Burping a Lake
One day in August 1986, catastrophe struck a patch of countryside in the African nation of Cameroon. Suddenly, mysteriously, people and animals died. Whole villages expired; herds of cattle collapsed where they stood. Concerned and puzzled scientists flocked to the area while people mourned more than 1700 dead neighbors and kinfolk.
The researchers found a surprising culprit. The victims had suffocated in a cloud of poison gas: carbon dioxide. Its source was beautiful Lake Nyos, which filled the crater of a dormant volcano. Since then, Lake Nyos has become a very well-studied body of water.
The lake is fed by geothermal springs at its bottom, The entering waters are warm and apparently are laden with dissolved gases, especially carbon dioxide. At least, the springs are the suspected source of the carbon dioxide; no one knows for sure where it comes from, but the gas is there in high concentrations. Warm water usually rises buoyantly, but in Lake Nyos the dissolved carbon dioxide increases the density of the bottom water. Because the gas-laden water is so dense, it stays down.
That is, the gas-rich water stays on the bottom until something happens to upset the lake's internal structure. In the 1986 disaster, that something seems to have been heavy rainfall. The rain cooled the surface water, increasing its density so much that it sank, displacing the original bottom water. Caught up in this convective overturn, the gas-rich water rose.
Water pressure is a function of depth. Rising to less depth, the gassy water was under less pressure. The gas began to come out of solution---like the fizzing of an opened can of soda pop. The water mass rose, stirring the lake, lifting yet more gas-rich bottom water, releasing more carbon dioxide bubbles as pressure decreased: if Lake Nyos were a can of soda pop, it would have been a well-shaken one. A froth of carbon dioxide, 240,000 metric tons of it, spilled over the lake's rocky rim and swept downslope into villages and pastures.
Measurements and calculations indicate that the amount of gas lost from the lake in 1986 will have been replaced by the year 2006. By then or even earlier, Lake Nyos will be ready to cause another disaster.
Foreseeing catastrophe is one thing, but figuring out how to prevent it is quite another. Draining the lake is theoretically possible but hopelessly impractical. That's also true for evicting the local population. Cameroon has little money and few technically trained people, so the country needs help from the international scientific and engineering community. The experts are trying; publications and meetings are being devoted to Lake Nyos and the other potentially gas-erupting lakes of Africa.
Among the more appealing ideas for safely releasing the gas is one proposed by Hugh Fletcher, who is affiliated with The Queen's University of Belfast in Northern Ireland. He suggests that Lake Nyos be fitted with what he calls a gas lift. It sounds rather like a drink straw.
Envision a thin tube extending from near the lake bed to the surface. Assume a pump begins lifting the gas-rich bottom water up the tube. At a certain level, the water in the tube would begin to effervesce as carbon dioxide bubbles out of solution. Bubbles and water would begin to rush up the tube. Once begun, the process would continue without further pumping until not enough gas remained at the bottom to drive the circulation---which would mean that the effort had succeeded.
Fletcher calculates that it would take a fair forest of tubes to exhale enough gas to prevent future eruptions. Then too, the degassed water would need careful disposal to avoid disturbing the lake's water layers. He thinks those concerns are easily handled.
Other proposals are being considered, and various laboratory-scale experiments are under way. But something will have to be tried soon; the gases are building up at the bottom of Lake Nyos.
