SITE INFORMATION AND CORRESPONDENCE FILE 2

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Former Chevron Service Station 9-0557 Page 3 139 South Center Street May 6, 2010 Stockton, California (southeast) direction. Impacted soil is well delineated toward the west-southwest to sixty feet bsg by data from the soil boring for MW-6, and is delineated toward the north-northwest to depths as great as twenty-five feet bsg by data from soil boring SB-5, and toward the east- northeast to forty-three feet bsg by data from soil borings SB-1 and thirty-six feet bsg by data from soil boring SB-6. In addition to the contaminants of concern noted for soil, groundwater was found to be impacted by TPH quantified as motor oil (TPH-mo), tertiary-butyl alcohol TBA), ethylene dibromide (EDB) and 1,2-dichloroethane (1,2-DCA). Impacted groundwater occurs from the water table (varying from approximately twenty-one to forty-seven feet bsg) to depths as great as one-hundred five feet bsg (MW-18D2) and one-hundred sixteen feet bsg (grab groundwater sample CPT5-W-116). The lateral extent of impacted groundwater in the down- gradient direction reaches as far as monitoring well MW-7, approximately one-hundred thirty- five feet from the former MW-9S. Laterally from the former USTs removed in 1990, the plume of impacted groundwater is delineated in the moderately shallow zones toward the west- southwest by data from monitoring well MW-6 screened thirty-five to sixty feet bsg; north- northwest by MW-12 screened seventeen to thirty-two feet bsg and MW-16 screened twenty to thirty feet bsg; north-northeast by MW-3 screened thirty-five to sixty feet bsg; east by MW-10 screened seventeen to thirty-two feet bsg and MW-14 screened twenty to thirty feet bsg; southeast by MW-5 screened thirty-five to sixty feet bsg; and south-southeast by MW-8 screened twenty-two to forty-two feet bsg, MW-7 screened thirty-two to fifty-two feet bsg, and MW-13 screened approximately eighteen to thirty-three feet bsg. HFA compared the various contaminant concentrations to the environmental screening levels (ESLs) established by the San Francisco Bay Regional Water Quality Control Board to evaluate the health risk and hazard posed by the residual contamination. Locally soil has been impacted by TPH-g and BTEX at concentrations that exceed the ESLs; groundwater is or has been impacted by the same contaminants of concern as soil and by TPH-mo, TBA, EDB, and 1,2-DCA at concentrations that exceed the ESLs but HFA argues that the exposure pathways are not complete and that the impacted soil and groundwater present little risk or hazard to human health and the environment. As the site is currently a vacant lot, zoned for commercial use, but with no development plans in place, HFA concluded that there is no completed exposure pathway and did not evaluate the potential for vapor intrusion. HFA states that Chevron will perform such an evaluation should the site be proposed to be put into use. Three field tests were conducted to evaluate potential remediation technologies to identify the most feasible and cost effective technology for possible use on the site to reduce contaminant mass in soil and groundwater and to bring the site more rapidly to regulatory closure. The tests reported in FS were an aquifer pumping test, an ozone injection test, and a two-phase extraction (TPE) test. Remediation by monitoring natural attenuation (MNA) was discussed. It was concluded in FS that groundwater extraction (GWE), TPE, and ozone injection would not be likely to be cost effective due to the contaminants of concern occurring predominantly in fine-grained soil in the saturated zone; HFA recommended use of MNA to remediate the impacted soil and groundwater.