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' Third Quarter 2000 Monitoring and Sampling Report 2 7-Eleven inc Store#19976,1399 North Main Street Manteca CA Au ust 24 2000 Two potentiometric 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 approximately 0 0045 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 was removed from each monitoring ' well unless the well was purged dry The groundwater samples were collected using disposable bailers Groundwater samples were placed in analysis-specific containers with analysis-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, DIPE, ETBE and TAME using EPA method 8260 ' Laboratory analytical results for quarterly groundwater samples 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 2,700 pg/L benzene, 68,000 pg/L TPH-G, and 310 pg/L MTBE Highest leveis of benzene and TPH-G were found in well MW-2, located near to/directly downgradlent from the hydrocarbon source area (the underground storage tanks/pump island area) Downgradient attenuation in the shallower water-bearing zone is evidenced by lower concentrations of benzene (22 pg/L)and ' TPH-G (<100 pg/L) in well MW-5 In the deeper water-bearing zone, hydrocarbon levels ranged up to 1 3 pg/L benzene in near-source well MW-1A Downlcrossgradient concentrations attenuate as observed by non-detectable benzene, TPH-G, and MTBE in well MW-5A t ' Well Search As mentioned in the previous quarterly report, sensitive receptor surveys completed in 1999 (IT Corporation, April 1999)and March 2000 indicate that there are no active public water supply wells within 2,000 feet downgradlent from the site, although there are two wells of unknown status within 2,000 feet upgradient/crossgradient of the site 1 19976030 RPT