A Watershed Study of Glyphosate Transport in Runoff

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Abstract
Glyphosate (N‐[phosphonomethyl]glycine)3 formulated as Round‐up® herbicide,4 was applied on 0.3‐ to 3.1‐ha watersheds at rates of 1.10‐, 3.36‐, and 8.96‐kg/ha as a preseeding herbicide in the no‐tillage establishment of rescue (Festuca arundinacea L.) and corn (Zea mays L.). Runoff from natural rainfall following early springtime treatments was measured and analyzed to define concentration and transport of glyphosate under these conditions.The highest concentration of glyphosate (5.2 mg/liter) was found in runoff occurring 1 day after treatment at the highest rate. Glyphosate (2 µg/liter) was detected in runoff from this watershed up to 4 months after treatment. For the lower application rates, maximum concentration of the herbicide in runoff was < 100 µg/liter for events occurring 9–10 days after application and decreased to µg/liter within 2 months of treatment. The maximum amount of glyphosate transport by runoff was 1.85% of the amount applied, most of which occurred during a single storm on the day after application. In each of the three study years, herbicide transport in the first runoff event following treatment accounted for 99% of the total runoff transport on one watershed. Glyphosate residues in the upper 2.5 cm of treated soil decreased logarithmically with the logarithm of time; they persisted several weeks longer than in the runoff water.