ALBERT

Description: In porous media, the nonwetting phase is trapped on water saturation due to capillary forces acting in a heterogeneous porous structure. Within the capillary fringe, the gas phase is trapped and released along with the fluctuation of the water table, creating a highly active zone for biological transformations and mass transport. We conducted column experiments to observe and quantify the magnitude and structure of the trapped gas phase at the pore scale using computed microtomography. Different grain size distributions of glass beads were used to study the effect of the pore structure on trapping at various capillary numbers. Viscous forces were found to have negligible impact on phase trapping compared with capillary and buoyancy forces. Residual gas saturations ranged from 0.5 to 10%, while residual saturation increased with decreasing grain size. The gas phase was trapped by snap-off in single pores but also in pore clusters, while this single-pore trapping was dominant for grains larger than 1 mm in diameter. Gas surface area was found to increase linearly with increasing gas volume and with decreasing grain size.

Description: Processes in capillary fringes (CFs) have a complex nature due to the interactions between the solid, liquid, and gaseous environments. Despite a considerable body of literature on CFs coming from different disciplines, the ongoing processes and their complex interactions are yet only partially understood.

Description: Water flow and solute transport in unsaturated porous media are affected by the highly nonlinear material properties and nonequilibrium effects. This makes experimental procedures and modeling of water flow and solute transport challenging. In this study, we present an extension to the well-known multistep-outflow (MS-O) and the newly introduced multistep-flux (MS-F) approaches to measure solute dispersivity as a function of water content under well-defined conditions (i.e., constant pressure head and uniform water content). The new approach is termed multistep-transport (MS-T) and complements the MS-O and MS-F approaches. Our setup allows for applying all three approaches in a single experimental setting. Hence, it provides a comprehensive data set to parameterize unsaturated flow and transport processes in a consistent way. We demonstrate this combined approach (MS-OFT) for sand (grain diameter: 0.1–0.3 mm) and complemented the experimental results with an analysis of the underlying pore structure using X-ray computed tomography (CT). The results show that dispersivity is a nonlinear function of water content, and a critical water content (0.2) exists at which dispersivity increased significantly. The results could be explained by marked change in the geometry of the flow field as derived from X-ray CT measurements. It is characterized by a reduced connectivity of the water phase. The results demonstrate the potential of a combined approach linking pore structure, hydraulic functions, and transport characteristic.

Description: The remarkable complexity of soil and its importance to a wide range of ecosystem services presents major challenges to the modeling of soil processes. Although major progress in soil models has occurred in the last decades, models of soil processes remain disjointed between disciplines or ecosystem services, with considerable uncertainty remaining in the quality of predictions and several challenges that remain yet to be addressed. First, there is a need to improve exchange of knowledge and experience among the different disciplines in soil science and to reach out to other Earth science communities. Second, the community needs to develop a new generation of soil models based on a systemic approach comprising relevant physical, chemical, and biological processes to address critical knowledge gaps in our understanding of soil processes and their interactions. Overcoming these challenges will facilitate exchanges between soil modeling and climate, plant, and social science modeling communities. It will allow us to contribute to preserve and improve our assessment of ecosystem services and advance our understanding of climate-change feedback mechanisms, among others, thereby facilitating and strengthening communication among scientific disciplines and society. We review the role of modeling soil processes in quantifying key soil processes that shape ecosystem services, with a focus on provisioning and regulating services. We then identify key challenges in modeling soil processes, including the systematic incorporation of heterogeneity and uncertainty, the integration of data and models, and strategies for effective integration of knowledge on physical, chemical, and biological soil processes. We discuss how the soil modeling community could best interface with modern modeling activities in other disciplines, such as climate, ecology, and plant research, and how to weave novel observation and measurement techniques into soil models. We propose the establishment of an international soil modeling consortium to coherently advance soil modeling activities and foster communication with other Earth science disciplines. Such a consortium should promote soil modeling platforms and data repository for model development, calibration and intercomparison essential for addressing contemporary challenges.