BEAVERTON — On May 17, if everything works well in tests up to then, fluoridated water will begin flowing to about 65,000 of Beaverton’s 79,000 residents.

At the same time, smaller amounts of fluoride will pass from Beaverton’s system to about 17,000 residences and businesses served by Tigard’s water system. Outsiders include residents of King City, Durham and unincorporated areas on Bull Mountain.

The Beaverton City Council, at Mayor Rob Drake’s urging, put the question to voters in November 2002 and found 53 percent favored adding fluoride to city water to combat tooth decay.

More than $900,000 later, Beaverton is mailing out customer notices today and Friday explaining fluoridated water’s arrival. Parts of the city served by Raleigh and West Slope water districts will not be affected. Nor will some of those supplied by the Tualatin Valley Water District.

Beaverton is adding fluoride at a concentration of nine-tenths part per million. The concentration is expected to fall off progressively the farther from Tigard’s intake point at the city line.

Tigard-area residents also have been notified. Dennis Koellermeier, Tigard’s acting public works director, said his city has no choice in the matter. It buys between 1 million and 4 million gallons of water daily from the Joint Water Commission, a consortium of water providers that includes Beaverton and Tigard.

The only pipe from the commission’s treatment plant near Forest Grove to Tigard’s system runs through Beaverton’s fluoridation operation. Portland, which does not fluoridate, fills the majority of Tigard’s needs, as much as 14 million gallons per day during summer, Koellermeier said.

In the next year, Tigard will continually test water at several locations to see how mixing the two sources affects fluoride concentration, but it will be stuck with whatever those concentrations are, Koellermeier said.

“To not use the Joint Water Commission water,” he said, “would put us into a state where we would not have enough water.” So far, about 100 Tigard water customers have responded to the city’s notices, with about 90 asking questions and 10 objecting to fluoride, he said.

Meanwhile, a contractor this week is putting together the fluoride-injection equipment in a small building west of Beaverton.

Its location is secret because of security requirements imposed by the U.S. Department of Homeland Security, said David Winship, Beaverton’s utility manager in charge of its water system.

The building is being remodeled to house the fluoridation operation. It will be surrounded with fence topped with barbed wire. Other security devices will instantly alert city workers to intrusions within the fence, said Charlie Harrison, a city engineering technician.

The tight security, Winship said, is necessary under federal regulations, because the setup injects chemicals into the water. Saboteurs could potentially introduce other substances into the water, he said.

The city’s $900,000-plus fluoridation outlay, including $588,375 for rebuilding and equipping the building, has been covered from the city’s $2.6 million contingency fund for water projects and should not affect water rates, said Patrick O’Claire, city finance director.