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114� S E C 0 R 2.0 SITE BACKGROUND AND CHARACTERIZATION This section discusses the site description, geology and hydrogeology, and results of previous environmental investigations at the Site (URS, Corporation [URS], 2006; Delta Environmental Consultants, Inc. [Delta], 2000), as presented in the following subsections. 2.1 SITE DESCRIPTION The Site is currently operating as a gasoline station, with AM/PM mini-mart. It is surrounded by E. Louise Avenue and S. Harlan Road, east of Interstate 5 (Figure 1). Current site structures (including three double-walled 12,000-gallon fiberglass underground storage tanks [USTs], associated piping, and one dispenser island), and locations of three monitoring wells MWA to MW-3 are depicted in Figure 2. 2.2 GEOLOGY The Sacramento Valley is part of the Great Central Valley (Central Valley) geomorphic province, located in a large structural trough in central California (U.S. Geological Survey [USGS], 2006). The Central Valley is bounded on the south by the Tehachapi and San Emigdio Mountains, the east by the Sierra Nevada, the north by the Klamath Mountains, the northeast by the Cascade Range and the west by the Coast Range. It is a structural trough about 400 miles long and from 20 to 70 miles wide and extends over more than 20,000 square miles. The trough is filled to great depths by marine and continental sediments, which are the results of millions of years of inundation by the ocean and erosion of the rocks that form the surrounding mountains. Sand and gravel beds in this great thickness of basin-fill material form an important aquifer system (USGS, 2006). The basin drains into the San Francisco Bay by the Sacramento and San Joaquin Rivers. The structure of the Central Valley is controlled by its tectonic evolution. As subduction of the Kula Plate ended during the Eocene, the formation of the San Andreas Fault became the major tectonic influence. Subsequently, numerous folds flanked by normal and reverse faults formed throughout the Central Valley. Central Valley depositional settings have ranged from predominantly marine to fluvial in origin. The Cretaceous ushered in seas that lasted until the Pleistocene. Uplift of the Coast Range from the Pleistocene to present caused the inland sea to withdraw. Following evacuation of the marine environment, freshwater swamps and lakes formed which were eventually replaced by the current system of rivers, deltas, and alluvial fans. 2.3 STRATIGRAPHY Based on the soil boring logs, the Site is predominantly made up of sand, with about 10-25 percent of fines (considered as loamy sand [USEPA, 2004b]) from ground surface to approximately 26 feet below ground surface (bgs), the total depth of all borings (Delta, 2000; URS, 2006). Clayey silt was 2