2021-09-03_PERMIT FILE - C1981019A (8)

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RULE 2 PERMITS The monitoring program described in the WMC (2005) report was continued through May 2006 to complete the prescribed baseline data collection. Monitoring of water levels, sampling to determine groundwater quality, and aquifer testing conducted during these studies provide a basis for identifying detailed hydrogeologic conditions as described in the following sub -sections. 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 operations 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 1,700 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 to be mined in the Collom permit expansion area is the Gb seam, which is about 460 ft above the base of the formation. A laterally -continuous, low -permeability clay bed called the Yampa Bed or KM layer is present about 165 ft above the base of the formation. Immediately below the Williams Fork Formation is the Trout Creek Sandstone Member 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 Collom 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 Hills Anticline, whose axis is about seven miles south of and approximately parallel to the axis of the Collom Syncline. 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 Collom Mine are presented on Table 2.04.7-38; the well locations are shown on Map l OB. 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 than the coal seams. In the saturated portion of the Williams Fork Formation, water levels vary from unit to unit, due to limited vertical movement through the lower -conductivity interbeds. Saturated valley fill also occurs in each of the main drainages including Straight Gulch, Collom Gulch, Little Collom Gulch, and Jubb Creek. Collom — Rule 2, Page 23 Revision Date: 4/7/17 Revision No.: RN -07