A proposal for mitigation of natural hazards by explosion of gassy lakes in Cameroon, West Africa

THE gas disasters at Lake Nyos in 1986 （1 746 casualties） and Lake Monoun in 1984 （37 casualties）,both in Cameroon, were caused by a sudden release of a large amount of CO2 stored in these lakes. TheCO2 is derived from a magmatic source, judging from the isotopic ratio of δ13C=-3.5±0.2‰ （PDB）and that of helium associated with the CO2, i.e. 3He/4He-8×10-6 or 5.7 Ra where Ra is the 3He/He ratio of the atmospheric helium. Follow-up surveys of Lake Nyos （209 m deep） revealed that thetemperature, concentration of dissolved ionic species （TDS） and free-CO2 generally co-vary and their profiles show a large decrease in the surface layer, limited change at mid depths and a significant increasetowards the lake bottom. This simultaneous increase in temperature, conductivity and free-CO2 in the bottom water supports the view that CO2 is being supplied to the lake in the form of warm, CO2-charged,mineralized water. Our recent observations （1993-1996） indicate that Lake Nyos still contains 12.4 gi-