Stormwater ponds are ubiquitous throughout the world, found around the properties of office buildings, airports, apartment complexes and other facilities. They collect runoff water from roads, roofs, lawns and parking lots and are designed to filter out pollutants before the water is released into local streams or rivers.

Given their prolific pock marking of countless urban landscapes, scientists are now suggesting that they are not a significant source of climate-warming nitrous oxide (N2O) emissions, a new Duke University-led study finds.

"Nitrous oxide is a greenhouse gas that can destroy the stratospheric ozone. Previous studies, however, suggested that stormwater ponds might be a problematic source of the gas, since they produce it as a by-product of the denitrification process that removes excess nitrogen from urban runoff," a Duke release notes.

The ponds can be polluted with oils, grime and chemicals picked up by the water that runs into them from street and other non-permeating surfaces, which are designed to pool the water into the man-made divots.

As designed, thus, the ponds are meant to remove is excess nitrogen and the other contaminants from the water, which, if left untreated, could spur oxygen-depleting algae blooms in downstream waters that can be deadly to marine life and other organisms.

"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," said Joanna Blaszczak, a 2018 doctoral graduate of Duke's Nicholas School of the Environment, who led the study. "Our research, which looked at 64 retention ponds in eight different cities and ecoregions across the nation, found no apparent trade-off.”

Apparently, the scientists wanted to question whether “there may be a trade-off to this benefit,” since the process by which the ponds reduce nitrogen in runoff also produces nitrous oxide.

The study included sediment samples from stormwater ponds in Boston; Baltimore; Miami; Minneapolis; Phoenix; Salt Lake City; Portland, Oregon; and Duke’s home of Durham, North Carolina, during summer 2014.

The experiment featured three samples from each of the eight ponds in each city; some ponds received runoff from heavily developed areas; some received runoff from moderately or lightly developed areas; and some were in largely undeveloped areas.

The researchers measured samples for nitrogen and metal concentrations and for the abundance of certain microbial genes that regulate the denitrification process in pond sediment. After that, they were then incubated and placed in water-filled glass bottles for six hours, so the researchers could measure how much nitrous oxide 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," Blaszczak said.

The nitrous oxide yield from most ponds — even in heavily developed drainages — was within the range of rates found in freshwater bodies draining undeveloped landscapes.

"This leads us to conclude that urban stormwater ponds are not likely to be important sources of nitrous oxide to the atmosphere," she said, and are not likely a major source of greenhouse gas emissions.

"Stormwater ponds are essentially black boxes," Blaszczak said. "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."

Stormwater pond tips for facilities managers

To keep the stormwater pond functioning properly, it may need to be periodically cleaned out to remove sediments that have accumulated over time. This can be an expensive and intensive process.

However, for those responsible for managing them, the following are a few tips for keeping the pond functioning properly:

• Keep litter, yard waste and pet waste out of drainage ditches and storm drains.
• Pesticides and fertilizers need to be used and disposed of properly. Fertilizers should never be broadcast over streets or sidewalks. Fertilizers and pesticides should only be applied at the label rate.
• Properly dispose of oil and antifreeze.
• Never hose chemicals off impermeable surfaces.
• Reduce erosion from the property by planting plants and grasses over exposed areas.
• Allow natural plants to grow up around your pond to help filter runoff water.
• Perform a stormwater inspection annually.

While the ponds don’t likely contribute to greenhouse gas production, they still serve and important purpose and require regular maintenance.

The peer-reviewed study was published June 29 in the journal Ecosphere.