Officials with Mosaic Fertilizer, which owns an unstable pile of waste phosphogypsum and a 140-acre lake of acidic water atop it, are encouraged by recent efforts to halt a potential environmental disaster in St. James Parish.

Unexpected movement along the northern face of the waste pile, also known as a "stack," has raised fears the pile could fail catastrophically and release hundreds of millions of gallons of acidic  water laced with heavy metals and trace radioactivity into swamps and waterways around the Uncle Sam plant near Convent.

The company says it doesn't have enough data to calculate the risk that a bulge in the pile wall will give way, but do believe the risk is less than it once was and is falling.

In recent weeks, Mosaic has been trying to drain a 140-acre lake of acidic water held atop the 200-foot-high pile and reduce pressure on the sliding wall of gypsum. The company is also trucking in dirt to the leading edge of the pile's northern side to act as a kind of brake, company officials have said. 

Monday, Mosaic finished construction of a new 500-million-gallon storage lake, which could be ready for use as early as next week, if state regulators approve. A reservoir the company has been using since late January to hold drained-off water is approaching its limit.

Between Jan. 11 and Monday, the company has cut the 140-acre acid lake's water volume by one-third, to 493 million gallons, according to Mosaic's latest reports to state regulators. 

Though the pile continues to shift — between a foot and 2 feet since Jan. 11 — it is moving more slowly, the company says. Its modeling shows that any failure would release the acidic water gradually and the water would not escape company property.

Hemmed in by elevated highways, now-sealed culverts and an earthen dam, the water would be trapped in a few hundred acres of cane fields the company owns, DEQ documents show.

"We are encouraged by the trend showing that the lateral movement on the north slope is slowing," said Callie Neslund, a Mosaic spokeswoman.

The risk to the nearby Blind River and the surrounding swamps is real should the water ever get out, several environmentalists have said, and Mosaic's modeling without attempts to block the water flow show it would escape far into surrounding swamp and waterways.

Philip Bucolo, an aquatic biologist and visiting assistant professor at Loyola University in New Orleans, said the water's high acidity would be harmful to freshwater plants it contacts, though waterways would dilute its acidity farther away from the plant. 

While he couldn't say for certain if the acidic water — if trapped in the cane fields — could penetrate the soil and escape underground, Bucolo said the company and regulators should be obligated to prove that it wouldn't.

Louisiana Department of Environmental Quality officials have pointed out that aquifers beneath the field are not used for drinking water, but they could not say Monday if they have modeled how a spill of a few hundred million gallons of acidic water would interact underground.

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Ron Yasurek, Mosaic general manager of  phosphate operations, stands Jan. 29, 2019, on a roadway atop a 200-foot-high pile of white waste called phosphogypsum. A byproduct of fertilizer production, the gypsum pile supports and encases giant cooling ponds for highly acidic process water at the company's complex next to the Mississippi River in Convent. The 140-acre lake, at left, is one of those cooling ponds and sits at the top of the waste stack 200 feet in the air. One of the gypsum walls supporting this lake has been shifting unexpectedly and company officials and regulators have begun draining the pond to try to halt the movement.   

Wilma Subra, a chemist and technical adviser to the nonprofit Louisiana Environmental Action Network, said the metals found in the water, including trace amounts of cadmium, iron, lead, aluminum, mercury and silver, and trace radioactive elements uranium and radium-226, don't break down. They can accumulate over time and concentrate in plants and animals if the contaminants escape, she said.

Company officials have acknowledged any spill in those fields would likely require a subsequent cleanup of the land.      

Daily reports from Mosaic to state regulators show portions of the northern face have apparent cracking but DEQ officials say those horizontal cracks are from settlement and aren't the more troublesome vertical, or transverse, cracks that could be a harbinger of a break in the gypsum wall.  

Those reports also appear to show slight movement in a handful of survey points along the pile's east and west walls but company officials believe that, too, is due to settlement as water is removed from the pile and not an indicator of new shifting on a different part of the gypsum pile. 

Like Mosaic, DEQ and the U.S. Environmental Protection Agency have not affirmatively said the gypsum wall won't give way eventually. They have also not said how much movement in the northern wall would be necessary for a failure to occur or how much drainage of the 140-acre lake is necessary to stop the movement. 

"Movement of the stack appears to have slowed and we don’t expect a catastrophic slope failure — however we cannot entirely rule out such an event," said David Gray, an EPA spokesman.

Early in the response, EPA had required that Mosaic provide failure probability estimates and best- and worst-case scenarios.

In a Feb. 1 letter to Mosaic, Cheryl T. Seagel, director of EPA's Compliance Assurance and Enforcement Division for Region 6, called the probability estimates "imperative" and gave the company until Feb. 4 to provide them.

Mosaic subsequently modeled what it thought a failure would look like and showed scenarios where the water would escape into the swamp and area waterways and, with steps to block drainage, how it wouldn't. But the agency never received a full probability estimate, documents EPA released Tuesday show.

More than two weeks of requests from The Advocate for comment and for a copy of the probability estimate drew not a single response from EPA via phone or email. But late Monday, after being told a story was being prepared for publication, Gray said he would try to find the estimate. After the story was posted online, he provided it and related correspondence Tuesday. 

Mosaic said it had "insufficient data" to compute the daily probability of a failure of the northern face of the gypsum pile or its rate of movement but added that its actions to drain the lake and bring in dirt to brake the wall's movement are raising the probability that the rate of movement will slow.

Russell Schweiss, Mosaic's vice president of mine permitting and land management, said in an interview that, as the water level in the lake lowers, any vertical crack that could break open the pile would have to penetrate deeper into an ever thicker wall of gypsum to release the acidic water. The huge lake, which is inside a portion of the pile, has a curved bottom shaped somewhat like a bowl.   

"The more you draw that (water) back, the less likely a crack is going to reach the water and, if the crack does reach the water, there's less (water) that will likely drain," Schweiss said.

In Mosaic's response to EPA, the company also suggested that historical data for other mining waste piles show that the annual probability of a sliding failure of the Uncle Sam pile can be expected to drop from less than 0.5 percent on Jan. 11 to 0.0001 percent at an undetermined future date when the safety factor is high enough.

It's not clear from the documents EPA provided Tuesday when that level of safety would be reached at Uncle Sam, but, in a followup letter Feb. 15, EPA told Mosaic that its response satisfied the agency's request.

Follow David J. Mitchell on Twitter, @NewsieDave.