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• Third Quarter 2001 Monitoring and Sampling Report t 2 7-Eleven, Inc,Store#21756,853 E Yosemite Avenue,Manteca,CA October 1,_2001 Groundwater Monitoring Data Before monitoring,the well caps were opened to allow the water in each well to stabilize to atmospheric ' pressure The DTW was measured to the nearest 0 01 foot from the top of the well casing using an Interface probe A potentlometric surface map was prepared using the quarterly groundwater elevation data (Figure 2) j ' Results Indicate that the groundwater flow Is toward the west-northwest at a gradient of about 0 002 ft/ft The groundwater monitoring results from the current monitoring and sampling event are presented In Table ' 1 (Attachment 2) Historical data are Included in Table 2 Field data sheets are Included in Attachment 3 Monitoring Well Sampling and Analytical Results 1 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 fob 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 Groundwater samples were analyzed for BTEX, MTBE, and TPH-G using EPA methods 8021/modified 8015 and for the oxygenates MTBE, TBA, DIPE, ETBE,and TAME using EPA method 8260 Dissolved oxygen (DO) concentrations were measured In the three wells prior to sampling Dissolved oxygen readings were generally less than 1 milligram per liter(mg/L.), indicating anaerobic conditions (Table 1) Quarterly analytical results are summarized In Table 1 and historical results are summarized In Table 2 Dissolved benzene was detected in all of the wells at levels of less than 1 microgram per liter(ug/L) Dissolved TPH-G levels ranged between non-detectable (wells MW-2 and MW-3) and 930 ug/L(well MWA) Dissolved MTBE concentrations ranged between 23 ug/L(well MW-3) and 2,800 ug/L (well MWA) The other oxygenates were not reported above detection limits In any of the wells sampled The laboratory analytical reports for the groundwater samples are included in Attachment 4 ' Sensitive Receptors As mentioned In previous quarterly reports, a well search database for public water supply wells located within one mile of the site Indicates that the nearest well is an active municipal well located 1,987 feet downgradient of the site, It Is 155 feet deep,with top of screen at approximately 65 feet below grade