William F. Spencer

The Concept of Convective Mass Transfer for Prediction of Surface-Runoff Pollution by Soil Surface Applied Chemicals

ABSTRACT Amodel describing chemical transfer from soil to surface runoff is presented as a convective mass transfer process through a boundary layer connecting a soil mixing cell to the runoff mixing cell. The boundary layer is assumed to be laminar above the soil surface, restricting chemical transport through this zone to molecular diffusion. The determined mass transfer coefficient is proportional to the chemical diffusion coefficient and inversely proportional to the laminar boundary layer depth. It is increasing with the increase of soil surface roughness, runoff hydraulic gradient and runoff hydraulic radius. A model for chemical runoff effluent concentration is proposed in which rainfall induced surface runoff is represented as a well-mixed reactor. Transfer of soil solutes to the reactor is assumed to occur by a rate limited process proportional to the soil concentration. This approach differs from others in the literature which assume instantaneous equilibrium between runoff and soil solution concentrations. Predicted outflow concentrations were calculated using both the equilibrium and the rate-limited models and the differences were analyzed. The main difference between the two models occurs during the early stages of runoff when the equilibrium model predicts greater loss of chemical to the outflow. The mass transfer model is characterized by two time scales that control the cumulative mass loss to the field outlet. One represents the runoff volume residence time and the other the diffusive soil mixing time.