Caratterizzazione chimica delle acque circolanti all'interno del complesso vulcanico del Monte Amiata

The Monte Amiata volcanic complex hosts one of the most important aquifers present in the Tuscany Region, which is used for drinking water supply. According to Barazzuoli et al. (2004) more than 150 cold springs draining the volcanic aquifer are distributed around the volcanic body. Locally, there are also some peculiar springs such as the thermal springs of Bagni San Filippo and several cold springs associated with CO2-rich gas emissions. Various studies were performed on the geochemical characterization of groundwaters and surface waters circulating in the Monte Amiata volcanic complex and nearby areas. In particular, a dataset comprising more than 190 chemical analyses was developed in the framework of the Significant Groundwater Bodies (SGB) study funded by the Tuscany Region and aimed at the geological, hydrogeological and geochemical characterization of different aquifers located in the Tuscan territory including the Monte Amiata aquifer, coded 99MM020 (Cerrina Feroni et al., 2009; Doveri et al., 2012). In this work, all available geochemical data of groundwaters and surface waters from previous studies were compiled producing a unique dataset comprising more than 250 complete chemical analyses. Most waters circulating in the Monte Amiata volcanic complex have Ca-(Na+K)-HCO3 chemical composition and are discharged at the periphery of the volcanic edifice, coinciding with the contact with the underlying impermeable clayey units belonging to the Ligurids s.l. The stoichiometry of the dissolution reactions involving Ca and Mg solid phases suggests that the mineral paragenesis typically encountered in the volcanic rocks of Monte Amiata complex plays a pivotal role during the water-rock interaction process, mainly generating the Ca-(Na+K)-HCO3 water type. Calcite, dolomite and gypsum and/or anhydrite are also present in the sedimentary rocks of the nearby area leading to the production of Ca-HCO3, Ca-(Mg)-HCO3, and Ca-(Mg)-SO4-HCO3 water types. Acidic aqueous solutions of Ca-(Na+K)-SO4 composition are locally produced through interaction of meteoric waters with volcanic rocks, but mineral dissolution is governed by sulphuric acid formed through O2-driven oxidation at relatively shallow levels of either deep-coming H2S or metals sulphides, e.g. pyrite. Similarly, reaction of H2SO4 and carbonate rocks produces acidic Ca-SO4 waters. The few Ca-(Na+K)-SO4-HCO3 waters are probably originated either by mixing of acidic Ca-(Na+K)- SO4 waters and Ca-(Na+K)-HCO3 waters or by water-rock interaction processes driven by both H2CO3 and H2SO4 but in different moments. Most waters of interest are undersaturated with calcite, excluding Ca-HCO3 and Ca(Mg)-HCO3 waters, which are close to the saturation, and the Ca-(Mg)-SO4-HCO3 thermal waters of Bagni San Filippo, which are saturated or oversaturated with calcite, probably due to CO2 loss at outlet conditions. All considered waters are undersaturated with gypsum apart from the thermal waters of Bagni San Filippo, which approach the equilibrium condition due to interaction with carbonate-evaporite units at depth. Waters of Ca-(Na+K)-HCO3 composition, which are hosted in Monte Amiata volcanics, have SiO2 contents bounded by saturation with chalcedony and with opal-CT. This means that several Ca-(Na+K)-HCO3 waters have relatively high SiO2 contents that favored the proliferation of diatoms in the numerous lacustrine basins which were present at the margin of the volcanic edifice in the past. The large variation range of Total Dissolved Inorganic Carbon (TDIC) suggests the involvement of more than one CO2 source, as proposed by Frondini et al. (2009) based on δ13C values and TDIC concentrations.