Effects of soil type and water flux on solute transport

Prediction of solute transport in soils requires appropriate transport models and model parameters. The most suitable solute transport model and its parameters are likely to depend on both soil type and water flux. This dependency was investigated by means of transport experiments that were carried out in three different soils (Plaggept, Hapludalf, and Glossudalf) for two water application rates (0.5 and 1.0 cm d(-1)). Solute concentrations were measured during the transport experiment at six different depths in undisturbed soil monoliths (1.0-m length, 0.8-m i.d.) using time domain reflectometry (TDR). For each depth, parameters of two solute transport models, (i) the convective lognormal transfer function (CLT) model assuming stochastic-convective solute transport or no solute mixing and (ii) the convective-dispersive (CDE) model assuming complete solute mixing, were derived. The transport in the Plaggept and Hapludalf soil was better modeled as a stochastic-convective process. Furthermore, solute transport in these two soils was very heterogeneous, resulting from considerable differences in local advection velocity. In the Glossudalf soil, solute transport was more homogeneous and characterized as a convective-dispersive process. The heterogeneity of solute transport in both Plaggept and Hapludalf soils increased with the applied water flux. This increase was more pronounced in the Hapludalf soil, as the higher water flux resulted in a larger contribution of macropores to flow and transport of solutes.