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Cape Cod Stormwater Best Management Practice (BMP) Retrofits for Control of Nitrogen

Hyannis Inner Harbor outfall at Gateway Marina (Hyannis, MA)Funded in 2015 by EPA under the Southern New England Program (SNEP; formerly, The Southern New England Coastal Watershed Restoration Program (SNECWRP)), two communities were selected for direct assistance projects to design and construct stormwater best management practices (BMP) for treatment of nitrogen. Each of these communities currently has waterbodies that are impaired for nitrogen. Construction took place from April to November, 2015.

The first BMP is an innovative subsurface gravel wetland retrofit constructed on an approximate 0.35 acre parcel of land at Hyannis Inner Harbor in Barnstable, MA. The BMP will intercept stormwater runoff discharging from a portion of the Town's municipal separate storm sewer system (MS4) to Hyannis Inner Harbor, a water body listed as impaired for total nitrogen and fecal coliform. The drainage area of the MS4 subcatchment is approximately 6.9 acres, of which some 4.6 acres (66%) is impervious cover (IC) (e.g., roofs, driveways and roadways). The subwatershed percent nitrogen reduction target (NRT) due to stormwater is 21%.

A second BMP, also a subsurface gravel wetland retrofit, was constructed on a currently undeveloped parcel of land along Oyster Pond Furlong in Chatham, MA. This BMP will intercept stormwater runoff from a portion of the Town's MS4 just before discharge to Oyster Pond in Chatham, MA, a water body similarly listed as impaired for nutrients and pathogens. This BMP is designed to treat stormwater runoff from a drainage area of 16.9 acres, of which 9.29 acres (54%) is IC. The subwatershed NRT due to stormwater is approximately 15%.

Both BMPs are based on a much smaller prototype developed by the University of New Hampshire Stormwater Center (UNHSC). The BMPs are characterized by two distinct functional areas: a bioretention area and subsurface gravel wetland cell:

The bioretention area consist of approximately 8-inches of a loam and compost mix (biosoil) planted with grass seed for nitrogen uptake through the root systems to provide partial stormwater treatment. This area also functions to oxidize sources of nitrogen, including total nitrogen, in preparation for treatment within the subsurface gravel wetland cell. The bioretention area is partially lined along the bottom with an impervious membrane, and thus stormwater is only allowed to infiltrate through and along the bioretention soil layer towards an infiltration zone located at the far end of the BMP. This infiltration zone consists of a permeable mix of gravel, sand, loam, and compost with augmented shredded newspaper to promote and ensure oxidation of nitrogen sources prior to the subsurface gravel wetland cell. The compost and newspaper provide a source of electrons for microbial-assisted oxidation of nitrogen (ideally to nitrate (NO32-)), which is subsequently used in the subsurface gravel wetland cell as the source of electrons for microbial-assisted reduction of nitrogen (i.e., denitrification) to nitrogen gas (N2).

Once stormwater passes through the infiltration zone, it enters the subsurface gravel wetland cell, an underlying gravel storage reservoir - or Internal Storage Reservoir (ISR). The ISR consists of a fully contained and isolated 24-inch deep zone of crushed stone (lined on the top by the bioretention area liner, and on the sides and bottom of the BMP with an additional impervious liner to prevent infiltration into native soils and contact between stormwater and groundwater). As stormwater flows through the ISR towards the BMP outlet, denitrification occurs. To date, information on the operation of gravel wetland cells indicates that some 24 to 33 hours of treatment time is required for complete denitrification. Typically, the 0.3 inch design storm event will ‘push out’ the prior 0.3 inch storm volume held within the ISR. In this way, the BMP operates not unlike a plug flow anaerobic bioreactor.

Both BMPs are designed to treat stormwater runoff from an approximate 0.3 inch storm. In New England, the majority of storms are small storms (84% of storm events less than 0.6 inches; 61% less than 0.2 inches), and because soluble forms of nitrogen are highly mobile, EPA expects the BMPs may capture and treat up to approximately 41-51% of the available nitrogen load from stormwater, particularly stormwater from IC.

BMP Site Date Task Status
2015 Project Schedule
Gateway Marina (Hyannis, MA) April 6 - 10 Pre-Construction meeting Completed
Equipment mobilization and materials stockpiling; site surveying and staking; sediment controls Completed
April 19 - 25 Topsoil Removal, excavation and grading Completed
Geotextile Lining and Stone Placement Completed
April 26 - May 2 Piping and structure placement Completed
Wetland soil, loam, seed, topsoil, seeding Completed
Cleanup and demobilization Completed
Fall 2016 - 2019 Performance Assessment Monitoring Scheduled
Oyster Pond Furlong (Chatham, MA) May 17 - 23 Kickoff meeting Completed
Mobilization Completed
Erosion controls, site clearing, topsoil removal Completed
May 24 - 30 Excavation and Grading Completed
May 31 - June 6 Geotextile lining and stone placement Completed
June 7 - 13 Wetland soil, loam, seed, topsoil, seeding Completed
June 14 - 20 Cleanup & demobilization Completed
October 11 - 17 Piping and structure placement Completed
October 18 - 24 Cleanup Completed
November   Site Landscaping   Completed
Fall 2016 - 2019 Performance Assessment Monitoring Scheduled

Supporting Documentation:


USEPA Project Technical Lead:
Ray Cody (
Surface Water Branch, Office of Ecosystem Protection
Phone: (617) 918-1366

USEPA SNEP Coordinator:
Karen Simpson (
Watersheds and Nonpoint Source Unit, Office of Ecosystem Protection
Phone: (617) 918-1672

Project Partners:

Town of Barnstable
Dale Saad, Ph.D.
Senior Project Manager
Water and Sewer
Barnstable DPW
382 Falmouth Road
Hyannis, MA 02601
Phone: (508) 790-6400 X4941

Town of Chatham
Robert A. Duncanson, Ph.D.
Director of Health & Natural Resources
Town of Chatham, 261 George Ryder Road, Chatham, MA 02633
Phone: (508) 945-5165
FAX: (508) 945-5163