How much of a surface pollutant will reach the groundwater is strongly determined by how the flow occurs in the unsaturated zone. The aim of this study was to investigate the flow pattern through a sandy, heterogeneous glacial deposit. Soil water was extracted frequently, using suction cups installed horizontally in a trench wall, during the 1 1/2 year period after solute application. Naturally infiltrating water (rain and meltwater) was followed down to a 220-cm depth using Cl−, Br− and 18O as tracers. Different results were obtained according to whether a surface applied solute (e.g., Cl− or Br−) or a variation in 18O/16O in the water itself was used for flow tracing. The meltwater (18O) was detected earlier than Cl−. Meltwater and intense rain percolated very rapidly along narrow, preferential flow paths, whereas the solutes may have been transported by a smaller amount of the infiltrating water in a more matrix-dominated flow. High spatial variation in flow velocities and solute concentrations were found. The results are discussed in light of different theories of flow mechanisms. The presence of preferential flow implies that a small soil volume may rapidly conduct a dissolved pollutant down to the groundwater.