Aquifer vulnerability and seawater intrusion risk using GALDIT, GQISWI and GIS: case of a coastal aquifer in Tunisia

Due to increase in population and agricultural activities, the Sfax coastal aquifer is under tremendous stress and seawater intrusion becomes a serious concern. This situation needs an assessment of aquifer vulnerability and seawater intrusion risk. A new approach for vulnerability mapping using GALDIT, groundwater quality index for seawater intrusion (GQI(SWI)), geostatistics, the Ghyben-Herzberg model and GIS was developed. The model is used to determine the trend of groundwater contamination by seawater intrusion in the coastal aquifer in Sfax basin (Tunisia). Vulnerability assessment method is recognized through assessing weight and ratings of the GALDIT relevant parameter. A second vulnerability map was realized based upon the groundwater quality index for seawater intrusion integrating the water quality parameters for delineating seawater intrusion in the aquifer. The validation of GQI(SWI) and GALDIT vulnerability maps was undertaken through comparison of areas of high Jones Ratio, chloride and TDS concentration and their relative vulnerability index. The Ghyben-Herzberg model has been used to predict the actual seawater intrusion extend and evaluate the freshwater-saltwater dynamics. The results reveal that the coastal area is currently undergoing seawater intrusion and its northern part is the most affected one by this contamination.