National Water Quality Assessment Program: Eastern Iowa Basins

Comparison of the Atmospheric Occurrence and Deposition of Glyphosate and Conventional Herbicides in Two Agricultural Areas of the United States

P. Capel, US Geological Survey;

F. Chang, Clarkson University;

M. Simcik, University of Minnesota;

M. Majewski, W. Foreman, S.Kalkhoff, R. Coupe, US Geological Survey

Program abstracts for 32nd annual North American meeting of the Society of Environmental Toxicology and Chemistry,November 13-17, 2011 Boston MA.

The use of herbicides to combat weeds is an important practice in modern agriculture. During the 1990s, crops which were genetically-modified (GM) to tolerate the effects of a specific herbicide, most commonly glyphosate, were introduced and widely accepted. In 2007, glyphosate-tolerant GM-soybeans, cotton and corn comprised 95, 80 and 59 percent of these crops, respectively. The widespread adoption of herbicide-tolerant GM crops has caused a major shift away from the use of conventional pre-emergent herbicides (triazines, acetanilides, etc.) towards glyphosate. During the 2007 growing season, the air and rain at agricultural sites in Iowa and Mississippi were monitored for 23 herbicides, including glyphosate. Glyphosate, a relatively non-volatile compound, enters the atmosphere primarily through application spray drift and wind erosion. In Iowa, the median glyphosate concentration in rain (0.20 mg/L) was 10 to 100 times greater than the median concentrations of metolachlor, acetochlor and atrazine, although their detection frequencies were similar (65, 76, 71 and 59% percent for glyphosate, metolachlor, acetochlor and atrazine, respectively). The results were similar at the site in Mississippi where median glyphosate concentration in rain was 0.25 mg/L, about 10 to 100 times greater than the median concentrations of atrazine, metolachlor, and propanil. In air, glyphosate was detected less frequently (61 and 86% in Iowa and Mississippi) than the most common conventional herbicides such as atrazine, trifluralin and metolachlor, but at similar median concentrations. Wet deposition of glyphosate during the growing season was calculated to be 50 and 130 ug/m2 compared to 90 and 104 ug/m2 for the sum of the other 22 targeted conventional herbicides in Iowa and Mississippi, respectively. us, although it is non-volatile, the atmospheric occurrence, concentration, and depositional fluxes of glyphosate are similar to or greater than the more volatile conventional herbicides due to the sheer magnitude of its use.

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