COMPLIANCE INFO_EW-5 INSTALL PERMIT PACKAGE 2/14/25

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Mr. Brenden Kenny Central Valley Regional Water Quality Control Board February 1, 2023 www.arcadis.com 2/6 General Information The expanded scope of work for the interim remedial action presented in this Second Work Plan Addendum is proposed to address the requirement in the 2017 Cleanup and Abatement Order (CAO) R5-2017-0703 for an interim remedial action to address the VOC plume mass beneath the site. The Work Plan (Arcadis 2019) provided a summary of the geology and hydrogeology for the surrounding area and presented 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 site. Concentrations of carbon tetrachloride and chloroform in groundwater monitoring well AMW-22S, the only shallow groundwater monitoring well on the site at the time, were generally increasing, and still 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-31S) had not, and still have not, indicated detections of these constituents; therefore, a potential source of carbon tetrachloride and chloroform appears to be present in the vicinity of AMW-22, on the site. A summary of the results of the additional investigations completed in the vicinity of the site since October 2019, and the results of CDCR historical as-built records review, and a geophysical utility survey on the site were presented in the Revised Work Plan Addendum (Arcadis 2022b). The expanded scope of work proposed in this Second Work Plan Addendum includes installation of three extraction wells (EW-2, EW-3 and EW-4), four piezometers (PZ-3, PZ-4, PZ-5 and PZ-6) and three monitoring wells (AMW-50, AMW-51 and AMW-52) on the CDCR property, which are presented in Figure 1.The extraction wells will be screened in the intermediate water bearing zone and the monitoring wells and piezometers will be screened in the shallow and intermediate zones. The rationale for the extraction locations and quantity of extraction wells was presented the Technical Memorandum submitted to the RWQCB on November 4, 2022 (Arcadis 2022a, Attachment 2). Drilling Details The expanded scope of work includes the installation of three extraction wells, four piezometers and three monitoring wells (Figure 1). A State of California C-57 licensed driller will be used to advance the boring(s) and construct the extraction wells, piezometers and monitoring wells. All drilling and well construction will be completed in general accordance with the requirements specified in California Well Standards (Department of Water Resources [DWR] Bulletins 74-81 and 74- 90), under the supervision of a California Professional Geologist. The groundwater extraction wells, piezometers and monitoring wells will be advanced using the appropriate drilling technology for the depth and diameter of installation, lithology and required well design (likely either hollow stem auger, sonic or air rotary drilling techniques). The borehole diameter will be at least 2 inches larger than the final outside casing diameter, installed to approximately 75 to 95 feet below ground surface (feet bgs) in the shallow zone and/or approximately 100-120 feet bgs to target the intermediate zone. The targeted drilling depth may be modified in the field based on the soil conditions encountered during drilling. Arcadis field personnel will describe the soil using the Unified Soil Classification System (USCS) and identify potential production intervals and confining layers/aquitards. Continuous soil sampling will be conducted from the water table down to the target depth. Soil sampling equipment will be decontaminated between each sample collected and drilling (rods and/or auger flights) and sampling equipment will be decontaminated between each borehole advanced . Decontamination will