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Water quality threatened by nutrient pollution in 40 U.S. states

We usually think of climate change in terms of summer heat waves, warming oceans and extreme weather events. But what is happening in winter, when the ground should be frozen or covered in snow? Statistics show that winter is actually the season that is warming fastest in the U.S., and this is having some serious and unforeseen consequences. 

In a first-of-its-kind national study, scientists from the Universities of Vermont, Colorado, Kansas, and Michigan have identified a significant new threat to U.S. water quality that is directly related to the warming associated with climate change. In cold regions, accumulated nutrients from farming activities, such as phosphorus and nitrogen from fertilizers, manure, animal feeds and other sources, have historically remained frozen in snowpack throughout winter. When the spring thaw has arrived, the nutrients have been carried away into rivers and lakes where plants have been able to use these nutrients for their growth.

But with winters becoming warmer, cold snaps becoming shorter and the number of days with temperatures below 32°F decreasing each year, the seasonal snowpack in much of the U.S. has become less stable. Increased rain-on-snow, snowmelt, and rainfall events now carry the nutrients into streams and rivers during winter instead, when the vegetation is dormant and cannot absorb the excess. As a result, nutrient pollution from winter runoff has quickly progressed from rare or non-existent to far worse than at other times of the year

“We are clearly seeing much larger amounts of cloudy water and sediment traveling through U.S. watersheds in winter,” said Carol Adair, a researcher at UVM’s Gund Institute for Environment, the Rubenstein School of Environment and Natural Resources, and Vermont EPSCoR. 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 – from algal blooms that make swimming dangerous to ‘dead zones’ that kill fish stocks.”

The researchers were particularly interested in the effects of so-called “rain-on-snow” events that occur when rain falls on existing snowpack, causing it to melt to some extent. They say that this can lead to large, economically and environmentally devastating floods, in addition to releasing frozen nutrients into the waterways. And as rainfall becomes more common than snowfall in some of these areas, the impacts of these events are likely to become more serious. 

The scientists used geospatial datasets to explore the impacts of rain-on-snow events in U.S. regions with large pools of nitrogen and phosphorous. The findings, published this week in Environmental Research Letters, indicate that nutrient pollution, unlocked by rising winter temperatures and rainfall, is putting water quality at risk in 40 percent of the contiguous U.S., including over 40 states. They show that rain-on-snow affects 53 percent of the contiguous U.S. and places 50 percent of the U.S. nitrogen and phosphorus pools at risk of being exported into groundwater and surface water reservoirs. 

Analyzing the Mississippi River floods of 2019, researchers found rain-on-snow events delivered a large pulse of nutrients and sediment into the river and Gulf of Mexico, at much greater levels than a similar growing season rainfall event would have done. This contributed to the Gulf of Mexico’s eighth largest dead zone on record. Dead zones occur when bacteria that thrive on excess nutrients remove too much oxygen from the water, causing massive die-offs of fish and other aquatic animals.

“We hope this study is a wake-up call for government agencies and researchers, because it reveals that 40 percent of the U.S. is producing winter pollution – but no one is tracking exactly how much, where it’s going, or the impacts on water quality and ecosystems,” said Adair. “That’s a big problem that urgently needs addressing.”

The study’s findings are visualized in several maps showing projected winter nutrient pollution across over 40 U.S. states, including large swathes of the Northeast, the northern Midwest and central plains, the Pacific Northwest, and the Sierra and Rocky Mountain ranges. The authors say the research marks the first large-scale study of the impact of rain-on-snow on nutrient runoff and water quality. The findings are conservative and do not include the additional risks from snowmelt and winter rainfall events, nor the impact of winter runoff on downstream ecosystems and communities.

“Climate change impacts on winter are often overlooked,” said co-author Aimee Classen, a Gund Affiliate from the University of Michigan. “If we care about our water quality, we can no longer ignore how climate change impacts winter precipitation.”

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By Alison Bosman, Staff Writer

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