In general, stormwater basins are either wet basins or dry basins. Water passes through dry basins in a few days, while wet basins have standing water for much longer. A variety of things can happen to the nitrate in the basin. It can pass into the groundwater and then to rivers, streams and lakes; it can be taken up by vegetation; or it can be converted to other compounds by microbes living in the basins.
While decay of basin vegetation places the nitrate back into the basin and the groundwater, some microbial assemblages can convert nitrate all the way to gaseous nitrogen, removing it. The researchers sampled the basins and checked microbial DNA for a gene that can allow conversion of nitrate to nitrogen gas. This gene produces an enzyme that can convert nitrate into gaseous nitrogen.
"Typically, the basins are designed to be dry, but as sediment from runoff and vegetation that grows in the basins builds up, they can become wet basins," said McPhillips.
They found that the capability of producing gaseous nitrogen was higher in wet basins than dry basins. However, they also found that partial conversion produced nitrous oxides and that consumption of organic matter produces methane, both greenhouse gases. The wet basins showed higher levels of the gene that allows complete conversion of nitrate to gaseous nitrogen.
According to McPhillips, designing the basins to hold water from the beginning could decrease production of nitrous oxides, because the longer the basins hold the water, the more complete the conversion from nitrate to gaseous nitrogen.
As for the methane, McPhillips suggests that engineering the basins so that the water retention layer is below ground and not on the surface of the basin could prevent the methane from releasing into the atmosphere. Trapped in the soil, oxygen would degrade the methane.
McPhillips is now looking at stormwater retention basins on Penn State's University Park campus for further research.
Also working on this project were M. Todd Walter, professor of biological and environmental engineering at Cornell University; and Natalie Morse, a recent doctoral recipient from Cornell University.
The National Science Foundation supported this work.