Fate of Pesticides in Surface Waters, Laboraty and Field Experiments

Fate of Pesticides in Surface Waters, Laboraty and Field Experiments

9 Conclusions from field experiments

Distinct time trends of decreasing water concentration levels were recorded for all the pesticides sprayed in the artificial ponds. A nearly complete removal was seen within the first two weeks. Bentazone, however, showed a rapid decrease during the first few days followed by a more stable water concentration during the next 130 days or more (the experiment lasted 130 days). The more hydrophobic substances such as pendimethalin and fenpropimorph dissipated rapidly from the water column during the first few days. The year-to-year variation for pendimethalin and fenpropimorph greatly exceeded the variation within the same year for ponds with and without macrophytes. One explanation may be the presence of sorption sites in suspension either as suspended solids or as organic matter. This indicates that the modelling of exposure levels in real systems needs to be done with care and, even for relatively simple systems, in which the spraying is well controlled, large variability in the results may occur.

The most rapid recorded decrease in water concentration indicates that a diffusion process takes place as this specific transfer process can introduce an initially infinitely high concentration decrease rate.

The field experiments have generated fate data for calibration and validation of the registration model for pesticides with a range of physico-chemical properties. The use of these data for calibration and validtion will improve the field of validity of the model.

It may take 1-2 days after pesticide application until the concentration of pesticide is evenly distributed in the ponds. This is contrary to the assumption of the model that there is instantly complete mixing.

Presence of macrophytes influence dissipation of pesticides from the water phase by reducing the hydraulic mixing and thereby delaying transport of pesticides to the sediment. Furthermore, pesticides may sorb to macrophytes thereby reducing the concentration in the water phase.