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

Abstract

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 tropic 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. This study illustrates that these two avenues of research prove to be complementary, justifying the use of common models and, in the final analysis, are indispensable to each other. The control of the salinity in these calcium and (then) sodium bicarbonate waters appears to be one of the most remarkable characteristics of the system. This control begins in the flood plains, where about 10% of the major constituents settle out. A regulation in time exists; the deviation between the average annual salinity of the yields to the lake and the long-term average does not exceed ± 10%. In spite of considerable dissolved salt inflow from rivers (ions 42 mg/l, 1.8·10 6 t/year; silica 25 mg/l, 1.1·10 6 t/year) and of a very high rate of evaporation, Lake Chad has a low salinity (ions 320 mg/l; silica 50 mg/l) and does not become noticeably more saline with time. Over periods of several years, a chemical steady-state exists as a result of sedimentation and infiltration. The removal of Ca 2+, Mg 2+, K +, CO 3H −, and SiO 2 by sedimentation in the lake is evaluated (using a simple model) at more than 58, 45, 23, 37 and 83% of the input, respectively. Removal of sodium, which is not evaluated, is the smallest. All these removals are the result of clay diagenesis (formation of montmorillonites), adsorption, and the physicochemical and biochemical precipitation of carbonates and silica. The salt budget provides a means of evaluating water losses by infiltration. These are between 4 and 7%, or less, of the inflow. 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 partly 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.