Seawater intrusion is a widespread process that is occurring with increasing frequency in coastal aquifers due to the higher demand of groundwater resources. The characteristics of the freshwater/seawater interface (FSI) and its dynamics are strongly conditioned by the lithology and among others the typology of the discharge density contrasts and tides. To increase the understanding of seawater intrusion system a multidisciplinary and multi)scale approach is necessary to build robust conceptual and numerical models. For that purpose we have developed an experimental field site in a coastal alluvial aquifer at the mouth of a temporary stream in the Maresme coast line (Barcelona Spain). This area is characterized by the discharge of a ephemeral flow (Argentona stream) and the Mediterranean seawater intrusion for more than 50 years due to over)pumping. The aquifer is formed by unconsolidated heterogeneous and polygenic alluvial sediments ranging from very fine to very coarse grained sand with discontinuous interfingering lenses of gravel and silt. All this sediments has above 20 m thickness and are overlying a weathered granitic basement. The proposed methodology consists in the installation of several boreholes drilled perpendicular and parallel to the shoreline at various depths to reach different conductivity areas along the mixing zone. Several geophysical techniques have been performed from the surface and within the boreholes in order to characterize the salinity gradient and distinguish sedimentary facies. Furthermore a fully hydrochemical investigation was carried out to define the initial groundwater composition using TOC/DOC measurements mayor and minor elements analysis nutrients identification and radium and radon isotopes quantifications. Also lithological description sedimentological correlation and geochemical analysis of the cores obtained after drilling (DRX CEC acid digestions) were integrated to fully characterize the initial stage of the experimental site. This integrated multidisciplinary and multi)scale methodology will allow understanding the local FSI dynamics and give new guidelines for the study of seawater intrusion processes in many other sites