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1 Third Quarter 2002 Monitoring and Sampling Report 3 • 7-Eleven, Inc,store 921756, 853 E Yosemite Avenue Manteca CA October 21 2002 Geology/Hydrogeology ' The shallow stratigraphy in the study area consists of Quarternary alluvium Materials underlying the site consist largely of fine-to medium-grained sand interbedded below about 13 feet bg with clayey silt ' Depth to groundwater is encountered during drilling at about 16 to 19 feet bg During the current sampling event, equilibrated depth to groundwater ranged from 17 12 to 20 57 feet below top of casing (TOC) ' Historic groundwater flow is to the northwest at a hydraulic gradient of about 0 002 foot per foot Sensitive Receptors A well search 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 bg The site is therefore considered to be of relatively low risk to sensitive receptors 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 potentiometric surface map prepared using the quarterly groundwater elevation data is shown in Attachment 1, Figure 2 Based on the groundwater elevation data, groundwater flow is toward the west- northwest at a gradient of 0 0017 foot per foot 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 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 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 Groundwater samples were analyzed for BTEX, MTBE, and TPH-G ' using EPA methods 8021/modified 8015 and for the oxygenates MTBE, TBA, DIPE, ETRE, and TAME using EPA method 8260