This paper presents a new model of dispersion of components in transport processes. It is suggested that the flow velocity may be specified by a Gauss function with relative dispersion of flow velocity beta = sigma(v)/v; the overall dispersion of inert component (in porous as well as in homogeneous medium) may be calculated from the equation sigma = (2Dt/tau(2) + (beta vt)(2))(0.5), where D is the diffusion coefficient, t is time, tau is tortuosity, and v is flow velocity. The most common range of variability of relative dispersion of flow velocity in natural soils is 8-32%. The model is well applicable to column and field experiments. (c) 2005 Elsevier Inc. All rights reserved.