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Rivers & Global Warming

It’s well documented that vehicle exhaust, fossil fuels and other forms of pollutions contribute to global warming. But so can more innocuous natural resources: Like rivers.

Rivers produce 10 percent of the entire human-caused nitrous oxide emitted in the world. Nitrous oxide is a greenhouse gas, air pollutant and contributor to climate change. Considered over a 100-year period, it has 298 times more impact per unit mass (global warming potential) than carbon dioxide.

“It didn’t use to be that way, and the main culprit is the use of nitrogen-based fertilizers that end up in rivers and waterways from agricultural runoff,” says Jeff Reeder, a civil engineering graduate student in the University of Idaho’s College of Engineering.

A key component of fertilizer, nitrate is transformed in nitrous oxide and di-nitrogen (N2) gas within the anaerobic zone of streambeds.

Not all rivers produce the same amount of nitrous oxide, and the reasons for this are not well understood. That’s where Reeder gets involved. He is conducting research at the Center Ecohydraulics Research’s (CER) at the University of Idaho Boise with Daniele Tonina, a professor of civil engineering. With a $500,000 National Science Foundation grant, they are working to understand why some rivers produce more nitrous oxide than others. This is an interdisciplinary research project in collaboration with chemists and microbiologists from Boise State University.

Located in downtown Boise at the Idaho Water Center building, U of I Boise’s CER flume is one of just a few ecohydraulic labs in the world. The flume can simulate river flows under the ideal conditions needed to conduct this type of research, which would be impossible in real rivers because of variables that cannot be controlled in nature.

Daniele Tonina, a professor of civil engineering at U of I Boise demonstrates the water flume.
Professor Daniele Tonina of U of I Boise demonstrates the flume research that is underway at the CER.

Researchers working at the flume have created two ecosystems that mirror the natural conditions of two streams. The two streams at the flume each display river beds with sandy dunes of different sizes and shapes populated with microorganisms.

“How the water is filtered through the dunes has an impact on the rates of nitrous oxide production in rivers and streams,” Tonina says.

To show the role the dunes’ shapes play in filtering, oxygen levels are monitored at the flume via water samples and optical fiber. Over 200 optical sensors are embedded in the stream bed and are connected to a network of fiber optic cables. Leveraging technology that was developed for fiber optic communication networks, each sensor is connected to an instrument that measures the dissolved oxygen concentration and logs it to a computer. This system enables continuous sampling of the oxygen throughout the stream bed. Because of the dynamics of how water filtering through the river beds carries oxygen, these measurements will help determine if the shape of the river beds controls the amount of nitrous oxide released.

A better understanding of the role of river beds in nitrous oxide production will help create strategies to reduce greenhouse gas emissions from natural and manmade water systems.

“The results could potentially be used to determine the impact of fertilizer use in global warming,” Tonina says. “We want to reduce our footprint on the environment by switching from fossil to corn-based fuels, but corn-based fuels might not be as environmental friendly as we think due to the amount of fertilizer used in the fields and drained to rivers. This research is a way to find out in advance the impact of such a switch.”

Article by Maria Ortega, University of Idaho Boise

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