A SIMPLE EQUATION FOR PREDICTING PREFERENTIAL FLOW SOLUTE CONCENTRATIONS

The transport of pesticides and other chemicals through macropores has been widely observed and predicting it is a challenge. This article c onsiders a simplified two-layer model, similar to overland flow models in which the processes of adsorption and desorption are separated. Fo r the layer near the surface, or the mixing layer, the solute concentr ation in the layer is equal to that in the percolating water (includin g preferentially moving water). In the lower profile, the flow is part itioned between matrix and preferential now. The solute concentration of the matrix flow is characterized by the soil condition near the out let point, whereas the preferential flow is represented by the solute concentration in the mixing layer. The closed form equation, exhibitin g exponentially decreasing macropore flow solute concentrations, is te sted against solute breakthrough curves using three independent sets o f experimental data. The predicted depths of mixing between 5 and 25 c m are physically realistic and the closed form is shown to reproduce t he form of experimental data, particularly under conditions of signifi cant macropore flow. Although highly simplified, the physically based model yields a framework for predicting solute concentration for prefe rentially moving water.