Geological, hydrodynamic and geochemical features of the volcanic aquifer of Mt. Amiata (Tuscany, central Italy): an overview

The Mt. Amiata Volcano represents the biggest and youngest volcano in Tuscany (Central Italy), whose products are mainly consisting of dacitic, rhyodacitic and olivine-latitic rocks. Mt. Amiata hosts one of the most important Significant Groundwater Bodies (sgb s) of Tuscany. This sgb is drained by several springs (more than 150), which are mostly located along the volcano boundary with an average flow rate that in some cases exceeds 100 L/s. It has a strategic role, this sgb providing drinkable water to most people living in the Mt. Amiata and surrounding areas. The aquifer is mostly under unconfined conditions. Its impervious substratum is mainly made up of clay rocks that also represent the caprock of the geothermal reservoir (the second largest one in Italy after Larderello) exploited in this area and whose origin is linked to the emplacement of the magmatic body that has originated the Mt. Amiata volcanic system. Fluids with geothermal features, i.e., thermal springs and gas discharges showing high CO2 concentrations (> 95% by volume), flow out a few kilometres away from the volcanic outcrops, and are located at se and ne of Mt. Amiata. In the framework of a project funded by the Tuscany Region Administration, the present work was carried out to characterize the sgb hosted in the Mt. Amiata volcanic rocks from a geological, hydrogeological, and geochemical point of view by elaborating and interpreting new data and previously published data. In this study the reconstruction of the top of volcanic products substratum was proposed via a multidisciplinary approach. In addition, the T and K hydraulic parameters in the north-east part of the aquifer (transmissivity = ~ 6.9E-03 m2/s and hydraulic conductivity = ~ 4.2E-05 m/s, respectively) were estimated. Moreover, some considerations on the hydrodynamic behaviour of sgb were provided and the location of the groundwater divide separating two main hydrogeological systems was recognized. The results suggested that along the flow path average direction the «Galleria Nuova» hydrogeological basin is ~5.5 km wider than that of «Ermicciolo». By geochemical point of view, most waters discharging from the Mt. Amiata volcanic complex have a Ca-Na-K-HCO3 composition with low salinity (1-3 meq/L), likely produced by the interaction of meteoric waters with the volcanic rocks at relatively shallow depths. In contrast, a few kms away from the volcanic outcrops, the CO2-rich thermo-mineral (Ca-SO4) waters of the Bagni San Filippo hydrothermal system represent a deeper groundwater circulation, whose discharge is controlled by the main faults and tectonic structures.