ARCHIVED REPORTS_SITE CHARACTERIZATION WORK PLAN

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Unconsolidated strata beneath the Stockton area consist primarily of interbedded clays, silts, and fine grained sands of the Quaternary age flood basin deposits. The flood basin deposits attain a thickness of approximately 1,000 feet in the Stockton area, and were deposited by fluvio-deltaic processes. There is considerable variability in the distribution and percentage of fine- and coarse-grained sediments beneath the STTC facilities. These deposits are interpreted to have been deposited by fluvial (water home) processes as overbank flood plain deposits outside the main channel of the San Joaquin River. The buildup of the sedimentary sequence illustrated on previously submitted geologic cross sections took place by addition of sediments during flood stages when the river overtopped its banks. The percentage of coarser- and finer-grained sedimentary sequences is principally controlled by distance from the main river channel, water depth, and velocity at the time of deposition. Higher water levels and higher flow velocities tend to transport more coarse-grained materials that tend to be primarily sandy. Lower velocity flow regimes are able to transport and deposit only fine-grained sediments consisting principally of silts and clays. The morphology of these sedimentary sequences tend to be ribbon-shaped bodies of coarser-grained sediments enclosed in finer-grained silts and clays. Since the majority of the coarser-grained deposits were deposited in relatively low energy environments, they typically contain a significant amount of fine-grained material in their matrix. The coarse-grained deposits also are discontinuous because they tend to be truncated as flow direction changed. The sand lenses are generally channel-shaped and the primary flow direction during deposition was principally north-south. This observation would suggest that preferential pathways should also trend in a north to south direction. The stratigraphic framework that has evolved from data collected from site investigations shows that the sedimentary sequence beneath the site can be divided into five units based on lateral and vertical distribution of fine- and coarse-grained sediments. The five zones identified beneath the terminals extend eastward and down-gradient of the site and characteristics of each zone are similar. The site stratigraphy is described in the following sections(IT,2001). Regional groundwater flow in the Stockton area is greatly affected by extensive pumping from groundwater production wells to the northeast and east of the Stockton area. This pumping and the resulting cones of depression have created regional flow patterns toward these pumping depressions. Regional flow patterns beneath the Port of Stockton area are to the northeast, and water quality is generally poor due to saltwater intrusion induced by over pumping in the previously mentioned area. As a result, water use from production wells in the Port of Stockton area is generally restricted to industrial and irrigation uses. The hydrogeologic units beneath the site correspond to the previously defined stratigraphic units, and are also described in the following sections. Site Characterization Work Plan,Final, May 18,2012 1007-012.002 2-3