Abstract
Gases dissolved in the deep water of lakes can pose a hazard when extreme concentrations are reached. A sudden release of large amounts of gas can cost the lives of humans living in the neighbourhood, as happened at Lake Nyos in 1986. Since 2001, the gas risk at Lake Nyos has been mitigated by induced degassing, but the lake continues to be supplied by CO2, and a regular survey needs to be implemented to guarantee safe conditions. Frequent sampling of this remote lake requires an enormous effort, and many analytical techniques are very difficult to run at the lake site. In this contribution, we combined a commercially available sound speed sensor with a CTD (electrical conductivity, temperature, depth) probe to obtain an indirect but quantitative estimate of carbon dioxide concentrations with fine depth resolution (decimetre scale). Dissolved carbon dioxide increases sound speed but does not contribute to electrical conductivity. Hence the difference between measured and calculated (on the base of electrical conductivity, temperature and pressure) sound speed gives a quantitative indication of dissolved carbon dioxide. We infer the vertical distribution of dissolved CO2 and hence continue the survey of the progress of the intended degassing. In conclusion, we present an easy to implement method for very high CO2 concentrations in deep lakes, and we highly recommend the implementation of the sound speed-CTD probe combination at Lake Nyos and at other gas-laden volcanic lakes, as such an approach could safeguard the people living in the area with acceptable cost and effort for the operators. In this manner, alarming CO2 concentrations in deep parts of lakes can be detected in a timely fashion.
Highlights
Extreme concentrations of dissolved gases in deep waters of lakes can pose a threat to the local population
The aim of this study was the field deployment of commercially available equipment to yield a profile of dissolved inorganic carbon (DIC hereafter) concentrations based on the excess sound speed in comparison to calculated sound speed
The CTD and sound speed measurements in March 2016 at Lake Nyos, Cameroon, provided a data set that correlated very well with the CO2 concentrations observed along a vertical depth profile from earlier measurements
Summary
Extreme concentrations of dissolved gases in deep waters of lakes can pose a threat to the local population. Extreme gas pressures have been detected and assessed for their danger to the local population 2016; Lake Vollert-Sued: Horn et al, 2017; Lago Monticchio Piccolo: Caracausi et al, 2009; Cabassi et al, 2013). All these lakes are meromictic, and accumulation of solutes in the deep water can occur over long periods (e.g., Boehrer et al, 2013; Boehrer et al, 2021). Artificial degassing is considered necessary to reduce risk around the lakes (Nyos, Monoun, Guadiana Pit Lake; Sánchez España et al, 2020)
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