Transport of pesticides via macropores (IUPAC technical report)

This report provides an overview of the transport of solutes via macropores focusing on the practical relevance of the phenomenon. After a description of matrix flow and preferential flow in soil, information related to macropores, including their formation and measurement techniques, is briefly presented. Then, the influence of experimental conditions and of environmental and agricultural factors and pesticide properties is discussed, based on a statistical evaluation of all published studies offering sufficient quantitative information. Most of the analyzed parameters do not significantly influence the experimental pesticide losses. The groundwater ubiquity score (GUS) index turned out to be the most important compound property to describe substance losses through macropore flow. In a third section, tools for modeling pesticide transport through macropores are presented and critically evaluated. Results of the computer model MACRO, which is also used in the EU pesticide registration process, are compared with experimental losses. For five out of seven investigated pesticides (A-D, F), the simulated losses are in agreement with the experimental data. However, for two compounds with very low K-OC values, MACRO overestimated the losses. Finally, the significance of pesticide transport via macropores for contamination of ground and surface water is assessed. Losses caused by macropore transport may considerably exceed losses caused by matrix transport at a specific site. Therefore, a site-specific assessment of pesticide leaching is needed.