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I 1]7 Lone Palm Avenue,Suite 201 <br /> V i.I Modesto,California 95351 <br /> ATC 209-579-2221 <br /> Fax:209-579-2225 . <br /> r Shaping the Future <br /> locations. No significant rebound effects were observed during the post remediation monitoring <br /> performed on. November 15, 2011. Details of these activities are summarized, in ATC s. Ozone <br /> Treatability Pilot Testing Report,dated June 8,2012. <br /> 2.0 GEOLOGY <br /> 2.1 Regional Geology <br /> Stockton, California is located in the San.Joaquin.Valley, the southern extension of the California Great <br /> Central Valley(Figure 1). The Great Central Valley is a deep alluvial plain extending nearly 500 miles.from <br /> the Siskiyou Range of the Cascade Mountains in the north to the Tehachapi Range to the south. At Stockton, <br /> fthe San Joaquin Valley is approximately 40,miles wide. The western Valley boundary consists of the low, <br /> rolling foothills of the California Coast Range Mountains (maximum elevation due west is less than 3,000 <br /> feet) and the eastern boundary consists of the more rugged foothills of the Sierra,Nevada (maximum <br /> r elevation due east is over 10,000 feet). <br /> L The Great Central Valley contains a thick.sequence;of sediment,.which, in places, reaches a depth of ten <br /> miles. These sediments range in geologic-age from Jurassic (205 million years before present) to Recent <br /> (present time) and include both marine and continental deposits. The site's immediate subsurface geology <br /> consists of Pleistocene and Recent alluvial deposits. These alluvial`deposits consist of heterogeneous <br /> sequences of sand and gravel originating from active stream channels, and silt and clay originating from <br /> overbank and marsh depositional environments. <br /> Recent Deposits <br /> Recent alluvium consists of sediments deposited on top of the.erosional surface that was formed near the end <br /> of the last glacial period. Recent, deposits are generally composed of alluvial deposits consisting of <br /> unconsolidated gravel, sand, silt, and clays. The subsurface lithology beneath the study area generally <br /> consists of silt with subordinate lenses of sand and clay to depths ranging from the ground surface to <br /> approximately 43 to 48 feet,bgs. A laterally extensive sand unit appears to be present beneath the entire study <br /> area, with the exception':of the location sof MW 10. The sand. unit was typically encountered from <br /> approximately 43 to 48 feet bgs and extended to the terminal depths explored(55 to 57 feet bgs): <br /> 3.0 HYDROGEOLOGY <br /> 3.1 Regional Hydrogeology <br /> The site is located within.the Great Central Valleygeomorphic.province. Groundwater in the Stockton <br /> area is present in Recent alluvium. Groundwater enters the Great Central Valley via: surface and <br /> subsurface flow and by direct percolation of precipitation; stream. flow, and applied water. <br /> Replenishment of the groundwater occurs by percolation of precipitation, stream flow,and applied water <br /> occurs in the forebay areas <br /> where permeable sediments are exposed at ground surface. <br /> According to the United States Geological Survey-(USGS) 7.5-Minute Series Topographic.Map of the <br /> Stockton West, California Quadrangle, the site is.located at an'approximate elevation of 24 feet above <br /> L <br /> i <br /> S:Enviromental\22493Uteports\Eggiman Closure Report.doc 4 <br /> c <br />