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Background and Site History <br /> Shell Stockton Terminal <br /> 3515 Navy Drive <br /> Stockton, California <br /> The following sections summarize available historical investigation and remediation activities at this site. <br /> Summaries of historical work that pre-date CRA's management were summarized referencing reports <br /> from Harding Lawson Associates (HLA), Pacific Environmental Group, Inc. (PEG), and RETEC. The <br /> site geology and hydrogeology, previous investigations, and remedial system operations are summarized <br /> below. <br /> Site Geology <br /> The sediments underlying the site are comprised of the Great Valley sedimentary sequence. Near-surface <br /> sediments are characterized as unconsolidated to semi-consolidated, mostly nonmarine, alluvium, lake, <br /> playa, and terrace deposits that are Quaternary in age(CDMG, 1977). <br /> Site investigation activities characterize soils beneath the site as primarily fine-grained silt and clay soils <br /> that average approximately 18 feet in thickness. Coarser-grained(sandy)soils have been observed,but to <br /> a lesser extent. Silty fine-grained sands were encountered to depths of approximately 2 feet below grade <br /> (fbg) at well MW-17, and to approximately 17 fbg at MW-9. The `coarser" sediments consist of fine- <br /> grained sand and silty sand, with minor clay lenses and some interbedded gravel. With the exception of <br /> well MW-9, these coarser grained soils are unsaturated (occur within the vadose zone) and overlie <br /> silt/clay horizons. Saturated coarse-grained soils were encountered in the southern portion of the site, at <br /> depths below 20 fbg in wells MW-23, MW-25, MW-26, MW-27, DW-1, and DW-2. Based on geologic <br /> cross sections prepared by previous consultants it does not appear that the saturated coarse-grained layers <br /> are horizontally continuous. <br /> Hydrogeology <br /> The San Joaquin River acts as a source of recharge to groundwater beneath the site and shows an average <br /> change in tidal flux of 3 feet (Port of Stockton). A tidal study conducted at the site in 1991 reported that <br /> tidal fluctuations of approximately 4.46 feet had a very slight effect (0.11 feet) on groundwater levels <br /> onsite (Harding Lawson Associates, 1991). Saltwater infiltration from the Stockton Deep Water Channel <br /> has been observed in the shallow groundwater beneath the site(URS,2003). <br />