The State of Sengekontacket Pond Report 2014 2015
For the last 26 years, the Friends of Sengekontacket, Inc. has carefully monitored the health of Sengekontacket Pond and its barrier beaches. In 2012, the Commonwealth classified Sengekontacket Pond as “somewhat impaired.” Although we are pleased that the health of the Pond has improved slightly, that classification is still valid. This report proposes an achievable and relatively low cost program for the healthy maintenance of the Sengekontacket Pond infrastructure.
It’s all about nitrogen!
The total amount of nitrogen in a column of water is a good indicator of the health of a body of water. Nitrogenisourcanary.Abodyofwaterrequires nitrogenforhealthandaquaticplantgrowth.Anexcessof nitrogen results in a heavy growth of the biomass, depletion of oxygen in the water column, loss of an essential aquatic habitat, a significant reduction of economic benefit, and place the Pond in a eutrophic state. In the eutopic state, our shell fish will die and fin fish will just swim away.
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 4,400 acre watershed enters the groundwater and travels at a rate of 13 feet a day toward the pond. The Sengekontacket 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.
How healthy is Sengekontacket Pond?
The purpose of the Massachusetts Estuaries Project (MEP) 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 Sengekontacket Pond. Although a number of measurements were used in the MEP, the amount of nitrogen in a water column (TDN or, Total Dissolved Nitrogen) is key. The Commonwealth recommended a maximum TDN reading of 0.35mg/L to define good pond health. When the report was published in 2011, the range of TDN readings in the pond ranged from 0.21kg/L to 0.61mg/L. The highest TDN readings were at Majors Cove (0.611mg/L) and at the Southern end at the Trapps Pond (0.601mg/L) culvert on Beach Road.
Since the two hotspots were at Majors Cove and Trapps, the Commonwealth selected those sites as sentinel stations to be monitored by the towns in the future. A third sentinel station at Farm Neck adjacent to the golf course has been added.
During May to December of 2014 and 2015, scientists from the Martha’s Vineyard Commission (MVC) tested the water in Sengekontacket Pond. The 2015 test results are unavailable, but 2014 showed a modest reduction of total dissolved nitrogen. The site at the southern tip of Trapps Pond was at the threshold of 0.5 kg/L and the sites at Majors Cove and Farm Neck never exceeded 0.36 kg/L. Since the “Little Bridge” at the north end of the pond was partially or fully closed during much of 2014 and part of 2015, higher nitrogen concentrations at Majors Cove were expected due to a loss of flushing.
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 indicates this mitigation effort is working.
During the winter of 20142015, 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.
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, and this recommendation is essential. Although the “Little Bridge” was dredged in the Spring of 2015. The October, 2015 storms have already started to close the channel. The Friends have met with town officials with town officials and are hopeful that a regular maintenance program will be included in the town budget.
● Going forward, we are encouraged that the MVC has committed to Sengekontacket testing in 2016 and beyond. A very big plus. FoS has also agreed to conduct additional baseline testing.
● There should be a continuation of the oyster and shellfish seeding program and the consideration of a creation of an oyster reef in Major’s 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 Cow Bay Association properties.
● 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 20152016.
E Carlson, Member of the FoS Board of Directors