Surface runoff contamination by soil chemicals: Simulations for equilibrium and first‐order kinetics

A model was developed to predict the potential contamination of overland flow by chemicals removed from soil water by rainfall on sloping soil. The model accounts for transient water infiltration and convective‐dispersive solute transport in the soil and also considers rate‐limited mass transfer through a laminar boundary layer at the soil surface/runoff water interface. Sorption‐desorption interactions between soil and chemicals are assumed to be subject to linear and nonlinear isotherms or to first‐order kinetics. The dissolved‐chemical concentrations at the soil surface and in the surface runoff were determined for different antecedent soil moistures and rainfall intensities. These concentrations are lower when the antecedent moisture is low because the time of ponding for drier soil is longer and because during that period soil solutes are displaced by greater volumes of infiltrating water. For a specified initial soil water content, higher rainfall rates cause higher dissolved‐chemical concentrations at the soil surface. The degree of nonlinearity of the equilibrium isotherm greatly affects the transient dissolved‐chemical concentrations and the linear isotherm cannot always be used as an alternative. These concentrations are also greatly affected by the value of the kinetics rate coefficient. In the first‐order kinetics model there is a recovery of the dissolved‐chemical concentration at the soil surface during the period between rainstorms. As a result, the initial concentration at the soil surface for the subsequent rainstorm is higher than that expected when equilibrium is assumed.