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

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.