2003-01-14_REVISION - M1977342 (3)

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• Design and construction of a gravity spillway along the north side of the impoundment that would direct excess waters (flood waters and releases) to the water treatment system or directly to Ute Creek if water quality met water quality standards. • Naturally colonizing and developed wetlands around the perimeter of the maximum water pool and cominually wetted area would provide wetlands values and, potentially, could be developed as high quality wetlands and incorporated into a high value mitigation banking program. • The damn face and approximately 30 percent of the eastern portion of the impoundment would be reclaimed as a dry cover per the specifications in the existing permit and the requested change in general }and use from open forest to rangeland as described in Section 2.2 herein. 2.4.2 Rationale for the New Alternative Generally, there are three principal reasons why Henderson wishes to change the reclamation plan for the tailings impoundment. • Hydrologic Stability: The present dry cover reclamation plan does not specifically address how the western water pool area would be dried or how it would be maintained in a dry condition since part of the plan would be to plug the decam lines. The natural tendency of the hydrologic system at this location will tend toward flooding and a persistent water pool along the low point of the west side of the impoundment. Hydrologically, the system will be statically unstable (with its tendency to flood) and dynamically unstable because failure of the pumping system that will be required will cause even greater loss of system control. Henderson's experience at the Urad site supports this principal. Urad, although not regulated under the Act, was reclaimed by Henderson in the 1970s as a dry cover using development rock from Henderson and wood chips/sewage sludge to develop a dry "soil" layer. The project was considered a success story and attained several awards. However, in the 25 years since the project was completed, water pools and associated wetlands have developed naturally, creating persistent wet covers over portions of the impoundments similar to what Henderson is proposing herein. Interestingly, this natural succession has provided habitat for the state endangered boreal toad, a species for which Henderson and the Colorado Division of Wildlife have developed a formal cooperative MOU. The permanent water pool scenario with the constructed spillway at Henderson's tailings facility accounts for the natural hydrological tendencies. The new scenario will allow the western 70 percent of the impoundment to flood to a predetermined level until static and dynamic stability is a