As winters warm, nutrient pollution threatens 40% of U.S.

Scientists are ringing alarm bells about a significant new threat to U.S. water quality: as winters warm due to climate change, they are unleashing large amounts of nutrient pollution into lakes, rivers and streams.

Now, a U.S. National Science Foundation-supported study finds that previously frozen winter nutrient pollution — unlocked by rising winter temperatures and rainfall — is putting water quality at risk in 40% of the contiguous U.S., including more than 40 states.

Nutrient runoff into rivers and lakes — from phosphorus and nitrogen in fertilizers, manure, animal feed and more — has affected water quality for decades. However, most research on nutrient runoff in snowy climates has focused on the growing season. Historically, cold temperatures and a continuous snowpack froze nutrients like nitrogen and phosphorous in place until the watershed thawed in the spring, when plants could help absorb excess nutrients.

But U.S. winters are fast warming, and the seasonal snowpack in much of the U.S. has become less stable. Increased rain-on-snow, snowmelt and rainfall events now carry nutrients and soil into streams and rivers during winter when dormant vegetation cannot absorb them. As a result, winter runoff impacts on nutrient pollution have quickly progressed from rare or nonexistent to worse than during other times of the year.

The study was published in Environmental Research Letters by scientists at the University of Vermont, the University of Colorado, the University of Kansas and the University of Michigan.

"We are clearly seeing much larger amounts of cloudy water and sediment traveling through U.S. watersheds in winter," said Carol Adair, a University of Vermont researcher. "The idea of winter nutrient pollution is new, because it's a relatively recent impact of climate change with the potential to cause significant problems for people and the environment."

Of particular concern are so-called "rain-on-snow" events, researchers say, which can cause large, economically and environmentally devastating floods. The team used geospatial datasets to explore the impacts of rain-on-snow events in U.S. regions with large pools of nitrogen and phosphorous.

The scientists found that rain-on-snow affects 53% of the contiguous U.S. and puts 50% of U.S. nitrogen and phosphorus pools at risk of export to groundwater and surface water. Where these factors converge, more than 40% of the contiguous U.S. is at risk of nutrient export and soil loss from these events.

"A rain-on-snow event along the Mississippi River, for example, shows that as climate warms and snowfall turns to rain, we may see increased flooding and more of the nutrients that cause coastal eutrophication," said Laura Lautz, a program director in NSF's Division of Earth Sciences. "This research highlights the cascading effects of climate change that impact water quality and ecosystem health."