Lake Tashmoo watershed plan could mean sewers, innovative septics

The town aims to have the plan ready by this summer. 

A map showing the potential places the Tashmoo watershed improvement plan can be implemented.

Tisbury intends to have a plan ready to tackle Lake Tashmoo nitrogen levels by the summer, which could include building sewers and upgrading septic systems. 

Water resources consultant Scott Horsley presented Tisbury officials with an update to the developing targeted watershed plan for Lake Tashmoo during a meeting hosted by the Tisbury water resources committee on Thursday, May 18. 

According to Horsley, the goal of the targeted plan is to “maintain and restore” Lake Tashmoo’s habitat, particularly through the reduction of nitrogen loads. The vast amount of nitrogen getting into Tashmoo is from backyard septic systems, mostly in Tisbury.

A preliminary plan is estimated to reduce nitrogen loads by 4,266 kilograms per year. 

The five main points of the plan include building a sewer connection to move existing septic systems out of the watershed. The plan also calls for upgrading septic systems to more innovative and alternative (I/A) systems in both Tisbury and West Tisbury, to cluster wastewater treatment at the Lake Street Park area, better manage fertilizer, and to retrofit stormwater systems at West Spring Street. Other options included a fertigation well, a type of irrigation system, at Mink Meadows, and to use plants to remove excess nitrogen from Tashmoo. 

“The core of this program is these upgrades to these enhanced I/A systems,” Horsley said, adding that the idea is to retrofit existing septic systems. He estimates 700 systems will need to be upgraded over a 20-year period.

Based on costs spent on enhanced innovative and alternative systems by the Buzzards Bay Coalition and the Barnstable Clean Water Coalition, Horsley gave estimates on how much different types of upgrades would cost per system. To reflect the increase in costs over time, he added $10,000 to the average actual costs for the estimates on the Vineyard. Retrofitting an existing Title 5 system is expected to cost $37,047 per system; a full upgrade, including both septic tank and leach field, is expected to cost $48,847. 

This targeted plan is meant to work in concert with the townwide, 20-year comprehensive water management plan Tisbury is developing. Past materials for the comprehensive water management plan can be found at

“The intent here is to try to get a plan together by this summer,” Horsley said about the Tashmoo plan, adding that looking for funding sources like the State Revolving Fund will be another step in the process. 

Horsley kicked off the information session by saying Lake Tashmoo has a “fair amount” of eelgrass compared with Cape watersheds he worked on. Eelgrass is an ecologically important species, but too much nitrogen in an estuary can lead to the degradation of eelgrass. 

According to the Martha’s Vineyard Shellfish Group, eelgrass protects shallow areas from erosion, creates habitat for marine animals, and improves water conditions. The health of eelgrass is an indication of the health of an estuary, as well.

“[Eelgrass in] many of the other estuaries on Cape Cod is completely gone,” he said. The Cape has been particularly impacted by excessive nitrogen pollution. 

Data that Horsley showed indicated a potential “stabilization” of the eelgrass population. Although eelgrass in Lake Tashmoo experienced a sharp decline, from 91 acres in 1995 to 38 acres in 2001, the plants have covered 47 acres of the area since 2015. 

Based on 2010 load numbers from the Massachusetts Estuaries Project (MEP), of the 9,154 kilograms of nitrogen entering Lake Tashmoo a year, 75 percent came from septic systems. The next highest sources were 8 percent from stormwater, and 5 percent each from natural sources and turf fertilizers. Other sources included agricultural fertilizers, landfill, agricultural animals, and the wastewater treatment facility. 

The new estimates, gained with help from the Martha’s Vineyard Commission, were that the nitrogen load increased to 9,831 kilograms a year for Lake Tashmoo in 2023. A 20-year projection anticipated 10,510 kilograms of nitrogen a year by 2043, if nothing changes. Horsley said the town should be aiming for an MEP threshold of 6,244 kilograms of nitrogen per year for Lake Tashmoo.

“The good news is, things are changing,” Horsley said. “You already are doing some very good things.” 

One of these improvements included some septic systems. Among the septic systems in the Tashmoo watershed, some are designed to reduce nitrogen loads, like “innovative or alternative systems.” 

While most of the septic systems in the Tashmoo watershed — 860 of them — are from Tisbury, 261 are from West Tisbury, and nine are from Oak Bluffs. Horsley said this means something like an intermunicipal agreement may be necessary in the future.

Time was also reserved for questions and comments from town officials and the public.


  1. so who pays for those septic system upgrades ?
    When I built my house on West Chop in 1999, I had to comply with the law and put in a title 5 septic system that I felt did nothing about nitrogen.
    I was not permitted to put in reasonable Anna Edey designed system.
    I’m not going to shell out $40,000 to do anything to the mandated system that is working according to design.

  2. Though the house I live in is not in the Tashmoo water shed area, I believe it is an exceptionaly unfair burden to put on people that did follow the laws when they installed their title 5 systems as did we. Then on the other hand I can’t reconcile the pollutants leaching into Tashmoo over time.
    Thinking outside the box for those that can prove they are unable to retrofit or upgrade their systems, I bet grants could be established. At the same time, construction in these zones should be limited and or zoned as larger parcels zoning, maybe 3 – 4 acres. That could mean lest buildings, less pollution.
    I am not an exclusivity person, yet one that looks ahead to encourage sustainability of the island and its inhabitants of all flora and fauna.

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