The Rocky Mountains are gradually recovering from acid rain

The Rocky Mountains are starting to recover from acid rain caused by traffic and industry, according to a new study from UC Boulder. The researchers found that nitric and sulfuric acid levels in the Green Lakes Valley region of Niwot Ridge have decreased over the past 30 years. 

Niwot Ridge is a high alpine area of the Rocky Mountains located west of Boulder and east of the Continental Divide. In the Rocky Mountain National Park to the north of Niwot Ridge, wildlife depends on limited levels of acidity in the water and soil to thrive.

The mountains also serve as the main water source for surrounding communities, and the health of these alpine ecosystems influences both water quality and quantity.

“It looks like we’re doing the right thing. By controlling vehicle emissions, some of these really special places that make Colorado unique are going back to what they used to be,” said study co-author Jason Neff, who is the director of the Sustainability Innovation Lab at Colorado (SILC).

Over the last two centuries, most of the world has been exposed to increased acidic nutrients such as nitrogen through rain and snow. Nitrate and other nitrogen oxides are primarily emitted from vehicles and energy production. 

While nitrogen is considered to be the most important nutrient for plant growth, excessive levels can have negative consequences.

In the summer sun, hot air rises and carries up nitrogen from cars, industry, and agriculture. As this air cools, explained Neff, it forms clouds over the Rocky Mountains and falls back down as afternoon thunderstorms – depositing contaminants.

In the 1970s, this type of “acid rain” affected lakes throughout North America. One of the areas that was hit particularly hard was upstate NewYork, where entire fish populations were completely wiped out from the acidic water. 

Scientists are still working to understand how increased levels of acidic nutrients affected other habitats, such as the alpine region of the Rockies, and how long these ecosystems take to recover.

To investigate, the UC Boulder team analyzed data from 1984 to 2017 on atmospheric deposition and stream water chemistry from the Mountain Research Station located on Niwot Ridge. The researchers discovered that levels of nitric and sulfuric acid stopped increasing in the early 2000s and started decreasing by the mid-2000s.

On the other hand, the data showed that levels of ammonium from fertilizer have more than doubled in rainfall across the area between 1984 and 2017. This indicates a need for continued monitoring and further investigation into the effects of fertilizer on mountain ecosystems.

“We used water quality modeling and statistical approaches to analyze the long-term datasets that Niwot researchers have been collecting for decades,” said study co-author Eve-Lyn Hinckley. “The data are available for anyone to download. Our modeling approaches allowed us to evaluate the patterns they hold in a rigorous way.”

Bill Bowman is the director of the Mountain Research Station and a professor of Ecology and Evolutionary Biology. Since 1990, Professor Bowman has been studying how nutrients like nitrogen affect plants in mountain ecosystems. He has determined that alpine environments are unique in how they respond to these nutrients.

“It’s a system that is adapted to low nutrients, as well as a harsh climate and a very short growing season – and frost in the middle of the season. These are very slow growing plants. And they just simply can’t respond to the addition of more nitrogen into the system.”

Professor Bowman has also found that these ecosystems recover quite slowly, even after acidic elements like nitrogen are no longer being introduced.

Despite some bad news, the researchers see their findings as encouraging overall. Even if it’s slow going, they said, the results show that the alpine ecosystem has a chance to recover.

“We still have air quality issues in the Front Range,” said Neff. “But even with those air quality issues, this research shows that regulating vehicle and power plant emissions is having a big impact.”

The study is published in the Journal of Geophysical Research: Biogeosciences. 

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By Chrissy Sexton, Earth.com Staff Writer