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HomeMy WebLinkAboutNC0002468_FINAL_CSA Sup 2_Report_20160901Comprehensive Site Assessment Supplement 2 Dan River Steam Station Ash Basin Site Name and Location Dan River Steam Station 900 South Edgewood Road Eden, NC 27288 Groundwater Incident No. Not Assigned NPDES Permit No. NC0003468 Date of Report September 1, 2016 Permittee and Current Property Owner Duke Energy Carolinas, LLC 526 South Church St. Charlotte, NC 28202-1803 704.382.3853 Consultant Information HDR Engineering, Inc. of the Carolinas 440 South Church St, Suite 900 Charlotte, NC 28202 704.338.6700 Latitude and Longitude of Facility 360 29' 28" N, 790 43' 16" W This document has been reviewed for accuracy and quality commensurate with the intended application. �.•••�,� •CAR01--. ••, �0 0GENSF�•.y� / 56L Ot FRAKc�SS.••• Malcolm F. Schaeffer, L.G. Senior Geologist Duke Energy Carolinas, LLC | CSA Supplement 2 Dan River Steam Station Ash Basin TABLE OF CONTENTS i Table of Contents Page Executive Summary ................................................................................................................... 1 Section 1 – Background ............................................................................................................. 3 1.1 Purpose of CSA Supplement ....................................................................................... 3 1.2 Site Description ............................................................................................................ 4 1.3 History of Site Groundwater Monitoring ........................................................................ 5 1.3.1 NPDES Sampling ................................................................................................. 6 1.3.2 CSA Sampling ...................................................................................................... 7 1.3.3 Post-CSA Sampling .............................................................................................. 7 1.3.4 NCDEQ Water Supply Well Sampling ................................................................... 8 Section 2 – CSA Review Comments .......................................................................................... 9 2.1 NCDEQ General Comments and Responses ............................................................... 9 2.2 NCDEQ Site-Specific Comments and Responses ....................................................... 9 2.3 Errata ........................................................................................................................... 9 Section 3 – Additional Assessment ...........................................................................................10 3.1 Additional Assessment Activities ................................................................................10 3.1.1 Well Installation ....................................................................................................10 3.1.2 Well Gauging and Sampling .................................................................................11 3.2 Additional Assessment Results ...................................................................................11 3.2.1 Groundwater Flow Direction .................................................................................12 3.2.2 Sampling Results .................................................................................................12 Section 4 – Background Concentrations ...................................................................................16 4.1 Methodology ...............................................................................................................16 4.2 Proposed Provisional Background Concentrations......................................................18 Section 5 – Anticipated Additional Assessment Activities ..........................................................19 5.1 Proposed Additional Assessment Monitoring Well ......................................................19 5.2 Implementation of the Effectiveness Monitoring Plan ..................................................19 Section 6 – Recommendations and Conclusions ......................................................................20 Duke Energy Carolinas, LLC | CSA Supplement 2 Dan River Steam Station Ash Basin TABLE OF CONTENTS ii FIGURES 1-1 Site Location Map 1-2 Sample Location Map 1-3 NCDEQ Water Supply Well Sampling 3-1 Potentiometric Surface – Shallow Flow Layer 3-2 Potentiometric Surface – Deep Flow Layer 3-3 Potentiometric Surface – Bedrock Flow Layer 3-4.1 Antimony Isoconcentration Contour Map – Shallow Wells (S) 3-4.2 Antimony Isoconcentration Contour Map – Deep Wells (D and BRU) 3-4.3 Antimony Isoconcentration Contour Map – Bedrock Wells (BR) 3-4.4 Arsenic Isoconcentration Contour Map – Shallow Wells (S) 3-4.5 Arsenic Isoconcentration Contour Map – Deep Wells (D and BRU) 3-4.6 Arsenic Isoconcentration Contour Map – Bedrock Wells (BR) 3-4.7 Beryllium Isoconcentration Contour Map – Shallow Wells (S) 3-4.8 Beryllium Isoconcentration Contour Map – Deep Wells (D and BRU) 3-4.9 Beryllium Isoconcentration Contour Map – Bedrock Wells (BR) 3-4.10 Boron Isoconcentration Contour Map – Shallow Wells (S) 3-4.11 Boron Isoconcentration Contour Map – Deep Wells (D and BRU) 3-4.12 Boron Isoconcentration Contour Map – Bedrock Wells (BR) 3-4.13 Hexavalent Chromium Isoconcentration Contour Map – Shallow Wells (S) 3-4.14 Hexavalent Chromium Isoconcentration Contour Map – Deep Wells (D and BRU) 3-4.15 Hexavalent Chromium Isoconcentration Contour Map – Bedrock Wells (BR) 3-4.16 Chromium (Total) Isoconcentration Contour Map – Shallow Wells (S) 3-4.17 Chromium (Total) Isoconcentration Contour Map – Deep Wells (D and BRU) 3-4.18 Chromium (Total) Isoconcentration Contour Map – Bedrock Wells (BR) 3-4.19 Cobalt Isoconcentration Contour Map – Shallow Wells (S) 3-4.20 Cobalt Isoconcentration Contour Map – Deep Wells (D and BRU) 3-4.21 Cobalt Isoconcentration Contour Map – Bedrock Wells (BR) 3-4.22 Iron Isoconcentration Contour Map – Shallow Wells (S) 3-4.23 Iron Isoconcentration Contour Map – Deep Wells (D and BRU) 3-4.24 Iron Isoconcentration Contour Map – Bedrock Wells (BR) 3-4.25 Manganese Isoconcentration Contour Map – Shallow Wells (S) 3-4.26 Manganese Isoconcentration Contour Map – Deep Wells (D and BRU) 3-4.27 Manganese Isoconcentration Contour Map – Bedrock Wells (BR) 3-4.28 Sulfate Isoconcentration Contour Map – Shallow Wells (S) 3-4.29 Sulfate Isoconcentration Contour Map – Deep Wells (D and BRU) 3-4.30 Sulfate Isoconcentration Contour Map – Bedrock Wells (BR) 3-4.31 Total Dissolved Solids Isoconcentration Contour Map – Shallow Wells (S) 3-4.32 Total Dissolved Solids Isoconcentration Contour Map – Deep Wells (D and BRU) 3-4.33 Total Dissolved Solids Isoconcentration Contour Map – Bedrock Wells (BR) 3-4.34 Vanadium Isoconcentration Contour Map – Shallow Wells (S) 3-4.35 Vanadium Isoconcentration Contour Map – Deep Wells (D and BRU) 3-4.36 Vanadium Isoconcentration Contour Map – Bedrock Wells (BR) 3-5.1 Site Cross Section Locations 3-5.2 Cross Section A-A’ (1 of 1) 3-5.3 Cross Section B-B’ (1 of 1) 3-5.4 Cross Section C-C’ (1 of 1) 3-5.5 Cross Section D-D’ (1 of 1) 3-6.1 Piper Diagram – Background Groundwater, Porewater, Areas of Wetness, and Surface Water Duke Energy Carolinas, LLC | CSA Supplement 2 Dan River Steam Station Ash Basin TABLE OF CONTENTS iii 3-6.2 Piper Diagram – Shallow Groundwater, Porewater, Areas of Wetness, and Surface Water 3-6.3 Piper Diagram – Deep Groundwater, Porewater, Areas of Wetness, and Surface Water 3-6.4 Piper Diagram – Bedrock Groundwater, Porewater, Areas of Wetness, and Surface Water TABLES 1-1 Well Construction Information 1-2 NPDES Historical Data 1-3 Range of 2L Groundwater Standard Exceedances from NPDES Sampling 2-1 Responses to General NCDEQ Comments 2-2 Total and Effective Porosity and Specific Storage by Flow Layer 3-1 Round 5 Analytical Results of Groundwater Monitoring 3-2 Round 5 Analytical Results of Porewater Monitoring 3-3 Round 5 Analytical Results of Surface Water Locations 3-4 Summary of Groundwater Elevations 3-5 Summary of Cation-Anion Balance Differences APPENDICES A Monitoring Well Logs and Core Photos B Field Sampling Forms and Slug Test Report C Laboratory Report, Chain-of-Custody Forms, and Validation Report Duke Energy Carolinas, LLC | CSA Supplement 2 Dan River Steam Station Ash Basin EXECUTIVE SUMMARY 1 Executive Summary Duke Energy Carolinas, LLC (Duke Energy) owns and formerly operated the Dan River Steam Station (DRSS), located on the Dan River in Rockingham County near Eden, North Carolina (Figure 1-1). DRSS began operation as a coal-fired generating station in 1949 and was retired from service in 2012. The Dan River Combined Cycle Station (DRCCS) natural gas generating facility was constructed at the site and began operations in 2012. Historically, coal ash residue from DRSS’s coal combustion process was disposed of in an ash basin located northeast of the station and adjacent to the Dan River. Discharge from the ash basin is currently permitted by the North Carolina Department of Environmental Quality (NCDEQ)1 Division of Water Resources (DWR) under the National Pollutant Discharge Elimination System (NPDES) Permit NC0003468. This Comprehensive Site Assessment (CSA) Supplement 2 report addresses the following: • Summary of groundwater, porewater, ash basin surface water, surface water, and area of wetness (AOW) monitoring data through April 2016; • Responses to NCDEQ review comments pertaining to the CSA; • Findings from assessment activities conducted since the submittal of the CSA report, including additional assessments previously identified in the CSA; • Update on the development of provisional background groundwater concentrations; and • Description of planned additional source area assessment activities. Boron, the primary site-derived constituent in groundwater, was detected at concentrations greater than the 15A NCAC (North Carolina Administrative Code) 02L.0202 Groundwater Quality Standards (2L Standards or 2L) beneath the ash basin as well as within the ash storage area. Boron has not been detected in porewater or in groundwater beyond the compliance boundary downgradient of the ash basin dam. Groundwater monitoring results from Round 5 of CSA well sampling (completed between March and April 2016) and NPDES groundwater monitoring data are presented herein. Updated summary tables, isoconcentration maps, cross sections, and other figures are included. In general, groundwater data reported from previous rounds of monitoring are consistent with recent groundwater data, specifically the extent of impact to groundwater from ash basin-related constituents (e.g., boron and sulfate). Presentation of site-specific proposed provisional background concentrations (PPBCs) was included in the Corrective Action Plan (CAP) Part 1 report and should be refined as more data becomes available and pending input from NCDEQ. With refinement of the PPBCs, the evaluation of whether the presence of constituents of interest (COIs) downgradient of the source areas is naturally occurring or potentially attributed to the source areas can be advanced in more detail when additional sampling data is available. 1 Prior to September 18, 2015, the NCDEQ was referred to as the North Carolina Department of Environment and Natural Resources (NCDENR). Both naming conventions are used in this report, as appropriate. Duke Energy Carolinas, LLC | CSA Supplement 2 Dan River Steam Station Ash Basin EXECUTIVE SUMMARY 2 The following conclusions and recommendations are offered: • Based on their locations either upgradient or across the Dan River from the ash basin and being hydraulically isolated, the water supply wells in the vicinity of the DRSS facility are not impacted by Coal Combustion Residuals (CCR) releases from the ash basin. • Groundwater monitoring results from Round 5 of sampling, including data from additional assessment groundwater monitoring wells, indicate consistency with previous sampling results, specifically the extent of impact to groundwater from ash basin-related constituents (e.g., boron and sulfate). • Groundwater monitoring results from Round 6 of sampling should be evaluated in the additional assessment wells, as several were not installed for inclusion within the Round 5 sampling event. In addition, surface water sampling results should be evaluated along with the additional assessment well results. • Refinement of PPBCs should be conducted once the minimum number of viable observations (e.g., eight) per background well is available. • An additional monitoring well (GWA-8BR) should be installed downgradient of Ash Storage 2 to refine the vertical delineation of groundwater impacts in these areas. • Duke Energy will implement the effectiveness monitoring plan in accordance with recommendations provided in the CAP Part 2 report as well as subsequent discussions with NCDEQ. Duke Energy Carolinas, LLC | CSA Supplement 2 Dan River Steam Station Ash Basin SECTION 1 – BACKGROUND 3 Section 1 – Background Duke Energy Carolinas, LLC (Duke Energy) owns and formerly operated the Dan River Steam Station (DRSS), located on the Dan River in Rockingham County near Eden, North Carolina (Figure 1-1). DRSS began operation as a coal-fired generating station in 1949 and was retired from service in 2012. The Dan River Combined Cycle Station (DRCCS) natural gas generating facility was constructed at the site and began operations in 2012. Historically, coal ash residue from DRSS’s coal combustion process was disposed of in an ash basin located northeast of the station and adjacent to the Dan River. Discharge from the ash basin is currently permitted by the North Carolina Department of Environmental Quality (NCDEQ)2 Division of Water Resources (DWR) under the National Pollutant Discharge Elimination System (NPDES) Permit NC0003468. The Comprehensive Site Assessment (CSA) for the DRSS was submitted to NCDEQ on August 14, 2015. Given the compressed timeframe for submittal, certain information was not included in the CSA report because the data was not yet available. Duke Energy committed to providing this information after submittal of the CSA report. In addition, NCDEQ’s review of the CSA report led to requests for additional information. As such, CSA Supplement 1, which was submitted to NCDEQ on February 10, 2016 as an appendix to the Corrective Action Plan (CAP) Part 2, provided information to address the temporal constraints, information requested by NCDEQ subsequent to submittal of the CSA report, additional data validation reporting, and a response to site-specific NCDEQ comments. 1.1 Purpose of CSA Supplement The purpose of this CSA Supplement 2 is to provide data obtained during additional well installation and sampling conducted between March and July 2016. These activities were conducted to refine the understanding of subsurface geologic/hydrogeologic conditions and the extent of impacts from historical production and storage of coal ash. This CSA Supplement 2 was prepared in coordination with Duke Energy and NCDEQ as a result of requests for additional information and areas identified for additional assessment. It includes the following information: • A brief summary and update of groundwater sampling data from the NPDES, CSA, and post-CSA monitoring well sampling events; • A brief summary of results of NCDEQ water supply well sampling activities; • A summary of NCDEQ comments on the CSA report and responses to those comments; • A description of additional assessment activities conducted since submittal of the CSA report and the findings of those assessment activities; • An updated approach for the refinement of proposed provisional background concentrations (PPBCs) for groundwater at the DRSS site; and • A description of additional planned assessment activities. 2 Prior to September 18, 2015, the NCDEQ was referred to as the North Carolina Department of Environment and Natural Resources (NCDENR). Both naming conventions are used in this report, as appropriate. Duke Energy Carolinas, LLC | CSA Supplement 2 Dan River Steam Station Ash Basin SECTION 1 – BACKGROUND 4 As a complement to the CSA report and the CSA Supplement 1, this CSA Supplement 2 provides an updated evaluation of the extent of impacts from the ash basin and related ash storage facilities based on existing CSA groundwater monitoring wells and monitoring wells installed subsequent to submittal of the CSA report (herein referred to as additional assessment wells). Additional groundwater monitoring wells are shown on Figure 1-2 with green text labels. 1.2 Site Description DRSS is a former coal-fired electricity generating facility along the Dan River approximately 4.5 miles from the city of Eden, North Carolina. Construction of DRSS commenced in 1948 and the station began commercial operation in 1949 with a single coal-fired unit. A second unit was added in 1950 and a third unit was added by 1955, resulting in a total installed capacity of 276 megawatts (MW). All three coal-fired units, along with three 28 MW oil-fired combustion turbine units, were retired in 2012. Construction of the DRCCS, a 620 MW combined cycle natural gas facility, commenced in December 2009 and commercial operations began on December 10, 2012. The DRSS ash basin is located adjacent to the Dan River and consists of a Primary Cell, a Secondary Cell, and associated embankments and outlet works, as further described in the CSA report (HDR 2015). The ash basin is impounded by earthen dams and an earthen/ash divider dam separates the Primary Cell from the Secondary Cell. Water levels within the Primary Cell have historically fluctuated approximately 11 feet from October 1984 until April 2013, ranging from approximately 526 to 537 feet. Since the DRSS facility was retired, processed water and stormwater inflow to the ash basin system have been rerouted from the Primary Cell to the Secondary Cell, resulting in decreased water levels in the Primary Cell. Water levels within the Secondary Cell have historically fluctuated approximately 8 feet from October 1984 until April 2013, ranging from approximately 518 to 526 feet. The ash storage areas are located upgradient of the ash basin and consist of Ash Storage 1, Ash Storage 2, a former dredge pond, and associated dredge dikes. The ash storage areas contain ash that was previously removed from the ash basin. Topography at the DRSS site generally slopes from northwest to southeast, ranging from an approximate high elevation of 606 feet near the northern property boundary just west of Edgewood Road to an approximate low elevation of 482 feet at the interface with the Dan River. Ground surface relief varies by approximately 124 feet over an approximate distance of 0.7 miles. Surface water drainage generally follows site topography and flows from northwest to southeast across the site except where drainage patterns have been modified by the ash basin or other construction. The groundwater system in the natural materials (alluvium, soil, soil/saprolite, and bedrock) at DRSS is consistent with the Piedmont regolith-fractured rock system and consists of an unconfined, connected system without confining layers. The groundwater system at DRSS is a two-layer system: shallow (regolith) and deep (bedrock). In general, groundwater at the site flows from the northern extent of the DRSS property boundary to the south/southeast toward the Dan River. Duke Energy Carolinas, LLC | CSA Supplement 2 Dan River Steam Station Ash Basin SECTION 1 – BACKGROUND 5 The primary source areas at DRSS are defined as the ash basin (Primary and Secondary cells) and the ash storage areas (Ash Storage 1 and Ash Storage 2). Source characterization was performed to identify physical and chemical properties of ash, ash basin surface water, porewater, and areas of wetness (AOW). The compliance boundary for groundwater quality at the DRSS site 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 waste boundary. As described in the CSA report, analytical results for groundwater samples were compared to the North Carolina Groundwater Quality Standards, as specified in 15A NCAC 2L.0202 (2L Standards or 2L) or Interim Maximum Allowable Concentration (IMAC) established by NCDEQ pursuant to 15A NCAC 2L.0202(c), or North Carolina Department of Health and Human Services (DHHS) Health Screening Level (HSL) (hexavalent chromium only) for the purpose of identifying constituents of interest (COIs). The IMACs were issued for certain constituents in 2010, 2011, and 2012; however, NCDEQ has not established a 2L Standard for those constituents as described in 15A NCAC 2L.0202(c). For this reason, the IMACs noted in this document are for reference purposes only. Areas of 2L Standard, IMAC, or DHHS HSL exceedances are limited to within or directly adjacent to the sources. The limited downgradient extent of impacts indicates that physical and geochemical processes (i.e., dispersion and sorption) beneath the DRSS site reduce the lateral migration of COIs. 1.3 History of Site Groundwater Monitoring Duke Energy has implemented voluntary and NPDES permit-required compliance groundwater monitoring at the DRSS site since 1993, in accordance with NPDES Permit NC0003468. From 1993 to 2010, voluntary groundwater monitoring was performed twice annually around the DRSS ash basin with analytical results submitted to NCDENR DWR. In October 2010, new compliance monitoring wells were installed and the existing monitoring wells became part of the voluntary monitoring program. Additional groundwater monitoring was required beginning in March 2011 with the frequency of sampling and parameters to be analyzed outlined in the NPDES permit. A review of voluntary monitoring well sampling results obtained between 1993 and 2011 indicates the following: • Boron exceeded the 2L Standard in the deep flow layer in MW -9D in all sampling events from April 2008 to September 2011. • Chromium 3 exceeded the 2L Standard in the shallow flow layer in well MW -12 during the April 2004 and October 2006 voluntary sampling events. Chromium exceeded the 2L Standard in the deep flow layer in well MW -11D during the April and October sampling events in 2008 and the April 2009 sampling event. • Iron and manganese have intermittently exceeded the 2L Standards in voluntary wells screened in the shallow and deep flow layers located across the site. However, these 3 Unless otherwise noted, references to chromium in this document indicate total chromium. Duke Energy Carolinas, LLC | CSA Supplement 2 Dan River Steam Station Ash Basin SECTION 1 – BACKGROUND 6 exceedances may be attributable to naturally occurring conditions and require additional evaluation as site-specific PPBCs are refined. • Lead exceeded the 2L Standard in the shallow flow layer in well MW -12 during the April 1998 and April 2000 voluntary sampling events. Lead exceeded the 2L standard in the deep flow layer in well MW-11D during the April 2008 sampling event. Construction details for voluntary monitoring wells are provided in Table 1-1. The location of the ash basin voluntary monitoring wells, the approximate ash basin waste boundary, and the compliance boundary are shown on Figure 1-2. As described in Section 1.2, the compliance boundaries for groundwater quality at the DRSS site are defined in accordance with Title 15A NCAC 02L .0107(a) as being established at either 500 feet from the waste boundaries or at the property boundary, whichever is closer to the waste boundary. After additional review, Duke Energy became aware that the compliance boundaries currently depicted should be revised to accurately reflect Duke Energy property ownership. The current property depiction was obtained from county GIS records and shows specific areas consisting of rights-of-way not under ownership of Duke Energy. Duke Energy is in the process of revising these areas and will submit revised compliance boundary figures to NCDEQ for approval. 1.3.1 NPDES Sampling NPDES compliance monitoring wells (compliance wells) were installed in December 2010. Compliance groundwater monitoring, as required by the NPDES permit, began in December 2011. From January 2011 through May 2016, compliance groundwater monitoring wells at the DRSS site have been sampled three times each year, resulting in 17 monitoring events during that time. One or more groundwater quality criteria (2L Standard and IMAC) has been exceeded in groundwater samples collected from the compliance monitoring wells. Exceedances have occurred in one or more wells during one or more sampling events for antimony, arsenic, boron, iron, manganese, sulfate, total dissolved solids (TDS), and thallium. A review of NPDES compliance well sampling results indicates the following: • Antimony exceeded the IMAC in the shallow flow layer in MW -20S in May 2012; in MW - 21S in January 2011, May 2011, September 2011, May 2012, and September 2015; and in MW -22S in January 2016. Antimony exceeded the IMAC in the deep flow layer in MW -20D in September 2015 and January 2016; in MW -21D in September 2015; in MW - 22D in January 2012, May 2012, January 2013, May 2013, May 2015, September 2015, and January 2016; in MW-23D in September 2015 and January 2016. • Arsenic exceeded the 2L Standard in the shallow flow layer in MW -21S in all sampling events from January 2011 through May 2016. • Boron exceeded the 2L Standard in the shallow flow layer in MW -22S in May 2013, September 2014, and May 2016. Boron exceeded the 2L Standard in the deep flow layer in MW -22D in all sampling events from January 2012 through May 2016 with the exception of September 2012 and May 2013. Duke Energy Carolinas, LLC | CSA Supplement 2 Dan River Steam Station Ash Basin SECTION 1 – BACKGROUND 7 • Iron and manganese have intermittently exceeded the 2L Standards in compliance wells screened in the shallow and deep flow layers located across the site during the NPDES sampling period. However, these exceedances may be attributable to naturally occurring conditions and require additional evaluation as site-specific PPBCs are refined. • Sulfate exceeded the 2L Standard in the deep flow layer in MW -21D in all sampling events from January 2011 through May 2016 with the exception of September 2014. • TDS exceeded the 2L Standard in the deep flow layer in MW -21D in all sampling events from January 2011 through May 2016. • Thallium exceeded the IMAC in the deep flow layer in MW -20D in September 2015. Historical analytical results and a summary of the range of exceedances within the NPDES groundwater monitoring program are provided in Tables 1-2 and 1-3, respectively. 1.3.2 CSA Sampling The CSA for the DRSS site began in February 2015 and was completed in August 2015. Sixty- three groundwater monitoring wells and three soil borings were installed/advanced as part of this assessment to characterize the ash, soil, rock, and groundwater at the DRSS site. One comprehensive round of sampling and analysis was included in the CSA report and included sampling of groundwater, ash, ash basin surface water, Dan River surface water, tributary surface water, porewater, and AOW (Figure 1-2). In addition, a hydrogeological evaluation was performed on newly installed wells. The following constituents were reported as COIs in the CSA report: • Soil: iron, manganese, arsenic, chromium, and selenium • Groundwater: antimony, arsenic, boron, chromium, cobalt, iron, manganese, sulfate, thallium, TDS, and vanadium • Surface Water: aluminum, arsenic, copper, lead, and vanadium Horizontal and vertical delineation of source-related soil impacts was presented in the CSA report. Where soil impacts were identified beneath the ash basin (beneath the ash/soil interface), the vertical extent of contamination was generally limited to the soil samples collected just beneath the ash. Groundwater impacts at the site attributable to ash handling and storage were delineated during the CSA activities with the following areas requiring refinement: • Vertical extent in the vicinity of Ash Storage 1 • Horizontal and vertical extent north of Ash Storage 1 1.3.3 Post-CSA Sampling Four additional rounds of groundwater sampling of the CSA wells have occurred since submittal of the CSA report. Round 2 of groundwater monitoring occurred in September 2015 and was reported in the Corrective Action Plan (CAP) Part 1 (submitted on November 12, 2015). Rounds 3 and 4 of background groundwater monitoring occurred in November and December 2015, and results were reported in the CSA Supplement 1 as part of the CAP Part 2 report (submitted on Duke Energy Carolinas, LLC | CSA Supplement 2 Dan River Steam Station Ash Basin SECTION 1 – BACKGROUND 8 February 10, 2016). Round 5 of groundwater monitoring was conducted in March and April 2016 and is the focus of the data evaluation presented in Section 3 of this CSA Supplement 2. 1.3.4 NCDEQ Water Supply Well Sampling Section § 130A-309.209 (c) of CAMA indicates that NCDEQ requires sampling of water supply wells to evaluate whether the wells may be adversely impacted by releases from CCR impoundments. NCDEQ required sampling of all drinking water receptors within 0.5 mile of the DRSS compliance boundary in all directions, since the direction of groundwater flow had not been determined at DRSS at the time of sampling. However, no private wells within a 0.5-mile radius of the compliance boundary were sampled at the DRSS sites, nor were “Do Not Drink” letters issued by the DHHS. Two reported private water supply wells are located at residences north of the DRSS within a 0.5-mile radius of the ash basin compliance boundary. Two additional reported private water supply wells are located at residences south of the DRSS within a 0.5-mile radius of the ash basin compliance boundary; they are located on the southern side of Dan River in Rockingham County (Wells 1 and 4 shown on Figure 1-3). Information evaluated as part of the CSA indicated that the identified water supply wells would not be impacted as they are either hydraulically upgradient or across the Dan River from the ash basin. Additional assessment wells GWA-20S and GWA-20D have been installed on the south side of the Dan River downgradient of the two private water supply wells (Wells 1 and 4). These two wells are not within the Dan River Triassic Basin, but are installed within alluvium and metamorphic rocks of the Milton terrane. Furthermore, Well 4 is upstream and upgradient of the Dan River ash basin. As such, the water supply wells in the vicinity of the DRSS facility are not impacted by CCR releases from the ash basin. Duke Energy Carolinas, LLC | CSA Supplement 2 Dan River Steam Station Ash Basin SECTION 2 – CSA REVIEW COMMENTS 9 Section 2 – CSA Review Comments Representatives of NCDEQ’s Central Office and Winston-Salem Regional Office (WSRO) met with Duke Energy and HDR on September 23, 2015, to present their review comments to the CSA report. Comments were organized in two categories: general comments applicable to all Duke Energy Carolinas CSA reports regardless of site and site-specific comments applicable to DRSS. 2.1 NCDEQ General Comments and Responses General comments applicable to the CSA reports and responses to the comments are presented in Table 2-1. 2.2 NCDEQ Site-Specific Comments and Responses Site-specific comments and responses were included in the CSA Supplement 1, which was submitted to NCDEQ on February 10, 2016 as part of the CAP Part 2 report. 2.3 Errata Editorial comments and corrections to the CSA report were included in the CSA Supplement 1, which was submitted to NCDEQ on February 10, 2016 as part of the CAP Part 2 report. Since the issuance of the CAP Part 2 report, additional evaluation of site data has occurred, resulting in refinement by flow layer of total porosity, secondary (effective) porosity, and specific storage for the lower hydrostratigraphic units (deep transition zone and bedrock), as provided in Table 2-2. Duke Energy Carolinas, LLC | CSA Supplement 2 Dan River Steam Station Ash Basin SECTION 3 – ADDITIONAL ASSESSMENT 10 Section 3 – Additional Assessment Additional assessment activities identified in the CSA report were addressed and the findings are discussed in the following sections. 3.1 Additional Assessment Activities Additional assessment activities included monitoring well installation and sampling, as discussed below. 3.1.1 Well Installation Additional monitoring wells were installed to refine understanding of groundwater flow direction and extent of exceedances at DRSS. Refinement of exceedances focused primarily on the extent of groundwater impacts in the vicinity of Ash Storage 1. To address the extent of exceedances near Ash Storage 1, monitoring well GWA-18D was installed. Subsequent discussions with Duke Energy and NCDEQ resulted in the installation of additional monitoring wells beyond described above. A summary of those additional assessment wells and installation dates, as well as the purpose for installation, is provided in the table below. Boring/Well Identification Installation Date Purpose for Installation Result GWA-5BRD 7/7/2016 Well requested by NCDEQ to further evaluate vertical extent of exceedances observed at GWA- 5BR Not installed in time for sampling inclusion during the Round 5 sampling event GWA-16S 6/14/2016 Wells requested by NCDEQ to further evaluate horizontal extent of exceedances observed at GWA- 14S/D and GWA-15S/D Not installed in time for sampling inclusion during the Round 5 sampling event GWA-16D 6/15/2016 Not installed in time for sampling inclusion during the Round 5 sampling event GWA-18D 6/9/2016 Wells requested by NCDEQ to further evaluate horizontal extent of exceedances north of Ash Storage 1 Not installed in time for sampling inclusion during the Round 5 sampling event GWA-20S 6/21/2016 Wells requested by NCDEQ to evaluate water quality across the Dan River Not installed in time for sampling inclusion during the Round 5 sampling event GWA-20D 6/23/2016 Not installed in time for sampling inclusion during the Round 5 sampling event MW-317BRL 7/12/2016 Wells requested by NCDEQ to further evaluate the vertical extent of exceedances observed between AS-8D/BR and MW -317BR Not installed in time for sampling inclusion during the Round 5 sampling event Duke Energy Carolinas, LLC | CSA Supplement 2 Dan River Steam Station Ash Basin SECTION 3 – ADDITIONAL ASSESSMENT 11 Boring/Well Identification Installation Date Purpose for Installation Result BG-10S 6/10/2016 To provide additional background groundwater quality for determination of PPBCs at the site Not installed in time for sampling inclusion during the Round 5 sampling event BG-10D 6/13/2016 Not installed in time for sampling inclusion during the Round 5 sampling event BG-10BR 6/22/2016 Not installed in time for sampling inclusion during the Round 5 sampling event Monitoring well logs and core photos, field sampling and slug test records, and analytical laboratory reports are included in Appendices A, B, and C, respectively. The following wells that were proposed as additional assessment wells were not installed because groundwater was not encountered above refusal depth: • GWA-17S/D – This well nest had been requested by NCDEQ to further evaluate horizontal extent of exceedances observed at GWA-14S/D and GWA-15S/D. • GWA-18S – This well had been requested by NCDEQ to further evaluate horizontal extent of exceedances north of Ash Storage 1. In addition, AOWs were not sampled during the Round 5 event due to dry and/or non-flowing conditions. Several monitoring wells exhibit pH levels of 9 or above, which may be indicative of cement leakage beyond the borehole seal. In addition, several monitoring wells had a turbidity value of greater than 10 NTU. The evaluation of groundwater quality data obtained from these wells during Round 5 sampling must be qualified and further evaluated. The additional assessment wells were installed by North Carolina-licensed drillers according to construction standards described in 15A NCAC 2C.0107. 3.1.2 Well Gauging and Sampling Round 5 of groundwater, porewater, ash basin surface water, surface water, and ash basin water sampling activities was completed between March 28 and April 26, 2016. Groundwater analytical parameters and methods for Round 5 were consistent with those used during previous sampling events, as presented in previous reports. A total of 60 groundwater and porewater monitoring wells were sampled during the Round 5 event. Sample locations are depicted on Figure 1-2. 3.2 Additional Assessment Results Findings and results from Round 5 of sampling and analysis and additional assessment activities are presented below. Note that the Round 3 and 4 monitoring events focused on sampling of background wells only; therefore, groundwater elevation and analysis results are compared to data previously obtained during Round 1 and 2 monitoring events. A summary of the analytical results is presented in Tables 3-1 through 3-3 for groundwater, porewater, and Duke Energy Carolinas, LLC | CSA Supplement 2 Dan River Steam Station Ash Basin SECTION 3 – ADDITIONAL ASSESSMENT 12 ash basin surface water, respectively. A summary of groundwater elevations measured during the Round 1 through 5 gauging events is presented in Table 3-4. 3.2.1 Groundwater Flow Direction On June 12, 2016, monitoring wells were manually gauged from the top of the PVC casing using an electronic water level indicator accurate to 0.01 foot. Groundwater elevations were generally higher than those measured during Rounds 1 and 2, which is likely attributable to seasonal variation of the water table. Groundwater flow direction was consistent with flow directions identified in Rounds 1 and 2, and generally flows to the northern extent of the DRSS property boundary to the south/southeast toward the Dan River. The groundwater flow direction is away from the direction of the nearest public or private water supply wells. Groundwater elevations and inferred contours for the shallow, deep, and bedrock flow layers are depicted on Figures 3-1, 3-2, and 3-3, respectively and show an upward gradient in the deep and bedrock flow layers at the Dan River, These groundwater flow directions are consistent with interpretations made in the CSA report. 3.2.2 Sampling Results 3.2.2.1 SUMMARY OF ROUND 1 AND 2 GROUNDWATER SAMPLING RESULTS As previously mentioned in Section 1.3.2, the following COIs were identified in groundwater as a result of sampling conducted during the CSA: antimony, arsenic, boron, chromium, cobalt, iron, manganese, sulfate, thallium, TDS, and vanadium. In addition, beryllium, hexavalent chromium, and pH exceeded applicable criteria during the Round 5 sampling event and may be considered additional COIs. 3.2.2.2 ROUND 5 POREWATER SAMPLING RESULTS A total of four porewater samples were collected from monitoring wells (AB-10S, AB-10SL, AB- 25S, and AB-5S) screened within the ash basin Primary and Secondary cells. Concentrations of antimony, arsenic, hexavalent chromium, cobalt, iron, manganese, selenium, thallium, vanadium, and pH that exceed the applicable 2L Standard, IMAC, or DHHS HSL were detected in porewater samples collected during the Round 5 sampling event. The range and number of exceedances of each COI in porewater is listed below. • Antimony: 4.3 µg/L; 1 exceedances/4 samples • Arsenic: 172 µg/L to 249 µg/L; 3/4 • Beryllium: no exceedances in porewater • Boron: no exceedances in porewater • Chromium: no exceedances in porewater • Hexavalent Chromium: 0.54 µg/L; 1/4 • Cobalt: 13.5 µg/L to 31.2 µg/L; 2/4 • Iron: 13,200 µg/L to 37,500 µg/L; 2/4 • Manganese: 371 µg/L to 1400 µg/L; 2/4 • Selenium: 21.6 µg/L; 1/4 • Sulfate: no exceedances in porewater • Thallium: 0.3 µg/L to 0.66 µg/L; 2/4 Duke Energy Carolinas, LLC | CSA Supplement 2 Dan River Steam Station Ash Basin SECTION 3 – ADDITIONAL ASSESSMENT 13 • TDS: no exceedances in porewater • Vanadium: 0.71 µg/L to 55.9 µg/L; 4/4 • pH: 6.3; 1/4 3.2.2.3 ROUND 5 GROUNDWATER SAMPLING RESULTS A total of 57 groundwater monitoring wells were sampled during March and April 2016 as part of the Round 5 sampling event. In addition, a total of three Field Duplicate (FD) samples were collected from the groundwater wells. The analytical results for the groundwater monitoring well samples are presented on Table 3-1. In general, the COIs identified during Round 5 groundwater sampling are consistent with the results obtained during Rounds 1 and 2. A summary of Round 5 sampling results per COI identified during the CSA is as follows: • Antimony: 1.1 µg/L to 2.3 µg/L; 5 exceedances/57 samples (5/57) o Antimony exceeded the IMAC in the deep flow layer in wells located within the ash basin and north of Ash Storage 1. Antimony exceedances in the bedrock flow layer were detected at concentrations that were similar to concentrations measured in the deep flow layer. Antimony was detected above the IMAC in background well BG-1D, indicating that antimony may be present as a naturally occurring constituent. In general, total and dissolved concentrations were consistent in each sample. There were no exceedances for antimony in wells screened within the shallow flow layer. • Arsenic: 11.3 µg/L to 319 µg/L; 2/57 o Arsenic exceeded the 2L Standard in the deep flow layer in one well (OW -308D) located within the Secondary Cell of the ash basin and one well (AB-35BR) in the bedrock flow layer. The measured exceedance in the deep flow layer was significantly higher than the exceedance measured in the bedrock flow layer. There were no exceedances for arsenic in wells screened within the shallow flow layer. • Beryllium: 7.7 µg/L; 1/57 o Beryllium exceeded the 2L Standard in one well (GWA-7S) screened within the shallow flow layer. There were no exceedances for beryllium in wells screened within the deep and bedrock flow layers. • Boron: 735 µg/L to 1640 µg/L; 6/57 o Boron exceeded the 2L Standard in one well (AS-12S) screened within the shallow flow layer. Boron was detected above the 2L Standard in four wells (MW - 318D, MW -9D, AB-25D, and AS-2D) in the deep flow layer within the ash basin and ash storage areas, and MW -311BR in the bedrock flow layer within the ash basin. • Chromium: 10.4 µg/L to 160 µg/L; 1/57 o Chromium exceeded the 2L Standard in the bedrock flow layer in one well (MW - 317BR). There were no exceedances for chromium in wells screened within the shallow and deep flow layers. • Hexavalent Chromium: 0.072J µg/L to 12.6 µg/L; 18/57 o Hexavalent chromium exceeded the DHHS HSL in the shallow flow layer in wells located across the DRSS site. Hexavalent chromium also exceeded the DHHS Duke Energy Carolinas, LLC | CSA Supplement 2 Dan River Steam Station Ash Basin SECTION 3 – ADDITIONAL ASSESSMENT 14 HSL in the deep flow layer with similar frequency as the shallow flow layer within and upgradient of the ash basin. Exceedances of hexavalent chromium in the bedrock flow layer wells occurred at two locations (MW-317BR and AB-25BR). Concentrations in six background wells (BG-5S, GWA-9S, GWA-12S, BG-1D, BG-5D, and MW-23D) also exceeded the DHSS HSL. Note that dissolved phase analyses were not performed on groundwater samples for hexavalent chromium. • Cobalt: 1.3 µg/L to 34.1 µg/L; 14/57 o Cobalt exceeded the IMAC in seven shallow flow layer wells. Cobalt exceedances in the deep flow layer occurred at six locations and in one FD, and were measured at similar concentrations as observed in the shallow flow layer. Cobalt exceedances in the bedrock flow layer were exhibited in one bedrock well (MW -308BR), which is located within the Secondary Cell of the ash basin. Total and dissolved concentrations were generally consistent across the sample set. • Iron: 322 µg/L to 31,400 µg/L; 21/57 o Iron exceeded the 2L Standard in the shallow flow layer in wells in multiple areas of the DRSS site. Exceedances of iron in the deep flow layer were more frequent throughout the site than in the shallow flow layer. Exceedances of iron in the bedrock flow layer exceeded the 2L Standard in four wells. Two FD wells, one in the deep flow layer and one in the bedrock flow layer, also indicated exceedances of the 2L Standard. Total and dissolved concentrations were generally inconsistent across the sample set. • Manganese: 76.3 µg/L to 7460 µg/L; 41/57 o Manganese exceeded the 2L Standard in the shallow, deep, and bedrock flow layers in wells located across the DRSS site. Total and dissolved concentrations were generally consistent across the sample set. • Selenium: No exceedances in groundwater • Sulfate: 388,000 µg/L to 811,000 µg/L; 3/57 o Sulfate exceeded the 2L Standard in one well within the deep flow layer (GWA- 8D) and two wells within the bedrock flow layer (MW -308BR and GWA-5BR). These wells are located upgradient from and within the ash basin. There were no exceedances for sulfate in wells screened within the shallow flow layer. • Thallium: No exceedances in groundwater • TDS: 522,000 µg/L to 1,400,000 µg/L; 5/57 o TDS exceeded the 2L Standard in three wells screened within the deep flow layer (GWA-1D, GWA-8D, and AS-2D) and two wells within the bedrock flow layer (MW -308BR and GWA-5BR). The bedrock flow layer wells with TDS exceedances are located within the ash basin at wells MW -308BR and GWA- 5BR. There were no exceedances for TDS in wells screened within the shallow flow layer. • Vanadium: 0.32 µg/L to 20.1 µg/L; 18/57 o Vanadium exceeded the IMAC in the shallow, deep, and bedrock flow layers in wells across the DRSS site, including one background well. Total and dissolved concentrations were generally consistent across the sample set. Duke Energy Carolinas, LLC | CSA Supplement 2 Dan River Steam Station Ash Basin SECTION 3 – ADDITIONAL ASSESSMENT 15 • pH: 4.8 to 12.2; 23/57 o PH exceeded the 2L Standard in the shallow, deep, and bedrock flow layers in wells across the DRSS site including three background wells (BG-5S, GWA-12S, and MW -23D). The horizontal extent of exceedances is presented in the form of isoconcentration figures (Figures 3-4.1 through 3-4.36). The vertical extent of boron and sulfate is presented on applicable cross sections (Figures 3-5.1 through 3-5.5). In addition, cross sections show hydrostratigraphic layers, rock lithology, rock core recovery (REC) and rock quality designation (RQD; a measure of rock mass discontinuities/fracturing) in response to comments received from NCDEQ. 3.2.2.4 COMPARISON OF POREWATER AND GROUNDWATER RESULTS Based upon review of data collected during Round 5 sampling, the concentrations of COIs identified in porewater were generally consistent with groundwater concentrations with the possible exceptions of beryllium, boron, selenium, and thallium within the shallow flow layer. Exceedances of beryllium and boron were exhibited in shallow groundwater wells, but not in porewater wells. Exceedances of selenium and thallium were exhibited in porewater wells, but not in groundwater wells. Piper diagrams presented in the CSA report provided evidence of mixing ash basin porewater and groundwater, and Round 5 analytical results are consistent with previous presentations. In general, geochemistry of groundwater and surface water at the DRSS site is predominantly rich in calcium, magnesium, and bicarbonate with the exception of downgradient groundwater monitoring wells, which trend closer to a calcium-, magnesium- and sulfate-rich geochemical composition. Piper diagrams with cation-anion balance differences < 10% are presented in Figures 3-6.1 through 3-6.4. In addition, overall cation-anion balance differences are summarized in Table 3-5. Duke Energy Carolinas, LLC | CSA Supplement 2 Dan River Steam Station Ash Basin SECTION 4 – BACKGROUND CONCENTRATIONS 16 Section 4 – Background Concentrations Presentation of site-specific PPBCs was included in the CAP Part 1 report and is pending refinement as the required minimum number of additional sampling results become available. Regulations providing North Carolina groundwater quality standards are provided in T15A NCAC 02L .0202. Section (b)(3) of the regulation provides that: Where naturally occurring substances exceed the established standard, the standard shall be the naturally occurring concentration as determined by the Director. The reference background concentrations determined by the methodology described below will be submitted to the NCDEQ DWR as the proposed naturally occurring site background concentrations for the specific constituents. A site-specific report documenting the procedures, evaluations, and calculations will be prepared and submitted to the NCDEQ. 4.1 Methodology As stated in the USEPA Unified Guidance (USEPA 2009) (Unified Guidance): The Unified Guidance recommends that a minimum of at least 8 to 10 independent background observations be collected before running most statistical tests. Although still a small sample size by statistical standards, these levels allow for minimally acceptable estimates of variability and evaluation of trend and goodness-of fit. However, this recommendation should be considered a temporary minimum until additional background sampling can be conducted and the background sample size enlarged.4 Once the required minimum number of samples is available, HDR will calculate PPBCs using the appropriate methods in the Unified Guidance, the USEPA ProUCL software, and guidance found in the North Carolina Division of Water Quality (NCDWQ) technical assistance document Evaluating Metals in Groundwater at DWQ Permitted Facilities. This process will also follow HDR’s proposed method to establish reference background concentrations for constituents according to the Environmental Protection Agency’s Hazardous and Solid Waste Management System; Disposal of Coal Combustion Residuals from Electric Utilities; Final Rule (EPA CCR).5 The proposed method will be developed in consultation with Synterra, Duke Energy’s groundwater assessment consultant for Duke Energy Progress sites, to ensure consistency in approach. 4 U.S. Environmental Protection Agency (USEPA) Unified Guidance (USEPA 2009), 5.2.1 Selecting Monitoring Constituents and Adequate Sample Sizes 5 HDR modified its earlier methods to establish reference background concentration so that both state and federal regulations are comparable. Having similar processes to address the two sets of regulations will minimize confusion. Duke Energy Carolinas, LLC | CSA Supplement 2 Dan River Steam Station Ash Basin SECTION 4 – BACKGROUND CONCENTRATIONS 17 As recommended by the USEPA Unified Guidance, HDR will calculate the 95 percent upper prediction limits (UPL95) as the proposed reference background concentration value for each constituent at the DRSS site.6 HDR will calculate UPL95 values for each of the constituents using their respective concentrations observed in the samples taken from the set of site-specific background wells once a minimum of eight observations per constituent is available. Samples will not be used to develop reference background concentrations whenever turbidity is 10 NTU or greater. Only non-filtered results will be utilized. HDR will review and evaluate the corresponding filtered results; however, they will not be used for compliance purposes at this time. The data collected from the background wells will be pooled prior to estimating the reference background concentration using the UPL95. When implementing this approach, HDR will consider that the background wells are screened in different hydrostratigraphic units (shallow, transition zone, or bedrock). While there are differences as described, the fundamental assumption is that the constituent concentrations sampled at these background wells, when pooled, will serve as an estimate of overall field conditions for a given constituent. HDR will test this assumption using statistical methods and if distinct sub-groups exist, separate background concentrations for each distinct sub-group of wells by hydrostratigraphic unit (shallow, transition zone (deep), and bedrock) will be calculated. The methodology used to calculate upper prediction limits (UPLs) for the constituents will be generally completed in three parts as follows: 1. Analyze Preliminary Data 2. Determine Differences Across Sub-Groups 3. Develop Background Threshold Values (UPLs) Part 1 of the process includes the preliminary data analyses used to assess and transform the data where necessary such that the data can be used to calculate UPLs. Statistical methods will be used to evaluate outliers, serial correlation, seasonality, spatial variability, trends, and appropriateness of the period of record (sampling period). Part 2 of the process includes describing the approach to test for sub-group differences. Types of sub-groups to test include seasonal sub-groups (winter, spring, summer, and fall) and well class sub-groups (shallow, transition zone (deep), and bedrock). If the groups are statistically different after testing, the same steps described in Part 1 can be applied to the partitioned data to better understand the distribution of the samples within a sub-group for each constituent. The 6 There are different methods that can be used to estimate the reference background concentrations such as the UPL and the upper tolerance limit (UTL). HDR selected the UPL as it is the statistic recommended by the USEPA Unified Guidance (page 2-15). The Unified Guidance recommends the UPL over the UTL for the following reasons, (1). The ability to estimate a UTL which can control for Type I error rates when simultaneously testing an exact number of multiple future or independent observations is not as precise as when estimating the appropriate UPL. (2) The mathematical underpinnings of UPLs under re-testing strategies are well established, while those for re-testing with tolerance limits are not. Re-testing strategies are now encouraged and sometimes required under assessment monitoring situations. (3) Statistically, the two limits are similar, especially under normal assumptions. Duke Energy Carolinas, LLC | CSA Supplement 2 Dan River Steam Station Ash Basin SECTION 4 – BACKGROUND CONCENTRATIONS 18 reference background concentration values using the UPL95 for a constituent will be produced for each sub-group of samples, provided the sub-groups represent distinct populations. Part 3 of the process involves describing the statistical analyses and presenting the resulting background threshold values (UPLs) for each constituent. 4.2 Proposed Provisional Background Concentrations Currently, the DRSS site has the following number of usable observations at background wells for implementation of the background concentration methodology described in Section 4.1: • BG-1D (CSA Monitoring Well) – 3 observations • BG-5S (CSA Monitoring Well) – 6 observations • BG-5D (CSA Monitoring Well) – 6 observations • BG-10S (Additional Assessment Monitoring Well) – 0 observations • BG-10D (Additional Assessment Monitoring Well) – 0 observations • BG-10BR (Additional Assessment Monitoring Well) – 0 observations • GWA-9S (CSA Monitoring Well) – 5 observations • GWA-9D (CSA Monitoring Well) – 5 observations • GWA-12S (CSA Monitoring Well) – 2 observations • GWA-12D (CSA Monitoring Well) – 6 observations • MW -23D (Compliance Monitoring Well) – 3 observations • MW -23BR (CSA Monitoring Well) – 5 observations It is expected that with interim monitoring implementation, the DRSS site will have the appropriate number of data points to perform the calculation of UPL95s. HDR is considering alternatives that will provide the required number of sample events and will provide an update on that evaluation at a later date. Duke Energy Carolinas, LLC | CSA Supplement 2 Dan River Steam Station Ash Basin SECTION 5 – ANTICIPATED ADDITIONAL ASSESSMENT ACTIVITIES 19 Section 5 – Anticipated Additional Assessment Activities Anticipated additional assessment activities are summarized below. 5.1 Proposed Additional Assessment Monitoring Well Based on review of site information and analytical data available at this time, there is one location at the site where additional groundwater assessment is warranted to refine delineation of the vertical extent of groundwater impacts associated with potential coal ash-related constituents. The following well is currently planned for installation during Fall of 2016: Proposed Additional Monitoring Wells Location Purpose Approximate Monitoring Well Depth(s) (ft) GWA-8BR Downgradient (southwest) of Ash Storage 2 Evaluate vertical extent of exceedances reported in GWA- 8D (TDS and sulfate). 70 5.2 Implementation of the Effectiveness Monitoring Plan The effectiveness monitoring plan proposed in CAP Part 2 provided detailed information regarding field activities to be performed during collection of groundwater, porewater, ash basin surface water, surface water, and AOW samples associated with the ash basin (Primary and Secondary cells) and the ash storage areas (Ash Storage 1 and Ash Storage 2) at the DRSS site. The monitoring plan is intended to evaluate the effectiveness of proposed corrective actions and address the need to evaluate baseline conditions and seasonal variation in groundwater, ash basin surface water, and AOWs. Duke Energy will implement the effectiveness monitoring plan in accordance with recommendations provided in the CAP Part 2 report as well as subsequent discussions with NCDEQ. Duke Energy Carolinas, LLC | CSA Supplement 2 Dan River Steam Station Ash Basin SECTION 6 – RECOMMENDATIONS AND CONCLUSIONS 20 Section 6 – Recommendations and Conclusions The following conclusions have been developed from the information presented in this CSA Supplement 2 report: • Based on their locations either upgradient or across the Dan River from the ash basin and being hydraulically isolated, the water supply wells in the vicinity of the DRSS facility are not impacted by CCR releases from the ash basin. • Groundwater monitoring results from Round 5 of sampling, including data from additional assessment groundwater monitoring wells, indicate consistency with previous sampling results, specifically the extent of impact to groundwater from ash basin-related constituents (e.g., boron and sulfate). Based on the conclusions presented above, the following recommendations are offered: • Groundwater monitoring results from Round 6 of sampling should be evaluated in the additional assessment wells as several were not installed for inclusion within the Round 5 sampling event. In addition, surface water sampling results should be evaluated along with the additional assessment well results. • Refinement of PPBCs should be conducted once the minimum number of viable observations (e.g., eight) per background well is available. • An additional monitoring well (GWA-8BR) should be installed downgradient of Ash Storage 2 to refine the vertical delineation of groundwater impacts in these areas. • Duke Energy will implement the effectiveness monitoring plan in accordance with recommendations provided in the CAP Part 2 report as well as subsequent discussions with NCDEQ. Figures Tables A Appendix A Monitoring Well Boring Logs Core Photos B Appendix B Field Sampling Forms Slug Test Report C Appendix C Laboratory Report and Chain-of-Custody Forms Validation Report