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ORNL study finds rivers play part in removing nitrogen
OAK RIDGE, Tenn., March 12, 2008 Tiny organisms play a powerful role in removing nitrate, a form of nitrogen pollution caused by human activity, in streams, according to a study by a team led by Oak Ridge National Laboratory and published in Nature.
In the first phase of the study, which involved 31 aquatic scientists from across the United States, researchers added small amounts of an uncommon non-radioactive isotope of nitrogen to 72 streams across the U.S. and Puerto Rico. Using this tracer, the team's objective was to measure how far downstream the nitrate traveled and how it was removed from water.
"We found that the nitrate was filtered from stream water by tiny organisms such as algae, fungi and bacteria," said Patrick Mulholland, lead author of the study and a member of Oak Ridge National Laboratory's Environmental Sciences Division. Mulholland has a joint appointment at the University of Tennessee.
The researchers also found that a considerable amount of nitrate was removed from streams by a bacterially mediated process known as denitrification, which converts nitrate to nitrogen gas. That gas then escapes harmlessly to the atmosphere, Mulholland said.
In the second phase of the study, the scientists developed a model to study nitrate removal from water within river networks. These networks develop as small streams flow into larger streams and rivers.
"Our model showed that the entire stream network is important in removing pollution from stream water," Mulholland said. "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 can help prevent nitrate pollution from reaching lakes and the coastal waters where this type of pollution can cause noxious algal blooms and lead to oxygen depletion and death of fish and shellfish.
The study was funded by a grant from the National Science Foundation to the University of Tennessee. UT-Battelle manages Oak Ridge National Laboratory for the Department of Energy.