Evaluating Pesticide Migration in Tile‐Drained Soils with a Transfer Function Model

Abstract

AbstractThe interpretation and prediction of chemical outflow in tile drains as a result of solute migration through both unsaturated and saturated soil can be an important tool for understanding and reducing groundwater pollution risk. A simple analytical solution for predicting pesticide transport in tile‐drained soils, based on the transfer‐function concept, is presented and tested in screening scenarios for chemicals with widely varying fate properties under different hypothetical transport characteristics for the individual soil layers. The inherent soil travel time variability of the two layers is embodied in probability density functions (pdf). The unsaturated zone is represented by either a unimodal or a bimodal pdf, the latter describing both preferential and matrix flow of solutes at the field scale. The saturated travel time pdf is derived from cumulative travel time distributions calculated by streamline functions. Sensitivity analysis focused on chemical outflow in tile drains as a function of travel time variability in the unsaturated and saturated soil, assuming linear equilibrium adsorption and first order decay in either the entire soil or the preferential and/or slow flow regions of the unsaturated soil, respectively. The tile drain outflow clearly reflects the time‐delayed travel time characteristics of the unsaturated soil. Compared with a Gaussian‐like pdf, a bimodal pdf for the unsaturated zone affects primarily the shortest travel times of highly retarded and short‐lived chemicals. Screening scenarios with 10 different pesticides exhibit a strong influence of preferential flow on both the early arrival time in the tile and the total residual mass fraction, whereas the total residence time is nearly independent of travel time characteristics of the unsaturated soil.