2018-02-21_REVISION - M2008017 (7)

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Derr Pit Project DRMS 112 Permit Amendment proposed mine site by Two Rivers Testing in October 2006. Groundwater monitoring wells have been installed at the proposed mining site in all fifteen borings by Two Rivers Testing. Monitoring data is attached for the monthly readings since the installation occurred in October 2006. During mining, the exposed surface area will include a recharge pond, planned to be at the southeast corner of the property for maximum benefit to the alluvial groundwater aquifer, and a recharge ditch on the south side of the pit that will be approximately 1,500 feet in length with a 5’ width. The recharge pond location and configuration may vary during the mining and dewatering operations for observed impacts to the area alluvial aquifer, monitored through the groundwater monitoring wells. The recharge pond is planned to have a total surface area of approximately 0.55 acres and the recharge ditch will have a total surface area of 0.2 acres. To enable dry mining at the site, dewatering trenches will be constructed around the perimeter of the pit in Phases 1 – 7. Typically, groundwater and storm water in the mine pit operation will flow along the dewatering trenches, and surface flow by gravity to collection basins. These flows will then be pumped into the southeast recharge pit some 70 to 90 ft vertically ‘above’ the collection basin in the mine bottom. During mining operations, less than 0.4 acres of water is expected to be exposed in less than 3,000 feet of 5’ to 6’ width dewatering trenches, at any one time in the mining operation. Water Requirements The annual consumptive use at this site during Phases 1 – 7 of the mining operation is estimated to be approximately of 16.0 ac-ft and will result from evaporative losses and moisture content of the material hauled off the site. Evaporative losses are dependent on the exposed water surface area, which may vary throughout the mining operation as discussed above. Exposed surface area at the Derr Pit will include groundwater exposed in dewatering trenches, a recharge pond, and recharge ditches. The maximum exposed surface area at the site during mining is estimated at 1.15 acres. Total annual evaporative loss is the product of net annual evaporation and total exposed groundwater surface area. Net annual evaporation is the difference between gross annual evaporation and effective annual precipitation. The NOAA Technical Report NWS 33, Evaporation Atlas for the Contiguous 48 United States (U.S. Department of Commerce), was used to determine gross annual evaporation. For this location, gross annual evaporation was estimated at 45 inches, or 3.75 feet. The Western Regional Climate Center climatic data for the City of Greeley weather station was used to estimate annual precipitation. For this station, the average annual precipitation was reported as 14.06 inches. Effective annual precipitation was assumed to be 70 percent of average annual precipitation and was calculated to be 0.82 ft. The