The amount of nutrients contained in fertilizer, livestock manure, municipal wastewater, atmospheric deposition, and legume residues were quantified in each of the major drainage basins within the Upper Mississippi River Basin study unit (fig. 1) as part of the U.S. Geological Survey's National Water-Quality Assessment Program. These sources of nutrients may potentially affect surface- and ground-water quality, so knowledge about the relative importance of each source may assist in the management of surface and ground waters within the study unit. The relative importance of each nutrient source was expected to vary among each of the four drainage basins due to differences in land use across the study unit.
Fertilizer and livestock manure were potentially large sources of nitrogen and phosphorus in each of the four drainage basins. However, nitrogen in legume residues was a more important source in the Upper Mississippi, St. Croix, and Lower Mississippi River Basins because hay comprised a larger part of the total acreage of crops grown in these basins. Atmospheric deposition comprised a larger percentage of the nitrogen sources in the St. Croix River Basin compared to the other three drainage basins probably because amounts of the other sources are relatively low. Nitrogen and phosphorus yields in streams were greatest in the Lower Mississippi River Basin and the Minnesota River Basin, where amounts of nonpoint sources of these constituents also were the greatest per square mile.
