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T 4 j 19 September 2001 AGE-NC Project No. 01-0872 Page 2 of 6 Lead was detected in the soil samples collected beneath UST TA3 and from one sample collected beneath the product lines (T4-3.5) at concentrations as high as 11 mg/kg, which is not outside the background, natural range of lead concentrations in California soils. 2.2. . 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). 2.3. 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 spring 1999 map, the estimated depth to ground water in the vicinity of the site is 20 to 30 feet bsg. Based upon data obtained from nearby sites, the prevailing ground water flow direction in the vicinity of the site is estimated to be north-northeast. However, flow direction may be both locally and seasonably variable. Advanced GeoEnvironmental,Inc.