A methodology is presented to assess the exposure of aquatic organisms resulting from pesticide applications by sideways and upward spraying in Dutch fruit crops. It is the intention that this methodology will be used in Dutch pesticide registration. The methodology is based on the principle that the endpoint concentration represents a 90th percentile of the statistical population of concentrations to be expected in ditches alongside fruit crops. Furthermore the methodology is founded on the principle that the user should be able to choose between different drift-reduction technology (DRT) classes and between different widths of the crop-free buffer zone. Spray drift is the only exposure route considered in the methodology. The 90th percentile concentration (PEC90) is based on a spatially distributed model that simulates the frequency distribution of the annual maximum concentration of more than 70,000 spatial units (i.e. ditches characterised by waterbody properties and their orientation with respect to the direction of the rows of the fruit trees and to the N-E-S-W direction) for 100 simulation years. This frequency distribution was calculated for different application patterns, different DRT classes and different widths of the crop-free buffer zone. Next, one of the 70,000 spatial units was selected which fulfilled the criterion that it could be used to calculate this PEC90 for all combinations of application patterns, DRT classes and widths by selecting a suitable percentile of its temporal distribution of concentrations. The TOXSWA model (coupled to the hydrological SWQN model simulating water depths and water flow rates) was parameterised for this selected spatial unit to run for 26 years. The first six years were used as a ‘warming-up’ period and the remaining 20 years for assessing the required temporal percentile. The scenario ditch was 300 m long of which only the 100 m in the middle received a drift load of pesticide. The direction of water flow may change on a daily basis and the water flowing into the 300-m ditch was assumed to be free of pesticide. The median residence time of a droplet of water in the 100-m ditch was about 1 day. The
spray drift deposition for the 99% DRT class appeared to be as low as 0.03-
0.07% for summer applications. At such low deposition levels the contribution of leaching from drain pipes and of atmospheric deposition may exceed that of the drift deposition.