A judge has ruled that Manchester city officials violated state law when they introduced fluoride into the regional water supply 18 months ago. He ordered it shut off by April 1, 2004.

In the meantime, Superior Court Judge Robert J. Lynn ruled on Tuesday, Manchester city officials could either get state law changed or hold referenda in six neighboring towns.

In his ruling, Lynn stuck closely to the wording of state law, which requires a public hearing and referendum in any town or city where fluoride is added to a public water supply.

When Manchester held a 1999 referendum on fluoridation, only Manchester residents voted on the issue, even though about 40,000 people who live outside the city rely on Manchester Water Works for their water.

Fluoridation opponents hailed the ruling.

“Obviously we’ve known all the time we were right,” said fluoride foe Lloyd Basinow, an original plaintiff who was later dropped from the suit. “It was a matter of the court going along with us. But there’s no reason for delay.”

Gerhard Bedding, executive director of the New Hampshire Pure Water Coalition, said the ruling could apply to water systems in Laconia, Dover and Portsmouth, which sell significant amounts of water outside their borders.

To a lesser extent, it could affect Durham and Rochester, he said. He called on Manchester officials to voluntarily stop fluoridation immediately.

The ruling took city and state officials by surprise. Manchester Mayor Robert Baines said he would confer with Water Works officials and others before discussing details.

But he said Manchester voters adopted fluoridation, so it is the responsibility of city officials to see the will of the voters is carried out.

When they filed a lawsuit last year, opponents of fluoridation sidestepped scientific issues about the benefit and safety of fluoride.

Rather, their main claim was that residents in portions of Auburn, Bedford, Derry, Goffstown, Hooksett and Londonderry were disenfranchised in the Manchester referendum.

Lynn agreed.

At one point in his ruling, Lynn wrote that the law “should be interpreted to mean what it says.”

“It necessarily follows that where a single supplier services more than one municipality, there must be a hearing and vote in each of said municipalities,” Lynn ruled.

But Lynn believes the Legislature never contemplated a situation where a single water supplier pipes water to more than one town.

The ruling “creates difficulties of its own,” he acknowledged.

For fluoridation to continue, each town would have to hold a referendum, Lynn hypothesized.

The referendum would have to be townwide, even if only a fraction of town residents are hooked up to Manchester Water Works. (In Auburn, for example, only 4 to 8 percent of town residents receive water from the city.)

And if only one of the seven municipalities were to deny fluoridation, the fluoridation would have to stop systemwide, Lynn ruled. That is because Water Works cannot practically separate fluoridated and unfluoridated water.

Lynn noted that at least three other multi-jurisdictional water suppliers have implemented fluoridation through the procedure that his ruling struck down.

Attorney Jed Z. Callen, who represented the 19 people who challenged the referendum, identified those communities as Portsmouth, Concord and Durham.

Bernie Lucey, senior water supply engineer at the state Department of Environmental Services, said some water companies have many customers outside their borders; others only a handful.

Durham’s well is in Lee and a handful of Lee houses along the pipeline are served. By contrast, Portsmouth has customers in Rye, Greenland, Newington, New Castle and Madbury.

Concord has fewer than a dozen customers in Bow.

Public health officials said it was too early to say what happens next.

“Certainly, the judge’s decision isn’t about the safety of fluoride; it was a matter of law,” said city Health Officer Fred Rusczek. Fluoridation continues to be one of the most effective measures possible to improve the dental health of the community, he said.

But Bedding noted that the hydrofluorisilic acid used by Manchester Water Works contains trace amounts of arsenic and lead. Goals set by the Environmental Protection Agency call for no lead or arsenic in drinking water, he said.

Brook Dupee, a public health official with the state Department of Health and Human Services, said the loss of fluoridation will hurt poor families that do not have access to dental care.

Twenty percent of individuals develop 80 percent of tooth decay, he said.

Dupee said it’s possible the state would join with the city to appeal the ruling.

The water users in the six towns receive water either directly from Manchester Water Works or from a wholesale water company.

Auburn has 315 water users; Bedford, 5,300; Derry, 15,000; Goffstown, 6,000; Hooksett, 6,200; Londonderry, 5,600.