Two dozen major corporations that have produced hazardous wastes agreed yesterday to pay $7.7 million to begin cleaning up one of the nation's biggest toxic dumps in what federal officials called the largest settlement of its kind.

The dump in the small city of Seymour, Ind. -- much larger, but less publicized than the Valley of the Drums and Love Canal dumps -- is considered a serious health and safety hazard, leaking toxic and flammable chemicals that have contaminated water and are threatening to cause explosions.

The Environmental Protection Agency announced the settlement with the 24 companies yesterday as a "major accomplishment," stressing that private money will finance the removal of some 60,000 drums of liquid and solid waste and 98 bulk storage silos from the 14-acre site.

"This negotiation has no equal in EPA history," said Rita M. Lavelle, director of EPA's hazardous waste program, standing in front of a color aerial photo of the sprawling dump site, showing disorderly piles of rusty storage drums. "It's just unbelievable the number of drums there."

Lavelle portrayed the settlement as an answer to environmentalists who charged recently that the agency is moving too slowly on its pledge to clean up the 400 worst toxic dumps in the country.

EPA officials estimated that 600 companies produced the toxic waste now stored at the dump, which was shut down by Indiana health authorities in 1980 and has been the target of EPA civil litigation ever since. Of the 600 users, 364 firms were identified as responsible for the damage, officials said.

Yesterday's settlement came after EPA urged the companies to band together and voluntarily pay cleanup costs. The 24 firms who joined the settlement -- among them, IBM, General Motors, Ford Motor Co., American Motors Corp., McDonnell Douglas Corp., Atlantic Richfield Co. and E.I. du Pont de Nemours & Co -- together produced 51 percent of the waste at the Seymour site, EPA officials said.

Their $7.7 million will pay for clearing the surface of the dump, and transporting the chemical waste to approved dump sites in Ohio, Illinois, Alabama and another site in Indiana. But the other 340 companies will have to cover the cost of cleaning up contaminated soil and water beneath the site--a project that will not begin for at least a year, officials said.

Attorneys for some of the remaining companies sharply criticized the agreement as unfair, noting that the second stage of the cleanup is expected to cost $15 million -- almost twice the settlement announced today -- while EPA has said their clients accounted for only 49 percent of the wastes at the site.

Environmentalists questioned the arrangement for the same reasons.

"We have no way of knowing whether EPA could have gotten more money out of these companies," complained Khristine Hall of the Environmental Defense Fund. "Here we have the administration announcing a major environmental settlement one week before the election. It's a tacit admission that the environment is a big issue in this election."

But federal officials involved in the settlement defended it as fair, arguing that the law makes all users of problem dumps liable for the full cleanup.

"Balance that question against the public interest of getting one of the largest, nastiest hazardous waste sites in the country cleaned up," said Steve Ramsey of the Justice Department.

The Seymour dump is often cited as an illustration of all that was wrong with toxic waste disposal in the days before federal regulation. Drums of polychlorinated biphenyls (PCBs), toluene and other poisonous industrial solvents were dumped there haphazardly for 11 years. At times, highly toxic chemicals were spilled indiscriminately, contaminating surface and underground water in the area, EPA officials said.

The operator of the dump, Seymour Recycling Corp., is now under receivership and many of the corporations that produced the wastes are bankrupt, according to EPA officials.

The cleanup was mandated by a 1976 federal law. Lavelle said the settlement was urgent because the deteriorating drums at the dump decay even faster in cold weather. Once winter begins, she said, leaking will probably increase, escalating the danger of explosions and fire and threatening the safety of the 14,000 residents of Seymour.