Lake Nyos degassing project : first results pertaning to the degassing under way.
M. Halbwachs, J.C Sabroux, J. Grangeon, G. Kayser, G. Tanyileke, European geophysical society, XXVIIth general assembly, Nice (France), april 2002 21-26.
The catastrophic August 1986 gas burst of lake Nyos, Cameroon, resulted in 1800 casualties asphyxiated by the huge amount of carbon dioxide released by an over-saturated water layer of the lake. Inspired by the proposal drawn by the French Ministry of Environment as early as 1987, a team of scientists and engineers has been working for 15 years to conceive, design, assemble and test an original system intended to remove, at an innocuous rate, the residual dissolved gas in order to eradicate the risk of a deadly lake overturn. After several experimental trials, a fully fledged degassing column has been successfully set up at lake Nyos: on January 30, 2001, a spectacular 50 m-high fountain soared above the lake surface.
Since March 2001, a powerful soda spray jet — stable and safe — composed of 90% carbon dioxide and 10% water (volume/volume) has been spurting up from a vertical 145 mm inner diameter pipe sunk into the lake. The lower end of the pipe taps water at a depth of 203 m. The process has been almost continuous, apart from the short periods of time needed for maintenance. Given a steady state CO2 flow-rate of 57000 STP m3/day, the recharge rate of the lake (assessed to be in the 9500 to 14000 m3/day range) is safely offset. The degassing device can be remotely-operated from France or Cameroon, through an INMARSAT satellite link, with a possibility of manual operation from the lake shore and instrumentation barge, as a backup.
This work has been done with the collaboration of the Université de savoie and coordination de la recherche volcanologique (CNRS, INSU), Euratom, and the Institut de recherches géologiques et minières MINREST, Yaoundé.
The satellite link provides also a real time access to technical parameters pertaining to the device operation, and to the physical and chemical lake response to the degassing. The thinning, by ca. 2 metres in nine months at 190 m depth, of the lowermost, gas rich layer of the lake is clearly visible on the continuous CTD measurements, and consistent with the gas and water flow-rates through the pipe. The observed subsidence of all the water layers is in good agreement with the calculated efficiency of the degassing system, taking into account the assessed gas recharge rate and the recycled water flow-rate through the degassing column. These results allow to extrapolate with confidence the single column experiment to the future five-column degassing scheme of lake Nyos: when operational, these columns will extract most of the dissolved carbon dioxide in a period of approximately four years of degassing.