Nonequilibrium solute transport results from both chemical and physical mechanisms. In saturated porous media the physical causes are usually taken to be film diffusion and intra-particle diffusion. Experimental data shows that nonequilibrium is affected by flow velocity, a dependence which would not be inferred from diffusion-based mechanisms. Dimensional analysis of a tracer moving in a porous medium leads to various Damköhler numbers which, in turn, suggest different physical causes of nonequilibrium. It is concluded that both diffusion/dispersion and local variations in the soil-water flow velocity lead to the velocity dependence observed in laboratory experiments. Based on the dimensional analysis, it is also shown that small scale physical modelling of a full scale prototype is feasible under certain conditions. For unsaturated flow, the situation is more complicated in that additional mechanisms, funnelling and fingering, affecting the soil- water flow come into play. These mechanisms are described in some detail, as is experimental data on their occurrence. Commonly, models of solute transport through the unsaturated zone rely on the assumption of steady flow. Then, funnelling and fingering will act as additional physical mechanisms leading to nonequilibrium solute transport.