Federal investigators have opened a probe of a 93-acre industrial landfill site in a residential area near Little Rock, Ark., where they say deadly pesticide waste has been dumped for the last 30 years.

Officials of the Environmental Protection Agency said yesterday that pesticide by products dumped into the landfill at Jacksonville, Ark., contain some of the highest levels of dioxin they have ever measured.

Dioxin is a waste product generated during the manufacturing of 2,4,5-T, a widely used and controversial pesticide also known as "Agent Orange" when it was used by the military as a defoliant during the Vietnam war. Some chemical experts have claimed that dioxin is one of the most deadly chemical compounds ever produced.

EPA officials, stung by recent criticism that they failed to move quickly enough on other hazardous waste dumps discovered in Michigan and New York, predicted that the agency would seek court action within days to force an emergency cleanup of the Jacksonville site.

"I anticipate that the agency will move the fastest on this one that it has ever moved," an EPA official connected with the investigation said yesterday.

According to Arkansas health officials, no reports have been received of any unusual health problems among people near the dump site. However, state officials banned fishing and swimming this week in a stream that runs alongside the site.

The landfill is part of a pesticide plant complex owned by the Vertac Corp., a Memphis firm that purchased it last November when the previous owner, the Transvaal Co., went bankrupt.

Robert Kirk, president of Vertac, said in a telephone interview that pesticides are still being manufactured at the site, although production of 2,4,5-T was discontinued earlier this year.

Robert Kirk Jr., who is general counsel for the firm, said yesterday that since 1972 Transvaal has been a subsidiary of Vertac. The plant site was originally operated by the Rezor Hill Co., a local firm, and then from 1964 to 1970 by the Hercules Chemical Co. of Wilmington, Del.

A Hercules spokesman yesterday said his firm dumped waste from its Agent Orange production at the Jacksonville site. He said he did not know the amount of waste that was dumped because company records have been destroyed. Hercules made 14.3 million pounds of Agent Orange for the military at the plant.

According to the Vertac counsel no pesticide waste has been dumped into the landfill since 1974. Since then, other drums stored above ground leaking at the site.

One official of the agency, who asked not to be identified because of the pending court action, said the investigators described "gunk" oozing through the surface of the dump site.

He said the "gunk" was probably dioxin containing waste buried in drums years ago.

Laboratory tests of 2,4,5-T waste from the pesticide plant done earlier this year for the EPA by Wright State University in Ohio, the University of Nebraska and EPA's own lab at Beltsville, Md., all showed dioxin levels of 40 parts per million. EPA officials said they could not recall ever seeing tests of waste samples with such high dioxin levels.

An EPA official in the agency's hazardous waste section said yesterday that dioxin si so potent it is difficult to get commercial testing facilities to handle it. "Dioxin in the hands of terrorists would be as dangerous as plutonium" he said.

Federal health officials and others claim that dioxin, even at levels as low as parts per trillion, causes severe reproductive defects, including miscarriage, and tumors in laboratory animals.

The federal official said EPA may require that some type of insulating material be injected into the landfill down to bedrock around dumpsites Kirk said, all the waste has been stored above ground in drums. According to Kirk, the company has not found any leakage off the site where pesticide waste was dumped.

EPA officials said, however, that an inspection team of federal and state officials wednesday and Thursday discovered drums containing the pesticide waste popping up out of the landfill where they had been buried and containing the hazardous material to contain any runoff or seepage. He said the agency may also demand that wells be dug to siphon off subsurface water containing dioxin or other contaminants.

"We are not talking about small dollars if our suspicions are correct," he said.

EPA sources said the agency actually tested samples from the site early last year at its Beltsville laboratory, but failed at that time to pick up any trace of dioxin. But a retesting of the same samples at Wright State and the University of Nebraska several weeks ago showed the 40 parts-per-million dioxin level.

EPA officials said they did not know why the traces were not identified the first time the samples were tested at Beltsville. CAPTION: Picture, The Rocky Branch Creek, where fishing and swimming have been banned because of dioxin waste in a nearby landfill. At right is a private vegetable garden. AP