Man-made wetlands filter water.

Creating wetlands for the nation's streams, rivers, and watersheds would go a long way toward ensuring cleanliness in local water supplies. Many communities have been building wetlands to process their waste water. Today, there are nearly 200 such systems across the U.S. Properly developed wetlands have proven more effective than many natural ones in filtering out sediments and phosphorus, two major pollutants from farm fields and other land, as well as nitrates, a concern in drinking water, indicates William Mitsch, professor of natural resources, Ohio State University. In some cases, the man-made wetlands can be as much as 30 times more efficient in controlling such substances. The reason is that they can be designed so water flows through them more slowly, giving more time for pollutants to be filtered. At the Des Plaines River north of Chicago, for example, man-made wetlands removed about 90% of the sediments from the water passing through, while Heron Pond, a natural wetland in southern Illinois, removed around three percent.

That natural wetland also retained about 4.5% of the phosphorus from flood water passing through. The Des Plaines man-made wetlands retained from 60% to more than 90% of the phosphorus.

Farmers use phosphorus - as well as nitrogen and potassium - on crops. Treatment plants are built to remove phosphorus from domestic wastewater before...