| Summary: | Yellowknife Bay, Northwest Territories, Canada, is a waterbody valued by surrounding communities for its subsistence, recreational, and cultural use. Located adjacent to the former Giant and Con Mines, Yellowknife Bay has received inputs from mine waste streams enriched in arsenic (As), antimony (Sb), and metals since the late 1930s. Lake sediments in Yellowknife Bay provided a record of metal(loid) contamination from aerially deposited roaster stack emissions, mine effluent, and direct disposal of Giant Mine tailings. A sediment sampling program was conducted in Yellowknife Bay to characterize both As and Sb mineralogy by scanning electron microscopy-mineral liberation analysis (SEM-MLA). Results from nine sediment cores collected in summer (August 2018, July 2019) and winter (March 2019) confirmed that As was mobile relative to layers of enrichment associated with peak mining emissions both downwards, where it precipitated as authigenic sulfides (interpreted to be realgar), and upwards where it was attenuated by Fe-oxyhydroxides and possibly roaster-generated Fe-oxides near the sediment water interface. Antimony minerals appeared to be stable in Yellowknife Bay sediments with no distinct evidence of post-depositional mobility identified. The observed prevalence of arsenic trioxide (As2O3) in near surface sediments proximal to Giant Mine suggested that As and Sb contamination is ongoing, likely from terrestrial weathering of contaminated soils and shoreline outcrops. Arsenic bearing oxide minerals were prevalent in near-surface sediments and may become unstable should redox conditions in the hypolimnion change; prolonged anoxia would destabilize the As phases and release As to bottom waters. Therefore, continual monitoring of hypolimnion conditions in Yellowknife Bay is necessary. M.E.S.
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