Many pollutants are washed by rainfall from urban and agricultural lands and are carried overland to water bodies. Pollutants include soil particles, fertilizers, pesticides, grease and oil from cars and trucks, and road salts. Wetlands can improve water quality by removing pollutants from surface waters. Three pollutant removal processes provided by wetlands are particularly important: sediment trapping, nutrient removal and chemical detoxification.As water from a stream channel or surface runoff enters a wetland, the water spreads out and flows through dense vegetation. The velocity of the flow is reduced, allowing suspended material in the water to settle to the wetland surface. The roots of wetland plants can then bind the accumulated sediments. As much as 90 percent of the sediments that are present in runoff or in streamflow may be removed if the water passes through wetlands. Also, because pollutants, such as heavy metals, are attached to soil particles, the settling of sediments in wetlands further improves water quality.Nitrogen and phosphorus from agricultural and lawn fertilizers, pet waste, sewer and septic systems, and other sources can act as plant fertilizers in natural water bodies and stimulate excessive plant, algae, and cyanobacteria growth. Such growth may produce toxic chemicals and choke out natural vegetation and wildlife. When runoff and stream flow pass through wetlands before entering a water body, these nutrients may be taken up by wetland plants and accumulate in less harmful chemical forms. When wetland plants die and decay, nutrients are recycled within the wetland. Wetlands are so effective at removing excess nutrients from water that many municipalities have built wetlands specifically for treating effluent from secondary sewage treatment plants. Natural wetlands are not suited for this purpose and for each wetland there is a limit to how much can be added before the natural plant and chemical processes are overloaded and break down.
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