Hydrogeochemistry of thermal and mineral water springs of the Azores archipelago (Portugal) |
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| Institution: | 1. University of Lisbon, Lisbon, Portugal;2. IDL, Lisbon, Portugal;3. Univ. Paris-Sud, Laboratoire IDES, UMR8148, Orsay F-91405, France;4. CNRS, Orsay F-91405, France;1. Department of Surface Waters Research and Management, Eawag: Swiss Federal Institute of Aquatic Science and Technology, Switzerland;2. Institute of Biogeochemistry and Pollutant Dynamics, ETH: Swiss Federal Institute of Technology, Switzerland;3. European Center for Geodynamics and Seismology, Luxembourg;4. Department of Geography, Vrije Universiteit Brussel, Belgium;5. Department of Earth Sciences, Royal Museum for Central Africa, Belgium;6. Renard Centre of Marine Geology, Department of Geology and Soil Science, Ghent University, Belgium;7. Institute of Geological Sciences and Oeschger Centre for Climate Change Research, University of Bern, Switzerland;1. IPL, ISEL - Instituto Superior de Engenharia de Lisboa, Physics Department, Lisboa, Portugal;2. Instituto Dom Luiz (IDL - Universidade de Lisboa), Portugal;3. Paléomagnétisme, Institut de Physique du Globe de Paris, Sorbonne Paris Cité, University Paris Diderot and UMR 7154 CNRS, 4 Av. de Neptune, 94107 Saint-Maur cedex, France;4. Faculdade de Ciências da Universidade de Lisboa, Lisboa, Portugal;5. GEOPS, Univ. Paris-Sud, CNRS, Université Paris-Saclay, Rue du Belvédère, Bât. 504, 91405 Orsay, France;6. Instituto Português do Mar e da Atmosfera, Lisboa, Portugal;7. Centro de Geofísica de Évora and Dep. de Geociências da Univ. de Évora, R. Romão Ramalho, 59, 7000-671 Évora, Portugal;8. Estrutura de Missão para a Extensão da Plataforma Continental, Paço de Arcos, Portugal;9. University of Azores, Department of Geosciences, Rua da Mãe de Deus, 9501-801 Ponta Delgada, Portugal;10. Institute of Geophysics, Acad. of Sci. of the Czech Republic, Prague, Czech Republic |
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| Abstract: | Mineral and thermal water chemistry from the Azores archipelago was investigated in order to discriminate among hydrochemical facies and isotopic groups and identify the major geochemical processes that affect water composition. A systematic geochemical survey of mineral and thermal water chemistry was carried out, incorporating new data as well as results from the literature. The Azores are a volcanic archipelago consisting of nine islands and samples were collected at São Miguel, Graciosa, Faial, São Jorge, Pico and Flores islands. Hydrothermal manifestations show the effects of active volcanism on several islands. Discharges are mainly related to active Quaternary central volcanoes, of basaltic to trachytic composition, but also some springs are related to older dormant or extinct volcanoes.Multivariate analysis – principal component and cluster analysis – enables classification of water compositions into 4 groups and interpretation of processes affecting water compositions. Groups 1 and 2 discharge from perched-water bodies, and mostly correspond to Na–HCO3 and Na–HCO3–Cl type waters. These groups comprise of cold, thermal (27 °C–75 °C) and boiling waters (92.2 °C–93.2 °C), with a wide TDS range (77.3–27, 145.7 mg/L). Group 3 is made of samples of dominated Na–SO4 from very acid boiling pools (pH range of 2.02–2.27) which are fed by steam-heated perched-water bodies. Group 4 is representative of springs from the basal aquifer system and corresponds to Na–Cl type fluids, with compositions dominated by seawater.Results are used to further develop a conceptual model characterizing the geochemical evolution of the studied waters. Mineral and thermal waters discharging from perched-water bodies are of meteoric origin and chemically evolve by absorption of magmatic volatiles (CO2) and by a limited degree of rock leaching. Existing data also suggest mixture between cold waters and thermal water. Water chemistry from springs that discharge from the basal aquifer system evolves by mixing with seawater; although, processes such as absorption of magmatic volatiles (CO2), rock leaching and mixture with hydrothermal waters are not excluded by the data because the actual composition of these waters deviates from that expected considering only conservative mixing between fresh and seawater. |
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