Hydrogeochemical and isotopic evidence of groundwater salinization in a coastal aquifer: A case study in Jerba Island, southeastern Tunisia

Detailed hydrogeochemical and isotopic data of groundwaters from Jerba unconfined aquifer are used to provide a better understanding of the natural and anthropogenic processes that control the groundwater mineralization as well as the sources of different groundwater bodies. The total dissolved solid contents of groundwaters are highly variable (0.5-20.3 g.L-1). Groundwaters with Na-K-Cl and Ca-Mg-Cl-SO4 facies, are mainly influenced by the dissolution of evaporites (especially in the central area of the island, where the substrate is mainly made of gypsum), the cation-exchange process in the southern parts of the study area, and seawater intrusion due to overexploitation of the aquifer, especially in the northern, western and southwestern coastal areas where the topographic and piezometric levels are the lowest, and complementary by anthropogenic processes in relation with sewage waters and uncontrolled tanks (in the northern part of Jerba island). Hydrogeochemical characteristics based on bivariate diagrams of major and minor ions show that changes in the chemical composition of groundwater are mainly controlled by the salinization process followed by cation-exchange reactions. Molar Li+/SO42- and Sr2+/Cl- ratios for the analysed samples strongly suggest that the source of salinity is seawater intrusion coupled to the dissolution of evaporites. The isotopic signatures permit to classify the studied groundwaters into two different groups. Evaporated groundwaters with enriched delta O-18 and delta H-2, are mainly encountered in the central areas of the Island Jerba. Saline water, from coastal areas, results from mixing of groundwater with seawater.