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Home My WebLink AboutNC0004961_CSA October 2017 Final_201710312017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx Explanation of altered or items not initialed: Item 1. The CSA was specifically designed to assess the coal ash management areas of the facility. Sufficient information is available to prepare the groundwater corrective action plan for the ash management areas of the facility. Data limitations are discussed in Section 11.2 of the CSA report. Continued groundwater monitoring at the Site is planned. Item 2. Imminent hazards to human health and the environment have been evaluated. The NCDEQ data associated with nearby water supply wells is provided herein and is being evaluated. Item 5. The groundwater assessment plan for the CSA as approved by NCDEQ was specifically developed to assess the coal ash management areas of the facility for the purposes of developing a corrective action plan for groundwater. Other areas of possible contamination on the property, if noted, are anticipated to be evaluated separately. 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx Work Performed by Others • HDR Engineering, Inc. (HDR) of the Carolinas prepared the reports referenced herein under contract to Duke Energy. • The reports were sealed by licensed geologists or engineers as required by the North Carolina Board for Licensing of Geologists or Board of Examiners for Engineers and Surveyors. • The evaluations of hydrogeologic conditions and other information provided in this updated Comprehensive Site Assessment (CSA) are based in part on the work contained in the HDR documents and on sampling activities performed by Pace Analytical Services after the submittal of the HDR documents. The evaluations described in this paragraph meet requirements detailed in 15A NCAC 02L .0106(g). • SynTerra relied on information from the HDR reports as being correct. SynTerra has proofread boring logs; monitoring well installation records; and data tables presenting chemical, physical, and hydraulic properties of ash, soil, rock, groundwater, and surface water, and has made corrections where mistakes were found. SynTerra did not perform additional validation activities concerning the HDR reports. SynTerra has found no reason to question geological interpretations of site hydrostratigraphic information and other information in the HDR reports. • The seal of the licensed geologist for this CSA applies to activities conducted and interpretations derived after the HDR reports were submitted. This submittal relies on the professional work performed by HDR and references that work. 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page ES-1 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx EXECUTIVE SUMMARY ES.1 Source Information Duke Energy Carolinas, LLC owns and operated the Riverbend Steam Station (RBSS), located near Mount Holly in Gaston County, North Carolina. Coal ash was historically generated and managed on the site. The Comprehensive Site Assessment (CSA) update was conducted to refine and expand the understanding of subsurface geologic/hydrogeologic conditions and evaluate the extent of impacts from historical management of coal ash. This CSA update contains an assessment of site conditions based on a comprehensive interpretation of geologic and sampling results from the initial site assessment, and geologic and sampling results obtained subsequent to the initial assessment. In accordance with the Coal Ash Management Act (CAMA), the coal ash at RBSS is currently being excavated and transported to off-site permanent storage solutions. RBSS began operation as a coal-fired generating station in 1929 and was retired in April 2013. During initial station operation, coal combustion residuals (CCR) from RBSS’s coal combustion process were deposited in a cinder storage area. Following installation of precipitators and a wet sluicing system, CCR was disposed of in the station’s ash basin system located adjacent to the station and the Catawba River (locally Mountain Island Lake). Discharge from the RBSS ash basin is currently permitted by the North Carolina Department of Environmental Quality (NCDEQ) 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 CCR at RBSS. The ash basin system consists of a primary cell, a secondary cell, and associated embankments and outlet works. An intermediate dam, constructed over ash deposited in the original single-cell ash basin, separated the primary and secondary cells until it was decommissioned in 2017 as part of ash removal activities. The ash basin cells are unlined. An ash storage area also existed to the southwest and sidegradient to the primary cell and consisted of ash relocated from the primary cell. A cinder storage area is located west and sidegradient of the primary cell and consists of cinders from station operations prior to construction of the ash basin in 1957. The ash storage and the cinder storage areas are unlined. The ash storage area was managed with a vegetative soil cap until commencement of ash removal activities that are ongoing. Assessment findings from the CSA determined that CCR accumulated in the ash basin, ash storage and cinder storage areas are the primary source of CCR influence in 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page ES-2 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx groundwater. The inferred general extent of constituent migration in groundwater based on evaluation of concentrations greater than both site background and groundwater quality standards is shown in Figure ES-1. A detailed evaluation of constituent migration is included in the CSA update report. 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. Coal ash from the site is in the process of being transported to off-site permanent storage solutions. Prior to commencement of rail operations for off-site ash transport, RBSS ash was transported by truck to the lined R&B Landfill in Homer, GA, to Duke Energy lined industrial landfills at Marshall Steam Station in Mooresville, NC, and to a lined structural fill at the former Brickhaven Mine (Brickhaven) in Chatham County, NC. The majority of excavated RBSS ash has been transported to Brickhaven, and all RBSS ash is currently being transported by rail to Brickhaven. This will allow ash, a valuable construction material, to replace soil in order to reclaim land that is currently unusable. Ash excavation and relocation began in 2015 and is anticipated to be complete by August 2019. ES.3 Receptor Information In accordance with NCDEQ direction, CSA receptor survey activities include listing and depicting all water supply wells (both public and private, including irrigation wells and unused wells) within a 0.5-mile radius of the ash basin compliance boundary. Results of the receptor survey indicate that the RBSS site and nearby developed properties are provided water service by Mount Holly Public Utilities Department. Mount Holly, as well as the Charlotte Metropolitan area and Gastonia, obtain drinking water from Mountain Island Lake. Water supply intakes are located approximately 3.4 miles (Charlotte) and 6.9 miles (Mount Holly and Gastonia) downstream from the RBSS site. ES.3.1 Public Water Supply Wells No public water supply wells (including irrigation wells and unused wells) or wellhead protection areas were identified within a 0.5-mile radius of the RBSS ash basin compliance boundary. 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page ES-3 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx ES.3.2 Private Water Supply Wells 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 hydraulically separated from the RBSS site by the Catawba River. ES.3.3 Surface Water Bodies RBSS is bordered by Mountain Island Lake, a part of the Catawba River system. Several surface water features that flow toward the Catawba River were identified within a 0.5-mile radius of the ash basin. The Mountain Island Lake section of the Catawba River is used for municipal drinking water supply. The nearest intake is located approximately 3.4 miles downstream from the RBSS site. ES.3.4 Human and Ecological Receptors A baseline human health and ecological risk assessment was performed in 2016 as a component of Corrective Action Plan (CAP) Part 2 (HDR, 2016a), concluding that potential risk existed from consumption of fish under hypothetical recreational and subsistence fisher scenarios from the off-site Mountain Island Lake portion of the Catawba River. Concentrations of cadmium, selenium, thallium, and zinc were estimated in fish tissue from conservative uptake models using on-site surface water and bioaccumulation factors, which overestimate risks. Concentrations of these constituents measured since conducting the risk assessment do not indicate that risks under the hypothetical fisher exposure scenario have increased. Additionally, potential risks were identified in 2016 to waterfowl exposed to selenium detected in surface water from the intake canal southwest of the ash basin. The constituents identified as associated with the intake canal do not appear to be attributable to groundwater migration from the ash management areas. One residential drinking water well was identified across the Catawba River, and to the north of RBSS. The Catawba River is considered a main stem river and hydraulically separates RBSS from properties to the north. Based on review and analysis of groundwater and surface water data collected since completing the human health and ecological risk assessment in 2016, there is no evidence of risks associated with groundwater migrating from the ash management areas at RBSS. 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page ES-4 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx ES.3.5 Land Use The land use in the area of RBSS is a mix of industrial, rural and residential properties. The ash basin system at RBSS is currently being excavated and closed. Other than this activity, no significant changes to land use in the area are anticipated. ES.4 Sampling/Investigation Results The CSA included evaluations of the hydrogeological and geochemical properties of soil and groundwater at multiple depths and distances from the ash management areas. ES.4.1 Background Concentration Determinations Naturally occurring background concentrations were determined using statistical analysis of inorganic constituents in soil and groundwater. Statistical determinations of provisional background threshold values (PBTVs) were performed in strict accordance with the revised Statistical Methods for Developing Reference Background Concentrations for Groundwater and Soil at Coal Ash Facilities (statistical methods document) (HDR and SynTerra, May 2017). The current background monitoring well network consists of wells installed within three flow zones – shallow, deep, and bedrock. Background datasets for each flow system used to statistically determine naturally occurring concentrations of inorganic constituents in soil and groundwater are provided herein. As of September 1, 2017, NCDEQ approved a number of the statistically derived background values; however, others are still under evaluation and thus considered preliminary at this time. Background results may be greater than the PBTVs due to the limited valid dataset currently available. The statistically derived background threshold values will continue to be adjusted as additional data become available. ES.4.2 Nature and Extent of Contamination Site-specific groundwater constituents of interest (COIs) were developed by evaluating groundwater sampling results with respect to 2L/IMACs and PBTVs, and additional regulatory input/requirements. The distribution of constituents in relation to the ash management areas, co-occurrence with CCR indicator constituents such as boron and sulfate, and likely migration directions based on groundwater flow direction are considered in determination of groundwater COIs. 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page ES-5 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx The following list of groundwater COIs has been developed for RBSS: Antimony Iron Arsenic Manganese Beryllium Strontium Boron Sulfate Chromium 1 Hexavalent Chromium Cobalt Total Dissolved Solids (TDS) Vanadium Concentrations of these constituents will be considered for the purpose of evaluating remedies in the CAP. Observations of COI occurrence based on groundwater sampling data from May 2017 considering 2L/IMAC and PBTVs indicate the following: Sampling performed in the cinder storage and former coal pile areas indicated elevated concentrations for beryllium, cobalt, iron, manganese, strontium, sulfate, and TDS. Boron concentrations were elevated above PBTVs within and downgradient of the ash management areas. The 2L for boron was exceeded in one sample from an upper bedrock well downgradient from the ash basin and inside the compliance boundary. Chromium concentrations greater than 2L and background were observed in the May 2017 groundwater sampling data in certain wells downgradient of the ash basin and upgradient of the ash storage area. However, most locations do not have a history of repeated elevated chromium detections, and the total metals (unfiltered) results are significantly higher than dissolved (filtered) results, indicating a less mobile particulate component creating an atypical sample result. The locations upgradient of the ash storage area do not historically indicate occurrence of boron or sulfate at levels that would reflect CCR influence. Therefore, the chromium concentrations are interpreted to be a result of naturally occurring conditions. Multiple areas indicated iron, manganese and vanadium concentrations in groundwater greater than background and 2L or IMAC values. Generally, 1 Unless otherwise noted, references to chromium in this document indicate total chromium. 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page ES-6 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx higher concentrations were detected downgradient of the cinder storage area and ash basin. Vanadium exceedances were widespread and do not always coincide with locations anticipated to be affected by CCR influence. For instance, elevated concentrations are observed in sidegradient areas where boron and sulfate concentrations are below background. Therefore, the elevated vanadium concentrations, while above the current PBTV, may be indicative of background groundwater quality. No exceedances of background, 2L or IMAC were observed for antimony, arsenic and hexavalent chromium for the May 2017 groundwater data set. Antimony and arsenic are retained as COIs based on historical observations and will continue to be evaluated. Figure ES-1 provides a conceptual representation of the site depicting the groundwater flow directions as well as the CCR influenced area. Elevated boron and sulfate concentrations best define the area of groundwater influenced by CCR. The shaded area incorporates the concentrations of boron and sulfate observed above their respective groundwater quality standards and demonstrates there are no receptors between the CCR influenced area and the Catawba River. ES.4.3 Maximum Contaminant Concentrations (Source Information) Wells monitoring ash porewater were installed within the ash basin and the cinder storage area. The ash stored at the ash storage area was managed at an elevation generally above the water table and no porewater wells were installed in that area. Due to ongoing coal ash removal activities, most ash porewater monitoring wells have been properly abandoned. The following summary of COI distribution in ash porewater is based on the most recent sample result available for ash porewater wells in the ash basin and in the cinder storage area. The location of maximum contaminant concentrations based on the most recent ash porewater samples are summarized below. Ash Basin – antimony, arsenic, boron, chromium, manganese, strontium, and vanadium Cinder Storage Area – beryllium, hexavalent chromium, cobalt, iron, sulfate, and TDS 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page ES-7 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx Soil samples collected from locations within the ash management areas indicate concentrations of arsenic, boron, chromium, cobalt, iron, manganese, nickel, and vanadium were greater than NCDEQ Protection of Groundwater Preliminary Soil Remediation Goals (POG PSRGs) and PBTVs. Nickel was limited to an exceedance in only one sample. Constituent concentrations in soil are considered as potential secondary sources and will be addressed as part of basin closure or evaluated in the CAP. ES.4.4 Site Geology and Hydrogeology The groundwater system in the natural materials (alluvium, soil, soil/saprolite, and bedrock) at RBSS is consistent with the Piedmont regolith-fractured rock system and is an unconfined, connected system without confining layers. The groundwater system at RBSS consists of a generally thick (greater than 100 feet) layer of regolith overlying bedrock. For discussion purposes in this report groundwater flow layers are divided into shallow (S wells), deep (D wells), and bedrock (BR and BRU wells). The shallow flow layer consists of surficial material such as soil, fill, and alluvium. The deep flow layer includes both saprolite and weathered rock which transitions to bedrock, while the bedrock layer is competent bedrock with limited fractures. Topography at the site slopes from south to north toward the Catawba River. Groundwater at the site follows the topographic gradient and flows northerly toward the Mountain Island Lake portion of the Catawba River. Horseshoe Bend Beach Road, present just south of RBSS, follows an east-west trending ridgeline which acts as a groundwater flow directional divide (Figure ES-1). Surface water samples collected in the Catawba River upstream and downstream of the site indicated no exceedances of the 2B surface water criteria or CCR influence in the Catawba River. ES.5 Conclusions and Recommendations The information provided in this CSA update presents the results of the assessments required by CAMA and 2L. Based on the data, the sources of the groundwater CCR influence include the ash management areas. The former coal pile area will be addressed separately. The assessment investigated the site geology and hydrogeology and determined the direction of groundwater flow from the ash management areas and determined the horizontal and vertical extent of impacts to groundwater and soil sufficient for preparation of the CAP. 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page ES-8 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx Areas of groundwater PBTV, 2L or IMAC exceedances of beryllium, boron, chromium, cobalt, iron, manganese, strontium, sulfate, TDS, and vanadium are beneath or immediately downgradient of the RBSS source areas. Ash removal from the source areas is ongoing and is anticipated to be completed by August 2019. Based on the receptor survey conducted in 2014 and updated in 2016, the nearest potential water well receptor, and the only one within one-half mile of the RBSS compliance boundary, is hydraulically separated from the ash management areas by the Catawba River, a main stem river. There are no well owners for whom permanent alternative water solutions are planned per North Carolina House Bill 630 requirements. Surface water sampling from the Catawba River does not indicate influence from the ash management areas. COI concentrations underneath and immediately adjacent to the ash management areas will be evaluated in more detail prior to the CAP as potential sources of impact to surface water in the Catawba River. A preliminary evaluation of groundwater corrective action alternatives is included in this CSA to provide insight into the CAP preparation process. For RBSS, the primary source control (closure) method has been determined to be dewatering and excavation of the ash, which is ongoing at this time. Groundwater corrective action by monitored natural attenuation (MNA) is anticipated to be a remedy further evaluated in the CAP. As warranted, a number of additional groundwater remediation technologies such as phytoremediation, groundwater extraction, migration barriers, in situ chemical immobilization, and permeable reactive barriers may be evaluated based upon short- term and long-term effectiveness, implementability, and cost. Results of the evaluation, including groundwater fate and transport modeling, and geochemical modeling, will be used for remedy selection in the CAP. RIVERBEND POWERHOUSE CATA W B A R I V E R ASH STORAGE CINDER STORAGE ASH BASIN 148 RIVER STREET, SUITE 220 GREENVILLE, SOUTH CAROLINA 29601 PHONE 864-421-9999 www.synterracorp.com PROJECT MANAGER: LAYOUT: DRAWN BY: JUDD MAHAN DATE:ADAM FEIGL ES1 - RBSS 10/09/2017 10/29/2017 2:38 PM P:\Duke Energy Progress.1026\00 GIS BASE DATA\Riverbend\MapDocs\CSA_Supplement_2\3D SCM\Riverbend_3D_ES1.dwg VISUAL AID ONLY - DEPICTION NOT TO SCALE CAROLINAS ASH BASIN WASTE BOUNDARY GENERALIZED GROUNDWATER FLOW DIRECTION NOTE: 1.OCTOBER, 2016 AERIAL PHOTOGRAPHY OBTAINED FROM GOOGLE EARTH PRO ON JULY 11, 2017, DATED OCTOBER 8, 2016. 2.STREAM FROM AMEC NRTR REPORT, 2015. 3.GENERALIZED GROUNDWATER FLOW DIRECTION BASED ON MAY 22, 2017 WATER LEVEL DATA. 4.PROPERTY BOUNDARY PROVIDED BY DUKE ENERGY. 5.GENERALIZED AREAL EXTENT OF MIGRATION REPRESENTED BY NCAC 02L EXCEEDANCES OF BORON AND SULFATE. APPROXIMATE STREAMS (NON-JURISIDICTIONAL) WITH FLOW DIRECTION DUKE ENERGY PROPERTY BOUNDARY LEGEND AREA OF CONCENTRATION IN GROUNDWATER ABOVE NC2L (SEE NOTE 5) NORTH HORSES H O E B E N D B E A C H R O A D SURFICIAL WEATHERED ROCK BEDROCK HORS E S H O E B E N D B E A C H R O A D FIGURE ES-1 APPROXIMATE EXTENT OF IMPACTS RIVERBEND STEAM STATION DUKE ENERGY CAROLINAS, LLC MOUNT HOLLY, NORTH CAROLINA 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page i P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx TABLE OF CONTENTS SECTION PAGE ES.1 SOURCE INFORMATION ..................................................................................... ES-1 ES.2 INITIAL ABATEMENT AND EMERGENCY RESPONSE .............................. ES-2 ES.3 RECEPTOR INFORMATION ................................................................................ ES-2 ES.3.1 Public Water Supply Wells ................................................................................... ES-2 ES.3.2 Private Water Supply Wells ................................................................................. ES-3 ES.3.3 Surface Water Bodies ............................................................................................. ES-3 ES.3.4 Human and Ecological Receptors ........................................................................ ES-3 ES.3.5 Land Use .................................................................................................................. ES-4 ES.4 SAMPLING/INVESTIGATION RESULTS ......................................................... ES-4 ES.4.1 Background Concentration Determinations ...................................................... ES-4 ES.4.2 Nature and Extent of Contamination .................................................................. ES-4 ES.4.3 Maximum Contaminant Concentrations (Source Information) ...................... ES-6 ES.4.4 Site Geology and Hydrogeology ......................................................................... ES-7 ES.5 CONCLUSIONS AND RECOMMENDATIONS ............................................... ES-7 1.0 INTRODUCTION ........................................................................................................ 1-1 1.1 Purpose of Comprehensive Site Assessment (CSA) ............................................ 1-1 1.2 Regulatory Background ........................................................................................... 1-2 1.2.1 Notice of Regulatory Requirements (NORR) ............................................... 1-2 1.2.2 Coal Ash Management Act Requirements .................................................... 1-2 1.3 Approach to Comprehensive Site Assessment ..................................................... 1-4 1.3.1 NORR Guidance ................................................................................................ 1-4 1.3.2 USEPA Monitored Natural Attenuation Tiered Approach ........................ 1-5 1.3.3 ASTM Conceptual Site Model Guidance ....................................................... 1-5 1.4 Technical Objectives ................................................................................................. 1-5 1.5 Previous Submittals .................................................................................................. 1-6 2.0 SITE HISTORY AND DESCRIPTION .................................................................... 2-1 2.1 Site Description, Ownership, and Use History..................................................... 2-1 2.2 Geographic Setting, Surrounding Land Use, Surface Water Classification ..... 2-2 2.3 CAMA-related Source Areas ................................................................................... 2-4 2.4 Other Primary and Secondary Sources .................................................................. 2-5 2.5 Summary of Permitted Activities ........................................................................... 2-5 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page ii P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx TABLE OF CONTENTS (CONTINUED) SECTION PAGE 2.6 History of Site Groundwater Monitoring .............................................................. 2-5 2.6.1 Ash Basin Voluntary Groundwater Monitoring .......................................... 2-6 2.6.2 Ash Basin NPDES Groundwater Monitoring ............................................... 2-6 2.6.3 Ash Basin CAMA Monitoring ......................................................................... 2-6 2.7 Summary of Assessment Activities ........................................................................ 2-7 2.8 Summary of Initial Abatement, Source Removal, or other Corrective Action 2-8 3.0 SOURCE CHARACTERISTICS ................................................................................ 3-1 3.1 Coal Combustion and Ash Handling System ....................................................... 3-1 3.2 General Physical and Chemical Properties of Ash............................................... 3-4 3.3 Site-Specific Coal Ash Data ..................................................................................... 3-6 4.0 RECEPTOR INFORMATION ................................................................................... 4-1 4.1 Summary of Receptor Survey Activities................................................................ 4-1 4.2 Summary of Receptor Survey Findings ................................................................. 4-3 4.2.1 Public Water Supply Wells .............................................................................. 4-4 4.2.2 Private Water Supply Wells ............................................................................ 4-4 4.3 Private and Public Well Water Testing Program ................................................. 4-5 4.4 Surface Water Drinking Water Sources ................................................................. 4-5 5.0 REGIONAL GEOLOGY AND HYDROGEOLOGY ............................................. 5-1 5.1 Regional Geology ...................................................................................................... 5-1 5.2 Regional Hydrogeology ........................................................................................... 5-2 6.0 SITE GEOLOGY AND HYDROGEOLOGY .......................................................... 6-1 6.1 Site Geology ............................................................................................................... 6-1 6.1.1 Soil Classification .............................................................................................. 6-1 6.1.2 Rock Lithology .................................................................................................. 6-3 6.1.3 Structural Geology ............................................................................................ 6-3 6.1.4 Soil and Rock Mineralogy and Chemistry .................................................... 6-4 6.2 Site Hydrogeology .................................................................................................... 6-4 6.2.1 Hydrostratigraphic Layer Development ....................................................... 6-5 6.3 Groundwater Flow Direction .................................................................................. 6-6 6.4 Hydraulic Gradient ................................................................................................... 6-6 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page iii P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx TABLE OF CONTENTS (CONTINUED) SECTION PAGE 6.5 Hydraulic Conductivity ........................................................................................... 6-7 6.6 Groundwater Velocity .............................................................................................. 6-8 6.7 Contaminant Velocity ............................................................................................... 6-8 6.8 Slug Test and Aquifer Test Results ........................................................................ 6-9 6.9 Fracture Trace Study Results ................................................................................. 6-12 6.9.1 Introduction ..................................................................................................... 6-12 6.9.2 Methods ............................................................................................................ 6-13 6.9.3 Results ............................................................................................................... 6-13 7.0 SOIL SAMPLING RESULTS ..................................................................................... 7-1 7.1 Background Soil Data ............................................................................................... 7-1 7.2 Facility Soil Data ....................................................................................................... 7-2 8.0 SEDIMENT RESULTS ................................................................................................ 8-1 8.1 Sediment (Surface Soil) Associated with AOWs .................................................. 8-1 8.2 Sediment in Major Bodies of Water ........................................................................ 8-3 8.3 Comparison of Exceedances to PSRGs and Background .................................... 8-3 9.0 SURFACE WATER RESULTS ................................................................................... 9-1 9.1 Comparison of Exceedances to 2B Standards ....................................................... 9-2 9.2 Discussion of Results for Constituents without Established 2B Standard ....... 9-3 9.3 Discussion of Surface Water Results ...................................................................... 9-4 10.0 GROUNDWATER SAMPLING RESULTS .......................................................... 10-1 10.1 Background Groundwater Concentrations ......................................................... 10-2 10.1.1 Background Dataset Statistical Analysis ..................................................... 10-3 10.1.2 Piper Diagrams (Comparison to Background) ........................................... 10-4 10.2 Downgradient Groundwater Concentrations..................................................... 10-5 10.2.1 Shallow Wells .................................................................................................. 10-5 10.2.2 Deep Wells ....................................................................................................... 10-6 10.2.3 Bedrock Wells .................................................................................................. 10-7 10.2.4 Piper Diagrams (Comparison with Downgradient Well Samples) ......... 10-7 10.2.5 Radiological Laboratory Testing ................................................................... 10-8 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page iv P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx TABLE OF CONTENTS (CONTINUED) SECTION PAGE 10.3 Site-Specific Exceedances (Groundwater COIs) ................................................. 10-9 10.3.1 Background Threshold Values (PBTVs) ...................................................... 10-9 10.3.2 Applicable Standards ..................................................................................... 10-9 10.3.3 Additional Requirements ............................................................................. 10-10 10.3.4 Riverbend COIs ............................................................................................. 10-11 11.0 HYDROGEOLOGICAL INVESTIGATION ........................................................ 11-1 11.1 Plume Physical and Chemical Characterization ................................................ 11-1 11.1.1 Chemical Characterization ............................................................................ 11-3 11.2 Pending Investigation(s) ...................................................................................... 11-16 12.0 RISK ASSESSMENT ................................................................................................. 12-1 12.1 Human Health Screening Summary .................................................................... 12-2 12.2 Ecological Screening Exposure ............................................................................. 12-3 12.3 Private Well Receptor Update ............................................................................... 12-4 12.4 Risk Assessment Update Summary ..................................................................... 12-4 13.0 GROUNDWATER AND GEOCHEMICAL MODELING ................................. 13-1 13.1 Summary of Fate and Transport Model Results................................................. 13-2 13.1.1 Flow Model Construction .............................................................................. 13-3 13.1.2 Transport Model Construction ..................................................................... 13-7 13.1.3 Summary of Flow and Transport Modeling Results to Date .................... 13-9 13.2 Summary of Geochemical Model Results ......................................................... 13-11 13.2.1 Model Construction ...................................................................................... 13-11 13.2.2 Summary of Geochemical Model Results to Date .................................... 13-14 13.3 Summary of Groundwater to Surface Water Evaluation ................................ 13-14 13.3.1 CAP 1 and 2 Surface Water Mixing Model Approach ............................ 13-15 13.3.2 Surface Water Quality .................................................................................. 13-15 13.3.3 Evaluation of Groundwater to Surface Water Conditions for Corrective Action .......................................................................................................................... 13-15 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page v P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx TABLE OF CONTENTS (CONTINUED) SECTION PAGE 14.0 SITE ASSESSMENT RESULTS .............................................................................. 14-1 14.1 Nature and Extent of Contamination ................................................................... 14-1 14.2 Maximum Source Constituent Concentrations .................................................. 14-4 14.3 Contaminant Migration and Potentially Affected Receptors ........................... 14-6 15.0 CONCLUSIONS AND RECOMMENDATIONS ................................................ 15-1 15.1 Overview of Site Conditions at Specific Source Areas ...................................... 15-1 15.2 Revised Site Conceptual Model ............................................................................ 15-1 15.3 Interim Monitoring Program ................................................................................. 15-3 15.3.1 IMP Implementation ....................................................................................... 15-3 15.3.2 IMP Reporting ................................................................................................. 15-4 15.4 Preliminary Evaluation of Corrective Action Alternatives............................... 15-4 15.4.1 CAP Preparation Process ............................................................................... 15-4 15.4.2 Summary .......................................................................................................... 15-6 16.0 REFERENCES ............................................................................................................. 16-1 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page vi P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx LIST OF FIGURES Executive Summary Figure ES-1 Approximate Extent of Impacts 1.0 Introduction Figure 1-1 Site Location Map 2.0 Site History and Description Figure 2-1 Site Layout Map Figure 2-2 1948 Paw Creek Topo Figure 2-3 Compliance and Voluntary Monitoring Wells Figure 2-4 Sample Location Map 3.0 Source Characteristics Figure 3-1 Site Features Map Figure 3-2 Elemental Composition for Bottom Ash, Fly Ash, Shale, and Volcanic Ash Figure 3-3 Coal Ash TCLP Leachate Concentration vs Regulatory Limits 4.0 Receptor Information Figure 4-1 Ash Basin Underground Features Map Figure 4-2 Ash Storage and Cinder Storage Area Underground Features Map Figure 4-3 USGS Map with Water Supply Wells Figure 4-4 Receptor Map Figure 4-5 Properties Contiguous to the Ash Basin Waste Boundary Figure 4-6 Surface Water Bodies Figure 4-7 Municipal Water Intakes Map 5.0 Regional Geology and Hydrogeology Figure 5-1 Tectonostratigraphic Map of the Southern and Central Appalachians Figure 5-2 Regional Geologic Map Figure 5-3 Interconnected, Two-Medium Piedmont Groundwater System Figure 5-4 Piedmont Slope-Aquifer System 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page vii P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx LIST OF FIGURES (CONTINUED) 6.0 Site Geology and Hydrogeology Figure 6-1 Site Geologic Map Figure 6-2 Cross Section Location Map Figure 6-3 General Cross Section A-A' Figure 6-4 General Cross Section B-B' Figure 6-5 General Cross Section C-C' Figure 6-6 General Cross Section D-D' Figure 6-7 General Cross Section E-E' Figure 6-8 Potentiometric Surface Early Excavation July 2015 - Shallow Wells Figure 6-9 Potentiometric Surface Later Excavation May 2017 - Shallow Wells Figure 6-10 Potentiometric Surface Early Excavation July 2015 - Deep Wells Figure 6-11 Potentiometric Surface Later Excavation May 2017- Deep Wells Figure 6-12 Potentiometric Surface Early Excavation July 2015 - Bedrock Wells Figure 6-13 Potentiometric Surface Later Excavation May 2017 - Bedrock Wells Figure 6-14 Topographic Lineaments and Rose Diagram Figure 6-15 Aerial Photography Lineaments and Rose Diagram 7.0 Soil Sampling Results Figure 7-1 Potential Secondary Source Soil Analytical Results 9.0 Surface Water Results Figure 9-1 Piper Diagram - AOW Water 10.0 Groundwater Sampling Results Figure 10-1 Piper Diagram - Shallow Flow Layer Figure 10-2 Piper Diagram - Deep Flow Layer Figure 10-3 Piper Diagram - Bedrock Flow Layer 11.0 Hydrogeological Investigation Figure 11-1 Isoconcentration Map - Antimony in Shallow Groundwater Figure 11-2 Isoconcentration Map - Antimony in Deep Groundwater Figure 11-3 Isoconcentration Map - Antimony in Bedrock Figure 11-4 Isoconcentration Map - Arsenic in Shallow Groundwater Figure 11-5 Isoconcentration Map - Arsenic in Deep Groundwater Figure 11-6 Isoconcentration Map - Arsenic in Bedrock Groundwater 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page viii P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx LIST OF FIGURES (CONTINUED) Figure 11-7 Isoconcentration Map - Beryllium in Shallow Groundwater Figure 11-8 Isoconcentration Map - Beryllium in Deep Groundwater Figure 11-9 Isoconcentration Map - Beryllium in Bedrock Groundwater Figure 11-10 Isoconcentration Map - Boron in Shallow Groundwater Figure 11-11 Isoconcentration Map - Boron in Deep Groundwater Figure 11-12 Isoconcentration Map - Boron in Bedrock Groundwater Figure 11-13 Isoconcentration Map - Chromium and Chromium (VI) In Shallow Groundwater Figure 11-14 Isoconcentration Map - Chromium and Chromium (VI) In Deep Groundwater Figure 11-15 Isoconcentration Map - Chromium and Chromium (VI) In Bedrock Groundwater Figure 11-16 Isoconcentration Map - Cobalt in Shallow Groundwater Figure 11-17 Isoconcentration Map - Cobalt in Deep Groundwater Figure 11-18 Isoconcentration Map - Cobalt in Bedrock Groundwater Figure 11-19 Isoconcentration Map - Iron in Shallow Groundwater Figure 11-20 Isoconcentration Map - Iron in Deep Groundwater Figure 11-21 Isoconcentration Map - Iron in Bedrock Groundwater Figure 11-22 Isoconcentration Map - Manganese in Shallow Groundwater Figure 11-23 Isoconcentration Map - Manganese in Deep Groundwater Figure 11-24 Isoconcentration Map - Manganese in Bedrock Groundwater Figure 11-25 Isoconcentration Map - Strontium in Shallow Groundwater Figure 11-26 Isoconcentration Map - Strontium in Deep Groundwater Figure 11-27 Isoconcentration Map - Strontium in Bedrock Zone Figure 11-28 Isoconcentration Map - Sulfate In Shallow Groundwater Figure 11-29 Isoconcentration Map - Sulfate In Deep Groundwater Figure 11-30 Isoconcentration Map - Sulfate In Bedrock Groundwater Figure 11-31 Isoconcentration Map - Total Dissolved Solids in Shallow Groundwater Figure 11-32 Isoconcentration Map - Total Dissolved Solids in Deep Groundwater Figure 11-33 Isoconcentration Map - Total Dissolved Solids in Bedrock Figure 11-34 Isoconcentration Map - Vanadium in Shallow Groundwater Figure 11-35 Isoconcentration Map - Vanadium in Deep Groundwater Figure 11-36 Isoconcentration Map - Vanadium in Bedrock Groundwater Figure 11-37 Vertical Gradients Map Figure 11-38 Concentration versus Distance from Source Various Constituents 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page ix P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx LIST OF FIGURES (CONTINUED) Figure 11-39 Concentration versus Distance from Source Various Constituents Figure 11-40 Concentration versus Distance from Source Various Constituents Figure 11-41 Antimony Analytical Results - Cross Section A-A' Figure 11-42 Arsenic Analytical Results - Cross Section A-A' Figure 11-43 Beryllium Analytical Results - Cross Section A-A' Figure 11-44 Boron Analytical Results - Cross Section A-A' Figure 11-45 Hexavalent and Total Chromium Analytical Results – Cross Section A-A' Figure 11-46 Cobalt Analytical Results - Cross Section A-A' Figure 11-47 Iron Analytical Results - Cross Section A-A' Figure 11-48 Manganese Analytical Results - Cross Section A-A' Figure 11-49 Strontium Analytical Results - Cross Section A-A' Figure 11-50 Sulfate Analytical Results - Cross Section A-A' Figure 11-51 Total Dissolved Solids Analytical Results - Cross Section A-A' Figure 11-52 Vanadium Analytical Results - Cross Section A-A' Figure 11-53 Antimony Analytical Results - Cross Section D-D' Figure 11-54 Arsenic Analytical Results - Cross Section D-D' Figure 11-55 Beryllium Analytical Results - Cross Section D-D' Figure 11-56 Boron Analytical Results - Cross Section D-D' Figure 11-57 Hexavalent and Total Chromium Analytical Results – Cross Section D-D' Figure 11-58 Cobalt Analytical Results - Cross Section D-D' Figure 11-59 Iron Analytical Results - Cross Section D-D' Figure 11-60 Manganese Analytical Results - Cross Section D-D' Figure 11-61 Strontium Analytical Results - Cross Section D-D' Figure 11-62 Sulfate Analytical Results - Cross Section D-D' Figure 11-63 Total Dissolved Solids Analytical Results - Cross Section D-D' Figure 11-64 Vanadium Analytical Results - Cross Section D-D' 12.0 Risk Assessment Figure 12-1 Ecological Exposure Areas 14.0 Site Assessment Results Figure 14-1 Time Vs Concentration Antimony in Shallow Zone Figure 14-2 Time Vs Concentration Antimony in Deep Zone Figure 14-3 Time Vs Concentration Antimony in Bedrock Figure 14-4 Time vs Concentration Arsenic in Shallow Zone 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page x P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx LIST OF FIGURES (CONTINUED) Figure 14-5 Time vs Concentration Arsenic in Deep Zone Figure 14-6 Time Vs Concentration Arsenic in Bedrock Figure 14-7 Time vs Concentration Beryllium in Shallow Zone Figure 14-8 Time vs Concentration Beryllium in Deep Zone Figure 14-9 Time vs Concentration Beryllium in Bedrock Figure 14-10 Time vs Concentration Boron in Shallow Zone Figure 14-11 Time vs Concentration Boron in Deep Zone Figure 14-12 Time vs Concentration Boron in Bedrock Figure 14-13 Time vs Concentration Total and Hexavalent Chrome in Shallow Zone Figure 14-14 Time vs Concentration Total and Hexavalent Chrome in Deep Zone Figure 14-15 Time vs Concentration Total and Hexavalent Chrome in Bedrock Figure 14-16 Time vs Concentration Cobalt in Shallow Zone Figure 14-17 Time vs Concentration Cobalt in Deep Zone Figure 14-18 Time vs Concentration Cobalt in Bedrock Figure 14-19 Time vs Concentration Iron in Shallow Zone Figure 14-20 Time vs Concentration Iron in Deep Zone Figure 14-21 Time vs Concentration Iron in Bedrock Figure 14-22 Time vs Concentration Manganese in Shallow Zone Figure 14-23 Time vs Concentration Manganese in Deep Zone Figure 14-24 Time vs Concentration Manganese in Bedrock Figure 14-25 Time vs Concentration Strontium in Shallow Zone Figure 14-26 Time vs Concentration Strontium in Deep Zone Figure 14-27 Time vs Concentration Strontium in Bedrock Figure 14-28 Time vs Concentration Sulfate in Shallow Zone Figure 14-29 Time vs Concentration Sulfate in Deep Zone Figure 14-30 Time vs Concentration Sulfate in Bedrock Figure 14-31 Time vs Concentration Total Dissolved Solids in Shallow Zone Figure 14-32 Time vs Concentration Total Dissolved Solids in Deep Zone Figure 14-33 Time vs Concentration Total Dissolved Solids in Bedrock Figure 14-34 Time vs Concentration Vanadium in Shallow Zone Figure 14-35 Time vs Concentration Vanadium in Deep Zone Figure 14-36 Time vs Concentration Vanadium in Bedrock Figure 14-37 Groundwater Concentration Trend Analysis - Antimony in All Flow Layers Figure 14-38 Groundwater Concentration Trend Analysis - Arsenic in All Flow Layers 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page xi P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx LIST OF FIGURES (CONTINUED) Figure 14-39 Groundwater Concentration Trend Analysis - Beryllium in All Flow Layers Figure 14-40 Groundwater Concentration Trend Analysis - Boron in All Flow Layers Figure 14-41 Groundwater Concentration Trend Analysis - Chromium (VI) In All Flow Layers Figure 14-42 Groundwater Concentration Trend Analysis - Chromium in All Flow Layers Figure 14-43 Groundwater Concentration Trend Analysis - Cobalt in All Flow Layers Figure 14-44 Groundwater Concentration Trend Analysis - Iron in All Flow Layers Figure 14-45 Groundwater Concentration Trend Analysis - Manganese in All Flow Layers Figure 14-46 Groundwater Concentration Trend Analysis - Strontium in All Flow Layers Figure 14-47 Groundwater Concentration Trend Analysis - Sulfate In All Flow Layers Figure 14-48 Groundwater Concentration Trend Analysis - Total Dissolved Solids in All Flow Layers Figure 14-49 Groundwater Concentration Trend Analysis - Vanadium in All Flow Layers Figure 14-50 Comprehensive Solids Data Figure 14-51 Comprehensive Groundwater Data Figure 14-52 Comprehensive Surface Water/AOW Data 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page xii P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx LIST OF TABLES 2.0 Site History and Description Table 2-1 Well Construction Data Table 2-2 Summary of Onsite Environmental Incidents 3.0 Source Characteristics Table 3-1 Range (10th percentile - 90th percentile) in Bulk Composition of Fly Ash, Bottom Ash, Rock, and Soil (Source: EPRI 2009a) Table 3-2 Soil/Material Properties for Ash, Fill, Alluvium, and Soil/Saprolite 4.0 Receptor Information Table 4-1 Public and Private Water Supply Wells within 0.5-mile Radius of Ash Basin Compliance Boundary Table 4-2 Property Owner Addresses Contiguous to the Ash Basin Waste Boundary 6.0 Site Geology and Hydrogeology Table 6-1 Soil Mineralogy Results Table 6-2 Soil Chemistry Results - Oxides Table 6-3 Soil Chemistry Results - Elemental Table 6-4 Weathered Rock Mineralogy Results Table 6-5 Chemical Composition of Weathered Rock Samples Table 6-6 Whole Rock Chemistry Results - Oxides Table 6-7 Whole Rock Chemistry Results - Elemental Table 6-8 Historic Water Level Measurements Table 6-9 Summary of Horizontal Hydraulic Gradient and Velocity Calculations Table 6-10 Hydraulic Gradients - Vertical Table 6-11 Hydrostratigraphic Layer Properties - Horizontal Hydraulic Conductivity Table 6-12 Hydrostratigraphic Layer Properties - Vertical Hydraulic Conductivity Table 6-13 Estimated (Effective) Porosity/Specific Yield, and Specific Storage for Upper Hydrostratigraphic Units Table 6-14 Total Porosity for Upper Hydrostratigraphic Units (A,F,S, M1, and M2) Table 6-15 Field Permeability Test Results Table 6-16 In-Situ Hydraulic Conductivity Results 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page xiii P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx LIST OF TABLES (CONTINUED) Table 6-17 Laboratory Permeability Test Results Table 6-18 Historic Laboratory Permeability Test Result 7.0 Soil Sampling Results Table 7-1 Soil, Sediment, and Ash Analytical Methods Table 7-2 Provisional Background Threshold Values for Soil Table 7-3 Background Soil Data Summary Table 7-4 Potential Secondary Source Soil Analytical Results 9.0 Surface Water Results Table 9-1 Ash Porewater, Groundwater, Surface Water, and Seep Analytical Methods 10.0 Groundwater Sampling Results Table 10-1 Background Groundwater Data Summary through February 2017 Table 10-2 Provisional Background Threshold Values for Groundwater Table 10-3 State and Federal Standards for COIs 11.0 Hydrogeological Investigation Table 11-1 Summary of Kd Values from Batch and Column Studies 15.0 Conclusions and Recommendations Table 15-1 Groundwater IMP Sampling Parameters and Analytical Methods Table 15-2 Interim Groundwater Monitoring Plan Sample Locations 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page xiv P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx LIST OF APPENDICES Appendix A Regulatory Correspondence NCDEQ Expectations Document (July 18, 2017) Completed NCDEQ CSA Update Expectations Check List NORR Letter NPDES Wastewater Permit #NC004961 NCDEQ Correspondence – Revised Interim Monitoring Plan DEQ Background Location Approvals July 7, 2017 Zimmerman to Draovitch September 1, 2017 DEQ PBTV Approval Attachments – September 1, 2017 Appendix B Comprehensive Data Table Appendix B Notes Table 1 - Groundwater Results Table 2 - Surface Water Results Table 3 - AOW Results Table 4 - Soil and Ash Results Table 5 - Sediment Results Table 6 - SPLP Results Appendix C Site Assessment Data HDR CSA Appendix H – Hydrogeological Investigation Soil Physical Lab Reports Mineralogy Lab Reports Slug Test Procedure Slug Test Reports Historic Permeability Data Field Permeability Data Fetter-Bear Diagrams – Porosity Historic Porosity Data Estimated Seasonal High Groundwater Elevations Calculation 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page xv P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx LIST OF APPENDICES (CONTINUED) Appendix C Site Assessment Data (continued) HDR CSA Supplement 2 Slug Test Report UNCC Soil Sorption Evaluation – HDR CAP 1 Appendix D Addendum to the UNCC Soil Sorption Evaluation – HDR CAP 2 Appendix C Appendix D Receptor Surveys Updated Receptor Survey Report – HDR CSA 2015 Drinking Water Supply Well and Receptor Survey – August 5, 2016 (HDR) Dewberry Report – Permanent Water Supply Proposal to DEQ Appendix E Supporting Documents Stantec Report WSP Maps Appendix F Boring Logs, Construction Diagrams, and Abandonment Records Boring Logs – CAMA Assessment Soil Boring Logs Historical Boring Logs and Construction Records Well Construction Records Abandonment Records Appendix G Sample Characterization Source Characterization – HDR CSA Appendix C Soil and Rock Characterization – HDR CSA Appendix D Surface Water and Sediment Characterization – HDR CSA Appendix F Groundwater Characterization – HDR CSA Appendix G Includes Duke Low Flow Sampling Plan Field, Sampling, and Data Analysis Quality Assurance / Quality Control - HDR CSA Appendix E 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page xvi P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx LIST OF APPENDICES (CONTINUED) Appendix H Background Determination Riverbend 2017 CSA PBTV Report HDR Background Determination Report – HDR CAP 1 Appendix B Appendix I Lab Reports HDR CSA Lab Reports – Appendix K CSA Supplement 1 Reports – Attachment to the HDR CAP Part 2 CSA Supplement 2 Reports – HDR CSA Supplement 2 Appendix C Lab Reports Post HDR CSA Supplement 2 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page xvii P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx LIST OF ACRONYMS 2B NCDEQ Title 15A, Subchapter 02B. Surface Water and Wetland Standards 2L NCDEQ Title 15A, Subchapter 02L. Groundwater Classification and Standards ADD Average Daily Dose AOW Area of Wetness APS NCDENR DWR Aquifer Protection Section AST Aboveground Storage Tank ASTM American Society for Testing and Materials CAMA Coal Ash Management Act CAP Corrective Action Plan CCR Coal Combustion Residuals CFR Code of Federal Register CMUD Charlotte-Mecklenburg Utilities Department COI Constituent of Interest CSA Comprehensive Site Assessment CT Combustion Turbine DO Dissolved oxygen Duke Energy Duke Energy Carolinas, LLC DWQ Division of Water Quality DWR Division of Water Resources EDR Environmental Data Resources EMP Effectiveness Monitoring Program EPRI Electric Power Research Institute FERC Federal Energy Regulatory Commission GIS Geographic Information Systems GMP Groundwater Monitoring Plan HAO Hydroxide Phases of Aluminum HFO Hydroxide Phases of Iron HSL Health Screening Level IMAC Interim Maximum Allowable Concentration IMP Interim Monitoring Plan meq/100/g milliequivalents per 100 grams MCL Maximum Contaminant Level 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page xviii P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx LIST OF ACRONYMS (CONTINUED) mg/L milligrams per liter mg/kg milligrams per kilogram MNA Monitored Natural Attenuation NAVD 88 North American Vertical Datum of 1988 NCAC North Carolina Administrative Code NCDENR North Carolina Department of Environment and Natural Resources NCDEQ/DEQ North Carolina Department of Environmental Quality NCDHHS North Carolina Department of Health and Human Services NORR Notice of Regulatory Requirements NPDES National Pollutant Discharge Elimination System NTU Nephelometric Turbidity Unit NURE National Uranium Resource Evaluation PBTV Provisional Background Threshold Values Plant/Site Riverbend Steam Station PMCL Primary Maximum Contaminant Level POG Protection of Groundwater PPBC Proposed Provisional Background Concentrations PSRG Preliminary Soil Remediation Goal PWR Partially Weathered Rock PWSS NCDENR Division of Water Resources Public Water Supply Section RBC Risk-based Concentrations RBSS Riverbend Steam Station RCRA Resource Conservation and Recovery Act REC Rock Core Recovery RQD Rock Quality Designation S.U. Standard Units SCM Site Conceptual Model SCS USDA Soil Conservation Service SMCL Secondary Maximum Contaminant Level SPLP Synthetic Precipitation Leaching Procedure SWAP NCDENR DWR Source Water Assessment Program TCLP Toxicity Characteristic Leaching Procedure 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page xix P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx LIST OF ACRONYMS (CONTINUED) TDS Total Dissolved Solids TOC Total Organic Carbon TRV Toxicity Reference Values µg/L micrograms per liter UNCC/UNC University of North Carolina at Charlotte USCS Unified Soil Classification System USDA U.S. Department of Agriculture USEPA U.S. Environmental Protection Agency USGS U.S. Geological Survey UTL Upper Tolerance Level Work Plan Groundwater Assessment Work Plan 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 1-1 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx 1.0 INTRODUCTION Duke Energy Carolinas, LLC (Duke Energy) owns and formerly operated the Riverbend Steam Station (RBSS) located adjacent to the Mountain Island Lake portion of the Catawba River near Mount Holly, Gaston County, North Carolina (Figure 1-1). RBSS began operation as a coal-fired generating station in 1929 and was retired from service in April 2013. Decommissioning of RBSS is ongoing. Until 1957, coal combustion residuals (CCR) from RBSS’s coal combustion process were managed at the cinder storage area. After installation of precipitators and a wet sluicing system in 1957, CCR was disposed of in the station’s ash basin located adjacent to the station and Mountain Island Lake. 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 NC0004961. 1.1 Purpose of Comprehensive Site Assessment (CSA) This Comprehensive Site Assessment (CSA) update was conducted to refine and expand the understanding of subsurface geologic/hydrogeologic conditions and evaluate the extent of impacts from historical management of coal ash. This CSA update contains an assessment of Site conditions based on a comprehensive interpretation of geologic and sampling results from the initial Site assessment and geologic and sampling results obtained subsequent to the initial assessment and has been prepared in coordination with Duke Energy and NCDEQ in response to requests for additional information, including additional sampling and assessment of specified areas. The CSA update includes information from the original CSA (HDR, 2015a). This CSA update was prepared in conformance to the most recently updated CSA table of contents provided by NCDEQ to Duke Energy on September 29, 2017. The NCDEQ Expectations Document (July 18, 2017) and the completed NCDEQ CSA Update Expectations Check List are included in Appendix A. In response to the NCDEQ request for an updated CSA report, this submittal includes the following information: Review of baseline assessment data collected and reported as part of CSA 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. 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 1-2 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx A summary of NPDES and Coal Ash Management Act (CAMA) groundwater monitoring information A summary of potential receptors including results from water supply wells A description of additional assessment activities conducted since submittal of the CSA Supplement report(s) An update on background concentrations for groundwater and soil; and Definition of horizontal and vertical extent of CCR constituents in soil and groundwater based upon NCDEQ approved background concentrations. 1.2 Regulatory Background The North Carolina Coal Ash Management Act of 2014 (CAMA) directs owners of CCR surface impoundments in North Carolina to conduct groundwater monitoring, assessment, and remedial activities, if necessary. The CSA was performed to collect information necessary to evaluate the horizontal and vertical extent of impacts to soil and groundwater attributable to CCR source area(s), identify potential receptors, and screen for potential risks to those receptors. 1.2.1 Notice of Regulatory Requirements (NORR) On August 13, 2014, North Carolina Department of Environment and Natural Resources (NCDENR) issued a Notice of Regulatory Requirements (NORR) letter notifying Duke Energy that exceedances of groundwater quality standards were reported at 14 coal ash facilities owned and operated by Duke Energy. Those groundwater quality standards are part of 15A NCAC 02L .0200 Classifications and Water Quality Standards Applicable to the Groundwaters of North Carolina. The NORR stipulated that for each coal ash facility, Duke Energy was to conduct a CSA. The NORR also stipulated that before conducting each CSA, Duke was to submit a Groundwater Assessment Work Plan (Work Plan) and a receptor survey. In accordance with the NORR requirements, a receptor survey was performed to identify all receptors within a 0.5-mile radius (2,640 feet) of the ash basin compliance boundary, and a CSA was conducted for each facility. The NORR letter is included in Appendix A. 1.2.2 Coal Ash Management Act Requirements The Coal Ash Management Act (CAMA) of 2014 — General Assembly of North Carolina Senate Bill 729 Ratified Bill (Session 2013) (SB 729) requires that ash from Duke Energy coal plant sites located in North Carolina either (1) be excavated and relocated to fully lined storage facilities or (2) go through a 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 1-3 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx classification process to determine closure options and schedule. Closure options can include a combination of excavating and relocating ash to a lined structural fill, excavating and relocating the ash to a lined landfill (on-site or off-site), and/or capping the ash with an engineered synthetic barrier system, either in place or after being consolidated to a smaller area on-site. As a component of implementing this objective, CAMA provides instructions for owners of coal combustion residuals surface impoundments to perform various groundwater monitoring and assessment activities. Section §130A-309.209 of the CAMA ruling specifies groundwater assessment and corrective actions, drinking water supply well surveys and provisions of alternate water supply, and reporting requirements as follows: (a) Groundwater Assessment of Coal Combustion Residuals Surface Impoundments. – The owner of a coal combustion residuals surface impoundment shall conduct groundwater monitoring and assessment as provided in this subsection. The requirements for groundwater monitoring and assessment set out in this subsection are in addition to any other groundwater monitoring and assessment requirements applicable to the owners of coal combustion residuals surface impoundments. (1) No later than December 31, 2014, the owner of a coal combustion residuals surface impoundment shall submit a proposed Groundwater Assessment Plan for the impoundment to the Department for its review and approval. The Groundwater Assessment Plan shall, at a minimum, provide for all of the following: a. A description of all receptors and significant exposure pathways. b. An assessment of the horizontal and vertical extent of soil and groundwater contamination for all contaminants confirmed to be present in groundwater in exceedance of groundwater quality standards. c. A description of all significant factors affecting movement and transport of contaminants. d. A description of the geological and hydrogeological features influencing the chemical and physical character of the contaminants. 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 1-4 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx 2) The Department shall approve the Groundwater Assessment Plan if it determines that the Plan complies with the requirements of this subsection and will be sufficient to protect public health, safety, and welfare; the environment; and natural resources. (3) No later than 10 days from approval of the Groundwater Assessment Plan, the owner shall begin implementation of the Plan. (4) No later than 180 days from approval of the Groundwater Assessment Plan, the owner shall submit a Groundwater Assessment Report to the Department. The Report shall describe all exceedances of groundwater quality standards associated with the impoundment. 1.3 Approach to Comprehensive Site Assessment The CSA has been performed to meet NCDEQ requirements associated with potential site remedy selection. The following components were utilized to develop the assessment. 1.3.1 NORR Guidance The NORR requires that site assessment provide information to meet the requirements of North Carolina regulation 15A NCAC 02L .0106 (g). This regulation lists the items to be included in site assessments conducted pursuant to Paragraph (c) of the rule. These requirements are listed below and referenced to the applicable sections of this CSA. 15A NCAC 02L .0106(g) Requirement CSA Section(s) (1) The source and cause of contamination Section 3.0 (2) Any imminent hazards to public health and safety, as defined in G.S. 130A-2, and any actions taken to mitigate them in accordance with Paragraph (f) of this Rule Sections ES.2 and 2.8 (3) All receptors and significant exposure pathways Sections 4.0 and 12.0 (4) The horizontal and vertical extent of soil and groundwater contamination and all significant factors affecting contaminant transport Sections 7.0, 10.0, 11.0 and 14.0 (5) Geological and hydrogeological features influencing the movement, chemical, and physical character of the contaminants Sections 6.0, 11.0, and 14.0 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 1-5 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx 1.3.2 USEPA Monitored Natural Attenuation Tiered Approach The assessment data is compiled in a manner to be consistent with “Monitored Natural Attenuation of Inorganic Contaminants in Groundwater” (EPA/600/R- 07/139). The tiered analysis approach discussed in this guidance document is designed to align site characterization tasks to reduce uncertainty in remedy selection. The tiered assessment data collection includes information to evaluate: Active contaminant removal from groundwater and dissolved plume stability, The mechanisms and rates of attenuation, The long-term capacity for attenuation and stability of immobilized contaminants, and Anticipated performance monitoring needs to support the selected remedy. 1.3.3 ASTM Conceptual Site Model Guidance The American Society for Testing and Materials (ASTM) E1689-95 generally describes the major components of conceptual site models, including an outline for developing models. To the extent possible, this guidance was incorporated into preparation of the Site Conceptual Model (SCM). The SCM is used to integrate site information, identify data gaps, and determine whether additional information is needed at the site. The model is also used to facilitate selection of remedial alternatives and effectiveness of remedial actions in reducing the exposure of environmental receptors to contaminants (ASTM, 2014). 1.4 Technical Objectives The rationale for CSA activities fall into one of the following categories: Determine the range of background groundwater quality from pertinent geologic settings (horizontal and vertical) across a broad area of the Site. Evaluate groundwater quality from pertinent geologic settings (horizontal and vertical extent of CCR leachate constituents in groundwater). Establish perimeter (horizontal and vertical) boundary conditions for groundwater modeling. Provide source area information including ash porewater chemistry, physical and hydraulic properties, CCR thickness and residual saturation within the ash management areas. 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 1-6 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx Address soil chemistry in the vicinity of the ash basins (horizontal and vertical extent of CCR leachate constituents in soil) compared to background concentrations. Determine potential routes of exposure and receptors. Compile information necessary to develop a groundwater CAP protective of human health and the environment in accordance with 2L. 1.5 Previous Submittals Detailed descriptions of the Site operational history, the site conceptual model, physical setting and features, geology/hydrogeology, and results of the findings of the CSA and other CAMA-related works are documented in full in the following: Comprehensive Site Assessment Report — Riverbend Steam Station (HDR, 2015a) Corrective Action Plan Part 1 — Riverbend Steam Station ( (HDR, 2015b) Corrective Action Plan Part 2 (included CSA Supplement 1 as Appendix A) — Riverbend Steam Station (HDR, 2016a) Comprehensive Site Assessment Supplement 2 — Riverbend Steam Station (HDR, 2016b) 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 2-1 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx 2.0 SITE HISTORY AND DESCRIPTION An overview of the RBSS setting and operations is presented in the following sections. 2.1 Site Description, Ownership, and Use History The RBSS site (the Site) was named for a bend in Mountain Island Lake on which it is located. The site is north of Horseshoe Bend Beach Road near the town of Mount Holly in Gaston County, North Carolina. The Site occupies approximately 340.7 acres of land. RBSS was a seven-unit, 454 MW, coal-fired, electricity-generating facility. The station began commercial operations in 1929 with Units 1–4. Units 5–7 began commercial operations sequentially from 1952 through 1954. Units 1–3 were retired from service in the 1970s, and Units 4–7 were retired from service on April 1, 2013. During its final years of operation, the plant was considered a cycling station, brought online to supplement energy supply when electricity demand was at its highest. Duke Energy also operated four combustion turbine (CT) units at the Site from 1969 until October 2012. The ash basin system at RBSS consists of a primary cell, a secondary cell, and associated embankments and outlet works. An ash storage area is located southwest and sidegradient of the primary cell, and a cinder storage area is located west and downgradient of the primary cell. The ash basin system is located approximately 2,400 feet to the northeast of the power plant, adjacent to Mountain Island Lake, as shown on Figure 2-1. The primary cell is impounded by an earthen dike located on the west side of the primary cell. The surface area of the primary cell is approximately 41 acres with an approximate maximum pond elevation of 724 feet.3 An intermediate dam, constructed over ash deposited in the original single-cell ash basin, separated the primary and secondary cells until it was decommissioned in 2017 as part of ash removal activities. The secondary cell is impounded by an earthen dike located along the northeast side of the secondary cell close to Mountain Island Lake. The surface area of the secondary cell is approximately 28 acres with an approximate maximum pond elevation of 714 feet. The full pond elevation of Mountain Island Lake is approximately 646.8 feet. The RBSS site contains two switchyards and associated transmission lines. While power production no longer exists at the site, these facilities continue to support power transmission on the Duke Energy system. 3 The datum for all elevation information presented in this report is NAVD88. Reported pond elevations are indicative of conditions prior to ash basin excavation and/or dewatering activities. 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 2-2 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx The Lark Maintenance Center is also located at the RBSS site west of the retired coal- fired units. This facility is an advanced machining and welding shop that supports various Duke Energy power plants in the region. The entire RBSS site is approximately 340.7 acres in area and is owned by Duke Energy (Figure 2-1). The earliest historical topographic map available for review was dated 1948 and indicated the power plant location. Topographic maps created prior to use of the property for power generation are not available for review to determine prior land use. In addition to the RBSS power plant property, Duke Energy owns and operates Mountain Island Lake as part of the Catawba-Wateree Hydroelectric Project (FERC Project No. 2232). Mountain Island Lake surrounds the RBSS site and is used for municipal water supply and recreation. In an August 25, 2017 letter to Duke Energy, NCDEQ provided direction related to establishment of compliance boundaries for various facilities under Title 15A NCAC Subchapter 02L Rule .0107. For instance, the compliance boundaries at Allen Steam Station and Marshall Steam Station along the Catawba River, will not extend into the river. Additionally, NCDEQ indicated that a compliance boundary was not applicable to CCR units that were not covered under an active NPDES permit. The compliance boundary for RBSS shown on Figure 2-1 and other figures prepared for this report reflect the NCDEQ direction in the August 2017 letter. 2.2 Geographic Setting, Surrounding Land Use, Surface Water Classification The RBSS site is situated on a peninsula which extends approximately three miles from the west and is surrounded by the Mountain Island Lake portion of the Catawba River on the north, east and south. A description of the physical setting for RBSS is provided in the following sections. Geographic Setting The RBSS site is generally forested along the Catawba River. The buildings and other structures associated with the power production facilities are located on the north side of Horseshoe Bend Beach Road, which extends from west to east and is generally located along a local topographic divide. The topography at the site generally slopes downward from this divide on the south to Mountain Island Lake on the north. A 1948 U.S. Geological Survey (USGS) topographic map depicting the site prior to construction 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 2-3 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx of the ash basin features is shown on Figure 2-2. For a detailed description of the RBSS ash basin and other ash storage facilities, refer to Section 3.2. Surrounding Land Use The area surrounding RBSS generally consists of residential properties, undeveloped land, and Mountain Island Lake (Figure 2-1). Properties north of Mountain Island Lake are located in the Town of Huntersville, Mecklenburg County, North Carolina. Those properties are zoned as Rural (R) and are primarily comprised of: a wildlife refuge located to the northwest; a nature preserve to the northeast; and a residential property also to the northeast of the ash basin and Mountain Island Lake. Properties south of Mountain Island Lake are located in Mount Holly, Gaston County, North Carolina. The majority of the property in that area is owned by Duke Energy and associated with the RBSS. The Duke property is zoned by the Town of Mount Holly as Heavy Industrial (H-1) to be used for industrial and commercial uses, and Light Industrial (L-1) to be used for certain kind of commercial uses that are located as neighbors of industrial use. Residential properties are located south and southeast of RBSS to the south of Horseshoe Bend Beach Road. Those properties are zoned Single Family Residential (R-12). The Mount Holly Public Utilities Department provides water service to the RBSS site and nearby properties located in Gaston County. Although water service is available, information gathered to date indicates there is no restriction on future installation of water wells for properties in the area. With the exception of the decommissioning of RBSS, future surrounding land uses are assumed to remain similar to their current uses (undeveloped land, wildlife refuge, nature preserve, heavy and light industrial, and residential). Meteorological Setting In winter, the average temperature for Gaston County is 43°F and the average daily minimum temperature is 32°F. In summer, the average temperature is 77°F and the average daily maximum temperature is 88°F (USDA SCS, 1989). The average annual precipitation in Mount Holly is 41.63 inches (over the 30-year period of record). Severe local storms occasionally occur in or near Gaston County. Those events, such as tropical depressions or remnants of hurricanes moving inland, can cause isolated heavy rainfall. The average annual precipitation in the Piedmont, where this site is located, ranges from approximately 42 inches to 46 inches (USDA SCS, 1989). The average relative humidity in midafternoon is approximately 70 percent, with humidity reaching higher levels at night. The prevailing wind is from the southwest, and average wind speed is highest (9 miles per hour) in spring (USDA SCS, 1989). 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 2-4 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx Surface Water Classification The Site is located in the Mountain Island Lake/Catawba River watershed, and the ash basin is adjacent to Mountain Island Lake. Surface water classifications in North Carolina are defined in 15A NCAC 02B. 0101 (c). The surface water classification for Mountain Island Lake is Class WS-IV, Class B, and Class C. Class WS-IV waters are protected as water supplies that are generally in moderately to highly developed watersheds. Class C are waters protected for uses such as secondary recreation, fishing, wildlife, fish consumption, aquatic life including propagation, survival and maintenance of biological integrity, and agriculture. Class B waters are protected for all Class C uses in addition to primary recreation (swimming). Surface water features located on the site are shown on Figure 2-1. Mountain Island Lake is used as water supply for the Charlotte Metropolitan area, as well as the towns of Gastonia and Mount Holly. The water supply intakes are located approximately 3.4 miles and 6.9 miles, respectively, downstream from the RBSS site (Figure 4-7). 2.3 CAMA-related Source Areas CAMA provides for groundwater assessment of CCR surface impoundments defined as topographic depressions, excavations, or diked areas formed primarily of earthen materials, without a base liner, and that meet other criteria related to design, usage, and ownership (Section §130A-309.201). At RBSS, groundwater assessment was conducted for the CCR surface impoundments, including the primary and secondary cells. The groundwater assessment incorporated the ash storage area located southwest and side- gradient of the primary cell, and the cinder storage area located west and side-gradient of the primary cell (Figure 2-1). Collectively, the primary and secondary cells, the ash storage area, and the cinder storage area are referred to herein as ash management areas. More detailed discussion of the ash management areas is provided in Section 3.0 of this report. As required by CAMA, Duke Energy plans to excavate the primary CAMA-related source, which is the coal ash contained in the ash basin (primary and secondary cells). In addition, Duke Energy is removing the ash stored in the ash storage area and cinder storage area. Excavated and removed material will be used beneficially off-site or will be relocated to a permitted off-site lined landfill. Regulation 15A NCAC 02L .0106 (f)(4) requires that the secondary sources that could be potential continuing sources of possible pollutants to groundwater be addressed in the CAP. At the RBSS site, the soil located below the ash basin could be considered a 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 2-5 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx potential CAMA-related secondary source. Soil potentially impacted by CCR is likely to be limited to a limited depth interval immediately below the ash/soil interface. 2.4 Other Primary and Secondary Sources CSA activities include an assessment of the horizontal and vertical extent of constituents related to the CCR impoundments and observed at concentrations greater than 2L/IMAC or background concentrations. If the CSA indicates constituent exceedances related to sources other than the CCR impoundments, these sources will be addressed as part of a separate process in compliance with the requirements of 2L. During operation as a coal-fired generating station, RBSS maintained a coal pile in the area located between the coal-fired units and the cinder storage area. As determined through discussions involving NCDEQ and Duke Energy, the coal pile area will be assessed as part of a separate process in compliance with the requirements of 2L. 2.5 Summary of Permitted Activities The NPDES program regulates wastewater discharges to surface waters to ensure that surface water quality standards are maintained. The most recent NPDES permit became effective December 1, 2016, and expires on February 29, 2020. As part of the permit renewal, the facility identified seeps and collected seep samples. Analysis of the samples to date indicates that there are no water quality violations. The seeps were incorporated into the permit as outfalls where appropriate. The permit requires surface water monitoring, including continued sampling of seep outfalls, as part of the permit conditions. In compliance with CAMA, RBSS is required to dewater the ash basins and excavate coal ash for removal from the site. It is anticipated that through this process existing seeps will dry out or significantly reduce in flow volume. 2.6 History of Site Groundwater Monitoring The location of the ash basin voluntary and compliance monitoring wells, the CSA wells, the approximate ash basin waste boundary, and the compliance boundary are shown in Figure 2-3. Construction details for site monitoring wells are provided in Table 2-1. At RBSS, monitoring wells are designated with an S, D, BRU, or BR identifier. These correspond to the flow layer the well is screened in. S refers to the shallow flow layer (alluvium and saprolite), D refers to the deep flow layer (saprolite and weathered rock), BRU and BR refer to the bedrock flow layer (sound, relatively unfractured rock). Some well labels included an “A” which indicates the well is a replacement for a previously installed well. The following sections discuss groundwater monitoring 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 2-6 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx activities prior to CSA activities through current CAMA-related monitoring activities. Groundwater monitoring results are presented in Section 10.0. 2.6.1 Ash Basin Voluntary Groundwater Monitoring Fourteen groundwater monitoring wells were installed by Duke Energy in 2006 as part of a voluntary monitoring system. Duke Energy implemented enhanced voluntary groundwater monitoring around the RBSS ash basin from December 2008 until June 2010. During this period, the voluntary groundwater monitoring wells were sampled two times per year, and the analytical results were submitted to NCDENR DWR. 2.6.2 Ash Basin NPDES Groundwater Monitoring Groundwater monitoring as required by the RBSS NPDES Permit NC0004961 began in March 2011. Through 2016, groundwater monitoring was conducted as described in NPDES Permit Condition A (11), Version 1.1, dated June 15, 2011. Groundwater monitoring events were conducted three times a year (February, June, and October). Compliance groundwater monitoring continued in 2017 in accordance with the NPDES Permit, but the guidelines were published separately in the Draft Groundwater Monitoring Plan (GMP), dated September 9, 2016. The Draft GMP included the compliance monitoring wells sampled from 2011 through 2016, and added monitoring well GWA-7S as well as monitoring wells BG-1S, BG-1D, BG- 2S, BG-2D, BG-2BR, BG-3S, and BG-3D to further establish background concentrations. The BG-1 through BG-3 series of wells is located in a forested area east of the secondary cell. The monitoring well network and frequency of sampling directed by DEQ under CAMA is anticipated to monitor groundwater conditions in and around the ash basin at the RBSS site. Until the CAMA monitoring plan is approved by DEQ, NPDES groundwater monitoring has continued in accordance with the September 2016 Draft GMP. 2.6.3 Ash Basin CAMA Monitoring A total of 78 groundwater monitoring wells were installed at RBSS between February and August 2015 as part of the CSA groundwater assessment program. One comprehensive round of sampling and analysis was conducted prior to, and reported in, the August 2015 CSA. A detailed discussion of well installation and sampling activities is provided in Section 2.7. 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 2-7 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx Nine additional rounds of groundwater sampling of the CSA wells have occurred since submittal of the CSA report. Prior to the CSA, there were no monitoring wells screened within or below the ash management areas at RBSS. Twenty-nine monitoring wells were installed within the waste boundaries of the ash basin, ash storage area, and cinder storage area in 2015 as part of the initial CSA to characterize the groundwater in the source areas. 2.7 Summary of Assessment Activities Several historical site investigations have been conducted due to fuel oil releases associated with the piping and above-ground storage tank (AST) that were part of the former combustion turbine system at the site. A summary of historical environmental incidents are provided in Table 2-2. In a letter dated March 16, 2012, the NCDENR Division of Water Quality (DWQ) Aquifer Protection Section (APS) requested that Duke Energy begin additional assessment activities at stations where measured and modeled concentrations of groundwater constituents exceed the 2L Standards at the compliance boundary. Duke Energy submitted the report Groundwater Assessment, Duke Energy Carolinas, LLC, Riverbend Steam Station Ash Basin, NPDES Permit NC0004961 (HDR, 2013) to address this request by NCDENR for RBSS. Duke Energy received comments on the May 31, 2013, report from NCDENR in a letter dated September 8, 2014. The NCDENR recommendations and requests were generally incorporated during the development of the CSA Work Plan. CSA Activities Groundwater monitoring wells were installed at multiple locations and depths across the site to monitor the horizontal and vertical constituent distribution. Monitoring wells were installed at the ash basin, the ash storage area, the cinder storage area, at locations inside and beyond the compliance area, and in background areas. Groundwater monitoring well locations are shown on Figure 2-4. Background monitoring wells included two pre-existing compliance groundwater monitoring wells (MW-7D and MW-7SR) and eight newly installed groundwater monitoring wells. The background wells are located hydrologically upgradient or cross- gradient and were strategically placed to maximize physical separation from the ash basin, ash storage area, and cinder storage area. 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 2-8 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx Additional Assessment 2016 Additional monitoring wells were installed in 2016 to refine and expand understanding of groundwater flow direction and extent of exceedances at RBSS. Several additional monitoring wells in downgradient areas provided better assessment of potential constituent migration. Two additional background well locations, BG-4 and BG-5, were installed to monitor background conditions to the south and west of the Site, respectively. Well Abandonments Several wells have been abandoned for various reasons since the submittal of the 2015 CSA. Wells within the footprint of the ash storage area, primary ash basin and secondary ash basin have been abandoned to allow for ash excavation. These wells include AB-5S/SL/D, AB-7S/I/D, AS-1S/D, AS-2S/D, and AS-3S/D. AB-3S/D/BR and AB- 4S/D were located along the intermediate dam between the primary and secondary ash basins. They were abandoned before removing the intermediate dam to continue ash excavation. Wells AB-6S/BRU were located along the primary cell main dam and were abandoned in August 2017 to facilitate continued ash excavation. Eight additional wells have been abandoned as part of additional assessment activities. These wells include C- 1BRU, GWA-3S, GWA-20D, GWA-22BR, GWA-23D, MW-7BR, MW-9BR, and MW-9D and were replaced either due to suspected grout contamination or because the well did not produce sufficient water to sample. 2.8 Summary of Initial Abatement, Source Removal, or other Corrective Action Initial decommissioning activities at RBSS began in the fall of 2013. Those activities involved demolition of gas-fired combustion turbine generation units, coal-fired turbine units, water tank, precipitators, and coal handling equipment, as well as initiation of asbestos removal. By mid-2014, asbestos removal activities were completed, relocation of electrical equipment had begun, and auxiliary buildings and structures were demolished. Demolition of the powerhouse building and chimneys is planned to begin in late 2017. In conjunction with decommissioning activities and in accordance with CAMA requirements, Duke Energy will permanently close the RBSS ash ponds per the North Carolina CAMA-required date associated with the risk ranking that the ash basins receive from the Coal Ash Management Commission. Ash removal activities began in May 2015 and are currently ongoing at the primary cell and ash storage areas. 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 2-9 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx Bulk dewatering of the Secondary Ash Basin was completed in January 2017, and interstitial dewatering began in February 2017. Dewatering flows are being treated by an onsite water treatment facility in accordance with the NPDES permit NC0004961. As of January 2016, ash is being transported to the Brickhaven Structural Fill site by rail. As of July 2, 2017, 2,416,323 tons of ash had been transported from the station, representing 46.4% of the total. The intermediate dam between the primary and secondary basins had been decommissioned and excavated as of March 9, 2017. The RBSS site is on track to comply with the CAMA deadline of complete excavation of the ash basins by August 1, 2019, with an expected completion date of March 3, 2019. 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 3-1 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx 3.0 SOURCE CHARACTERISTICS For purposes of this assessment, the source area is defined by the ash waste boundary as depicted on Figure 2-1. For the RBSS site, sources include the ash management areas (the ash basin, ash storage area, and cinder storage area). Source characterization was performed through the completion of soil borings, installation of monitoring wells, and associated solid matrix and aqueous sample collection and analysis. Ash samples were collected for analysis of physical characteristics (e.g., grain size, porosity, etc.) to provide data for evaluation of retention/transport properties within and beneath the ash basins and ash storage areas. Ash samples were collected for analysis of chemical characteristics (e.g., total inorganics, leaching potential, etc.) to provide data for evaluation of constituent concentrations and distribution. 3.1 Coal Combustion and Ash Handling System Coal ash is produced from the combustion of coal. The coal is dried, pulverized, and conveyed to the burner area of a boiler. The smaller particles produced by coal combustion, referred to as fly ash, are carried upward in the flue gas and are captured by an air pollution control device, such as an electrostatic precipitator. The larger particles of ash that fall to the bottom of the boiler are referred to as bottom ash. During operation, RBSS produced approximately 100,000 tons of ash per year. Coal ash residue from the coal combustion process was sluiced to the RBSS ash basin from approximately 1957 until the last coal-fired generating units were retired in April 2013. After collection, both fly ash and bottom ash/boiler slag were sluiced to the ash basin using conveyance water withdrawn from the Catawba River. The sluice lines conveyed the water/ash slurry and other flows to the southwest corner of the primary cell. Refer to Figure 3-1 for a depiction of these features. The ash at RBSS was originally stored in an area known as the cinder storage area. In 1957, a single-cell ash basin was commissioned to serve as an effective treatment system for wastewater containing coal ash. The ash basin was expanded in 1979 and divided by constructing an intermediate (divider) dike to form two separate cells. These cells, known as the primary and secondary cells, were previously used to retain and settle ash generated from coal combustion at RBSS. The cinder storage area stored ash from station operations prior to the construction of the ash basin. The ash storage area was used to store ash generated from ash basin cleanout projects. The ash management areas are unlined. 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 3-2 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx Ash Basin The ash basin is located approximately 2,400 feet to the northeast of the power station as shown on Figure 2-1. The primary cell is impounded by an earthen embankment dam, referred to as Dam #1 (primary), located on the west side of the primary cell. The secondary cell is impounded by an earthen embankment dam, referred to as Dam #2 (secondary), located along the northeast side of the secondary cell. The toe areas for both dams are in close proximity to Mountain Island Lake. The intermediate dike built in 1979 was constructed of soil on top of the existing ash in the basin. The construction provided hydraulic connection between the primary cell and secondary cell. The intermediate dike was removed in 2017 as part of excavation of the basin. Borrow areas within the ash basin are depicted in some RBSS site drawings but are omitted from others. Based on information provided by Duke Energy, a dredge pond was previously located south of the primary cell in the approximate location of the current ash storage area, as described below in Section 3.2.2. From 1993 to 2000, the dredged ash was allowed to dry and was hauled offsite for reuse at least once. The surface area of the primary cell is approximately 41 acres with an approximate maximum pond elevation of 724 feet. Prior to ongoing ash removal activities, the primary cell contained approximately 1.9 million cubic yards of ash. The surface area of the secondary cell is approximately 28 acres with an approximate maximum pond elevation of 714 feet. Prior to commencement of ash removal activities, the secondary cell contained approximately 700,000 cubic yards of ash. At a full pond elevation of 646.8 feet, Mountain Island Lake measures approximately 77 feet to 67 feet lower than the historical full pond elevations of the primary and secondary cells, respectively. During operation of the coal-fired units, the ash basin system was operated as an integral part of the site’s wastewater treatment system. This system predominantly received inflows from the ash removal system, station yard drain sump, and stormwater flows. During station operations, inflows to the ash basin were highly variable due to the cyclical nature of station operations. The inflows from the ash removal system and the station yard drain sump were conveyed through sluice lines into the primary cell. Discharge from the primary cell to the secondary cell was conveyed through a concrete discharge tower located near the northwestern end of the divider dike. 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 3-3 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx Prior to the station being retired, wastewater effluent from other non-ash-related station activities flowed to the ash basin and were discharged from the secondary cell through a concrete discharge tower, to NPDES Outfall #002. The concrete discharge tower historically drained through a 30-inch-diameter corrugated metal pipe into a concrete- lined channel that conveys to Mountain Island Lake. Ash Storage Area An unlined ash storage area is located topographically cross-gradient/upgradient and adjacent to the southwest side of the primary cell (Figure 2-1). The footprint is approximately 29 acres and was estimated to contain approximately 1.5 million tons of ash prior to ongoing ash removal activities. The ash storage area was constructed during two ash basin clean-out projects: one that occurred around 2000-2001 and another that occurred from late 2006 to early 2008. The clean-out projects were performed to provide additional capacity in the ash basin for future sluiced ash. For the purpose of water management, the storm water runoff from the ash storage area is routed to the ash basin system. As required by CAMA and per Duke Energy’s November 13, 2014 (revised December, 2016), proposed Coal Ash Excavation Plan for RBSS, Duke Energy will permanently close the RBSS ash ponds by August 1, 2019. The first phase of this plan includes the dewatering, excavation, and removal of approximately 1.0 million tons of ash from the storage area. A modified NPDES Wastewater Permit became effective on December 1, 2016, allowing for the dewatering of the ash basins. Subsequent phase(s) of excavation will remove the remaining ash in the ash storage area of the site. Ash removed from the site was previously transported by truck to the Brickhaven Structural Fill in Moncure, NC. As of first quarter 2016, ash is being transported by rail to the Brickhaven Structural Fill and over 2 million tons of ash has been excavated to-date. The RBSS ash management areas are in the process of being dewatered. Water from the basins is pumped to the onsite waste water treatment facility and discharged to the Catawba River via the permitted outfall SW002. Cinder Storage Area The unlined cinder storage area is located topographically cross-gradient and immediately west/southwest of the primary cell, and northwest of the ash storage area (Figure 2-1). The footprint is approximately 13 acres and forms a roughly triangular area northeast of the coal pile and northwest of the rail spur. After initial station operation in 1929 and prior to initial ash basin operation in 1957, bottom ash (cinders) generated as part of the coal combustion process was deposited in the cinder storage 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 3-4 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx area and other areas near the cinder storage area and coal pile. This area was also used for storage of ash material at the station prior to the installation of precipitators and a wet sluicing system around 1958. The storage area is estimated to contain approximately 300,000 tons of ash. Per Duke Energy’s November 13, 2014, proposed Coal Ash Excavation Plan for RBSS, the ash contained within the cinder storage area will be removed. 3.2 General Physical and Chemical Properties of Ash Coal ash consists of fly ash and bottom ash produced from the combustion of coal. The physical and chemical properties of coal ash are determined by reactions that occur during the combustion of the coal and subsequent cooling of the flue gas. Physical Properties Approximately 70 percent to 80 percent of the ash produced during coal combustion is fly ash (Young, S.C., 1993). Typically 65 percent to 90 percent of fly ash has particle sizes that are less than 0.010 millimeter (mm). In general, fly ash has a grain size distribution similar to that of silt. The remaining 20 percent to 30 percent of ash produced is considered to be bottom ash. Bottom ash consists of angular particles with a porous surface and is normally gray to black in color. Bottom ash particle diameters can vary from approximately 0.05 to 38 mm. In general, bottom ash has a grain size distribution similar to that of fine gravel to medium sand (Zachara, Rai, Moore, & Resch, 1995). Based on published literature not specific to the site, the specific gravity of fly ash typically ranges from 2.1 to 2.9 and the specific gravity of bottom ash typically ranges from 2.3 to 3.0. The permeability of fly ash and bottom ash vary based on material density but would be within the range of a sand-gravel with a similar gradation, grain- size distribution and density (Zachara, Rai, Moore, & Resch, 1995). Chemical Properties In general, the specific mineralogy of coal ash varies based on many factors, including the chemical composition of the coal, which is directly related to the geographic region where the coal was mined, the type of boiler where the combustion occurs (i.e., thermodynamics of the boiler), and air pollution control technologies employed. The overall chemical composition of coal ash resembles that of siliceous rocks from which it was derived, particularly shale. Oxides of silicon, aluminum, iron, and calcium make up more than 90 percent of most siliceous rocks, soils, fly ash, and bottom ash. Other major and minor elements (sulfur, sodium, potassium, magnesium, titanium) make up an additional 8 percent, while trace constituents account for less than 1 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 3-5 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx percent. The following constituents are considered to be trace elements: arsenic, barium, cadmium, chromium, lead, mercury, selenium, copper, manganese, nickel, vanadium, and zinc (Pugh, Whetstone, & Redwine, September 2010). According to Duke Energy, the primary source of coal burned at RBSS was bituminous coal from Eastern Kentucky. The majority of fly ash particles are glassy spheres mainly composed of amorphous or glassy aluminosilicates, crystalline matter, and carbon. The glassy spheres themselves are generally resistant to dissolution. During the later stages of the combustion process and as the combustion gases are cooling after exiting the boiler, molecules from the combustion process condense on the surface of the glassy spheres. These surface condensates consist of soluble salts (e.g. calcium (Ca2+), sulfate (SO42-), metals (copper (Cu), zinc (Zn), and other minor elements (e.g. boron (B), selenium (Se), and arsenic (As)) (Zachara, Rai, Moore, & Resch, 1995). The major elemental composition of fly ash (approximately 95 percent by weight) is composed of mineral oxides of silicon, aluminum, iron, calcium. Oxides of magnesium, potassium, titanium, and sulfur comprise approximately 4 percent by weight (Zachara, Rai, Moore, & Resch, 1995). Trace elemental composition typically is approximately 1 percent by weight and may include antimony, arsenic, barium, boron, cadmium, chromium, copper, manganese, mercury, nickel, lead, selenium, silver, thallium, zinc, and other elements. For comparison, Figure 3-2 shows the elemental composition of fly ash and bottom ash compared with typical values for shale and volcanic ash. Table 3-1 shows the bulk composition of fly ash and bottom ash compared with typical values for soil and rock. In addition to these constituents, fly ash may contain unburned carbon. Bituminous coal ash typically yields slightly acidic to alkaline solutions with pH levels ranging from approximately 5 to 10 on contact with water. The geochemical factors controlling the reactions associated with leaching of ash are complex. Factors such as the chemical speciation of the constituent, solution pH, solution-to-solid ratio, and other factors control the chemical concentration of the resultant solution. Constituents that are held on the glassy surfaces of fly ash such as boron, arsenic, and selenium may initially leach more readily than other constituents. As noted in Table 3-1, aluminum, silicon, calcium, and iron represent the larger fractions of fly ash and bottom ash by weight. The presence of calcium may limit the release of arsenic by forming calcium-arsenic precipitates. Formation of iron hydroxide compounds may also sequester arsenic and retard or prevent release of arsenic to the environment. Similar processes and reactions 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 3-6 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx may affect other constituents of concern; however, certain constituents such as boron and sulfate will likely remain highly mobile. In addition to the variability that might be seen in the mineralogical composition of the ash, which is based on different coal types, different age of ash in the basin, and other factors, it is anticipated that the chemical environment of the ash basin varies over time, distance, and depth. EPRI (Pugh, Whetstone, & Redwine, September 2010) reported that 64 samples of coal combustion residuals (including fly ash, bottom ash, and flue gas desulfurization residue) from 50 different power plants were subjected to EPA Method 1311 Toxicity Characteristic Leaching Procedure (TCLP) leaching and no TCLP result exceeded the TCLP hazardous waste limit. Figure 3-3 provides the results of that testing. 3.3 Site-Specific Coal Ash Data Source characterization was performed to identify the physical and chemical properties of the ash in the source areas (the ash basin, ash storage area and the cinder storage area). The source characterization involved developing selected physical properties of ash, identifying the constituents found in ash, measuring concentrations of constituents present in ash porewater, and performing laboratory analyses to estimate constituent concentrations resulting from the leaching process. The physical and chemical properties evaluated as part of this characterization will be used to better understand impacts to soil and groundwater from the source area and will also be used as part of groundwater model development in the CAP. At the RBSS site, source characterization was performed through the completion of soil borings, installation of monitoring wells, and collection and analysis of associated solid matrix and aqueous samples. Ash samples were collected for analysis of physical characteristics (e.g., grain size, porosity, etc.) to provide data for evaluation of retention/transport properties within and beneath the ash basin primary and secondary cells, ash storage area, and cinder storage area. Ash samples were collected for analysis of chemical characteristics (e.g., total inorganics, leaching potential, etc.) to provide data for evaluation of constituent concentrations and distribution. Samples were collected in general accordance with the Work Plan. Physical Properties of Ash Physical properties (grain size, specific gravity, and moisture content) were performed on six fly ash samples from the ash basin. Physical properties were measured using ASTM methods (Appendix G). Ash is generally characterized as a non-plastic silty 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 3-7 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx (medium to fine) sand or silt. Compared to soil, fly ash exhibits a lower specific gravity with three values reported from AS-1S (2.1), AS-2S (2.2), and AS-3S (2.3). Moisture content of the fly ash samples ranges from 18.6 to 52.4% (Table 3-2). Ash was generally described as gray to dark gray, non-plastic, loose to medium density, dry to wet, fine- to coarse-grained sandy silt texture. Chemical Properties of Ash Ash samples were collected during the installation of the monitoring wells installed inside the waste boundary of the ash basin as part of the 2015 CSA investigation. Concentrations of antimony, arsenic, chromium, cobalt, iron, manganese, selenium, thallium, and vanadium were reported above the North Carolina PSRGs for Industrial Health and/or Protection of Groundwater for ash samples collected within the ash basin waste boundary (see Appendix B, Table 4). Concentrations of arsenic, chromium, cobalt, iron, manganese, selenium, and vanadium were reported above the North Carolina PSRGs for Industrial Health and/or Protection of Groundwater Standards within the cinder storage area waste boundary (see Appendix B, Table 4). In addition to total inorganic testing of ash samples, eight ash samples collected from borings completed within the ash basin Primary and Secondary Cells, ash storage area, and cinder storage area were analyzed for leachable inorganics using Synthetic Precipitation Leaching Procedure (SPLP) (see Appendix B, Table 6). The coal ash managed at RBSS is currently being excavated. Coal ash removal for off-site re-use will be completed by the target date of August 1, 2019. The evaluation of leaching potential is helpful for understanding historical processes at the site. The purpose of the SPLP testing is to evaluate the leaching potential of constituents that may result in impacts to groundwater above 2L or IMACs. The results of the SPLP analyses indicated that antimony, arsenic, chromium, cobalt, iron, lead, manganese, selenium, thallium, vanadium, and nitrate exceeded their respective 2L or IMAC. Although SPLP analytical results are being compared to 2L or IMAC, these samples do not represent groundwater samples (for comparative purposes only). Chemistry of Ash Porewater Porewater refers to water samples collected from wells installed within the ash basins and screened in the ash layer. Five porewater monitoring wells (AB-3S, AB-4S, AB-5S, AB-5SL, and AB-7S) were installed within the waste boundary of the ash basin primary and secondary cells (Figure 2-4). At the time of the initial CSA in 2015, concentrations of antimony, arsenic, boron, cobalt, iron, manganese, pH, thallium, vanadium, and total dissolved solids (TDS) were 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 3-8 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx reported above 2L or IMAC in porewater samples collected from wells within the waste boundary of the ash basin primary and secondary cells. These wells were sampled as part of the CAMA monitoring program until the second quarter of 2016 when the wells were abandoned as part of ash removal activities. Sampling results show a decrease in constituent concentrations at some locations with most locations showing stable concentrations. No significant increases in constituent concentrations were observed. One porewater monitoring well (C-1S) was installed within the cinder storage area waste boundary. Concentrations of arsenic, cobalt, iron, manganese, nickel, pH, sulfate, thallium, TDS, and vanadium have been reported above 2L or IMAC in porewater samples from well C-1S. The well has been sampled periodically over the past two years and concentrations of the aforementioned constituents have decreased over time. The porewater sample locations are shown on Figure 2-4 and results are listed in Appendix B, Table 1. Ponded Ash Basin and Cinder Storage Area Water Samples Samples SW-1, SW-2, and SW-3 were collected from ponded water in the ash management areas (source water) for source characterization purposes. Locations SW-1 and SW-2 are in the ash basin secondary cell. Location SW-3 is in the cinder storage area. The locations were sampled periodically (6-7 total events) from June 2015 through May 2017 (Appendix B, Table 3). Observations from the source water samples are as follows: Boron concentrations ranged from non-detect at SW-2 in March 2016 to 1,230 µg/L at SW-3 in November 2016. Sulfate concentrations ranged from multiple non-detect values to 126 mg/l at SW- 3 in September 2015. Arsenic concentrations ranged from 0.34 µg/L at SW-3 in November 2015 to 229 µg/L at SW-1 in June 2015. Zinc concentrations ranged from non-detect at SW-1 in November 2015 to 220 µg/l in SW-3 in June 2015. 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 4-1 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx 4.0 RECEPTOR INFORMATION Section §130A-309.201(13) of the CAMA defines receptor as “any human, plant, animal, or structure which is, or has the potential to be, affected by the release or migration of contaminants. Any well constructed for the purpose of monitoring groundwater and contaminant concentrations shall not be considered a receptor.” In accordance with the NORR CSA guidance, receptors cited in this section refer to public and private water supply wells (including irrigation wells and unused wells) and surface water features. Refer to Section 12.0 for a discussion of receptors that were evaluated as part of this CSA effort. The NORR CSA receptor survey guidance requirements include listing and depicting water supply wells, public or private, including irrigation wells and unused wells (other than those that have been properly abandoned in accordance with 15A NCAC 2C .0100) within a minimum of 1,500 feet of the known extent of contamination. In NCDENR’s June 2015 response to Duke Energy’s proposed adjustments to the CSA guidelines, NCDENR DWR acknowledged the difficulty with determining the known extent of contamination at this time and stated that it expected all drinking water wells located 2,640 feet (0.5-miles) downgradient from the established compliance boundary to be documented in the CSA reports as specified in the CAMA requirements. The approach to the receptor survey in this CSA includes listing and depicting all water supply wells (public or private, including irrigation wells and unused wells) within a 0.5-mile radius of the ash basin compliance boundary. The NORR CSA guidance requires that subsurface utilities be mapped within 1,500 feet of the known extent of contamination in order to evaluate the potential for preferential pathways. Identification of piping near and around the ash basin was conducted by Stantec in 2014 and 2015 (Appendix E). It is anticipated that any underground utilities present at the site would not act as potential preferential pathways for contaminant migration through underground utility corridors to water supply well receptors. The flow of groundwater from the ash basin, ash storage area, and cinder storage area is to the Catawba River. Therefore, the mapping of underground features that serve as potential preferential pathways was limited to underground piping and drains located between the ash basin waste boundary and the Catawba River depicted in Figures 4-1 and 4-2. 4.1 Summary of Receptor Survey Activities Duke Energy completed and submitted a receptor survey to NCDENR (HDR, 2014a) in September 2014. Duke Energy subsequently submitted to NCDENR a supplement to 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 4-2 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx the receptor survey (HDR, 2014b) in November 2014, updated the receptor survey as part of the 2015 CSA, and again as part of the Drinking Water Supply Well and Receptor Survey (Draft, HDR 2016, included in Appendix D). The purpose of the receptor survey was to identify the potential exposure locations that are critical to be considered in the groundwater transport modeling and human health risk assessment. The supplementary information provided in November 2014 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. 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 (DWR) Public Water Supply Section’s (PWSS) most current Public Water Supply Water Sources Geographic Information Systems (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 USGS National Hydrography Dataset. In addition, a field reconnaissance was performed on January 27, 2014, to identify public and private water supply wells (including irrigation wells and unused or abandoned wells) and surface water features located within a 0.5-mile radius of the RBSS ash basin compliance boundary. A windshield survey was conducted from public roadways to identify water meters, fire hydrants, valves, and any potential well heads/well houses. Duke Energy site personnel provided information regarding water supply wells located on Duke Energy property. 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 4-3 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx During the week of October 8, 2014, 328 water supply well survey questionnaires were mailed to property owners within a 0.5-mile radius of the RBSS ash basin compliance boundary requesting information on the presence of water supply wells and well usage information for each property. The mailing list was compiled from a query of the parcel addresses included in the Gaston and Mecklenburg counties’ GIS databases utilizing the 0.5-mile offset. Between July 8 and July 23, 2015, the agencies/records listed above were contacted to provide additional update information. Updated information is provided in Appendix D. 4.2 Summary of Receptor Survey Findings As part of the 2015 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. The identified water supply wells are shown on the USGS receptor map on Figure 4-3 and on an aerial photograph on Figure 4-4. Available property and well information for the identified water supply wells is provided in Table 4-1. Underground features including underground piping and drains identified in the vicinity of the ash basin are shown in Figure 4-1. Underground features including underground piping and drains identified in the vicinity of the cinder storage and ash storage areas are shown in Figure 4-2. The dams associated with the ash basin contain engineered drainage features associated with dam drainage and stability. These features are internal or adjacent to the dams and are not included in the underground features mapping. The underground piping and drains on the site appear to convey water from various portions of the RBSS site toward Mountain Island Lake. The underground piping or drains do not act as preferential pathways between the impacted portions of the RBSS site to the identified water supply well on the north side of the lake. Table 4-2 presents names and addresses of property owners contiguous to the ash basin waste boundary which correspond to the parcels depicted on Figure 4-5. 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 4-4 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx 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 the Catawba River 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. 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 the Catawba River were identified within a 0.5-mile radius of the ash basin (Figure 4-6). Based on the returned water supply well questionnaires since October 31, 2014, no additional receptors were identified. Further details of HDR’s receptor survey activities and findings are presented in Appendix D. The 2016 receptor survey activities confirmed that the Mount Holly Public Utilities Department provides municipal water service to the RBSS site and the properties located in Gaston County within 0.5-mile radius of the RBSS ash basin compliance boundary. The Charlotte-Mecklenburg Utilities Department (CMUD) does not provide water service to properties in Mecklenburg County within 0.5-mile radius of the RBSS ash basin compliance boundary. Additional details were gathered as provided below for water supply wells within 0.5-mile radius of the ash basin compliance boundary. 4.2.1 Public Water Supply Wells No public water supply wells (including irrigation wells and unused wells) or wellhead protection areas were identified within a 0.5-mile radius of the RBSS ash basin compliance boundary. 4.2.2 Private Water Supply Wells 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. The Catawba River, a main stem river, separates the well location from the RBSS site. 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 4-5 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx 4.3 Private and Public Well Water Testing Program Section § 130A-309.209 (c) of the CAMA requires the owner of a coal combustion residuals surface impoundment to conduct a Drinking Water Supply Well Survey that identifies all drinking water supply wells within one-half mile downgradient from the established compliance boundary of the impoundment and submit the Survey to the Department. NCDEQ coordinated sampling of drinking water receptors within 0.5 mile of the ash basin compliance boundary at a number of CCR facilities. The one reported private water supply well located northeast of RBSS within a 0.5-mile radius of the ash basin compliance boundary is separated from the ash management areas by the Catawba River, considered a main stem river. (Well 1 shown on Figures 4-3 and 4-4). The NCDEQ private well sampling program did not include any wells within 0.5 mile of the compliance boundary at the RBSS site, and “Do Not Drink” letters were not issued by the North Carolina Department of Health and Human Services (NCDHHS). 4.4 Surface Water Drinking Water Sources The surface water classification for Mountain Island Lake is Class WS-IV, Class B, and Class C. Class WS-IV waters are protected as water supplies which are generally in moderately to highly developed watersheds. Class C are waters protected for uses such as secondary recreation, fishing, wildlife, fish consumption, aquatic life including propagation, survival and maintenance of biological integrity, and agriculture. Class B waters are protected for all Class C uses in addition to primary recreation (swimming). Mountain Island Lake is used as water supply for the Charlotte Metropolitan area, as well as the towns of Gastonia and Mount Holly, North Carolina. The Charlotte intakes, Charlotte-Mecklenburg, Franklin and Vest WTP 1 and WTP, are located 3.4 miles downstream from the RBSS site. The Gastonia and Mount Holly intakes, Two Rivers Utilities (City of Gastonia), Gastonia WTP and City of Mount Holly, Mount Holly WTP, are located approximately 6.9 miles downstream from the RBSS site. These surface water intake locations are shown in Figure 4-7. 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 5-1 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx 5.0 REGIONAL GEOLOGY AND HYDROGEOLOGY North Carolina is divided into three physiographic provinces: the Atlantic Coastal Plain, Piedmont, and Blue Ridge (Fenneman, 1938). The RBSS site is located in the Piedmont province. The Piedmont province is bounded to the east and southeast by the Atlantic Coastal Plain and to the west by the escarpment of the Blue Ridge Mountains, with a width ranging from 150 miles to 225 miles in the Carolinas (LeGrand, 2004). A discussion of geology and hydrogeology relevant to the RBSS site is provided below. 5.1 Regional Geology The topography of the Piedmont region is characterized by low, rounded hills and long, rolling, northeast-southwest trending ridges (Heath, 1984). Stream valley to ridge relief in most areas ranges from 75 feet to 200 feet. From the Coastal Plain boundary, the Piedmont region rises from an elevation of 300 feet above mean sea level to an elevation of 1,500 feet, at the base of the Blue Ridge Mountains (LeGrand, 2004). The RBSS site lies within the Charlotte terrane, one of a number of tectonostratigraphic terranes that have been defined in the southern and central Appalachians. The Charlotte terrane is in the western portion of the larger Carolina super terrane (Figure 5-1; (Horton, Jr., Drake, Jr., & Rankin, 1989); (Hibbard, Stoddard, Secor, & Dennis, 2002); (Hatcher, Jr., Bream, & Merschat, 2007). On the northwest side, the Charlotte terrane is in contact with the Inner Piedmont zone along the Central Piedmont suture. The Charlotte terrane is distinguished from the Carolina terrane to the southeast by its higher metamorphic grade. Portions of the boundary may be tectonic (Secor, Balinsky, & Colquhoun, 1998); (Dennis, Shervais, & Secor, 2000). The Charlotte terrane is dominated by a 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. The terrane includes minor occurrences of metasedimentary rocks including phyllite, mica schist, biotite gneiss, and quartzite with marble along its western portion ( (Butler & Secor, 1991); (Hibbard, Stoddard, Secor, & Dennis, 2002). The general structure of the belt is primarily a function of plutonic contacts. A geologic map of the area around the RBSS site is shown in Figure 5-2. 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 and mechanical weathering of the underlying bedrock, is typically composed of clay and coarser granular material up to 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 5-2 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx boulder size. Saprolite reflects the texture and structure of the rock from which it was formed. For example, the weathering products of granitic rocks are quartz-rich and sandy textured. Rocks poor in quartz and rich in feldspar and ferro-magnesium minerals form a more clayey saprolite (LeGrand, 2004). The degree of weathering decreases with depth and partially weathered rock (PWR) is commonly present near the top of the bedrock surface. The transition zone from the regolith and the PWR and competent bedrock is often gradational and difficult to differentiate. 5.2 Regional Hydrogeology The groundwater system in the Piedmont province, in most cases, is described as being comprised of two interconnected layers, or as having a two-medium system. The parts include (1) residual soil/saprolite and weathered fractured rock (regolith and PWR) overlying (2) fractured crystalline bedrock (Heath R. , 1980); (Harned & Daniel, 1992); Figure 5-3). The shallow regolith layer is a thoroughly weathered and structureless residual soil that occurs near ground surface with the degree of weathering decreasing with depth. Residual soil grades into saprolite. Beneath the saprolite, partially weathered/fractured bedrock occurs with depth until competent bedrock is encountered. This mantle of residual soil, saprolite, and weathered/fractured rock (transition zone) is a hydrogeologic unit that covers and crosses various types of rock (LeGrand, 1988). This layer serves as the principal storage reservoir and provides a granular medium through which the recharge and discharge of water from the underlying fractured rock occurs. A transition zone at the base of the regolith is present in many areas of the Piedmont. The zone consists of partially weathered/fractured bedrock and lesser amounts of saprolite that grades into competent bedrock and has been described as “being the most permeable part of the system, even slightly more permeable than the soil zone” (Harned & Daniel, 1992). The zone thins and thickens within short distances and its boundaries may be difficult to distinguish. Where present, the zone may serve as a conduit of rapid flow and transport of impacted groundwater (Harned & Daniel, 1992). Daniel and Dahlen provide a summary of the nature and occurrence of groundwater in fractured rock (Daniel & Dahlen, 2002). Within the fractured crystalline bedrock, fracture apertures, connectivity, etc. control groundwater movement and storage capacity. The bedrock is broken and displaced by faults and shear zones, some of which extend for miles. Joints, rock breaks without accompanying displacement, are common, and the joints typically occur in groups oriented in preferred directions. Weathering and erosion have resulted in fracturing in the form of stress-relief fractures, as well as expansion of existing fractures, and it is through these fractures that groundwater 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 5-3 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx flows. Planes and bedding of metamorphic foliation, as well as breaks and folds in these rocks, are areas of higher permeability (Daniel & Dahlen, 2002). LeGrand’s conceptual model of the groundwater setting in the Piedmont incorporates the above two-medium system into a single feature that is useful for the description of groundwater conditions (LeGrand, 1988) (LeGrand, 1989). That feature is the surface drainage basin that contains a perennial stream (LeGrand, 1988). Each basin is similar to adjacent basins and the conditions are generally repetitive from basin to basin. Within a basin, movement of groundwater is generally restricted to the area extending from the drainage divides to a perennial stream (Slope-Aquifer System; Figure 5-4; (LeGrand, 1988), (LeGrand, 1989), (LeGrand, 2004). Rarely does groundwater move beneath a perennial stream to another more distant stream or across drainage divides (LeGrand, 1989). The crests of water table undulations represent natural groundwater divides within a slope-aquifer system and may limit the area of influence of wells or contaminant plumes located within their boundaries depending on the depth of the impacted groundwater. The concave topographic areas between the topographic divides may be considered as flow compartments that are open-ended down slope. Therefore, in most cases in the Piedmont, the groundwater system is a two-medium system (LeGrand, 1988) restricted to the local drainage basin. Groundwater within the area exists under unconfined (water table) conditions within the saprolite, PWR/transition zone, and in the fractures and joints of the underlying bedrock. The water table and bedrock aquifers are often interconnected. Typically, the residual soil/saprolite is partially saturated and the water table fluctuates within it. The saprolite and PWR/transition zone acts as a reservoir for water supply to the fractures and joints in the underlying bedrock. The near-surface fractured 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 bedrock modifies both transmissivity and storage characteristics. Groundwater flow paths in the Piedmont are almost invariably restricted to the zone underlying the topographic slope extending from a topographic divide to an adjacent stream. Under natural conditions, the general direction of groundwater flow can be approximated from surface topography (LeGrand, 2004). Shallow groundwater generally flows from local recharge zones in topographically high areas, such as ridges, toward groundwater discharge zones, such as stream valleys. Ridges and topographically higher areas may serve as groundwater recharge zones. Groundwater flow patterns in recharge areas tend to develop a somewhat radial pattern from the 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 5-4 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx center of the recharge area outward toward the discharge areas and are expected to mimic surface topography. 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 6-1 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx 6.0 SITE GEOLOGY AND HYDROGEOLOGY Information from previous site investigations and information from the soil borings and monitoring wells installed for the CSA and additional assessment were used to characterize site geology and hydrogeology. 6.1 Site Geology 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, Stoddard, Secor, & Dennis, 2002) that range from intermediate to mafic in composition (Butler & Secor, 1991). A bedrock geologic map displaying the site and boring locations is presented as Figure 6-1. The installed well and sample locations are shown on Figure 2-4. 6.1.1 Soil Classification The following soils/materials were encountered in the boreholes: Ash – Ash was encountered in borings advanced within the ash basin, ash storage area, and cinder storage area, as well as in one boring at the ash storage area perimeter. Ash was generally described as gray to dark bluish gray with a silty to sandy texture, consistent with fly ash and bottom ash. Fill – Fill material generally consisted of reworked sandy silts, clays, and sands that were re-distributed from one area of the site to other areas. Fill was classified in the boring logs as sandy silt, clay with sand, clay, sandy clay, and clay with gravel. Fill was used in the construction of dikes and as cover for the ash storage area. Alluvium – Alluvium is unconsolidated soil and sediment that has been eroded and redeposited by streams and rivers. Alluvium may consist of a variety of materials ranging from silts and clays to sands and gravels. During site construction and plant operation, alluvial deposits have been removed or covered. Alluvium was encountered in borings along Mountain Island Lake during the project subsurface exploration activities. Alluvium was logged in GWA-5D and MW-15D. Differentiations between alluvium and fill are approximate; similarities in material made such differentiation challenging. Residuum (Residual soils) – Residuum is the in-place weathered soil that consists primarily of sandy silt, clay, silt, clayey sand, sand with silt and gravel, and clay with sand and gravel at the RBSS site. Residuum varied in 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 6-2 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx thickness and was relatively thin compared to the thickness of saprolite. Differentiations between residuum and fill are approximate; similarities in material made such differentiation challenging. Saprolite – Saprolite is soil developed by in-place weathering of rock that retains remnant bedrock structure. The primary distinction that sets it apart from residuum is that saprolite typically retains some structure (e.g., mineral banding) from the parent rock. Saprolite at the RBSS site consists primarily of lean clay, sand with clay, silt with sand, sand, sandy silt (variably micaceous), silty sand, silty sand with gravel, and silt with gravel. Sand particle size ranges from fine to coarse grained. Saprolite is typically logged as micaceous. Undisturbed and split-spoon samples collected during the field investigation were tested for various geotechnical parameters, including natural moisture content, Atterberg Limits (undisturbed samples only), grain size with hydrometer, total porosity (undisturbed samples only), and vertical hydraulic conductivity (undisturbed samples only). The samples were classified using the Unified Soil Classification System (USCS). Geotechnical index property testing was completed for disturbed and undisturbed samples in accordance with ASTM standards. Thirty-five (35) undisturbed (Shelby Tube) samples were submitted for geotechnical index testing. Index property testing for undisturbed samples included USCS ASTM D 2487 (ASTM, 2001), natural moisture content ASTM D 2216 (ASTM, 2010a), Atterberg Limits ASTM D 4318 (ASTM, 2010b), grain size distribution, including sieve analysis and hydrometer ASTM D 422 (ASTM, 2007), total porosity calculated from Specific Gravity ASTM D 854 (ASTM, 2010d), and hydraulic conductivity ASTM D 5084 (ASTM, 2010c). Four undisturbed samples were not subjected to the full suite of index property tests due to low recovery, wax and gravel mixed in the tube, loose material, or damaged tubes. Twenty (20) disturbed (split spoon, or jar) samples were analyzed for grain size distribution with hydrometer ASTM D 422 (ASTM, 2007) and natural moisture content ASTM D 2216 (ASTM, 2010a). Results from the geotechnical property testing show background soil samples collected at RBSS range from silty sand and sandy silt to clayey sand. Natural moisture content in the background soil samples is as low as 16.2% (clayey sand) and as high as 36.2% (silty sand). Specific gravity values for the background soil samples are between 2.657 and 2.689. High levels of fine sand, silt, and clay are present in all background soil samples, while little to no levels of gravel are 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 6-3 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx present in the samples. Soil samples collected from downgradient boreholes are characterized as silty sand and sandy silt to clayey sand. Downgradient soil samples have natural moisture content levels from 11.3% to 52.6%. Similar to the background soil samples, the downgradient soil samples have a specific gravity range of 2.603 to 2.923. While the downgradient samples are mainly comprised of fine sand, silt and clay, fine gravel is also present in many of the samples. All soil property results are shown in Table 3-2. 6.1.2 Rock Lithology Bedrock at the RBSS site consists primarily of meta-quartz diorite and lesser occurrences of meta-diabase. Based on rock core descriptions, the meta-quartz diorite color typically is a white to light gray matrix with dark greenish gray, dark gray, and black phenocrysts. The texture is described as phaneritic, fine to coarse grained, non-foliated and massive. Foliation is rarely noted. The meta- quartz diorite is composed dominantly of plagioclase, quartz, biotite, hornblende, and epidote. The meta-diabase is very dark to dark greenish gray black to very dark greenish gray, is mostly non-foliated, and is mostly noted as aphanitic, but a porphyritic texture with phenocrysts is noted on some of the boring logs. Banding and foliation is noted in the meta-diabase and generally parallel the dike contacts. Vugs and fractures with calcite mineralization are occasionally noted in the meta-diabase. 6.1.3 Structural Geology The Charlotte terrane is a meta-igneous terrane consisting of volcanic and plutonic rocks that have been subjected to deformation and high grade metamorphism due to tectonic stress during and after intrusion of the igneous units. Foliation is noted only in some of the rock core and is not dominant with respect to the structure of the rock mass. Data from the rock core also show a number of joint dip angles that cannot be properly defined as joint sets since there is no strike orientation information. For the purpose of this discussion, the joints have been assessed based on dip angle alone. The most prevalent dip angles are from 10 to 20 degrees and from 30 to 40 degrees. These two sets are predominant based on the number of joints noted on the boring logs. Iron and manganese staining is noted on joints, indicating that the joints are pathways for groundwater flow. The degree of openness (aperture) of any of the joints is difficult to assess from rock core since the core is often 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 6-4 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx broken at a joint and no longer retains its actual in-place aperture. However, some of the logs describe some joints as open. Geologic mapping was not successful in defining any joint sets in outcrop that could help define the dip strike of any of the joint sets. 6.1.4 Soil and Rock Mineralogy and Chemistry Soil and rock mineralogy and chemistry analyses conducted for the 2015 CSA are shown in Table 6-1 (mineralogy), Table 6-2 (chemistry, percent oxides), and Table 6-3 (chemistry, elemental composition). The mineralogy and chemical composition of partially weathered bedrock materials are presented in Table 6-4 (mineralogy) and Table 6-5 (chemistry). Whole rock chemistry results (percent oxides and elemental composition) are shown in Tables 6-6 and Table 6-7, respectively. The dominant mineral constituents in the soils are quartz, feldspar (both alkali and plagioclase feldspars), kaolinite, and illite. Soils exhibiting a higher degree of weathering show an increase in kaolinite and illite content. Other minerals identified include chlorite, biotite, muscovite, and amphibole. The major oxides in the soils are SiO2, Al2O3, and Fe2O3. Major weathered rock minerals are quartz, feldspar, muscovite/vermiculite/illite, kaolinite, chlorite, and smectite. 6.2 Site Hydrogeology The CSA included a site hydrogeological investigation to characterize site hydrogeological conditions, including groundwater flow direction, hydraulic gradient and conductivity, groundwater and contaminant velocity, and slug and aquifer test results. The hydrogeological investigation was performed in general accordance with the procedures described in the Work Plan. According to LeGrand, the soil/saprolite regolith and the underlying fractured bedrock represent a composite water-table aquifer system (LeGrand, 2004). The regolith provides the majority of water storage in the Piedmont province, with porosities that range from 35 percent to 55 percent (Daniel & Dahlen, 2002). Calculated porosities specific to the Site (36.5 percent to 55.4 percent) are consistent with this range. The soil and saprolite regolith layer at RBSS is thickest underneath the ash basin and in upgradient areas (147 feet to 245 feet for AB-4, AB-7 and GWA-20), and thins in downgradient areas (35 feet to 67 feet for GWA-3, GWA-9 and MW-11) near the Catawba River. 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 6-5 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx 6.2.1 Hydrostratigraphic Layer Development Based on the CSA site investigation, the groundwater system is consistent with the two-layer regolith-fractured bedrock system discussed in Section 5.2. Regolith, generally consisting of a thick (greater than 100 feet) layer of saprolite, is underlain by a limited zone of weathered rock that transitions to competent bedrock. A hydrostratigraphic confining layer was not observed at RBSS. The hydrostratigraphic classification system of Schaeffer (2014a) was used to evaluate natural system (except alluvium) hydrostratigraphic layer properties. The classification system is based on Standard Penetration Testing values (N) and the Recovery (REC) and Rock Quality Designation (RQD) collected during the drilling and logging of the boreholes (Borehole/Well logs in Appendix F). The Schaeffer classification system uses the terms M1 and M2 to classify saprolite material with the M2 designation indicating greater competency. A zone of weathered, fractured rock is delineated between overlying saprolite and underlying bedrock based on rock core recovery (REC) and rock quality designation (RQD). Where that zone of weathered rock is found to be of greater permeability than the overlying saprolite it is identified as the transition zone. The bedrock zone is classified as having REC greater than 85 percent and RQD greater than 50 percent. As noted, the regolith and saprolite overlying bedrock at RBSS consists of a relatively thick layer of unsaturated and saturated material. For discussion purposes, hydrostratigraphic units will be recognized in the text and supporting documents as follows: Shallow Unit – Alluvium/Saprolite (S wells) Deep Unit – Saprolite and weathered rock (D wells) Bedrock – Sound rock, relatively unfractured (BR and BRU wells) The shallow zone generally corresponds to the M1 unit, and the deep zone incorporates the M2 and weathered, fractured rock layers. Bedrock is identified per the REC and RQD criteria. The designations saprolite, weathered rock, and bedrock are used on the generalized geologic cross-sections presented in Figures 6-3 to 6-7 showing site geology, groundwater flow directions, and current state of ash excavation. 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 6-6 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx Locations of the geologic cross-sections presented in Figures 6-3 to 6-7 are shown on Figure 6-2. 6.3 Groundwater Flow Direction Based on the CSA site investigation, groundwater flow is generally to the northwest, north, and northeast in the direction of the Catawba River (Mountain Island Lake). Accessible voluntary, compliance, and ash basin assessment monitoring wells are periodically gauged for depth to water at the Site (Table 6-8). Coal ash removal was initiated at RBSS in May 2015 and is ongoing as of October 2017. As of July 2017, nearly half (46.4 percent) of the coal ash managed at the site had been removed. The intermediate dam separating the primary and secondary cells was excavated as of March 2017. Due to these significant site developments that have the potential to influence groundwater flow conditions, depth to water gauging events early (July 8-9, 2015) and later during the ash removal process (May 22, 2017) were selected to illustrate the potentiometric surface at the site. Depths to water measurements were subtracted from surveyed top-of-well-casing elevations to produce groundwater elevations in shallow, deep, and bedrock monitoring wells. Groundwater flow direction was estimated by contouring those groundwater elevations. In general, groundwater at the site flows to the northwest, north, and northeast 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 generally in a northerly direction from the south side of the site to the Catawba River. Water levels were lower in most wells around the basin in May 2017 when compared with measurements from July 2015. Review of the flow paths observed in July 2015 and May 2017 do not indicate significant changes in groundwater flow direction. The shallow, deep, and bedrock water level maps are included as Figures 6-8 through 6- 13. 6.4 Hydraulic Gradient Horizontal hydraulic gradients were derived for the shallow, deep, and fractured bedrock wells by calculating the difference in hydraulic heads over the length of the flow path between two wells with similar well construction (e.g., both wells having 15- 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 6-7 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx foot screens within the same water–bearing unit). The following equation was used to calculate horizontal hydraulic gradient: i = dh / dl where i is the hydraulic gradient; dh is the difference between two hydraulic heads; and dl is the flow path length between the two wells. Applying this equation to wells installed during the CSA investigation yields the following average horizontal hydraulic gradients (measured in feet/foot): S wells: 0.031 D wells: 0.023 BR wells: 0.028 A summary of hydraulic gradient calculations is presented in Table 6-9. Vertical hydraulic gradients were calculated by taking the difference in groundwater elevation in a deep and shallow well pair over the difference in total well depth of the deep and shallow well pair. A negative output indicates upward flow and a positive output indicates downward flow. Thirteen (13) well pair locations, each consisting of a shallow, deep, or bedrock groundwater monitoring well, were used to calculate vertical hydraulic gradient across the site. Based on review of the results, vertical gradients were mixed across the site. More upward values were noted near the Catawba River downgradient from the primary and secondary cells (GWA-2, GWA-1, MW-3, and MW- 5 locations). Vertical gradient calculations are summarized in Table 6-10. 6.5 Hydraulic Conductivity Hydraulic conductivity values for hydrostratigraphic units were developed by grouping data from multiple coal ash facilities in the Piedmont into their respective hydrostratigraphic units and calculating the geometric mean, median, and standard deviation. Horizontal and vertical hydraulic conductivities are presented in Tables 6-11 and 6-12. Site specific hydraulic conductivities from slug tests are presented in Section 6.8. 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 6-8 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx 6.6 Groundwater Velocity Darcy’s Law is an equation that describes the flow rate or flux of fluid through a porous medium. To calculate the velocity that water moves through a porous medium, the specific discharge, or Darcy flux, is divided by the effective porosity, ne. The result is the average linear velocity or seepage velocity of groundwater between two points. The following equation was used to calculate seepage velocities through each hydrostratigraphic unit present at the site: v = Ki/ne where v is velocity; K is horizontal hydraulic conductivity; i is horizontal hydraulic gradient; and ne is the effective porosity Seepage velocities for groundwater were calculated using horizontal hydraulic gradients established in Section 6.2.2, horizontal hydraulic conductivity values for each hydrostratigraphic unit established in Table 6-11, and effective porosity values established in Table 6-13. Hydrostratigraphic layers are defined in Section 11.1. Average groundwater velocity results are summarized in Table 6-9. 6.7 Contaminant Velocity The degree of migration, retardation, and attenuation of constituents in the subsurface is a function of physical and chemical properties of the media through which the groundwater passes. Contaminant velocity depends on factors such as the rate of groundwater flow, the effective porosity of the aquifer material, and the soil-water partitioning coefficient, or Kd term. Soil samples were collected and analyzed for grain size, total porosity, soil sorption (Kd), and anions/cations to provide data necessary for completion of a fate and transport model. Constituents enter the ash basin system in both dissolved and solid phases and may undergo phase changes, including dissolution, precipitation, adsorption, and desorption. Dissolved phase constituents may undergo these phase changes as they are transported in groundwater flowing through the basin. Phase changes are collectively addressed by specifying a linear soil-groundwater partitioning coefficient (sorption coefficient [Kd]). In the fate and transport model, the entry of constituents into the ash basin is represented by a constant concentration in the saturated zone (porewater) of the basin, and is continually replaced by infiltrating recharge from above. Laboratory Kd terms were developed by University of North Carolina at Charlotte UNCC via column testing of 14 site-specific samples of soil. The methods used by UNCC and Kd results 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 6-9 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx obtained from the testing are presented in Appendix C. The Kd data were used as an input parameter to evaluate constituent fate and transport through the subsurface. Boron is relatively mobile in groundwater and is associated with low Kd values. This is primarily because boron is mostly inert, has limited potential for sorption, and lacks an affinity to form complexes with other ions. In general, the low Kd measured for boron allows the constituent to move at a similar velocity as that for groundwater. The higher Kd values measured for other constituents, like arsenic and cobalt, are consistent with the observed, limited migration of these constituents. Constituents like cobalt and arsenic have much higher Kd values and will move at a much slower velocity than groundwater as it sorbs onto surrounding soil. Geochemical mechanisms controlling the migration of constituents are discussed further in Section 13.0. Groundwater modeling to be performed in the CAP will include discussion of contaminant velocities for the modeled constituents. 6.8 Slug Test and Aquifer Test Results The material properties required for the groundwater flow and transport model (total porosity, effective porosity, specific yield, and specific storage) for ash, fill, alluvium, and soil/saprolite were developed from laboratory testing (Table 6-13 and Table 6-14; test data in Appendix C) and published data. Effective porosities were calculated using laboratory testing and physical soil data presented in Appendix B, Table 4 (soils table) and estimating them on a Fetter-Bear diagram, as described in (Johnson, 1967). This technique provides a simple method for estimating specific yield; however, there are limitations to this method that may not provide an accurate determination of the specific yield of a single sample (Robson, 1993). Hydraulic conductivity (horizontal and vertical) of all layers, except the weathered rock and bedrock (BR), was developed using existing site data and historic data, in-situ permeability testing (falling head, constant head, and packer testing where appropriate), slug tests in completed monitoring wells, and laboratory testing of undisturbed samples (ash, fill, soil/saprolite: test results in Appendix C). Borehole In-Situ Tests In-situ horizontal (open hole) and vertical (flush bottom) permeability tests, either falling or constant head as appropriate for field conditions, were performed in each of the hydrostratigraphic units above refusal, ash, fill, alluvium, and soil/saprolite. In-situ borehole horizontal permeability tests, either falling or constant head tests as appropriate for field conditions, were performed just below refusal in the first 5 feet of a rock cored borehole. 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 6-10 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx The flush bottom test involves advancing the borehole through the overburden with a casing advancer until the test interval is reached. The cutting tool is removed from the casing, and the casing is filled with water to the top. The top of the water level in the casing is then measured over a period of 60 minutes. In the open hole test, after the top of the test interval is reached, the cutting tool (but not the casing) is advanced (5 feet in the majority of tests) and top of the water level in the casing is measured over a time period of 60 minutes. The constant head test is similar except the water level is kept at a constant level in the casing and the water flow-in is measured over a period of 60 minutes. The constant head test was used only when the water level in the borehole was decreasing exponentially back to the static water level, such that the time interval was insufficient to calculate the hydraulic conductivity. The results from the field permeability testing are summarized in Table 6-15, and the worksheets are provided in Appendix C. Packer tests (shut-in and pressure tests) were conducted in a minimum of five boreholes. The shut-in test is performed by isolating the zone between the packers (in effect, a piezometer) and measuring the resulting water level over time until the water level is stable. The shut-in test provides an estimate of the vertical gradient during the test interval. The pressure test involves forcing water under pressure into rock through the walls of the borehole providing a means of determining the apparent horizontal hydraulic conductivity of the bedrock. Each interval is tested at three pressures with three steps of 20 minutes up and two steps of 5 minutes back down. The pressure test results are summarized in Table 6-15 and the shut-in and packer tests worksheets are provided in Appendix C. Where possible, tests were conducted at borehole locations specified in the Work Plan and at test intervals based on site-specific conditions at the time of the groundwater assessment work. The U.S. Bureau of Reclamation (1995) test method and calculation procedures, as described in Chapter 10 of their Ground Water Manual (2nd Edition), were used for the field permeability and packer tests. Monitoring Well and Observation Well Slug Tests Hydraulic conductivity (slug) tests were completed in monitoring wells and observation wells. Slug tests were performed to meet the requirements of the May 31, 2007 NCDENR Memorandum titled, Performance and Analysis of Aquifer Slug Tests and Pumping Tests Policy. Slug test field data were analyzed using the Aqtesolv (or similar) software and the Bouwer and Rice method. 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 6-11 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx The slug test results are presented in Table 6-16, and the Slug Test Reports are provided in Appendix C. The geomean for hydraulic conductivity in the shallow zone wells was 2.79E-04 centimeters per second (cm/sec), while the deep and bedrock layers were 3.03E-04 cm/sec and 2.60E-05 cm/sec, respectively. Monitoring well logs and core photos, field sampling and slug test records, and analytical laboratory reports for the 2015 CSA, CAP Part 1 and 2, and the 2016 CSA Supplement 2 are included in Appendices A, B, and C, respectively. From June 2016 to October 2016, a total of eight additional monitoring wells were also installed after submittal of the 2016 CSA Supplement 2 report. The additionally installed monitoring wells included BG-4BR, GWA-15S/D, MW-9BRA, GWA-20DA, GWA-23DA, and MW-7BRA. Laboratory Permeability Tests Laboratory permeability tests were conducted on undisturbed samples (Shelby Tubes) of ash, fill, soil, and saprolite collected during the field investigation. The tests were performed in accordance with ASTM D 5084 (ASTM, 2010c). Results of the laboratory permeability tests are presented in Table 6-17, and a historical laboratory permeability test result is presented in Table 6-18. The soil material parameters for the A (ash), F (fill), S (alluvium), M1 (soil/saprolite), and M2 (saprolite/weathered rock) were developed by grouping the data into their respective hydrostratigraphic units and calculating the mean, median, and standard deviation of the different parameters. Estimated values for total porosity for hydrostratigraphic layers are presented in Table 6-14. Geologic Mapping Geologic mapping was conducted in April 2015 to map outcrops at the site and within a 2-mile radius of the site to identify rock types. A Brunton compass was used in an attempt to characterize the orientation (strike and dip) of structure such as foliation, joint sets, folds, and shear zones. Due to limited outcrop locations, geologic mapping was not successful in defining major rock structure. Figure 6-1 shows the locations where outcrops were mapped. The site location and well locations are overlaid on the Geologic Map of the Charlotte 1° x 2° Quadrangle, North Carolina and South Carolina (Goldsmith, Milton, & Horton, Jr., 1988). Field mapping and use of all of the borehole data confirm the geologic units and location of contacts of the units with no evidence for differing geologic units or contact locations. 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 6-12 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx 6.9 Fracture Trace Study Results 6.9.1 Introduction Fracture trace analysis is a remote sensing technique used to identify lineaments on topographic maps and aerial photography that may correlate to locations of bedrock fractures exposed at the earth’s surface. Although fracture trace analysis is a useful tool for identifying potential fracture locations, and hence potential preferential pathways for infiltration and flow of groundwater near a site, results are not definitive. Lineaments identified as part of fracture trace analysis may or may not correspond to actual locations of fractures exposed at the surface, and if fractures are present, it cannot be determined from fracture trace analysis whether these are open or healed. Healed fractures intruded by diabase are common in the vicinity of the site. Strongly linear features at the earth’s surface are commonly formed by weathering along steeply dipping to vertical fractures in bedrock. Morphological features such as narrow, sharp-crested ridges, narrow linear valleys, linear escarpments, and linear segments of streams otherwise characterized by dendritic patterns are examples. Linear variations in vegetative cover are also sometimes indicative of the presence of exposed fractures, though in many cases these result from unrelated human activity or other geological considerations (e.g., change in lithology). Straight (as opposed to curvilinear) features are commonly associated with the presence of steeply dipping fractures. Curvilinear features in some cases are associated with exposed, moderately dipping fractures, but they also can be a result of preferential weathering along lithologic contacts, or along foliation planes or other geologic structure. As part of this study, only strongly linear features were considered, as they are far more commonly indicative of steeply dipping or vertical fractures. The effectiveness of fracture trace analysis in the eastern United States, including in the Piedmont, is commonly hampered by the presence of dense vegetative cover, and oftentimes extensive land-surface modification owing to present and past human activity. Aerial-photography interpretation is most affected, as identification of small-scale features can be rendered difficult or impossible in developed areas. Substantial surface alteration occurs over an estimated 30 percent of the aerial-photography study area for the RBSS site. 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 6-13 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx 6.9.2 Methods To identify lithologies and structures in the area, as well as likely orientations, available geologic maps, aerial photography and topographic map for the area were reviewed. Both low-altitude aerial photography — provided by Duke Energy (from WSP Global, Inc.,), covering approximately 4 square miles, — and USGS 1:24000 scale topographic maps covering an area of approximately 20 square miles were examined. Maps examined included portions of the Mountain Island Lake, N.C., and Lake Norman South N.C. USGS 7.5’ (1:24,000 Scale) topographic quadrangles. Digital copies of the quadrangles were obtained and viewed on a monitor at up to 7x magnification. Lineaments identified were plotted directly on the digital images. Lineaments identified from topographic maps and lineament trends indicated by a rose diagram are included on Figure 6-14. Photography provided for review included 1 inch=600 feet scale, 9-inch x 9-inch black-and-white (grayscale) contact prints dated April 17, 2014. The photography was examined using a Lietz Sokkia MS-27 mirror stereoscope with magnifying binocular eyepiece. Lineaments identified on the photographs were marked on hard copies of scanned images (600dpi resolution), and subsequently compiled onto a photomosaic base. Rose diagrams were prepared for lineament trends identified from both aerial photography and topographic map interpretation, and are included as inserts on the respective figures. 6.9.3 Results A total of 17 well-defined lineaments were identified. Trends are predominantly toward the northeast in accordance with the predominant structural trend in polydeformed rocks beneath the study area. Less pervasive northwest-trending lineaments are also relatively common. Fractures in the area may have formed as a result of non-ductile deformation postdating the peak of metamorphism, as lineaments suggestive of fracture traces are present in an Ordovician to Devonian granitoid body that intrudes older kyanite to sillimanite grade rocks in the southwest part of the study area. Lineaments identified from aerial photography are shown and lineament trends indicated by a rose diagram are included on Figure 6-15. A total of nine lineaments were identified from aerial photography interpretation. Three of 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 6-14 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx these correspond to lineaments identified from topographic map interpretation as shown on Figure 6-14. The remaining lineaments are small-scale features not visible at the scale of the topographic maps. Similar to the lineaments identified on topographic maps, the dominant trend is toward the northeast, and to a lesser extent toward the northwest. 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 7-1 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx 7.0 SOIL SAMPLING RESULTS The purpose of soil and rock characterization is to evaluate the physical and geochemical properties in the subsurface with regard to constituent presence, retardation, and migration. Soil, PWR, and bedrock samples were collected from background locations, beneath the ash basin, ash storage area, and cinder storage areas, and from locations beyond the waste boundary. Soil sampling was performed in general accordance with the procedures described in the Work Plan (Appendix G). Variances from the proposed Work Plan are also presented in Appendix G. The boring locations are shown on Figure 2-4. Table 7-1 summarizes the parameters and constituent analytical methods for soil, PWR, and bedrock samples collected. Total inorganic results for background soil, PWR, and bedrock samples are provided in the comprehensive data table (Appendix B, Table 4). 7.1 Background Soil Data Because some constituents are naturally occurring in soil and are present in the source areas, establishing background concentrations is important in assessing potential influence from source areas. Background boring locations were identified based on the SCM at the time the Work Plan was submitted. The initial 2015 CSA background soil borings were co-located with proposed background monitoring well locations BG-1, BG-2 and BG-3 (Figure 2-4). NCDEQ later determined the BG-2 monitoring well location to be potentially influenced by non-ash basin anthropogenic sources, leaving soil samples from BG-1 and BG-3 for inclusion in the background dataset. An updated background soil dataset for RBSS was provided to NCDEQ on May 26, 2017. Additional soil samples associated with the borings for monitoring wells GWA- 5D, GWA-6D, GWA-21D, MW-7BR, and OB-2 were proposed for use as background. The additional locations were approved; however, due to reporting limits below the regulatory comparative values, antimony, selenium, and thallium datasets were insufficient to calculate background. In order to collect additional samples to meet the lower reporting limits threshold, and to increase the overall background dataset, additional background samples ( locations BGSB-4 through BGSB-9) were collected in July 2017 (Figure 2-4). Where soil samples were obtained as part of drilling for well installation, the monitoring well label (for instance GWA-21D) serves to denote the soil sample location. The BGSB locations were soil borings conducted in July 2017 specifically for background soil sample collection and are denoted separately from monitoring well labels. It is assumed that the areas of background monitoring were not impacted by the ash basin, ash storage, and cinder storage areas. The background areas 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 7-2 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx are considered upgradient or sidegradient from the ash basin, ash storage, and cinder storage areas. The background soil dataset (Table 7-3) was approved by NCDEQ on September 1, 2017 (Appendix A) and provisional background threshold values (PBTVs) for RBSS soil were calculated (Table 7-2). Sample intervals for each location are provided in Appendix B, Table 4. Background SPLP Results SPLP results for background soil samples can be found in Appendix B, Table 6. SPLP results for soil samples are shown in Appendix B, Table 6. Although SPLP analytical results are compared with 2L or IMAC, these samples do not represent groundwater samples. The analysis revealed two constituents (iron and vanadium, both for sample BG-3D (23-24)) were present at concentrations greater than the 2L and IMAC in the leachate from background soil. 7.2 Facility Soil Data Soil samples were collected at the time borings were conducted for CSA monitoring well installations. Comparison of soil analytical results with background, based on the area of the site, is discussed below. Soil beneath Ash Basin and within the Waste Boundary Soil samples beneath the ash basin (primary and secondary cells) were obtained from AB-1D, AB-2D, AB-3D, AB-4S/D, AB-5D, AB-6D, AB-7S/D, and AB-8S/D. Constituent concentrations for soils are presented in Appendix B, Table 4. Concentrations of arsenic, boron, chromium, cobalt, iron, manganese, nickel, and vanadium were greater than POG PSRGs and PBTVs at multiple sample locations within the ash basin. Nickel was limited to an exceedance a single location (AB-4D). Estimated antimony and selenium concentrations (J flagged results reported between the laboratory method detection limit and the method reporting limit) were reported over the POG PSRG and the PBTV for a limited number of samples. Soil beneath Ash Storage Area Soil samples beneath the ash storage area were obtained from AS-1D, AS-2S/D, AS-2S, AS-3D, and GWA-23D. Constituent concentrations for soils are presented in Appendix B, Table 4. Concentrations of arsenic were greater than POG PSRGs and PBTVs at three sample locations (AS-1D, AS-2D and AS-3D) beneath the ash storage area. Boron concentrations were generally low in soil beneath the ash storage and did not exceed the POG PSRG. 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 7-3 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx Soil beneath Cinder Storage Area Soil samples beneath the cinder storage area were obtained from C-1D, C-2S, and C-2D. Constituent concentrations for soils are presented in Appendix B, Table 4. A limited number of constituent results in soil samples from beneath the cinder storage were greater than the POG PSRGs. No concentrations were reported greater than background. Boron concentrations in soil below the cinder storage were less than PBTVs and POG PSRGs. Soil outside the Waste Boundary and within the Compliance Boundary Soil samples were collected from monitoring well locations outside the waste boundary and within the compliance boundary around the perimeter of the ash management areas. Soil sampling results indicated concentrations of chromium, cobalt, iron, and vanadium were higher than POG PSRGs and background in multiple samples. The manganese concentration was higher than both the POG PSRG and PBTV at the GWA-1 location. The nickel concentration was higher than both the POG PSRG and PBTV at the GWA-8 location. Soil outside the Compliance Boundary Soil samples were collected during installation of the MW-9 well cluster located outside the compliance boundary northwest of the ash basin. Only chromium was detected above the POG PSRG and PBTV in the MW-9 soil samples. Soil Samples from Other Areas A number of soil samples included on the summary table in Appendix B, Table 4 do not fall into the categories described in the previous sections. Those include: (1) soil samples from background soil locations BG-1, BG-3, and GWA-21D that were collected below the water table, and therefore not approved for use in PBTV determination, and (2) soil samples from the BG-2 location, which was proposed as background but not approved, due to potential off-site influence indicated by elevated nitrite/nitrate concentrations in groundwater samples. Review of the soil analysis results for these samples indicated chromium concentrations were above both the POG PSRG and PBTV in samples from the BG-2 location. No other exceedances of both POG PSRGs and PBTVs were indicated. 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 7-4 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx Soil SPLP Results SPLP was used to determine the ability of simulated rainwater to leach specific constituents out of the soil. The analysis indicated that antimony, arsenic, cobalt, iron, manganese, nitrate, and vanadium were present at concentrations greater than the 2L or IMAC in the leachate from soil underlying, or adjacent to, the ash management areas (Appendix B, Table 4). Of those constituents, both iron and vanadium leached from background soil at concentrations greater than 2L or IMAC in SPLP analysis. Comparison of PWR and Bedrock Results to Background In addition to comparison of soil results to regulatory criteria, PWR and bedrock sample results have also been compared to background threshold values as discussed below. Background PWR and Bedrock Background PWR and bedrock sample locations were obtained from MW-7BR. Background PWR and bedrock sample concentration ranges are listed below for constituents that exceeded the POG PSRGs. Results with a J qualifier are estimated concentrations less than the laboratory method reporting limit. Chromium 1.3 mg/kg to 4.6 mg/kg Cobalt 2.6 J mg/kg to 7.3 mg/kg Iron 4,100 mg/kg to 7,550 mg/kg Manganese 80.1 mg/kg to 110 mg/kg Vanadium 8.5 mg/kg to 17.7 mg/kg PWR and Bedrock beneath Ash Basin and within Ash Basin Waste Boundary One PWR sample within the waste boundary was obtained from AB-8D. Additional PWR and bedrock samples were not collected within the ash basin. Constituent concentrations for PWR and bedrock samples are presented in Appendix B, Table 4. PWR concentrations of manganese within the waste boundary were greater than POG PSRGs and background PWR, but less than the approved soil PBTV. PWR and Bedrock beneath Ash Storage Area PWR and bedrock samples collected beneath the ash storage area were obtained from GWA-23BR. Constituent concentrations for PWR and bedrock samples are presented in Appendix B, Table 4. 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 7-5 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx PWR concentrations of cobalt, iron, and manganese within the waste boundary were greater than POG PSRGs and background PWR, but less than the approved soil PBTVs. PWR and Bedrock outside the Waste Boundary and within the Compliance Boundary PWR and bedrock samples collected outside the waste boundary and within the compliance boundary were obtained from locations shown in Figure 2-4. Constituent concentrations for PWR and bedrock samples are presented in Appendix B, Table 4. Concentrations of antimony, barium, chromium, cobalt, iron, manganese, and vanadium in PWR and bedrock outside the waste boundary, and within the compliance boundary, were greater than the POG PSRGs and background PWR. Secondary Sources Concentrations of arsenic, boron, chromium, cobalt, iron, manganese, nickel, and vanadium in soils were found to be higher than the POG PSRGs and background for one or more sample locations beneath and around the ash management areas. A large number of soil samples were collected during assessment activities from areas outside the ash management areas but within or near the compliance boundary. Constituent occurrences in these areas may exceed site-specific PBTVs but reflect natural conditions. Soil beneath the ash basin generally indicated higher concentrations for certain constituents than other sample areas, and also yielded boron concentrations that may indicate association with coal ash (Figure 7-1, Table 7-4). Constituent concentrations often decrease with depth in borings below the ash basin. Saturation and other factors may also affect constituent occurrence in the samples. Geochemical modeling conducted for the CAP is anticipated to help in determining constituent association with coal ash and an appropriate site remedy if necessary. 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 8-1 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx 8.0 SEDIMENT RESULTS The purpose of sediment characterization is to evaluate whether storage of ash has resulted in impacts to sediment or soils associated with Areas of Wetness (AOWs) in the vicinity of the ash basin, ash storage area, and cinder storage area. The sediment characterization was performed in general accordance with the procedures described in the Work Plan. Sampling methodology and variances to that methodology are described in Appendix G. Sediment samples were collected from seep and surface water locations (areas of wetness (AOWs)) near the ash basin, and near the cinder storage area in July 2015. Seeps S-1, S-3, S-10, and S-12 were noted to be dry at the time of sample collection. Seep S-5 is routinely dry and no sediment sample was collected from that location. Sediment samples were analyzed for the constituent and parameter list used for solid matrix characterization (Table 7-1). In order to assess the potential for contribution to groundwater impacts, the sediment sample results were compared with North Carolina Protection of Groundwater Preliminary Soil Remediation Goals (POG PSRGs) and soil preliminary background threshold values (PBTVs) (Appendix B, Table 5). Sediment sample locations are shown on Figure 2-4. 8.1 Sediment (Surface Soil) Associated with AOWs All 11 of the sediment samples were co-located with designated AOWs. For those locations, solid material was collected at or near the point of emergence or flow of water. In most cases, the “sediment” that was collected was actually surface soil over which water originating at the AOW was flowing or seeping. Descriptions of the AOWs and the results of sediment analysis are provided as follows: S-1: Sediment collected from wetland area where seepage from multiple locations collects west of the toe of the primary cell, and north of cinder storage area dikes. The seep location is usually dry. Cobalt, iron, manganese and vanadium concentrations exceeded the POG PSRGs but did not exceed PBTVs. The chromium result, 21 mg/kg, exceeded the POG PSRG of 3.8 mg/kg and slightly exceeded the PBTV of 20 mg/kg. S-2: Sediment collected from an AOW that emerges from the end of a rip-rap lined channel that collects seepage from the toe of the primary cell dam. Chromium and vanadium concentrations exceeded the POG PSRGs but not the PBTVs. Arsenic, cobalt, and manganese concentrations exceeded both the POG PSRGs and PBTVs. 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 8-2 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx S-3: Sediment collected from a dry AOW located in a heavily wooded area to the west of the secondary cell. Iron and vanadium concentrations exceeded the POG PSRGs but did not exceed background. Antimony, arsenic, chromium, cobalt, and manganese exceeded both the POG PSRGs and PBTVs. S-4: Confluence of S-09 and S-05, this AOW flows through wetlands toward the Catawba River from the north side of the secondary cell. Sediment was collected from the channel. The vanadium result exceeded the POG PSRG. Chromium, cobalt, iron, and manganese exceeded POG PSRGs and PBTVs. S-6: Location is often dry. Seepage follows a poorly defined channel through wetland area toward the Catawba River. It is located to the east of the secondary cell. Sediment collected from channel. Vanadium concentrations exceeded the POG PSRG. Barium, chromium, cobalt, iron, and manganese exceeded both POG PSRGs and PBTVs. S-7: Seep located in a heavily wooded area to the east of the secondary cell. Sediment collected from channel. Iron and vanadium concentrations exceeded the POG PSRGs. Concentrations of chromium and manganese exceeded both POG PSRGs and PBTVs. S-8: Perennial stream-tributary to the Catawba River located to the east of the secondary cell. Sediment collected from channel. The POG PSRGs were exceeded by concentrations of chromium, cobalt, iron, manganese, and vanadium. Results did not exceed PBTVs. S-9: Seep in a well-defined channel flowing through a wetland area north of the secondary cell and downgradient of AOW S-5. Sediment collected from channel. Manganese and vanadium concentrations exceeded the POG PSRGs. Chromium and cobalt concentrations exceeded POG PSRGs and PBTVs. S-10: A typically dry AOW, this seep is located in a wooded area to the north of the secondary cell. Chromium and cobalt concentrations exceeded both the POG PSRGs and PBTVs. S-11: Sediment collected from a seep location north of the secondary cell. Vanadium exceeded the POG PSRG. Chromium, cobalt, iron, and manganese exceeded both POG PSRGs and PBTVs. S-12: The AOW is an engineered concrete channel extending along the groin of the secondary cell dike terminating in rip rap near the Catawba River. It was dry when the sediment sample was collected. Arsenic, chromium, cobalt, iron, manganese, and vanadium concentrations exceeded POG PSRGs and PBTVs. 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 8-3 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx 8.2 Sediment in Major Bodies of Water No sediment samples were collected from major bodies of water, including the Catawba River (Mountain Island Lake). 8.3 Comparison of Exceedances to PSRGs and Background Sediment results for seep locations S-1 through S-4, S-6, S-7, S-9, S-11, and S-12 included one or more constituents with concentrations greater than both POG PSRGs and background (PBTVs). Arsenic, chromium, cobalt, iron, manganese, and vanadium were often observed at concentrations greater than the comparative values in one or more samples. The samples also commonly indicated boron, calcium and sulfate concentrations greater than background. These samples were located in areas downgradient from the ash basin and cinder storage area. The S-8 seep location east of the secondary cell did not yield constituent results greater than background concentrations, and also yielded low concentrations for boron, calcium, and sulfate. This is consistent with water samples from seeps, wherein seeps downgradient from the ash management areas often indicated elevated concentrations of boron, calcium, and sulfate, while the S-8 location east of the secondary cell, and the background seep location, S-13, indicated lower concentrations for these constituents. The seep sediment locations with sample results greater than both POG PSRGs and PBTVs may represent secondary source areas for potential contribution to groundwater impacts. Seep locations S-1 through S-4, S-6, S-7, S-9, S-11, and S-12 will be evaluated in the CAP and addressed as part of the site remedy if necessary. 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 9-1 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx 9.0 SURFACE WATER RESULTS The purpose of surface water characterization is to evaluate whether storage of ash has resulted in impacts to surface waters in the vicinity of the ash basin, ash storage areas, and cinder storage area. Surface water parameters and laboratory methods used for analysis are presented in Table 9-1. Surface water sample results for total and dissolved fractions of constituents are presented in the Comprehensive Table (Appendix B, Table 2). Surface water sample locations are shown on Figure 2-4. The RBSS site is located adjacent to Mountain Island Lake, which is a part of the Catawba River system. Samples discussed within the following sections include four distinct types: 1) Areas of wetness (AOWs)/seeps; 2) wastewater conveyance (effluent channels); 3) industrial stormwater, and; 4) Named Surface Waters. For the scope of this CSA, it is only appropriate to compare Surface Waters to 2B standards because AOWs/seeps, wastewater conveyances (effluent channels), and industrial stormwater are evaluated and governed wholly separate in accordance with the NPDES Program administered by NCDEQ DWR. This process is on-going in a parallel effort to CSA document development and subject to change. AOWs, including known discharges from drain outfalls and seeps were identified as part of the NPDES reapplication process in 2013 and 2014. Sample locations S-1 through S-12, and Catawba River upstream sample location (278.0-0.3m) and downstream sample location (277.5-0.3m) were subsequently documented in a discharge assessment plan (HDR, 2014c). Representative, previously identified AOW locations and a background location (S-13) were sampled as part of the 2015 CSA. Samples associated with AOWs were collected for water-quality analysis from the following locations: S-1, AOW area where seepage from multiple locations collects west of the toe of the primary cell dike and north of cinder storage area dikes S-4, a confluence location of AOWs S-05 and S-09 that flows through a wetland north of the secondary cell S-6, a poorly defined, often dry AOW east of the secondary cell that flows through a wetland area toward the Catawba River S-7, located in a heavily wooded area east of the secondary cell 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 9-2 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx S-8, an AOW located east of the secondary cell, S-10, an AOW, often dry, located in a wooded area to the north of the secondary cell S-13, a background surface water sample from a small stream/tributary to the Catawba River located east of the BG-3 monitoring well cluster. Other water samples used for the 2015 CSA characterization are as follows: As reported in the 2015 CSA, NCDENR collected two sidegradient surface-water samples (RBSW001 and RBSW002) from the west side of the RBSS intake canal. Two locations in the Catawba River were sampled (278.0-0.3m, upstream, and 277.5-0.3m, downstream of RBSS). Those locations were sampled periodically (about every six months) from 2010 through 2014. AOW locations S-14 through S-23, and S-13A and S-21A were identified after the 2015 CSA. Labels S-13A and S-21A refer to alternate sampling locations near, and along the same reach, as the associated AOW location number. The current NPDES permit incorporated a number of AOW locations as outfalls. The outfalls are labeled according to the “S” designation. For instance, location S-3, is incorporated into the permit as Outfall 103. Some AOWs were identified after the 2015 CSA seep assessment and sampling events. Under the NPDES permit, AOW water samples are collected at Outfall 101 through 112 on a monthly basis for 12 months at which point sampling will be reduced to a quarterly basis. AOW areas S-13, S-13A, S-19, S-20 and S-21A are inspected semi-annually. New seeps identified during AOW inspections are documented and sampled. Sample results from the locations and activities described above from 2010 and 2017 are tabulated and compared with 2B standards for surface water (Appendix B, Table 3). 9.1 Comparison of Exceedances to 2B Standards Background Locations Sample locations 278.0-0.3m and S-13 represent background conditions for the Catawba River and a Catawba River tributary, respectively. Samples with high turbidity (exceeding the 2B of 50 Nephelometric Turbidity Units (NTUs) were not included in analysis or discussion as high turbidity can affect metals detection. Observations from background sample results are provided below: 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 9-3 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx pH ranged from 6.1 to 7.3 S.U. Boron concentrations ranged from non-detect to 65.9 µg/L Sulfate concentrations ranged from 3.8 to 41 mg/L Iron concentrations ranged from 74.6 µg/L to 13,200 µg/L Manganese concentrations ranged from 13.1 µg/L to 352 µg/L Mercury concentrations ranged from non-detect to 0.0146 µg/L Nickel concentrations ranged from non-detect to 0.76 µg/L Intake Channel Samples NCDEQ collected surface water samples from two locations (RBSW001 and RBSW002) on the west side of the intake canal in July 2015. Boron and sulfate concentrations in the samples were consistent with the background sample results. Total selenium exceeded 2B in two samples. Cadmium, copper, lead, and zinc results exceeded 2B in at least one of the July 2015 samples. A March 2017 sample from the RBSW0002 location indicated no 2B exceedances. Analytical results are provided in Appendix B, Table 2. The locations and analytical results for those samples were provided by NCDEQ to Duke Energy and are assumed to be accurate. Comparison of Upstream (278.0-0.3m) and Downstream (277.5- 0.3m) Catawba River Samples The Catawba River surface water samples collected at 278.0-0.3m (upstream, background) and 277.5-0.3m (downstream) were compared to 2B. Of the 17 samples collected over five years (2010-2014), no surface water 2B criteria were exceeded. Piper Diagrams Representative water samples collected from May 2016 through May 2017 are plotted on a piper diagram as Figure 9-1. Observation of the groundwater facies diamond indicates that most of the AOW samples plot as calcium-sulfate waters. The sample from location S-8 east of the ash basin plots as a calcium-bicarbonate water, which is indicative of a background water type not influenced by the ash basin. Further discussion of piper diagrams is provided in Section 10.0. 9.2 Discussion of Results for Constituents without Established 2B Standard A 2B value has not been established for a number of constituents. A summary of results for select constituents without associated 2B values follows. 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 9-4 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx Antimony was detected in only one sample with a concentration of 2.1 µg/L (Outfall 112/S-12). Beryllium was detected in a limited number of samples. Boron was detected in most AOW and surface water samples. Concentrations from the upstream (sample 278.0-0.3m) and downstream (sample 277.5-0.3m) Catawba River samples, and the samples from a small background location stream (S-13) were below 100 µg/L. Concentrations from AOW locations near the ash management areas were generally higher, up to 704 µg/L at Outfall 112/S-12. Total chromium was detected in most samples collected during the assessment. Concentrations ranged from 0.51 µg/L (Outfall 109/S-9) to 10.4 µg/L (Outfall 107/S-7). Hexavalent chromium was only detected in a few of the seep and surface water samples. Detections ranged from 0.035 µg/L (Outfall 106/S-6) to 2 µg/L (Outfall 112/S-12). Cobalt was detected in a number of AOW and surface water samples. The concentrations ranged from 0.1 µg/L (Outfall 108/S-8) to 46.1 µg/L at (Outfall 105/S-5). Iron was detected in most samples. Concentrations ranged from 28 µg/L (Outfall 102/S-2) to 30,100 µg/L (Outfall 101/S-1). Manganese was detected in most samples. Concentrations ranged from 16.5 µg/L (277.5-0.3m, downstream Catawba River) to 8,650 µg/L (Outfall 106/S-6). Strontium was detected in a few samples with concentrations ranging from 17.5 µg/L (Outfall 107/S-7) to 956 µg/L (Outfall 112/S-12). Vanadium was detected in several samples. Concentrations ranged from 0.32 µg/L (Outfall 109/S-9) to 6.2 µg/L (Outfall 107/S-7). 9.3 Discussion of Surface Water Results Surface water and AOWs were sampled in order to evaluate potential influence from the ash management areas. Dewatering of the ash basin is ongoing at the site, and flow to AOW locations may diminish as a result. Review of the Catawba River upstream (278.0-0.3m) and downstream (277.5-0.3m) samples indicated no exceedances of the 2B standards. Observation of boron concentrations is useful as an indicator of potential CCR impact. Boron concentrations in the background stream location, S-13, and the upstream Catawba River samples, 278.0-0.3m, ranged from non-detect to 65.9 µg/L. AOW sample 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 9-5 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx locations downgradient of the ash management areas generally yielded boron concentrations greater than background. This is consistent with results from groundwater monitoring wells in the same areas. AOW sample locations east of the ash basin, S-7 and S-8, yielded non-detect to very low boron results, which is also consistent with groundwater monitoring well results from the same area. AOW sample location S-21A indicated elevated TDS and sulfate. The GWA-3 groundwater monitoring well cluster is located upgradient and near the S-21A location. As with a number of other downgradient seeps, and nearby monitoring wells, groundwater sampling results for the shallow well, GWA-3SA, are consistent with results from S-21A. Review of surface and seep water sampling indicates the groundwater monitoring well network provides necessary data for evaluating potential groundwater/surface water interaction. 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 10-1 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx 10.0 GROUNDWATER SAMPLING RESULTS This section provides a summary of groundwater analytical results for the most recent monitoring event (2Q2017) with discussion of historical data results and trends. A comprehensive table with all media analytical results is provided in Appendix B. As directed by NCDEQ, the data with turbidity greater than 10 NTUs, pH greater than 8.5 that may be a result of grout intrusion, and data that may be auto-correlated because it was collected within 60 days of a previous sampling event, are excluded for statistics and other evaluation methods. The most recent data collected is shown on the pertinent maps. The most recent data collected and reported herein is from May 2017 (second quarter 2017). One comprehensive round of sampling and analysis was conducted prior to and reported in, the August 2015 CSA. In addition, the following sampling and analysis events were conducted: Comprehensive Round (2) – September 2015 (reported in November 2015 CAP Part 1) Limited Round (3) – November 2015 (background wells only, reported in the CSA Supplement 1 as part of the February 2016 CAP Part 2 report) Limited Round (4) – December 2015 (background wells only, reported in the CSA Supplement 1 as part of the February 2016 CAP Part 2) Comprehensive Round (5) – February, March, and April 2016 (reported in August 2016 CSA Supplement 2) Comprehensive Round (6) – June 2016 Comprehensive Round (7) – September 2016 Comprehensive Round (8) – November 2016 Comprehensive Round (9) – February 2017 Comprehensive Round (10) – May 2017 Groundwater sampling methods and the rationale for sampling locations were in general accordance with the procedures described in the Work Plan (HDR, 2014c) and are included in Appendix G. Variances from the proposed well installation locations, methods, quantities, and well designations are presented in Appendix G. 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 10-2 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx As described in the approved Work Plan, both unfiltered and filtered (0.45 um filter) samples were collected for analyses of constituents whose results may be biased by the presence of turbidity. Unless otherwise noted, concentration results discussed are for the unfiltered samples and represent total concentrations. 10.1 Background Groundwater Concentrations Well clusters, including wells installed in different flow layers, at the BG-1, BG-2, BG-3 and MW-7 locations were proposed as background locations in the 2015 CSA (Figure 2- 4). All of those locations are east of, and upgradient-to-sidegradient from, the ash basin. Evaluation of the suitability of each of those locations for background purposes was conducted as part of the CAP 1 (Appendix H) and in technical memoranda (Ruffing, December 12, 2016) and (Ruffing, May 26, 2017). Factors such as horizontal distance from the waste boundary, the relative topographic and groundwater elevation difference compared to the nearest ash basin surface or porewater, and the calculated groundwater flow direction were considered to determine whether the locations represent background conditions. Based on those criteria, the BG-2 and BG-3 locations were determined to be potentially influenced by the ash basin and were removed from the background dataset. In order to expand the background well dataset, well clusters BG-4 and BG-5 were installed in areas south of the Site outside of potential ash basin influence in March and April 2016. The wells at the BG-4 and BG-5 locations, as well as wells at the upgradient monitoring location GWA-14, were evaluated as to whether they represent background conditions as part of the December 12, 2016, Technical Memorandum. The BG-4 and BG-5 locations were determined to represent background conditions. The shallow well (GWA-14S), at the GWA-14 well cluster, was determined to represent background. However, the deep well at that location, GWA-14D, exhibited elevated pH and was not included in the background dataset (Technical Memorandum, May 26, 2017, HDR). Elevated pH attributed to grout contamination has negated the use of certain other wells at the background locations. Monitoring wells at the BG-1, BG-4, BG-5, MW-7, and GWA-14 locations are considered to represent background groundwater conditions at RBSS. The deep well at GWA-14, and wells at the locations indicating elevated pH are excluded. These background well locations were approved by NCDEQ in a letter dated July 7, 2017 (Appendix A). Background groundwater monitoring wells are depicted on Figure 2-4. 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 10-3 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx 10.1.1 Background Dataset Statistical Analysis The revised background groundwater datasets and statistically determined PBTVs are presented below. The current background monitoring well network consists of wells installed within three flow zones — shallow, deep, and fractured bedrock. For groundwater datasets with less than 10 valid samples available for determination of PBTVs, no formal upper tolerance limit (UTL) statistics will be run and the PBTV for a constituent and groundwater flow system will be computed to be either: The highest value, or If the highest value is above an order of magnitude greater than the geometric mean of all values, then the highest value is considered an outlier and removed from use and the PBTV is computed to be the second highest value. NCDEQ requested that the updated background groundwater dataset exclude data associated with one or more of the following conditions: Sample pH is greater than or equal to 8.5 standard units (S.U.) unless the regional DEQ office has determined an alternate background threshold pH for the Site; Sample turbidity is greater than or equal to 10 Nephelometric Turbidity Units (NTUs). Result is a statistical outlier identified for background sample data presented to NCDEQ on May 26, 2017; Sample collection occurred less than a minimum 60 days between sampling events. Non-detected results are greater than 2L/IMAC. Statistical determinations of PBTVs were performed in accordance with the revised Statistical Methods for Developing Reference Background Concentrations for Groundwater and Soil at Coal Ash Facilities (statistical methods document) (HDR and SynTerra, 2017). The background datasets for each flow system used to statistically determine naturally occurring concentrations of inorganic constituents in groundwater are provided in Table 10-1. The following sections summarize the refined background datasets along with the results of the statistical evaluations for determining PBTVs. 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 10-4 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx Shallow Flow System Four wells — BG-1S, MW-7SR, BG-4S, and GWA-14S — monitor background groundwater quality within the shallow (surficial) flow system. The shallow background groundwater dataset meets the minimum requirement of 10 samples for all constituents. PBTVs were calculated for constituents monitored within the shallow flow zone using formal UTL statistics (Table 10-2). Deep Flow System Three wells — BG-4D, BG-5D, and MW-7D—monitor background groundwater quality within the deep flow zone. The background groundwater dataset meets the minimum requirement of 10 samples for all constituents. PBTVs were calculated for constituents monitored within the deep flow system using formal UTL statistics (Table 10-2). Bedrock One well, BG-5BR, monitors background groundwater quality within fractured bedrock. Currently, the dataset for bedrock does not meet the minimum requirement of 10 samples. Replacement well MW-7BRA was installed on June 1, 2017 to increase the bedrock background dataset. Results from this well are not available for this report, but it is anticipated that they will be available for use with the CAP update. PBTVs for constituents in bedrock were computed with either the maximum value or if the maximum value was above an order of magnitude greater than the geometric mean of all values, the second highest value (Table 10-2). 10.1.2 Piper Diagrams (Comparison to Background) A Piper diagram, also referred to as a trilinear diagram, is a graphical representation of major water chemistry using two ternary plots and a diamond plot. One of the ternary plots shows the relative percentage of major cations in individual water samples and the other shows the relative percentage of the major anions. The apices of the cation plot are calcium, magnesium, and sodium plus potassium. The apices of the anion plot are sulfate, chloride, and carbonates. The two ternary plots are projected onto the diamond plot to represent the major ion chemistry of a water sample. The ion composition can be used to classify groundwater of particular character and chemistry into sub-groups known as groundwater facies. For this reason, the diamond of the piper plot is sometimes referred to as the groundwater facies diamond. Percentages of major anions and cations are based on concentrations expressed in meq/L (EPRI, 2006). Plots of 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 10-5 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx shallow, deep, and bedrock groundwater including background locations are shown on Figure 10-1, Figure 10-2, and Figure 10-3. Background water types at RBSS are consistent with findings from a five-year study of groundwater flow and quality conducted at the Langtree Peninsula Research Station, located in a similar geologic setting approximately 20 miles north of the Site. Samples collected from background wells at RBSS generally indicate a calcium-bicarbonate water. 10.2 Downgradient Groundwater Concentrations In order to best reflect current conditions at the Site, and changes that might be related to ongoing coal ash removal activities (source removal), the second quarter 2017 groundwater sample results provide a focus for data evaluation in this report. Results from prior events are incorporated in data evaluation and summarized as appropriate. The second quarter 2017 data is the primary dataset used for generating isoconcentration maps and graphical representation of data such as Piper diagrams. A total of 86 groundwater monitoring wells were sampled during May 2017 as part of the second quarter 2017 sampling event. Of the 86 groundwater samples, 29 are not valid due to a pH greater than or equal to 8.5 S.U. or turbidity greater than or equal to 10 NTUs. Measurements of pH indicated a number of locations with pH less than the 2L lower limit of 6.5, or higher than the 2L upper limit of 8.5. The PBTV for pH ranges from a low of 4.9 in the shallow flow layer to 7.8 in bedrock. In general, elevated pH measurements are interpreted as the result of grout contaminated wells and in accordance with DEQ guidance, the associated groundwater samples are not used for evaluation of constituent concentrations. Boron and sulfate at RBSS are observed in concentrations that are generally higher than background (but usually below 2L), in and around the ash management areas. As such, their presence may be indicative of CCR influence and is included in this section’s discussion of May 2017 groundwater sampling results. 10.2.1 Shallow Wells Boron concentrations in downgradient shallow zone wells ranged from below detection limit to 517 µg/L for the May 2017 sampling event. No results were reported above 2L. The higher observed concentrations (greater than 100 µg/L) were from wells located within the footprint of the ash management areas, or along the northern waste boundary, from the cinder storage area to the east, 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 10-6 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx along the north side of the primary and secondary cells. Boron was not observed in samples from areas east of the secondary cell (e.g., GWA-7S, BG-2S, and BG- 3S). Neither boron nor sulfate was observed in wells south of the primary cell and ash storage area (GWA-20S, GWA-21S, and GWA-23S). Iron and manganese were observed above PBTVs and 2L in multiple shallow, downgradient wells. Higher concentrations occurred near the former coal pile and cinder storage (GWA-3SA, GWA-11S, and GWA-12S) and north of the primary cell (GWA-2S). The sample from monitoring well GWA-2S also indicated elevated concentrations of chromium, nickel, molybdenum and vanadium. The chromium, nickel, and molybdenum concentrations from the GWA-2S sample in May 2017 were significantly higher than previous sampling events for the same well and appear to be anomalous. For instance, the total nickel concentration was 158 µg/L while previous results ranged from 0.77 µg/L to 5.8 µg/L; and the total chromium result was 314 µg/L while previous results ranged from 2.0 µg/L to 7.1 µg/L. Chromium was also detected in shallow wells GWA-1S, GWA-6S, GWA-20S, and GWA-21S at concentrations greater than 2L and PBTVs. As with GWA-2S, most locations do not have a history of repeated elevated chromium detections, and the total results are significantly higher than dissolved results, indicating a less mobile particulate component creating an atypical sample result. The locations upgradient of the ash storage area do not historically indicate occurrence of boron or sulfate at levels that would reflect CCR influence, and chromium concentrations there are interpreted as a result of naturally occurring conditions. Cobalt was observed above IMAC and background in wells near the former coal pile, cinder storage and the secondary cell. Beryllium, sulfate, and TDS were observed above 2L/IMAC and background in wells near the former coal pile and cinder storage. 10.2.2 Deep Wells Boron concentrations in downgradient deep-layer wells ranged from below detection limit to 690 µg/L for the May 2017 sampling event. No results were reported above 2L. The highest observed concentrations were from wells located beneath the footprint of the ash management areas, or along the northern waste boundary, from the cinder storage area to the east, along the north side of the 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 10-7 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx primary and secondary cells. Boron was not observed in samples from areas east of the secondary cell (e.g., GWA-7D and BG-3D). Iron and manganese were observed above PBTVs and 2L in multiple deep, downgradient wells. Higher concentrations of those constituents occurred near the former coal pile and cinder storage (GWA-3D, GWA-11D, and GWA-12D). Vanadium was observed above background and IMAC in one deep downgradient well (GWA-7D). Strontium and thallium were observed above background and/or IMAC in one deep well near the former coal pile (GWA-11D). Sulfate was observed above 2L and background in multiple wells near the former coal pile and cinder storage. 10.2.3 Bedrock Wells Boron was not detected above 50 µg/L in valid samples from downgradient bedrock zone wells for the May 2017 sampling event. A concentration of 777 µg/L was reported from a bedrock well, GWA-2BRU, with elevated pH. Chromium and iron were detected above 2L and preliminary background in GWA-1BRU north of the secondary cell; however both results were significantly higher than past sampling events and the associated dissolved results were potentially due to particulate influence. Iron, manganese, molybdenum, strontium, sulfate, and TDS were detected above 2L and preliminary background from GWA-3BR downgradient of the cinder storage area and the former coal pile. Vanadium was observed in two downgradient bedrock wells above IMAC and preliminary background. 10.2.4 Piper Diagrams (Comparison with Downgradient Well Samples) In contrast to background water samples that often plot as calcium-bicarbonate water, a 2006 EPRI study of 40 ash leachate water samples collected from 20 different coal ash landfills and impoundments characterized bituminous coal ash leachate as calcium-magnesium-sulfate water type and subbituminous coal ash leachate as sodium-calcium-sulfate water type. Shallow downgradient locations characterized by calcium-magnesium-sulfate water type include: AB-8S, C-2S, GWA-3SA, GWA-4S, GWA-9S, GWA-10S, GWA-12S, GWA-15S, MW-2SA, MW-5S, and MW-15. Nine (9) of these 11 wells indicated boron concentrations greater than 100 µg/L for the May 2017 sampling event. The locations include the downgradient areas from the ash and cinder 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 10-8 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx storage areas to the primary and secondary cells and are generally near the waste boundaries. Locations that indicate potential mixing between background and potentially CCR-impacted water include GWA-2S (north of primary cell) and MW-06S (northeast of secondary cell). Both locations are generally near the Catawba River. Downgradient locations MW-3S, MW-9, GWA-7S, GWA-11S, and GWA- 1S and sidegradient location BG-3S are characterized as calcium-bicarbonate type indicating little influence from source areas or a high degree of mixing with background groundwater. Five of those six wells contained boron concentrations below detection limit or less than 100 µg/L for the May 2017 sampling event. Those wells are also located very near the Catawba River. Samples from monitoring wells BG-1S, BG-2S, BG-9S, GWA-14S, GWA-20S, GWA-21S, GWA-23S, and MW-7SR exhibit ion charge balance of greater than 10 percent (10.717 percent to 49.726 percent) and are therefore not represented on the piper diagrams. Deep groundwater locations characterized by calcium-magnesium-sulfate type water include: AB-8D, GWA-12D, MW-6D, GWA-9D, MW-1D, GWA-7D, and GWA-11D. Locations that indicate potential mixing between background groundwater and impacted groundwater include MW-15D, MW-5D, GWA-8D, and MW-3D. Downgradient location GWA-7D is characterized as calcium- bicarbonate type water consistent with unimpacted background water. AB-2D beneath the ash basin is characterized as calcium-bicarbonate type, indicating little influence from potential source areas. Deep well GWA-22D is characterized as a sodium-bicarbonate type water. One bedrock location, GWA-3BR (north of the former coal pile and cinder storage), is characterized as a calcium-magnesium-sulfate type water. Bedrock locations (BG-2BR, GWA-2BR, and GW-7BR) were generally characterized as a calcium-bicarbonate type water, consistent with background groundwater. 10.2.5 Radiological Laboratory Testing Radionuclides may exist dissolved in water from natural sources (e.g. soil or rock). The USEPA regulates various radionuclides in drinking water. Radium- 226, radium-228, total radium, uranium-238, and total uranium were analyzed in 29 wells as part of the CAMA sampling event in May of 2017. Results for radiological laboratory testing are presented in Table 1 in Appendix B. Radium 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 10-9 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx and uranium isotopes were detected below USEPA Maximum Contaminant Levels (MCLs) in all samples analyzed. 10.3 Site-Specific Exceedances (Groundwater COIs) Site-specific Constituents of Interest (COIs) were developed by evaluating groundwater sampling results with respect to PBTVs, applicable regulatory standards, and additional regulatory input/requirements. The approach to determining those constituents, which should be considered COIs for the purpose of evaluating a Site remedy, is discussed in the following section. 10.3.1 Background Threshold Values (PBTVs) As presented in 15A NCAC 02L .0202 (b)(3), “Where naturally occurring substances exceed the established standard, the standard shall be the naturally occurring concentration as determined by the Director.” Statistical Methods for Developing Reference Background Concentrations for Groundwater and Soil at Coal Ash Facilities (HDR and SynTerra, 2017) was provided to NCDEQ. NCDEQ (July 7, 2017) addressed each Duke Energy coal ash facility and identified soil and groundwater data appropriate for inclusion in the statistical analysis to determine background threshold values (PBTVs) and provisional background threshold values (PBTVs). A revised and updated technical memorandum that summarized revised background groundwater datasets and statistically determined PBTVs for RBSS was submitted to NCDEQ on August 16, 2017. NCDEQ approved the PBTVs for RBSS in a letter dated September 1, 2017. Minor changes were noted in the PBTVs based on NCDEQ evaluation of the data. 10.3.2 Applicable Standards As part of CSA activities at the site, multiple media — including coal ash, ponded water in the ash basin, ash porewater, seeps, soil, and groundwater downgradient of the ash management areas and in background areas — have been sampled and analyzed for inorganic constituents. Based on comparison of the sampling results from the multiple media to applicable regulatory values, potential lists of COIs were developed in the 2015 CSA, CAPs, and CSA Supplement. For the purpose of developing the groundwater COIs, constituent exceedances in downgradient groundwater of PBTVs and 2L or IMAC are considered a primary focus. The COI list has been developed based on site- specific conditions and observations. Certain constituents, such as boron and sulfate, are included due to their usefulness as indicators of coal ash influence. Additionally, NCDEQ requested that hexavalent chromium be included as a COI at each CAMA-related site due to public interest and receptor wells. As directed 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 10-10 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx by July 14, 2017 NCDEQ correspondence a comparative value of 10 µg/L is used for both total and hexavalent chromium. Molybdenum and strontium do not have 2L or 2B criteria, or IMACs established; however, these constituents are considered potential contaminants of concern with regard to Coal Combustion Residuals (CCRs) and are evaluated as potential COIs for the Site at the request of NCDEQ. The following constituents do not have a 2L, IMAC, or Federal MCL established: alkalinity, bicarbonate, calcium, carbonate, magnesium, methane, potassium, sodium, sulfide, and TOC. Results from those constituents are useful in comparing water conditions across the Site. For example, calcium is listed as a constituent for detection monitoring in Appendix III to 40 CFR Part 257. Although those constituents will be used to compare and understand groundwater quality conditions at the Site, because there are no associated 2L criteria, IMACs, or MCLs, the constituents are not evaluated as potential COIs for the Site. 10.3.3 Additional Requirements NCDENR requested that figures be included in the CSA that depict groundwater analytical results for the constituents in 40 CFR 257, Appendix III detection monitoring, and in 40 CFR 257, Appendix IV assessment monitoring (USEPA CCR Rule, 2015). Detection monitoring constituents in 40 CFR 257 Appendix III are: Boron Calcium Chloride Fluoride (limited historical data, not on assessment constituent list) pH Sulfate Total dissolved solids (TDS) Constituents for assessment monitoring listed in 40 CFR 257, Appendix IV include: Antimony Arsenic 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 10-11 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx Barium Beryllium Cadmium Chromium Cobalt Fluoride (limited historical data, not on assessment constituent list) Lead Lithium (not analyzed) Mercury Molybdenum Selenium Thallium Radium 226 and 228 combined Aluminum, copper, iron, manganese, and sulfide were originally included in the Appendix IV constituents in the draft rule; USEPA removed those constituents in the final rule. Therefore, those constituents are not included in the listing above; however, they are included as part of the current Interim Monitoring Plan (IMP). In addition, NCDEQ requested that vanadium be included. 10.3.4 Riverbend COIs Exceedances of 2L/IMACS and PBTVs, the distribution of constituents in relation to the ash management areas, co-occurrence with CCR indicator constituents such as boron and sulfate, and likely migration directions based on groundwater flow direction are considered in determination of groundwater COIs. Based on site-specific conditions, observations, and findings, the following list of COIs has been developed for RBSS: Antimony* Arsenic* Boron Beryllium Chromium (total) Chromium (hexavalent)* Cobalt Iron Manganese Strontium Sulfate TDS Vanadium 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 10-12 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx The constituents listed with an asterisk (antimony, arsenic, and hexavalent chromium) were not observed in valid groundwater results for May 2017 in excess of their respective comparative values. Antimony and arsenic were included in previous reports as COIs due to occurrences above 2L/IMAC and background in ash porewater wells. Due to well abandonments in the basin footprint and the resulting limited dataset, those constituents are retained as COIs until further evaluation. Hexavalent chromium is retained as a COI at the direction of NCDEQ. Review of groundwater sampling results for molybdenum indicated that it commonly occurred in background locations (BG-4 and BG-5) at concentrations equal to, or greater than, concentrations observed in and around the ash management areas. It was not retained as a COI. Groundwater results for thallium indicated one relatively low exceedance of the IMAC (C-1S). Most thallium results across the site were below detection limits. A number of other results were low, laboratory-qualified values. Thallium was not retained as a COI. Nickel was observed in one sample (GWA-2S) for the May 2017 sampling event at a concentration greater than 2L. Historical nickel concentrations from samples at GWA-2S are significantly lower and below 2L; therefore, the May 2017 nickel result is interpreted as anomalous. Nickel was not retained as a COI. Table 10-3 lists the COIs for RBSS along with their associated 2L groundwater standards, IMACs, and federal drinking water standards (Primary Maximum Contaminant Levels [PMCLs] and Secondary Maximum Contaminant Levels [SMCLs]). NC 2L standards are established by NCDEQ, whereas federal MCLs and SMCLs are established by the USEPA. Primary MCLs are legally enforceable standards for public water supply systems set to protect human health in drinking water. Secondary MCLs are non-enforceable guidelines set to account for aesthetic considerations, such as taste, color, and odor (USEPA, 2014). 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 11-1 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx 11.0 HYDROGEOLOGICAL INVESTIGATION Results from the hydrogeological assessment of the RBSS site, summarized in this section, are primary components of the SCM. 11.1 Plume Physical and Chemical Characterization Plume physical and chemical characterization is detailed below for each groundwater COI. The horizontal and vertical extent of constituent concentrations is presented on the isoconcentration maps and cross sections. These descriptions are primarily based on the most recent groundwater sampling event (May 2017). As discussed in Section 10.1, concentrations of boron in background monitoring wells are below detection limits. Boron has a very low Kd value, making the constituent highly mobile in groundwater (Table 11-1). The approximate distribution of boron at concentrations in groundwater greater than 2L and background concentrations best represents the area of potential CCR influence. Review of groundwater sampling results indicates that sulfate, which shows very low concentrations in background monitoring wells at RBSS, is also useful as an indicator of potential CCR influence to groundwater. Other COIs (defined in Section 10.0) are used to help refine the extent and degree of CCR influence from the ash management areas. Groundwater sampling results (Appendix B, Table 1) and evaluation of groundwater flow direction (Section 6.3, Figure 6-8 to 6-13) were used to define the horizontal and vertical extent of potential CCR impact. The geochemical discussion provided in Section 13.2 indicates that not all constituents with PBTV exceedances can be attributed to the ash management areas. Examples of these constituents include alkalinity, aluminum, barium, bicarbonate, carbonate, chloride, copper, lead, magnesium, methane, nitrate + nitrite, potassium, selenium, sodium, total organic carbon (TOC), zinc, and total uranium. Exceedances of these constituents do not necessarily demonstrate horizontal or vertical patterns that would indicate impact from the ash management areas. Isoconcentration Maps The horizontal extent of the constituent concentrations in each flow unit is shown on the isoconcentration maps, Figure 11-1 to 11-36. These maps use groundwater analytical data to spatially define areas where groundwater concentrations are above the respective constituent PBTV and 2L or IMAC. The Catawba River is a downgradient receptor at RBSS and obscures the monitoring of the CCR extent due to the close proximity of the ash management areas. Boron is detected above background on the downgradient side of the cinder storage area and ash basin in the shallow, deep, and bedrock flow layers. 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 11-2 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx The horizontal extent of boron concentrations in downgradient areas is shown on Figures 11-10 to 11-12. The background contour in the surficial unit roughly follows the perimeter of the ash basin and extends to the Catawba River. In the deep zone, the background contour encompasses the downgradient areas of the site. As noted in Section 6.2.1, there is no hydrogeologic confining unit at RBSS. Observation of the vertical gradients isopleth map indicates mixing of the shallow, deep and bedrock flow units (Figure 11-37). The regolith, which makes up much of the shallow and deep flow zones, consists largely of saprolite and is relatively thick (greater than 100 feet) at the RBSS site. Boron is only observed at concentrations greater than 2L in shallow bedrock (GWA-2BRU). Concentrations of boron and sulfate greater than 2L are limited to the areas beneath the ash storage area and primary cell of the ash basin, and downgradient from the primary cell and cinder storage area. Other COIs such as chromium and iron, are observed at concentrations greater than 2L on the east side of the ash basin (secondary cell), but not coincident with boron or sulfate concentrations above 2L. Concentration versus Distance Plots Figures 11-38 to 11-40 depict COI concentration versus distance graphs from the sources to various downgradient monitoring wells. In the shallow flow unit AB-5S is used as the source well and MW-2S-A, MW-3S, and GWA-10S are used as downgradient wells. In the deep flow unit AB-7I is used as the source well and AB-2D, MW-3D, and MW-15D are used as downgradient wells. In the bedrock flow unit AB- 6BRU is used as the source well and GWA-2BR, GWA-1BRU, and MW-15BR are used as downgradient wells. While PBTV values could not be distinguished on these graphs because values differ by flow unit, the graphs show constituent concentrations in source areas and downgradient, and aid in understanding constituent distribution. Most constituents concentrations in the shallow flow layer including antimony, arsenic, beryllium, boron, strontium, sulfate, TDS, and vanadium decrease significantly with distance from the source area. Other constituents in the shallow flow layer such as chromium, cobalt, iron, and manganese do not exhibit a clear trend, potentially due to naturally occurring concentrations of these constituents. Less reduction in concentration with distance for the deep and bedrock flow layers is likely the result of less CCR influence in deeper flow zones. Vertical Extent Cross-Sections The vertical extent of constituents is illustrated on the A-A’ Cross Sections (Figures 11- 41 to 11-52) and D-D’ Cross Sections (Figures 11-53 to 11-64). Cross-section A-A’ – Boron Analytical Results, Figure 11-44, shows a cross-section from southwest to northeast. This transect includes both the ash storage area and the ash basin. Boron concentrations, greater than background, but below 2L, are present in shallow and deep 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 11-3 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx wells with greater concentrations below and downgradient of the ash basin (AB-2D and GWA-9D). Abandoned source area (AS and AB) wells are shown on the cross section for reference. No recent data is available for the abandoned wells; therefore the most recent data available is posted. Cross-section D-D’ – Boron Analytical Results, in Figure 11-56, shows a south-north transect (from Horseshoe Bend Beach Road to Mountain Island Lake) across the ash basin. There are nine groundwater monitoring wells along the centerline. These wells are background, across the basin, to downgradient of the ash basin. Abandoned source area wells are shown on the cross section for reference. Cross- section D-D’ illustrates the thick regolith present at RBSS and the shallowing of bedrock near the Catawba River. Boron is below detection limits in the upgradient wells (GWA- 14S/D and GWA-6S/D), but is present downgradient from the ash basin (GWA- 2SA/BRU). The highest concentration is present at a bedrock well (GWA-2BRU) screened just below the saprolite/bedrock interface. A second bedrock well at the location (GWA-2BR) screened approximately 44 feet below GWA-2BRU, indicates boron is below detection limits. Concentrations of COIs exceed PBTVs and certain 2L/IMAC values in porewater; surficial groundwater beneath and downgradient of the ash basin; in the deep flow layer beneath and downgradient of the ash basin; and in bedrock downgradient of the ash basin. The well screens in the CAMA wells accurately monitor groundwater conditions and impact to the groundwater flow zones for the shallow and deep units. COI impacts to the bedrock flow unit are generally confined to downgradient wells installed along the thin strip of land between the ash management areas and Mountain Island Lake (GWA-2 and GWA-3 locations). Other bedrock COI occurrences, such as manganese in BG-2BR, are not accompanied by boron or sulfate concentrations which indicate CCR influence. In summary, the horizontal and vertical extent of the plume has been defined. Monitoring wells across the site are appropriately placed and screened to the correct elevations to monitor groundwater quality. Monitoring wells installed for other regulatory programs have added additional details about the orientation and extent of the downgradient plume and have helped refine an understanding of the vertical and horizontal distribution of the plume. 11.1.1 Chemical Characterization Plume chemical characterization is detailed below for each COI. Analytical results are based on the March-April 2017 groundwater sampling event. Eighty- six groundwater monitoring wells including ten background monitoring wells 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 11-4 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx were sampled as part of the event. Twenty-nine samples were determined not valid due to elevated pH or turbidity. The summary below is based on valid samples excluding background for a total of 47 samples. Antimony Detected Range: 0.81 µg/L; 1 detection/47 samples (1/47) No IMAC exceedances in groundwater Antimony was not observed above IMAC or PBTVs in valid samples from downgradient and upgradient areas around the ash basin. A number of elevated pH samples indicated antimony detections. Soil concentrations beneath the ash basin exceed the POG PSRG and PBTV. Antimony is a silvery-white, brittle metal. Small amounts of antimony are naturally present in rocks, sediments, soils and water. Since no 2L value has been established for this constituent by NCDENR antimony is compared to IMAC. Antimony was detected above the IMAC in one ash porewater well (ABMW-1), one transition zone well below the ash (ABMW-4D), and one background transition zone well (MW-12D). Antimony can occur in pyrite and sulfides associated with pyrite in coal (Finkelman, 1995). Antimony is a minor trace element in the crust that occurs in concentrations of 0.26 milligrams per kilogram in felsic (light colored, silica rich) rocks to 2.0 mg/kg in clays and shales (Parker, 1967), Table 19). Arsenic Detected Range: 0.10 µg/L to 9.0 µg/L; 22/47 No 2L exceedances in groundwater Arsenic exceeded the PBTV for the shallow flow layer in C-1S screened within the cinder storage area and GWA-11S screened downgradient of the cinder storage area and near the former coal pile. Arsenic exceeded the PBTV for the deep flow layer in GWA-12D screened sidegradient of the cinder storage area and former coal pile. Arsenic exceeded the PBTV for the bedrock flow layer in GWA-3BR located downgradient of the cinder storage area and former coal pile. 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 11-5 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx Some soil concentrations from immediately beneath the ash basin exceed the POG PSRG and PBTV. Arsenic is a trace element in the crust, with estimated concentrations ranging from less than one mg/kg in mafic igneous rocks to 13 mg/kg in clay-rich rocks (Parker, 1967). It occurs in multiple valence states (As5+, As3+, and As3-). Arsenic in coal occurs primarily in pyrite (iron sulfide, with arsenic replacing iron in the crystal structure) (Finkelman, 1995). Arsenic condenses on fly ash as arsenate (As5+) (Goodarzi, Huggins, & Sanei, 2008). Leaching tests on ash indicate that trace quantities of up to 50 percent of the arsenic present can be leached. In addition to the solubility of the source, the concentration of calcium and presence of oxides appear to limit the mobility of arsenic (Izquierdo & Querol, 2012). The USEPA estimates that the amount of natural arsenic released into the air as dust from the soil is approximately equal to the amount of arsenic released by all human activities (EPRI, 2008a). Beryllium Detected Range: 0.10 µg/L to 5.2 µg/L; 11/47 Beryllium exceeded the IMAC of 4 µg/L and PBTV of 0.144 µg/L in one well (GWA-12S) screened within the shallow flow layer. There were no exceedances for beryllium in wells screened within the deep and bedrock flow layer. Beryllium also exceeded the PBTV for the shallow flow layer in several wells downgradient and within the cinder storage area. Soil concentrations beneath the ash basin were below the POG PSRG and PBTV. Beryllium is a hard, gray metal that is very lightweight. In nature, it combines with other elements to form beryllium compounds. Small amounts of these compounds are naturally present in soils, rocks, and water. Emeralds and aquamarines are gem-quality examples of a mineral (beryl) that is a beryllium compound. Beryllium combines with other metals to form mixtures called alloys. Beryllium and its alloys are used to construct lightweight aircraft, missiles, and satellite components. Beryllium is also used in nuclear reactors and weapons, X-ray transmission windows, heat shields for spacecraft, rocket fuel, aircraft brakes, bicycle frames, precision mirrors, ceramics, and electrical switches (EPRI, 2008b). 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 11-6 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx Most of the beryllium occurring in North Carolina is along the south and southwest sides of the Blue Ridge Mountains. The most notable mines include the Biggerstaff, Branchand, and Poteat mines in Mitchell County; the Old Black mine in Avery County; and the Ray mine in Yancey County. Beryllium forms golden or pale-green well-formed prismatic crystals ranging in size from a fraction of an inch to about 3 inches in diameter. It is generally found near the cores of bodies of pegmatites of moderate size that contain considerable amounts of perthitic microcline. Production has been negligible, and no regular production appears possible. Green beryllium (aquamarine and emerald) was mined commercially many years ago at the Grassy Creek emerald mine and the Grindstaff emerald mine on Crabtree Mountain in Mitchell County. The Ray mine in Yancey County has also produced some golden beryllium and aquamarine (Brobst 1962). Beryllium-containing minerals are also common in granites and pegmatites throughout the Piedmont; however, to a much lesser degree than the Blue Ridge Mountains Province (Brobst, 1962). Beryllium is concentrated in silicate minerals relative to sulfides and in feldspar minerals relative to ferromagnesium minerals. The greatest known naturally occurring concentrations of beryllium are found in certain pegmatite bodies. Beryllium is not likely to be found in natural water above trace levels due to the insolubility of oxides and hydroxides at the normal pH range (Brobst, 1962). Boron Detected Range: 73.1 µg/L to 690 µg/L; 27/47 No 2L exceedances in valid groundwater samples. Boron was observed above the 2L of 700 µg/L at a concentration of 777 µg/L in a pH-elevated sample from bedrock unit well GWA-2BRU. Boron has periodically been observed at concentrations greater than 2L in AOW/seep samples which represent an expression of groundwater at the surface. Boron exceeded the PBTV for the shallow and deep flow layers (50 µg/L) in wells across the RBSS site. The concentrations were generally highest in wells located downgradient of the ash management areas in the shallow and deep flow zones. Some soil concentrations beneath the ash basin exceed the POG PSRG and PBTV. 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 11-7 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx Boron is a trace element in the crust, with estimated concentrations ranging from as little as 1 mg/kg in mafic igneous rocks to hundreds of milligrams per kilogram in clay rich rocks (Parker, 1967). It occurs only in the trivalent form (B3+) and is concentrated in sedimentary rocks (Urey & Mem, 1953). This observation indicates that a mechanism exists to concentrate boron in minerals because the oceans could dissolve all of the boron estimated to be present in the crust (Fleet, 1965). Fleet (1965) presents both biogenic and mineralogical processes to account for the preferential concentration of boron in the crust. Boron is a micronutrient (Goldberg, 1997) that is concentrated in plant tissue, including the plants from which coal formed. While boron is relatively abundant on the earth’s surface, boron and boron compounds are relatively rare in all geological provinces of North Carolina. Natural sources of boron in the environment include volatilization from seawater, geothermal vents, and weathering of clay-rich sedimentary rocks. Total contributions from anthropogenic sources are less than contributions from natural sources. Anthropogenic sources of boron include agriculture, refuse, coal and oil burning power plants, by-products of glass manufacturing, and sewage and sludge disposal (EPRI, 2005). Because boron is associated with the carbon (fuel) in coal, it tends to volatilize during combustion and subsequently condense onto fly ash as a soluble borate salt (Dudas, 1981). Boron leaches readily (up to 50 percent of total present) and rapidly from fly ash (Cox, Lundquist, Przyjazny, & Schmulbach, 1978). Boron is considered a marker COI for coal ash because boron is rarely associated with other types of industrial waste products. Boron is the primary component of a few minerals including tourmaline, a rare gem mineral that forms under high temperature and pressure (Hurlbut, 1971). The remaining common boron minerals, including borax that was mined in the Mojave desert, in Boron, California, form from the evaporation of seawater in deposits known as evaporites. Boron mobilized into the environment will remain in solution until incorporation into plant tissue or adsorption by a mineral. Fleet describes sorption of boron by clays as a two-step process. Boron in solution is likely to be in the form of the borate ion (B(OH)4-). The initial sorption occurs onto a charged surface. Observations that boron does not tend to desorb from clays indicates that it migrates rapidly into the crystal structure, most likely in substitution for aluminum. Goldberg et al. determined that boron sorption 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 11-8 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx sites on clays appear to be specific to boron (Goldberg, Forster, Lesch, & Heick, 1996). For this reason, there is no need to correct for competition for sorption sites by other anions in transport models. Goldberg lists aluminum and iron oxides, magnesium hydroxide, clay minerals, calcium carbonate (limestone), and organic matter as important sorption surfaces in soils (Goldberg, 1997). Boron sorption on oxides is diminished by competition from numerous anions. Boron solubility in groundwater is controlled by adsorption reactions rather than by mineral solubility. Goldberg concludes that chemical models can effectively replicate boron adsorption data over changing conditions of boron concentration, pH, and ionic strength. Chromium Detected Range: 0.52 µg/L to 314 µg/L; 29/57 Chromium exceeded 2L and the PBTV in five shallow flow layer wells around the ash storage area and the ash basin. There were no exceedances for chromium in the deep flow layer. Chromium exceeded 2L and the PBTV in the bedrock flow layer in one well (GWA-1BRU) located downgradient of the ash basin primary cell. A number of elevated chromium concentrations from the May 2017 sampling event were not indicative of past sampling events which reflected significantly lower concentrations. Comparison of total and dissolved results indicates that the dissolved fractions were often much lower indicating a less mobile particulate influence. Soil concentrations beneath the ash basin exceed the POG PSRG and PBTV. Chromium is a blue-white metal found naturally occurring in combination with other substances. It occurs in rocks, soils, plants, and volcanic dust and gases (EPRI, 2008c). Background concentrations of chromium in groundwater generally vary according to the media in which they occur. Most chromium concentrations in groundwater are low averaging less than 1.0 µg/L worldwide. Chromium tends to occur in higher concentrations in felsic igneous rocks (such as granite and metagranite) and ultramafic igneous rocks; however, it is not a major component of the igneous or metamorphic rocks found in the North Carolina Piedmont or the Blue Ridge (Chapman, Cravotta, III, Szabo, & Lindsey, 2013). 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 11-9 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx In a statistical summary of groundwater quality in North Carolina, the Superfund Research Program at UNC analyzed 1,898 private well water samples in Gaston and Mecklenburg Counties. The samples were tested by the North Carolina State Laboratory of Public Health from 1998 to 2012. The average chromium concentrations were 5.1µg/L and 5.2 µg/L in Gaston and Mecklenburg Counties respectively. Hexavalent Chromium Detected Range: 0.031µg/L to 6.7 µg/L; 34/47 No exceedances of the regulatory comparative value of 10 µg/L in groundwater. The PBTV for hexavalent chromium in the shallow flow layer (1.1 µg/L) was exceeded in wells screened within and downgradient of the cinder storage area and in wells downgradient of the ash basin. The PBTV for hexavalent chromium in the deep flow layer (1.2 µg/L) was exceeded in wells downgradient of the ash basin and in one well located sidegradient of the ash storage area. The PBTV for hexavalent chromium in the bedrock flow layer (0.16 µg/L) was exceeded in two wells (GWA-2BR and GWA-7BR) located downgradient of the ash basin. Hexavalent chromium values were generally less than total chromium values. Cr (VI) is the dominant form of chromium in shallow aquifers where aerobic conditions exist. Chromium can also occur in the +III oxidation state, depending on pH and redox conditions. Cr (VI) can be reduced to Cr (III) by soil organic matter, S2- and Fe2+ ions under anaerobic conditions often encountered in deeper groundwater. Major Cr (VI) species include chromate (CrO42-) and dichromate (Cr2O72-) which precipitate readily in the presence of metal cations (especially Ba2+, Pb2+, and Ag+). Chromate and dichromate also adsorb on soil surfaces, especially iron and aluminum oxides. Cr (III) is the dominant form of chromium at low pH. Chromium mobility depends on sorption characteristics of the soil, including clay content, iron oxide content, and the amount of organic matter present. Chromium can be transported by surface runoff to surface waters in its soluble or precipitated form. Soluble and un-adsorbed chromium complexes can leach 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 11-10 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx from soil into groundwater. The leachability of Cr(VI) increases as soil pH increases. Most of chromium released into natural waters is particle associated, however, and is ultimately deposited into the sediment (Smith, Means, Chen, & others, 1995). Cobalt Detected Range: 0.10 µg/L to 68.4 µg/L; 38/47 Cobalt exceeded the IMAC and PBTV in the shallow flow layer in wells across the RBSS site. The concentrations were generally the highest in wells located at or near the cinder storage area and downgradient of the ash basin. The IMAC and PBTV were exceeded in the deep flow layer in two wells (GWA-3D and GWA-12D). Cobalt exceedances in the deep flow layer were significantly less frequent and at lower concentrations than were observed in the shallow flow layer. There were no cobalt exceedances in the bedrock flow layer. Soil concentrations beneath the ash basin exceed the POG PSRG and PBTV. Cobalt is a base metal that exhibits geochemical properties similar to iron and manganese, occurring as a divalent and trivalent ion. Cobalt can also occur as Co-1. In terms of distribution in the crust, all three metals exhibit a strong affinity for mafic igneous and volcanic rocks and deep-sea clays (Parker, 1967). Cobalt occurs in clay minerals and substitutes into the pyrite crystal structure. There is also evidence that it is organically bound in coal (Finkelman, 1995). Izquierdo and Querol report limited leaching of cobalt from coal, attributing this observation to incorporation into iron oxide minerals (Izquierdo & Querol, 2012). The concentration of cobalt in surface and groundwater in the United States is generally low— between 1 and 10 parts of cobalt in 1 billion parts of water (ppb) in populated areas. The concentration may be hundreds or thousands times higher in areas that are rich in cobalt containing minerals or in areas near mining or smelting operations. In most drinking water, cobalt levels are less than 1 to 2 ppb (USGS, 1973). Cobalt is compared to IMAC since no 2L standard has been established for this constituent by NCDEQ. 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 11-11 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx Iron Detected Range: 51.4 µg/L to 40,500 µg/L; 36/47 The 2L (300 µg/L) and PBTV (593 µg/L) for iron in the shallow flow layer were exceeded in wells screened both within and downgradient of the cinder storage area, downgradient of the ash basin, and sidegradient of the ash storage area. The 2L and PBTV (56.3 µg/L) were exceeded in deep wells associated with the cinder storage area, former coal pile, and downgradient of the ash basin. The 2L and PBTV (125 µg/L) for iron in the bedrock flow layer was exceeded in GWA-3BR located downgradient of the cinder storage area. Soil concentrations beneath the ash basin exceed the POG PSRG and PBTV. Iron is a naturally occurring element that may be present in groundwater from the erosion of natural deposits (NCDHHS, 2010). A 2015 study by DENR (Summary of North Carolina Surface Water Quality Standards 2007-2014) found that while concentrations vary regionally, “iron occurs naturally at significant concentrations in the groundwaters of NC,” with a statewide average concentration of 1320 µg/L. Iron is estimated to be the fourth most abundant element in the Earth’s crust at approximately 5 percent by weight (Parker, 1967). Only Oxygen (46.60 weight percent), silicon (27.72 weight percent), and aluminum (8.13 weight percent) occur in higher concentrations. Iron occurs in divalent (ferrous, Fe2+), trivalent (ferric, Fe3+), hexavalent (Fe+6), and Fe-2 oxidation states. Iron is a common mineral-forming element, occurring primarily in mafic (dark colored) minerals including micas, pyrite (iron disulfide), and hematite (iron oxide), as well as in reddish-colored clay minerals. Clay minerals and pyrite are common impurities in coal. Under combustion conditions in a coal-fired boiler, clay minerals would be dehydrated to mullite or gibbsite, possibly liberating iron, and pyrite would oxidize to hematite or magnesioferrite. Research summarized by Izquierdo and Querol indicates that iron leaching from coal ash is on the order of 1 percent of the total iron present due to the low pH required to solubilize iron minerals (Izquierdo & Querol, 2012). Despite the low apparent mobilization percentage, iron is often one of the COIs detected in the highest concentrations in ash porewater. Ferric iron is soluble at pH less than 2 at typical surface conditions (25°C and 1 atmosphere 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 11-12 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx total pressure, (Schmidt, 1962)). For this reason, dissolved iron in surficial waters is typically oxidized to the trivalent state resulting in formation of ferric iron oxide flocculation that exhibits a characteristic reddish tint. Manganese Detected Range: 6.7 µg/L to 15,800 µg/L; 39/57 Manganese exceeded the 2L and PBTV in the shallow, deep, and bedrock flow layers in wells across the RBSS site. Soil concentrations beneath the ash basin were less than the POG PSRG and PBTV. Manganese is a naturally occurring silvery-gray transition metal that resembles iron but is more brittle and is not magnetic. It is found in combination with iron, oxygen, sulfur, or chlorine to form manganese compounds. High manganese concentrations are associated with silty soils, and sedimentary, unconsolidated, or weathered lithologic unit and low concentrations are associated with non- weathered igneous bedrock and soils with high hydraulic conductivity (Gillespie, 2013), (Polizzotto, 2014). Manganese is most readily released to the groundwater through the weathering of mafic or siliceous rocks (Gillespie, 2013). When manganese-bearing minerals in saprolite, such as biotite, are exposed to acidic weathering, the metal can be liberated from the host mineral and released to groundwater. It then migrates through pre-existing fractures during the movement of groundwater through bedrock. If this aqueous-phase manganese is exposed to higher pH in the groundwater system, it will precipitate out of solution. This results in preferential pathways becoming “coated” in manganese oxides and introduces a concentrated source of manganese into groundwater (Gillespie, 2013). Manganese(II) in suspension of silt or clay is commonly oxidized by microorganisms present in soil, leading to the precipitation of manganese minerals (ATSDR, 2012). Roughly 40 percent to 50 percent of North Carolina wells have manganese concentrations higher than the state drinking water standard (Gillespie, 2013). Concentrations are spatially variable throughout the state, ranging from less than 0.01 mg/L to more than 2 mg/L. This range of values reflects naturally derived concentrations of the constituent and is largely dependent on the bedrock’s mineralogy and extent of weathering (Gillespie, 2013). 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 11-13 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx Manganese is estimated to be the 12th most abundant element in the crust (0.100 weight percentage, (Parker, 1967). Manganese exhibits geochemical properties similar to iron with Mn7+, Mn6+, Mn4+, Mn3+, Mn2+, and Mn1- oxidation states. Manganese substitutes for iron in many minerals. Similar to iron, manganese leaching from coal ash is limited to less than 10 percent of the total manganese present due to the low pH required to solubilize manganese minerals (Izquierdo & Querol, 2012). Despite the low apparent mobilization percentage, manganese can be detected in relatively high concentrations in ash porewater. Strontium Detected Range: 8.6 µg/L to 3,000 µg/L; 47/47 The PBTV for strontium in the shallow flow layer (293 µg/L) was exceeded in wells associated with the cinder storage area and former coal pile. The PBTV for strontium in the deep flow layer (697 µg/L) was exceeded at wells located downgradient of the cinder storage area and former coal pile. The PBTV for strontium in the bedrock flow layer (961 µg/L) was exceeded in GWA-3BR located downgradient of the cinder storage area and near the former coal pile. Strontium is a soft silver-yellow alkaline earth metal. It is highly chemically reactive and forms a dark oxide layer when it interacts with air. It is chemically similar to Ca and replaces Ca or K in igneous rocks in minor amounts. Strontium is generally present in low concentrations in surface waters but may exist in higher concentrations in some groundwater (Hem, 1985). Sr is present as a minor coal and coal ash constituent. Sr has been observed to leach from coal cleaning rejects more in neutral conditions than acidic, unlike many other metals (Jones & Ruppert, 2017). It has been shown to behave conservatively in surface waters downstream of coal plants (Ruhl, et al., 2012). Sulfate Detected Range: 2.1 mg/L to 1,780 mg/L; 38/47 Sulfate exceeded the 2L (250 mg/L) and PBTV (1.2 mg/L) in one well screened within the shallow flow layer (GWA-3SA). The result was flagged with laboratory qualifier (M6) indicating matrix interference. 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 11-14 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx Sulfate exceeded the 2L and PBTV (21.9 mg/L) in three wells within the deep flow layer. One sample GWA-3BR, exceeded the 2L and PBTV in the bedrock flow layer. The PBTV for the shallow flow layer was exceeded in most wells downgradient of the ash basin and cinder storage. Concentrations were highest sidegradient and downgradient of the cinder storage area. Sulfate is a naturally occurring substance found in minerals, soil, and rocks. It is present in ambient air, groundwater, plants, and food. Primary natural sources of sulfate include atmospheric deposition, sulfate mineral dissolution, and sulfide mineral oxidation. The principal commercial use of sulfate is in the chemical industry. Sulfate is discharged into water in industrial wastes and through atmospheric deposition (USEPA, 2003). Anthropogenic sources include coal mines, power plants, phosphate refineries, and metallurgical refineries. While sulfate has an SMCL, and no enforceable maximum concentration set by the USEPA, ingestion of water with high concentrations of sulfate may be associated with diarrhea, particularly in susceptible populations, such as infants and transients (USEPA, 2012). However, adults generally become accustomed to high sulfate concentrations after a few days. It is estimated that about 3 percent of the public drinking water systems in the United States may have sulfate concentrations of 250 mg/L or greater (Miao, Brusseau, Carroll, & others, 2012). Sulfate is on the list of enforced regulated contaminates that may cause cosmetic effects or aesthetic effects in drinking water (USEPA, 2014). TDS Detected Range: 28 mg/L to 2,770 mg/L; 47/47 TDS exceeded the 2L of 500 mg/L in one well, GWA-3SA screened within the shallow flow layer, three wells screened within the deep flow layer, and one well screened within the bedrock flow layer. The wells with highest concentrations are located downgradient of the cinder storage area and near the former coal pile area. Groundwater contains a wide variety of dissolved inorganic constituents as a result of chemical and biochemical interactions between the groundwater and the elements in the soil and rock through which it passes. Total Dissolved Solids (TDS) mainly consist of cation and anion particles (e.g., calcium, chlorides, 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 11-15 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx nitrate, phosphorus, iron, sulfur, and others) that can pass through a 2 micron filter (USEPA, 1997). TDS is therefore a measure of the total amount of dissolved ions in the water, but does not identify specific constituents or explain the nature of ion relationships. TDS concentrations in groundwater can vary over many orders of magnitude and generally range from 0 – 1,000,000 µg/L. The ions listed below are referred to as the major ions as they make up more than 90 percent of the TDS in groundwater. TDS concentrations resulting from these constituents are commonly greater than 5,000 µg/L (Freeze & Cherry, 1979). Sodium (Na+) Magnesium (Mg2+) Calcium (Ca2+) Chloride (Cl-) Bicarbonate (HCO3-) Sulfate (SO42-) Minor ions in groundwater include: boron, nitrate, carbonate, potassium, fluoride, strontium, and iron. TDS concentrations resulting from minor ions typically range between 10 – 1,000 µg/L (Freeze & Cherry, 1979). Trace constituents make up an even smaller portion of TDS in groundwater and include: aluminum, antimony, arsenic, barium, beryllium, cadmium, chromium, cobalt, lead, manganese, nickel, selenium, thallium, vanadium, and zinc among others. TDS concentrations resulting from trace constituents are typically less than 100 µg/L (Freeze & Cherry, 1979). In some cases, contributions from anthropogenic sources can cause some of the elements listed as minor or trace constituents to occur as contaminants at concentration levels that are orders of magnitude above the normal ranges indicated above. TDS in water supplies originate from natural sources, sewage, urban and agricultural run-off, and industrial wastewater. Salts used for road de-icing can also contribute to the TDS loading of water supplies. Concentrations of TDS from natural sources have been found to vary from less than 30 mg/L to as much as 6,000 mg/L. Water containing more than 2,000 – 3,000 mg/L TDS is generally too salty to drink (the TDS of seawater is approximately 35,000 mg/L) (Freeze & Cherry, 1979). Reliable data on possible health effects associated with the ingestion of TDS in drinking water are not available. (WHO, 1996) TDS is on the list of “National Secondary Drinking Water Regulations” (NSDWRs) which are 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 11-16 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx non-enforced regulated contaminates that may cause cosmetic effects or aesthetic effects in drinking water (USEPA, 2014). Vanadium Detected Range: 0.55 µg/L to 11.7 µg/L; 24/47 The PBTV for vanadium in the shallow flow layer (2.02 µg/L) was exceeded in two wells (GWA-2S and GWA-1S) located downgradient of the ash basin. The PBTV for vanadium in the deep flow layer (12 µg/L) was exceeded in GWA-7D located east of the ash basin. The PBTV for vanadium in the bedrock flow layer (0.33 µg/L) was exceeded in two wells, GWA-2BR and GWA-7D. Soil concentrations beneath the ash basin exceed the POG PSRG and PBTV. Vanadium is estimated to be the 22nd most abundant element in the crust (0.011 weight percent, (Parker, 1967)). Vanadium occurs in four oxidation states (V5+, V4+, V3+, and V2+). It is a common trace element in both clay minerals and plant material. The National Uranium Resource Evaluation (NURE) program was initiated by the Atomic Energy Commission in 1973 with a primary goal of identifying uranium resources in the United States (http://pubs.usgs.gov/of/1997/ofr-97- 0492/, accessed on June 8, 2015 (Smith, 2006). The Hydrogeochemical and Stream Sediment Reconnaissance program (initiated in 1975) was one component of NURE. Planned systematic sampling of the entire United States began in 1976 under the responsibility of four Department of Energy national laboratories. Samples were collected from 5,178 wells across North Carolina. Of these, the concentration of vanadium was equal to or higher that the former IMAC of 0.0003 mg/L in 1,388 well samples (27 percent). 11.2 Pending Investigation(s) Additional metal oxy-hydroxide phases of iron (HFO) and aluminum (HAO) data are needed to support geochemical modeling conducted as part of the CAP. Soil and rock samples will be collected from previously installed borings or from additionally drilled boreholes along the primary groundwater flow transects. The samples will be located: Directly beneath the ash basin 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 11-17 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx Downgradient locations north of the ash management areas The samples will be collected at vertical intervals that coincide with nearby well screen elevations. Analysis results of collected samples will be used to improve input parameters for the updated geochemical model. To help determine potential routes of exposure and receptors related to the ash management areas, additional surface water samples will be collected from the Catawba River at locations along the river bank most likely to be impacted by potentially contaminated groundwater discharge. Surface water samples to be collected will include two upstream locations and up to 12 downgradient sample locations spaced at intervals along two segments of the river bank downgradient from the cinder storage area and the primary ash basin. One segment of the riverbank will total approximately 800 feet and be downgradient from monitoring well location GWA-2. The second segment will measure approximately 1,200 feet and be located downgradient from monitoring wells GWA-3, and GWA-11. Groundwater samples from those wells indicate elevated concentrations of the CCR indicator constituents boron and sulfate above 2L and PBTVs. Locations will be sampled at a frequency and at the same physical location to allow an assessment with 2B water quality standards. At each location, two samples will be collected within one hour to be evaluated for acute instream metals standards and the remaining two samples will be collected within the following 95 hours to be evaluated, using an average of a minimum of four samples, for chronic instream metals standards. 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 12-1 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx 12.0 RISK ASSESSMENT A baseline human health and ecological risk assessment was conducted in 2016 as a component of CAP Part 2 (HDR, 2016a). The 2016 risk assessment characterized potential effects on humans and wildlife exposed to coal ash constituents present in environmental media for the purpose of aiding corrective active decisions. Implementation of corrective action is intended to achieve future site conditions protective of human health and the environment, as required by the North Carolina CAMA. This update to the 2016 risk assessment evaluates groundwater and surface water results collected since the 2016 risk assessment in order to confirm or update risk conclusions in support of remedial actions. Data used in the 2016 risk assessment included groundwater, surface water, sediments, AOW water and soil collected from January 2011 to October 2015 (HDR, 2016a). This risk assessment update uses sampling locations described in Attachment A of the CAP 2 risk assessment (HDR, 2016a). AOW locations are outside the scope of this risk assessment because AOWs, wastewater, and wastewater conveyances (effluent channels) are permitted under the NPDES Program administered by NCDEQ DWR. No new sediment or soil samples have been collected that are applicable to the risk assessment, so risk estimates associated with those media have not been re-evaluated. As part of the 2016 risk assessment, human health and ecological conceptual site models (CSMs) were developed to guide identification of exposure pathways, exposure routes, and potential receptors for evaluation in the risk assessment. The CSMs (CAP Part 2, Appendix F, Figures 2-3 and 2-4) describe the sources and potential migration pathways through which groundwater beneath the ash management areas may have transported coal ash-derived constituents to other environmental media (receiving media) and, in turn, to potential human and ecological receptors. Exposure scenarios and exposure areas were presented in detail in Sections 2 and 5 of the 2016 risk assessment (CAP Part 2, Appendix F). This risk assessment update included the following: Identification of maximum constituent concentrations per groundwater and surface water Inclusion of new groundwater and surface water data to derive overall average constituent concentrations for exposure area 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 12-2 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx Comparison of new maximum constituent concentrations to the risk assessment human health and ecological screening values Comparison of new maximum constituent concentrations to human health Risk- Based Concentrations (RBCs) Incorporation of new maximum constituent concentrations into wildlife Average Daily Dose (ADD) calculations for comparison to ecological Toxicity Reference Values (TRVs) Results of the evaluation of new groundwater and surface water data and the influence on the 2016 risk assessment are summarized as follows by exposure area at the RBSS Site (Figure 12-1). 12.1 Human Health Screening Summary On-Site Groundwater Groundwater analytical data used in the 2016 human health risk assessment include samples collected from 88 locations across the site. The data from these wells were evaluated because it represents the potential trespasser/worker exposure area as determined in the 2016 risk assessment. The 2016 risk assessment concluded that groundwater posed no unacceptable risks to human receptors evaluated under the trespasser and worker exposure scenarios. Analytical results are included in Appendix B, Table 1. No new maximum detected concentrations exceeded human health RBCs; therefore, no evidence of potential risks to humans exposed to on-site groundwater was indicated. On-site – Surface Water Surface water samples collected in two locations (RBSW001 and RBSW002) are considered in this risk assessment update. The 2016 risk assessment concluded that surface water posed no unacceptable risks to human receptors evaluated under the trespasser and worker exposure scenarios. Analytical results are included in Appendix B, Table 2. No new maximum detected constituent concentrations in RBSW001 and RBSW002 exceeded human health RBCs; therefore, no evidence of potential risks to humans exposed to surface water at this location was indicated. Off-site – Surface Water On-site surface water samples were used as a surrogate for off-site evaluations. The 2016 risk assessment concluded that, with the exception of a recreational and 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 12-3 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx subsistence fisher, off-site surface water posed no unacceptable risk to human receptors evaluated under the recreational scenarios. The 2016 risk assessment concluded that although recreational fisher and subsistence fisher resulted in a hazard index (HI) greater than unity for a few constituents, the use of on-site data to evaluate these potential exposure scenarios likely over-estimated risks. Analytical results are included in Appendix B, Table 1. No new maximum detected constituent concentrations in surrogate samples (on-site surface water) exceeded human health RBCs; therefore, no evidence of potential risks to humans exposed to off-site surface water was indicated. Off-Site – Groundwater No off-site groundwater samples were collected. It is not anticipated that the RBSS property will be used for residential development and therefore, it is not anticipated that the site groundwater will be used as a source of drinking water. In addition, there are no private residences located hydraulically downgradient of on-site groundwater; therefore, off-site residential exposure to groundwater was not evaluated in the 2016 risk assessment. 12.2 Ecological Screening Exposure Exposure Area 1 – Surface Water Selenium measured in surface water from Ecological Exposure Area 1 (Figure 12-1) resulted in a lowest-observed-adverse-effects-level (LOAEL) hazard quotient (HQ) of 2 for the piscivorous bird (great blue heron) in the 2016 risk assessment. All other aquatic receptors had HQs less than unity. Analytical results are included in Appendix B, Table 2. The maximum concentration of selenium detected in Exposure Area 1 in the 2016 risk assessment was 35.4 µg/L, while the maximum concentration in recent samples is 0.4 µg/L. Incorporating recent selenium results into exposure calculations has resulted in an updated HQ less than unity, indicating no evidence of risks to ecological receptors exposed to selenium in Exposure Area 1. Since the 2016 risk assessment, boron is the only constituent detected in this exposure area at a greater concentration than previously evaluated (59 µg/L, compared to 50 µg/L in 2016), but boron did not exceed the surface water ecological screening value of 7,200 µg/L, and therefore does not pose unacceptable risks. According to the delineated groundwater plume (Figure 12-1), Exposure Area 1 does not appear to be affected by constituent migration in groundwater from the ash basin. 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 12-4 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx 12.3 Private Well Receptor Update One residential water supply well was located as part of the receptor survey for the RBSS site. The well is located on the north side of the Catawba River, which hydraulically separates the well from the RBSS site. A complete exposure pathway does not exist; therefore, there is no evidence unacceptable risks to humans exposed to off– site groundwater exist. 12.4 Risk Assessment Update Summary Based on groundwater and surface water results collected since completing the risk assessment in 2016, there is no evidence of risks to humans and wildlife potentially exposed to groundwater and surface water. Possible ecological risks previously identified in the 2016 risk assessment either do not appear to be attributable to groundwater migration from the ash basin or are now governed as permitted outfalls (AOWs) under the current NPDES permit. There are no indications that risks to humans and wildlife exposed to environmental media associated with the ash management areas at RBSS exist. 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 13-1 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx 13.0 GROUNDWATER AND GEOCHEMICAL MODELING Groundwater flow, and transport, and geochemical models are being developed to simulate movement of COIs through the subsurface to support the evaluation and design of remedial options at RBSS. The models will provide insights into: 1. COI mobility: Geochemical processes affecting precipitation, adsorption and desorption onto solids will be simulated based on lab data and thermodynamic principles to predict partitioning and mobility in groundwater. 2. COI movement: Simulations of the groundwater flow system will be combined with estimates of source concentrations, sorption, effective porosity, and dispersion to predict the paths and rates of constituent movement at the field scale. 3. Scenario screening: The flow, transport, and geochemical models will be adjusted to simulate how various ash basin closure design options and groundwater remedial technologies will affect the short-term and long-term distribution of COIs. 4. Design: Model predictions will be used to help design basin closure and groundwater corrective action strategies in order to achieve compliance with 2L at a reasonable cost and timeframe. The groundwater flow model linked with the transport model will be used to establish transport predictions that best represent observed conditions at the site particularly for the constituents, such as boron, that tend to be negligibly affected by geochemical processes. The geochemical model information will provide insight into the complex processes that influence constituent mobility, which will be used to refine constituent sorption within the transport model. Once the flow, transport and geochemical models for the site accurately reproduce observed site conditions, they can be used as predictive tools to evaluate the conditions that will result from various groundwater remedial options in addition to ash excavation. The site-specific groundwater flow and transport model and the site-specific geochemical model are currently being updated for use in the CAP. The CAP will further discuss the purpose and scope of both the groundwater and geochemical models. It will detail model development, calibration, assumptions and limitations. The CAP will also include a detailed remedial option evaluation, based on observed conditions and the results of predictive modeling. The evaluation of the potential 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 13-2 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx remedial options will include comparisons of predictive model results for long-term source concentration and plume migration trends toward potential receptors. The model predictions will be used in combination with other evaluation criteria to develop the optimal approach for groundwater remediation. The following sections provide a brief summary of modeling efforts completed to date for RBSS. 13.1 Summary of Fate and Transport Model Results The initial groundwater flow and transport model was developed by HDR in conjunction with the University of North Carolina at Charlotte (UNCC) to gain an understanding of COI migration after closure of the ash management areas. The initial groundwater model in the CAP Part 1 (HDR, 2015b) included a calibrated steady-state flow model of July 2015 conditions; a calibrated historical transient model of constituent transport to match June/July 2015 conditions; and three potential basin closure scenarios. Those basin closure simulation scenarios included: No change in site conditions (basin remains open, as is) Cap-in-place Ash removal (excavation) The initial model used antimony, chromium and sulfate as primary modeling constituents. Remedial alternative evaluation simulations were run to a total time of 250 years. As part of the CAP Part 2 (HDR, 2016a), the model was revised to include arsenic, boron, cobalt, hexavalent chromium, thallium, and vanadium. The revised model in the CAP Part 2 (HDR, 2016a) included a calibrated steady-state flow model of June 2015 conditions; a calibrated historical transient model of constituent transport to June/July 2015 conditions; and two potential basin closure scenarios. Those basin closure simulation scenarios included: No change in site conditions Ash removal (excavation) The flow and transport model is currently being modified as a part of the updated CAP and will include: development of a calibrated steady-state flow model that includes data available through November 2017; development of a historical transient model of constituent transport; and predictive simulations of basin closure plus groundwater corrective action scenarios. The updated fate and transport model will consider boron and potentially additional COIs that are hydraulically driven. Predictive simulations 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 13-3 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx will have simulation times that continue until modeled COI concentrations are below 2L at the compliance boundary. The following sections provide a brief summary of the groundwater modeling that was presented in the CAP Part 2 and a general outline for the updated modeling effort. The summary of the groundwater modeling presented in the CAP Part 2 was compiled to address specific questions regarding model set-up and calibration. A complete updated groundwater flow and transport model report is being developed and will be submitted as part of the updated CAP. The model was developed using the MODFLOW-NWT version (Niswonger, Panday, & Motomu, 2011). This version provides improved numerical stability and accuracy for modeling problems within a variable water table. The improved numerical stability and accuracy can provide better estimates of the water table fluctuations that result from ash basin operating conditions and potential closure and groundwater corrective action activities. MT3DMS was used to simulate fate and transport of selected COIs. MT3DMS uses the groundwater flow field from MODFLOW to simulate 3D advection and dispersion of the dissolved COIs, including the effects of retardation due to the soil matrix adsorption of COIs. 13.1.1 Flow Model Construction The flow and transport model was built through a series of steps. The first step was to build a three-dimensional (3D) model of the Site hydrostratigraphy based on the SCM. The next steps were to determine the model dimensions and the construction of the numerical grid. The numerical grid was then populated with flow parameters, which were calibrated in the steady-state flow model. Once the flow model was calibrated, the flow parameters were used to develop a transient model of the historical flow patterns at the site. The historical flow model was then used to provide the time-dependent flow field for the constituent transport simulations. Some model construction parameters — such as the model domain, grid density, and layer structure — may be modified for the updated model. Hydraulic parameters such as hydraulic conductivity values may be adjusted within reasonable site-specific conditions to achieve hydraulic head calibration error of less than 10 percent. 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 13-4 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx Flow Model Domain and Grid Layers The current model has dimensions of approximately 1 mile-by-1 ½ miles, with the ash basin at the center of the model domain. The model domain runs parallel to the Catawba River. The shortest distance between the ash basin and the current model boundary is approximately 250 feet. The lower limit of the model domain coincides with an assumed maximum depth of water yielding fractures in bedrock. This was assumed to be 80 feet below the base of the weathered bedrock (transition zone) upper limit across the site based on a review of boring logs included in the CSA. The updated model domain will be extended south to the Catawba River, and will be extended significantly in both the eastern and western directions. The hydrostratigraphic model consists of six units: ash, dike/ash storage material , alluvium, saprolite, weathered bedrock (transition zone), and fractured bedrock. Those units were determined by interpolating boring log data from historical data, the CSA, and the CAP reports ( (HDR, 2015b), (HDR, 2016a)). Flow Model Boundary Conditions The north, east, and west boundaries along the Catawba River were set to a constant head using the specified head boundary. Drainage features were set to the east, and to the south a presumed topographic groundwater divide was approximated by the route of Horseshoe Bend Beach Road. Other boundaries include drainage features and no flow boundaries at topographic divides. Seeps (areas where the water table intersects the ground surface and groundwater is discharged) are defined as drain-type boundaries. It is assumed that surface drainage is ultimately conveyed to an outfall to the Catawba River. Sources and Sinks Water can enter the model or leave the model through the use of sources and sinks. MODFLOW uses point sources/sinks as well as aerial sources/sinks. Point sources/sinks include rivers, wells, drains, and general head. Aerial sources/sinks considered are limited to recharge. Source (Recharge) Model recharge sources in the current model include: Recharge infiltrates through the ash basin (set at 21.5 inches per year) 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 13-5 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx Rainwater that infiltrates through the rest of the model domain (6.5 inches per year) Constant head boundaries Lakes were represented as specified head boundaries with the head set to their stage. This includes the Mountain Island Lake portion of the Catawba River, and the ash basin. The stage of Mountain Island Lake was 646.8 feet. The primary stage of the ash basin was set a maximum pond elevation of 724 feet. The secondary ash basin was set at a maximum pond elevation of 714 feet. Model Sinks (Drains) Model sinks in the current model include: Streams within the model domain Drains located at the base of the ash basin dam Areas of wetness (seeps) at the toe of the dam Water Supply Wells At the time the model was constructed only one private well had been identified within the half-mile radius of the site and no public water supply wells had been identified near the site. The well is located on the other side of the Catawba River which is a regional groundwater discharge point and hydraulically separates the well from RBSS. Hydraulic Conductivity The horizontal hydraulic conductivity and the horizontal-to-vertical hydraulic conductivity anisotropy ratio (anisotropy) are the main variable hydraulic parameters in the model. The distribution of those parameters is based primarily on the model hydrostratigraphy, with some local variations. The values can be adjusted during the calibration process to provide a best fit for observing water levels in wells. Flow Model Calibration Targets The steady state flow model calibration data for June 2015 were presented in the CAP Part 2. In the updated CAP, calibration target data may be incorporated by taking the mean of the hydraulic head data for each well and applying a 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 13-6 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx standard deviation to reflect the seasonal changes in the hydraulic heads. Hydraulic head data is projected to include measurements through November 2017. Mass Balance The previous model had a mass balance error of less than or equal to 1%. The updated model will have a similar numerical accuracy. Flow Model Sensitivity Analysis A parameter sensitivity analysis for the preliminary calibrated model showed the highest degree of sensitivity to hydraulic conductivities in the shallow aquifer, then reduction in recharge outside of the basin. The model was least sensitive to reduction in recharge within the basin. A final sensitivity analysis will be presented in the final CAP. Particle Tracking A primary concern is the potential impact to domestic and public wells from COIs emanating from the Site. The final calibrated groundwater flow model will be used to assess potential impacts by considering pumping from domestic and public wells within the model domain. Flow Model Assumptions and Limitations The groundwater model is currently being updated/refined and assumptions and limitations are subject to change. Based on the preliminary modeling results, the assumptions and limitations include the following: The steady-state flow model was calibrated to hydraulic heads measured in monitoring wells in June 2015. The model was not calibrated to transient water levels over time, recharge, or stream flow. MODFLOW simulates flow through porous media. A single domain MODFLOW modeling approach for simulating flow in the primary porous groundwater zones and bedrock was used for contaminant transport. Flow in fractured bedrock is simulated using the equivalent porous media approximation. Predictive simulations were performed and steady-state flow conditions were assumed from the time that the ash basin was placed in service through the current time until the end of the predictive simulations (2265). The uncertainty in model parameters and predictions has not been quantified; therefore, the error in model predictions is not known. It was 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 13-7 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx assumed the model results are suitable for a relative comparison of closure scenario options. 13.1.2 Transport Model Construction Modular 3-D Transport Multi-Species (MT3DMS) is being used to simulate constituent transport. MT3DMS simulates 3D advection and dispersion of the dissolved COIs, including the effects of retardation due to the soil matrix adsorption of COIs based on flow fields established by MODFLOW. The initial model used antimony, arsenic, boron, chromium, cobalt, hexavalent chromium, sulfate, thallium, and vanadium as primary modeling constituents. The updated fate and transport modeling will focus on boron and potentially additional COIs that are hydraulically driven. Other constituents will be considered using the geochemical model. Transport Model Parameters The key transport model parameters (besides the flow field) are the constituent source concentrations in the ash basin and the constituent soil-water distribution coefficients (Kd). Secondary parameters are the longitudinal, transverse, and vertical dispersivity, and the effective porosity. Transport Model Boundary Conditions In the current model, the transport model boundary conditions have an initial concentration of zero where water leaves the model. The background concentration used as initial concentrations for each COI is specified as the proposed provisional background concentration (PPBC) identified in the RBSS Corrective Action Plan (CAP) Part 1 (HDR, 2015b). Exceptions are hexavalent chromium and vanadium where 0.07 µg/L and 0.9 µg/L were used, respectively. Contaminants are assumed to leave the model when they reach a drain or are removed by flow that enters a constant head boundary. In the current model, the concentrations from the June 2015 sampling event were set as specified concentrations within the ash basin. These values are anticipated to be updated to use more recent concentration data. Transport Model Sources and Sinks Transport model sources include: The ash basin, ash storage area and cinder storage area are considered the source of COIs in the model. The sources are simulated by applying a constant COI concentration within the cells of the ash basin and were 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 13-8 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx applied at the ash model layers in the ash basin cells. This allows infiltrating water to carry dissolved constituents from the ash porewater into the groundwater underneath the ash basin. As the COIs migrate beneath and away from the coal ash, zones of soil and potentially a limited area of the upper fractured rock become impacted. Those impacted zones can serve as secondary sources, and are fully accounted for in the transport models. For simulations that involve ash excavation, the constant concentration sources in the ash zones are removed, but the secondary sources in the impacted soil and limited area of fractured bedrock remain. The longevity of these secondary sources depends on the COI Kd, and on the degree of flushing by infiltration and groundwater flow. Transport model sinks include: Mountain Island Lake (Catawba River) Drains Transport Model Calibration Targets and Sensitivity The initial transport model calibration targets were COI concentrations measured in monitoring wells in June/July 2015. The updated model calibration targets are anticipated to include COI concentrations measured in monitoring wells in 2017. COIs not amenable to simulation in the fate and transport model will be addressed in the geochemical model. Transport Simulation The updated model is projected to be calibrated to include data through November 2017 and will extend until modeled COI concentrations are below the 2L standard at the compliance boundary. The following is a summary of the basin closure options to be modeled: No Action – Leave the ash basin open, with residual ash, to evaluate whether groundwater quality would be restored by natural attenuation under previous conditions. Ash Removal – Remove the ash from the basin. This scenario assumes that the ground surface would be restored to its initial grade (prior to construction of the ash basin). 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 13-9 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx The results of these simulations will be included as part of the update CAP submittal. 13.1.3 Summary of Flow and Transport Modeling Results to Date The simulated June/July 2015 concentration distributions described in the CAP 1 (UNCC) (HDR, 2015b) were used as initial conditions in a predictive simulation of future flow and transport at the Site and modeled antimony, chromium and sulfate. Predictive simulations of future flow and transport for (CAP 2 (UNCC) (HDR, 2016a) arsenic, boron, cobalt, hexavalent chromium, thallium, and vanadium under the “no action” and excavation scenarios were run for a 250- year projection. No action The CAP 1 (UNCC) (HDR, 2015b) groundwater simulations indicated that north of the ash basin primary cell at GWA-2, antimony in all groundwater zones will remain above the 2L standard throughout the modeling period. For MW-15 (located north of the ash basin secondary cell), antimony in shallow and deep groundwater zones is predicted to remain below the 2L standard, but groundwater in the bedrock zone will exceed the 2L standard. For MW-6 (on the western edge of the ash basin primary cell), antimony concentrations remain above the 2L standard in all groundwater zones. Antimony exits the model domain and discharges to the Catawba River/Mountain Island Lake to the north, northwest, and northeast in all groundwater zones. The highest concentrations of antimony leave the model northeast of the ash basin secondary cell. Chromium concentrations for GWA-2 in the shallow and deep zones were predicted to increase and remain above the 2L standard. For MW-15 no impact is predicted. For MW-6 chromium is predicted to remain below the 2L standard until 2045 and at that time groundwater in the shallow and deep zones will exceed the 2L standard. Groundwater in the shallow zone exits the model domain to the northwest and northeast and discharges into the Catawba River. Groundwater in the deep zone exits the model to the northwest into the Catawba River. The predicted concentration of sulfate in monitoring wells GWA-2, MW-15, and MW-6 remains below the 2L standard in all groundwater zones. However, sulfate remains above the 2L standard in other areas of the model domain even after 250 years. Groundwater containing dissolved sulfate exits the model and discharges to the Catawba River 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 13-10 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx The CAP 2 (UNCC) (HDR, 2016a) modeling scenario consists of modeling each COI using the calibrated model for steady-state flow and transient transport under the Existing Conditions across the site to estimate when steady state concentrations are reached at the compliance boundary. The model indicates that antimony, cobalt, thallium and vanadium will continue to be greater than the IMAC at the Catawba River Compliance Boundary after 100 years in all groundwater zones. Arsenic, boron and chromium remain less than the 2L standard at Mountain Island Lake. Excavation Scenario The Excavation scenario presented in the CAP 1 (UNCC) (HDR, 2015b) simulates the effects of removing the ash basins, the dikes, and ash storage areas. At the beginning of this scenario, antimony in GWA-2 is predicted to increase slightly until 2025 and then decrease steadily, falling below the 2L standard beginning in 2070 for all groundwater zones. For MW-15 the shallow and deep groundwater zones will remain below the 2L limit for antimony, and the bedrock groundwater zone will decrease to the 2L by 2055. For MW-6, antimony will increase slightly until 2025 then decrease steadily over time in all groundwater zones and in 2070 be below the 2L. In all scenarios, antimony will be nearly non-detectable by 2155. Chromium concentrations will increase in the shallow and deep groundwater zones in GWA-2 and exceed the 2L in 2020 in the shallow groundwater zone, and 2060 in the deep groundwater zone. Chromium will remain above the 2L through 2260. Chromium will not be detectable in MW-15 in any zone. In MW-6, chromium concentrations will increase to levels above the 2L in 2060 in shallow groundwater and 2080 in deep groundwater. Chromium will continue to rise through 2265. Chromium in groundwater will exit the model and discharge to the Catawba River by 2115. Sulfate concentrations at wells GWA-2, MW-15, and MW-6 will be below the 2L standard. The CAP 2 Excavation (UNCC) (HDR, 2016a) Scenario results indicate that antimony, cobalt, thallium, and vanadium will continue to be greater than the IMAC at the Catawba River Compliance Boundary after 100 years in all groundwater zones. For these COIs, the background concentrations used for modeling are also above their respective IMAC or other comparative value, so the actual impact of the site sources on groundwater quality is unknown. Model predictions do not show that COI concentrations will be effectively reduced by 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 13-11 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx ash removal under the excavation scenario (UNCC) (HDR, 2016a). The COIs that are predicted to exceed 2L, or to be greater than the IMAC or other applicable comparative value, will not achieve compliance with the standards within the time period modeled (2015-2265) (UNCC) (HDR, 2016a). Arsenic, boron and chromium remain less than 2L at the Catawba River. 13.2 Summary of Geochemical Model Results The RBSS geochemical model investigates how variations in geochemical parameters affect movement of constituents through the subsurface. The geochemical SCM will be updated as additional data and information associated with Site constituents, conditions, or processes are developed. The geochemical modeling approach presented in the following subsections was developed using laboratory analytical procedures and computer simulations to understand the geochemical conditions and controls on groundwater concentrations at RBSS in order to predict how remedial action and/or natural attenuation may occur at the site. The final geochemical model will be presented in the updated CAP. 13.2.1 Model Construction The geochemical model in the CAP Part 2 (HDR, 2016a) included: Eh-pH (Pourbaix) diagrams showing potential stable chemical phases of the aqueous electrochemical system, calibrated to encompass conditions at the Site, Sorption model where the aqueous speciation and surface complexation are modeled using the USGS geochemical modeling program PHREEQC, Simulations of the anticipated geochemical speciation that would occur for each COI in the presence of adsorption to soils and in response to changes in Eh and pH, and Attenuation calculations where the potential capacity of aquifer solids to sequester constituents of interest were estimated Laboratory Determination of Distribution Coefficient HDR retained researchers from UNCC to determine site-specific distribution coefficients (Kd) for the primary hydrostratigraphic units. The UNCC Soil Sorption Evaluation and Addendum to the UNCC Soil Sorption Evaluation reports are provided in Appendix C. Selected soil samples were analyzed using batch and column experiments to determine Kd values for COIs (Table 11-1). In addition to these analyses, metal oxy-hydroxide phases of iron (HFO), 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 13-12 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx manganese (HMO), and aluminum (HAO) in soils were measured. HFO, HMO, and HAO are considered to be the most important surface reactive phases for cationic and anionic constituents in many subsurface environments (Ford, W., & Puls, 2007). Quantities of these phases in soil can thus be considered as a proxy for the presence of ferrihydrite (HFO) and gibbsite (HAO), which can be used to model COI sorption capacity for a given soil (Dzombak & Morel, 1990); (Karamalidis & Dzombak, 2010). Geochemical Model Construction To examine the sorption behavior of multiple ions of interest in the subsurface environment surrounding coal-fired power plants, a combined aqueous speciation and surface complexation model was developed using the USGS geochemical modeling program PHREEQC. Equilibrium constants for aqueous speciation reactions were taken from the USGS WATEQ4F database. This database contained the reactions for most elements of interest except for Co, Sb, V, and Cr. Constants for aqueous reactions and mineral formation for these elements were taken from the MINTEQ v4 database which is also issued with PHREEQC. The constants were all checked to provide a self-consistent incorporation into the revised database. The source of the MINTEQ v4 database is primarily the well-known NIST 46 database (Martell & Smith, 2001). Sorption reactions were modeled using a diffuse double-layer surface complexation model. For self-consistency in the sorption model, a single database of constants was used as opposed to searching out individual constants from literature. The diffuse double-layer model describing ion sorption to HFO and HAO by (Dzombak & Morel, 1990) and (Karamalidis & Dzombak, 2010), respectively, was selected for this effort. Geochemical Controls on COI As described in previous geochemical model reports (HDR, 2016a), pH, Eh, and solubility are the primary geochemical parameters affecting constituent mobility. In the updated geochemical model that will be submitted in 2018 as part of the CAP, hydraulically significant flow transects will be used to evaluate the conceptual model of COI mobility in the subsurface. It will compare trends in the concentrations of each COI along the transect with the model output to verify that the conceptual and qualitative models can predict COI behavior. Then the model will be used to evaluate the potential impacts of remediation activities. The model will relate the COI concentrations observed in groundwater along 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 13-13 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx flow transects to key geochemical parameters influencing constituent mobility (i.e., Eh, pH, and saturation/solubility controls). Geochemical Model Assumptions Several key assumptions will be applied to the planned geochemical modeling effort: The thermochemical sorption constant reactions describe ion sorption to ferrihydrite and gibbsite (HFO and HAO). The model will use the same or more conservative site density assumptions as those used by Dzombak and Morel (1990) and Karamalidis and Dzombak (2010) to constrain the surface sites. HAO and HFO (i.e., gibbsite and ferrihydrite) are used as the primary reactive minerals due to the availability of surface complexation reactions. Differences between the sorption behaviors at each site will be primarily due to 1) differences in the pH, Eh, and ion concentrations at each site, and 2) differences in the extractable iron and aluminum concentrations from Site specific solids. Additional reactive minerals will be incorporated into the model as needed on a Site specific basis. Updated Geochemical Model Development The updated geochemical site investigation to accompany the CAP will develop parameters for each aquifer or geologically derived flow zone (geozone) by considering the bulk densities, porosities, and hydraulic gradients used in the fate and transport model. These parameters are used to constrain the sorption site concentrations in the model input and will be incorporated in the 1-D ADVECTION model to accompany the capacity simulations. The objective of these capacity simulations is to determine the mass balance on iron and aluminum sorption sites when simulating flow through a fixed region. Groundwater concentrations and initial solid phase iron and aluminum concentrations will be fixed based on site-specific data. Thus, the model will be able to simulate the stability of the HFO and HAO phases assumed to control constituent sorption. The updated geochemical model report will include a site specific discussion of: The model description The purpose of the geochemical model 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 13-14 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx Modeling results with comparison to observed conditions COI sensitivity to pH, Eh, iron/aluminum oxide content Model limitations The updated geochemical modeling will also present multiple methods of determining constituent mobility at the Site. Aqueous speciation, surface complexation, and solubility controls will be presented in the revised report. These processes will be modeled using: Pourbaix diagrams created with the Geochemist Workbench v10 software using site-specific minimum and maximum constituent concentrations PHREEQC’s combined aqueous speciation and surface complexation model and the 1-D ADVECTION function to gain a comprehensive understanding of current geochemical controls on the system and evaluate how potential changes in the geochemical system might affect constituent mobility in the future. 13.2.2 Summary of Geochemical Model Results to Date The relationship between aqueous and sorbed COI concentrations is an equilibrium process. However, redox conditions vary widely across the Site indicating the site soils (or ash) may not have reached equilibrium with the groundwater which may affect the results of the model. The geochemical model results verified the geochemical behavior of the constituents of interest. Constituents found to be relatively mobile with low distribution coefficients included boron, barium, and antimony. Low distribution coefficients were predicted for some additional species such as iron, magnesium, manganese, nickel, and sulfate. High distribution coefficients indicating low mobility were found for arsenic, beryllium, and chromium. 13.3 Summary of Groundwater to Surface Water Evaluation Regulation 15A NCAC 02L requires that groundwater discharge will not possess constituent concentrations that would result in exceedances of standards for surface waters contained in 15A NCAC 02B .0200. The RBSS ash management areas are located 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 13-15 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx adjacent to the Catawba River. The Catawba River represents a groundwater discharge feature for the ash management areas. 13.3.1 CAP 1 and 2 Surface Water Mixing Model Approach CAP 1 and CAP 2 provided an evaluation of the COI concentrations in surface water due to groundwater discharge from the ash basin. The evaluation was performed by using a mixing model approach. The groundwater fluxes from the fate and transport model were used to represent the groundwater discharge. For each groundwater COI that discharges to surface waters at a concentration exceeding its applicable groundwater quality standard or criteria, the appropriate dilution factor and upstream (background) concentration were applied to calculate the surface water concentration. This concentration was then compared to the applicable water quality standard or criteria to determine surface water quality standard compliance. 13.3.2 Surface Water Quality Surface water, AOWs and ponded ash basin water were sampled in order to evaluate potential influence from the ash management areas. Boron concentrations in the background seep location, S-13, and the upstream Catawba River samples, 278.0-0.3m, ranged from non-detect to 65.9 µg/L. Seep sample locations downgradient of the ash management areas generally yielded boron concentrations greater than background, but rarely above 2L. This is consistent with results from groundwater monitoring wells in the same areas. AOW sample locations east of the ash basin, S-07 and S-08, yielded non-detect to very low boron results, which is also consistent with groundwater monitoring well results from the same area. Review of the Catawba River upstream (278.0- 0.3m) and downstream (277.5-0.3m) samples indicated no exceedances of the 2B criteria. Surface water data is discussed in detail in Section 9.0. Review of surface and seep water sampling data indicates the groundwater monitoring well network provides information to evaluate potential groundwater/surface water interaction. 13.3.3 Evaluation of Groundwater to Surface Water Conditions for Corrective Action To help determine potential routes of exposure and receptors related to the ash management areas, additional surface water samples will be collected from the Catawba River near the stream/river bank most likely to be impacted by 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 13-16 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx potentially contaminated groundwater discharge. The additional surface water sampling effort is described in detail in Section 11.2. 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 14-1 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx 14.0 SITE ASSESSMENT RESULTS A site conceptual model (SCM) is an interpretation of processes and characteristics associated with hydrogeologic conditions and constituent interactions at the Site. The site assessment results provide the information to evaluate distribution of constituents with regard to site-specific geological/hydrogeological properties. Coal ash removal activities began in May 2015 and are currently ongoing at RBSS. Bulk dewatering of the secondary cell was completed in January 2017, and interstitial dewatering began in February 2017. As of July 2, 2017, 2,416,323 tons of ash had been transported from the station, representing 46.4% of the total. The ash removal activities limit current data available in and beneath the ash management areas. Ongoing source removal is a significant activity and will be a primary component of site evaluation in the CAP. 14.1 Nature and Extent of Contamination The site assessment described in the CSA presents the results of investigations required by CAMA and 2L regulations. The ash basin porewater was determined to be a source of impact to groundwater. The site assessment investigated the site hydrogeology, determined the direction of groundwater flow from the ash basin, and determined the horizontal and vertical extent of impacts to groundwater and soil sufficient to proceed with preparation of a CAP. Constituents of Interest Soil and groundwater beneath the ash management areas downgradient to the Catawba River have been influenced by ash storage at the RBSS site. COIs in groundwater identified as associated with RBSS ash management areas include antimony, arsenic, beryllium, boron, chromium, hexavalent chromium, cobalt, iron, manganese, strontium, sulfate, TDS, and vanadium. Not all COIs were observed at concentrations greater than 2L/IMAC or PBTVs in recent groundwater monitoring. For instance, antimony and arsenic are retained as COIs due to historical observations, and hexavalent chromium is evaluated as a COI at NCDEQ direction, but is not observed above the regulatory comparative value. Hydrogeologic Conditions Groundwater COIs migrate laterally and vertically into and through regolith, weathered rock, and shallow bedrock. Regolith at RBSS is generally thick (>100 feet) and saturated. A highly permeable transition zone between saprolite and bedrock was not encountered during the CSA investigation. Constituent migration in groundwater occurs at variable rates depending on constituent sorption properties and geochemical 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 14-2 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx conditions (e.g., redox state, pH, etc.). Some COIs, such as boron, readily solubilize and migrate with minimal retention. In contrast, some COIs, such as arsenic, readily adsorb to aquifer materials, do not readily solubilize, and thus are relatively immobile. Site hydrogeologic conditions were evaluated by installing and sampling groundwater monitoring wells and piezometers; conducting in-situ hydraulic tests; sampling soil for physical and chemical testing; and sampling surface water, seeps, and sediment. Monitoring wells were completed in each hydrostratigraphic unit. The groundwater flow system serves to store and provide a means for groundwater movement. The porosity of the regolith is largely controlled by pore space (primary porosity); whereas, in bedrock, the effective porosity is largely secondary and controlled by the number, size, and interconnection of fractures. The nature of groundwater flow across the Site is based on the character and configuration of the ash basin relative to specific Site features such as man-made and natural drainage features, engineered drains, streams, and Mountain Island Lake; hydraulic boundary conditions; and subsurface media properties. The majority of groundwater flow across the Site appears to be through the regolith and shallow bedrock toward Mountain Island Lake. Four hydrostratigraphic units were identified at RBSS and were evaluated during the CSA: Ash – The ash porewater unit consisted of saturated ash material. Ongoing ash excavation activities have removed nearly half of the coal ash stored at RBSS since CSA activities were initiated. Observed ash depths range from a few feet to approximately 60 feet. Shallow/Surficial – The shallow/surficial unit consists of soil, saprolite, and alluvial material that overlie the thick section of saprolite regolith. Alluvium is found in areas near the Catawba River and directly overlies saprolite. The first occurrence of groundwater is represented by this unit. Un-saturated regolith varies from approximately five to 65 feet. The shallow wells monitor conditions near the top of the water table. Saturated thickness averages 10 feet. Deep Saprolite/Weathered Rock – The deep flow layer includes saprolite below the surficial zone and partially weathered bedrock present above competent bedrock. This unit lies directly above competent bedrock. Deep wells monitor a saturated zone of regolith approximately 100 feet thick. Bedrock – Top of competent bedrock was defined during the CSA investigation REC>85 percent and RQD>50 percent. Secondary porosity through weathering and subsequent fracturing of bedrock control groundwater flow through the 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 14-3 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx deepest hydrostratigraphic unit beneath RBSS. The depth of bedrock well screens below the top of competent bedrock ranges from 50 to 75 feet. Horizontal and Vertical Extent of Impact Boron and sulfate are the primary CCR-derived constituents in groundwater. They are detected at elevated concentrations above background, with only limited exceedances of 2L, beneath and downgradient (northwest-north-northeast) of the ash basin, ash storage and cinder storage areas. Boron is not generally detected in background groundwater. Sulfate background concentrations are significantly lower than those observed in and around the ash basin, cinder storage area and former coal pile. Boron, in its most common form is soluble in water, and has a very low Kd value, making the constituent highly mobile in groundwater. Therefore, the presence/absence of boron in groundwater provides a close approximation of the distribution of CCR-impacted groundwater. The detection of boron at concentrations in groundwater greater than applicable 2L and PBTVs best represents the extent of CCR influence downgradient from the source area (ash management areas). Groundwater flows from the highest topographic portion of the Site near Horseshoe Bend Beach Road toward the ash management areas and into the Catawba River. The extent of CCR-influence groundwater is defined as any location (in three-dimensional space) where groundwater quality is impacted by the ash management areas. Naturally occurring groundwater contains varying concentrations of a number of constituents (e.g., alkalinity, aluminum, magnesium, sodium, zinc, etc.). Sporadic and low- concentration exceedances of these constituents in the groundwater data do not necessarily demonstrate horizontal or vertical distribution in groundwater that indicates impact from the ash management areas. Observations on the extent of CCR influence are as follows: Based on the extent of boron and sulfate concentrations above background, CCR influence is assessed to the south, east and southwest of the ash management areas. Monitoring wells on the north side of the Site monitor groundwater conditions along the thin strip of land between the ash management areas and the Catawba River. Constituents observed in groundwater northwest of the ash basin and cinder storage area - between the cinder storage area and former coal pile - will be further evaluated as part of coal pile assessment. Boron was not observed above 2L in valid samples for the May 2017 groundwater sampling event. Concentrations above the PBTV were from wells located within the footprint of the ash basin, or along the northern waste 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 14-4 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx boundary, from the cinder storage area to the east, along the north side of the ash basin. Boron was not observed in shallow or deep samples from areas east of the ash basin (e.g., GWA-7S, BG-2S, BG-3S, GWA-7D and BG-3D). Boron was not observed in shallow or deep samples from areas south of the ash basin (GWA-5, GWA-6, GWA-13, GWA-20, GWA-21, GWA-22, GWA-23, and MW-8). Boron was not detected above 50 µg/L in valid samples from downgradient bedrock zone wells from the May 2017 sampling event. Boron was observed at a concentration above 2L in a pH-elevated sample from a shallow bedrock well (GWA-2BRU). Chromium and iron were detected above background and 2L in bedrock (GWA- 1BRU) north of the secondary cell; however the results were significantly higher than past sampling events, and total results were significantly higher than dissolved results indicating a less mobile particulate component. Manganese, sulfate, and TDS are observed above 2L/IMAC and PBTVs in the shallow, deep and bedrock flow units northwest of the cinder storage area beyond the compliance boundary. There are currently no bedrock wells installed in the footprint of the ash basin. Bedrock wells in the footprint of the ash basin that were abandoned as part of coal ash removal activities yielded no valid samples. There is a bedrock well in the cinder storage area, C-1BRU; however, the well exhibits high pH and does not yield valid samples. Bedrock wells on the south side of the ash storage area (GWA-4BR, GWA-20BR, GWA-21BR, GWA-22BR-A and GWA-23BR) exhibit high pH and do not yield valid samples. Beryllium was observed above the IMAC in one sample, GWA-12S, near the former coal pile. The location is sidegradient to the ash and cinder storage areas. Beryllium has historically been observed above the IMAC in a shallow well at the cinder storage area, C-1S; however, the beryllium concentration in May 2017 was below the IMAC. 14.2 Maximum Source Constituent Concentrations Source removal (coal ash excavation) is ongoing at RBSS. Ash removal activities are anticipated to be complete by August 2019. Wells monitoring ash porewater were installed within the ash basin and the cinder storage area. The ash stored at the ash 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 14-5 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx storage area was managed at an elevation generally above the water table and no porewater wells were installed in that area. Due to ongoing coal ash removal activities, most ash porewater monitoring wells have been properly abandoned. The following summary of COI distribution in ash porewater is based on the most recent sample result available for ash pore water wells in the ash basin and in the cinder storage area. Five porewater monitoring wells (AB-3S, AB-4S, AB-5S, AB-5SL, and AB-7S) were installed within the waste boundary of the ash basin primary and secondary cells. One pore water monitoring well (C-1S) was installed within the cinder storage area waste boundary. The location of maximum contaminant concentrations based on the most recent ash porewater samples are summarized below. Ash Basin – antimony, arsenic, boron, chromium, manganese, strontium, and vanadium Cinder Storage Area – beryllium, hexavalent chromium, cobalt, iron, sulfate, and TDS The porewater sample locations are shown on Figure 2-4 and results are listed in Appendix B, Table 1. Based upon the results of the CSA and the determination of provisional background concentrations, boron and sulfate are the primary constituents in groundwater detected at concentrations greater than background albeit with limited 2L exceedances. Boron is detected at concentrations greater than background beneath and downgradient of the ash basin in the shallow, deep and bedrock flow layers. COI exceedances of 2L in each hydrostratigraphic unit are depicted in Figures 11-1 through 11-36. Many constituents present in ash porewater are inhibited by geochemical mechanisms such as sorption (measured by the distribution coefficient Kd) and precipitation. Boron is relatively mobile in groundwater and is associated with low Kd values. This is primarily because boron is mostly inert, has limited potential for sorption, and lacks an affinity to form complexes with other ions. The low Kd measured for boron and sulfate is consistent with observations of migration from the ash basin (Figure 11-10 through 11-12), and the higher Kd values measured for other constituents is consistent with the limited migration of these constituents. Geochemical mechanisms controlling the migration of constituents are discussed further in Section 13.0. Time-series graphs showing changes in COI concentrations over time are included as Figures 14-1 through Figure 14-36. Representative groundwater monitoring locations including background, source, and downgradient areas were selected to illustrate site 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 14-6 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx conditions. COI concentrations are generally stable, often indicating slight variation which may be attributed to natural fluctuations. Figure 14-10 for boron in shallow wells illustrates boron concentrations as stable and below 2L. 14.3 Contaminant Migration and Potentially Affected Receptors Contaminant Migration The ash management areas (primary and secondary ash basin, ash storage area and cinder storage area) at RBSS are situated on a north-facing slope bordered to the north by the Catawba River. Groundwater flows to the northwest, north, and northeast underneath the ash management areas to the Catawba River. There are no drinking water wells located between the ash management areas and the Catawba River. CCR indicator constituents boron and sulfate are absent, or observed at low concentrations, in background areas. In contrast, both boron and sulfate are observed above PBTVs, and less commonly above 2L, in groundwater beneath the ash management areas and in downgradient areas. Groundwater sampling data gathered during the CSA (June 2015 to May 2017) was used to generate colored-coded figures depicting whether analytical concentrations seem to be increasing, decreasing, stable, or a trend could not be determined (Figures 14-37 to 14-49). Surface water, AOW, and outfall analytical results are incorporated on the trend figures. The majority of figures show concentrations for most COIs are stable, with a few notable exceptions. Boron concentrations are generally non-detect in upgradient areas south of the ash basin, and below 2L, but greater than background in downgradient areas. Figure 14-40 indicates monitoring locations in downgradient areas with low concentrations of boron show potentially increasing trends for the limited time period of data available. For other constituents such as chromium and iron (Figure 14-42 and 14-44), there is mix of stable, increasing and decreasing trends across upgradient and downgradient areas. The inconsistency in concentrations may reflect natural variation in site conditions. Summary figures presenting results associated with CSA sampling activities were prepared. Figure 14-50 provides solid media data including soil and ash. Figure 14-51 summarizes the most recent groundwater sample results and Figure 14-52 includes other water sample results including AOWs and outfalls. A previously completed human health and ecological risk assessment identified no imminent threat to humans or the environment as part of the scope of this assessment for the RBSS. A screening level risk assessment has been completed (Section 12.0) that evaluated recent analytical data for their potential to influence risk assessment 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 14-7 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx conclusions. No additional potential risks to humans and wildlife at the RBSS site were determined. No public or private drinking water wells or wellhead protection areas were found to be located downgradient of the ash management areas. There are no water supply wells located downgradient of the ash management areas. One residential drinking water well was identified across the Catawba River, to the north, from RBSS. The Catawba River is considered a main stem river and hydraulically separates RBSS from properties to the north. Results of the receptor survey indicate that the RBSS site and nearby vicinity are provided water service by Mount Holly Public Utilities Department. Mount Holly, as well as the Charlotte Metropolitan area and Gastonia, obtain drinking water from the Mountain Island Lake portion of the Catawba River. Water supply intakes are located approximately 3.4 miles and 6.9 miles downstream from the RBSS site. 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 15-1 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx 15.0 CONCLUSIONS AND RECOMMENDATIONS A discussion of preliminary corrective action alternatives that may be appropriate to consider during the updated CAP development are presented in this section. 15.1 Overview of Site Conditions at Specific Source Areas The ash management areas at RBSS include the ash basin, ash storage area and cinder storage area. Coal ash removal is ongoing at the site and is anticipated to be complete by August 2019. Coal ash has been removed from the ash storage area and a portion of the ash basin. Dewatering at the ash basin began in February 2017 and is ongoing. Review of groundwater data indicates that boron (Figures 11-10 through 11-12), and sulfate (Figures 11-28 through 11-30) are not generally observed in background locations. Those constituents are observed at concentrations above PBTVs, and to a lesser degree, above 2L, within the ash management areas and downgradient. That spatial distribution is useful as an indicator of CCR influence from the ash management areas. Boron and sulfate concentrations are greatest near, and downgradient from, the west side of the ash basin primary cell and the cinder storage area (Figure ES-1). Boron is historically observed above 2L in ash porewater, the shallow (upper) flow layer, and in one recent shallow bedrock sample (GWA-2BRU). Sulfate is observed above 2L in the shallow, deep, and bedrock flow layers downgradient from the cinder storage area in the vicinity of the former coal pile. Other groundwater COIs—including beryllium, cobalt, chromium, iron, manganese, strontium, and TDS—are observed above 2L/IMAC and/or PBTVs within the area of elevated boron and sulfate concentrations. Only chromium and iron were observed at elevated concentrations in areas without coincident boron and sulfate above 2L and PBTVs and may be attributable to particulate influence or other natural conditions. The horizontal and vertical extent of CCR exceedances has been defined for the ash management areas. The former coal pile is located west of the cinder storage area. Constituent concentrations will be assessed at the former coal pile as part of a separate process. 15.2 Revised Site Conceptual Model Site Conceptual Models (SCMs) are developed to be a representation of what is known or suspected about a site with respect to contamination sources, release mechanisms, transport, and fate of those contaminants. SCMs can be a written and/or be a graphic presentation of site conditions to reflect the current understanding of the site, identify data gaps, and be updated as new information is collected. SCMs can be used to develop an understanding of the different aspects of site conditions, such as a 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 15-2 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx hydrogeologic site conceptual model to help understand the site hydrogeologic conditions affecting groundwater. SCMs can also be used in a risk assessment to understand contaminant migration and pathways to receptors. In the initial site conceptual hydrogeologic model presented in the Work Plan (Appendix A, HDR 2015a) the geological and hydrogeological features influencing the movement, chemical, and physical characteristics of contaminants were related to the Piedmont hydrogeologic system present at the Site. A SCM was developed from data generated during previous assessments, existing groundwater monitoring data, and CSA activities. Spatially, the SCM for RBSS is bounded by the Catawba River to the north and west, and topographic divides to the east and south. Groundwater flows from the ash management areas (ash basin, ash storage area and cinder storage area) to the northwest, north, and northeast and discharges to the Mountain Island Lake portion of the Catawba River. CCR and the porewater in the ash management areas is the source of constituents detected above 2L/IMAC and PBTVs in groundwater samples in the vicinity of the ash management areas. Site-specific groundwater constituents of interest (COIs) were developed by evaluating groundwater sampling results with respect to 2L/IMAC and PBTVs, and additional regulatory input/requirements. The distribution of constituents in relation to the ash basin, co-occurrence with CCR indicator constituents such as boron and sulfate, and likely migration directions based on groundwater flow direction were considered in determination of groundwater COIs. Wells monitoring the shallow, deep, and bedrock flow units were installed beneath the ash management areas. Boron and sulfate concentrations in groundwater above 2L and PBTVs indicate areas beneath and downgradient of the ash management areas that are potentially influenced by constituent migration. Wells completed in the shallow and deep zones beneath the ash management areas have PBTV and 2L/IMAC exceedances for boron, iron, manganese, and strontium. Evaluation of data from the bedrock monitoring wells installed within the ash management areas is hampered by elevated pH indicative of grout contamination. Review of the available data for boron indicated no 2L exceedances for the deep or bedrock wells within the footprint of the ash management areas. The area downgradient from the ash management areas consists of a thin, generally 1,000 feet wide or less, strip of land along the Catawba River. Monitoring wells installed in the downgradient area indicated concentrations of beryllium, cobalt, chromium, iron, 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 15-3 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx manganese, strontium, sulfate and TDS above 2L/IMAC and/or PBTVs. Boron was observed in a pH-elevated sample from a downgradient well screened within upper bedrock (GWA-2BRU). Surface water sampling data from the Catawba River upstream and downstream of the RBSS site do not indicate 2B exceedances, or CCR influence from the ash management areas. One residential drinking water well was identified across the Catawba River, to the north, from RBSS. The Catawba River is considered a main stem river and hydraulically separates RBSS from properties to the north. Results of the receptor survey indicate that the RBSS site and nearby vicinity are provided water service by Mount Holly Public Utilities Department. Coal ash is currently being excavated and removed from RBSS to off-site, permanent storage solutions. Ash basin closure activities are anticipated to be completed by August 2019. The SCM will continue to be refined following evaluation of the completed groundwater models to be presented in the CAP and additional information obtained in subsequent data collection activities. 15.3 Interim Monitoring Program An Effectiveness Monitoring Program (EMP) is required by CAMA §130A-309.209 (b)(1)e. The EMP for RBSS is anticipated to begin once the basin closure and groundwater CAP have been implemented. In the interim, an IMP has been developed at the direction of NCDEQ. The CAP, and a proposed EMP, will be submitted at a future date; therefore, this section presents details concerning the IMP only. 15.3.1 IMP Implementation An IMP has been implemented in accordance with NCDEQ correspondence (NCDEQ, October 19, 2017, Appendix A) that provided an approved “Revised Riverbend Steam Station Interim Monitoring Plan.” Sampling will be conducted quarterly until approval of the CAP or as otherwise directed by NCDEQ. Groundwater samples will be collected using low-flow sampling techniques in accordance with the Low Flow Sampling Plan, Duke Energy Facilities, Ash Basin Groundwater Assessment Program, North Carolina, June 10, 2015 (Appendix G) conditionally approved by NCDEQ in a June 11, 2015 email with an attachment summarizing its approval conditions. Samples are analyzed by a North Carolina certified laboratory for the parameters listed in Table 15-1. The table includes targeted minimum detection limits for 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 15-4 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx each listed constituent. Analytical parameters and detection limits for each medium were selected so the results could be used to evaluate the effectiveness of a future remedy, conditions within the aquifer that may influence the effectiveness of the remedy, and migration of constituents related to the ash basin. Laboratory detection limits for each constituent are targeted to be at or below applicable regulatory values (i.e., 2L, IMAC, or 2B). Monitoring wells and surface water locations that are sampled and monitored as part of the IMP are included in Table 15-2. 15.3.2 IMP Reporting Currently, data summary reports comprised of analytical results received during the previous month are submitted to NCDEQ on a monthly basis. In addition, NCDEQ (May 1, 2017) directed that an annual IMP report be submitted by April 30 of the following year of data collection. The reports shall include materials that provide “an integrated, comprehensive interpretation of site conditions and plume status.” The initial report was to be submitted to NCDEQ no later than April 30, 2018; however, the October 19, 2017 correspondence provides that the required date for an annual monitoring report will be extended to a date in 2018 to be determined later. 15.4 Preliminary Evaluation of Corrective Action Alternatives This preliminary evaluation of corrective action alternatives is included to provide insight into the groundwater CAP preparation process. This preliminary evaluation is based on data available and the current understanding of regulatory requirements for the Site. Source control via dewatering and excavation of the ash is ongoing. The groundwater currently presents minimal, if any, risk to receptors. It is currently assumed that source control and monitored natural attenuation (MNA) will be substantial elements of the groundwater corrective action strategy. However, additional groundwater remedial strategies will also be evaluated as necessary in the CAP. 15.4.1 CAP Preparation Process The CAP preparation process is designed to identify, describe, evaluate, and select remediation alternatives with the objective of bringing groundwater quality to levels that meet applicable standards, to the extent that the objective is economically and technologically feasible, in accordance with 2L .0106 Corrective Action. Sections (h), (i), and (j) regarding CAP preparation read as follows: 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 15-5 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx (h) Corrective action plans for restoration of groundwater quality, submitted pursuant to Paragraphs (c), (d), and (e) of this Rule shall include: (1) A description of the proposed corrective action and reasons for its selection; (2) Specific plans, including engineering details where applicable, for restoring groundwater quality; (3) A schedule for the implementation and operation of the proposed plan; and (4) A monitoring plan for evaluating the effectiveness of the proposed corrective action and the movement of the contaminant plume. (i) In the evaluation of corrective action plans, the Secretary shall consider the extent of any violations, the extent of any threat to human health or safety, the extent of damage or potential adverse impact to the environment, technology available to accomplish restoration, the potential for degradation of the contaminants in the environment, the time and costs estimated to achieve groundwater quality restoration, and the public and economic benefits to be derived from groundwater quality restoration. (j) A corrective action plan prepared pursuant to Paragraphs (c), (d), or (e) of this Rule shall be implemented using a remedial technology demonstrated to provide the most effective means, taking into consideration geological and hydrogeological conditions at the contaminated site, for restoration of groundwater quality to the level of the standards. Corrective action plans prepared pursuant to Paragraphs (c) or (e) of this Rule may request an exception as provided in Paragraphs (k), (l), (m), (r), and (s) of this Rule. To meet these requirements and to provide a comprehensive evaluation, it is anticipated that the CAP will include: Corrective action objectives and evaluation criteria Technology assessment Formulation of remedial action alternatives Analysis, modeling, selection, and description of selected remedial action alternative(s) Conceptual design elements, including identification of pre-design testing such as pilot studies Monitoring requirements and performance metrics Implementation schedule 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 15-6 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx The preliminary screening of potential groundwater corrective action includes the following: Source control by excavation and monitored natural attenuation will be vital components to the CAP. Secondary source remediation via phytoremediation in the footprint of the former basins and potentially in seep areas may be a viable option. Groundwater migration barriers. The depth and heterogenitity of the impacted zone may limit the feasibility of this technology. Insitu chemical immobilization. This technology has not been demonstrated to be effective for a primary COI, boron. It may be applicable for other COIs. Permeable reactive barrier. Similar to insitu chemical immobilization, permeable reactive barrier technology has not been demonstrated to be effective for boron. Groundwater extraction. Groundwater extraction is an easily adaptable technology using ‘tree –wells’ or conventional vertical wells. Following basin excavation, groundwater extraction could be conducted within the ash basin footprint or around the perimeter, with possible continued use of the current treatment system prior to the NPDES outfall. The thickness of the regolith and depth of bedrock may limit groundwater extraction. Potentially viable options will be further evaluated in the CAP with updated fate and transport and geochemical modeling. 15.4.2 Summary This preliminary evaluation of corrective action alternatives is intended to provide insight into the revised CAP preparation process, as outlined in 2L. It is based on data available and the current regulatory requirements for the Site. It addresses potentially applicable technologies and remedial alternatives. Potential approaches are based on the currently available information about site hydrogeology and COIs. In general, three hydrogeologic units or zones of groundwater flow can be described for the site: shallow zone, deep zone, and bedrock flow zone. The site COIs include a list of common coal ash related constituents such as boron and sulfate. Potentially viable options will be evaluated in the CAP with updated fate and transport and geochemical modeling. The CAP will evaluate options that could 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 15-7 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx be deployed (in combination with source control via excavation and groundwater MNA) to reduce the potential impacts to human health or the environment. The options analysis will include short- and long-term effectiveness; implementability; potential for attenuation of contaminants; time and cost to achieve restoration; public and economic benefits; and compliance with applicable laws and regulations. 2017 Comprehensive Site Assessment Update October 2017 Riverbend Steam Station SynTerra Page 16-1 P:\Duke Energy Carolinas\15. Riverbend\05.EHS CAMA Compliance Support\CSA Supplement 2017\Report\Riverbend CSA October 2017 Final.docx 16.0 REFERENCES ASTM. (2001). D2487: Standard Practice for Classification of Soils for Engineering Purposes (Unified Soil Classification System). West Conshohocken, PA: American Society of Testing and Materials International, DOI:10.1520/D2487-11. ASTM. (2007). D422: Standard Test Method for Particle-Size Analysis of Soils. West Conshohocken, PA: American Society of Testing and Materials International, DOI:10.1520/D0422-63R07. ASTM. (2010a). 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