Water quality benefits of a nature-based solution: Constructed floating wetlands tested on two urban waterways

In urbanized rivers and waterways, Constructed Floating Wetlands (CFWs) offer a nature-based solution for ecosystem restoration, particularly in areas with hardened shorelines and limited space for traditional land-based interventions. This four-year study deployed CFWs in two urban waterways in Seattle, Washington (the Lower Duwamish Waterway and the Lake Washington Ship Canal) to enhance salmon habitat by reintroducing wetland functions. Monitoring data from the organic plant root substrate revealed that CFWs effectively removed heavy metals and nutrients from the water in both freshwater and brackish environments. Importantly, no adverse effects were observed on key water quality parameters such as temperature, dissolved oxygen, or conductivity. While results related to carbon sequestration were mixed, the findings suggest that even small-scale CFW installations can contribute positively to water quality. Although the limited surface area of the CFWs constrained their overall treatment capacity, the study suggests that scaling up deployment could contribute to reducing contaminant and nutrient loading in urban waterways, while also benefiting aquatic organisms. These results offer support for the use of CFWs as a viable nature-based solution for retrofitting hardened shorelines and rehabilitating highly urbanized or industrialized aquatic environments; especially where land-based restoration is impractical due to cost or space constraints.