PASSIVELY ADDRESSING FERRUGINOUS LEAD-ZINC MINE WATERS THROUGH HYDRO-BIOGEOCHEMICAL AND ECOLOGICAL ENGINEERING: THE FIRST FULL-SCALE MINE WATER TREATMENT SYSTEM IN THE TRI-STATE DISTRICT

Artesian discharges and mine waste runoff cause considerable surface water degradation at the Tar Creek Superfund Site, part of the historic Tri-State Lead-Zinc Mining District of OK, KS and MO. At the study site, net-alkaline, ferruginous, perennial borehole discharges flowed unabated for 30 years from underground mines and considerably degraded the physical, chemical, and biological integrity of a first-order tributary and Tar Creek. Passive treatment technologies relying on natural hydro-biogeochemical and microbiological processes were determined to provide a viable alternative to costly and laborious active treatment technologies. Based on a comprehensive water quality and quantity characterization study, a large multi-cell passive treatment system was designed to receive approximately 1000 L/minute of mine water flowing from these abandoned boreholes (pH 5.95±0.06, total alkalinity 393±18 mg/L CaCO3, total acidity 364±19 mg/L CaCO3, Fe 192±10 mg/L, Zn 11±0.7 mg/L, Cd 17±4 ug/L, Pb 60±13 ug/L and As 64±6 ug/L). The passive treatment system (~ 2 ha) includes an initial oxidation pond followed by parallel treatment trains consisting of aerobic wetlands, vertical flow bioreactors, re-aeration ponds (including wind- and solar-powered aeration), and horizontal-flow limestone beds. Waters from the parallel trains are recombined in a polishing wetland prior to final discharge. All flows are via gravity and no fossil fuels are needed for operation. In the first year of operation, mean final effluent waters had pH >7, were net alkaline and contained ~ 1 mg/L total Fe and 0.2 mg/L total Zn, with concentrations of Cd, Pb and As below detectable limits. Through an engineered combination of oxidative and reductive processes, the treatment system is successfully removing nearly 100 kg Fe/d, 4.5 kg Zn/d, 42 g Pb/d, 11 g Cd/d and 45 g As/d. The system addresses approximately 20% of the contaminant mass loading from artesian mine pool discharges in the watershed and has a 30-year design life requiring limited operation and maintenance. Despite nearly a century of neglect, this project, along with comprehensive stream and targeted soil/mine waste monitoring efforts, indicates that ecological engineering technologies hold considerable promise to provide positive environmental benefit in this watershed.