Wetlands are filters for water coming off the land, reducing sediment and chemicals in run off before it gets into open water. These chemicals and sediment could kill fish and amphibian eggs, smother bottom feeding wildlife and plants, and clog waterways. In wetlands, water flow slows so suspended sediment drops out and settles to the wetland floor. Nutrients from manure, sewage systems, and other sources are absorbed for leaf and stem growth of plants. Others are trapped in the soil and used by microorganisms there. (Environmental Protection Agency b, 2001, p.1)
In some small cities, wetlands are being built as wastewater treatment plants. “A study by the Georgia Water Quality Control Board… showed that water heavily polluted with human sewage and chicken offal was designated clean after passing through 2.75 miles of swamp forest,” (U.S. Government Council on Environmental Quality, 1978, p. 23). The Environmental Protection Agency (2006, p.1) has data showing the Congaree Swamp in South Carolina removes a quantity of pollutants from the watershed equivalent to that which would be removed by a $5 million treatment plant. It should be noted that the swamp provides far more recreation opportunities than the average municipal water treatment facility.
There are limits to how much and what kinds of pollution wetlands can handle. Heavy metals in contaminated water are taken up by plants and enter the food web of other species this way. The contaminated plants break down, releasing the heavy metals. Some wetland bacteria, however, can process some inorganic molecules. A growing industry of bioremediation is being developed from these wetland bacteria which “eat” pollution.