Abstract

A new method to measure the CO₂ concentration in lakes Nyos and Monoun using sound speed (SS) and electrical conductivity (EC) of the lake water was developed. Limnic eruptions at lakes Monoun and Nyos in Cameroon, characterized by a risk related to possible sudden release of magmatic CO₂ gas dissolved in the lake water, occurred in 1984 and 1986 respectively, killing about 1800 residents. CO₂ monitoring of the lake waters was initiated to prevent further disasters. Classical methods of CO₂ measurement, which require chemical analysis of water samples, are not suitable for frequent and multi-point monitoring. Inspired by the method of measuring salt concentration using SS, we obtained data for dissolved CO₂ (CO_2(aq)) assuming the following formula , where Δv is additional SS due to dissolved ions and k₁ and k₂ are coefficients determined through experiments. The coefficient k₁ was determined with temperature T (°C) dependence as 0.0323–0.000523T [l m s⁻¹ mmol⁻¹]. k₂ was estimated to be 0.132 at Lake Nyos and 0.0930 at Lake Monoun. Once k₁ and k₂ values are obtained, [CO_2(aq)] and are calculated using the SS, T and EC.