National Water Quality Assessment Program: Eastern Iowa Basins

Selected herbicides and herbicide metabolites in groundwater in Iowa

by Kalkhoff, S.J., Kolpin, D.W., Sneck-Fahrer, D.A., and Goolsby, D.A.

Summer 1995: Proceedings of the Geological Society of America North-Central Section meeting, May 2-3, 1996, Ames Iowa.

A study to document the occurrence of selected herbicides commonly used in Iowa and some of their metabolites was conducted by the U.S. Geological Survey in coopeation with the Iowa Department of Natural Resources, Geological Survey Bureau and the U.S. Environmental Protection Agency during the summer of 1995. Samples were collected from 106 municipal wells in Iowa that are screened in unconsolidated alluvial and Pleistocene aquifers and in the consolidated bedrock aquifers. Samples were analyzed for 13 herbicides, two triazine metabolites, one alachlor metabolite, and one cyanazine metabolite by gas chromatography/mass spectrometry following solid-phase extraction on C-18 cartridges.

At least one of the herbicides or herbicide metabolies analyzed were detected in 70% of the wells sampled. Commonly more than one herbicide compound was detected in a given water sample (44 of 106) , and as many as nine compounds were detected in one sample. The maximum total herbicide compound concentration for this study was 29 ug/L. Herbicides metabolites were found to be important constituents in groundwater, being 3 of the 4 most frequently detected compounds. Alachlor-ESA was detected almost 9 times more frequently than alachlor. Cyanazine amide was detection over 3 times more frequently than cyanazine. Cyanazine or cyanazine amide was detected coincidentially with 62% of the DIA detections (10 of 16). This result suggests that, although most of the DIA was derived from atrazine because of its much greater frequency of detection in Iowa's groundwater, some portion of the DIA likely has originated from cyanazine.

Herbicide compounds generally were detected more frequently in shallow (less than 150 feet deep) than deep wells; in the alluvial and bedrock aquifers than in the Pleistoce aquifer; and in waters that were oxygenated.

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