Report (HDR, 2015a) (see Appendix B). Radionuclides including radium-226, radium-228, natural uranium, uranium-233, uranium-234, and uranium-236 were analyzed from samples collected at four locations, BG-1D, BG-1S and MW -13. Results for the radiological laboratory testing are presented in Table 10-6 of the CSA Report (HDR, 2015a) (see Appendix B). Amec Foster Wheeler Environment & Infrastructure, Inc. December 2016 Duke Energy Coal Combustion Residuals Management Program Riverbend Steam Station Site Analysis and Removal Plan Revision 0 35 The objective of the CAP geochemical modeling for the Riverbend Steam Station was to describe the expected partitioning of COIs between the aqueous and solid phases (i.e., between groundwater and soil and between ash porewater and ash), and to anticipate changes in phase distributions given variations in dissolved oxygen (DO), pH and TDS. COIs evaluated for the geochemical modeling included: antimony, arsenic, boron, chromium, cobalt, iron, manganese, pH, selenium, sulfate, TDS, thallium and vanadium. Evaluations of COIs were performed for each monitoring well using the United States Geological Survey PHREEQC (v3.3.3.) geochemical speciation code (Parkhurst and Appelo 2013) and PhreePlot (Kinniburgh and Cooper 2011). Model input parameters included the concentration of the COIs, ORP, alkalinity, sodium, and other ions in groundwater for monitoring wells at the site. Simulations were performed to predict geochemical speciation for COIs in the presence of adsorption to soils and response to changes in DO, pH and TDS. Hydrous ferric oxides represented weak binding sites and hydrous aluminum oxides represented strong binding sites. The CAP Part 2 (HDR, 2016) provided the following geochemical modeling observations:  Because redox conditions varied widely across the site, equilibrium was not achieved or data are not representative of the conditions sampled. HDR recommended that additional groundwater results be added to the model to further refine the model and to confirm findings if data are not representative of actual groundwater conditions.  Sorption of all of the aqueous groundwater species identified by the CSA would consume only a fraction of the hydrous ferric oxides and hydrous aluminum oxides sorption sites available in site soils. This will be evaluated further under the Tier Ill monitored natural attenuation (MNA) evaluation to be completed after this CAP.  The limited solubility of arsenic, chromium, cobalt and selenium in site groundwater was confirmed by geochemical modeling.  pH, Eh and TDS can be further evaluated to address MNA or remediation options. 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 capacity for COIs such as boron were lower than for COIs such as arsenic. 5.3 Groundwater Trend Analysis Methods COIs under the influence of certain physical and geochemical processes may leach and migrate into groundwater. In order to evaluate COI migration and predict potential impacts that could result following closure of the CCR units at the site, HDR performed modeling of groundwater flow, COI fate and transport, and groundwater to surface water mixing. Groundwater models were run to simulate groundwater elevations in the ash and underlying groundwater flow layers and to simulate COI concentrations at the compliance boundary or other downgradient locations of interest over time for closure scenarios. Amec Foster Wheeler Environment & Infrastructure, Inc. December 2016 Duke Energy Coal Combustion Residuals Management Program Riverbend Steam Station Site Analysis and Removal Plan Revision 0 36 The University of North Carolina at Charlotte (UNCC) developed a site-specific, 3-D, steady- state groundwater flow and fate and transport model for the Riverbend Steam Station using MODFLOW (Niswonger et. al., 2011) and MT3DMS (Zheng and Wang, 1999). The model domain included the site, a section of the Catawba River, and site features relevant to the assessment of groundwater. The model domain was bounded by the following hydrologic features:  Mountain Island Lake to the north and west (Catawba River shoreline)  Topographic divide to the east and south of the station Initially, as part of the CAP Part 1 (HDR, 2015b), groundwater elevations and COI concentrations were evaluated for each of the closure scenarios using model layers divided among the hydrostratigraphic units at the station. COI velocity and flow direction to potential off- site receptors were simulated by assigning geologic units, hydrologic features, and flow boundaries within the COI source areas. For the site, a laboratory determination of the partition coefficient (Kd) was performed by UNCC on soil samples collected during the CSA. Soil samples were tested in flow-through columns to measure sorption of COIs at varying concentrations. The resulting Kd data was used as input parameters to evaluate fate and transport through the subsurface. Sorption studies on soil samples obtained during the CSA indicated that Kd values for COIs in native soil surrounding the ash basins and ash storage areas are higher than the values used in modeling. Subsequent to the submittal of the CAP Part 1 (HDR, 2015b), UNCC and Geochemical, LLC recalculated Kd values using linear Freundlich isotherm. Use of the refined COI Kd values in the fate and transport model resulted in improved model calibration of source concentrations to measured concentrations in downgradient wells. Additionally, the model was refined to incorporate proposed provisional background concentrations. Finally, the refinements were made to better represent measured source area porewater concentrations. Two closure scenarios were modeled for the Riverbend Steam Station. The existing conditions scenario assumed ash sources were left in place. The excavation scenario assumed accessible ash was removed from the site. No modifications were made to the previously modeled existing conditions scenario hydrogeologic parameters or structure between each modeling phase. Existing Conditions Scenario One of the purposes of modeling the existing conditions scenario was to predict when steady- state concentrations would be achieved at the compliance boundary. The model was calibrated for steady-state groundwater flow conditions and transient transport of COIs under existing conditions. The simulation revealed that COI concentrations remain the same or increase initially with source concentrations held at their constant value over time. Concentrations and discharge rates were found to remain constant thereafter. According to HDR, the existing conditions scenario represented the most conservative case in terms of groundwater concentrations onsite and offsite, with COIs discharging to surface water at steady-state. Areas close to the compliance boundary were predicted to reach steady-state concentrations sooner than areas further away from the compliance boundary. Sorptive COIs were predicted to Amec Foster Wheeler Environment & Infrastructure, Inc. December 2016 Duke Energy Coal Combustion Residuals Management Program Riverbend Steam Station Site Analysis and Removal Plan Revision 0 37 remain transient for a longer period of time as their peak breakthrough concentrations travel at rates less than groundwater pore velocity. Excavation Scenario The excavation scenario assumed water and/or ash would be removed from the CCR units and transported offsite. The model did not account for backfilling of excavation areas and the constant concentration of COIs in the source areas above and below the water table being removed. An assumed recharge rate of 6.5 inches per year was used. The simulation revealed that COIs already present in groundwater continued to migrate downgradient as water infiltrated and recharged the aquifer. COIs also moved through the saturated zone beneath the source areas at rates dependent on physical and geochemical interactions of the COI and groundwater. If the area became unsaturated, COIs were observed to decrease over time without a contributing source. COI migration slowed relative to porewater velocity with sorptive COIs attenuated by site materials. Summary of Groundwater Modeled Scenario Predictions A summary of the modeled results for both scenarios at the compliance boundary as adopted from the CAP Part 2 (HDR, 2016) is provided in the Table 5.1 below. Table 5-1 Summary of Modeled COI Results at the Compliance Boundary Constituent (Standard) Flow Layer Existing Conditions Scenario Excavation Scenario Year 0 Year 100 Year 0 Year 100 Antimony IMAC (1 µg/L) Shallow + + + + Deep + + + + Bedrock + + + + Arsenic 2L (10 µg/L) Shallow - - - - Deep - - - - Bedrock - - - - Boron 2L (700 µg/L) Shallow - - - - Deep - - - - Bedrock - - - - Chromium 2L (10 µg/L) Shallow - - - - Deep - - - - Bedrock - - - - Amec Foster Wheeler Environment & Infrastructure, Inc. December 2016 Duke Energy Coal Combustion Residuals Management Program Riverbend Steam Station Site Analysis and Removal Plan Revision 0 38 Constituent (Standard) Flow Layer Existing Conditions Scenario Excavation Scenario Year 0 Year 100 Year 0 Year 100 Cobalt IMAC (1 µg/L) Shallow + + + + Deep + + + + Bedrock + + + + Hexavalent Chromium NCDHHS HSL (0.07 µg/L) Shallow + + + + Deep + + + + Bedrock + + + + Sulfate 2L (250,000 µg/L) Shallow - - - - Deep - - - - Bedrock - - - - Thallium IMAC (0.2 µg/L) Shallow + + + + Deep + + + + Bedrock + + + + Vanadium IMAC (0.3 µg/L) Shallow + + + + Deep + + + + Bedrock + + + + Notes: “+” indicates that concentration of a given COI has exceeded its applicable 2L Standard, IMAC or NCDHHS HSL. “-“ indicates that concentration of a given COI is below its applicable 2L Standard, IMAC or NCDHHS HSL. “Year 0” represents initial concentrations observed in 2015. “Year 100” represents the observed 100 year post implementation of each scenario in 2115. Based on the model prediction results, the CAP Part 2 (HDR, 2016) provided the following observations:  A CAP may be approved by the NCDEQ without requiring groundwater remediation to the 2L Standards if seven requirements are met (15A NCAC 02L. 0106[k]). One requirement is that the 2L Standards must be met at a location no closer than one year time to travel upgradient of an existing or foreseeable receptor. Mountain Island Lake is considered the receptor for the site. To evaluate this requirement, HDR and UNCC conducted particle tracking for the excavation steady-state flow field scenario to identify the one-year travel time boundary using six select wells located near Mountain Island Lake and side-gradient wells near the ash basins. A particle tracking simulation was also performed to demonstrate steady-state effects of pumping the six wells at a rate of 3 Amec Foster Wheeler Environment & Infrastructure, Inc. December 2016 Duke Energy Coal Combustion Residuals Management Program Riverbend Steam Station Site Analysis and Removal Plan Revision 0 39 gallons per minute. Results of the simulation show that the pumping wells would capture a portion of the groundwater in the shallow zone that has been impacted by the ash basin and other source areas. If Duke 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 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 source areas at the station. If Duke 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 areas.  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 250-year time period modeled (2015 to 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 K, values (e.g. rapid and nearly complete reduction to below the respective standard or IMAC under the Excavation scenario). Groundwater to Surface Water Interaction Modeling As part of the CAP Part 1 (HDR, 2015b), a simulation model was performed to estimate groundwater flow and constituent loading to Mountain Island Lake. For the groundwater-surface water interaction simulation, fate and transport output data were applied using a Mixing Model Approach. River flow data from the USGS (or other suitable gauges) were used to design upstream river design flows and constituent compliance with 2B Standards. 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 located on the downstream (east) side of the station. Groundwater loading of COIs to Mountain Island Amec Foster Wheeler Environment & Infrastructure, Inc. December 2016 Duke Energy Coal Combustion Residuals Management Program Riverbend Steam Station Site Analysis and Removal Plan Revision 0 40 Lake and the semi-enclosed basin were calculated as the product of volumetric groundwater fluxes and corresponding COI concentrations calculated with the groundwater model. The mixing model results indicate that impacts from groundwater exceedances within the sources areas at the Riverbend Steam Station do not cause violation of 2B surface water quality standards at the edge of the mixing zones. The calculated surface water concentrations of COIs in Mountain Island Lake downstream of the station and for the semi-enclosed basin were below applicable human health and water supply regulatory criteria. Amec Foster Wheeler Environment & Infrastructure, Inc. December 2016 Duke Energy Coal Combustion Residuals Management Program Riverbend Steam Station Site Analysis and Removal Plan Revision 0 41 6. BENEFICIAL USE AND FUTURE USE 6.1 CCR Material Use Duke considers CCR beneficial use in an environmentally responsible manner for ash that is produced at its plants or is removed from existing ash basins. Ash basin closure by removal presents the opportunity for CCR beneficial use. Duke has a team dedicated to identifying beneficial use opportunities and evaluating their feasibility. Consistent with North Carolina CAMA requirements, Part III, Section 4.(e), Duke issued a request for proposals to conduct a beneficial use market analysis, study the feasibility and advisability of installing existing beneficiation technologies, and examine innovative technologies. The selected beneficial use for the majority of CCR being removed from the Riverbend Steam Station is placement of the material as structural fill at the Brickhaven Mine facility in Moncure, North Carolina. Section 9.0 of this Removal Plan summarizes the final disposition of the CCR materials at the station. Findings indicate that large-scale beneficiation technologies are not feasible to install at the Riverbend Steam Station at this time. In light of the August 1, 2019 CAMA closure deadline and the large investment that would be required, large-scale beneficiation is unsupportable on the basis of economic and business criteria. 6.2 Site Future Use The closure of the Riverbend Steam Station ash storage areas involves excavation of the CCR materials with removal from the site. The grading plan provides for breaching and removal of the impoundment structures, and establishing relatively gentle final grades for controlled runoff velocities and positive drainage from the site. Upon establishing final grades, the site will be seeded to establish grassy ground cover per the approved grading plan drawings and specifications. After ash and dam removal, the ash basin area will be stabilized with permanent vegetation (consisting of grasses) and will be maintained throughout the post-closure period. Amec Foster Wheeler Environment & Infrastructure, Inc. December 2016 Duke Energy Coal Combustion Residuals Management Program Riverbend Steam Station Site Analysis and Removal Plan Revision 0 42 7. CLOSURE DESIGN DOCUMENTS Ash basin closure design has been developed and documented through engineering evaluations and analyses, drawings, specifications, and a construction quality assurance plan. The closure design documents are summarized in the following sections. In addition to this Removal Plan, these documents will support the decommissioning plan for submittal to the NCDEQ Division of Energy, Mineral, and Land Resources, Safe Dams Program (Dam Safety) to decommission the ash basin regulated dams. The Ash Pond CCR Removal Grading Plan is included as Appendix D of this Removal Plan. 7.1 Engineering Evaluations and Analyses Engineering evaluations and analyses for ash basin closure focus on stormwater management, ash inventory, and earthworks quantities. Geotechnical stability analyses are not necessary to support ash removal and dam decommissioning and are not provided. Engineering evaluations and analyses of the Riverbend Ash Pond CCR Removal Grading Plans are included in Appendix E of this Removal Plan and summarized below. 7.1.1 Freeboard During Dam Decommissioning The existing ash basin embankments and outlet structures can be lowered as CCR removal progresses. Adequate freeboard will be maintained during dam decommissioning activities. The freeboard requirements during dam decommissioning are specified for the Primary Ash Basin Dam (GASTO-097) and Secondary Ash Basin Dam (GASTO-098) as conveyed in the dam decommissioning plan request letter, supporting drawings, and specifications. During excavation of the CCR materials from within the ash basin, a minimum grade differential of 10 and 20 feet will be maintained, respectively, for Dams GASTO-097 and GASTO-098 between the elevation of ash within the basin and the lowered dam crests. In addition, the contractor is to maintain a minimum elevation differential of 10 feet between the maximum CCR excavation grade and minimum embankment crest elevation for crest elevations 687 and above. The contractor is to maintain a minimum elevation differential of 22 feet between maximum CCR excavation grade and minimum embankment crest elevation for crest elevations below 687. The more stringent criterion will be applied. Piezometers and observation wells located on the dams will continue to be routinely monitored for groundwater elevations. 7.1.2 Stormwater Management During Interim Conditions The existing ash basin embankments can be lowered during removal of CCR materials, and the CCR removal phase is referred to as Phase 1. A section of embankment at least 10-feet in height will temporarily remain upon completion of CCR removal, and this configuration is referred to as interim conditions or Phase 2. After completion of CCR removal, two sediment basin outlets will be installed within the ash basin footprint to discharge clean stormwater from the remaining impounded area to Mountain Island Lake, and this configuration is referred to as Amec Foster Wheeler Environment & Infrastructure, Inc. December 2016 Duke Energy Coal Combustion Residuals Management Program Riverbend Steam Station Site Analysis and Removal Plan Revision 0 43 final conditions. The interim condition will exist during fine-grading within the ash basin footprint and establishment of vegetation within the newly graded area. Proposed stormwater channels, and culverts for interim conditions (as shown on Drawing RVB_C901.004.007 in Appendix D) were designed for flow capacity and lining stability for a 25- year, 24-hour design storm assuming interim conditions consisting of newly graded tributary drainage areas and channels temporarily stabilized with erosion control matting or riprap. The design calculations for interim conditions are presented in the calculation entitled “Stormwater Evaluation” in Appendix E. Note that each of the two sediment basins within the ash basin area was sized to handle approximately the full ash basin drainage area in order to accommodate a wide range of possible temporary configurations during grading of the area. Also note that this calculation was prepared in 2015 and includes outdated statements not germane to the calculation results, such as a reference to an outlet from the secondary riser which has since been grouted and a statement that means and methods for dewatering have not been determined. 7.1.3 Stormwater Management During Final Conditions The remaining 10-foot high section of embankment and two temporary sediment basins will be removed upon stabilization of the tributary drainage area such that water will not be impounded within the abandoned ash basin footprint. Stormwater flows will discharge to Mountain Island Lake by overland flow through the proposed dam breaches coinciding with the location of the temporary sediment basins. For final conditions (as shown on Drawing RVB_C901.004.053 in Appendix D), stormwater conveyances passing through dam breaches were designed for flow capacity and lining stability for a 50-year, 24-hour design storm in response to a request from NCDEQ Dam Safety. These design calculations are presented in the calculation entitled “Stormwater Analysis of Final Grades with Dam Breach Openings” in Appendix E. These calculations supersede the “post- construction conditions” calculations presented in the calculation entitled “Stormwater Evaluation” in Appendix E. Differences between runoff curve numbers (CN) may be observed between the “Stormwater Evaluation” post-construction calculations and “Stormwater Analysis of Final Grades with Dam Breach Openings” calculations. The earlier post-construction stormwater analysis was a conservative approach. Originally, the soils were identified as Hydrological Soil Group (HSG) C with a CN value of 79 for fair grassed open space condition and drainage areas were delineated not including permanent diversions (resulting in areas larger than actual). Also, the time of concentrations did not include channel flow through breach channels. In the more recent “Stormwater Analysis of Final Grades with Dam Breach Openings,” the delineation of drainage areas take into account permanent diversions that cause the contributing drainage areas to be smaller and the final condition soils are identified as HSG B soils which causes the CN values to lower to 69 for fair grassed open space condition. With the revision of the final grades to include breach channels and low flow channels, the time of concentrations for the drainage areas decreased due to the addition of the channel flow segment. Amec Foster Wheeler Environment & Infrastructure, Inc. December 2016 Duke Energy Coal Combustion Residuals Management Program Riverbend Steam Station Site Analysis and Removal Plan Revision 0 44 The net result of these differences is a slight increase in the peak flows for a given design storm compared to the older post-construction calculation. 7.2 Removal Plan Drawings Removal Plan drawings have been developed to support the decommissioning plan for submittal to the NCDEQ Dam Safety. The Removal Plan drawings are provided in Appendix D and include the following series of drawings:  Cover Sheet - Drawing Index, Location Map  General Notes and Legends  Existing Conditions Plan - Aerial Photography Map  Existing Conditions Plan - Topographic Map  Boring Location Plan  Demolition Plans  Erosion and Sedimentation Control Plans  Erosion and Sedimentation Control Details  Erosion and Sedimentation Control Sequence and Notes  Proposed Excavation Grades  Final Grades  Grading Profile Alignments  Grading Profiles  Project Boundaries  Wetland Boundaries  Wetland Point Tables 7.3 Construction Quality Assurance Plan The Construction Quality Assurance (CQA) Plan is included in Appendix F of this Removal Plan. The purpose of the CQA Plan is to identify the quality assurance procedures, standards, and methods that will be employed during the project to provide that the requirements of the drawings, specifications, regulatory permits, and owner specified health, safety and environmental requirements are met. The CQA Plan is specific to the Riverbend Steam Station ash basin closure project and has been prepared in compliance with the CAMA requirements. Amec Foster Wheeler Environment & Infrastructure, Inc. December 2016 Duke Energy Coal Combustion Residuals Management Program Riverbend Steam Station Site Analysis and Removal Plan Revision 0 45 8. MANAGEMENT OF WASTEWATER AND STORMWATER Management of wastewater and stormwater discharges at the Riverbend Steam Station is discussed below. Existing plant and site-wide stormwater and wastewater will be diverted from the ash basins and be treated as necessary and discharged in accordance with the respective NDPES Permits. 8.1 Stormwater Management The Riverbend Steam Station ash basins have historically collected the wastewater (industrial and septic) from plant operations and from rainfall runoff before discharging it through the basins’ outlet structures. The ash basins currently only receive stormwater since sluicing operations were ceased when the steam plant was retired. The stormwater that enters the basin area ispumped to the on-site wastewater treatment plant, treated and discharged to the Catawba River as part of the permitted industrial wastewater discharge. As mentioned previously, the riser structure and discharge outlet pipe within the Secondary Basin have been taken out of service and a portable pump system has been installed. Stormwater that accumulates elsewhere on the Riverbend Steam Station property, that does not enter the ash basins, is directed to several permitted outfalls and monitored as part of the NPDES Industrial Stormwater Permit. Upon completion of ash removal, clean stormwater from within the former ash basin areas will be conveyed and controlled in a stable manner and discharged to the Catawba River. Details of the stormwater management measures after completion of ash removal are part of the erosion and sedimentation controls drawings in Appendix D of this Removal Plan along with detail sheets and technical specifications to be prepared for the decommissioning of the ash basins. The Duke Coal Combustion Products (CCP) Closure Team will coordinate with the Duke CCP Environmental Permitting and Compliance Specialists for any NPDES Industrial Stormwater Permit modifications. 8.2 Wastewater Management Management of wastewater at the Riverbend Steam Station will be addressed using a temporary on-site wastewater treatment plant. The goal of the decommissioning is to remove CCR materials from the site and return the former ash basins back to a natural state where stormwater is discharged via sheet flow to the receiving water(s) such as the Catawba River. To accomplish this, multiple phases of decommissioning work are required and these are detailed in the stormwater diversion portion of the decommissioning plan for the site. NPDES permit modifications will be submitted to address the fully decommissioned site when wastewater discharges are eliminated at the site. The Duke CCP Closure Team will coordinate with the Duke CCP Environmental Permitting and Compliance Specialists for any NPDES Wastewater Permit modifications. Amec Foster Wheeler Environment & Infrastructure, Inc. December 2016 Duke Energy Coal Combustion Residuals Management Program Riverbend Steam Station Site Analysis and Removal Plan Revision 0 46 9. DESCRIPTION OF FINAL DISPOSITION OF CCR MATERIALS The CCR materials at the Riverbend Steam Station are being excavated and removed from the site. Removal of the materials is being conducted using conventional excavation equipment. Transportation of the CCR materials started by using over-the-road trucks and transitioned to railcar after the initial phases of the project. It is anticipated that the majority of the material will be transported offsite by railcar. Some of the ash from the ash stack at the retired Riverbend Steam Station has been relocated to a fully lined landfill in Homer, Georgia as well as to fully lined landfills at Duke’s Marshall Steam Station in Terrell, North Carolina. Relocating ash to these other permanent storage solutions allows Duke to proceed with ash excavation to meet the August 1, 2019 CAMA closure deadline. The majority of Riverbend ash will be transported by rail to the Brickhaven clay mine in Chatham County, North Carolina for beneficial use. This will allow ash, a valuable construction material, to replace virgin soil to reclaim land that is currently unusable. Duke will continue to evaluate additional storage options, as needed, to meet project deadlines. Table 9.1 provides a list of the sites to be used for final deposition of the CCR materials removed from the Riverbend Steam Station. Other permitted locations may be added in the future. Table 9-1 List of Approved Lined Landfills and Structural Fills for Riverbend CCR Materials Site Permit Address Town County State R&B Landfill 006-009D 610 Frank Bennett Road Homer Banks Georgia Marshall Steam Station Landfill 1812-INDUS-2008 8320 East NC Highway 150 Terrell Catawba North Carolina Marshall Steam Station Landfill (FGD Residuals) 1809-INDUS (Inactive) 8320 East NC Highway 150 Terrell Catawba North Carolina Brickhaven Mine 1910-STRUCT-2015 1473 Moncure- Flatwood Road Moncure Chatham North Carolina Amec Foster Wheeler Environment & Infrastructure, Inc. December 2016 Duke Energy Coal Combustion Residuals Management Program Riverbend Steam Station Site Analysis and Removal Plan Revision 0 47 10. APPLICABLE PERMITS FOR CLOSURE Implementation of the ash basin closure at the Riverbend Steam Station will require several permits issued by regulatory authorities. Below is a list of the applicable permits required for closure:  Dam Breach Approval for Decommissioning Dam Structures  Discharge Permits for Wastewater and Stormwater  Solid Waste Permits for Landfills and Structural Fills  Erosion and Sedimentation Control Permits  Section 401/404 Water Quality certification if applicable Note that air permits or air permit modifications are not anticipated for the closure of the CCR facilities at the Riverbend Steam Station. The Title V air permit for operating the Riverbend Steam Station was rescinded when the plant was retired in 2013. The Lark Maintenance Center, located on the station property, operates under NCDEQ Division of Air Quality Permit 07248R054, with effective dates from June 9, 2015 through May 31, 2023. No new air permits are anticipated for closure of the CCR facilities at the Riverbend Steam Station. 10.1 Decommissioning Request and Approval The plans, specifications, design data and calculations for decommissioning of the Intermediate Dam (GASTO-099) has been submitted and approved. The plans, specifications, design data and calculations for decommissioning of the Primary and Secondary dams (GASTO-097 and GASTO-098) will be prepared and submitted to the Dam Safety Section of the NCDEQ Division of Energy, Mineral and Land Resources as part of the dam breach permit application (decommissioning plan). The decommissioning plan and accompanying design package will be a separate submittal from this Removal Plan. The design package for the permit application will generally consist of drawings and technical specifications. The drawings will provide grading plans and erosion and sedimentation control measures to be implemented for the removal of CCR materials from the site. The drawings will also establish final grades after the dams are breached. The design package will also provide sequencing of construction activities, controls and restrictions on dewatering, and dam breach sequencing and restrictions. The intent of the dewatering restrictions are to control the rate of drawdown as well as maintaining dewatering levels during the project. The purpose of the dam breach restrictions is to maintain adequate freeboard during construction for containment of CCR materials and precipitation during construction. Following review of the decommissioning plan by the Dam Safety Section, an approval to breach the dams will be issued by the Director of the Division of Land Resources with any applicable stipulations. Once the dams are breached under the supervision of a professional engineer, as-built drawings, engineer’s certification, and the owner’s certification will be prepared and submitted to the Dam Safety Section. Subsequently, the dams will be inspected Amec Foster Wheeler Environment & Infrastructure, Inc. December 2016 Duke Energy Coal Combustion Residuals Management Program Riverbend Steam Station Site Analysis and Removal Plan Revision 0 48 by the NCDEQ Land Quality Section to confirm that the as-built drawings are accurate. An approval will be granted by the Division of Energy, Mineral and Land Resources and the dams will be removed from the state dam inventory. Amec Foster Wheeler Environment & Infrastructure, Inc. December 2016 Duke Energy Coal Combustion Residuals Management Program Riverbend Steam Station Site Analysis and Removal Plan Revision 0 49 11. POST-CLOSURE MONITORING AND CARE The Post-Closure Operations Maintenance and Monitoring (OM&M) Plan is provided as Appendix G of this Removal Plan. The default post-closure period is 30 years, however opportunities to modify and reduce the post closure period for various requirements including groundwater and surface water monitoring are possible. The Post-Closure OM&M Plan addresses the following:  Description of the closure components  Regular inspections and maintenance of the stormwater and erosion control measures  Post closure inspection checklist to guide post-closure inspections  Continuation of the groundwater and surface water monitoring and assessment program  Provide means and methods of managing affected groundwater and stormwater  Maintaining the groundwater monitoring system  Facility contact information  Description of planned post-closure uses 11.1 Groundwater Monitoring Program Post-closure groundwater monitoring requirements will be established in the Groundwater Monitoring Plan, to be submitted under separate cover. The CSA Report (HDR, 2015a) provides an interim groundwater monitoring plan to bridge the gap between completion of CSA Report (HDR, 2015a) activities and implementation of the pending Groundwater Monitoring Plan and CAP. Two comprehensive sampling events and two background-only sampling events were conducted in 2015. There have been two comprehensive sampling events so far in 2016. The proposed constituents and parameters for the interim groundwater monitoring plan are presented in Table 16-1 of the CSA Report (HDR, 2015a), and the proposed sampling locations are presented in Table 16-2 of the CSA Report (HDR, 2015a) (see Appendix B). The interim groundwater monitoring plan includes sampling background wells during the additional interim groundwater sampling event in 2015. Amec Foster Wheeler Environment & Infrastructure, Inc. December 2016 Duke Energy Coal Combustion Residuals Management Program Riverbend Steam Station Site Analysis and Removal Plan Revision 0 50 12. PROJECT MILESTONES AND COST ESTIMATES 12.1 Project Schedule The North Carolina CAMA deems Riverbend a “high-priority” site and specifically requires closure by August 1, 2019. The CAMA defined closure definition of dewatering to the maximum extent practicable and removing and transferring CCRs to a landfill or structural fill is addressed in the proposed schedule. The CAMA defined closure definition for providing corrective action to restore groundwater quality (if needed) is not addressed in the schedule included herein. Groundwater assessment and corrective action is currently on-going and the need and timeframe for restoring groundwater quality is currently unknown. The milestones tracked will include the following items:  Removal Plan Submittal (milestone)  Removal Plan Concurrence (milestone)  Dam Decommissioning Plan Submittals (milestones)  Dam Decommissioning Plan Approvals (milestones)  Start Date of Ash Removal (milestone)  Completion of Ash Removal (milestone)  Completion of Dam Decommissioning (milestone)  Dam Decommissioning Letter Issued (milestone)  Beginning of Post Closure Care Period 12.2 Closure and Post-Closure Cost Estimate Closure Cost Estimate Duke is preparing closure and post-closure care cost estimates at a level of detail and from the perspective that sufficient funding will be set-aside in a financial assurance mechanism for a third-party (other than the owner) to complete the scope of work. The cost estimates will be included as Appendix H of this Removal Plan at a later date. Amec Foster Wheeler Environment & Infrastructure, Inc. December 2016 Duke Energy Coal Combustion Residuals Management Program Riverbend Steam Station Site Analysis and Removal Plan Revision 0 51 13. REFERENCED DOCUMENTS AECOM (URS), 2015, Phase 2 Reconstitution of Ash Pond Designs Report, June 2015. AMEC, 2014, Annual Ash Basin Dam Inspection, Riverbend Steam Station, December 2014. Amec Foster Wheeler, 2016a, Coal Combustion Residuals (CCR) Annual Surface Impoundment Report (October 2015) Inspection, Riverbend Steam Station, February 2016. Amec Foster Wheeler, 2016b, Coal Combustion Residuals (CCR) Annual Surface Impoundment Report, May 2016 Inspection, Riverbend Steam Station, June 2016. CHA, 2009, Assessment of Dam Safety, Coal Combustion Surface Impoundments Draft Report, Riverbend Steam Station, July, 2009. Duke Energy, 2014a, Coal Ash Excavation Plan, November 2014. Duke Energy, 2014b, Permit Application for Renewal, May 2014. Duke Energy, 2009, Permit Application for Renewal, August 2009. Duke Energy, 2015, Riverbend Steam Station CCP Disposal Operations & Maintenance Manual, March 2015. Geosyntec Consultants, 2014, Dewatering Plan for Coal Combustion Residuals (CCR) Basins Riverbend Steam Station, September 2014. Hall, 2014, Stormwater Drainage Pipe Inspection Videos and Observations, February 2014. HDR, 2014, Revised Groundwater Assessment Work Plan, December 2014. HDR, 2015a, Comprehensive Site Assessment Report, Riverbend Steam Station Ash Basin, August 2015. HDR, 2015b, Corrective Action Plan – Part 1, Riverbend Steam Station Ash Basin, November 2015. HDR, 2016, Corrective Action Plan – Part 2, Riverbend Steam Station Ash Basin, February 2016. Kleinfelder, 2010, Report of Hydrologic and Hydraulic Engineering Services, Primary and Secondary Ash Ponds at Riverbend Steam Station, June 2010. MACTEC Engineering and Consulting, Inc., 2004, Independent Consultant Inspection, Ash Basin Dams, November 2004. NCDEQ, 2014, Letter on Weekly and Annual Dam Inspections, August 2014. Amec Foster Wheeler Environment & Infrastructure, Inc. December 2016 Duke Energy Coal Combustion Residuals Management Program Riverbend Steam Station Site Analysis and Removal Plan Revision 0 52 NCDEQ, 2015a, Letter on Certificate of Approval, August 2015 NCDEQ, 2015b, NPDES Discharge Permit NCS000549 for Stormwater, May 2015 NCDEQ, 2015c, NPDES Discharge Permit NC0004961 for Wastewater (draft), March 2015 S&ME, Inc., 2009, Annual Ash Basin Dike Inspection Report, May 2009. Trigon Engineering Consultants, 1989, Independent Consultant Inspection, Ash Basin Dams, June 1989. URS, 2014a. Letter report, Subject: Draft Ash Storage Basin Phase 1 Evaluation, Duke Energy – Riverbend Steam Station. May 30, 2014. URS, 2014b. Letter report, Subject: Draft Hydrogeologic Assessment report, Duke Energy - Riverbend Steam Station. June 20, 2014. Amec Foster Wheeler Environment & Infrastructure, Inc. December 2016 Duke Energy Coal Combustion Residuals Management Program Riverbend Steam Station Site Analysis and Removal Plan Revision 0 53 NCDEQ, 2015a, Letter on Certificate of Approval, August 2015 NCDEQ, 2015b, NPDES Discharge Permit NCS000549 for Stormwater, May 2015 NCDEQ, 2015c, NPDES Discharge Permit NC0004961 for Wastewater (draft), March 2015 S&ME, Inc., 2009, Annual Ash Basin Dike Inspection Report, May 2009. Trigon Engineering Consultants, 1989, Independent Consultant Inspection, Ash Basin Dams, June 1989. URS, 2014a. Letter report, Subject: Draft Ash Storage Basin Phase 1 Evaluation, Duke Energy – Riverbend Steam Station. May 30, 2014. URS, 2014b. Letter report, Subject: Draft Hydrogeologic Assessment report, Duke Energy - Riverbend Steam Station. June 20, 2014. Amec Foster Wheeler Environment & Infrastructure, Inc. December 2016 Duke Energy Coal Combustion Residuals Management Program Riverbend Steam Station Site Analysis and Removal Plan Revision 0 TABLES December 2016 Table 2-1: NC CAMA Closure Plan Requirements Summary and Cross Reference Table Site Analysis and Removal Plan - Riverbend Steam Station Duke Energy No.Description Corresponding Closure Plan Section 1 Site history and history of site operations, including details on the manner in which coal combustion residuals have been stored and disposed of historically.3.1.1 2 Estimated volume of material contained in the impoundment.3.1.2 3 Analysis of the structural integrity of dikes or dams associated with impoundment.3.1.3 4 All sources of discharge into the impoundment, including volume and characteristics of each discharge.3.1.4 5 Whether the impoundment is lined, and, if so, the composition thereof.3.1.5 6 A summary of all information available concerning the impoundment as a result of inspections and monitoring conducted pursuant to this Part and otherwise available. 3.1.6 1 All structures associated with the operation of any coal combustion residuals surface impoundment located on the site. For purposes of this sub-subdivision, the term "site" means the land or waters within the property boundary of the applicable electric generating station. 3.2.1 2 All current and former coal combustion residuals disposal and storage areas on the site, including details concerning coal combustion residuals produced historically by the electric generating station and disposed of through transfer to structural fills. 3.3 3 The property boundary for the applicable site, including established compliance boundaries within the site.3.3 4 All potential receptors within 2,640 feet from established compliance boundaries. 3.2.2 5 Topographic contour intervals of the site shall be selected to enable an accurate representation of site features and terrain and in most cases should be less than 20-foot intervals.3.3 6 Locations of all sanitary landfills permitted pursuant to this Article on the site that are actively receiving waste or are closed, as well as the established compliance boundaries and components of associated groundwater and surface water monitoring systems.3.2.3 7 All existing and proposed groundwater monitoring wells associated with any coal combustion residuals surface impoundment on the site.3.3 8 All existing and proposed surface water sample collection locations associated with any coal combustion residuals surface impoundment on the site.3.3 1 A description of the hydrogeology and geology of the site.4.1 2 A description of the stratigraphy of the geologic units underlying each coal combustion residuals surface impoundment located on the site. 4.2 3 The saturated hydraulic conductivity for (i) the coal combustion residuals within any coal combustion residuals surface impoundment located on the site and (ii) the saturated hydraulic conductivity of any existing liner installed at an impoundment, if any. 4.3 4 The geotechnical properties for (i) the coal combustion residuals within any coal combustion residuals surface impoundment located on the site, (ii) the geotechnical properties of any existing liner installed at an impoundment, if any, and (iii) the uppermost identified stratigraphic unit underlying the impoundment, including the soil classification based upon the Unified Soil Classification System, in-place moisture content, particle size distribution, Atterberg limits, specific gravity, effective friction angle, maximum dry density, optimum moisture content, and permeability. 4.4 5 A chemical analysis of the coal combustion residuals surface impoundment, including water, coal combustion residuals, and coal combustion residuals-affected soil. 4.5 6 Identification of all substances with concentrations determined to be in excess of the groundwater quality standards for the substance established by Subchapter L of Chapter 2 of Title 15A of the North Carolina Administrative Code, including all laboratory results for these analyses.4.6 7 Summary tables of historical records of groundwater sampling results.4.6 8 A map that illustrates the potentiometric contours and flow directions for all identified aquifers underlying impoundments (shallow, intermediate, and deep) and the horizontal extent of areas where groundwater quality standards established by Subchapter L of Chapter 2 of Title 15A of the North Carolina Administrative Code for a substance are exceeded.4.7 9 Cross-sections that illustrate the following: the vertical and horizontal extent of the coal combustion residuals within an impoundment; stratigraphy of the geologic units underlying an impoundment; and the vertical extent of areas where groundwater quality standards established by Subchapter L of Chapter 2 of Title 15A of the North Carolina Administrative Code for a substance are exceeded.4.8 Part II. Provisions for Comprehensive Management of Coal Combustion Residuals § 130A-309.214(a)(4) Closure Plans for all impoundments shall include all of the following: a. Facility and coal combustion residuals surface impoundment description. – A description of the operation of the site that shall include, at a minimum, all of the following: b. Site maps, which, at a minimum, illustrate all of the following: c. The results of a hydrogeologic, geologic, and geotechnical investigation of the site, including, at a minimum, all of the following: 1 OF 2 December 2016 Table 2-1: NC CAMA Closure Plan Requirements Summary and Cross Reference Table Site Analysis and Removal Plan - Riverbend Steam Station Duke Energy No.Description Corresponding Closure Plan Section d. 1 An account of the design of the proposed Closure Plan that is based on the site hydrogeologic conceptual model developed and includes (i) predictions on post-closure groundwater elevations and groundwater flow directions and velocities, including the effects on and from the potential receptors and (ii) predictions at the compliance boundary for substances with concentrations determined to be in excess of the groundwater quality standards for the substance established by Subchapter L of Chapter 2 of Title 15A of the North Carolina Administrative Code. 5.1 2 Predictions that include the effects on the groundwater chemistry and should describe migration, concentration, mobilization, and fate for substances with concentrations determined to be in excess of the groundwater quality standards for the substance established by Subchapter L of Chapter 2 of Title 15A of the North Carolina Administrative Code pre- and post-closure, including the effects on and from potential receptors.5.2 3 A description of the groundwater trend analysis methods used to demonstrate compliance with groundwater quality standards for the substance established by Subchapter L of Chapter 2 of Title 15A of the North Carolina Administrative Code and requirements for corrective action of groundwater contamination established by Subchapter L of Chapter 2 of Title 15A of the North Carolina Administrative Code.5.3 e.A description of any plans for beneficial use of the coal combustion residuals in compliance with the requirements of Section .1700 of Subchapter B of Chapter 13 of Title 15A of the North Carolina Administrative Code (Requirements for Beneficial Use of Coal Combustion By-Products) and Section .1205 of Subchapter T of Chapter 2 of Title 15A of the North Carolina Administrative Code (Coal Combustion Products Management).6.1 f.All engineering drawings, schematics, and specifications for the proposed Closure Plan. If required by Chapter 89C of the General Statutes, engineering design documents should be prepared, signed, and sealed by a professional engineer.7.1, 7.2 g.A description of the construction quality assurance and quality control program to be implemented in conjunction with the Closure Plan, including the responsibilities and authorities for monitoring and testing activities, sampling strategies, and reporting requirements. 7.3 h.A description of the provisions for disposal of wastewater and management of stormwater and the plan for obtaining all required permits. 8 i. A description of the provisions for the final disposition of the coal combustion residuals. If the coal combustion residuals are to be removed, the owner must identify (i) the location and permit number for the coal combustion residuals landfills, industrial landfills, or municipal solid waste landfills in which the coal combustion residuals will be disposed and (ii) in the case where the coal combustion residuals are planned for beneficial use, the location and manner in which the residuals will be temporarily stored. If the coal combustion residuals are to be left in the impoundment, the owner must (i) in the case of closure pursuant to sub-subdivision (a)(1)a. of this section, provide a description of how the ash will be stabilized prior to completion of closure in accordance with closure and post-closure requirements established by Section .1627 of Subchapter B of Chapter 13 of Title 15A of the North Carolina Administrative Code and (ii) in the case of closure pursuant to sub-subdivision (a)(1)b. of this section, provide a description of how the ash will be stabilized pre- and post-closure. If the coal combustion residuals are to be left in the impoundment, the owner must provide an estimate of the volume of coal combustion residuals remaining. 9 j.A list of all permits that will need to be acquired or modified to complete closure activities.10 k. A description of the plan for post-closure monitoring and care for an impoundment for a minimum of 30 years. The length of the post-closure care period may be (i) proposed to be decreased or the frequency and parameter list modified if the owner demonstrates that the reduced period or modifications are sufficient to protect public health, safety, and welfare; the environment; and natural resources and (ii) increased by the Department at the end of the post-closure monitoring and care period if there are statistically significant increasing groundwater quality trends or if contaminant concentrations have not decreased to a level protective of public health, safety, and welfare; the environment; and natural resources. If the owner determines that the post-closure care monitoring and care period is no longer needed and the Department agrees, the owner shall provide a certification, signed and sealed by a professional engineer, verifying that post-closure monitoring and care has been completed in accordance with the post-closure plan. If required by Chapter 89C of the General Statutes, the proposed plan for post-closure monitoring and care should be signed and sealed by a professional engineer. The plan shall include, at a minimum, all of the following: 11 1 A demonstration of the long-term control of all leachate, affected groundwater, and stormwater.11 2 A description of a groundwater monitoring program that includes (i) post-closure groundwater monitoring, including parameters to be sampled and sampling schedules; (ii) any additional monitoring well installations, including a map with the proposed locations and well construction details; and (iii) the actions proposed to mitigate statistically significant increasing groundwater quality trends. 11.1 l.An estimate of the milestone dates for all activities related to closure and post-closure. 12.1 m.Projected costs of assessment, corrective action, closure, and post-closure care for each coal combustion residuals surface impoundment. 12.2 n. A description of the anticipated future use of the site and the necessity for the implementation of institutional controls following closure, including property use restrictions, and requirements for recordation of notices documenting the presence of contamination, if applicable, or historical site use.6.2 The results of groundwater modeling of the site that shall include, at a minimum, all of the following: § 130A-309.214(b)(3) No later than 60 days after receipt of a proposed Closure Plan, the Department shall conduct a public meeting in the county or counties proposed Closure Plan and alternatives to the public. § 130A-309.214(d) Within 30 days of its approval of a Coal Combustion Residuals Surface Impoundment Closure Plan, the Department shall submit the Closure Plan to the Coal Ash Management Commission. 2 OF 2 Amec Foster Wheeler Environment & Infrastructure, Inc. December 2016 Duke Energy Coal Combustion Residuals Management Program Riverbend Steam Station Site Analysis and Removal Plan Revision 0 FIGURES Ramp R a m p R a m p R a m p R o c k y UV273 UV27 UV1924 UV1923 UV1922 UV2074 UV2136 UV16 UV73 UV2128 UV16 PREPAREDBY JMS DATE 10/15/15 CHECKEDBY GM DATE 11/13/15 JOB NUMBER FIGURE 17810-15-0384 SITE VICINITY MAPDUKE ENERGY RIVERBEND STEAM STATIONGASTON COUNTY, NORTH CAROLINA ¯ ^_ NORTH CAROLINA Project Location SecondaryPond PrimaryPond AshStack CinderPit 0 5,000 10,000Feet Service Layer Credits: Sources: Esri, HERE, DeLorme, Intermpa, increment P Corp., GEBCO, USGS, FAO, NPS, NRCAN, GeoBase, IGN, Kadaster NL, Ordnance Survey, Esri Japan, METI, Esri China (Hong Kong), swisstopo, MapmyIndia, OpenStreetMap contributors, and the GIS User community Copyright: 2013 National Geogrpahic Society, i-cubed SecondaryPond PrimaryPond AshStack CinderPit Source: Esri, DigitalGlobe, GeoEye, Earthstar Geographics, CNES/AirbusDS, USDA, USGS, AEX, Getmapping, Aerogrid, IGN, IGP, swisstopo, andthe GIS User CommunitySITE AERIAL MAP - CCR UNITSDUKE ENERGY CAROLINAS, LLCRIVERBEND STEAM STATIONGASTON COUNTY, NORTH CAROLINA F :\A M E C _P r o j e c t s \2 0 1 5 \7 8 1 0 -1 5 -0 3 8 4 R i v e r b e n d \F i g u r e s \F i g u r e 2 _R i v e r b e n d .m x d , U s e r : m a d d i s o n .s u t t o n ; D a t e : 1 1 /1 1 /2 0 1 5 1 1 :4 3 :3 3 A M , C h e c k e d b y : G M D a t e : 1 1 /1 3 /2 0 1 5 PROJECT NO:7810-15-0384 FIGURE NO:2Note: This figure is for reference only. ¯0 400 800 Feet LEGEND PROPERTY BOUNDARY CCR UNIT BOUNDARIES MOUNTAIN ISLAND LAKE Source: Esri, DigitalGlobe, GeoEye, Earthstar Geographics, CNES/AirbusDS, USDA, USGS, AEX, Getmapping, Aerogrid, IGN, IGP, swisstopo, andthe GIS User Community SITE AERIAL MAP - COMPLIANCE BOUNDARYDUKE ENERGY CAROLINAS, LLCRIVERBEND STEAM STATIONGASTON COUNTY, NORTH CAROLINA F :\A M E C _P r o j e c t s \2 0 1 5 \7 8 1 0 -1 5 -0 3 8 4 R i v e r b e n d \G I S \M X D s \F i g u r e 3 _R i v e r b e n d .m x d , U s e r : m a d d i s o n .s u t t o n ; D a t e : 1 0 /1 5 /2 0 1 5 4 :0 9 :5 2 P M , C h e c k e b y : S S D a t e : 1 0 /1 5 /2 0 1 5 PROJECT NO:7810-15-0384 FIGURE NO:3Note: This figure is for reference only. ¯0 500 1,000 Feet LEGEND LIMIT OF WASTE BOU NDARY COMPLIANCE BOUNDARY MOUN TAIN ISLAND LAKE