Investigating the mobility of arsenic in subarctic mining pollution-affected peatlands near Yellowknife, Northwest Territories

Arsenic (As) is a pollutant of global concern that is detrimental to human and environmental health. Between 1948 and 2004, mining operations near Yellowknife, NT released over 20,000 tonnes of As trioxide (As2O3) that continues to persist in the surrounding landscape. Although much effort has been put into studying the chemical forms, spatial distribution, ecological impacts, and environmental cycling of As in the area, more work is needed to determine the controls of As mobility between the terrestrial and aquatic components of the landscape. Soils are known reservoirs of legacy As that can release it downslope to lakes. With climate change rapidly warming and changing the precipitation patterns of the area to include more rain in late summer and fall, soils may be more susceptible to As release than they were when most runoff occurred over frozen soils in the spring. Peat soils are of particular interest as peatlands tend to exist on the boundary between terrestrial and aquatic systems, act as hot spots of biogeochemical activity, and provide a major source of dissolved organic carbon (DOC), allowing them to act as moderators of solute movement into adjacent waters. In this study, I investigated the mobility of As in peatlands through field observations and diffusive flux modelling of four subarctic peatlands, performed a sequential extraction protocol to determine forms of As present in surficial peat, and conducted a wetting/drying experiment to simulate the effects of prolonged saturation or drought.\nPeat porewater chemistry remained relatively consistent throughout the summer at each of the four study sites despite hydrologic evidence that advective fluxes should export As from three of them into adjacent lakes. As may have been retained at the peat-lake interface due to interactions between complexes of As-Fe-DOC, pH, and redox conditions. As and DOC from the surrounding catchment did accumulate in porewater at the base of an upslope ravine before being flushed by a sizeable mid-August rain event, indicating that depressions in upslope bedrock could act as collectors and sources of As. Further, diffusive fluxes modelled to provide a low-end estimate of As export into adjacent lakes were an order of magnitude larger than the expected fluxes from lake sediments, providing evidence that As fluxes from peatlands may be more important than currently known.\nThe majority (50-70%) of As stored within surficial peat was found to be in residual non-extractable forms that would be expected to be immobile. Much of this As is likely in the form of As2O3, as it was originally deposited decades ago, which underscores the remarkable stability of As in legacy mining emissions in the area. The remaining 30-50% of As was in extractable forms with 10-25% in forms that would be considered more readily mobile. Peat that was subjected to prolonged experimental saturation released more As and DOC than peat that remained dry. The wet treatment released As quantities that would exhaust the pool of readily mobilized As after only a few periods of saturation. This suggests that prolonged saturation, and resulting decomposition and reducing conditions facilitates the release of As held in other fractions including As bound to organic matter or Fe/Mn oxyhydroxides, or As within As2O3. The effect of the wet treatment was particularly pronounced in peat from an upland ravine site that contained more easily biodegradable vascular plants and feather mosses (Order Hypnales) than more resistant Sphagnum mosses that tend to dominate the three other sites. Therefore, upland sites could be more prone to decomposition and resulting As release due to prolonged saturation.\nThese results suggest that peatlands may be larger sources of As to lakes than they are currently known to be and could become even greater sources as climate change makes late summer rainfall more common. Upslope depressions can accumulate As throughout the summer before being flushed downstream with rain, while the saturation of peat from these rains may promote further release of As through changing redox conditions and enhanced decomposition of the peat.

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