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• 2. SITE BACKGROUND ' 2.1 SITE LOCATION AND LAND USE Former Exxon RS 7-3942 is currently a vacant lot located on the southeast comer of the t intersection of North Pershing Avenue and Rosemarie Lane (Figure 2) Used oil and three grades of gasoline were stored in four underground storage tanks (USTs), which were removed ' from the site in July 1986 Land use in the vicinity of the site is primarily commercial and residential A Shell service ' station and a 7-Eleven food and gasoline service store are located on the southwest and northwest corners, respectively, of the intersection of North Pershing Avenue and Rosemarie Lane The Chateau Hospital convalescent home is located approximately 200 feet northeast of ' the site across Rosemarie Lane There is an apartment complex to the east of the site and a commercial building to the south Stagg High School is located approximately 1,500 feet southwest of the site ' 2.2 SITE GEOLOGY AND HYDROGEOLOGY ' The geology and hydrogeology of the site were evaluated using existing boring logs from previous site investigations and monitoring data The site is generally underlain by silty-to-clayey sediment to a total depth of approximately 50 feet below ground surface (bgs), with laterally generally discontinuous sand to silty sand lenses at approximately 10-20 feet bgs and at approximately 25-35 feet bgs Cross-sections are shown in Appendix A ' As presented on Figure 1, the nearest body of surface water is the Calaveras River, which is located approximately 2,000 feet south and upgradient of the site The East Bay Municipal ' Utilities District's March Lane Aqueduct piping is located approximately 1,000 feet north of the site ' The groundwater flow direction has generally been to the northeast with a historical average of N48E and an average gradient of 0 006 Historically, groundwater has been encountered at the site at depths ranging from approximately 13 to 41 feet bgs Groundwater has risen ' approximately 25 feet over time at the site, from -30 55 feet (40 70 feet bgs) in October 1992 to -5 24 feet (12 72 feet bgs) in June 2001 In December 2004 groundwater elevations ranged from -1178 feet (18 20 feet below top-of-casing) and -14 46 feet (22 54 feet below top-of-casing) As ' presented on the cross-sections, petroleum hydrocarbon impacted soil occurs between approximately 20 and 46 feet bgs Currently groundwater is at approximately 20 feet bgs, which means that these petroleum hydrocarbons entrapped in soil at the site are submerged 1 2.3 PREVIOUS ENVIRONMENTAL ACTIVITIES AT THE SITE lo Between January 1987 and April 1988, groundwater monitoring wells MW1—MW7 were installed onsite, and wells MWS—MW10 were installed offsite Groundwater recovery wells 1 ' G 1Projects1739421MASTERI WPIIPR I20411PRtext doc 3