English

Abstract

A 12-cell passive treatment system was constructed to treat an abandoned coal mine discharge near Hartshorne, Oklahoma. The system was designed to treat a net-acidic discharge (1400 mg/L) characterized by elevated concentrations of metals (Fe 765 mg/L; Mn 18 mg/L; Na 1800 mg/L), anions (Cl - 225 mg/L; SO4 2- 7800 mg/L), with pH 5.4, and flow ~40 L/min. The mine water was discharging from an existing air shaft, which was converted into a vertical anoxic limestone drain (VALD) to generate alkalinity. The VALD is followed by an alternating series of oxidation ponds and vertical flow cells, a polishing wetland, and finally an existing farm pond. System construction was completed in late 2005, but discharge from the VALD did not occur until January 2007 due to a prolonged regional drought. In the first 1.5 years of operation and monitoring, the VALD has consistently produced water with alkalinity concentrations in excess of 400mg/L as CaCO3. More importantly, the system has been effective in removing iron and manganese, with final concentrations of 0.67±0.62 mg/L and 4.39±4.71 mg/L, respectively. There has also been significant removal of trace metals such as cadmium and lead. The system is now discharging net-alkaline waters to the receiving stream. At current rates, the system generates ~4 tons/yr net alkalinity as CaCO3 and retains ~12.5 tons/year iron.