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1 Second Quarter 2002 Monitoring and Sampling Report and Request for Closure 3 16 7-Eleven Inc Store#17647 1048 West Yosemite Avenue Manteca CA July 25 2002 ' During drilling, groundwater was encountered at approximately 15 feet bg During the current sampling event, depth to water ranged from 14 00 to 14 78 feet bg Groundwater flow directions are generally westward (ranging from north to southwest), at a gradient of about 0 002 foot per foot Sensitive Receptors ' A sensitive receptor evaluation (IT Corporation, May 2000) Indicates that there are approximately 14 wells within a one-mile radius of the site, but few within 1,000 feet of the site The nearest public supply wells ' are located approximately 993 feet north (crossgradient) of the site Two wells are also located approximately 4,956 feet southwest(downgradlent) of the site 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 potentiometrlc surface map prepared using the quarterly groundwater elevation data is shown on Attachment 1, Figure 2 Based on the groundwater elevation data, groundwater flow was toward the northwest at a hydraulic gradient of approximately 0 002 foot per foot, consistent with historic trends Quarterly groundwater monitoring results and historical data are summarized in Tables 1 and 1A, respectively (Attachment 2) Field data sheets are Included in Attachment 3 ' Monitoring Well Sampling and Analytical Results ' Before sampling, each of the wells was purged of three volumes of water by hand bailing Dissolved oxygen (DO) concentrations were also measured prior to sampling The wells were sampled 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, DIPS, ETBE, and TAME using EPA method 8260 Quarterly analytical results are presented in Table 1 Dissolved benzene was not detected In any of the ' wells Dissolved TPH-G was not detected in any of the wells Dissolved MTBE was detected in wells MW-1 and MW-3 at 0 82 pg/L and 9 5 pg/L, respectively The other oxygenates were not detected, consistent with a trend established during seven previous quarterly sampling events (Table 1A) ' Dissolved 1,2 DCA was not detected above 1 pg/L Dissolved EDB levels were non-detectable Laboratory analytical reports for the groundwater samples are Included in Attachment 4 1 1 21756g4 rpt