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RULE 4 PERFORMANCE STANDARDS <br /> <br />Rule 4 Performance Standards 4-17 Revision Date: 8/21/24 <br /> Revision No.: MR-259 <br />the only source of recharge for the mined units in the pit areas of the Colowyo Mine is <br />precipitation. <br /> <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 /> <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 /> <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 approximately <br />400 feet below the bottom of the active pits. The tonstein bed has an approximate thickness <br />of 2.5 feet. Permeability tests of this material show it has permeabilities greater than 1x10- <br />10 centimeters per second. Thus, this bed is an effective aquiclude and prevents downward <br />movement of any ground water to the underlying Trout Creek Sandstone. <br /> <br />Water Quality <br /> <br />The quality of the water in the area of the Colowyo Mine has been rated as poor by the <br />USGS and designated for limited agricultural use. Since USGS testing in 1978, no water <br />quality analysis performed at monitoring points at the Colowyo Mine have shown any <br />significant difference in water quality compared with what the initial USGS work. The <br />water is slightly saline, alkaline and definitely classified as ‘hard’ water. This can be seen <br />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 standards. <br /> <br />Since the water is alkaline, the pH is above 7, but rarely exceeding 8.4. Concentrations of <br />heavy metals rarely exceed health limits, as stated in the USGS report. This has also been <br />backed up by the shallow ground water monitoring performed since the Colowyo Mine <br />began operation. <br /> <br />Conclusions <br /> <br />The Colowyo Mine has no single or multiple continuous geologic units in the mine that <br />contains ground water under unconfined or confined conditions. The only ground water