Hydrologic conditions drive denitrification and greenhouse gas emissions in stormwater detention basins

Stormwater detention basins are primarily designed to detain large volumes of storm runoff and trap suspended sediments and associated pollutants. Detaining and retaining nutrients are often not a design focus. The combination of variable moisture patterns in stormwater basins along with potential nutrient influxes may make these basins hotspots for nitrogen transformations such as denitrification, as well as potential sources of greenhouse gases nitrous oxide (N2O) and methane (CH4). Nitrous oxide and CH4 emissions were measured using static chambers in four stormwater detention basins - two fast-draining or 'dry' basins and two slow-draining or 'wet' basins. Denitrification potential of soils collected from these basins was also measured using the denitrification enzyme assay (DEA). While N2O emissions were low, CH4 emissions were higher in both wet basins, averaging 5667 μg C m−2 h−1 in the wettest basin. Denitrification potential was higher in the wet basins (2.27 mg N kg−1 h−1) compared to the dry basins (0.23 mg N kg−1 h−1). Overall, wet detention basins had higher greenhouse gas emissions but also had higher potential for nitrate removal via denitrification. Designing future stormwater control measures to maintain a subsurface saturated zone rather than fully saturated soils should be considered to promote denitrification while also reducing CH4 emissions at the surface.