ESTABLISHING BACKGROUND ARSENIC CONCENTRATIONS IN AN AREA IMPACTED BY ~75 YEARS OF GOLD MINING IN SUBARCTIC CANADA

We obtained near total element geochemistry on 211 near-surface sediment samples from lakes along a transect across the Western Interior Platform to the central portion of the Slave Geological Province in the Northwest Territories, Canada, to document regional concentrations of arsenic (As) and other elements in lake sediments. We focused sampling near the City of Yellowknife, Northwest Territories, where ~75 years of gold mining occurred and background metal(loid) concentrations are poorly known. Concentrations of major and trace elements, including elements of potential human and ecological concern (As, Cd, Cr, Cu, Pb, Zn), were extracted from sediments using a modified aqua regia digestion. Concentrations of As exceed Canadian federal guidelines for the protection of aquatic life in most of the lakes sampled in the Slave Geological Province. Seventy one percent (n=149) of all sediment samples contain As concentrations higher than the Canadian Council of Ministers of the Environment (CCME) Interim Freshwater Sediment Quality Guideline of 5.9 mg/kg and 54% (n=114) of the samples contain As concentrations that exceed the CCME Probable Effect Level of 17 mg/kg. Sediments with the highest As concentrations are from lakes near the City of Yellowknife and likely reflect a combination of contamination associated with gold mining and geogenic input from mineralized bedrock and derived surficial materials (median As concentration 107.9 mg/kg, range 6.30->10,000+, n=95). Synchrotron-based µXRD demonstrates that the anthropogenically-derived mineral phase arsenolite (As₂O₃) is present in sediments of several lakes near Yellowknife. Arsenic concentrations in lake sediments we sampled elsewhere in the Northwest Territories are lower (Ingraham Trail region, median As concentration 10.6 mg/kg, range 1.9-101.6, n=27; Tibbitt to Contwoyto Winter Road region, median As concentration 7.9 mg/kg, range 0.3-101.4, n=52; Western Interior Platform, median As concentration 1.1 mg/kg, range 0.1-7.1, n=37). Based on our data and a review of existing literature background As concentration in lake sediments appear to be ~25 mg/kg for the Yellowknife region, and lower for the other regions sampled. Other elements (Cr, Cu, Pb, Zn) are below sediment quality guidelines in the majority of lake sediments sampled.