2013-04-10_PERMIT FILE - C1981019 (82)

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RULE 2 PERMITS The volume of materials that must be resaturated to bring the water level in the pit up to 7,275 feet amsl is calculated to be about 5.2 x 109 cubic feet (ft). Since recharge will occur rapidly up an estimated elevation of 7150 feet, the pit spoil aquifer will develop on top of this level in the northern portion of the Collom Lite pit. The volume of materials below this level is calculated to be 1.0 x 109 ft3. This decreases the volume of materials to be saturated to 4.2 x 109 ft3. Assuming 20 percent effective porosity, 8.4 x 108 ft3 of water (19,284 acre -feet) must infiltrate from the surface and from the Williams Fork Formation to fill the pit to this level. WMC (2006) estimated groundwater recharge from precipitation in the Collom Lite pit area to be 1.1 inches per year. Other studies show a potential infiltration rate of up to 18 percent of annual precipitation. This would translate to an estimated annual recharge rate of up to 3.24 inches per year. Using the highest potential infiltration rate for the Collom Lite pit area of 880 acres that translates to approximately 28,255 cubic feet/day (0.65 acre -feet /day) of water infiltration once the pit is reclaimed. WMC (2005) also estimated the horizontal groundwater flow rate through the Collom Gulch drainage area to be about 19,000 cubic feet per day (cfd). Since the Collom Lite pit is near the high point of the topography for the area and the pit will be entirely dewatered of perched aquifers when mining is completed, this flow through number will not occur for some time in the Collom Lite pit. In addition, since the pit is on one side of Collom Gulch, only part of the flow through should be considered as flowing through the Collom Lite pit. Therefore, only 9,500 cfd is assumed to enter the upgradient side of the reclaimed Collom Lite pit. The infiltration of water (28,255 cfd) plus the flow through seepage (9,500 cfd), equals approximately 37,755 cfd (0.87 acre - feet/day). Including the potential surface water seepage of 0.016 cfs (1382 cfd) equals a total of 39, 137 cfd (0.9 acre - feet). This equals the total recharge after mining is complete before any spoil aquifer water can flow down Little Collom Gulch. Given the above numbers, it would take approximately 58 years to resaturate the pit backfill to an elevation of 7,275 feet amsl. If the water- bearing units updip of the pit area are considered to have been completely dewatered and therefore not be a source of groundwater inflow to the backfilled pit after mining, the time to resaturate the pit backfill from precipitation recharge and surface water seepage alone would be about 80 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 fie X34 seam (and the Y25 as geologic material above the X34 is exposed). This seam is exposed on Tooth sides of the Little Collom. The current mining scenario plans to remove overburden and oxidized coil and then mine fresh coal. This mining will continue until there Collom — Rule 2, Page 137 Revision Date: 9/28/11 Revision No.: PR -03