2010-12-17_REVISION - C1981019 (127)

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Mine water management 93 Water produced from the dewatering wells is probably adequate for portable use after treatment for high TDS, and as long as the wells remain unaffected by mining. Water for dust control can be obtained from both contact and non - contact water. The preliminary water balance indicates that there may be surplus water available (e.g., up to 275 gpm) that must be stored, used elsewhere, or discharged. The water balance also indicates that additional sources of water up to about 420 gpm may be needed. Additional sources may include surface diversion from the streams or additional water supply wells. As mining in Phase 1 advances, it is anticipated that long -term pumping rates will be reduced from those estimated in the box cut. Therefore, the requirement for contingency water supplies is expected to increase as the mine advances south. 7.2.5 Surface water management The mine plan under Option 1 will not intersect the larger intermittent surface drainages in the Collom study area including Straight Gulch, Collom Gulch and Jubb Creek. These drainages would require significant diversion features if they were intersected by the pit. Also, they contain saturated valley fill deposits that would require additional dewatering measures if they were intersected. The Phase 1 pit will intersect Little Collom Gulch in the box cut area. The 10 -yr, 24 -hr and 100 -yr, 24 -hr precipitation amounts are 2.0, 2.3, and 2.8 inches, respectively. The 10 -yr, 25 -yr, and 100 -yr storm flows in this drainage at the point it intersects the pit are C estimated to be 41, 73, and 144 cfs, respectively. The corresponding runoff volumes are 8.5, 12.9, and 21.7 ac -ft, respectively. These estimates are made using the HEC -1 runoff model with a curve number of 70 and a lag time of 0.47 hours. The mine plan will need to include a diversion to prevent flows in Little Collom Gulch from entering the pit. Other minor drainages will also be intersected by the pit. To prevent flow from these minor drainages and sheet flow from upstream areas from entering the pit, a diversion berm should be maintained at all times on the south side of the advancing wall to divert surface water around the pit. This will reduce the need for in -pit sumps, prevent excessive erosion of the pit wall, and reduce the potential for surface water to infiltrate into fractures and cause a buildup of water pressure. 7.3 Conceptual dewatering design for Option 2 (south to north mine plan) 7.3.1 Mine plan The preliminary mine plan for Option 2 used as the basis for dewatering evaluations was provided by KEC. Mining will be initiated on the southern boundary of the mine footprint area as shown in Figure 7.3 and will advance towards the north. Phases 1 and 2, as defined in Option 1, are combined and will be mined together in one pass. Under Option 2, the mining will advance down -dip and spoil material will be placed on the south side of the pit. The estimated rate of advance of the mine to the north is about 1,000 ft per year based on discussions with KEC' For purposes of this preliminary dewatering plan, it is estimated that Option 2 has about a 20 -yr mine life. 2572 -R2 Colowyo Coal Company Water Management Consultants