Experimental study on diffusion of chloride ions in unsaturated soils

Advection and diffusion are the mechanisms involved in contaminant transport in soils. The diffusion will be the dominant process when seepage velocity is low. At present the unsaturated hydraulic conductivity has been studied intensively, however, the study on diffusion mechanism in unsaturated soils is limited. Controlling of water content in unsaturated soils is a technical bottleneck that restricts the study. Here, an advection-diffusion column test apparatus is developed for the precise control of water content during the tests. Chloride ion is selected as the tracer in the column tests and the slice method is used to get the concentration distributions in the soil column. The results are fitted by an advection-diffusion analytical solution to determine the hydraulic dynamic dispersion coefficient. The effective diffusion coefficient is then calculated, and the relationship between water content and effective diffusion coefficient is examined. The unsaturated hydraulic conductivity is obtained by considering seepage in the three-layered soils. The results show that the influences of the osmotic suction on the water content are insignificant and that the control of water content in the test is reliable. The effective diffusion coefficients corresponding to matric suctions of 0 to 100 kPa range from 1.59×10^-6 to 5.22×10^-6 cm²/s. The effective diffusion coefficient decreases linearly with the decreasing dimensionless water content. The dispersivity increases with the decreasing water content. The logarithmic unsaturated hydraulic conductivity decreases linearly with the decreasing dimensionless water content.