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27 December 1999 AGE-NC Project No. 99-0683 Page 2 of 6 During the UST removal, a total of four soil samples from the floor of the UST excavation and three composite soil samples from the stockpiled soil. Total petroleum hydrocarbons quantified as diesel (TPH-d) was reported in two soil samples collected from the east end of the excavation floor at concentrations of 13,000 milligrams per kilogram (mg/kg) and 230 mg/kg. TPH-d was also reported in all three composite stockpiled soil samples at concentrations as high as 3,000 mg/kg. Toluene was reported in soil sample T2E, collected from the east end of the excavation, at a concentration of 0.005 mg/kg. No benzene, ethyl benzene, xylenes nor methyl tertiary butyl ether (MTBE) were reported in any of the soil samples submitted for analysis. A summary of soil analytical results are presented in Table 1. 2.3. REGIONAL GEOLOGIC/HYDROGEOLOGIC 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 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). 2.4. GROUND WATER DEPTH AND FLOW DIRECTION Based on the San Joaquin County Flood Control District and Water Conservation District Lines of Equal Depth of Water Wells Full 1998 map, the estimated depth to ground water in the vicinity of the site is 50 to 60 feet below surface grade (bsg). Based upon data obtained from nearby sites, the prevailing ground water flow direction in the vicinity of the site is estimated to be west. However, flow direction may be both locally and Advanced Geo£nironmental,Inc.