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9 Second Quarter 2000 Monitoring and Sampling Report June 29 2000 7-Eleven Inc Store#19976 1399 North Main Street Manteca CA Groundwater Monitoring Data Before monitoring, each well cap was opened to allow the water level in each well to stabilize to ' atmospheric pressure The DTW was measured to the nearest 0 01 foot from the top of the well casing using an Interface probe Two potentiometric surface maps were prepared using the quarterly groundwater elevation data Based on the groundwater elevation data, groundwater flow In the shallower water-bearing zone Is toward the northwest at a gradient of approximately 0 003 ft/ft (Figure 2) Groundwater flow In the deeper part of the ' water-bearing zone is toward the north-northwest at a gradient of about 0 003 ft/ft (Figure 3) Groundwater monitoring results are presented in Table 1 Field data sheets are Included in Attachment 3 Monitoring Well Sampling And Analytical Results tBefore 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 fob 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 5,600 pg/L benzene, 58,000 pg/L TPH-G, and 1,400 pg/L MTBE Highest levels were found in well MW-2, located near to/directly downgradient 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 (86 lag/L benzene, 1,200 pg/L TPH-G, and 130 lag/L MTBE) in well MW-5 In the deeper water-bearing zone, hydrocarbon levels ranged up to 2 9 tag/L benzene in ' upgradient well MW-4A Down/crossgradient concentrations attenuate as observed by non-detectable benzene, TPH-G, and MTBE in well MW-5A Well Search tSensitive receptor surveys completed in 1999 (IT Corporation, April 1999) and March 2000 (Attachment 5) indicate that there are no active public water supply wells within 2,000 feet downgradient from the site, although there are two wells of unknown status within 2,000 feet upgradient/crossgradient of the site 19976020 RPT