PORT ALLEN — Heated asphalt mixed with a water-based additive being offloaded from a barge Monday night, produced a foul smelling steam that drew complaints from residents and prompted emergency responders into action, Fire Chief Rick Boudreaux said.

The barge, carrying the type of asphalt used to construct roads, was offloading its cargo into a bulk storage tank at the Center Point Terminal tank Farm in Port Allen, Monday evening, Boudreaux said.

The asphalt has to be liquefied — heated to about 260 degrees Fahrenheit — in order for it to be pumped into a tank, Boudreaux said.

Companies generally mix an oil-based additive with the asphalt before pumping it into tanks, he said.

On Monday, however, the asphalt was mixed with ammonia and the water-based additive, the chief said.

As the mixture rose in temperature it produced a thick, pungent steam that wafted throughout the city around 6:30 p.m., Boudreaux said.

Emergency personnel responding to the scene determined the steam was non-hazardous, Boudreaux said.

The fire chief and other responders were back at the scene Tuesday morning to continue their investigation.

With no public safety risk, Boudreaux said, “We’re about ready to let them get pumping again.”