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Second Quarter 2001 Mondonng and Sampling Report Z k 7-Eleven Inc Store#19976,1399 North Main Street Manteca CA July 9 2001 Two potenttometnc surface maps were prepared using the quarterly groundwater elevation data Based on the groundwater elevation data, groundwater flow in both water-bearing zones is toward the northwest at a gradient of between about 0 001 and 0 002 ft/ft(Figures 2 and 3) Groundwater monitoring results are ' presented in Table 1 Field data sheets are Included in Attachment 3 ' Monitoring Well Sampling And Analytical Results ' Before sampling,at least four well-casing volumes of groundwater were removed from each monitoring well unless the well was purged dry The groundwater samples were collected using disposable bailers Groundwater samples were placed in analysts-specific containers with analysts-specific preservative The ' samples were then sealed using Teflon septum caps, labeled with the job identification number,the sample number,date,time, and type of laboratory analysis required The samples were stored on ice and transported with a chain-of-custody record to a state-certified laboratory for analysis for BTEX and TPH-G ' using Environmental Protection Agency (EPA) method 8020/modified 8015 and for oxygenates TBA, MTBE, DIPS,ETBE and TAME using EPA method 8260 ' Laboratory analytical results for quarterly groundwater samples are included in Table 1, and historical results are summarized in Table 2 Analytical results for the shallower part of the water-bearing zone indicated dissolved concentrations of up to 5,900 pglL benzene,44,000 pg/L TPH-G, and 5,100 pg/L MTBE Highest levels were found in well MW-2, located immediately adjacent to and downgradient from the hydrocarbon source area (the underground storage tanks/pump island area) Historically, downgradtent attenuation in the shallower water-bearing zone has been evidenced by sharply lower concentrations of benzene and TPH-G in well MW-5 relatively to the source well MW-2 In the deeper water-bearing zone, hydrocarbon levels ranged up to 2 4 pg/L benzene and 16 ug/L MTBE in near-source well MW-1A Down/crossgradient concentrations attenuate as observed by low to non-detectable benzene, TPH-G, and MTBE in wells MW-5A, MW-4/4A,MW-6, and MW-7 ' As mentioned in previous quarterly reports, there are no active public water supply wells within 2,000 feet downgradtent from the site, although there are two wells of unknown status within 2,000 feet upgradient/crossg radio nt of the site 1 y997OW2 wPD