Nitrogen Mitigation Strategies
Scroll down to learn about the issue and
what’s happening in the Town of Tisbury to address it.
The Problem
High nitrogen levels in our ponds can cause loss of eel grass, macroalgal accumulation, and subsequently low oxygen levels and potential fish kills (eutrophication). Beyond the environmental concerns, the financial impact of pond eutrophication can include loss of tourism, loss of fisheries, and ultimately, loss of property values. Solutions to combat nitrogen enrichment come in many forms. Two are described below. TWI will keep you up to date on the strategies applied in Tisbury.
Denitrifying Septic Systems
Nitrogen in human wastewater from standard Title V septic systems constitutes approximately 80% and 76% of the overall controllable nutrient loading for Lake Tashmoo and Lagoon Pond, respectively. Standard Title V septic systems were not designed – nor do they have the potential – to address the ever-growing nutrification of our Island waterbodies.
In 2016 the Town of Tisbury first adopted a regulation requiring the installation of enhanced de-nitrification technology capable of addressing both the Town’s sanitation and denitrification needs. At that time, the best-in-class denitrification technology removed less than 50% of the nitrogen from household sanitary waste.
Since 2016 MassDEP has approved enhanced de-nitrification technologies that remove up to 85% of septic nitrogen and meet a groundwater discharge standard of not more than 12 mg/liter of nitrogen from septic waste.
Consistent with its commitment in 2016, the Board of Health continues to review new technologies as they become available and evaluates the appropriateness of amending their regulation to lower the nitrogen discharge and/or removal standard.
In 2021 the Tisbury Board of Health amended their regulation to require certain properties to install on-site de-nitrification wastewater disposal technology that meets a nitrogen groundwater discharge standard of not more than 13 mg/liter of nitrogen from septic waste or removes 75% of septic nitrogen, and is approved by the Massachusetts Department of Environmental Protection for general use, provisional use, or piloting use for nitrogen reduction.
TWI has supported the development of new technologies to reduce nitrogen output, and will provide updates as these developments occur.
Permeable Reactive Barriers
As a pilot project to help reduce nitrogen levels flowing into Lagoon Pond, the MVC received a grant to install a Permeable Reactive Barrier (PRB) near Lagoon Pond. Click here to read a 2020 article on the project in the Vineyard Gazette. Click Here to watch a video of a September 2021 presentation, sponsored by TWI, on the siting of the PRB and preliminary results. (From the MVC YouTube Channel)
Photo to the right shows the design of the Lagoon Pond Road installation: Injection sites (PRB) shown as green circles, monitoring wells shown as blue circles