Turning a risk into a resource

Lake Kivu, lying between Rwanda and the Democratic Republic of the Congo, is about one and a half times the size of Canton Zurich and almost 500 metres deep. The landscape around the lake is reminiscent of the foothills of the Swiss Alps, although banana and cassava plants grow on the slopes, rather than beech and pine trees.

Belying the idyllic setting, however, is a serious hazard in the depths of the lake: approx. 250 billion m3 of carbon dioxide and 55 billion m3 of methane are dissolved in the water. In recent years, the Swiss researchers have shown that the gas concentrations are increasing, with a rise of up to 20 per cent since the 1970s in the case of methane. At present, the gas remains dissolved in the bottom layers as a result of the high water pressure at this depth and the extremely stable stratification of the lake, which means that exchanges between the bottom and surface waters are very limited. However, if gas concentrations continue to increase or if a severe disruption occurred – e.g. following a volcanic eruption or a major earthquake – the situation could change rapidly.

Large quantities of gas bubbles could rise to the surface, triggering a chain reaction that could lead to a massive gas eruption. The release of a mixture of carbon dioxide and methane gases could have catastrophic consequences on the densely populated shores of Lake Kivu, where roughly 2 million people live. Hundreds of thousands could be asphyxiated. In 1986, a disaster of this kind occurred on Lake Nyos in Cameroon, with 1800 people dying after a gas eruption.

The Rwandan government now plans to exploit the gas reserves in Lake Kivu for power generation. It recently awarded the South African engineering company Murray & Roberts a contract to construct a power station. This pilot project is to be initiated in 2008. The principle is simple: if a pipe extending into the depths of the lake is installed, water rises spontaneously as a result of the gas bubbles forming in the pipe. At the surface, the water effervesces – like carbonated water from a bottle that has been shaken before being opened. The methane then has to be separated from the carbon dioxide before it can be used. Professor Alfred Wüest, Head of the Surface Waters Department at Eawag, points out: “It makes sense to use the gas, especially if the risk of an eruption can thereby be reduced at the same time. But because nobody knows exactly how the lake will respond to this extraction, even small-scale pilot studies have to be performed and monitored extremely carefully.”

Wüest and his team have been requested by the Rwandan government and the Netherlands Commission for Environmental Impact Assessment (NCEIA) to oversee the planning of methane recovery on Lake Kivu. This week, several workshops involving international experts are being held to establish a framework which will ensure that the stability of stratification and the ecology of the lake are closely monitored. One controversial question, for example, concerns the depth at which the degassed water should be returned to the lake so as to prevent disruption of the stratification. Also under discussion is whether at least some of the carbon dioxide can be piped back into the deep water, so that greenhouse gas emissions to the atmosphere from methane exploitation are kept to a minimum. Another key question is how methane recovery will affect the growth of algae in the lake. Errors in planning could have a disastrous impact on the sensitive ecosystem and people’s livelihoods. As well as a computer model for simulating processes in the lake, the researchers are therefore also developing a continuous monitoring programme. Any ominous changes occurring in the depths are not to go unnoticed.