A malfunctioning valve at a local water station caused nearly six times the normal amount of fluoride to flow into drinking water in parts of the city yesterday, state and local officials said.

Residents and businesses along Millham Street, Boundary Street and lower sections of Robin Hill Road and the surrounding area were instructed to flush the hot and cold taps in their faucets for at least 10 minutes.

Martin J. Suuberg, Central Massachusetts regional director for the state Department of Environmental Protection, said local authorities told the DEP that as of last night, there were no reports of illness or injury caused by the affected water.

Officials were confident yesterday that, along with discarding any ice cubes, baby milk, beverages and meals that may have been made with the affected water, flushing out the pipes was all that was needed to restore normal levels of fluoride.

A DEP flier distributed to residents and businesses advised them to take extreme care when flushing their pipes, and not to come into contact with the water, which could cause burning, skin irritation or both. The flier assured residents that the problem had been fixed, but still requested that they take proper precautions.

Mr. Suuberg said that from around 10 a.m. to noon yesterday, there was a “chemical overfeed of fluoride” into the water system.

He said that officials at the Millham Water Treatment Plant told the DEP that a valve malfunction allowed a concentrated level of fluoride – which is normally used in the water to fight tooth decay in the city – to flow into the water system.

“A regular level of fluoride in the system is under four parts per million,” Mr. Suuberg said. “After two hours, tests showed the fluoride level at 24 parts per million.”

Mr. Suuberg said it is the first time he has seen such a concentrated amount of fluoride flow into a community’s water.

Doran Crouse, assistant commissioner of the city’s Public Works Department, said he was pleased with how the city handled the situation. He said there was a potential for disaster, but he said the problem was contained rather quickly.

Mr. Crouse said he estimated that 30 to 50 customers in an area about one mile square were affected. He said workers from the plant went from door to door yesterday afternoon, making residents and businesses aware of the situation and giving them instructions on what to do with their water.

Workers flushed affected water mains in the area, and Mr. Crouse said that yesterday afternoon fluoride levels were returning to normal. The DEP flier said discoloration of drinking water after the mains were flushed could be expected, but it posed no immediate health risks.

The plant – which is not the city’s only source of water – was shut down, and Mr. Crouse said that as workers tested the water farther and farther away, the fluoride level returned to normal.

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