IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

4A1_3I-O4Room A4

Date/Time: July 24 9:30-9:45

Estimation of vertical profile of dissolved CO2 concentration in Cameroonian volcanic
lakes using sound velocity of lake water
Kazuto Saiki1 , Mitsuhisa Sanemasa1 , Katsuya Kaneko2 , Takeshi Ohba3 , Minoru Kusakabe4 , Gregory
Tanyileke5 , Joseph V. Hell5
1Osaka Univ., Japan, 2 Kyoto Univ., Japan, 3 Tokai Univ., Japan, 4 Toyama Univ., Japan, 5 IRGM,Cameroon
E-mail: ksaiki@ess.sci.osaka-u.ac.jp

Limnic eruptions in 1984 and 1986 at Lakes Monoun and Nyos in Cameroon were caused by sudden degassingof magmatic CO2 dissolved in the lake water. The disasters killed about 1800 residents around the lakes. To
prevent further disasters, monitoring of CO2 in the lake waters is essential. Until today CO2 measurement has
been done only once or twice a year because the methods of CO2 measurement require chemical analysis ofwater samples, and are not suitable for more frequent measurement. For this reason, we are trying to develop
a simple and convenient method of CO2 monitoring as part of SATREPS project supported by JICA and JST. In
the field of engineering sound velocity (SV) has been proposed to measure salt concentration (Kleis and Sanchez,1990). We applied the method to dissolved CO2 (CO2(aq)) assuming the following formulav+ =k1[CO2(aq)]+k2[HCO3-], where v+ is a term additional to SV due to dissolved ions, and k1 and k2 are theempirical coefficients that we should determine by experiments.
Laboratory experiments
To determine k1, a SV profiler (Minos X) with a SV sensor, thermometer, and pressure sensor were placed in acylindrical stainless vessel filled with pure water. Then, high-pressure CO2 gas was injected into the vessel toproduce carbonated water. Additional term v+ was defined as the difference of SV between carbonated water andpure water under the same temperature and pressure. CO2(aq) concentration was calculated using Henry’s law.
The result indicated that v+ [m/s] was proportional to CO2(aq) concentration [mmol/kg], and the coefficient k1 wasfound temperature (T) dependent with a regression equation of k1 = 0.033-0.0005*T [m kg/s/mmol].
Field survey
Depth profiles of SV, pressure, T, and electric conductivity of Lakes Nyos and Monoun were measured in March2012 using the SV profiler. The profiles of total CO2 concentration was determined by the syringe method(Kusakabe et al., 2008). Using these data and the correlation between [HCO3-] and electric conductivity proposedby Kusakabe et al. (2008), k2 was determined to be 0.091 at Lake Nyos and 0.067 at Lake Monoun. Then,CO2(aq) concentration was calculated using the k1, k2, SV, T, and electric conductivity. Comparison of the CO2(aq)profiles between the SV method and the syringe method indicated the accuracy of the SV method was better than+-10 mmol/kg. The accuracy is good enough for practical CO2 monitoring. We confirmed that the SV method isapplicable to detect an abrupt change of the CO2(aq) profile that may be caused by sudden CO2 injection to thelakes. Concerning the set of SV, pressure, and T, we measured at 19 points at Lake Nyos and 14 points at Lake
Monoun. All the SV profiles at Nyos were almost identical suggesting that lake water was stably stratified. On theother hand, the SV profiles at Monoun can be grouped into two patterns. It suggests that lake water in the west
basin contains higher CO2(aq) than water in the east basin.

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