The Mitidja Plain is situated in the north of Algeria, the Eastern part (575km2) lies by the sea in the north and is limited by the Atlas Monntains in the south. the region has two main aquifers separated in some places by the El-Harrach Formation (Calabrian) , The Plaisancian marls (lower Pliocene) are the substratum of the whole area , the second and most important aquifer , Mitidja formation , is alluvial , coastal. The Eastern Mitidja plain constitutes a subterranean water reservoir essential for the agricultural, urban and industrial development of the capital Algiers. Recently, industrial activities, the spreading of exhaustive agricultural practices and the urbanization of the plain have lowered the quality of water. As many of the Mediterranean countries, Algeria has known a severe dryness during the last two decades. Unfavourable climatic conditions causing long period droughts have predictably led to a contamination of coastal groundwaters along the Mediterranean by ingressive seawater. Intensive pumping practices in use for the sake of securing water allocation for both populations and agriculture have drastically affected the groundwater reserves through overexploitation of the resource creating a consequent drawdown in the water table. During the dry season, the mobile fresh/sea water interface moves forward farther inland contaminating wells and boreholes. Two approaches making use of both hydrochemical and isotopic tools were applied to assess the extent of seawater intrusion. The Br vs. Cl plot showed that the points align in a parallel way to seawater dilution line confirming thus a marine origin for those elements. Na/Cl ratio vs. Cl plot brings to the fore two poles of points: one composed of shallow unaffected groundwater and a second one composed of deeper boreholes and wells in which seawater is present to different extents. This is further confirmed by isotopes which exhibited a wide range of values mirroring the affected and unaffected areas as well as those points submitted to intermingling between different endmembers.
Keywords: Seawater, Groundwater, Saline Intrusion, Environnemental isotopes, Hydrochemistry, Algeria.