Résumé
The coastal plain aquifer of Oussja-Ghar El Melah close to Bizerte in Tunisia (P = 500 mm/year) is composed of a detrital fluvial Plio-Quaternary basin, surrounded by a mountainous relief, the Mediterranean Sea and a marshy area. During recent decades, the urbanization of the plain, the development of the agricultural activities and the succession of dry years have led to a water-level lowering and groundwater quality degradation. Preliminary results from a geochemical and isotopic ( (super 18) O, (super 2) H and (super 3) H) study show different origins of groundwater with different geochemical characteristics indicating various possible origins in the acquisition of the mineralization. The degradation of water quality is larger in the most irrigated areas marked by high nitrate contents. The tritium data show a recent groundwater renewal with two different recharge modes: fast recharge due to the infiltration of irrigation water and runoff water, and diffuse recharge by direct infiltration of precipitation.