Groundwater is the main source of water supply on São Miguel. Groundwater abstraction (25 hm³ a⁻¹) relies almost exclusively on springs discharging from perched aquifers. A total of 1100 springs (1.5 springs/km²) are located on São Miguel, mainly in the Água de Pau and the Furnas–Povoação groundwater bodies. Only 26 wells have been drilled, distributed principally in the west-central zone. Specific capacity ranges between 0.49 and 100 l (s m)⁻¹ (median=1.11 l (s m)⁻¹), while transmissivity ranges between 5.98×10⁻⁴ and 1.22×10⁻¹ m² s⁻¹ (median=1.35×10⁻³ m² s⁻¹). Groundwater is predominantly of the Na-Cl and Na-HCO₃ types and electrical conductivity in springs ranges from 36 to 440 μS cm⁻¹ (median=152 μS cm⁻¹), which is lower than in wells (211–9670 μS cm⁻¹; median=515 μS cm⁻¹). These values result from seawater spray, a CO₂ soil contribution and limited dissolution of primary silicate minerals from volcanic rocks. In the case of wells, seawater intrusion may also influence electrical conductivity. One of the main problems in groundwater quality is total and faecal coliform microbial contamination. A pollution risk map constructed for São Miguel as a whole showed that risk in the majority of the groundwater bodies is only very low to moderate.