Process-Based Assessment of Pesticide Fate and Mitigation Strategies in Drained Agricultural Landscapes

The use of pesticides poses a risk to the ecological health of surface waters, particularly in lowland catchments where drainage systems facilitate rapid transport from fields to streams. Short-term pesticide peaks and the formation of transformation products (TPs) indicate that transport processes in drained agricultural systems remain insufficiently understood. This dissertation aims to improve the understanding of pesticide and TP fate and to assess scenarios for reducing losses using the ecohydrological model SWAT+. The model was applied to a small, drained agricultural catchment (100 ha) in the northern German lowlands. Three herbicides with contrasting environmental behaviour and two TPs were analysed.

The results show that pesticide transport is mainly controlled by precipitation, soil properties and tile drain connectivity. Tile drains contribute to short-term pesticide peaks and long-term background contamination. Field data indicate that even non-mobile pesticides can reach surface waters via tile drains in considerable quantities, suggesting that preferential flow contributes to contamination.

A temporal sensitivity analysis shows that hydrological conditions during high-discharge events can influence transport more strongly than half-life or adsorption coefficient. The scenario analysis reveals that field and drainage management measures varied in effectiveness across fields, depending on pesticide properties, field structure and hydrological dynamics. Interface measures, such as reactive ditches, can act as additional barriers to reduce short-term pesticide concentration peaks before they reach surface waters.

Overall, mitigation measures can modify hydrological dynamics and reduce transport velocity, but cannot fully eliminate the risk of contamination. Long-term reduction or avoidance of pesticide use remains the most effective strategy for reducing pesticide losses.