The authorities in Florida said on Sunday that they were making progress in their efforts to drain a leaking reservoir holding more than 300 million gallons of wastewater but warned that were it to breach, it could result in a 20-foot wall of water.
“What we’re looking at now is trying to prevent and respond to, if need be, a real catastrophic flood situation,” Gov. Ron DeSantis said at a news conference on Sunday morning.
The governor issued an executive order on Saturday declaring a state of emergency for three counties that could be affected by the leaking 79-acre reservoir.
Controlled releases from the reservoir to reduce the chances of a full-fledged breach started on Friday, officials said, resulting in an average of 35 million gallons a day being siphoned.
Still, Mr. DeSantis, a Republican, warned that residents needed to be prepared for “further degradation” of the reservoir, which is part of a system of ponds connected to a former phosphate mine in Piney Point, Fla., south of Tampa.
Scott Hopes, the acting administrator for Manatee County, said the reservoir was down to about 340 million gallons but warned that models suggest that if the reservoir were to give way at that volume, it could result in a “20-foot wall of water” cascading across residential and commercial areas.
“If you are in an evacuation area, and you have not heeded that, you need to think twice and follow the orders,” he said.
Mr. Hopes reiterated those warnings at another news conference on Sunday afternoon. By Tuesday, the situation should be in a “much better position,” he said, though he cautioned that “we are not out of the critical area yet.”
On March 26, when the initial leak was reported, the reservoir held about 480 million gallons of water. Before officials started to pump water to reduce the threat of a breach, the reservoir was leaking at a rate of two million to three million gallons per day but conditions deteriorated in recent days, officials said.
The “steady drawdown” at the reservoir has continued, Vanessa Baugh, chairwoman of the Manatee County Commission, said on Sunday afternoon, adding that she was “happy to report that no news is good news.”
More than 300 homes were under a mandatory evacuation order and arrangements had been made to put displaced residents in hotels and shelters. The Florida National Guard was bringing more pumps to augment the 20 pumps already deployed, officials said on Sunday.
The Manatee County Jail, which Mr. Hopes said is a two-story building, is in the evacuation zone. Inmates and staff members had been moved to the second floor — about 10 feet above expected flood levels — and sandbags had been placed at the ground level of the jail, Mr. Hopes said.
Ms. Baugh said she had spoken with the sheriff, who told her that “everything is under control” at the jail and that there was “no need to be concerned — they have it all handled.”
The water being discharged from the reservoir is seawater — primarily saltwater from a dredging project — “mixed with legacy process water and storm water runoff/rainfall,” according to a website tracking developments about the reservoir.
“The water meets water quality standards for marine waters with the exception of pH, total phosphorus, total nitrogen and total ammonia nitrogen,” the Florida Department of Environmental Protection said. “It is slightly acidic, but not at a level that is expected to be a concern.”
Officials said the primary concern about the discharged water was the concentration of nitrogen and phosphorus but emphasized that the water was not radioactive.
A sudden, uncontrolled breach could upend stacks of phosphogypsum, a waste product of phosphate mining, that hold the ponds. Phosphogypsum contains “appreciable quantities” of radioactive materials, like uranium and radium, according to the federal Environmental Protection Agency.
“If there were to be a full breach, a section of the gypsum stack would be part of that breach,” Mr. Hopes said.
Florida is the world’s largest phosphate-producing area, according to the E.P.A., and accounts for approximately 80 percent of the national phosphate mining activity. The United States each year mines and consumes about 23 million tons of phosphate, which is mined mostly for use in fertilizers.
The phosphogypsum created in the mining process is “transferred as a slurry” to the stacks, where the slurry solidifies and water pools on the surface of the stack, according to the E.P.A. Gypsum is dredged to build up the sides of the stack, which is then used to hold the wastewater from the mining process.
The authorities said there were no public water supply wells in the evacuation zone. Ms. Baugh said that water customers in Manatee County “can rest assured that their drinking water is completely safe to drink,” and that people who rely on well water “also at this point have no need for concern.”
“If a breach occurs,” Ms. Baugh added, “we believe that the surface layers of dirt and earth will safely filter any harmful nutrients near the surface.”
The Florida Department of Health would issue water quality advisories if necessary, she said.
Mr. Hopes said it was unlikely that officials would seek to repair a liner in the leaking reservoir. He suggested instead that efforts would be made to deplete the holding ponds and then move to a permanent solution, like filling and capping them.
Bryan Pietsch contributed reporting.