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Site Background Information i STALLWORTH AUTO SALES 602 North California Street, Stockton, California i A small, single-story office facility with a garage (Figure 2) presently occupies the site, which is located at the intersection of North California Street and Oak Street in Stockton, California (Figure 1 ), in the southwest portion of Section 2, Range 6 East, Township 1 North, Mount Diablo Base and Meridian (MDB&M). The site is in an area of mixed residential and commercial use, and was the former location of a gasoline service station; it is presently utilized for used auto sales. REGIONAL GEOLOGICMDROGEOLOGIC SETTING The site is located within the northern 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 Siena 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) . Data obtained from monitoring wells located at the site indicate that depth to ground water on-site is between 28 and 32 feet below surface grade (bsg). Ground water flow direction at the site is towards the northeast. UNDERGROUND STORAGE TANK REMOVAL On 27 May 1991 , one 3,000-gallon diesel UST, one 3,000-gallon unleaded gasoline UST, one 3 ,000-gallon leaded gasoline UST and one 550-gallon waste oil UST were removed from the site by Otto Watkins, General Contractors; the three 3,000-gallon USTs were located at the southern end of the site; the 550-gallon waste oil UST was located at the northern end of the property. Approximate locations of the former fuel USTs are shown on Figure 2 . Advanced GeoEnvironmental, Inc.