Technical Abstract: Modeling chemical transfer from soil to surface runoff is essential for developing a surface water quality model which can be used to assess pollution potentials of agricultural chemicals. Chemical transport to runoff can be modeled as a two-rate process. A fast rate subprocess, which prevails at early stages of rainfall, causes an exponential depletion of chemicals from the mixing zone. A slow rate subprocess, which becomes significant under poorly drained conditions, transports chemicals into the mixing zone from the soil below. The two-rate process can be described by coupling the mixing zone concept with the convection diffusion equation (CDE). We evaluated this coupling approach by comparing predicted results with measured bromide concentration data. A finite element scheme was developed to solve the CDE in conjunction with a near-surface boundary condition derived from the mixing concept. Overall results showed the coupling approach satisfactorily predicted bromide concentrations in both surface runoff and soil solution. The coupling approach, while allowing for direct use of the mixing concept under free infiltration conditions, refines the mixing concept for use under poorly drained conditions.