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In the Grombalia region, Cap Bon Peninsula, northeastern Tunisia, increasing population and development of agricultural activity during the last few decades have engendered large expansion of groundwater pumping from the shallow, unconfined aquifer. Recently, the water table has displayed some signs of overexploitation such as decline in the water level in boreholes and considerable deterioration of water quality. Hydrochemical and isotopic data were used in conjunction with hydrogeological characteristics to investigate the groundwater composition in this aquifer. It has been demonstrated that groundwaters, recharged mainly in the surrounding foothills, acquire their mineralization principally by water-rock interaction, i.e., dissolution of evaporites and reverse cation exchange, and secondarily by return flow of irrigation waters. The isotopic signature of groundwaters permits identification of two different types. Water with depleted d 18 O and d 2 H contents is interpreted as recharge by non-evaporated rainfall, originating from a mixture of Atlantic and Mediterranean air masses. However, water with relatively enriched d 18 O and d 2 H contents is thought to reflect contamination by return flow of irrigation waters. Tritium contents, ranging between 1.2 and 4.5 TU indicate that groundwaters in an unconfined aquifer derive from post-nuclear recharge or contain a significant component of post-1950s water. The recent origin of these groundwaters is confirmed by the high carbon-14 activities, exceeding sometimes 100%, indicating the existence of a significant fraction of organic 14 C. This younger source of radiocarbon is, however, highlighted by the depleted d 13 C contents, corresponding to a system where CO 2 of C4 plants predominates.

Detailed hydrogeochemical and isotopic data of groundwaters from the Hammamet-Nabeul unconfined aquifer are used to provide a better understanding of the natural and anthropogenic processes that control the groundwater mineralization as well as the sources of different groundwater bodies. It has been demonstrated that groundwaters, which show Na-Cl and Ca-SO 4 -Cl water facies, are mainly influenced by the dissolution of evaporates, the dedolomitization and the cation-exchange process; and supplementary by anthropogenic process in relation with return flow of irrigation waters. The isotopic signatures permit to classify the studied groundwaters into two different groups. Non-evaporated groundwaters that are characterized by depleted d 18 O and d 2 H contents highlighting the importance of modern recharge at higher altitude. Evaporated groundwaters with enriched contents reflecting the significance infiltration of return flow irrigation waters. Tritium data in the studied groundwaters lend support to the existence of pre-1950 and post-1960 recharge. Carbon-14 activities in shallow wells that provide evidence to the large contamination by organic 14 C corroborate the recent origin of the groundwaters in the study area.

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L'investigation hydrogéochimique des eaux de la nappe phréatique de la basse vallée de Medjerda a permis de préciser les conditions d'acquisition de la charge saline des eaux dans un contexte méditerranéen. En effet, la chimie des éléments majeurs a montré que la minéralisation des eaux souterraines est contrôlée principalement par la dissolution des minéraux évaporitiques, la dédolomitisation et les échanges de base avec les argiles, relativement abondants dans les environnements semi-arides à arides. D'autre part, l'utilisation excessive des engrais chimiques est à l'origine de la contamination des eaux par les fortes teneurs en nitrates en relation avec le phénomène de retour des eaux d'irrigation. Par ailleurs, l'utilisation d'autres paramètres tels que la salinité traduite par la conductivité électrique et le SAR (indice d'adsorption du sodium, aussi appelé pouvoir alcalinisant) ont permis de déterminer l'aptitude des eaux étudiées à l'irrigation. Ces deux paramètres ont montré, généralement, une dégradation de la qualité des eaux pour l'irrigation et par conséquent un risque de salinisation des sols.