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Mimicking Nature - Natural Wetlands
Natural Wetlands purify water by acting like a sponge, soaking up rainwater that runs off the land before it enters rivers and streams. The extensive root mass of wetland plants and the soil itself work together to extract contaminants and nutrients from water. These natural cleaning processes of wetlands help keep our rivers, streams and oceans clean.
[Waterwise, Winter 1995].
Section 404 of the Clean Water Act defines wetlands as:
Those areas that are inundated or saturated by surface or ground water (hydrology) at a frequency and duration sufficient to support, and that under normal circumstances do support, a prevalence of vegetation (hydrophytes) typically adapted for life in saturated soil conditions (hydric soils). Wetlands generally include swamps, marshes, bogs, and similar areas (40 CFR 232.2(r)).
Natural Marshes are lowland areas created in nature by flooding. Typically, these wetlands have soft vegetation, such as cattails and reeds. Constructed wetlands mimick two types of natural marshes:
Low marshes typically occur next to open water and are flooded regularly by tides and irregularly but wind and rain. Examples are marshes along coastal sounds and estuaries. In low marsh constructed wetlands, the coarse fill of sand and stone, and plant roots are the primary filters of pollutants. These wetlands are effective in the anaerobic second step of nitrogen treatment, because the wastewater flows below the surface.
High marshes are flooded periodically in the transition zone between low marshes and higher upland grounds. In constructed wetlands mimicking High marshes, wastewater is flooded onto the top of the marsh and allowed to flow down through the sand. Dry periods make oxygen plentiful for the first step of nitrogen treatment. As with subsurface wetlands, plant roots and soil are effective filters, and a microbial treatment area usually forms around the roots.
Soil filters mimic permeable upland areas that soak up and cleanse runoff as it travels through the soil toward groundwater. The soil acts as a filter by removing sediment and other pollutants. Oxygen inside the soil filter aerates the wastewater and fuels the microbes that break down pollutants. Soil filters have been widely used since the 1970's to treat household wastewater at sites where soil conditions (high water table and slowly permeable clays) hinder the performance of a standard septic system. Sand, gravel, brick, and earth all act as soil filters. Combinations of these substrates can provide more extensive filtering.