A comparison of water quality and greenhouse gas emissions in constructed wetlands and conventional retention basins with and without submerged macrophyte management for storm water regulation

Abstract

The city of Winnipeg (Manitoba, Canada) has numerous conventional stormwater retention basins to attenuate stormwater runoff. Significant operational resources are applied to control the proliferation of aquatic macrophytes and filamentous algal mats in these conventional stormwater retention basins to meet the aesthetic expectations of surrounding residents. Recent urban developments in the city of Winnipeg have used constructed wetlands to handle stormwater as opposed to conventional basins. We examined the differences in water quality, greenhouse gas emissions and cumulative greenhouse gas fluxes between conventional stormwater retention basins and urban constructed wetlands. Additionally, we examined how water quality and greenhouse gas emissions were impacted by the removal of submersed aquatic vegetation in conventional stormwater retention basins. Our results indicate large phytoplankton blooms were consistently reduced both pre and post treatment in the urban constructed wetlands (chlorophyll-a = 13 µg l−1 and 33 µg l−1) relative to the untreated conventional stormwater retention basins (chlorophyll-a = 56 µg l−1 and 128 µg l−1) and treated conventional stormwater retention basins (chlorophyll-a = 58 µg l−1 and 405 µg l−1). Mean daily greenhouse gas fluxes were lower pre and post treatment in the urban constructed wetlands (CO2eq = 26.5 kg ha−1 d−1 and 53.1 kg ha−1 d−1) relative to the untreated (CO2eq = 85.5 kg ha−1 d−1 and 89.0 kg ha−1 d−1) and treated conventional stormwater retention basins (CO2eq = 98.5 kg ha−1 d−1 and 129.8 kg ha−1 d−1). Over the 22 week period, mean total basin cumulative CO2eq flux was lowest in the urban constructed wetlands (6,447 g ha−1 d−1) compared to the untreated (12,631 g ha−1 d−1) and treated (17,633 g ha−1 d−1) conventional stormwater retention basins. The combined treatments (herbicides and harvesting) applied to the conventional stormwater retention basins to remove aquatic vegetation result in poorer water quality and appear to stimulate methane emissions resulting in noticeably higher cumulative fluxes of greenhouse gases from these basins. The use of urban constructed wetlands in new residential areas should result in enhanced water quality and reduced cumulative greenhouse gas fluxes, while avoiding the cost associated with macrophyte removal.