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t, ' Third Quarter 2002 Monitoring and Sampling Report 4 7-Eleven, Inc, Store#14117,2725 Country Club Boulevard Stockton,CA October 23,2002 1 Dissolved benzene, TPH-G, and MTBE concentrations ranged up to 1,600 micrograms per liter(pg/L), 13,000 pg/L and 1,300 pg/L, respectively Highest benzene and MTBE levels were reported In well MW-4, located crossgradrent next to the UST area, whereas highest TPH-G concentrations were detected In well MW-3, also located crossgradrent to the tank area and pump dispenser island Well MW-2, which formerly contained up to 36,000 pg/L MTBE, has since experienced concentration declines of up to 99% and currently contains 24 pg/L MTBE Dissolved TBA levels in this well declined by over 60% from first quarter 2002 levels of 141,000 pg/L to ' current levels of 47,000 pg/L As postulated In the CAP, Intrinsic (anaerobic) biodegradation may be occurring at the site The SJCPS has Indicated that heterotrophic plate count(HPC) data documented as evidence for ongoing Intrinsic biodegradation In the CAP was not representative of the In-situ conditions, as It was collected prior to well purging It is proposed that this site parameter be re-evaluated during the fourth quarter 2002 by collection of HPC samples from wells MW-1, MW-2, MW-3, and MW-4 subsequent to routine well-purging operations Results of In-situ bacterial population analysis will be submitted with fourth quarter 2002 monitoring data The plume Is currently delineated to the south (downgradient) and the west (crossgradrent) Quarterly and historical analytical results for groundwater samples are summarized In Tables 1 and 1A, respectively (Attachment 2) The laboratory analytical reports for the groundwater samples are Included In Attachment 4 1 Dissolved oxygen data taken from the monitoring wells Indicate generally anaerobic conditions, with dissolved oxygen levels at or near 1 milligram per liter(mg/L) (Table 1) Purge Water Transportation and Disposal I All purge water generated during this quarterly monitoring and sampling event was stored in 55-gallon Department of Transportation (DOT)-approved drums pending characterization and proper disposal Waste disposal documentation will be forwarded if requested Conclusion and Recommendations Results of the current quarterly sampling event conform to plume delineations presented In the December 12, 2001 additional assessment report, and verify apparent ongoing intrinsic biodegradation outlined In the CAP (as documented by apparent biodegradation of MTBE to TBA, followed by apparent ' biodegradation of TBA to [assumed] biomass) In-situ bacterial populations will be evaluated during the fourth quarter 2002 by collection of HPC samples from groundwater monitoring wells MW-1, MW-2, MW-3, and MW-4 after purging of the wells, concurrent with collection of samples for volatile organic constituent analyses Benzene and TPH-G concentrations have remained at tow levels or have continued to decline in well MW-2 (located In the UST pit area), and in crossgradient wells MW-1 and MW-3 14117023 doe