Investigation of spatial and temporal dynamics of ebullition in impoundments

Orientador: Dr.-Ing Tobias BleningerCoorientadores: D.Sc. Michael Männich e Dr. Andreas LorkeTese (doutorado) - Universidade Federal do Paraná, Setor de Tecnologia, Programa de Pós-Graduação em Engenharia de Recursos Hídricos e Ambiental e Universidade RPTU Kaiserslautern-Landau (Alemanha). Defesa : Curitiba, 18/07/2023Inclui referênciasResumo: As aguas interiores, como os reservatorios de agua doce, sao fontes significativas e variaveis do gas de efeito estufa metano para a atmosfera. Em corpos d'agua, o metano e principalmente produzido no sedimento de fundo rico em materia organica, onde o metano pode se acumular, formar vazios de gas e ser transportado para a atmosfera por bolhas que escapam do sedimento. O transporte de metano mediado por bolhas, tambem conhecido como ebulicao de metano, e uma via comumente dominante de emissoes de metano em reservatorios de agua doce. A ebulicao resulta de uma interacao complexa de varios processos fisicos e biogeoquimicos simultaneos que atuam em diferentes escalas de tempo, resultando em fluxos variaveis no espaco e no tempo. Embora a maior parte da producao e do acumulo de gas acontece no sedimento, ha uma falta de dados in situ sobre o armazenamento de gas em reservatorios e sobre a interacao entre o armazenamento de gas em sedimentos com o balanco e com a ebulicao de metano. Diversas variaveis ambientais sao determinantes na ebulicao; no entanto, ainda e um desafio simular a dinamica temporal da ebulicao e identificar os fatores determinantes em diferentes sistemas. Portanto, o principal objetivo desta tese foi investigar o efeito de diferentes fatores na regulacao da variabilidade espacial e da dinamica temporal da ebulicao do metano em reservatorios. Foram investigados dois reservatorios contrastantes, um subtropical e outro temperado. Medicoes de alta frequencia de fluxos de ebulicao, variaveis ambientais e mapeamento do gas no sedimento baseado em acustica foram realizadas em ambos os reservatorios e compoem o conjunto de dados para este estudo. Os principais resultados foram apresentados em tres manuscritos cientificos. A distribuicao espacial de gas no sedimento foi controlada principalmente pela deposicao de sedimentos e pela profundidade da agua, em que as regioes rasas com alta deposicao de sedimentos tiveram elevado acumulo de gas. As mudancas temporais na quantidade de gas no sedimento foram influenciada pelos componentes do balanco de metano no reservatorio e pela dinamica temporal da ebulicao. Embora o sedimento possa armazenar dias de producao potencial acumulada de metano, o que poderia manter meses de fluxo medio de ebulicao, os periodos de ebulicao intensificada levam a um esgotamento do gas armazenado no sedimento. Os fatores que agem em grande escala espacial e que controlam a ebulicao, como mudancas na pressao, resultaram na sincronizacao de eventos de ebulicao em diferentes locais de monitoramento. No entanto, o grau de correlacao da ebulicao com as variaveis ambientais variou de um sistema para outro e com o tempo. A estratificacao termica foi considerada um modulador importante na relacao entre a ebulicao e outras variaveis ambientais, como as correntes de fundo e a turbulencia. A dinamica temporal da ebulicao pode ser capturada e reproduzida por modelos empiricos em funcao de variaveis ambientais conhecidas; no entanto, os modelos nao conseguiram reproduzir as variabilidades subdiarias da ebulicao e apresentaram baixo desempenho quando transferidos de um sistema para outro. Por fim, embora ainda haja questoes em aberto, os resultados deste trabalho contribuiem para o avanco do conhecimento da complexa dinamica da ebulicao do metano e seus controles em reservatorios de agua doce.Abstract: Inland waters, such as freshwater impoundments, are significant and variable sources of the greenhouse gas methane to the atmosphere. In water bodies, methane is mainly produced in the organic-matter rich bottom sediment, where it can accumulate, form gas voids, and be transported to the atmosphere by gas bubbles escaping the sediment. The bubble mediated transport of methane, known as methane ebullition, is a commonly dominant pathway of methane emissions in freshwater reservoirs. Ebullition results from a complex interplay of several simultaneous physical and bio-geochemical processes acting at different timescales, leading to highly variable fluxes in both space and time. Although the sediment matrix is a hot spot for gas production and accumulation, there is a lack of in-situ data on free gas storage in reservoirs and the interaction among sediment gas storage, methane budget, and methane ebullition. Several environmental variables are known to be ebullition drivers; however, simulating the temporal dynamics of ebullition and identifying the governing factors across different systems remains challenging. Therefore, the main goal of this thesis was to investigate the effect of different drivers on the spatial variability and temporal dynamics of methane ebullition in impoundments. Two contrasting reservoirs, one subtropical and one temperate, were investigated. High-frequency measurements of ebullition fluxes and environmental variables, and acoustic-based mapping of gas content in the sediment were performed in both reservoirs, constituting the dataset for this study. The main findings were presented in three main scientific manuscripts. The spatial distribution of gas content in the sediment was primarily controlled by sediment deposition and water depth, with shallow regions of high sediment deposition were hot spots of free gas accumulation in the sediment. Temporal changes in gas content in the sediment were linked to the methane budget components in the reservoir and further influenced by the temporal dynamics of ebullition. While the sediment could store days of accumulated potential methane production, which could sustain months of mean ebullition flux, periods of intensified ebullition led to a depletion of gas stored in the sediment. Large spatial scale ebullition drivers, such as pressure changes, resulted in the synchronization of ebullition events across different monitoring sites. Nevertheless, the degree of correlation between ebullition and environmental variables varied from one system to another and over time. Thermal stratification was an important modulator in the relationship between ebullition and other environmental variables, such as bottom currents and turbulence. The temporal dynamics of ebullition could be captured and reproduced by empirical models based on known environmental variables. However, these models failed to reproduce the sub-daily variabilities of ebullition and demonstrated poor performance when transferred from one system to another. Lastly, although some questions remain unanswered, the findings from this study contribute to advancing the understanding of the complex dynamics of methane ebullition and its controls in freshwater reservoirs.ZUSAMMENFASSUNG: Binnengewasser, wie z. B. Talsperren, sind wichtige sich stetig verandernde Quellen des Treibhausgases Methan. In Gewassern wird Methan hauptsachlich in Bodensedimenten, welche reich an organischen Substanzen sind, produziert. Dort sammelt sich Methan an, bildet gasgefullte Hohlraume und wird durch aus dem Sediment entweichende Gasblasen in die Atmosphare transportiert. Der Transport von Methan in aufsteigenden Gasblasen, auch als Methan-Ebullition bezeichnet, ist ein haufig vorherrschender Mechanismus in Suswasserreservoirs. Die Ebullition resultiert aus einem komplexen Zusammenspiel mehrerer physikalischer und biogeochemischer Prozesse, die auf unterschiedlichen Zeitskalen ablaufen. Das fuhrt zu raumlich und zeitlich sehr variabeln Gasflussen. Obwohl die Sedimentmatrix ein Hotspot der Gasproduktion und -akkumulation ist, mangelt es fur Talsperren an In-situ-Daten uber die freie Gasspeicherung in Sedimenten und die Wechselwirkung zwischen Gasspeicherung, Methanbudget und Methan-Ebullition. Man weis, dass mehrere Umweltvariablen die Ebullition beeinflussen. Es ist jedoch nach wie vor schwierig, die zeitliche Dynamik der Ebullition zu simulieren und die bestimmenden Faktoren in verschiedenen Systemen zu ermitteln. Das Hauptziel dieser Arbeit bestand somit darin, die Auswirkungen verschiedener Einflussfaktoren auf die raumliche Variabilitat und die zeitliche Dynamik der Methanemissionen aus Stauseen zu untersuchen. Es wurden zwei verschiedene Stauseen, einer in der subtropischen und einer in der gemasigten Klimazone, untersucht. In beiden Stauseen wurden Hochfrequenz-Messungen der Methanflusse und der Umweltvariablen, sowie eine akustische Kartierung des Gasgehalts im Sediment durchgefuhrt, welche gemeinsam den Datensatz fur diese Studie bilden. Die wichtigsten Ergebnisse wurden in drei wissenschaftlichen Manuskripten veroffentlicht. Die raumliche Verteilung des Gasgehalts im Sediment wurde hauptsachlich durch die Sedimentablagerung und die Wassertiefe beeinflusst, wobei flache Regionen mit grosen Mengen an Sediment Hotspots der Ansammlung von freiem Gas im Sediment waren. Es wurde aufgezeigt, dass zeitliche Veranderungen des Gasgehalts im Sediment mit den verschiedenen Komponenten des Methan-Budgets der Talsperre zusammenhangen und durch die Dynamik der Ebullition beeinflusst werden. Obwohl das Sediment die potenzielle Methanproduktion von mehreren Tagen speichern konnte, was wiederum die Aufrechterhaltung eines durchschnittlichen Ebullitionsflusses uber Monate ermoglicht, fuhren Perioden verstarkter Ebullition zu einer Erschopfung des im Sediment gespeicherten Gases. Einflussfaktoren mit groser raumlicher Ausdehnung, wie z. B. Druckveranderungen, fuhrten zu einer Synchronisierung von Ebullitionsereignissen an verschiedenen Messstellen. Das Ausmas, in dem die Ebullition mit Umweltvariablen korreliert, variierte jedoch von einem System zum anderen und zeitlich. Die thermische Schichtung erwies sich als wichtiger Parameter in der Beziehung zwischen Ebullition und anderen Umweltvariablen, wie z. B. Bodenstromungen und Turbulenzen. Die zeitliche Dynamik der Ebbullition konnte durch empirische Modelle in Abhangigkeit von bekannten Umweltvariablen erfasst und reproduziert werden. Jedoch konnten die Modelle auf Schwankungen fur Zeitraume, welche kurzer als ein Tag waren, nicht angewandt werden und lieferten bei der Ubertragung zwischen unterschiedlichen Systemen schlechte Resultate. Obwohl einige Fragen unbeantwortet blieben, tragen die Ergebnisse dieser Studie zum Verstandnis der komplexen Dynamik der Methan-Ebullition und ihrer Einflussfaktoren in Talsperren bei

High temporal resolution measurement of ebullition in a subtropical reservoir

Orientador : Dr.-Ing Tobias BleningerCoorientador : D. Sc. Michael MännichDissertação (mestrado) - Universidade Federal do Paraná, Setor de Tecnologia, Programa de Pós-Graduação em Engenharia de Recursos Hídricos e Ambiental. Defesa : Curitiba, 16/02/2018Inclui referências : p. 76-80Resumo: Corpos d'água, tanto natural quanto artificial, são fontes de metano para a atmosfera. O metano é formado no sedimento de reservatórios a partir da decomposição da matéria orgânica em condições anóxicas. Apesar do metano ser liberado para a atmosfera por três processos principais: ebulição, difusão e através de plantas com raiz, a ebulição é o caminho predominante. A ebulição é um processo complexo. Depende espacialmente de fatores que atuam sobre grandes distâncias (mudanças de pressão atmosférica, incidência de ventos, entre outros), fatores que agem localmente (características do sedimento, produção de gás, biota, etc.) e varia temporalmente devido às oscilações dos parâmetros com o tempo. O objetivo principal desta pesquisa foi obter medições contínuas de ebulição em um reservatório de água localizado em Curitiba - PR, Brasil, avaliar a variabilidade temporal das emissões e investigar possíveis correlações com condições ambientais. Três armadilhas automáticas de captura de gás (ABT), que são funis invertidos que permanecem submersos na coluna d'água, foram utilizados para realizar medições contínuas do fluxo de gás com registros em intervalos de 15 segundos. Dados auxiliares foram obtidos com medições realizadas diretamente no reservatório ou a partir de estações instaladas nas proximidades da área de estudo. Os resultados obtidos pelos ABTs estiveram de acordo com medições feitas com funis convencionais. A serie temporal de dados de ebulição evidenciou a grande variabilidade temporal existente, os fluxos com menor intensidade ocorreram com maior frequência e predominando intervalo entre eventos menor que 10 minutes. As emissões variaram espacialmente, sendo os maiores fluxos registrados no ponto de média profundidade (P2). Foi observado nos fluxos uma forte variação sazonal. O fluxo médio registrado durante o verão no ponto P2 foi 16 vezes superior ao fluxo médio que ocorreu durante final do outono e início do inverno. Essas variações foram atribuídas a uma combinação de alterações das condições químicas da coluna d'água e às características de armazenamento do gás no sedimento. Além disso, fluxos de maior intensidade estiveram correlacionados à períodos de redução na pressão atmosférica e à incidência de ventos com maior intensidade. Por fim, foi mostrado que medições realizadas durante curtos períodos tendem a subestimar o fluxo médio de gás. Palavras-chaves: Variação temporal. Armadilha automática para bolhas. Emissão de metanoAbstract: Water bodies, either natural or man-made impounded areas, are sources of methane to the atmosphere. Methane can be formed in the anoxic region at bottom sediments of reservoirs by decomposition of organic matter. Although there are three main ways through which methane reach the atmosphere: ebullition flux, diffusion flux and through rooted plants, the ebullition flux is the dominant pathway. Ebullition is a complex process spatially dependent on factors acting over large distances (atmospheric pressure changes, wind, so forth) and factors acting locally (sediment characteristics, gas production, biota, etc), and temporal variable due to parameters' oscillation with time. As a consequence, the temporal resolution of measurements and their sampling time have an effect on understanding the process and in determining ebullition rates. The main objective of this research was to obtain continuous measurements of ebullition in a freshwater reservoir located in Curitiba - PR, Brazil, to evaluate its temporal variability, and to investigate possible correlation with ambient conditions. Three Automated Bubble Traps (ABT), which are inverted funnels that stay submerged in the water column, had been used for continuous measurement of gas flux in time steps of 15 seconds. Auxiliary data were obtained either through direct measurement in the reservoir or from stations nearby the study site. ABTs measurements agreed with measures from conventional funnels. The time series data showed the large temporal variability in ebullition, the less intense fluxes occurred with higher frequency and prevailing a time interval between events smaller than 10 minutes. Emissions fluxes varied spatially; the largest mean fluxes were recorded from the middle depth location (P2). A strong seasonal variation was observed in the fluxes. The mean gas emission recorded during summer time at P2 was sixteen-fold the mean flux of late fall and beginning of winter period. The variations were attributed to a combination of changes in the water column chemical conditions and sediment characteristics of gas storage. In addition, high flux events were correlated with reductions in the atmospheric pressure and increased winds intensities. Lastly, it is showed that short-period of sampling time tends to underestimate the mean gas emission flux. Key words: Temporal variability. Automated Bubble Trap. Methane emission

Acoustic Mapping of Gas Stored in Sediments of Shallow Aquatic Systems Linked to Methane Production and Ebullition Patterns

Bubble-mediated transport is the predominant pathway of methane emissions from inland waters, which are a globally significant sources of the potent greenhouse gas to the atmosphere. High uncertainties exist in emission estimates due to high spatial and temporal variability. Acoustic methods have been applied for the spatial mapping of ebullition rates by quantification of rising gas bubbles in the water column. However, the high temporal variability of ebullition fluxes can influence estimates of mean emission rates if they are based on reduced surveys. On the other hand, echo sounding has been successfully applied to detect free gas stored in the sediment, which provide insights into the spatial variability of methane production and release. In this study, a subtropical, midsize, mesotrophic drinking water reservoir in Brazil was investigated to address the spatial and temporal variability of free gas stored in the sediment matrix. High spatial resolution maps of gas content in the sediment were estimated from echo-sounding surveys. The gas content was analyzed in relation to water depth, sediment deposition, and organic matter content (OMC) available from previous studies, to investigate its spatial variability. The analysis was further supported by measurements of potential methane production rates, porewater methane concentration, and ebullition flux. The largest gas content (above average) was found at locations with high sediment deposition, and its magnitude depended on the water depth. At shallow water depth (12 m), the gas stored in the sediment is released episodically during short events. An artificial neural network model was successfully trained to predict the gas content in the sediment as a function of water depth, OMC, and sediment thickness (R$^2$ = 0.89). Largest discrepancies were observed in the regions with steep slopes and for low areal gas content (<4 L m$^{−2}$). Although further improvements are proposed, we demonstrate the potential of echo-sounding for gas detection in the sediment, which combined with sediment and water body characteristics provides insights into the processes that regulate methane emissions from inland waters

Linking Sediment Gas Storage to the Methane Dynamics in a Shallow Freshwater Reservoir

Freshwater reservoirs are globally relevant sources of the greenhouse gas methane. Organic matter rich sediments are hot spots of methane production and can store large amounts of methane dissolved in porewater and as free gas. Yet, in situ data on the gas storage as free gas (bubbles) in freshwater sediments are scarce. Here, an acoustic approach was tested and used to map the gas content in the sediment of a shallow temperate reservoir. The sediment gas storage was linked to the methane budget obtained from almost 2 years of in situ monitoring. The emission fluxes were dominated by ebullition and degassing at the reservoir outlet, which combined accounted for 93% of the total methane emissions. 66% of the ebullition variability was explained by a combination of environmental parameters. Mappings of sediment gas content using echo sounder surveys revealed the accumulation of free gas in regions of elevated sediment deposition. Temporally, the gas storage in the sediment was related to methane emissions, in which a period of intensified emissions resulted in a reduction of sediment gas storage. The sediment could store an equivalent of 4 to 13 days of accumulated potential methane production, which could supply the mean ebullition flux for more than 2 months. We suggest that sediment gas storage plays an important role in buffering and modulating methane emissions in aquatic systems and need to be accounted for in process-based models

Investigation of spatial and temporal dynamics of ebullition in impoundments

Inland waters, such as freshwater impoundments, are significant and variable sources of the greenhouse gas methane to the atmosphere. In water bodies, methane is mainly produced in the organic-matter rich bottom sediment, where it can accumulate, form gas voids, and be transported to the atmosphere by gas bubbles escaping the sediment. The bubble mediated transport of methane, known as methane ebullition, is a commonly dominant pathway of methane emissions in freshwater reservoirs. Ebullition results from a complex interplay of several simultaneous physical and bio-geochemical processes acting at different timescales, leading to highly variable fluxes in both space and time. Although the sediment matrix is a hot spot for gas production and accumulation, there is a lack of in-situ data on free gas storage in reservoirs and the interaction among sediment gas storage, methane budget, and methane ebullition. Several environmental variables are known to be ebullition drivers; however, simulating the temporal dynamics of ebullition and identifying the governing factors across different systems remains challenging. Therefore, the main goal of this thesis was to investigate the effect of different drivers on the spatial variability and temporal dynamics of methane ebullition in impoundments. Two contrasting reservoirs, one subtropical and one temperate, were investigated. High-frequency measurements of ebullition fluxes and environmental variables, and acoustic-based mapping of gas content in the sediment were performed in both reservoirs, constituting the dataset for this study. The main findings were presented in three main scientific manuscripts. The spatial distribution of gas content in the sediment was primarily controlled by sediment deposition and water depth, with shallow regions of high sediment deposition were hot spots of free gas accumulation in the sediment. Temporal changes in gas content in the sediment were linked to the methane budget components in the reservoir and further influenced by the temporal dynamics of ebullition. While the sediment could store days of accumulated potential methane production, which could sustain months of mean ebullition flux, periods of intensified ebullition led to a depletion of gas stored in the sediment. Large spatial scale ebullition drivers, such as pressure changes, resulted in the synchronization of ebullition events across different monitoring sites. Nevertheless, the degree of correlation between ebullition and environmental variables varied from one system to another and over time. Thermal stratification was an important modulator in the relationship between ebullition and other environmental variables, such as bottom currents and turbulence. The temporal dynamics of ebullition could be captured and reproduced by empirical models based on known environmental variables. However, these models failed to reproduce the sub-daily variabilities of ebullition and demonstrated poor performance when transferred from one system to another. Lastly, although some questions remain unanswered, the findings from this study contribute to advancing the understanding of the complex dynamics of methane ebullition and its controls in freshwater reservoirs.Binnengewässer, wie z. B. Talsperren, sind wichtige sich stetig verändernde Quellen des Treibhausgases Methan. In Gewässern wird Methan hauptsächlich in Bodensedimenten, welche reich an organischen Substanzen sind, produziert. Dort sammelt sich Methan an, bildet gasgefüllte Hohlräume und wird durch aus dem Sediment entweichende Gasblasen in die Atmosphäre transportiert. Der Transport von Methan in aufsteigenden Gasblasen, auch als Methan-Ebullition bezeichnet, ist ein häufig vorherrschender Mechanismus in Süßwasserreservoirs. Die Ebullition resultiert aus einem komplexen Zusammenspiel mehrerer physikalischer und biogeochemischer Prozesse, die auf unterschiedlichen Zeitskalen ablaufen. Das führt zu räumlich und zeitlich sehr variabeln Gasflüssen. Obwohl die Sedimentmatrix ein Hotspot der Gasproduktion und -akkumulation ist, mangelt es für Talsperren an Insitu-Daten über die freie Gasspeicherung in Sedimenten und die Wechselwirkung zwischen Gasspeicherung, Methanbudget und Methan-Ebullition. Man weiß, dass mehrere Umweltvariablen die Ebullition beeinflussen. Es ist jedoch nach wie vor schwierig, die zeitliche Dynamik der Ebullition zu simulieren und die bestimmenden Faktoren in verschiedenen Systemen zu ermitteln. Das Hauptziel dieser Arbeit bestand somit darin, die Auswirkungen verschiedener Einflussfaktoren auf die räumliche Variabilität und die zeitliche Dynamik der Methanemissionen aus Stauseen zu untersuchen. Es wurden zwei verschiedene Stauseen, einer in der subtropischen und einer in der gemäßigten Klimazone, untersucht. In beiden Stauseen wurden Hochfrequenz-Messungen der Methanflüsse und der Umweltvariablen, sowie eine akustische Kartierung des Gasgehalts im Sediment durchgeführt, welche gemeinsam den Datensatz für diese Studie bilden. Die wichtigsten Ergebnisse wurden in drei wissenschaftlichen Manuskripten veröffentlicht. Die räumliche Verteilung des Gasgehalts im Sediment wurde hauptsächlich durch die Sedimentablagerung und die Wassertiefe beeinflusst, wobei flache Regionen mit großen Mengen an Sediment Hotspots der Ansammlung von freiem Gas im Sediment waren. Es wurde aufgezeigt, dass zeitliche Veränderungen des Gasgehalts im Sediment mit den verschiedenen Komponenten des Methan-Budgets der Talsperre zusammenhängen und durch die Dynamik der Ebullition beeinflusst werden. Obwohl das Sediment die potenzielle Methanproduktion von mehreren Tagen speichern könnte, was wiederum die Aufrechterhaltung eines durchschnittlichen Ebullitionsflusses über Monate ermöglicht, führen Perioden verstärkter Ebullition zu einer Erschöpfung des im Sediment gespeicherten Gases. Einflussfaktoren mit großer räumlicher Ausdehnung, wie z. B. Druckveränderungen, führten zu einer Synchronisierung von Ebullitionsereignissen an verschiedenen Messstellen. Das Ausmaß, in dem die Ebullition mit Umweltvariablen korreliert, variierte jedoch von einem System zum anderen und zeitlich. Die thermische Schichtung erwies sich als wichtiger Parameter in der Beziehung zwischen Ebullition und anderen Umweltvariablen, wie z. B. Bodenströmungen und Turbulenzen. Die zeitliche Dynamik der Ebbullition konnte durch empirische Modelle in Abhängigkeit von bekannten Umweltvariablen erfasst und reproduziert werden. Jedoch konnten die Modelle auf Schwankungen für Zeiträume, welche kürzer als ein Tag waren, nicht angewandt werden und lieferten bei der Übertragung zwischen unterschiedlichen Systemen schlechte Resultate. Obwohl einige Fragen unbeantwortet blieben, tragen die Ergebnisse dieser Studie zum Verständnis der komplexen Dynamik der Methan-Ebullition und ihrer Einflussfaktoren in Talsperren bei