Microbial populations associated with the generation and treatment of acidmine drainage

Bacterial populations and water chemistry were profiled throughout the grou ndwater flow system associated with the Nickel Rim mine tailings impoundmen t Ontario, Canada. Groundwater containing high concentrations of sulfate (2 000-12,000 mg/l) and iron (500-4000 mg/l) flows from the tailings into an a djacent aquifer, A portion of the plume then discharges to the surface wher e ferrous iron is oxidized creating low pH (pH < 3) conditions. The remaini ng groundwater passes through a permeable reactive barrier which induces su lfate reduction and metal sulfide precipitation. Elevated populations of ir on-oxidizing bacteria (IOB) and sulfur-oxidizing bacteria (SOB) are restric ted to hydrologically defined zones of recharge and discharge. Sulfur oxidi zers are highest in the tailings (1.27 X 10(3) most probable number (MPN)/g ) where sulfide minerals an exposed to oxygen and oxygen-rich recharge wate r. IOB were highest (9.56 X 10(5) MPN/g) where tailings-derived effluent, r ich in Fe(II), discharges to the aerobic surface water environment. Populat ions 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 oxidat ion in the Nickel Rim tailings is occurring immediately above the water tab le where the water content is high. The high water content limits oxygen in gress acid sulfide oxidation, and the associated populations of oxidizing b acteria are low. Populations of sulfate-reducing bacteria (SRB) are elevate d in the tailings and in portions of the down-gradient aquifer where organi c carbon concentrations are high. The highest population (3.73 X 10(7) MPN/ g) of SRB were found where sulfate-bearing water migrates through the organ ic carbon-rich permeable reactive barrier. At locations with high populatio ns of SRB, elevated populations of SOB were also found, suggesting SOB in t hese zones are metabolizing the reduced sulfur species produced by the SRB in adjacent, but disparate, redox microenvironments. (C) 2000 Elsevier Scie nce B.V. All rights reserved.