The Cameroon volcanic line (CVL) hosts numerous volcanic lakes whose internal processes and hydrological functioning remain poorly documented. A detailed understanding of these hydro-systems is however essential, both for the consideration of these lakes as sentinels of the regional hydro-climatic changes and for the calibration of palaeoenvironmental proxies. Here, we present a hydrological and geochemical investigation of five of these lakes (Mbalang, Tabere, Tizon, Gegouba and Baledjam) around Ngaoundere on the Adamawa Plateau, based on repeated sampling of water profiles and monthly monitoring of rain and lake water samples over two seasonal cycles. We show that each of these throughflow lakes bears a distinct geochemical and isotopic signature, despite quite similar morphometric characteristics and a common climatic regime, due to varying contribution of the watersheds to the watermass balance and different partitioning between evaporation (E) and outflow (Inflow minus E). We use these differences as a benchmark for a sensitivity analysis of the classical budget equations of conservative tracers. The results demonstrate that to reconcile chloride and stable isotope data with the standard single-box steady state model would require unusual values of the physical parameters of Craig and Gordon's equation such as the n.θ term that would have to be significantly lower than its usual value (n.θ = 0.5). We also show that the data can be simulated more easily by including the inflow from the watershed while assuming that transpiration exceeds evaporation for this compartment. Using this conceptualization of the throughflow lakes, we were able to constrain the different fluxes. Transpiration from the watershed and evaporation from the lake are on the same order of magnitude, or slightly in favour of † Died October 22, 2022. This work is dedicated to the memory of our colleague and friend, Jean-Luc Michelot, who passed away too early on October 22, 2022.

<p>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 CO<sub>2</sub> 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 (<sup>36</sup>Cl) and stable isotopes of water.</p><p><sup>36</sup>Cl is a cosmogenic isotope of chlorine produced naturally in the stratosphere by spallation of <sup>40</sup>Ar induced by cosmic-rays and has been massively injected into the atmosphere by nuclear tests during the 1950s. This pulse of bomb-<sup>36</sup>Cl 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 <sup>36</sup>Cl for the same purpose in tropical areas.</p><p>In this study, together with major elements and stable isotopes, we analyzed <sup>36</sup>Cl contents in water from lakes Mbalang, Tabere, Tizon, Gegouba and Baledjam around Ngaoundere, to assess residence time in these lacustrine systems. <sup>36</sup>Cl/Cl ratios range from 1400.10<sup>-15</sup> to 2800.10<sup>-15</sup> 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 (<sup>36</sup>Cl/Cl ~200.10<sup>-15</sup> at/at, see Bouchez et al., Scientific Reports, 2019). These <sup>36</sup>Cl/Cl ratios above the natural baseline are clearly tagged with the bomb-<sup>36</sup>Cl 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 <sup>36</sup>Cl/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.</p>