The equilibrium of the pollutant between the groundwater and the solid phase of the soil is described by a Freundlich isotherm. This nonlinear sorption isotherm does not vary in time. Sorption takes place at the so-called "sorption sites" of the soil. Two classes of sorption sites are distinguished (Boesten, 1986; Brasseau, 1992b). The sorption sites of class 1 are continuously in equilibrium. The class 2 sorption sites are not continuously in equilibrium with the soil solution. The rate of (de)sorption at class 2 sites is driven by the shortage (or excess) of the sorbed amount relative to the concentration in.
