Wetlands: Nature's Pollution Solution?

Wetlands are vital for the health of our planet. They are highly effective at removing excess nutrients from water, such as nitrogen, phosphorus, nitrates, and ammonia, which can harm the health of a river. Wetlands can also remove up to 60% of metals and eliminate up to 90% of sediment runoff. They act as natural filters, improving water quality and providing a habitat for a diverse array of plants and animals. However, wetlands are also under threat from pollution, particularly from fertilisers and pesticides used in modern intensive farming, and toxins from factories.

Wetlands can remove up to 60% of metals from water

Wetlands are nature's way of purifying water. They are highly productive ecosystems that store large volumes of water

Wetlands can trap and retain up to 90% of sediment runoff

Constructed wetlands are engineered ecosystems that use plant, microbe, and substrate systems to naturally treat wastewater, removing pollutants and allowing cleaner water to be returned to rivers, streams, or estuaries. They can also create habitats for biodiversity, act as carbon sinks, and control surface water flows.

Dense vegetation cover reduces surface water velocities, allowing for greater infiltration and filtration of particulates, and soils are less likely to erode. Vegetation structure also influences the ability of the wetland to trap sediment, both from water and wind erosion. The removal of particulates reduces the load of particle-bound nutrients, heavy metals, pesticides, and other pollutants into groundwater, and nearby rivers and streams.

Wetlands can eliminate up to 90% of nitrogen

Constructed wetlands can be a cost-effective and sustainable alternative to traditional infrastructure for treating wastewater. They can remove up to 90% of nitrogen, along with phosphorus and ammonia, which are all harmful to rivers.

Constructed wetlands are 'engineered eco-systems' that use natural processes to treat wastewater. The wetlands' plant, microbe, and substrate systems work together to remove pollutants from wastewater.

Nitrogen removal in wetlands occurs through a combination of physical, chemical, and biological processes. These processes include particle settling (sedimentation), volatilization (releasing gas into the atmosphere), and sorption (a nutrient adhering to a solid or diffusing into another liquid or solid).

The main biological process for nitrogen removal is uptake by plants, algae, and bacteria. These organisms uptake inorganic nitrogen forms (nitrate, ammonia, and soluble reactive phosphate) through their roots and foliage during spring and summer and convert them into organic compounds for growth. However, as plants grow old and decompose, most of the assimilated nutrients are released back into the water and soil.

Denitrification is the dominant, sustainable removal process in wetlands that receive high nitrate loadings from agricultural runoff or wastewater treatment plant discharge. Denitrification is performed by heterotrophic bacteria, which require a carbon source for growth and energy. Wetland plants are a key source of this carbon.

The rate of nitrogen removal depends on various factors, including the presence or absence of oxygen, season, temperature, water inflow rate, nutrient loading rate, and retention or holding time of the water within the wetland.

Wetlands can reduce the impact of flooding

The ability of wetlands to reduce flooding is well recognised. For example, the U.S. Army Corps of Engineers deemed the floodplain wetlands along the Charles River in Massachusetts so effective that they purchased them instead of constructing expensive flood control structures to protect Boston. Wetlands within and upstream of urban areas are particularly valuable in this regard, as the impervious surfaces in cities tend to increase the rate and volume of runoff, thereby elevating the risk of flood damage.

The significance of wetlands in flood mitigation is further emphasised by the Vermont Wetland Rules, which acknowledge the role of wetlands in reducing risks to public safety, minimising damage to property, and decreasing downstream erosion. Additionally, wetlands contribute to the stability of habitats for aquatic life by attenuating peak flood flows and reducing the scouring and erosion of stream banks.

Wetlands' effectiveness in flood control can be enhanced through human intervention. By fortifying wetlands with human-made constructions that enclose them, more water can be trapped during flooding seasons and then gradually released or utilised for irrigation during droughts. This dual benefit of wetlands in addressing both flooding and drought conditions is invaluable, especially in regions susceptible to extreme weather events due to climate change.

The preservation of wetlands is, therefore, crucial for flood mitigation and brings significant financial benefits. The University of Waterloo estimated that maintaining wetlands intact could reduce the financial costs of floods by up to 38%. Thus, recognising the economic value of wetlands and their ecosystem services is essential to encourage their conservation and prevent their destruction.

Wetlands can act as carbon sinks

Wetlands are effective at reducing pollution, particularly nutrients like phosphorus, nitrates, and ammonia, which can harm rivers. They can also act as carbon sinks, absorbing and storing carbon from the atmosphere.

Wetlands include marshes, swamps, and bogs, which are periodically saturated with or covered by water. They provide food and habitat for a wide range of plants and animals and act as buffers against flooding and erosion. They are also highly productive ecosystems, with microbial activity that enriches the water and soil with nutrients. This microbial activity, along with the plant growth in wetlands, provides a "sink" for many chemicals, including atmospheric carbon.

Wetland soils contain some of the highest stores of soil carbon in the biosphere. They are anoxic (oxygen-poor), which slows decomposition and leads to the accumulation of organic matter. This makes wetlands important sinks for atmospheric carbon dioxide, holding up to 40% soil carbon, which is significantly more than the 0.5-2% carbon commonly found in agricultural soils.

The magnitude of carbon storage in wetlands depends on several factors, such as wetland type and size, vegetation, the depth of wetland soils, groundwater and nutrient levels, pH, and other factors. Freshwater inland wetlands, due to their substantial areal extent, hold nearly ten times more carbon than tidal saltwater sites. Peatlands in forested regions of the East and Upper Midwest store the most carbon, accounting for nearly half of the wetland carbon in the United States.

Protecting and restoring wetlands is crucial for maintaining these carbon sinks. The loss of a wetland means the loss of a carbon sink, and the stored carbon can be released into the atmosphere. When disturbed or warmed, wetlands can release greenhouse gases like carbon dioxide, methane, and nitrous oxide. Therefore, it is essential to implement best practices to protect carbon stores in wetlands, such as reducing wetland drainage, controlling fires, allowing natural revegetation, and controlling peat harvesting.

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