SITE INFORMATION AND CORRESPONDENCE

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No Further Action Required Request Report ' Mountain House Development Site#6 September 15,2011 2. Site geology and hydrogeology. 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-Stani slaus 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 (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 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 2008 and 2010 site investigations, depth to groundwater was observed between 4.5 and 10.2 feet bgs. Regional groundwater flow-direction information and local topography on the relevant United ' States Geological Survey (USGS)topographic map indicate that the inferred groundwater flow in the vicinity of the Site is to the northeast (DWR, 2006; USGS, 1978). 3. Sensitive potential receptors including water supply wells and surface water. SAIC performed a search of DWR files in 2008 to identify existing water-supply wells within a 2,000-foot radius of the Site. The search covered Township 2 South, Range 4 East, Sections 2, 3, 10, and 11. SAIC identified seven domestic water-supply wells within the search area. Five of the wells are located northwest and cross gradient of the Site. The remaining two wells are located southeast and cross gradient of the Site. ' Affected groundwater at the Site has only been identified in the shallow water-bearing zone, typically found at 4.5 to 10.2 feet bgs. There are no water-supply wells within 2,000 feet of the Site in the down-gradient direction. A summary of the water well search results is provided in Table 2 and the identified well is shown on Figure 1. As all of the wells identified (with the exception of the well owned by Arnold and Joan Fish [trap 1D 7]) have sanitary seals to at least 50 feet bgs, it is unlikely that the wells are affected by crude-oil residuals. The well construction report provided by DWR for the well owned by Arnold and Joan Fish did not include information about the depth of the sanitary seal. SAIC sampled two of the wells located northwest (approximately 1,500 feet cross gradient) as part of investigations at Mountain House Development Site #3 for analysis of COPCs, including the well owned by Arnold and Joan Fish. 7 SAIC 1