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Streams natural filters, if not overloaded

Blacksburg, Va. Streams are natural filters that help remove and transform pollutants that drain from surrounding watersheds, including excess nitrogen from human activities. Scientists know this as a result of many hours of getting their hands if not dirty, at least very wet -- monitoring streams nationwide.

Nitrogen removal in streams is important because it reduces the potential for eutrophication the excessive growth of algae and aquatic plants in downstream lakes and coastal marine waters, said Jack Webster, professor of biology at Virginia Tech. Eutrophication in the Chesapeake Bay has damaged the oyster industry in Virginia and in the Gulf of Mexico, the Mississippi River has created a vast zone of oxygen depletion with adverse effects on fisheries.

Webster, two of his Virginia Tech colleagues, and four former Virginia Tech students are among 31 authors of an article in the March 13 issue of Nature that reports the researchers findings on how stream systems are able to remove nitrogen.

The study, lead by Oak Ridge National Laboratory (ORNL), looked at 72 streams in the U.S. and Puerto Rico over the course of three years. Virginia Techs Stream Team conducted measurements on nine streams in North Carolina including forest streams in the southern Appalachian Mountains, agricultural streams where they had to protect equipment from curious cows, and urban streams, including one that ran through a golf course and another that ran through a construction site. Eight other teams worked at the other 63 streams.

The research process meant 24-hour monitoring. The Stream Team involvement was very important, said Webster.

In the first phase of the study, the scientists added small amounts of a non-radioactive isotope of nitrogen to streams as nitrate, the most prevalent form of nitrogen pollution. They then measured how far downstream the nitrate traveled and how what processes removed it from the water.

The scientists found that the nitrate was taken up from stream water by algae and microorganisms. In addition, a fraction was permanently removed from streams by denitrification, a bacterial process that converts nitrate to nitrogen gas, which harmlessly joins an atmosphere already predominantly composed of nitrogen gas.

In the second phase of the study, the scientists developed a model that predicts nitrate removal as water flows through small streams and into larger streams and rivers. Our model showed that the entire stream network is important in removing pollution from stream water, said Patrick Mulholland, lead author of the study, a member of ORNLs Environmental Sciences Division, and a faculty member at the University of Tennessee. In addition, the effectiveness of streams to remove nitrate was greatest if the streams were not overloaded by pollutants such as fertilizers and wastes from human activities.

The largest removal occurred when nitrate entered small healthy streams and traveled throughout the network before reaching large rivers. The scientists concluded from their research that streams and rivers are effective filters that help reduce the amount of nitrate pollution exported from landscapes and thereby reduce eutrophication problems, Webster said.


Contact: Susan Trulove
Virginia Tech

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