No injuries were reported, and the cloud was later found to be harmless.
James Staples, spokesman for the state Department of Environmental Protection, said that "the people in the area were lucky."
"Given a different set of atmospheric conditions, and stronger concentration of acid, it could have been a major problem," he said. ''. . . We thought we had a real mess on our hands."
Hydrochloric acid in vapor form can cause damage and pain to the tissues of the nose, eyes, lungs and skin if inhaled in sufficient concentration, Staples said. In certain situations, it can be lethal, he said.
Staples said the close call occurred when the tank ruptured about 10:15 p.m. Thursday.
According to local officials, a Mount Laurel police officer discovered the spill when he noticed a large vapor cloud over the tanks. Authorities immediately ordered 50 people, including 25 from a nearby factory, evacuated
from the area, and closed a stretch of Route 38 from Hartford Road to Marter Avenue.
By about 4 a.m., the situation was under control. Residents were allowed to return to their homes, and Route 38 was reopened.
Staples said the DEP set up a command post behind a church and took numerous air samples up to a mile downwind from the spill, but found no detectable readings of acid in the vapor.
The cause of the rupture, which produced several cracks and a huge gash in the bottom of one of three cylindrical tanks on the grounds of Halliburton, is still under investigation, according to Col. Saunders Weinstein, Mount Laurel director of public safety.
According to Staples, it is too early to speculate whether there will be any fines resulting from the spill until its cause has been determined.
"We know what happened, but we don't know why," said Staples. "The acid leaked from a fiberglass outdoor tank containing 30 percent strength hydrochloric acid."
He said Halliburton, which does industrial cleaning, uses the acid for cleaning out boilers and tanks of industrial firms.
Staples said the DEP was notified by Burlington County authorities 15 minutes after the spill was detected, and dispatched a seven-member emergency response team, which took air samples and determined that the cloud was harmless.
Nearly 9,000 gallons of acid spilled from the tank, about 7,000 gallons of which was contained inside a concrete dike surrounding the tank. Some of the chemical splashed over the dike and contaminated a 500-square-foot patch of soil that had to be neutralized by firefighters shoveling mounds of soda ash to counteract the deadly liquid.
The excess acid was pumped into an empty tank.
Most of those evacuated went to the homes of family and friends, but officials also set up a shelter in a nearby firehouse, according to Weinstein.
John Matraszek, 41, night supervisor at Johnson & Towers, a diesel-engine repair firm across the street from Halliburton, said that 25 of his employes were evacuated about 10:40 p.m., and that the employees had to drive through the cloud to leave.
He said that none of his employees apparently suffered ill effects, but some gagged as they made their way to their cars.