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Fourth Quarter 1999 Monitoring and Sampling Report 2 7-Eleven Inc Store#19976 1399 North Main Street,Manteca CA December 27 1999 ' Groundwater Monitoring Data ' Before monitoring, each well cap was opened to allow the water 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 0 002 ft/ft (Figure 2) Groundwater flow in the deeper part of the water-bearing zone is toward the north at a gradient of about 0 007 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 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, RIPE, ETBE and TAME using EPA method 8260 Laboratory analytical results for quarterly groundwater samples and historical results are summarized in Table 1 Analytical results in the shallower part of the water-bearing zone indicated dissolved concentrations of up to 2,280 fig/L benzene, 46,400 pg/L TPH-G, and 216 lag/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 (20 8 lag/L benzene, 702 lag/L TPH-G, and 216 lag/L MTBE) in ' well MW-5 In the deeper water-bearing zone, hydrocarbon levels ranged up to 1 26 lag/L benzene, in well MW-1A (adjacent to/crossgradient from the underground storage tank area) Down/crossgradient ' concentrations attenuate as observed by non-detectable TPH-G, MTBE, and <1 lag/L benzene in well MW-5A t 19976049 RPT I