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concrete covered lot, at an elevation of about 20 ft. above mean sea level (USGS, 1980). Topography in the area slopes very gently (less than 1 percent) toward the north There are currently four single-wail USTs at the Site, three 8,000-gallon steel USTs and one 6,000-gallon steel UST The USTs are located in a common tank cavity southeast of the ARCO AM/PM building (Figure 2). ARCO plans to install four new double-wall 10,000-gallon fiberglass USTs in the same approximate location occupied by the existing USTs The existing single-wall underground piping will be replaced with new double- wall fiberglass underground piping 3.0 GEOLOGY AND HYDROGEOLOGY The Site is located in the Central Valley of California, near the southern margin of the San Joaquin River Delta Shallow subsurface deposits near the Site consist of Quaternary alluvium, which has been described as Pleistocene nonmanne sedimentary deposits and Recent alluvial fan deposits, consisting mostly of sand and silt, as well as Recent basin deposits consisting of fine grained silts and clays (Rogers, 1966) Native soils encountered in soil borings drilled during previous investigations at the Site consist of silt with interbedded sand lenses The depth to ground water at the Site is about 10 ft below ground surface (bgs) The direction of ground water flow is toward the north and northeast at a gradient of about 0 002 (Brown and Caldwell, 1993) 4.0 PREVIOUS INVESTIGATIONS Gasoline range total petroleum hydrocarbons have been detected in soil and ground water at the Site dunng previous investigations by Brown and Caldwell Consultants of • Sacramento, California (Brown and Caldwell). Petroleum hydrocarbons are present m the soil around the existing tank cavity and in the capillary fringe beneath the Site 2 Doc #A159W01 15