Wetlands and Wet Ponds

Page Contents:

• Advantages and Disadvantages
• Considerations
  • Siting
  • Permitting
  • Funding Sources
  • Factors that Control the Level of Nitrogen Reduction
• Treatment Details
• Cost per Pound of Nitrogen Removed

This BMP includes systems that are designed and constructed to remove nitrogen and other pollutants from stormwater by incorporating the functions of natural wetland habitats. This BMP is distinct from both constructed wetlands for wastewater treatment and natural wetlands. Stormwater wetlands or wet ponds rely on engineered hydrology (Simpson and Weammert 2009). There are a wide variety of wetlands created to manage and treat stormwater, ranging from surface ponds with a permanent pool to wetland basins with a sediment forebay and permanent pool, and include emergent wetland vegetation in shallower areas over a significant portion of the wetland surface (NYS 2015; Simpson and Weammert 2009).

Advantages and Disadvantages

Considerations

The creation of stormwater management systems is largely dependent on availability of land. In addition to locating surface runoff BMPs themselves, surface flow routing options need to be considered. Opportunity costs for the loss of land are not included in the cost estimates for this assessment. Stormwater wetlands should not be located within jurisdictional wetlands but permits may be granted for restoration of degraded waterbodies (NYS 2015).

Siting

In New York State, stormwater collection systems must follow a five-step process (NYS 2015):

1. Site planning to preserve natural features.
2. Calculation of the water quality volume for the site.
3. Incorporation of runoff reduction techniques and standard stormwater management practices (SMPs) with runoff reduction volume (RRv) capacity.
4. Use of standard SMPs, where applicable, to treat the portion of water quality volume not addressed by runoff reduction techniques and standard SMPs with RRv capacity.
5. Design of volume and peak rate control practices where required.

Detailed information on the SPDES permit requirements for SMPs are available on the NYSDEC website at https://www.dec.ny.gov/chemical/8468.html.

Permitting

Permitting would depend on the type, location, and size of the surface runoff BMP. The following permits are required for installation of a wetland, representing the BMP reviewed during this study requiring the most regulatory oversight:

• U.S. Army Corp of Engineers
• New York State Department of Environmental Conservation (NYSDEC) Freshwater Permit and/or Tidal Wetland Permit
• NYSDEC Water Quality Certification
• NYSDEC or Town and Village setbacks in close proximity to surface waters or wetlands
• Town and Village Trustee Permit
• NYSDEC State Pollutant Discharge Elimination System (SPDES)
• New York State Department of Transportation
• Town and Village Building Construction

Funding Sources

The SuffoThe Suffolk County Water Quality Improvement Division promotes stormwater quality improvements through the Water Quality Protection and Restoration Program and Land Stewardship Initiatives (WQPRP) grant funding program. New York State provides funding through the Water Quality Improvement Project (WQIP) Program.

Factors that Control the Level of Nitrogen Reduction

The effectiveness of surface runoff BMPs in reducing nitrogen range widely and are a function of both watershed properties and system design. In terms of the watershed, the drainage area and land use (including the amount of impervious area) will determine the nitrogen loading to the basin (USEPA 1999). In terms of design, the hydraulic residence time, size, and design features such as soil type, substrate, and types of vegetation will have a large impact on the nitrogen removal efficiency of the BMP (Koch et al. 2014), particularly in wetlands (Ooi et al. 2020; Kaplan et al. 1979). Sequestration of nitrogen by wetland plants is not the primary removal mechanism in wetlands (Craft et al. 2009; Ooi et al. 2020). Nitrogen removal in wetlands is dominated by microbial nitrification of ammonia in aerobic zones and anaerobic denitrification in soils (Ooi et al. 2020; Woltemade 2000). However, denitrification rates vary across wetland vegetation zones because they are good indicators of soil characteristics and functions, and higher rates have been measured in high marsh versus low marsh communities (Ooi et al. 2020; Kaplan et al. 1979).

Nitrogen removal efficiency increases with the length of time stormwater is retained within a wetland (Jordan et al. 2003), and thus system design to reduce annual flow variability enhances the nitrogen removal effectiveness of created wetlands (Simpson and Weammert 2009). Design features that distribute water flow over the entire wetland also maximizes the removal efficiency (Persson et al. 1999). Additionally, removal efficiencies in wetlands are proportional to wetland size and the ratio of wetland size to its drainage area (Simpson and Weammert 2009). Removal efficiencies are also improved for wetlands created within the floodplain or fringe wetlands compared with those placed in depressions, and removal efficiency also increases with higher nitrogen loading rates (Simpson and Weammert 2009).

Treatment Details

The influent source for the NRB is septic tank effluent, with a concentration of 65 mg/L (CDM Smith 2020). The low and The stormwater wetland BMP considers two types of wetlands. The first is a simplistic 1-acre wetland buffer zone with a 25-acre watershed and an average impervious cover of 37% (New York State average; Chesapeake Bay Program 2018). The second represents a more complex stormwater wetland based on a 1.2-acre wetland proposed for Meetinghouse Creek in Aquebogue (Land Use 2019). The wetland receives stormwater from a 5.6-acre drainage area that has an impervious cover of 60% (Land Use 2019). For this assessment, the watershed was scaled down to 4.7 acres for a 1.0-acre wetland. The influent concentration in stormwater is based on the national mean stormwater nitrogen concentration of 2.0 milligrams per liter (mg/L; NYS 2015). An average percent nitrogen reduction of 45.6% for constructed surface flow wetlands was used for both scenarios (Hammer and Knight 1994). The annual nitrogen load removal estimates for wetland BMPs reflect the load estimated for these drainage areas, taking the impervious cover into account.

Stormwater Wetland Nitrogen Effluent Concentrations, Nitrogen Reduction, Drainage Area, and Annual Removal Rates

Cost Per Pound Nitrogen Removal

The simple wetland and wet pond BMP capital and operation and maintenance (O&M) costs are the average of the median costs estimated by the Chesapeake Assessment Scenario Tool (CAST) for new/redevelopment and retrofit projects (Chesapeake Bay Program 2020). Capital costs include design and engineering as well as constructions costs and are the average of costs estimates developed by the U.S. Environmental Protection Agency (USEPA; 2014), Price et al. (2019), Wieland et al. (2009), Herrera Environmental Consultants (2012), and the Charles River Watershed Association (2010). The complex stormwater wetland capital costs include the construction and design and engineering costs estimated for a retrofit construction of a 1.2 wetland proposed for Meetinghouse Creek (Land Use 2019) scaled to 1 acre. The proposed wetland is located within an existing jurisdictional wetland, thus representing an upper bound scenario in terms of permitting requirements and design complexity. O&M costs for the complex stormwater wetland were estimated at 3% of the construction costs, with a 2% inflation rate applied. The annualized total costs include capital and O&M costs adjusted to a per acre treated basis by multiplying by the average impervious cover for New York (0.37) and the Meetinghouse Creek drainage area (0.6) for the simple wetland and wet pond and complex stormwater wetland, respectively, and capital costs are annualized over a 30-year lifespan, which is an average of the life spans of the projects included in the CAST cost estimates for the simple wetland and wet pond, with a 5% discount rate and the annual average O&M cost with a 2% inflation rate applied.t with a 2% inflation rate applied.

Wetland Buffer Zone Capital, O&M, Total Annualized Costs, and Costs per Pound of Nitrogen Removed