Salinity of Senegalese phreatic aquifers: the role of the geomorphology?

Salinity is a common feature in amphibious environments. However, in Senegal (West Africa), elevated salinity levels are observed in groundwater hundreds of kilometres inland from the coastline – far beyond the marine influence. This raises questions about the origin of the salinity, decoupled from seawater intrusion. A spatial alignment of high (and, conversely, low) salinity values – parallel to a dune system – suggests a potential link to these aeolian formations. Field observations in four distinct regions of northwestern Senegal reveal sharp salinity contrasts between dunes and adjacent interdune depressions, even at scales of just a few hundred metres. Electrical conductivity (EC) values can shift abruptly from low (EC < 0.5 mS/cm) to high (EC > 5 mS/cm). The findings, reinforced by a principal component analysis based on data from over 500 wells, support a geomorphological hypothesis to explain the spatial distribution of salinity, highlighting the influence of palaeoenvironmental features on present-day hydrogeochemistry.