Measurement of Molecular Diffusion of Salt in Unsaturated Soils

The molecular diffusion coefficients of salt were measured in Shonai sand dune sand and Kanto loam soils over a wide range of water contents using a transient state method. The measured relative diffusion coefficient, Dr, the ratio of diffusivity of salt in soil to the diffusivity in bulk water, varies from 0.31 to 2.98 × 10-3 for Shonai dune sand as volumetric water content decreases from 31.74% (0.00215 MPa) to 2.13% (5 MPa). For Kanto loam soil it varies from 0.17 to 6.25 × 10-3 as water content decreases from 60.89% (0.0021 MPa) to 20.22% (3.11 MPa suction). The relationship of Dr with water content in soil shows a linear relationship on log-log scale at higher water contents for both soils. However, it shows a break point at the water content that corresponds to a matric suction of about 0.09 MPa for both soils. The respective slopes of Dr(θ) for Shonai dune sand and Kanto loam are 0.75 and 1.25 at higher water contents (> break point) and 3.99 and 4.1 at lower water contents. The relationship of Dr with matric suction is found to be less dependent on the soil texture.