Arsenic Mobility and Attenuation in a Natural Wetland at Terra Mine, Northwest Territories, Canada
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Elevated arsenic (As) concentrations in surface water from storing mine tailings in lakes can have a negative impact on local and downstream vegetation and aquatic life. At Terra Mine, an abandoned silver and copper mine in the Northwest Territories, tailings storage in Ho-Hum Lake has resulted in dissolved As concentrations of 50-80 μg/L, exceeding the 5 μg/L maximum guideline for aquatic life. A natural wetland located downstream appears to be attenuating As from surface water. The objectives of this study was to understand the sources of As to the wetland, the effectiveness of the wetland to sequester As, the form and stability of As in the sediments, the processes controlling As mobility, and the effect of seasonal changes in the wetland in the dissolved phase. Arsenic bound to the sediments was determined by analyzing for bulk composition, and As speciation and element association were identified using synchrotron-based bulk XANES and ESEM analysis. Arsenic enters the wetland by surface flow from Ho-Hum Lake, subsurface flow through the waste rock airstrip, and by windblown dust. In spring, dissolved As concentrations in surface water increased downstream. In late summer, a decrease in concentration was observed in the upstream portion of the wetland, however As returned to lake concentrations further downstream. Sediment As concentrations increased over the summer. ESEM and bulk XANES indicate that As was associated with (oxy)hydroxides and secondary sulphides. In the spring, when water levels were high from snow melt, (oxy)hydroxides formed and captured As, while sulphide oxidation in the sediments lead to the release of As into surface water. Over the summer, the onset of reducing conditions from microbial activity drove the formation of As-bearing sulphides and dissolution of (oxy)hydroxides. While As was accumulating in the sediments at most sites in the wetland over the summer, these results suggest that the wetland was not effectively sequestering dissolved As from the surface water, and that sediment-water cycling of As in the wetland as a result of seasonal redox variations were contributing As in the surface water.