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e A 1 i gasoline) and total lead was nondetectable for each sample. The water from the tanks was discharged to the sanitary server by permission of the City of Stockton Municipal Utilities Department. Upon removal of the tanks, the soil beneath the tanks was found to be contaminated with,gasoline. An obvious source for fuel leakage (i.e., pipe breaks or tank holes) was not located, but gasoline vapors emanated from the excavation. Soil samples below the tank excavation were sampled and analyzed. Table 2-2 presents the results of the analyzed soil samples. Based on the results presented in Table 2-2, it was assumed that the soil contamination was limited to a depth of approximately seven feet below the tanks (Exceltech, 1985). The contaminated soil was excavated and aerated for a 2-day period to reduce volatile �. organic levels below.l,0(l0 mg/kg.. After:the aeration process,two random soil stamples .reve.aledvt;latile l� _roca�oon�cconcentration s of 183 and 20 mg/kg. The excavated soil _- was:backftlled into the tank pit. 2.4 ENVIR.OPMENTAL SETTING 2.4.1 REGIONAL GEOLOGY Plant 33 is located just east of the Sacramento-San Joaquin delta, near the margin of the Great Central Valley of California. The valley is a sedimentary basin consisting of a series of nearly flat Iying beds of clay, silt, sand, and gravel. The upper 300 feet of sediments in the vicinity of the site include Recent Alluvium, the Pleistocene Victor Formation, and the Plio-Pleistocene Tulare Formation. These units are laterally and c vertically heterogeneous, and it is difficult to distinguish between them in the subsurface L (California Department of Water Resources (CDWP:1 1967). Both the Recent Alluvium and the Victor Formation consist of discontinuous beds of sand, gravel, clay, and silt, deposited in a continental alluvial fan environment. Sand and gravel dominate in fan areas,while clay and silt are the dominant interfan deposits. As the lithologies of the Recent Alluvium and Victor Formation are similar, these two E units are not distinguishable on driIler's or geophysical logs. Their combined thickness �n the site area is probably between 100 and 150 feet (CDWR 1967). 4 -- The Tulare Formation is the major source of groundwater in the Stockton area. It consists of semiconsolidated, poorly sorted, discontinuous deposits of sand,gravel, clay, and silt. A regionally extensive clay layer,the Corcoran clay, occurs near the top of the Tulare Formation in areas west of the San Joaquin River. 5F03160.11wP\006.51 2-4 d