Geochemistry of clay dunes and associated pan in the Senegal delta, Mauritania
Résumé
Since the construction of the anti-salting Diama dam, a ressumption of aeolian deflation is observed in the Senegal delta, and particularly in the Diawling National Park. It results in the formation of many clay dunes with high salt concentrations. The soft powdery structure observed on the soil surface, affected by aeolian deflation is due to rapid salt crystallisation. In order to identify the mineralogy of the salt involved in this environment, the chemistry of soil and groundwater was studied along a transect from a pan to the associated clay dune. Equilibria computation and field and SEM observations indicated that the changes observed in the chemical facies of the solution were due to the following precipitation sequence: calcite > gypsum > halite. A progressive but significant fixing of Na + onto the clay particles was observed. The calcite deposition in the soil was limited by exhibition of the potential acidity arising from the former mangrove site soils. It resulted in high equilibrating pCO2. This study provided to test a new extension to the Debye-Hückel law for concentrated chlorurated sodic waters. The relationship between chloride amounts and residual alkalinity concept confirmed that calcite and gypsum formation, and the exchanges between Na and Ca are the main phenomena responsible for the changes observed in the soil solution (r² = 0.99).
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