Gilbert has put two top administrators on leave and launched an internal investigation after officials discovered fluoride has been withheld from part of the town’s water supply for at least 13 months.

Gilbert voters approved fluoridation 12 years ago after a heated debate on the controversial issue, and the task of adding the cavity-fighting chemical fell to Public Works Director Lonnie Frost, whose wife Shelley had led the opposition against fluoride.

Frost, along with Water Manager Chris Ochs, are now on paid leave while the town investigates how and why fluoride was not being added at Gilbert’s North Water Treatment Plant. Town Manager Patrick Banger could not immediately say whether it was done intentionally or by mistake.

“We just want to make it clear that we’re taking it very seriously, but it’s not a matter of a public health hazard in that the water was being over-fluoridated,” Banger said. “We weren’t meeting the minimum levels.”

The town has ordered an independent verification of its water system and will look to get the fluoridation system online as soon as possible, said Dana Berchman, Gilbert’s communications director.

“This is unacceptable and we are taking immediate steps to remedy the situation,” Berchman said.

Gilbert Municipal Code requires the public works director to assure compliance with all laws, regulations and ordinances related to drinking water quality. Frost, who has been public works director for 14 years, declined to comment due to the ongoing investigation.

Deputy Town Manager, Marc Skocypec, first became aware of a possible problem on Aug. 14, Berchman said. The town conducts regular water testing at the plant and files an annual water quality report.

Reports from 2009 and 2010 indicate the amount of fluoride in Gilbert’s water varied between 0.42 and 1.1 parts per million, well below the maximum allowable level of 4 parts per million. The 2011 report, however, indicates fluoride levels had dropped, ranging from “not detected” to 0.92 parts per million.

Gilbert dentist William Fulcher said a concentration of 1 part per million has “astronomical benefits for dental health.” He said he would be concerned if fluoride is being withheld from some residents’ tap water.

“There’s a lot of anti-fluoridation groups out there,” Fulcher said. “For us as dentists, if you do it, that’s great … and if you don’t, oh well, you’ll just have more cavities and we’ll do more dental treatment.”

Fluoride is still being added to water at the Santan Vista Water Treatment Plant in south Gilbert, which is operated through a joint partnership with Chandler, officials said.

Flouoride apparently had been withheld from the North Water plant beginning around the same time three new Town Council members were elected and sworn in; at least two have direct ties to the conservative “tea party” movement.

Tea party activists have been vocal opponents of municipal water fluoridation, and other conservative organizations such as the John Birch Society have opposed it for decades.

Critics dispute some of the health benefits attributed to fluoride and worry about potential side effects, such as muscle pain, discolored teeth or bone damage.

Meanwhile, Gilbert Councilman Eddie Cook, elected to his first term last spring, has requested that Town Council discuss eliminating water fluoridation during its annual retreat this weekend. That agenda item is unrelated to the investigation at the North Water Treatment Plant, Banger said.

Despite the claims from fluoride critics, the U.S. Centers for Disease Control and Prevention has proclaimed it one of the 10 greatest public-health achievements of the 20th century, and every U.S. surgeon general since the 1950s has endorsed the practice.

In Gilbert, the Town Council first approved fluoridation in early 2000, but a resident referendum forced the issue onto a November ballot. About 54 percent of voters approved the measure, and fluoridation commenced in 2001.