River Monitoring stations and major regional watersheds in the Mississippi river basin Coastal zone managers have long suspected that much of the nitrogen pollution that enters the Gulf of Mexico via the Mississippi River comes from the northern Midwest and Ohio Valley states and is the result of high intensity agricultural activity in those regions. The percentage of nitrogen that reaches the Gulf from interior watersheds was highest when nitrogen was carried quickly to larger rivers such as the Ohio and Missouri Rivers. The location of the major nitrogen sources has been difficult to accurately determine because of the lack of information on the rates at which nitrogen is naturally removed by denitrification and other processes in rivers. The USGS researchers used data from 374 monitoring stations on rivers and streams in the United States, including 123 stations in the Mississippi River basin, to quantify nitrogen loss rates in streams and rivers.
Livestock wastes account for about a quarter of the agricultural sources.
The percentage of nitrogen delivered to the Gulf of Mexico has generally been assumed to decrease gradually with increasing distance from the coast. However, because nitrogen is naturally removed more rapidly in small streams than in large rivers, large differences in the percentage delivered to the Gulf may occur among neighboring watersheds. The results of the USGS study have relevance to basic science as well as policy issues. Theoretically, denitrification will be more effective in small, shallow streams because water in these streams has more contact with the bottom sediments than water in deep, large rivers. With respect to policy matters, the findings of the study suggest that a cost-effective strategy for reducing nutrient pollution in coastal watersheds may be to emphasize the control of nutrient sources near large rivers.