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HomeMy WebLinkAboutNC0004961_RBSS CSA Executive Summary_20150818Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report Riverbend Steam Station Ash Basin EXECUTIVE SUMMARY ES-1 Executive Summary On August 20, 2014, the North Carolina General Assembly passed Senate Bill 729, the Coal Ash Management Act of 2014 (CAMA). Section § 130A-309.209 of the bill requires the owner of a coal combustion residuals surface impoundment to submit a Groundwater Assessment Plan (Work Plan) to the North Carolina Department of Environment and Natural Resources (NCDENR) no later December 31, 2014 and a Groundwater Assessment Report (herein referred to as a Comprehensive Site Assessment (CSA)) no later than 180 days following approval of the Work Plan. Duke Energy Carolinas, LLC (Duke Energy) submitted a Work Plan to NCDENR for assessment and characterization of the Riverbend Steam Station (RSS) ash basin, ash storage, and cinder storage areas on December 30, 2014. The Work Plan was subsequently conditionally approved by the NCDENR in correspondence dated February 19, 2015. This CSA Report was prepared to comply with the CAMA and is submitted to NCDENR within the allotted 180 day timeframe. Data generated during the CSA will be used in development of the Corrective Action Plan (CAP), which is due 90 days after submittal of the CSA. The purpose of this CSA is to characterize the extent of contamination resulting from historical production and storage of coal ash, evaluate the chemical and physical characteristics of the contaminants, investigate the geology and hydrogeology of the site including factors relating to contaminant transport, and examine risk to potential receptors and exposure pathways. This CSA was prepared in general accordance with requirements outlined in the following regulations and documents:  Classifications and Water Quality Standards Applicable to the Groundwaters of North Carolina in Title 15A NCAC 02L .0106(g),  Coal Ash Management Act in G.S. 130A-309.209(a),  Notice of Regulatory Requirements (NORR) issued by NCDENR on August 13, 2014,  Conditional Approval of Revised Groundwater Assessment Work Plan issued by NCDENR on February 16, 2015, and  Subsequent meetings and correspondence between Duke Energy and NCDENR. The assessment addresses the horizontal and vertical extent of contamination in soil, groundwater, and surface water. If a constituent1 concentration exceeded the North Carolina Groundwater Quality Standards, as specified in T15A NCAC .0202L (2L Standards) or Interim Maximum Allowable Concentration (IMAC)2, it has been designated as a “Constituent of Interest” (COI). Some COIs (e.g., iron and manganese) are also present in background monitoring wells and thus require careful examination to determine whether their presence 1 Constituents are elements, chemicals, or compounds that were identified in the approved Work Plan for sampling and analysis, and include antimony, arsenic, boron, chromium, cobalt, iron, manganese, selenium, thallium, vanadium, sulfate, and total dissolved solids (TDS). 2 Appendix #1 of 15A NCAC Subchapter 02L Classifications and Water Quality Standards Applicable to The Groundwaters of North Carolina, lists Interim Maximum Allowable Concentrations (IMACs). The IMACs were issued in 2010 and 2011; however, NCDENR has not established a 2L Standard for these constituents as described in 15A NCAC 02L.0202(c). For this reason, IMACs noted in this report are for reference only. Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report Riverbend Steam Station Ash Basin EXECUTIVE SUMMARY ES-2 downgradient of the ash basin or ash storage areas is naturally occurring or a result of ash handling and storage. In addition to evaluating the distribution of constituents across the RBSS site, significant factors affecting constituent transport, and the geological and hydrogeological features influencing the migration and chemical and physical character of the COIs were also evaluated. The assessment consisted of the following activities:  Completion of soil and rock borings and installation of groundwater monitoring wells to facilitate collection and analysis of chemical, physical, and hydrogeological parameters of subsurface materials encountered within and beyond the waste and compliance boundaries;  Evaluation of testing data to supplement the Conceptual Site Model (SCM);  Revision to the Receptor Survey previously completed in 2014; and  Completion of a Screening-level Risk Assessment. Based on scientific evaluation of historical and new data obtained during completion of the above-referenced activities, the following conclusions can be drawn:  No imminent hazard to human health or the environment has been identified as a result of groundwater migration from the ash basin or ash storage areas.  Upgradient, background monitoring wells contain naturally occurring metals and other constituents at concentrations that exceeded their respective 2L Standards or IMACs. This information is used to evaluate whether concentrations in groundwater downgradient of the ash basin and ash storage areas are also naturally occurring or might be influenced by migration of constituents from the ash basin and ash storage areas. Examples of naturally occuring metals and consituents include cobalt, iron, manganese, and vanadium. These constituents were detected in background groundwater samples at concentrations greater than 2L Standards or IMACs.  Under the RBSS ash basin shallow groundwater flows to the north, east and west and discharges to the Catawba River. Groundwater in the bedrock flows in a northern to northeasterly direction from the southern (upgradient) area of the site to the Catawba River. This flow direction is away from the direction of the nearest public or private water supply wells. The Catawba River serves as a hydrologic boundary for shallow groundwater flow layera, prohibiting the shallow groundwater flow from the ash basin to properties north and east of the RBSS site.  There are no water supply wells located between the ash basin and the Catawba River.  Groundwater in the southwest portion of the site under the ash storage area flows to the northwest, under the cinder storage area, to the Catawba River.  The geological and hydrogeological features influencing the migration, chemical, and physical characteristics of contaminants are related to the Piedmont hydrogeologic system present at the site.  Regional groundwater flow in the vicinity of the RBSS is east toward the Catawba River.  Exceedances of the 2L Standards have been measured in samples from monitoring wells south of the ash storage area between the ash storage area and the property boundary; however, groundwater elevation measurements indicate that flow direction in this area is to the northwest. Residences located south of the ash basins and ash storage areas are served by municipal water. Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report Riverbend Steam Station Ash Basin EXECUTIVE SUMMARY ES-3  Ongoing NPDES surface water quality data for samples collected upstream and downstream of the ash basin within the Catawba River do not indicate that the ash storage onsite has resulted in increased constituent concentrations above the North Carolina Surface Water Quality Standards (2B Standards).  Boron and sulfate are the primary ash related constituents of interest identified in the groundwater at concentrations that exceed the background concentrations and 2L Standards. The constituents are detected above the 2L Standards beneath the ash basin and ash storage areas in the groundwater.  The horizontal migration of boron best represents the dominant flow and transport system. Boron is highly soluble and was identified by the USEPA as one of the leading indicators for releases of contaminants from ash. Because of these characteristics, boron can be used to represent the general extent of the groundwater impacted by the ash basin and ash storage areas.  USEPA has identified constituents for groundwater detection monitoring programs that can be used as indicators of groundwater contamination from CCR. Specifically boron and sulfate would be expected to migrate rapidly in groundwater, and that would provide early detection as to whether contaminants were migrating from the ash basin system. Figure ES-1 indicates the impacted groundwater in the shallow monitoring wells onsite as it relates to boron and sulfate.  Cobalt, iron, manganese and vanadium were the primary constituents detected in background wells and groundwater at concentrations that exceed 2L Standards. These constituents were detected above the 2L Standards beneath the ash basins and ash storage areas in the shallow aquifer.  Aluminum, lead, and zinc exceeded the 2B standards in the surface water sample (SW- 3) collected from the cinder storage area. Additionally, seep samples collected from surface water features north and east of the ash basin (S-4, S-6, S-7, S-8 and S-13) had aluminium, cobalt, copper, and lead in excess of the 2B standards, which can be naturally occurring in the groundwaters of the Piedmont Carolinas.  The CSA serves to characterize the horizontal and vertical extent of ash-related constituents and evaluate groundwater gradients which facilitate the development of the Site Conceptual Model. This then facilitates developemnt of the Corrective Action Plan, due 90 days after submittal of this CSA. Note that as required to by CAMA, Duke Energy has agreed to remove the ash in the ash basin and ash storage areas via excavation. Approximately 4.6 million tons of ash will be transported to permitted lined landfills in Homer, Georgia and Marshall Steam Station. The majority of ash at Riverbend is anticipated to be transported by rail to a lined clay mine reclamation project in central North Carolina, pending permitting and approval. Final removal of ash is anticipated to be completed no later than August 2019. ES.1 Source Information Duke Energy owns and operated the Riverbend Steam Station (RBSS), located near Mount Holly in Gaston County, North Carolina. RBSS began operation as a coal-fired generating station in 1929 and was subsequently retired in April 2013. Following initial station operation, coal ash residue from RBSS’s coal combustion process was deposited in a cinder storage area and other areas near the cinder storage area and coal pile on site. Following installation of Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report Riverbend Steam Station Ash Basin EXECUTIVE SUMMARY ES-4 precipitators and a wet sluicing system, coal ash residue was disposed of in the station’s ash basin system located adjacent to the station and Mountain Island Lake. Discharge from the RBSS ash basin is currently permitted by the North Carolina Department of Environment and Natural Resources (NCDENR) Division of Water Resources (DWR) under the National Pollutant Discharge Elimination System (NPDES) Permit NC0004961. The ash basin system at the plant was used to settle and retain ash generated from coal combustion at RBSS. The ash basin system is located adjacent to the Catawba River (Mountain Island Lake) and consists of a Primary Cell, a Secondary Cell, and associated embankments and outlet works. The ash basin cells are unlined and are in the process of being closed. An ash storage area also exists to the southwest and side-gradient to the Primary Cell and consists of ash relocated from the Primary Cell. In addition, a cinder storage area is located west and is down-gradient of the Primary Cell and consists of ash from station operations prior to the construction of the ash basin in 1957. The ash storage areas are unlined and have a vegetative soil cap. The ash basin was operated as an integral part of the site’s wastewater treatment system. During operation of the coal-fired units, the ash basin received permitted variable inflows of fly ash, bottom ash, pyrites, stormwater runoff (including runoff from the coal pile), cooling water, powerhouse floor drains, sanitary waste effluent, station yard drainage sump, and boiler chemical cleaning wastes. The coal ash was historically sluiced to the southwest corner of the Primary Cell on a variable basis (i.e., dependent on RBSS operations) via sluice pipes. The CSA found that exceedances of ash-related constituents in soil, groundwater, and surface water are likely the result of leaching from the coal ash contained in the ash basins and ash storage areas. However, some exceedances may be due in part to naturally occurring conditions based on a review of background groundwater quality data. ES.2 Initial Abatement and Emergency Response The coal-fired units at the plant have been decommissioned. The ash management areas are no longer in use. Duke is in the process of excavating the ash from the site. No imminent hazard to human health or the environment has been identified ES.3 Receptor Information The purpose of the receptor survey was to identify the exposure locations that are critical to be considered in the groundwater transport modeling and human health risk assessment. Duke Energy completed and submitted a receptor survey to NCDENR (HDR 2014a) in September 2014, and subsequently submitted to NCDENR a supplement to the receptor survey (HDR 2014b) in November 2014. The supplementary information was obtained from responses to water supply well survey questionnaires mailed to property owners within a 0.5-mile (2,640-foot) radius of the RBSS ash basin compliance boundary requesting information on the presence of water supply wells and well usage. Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report Riverbend Steam Station Ash Basin EXECUTIVE SUMMARY ES-5 The survey activities included contacting and/or reviewing the following agencies/records to identify public and private water supply sources, confirm the location of wells, and/or identify any wellhead protection areas located within a 0.5-mile radius of the RBSS ash basin compliance boundary:  NCDENR Division of Water Resources Public Water Supply Section’s (PWSS) most current Public Water Supply Water Sources GIS point data set;  NCDENR DWR Source Water Assessment Program (SWAP) online database for public water supply sources;  Environmental Data Resources (EDR) local/regional water agency records review;  Mecklenburg County’s Groundwater and Wastewater Services Well Information System online database;  Gaston County Environmental Health Department;  Charlotte-Mecklenburg Utilities Department (CMUD);  Mount Holly Public Utilities Department; and  USGS National Hydrography Dataset. The review of these records identified one private water supply well and several tributaries to Mountain Island Lake within a 0.5-mile radius of the ash basin compliance boundary. The water supply well is located across Mountain Island Lake from the ash basin system. No public water supply wells or wellhead protection areas were identified within a 0.5-mile radius of the ash basin compliance boundary. In addition, no water supply wells (including irrigation wells and unused wells) were identified within the ash basin potential area of interest. No information gathered as part of this assessment suggests that any water supply is impacted by the Riverbend ash basin system. As part of this CSA report, the previously completed Receptor Survey activities were updated based on the CSA Guidelines. The update included contacting and/or reviewing the agencies/records to identify public and private water supply sources identified in Section 4.1 and reviewing any questionnaires that were received after the submittal of the November 2014 supplement to the September 2014 receptor survey (i.e. questionnaires received after October 31, 2014). A summary of the receptor survey findings is provided below.  One reported private water supply well is located at a residence located northeast of RBSS within a 0.5-mile radius of the ash basin compliance boundary. This well is located across Mountain Island Lake in Mecklenburg County (Well 1).  No public water supply wells (including irrigation wells and unused wells) were identified within a 0.5-mile radius of the RBSS ash basin compliance boundary. According to Duke Energy, the two private water supply wells and one public water supply well previously identified on the RBSS property were properly abandoned in June 2015. Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report Riverbend Steam Station Ash Basin EXECUTIVE SUMMARY ES-6  No wellhead protection areas were identified within a 0.5-mile radius of the ash basin compliance boundary.  Several surface water features that flow toward Mountain Island Lake were identified within a 0.5-mile radius of the ash basin (Figure 4-5). Based on the returned water supply well questionnaires since October 31, 2014, no additional receptors were identified. ES.4 Sampling / Investigation Results ES.4.1 Background Findings MW-7D and MW-7SR are designated as part of the RBSS compliance groundwater monitoring program as background wells for comparison based on historical data. Analyses of groundwater samples collected from wells MW-7D/SR indicated that the following naturally occurring metals exceeded 2L Standards in background locations: chromium, iron, manganese and pH. As part of the CSA, Duke Energy installed additional nested wells (three shallow, three deep, and one bedrock monitoring well) to provide background soil and groundwater quality data. The background locations selected at locations not expected to have been impacted by site activities and would not be impacted by groundwater flow from areas potentially impacted by ash. The additional background wells are designated BG-1S/D, BG-2S/D/BR, BG-3S/D, and MW-7BR. Analyses of groundwater samples collected from these wells indicated that the following naturally occurring constituents exceeded 2L standards in background locations: antimony, boron, chromium, cobalt, iron, manganese, and vanadium. The results for all other constituents were reported below 2L Standards. A summary of COI, 2L Standard or IMAC values and ranges of exceedances is provided as follows. Constituent of Interest Groundwater 2L Standard or IMACs (µg/L) Background Wells Range of Results Antimony 1 <0.5 to 3.9 Chromium 10 0.25J+ to 27.5 Cobalt 1 <0.5 to 3 Iron 300 28J to 2,200 Manganese 50 <5 to 370 pH 6.5-8.5 5.0 to 5.9 Total Dissolved Solids (TDS) 500,000 < 25,000 to 1,180,000 Vanadium 0.3 0.35J to 29.9 ES.4.2 Nature and Extent of Contamination Soil and groundwater beneath the ash basin and ash storage areas (within the compliance boundary) have been impacted by ash handling and storage at the RBSS site as described in detail below. Concentrations of several constituents exceed their respective 2L Standards or IMACs in groundwater across the site. The extent of the exceedances is as noted below: Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report Riverbend Steam Station Ash Basin EXECUTIVE SUMMARY ES-7  Antimony concentrations in shallow monitoring wells exceeded the IMAC in wells associated with the ash basin, in GWA-22S located south of the ash storage area, and in BG-3S, located east of the ash basin compliance boundary near Mountain Island Lake. Antimony concentrations in the deep monitoring wells exceeded the IMAC in wells associated with the ash storage area; the ash basins; GWA-1D and GWA-10D, located north of the ash basin Secondary Cell; MW-9D, located north of the cinder storage area; and BG-1D, located east of the ash basin compliance boundary. Antimony concentrations in the bedrock monitoring wells exceeded the IMAC on the ash basin Primary Dam (AB-6BRU), west of the ash basin Primary Cell and north of the cinder storage area (MW-9BR), and GWA-9BR located immediately east of the ash basin Secondary Cell.  Arsenic was not reported above 2L Standards in the shallow or deep monitoring wells. Arsenic concentrations exceeded the 2L Standards in bedrock monitoring well GWA- 9BR, located east of the ash basin Secondary Cell.  Boron concentrations in shallow monitoring wells exceeded 2L Standards in monitoring well AS-1S, located within the ash storage area. No other exceedances of boron were reported in the deep or bedrock monitoring wells.  Chromium concentrations in shallow monitoring wells exceeded 2L Standards in monitoring well AS-2S and GWA-20S, located in and adjacent to the ash storage area, and in monitoring well GWA-1S, located northwest of the ash basin Secondary Cell. Chromium concentrations exceeding the 2L Standards were reported in deep monitoring wells located beneath the ash basin, north of the cinder storage area, and in the newly installed background well results. Chromium exceedances of the 2L Standards were reported in the monitoring wells to the south of the ash storage area, northwest of the ash basin Primary Cell, and in background well location MW-7BR.  Cobalt concentrations in shallow monitoring wells exceeded the IMAC throughout the majority of the site, including results at newly installed background well BG-1S. Cobalt concentrations in deep monitoring wells exceeded the IMAC in monitoring wells AB-1D, associated with ash basin Secondary Cell; GWA-3D, located northwest of the cinder storage area; GWA-20D, GWA-22D, and MW-8D, located south of the ash storage area; and newly installed background well BG-1D, located east of the ash basin compliance boundary. Cobalt concentrations in deep monitoring wells exceeded the IMAC in monitoring wells AB-6BRU and AB-3BR, beneath the ash basin.  Iron concentrations in shallow monitoring wells exceeded 2L Standards throughout the majority of the site, including results from newly installed background well BG-1S; however, the dissolved phase iron result in BG-1S was below the laboratory reporting limit. Iron concentrations in the deep monitoring wells exceeded 2L Standards in monitoring wells AB-1D, associated with ash basin Secondary Cell; GWA-20D, GWA- 22D, and MW-8D, located south of the ash storage area; and GWA-8D and GWA-7D. Iron concentrations also exceeded the 2L Standards in newly installed deep background wells BG-1D and BG-3D, located east of the ash basin compliance boundary; however, the dissolved phase iron results in the background groundwater samples were reported as less than the laboratory reporting limit. Iron concentrations in the bedrock monitoring wells exceeded 2L Standards in monitoring wells AB-6BRU and AB-3BR, associated Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report Riverbend Steam Station Ash Basin EXECUTIVE SUMMARY ES-8 with the ash basin; GWA-9BR located east of the ash basin Secondary Cell; and newly installed background well BG-2BR; however, the dissolved phase iron results in the background groundwater sample was reported as less than the laboratory reporting limit. The dissolved iron concentrations varied significantly from the totals concentrations for the majority of the groundwater samples collected. Dissolved iron concentrations in the shallow monitoring wells exceeded the 2L Standards in AS-2S located in the ash storage area; MW-13 located adjacent to Mountain Island Lake northeast of the ash basin Secondary Cell; and MW-1S located at the western toe of the ash basin Primary Cell. All other dissolved iron concentrations (where data is available) in the shallow, deep, and bedrock wells were below the 2L Standards.  Manganese concentrations in the shallow monitoring wells exceeded 2L Standards throughout the majority of the site, including BG-1S, BG-2S and BG-3S. Manganese concentrations in deep monitoring wells exceeded 2L Standards in monitoring wells AB- 8D, associated with the ash basin Primary Cell; MW-1D, and GWA-3D, located west of ash basin Primary Cell; GWA-9D and GWA-8D located east of the ash basin Secondary Cell, BG-3D, located east of the ash basin compliance boundary; and GWA-22D, located south of the ash storage area. Manganese concentrations in the bedrock monitoring wells exceeded 2L Standards in AB-3BR and in GWA-2BRU located northwest of the ash Primary Cell. The dissolved phase results for manganese in these wells were below the laboratory reporting limits.  Sulfate concentrations in the shallow monitoring wells exceeded 2L Standards in monitoring well GWA-3SA located northwest of the cinder storage area. Sulfate concentrations in the deep monitoring wells exceeded 2L Standards in monitoring wells GWA-3D, located northwest of the cinder storage area, and GWA-20D, located south of the ash storage area. Sulfate did not exceed 2L Standards in the bedrock monitoring wells.  Thallium was not reported above 2L Standards in shallow monitoring wells. Thallium concentrations in the deep monitoring wells exceeded 2L Standards in monitoring well GWA-20D, located south of the ash storage area. Thallium was not reported above 2L Standards in the bedrock wells.  TDS concentrations in the shallow monitoring wells exceeded 2L Standards in monitoring wells AS-1S, located in the western portion of the ash storage area; and GWA-3SA, located northwest of the cinder storage area. TDS concentrations in the deep monitoring wells exceeded 2L Standards in monitoring wells AB-3D, located in the ash basin; GWA-20D, located south of the ash storage area; GWA-3D, located northwest of the cinder storage area; and BG-1D, located in the background location east of the ash basin compliance boundary. TDS concentrations in the bedrock monitoring wells exceeded 2L Standards in monitoring well GWA-2BR, located west of the ash basin Primary Cell; GWA-23BR, located south of the ash storage area; MW-7BR located southeast of the ash basin; and GWA-4BR located southwest of the ash storage area.  Vanadium concentrations exceeded 2L Standards in all of the shallow, deep, and bedrock monitoring wells, including all background monitoring wells. The relative concentrations of vanadium are generally higher in the deep and bedrock wells than in Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report Riverbend Steam Station Ash Basin EXECUTIVE SUMMARY ES-9 the shallow wells. The vanadium method reporting limit provided by the analytical laboratory was 1.0 ug/L. The IMAC for vandium is 0.3 ug/L. The vanadium results reported at concentrations less than the laboratory method reporting limit are estimated. During subsequent monitoring events, a laboratory method reporting equal to or less than the IMAC should be utilized. ES.4.3 Maximum Contaminant Concentrations The maximum contaminant concentrations reported in groundwater, ash porewater, seep water, and ash basin surface water samples collected during the CSA are listed below. COI Maximum Constituent of Interest (COI) Concentrations Groundwater (µg/L) Ash Porewater (µg/L) Seep Water (µg/L) Ash Basin Surface Water (µg/L) Aluminum 7,120 6,370 330 310 Antimony 18.2 21.6 0.5U 0.2J Arsenic 10.7 651 0.5U 2.4 Barium 370 2,400 69 100 Beryllium 1.9 0.4 0.2U 0.1J Boron 2,200 2,400 490 530 Cadmium 0.21 0.1 0.08U 0.32 Chromium 903 4.3 1.8 10.4 Cobalt 66.8 102 46.1 11.7 Copper 68 J+ 8.2 2.2 35.9 Iron 30,800 175,000 4600 2,200 Lead 7.5 1.7 0.52 1.1 Manganese 12,700 3,200 3100 4,300 Nickel 61.3 30.8 7.1 10.5 Selenium 15.4 4 0.5U 35.4 Sulfate 1,420,000 559,000 89300 119,000 TDS 23,000,000 1,110,000 195000 33,200 Thallium 3.2 0.48 0.1U 0.22 Vanadium 40.5 231 3.2 2.8 Zinc 200 J 91 10U 750 Notes: 1. N/A indicates that a constituent was not detected above the reporting detected limit. 2. J indicates an estimated concentration. 3. J+ indicates an estimated concentration, biased high. ES.4.4 Source Characterization Source characterization was performed through the completion of borings and installation of groundwater monitoring wells within the footprint of the ash basin cells and ash and cinder storage areas, and associated solid matrix (ash) and aqueous sample (ash pore water) collection and analysis. Ash samples were collected for analysis of physical characteristics (e.g., grain size, porosity) to provide data for evaluation of retention/transport properties within and beneath the ash basin and ash storage areas. Ash samples were collected for analysis of chemical characteristics (e.g., total inorganics, leaching potential). The results of the characterization will be used to refine the site conceptual model and to provide data for use in the CAP. Ash porewater refers to water samples collected from wells installed within the ash basins or ash storage area screened in the ash layer. HDR does not consider ash porewater results to be representative of groundwater. Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report Riverbend Steam Station Ash Basin EXECUTIVE SUMMARY ES-10 Review of laboratory analytical results of ash samples collected from the ash basin and ash storage areas identified eight COIs which include antimony, arsenic, boron, cobalt, iron, manganese, selenium and vanadium within the ash basin boundary. COIs identified in porewater in the ash basin include antimony, arsenic, boron, cobalt, iron, manganese, thallium vanadium and TDS. COIs identified in surface water include aluminum, antimony, arsenic, beryllium, cadmium, chromium, cobalt, copper, lead, nickel, thallium and zinc. SPLP (Synthetic Precipitation Leaching Procedure) testing was conducted to evaluate the leaching potential of COIs from ash. Although SPLP analytical results are being compared to the 2L Standards and IMACs, these samples do not represent groundwater samples. The results of SPLP analyses indicated that the following COIs exceeded their 2L Standards: antimony, arsenic, chromium, cobalt, iron, lead, manganese, nitrate, selenium, thallium, and vanadium. However, many factors influence the transport of these COIs and any potential impacts to groundwater over time will be investigated through modeling as part of the CAP. Four seeps (S-2, S-5, S-9 and S-11) were identified to be associated with the ash basin at RBSS. Constituents identified in seeps include aluminum, antimony, chromium, cobalt, copper, iron, manganese, selenium and vanadium, but some of these COIs are naturally occurring. ES.4.6 Regional Geology and Hydrogeology The RBSS site is located within the Charlotte terrane, one of a number of tectonostratigraphic terranes that have been defined in the southern and central Appalachians and is in the western portion of the larger Carolina superterrane (Figure 5-1; Horton et al. 1989; Hibbard et al. 2002; Hatcher et al. 2007). On the northwest side, the Charlotte terrane is in contact with the Inner Piedmont zone along the Central Piedmont suture along its northwest boundary and is distinguished from the Carolina terrane to the southeast by its higher metamorphic grade and portions of the boundary may be tectonic (Secor et al. 1998; Dennis et al. 2000). The Charlotte terrane is dominated by complex sequence of plutonic rocks that intrude a suite of metaigneous rocks (amphibolite metamorphic grade) including mafic gneisses, amphibolites, metagabbros, and metavolcanic rocks with lesser amounts of granitic gneiss and ultramafic rocks with minor metasedimentary rocks including phyllite, mica schist, biotite gneiss, and quartzite with marble along its western portion (Butler and Secor 1991; Hibbard et al. 2002). The general structure of the belt is primarily a function of plutonic contacts. The fractured bedrock is overlain by a mantle of unconsolidated material known as regolith. The regolith includes residual soil and saprolite zones and, where present, alluvial deposits. Saprolite, the product of chemical weathering of the underlying bedrock, is typically composed of clay and coarser granular material and reflects the texture and structure of the rock from which it was formed. The groundwater system is a two-medium system restricted to the local drainage basin. The groundwater occurs in a system composed of two interconnected layers: residual soil/saprolite and weathered rock overlying fractured crystalline rock separated by the transition zone. Typically, the residual soil/saprolite is partially saturated and the water table fluctuates within it. Water movement is generally preferential through the TZ (i.e., enhanced permeability zone). Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report Riverbend Steam Station Ash Basin EXECUTIVE SUMMARY ES-11 The near-surface fractured crystalline rocks can form extensive aquifers. The character of such aquifers results from the combined effects of the rock type, fracture system, topography, and weathering. Topography exerts an influence on both weathering and the opening of fractures, while the weathering of the crystalline rock modifies both transmissive and storage characteristics. ES.4.7 Site Geology and Hydrogeology The RBSS site and its associated ash basin, ash storage area, and cinder storage area are located in the Charlotte terrane. The Charlotte terrane consists of an igneous complex of Neoproterozoic to Paleozoic ages (Hibbard et al. 2002) that range from intermediate to mafic in composition (Butler and Secor 1999). The Charlotte terrane is bordered on the east and southeast by the Carolina terrane and to the west and northwest by the Inner Piedmont (Cat Square and Tugaloo terranes) and the Kings Mountain terrane. The structural contact of the Inner Piedmont and Charlotte terrane is the Central Piedmont Shear Zone. The most important effects of structural geology on groundwater flow are the contacts of the meta-diabase and the meta-quartz diorite, and the likely interconnected joint sets discussed in Sections 6.1.3 and 6.1.4. Since it is difficult to define joint sets based on dip angle alone, it is also difficult to define which joints are most relevant with respect to groundwater flow. Based on the CSA site investigation, the groundwater system in the natural materials (alluvium, soil, soil/saprolite, and bedrock) at RBSS is consistent with the regolith-fractured rock system and is an unconfined, connected system without confining layers. However, the hydraulic conductivity data collected during the investigation and discussed in Section 11.2 indicates that a distinct transition zone of higher permeability does not exist at the site. This is consistent with Harned and Daniel’s (1992) concept of the two types of rock structure (foliated/layered and massive) in the Piedmont province discussed in Section 5.2. The RBSS is underlain by a relatively massive meta-plutonic complex of the type that they believe may develop an indistinct transition zone. The groundwater system at RBSS is a two-layer system: shallow (regolith) and deep (bedrock). In general, groundwater at the site flows to the north, east, and west and discharges to the Catawba River. Groundwater in the southwest portion of the site under the ash storage area flows to the northwest, under the cinder storage area to the Catawba River. Flow contours developed from groundwater elevations measured in the shallow and deep wells in the southeastern portion of the site depict groundwater flow generally to the northeast to the Catawba River. Groundwater contours developed from the groundwater elevations in the bedrock wells show groundwater moving in a northeasterly direction from the south side of the site to the Catawba River. ES.4.8 Existing Groundwater Monitoring Data Groundwater monitoring prior to 2015 consisted of a voluntary groundwater monitoring program in 2006 with the installation of an initial set of twelve monitoring wells adjacent to the RBSS active ash basin. In December, 2008, Duke Energy implemented an expanded voluntary groundwater monitoring around the RBSS active ash basin until June 2010. During this period, Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report Riverbend Steam Station Ash Basin EXECUTIVE SUMMARY ES-12 the voluntary groundwater monitoring wells were sampled two times per year and the analytical results were submitted to NCDENR DWR. Compliance groundwater monitoring as required by the NPDES Permit began in December 2010. From December 2010 through June 2015, The compliance monitoring wells are sampled three times a year (February, June, and October) and 15 sampling events have been conducted to date. Background monitoring wells MW-7D and MW-7SR were installed in December 2006 and November 2010, respectively, as a part of the compliance monitoring program to evaluate background water quality at the site. New background monitoring well locations (BG) were identified based on the SCM at the time the Work Plan was submitted. Background monitoring wells include two existing compliance groundwater monitoring well (MW-7D and MW-7SR) and eight newly installed groundwater monitoring wells (MW-7BR which is located near MW-7D and MW-7SR, as well as BG-1S/D, BG-2S/D/BR, and BG-3S/D which is located near the eastern property boundary). Background groundwater monitoring wells are depicted in Section 10. Groundwater flow in the vicinity of MW-7D and MW-7SR is to the northeast toward Mountain Island Lake. Historical groundwater data dates back to December 2008 for MW-7D and December 2010 for MW-7SR. Newly installed background monitoring wells BG-1S/D, BG-2S/D/BR, BG-3S/D, and MW-7BR were installed to evaluate background water quality in the regolith and within the bedrock at the site. Groundwater flow in the vicinity of these monitoring wells is generally to the north or northeast towards Mountain Island Lake. Currently, insufficient data are available to qualify BG- 1S/D, BG-2S/D/BR, and BG-3S/D as background monitoring wells and provide associated statistical analysis. ES.4.9 Screening Level Risk Assessments The prescribed goal of the human health and ecological screening level risk assessments is to evaluate the analytical results from the COI sampling and analysis effort and using the various criteria taken from applicable guidance, determine which of the COIs may present an unacceptable risk, in what media, and therefore, should be carried through for further evaluation in a baseline human health or ecological risk assessment or other analysis, if required. Constituents of Probable Concern (COPCs) are those COIs that have been identified as having possible adverse effects on human or ecological receptors that may have exposure to the COPCs at or near the site. The COPCs serve as the foundation for further evaluation of potential risks to human and ecological receptors. To support the CSA effort and inform corrective action decisions, a screening level evaluation of potential risks to human health and the environment to identify preliminary, media-specific COPCs has been performed in accordance with applicable federal and state guidance, including the Guidelines for Performing Screening Level Ecological Risk Assessments within the North Carolina Division of Waste Management (NCDENR, 2003). The criteria for identifying COPCs vary by the type of receptor (human or ecological) and media, as shown in the comparison of Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report Riverbend Steam Station Ash Basin EXECUTIVE SUMMARY ES-13 contaminant concentrations in various media to corresponding risk-based screening levels presented in Tables 12-1 through 12-9. COIs were not screened out as COPCs based on a comparison to background concentrations, as NCDENR SLERA guidance does not allow for screening based on background. Site-specific background concentrations, discussed above in Section 12.1.3 will be considered in the uncertainty section of the baseline ecological risk assessment, if determined to be necessary. This initial screening, does not specifically identify that health or environmental risks are present, rather the results indicate constituents in the environmental media for further investigation by a site-specific risk assessment. It should be noted that the observed levels of certain COIs in the naturally occurring background at Riverbend would also warrant consideration of a BERA. ES.4.10 Development of Site Conceptual Model The human health and ecological risk assessment conceptual site models, illustrating potential pathways of exposure from source to receptors, are provided in this report. In the initial site conceptual hydrogeologic model presented in the Work Plan dated December 30, 2014, the geological and hydrogeological features influencing the migration, chemical, and physical characteristics of contaminants were related to the Piedmont hydrogeologic system present at the site. A hydrogeological site conceptual model was developed from data generated during previous assessments, existing groundwater monitoring data, and modified based on the results of the 2015 groundwater assessment activities. The CSA found the ash basin source areas discharge porewater to the subsurface beneath the basins and via seeps through the embankments. Groundwater flows in a generally northern, western, and easterly direction from the vicinity of the primary and secondary cells to Mountain Island Lake. ES.4.11 Identification of Data Gaps HDR has identified data gaps that will require further evaluation to refine the CSM through completion of additional groundwater assessment field activities and evaluation of data collected during those activities. The data gaps have been separated into two groups: 1) data gaps resulting from temporal constraints and 2) data gaps resulting from evaluation of data collected during the CSA. ES.4.11.1 Data Gaps Resulting from Temporal Constraints Temporal data gaps identified in this category are generally present due to insufficient time to collect, analyze, or evaluate data collected during the CSA activities. It is expected that the majority of these data gaps will be remedied in supplemental information to the CSA report to be submitted to NCDENR following completion of the second comprehensive groundwater sampling event.  Mineralogical characterization of soil and rock: a total of 17 soil, three TZ, and eight bedrock samples were submitted to three third-party mineralogical testing laboratories Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report Riverbend Steam Station Ash Basin EXECUTIVE SUMMARY ES-14 for analysis of soil and rock composition. As of the date of this report, Duke Energy has not received results of this testing; however, results should be available for inclusion in the CSA supplement.  Horizontal Delineation of Groundwater Contamination: as part of Work Plan development prior to field mobilization, Duke Energy reviewed existing groundwater quality data from compliance monitoring wells MW-8S/I/D to evaluate the potential for off-site migration of COIs and the potential need for addional on-site and off-site wells. This evaluation prompted the installation of groundwater monitoring wells GWA- 20S/D/BR, GWA-22S/D/BR, and GWA-23S/D/BR on the RBSS property south of the ash storage area, and GWA-21S/D/BR on the adjacent property to the south of MW-8S/I/D and south of Horseshoe Bend Beach Road, to better define groundwater flow in this area and the distribution of COIs . The sampling results from these wells was not received in time for the evaluation of the results to be incorporated in this report. This evaluation will be included in the submittal of the CSA supplement.  Additional Speciation Analyses: In order to meet the requirements of the NORR, Duke Energy conducted speciation of samples for arsenic, chromium, iron, manganese, and selenium along flow transects, at ash basin water sample locations, and at compliance wells with historical exceedances of the 2L Standards for speciation constituents. Duke Energy and NCDENR are currently conducting discussions concerning the specifics of the requirements for sampling associated with additional speciation sampling.  Groundwater analytical results from monitoring wells AS-3SA and GWA-4BR, and bedrock analytical results from boring GWA-21D, were not received with sufficient time to include the results in this report. These groundwater analytical results will be included in the CSA supplement. ES.4.11.2 Data Gaps Resulting from Review of Data Obtained During CSA Activities  Additional refinement is needed for the horizontal and vertical extent of groundwater impacts to the west of the ash and cinder storage areas near well GWA-3SA/D with sulfate, manganese, and TDS concentrations exceeding the 2L Standards reported at this location. Additional groundwater monitoring wells may be required to delineate exceedances in this area.  Additional refinement is needed for the horizontal and vertical extent of groundwater impacts to to the south of the ash storage area. Sulfate and TDS 2L Standard exceedances were reported in monitoring wells south of the ash storage area. However, groundwater flow direction in this location is from the south to the north/northwest. To address this data gap, monitoring wells GWA-21S/D/BR are currently being installed southeast of MW-8S/I/D and will refine the understanding of groundwater flow direction in this area, and provide information regarding potential concentrations of sulfate and TDS south of Horseshoe Bend Beach Road. The analytical results from GWA- 21S/D/BR will be included in the CSA supplement and a determination will be made at that time whether additional groundwater monitoring wells are needed south of the ash storage area. Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report Riverbend Steam Station Ash Basin EXECUTIVE SUMMARY ES-15  Groundwater samples were inadvertently not collected from compliance background monitoring wells MW-7D and MW-7SR. Although historical analytical results are available for these wells, groundwater from these wells was not analyzed for the full list of parameters and constituents used during the assessment activities. Monitoring wells MW-7D and MW-7SR should be sampled and analyzed along for the same parameter and constitiuent list as the assessment wells onsite during any future monitoring events.  The vanadium method reporting limit provided by the analytical laboratory was 1.0 ug/L. The IMAC for vandium is 0.3 ug/L. The vanadium results reported at concentrations less than the laboratory method reporting limit are estimated. During subsequent monitoring events, a laboratory method reporting equal to or less than the IMAC should be utilized.  Newly installed monitoring wells GWA-9BR and C-1BRU, and voluntary monitoring wells MW-2S and MW-4S were noted as dry at the time of the sampling event. A groundwater sample was not able to be collected from these well. An attempt should be made to collect a groundwater sample from this well during subsequent monitoring events. If the well remains dry NCDENR will be contacted regarding the potential replacement of the well. Review of Non-Ash Contamination Information: Review of information regarding areas of non-ash contamination (i.e., petroleum-contaminated areas) is needed to evaluate potential interferences with possible future remedial actions, if applicable. ES.5 Conclusions The CSA identified the horizontal and vertical extent of groundwater contamination within the compliance boundary at RBSS, and found that the source and cause of the contamination within that boundary is the coal ash contained in the ash basin and ash storage areas. The cause of contamination is leaching of constituents from the coal ash into the underlying soil and groundwater. Background monitoring wells contained naturally occurring metals and other constituents at concentrations that exceeded their respective 2L Standards or IMAC. These included antimony, chromium, cobalt, iron, manganese, and vanadium. The presence of these metals are used to evaluate if concentrations detected downgradient of the ash basin and ash storage areas are naturally occurring. The CSA identified arsenic, cobalt, iron, manganese, selenium thallium and vanadium as soil COIs. Groundwater COIs were identified as antimony, arsenic, boron, chromium, cobalt, iron, lead, manganese, sulfate, thallium, TDS, and vanadium. Antimony, cobalt, chromium, iron, manganese, and vanadium are constituents that may be naturally occurring in regional groundwater. The CSA identified the horizontal and vertical extent of soil contamination, with exception of off-site areas east and north of Ash Storage 1 (as described in Section 14.1.1). Migration of each contaminant is related to the groundwater flow direction, the groundwater flow velocity, and the rate at which a particular contaminant reacts with materials in the aquifer. The data confirm that geologic conditions present beneath the ash basins impedes the vertical migration of contaminants. The CSA found that the direction of mobile contaminant transport is Duke Energy Carolinas, LLC | Comprehensive Site Assessment Report Riverbend Steam Station Ash Basin EXECUTIVE SUMMARY ES-16 generally in north, west, and east direction towards the Catawba River, as anticipated, and not towards other off-site receptors. The human health and ecological screening-level risk assessments did not specifically identify the presence of health or environmental risks; however, the results indicate that constituents in environmental media could be of concern and further investigation by a site-specific risk assessment may be warranted. No imminent hazards to human health and the environment were identified as a result of the assessment. Duke Energy is required per CAMA and has committed to removing the ash in the ash basin and ash storage areas via excavation. In conjunction with decommissioning activities and in accordance with CAMA requirements, Duke Energy will permanently close the RBSS ash ponds by August 1, 2019. Closure of the RBSS ash ponds was defined in CAMA as excavation of ash from the site, and beneficial reuse of the material or relocation to a lined structural fill or landfill. As part of the RBSS closure process, Duke Energy submitted a coal ash excavation plan to NCDENR in November 2014. The excavation plan detailed a multiphase approach for removing coal ash from the site with an emphasis on the first 12 to 18 months of activities. Based on the results of soil and groundwater samples collected beneath the ash basins and the ash storage areas, some residual contamination will remain after excavation (if required); however, the degree of contamination and the persistence of this contamination over time cannot be determined at this time. Duke Energy will pursue corrective action under 15A NCAC 02L .0106. The approaches to corrective action under rule .0106(k) or (l) will be evaluated along with other remedies depending on the results of groundwater modeling and evaluation of the site’s suitability to use Monitored Natural Attenuation or other industry-accepted methodologies. .