(Senegal) coastal aquifer

Seawater has entered the Saloum River system and encroached up to 100 km inland, contaminating surface water and the superficial groundwater resources. A hydrogeochemical study reveals four major water types occurring in the study area, namely Na-rich saline groundwater, Ca-Na-rich saline groundwater, Na-dominant fresh groundwater and Ca-dominant fresh groundwater. A hydrochemistry zonation model of the aquifer, based on the presence of different water types and on the groundwater flow, has led to the identification of the main processes controlling the groundwater chemistry. These are cation exchange, reverse cation exchange, dissolution of calcite, and to a lesser extent, gypsum dissolution. Results of δ18O and δD analysis indicate that the groundwater has been partly affected by evaporation, as groundwater isotopic composition is lighter as depth of the water table increases. δ18O and chloride data indicate sources of high salinity other than simple mixing between of modern seawater and native groundwater