Passive Aerobic Treatment of Net-Alkaline, Iron-Laden Drainage from a Flooded Underground Anthracite Mine, Pennsylvania, USA

Authors

Charles A. Cravotta III, US Geological Survey, PA Water Science Center, 215 Limekiln Rd, New Cumberland, PA, USAFollow

Date of this Version

2007

Comments

Published in Mine Water Environ (2007) 26:128–149.

Abstract

This report evaluates the results of a continuous 4.5-day laboratory aeration experiment and the first year of passive, aerobic treatment of abandoned mine drainage (AMD) from a typical flooded underground anthracite mine in eastern Pennsylvania, USA. During 1991–2006, the AMD source, locally known as the Otto Discharge, had flows from 20 to 270 L/s (median 92 L/s) and water quality that was consistently suboxic (median 0.9 mg/L O₂) and circumneutral (pH ~ 6.0; net alkalinity >10) with moderate concentrations of dissolved iron and manganese and low concentrations of dissolved aluminum (medians of 11, 2.2, and <0.2 mg/L, respectively). In 2001, the laboratory aeration experiment demonstrated rapid oxidation of ferrous iron (Fe²⁺) without supplemental alkalinity; the initial Fe²⁺ concentration of 16.4 mg/L decreased to less than 0.5 mg/L within 24 h; pH values increased rapidly from 5.8 to 7.2, ultimately attaining a steady-state value of 7.5. The increased pH coincided with a rapid decrease in the partial pressure of carbon dioxide (PCO₂) from an initial value of 10^-1.1 atm to a steady-state value of 10^-3.1 atm. From these results, a staged aerobic treatment system was conceptualized consisting of a 2 m deep pond with innovative aeration and recirculation to promote rapid oxidation of Fe²⁺, two 0.3 m deep wetlands to facilitate iron solids removal, and a supplemental oxic limestone drain for dissolved manganese and trace-metal removal. The system was constructed, but without the aeration mechanism, and began operation in June 2005. During the first 12 months of operation, estimated detention times in the treatment system ranged from 9 to 38 h. However, in contrast with 80–100% removal of Fe²⁺ over similar elapsed times during the laboratory aeration experiment, the treatment system typically removed less than 35% of the influent Fe²⁺. Although concentrations of dissolved CO₂ decreased progressively within the treatment system, the PCO₂ values for treated effluent remained elevated (10^-2.4 to 10^-1.7 atm). The elevated PCO₂ maintained the pH within the system at values less than 7 and hence slowed the rate of Fe²⁺ oxidation compared to the aeration experiment. Kinetic models of Fe²⁺ oxidation that consider effects of pH and dissolved O₂ were incorporated in the geochemical computer program PHREEQC to evaluate the effects of detention time, pH, and other variables on Fe²⁺ oxidation and removal rates. These models and the laboratory aeration experiment indicate that performance of this and other aerobic wetlands for treatment of net-alkaline AMD could be improved by aggressive, continuous aeration in the initial stage to decrease PCO₂, increase pH, and accelerate Fe²⁺ oxidation.