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

Temporal Distributions of Herbicide and Nitrite-Nitrate Concentrations and Loads in the West Fork of the Big Blue River, Nebraska, 1990 and 1991-92

Abstract

The West Fork of the Big Blue River in Nebraska Drains a relatively flat basin of about 1,200 square miles in which the dominant land use is rowcrop agriculture. From April through July 1990 and April 1991 through March 1992 water samples were collected from the West Fork of the Big Blue River and analyzed principally for concentrations of selected herbicides, nitrite-nitrate, and suspended sediment. The purpose of the investigation was to determine the occurrence, temporal distribution, and persistence of selected herbicides and nitrite-nitrate in surface water within the West Fork of the Big Blue River Basin and to determine the effects of agricultural practices on the concentrations of these constituents.

Atrazine, alachlor, metolachlor, and cyanazine were the herbicides most commonly detected and present in the largest concentrations in the West Fork of the Big Blue River. Large concentrations of these herbicides were present in the river during the first major runoff events after spring chemical application in the fields. The magnitude of the concentrations diminished with successive runoff events. The maximum observed concentrations of the four herbicides were 116, 43, 26, and 8.6 micrograms per liter, respectively. Persistent concentrations of atrazine and some of its daughter products in base flow indicate that ground water is contributing atrazine to the river. On the basis of estimated herbicide application and estimated herbicide loads in the river in 1990 and 1991-92, about 0.4 to 1.3 percent of the total amount of herbicides applied are transported annually in the West Fork of the Big Blue River. The 3- to 4-fold increase in concentrations and loads of selected herbicides from the 1990 to 1991-92 sampling periods probably reflect the timing of chemical application and periods of major precipitation.

Nitrite-nitrate concentrations in the West Fork of the Big Blue River averaged about 2.2 milligrams per liter and unlike herbicides showed limited variation between periods of high and low-flow. The timing of nitrite-nitrate loads in the river indicates that the principal source of nitrogen is probably surface runoff of nitrogen fertilizer. Ground water also may be contributing to low-flow concentrations of nitrite-nitrate observed in the river. The load of nitrite-nitrate in the river is estimated to be about 0.6 percent of the nitrogen applied to croplands in the basin during each of the two years.