When pollution levels rise in the rivers that supply drinking water to Iowa’s largest city, Des Moines must spend roughly $16,000 each day to operate a specialized system that removes hazardous nitrates. This recurring expense has become part of life in an agriculture-driven state — and climate change is intensifying the challenge.

Nitrates originate from fertilizers and pesticides that seep into the soil and eventually flow into waterways such as the Raccoon and Des Moines rivers. Typically, winter does not pose major issues. Yet this year, the city was forced to activate its filtration system in January and February — only the second time in over three decades that winter treatment was required. The added costs are likely to translate into higher water bills for residents in a state whose waterways are already among the most vulnerable to nitrate contamination.

Specialists point to shifting weather patterns, including milder winters, as a key factor behind the growing and expensive problem — one they warn is likely to spread across agricultural regions.

“Winter nitrate events are expected to become more common,” said Iowa State Climatologist Justin Glisan. “They may not happen every year, but the conditions that produce them are appearing more frequently.”

Why warmer winters lead to more water pollution

A field used for corn silage on Blue Spruce Farm is pictured on Tuesday, March 24, 2026, in Bridport, Vt. (AP Photo/Amanda Swinhart)

The fertilizers and pesticides applied to farm fields leave behind nitrogen and phosphorus. When rain falls or snow melts, these nutrients are washed into streams and rivers, threatening drinking water supplies. High nitrate levels have been linked to serious health risks, including certain cancers and “blue baby syndrome,” a condition that reduces oxygen levels in infants.

As the planet warms due to human-driven climate change, frozen ground is becoming less consistent in many regions. Snow increasingly melts quickly or falls as rain onto unfrozen soil. This shift creates more winter days when nitrates can move freely into waterways and reach unsafe concentrations.

Researchers also note that a warming atmosphere contributes to more extreme weather, including prolonged droughts and intense downpours. Because warmer air holds more moisture, storms can release heavy bursts of rain.

When dry periods are followed by heavy rainfall, large volumes of water surge through the soil, carrying nitrogen and other farm chemicals along with it, Glisan explained.

Dairy cows feed at Blue Spruce Farm on Saturday, March 28, 2026, in Bridport, Vt. (AP Photo/Amanda Swinhart)

In addition, warming temperatures in polar regions are contributing to erratic winter patterns that swing between bitter cold and milder, less snowy conditions.

Although some winter storms still bring substantial snowfall, the snow often does not remain long. In some areas, snow cover insulates the soil, preventing it from freezing deeply. When rapid thaws occur, melting snow — followed by heavy rain — can percolate through the ground and carry nutrients into nearby streams.

When soil does not stay frozen, nutrients are no longer effectively “locked” in place.

“In central and southern Illinois, we’ve long experienced freeze-thaw cycles,” said Illinois State Climatologist Trent Ford. “What’s changing is that these patterns are now extending farther north.”

Stakes are high for low-income and rural communities

University of Vermont graduate student Delaney Bullock gathers runoff samples from two agricultural fields to be analyzed for nutrient concentrations on Thursday, March 12, 2026, in Bridport, Vt. (AP Photo/Amanda Swinhart)

Nitrate contamination presents serious challenges for low-income and rural communities nationwide, said Samuel Sandoval Solis, a water resources management specialist at the University of California, Davis.

While some municipalities have advanced treatment systems to remove nitrates, many smaller communities lack such infrastructure. About 15% of Americans rely on private wells for drinking water, according to federal data, and these wells are vulnerable to nitrate seepage.

Routine testing and installing proper filtration systems can cost households hundreds of dollars annually. For small towns whose treatment facilities are not equipped to handle nitrate removal, upgrading systems may require significant financial investment, Sandoval said.

More research is connecting climate change, runoff and nutrient loss

University of Vermont graduate student Delaney Bullock gathers runoff samples from two agricultural fields to be analyzed for nutrient concentrations on Thursday, March 12, 2026, in Bridport, Vt. (AP Photo/Amanda Swinhart)

States have long grappled with nitrate pollution, but officials are increasingly recognizing how warmer winters complicate mitigation efforts. In Illinois, for example, annual reports now more clearly acknowledge climate change as a contributing factor, said Joan Cox, who manages the state’s Nutrient Loss Reduction Strategy.

Scientists have observed that more nitrogen is flowing downstream during winter months, though research continues to determine whether this results in greater total pollution, said Carol Adair, a University of Vermont professor who studies how rain-on-snow events influence nutrient runoff.

The broader ecological consequences remain uncertain. With fewer plants actively growing in winter to absorb nitrogen, more of the nutrient may travel downstream. That could worsen problems such as the Gulf of Mexico’s “dead zone,” an oxygen-depleted area fueled in part by fertilizer runoff that harms fish and marine ecosystems.

Dani Replogle, a staff attorney with the nonprofit Food and Water Watch, said large-scale livestock operators often attempt to time manure and fertilizer applications to avoid rainfall. However, increasingly unpredictable weather patterns make that strategy less reliable.

Regulating nutrient pollution has proven difficult

Joshua Faulkner, left, research associate professor and director of the Agricultural and Environmental Testing Lab at the University of Vermont, and graduate student Delaney Bullock check on flumes used to collect runoff from two agriculture fields for analysis on Thursday, March 12, 2026, in Bridport, Vt. (AP Photo/Amanda Swinhart)

Efforts to require farmers to reduce nutrient runoff have faced resistance, particularly in states like Iowa where agricultural interests oppose mandatory regulations.

In a recent move, the Environmental Protection Agency removed seven Iowa waterways from the federal Impaired Waters List. Previously, their inclusion would have required the state to establish limits on pollution entering those rivers under the Clean Water Act. Advocacy groups have indicated plans to challenge the decision.

Meanwhile, Iowa’s water utilities are developing resilience strategies to cope with increasing winter nutrient spikes, said Amy Kahler, CEO and general manager of Des Moines Water Works. Still, she believes upstream polluters should bear more responsibility.

“There are two options,” Kahler said. “We can invest in conservation and responsible watershed management — or we can spend hundreds of millions of dollars on advanced treatment systems.”

Canada Geese wade in the waters and ice of Lake Champlain on Tuesday, March 24, 2026, in Addison, Vt. (AP Photo/Amanda Swinhart)

Kahler favors preventive conservation measures, noting they also enhance overall environmental quality and community well-being.

In 2015, Des Moines Water Works filed a lawsuit seeking compensation for the high costs of removing unsafe nitrate levels from river water. The case was ultimately dismissed by a judge.

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This article was produced as part of ongoing coverage of climate and environmental issues.