Ye Ma

Coastline changes in estuarine areas can result from a combination of natural processes such as erosion, sedimentation, and sea-level rise, as well as human activities, including urbanization and infrastructure development. These changes have the potential to affect the local environment, including submarine groundwater discharge, wetlands, and navigation routes. The Yalu River Estuary (YRE), situated on the border between China and North Korea, has been experiencing significant changes in its coastline over recent years. This study aims to investigate the coastline dynamics in the YRE from 2000 to 2020. The study employs Landsat 5/7/8 satellite data and proposes a modified Normalized Difference Water Index (NDWI) to accurately delineate the coast boundary, particularly in areas with extensive tidal flats like the YRE. The research findings indicate that from 2010 to 2020, significant changes occurred in the YRE shoreline, with erosion being the dominant trend. Human activities and alternations in hydrological conditions are important factors affecting the YRE coastline changes, contributing to the formation of distinctive spatiotemporal patterns. An extreme flooding event in the year 2010 also altered the inner estuarine coastline in the YRE, indicating the impact of strong natural drivers. Findings from this study provide a comprehensive understanding of the evolving coastal environment, considering natural and anthropogenic drivers, and highlight the importance of continuous monitoring in a region of ecological and geopolitical significance.

This work investigated the bubble size variation under various aqueous conditions, including saline and surfactant solutions, with different gas injection rates using a commercial bubble analyzer. The results show that salt and surfactant can minimize bubble size with increasing solute concentration, and the surfactant outperforms salt. In addition, the critical coalescence concentrations (CCC) of salt and surfactant could be determined at 20 g/l and 20 mg/l, respectively, over which bubble mean size cannot be further reduced. On the other hand, the gas injection rate, compared to the solute concentration, has minor effects on bubble size variation. Nonetheless, there is a critical coalescence injection rate (CCIR) of 30 ml/min for surfactant solution, over which the standard deviation of the bubble size distribution (BSD) cannot be further increased. In principle, this work improves the accuracy and efficiency of the bubble analyzer. It also presents a sound understanding of two influential factors rather than a single-factor controlling bubble size. Most importantly, it is the first time to observe and propose the concept of CCIR to describe how the gas injection rate influences the standard deviation of BSD. Based on those results and findings, it is able to conclude that bubble size control, whose mechanism has been previously identified as the bubble collision and coalescence rather than the surface tension of water solutions, is actually dominated by not only the solute concentration but also the gas injection rate when a porous air sparger is used to generate bubbles. It is expected that this work could contribute to laboratory modeling bubbly flow in a porous medium in order to bring more insights into the mechanism of soil gas leaking through soil strata.

The coal seam gas industry has raised public concerns about the potential risk of groundwater contamination, where gases leaked from coal seams are thought to pollute groundwater. However, the basic principles and controlling parameters for gas seepage from deep ground formations to the ground surface have not been fully understood. As a possible mechanism for gas transport in the subsurface environment, discrete bubble flow was previously investigated using laboratory experiments by Ma et al. (Water Resour. Res, 2015, 51 (6), 4359–4373). This study developed a multiphase computational fluid dynamic (CFD) model to simulate discrete bubbly flow in a two-dimensional granular porous media at the pore scale. Following the experimental setup from Ma et al. (Water Resour. Res, 2015, 51 (6), 4359–4373), a “point source” with preset bubble fluxes was specified in a simulating domain representing the flume size in the earlier experiments. There were around 7,000 granular particles within this domain to model the porous media. This numerical model was validated by conserving the gas mass in the simulating domain. The simulation results provide more physical insights into complex bubble transport behaviour in porous media through specific plume parameters. The breakthrough time of the bubble plume and the cross-sectional averaged velocity of ambient pore water flow were manifested to be proportional to the gas release rates in the logarithmic scales. Also, the bubble plume width was also observed to be proportional to the gas release rates. Moreover, the gas distribution on the top boundary could be observed. The outcomes were further tested against the scaling solutions proposed by Ma et al. (Water Resour. Res, 2015, 51 (6), 4359–4373) with disagreements. The limitations of this multiphase computational fluid dynamic model were finally discussed.

With the presence of highly permeable pathways, such as faults and fractures zones, coal seam gases, particularly CO₂, could potentially migrate upwardly from the coal deposits into the shallow subsurface and then to the atmosphere. This letter reports soil gas mapping and gamma ray survey in coal basin of Hunter River Valley, Australia. The survey facilitated the delineation of fault structures across the sampling regions, where the identified faults were confirmed by an independent drilling investigation later. Furthermore, to evaluate the gas emission fluxes from coalbeds through fault zones, the measured CO₂ concentrations, coupled with an inverse modeling, enable the estimation of the width of the fault zone and associated CO₂ emission flux in the range of 2 × 10⁻⁵–6 × 10⁻⁵ mol/m²/s at the study site. Our new approach provides a way to determine emissions of gases from deep formations, which may contribute considerably to the greenhouse gases cycles.