Due to their geostrategic location, the aquifers on the Algerian coastline are of the utmost socio-economic interest because they satisfy water demand from the industrial and agricultural sectors and the population. The latter concentrates a lot on the northern fringe of the country as the climate is much more tolerable than anywhere else, knowing that more than 80% of the surface area of Algeria is desert. Such conditions provide an appropriate field of research in determining and studying the mobility and extension of the freshwater/saltwater interface and its impacts on groundwater resources. The aquifer of the coastal strip of the Bay of Algiers, part of the hydrogeological unit of the Mitidja contained in a heterogeneous quaternary filling, has been heavily used through intensive pumping for drinking water supply and irrigation. This led to decreased piezometric levels, contamination of fresh waters by marine waters, a reversal of the hydraulic gradient, and, consequently, the inland advance of the saline bevel over about 5 km. The hydrochemical study using the contents of major ions (Ca2+, Mg2+, Na+, K+, SO4–, Cl–, HCO3–, NO3–) and traces (Br–, Sr++) coupled with stable isotopes (18O, 2H), allows the identification of the processes and chemical reactions that govern the salinization of water, their mixtures, their origins as well as the limit and extension of the freshwater/Saltwater interface. The interpretation of the results is based on the spatio-temporal study of chemical parameters, the correlations of major elements with chlorides, and the variation of the Mg2+/Ca2+ ratios. The saturation in certain minerals (NaCl, Gypse, Sylvite) is calculated by PHREEQC 2.8 software, which models the saturation state of minerals as a fresh water-seawater mixture in an aquifer system. The vertical measurements of electric conductivity carried out (conductivity profiles) for several boreholes confirm and specify, on the one hand, the phenomenon and, on the other hand, the extension and the limit of the interface.