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• 10°f5°. The Colowyo Mine is located on the south flank of the Collom Syncline. <br />Therefore, based on the geologic structure of the area, the coal seams and non-coal <br />beds of the Colowyo Mine dip to the north-northeast at approximately 10°. The <br />southern portion of the Colowyo Mine is located on a stmctural high, an unnamed <br />anticline, which is an offshoot of the Danforth Hills anticline. <br />Topographically, the mine is located on a topographic high, bordered on the east and <br />west by deeply incised valleys. These valleys are Good Spring Creek and Wilson <br />Creek, on the east and west respectively. The valleys slope from south to north, <br />similar to the topographic slope at Colowyo Mine. The topography in the area of the <br />Colowyo Mine ranges from 8000 feet on the south to 7150 feet on the north. The <br />valleys have elevation ranges from approximately 7100 feet on the south to 6550 feet <br />on the north. On the south, south of the Section 16 mine area, the topography drops <br />off into the West Fork Good Spring Creek, a small tributary to Good Spring Creek. <br />Hvdroloev <br />Based on the above, the Colowyo Mine is located on both a topographic and <br />structural high. Thus, these highs cause the mined units of the Colowyo Mine to be <br />above any significant recharge source, e.g., surface water. This is because the <br />bottoms of the pits are at an elevation higher than the elevation of the surface water in <br />the creeks. Only when the units are at an elevation lower than the valleys does any <br />significant recharge occur. Thus, the only source of recharge for the mined units in <br />the pit areas of the Colowyo Mine is precipitation. <br />• Precipitation is less than 22 inches (on average) per year. Evaporation rates approach <br />30 inches per year, with recharge rates in the Goodspring Creek and Taylor Creek <br />basins being less than 0.35 inches per year. In addition, any surface <br />water/precipitation on this topographic high has to percolate through the clayey soils, <br />prevalent in the area of the Colowyo Mine, into the underlying bedrock. Any water <br />that recharges the bedrock units tends to accumulate along unit contacts since these <br />tend to be areas of least flow resistance. This is exhibited in the highwall of both pits <br />of the Colowyo Mine, where any discharge is easily seen as issuing primarily from <br />these contacts and has been the case since 1981. <br />Any ground water that has been discharged from the mine highwall has been found to <br />evaporate from the pit floor or be consumed by pit highwall. Past hydrological <br />studies also reveal the mined units tend to have low permeabilities (even the <br />sandstones) and do not allow for large water movement, even if the ground water is <br />present. This is also the case where the ground water is under confined or unconfined <br />conditions (i.e. below the elevation ofthe valley bottoms). <br />If any ground water does percolate vertically through the discordant geologic units, it <br />encounters a tonstein bed near the base of the Williams Fork Formation. This bed is <br />approximately 150 feet above the top of the Trout Creek Sandstone and is <br />approximately 400 feet below the bottom of the active pits. The tonstein bed has an <br />approximate thickness of 2.S feet. Permeability tests of this material show it has <br />• permeabilities greater than 1x10-10 centimeters per second. Thus, this bed is an <br />effective aquiclude and prevents downward movement of any ground water to the <br />underlying Trout Creek Sandstone. <br />4.05-9b Revision Date: 11/17/05 <br />Revision No.: MR-7S <br />