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SECTIONTWO 30 8ackwoof The following section provides a description of the site history, land use, climatology of the site and the surrounding area Summaries of previous environmental investigations are included in this section Information presented in tlus section was used to help evaluate potential chemical exposures and corresponding potential health risk in subsequent sections 2.9 SITE DESCRIPTION AND HISTORICAL USE Shop 10 has been operated as a state highway repair and maintenance yard since approximately I the late 1940s Shop 10 is located adjacent and to the south of Caltrans District 10 headquarters in Stockton, California (Figures 1-1 and 1-2) To the west of the site are the Stockton Fairgrounds, to the north are the District headquarters buildings, to the east is the Caltrans Maintenance Yard, and to the south are open fields Four 5,000-gallon USTs that contained diesel/gasoline were removed from the site in 1989 The predominant structures located on site include a h p sop building, a storage building, and an office building A large shop building in the center of the site, near the former UST location, is currently under construction The shop will be used for equipment maintenance and will have large open bays for vehicles The majority of the site is overlain with b to 10 inches of concrete and asphalt Irrigated landscaping consisting of lawn, trees, and shrubs is located on the eastern side of the property near South B Street 2.2 GEOLOGY AND HYDROGEOLOGY 2.2.9 Soils The USDA classified soil in the vicinity of the site as Fine, Montmorillomtic, Thermic Typic Pelloxerert The soils are vertisols, which indicates they are clayey soils that have cracks at some time of the year and have high bulk densities(USDA, 195 1) The soil is formed from transported material of mixed origin and occupies nearly level or very gently sloping plains Erosion is not a problem, but because of the basin position, some areas are subject to flooding Internal drainage is poor as a result of the heavy-textured soil which becomes plastic and swells when wet, and movement of moisture is further retarded because the substratum is comparatively impervious When the soil is dry it forms cracks When the soil becomes wet the cracks close and the soil is then only slowly pervious to moisture(USDA, 195 1) Based on boring logs from previous investigations, the soil predominately consists of a fill material overlying alluvial deposits The fill soil was generally a sandy gravel to a silty sand and was encountered to depths of 3 to 15 feet below ground surface (bgs) In the boring logs, the soil W0*dwzxWU ft TkIS97 T3009NALSfOCKTOMREPORTISHOREP#1 D0C%&FE8-wj73008NAvsNA 2-1