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Geosyntec <br /> consultants <br /> Some of the metals concentrations at Site 4 have historically exceeded the MCLs and/or DTSC <br /> Tap Water screening criteria. Whether these metals exceedances are naturally occurring due to <br /> the site geology and geochemistry or whether the metals are COCs associated with historical <br /> activities conducted at Site 4 remains an outstanding data gap that will be addressed though the <br /> Work Plan to Evaluate Background Concentrations in Groundwater(Geosyntec, 2021 c). <br /> Other groundwater data gaps previously identified for Site 4 have been addressed by the <br /> sampling conducted between 2018 and 2021. Once the two outstanding data gaps listed above <br /> have been addressed, the characterization of Site 4 groundwater will be adequate to proceed to <br /> the next step of a REFS. <br /> 3.3 Central Area <br /> In the Central Area, grab-groundwater sampling was conducted in 2019 and 2020, and new <br /> monitoring wells were installed in 2020 that defined the vertical and lateral extent of VOCs and <br /> TPH in groundwater, with the exception of the downgradient western lateral extent. The <br /> concentrations of cis-1,2-DCE in the furthest downgradient grab-groundwater samples (GGW-29 <br /> and GGW-30) collected between 25 and 30 feet bgs in 2020 exceeded the MCL. In 2021, a <br /> surveyor marking the historical coordinates and using hand tools to dig at marked locations <br /> attempted to locate monitoring wells W-274 and W-275 further downgradient of GGW-29. <br /> However, W-274 and W-275 were not located during these field efforts. Therefore, monitoring <br /> the extent of VOCs and TPH in groundwater downgradient to the west of the former source area <br /> remains a data gap for the Central Area. <br /> Some of the metals concentrations in the Central Area have historically exceeded the MCLs <br /> and/or DTSC Tap Water screening criteria. Whether these metals exceedances are naturally <br /> occurring due to the site geology and geochemistry or whether the metals are COCs associated <br /> with historical activities conducted in the Central Area remains an outstanding data gap that will <br /> be addressed though the Work Plan to Evaluate Background Concentrations in Groundwater <br /> (Geosyntec, 2021c). <br /> Other groundwater data gaps previously identified for the Central Area have been addressed by <br /> the sampling conducted between 2018 and 2021. Once the two outstanding data gaps listed <br /> above have been addressed, the characterization of the Central Area groundwater will be <br /> adequate to proceed to the next step of an RI/FS. <br /> RRI Groundwater Monitoring Work Plan 16 June 2022 <br />