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� r�r_2 First Quarter 2002 Monitoring, Sampling,and Air Sparge Test Report f; ar,-f, 7-Eleven,Inc,Store#19976, 1399 North Main Street, Manteca, CA May 10, 2002 f r� 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 both water-bearing zones is toward the northwest at a gradient of about 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 ' 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, 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 result for the shallower part of the water-bearing zone ft % ` n� indicated dissolved concentrations of up 0AVVIR mrograMs�€r liter(dig/L) benzene, 5,500 tag/L TPH-G, and 9,200 lag/L MTBE Highest benzened levels were found in well MW-2, located immediately ' adjacent to and downgradient from the hydrocarbon/MTBE source area (the underground storage tanks/pump Island area) Well MW-1, located crossgradient near to the MTBE source area, had the �a5 ' highest MTBE concentrations Historically, downgradient attenuation in the shallower water-bearing zone has been evidenced by sharply lower concentrations of hydrocarbons/oxygenates in well MW-5 relative to h 3x5 the source well MW-2 Furthest downgradient well MW-7 contains low MTBE concentrations, documenting lateral plume delineation in the shallow water-bearing zone Concentrations in well MW-2 ��;�5)o�OA have declined significantly since last quarter(from 690 pg/L to 170 pg/L) GGG��r-"in ' In the deeper water-bearing zone, hydrocarbon levels ranged up to 0 82 tag/L benzene and no detectable u1 Y� MTBE in near-source well MWAA Down/crossgradient concentrations attenuate as observed by ��{►� ' non-detectable benzene, TPH-G, and MTBE in wells MW-5A and MW-4A ' As mentioned In previous quarterly reports, there are no active public water supply wells within 2,000 feet downgradient from the site, although there are two reported wells of unknown status within 2,000 feet . upgradient/crossgradient of the site ' 19976021 ooc