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Mr. Brenden Kenny Central Valley Regional Water Quality Control Board March 7, 2022 an interim remedial action on the CDCR property, to be followed by the completion of the proposed SI activities on the CDCR property to determine the source and extents of VOC mass(es) beneath the site. Details regarding pre-field activities, groundwater and vapor sampling methodologies, laboratory analysis, and investigation-derived waste disposal are presented in the Work Plan (Arcadis 2019). The following sections present the updated interim remedial action, followed by the multi-phased approach to the CDCR SI. Overall Objectives and Approach: Interim Remedial Design The treatment remedy described in the CAO R5-2017-0703 (RWQCB 2017) includes groundwater extraction and treatment to address VOC concentrations in groundwater exceeding 25 ug/L in the vicinity of AMW-22S. With the existing plume knowledge at the CDCR facility limited to a single shallow groundwater well (AMW-22S), the initial shallow remedial design has been required to focus on this data point. The initial remedial groundwater extraction design is based on desktop hydraulic calculations using the lithological interpretation from borehole logs AMW-22 and AMW-22S, and well development hydraulic responses from recently installed monitoring wells (AMW-44, AMW-45 and AMW-46) located approximately 1000 feet south of AMW-22S. These indicate that a well screened in the shallow permeable zone should produce approximately 3 gallons per minute (gpm)with a shallow groundwater capture width of approximately 2,500 feet. These estimations will be assessed with hydraulic testing after the extraction well installation. The proposed interim remedial design includes the installation of one extraction well and two piezometers. A State of California C-57 licensed driller will be used to advance the boring(s) and construct the extraction well and piezometers. 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). The groundwater extraction well and piezometers will be advanced using and appropriate drilling technology for the depth and diameter of installation, lithology and required well design (likely using either hollow stem auger, sonic or air rotary drilling techniques). The borehole diameter will be at approximately 6 inches larger than the final casing diameter, installed to approximately 75 to 95 feet below ground surface (feet bgs). The targeted drilling depth may be modified in the field based on the soil 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. Extraction Well Design The proposed extraction well is located on Figure 1, with the well design presented in Table 1 and in Attachment 1. The design will consist of a 6-inch schedule 80 polyvinyl chloride (PVC) riser and stainless-steel wire wrap screen, with 20-30 feet length of screen, 0.02 inch slot size and a 5-foot blank sump, with the sump installed into lower permeable materials. The well screen will be positioned in the shallow water bearing zone targeting the most permeable materials. The actual depth and well design will depend on the materials encountered. The screen interval will be backfilled with silica sand to match the respective screen width, ensuring that a minimum of 5 ft of sand extends above the screen. The well will be periodically surged during placement of the filter pack to consolidate the sand. Centralizers will be placed at the top and bottom of the screen and close to the surface. The extraction well annulus above the sand interval will be backfilled with a minimum of three feet of hydrated bentonite chips. Depth soundings will be utilized to evaluate the thickness and uniformity of the sand pack and www.arcadis.com 4/9