Cameroon clouds: soda source?

Cameroon Clouds: Soda Source?

With new evidence in hand, two U.S.scientists have recently returned from a follow-up study of last year's Lake Nyos disaster in Cameroon, in which the lake had expelled a large cloud of carbon dioxide that spread into surrounding valleys, asphyxiating livestock and killing 1,746 people. This event, and a similar but less damaging one at Lake Monoun in 1984, had prompted much concern about the hazard posed by other crater lakes in the region (SN: 9/20/86, p. 180). However, the researchers, who have just completed a survey of Cameroon's 33 crater lakes, report that only Nyos and Monoun contain large amounts of dissolved carbon dioxide and therefore are the only lakes to pose any danger of future gas releases.

Also emerging from the study is evidencethat will help determine how carbon dioxide entered the two lakes and how quickly this process is occurring. The new findings, which the researchers discussed this week in interviews with SCIENCE NEWS, support a theory proposed earlier that carbon dioxide rises from the earth's interior and becomes dissolved in groundwater, which then feeds into the lakes.

"We strongly suspect that the carbondioxide enters Lakes Nyos and Monoun through submerged soda [carbonated] springs,' says William C. Evans of the U.S. Geological Survey in Menlo Park, Calif., one of the participants in this study.

Numerous springs of carbonated,slightly heated water bubble up to the surface along a line of past and present volcanic activity in Cameroon. In a survey of these surface springs, Evans and George Kling from Duke University in Durham, N.C., found one that was discharging enormous quantities of carbon dioxide. This spring, says Evans, "is potent enough to supply one of those lakes with enough gas to saturate it in a matter of, say, thousands of years,' a number consistent with estimated ages for the lakes. Once it enters these deep, highly stratified lakes, the carbon dioxide dissolves and is trapped until some event upsets the delicate equilibrium and the gas comes out of solution.

Evans says that finding such a potentspring at the surface lends credence to the possibility that submerged springs are actively supplying Lakes Nyos and Monoun with carbon dioxide.

The researchers also measured slightincreases in the alkalinity and temperature of the water at the bottom of Nyos. An influx of carbonated, heated ground-water would account for these changes, the researchers say in a report presented to the Cameroonian government on June 4. However, Evans and Kling say it is possible that other processes might be creating these observed trends.

Laboratory tests in the coming monthswill help determine whether ground-waters are causing the increase in alkalinity, says Kling. In particular, U.S. researchers will be analyzing ion and isotope concentrations in samples from Monoun and Nyos. They will also compare samples from the surface soda springs and from the bottom of Nyos.

With all the other Cameroonian craterlakes tested and declared safe, most of the danger to the region comes from a weak dam on the northern shore of Nyos, says Kling. Since the Cameroonian government has evacuated most people from the valley surrounding Nyos, a second release of gas would not be as disastrous as the first; however, a flood from a dam break would cause significant damage to the homes in the region.

Although scientists are unsure howmuch carbon dioxide remains dissolved in Monoun, this lake's relatively small size prevents it from holding as much gas as Nyos, and it is therefore less dangerous, says Kling. As well, preliminary analyses of the bottom samples from both lakes indicate that the rate of recharge for carbon dioxide is quite small. However, Kling stresses, "We're working with a phenomenon that occurs on a geologic time scale and we're trying to make measurements on a nine-month time scale.'

Photo: Lake Nyos (left) and the prodigious Ahio-Ekenzu soda springwith CO2 bubbles.