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Mr. Brenden Kenny Central Valley Regional Water Quality Control Board March 7, 2022 contamination. The initial SI phase includes proposed sampling locations based on relative proximity to the above AOIs and may include installation of permanent groundwater monitoring wells. Note the location of CDCR-SB-03 proposed in the Work Plan Addendum (Arcadis 2021), has been updated following direction from the RWQCB in the virtual meeting conducted on February 7, 2022, as located in Figure 2. The subsequent SI phase(s)will use a data-based approach for decision making, data evaluation, identification of additional data needs and potential source area delineation. The final phase will include evaluation of the required interim remedial action design based on the results of the proposed SI activities. Background The Work Plan (Arcadis 2019) provides a summary of the geology and hydrogeology for the surrounding area and presents the known VOCs impacts based on available data at that time, specifically carbon tetrachloride and its degradation product chloroform in the shallow groundwater zone beneath the CDCR property. Concentrations of carbon tetrachloride and chloroform in groundwater monitoring well AMW-22S, the only shallow groundwater monitoring well on the CDCR property, are generally increasing, and currently comprise the majority of VOC mass in groundwater at this location. However, shallow and intermediate wells located hydraulically upgradient and downgradient of AMW-22S (AMW-13, AMW-13B, AMW-14, AMW-21, AMW-21S, AMW-23S, AMW-23M, AMW-31 and AMW-31 S) have not indicated detections of these constituents; therefore, a potential source of carbon tetrachloride and chloroform may be present in the vicinity of AMW-22, on the CDCR property. A summary of the results of the additional investigations completed in the vicinity of the CDCR property since October 2019, and the results of CDCR historical as-built records review, and a geophysical utility survey on the CDCR property are presented in the sections below: 7833 Newcastle Road: From February to August 2020, two phases of investigation were completed at 7833 Newcastle Road to investigate the source of trichloroethylene (TCE) impacts to the domestic well on the property (Arcadis 2020a, 2020b). In total, 37 soil samples, 15 groundwater samples, and 49 soil vapor samples were collected during both phases of investigation. Based on the available data, specifically the lateral attenuation in all directions of the highest TCE concentrations in soil vapor at the residential property, including in the direction of the Landfill, and geochemical and isotopic analysis of the groundwater samples collected from the domestic well on the residential property compared to shallow wells associated with the Landfill, the TCE impacts beneath 7833 Newcastle Road have been interpreted as not a result of VOCs migrating from the Landfill in groundwater or soil vapor, and are instead likely the result of a localized VOC source in the southeast corner of the residential property(Arcadis 2020a, 2020b). In addition, there were no detections of carbon tetrachloride in groundwater or soil vapor at the 7833 Newcastle Road property, which further indicates a potential source of carbon tetrachloride may be present in the vicinity of AMW-22 on the CDCR property. The Landfill and CDCR Property: A groundwater plume of tetrachloroethylene (PCE), TCE and degradation products of these compounds was determined to have originated from the Landfill and has migrated downgradient offsite mainly in the intermediate groundwater zone. The landfill groundwater plume is currently captured along the northern boundary of the Landfill at the Point of Compliance (POC). Groundwater treatment along the POC has involved the sequential installation of groundwater extraction wells EW-1, EW-2, EW-3 (later replaced by EW-3R), EW-4, and EW-5 to refine and obtain groundwater capture to prevent downgradient migration of VOC-impacted groundwater from the www.arcadis.com 2/9