Microbial populations associated with the generation and treatment of acid mine drainage

Abstract

Bacterial populations and water chemistry were profiled throughout the groundwater flow system associated with the Nickel Rim mine tailings impoundment Ontario, Canada. Groundwater containing high concentrations of sulfate (2000–12,000 mg/l) and iron (500–4000 mg/l) flows from the tailings into an adjacent aquifer. A portion of the plume then discharges to the surface where ferrous iron is oxidized creating low pH (pH<3) conditions. The remaining groundwater passes through a permeable reactive barrier which induces sulfate reduction and metal sulfide precipitation. Elevated populations of iron-oxidizing bacteria (IOB) and sulfur-oxidizing bacteria (SOB) are restricted to hydrologically defined zones of recharge and discharge. Sulfur oxidizers are highest in the tailings (1.27×10 3 most probable number (MPN)/g) where sulfide minerals are exposed to oxygen and oxygen-rich recharge water. IOB were highest (9.56×10 5 MPN/g) where tailings-derived effluent, rich in Fe(II), discharges to the aerobic surface water environment. Populations of both iron and SOB bacteria in the zone of active oxidation are low compared to those found at other, less mature, tailing sites. Active oxidation in the Nickel Rim tailings is occurring immediately above the water table where the water content is high. The high water content limits oxygen ingress and sulfide oxidation, and the associated populations of oxidizing bacteria are low. Populations of sulfate-reducing bacteria (SRB) are elevated in the tailings and in portions of the down-gradient aquifer where organic carbon concentrations are high. The highest population (3.73×10 7 MPN/g) of SRB were found where sulfate-bearing water migrates through the organic carbon-rich permeable reactive barrier. At locations with high populations of SRB, elevated populations of SOB were also found, suggesting SOB in these zones are metabolizing the reduced sulfur species produced by the SRB in adjacent, but disparate, redox microenvironments.