London, September 30 : A new research by scientists has found that a particular dam in China could be producing tonnes of methane, a potent greenhouse gas.

According to a report in Nature News, marshes in the drawdown area of the Three Gorges Reservoir in China could be a significant source of methane.

Scientists have become increasingly concerned about the greenhouse gases released by submerged grass and trees when land is flooded to create dams.

When such organic matter decays, it releases methane and carbon dioxide, which contribute to global warming.

Methane is particularly troublesome as it has more than 20 times the warming impact of CO2.

Previous studies of methane emission have focused on dam components themselves, such as the power-generating turbines — where the gas fizzes out of solution as reservoir water is fed into them — and also on the reservoir surface, where methane is released either through slow diffusion or as bubbles.

But many dams are partially drained at different times of year for maintenance purposes, such as silt removal, exposing ‘drawdown’ areas that have previously been underwater.

“Little attention has been paid to the drawdown regions, probably because they usually constitute a small part of the reservoir surface area,” said Huai Chen, an ecologist at Chongqing University in China, who was first author of the study.

The Three Gorges Reservoir, however, has a large drawdown area of 350 square kilometres — or about one-third of the total flooded reservoir area when the dam is in full operation.

That led Chen and his colleagues to suspect that methane emission from such areas might not be negligible.

In a pilot study, the researchers focused on an area of 37 square kilometres near the Pengxi River, a branch of the Yangtze River, that becomes marshland when the water level is lowered in the summer to clear silt.

They covered a fixed area of the wetland with a plastic chamber to trap escaping gases, which they then tapped and analyzed.

After taking measurements from four types of vegetation every ten days from July to September in 2008, the team found that different plants emitted varying amounts of methane at different times of the year.

The researchers calculated that, on average, the newly created marshes along the Pengxi River emitted 6.7 milligrams of methane per square metre per hour — higher than the rate of methane emission from the surface of reservoirs in the tropics, such as the Petit Saut reservoir in French Guiana or the Balbina and Samuel reservoirs in Brazil. (ANI)