Hydrogeochemistry of thermal and mineral water springs of the Azores archipelago (Portugal)

摘要

Mineral and thermal water chemistry from the Azores archipelago was investigated in order to discriminate among hydro-chemical 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 Sao Miguel, Graciosa, Faial, Sao 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-HCO_3 and Na-HCO_3-Cl type waters. These groups comprise of cold, thermal (27℃-75℃ ) and boiling waters (92.2℃-93.2℃), with a wide TDS range (77.3-27,145.7 mg/L). Group 3 is made of samples of dominated Na-SO_4 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 (CO_2) 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 (CO_2), 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.