Transport, sorption and degradation of atrazine in two clay soils from Mexico : Andosol and Vertisol

Although atrazine has been banned in the European Union, it is still one of the most widely used herbicides in the world. It is has been detected in surface and groundwater and has been shown to be associated with major human health problems. Atrazine fate in the environment, e.g. sorption, leaching and degradation depends, inter alia, on soil characteristics. Independent static and dynamic experiments were conducted to identify and uncouple the processes governing the fate of atrazine. Two agricultural soils from Mexico with contrasting characteristics in terms of organic matter content and degree of decomposition as well as clay types were selected. Soil organic matter was the main sorbent for atrazine, followed by montmorillonite clays in the Vertisol, and iron oxides and allophanes in the Andosol. Humic acids were predominant in the Andosol's organic matter and favored atrazine sorption, compared to more recalcitrant organic matter such as humin fractions in the Vertisol. Atrazine mobility was enhanced because of the occurrence of preferential flow in the upper layer of the Vertisol and because of the formation of mobile complexes between dissolved organic carbon and atrazine in the Andosol. Our detailed soil characterization and the independent sorption, degradation and transfer experiments allowed identifying the main processes affecting atrazine's fate in the two contrasting soils: In the Vertisol, preferential flow was enhanced because of the lower affinity of the soil's organic matter for atrazine compared to the Andosol. The application of atrazine to Vertisol soils poses a higher risk for groundwater contamination than its application to Andosols