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f I 1 7810 Thornton Road,Stockton,California { 3.0 INTRODUCTION 3.1 Site Background The site is located on the east side of Thornton Road, south of Hammer Lane in Stockton, California (Figure 1). The property was formerly occupied by a Chevron service station. There are currently no underground fuel tanks on the property. The site covers an irregular shaped area of about 0.48 acre. A single structure presently occupies the site, including an office, vault and cashier areas. Parking facilities, signpost and a drive-through banking areas surround the building. Site features and dimensions are shown on Figure 2. Underground fuel storage tanks were installed by the Chevron Oil Company of California in 1958. The tanks were removed during building demolition in 1983 or 1984. No investigation of potential soil contamination was conducted at the time of tank removal. 3.2 Geology and Hydrogeology The exploratory borings drilled during this investigation encountered uncon- solidated alluvial deposits consisting primarily of clay and silt with occasional layers of poorly-sorted fine sand to silty sand. These native soils were encountered in all borings_The upper 15 feet of the soil column consists of Silty clay and fill materials overlying 15 to 30 feet of interbedded silty sand and silty clay. The native soils encountered below a depth of 20 feet were damp and firm to stiff. These soils became stiffer and lighter in color with depth. The upper(fill) soils consisted of mixed clay and sandy soils that were soft to firm.Occasional thin sandy seams were encountered and a uniform clean(clay and silt free) sand layer was identified in SB-3. This layer was four feet thick and appeared to resemble imported backfill. Below this Iayer the sand was randomly mixed with a darker clayey soil to a depth of 15 feet. Project Number. USM0192 Page 4 of 9 i