Solid-phase speciation and post-depositional mobility of arsenic in lake sediments impacted by ore roasting at legacy gold mines in the Yellowknife area, Northwest Territories, Canada

Abstract

Arsenic (As) concentrations in lake sediments in the Yellowknife region, Northwest Territories, Canada, are elevated from the weathering of mineralized bedrock and/or from the aerial deposition of arsenic trioxide (As2O3) generated via the roasting of arsenical gold ores at the now-defunct Giant Mine and other historical roasting operations in the area. Sediment cores were collected from shallow- and deep-water sites in two lakes located downwind of Giant Mine to determine the origin of As in sediments and understand how As concentrations and solid-phase speciation vary with sediment depth and spatially with changes in water depth and sediment texture. Deep-water cores were dated using 210Pb and 137Cs methods. Select sediment intervals were chosen for polished section preparation and analyzed using scanning electron microscopy (SEM)-based automated mineralogy, electron microprobe analysis (EMPA), and synchrotron-based microanalyses to characterize As-hosting solid phases. Sediment redox conditions and the post-depositional mobility of As at the shallow-water sites were assessed by sampling sediment porewaters using dialysis arrays (peepers). At the shallow-water sites, maximum sediment As concentrations occur at the sediment-water interface. In contrast, maximum As concentrations in the deep-water cores occur at sediment depths >15 cm. Dating using 210Pb and 137Cs methods indicates that the As-rich horizons at both deep-water sites are coincident with the period of maximum emissions from the Giant Mine roaster. Four predominant As-hosting solid phases were identified: anthropogenic As2O3 from stack emissions, and authigenic realgar, As-bearing Fe-oxyhydroxide, and As-bearing framboidal pyrite. The contribution of arsenopyrite to total sediment As concentrations was negligible, suggesting that elevated As concentrations are largely derived from ore roasting. In near-surface sediments at the shallow-water sites, the dominant host of As is Fe-oxyhydroxide. Congruent porewater profiles for As and Fe indicate that the post-depositional mobilization of As is governed by the reductive dissolution of As-bearing Fe-oxyhydroxide. Deep-water sediments are enriched in As2O3 that has persisted for more than 60 years. The presence of authigenic As-bearing sulphides suggests that the partial dissolution of As2O3 leads to the formation of less bioaccessible phases where reduced sulphur (S) is available. The distributions of As-hosting solid phases at the shallow- and deep-water sites are interpreted to be both directly and indirectly influenced by water depth. Fine-grained As2O3 particles are less likely to persist in shallow-water areas and typically accumulate in deep-water zones as a result of sediment-focusing processes. Fine-grained organic matter also preferentially accumulates in deeper areas, which influences redox gradients in porewater, the stability field for As-bearing Fe-oxyhydroxide, and the depth of authigenic As-bearing sulphide precipitation.