Concern exists about the occurrence of agricultural chemicals, primarily nitrate and pesticides, in shallow ground water. In response to this concern, the focus of the Iowa ground-water-quality-monitoring program has shifted to emphasize nonpoint contaminants. The program, which began in 1982, is operated by the U.S. Geological Survey in cooperation with the University of Iowa Hygienic Laboratory and the Iowa Department of Natural Resources, Geological Survey Bureau and Environmental Protection Division. The objectives are to describe and assess the long-term chemical quality of the principal aquifers in Iowa and to direct water-quality assessment and sampling toward regional ground-water-quality concerns.

In the spring of 1985, emphasis was placed on the analysis of pesticides in water samples collected from wells completed in surficial aquifers in Quaternary deposits or from wells less than 200 feet deep or both. Samples from these wells were analyzed for nitrate and selected pesticides. In water samples collected from 515 individual shallow wells, 6 percent had nitrate concentrations larger than the maximum contaminant level for public drinking water of 10 milligrams per liter as nitrogen. The concentrations ranged from less than 0.1 to 53 milligrams per liter as nitrogen. Eighteen percent of the samples had concentrations of nitrate between 5 and 10 milligrams per liter as nitrogen. Water samples from 355 individual municipal wells less than 200 feet deep have been analyzed for pesticides at least once. Detectable concentrations of at least one pesticide, including alachlor, atrazine, cyanazine, dicamba, metolachlor, or metribuzin, were detected in 20 percent of the samples. Atrazine was the most prevalent pesticide; concentrations, which ranged from 0.10 to 21 micrograms per liter, were detected in 18 percent of the samples.

The results of the monitoring program indicate a relation between decreasing well depth and the presence of detectable concentrations of nitrate and pesticides. Most nitrate and pesticide detection was in areas where surficial aquifers in Quaternary deposits are the principal source of ground water. Pesticides were detected in samples collected throughout the year; samples collected in late spring and early summer more frequently contained pesticides than the other samples.