Metals leachability from mine drainage passive treatment systems vertical flow bioreactor substrates

Vertical flow bioreactors (VFBRs) are an important form of biological passive treatment of mine drainage. The organic substrate in VFBRs provides an anaerobic environment for sulfate reducing bacteria which facilitate the precipitation of metal sulfides from mine impacted waters, effectively removing ecotoxic trace metals from the effluent of the treatment units. However, these treatment units can become clogged and require rehabilitative maintenance (e.g., stirring or replacing the organic substrate) after years to decades in use. Spent substrate from VFBRs may carry a large metal load and must be characterized before disposal or consideration for beneficial reuse. In this study, VFBR substrate from two hard rock mine drainage passive treatment systems in the Tri-State Lead-Zinc Mining District were evaluated for environmental availability of metals through three leaching tests. The leachates used in these tests have varying pH and composition and represent different potential disposal or reuse scenarios for the substrates. Concentrations of metals in leachates did not exceed Resource Conservation and Recovery Act limits but did exceed some Oklahoma Water Resources Board guidelines. Additionally, total metals concentrations exceeded consensus-based toxicity benchmarks for soil and sediment. These factors indicate that, although the substrates are not considered hazardous, caution must be exercised when exploring reuse options for these organic materials.