Linear Model to Predict Soil-Gas Diffusivity from Two Soil-Water Retention Points in Unsaturated Volcanic Ash Soils

Risk assessment and design of remediation methods at soil sites polluted with gaseous phase contaminant require an accurate description of soil-gas diffusion coefficient (Dp) which is typically governed by the variations in soil air-filled porosity (va). For undisturbed volcanic ash soils, recent studies have shown that a linear Dp(va) model, taking into account inactive air-filled pore space (threshold soil-air content, va, th), captured the Dp data across the total soil moisture range from wet to completely dry conditions. In this study, we developed a simple, easy to apply, and still accurate linear Dp(va) model for undisturbed volcanic ash soils. The model slope C and intercept (interpreted as va, th) were derived using the classical Buckingham (1904) Dp(va) power-law model, vaX, at two soil-water matric potentials of pF 2 (near field capacity condition) and pF 4.1 (near wilting point condition), and assuming the same value for the Buckingham exponent (X=2.3) in agreement with measured data. This linear Dp(va) prediction model performed better than the traditionally-used non-linear Dp(va) models, especially at dry soil conditions, when tested against several independent data sets from literature. Model parameter sensitivity analysis on soil compaction effects showed that a decrease in slope C and va, th due to uniaxial reduction of air-filled pore space in between aggregates markedly affects the magnitude of soil-gas diffusivity. We recommend the new Dp(va) model using only the soil-air contents at two soil-water matric potential conditions (field capacity and wilting point) for a rapid assessment of the entire Dp-va function.