Its depth still a mystery, Mosaic plans to plug sinkhole

A few days of bouncing lasers off mirrors led Mosaic Company to understand its New Wales facility’s sinkhole problem a little bit better.

Now, company officials know that the sinkhole’s airspace, where the acidic water byproduct filtered through, is 220 feet deep. They now know how they can safely tunnel a way to plug up the passage to the Floridan Aquifer.

But they still don’t know how deep the sinkhole goes.

The results of Mosaic’s investigation come just two days after reporting to the state the spill of 50,000 gallons of phosphoric acid at its Plant City facility after a piece of machinery spilled, according to the Tampa Bay Times.

On Aug. 27, a sinkhole appeared under a phosphogypsum stack, a large structure built 190 feet above ground and made of the solid byproduct of fertilizer-making that holds the acidic water byproduct of the process. More than 215 million gallons of that acidic water leaked past limestone layers under ground and into the Floridan Aquifer, a drinking source for thousands of people. The public wasn’t notified until three weeks later.

Contractors and Mosaic employees were able to survey the airspace of the hole by moving a LiDAR device, or light detection and radaring, over and into the hole with a wire like those used to record NFL games. Lasers bounced off mirrors in every direction and, since the speed of light is calculable, the time it takes for the laser to return after bouncing off the cavity’s walls could be used to measure the size.

The technology cut down what could have been months of manually surveying the hole to just two or three days.

About 30 feet of the 220-foot depth is considered underground, but the material that represents the bottom of the airspace isn’t the actual bottom of the sinkhole and Mosaic doesn’t know how deep it goes.

“We know it was connected to the aquifer by the rate (the water) went in,” Mosaic vice president of operations Herschel Morris said Monday.

Now that employees know what part of the sinkhole looks like, they can start to pump grout underground (not the entire airspace) to seal the damage.

Morris said they hope to start the plugging process in December and finish just before the rainy season starts next June.

Mosaic won’t know how much the sealing process will cost until they’re able to figure out what’s underneath the area they surveyed.

“It’ll be in the millions, for sure,” Morris said.

The company is continuing to retrieve the acidic water in the aquifer by using a recovery well and said it’ll be years before they’re finished. The Florida Department of Environmental Protection’s most recent daily New Wales update states there are still no off-site impacts of the sinkhole, but the company has tested 763 residential wells as of noon on Oct. 17 and is still providing well tests to those in the area who want them.

In Plant City, Morris said that two-thirds of the spilled material was recovered to the plant and soil neutralization efforts were made on affected areas.

“I would say the spill over the weekend is a rare event for us,” Morris said. He added that he used to work at the Plant City facility for at least two decades and there was never a spill. “This is very abnormal for us.”

Though sinkholes can’t be predicted, Morris said the company is going to work on a way to be proactive rather than reactive.

“We’re going to put all our minds together and see what all we can do to better predict if there’s a potential for a sinkhole,” he said.

This story was originally published October 17, 2016 at 3:06 PM with the headline "Its depth still a mystery, Mosaic plans to plug sinkhole."