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Work Plan For Plume Evaluation E.� Quik Stop Market No. 132 F.d 3555 West Hammer Lane Stockton, California In,March 1996, groundwater samples obtained from groundwater monitoring wells MW-1R, MW-2, MW-5, MW-6 and MW-7 were first analyzed for methyl tert-butyl ether (MTBE) in accordance with EPA test method 8020. MTBE was present in groundwater samples obtained from MW-IR, MW-2 and MW-5. The maximum MTBE concentration was present in a groundwater sample obtained from MW-5 (I,100 ug/L). MTBE concentrations in MW-5 subsequently decreased steadily for five quarters until September 1997, when the MTBE concentrations rose back to 1,100 ug/L. In a letter to Quik Stop Markets Inc., dated April 1, 1997, the San Joaquin County Public Health Services Environmental Health Division (SJCPHS-EHD) stated that "the only remaining issue at the site is the MTBE currently being detected in Monitoring Well MW-5". The SJCPHS-EHD E recommended the sampling of only Monitoring Well MW-5 for future quarterly monitoring events, therefore, no other groundwater monitoring wells at the site were sampled during the first quarter 1998 sampling event. However, in a letter to Quik Stop Markets Inc. dated April 27, 1998, the SJCPHS-EHD requested that "each quarter sample and analyze water from all of the MWs until further direction from the San Joaquin County Public Health Services Environmental Health Division". l A Work Plan for additional groundwater investigation dated October 19, 1998 was prepared by ATC at the request of the SJCPHS-EH. The Work Plan included a well survey and an operating history of the site USTs. The well survey was conducted to locate any domestic, municipal, and agricultural wells within 2,000 feet of Quik Stop No. 132. No drinking water wells were ( I identified within the search radius. Two irrigation wells were identified within 2,000 feet of Quik �1 Stop 132. The operating history of the site USTs includes installation of new double-walled USTs and product lines, dispensers, and canopies in March 1991 (ATC, October 1998). To further assess groundwater conditions beneath the site; ATC completed six soil borings on March 1, 1999. The purpose of this investigation was to further assess the vertical and lateral (� downgradient extent of methyl tert-butyl ether (MTBE) concentrations in soil and groundwater �! related to the site. TPH-G and BTEX were not detected in any of the soil samples. Based on the results of the investigation, it appears that MTBE has migrated a short distance generally down- E� gradient of the site. The highest MTBE concentration detected in the borings was 2,900 E� micrograms per liter (ugA) at a depth of 20 feet bgs. A depth discrete groundwater sample was obtained in order to define the extent of petroleum hydrocarbons vertically. Based on the results it appears that petroleum hydrocarbon impact is limited primarily to shallow groundwater (ATC, April 1999). In order to further define the lateral extent of MTBE concentrations in groundwater related to the site, an additional groundwater monitoring well, MW-8 was installed off site. The SJCPHS-EHD requested the work in a letter dated April 16, 1999. Monitoring Well MW-8 was screened in the local lower aquifer (ATC, December 1999). All of the site wells were sampled in December 1999; no concentrations of petroleum hydrocarbons were detected in monitoring well MW-8 (ATC, January 2000). I:W Nagm\QNKMI QS 132(000])\UE RTS\2WWORKPLAN(%-uc IT"WtxR).doc 3