Modeling Fate and Transport of Roadside-Applied Herbicides

Abstract

A physically based, one-dimensional, mathematical model is developed to simulate transport of herbicides applied to roadsides into adjacent surface water. Herbicide transport by overland flow, infiltrating water, and subsurface flow is considered. The model accounts for advection, dispersion, equilibrium linear sorption in the soil, and first-order decay of the herbicides. Chemical transport between soil and overland flow is described using a rate-limited mass exchange between the two compartments that is proportional to the difference between the chemical concentration in overland flow and the pore water with a proportionality constant that is a function of overland flow characteristics and rain intensity. For simulating overland and subsurface flow, the kinematic wave equation is solved using a Crank-Nicolson finite difference scheme. Richards' equation is solved using a Galerkin finite element method to model infiltration, and the transport equation is solved using a finite difference method. The model is calibrated and verified using extensive field data on the runoff of five herbicides with a range of physical-chemical properties at two field sites. Good agreement is obtained between predicted and measured event-mean concentrations in runoff and the herbicide concentration remaining in the application-zone soil.