U.S. Environmental Protection Agency
EPA Science Advisory Board
Background Information

Document NameNitrogen Flux and sources in the Mississippi River Basin.
Document AuthorGoolsby, D.A.
Battaglin, W.A.
Aulenbach, B.T.
Hooper, R.P.
Short DescriptionScience of the Total Environment 248 (2-3): 87-100.
CategorySubgroup 2: Characterization of Nutrient Fate-Transport-Sources
Publication Year2000

Abstract: Nitrogen from the Mississippi River Basin is believed to be at least partly responsible for the large zone of oxygen-depleted water that develops in the Gulf of Mexico each summer. Historical data show that concentrations of nitrate in the Mississippi River and some of its tributaries have increased by factors of 2 to more than 5 since the early 1900s. We have used the historical streamflow and concentration data in regression models to estimate the annual flux of nitrogen (N) to the Gulf of Mexico and to determine where the nitrogen originates within the Mississippi Basin. Results show that for 1980-96 the mean annual total N flux to the Gulf of Mexico was 1,568,000 metric tons per year (t/yr). The flux was about 61% nitrate as N, 37% organic N, and 2% ammonium as N. The flux of nitrate to the Gulf has approximately tripled in the last 30 years with most of the increase occurring between 1970 and 1983. The mean annual N flux has changed little since the early 1980s, but large year-to-year variations in N flux occur because of variations in precipitation. During wet years the N flux can increase by 50 percent or more due to flushing of nitrate that has accumulated in the soils and unsaturated zones in the basin. The principal source areas of N are basins in southern Minnesota, Iowa, Illinois, Indiana, and Ohio that drain agricultural land. Basins in this region yield 800 to more than 3,100 kilograms total N per square kilometer per year (kg/km2/yr) to streams, several times the N yield of basins outside this region. Assuming conservative transport of N, streams draining Iowa and Illinois contribute on average about 35% of the total N discharged by the Mississippi River to the Gulf of Mexico. In years with high precipitation they can contribute a larger percentage.