By Tom Henderson, Andrew Ray, Pete Penoyer, Ann Rodman, Mary Levandowski, Alysa Yoder, Shane Matolyak, Mary Beth Marks, and Autumn Coleman Mary Beth Marks, and Autumn Coleman
Given the ongoing contamination of Soda Butte Creek and the risk of a catastrophic tailings release, Montana DEQ’s preferred alternative was the complete removal of the tailings impoundment from the Soda Butte Creek floodplain (MTDEQ 2002a). The tailings impoundment covered an area of approximately 10 acres (4 ha) and included approximately one-half million tons of mine tailings and approximately one million gallons of contaminated groundwater. Groundwater discharging from the impoundment contaminated the underlying aquifer, which contained groundwater under confined pressures (MTDEQ 2009).
In order to meet the objective of complete removal of the impoundment, project design and construction work included a significant groundwater pumping and treatment effort. Dewatering the tailings was achieved by capturing uncontaminated groundwater around the perimeter of the tailings impoundment and from the aquifer below the impoundment. The pumped water was sent to a water treatment plant to treat the water to achieve Montana water quality standards. More than 110 million gallons (0.4 million m3) of contaminated water were pumped and treated during active reclamation. The contaminated water was treated using calcium hydroxide to increase the pH and precipitate dissolved metals. Treated water was discharged to Soda Butte Creek with daily field monitoring and weekly laboratory analysis to document water quality (MTDEQ 2015). Calcium oxide was mixed with tailings to neutralize acidity in the tailings and dry the tailings for compaction in the constructed repository. Approximately 1,800 lineal feet (550 m) of Soda Butte Creek and Miller Creek channels were reconstructed in their approximate pre-mining locations following the removal of the tailings impoundment in 2013. The project was covered with compost-amended soil and seeded in 2014. Construction costs, including water treatment and site reclamation work, totaled $21,897,249 (MTDEQ 2015). The work was funded by annual grants from the Department of the Interior Office of Surface Mining Reclamation and Enforcement and a grant from the Montana Department of Natural Resources and Conservation Reclamation and Development Grants Program. The project earned a National Recognition Award from the American Council of Engineering Companies in 2015.