Geochemistry of clay dunes and associated pan in the Senegal delta, Mauritania

Since the construction of the anti-salting Diama dam, a resumption of aeoli an deflation has been observed in the Senegal delta and particularly in Dia wling National Park. It results in the formation of many clay dunes with hi gh salt concentrations. The soft powdery structure observed on the soil sur face, affected by aeolian deflation is due to rapid salt crystallization. I n order to identify the mineralogy of the salt involved in this environment , the chemistry of soil and 91 groundwater was studied along a transect fro m a pan to the associated clay dune. Equilibria computation and field and s canning electron microscope (SEM) observations indicated that the changes o bserved in the chemical facies of the solution were due to the following pr ecipitation sequence: calcite > gypsum > halite. A progressive but signific ant fixing of Na+ onto the clay particles was observed. The calcite deposit ion in the soil was limited by exhibition of the potential acidity arising from the former mangrove site soils. It resulted in high equilibrating pCO( 2),. This study intended to test a new extension to the Debye-Huckel law fo r concentrated chlorurated sodic waters. The relationship between chloride amounts and the residual alkalinity concept confirmed that calcite and gyps um formation and the exchanges between Na and Ca are the main phenomena res ponsible for the changes observed in the soil solution (r(2) = 0.99).