## 4 Results
### Hydrogeophysical Basis for VS and ER Models Interpretation
In this case, in time-lapse ER models, transient GER changes ultimately depend on the variable combination of the non-evaporative fractions of precipitation and atmospheric bulk deposition salinity reaching the water table over time, which are predictable, measurable variables. For instance, atmospheric bulk deposition EC decreases inland, so inland GEC (recharge zone) is typically less than coastal GEC [5; 39]. How the GER changes extend over space and time is crucial to define the hydraulic boundaries of aquifer H.  
In the SAL coastal fringe, atmospheric bulk deposition EC was deduced from the long-term Monte Velho meteorological station (Figure 1a), which belongs to the _Co-operative Programme for Monitoring and Evaluation of the Long-Range Transmission of Air Pollutants in Europe_ [EMEP network: [[http://www.emep.int/](http://www.emep.int/)]([http://www.emep.int/](http://www.emep.int/))] aimed at providing chemical analysis of precipitation [5; 39]. In year 2014, monthly precipitation-weighed EC varied in the 39-138 \(\mu\)S cm\({}^{-1}\) range with typical values of 40-80 \(\mu\)S cm\({}^{-1}\) in the rainy phase and higher than 100 \(\mu\)S cm\({}^{-1}\) in the dry phase (Figure 4b). The values in the rainy phase determine the expected GEC in aquifer H since aquifer recharge from P in the dry phase is negligible (Figure 4a). Applying the monthly E-to-P ratios deduced from daily P and T time series (Figure 3c) to the atmospheric bulk deposition EC in the rainy phases, monthly GEC results in the 200-400 \(\mu\)S cm\({}^{-1}\) range; GER being in 25-50 \(\Omega\) m range using Equation (2). In addition to this theoretical appraisal, GEC was measured in pristine (without apparent human influence) Pliocene (W1 and W6) and Pleistocene (W5) wells to corroborate this regional GEC baseline (Table 3). GEC varied in the 200-500 \(\mu\)S cm\({}^{-1}\) range, which is similar to the theoretical range described above, thus GER being in 20-50 \(\Omega\) m range. Unfortunately, there are no inland atmospheric bulk deposition EC data, although values 0.2-fold of the magnitude reported in the coastal fringe can be assumed after the decreasing inland gradient \begin{table}
\begin{tabular}{c c c} \hline \hline
**Geomaterial** & **VS, m s\({}^{-1}\)** & **Reference** & **Equivalence 1** \\ \hline Soft clay & 80–200 & [34] & Holocene clay \\ Loose sand & 80–250 & [34] & Holocene sand \\ Loose sand and gravel & 100–200 & [59] & Holocene sand and gravel \\ Anthropogenic filling & 50–100 & [59] & Holocene floodplain \\ Cropland and organic soil & 50–150 & [59] & Holocene floodplain \\ Stiff clay & 200–600 & [34] & Pleistocene clay \\ Dense sand & 150–500 & [34] & Pleistocene sand dunes \\ Soft-stiff sand & 300–500 & [59] & Pleistocene sand \\ Stiff gravel & 300–600 & [34] & Pleistocene conglomerate \\ Cemented clay & 600–1000 & [59] & Pliocene marl \\ Cemented sand & 500–900 & [59] & Pliocene calacrenite \\ Cemented gravel & 500–900 & [34] & Pliocene conglomerate \\ Weathered carbonate bedrock & 600–1000 & [34] & Jurassic marls \\ Weathered crystalline bedrock & 800–1200 & [59] & Variscan weathered metapelites \\ Hard carbonate bedrock & 1200–2500 & [34] & Jurassic carbonates \\ Hard crystalline bedrock & 1500–2500 & [59] & Variscan metapelites \\ \hline \hline \end{tabular} \({}^{1}\) Geological description and location in Figure 1c.  
\end{table} 
Table 4: Summary of some reference VS ranges compiled from the scientific literature for different sediments and rocks, and its equivalence to the geological materials described in the SAL area. documented by [39]. Applying this decreasing gradient to the theoretical coastal GEC and GER, inland GEC and GER can be tentatively proposed in the 40-100 \(\mu\)S cm\({}^{-1}\) and 100-250 \(\Omega\) m ranges, respectively. This theoretical GER baseline is a reference for interpreting experimental ER models in the study area. Piezometry in wells W1 to W6 enables us to deduce how rainy phases and human actions determine GER changes in each geological formation (Figure 4; Table 3).