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a Fourth Quarter 2002 Mondonng and Sampling Report 2 7-Eleven Inc Store No 20632,4627 DaVina Drive,Stockton, CA December 17 2002 Additional lateral and vertical assessment of dissolved MTBE and hydrocarbons has been conducted as required by the SJCPHS There are currently seven site-related monitoring wells Most recent assessment activities Included the April 2002 installation of three onsite monitoring wells (MW-4R, MW-6, and MW-7), two dual-nested soil vapor extraction/air sparge (SVEIAS)wells (SPV-1 and SPV-2), and vertical soil delineation boring SB-1 { j Groundwater data since 1998 Indicate concentrations of dissolved MTBE at levels of up to 31,000 u micrograms per liter (ug/L), with highest levels typically observed near the UST/pump island areas The most recent sampling data indicate additional delineation is needed downgradient(south) of the existing well set Additional offsite drilling on private property downgradient from the site is planned for early 2003 to complete lateral plume delineation as required by the SJCPHS Geology/Hydrogeology a The relatively flat site is located at an elevation of approximately 2 feet above mean sea level Quaternary alluvial deposits underlay the site Materials encountered during previous drilling at the site generally consisted of silts and clays to maximum exploratory drilling depths to 21 5 feet below grade (bg) Historically, groundwater has been encountered at depths ranging from 10 to 15 feet bg During the current sampling event, equilibrated depth to water ranged from 6 73 to 7 95 feet below top of casing LW (TOC) Historical groundwater flow has been to the south-southwest The potentiometric surface for the quarter is shown on Figure 2 Sensitive Receptors Previous reports Indicate that there are no public water supply wells located within one mile 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 potentiometric surface map prepared using the quarterly groundwater elevation data is shown on Attachment 1, Figure 2 Based on the groundwater elevation data, groundwater flow is toward the south- southwest at an approximate hydraulic gradient of 0 003 foot per foot Monitoring data from the two recently Installed monitoring wells MW-6 and MW-7 indicate that the groundwater flow direction is more southerly than estimated prior to installation of these two wells Quarterly groundwater elevations for the quarter and historical monitoring results are presented in Tables 1 and 1A, respectively (Attachment 2) Field data sheets are included in Attachment 3 20632Q24 QMR DOC