Water supplied to the city of Ponta Delgada is subjected to stringent monitoring and quality control, in accordance with Portuguese legislation despite the overall tendency of a very low number of values that did not comply with normative limits (0.69% in 2010). Some problems were detected regarding total and fecal coliforms as well as nitrates in the water. Therefore, water quality control, which is undertaken following sample collection in the houses of consumers, exhibits the same problems detected by the monitoring of groundwater bodies that are made in order to fulfill the EU Water- Framework Directive requirements (Cruz et al., 2010b). However, abnormal high fluoride contents have been detected in the water supply system of Ponta Delgada, with average values in the period 2004–2010 exceeding or equalizing the normative value of 1.5 mg/L in the supply zones of Mosteiros and Bretanha (average value=1.7 mg/L), Sete Cidades (average value=1.6 mg/L), Bretanha and Remédios (average value=1.5 mg/L), Fajã de Baixo (average value=1.5 mg/L) and Fajã de Cima (average value=1.5 mg/L), as well as the water transfer to Ribeira Grande municipality (average value=1.5 mg/L). Considering the maximum fluoride content measured in this period, 12 of the supply zones present values higher than 1.5 mg/L, indicating that the Municipal Water and Sanitation Services have implemented harmonization procedures, such as the mixing of waters abstracted in different origins to dilute fluoride ion