[C-14]glyphosate transport in undisturbed topsoil columns

Although glyphosate (N-(phosphonomethyl) glycine) is one of the most freque ntly used herbicides, few controlled transport experiments in undisturbed s oils have been carried out to date. The aim of this work was to study the i nfluence of the sorption coefficient, soil-glyphosate contact time, pH, pho sphorus concentration and colloid-facilitated transport on the transport of [C-14]glyphosate in undisturbed top-soil columns (20cm height x 20cm diame ter) of a sandy loam soil and a sandy soil. Batch sorption experiments show ed strong Freundlich-type sorption to both soil materials. The mobility of glyphosate in the soil columns was strongly governed by macropore flow. Con sequently, amounts of glyphosate leached from the macroporous sandy loam so il were 50-150 times larger than from the sandy soil. Leaching rates from t he sandy soil were not affected by soil-glyphosate contact time, whereas a contact time of 96h strongly reduced the leaching rates from the sandy loam soil. The role of pH and phosphorus concentration in solution was relative ly unimportant with respect to total glyphosate leaching. The contribution of colloid-facilitated transport was <1 to 27% for the sandy loam and <1 to 52% for the sandy soil, depending on soil treatment. The risk for glyphosa te leaching from the top-soils seems to be limited to conditions where pron ounced macropore flow occurs shortly after application. (C) 2000 Society of Chemical Industry.