When pollution gets bad enough in the rivers supplying Iowa’s largest city with drinking water, it costs Des Moines around $16,000 a day to run a special system to filter out dangerous nitrates. It’s a fact of life in the agriculture-dependent state — and climate change is making the water quality problem even worse.

The nitrates come from fertilizer and pesticides that make their way into the soil and then waterways like the Raccoon and Des Moines rivers. It’s not usually a problem in winter, but this year Iowa’s capital had to filter in January and February — just the second time that’s happened in more than 30 years. That’s likely going to mean higher water bills for people who live in a state with some of the nation’s waterways that are most vulnerable to nitrate pollution.

Experts blame weather conditions, including warming winters, for a costly problem they say will only grow across farm country.

When it comes to winter nitrate pollution events, “We are more apt to see these in the future. Are they going to occur every year? No. But the ingredients are there for them to potentially occur more often,” said Justin Glisan, Iowa’s state climatologist.

The fertilizers and pesticides that farmers use leave nitrogen and phosphorus in their fields. Rain or snowmelt then carries the chemicals into drinking water, which is dangerous. Ingesting too many nitrates can cause health issues like cancer or blue baby syndrome, low oxygen levels in infants.

As Earth warms due to human-caused climate change, the ground isn’t staying frozen as consistently in many places, and snow is often melting or falling as rain on thawed ground. That all adds up to more winter days when nitrates are likely to reach unhealthy levels.

Scientists say one effect of Earth’s warming is more frequent extreme weather events, including drought and intense bursts of rainfall from an atmosphere that now holds more moisture than in the past.

Intense dryness followed by intense wetness means massive amounts of water moving through the soil, bringing farm chemicals like nitrogen with it, Glisan said.

And a warmer atmosphere is thawing Earth’s polar regions and causing more of those winter flip-flops from frigid polar air to warmer, less snowy weather, he said.

Even though some storms brought a lot of snow this winter, it didn’t stay on the ground for very long. Instead, snow insulated the soil in some areas from freezing too deep, and a quick thaw let melting snow, followed by pounding rain, travel down through the soil and eventually into streams.

Where the ground isn’t consistently frozen, nutrients aren’t as “locked in” to the soil frost.

“In central and southern Illinois, we’ve always dealt with a sort of ephemeral freeze-thaw, freeze-thaw process. What we’re seeing is that’s really tracking farther north,” said Trent Ford, Illinois’ state climatologist.

Nitrate pollution is a big problem for low-income, rural residents across the United States, said Samuel Sandoval Solis, a professor at the University of California-Davis and an extension specialist in water resources management.

While some communities already have the infrastructure to manage nitrate levels in drinking water, like filtration systems, many others don’t. Around 15% of the U.S. population relies on drinking water wells that are private, according to the U.S. Geological Survey. Nitrates can seep into those wells.

Testing well water regularly and correctly filtering it in a home can cost hundreds of dollars a year. Small communities whose water treatment facilities aren’t yet equipped to filter nitrates will also have expensive decisions to make, Sandoval said.

States have been wrestling with nitrate pollution for years, but they’re starting to realize increasingly warm winters are making that tougher — like in Illinois, where yearly reports on the issue have started to more explicitly mention the role of climate change, said Joan Cox, program manager for the Illinois Nutrient Loss Reduction Strategy.

Scientists know there’s more nitrogen going downstream in the winter, but they’re still trying to figure out whether that means more pollution overall, said Carol Adair, a professor at the University of Vermont who has studied how rain-on-snow events could worsen nutrient pollution.

Either way, there’s little known about the consequences of those changes on ecosystems, Adair said. She thinks because there’s less plant life to suck up nitrogen in the winter, more could end up further downstream, like in the Gulf’s “dead zone” where fertilizer pollution contributes to an area of low to no oxygen, which kills fish and marine life.

Dani Replogle, a staff attorney for Food and Water Watch, a nonprofit for sustainable food and clean water, said factory farm operators try to plan manure and fertilizer applications when precipitation is unlikely. But that is “increasingly not a successful strategy because everything is becoming so unpredictable,” she said.

Mandating that producers curb farm chemicals in water has proven difficult in agricultural areas, especially in Iowa, where the state’s farm lobby has opposed mandatory rules.

Trump’s EPA has delisted seven Iowa waterways from the federal Impaired Waters List, which under the Clean Water Act would have required the state to set limits on how much pollution gets into them. Food and Water Watch has announced an intent to sue.

As for Iowa’s water treatment facilities, they are preparing resiliency plans for a future with more winter nutrient pollution, said Amy Kahler, CEO and general manager at Des Moines Water Works. But she thinks polluters upstream should clean up their acts.

“There really are two paths. One is conservation efforts and responsible watershed practices. And the other is spending hundreds of millions of dollars in treatment solutions,” Kahler said.

She thinks the best solution is the former, since it also has positive impacts on quality of life.

In 2015, the agency sued for the millions of dollars it was being forced to spend to filter unsafe levels from drinking water taken from the Des Moines and Raccoon rivers. A judge ultimately dismissed the lawsuit.

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Follow Melina Walling on X @MelinaWalling and Bluesky @melinawalling.bsky.social.

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