Metal uptake in northern constructed wetlands

Constructed wetlands are complex biogeochemical systems where an effluent flows through a plant-soil matrix and natural processes reduce pollutant levels to a given discharge limit. The goal of this study was to evaluate the performance of laboratory-scale constructed wetlands for the removal of metals such as Cd, Cu, Fe, Se and Zn, as well as, to evaluate Cu and Se uptake in two northern plant species (Carex aquatilis and Juncus balticus).

Due to their low maintenance and operational cost requirements and high removal capacity, constructed wetlands have been employed as passive treatment systems for metal contaminated mine drainage in Canada. However, relatively few constructed wetlands have been documented in northern environments and further studies are needed to understand the metal removal mechanisms in wetlands in cold climates.

Project Overview

Eight laboratory-scale wetlands were constructed using local materials, including locally harvested plant species and wetland substrate and were operated under northern summer conditions for 14 weeks. The constructed wetlands had a 3 day retention time and were fed continuously with synthetic effluent containing Cd, Cu, Fe, Se and Zn at concentrations predicted at mine closure. Two wetlands, one with each northern plant species, were fed tap water as a control and two were fed with methanol in addition to the synthetic effluent. Metal removal efficiencies were compared between the wetlands.

After 14 weeks of operation, wetland substrate and plant biomass were analyzed for Cu and Se to examine metal partitioning in these water-soil-plant systems. Microbial sulfate reduction processes and metal precipitation is considered a major mechanism for metal sequestration in constructed wetlands. Metal bioaccumulation in plants is another metal removal pathway, however metal uptake in aboveground biomass it is not desired for mine closure due to the risk for contaminant uptake by wildlife foraging. 

The removal efficiencies and metal partitioning in these laboratory-scale constructed wetlands provided important insights into the efficacy of constructed wetlands in northern environments. The results of which can be found in the final report.

Partners

Casino Mining Corporation

Clear Coast Consulting

Funders

Natural Science and Engineering Research Council of Canada

Casino Mining Corporation