This study focused on 13 water samples collected from two of the main active volcanoes (Furnas and Fogo) at São Miguel, Azores. Based on the major element composition, the waters are classified into Na-HCO3 and Na-Cl types. While the concentrations of chloride seem to reflect the contribution of sea salt aerosols, the behavior of the main cationic species and Sr in the analyzed waters appear to have been largely controlled by the interaction between meteoric waters and the underlying bedrock. The temperature and input of CO2 from the secondary volcanic activity are enhancing the silicate leaching. The stable isotopic data show that these waters have a meteoric origin (δ18O = −2.03 to −4.29‰; δ2H = −7.6 to −17.4‰) and are influenced by a deep hydrothermal/volcanic carbon source (δ13C = −4.36 to −7.04‰). The values of δ34S (0.13 to 12.76‰) reflects a juvenile sulfur source derived from the leaching of volcanic rocks. The Sr isotopic ratios show a slight difference between the values from Furnas (87Sr/86Sr = 0.705235–0.705432) and Fogo (87Sr/86Sr = 0.705509–0.707307) whereas the Furnas waters are less radiogenic. The Sr isotope also shows that the hydrochemical signatures of the groundwater was controlled by the rock leaching, and the samples Furnas reached water-rock isotopic equilibrium.
