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A methodology was developed and applied to the Tindouf basin (south-western Algeria) to understand the hydrogeology of a complex aquifer system with a limited number of data, to identify the favorable areas for the design and building of new wells, and to know whether there is still current recharge of these aquifers. The principal components analysis (PCA), diagram of deuterium versus oxygen-18, and equilibrium diagrams Mg/Na and Ca/Na were the techniques used to combine different datasets in order to identify chemical and isotopic groups, which were in turn used to define the groundwater flow paths. In addition, on the basis of thermodynamic equilibrium, it is possible to define the chemical evolution of the Tindouf basin aquifer. The results of this study are consistent with the generally accepted hydrogeological conceptual model. The combination of the different methods made possible to define and and to characterise the main groundwater flow paths from their sources to the discharge zones. These flow paths are defined by water categories, which are represented by salinity and groundwater origin. This approach can be used to analyze aquifers characterized by a lack of data and can also be useful for studying other complex groundwater basins.

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Seawater intrusion represents a high risk for the water supply, the agriculture and industry activities in the lower Seybouse region of North-Eastern Algeria. In order to analyze this risk, a three-dimensional model was developed using the MODFLOW and MT3DMS codes to predict seawater intrusion in the coastal aquifer. The application of this model indicates that the groundwater withdrawals result in a continuous decrease of the water level and in an increase of chloride concentration. Moreover, the salt front could progress by 300 to 2500 m in the land. These results show the necessity of adequate measures for the protection of the aquifer. Numerical predictions for 2045, considering an increase of groundwater withdrawals by 20%, show a fairly significant decrease in water levels, up to -6 m with respect to the mean sea level, and an increase of Cl- concentrations up to about 10 km inland.