Pesticides Research, 63. – Scenarios and Model Describing Fate and Transport of Pesticides in Surface Water for Danish Conditions – 9 Conclusions Concerning the Appropriateness of the Model Tool for Evaluation of the Fate and Transport of new Pesticides to Surface Water in Connection with the Registration Procedure

Scenarios and Model Describing Fate and Transport of Pesticides in Surface Water for Danish Conditions

9 Conclusions Concerning the Appropriateness of the Model Tool for Evaluation of the Fate and Transport of new Pesticides to Surface Water in Connection with the Registration Procedure

Despite the uncertainties, the results of the calibration runs showed that simulated and observed concentrations were within the same order of magnitude. The general levels of exposure simulated thus appear to be correct. However, the timing of the peaks is less convincingly simulated. In Odder Bæk, where macropores are not present, the simulated peaks are too low and too wide, while the peaks in the Lillebæk catchment appear more realistic. The baseflow concentrations in the Lillebæk catchment appear to be exaggerated. In general, the concentrations are over-simulated in Lillebæk and underestimated in Odder Bæk, making Lillebæk the best choice for a conservative estimate.

Tests with the model has shown that the highest concentrations do not always occur in the wettest year, and the selection of climatic conditions appear to fulfill its purpose. Also the variations in space appear realistic, with zones of natural vegetation providing some protection to the watershed in general.

The model is aimed at a tier-two level in the national registration procedure. Some of the assumptions in the model are worst case, such as the fact that the total agricultural area is sprayed with pesticide within a short time (30 minutes) and the fact that a large part of the stream and pond boundary may be subject to drift and deposition. In addition, it was found that one rainfall event in the Lillebæk scenario represents a one in 10-year or one in 20-year event with respect to 24 and 48 hours of rainfall, respectively. This should be considered when judging the simulation results.

The peaks generated by wind drift are by far the largest exposure seen in the Odder Bæk-model, at least when the buffer zones are small. In Lillebæk, the contribution via drain often exceeds the calculated wind drift.

For pesticides that sorb strongly, the transport is underestimated, particularly in the Odder Bæk catchment. This is due to the problems described by Holm et al., 2003, regarding generation of pesticide-loaded colloids and the presence or absence of macropores in the scenarios.

In the ponds the fate of hydrophobic pesticides mainly depend on the size of an effective diffusion coefficient, which varies with at least a factor of 10. However, the cause for this variation is not known and the effective diffusion coefficent have therefore been set to a fixed value. Another fundamental assumption for the surface water model (MIKE 11) is that the surface water is vertically well mixed, which not is the case within the first couple of hours after a pulse of pesticide have entered the pond through for instance wind drift. To evalute the consequences of these and other uncertainties of the pond and stream models (MIKE 11) a coupling of the fate and transport model to an exposure and effect model seems relevant.

To conclude, the Lillebæk model provides a conservative estimate of pesticide occurrence under moraine clay conditions. The peaks modelled are comparable to what is observed in monitoring, but they do not necessarily fit the observed pattern satisfactorily in time. For Lillebæk, the high peaks appear flashy in the simulations as in the observations, indicating that the exposure time may be in the right order of magnitude. However, the baseflow concentrations simulated (particularly during summer) are clearly too high and should not be used as the sole argument for not registering an active substance.

The framework for the registration model is now in order, but there is no doubt that the model would benefit from further improvements in the areas listed in Section 7.6.