## 4 Results  
Groundwater samples have \(5^{18}\)O, as reported in Table 2, contents between a minimum of \(-6.74\%\) with respect to VSMOW (29 May 2019) and a maximum of \(-7.37\%\)% with respect to VSMOW (01 October 2018 and 26 June 2019). The isotope concentration values were plotted (Figure 4) together with the Global Meteoric Water Line (GMWL), the Mediterranean meteoric Water Line (MMWL) and two local meteoric water lines for central Italy, i.e., the two Central Italy Meteoric Water Lines (CIMWL), as proposed by [PERSON] and [PERSON]. [19] and [PERSON], [PERSON] et al. [20]. Figure 4 shows two regression lines, having different slopes. Besides the main trend line, whose slope is parallel to meteoric lines, there is one with a lesser slope that is relative to the waters sampled in the summer months. In this regard, the evaporation process can affect the isotopic composition of precipitation, atmospheric moisture and surface waters Evaporation of rainfalls results in the  
Figure 4: Plot of \(\delta^{2}\)H vs. \(\delta^{18}\)O for Capodacqua di Spigno samples. Figure 3: Comparison between the monthly rainfall trend and the flow rates exploited and supplied by Capodacqua di Spigno Spring. The monthly discharge values were reported at the half of each month by using the interpolation method. enrichment of the heavy isotopic species along the so-called evaporation line; it usually has a slope of less than 8 in \(\delta\)(D) vs. \(\delta\)(18O) space. According to this effect in this specific study case, Figure 4 shows higher \(\delta\)-values for summer months' samples. These enriched rainfall waters, due to a possible evaporation of rainwater, transfer their isotopic signature to groundwater at the moment of infiltration. Anyway, the hypothesis of re-evaporated effect on the precipitation and therefore on the contribution to spring discharge, can be validated by the knowledge of the rainfall water isotopic composition [14]. However, the main trend common to all values (Figure 4) depends on the variation of the infiltration altitude. On this base, the hydrological applications of stable isotope measurements are strictly related to an accurate knowledge of the vertical isotopic gradients that are generally used to calculate the mean elevation of the recharge area of aquifers. Air masses, rising at progressively higher elevations, expand adiabatically; subsequently, fractional condensation of water vapor takes place at decreasing temperatures from a water vapor that is progressively depleted in heavy molecules. This composite effect causes isotopically lighter precipitations with increasing elevations and is generally referred to as the \"vertical isotopic gradient\" [19]. Groundwater samples collected in 29 May 2019 and in 18 December 2018, regarding the events of maximum precipitation, show the similar highest content in heavy isotopes, despite the different rainfall regime, that characterized these two events. Therefore, this effect can be mainly attributed to the infiltration of the precipitation at a lower elevation than the water collected in all the other samples. As for the seasonality effect on groundwater, the enrichment in both heavy isotopes of meteoric waters is irrelevant in the warm months. In fact, in warm months (Figure 4) spring waters show lesser \(\delta\)-values than the others. This effect is not in accordance with the seasonal effect characterizing the rainfall water which is found by [PERSON] et al. [21] in a coastal area about 100 km away from Capodacqua di Spigno Spring. The effect of variation in isotopic concentration of the rainfall water is lesser on the isotopic concentration of the spring water than the altitude effect (Figure 4). This condition is probably due to the fact that spring waters are the result of several rain events occurring in different periods. Considering the natural attitude of the isotopic signature in depending on the infiltration altitude, groundwater sources may be studied using isotopes, e.g., \({}^{2}\)H and \({}^{18}\)O.