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George's Service Remedial Action Work Plan Project No.425.2 June 30,2011 The Site is located on the gradually sloped floor of the northern San Joaquin Valley. The geologic composition of the area is characterized as fluvial deposits of the San Joaquin - Sacramento River delta system that overlay continental rocks and deposits derived from erosion of the Sierra Nevada Mountain Range located approximately 50 miles northeast of Modesto. The southwest tilting Siena Nevada fault block underlies the northern San Joaquin Valley area. Overlying the consolidated rocks are unconsolidated sediments. The lower consolidated batholithic and metamorphic rocks are poorly transmissive; however the overlying sedimentary layers are often quite permeable. These unconsolidated layers include Pliocene/Pleistocene age continental deposits, Pleistocene age lacustrine and marsh deposits, and Holocene older and younger alluviums and flood-basin deposits. 3.2. Regional Stratigraphy The following information comes from the United States Department of Agriculture — Soil Conservation Service (USDA). Soil Survey of San Joaquin County, California (McElhiney, M. A., 1992) indicates the area surrounding the site is predominately made up of one general soil type: • Capay—Stomar—Zacharias: A gentle 0 to 2 percent slope; moderately well drained and well drained, moderately fine textured, gravelly moderately fine textured, and fine textured soils that are very deep; in interfan basins and on alluvial fans and stream terraces. • The USDA Soil Survey of San Joaquin County, California (McElhiney, M. A., 1992), states the soils beneath and adjacent to the site are categorized as: o Capay clay: Wet 0 to 2 percent slope; very deep, moderately well drained, nearly level soil is in interfan basins. It formed in alluvium derived from mixed rock sources. Typically the surface layer is grayish brown and dark grayish brown clay about 20 inches thick and the next 60-inches is typically pale brown silty clay loam and clay loam. Slow permeability, high shrink- swell potential, low strength, and high water table. 3.3. Lithology and Soil Characteristics The following summary discusses the subjective field observations and geology documented during various investigations based on the interpretations of the field geologist. • The lithology of the Site is predominantly alternating layers of clay-sand mixtures and sand-gravel mixtures. • The lithology between the surface and 20-feet bgs is predominately clays and sands. • The lithology between 20 and 40-feet bgs is predominately sands and gravels. • The lithology between 40 and 80-feet bgs is comprised of alternating layers of clay- sand mixtures and sand-gravel mixtures approximately 2 to 8 feet in thickness. geologicalTecltatcs6rc. 5 4252 RAP.docx