The present study is concerned with the Sfax Sahelian aquifer (in Tunisia), particulary the Sfax deep aquifer lodged in the upper Miocene sand series. Its important extension (14000 km ) offers considerable resources. A pluridisciplinary study based on hydrogeology, hydrochemistry and isotopic (18O, 2H, 3H, 14C, 13C, 34S) was carried out in order to know the hydrodynamic functioning of this deep aquifer.
The hydrogeological study has enabled a better definition of the aquifer's structure and geometry and allowed groundwater circulation diagram establishment.
The hydrochemical results have helped to understand the groundwater mineralization origin and mechanism. This water's chemical facies is Na-Cl type. It salt content rangers from 3 to 10g.l-1. The high salinity observed in the south of the basin is likely to be due to a partial intrusion of sea water. This hypothesis is in keeping zith isotope dqtq of 34S and 18O. Elsewhere in the basin, groundwater salinization is due to evaporate dissolution.
The study of isotopic composition (18O and 2H) of groundwater and the present precipitation has made it possible to precise the behavior of the different aquifers. The groundwater in the deep aquifer is show to be old, probably slow moving, and recharged under a climate cooler than the current one. This hypothesis of paleorecherge is in good agreement with the lower 14C contents of deep groundwater, which correspond to "ages" superior to 10 000 years. The homogeneity of the stable isotope and 14C contents suggests a slow flow, with very weak fluxes in the system The "almost stagnation" state of the aquifer in the coastal refion could be linked to sea level variation during the recent Quaternary.
This study of isopic compositions (18O and 2H) of groundzqter collected in the Sfax deep aquifer to the phratic water table. This participation, quantified by isotope balance, is very variable and may sometimes reach 100%.