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

Abstract

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).

Polyphasic origin of salinity in the Senegal delta and middle valley

“…When the topsoil dries, the increasing salinity and rapid crystallization of salts such as gypsum and halite cause the mud to flocculate forming silt and sand size aggregates, which are vulnerable to wind erosion. These aggregates accumulate on leeward sides of all wind-breaking obstacles to form saline clay dunes (Mohamedou et al, 1999). The environmental conditions favouring formation of clay dunes are very specific.…”

“…Saturated extracts from these samples were analysed by the methods described in 3.1. The chemical results from the second sequence were presented by Mohamedou et al (1999). Salt efflorescences were collected for SEM (XL20 Philips) observations and chemical analysis (LINK Analytical eXL energy dispersive X-ray system).…”

Modeling the Spatial Variability of the Electrical Conductivity of the Soil Using Differents Spatial Interpolation Methods: Case of the Dawling National Park in Mauritania

“…It covers an area of 353 km 2 . It is a sub-Sahelian zone with low rainfall (170 mm / year) (Mohamedou et al, 1998) . The park consists of several basins that develop from north to south: Diawling, Bell, Gambar and N'Tiallakh.…”

Soil cover and landscape evolution in the Senegal floodplain: a review and synthesis of processes and interactions during the late Holocene

“…The process of aeolian deflation and clay dune (or lunette) formation has been studied by numerous authors in saline alluvial valleys and particularly in the deltas of West African rivers, such as in Senegal and Casamance (Maymard, 1952; Tricart, 1954; Vieillefon, 1967; Michel, 1973; Mougenot, 1982; Mohamedou et al , 1999). Clay dunes are a few metres wide with vertical dimensions of a few decimetres, but they can reach several kilometres long.…”