A sinkhole has opened up at a Florida fertilizer plant, causing about 980 million litres of radioactive water to leak into one of the state’s main underground sources of drinking water, state officials say.

The huge sinkhole opened up under a phosphate fertilizer plant near Tampa, damaging the stack where waste water was stored.

The water contained phosphogypsum, a slightly radioactive by-product from the production of fertilizer.

The phosphate company Mosaic said the leak posed no risk to the public.

The company added the contaminated water had not reached private supplies and it was recovering it using pumps.

Mosaic official David Jellerson was quoted as saying by the AP: “Groundwater moves very slowly.”

However, Jacki Lopez, Florida director of the Center for Biological Diversity, told Reuters: “It’s hard to trust them when they say <<Don’t worry,>> when they’ve been keeping it secret for three weeks.”

Dee Ann Miller, a spokeswoman for Florida’s Department of Environmental Protection, said Mosaic was updating state and federal agencies on the situation.

She said her agency was doing frequent site visits to safeguard public health.

“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,” Dee Ann said in an email to Associated Press.

The sinkhole – about 45ft in diameter – at Mosaic’s New Wales facility in the town of Mulberry was discovered by a company worker on August 27.

The sinkhole later caused the waste pond to drain, and the contaminated water has now seeped into the aquifer.

Aquifers are massive underground systems of porous rocks that hold water.