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Taj Bahadori Page 4 of 9 December 23,2014 The Mehrten is moderately to well indurated and is approximately 400 ft thick in eastern surface outcrops to over 600 ft thick in the subsurface near Stockton. It is reported to be 1,300 ft thick at McDonald Island in the central part of the Central Valley. (California Department of Water Resources, 1980, Ground Water Basins in California, Bulletin 118). In many areas, basal deposits of the Mehrten can contain reworked detritus of the Valley Springs and older formations. The dark gray to black sandstones are very widespread and compose approximately 50 percent of the Mehrten, as evidenced by the material descriptions on the boring logs for the monitoring wells installed at the landfill. Well-cemented siltstones and claystones of the Mehrten are generally gray to dark brown. The Mehrten Formation unconformably overlies the Valley Springs Formation (TVs) also shown on the Geologic Map. The Valley Springs Formation is composed of andesite and basalt lava flows with lahars.In general,the clayey layers of the Valley Springs Formation are greenish in color. Since no greenish colored clayey layers have been described in the boring logs for the monitoring wells, it is unlikely that the Mehrten Formation has been penetrated by the existing site wells. Geologic Cross Sections To help deduce the hydrogeologic conditions that underlie the Foothill site,two geologic cross sections were prepared using the boring logs for the monitoring wells installed at the site. The location of these cross sections is shown on Figure 1. Cross Section A-A' generally runs from north to south across the site including monitoring wells MW-4, MW-3, MW-2R and MW-5 as shown on Figure 2. Cross Section B-B' generally runs east to west across Module I including monitoring wells MW-IA and MW-3 as shown on Figure 2. These two geologic cross sections generally portray a typical view of what would be expected for the Mehrten Formation. At the Foothill Landfill, as shown on these cross sections, the Mehrten contains a high percentage of continuous sandstone units with thinner and less continuous claystone and siltstone units.The upper subsurface of the site is generally clayey in texture. There appears to be a thin relatively continuous claystone unit between elevations 280 to 290 ft, mean sea level (msl) and another thicker relatively continuous claystone unit between elevations 90 to 150 ft,msl. Groundwater is encountered at approximately elevation 50 ft, msl. All of the monitoring wells are screened in the same continuous sandstone unit, which is encountered below elevation 100 ft,msl. Considering that the Mehrten is approximately 600 ft thick in the Stockton area, it is likely the same thickness at the Foothill site. Since the existing wells at the site have penetrated approximately 350 ft of Mehrten,it is also likely that the underlying Valley Springs Formation will not be encountered above an elevation of approximately -200 ft, msl. However, considering that the Valley Springs Formation is also derived from andesitic volcanic debris, the water bearing characteristic will likely be similar to the Mehrten. Although the likelihood for water levels to drop into the Valley Springs at the Foothill site are remote and would likely MW-6 Design.docx