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Third Quarter 2001 Monitoring and Sampling Report and Pre-Closure Sampling Workplan 2 } 7-Eleven Inc Store#17647 1048 West Yosemite Avenue Manteca CA October 4 2001 4� 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 Groundwater monitoring results for the quarter are presented in Table 1 (Attachment 2) Historical data is Included as Table 1A A potentiometric surface map was prepared using the quarterly groundwater elevation ' data (Figure 2),which indicates that as of the date of the sampling event, groundwater flow was toward the north at a gradient of approximately 0 007 ftlft Field data sheets are included In Attachment 3 iMonrtonng Well Sampling and Analytical Results Groundwater samples were collected from the three monitoring wells Dissolved oxygen (DO) concentrations were measured in the three wells prior to sampling 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 and TPH-G using EPA methods 8021/modified 8015, and for the oxygenates MTBE,TBA, DIPE, ETRE, and TAME using EPA method 8260 Dissolved oxygen data indicate relatively uniform, generally anaerobic conditions Dissolved oxygen levels ranged between about 0 9 and 1 0 milligrams per liter(mglL) Laboratory analytical results for quarterly groundwater samples are summarized In Table 1 Dissolved benzene levels ranged between 0 74 and 2 4 micrograms per liter(ug/L) Dissolved TPH-G concentrations ' were not detected Dissolved MTBE concentrations were detected in wells MW-2 and MW-3 at levels of between 3 6 and 5 ug/L The other oxygenates were not detected, as has been the case for a total of six quarterly sampling events (Table 1A) Normally,groundwater flow is to the northwest,with MW-3 lying furthest downgradient from the farmer tank/pump Island locations MTBE concentrations in well MW-3 have declined since the first quarter 2000, when levels were at 108 ug/L Low and non-detectable dissolved gasoline-constituent concentrations with declining dissolved oxygen levels(MW-3) Indicate that natural attenuabon/biodegradation is occurring The laboratory analytical reports for the groundwater samples are included in Attachment 4 1 {' 2175684 rpt