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&ASECOR www secor com INTERNATIONAL 3017 Kilgore Road Suite 100 ECOR INCORPORATED Rancho Cordova CA 95670 916 861-0400 TEL 916 661-0430 FAX April 19, 2004 ,r APR 2 2 2004 Mr Michael Infurna ENJ1RU;\i H I I San Joaquin County Environmental Health Department Pil �SCRViLi S Unit IV—Site Mitigation 304 East Weber Avenue Stockton, CA 95202 RE Work Plan for Interim Remediation and Oxygen Injection Feasibility Test 7-Eleven Store #14117 2725 Country Club Boulevard Stockton, California 95204-3937 SECOR Project No 77EL 14117 04 0104 Dear Mr Infurna This work plan, prepared by SECOR International Incorporated (SECOR) on behalf of 7-Eleven Inc , proposes interim remediation via periodic groundwater extraction at the subject site (Figures 1 and 2) SECOR received a telephone call from Mr Mike Infurna of the San Joaquin County Environmental Health Department (SJCEHD) expressing concerns over the increasing concentrations of tert-butyl alcohol (TBA) in the adjacent Exxon/USA gas station well MW7 He • requested that SECOR submit a workplan for interim remediation to control the migration of dissolved TBA away from the 7-Eleven Property SECOR also proposes the installation of 3 oxygen injection test wells, and oxygen (02) injection testing on these wells to evaluate the effectiveness of 02 injection for enhancing in-situ biodegradation of the residual hydrocarbons in soil and groundwater The site is an active 7-Eleven convenience store and gasoline station The retail gasoline station consists of one dispenser island and two 10,000-gallon underground storage tanks (USTs) (Figure 2) BACKGROUND On July 17, 1998, three 10,000-gallon single-walled USTs were removed from the site The UST excavation was extended to the east, and two new 10,000-gallon double-walled steel USTs were installed (Figure 3) The new tanks were installed in a tank pit located east of the original UST locations One ten-inch diameter Schedule 40 PVC conductor casing was installed in the new UST pit between the new tanks for future installation of well MW-2 On September 29 and October 2, 1998, Fluor-Daniel GTI advanced six offsite (OSGP-1 through OSGP-6) and 11 onsite (GP-7 through GP-17) Geoprobe soil borings (Table 1) The offsite borings did not contain total petroleum hydrocarbons as gasoline (TPHg), benzene, toluene, ethylbenzene, and total xylenes (BTEX), or methyl-tert-butyl ether (MtBE) in soil above laboratory reporting limits (Table 1), except for one detection of total xylenes in the shallow sample (4 foot below ground surface) In OSGP-2 (Table 1) The onsite borings reported benzene, TPHg, and MtBE at maximum concentrations of 12 5 milligrams per kilogram (mg/kg) (GP-17@20'), 42,900 mg/kg (GP-15@15'), and 234 mg/kg (GP-14@10'), respectively (Fluor Daniel GTI, 1998) TBA was detected at two . samples at a concentration of 166 mg/kg (GP-14@10') and 23 4 mg/kg (GP-17@15') 1 17-Elevenl7-11 Stores1141171Workplans114117 WP for Intenm Remediation doe