Posted on May 29, 2023
Like Brigadoon, the mythical Scottish highland village that reemerges to great fanfare once every century, a half-dozen small islands rise from the waters of Neponset Reservoir each year, usually in late summer when water levels can dwindle and aquatic plant growth mushrooms.
There’s nothing enchanted about this seasonal phenomenon, however.
According to Richard Lewis, treasurer of the non-profit Neponset Reservoir Restoration Committee, the phantom islands result, not from falling water levels, but methane gases trapped below shifting sediments which literally push the lake bottom upwards in certain areas — sometimes as much as two to four feet.
Appearing before the conservation commission Monday night, Lewis said the islands, which can reach 15-feet by 20-feet in size, emerge in a half-dozen different locations in a central portion of the reservoir loosely framed by Beach Street, Shoreline Drive and Ridge Road.
He suggested that related methane discharges are probably linked to the depth of peat layers beneath the man-made lake, which in some areas can be more than 30 feet thick.
To address the situation, a consulting firm hired by the NRRC is recommending targeted dredging in areas where the islands typically to allow methane to more easily disperse.
“They pose a hazard for people out in boats,” Lewis said. “If you don’t know where they are you can do all kinds of damage.”
Lewis stressed that further studies are needed before a detailed plan can be filed with state regulators, who would need to approve any dredging operation.
In a related matter, Lewis reported that herbicide treatments in the reservoir will resume this year, primarily in an effort to control invasive milfoil — the first application since an extensive weed treatment program in 2021.
The treatment, marketed under the brand name ProcellaCOR, does not typically eradicate milfoil, but is able to control the species for three years or more, during which time efforts can focus on other offending plants, like fanwort.
ProcellaCOR was used last year to treat invasive milfoil in Cocasset Lake with impressive results. The product, considered so effective that a 20-ounce soft drink bottle could treat an acre, is typically applied via boat, with sub-surface hoses connected to a holding tank.
Areas designated for treatment with ProcellaCOR are situated largely on the eastern side of the reservoir, Lewis said. On the western side, weed control measures will involve chemicals previously used to better compare the effectiveness of different products.
“Never having done this before with this new chemical — and also the additional cost — we thought it prudent to try it this way,” he explained.
