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1 First Quarter 2002 Monitoring and Sampling Report 3 7-Eleven Inc Store No 17334 4501 North Pershing Avenue Stockton,CA April 22, 2002 tspecific 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 80216/modified 8015, and for oxygenates MTBE, TBA, DIPE, ETBE, and TAME using EPA method 8260B ' Analytical results for the quarterly sampling event are presented in Table 1, and historical laboratory analytical results are summarized in Table 2 Dissolved benzene concentrations were not detected above the reporting limits from any of the wells during the most recent monitoring event Dissolved MTBE ' concentrations were highest In well MW-4 (3,230 ug/L) Other monitoring wells contained less than 10 ug/L MTBE Selected dissolved constituents are plotted on Figure 2 The laboratory analytical reports are ' Included in Attachment 4 Prior to purging, dissolved oxygen (DO) levels were recorded in each well Dissolved oxygen levels ' indicate generally anaerobic conditions (less than 0 9 milligrams per liter(mg/L)) (Table 1) Purgewater Disposal All purgewater generated during this quarterly monitoring and sampling event was stored in 55-gallon Department of Transportation (DOT)-approved drums, pending proper transport and disposal Waste ' disposal documentation will be forwarded once received Conclusion and Recommendations 1 As reported previously, field records 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 underground storage tanks Wells MW-2, MW-3, and MW-5B were non-detect for all analytes during the current monitoring event, suggesting a decline in concentrations and geographic extent of plume constituents Dissolved ' plume constituent concentrations in the deeper nested well MW-513 have also been used to Indicate vertical plume delineation near the source area Plume delineation will be reviewed in the context of additional site ' assessment • ' 17334Q21 QMR DOC