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2. SITE BACKGROUND • 2.1 SITE LOCATION AND LAND USE Former Exxon RS 7-3942 as currently a vacant lot located on the southeast comer of the intersection of North Pershing Avenue and Rosemarie Lane (Figure 2) Used oil and three grades of gasoline were stored in four underground storage tanks, which were removed from the site in July 1986 Land use in the vicinity of the site as primarily commercial and residential A Shell service station and a 7-11 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 convalescent home Chateau Hospital is located approximately 200 feet northwest of the site across Rosemarie Lane There as an apartment complex to the east of the site and a commercial building to the south (Figure 2) Stagg High School is located approximately 1,500 feet southwest of the site as presented on Figure 1 2 2 SITE GEOLOGY AND HYDROGEOLOGY The geology and hydrogeology of the site have been evaluated using existing boring logs from previous site investigations and monitoring data The site is generally underlain by salty-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 30-40 feet bgs Four cross-section locations are shown on Figure 3 and cross- sections A-A', B-B', C-C' and D-D' are shown as Figures 4a, 4b, 4c, and 4d, respectively 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 March Lane Aqueduct Piping as located approximately 1,000 feet north of the site The groundwater flow direction has generally been to the northeast with a historical average of N53E and an average gradient of 0 006 Historical groundwater elevation data are presented in Table 1, and a rose diagram showing historical groundwater flow direction and gradient trends is presented on Figure 2 Historically, groundwater has been encountered at the site at depths ranging from approximately 13 to 41 feet bgs Hydrographs presenting well construction details, groundwater elevations, and concentrations of benzene, methyl t-butyl ether (MTBE), and Total Petroleum Hydrocarbons as gasoline (TPH-g) over time have been prepared for wells IWI, MW3, MW6, MW10, MWI1, RWl, WB-MW1, UVB-MW2, V1, V2, and V4, and are included in Appendix B A review of these hydrographs reveals an approximately 25-foot rise in groundwater elevations over time at the site from —30 55 feet above mean sea level (msl) in October 1992 to —5 24 feet msl an June 2001 These historical groundwater elevation high and low points are also presented on the geologic cross-sections (Figures 4a-4d), as are the analytical results for soil samples collected during this ongoing hydrogeologac investigation As presented on these cross-sections, petroleum hydrocarbons occur an soil at the site between approximately 17 and 42 feet bgs Currently the G%No}ee[s1739421MA5TEMWPlRa080 I�IexQ da 2