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1 Third Quarter 2002 Monitoring and Sampling Report 4 7-Eleven, Inc Store No 17334 4501 North Pershing Avenue,Stockton, CA October 29,2002 Monitoring Well Sampling and Analytical Results Prior to purging, dissolved oxygen (DO) levels were recorded in each well Quarterly groundwater samples ' were collected after purging at least four well-casing volumes of groundwater from each monitoring well, unless the well was purged dry The groundwater samples were collected using disposable bailers Groundwater samples were placed in analysts-specific containers with analysts-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 Groundwater samples were analyzed for BTEX and TPH-G using Environmental Protection Agency (EPA) methods 8021 B/modified 8015, and for oxygenates MTBE, TBA, DIPS, ETBE, and TAME using EPA method 8260E Dissolved benzene concentrations were detected at 2 200 /L in well el! MW-5A Dissolved MTBE concentrations were highest in well MW-4 (2,800 µg/L) Well MW-5A contained 500 µg/L MTBE, while in well MW-513, MTBE was detected at 0 52 µg/L Quarterly and historical analytical results for the groundwater samples are presented in Tables 1 and 1A, respectively (Attachment 2) The laboratory analytical reports are included in Attachment 4 Dissolved oxygen levels ranged from 0 8 milligrams per liter(mg/L) in well MW-513 to 1 2 mg/L in well MW-4, indicating generally anaerobic conditions Purgewater Disposal Allewur p g ater generated during this quarterly monitoring and sampling event was stored in 55-gallon Department of Transportation (DOT)-approved drums, pending proper transport and disposal at a licensed recycling facility Waste disposal documentation will be forwarded if requested ' Conclusion and Recommendations As reported previously, field observations indicate the presence of a possible offsite contaminant source, a remediation system is present on the property located directly across North Pershing Avenue from the 7-Eleven site This finding may indicate plume mixing in the area of well MW-4 with another hydrocarbon source (not 7-Eleven) Other quarterly data obtained during this sampling event are consistent with previous findings, indicating the onsite hydrocarbon source is in the vicinity of the USTs Wells MW-2, MW-3, and MW-56 were reported at low to non-detectable concentrations for all analytes during the current monitoring event, suggesting stable plume conditions in general ' t Nan-deectable levels of dissolved plume constituents in the deeper - • p nested well MW 5B indicate vertical plume ' delineation near the source area ' 17334Q23 QMR DOC