National Water Quality Assessment Program: Eastern Iowa Basins

EIWA Abstracts

Map showing the Eastern Iowa Basins NAWQA study area

The occurrence of chloroacetanilide and triazine herbicide metabolites in streams in eastern Iowa

By S.J. Kalkhoff and E.M.Thurman

Abstracts of the Seventh Symposium on the Chemistry and Fate of Modern Pesticides, Lawrence KS, September 14-16, 1999

The occurrence of chloroacetanilide and triazine herbicides and metabolites were investigated from 1996 to 1998 as part of the U.S. Geological Survey's National Water-Quality Assessment Program. A total of 355 water samples were collected from 12 stream sites in the eastern Iowa. Monthly samples were collected for one year beginning in March 1996 and during the growing season (April to September) in 1998. Samples were collected weekly or biweekly at three sites in 1997 to determine temporal variability. Samples were analyzed for the chloroacetanilide herbicides, acetochlor, alachlor, and metolachlor, and their sulfonic (ESA) and oxanilic acid (OA) metabolites. Samples were also analyzed for the triazine herbicides, atrazine and cyanazine, and three of their common metabolites.

Chloracetanilide and triazine herbicide metabolites are substantial components of the pesticide load in eastern Iowa streams. Metabolites are detected more frequently and at greater concentrations than the parent compounds. When adjusted to a common detection threshold of 0.20 micrograms per liter, the four most frequently detected compounds were metolachlor ESA (detected in 99.7 % of the samples), alachlor ESA (99.1%), metolachlor OA (94.3 %), and acetochlor ESA (71.8 %). Metolachlor, the most commonly occurring parent compound, was detected in 54.1 % of the samples. On average, metabolites accounted for most (93 %) of the total pesticide mass measured in each sample. Metolachlor compounds account for 63 %; alachlor for 19 %; acetochlor for 9.5 %; atrazine for 6.9 % and cyanazine for 1.2 % of the metabolites. The monthly median total metabolite concentration ranged from 4.6 micrograms per liter in February to 10.0 micrograms per liter in June. In contrast, the monthly median total pesticide concentration ranged from 0.16 micrograms per liter in October to 3.8 micrograms per liter in June. The data suggest that differing chemical properties, application rates, and precipitation may affect metabolite concentrations in streams.

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