2011-08-09_REVISION - C1981019 (10)

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• • • RULE 2 PERMITS to resaturate the pit backfill from precipitation recharge and surface water seepage alone would be about 100 years. Calculation of Annual Seepage of Spoil Water Into Downdip Pitwalls in the Little Collom X and Collom Lite Pits The two mining areas planned by Colowyo are located on the Collom Syncline. The Little Collom X mining area is located above the synclinal axis of the syncline and is transected by the axial trough of the Collom syncline. Collom Lite is located on the southern flank of the syncline. Their location and the geologic environment impact the ground water conditions from the pits currently, during, and after mining of the coal. The geology of this area is comprised of discordant beds of sandstone, siltstone and mudstone with coal seams of varying thicknesses. These are all part of the Williams Fork formation. The beds and seams have a dip similar to the slope of the topography. The area does not support any significant water bearing zones above the saturated water table. The saturated water table has been identified as usually being in excess of 100 feet in depth and more commonly several hundred feet in depth. The Little Collom X mining area is mining the X34 seam (and the Y25 as geologic material above the X34 is exposed). This seam is exposed on both sides of the Little Collom. The current mining scenario plans to remove overburden and oxidized coal and then mine fresh coal. This mining will continue until there are benches on each side of Little Collom. Then each area will be mined with a high wall miner. When the mining is complete the area will be reclaimed. This entire mining sequence will take up to 3 to 4 years. The mining area will have highwalls created during the rock removal above the X34 seam. These highwalls will be backfilled and the area reclaimed to the proposed post mine topography. The floor of the Little Collom X mining area is several hundred feet above any saturated water zone, based on exploratory drill holes in the area. No springs or seeps have been identified in the area. In addition, the area has dips toward Little Collom Gulch (due to the Collom syncline) which precludes any seepage into the pitwalls. Based on the above, there is no potential for seepage from recharge into any downdip pitwall of the Little Collom X mining area. For Collom Lite, there are two separate ground water conditions in the mining area — the saturated groundwater zone and the unconfined perched aquifers above the deeper zone. The saturated ground water conditions exist at a depth of approximately 7150 feet. Any pitwall seepage below this depth is to be expected since the ground water at this depth will be coming to equilibrium with the surrounding ground water levels (water table /piezometric surface) following the cessation of dewatering operations. Based on numerous previous studies in the area, this water table level is at or below the bottom elevations of the creeks and gulches in the area of Collom Lite. After the pit is reclaimed and recharge of the pit begins, seepage will occur through the pitwalls in a northerly direction due to the hydraulic gradient in the area of Collom Lite. The saturated water level is below the F /F seam, so pit wall seepage will occur above this level. For the zone above the saturated water table, there are a variety of geologic beds and coal seams, all of different thicknesses and aerial extent. Pitwall seepage will be predominantly through the coals and to a lesser extent sandstones and their associated fractures. Some seepage will occur along fracture planes in the mudstones and siltstones but will be minimal. Groundwater movement in the bedrock is largely controlled by the existence of joints or fractures rather than by permeability within the rock strata. The typically low permeability and discontinuous and lenticular nature of the strata restrict the ability of the bedrock to transmit water. No hydraulic conductivities (K) have been obtained for these units since they Collom — Rule 2, Page 137 Revision Date: 8/5/11 Revision No.: PR -03