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0 0 Site Background Information IDEALEASE 1137 South Stockton Street, Stockton, California The site is situated in an industrial and commercial area of west Stockton, San Joaquin County, California. Prominent features on the site include an office building, truck wash facility, fuel dispenser area and a single12,000-gallon diesel above ground storage tank (AST). The surface primarily consists of asphalt and cement. A canopy covers the truck wash and dispenser area (Figure 2). During the removal and replacement of concrete southeast of the fueling islands, petroleum-impacted pea gravel was encountered, suggesting a subsurface leak of diesel from the AST fueling system. Elite IV Contractors was contacted to remove concrete overlying the subsurface product piping and evaluate for leakage. Upon removal of the concrete and exposure of the subsurface product piping, Elite IV noted leaking "clamshell' couplings in the area of the dispensers. Free product was noted within the shallow excavations and removed via a sump pump. It is our understanding that more than 60- gallons of free product was pumped from the shallow excavations. Elite IV subsequently repaired the piping and backfilled the excavations. REGIONAL GEOLOGIC/HYDROGEOLOGIC SETTING The site is located within the southern San Joaquin Valley which comprises part of the Great Valley geomorphic province of California. The San Joaquin Valley is formed by the Great Valley geosyncline, which is a large, elongate, northwest-trending asymmetrical structural trough (basin). It is bordered by the Coast Ranges to the west, the Klamath Mountains and Cascade Range to the north, and the Sierra Nevada to the east. This trough has been filled with sediments derived from both marine and continental sources. Thickness of the sedimentary fill ranges from thin veneers along the valley edges to more than 20,000 feet in the south central portion of the valley. The sedimentary formations range in age with the older deposits being primarily marine in origin and the younger deposits being primarily continental. Continental-derived sediments were primarily deposited in lacustrine, fluvial, and alluvial environments with sediment sources being the mountain ranges surrounding the valley(Olmsted and Davis, 1961). The site is located on unconsolidated and semi-consolidated alluvium, lake, playa and terrace deposits of Quaternary age (California Division of Mines and Geology, 1977). The Modesto, Riverbank and Turlock Lake Formations and overlying Recent alluvium are the principal sources of domestic ground water in the 13,500-square mile San Joaquin Valley Ground Water Basin (Basin 5-22). Advanced GeoEnvironmental,Inc.