For a number of years, engineers from the Swiss Federal Institute of Aquatic Science and Technology (Eawag) have been taking a close interest in Lake Kivu in East Africa. Their concern is focused on the hazard posed by billions of cubic metres of gases dissolved in the deep waters.

In a project sponsored by the Swiss National Science Foundation, the engineers are trying to harness a two-fold benefit from this methane resource: ensuring secure power supplies in the region for decades and reducing the risk of a deadly gas eruption.

Lying between Rwanda and the Democratic Republic of the Congo, Lake Kivu is about one and a half times the size of Canton Zurich and almost 500metres deep (see ‘Focus on Lake Kivu’, over). 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 (Fig.1).

Despite the idyllic setting, however, there lurks a serious hazard in the depths of the lake: approximately 250billionm3 of carbon dioxide and 55billionm3 of methane are dissolved in the water.

In recent years, the Eawag researchers have shown that the gas concentrations are increasing, with a rise of up to 20percent 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 – following a volcanic eruption or a major earthquake, for example – 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 two 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 early 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 (Fig.2).

Several workshops involving international experts have already been held to establish a framework that 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,” they conclude.

Focus on Lake Kivu

Lake Kivu lies on the border between Rwanda and the Democratic Republic of the Congo, at an altitude of almost 1500 metres above sea level. Covering an area of about 2400km2, it has a maximum depth of 500metres. Worldwide, only two other lakes are known to harbour similar quantities of gases – Lakes Monoun and Nyos in Cameroon. In both of these cases, however, carbon dioxide predominates; the quantities of methane are much too small for exploitation to be worthwhile.

The carbon dioxide in the deep waters of Lake Kivu mainly derives from volcanic activity; the methane is produced by bacteria decomposing dead organic matter (algae) in the anoxic bottom waters. The value of Lake Kivu’s gas reserves is currently estimated by experts at around CHF16billion (US$14.3billion).

According to Eawag researcher Martin Schmid, the rise in methane concentrations observed over the past 20–30 years may be attributable to two factors: a huge increase in nutrient inputs, associated with population growth around the lake, and the introduction of a sardine species that has had a major impact on nutrient cycles.

The development of nutrient fluxes and balances in Lake Kivu is therefore now being investigated by Eawag in this project sponsored by the Swiss National Science Foundation.

As well as answering scientific questions, this collaborative effort is designed to promote research activities at local universities.