Contribution of methane to total gas pressure in deep waters at lakes Nyos and Monoun (Cameroon, West Africa)

Abstract

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.