The hydrodynamic of the multi-aquifer system (the Continental Intercalaire " C.I " and the Complex Terminal " C.T ") of the North Gafsa basin is largely determined by tectonics (Tebessa-Gafsa fault). The composition of groundwater is controlled by complex reactions at gas-liquid-solid "mineralogical composition of associated rocks" interfaces, which depend on the natural surrounding and potential anthropogenic impact. The hydrochemical data (major ion geochemistry) indicate that these groundwaters are characterized by the dominance a Ca-Mg-HCO 3 /SO 4 and Na-Cl-NO 3 water types. Geochemical pattern is mainly controlled by the