The Mississippi River winding through marshlands in Minnesota

Marshes and farmland border a stretch of the Mississippi River near its headwaters in Minnesota. Credit: Nathan Benn/Corbis/Getty

Environmental sciences

How Mother Nature cleans dirty water

Wetlands beat pasture land at scrubbing nitrate pollution from rivers.

Wetlands are natural water purifiers on a vast scale, reducing levels of one common pollutant in rivers much more effectively than a gold-standard method.

Many waterways in the US Midwest are awash in harmful fertilizer by-products called nitrates. Wetlands are known to reduce nitrates, but Amy Hansen at the University of Minnesota in Minneapolis and her colleagues wanted to find out how wetlands compare to other measures for controlling nitrates. The team sampled river water at more than 200 sites in the heavily farmed Minnesota River basin and measured the extent of both crops and wetlands in the area.

The team’s calculations showed that wetlands decrease nitrate pollution up to five times more efficiently than the conversion of farmland to pasture — the best land-based method for reducing nitrates.

The authors say that restoring wetlands could lower pollution levels across an entire watershed.