Pesticide transport under runoff-erosion potentially dominated by small sediments: A glyphosate and AMPA experiment

Abstract

Glyphosate and its degradation product AMPA, are ecotoxic, recurrent and persistent in agricultural soils, susceptible to overland transport by runoff, and sediment erosion due to their strong sorption affinities. We hypothesize that eroded sediments of different sizes have differing sorbed concentrations and relative contributions to glyphosate and AMPA transport, due to different specific surface areas and adsorption site abundances. Hence, we conducted a flume experiment of glyphosate-polluted sediment erosion in a rainfall simulator. After 1 h of rainfall (90 mm water), 10 % of applied glyphosate degraded to AMPA. The top 2 cm of soil retained 68 % of the total glyphosate-equivalent (including AMPA) mass, while runoff and eroded sediment accounted for 8 % and 10 % respectively. Amongst pesticides transported overland, runoff (61 %) and eroded sediment (39 %) were similarly important for glyphosate, but eroded sediment (95 %) transported remarkably more AMPA than runoff (5 %), although glyphosate sorption affinities are typically larger. Small sediments (<0.25 mm) constituted 75 % of eroded sediment counts, but carried 60 % of sediment-phase glyphosate and 85 % of sediment-phase AMPA mass. In < 0.25 mm sediments, unlike glyphosate, AMPA breakthrough concentrations were substantially greater than in larger sediments. As water and pollutant mass exchanges between various environmental compartments (soil moisture, soil grains, runoff, eroded sediment, biodegradation) are highly dynamic, equilibrium sorption affinities alone may not fully characterize the predominant modes of pollutant transport, which may vary across spatio-temporal scales. Therefore, pollutants that are preferentially transported by eroded sediments, particularly small sediments, should be identified and prioritized in future research, due to potentially amplified environmental impacts.