Faced with rainy skies and shrinking room in a key reservoir, Mosaic Fertilizer has halted pumping out a lake filled with acid water, at least for now.

The pumping has been put on hold, company and state officials said, until a new pond can be opened to hold as much as 500 million more gallons of the hazardous fluid. The company is trying to reduce pressure on the lake's north wall to prevent it from breaking through and spilling out into nearby freshwater swamps.

The company has been trying to pump down the 140-acre lake that sits atop a 200-foot-high section of its slipping mountain of waste gypsum in St. James Parish to avoid the potential for a catastrophic failure of the water body.

But rain in recent days and the forecast of more to come have complicated those efforts, officials said, and contributed to the halt in pumping Tuesday as Mosaic awaits the opening of the new storage pond at the "Uncle Sam" site near Convent. 

Mosaic spokeswoman Callie Neslund said just over three inches of rain fell on the plant site next to the Mississippi River between Feb. 18 and Monday. Another 0.61 inches was projected through Sunday, according to the National Weather Service.

So for the past few days, Mosaic hasn't been pumping water, and the 140-acre lake has taken in another 6 million gallons of water through Thursday after the volume had been dropping for weeks. As of Thursday, the lake had 493 million gallons, about a third of what it contained when the emergency started in early January, according to daily reports Mosaic has been supplying to DEQ.

Company officials and federal regulators have said early fears that a major collapse could happen and release the lake's hazardous contents are very unlikely. The concern had been that the highly acidic process water laced with heavy metals and radioactive elements might find its way into freshwater swamps and harm aquatic life.

In addition to pumping out water from the lake to lower pressure on the wall containing it, the company also has been dumping loads of dirt, known as a "stability berm," at the toe of the moving gypsum wall to act as a brake.

"The volume of water we’ve removed, coupled with the stability berm that’s been constructed, has reduced the risk of failure," Neslund said.

Despite those assurances, Mosaic has not been able to estimate the probability of a failure because it does not have enough data and can't say when the north face of the pile of gypsum will stop slipping. U.S. Environmental Protection Agency officials won't completely rule out a major failure, though they say the risk is "negligible."

Mosaic produces gypsum as a waste byproduct from its creation of phosphoric acid used to make fertilizer for crops. The acid is made from phosphate rock mined in Florida and elsewhere.

The company and its predecessors have been stacking the waste gypsum for decades. The trace levels of heavy metals and radioactive elements from the source rock make the byproduct hard to reuse.

Mosaic officials said they submitted paperwork Friday to have the state Department of Environmental Quality inspect the new pond, known as the East Cell, for permits so it can open. The company finished construction earlier in the week.

Greg Langley, a DEQ spokesman, said the agency is hopeful it can issue the permit in a few days after Mosaic submits that request.

"We're hoping to get them ready on the East Cell. That will give them a lot more" space, Langley said.

The reservoir previously being used to hold water pumped from the 140-acre lake recently ran out of available capacity. Its remaining space is being reserved as a cushion for rainfall runoff, Langley said.

The halt in pumping highlights Mosaic's complicated juggling act since problems with the gypsum pile's wall came to light at the turn of the year. The company has tried to get pressure off the moving north wall by draining the 140-acre lake but must also contain those hazardous contents in other ponds.

Further complicating the response, the 960-acre pile isn't only a waste dump for gypsum but is a cog in a complicated system at Mosaic to collect, store and recycle water to and from its production plant that has been hampered by above average rain over the past few years and the recent emergency with the slipping wall.

Mosaic's phosphoric acid production consumes water as part of its chemical process. The slipping 140-acre lake is part of that collecting and recycling system for the plant, serving both as a kind of giant rain-gathering cistern and a cooling pond to recycle water.

The waste pile also creates a lot of rainfall runoff that must be handled. One inch of rain over the entire pile generates 20 million gallons of hazardous process water by falling on or running off into a series of "active" ponds and trenches, DEQ reports say. That water must eventually be used by Mosaic's plant, injected deep underground or evaporate naturally to be disposed of properly, company officials said.

One inch of rain creates 4.6 million gallons of additional water inside the 140-acre lake alone. Once the rain touches the process water in the lake, it is considered equally hazardous, company officials have said.

About 5.5 pounds of gypsum are produced for every pound of phosphoric acid made in Mosaic's plant, company permit documents show.

Follow David J. Mitchell on Twitter, @NewsieDave.