| Summary: | In this study, total As (T-As) levels in superpermafrost groundwater at a site in the High Arctic doubled over the course of the summer thaw. This increase was not due to snow input, as levels in snow were negligible. This increase in T-As did not correspond with a decrease in Eh, nor a rise in soluble Fe(II). It did, however, correspond with a shift in As speciation from arsenate to arsenite suggestive of reducing conditions. In the absence of predominant reducing conditions, the highly alkaline nature of the melting snow and concomitant large input of HCO3- may have played an important role in the increase of As in groundwater during the summer thaw. Laboratory studies found that dissolved As (D-As) release under anaerobic conditions depended on the organic matter content of soil, with organic soils releasing D-As under reducing conditions and mineral soils sequestering D-As. In temperate soils, the release of D-As from organic soils is greatly accelerated due to the activity of anaerobic microbes. In northern soils, the same phenomenon may occur, with greater microbiological activity in organic soils where there is more labile C and nutrients than in mineral soil. The sequestration of As in mineral soil is postulated to have occurred due to preferential sorption of arsenite to ferrihydrite or possibly to green rust minerals present under anaerobic conditions. Supporting this, arsenite sequestration occurred to a greater extent compared to arsenate, which is in agreement with the relative affinities of these two species for ferrihydrite. Evidence from this study suggests that the As cycle on Truelove Lowland is dominated by the desorption of As due to HCO3- input each year during the spring melt linked to the sorption of As to ferrihydrite or green rust present in underlying mineral soils. The sequential thawing of the soil’s active layer and large inputs of HCO3- are unique to northern environments. Thus, this delicate balance of two sorbing processes should be born in mind in northern development. Large inputs of soluble organic matter or nutrients could cause increased solubilization and mobility of D-As during the summer thaw when soils become flooded.
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