In this chapter, an analysis is presented of a kinetic second-order approach for describing mechanisms for retention of reactive chemicals in the soil environment. The basis for this approach is that it accounts for the sites on the soil matrix that are accessible for retention of reactive chemicals in solution. The second-order approach will be incorporated into the nonequilibrium two-site model for the purpose of simulation of the potential retention during transport of reactive chemicals in soils. As will be described in a subsequent chapter, this second-order approach will be extended to the diffusion-controlled mobile-immobile (or two-region) transport model.