Stream size major factor in nitrogen reaching the Gulf of Mexico

A U.S. Geological Survey study supports previous findings that most of the nitrogen pollution delivered to the Gulf of Mexico by the Mississippi River originates far upstream in the upper Midwest and Ohio Valley states. But this new USGS study also finds that within these regions there are large differences in the percentage of nitrogen reaching the Gulf, depending on the relation of the location of nitrogen sources to streams of different sizes in the watershed.

The study finds that the rates of nitrogen reaching the Gulf from upstream areas near large rivers in such states as Ohio and Minnesota are much higher than those in neighboring areas near small streams. Moreover, areas near large rivers in these same states, located more than 1,500 miles from the Gulf, also deliver more nitrogen to the Gulf than areas near small streams in the states of Mississippi and Arkansas, located only a few hundred miles from the Gulf.

Previously it was not clear whether a unit of nitrogen released in different areas of the Mississippi River drainage basin has an equal chance of reaching the Gulf. It had generally been assumed that the percentage of nitrogen traveling downstream to the Gulf decreased as the distance increased. However, this study, to be published in the February 17 issue of Nature, finds that nitrogen pollution is naturally removed from water much more rapidly in small streams than in large rivers. As a result, nitrogen delivery from point and nonpoint sources in a stream drainage basin is not simply a function of the distance between the Gulf and the nitrogen source, but a function of the amount of time the nitrogen travels through small streams.

"We found that nitrogen is naturally removed from small streams much more quickly than in large rivers, such as the Mississippi River and its major tributaries," said USGS scientist Richard Alexander, one of the authors of the study. "One of the most important ways that nitrogen is removed

Contact: Richard Alexander
United States Geological Survey

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