SITE INFORMATION AND CORRESPONDENCE

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Soil and Groundwater Investigation Report Mountain House Development Site#6 February 25,2010 properties located to the south are in unincorporated San Joaquin County. Nearby development of the area consists of agricultural and light-industrial facilities. Mountain House is a master- planned development where single-family homes are planned or under construction in areas west of the site. 2.4 HYDROGEOLOGIC SETTING The site is in the San Joaquin Valley Groundwater Basin, which is divided into nine sub-basins. The site lies within the Tracy Sub-basin(Groundwater Basin Number 5-22.15), which is defined by the areal extent of unconsolidated to semi-consolidated sedimentary deposits bound by the Diablo Range to the west, the Mokelumne and San Joaquin rivers to the north, the San Joaquin River to the east, and the San Joaquin-Stanislaus county line to the south. The Tracy Sub-basin is adjacent to the eastern San Joaquin Sub-basin to the east and the Delta-Mendota Sub-basin to the south (California Department of Water Resources [DWR], 2003). The Tracy Sub-basin is comprised of continental deposits of late Tertiary to Quaternary age, which includes the Tulare Formation, Older Alluvium, Flood Basin Deposits, and Younger Alluvium. The thickness of these formations ranges from a few hundred feet in the western foothills to about 3,000 feet near the eastern margin of the basin. The Tulare Formation consists of semi-consolidated,poorly sorted, discontinuous deposits of clay, silt, and gravel. The Corcoran Clay is situated near the top of the Tulare Formation, separating groundwater in the basin in what has been reported as two primary aquifers. The upper aquifer is reported at 15 to 250 feet below ground surface (bgs) and the lower aquifer is reported at 600 feet bgs (DWR, 2003). Given the location of the site within the sub-basin, the site is likely immediately underlain by Younger Alluvium, Older Alluvium, and/or Flood Basin Deposits. Data from previous investigations at an adjacent site indicate soils encountered in this vicinity are low-permeability, unconsolidated silts and clays with minor amounts of poorly graded sand and gravel. Groundwater was first observed at depths ranging from 7 to 17 feet bgs during the 2001 site investigation by Condor Earth Technologies, Inc. (Condor, 2001). During the current site investigation, depth to groundwater was observed between 6.5 and 10 feet bgs. Regional groundwater flow-direction information and local topography on the relevant United States Geological Survey(USGS)topographic map indicates that the inferred groundwater flow in the vicinity of the site is to the northeast(DWR, 2006; USGS, 1978). 2.5 PREVIOUS ENVIRONMENTAL ACTIVITIES Environmental investigations have been performed in the site area and at the site; a summary of these site investigation activities and results are included as Appendix B. The RWQCB reviewed the results of previous investigations and the recommendations presented in the July 2008 SGIR(SAIC, 2008), and concurred that additional lateral delineation of affected soil and groundwater was necessary to the northeast(inferred down-gradient direction) and between the affected areas at the site, as well as to the southwest(inferred up-gradient direction) of the site (RWQCB, 2008a). It should be noted that the results of the previous investigation indicated that there were three affected areas at the site. Based on the results of the 2009 investigation, affected Area 3 was combined with Area 2 resulting in two remaining affected areas, as shown on Figure 3. 3 Fmin Science to Solutions