Assessment of the bioaccessibility of glyphosate in soil using a physiologically based extraction test

Abstract

The herbicide glyphosate (N-(phosphonomethyl) glycine) is among the most extensively used agrichemicals worldwide. The environmental fate of glyphosate has been well studied, revealing that glyphosate rapidly binds to soil. The objective of this study was to provide an initial assessment of whether this binding also reduces the bioaccessibility of glyphosate under simulated gastrointestinal tract conditions, and therefore might diminish bioavailability and risk from incidental ingestion of glyphosate contaminated soil in humans. Five soil samples were constructed to vary in composition with respect to aluminum, iron, phosphate, clay, and carbon content —factors that have been shown to influence glyphosate soil binding. Bioaccessibility of glyphosate at 4 or 40 mg/kg was evaluated in each soil sample using a Physiologically Based Extraction Test (PBET). Bioaccessibility of glyphosate at 4 mg/kg was significantly decreased by elevated aluminum and iron (p < 0.001) in the stomach phase, but not at the higher concentration or in the intestinal phase. Bioaccessibility of glyphosate was essentially complete in all other soils at both glyphosate concentrations. Although there are acknowledged limitations in the study, the results suggest that the relative oral bioavailability (RBA) of glyphosate from soil may be at or near 100% for human health risk assessment purposes.