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1117 Lone Palm Ave,Suite 201 ppp Modesto,CA 91 (YVJ%TCPhone:209-579-2221 Fax: 209-579-2225 L.� h 2.2 Stratigraphy Stockton, California is located in the San Joaquin Valley, the southern extension of the California Great Valley Province (Figure 1). The Great Valley is a deep alluvial plain extending nearly 500 miles from the Siskiyou Range of the Cascade Mountains in the north to the Tehachapi Range to the south. At Stockton, the San Joaquin Valley is approximately 40 miles �. wide. The western Valley boundary consists of low, rolling foothills of the California Coast Range Mountains (maximum elevation due west is less than 3,000 feet) and the eastern boundary consists of the more rugged foothills of the Sierra Nevada(maximum elevation due east is over 10,000 feet). The Great Valley contains a thick sequence of sediment, which, in places, reaches a depth of ten miles. These sediments range in geologic age from Jurassic (205 million years before present)to Recent (present time) and include both marine and continental deposits. The site's immediate subsurface geology consists of Pleistocene and Recent age alluvial deposits. These alluvial deposits consist of heterogeneous sequences of sand and gravel originating from active stream channels, and silt and clay from overbank and marsh depositional environments. 2.2.1 Recent Deposits Recent (Holocene) alluvium consists of sediments deposited on top of the erosional surface that was formed near the end of the last glacial period. Recent deposits are generally composed of alluvial deposits consisting of unconsolidated gravel, sand, silt, and clay. The subsurface lithology beneath the study area generally consists of silt with subordinate lenses of sand and clay to depths ranging from the ground surface to approximately 43 to 48 feet bgs. A laterally extensive sand unit appears to be present beneath the entire study area, with the exception of the location of MW 10. The sand unit was typically encountered from approximately 43 to 48 feet bgs and extended to the terminal depths explored(55 to 57 feet bgs). 2.3 Investigation Results The following paragraphs summarize the results of the subsurface investigations performed at the project site. 1. The vadose zone across the center of the site, from 0 to approximately 40 feet bgs, generally consists of silt to approximately 22 feet bgs and is underlain by a layer of clay from 22 to 27 feet bgs, more silt from 27 to 39 feet bgs, an apparently perched sand layer from 39 to 42 feet bgs; silt from 42 to 50 feet bgs, and an extensive sand layer below 50 feet bgs. 2. Groundwater in the uppermost water bearing zone appears td be semi-confined. Groundwater flow in the shallow zone beneath the site tends to be somewhat variable s:\environmental\22493\reports\fcasibility.doc 4