Even Small Dams a Potential Hotbed for the Greenhouse Gas Methane

Dams, it turns out, may take an even greater toll on the environment than previously thought.

Already scientists have shown that the world's 50,000 large dams are a source of methane, but a new study, published in the journal Environmental Science & Technology, provides new evidence that smaller dams are a problem as well.

Methane is a known greenhouse gas with a warming effect 25 times more powerful than the headline-grabbing carbon dioxide. Among the largest contributors of emissions in the United States is industry, followed by domesticated animal and landfill waste, the Environmental Protection Agency (EPA) reports on its site.

In regards to natural sources, wetlands lead the way, emitting methane from bacteria that decompose organic materials in the absence of oxygen. Other smaller contributors are termites, oceans, sediments, volcanoes and wildfires.

In the case of dams, methane comes from organic matter in the sediments that accumulated behind them -- a fact that has led to questions about hydroelectric's reputation as "green."

Knowing this, a team of researchers led by Andreas Maeck, a graduate student at the University of Koblenz in Germany, decided to take a look at methane releases from six water impoundments found behind dams storing less than 50 feet of water.

Though a small sample, the researchers were able to detect a trend nevertheless.

"Our results suggest that sedimentation-driven methane emissions from dammed river hot spot sites can potentially increase global freshwater emissions by up to 7 percent," the authors report.

Based on this, they note that emissions of the worrisome gas are likely to increase due to a boom in dam construction encouraged both by the quest for new energy sources as well as water shortages.

In the case of the United States, methane emissions have hit a low of late, according to the EPA, having decreased by 8 percent between 1990 and 2011 due to a downward trend in emissions associated with the exploration and production of natural gas and petroleum products. This drop would have been greater, the US agency reports, if it hadn't been for an accompanying rise in emissions due to agricultural activities.