Artificial circulation of a mine pit lake

We describe the artificial circulation (destratification) of a pit lake at the Colomac mine site, 220 km north of Yellowknife, NWT, Canada. The pit lake, with a surface area of 17 ha and depth of 110 m, contained relatively saline water contaminated with the mining-related cyanide by-products, thiocyanate and ammonia. As a result of snow and ice melt, the pit lake was intermittently meromictic and cycling of contaminants through the oxygenated surface layer would have taken many years. To accelerate the oxidation process, the pit was artificially circulated using two air diffusers at a depth of 57 m. The pit lake was circulated during the open-water season, from 12 July 2006 for 57 days, and from 17 June 2007 for 89 days. After the first 12 h of operation of the circulation system, the top 37 m of the pit lake was mixed completely; after 34 h, the pit lake was mixed to the depth of the diffusers (57 m); and after approximately 6.5 days, the pit lake was mixed to the bottom. Upon complete mixing, the temperature and conductivity remained uniform during operation of the circulation system, and the concentration of oxygen in the deep water of the pit lake increased significantly. All thiocyanate was oxidized by the end of the first summer, producing ammonia that was depleted after the second summer of circulation.