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Geological Technics Inc. Page 4 Groundwater Monitoring Report Project No.750.2 April 1,2005 y This report documents the activities performed at the site during the 0 quarter of 2005. 2.0 HYDROGEOLOGIC SYSTEM u 2.1 Regional Structure The COE site is located in the northwest quarter, of the northwest quarter of Section 4, T2S, R9E, related to the Mount Diablo B&M, on the United States Geological Survey (USGS) topographic map, Escalon Quadrangle, California, 7.5 minutes series. A review of the USGS Topographic Map reveals that the elevation of this site is approximately 115-feet above mean seal level (AMSL). 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 Sierra 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. 2.2 Regional Stratigraphy The following information comes from the United States Department of Agriculture — Soil Conservation Service (USDA). Soil Survey of San Joaquin County, California (1992) indicates the area surrounding the COE site is predominately made up of one general soil type: • DELHI — VERITAS — TINNIN: A gentle 0 to 2 percent slope; moderately well drained to somewhat excessively drained, coarse textured and moderately coarse textured soils that are deep to a cemented hardpan or are very deep; on dunes, alluvial fans and low fan terraces. The USDA Soil Survey of San Joaquin County, California (1992), states the area beneath the COE is categorized as Delhi loamy sand with the following description: o Delhi loamy sand—0 to 2 percent slopes. Very deep, somewhat excessively drained, nearly level soil on dunes. It formed in wind-modified alluvium derived from granitic �. rock sources. The average annual precipitation is about 11-inches, the average air temperature is about 60° F.