We conducted a column experiment in the field, for 1 yr, to determine the chemical effects of the evaporation of a shallow aquifer through a homogeneous sandy clay loam soil. The experimental arrangement simulated conditions found in the polders surrounding Lake Chad in Central Africa. To characterize the nature of the water transfer, oxygen isotopes were determined in the soil solution. Major soil elements, as well as soluble species, were also measured as a function of depth. The data obtained indicated that the water, moving upward by capillarity through the soil profile induced the transfer of soluble ions from the lower column depths. Evaporation took place just below the soil surface, where soluble salts tended to accumulate: the concentration of the soil solution increased drastically, while some soluble species tended to precipitate. Thermodynamic simulation of direct evaporation of the aquifer water and mass balance calculations of the mass transfer during experimentation suggested the formation of Mg-rich clay minerals at the surface of the soil. Detailed transmission electron microscope observations confirmed these results in showing tiny fibers around detrital particules and longer rigid palygorskitelike laths. Thus, even after a short time, water evaporation of a nonsaline shallow aquifer affected strongly the chemistry of the soil above.
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