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

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RULE 2 PERMITS 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. The Trout Creek Sandstone was dry in monitoring well C -05 -33 in the southern part of the Collom permit expansion area (Map l OB) but was saturated at wells DH -76 -8 about a half -mile northeast of the proposed Collom Lite pit and W -95 -02 about two miles east of the proposed Collom Lite pit. At the latter two locations, groundwater in the Trout Creek Sandstone is present under confined (artesian) conditions. At well DH -76 -8, the top of the aquifer is at approximately 6,200 ft amsl and the potentiometric approximately 7,160 ft amsl. At well W- 95 -02, the top of the aquifer is at about 6,150 ft amsl and the potentiometric surface is above the land surface elevation of approximately 6,780 ft amsl. Farther north and downdip, near the Little Collom X pit, the top of the aquifer is at elevations between about 5,500 and 5,600 ft amsl (Reheis, 1981; Brownfield et al., 2000). Along the southern reaches of the stream channels, the deeper units of the Williams Fork Formation, down to the I coal sequence, outcrop. The stream channels cut through successively higher stratigraphic units from south to north on the southern limb of the Collom Syncline. The valleys provide drainage for the shallower coal and sandstone units. As a result of these conditions, the southern portion of the groundwater basin consists of a sequence of perched groundwater zones. In general, the groundwater levels occur within the respective water - bearing units. Conditions in the southern portion of the Collom permit expansion area are therefore similar to those at the existing Colowyo mine. Groundwater is present as perched zones in the upper coal seams and sandstone units, and seeps are typically associated with perched water tables in the uppermost coal seams. At the existing mine, these seeps normally drain within a few weeks, although sustained seeps recharged by upgradient drainages also occur, with a total discharge of approximately 15 to 25 gallons per minute (gpm). With increasing depth below the elevations of the adjacent stream channels, the bedrock units become fully saturated and groundwater is present under confined conditions. In the northern portion of the proposed Collom Lite pit, the units are continuously saturated below a depth of approximately 300 ft below ground surface. This generally includes the F sandstone and lower stratigraphic units. Hydraulic Characteristics - Previous studies by CDM (1985a), Dennis (2001, 2006) and WMC (2005) determined the hydraulic characteristics of the bedrock aquifers in the Williams Fork Formation and the Trout Creek Sandstone. The results of these studies are presented in Table 2.04.7 -39, Table 2.04.7 -40 and Figure 2.04.7 -33 and are discussed below. The valley -fill aquifers have moderate to high permeability where encountered and exhibit a wide range of hydraulic conductivity values. They exhibited the highest geometric mean hydraulic conductivity (3.2 feet per day (ft/day)) but also showed a very wide range (0.001 to 196 ft /day). Because of their limited thickness, however, they typically have relatively low transmissivity. Storativity was not determined by the testing, but values in the range typical for unconfined aquifers can be expected. The most permeable bedrock units are the F/G sequence of the Williams Fork Formation (F sandstone through G coal seams) and the Trout Creek Sandstone. The sandstone units of the Williams Fork Formation typically have lower permeabilities than the coal seams. The coal seams have hydraulic conductivities ranging from approximately 0.002 to 0.77 ft/day and a geometric mean of 0.14 ft /day. The highest hydraulic conductivities were observed in the Fab and G seams. The sandstone units have geometric mean hydraulic conductivity values ranging from approximately 0.002 ft /day (H sandstone) to 0.008 ft/day (F sandstone). The geometric mean hydraulic conductivity for all the sandstones of the Williams Fork Formation is 0.006 ft/day. The Trout Creek Sandstone exhibited a range of hydraulic conductivity values between 0.006 and 0.09 ft/day and a geometric mean of 0.02 ft/day, indicating that it Collom — Rule 2, Page 37 Revision Date: 9/28/11 Revision No.: PR -03