Simulating the Gas Diffusion Coefficient in Macropore Network Images: Influences of Soil Pore Morphology

Authors: LIU, GANG - CHINA AGRI UNIVERSITY; LI, BAOGUO - CHINA AGRI UNIVERSITY; HU, KELIN - CHINA AGRI UNIVERSITY; Van Genuchten, Martinus

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/20/2006
Publication Date: 6/21/2006
Citation: Liu, G., Li, B., Hu, K., Van Genuchten, M.T. 2006. Simulating the Gas Diffusion Coefficient in Macropore Network Images: Influences of Soil Pore Morphology. Soil Science Society of America Journal. Vol 70:1252-1261

Interpretive Summary: Gaseous diffusion controls many transport phenomena in soils, such as soil aeration, fumigant emissions, volatilization of volatile organic chemicals from contaminated sites, and the sequestration or emission of greenhouse gases such as methane into or from soils. Knowledge of diffusion is critical to developing accurate predictive models for gas flow in porous media, and to improving our understanding of the basic transport processes involved. The gasous diffusion coefficient (D) in soils, which determines the overall rate of diffusion, has long been correlated empirically with porosity or the soil air content. Unfortunately, D is known to be affected also by other parameters including especially pore structure and soil type. To investigate the effects of pore morphology on D, we carried out a large number of numerical simulation of diffusion in idealized soil pore systems estimated from digitized images of soil thin sections. Results of the pore-scale simulations confirm that the diffusion coefficient is not a unique function of porosity, but depends also on pore structure (especially macroporosity) as well as the direction of the concentration gradient leading to gaseous diffusion, especialy for very heterogeneous (macroporous or fractured) media . The calculations furthermore indicate that accurate measurements of the diffusion coefficient in the laboratory requires careful packing of soil cores, and that relatively large samples should be used (this to minimize boundary and heterogeneity effects). If small cores are to be used, the diffusion experiments may need to be carried out with concentration gradients in different directions. Results of this study are important for improved predictive modeling of gasous transport in unsaturated soils.

Technical Abstract: Knowledge of the diffusion coefficient is necessary for modeling gas transport in soils and other porous media. This study was conducted to determine the relationship between the diffusion coefficient and pore structure parameters, such as the fractal dimension of pores (Dmp), the shortest path length through the medium, and the fractal dimension of the shortest path (Dmin). The finite element method was used to simulate the gaseous diffusion process in an idealized soil system with a highly connected macropore network. The analysis was performed on binary images of soil thin sections. We show that the ratio of the diffusion coefficient in soil to that in free air is a function of not only the air-filled porosity, but also of other parameters, and hence no universal relationship exists between the ratio and Dmp and Dmin. Furthermore, the ratio is shown to be strongly related to the pore-space structure and the direction of the concentration gradient. The tortuosity furthermore was found to be related to the weighted path length along the main diffusion direction.