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" variability until the fall of 2003 , and has remained relatively low since. MW-3 (southernmost <br /> well) showed diesel contamination in late 1999, early 2000, motor oil contamination in late <br /> 2001 , early 2002, and has remained free of contamination since. MW-4 (Sidewalk along S . El <br /> Dorado Street) showed relatively high contamination from gasoline and significant levels of <br /> BTEX and oxygenates from late 2000 through late 2003 , when gasoline levels dropped to more <br /> y � " x DGA <br /> moderate levels. However, BTEX and oxygenates did not show similar reductions. <br /> The Environmental Health Department of San Joaquin County issued a letter dated May 22, <br /> 2006, noting that BTEX and 1 ,2-DCA have consistently been detected above the California <br /> primary maximum contaminant levels for drinking water in MW-4, and directing R. Ellis Jr. to <br /> i <br /> submit a Work Plan by August 1 , 2006, to conduct additional investigations at the site to assess <br /> the vertical and lateral extent of impacted soil and groundwater. CTL contracted EI to complete <br /> the necessary work plan. <br />" <br /> 2.0 SITE DESCRIPTION <br /> The subject site is located in the Southern part of the City of Stockton, California (Figures 1 and <br /> 2). The site is an essentially level commercial trucking facility, which contains an office <br /> building, two truck maintenance shops, a truck-washing rack, a small above-ground storage tank <br /> compound, a vehicle fueling facility and a large parking area for tank trailers (see aerial photo <br /> Figure 3A) . . <br /> 2A Geologic Conditions <br /> fi <br /> The site is situated in the northern part of the San Joaquin Valley, in an area underlain by several <br /> thousand feet of terrestrial and marine sedimentary rocks. The northern San Joaquin Valley has <br /> been a lowland area subject to marine intrusion, flooding and incremental sedimentation for <br /> millions of years. The uppermost 1000 feet of sedimentary strata consist of Holocene, <br /> Pleistocene and Pliocene sediments, including the Laguna Formation, the Mehrten Formation <br /> c <br /> p and the Valley Spring Formation. The sediments are interbedded mixed-fluvial and deltaic <br /> r. <br /> sediments, dominated by medium-grained sand and silts. Generally, thin beds of unconsolidated <br /> clayey silt, silty clay and sandy clay are abundant in this upper zone, which are laterally <br /> discontinuous over distances of several tens of feet. <br />. 2 <br /> rl <br />