Macropore flow in soils and pesticide risk assessment

Agricultural use of pesticides may result in contamination of groundwater being used as a drinking water source. Additionally, the emission of these compounds via drainage into surface waters can adversely affect aquatic ecosystems. Pesticide risk assessment is applied to evaluate the potential for health and ecological effects of a pesticide and is commonly performed using mechanistic models. Pesticides currently used in agriculture are soluble in water. Hence, both pesticide transport and water flow through the soil must be studied together through an environmental risk assessment of pesticides.Water flow and pesticide transport can be described for field conditions as uniform or preferential. While uniform flow leads to stable wetting fronts parallel to the soil surface, preferential flow generates unstable wetting fronts, differences in water pressure and solute concentrations, and rapid flow through parts of the soil matrix. This Ph.D. research project focuses on one source of preferential flow; macropore flow. Macropore flow produces fast vertical water flow and pesticide transport in a small soil volume, bypassing the reactive soil matrix. One effect of this is that some of the applied pesticides cannot degrade in the soil and instead arrives in groundwater or surface waters, negatively affecting water quality. Therefore, mechanistic models must incorporate macropore flow in pesticide risk assessment to ensure accurate simulations.Macropores are originated mainly by biological activity, drying and wetting cycles, and shrinking clays, which results in natural variation of the number of macropores in time and space. Some macropores directly end within the soil matrix, referred to as ‘dead-end macropores.’ We have designated the spatial variation of macropores over depth as ‘heterogeneous macropore geometry.’Dual-permeability models such as HYDRUS and SWAP are conventional mechanistic models utilized in pesticide risk assessment studies. The parametrization of t