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Geosyntec L'011S Il lit" 2. BACKGROUND 2.1 Site Location The Site comprising the former OCC facility (currently owned and operated by J.R Simplot [Simplot]) is located at 16777 Howland Road in Lathrop, California (Figure 2). The Site is generally surrounded by areas developed for residential, commercial, and agricultural usage. Several water supply wells are located in the area surrounding the Site that serve as water supply wells for potable water, irrigation, and industrial use. The supply wells of relevance to this Work Plan are the Simplot water supply wells (the primary supply well [PSW] and the BSW) and, the City of Lathrop (Lathrop) municipal water supply wells, LWD-6, LWD-7, LWD-8, LWD-9 and LWD-10 located to the north and east of the Site(Figure 2). 2.2 Site Hydrostratigraphy Sediments beneath the Site generally consist of interbedded clays, silts, and sands that transition with depth to a more transmissive, well-graded sand and gravel layer overlying the regionally extensive Corcoran Clay ayuitard. The depth of the Corcoran Clay near the Site is currently being investigated and based on the supplemental characterization work completed to date has been encountered at a depth of approximately 270 ft bgs in the vicinity of BSW and 260 ft bgs in the vicinity of PSW. Preliminary geologic logs from the ongoing supplemental characterization in the eastern portion of the Site confirm the presence of the highly transmissive, well-graded sand and gravel layer that is approximately 20 to 30 feet thick, immediately overlying the Corcoran Clay. The conceptual cross section of Site stratigraphy based on preliminary data from the ongoing supplemental characterization work is shown in Figure 3. 2.3 Groundwater Flow 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 Lathrop 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 rates they are pumping.Preliminary data from water level transducers installed in monitoring wells to the east of the Site suggest that the groundwater extraction at the Lathrop water supply wells dominates the horizontal gradient in the transmissive zone. 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 LWD-6. Downward vertical gradients are observed both north and east of the Site and increase toward the water supply wells as a result of the pumping from the transmissive zone. Due to pumping, groundwater elevations near the BSW and PSW are also highly dynamic and range from approximately 25 to 50 feet bgs in the area. In-Situ Bioremediation Pilot Test Work Plan—GSH Lathrop 3