National Water Quality Assessment Program: Eastern Iowa Basins

Herbicide loads in streams of eastern Iowa: The importance of herbicide metabolites in fate and transport

D. J. Schnoebelen and S.J. Kalkhoff

Geological Society of American 1999 Annual Meeting, October 25-28, 1999
Denver, Colorado

The intense use of soluble and mobile herbicides in the Midwest poses potential problems for nonpoint source contamination to streams and reservoirs. Numerous studies have been conducted to investigate the fate and transport of herbicides, but relatively few studies have considered the herbicide breakdown products (metabolites) in their investigations. Water samples were collected monthly from 11 stream sites in eastern Iowa during 1996-98 and analyzed for commonly used herbicides (acetochlor, alachlor, atrazine, cyanazine, and metolachlor) and selected metabolites. Metabolites analyzed included the sulfonic and oxanilic acids of acetochlor, alachlor, and metolachlor, and deisopropylatrazine, hydroxy-atrazine, and cyanazine-amide.

Herbicide metabolites were detected from 2 to 100 times more frequently than their parent compounds at all sites. Median concentrations of parent compounds ranged from 0.01 to 0.22 micrograms per liter (mg/L) with maximum concentrations ranging from 10.6 to 48 mg/L. Median concentrations of metabolites were higher than their parent compounds and ranged from 0.10 to 3.6 mg/L, but typically had lower maximum concentrations (1.4 to 12.4 mg/L) than their parent compounds. Herbicide and metabolite loads were estimated using a minimum variance unbiased model. Loads of parent herbicides varied seasonally from several hundred kilograms per day in the spring to near zero during the winter, whereas metabolite loads were often several hundred kilograms per day throughout the year. Monthly loads for metabolites of acetochlor, alachlor, and metolachlor were generally 30 to 100 percent larger than loads for their parent compounds whereas loads for metabolites of atrazine and cyanazine were generally 10 to 60 percent higher than their parent compounds.

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