Several studies showed that sorption-desorption of dissolved chemicals on different soils was not adequately described by use of a single reaction of the equilibrium or kinetic type. Failure of single reactions is not surprising since they only describe the behavior of one species, with no consideration given to the simultaneous reactions of others in the soil system. Multisite or multireaction models deal with the multiple interactions of any one species in the soil environment. Such models are empirical in nature and are based on the assumption that retention sites are not homogeneous in nature; rather, the sites are heterogeneous and thus have different affinities to individual solute species. Figure 5.1 clearly illustrates the heterogeneous make up of the soil matrix.