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rr r • Fourth Quarter 2000 Monitoring and Sampling Report 2 7-Eleven Inc Store No 17334 4501 North Pershing Avenue Stockton CA December 28 2000 Monitoring Well Sampling and Analytical Results Quarterly groundwater samples were collected after purging at least 4 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 analysis-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 analysts required The samples were stored on ice and transported with a chain-of-custody record to a state- certified laboratory for analysts Groundwater samples were analyzed for BTEX and TPH-G using Environmental Protection Agency (EPA) methods 8021B/modified 8015, and for oxygenates MTBE, TBA, DIPS, ETBE, and TAME using EPA method 8260B Analytical results for the quarterly sampling event are presented in Table 1 and Figure 1, and historical laboratory analytical results are summarized in Table 2 Highest dissolved benzene and TPH-G concentrations were reported in well MW-5A at 5,500 micrograms per liter(pg/L) and 48,000 pg/L, respectively Elevated MTBE concentrations were detected in groundwater samples from wells MW-4 and MW-5A at 4,700 pg/L and 2,800 pg/L, respectively Dissolved benzene, TPH-G, and MTBE concentrations are plotted on Figure 2 The laboratory analytical reports are included in Attachment 4 Low concentrations of BTEX components and relatively high MTBE levels in offsite well MW-4 as compared with onsite source well MW-SA may indicate an offsite source of MTBE in well MW-4 Quarterly data obtained during this sampling event are consistent with previous findings, indicating the hydrocarbon source is in the vicinity of the underground storage tanks Concentrations of benzene, TPH-G, and MTBE in the deeper nested well MW-5B, are more than 99% lower than those observed in the shallow well MW-5A Prior to purging, dissolved oxygen (DO) levels were recorded in each well Dissolved oxygen levels ranged from 0 4 milligrams per liter (mg/L) to 0 7 mg/L (Table 1) As mentioned in previous quarterly reports, a well search database for public water supply wells located within one mile of the site indicates that there are no public water supply wells located within 4,000 feet downgradient from the site The nearest crossgradient/upgradient water supply well of unreported status is located more than 1,500 feet southeast of the site 17334Q40 QMR