About 980 million litres of contaminated water have leaked into Florida’s main underground source of drinking water, state officials say.
The leak occurred after a huge sinkhole opened up under a phosphate fertiliser plant near Tampa, damaging the stack where waste water was stored.
The water contained phosphogypsum, a slightly radioactive by-product from the production of fertiliser.
The phosphate company Mosaic said the leak posed no risk to the public.
It added the contaminated water had not reached private supplies and the firm was recovering it using pumps.
“Groundwater moves very slowly,” senior Mosaic official David Jellerson was quoted as saying by the Associated Press news agency.
However, Jacki Lopez, Florida director of the Center for Biological Diversity, told Reuters news agency: “It’s hard to trust them when they say ‘Don’t worry,’ when they’ve been keeping it secret for three weeks.”