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Home My WebLink About3612_Allen_ModifiedClosurePlan_Rev2_Narrative_FID1897203_20230731DUKE ENERGY ALLEN STEAM STATION COAL COMBUSTION RESIDUALS SURFACE IMPOUNDMENT CLOSURE PLAN (AMENDED) CLOSURE BY EXCAVATION Prepared for: ('DUKE ENERGY.. Duke Energy (Duke) 400 South Tryon Street Charlotte, North Carolina 28202 January 30, 2020 (Rev 0) July 31, 2020 (Rev 1) July 31, 2023 (Rev 2) Prepared by AL=com 1600 Perimeter Park Dr. Morrisville, North Carolina 27560 AECOM July 31, 2023 Duke Energy -Allen Steam Station CAMA Closure Plan (Closure by Excavation) - Retired Ash Basin and Active Ash Basin DESCRIPTION OF REVISIONS The following table provides a brief description of the revisions to this Closure Plan. This Closure Plan was originally submitted to the North Carolina Department of Environmental Quality (NCDEQ) in January 2020 and has been modified as shown in the following table: Revision Date of Document Description of Revisions Initial Issue January 30, 2020 Initial issuance of document. Rev 1 July 31, 2020 Revised document to incorporate additional proposed landfill. Rev 2 July 31, 2023 Revised document to incorporate additional proposed landfill (Canal Road Landfill). A=COM CAMA CLOSURE PLAN AECOM July 31, 2023 Duke Energy —Allen Steam Station CAMA Closure Plan (Closure by Excavation) — Retired Ash Basin and Active Ash Basin TABLE OF CONTENTS EXECUTIVE SUMMARY "' 1. INTRODUCTION...................................................................................................................6 1.1 Background..............................................................................................................................6 1.2 Closure Plan Objectives......................................................................................................... 6 1.3 Report Organization................................................................................................................7 2. GOVERNING LAWS.............................................................................................................7 3. FACILITY DESCRIPTION AND EXISTING SITE FEATURES...............................................8 3.1 Surface Impoundment Description........................................................................................ 8 3.1.1 Site History and Operations................................................................................... 8 3.1.2 Estimated Volume of CCR in Surface Impoundments...........................................9 3.1.3 Description of Surface Impoundment Structural Integrity....................................10 3.1.4 Sources of Discharges into Surface Impoundments ............................................ 11 3.1.5 Existing Surface Impoundment Liner Systems .................................................... 11 3.1.6 Inspection and Monitoring Summary................................................................... 11 3.2 Site Maps ................................................................................................................................ 12 3.2.1 Existing Surface Impoundment -Related Structures ............................................. 12 3.2.2 Receptor Survey.................................................................................................. 12 3.2.3 On -Site Landfills................................................................................................... 12 3.3 Monitoring and Sampling Location Plan.............................................................................13 4. RESULTS OF HYDROGEOLOGIC, GEOLOGIC, AND GEOTECHNICAL INVESTIGATIONS 13 4.1 Background............................................................................................................................13 4.2 Hydrogeology and Geologic Descriptions..........................................................................13 4.3 Stratigraphy of the Geologic Units Underlying Surface Impoundments .........................14 4.4 Geotechnical Properties.......................................................................................................14 4.4.1 CCR Within the RAB and AAB.......................................................................... 14 4.4.2 Liner Material Properties................................................................................... 14 4.4.3 Subsurface Soil Properties............................................................................... 15 4.4.4 AAB and RAB Dam Soil Properties.................................................................. 16 4.5 Chemical Analysis of Impoundment Water, CCR, and CCR-Affected Soil......................16 /Ocom CAMA CLOSURE PLAN iii AECOM July 31, 2023 Duke Energy -Allen Steam Station CAMA Closure Plan (Closure by Excavation) - Retired Ash Basin and Active Ash Basin 4.6 Historical Groundwater Sampling Results.........................................................................16 4.7 Groundwater Potentiometric Contour Maps.......................................................................16 4.8 Estimated Vertical and Horizontal Extent of CCR Within the Impoundments.................16 5. GROUNDWATER MODELING ANALYSIS.........................................................................17 5.1 Site Conceptual Model Predictions.....................................................................................17 5.2 Groundwater Chemistry Effects...........................................................................................17 5.3 Groundwater Trend Analysis Methods................................................................................18 6. BENEFICIAL AND FUTURE USE.......................................................................................18 6.1 CCR Use.................................................................................................................................18 6.2 Site Future Use......................................................................................................................18 7. CLOSURE DESIGN DOCUMENTS....................................................................................18 7.1 Engineering Evaluations and Analyses..............................................................................18 7.2 Closure Plan Activities..........................................................................................................19 7.3 Design Drawings................................................................................................................... 20 7.4 Description of the Construction Quality Assurance Plan ................................................. 21 8. MANAGEMENT OF WASTEWATER AND STORMWATER................................................21 8.1 Anticipated Changes in Wastewater and Stormwater Management ................................ 22 8.2 Wastewater and Stormwater Permitting Requirements.................................................... 23 9. DESCRIPTION OF FINAL DISPOSITION OF CCR............................................................23 10. APPLICABLE PERMITS FOR CLOSURE..........................................................................23 11. DESCRIPTION OF POST -CLOSURE MONITORING AND CARE......................................23 11.1 Groundwater Monitoring Program................................................................................ 24 12. PROJECT MILESTONES AND COST ESTIMATES...........................................................24 12.1 Project Schedule............................................................................................................. 24 12.2 Closure and Post -Closure Cost Estimate..................................................................... 24 13. REFERENCED DOCUMENTS............................................................................................26 A=COM CAMA CLOSURE PLAN iv AECOM July 31, 2023 Duke Energy -Allen Steam Station CAMA Closure Plan (Closure by Excavation) - Retired Ash Basin and Active Ash Basin Figures Figure ES-1 Current Condition View Figure ES-2 Post -Closure Condition View Figure 1-1 Vicinity Map and Site Plan Figure 3-1 Existing Conditions Plan /Ocom CAMA CLOSURE PLAN AECOM July 31, 2023 Duke Energy —Allen Steam Station CAMA Closure Plan (Closure by Excavation) — Retired Ash Basin and Active Ash Basin Tables Table 2-1 NC CAMA Closure Plan Requirements, Summary and Cross Reference Table Table 4-1 Summary of Typical Geotechnical Index Properties Table 10-1 Allen Steam Station Regulatory Permits, Approvals, or Requirements for Basin Closure by Excavation /Ocom CAMA CLOSURE PLAN ii AECOM July 31, 2023 Duke Energy —Allen Steam Station CAMA Closure Plan (Closure by Excavation) — Retired Ash Basin and Active Ash Basin Appendices Appendix A Estimated Volume of CCR in Impoundment Appendix B Geotechnical Data and Properties Appendix B-1 Logs of Boring and Monitoring Wells Appendix B-2 Results of Seismic Cone Penetration Testing (SCPTu) Appendix B-3 Results of Laboratory Testing Appendix B-4 Additional CPT Data (AAB) Appendix C Engineering Evaluations and Analyses Appendix C1 Stormwater Management Appendix C2 Borrow Soil Quantities Appendix D Closure Plan Drawings Appendix E Excavation Soil Sampling Plan A=COM CAMA CLOSURE PLAN ii AECOM July 31, 2023 Duke Energy -Allen Steam Station CAMA Closure Plan (Closure by Excavation) - Retired Ash Basin and Active Ash Basin EXECUTIVE SUMMARY As required by the North Carolina Department of Environmental Quality's (NCDEQ) April 1, 2019 "Coal Combustion Residuals Surface Impoundment Closure Determination," (Closure Determination) and Paragraph 12 of that certain Settlement Agreement by and between Duke Energy, NCDEQ, and various community groups dated December 31, 2019 (Settlement Agreement), Duke Energy has prepared this Closure Plan to describe the closure of the Retired Ash Basin (RAB) and Active Ash Basin (AAB) at the Allen Steam Station (Allen Station). This plan details closure -by -excavation of the RAB and AAB (collectively, Basins), and placement of the excavated coal combustion residuals (CCR) in a new permitted on -site lined landfill within the existing footprint of the AAB, two additional, new permitted on -site lined landfills within the partial existing footprints of the RAB and AAB, and one additional, new permitted on -site landfill north of the plant in the current Lined Retention Basin (LRB) area. The excavation of CCR and the closure of the RAB and AAB will be in accordance with applicable provisions of the North Carolina Coal Ash Management Act of 2014, as amended (CAMA), (codified at N.C.G.S. § 130A-309.200 et seq.), and the federal Disposal of Coal Combustion Residuals from Electric Utilities rule (CCR Rule) (codified at 40 C.F.R. § 257.50 et seq.). The Allen Station is owned and operated by Duke Energy Carolinas, LLC (Duke Energy) and is located in Gaston County, North Carolina along the west shore of Lake Wylie (Catawba River), near the town of Belmont, North Carolina. Allen Station began operations in 1957 as a coal-fired electric generating station and is currently in active operation. The RAB was operational from 1957 until 1973 when it reached capacity and was retired. The RAB is estimated to contain approximately 8.88 million tons of CCR (or an estimated 7.59 million cubic yards) that are subject to this Closure Plan. The RAB includes two embankment structures functioning as dams — RAB-North Dike and East Dike (regulated by NCDEQ as Gasto-016). The AAB was commissioned in 1973 to divert wet sluiced CCR from the RAB after it was retired and operated as a wet sluiced pond until flows were discontinued in February 2019. Based on topographic and bathymetric surveys performed in July 2014 and February 2015, the AAB is estimated to contain approximately 10.48 million tons of CCR (an estimated 8.73 million cubic yards). The AAB includes two embankment structures functioning as dams — AAB-East Dike and AAB-North Dike (regulated by NCDEQ as Gasto-061). Based on CCR inventory data provided by Duke Energy as of June 30, 2023, the RAB and AAB are estimated to contain a total of approximately 19.42 million tons of CCR (an estimated 16.39 million cubic yards). This approximate total includes all historically sluiced CCR as well as the CCR contained within the RAB Landfill footprint (both above and below the RAB Landfill's bottom liner system). This also includes the unlined Distribution of Residual Solids (DORS) facilities located to the west that were constructed over the RAB. Under this plan, all CCR except for a small amount necessary for transmission tower stability will be removed and placed into four new lined landfills within plant property, three of which are located at least partially within the prior footprint of the Basins. Closure activities for the Basins have already begun with the completion of decanting under the Special Order by Consent (SOC), continued dewatering and removal of interstitial water to A=CQM CAMA CLOSURE PLAN iii AECOM July 31, 2023 Duke Energy -Allen Steam Station CAMA Closure Plan (Closure by Excavation) - Retired Ash Basin and Active Ash Basin support CCR excavation, construction of the North Starter Landfill (NSLF), and construction of the South Starter Landfill (SSLF). Upon approval of this revised closure plan by NCDEQ, additional actions will commence, including finalization of detailed designs, additional dewatering and removal of interstitial water, development of the Ash Basin Landfill (ABLF) and Canal Road Landfill (CLF) in conjunction with excavation of the CCR, final grading of the site and landfills, and development of stormwater features and vegetative covers. Figures ES-1 and ES-2 illustrate the current state, and post -closure state of the Basins and Allen Station plant area as detailed by this Closure Plan. Figure ES-1 Current View Figure ES-2 Post Closure View AAB and RAB CCR will be removed and placed into four new lined landfills located within plant property — the Ash Basin Landfill (ABLF), the North Starter Landfill (NSLF), the South Starter Landfill (SSLF), and Canal Road Landfill (CLF). The ABLF will be located within the prior footprint of the AAB. The ABLF would rise approximately 110 feet above Southpoint Road, which has an approximate elevation of 690 feet. The smaller NSLF will be located partially within the prior footprint of the RAB, rising approximately 50 feet above Southpoint Road. The smaller SSLF will be located partially within the prior footprint of the AAB, rising approximately 40 feet above Southpoint Road. The CLF will be located north of the AAB, RAB, and the plant, rising approximately 85 feet above Southpoint Road and 125 feet above Boat Club Road. For reference, the existing RAB Landfill (Phase 1) is currently at an elevation of 680 feet and is permitted to a maximum elevation of 816 feet at the final buildout of Phase 2, or approximately 126 feet above Southpoint Road. Post -excavation, the AAB and RAB sites will restore some of the land's valley shape before the Basins were created. Soil will be graded to restore contours for stormwater flows, then either planted with native grasses or armored with riprap for erosion control. Portions of the existing basin dams will be removed, pursuant to a NCDEQ Dam Safety permit approval, allowing stormwater flows to Lake Wylie. Excavation and placement of the ash in the originally proposed three landfills was found to be difficult to sequence and operate. By evaluating likely excavation sequencing, it was determined that constructing the NSLF, SSLF, and CLF to support AAB CCR excavation and constructing the ABLF to support RAB CCR excavation would better facilitate the closure process and ease staging landfill cell construction. This document also includes a description of the Post -Closure Plan, which provides a description of the inspection, monitoring, and maintenance activities required to be performed throughout the 30-year post -closure care period for the closed Basins at the Allen Station. A=CQM CAMA CLOSURE PLAN iv AECOM July 31, 2023 Duke Energy -Allen Steam Station CAMA Closure Plan (Closure by Excavation) - Retired Ash Basin and Active Ash Basin This document provides a summary of properties of the site, as well as geotechnical properties of CCR and natural soils to support engineering analyses of the closure design. These analyses indicate that closure by excavation, as detailed in the Closure Plan, meets regulatory requirements for the stability of the site, management of stormwater run-off, and access for effective maintenance over the post -closure care period. In accordance with the requirements of N.C.G.S. § 130A-309.211(b)(1), Duke Energy separately submitted to NCDEQ an updated groundwater Corrective Action Plan (CAP) on December 31, 2019; the updated CAP is herein incorporated in its entirety by this reference. Neither the updated CAP nor its content is the work product ofAECOM. Although the Closure Plan contains references to the updated CAP, all specific relevant details to groundwater and related actions are found in the updated CAP itself and not in this Closure Plan. The updated CAP evaluates the extent of, and remedies for, constituents of interest (COls) in groundwater associated with the ash basins and coal pile area, focusing on constituent concentrations detected above the applicable 02L Standards, Interim Maximum Allowable Concentrations, or background threshold values at or beyond the Geographic Limitation (as defined in the Settlement Agreement) to north and north-northeast of the Retired Ash Basin and coal pile, and east of the Active Ash Basin. The CAP Update was conditionally approved by the NCDEQ on July 8, 2021. On February 10, 2020, the NCDEQ approved Duke Energy's request to expedite the corrective action process by implementing a CAP pilot test at the Allen Station. The pilot system was designed to address groundwater proximate to the northern portion of the Retired Ash Basin. The system was activated in September 2021 and has been operated since that time. The second phase of CAP system installation is currently in progress and addresses groundwater conditions downgradient of the Retired Ash Basin and Active Ash Basin. The installation of the phase two CAP system is currently on schedule to be completed in the fourth quarter of 2023. In addition, the updated CAP considers the federal groundwater corrective action requirements at 40 C.F.R. §§ 257.96. As detailed in the updated CAP, Duke Energy has begun to implement, and will continue implementing, source control measures at the site, including (i) complete decanting of the Basins to remove the hydraulic head, thereby reducing hydraulic gradients, groundwater seepage velocities, and COI transport potential; and (ii) complete closure of the ash basins. In addition, Duke Energy intends to implement a robust groundwater remediation program that includes actively addressing COI in groundwater above applicable standards at or beyond the [Geographic Limitation] using groundwater extraction combined with clean water infiltration and removal of the low pH area source proximate to the coal pile area. The CAP provides that these corrective action measures will most effectively achieve remediation of the groundwater through the use of extraction wells to the north, northeast, and east of the ash basins and coal piles, and strategically located clean water infiltration wells. The CAP further provides that groundwater modeling simulations indicate (i) these measures will control COI at or beyond the [Geographic Limitation]; and (ii) at such time the site -specific considerations detailed within the CAP have been satisfied, including, but not limited to, securing all required state approvals, installing the necessary equipment, and commencing full-scale system operation, COI at or beyond the [Geographic Limitation] will meet the remedial objectives in nine years (SynTerra, 2019). A=CQM CAMA CLOSURE PLAN v AECOM July 31, 2023 Duke Energy -Allen Steam Station CAMA Closure Plan (Closure by Excavation) - Retired Ash Basin and Active Ash Basin 1. INTRODUCTION 1.1 Background Allen Station is located at 253 Plant Allen Road in Gaston County, North Carolina. Allen Station is a five -unit, 1,140-megawatt, coal-fired power generation facility that began commercial operation in 1957 with Units 1 and 2. Unit 3 began operation in 1959, Unit 4 in 1960, and Unit 5 in 1961. Allen Station historically wet sluiced CCR into two surface impoundments located on the property known as the RAB and the AAB. The RAB received CCR from initial operation for approximately 16 years beginning in 1957 until 1973, when it reached capacity and was retired. Allen Station then commissioned the AAB and began wet sluicing CCR into this new basin. In 2009, when Allen Station replaced its wet fly ash sluicing operation with a flue gas desulfurization (FGD) facility, the RAB Landfill was constructed over the southeastern portion of the RAB for placement of dry fly ash (Permit No. 3612) (NCDEQ, 2009; S&ME, 2014). Although Allen Station previously wet sluiced bottom ash into the AAB, a dry bottom ash system is now operational and sluicing of bottom ash and all process water to the AAB ceased in February 2019 and has been redirected to the Lined Retention Basin (LRB) system. The Allen Station is scheduled to be retired in 2024. The LRB is one of the station assets to be demolished, and its demolition will be sequenced with the CLF construction. The only remaining wastewater flows will be landfill leachate and CAP flows, which will be redirected to and treated at the water management system (WMS) near the southern end of the AAB East Dike (GASTO-061) and discharged to NPDES wastewater Outfall 002. Figure 1-1 presents a Vicinity Map and Site Plan of Allen Station. Duke Energy uses two facilities to manage CCR at the Allen Station that include dams and dikes regulated by NCDEQ: 1. The AAB-North Dike and East Dike (NCDEQ ID: GASTO-061); and 2. The RAB-North Dike and East Dike (NCDEQ ID: GASTO-016). As further discussed in Section 2 below, the closure method mandated by order of NCDEQ for the AAB and RAB is closure by excavation. 1.2 Closure Plan Objectives The objective of this Closure Plan is to address the closure by excavation of CCR from the RAB and AAB as directed by order of NCDEQ. Duke Energy notes that approval from NCDEQ is required to proceed and develop the additional details as described further within this Closure Plan to complete the necessary working documents to complete the closure actions. Duke Energy submits this Closure Plan with the knowledge that other details will follow, as necessary. This Closure Plan describes and communicates the key actions and activities necessary to close the Basins in accordance with the requirements for written Closure Plans for CCR surface impoundments presented in N.C.G.S. §130A-309.214(a)(4). Planned closure activities include: • Continued operation of the WMS, originally constructed to facilitate decanting of the AAB, to manage all discharges in compliance with the NPDES permit during closure; AECOM July 31, 2023 Duke Energy -Allen Steam Station CAMA Closure Plan (Closure by Excavation) - Retired Ash Basin and Active Ash Basin • Dewatering to support safe excavation of CCR from the Basins; • Construction of four CCR landfills within plant property to permanently store the excavated CCR; • Excavation of the CCR and establishing post -excavation final grades using soils where required; • Breaching of the Basin dams; and • Restoration of disturbed areas. 1.3 Report Organization This document is structured to follow the requirements provided in N.C.G.S. §130A-309.214(a)(4). 2. GOVERNING LAWS In August 2014, the North Carolina General Assembly enacted CAMA, which contains specific statutory requirements applicable to the Basins. Subsequently, in July 2016, the North Carolina General Assembly enacted H.B. 630, Session Law 2016-95, which provides that impoundments be classified as "low -risk" if, by certain deadlines, the owner has established permanent alternative water supplies, as required, and has rectified any deficiencies identified by, and has otherwise complied with requirements of, any dam safety order. NCDEQ determined that Duke Energy met these criteria on November 13, 2018, and officially classified the RAB and AAB at Allen Station as "low -risk." On April 1, 2019, NCDEQ issued its Closure Determination mandating that the RAB and AAB be closed by excavation pursuant to N.C.G.S. § 130A-309.214(a)(3)a. On April 26, 2019, Duke Energy filed a Petition for Contested Case Hearing before the North Carolina Office of Administrative Hearings appealing this determination. On December 31, 2019, Duke Energy, NCDEQ, and certain community groups entered into a Settlement Agreement, which culminated in a consent order entered in North Carolina superior court on February 5, 2020. The consent order obligates Duke Energy to excavate all coal ash from the RAB and AAB and dispose of it in lined on -site landfills. A closure plan is required for each CCR surface impoundment regardless of the risk classification. CAMA's closure plan requirements and cross-referenced sections of this Closure Plan are summarized in Table 2-1. In addition to the closure plan requirements, CAMA sets out groundwater assessment and corrective action requirements. A Comprehensive Site Assessment report was submitted to NCDEQ in August 2015 with supplemental reports submitted August 2016 and January 2018. Duke Energy separately submitted to NCDEQ an updated Corrective Action Plan (CAP) on December 31, 2019. The CAP was conditionally approved by the NCDEQ in July 2021. In addition to the above requirements, National Pollutant Discharge Elimination System (NPDES) permit program compliance, SOC (which committed Duke Energy to initiate and complete decanting of the Basins by dates certain) compliance, dam safety approvals for modifications to regulated CCR basin dams, and environmental permitting requirements must be considered as part of closure. 7 AECOM July 31, 2023 Duke Energy —Allen Steam Station CAMA Closure Plan (Closure by Excavation) — Retired Ash Basin and Active Ash Basin 3. FACILITY DESCRIPTION AND EXISTING SITE FEATURES 3.1 Surface Impoundment Description This section provides details on the CCR-related features at Allen Station. 3.1.1 Site History and Operations Allen Station is located near the town of Belmont in Gaston County, North Carolina and is situated along the west shore of Lake Wylie, a man-made reservoir created by the impoundment of the Catawba River. A layout plan of the site is shown in Figure 1-1. The facility, which is owned and operated by Duke Energy, is approximately 1,009 acres. CCR facilities located on -site include the RAB, AAB and additional facilities as described in more detail below. Retired Ash Basin The RAB is located south of the Allen Station plant and adjacent to Lake Wylie/the Catawba River. It is situated along Plant Allen Road to the north, Lake Wylie/the Catawba River to the east, and the Duke Energy property boundary to the west. The natural topography at the site generally slopes downward from the western property line to the RAB and subsequently toward Lake Wylie/the Catawba River. The RAB includes three dams — RAB-North Dike, RAB-East Dike (collectively GASTO-016), and RAB-South Dike (now referred to as the AAB-North Dike) — that were constructed to form a U-shaped basin. The RAB is comprised of dikes that impound CCR and several either active or inactive CCR fill areas, which are described later in this section. The RAB CCR boundary currently occupies an area of approximately 123 acres. The RAB received sluiced CCR from initial operation in 1957 until 1973 when it reached capacity and was retired. The facility no longer retains sluiced process water or free water. Non -contact stormwater from the RAB and the RAB Landfill discharges through the primary spillway via a headwall structure (Outfall SW015) located in the ditch in the southeast corner of the basin. The primary spillway was constructed in 2016 as a replacement to the prior reinforced concrete riser structure and associated reinforced concrete piping (RCP). The discharge flow is currently conveyed through a 54-inch and 42-inch diameter high -density polyethylene HDPE pipe system. The primary spillway ties into the existing 36-inch diameter RCP near the downstream toe -of -slope via a reinforced concrete manhole structure (NPDES Outfall SW015). In addition to the primary spillway, an emergency spillway was installed for the RAB in 2015 (NPDES Outfall 008). Active Ash Basin The AAB is located south of the Allen Station plant and adjacent to Lake Wylie/the Catawba River. The AAB is located immediately south of the RAB and the two Basins share a dike (the AAB-North Dike). The natural topography at the site generally slopes downward from the property line in the west to the AAB and subsequently toward Lake Wylie/the Catawba River. The AAB includes two dams —AAB-East Dike and AAB-North Dike (collectively GASTO-061) — that were constructed to form an L-shaped basin. The AAB is comprised of deposited CCR and the AAB CCR boundary currently occupies an area of approximately 170 acres. The AAB formerly received flows from the E:3 AECOM July 31, 2023 Duke Energy -Allen Steam Station CAMA Closure Plan (Closure by Excavation) - Retired Ash Basin and Active Ash Basin ash removal system, coal pile runoff, landfill leachate, FGD wastewater, the station yard drain sump, and site stormwater. Bottom ash sluicing and all other process flows to the AAB ceased in February 2019. Until decanting began in 2019, storm and process water from the AAB discharged through a weir box outlet structure (Outfall 002) located in the southeast corner of the basin, which is permitted by NCDEQ under NPDES Permit NC0004979 (NCDEQ, 2021). The weir box and spillway system were constructed in 2016 as a replacement to the prior reinforced concrete riser structure and associated RCP. The discharge flow is currently conveyed through a 48-inch HDPE pipe system which ties into the pre-existing 42-inch diameter RCP near the downstream toe -of -slope via a reinforced concrete manhole structure. Additional Facilities Other CCR facilities within the RAB and AAB areas are shown in the attached Figure 3-1 and are described in more detail below. The DORS areas (DORS 1 through DORS 4) are retired CCR stacks/fills located above the western portion of the RAB footprint. The DORS areas, the closures of which are addressed in this Closure Plan, will be closed by excavation in conjunction with the remainder of the RAB. CCR was removed from the AAB and placed in the DORS fills at various times over a 10-year period under permit W00003255. These dry CCR stacks/fills are unlined with a vegetated cover soil. The DORS area was partially excavated to support the ongoing NSLF construction. The 25-acre RAB Landfill is an active overfill landfill (NCDEQ Permit No. 3612) wholly contained within the limits of the RAB and is constructed over the southeastern portion of the RAB. Its permit also includes a strip of the RAB immediately south of the landfill for management of non -contact stormwater. (NCDEQ, 2009, S&ME, 2008). The landfill is permitted to receive residual waste, including fly ash, bottom ash, FGD residual, boiler slag, mill rejects, and other non-CCR wastes generated at Allen Station. 3.1.2 Estimated Volume of CCR in Surface Impoundments Based on CCR inventory data provided by Duke Energy as of June 30, 2023 and upon a surface comparison calculation, performed within AutoCAD Civil 3D, comparing the approximate pre - development topography to the existing topographic and bathymetric survey, the approximate volume of CCR in the basins is listed in the table below. To compute the estimated mass of CCR in place an assumed density of 1.2 tons per CY was used, which is the Duke Energy fleet wide assumption, with the exception of CCR contained within the RAB Landfill above the liner, in which case an assumed density of 1.0 tons/cy was used. See Appendix A for the Estimated Volume of CCR in Impoundment calculation. �9 AECOM July 31, 2023 Duke Energy -Allen Steam Station CAMA Closure Plan (Closure by Excavation) - Retired Ash Basin and Active Ash Basin Impoundment Estimated CCR Weight Estimated CCR Volume (Tons) (CY) AAB 11,022,200 9,185,200 RAB (excludes DORS) 7,404,400 6,376,200 DORS No. 1 562,800 469,000 DORS No. 2 428,400 357,000 Total CCR 191417,800 16,387,400 Subject to Closure Plan 3.1.3 Description of Surface Impoundment Structural Integrity The purpose of this section is to summarize the Basins' structural integrity evaluations based on current existing information. This section includes the geotechnical, and hydrology and hydraulics capacity analyses results. In summary, the structural integrity of the Basins and subsequent dam inspection reports meets the regulatory requirements of EPA's CCR Rule (40 § CFR 257.73). Duke Energy's certifications of these requirements are available on Duke Energy's publicly accessible CCR Rule Compliance Data and Information website. • Slope stability For the AAB and RAB embankments, slope stability analysis results for the existing conditions global factors of safety for static long-term maximum storage pool, static maximum surcharge pool, sudden drawdown conditions, and pseudo -static seismic conditions meet regulatory and programmatic criteria. Slope stability results for the RAB at two selected sections and for the AAB at six selected sections under the loading conditions mentioned above meet regulatory requirements. • Liquefaction conditions (where susceptible) and Liquefaction potential Data from the PSHA and QUAD4M analyses indicate that the foundation and embankment soils for the RAB and AAB embankments meet liquefaction screening programmatic criteria for minimum factor of safety (FS >- 1.20). It is concluded that these soils are not subject to liquefaction during the 2,500-year return period seismic event. Liquefaction data for the fill material placed for the railroad tracks at the toe of the RAB east dike and the CCR in the AAB suggested that these materials have limited susceptibility to liquefaction. 10 AECOM July 31, 2023 Duke Energy -Allen Steam Station CAMA Closure Plan (Closure by Excavation) - Retired Ash Basin and Active Ash Basin • Hydrology and Hydraulics (H&H) Capacity Analyses The existing RAB outlet structure passes the required spillway design flood (SDF) (i.e., the flood caused by the full Probable Maximum Precipitation (PMP) based on NCDEQ and 1,000-year storm based on the federal CCR Rule) without the embankment overtopping. The existing AAB outlet structure can safely pass the required SDF (i.e., the flood caused by the full PMP based on NCDEQ and full PMP based on the federal CCR Rule) without the embankment overtopping. Also, a time -to -drain analysis indicates that the existing spillway systems for the RAB and AAB are capable of removing at least 80% of the water temporarily detained in the reservoir within 15 days following the design storm event, as required by the North Carolina Dam Safety Regulations. 3.1.4 Sources of Discharges into Surface Impoundments Historically, runoff and process water streams from two yard -drain sumps, the ash removal system, the RAB Landfill leachate collection system, low volume wastes, and stormwater runoff were discharged into the Basins. Process flows no longer discharge into the Basins. Process flows are directed toward the newly constructed holding basin and lined retention basin. Upon demolition of the lined retention basin, landfill leachate and CAP flows will be conveyed to the WMS while station process flows will no longer exist. The Allen Station currently employs a dry ash handling system. Since 2009, fly ash has been dry -handled and disposed of in the RAB Landfill, which is constructed on the eastern portion of the RAB (described in Section 3.1.1). The dry bottom ash system became operational in 2019. 3.1.5 Existing Surface Impoundment Liner Systems The RAB and AAB located at the Allen Station are unlined surface impoundments and were constructed over natural existing ground. 3.1.6 Inspection and Monitoring Summary Weekly inspections of the AAB and RAB have been on -going since 2014, and include observation of upstream slopes and shorelines, crest, downstream slopes, toes, abutment contacts, adjacent drainage way(s), spillway(s), and associated structure(s), and other structures and features of the dams. Monthly inspections of the AAB and RAB 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 of the dry CCR stacks. Daily inspections of basins are not routinely required; however, on a case -by -case basis, the basins may be inspected daily beginning at such times and continued for the duration as specified by plant management. Such daily inspections might be initiated during a repair activity on the dam or in response to a specific imposed regulatory agency requirement. The Basins are inspected annually by an independent third -party consultant. In a letter dated August 13, 2014, NCDEQ requires these inspections to be conducted annually at all of Duke Energy's CCR impoundments in North Carolina. These inspections are intended to confirm 11 AECOM July 31, 2023 Duke Energy -Allen Steam Station CAMA Closure Plan (Closure by Excavation) - Retired Ash Basin and Active Ash Basin adequacy of the design, operation, and maintenance of the surface impoundments in accordance with accepted engineering standards. Reports are to be submitted to NCDEQ within 30 days of the completion of the inspection. The results of the annual inspections are used to identify needed repairs, repair schedules, to assess the safety and operational adequacy of the dam, and to assess compliance activities regarding applicable permits, environmental, and dam regulations. Annual inspections are also performed to evaluate previous repairs. The 2015 through 2023 annual inspections did not identify features or conditions in the Basin dams, or 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 underway. Special, episodic inspections of the Basins may be performed during episodes of earthquake, emergency, or other extraordinary events. Visual inspections are performed after a heavy precipitation event when accumulation of four inches of rainfall or greater occurs within a 24-hour period. An internal inspection will be performed if an earthquake is felt locally or detected by the US Geological Survey measuring greater than a Magnitude 3 and with an epicenter within 50 miles of the dams. 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 cleanout) are planned on or near the dams. Special inspections are also conducted when the ongoing surveillance program identifies a condition or a trend that warrants special evaluation. 3.2 Site Maps 3.2.1 Existing Surface Impoundment -Related Structures A site map showing property boundary, location of the Allen Station, Basins with their boundaries and topographic and bathymetric contours is shown on Figure 3-1. 3.2.2 Receptor Survey This information is included as part of the updated CAP that was prepared separately by SynTerra for Duke Energy. Duke Energy submitted the updated CAP to NCDEQ on December 31, 2019. The CAP was conditionally approved by the NCDEQ on July 8, 2021. The CAP is herein incorporated by this reference but its content is not the work product of AECOM. 3.2.3 On -Site Landfills There are five either existing or future on -site landfills at the Allen Station as identified in the table below, along with the status (active, under construction, in design, etc). The existing RAB Landfill is shown on Figures ES-1 and 3-1 and is described in more detail in Section 3.1.1. The remaining four landfills are shown conceptually on Figure ES-2 and in Appendix D and are described in more detail in Sections 7 and 9. 12 AECOM July 31, 2023 Duke Energy -Allen Steam Station CAMA Closure Plan (Closure by Excavation) - Retired Ash Basin and Active Ash Basin Landfill Permit Number Comment RAB Landfill NCDEQ Permit No. 3612 Active North Starter Landfill NCDEQ Permit No. 3619 Under Construction South Starter Landfill NCDEQ Permit No. 3620 Under Construction Ash Basin Landfill NCDEQ Permit No. 3621 In Design Canal Road Landfill NCDEQ Permit No. 3623 In Design 3.3 Monitoring and Sampling Location Plan This information is included as part of the updated CAP that was prepared separately by SynTerra for Duke Energy. Duke Energy submitted the updated CAP to NCDEQ on December 31, 2019. The CAP was conditionally approved by the NCDEQ on July 8, 2021. The site monitoring plan has been updated periodically based on site conditions and approved by the NCDEQ. The CAP is herein incorporated by this reference but its content is not the work product of AECOM. 4. RESULTS OF HYDROGEOLOGIC, GEOLOGIC, AND GEOTECHNICAL INVESTIGATIONS 4.1 Background An overall boring and existing monitoring well location plan indicating the locations of recent and historical borings, monitoring wells, piezometers, and Cone Penetration Test (CPT) locations is shown on Drawings ALN_C999.001.018 through ALN_C999.001.020 included in Appendix D. Logs of the borings and monitoring wells are included in Appendix B-1, results of Seismic Cone Penetration Testing (SCPTu) are included in Appendix B-2, a summary of laboratory tests data performed in support of the closure design is presented in Appendix B-3, and additional recent CPT data obtained to support CCR excavation in the AAB are included in Appendix B-4. This section summarizes the site geology and hydrogeology, site stratigraphy of the geologic units underlying the surface impoundments, geotechnical properties of the CCR, and the uppermost stratigraphic unit under the surface impoundment. 4.2 Hydrogeology and Geologic Descriptions This information is included as part of the updated CAP that was prepared separately by SynTerra for Duke Energy. Duke Energy submitted the updated CAP to NCDEQ on December 31, 2019. The CAP was conditionally approved by the NCDEQ on July 8, 2021. The CAP is herein incorporated by this reference but its content is not the work product of AECOM. 13 AECOM July 31, 2023 Duke Energy -Allen Steam Station CAMA Closure Plan (Closure by Excavation) - Retired Ash Basin and Active Ash Basin 4.3 Stratigraphy of the Geologic Units Underlying Surface Impoundments This information is included as part of the updated CAP that was prepared separately by SynTerra for Duke Energy. Duke Energy submitted the updated CAP to NCDEQ on December 31, 2019. The CAP was conditionally approved by the NCDEQ on July 8, 2021. The CAP is herein incorporated by this reference but its content is not the work product of AECOM. 4.4 Geotechnical Properties This section provides a summary of geotechnical conditions and properties found from investigations performed within the RAB and AAB and dam areas. The presented information was obtained from previous geotechnical investigations at the site and recent investigation activities conducted to support the Closure Plan development. The geotechnical conditions within the Basins generally consist of CCR (interbedded layers of fly ash and bottom ash) placed in the basin primarily by hydraulic sluicing underlain by residual soil, saprolite, partially weathered rock (PWR), and bedrock. For purposes of discussion of the geotechnical properties of the materials, the saprolite material is described as residual material. General properties of the various materials encountered within and surrounding the Basin are described below. A range of measured material properties of laboratory tests performed by AECOM, SynTerra, and MACTEC for the subsurface explorations completed within the Basin is presented in Table 4.1. 4.4.1 CCR Within the RAB and AAB The CCR encountered within the AAB complex generally consisted of dry to wet, dark gray to gray fly ash as sandy non -plastic silt (ML), and dark blueish gray fly ash as silty sand (SM). In recent years, bottom ash has been sluiced to the AAB area in addition to fly ash. Consequently, samples of bottom ash were intermittently observed and collected during the AECOM investigation. Both stacked and sluiced CCR are present within RAB. In the DORS 1 and 2 areas, stacked CCR primarily consists of dry to wet, dark gray to gray bottom ash as silty sand (SM), and lesser quantities of fly ash as silt (ML), with varying quantities of sand, gravel, and slag. This stacked CCR is underlain by sluiced CCR primarily consisting of moist to wet, dark gray to gray bottom ash as silty sand (SM), and fly ash as silt (ML). Elsewhere in the RAB, stacked CCR within the DORS 3 and 4 areas was classified as dry, gray and dark gray, fly ash as sandy silt (ML), and silty sand (SM). Sluiced CCR within these areas is generally classified as dry to wet, gray and dark gray, fly ash as sandy silt (ML), and silty sand (SM). 4.4.2 Liner Material Properties The AAB and RAB are unlined so there are no associated material properties. 14 AECOM July 31, 2023 Duke Energy -Allen Steam Station CAMA Closure Plan (Closure by Excavation) - Retired Ash Basin and Active Ash Basin 4.4.3 Subsurface Soil Properties Alluvium: Alluvial soils were encountered in seven borings within the AAB. The alluvium generally consisted of wet, orange or yellow silty sand (SM), and brown or red sandy clay (CUCH), with quartz rock fragments. Residual Soils (Residuum and Saprolite): Residual soils within the AAB were encountered in multiple borings. This stratum generally consisted of moist to wet, reddish brown to light brown mottled, light gray and greenish gray silty sand (SM), sand with silt (SW-SM), poorly -graded sand (SP), and sandy non -plastic silt (MIL), with varying quantities of mica and gravel. Occasionally, soils in this stratum were classified as sandy lean clay (CL), clayey elastic silt (MIL), and sandy fat clay (CH). Within the RAB in DORS 1 and 2, residual soils primarily consisted of wet, reddish brown to light brown and light gray, silty sand (SM), sandy non -plastic silt (MIL), and clayey sand (SC), with varying quantities of quartz and mica. Occasionally, soils in this stratum were classified as lean clay (CL) and sandy fat clay (CH). Within DORS 3 and 4, the residual soils can be divided into two distinct strata: the upper 10 to 17 ft of the residual soil stratum generally consisted of moist, brownish yellow, reddish gray, and dark gray, native lean clay (CL) and fat clay (CH) soils, underlain by up to 37 ft of moist to wet, yellowish brown, orange, and olive gray coarse -grained sand with silt (SP-SM), well graded sand (SW), silty sand (SM), sand with clay (SP-SC), and non - plastic silt (ML). PWR: PWR was encountered within several borings in the AAB. In these locations, PWR primarily consists of wet, light gray to reddish brown to light brown mottled sand (SP), silty sand (SM), sandy non -plastic silt (MIL), sand with clay (SP-SC), lean clay (CL), and weathered meta -quartz diorite, meta-granodiorite, and meta-diabase bedrock. In the RAB within DORS 1 and 2, the PWR below the residual soils primarily consists of wet, light gray to light brown mottled coarse -grained sand (SM), greenish gray non -plastic silt (MIL), and meta -quartz diorite. Within DORS 3 and 4, PWR consisted of moist to wet, yellowish brown, grayish brown and olive brown, silty sand (SM), well graded sand with gravel (SW), clayey sand (SC), sandy clay with gravel (CL), sandy silt with gravel (MIL), sand with clay (SP-SC) and minor layers of fat clay (CH). Bedrock: Bedrock was cored in four of the soil borings conducted in the interior of the AAB. The bedrock primarily consisted of dark greenish gray, very strong, coarse grained, massive Meta - Quartz diorite, and black and white, strong, fine to coarse grained Meta-Diabase bedrock. In the RAB, bedrock was cored in two borings within DORS 1 and 2. The bedrock encountered in the borings primarily consisted of light gray to dark gray, strong, massive, Meta -Quartz diorite bedrock. Bedrock was also encountered within DORS 3 and 4 consisted of black and white to dark gray and white, very strong, massive, moderately to slightly fractured, fresh Meta -Quartz diorite bedrock. 15 AECOM July 31, 2023 Duke Energy -Allen Steam Station CAMA Closure Plan (Closure by Excavation) - Retired Ash Basin and Active Ash Basin 4.4.4 AAB and RAB Dam Soil Properties The RAB and AAB dam embankments consist primarily of loose to medium dense clayey to silty sand (SC and SM), sandy silt (ML) and stiff to very stiff elastic silt with sand (MH), and sandy lean clay to fat clay (CL and CH). The dam embankment fill was described as moist, red, and reddish brown to brown, with varying quantities of sand and gravel. 4.5 Chemical Analysis of Impoundment Water, CCR, and CCR-Affected Soil This information is included as part of the updated CAP that was prepared separately by SynTerra for Duke Energy. Duke Energy submitted the updated CAP to NCDEQ on December 31, 2019. The CAP was conditionally approved by the NCDEQ on July 8, 2021. The CAP is herein incorporated by this reference but its content is not the work product of AECOM. 4.6 Historical Groundwater Sampling Results This information is included as part of the updated CAP that was prepared separately by SynTerra for Duke Energy. Duke Energy submitted the updated CAP to NCDEQ on December 31, 2019. The CAP was conditionally approved by the NCDEQ on July 8, 2021. The CAP is herein incorporated by this reference but its content is not the work product of AECOM. Duke Energy has continued to provide additional groundwater sampling results received since the CAP preparation to NCDEQ via quarterly data submittals. The most current groundwater monitoring evaluation at the time of this closure plan update can be found in the CAMAAnnual Groundwater and Surface Water Monitoring Report (Annual GW/SW Report) submitted to NCDEQ on November 1, 2022. 4.7 Groundwater Potentiometric Contour Maps This information is included as part of the updated CAP that was prepared separately by SynTerra for Duke Energy. Duke Energy submitted the updated CAP to NCDEQ on December 31, 2019. The CAP was conditionally approved by the NCDEQ on July 8, 2021. The CAP is herein incorporated by this reference but its content is not the work product of AECOM. The most current groundwater potentiometric contour maps at the time of this closure plan update can be found in the Annual GW/SW Report submitted to NCDEQ on November 1, 2022. 4.8 Estimated Vertical and Horizontal Extent of CCR Within the Impoundments This information is included as part of the updated CAP that was prepared separately by SynTerra for Duke Energy. Duke Energy submitted the updated CAP to NCDEQ on December 31, 2019. The CAP was conditionally approved by the NCDEQ on July 8, 2021. The CAP is herein incorporated by this reference but its content is not the work product of AECOM. 16 AECOM July 31, 2023 Duke Energy -Allen Steam Station CAMA Closure Plan (Closure by Excavation) - Retired Ash Basin and Active Ash Basin 5. GROUNDWATER MODELING ANALYSIS In accordance with the requirements of N.C.G.S. § 130A-309.211(b)(1), Duke Energy separately submitted an updated Corrective Action Plan (CAP) on December 31, 2019. The CAP was conditionally approved by the NCDEQ on July 8, 2021. The updated CAP is herein incorporated in its entirety by this reference. Neither the updated CAP nor its content is the work product of AECOM. Although the Closure Plan contains references to the updated CAP, all specific relevant details to groundwater and related actions are found in the updated CAP itself and not in this Closure Plan. The updated CAP evaluates the extent of, and remedies for, constituents of interest (COls) in groundwater associated with the ash basins and coal pile area, focusing on constituent concentrations detected above the applicable standards at or beyond the Geographic Limitation (as defined in the Settlement Agreement) to north and north-northeast of the Retired Ash Basin and coal pile, and east of the Active Ash Basin. As detailed in the updated CAP, Duke Energy has begun to implement, and will continue implementing, source control measures at the site, including (i) complete decanting of the Basins to remove the hydraulic head, thereby reducing hydraulic gradients, groundwater seepage velocities, and COI transport potential; and (ii) complete closure of the Basins. In addition, Duke Energy intends to implement a robust groundwater remediation program that includes actively addressing COI in groundwater above applicable standards at or beyond the [Geographic Limitation] using groundwater extraction combined with clean water infiltration and removal of the low pH area source proximate to the coal pile area. The CAP provides that these corrective action measures will most effectively achieve remediation of the groundwater through the use of extraction wells to the north, northeast, and east of the Basins and coal piles, and strategically located clean water infiltration wells. The CAP further provides that groundwater modeling simulations indicate (i) these measures will control COI at or beyond the [Geographic Limitation]; and (ii) at such time the site -specific considerations detailed within the CAP have been satisfied, including, but not limited to, securing all required state approvals, installing the necessary equipment, and commencing full-scale system operation, COI at or beyond the [Geographic Limitation] will meet the remedial objectives in nine years. 5.1 Site Conceptual Model Predictions This information is included as part of the updated CAP that was prepared separately by SynTerra for Duke Energy. Duke Energy submitted the updated CAP to NCDEQ on December 31, 2019. The CAP was conditionally approved by the NCDEQ on July 8, 2021. The CAP is herein incorporated by this reference but its content is not the work product of AECOM. 5.2 Groundwater Chemistry Effects This information is included as part of the updated CAP that was prepared separately by SynTerra for Duke Energy. Duke Energy submitted the updated CAP to NCDEQ on December 31, 2019. The CAP was conditionally approved by the NCDEQ on July 8, 2021. The CAP is herein 17 AECOM July 31, 2023 Duke Energy -Allen Steam Station CAMA Closure Plan (Closure by Excavation) - Retired Ash Basin and Active Ash Basin incorporated by this reference but its content is not the work product ofAECOM. The most current groundwater quality evaluation at the time of this closure plan update can be found in the Annual GW/SW Report submitted to NCDEQ on November 1, 2022. 5.3 Groundwater Trend Analysis Methods This information is included as part of the updated CAP that was prepared separately by SynTerra for Duke Energy. Duke Energy submitted the updated CAP to NCDEQ on December 31, 2019. The CAP was conditionally approved by the NCDEQ on July 8, 2021. The CAP is herein incorporated by this reference but its content is not the work product ofAECOM. The most current groundwater trend analysis at the time of this closure plan update can be found in the Annual GW/SW Report submitted to NCDEQ on November 1, 2022. 6. BENEFICIAL AND FUTURE USE 6.1 CCR Use At this time, Duke Energy has not identified a beneficial use of CCR from the Basins at Allen Station. 6.2 Site Future Use At this time, Duke Energy has not identified any future use of the land reclaimed by the dewatering and excavation of the AAB and RAB, except that three of the four proposed new CCR landfills are proposed to be built at least partially within the footprints of the excavated RAB and AAB. 7. CLOSURE DESIGN DOCUMENTS 7.1 Engineering Evaluations and Analyses Engineering evaluations and analyses to support closure of the AAB and RAB at the Allen Station, as detailed in this Closure Plan, are provided in Appendix C. Geotechnical calculations for the proposed NSLF and SSLF landfill designs were previously submitted to NCDEQ in Permit to Construct (PTC) submittals. Geotechnical calculations for the proposed ABLF and CLF will be performed separately as part of their permit application(s), which will follow NCDEQ approval of this Closure Plan. It is noted that the conceptual layout of the landfills may be optimized during subsequent design analysis and that the permit submittals may vary in some respects from that proposed in this Plan. Dam removal -related calculations will be included in the dam modification permit applications, which will follow NCDEQ approval of this Closure Plan. Basin access and excavation activities are controlled by Duke Energy Procedure: CCP-PRC-NA- PMI-021, CCR Excavation, Planning and Permitting, for Work Activities and Access Control within Basins, Impoundments, Ponds, and Stacks. This procedure governs ash excavation and dewatering to ensure ash stability is evaluated and access is controlled to prevent personnel from EK AECOM July 31, 2023 Duke Energy -Allen Steam Station CAMA Closure Plan (Closure by Excavation) - Retired Ash Basin and Active Ash Basin entering or creating unsafe conditions. This procedure is Duke Proprietary and was submitted previously to NCDEQ under separate cover. For closure by excavation of the RAB and AAB, all CCR will be removed, with the exception of CCR integral with the foundations of overhead transmission line support towers currently located within or adjacent to the RAB limits. Ash will remain for structural stability around the supporting structures of these transmission towers and will be capped with geomembrane. The estimated volume of ash that will remain surrounding the overhead transmission line towers is approximately 20,000 tons (16,600 cubic yards). The ash to remain in place at the towers is and would continue to be unsaturated based on comparing historical bottom of ash elevations and groundwater elevations. Areas for stockpiling or conditioning of CCR are generally needed. These areas must be established within the limits of the CCR unit and require placement or stacking of CCR excavated from other areas of the basin. They can be established in areas where all or most of the CCR has been removed, or on areas where a significant depth of CCR remains in place. Sluiced CCR forming the foundation of stockpiles or conditioning areas may be subject to bearing capacity or slope failures from the additional vertical compressive stress imparted by the stacked CCR and hauling equipment. CCR stockpiling will comply with Duke Energy Procedure CCP-PRC-NA-PMI- 021, which complies with recommendations from a letter from NCDEQ dated January 15, 2021 and entitled "NCDEQ Recommendation Regarding Proposed Stacks and Piles of CCR Within Existing CCR Surface Impoundments in North Carolina". During excavation of CCR, interim or temporary excavated CCR slopes are commonly created. These slopes vary in height and the duration they will have to stand. Some slopes are subject to potential loading from hauling or stockpiling operations. The location and geometry of such slopes cannot be established during design. These elements depend on the means and methods employed by the construction contractor, site conditions, schedule and other site conditions. Excavation in a deep valley fill creates safety risks that need further evaluation and will require means and methods inputs from a contractor to fully address before closure excavation work commences. CCR excavation will comply with Duke Energy Procedure CCP-PRC-NA-PMI-021. 7.2 Closure Plan Activities The primary activities associated with closure by excavation are as follows: • Continued operation of the WMS, originally constructed to facilitate decanting of the AAB, to manage all discharges in compliance with the NPDES permit during closure. • Dewater the CCR to allow for access, CCR excavation and conditioning (drying) prior to placement in the proposed on -site landfills. • Start CCR excavation from the basins, with sequencing determined for optimal progression. Manage and control dust -generating activities through specific site planning and mitigation. Construct landfill cells in coordination with CCR excavation. Place the excavated CCR in the on -site landfills and compact. Instrumentation and monitoring requirements to be developed prior to construction will be followed to verify construction 19 AECOM July 31, 2023 Duke Energy -Allen Steam Station CAMA Closure Plan (Closure by Excavation) - Retired Ash Basin and Active Ash Basin phase stability. Construction dewatering to be used as needed to provide stable work areas and slopes. Maintain required hydraulic storage capacity though the excavation process and progressively breach the AAB and RAB dams as excavation advances. • Complete closure by excavation verification. Grade the area to promote positive drainage and seed for vegetative growth. • Sequence final dam breaches with construction of proposed stormwater detention basins and inflow design flood management. Additional information and details pertaining to the closure design are provided in the Closure Plan drawings, which can be found in Appendix D. 7.3 Design Drawings The Closure Plan drawings found in Appendix D include the following: • Cover sheet • General notes • Existing conditions plans with aerial photograph • Existing conditions plans with topography • Subsurface exploration location plans • Estimated bottom of CCR contour plans • Demolition plans • Final closure grading plans • Final surface water management plan • Final closure grading cross section layout plan • Final closure grading cross sections • Final closure grading details • On -site landfill schematic plan • On -site landfill schematic cross sections These Closure Plan drawings will be further developed and refined to develop construction -level drawings during subsequent stages following NCDEQ approval of the Closure Plan. In addition, supplemental drawing sets will be prepared on an as -needed basis to support dam modification and/or decommissioning permits, erosion and sediment control permits, NPDES permit modifications, and other related permits. Once the excavation grades shown on the Closure Plan drawings have been achieved, the procedures described in the Duke Energy Excavation Soil Sampling Plan (Appendix E) will be followed to confirm that closure by excavation has been achieved. 017 AECOM July 31, 2023 Duke Energy -Allen Steam Station CAMA Closure Plan (Closure by Excavation) - Retired Ash Basin and Active Ash Basin 7.4 Description of the Construction Quality Assurance Plan The CQA Plans developed for the ongoing construction of the North Starter and South Starter Landfill, as well as the Excavation Soil Sampling Plan provided in Appendix E comply with N.C.G.S. §130A-309.214(a)(4)(g). The plans provide a description of the CQA program to be adhered to in execution of closure activities. The plans present a description of the roles and responsibilities for monitoring and testing activities and provides guidance on the methodology to be used for evaluating whether the construction has been performed in accordance with the approved Closure Plan. The plans also detail the material properties and specifications; methods for transportation, handling, and storage of materials; test methods and verifications; manufacturer, field, and laboratory testing; field activities for construction monitoring and oversight; and reporting and documentation requirements. The plans address materials and CQA activities associated with the following components: • Earthwork o CCR Excavation o Structural Soil Fill • HDPE Piping • Vegetation • As -Built Conditions • Record Documentation Report 8. MANAGEMENT OF WASTEWATER AND STORMWATER The Allen Station manages wastewater and stormwater under two NPDES permits issued by NCDEQ. Permit number NC0004979, issued on July 9, 2021 (NCDEQ, 2021), permits the discharge of various process -related wastewaters in accordance with specified limits and monitoring requirements. Permit number NCS000546, issued April 12, 2022 (NCDEQ, 2022), provides monitoring and best management practice requirements for industrial stormwater discharges from the Allen Station. Discharges of stormwater and treated process -related wastewaters flow directly or indirectly to Lake Wylie/the Catawba River. Plant discharges to the AAB have stopped as of February 2019. The AAB will continue to operate during closure to meet the NPDES permit discharge requirements as it goes through the phases of decanting and dewatering. A water management system (WMS) to treat wastewater prior to discharge via NPDES wastewater Outfall 002 was constructed near the southern end of the AAB East Dike (GASTO-061) initially to facilitate decanting of the AAB and remains operational to facilitate ongoing Basin dewatering. The AAB must continue to safely pass the SDF to meet dam safety requirements. Decanting of the AAB has been performed. Because the free water has been drawn down, discharges from the basin via the passive weir box outlet gravity discharge system are not expected to occur. The pumping system used to decant the AAB remains in place and is expected to draw down the stored water after storm events, route it through the WMS, and discharge the 21 AECOM July 31, 2023 Duke Energy -Allen Steam Station CAMA Closure Plan (Closure by Excavation) - Retired Ash Basin and Active Ash Basin water via the permitted outfall. Dewatering began on June 2, 2020. Wastewater flows are being routed through the WMS to meet the permitted discharge limits. The AAB currently has the capacity to contain the PMP storm event if the pre -storm water surface elevation is maintained at or below El. 632.10 ft. The normal water surface elevation is maintained between El. 615 ft and El. 620 ft. As part of closure, a new, deeper stormwater outfall channel will be created to Lake Wylie/the Catawba River, but the final connection to the stormwater outlet channeling will be timed to avoid exposure to contact wastewater leaving the AAB work site. Dewatering is currently being performed to remove the interstitial or pore water from the CCR to facilitate excavation, to access in -place CCR and to establish safe slopes prior to and after CCR excavation. It is anticipated that performance criteria will be established in the construction -level documentation to identify required vertical and horizontal limits of interstitial water removal at critical locations and for critical conditions during closure. Wastewater from the ash basins will be pumped, treated as needed and discharged in two phases: the decanting phase and dewatering phase. In the completed decanting phase, free water above the settled CCR layer was removed from the basins without the mechanical disturbance of the CCR. The decanting phase was completed on June 2, 2020, and the Allen site is currently in the dewatering phase to remove interstitial water from the Basins. During this phase, interstitial water is sent to the WMS to maintain compliance with the requirements of the discharge permit. The WMS is planned to be retrofit to also treat Ieachate and CAP flows. The post -closure grades restore the historical flows from the surrounding landscape and route that flow toward the proposed detention basins. The detention basins will be designed with a culvert to attenuate stormwater discharges under a large rain event that will result in a slower discharge release to Lake Wylie/the Catawba River. Up to and including the last phase of closure before the AAB and RAB dams are breached, the Basins will maintain the capacity to contain the required storm size/flows. The detention basin design criteria will be further refined for the construction -level documents based on actual field elevations reached in the excavated areas and discussions with NCDEQ with regards to the embankment heights, which will follow NCDEQ approval of this Closure Plan. The designs for the detention basins are limited to conceptual level at this time. These designs for the detention basins are based conservatively on 100-year storms. Appendix C1 presents the results of the post -closure stormwater management calculations and the design of these detention basins. Detailed stormwater design for the proposed CCR on -site landfills will be developed as part of their permit designs and is not covered herein. 8.1 Anticipated Changes in Wastewater and Stormwater Management All CCR and wastewater flows historically sent to the AAB have been diverted to the holding basin and lined retention basin in the coal pile area and to the north of the station. Prior to station retirement and demolition of the lined retention basin, Ieachate and CAP flows will be diverted to the WMS for treatment and discharge. 22 AECOM July 31, 2023 Duke Energy -Allen Steam Station CAMA Closure Plan (Closure by Excavation) - Retired Ash Basin and Active Ash Basin The WMS will be utilized such that the NPDES Outfall 002 effluent discharge limits will be met throughout the duration of dewatering and closure. The WMS will remain in place to support long term leachate and CAP flows. Erosion and sediment control plans for different phases of the construction will be developed as part of the construction -level packages and formal erosion and sediment control plan permit submittal. The details for the erosion and sediment control measures depicted on the drawings in this Closure Plan submittal will be re-evaluated after the specific construction phasing is established, which will follow NCDEQ approval of this Closure Plan. In addition, erosion and sediment control measures may be installed and removed in phases as stabilization is achieved. 8.2 Wastewater and Stormwater Permitting Requirements Additional information on required permits is described in Section 10. 9. DESCRIPTION OF FINAL DISPOSITION OF CCR CCR will be dispositioned by placement into four proposed lined CCR landfills located on plant property. Duke Energy intends to construct four new on -site landfills to accommodate CCR dispositioned in completion of the NCDEQ-mandated closure by excavation. Permit applications for construction of the on -site landfills will be prepared and submitted to NCDEQ Division of Waste Management following approval of this Closure Plan. The permitting process began earlier with the Permit to Construct submittals for the NSLF and SSLF that NCDEQ previously approved. The permitting process would continue to the CLF then the ABLF, since ABLF construction will begin much later in the CCR excavation process. Vegetation encountered or removed during the progression of the work will be managed in accordance with state regulations for handling and disposal. 10. APPLICABLE PERMITS FOR CLOSURE Refer to Table 10-1 for detailed information on the potential and applicable permitting/approval needed to implement this Closure Plan. Development of permitting package submittals and/or regulatory approval requests would follow NCDEQ approval of the Closure Plan. 11. DESCRIPTION OF POST -CLOSURE MONITORING AND CARE A post -closure plan will be developed and submitted to NCDEQ for approval at a later date. The purpose of the post -closure plan will be to provide a description of the inspection, monitoring, and maintenance activities required to be performed throughout the 30-year post -closure care period for the closed AAB and RAB. The post -closure care plan will be developed to meet the requirements of N.C.G.S. §130A- 309.214(a)(4)(k). The items that will be in the post -closure plan for the Allen site include: • Name, address, phone number, and email address of the responsible office or person; • Means and methods of managing affected groundwater and stormwater; 23 AECOM July 31, 2023 Duke Energy -Allen Steam Station CAMA Closure Plan (Closure by Excavation) - Retired Ash Basin and Active Ash Basin • Maintenance of the groundwater monitoring systems; • Regular inspection and maintenance of the final cover system of the on -site landfills; • Groundwater and surface water monitoring and assessment program (included as a part of the CAP); • Post -closure inspection checklist to guide post -closure inspections; • Description of planned post -closure uses; and • Financial assurance estimates for post -closure operations and maintenance and remedial action. 11.1 Groundwater Monitoring Program This information is included as part of the updated CAP that was prepared separately by SynTerra for Duke Energy. Duke Energy submitted the updated CAP to NCDEQ on December 31, 2019. The CAP was conditionally approved by the NCDEQ on July 8, 2021. The CAP is herein incorporated by this reference but its content is not the work product of AECOM. In addition, Duke Energy has periodically updated the Allen monitoring plan based on site -specific data. At the time of this closure plan update, an Interim Monitoring Plan (IMP) update approved by NCDEQ in August 2020 is being implemented at the Allen Site. An Optimized Groundwater Monitoring Plan was submitted to NCDEQ in March 2023, and is still pending review at the time of this closure plan update. 12. PROJECT MILESTONES AND COST ESTIMATES 12.1 Project Schedule A Closure project high-level milestone schedule has been prepared by Duke Energy and is provided below. The schedule defines the following anticipated activities and milestones: Engineering and dewatering Submit plan and design for NSLF/SSLF construction permits Start CCR excavation Landfill permit approvals Start new landfill construction New landfill in service (NSLF) Complete CCR excavation Complete final closure and cover system of new landfills Site final grading and vegetative cover 12.2 Closure and Post -Closure Cost Estimate Ongoing Q4-2020 Q3-2021 Q2-2021 Q2-2021 Q 1-2024 Q4-2037 Q4-2038 Q4-2039 Cost estimates for closure and post -closure of the Basins at Allen Station were developed by Duke Energy and provided to AECOM. These cost estimates are not a work product of AECOM. These are Class 3 estimates as the detailed and final design is not developed at this stage of the I AECOM July 31, 2023 Duke Energy -Allen Steam Station CAMA Closure Plan (Closure by Excavation) - Retired Ash Basin and Active Ash Basin closure project. Following approval of this Closure Plan by NCDEQ and further development of the project plans and engineering designs the cost estimate will be refined and updated. The cost to complete closure by excavation, including the new CCR landfills, is estimated to be $501 million. The cost to perform the 30-year post -closure activities and monitoring is estimated to be $38 million. The cost estimates prepared by Duke Energy includes the following major activities: • Mobilization and Site Preparation • Dewatering, earthwork, and subgrade preparation • On -site landfill construction • CCR excavation • Stormwater management, erosion and sediment control, and site restoration • Engineering support (design and CQA) • Post closure — groundwater monitoring • Post closure — operations and maintenance • Contingency Corrective action costs are included as part of the CAP that was prepared separately by SynTerra for Duke Energy and was submitted to NCDEQ on December 31, 2019. The CAP was conditionally approved by the NCDEQ on July 8, 2021. The CAP is herein incorporated by this reference, but its content is not the work product of AECOM. 25 AECOM July 31, 2023 Duke Energy -Allen Steam Station CAMA Closure Plan (Closure by Excavation) - Retired Ash Basin and Active Ash Basin 13. REFERENCED DOCUMENTS Duke Energy Corporation (Duke Energy), Misc. Dates. Allen Steam Station Design Plans. Drawing numbers A-3330, A-3350, A-3350-01, A-3350-02, A-3350-03, A-3350-A-01, A- 3350-B-01, A-3352-01, A-3352-02, A-3352-03, A-3350-2014-0507. 1965, 1972, 1973, 1975, 1983 and 2014. NCDEQ, 2022. Permit No. NCS000546 to Discharge Stormwater under the National Pollutant Discharge Elimination System. April 12, 2022. NCDEQ, 2021. Permit No. NC0004979 to Discharge Wastewater under the National Pollutant Discharge Elimination System. July 9, 2021. NCDEQ, 2022. Industrial Solid Waste Landfill Facility Permit No.'s 3612, 3619, and 3620, Issued April 27, 2022. SynTerra Corporation (SynTerra), 2019. Corrective Action Plan Update. December 31, 2019 SynTerra Corporation (SynTerra), 2022. Annual Groundwater and Surface Water Monitoring Report. November 1, 2022. 0 PROFESSIONAL ENGINEER CERTIFICATION I, Jay Mokotoff, being a registered Professional Engineer in the state of North Carolina, do hereby certify to the best of my knowledge, information, and belief, that the information contained in this Closure Plan dated July 31, 2023, was developed pursuant to the requirements of N.C.G.S. § 130A-309-214(a)(4) and has been prepared pursuant to recognized and generally accepted good engineering practices. SIGNATURE Alf, kvDATE 7/31/2023 AECOM Technical Services of North Carolina, Inc. (License: F-0342)