Small, isolated wetlands adept at catching pollutants, study finds

What the study’s authors call “transient hydrologic dynamics” can ultimately increase nitrogen retention by as much as 130%. Photo credit: IzzetNoyan,

University of Waterloo researchers have determined that small, geographically-isolated wetlands make more efficient filters for nitrogen.

The new University of Waterloo study is one of the first to utilize computer modelling and satellite data to quantify nutrient retention dynamics in wetlands.

Geographically-isolated wetlands are defined as wetlands that are completely surrounded by uplands, lack a persistent surface water connection to navigable streams, and may contain water for only part of the year.

“Also, while the lack of apparent connection to surface waters increases their ability to be most effective as nutrient filters, it is this lack of connection that excludes them from the Clean Water Act and makes them most vulnerable to loss,” the study says.

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Pollution is a particular concern in regions like southern Ontario, which has already lost more than 70% of its wetlands, says Dr. Nandita Basu, a professor at UWaterloo, Canada Research Chair in Global Water Sustainability and Ecohydrology, and study co-author.

In a 2022 study, UWaterloo estimated that filtration in southern Ontario’s wetlands could provide $4.2 billion worth of sediment filtration and phosphorus removal every year.

The wetlands not only act like sponges, but provide flooding protection. They retain excess nutrients from agricultural and urban runoff and protect downstream waters. What the study’s authors call “transient hydrologic dynamics” can ultimately increase nitrogen retention by as much as 130%. In particular, the anoxic conditions and high organic carbon content in wetlands promotes the removal of nitrogen through denitrification.

“If pollutants aren’t caught by small wetlands, then they’ll run into our lakes, beaches and eventually impact our supply of drinking water and ability to use the beaches for recreation,” Basu said in a statement.

According to the study, the researchers used 30 years of satellite imagery from across the U.S. to determine how 3,700 wetlands were filling up and draining as a function of seasons and climate. Then they estimated how much nitrogen would be removed by these water bodies.

“Being disconnected can actually be better because they are catching the pollutants and retaining them as opposed to leaking them back to the stream waters,” said Dr. Frederick Cheng, first author of the study and a postdoctoral fellow at Colorado State University.

The research team intends to apply these techniques to Canadian wetlands across the Great Lakes basin, as well as the prairie region in Western Canada. That work will be supported in part by a $6.8-million grant from Environment and Climate Change Canada.

This article appears in ES&E Magazine’s April 2023 issue:

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