2015-10-06_PERMIT FILE - C1981019A (3)

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RULE 4 PERFORMANCE STANDARDS With respect to ground water monitoring of the Williams Fork, the current mined sequence of non -coal and coal beds in the West and South Taylor Pits are above any significant recharge source, i.e., surface water recharge to the bedrock in the valleys. This is because the bottoms of the current pits are at an elevation higher than the elevation of the surface water in the valleys. Precipitation in this area is less than 22 inches (on average) per year. Evaporation rates approach 30 inches per year. Any surface water/precipitation on the topographic highs has to percolate through the clayey soils, prevalent in the area of the CM and Collom, into the underlying bedrock. The recharge rates in the Good Spring Creek and Taylor Creek basins are estimated to be 0.35 inches per year, based on past studies. Any water that does recharge the bedrock units tends to accumulate along unit contacts since these tend to be zones of least flow resistance. This was and is exhibited in the highwall of both pits of the Colowyo Mine, where any discharge is seen as issuing primarily from these contacts and has been the case since 1981. Any ground water that has been discharged from the mine highwall has been found to evaporate from the pit floor or be consumed by down slope (usually northern) pit highwall. Past hydrological studies also reveal the current mined units tend to have low permeabilities (even the sandstones) and do not allow for large water movement, even if the ground water is present. This is the situation whether ground water is under unconfined or confined conditions. The projected bottom of the current South Taylor pit will be at an elevation higher than the majority of the coal seams mined in the West Pit, where only perched water has been encountered in the last 14 years. The only source of water via recharge for the mined units in the South Taylor pit is precipitation and the storage ponds that were used for cattle and sheep watering. Therefore, no monitoring of a continuous non -perched aquifer is possible, even if the beds of the Williams Fork formation were continuous over a large area. The same geologic setting is also found in the proposed Collom mining area. The southern portion of the proposed permit area has the top portion of the Iles formation and the bottom of the Williams Fork formation exposed above Wilson Creek. Drilling in this area down to the TCSS revealed dry conditions and long term monitoring of a TCSS monitoring well revealed no water. Thus in the Collom permit area, like at the active Colowyo mining area, there is no up -gradient bedrock ground water in a continuous aquifer, which can monitored down gradient of the proposed mining area. If any ground water does percolate vertically through the discordant geologic units, it encounters a tonstein bed near the base of the Williams Fork Formation. This bed is approximately 150 feet above the top of the Trout Creek Sandstone and is approximately 400 feet below the bottom of the active pits. The tonstein bed has an approximate thickness of 2.5 feet. Permeability tests of this material show it has permeabilities greater than 1x10-10 centimeters per second. Thus, this bed is an effective aquiclude and prevents downward movement of any ground water to the underlying Trout Creek Sandstone. The monitoring of deep ground water above and below any mined area or proposed mine area is not possible. With no significant recharge of the Williams Fork formation from the south (up - dip), no ground water can be transmitted down gradient through a continuous unit, which could then be monitored. Valley Fill Ground Water Collom — Rule 4, Page 19 Revision Date: 9/28/11 Revision No.: PR -03