TAMPA — In an unprecedented move, federal regulators announced Thursday they will allow Florida to dispose 500-million gallons of acidic wastewater into the Gulf of Mexico.

The waste from the abandoned Piney Point phosphate mine in Manatee County could be loaded onto barges and sprayed into the gulf starting in two weeks, said Allan Bedwell, deputy secretary of the state Department of Environmental Protection.

The disposal will be allowed through summer and into fall, ending in November. The amount of waste to be disposed would fill more than 700 Olympic-size swimming pools.

Although some environmentalists criticized the plan, state and federal officials said doing nothing risks an environmental disaster in Tampa Bay.

The earthen mound now containing the waste water is unstable. Too much rain could breach the mound and send the waste cascading into Bishop Harbor, an aquatic preserve at the mouth of the bay just south of the Hillsborough County line.

During a 20-minute closed-door meeting in Tampa Wednesday, U.S. Environmental Protection Agency Administrator Christie Todd Whitman told state DEP Secretary David Struhs that she had approved the plan and would issue an emergency permit in a few days.

Whitman said her agency spent four weeks wrestling with the proposal, finally agreeing it is the only way.

“We wanted to be sure every possible alternative had been explored,” Whitman said.

The DEP already has discharged several million gallons of the treated waste into Bishop Harbor and even that comparatively small amount has produced algae blooms that could lead to fish kills and other problems.

Bedwell said state and federal officials have wracked their brains trying to come up with other alternatives, but nothing else gets rid of the wastewater fast enough. Speed is of the essence after a rainy winter and spring, especially with hurricane season starting June 1.

Disposing of so much waste in the gulf “is not option A, it’s option Z,” Bedwell said. And it won’t be cheap. DEP officials figure it will cost between $15-million and $37.5-million.

The barges carry coal from New Orleans to Tampa Electric Co.’s two power plants in Hillsborough County and return empty.

The waste will be sprayed across a 20,000 square mile area at least 40 miles from the Florida coast by one of two methods. One plan calls for the empty barges to carry up to 10-million gallons of the waste in their ballast tanks and spray it into the gulf as the ships return to Louisiana. The other calls for using just one barge, which would repeatedly circle around the designated area.

Initially, trucks will carry the waste from Piney Point up to TECO’s plant at Apollo Beach, Bedwell said. But DEP also will build a 2-mile-long pipeline across U.S. 41 to Port Manatee to send the waste to the port to be loaded onto the barges.

The wastewater will be treated before it is shipped to the barges. That will adjust its acid content, but will still leave it with more ammonia than EPA guidelines allow, Bedwell said.

However, both Bedwell and EPA regional administrator Jimmy Palmer said that by spraying the waste across a large expanse of the gulf, the ammonia will be diluted so it will not harm marine life.

Nevertheless, environmental advocates still questioned the wisdom of using the gulf to solve a pollution disposal problem.

“Everyone turns to the ocean to solve the problems of their poor planning,” said David White of the Ocean Conservancy. “And as usual the taxpayers are stuck paying to clean up the pollution.”

Quenton Dokken, executive director of the Gulf of Mexico Foundation in Corpus Christi, Texas, questioned assertions that diluting the waste would render it benign.

“You know, this is the whole attitude toward the Gulf of Mexico — dilution is the solution,” said Dokken, a marine research scientist at Texas A&M University. “You dump a little of this and a little of that, and when you throw them together and measure the total impact, it is having an effect.”

Paul Johnson, spokesman for an ocean protection group called Reef Relief, said he was surprised such a major environmental policy happened so quickly.

“It took the U.S. Army Corps of Engineers and the Port of Tampa almost a decade to get a permit to dump spoil from channel maintenance dredging into the gulf,” Johnson said. “My main concern about dribbling this around the Gulf of Mexico is that you could get a whole class of egg species in the water column, and I don’t know what that phosphate waste will do to them.”

Whitman said she would not allow any waste to be sprayed in areas of “critical marine habitat.” It will be disposed of in “an area that’s been determined to have no significant endangered fish life.”

However, White of the Ocean Conservancy questioned whether there is such an area in the gulf. And he noted that the gulf’s currents could carry the waste where it is not supposed to go: back onto a Florida beach, or into an estuary like Tampa Bay.

Initially the EPA will send a research vessel behind the barges to monitor what the waste does to the gulf’s water quality “to ensure that dilution is achieved,” EPA spokesman Carl Terry said. After that it will be up to the DEP to monitor the disposal.

The EPA announcement caught some by surprise. “This is the first I’ve heard of it,” said Rick Leard of the Gulf of Mexico Fisheries Management Council. “Our agency usually, in concert with the National Marine Fisheries Service, comments on any types of projects that would allow ocean dumping or filling. There are laws against ocean dumping.”

In fact, Palmer of the EPA acknowledged that dumping any sort of waste in the ocean violates international treaties. The only exceptions are for cases involving a risk to human life and limb, he said — not for avoiding an environmental catastrophe.

“This is probably an unprecedented situation,” Palmer said.

Before EPA can issue the emergency permit to Florida, he said, the U.S. State Department must notify more than 80 countries that signed those treaties. But they cannot stop it, Whitman noted.

“It’s not a question of getting permission,” Whitman said. “It’s just a question of notification.”

Although it may have no effect on the EPA’s decision, the agency will probably invite public comment on the gulf disposal plan.

— Times researcher Caryn Baird contributed to this report