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Second Quarter 2000 Monitoring and Sampling Report 2 7-Eleven Inc Store No 17334 4501 North Pershing Avenue Stockton CA June 9 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 analysts-specific containers with analysis- specific preservative The samples were then sealed using Teflon septum caps, labeled with the lob 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 analysis Groundwater samples were analyzed for BTEX and TPH-G using Environmental Protection Agency(EPA) methods 8021 B/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 to Table 2 Highest dissolved benzene and TPH-G concentrations were reported in well MW-5A at 7,900 micrograms per liter (Ng/L) and 18,000 pg/L, respectively MTBE was detected in groundwater from wells MW-4 and MW-5A at concentrations of 5,700 pg/L and 4,600 pg/L, respectively Dissolved benzene, TPH-G, and MTBE concentrations are plotted on Figure 2 The laboratory analytical reports are included in Attachment 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 5 milligrams per liter (mg/L)to aerobic conditions In the deep well MW-513 at 4 7 mg/L (Table 1) Well Search ' 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 includes (where available)well ' ownership, depth of sanitary seal, well type, length of screen, location, operating pumping rate, status, and any available testing Information The database for this site (Attachment 5) indicates that there are no public water supply wells located within 4,000 feet downgradient from the site The nearest cross grad i ent/upgrad ient water supply well of unreported status is located more than 1,500 feet southeast of the site \ 17334Q20 QMR