The owner of Piney Point, HRK Holdings, bought the site after it was abandoned by the Mulberry Corporation, which operated the phosphate plant for more than 40 years.
More than 300 homes in the area have been ordered to be evacuated, and a motorway near the Piney Point reservoir has been closed off.
"'What we are looking at now is trying to prevent and respond to, if need be, a real catastrophic flood situation", Mr DeSantis said at a press conference after flying over the old Piney Point phosphate mine.
County officials say well water remains unaffected and there is no threat to Lake Manatee, the area's primary source of drinking water.
"This could have been resolved over two decades ago", Hopes said. The water is slightly acidic but not to a concerning or toxic level, officials said.
"The pond is basically salt water".
For all of these reasons, Manatee County made finding a solution to drain the toxic water their number one priority in 2021.
The executive order declaring the state of emergency said the breached structure has 1.8 billion liters of seawater mixed with process water and the embankment materials from the old fertiliser manufacturing plant. "So, again this is not water we want to see leaving the site", he said. As of Saturday morning, an estimated 390 million gallons remained in the reservoir, Florida's Department of Environment and Protection said.
In March, the county decided the best environmental option is to move the water from above ground to a deep water injection well 3000 feet below but that idea is still waiting for state approval and funding.
Phosphogypsum, the radioactive waste seeping from the site, is formed as a by-produce from fertilizer production, according to the U.S. Environmental Protection Agency.
Elevated ponds called gypsum stacks are full of contaminated water produced as a chemical byproduct of mining.
There are at least 70 gypsum stacks in the USA and about 27 in Florida, mostly in the region of west-central Florida.
"We hope the contamination is not as bad as we fear, but are preparing for significant damage to Tampa Bay and the communities that rely on this precious resource", Justin Bloom, founder of the Sarasota-based non-profit organization Suncoast Waterkeeper, said in a statement.