A new study finds as winter weather warms across the Midwest, fertilizers that once froze are now finding their way into streams and rivers.

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The Midwest is at-risk of increased water pollution from fertilizer run-off during the winter, according to a new study from the Gund Institute for Environment at the University of Vermont.

“We figured out that we've got more than 40% of the United States that's potentially creating this winter pollution every time it rains or snow melts,” said Carol Adair, a University of Vermont researcher who worked on the study.

Nitrogen and phosphorus run-off has been known to pollute waters. It can cause algae blooms that make beaches dangerous and creates dead zones that kill fish.

In the past, much of the focus on nutrient run-off has been during the growing season.

Nitrogen and phosphorus freezes on farmland during winters, keeping it from polluting waters until the plants can absorb it in the spring. Now winters are warming faster than any other season across the Midwest. Warmer temperatures are thawing the soil and heavy rainfall events are washing those chemicals into waterways.

Read more of our reporting on fertilizer runoff

“Winter events tend to carry a little bit more pollution than the same size event in the growing season,” said Adair. “That's largely because there are no plants around taking things up.”

The absence of plants in the winter means the soil stores more nutrients, which are now being released.

The Upper Midwest is particularly at-risk because it is both rich in nutrients from the agriculture in the region and it is seeing more heavy rainfall events on top of snow during the winter. In the past, winter rainfall was more common in southern parts of the Midwest, such as Missouri and Kansas, according to Dennis Todey, the director of the USDA Midwest Climate Hub.

“The further north you go, snow was more predominant during the wintertime,” said Todey. ”But that line has moved northwards so that we can see rainfalls throughout a good portion of the Midwest throughout the winter.”

The study looked in particular at the Mississippi River floods that occurred during the winter and early spring of 2019 and found that “rain-on-snow” events released a massive amount of nutrients and sediment into the river, which contribute to the dead zone that’s killing fish in the Gulf of Mexico.

“If we know where this is coming from – even just using these maps – we could start to think about where to pinpoint in order to try to control it,” said Adair.

Riparian buffer strips and cover crops can help keep these chemicals out of the waterways. Farmers can also help mitigate the problem by not applying manure and nitrogen to fields in the fall.

“If you're losing the nitrogen to the water, that's nitrogen that you don't have available for your crop, so it's a lose-lose in that situation,” Todey said.

This story is a product of the Mississippi River Basin Ag & Water Desk, an editorially independent reporting network based at the University of Missouri School of Journalism in partnership with Report For America and the Society of Environmental Journalists, funded by the Walton Family Foundation.

Top image: Warmer temperatures and heavy rainfall due to climate change are causing increased run-off of fertilizers during the wintertime in the Midwest. Credit: Sam Zeff, KCUR