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` SECOR WWW-Secor.com <br />' INTERNATIONAL 3017 Kilgore Road,Suite 100 <br /> S E C O R INCORPORATED Rancho Cordova,CA 95670 <br /> 916-661-0400 TEL <br /> 916-661.0430 FAX <br /> September 13, 2004 <br /> Mr. Mike Infurna 5 2004 <br /> San Joaquin County Environmental Health Department SSP <br /> 304 Webber Avenue, 3rd Floor <br /> Stockton, California ENVIRONMENT HEALTH <br /> PERMIT/SERVICES <br /> i I <br /> RE: Work Plan for Additional Site Assessment <br /> 76 Station 11195 <br /> 16500 North 1-5 �f <br /> Lathrop, California <br /> Dear Mr. Infurna: <br /> SECOR International Incorporated (SECOR) has prepared this letter on behalf of <br /> ConocoPhillips to present a work plan for the installation of three additional groundwater <br /> monitoring wells (Figure 1) at the site referenced above. This work is being proposed as <br /> based on 3rd quarter 2004 groundwater monitoring results. The purpose of the work is to <br /> complete delineation of the dissolved petroleum and methyl tertiary butyl ether (MtBE) <br /> plume to the south and southeast of the site. <br /> ` A brief discussion of site background, proposed scope of work, and schedule are presented <br /> below. <br /> SITE BACKGROUND <br /> A retail gasoline outlet has been located at the subject site since at least 1983. BP Oil <br /> Company (BP) acquired the site from Mobil Oil Corporation in 1989, then sold the site to <br /> Tosco in 1994 (Alisto, Additional Site Investigation Report, dated January 25, 1999). <br /> PREVIOUS INVESTIGATIONS <br /> Five onsite groundwater monitoring wells (MW-1 through MW-5) were installed in December <br /> 1992. Soil samples collected from the well borings were reported to contain petroleum <br /> hydrocarbons at concentrations of up to 120 parts per million (ppm) of Total Petroleum <br /> Hydrocarbons as gasoline (TPHg), and 16 ppm of benzene. The initial groundwater samples <br /> collected from wells MW-3 and MW-4 contained concentrations of up to 10,000 parts per <br /> billion (ppb) of TPHg (Alisto, 1999). <br /> Two additional wells (MW-6 onsite and MW-7 offsite) were installed in September 1993. Soil <br /> samples collected at a depth of 6 feet below ground surface (bgs) were non-detect for <br /> petroleum hydrocarbons (Alisto, 1999). <br />