Relating the Air Permeability of Mayon Sand with Its Soil-Water Retention Curve: Measurements and Modeling.

Air permeability (k_a) and soil-water retention curve are two important parameters that are needed when simulating the movement of volatile organic chemicals in the subsurface. In this study, we measured the air permeability and soil-water retention curve of sand obtained within the vicinity of Mayon Volcano in the Philippines, across a wide range of matric potentials (ψ) and soil-water contents.  Samples were prepared (1) by adding a pre-determined amount of water to sand, left to equilibrate in a sealed bag, and repacked at a bulk density of 1.5 g/cm³ for air permeability; or (2) by saturating repacked 1.5 g/cm³ sands and draining to a desired matric potential in a sandbox or a pressure plate apparatus for the soil-water retention curve. In addition, independent data sets of air permeability where obtained on 1.5 g/cm³ sand samples drained to different matric potentials. The variation of k_a with soil-air content (ε) showed an increase in k_a during the drainage of the bigger pores (effective diameter >  0.03 mm) while no further significant increase in air permeability when effective pore size of less than 0.03 mm has been drained. A power law k_a(ε) model was fitted into the k_a(ε) data for ψ ≥ -100 cm H₂O (pF 2 where pF = log(-ψ; in cm H₂O) while a constant value described the k_a(ε) in the dry region where ψ < -100 cm H₂O. A k_a(pF) model was developed by combining the developed k_a(ε) model with the van Genuchten soil-water retention model and tested on the independent k_a(pF) data sets. The developed model reasonably captured the behavior of the air permeability as a function of matric potential.