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�a Third Quarter 2000 Monitoring and Sampling ReportI 2 7-Eleven Inc Store No 17334 4501 North Pershing Avenue Stockton,CA Au ust�l �200D 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 analysis- specific preservative The samples were then sealed using Teflon septum caps, labeled with the job 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 8021 Blmodified 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 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 5,700 micrograms per liter(pg/L)and 60,000 pg1L, respectively MTBE was detected in groundwater from wells MW-4 and MW-5A at concentrations of ' 6,000 pg/L and 2,900 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-5A 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-513, 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 3 milligrams per liter (mg/L)to 1 6 mg/L in the deep well MW-513 (Table 1) As mentioned in the previous quarterly report, IT Corporation performed a well search using multiple sources of information to assemble a database for public water supply wells located within one mile of the site The database for this site Indicates that there are no public water supply wells located within 4,000 feet downgradient from the site The nearest crossgradientlupgradient water supply well of unreported ' status is located more than 1,500 feet southeast of the site 17334Q30 QMR 1