An evaluation of hydrodynamic dispersivity and dispersion pattern in soils.

Abstract

The purpose of this study was investigating the dispersivity in a range of flow rates under steady state conditions. In this investigation, two models of dispersion - mass and stochastic were assessed to justify the solute transport processes in unsaturated soils. Soil samples with different textures were collected from different region of the North West province of Zanjan, Iran. To identify the mixing regime of solute in the unsaturated soils, series of solute transport experiments were conducted on 9 soil columns at steady - state unsaturated condition. Then, solute transport models, convective - dispersive Equation (CDE) and convective lognormal transfer (CLT) were fitted to experimental breakthrough curves (BTC’s). The results showed that for homogeneous and undisturbed soils, the CDE model can explain the BTCs more reliably than the CLT model and the soil physical properties as well the conditions of experiment are critically determining the appropriate model which can characterize the BTCs. The results indicated that the travel time distribution decreased with an increase in flow rate. It was further concluded that the hydrodynamic mixing regime is determined with two antagonist mechanisms, the effect of solute flow tortuosity and contribution of larger pores in high flow rates. It was also observed that the variance of solute molecule velocities varies with flow rate.