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 (D-p) which is typically governed by the variations in soil air-filled porosity (nu(a)). For undisturbed volcanic ash soils, recent studies have shown that a linear D-p(nu(a)) model, taking into account inactive air-filled pore space (threshold soil-air content, nu(a), (th)), captured the D-p 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 D-p(nu(a)) model for undisturbed volcanic ash soils. The model slope C and intercept (interpreted as nu(a), (th) were derived using the classical Buckingham (1904) D-p(nu(a)) power-law model, nu(x)(a), 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 D-p(nu(a)) prediction model performed better than the traditionally-used non-linear D-p(nu(a)) 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 nu(a), (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 D-p(nu(a)) 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 D-p-nu(a) function.