Pesticide leaching in polders : field and model studies on cracked clays and loamy sand

This thesis reports on a study of pesticide leaching in polder areas. The study comprises two aspects: a data collection program and the development, calibration and application of the model <em>SWACRO</em> for the simulation of pesticide transport.Field data were collected at three experimental fields, all situated in the IJsselmeerpolders. The experimental fields were situated on loamy sands and cracked clay soils. Four different pesticides were incorporated in the research program: 1,3-dichloropropene, metamitron, aldicarb and simazine. The inert ion bromide was used as a tracer and applied with the pesticides. The soil physical properties of the experimental fields and the characteristics of the pesticides involved were determined in laboratory experiments.The model <em>SWACRO</em> was selected to describe water and solute flow in the (un)saturated zone. 'Me model was extended to transport of pesticides and preferential flow. Preferential flow was calculated according to the mobile/immobile concept (loamy sand) or the imaginary drain concept, conceivably extended with segmentation of the upper boundary conditions (cracked clays). Calibration of the model was performed in three steps: (1) calibration of soil water fluxes, (2) calibration of the concentration of conservative solutes in drainage water and the concentration profiles of conservative solutes, (3) calibration of the concentration of pesticides in drainage water and transformation of pesticides. The model was calibrated for the three experimental fields. Generally, soil water fluxes and the concentration levels of bromide and pesticides in drain pipes could be reasonably well simulated.The extended model <em>SWACRO</em> was used to formulate design and management criteria for polder areas in order to develop an integrated pest management. With the calibrated model the effects of several design and management scenarios have been evaluated. Generally, scenario analysis showed that pesticide characteristics, time of application of the pesticide, soil characteristics, lateral boundary conditions and weather conditions define the concentration of the pesticide in the drain pipes and thereby the fraction of the dosage leached. Generally, the fraction of the dosage leached is higher for cracked clay soils than for loamy sands. Sensitivity for leaching is low to moderate for areas without preferential flow and a high organic matter content and high to very high for areas with preferential flow and a low organic matter content. For cracked clay soils it was calculated that leaching is mainly caused by preferential flow through cracks. The scenario studies showed that several measures can be taken in order to reduce leaching: (1) Reduction of leaching by allowing pesticide application only during a certain period (2) Introduction of new drainage criteria with increasing drain depth (3) Decreasing preferential flow by increasing the ploughing depth.For the IJsselmeerpolders the present legislation for the registration of pesticides should be adapted, taking into account that: (1) Leaching may contribute significantly to the presence of pesticides in surface waters (2) Leaching is highly influenced by preferential flow.