Northwest Territories Geological Survey

A two year study (2018 to 2020) is investigating the present-day and future stability of legacy arsenic contamination in Yellowknife Bay sediments that originated from local gold mining activities. Using a combination of quantitative methods (field and laboratory experiments, paleolimnology, and mass balance modelling), we will estimate the diffusion of arsenic from sediments to overlying water in the bay, investigate environmental factors that control this process, and develop predictions of how long-term environmental change from climate warming during the 21st century could impact arsenic cycling and levels of water arsenic in Yellowknife Bay. This information will inform stakeholders and decision makers who are involved in the environmental management of this important waterbody for residents of Ndilo, Detah, and Yellowknife.

This presentation will focus on initial field incubation experiments that were conducted in Yellowknife Bay in August of 2018. Twelve sediment cores were collected at sites of varying water depth and distance from the Giant Mine site. The cores were incubated in holders near shore or at the lake bottom for six days to maintain in situ temperature and light conditions. Overlying water in each of the cores was sampled on days 0, 2, 4 (or 5) and 6 to measure short-term fluxes of arsenic from contaminated sediment.

Both positive and negative exchange of arsenic was observed between overlying water and sediment in the cores. Three cores from shallow water sites (depth of 1-3 m) showed a small loss of arsenic from the overlying water at a rate of -49±3 µg/m2/day. In contrast, offshore cores collected in deeper water (depth of 7.5-19 m) showed an increase of arsenic in overlying core water, with a wide range in sediment flux of 30-1520 µg/m2/day and a mean of 525±522 µg/m2/day. The sediment flux of arsenic from a core collected at the tailings beach at the north end of Yellowknife Bay was minimal (45 µg/m2/day), possibly due to the low porosity of the tailings.

Another mining contaminant, antimony, was also measured to determine sediment fluxes for comparison with arsenic. In five of the cores, there was no detectable change in antimony concentration in overlying water. In the remaining seven cores, a small flux of antimony was observed between sediment and overlying water (37±34 µg/m2/day).

Sediment flux measurements of arsenic will be discussed in relation to sediment characteristics, sediment porewater profiles, and site location. Next steps in the study will also be outlined. Preliminary results showed there was substantial short-term flux of arsenic from contaminated sediment to overlying water during summer near the Giant Mine site. Sediments of Yellowknife Bay are a leaky reservoir of legacy arsenic.