## 5 Hydrogeological Model  
Geological, hydrogeological, isotopic and biomolecular investigations allowed characterisation of the hydrogeological functioning of the studied heterogeneous low-permeability system, as well as refinement of knowledge about the residence time of groundwater within such systems. As expected, taking into consideration previous studies in similar geologic settings [16], the studied system is characterised by vertical and discontinuous heterogeneity [81], caused by various factors such as (i) the coexistence of different geological formations, (ii) their folded regular sequence, and (iii) the fractured (karstified) evaporitic intrastrata/lenses that locally increase the bulk permeability of the low-permeability system. In contrast with other heterogeneous systems, no fault zones influence fluid flow, neither enhancing [82; 83; 84] nor partially or totally impeding it [85; 86; 87]. When the hydrogeological system is analysed at the basin scale, the whole system can be depicted as a continuous medium, and the groundwater flow net is a smoothed replica of the local topography. Groundwater flow clearly occurs from the highlands towards the topographic depressions. The vertical heterogeneity of the system causes the hydraulic head to significantly vary with depth. Close to the C5a-c cluster, the hydraulic head decreases with increasing depth, with the lowest head into the karstified gypsum layer (C5c). This head distribution agrees with that observed by [PERSON] et al. [16] within the peninsular Southern Italy, where the more transmissive and discontinuous localised media drain the surrounding lower permeability rocks up-gradient (lower hydraulic head), whereas they are characterised by higher head down-gradient and feed the surrounding lower permeability media (Figure 12). Because the vertical heterogeneity is discontinuous, it is unable to provide a continuous solution within the stratigraphic sequence over an extended area. Therefore, in contrast with other systems characterised by vertical heterogeneity, from the hydraulic point of view [64; 88; 89; 90], no perched aquifers overlie deeper ones, and a unique saturated zone can be assumed. Overall, the heterogeneous distribution of lower- and higher-permeability rocks within the studied system, as well as the groundwater flow field described above (Figure 12), cause the mean residence time of groundwater to not necessarily be in agreement with the only groundwater flow net depicted in Figure 8. The coexistence of slower and faster flow velocities and the existence of vertical flow, cause a sort of patchwork distribution of tritium content, with higher levels detected at depths extending into evaporitic interstrata/lenses, linked mainly to rainwater precipitated during the 1950s and 1960s, when tritium was spiked in the atmosphere by nuclear weapons testing. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
The stable isotopes confirmed the local origin of the groundwater. However, as indicated above, the isotopic features of the samples coming from C5a-c cluster are unusual for groundwaters, and may be due to the phenomena discussed hereafter. Based on data reported by [91] for partitioning between liquid and gaseous water and by [PERSON] et al. [92] for gaseous water and H\({}_{2}\)S gas, the following approximate value for the fractionation factor of H\({}_{2}\)O liquid vs. H\({}_{2}\)S gas at 21 \({}^{\circ}\)C is obtained:  
\[{}^{2/1}\alpha_{\rm H_{2}O-\ H_{2}S}\ =\ \frac{{}^{2/1}\delta_{\rm H_{2}O+1}}{{}^{2/1} \delta_{\rm H_{2}S+1}}\ =\ \ 2.587\ \ \rm at\ 21\ ^{\circ}C \tag{2}\]  
However, based on the data reported by [PERSON] and [PERSON] [93], for H\({}_{2}\)O liquid vs. CH\({}_{4}\) gas, the value:  
\[{}^{2/1}\alpha_{\rm H_{2}O-\ CH_{4}}\ =\ \frac{{}^{2/1}\delta_{\rm H_{2}O+1}}{{}^{2/1} \delta_{\rm CH_{4}S+1}}\ =\ \ 1.283\ \ \rm at\ 21\ ^{\circ}C \tag{3}\]  
Thus, liquid water is strongly enriched in heavy isotopes with respect to H\({}_{2}\)S and CH\({}_{4}\).