The globally used herbicide glyphosate [N-(phosphonomethyl)glycine] and its most frequently detected metabolite, aminomethylphosphonic acid (AMPA), were studied in a unique 12-yr field-scale monitoring program. The leaching of glyphosate, AMPA, and soil particles was studied in a shallow drainage system beneath a 1.26- ha field. Five annual glyphosate applications were applied with different autumn application dates. Solute mass flux from the drain system following the five glyphosate applications were compared to determine how different factors affect the leaching of glyphosate, AMPA, and particles. Glyphosate and AMPA leaching were highly event driven, controlled by the time and intensity of the first rainfall event after glyphosate application. A high similarity in cumulative drainage and leached pesticide masses with time suggests near-constant drainage and leaching rates. There was no clear relationship between particle-facilitated transport and the transport of glyphosate or AMPA. However, soil particles, glyphosate, and AMPA all showed distinct, simultaneous concentration curves, indicating common dominant transport mechanisms. Also, soil-water content at the time of application and the level of the groundwater table relative to the drain depth exerted clear controls on detection of solutes in the drainage water. To summarize our findings, we present a leaching risk chart to illustrate the dependence of glyphosate, AMPA, and soil particle leaching based on rainfall intensity and the timing of rainfall events after glyphosate application.

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