## 4 Discussion  
Groundwater modelling was carried out for the Caposele karst aquifer, which has a hydraulic behavior similar to a porous-equivalent medium and an EPM approach was used. Nevertheless,  
Figure 13: (**a**) 2D vertical groundwater model along the cross-section trace B’S’; (**b**) Caposele spring catchment zoning in flow pipe areas (FPA) for a model thickness of 2000 m; each flow area is marked by different colors and the related item. \begin{table}
\begin{tabular}{c c c c c} \hline  & \multicolumn{2}{c}{**FPA**} & \multicolumn{2}{c}{**Q**} \\
**item** & **km2** & **_R_ **(Mm3/y)** & \(R\) **(\%)** & **l/s** \\ \hline
1 & 4.1 & 4.3 & 4 & 147 \\
2 & 5.2 & 5.5 & 5 & 185 \\
3 & 7.9 & 8.4 & 7 & 259 \\
4 & 11.3 & 11.8 & 10 & 368 \\
5 & 17.1 & 18.0 & 15 & 554 \\
6 & 33.9 & 35.9 & 31 & 1142 \\
7 & 30.5 & 32.4 & 28 & 1032 \\ TOT & 110 & 116.5 & 100 & 3687 \\ \hline \end{tabular}
\end{table} 
Table 3: The main results obtained from aquifer zoning in flow pipe areas for a thickness of the aquifer’s saturated zone TSZ = 2000 m. FPA, flow pipe area; \(R\), recharge; \(Q\), discharge outlet. a groundwater numerical simulation was also successfully applied to karstified aquifers [10; 48; 49; 50], with satisfying results. The input parameters in the flow simulation, together with an accurate modelling strategy [51], play a crucial role for groundwater analysis, affecting the efficiency and then the interpretation of the model. [PERSON] et al. [52] exploited chemical and isotopic tracers to evaluate the contribution of different water components discharging at the Fontaine de Vaucluse karst spring near Avignon (Southeastern France), considering two different hydrodynamic phenomena of the aquifers: (i) a perfect piston flow (PF), where each recharge event pushes the previous one towards the hydrological outlet; (ii) a well-mixed model (WMM), where the discharged water is a mixture between all aquifer components [52; 53]; the aforementioned systems point out two extreme flow modalities; in nature in particular, the majority of aquifers highlight an intermediate behavior between the PF and WMM modalities. The numerical simulation performed for the Caposele aquifer relies on the boundary conditions of the conceptual model subsequently implemented with the MODFLOW numerical code. The accuracy of the model depends on the number and kind of the assumptions, together with the type of mathematical and physical conditions on which it is based. A numerical flow simulation using the EPM approach is able to simplify the modelling of the heterogeneous structure as an aquifer system [53; 54; 55; 56]. The methodology described in this paper, considering all workflow steps shown in Figure 6, led to an understanding of the results obtained by both a travel time analysis and aquifer zoning in flow pipes, also allowing for the detection of a hydraulic phenomenon of ascendant flow near the discharge zone. The groundwater flow simulation was carried out considering a combined 2D-3D approach because the model was run both along a cross-section (2D view) and a horizontal view (3D), considering a TSZ that was variable from 100 m up to 2000 m. The preliminary results obtained from the travel time assessment gave an overview of the behavior of the fluid flow in the karst aquifer, in this case comparable to a porous medium. The flow net drawn for the generic model, then extendable to the Caposele aquifer, points out three different groundwater paths: a descendant groundwater behavior in the farthest zone from the spring (main recharge area), a sub-horizontal path in the central zone, and a prevalent ascendant flow near the spring zone, in accordance with the previous results deduced from the analysis of other karst aquifers in southern Italy [47]. As the actual TSZ is unknown and specific hydrogeological data are not available, the approach followed in the modelling regarded the design of several scenarios that varied the aquifer depth, in order to find a reliable range representative of the TSZ of the Caposele spring catchment.