2010-12-17_REVISION - C1981019 (108)

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RULE 2 PERMITS Any groundwater present in the shallow mining sequence in the Little Collom X pit is expected to be of very limited quantity and areal extent, as the coal seams and associated rocks to be affected by mining outcrop along the adjacent valley walls and have been removed from the valley areas by erosion. Perched aquifers in a similar setting have been encountered in the existing Colowyo operations, and these aquifers drain rapidly. Once drained, they do not produce any significant water to the current pits. The Trout Creek Sandstone aquifer is separated from the lowest coal seam to be mined by approximately 400 feet in the Collom Lite pit area. Between this coal seam and the Trout Creek Sandstone is a mudstone /shale, sandstone, siltstone, and coal sequence of the Williams Fork Formation. About 200 feet above the Trout Creek Sandstone, a laterally- continuous, smectite clay layer known as the Km bed exists. This layer has very low permeability and therefore would be an additional impediment to downward or upward groundwater flow. WMC (2005) conducted evaluations of pit dewatering scenarios for an earlier mine plan that included mining in the same general area as the proposed Collom Lite pit, but that encompassed an area much larger than the proposed Collom Lite pit. Pit Inflow and Pit Surface Water Recharge Impacts Five springs within Little Collom Gulch will be eliminated by the mining operations: SPRLC -01, SPRLC -02, SPRLC -03, VI I, and V29. Two of those, VI I and V29, were identified by previous investigations, but proved to be dry during baseline monitoring efforts. Little Collom Gulch had no flow during the 18 months of baseline monitoring; therefore, elimination of these springs will have no measurable effect on streamflow. The Collom Lite pit is likely to be within the watersheds for springs V1, V10, and V32 (West Fork of Jubb Creek). These springs collectively produce about 0.024 cfs during September, during which monitoring location WFJC was dry, indicating that the spring flow reinfiltrates into the valley fill, or is captured in stock ponds before reaching the monitoring point. The springs contributed about 0.042 cfs in May 2006 to the West Fork of Jubb Creek. Stream flow was measured in May 2005 as 0.10 cfs. No conclusive estimate of the percentage contributed by the springs can be made given the likely difference between hydrologic conditions in those two years. Considering that 0.024 cfs was lost to valley fill or ponds during September, it appears that the contribution of V1, V10, and V32 to the peak flow at WFJC could be considerably less than the 42 percent implied by a year -to -year comparison. At monitoring point CJC, the May 2005 discharge was 0.15 cfs, implying that the affected springs contribute 28 percent of the flow at that location. Since much of the recharge is from undisturbed areas outside of the Collom Lite pit, the probable reduction is likely to be less than half of this amount and not expected to be measurable or statistically significant. Once the mining has been completed and the pit has been saturated, the contributions to surface water from springs originating from infiltration into the Collom Lite pit would return to normal. Collom Lite Pit Hydrology — The Collom Lite pit will have a reclaimed surface area of approximately 880 acres and a pit bottom that inclines predominantly toward the north. The low point in the reclaimed pit topography will be at its intersection with the valley of Little Collom Gulch, at an elevation of approximately 7,290 feet amsl. Resaturation of the reclaimed pit backfill will occur from infiltrating precipitation, seepage of surface water, and groundwater inflow from upgradient areas. The water entering the reclaimed pit backfill will accumulate on the pit floor and flow down the slope of the pit floor until it encounters the downdip side of the pit. Because the mining activity will not cause any decrease in the hydraulic conductivity or transmissivity of the unmined bedrock units downdip (north) of the pit, the Collom — Rule 2, Page 128 Revision Date: 1/23/09 Revision No.: PR -03