Concentrations and mass transport of herbicides in streams throughout the midwestern United States follow an annual cycle. During storm runoff in May, June, and July, following herbicide application, a small fraction, generally less than 3 percent of the herbicide mass applied to cropland, is transported into streams. However, this mass is large enough to produce concentrations of some herbicides in excess of 3 micrograms per liter in the Mississippi River for a short period of time. Concentrations typically are an order of magnitude larger in tributary streams. Herbicide concentrations quickly decrease in mid to late summer, but some herbicides are detected at low concentrations year round, probably due to storage in surface-and groundwater reservoirs. Herbicide concentrations and mass transport measured in streams generally are related to the amounts of herbicides applied. Insecticides follow the same general pattern of occurrence and transport as herbicides, but they are less persistent, concentrations are lower, and peak concentrations occur later than herbicides. The cycle for nitrate-nitrogen differs from that of herbicides in that the highest concentrations and mass transport occur in the winter and spring. Low nitrate concentrations in the summer probably are due to increased use of nitrate by terrestrial and aquatic plants, and reduced leaching from soils to streams.

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