Particle-facilitated transport of prochloraz in undisturbed sandy loam soil columns

The mobility of strongly sorbing pesticides in soils may be higher than formerly assumed, due to facilitated transport along with mobilized colloids. Our objective was to verify whether facilitated transport enhances the vertical movement of prochloraz (N-propyl-N-[2-(2,4,6-trichlorophenoxy)ethyl] imidazole-l-carboxamide). Experiments were carried out using 20 cm diam. by 20 cm long undisturbed soil columns taken from the topsoil (sandy loam, Typic Hapludalf) from a held under barley/grass (Hordeum vulgare L./Lolium perenne L.) cultivation near Rogen, Denmark. Prochloraz was applied to the surface as a pulse in solution. The irrigation intensity was 10 mm h(-1) and the lower boundary of the columns was at atmospheric pressure. Three treatments each with four replicates were performed to study the effect of ionic strength and pH on pesticide and particle transport. Leaching of particles and pesticide was promoted by decreasing the ionic strength of the irrigation solution, and increasing pH by ammonia application. This is in line with the theory of electrostatic interactions. Preferential flow and particle transport were the two most important factors determining the amount of pesticide leached. A large variability in amount leached was observed among the replicates (standard deviations of 38-141%) mainly due to differences in water transport in the columns. Particle-facilitated transport was significant, but did not dominate prochloraz transport. Depending on the pH and ionic strength of the applied solution, 2.5 to 13.1% of the leached pesticide was sorbed to particles with diameter d > 0.24 mu m, and 3 to 9% to particles with 0.02 < d < 0.24 mu m.