GEOLOGICAL, HYDRODYNAMIC AND GEOCHEMICAL FEATURES OF THE VOLCANIC AQUIFER OF MT . AMIATA (TUSCANY , CENTRAL ITAL Y ): AN OVERVIEW

The Mt. Amiata Volcano represents the biggest and youngest volcano in Tuscany (Central Italy), whose products are mainly consis t- ing 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 av- erage 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) exploit- ed in this area and whose origin is linked to the emplacement of the magmatic body that has originated the Mt. Amiata volcanic sys- tem. 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 oftop of volcanic products substratum was proposed via a multidisciplinary approach. In a ddi- tion, the T and K hydraulic parameters in the north-east part of the aquifer (transmissivity = ~ 6.9E-03 m2/s and hydraulic con ductiv- ity = ~ 4.2E-05 m/s, respectively) were estimated. Moreover, some considerations on the hydrodynamic behaviour of sgb were pro- vided and the location of 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»