## 1 Introduction  
Groundwater hydrogeochemistry is closely related to the nature of the aquifer and to the length and depth of the groundwater flow paths, and it strongly depends on the residence time [1, 2]. However, structural-geological features (e.g., faults and the tectonic pattern distribution) can also affect the water chemistry, as they allow and facilitate the mixing of groundwater of different origins [3, 4]. While down-going fluids may be of meteoric (or even karstyl) origin, uprising fluids may connect the lowermost parts of a stratified aquifer to the surface. Direct field evidence of exhumed seismogenic zones in carbonate rocks furnishes a fossilized example of how high pore-fluid pressure currently is at depth [5, 6]. In these contexts, the vein and fault record demonstrated that isotopic equilibrium can be interrupted during exhumation by transient thoroughgoing faulting, allowing meteoric fluids to reach otherwise rock-buffered fluid systems [7, 8, 9]. Additionally, deep fluids play an active role in the seismic cycle and fault activation mechanisms, which are increasingly highlighted in geoscience studies [10, 11, 12, 13, 14, 15, 16, 17, 18]. For this reason, in recent decades, works aimed at identifying signals that can anticipate strong seismic events in field and laboratory activities have been intensified [3, 19, 20, 21, 22, 23, 24, 25]. These studies have identified variations in the concentration of gases such as He, Rn, CO\({}_{2}\), H, and CH\({}_{4}\)[26; 27], groundwater chemical content [3; 21; 22], and the physical properties of rocks [28; 29]. However, determining the origin of fluids in shallow aquifers and answering the questions of where and how these fluids originate, and determining the possible water-gas-rock interaction processes during their migration, can provide new constraints for understanding the processes related to seismogenic processes. Research in this direction has mainly deepened the understanding of fluid origins, focusing on the determination of quantities useful to define their origin and mixing. In detail, some conservative elements in waters (e.g., As, V, Fe, Li, Cs, and Rb) and isotopes of some gaseous species (e.g., He and Ar) have been recognized and used as tracers that are often not bound to a circulation model [21; 30; 31; 32; 33]. In this frame, the central Apennines mountain belt in central Italy represents a natural laboratory to efficiently examine these topics, both for its tectonic-structural framework [34], the abundance of groundwater resources [35], and the rise of deep fluids along normal faults [11; 12; 13; 36; 37]. In addition, the nature of the regional aquifers hosted by fractured Meso-Cenozoic carbonates with limited karst development [38] enhances the response to deep fluids uprising through the shallow crustal layers, possibly allowing fast and concentrated changes in groundwater close to high-angle faults, coupled with smoothed and aquifer-wide less evident effects on hydrogeochemistry. Thus, to improve the comprehension of the relationships of both deep and shallow fluids in regional aquifers, we conducted an integrated study of the regional hydrostructures and water composition in the San Vittorino Plain, an intramontane basin in the central Apennines. In fact, this plain is characterized by an abundance of water resources [38; 39] and large diffuse active degassing focused along high-angle faults [36]. Overall, the area is defined by different fault systems related to the different steps of the regional structural history [40], and there are traces of Quaternary volcanism [41; 42]. In addition, according to a recent study [37] which aimed to evaluate the aquifer's potential susceptibility to be affected by detectable chemical-physical anomalies during seismogenic processes, the San Vittorino Plain has been placed at the \"top-ranking position\" as one of the most suitable areas for the monitoring of seismic activity in central Italy by groundwater monitoring.