Variable Pore Connectivity Model Linking Gas Diffusivity and Air-Phase Tortuosity to Soil Matric Potential

Soil-gas diffusivity (D_p/D_o) and its dependency on soil matric potential (ψ) is important when taking regulative measures (based on accurate predictions) for climate gas emissions and also risk-mitigating measures (based on upper-limit predictions) of gaseous-phase contaminant emissions. Useful information on soil functional pore structure, e.g., pore network tortuosity and connectivity, can also be revealed from D_p/D_o–ψ relations. Based on D_p/D_o measurements in a wide range of soil types across geographically remote vadose zone profiles, this study analyzed pore connectivity for the development of a variable pore connectivity factor, X, as a function of soil matric potential, expressed as pF (=log |−ψ|), for pF values ranging from 1.0 to 3.5. The new model takes the form of X = X* (F/F*)^A with F = 1 + pF⁻¹, where X* is the pore network tortuosity at reference F (F*) and A is a model parameter that accounts for water blockage. The X–pF relation can be linked to drained pore size to explain the lower probability of the larger but far fewer air-filled pores at lower pF effectively interconnecting and promoting gas diffusion. The model with X* = 2 and A = 0.5 proved promising for generalizing D_p/D_o predictions across soils of wide geographic contrast and yielded results comparable to those from widely used predictive models. The X–pF model additionally proved valuable for differentiating between soils (providing a unique soil structural fingerprint for each soil layer) and also between the inter- and intraaggregate pore regions of aggregated soils. We further suggest that the new model with parameter values of X* = 1.7 and A = 0 may be used for upper limit D_p/D_o predictions in risk assessments of, e.g., fluxes of toxic volatile organics from soil to indoor air at polluted soil sites.