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L A S S O C I A T E S I N C . The near surface of the geology underlying the site is comprised of unconsolidated alluvial deposits of Pleistocene to Holocene age. These deposits, sometimes referred to as "older alluvium", consist of intercalated beds of gravel, sand, silt, and clay. Underlying the older alluvium are PliolPleistocene continental deposits of similar derivation and lithology. The older alluvium functions as the most important in the site area (USGS Professional Paper 1401-C, 1986). Soils developed on the alluvium are generally well drained, differing from the parent material only in the increased volume of organic matter(DWR Bulletin No. 146, 1967). imw In the extreme western part of the San Joaquin Valley and along most of it's western flank, the continental deposits make up the Tulare Formation, which in many places is overlain by younger deposits. This formation outcrops as a thin band in the lower foothills of the Coast Range and ` plunges rapidly into the subsurface. It is the major reservoir for subsurface pumpage of water for the west side of San Joaquin County. The formation dips in the order of 15 to 20 degrees into the subsurface from its outcrop area and flattens rapidly in the vicinity of Tracy. It varies in thickness at the outcrop, to approximately 1,400 feet in the thickness area of the delta (DWR Bulletin No. 146, 1967). .. A regionally extensive clay layer, termed the Corcoran Clay of the Tulare formation occurs near the top of the Tulare formation and confines the underlying fresh water deposits. The thickness of �.�,. the Corcoran Clay in the Tracy area varies from near zero to about 150 ±feet. Its eastern limit in San Joaquin County is about at the San Jaquin River(DWR Bulletin No. 146, 1967). The USGS has estimated the Corcoran Clay in the Tracy area lies approximately 200 to 250 feet bgs. V The Tulare formation is moderately permeable in both its confined portion below the Corcoran Clay and unconfined above the Clay. Most of the larger irrigation wells, the industrial and the municipal wells obtain their water supply from below the Corcoran Clay. The small domestic wells often obtain their water supply from above the confining layer (DWR Bulletin No. 146, 1967). The water quality differences above and below this confining layer are discussed in further detail later in this report. A review of soil boring logs of boreholes completed at the site indicates the upper ten feet of the site's soil column consists of clay. A predominantly sandy water-bearing unit is present at approximately ± 10 feet to ± 20 feet bgs. This sandy unit appears to be laterally continuous across the site and may be a horizontal preferential flow path for groundwater beneath the site. Below the sand unit is an additional clay unit. A completed Water Well Drillers Report of the on- site domestic well, which was submitted to California Department of Water Resources (DWR), is indicates clay is present from 22 to 51 feet bgs; gravel from 51 to 70 feet bgs; clay from 70 to 131 feet bgs; and gravel from 131 to 139 feet bgs, the maximum depth explored at the site. Hydrogeologic cross-sections have been prepared exhibiting the sites underlying geology. Cross section orientations are shown on Figure 3. Cross sections of the site following an east-west and north-south axis are shown on Figures 4 and 5, respectively. wA95122\reports\ReWest for NFAdm 4 r