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First Quarter 2001 Monitoring and Sampling Report 2 7-Eleven,Inc,Store#19976 1399 North Main Street,Manteca CA March 30 2001 ' 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 between about 0 002 and 0 003 fttft (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 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 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 29 pg/L benzene, 1,000 pg/L.TPH-G, and 610 pg/L MTBE Highest levels of benzene and TPH-G were found in well MW-5, located downgradient from the hydrocarbon source area (the underground storage tanks/pump Island area) Historically, downgradient 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 Inexplicably, groundwater In well MW-2,which historically has contained the highest BTEX and TPH-G concentrations, did not have reportable levels of BTEX and TPH-G this quarter In the deeper water-bearing zone, hydrocarbon levels ,} ranged up to 1 pg/L benzene and 8 4 ug/L MTBE in near-source well MW-1A Down/crossgradlent concentrations attenuate as observed by low to non-detectable benzene, TPH-G, and MTBE in wells MW-5A, MW-4, MW-6, and new well MW-7 ' As mentioned in previous quarterly reports, there are no active public water supply wells within 2,000 feet p q Y P downgradient from the site, although there are two wells of unknown status within 2,000 feet upgradlenttcrossgradient of the site 1 19976Q-1 WPB I