2002-07-26_PERMIT FILE - M2002052 (5)

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Attachment B - Hydrostrstigraphic Information Proposed lrvo Rivers Mine Site Reeional Geoloeic Setting The proposed Two Rivers Mine site is located in valley fill alluvium associated with the Big Thompson River. The alluvium in the Big Thompson Valley represents several ages of river deposition during the Quaternary Period (Barnett and Ellefsen 2000). Alluvial deposits near the project site have been categorized into several types of material, all composed of unconsolidated sand and gravel with varying amounts of clay and silt. Alluvial material in the valley is generally from 0 to 30 feet thick and typically overlain by organic matter and weakly developed soil. The valley alluvium is underlain by the Pierre Shale Formation, or transition beds from the underlying Pierre Shale to the overlying Fox Hills Sandstone Formation (Barnett and Ellefsen 2000). The underlying beds aze composed of material that ranges from soft siltstone/claystone to friable sandstone and soft, shaly sandstone. Site Conditions Information on the subsurface conditions for the project site was derived from boring logs and samples collected from solid stem auger exploration holes (Temdcon, 2002). The exploration holes were drilled in November 2001 by Tetracon geologists to define sand and gravel reserves for the proposed mining operation. The drilling included a total of nine (9) holes spaced throughout the proposed project site. The exploration drilling also included lithologic descriptions, sieve analyses of representative samples, and water level measurements. The exploration borings were used to construct a conceptual hydrogeologic model of the site. For the purpose of characterizing the hydrogeologic conditions for the proposed site, the subsurface materials can be subdivided into three units: a sandy sih overburden, permeable unconsolidated sand and gravel deposits, and low permeability bedrock. Bedrock is covered by alluvium throughout the property. In general, the thickness of the alluvium thins from approximately 30 feet to 10 feet from the southeast to the northwest across the site. Local fine- grained silt or gravel and cobble canes occur as lenses or discontinuous beds within the alluviurr~ but none of the layers appear to be laterally corninuous. Therefore, the subsurface information suggests that the alluvium can conceptually be considered one aquifer. Sieve analyses for three (3) exploration borings were used to evahrate the general grain size distributions, and range of materials within the alluvium below the approximate water table. All of these holes extended completely through the alluvium. The saturated alluvium consists of predominantly medium to coarse sand with variable amounts of gravel and fines. The saturated alluvium in the three evaluated borings contained between 57-91 percent sand, 5-31 percent gravel, and 3-11 percent fines. Overall, the samples indicate that the characteristics of the alluvium aze somewhat variable across the site, ranging from poorly graded sands to poorly graded sands with sih and gravel The borings extended through the alluvium and bottomed in several feet of moist, grey, sihsone/claystone. This clay layer is interpreted to be the transition bed of the Pierre Shale Formation. The description of the material suggest that the top of bedrock behaves as an aquitazd (low permeable barrier) for the overlying alluvial aquifer. 1