Geochemical Journal, Vol. 42 (No. 1), pp. 93-118, 2008
Minoru Kusakabe,1 Takeshi Ohba,2 Issa,2 Yutaka Yoshida,3 Hiroshi Satake,4 Tsuyoshi Ohizumi,5 William C. Evans,6 Gregory Tanyileke7 and George W. Kling8
1Institute for Study of the Earth's Interior, Okayama University, Misasa 682-0193, Japan
2Volcanic Fluid Research Center, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152-8551, Japan; Issa is on leave from Institute of Research for Geology and Mining, Yaounde, Cameroon
3Yoshida Consulting Engineer Office, Tsukigaoka, Morioka 020-0121, Japan
4Department of Environmental Biology and Chemistry, University of Toyama, Toyama 930-8555, Japan
5Niigata Prefectural Institute of Public Health and Environmental Sciences, Niigata 950-2144, Japan
6U.S. Geological Survey, Menlo Park, California, CA 94025, U.S.A.
7Institute of Research for Geology and Mining, Yaounde, Cameroon
8Department of Ecology and Evolutionary Biology, University of Michigan, Ann Arbor, MI 48109, U.S.A.
(Received June 14, 2007; Accepted November 9, 2007)
Evolution of CO2 in Lakes Monoun and Nyos (Cameroon) before and during controlled degassing is described using results of regular monitoring obtained during the last 21 years. The CO2(aq) profiles soon after the limnic eruptions were estimated for Lakes Monoun and Nyos using the CTD data obtained in October and November 1986, respectively. Based on the CO2(aq) profiles through time, the CO2 content and its change over time were calculated for both lakes. The CO2 accumulation rate calculated from the pre-degassing data, was constant after the limnic eruption at Lake Nyos (1986- 2001), whereas the rate appeared initially high (1986-1996) but later slowed down (1996-2003) at Lake Monoun. The CO2 concentration at 58 m depth in Lake Monoun in January 2003 was very close to saturation due to the CO2 accumulation. This situation is suggestive of a mechanism for the limnic eruption , because it may take place spontaneously without receiving an external trigger.
The CO2 content of the lakes decreased significantly after controlled degassing started in March 2001 at Lake Nyos and in February 2003 at Lake Monoun. The current content is lower than the content estimated soon after the limnic eruption at both lakes. At Monoun the degassing rate increased greatly after February 2006 due to an increase of the number of degassing pipes and deepening of the pipe intake depth. The current CO2 content is ~40% of the maximum content attained just before the degassing started. At current degassing rates the lower chemocline will subside to the degassing pipe intake depth of 93 m in about one year. After this depth is reached, the gas removal rate will progressively decline because water of lower CO2(aq) concentration will be tapped by the pipes. To keep the CO2 content of Lake Monoun as small as possible, it is recommended to set up a new, simple device that sends deep water to the surface since natural recharge of CO2 will continue.
Controlled degassing at Lake Nyos since 2001 has also reduced the CO2 content. It is currently slightly below the level estimated after the limnic eruption in 1986. However, the current CO2 content still amounts to 80% of the maximum level of 14.8 giga moles observed in January 2001. The depth of the lower chemocline may reach the pipe intake depth of 203 m within a few years. After this situation is reached the degassing rate with the current system will progressively decline, and it would take decades to remove the majority of dissolved gases even if the degassing system keeps working continuously. Additional degassing pipes must be installed to speed up gas removal from Lake Nyos in order to make the area safer for local populations.
Key words: Lake Nyos, Lake Monoun, hazard mitigation, CO2 evolution, natural recharge