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III. Environmental Setting, Impacts,and Mitigations J. Geology,Soils and Seismicity prudent to attempt to correlate individual units for distances more than a few tens of feet. However,a generalized description into zones of predominant sediment type has been made (Kleinfelder, 1988). The first distinct unit extends from the surface to about 20 to 40 feet below the surface,and consists of silty clay and sandy or clayey silt,with only minor discontinuous thin interlayers of sand. In the eastern portion of the site,a thick sand lens occurs in the upper clay- silt unit. The second sediment unit is predominantly sand,extending from the base of the first unit to depths of 55 to 60 feet below the surface. The sand unit is underlain by more silt and clay deposits Soils Soils on the existing Austin Road Landfill site are extracted from the borrow pit and used as cover material for the landfill. Soils on the proposed expansion areas are used for agricultural production. The following descriptions of soil characteristics for the project site are summarized from the U.S.D.A. Soil Conservation Service soil survey(SCS, 1992). Dumps This map unit occurs on the area of the existing landfill, and is described as smoothed or uneven accumulations of refuse that cannot support plants unless major reclamation measures are applied. Soil properties, such as permeability,drainage,runoff,effective rooting depth,and available water capacity, vary from one area to another. Jacktone Clay,0 to 2 Percent Slopes The Jacktone clay occurs along the northern border of the proposed expansion area by the CYA facility, and in the southeast corner of the proposed expansion area near Austin Road. This somewhat poorly drained, nearly level soil is in basins,formed in alluvium from mixed rock sources. It is moderately deep to a hardpan. Mottles in the profile indicate a somewhat poorly drained soil;however,drainage has been improved by levees and reclamation projects. A few areas are dissected by intermittent sloughs that have been filled as a result of land leveling. Typically,the surface layer of 22 inches of clay is underlain by 12 inches of clay and clay loam and a three inch thick strongly cemented to indurated hardpan. A weakly cemented hardpan occurs between 46 and 60 inches in the profile. Permeability is slow and shrink-swell potential is high. Depth to the water table is more than five feet,but water may be briefly perched above the hardpan after periods of heavy rainfall or irrigation. The soil is subject to rare flooding,in III.J.2