What is Water Purification?

Over time, ecosystems have developed sophisticated ways to process and store natural waste products such as sediments, nutrients, heavy metals, and bacteria. Water is the primary medium for transporting these materials through most ecosystems. The "water purification" process is accomplished to varying degrees in a variety of ecosystems including streams, wetlands, estuaries, and forests. Wetlands and riparian (streamside) plant communities are particularly efficient at improving water quality. These communities act as buffers that filter and process sediment and debris from floods, waste products from aquatic and terrestrial plants and animals, and, minerals, metals, and microorganisms before they can reach faster moving water such as rivers (17).

Streamside (Riparian) Plant Communities
Streamside, or riparian, plant communities occur adjacent to streams, rivers, and wetlands and often serve as transition zones between aquatic and terrestrial habitats. Riparian areas along rivers and streams act as "living filters" that intercept and absorb sediments, and store and biogeochemically transform nutrients and pollutants carried in runoff from adjacent lands. Living and dead vegetation slow down the rate of water that runs off of land surfaces, allowing adsorption of nutrients, metals, and other contaminants on sediment surfaces and providing an opportunity for microbial breakdown of chemicals and uptake of nutrients for growth. Sediments are trapped and excess nutrients (such as nitrogen and phosphorus), heavy metals, and other materials may be incorporated into living plant tissue or broken down to less harmful substances by soil microbes and other organisms (13).

Wetlands act in similar ways to riparian plant communities. Wetlands are areas where water covers the soil or is present either at or near the surface of the soil, all year or for varying periods of time during the year, including during the growing season. Some wetlands are wet constantly, while others, such as those along the Mississippi River and in the Everglades, only hold water seasonally (5). Each species that occurs in a wetland, river, or forest has evolved to deal with the level of nutrients and other materials that naturally occur within its habitat. Although considerable variability exists within an ecosystem, there is a range of conditions under which the system can function. If this range is exceeded, the capacity of the system to function in a normal manner may break down. Even nutrients, essential to all living organisms, can disrupt the function of the system when present in excess.

The ability of a particular wetland or riparian forest to store materials depends on several factors, including how fast the water flows (e.g., residence time of the water within the system), size of the area, and type of soil and vegetation it contains. For improving water quality, the single most important attribute of a wetland is probably how hydrologically open or closed it is (14). In hydrologically open wetlands, such as the Everglades, large amounts of groundwater and/or surface water flow through the system, and the residence time of the water is short. This means that there is little time for the purification processes to take place, and a high potential for the system to be overwhelmed if, for example, high nutrient loads enter the system. In contrast, closed wetlands have long water retention times and are highly efficient at removing pollutants. However, the low influx of water from outside of the closed system means they may become saturated if too much of a pollutant enters the system.