Use of constructed wetlands for urban stream restoration: a critical analysis

Investigation of a delta marsh restoration project proposed for the Don River in Toronto, Ontario, underlines several concerns about constructed wetland projects designed for water quality improvement and aquatic habitat enhancement. The Don is a highly urbanized river that has undergone significant physiographic modifications and continually receives a complex mixture of conventional, metallic, and organic contaminants from multiple point and nonpoint sources. Rather than providing permanent removal of urban contaminants, wetland processes offer a limited capacity for temporary storage of contaminant inputs and potential reactions may actually produce more toxic and/or bioavailable forms of some chemicals. These processes tend to result in the concentration of watershed contaminants in wetland vegetation and sediments. As the restored marsh would be available for spawning and feeding by aquatic fauna, the potential exists for chemical bioconcentration and biomagnification through the aquatic community. Accordingly, wetland systems are not suited to the dual purposes of water quality improvement and aquatic habitat enhancement. Upstream controls, including source reduction of contaminant inputs, are recommended as essential components of all constructed wetland projects.