A survey of constructed wetlands for acid coal mine drainage treatment in the Eastern United States

Abstract

The oxidation of pyritic minerals, exposed to oxygen and water during the mining of coal, results in the formation of acid mine drainage (AMD), which is characterized by low pH and high concentrations of dissolved sulfate, iron, and other metals. Federal and State regulations require that discharges from coal surface mines meet water quality criteria. Toward that end, chemical treatment of AMD, usually with soda ash briquettes, lime, limestone, or sodium hydroxide, is effective but expensive. Recently, man-made wetlands have been proposed as a low-cost, low-maintenance alternative to chemical treatment of AMD. To assess the status of man-made wetland treatment of AMD in the eastern U.S., a survey was conducted by the U.S. Office of Surface Mining, Reclamation, and Enforcement. As of May 1988, 142 wetlands had been constructed for AMD treatment. In 50% of the constructed wetlands, treatment efficiencies (reductions in concentration) for H+, acidity, Fe, Al, Mn, and SO4 2 of at least 68, 67, 81, 48, 34, and 8%, respectively, were obtained. However, over 11% of the constructed wetlands yielded greater concentrations in the effluent from the wetland than were present in the influent AMD for one or more of these 6 chemical parameters. Treatment efficiency generally was not correlated with design criteria (e.g., area of wetland, depth of the organic substrate in the wetland, AMD flow rate, metal loading rates). Also, treatment efficiency was generally not affected by either the type of organic substrate used in wetland construction or the addition of lime and/or fertilizer to the constructed wetland. The effectiveness of wetland treatment of AMD is not only extremely variable, but also presently not predictable.