It has been established that intensive agricultural activities, usually, increase the risk of groundwater quality degradation through high groundwater pumping rates. In fact, the uncontrolled groundwater extraction causes a modification of natural flow systems and induces seawater intrusion from the coast and causes the groundwater quality deterioration. The Korba aquifer is located in the North-East of Tunisia, where a semi-arid Mediterranean climate prevails. The dry season is pronounced and this aggravates the situation, given that the highest water demand usually coincides with the drought period (dry weather conditions). The principal aim of this study is to characterize the hydrochemistry of this coastal aquifer, identifying the main processes that occur in the system, and to determine the extent of marine intrusion in the aquifer. In order to achieve this aim, geophysical and chemical parameters were measured, such as vertical electrical soundings (VES), electrical conductivity, pH, temperature, anions and cations concentrations. The analytical results obtained in the hydrochemistry study were interpreted using ion correlations with chloride and SO 4 2-/Cl À and Mg 2+ /Ca 2+ ratios, in conjunction with calculations of the ionic deviations and the saturation indexes. Saturation indexes are calculated with the PHREEQC 2.8 software used for mineral saturation modelling of the aquifer seawater-freshwater mixture. The high groundwater salinity anomaly observed in Diar El Hajjej, Garaet Sassi and Takelsa-Korba zones was explained by the presence of seawater intrusion in these areas. This hypothesis is based on high chloride concentrations, the inverse cation exchange reactions, and the lower piezometric level compared to sea level.

The groundwater recharge zone mapping often requires a large amount of spatial information and criteria. Geographic information systems are capable of managing large amount of spatially related information, providing the ability to integrate multiple layers of information for multi-criteria analysis. To show the capabilities of GIS techniques for mapping groundwater refill zone in arid area, a study was carried out in the Maknassy basin located in Central Tunisia. This evaluation incorporates historic rainfall data analysis, watershed drainage density, surficial geology and aquifer boundary conditions. The study basin is categorized according to the previous criteria. Multi-criteria analysis is performed to evaluate suitability to the groundwater recharge for each factor, according to its associated weight. The thematic layers were integrated with one another using the weighted aggregation method to derive the groundwater recharge map. The results demonstrated that the GIS methodology has good functionality for mapping groundwater recharge zone.

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