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S' <br /> i <br /> July 28, 1988 <br /> AGS Job TTo. 78012-2 <br /> Page 2 <br /> Health District, contaminated soil - ds removed to North Cal <br /> construction's yard in Stockton for aeration. Results of <br /> chemical analyses of samples taken from this soil indicate that <br /> hydrocarbon contaminants are now present at levels below <br /> laboratory detection limits. <br /> The location of the wells installed during this study were chosen <br /> > s to further delineate the vertical and lateral extent of the soil <br /> ccntamination initially detected and to evaluate the degree of <br /> ground-water contamination which has occurred at the site. Soil <br /> samples from borings B-1 and B-3 contained levels of hydrocarbon <br /> contamination below laboratory detection limits. Soil from <br /> immediately above the ground-water surface in boring S-2 <br /> contained 28 parts per million (ppm) total petroleum hydrocarbons . <br /> and 0.55 ppm benzene. x Ground water_ from monitoring well 1414-1 <br /> contained hydrocarbon contamination below laboratory detection <br /> limits. Water from monitoring well Mw-3 contained 0.001 ppm <br /> benzene, which is slightly above the sate drinking water limit of <br /> 0.0007 ppm. Water from monituring well M59-2 contained 99 ppm <br /> total petroleum hydrocarbons and 5.7 ppm benzene which is <br /> significantly above recommended limits. calculation of ground- <br /> water gradient and direction of ground-water flow indicates that <br /> hydrocarbon contaminants have probably migrated offsite. <br /> Applied GeoSystems recommends performing a soil vapor survey to <br /> *a identify areas containing elevated levels of hydrocarbons in <br /> soil. If remediation of residual levels of hydrocarbon <br /> contamination in soil is necessary, a soil vapor survey can <br /> provide information useful in selecting the most efficient and <br /> effective remedial alternative. In addition, Applied GeoSystems <br /> recommends installing two additional offsite ground-water <br /> monitoring wells to further delineate the extent of the <br /> hydrocarbon contamination plume. <br /> A system to remediate ground-water contamination cannot be <br /> designed until the limits of contamination have been defined; <br /> however, because of the significantly elevated levels of <br /> hydrocarbons in well MW-2, it is likely that some remediation of <br /> i.J <br /> ground-water hydrocarbon contamination will be required. <br /> - - - . ...--- -- - - <br /> j <br /> AP,01led GeOSYSMms - <br /> r, <br />