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SECOR vvww secor corn ® INTERNATIONAL 3017 Kilgore Road Suite 100 INCORPORATED Rancho Cordova CA 95670 916-661-0400 TEL 916-861-0430 Fax t December 2, 2003 non E VE VD' Mr Harlin Knoll DEC 0 4 2003 San Joaquin County ZMV1RONMENT HEALN Environmental Health Department pERMITISERVICES 304 East Weber Avenue, Third Floor Stockton, CA 95202 RE Request for Reduction in Groundwater Sampling Frequency 7-Eleven Store #21756 853 East Yosemite Avenue Manteca, CA SECOR Project No 77EL 21756 03 0104 I Dear Mr Knoll This report was prepared by SECOR International Incorporated (SECOR), on behalf of Mr Ken Hilliard of 7-Eleven Inc , to request a reduction in the groundwater sampling frequency, from the current quarterly program, to sampling groundwater on an annual basis This report Includes a brief discussion of the site background and previous Investigations, and discusses the rational behind SECOR's request for a reduction in the groundwater sampling frequency The site is located at 853 East Yosemite Avenue in Manteca, California (Figure 1) SITE LOCATION AND FACILITY DESCRIPTION The site is an active 7-Eleven convenience store Three underground storage tanks (USTs) were removed from the site in 1998 The locations of the previous USTs, dispensers, and other pertinent site features are shown on Figure 2 SITE BACKGROUND AND PREVIOUS INVESTIGATIONS r On December 9, 1998, three 10,000-gallon steel single-walled USTs were removed from the site and not replaced (IT Corporation, January 1999) Six soil samples were collected from the base of the UST pit approximately 4 feet below ground surface (bgs), and two soil samples were collected from beneath the pump Islands and associated fuel dispensing piping at approximately 14 bgs Total petroleum hydrocarbons as gasoline (TPHg), TPH as diesel (TPHd), and benzene, toluene, ethylbenzene, and total xylenes (BTEX)were not detected above laboratory reporting limits Methyl- tert-butyl ether(MtBE)was reported at a concentration of 0 18 milligrams per kilogram (mglkg) In the northwest corner of the UST pit Initial site assessment was conducted in June 1999 (IT Corporation, 1999) Five direct-push borings (GP1 through GP5) were advanced to total depths of up to 25 feet bgs Soil samples collected during drilling contained up to 0 167 mg/kg MtBE, benzene and TPHg were not detected above laboratory reporting limits Grab groundwater samples collected at the base of the borings contained up to 607 micrograms per liter (uglL) benzene, 32,200 ug/L TPHg, and 46,300 uglL MtBE The highest concentrations were observed in boring GP-3, located near the western end of the former dispenser island location On December 20, 1999, three groundwater monitoring wells (MW-1 to MW-3) were installed to depths of 25 feet bgs, and screened from 10 to 25 feet bgs (IT Corporation, February 2000) Benzene and TPHg In soil were not detected above laboratory reporting limits Groundwater File 7-Eleven#217561Reports121756 Request for Sample Freq Reduction doc