Noranda Alumina won’t have to undertake immediate modeling to show how far mercury is spreading into the air from its aluminum ore processing plant in St. James Parish, state regulators said.

Louisiana Department of Environmental Quality officials had ordered that modeling in late February after company officials acknowledged last year that mercury air emissions had been occurring, possibly for years, without a permit.

But late last month, DEQ changed a compliance order that had required the modeling. Instead, DEQ, at Noranda’s request, ordered the company identify possible sources of mercury and figure out how much is coming from those sources.

The revisions came after Noranda Alumina officials told DEQ in a meeting March 17 that the Gramercy complex along the Mississippi River is emitting far less mercury into the air than company officials had first feared and likely falls under the minimum rate of 25 pounds per year, DEQ filings show.

Below that rate, state law views the emissions as insignificant and, on a case-by-case basis, DEQ can choose not to require a permit.

DEQ officials are reviewing Noranda’s plans to spend up to three months estimating the total amount of mercury the plant is releasing into the air, narrowing down where the mercury is coming from and then directly measuring the emissions, an agency spokesman said.

Greg Langley, DEQ spokesman, said Monday the plan must be reviewed by DEQ’s Enforcement and Air Permit divisions. He said he does not yet know how long that will take.

“It’s complex and it’s complicated, but it’s movement on their part in response to the revised compliance order,” Langley said.

Chance McNeely, DEQ assistant secretary for environmental compliance, signed the revised order March 25. Noranda’s plans arrived the next day when they were due, DEQ online records show.

Separately, DEQ sent out its testing vehicle, known as the Mobile Air Monitoring Lab, last month to measure off-site mercury emissions from the Gramercy complex. But Langley said the unannounced visit, originally scheduled for three days, was cut short due to mechanical problems. He said the measurements are not yet available.

Whether and when the RV-like testing vehicle returns remains a decision for McNeely, Langley said.

Noranda reported the mercury emissions last April after maintenance workers uncovered mercury scale in tubing for a plant heat exchanger and drops of mercury under the heater. Mercury is a heavy metal that in higher concentrations can harm the human nervous system, heart, kidney, brain and lungs.

Even in small amounts, mercury is a worry because it doesn’t break down but accumulates in fish and shellfish that people eat.

The Noranda plant, which is near the Veterans Memorial Bridge and across La. 3213 from Gramercy, makes alumina from bauxite ore, a suspected primary source of the mercury. Alumina is turned into aluminum at the company’s smelter near New Madrid, Missouri.

DEQ and Noranda officials have said they do not believe the emissions rates are high enough to be unsafe.

Last year, Noranda tested steam vents and found their emissions had mercury concentrations of 0.5 parts per billion. The concentrations, if accurate, would be more than three times Louisiana’s eight-hour ambient air standard for mercury. But DEQ does not enforce that standard until emissions leave a plant’s grounds where the mercury-laced air would likely be more diffuse, agency officials have said.

Noranda also immediately questioned the validity of its own measurements due to limitations in the equipment the company used, company officials have told DEQ.

Celena Cage, DEQ Enforcement Division administrator, said the earlier modeling requirements, which would have looked at how widely mercury emissions spread off-site, were based on Noranda’s request for a permit to emit up to 250 pounds of mercury per year into the air.

That requested permit limit would have made Noranda Louisiana’s second-largest annual emitter of mercury into the air in 2013.

But Cage said there was no scientific basis for the emissions limit Noranda officials had requested.

“They could not tell us or explain where they got that number from,” Cage said.

When asked, Cage did say if emissions prove to be higher than Noranda officials now believe, DEQ could revisit its compliance order and require more testing.

Wilma Subra, a chemist and technical adviser for the Louisiana Environmental Action Network, said Noranda should do the off-site modeling anyway.

“They need to do the testing. And as result of the testing, they need to do the modeling, whether it’s high or low, so we can really see how far it (the mercury) goes,” she said.

But, in a March 19 post-meeting letter to DEQ, Noranda’s attorneys argued the company did not release enough toxic air pollutants overall, including mercury, to be classified as a “major source.”

Under state law in this instance, Noranda’s lawyers say, DEQ cannot require the company to do the off-site modeling. Noranda, instead, proposed the alternative testing regime.

To support their contention that the mercury emissions are much lower than thought, Noranda officials point to self-reported emissions data from a similar facility in Texas owned by the worldwide Swiss mining concern Glencore.

The Sherwin Alumina complex near Corpus Christi, Texas, has reported emitting an average of 6.36 pounds of mercury compounds into the air per year between 2005 and 2013, U.S. Toxics Release Inventory data show. In 2013, air releases hit 8.72 pounds.

Sherwin Alumina buys its bauxite from Noranda, which has a mine in Jamaica, and has a plant with a design similar to Noranda’s, company officials said.

Bauxite is the source ore for aluminum and naturally has small amounts of mercury, but other chemicals in alumina processing contain trace amounts too.

Noranda officials have theorized that mercury compounds in bauxite can be reduced to mercury’s elemental form under their plant’s pressures and temperatures and, in rare cases, sent up as vapor.

The statement says the mercury vapor should remain trapped in plant equipment until routine maintenance work, when it could be released in small amounts.

John Parker, Noranda spokesman, said company officials can’t yet say if the circumstances that may cause that problem can be halted.

“The first step to figuring that out,” he said, “is the work that we have proposed with DEQ to quantify and identify potential locations of the emissions.”

Follow David J. Mitchell on Twitter, @NewsieDave.