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1.0 INTRODUCTION <br /> This draft Corrective Action Plan(CAP)review presents the results ofthe 8-year review ofthe January <br /> 1998 CAP prepared by Weston Solutions,Inc.(WESTON). This review is performed on behalf ofthe <br /> San Joaquin County Capital Projects Division(Capital Projects)in response to requirements ofthe San <br /> Joaquin CountyPublic Health Services Environmental Health Division(EHD)for the site located at the San <br /> Joaquin County Human Services Agency(HSA)building. The HSA building is located at 145 South Sutter <br /> Street, Stockton, California. The location of the project site is shown in Figure 1-1. <br /> The Federal government is actively pursuing closing sites contaminated from underground storage tank <br /> (UST)releases. Asa result,the State government is informing local governments that they are to perform <br /> ` the corrective actions necessary to achieve site closures. Quarterly groundwater sampling has been <br /> conducted at the site through December 2005. Concentrations of contaminants in the source area have <br /> ` not declined to a level that would indicate the site should be closed. In addition,the vertical extent ofthe <br /> groundwater contamination in the source area has not been fully defined. The purpose ofthe CAP review <br /> is to review the success ofthe remedial action selected in the January 1998 CAP;to review the mass of <br /> contaminants remaining at the site;and to determine if active remediation can be performed or if site <br /> closure, leaving the remaining mass of contaminants in place, can be justified(EHD, 2005). <br /> 1.1 Site History <br /> In April 1988,WESTON supervised the removal ofnine USTs from a former gasoline station located on <br /> the northwest corner of W ashington and Sutter Streets. Petroleum hydrocarbons were detected in soil <br /> samples collected from beneath the tanks. <br /> An initial soil and groundwater investigation(Phase n was performed at the site by WESTON in December <br /> 1988. Dissolved hydrocarbons were detected in two of three monitoring wells installed during this <br /> investigation(MW-2 and MW-3). The highest concentrations ofpetroleum hydrocarbons were detected <br /> in monitoring well MW-2. <br /> In 1989,approximately 2,000 cubic yards ofpetroleum hydrocarbon contaminated soil and one additional <br /> UST were excavated and disposed ofoffsite. Soil was excavated until groundwater was encountered at <br /> ` approximately 29 feet below ground surface(bgs). Confirmation sampling of the side walls of the <br /> excavation verified that the target cleanup levels(10 milligrams per kilogram[mg/kg]oftotal petroleum <br /> hydrocarbon[TPH]residues)were met except in one area. Soil samples taken from the southwest portion <br /> ofthe excavation indicated that a limited volume of soil in excess ofthe target cleanup level remained but <br /> could not be removed because of site constraints. Following the excavation,the pit was backfilled with <br /> clean soil (WESTON, 1989). <br /> A Phase II groundwater investigation was completed in August 1991,and consisted of the installation of <br /> ` five additional monitoring wells(MW-4 through MW-8),aquifer testing,and a tidal influence study. During <br /> 1 <br />