Protecting & Restoring Long Island's Peconic Bays
Wetlands slow the movement of water and can filter and remove nitrogen and other pollutants from agricultural runoff. This BMP describes the nitrogen removal benefits and costs for conserving, enhancing, and restoring natural wetlands. While the inclusion of wetland as a potential buffer to reduce nitrogen is explored here, it is important to recognize that the cost of land is high and developing wetland area for water quality improvement would likely result in a financial loss that would make this option unattainable for most farms. It is included here as an option to explore in specific cases where sea level rise has created a situation where land is no longer able to sustain crops or if there is available land adjacent to farmland that may be publicly owned and available for wetland conversion. Additionally, any such parcel would need to have the correct existing conditions for a constructed wetland to be successful.
Wetland conservation and restoration is largely dependent on availability of existing or former wetland on agricultural land. Crop loss needs to be factored into the cost of BMP implementation if excess land is not available. However, areas predicted to become inundated due to sea level rise (SLR), may provide an opportunity for marsh migration and restoration and be candidates for conservation easements and other agricultural preservation mechanisms. Costs for the loss of land are not included in the cost estimates for this assessment due to the variation in market prices but need to be carefully considered.
Maximum nitrogen removal benefits are realized for wetland down gradient to agricultural fields so they may intercept and filter the maximum amount of agricultural drainage. Wetlands areas within riparian buffer strips may provide enhanced nitrogen removal (Woltemade 2000).
Permitting would depend on location and size. The following permits categories are required for enhancement and restoration of a wetland:
The NRCS Wetland Reserve Easement component of the Agricultural Conservation Easement Program (ACEP) provides financial and technical assistance to protect, enhance, and restore wetlands through the purchase of a wetland reserve easement. The NRCS Environmental Quality Incentives Program (EQIP) program provides benefits for runoff BMPs, provided guidelines are followed. The Suffolk County Water Improvement Division also promote 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.
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 water is retained within a wetland, resulting in higher removal efficiency in dry years compared with wet (Jordan et al. 2003). Thus, wetland enhancement to reduce annual flow variability enhances the nitrogen removal effectiveness of created wetlands (Simpson and Weammert 2009). Additionally, features that distribute water flow over the entire wetland maximizes the removal efficiency (Persson et al. 1999). Further, removal efficiencies increase with wetland age and are proportional to wetland size and the ratio of wetland size to its drainage area (Woltemade 2000; Simpson and Weammert 2009).
The wetland conservation and restoration BMP considers an agricultural watershed with and impervious cover of 2% (NYS 2015) and an average influent concentration of 9.9 milligrams per liter (mg/L) total nitrogen in runoff based on the average concentrations of nitrogen in shallow groundwater beneath agricultural areas in Suffolk County (USGS 2001); this value is similar to the national mean stormwater nitrogen concentration of 9.1 mg/L for lawns (NYS 2015) and falls within the range of stormwater nitrogen concentrations in agricultural areas (e.g., Kovacic et al. 2000).
Restored wetland nitrogen removal rates are based on average values from a comprehensive literature review conducted by Land et al. (2016); the lower bound removal efficiency was set at 28% based on the average for restored, formerly drained cropland wetlands and the upper bound value was set to 39% based on the average value for restored, formerly other land use wetlands.
The drainage areas presented in Table 4-1 are based on wetland size of 1 acre and a 2% wetland to watershed ratio (Tyndall and Bowman 2016).
Wetland Restoration Nitrogen Effluent Concentrations, Nitrogen Reduction, Drainage Area, and Annual Removal Rates
| BMP | Effluent Nitrogen Concentration (mg/L) | Nitrogen Reduction (%) | Drainage Area (acres) | Annual Nitrogen Removal (lb/yr) |
|---|---|---|---|---|
| Wetlands | 6.0–7.1 | 28–39 | 50 | 103–144 |
The lower bound wetland enhancement and restoration capital cost is based on the total project cost estimated for the tidal wetland restoration of Fireplace Neck Marsh in Brookhaven of $810,000 divided by the total wetland size of 115 acres (USFWS 2020). The upper bound capital cost is based costs estimated for restoration of the Robinson Duck Farm in Brookhaven (Suffolk County Department of Planning 2010) adjusted to 2020 equivalent costs and include per acre costs of $3,168 and $2,640 for construction and planning, respectively, and design and engineering and incidental costs of $2,650 and $3,533, respectively. Operations and maintenance (O&M) costs were set at 3% of capital costs for both restoration scenarios. The annualized total cost includes capital costs annualized over a 40-year lifespan (Tyndall and Bowman 2016), with a 5% discount rate and the annual average O&M cost with a 2% inflation rate applied.
Lost income from farming and property value conversion to wetland use that would render land use ineligible for use as farmland or development due to conservation and wetland restoration of agricultural land is not included in this study. In order to accurately reflect the cost of this nutrient reduction option, the market real estate cost needs to be assessed. The high level of uncertainty around market prices from 2020 to 2023 is well documented with sale prices for farms ranging from $13,000 per acre to upward of $50,000 per acre according to landgate.com. A thorough assessment of current conditions needs to be carried out when including lost income when active farmland is converted to wetland.
Wetland Buffer Zone Capital, O&M, Total Annualized Costs, and Costs per Pound of Nitrogen Removed
| BMP Type | Capital Cost | Average Annual O&M Costs | Annualized Total Cost | Cost per Pound N Removed |
|---|---|---|---|---|
| Wetland Restoration | $7,044–$11,991 | $425–724 | $1,182–$1,504 | $8-$15 |
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