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Influência da sedimentação de sólidos em suspensão na dinâmica da contaminação fecal no estuário do Tejo
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Uma das principais ameaças à qualidade das águas de sistemas aquáticos costeiros é a contaminação microbiológica, que pode afectar negativamente as diversas actividades recreativas, comerciais ou industriais realizadas nestes sistemas. As bactérias indicadoras de contaminação fecal, FIB, são comummente utilizadas na avaliação da qualidade da água destes sistemas. Ao longo dos anos vários estudos têm sido desenvolvidos visando perceber como diversos factores bióticos e abióticos afectam a sobrevivência das bactérias entéricas em meios aquáticos. Estes estudos têm suportado o desenvolvimento de modelos que incorporam as principais variáveis que interferem no decaimento das FIB. Contudo, existem ainda algumas questões por responder, sendo uma das áreas onde ainda existem lacunas de conhecimento a interacção entre os sedimentos e as bactérias. Com vista a melhorar a compreensão da influência dos sedimentos em suspensão na dinâmica das bactérias entéricas na coluna de água, esta investigação analisa a agregação e transporte de Escherichia coli (E. coli) por sedimentos em suspensão, tendo como caso de estudo o estuário do Tejo. As amostras de água e sedimentos em suspensão analisadas foram recolhidas em diferentes locais do estuário e a experimentação desenvolveu-se em laboratório, nas instalações do Laboratório Nacional de Engenharia Civil (LNEC), reproduzindo-se tanto quanto possível as condições do meio em que foi feita a amostragem. A experimentação realizada visou perceber a capacidade dos sedimentos em suspensão para adsorver e transportar células de E. coli., avaliar a influência dos sedimentos na intensidade e penetração da radiação UV-B, estudar as principais características dos sedimentos em suspensão em determinadas zonas do estuário e obter bases para melhorar a descrição e previsão da evolução de plumas de contaminação fecal em águas de recreio e balneares. O trabalho desenvolvido compreendeu a amostragem de sedimentos em suspensão no estuário do Tejo e o estudo em laboratório das características da água e dos sedimentos amostrados, uma análise do Kd (coeficiente de atenuação vertical da radiação solar) de UV-B nas águas amostradas e a determinação das velocidades de queda dos sedimentos em suspensão. No estuário do Tejo observaram-se concentrações mais elevadas de E. coli durante a baixa-mar face às concentrações de preia-mar, pondo em evidência a importância da maré na dinâmica do transporte de plumas de E. coli no estuário. Salientam-se também as correlações positivas encontradas entre a concentração de E. coli e a concentração de sólidos suspensos totais (SST) e entre a concentração de SST e a turvação na análise exploratória realizada. Os resultados obtidos sugerem a existência de agregação de bactérias entéricas aos sedimentos em suspensão, caracterizada por uma variabilidade espacial associada às características dos mesmos, em particular, verificou-se a agregação de E. coli aos sedimentos em suspensão que têm tendência a agregarem e formarem flocos. Observou-se também uma diminuição do efeito da radiação solar com concentrações mais elevadas de SST e que a extinção de 90 % da radiação UV-B ocorre até cerca de 1 metro de profundidade, pelo que este efeito deverá ser pouco significativo para o decaimento das bactérias entéricas nas zonas mais profundas do estuário. As diferenças observadas na estrutura e granulometria dos sedimentos em suspensão nos locais amostrados influenciaram também os comportamentos observados no que se refere às velocidades de queda, sugerindo variações das velocidades de queda ao longo do estuário e a influência de processos de floculação em algumas amostras. A velocidade de queda mediana estimada foi de 1,2 mm/s em Algés, 0,24 mm/s em Alcântara e de 0,45 mm/s no Parque das Nações. Os resultados obtidos contribuem para melhorar a compreensão da influência dos sedimentos em suspensão no transporte de E. coli, em particular no estuário do Tejo, e poderão contribuir para a implementação de planos de monitorização mais eficazes e de sistemas de previsão mais fiáveis que permitam antecipar eventos de contaminação fecal. De modo a complementar, aprofundar e validar os resultados obtidos em desenvolvimentos de investigação futura seria útil a realização de estudos com maior abrangência espacial e temporal e de uma caracterização mais extensiva dos parâmetros ambientais da água e dos sedimentos em suspensão.
One of the major threats to the water quality of coastal aquatic systems is microbiological contamination, which can adversely affect the various recreational, commercial or industrial activities that are performed in these systems. Fecal indicator bacteria, FIB, are commonly used in the evaluation of water quality of aquatic systems. Over the years, several studies have been developed to understand how various biotic and abiotic factors affect the survival of enteric bacteria in aquatic environments. These studies have supported the development of models that incorporate the main variables that interfere in the decay of FIB. However, there are still some unanswered questions, and one of the areas where knowledge gaps still exist is related whit the interaction between sediments and bacteria. In order to improve the understanding of the influence of suspended sediments on the dynamics of enteric bacteria in the water column, this investigation analyzes the aggregation and transport of Escherichia coli (E. coli) by suspended sediments. The study is developed in the Tagus estuary. The water samples and suspended sediments were collected at different locations in the estuary and the experiments were carried out at the Laboratório Nacional de Engenharia Civil (LNEC), reproducing as much as possible the conditions of the environment in which was sampled. The objective of these experiments were to understand the ability of suspended sediments to adsorb and transport E. coli, to evaluate the influence of sediments on the intensity and penetration of UV-B radiation, to study the main characteristics of suspended sediments in certain areas of the estuary and to obtain bases to improve the description and prediction of the evolution of fecal contamination plumes in recreational and bathing waters. The work involved the sampling of suspended sediments in the Tagus estuary and the laboratory study of water characteristics and sediments sampled, an analysis of the UV-B Kd (coefficient of vertical attenuation of solar radiation) in the sampled waters and the determination of the sediment settling velocities. In the Tagus estuary, higher concentrations of E. coli were observed during low tide compared to high tide concentrations, highlighting the importance of the tide in the dynamics of the transport of microbial populations in the estuary. In the exploratory analysis, positive correlations between the concentration of E. coli and the concentration of TSS (total suspended solids) and between the concentration of TSS and the turbidity were also observed. The results obtained suggest the existence of aggregation of enteric bacteria in suspended sediments, characterized by a spatial variability associated to their characteristics, in particular, the aggregation of E. coli was verified to suspended sediments that tend to aggregate. There was also a decrease in the effect of solar radiation with higher concentrations of SST and the extinction of 90 % of UV-B radiation occurs near the surface until 1 m deep, so this effect should be less significant for the decay of bacteria in the deepest areas of the estuary. The observed differences in the structure and granulometry of suspended sediments at the sampled sites also influenced the observed velocity behavior, suggesting variations in the settling velocity along the estuary and the influence of flocculation processes in some samples. The estimated median settling velocity was 1,2 mm/s in Algés, 0,24 mm/s in Alcântara and 0,45 mm/s in Parque das Nações. The results obtained contribute to a better understanding of the influence of suspended sediments on E. coli transport, in particular in the Tagus estuary, and may contribute to the implementation of more effective monitoring plans and more reliable forecasting systems to anticipate events of fecal contamination. In order to complement, deepen and validate the results obtained further studies covering larger spatial and temporal scales and a wider characterization of the environmental parameters of water and suspended sediments would be useful in future research developments.
One of the major threats to the water quality of coastal aquatic systems is microbiological contamination, which can adversely affect the various recreational, commercial or industrial activities that are performed in these systems. Fecal indicator bacteria, FIB, are commonly used in the evaluation of water quality of aquatic systems. Over the years, several studies have been developed to understand how various biotic and abiotic factors affect the survival of enteric bacteria in aquatic environments. These studies have supported the development of models that incorporate the main variables that interfere in the decay of FIB. However, there are still some unanswered questions, and one of the areas where knowledge gaps still exist is related whit the interaction between sediments and bacteria. In order to improve the understanding of the influence of suspended sediments on the dynamics of enteric bacteria in the water column, this investigation analyzes the aggregation and transport of Escherichia coli (E. coli) by suspended sediments. The study is developed in the Tagus estuary. The water samples and suspended sediments were collected at different locations in the estuary and the experiments were carried out at the Laboratório Nacional de Engenharia Civil (LNEC), reproducing as much as possible the conditions of the environment in which was sampled. The objective of these experiments were to understand the ability of suspended sediments to adsorb and transport E. coli, to evaluate the influence of sediments on the intensity and penetration of UV-B radiation, to study the main characteristics of suspended sediments in certain areas of the estuary and to obtain bases to improve the description and prediction of the evolution of fecal contamination plumes in recreational and bathing waters. The work involved the sampling of suspended sediments in the Tagus estuary and the laboratory study of water characteristics and sediments sampled, an analysis of the UV-B Kd (coefficient of vertical attenuation of solar radiation) in the sampled waters and the determination of the sediment settling velocities. In the Tagus estuary, higher concentrations of E. coli were observed during low tide compared to high tide concentrations, highlighting the importance of the tide in the dynamics of the transport of microbial populations in the estuary. In the exploratory analysis, positive correlations between the concentration of E. coli and the concentration of TSS (total suspended solids) and between the concentration of TSS and the turbidity were also observed. The results obtained suggest the existence of aggregation of enteric bacteria in suspended sediments, characterized by a spatial variability associated to their characteristics, in particular, the aggregation of E. coli was verified to suspended sediments that tend to aggregate. There was also a decrease in the effect of solar radiation with higher concentrations of SST and the extinction of 90 % of UV-B radiation occurs near the surface until 1 m deep, so this effect should be less significant for the decay of bacteria in the deepest areas of the estuary. The observed differences in the structure and granulometry of suspended sediments at the sampled sites also influenced the observed velocity behavior, suggesting variations in the settling velocity along the estuary and the influence of flocculation processes in some samples. The estimated median settling velocity was 1,2 mm/s in Algés, 0,24 mm/s in Alcântara and 0,45 mm/s in Parque das Nações. The results obtained contribute to a better understanding of the influence of suspended sediments on E. coli transport, in particular in the Tagus estuary, and may contribute to the implementation of more effective monitoring plans and more reliable forecasting systems to anticipate events of fecal contamination. In order to complement, deepen and validate the results obtained further studies covering larger spatial and temporal scales and a wider characterization of the environmental parameters of water and suspended sediments would be useful in future research developments.
Descrição
Tese de mestrado em Ecologia e Gestão Ambiental, apresentada à Universidade de Lisboa, através da Faculdade de Ciências, 2017
Palavras-chave
FIB Floculação Sedimentos coesivos Velocidade de queda Radiação UV Teses de mestrado - 2017