Drinking water supplies in the municipality of Ponta Delgada (São Miguel, the Azores) have presented, in some areas of its network, high concentration of fluoride. These concentrations reach 2.0 mg/L whereas the admitted content is 1.5 mg/L. The origin of the water is entirely groundwater extracted from 45 springs and four wells, located in Ponta Delgada, Lagoa and Vila Franca municipalities. Considering that São Miguel is a volcanic island, the study was carried out through the characterization of 30 groups of springs and three wells that were duly analyzed in order to understand the mechanisms that lead to fluoride enrichment. In addition to the above-mentioned sites, two more sites from Furnas volcano (Povoação) were also included. Sampling was performed between November 2009 and July 2010. Waters are cold (temperature 9 to 18.2 °C), slightly acid to slightly alacaline (pH 6.50 to 7.60), low electrical conductivity (springs, 87 to 502 µS/cm, and wells from 237 to 1761 µS/cm) and the predominant hydrochemical facies is bicarbonate sodium (HCO3-Na). Sodium Chloride facies (Cl-Na) and bicarbonate chloride sodium facies (HCO3-Cl-Na) are also present in some extent. Results suggest two geochemical evolution trends: (1) dissolution of silica rich minerals and CO2 dissolution and (2) sea aerosols contained in the rain. Both tendencies justify the enrichment in sodium, although only sea aerosols account for chloride contribution. Fluoride content in the water ranges 0.17 to 2.0 mg/L and no seasonal variations were detected. These values are much lower than the ones obtained in Furnas, were they reach a maximum of 5.09 mg/L. These facts confirm the influence of secondary volcanism manifestations in water chemistry, namely the eě ect of the fluoride rich gaseous emanations that exist where the springs are located. On the contrary, the origin of fl uoride in the drinking waters supplied by Ponta Delgada network is mainly due to water–rock interaction processes, considering that rocks in the aquifers have silicate minerals that contain fluorine. The comprehension of the obtained results is of paramount importance for better water management in the municipality, namely in what concerns the separation of supply areas and the meaningful reduction of end users consuming water with fluoride in excess.