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

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.

Methane emission via sediment and water interface in the Bohai Sea, China

Sediment is recognized as the largest reservoir and source of methane (CH₄) in the ocean, especially in the shallow coastal areas. To date, few data of CH₄ concentration in sediment have been reported in the China shelf seas. In this study, we measured CH₄ concentration in sediment and overlying seawater columns, and conducted an incubation experiment in the Bohai Sea in May 2017. CH₄ concentration was found to be ranged from 3.075 to 1.795 μmol/L in sediment, which was 2 to 3 orders of magnitude higher than that in overlying seawater columns. The surface sediment was an important source of CH₄, while bottom seawater acted as its sink. Furthermore, the net emission rate via sediment water interface (SWI) was calculated as 2.45 μmol/(m²∙day) based on the incubation experiment at station 73, and the earthquake may enhance CH₄ release from sediment to seawater column in the eastern Bohai Sea.

Ebullition enhances chemical mass transport across the tailings-water interface of oil sands pit lakes

Base Mine Lake (BML) was the first commercial-scale demonstration oil sands pit lake established in northern Alberta, Canada. Recent studies indicate that ebullition enhances internal mass loading of dissolved constituents during settlement and dewatering of methanogenic fluid fine tailings (FFT) below the overlying water cap. Here, we describe results of integrated field measurements and numerical modelling to (i) determine potential for ebullition and enhanced mixing within BML, and (ii) assess impacts on chemical mass transport across the tailings-water interface. We observed sharp increases in [CH_4(aq)] with depth from <0.1 mg L^-1 immediately above the interface to >60 mg L^-1 over the upper 1.5 to 3.0 m of FTT. Thermodynamic modelling revealed that maximum [CH_4(aq)] values represent 60 to 80% of theoretical saturation, and corresponding total dissolved gas pressures approach or exceed fluid pressures. These findings supported integration of enhanced mixing into one-dimensional (1-D) advective-dispersive transport models, which substantially improved upon previous simulations of conservative tracer (i.e., Cl^-) profiles and chemical mass fluxes. The models revealed a positive relationship between CH_4(aq) saturation and enhanced mixing, showing that ebullition enhances internal mass loading. This information has potential to inform ongoing assessments of pit lake performance and support improved closure and reclamation planning at oil sands mines.

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.

An Experimental Study on Bubble Growth in Laponite RD as Thixotropic Yield Material

The growth and release of the leading major bubble at the tip of a needle in the thixotropic yield material Laponite RD was different from subsequent minor bubbles. The gas injection experiments combined with high-speed camera were conducted. The results showed that the shape of the major bubbles transformed from an inverted carrot shape to an inverted teardrop shape, while the shape of the minor bubbles tended to be elliptical. In addition, the pressure of bubble emergence consisted of hydrostatic pressure, capillary pressure, and cracking pressure. The major and minor bubbles differed only in the cracking pressure. The pressure when the minor bubble detached could be estimated from the lateral hydrostatic pressure. It can be deduced from dimensionless numbers that buoyancy and viscous forces were, respectively, the main driving force and resistance of bubble growth. The yield stress of Laponite RD and inertial force at the initial moment resulted in distinctive behavior of the major bubble. In addition to the viscosity resistance, surface tension, and hydrostatic pressure had a non-negligible influence on minor bubbles and still accounted for 10-20% of the total resistance in the later stage but less than 5% in major bubble growth.

Reliance and effect of sediment bulking on the physicochemical properties of sediments in aquatic environments

Sediment bulking is intently related to the occurrence of black water agglomerate, sediment resuspension and erosion in aquatic environments. In this study, five different lake sediments were sampled to study effects of sediment characteristics on sediment bulking and then investigate how sediment bulking affected in turn sediment physicochemical properties. Within 30 days of experiments, the sediment properties showed an obvious influence on variation in sediment height (VSH) ranging from only 0.03 to 1.26 cm for five sediment samples. It was found that labile nutrients were closely related to the VSH (P < 0.05) during sediment bulking. In addition, the high-throughput sequencing revealed that the microbial communities in sediments associated with degradation of organic matter and anaerobic environments, were also related to sediment bulking. Through comparing sediments with and without bulking, it was found that sediment bulking would clearly increase the proportion of air around 2.14 times, and reduce the critical shear stress of sediment with a decrease by 67.33% after 30 days, which favored sediment resuspension and erosion. Thus, this study could provide a deep insight in the key factors and the environmental effects of sediment bulking, and then be helpful in protecting the aquatic environments against ecological disasters.

Sediment Microstructure and the Establishment of Gas Migration Pathways during Bubble Growth

Soft sediments exhibit complex and varied deformation behavior during in situ bubble growth; however, the sediment microstructure is often neglected when predicting bubble networking or fracture propagation dynamics. This study considers three chemically similar Mg(OH)₂-rich sediments, which differ slightly in their particle size distributions and morphologies but exhibit significant differences in their porosity, stiffness, and pore throat dimensions at equivalent yield strengths. At low yield strengths, microstructure greatly influenced the size distribution and connectivity of spherical bubble populations, with narrow sedimentary pore throats promoting coarser bubbles with diminished connectivity. Increased connectivity of the bubble population appeared highly significant in limiting bed expansion, either by establishing pathways for gas release or by dissipating excess internal bubble pressure, thereby diminishing further growth. During in situ gas generation, each sediment demonstrated a critical fracture strength, which demarcated the populations with high void fractions (0.27 < ν < 0.4) of near-spherical bubbles from a fracturing regime supporting reduced void fractions (ν ≈ 0.15) of high aspect ratio cracks. However, critical fracture strengths varied significantly (in the 60-1000 Pa range) between sediments, with coarser-grained and higher porosity sediments promoting fracture at lower strengths. Fracture propagation greatly enhanced the connectivity and diminished the tortuosity of the void networks, thereby augmenting the continuous gas release flux.