Leaching of fertilizer and pesticide compounds from agricultural lands into ground and surface waters is a major environmental problem. To study this phenomenon, a bromide tracer experiment was performed on a 1.6-ha area of a tile-drained arable field to determine the contribution of preferential flow to solute leaching, and to investigate the factors triggering the onset of such events. During the 2 yr following the application of Br−, concentration of the drainage effluent exhibited a pattern of peaks coinciding with the discharge peaks. The concentration peaks, on top of a smooth base flow, were interpreted as being the contribution of preferential flow. The separation of peak and base flow indicated that 73% of the Br− leached during the 2 yr of study was exported through preferential flow. A simple leaching bucket model was able to accurately predict the occurrence of solute peaks in the drain discharge. The analyses indicated that preferential flow originated primarily at the boundary between top- and subsoil as soon as the topsoil became sufficiently saturated. Thus, low intensity, high duration events could also trigger preferential flow.