The day of reckoning is at hand for the nation's operating toxic garbage dumps. In three days, on what is already becoming known as "Black Friday," one-third or more of the nation's 1,600 hazardous-waste landfills, pits, ponds, lagoons and deep-injection wells are expected to go out of business.

Federal law sets Friday as the deadline by which hazardous-waste facilities must have monitoring wells in place to check for leaks and enough liability insurance to cover up to $6 million a year in cleanup costs.

According to Environmental Protection Agency projections, about 450 of the facilities will not be in compliance with the leak-detection rules, and 50 more will not be able to secure insurance.

Those facilities will have to stop accepting waste immediately, and within two weeks file a plan for sealing the site.

Industry officials say they believe the EPA has underestimated the impact of the deadline, and some predict that the nation could lose as many as half of its toxic disposal sites in a single day -- with dire consequences for manufacturers. The EPA agrees that the move is likely to cause shortages of land-disposal capacity in some areas, particularly the West.

But there isn't much sympathy floating around for the soon-to-be-defunct disposal sites. The monitoring rules have been in place, after all, since November 1981.

"The chickens are coming home to roost," said Richard Fortuna, executive director of the Hazardous Waste Treatment Council, which represents companies that recycle or treat waste rather than dispose of it in the ground. "What you're hearing is not a lot of cries but a lot of clucking."

Environmentalists, meanwhile, have the champagne on ice. "It's the first piece of news about hazardous waste in a long time that people can feel good about," said Linda Greer of the Environmental Defense Fund.

It will be up to the EPA to make sure that facilities unable to comply with the rules actually close the sites. Agency officials said they intend to start by comparing lists of sites known to lack monitoring wells with the list of companies that "certify" themselves in compliance with the law.

The agency is also counting on "competitors' tips" to help it ferret out dumps that continue to operate illegally -- a criminal violation that can result in heavy fines and up to two years in prison.

AND A SIX-PACK TO GO . . . It was one of those compelling statistics that beggar explanation, and for more than two years it popped up in virtually every story about the EPA's progress -- or lack of same -- under the Superfund toxic-cleanup law. In its years of operation, Superfund had managed to clean up only six -- count 'em, six -- of the nation's most dangerous dumps.

The agency argued mightily that the figure didn't tell the whole story, but not once did it suggest that it didn't deserve credit for at least those six. Until recently, that is.

Last month, William Hedeman, former head of the Superfund program, informed a House subcommittee that the six sites were never really on the Superfund cleanup list. They were merely on an "interim" list before the real, final list was completed.

EPA's new explanation is prompted in large part by the fact that one of the "cleaned up" sites, the Butler Tunnel near Pittston, Pa., sprang a leak several weeks ago, spilling more than 100,000 gallons of contaminated water into the Susquehanna River.

The National Campaign Against Toxic Hazards, a national coalition of citizen groups that investigated EPA's cleanup record, warned more than a year ago that the Butler Tunnel would erupt again. The group contends that at least two of the other supposedly clean sites also pose continuing threats to public health and the environment.

Now facing a new round of criticism for taking dangerous sites off the list, the EPA apparently finds it more convenient to simply relinquish the six cleanups.

Hence, a new statistic: In its years of operation, Superfund has managed to clean up zero -- count 'em, zero -- of the nation's most dangerous toxic dumps.