More than 15 inches of unrelenting rain from Hurricane Milton has caused a spill at Mosaic's Riverview processing plant.
As a result, an unknown amount of wastewater flowed into Tampa Bay.
Company officials said "the volume may have been greater than the 17,500-gallon reporting standard."
A company statement said a water collection system at its phosphate gypstack became overwhelmed by the rain, causing it to flow out of a manhole.
"At this time, we believe some of that impacted stormwater made its way to an outfall which discharges into Tampa Bay. The issue was addressed yesterday (Sunday) and is not continuing. We expect water quality impacts, if any, to be modest."
State environmental regulators were on the site Monday.
The gypstack is not active and holds slightly radioactive byproducts of phosphate fertilizer production.
Last week, the company halted all its operations in Florida ahead of Milton. It says power has been restored to all of its facilities.
Mosaic previously announced a production halt at its Riverview facility due to Hurricane Helene.
Milton made landfall in Siesta Key as a Category 3 hurricane before crossing the state.
Also, state environmental regulators reported a spill at a wastewater treatment plant reuse pipeline from Mosaic in Fort Meade, in Polk County. About 70,000 gallons of highly treated effluent flowed into a dry area after it was damaged by firefighters cutting a fire line. No rivers or lakes were impacted.
Mosaic has phosphate mining and fertilizer operations in Hillsborough, Polk, Manatee and Hardee counties.
