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 degrees 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 degrees 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.