XA9950079 IAEA-SM-361/29 Isotopic approach to the behaviour of two multilayered aquifer systems (Gafsa North and Hajeb el AToun-Jilma) in central Tunisia K. Zouari (I\ A. Mamou a\ B. Ouda(:), M. Yerrnania), E. Gibert-Massault(3), J.L.MicheIot(3) (1> Lab. de Geocbimie Isotopique et de Paleoclimatologie, Ecole Nationale d'Inge'nieurs de Sfax, Tuaisie m Direction Generate des Ressources en Eau, Ministere de 1'Agriculture, Tunis, Tunisie (3) Lab. d'HydroIogie et de Geochimie Isotopique, CNRS EP 1748, Universite de Paris-Sud, Orsay, France In central Tunisia, the hydrogeological behaviour of numerous multilayered aquifers is controlled by tectonics, because major faults act as natural underground hydraulic sills. This tectonic complexity limits the application of the classical hydrogeological methods and it appeared interesting to examine how environmental isotopes techniques could contribute to the understanding of the general behaviour of such systems. For this study, undertaken with the support of of IAEA, two aquifer systems, corresponding to Gafsa North and Hajb el AToun-Jilma basins, were selected because they are particularly affected by water managing poblems, such as lowering of piezometric levels probably due to overexploitation of the aquifers. The behaviours of the studied environmental isotopes (18O, 2H, 3H, 14C, 13C) are very different in the two systems. In Gafsa North basin, recent recharge (evidenced by tritium contents) is largely spread all over the basin and is locally identified in the different levels of the aquifer, even close to the main outlet of the system ("Gafsa sill"). Besides, in this region, groundwaters seem to be stratified. For these reasons, it is difficult to reconstitute in details the lateral evolution of ionic and isotopic contents. At the contrary, in the multilayered system of Hajeb el AToun-Jilma basin, the evolution of the environmental tracers is quite continue, showing a relay between the different aquifer levels from recharge areas to the main outlet ("Hajeb sill"). Present day water uptakes from the aquifers are likely to exceed recent recharge; however, the comparaison between results obtained in the present study and those of a previous study (1980), shows that over-exploitation is limited compared with the reserves stored in the system. 65 / / .^>": FIG.l. Location map Reference R. KOSCHEL (1980): Etude hydrogeologique de la nappe de Hajeb el Aioun Jilma Ouled Asker, Tunis: Division des Ressources en Eau, 245p. 66.