Constructed Denitrifying Wetland
Practice Overview
Wetlands are strategically placed on low-land areas of a watershed where farm field runoff naturally flows and where tile water can outlet into. Constructed on agriculturally unproductive acres, the vegetation in these wetlands naturally denitrifies the water. While other best practices, like denitrifying bioreactors and saturated buffers, improve downstream water quality, constructed wetlands have the added benefit of creating habitat for wildlife and stormwater capture to reduce flooding.
Benefits
Constructed wetlands can offer a variety of benefits for agricultural land, including:
- Denitrification (20% to 50%)
- Phosphorus reduction
- Sediment capture
- Habitat for wildlife
- Flood water storage and management
Components of a Denitrifying Wetland
These wetlands use natural processes to handle the denitrification of drainage water. The components are:
- Field Drainage Outlet
- Shallow Pool in Low-Lying Area
- Control Structure or Dam
- Wetland Outlet
How It Works
Tile (county or private) outlets into the wetland where vegetation in the shallow pool provides an ideal environment for nutrient consumption. The water level is controlled by an outlet weir or control structure, depending on the design. Low, wet, unproductive acres are targeted as natural locations for wetlands because they are effective at capturing runoff and drainage water while allowing the natural ecosystem to treat the nutrients. Upland areas can be sufficiently drained while limiting the impact of downstream water quality or quantity.
Tile zone wetlands can outlet back into the county or district tile. Break point wetlands collect and treat drainage (tile or open ditch) water before it enters a larger ditch system.
Installation Considerations
- Multiple styles can be utilized depending on watershed characteristics
- Wetland area is typically 0.5% to 2% of watershed/drainage area
- For example, a 500-acre watershed would need about a 5-acre wetland
- Shallow pool depth
- Half of pool area must be two feet deep or shallower
Costs
- Installation: $42,000 to $80,000
- $1.32 per lb of N removed