The state of Sengekontacket Pond, 2014–15


For the past 26 years, the Friends of Sengekontacket, Inc., has carefully monitored the health of Sengekontacket Pond and its barrier beaches. The directors are pleased to report that the pond is in slightly better condition than it was five years ago. A series of reports published by the Commonwealth of Massachusetts in 2012, commonly known as the Massachusetts Estuaries Project (MEP), has been used as the base for evaluation.

In 2012, the commonwealth classified Sengekontacket Pond as “somewhat impaired.” Unfortunately, that classification is still valid. This State of Sengekontacket Pond report proposes an achievable and relatively low-cost program for the healthy maintenance of the Sengekontacket Pond infrastructure.

The Friends applaud the efforts of the many people and officials who love the pond. However, we can do better.

It’s all about nitrogen

The total amount of nitrogen in a column of water is the best indicator of the health of a body of water. Nitrogen is our canary. A body of water requires nitrogen for health and aquatic plant growth. However, too much nitrogen will cause the plant population to explode and place the pond in a eutrophic state. An excess of nitrogen results in a heavy growth of the biomass, depletion of oxygen in the water column, the loss of an essential aquatic habitat, and a significant reduction of the economic benefit of the pond.

About 40 percent of the nitrogen in Sengekontacket Pond originates from the atmosphere, minerals in the soil, and other sources that cannot be managed. Of the remaining amount, 80 percent is from septic systems, 12 percent from fertilizer, 1 percent from agricultural waste, and 7.5 percent from runoff. Presently, almost all of the septic waste from Sengekontacket Pond’s 4,400-acre watershed enters the groundwater and travels at a rate of 1 to 3 feet a day toward the pond. The watershed is primarily in Edgartown and Oak Bluffs, but stretches to West Tisbury. Almost none of the septic waste in the watershed is presently treated to remove nitrogen and other mineral pollutants, and becomes part of an underground ocean of waste heading to Sengekontacket Pond. For some septic waste, the journey to Sengekontacket Pond can take up to 15 years. Therefore, it makes sense to have a nitrogen pollution program that includes both mitigation and prevention.

The goal for a healthy pond

The Massachusetts Estuaries Project (MEP) was based on data from a multiyear study period as well as research sponsored by the Friends. The purpose of the report was to set the standard, or goal, of a healthy pond, and enable the towns of Edgartown and Oak Bluffs to manage the infrastructure of the pond. The study defined Sengekontacket Pond as “somewhat impaired.” Although a number of measurements were used in the MEP, the daily loading of nitrogen (MDL, or maximum daily load) and the amount of nitrogen in a water column (TDN, or total dissolved nitrogen) are key. The range of TDN readings in the pond ranged from 0.21kg/L to 0.61kg/L when the report was published. The highest TDN readings were at Majors Cove (0.611kg/L), and at the southern end, at the Trapps Pond culvert on Beach Road (0.601kg/L).

The commonwealth recommended a maximum TDN reading of 0.35kg/L to define good pond health. Since the two hotspots were at Majors Cove and Trapps, they selected those sites as sentinel stations to be monitored in the future. A third sentinel station at Farm Neck adjacent to the golf course has been added. Many of the other tests used by aquatic scientists measure the effect of nitrogen on the water column, but total dissolved nitrogen (TDN) is a reliable basic measurement that can indicate either success or failure.

Better than expected

During May to December of 2014 and 2015, scientists from the Martha’s Vineyard Commission tested the water in this pond and other ponds on Martha’s Vineyard looking for nitrogen pollution, including from pharmaceuticals. It’s little known, but nitrogen is an almost universal element in most pharmaceuticals, and the byproducts from those drugs enter the groundwater through septic runoff. Readings were taken at the three sentinel sites at Majors Cove, Trapps, and Farm Neck. The 2015 test results are unavailable at this time, but 2014 showed a modest reduction of total dissolved nitrogen. The site at the southern tip of Trapps Pond was at the threshold of 0.5kg/L and the sites at Majors Cove and Farm Neck never exceeded 0.36kg/L.

“Little Bridge” at the north end of the pond was partially or fully closed during much of 2014 and part of 2015, and higher nitrogen concentrations at Majors Cove were expected due to a loss of flushing. However, in a nitrogen mitigation effort, Edgartown and Oaks Bluffs have spread about a million and a half oysters a year in the pond, with a heavy concentration in Majors Cove. Oysters, and other shellfish, consume nitrogen to grow, and return some free nitrogen to the atmosphere. Similar programs have been used successfully throughout the world to reduce unacceptable nitrogen levels in a water body. Although it is difficult to quantify, the nearly acceptable TDN readings in the cove indicate this mitigation effort is working. A dry 2014 summer with less runoff may have also helped to reduce nitrogen pollution.

During the winter of 2014–15, the impressive and cooperative efforts of the Vineyard towns and interested parties have resulted in an Island-wide adoption of fertilizer regulations. Since fertilizer is a 12 percent contributor of manageable nitrogen pollution, we expect to see some additional benefit in late 2015 and in 2016. We have been told that over 120 landscapers have been certified to use the new regulations. For everyone, fertilizer management is a win/win situation.

Continued efforts are needed. They include:

The town of Oak Bluffs must budget regular maintenance dredging of the “Little Bridge.” The channel should never be allowed to become restricted or totally closed; this recommendation is essential.

There should be a continuation of the oyster and shellfish seeding program, and consideration of the creation of an oyster reef in Majors Cove.

The dredging of Majors Cove should be studied, as well as the channel between the Big and Little Bridges.

We encourage the Cow Bay Association to work cooperatively with the commonwealth to reduce or eliminate nitrogen pollution emanating from the Trapps system. The MEP proposed the removal of 100 percent of septic waste from Trapps by sewering as well as removing the several tidal restrictions within the Trapps system. In the interim, we are pleased that attempts have been made to use aeration to mitigate nitrogen pollution from Trapps Pond.

The towns should be encouraged to require nitrogen-reduction septic systems for all replacement and new septic systems within the Sengekontacket Pond watershed, and certainly within the Majors Cove sub watershed. The Friends will recommend a cost-effective program of replacement during 2015–16.

Going forward, the Friends will work with the MVC to continue testing at least the three sentinel locations; work with the MVC and the communities to educate the public on the benefits of the new fertilizer regulations; and continue to monitor bacterial pollution from septic sources and the bird population.

Most importantly, thanks to everyone involved in caring for the long-term health of Sengekontacket. It’s working.

Charles Carlson

Member, Board of Directors

Friends of Sengekontacket, Inc.