5.4.24 Stormwater Treatment Wetlands

5.4.24.1 BMP Description

Stormwater treatment wetlands (BMP T10.30 in Volume V, Chapter 8 of the Ecology Manual) are similar to wetponds, but also provide a shallow marsh area to allow the establishment of emergent wetland aquatic plants, which improves pollutant removal. In land development situations, wetlands are usually constructed for two main reasons: to replace or mitigate impacts when natural wetlands are filled or impacted by development (mitigation wetlands), and to treat stormwater runoff (stormwater treatment wetlands). Stormwater treatment wetlands are shallow manmade ponds that are designed to treat stormwater through the biological processes associated with emergent aquatic plants.

5.4.24.2 Performance Mechanism

Stormwater treatment wetlands remove sediment, metals, and pollutants that bind to humic or organic acids primarily through settling and biological uptake. Secondary performance mechanisms include filtration and soil adsorption. Phosphorus removal in stormwater wetlands is highly variable; therefore, stormwater treatment wetlands are not expected to provide phosphorus control as a standalone BMP and must be paired per the treatment train approach.

5.4.24.3 Application and Limitations

This stormwater wetland design occupies about the same surface area as wetponds but has the potential to be better integrated aesthetically into a site because of the abundance of emergent aquatic vegetation. The most critical factor for a successful design is the provision of an adequate supply of water for most of the year. Careful planning is needed to be sure sufficient water will be retained to sustain good wetland plant growth. Since water depths are shallower than in wetponds, water loss by evaporation is an important concern. Stormwater wetlands are a good runoff treatment BMP choice in areas with high winter ground water levels.

Stormwater treatment wetlands can be combined with detention to provide flow control as well as runoff treatment; see Vol II–5.4.29 Combined Detention and Wetpool Facilities. As shown in Stormwater Treatment Wetland Applicability., this BMP can be designed to meet basic, and enhanced requirements. If used as part of a two-BMP treatment train, stormwater treatment wetlands can also be designed to meet phosphorus runoff treatment requirements.

Table II-5.27. Stormwater Treatment Wetland Applicability.

BMP	MR #5: On-site Stormwater Management		MR #6: Runoff Treatment				MR #7: Flow Control
	List	LID Performance Standard	Basic	Enhanced	Oil Control	Phosphorus
Stormwater Treatment Wetlands			X	X		Xa
Notes: Can be used to meet phosphorus runoff treatment requirement as part of a two-BMP treatment train; refer to Volume III, Section 1.2 in the Ecology Manual.

5.4.24.4 Site Considerations

The following site considerations can help determine the feasibility of a stormwater treatment wetland for a particular site:

• Location of the stormwater wetland relative to site constraints (e.g., buildings, property lines) shall be the same as for detention ponds; see Vol II–5.4.26 Detention Ponds.

• Access and maintenance roads shall be provided and designed according to the requirements for detention ponds (see BMP D.1 in Volume V, Chapter 12 of the Ecology Manual). Access and maintenance roads shall extend to both the wetland inlet and outlet structures. An access ramp (7H minimum:1V) shall be provided to the bottom of the first cell unless all portions of the cell can be reached, and sediment loaded from the top of the wetland side slopes.

• If the dividing berm is also used for access, it shall be built to sustain loads of up to 80,000 pounds.

5.4.24.5 Design Information

When used for stormwater treatment, stormwater wetlands employ some of the same design features as wetponds. However, instead of gravity settling being the dominant treatment process, pollutant removal mediated by aquatic vegetation and the microbiological community associated with that vegetation becomes the dominant treatment process. Thus, when designing wetlands, water volume is not the dominant design criteria. Rather, factors that affect plant vigor and biomass are the primary concerns.

See BMP T10.30 in Volume V, Chapter 8 of the Ecology Manual for detailed design guidance and criteria.

5.4.24.6 Minimum Construction Requirements

• Construction and maintenance considerations are the same as for wetponds (see Vol II–5.4.22 Wetponds).

• Construction of the naturalistic alternative (see BMP T10.30 in Volume V, Chapter 8 of the Ecology Manual) can be easily done by first excavating the entire area to the 1.5‑foot average depth. Then soil subsequently excavated to form deeper areas can be deposited to raise other areas until the distribution of depths indicated in the design is achieved.

5.4.24.7 Operations and Maintenance

• Stormwater treatment wetlands shall be inspected at least twice per year during the first 3 years during both growing and non-growing seasons to observe plant species presence, abundance, and condition; bottom contours and water depths relative to plans; and sediment, outlet, and buffer conditions. Coordinate with Kitsap County to develop a monitoring plan.

• Maintenance shall be scheduled around sensitive wildlife and vegetation seasons.

• Plants may require watering, physical support, mulching, weed removal, or replanting during the first 3 years.

• Nuisance plant species shall be removed, and desirable species shall be replanted.

• The effectiveness of harvesting for nutrient control is not well documented. There are many drawbacks to harvesting, including possible damage to the wetlands and the inability to remove nutrients in the below-ground biomass. If harvesting is practiced, it shall be done in the late summer.