2012-01-24_PERMIT FILE - M2011054 (4)

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Mr. Alex Schatz December 12, 2008 Page 2 of 11 to excavation. These mine pit locations and extents were provided to us by Banks and Gesso. Based on geotechnical borehole data that has been provided to us, the gravel deposit at the mining site extends 26 -40 feet below the land surface. The current ground water level reportedly ranges from approximately 5 to 11 feet beneath the ground surface elevation. Water produced from the dewatering activities will be routed to nearby irrigation ditches or to the Colorado River. Geology and Hydrogeology The proposed gravel pits are located within the unconsolidated alluvial sediments adjacent to and associated with the Colorado River. The surficial geology at the site was mapped by the United States Geological Survey (USGS) from 1963 to 1966. A geological map showing the surficial deposits was published in 1992 (Donnell et al., 1992). Figure 2, the Geologic Map Alluvial Extent Map, presents the geologic mapping described by Donnell et al. within the subject study area. The map of the alluvial sediments was used to approximately define the boundaries of the valley where the Debeque Mine is located. Though unnamed, an area referred to as the Debeque Area Valley (DAV) was defined. The DAV contains a relatively wide swath of alluvial sediments on the eastern bank of the Colorado River. The maximum width of the DAV is approximately 2 miles. The northern and southern borders of the DAV are defined by narrow canyons where there is minimal alluvium in the canyons through which the Colorado River flows. As described by Donnell et al., the quaternary alluvium at the site includes "stream -bed, low alluvial terrace, and floodplain deposits. Unconsolidated clay, silt, sand, gravel, cobbles and boulders deposited along Colorado River and its tributaries. Well- rounded and well -sorted clasts of crystalline rock are common in alluvium along [the] Colorado River." The alluvial sediments are underlain by the Paleocene (Tertiary) aged Atwell Gulch Member of the Wasatch formation. According to Donnell et al., "the upper part of this formation is composed predominantly of yellowish -gray, yellowish- brown, and black mudstone or claystone and commonly contains carbonaceous shale or mudstone and some coal locally." It is not believed that the Wasatch formation contributes any significant quantity of ground water to the overlying alluvial sediments. However, wells drilled into the Wasatch formation may provide enough water for household or domestic uses. Site - specific hydrogeologic data (hydraulic conductivity and specific yield) for the alluvial sediments in the DAV were unavailable and were therefore estimated based on a range of values provided in the literature and hydraulic conductivity estimates for similar alluvial sediments in Grand Junction, Colorado. In the Ground Water Compliance Action Plan for the Grand Junction, Colorado UMTRA Project Site, •