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t ' Third Quarter 2002 Morntonng and Sampling Report 2 7-Eleven, Inc,Store#14117,2725 Country Club Boulevard,Stockton, CA October 23 2002 The dissolved MTBE plume is currently delineated to the south and the west A corrective action plan (CAP) (Shaw, June 2002) was submitted to the SJCPHS Agency response Indicated that completion of lateral delineation to the east and north of the site will be required prior to CAP review by the SJCPHS In addition, site hydraulic parameters applied In the fate and transport evaluation submitted with the CAP need to be further verified (previous data were based on llthologlc determination and literature values for hydraulic parameters) A synopsis of regional assessment data Including Investigations north and east of the site pursued by the neighboring service station facility Is In preparation for submittal by January 2003 Included will be a more detailed hydraulic parameter evaluation relative to the fate and transport model Geology/Hyd rogeo logy 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- grained sand, silty clay, and clayey silt, with a laterally-continuous layer of silty to very fine-grained sand encountered from about 18 to 28 feet below grade (bg) In some of the wells, fine-to medium-grained sand is encountered from 38 to 51 feet bg (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 bg, 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 bg, static levels are at similar depths iDuring the current sampling event, equilibrated depth to groundwater ranged from 6 82 to 7 47 feet below top of casing (TOC) Historic flow gradients have fluctuated, with flow directions ranging from the south to the east, to the northwest The fluctuation of flow directions may be due to tidal action, thus the net gradient may be near zero The shallow groundwater and proximity of some of the monitoring wells to the former and current UST pits are likely responsible for local perturbations and fluctuations in groundwater flow direction, which may not reflect regional flow direction i 14117Q23 doe