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Sminja plain is an inland aquifer in Zaghouan prefecture, northeast of Tunisia. In some parts of the aquifer, salinity exceeds 13 g L-1, which poses serious problems to the safe use of the groundwater in irrigation and drinking purposes.

The present work aims to delineate the saline water plumes and to understand their salinization processes. A field survey including surface water and groundwater sampling was conducted. The taken samples were analyzed for major chemical elements (Na+, Ca2+, Mg2+, K+, Cl-, SO42- and NO3-) and stables isotopes composition (O-18 and H-2). The geochemical data were used to characterize and classify the water samples. Stable isotopes are useful tools to understand recharge processes and to differentiate between salinity origins. Indeed, the saline groundwater zones are concentrated in the center, the north-west (Aziz mountain) and south-west parts of the plain (City Al Fahs). The evolution of the groundwater salinization according to the main flow direction is principally caused by rock-water interaction which is mainly represented by the dissolution/precipitation of minerals dispersed in the saline reservoir formation and ion exchange reaction. Stable isotopes signature indicates that some water samples are originated from the infiltration of modern precipitation. Non evaporated water is rapidly infiltrated to the saturated zone with relatively lower contents of delta O-18 and delta H-2. Thus, most of the samples are presenting enrichment in heavy isotopes which highlight the occurrence of evaporation process of meteoric water prior to groundwater recharge. Furthermore, anthropogenic phenomenon represented by the return-flow of irrigation water and the use of nitrogen fertilizers were identified as non negligible contamination sources in the northeastern part of the aquifer near the irrigated perimeters. These wide variations in water salinity indicate variability in geochemical process controlling aquifer water mineralization in Sminja plain.