2018-10-12_REVISION - C1982056 (2)

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Executive Summary Twentymile Coal (TC), the current operator of the Foidel Creek Mine (the mine) located in northwest Colorado, has developed and is proceeding with mining of a coal seam which underlies previously mined areas. These mining activities will require drainage and treatment of water which has accumulated in the previously mined areas to remove mine water contaminants including iron, sulfate and total dissolved solids. TC intends to pump the mine water through a passive treatment system to improve the water quality, as part of the mine water treatment and handling system. Passive treatment systems have been shown to provide effective treatment of similar composition in other applications. TC had requested CH2M HILL (CH2M) to complete a conceptual study on passive treatment alternatives in December 2015. This study, "Twentymile Coal Mine Discharge Passive Treatment Evaluation" referenced in Appendix A, had identified that a constructed free water surface wetland system consisting of five cells in series, including the 61VIN Pond as the final polishing cell, would provide cost effective removal of mine water contaminants. At the conclusion of the treatment project, the wetland area would be removed and reclaimed, although TC is further evaluating the potential to retain the wetlands to continue to function passively as a wildlife habitat, fed by natural precipitation and runoff. The 2016 design ran into permitting obstacles that ultimately required the system to be moved to anew location. CH2M HILL was asked to update the system to conform to the new location while making minimal changes to the design. In general, the basis of design factors that influence sizing of the system remained similar or in most case led to the original design being conservatively sized. The system modeling for cell sizing and hydraulics remained as completed in the 2016 design. As a new location, TC will need to verify the characteristics of the foundation and construction materials, although given proximity and similar topography, it is reasonable to assume that they will be very similar to the original location.