Occurrence and Fate of the Herbicide Glyphosate in Tile Drainage and Receiving Rivers in East Central Illinois

Agricultural fields in the Midwestern United States are commonly tile-drained. This drainage system is used to remove excess water from the soil profile to increase crop production and promote soil conservation. However, subsurface tile drainage can readily deliver nutrients and pesticides from agricultural fields into surrounding watersheds. Glyphosate is a widely used pesticide in Midwestern agricultural fields, especially for modified glyphosate-resistant soybean and corn cultivars. The goal of this project was to develop a reliable and accurate analytical method to monitor the occurrence of glyphosate and its metabolite, aminomethylphosphonic acid (AMPA), in tile drain water and receiving river water collected in east central Illinois agricultural lands. In this study, an isotopic dilution method was developed to analyze trace levels of glyphosate and AMPA in water using liquid chromatography-tandem mass spectrometry (LC-MS/MS), combined with pre-column derivatization and solid phase extraction for sample preparation. The method recoveries of glyphosate and AMPA during the whole monitoring period ranged from 85 to 120% and 83 to 147%, respectively. The limit of detection of the developed methods for glyphosate and AMPA was 0.10 μg/L with a relative standard deviation (RSD) of <10%. The developed method was used to monitor glyphosate and AMPA in tile drainage and their receiving watersheds (e.g., the Spoon River and Salt Fork). Glyphosate and AMPA were frequently detected in river water samples at concentrations ranging from 0.13 to 2.85 μg/L and 0.13 to 1.30 μg/L, respectively. In contrast, the occurrence and concentrations of glyphosate and AMPA in subsurface tile drainage were much less than those in the receiving waters. Study results suggest that surface runoff and soil erosion could be the major transport pathways for glyphosate and AMPA losses from agricultural fields to receiving surface water.