/AnMtgsAbsts2009.55079 Excluded-Volume Analysis of Tortuosity and Diffusivity in the Gaseous Phase of Unimodal and Bimodal Porous Media.

Wednesday, November 4, 2009
Convention Center, Exhibit Hall BC, Second Floor

Augustus Resurreccion, Institute of Civil Engineering, Univ. of the Philippines, Diliman, Quezon City, Philippines, Per Moldrup, Aalborg Univ., Aalborg, DENMARK, Shoichiro Hamamoto, Graduate School of Science and Engineering, Saitama Univ., Saitama 338-8570, Japan, Ken Kawamoto, JAPAN, Saitama Univ., Saitama, Japan and Toshiko Komatsu, Graduate School of Science and Engineering, Saitama University, Saitama Univ., Saitama, Japan
Abstract:
The soil-gas diffusivity, Dp/Do, is a key parameter for describing the fate and transport of gaseous phase chemicals through soil. The soil pore structure largely influences the magnitude and variation of Dp/Do as a function of soil-air content, ε. As soil-water fills up the pore spaces, certain amount of air-filled pores are entrapped thus becoming inactive air-filled pore spaces where gas diffusion cannot take place.  In this study, we present an inactive pore volume analysis where we calculate the amount of the air-filled pores that is rendered inactive due to inter-connected water films between solid particles/aggregates and inside aggregates. We assume a general power law model, Dp/Do=F(ε-εin)X, where εin is inactive soil-air content, and F and X are tortuosity-connectivity parameters for the air-filled pore networks. F and X can be estimated from measurements at dry conditions or at a given soil-water potential where εn can be assumed zero. The εn was subsequently obtained by inverse calculations based on measured Dp/Do in unimodal and bimodal soils. The εin is zero at full water saturation, increases linearly to a maximum when e equals the percolation threshold, εth. At ε >εth, data suggested a linear (for soils) or non-linear (for sands) decrease of εin with ε down to zero at totally dry conditions for both unimodal and bimodal media, and additionally at around -1000cm H2O matric potential for bimodal media (corresponding to a potential where inter-aggregate pores have been drained completely). At drier conditions than -1000cm H2O for bimodal media (intra-aggregate pores become increasingly air-filled), the estimated εin was small and often negligible, suggesting sequential drainage of well-connected intra-aggregate pores without significant creation of inactive air-filled porosity. A model for the gradual increase/decrease in inactive pore space with fluid saturation is developed and linked to matric potential for unimodal and bimodal porous media.