Field investigations and modeling of particle-facilitated pesticide transport in macroporous soil

Physical and chemical non-equilibrium processes may facilitate the transpor t of pesticides and other chemicals through structured and macroporous soil s. For sorbing pesticides, transport associated with a mobile colloidal or particulate phase represents an additional transport mechanism in structure d soils that is not well understood, We investigated particle-facilitated t ransport of a sorbing pesticide (pro-chloraz, N-propyl-N-[2-(2,4,6-trichlor ophenoxy)ethyl]imidazole-1-carboxamide) in a 5- by 5-m subsurface-drained f ield plot in a structured sandy loam in Denmark, Following pesticide applic ation, three simulated rainfall events during an 8-d period were monitored in terms of drainage now rate, content of particulate matter (>0.24 mu m), and pesticide concentration in the solution and in the particulate phases i n the drainage water. The fraction of pesticide loss to the drain was 0.2% of the applied mass, of which 6% was associated with the particulate phase. Macroporous now paths appeared to be major routes of pesticide and particl e transport. Preferential sorption to particles in the drainage water relat ive to bulk soil, and possibly also slow desorption from the particles, wer e assumed to influence the pesticide Leaching in the particulate fraction. Based on experimental and reported data, the dual-porosity model MACRO, mod ified to account for particle mobilization and transport, could be calibrat ed to simulate the observations, Sensitive parameters for the particle and pesticide descriptions were identified.