Meandering rivers fight pollution better

March 13th, 2008 - 1:46 pm ICT by admin  

Washington, March 13 (IANS) Do not be misled by the size of the rivers. Small rivers can be as efficient as the longer ones when it comes to cleansing themselves - provided they are allowed to maintain their natural flow. Scientists at Michigan State University found this after studying several streams and rivers in an effort to understand what happens to the nitrogen that is washed into the water.

“This study presents a picture of unprecedented detail of the extent to which streams can remove nitrate,” scientist Steven Hamilton said. “We also now have a better idea of what makes one stream more efficient at nitrate removal than another.”

The results of the study have been published in the latest issue of the journal Nature.

Nitrogen gets into the water as runoff from fertilisers and wastes from human activities. Too much nitrogen can cause noxious algal blooms and lead to oxygen depletion and death of fish and shellfish, as has been recently reported in the Gulf of Mexico.

Hamilton and his team spent three years conducting experiments by adding small amounts of a harmless, non-radioactive isotope of nitrogen, N-15, into streams. They then were able to track the isotope as it travelled downstream and record what processes removed it from the water.

What they found, which was supported by experiments across 72 streams, was that the nitrate was taken up from water by tiny organisms such as algae, fungi and bacteria.

Hamilton said they also learned that not all streams are created equal. Streams that are allowed to meander naturally through a complex channel were more efficient at filtering pollutants than streams that had been engineered to quickly convey water away from farmland or urban sprawls.

“What we often do to streams to make them more like drains diminishes their ability to reduce pollutants,” Hamilton said. “Complexity - both biological and physical - helps streams be more effective at removing nitrogen.”

This appears to put two imperatives at odds - removing water quickly from urban areas or agricultural fields versus trying to reduce pollutants. But Hamilton said there are ways to satisfy both goals, such as directing waters into wetland ponds or buffer strips that allow nature time to gobble the nitrates.

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