Glyphosate (N-(phosphonomethyl)glycine) is the most widely used herbicide in the world. Despite strong soil sorption and rapid microbial degradation, it is increasingly detected in agricultural runoff and downstream drainage. Due to their propensity for producing outflow from fields, runoff source areas contribute disproportionally to agrichemical transport. We used our 4.9 ha agricultural catchment (primarily perennial switchgrass, Panicum virgatum L.) to determine the extent of glyphosate transport and its underlying mechanisms. Due to soil restrictive layers and topography, the site is prone to generating runoff, including during wet spring and summer months. Intensive annual field sampling campaigns since 2015 monitored dissolved glyphosate in runoff at the field’s outlet weir. We later also measured chloride (from winter road salt at the field’s upper bound) and specific electrical conductance (EC) to segregate slow-moving matrix baseflow vs. rapid overland flow. Field site characterizations (since 2011) included relative drainage status and perched water table dynamics. These data streams have been reported independently, but we here integrate these factors to interpret observed trends. Transport following spray applications (prior to switchgrass emergence in late April/early May) varied in extent and timing as a function of rainfall-induced runoff patterns. Significant runoff events within 1-2 days of spraying resulted in maximum cumulative losses of applied glyphosate of 1.5 percent or more, with peak concentrations of up to nearly 400 µg/L. In contrast, delays of a week or more until the first major event limited losses to as low as 0.2 percent. Glyphosate concentrations were dynamic during each flow event, but also rebounded somewhat at the onset of later peaks. The comparison between concentrations of surface-applied glyphosate vs. other analytes contributing to EC showed that the latter traveled via matrix flow and were diluted in the cumulative field outflow during storms as the fast flow component increased. In contrast, glyphosate concentration increased with the extent of fast flow. We also found that the ratio of glyphosate concentration to EC during a given flow event increased linearly with discharge at the outlet and with the water table height, indicating that overland flow increased as the water table level increased. With these insights we can better posit mitigation measures to reduce losses from runoff-prone agricultural areas.

Glyphosate transport from hydrologically sensitive agricultural fields

ACS Spring 2022

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.