A constructed wetland (CW) is a low-cost and sustainable, engineered, best management practice (BMP) designed to reduce stormwater pollution. Constructed wetlands are considered to be one of the most reliable stormwater treatment practices. They are designed to function similarly to a self-sustaining natural wetland, and should require only moderate maintenance to function (figure 1).
An adequate, consistent water supply and the soil texture are two primary considerations when determining where CWs can be located. A constant baseflow from an upstream source is necessary to maintain an appropriate environment for wetland plant growth.
Sandy soils should be avoided if possible; otherwise, the wetland may not hold water. Soils in a wetland must be saturated for certain periods during a year. If the soil where the CW is needed is too coarse (i.e., too much sand), an impermeable liner can be used to help it hold water and minimize seepage losses.
Management of stormwater focuses on two objectives: water quantity control and water quality improvement.
“Water quantity control” is provided by constructed wetlands in the form of storage. This storage is the space that begins at the normal water surface elevation and ends at the top of the dam or control elevation. CWs effectively reduce and retard peak stream flows by acting like a buffer or shock absorber for flows in the stream.
“Water quality improvement” in CWs is provided by natural processes, including biological uptake, microbial decomposition, and settling. CWs are most effective at removing excess nutrients and sediment.
• Constructed wetlands are large — about 3 to 5 percent of the watershed that drains to the CW. If land is expensive, this can mean a high treatment cost.
• A minimum of 4 inches of hydric soils are needed to support plant growth.
• May export nutrients when vegetation is not actively growing or is dying back.
• Must maintain water levels to limit dry periods to a maximum of 30 days to maintain vegetation.
• May raise water temperatures due to long standing time.
• Must consider mosquito control.
• Increased infiltration may result in groundwater contamination.
• Monitor sediment levels. Excess sediment can fill in the CW, harming vegetation and reducing wetland performance.
• Monitor and replace wetland plants as needed.
• Inspect the CW regularly (e.g., are the inlet and outlet structures functioning and clear of debris?) and repair as needed.
• Remove trash and debris.
• Remove excess accumulated sediment (see above).
• Control invasive species and thin woody growth to keep vegetation manageable.
Constructed wetlands are effective at removing multiple pollutants from incoming water flow. A typical CW is expected to reduce total phosphorus by 50 percent and total nitrogen by 25 percent.
In a more advanced design, the constructed wetland is much larger (figure 2). The larger size helps provide a longer residence time. Advanced designs also provide for variable bottom topography, which promotes dense and diverse vegetation and enhances treatment. Advanced CW designs can improve the expected reduction of total phosphorus to 75 percent and of total nitrogen to 55 percent.
The use of constructed wetlands is a relatively inexpensive stormwater treatment practice when compared to other alternatives. A very preliminary estimate of the cost of a CW that would provide treatment of 20 to 30 acres is $60,000 to $70,000. This is based on similar projects in the mid-Atlantic region. Annual maintenance cost is estimated to be 3 to 5 percent of the construction cost, or $1,800 to $3,500. The value of land is not included in this analysis.
The Virginia Department of Conservation and Recreation is the state agency that controls nonpoint source pollution and stormwater. The VA-DCR regulates urban stormwater through the Virginia Stormwater Management Program.
Additional information on best management practices can be found at the Virginia Stormwater BMP Clearinghouse website at http://vwrrc.vt.edu/swc. The BMP Clearinghouse is jointly administered by the VA-DCR and the Virginia Water Resources Research Center, which has an oversight committee called the Virginia Stormwater BMP Clearinghouse Committee. Committee members represent various stakeholder groups involved with stormwater management.
Charles River Watershed Association – www.crwa.org/projects/stormwater/stormwaterBMPs.html
ConstructedWetlands.org – www.constructedwetlands.org/cw/index.cfm
Metrocouncil.org (Metropolitan Council of Minneapolis) – http://metrocouncil.org/environment/Water/BMP/CH3_STConstWLSwWetland.pdf
National Environmental Services Center – http://nesc.wvu.edu/nsfc/Articles/SFQ/SFQ_f04_PDF/Wetland_f04.pdf
Natural Systems International – www.natsys-inc.com/resources/about-constructed-wetlands/
New Jersey Department of Environmental Protection – http://state.nj.us/dep/stormwater/bmp_manual2.htm
North Carolina State University – http://soil.ncsu.edu/lockers/Broome_S/vmmiller/stormwater.html
Oregon Department of Environmental Quality – http://deq.state.or.us/wq/stormwater/nwrinfo.htm
University of Minnesota Extension – http://extension.umn.edu/distribution/naturalresources/DD7671.html
U.S. Environmental Protection Agency – http://water.epa.gov/type/wetlands/restore/cwetlands.cfm and http://epa.gov/owow/wetlands/pdf/Design_Manual2000.pdf
Virginia Stormwater BMP Clearinghouse – http://vwrrc.vt.edu/swc/
Daniels, W., G. Evanylo, L. Fox, K. Haering, S. Hodges, R. Maguire, D. Sample, et al. 2011. Urban Nutrient Management Handbook. Edited by J. M. Goatley. VCE Publication 430-350.
Gilland, T., Fox, L., Andruczyk, M., French, S., and Swanson, L. 2009. Urban Water Quality Management: What Is a Watershed? VCE Publication 426-041.
The authors would like to express appreciation for the review and comments provided by the following individuals: Brian Benham, associate professor, biological systems engineering, Virginia Tech; Jon Hathaway, Biohabitats Inc.; Thomas Bolles, Extension agent, Virginia Tech; and Adria Bordas, Extension agent, Virginia Tech.
Stormwater Manager’s Resource Center. 2010. Stormwater Management Fact Sheet: Stormwater Wetland. www.stormwatercenter.net.
Virginia Department of Conservation and Recreation (VA-DCR). 2011. Virginia DCR Stormwater Design Specification No.13: Constructed Wetlands, Version 1.9. http://vwrrc.vt.edu/swc/NonPBMPSpecsMarch11/VASWMBMPSpec13CONSTRUCTEDWETLAND.html.
Anaerobic – Chemical reactions that proceed without the presence of oxygen.
Baseflow – The portion of flow in a stream that continues even during extended dry periods.
Best management practice – Any treatment practice for urban lands that reduces pollution from stormwater. A BMP can be either a physical structure or a management practice. A similar but different set of BMPs is used to mitigate agricultural runoff.
Biological uptake – The process by which plants absorb nutrients for nourishment and growth.
Constructed wetland – A wetland that is designed to provide water quality treatment of stormwater. CWs have been used to treat domestic wastewater.
Erosion – the movement of soils and rock through weathering from water and wind.
Groundwater contamination – The presence of unwanted chemical compounds in groundwater. In this case, we would normally be referring to dissolved nitrogen compounds, such as nitrates.
Habitat – The environment where organisms, like plants, normally live.
Hydric soils – Soils that form under saturated conditions. When saturated conditions exist, anaerobic chemical processes dominate and unique chemical properties develop. A common characteristic of hydric soils is the presence of a rotten-egg-smelling odor, indicating the presence of hydrogen sulfide (H2S) gas.
Impervious surface – A hard surface that does not allow infiltration of rainfall into it; not pervious.
Impermeable liner – A material designed to retard seepage from ponds and wetlands.
Infiltration – The process by which water (surface water, rainfall, or runoff) enters the soil.
Invasive species – Nonnative species that can cause adverse economic or ecological impacts to the environment, usually due to the tendency of these introduced species to dominate local habitats and replace native ecological communities.
Microbial decomposition – The breakdown of compounds or organic matters into smaller one with the aid of microorganisms.
Nutrients – Substances that are required for growth of all biological organisms. When considering water quality, the nutrients of most concern in stormwater are nitrogen and phosphorus. Excessive amounts of these substances are pollution and can cause algal blooms and dead zones to occur in streams and estuaries.
Peak stream flows – The highest water flows within a stream during a storm event.
Pervious – A ground surface that is porous and allows infiltration.
Residence time – The average time it takes water to travel through a treatment system such as a CW. Residence time can also be called “detention time.”
Sediment – Soil, rock, or biological material particles formed by weathering, decomposition, and erosion. In water environments, sediment is transported across a watershed via streams.
Seepage – Water that is lost through the bottom of a lake or wetland.
Settling – The process by which particles that are heavier than water fall to the bottom under the influence of gravity.
Soil texture – Describes the composition of soil based on its particle sizes. According to the U.S. Department of Agriculture’s classification, soils are classified as sands (larger than 0.05 millimeter, or mm), silts (0.002 to 0.05 mm), and clays (smaller than 0.002 mm).
Stormwater – Water that originates from impervious surfaces during rain events, often associated with urban areas; also called “runoff.”
Stormwater treatment practice – A type of BMP that is structural and reduces pollution in the water that runs through it.
Sustainable – The ability of the system to endure and remain productive over a long time.
Sustaining – The act of enduring (see “sustainable”).
Watershed – A unit of land that drains to a single “pour point.” Boundaries are determined by water flowing from higher elevations to the pour point. A pour point is the point of exit from the watershed, or where the water would flow out of the watershed if it were turned on end.
Wetland – Land that has hydric soil and wetland vegetation and is periodically saturated with water.
Virginia Cooperative Extension materials are available for public use, re-print, or citation without further permission, provided the use includes credit to the author and to Virginia Cooperative Extension, Virginia Tech, and Virginia State University.
Issued in furtherance of Cooperative Extension work, Virginia Polytechnic Institute and State University, Virginia State University, and the U.S. Department of Agriculture cooperating. Alan L. Grant, Dean, College of Agriculture and Life Sciences; Edwin J. Jones, Director, Virginia Cooperative Extension, Virginia Tech, Blacksburg; Jewel E. Hairston, Administrator, 1890 Extension Program, Virginia State, Petersburg.
August 22, 2011