The 36-inch main break happened on Route 206 just before 7 a.m. Wednesday, shutting down northbound lanes between South Bridge and Orlando Drive.
Schools closed on Wednesday but reopened on Thursday with students being told to bring bottled water with them for drinking.
Northbound 206 is closed from South Bridge Street to Somerset Street for at least the next three days while crews continue to make repairs.
Water pressure and service have been restored to the system.
Residents are being asked to conserve water for the time being and a mandatory boil water advisory issued by American Water for customers residing in Bridgewater, Raritan Borough and Somerville, is still in effect.
Boiled or bottled water should be used for drinking, preparing foods, mixing baby formula, food, juices or drinks, washing vegetables and fruit, cooking, making ice, brushing teeth, and washing dishes until further notice.
Somerville Mayor Brian Gallagher said Wednesday morning's water main break impacted 18,000 people and he says his top priority is always safety.
"We're home to government, to hospitals, to a lot of schools, public and private and so the schools were closed," Gallagher said. "Our medical center, we brought in tankers from sister agencies in case of fire suppression being needed."
American Water says that crews will work continuously to repair the main.
New Jersey American Water officials said there are plans approved to run new water lines and bypasses and that work is expected to start in early 2025.
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