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Second Quarter 2002 Monitoring and Sampling Report 3 7-Eieven, Inc, Store#14117 2725 Country Club Boulevard, Stockton, CA July 25, 2002 116 ' A potentiometric surface map generated using the quarterly groundwater elevation data is shown in Attachment 1, Figure 2 Based on the groundwater elevation data, the estimated groundwater flow is southeasterly at a gradient of about 0 003 foot per foot Quarterly groundwater elevations and historical ' data are presented in Tables 1 and 1A, respectively (Attachment 2) Field data sheets are Included in Attachment 3 ' Monitoring Well Sampling and Analytical Results ' Before sampling, at least four well-casing volumes of water were removed from each well The groundwater samples were collected using disposable bailers Groundwater samples were placed in 40- milliliter glass vials, which were acidified with hydrochloric acid to preserve the samples The samples ' were then sealed using Teflon septum caps, labeled with the lob 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 The groundwater samples were analyzed for TPH-G, BTEX, and MTBE using Environmental Protection Agency (EPA) methods 8021B/modified 8015, and oxygenates MTBE, TBA, DIPS, ETRE, and TAME using EPA method ' 8260B Dissolved benzene, TPH-G, and MTBE concentrations ranged up to 620 micrograms per liter(µg/L), 14,000 µg/L and 910 µglL, respectively ' Highest MTBE levels were reported in well MW-4, located crossgradient next to the UST area, whereas highest benzene concentrations were detected In well MW-3, located crossgradient to the tank area and pump dispenser island Well MW-2, which formerly contained up to 36,000 fcglL MTBE, has since ' experienced concentration declines of up to 99% and currently contains 260 ug/L MTBE Dissolved TBA levels in this well declined by over 50%from first quarter 2002 levels of 141,000 [tg/L to current levels of 67,000 pg/L The plume is currently delineated to the south (downgradienk) and the west(crossgradient) 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 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 ' 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 ' 14117022 doc