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2.3 Geology and Hydrogeology <br /> Vernalis is located within the Central Valley Physiographic Province of <br /> California. The valley is approximately 40 miles wide and is bounded by the Coast Ranges to <br /> the west and the Sierra Nevada Range to the east. In the vicinity of Vernalis, unconsolidated <br /> Quaternary alluvial and fluvial sediments are present to a depth of at least 400 feet (WHF, 1996). <br /> Most of these sediments were deposited in alluvial fans associated with rivers draining the Sierra <br /> Nevada Range. Coarser grained soils are associated with stream channel and natural levee <br /> deposits, and fine-grained soils are associated with floodplains and marshes. These fine-grained <br /> soils are associated with rivers extending along the axis of the Central Valley (WHF, 1996). <br /> The soil underlying the site is composed primarily of silty sand (GTI, 1995). Soil <br /> boring data show that a sandy gravel layer was encountered beneath the silty sand at an <br /> approximate depth of 25 feet below ground surface. No groundwater information such as the <br /> groundwater depth, flow direction, and gradient has been obtained at this site to date. During <br /> GTI's investigation, saturated soils were encountered at a depth of 33 feet below ground surface. <br /> Preliminary Site Investigation Work Plan-Final July 9,1998 <br /> Roberts Property(Former Vernalis Pump Station) <br /> Vernalis,California <br /> 4 <br />