By Rod Nickel

(Reuters) - A sinkhole spanning 45 feet (13.7 meters) in diameter opened at a Mosaic Co phosphate fertilizer facility in Florida, leaking 215 million gallons of "slightly radioactive water," a company spokesman said on Friday.

Mosaic said the monitoring system at its New Wales facility at Mulberry, Florida, showed a decline in water levels on Aug. 27 from the retention pond of a phosphogypsum stack, a hill of hazardous waste. Phosphogypsum is a radioactive byproduct resulting from the production of phosphate.

The Minnesota-based company immediately reported the incident to state and federal environmental authorities, Mosaic spokesman Ben Pratt said on Friday. But it did not otherwise report it publicly until posting information on its website on Thursday, he said.

The leaked water is enough to fill more than 300 Olympic swimming pools.

The nearly three-week gap between detecting the sinkhole and reporting it to the public is alarming, said Jacki Lopez, Florida director of the Center for Biological Diversity.

"It’s hard to trust them when they say 'Don’t worry,' when they’ve been keeping it secret for three weeks," she said.

The sinkhole, located about 30 miles from Tampa, damaged the liner system at the base of the stack, causing the pond on top to drain. Seepage continued and the sinkhole reached Florida's aquifer, Mosaic said on its website.

Specific environmental and health concerns are the release of uranium, radium and radon gas, Lopez said. Once contaminants reach the aquifer, which extends from central Florida to Georgia, they can potentially travel hundreds of miles, she said.

"We don't know what the long-term effects will be," said Lopez. "If I were living in this area, and I had well water, I would be worried about my health."

Mosaic said it had increased monitoring and sampling of groundwater and found no offsite impact. It also said it pumped water out of the affected pond to reduce the volume of leakage.

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The company said it is attempting to recover the water through production wells on site.

The incident has not interrupted operations at the facility.

Mosaic shares fell 1.7 percent, declining with other major fertilizer producers.

In 2015, Mosaic reached an $800 million settlement with U.S. regulators over its waste management practices at plants in Florida and Louisiana.