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3 First Quarter 21102 Wnitonng and Sampling Report Ani 29 2002 16 7-Eleven Inc Store#17647 1048 West Yosemite Avenue Manteca CA Geology/Hyd rogeology The site is located in the central part of the San Joaquin Valley The shallow stratigraphy in the area consists of Quaternary alluvium and lake deposits The materials encountered during recent drilling at the site generally consisted of silty fine sand grading downward to medium-grained sand to a depth of about 15 to 20 feet below grade, with scattered interbedded clay strata ' During drilling, groundwater was encountered at approximately 15 feet below grade Equilibrated groundwater levels are about 13 feet below grade During the current sampling event, depth to water ranged from 14 00 to 14 78 feet below grade Groundwater flow directions are generally westward (ranging from north to southwest), at gradients ranging from 0 0018 to 0 03 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 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 ' (downgradient) 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 potentiometnc 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 southwest at a hydraulic gradient of approximately 0 006 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 1 for the oxygenates MTBE, TBA, DIPE, ETBE, and TAME using EPA method 8260 is 217580h rpt