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1 First Quarter 2002 Monitoring and Sampling Report 2 7-Eleven, Inc, Store#14117 2725 Country Club Boulevard,Stockton, CA April 25,2002 ' The dissolved MTBE plume is currently delineated to the south (downgradient) and the west (crossgradlent) ' Geology/Hydrogeology The site is located In the northern part of the San Joaquin Valley at an elevation of approximately 1 foot above mean sea level Shallow stratigraphy in the area consists of Quaternary alluvial/deltaic deposits Materials encountered during previous drilling at the site generally consist of interbedded clayey fine sand, silty clay, and clayey silt, with a laterally-continuous layer of silty to very fine sand encountered from about 18 to 28 feet below grade In some of the wells, fine-to medium-grained sand is encountered from 38 to ' 51 feet below grade (the maximum drilling depth) The sand units are generally fresh-water-bearing east and southeast of Stockton, but increasingly brackish or saline water has been reported westward The saline and brackish-water/fresh-water boundary trends north to south across the western part of Stockton The brackish/saline water west of this boundary is often overlain by a layer of fresh groundwater to about 50 feet below grade, therefore, shallower groundwater monitoring wells may encounter fresh water on both sides of the boundary Historically, groundwater has been encountered at the site at about 10 feet below grade, static levels are at similar depths Historic flow gradients have fluctuated, with flow directions ranging between to the south and to the northwest The shallow groundwater and proximity of some of the monitoring wells to the former and current UST pits are probably responsible for local perturbations and fluctuations in ' groundwater flow direction, which may not reflect regional flow direction ' Sensitive Receptors A formal 2,000-foot sensitive receptor investigation (to identify all municipal and/or domestic wells in the area) has not been conducted for this site and is scheduled to be completed prior to the end of June 2002, in accordance with SJCPHS requirements (letter dated May 14, 2001) Previous well search results (Fluor Daniel GTI, Onsite and Offsite Assessment Report, November 30, 1998) indicate that there are few active wells within a 2,000-foot radius of the site, and that most wells included in the list are perforated at depths of about 100 feet below grade or deeper No public water supply wells are presently known to be located within one mile downgradlent or crossgradient from 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 Quarterly results and historical data are presented in Table 111A, respectively A 1 potentiometric surface map was generated using quarterly top of casing elevations and DTW data (Attachment 1, Figure 2) Based on the monitoring data, the estimated groundwater flow is southwesterly ' at a gradient of about 0 008 foot per foot Field data sheets are included in Attachment 3 ' 14117Q21doc