CONVENT — Hundreds of trucks a day will be traveling this week through a mushy cane field north of Mosaic Fertilizer's huge phosphogypsum waste pile in St. James Parish to dump 150,000 cubic yards of earth in hopes of blunting a slow-moving, potential environmental disaster.

Mosaic officials said the mounds of dirt to shore up a failing retaining wall of the gypsum waste pile are one half of a two-part strategy to halt the incremental creep of a portion of the towering outer wall surrounding the waste and to prevent a potentially catastrophic failure of a giant lake of highly acidic water contained at the top of the pile nearly 200 feet in the air and visible from miles around. 

Mosaic's waste byproduct at its Uncle Sam complex on the Mississippi River has been in existence and ever-growing since 1975. The now nearly 960-acre waste pile outside Convent contains gypsum, an unwanted byproduct from phosphate fertilizer production with trace radioactivity and limited ability to be recycled.

Russell Schweiss, Mosaic's vice president of mine permitting and land permitting, said the process water and rainwater inside the waste pile is a hazardous material with an acidity equivalent to lemon juice or vinegar, but company officials are hopeful their efforts will prevent any release.  

"It's not something we want to see moving off-site," Schweiss said, "and we're going to take the countermeasures necessary to ensure that doesn't happen."

In addition to bringing in dirt, the other step has been happening since Jan. 14. Using pumps and siphons, Mosaic has been draining the 140-acre lake at the top of the gypsum pile into other ponds on the site. The lake, which is being drained of about 10 million gallons, or 15 Olympic-sized swimming pools, per day, is believed to contain 720 million gallons of process water.

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The twin measures are designed to attack the problem from the back and the front of the moving wall, company officials said: Alleviate the weight from the lake water pushing on the inching wall and provide a counterweight at the front of the shifting gypsum wall to brake its leading edge.  

Yet, neither Schweiss and Ron Yasurek, Mosaic's general manager of phosphate operations, nor state regulators could say for certain how long it would take for those measures to work, or if they definitively would.

"I want to see a couple weeks of stability before we say, 'Hey, we won,'" Yasurek said.

Schweiss said company official are also waiting on an analysis of the lake's underwater bottom for a better idea about how much water would need to be removed to halt the wall's movement.

Greg Langley, spokesman for the state Department of Environmental Quality, said the wall is moving about a half inch, on average, per day; but the movement is uneven: Some wall parts move and stop and then other parts shift.

DEQ remains on an emergency footing over the waste pile and has been monitoring it daily. Secretary Chuck Carr Brown visited the site Monday.

Mosaic officials said a 1,500- to 2,000-foot length in the nearly mile-long north wall of the gypsum pile was moving out horizontally toward La. 3214.

The wall is actually warping outward with a bowed or curved shape. Schweiss and Yasurek said company officials believe the movement has been happening since the spring and the farthest shift in the wall has gone nearly 13 feet to the north.

Yet, the shifting was imperceptible against the huge pile's bulk, the officials said, and wasn't caught until sometime in late December and early January. A farmer leasing Mosaic land along the north face of the pile cut his sugar cane and found a 4- to 5-foot-high "bulge" in the field along the north face of the waste pile.  

Company engineers believe the moving wall is causing the earth to rise in that area. This is where the mounds of dirt are expected to go later this week.

Schweiss said engineers have discovered that a layer of the subsoil at 95 feet deep has sheered off from the lower depths and is bringing the gypsum wall with it. The ultimate cause of that movement remains unclear and will be the subject of later analysis, company officials said. 

Meanwhile, the Mosaic officials said they have hired a firm to assess where water would drain from the pile if there were a catastrophic release into surrounding fields, which the company owns. They have also positioned earth-moving equipment to try to divert escaping water to nearby rainfall runoff ponds.

There are neighborhoods to the southwest — opposite the direction of the sliding wall — though company officials said they doubt escaping water could reach that area. 

It's hard to grasp the size of Mosaic's lake at the top of the waste pile, which offers a startling view of the Mississippi, chemical plants and farming communities.

The lake covers about half the surface area of the LSU Lakes, and company officials had to account for wave action from wind when they designed how high berms immediately around the lake should be built to prevent overtopping, DEQ permit records show. 

On a tour Tuesday of Mosaic's operations inside its 3,300-acre site, Schweiss and Yasurek explained that the ponds built into the gypsum pile are really cooling and storage areas for the process water that the company manages to supply its fertilizer production.  

That chemical process consumes water, so Mosaic makes sure it has a supply with the massive lakes built into the waste pile and has designed a loop between the plant and the lakes in the pile.

In addition to rainfall, the company pumps gypsum slurry from plant operations up into the lake to resupply its water. Once up there, the gypsum is dewatered, further building up the gypsum pile while the process water is eventually sent back to the plant.

Follow David J. Mitchell on Twitter, @NewsieDave.