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i 1 .3 Site History ' The Stockton Service Center is an operations and maintenance yard that formerly had three USTs and a pump dispenser for refueling vehicles located ' adjacent to the shop and garage. These USTs (one containing leaded gasoline, and two containing unleaded gasoline) were removed by the District between 1987 and ' 1989, and were replaced by two aboveground tanks During the removal of one unleaded gasoline UST in 1987, evidence of leakage was observed. Several intrusive soil and groundwater investigations have been performed at the site to determine the presence and extent of petroleum hydrocarbon contamination Based on these investigations, a petroleum hydrocarbon groundwater plume has been identified at the site The plume is estimated to be elliptically-shaped, approximately 85 feet along the long axis, and situated around ' MW-8 (Figure 2) 1 .4 Site Geology and Hydrogeology Based on soil boring logs from the investigations performed at the site, the geology of the site primarily consists of two sedimentary units The upper unit extends from the surface to approximately 10 to 15 feet below ground surface (bgs) and consists of clay and silty clay The lower unit extends from 10 feet bgs to ' approximately 25 feet bgs and consists of fine grained sand, silty sand, and sandy silt The geology is fairly continuous throughout the site Two geologic cross sections ' have been generated using the monitoring well boring logs (Figures 3, 4, and 5) From the available geologic data, the upper aquifer at the site appears to be i ' semi-confined, with a clay layer on the bottom and a silty clay layer on top. The aquifer itself consists of silty sand, with a typical porosity of approximately 0 25 Bonng logs and monitoring well installation logs from the site investigations are provided in Appendix B The site is basically flat and at an elevation of approximately 5 feet above mean ' sea level (msl) Surface water drains to the east Since the wells were installed in 1989, the groundwater elevation has fluctuated seasonally from approximately 2 to 13 feet bgs The groundwater gradient is approximately 0 002 feet per foot (ft/ft) in the southwest direction The groundwater elevations and flow beneath the site are susceptible to seasonal fluctuations, and in wet conditions, the water levels rise to within a few feet of the surface When these conditions occur, the measured gradient ' 96R7 East Bay Municipal Utility District 40429700 fer 3 Final Closure Report