Abstract We simulate the accumulation of a chemical transported from soil to ponding water. We consider one-dimensional convective—diffusive solute transport in water and soil. The results are compared with those of models which simulate an instantaneous mixing in the surface water layer and the surface solute flux dependent on the difference in solute concentrations between soil and water. Numerical inversion of Laplace transforms is used to obtain the simulation results for all models. Results of this work show that the relative effect of diffusion on the accumulation of a solute in ponding water may be significant at infiltration rates that are not uncommon in agriculture practice. The results also imply that locations with low infiltration rates within fields may have a large effect on the overland accumulation of chemicals.