Hydrogeochemical evolution and characterization study in volcanic lakes of the Azores archipelago (Portugal)

A hydrogeochemical study was performed on 45 lakes spread across six islands of the Azores archipelago (Portugal). Water temperature of the studied lakes is cold (mean = 16.2 °C), and the deeper lakes depict a monomictic behaviour, with water column stratification during summer resulting in higher carbon dioxide concentrations in the hypolimnion. Samples present an acid to neutral pH (mean = 7.04), and in general correspond to diluted and weakly mineralized waters (mean electrical conductivity = 107 μS/cm).The prevailing hydrogeochemical facies are of the Na–Cl type, which derives from atmospheric marine salts contamination, and, albeit not to the same extent, Na–HCO3, suggesting a cumulative effect of water-rock interaction. Water from Furna do Enxofre Lake exhibits a dissimilar Mg–HCO3 type. The neutralization of water acidity is promoted by water-rock interaction, which also leads to a bicarbonate enrichment in many lakes in the Azores. The HCO3− enrichment shows that other processes are controlling the water composition, such as the dissolution of CO2 from various origins, of which CO2-rich gas emissions from volcanic sources. A fraction of the emitted CO2 at Furna do Enxofre Lake (Graciosa) and Furnas Lake (on São Miguel) is of deep origin as showed by the δ13C content. In the remaining lakes, where these isotopic analyses (δ13C) were carried out, the emitted CO2 has a biogenic origin, derived from the decomposition of organic matter and eutrophication, a process that affects several lakes in the Azores. The study of other stable isotopic contents (δ18O and δ2H) allowed also the confirmation of the meteoric origin of studied waters. Excluding Furna do Enxofre lake, classified in class 5 (i.e. low activity lake), all lakes are classified as no activity (class 6).