Herbicide movement in soils: principles, pathways and processes

European legislation concerning ground- and surface-water quality and the protection of non-target organisms in surface-water from pesticide contamination has initiated more stringent data requirements from regulatory authorities concerning the movement of all pesticides in soils. Other interested parties, such as water companies, environment agencies and consumer-driven organizations, have sought to influence the use of herbicides and their impact on the environment. The resulting studies and associated research have led to a better understanding of the fate and behaviour of herbicides in the soil environment. The amount of herbicide that moves away from the area of application will depend on the physico-chemical properties of the chemical and the agroclimatic characteristics of the target site. Under average conditions, the amount of herbicide lost by movement from a soil profile is typically <0.1% to 1% of the applied mass but, under certain localized circumstances, can reach up to 5% or greater. Leaching, drain-flow and surface run-off are the main pathways responsible for herbicide movement within soils. The soil/herbicide processes determining the losses are also variable in both time and space. It is therefore necessary to understand the spatial characteristics of soils, their hydrology and the associated herbicide use patterns.