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SECOR www.secor.com <br /> E C ® R INTERNATIONAL <br /> INCORPORATED 3017 Kilgore Road,Suite 100 <br /> Rancho Cordova,CA 95670 <br /> 916-861-0400 TEL <br /> � i 916-861-0430 FAx <br /> September 7, 2004 <br /> SEP 0 9 2004 <br /> ENVIRONMENT HEALTH <br /> Mr. Michael Infurna PERMIT/SERVICES <br /> San Joaquin County Environmental Health Department <br /> 304 East Webber Avenue, Third Floor <br /> Stockton, CA 95202 <br /> �l <br /> � <br /> Re: Work Plan for Well Installation and Vertical Assessment " V a <br /> Circle K Store 1205 <br /> 16470 Cambridge Street <br /> Lathrop, California <br /> Dear Mr. Infurna <br /> SECOR International Incorporated (SECOR) has prepared this letter on behalf of ConocoPhillips to <br /> present a work plan for installation of three groundwater monitoring wells (wells MW-17, MW-18, and <br /> MW-19) to complete lateral delineation and two soil borings to complete vertical assessment at the <br /> site referenced above (Figure 1). The work plan is being prepared based on recommendations in <br /> Results of Geoprobe Assessment Report dated August 13, 2004 (for lateral delineation of the <br /> dissolved plume) and previous communications with San Joaquin County Environmental Health <br /> Department regarding the need for vertical delineation. This work plan includes a brief discussion of <br /> site background and previous investigations, proposed scope of work, and schedule. <br /> a <br /> SITE BACKGROUND AND PREVIOUS INVESITGATIONS <br /> The site is located on northeast corner of Cambridge Street and Louise Avenue in Lathrop, California, A <br /> and is an operating Circle K Mini Mart and 76 service station. <br /> In November 1997, Pacific Environmental Group (PEG) performed oversight and sampling during the <br /> removal of one 4,000-gallon and two 10,000-gallon underground storage tanks (UST's), product <br /> dispensers, and associated product piping from the site during facility upgrade activities. <br /> Approximately 1,241 tons of hydrocarbon-impacted soil was excavated from beneath the UST's and <br /> from the new tank pit excavation. Additionally, approximately 17,000 gallons of hydrocarbon- <br /> impacted groundwater was pumped from the new tank pit excavation. Soil samples collected from <br /> the excavated former tank pit contained total petroleum hydrocarbons as gasoline (TPHg) at <br /> concentrations ranging from 130 ppm to 5,800 ppm. Methyl tert-butyl ether (MtBE) was reported in <br /> the excavation soil samples at concentrations ranging from 0.19 ppm to 3.6 ppm (PEG, 1998). <br /> In July 1998, one Geoprobe "'boring was advanced to a depth of 40 feet below ground surface (bgs) <br /> downgradient (north) of the former UST excavation. Laboratory analysis of soil samples collected <br /> from GP-1 at 10 feet bgs reported maximum concentrations of TPHg and MtBE at 1,900 ppm and <br /> 0.31 ppm, respectively. Laboratory analysis of grab groundwater samples collected from GP-1 <br /> contained concentrations of TPHg and MtBE of 52,000 ppb and 61 ppb, respectively (PEG, 1999). <br /> From August through September 1998, IT Corporation (IT) drilled and installed four groundwater <br /> monitoring wells (MW-1 through MW-4) to approximate depths of 26 to 28 feet bgs. During the <br /> • installation of well MW-4, the drill auger broke through a City of Lathrop water main releasing <br /> approximately 600,000 to 1,000,000 gallons of water. Laboratory analysis of soil boring samples <br />