2015-10-06_PERMIT FILE - C1981019 (14)

Home Browse Search

RULE 2 PERMITS 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 three to four 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 FI/Fab 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 do not contain enough water to perform long -term hydrologic tests. Therefore, the hydraulic conductivities used in any pit wall seepage calculation are derived from tests in wells (WMC, 2005) where saturated water table and /or confined conditions exist. As the pit is reclaimed to post mining topography, it is likely that the discharge point of any spoil water will be at the lowest topographic point on the northern pitwall. This is the drainage channel of Little Collom Gulch. The elevation of this potential discharge point is approximately 7275 feet (based on the current mining plan and topographic data). Using an estimated saturated water table /piezometric surface of approximately 7150 feet and a discharge elevation of 7275 feet, any water in the spoils would have to rise a minimum of 125 feet before any water could be discharged into the Little Collom Gulch drainage. During this time, some pitwall seepage would occur thus increasing the time for a potential spoil aquifer to develop. As previously noted, since the coals and sandstones will transmit the majority of water from the spoils, the estimate of pitwall seepage will be based on the hydraulic conductivities and estimated total thickness of the sandstones and coals in the northern pitwall. The majority of the geologic beds above the saturated zone are siltstones and mudstones with lesser amounts of sandstones and then coals. The coal seams have hydraulic conductivities ranging from approximately 0.002 to 0.77 ft/day, with a geometric mean of 0.14 ft/day. The sandstone units ha -e hydraulic conductivity values ranging from Collom — Rule 2, Page 138 Revision Date: 9/28/11 Revision No.: PR -03