## 2 Geological, Hydrogeological, and Geothermal Settings  
The AVGA is dominated by the presence of a Quaternary volcanic complex lava dome field massif (231-305 ka) [2], which is emplaced on the structural high geology of Mt. Amiata-Mt. Razzano in correspondence with a sinistral transcurrent fault. To the east of the volcano, the Radicofani graben develops with a homonymous trachybasaltic volcano in the center. A remarkable uplift (about 800-900 m) affected the volcanic geothermal area because of the isostatic deformations of the low-density granitic bodies from an intrusion/magm chamber seated at about 4-5 km depth. Vertical normal faults separate the Mt. Amiata structural high from the deep Neogene basin. The southern Tuscany succession created by phyllites from a metamorphic basement and by the tectonic sedimentary covers characterizes the area [17]. The subsoil of this volcanic geothermal system is well known because of the deep wells that allow the geothermal fluid production of the Mt. Amiata geothermal fields. The Quaternary magmatism induces a very strong shallow thermal anomaly because of the superimposition of the high regional heat flow due to the regional Plio-Quaternary stretching, local Pleistocene regional intrusions, and fluid convection in shallow fractured geothermal reservoirs in the upflow areas of Piancastagnaio-Bagnore and in correspondence with the main fault systems. Surface thermal manifestations, thermal springs, concentrated gas emissions, and general diffuse natural soil gas emissions, mainly of CO\({}_{2}\), are ascribed to high thermal anomalies. A three-dimensional (3D) reconstruction of the subsurface of the whole AVGA [16, 18, 19, 20] was created using data from the intense geothermal exploration drilling and geophysics studies carried out in the area since the 1960s [12]. Based mostly on the geophysical evidence, a granitoid intrusion mingled with mafic magmas located at a depth of about 4 km is supposed to be the heat source for the geothermal system [6, 8, 21]. Gravimetric models [7, 8, 22] indicate the presence of a negative NE-SW anomaly centered in correspondence with the volcanic massif and the geothermal fields, interpreted as a low-density body located at shallow crustal levels. Seismic reflection surveys highlight a strong reflector (4-7 km depth), known in the literature as the K-horizon [23, 24, 25], whose depth is lower in correspondence with the geothermal fields and the NE outer sector of the volcanic massif. The K-horizon shows an elongated NE-SW shape, whose culmination is centered on the volcanic edifice [22, 25] and coincides with the granitoid intrusion (Mt. Amiata magma chamber) and its thermometamorphic contact aureole [6, 8, 26]. The K-horizon is associated with the 400-450 \({}^{\circ}\)C isotherm [24, 26]; therefore, it represents the roots of the geothermal systems at depth. Two water-dominated high-enthalpy geothermal reservoirs are present at increasing depths: i.e., a shallower reservoir hosted in the carbonate and evaporitic formations of the Tuscan Nappe (with temperatures ranging from 150-230 \({}^{\circ}\)C) and a deeper reservoir hosted in a fractured layer of the thermal aureole in the metamorphic basement where temperatures are higher than 300 \({}^{\circ}\)C and up to 350 \({}^{\circ}\)C [18, 20, 27]. The two geothermal reservoirs are separated by low permeability phyllite layers of the regional metamorphic basement [18; 20]. Nowadays, only the deeper reservoir is exploited for power generation. Mt. Amiata geothermal reservoirs are both characterized by high gas contents, with CO\({}_{2}\) being the main component (94-95%), followed by CH\({}_{4}\) (1-3.8%) and H\({}_{2}\)S (0.5-2.4%) [15; 28]. Dome-shaped structures (i.e., gas caps) develop in the shallow reservoir at the contact with the above impermeable formations, where steam and gases produced at depth can accumulate over the liquid phase [18].