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Análise hidrogeoquímica evolutiva e processos reativos no Sistema Aquífero Bacia Tejo-Sado/Margem Direita (T1)
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Resumo(s)
A presente dissertação teve como objeto de estudo o Sistema Aquífero Bacia Tejo-Sado/Margem Direita
(T1). Este, nunca foi alvo de um estudo pormenorizado, encontrando-se essencialmente descrito no livro
“Sistemas Aquíferos de Portugal Continental” e nos “Planos de Gestão da Região Hidrográfica do Tejo”.
Neste trabalho procurou-se aprofundar o conhecimento deste sistema aquífero, dando especial ênfase à
evolução hidrogeoquímica da massa de água associada ao mesmo. Recorrendo a modelos geoquímicos
de balanço de massa e modelos diretos, procurou-se validar hipóteses que permitissem explicar a
ocorrência de fácies pouco comuns no ambiente que carateriza o sistema.
O Sistema Aquífero Bacia Tejo-Sado/Margem Direita (T1), engloba parte das áreas dos distritos de
Lisboa e Santarém, intersetando território de 14 concelhos. Geologicamente, é composto por formações
com idades compreendidas entre o Paleogénico e o Quaternário, destacando-se, pela sua predominância,
as formações de Alcoentre e de Almoster.
Para a obtenção de dados, começou-se por identificar pontos de observação, dentro dos limites do
sistema aquífero, e posterior seleção com vista à recolha de amostras de água. O trabalho de campo foi
desenvolvido em duas campanhas de campo, tendo sido recolhidas amostras de água em nove pontos,
com o objetivo de realizar uma análise de elementos maiores. Os dados obtidos, após a análise destas
amostras, foram completados recorrendo a outras bases de dados existentes.
A análise dos dados permitiu verificar que as águas apresentam uma grande variabilidade de fácies,
predominando as bicarbonatadas cálcicas e/ou magnesianas, com uma forte presença de fácies
bicarbonatada sódica. Estes dados químicos foram sujeitos a tratamento estatístico, (análise de cluster
hierárquica e análise fatorial utilizando o método de componentes principais). A análise de cluster
hierárquica permitiu dividir as amostras em dois grupos, considerando uma determinada distância.
Contudo, quando analisadas as amostras de um mesmo grupo, verifica-se que estas não apresentam
coerência no que diz respeito as formações captadas, pelo que, não é possível associar cada um dos
grupos a uma formação geológica em concreto. A análise fatorial possibilitou a identificação de três
grupos de pares de iões que se correlacionam bem entre si, especificamente, o Ca2+/HCO3
-
, Mg2+/Cl-
e
Na+
/Cl-
. No decorrer desta análise ainda foi possível determinar que 89% da variância das amostras é
explicada por três componentes, a que se associam os elementos Ca2+/HCO3
-
, Cl-
/Na+
e SO4
2-
.
Tendo por base o contexto geológico da área em estudo, considerou-se que os processos reativos que
melhor explicariam a variabilidade das fácies presentes, seria a dissolução de carbonatos e processo de
troca catiónica, mais concretamente, entre o Ca2+ e o Na+
. O software PHREEQC foi usado para modelar
um conjunto de amostras tendo com premissa o descrito anteriormente. Os resultados da modelação
revelaram-se satisfatórios, pois explicam bem a variabilidade de fácies que ocorrem no sistema aquífero.
Contudo, como sempre acontece em exercícios de modelação, é possível existirem outros processos que
não foram considerados, nomeadamente a transição entre ambientes marinhos e continentais. A partir
da análise dos dados hidroquímicos, não foi possível confirmar a separação do sistema aquífero em dois
aquíferos com litologias distintas.
The main object of this dissertation was the study of the Tagus-Sado Basin/Right Bank Aquifer System (T1). This aquifer system has never been subject of a detailed study, being mainly described in the book "Sistemas Aquíferos de Portugal Continental" and in the Management Plans of the Tagus River Hydrographic Region. In this work, we studied this aquifer system giving special emphasis to the hydrogeochemistry, more specifically, to the hydrogeochemical evolution of the water mass associated with it. The Tagus-Sado Basin/Right Bank Aquifer System (T1) is in mainland Portugal, covering part of Lisboa and Santarém districts and intersecting the territory of 14 municipalities. Geologically, it is composed of Palaeogene to Quaternary formations, with the predominance of Alcoentre and Almoster formations. The data collecting, starts by identifying water points, within the limits of the system and subsequent selection to collect water samples. The fieldwork was done in two campaigns, having collected water samples in nine points, to perform a major element analysis. The data obtained, after the analysis of these samples, were completed using other existing databases. The data analysis allows to verify that the waters present a great variability of facies, predominantly calcium bicarbonate and/or magnesian, with a strong presence of sodium bicarbonate facies. These chemical data were also subjected to statistical treatment, namely, hierarchical cluster analysis and factor analysis, using the principal components method. Through the hierarchical cluster analysis, it was possible to divide samples into two groups, considering a certain distance. However, when the samples of the same group are analysed, it is possible to note that they do not present coherence in what concerns the producing formations, so it is not possible to associate each group with a specific geological formation. The factor analysis enabled the identification of three groups of ion pairs that correlate well with each other, specifically, Ca2+/HCO3 - , Mg2+/Cl- and Na+ /Cl- . This analysis shows that 89% of the variance of the samples could be explained by three components, associated with the pairs of species Ca2+/HCO3 - , Cl- /Na+ and SO4 2- . Considering the geological context, the reactive processes that best explained the variability of the facies, present in the aquifer system, will be the dissolution of calcite and the cationic exchange, more specifically, between Ca2+ and Na+ . The PHREEQC software was used to model a set of samples, under the premise described above. The modelling results were satisfactory, as they explain well the variability of the facies that occur in the aquifer system. However, as it occurs in any modelling exercise, the considered processes may not be the only ones responsible for the observed variability. The task related to hydrogeochemistry does not allow us to confirm the separation of the aquifer system into two aquifers with distinct lithologies. The variability of the facies present in the aquifer system may be attributed to the different lithologies present, but it does not explain, by itself, the water composition variations which, may be justified by changes in the local characteristics, such as the transition between marine and continental environments.
The main object of this dissertation was the study of the Tagus-Sado Basin/Right Bank Aquifer System (T1). This aquifer system has never been subject of a detailed study, being mainly described in the book "Sistemas Aquíferos de Portugal Continental" and in the Management Plans of the Tagus River Hydrographic Region. In this work, we studied this aquifer system giving special emphasis to the hydrogeochemistry, more specifically, to the hydrogeochemical evolution of the water mass associated with it. The Tagus-Sado Basin/Right Bank Aquifer System (T1) is in mainland Portugal, covering part of Lisboa and Santarém districts and intersecting the territory of 14 municipalities. Geologically, it is composed of Palaeogene to Quaternary formations, with the predominance of Alcoentre and Almoster formations. The data collecting, starts by identifying water points, within the limits of the system and subsequent selection to collect water samples. The fieldwork was done in two campaigns, having collected water samples in nine points, to perform a major element analysis. The data obtained, after the analysis of these samples, were completed using other existing databases. The data analysis allows to verify that the waters present a great variability of facies, predominantly calcium bicarbonate and/or magnesian, with a strong presence of sodium bicarbonate facies. These chemical data were also subjected to statistical treatment, namely, hierarchical cluster analysis and factor analysis, using the principal components method. Through the hierarchical cluster analysis, it was possible to divide samples into two groups, considering a certain distance. However, when the samples of the same group are analysed, it is possible to note that they do not present coherence in what concerns the producing formations, so it is not possible to associate each group with a specific geological formation. The factor analysis enabled the identification of three groups of ion pairs that correlate well with each other, specifically, Ca2+/HCO3 - , Mg2+/Cl- and Na+ /Cl- . This analysis shows that 89% of the variance of the samples could be explained by three components, associated with the pairs of species Ca2+/HCO3 - , Cl- /Na+ and SO4 2- . Considering the geological context, the reactive processes that best explained the variability of the facies, present in the aquifer system, will be the dissolution of calcite and the cationic exchange, more specifically, between Ca2+ and Na+ . The PHREEQC software was used to model a set of samples, under the premise described above. The modelling results were satisfactory, as they explain well the variability of the facies that occur in the aquifer system. However, as it occurs in any modelling exercise, the considered processes may not be the only ones responsible for the observed variability. The task related to hydrogeochemistry does not allow us to confirm the separation of the aquifer system into two aquifers with distinct lithologies. The variability of the facies present in the aquifer system may be attributed to the different lithologies present, but it does not explain, by itself, the water composition variations which, may be justified by changes in the local characteristics, such as the transition between marine and continental environments.
Descrição
Tese de Mestrado, Geologia do Ambiente, Riscos Geológicos e Ordenamento do Território , 2022, Universidade de Lisboa, Faculdade de Ciências
Palavras-chave
Hidrogeologia Hidrogeoquímica Modelação hidrogeoquímica Troca catiónica Teses de Mestrado - 2022