Biogeochemical Processes Contributing to Nickel Dynamics Within a Mine Tailings Impacted Lake

Nickel mining in the Sudbury area in Ontario, Canada has been pursued since the late 1920's by Falconbridge and INCO. Large tailings deposits have therefore been generated and require remediation. At the Onaping mine site, Moose Lake is used as the treatment pond for tailings. The drainage released has had a profound effect on Moose Lake's geochemistry, rendering it highly acidic (pH below 3.5), metal impacted, and chemically stratified. These conditions removed higher trophic levels, thus making microbial processes dominant. Since Moose Lake discharges into the Onaping River system, waters from its upper basin need to be treated. Presently, chemical treatment is performed, however this procedure is not useful for long-term remediation. Rather, an effective remediation strategy for Moose Lake requires an understanding of metal transport through, and cycling within, its water column and particularly of the role that microbial processes play in influencing metal fate. Since the prevailing geochemical conditions and processes occurring within this lake are not well characterized, our aims are to: determine metal concentrations through the water column; identify potential solid phases retaining metals; and to identify biogeochemical processes controlling the dynamics of their partitioning. Initial samples were collected from June - Sept. 2001 for water column metals (particulate (above 0.45 um), colloidal (0.2-0.45 um) and dissolved (lower than 0.2um), iron (Fe3+ and Fe2+) sulfate and sulfide, microbial community structure and physico-chemical parameters (pH, temperature, O₂, redox, conductivity). Results indicate that the water column is chemically stratified at a depth of 3.5 m (25 m max. depth). Water column pH is less than 3.5 and shows low to anoxic conditions below the chemocline. Metal analyses indicate high dissolved nickel concentrations (700 uM). A depth related decrease of Ni levels was observed near the sediment-water interface, probably due to solid partitioning in the lower depths. High concentrations of hydrogen sulfide and ferrous iron were also detected at this depth, likely indicating sulfate and iron reduction. Metal analyses in the colloidal and particulate phases are ongoing and will be presented. Samples collected throughout the water column by slide samplers reveal two distinct zones rich in microbial cells; one at the chemocline (3.5-4m); and the second coinciding with the zone of sulfate and iron reduction near the sediment-water interface. Observations of DAPI-stained cells have shown morphologically distinct samples from these two zones. Cells from the chemocline zone also displayed a characteristic green pigmentation, which combined to a peak in oxygen observed at the same depth, suggests photosynthetic properties. Cells from the sediment-water interface were coated with black (sulfidic) particles. The taxonomic makeup of these assemblages is currently analyzed by molecular techniques (FISH). Early analyses show that two microbially distinct and active zones occur in this acidified system. Dissimilar processes are likely occurring in these geochemically differing zones with potentially distinct impacts for metal cycling and ultimate fate. Diurnal differences in metabolic activities may influence dynamic processes particularly at the chemocline where photosynthesis occurs. These populations might differently affect metal cycling by their biomineralization capacities or by sorption through microbial-mineral complexes. Results of ongoing analyses will be presented in the context of the long-term goal of this project, aimed at untangling the role of these distinct microbial consortia on metal cycling within Moose Lake.