National Water-Quality Assessment (NAWQA) Project
The delivery of phosphorus (shown in map) and nitrogen to the Gulf of Mexico is highest from watersheds in the central and eastern portions of the Mississippi River Basin. Animal manure on pasture and range lands contribute nearly as much phosphorus as cultivated crops, 37 versus 43 percent. About 65 percent of nitrogen originates primarily from cultivated crops, mostly corn and soybean, with animal grazing and manure contributing about 5 percent. Findings are based on innovative geo-spatial modeling that integrates long-term monitoring data with spatially extensive geographic maps of watershed characteristics and contaminant sources.
Full study and supporting information are accessible at http://water.usgs.gov/nawqa/sparrow/gulf_findings. A ranking and uncertainty for 818 watershed with delivered yields to the Gulf are accessible at: http://water.usgs.gov/nawqa/sparrow/nutrient_yields/.