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September 14, 1998 <br /> Page 2 <br /> Topography at the site is relatively flat Site elevation is approximately 15 feet above mean <br /> sea level (MSL) Surface water drainage is generally to the southeast The nearest surface <br /> water is Fivemile Creek, approximately 3,000 feet northwest of the site Fivemile Creek <br /> flows southwest toward Fiverrule Slough, which flows into the San Joaquin Delta <br /> Regional groundwater flow is to the southeast Depth to groundwater beneath the site <br /> ranges from approximately 38 to 45 feet below ground surface (bgs) <br /> PREVIOUS INVESTIGATIONS <br /> Soil and groundwater investigations at the site have been conducted by Groundwater <br /> Technology, Incorporated (GTI), Dames and Moore (D&M), S i erra'Environmental <br /> Services (Sierra), and PEG Investigative work included a soil vapor survey, installation of <br /> groundwater monitoring wells (MW-1 through MW-7), soil sampling beneath the former <br /> UST excavation, well abandonment (MW-1), and a linuted off-site source search Results <br /> of these investigations have indicated the presence of petroleum hydrocarbons in soil and <br /> groundwater beneath the site Additionally, relatively low dissolved concentrations of <br /> volatile organic compounds 1,2-Dichloroethane (1,2-DCA), 1,1-dichloroethane (1,1-DCA), <br /> and trichloroethene (TCE), and elevated concentrations of tetrachloroethylene PCE at <br /> 10,000 ppb) have been reported in groundwater beneath the site The detection of these <br /> compounds in site groundwater is thought to result from migration of a regional upgradient <br /> source plume <br /> In December 1986, the station was remodeled and the product distribution facilities <br /> c,cnsisting of three USTs, a fuel island, and product piping were replaced The farmer tank <br /> corriolex was backfilled and the new USTs were iocated in the southwestean t orner of the <br /> site (see Figure 2) At that time, the station was upgraded and an additional fuel island was <br /> installed Additionally, a former 550-gallon waste oil tank was replaced with a <br /> 1,000-gallon tank <br /> Results of previous investigations suggest that vadose zone hydrocarbon impact was <br /> negligible based on soil samples collected during installation of the seven on-site wells <br /> (MW-1 through MW-7) Analyses of soil samples collected at 5-foot depth intervals from <br /> these wells did not reveal detectable hydrocarbons Th--se samples were collected at depths <br /> ranging from 5 to 65 feet bgs Hydrocarbons were defected n soil sarttp!es collected in the <br /> foriier tank complex excavation These hydrocarbons however, appeared to be limited to <br /> soils directly underlying the former USTs and were not laterally or ver`ucally continuous <br /> This ccnclusion was based on the results of soil samples collected during the drilling of <br /> Well MW-1, located directly within the former tank complex, and the results of other soil <br /> samples that were collected in the tank excavation <br /> A well survey (Sierra 1992) was performed for the site using data from the California State <br /> Department of Water Resources (DWR) Thirty water wells were identified within <br /> 1/2-male of the site Two of the wells identified in the well survey, a domestic well and a <br /> 16 municipal weR, are potentially downgradient of the site These wells are both more than <br /> 1/4-male southeast of the site <br /> 920-119 1 AlrKPULRPT DOC <br />