The acidic waters in Italy: a brief overview

The present study is aimed at providing a brief overview of the Italian acidic waters based on literature and unpublished data. Acidic waters in Italy, as elsewhere, are relatively common and associated with extremely variable geological settings. Owing to their peculiar features, these waters may seriously affect the environment and the ecosystems. Along the Apennine belt, the western and inner sectors of the Italian peninsula record an anomalous geothermal gradient, mostly overlapping with the Neogene-to-present magmatism, that explains the presence of a huge amount of CO2(H2S)-rich gas and thermal water discharges, geothermal fields (e.g. Larderello and Mt. Amiata) and ore deposits (e.g. Fe- and polymetallic sulfides, e.g. Elba Island and Colline Metallifere). Acidic waters (pH ≤5) from volcanic and geothermal areas show outlet temperatures and Total Dissolved Solids (TDS) from 10 to 96 °C and <1 to ≈30 g/L, respectively, with a chemical composition usually belonging to the Ca-SO4, NH4-SO4 or Na-Cl facies. Frequently, they are related to bubbling and boiling pools due to the interaction between deep sourced gases and shallow aquifers or meteoric waters. Concentrations of heavy metals and metalloids are in most cases high. Extremely high contents of metals are also recorded for those acidic waters that characterize the main Italian mining districts, mostly located in Sardinia (not included in the present study), Tuscany and NW Alps, where they are related to Acid Mine Drainage l.s. or Acid Rock Drainage. The pH values are as low as 2.08, with variable TDS concentrations. Compositionally, they are Na-SO4, Ca(Mg)-SO4, and/or Mg(Ca)-SO4 waters, prevalently due to oxidative processes affecting polymetallic sulfides.