GLYPHOSATE IN SUSPENDED SEDIMENT IN STREAMS FROM TWO AGRICULTURAL AREAS OF THE UNITED STATES

Glyphosate-based herbicides are used extensively used in agricultural areas for control of weeds on glyphosate-resistant crops (mainly corn, soybeans, and cotton), as well as in pre-plant weed removal. In this study we measured the concentrations of glyphosate in suspended sediment collected from two small streams in predominantly agricultural areas of Iowa and Mississippi. Time-integrating sediment samplers were used to collect suspended sediment during 27 - 70 day sampling events from July through September 2007. The use of such samplers facilitated the collection of suspended sediment during major storm-runoff at each location. Concentrations of glyphosate in suspended sediment were 37 - 172 ng/g in Iowa, and 437 - 1755 ng/g in Mississippi. Based on median concentrations of glyphosate in suspended sediment and in stream samples collected during these sampling events, the glyphosate in suspended sediment represented a small fraction (1 - 12 percent) of the total glyphosate load in the stream. During storms, glyphosate concentrations increased in suspended sediment, and the amount of glyphosate increased relative to aminophosphomethylamine (a glyphosate degradate), suggesting shorter residence times in the soil before transport to the streams. Calculated distribution coefficients (Kd) for glyphosate in the suspended sediment in streams ranged from 247 to 1,505 L/kg, one order of magnitude higher than those from batch equilibrium studies of glyphosate sorption to soils. The results from this investigation suggest that the transport of glyphosate on suspended sediment in agricultural streams may be greater than that predicted from soil and microcosm studies, and aquatic exposures are less than predicted. These results also indicate that field studies of glyphosate in streams, and potential interaction with sediments, are an important component of aquatic-risk assessments for this and other herbicides.