More than 200 million gallons of contaminated waste water from a fertiliser plant leaked into one of Florida's main underground sources of drinking water after a massive sinkhole opened up beneath a storage pond.
Mosaic, the world's largest supplier of phosphate, said the hole opened up beneath a pile of waste material called a "gypsum stack".
The 215-million gallon storage pond sat on top of the waste mineral pile. The company said the sinkhole is about 45 feet in diameter.
Mosaic says it is monitoring groundwater and has found no offsite impacts.
"Groundwater moves very slowly," said David Jellerson, Mosaic's senior director for environmental and phosphate projects. "There's absolutely nobody at risk."
The water had been used to transport the gypsum, which is a byproduct of fertiliser production, the company said.
The sinkhole, discovered by a worker on August 27, is believed to reach down to the Floridan aquifer.
Aquifers are vast, underground systems of porous rocks that hold water and allow water to move through the holes within the rock.
The Floridan aquifer is a major source of drinking water in the state. One of the highest producing aquifers in the world, it underlies all of Florida and extends into southern Alabama, Georgia, and South Carolina.
According to the University of Florida, it is the principal source of groundwater for much of the state, and the cities of Tallahassee, Jacksonville, Gainesville, Orlando, Daytona Beach, Tampa, and St. Petersburg all rely on it.
The aquifer also supplies water to thousands of domestic, industrial and irrigation wells throughout the state.
Mosaic began diverting the pond water into an alternate holding area to reduce the amount of drainage when the problem was first detected. The company said it has been "recovering the water by pumping through onsite production wells".
Mr Jellerson said: "We have an extensive monitoring system. It's already indicating that it's recovering the material, but it will take some time for that process to complete."
Dee Ann Miller, spokeswoman for the state's department of environmental protection, said the company is updating state and federal agencies on the situation.
"Along with reviewing daily reports, DEP is performing frequent site visits to make sure timely and appropriate response continues in order to safeguard public health and the environment," she said.
"While monitoring to date indicates that the process water is being successfully contained, groundwater monitoring will continue to ensure there are no offsite or long-term effects."
The Polk County phosphate plant is still running.
The incident comes less than a year after Mosaic, one of the world's largest fertiliser makers, settled a massive environmental lawsuit with the US environmental protection agency in which the company agreed to nearly $2 billion (€1.8 billion) in improvements and clean-ups at its plants.
Environmental groups said the damage from the sinkhole could be severe, and adds to decades of pollution from the phosphorous fertiliser industry. Florida is a key centre of phosphate mining.
"I wish we could say that watching an environmental tragedy unfolding at a Florida phosphate mining site was a new occurrence, but sadly it's happened repeatedly," said Tania Galloni, a lawyer with the Florida office of Earthjustice.