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SECTIONTWO Physical Setting <br /> This section provides a general physical setting of the subject property and surrounding areas. <br /> 2.1 TOPOGRAPHY <br /> The site is located in Tracy, California. Based on the USGS 7.5-Minute topographic quadrangle <br /> for Tracy, California, the site and surrounding area is relatively flat. The topography across the <br /> site ranges from approximately 110 feet to 115 feet above mean sea level (msl) from north to <br /> south. The overall topographic gradient is to the north. The subject property is not located <br /> within a 100-year floodplain (ERM 2000). <br /> 2.2 GEOLOGY <br /> The surficial geology in the vicinity of the subject property is mapped as Quaternary alluvial fan <br /> deposits consisting of sands, silts, gravels, and some clay deposits. The subject property is <br /> located in the Great Valley geologic province, which is characterized by a large northwest- <br /> trending valley. The valley floor ranges from below sea level to approximately 400 feet above <br /> sea level. The Great Valley province is underlain by a thick (up to 80,000 feet) sequence of <br /> sedimentary units (the Great Valley Sequence) that is Jurassic in age or younger. The valley is <br /> an asymmetrical synclinal trough with a more gently dipping eastern limb. The only drainage <br /> from the province is into San Francisco Bay and through the Golden Gate (Oakshott, 1978). <br /> Based on CPT data and information from the on-site soil borings, the lithology underlying the <br /> subject property generally consists of gravelly sands interbedded with sandy silts and silts. <br /> 2.3 HYDROGEOLOGY <br /> Local hydrogeology consists of river flood plains and channels. The unconsolidated deposits <br /> consist chiefly of alluvium, but in some areas include widespread lacustrine and marsh or <br /> estuarine sediments. The San Joaquin River and the Sacramento River drain the Great Valley <br /> Province. The San Joaquin Valley (to the south) and the Sacramento Valley (to the north) make <br /> up the Great Valley province. Tracy lies between the two valleys, in the flood plains of the <br /> rivers that drain them (California Division of Mines and Geology [CDMG], 1966). <br /> Groundwater at the subject property is encountered at approximately 70 feet below ground <br /> surface (bgs). The general groundwater flow direction is north/northwest, which is consistent <br /> with regional flow directions. <br /> UMVADow\Celotex\Soil Gas WP\So11 Gas WP.doc 2-1 <br />