ARCHIVED REPORTS XR0003381 - Laserfiche WebLink

Home Browse Search

Second Quarter 2002 Monitoring and Sampling Report 2 16 7-Eleven, Inc Store#14117,2725 Country Club Boulevard Stockton, CA July 25, 2002 The dissolved MTBE plume is currently delineated to the south (downgradient) and the west (crossgradient) ' A corrective action proposal (Shaw, June 2002) has been submitted to the SJCPHS Remediation will commence within 30 days of agency concurrence with the corrective action proposal ' 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 alluvialldeltaic 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-waterlfresh-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 During 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 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 September 2002 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 bg or deeper No public water supply wells ' are presently known to be located within one mile downgradlent or crossgradlent from the site Groundwater Monitoring Data I 1 Before monitoring, the well caps were opened to allow the water in each well to stabilize to atmospheric t pressure The DTW was measured to the nearest 0 01 foot from the top of the well casing using an interface probe ' 14117Q22 doc