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I IMWI1, and MW13 are shown in Figure 2 Well construction details are presented in Table 2 Historical analytical results for soil samples are summarized in Appendix B ' Aquifer tests as well as injection/percolation tests were performed at the site to gain a better understanding of the hydraulic properties of soil underlying the site Aquifer tests were ' performed to estimate aquifer parameters,radii of influence, and sustainable extraction rates The injection/percolation tests were performed to assess if a hydraulic barrier could be created which would prevent the migration of upgradient hydrocarbon-impacted groundwater from the Shell Iservice station to the former Exxon site during groundwater extraction onsite Due to problems in securing the permit for reinfection of treated groundwater, this remedial option was not implemented ' From 1988 to ,resentliquid-phase hydrocarbons (LPH) have been found intermittently in wells p MW3, MW6, RW1, RW3, WB-MW1, V1, V2, V3, and V4, as documented in quarterly ' groundwater monitoring reports From 1992 to 2000, three remedial approaches were implemented to address petroleum Ihydrocarbons detected in soil and water Details concerning the remedial technologies implemented at the site are presented in Section 2 7 A Request for No Further Remedial Action ' Report (ETIC 2001) was submitted which included both an evaluation of remedial alternatives considering site-specific conditions and a risk analysis In a letter to ExxonMobil dated 27 January 2003, the SJCEHD (1) denied the request of no further remedial action "based upon risk- based corrective action (RBCA) analysis", (2) requested additional groundwater cleanup, and (3) requested vertical assessment of the hydrocarbon-impacted groundwater In a meeting on 17 April 2002 with representatives from ETIC, the SJCEHD, and the Regional Water Quality ' Control Board (RWQCB), ozone sparging was suggested as a remedial alternative by a representative of the RWQCB 2.3 SUMMARY OF GEOLOGY AND HYDROGEOLOGY 1 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 silty-to- clayey sediment to a total depth of approximately 50 feet bgs, with laterally generally discontinuous sand to silty sand lenses at approximately 10-20 feet bgs and at approximately 25- 35 feet bgs Four cross-section locations are shown on Figure 4 and cross-sections A-A', B-B', C-C' and D-D' are shown as Figures 5a, 5b, 5c, and 5d,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 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 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 6 Historically, groundwater has been encountered at the site at depths ' GSProgacts1739421MASTERSW13IWP03o3�WYtx doc 3