Sorptive diffuse pollutant transport under runoff-erosion potentially dominated by small sediments

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 hour 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 (