## 4 Results and Discussion  
### Hydrogeological Features  
The hydraulic head fluctuated over the year from tens of centimeters to several meters, depending on the area. A recession period was typically observed from late spring to early autumn, whereas recharge was typically observed from early autumn to spring (Figure 7). Overall, the head measurements suggested relatively slow recharge, according to the low bulk permeability of the studied system, which does not favor rapid percolation of fresh-infiltration waters within the unsaturated zone and corresponding short-term fluctuations of the groundwater surface. The low bulk permeability of the studied medium is in agreement with the findings of other authors who performed pumping tests in similar Messinian successions in Southern Italy (transmissivity on the order of 10\({}^{-5}\) m\({}^{2}\)/s; [14; 16]).  
Based on the hydrogeological monitoring and survey results, the general groundwater flow scheme in the area was reconstructed (Figure 8a). The groundwater flows from the highlands towards the topographic depressions, with the hydraulic gradient ranging from \(\sim\)30% to \(\sim\)3%, respectively. The lowest head was consistently measured within the eastern depression, in the area of piezometers S3-S5, and no significant modifications of the groundwater flow net were observed over time, during the observation period. Therefore, all the piezometers used to analyse the isotopic signature from a hydrogeological perspective maintained the same hydraulic relationships throughout the year This finding implies that all the results discussed in the isotopic section were not influenced at all by modifications of the groundwater flow field.  
Figure 8: (**a**) Hydrogeological map of the study area (December 2019); (**b**) relationship between the hydraulic head and the depth below ground observed at the C5 cluster.  
Figure 7: Hydraulic head variations in piezometer S4 (blue dots) vs. daily rainfall (light blue bars). Dates are given in day/month/year format.  
In light of the sedimentological features and common lateral inhomogeneity of the evaporitic layers (Figure 3), simple expected hydrogeological and hydrochemical behaviors are difficult to define for the sequence that outcrops at the key site of the C5 cluster. In fact, the RNZ layer, which overlies the second cycle gypsum evaporitic bodies, is composed of a very chaotic sandy-clayey deposits, including dispersed gypsum blocks; consequently, a moderate primary permeability is expected, despite the high clay content. In addition, the first cycle evaporitic layers and the basal Tortonian clays are expected to be characterised by high and low permeability, respectively, whereas the role of the second cycle gypsum bodies, which are to be classified as permeable on a small scale, could be strongly controlled, overall, by the lack of lateral continuity, resulting in very slow hydraulic circuits.  
As observed at the C5 cluster, the hydraulic head significantly varied with depth (Figure 8b). In greater detail, it progressively decreased with increasing depth, showing the highest head in the shallow system and the lowest one into the piezometer C5c, screened within a gypsum horizon. The head zonation was confirmed based on the seasonal groundwater oscillation, with coherent hydraulic heads shifts, consistent with the fact that the analysed medium behaves as a continuum at the observation scale of the present study. This finding is also in agreement with the expectation for saturated heterogeneous media comparable with the studied media. In these media, if a lower geological formation has a higher hydraulic conductivity than the overlying layer, it acts as the major conduit of flow [54]. The highest head in C5a was compatible with the phreatic surface reconstructed within the whole studied area, the morphology of which reflects the local topography (Figure 8a), as expected in such low-permeability systems.