Introduction: Resource extraction activities can impact water resources for decades to millennia. Sustainable water management strategies utilizing natural energies with limited maintenance and operation requirements are needed for abandoned or derelict sites producing legacy mine waters. Passive treatment, based on provision of the appropriate conditions for natural biogeochemical processes to occur, has been demonstrated to have positive environmental impacts. The objectives of this study were to address impacts to surface waters once deemed due to "irreversible man-made damages" that resulted in minimal efforts to address ecological and human health risk from legacy mine waters for over 30 years. The study documents the six year performance of a full-scale passive treatment system and provides design guidance for future systems.
Methods: In the Tar Creek (Oklahoma, USA) watershed of the abandoned Tri-State Lead-Zinc Mining District, a full-scale passive treatment demonstration project, coupled with recent watershed-scale environmental monitoring efforts, indicate that ecological engineering (the design and construction of sustainable ecosystems that integrate human society with the natural environment for the benefit of both) solutions exist for metal-contaminated waters. For the targeted mine water discharges, a large multi-cell passive treatment system (~2 ha surface area), designed to receive 1000 liters per minute of abandoned mine water (pH 5.95Â±0.06, Fe 192Â±10 mg/L, Zn 11Â±0.7 mg/L, Cd 17Â±4 g/L, Pb 60Â±13 g/L and As 64Â±6 g/L), includes an initial oxidation pond followed by parallel treatment trains of aerobic wetlands, vertical flow bioreactors (incorporating a compost/wood chip substrate), re-aeration ponds, and horizontal-flow limestone beds, and a single final polishing wetland/pond. Inflows to the system are artesian and flow through the system is via gravity-driven head differences. No fossil fuels are expended in system operation. Water quality and quantity have been regularly monitored for six years. The extent and magnitude of specific biogeochemical and microbiological treatment mechanisms, as well as indicators of ecosystem structure and function, have been evaluated in all process units.
Results and Discussion: Overall, final effluent waters had pH >7, were net alkaline and contained <0.15 mg/L total iron and <0.25 mg/L zinc, with concentrations of cadmium, lead and arsenic below detectable limits. Iron was removed predominantly via aerobic means in the first process unit at a rate of 20.4 +/- 5.4 grams square meter per day. Substantial arsenic, cadmium and lead removal occurred via sorption and co-precipitation to ferric oxyhydroxide solids in the initial aerobic units. Degassing of excess carbon dioxide (> 0.3 atmosphere) present in the upwelling mine waters (due to dissolution of carbonate host rock) resulted in increased pH in the first process unit, despite proton production due to iron hydrolysis. Zinc, nickel, cadmium and lead were further removed via bacterial sulfate reduction, organic complexation and carbonate formation in vertical flow bioreactors and, to a lesser extent, limestone beds. Sulfate reduction rates demonstrated seasonality but were consistently greater than 300 mmol per cubic meter per day in the vertical flow bioreactors. Initial growing season sulfide production resulted in nuisance odor problems. Off-the-grid solar- and wind-powered re-aeration of the reduced waters increased dissolved oxygen and stripped biochemical oxygen demand and sulfide produced by these reactors. Full system mass retention of specific contaminants of concern are estimated to be 57,000 kg iron, 3,300 kg zinc, 300 kg nickel, 18 kg arsenic, 17 kg lead and 5 kg cadmium on an annual basis. Ecological data demonstrate that richness and diversity of specific indicator taxa (vegetation, amphibians, and Odonata) are equal to or greater than reference conditions. Fish community recovery has been demonstrated in the receiving stream, with catch per unit effort increasing from 3 - 300 times for six species present before and after construction. Four additional species, initially absent, were documented in post-construction stream sampling. Contaminated waters were shown to be eminently treatable via sustainable passive treatment technologies and, furthermore, several indicators of ecosystem structure and function demonstrate ecological recovery. Conceptual designs for watershed-scale passive treatment have been developed and a second system is slated for construction in summer 2015.
Conclusions: Passive treatment, an ecological engineering technique focused on exploitation of natural processes in constructed ecosystems, provides effective and environmentally relevant solutions to mine water pollution. In this study, waters once deemed untreatable are being addressed by installation of a passive treatment system with quarterly (four times per year) operational and maintenance visits, mainly focused on vegetation and piping maintenance. Legacy mine water pollution, especially from abandoned and derelict locations, is a chronic problem throughout the world. Although designs are site- and region-specific, passive treatment systems provide an alternative to traditional labor- and chemical-intensive active technologies.