National Water Quality Assessment Program: Eastern Iowa Basins

Pesticides In Shallow Alluvial Aquifers In Agricultural And Urban Areas In Eastern Iowa And Southern Minnesota

Eric .M. Sadorf

Program abstract of the 43rd Annual Midwest Ground Water Conference, October 12-14, 1998, Lawrence, Kansas

Ground water is an important source for domestic and municipal urban water supplies in the Cedar, Iowa, Skunk and Wapsipinicon watersheds in eastern Iowa and southern Minnesota. About 190 million gallons per day are pumped from ground-water aquifers in these watersheds and about 44 percent, or 83 million gallons per day, are pumped from the alluvial aquifers. Alluvial aquifers generally consist of variable thicknesses of sand and gravel deposits in the valleys of most streams and rivers. The shallow depth to ground-water and permeable materials result in these aquifers being potentially vulnerable to contamination by pesticides used to control vegetation and insects in agricultural and urban areas.

The U.S. Geological Survey, National Water-Quality Assessment Program, conducted a water-quality study in the summer of 1997 to determine the occurrence of selected pesticides in alluvial aquifers in the Eastern Iowa Basins study unit. Thirty one sites in agricultural areas and 30 sites in urban areas with underlying alluvial aquifers were randomly selected for installation of monitoring wells. The monitoring wells were sampled during June through August 1997.

The sum of the concentrations of all detected pesticides was significantly greater in agricultural samples (average of 1.3 ug/l) than in urban samples (average of 0.17 ug/l). Although urban ground- water samples generally had lower pesticide concentrations, a larger number of pesticides were detected. Twenty of 88 pesticide compounds analyzed were detected in water samples. Atrazine was the most commonly detected pesticides in both agricultural and urban samples. Metolachlor was the next most commonly detected compound in agricultural samples and prometon in urban samples. Seventeen of the 20 pesticides detected were herbicides and 3 were insecticides.

Degradation products comprised a substantial part of the pesticide occurrence in both agricultural and urban samples. Concentrations of selected degradation products were 7.9 times greater in agricultural samples and 7.5 times greater in urban samples than the sum of all the parent compound concentrations. Metolachlor ethanesulfonic acid was the most commonly detected metabolite in both agricultural and urban samples. Deethylatrazine was the next most commonly detected metabolite in agriculture samples and alachlor ethanesulfonic acid in urban samples.

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