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SECOR www secor com INTERNATIONAL 3017 Kilgore Road Suite 100 S E C ® !!i O IM t Rancho Cordova CA 95670 r 4 i fst A 916-661 0400 TEL f Y • II f f7 916-861 0430 Fm 03 OCT 15 AM 9` 44 October 13, 2003 r Mr Mike Infurna • San Joaquin County Environmental Health Department 304 East Weber Ave, Third Floor Stockton, California 95202 Re Addendum to Work Plan for Off-Site Assessment dated April 7, 2003 Circle K Store 41205 16470 Cambridge Road, Lathrop, CA ; SECOR Project No Dear Mr Infurna A This letter, prepared by SECOR International Incorporated (SECOR) on behalf of ConocoPhillips, presents an addendum to Work Plan for Off-site Assessment dated April 7, 2003 at the site referenced above (Figure 1) The addendum is being submitted to propose additional assessment to define the lateral extent of the petroleum hydrocarbon plume beneath the site As outline in the Work Plan for Off-site Assessment, well MW-16 was installed northeast of well MW-12 to delineate the downgradient extent of petroleum hydrocarbons, particularly MtBE, in groundwater MW-16 was installed on September 11, 2003 and developed and sampled on September 15, 2003 TPHg, MtBE, and ethanol were reported at concentrations of 52 ppb, 140 ppb, and 68 ppb, respectively in the groundwater sample collected September 15, 2003 Additionally, quarterly monitoring and sampling conducted on July 25, 2003, reported 6 6 ppb in downgradient well MW-14 Based on the recent data from wells MW-14 and MW-16, additional assessment east of MW-14 and north and northeast of well MW-16 is needed to delineate the hydrocarbon plume SECOR proposes to delineate the hydrocarbon plume by completing the following two phases 1) drilling five Geoprobe® soil borings, and 2) installing groundwater monitoring wells in locations determined to delineate groundwater from 4 the prior phase The number and location of the wells to be installed in phase two will be determined after the Geoprobe® locations have been completed and will be presented to San Joaquin County Environmental Health Department in a second addendum PROPOSED SCOPE OF WORK The scope of work for this addendum includes additional offsite assessment (five Geoprobe®soil borings)to delineate the lateral extent of petroleum hydrocarbons and MtBE in groundwater to the east of well MW-14 and north and northeast of well MW-16 The proposed boring locations are ~. • shown on Figure 1 The specific scopes of work are discussed below Soil and Groundwater Assessment • Site Safety Plan As required by the Occupational Health and Safety Administration (OSHA) Standard "Hazardous Waste Operations and Emergency Response" guidelines (29 CFR 1910 120), and by California Occupational Health and Safety Administration (Cal-OSHA) "Hazardous Waste Operations and Emergency Response" guidelines (CCR Title 8, Section 5192), SECOR prepared a site-specific Health and Safety Plan (HASP) that was previously {• presented in Work Plan for Off-site Assessment dated April 7, 2003 Field staff and contractors will review the HASP before field operations at the site begin