The northern Djeffara aquifer system is located in the southeast of Tunisia and extends over an area of 400 km 2 . This multilayer system of aquifers is essentially recharged by infiltration of rain and by groundwater of the Continental Inter-calaire aquifer, and occurs mainly at two levels between 0 and 180 m depth that belong to the Mio-Pliocene to form the aquifers. A combined hydrogeologic and isotopic investigation using several chemical and isotopic tracers', i.e., major ions, δ 18 O, δ 2 H and tritium, was carried out in order to determine the sources of water recharge to the aquifer and the origin of salinity. The results of geochemical and isotopic studies show that the groundwater in the south of the study area represents a mixture of the Djeffara aquifer groundwater and locally infiltrates modern recharge. In the northern part, the groundwater which resembles that of the Sekhira aquifer originates from locally infiltrated rain and runoff. The salinity of the groundwater is caused by dissolution of evaporate rocks (gypsum and halite minerals) in the aquifer system. The stable isotopes data do not support the hypothesis of mixing with seawater.