THAW FEATURES IN CONTINUOUS PERMAFROST CONTRIBUTE MINOR AMOUNTS OF MERCURY TO DOWNSTREAM ENVIRONMENTS IN THE OLD CROW RIVER, YUKON

Anthropogenic climate warming is leading to the acceleration of permafrost degradation and potential release of legacy contaminants, such as mercury, to aquatic systems. Point thaw features such as thermokarst lakes and retrogressive thaw slumps are potential hot spots of legacy mercury flux to downstream systems. We studied the temporal evolution of mercury and other trace element concentrations and fluxes in the Old Crow River, a pristine watershed underlain by continuous permafrost in the western Canadian Arctic. We sampled river water chemistry up- and downstream of four retrogressive thaw slumps and along eight tributaries with variable thermokarst lake connectivity during peak ground thaw, in addition to ~weekly sampling at the river mouth across the open water season. Mercury and methylmercury concentrations did not vary significantly between samples up- and downstream of the thaw slumps, nor between physiographically varied tributaries. At the river mouth, contaminant concentrations peaked during the spring freshet and were low throughout the summer to early-autumn ground thaw season. Mercury concentrations were low (< 8 ng/L), and were controlled by suspended sediment with low bioavailability to aquatic organisms; dissolved mercury and methylmercury concentrations were very low. Annual flux and yield were modeled using simple linear regression with daily discharge (r > 0.8). The annual mercury yield (0.8 g/km²/yr), and methylmercury yield (0.01 g/km²/yr) were low and comparable to non-permafrost-influenced wetland and forested rivers. Our results suggest that contaminant flux in this permafrost-dominated Arctic watershed is less controlled by direct release of legacy contaminants from thawing ground. Rather, the magnitude of spring freshet and—to a lesser extent—high-precipitation events in the summer control high-discharge conditions that, in turn, promote erosion of permafrost bank material and thawed slump debris in valley bottom settings.