Interaction between groundwater and seawater in the coastal aquifer of Cap-Bon in the semi-arid systems (north-east of Tunisia)

Groundwater investigations were carried out in June 2005 in the Korba unconfined aquifer (north-east of Tunisia) to identify the potential origins of groundwater salinization and to study the associated processes. They involved groundwater sampling, piezometric measurements and electrical resistivity tomography imaging. Ionic deltas, conventional diagrams (piper and stiff) and geochemical modeling using PHREEQC were also used. The intensive extraction of groundwater from aquifer reduces freshwater outflow to the sea, creates several drawdown cones and lowering of the water table to as much as 12 m below mean sea level in the center part of the study area, especially between Diarr El Hojjaj and Tafelloun villages, causing seawater migration inland and rising toward the wells. Geochemical investigation reveals that other processes are contributing to groundwater mineralization and thus salinization: pure mixing in a seawater intrusion setting by ion-exchange, dissolution processes and agricultural pollution. The mixing zone between freshwater and saltwater was clearly identified from the result of electrical conductivity investigation both laterally and vertically where a strong increase in electrical conductivity with depth was observed, corresponding to the freshwater and saltwater interface. Furthermore, the results of electrical resistivity tomography show a transition zone between conductive seawater and resistive freshwater confirming seawater intrusion process. High salinity in groundwater may also be due to the recent intrusion of saltwater from the salt lakes dispersed in the study area, solute recycling from irrigation and/or to the existence of connate fossil water from ancient seawater incursion that occurred in the past.