2011-05-23_REVISION - C1981019 (2)

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Colowyo Response - PR3 adequacy No. 2 20 May 6, 2011 Specific mitigation proposals are still under review by CDOW and referenced in this section of the text. Once Colowyo receives communication from CDOW on specific agreement with Colowyo's mitigation proposals, the appropriate language will either be added to this section or the specific communications will be added to an exhibit and referenced in the text. Rule 2.05.6(3)(a) Protection of the Hydrologic Balance 1. The application contains limited permit text narrative with regard to impacts to registered ground water users due to pit dewatering and development and is deficient with regard to impacts to ground water users (see discussion above under Rule 2.04.7(3)). Please expand the permit text narrative under the section discussing probable hydrologic consequences to ground water quantity, with particular emphasis on discussion of impacts to wells owned by others. In order to recognize that most postmining spoil leachate seepage into the downdip pitwall probably will occur in the more permeable beds (the coal seams and cleaner sandstone beds), please use in the pitwall seepage calculation for the Collom Lite Pit a value for K that is within the range of most values for the coal seams and sandstone beds (for example, 0.01 ft/day, rather than the 0.0001 ft/day in the submittal). Also, please use in the calculation of the cross- sectional area of the downdip pitwall only the aggregate net thickness of sandstone and coal beds (that is, exclude the shale and siltstone beds from the measurement of the pitwall height because those beds are not capable of transmitting a significant amount of spoil leachate). The aggregate net thickness can be measured on a geophysical log. Please refer to question 2.04.7(3) above under the heading Alternative Water Supply. Colowyo's Response: As noted in the current response to Rule 2.04.7 (3), there are no registered ground water wells in the vicinity of the Collom Lite that will be impacted by the dewatering of the geologic units around the pit. The potential for pitwall seepage will only occur if a spoil aquifer develops. Before this can occur the water level in the pit area that was dewatered will begin to rise and recharge the spoils after the dewatering pumps cease operation. This will continue until static conditions are reached and the water level in the saturated spoils will be similar to that found pre-mining. This saturated water table/piezometric surface will be at approximately 7150±50 feet (based on the 2005 WMC report). The water beneath this piezometric/water table surface will flow in a northern direction at the rate of normal ground water flow for the area. The inflow of any perched water from the pitwalls may also provide some minimal recharge. As the pit is reclaimed to post mining topography, it is likely that the discharge point of any spoil water will beat 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