Perchlorate in Lake Water from an Operating Diamond Mine

Mining-related perchlorate [ClO4 –] in the receiving environment was investigated at the operating open-pit and underground Diavik diamond mine, Northwest Territories, Canada. Samples were collected over four years and ClO4 – was measured in various mine waters, the 560 km2 ultraoligotrophic receiving lake, background lake water and snow distal from the mine. Groundwaters from the underground mine had variable ClO4 – concentrations, up to 157 μg L–1, and were typically an order of magnitude higher than concentrations in combined mine waters prior to treatment and discharge to the lake. Snow core samples had a mean ClO4 – concentration of 0.021 μg L–1 (n=16). Snow and lake water Cl–/ClO4 – ratios suggest evapoconcentration was not an important process affecting lake ClO4 – concentrations. The multiyear mean ClO4 – concentrations in the lake were 0.30 μg L–1 (n = 114) in open water and 0.24 μg L–1 (n = 107) under ice, much below the Canadian drinking water guideline of 6 μg L–1. Receiving lake concentrations of ClO4 – generally decreased year over year and ClO4 – was not likely [biogeo]­chemically attenuated within the receiving lake. The discharge of treated mine water was shown to contribute mining-related ClO4 – to the lake and the low concentrations after 12 years of mining were attributed to the large volume of the receiving lake.