Methane gas dynamics in sediments of Lake Kinneret, Israel, and their controls: Insights from a multiannual acoustic investigation and correlation analysis

Methane (CH4) is the simplest and most common hydrocarbon in nature. CH4 gas content is accommodated in discrete bubbles in shallow aquatic sediments. The bubble dynamics there are controlled by a diversity of physical, mechanical and biogeochemical processes that vary spatially and temporally over the aquatic ecosystem. Previous studies explored these controls on gas dynamics in shallow aquatic sediments mostly separately, despite of their coupled nature. In this study, a multiannual (2015-2021) acoustic database on gas content in sediments of Lake Kinneret, Israel is compiled. Gas content is evaluated by acoustic applications based on the sound speed inferred from the reflection coefficient. A multivariate linear regression is fitted and a closed form expression of gas content dependence on the following predictors, which change spatially and temporally over the lake, is obtained: 1) water depth; 2) short-leaving CH4 production rate peaks fueled by punctuated phytoplankton bloom crashes; and 3) CH4 bubble dissolution rates. Our comprehensive multidisciplinary analysis indicates that short-leaving CH4 production peaks act as major controls on sediment gas content in Lake Kinneret, where the hydrodynamic regime and sloping bottom transport the autochthonous organic matter toward the profundal lake zone. In contrast, the water depth predictor has the least significance, which is explained mainly by lack of ebullition in the deepest part of the lake. Our novel process-based correlation analysis enables quantification and prediction of gas content dynamics in sediments of Lake Kinneret under changing spatial and temporal conditions. Our modeling could be extended to other marine and lacustrine ecosystems with different predictors and temporal variability. Predicting CH4 gas content dynamics is important for accurate evaluation and even reduction of a long-persisting uncertainty related to CH4 flux from aquatic sediments and for assessment of sediment load-bearing capabilities affected by gas presence.

Radon transport carried by geogas: prediction model

This paper provides an overview and information on radon migration in the crust. In the past several decades, numerous studies on radon migration have been published. However, there is no there is no comprehensive review of large-scale radon transport in the earth crust. A literature review was conducted to present the research on the mechanism of radon migration, geogas theory, investigation of multiphase flow, and modeling method of fractures. Molecular diffusion was long considered the primary mechanism for radon migration in the crust. However, a molecular diffusion mechanism cannot explain the understanding of anomalous radon concentrations. In contrast with early views, the process of radon migration and redistribution within the Earth may be determined by geogas (mainly CO2 and CH4). Microbubbles rising in fractured rocks may be a rapid and efficient way of radon migration, as reported by recent studies. All these hypotheses on the mechanisms of geogas migration are summarized into a theoretical framework, defined as "geogas theory." According to geogas theory, fractures are the principal channel of gas migration. The development of the discrete fracture network (DFN) method is expected to supply a new tool for fracture modeling. It is hoped that this paper will contribute to a deeper understanding of radon migration and fracture modeling.

Gas ebullition from petroleum hydrocarbons in aquatic sediments: A review

Gas ebullition in sediment results from biogenic gas production by mixtures of bacteria and archaea. It often occurs in organic-rich sediments that have been impacted by petroleum hydrocarbon (PHC) and other anthropogenic pollution. Ebullition occurs under a relatively narrow set of biological, chemical, and sediment geomechanical conditions. This process occurs in three phases: I) biogenic production of primarily methane and dissolved phase transport of the gases in the pore water to a bubble nucleation site, II) bubble growth and sediment fracture, and III) bubble rise to the surface. The rate of biogenic gas production in phase I and the resistance of the sediment to gas fracture in phase II play the most significant roles in ebullition kinetics. What is less understood is the role that substrate structure plays in the rate of methanogenesis that drives gas ebullition. It is well established that methanogens have a very restricted set of compounds that can serve as substrates, so any complex organic molecule must first be broken down to fermentable compounds. Given that most ebullition-active sediments are completely anaerobic, the well-known difficulty in degrading PHCs under anaerobic conditions suggests potential limitations on PHC-derived gas ebullition. To date, there are no studies that conclusively demonstrate that weathered PHCs can alone drive gas ebullition. This review consists of an overview of the factors affecting gas ebullition and the biochemistry of anaerobic PHC biodegradation and methanogenesis in sediment systems. We next compile results from the scholarly literature on PHCs serving as a source of methanogenesis. We combine these results to assess the potential for PHC-driven gas ebullition using energetics, kinetics, and sediment geomechanics analyses. The results suggest that short chain <C₁₀ alkanes are the only PHC class that alone may have the potential to drive ebullition, and that PHC-derived methanogenesis likely plays a minor part in driving gas ebullition in contaminated sediments compared to natural organic matter.

Methane Bubble Ascent within Fine-Grained Cohesive Aquatic Sediments: Dynamics and Controlling Factors

Methane (CH₄) is a potent greenhouse gas. Its release from aquatic sediments to the water column and potentially to the atmosphere, is a subject of great concern. A coupled macroscopic single-bubble mechanical/reaction-transport numerical model was used to explore the ascent of a mature CH₄ bubble toward the seafloor in muddy aquatic sediment. Two bubble ascent scenarios were demonstrated: stable and dynamic. For small effective overburden loads (≤11 kPa), stable ascent is followed by dynamic ascent (which has not been previously demonstrated to the best of the our knowledge). This ultimately leads to the bubble being released to the water column. Higher effective overburden loads induce only stable bubble ascent, which stops at the gas horizon frequently observed below the seafloor. The depth of the gas horizon increases, while bubble rise velocity decreases with an increase in the overburden load. It is shown that the bubble migration scenario is managed predominantly by inner bubble pressure, which defines a bubble solute exchange with ambient porewaters. Predicting a bubble ascent scenario in muddy sediment will further allow estimation of CH₄ emission to the atmosphere and evaluation of changes in the effective mechanical properties of aquatic sediment due to the ascending bubbles.

High-frequency measurements of gas ebullition in a Brazilian subtropical reservoir-identification of relevant triggers and seasonal patterns

Water bodies, either natural or constructed impoundments, are sources of methane to the atmosphere, in which ebullition is frequently mentioned to be the dominant pathway. Ebullition is a complex process that is spatially dependent on factors acting over large distances (atmospheric pressure changes, wind) and factors acting locally (sediment characteristics, gas production) and is temporally variable due to the parameters' oscillation with time. Its quantification through measurements is still limited, as is the identification of production processes and triggers for ebullition. This research focused on obtaining high temporal resolution measurements of gas ebullition from a water supply reservoir located in Brazil, to compare its temporal variability with changes in reservoir conditions, and obtain insights on its spatial patterns. Three automated bubble traps were deployed in the reservoir and measured gas flux from February 2017 to March 2018. The time series data showed a large temporal variability in ebullition. Less intense fluxes occurred with higher frequency, and short-duration events made a larger contribution to the total amount of gas emitted. A strong seasonal variation was observed, in which the mean flux recorded during periods when the reservoir was stratified was 2-16 fold the bubbling rates recorded during colder months and mixed water column. In addition, high flux events were correlated with decreasing atmospheric pressure and increased wind intensities. Lastly, we show that the mean gas emission flux tends to be underestimated during short sampling periods (probability > 41% for sampling periods shorter than 10 days).

Methane Bubble Growth and Migration in Aquatic Sediments Observed by X-ray μCT

Methane bubble formation and transport is an important component of biogeochemical carbon cycling in aquatic sediments. To improve understanding of how sediment mechanical properties influence bubble growth and transport in freshwater sediments, a 20-day laboratory incubation experiment using homogenized natural clay and sand was performed. Methane bubble development at high resolution was characterized by μCT. Initially, capillary invasion by microbubbles (<0.1 mm) dominated bubble formation, with continued gas production (4 days for clay; 8 days for sand), large bubbles formed by deforming the surrounding sediment, leading to enhanced of macropore connectivity in both sediments. Growth of large bubbles (>1 mm) was possible in low shear yield strength sediments (<100 Pa), where excess gas pressure was sufficient to displace the sediment. Lower within the sand, higher shear yield strength (>360 Pa) resulted in a predominance of microbubbles where the required capillary entry pressure was low. Enhanced bubble migration, triggered by a controlled reduction in hydrostatic head, was observed throughout the clay column, while in sand mobile bubbles were restricted to the upper 6 cm. The observed macropore network was the dominant path for bubble movement and release in both sediments.

Non-Fickian diffusion and the accumulation of methane bubbles in deep-water sediments

In the absence of fractures, methane bubbles in deep-water sediments can be immovably trapped within a porous matrix by surface tension. The dominant mechanism of transfer of gas mass therefore becomes the diffusion of gas molecules through porewater. The accurate description of this process requires non-Fickian diffusion to be accounted for, including both thermal diffusion and gravitational action. We evaluate the diffusive flux of aqueous methane considering non-Fickian diffusion and predict the existence of extensive bubble mass accumulation zones within deep-water sediments. The limitation on the hydrate deposit capacity is revealed; too weak deposits cannot reach the base of the hydrate stability zone and form any bubbly horizon.

Methane production and ebullition in a shallow, artificially aerated, eutrophic temperate lake (Lake Elsinore, CA)

Methane is an important component of the gases released from lakes. Understanding the factors influencing the release is important for mitigating this greenhouse gas. The volume of methane (CH4) and other gases in sediments, and the rate of CH4 ebullition, were determined for an artificially aerated, shallow, eutrophic freshwater lake in Southern California. Gas volume was measured at 28 sites in July 2010, followed by monthly sampling at 7 sites through December 2011. Gas volumes measured in July 2010 at the 28 sites exhibited a complex dependence on sediment properties; the volume of CH4 and other gases was negligible in very coarse-textured sediment with low water and organic carbon contents. Gas volumes increased strongly with increased silt content, and were highest in sediments with intermediate water contents (60 to 70%), organic carbon contents (2 to 3%) and depths (approximately 4m). Methane was the dominant gas collected from sediment (80 to 90%), while carbon dioxide comprised roughly 2 to 3% of sediment gas in the lake. Gas sampling during cool winter months revealed very low or undetectable volumes of gas present, while sediment gas volumes increased markedly during the spring and early summer months, and then declined in late summer and fall. The rate of CH4 ebullition, quantified with an echosounder, also varied markedly across the lake and seasonally. High rates of ebullition were measured at all 7 sites in July 2011 (up to 96mmolCH4m(-2)d(-1)), while the rates were >50% lower in September and negligible in December 2010. Ebullition rates were inversely correlated with depth and most other sediment properties, but strongly positively correlated with sand content. No simple relationship between ebullition rate and sediment gas volume across the set of sites was found, although ebullition rates at individual sites were strongly related to gas volume.