Cape Cod Stormwater Control Measure (SCM) Retrofits for Control of Nitrogen

Funded in 2015 by EPA under the Southern New England Program (SNEP), two communities were selected for direct assistance projects to design and construct subsurface gravel wetland (SGW) stormwater control measures (SCM) (i.e., 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 SCM is an innovative biofilter subsurface gravel reservoir system retrofit constructed on an approximate 0.35 acre parcel of land at Hyannis Inner Harbor in Barnstable, MA. The SCM intercepts 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 sub-catchment 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 SCM, also a biofilter subsurface gravel reservoir system retrofit, was constructed on a currently undeveloped parcel of land along Oyster Pond Furlong in Chatham, MA. This SCN intercepts 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 SCM 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 SCMs are based on a much smaller sizing approach developed by the University of New Hampshire Stormwater Center (UNHSC) and supported by the EPA Region 1 Performance Curves. The SCMs are characterized by two distinct functional components: a surface biofilter area overlying a subsurface gravel reservoir cell:

The surface biofilter area consists of a grass-vegetated upper biosoil mix layer (sand, loam/topsoil, organic material), a highly permeable hydraulic inlet consisting of sand-soil-gravel mix layer augmented with shredded newspaper, and a stone reservoir consisting of ¾" stone. Refer to the design schematics provided below in the section entitled "Additional Supporting Documentation". The biofilter layer provides treatment of SW pollutants though filtering and nitrogen uptake through the vegetation root systems. This upper filter area also functions to oxidize total organic nitrogen in preparation for treatment within the subsurface gravel reservoir cell. The compost and newspaper provide an effective carbon source for microbial-assisted oxidation of nitrogen (ideally to nitrate (NO₃^-)), 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).The biofilter area is partially lined along the bottom with an impervious membrane in order to direct the filtered stormwater to one end of the subsurface gravel reservoir cell.

The subsurface gravel reservoir cell, referred to as the Internal Storage Reservoir (ISR), consists of a fully contained 24-inch deep zone of crushed stone lined on the top, bottom and sides with an impervious liner. The liner is necessary to prevent exchange between the filtered stormwater and the surrounding soils and to maintain anaerobic conditions necessary for microbial mediated denitrification. The outlet structure is located at the far end of the ISR and consists of a vertical pipe with a small diameter orifice elevated to maintain saturated conditions and designed to control the rate of outflow and to ensure an adequate residence time is provided within the ISR. As stormwater flows through the ISR towards the outlet, denitrification occurs. To date, information on the operation of ISR cells indicates that 24 to 33 hours of treatment time is required for complete denitrification. Each new storm event will 'push out' the prior stored storm volume held within the ISR. In this way, the SCM operates not unlike a plug flow anaerobic bioreactor.

Both SCMs are designed to treat the first 0.3 inches of stormwater runoff depth from the contributing impervious area In New England, the majority of storms are small (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 SCMs may capture approximately 41-51% of the annual stormwater nitrogen load from IC.

2015 Project Schedule

SCM Site	Date	Task	Status
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 - Present	Performance Assessment Monitoring	Ongoing

Performance Assessment Summary

A stated goal of the Project was to monitor the performance of the SCM. EPA is interested in testing new implementation technologies. The data collected from this Project would help establish the feasibility and practicality of SCM implementation, and it was possible the data could be utilized for EPA's Performance Curves.

The design and construction for each SCM incorporated monitoring structures located at each inlet and outlet. Incorporation of these structures complicated the design and construction. For instance, in the case of Hyannis, stormwater flowing downhill through the MS4 was required to discharge into a relatively constrained area; the turbulence of the flow made monitoring flow at the inlet quite challenging.

Due to the SCMs being quite distant from EPA Region 1 offices, the performance assessment of the SCMs was initially predicated on having the municipalities conduct the monitoring. As such, EPA acquired monitoring equipment, retrofit the equipment into each SCM, developed Standard Operating Procedures (SOP) and allocated time training the municipalities on the procedures required for monitoring the SCMs.

It was important for EPA to understand whether SCM monitoring by a municipality was practicable; because, if so, then monitoring could potentially be incorporated into next-generation Small MS4 General Permits. Through no fault of the municipalities, a primary finding of this Project was that performance monitoring of SCMs, such as these innovative subsurface gravel wetland retrofits, is far too complicated a matter for municipalities to conduct with limited time and resources and the general lack of required expertise.

As EPA documented in a more recent policy memo from another Project (refer to Task 5 Technical Support Document (TSD) available at https://www.epa.gov/snep/tisbury-ma-impervious-cover-disconnection-icd-project-integrated-stormwater-management-approach) this Project demonstrated that, as a general rule, municipalities should not monitor SCMs for performance; rather, the care and effort of the municipality / practitioner should be on designing and constructing an SCM to specification and thereafter maintaining the SCM over its operational lifetime.

Additional important findings from this project include:

• particular additional care will be required for design and construction of SGW SCMs in locations where the groundwater table is high: notably, an increased level of care for placement / construction of SWG SCM liners, potentially requiring even the sealing together of the top and bottom liners along the outer circumference of the SCM;

• there are potential implications for operation of SCM due to fluctuating groundwater levels and/or for implementation of SCMs in close proximity to tidally influenced coastal waters (e.g., flooding; flooding exacerbated by high tide and/or more severe and frequent storm events; potential back-charging of saline-containing coastal waters);

• not all SCM are suitable for performance monitoring and careful pre-monitoring evaluation of feasibility should be conducted prior to deciding whether a monitoring program should be conducted. Furthermore, if performance monitoring is desired for a planned SCM, it is prudent to conduct the feasibility evaluation during the SCM design phase in case design elements can be included to improve the feasibility of a monitoring program; and

• the importance of employing a simple drought-resistant grass for SCM landscaping instead of more aesthetic landscaping plans incorporating less multiple plant species – because more robust or invasive species eventually crowd out and dominate. In the case of Hyannis, the dominant specie was Typha (cattail) and the Typha crowded out all other species, and the Typha root systems tended to decrease the permeability of the aerobic zone leading to the "clogged" condition noted in the UNH reports.

2018 UNH Report. Over time, and as the reality of many of the findings noted above became clearer, EPA determined a fundamental change in approach to performance assessment was necessary, and EPA retained the expertise of the University of New Hampshire's Stormwater Center (UNHSC). Over the course of about one year, UNHSC developed a preliminary assessment (2018 Report; available below). This report includes the original Quality Assurance Project Plan (QAPP). In brief, this report concluded:

• the Hyannis SCM required further hydraulic assessment and rehabilitation maintenance,
• the Chatham SCM was operating in accordance with its design, and
• additional testing and evaluation of the SCMs was recommended.

2020 UNH Report. On the basis of the findings and recommendations in the 2018 UNHSC Report, EPA again retained UNHSC for a second and more robust performance assessment. Coincidentally, this timing of this work coincided with the availability of real-time ultraviolet optical spectrometers (UV-Vis) for water quality monitoring. These spectrometers were employed as an assessment of the utility and status of the older monitoring equipment was evaluated. As a result of this work, UNHSC produced a Final Report on its work available below. In brief, the findings of the 2020 Final Report are:

• Barnstable (Hyannis) SCM. Consistent with the 2018 Report, the Hyannis SCM is under-performing hydraulically due to clogging and excessive vegetative growth of one or more dominant plant species (e.g., Typha, phragmites) in the upper/aerobic zone. The data also indicates the SCM intercepts and is influenced by the groundwater table which is near or at surface elevation of the SCM. In addition, diurnal patterns from groundwater wells installed upgradient and downgradient of the SCM indicate that groundwater is influenced by the tidal cycle of Hyannis Inner Harbor. The hydraulic performance of the SCM explains the inability of the SCM to effectively denitrify nitrogen. UNH provided a number of recommendations for rehabilitation maintenance of the SCM, including elevating the system surface, incorporating wood chips to provide a carbon source for denitrification, and replanting with desired native vegetation if and when access to the SCM is possible. The highly challenging conditions associated with retrofit of this SCM and in such close proximity to an estuarine water has provided invaluable experience for advancement of small-scale green infrastructure-based stormwater control practice.

• Chatham SCM. The data demonstrates the Chatham SCM is operating according to design and is effective at treating nitrogen. The data presented in Table 6 on page 35 evidences a very high median mass removal efficiency (RE) for the SCM. The Chatham SCM most closely matches and even outperforms the infiltration basin except that the RE of TN is slightly lower. It is possible some exfiltration is occurring between the bottom and top liners. UNH recommendations include next-generation work to forensically assess possible exfiltration/infiltration.

• UV Sensors. The Project supports the potential for developing global calibration curves specific to stormwater runoff for real-time ultraviolet sensors. The results are encouraging that real-time ultraviolet sensors are capable of advancing stormwater quality monitoring and hold the potential to deliver more accurate laboratory quality data instantaneously with greater efficiency and at a lower overall cost than conventionally available methodologies.

October 2018 UNHSC Report (pdf) (14 MB)

December 2020 UNHSC Final Report (pdf) (41 MB)

Additional Supporting Documentation

• Gateway Marina (Barnstable)
  • Final Design (pdf) (2 MB)
  • Aerial Overview Map (pdf) (3 MB)
  • O&M Plan (pdf) (3 MB)
  • Landscape Plan (pdf) (620 K)
  • Photos
• Oyster Pond Furlong (Chatham)
  • Final Design (pdf) (3 MB)
  • Aerial Overview (pdf) (3 MB)
  • Oyster Pond Furlong O&M Plan (pdf) (3 MB)
  • Landscaping Plan (pdf) (839 K)
  • Photos

Contacts

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

USEPA SNEP Coordinator:
Ian Dombrowski ([email protected])
Watersheds and Nonpoint Source Unit, Office of Ecosystem Protection
Phone: (617) 918-1672

Project Partners

Town of Barnstable
Dale Saad, Ph.D. (Retired)
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