Ecological development of constructed wetlands built for treating mine tailings water at Tara Mines, Ireland
Mine associated wastewater is characteristically elevated in metals and other contaminants and has been conventionally treated with costly chemical applications. The development of passive treatment systems such as wetlands, which employ both biotic and abiotic processes, has been recognized as an economically feasible, ecologically acceptable treatment technology in the last decade. Not only can constructed wetlands provide an efficient facility for treating wastewater, they can also offer ancillary benefits such as ecological niches and therefore be of educational and often recreational value to society as well. Two experimental-scale treatment wetlands were constructed at an active lead/zinc mine near Navan, Ireland in 1997 to treat water enriched with sulfate and metals. Each system comprised three 12 m2 (2 m depth) in-series surface-flow cells viz., inflow, vegetated and outflow. Sulfate-reducing bacteria were indigenous in the anaerobic spent mushroom substrate used, where biological reduction of sulfate to sulfide occurred. Sulfide subsequently precipitated with metals from the water. The treatment efficiency of the wetlands was promising with concentrations of sulfate (up to 29 g m-2 day -1 (69%)), lead (6.6 mg m-2 day -1 (64%)) and zinc (70 mg m-2 day -1 (98%)) successfully removed from the wastewater. The ecological functioning of these constructed wetlands was also demonstrated with food webs, nesting niches and refuge sites afforded by colonizing communities of macroinvertebrates, macrophytes, microorganisms and other visiting wildlife. By 15 months following construction of the treatment wetlands, 30 species of macroinvertebrates were identified in system 1 and 21 species in system 2, while 3 plant species, 3 algae species and 1 moss had also colonized the ecosystems. Sulfate reducing bacteria genera included Desulfotomaculum, Desulfovibrio, Desulfococcus and Desulfobulbus. Annual dieback of planted species Typha latifolia and Phragmites australis contributed substantial amounts of biomass to the ecosystems, which led to a renewal of the carbon supply that drove the biologically mediated treatment process. It is speculated that the ecological diversity of the wetlands contributed to their treatment success based on inherent ecosystem complexity.