Remobilization of arsenic from contaminated sediments in a subarctic lake: field and laboratory measurements of sediment fluxes and environmental controls

Abstract:

Yellowknife Bay, on the north shore of Great Slave Lake (Northwest Territories, Canada), received loadings of arsenic from gold mining during the 20th century. Although most of the arsenic released into the bay occurred during earlier years of mining operation (1940s to 1970s) when few pollution controls were in place, present-day surface layers of lake sediments contain high concentrations of solid-phase arsenic (up to 3,200 µg/g dry weight) and porewater arsenic (up to 3 mg/L). Legacy arsenic contamination in Yellowknife Bay sediments has not been buried efficiently over time, and in the future, potential shifts in temperature and dissolved oxygen regimes with climate change could potentially affect arsenic mobility. The objective of this research was to evaluate the sediment arsenic flux from Yellowknife Bay sediments and potential influences of environmental conditions in this subarctic ecosystem.

Both field and laboratory measurements were conducted to estimate the arsenic flux from contaminated sediment in Yellowknife Bay. Using undisturbed sediment cores, short-term (48 hour) field measurements showed arsenic diffused from sediment to overlying water at the majority of sites (20 out of 28) sampled over three sampling dates (August 2018, March 2019 and July 2019). Sediment arsenic flux varied from -65 to 1,520 µg/m²/day, was correlated with the bulk solid-phase concentration of arsenic in the surface layer, and was higher offshore compared to nearshore sites. The surface sediment layer (approximately 0-3 cm depth) was oxic, and porewater concentrations of arsenite (As³⁺) increased with depth below that layer. Laboratory experimental incubations of mixed sediment (slurries) from Yellowknife Bay provided similar arsenic flux estimates over longer time periods (weeks). Sediment temperature (5-22 °C) had little influence on arsenic diffusion from contaminated sediment under well-oxygenated conditions but an order of magnitude increase in sediment arsenic flux occurred following the removal of dissolved oxygen (onset of anoxia) and sustained reducing conditions in the experimental incubations. These findings highlight the dynamic behaviour of arsenic in mining-contaminated lake sediments and the importance of redox conditions in the surface layer for controlling sediment arsenic mobility.