Yellowknife Bay, on the north shore of Great Slave Lake, received loadings of arsenic from gold mining during the 20th century. Previous studies have demonstrated that legacy arsenic in Yellowknife Bay sediments has not been buried efficiently over time and dissolved arsenic continues to diffuse from sediment back into overlying water. This poster will present field and laboratory measurements of sediment arsenic flux from Yellowknife Bay sediments and examine influences of environmental conditions on arsenic mobility. Field measurements on undisturbed sediment cores showed arsenic diffused from sediment to overlying water at the majority of sites sampled over three sampling dates. Sediment arsenic flux was correlated with the bulk solid-phase concentration of arsenic in the surface layer and was higher offshore compared to nearshore. The surface sediment layer was oxic, and porewater concentrations of arsenite (As3+) increased below 3 cm depth. Experimental incubations of mixed sediment (slurries) from Yellowknife Bay showed that the sustained removal of dissolved oxygen resulted in an order of magnitude increase in arsenic flux from sediment. 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.