Shallow aquifer dynamics in a volcanic-hydrothermal environment: geochemical evidences from Bagnore spring, Mt. Amiata, Italy.

Since 2004, the Bagnore spring is monitored with an automatic station for geochemical precursors of earthquakes [Pierotti et al. 2017]. The spring emerges at an altitude of 765 m a.s.l. on the southwestern slopes of the Mt.Amiata extinct volcano (Southern Tuscany, Italy). It drains a local, shallow, short-circuiting aquifer hosted in the volcanic rocks that mantle the upper part of the Mt.Amiata edifice [Pierotti et al., 2016], and is located above the structural high of the upper carbonate reservoir of the Bagnore geothermal field. The Bagnore spring has flow rate of about 0.5 l/s, and an average temperature of 21.2°C, which is about 8 °C above mean ambient temperature in the area (13.4 °C; data from Hydrological Service of the Tuscan Region; http://www.sir.toscana.it/). The current geochemical model of the aquifer drained by the Bagnore spring [Pierotti et al., 2016], considers that chemicals in solution are mainly acquired by leaching of primary solid constituents (volcanic glass and anorthite) and secondary carbonate cement, after absorption of hydrothermal gases. Hydrothermal gases are supposed to be prevalently conveyed to the surface in correspondence to the intersection of the Bagnore fault (one of the main faults of the volcano), and a NNW-SSE-trending fault/fracture system located SW of the volcano [Pierotti et al., 2017]. The presence of Tritium, the low dissolved salinity (< 250 mg/L) and Cl content (< 20 mg/L), along with their near-meteoric ?18O signature, indicate that spring waters are not affected by mixing with geothermal waters hypothetically coming from the deep reservoir currently exploited for electricity generation.