​High fluoride contents in the water supply of the city of Ponta Delgada, located in the volcanic island of São Miguel (Azores, Portugal) have been reported. Dental fluorosis in São Miguel has been identified and described in several medical surveys. The water supply in Ponta Delgada consists entirely of groundwater. A study was carried out in order to characterize the natural F-pollution of a group of springs (30) and wells (3), that are associated to active central volcanoes of a trachytic nature. Two springs known for their high content in fluoride were sampled, both located in the central volcano of Furnas.

The sampled waters are cold, ranging from slightly acidic to slightly alkaline (pH range 6.53–7.60), exhibiting a low electrical conductivity (springs range 87–502 μS/cm; wells range 237–1761 μS/cm), and are mainly from the Na–HCO₃, Na–HCO₃–Cl and Na–Cl–HCO₃ water types. Results suggest two main trends of geochemical evolution: silicate weathering, enhanced by CO₂ dilution, and seawater spraying. Fluoride contents range between 0.17 mg/L and 2 mg/L, and no seasonal variations were detected. Results in the sources of the water supply system are lower than those of the Furnas volcano, which reach 5.09 mg F/L, demonstrating the effect of F-rich gaseous emanations in this area. Instead, the higher fluoride contents in the water supply are mainly due to silicate weathering in aquifers made of more evolved volcanic rocks.