2011-08-08_REVISION - M1977493 (40)

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Robinson Lake Operating Plan Climax Molybdenum Company Version: R1 Permit M -1977 -493 • 1.0 OVERVIEW • In accordance with the requirements described in the Climax Mine Environmental Protection Plan (EPP), Robinson Lake is designed and operated as a zero discharge facility to preclude overflow to Eagle Park Reservoir and the East Fork of the Eagle River. To fulfill this commitment, the water level in Robinson Lake is maintained well below the spillway control crest to provide sufficient surcharge capacity to store, without release, runoff from the probable maximum precipitation (PMP) event, plus runoff from the 100 -year snowmelt event, plus seepwater from the Robinson Dam, 1 Dam, and 4 Dam seepage collection systems, plus operating return flows from the Tenmile Tunnel. The following technical discussion provides the basis and assumptions used to establish the Robinson Lake Operating Plan. 2.0 BASIS AND ASSUMPTIONS 2.1 Probable Maximum Precipitation A site - specific PMP study for the Climax Mine was completed by Applied Weather Associates in 2005 and revised in 2009. This study concluded that the PMP event at Climax would produce 4.41 inches of total rainfall. A discussion of the analyses and conclusions are presented in Permit Amendment AM -06 Exhibit G. 2.2 100 -Year Snowmelt • As part of the Climax flood hydrology study completed in 2005, Wheeler developed 100 -year snowmelt runoff rates for the Climax site. These rates were based on an analysis of the Black Gore Creek drainage basin near Vail and adjusted for the Climax site. The snowmelt study determined the 100 -year 1 -day maximum snowmelt runoff rate to be 15.28 gallons per minute (gpm) per acre (0.0675 acre -feet per acre in one day). Correspondingly, the peak 100 -year 5- day- average snowmelt runoff rate was determined to be 12.40 gpm per acre (0.2740 acre -feet per acre in five days). 2.3 Seepwater Inflow The Robinson Dam and 4 Dam seepwater systems collect seepage below the respective dams and pump that water into Robinson Lake. The seepage rate from Robinson Dam has historically been less than about 25 gpm. The pump system that pumps this seepwater back to Robinson Lake has a nominal capacity of 480 gpm and is therefore only run on an intermittent basis. The seepage rate from 4 Dam has historically been less than 25 gpm. This pump system also has a pumpback capacity of 480 gpm. Seepage from 1 Dam is conveyed to Warren's Pump System where it is normally pumped into the Tenmile Creek basin for treatment. The rate of 1 Dam seepage has historically been less than 1,000 gpm. Warren's Pump System has a nominal capacity of 4,500 gpm. 2.4 Operating Return Flows During operation, approximately 10,000 gpm of process water will be pumped from Robinson Lake to supply the demands of the mine and mill. Whenever the natural inflow to the lake is not sufficient to maintain the desired operating water levels, water can be diverted into the lake from the Tenmile Creek basin through the Tenmile Tunnel. Water from the Tenmile Tailings Storage EPP — Appendix D August 2011 1