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Mineral water occurrence and geochemistry in the Azores volcanic archipelago (Portugal): insight from an extended database on water chemistry

Freire, P., Andrade, C., Viveiros, F., Silva, C., Coutinho, R., Cruz, J. V.
Environmental earth sciences 2015 v.73 no.6 pp. 2749-2762
aquifers, cold, data collection, databases, electrical conductivity, geochemistry, hydrochemistry, islands, mineral water, mixing, pH, principal component analysis, seawater, water temperature, weathering, Azores, Portugal
The occurrence and geochemistry of mineral water discharges in the Azores (Portugal) have been studied. Due to the volcanic nature of the archipelago, mineral water discharges occur in seven of the nine islands, namely at São Miguel, Terceira, São Jorge, Pico, Faial, Graciosa and Flores, both associated to perched-water and basal aquifers, and mainly corresponding to springs (72.4 %). The majority of the 116 discharges with water geochemistry data occurs in São Miguel island (73.3 %). Water temperature ranges between 13.2 and 99.8 °C (average 37.1 °C), being 63.8 % of the samples in the dataset classified as thermal. Waters are mainly acid to slightly alkaline, with pH values ranging between 2.1 and 7.9 (average 5.85). Despite the wide range of electrical conductivity values (122–53,000 μS/cm), the median value (876 μS/cm) suggests that in general waters in the dataset are poorly mineralized, being mainly from the Na–HCO₃and Na–HCO₃–Cl types, besides a few samples are from Na–SO₄and Na–SO₄–HCO₃types. Instead, waters abstracted in basal aquifers are mainly from the Na–Cl type. Principal component analysis, as well as a classic hydrogeochemistry approach, suggests that mineral water chemistry evolves as function of several processes, as silicate weathering, enhanced by water temperature, not excluding mixing mechanisms between cold and hot waters. Regarding the SO₄-rich thermal waters, steam-heating inputs are also to be considered. Dilution of volcanic CO₂may also influence the chemical evolution of mineral waters. The influence of seawater over the mineral water chemistry in basal aquifers is shown by the much higher conductivity and Cl content.