Evaluating sources of mercury in Canada's Mackenzie River.

Arctic rivers may be the largest net sources of mercury (Hg) to the Arctic Ocean, yet riverine sources of Hg remain poorly characterized compared to atmospheric processes. This article reviews the current state of knowledge on Hg inputs to the Mackenzie River and Valley in Northern Canada from six point and non-point sources. Point sources include the locations of mines, fossil fuel extraction facilities, and retrogressive permafrost thaw slumps. Non-point sources are assessed through models of Hg release from anthropogenic and wildfire-derived atmospheric Hg deposition (GEM-MACH-Hg), permafrost thaw (SiBCASA), and rainfall-induced soil erosion (RUSLE). Ongoing anthropogenic activity is likely a minor contributor to Hg levels in the Mackenzie Valley as production from the fossil fuel and mining industries have steadily declined over the past two decades. Conversely, Hg inputs from atmospheric deposition, permafrost thaw, and permafrost thaw slumps have increased due to climate change and the re-emission of legacy Hg. The widespread influence of atmospheric Hg deposition makes it the dominant source of Hg to both aquatic and terrestrial systems in the Mackenzie Valley, although soil erosion inputs, while higher, are restricted to regions of steep terrain. Climate-driven increases in terrestrial Hg release, particularly from permafrost degradation and erosion, are emerging as key localized drivers of Hg inputs in the Mackenzie Valley.