Effect of three hydrodynamic parameters on the transfer of four pesticides in an experimental flume

This study aimed to better understand the effect of three hydrodynamic parameters on the transfer of four pesticides in agricultural ditches during permanent flows. The three parameters considered were i) the surface water speed, ii) the submergence, defined as the ratio between the form height and the water depth, and iii) the bed form geometry. The effect of these three parameters on the transfer of diuron, isoproturon, azoxystrobin and tebuconazole has been studied using an experimental flume, which bottom was filled with a simplified substrate - hemp fibres - as a standard of the natural substrates often found in the bed of natural ditches. The use of this standard has been validated with preliminary studies which have shown the adsorption of the selected pesticides onto the hemp was close to the adsorption onto dead leaves from natural ditches. The different experiments performed in the flume show that, for the selected range of water speed (from 2 to 8 cm/s), an increase of the surface water speed favours the transfer of the pesticides into the bed, and also increases the kinetics of this transfer. The effect is the same with an increase of the submergence. Finally, we observed that a variation of the geometry of the bed forms could greatly affect the penetration of the four pesticides. We also show that the influences of the three parameters were linked. From these results some management advices are formulated so as to improve the mitigation of pesticides in agricultural ditches.