Water flow and pesticide transport in cultivated sandy soils : experimental data on complications

The risk of leaching of agricultural pesticides from soil to groundwater and water courses has to be evaluated. Complications in water flow and pesticide transport in humic-sandy and loamy-sandy soil profiles can be expected to increase the risk of leaching. Much of the precipitation water is intercepted by the crop canopy, after which stemflow and leafdrip lead to peaks in water load at the soil surface. Water infiltration at the soil surface can be highly non-uniform, e.g. due to differences in soil structure, presence of micro-depressions and water-repellency. Water flow in the soil profile is affected by sedimentation pattern, soil forming processes, differences in soil structure, antecedent soil moisture content, etc. The heterogeneous transport of pesticides in soils is reflected by the wide range of concentrations measured at the various depths. The leaching of substances from sandy soil profiles can occur in 'hot spots', which indicates that there are preferential flow and transport paths. A special situation is presented by fields with ridges and furrows (e.g. used for growing potatoes), in which much of the precipitation water can infiltrate in the furrow soil. In view of the conditions inducing preferential pesticide transport in sandy soils, the possibilities to describe/predict this type of transport by modelling should be investigated further.