Granite- and Andesite-Hosted Thermal Water: Geochemistry and Environmental Issues in Northern Sardinia, Italy

Groundwater quality can be compromised by its interaction with deep thermal waters. In northern Sardinia, two different deep thermal-water-flow systems have recently been recognized on the basis of thermal and isotopic features. One system (GW) is hosted in deep, mainly granitic, fractured reservoirs with water temperatures of 30–45 °C and Cl–Na hydrofacies. These waters have high fluoride contents owing to their alkalinity and water–rock interactions during long residence times in the reservoirs. Their bulk chemistry indicates chemistry by concentrating elements of environmental concern through adsorption, such as first-row transition metals and some chalcophile elements. The other system (AW) involves volcano-sedimentary rocks with water temperatures of 20–30 °C. These waters have neutral pH and are categorized as bicarbonate-alkaline and alkaline-earth hydrofacies. Their relatively high contents of chalcophile elements are consistent with their high dissolved CO2 contents; major elements are in equilibrium with kaolinite, which has a low cationic exchange capacity. In both of these flow systems, B speciation depends on pH. Furthermore, Mn and Fe occur as Mn2+ and Fe2+ species, respectively, depending on Eh–pH conditions. High Mn and Fe concentrations are derived from the reduction of Fe oxyhydroxides, releasing adsorbed Mn2+. The direct reduction of Mn4+ phases of Tertiary volcanic rocks also produces high Mn2+ concentrations. Fe oxyhydroxide reduction, at near-neutral pH, can also promote mobilization of As as HAsO42− or, with increasing pH, as H2AsO4− species. Trace-element analyses and speciation modeling indicate marked differences between GW and AW thermal-water systems, but neither contains metal concentrations at sufficient levels to cause health concerns via water consumption.