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The Site is situated in the northwestern portion of the San Joaquin Valley in the southern Great Valley physiographic province The Great Valley is an elongate northwest trending asymmetric structural basin that has a wide eastern flank and a short western flank expressed by the upturned edges of the sediments that have accumulated in the basin The basin has a regional southward tilt, which is interrupted by the Stockton fault The Stockton fault is the boundary that separates the Great Valley into a northern portion known as the Sacramento Valley, and a southern portion referred to as the San Joaquin Valley The San Joaquin Valley is bordered on the north by the Stockton fault, on the east by the Sierra Nevada Range, on the south by the Transverse Ranges, and on the west by the Coast Ranges The San Joaquin Valley is filled with a thick sequence of marine and continental sedimentary rocks ranging in age from the Jurassic to Recent The thickest accumulations underlie the western portion of the valley The Sierra Nevada Range is the primary provenance for this 1 sequence of sedimentary rocks 4.0 BACKGROUND The intent of this section is to summarize the removal of underground storage tanks and appurtenances from the Site, and to summarize the conduct and results generated by soil and ' groundwater contamination assessment activities performed at the Site by others 4.1 UNDERGROUND STORAGE TANK REMOVAL On August 14, 1986, American Environmental Management Corporation of Rancho Cordova, California removed a 1,000- Gallon underground storage tank (identified as TKI-86 on Figure 2) from the Site The tank historically contained only leaded gasoline Analytical results of the soil sample collected from beneath the former tank location indicated the presence of diesel, motor ' oil, and total lead No apparent source for the diesel was determined AEMC reported encountering rubble fill material during the tank removal AEMC conducted additional excavation in August, 1986 Two soil samples were collected from the west and east side of the ' excavation Total lead and low levels of aromatic volatile organics were detected in both samples, and 940 mg/kg diesel was detected in the east sample On December 11, 1990 Oil Equipment Services of San Andreas, California, removed a 2000- gallon underground storage tank (identified as TKI-90 on Figure 2) from the Site The tank historically contained only unleaded gasoline Soil samples collected from beneath the former ' tank indicated gasoline concentrations in the soil Tetraethyl lead and ethyl dibromide were not detected in the soil samples A report dated January 2, 1991 was prepared by Condor 1 summarizing the work I366-001 -4-