Laboratory mesocosm studies of Fe, Al, Mn, Ca, and Mg dynamics in wetlands exposed to synthetic acid coal mine drainage

Abstract

To evaluate the potential for constructed wetlands to treat acid coal mine drainage, six model wetland mesocosms (each 2.4 m × 15 cm) were filled with Sphagnum peat (15 cm deep), planted either with cattails (Typha latifolia) and living Sphagnum, living Sphagnum only, or left as bare peat (2 mesocosms per treatment). The model wetlands were exposed to synthetic acid coal mine drainage (pH 3.5, concentrations of Fe2+, Al3+, Mn2*, Ca2+, and Mg2+ of 78.8, 10.0, 5.2, 12.0, and 4.5 mg L−1, respectively) at a rate of 90 mL min−1, 6 hr d−1, 5 d wk−1, over a 16 week period. Chemical analysis of peat at periodic intervals indicated that the model wetlands were net sources of Al3+, Mn2+ Ca2+ and Mg2+, but net sinks for Fe2+. Type of vegetation had no significant effect on Fe2+ retention; of the 204 g of Fe2+ added to the model wetland systems, 162 g were retained. Formation of Fe oxides accounted for 73 to 86% of the Fee' retention, with exchangeable Fe contributing 0.2 to 1.2%, organically bound Fe contributing 4 to 19%, and residual Fe contributing 7 to 15% of total Fee' retention. Fe retention was greatest at the inflow ends of the model wetlands where Fe retention appeared to reach saturation at a final Fe concentration in the peat of 235 mg g−1. At the rate of application of the synthetic acid mine drainage, we estimated that the model wetland systems would have reached complete Fe saturation after 157 days. We suggest that the mesocosm approach could be useful in generating site-specific data that can be applied to the formulation of cost-benefit analyses that can compare a proposed wetland treatment system with alternative conventional chemical methods for treating acid mine drainage.