Compaction and Soil Type Effects On Gas Diffusivity and Air Permeability in Vadose Zone Profiles.

Accurate prediction of gas diffusivity (D_p/D_o) and air permeability (k_a) and their variations with air-filled porosity (ε) in soil are critical  for simulating the migration and fate of gaseous phase compounds, for example in relation to climate gas emissions or uptake in soil and  indoor and outdoor air pollution at contaminated sites. Soil type/texture and soil compaction are two important parameters controlling gas movement in soils. This study investigates the effect of soil type and compaction on D_p/D_o and k_a and introduces two new compaction-corrected predictive models. We used D_p/D_o and k_a measurements from Danish soil profile data (total of 150 undisturbed soil samples) representing different soil types/textures and compaction levels (total porosities). The measurements were within a given range of matric potentials (-10 to -500 cm H₂O) typically representing natural field conditions in subsurface soil. Data were regrouped into 4 categories based on soil compaction (total porosity (Φ) <0.4 and Φ >0.4 m³ m^-3) and soil texture (volume-based percentage of clay, silt and organic matter (CSOvol) >15% and CSOvol <15%).Results revealed that both gas diffusivity and air permeability significantly depend on soil compaction. We developed a new D_p(ε)/D_o model that is generalized from a previous macroporosity (ε₁₀₀,  air-filled porosity at -100 cm H₂O)-dependant model developed specifically for D_p/D_o at -100 cm H₂O of matric potential. The model was further modified for compaction effects and performed well across different soil types, moisture conditions (pF 1-3) and compaction levels when compared to existing models. A similar compaction-corrected model was introduced to estimate reference-point air permeability (k_a at -100 cm H₂O), and combined with a power law model with an exponent linked to the exponent of gas diffusivity. A test against independent D_p/D_o and k_a data was promising and suggests that we have taken a step towards a unified compaction-corrected model concept for gas diffusivity and air permeability in undisturbed, variably saturated soils.