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Evaluation of Hydrologic Impacts <br />In accordance with Section 3.03.2(2), the Division has evaluated whether pollution of <br />surface or subsurface water is occurring, and the probability of future occurrence of such <br />pollution. The evaluation considered information in the following documents: the <br />Probable Hydrologic Consequences section of the permit application, Annual Hydrology <br />Reports submitted by the operator, the Division's Annual Hydrology Report review <br />memoranda, documents in the SL-2 bond release application and file, and the Division's <br />Yampa River Cumulative Hydrologic Assessment. Details of the evaluation are described <br />in a memorandum contained in the SL-2 file, dated April 4, 2005 (the "hydrology <br />evaluation memorandum"). The results of the evaluation are summarized below. <br />Ground Water Impacts: <br />Flooding of workings. Currently, underground mine water is approved for pumping <br />from the workings at two locations and then is discharged into the Williams Fork River at <br />NPDES outfallls 003 and 024. In recent years, the level of flooding inside the workings <br />has been maintained near the elevation 5,825 ft. as shown by sumped water level <br />measurements made annually in the 7 North Angle dewatering borehole. The mine <br />operation currently is in temporary cessation status, indicating future mining operations <br />are anticipateci. If mining operations are repeatedly resumed and curtailed, then there <br />could be cycles of flooding and dewatering of the workings. After pumping permanently <br />ceases at the mine, the water flowing into the workings can be expected to fill the <br />workings to a maximum elevation of approximately 6,300 feet during the 16-year period <br />following cessation of pumping, based on a calculation on page 2.05-44 of permit <br />application. T'he predicted fill-up time could be significantly less than 16 years if mining <br />does not expand the volume of the open workings beyond the currently existing amount, <br />which is significantly less than what is approved for ultimate development. After the <br />workings are :fully flooded, the water level in the workings can be expected to fluctuate <br />no more than a few feet over several years of time because the predicted rate of inflow <br />into the workings of approximately 750 gpm is significantly more than the predicted total <br />outflow rate o:f 0.042 cfs. <br />Potential for migration of mine water from workings to aquifers through faults and <br />fractures. The- Middle sandstone (above the workings) is the aquifer nearest the workings <br />in the stratigraphic column and the Trout Creek sandstone (below the workings) is the <br />next nearest.:iignificant flow of mine water to aquifers in the Trout Creek sandstone or <br />Middle sandstone via faults or fractures is improbable because water sumped in the <br />workings will not develop enough head to overcome hydraulic pressures within the <br />aquifers. Mint; water could not migrate to the Twentymile sandstone (above the Middle <br />sandstone) wil:hout first migrating through the Middle sandstone. <br />Potential for mine water migration to land surface and alluvium of Williams Fork <br />River and Yampa River. The E and F coal seams subcrop beneath the Williams Fork <br />alluvium in S~ 1 /2 S W 1 /4 Sec. 31-6N-91 W and beneath the Yampa River alluvium in <br />NE1/4 Sec. 36-6N-92W. These subcrops are lower in elevation than the presumed <br />