Do retention ponds do what they're supposed to?

New research shows that although retention ponds may look nice above the water, they aren’t working the way they are supposed to underwater.

Retention ponds are pretty common sites in urban areas. The ponds usually are located next to shopping centers, apartment complexes, or anywhere with a large paved area. Many are nicely landscaped with fountains. But did you ever wonder what lies beneath the water?

If you use a GoPro camera to take a look around on the bottom, you’ll find it’s a pretty silty and sometimes slimy world underwater. And that’s a problem. New research shows retention ponds may look nice above the water but aren’t working the way they are supposed to underwater.

“The water when it rains flows into these wet ponds, which are one of a number of stormwater controls measures,” explains Michael Piehler, Ph.D., director, UNC Institute for the Environment.“And because the law requires developers to mitigate for stormwater runoff, the ponds are now part of the landscape.”

Our story begins with a question: what can be done to reduce the amount of rainwater sitting on roads and on developed land. That stormwater carries pollutants such as fertilizers, yard and pet waste and even motor oil. And if stormwater carried all of that pollution into streams and rivers, the water supply, along with people and the environment would be seriously harmed.

“The presumption is that everything gets captured and fewer things leave,” adds Piehler. “And if you use that in a general sense that can mean water, which it does, it captures the water and takes the peak flow off, so that you don’t have water rushing off into streams after storm. But if you think of everything else, sediments, nutrients, it not as simple as everything gets captured and just stays there.”

State law requires developers to reduce the impact of stormwater runoff. The ponds are specifically designed to not only hold the water and let solid pollutants settle out, but also to remove 30 percent of the nitrogen and phosphorus in the runoff. That happens through a process called denitrification; where an available form of nitrogen, which is a nutrient that can stimulate algae blooms, comes in and is removed through a microbial mediated process. Essentially, the microbes and plants in the water are supposed to take up the nitrogen. Some is consumed and some is released into the air as N2 gas, which animals, people and most plants can’t use.

You’d think that would solve the problem because a lot of those nutrients are now gone. But that’s not always what happens.

“Some of our early work found that not only was denitrification not happening, nitrogen fixation was happening.,” said Piehler. “And that’s the opposite of what the ponds are supposed to do. New nitrogen from the atmosphere was being added.”

To get some answers to what was happening in the wet retention ponds, researchers surveyed dozens of ponds by taking core samples of the bottom. All of the action in a wet pond happens at what researchers call the surface-water interface, where all of the solid nutrients and organic matter settle. Researchers hooked up the core samples to a monitor to find out what was happening to the nitrogen.

“So what we have is our line connected to core, where the water is being pumped in at the top and it’s being taken out at the sediment water interface,” explains Adam Gold, a Ph.D. candidate in Environmental Sciences at UNC-Chapel Hill. “As the water flows, we can get a signal of what the sediments are doing, whether they are removing nitrogen or adding nitrogen.”

“What we want to happen is denitrification, where the sediments are converting nitrate, which is a nutrient, and converting it to N2, which organisms can’t use to grow,” Gold continued. “But what we’re seeing is net nitrogen fixation, which means n2 gas is being taken from the water and being turned into nutrients which they can use to grow.”

Which explains why some ponds have a think layer of algae on the surface and on the bottom. Instead of removing nitrates, the ponds are actually adding nitrogen into the system through nitrogen fixation. And the findings show that the problem is worse in the summer, when the water is warmer and rains aren’t as frequent. That additional rainfall helps to flush out the retention ponds.

“It’s bad because it’s a net influx of nitrogen into the system,” Gold added. “So instead of being a nitrogen sink, where nitrogen is removed and things can’t use it, it’s actually a net influx, there’s actually more nitrogen coming into the system than before.”

And those explosions of algae can threaten streams and rivers downstream. “It’s important to understand the problem because in a world where we have a lot of development, and a lot more impervious surfaces and stormwater runoff increasing, its important to think about where that water is going,” said Peggy Mullin, a lab technician and a senior at UNC-Chapel Hill. “But then you need to take a bigger view and see how it’s impacting the wildlife community with the kind of microbes you don’t see or the water flow you don’t see. It’s a key component.”

The solution, researchers say, is to routinely excavate the pond to prevent the buildup of organic material. It’s also important to installing aerators that mix the water in the pond, because that will promote nitrogen removal. And that aeration can include those landscaping fountains.

“As the ponds receive more and more water they receive more and more sediment and organic material the pond physically fills up so you need to clean it out,” Piehler said, explaining what has been learned from the lab work. “But that excavation not only empties the pond, it also changes the composition of the sediment. Because if the sediment gets too organic rich, that doesn’t favor the removal process that the microbes do, that favors the recycling of nutrients into the water column and more nutrients passing through. Promoting conditions that are favorable to nitrogen removal is the best way to make sure the pond is doing what it is supposed to do.”