Mr DeSantis tweeted: "Due to a possible breach of mixed saltwater from the south reservoir at the Piney Point facility, I have declared a state of emergency for Manatee County to ensure resources are allocated for necessary response and recovery".
An estimated 390 million gallons of water remained in the pond as of Saturday morning, according to the Florida Department of Environmental Protection.
The Manatee County Public Safety Department said that 316 households were within the full evacuation area as of Saturday evening.
Officials are working to prevent a collapse but according to Scott Hopes, Manatee's acting county administrator, attempts to plug the leak with rocks and other earth materials have failed.
Florida officials are urging residents in the surrounding areas of a wastewater storage pond to heed evacuation orders ahead of a potential overflow of polluted water.
According to Florida authorities, water at the Piney Point site is not radioactive, although it has high levels of phosphorus and nitrogen.
Crews have been discharging water since the pond began leaking in March. The Florida governor declared a state of emergency on Saturday for Manatee County, where the plant is located.
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"We won't be repairing the liner, we will be depleting the holding ponds of their water and then we will be moving forward to a permanent solution into the future once we mitigate the current risk", Hopes told the press conference.
"The radiologicals are still below surface water discharge standards".
"So if you are in an evacuation area and you have not heeded that, you need to think twice and follow the orders", said Hopes at a news conference with DeSantis.
Emergency crews in central Florida were working Sunday to prevent an environmental catastrophe at a leaking reservoir that risked sending millions of gallons of contaminated wastewater toward nearby homes and into the Tampa Bay. "We are not out of the critical area yet", he nonetheless warned. "So, again this is not water we want to see leaving the site", he said.
The leaking pond sits in a stack of phosphogypsum, a waste product from manufacturing fertiliser, and contains "small amounts of naturally occurring radium and uranium". For example, too much nitrogen in the water can lead to accelerated growth of algae and therefore the death of fish. According to state authorities, the water in the breached pond is non-radioactive.
Manasota-88, an environmentalist group, lambasted officials saying that the gyp stacks at Piney Point have been mismanaged for decades.