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2.4 Geology/Hyd rogeology <br /> Subsurface investigations indicate that lithology beneath the site consists of interbedded sedi- <br /> ments primarily of low to moderate permeability clayey silt, silty clay, clay, and silt, and higher <br /> permeability sand, silty sand, and sandy clay. <br /> Groundwater elevation trends were analyzed to provide information regarding the association <br /> between groundwater elevation changes and dissolved hydrocarbon concentration changes. <br /> The analysis was performed using average groundwater elevations from September 1990 to <br /> October 1994. Historically, average groundwater elevations have ranged from -43.47 to <br /> -35.61 feet MSL. <br /> Between September 1990 and June 1992, minimum and maximum groundwater elevation <br /> events closely followed the cycle of dry and wet seasons typical of northern California; aver- <br /> age groundwater elevations demonstrated seasonal fluctuations between -41 and -44 feet <br /> MSL. Since June 1992, groundwater elevations have been steadily increasing to a maximum <br /> of-35.61 feet MSL in June 1994. An association between changes in groundwater elevation <br /> and changes in dissolved hydrocarbon concentrations is demonstrated at Monitoring <br /> Well MW-2. Typically, dissolved hydrocarbon concentrations increase during periods of <br /> higher groundwater elevation and decrease during periods of lower groundwater elevation. <br /> 2.5 Remediation <br /> Site remediation has included the removal of fill materials surrounding the product storage <br /> tanks and associated piping, and the waste oil storage tank. These facilities were removed in <br /> December 1986. New product storage tanks were installed in the southwestern corner of the <br /> site, and a new waste oil storage tank was placed in the northwestern corner of the site. <br /> 2.6 Exposure Pathway <br /> Since gasoline constituents have been detected in groundwater beneath the site, it is reason- <br /> able to expect groundwater flow to be the primary exposure pathway. The most likely expo- <br /> sure point would be a downgradient water-supply well located near the site and the primary <br /> exposure route would be ingestion. Two wells identified in the well survey, a domestic well <br /> and a municipal well are potentially downgradient of the site. The domestic well is located <br /> 0.31 miles downgradient of the site, and the municipal well is located 0.47 miles downgradient <br /> of the site. <br /> 3201357B/CAPREVI 5 June 29, 1995 <br />