Antecedent and Post-Application Rain Events Trigger Glyphosate Transport from Runoff-Prone Soils

Recent environmental surveys report widespread detections of the herbicide glyphosate [N-(phosphonomethyl)glycine] in surface waters, despite its strong immobilization and rapid biodegradation in soils. We performed four high-frequency sampling campaigns (from 2015 to 2017) following controlled spray applications on an experimental perennial grass field site with wetness-prone marginal soils. We monitored dissolved glyphosate concentrations in the outflow (runoff and shallow drainage) using liquid chromatography mass spectrometry and enzyme-linked immunosorbent assays. Rainfall-triggered outflow events occurred between 3 and 13 days following spray application. Outflow concentrations varied widely from nondetectable levels to 90 mu g L-1, peaking during the first significant outflow event in each campaign and diminishing as flows subsided. Subsequent outflow peaks caused concentrations to increase again but to a lesser extent. Cumulative mass efflux in outflow across the different campaigns ranged from 0.06 to 1.0% of applied glyphosate. Cumulative glyphosate losses in the outflow were not during the postspray sampling period, but rather with soil hydrologic conditions at the time of spraying as reflected by the 7 day cumulative prespray rainfall, with wetter antecedent conditions favoring greater cumulative mobilization. Avoiding spraying under such conditions may mitigate potential glyphosate mobilization.