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INI r� t� SECOR wwwsecorcam S E C ® R Ran INTERNATIONAL Kilgore Road 100 v INCORPORATED Rancho Cordova GAA 95 96670 4�C 916-861-0400 TEL APR 10 2003 916 861-0430 FAX 09 ENVIRONMENT HEALTH PERMIT/SERVICES 44 April 7, 2003 x Mr Mike Infurna San Joaquin County Environmental Health Department § 304 East Weber Ave, Third Floor Stockton, California 95202 Re Work Plan for Off-Site Assessment Circle K Store #1205 16470 Cambridge Road, Lathrop, CA SECOR Project No Dear Mr Infurna r� This letter, prepared by SECOR International Incorporated (SECOR) on behalf of ConocoPhillips, presents a work plan for off-site assessment at the site referenced above (Figure 1) The work plan 4 is being submitted as proposed in SECOR's Results of Additional Site Assessment report dated u> February 12, 2003 The purpose of this assessment is to delineate the lateral extent of MtBE to the northeast of well MW-12 This work plan also includes a brief discussion of site background and previous Investigations, proposed scope of work, quality assurance plan, and schedule 4 SITE BACKGROUND The site Is located on the northeast corner of Cambridge Street and Louise Avenue in Lathrop, California, and is an operating Circle K Mini Mart and 76 service station In November 1997, Pacific Environmental Group (PEG) performed oversight and sampling during the removal of one 4,000-gallon and two 10,000-gallon underground storage tanks (UST's), product dispensers, and associated product piping from the site during facility upgrade activities Approximately 1,241 tons of hydrocarbon-impacted soil was excavated from beneath the UST's and from the new tank pit excavation Additionally, approximately 17,000 gallons of hydrocarbon- impacted groundwater was pumped from the new tank pit excavation Soil samples collected from 4 the excavated former tank pit contained total petroleum hydrocarbons as gasoline (TPHg) at concentrations ranging from 130 ppm to 5,800 ppm Methyl tert-butyl ether (MtBE) was reported In the excavation soil samples at concentrations ranging from 0 19 ppm to 3 6 ppm (PEG, 1998) a In July 1998, one Geoprobe'boring was advanced to a depth of 40 feet below ground surface (bgs) downgradient (north) of the former UST excavation Laboratory analysis of soil samples collected from GP-1 at 10 feet bgs reported maximum concentrations of TPHg and MtBE at 1,900 ppm and 0 31 ppm, respectively Laboratory analysis of gran groundwater samples collected from GP-1 contained concentrations of TPHg and MtBE of 52,000 ppb and 61 ppb, respectively(PEG, 1999) From August through September 1998, IT Corporation (IT) drilled and installed four groundwater monitoring wells (MW-1 through MW-4) to approximate depths of 26 to 28 feet bgs During the installation of well MW-4, the drill auger broke through a City of Lathrop water main releasing • approximately 600,000 to 1,000,000 gallons of water Laboratory analysis of soil boring samples reported maximum concentrations of 6,800 ppm TPHG in MW-3-16' and 7 2 ppm MtBE In MW-2-11'