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Colowyo Response - PR3 adequacy No. 2 22 May 6, 2011 <br />(elevation 7275 feet), all pitwall discharges will be below this elevation and thus below the <br />Little Collom Gulch drainage to create spoil springs. <br />Colowyo has revised the text in Section 2.05.6(3)(b)(iii) under the sub-heading "Calculation <br />Annual Seepage of Spoil Water Into Downdip Pitwalls in the Little Collom X and Collom Lite <br />Pits" to reflect this response. <br />2. Discussion is also limited with regard to the potential for development of a spoil aquifer and <br />potential long-term discharge of the spoil aquifer to the surface water system. Proposed text <br />narrative provides an analysis indicating that development of a spoil spring, if it occurs, would <br />take 100 years or more to develop to the point where it would discharge to the surface water <br />system. The Division believes a spoil aquifer will develop over time within the confines of the <br />backfilled Collom Lite pit, based on past experience in northwestern Colorado. Long-term <br />discharge of degraded spoil water could therefore impact areas downstream. Alluvial valley <br />floors, as defined by the Coal Rules, are located downstream of the permit area in Collom <br />Gulch. Please expand the permit text narrative regarding the potential for spoil water discharge <br />and impacts to downstream areas. <br />Please revise the prediction of spoil spring discharges to the surface for the Collum Lite Pit to <br />reflect the requested re-calculation of annual pitwall seepage. <br />Colowyo's Response: <br />The recalculation of potential pitwall seepage is discussed in Colowyo's response to <br />item I in Rule 2.05.6(3) (a) Protection of the Hydrologic Balance. <br />For Collom Lite, there are two separate ground water conditions in the mining area - the <br />saturated groundwater zone and the unconfined perched aquifers above the deeper zone. The <br />saturated ground water conditions exist at a depth of 7150±50 feet (based on the 2005 WMC <br />report). Any pitwall seepage from the 7150 feet level and below is to be expected since the <br />ground water at this depth will be coming to equilibrium with the surrounding ground water <br />levels (water table/piezometric surface). The water beneath this piezometric/water table <br />surface will flow in a northern direction at the rate of normal ground water flow for the area. <br />Based on numerous previous studies in the area, this saturated water table level is at or below <br />the bottom elevations of the creeks and gulches in the area of Collom Lite. This creates an <br />outflow from the creeks to the groundwater. <br />Before any spoil aquifer can develop, the water level in the pit area that was dewatered will <br />begin to rise and recharge the spoils after the dewatering pumps cease operation. This will <br />continue until static conditions are reached and the water level in the saturated spoils will be <br />found to be similar to pre-mining conditions in the area. This saturated water <br />table/piezometric surface will be at approximately 7150ZL50 feet. The inflow of any perched <br />water from the pitwalls may also provide some minimal recharge. <br />After the premining piezometric water level is reached, further recharge of ground water in the <br />reclaimed pit will be primarily from precipitation. Precipitation in the area of Collom Lite pit <br />is estimated at an annual average of less than 18 inches per year. A study for Colowyo by <br />Leonard Rice Consulting Water Engineers, Inc. (1979) and discussed by WMC (2005),