2011-03-01_REVISION - M1977493 (6)

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Mr. Eric Scott Climax Mine - Response to DRMS Adequacy Review Comments • Permit M-1977-493; Amendment AM-06 March 1, 2011 Page 11 of 16 Exhibit U/Section (6) Comment: Please justify the proposal of the "wet cap" reclamation of the remaining TSF areas, as opposed to the more standard practice of reclamation of TSFs using a low permeability cap with positive drainage. It would seem that using the "wet cap" reclamation would create a setting where surface runoff is actually captured on top of the TSF, rather than shed, resulting in a greater volume of impacted water requiring treatment in perpetuity prior to discharge. As it is generally understood that this facility will require post-mining water treatment in perpetuity to prevent adverse impact to surface water, we believe that every attempt should be made to minimize the amount of impacted water that will require future treatment. Response: As described in Exhibit E - Reclamation Plan, both Tenmile and Mayflower TSFs will be reclaimed with a combination of dry and wet covers. Due to concerns regarding water treatment, flood control, constructability, and dam safety, a wet cap is needed for the Climax Mine site. The flow rate of impacted water requiring treatment at CIVIC currently ranges from about 3,000 gpm during the winter to about 40,000 gpm during the peak of the snowmelt period. CIVIC manages water to reduce the volume and flow rate of water requiring treatment. This management includes the use of interceptor canals, diversion ditches, surface treatments, and reclamation of formerly impacted land. The seepage from TSF dams and water pumped from the open pit or underground mine workings, however, will continue to require treatment well into the • future. These future sources of impacted water are expected to follow a similar pattern of significant seasonal variability. The use of a wet cap reclamation scenario provides detention storage capacity that allows for seasonal attenuation during the peak flow periods and promotes a more constant flow rate for water treatment. This detention storage dampens the peaks of the impacted water hydrographs and reduces the required capacity of the treatment system. In addition, an active reservoir to store impacted water provides flexibility in the timing and rate of water treatment and therefore reduces the risk of releasing impacted water during an upset scenario in the water treatment system or under unusual hydrologic conditions. Hydrologic flood studies indicate that peak flow rates onto the TSFs are generally in the thousands of cfs during a PMP event. If there is no detention storage capacity within the TSF to attenuate these flows, which would be the case in a dry cover scenario, then very large and heavily armored channels would be required to convey the flood flows safely beyond the site. The detention storage capacity provided by the wet cover configuration allows for much lower capacity flood conveyance facilities. The detention concept is less sensitive to peak flood flow estimation and results in lower risk from erosion, channel failure, and potential dam breach. The wet cap configuration also is consistent with that presented to DRMS and approved in Technical Revision TR-13. The wet cover already established on the Robinson TSF is currently developing into a sustainable mosaic of mesic, wet meadow, and emergent marsh habitats which support a variety of wildlife and waterfowl species. In addition, previously constructed infrastructure at the Robinson TSF, including the existing East Side Channel and 2 Dam Spillway will allow the established pond on the Robinson TSF to be utilized for detention storage capacity at closure.