The Delaware Memorial Bridge — a heavily traveled span that serves as a key conduit for East Coast traffic — reopened Sunday evening after being closed more than six hours following a potentially “catastrophic” chemical leak at a nearby chemical plant.

The closure brought vehicles to a standstill in both directions on one of the busiest travel days of the year.

The leaked chemical, ethylene oxide, is a highly flammable gas that is a finished product stemming from methanol, Holloway Terrace Fire Company Public Information Officer George Greenley said.

“If that flume would have had an ignition source it could have been catastrophic with the bridge traffic,” he said.

The Delaware chemical production facility Croda Atlas Point was recently expanded to produce two tons of ethylene oxide per hour, the Philadelphia Inquirer reported.

The bridge closed around 5 p.m. and reopened about 11:30 p.m. Croda operators requested it be shut down, the Delaware River and Bay Authority tweeted.

Greenley said prior to the reopening that fire officials were waiting for pressure to dissipate in the pipes at the facility before allowing travel to resume on the span.

During the closure, traffic was diverted to other crossings, including the Commodore Barry Bridge to the north, causing what the authority called a “parking lot” situation to disperse to other clogged roadways.

The bridge, where more than 80,000 vehicles make the crossing daily, carries Interstate 295 traffic on eight lanes over the southern reaches of the Delaware River between northern Delaware and southern New Jersey.

Virginia House Delegate Lashrecse Aird was stuck in the traffic jam on her way home to Virginia from New York. She said she had surpassed the usual six-hour holiday commute, and her GPS told her she had more than three hours to go.

A police vehicle and a hazardous-materials truck were blocking the bridge when she approached. She was at a standstill for an hour before being redirected, she said.

Greenley said emergency responders are monitoring air levels on top of the bridge. It wasn’t immediately clear when the plant would be returned to full production, according to the Inquirer.