Tisbury has a plan to tackle nitrogen levels in Lake Tashmoo, but it still needs state approval.
The Tisbury Select Board unanimously approved on Wednesday, July 26, to accept and send a targeted watershed management plan for Lake Tashmoo, as presented by water resources consultant Scott Horsley, to the state for review.
The plan is designed with steps to maintain and restore Lake Tashmoo’s environment, particularly through nitrogen load reduction, over a 20-year period. Although there are various sources of nitrogen loading in Tashmoo, 75 percent of it comes from septic systems (76 percent from Tisbury, 23 percent from West Tisbury, and 1 percent from Oak Bluffs). The percentage was based on 2010 load numbers from the Massachusetts Estuaries Project (MEP) that showed 9,154 kilograms of nitrogen entered the Tashmoo watershed a year. The MEP threshold for Lake Tashmoo is 6,244 kilograms of nitrogen a year.
The load amount increased to 9,831 kilograms of nitrogen a year in 2023, which is projected to rise to 10,605 kilograms of nitrogen a year by 2043 if nothing changes — a slight increase from the projected 10,510 kilograms of nitrogen a year by 2043 projected when the data was presented in May.
“Things are going to get a little worse before they get better,” Horsley said. “This is what all the towns I’ve worked with on the Cape and Islands [see]. We wait, things are going to get worse.”
Additionally, Horsley showed a projection of the number of septic systems that are anticipated to be added between 2020 and 2023 and contribute to the nitrogen load in Lake Tashmoo: seven systems in Oak Bluffs, adding 45 kilograms of nitrogen a year; 110 systems in West Tisbury, adding 684 kilograms of nitrogen a year; and 156 systems in Tisbury, adding 296 kilograms of nitrogen a year. Horsley said West Tisbury is anticipated to have a higher load output because it doesn’t have the types of regulations Tisbury has in place.
Some of the proposed changes in the plan for Tisbury include upgrading septic systems to more innovative and alternative (I/A) systems, to cluster wastewater treatment at the Lake Street Park area, retrofitting stormwater systems at West Spring Street, making a sewer connection in the State Road area (B2 district), and creating a fertigation well, a type of irrigation system, at Mink Meadows.
Additionally, Horsley pointed out that while some people have Title 5 septic systems in place, which were meant to have a lower environmental impact than regular septic systems, an enhanced I/A system will further decrease the output of nitrogen loads. Depending on the type of enhanced I/A system, it can reduce 28 percent to 81 percent of the loads if it replaces a Title 5 system. Horsley said the I/A systems being proposed for the plan would decrease nitrogen loads by around 70 percent. Upgrading to an I/A system can cost around $45,000 or more.
Horsley said implementing these plans can reduce a total of 3,587 kilograms of nitrogen a year. Using the same strategy but at an increased rate, the strategy could reduce a total of 4,361 kilograms of nitrogen a year by 2043.
If the board approved the plan, Horsley said, a finalized version could be sent to the state, and the town could proceed to try acquiring funding for things like upgrades.
“Ultimately, the town gets what’s called a watershed permit,” he said. Watershed permits allow municipalities to design ways to improve and restore waterways. The state Department of Environmental Protection recently announced regulations meant to reduce nitrogen levels in Cape Cod watersheds, including the usage of watershed permits, although the Island was not included in these regulations.
“There’s only been one watershed permit that has been issued, and that’s for Pleasant Bay in Cape Cod, to relieve some of the surrounding towns,” Horsley continued.
Horsley said the presented plan is similar to a watershed management plan he worked on for Wellfleet, which is waiting for state approval.
The board took some time to ask some clarifying questions before giving its approval.
