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' Fourth Quarter 1999 Monitoring and Sampling Report 2 7-Eleven,Inc Store No 17334 4501 North Pershing Avenue,Stockton CA December 27 1999 tMonitoring 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 80216/modified 8015, and for oxygenates MTBE, TBA, DIPE, ETRE, 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 7,230 micrograms per liter (pg/L) and 1,120,000 pg/L, respectively MTBE was detected in groundwater from wells MW-3, MW-4, MW-5A and MW-5B at concentrations of 5 47 pg/L, 7,470 pg/L, 7,580 lag/L and 88 2 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 plume is primarily centralized in the vicinity of the underground storage tanks Concentrations of TPH-G in downgradient well MW-4 are about 25% lower than those observed in this well during the previous quarter ' Prior to purging, dissolved oxygen (DO) levels were recorded in each well Dissolved oxygen levels ranged from 1 36 milligrams per liter(mg/L) in the deeper well MW-513 to 2 68 mg/L in well MW-2 (Table 1) ' Purgewater Disposal All purgewater generated during this quarterly monitoring and sampling event was stored in 55-gallon ' Department of Transportation (DOT)-approved drums, pending proper transport and disposal Waste disposal documentation will be forwarded once received r r I . 17334g49 qmr r r