Killer Lake

The last thing you would expect of a body of water is that it would explode. Yet one of Africa’s largest lakes threatens to erupt, with dire consequences for 2 million people who live around it.

The lake is Kivu, on the border of Rwanda and the Democratic Republic of Congo. Volcanic vents on the lake floor cause the build up of terrific concentrations of carbon dioxide. With the right trigger—a volcanic eruption, an earthquake or simply the ongoing production of gas—the CO2 might explode out of the water, like the fizz gushing from a giant can of Coke. The invisible, heavier-than-air gas would settle on the shores and asphyxiate residents of lakeshore villages, towns and cities.

Lakes have exploded with fatal results before. In 1984 Lake Monoun in Cameroon released its buildup of CO2, asphyxiating 37 nearby residents. Two years later, Lake Nyos in Cameroon erupted, killing more than 1,700 up to 15 miles away.

Kivu is more than 1,000 times larger than Nyos, with far more people living in its basin. “Because of its size, it is absolutely unique in the world,” says Robert Hecky, professor at the University of Minnesota Duluth’s Large Lakes Observatory and head of the Institute on the Environment-funded Global Great Lakes project. Kivu is unique for another reason: It is the only lake known to harbor large concentrations of not only CO2 but also methane.

Here’s how it works. Kivu straddles a volcanic rift. Water seeping through the volcanic sediments dissolves CO2 as well as various salts. Even though this water is warm, the dissolved materials increase its density enough that it settles on the bottom, up to 1,600 feet deep. “It’s a very tenuous kind of stratification,” says Hecky. “You have a delicate balance in the deep water.” Bacteria decompose organic muck in the sediment, consume CO2 and produce methane, which also dissolves in the water. As long as the lake remains stratified, the tremendous pressure of the water above keeps the gases in solution.

Scientists hypothesize that the heat of a volcanic eruption or sudden uplift of an earthquake will force gas-saturated water upward. With the drop in pressure as the water rises to the surface, the gas will boil free. Once a bubble-plume begins, it will strip other gas out of solution.

If a large portion of the gas erupts, the lake will bulge at the surface from the violent upheaval. Tsunami-like waves will crash the shore and rebound across the lake. Released CO2 will settle on the shores and suffocate those who can’t escape.

Hecky first studied Kivu in the 1970s. He noticed that in at least five intervals in the past 5,000 years, lake-bottom sediments were missing common microfossils. The absence puzzled him until the explosion of the Cameroon lakes, when he wondered if similar explosions in Kivu had temporarily destroyed or prevented deposition of the plankton. He’s now looking for further clues as to whether Kivu underwent past eruptions.

Meanwhile, energy developers are hoping to harvest dissolved methane to reduce danger and provide power. In a technique already being used to provide gas for a small brewery boiler, engineers are designing a platform and pipes to bring deep water to the surface and capture the CO2 and methane. An existing plant has plans to expand. And Rwanda and the Democratic Republic of Congo are trying to arrange construction of a methane-powered plant of up to 200 megawatts.

“There’s a huge demand for electricity,” says Hecky. “If you could do this in a secure engineering fashion, it would relieve the gas pressures and make them even less likely to build up over time—though you’d never eliminate that possibility.”

GREG BREINING writes about travel, science and nature for Momentum, the New York Times and many other publications. He paddled around Lake Superior to write the book Wild Shore: Exploring Lake Superior by Kayak.

Kivu Lake, Rowanda and the Democratic Republic of Congo