(Authors and Abstract from a related journal paper of similar title: Brian K. Richards, Steven Pacenka, Michael T. Meyer, Julie E. Dietze, Anna L. Schatz, Karin Teuffer, Ludmilla Aristilde, Tammo S. Steenhuis - all were with Cornell University during 2017 except Meyer and Dietze with US Geological Survey, Organic Geochemistry Research Laboratory, Lawrence, Kansas) 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 carried out four sampling campaigns (during 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 0.01 µg L-1 up to 90 µg L-1, peaking during the first significant outflow event in each campaign and diminishing as flows subsided. Subsequent outflow peaks caused concentrations to again rise but to a lesser extent. Cumulative mass efflux in outflow ranged among campaigns from 0.06 to 1.0 percent of applied glyphosate. Cumulative glyphosate losses in outflow were not associated with total rainfall during the post-spray sampling period but rather with soil hydrologic conditions at the time of spraying as reflected by the 7-day cumulative pre-spray rainfall, with wetter antecedent conditions favoring greater cumulative mobilization. Avoiding spraying under such conditions may mitigate potential glyphosate mobilization.