Uptake and translocation of iron and manganese by native tree species in a constructed wetland

Landfills are physical facilities constructed in the surface soils of the earth for the purpose of solid waste disposal. Historically, landfills have been the most economical and environmentally acceptable method for the disposal of solid wastes throughout the world. However, landfills generate leachate, which can potentially impact the quality of groundwater, surface water and soil. Phytoremediation is a passive technology that takes advantage of the natural processes of aquatic and terrestrial plants and microbial populations in the soil to stabilize, sequester, accumulate and/or metabolize organic and inorganic contaminants in polluted soils and wastewaters. Field work was conducted at a surface flow constructed wetland located in the Burnside Industrial Park, Dartmouth, Nova Scotia, in which phytoremediation is being used to remediate landfill leachate. The wetland consists of 9 deep open water cells and 2 naturally vegetated islands with various tree species. The water entering the wetland contains high concentrations of Fe and Mn. The objectives of this study were: (a) to investigate the uptake of Fe and Mn by native tree species, (b) to determine the concentration of Fe and Mn in plant tissues, (c) to compare Fe and Mn concentrations in trees grown in the polluted wetland site with those in trees grown in an unpolluted site and (d) to determine the most efficient tree species for the removal of Fe and Mn from the wetland site.;The removal efficiencies of the constructed wetland ranged from a low of 35.56% to a high of 78.49% for Fe and from a low of 0.0% to a high of 33.33% for Mn. The average pH values of the inlet and the outlet water were 6.26 and 6.70, respectively. The average total and plant available Fe concentrations in the soil of the constructed wetland and the reference site were 52 290 mg/kg and 407.6 mg/kg and 21 292 mg/kg and 132.1 mg/kg, respectively. The average total and plant available Mn concentrations in the soil of the constructed wetland and the reference site were 1849 mg/kg and 203.7 mg/kg and 154.5 mg/kg and 7.63 mg/kg, respectively. The average pH values of the soil in the constructed wetland and the reference site were 4.90 and 3.22, respectively. On the wetland site, Fe accumulated in the roots of trees while on the reference site, Fe accumulated in different compartments. Mn accumulated in the trunk bark and either the leaves or twigs of trees in the constructed wetland and reference sites. Based on average total concentrations in August 2006, white ash trees in the constructed wetland accumulated the highest iron (998.8 mg/kg) in their aboveground tissues followed by red spruce (885.5 mg/kg), red maple (651.2 mg/kg), white birch (607.8 mg/kg) and green alder (571.5 mg/kg). Based on average total concentrations in August 2006, white birch trees in the constructed wetland accumulated the highest manganese (1565.8 mg/kg) in their aboveground tissues followed by green alder (900.4 mg/kg), red maple (819.0 mg/kg), red spruce (694.4 mg/kg) and white ash (390.1 mg/kg). The most appropriate species to plant in the constructed wetland would be red maple and green alder trees because they appeared to be the most flood tolerant species.