The 36-inch main break happened on Route 206 just before 7 a.m., shutting down northbound lanes between South Bridge and Orlando Drive.
While water pressure is slowly returning to most of Somerville, many places are still closed, including schools in Somerville and the Bridgewater-Raritan School District.
Borough Hall and the Somerville library will also be closed for the rest of the day.
Residents are being asked to conserve water for the time being and American Water has issued a mandatory boil water advisory for customers residing in Bridgewater, Raritan Borough and Somerville.
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."
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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