A constructed wetland is an ecosystem that can hold water and contains water-tolerant native or non-invasive vegetation.
- Best suited in areas needing treatment of wastewater and contaminated runoff from agricultural processing, livestock, and aquaculture facilities, or for improving the quality of nonregulated storm water runoff.
- The outlet of a tile drainage system.
- Locations that do not cause surface or subsurface flow to encroach upon an upstream neighboring property unless a flowage easement is signed and recorded.
- Where effluent can be irrigated onto cropland, or discharged into a vegetative treatment area.
- Outflow from the wetlands may be stored for land application, recycled through the wastewater management system, or otherwise used in the agricultural operation.
- Captures and stores seasonal surface and subsurface runoff from upstream for use when it is needed or appropriate.
- Reduces downstream exports of nutrients and sediment. Can be combined with a sediment basin and vegetative buffers for additional filtering of pollutants.
- Wetlands must be able to be drained completely for maintenance as needed.
- Additional design criteria must be met if sited in a floodplain.
- Cannot be located within a jurisdictional existing wetland.
- Primary use cannot be for wildlife habitat.
- Case studies in Maryland, Illinois, and Iowa indicate wetlands can remove 68% of nitrate-nitrogen and 43% of phosphorus can be retained from drainage water. (Woltemade, 2000)
- A Minnesota study on constructed wetlands found a 68% and 73% reduction in nitrate-nitrogen and phosphorus, respectively.
- Crumpton et al. (1993 Iowa) found that constructed wetland mesocosms could reduce N03-N concentrations ranging from 3 to 15 mg/L by >80% during residence times of approximately 1 week.
- Costs will vary based on the size of the wetland and existing site conditions.
- The highest costs are associated with site planning and design, excavation activities, control structures required and the opportunity cost of any land removed from agricultural production.
- Cost analyses should take into account the cost of the wetland as well as the acres treated. Estimated life spans of constructed wetlands can be 50 years or longer.
- Average 2016 costs (first year costs and per acre per year) of 1-acre constructed wetland located on land with a high Corn Suitability Rating (CSR 80) treating about 100 acres of drainage, would cost just over $10,000 for design and installation, or just under $800 per acre per year when annualized over a 40-year time period; (Tyndall)
- Generally considered cost efficient with a cost per unit of nutrient reduction range from $1.60 to $5.00/Kg of N removed.
Ideally, constructed wetlands are placed where they can receive inflow from upstream areas and outflow to a ditch or stream by gravity. Vegetation and microorganisms that thrive in the wetland environment utilize and remove components of runoff and wastewater that are concerns for conservation and water quality, such as nitrogen and phosphorus. Constructed wetlands help to have a holding place for water for reuse or better control of runoff in a watershed. In some cases, tile flow can be pumped into the wetland. Water control structures can be used to maintain water levels or drain wetlands as needed.
Operation requirements most commonly include:
- Hydraulic control structures necessary for maintaining the desired water depth and stormwater management,
- planting, care and handling of the seed to ensure that planted materials have an acceptable rate of survival, and
- site preparation such as stabilizing crop, mulching, or mechanical means of stabilizing, fertilizer, and lime requirements sufficient to establish and grow selected species.
Maintenance requirements most commonly include:
- Repair of embankments,
- control of non-native or invasive vegetation and unwanted animals or mosquitoes
- repair of fences or other ancillary features,
- repair of pipelines and spillways
Consult your SWCD, NRCS or a drainage contractor as proper sizing and construction are critical.
General design criteria are established for all purposes that establish minimum embankment height, spillway requirements, and excavated side slopes and protective cover on disturbed soils.
When used to treat wastewater design the wetland so that:
- It will return to design operating levels within 96 hours after a 25 year, 24 hour storm event.
- Maximum daily loading does not exceed 65 lb BOD5 per acre of surface area.
- A sufficient size can be built to provide the minimum hydraulic retention time of 12 days.
When used to improve the water quality of surface water runoff, design the wetland so that:
- It will return to design operating levels within 72 hours after a 10 year, 24 hour storm event.
- The minimum surface area of wetlands for treatment of surface or subsurface crop production runoff is 0.5% to 2% of the contributing drainage area.
- A minimum of 75% of the surface area of the treatment wetland must be less than 2 feet deep.