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D ! <br /> August 26, 2004 <br /> 00548 <br /> 06.a3 <br /> Ms. Lori Duncan <br /> San Joaquin County <br /> Environmental Health Department <br /> 304 East Weber Avenue, Third Floor <br /> Stockton, California 95201 <br /> RE: Site Conceptual Model Report <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- <br /> affiliated company) has prepared this Site Conceptual Model (SCM)for the former ARCO Service <br /> Station No. 548 located at 1612 West Hammer Lane in Stockton, California(site). This report was <br /> prepared in response to the San Joaquin County Environmental Health Department(SJCEHD) <br /> letter dated June 21, 2004 (Appendix A), in which an SCM was requested. <br /> The purpose of this SCM is to provide detailed modeling of the subsurface and an evaluation of <br /> 1 recent groundwater analytical data from existing site wells so that future efforts at the site are <br /> focused where most needed. <br /> BACKGROUND <br /> Site Setting <br /> The site is at the southwestern corner of the intersection of West Hammer Lane and Brentwood <br /> Drive in Stockton, California(Figures I and 2). The site was formerly operated as an ARCO <br /> service station and AM/PM mini-mart. Currently, the site is operating as a Valero gasoline station. <br /> �t Site Geology and Hydrogeology <br /> Subsurface sediments generally consist of silt,clayey silt, and sandy silt, with minor beds and <br /> lenses of sand and silty sand from 0 to 65 feet below ground surface(bgs), the maximum depth <br /> explored. Geologic cross-sections of the site are included as Figures 3 and 4, and boring logs are <br /> included in Appendix B. The sand lenses are stream channel deposits and are not laterally <br /> continuous across the site; Figures 3 and 4 illustrate the discontinuous nature of the site lithologies. <br /> Depth to groundwater in site monitoring wells has ranged from approximately 50 feet bgs, when <br /> the wells were first installed in 1991, to approximately 28 feet bgs during the first quarter of 2004. <br /> Increasing groundwater elevations have caused the screen intervals of the 6 groundwater <br /> monitoring wells to become submerged (well screens range from 40 to 63 feet bgs). Soil vapor <br /> URS Corporation <br /> Crown Corporate Center <br /> 2870 Gateway Oaks Drive, Suite 300 <br /> Sacramento,CA 95833 <br /> Tel;916.679.2000 KAWprocessU5 13P ARC0100549\2004 SCM1Site Conceptual Model V2.doc <br /> Fax: 916.679.2900 <br /> I <br />