Modelling herbicides movement in railway embankments in Germany.

Railway operations have long relied on herbicides to keep tracks free of vegetation and ensure safety and track stability. In Germany, regulatory changes introduced in 2020 have increased concern about the long-term environmental effects of herbicide use along railway corridors. This study examines the fate and transport of glyphosate, its metabolite aminomethylphosphonic acid (AMPA), flumioxazin, and flazasulfuron beneath railway embankments using HYDRUS-2D simulations and field monitoring data from five German sites. Simulations were carried out under identical application scenarios, focusing on the influence of local hydrogeological conditions, precipitation patterns, and groundwater depth. Results show that the standardized embankment geometry produces a consistent internal flow regime, characterized by vertical percolation through the ballast, lateral redirection at the subgrade protective layer (PSS), and convergent flow toward the embankment slopes. The ballast layer was identified as the dominant retention zone, while site-specific differences in contaminant fate were primarily controlled by the geological boundary conditions beneath the embankment. Three distinct hydrogeological regimes emerged: sites without a geological barrier showed retention comparable to that of the reference scenario; a site with a low-permeability silt layer exhibited anomalously high retention; and a site with a shallow water table over permeable deposits showed the highest potential groundwater vulnerability. The interaction between climatic forcing and geological conditions was multiplicative, implying that site-specific risk assessments must consider both factors simultaneously. AMPA shows highest peak concentrations, exceeding those of glyphosate, flazasulfuron the deepest plumes and railway-specific sorption parameters was essential to reproduce observed concentration patterns.