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IN Environniental, Setting Regional Geology Plant 33 is located just east of the Sacramento-San Joaquin delta, near file margin of the Great Central Valley of California. The valley is a sedimentary basin consisting of a series of nearly flat lying beds he y�[e inc 1ud� sand, e d,n�ndAlgueet of viuel.m' hie upper e Pleistocene tVictor sediments in the vicinity Formation, and the Plio-Pleistocene Tulare Formation. These units are laterally and vertically heterogeneous, difficult Re c urce,,, 1967( CDWR).between them in the subsurface (California Department of Water nuous Bath the Recent Alluvium and the Victor Formation ntal alluvial fan ranmentheSanrl of sand, gravel, clay, and silt, deposited In a and gravel dominate in fan areas,while clay and silt are he dominant interfanhe eotwo As the lithologies of the recent Alluvium and Victor Formation are similar, t units are not distinguishable on driller's or geophysical logs. Their combined thickness in the site area is probably between 100 and 150 feet (CDWR 1967). The Tulare F,,mation 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 day layer, the Corcoran Clay, occurs near the top of the Tulare Formation in areas west of the San Joaquin River. Local Geology r As described in the workplan (CH2M HILI.. March 1991), inns 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 run6waer is extensively developby hih Plant 33 is located within an arectseveralthundred feet deep and peurrfp water where g capacity wells. Nearly all of these wills are 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 flaw 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 in the area. _ -_-_-_-Three- groundwater production wells are operated at Plant 3; and are shown on Figure 2-2• Table 2-1 describes the three wells.- 2-6 e s.2-6 SF03105AP1002.51