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2. 2 DISCUSSION OF CHEMICAL CONTAMINATION ANALYSIS RESULTS <br /> There is no evidence that waste oil was ever released to the <br /> subsurface environment. The soil ,sample taken from the area of <br /> the waste oil tank (Sample 4 in Table 3) did not contain detect- <br /> able levels of benzene, toluene, xylenes, PCB, or halogenated <br /> volatile hydrocz.' zons. Total petroleum hydrocarbons (TPH) were <br /> found in the soil at 14 mg/kg, but this level is very minor and <br /> should not be the source of significant ground-water contamin- <br /> ation. <br /> The vapor, soil, and ground--water data for samples in the area of <br /> the former tank field indicate that a release may have occurred <br /> in the area, but the low concentrations detected suggest that the <br /> volume of the release was small. In addition, since the soils <br /> collected during the construction of the monitoring wells did not <br /> t have detectable levels of contaminants, the extent of contamina- <br /> tion was limited. Based on this observation, most of the con- <br /> taminated soil was probably removed during the tank excavation. � 7 <br /> Ground water in contact with a layer of floating petroleum hydro- <br /> carbons becomes saturated with hydrocarbon constituents. work <br /> sponsored by the San Francisco Regional water Quality Control <br /> Board (1985) found that water in contact with gasoline may, at <br /> equilibrium, contain 40 mg/L benzene, 9-76 mg/L toluene, and 30-- <br /> 75 mg/L short-chain hydrocarbons. Another study of petroleum <br /> hydrocarbon solubility (API 1985a) found similar results; this <br /> study suggested that ground water in contact with a layer of <br /> floating hydrocarbon would have a minimum combined concentration <br /> of 10--20 mg/I, dissolved benzene, toluene, xylenes and ethylben- <br /> zene. At the Stockton site, the groundwater concentrations are <br /> well below these saturation concentrations: the sum concentra- <br /> tions of benzene, toluene, xylene, and ethylbenzene in MW1, MW2, <br /> and MW3 are 0.5 mg/L,. 1.5 mg/L, and 4.2 mg/L, respectively. - This <br /> implies that the ground water was not in contact with a layer of <br /> t free product . <br /> 12 <br />