Geochemistry and natural ionic and isotopic tracing; two complementary ways to study the natural salinity regime of the hydrological system of Lake Chad

As a consequence of intense evaporation and transit through a succession of sedimentary traps (flood plains, lake, dune aquifer and interdune depressions), the isotopic composition of the water and the characteristics of the dissolved and suspended loads change downstream in a regular manner resembling chromatography. The migration of the water and chemical constituents, from one end to the other of this endorheic basin extending from the humid tropics to the desert margins, is outlined and quantified. The erosion, transport and sedimentation phenomena, and the natural saline and isotope tracing methods are studied simultaneously. The isotopic composition of the waters shows a large contribution (300 mm/year) to the groundwater by rain falling on the dunes which border the lake, The lake water infiltrates into the banks and is then partially lost by evaporation. The dissolved substances are taken up again by the rains on the banks. Major floods at intervals of several years also evacuate some of the dissolved salts from the vicinity of the lake towards the land bordering it. In the interdune depressions, these solutes give rise to evaporite parageneses where trona is predominant