Assessing seawater intrusion in arid and semi-arid Mediterranean coastal aquifers using geochemical approaches

The Plio-Quaternary aquifer of Jerba Island (Tunisia), the Upper Miocene-Pliocene-Quaternary aquifer of Tripoli (Libya), the Plio-Quaternary aquifer of Nador plain (Algeria), the Bou Areg Plio-Quaternary aquifer (Morocco), the Miocene carbonate aquifer of Porto Torres (NW Sardinia, Italy) and the Pleistocene oolitic aquifer of the Baghoush area (Egypt), are essential contributor sources of drinking water and irrigation water for agricultural needs. They are increasingly being degraded by salinization processes, essentially allied with groundwater over-exploitation. In this study, a hydrogeochemical investigation, by interpretations of conservative dissolved ions was used to probe the main mechanisms controlling groundwater salinization and to characterize the associated hydrogeochemical processes occurring in Mediterranean coastal aquifers. Groundwaters of the different aquifers are of meteoric origin and they mostly flow toward the coast. Based on EC and ionic ratios, a hydro-chemical zonation of water types (ranging from Ca-HCO3, Na-HCO3, Ca-Mg-Cl-SO4, Ca-Cl to Na-Cl) from recharge areas toward the coast is revealed. This zonation is mainly controlled by migration and mixing of saline waters with regional groundwaters. The behaviour of the major ions, the adherence of the majority of ion/Cl ratios and molar ratios characterising salinity in function of [Cl-] indicate that besides mixture with seawater, groundwater chemistry of the six aquifers is also influenced by evaporation, weathering of silicates, cation exchange and anthropogenic pollutants. The study provides insights into the hydrodynamic and geochemical relationships between freshwater and seawater at different Mediterranean coastal aquifers and provides regional authorities with a basic tools for sustainable water management aiming to limit seawater intrusion.