PREDICTION OF SOLUTE TRANSPORT USING A TRANSFER FUNCTION MODEL AND THE CONVECTION-DISPERSION EQUATION

Abstract

The convection-dispersion equation (CDE) is the most commonly used process representation for describing solute movement through soils. A transfer function is a model for studying complex soil systems in a simple way by characterizing the output flux as a function of the input flux. Extensive solute transport data from a laboratory experiment conducted through long soil columns were used to validate the intrinsic assumptions of the CDE and the transfer function. Analyses of the experimental data showed that the observed dispersion coefficient increased with the travel distance for both homogeneous and heterogeneous soils. The dispersion coefficients had a two-order change within the 1250-cm soil columns and were described using a linear function of the solute travel distance. At the same travel distance, the dispersion coefficients in the heterogeneous soil column were about 50 times higher than those in the homogeneous soil. The experimental data better supported the assumptions of the transfer function than those of the CDE. An excellent linear relationship was found between the mean of log of the travel time and log of the travel distance. In turn, the relationship well described the scale-dependent dispersion coefficients of the homogeneous and heterogeneous soil columns. The convection-dispersion equation and the transfer equation were utilized to predict solute transport in homogeneous and heterogeneous porous media. Compared with the experimental data, the transfer function predicted solute transport in the porous media much more accurately than did the CDE for these examples. In addition, the soil spatial variability had a less profound effect on the transfer function predictions than on the CDE predictions.