Geophysical and hydrochemical study of the seawater intrusion in Mediterranean semi arid zones. Case of the Korba coastal aquifer (Cap-Bon, Tunisia)

Abstract

Coastal aquifers serve as major sources for freshwater supply in many countries around the world, especially in arid and semi arid zones. The fact that coastal zones contain some of the densely populated areas in the world makes the need for freshwater even more acute. The intensive extraction of groundwater from coastal aquifers reduces freshwater outflow to the sea and creates local water aquifer depression, causing seawater migration inland and rising toward the wells. This phenomenon, called seawater intrusion, has become one of the major constraints imposed on groundwater utilization. As seawater intrusion progresses, existing pumping wells become saline and have to be abandoned. In this paper, we have the results of the seawater intrusion study of the Korba aquifer by the geophysical and hydrochemical methods. In order to locate the zones affected by saltwater intrusion, 38 Vertical electrical sounding (VES) were distributed over the coastal area between Korba and Oued Lebna. The interpretation of these electric soundings using Winsev software, based on mechanical boreholes, carry out iso-resistivity and iso-depth maps of seawater intrusion. The maps of apparent iso-resistivity having different lengths of line and the pseudosections differentiate dry grounds, grounds saturated with fresh water and those saturated with brackish water and saltwater. Mapping of the boundaries between freshwater and saltwater is an ideal application for resistivity surveys because of the high electrical conductivity of the saltwater and its contrast with that of fresh water. The correlation of the different electric surveys allowed realizing geo-electric sections showing the vertical configuration of seawater intrusion. It comes out from this study that saltwater intrusion reached approximately a distance of 3 km inland. The high groundwater salinity anomaly observed in Diar El Hajjej, Garaet Sassi and Takelsa-Korba zones was explained by the presence of seawater intrusion in these areas. This hypothesis is based on high chloride concentrations, the inverse cationic exchange reactions, and the lower piezometric level compared to sea level.