A 2016 aerial photo of the Mosaic reservoir in St. James Parish. Credit: Jonathan Henderson / VanishingEarth.org/Southwings.org

The U.S. Environmental Protection Agency has asked Mosaic Fertilizer to resume removing wastewater from its reservoir atop its Gypsum Stack No. 4 in St. James Parish, despite the company reaching its objective of lowering that water below an elevation of 180 feet. But Mosaic told The Lens that the company has not decided to do so — even as the EPA cast doubt on the company’s model used to determine the stability of the reservoir itself.

Mosaic, the EPA and Louisiana’s Department of Environmental Quality have been in emergency mode since Jan. 10, when the company first alerted the agencies to a bulge in some farmland neighboring the northern slope of the 200-ft-tall gypsum stack, which at the time held roughly 750 million gallons of highly acidic wastewater left over from production processes at the plant. It indicated shifting in the layers of soil beneath the stack and raised the possibility that the northern wall of the gypsum dam could breach.

The company had been removing much of that water from the top of Stack No. 4, with the aim of lowering the water level’s elevation, easing the weight of the water stored and reducing the chance that a wall breach could pour much of the wastewater into nearby waterways, including the Blind River.

In a Mar. 15 letter to Mosaic, Cheryl Seager, EPA Region 6’s director of compliance assurance and enforcement, noted that the company had worked to reduce the water levels to a level of 180 feet. Based on a stability evaluation performed by a third party, the engineering firm Ardaman & Associates, the company said that level would be adequate to stabilize the stack.

Ardaman believes, based on its modeling, that “a breach at or below the 180-foot elevation will not result in a release of process water outside the stack,” the letter stated.

As of Mosaic’s daily situation report on Tuesday, the latest available in LDEQ records, the level of the water atop Stack No. 4 was 179.9 feet, and a little over 402 million gallons in volume.

But Seager wrote that EPA and LDEQ remain concerned about the stability of Stack No. 4.

“Although stack movement has slowed over the past few weeks, it still continues at a rate such that movement expected in one year (1/2″) is observed in just 2 or 3 days,” she said. “Extrapolation from the current data suggests that movement may not stop for another 100 days.”

Seager went on to say that such a rate of underground movement means the stack “remains in a slow failure mode,” even with the water at a 180-foot level.

“Because the stability model does not appear to be correctly predicting stack stability,” Seager wrote, “and we still have not physically identified the location where the failure surface encounters the surface” — an apparent reference to the point underground where the clay layers are shifting — “there remains the possibility that the failure surface starting point is farther south than predicted,” she wrote. “For these reasons, EPA, with the concurrence of LDEQ, is asking that Mosaic continue to decant water from Stack 4 until the elevation is below 179 feet. We will evaluate the data again when the elevation reaches that level or below.”

It doesn’t appear that Mosaic is immediately agreeing to that request.

“We are in constructive discussions with the agencies about possible further water transfers,” wrote Callie Neslund, Mosaic’s director of public and governmental affairs, in a Thursday statement to The Lens. “No decision has been made yet, and those discussions continue.”

Throughout the crisis at Stack No. 4, Mosaic has shifted millions of gallons to other ponds at its plant site in Convent. It has at least five such ponds of assorted sizes now, including the new “East Cell” pond, which has a capacity of about 483 million gallons and was completed in early March.

Mosaic began transferring water from the pond atop Stack No. 4 to the East Cell on March 7, but suspended the transfer on Friday, the same day that EPA dated its letter requesting that the transfer operation be resumed, according to the daily situation report.

The EPA referenced its view that the “stack remains in a slow failure mode, implying a safety factor below unity.” Ardaman’s modelling suggests, with the Stack No. 4 water level at a 180-ft. elevation, that the safety factor is at 1.10.

Such safety factors in the engineering sense represent calculations of the forces that a dam such as the gypsum wall can sustain, according to Dr. John Christian, a professor of civil and environmental engineering at the University of Massachusetts Lowell.

“You take the strength, if you will, of the dam and divide it by what the loads are that you expect to have to carry,” he told The Lens. “And if that number is greater than 1, that indicates that you think the available resistance is larger than the force that it’s going to have to carry.”

A safety factor below unity is less than 1.0. “Obviously, you don’t want to get there,” Christian said. “Getting down to 1.0 clearly indicates that you’re in trouble.”

Concern about a potential dam breach has focused on the potential environmental impact on Blind River, which biologists and other scientists have warned could kill a wide range of animals and vegetation as the river drains into Lake Maurepas in the Lake Pontchartrain Basin. The water itself has a pH factor between 2 and 3, which is a dangerous level of acidity to put into an environment that has not evolved to handle it.

Mosaic’s worst-case scenario assumed that any breach would likely occur toward the top of the northern wall, about 185 feet above grade. Their modeling assumed that up to 159 million gallons might escape over a seven-day period, but that such a volume would be held on site by berms and blocked culverts along two nearby highways, LA 3214 and LA 3125.