EGU General Assembly 2021
© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Assessing the residence time of water in volcanic lakes of north Cameroon using bomb-36Cl and stable isotopes

Souleyman Abba1,2, Bruno Hamelin1, Pierre Deschamps1, Yannick Garcin1, David Badoga2, Roger Tamonkem Adzeh2, Bouba Djangue Moustapha2, and Benjamin Ngounou Ngatcha2
Souleyman Abba et al.
  • 1Aix Marseille Université, IRD, CEREGE, France (
  • 2The University of Ngaoundéré, Faculty of Science, Department of Earth Sciences, Cameroon

The Cameroon Volcanic Line (CVL) in Central Africa hosts numerous volcanic lakes. While Nyos and Monoun lakes in western Cameroon were well studied following the catastrophic release of CO2 that occurred in 1980s, other volcanic lakes such as those of the Adamaoua Plateau remain less documented. Although some of these (Mbalang and Tizon) have been investigated through their sedimentary archives in order to reconstruct past-environments, the functioning of these hydro-systems located in the northern part of the CVL is not well constrained. Here, we characterize the hydrological functioning of five volcanic lakes by coupling classical hydrology methods and isotope tracers. Specifically, we assess water residence time in these lakes using radioactive (36Cl) and stable isotopes of water.

36Cl is a cosmogenic isotope of chlorine produced naturally in the stratosphere by spallation of 40Ar induced by cosmic-rays and has been massively injected into the atmosphere by nuclear tests during the 1950s. This pulse of bomb-36Cl can thus be used as a tracer to estimate recharge rates in the unsaturated zone and to constrain water transit times at a regional scale. While water stable isotopes have been widely used to establish lakes hydrological balance in Sahelian regions, only a few studies have been reported to date using 36Cl for the same purpose in tropical areas.

In this study, together with major elements and stable isotopes, we analyzed 36Cl contents in water from lakes Mbalang, Tabere, Tizon, Gegouba and Baledjam around Ngaoundere, to assess residence time in these lacustrine systems. 36Cl/Cl ratios range from 1400.10-15 to 2800.10-15 at/at and are significantly higher than the natural baseline as assessed by data obtained in local groundwater or at a larger scale in the Lake Chad Basin (36Cl/Cl ~200.10-15 at/at, see Bouchez et al., Scientific Reports, 2019). These 36Cl/Cl ratios above the natural baseline are clearly tagged with the bomb-36Cl footprint. We will illustrate at the meeting how a simple transient-state one-box model can be used to explain why these lakes have different 36Cl/Cl ratios, and how these results can help to constrain the E/I ratios of the lakes, and be compared with their hydrological characteristics and stable isotopes signatures.

How to cite: Abba, S., Hamelin, B., Deschamps, P., Garcin, Y., Badoga, D., Tamonkem Adzeh, R., Djangue Moustapha, B., and Ngounou Ngatcha, B.: Assessing the residence time of water in volcanic lakes of north Cameroon using bomb-36Cl and stable isotopes, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9821,, 2021.