Hidralia is the company responsible for the supply of drinking water to the population of Estepona, a 65,000 inhabitant’s city located in the western Costa del Sol (Málaga province, southern Spain). Water resources come from both surface (reservoirs) and ground (aquifers) sources. One of the most strategic groundwater extraction zones corresponds to Padrón sector, where Hidralia has got two pumping wells 114 and 146 m deep, respectively. Despite the fact that the two wells are situated close by (350 m) and go through the same deposits, the quality of water is quite different. The easternmost well produces water of very good quality with values of electrical conductivity (EC) around 700-800 µS/cm, whereas the westernmost one regularly suffers from salinization problems leading to significant increases of EC values up to 4,000 µS/cm. Besides, the latter eventually becomes an artesian well -usually during or immediately after the recharge period- and the salinity of water does not remain stable along the year. In consequence, the current pumping strategy in this sector depends entirely on the easternmost well and in case it would become inoperative, there would not be any operational alternative. The objective of this contribution is to clarify the origin of the salinization and characterize the factors explaining its spatial and temporal variability, making use of a combination of hydrochemical, hydrodynamic and isotopic techniques. Methodology included monthly field campaigns for in situ EC, temperature (T), pH and oxidation-reduction potential (ORP) measurements, groundwater sampling and laboratory analyses of water samples for determination of major components concentrations and water isotopic signature (d18O-d2H). CTD-Diver devices were deployed in two monitoring points for continuous EC, T and groundwater depth recording. Vertical EC profiles in selected wells were periodically carried out. Finally, electrical resistivity tomography and induced polarization was used to characterize the freshwater-saltwater interface and provide information about the subsurface geometry of the aquifer. Results show the existence of, at least, three independent permeable layers in the study area made up of Quaternary and Pliocene sands, gravels and conglomerates. Water stored in the deepest layers shows low-to-intermediate EC values (<850 µS/cm) all throughout the year, whereas the shallower levels can become salinized because of their hydrogeological connection with seawater and extensive pumping, especially during the summer season. Changes of the piezometric pressure in the various permeable layers generate longitudinal flows inside the wells which modify the vertical distribution of EC. The different salinity of water from the two wells is attributed to the configuration of the filtering screen, in such a way that the non-salinized -easternmost- well would be disconnected from the upper levels and would only pump water from the deeper aquifer. This aspect remains to be validated through borehole video inspection.