###### Abstract
Water percolation through infiltration basins is currently creating significant synergies for the broad adoption of water reuse as an alternate water supply and could foster sustainable water consumption in a transition to a circular economy. Despite the apparent simplicity of the surface SAT approaches, the complexity of site-specific hydrogeological conditions and the processes occurring at various scales, combined with different objectives, requires an excellent understanding of the system's response [18; 19]. A groundwater model is required according to published guidelines for managed aquifer recharge and water recycling [20]. Groundwater modeling tools involve three general stages for aquifer conceptualization: (1) geometry definition, (2) acquisition of hydraulics data, and (3) evaluation of water balance components [21]. The paper's general objective is advancing the first stage, i.e., aquifer geometry definition, and qualifying some transient groundwater features subordinately (saltwater intrusion). Classical hydrogeological approaches such as using lithological logs, aquifer water sampling, and measuring water tables require the existence of available detailed drilling reports of wells and piezometers and/or drill and equipping new ones. Besides, such logs provide only punctual assessments of the aquifer geometry and hydraulic data. In flat coastal areas, near-surface geophysical techniques have been widely used in groundwater research to acquire this basic information. Geophysical techniques are high-resolution tools that provide information on the spatial distribution of the physical parameters of the subsoil. These techniques are non-invasive and become useful when lithological data are sparse or not able to provide detailed subsurface information required for groundwater modeling [21; 22]. The advantage of the surface geophysical methods, compared with borehole methods, is that they allow denser and faster data coverage at a much lower cost [23], reduce risks to interconnect different water bodies during drilling operations, and can provide a way to improve the subsurface characterization from a set of boreholes. The most widely used method for hydrological applications and shallow aquifers characterization is probably the direct-current (DC) resistivity, which is highly responsive to detailed subsurface electrical conductivity changes related to geological and hydrological heterogeneities [24]. Electrical resistivity (also called DC resistivity) methods measure the apparent electrical resistivity of the formation and have been widely used to delineate the geometry of shallow aquifers [25; 26; 27] and saltwater intrusion [28; 29; 30; 31; 32]. However, ERT has rarely been used in SAT projects framework and it could define better the boundary conditions in its hydrogeological models, which is important for understanding and predicting subsurface flow and transport. The specific aim of our research was to develop a fast and non-invasive methodology to improve the characterization of the Port de la Selva shallow coastal aquifer and delineate the saltwater intrusion. To this end, we propose an approach using electrical resistivity tomography and implicit 3D modeling tools [33] to define the aquifer geometry and the saltwater intrusion. Particularly, a 2D ERT campaign was conducted to assess the thickness and lateral extent of the alluvial formation below the profiles acquired and to gather data for subsequent 3D modeling. Placing ERT data in the same tool framework allowed to delimit a saltwater intrusion phenomenon. The information from the geological map and research boreholes was also integrated to define boundary model conditions and to obtain the aquifer geometry of the whole studied area respectively.