## 6 Conclusions  
The geometry of the Port de la Selva shallow aquifer is irregular and the correlation between the data from boreholes and electrical resistivity tomography shows that the latter geophysical prospecting technique is a suitable tool for in-depth analysis of shallow aquifers geometry. It provides a high-resolution geological correlation as well as uninterrupted monitoring of the thickness of aquifer zones. The 17 obtained geoelectric cross-sections had variable values of resistivity, both laterally and in-depth. In the vertical dimension, three layers can be distinguished. The superficial level had high resistivity values, which correlated with gravels and sands above the water table. The second level had very low \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Physicochemical Parameter**} & \multirow{2}{*}{**Units**} & \multirow{2}{*}{**2017**} & \multirow{2}{*}{**2018**} & \multirow{2}{*}{**2019**} & \multicolumn{3}{c}{**WHO Range for Aquifers 1**} & \multirow{2}{*}{**RD 14/2013 2**} \\ \cline{5-6}  & & & & & & **Fresh** & & **Brackish** & **Saline** \\ \hline Na\({}^{+}\) & (mg/L) & 361 & \textgreater{}1000 & \textgreater{}1000 & \textgreater{}200 & 200-400 & \textgreater{}400 & 200 \\ K\({}^{+}\) & (mg/L) & 9 & 67 & 30 & \textgreater{}55 & 55–70 & \textgreater{}70 & — \\ Cl\({}^{-}\) & (mg/L) & 683 & 8518 & 4329 & \textgreater{}250 & 250-1000 & \textgreater{}1000 & 250 \\ SO\({}_{4}\)2\({}^{-}\) & (mg/L) & 129 & 1008 & 536 & \textgreater{}200 & 200–500 & \textgreater{}500 & 250 \\ EC 3 & (uS/cm) & 2315 & 24,271 & 12,342 & \textgreater{}1500 & 1500–3000 & \textgreater{}3000 & 2500 \\ \hline \hline \end{tabular}  
* World Health Organization (WHO) suggested groundwater classification [78]. 2 Spanish sanitary guidelines for the quality of water for human consumption [37]. 3 Laboratory electrical conductivity at 20 \({}^{\circ}\)C.  
\end{table} 
Table 2: Salinity indicators results of Port de la Selva aquifer control point compared to World Health Organization suggested groundwater classification and Spanish sanitary guidelines for the quality of water for human consumption. resistivity values, corresponding to water-saturated aquifer sediments. The thickness of this intermediate area is highly variable, ranging from 3 to 45 m at nearby points. Finally, the lowest level corresponds to the Paleozoic substrate and was characterized by a progressive increase in resistivity values. The lithological data from borehole and resistivity data from ERT have been progressively and fast incorporated into the 3D aquifer geometry and saltwater intrusion models using implicit modeling tools. We have used implicit modeling software for deriving the aquifer geometry model from discrete variables such as lithological information and ground resistivity and a saltwater intrusion model from the same resistivity data. The aquifer geometry model shows the contact between the shallow aquifer and aquilude unit and infer the potential aquifer thickness all over the studied area. The geometry of the contact between the aquifer and the metamorphic bedrock is quite irregular and located at an average depth of about 18 m. The 3D saltwater intrusion model shows a general electrical resistivity increasing trend towards the hills. An inward feature wedge-shaped with low resistivity values (below 10 Ohm\(\cdot\)m) is identified at the seaside which highlights possible marine intrusion. Electrical resistivities in the range of 10-100 Ohm\(\cdot\)m are also mainly located close to the coastline and should include a brackish or mixed water zone. The model also showed a low resistivity anomaly 1-100 \(\Omega\cdot\)m beneath and towards the municipality well position reflecting an upcoming that is believed to be triggered by water over-pumping. The geological model obtained can easily incorporate data from other complementary geophysical techniques, such as electromagnetic and seismic methods.