Herbicides in streams. Midwestern United States

Results from a 2-year study of 149 streams geographically distributed across the corn-producing region of 10 midwestern States show that detectable concentrations of herbicides persist year round in most streams. Some herbicides exceeded proposed maximum contaminant levels for drinking water for periods of several weeks to several months following application. Atrazine was the most frequently detected and most persistent herbicide measured, followed by desethylatrazine and metolachlor. The seasonal distribution of atrazine indicates that aquifers contributing base flow to many of the streams are contaminated with herbicides.