Influence of Dispersion on Transport of Tracer through Unsaturated Porous Media

Abstract

The dispersion phenomenon has resulted from the various water flow magnitude and direction in porous media. The dissolved tracer tends to spread due to dispersion and then travel time of tracer through the porous media increases. In unsaturated porous media, dispersion coefficient varies with non-linear Darcy’s velocity and the water content. These nonlinear dispersions were observed in both of the laboratory scale sand and soil columns (20 cm). The unsaturated infiltration column and tracer tests have been used to interpret the relationships between Darcy’s velocity and the water content together with the dispersion coefficient. However, the dispersivity coefficient cannot be measured directly; it has to be determined from advection-dispersion equation (ADE), which can be used to model the tracer transport in unsaturated porous media. The model was used to describe the non-linear functions of water contents and dispersivities for both porous media. The simulations have been verified that the dispersion of tracer through soil is higher than that of sand column and also the travel time of tracer through soil is longer than that of sand column. Even though, soil has very low degree of pore velocity, high dispersivity is observed in the simulations. The water content and tracer concentration profiles reveal that the increase of dispersivity induces the increase of flow path distance and the decrease of pore velocity. The maximum dispersivity was observed when the water content of porous media is relatively low; this leads to the maximum of spreading of tracer.