MODELING SURFACE RUNOFF CONTAMINATION BY SOIL CHEMICALS UNDER TRANSIENT WATER INFILTRATION

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, overland flow and convective-dispersive solute transport in the soil. Solutes are assumed to be subjected to linear equilibrium sorption onto the soil's solid phase. Numerical simulations indicated that the value of the coefficient for soil chemical transfer to surface runoff does not affect the soil surface concentration when soil is initially dry, but is linearly related to the dissolved chemical flux to overland flow. This enables an approximate solution to the convective-dispersive equation for determining soil surface concentration, independent of its determination from rainfall-runoff relationships. Simulated dissolved chemical concentrations at the soil surface and in surface runoff were determined for different antecedent soil moistures and rainfall intensities. These concentrations were found to be lower when antecedent moisture was low, because ponding time for drier soils is longer and during this period soil solutes are therefore displaced downward by greater volumes of infiltrating water. For a specified initial soilwater content, higher rainfall rates cause higher dissolved chemical concentrations at the soil surface and in runoff water. The hydrological parameters (e.g. soil-surface slope, length and roughness) affecting the residence time of surface water on the field, greatly affect runoff concentration for a short time after the initiation of overland flow. The runoff concentration for longer periods is mainly affected by soil surface concentration, which is in turn controlled by the dispersion of the chemicals in the soil. The distribution coefficient between the liquid and solid phases of the soil controls the release of adsorbed chemicals to the soil solution and therefore affects the runoff concentration.