Remobilization of legacy arsenic from sediment in a large subarctic waterbody impacted by gold mining

Abstract

Arsenic contamination from mining poses an environmental challenge due to the long-term mobility of this redox-sensitive element. This study evaluated arsenic mobility in sediments of Yellowknife Bay (Canada), a large subarctic water body impacted by gold mining during the 20th century. Short-term measurements of arsenic fluxes from sediment, arsenic profiling of the water column and sediment porewater, and mass balance modelling were conducted to assess the importance of sediment as an arsenic source. Sediment arsenic fluxes ranged from -65 to 1520 μg m^-2 day^-1 and a positive flux (to overlying water) occurred at 13 of 20 sites. Elevated fluxes at sites adjacent to the mine site were among the highest published for lakes with a well-oxygenated water column. Redox boundaries were typically 2-3 cm below the sediment surface as indicated by porewater profiles of iron, manganese, and arsenic, with arsenic maxima of 65-3220 μg L^-1 predominately as arsenite. Arsenic flux was positively related to its solid-phase concentration in sediment (10-3206 μg g^-1). Modelled loads indicated sediment was a principal source of arsenic to the water column. Internal recycling of legacy arsenic between sediment and surface water will continue to impede a return to background conditions in Yellowknife Bay for decades.