EarthFax developed a plan to reclaim land affected by molybdenum mine and mill located at an elevation of over 10,000 feet in Colorado. The ore body represented the largest molybdenum deposit in the world. The mine had been in operation for over 90 years and plans were in place to expand the operation in response to world demand for molybdenum.
Expansion of the mining operation was projected to result in an affected area of over 6,400 acres, with nearly 300 acres of open-pit mine, 1,870 acres of tailings impoundments, a 620-acre waste-rock disposal site, a 330-acre mill site, and several ancillary facilities. Critical issues affecting the design of the reclamation plan included the high-altitude location in montane and alpine ecological zones, dealing with a short growing season and an average annual snowfall of over 20 feet, the control of acid-mine drainage, the location of the site on the continental divide at the headwaters of three drainages, and the fact that much of the surface water from the site discharged into watersheds that provided a portion of the drinking-water supply for Denver, Colorado.
We evaluated areas to be affected by the expansion project and developed topsoil salvaging plans for those areas that had not yet been disturbed. We also designed reclamation channels to safely convey the peak flow from the probable maximum precipitation event around and across the tailings in a non-erosive manner. We considered both wet and dry covers for final reclamation of the tailings, with the goal of minimizing long-term acid generation while cost-effectively achieving adequate revegetation and erosion protection of the site.
Our reclamation plan included demolition of site structures, placement of the demolition debris in the tailings impoundments, construction of a cover system for the tailings, recontouring of selected areas, incorporation of lime into the surface of acid-generating material, placement of topsoil, and site revegetation. The client had conducted field trials for the prior 10 years to evaluate alternatives for effective reclamation of the acid-generating tailings. We reviewed the results of these field trials and developed a tailings reclamation plan that included placement of about 2 feet of waste rock on top of the tailings, incorporation of several inches of lime and composted biosolids into the waste rock, and revegetation of the resulting surface.
Following development of the plan, we estimated the cost to reclaim the site. This estimate, which served as the basis for the reclamation bond with the State, included the cost for demolition, disposal of demolition debris, earthwork, revegetation, long-term operation of a plant to treat leachate from the tailings impoundments, and miscellaneous other items.