COLLOID-FACILITATED TRANSPORT OF PESTICIDES IN THE MACROPORES OF A DANISH ALFISOL

The leaching of two pesticides (pendimethalin and glyphosate) through macropores in the unsaturated zone of a Danish alfisol was investigated in the laboratory. The soil was sampled as 0.3 m diameter and 0.5 meter long cores and the pesticides were added as radioactive labelled compounds (on carbon). Leaching was controlled by artificial rain water (3 consequtive additions). Each pesticide was added to 4 columns sampled from the soil surface to 50 cm's depth. Of these, 2 columns were sampled on a ploughed field and 2 from a near by minimally tilled field. The amounts of colloids generated differ significantly between the two treatments (50-97 mg colloid /l leached from the minimum tilled field and 183-295 mg/l from the ploughed field). Interestingly, we found no detectable difference in the composition of the colloidal fractions as investigated by infrared spectroscopy. Although the detailed development of geochemical properties of the pore water remains unknown there is ample evidence from macroscopic parameters that the properties of the pore water changes as it interact with the soil material. That would also potentially change the adsorption equilibria involving the pesticides. Experiments carried out to evaluate the sorption/desorption kinetics for glyphosate showed that a considerable desorption of pesticide from takes place from the colloids in the effluent, if colloid facilitated transport was dominating. I.e. transport can be colloid-facilitated initially and then at later stages it has the characteristics of mass transport in a solution.