Single- and Two-Individual-Region Models for Gas Diffusivity in Structureless or Aggregated, Unsaturated Soil.

An accurate evaluation of the fate and transport of gaseous phase contaminants requires the understanding of gas diffusion coefficient (D_p) of soils as a function of soil-air content, e.  Recent studies have shown that the modality of the pore size distribution, as a result of soil structure, largely influence the magnitude of the soil-gas diffusivity (D_p/D_o, where D_o is the gas diffusion coefficient in free air) and its variation with e. In this study, flexible D_p/D_o models for one-region (structureless) and two-region (aggregated or structured) soils were presented. For one-region soils, we modified the recently-developed Water-induced Linear Reduction (WLR) factor of Moldrup et al. (2000) to become a nonlinear description of water blocking effects of connected water films on soil-gas diffusion. This Water-induced Non-linear Reduction (WNR) D_p/D_o model performs well for structureless soils ranging from sand to clay. For two-region soils, WNR factors were applied separately to a power-law model in the inter-aggregate pore space region and to a linear model in the intra-aggregate pore space region to become Two-Individual Region (TIR) D_p/D_o model. This TIR model described well the D_p/D_o of pure aggregates and highly structured soils at different compaction levels.  The impedance and tortuosity factors were observed to be almost constant in the intra-aggregate pore space region suggesting small water blocking effect that, in most cases, verified the observed linear variation of D_p/D_o with e in the intra-aggregate region. By merely using standard parameter values for the TIR D_p/D_o model and with the inter-aggregate porosity estimated as half of the total porosity, a better prediction of D_p/D_o was observed within the total range of soil-air content than the widely-used classical D_p/D_o models.