Wetlands for minewaters : constructed wetland systems for biological treatment of mining wastewaters in western Tasmania

Mining and lime-treated mineral processing wastewaters from the Hellyer Zn/Pb Mine in western Tasmania, bearing Pb after tailings dam treatment, pass through a series of pilot wetlands. This field-based study focuses on identifying mechanisms behind wetland removal of residual Pb from wastewaters, performance improvement and catchment issues. The background, aims and significance of the industry-sponsored study are outlined. The study site for this applied research project is described and defined. The environmental uncertainties and practical challenges dictated the need for field-based observations supplemented by limited scope experimentation. Problems with sampling are identified and overcome by innovation. Surprisingly, wastewater Pb sinks in wetlands almost exclusively in the form of PbS in the mud. The 50% suspended fraction of Pb uptake is removed from wastewaters by sedimentation, dependent on quiescent wetland residence time. Several native emergent wetland plants suitable for treatment of mine wastewaters are identified using several criteria, bulk and experimental plantings. These include Eleocharis, Juncus, Restio and Triglochin. The wetland plants contribute little to direct removal of Pb from wastewaters; rather, provide structural stability and serve as in situ photosynthetic generators of organic matter. Falling into the mud, the organic matter maintains a decomposer-rich, low-Eh anaerobic mire, and fuels a microbial consortium including three genera of sulphate-reducing bacteria (SR B). By dissimilatory respiration, SRB reduce sulphate and thiosalts diffusing into the mud from the wastewaters to H2S. The \dissolved\" (filterable) 50% fraction of wastewater Pb uptake precipitates as PbS by chemical demand of sulphide acting on various complexed and colloidal forms of Pb present in the wastewater stream. This continues to a lesser degree downstream into the catchment with streambed colonisation by SRB consortia. Wetland operating parameters are measured and evaluated. Key changes to minesite operation and wetland management including optimising conditions for maximum SRB activity waste co-treatment and improving wetland hydrology are implemented or recommended to enhance wetland treatment performance. The river system affected by mine operations is examined applying principles of total catchment management. Catchment areas need a multidisciplinary approach and cooperative proactive management by stakeholders to minimise disturbance degradation and water quality problems and apply remedial strategies."