TERRAPUB Geochemical Journal

Geochemical Journal, Vol. 47 (No. 3), pp. 349-362, 2013

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

Issa,1,2 Takeshi Ohba,1 Wilson Fantong,2 Alain Fouepe,2 Boris Chako Tchamabe,1 Yutaka Yoshida,3 Minoru Kusakabe,4 Nkamdjou Sigha,2 Urumu Tsunogai,5 Yu Oginuma,1 Gregory Tanyileke,2 Hiroshi Satake4 and J. V. Hell4

1Laboratory of Volcanology and Geochemistry, Department of Chemistry, School of Science, Tokai University, 4-1-1 Kitakaname, Hiratsuka, Kanagawa 259-1211, Japan
2Institute for Geological and Mining Research (IRGM), Yaoundé, Cameroon
3Yoshida Consulting Engineer Office Morioka, Iwate 020-0121, Japan
4Department of Environmental Biology and Chemistry, University of Toyama, Gofuku 3190, Toyama 930-8555, Japan
5Graduate School of Environmental Studies, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan

(Received July 2, 2012; Accepted February 2, 2013)

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.
Key words: methane, Cameroon volcanic line, Lake Nyos, Lake Monoun, gas-analyser

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