U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings of the Technical Meeting, Colorado Springs, Colorado, September 20-24, 1993, Water-Resources Investigations Report 94-4015

Pesticides in Near-Surface Aquifers: Results of the Midcontinental United States Ground-Water Reconnaissance, 1991-92

Abstract

Selected pesticides and metabolites were examined in near-surface aquifers in the corn- and soybean- producing region of the midcontinental United States to determine their hydrogeologic, spatial, and seasonal distribution. At least one herbicide or atrazine metabolite was detected in 28.4 percent of the 303 wells sampled during the spring and late summer of 1991. During 1991, deethylatrazine was the most frequently detected compound followed by atrazine, deisopropylatrazine, and prometon. No herbicide concentration exceeded the U.S. Environmental Protection Agency's maximum contaminant levels for drinking water. During 1991, the highest frequency of herbicide detection was in the western part of the study region and lowest frequency of detection was in the eastern part of the study region. Unconsolidated aquifers were found to be more susceptible to herbicide contamination than bedrock aquifers on the basis of results of the 1991 study. During the summer of 1992, additional water samples were collected from 101 wells to examine the occurrence of agricultural chemicals not analyzed during the previous year. Water from 62 percent of the resampled wells had detectable concentrations of either a pesticide or pesticide metabolite; 27 different compounds were detected. During the 1992 study, five of the six most frequently detected compounds were herbicide metabolites.