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November 23, 2005 [NEC E <br /> ii <br /> LI , 00548 <br /> DIE "x''.005 <br /> 06.a3 <br /> Ms. Lori Duncan <br /> San Joaquin County BViRON%IENT HEALTH <br /> Environmental Health Department PERMIT/SERVICES <br /> 304 East Weber Avenue,Third Floor <br /> Stockton, California 95201 <br /> RE: Site Conceptual Model U date Report <br /> P P P <br /> Former ARCO Service Station 548 <br /> 1612 West Hammer Lane <br /> Stockton, California 95209 <br /> Dear Ms. Duncan:. <br /> URS Corporation Americas (URS), on behalf of Atlantic Richfield Company (ARCO, a BP-affiliated <br /> company)has prepared this Site Conceptual Model(SCM)Update for the former ARCO Service Station No. <br /> 4 548 located at 1612 West Hammer Lane in Stockton,California(site).This report was prepared'in response <br /> to the San Joaquin County Environmental Health Department (SJCEHD) letter dated October 13, 2005 <br /> (Appendix A), in which an SCM Update was requested. <br /> The purpose of this SCM Update is to provide updates to the detailed modeling of the subsurface and an <br /> evaluation of recent groundwater analytical data from existing site wells so that future efforts at the site are <br /> focused where most needed.The previous SCM was provided on August 26,2004.This update is based on <br /> site investigation work conducted in December 2004 and groundwater sampling conducted to date in 2005. <br /> / Only those areas with updated information are presented in this report; the previous SCM should also be <br /> n reviewed for additional details. <br /> u� BACKGROUND <br /> Site Setting <br /> The site is at the southwestern corner of the intersection of West Hammer Lane and Brentwood Drive in <br /> Stockton, California (Figures 1 and 2). The site was formerly operated as an ARCO service station and <br /> AM/PM mini-mart. Currently, the site is operating as a Valero gasoline station. <br /> Site Geology and Hydrogeology <br /> Subsurface sediments generally consist of silt,clayey silt,and saiidy silt,with!minor beds and lenses of sand <br /> and silty sand from 0 to 100 feet below ground surface(bgs),the maximum depth explored. The geologic <br /> cross-sections of the site are mapped on Figure 2 and illustrated on Figures 10 through 15.with contaminant <br /> concentrations also shown. Two new boring logs for MW-7 and MW-8 are included in Appendix B. The <br /> sand lenses are stream channel deposits and are not laterally continuous across the site;Figures 10 through 15 <br /> n illustrate the discontinuous nature of the site lithologies. <br /> Currently, groundwater is being monitored and sampled,quarterly from all existing monitoring, soil vapor. <br /> extraction(SVE),and air sparge(AS)wells.Groundwater flow across the site is generally eastward,ranging <br /> URS Corporation <br />' Crown Corporate Center <br /> 2870 Gateway Oaks Drive,Suite 150 <br /> Sacramento,CA 95833 <br /> Tel:916.679.2000 <br /> Fax: 916.679.2900 KAWprocessUS SP ARCO\0054812005 SCM UpdatelSCM Update Text.doc <br />