A portable gas-analyser was used to perform on-site gas analysis at lakes Nyos and Monoun. Results were used to evaluate the effect of the dissolved CH4 on the gas saturation level. Although CH4 represents marginal proportion in term of the total gas concentration (~2.2 mol % in Lake Monoun and ~0.6 mol % in Lake Nyos) it contributes ~37% and ~19% to the total dissolved gas pressure and ~23% and ~13% to saturation level in lakes Monoun and Nyos respectively. The risk due to CH4 increase is evaluated and monitoring and surveillance measures proposed. Because methane is less more soluble than carbon dioxide, its increase may accelerate saturation of the lake waters with dissolved gas making gas eruptions more frequent. It is therefore recommended to regularly monitor the lakes in order to assess CH4 evolution for hazard mitigation purposes. If future monitoring surveys confirm current CH4 increase, a device to extract deep rich-gas water must be experimented and put in place without delay.
2013 by The Geochemical Society of Japan