2013-04-10_PERMIT FILE - C1981019A (11)

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RULE 2 PERMITS Williams Fork Formation about 200 ft above the Trout Creek Sandstone. It is referred to as the Yampa Bed by Johnson (2008) and is considered equivalent to the KM Layer in the Colowyo mine area. The Williams Fork Formation is not generally considered to be a major aquifer in the region (CDM, 1985a). In the Danforth Hills and the Colowyo revised permit area, the exposures of the Iles and Williams Fork Formations have been dissected by erosion along the stream valleys, and the exposed stratigraphic units are therefore laterally discontinuous. Mixed alluvium and colluvium, referred to as valley fill, is present along the stream valleys in the area. It consists of a mixture of clay, silt, sand, gravel and cobbles with highly variable sorting. The thickness of the valley fill ranges from a feather -edge to many tens of feet, depending on the location and geologic setting. The valley fill generally contains and transmits groundwater The only regional aquifer in the Collom permit expansion area is the Trout Creek Sandstone. However, the portion of the Trout Creek Sandstone in the general area is physically and hydrologically isolated from the Trout Creek Sandstone elsewhere in the region, as explained below. The northern limit of the Trout Creek Sandstone in the general area of the Collom permit expansion area is its outcrop along the northern limb of the Collom Syncline; north of there it has been removed by erosion. To the south and west also, along most of the crest of the Danforth Hills Anticline, the Trout Creek Sandstone has been removed by erosion. On the south, it is absent from the Wilson Creek valley upstream of the southern boundary of the Colowyo revised permit area. On the west, it is absent starting about two miles west of the southwestern side of the Collom permit expansion area. On the east side of the general area, it has been removed by erosion along the upturned margin of the Elkhorn Syncline basin. Except for the Trout Creek Sandstone, the rocks in the general area are primarily delta or swamp deposits of sandstone, coal, mudstone and shale. The discontinuous, lenticular sandstones and the coal beds form local aquifers in some areas, and the finer- grained mudstones and shales form local confining layers. Groundwater movement in the bedrock is largely controlled by the existence of joints or fractures rather than by primary permeability within the rock strata. The typically low permeability and typically discontinuous and lenticular nature of the strata restrict the ability of the bedrock to store and transmit water. In addition to joints and fractures, structural features in the area influence the limited movement and occurrence of groundwater. Groundwater tends to occur in the synclinal axes of the folds in the area as a result of the increased fracturing in these areas and the general movement of groundwater in the down -dip direction. The dissected topography of the area results in the uplands between the stream valleys supporting only isolated, discontinuous and laterally - limited water - bearing zones. Groundwater conditions in the Collom permit expansion area are controlled by a combination of geology and topography. The permit expansion area is located in a groundwater basin generally bounded by the outcrops of the Trout Creek Sandstone. Elevations at the base of the Trout Creek Sandstone range from about 8,000 ft amsl where it outcrops near the southern boundary of the Collom permit expansion area to about 5,500 ft amsl along the axis of the Collom Syncline just west of the northwestern part of the Collom permit expansion area (Utah International, 1975; Brownfield et al., 2000). The water table in this basin varies from about 7,800 ft amsl on the south to about 6,500 ft amsl on the north. The overall direction of groundwater flow in the Collom synclinal basin is from south to north down the dip of the southern limb of the syncline. Eastward or westward flow occurs locally where the units drain into the adjacent stream drainages. Numerous small springs and seeps have been identified in the drainages of the area; however, none of these represents a significant groundwater discharge area. The primary discharge of groundwater from the bedrock units including and stratigraphically above the Trout Creek Sandstone in the Collom permit expansion area is to the valley fill aquifers in the northern reaches of the valleys crossing the area. General Groundwater Quantity — Robson and Stewart (1990) irdicate that aquifers in the upper part of the Mesaverde Group in northwestern Colorado generally product; such small sustained yields, about 0— Collom — Rule 2, Page 34 Revision Date: 9/28/11 Revision No.: PR -03