MULBERRY

A massive sinkhole that opened underneath a gypsum stack at a Mosaic phosphate fertilizer plant in Mulberry may have dumped at least 215 million gallons of contaminated water into the Floridan Aquifer over the past three weeks, company officials say.

And it could be months before the hole is plugged, the officials acknowledge.

The 45-foot-wide sinkhole opened at the New Wales plant, where phosphate rock mined elsewhere is converted into fertilizer.

It drained millions of gallons of acidic water laced with sulfate and sodium from a pool atop a 120-foot gypsum stack. An unknown amount of gypsum, a fertilizer byproduct with low levels of radiation, also fell into the sinkhole, which is believed be at least 300 feet deep.

The pond is now drained, but aerial video taken Friday shows polluted water is still seeping from the gypsum stack and plunging like a waterfall into the sinkhole. More contaminated water will leak with every new rainfall until the sinkhole is filled. The acidic level of the water is roughly equivalent to vinegar or lemon juice.

Mosaic workers became aware of the leak when water levels in the pond dropped 2 feet between readings on Aug. 27. They began diverting water from the pond, which can hold up to about 250 million gallons.

Wells were used to monitor groundwater around the sinkhole. No off-site contamination has been detected, Mosaic officials said.

The company notified the Florida Department of Environmental Protection, the U.S. Environmental Protection Agency and Polk County about the leak.

But neither Mosaic nor DEP officials notified the public about the spill.

David Jellerson, senior director of environment and phosphate projects, said Mosaic decided there was no need to alert neighbors — many of whom use well water — because testing has shown that the contamination has not spread from the 1,600-acre site.

The DEP was notified Aug. 28, spokeswoman Dee Ann Miller said. Staffers were on site the next day and are making frequent visits to monitor cleanup efforts.

"The department's focus at this time is on the oversight of Mosaic's first-response efforts in order to safeguard public health and the environment," Miller said in an email.

Mosaic has agreed to pay for the testing of well water at surrounding properties and has received 14 requests so far. Testing will be conducted by a contractor hired by Mosaic and is expected to start next week.

To tackle the contamination in the aquifer, Mosaic is using a recovery well that pumps 3,500 gallons of water per minute from the aquifer back to the surface, where it will be reused for phosphate processing.

"We're confident the recovery well is effectively collecting the contaminants," Jellerson said.

Water in the aquifer is laden with sand and sediment and flows slowly. Nonetheless, extracting all of the contaminated water and sediment before it spreads will be extremely difficult, said Robert Brinkmann, a professor of geology and environmental sustainability at Hof­stra University and author of Florida Sinkholes: Science and Policy.

"That's the bad thing about this: The aquifer is like Swiss cheese and it's interconnected," Brinkmann said. "Contamination can be very rapid. They must be working very hard to figure out where this is going."

Longer term, Mosaic will likely fill the sinkhole with a concrete-like grout and repair the heavy-duty plastic liner beneath the gypsum stack.

First, though, it will have to drop sonarlike equipment into the hole to map its dimensions.

It isn't yet known whether the DEP will insist that Mosaic remove phosphogypsum from the sinkhole before filling it or whether Mosaic will face fines for the spill.

"Once the initial response phase is complete, we will have a more complete understanding of all circumstances surrounding the event," Miller said. "At that time, we will turn our attention to determining the best next steps in DEP's enforcement process."

The incident hasn't affected the plant's daily operations. It produces roughly 4 million tons a year of fertilizer and animal feed.

But the spill has angered environmentalists, who question Mosaic's ability to prevent the leak from contaminating the aquifer beyond Mosaic's property line.

"We don't know how deep the monitoring wells are," said Andy Mele, conservation chairman for the Sierra Club's Manatee-Sarasota group. "I trust nothing that Mosaic says."

Mosaic has run afoul of environmental regulators before.

Last October, it agreed to pay nearly $2 billion to settle a federal lawsuit over hazardous waste and to clean up operations at six Florida sites and two in Louisiana.

The EPA said it had discovered Mosaic employees were mixing highly corrosive substances from its fertilizer operations with the solid waste and wastewater from mineral processing, in violation of federal and state hazardous waste laws.

The sinkhole is the second to open at the New Wales plant, following a 120-foot-wide one in 1994 at a different section of the facility.

At that time, the plant was owned by IMC, which became Mosaic after merging with Cargill. It cost IMC about $7 million to remediate the site, according to a Sarasota Herald-Tribune report.

The same approach of pumping water out of the aquifer was used then and was successful, Jellerson said.

Still, a second potential contamination of the aquifer shows that phosphate production puts water supplies at risk, said Tania Galloni, managing attorney for the Florida Office of Earthjustice, an environmental nonprofit group.

"These phosphate companies are playing roulette with our public waters," Galloni said. "They cause devastation so severe that the scars can be seen clearly from space."

Contact Christopher O'Donnell at codonnell@tampabay.com or (813) 226-3446. Follow @codonnell_Times.