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i <br /> December 16, 1992 AmAxAm% <br /> SUnocal Service Station No 6981, Stockton, California Working To Restore Nature <br /> i <br /> varied from 0 004 to 0 007 in August 1992 to 0 010 to 0 013 in March 1992 Potentiometric <br /> Maps showing the gradient and direction of groundwater flow for March 9, August 27, and <br /> September 9, 1992, are included as Plates 8, 9, and 10 <br /> r <br /> Residual Hydrocarbons in Soil <br /> The results of subjective and laboratory analyses of soil samples collected from the onsite <br /> and offsite soil borings and the tank excavation were used to evaluate the lateral and <br /> vertical extent of residual hydrocarbons in soil The sample locations and the concentrations <br /> of total petroleum hydrocarbons as gasoline (TPHg) are presented on the TPHg <br /> Concentration Maps (Plates 11, 12, and 13) Tables 4, 5, and 6 show the analytical results <br /> for soil samples collected from the tank and product-line excavations and soil borings <br /> Results of laboratory analyses of soil samples indicated that soil with residual gasoline <br /> hydrocarbons remains in place beneath the tank pit and the dispenser islands The lateral <br /> and vertical extent of residual hydrocarbons has been delineated, soil with residual <br /> hydrocarbons appears to be predominately onsite Soil containing hydrocarbons may extend <br /> offsite northeast and east of the dispenser islands at depths greater than 25 feet below <br /> surface Because of the lowering of ground-water in this area, samples collected from the <br /> 45 to 56 foot depth have probably been within the zone of capillary fringe during the past <br /> Using the available data, this trend is shown in Plate 13 as the soil containing TPHg within <br /> this interval is elongate in the approximate direction of the ground-water gradient <br /> Residual Hydrocarbons In Tank Pit Area <br /> Soil samples collected from the limits of the tank pit and from soil borings drilled near the <br /> pit (B-1, B-5, and B-12) contained gasoline hvdrocarbons, TPHg concentrations ranged from <br /> 1 below laboratory detection limits to 1,900 parts per million (ppm) (Tables 5 and 6) <br /> iBenzene concentrations in these samples ranged from below laboratory detection limits to <br /> 38032-10 11 <br /> 1 <br />