## 2 Geological and Hydrogeological Setting
### Hydrogeological Setting  
The carbonate ridges in the central Apennines correspond to wide recharge areas of the main fractured regional aquifers that can store large quantities of groundwater. Groundwater flow in fractured carbonate aquifers is laterally limited by continental depositional sequences in the intramontane plains, and tectonic activity induces continuous base-level changes in the aquifers [39; 62]. These conditions have possibly hindered the development of a mature karst network in the discharge areas of the aquifers [63]. Therefore, in such wide aquifers (up to 1000 km\({}^{2}\)), an impulsive response to seasonal recharge is not recognizable, as stated by very high and steady spring discharges [39], located at the boundaries of the carbonate aquifers limited by recent clastic and alluvial deposits. The San Vittorino Plain hosts the discharge of large carbonate aquifers; for example, the Giano-Nuria-Velino Mts hydrogeological system [38; 39] (Figure 1). These fractured ridges, belonging to the carbonate platform domain, are highly permeable due to fracturing and karst processes in the recharge zone, with a total recharge area of approximately 1016 km\({}^{2}\), and effective infiltration of 880 mm/yr vs. the average precipitation of over 1200 mm/yr [35; 64]. These carbonate aquifers feed spring groups with a total discharge of approximately 30 m\({}^{3}\)/s [39]. The main springs are located on the southern and northern boundaries of the San Vittorino Plain and include the Peschiera springs (S10 (Figure 1), 18 m\({}^{3}\)/s), partially exploited for drinking purposes; the streambed springs of Antrodoco (approximately 2 m\({}^{3}\)/s), the spring group of Canetra (S4, 4.5 m\({}^{3}\)/s), San Vittorino (S14, 0.5 m\({}^{3}\)/s), and the highly mineralized springs of Terme di Cotilia (S11, 0.25 m\({}^{3}\)/s). On the other hand, the Paterno perched spring, located on the right slope of the Velino River, is fed by the Mt. Paterno-Canetra ridge (S7, Figure 1). This minor hydrogeological unit also contributes to regional groundwater recharge.  
In addition, spring locations and hydrogeochemistry are influenced by tectonics and the different permeabilities of the alluvial/clastic layers of the plain, having variable widths, up to approximately 200 m. Groundwater from carbonate aquifers can undergo chemical-physical variations upon its transfer towards the alluvial shallow aquifer. Consequently,the springs of the area have different hydrogeochemical characteristics that are also the result of active degassing in the middle valley of the Velino River [31; 36; 38; 39; 43; 65]. Hence, mixing induced by deep fluid inputs, whose upward movements occur along tectonic discontinuities and are partially buried by recent clastic deposits, may cause progressive and significant hydrogeochemical changes in the springs.  
Although the San Vittorino Plain is far from having any evidence of active volcanism, it is characterized by relevant gas emissions, with a maximum CO\({}_{2}\) flux of \(5.7\times 10^{-5}\) kg m\({}^{-2}\) d\({}^{-1}\)[31; 38; 66]. In some cases, the aggressiveness of the waters conferred by gaseous contributions can promote the development of collapse phenomena, so-called sinkholes [67]. Through the assessment of the chemical composition of the fluids associated with the sinkholes of the plain, the origin of this CO\({}_{2}\) was explained as being the combination of both a mantle source and a product of thermometamorphic reactions within the buried Meso-Cenozoic limestone [68].