Modelling the leaching and drainage of pesticides in the Netherlands: the GeoPEARL model

A one-dimensional pesticide leaching model, PEARL, in combination with a geographical information system was used to calculate the leaching potential of pesticides into local surface waters and the regional groundwater. Calculations were performed for6405 plots, which are unique combinations of spatially distributed model inputs. To have the seepage and drainage fluxes correctly described, the model was loosely coupled with a regional groundwater model. Simulations were carried out for four pesticides with different properties. Results showed that, generally, the average fluxes of pesticide into local surface waters were higher than the average fluxes of pesticide into the regional groundwater. Discharge by rapid drainage mechanisms (i.e. tube drainage and surface drainage) dominated. For the four pesticides, different spatial patterns of leaching and discharge by drainage water were predicted. It was shown that the spatial pattern was affected by a large number of interacting processes, and that the relative importance of these processes differed between the four example pesticides. This should be kept in mind when applying the USES system, which describes the leaching of pesticides on the basis of a limited number of pesticide parameters. Frequency distributions of the leaching concentration were compared with results from the first tier of the Dutch pesticide registration procedure, which comes down to the application of a single standard scenario. For two pesticides, the first tier was not strict enough. It was shown that it is not possible to find one single standard scenario, which applies to the full range of registered pesticides. This means that the number of standard scenarios should be increased. Direct application of a regional-scalemodel, however, is to be preferred, because it provides the user with frequency distributions and gives information about areas of safe usage.