Geochemistry of Clay Dunes and Associated Pan in the Senegal Delta, Mauritania

Abstract

Since the construction of the anti - salting Diama dam , a resumption of aeolian deflation has been observed in the Senegal delta and particularly in 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 defla tion is due to rapid salt crystallization . 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 scanning electron microscope (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 man grove site soils . It resulted in high equilibrating p CO2 . This study intended to test a new extension to the Debye - Huckel law for concentrated chlorurated sodic waters . The relationship between chloride amounts and the residual alkalinity concept con firmed 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). Keywords: Aeolian DeflationLunettesSaline SoilsSenegal Delta