## 5 Discussion
The samples of the San Vittorino Plain have very low contributions of mantle-derived He, which is no higher than \(\sim\)1-2% (Figure 8). However, the mantle below Italy is contaminated by subduction processes, decreasing the pristine signature of the He isotopic ratio [129]. Hence, if we assume a He isotopic signature lower than the typical MORB value (\(8\pm 1\) Ra), the mantle contribution would be higher than the above estimated values. Considering the existence of the local tectonic discontinuities that were able to drive magma batches towards the surface [130], (Figure 1), the San Vittorino graben can still act as an efficient structure that allows bottom-up interconnections for the transfer of deep-sourced volatiles to stratified regional aquifers. However, we cannot determine whether He is sourced by shallow reservoirs of mantle He stored in crustal layers [131] or directly by mantle ([73] and references therein). Overall, the mixing of prevailing crustal components with mantle-derived volatiles at the regional scale has also been evidenced by previous studies [11; 13; 20; 31; 32; 36; 65] based on He isotopes and the \(\delta^{13}\)C of CO\({}_{2}\). This isotopic ratio can be ascribed to different sources: limestone (\(\delta^{13}\)C = 0%), organic matter (\(\delta^{13}\)C = \(-\)30%), or mantle (\(\delta^{13}\)C = \(-\)6.5%) [133]. Most of the sampled groundwater exhibits \(\delta^{13}\)C\({}_{\rm ext}\) values between \(-\)5% and +5 %, which are near the values proper of the deep fluid contribution (\(\delta^{13}\)C\({}_{\rm ext}\) = \(\delta^{13}\)C\({}_{\rm deep}\) = \(-\)0.35%; Figure S2), which is different from spring S7, which shows a mean value of approximately \(-\)25% and is confirmed to be represented only by \"infiltrating waters\" (\(\delta^{13}\)C\({}_{\rm inf}\)) (Figure S2). Concerning the more negative values of the deep source CO\({}_{2}\), while \(\delta^{13}\)C\({}_{\rm ext}\) values near \(-\)6% might result from the simple infiltration of magmatic carbon, they do not prove the existence of a mantle-derived fluid [134]. In contrast, according to [31; 36], the highest/positive calculated \(\delta^{13}\)C\({}_{\rm ext}\) is due to a fractionation effect related to degassing, which in the study area could reach values close to +8% (Figure S2). The effect of diffuse CO\({}_{2}\) degassing in the study area is also detectable in the Peschiera spring, where \(\delta^{13}\)C\({}_{\rm ext}\) and the alkali/alkaline earth metal ratio showed concomitant variations (Figures S3 and S4). The highest value of \(\delta^{13}\)C\({}_{\rm ext}\) = \(-\)4.7%, corresponding to \(\delta^{13}\)C\({}_{\rm deep}\) = +8.8% [36], was calculated on a sample collected just a day after the main shock occurred in Norcia (30 October 2016, magnitude moment M\({}_{\rm w}\) = 6.5; Figures S3 and S4 show the recalculated data of the Pe spring from [90]). In summary, the groundwater in the San Vittorino Plain derives from complex geochemical balances that are established between water, rock, and gas. Each component of this ternary system substantially and differently affects the final geochemistry of the  
Figure 8: Diagram of \({}^{3}\)He/\({}^{4}\)He (R/Ra) versus \({}^{4}\)He/\({}^{20}\)Ne ratios. Mixing lines between the atmosphere and upper mantle, and the atmosphere and crust were calculated using the end members: air (\(\delta^{13}\)He/\({}^{4}\)He = 1.4 \(\times\) 10\({}^{-6}\), \({}^{4}\)He/\({}^{20}\)Ne = 0.318) [86], upper mantle (\(\delta^{13}\)He/\({}^{4}\)He = 12 \(\times\) 10\({}^{-6}\), \({}^{4}\)He/\({}^{20}\)Ne = 100,000) [132], and old continental crust (\(\delta^{13}\)He/\({}^{4}\)He = 0.02 \(\times\) 10\({}^{-6}\), \({}^{4}\)He/\({}^{20}\)Ne = 100,000) [131]. Samples of dissolved and free gases are named (D) and (F), respectively. See Figure 1 for spring location. groundwater, depending on the concentration of rising gases, the lithology of the aquifer, and the chemical-physical characteristics of the infiltrating water. No contributions of surface runoff to groundwater hydrogeochemistry were detected.