Abstract
The Lake Chad Basin (LCB) is one of the main endorheic basins in the world and has undergone large-level and surface variations during the last decades, particularly during the Sahelian dry period in the 1970s and the 1980s. The Chari–Logone River system covers 25% of the LCB but accounts for up to 82% of the Lake Chad water supply. The aim of this study is to investigate the dissolved phase transported by the Chari–Logone system, in order (1) to elucidate the origin and the behavior of major elements and the weathering processes in the watershed; (2) to estimate the total dissolved flux, its variability over the last decades and the driving factors. To do so, samples were collected monthly between January 2013 and November 2016 at three representative sites of the basin: in the Chari River in “Chagoua,” in the Logone River in “Ngueli” just before the confluence of both rivers, and at a downstream site in “Douguia,” 30 km after the confluence. Concentrations in major elements displayed significant seasonal variations in the Chari and Logone waters. At the seasonal time scale, the comparison between the concentrations of chemical elements and the flow rates showed a hysteresis loop. This hysteresis behavior corresponds to a variable contribution over time of two water bodies, fast surface water, and slow groundwater, the latter carrying higher concentrations and Ca/Na ratio, which may result from the contribution of pedogenic carbonate weathering to the dominant signature of silicate weathering. At the annual time scale, similar average concentrations are observed in the Chari and Logone Rivers, despite contrasted annual runoff. In addition, an interannual stability of ionic concentrations was observed in the Chari–Logone River during the flood regime, both during the years covered by our monitoring (2013–2016) and during the pre-drought period (1969, 1972 and 1973). This situation corresponds to a chemostatic behavior, where the annual river discharge is the main factor controlling the interannual variation of chemical fluxes.
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Acknowledgements
This work was supported by the French National Research Institute for Sustainable Development (IRD) in the framework of the project “Préservation du Lac Tchad: Contribution à la stratégie de développement du lac” funded by “Fond Français de l’Environnement Mondial” and by “Agence Française pour le Développement.” This work also benefit from the support of Equipex project ASTER-CEREGE and ofthe LABEX OT-Med program of the French Investissement d’Avenir program. The authors are grateful to the University of N’Djamena, the Centre National de la Recherche pour le Développement of Chad (CNRD) and the French Embassy in Chad for their logistical support. We thank Hélène Mariot and Wulfram Barthelemy for the preparation of the chemistry and the analysis of the major elements and Abel Guihou for the analysis of the isotopic composition of strontium.
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Mahamat Nour, A., Vallet-Coulomb, C., Bouchez, C. et al. Geochemistry of the Lake Chad Tributaries Under Strongly Varying Hydro-climatic Conditions. Aquat Geochem 26, 3–29 (2020). https://doi.org/10.1007/s10498-019-09363-w
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DOI: https://doi.org/10.1007/s10498-019-09363-w