Hydro-geochemical and isotopic composition of groundwater, with emphasis on sources of salinity, in the aquifer system in Northwestern Tunisia

Abstract

The hydro-geochemical and isotopic data of groundwater of the aquifer of the El Kef area, North-Western Tunisia, were examined to determine the main factors controlling the groundwater chemistry and salinity as well as its hydro-geochemical evolution. This study area has a complex geological structure which is mainly controlled by halokinesis movements and inherited structures. Groundwater occurs in different water bearing formations belonging to Upper Cretaceous, Paleogene (Eocene) and Mio-Plio-Quaternary (MPQ). Different geochemical interpretation methods were used to identify the geochemical characteristics. Groundwater of the MPQ aquifer has the highest salinity values (0.3–7.0gl−1) in the study area due to the impact of agricultural activities. Piper diagram showed that Cl− and SO42- are the dominant anions, where as Na+ is the most dominant cation, where it is sometimes replaced by Ca2+ and/or Mg2+ in the hydro-chemical facies of the groundwater. Dissolution of carbonate and sulfate minerals in the aquifer matrices and recharge areas as well as cation exchange are shown to modify the concentration of ions in groundwater. The groundwaters are depleted in 2H and 18O and displayed an isotopic signature close to that of meteoric water with d-excess values indicating present-day precipitation over the region and reflect the contribution of vapor masses from Mediterranean and Atlantic origins. The isotopic features suggest that most of the groundwater at the study area result from mixing between recent recharge and an older component recharged under climatic conditions cooler than at present.