## 4 Results and Discussion
### Hydrogen and Oxygen Isotopes  
Waters from all the piezometers P, S, and C shown in Figure 8a were sampled for \(\delta^{18/16}\)O and \(\delta^{2/1}\)H measurements. The values of the analysed samples ranged from \(-\)37.0% to 2.2% for hydrogen and from \(-\)6.8% to \(-\)0.2% for oxygen. Three different main groups could be identified: Group A (P1, P2, P3 and S6) exhibited the highest \(\delta^{2/1}\)H and \(\delta^{18/16}\)O values in the intervals from \(-\)5.2% to 2.2% and from \(-\)1.1% to \(-\)0.2%, respectively; Group B (wells P4, P5, P7, S1, S2, S3, and S5) had intermediate values in the ranges from \(-\)16.0% to \(-\)10.6% for \(\delta^{2/1}\)H, and from \(-\)3.4% to \(-\)2.4% for \(\delta^{18/16}\)O; Group C (wells P6, S4, C1, C3, and C4) included the lowest \(\delta^{2/1}\)H and \(\delta^{18/16}\)O values, from \(-\)26.3% to \(-\)20.5% and from \(-\)5.5% to \(-\)4.2%, respectively (Figure 9). For all the piezometers, the isotopic values did not change in the different sampling periods, with the exception of C2, C6 and C7, which showed swings across groups B and C (C2: \(\delta^{2/1}\)H = \(-\)22.9% to \(-\)8.4% and \(\delta^{18/16}\)O = \(-\)4.6% to \(-\)2.0%; C6: \(\delta^{2/1}\)H = \(-\)25.0% to \(-\)9.8% and \(\delta^{18/16}\)O = \(-\)5.1% to \(-\)2.3%; C7: \(\delta^{2/1}\)H = \(-\)37.0% to \(-\)24.1% and \(\delta^{18/16}\)O = \(-\)6.8% to \(-\)5.0%). The groundwater samples collected in the C5 cluster exhibited a very unusual distribution. In the \(\delta^{2/1}\)H vs. \(\delta^{18/16}\)O diagram shown in Figure 9, their \(\delta^{2/1}\)H values are distributed vertically ranging from about \(-\)23.5 to \(-\)8.5% (see below for more details). Regarding the affinity of the sampled groundwaters, all groups plotted along the local meteoric water lines available in the scientific literature [55; 56; 57; 58; 59; 60; 61; 62], being consistent with those of rainwater collected in the study area during the observation period; this finding suggests a meteoric origin for the analysed waters (Figure 9), with no influence of ascending fluids coming from deep reservoirs, the isotopic signature of which are notably different from those of actual precipitations [35]. The \(\delta^{2/1}\)H and \(\delta^{18/16}\)O values are correlated with the depth of the phreatic surface (Figure 10). The piezometers showing the deepest phreatic surface (\(>\)25 m below ground, m b.g.) belong to Group A; Group B includes phreatic surface at 23.5 to 17 m b.g. ; Group C may be divided into two different sub-groups: one with a depth lower than 6 m b.g., and another one with a level ranging from 14.5 to 17 m b.g. Wells C5b and C5c are excluded because their water levels are representative of the hydraulic head measured at different depths below ground and is not referable to the local phreatic surface (coinciding with the water level measured in C5a). Regarding tritium, the rainwater samples had high \({}^{3}\)H contents (6.2 to 10.8 T.U. ), whereas the groundwater samples showed a wide range of tritium contents (Figure 11). Taking into consideration the tritium content detected in local rainwater during the observation period, as well as data already available for Southern Italy (e.g., 4.5 T.U. in [63]; 5.0 T.U. in [16]), three groundwater types have been detected at the study site: (i) groundwater related to rapid and/or short pathways within the aquifer system (4 to 10 T.U. ), (ii) groundwater related to longer and/or prolonged pathways, with higher mean-residence times (<4 T.U. ), (iii) groundwater related to very prolonged pathways (>10 T.U., corresponding to at least 300 T.U. at the beginning of 1960s), the recharge of which, according to findings in other sites in Italy [64], is linked mainly to rainwater precipitated during the 1950s and 1960s, when tritium was spiked in the atmosphere by nuclear weapons testing [65], reaching up to several thousand T.U. in rainwater (e.g., [66]). Figure 10: Relations between the hydraulic head and \(10^{3}\)\(\delta^{2/1}\)H and \(10^{3}\)\(\delta^{18/16}\)O for the investigated waters.