About 980 million litres of radioactive water have leaked into Florida's main underground source of drinking water, officials in the US state say.
The leak occurred after a huge sinkhole opened up under a phosphate fertiliser plant near Tampa, damaging the stack where wastewater was stored.
The roughly 14m-wide sinkhole at Mosaic's New Wales facility in the town of Mulberry was discovered by a company worker on 27 August.
The water contained phosphogypsum, a slightly radioactive by-product from the production of fertiliser.
Phosphate company Mosaic, which was was recovering the water using pumps, said the leak posed no risk to the public.
It said it believed the contaminated water had not reached private supplies.
"Groundwater moves very slowly," senior Mosaic official David Jellerson was quoted as saying by the Associated Press news agency.
However, Florida director of the Center for Biological Diversity Jacki Lopez said it was hard to trust the company which had been keeping the contamination secret for three weeks.
The sinkhole later caused the waste pond to drain, and the contaminated water has now seeped into the aquifer.