According to a new Duke University-led study, storm water retention ponds, an ever-present feature in developed landscapes globally, are not a major source of climate-warming nitrous oxide (N2O) emissions.
Several office buildings, factories, airports, retail centers, and apartment complexes, among other sites, utilize the ponds to collect runoff from roofs, roads, parking lots, and lawns and filter out pollutants prior to releasing the water into local rivers or streams.
The ponds help eliminate pollutants such as excess nitrogen, which, if left untreated, could encourage the growth of oxygen-depleting algae blooms in downstream waters.
However, some researchers question whether there may be a downside to this advantage, because the process by which the ponds lower nitrogen in runoff also generates nitrous oxide as a by-product. Nitrous oxide is a potent greenhouse gas that destroys the stratospheric ozone.
Previous studies have suggested we might find elevated nitrous oxide emissions from these ponds, especially urban ponds where high levels of metal contaminants from road runoff might interfere with the complete reduction of the nitrogen. Our research, which looked at 64 retention ponds in eight different cities and ecoregions across the nation, found no apparent trade-off.
Joanna Blaszczak, Doctoral graduate, Duke’s Nicholas School of the Environment
The peer-reviewed work performed by Blaszczak and her colleagues was published in June 29 in the journal Ecosphere.
During summer 2014, the team collected and examined sediment samples from stormwater ponds in, Baltimore, Boston, Minneapolis, Miami, Salt Lake City, Phoenix, Durham, N.C., and Portland, Oregon in order to perform the study. They collected three samples each from eight ponds in each city. Certain ponds received runoff from heavily developed regions; some received runoff from lightly or fairly-developed regions; and some were located in largely undeveloped regions.
The researchers quantified the samples for metal and nitrogen concentrations and for the abundance of specific microbial genes that control the denitrification process in pond sediment. The researchers subsequently incubated and placed the samples in glass bottles filled with water for six hours, so that they could calculate the amount of nitrous oxide that was created and emitted.
We found there was almost no correlation, no single and simple link, between the intensity of nearby urban land cover and potential denitrification rates, across and within all cities. This leads us to conclude that urban stormwater ponds are not likely to be important sources of nitrous oxide to the atmosphere.
The nitrous oxide yield from the majority of ponds—including the ones in highly developed drainages—was within the range of rates found in freshwater bodies draining undeveloped sites.
While the study’s inferences should help reduce concerns that the ponds could be a key source of greenhouse gas emissions, there are other concerns which are yet to be addressed.
“Stormwater ponds are essentially black boxes,” Blaszczak observed. “We understand what goes into them and what flows out of them, but still have limited understanding of the chemical and biophysical processes that occur within them.”
“Many of our team’s starting assumptions about how sediment chemistry would change with changing urban land use proved to be untrue,” she said. “That’s probably because urban ponds reflect previous land-use history as well as current land-use strategies. We’re only beginning to figure it all out.”
At present, Blaszczak is a postdoctoral research associate at the University of Montana’s Flathead Lake Biological Station.
She carried out the new study with colleagues from the Virginia Polytechnic Institute and State University, Portland State University, the University of Minnesota, Arizona State University, the University of Utah, Woods Hole Research Center, and the Cary Institute of Ecosystem Studies.
Co-authors of this study were Jim Heffernan, assistant professor of ecosystem ecology and ecohydrology at Duke’s Nicholas School, and Emily Bernhardt, Jerry G. and Patricia Crawford Hubbard Professor of Biogeochemistry at the Nicholas School and Duke’s Department of Biology.
The National Science Foundation Macrosystems Biology Program (#NSF EF-1065785) and the National Science Foundation Graduate Research Fellowship Program (#NSF GDE-1644868) funded the study.