Green Infrastructure benefits of constructed wetlands

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Caution: This discussion includes wetlands constructed for purposes of managing stormwater. It does not include natural wetlands. Natural wetlands should not be used for stormwater management.

Example of a stormwater wetland in a suburban area.

Stormwater wetlands are similar in design to stormwater ponds and mainly differ by their variety of water depths and associated vegetative complex. They require slightly more surface area than stormwater ponds for the same contributing drainage area. Stormwater wetlands are constructed stormwater management practices, not natural wetlands. Like ponds, they can contain a permanent pool and temporary storage for water quality control and runoff quantity control.

Green Infrastructure benefits of constructed wetlands

Benefit	Effectiveness	Notes
Water quality	◕	Primary benefit is retention of sediment; may export phosphorus if not designed and maintained properly.
Water quantity/supply	◑	Rate control benefit.
Energy savings	◔
Climate resiliency	◕	Provides some rate control. Impacts on carbon sequestration are uncertain.
Air quality	◑
Habitat improvement	●	Use of perennial vegetation and certain media mixes promote invertebrate communities.
Community livability	◕	Aesthetically pleasing and can be incorporated into a wide range of land use settings.
Health benefits	◑
Economic savings	◔	Generally provide cost savings vs. conventional practices over the life of the practice.
Macroscale benefits	◑	Individual practices are typically microscale, but multiple practices, when incorporated into a landscape design, provide macroscale benefits such as wildlife corridors.
Level of benefit: ◯ - none; ◔ - small; ◑ - moderate; ◕ - large; ● - very high

• Water quality: Pollutants are removed from stormwater runoff in a wetland through uptake by wetland vegetation and biota (algae, bacterial), vegetative filtering, soil adsorption, and gravitational settling in the slow moving marsh flow. Volatilization and chemical activity can also occur, breaking down and assimilating a number of other stormwater contaminants such as hydrocarbons. Wetlands effectively remove solids and pollutants associated with solids. They are only moderately effective at removing nitrogen and phosphorus. Some designs or poorly designed and maintained wetlands may export phosphorus.
• Water quantity and hydrology: Stormwater wetlands temporarily store water and therefore effectively control runoff rates. They are well-suited to provide channel protection and overbank flood protection. This is accomplished with live storage (extended detention) above the permanent pool. Properly designed wetlands do not contribute significantly to infiltration.
• Climate resiliency: Wetlands improve ecosystem services, alleviate water shortages through water re-use, and provide flood control. Rates of carbon accumulation measured in the soils of other constructed wetlands suggest that these systems provide sequestration benefits.
• Habitat improvement: Constructed wetlands, because they retain a permanent water pool, provide excellent wildlife habitat. Many wildlife species are dependent on or otherwise utilize wetland habitats, including waterfowl, wading birds, shorebirds and songbirds, furbearers such as beaver, muskrat and mink, and a variety of reptiles and amphibians like turtles, snakes, frogs, salamanders, and toads. An important factor affecting the habitat value of a constructed wetland is the surrounding landscape. A complex of wetland types interspersed with upland nesting cover provides optimum habitat. Isolated wetlands provide significantly habitat value. (References: [1]; [2]; [3]; [4]; [5])
• Community livability: Constructed ponds are an aesthetically pleasing practice. However, they require space and are difficult to incorporate in urban landscapes. A variety of vegetation can also be used, including perennial plants, shrubs, and trees.
• Health benefits: Green spaces may also improve mental and physical health for residents and reduce crime (Barton and Rogerson, 2017).
• Economic savings: Properly designed and integrated constructed wetland practices provide life cycle cost savings. Well designed and maintained constructed wetland practices increase property values.

Design considerations

Example of a stormwater wetland in a largely undeveloped area. Constructed wetlands in developing areas offer potential to incorporate many of the design features discussed in this section.

Maximizing specific green infrastructure (GI) benefits of constructed ponds requires design considerations prior to constructing the practice. While site limitations cannot always be overcome, the following recommendations maximize the GI benefit of constructed ponds.

• Water quality (Balderas-Guzman, 2007)
  • Distribute constructed wetlands systemically throughout a watershed to increase potential for delivering networked benefits
  • Design to maximize retention time and prevent short-circuiting
  • Create ecological diversity within the wetland to expose water to a variety of conditions where different treatment processes can take place
  • Create shallow zones were water will come into contact with plant roots and microbes and deeper zones where anaerobic processes can take place
  • Utilize a length to width ratio of 20:1
  • Construct multiple wetland cells
• Water quantity/supply
  • Distribute constructed wetlands systemically throughout a watershed to increase potential for delivering networked benefits
• Climate resiliency
  • Moore and Hunt (2012) determined that most carbon accumulated in constructed wetlands was autochthonous (derived in situ rather than imported). Thus, vegetation establishment is critical to carbon sequestration, with dense emergent communities being favored. Ensure water levels are shallow enough to support emergent macrophytes. Adjustable outlet structures and proper construction and maintenance are tools for ensuring shallow water levels that favor emergent vegetation.
  • To avoid or minimize the potential for methane release, construct wetlands to avoid permanent inundation and properly maintain wetlands (Altor and Mitsch, 2006).
• Habitat
  • Include a littoral shelf that promotes emergent macrophytes (Moore and Hunt, 2012)
  • Vegetative biodiversity may be enhanced by planting littoral areas rather than relying upon natural colonization (Moore and Hunt, 2012)
  • Promote a diversity of predators to control mosquito populations (Greenway, 2010)
• Community livability
  • Maximize the size of wetlands to enhance recreational opportunities (hiking, boating, fishing)(Serrano and DeLorenzo, 2008)
  • Couple wetlands with open space to enhance recreation and social activities (Schroeder and Louviere, 1999)
  • Include recreational infrastructure and interpretative signs (Greenway, 2010)
  • Ensure safety and perceived safety of wetlands and adjacent areas, which typically entails creating more open space and less wooded area (Gobster and Westphal, 2004; Schroeder and Anderson, 1984)
  • Conduct surveys prior to and after development to identify features that enhance education, recreation, and other benefits of wetlands
• Health benefits
  • See community livability above for recreation opportunities that promote health
• Economic benefits

Recommended reading

• Ecosystem service provision by stormwater wetlands and ponds e A means for evaluation?. Moore and Hunt (2012). An excellent article covering carbon sequestration, biodiversity, edication, and cultural services provided by wetlands. Includes an extensive reference list.

References

• Altor, A.E., Mitsch, W.J., 2006. Methane flux from created wetlands: relationship to intermittent versus continuous inundation and emergent macrophytes. Ecological Engineering 28, 224-234.
• Balderas-Guzman, Celina. 2013. Strategies for Systematic Urban Constructed Wetlands. M.S. Thesis. Massachusetts Institute of Technology.
• Gobster, P.H., Westphal, L.M., 2004. The human dimensions of urban greenways: planning for recreation and related experiences. Landscape and Urban Planning 68 (2-3), 147-165.
• Greenway, M., 2010. Wetlands and ponds for stormwater treatment in subtropical Australia: their effectiveness in enhancing biodiversity and improving water quality? Journal of Contemporary Water Research & Education 146, 22-38.
• Greenway, M., Dale, P., Chapman, H., 2003. An assessment of mosquito breeding and control in four surface flow wetlands in tropical-subtropical Australia. Water Science and Technology 48 (5), 249-256.
• Moore, T.L.C., W.F. Hunt. 2012. Ecosystem service provision by stormwater wetlands and ponds e A means for evaluation?. Water Research. 46:6811-6823.
• Schroeder, H.W., Anderson, L.M., 1984. Perception of personal safety in urban recreation sites. Journal of Leisure Research 2, 178-194.
• Schroeder, H.W., Louviere, J., 1999. Stated choice models for predicting the impact of user fees at public recreation sites. Journal of Leisure Research 31 (3), 300-324.
• Serrano, L., DeLorenzo, M.E., 2008. Water quality and restoration in a coastal subdivision stormwater pond. Journal of Environmental Management 88, 43-52.