In the case of Lake Kivu, if all the methane and CO₂ currently dissolved in these waters were to be released on the surface, a gas cloud would form up to more than 100 metres above the current level of the lake. This cloud would cover the entire region, including the big Congolese towns of Goma and Bukavu. The cloud would affect at least two million people. They would die either as a result of the eruption itself or from the gas cloud. Besides its direct toxicity, CO₂ being heavier than air, it would stagnate at ground level and send the oxygen upwards, asphyxiating all aerobic forms of life, including human beings. 

However, the researcher immediately reassures us by saying that there is no point dwelling on the worst case scenario. The water at a depth of 275 metres currently contains a concentration of dissolved gas at a 50 to 60% of saturation rate. This concentration increases with the depth, but the hydrostatic pressure also increases allowing a greater quantity of dissolved gas. The most critical point lies at a depth of 275 metres. In the case of a gas eruption, this is where the first bubbles would form. A recent study shows that the concentration of CH₄ in the lake increased from 10 to 15 % between 1974 and 2004. Despite a certain margin of error, it is possible to estimate the time remaining before the waters of the lake reach saturation point to be a century. By then, the majority of the water will have been degassed by the industrial extraction that will have undoubtedly been set up. At the moment, only a major event that would cause the waters to rise some several hundred metres could trigger significant natural degassing. And these events are particularly rare. “In principle, according to the study of sediments, the last gas eruption of Lake Kivu dates back almost 5 000 years”, the researcher reassures us. However, Kabuno bay situated to the north-west of the lake, close to the town of Goma, is of more immediate concern. No recent study has provided information on the evolution of the gas concentrations it may contain. It should be possible to carry out this urgent study in the very near future.

Extracting the gas and the problem of the lake’s eutrophication

The risk of a gas explosion must nevertheless be taken seriously. Extracting the gas could defuse this time bomb. At the same time, extracting such a quantity of methane offers the prospect of ensured profitability, and some Rwandan and American industrialists have got their beady eyes on these waters. “At the moment, there are several pilot projects, which don’t have a very powerful extraction rate and which currently don't have a very great impact on the lake's ecosystem", explains François Darchambeau. “For now, their purpose is more to develop and test technologies rather making a profit out of the extraction. But a big American project should be operational by the end of 2012. Subsequently, we must already start thinking about how to extract this gas.”

The extraction itself is relatively simple because as the water rises to the surface, it gradually reaches an atmospheric pressure that is no longer sufficient to maintain the dissolved gas in such quantities. Similar to opening a fizzy drink, the gas escapes from the liquid, decompressing to achieve its normal volume. It thereby sucks the liquid upwards, which explains the risk of eruption.  Extraction uses the same natural process. “As it rises, the water naturally degasses. All that needs to be done is to place a long tube in the lake. Furthermore, the developers can take advantage of what is called the siphon effect. As the water rises, the bubbles created will, in turn, cause the water to rise, rather like a water filter system in an aquarium. It simply needs to be primed in the beginning, and then after rising a hundred metres, the bubbles form and the water rises by itself.”

A mixture of water and gas is left on the surface, with one litre of water for two to three litres of gas. This means that these two to three litres of gas, in the deep waters, are compressed and dissolved in just one litre of water, and that they assume their natural volume when they come into contact with the atmospheric conditions at the surface. But the mixture remains wet. It has therefore gone through a separator which separates the gas from the liquid. The gaseous phase is retained and the liquid phase eliminated. This gaseous phase contains 80% CO₂, 15% CH₄ and 5% nitrogen. To obtain the greatest percentage of methane possible, this gas is washed: the flow of gas passes through a water column in which the extremely soluble CO₂ naturally dissolves. The mixture obtained at the end contains between 80 and 90% CH₄, and can be used as fuel to produce electricity, for instance.