The Tisbury board of health this week sought to tie up loose ends at a third and final public hearing on a controversial policy proposal that would charge new homeowners for the wastewater they produce.

About 20 people attended the hearing at the Tisbury Emergency Services Building Monday. As evening sunlight streamed in through the windows, residents took turns attacking the proposal, first unveiled in May at a daylong conference dealing with the health of Island ponds.

The proposed rules, which the board of health has the power to enact on its own, seek to limit the amount of new nitrogen entering Lagoon Pond and Lake Tashmoo by charging homeowners a semiannual fee based on how much wastewater they produce. The fee for a three-bedroom house, for example, would range from around $320 to $3,200, depending on the type of wastewater treatment installed. The rules would apply only to new construction in the watersheds.

A series of three public hearings began on June 16. The draft policy has been roundly criticized at every one.

On Monday the three board of health members were prepared to answer questions that came up last week. One focused on whether eutrophication — a problem facing most Island estuaries — poses a hazard to human health. Board member Michael Loberg cited a number of official studies suggesting that it does. The studies indicate, among other things, that algal blooms caused by too much nitrogen may produce toxins that can cause fever, headache and vomiting. Mr. Loberg was unaware of any cases on the Vineyard.

Critics remained unconvinced.

“Why is the board of health spearheading this?” said Doug Dowling, an engineer and outspoken critic of the proposal. “It’s definitely not a health issue.” He argued that the town lacks the required data to be certain of where the nitrogen originates — whether from groundwater or elsewhere. Most Island officials working to address the problem agree that excess nitrogen comes mostly from septic tanks leaching into the groundwater.

Some again took aim at the Massachusetts Estuaries Project, a long-term collaboration between the state Department of Environmental Protection and the University of Massachusetts at Dartmouth that provides much of the baseline data for nitrogen mitigation efforts in the state. Donald Muckerheide believes mechanical aeration would be more cost effective than fees or wastewater management. He also believes rainfall adds far more nitrogen to the watersheds than septic tanks.

Kent Healy, a well-known civil engineer, claims the MEP numbers for nitrogen entering Lake Tashmoo don’t add up. Using figures in the MEP report, he calculated that the pond receives only up to 24 pounds of nitrogen per day, not the 55 pounds cited in the report.

“Before you start making all these plans, you have to have a good handle on the numbers,” Mr. Healy said.

There were questions about why the MEP does not appear to include groundwater and well monitoring information in its reports. Island surveyor Reid Silva noted levels of around 0.2 milligrams of nitrogen per liter in well water, which he said do not correspond to higher concentrations in the ponds. “If it’s septics and it’s groundwater, how is the groundwater not diluting the pond, as opposed to increasing concentrations?” he said.

Mr. Loberg countered that most of the food coming onto the Island ends up in septic tanks, amounting to about 50 milligrams of nitrogen per liter. “There is no way that’s not the main source of nitrogen here,” he said. But he agreed the town could do a better job of monitoring its wells.

Health agent Maura Valley confirmed the existence of monitoring wells in town, but did not know exactly where they were.

The meeting also touched on funding for wastewater projects. The Martha’s Vineyard Commission, working with the town, applied this year for a major grant from the EPA for a permeable reactive barrier to stop nitrogen on its way to Lagoon Pond. The project would have helped with groundwater monitoring efforts in town, but the grant was not awarded. A separate grant may soon allow the town to test a new type of denitrifying septic system, although Mr. Loberg said those systems are generally less reliable than denitrifying toilets, as reflected in the proposed fees.

There were questions about the fairness of targeting only new homeowners for fees.

“If this is a townwide issue, a regional issue . . . it has to be the responsibility of the entire community, and not just the person that walked in the door last,” said Mr. Silva. “And this Island, I have to say, has been very quick to shut that person out.” He said if the MEP numbers can be confirmed, then the town should move forward with regulations that apply across the board.

Mr. Loberg reacted favorably to the suggestion. “If that’s the outcome of spending six hours together doing this, that’s fairly valuable,” he said, looking back on the hearings.

Other concerns focused on how the fees collected will be used. Current guidelines call for spending the fees on nitrogen mitigation in the watersheds, subject to town meeting approval. Board of health chairman Jeff Pratt agreed that the rules should clarify how much can be spent on sewers, and he assured residents that the money would be used fairly. “There is no slush fund,” he said.

The board plans to deliberate further and present its final proposal to the selectmen. Mr. Loberg said it was too soon to know whether the board might also want to bring the proposal to a town meeting vote, but noted that town meeting approval is not required for public health regulations.

Dylan Seder worried that comments during the public hearings would not make a difference in the end. “What is our voice going to have to do with this decision?” he said. “People are not into this.”

But Mr. Loberg said he hoped the public process would encourage people to begin thinking about their own role in protecting the health of the ponds. “We felt that this discussion needed to be held,” he said. “This was one way to handle it.”