Leaching of glyphosate and AMPA as affected by soil properties and precipitation distribution

The leaching of glyphosate was evaluated under field conditions at one sandy and two loamy sites in Denmark. Pesticides were applied to the test fields as part of standard agricultural practice and in accordance with current regulation. Samples of drainage water, groundwater, and soil water taken from the unsaturated zone were analysed for pesticide contents. At one loamy site, glyphosate and its metabolite, aminomethylphosphonic acid (AMPA), leached from the root zone to the tile drains in average concentration considerably exceeding 0.1 µg/litre. The average concentration of glyphosate and AMPA in the drainage water during the leaching period of 2000/01 was 0.54 and 0.17 µg/litre, respectively. The leaching appeared to be governed by a combination of pronounced macropore flow occurring shortly after application, as well as a limited sorption and degradation capacity allowing both compounds to leach continuously throughout the six-month leaching period. Evidence of glyphosate and AMPA leaching was restricted to the depth of the tile drains, with neither of these compounds detected in groundwater monitoring screen situated beneath the depth of the tile drains. At the two remaining sites, the leaching of both glyphosate and AMPA was negligible.