Deadly Gas, Cheap Power

Magma A stream of magma from a nearby volcano flowed into Lake Kivu in 2002. Lava like this could cause the lake to release a huge cloud of toxic gas. J. naegelen/reuters/scanpix

To live on the banks of Africa’s Lake Kivu is to risk your life every day. Large amounts of methane, carbon dioxide and hydrogen sulfide gas are dissolved in various layers of the lake’s deep waters. Scientists warn that a disturbance such as a volcanic eruption or earthquake could cause a redistribution of the lake’s waters and the gases in them. This shuffling, known as an overturn, could unleash an invisible, suffocating cloud of these compounds—a rare event known as a limnic eruption—killing as many as two million people nearby. Now, rather than simply wait for disaster, scientists hope to put some of that gas to use, as cheap energy.

Lake Kivu, located on the border between Rwanda and the Democratic Republic of the Congo, is one of Africa’s largest lakes. It covers 1,040 square miles and is nearly 1,600 feet deep. The region is particularly volcanically active, and magma and groundwater feed gases and minerals into the lake.

Carbon dioxide dissolves best in high-pressure, low-temperature water like that at the bottom of the lake—and the CO₂ makes the water more dense, so it stays at the deepest levels. Dissolved methane, hydrogen sulfide and minerals also increase the water’s density. Organic materials, such as dead plants and fish, decompose on the lakebed, producing additional methane. As a result, the bottom of the lake contains layers of pressurized solutions of water and gas—a lot like a bottle of soda—while the less-dense layers of water on top act like a cap.

Over time, the deepest waters of the lake have accumulated about 60 cubic miles of CO₂ and about 14 cubic miles of methane. With these gases mostly trapped at the bottom, the lake’s dark depths are a time bomb.

Do Not Disturb

A disturbance of the lake could have the same explosive effect that occurs when you open the top of a soda bottle or pop a champagne cork—a bubbling over of liquid and gas. In the lake, the cause of the sudden release might be an earthquake or the immersion of hot lava that disturbs the layers of water in the lake. Large amounts of gas would bubble up and, just like it happens in the bottles, the contents of the lake would surge up and spill over.

The lake lies in the Great Rift Valley, where Africa is splitting apart as two tectonic plates move away from each other, making it an earthquake hot zone. Early this year, there were two serious quakes, which killed a total of 40 people.