Mineral waters characterization in the Azores archipelago (Portugal)

A study of the hydrogeochemistry of more than 100 mineral waters in the Azores archipelago (Portugal) was conducted on São Miguel, Terceira, São Jorge, Faial, Pico Graciosa and Flores islands. About 75% of the discharges are located at São Miguel, whereas the rest are more equally distributed over the other islands. Springs are dominant (74%), but drilled wells and boiling pools comprise about 10% of the studied discharges. Only 2% of discharges represent hand-dug wells. Results show a large range of chemical characteristics and diverse water types: discharge temperature range between 15 °C and 99.5 °C (median = 35 °C), conductivity between 139 and 43,100 μS/cm (median = 906 μS/cm), and pH between 2.2 and 7.82 (median = 5.78). The dominant water types are Na-HCO3 and Na-Cl, representing two main trends (“hydrothermal” and “marine”), despite a few samples of the Na-SO4 type. Relative minor and trace element contents reinforce conclusions drawn from major-ion compositions that major hydrogeochemical processes are: evolution of deep hydrothermal fluids, adsorption of magmatic volatiles, steam-heating of perched-aquifers, and mixing of deep fluids with marine or near-surface hydrothermal fluids.