2017-03-23_PERMIT FILE - C1981019A

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RULE 2 PERMITS Changes in groundwater levels during mining The mine pit will be excavated to a depth below the groundwater table and dewatering is needed to achieve stable pit slopes and safe mining conditions. Groundwater levels in the bedrock units within the pit footprint and in the immediate vicinity of the pit will be lowered ahead of mining by pumping from the dewatering wells. Colowyo will submit to the Colorado Division of Water Resources (CDWR) dewatering well/system application(s) to install dewatering wells and remove the groundwater to facilitate the mining. The Colowyo permit area is not in a designated groundwater basin as defined by the CDWR. The bottom of the pit in the northern area of the mine will be about elevation 6900 to 6950 ft. The bedrock groundwater level in this area is at about elevation 7100 to 7150 ft. Therefore, there will be in the range of 200 to 250 ft of drawdown in groundwater levels in this part of the pit, assuming that the groundwater level is dropped to just below the bottom of the pit. In the southern area of the mine, the bottom of the pit is at about elevation 7500 to 7600 ft. The bedrock groundwater level in this area is at about elevation 7640 ft so the drawdown in groundwater levels will be in the range of 40 to 140 ft. A dewatering well system supplemented as needed by horizontal and vertical drain holes is planned to intercept groundwater before it enters the pit and to achieve the depressurization needed to maintain stable pit slopes. Based on the field work and computer modeling performed by WMC (2005), dewatering wells will be installed to dewater the rock strata from below the Fab coal seam, with a projected depth to about 100 ft below the bottom of the Gab seam. Based on this previous work, the wells will be placed on about 1,000 foot centers. This will allow for the drawdown cones for each well to compound with another within 500 feet and allow the water level to be dropped in and around the pit footprint in a reasonable timeframe. The current dewatering plan includes perimeter wells around the initial box cut and two rows of wells installed in the initial box cut area. Additional perimeter wells will be installed as the pit advances to the south. The drawdown of the groundwater will be monitored using vibrating wire piezometers (VWP) placed in monitoring holes. It is planned to place several holes outside the perimeter dewatering wells on the east and west sides to monitor any water level decline at some distance from the pit (more than 500 ft but less than 1,500 ft.) Monitoring on the south is not necessary since the hydrogeology of the pit tends to unconfined water condtions and will be drained with the dewatering wells. Monitoring on the north will be limited to next to the pit. Since groundwater flow is to the north, this portion of the groundwater system will tend to `drain' away from the pit with no ground water flow. The dewatering system is designed in three phases with a total of about 30 to 40 wells planned. Each well is expected to pump between about 10 to 15 gpm and the total pumping rate is estimated to be in the range of 200 to 300 gpm for the first three years of mining, decreasing to between 100 and 200 gpm during mine years 4 to 8 and further decreasing to less than 100 gpm after year eight. The groundwater removed during dewatering will be used in the daily operation of the Collom mine. Water produced from the mine dewatering well system will be pumped via pipeline to a one million gallon capacity freshwater storage tank complex (five 20,000 gallon tanks) located in the facility area. The water will not be used for potable use. A properly permitted commercial well will be drilled, and with a permitted treatment system, be used for potable water. Collom — Rule 2, Page 116 Revision Date: 11/16/16 Revision No.: MR -166