Wellesley officials urged residents to boil tap water before drinking it after E. coli was detected in the town’s water supply Thursday afternoon.
Officials were investigating the cause of the contamination, but said they expected it could be a few days before the town’s drinking water proves safe again.
“While we believe at this time that the potential health risk is relatively small, we take any contamination seriously and are taking every precaution to protect public health,” town officials said in a statement.
It was unclear how long the water has been infected, but Hans Larsen, Wellesley’s executive director, said the town’s water is tested about 36 times each month.
He did not know exactly how long it would take for the town to resolve the issue, but noted that it could take days.
“As we work through our investigation, there’s the need to do continued sampling, and there’s some turnaround time associated with seeing the results of those samples,” he said.
The town has turned off the water storage tank where the bacteria was found and was flushing nearby hydrants to get water with more chlorine into the system.
“Town staff will work tirelessly to figure out the source of the issue and get the water supply back on line as quickly and safely as possible,” Larsen said. “We recognize it’s a significant inconvenience to residents and want to address it as soon as we can.”
Michael Pakstis, director of the town’s Department of Public Works said testing will be a challenge because the tank where it was discovered is underground, so “investigating it is a little more difficult.”
The department was notified about contamination in a storage tank in the Pierce Hill Reservoir at about 8 a.m., and the order was issued at about 11 a.m. Pakstis said it was the first time such contamination had been detected in Wellesley in decades.
“Best case scenario, the order will be lifted by Saturday,” he said.
Wellesley is working with the state Department of Environmental Protection and the Department of Public Health to find the root of the problem and to monitor the situation.
The presence of E. coli suggests that human or animal waste contaminated the water, officials said. The bacteria can cause diarrhea, cramps, nausea, and headaches and poses a heightened risk for infants, young children, some elderly people, and those with severely compromised immune systems.
“These bacteria can make you sick and are a particular concern for people with weakened immune systems,” officials wrote.
Larsen said he was not aware of any residents contracting the bacteria by late Thursday afternoon.
Among possible causes of contamination are a break in the pipes, a failure in the water treatment process, or an increase in runoff, such as following a heavy rain, the statement said.
The town has notified residents through a reverse 911 call and reached out to restaurants and food services, day-care centers, and health care providers.
With school set to start next week, School Superintendent David Lussier said every school has publicized the contamination, turned off water fountains, and placed signs and hand sanitizer in every school bathroom recommending hand-washing.
“We will continue to monitor this situation closely,” he said in an e-mail.
Wellesley residents should boil any tap water they plan on using to drink, cook, or wash dishes, make ice, or brush their teeth, the statement said. Or, they should use bottled water.
Residents should bring the water to a rolling boil and keep it there for at least one minute before using.
According to the town’s website, 85 percent of Wellesley’s water supply is derived from local wells, with the rest coming from the Massachusetts Water Resources Authority system.
Larsen noted the town has “no data to lead us to believe the contamination source is from the MWRA.”
The contamination does not affect any other nearby communities, he said.Globe correspondent Trisha Thadani contributed to this report. Jaclyn Reiss can be reached at jaclyn.reiss@
Correction: An earlier version of this story had the incorrect time that the Department of Public Works was notified about contamination in a storage tank.