National Water Quality Assessment Program: Eastern Iowa Basins

Pesticides in Rain in the Mississippi River Valley: A Comparison Between 1995 and 2007

M.S. Majewski, S.J. Kalkhoff, R.H. Coupe, P.D. Capel, US Geological Survey;

W.T. Foreman, US Geological Survey, National Water Quality Laboratory.

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

Weekly composite rainfall samples were collected during the 1995 and 2007 growing seasons at two agricultural locations in both Iowa and Mississippi, and analyzed for 47 pesticides and transformation products (TPs) in 1995, and 76 in 2007. Thirty two analytes were detected at least once in one or both years; 11 analytes were detected both years. Herbicides were the predominant type of pesticide detected in both years and states. In Iowa, 15 herbicides, 7 insecticides, and 2 TPs were detected in 1995 with total flux of 445 ug/m2/season. Nine pesticides detected in 50% or more of the samples accounted for 94% of the deposited mass. In 2007, 8 herbicides, 3 insecticides, 2 fungicides, and 4 TPs were detected with a total flux of 186 ug/m2/season. Six pesticides detected in 50% or more of the samples accounted for 86% of the flux. Acetochlor, alachlor, atrazine, CIAT (an atrazine TP), DCPA, and chlorpyrifos were among the top 10 most frequently detected compounds in both years. In Mississippi,16 herbicides, 9 insecticides, and 1 TP were detected in 1995 with a total flux of 1981 ug/m2/season, but methyl parathion accounted for 90% of this flux. Nine pesticides that were detected in 50% or more of the samples accounted for 98% (83% not including methyl parathion) of the deposited mass. In 2007, 11 herbicides, 6 insecticides, 2 fungicides, and 6 TPs were detected with a total flux of 436 ug/m2/season. Five pesticides detected in 50% or more of the samples accounted for 54% of the flux. Atrazine, metolachlor, and propanil were among the top 5 compounds detected in rainin both years in Mississippi. e amount, timing, and frequency of rainfall play a critical role in when and which pesticides are detected, their concentrations, and their seasonal flux. During the spring when most herbicides are applied, rain events in Iowa were more frequent in 1995 than in 2007. is was reflected by higher detection frequencies, mean concentrations, and total flux in 1995. In Mississippi, rain events occurred more uniformly throughout the growing season in both years, yet the total pesticide flux increased from 1995 to 2007 while the number of compounds detected at greater than 50% decreased by about a half. Much of the increased 2007 deposition amount is attributed to the considerable increase in use of atrazine and glyphosate and the number of compounds detected below 50% frequency that contributed significantly to the total flux, 7 in 2007 versus 2 in 1995.


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