2023-12-26_GENERAL DOCUMENTS - C1981044

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Page 18 of 27 the Twentymile Sandstone. 4. Mining in the 5 and 9 Mines was not expected to significantly impact the only domestic well near the permit area that may be completed in the Twentymile Sandstone (Lux well). The predicted drawdown of this aquifer where the well is located would not significantly impair the usefulness of the well. 5. Mining,mine dewatering discharge would cause a net increase in stream flow in the Williams Fork River. 6. Underground mining in the Williams Fork Mines was not expected to directly affect ground water in the Williams Fork River alluvium. 7. Underground entries in the Williams Fork Mines located beneath alluvium would not directly affect ground water in the Yampa River alluvium. 8. Development mining beneath the Big Bottom alluvial valley floor of the Yampa River would not impact the alluvial valley floor. 9. During mining of the 5 and 6 Mines,underground mine discharge from those mines, combined with spoil spring discharge from the Strip Pit,would increase dissolved solids concentration in the Williams Fork River. During low flow of the river and assuming worst-case mine discharges,the concentration was expected to increase by 224 mg/l, from the historical mean of 332 mg/1 to 576 mg/l. SAR was expected to increase from the historical mean of 0.44 to 5.05. The Williams Fork River would have a medium salinity hazard during low flow and would have a low to moderate sodium hazard at other times. Based on past discharges,the worst-case discharges were not expected to be reached therefore,the medium salinity hazard was not expected to be achieved. Impacts from dissolved solids loading of the Williams Fork during irrigation season would be minimal due to dilution resulting from high river stage. 10. At this point all the mine pumps are shut off, and the water levels in the 45 and#6 Mines are gradually rising. It is estimated that at the end of the life of the mine, it may take on the order of 16 years for the mines to completely fill. At the writing of this document,the pumps are permanently shut off. 11. After the 45 and 46 Mines refill,water may seep from the coal subcrop into the Williams Fork alluvium. The seepage would be driven by a maximum pressure developing in the subcrop equal to a head of 100 ft. above the ground surface. This head would cause maximum seepage of 20 gpm. In a worst-case scenario,the 19.5 gpm discharge would raise SAR in the Williams Fork River alluvial water from the historical mean of 3.6 to 9.4. The alluvial water is naturally higher in dissolved solids and metals than the mine discharge water;therefore,increased SAR is the only Williams Fork Mines Prepared by: R ReiRey M.S. GISP C1981044 December 2023