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Fourth Quarter 2002 Monitoring Sampling,and Remediation Progress Report 5 ' 7-Eleven, Inc,Store#19976, 1399 North Main Street, Manteca, CA December 24,2002 • ' 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 0 003 foot per foot(Figures 2 and 3, Attachment 1) Quarterly and historical groundwater ' monitoring results are summarized in Tables 1 and 1A, respectively (Attachment 2) 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, DIPE, ETBE and TAME using ' EPA method 8260 Dissolved concentrations of up to 5,400 pg/L.benzene, 46,000 pg/L TPH-G, and 1,600 pg/L MTBE were detected in the shallower water-bearing zone Highest benzene, TPH-G, and MTBE levels were found in well MW-2, located immediately adjacent to and downgradient from the hydrocarbon/MTBE source area (the UST/pump island area) Historically, downgradient attenuation in the shallower water-bearing zone has been evidenced by much lower concentrations of hydrocarbons/oxygenates in well MW-5 relative to source area well MW-2 These conditions remain demonstrated by 160 pg/L MTBE in well MW-5 as of the October 2002 sampling date Furthest downgradient well MW-7 usually contains less than 10 pg/L MTBE concentrations, documenting lateral plume delineation in the shallow water-bearing zone These conditions are verified by the current data (2 6 pg/L MTBE in well MW-7 as of Novemfi6er 2002) Vyyv MTBE concentrations in well MW-2 have decreased by 85% since the second quarter of 2002 (from ' 11,000 pg/L to 1,600 pg/L) Quarterly and historical analytical results for groundwater samples are summarized in Tables 1 and 1A, respectively(Attachment 2) Laboratory reports are included in Attachment 4 ' In the deeper water-bearing zone, hydrocarbon concentrations are non-detectable for all analytes ' DO levels ranged from 0 9 milligrams per liter(mg/L) in well MW-5A to 1 3 mg/L in wells MW-1A and MW-4, indicating generally anaerobic conditions • 1 ' 19976024 POC