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SS No. 6348, Tracy <br /> PIER, 1/31/92 <br /> site consist of a monotonous sequence of clay and silt , in <br /> varying proportions. Plant root-hairs and other plant fibers <br /> were abundant within discrete sections. The sediments were <br /> gray or brown, with the gray attributed either to hydrocarbon <br /> discoloration or to the presence of vegetable material. Very- <br /> fine gravel-bearing lenses were encountered within a few <br /> discrete horizons, always in gravel abundance less than 5% . <br /> In addition to the naturally-occurring sediments, the <br /> property is overlain by imported sand up to three feet thick <br /> which raises and levels the pad below the asphalt top. <br /> The measured depth to water at the site on July 15, <br /> 1991 was approximately 10 feet below the surface. Groundwater <br /> was initially encountered at depths of 11 to 12 feet. <br /> Based on surveyed elevations of the wells and depth <br /> of groundwater in the wells, the groundwater flow direction on <br /> July 15, 1991 was to the N20W, having a gradient of 0. 0024 . <br /> According to Mr. Harlin Knoll of San Joaquin County Department <br /> of Public Health services, groundwater flow direction may vary <br /> as much as 110 degrees within a generally northward trend. <br /> The wells developed readily and, although cloudy at <br /> first, soon produced clear water. This aquifer is not utilized <br /> for water supply purposes because of poor quality; an aquifer <br /> at the 300 to 400 foot depth is utilized by wells. Irrigation <br /> to fields in this area may influence this groundwater body; <br /> however, agricultural areas with irrigated fields are down <br /> gradient for the most part. <br /> DISCUSSION <br /> Three areas appear to have been impacted by petroleum <br /> hydrocarbons: (1) the unexcavated contaminated soil surrounding <br /> the pit of the former underground waste-oil storage tank, (2) <br /> the shallow-soil contamination underlying the present <br /> storage area of waste-oil drums, (3) gasoline-contaminated soil <br /> south of the UST fill cap, which may have affected soil <br /> underlying the tank. <br /> Low levels of petroleum hydrocarbons were detected in <br /> groundwater in MW-3 which is located immediately adjacent to <br /> the USTs in the down gradient direction . Based on the <br /> findings of Non Detect petroleum in the groundwater from MW-4 <br /> and the findings of Non Detect in soil from the downgradient <br /> borings, it is unlikely that hydrocarbons have migrated from <br /> • -7- <br />