The role of drainage ditches in pesticide retention: a three scale study

Agricultural non-point source pollution may lead to water quality deterioration. In this study, natural ditches were selected to demonstrate their potential effect on pesticide removal. The nature of bottom materials and hydrodynamic conditions had an influence on the reduction of both pesticide concentration and mass with distance. In ditches a great variety of materials may act as sorbents: grasses, leaves, or sediments. As sorption to organic materials is a major process that may influence agrochemical concentration in water, batch experiments have been led to determine sorption characteristics for three herbicides with different physicochemical properties (isoproturon, diuron and diflufenican) on sediments and leaves in decay. An experimental channel has also been developed to test the effect of hydrodynamic parameters combined with bottom substrate nature on pesticide retention. Results revealed that adsorption is more important on leaves than on sediments. This can be explained by the high organic content of the leaves, combined with the relative hydrophobicity of the compounds. In the channel, sorption equilibrium was not reached after a five hour contact time between a pesticide solution in recirculation and dead leaves, which highlighted the necessity to consider kinetics to assess potential pesticide retention level in ditches.