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SHALLOW INJECTION WELL INSTALLATION WORK PLAN arcadis.com 2022.02.15 OCC Lathrop_Shallow Injection Well Installation Work Plan_final 1 1 INTRODUCTION On behalf of Glenn Springs Holdings, Inc., Arcadis U.S., Inc. (Arcadis) has prepared this Shallow Injection Well Installation Work Plan (Work Plan) for the former Occidental Chemical Corporation (OCC) manufacturing facility, located at 16777 Howland Road near the southwest corner of Louise Avenue and Howland Road in Lathrop, San Joaquin County, California (site; Figures 1 and 2). The project objective is to provide additional injection capacity to be able to meet the target extraction volume for the site. Therefore, this Work Plan proposes to install two shallow injection wells. The summary of the site background, work plan objectives, and proposed scope of work are described in the following sections. 2 SITE BACKGROUND A summary of site background including lithology and hydrogeology, the current remediation system and system expansion, the current injection well network and injection capacity are presented in the sections below. 2.1 Lithology and Hydrogeology Sediments beneath the site consist of unconsolidated alluvial deposits, composed of sands, gravelly sands and silts, with lesser amounts of silty clays, underlain regionally by the Corcoran Clay. Based on site boring logs, the Corcoran Clay is present at a depth of approximately 271 feet below ground surface (bgs). On-site, the Corcoran Clay is overlain by a low permeability silt and clay (Brown Clay) which is encountered at a depth of approximately 210 to 220 feet below ground surface. However, toward the north and east of the site, the Brown Clay pinches out, and a highly transmissive well-graded sand overlies the Corcoran Clay (Geosyntec 2019). Below the Corcoran Clay, zones of clay and sandy and/or gravelly alluvial deposits were observed to approximately 700 feet. The aquifer system above the Corcoran Clay is divided into five zones. These include the water table (0 to 50 feet bgs), the shallow zone (50 to 100 feet bgs), intermediate zone (100 to 150 feet bgs), Deep Zone 1 (150 to 220 feet bgs) and Deep Zone 2 (220 feet bgs to the top of the Corcoran Clay). The Deep Zone 2 is only present in the areas north and east of the site, where there is a highly transmissive sandy zone located immediately above the Corcoran Clay. 2.2 Remediation Systems and System Expansion OCC has operated a Groundwater Extraction and Treatment System (GETS) since 1982. However, to expedite the removal of sulfolane-affected groundwater and provide additional hydraulic control, the remediation strategy was re-evaluated and expanded in two phases. In 2019, the first phase of the GETS expansion was completed. During the first phase of expansion, three extraction wells (EW-18A, EW- 18BR, and EW-19) were installed to increase flow capacity, and the GETS was upgraded to allow treatment of up to 750 gallons per minute (gpm). A second phase of expansion included the installation of a Moving Bed Bioreactor (MBBR) treatment system. The MBBR was brought online in April of 2021.