Preferential solute movement along the interface of soil horizons

Preferential flow significantly contributes to groundwater contamination due to rapid bypass of large fractions of applied agrochemicals through the vadose zone. Transport of surface-applied non-reactive tracer (bromide) and pesticides was studied from 1991/92 to 1994/95 in an experimental drain plot in northern Germany. The soil of the study site was sandy loam in texture with high bulk density and a distinct structural change from sub-angular to angular at the interface of two soil horizons at 30-40 cm. Each year, bromide and pesticides appeared in drain water with the first drainage event followed by steep concentration peaks indicating preferential solute movement. The general shape of the breakthrough curves, magnitude and time of occurrence of the main concentration peaks of bromide and pesticides were comparable in different years despite notable differences in rainfall, drain discharge and areas of chemical application. Further, it was observed that variable drain discharges occurred without significant changes in water table levels, and a major fraction of surface-applied bromide and pesticides appeared in drainage effluent without apparently leaching through the deeper layers. It appears that water and solutes, after vertical movement through the upper plough layer, moved horizontally along the interface of two soil horizons towards and through the porous drain trench to the drain.