Trends of glyphosate residue in US Midwest nested watersheds

Glyphosate, a non-selective herbicide, is widely used in US agriculture, raising concerns about its off-site transport into streams. This study investigated glyphosate concentrations in streams within nested agricultural watersheds in the US Midwest, dominated by corn (Zea mays)-soybean (Glycine max) rotations. Over a 7-year period (2005-2011), water samples from three nested watersheds were collected daily or every other day and analyzed for glyphosate. Results showed no detectable glyphosate in over 80% of samples (detection limit: 0.62 µg L^{- 1}), with the highest concentrations observed during rainstorms. The maximum glyphosate concentration observed (102 µg L^{- 1}) was below the US Environmental Protection Agency drinking water standard (700 µg L^{- 1}) and the threshold for protecting aquatic life (12,000 µg L^{- 1}). Regressions indicated that glyphosate concentrations correlated strongly with phosphate concentrations, precipitation, and seasonal variations. Additionally, the watershed with the most susceptibility to erosion showed the highest glyphosate off-site transport. These findings highlight the impact of hydrological and chemical factors on glyphosate transport under the conditions evaluated in this study.