Transfer of chemicals from soil solution to surface runoff: A diffusion-based soil model

A physically-based diffusion and transport model is developed to describe chemical outflow concentrations during chemical removal from soil to overlying runoff water induced by continuous rainfall over the soil surface. In contrast to earlier models, movement from the soil to the runoff water is described as a liquid diffusion process to the surface, coupled to the runoff zone through a laminar boundary layer at the runoff interface with the soil surface. Within the soil, diffusion is moderated by equilibrium adsorption to solid surfaces characterized by a partition coefficient. The runoff concentration at the outlet is derived by treating the runoff zone as a well-mixed reactor, characterized by a residence time. The model was used to predict the results obtained in the experimental study of L.R. Ahuja and O.R. Lehman (1983) where infiltration was suppressed, with good agreement obtained between predicted and measured outflow concentrations when the model parameters were estimated independently using standard engineering equations from channel flow hydraulics. The model also predicted the final soil concentrations satisfactorily after runoff ceased.