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concrete covered lot, at an elevation of about 20 ft. above mean sea level (USGS, 1980). <br />Topography in the area slopes very gently (less than 1 percent) toward the north. <br />There are currently four single-wall USTs at the Site, three 8,000 -gallon steel USTs and <br />one 6,000 -gallon steel UST. The USTs are located in a common tank cavity southeast <br />of the ARCO AM/PM building (Figure 2). ARCO plans to install four new double-wall <br />10,000 -gallon fiberglass USTs in the same approximate location occupied by the existing <br />USTs. The existing single-wall underground piping will be replaced with new double- <br />wall fiberglass underground piping. <br />The Site is located in the Central Valley of California, near the southern margin of the <br />San Joaquin River Delta. Shallow subsurface deposits near the Site consist of Quaternary <br />alluvium, which has been described as Pleistocene nonmarine sedimentary deposits and <br />Recent alluvial fan deposits, consisting mostly of sand and silt, as well as Recent basin <br />deposits consisting of fine grained silts and clays (Rogers, 1966). Native soils <br />encountered in soil borings drilled during previous investigations at the Site consist of <br />silt with interbedded sand lenses. <br />The depth to ground water at the Site is about 10 ft. below ground surface (bgs). The <br />direction of ground water flow is toward the north and northeast at a gradient of about <br />Gasoline range total petroleum hydrocarbons have been detected in soil and ground water <br />at the Site during previous investigations by Brown and Caldwell Consultants of <br />Sacramento, California (Brown and Caldwell). Petroleum hydrocarbons are present in <br />the soil around the existing tank cavity and in the capillary fringe beneath the Site. <br />2 Doc #A159W01.1.5 <br />