Simulation of Pesticide Transport in Macroporous Soils Using a Dual-Porosity Approach

A conceptual model, based upon the bicontinuum double porosity approach, has been developed to simulate water flow and reactive chemical transport in both the soil matrix and the macropore region. Flow of water in both regions is governed by Richards’ equation and chemical transport is based upon convective-dispersive mechanisms with linear kinetic sorption and first-order decay. The pressure-water content and the pressure-conductivity relationship for the unsaturated soil in each of the two regions are based upon the van Genuchten-Mualem relationships. The developed model was in one- and two-space dimensions and it contained several improvements over the previously proposed one-dimensional conceptual flow and transport models. Convergence was a problem when the magnitude of coupling or the size of the time step was large. Analyses of the magnitudes of the flow and transport parameters on the prediction of pressure and concentration profiles were presented. These include fluid and solute mass transfer coefficients, saturated hydraulic conductivity of the interface through which mass transfer takes place, sorption distribution coefficient and reaction rate, decay rate, and root extraction of water. From the pressure and the concentration profiles, it was observed that the fluid and the solute mass transfer coefficients have the greatest impact on model predictions. Sorption distribution coefficients and reaction rates appeared to have moderate impact on concentration profiles. For short durations of simulation (e.g. minutes to a few hours), decay rates and root uptake of water appeared to have negligible effect. The model was used to study the impact of agricultural management practices such as row versus nonrow crops, conventional versus conservation tillage, organic amendment, and furrow irrigation on the prediction of pressure and concentration profiles for a local soil with a given degree of macroporosity. With the chosen set of parameters, the importance of macropores on the deep movement of water and chemicals was observed. Once the model parameters for a given soil type are estimated and the model is validated, it can effectively be used as a management tool to evaluate the impact of agricultural management practices.United States Department of Agriculture, Illinois Groundwater Consortium: grants 1-5-27487 AG-RAY ANTIC and 1-5-28082 SIU-92; Illinois Groundwater Association (Champaign, IL); American Water Foundation (Denver, CO