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RULE 4 PERFORMANCE STANDARDS <br />Syncline, located approximately 0.5 miles north of the north edge of the reclaimed East <br />Pit, trends west-northwest (approximately N60°W) with a slight dip in the axis to the west- <br />northwest. The Collom Syncline is sub -parallel to the Axial Anticline on the north and the <br />Danforth Hills Anticline on the south. The Collom Syncline is asymmetrical, with the <br />north flank of the syncline steeply dipping (20°-40°) to the south-southwest. The south <br />flank dips to the north-northeast at around 10°±5°. The Colowyo Mine is located on the <br />south flank of the Collom Syncline. Therefore, based on the geologic structure of the area, <br />the coal seams and non -coal beds of the Colowyo Mine dip to the north-northeast at <br />approximately 10°. The southern portion of the Colowyo Mine is located on a structural <br />high, an unnamed 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 west <br />by deeply incised valleys. These valleys are Good Spring Creek and Wilson Creek, on the <br />east and west respectively. The valleys slope from south to north, similar to the <br />topographic slope at Colowyo Mine. The topography in the area of the Colowyo Mine <br />ranges from 8000 feet on the south to 7150 feet on the north. The valleys have elevation <br />ranges from approximately 7100 feet on the south to 6550 feet on the north. On the south, <br />south of the Section 16 mine area, the topography drops off into the West Fork Good Spring <br />Creek, a small tributary to Good Spring Creek. <br />Hydrology <br />Based on the above, the Colowyo Mine is located on both a topographic and structural <br />high. Thus, these highs cause the mined units of the Colowyo Mine to be above any <br />significant recharge source, e.g., surface water. This is because the bottoms of the pits are <br />at an elevation higher than the elevation of the surface water in the creeks. Only when the <br />units are at an elevation lower than the valleys does any significant recharge occur. Thus, <br />the only source of recharge for the mined units in the pit areas of the Colowyo Mine is <br />precipitation. <br />Precipitation is less than 22 inches (on average) per year. Evaporation rates approach 30 <br />inches per year, with recharge rates in the Goodspring Creek and Taylor Creek basins being <br />less than 0.35 inches per year. In addition, any surface water/precipitation on this <br />topographic high has to percolate through the clayey soils, prevalent in the area of the <br />Colowyo Mine, into the underlying bedrock. Any water that recharges the bedrock units <br />tends to accumulate along unit contacts since these tend to be areas of least flow resistance. <br />This is exhibited in the highwall of both pits of the Colowyo Mine, where any discharge is <br />easily seen as issuing primarily from 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 studies also <br />reveal the mined units tend to have low permeabilities (even the sandstones) and do not <br />allow for large water movement, even if the ground water is present. This is also the case <br />where the ground water is under confined or unconfined conditions (i.e. below the elevation <br />of the valley bottoms). <br />Rule 4 Performance Standards 4-16 Revision Date: 5/25/18 <br />Revision No.: RN -07 <br />