Résumé
In the Merguellil valley (central Tunisia), the El Haouareb Dam is constructed over a karstic sill. Its construction completely changed the surface water and groundwater flows. Now water massively infiltrates under the dam reservoir and mixes with the Ain El Beidha groundwater, from the upper catchment, and feeds the karst that later flows into the great alluvial aquifer in the Kairouan Plain. The isotopic analysis ( (super 2) H, (super 3) H, (super 18) O) of these different compartments gives a clearer idea of the regional water budget and significantly supplements the available hydrodynamic information. For the sampled times, contribution of dam water to the karst varies between 21 and 66%; it increases with the reservoir water level. Water infiltrated since the dam construction has greatly "contaminated" the first 7 km of the alluvial water table where traces of the previous recharge process (in-wadi infiltration of heavy floods) are still observable.