## 5 Discussion
### Saltwater Intrusion  
Previous modeling studies on the area were focused on simulating aquifer response to rainfall events and pumping in water-supply wells and testing the sensitivity of travel time and dilution rate to several aspects such as rainfall scenarios, infiltration rates, pumping rates in water supply wells, and (uncertain) aquifer parameters such as porosity and hydraulic conductivity [35]. Specific studies delineating saltwater intrusion have not been previously published. The delineation generally requires multiple depth sampling at different water control points or implies combining flow and solute transport equations which are not easy to model even though numerically.  
The saltwater intrusion model was obtained by combining ERT data from two different acquisition campaigns and unveiled an image of the issue. The model of Port de la Selva's shallow aquifer shows an area of low resistivity values close to the coastline. The existence of current groundwater salinity at this area is confirmed by the results provided by the town council from supply water wells and the Catalan Water Agency (ACA) from  
Figure 8: (**a**) P11 and P12 inverted resistivity cross-section and its lithological interpretation; (**b**) location sketch of boreholes S3 and S8 and ERT cross-sections P11 and P12 (Port de la Selva).  
the groundwater control point (Figure 7). The control point is sampled once a year--in September--coinciding with the end of the main holiday season. The indicators of salinity used are some ionic concentrations and the electrical conductivity of water samples (Table 2). Additionally, chloride concentration's up to 700 mg/L have been reported during summer 2018. Therefore, it was necessary to contract a portable desalination plant to guarantee standard quality for drinking water. Periodically, groundwater analysis clearly exceed the local regulations for human water supply without any additional treatment and the range for being considered freshwater according to World Health Organization [31].  
However, the delineation must be used as a qualitative image of the saltwater intrusion and to assist the straightforward design of optimal and cost-effective acquisition future field surveys. At this stage, the use of a constant resistivity threshold to improve the delineation of seawater intrusion must include additional information about the geological heterogeneity of the aquifer and direct estimation of TDS in the low salinity (high resistivity) region should be avoided as it is highly sensitive to clay content which is not properly defined [22].  
Future research results could be fast incorporated such as other data from other geophysical techniques such as electromagnetic techniques, and seismic methods successfully applied as by the authors of [21] in characterizing coastal shallow aquifers and new research boreholes specifically drill on Port de la Selva coastal area for saltwater delineation scope. Moreover, the use of the ERT methodology could be implemented to other shallow coastal aquifer sites for qualitative saltwater identification and monitoring and in groundwater models used for decision-making management of freshwater resources.  
Finally, we are convinced that the methodology has yielded information that can greatly help us to define better the aquifer salimization extension and apply the geometry coupled with numerical groundwater model, major ions, and isotope data to determine travel time and dilution rate with more acceptable reliability [38]. Furthermore, in projects where it is necessary to follow up on a timeline, it would be possible to re-evaluate new conditions (salinity, phreatic changes, new geophysics data, water environment changes... ) and remediation or response time could be drastically reduced. All the proposed approaches could improve the commitment of stakeholders to the benefits of SAT and/or adopting the methodology on a larger scale.