Leaching of herbicides and their metabolites in lysimeters filled with soils from railway tracks

Glyphosate currently is the most-widely used herbicide for weed control on railway tracks. However, railway companies look for non-chemical and chemical alternatives. Glyphosate has a low potential for leaching to groundwater and alternative herbicides should ideally share this behavior. We investigated the leaching of glyphosate and five potential alternatives in lysimeters filled with three different soils from railway tracks. Herbicides were applied in three consecutive years to 4-7 lysimeters. Leachate was collected and analysed for herbicides and major metabolites. Highest annual average concentrations of 23 μg/L were found for atrazine, which was selected as mobile reference compound. 2,4-D, flazasulfuron, and its major metabolite DTPU reached annual average concentrations of 0.60, 0.43, and 0.50 μg/L, respectively. Pelargonic-acid was detected in single samples shortly after application and heavy rainfall. Quizalofop-P, glyphosate, and metabolites were only sporadically found in leachate at annual average concentrations ≪0.1 μg/L. Highest leachate concentrations were generally observed in lysimeters with a high proportion of sandy fine material and little organic carbon, except for the quickly-degradable pelargonic-acid and 2,4-D that reached highest concentrations in lysimeters with high gravel content and thus small effective transport volume. After three years, up to 30, 6.5, and 0.56 % of the applied mass of flazasulfuron, atrazine, and 2,4-D, respectively, was recovered in leachate in form of the active substances and/or their metabolites (expressed in parent equivalents). Although recoveries of flazasulfuron exceeded those of atrazine, concentrations were lower due to the 100-fold lower application rate. From the perspective of groundwater protection, the currently used glyphosate still is by far the best (chemical) option while all tested possible glyphosate alternatives did show some potential for leaching which may, however, be manageable with certain restrictions (e.g., spot spraying). These findings are likely applicable to other uses in non-agricultural settings with gravel or paving stone surfaces.