Metal uptake in northern constructed wetlands

Constructed wetlands (CWs) have been employed as passive treatment systems for metal contaminated mine drainage in Canada. However, relatively few CWs have been documented in northern environments and further studies are needed to understand the metal removal mechanisms in wetlands operating under cold climates, with short growing seasons. The goal of this study was to evaluate the performance of laboratory-scale CWs for the removal of Cd, Cu, Se and Zn, as well as, to evaluate Cu and Se uptake in two northern plant species (Carex aquatilis and Juncus balticus). Eight laboratory-scale wetlands were constructed using local materials, including locally harvested plant species and microorganisms and operated under northern summer conditions for 10 weeks. The CWs were fed continuously with synthetic influent containing Cd, Cu, Fe, Se and Zn at concentrations predicted at mine closure. Average removal efficiencies of 96%, 99%, 79% and 97% were observed for Cd, Cu, Se and Zn respectively. There were no significant differences in plant establishment or growth between our CW treatments, or any evidence of increasing Cu uptake with increasing contaminant availability in either northern plant species. Increased belowground uptake of Se was observed at the higher influent concentration in the Pit treatment. However, overall our study suggests that uptake of contaminants by these two northern species is very minor (<0.06% Cu and <0.11% Se, except for C. aquatilis in one treatment <0.2% Cu and <0.4 % Se) and likely does not pose a risk to the surrounding environment. We conclude that CWs could operate as successful passive treatment solutions in a northern environment, at least during the summer months, pending further studies on winter treatment. Further studies are required to examine seasonal metal removal rates in relation to rates of sulfate reduction, carbon consumption, metal precipitation and sorption. In addition, potential contaminant uptake and the influence of functional plant characteristics on metal removal in a suite of northern plant species would further assist in the development of large-scale long-term northern CWs.