Radon (222Rn) occurrence in groundwater bodies on São Miguel Island (Azores archipelago, Portugal)

A set of springs were sampled along São Miguel Island (Azores, Portugal) to proceed to the characterization of the major-ion and radon (²²²Rn) content. According to the selection criteria a total of 42 discharges were sampled, both during winter and summer periods, along major geological units, namely 3 springs at Sete Cidades Volcano, 4 at Nordeste Volcano, 17 at Fogo Volcano, and 18 at Furnas Volcano. Spring water temperature ranges from 12.8 to 78.1 °C in winter (mean = 23.6 °C), and from 12.6 to 76.7 °C in summer (mean = 23.9 °C), and the dataset comprehends 26 (winter) to 21 (summer) cold springs, 3 to 8 ortothermal springs, as well as 13 thermal discharges. Mean electrical conductivity is equal to 552.9 μS/cm (winter) and 550 μS/cm (summer), suggesting a short residence time, and waters are acidic to slightly basic in character (pHwᵢₙₜₑᵣ: 4.16–7.16; pHₛᵤₘₘₑᵣ: 4.05–7.35). Groundwater chemistry is manly controlled by silicate leaching, enhanced by the water temperature, seawater spraying and by the active volcanic environment. ²²²Rn content ranges between 0.99 and 551.64 Bq/L (mean = 47.8 Bq/L) in winter and between 1.42 and 559.67 Bq/L (mean = 49.4 Bq/L) in summer. All the springs with anomalously higher values are located in Fogo Volcano, while the lowest radon concentrations are observed at Furnas Volcano. Groundwater temperature exerts a control over the ²²²Rn concentration; thus, the lower content corresponds to the most mineralized thermal waters. Springs with the higher radon content show the radiogenic contribution from uranium-rich peralkaline syenite intrusive body. Furthermore, data reveal that besides this lithologic control some waters are influenced by ²²²Rn migration from a deeper source towards shallower aquifers, carried by a volatile flux of CO₂.