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' Second Quarter 1999 Monitoring and Sampling Report 2 7-Eleven, Inc Store No 17334,4501 North Pershing Avenue,Stockton,CA July 6, 1999 ' Prior to purging,dissolved oxygen (DO) levels were recorded in each well Dissolved oxygen levels ranged from 0 2 milligrams per liter(mg/L) in well MW-5B to 0 8 mg/L in well MW-2 (Table 1) ' Dissolved oxygen levels are included in Table 1 The overall DO distribution indicates significant localized depletion of DO in hydrocarbon source areas (the former underground storage tank pit) ' Monitoring Well Sampling and Analytical Results On May 21, 1999,groundwater samples were collected from five monitoring wells Before ' sampling, at least 4 well-casing volumes of groundwater was removed 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 analysts-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 state-certified ' Core Laboratories, in Anaheim, California Groundwater samples were analyzed for BTEX and TPH-G using Environmental Protection Agency(EPA) methods 8020/modified 8015, and for oxygenates MTBE,TBA, DIPS, ETBE, and TAME using EPA method 8280 Analytical results for the May 21, 1999, sampling event and historical laboratory analytical results are summanzed in Table 1 (Figure 2) Highest dissolved benzene and TPH-G concentrations were reported in well MW-5A at 7,010 micrograms per liter(pg/L) and 103,000 fig/L, respectively MTBE was detected in groundwater from wells MW-3, MW-4, MW-5A and MW-5B at concentrations of ' 14 pg/L, 4,710 pg/L, 10,400 pg/L and 30 pg/L., respectively Dissolved benzene, TPH-G, and MTBE concentrations are plotted on Figure 2 The laboratory analytical reports are included in Attachment 4 ' Dissolved benzene concentrations are moderate (100 pg/L) to low(0 91 pg/L) in all wells except source area well MW-5A, where concentrations have continued to fluctuate TPH-G concentrations ' in well MW-5A have remained relatively stable Dissolved MTBE concentrations have increased slightly in well MW-5B from last quarter's results, ' yet remain low at 30 pg/L Concentrations of MTBE in source area well MW-5A have increased since the first quarter of 1999 but remain below highest recorded levels Concentrations of MTBE have remained stable in downgradient well MW-4 1 le, 1733482 qmr