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

Annual Use and Transport of Agricultural Chemicals in the Mississippi River, 1991-92

Abstract

The presence of agricultural chemicals (herbicides, insecticides, and nutrients) in the Mississippi River and several tributaries follows an annual cycle. Herbicide concentrations are generally highest during periods of storm runoff following application in the spring and early summer. Nitrate concentrations are generally highest in the winter and spring and lower during the summer and fall. Concentrations and mass transport of agricultural chemicals in rivers are generally related to the amounts of the chemicals used within the river drainage basins. A geographic information system (GIS) was used to analyze relations among county-level estimates of the mass of agricultural chemicals applied (kilograms of active ingredient applied per county) and the annual transport of dissolved chemicals in rivers. Relations were studied for the Mississippi River and several tributaries in the midwestern United States. County-level estimates of herbicide and nitrogen use were developed into a series of GIS data layers and used to estimate the mass of agricultural chemicals used annually within specific drainage basins. Analytical data from periodic water-quality sampling and daily streamflow data were used to estimate the mass of agricultural chemicals transported out of specific drainage basins by rivers. These data sets were used to develop statistical models for estimating annual transport of agricultural chemical as a percentage of estimated annual agricultural chemical use in the basins of the Mississippi River and several tributaries. Results indicated that, in 1991, estimated masses equivalent to about 15 percent of the commercial nitrogen fertilizer, 1.6 percent of the atrazine and cyanazine, 0.8 percent of metolachlor, and 0.2 percent of the alachlor applied in the drainage basins studied were transported out of the basins by rivers.