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HomeMy WebLinkAboutBelews Additional BG Wells Tech Memo_REV1_20170526F)� Technical Memorandum Date: Friday, May 26, 2017 Project: Belews Creek Steam Station To: Melonie Martin From: Jacob Ruffing Subject: Additional Background Monitoring Well Determinations — Revision 1 The purpose of this revised technical memorandum (TM) is to address the North Carolina Department of Environmental Quality's (NCDEQ's) requirement identified below. In addition, this revised TM includes the justification for additional monitoring wells that should be considered as background wells for the Belews Creek Steam Station (BCSS) ash basin system as originally presented in the TM dated December 12, 2016. Following submittal of the TM dated December 12, 2016, Duke Energy submitted the Statistical Methods for Developing Reference Background Concentrations for Groundwater and Soil at Coal Ash Facilities (Background Methodology) to NCDEQ on January 20, 2017. NCDEQ subsequently requested additional information regarding the Background Methodology via a letter dated April 28, 2017. As part of the April 28, 2017 letter, NCDEQ requested the following information: • Provide up-to-date digital spreadsheets of raw background groundwater data for each facility by May 26, 2017, and include, within the raw background data spreadsheet as "strike- throughs", the individual monitoring data results that Duke Energy believes should be omitted from the background dataset due to (a) high pH, (b) high turbidity, (c) auto -correlation, (d) outlier designation, (e) non -detect values that are above 2L/IMAC, or (f) other reasons. The up-to-date raw background groundwater data for background determinations at BCSS are provided in Table 2 of this revised TM. Note the remainder of the requirements in the April 28, 2017 NCDEQ letter are addressed in a revision to the Background Methodology under separate cover. Monitoring wells BG -1 D, BG -2S, BG -2D, BG-2BR, BG -3S, BG -3D, MW -3, MW -202S, MW -202D, and MW-202BR were previously evaluated and will continue to be used for evaluation of background groundwater quality, as documented in Appendix B of the BCSS Corrective Action Plan (CAP) Part 1, dated November 16, 2015. Well locations are shown on Figure 1. Based on the current review of groundwater flow direction, groundwater elevation as compared to the nearest ash basin surface water or porewater elevation, historical analytical results, and horizontal distance to the ash source area; no additional wells are proposed for use as background groundwater monitoring wells. Wells BC -23A, BC -28, and CW -10 were previously considered as background wells, but have been removed from consideration based on evaluation of horizontal distance to the ash source area and the geology at the location of the wells. Distances and elevations pertinent to the evaluation are provided on Table 1. Additional details are provided below for each monitoring well . F)� Historical analytical data for previously evaluated background wells (CAP Part 1) listed above are provided in Table 2. The background wells and analytical data are separated by groundwater flow layer (shallow, deep and bedrock) and a valid sample count is provided for each constituent. Sample data associated with turbidity reported to be greater than 10 nephelometric turbidity units (NTU), samples without a recorded turbidity, samples with a pH greater than 8.5, non -detect samples with a method detection limit greater than the respective 2L Standard or IMAC, or samples with less than a 60 -day interval between sample events are denoted with "strike-throughs" and were not included in the valid sample counts, as requested by NCDEQ. BC -23A, BC -28, and CW -10 Monitoring wells BC -23A, BC -28, and CW -10 were installed as background wells for the Flue Gas Desulfurization and Craig Road landfills. Wells BC -23A, BC -28, and CW -10 are installed more than one mile southeast of the ash basin and in a different geologic environment than the ash basin. These wells should not be used for background water quality near the ash basin. 2 NOTES: WELL LOCATION MAP DATE 1. WASTE BOUNDARY AND ASH STORAGE AREA BOUNDARIES ARE APPROXIMATE 2. THE COMPLIANCE BOUNDARY IS ESTABLISHED ACCORDING TO THE DEFINITION FOUND IN SCALE(FEET) DUKE ENERGY CAROLINAS, LLC DECEMBER 2016 15A NCAC 02L.0107(a) 1,000 0 1,000 2,000 3. AERIAL IMAGERY IS A COMBINATION OF A WSP AERIAL SURVEY (2014) AND NC ONEMAP(2014) BELEWS CREEK STEAM STATION ASH BASIN FIGURE 4. HYDROGRAPHY WAS DIGITIZED FROM USGS 1:24,000 SCALE TOPOGRAPHICAL MAPS F)l1 5. AS-BUILT CAMA ASSESMENT AND NPDES SERIES WELLS HAVE BEEN PROVIDED BY WSP AND STOKES COUNTY, NORTH CAROLINA DUKE ENERGY Table 1. Monitoring Well Information Ground Elevation of Elevation of Closest Pore Distance and Direction Groundwater Surface Screened Groundwat Water Well ID Elevation to from Active Ash Basin Flow Direction Elevation Interval er Well (feet) (feet) (feet) feet BC -23A 5,700 feet SE NE 831 783.5-798.5 791 738 BC -28 5,700 feet SE E and NW 774 737-747 751.42 747 CW -10 6,500 feet SE NE 831 740-745 790 738