Climate Change Is Hurting Forests’ Ability to Filter Agricultural Nitrate Pollution

May 13, 2019, 6:45 a.m. ·

The Yeltin Spring, located deep in the Mark Twain National Forest, overflowed after heavy spring rains. When water moves too fast, the forest can’t remove pollutants like nitrate. (Photo by Jonathan Ahl, Harvet Public Media)

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Animal waste and nitrogen-based agricultural fertilizers contribute to nitrate runoff, which ends up in creeks, streams, rain and, eventually, water systems. Nitrate, that mix of nitrogen and oxygen, can cause serious health problems if it’s too concentrated.

The best defense is filtering, which forests are great at doing. But a new study from the U.S. Department of Agriculture’s Forest Service suggests forests are falling behind, and heavy rains brought on by climate change are making it worse.

“They always say the solution to pollution is dilution,” said Theresa Davidson, a USDA Forest Wildlife and Fisheries Biologist at the Mark Twain National Forest in Missouri. When water enters a forest, either through rain or runoff, the soil, leaves and trees absorb things and make the water cleaner than when it came in.

But 21 years of data from more than 100 streams across 20 states — including Missouri, Iowa, Illinois and Indiana — showed an increasing amount of severe storms and floods created times when the water moved too fast and hampered forests’ ability to filter nitrate, according to Stephen Sebestyen, a research hydrologist with the Forest Service.

“There were some, in particular, short-term duration events, rainfall or snowmelt events, when some of that atmospheric nitrate rapidly reached the streams,” Sebestyen said. “And those amounts were rather large.”

Heavy rains this spring across the Midwest created similar conditions in the Mark Twain National Forest in Missouri. The Yeltin Spring was overflowing, clear water erupting out of a cave-like opening and rushing down a hill. It turned what was a dry creek bed into a quick-moving, two-foot-deep stream.

Nitrate’s effects

Here’s the issue with nitrate: It’s common and potentially dangerous. Nitrate plays a major role in producing food in the U.S., but it can cause problems in drinking water.

The Environmental Working Group, a nonpartisan advocacy organization, actively monitors nitrate levels in drinking water. Senior Analyst Anne Schechinger said 10 milligrams per liter are considered an acceptable amount of nitrate in drinking water, but she believes even that is major threat to public health.

“Many newer studies have shown that drinking water with levels of nitrate at just 5 milligrams per liter or more can cause increased rates of cancer such as colon, bladder and ovarian cancer as well as birth defects,” Schechinger said.

It’s not cheap to clean nitrate out of drinking water, either. Des Moines, Iowa, spent more millions of dollars to do that, and a recent study showed Iowa well water contains significant amounts of nitrate.

The nitrate levels are bad for forests, too, Sebestyen said, to the point that long-term damage is possible.

“It affects the productivity, how things grow, how quickly they grow,” he said. “If the levels get high enough, it can cause decline in forests. It can cause tree species to die.”

Possible fixes

Municipalities and farmers should be focusing on preventive measures, which Schechinger said starts with better agriculture land use.

“We really believe that implementing agricultural conservation practices like edge-of-field buffers or cover crops or grassed waterways could prevent nitrate into drinking water and just getting into our water bodies in the first place,” Schechinger said.

Schechinger also said there needs to be more studies on the effectiveness of fertilizers, and how much nitrogen needs to be in the formula.

Sebestyen wants policymakers to ask basic questions about forests, water and agriculture, such as “what we want forests to produce, how effective we want them to be in their production and how much nitrate do we want to move through the forest.”

Then, he said, the people in power should figure out how to scale back on nitrate that’s entering the water system.


Harvest Public Media is a reporting collaboration focused on issues of food, fuel and field. Harvest covers these agriculture-related topics through an expanding network of reporters and partner stations throughout the Midwest.