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Situated in semi-arid regions of northern Africa, the Nabeul-Hammamet basin, North-eastern Tunisia, is a typical alluvial plain, where rural population relies exclusively on groundwater as a water-supply source. Major ions hydrochemistry was used in conjunction with hydrogeological data to understand the aquifer hydrodynamic functioning and to identify natural and anthropogenic salinisation processes. An attempt has been made to recognize these processes using Principal Components Analysis (PCA) and bivariate diagrams of major element data. The groundwater composition is extensively modified by the water-rock reaction in the subsurface, that is, dissolution of halite, gypsum, and/or anhydrite; and ion-exchange with phyllosilicates. Nevertheless, overall groundwater samples are characterized by relatively high nitrate contents suggesting that the return flow of irrigation waters is a significant source of the groundwater contamination.