Samples collected from wells in Carroll and Guthrie Counties, Iowa, were analyzed to describe the chemical quality of the ground water in shallow aquifers. The study began in June 1986 with the selection of 20 shallow wells in Carroll County and 22 shallow wells in Guthrie County. These wells were sampled in the summer and fall of 1986 and 1987. Samples from the wells were analyzed for major ions, nutrients, trace elements, radiochemicals, nitrate, and pesticides.

Median and mean nitrate concentrations were 95 and 11 milligrams per liter. About 60 percent of the samples had nitrate concentrations greater than 5.0 milligrams per liter. Detectable concentrations of a least one pesticide were in 27 percent of the samples. Atrazine, the most prevalent pesticide, was detected in 25 percent of the samples; alachlor was detected in 6 percent; and cyanazine was detected in 5 percent. Dicamba, metolachlor, trifluralin, and2,4-D were detected in 2 percent or less of the samples.

In the summer and fall 1986,43 and 45 percent of the water samples had nitrate concentrations larger than 10 milligrams per liter, the maximum contaminant level for public drinking water specified by the State of Iowa; in the summer and fall 1987,41 and 55 percent of the samples had nitrate concentrations larger than the maximum contaminant level. In the summer and fall 1986,23 and 18 percent of the samples contained detectable concentrations of at least one pesticide; in the summer and fall 1987,21 and 47 percent of the samples contained detectable concentrations of at least one pesticide. Detection frequencies ofatrazine, alachlor, and cyanazine increased for samples collected in October 1987. Pesticides were more frequently detected along Brushy Creek, and the East Nishnabotna, Middle, North Raccoon, and South Raccoon Rivers. However, pesticides also were detected along the Middle Raccoon and West Nishnabotna Rivers. Detection frequencies were greatest (50 percent) for the East Nishnabotna River and Middle River alluvial settings.

Increased nitrate concentrations generally were observed in the shallower wells and were larger than those previously reported for shallow municipal wells. Pesticide detection relative to well depth was inconsistent with the trend observed for nitrate. Pesticide detection frequencies were greater for wells that were 40 to 80 feet deep (about 40 percent) than for wells less that 40 feet deep (about 20 percent).

Of the water samples with nitrate concentrations greater than or equal to 5.0 milligrams per liter, 33 percent had detectable pesticide concentrations. Conversely, for those samples with detectable pesticide concentrations, 76 percent had nitrate concentrations larger than or equal to 5.0 milligrams per liter and 67 percent were larger than or equal to 10 milligrams per liter. The median concentration of nitrate for samples containing at least one detectable pesticide was 15 milligrams per liter and was greater than the overall median nitrate concentration of 95 milligrams per liter. The median for samples that did not contain a detectable pesticide was 73 milligrams per liter.