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' Workplan for Additional Offsite Assessment, Fate and Transport Evaluation Addendum, 7 Sensitive Receptor Survey,and Fourth Quarter 2002 Monitoring And Sampling 7-Eleven Store No 14117, 2725 Country Club Blvd, Stockton,CA January 30, 2403 ' Dissolved benzene, TPH-G, and MTBE concentrations ranged up to 3,600 µg/L, 8,300 µg/L and 210 µg/L, respectively Highest benzene and TPH-G levels were reported in well MWA, located 40 feet northeast of the UST area, whereas highest concentrations were detected in well MW-4, located adjacent to the tank area and pump dispenser island The elevated concentration of benzene in well MW-1 is anomalous relative to previous data Well MW-2, which formerly contained up to 36,000 gg/L MTBE, has since ' experienced concentration declines of up to 99% and currently contains non-detectable levels of MTBE (by EPA 8260) Dissolved TBA levels in this well declined by over 78%from first quarter 2002 levels of 141,000 µg/L to current levels of 11,000 µg/L (Figure 5) As postulated in the CAP, intrinsic(anaerobic) ' biodegradation may be occurring at the site 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) 732 Heterotrophic Plate Count Verification Sampling The SJCPS has indicated that heterotrophic plate count(HPC)data documented as evidence for ongoing intrinsic biodegradation in the CAP were not representative of in-situ conditions, as they were collected prior to well purging This site parameter was 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 (Table 3) verify that microbial populations are high, ranging from 25,000 to 94,000 colony-forming units per milliliter(CFU/ml) Therefore, biodegradation may be ' occurring either through anaerobic processes or depleted aerobic (anoxic) processes 74 Purge Water Transportation and Disposal ' 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 80 CONCLUSION AND RECOMMENDATIONS Results of the current quarterly sampling event conform to plume delineations presented in the December 12 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) Evaluation of HPC samples from groundwater monitoring wells MWA through MW-4 confirms the presence of significant microbial populations Benzene and TPH-G concentrations have remained at low levels or have continued to decline in well MW-2 (located in the UST pit area) • ' 14117024 SAR doc