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Bonngs TH-2, TH-4b, TH-5, and TH-6b were advanced at a distance of about 40 to 45 feet southeast, southwest, northwest, and northeast, respectively, from TH-1 Field screening techniques detected hydrocarbons concentrations ranging from non-detect to 40 ppmv Likewise, no stains, and generally no odors, were observed or detected in the soil samples with the exception of TH-fib However, an unidentified odor was detected at TH-fib from the soil samples collected below groundwater (30 feet, bsg), registering 200 ppmv on the FID meter Selected soil samples from all soil bonngs were chemically analyzed for hydrocarbon concentrations Laboratory analyses were conducted on soil samples collected every five feet between the depth interval of 10 5 to 30 feet (bsg) in TH-1 , and on two to three select soil samples from each remaining boring Laboratory analyses detected the highest concentrations of hydrocarbons in TH-1 , ranging from 440 to 4700 mg/kg for TPHg, and non-detect to 0 54 mg/kg for benzene Soil boring TH-3, located approximately 20 feet southwest of TH-1, encountered impacted soil from 20 feet bgs to groundwater indicating the southern extent of the plume is not fully defined The analytical results from soil samples collected in soil borings TH-2, TH-4b, TH-5, and TH-6b were reported as at or near detection limits for all constituents These results indicate the soil plume has been defined to the southeast, southwest, northwest, and northeast, respectively 8 0 SITE GEOLOGY • The soil bonngs penetrated a sequence of unconsolidated sediments to a maximum depth of 50 feet (bsg) and are shown in cross-section on Exhibits 5 and 6 The unconsolidated soils are Pleistocene non-marine flood-plain deposits (California Division of Mines and Geology, 1966 ) Drilling encountered a section of brown, fine- grained silty sand to sandy silt at a depth of 38 to 50 feet (bsg), interbedded with a three-foot thick blue-gray clay Poorly graded, medium- to coarse-grained sand overlies the silty sand to a depth of 20 to 25 feet (bsg) An olive-gray, medium dense, sandy silt is next in the sequence to a depth of approximately 13 feet (bsg) The silt may vary between 5 to 10 feet thick and appears to be generally continuous across the site A loose, very fine to fine, silty sand completes the section to ground surface 7 0 SITE HYDROGEOLOGY Free-standing groundwater was encountered during the site investigation at a depth of approximately 29 feet (bsg) under unconfined conditions San Joaquin County Flood Control Map of Spring 1986 indicates the historic groundwater level was once as high as approximately 20 feet (bsg) On August 25, 1993 the depth to groundwater was measured to range between approximately 29 15 feet to 29 46 feet below the top of the well casing (Table 4) The depth to groundwater in each of the three monitoring wells was compared with the well's surveyed elevation at the top and north-side of the California Division of Mines and Geology 1988 Geologic Map of California San Jose Sheet 14 of 17