Internal WaterStorage as a Means of Improving NitrogenRetention and Reducing Greenhouse Gas Emission in Stormwater Treatment

Internal water storage (IWS) is gaining interest as a design element in stormwater control measures. It is implemented via an upturned elbow or elevated underdrain to create a subsurface storage zone with saturated conditions conducive to nitrogen removal via denitrification. However, IWS can potentially alter emissions of microbially produced greenhouse gases due to changes in subsurface redox conditions. These greenhouse gases include carbon dioxide, methane, and nitrous oxide. We investigated these biogeochemical dynamics using mesocosms mimicking free draining, IWS, and fully saturated stormwater treatment basins. In a series of simulated storm events, we quantified nitrogen removal, dissolved gas concentrations in the outflow, and surface soil emissions of greenhouse gases. IWS and fully saturated mesocosms had the best nitrate reduction, although fully saturated mesocosms exported other forms of nitrogen. Regarding greenhouse gas emissions, fully saturated mesocosms had the highest methane concentrations in outflow water and higher overall greenhouse gas fluxes from the soil surface compared with IWS. Free draining mesocosms sometimes had significantly higher nitrous oxide emissions, particularly after induced drought periods. These results suggest that stormwater basins with IWS have the potential to enhance nitrogen removal while minimizing biological greenhouse gas emissions compared with other stormwater basin drainage configurations.