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Glyphosate (N-(phosphonomethyl)glycine) is one of the most widely used herbicides in the world to control weeds in agricultural and urban areas. Its increasing use requires special attention to its transfer from terrestrial to aquatic environments. However, knowledge on the leaching of glyphosate and its metabolite aminomethylphosphonic acid (AMPA) is scarce. Here we aimed to assess the dynamic interactions between glyphosate sorption and leaching; and to identify the main factors that influence the two processes in three undisturbed agricultural soils using microlysimeters under outdoor conditions. We studied the sorption, desorption and leaching of 14 C-labelled glyphosate on three soils using batch experiments in the laboratory and lysimeters under natural conditions for 11 months. The laboratory results showed that glyphosate was strongly adsorbed, yielding empirical constants of Freundlich sorption isotherms (K f) of 16.6 for the clay loam soil, 33.6 for the silt clay loam soil and 34.5 for the sandy loam soil, with n f close to 1 in all three cases. Glyphosate was also weakly desorbed, i.e. 5 to 24% (w) of initially sorbed glyphosate. Sorption and desorption were only pH-dependent. The outdoor results showed that nearly 70% of the initial glyphosate was present in the soil in a non-extractable form at the beginning of the experiment. Conversely, only less than 20% of the initial glyphosate is present in the soil in a non-extractable form after 11 months. These findings suggest that the non-extractable residues become available and take part in biodegradation and leaching. The amounts of 14 C-glyphosate derivatives leached were less than 0.28% of the initially applied glyphosate. HPLC analyses showed that the AMPA metabolite generally represented up to 100% of the residues present in the leachates. The results of leaching were highly influenced by the hydrodynamic properties and the biodegradation capacities of the soils. Although glyphosate residues were found in low concentrations in the leachates for almost a year, the contamination of groundwater does not seem to be a concern, regardless of the soil type, if the herbicide is used in accordance with good agricultural practice.

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We studied the sorption and desorption of 14 C-labelled phenanthrene, a polycyclic aromatic hydrocarbon, on soils, both in the laboratory using batch experiments and during 6 months of natural conditions using lysimeters. The laboratory results show that phenanthrene is strongly adsorbed, yielding empirical constants of sorption isotherms K f of 163 for the clay soil and of 77 for the sandy loam soil. This result is unexpected because phenanthrene has a relatively high water solubility of 1.1 mg/L. Phenanthrene is also poorly desorbed, as shown by leaching of only 30% of initially sorbed phenanthrene. The natural condition results show that 27% of the initial phenanthrene occurs as nonavailable, bound residues after 6 months. The leaching of 14 C phenanthrene derivatives amounts to less than 2% of the applied phenanthrene. The 14 C phenanthrene derivatives leached included 80% of polar products. Analysis of the extractable residues from soils by high-performance liquid chromatography (HPLC) showed the occurrence of 9,10-phenanthrenedione, a toxic degradation product, in notable quantities of 2-3% of extractable compounds.