English

Abstract

State and federal reclamation programs, mining operators, and citizen- based watershed organizations have constructed hundreds of passive systems in the eastern United States over the past 20 years to provide reliable, low cost, low maintenance mine water treatment in remote locations. In 2000, we evaluated 116 systems comprised of eight system types in eight states. We revisited 14 of these sites in 2004 to confirm results from the earlier study. Each system was monitored for influent and effluent flow, pH, net acidity, and metal concentrations. Performance was normalized among types by calculating acid load removed, and also by converting construction cost, projected service life, and metric tonnes of acid load treated into cost per tonne of acid treated. Of the 116 systems, 105 reduced acid load (90%). Average acid load reductions were 0.8 t/yr for Ponds; about 9 t/yr for open limestone channels (OLC), anaerobic wetlands (AnW), aerobic wetlands (AeW), and vertical flow wetlands (VFW); 76 t/yr for slag leach beds (SLB), and about 15 t/yr for limestone leach beds (LSB) and anoxic limestone drains (ALD). Average removal rates ranged from 18 to 2,334 g/day/t for the limestone systems, and 1.7 to 87 g/m 2 /day for the Ponds and wetlands. Average costs for acid removal varied from $36/t/yr for SLB to $1,468/t/yr for Ponds. The 2004 data showed slightly greater removal efficiencies for two Ponds, two VFWs, and one LSB. Large declines in removal were found for one AnW, two VFWs, one ALD, and one OLC. Two OLCs greatly increased efficiency. Most passive systems were effective for >5 yrs, yet there was wide variation in performance within each system type.