The McLaren Tailings Site is located near Cooke City, Montana, in a valley drained by Soda Butte Creek, which runs through the site and through Yellowstone National Park, approximately five miles downstream. From 1934 until 1953, the McLaren Mill processed gold and copper ore from the New World Mining District. During the operation of the mill, Soda Butte Creek’s channel was filled with mine waste and the stream was diverted into a ditch and culvert that ran along the south side of the impoundment. Discharges of polluted water from the McLaren Tailings have been identified since the 1960s as a significant source of metals contamination to Soda Butte Creek. Following the Yellowstone fires of 1988, the proximity of the tailings dam to Soda Butte Creek was identified as a significant concern because a high runoff could erode or saturate the dam, causing it to fail. This would have resulted in a catastrophic release of the tailings into the creek. The McLaren Tailings site was designated an Emergency Response Action Site by the U.S. Environmental Protection Agency in 1988, resulting in efforts to stabilize the dam.
DEQ began work on the McLaren Tailings in June 2010. Excavation of the tailings required extensive construction dewatering and water treatment to excavate the tailings safely. The excavated tailings were mixed with quicklime to remove excess moisture and to promote compaction of these materials in the repository. Approximately 250,000 cubic yards of mine waste was removed, mixed with lime, and placed in the repository. Over 100 million gallons of contaminated water were pumped from dewatering wells surrounding the area and treated to meet DEQ water quality standards. Approximately 2,000 feet of Soda Butte Creek has been reconstructed in the area that was covered by the mine waste and the creek has been returned to its historical location. Revegetation of the site occurred during the 2014 construction season and major work at the project was completed in October 2014.
Soda Butte Creek Water Quality Assessment
Annual Project Status Updates
Construction Completion Reports