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Geosyntec". 1, INTRODUCTION Geosyntec Consultants, Inc. (Geosyntec)has prepared this Extraction Well Installation Work Plan (Work Plan) on behalf of Glenn Springs Holdings, Inc. (GSH), for the Former Occidental Chemical Corporation (OCC) Facility located at 16777 Howland Road in Lathrop, California (Site; Figure 1). This Work Plan describes the proposed installation of groundwater extraction wells east of Howland road on the J.R. Simplot Company (Simplot) property (Figure 2). The purpose of the new groundwater extraction wells is to increase the containment of sulfolane in groundwater between the Site and the Simplot and Lathrop Water District(LWD) water supply wells located east of the Site. Previous assessment work identified the fumigants 1,2-dibromo-3-chloropropane (DBCP) and ethylene dibromide (EDB)as constituents of concern (COCs) for the former OCC Facility. A groundwater extraction and treatment system (GETS) for DBCP and EDB was installed at the Site in 1982 (Figure 2; Conestoga-Rovers& Associates [CRA], 2008). Environmental investigations identified sulfolane as a COC from the former OCC operations, and in 1992 the GETS was modified to include the treatment of sulfolane (Ying, 1994). This Work Plan is for the installation of three new groundwater extraction wells (EW-18A, EW- 18B, and EW-19)to provide better hydraulic containment east of the Site. 2. BACKGROUND 2.1 Geology The Site is located in the San Joaquin Valley Groundwater Basin and is underlain by the Corcoran Clay, which is a former lake deposit that forms a regionally extensive aquitard in the San Joaquin Valley (Department of Water Resources [DWR], 2016; DWR, 1981). During the Site characterization work completed to date,the Corcoran Clay has been encountered at depths ranging from approximately 255 to 270 feet below ground surface (bgs). Overlying the Corcoran Clay are alluvial deposits made up of interbedded clays, silts, and sands that transition with depth to a more transmissive, well-graded sand and gravel layer immediately above the Corcoran Clay. 2.2 Groundwater Flow and Sulfolane Transport The groundwater elevations, flow direction, and gradients in the vicinity of the Site are highly influenced by groundwater pumping from the on-site GETS and the Simplot and LWD water supply wells. Within the well-graded sand and gravel transmissive zone immediately above the Corcoran Clay, where the Simplot and LWD water supply wells are screened, the groundwater elevations and flow directions are highly dynamic depending on which water supply wells are operating and what flow rates they are pumping at. Preliminary data from water level transducers installed in monitoring wells to the east of the Site suggest that the groundwater extraction at the LWD wells dominates the horizontal gradient in the transmissive zone overlying the Corcoran Clay. When LWD-9 and LWD-10 are operational,the gradient in this zone to east of the Site appears to be primarily eastward. When LWD-10 is not operating and LWD-6 is operating, the horizontal gradient in this zone to the east of the Site appears to be primarily northward towards Extraction Well Installation Work Plan I November 30,2018