The Revival of a Failed Constructed Wetland Treating a High Fe Load AMD

Abstract

Acid mine drainage (AMD) from abandoned mines has significantly impaired water quality in eastern Kentucky A small surface flow wetland constructed ill 1989 to reduce AMD effects and subsequently failed after six months of operation was renovated by incorporating anoxic limestone drains (ALDs) and anaerobic subsurface drains promoting vertical flow through successive alkalinity producing (SAPS) limestone organic compost beds. Two years of post renovation monitoring indicate that mean iron concentrations have decreased from 787 to 391mL(-1), pH increased from 3.38 to 6.46 and acidity has been reduced from 2,244 to 199mgL(-1), (CaCO3 equivalent). Mass removal rates averaged 98% for Al, 95%, for Fe, 94% for acidity, 55% for sulfate and 49% for Mn during the study period. The combination of ALDs and SAPS technologies used in the renovation and the sequence ill which they were implemented within the wetland system appeared to provide sufficient buffering and longer residence time rendering a promising design for treatment of this and other sources of high metal load AMD. Characterization of sediments from abiotic/aerobic zones within the treatment system showed low SO4/Fe ratios in initial abiotic treatment basins, supporting the formation of jarosite and goethite. As the ratios increased due to treatment and subsequent reductions in iron concentration, jarosite was transformed to other Fe-oxyhydroxysulfates and goethite formation was inhibited. Amorphous iron minerals such as ferrihydrite and Fe(OH)(3) were dominant in biotic wetland cell substrates. However, Low Fe3+ activity, redox potential, and oxygen diffusion rates in the anaerobic subsurface environment inhibited crystalline iron precipitation. The formation of gypsum, rhodochrosite, and siderite as byproducts of alkalinity-generating reactions in this system also appeared to have an impact on S, Mn, and Fe solubility controls. Sustaining alkaline conditions within the wetland was necessary for maintaining metal retention consistency and long-term treatment efficiency.