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Colowyo Coal Company L.P. <br />Neilson Pit <br />elevation of the surface water in the creek. Only when the units are at an elevation lower <br />than the valleys does any significant recharge occur. Thus, the only source of recharge <br />for the mined unit in the pit areas is precipitation. <br />Precipitation is 15 to 18 inches (on average) per year. Evaporation rates approach 30 <br />inches per year, with recharge rates in the Goodspring Creek and Taylor Creek basins <br />being 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 />Neilson Pit, 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 <br />resistance. <br />Any groundwater 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 <br />also reveal the mined units tend to have low permeabilities (even the sandstones) and do <br />not allow for large water movement, even if the groundwater is present. This is also the <br />case where the groundwater is under confined or unconfined conditions (i.e. below the <br />elevation of the valley bottoms). <br />If any groundwater 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 approximately <br />400 feet below the bottom of the active pit. The tonstein bed has an approximate <br />thickness of 2.5 feet. Permeability tests of this material show it has permeabilities greater <br />than 1x10 "10 centimeters per second. Thus, this bed is an effective aquiclude and prevents <br />downward movement of any groundwater to the underlying Trout Creek Sandstone. <br />All surface water runoff is contained at the base of the Neilson Pit by a ditch and small <br />containment area. The ditch runs north to south below the entire Neilson Pit and directs <br />all surface runoff to a small containment area at the north end of the disturbance area. <br />This ditch will be maintained to catch all surface water runoff from exiting the <br />disturbance area. Furthermore, any runoff that flows down the access road to the <br />Neilson Pit is contained in a ditch on the north side of the road to contain and transfer <br />runoff to Colowyo's existing ditches and containment sumps. <br />Water Quality <br />The quality of the water in the area of the Neilson Pit has been rated as poor by the USGS <br />and designated for limited agricultural use. Since USGS testing in 1978, no water quality <br />analysis performed at monitoring points at the adjacent Colowyo Coal Mine have shown <br />any significant difference in water quality compared with what the initial USGS work. <br />The water is slightly saline, alkaline and definitely classified as `hard' water. This can be <br />seen in the water quality measurements for total dissolved solids (TDS) and electrical <br />conductivity (eC). Both TDS and eC exceed the EPA secondary drinking water <br />standards. <br />5 <br />