U study: Wetlands key to keeping harmful nitrates out of waters

A new University of Minnesota study found wetlands are providing a significant water quality benefit by keeping nitrates from crop fertilizer out of rivers.

The study from the U's St. Anthony Falls Laboratory and the College of Biological Sciences found that when stream flows are high, wetlands are actually more effective at removing nitrates than conventional land conservation practices.

"What our study shows is that multiple wetlands in the watersheds are having a measurable effect on the nitrate concentration in the river," said Amy Hansen, a research associate at the university and the study's lead author.

High nitrate levels can be harmful to ecosystems and human health, contaminating drinking water and creating the Dead Zone in the Gulf of Mexico.

It's not news that wetlands capture pollutants, slow runoff and help improve water quality.

However, Hansen said past studies haven't demonstrated that wetlands reduce nitrate concentrations on a watershed scale. That's probably because other Midwest states like Iowa and Illinois don't have enough remaining wetlands to measure an impact, she said.

Hansen's study collected water samples from more than 200 waterways in the 17,000-square-mile Minnesota River basin in southwestern Minnesota, where farming is prevalent.

The study found the basin's wetlands are on average five times more effective at reducing nitrate pollution than other conservation methods such as taking land out of production, Hansen said.

"The reason for that is because wetlands are actually intercepting water from a much larger area than they occupy," she said. "Water is running off and into wetlands, where it's being acted on by the biology within the wetland."

The study found that temporary wetlands that appear after heavy rains are also effective at removing nitrates, when other conservation practices are usually overwhelmed, Hansen said.

Hansen said the findings show that in places with few remaining natural wetlands, constructed wetlands would be an effective way to improve water quality in heavily farmed areas.

"Locally, what this study does is shows us that the wetlands that we have have value and are probably worth protecting," she said.

The study was published in the journal Nature Geosciences.

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