Polyphasic origin of salinity in the Senegal delta and middle valley

Saline areas are a major obstacle to the development of sustainable irrigated agriculture in the Senegal valley. They have been attributed to the incorporation of marine salts in the sediments during the last marine transgression. However, this does not explain their geomorphological situation and geochemical features. They are distributed as strips about 100–200 m wide and several kilometers long, which are composed of two parallel substrips, one located in the depressions of former creek beds, and the other on higher ground on the southern bank. In these two substrips, the chemical composition of salinity and its distribution in the soil suggest that it arises from more than one source. Comparison of saline areas of the middle valley with present-day salt accumulation in the delta suggests a four-stage salinization process, which involves evaporation from a shallow water table, aeolian salt accumulation as clay dunes, deep transformation of shell beds into gypseous layers under temporary acid conditions induced by oxidation of pyrite under the mangrove vegetation, and secondary salinization by runoff. The four-stage model agrees with a regional salinity chemical database and the geomorphology of the saline areas. In the Senegal valley, the lower middle valley and the delta can therefore be regarded as a chronosequence, the recent salinity features occurring in the delta and the more developed ones upriver at the limit of the marine transgression.