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Case Closure Report Former Tosco(Unocal) SS No 5421, Stockton,California July 8, 1998 MW-2, all analytes were reported as non detected for TPH as gasoline, BTEX, MtBE, and the five additional oxygenates The site monitoring wells have been non detect since the December 1995 sampling event Based on historical monitoring data collected through December 1997, no evidence of free product or sheen has been observed in any of the wells, and a consistent groundwater flow direction (northeast) has been verified at the site Geology and HydrogeoIogy During the drilling and exploratory borings EBI through EB4, groundwater was encountered at depths of 58 0 to 58 5 feet bgs In the four existing monitoring wells (MW-1 through MW-4) at the site, the measured depth to groundwater on June 22, 1992, ranged between 56 63 and 57 43 feet bgs Based on water level data gathered on June 22, 1992, the groundwater flow direction at the site appears to be to the east-southeast The subject site is located adjacent to the Sacramento-San Joaquin delta in the Central Valley geomorphic province The Central Valley is a large, northwest-trending, asymmetric structural trough that has been filled with as much as six vertical miles of sediment in the San Joaquin Valley Based on review of regional geologic maps (U S Geological Survey Miscellaneous Field Studies Map MF-1401 "Geologic Maps of the Sacramento-San Joaquin Delta, California" by Brian F Atwater, 1982), the subject site is underlain by Holocene and/or upper Pleistocene alluvium identified as Alluvium of Calaveras River and vicinity (Qcr) The Calaveras River alluvium was deposited by the Calaveras River, Bear Creek, and several lesser streams, and has not been described in detail The results of our subsurface study indicate that the site is underlain by fill materials that are about 0 5 to 2 5 feet thick, except in the vicinity of MW-3 at the former fuel tank pit where pea gravel backfill materials were encountered to a depth of approximately 27 feet below grade The fill is in turn underlain by alluvium to the maximum depth explored (75 feet below grade) The unsaturated zone beneath the site is approximately 58 feet thick and consists predominantly of clayey silt and/or silt, silty clay, sand, silty sand and sandy gravel, in order of decreasing abundance Mostly silt and sand units characterize the base of the unsaturated zone The first water bearing unit beneath the site (first aquifer) consists largely of silty sand and sand with slightly lesser amounts of clayey silt and silt. Silty clay, clayey silt and silty sand characterize the units immediately below the water table i 140162 03 11