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Instituto de Investigação
em Vulcanologia e Avaliação de Riscos
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Artigos em livros de actas ► Internacionais


Referência Bibliográfica

CRUZ, J.V., ANTUNES, P., AMARAL, C. (2004) - Volcanological setting and hydrogeochemistry of volcanic lakes in the Azores archipelago (Portugal). Abstract Book, IAVCEI - General Assembly 2004, Pucón, Chile.


A set of 30 lakes from the Azores archipelago (Portugal) has been studied to characterize their volcanic setting and water composition. The majority of the lakes present small surface area (8.94x10-4-4.35 km2) and volume (2.4x103-4.78x107 m3; median=3.99x104 m3), and the maximum depth range between 0.8 and 33 m. Lakes are mainly located inside volcanic craters. Waters are cold (11.9ºC-24.6ºC; summer period), very diluted (TDS range between 11.0 and 356.2 mg.L-1; median=36.8 mg.L-1) and from the Na-Cl to Na-HCO3 type, with the exception of the Furna do Enxofre lake (Graciosa) that present a Mg-HCO3 trend. Two major trends are revealed: Na-Cl waters, with a Na/Cl ratio close to seawater ratio, which evolves by marine contribution, and Na-HCO3 waters, influenced by other processes enhanced by the active volcanic processes, such as water-rock interaction and hydrothermal seepage. Furna do Enxofre lake chemistry (total CO2 content=365.1 mg.L-1; SiO2 content=74.9 mg.L-1) is an example of the latter trend. Vertical profiles made at the deepest lakes of São Miguel reveal that major-ion content is rather stable with depth, independently of the thermal driven density stratification observed in summer for some lakes. In summer period dissolved CO2 increases with depth, and is kept in the hypolimnion, while HCO3 -content is stable, suggesting that CO2 release exceeds acidity neutralization by water-rock interaction. In winter period, dissolved CO2 increase is not so sharp for the majority of the lakes. The CO2 enrichment observed for certain lakes is suggested to be due to a volcanic input.