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TECHNICAL REPORTS

Vadose Zone Processes and Chemical Transport

Leaching of Glyphosate and Amino-Methylphosphonic Acid from Danish Agricultural Field Sites

^a Geological Survey of Denmark and Greenland, Øster Vold 10, DK-1350 Copenhagen, Denmark
^b Danish Institute of Agricultural Sciences, Research Centre Foulum, DK-8830 Tjele, Denmark
^c National Environmental Research Institute, Vejlsøvej 25, DK-8600 Silkeborg, Denmark

^* Corresponding author (jkj{at}geus.dk)

Received for publication November 12, 2003. Pesticide leaching is an important process with respect to contamination risk to the aquatic environment. The risk of leaching was thus evaluated for glyphosate (N-phosphonomethyl-glycine) and its degradation product AMPA (amino-methylphosphonic acid) under field conditions at one sandy and two loamy sites. Over a 2-yr period, tile-drainage water, ground water, and soil water were sampled and analyzed for pesticides. At a sandy site, the strong soil sorption capacity and lack of macropores seemed to prevent leaching of both glyphosate and AMPA. At one loamy site, which received low precipitation with little intensity, the residence time within the root zone seemed sufficient to prevent leaching of glyphosate, probably due to degradation and sorption. Minor leaching of AMPA was observed at this site, although the concentration was generally low, being on the order of 0.05 µg L^–1 or less. At another loamy site, however, glyphosate and AMPA leached from the root zone into the tile drains (1 m below ground surface [BGS]) in average concentrations exceeding 0.1 µg L^–1, which is the EU threshold value for drinking water. The leaching of glyphosate was mainly governed by pronounced macropore flow occurring within the first months after application. AMPA was frequently detected more than 1.5 yr after application, thus indicating a minor release and limited degradation capacity within the soil. Leaching has so far been confined to the depth of the tile drains, and the pesticides have rarely been detected in monitoring screens located at lower depths. This study suggests that as both glyphosate and AMPA can leach through structured soils, they thereby pose a potential risk to the aquatic environment.

Abbreviations: AMPA, amino-methylphosphonic acid • BGS, below ground surface

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