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{ i Environmental Setting 1 Regional Geology Plant 33 is located just east of the Sacramento-San Joaquin delta, near the margin of the Great Central Valley of California. The valley is a sedimentary basin consisting of a series of nearly flat lying beds of clay, silt,.sand, and gravel. The upper 300 feet of sediments in the vicinity of the site include Recent Alluvium, the Pleistocene Victor Formation, and the Plio-Pleistocene Tulare Formation. These units are laterally and vertically heterogeneous, and it is difficult to distinguish between them in subsurface (California Department of Water Resources, 1967 [CDWR]). Both the Recent Alluvium and the Victor Formation consist of discontinuous beds of sand, gravel, clay, and silt, deposited in a continental alluvial fan environment. Sand and gravel dominate in fan areas, while clay and silt are the dominant_ interfan deposits. As the lithologies of the recent Alluvium and.Victor Formation are similar, these two units are not distinguishable on driller's or geophysical logs. Their combined thickness in the site area is estimated to be between 100 and 150 feet (CDWR 1967). The Tulare Formation is the major source of groundwater in the Stockton area. It consists of semiconsolidated, poorly sorted, discontinuous deposits of sand, gravel, clay, and silt. A regionally extensive clay layer, the Corcoran Clay, occurs near the top of the Tulare Formation in areas west of the San Joaquin River. Local Geology As described in the workplan (CH2M HILL, March 1991), logs of borings drilled within 1/2 mile of Plant 33 indicate that the upper 60 feet of soil is generally silt with sandy intervals. This material is probably Recent Alluvium and/or Victor Formation. Regional Groundwater Occurrence and Flow Plant 33 is located within an area where groundwater is extensively developed by high- capacity wells. Nearly all of these wells are several hundred feet deep and pump water from sands and gravels below about 200 feet bgs. The large withdrawals from these depths have created an extensive cone of depression, as shown on the San Joaquin County Flood Control and Water Conservation District (SJCFCWCD) groundwater level maps (SJCFCWCD, 1989 and 1990). Lateral groundwater flow in this area is often controlled by the locations of the nearest pumping wells. Groundwater typically occurs at depths of 50 to 70 feet below ground surface (bgs). Three groundwater production wells are, operated at Plant 33 and are shown on Figure 1-2. Table 1-1 describes the three wells. Del Monte Plant 33 personnel estimate the following yearly percentage contribution of each well: W-1, 22 percent; W-2, 18 percent; W-3, 60 percent. Table 1-3 presents total Plant 33 groundwater SF0316041IthM.51 1-6 i