While wetlands are known to be the kidneys of the landscape by removing excess nutrients, toxins, and sediment, this has made wetlands, and derivatives of, a popular choice for managing stormwater in urban areas. In the Greater Cincinnati region, stormwater runoff is a top concern for water quality of streams and rivers and contributes to frequent combined and sanitary sewer overflows. Green infrastructure that may mimic the ecosystem processes of natural wetlands, such as stormwater wetlands, detention/retention basins, bioswales and even green roofs are being implemented as a solution for managing the excess stormwater runoff. To better understand if the green infrastructure “wetlands” being implemented around the region are effective and functioning as a wetland, we studied four distinct stormwater wetlands. In addition, while it is predicted that green infrastructure may be successful at filtering water, we were curious if there would be trade-offs in ecosystem processes – such as removing nitrogen and phosphorus from stormwater, but emitting nitrous oxide and methane to the atmosphere. To begin investigations to these questions, we studied greenhouse gas flux from six green roofs, two stormwater wetlands, and a retention basin in the region.
Results/Conclusions
We found the stormwater wetlands studied were effective at retaining water and reducing flow rates during storm events. Retention times during storm events of varying sizes ranged from 6.75 hours to 8 days and flow rates were reduced between 25-135%. The wetlands were also effective at nutrient removal, with one wetland reducing nitrate concentration from 1.45 ppm to 0.001 ppm. The sediment removal results were not as hopeful with an increase in sediment load due to management challenges with a beaver family. This highlights the importance of adaptive management with the challenges in urban wetland systems. We learned through these studies that the effectiveness of a stormwater wetland’s functioning can be highly dependent on seasonal and pre-storm event conditions. While the urban stormwater wetlands were effective in their functioning, we learned most were releasing small amounts of methane and nitrous oxide. However, like the green roofs, these amounts were negligible compared to other managed ecosystems. We found green roofs to function as a minor sink for methane and a minor source of nitrous oxide. Overall, stormwater green infrastructure was functioning similarly to natural wetlands by effectively retaining stormwater and removing nutrients with potentially minor trade-offs in greenhouse gas emissions.