The effects of migration on ca. 100-year-old arsenic-rich mine tailings in Cobalt, Ontario, Canada

Abstract

Century-old tailings at the Beaver Mine site, Cobalt, Ontario, Canada are migrating downstream to previously uncontaminated Kirk Lake, resulting in metal dispersal into the environment. These metal-rich tailings have high arsenic concentrations and exceed the Ontario regulatory standards for arsenic in soil (18 mg/kg) by up to 240 times. Arsenic concentrations in surface water exposed to the tailings and waste rock were found to be up to 268 times the acceptable limit of 5 µg/L for the protection of aquatic life. At the Beaver Mine Site, where the tailings have sat largely undisturbed for the last century, prevailing redox conditions play an important role in the behavior of arsenic and other metals. Surface enrichment of metals was evident in tailings located above the water table and at both the Beaver Mine and Kirk Lake sites. Arsenic enrichment was evident in clay layers within the tailings. A principal component analysis and an analysis of similarity suggest that the migration process did not significantly alter the metal composition of the tailings (ANOSIM p = 0.21), likely because the migration process is relatively rapid compared to leaching mechanisms that would alter metal composition during migration. Our results indicate that the transportation of these century-old, metal-rich, tailings is having minimal impact on reducing their metal concentrations and as these largely unmanaged tailings continue to be transported through the environment they pose a risk to previously uncontaminated lake and stream ecosystems.