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O Arquipélago dos Açores encontra-se situado no Atlântico Norte, na proximidade da junção tripla das placas litosféricas Euroasiática, Núbia e Norte Americana, originando um contexto geológico, hidrológico e hidrogeoquímico único. A configuração geológica complexa da região é evidenciada por uma atividade sísmica e vulcânica significativa, acompanhada por diversas manifestações secundárias de vulcanismo, observadas em várias ilhas, tais como campos fumarólicos, nascentes de águas termais e/ou ricas em CO₂, bem como desgaseificação difusa a partir de solos e de massas de água de superfície.
O arquipélago é abundante em recursos de água mineral, com nascentes deste tipo distribuídas por sete das nove ilhas dos Açores (São Miguel, Terceira, São Jorge, Pico, Faial, Graciosa e Flores), sendo a maioria localizada em São Miguel, particularmente nas áreas dominadas pelos vulcões das Furnas e do Fogo. A caracterização das variações composicionais das descargas de águas minerais do arquipélago, especialmente na ilha de São Miguel, tem sido relevante para vários programas de monitorização. Este estudo centrou-se nas descargas de águas minerais de dois dos três vulcões traquíticos ativos de São Miguel (vulcões das Furnas e do Fogo), com especial ênfase no Vulcão das Furnas. No Vulcão das Furnas foram conduzidas sete campanhas ao longo de um ano, envolvendo a recolha de amostras em trinta e nove pontos de água, enquanto no Vulcão do Fogo foram realizadas duas campanhas de amostragem em dez pontos de água mineral, uma no inverno e outra no verão. O estudo integra a investigação das assinaturas hidrogeoquímicas, compreendendo elementos principais, menores e em traço, incluindo as terras raras, e a determinação da razão isotópica de estrôncio.
Dada a importância da água da chuva como principal fonte de recarga dos aquíferos em estudo, foi realizada uma campanha para avaliar a sua composição química. A composição das águas da chuva apresenta padrões semelhantes aos da água do mar, sugerindo que os aerossóis marinhos podem constituir a principal fonte. No entanto, a presença de outros componentes, resultantes das emissões vulcânicas e poeiras minerais, destaca a complexidade do sistema e a necessidade de investigação adicional. A proximidade de um campo fumarólico revelou ter um impacto significativo na química da água da chuva.
Apesar de alguns estudos anteriores baseados na composição química das águas no Vulcão das Furnas, a integração de dados relativos aos elementos menores e em traço e às razões isotópicas de Sr ainda era muito limitada. Neste contexto, a integração de dados hidrogeoquímicos permitiu revelar uma maior complexidade do sistema. As águas refletem a influência da interação com as rochas encaixantes e com gases vulcânicos. A contribuição vulcânica é evidenciada pela variação de temperatura, com águas com temperaturas entre os 90ºC e 100ºC, ou pela presença de teores elevados de CO₂ dissolvido em algumas nascentes de água mineral. A aplicação de um conjunto abrangente de ferramentas hidroquímicas possibilitou a identificação de múltiplas influências vulcânicas. A composição de algumas águas de alta temperatura tem origem no aquecimento por vapor, resultante da interação entre um gás ácido e os aquíferos mais superficiais. A composição de outras águas minerais resulta da contribuição de um reservatório mais profundo, em que águas subterrâneas mais superficiais se misturam com fluidos cloretados neutros provenientes de profundidade.
As águas minerais do Vulcão do Fogo apresentam influência da dissolução mineral e da desgaseificação vulcânica. Ambos os contributos são visíveis no conteúdo dos principais iões em solução, com os catiões a evidenciarem a dissolução da rocha, enquanto a influência vulcânica é demonstrada pela presença de elevadas concentrações de SO₄2-, associadas a valores de pH muito reduzidos em águas termais, e pela elevada concentração de CO₂ dissolvido em algumas águas frias. Os principais constituintes apresentam estabilidade ao longo do tempo, sem variações sazonais significativas. No entanto, a sazonalidade é observada nos padrões dos elementos de terras raras e nas razões ⁸⁷Sr/⁸⁶Sr de algumas amostras, estando relacionada com diferentes proporções de alteração da rocha, causada pela dissolução incongruente de alguns minerais. Para além disso, a integração dos dados hidrogeoquímicos com a informação geológica de profundidade da área do Vulcão do Fogo permitiu a identificação de múltiplos sistemas aquíferos nesta região.
Os padrões de desgaseificação difusa de CO₂ no solo das Furnas, monitorizados através de estações permanentes, foram também analisados. Estes padrões demonstram a influência de variáveis meteorológicas e parecem ainda sugerir uma possível correlação entre o processo de desgaseificação e as variações no nível do aquífero. Os dados revelaram também algumas alterações em comparação com estudos anteriores, indicando que os modelos de regressão aplicados para a monitorização sismovulcânica necessitam de atualização regular.
Os resultados do presente estudo representam um avanço significativo na compreensão dos sistemas hidrotermais em análise. Reforçam a necessidade da aplicação de um conjunto diversificado de técnicas hidrogeoquímicas para esclarecer as reações complexas que ocorrem nos aquíferos. Estes resultados poderão ainda servir como valores de referência para análises futuras, especialmente no âmbito da monitorização sismovulcânica nos Açores, considerando que alterações nas concentrações e padrões hidroquímicos podem refletir mudanças ocorridas em profundidade, particularmente nos sistemas vulcânicos estudados.
ABSTRACT: The Azores Archipelago is located on the North Atlantic Ocean, at the vicinity of the triple junction between the Eurasian, the Nubian and the North American plates, creating unique geological, hydrological, and hydrogeochemical settings. The complex structural framework of the region is evidenced through significant seismic and volcanic activity, accompanied by a variety of secondary manifestations of volcanism, such as fumarolic fields, thermal and/or cold CO2-rich springs, as well as diffuse degassing through soils and surface of water bodies, observed across several islands. The archipelago has abundant mineral water resources, with springs occurring in seven of the nine islands (São Miguel, Terceira, São Jorge, Pico, Faial, Graciosa and Flores); these springs are mostly located in São Miguel, particularly in the area dominated by the Furnas and Fogo volcanoes. The characterization of the compositional variations of the mineral water discharges over the archipelago, especially at the São Miguel Island, has been relevant for several monitoring programs. The present study focuses in the mineral water discharges from two of the three active trachytic central volcanoes (Furnas and Fogo volcanoes), with a particular emphasis in Furnas Volcano. Seven surveys were carried out over a year in the Furnas area, involving the collection of samples from thirty-nine mineral water discharges, while in the Fogo Volcano two sampling surveys, one in winter and another during summer, were made on ten discharges. This study comprises research on major and trace element compositions, including rare earth elements, and strontium isotopic ratio. Given the significance of rainwater as the primary source of recharge for the aquifers under study, a survey was also made to assess the chemical composition of the rainwater. The composition of the rainwater exhibited similar patterns as the seawater, suggesting that marine sprays may constitute the predominant chemical source. However, the presence of other components, suggesting the influence of volcanic emissions and mineral dust, underscores the complexity of the system and the need for further research. The proximity to a fumarolic field showed a significant impact on the chemistry of rain waters. Despite the numerous studies based on the major composition developed in the Furnas Volcano, the combination of trace elements and Sr isotopes remained limited. The integration of hydrogeochemical data, allowed to uncover further complexity of the system. These waters depict an influence of the wall rock and of volcanic gases. The volcanic contribution is observed by the different range of temperatures, as the waters reach temperatures between 90ºC and 100ºC, or by the presence of high amount of dissolved CO2 in some cold waters. The application of a comprehensive array of hydrochemical tools facilitated the discrimination of multiple volcanic inputs. The composition of some high temperature waters is attributed to steam heating, resulting from the mixture between an acid gas and shallower aquifers. The composition of others discharges result from an in-depth input, where shallower groundwater mixes with neutral chloride fluids from a deeper reservoir. Fogo Volcano mineral waters depict a contribution of mineral dissolution and volcanic degassing. Both inputs could be observed in the major ion composition, with the major cations showing evidence of rock dissolution, while the volcanic input was observed by the presence of high SO42- accompanied by low pH in thermal waters, and also high dissolved CO2 in some cold waters. The main components demonstrate stability, with no significant seasonal variation. However, seasonality is observed in rare earth elements patterns and 87Sr/86Sr ratios of some samples, related to different proportions of rock weathering, caused by incongruent dissolution of some minerals. Besides that, the integration of hydrogeochemical analysis with a geological investigation of the Fogo area has allowed the identification of multiple aquifer systems within this region. The Furnas soil CO2 diffuse degassing patterns from permanent stations were also analyzed. The degassing patterns depict the influence of meteorological variables and seem to point out a possible correlation between the degassing and variations in the hydraulic head of the aquifer. The data also showed some changes when compared to previous analysis, revealing that the filtered regression models applied for seismovolcanic monitoring require regular updating. The results of the present work represent a significant advancement in the comprehension of the hydrothermal systems under study. They emphasize the need to implement a range of hydrogeochemical techniques to characterize the processes occurring in the aquifers. These results can also be used as reference values for future analysis, especially concerning the seismovolcanic monitoring in Azores, considering that changes in the hydrochemical concentrations and patterns may reflect changes deeply-derived in the studied volcanic systems.
ABSTRACT: The Azores Archipelago is located on the North Atlantic Ocean, at the vicinity of the triple junction between the Eurasian, the Nubian and the North American plates, creating unique geological, hydrological, and hydrogeochemical settings. The complex structural framework of the region is evidenced through significant seismic and volcanic activity, accompanied by a variety of secondary manifestations of volcanism, such as fumarolic fields, thermal and/or cold CO2-rich springs, as well as diffuse degassing through soils and surface of water bodies, observed across several islands. The archipelago has abundant mineral water resources, with springs occurring in seven of the nine islands (São Miguel, Terceira, São Jorge, Pico, Faial, Graciosa and Flores); these springs are mostly located in São Miguel, particularly in the area dominated by the Furnas and Fogo volcanoes. The characterization of the compositional variations of the mineral water discharges over the archipelago, especially at the São Miguel Island, has been relevant for several monitoring programs. The present study focuses in the mineral water discharges from two of the three active trachytic central volcanoes (Furnas and Fogo volcanoes), with a particular emphasis in Furnas Volcano. Seven surveys were carried out over a year in the Furnas area, involving the collection of samples from thirty-nine mineral water discharges, while in the Fogo Volcano two sampling surveys, one in winter and another during summer, were made on ten discharges. This study comprises research on major and trace element compositions, including rare earth elements, and strontium isotopic ratio. Given the significance of rainwater as the primary source of recharge for the aquifers under study, a survey was also made to assess the chemical composition of the rainwater. The composition of the rainwater exhibited similar patterns as the seawater, suggesting that marine sprays may constitute the predominant chemical source. However, the presence of other components, suggesting the influence of volcanic emissions and mineral dust, underscores the complexity of the system and the need for further research. The proximity to a fumarolic field showed a significant impact on the chemistry of rain waters. Despite the numerous studies based on the major composition developed in the Furnas Volcano, the combination of trace elements and Sr isotopes remained limited. The integration of hydrogeochemical data, allowed to uncover further complexity of the system. These waters depict an influence of the wall rock and of volcanic gases. The volcanic contribution is observed by the different range of temperatures, as the waters reach temperatures between 90ºC and 100ºC, or by the presence of high amount of dissolved CO2 in some cold waters. The application of a comprehensive array of hydrochemical tools facilitated the discrimination of multiple volcanic inputs. The composition of some high temperature waters is attributed to steam heating, resulting from the mixture between an acid gas and shallower aquifers. The composition of others discharges result from an in-depth input, where shallower groundwater mixes with neutral chloride fluids from a deeper reservoir. Fogo Volcano mineral waters depict a contribution of mineral dissolution and volcanic degassing. Both inputs could be observed in the major ion composition, with the major cations showing evidence of rock dissolution, while the volcanic input was observed by the presence of high SO42- accompanied by low pH in thermal waters, and also high dissolved CO2 in some cold waters. The main components demonstrate stability, with no significant seasonal variation. However, seasonality is observed in rare earth elements patterns and 87Sr/86Sr ratios of some samples, related to different proportions of rock weathering, caused by incongruent dissolution of some minerals. Besides that, the integration of hydrogeochemical analysis with a geological investigation of the Fogo area has allowed the identification of multiple aquifer systems within this region. The Furnas soil CO2 diffuse degassing patterns from permanent stations were also analyzed. The degassing patterns depict the influence of meteorological variables and seem to point out a possible correlation between the degassing and variations in the hydraulic head of the aquifer. The data also showed some changes when compared to previous analysis, revealing that the filtered regression models applied for seismovolcanic monitoring require regular updating. The results of the present work represent a significant advancement in the comprehension of the hydrothermal systems under study. They emphasize the need to implement a range of hydrogeochemical techniques to characterize the processes occurring in the aquifers. These results can also be used as reference values for future analysis, especially concerning the seismovolcanic monitoring in Azores, considering that changes in the hydrochemical concentrations and patterns may reflect changes deeply-derived in the studied volcanic systems.
Descrição
Tese de Doutoramento, Geologia, 28 de novembro de 2024, Universidade dos Açores.
Palavras-chave
Água mineral Vulcão Furnas Vulcão do fogo Hidrogeologia Fluxos de iões metálicos
Contexto Educativo
Citação
FERREIRA, Letícia Resende. (2025). "Geochemistry of mineral waters in Furnas Volcano (São Miguel, Azores): source and fluxes of metal ions and 87Sr/86Sr ratios". Ponta Delgada: Universidade dos Açores, 2025. 280 p. Tese de Doutoramento em Geologia. Disponível em http://hdl.handle.net/10400.3/8863