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

Occurrence, Deposition, and Long Range Transport of Herbicides in Precipitation in the Midwestern and Northeastern United States

Abstract

Herbicides were detected in precipitation throughout the midwestern and northeastern United States during late spring and summer of 1990 and 1991. During May and June atrazine, alachlor, or both were detected in 60 to 75 percent of weekly accumulations of precipitation collected at 81 sampling sites in 23 States. Atrazine or alachlor were detected in at least one sample from all States, including samples from remote wilderness areas such as Isle Royale in northern Lake Superior. During the remainder of the year, herbicides generally were detected in weekly samples at less than about 20 percent of the sites. Atrazine was the most frequently detected herbicide, followed by alachlor, desethylatrazine, and metolachlor. Herbicide concentrations and frequency of detection were much higher in the Midwest than elsewhere. Precipitation-weighted herbicide concentrations for mid-April through mid-July typically were 0.2 to 0.4 µg/L (micrograms per liter) in the Midwest and weighted concentrations as large as 0.9 µg/L were present at a few sites. Concentrations of 1 to 3 µg/L were measured in a few individual samples representing small amounts of precipitation. Deposition rates for both atrazine and alachlor ranged from more than 200 (µg/m²)/yr (micrograms per square meter per year) at a few sites in the Midwest to less than 10 (µg/m²)/yr in the Northeast. These amounts represent less than 1 percent of the atrazine and alachlor applied annually to crops. The geographic pattern of herbicide deposition provides evidence for long-range atmospheric transport.