The Bagni di Lucca thermal waters (Tuscany, Italy): an example of Ca-SO4 waters with high Na/Cl and low Ca/SO4 ratios

The waters are Ca-sulphate and characterised by high Na/Cl (≌2) and low Ca/SO4 (≌0.7) mole ratios, high Sr (≤12 mg/L), F (≤4.2 mg/L), As (≤80 μg/L), Cs (≤70 μg/L), Rb (≤107 μg/L) and salinity (up to 3087 mg/L of total dissolved solids), low alkalinity, and pH in the range 6.3-7.5. Assuming equilibrium of the waters with anhydrite and chalcedony, the evaluated temperature and pressure are about 70-75 °C and 200-300 bar; considering the pressure as hydrostatic, the evaluated depth of equilibration approaches that of the metamorphic basement (about 3.0-3.8 km) underlying the carbonate-evaporite formations occurring in the area. Thermodynamic computations indicate that the high Na/Cl ratio and the low Ca/SO4 ratio in the waters may be due to progressive dissolution of albite, anhydrite, dolomite and carbon dioxide accompanied by calcite precipitation. The linear correlations between the major and trace components, including 2H and 18O, demonstrate that thermal springs undergo dilution by low-salinity shallow waters which are recharged at low elevation.