Wastewater planning for the Waquoit Bay watershed is complicated by the fact that the 1,623-acre area has nearly 50 sub-watersheds, many of which are shared by Falmouth, Mashpee and Sandwich, the Falmouth Water Quality Management Committee said in its meeting last Thursday, June 6.

Through an inter-municipal agreement, the three towns are looking to create a special committee to determine each town’s allocation for nitrogen removal in Waquoit Bay.

As part of the 2019 update to Falmouth’s comprehensive wastewater management plan for the Massachusetts Department of Environmental Protection, Falmouth’s water quality committee is reviewing a set of draft scenarios for the next five years to achieve total maximum daily load, or TMDL, compliance in all of Falmouth’s watersheds.

A TMDL is the maximum amount of a pollutant that a body of water can receive while still meeting water quality standards.

The three sections of the Waquoit Bay watershed are Eel Pond, Childs River and Waquoit Bay’s main basin, and the three residential peninsulas within these sections in Falmouth are Antlers Shores, Seacoast Shores and Seapit.

Committee member John B. Waterbury said the Waquoit Bay system is “unbelievably dynamic compared to all the other estuaries that we’ve got.”

“The tidal flow through the Seapit River is enormous, so there’s all sorts of really strong circulation that occurs in that system,” he said.

“Waquoit Bay has a lot of challenges, the first being that there are 48 sub-watersheds within the Waquoit Bay system,” Kristen Rathjen, a water quality technical consultant from Science Wares Inc., told the committee. “These sub-watersheds do not abide by town boundaries, so that’s part of what’s making the allocation difficult.”

The main source of nitrogen in the whole system comes from wastewater, with lawn fertilizers and storm runoff as additional sources.

In 2013, the Massachusetts Estuaries Project, or MEP, assessed nitrogen levels in Waquoit Bay, which has three sentinel stations and several monitoring stations. MEP is a collaboration of MassDEP and the School for Marine Science and Technology at the University of Massachusetts at Dartmouth to restore and protect estuaries.

“The Childs River and Eel Pond watersheds need to be considered together because of the geology and the water flow,” committee vice chairman Virginia Valiela said.

Eel Pond watershed is entirely within Falmouth’s boundaries, Ms. Rathjen said.

“It could have been its own MEP report, with the exception that the Seapit River connects Eel Pond to Waquoit Bay,” she said.

Childs River discharges into Eel Pond and not directly to Nantucket Sound as do the other south coast estuaries.

Nitrogen loads from the Eel Pond and Childs River watersheds can be calculated, Ms. Rathjen said. But she added that nitrogen removal cannot be quantified accurately because of movement of the tidal waters.

One nontraditional option for nitrogen removal is state-approved nitrogen reduction credits from fertilizer and stormwater runoff. Another option is shellfish propagation. Nine acres in Eel Pond have been allocated for aquaculture.

“The nontraditional options get you about two-thirds of the way there, and sewering alone should get you what you need for the nitrogen removal number,” Ms. Rathjen said of Eel Pond.

The Childs River watershed extends into Mashpee.

Nontraditional options for nitrogen removal are stormwater and fertilizer credits, shellfish propagation and wetland restoration in Farley Bog and along the upper Childs River.

“It is too early in the restoration project to assign any kind of nitrogen removal benefits,” Ms. Rathjen said.

The traditional option is sewering in the northern part of the Seacoast Shores peninsula, which contains 394 parcels, and the Seapit Peninsula, which includes 101 parcels.

After factoring in the nontraditional and traditional options, there is still a nitrogen removal deficit of 2,007 kilograms per year, Ms. Rathjen said.

“With sewering farther south in Seacoast Shores, because of tides, there is a benefit to the Childs River watershed, but it gets credited to the Eel Pond watershed. The tide is going to carry the clean water northward, so there is going to be a benefit,” Ms. Valiela said.

Following a discussion of the need for scientific modeling, Eric T. Turkington, the committee’s chairman, said the draft scenario is only for the next five years.

“We know we’re not going to be putting in sewers there in the next five years. What we might do in the next five years is sewer the Acapesket peninsula. All we can do about these other peninsulas is say, ‘We have this scenario that we think is valid. It says this one may need sewering. This one we could get away without sewer. And we’ll see how it goes. Come back and talk to us in five years.’ Right now it’s a five-year plan, and we’re not going to be doing model runs on Waquoit Bay in the next five years,” he said.

Ms. Rathjen completed her presentation by describing the shared watersheds—Waquoit Bay’s main basin, Hamblin Pond, Red Brook, and the Quashnet River—that are shared with Mashpee and/or Sandwich.

“The shared watersheds are so varied in terms of land use that a special committee of the three towns will be needed to develop a workable scenario. The shared sub-embayment options are too complex to explore until the formal allocation has been decided,” she said.

In a worst-case scenario, if Falmouth were to assume 100 percent of the nitrogen removal requirements for Eel Pond, Childs River, Waquoit Bay’s main basin, Hamblin Pond, Red Brook and the Quashnet River, the total nitrogen to be removed would be 14,966 kilograms per year, Ms. Rathjen said.

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