Equation for describing solute transport in field soils with preferential flow paths

Modeling the transport of chemicals is challenging due to the presence of preferential flow paths and the input data needed to describe these paths. We propose a simple equation that can predict the breakthrough of solutes without excessive data requirements under steady state flow conditions. In our generalized preferential flow model (GPFM), the soil is conceptually divided into a distribution zone and a conveyance zone. The distribution zone acts as a linear reservoir resulting in an exponential loss of solutes through preferential flow paths from this zone. In the conveyance zone, the transport of solutes is described with the convective-dispersive (CD) equation. Input data required are the apparent water content of the distribution zone, and solute velocities and dispersivities in the conveyance zone. The model is tested with chloride breakthrough curves (BTCs) resulting from three sets of experiments performed with steady state artificial acid rainfall at different intensities applied sequentially using duplicate columns filled with undisturbed soil or sand both exhibiting preferential flow. The model is able to describe the breakthrough of solutes with physically meaningful parameters through coarse sand with fingered flow and through undisturbed soil cores. In coarse sand, water and solutes flow only through the fingered flow paths in the conveyance zone. In undisturbed columns there is both flow through the matrix and preferential flow paths, and two velocities in the conveyance zone are needed to simulate the breakthrough curves.