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410 <br /> 1 .2 Site History <br /> The Stockton Service Center is an operations and maintenance yard that <br /> formerly had three USTs and a pump dispenser for refueling vehicles located <br /> adjacent to the shop and garage. These USTs (one containing leaded gasoline, and <br /> two containing unleaded gasoline) were removed by the District between 1987 and <br /> 1989, and were replaced by two aboveground tanks. <br /> During the removal of one unleaded gasoline UST in 1957, evidence of leakage <br /> �. was observed. Several intrusive soil and groundwater investigations have been <br /> performed at the site to determine the presence and extent of petroleum <br /> hydrocarbon contamination. Based on these investigations, a petroleum hydrocarbon <br /> groundwater plume has been identified at the site. The plume is estimated to be <br /> elliptically-shaped, approximately 85 feet along the long axis, and situated around <br /> Monitoring Well MW-8 (Figure 2). <br /> La 1 .3 Site Geology and Hydrogeology <br /> Based on soil boring logs from the investigations performed at the site, the <br /> 4 geology of the site primarily consists of two sedimentary units. The upper unit <br /> extends from the surface to approximately 10 to 15 feet below ground surface (bgs) <br /> and consists of clay and silty clay. The lower unit extends from 10 feet bgs to <br /> L' approximately 25 feet bgs and consists of fine grained sand, silty sand, and sandy silt. <br /> The geology is fairly continuous throughout the site. Two geologic cross sections <br /> 160 have been generated using the monitoring well boring logs (Figures 3, 4, and 5). <br /> From the available geologic data, the upper aquifer at the site appears to be <br /> L. semi-confined, with a clay layer on the bottom and a silty clay layer on top. The <br /> aquifer itself consists of silty sand,with a porosity of approximately 0.25. Boring logs <br /> and monitoring well installation logs from the site investigations are provided in <br /> LO <br /> Appendix B. <br /> The site is basically flat and at an elevation of approximately 5 feet above mean <br /> LO sea level (msl). Surface water drains to the east. Since the wells were installed in <br /> 1989, the groundwater elevation has fluctuated seasonally from approximately 2 to <br /> 4W 13 feet bgs. The groundwater gradient is approximately 0.002 feet per foot (ft/ft) in <br /> the southwest direction. The groundwater elevations and flow beneath the site are <br /> w.. <br /> susceptible to seasonal fluctuations, and in wet conditions,water levels rise to within <br /> a few feet of the surface. When these conditions occur, the measured gradient and <br /> flow direction differ from the typical gradient and southwesterly flow direction. <br /> 96127 East Bay Municipal Utility District <br /> 40429700.fcr 2 Final Closure Report <br /> V <br />