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A. <br /> Work Plan Evaluation of Soil and Ground Water December 4, 1989 <br /> Unocal Station No. 6981, Stockton, California AGS 38032-3W <br /> The results of the soil-vapor survey indicated that 2 ppm of benzene were detected at 12 <br /> feet below grade in vapor probe location VP-7, but detectable concentrations of BTX <br /> r-� <br /> constituents were not present in vapor samples from the other 12 vapor probe locations. <br /> The results of the soil vapor survey are shown on Table 4. Because these locations are <br /> near the boundaries of the site,it appears that contamination previously detected onsite has <br /> not migrated offsite. <br /> SITE. GEOLOGY <br /> Unocal Service Station No.6981 is in the Central Valley geornorphic province of California. <br /> Areas adjacent to the site are flat with little topographic relief. The area is underlain by <br /> E <br /> a thick sequence of continental and marine sediments ranging in age from Recent to pre- <br /> Cretaceous (Department of Water Resources, 1967), Most of the pre-Cretaceous to mid- <br /> Tertiary sediments in this portion of the valley are of,marine origin while the late Tertiary <br /> to Recent sediments consist primarily of continental deposits (Department of Water <br /> Resources, 1967). Depositional and erosional sequences were primarily controlled by <br /> ' fluctuations in sea level (Department of Water Resources, 1967). <br />