Of the five largest public water utilities operating in West Marin, M.M.W.D. is the only one that fluoridates its water supply. North Marin Water District, the Inverness Public Utility District, the Bolinas Community Public Utility District and the Stinson Beach County Water District do not fluoridate their water, though slight naturally occurring levels are present at about 0.1 milligrams per liter in each supply.

Fluoride is viewed by groups such as the American Dental Association as a way to prevent tooth decay; it has been added to M.M.W.D.’s drinking water since the 1970s, when voters approved measures supporting fluoridation. But critics, like Clean Water Sonoma Marin, cite recent studies, including one by a former Harvard Medical School researcher, that show a potential link between fluoridated water and hypothyroid conditions, bone cancer and early childhood brain development.

“The worst part of all this is that it isn’t even a pharmaceutical-grade product that they’re putting in the water,” said Clean Water’s director, Dawna Gallagher-Stroeh. “They’re putting in an industrial-grade byproduct of the fertilizer industry.”

The district’s environmental and engineering services manager, Michael Ban, said that even if the petition lands on the ballot, a 1995 state law that requires fluoridation for districts with more than 10,000 customers may make the effort moot. M.M.W.D. serves 186,000 customers.

“We can’t not do it,” Mr. Ban said. “We cannot unilaterally stop fluoridating the water supply.”

Mr. Ban describes the amount of acid-based fluoride compounds added to the water as about “one drop in every 18
gallons.”

M.M.W.D. customers narrowly supported water fluoridation through two ballot measures—in 1972 and in 1978—that passed by slightly more than 50 percent. Neither of those measures can be overturned without another ballot measure. And according to the district, even then the state Attorney General’s Office could bring a lawsuit against the district if they were overturned.

Ms. Gallagher-Stroeh, however, pointed to a loophole in a state health code that would allow M.M.W.D. to be exempted from the state’s water fluoridation requirement if ratepayers are funding the fluoride program’s service and maintenance costs. She claims that fluoridation is paid for out of the district’s general fund, which includes ratepayer
contributions.

“They think they have to adhere to the state mandate,” Ms. Gallagher-Stroeh said. “If that were true, then they would have to stop [using] ratepayer money for fluoridation.”

But Mr. Ban, in a 2013 memo to the district, said that the approximately $140,000 needed to fluoridate the water comes from sources other than ratepayers, such as property leases. Nonetheless, he concedes that the state law on fluoridation is fuzzy and, should district voters overturn the old ballot measures, that the end result is presently unknowable.

“I think we’d still have to figure out what the outcome would be,” Mr. Ban said. “It’s not entirely clear.”