Atrazine retention and transport in soils.

No pesticide has been studied as extensively as atrazine. The study of atrazine has contributed to our general understanding of the behavior of pesticides in soils. New knowledge and concepts were evaluated, such as atrazine adsorption kinetics, desorption hysteresis, and preferential flow. Corresponding conceptual models were also proposed to explain the behavior of atrazine in soils. Atrazine adsorption-desorption is the major process affecting atrazine behavior in soils and is mainly affected by organic matter and soil pH. Atrazine in soils is subject to biological and chemical degradations. Hydroxyatrazine, the chemical degradation product, is more strongly adsorbed to soil than atrazine. Deethylatrazine and deisopropylatrazine, the major biological degradation products, are more mobile than atrazine. Hydrolysis of atrazine is soil-surface catalyzed and favored by low soil pH. The overall dissipation rate of atrazine was found to be pseudo first-order. Two distinct and different processes are involved in atrazine movement: slow transport through the soil matrix and rapid movement through macropores. The first process is controlled by adsorption kinetics and degradation reactions and can be well explained by models based on chemical heterogeneity, such as the two-site models and second-order models. The second flow process results from preferential flow through large pores and can be explained by physical nonequilibrium models such as the mobile-immobile and two-flow domain models. Because both processes coexist in atrazine transport, coupling of physical and chemical nonequilibrium models is often necessary and has shown promise in atrazine transport modeling. However, more efforts are needed in estimating model parameters and in developing management-oriented models.