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

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RULE 2 PERMITS Groundwater Characteristics — The Collom permit expansion area extends from Colowyo's existing western permit boundary along Wilson Creek / Moffat County Road 51 westward to approximately the ridgeline between Collom Gulch and Straight Gulch. The intervening drainages of Collom Gulch, Little Collom Gulch, and Jubb Creek will be affected by mining operations. Colowyo's existing mining operat ;ons exist primarily east of the Collom permit expansion area, in the Taylor Creek, Wilson Creek, and Good Spring Creek drainages, and are described in previous permit documents. The groundwater conditions in the principal drainages within the Collom permit expansion area described below. The descriptions below are modified from WMC's 2005 report to reflect data collected after publication of that report and development of the current mine plan. Hydrogeologic Conditions — In the Colowyo revised permit area, the Williams Fork and Iles Formations are the main stratigraphic units of hydrologic concern. The Williams Fork Formation is up to about 1,200 ft thick and contains 18 different coal groups (A through O, TC, X, and Y). The coal beds are intercalated and interbedded with siltstone, sandy siltstone and carbonaceous shale (Hildebrand et al., 1981; Brownfield et al., 2000). The lowest coal bed proposed to be mined in the Collom permit expansion area is the G seam, which is about 400 ft above the base of the formation. A laterally- continuous, low - permeability clay bed called the Yampa Bed or KM Layer is present about 200 ft above the base of the formation. Immediately below the Williams Fork Formation is the Trout Creek Sandstone of the Iles Formation. Geologic structure along the north side of the Colowyo revised permit area is dominated by the Collom Syncline, an asymmetric fold that strikes generally about N60 °W through the Collom permit expansion area. The beds on the north limb of the syncline dip toward the south at up to 40 °, whereas the beds on the southern limb dip from 2° to 8° to the north. In the Collom permit expansion area, the proposed Little Collom X pit will be located along the axis of the Collom Syncline, and the proposed Collom Lite pit will be located on the southern limb of the syncline. The southern limb of the Collom Syncline is also the northern limb of the Danforth Hillls Anticline, whose axis is about seven miles south of and approximately parallel to the axis of the Collom Syncline. Faulting is not common in the area, and there are very few mapped structures with vertical displacement. A reverse fault about two miles long, with downthrown side on the north and a vertical displacement of about 25 ft strikes N55 °W, passes about 1,000 ft north of the Little Collom X pit ( Reheis, 1981). A similar fault, with about the same strike and displacement, passes through the Little Collom X pit (Dodd & Dodd Consulting, 2006). Reheis (198 1) also shows a normal fault about one mile long, downthrown to the north, with a strike of about N70 °W and small vertical displacement, present about a half mile west of the Little Collom X pit. Two major joint sets in the mine area are nearly vertical and strike N66 °W and N19 °E. Jointing appears to be more pervasive in the sandstone units of the Williams Fork Formation than the other lithologic units and, based on aquifer testing in the area, contributes to directionally- dependent permeabilities. Groundwater Occurrence — Monitoring wells have been established in the valley fill materials, the Williams Fork Formation interburden and coal, and the Trout Creek Sandstone. A summary of the bedrock and valley fill monitoring wells relevant to the permit revision is presented in Table 2.04.7 -38; the well locations are shown on Map l OB with other hydrologic monitoring points and on Map 11 C along with the locations of all known wells and test boreholes within the permit area and adjacent area. Groundwater is found in the Trout Creek Sandstone, within the coal beds, interbedded and lenticular sandstones of the Williams Fork Formation, on the contacts between different lithologic units of the Williams Fork Formation, and in the valley fill along the stream valleys. The primary water-bearing units are the coals. The sandstone units and valley -fill aquifers also transmit water, but on a more limited basis Col om — Rule 2, Page 36 Revision Date: 9/28/11 Revision No.: PR -03