A study of suspended and colloidal matter in the leachate from lysimeters and its role in pesticide transport

The relative importance of suspended (>0.45 mu m) and organic colloidal (<0.45 mu m) matter in the transport of pesticide through and across a soil was studied by monitoring suspended and colloidal matter from lysimeters over a winter (1994-1995) and relating it to the leaching of pesticides. The major results were: (i) The organic colloidal matter and suspended sediment appeared at different times in the leachate. (ii) The suspended sediment was enriched in total organic carbon (TOC). (iii) The leaching of pesticide occurred largely in the first few samples following rainfall, coinciding with the occurrence of the colloidal matter. (iv) Modelling the distribution of pesticide between the colloids, suspended matter, and pure solution suggests that negligible amounts of isoproturon are carried on suspended sediment and the organic colloids are more important. The importance of organic colloids may be even more significant for compounds more hydrophobic than those studied here: this has implications for the management of soils and pesticides in relation to the release of organic colloids into solution.