Norwich — Water in the Norwich Public Utilities drinking water system has exceeded federal standards slightly for a contaminant that forms when organic material interacts with chlorine used to purify the water.

Water tests revealed that the level of trihalomethanes (TTHMs) in the NPU water system exceeded the recently revised federal Environmental Protection Agency standard at times during the past six months.

According to information released by NPU, the TTHM levels ranged from 0.087 to 0.093 parts per million in a liter of water. The EPA limit is 0.080.

“It’s vitally important to us for the people to realize the water is safe to drink,” NPU spokesman Chris Riley said, “and that means the level we violated the standard doesn’t pose any health hazards whatsoever.”

Mark Decker, water integrity manager for NPU, said NPU officials were notified by state officials April 8 of the violations, and the utility has worked out a plan already approved by the state Department of Public Health to correct the issue.

TTHMs are created when methane from decaying organic leaves and plant matter in the reservoirs interacts with chlorine used to disinfect the water, NPU officials said.

To address the problem in the short term, NPU will flush the water system starting May 11 by opening fire hydrants. NPU will announce a schedule of flushing next week. The flushing could cause some discoloration of water for short periods, Decker said. Running faucets for several minutes is expected to clear up the discoloration, officials said.

NPU regularly flushes the system to remove dirt and sediment from the pipes, Decker said.

NPU will send a required notification letter to all water customers within the next few days describing the contamination issue and the utility’s response. A fact sheet about the problem and a list of “frequently asked questions” and answers is posted on the utility website, www.norwichpublicutilities.com.

Department of Public Health spokesman William Gerrish said the Drinking Water Section of the department has reviewed and approved NPU’s plan to address the problem both short and long term. He said the agency will send a formal letter to NPU announcing the approval soon.

Gerrish described TTHMs as a byproduct of the disinfection process in the water system.

“This exceedance was discovered as a result of Norwich’s routine water quality monitoring within Norwich Utilities water distribution system,” Gerrish said in a written statement. “This violation requires that Norwich Utilities issue a public notice to their customers within 30 days of learning of the violation. Norwich is working on a number of water treatment and distribution projects that will assist to reduce the levels of TTHMs in their water distribution system.”

Several long-term projects have been in the works at NPU to improve water quality and also will reduce likelihood that TTHMs would build up, Decker said. Construction on a new system of pumps and drives at NPU’s Deep River water treatment plant in Lebanon is expected to begin this month. A new water tank, also in Lebanon, already is in service with improved water circulation systems.

Riley said ensuring water “moves” through the system more efficiently also would reduce the problem of water standing in one place within the system for too long.

“This is a complex problem that will require a very complex solution,” NPU’s fact sheet posted on the website said. “We will continue to monitor this situation very closely and to measure the levels of TTHMs in our system. Our results will be reported to the state of Connecticut Department of Public Health in three months as required by state regulation.”

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