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�i 0 <br /> City of Stockton A68101.02C <br /> February23 1996 <br /> � Page 2 <br /> and Mesozoic metamorphic and igneous rocks. The shallow subsurface geology in the site <br /> vicinity is a heterogeneous mix of gravel, sand, silt, and clay (Hackel, 1966). <br /> 2.2.2 Hydrology: First encountered groundwater in the site vicinity occurs under <br /> unconfined (water table) conditions at a depth of approximately 14 feet below site grade <br /> (BSG). Groundwater flow direction has been towards the north between June and <br /> December, 1994 (Twining, 1995). <br /> Sediments containing fresh groundwater are largely unconsolidated silts and sands which <br /> were derived from river channel, flood plain, and alluvial fan deposits of Pliocene to Recent <br /> age (Davis and others, 1959). Local groundwater flow may be influenced by pumping of* <br /> water wells and recharge from various sources. Evaluation of the effects of these conditions <br /> on groundwater beneath the site is beyond the scope of this investigation. <br /> 2.3 Project History: Twining understands that a 550-gallon gasoline UST was removed <br /> from the site in April 1991 and that analysis of a soil sample collected from four feet BSG <br /> in the vicinity of the former gasoline fuel dispenser detected 350 milligrams per kilogram <br /> (mg/kg) total petroleum hydrocarbons - gasoline range constituents (TPH-G) and 7,200 <br /> mg/kg total petroleum hydrocarbons - diesel ranged constituents (TPH-D). Twining also <br /> understands that a 1,000-gallon diesel UST was removed from the site in January 1994. <br /> Analysis of a soil sample collected from 11 feet BSG at the west end of the UST removal <br /> excavation detected 135 mg/kg TPH-D, 435 mg/kg TPH-G, 5.8 mg/kg benzene, 29 mg/kg <br /> toluene, 8.2 mg/kg ethylbenzene, and 44 mg/kg total xylenes. The diesel UST excavation <br /> was left open. Drawing 2 shows the approximate locations of the USTs, fuel dispensers and <br /> the diesel UST excavation. <br /> On June 1 and 2, 1994, Twining drilled three soil borings, collected soil samples for <br /> chemical analyses, and installed three monitoring wells in the soil borings. Analytical results <br /> for the soil samples are summarized in Table 1 (Twining, 1994). The approximate locations <br /> of the monitoring wells are shown on Drawing 2. <br /> • <br />