Predicting the Soil-Gas Diffusion Coefficient: Universal Water-Induced Linear Reduction (U-WLR) Model for Repacked and Intact Soil.

The soil-gas diffusion coefficient (Dp) is a major control of transport, reactions, emissions, and uptake of vadose zone gases, including oxygen, greenhouse gases, applied fumigants, and spilled volatile organics. The Dp depends on soil moisture, texture, aggregation, compaction and, not at least, on the local-scale variability of all of these. This likely explains why different predictive models have been developed and used for Dp in intact and repacked soil, respectively. In this study, the model exponent of the frequently used Water-Induced Linear reduction (WLR) model for Dp/Do (where Do is the gas diffusion coefficient in free air) was modified with a porosity term including a coefficient of local-scale (sample-scale) complexity and heterogeneity (Cm). With Cm = 1, the universal WLR model (U-WLR) accurately predicted gas diffusivity (Dp/Do, where Do is the gas diffusion coefficient in free air) in repacked soils with between 0 and 54% clay. With Cm = 2, the model on the average gave excellent predictions for 287 intact soils grouped into 3 data bases, hereunder performed well for sub-groupings with respect to soil depth, texture, and compaction (density). In general, the U-WLR model outperformed similar Dp/Do models also depending only on total and air-filled porosity. Finally, Cm values for high-organic materials like peat, and possible extension of the U-WLR model to highly bimodal (e.g. aggregated) soils are discussed.  Representing both repacked and intact soil conditions well, the U-WLR model is recommended instead of the commonly used WLR and Millington and Quirk type models for predicting gas transport and fate in soil, with Cm = 1 for repacked soil and Cm = 2 for intact soil. Additionally, for risk assessment and uncertainty analyses of soil-gas transport, the U-WLR model with Cm = 1 and 3, respectively, represent likely upper- and lower-limit Dp/Do predictions for intact soil.