Spatial and temporal variations in riverine mercury in the Mackenzie River Basin, Canada, from community-based water quality monitoring data

Arctic rivers deliver similar to 40 t yr(-1) of mercury (Hg) to the Arctic Ocean, similar to 6% of which is from the Mackenzie River Basin (MRB), a region warming at similar to 3 times the mean hemispheric rate. How this will affect Hg transfer to ecosystems of the Beaufort Sea is a worrying issue. To help address this question, we analyzed >500 measurements of Hg and other water properties from 22 rivers collected in 2012-2018 by communities of the MRB. This new dataset provides a more comprehensive view of riverine Hg variations across the basin than was previously available. We find that rivers issued from mountains in the western MRB contribute the largest share of Hg in the Mackenzie River, 60-95 % of it being carried as fine suspended solids and probably sourced from riverbank erosion and thaw slumps. In contrast, lowland rivers of the central and eastern MRB contribute larger shares of dissolved Hg (up to 78 %), likely from recent atmospheric deposition through precipitation. Using load modelling constrained by the new water quality dataset, we estimate that the three largest western tributaries (Liard, Peel and Arctic Red rivers) of the Mackenzie contribute 60 % of the annual MRB THg export and DHg export to the Beaufort Sea during freshet, as well as 51 % of DHg export, while supplying 60% of freshet discharge. Load modelling also reveals a sustained decline in DHg loads of similar to 13 kg yr(-1) between 2001 and 2016 in the lower Mackenzie River, which likely reflect a decreasing trend in atmospheric Hg deposition over most of northwestern Canada during this period. This study highlights the value of community-based water quality monitoring in helping to support assessments of riverine Hg in the MRB in support of the Minamata Convention on Mercury.

Automated summary

Arctic rivers deliver about 40 tons of mercury to the Arctic Ocean each year, with 6% coming from the Mackenzie River Basin (MRB). This study analyzed over 500 water quality measurements from 22 rivers in MRB, showing that the mercury pollution in rivers mainly comes from riverbank erosion and thaw slumps in mountainous areas, as well as atmospheric deposition in lowland rivers. The study estimated that the three largest western tributaries of the Mackenzie River contribute significantly to the overall mercury export to the Beaufort Sea during freshet, while the dissolved mercury loads in the lower Mackenzie River have been declining.