Cameco Corporation's Contact Lake Operation was a small gold mine and mill located in central Saskatchewan approximately 45 km northeast of La Ronge. Untreated tailings slurry from the cyanide leach, carbon in pulp (CIP), mill were deposited in a partially dewatered lake (Turtle Lake) from December 1994 until June 1998. As expected, concentrations of cyanide and copper, the primary contaminants of concern, gradually built up over the first seventeen months of operation. There were no discharges from the tailings pond throughout this period and tailings supernatant was recycled to the mill for re-use as process water throughout the entire operating life of the mill. After cyanide and copper built up to approximately 45 - 50 mg/L in the tailings supernatant water, a dramatic decrease in both cyanide and copper concentrations occurred over a four month period. These reductions occurred despite the continued loadings being received from untreated mill tailings and the recycling of supernatant to the mill. Concentrations of cyanide and copper continued to decline over the rest of the operating period of the mill and continue to decline to this date. Microbial degradation was investigated as a possible explanation for the dramatic and continuing decrease in cyanide and copper concentrations in the tailings pond.