Selenium (Se), an essential micro nutrient, is toxic to aquatic life at slightly higher water concentrations. Watersheds receiving leachate from selenium rich sources require large-scale, long-term treatment to mitigate Se toxicity. We applied the principles of anaerobic bacterial bioreactors, previously successful in small scale Se mitigation, to a whole end-pit lake ecosystem. Fertilization of the lake with N and P increased primary production, creating a meromictic, anoxic layer, and enhanced the habitat for locally present, anaerobic, Se and sulfur reducing bacteria. Within two years, Se concentrations were reduced ten-fold, reaching water-quality guideline values. The successful experiment demonstrated a novel treatment of large volumes of Se-contaminated water, and introduced an inexpensive method to mitigate a persistent aquatic pollutant of global concern.
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