REVERSING THE IRREVERSIBLE: ECOTOXIC METAL RETENTION IN ECOLOGICALLY ENGINEERED PASSIVE TREATMENT SYSTEMS CONTRIBUTES TO STREAM RECOVERY IN A MINING-DISTURBED WATERSHED

In the mid-1980s, surface water impacts at the Tar Creek Superfund Site, the Oklahoma portion of the historic Tri-State Lead-Zinc Mining District, were deemed due to “irreversible man-made damages”. Two full-scale, ecologically engineered mine water passive treatment systems (PTS) were installed to address some of these waters, contaminated by elevated concentrations of iron (138-192 mg/L), zinc (6-11 mg/L), lead (60-81 μg/L), cadmium (17-20 μg/L) and arsenic (40-64 μg/L). The Mayer Ranch PTS (since 2008) and Southeast Commerce PTS (since 2017) produce circumneutral pH, net alkaline effluents containing ecotoxic metals concentrations meeting in-stream water quality criteria. Each PTS includes multiple process units designed for specific biogeochemical functions. Iron is primarily retained via oxidative mechanisms in aerobic ponds and wetlands. Resulting iron oxyhydroxide solids retain trace metals (especially arsenic) via sorption. Primary lead, zinc and cadmium removal occurs in sulfate-reducing vertical flow bioreactors. Given the relatively flat landscape of the region, off-the-grid aeration technologies (solar- and wind-powered) were implemented to increase oxidative iron removal rates and to address nuisance constituents (biogenic sulfide and oxygen demand) produced by the bioreactors. Annually, MRPTS and SECPTS retain approximately 57,000 and 27,000 kg of iron, 3,300 and 2,200 kg of zinc, 18 and 12 kg of lead, 19 and 8 kg of arsenic, and 5 and 7 kg of cadmium, respectively. The receiving stream has demonstrated substantial water chemical composition improvement and ecological recovery, with documented increases in both fish species richness and abundance, as well as the return of other fauna. Although considerable water quality improvement has occurred in the first-order tributary into which these PTS discharge, additional artesian discharges and substantial tailings pile and pond runoff still pollute the main stem of Tar Creek.