The Hydrogeological Unit Migjorn is composed of an Upper Miocene carbonate platform and represents the main water resource in the island of Menorca. This unit is affected by general seawater intrusion process preferably in the west. This study, integrating techniques of Electrical Resistivity Tomography (ERT) with hydrochemical data analysis from wells of urban water supply and correlation of borehole logs of these wells, allows new contributions to the knowledge of the hydrogeological problem in the town of Ciutadella de Menorca. Analysis of hydrochemical facies and its temporal evolution indicate all samples show chloride facies without exception, but with a very different distribution of chloride ion concentrations. This distribution is justified by the presence of collapse structures and karstifications detected by Electrical Resistivity Tomography, as well as textural variability of materials where the pumping occurs