Workers unclogged a backed-up line early Wednesday afternoon and stopped an ammonia leak at the PCS Nitrogen complex in Ascension Parish that forced the closure of a state highway for nearly eight hours, state officials said.

Once the blockage was cleared, pressure in the line, which is tied to an ammonia storage tank, was reduced and a pressure relief valve closed, halting the flow of the noxious gas to the atmosphere, state environmental and police officials said.

Garry Hiebert, human resources manager for PCS Nitrogen, said there were no injuries on- or off-site. He said company engineers are trying to calculate how much ammonia escaped and are investigating the root cause of the leak.

The State Police and the state Department of Environmental Quality representatives remained at PCS Nitrogen Wednesday afternoon.

“We are still in place to monitor and assess the event,” said Greg Langley, DEQ spokesman.

Langley said Wednesday afternoon that the last air tests taken in the area detected no ammonia.

Air readings taken from La. 3115 early Wednesday showed elevated amounts of ammonia gas, Langley said. But he could not say how high those levels were.

He added that calculations for how much ammonia was released were still being made Wednesday afternoon.

Hiebert said workers spotted the leak from the storage tank in the plant’s ammonia production unit about 4 a.m. and notified authorities.

He said company officials worked with State Police to close La. 3115 when the wind began shifting toward the highway and used the plant’s water suppression system to reduce the amount of ammonia gas escaping.

Lee said State Police closed La. 3115 in Iberville Parish about 6 a.m. between La. 30 and the Mississippi River.

Hiebert said the pressure relief valve closed about 1:30 p.m., which allowed La. 3115 to reopen.

PCS Nitrogen’s entrance faces La. 75, which is River Road, but the facility stretches north along La. 3115 and backs up against the Honeywell and Williams Olefins.

The section of La. 3115 that was closed is near the Ascension-Iberville Parish line and inside St. Gabriel’s corporate limits in Iberville Parish.

Langley and Lee said the leak did not prompt shelter in place orders nor pose a threat to the general public, but the highway closure was precautionary. Non-essential workers at PCS Nitrogen and other plants in the vicinity of the leak were allowed to go home, however.

The complex has 175 PCS Nitrogen employees and another 250 contractors who work there.

Ammonia, which has a pungent odor even at low concentrations, immediately burns the nose, throat and respiratory tract at higher concentrations.

Even at low concentrations, ammonia, one of the most widely produced industrial chemicals that is commonly used in the fertilizer industry, can cause coughing and irritate the nose and throat.

The PCS Nitrogen Fertilizer LP plant is a subsidiary of Potash Corp., a Canadian-based company that claims to be the world’s largest fertilizer company by capacity.

The company, based in Saskatoon, Saskatchewan, mines large deposits of potash across Canada but also produces ammonia and phosphate. All three are used in agricultural production.

The Geismar plant, which is one of Potash’s four nitrogen complexes worldwide, produces 500,000 tons of ammonia per year in addition to urea, nitrogen solutions, nitric acid and ammonium nitrate, the company website says.

PCS Nitrogen opened its once-shuttered ammonia unit in Geismar in 2011 to take advantage of low natural gas prices from the U.S. shale gas boom. The company sunk $158 million to restart the unit, which had been closed since 2003 due to previously high natural gas prices.

Follow David J. Mitchell on Twitter, @NewsieDave.