PERCHLORATE IN GROUNDWATER AND LAKE WATER RELATED TO MINING ACTIVITIES?

Perchlorate [ClO₄^-] is an emerging contaminant of concern. Perchlorate salts used as an oxidant for propellants, including explosives, can potentially contaminate water resources. Water samples were collected in 2008, 2009, 2011 and 2013 from various mine waters and lake water surrounding the operating Diavik Diamond Mine in the Northwest Territories, Canada. Snow samples were also collected in 2013. Mine water samples were collected form open pit sumps, underground seeps and inflows, the mine water retention pond (prior to treatment), outflow from the mine water treatment plant, and sample stations 60 m from the treatment plant outflow diffuser. The highest concentration of ClO₄^- (157 µg L^-1) was measured in water collected from the underground mine, and was associated with an area of active blasting. The mine is located on an island within an oligotrophic lake. Lake water samples also were collected in 2008, 2009, 2011 and 2013 at near-field, mid-field, far-field and background stations, relative to the mine water discharge, to assess temporal and spatial trends within the lake. Samples were collected in open water conditions in 2008, 2011 and 2013 and under-ice in 2009 and 2013. The mean ClO₄^- concentrations in open water were 0.30 µg L^-1 (n=114), and 0.24 µg L^-1 (n=107) under ice. The Canadian drinking water guideline is 6 µg L^-1. There were no statistical differences in mean ClO₄^- concentrations among sample stations or sample depths for samples collected in open water conditions. Statistically significant differences were observed among samples collected at varying depths below an ice cover. The differences between mean ClO₄^- concentrations in the lake under open water and under ice conditions were statistically significant during some sampling years. Snow core samples collected in 2013 had a mean ClO₄^- concentration of 0.021 µg L^-1 (n=16). There were no correlations between ClO₄^- and the blasting residuals NO₃-N or NH⁴⁺-N for the lake samples. Perchlorate concentrations within the lake remain low after 12 years of continuous mining and no mining-related ClO₄^- impacts to the lake could be established with certainty.