A major chemical weapons research plant at the Army's Aberdeen Proving Ground northeast of Baltimore has become the focus of investigation by Maryland environmental authorities and, for the first time, forced federal officials to acknowledge decades-old hazardous waste disposal problems at the sprawling post.

State environmental officials were allowed last fall to visit Aberdeen, one of eight Army posts that stockpile chemical weapons, after the Army reported a 200-gallon spill of concentrated sulfuric acid from a 45-year-old pilot plant. The spill killed dozens of fish in a tributary of the Chesapeake Bay.

In past years, Army officials had restricted state and federal environmental officials' access to Aberdeen's 83,000 acres, citing national security reasons.

Once at Aberdeen, state inspectors discovered problems beyond the plant: corroded sewer lines leaking waste, 40 illegally dumped barrels of ash from burned chemicals and six 50-to-75-pound bags of asbestos improperly dumped in one rubble heap, and chemical sludge from sewage treatment plants illegally dumped into another, according to Army and state officials. Another 134 barrels of ash, which the Army called "nonhazardous," also were found buried in a landfill, in violation of state laws.

Extensive chemical contamination was detected in soil and ground water at three other sites that were tested.

According to Army reports, Aberdeen also is home to an unknown quantity of undetonated explosives and munitions buried or stored there, and the generator of an explosive, called white phosphorous, which is buried in the Chesapeake Bay. Such burials may pose potential long-term health threats to the more than 40,000 people who live on or near the post and the more than 14,000 who work there, testing chemical weapons and equipment, officials say.

Maryland's investigation of Aberdeen has "detailed quite a history of safety concerns, implied environmental problems, leaks and hazardous discharges into public sewers," said John Koontz, the state's chief of hazardous waste enforcement. "We are going to continue to investigate what was, what is and what went where. And it was directed by whom? It's a big city and who's running it?"

In response to such concerns, state inspectors have begun daily visits to Aberdeen and forced the Army to begin cleanup procedures. U.S. Environmental Protection Agency officials, who earlier told The Washington Post that environmental practices at Aberdeen were sound, spent three days last week at the post.

The Maryland delegation to Congress is holding a hearing today on Aberdeen.

And the Maryland state attorney general's office is looking into waste disposal practices for possible criminal violations, following reports in The Baltimore Sun.

For its part, the Army conducted an audit of the facility in February. It found no criminal wrongdoing, but listed a series of recommendations, according to Army officials who declined to detail the report. On March 28, the Army closed the pilot plant at Aberdeen that was used for proofing laboratory experiments on a larger, industrial scale. Shutting the plant left 170 buildings still in operation.

Last week, Department of Army officials said that more than 50 individual dump sites at Aberdeen are being studied for cleanup possibilities.

Most of the dumping, they said, took place at a time when environmental protection was not a national concern.

"In some senses, Aberdeen is a mess -- there's no other way to characterize it," said Maj. Philip Souci, a Department of Army spokesman at the Pentagon. "But it's a mess that didn't get started last week and it is getting cleaned up. "It's sort of like finding a dead body in your back yard and someone says, 'We'll get it next June.' Wrong answer."

One day last week, Army technicians were working until dark to clean the rust-stained tanks and pipes on the grounds of the four-story pilot plant, about a mile from Army housing and the town of Edgewood.

Deep gouges caused by the sulfuric acid spill peppered the concrete walkways of the plant grounds. Piles of sand and gravel had been spread in several large areas to absorb and cover chemical spills. Two large holes gaped open in a rusted pipe, exposing murky liquid.

Army officials declined to let a reporter enter the plant itself, explaining that full protective gear and safety courses in dealing with hazardous materials would be required first.

A noxious odor stung the eyes and the nose. An Army colonel linked the discomfort to a chemical used in making a nerve agent that spilled from steel drums in August 1983, when an 8-by-8-foot section of the plant roof collapsed, crushing at least four drums. Army records show that those drums were not removed for more than two years. The smashed barrels were removed last year after the plant's ventilation system failed and workers complained about the stinging and the odor.

Army safety inspection reports depict a troubled past at Aberdeen, 75 miles from Washington in Harford County:

*The O-Field is an old landfill where uncounted chemical agents and explosives, including mustard gas and nerve agents from the United States, Great Britain, Japan and Germany were burned or buried from the early 1930s through the late 1960s. In recent years, explosives, laboratory wastes and other wastes also have been buried there.

Unexplained explosions have occurred in several of these pits, according to a 1982 Army report. Ground water at the four-acre field, located three miles from residential neighborhoods, is highly contaminated, Army officials said.

Contamination also has been found in Watson Creek, a tributary of the Gunpowder River that flows into the Chesapeake.

"We're talking about the potential release of agents or explosions -- it's not your average hazardous waste dump," said Col. Francis Durel, deputy commander of the research center.

*An unknown quantity of munitions containing white phosphorous -- a chemical that bursts into flames upon exposure to the air -- was buried in the tidal flats of the Chesapeake Bay between 1922 and 1925, according to officials.

The site is within 2.5 miles of post housing and within 4.5 miles of civilian housing areas.

Recent environmental studies have found that elemental phosporous is extremely toxic to aquatic life.

Dredging operations and storms could loosen the white phosphorous, posing the threat of explosion if it dries on contact with the air.

*Dozens of toxic chemicals were found by the Army in 1982 and 1983 in the ground water and soil at the Old Bombing Field, about three miles from housing, and at the J-Field, about five miles from housing. Both fields were used for the burning of chemical warfare agents, including mustard and napalm, and equipment contaminated with them. J-Field also was used as a rocket test site from 1951 to 1954 and from 1961 to 1973.

Today J-Field is used as a training ground for soldiers handling hazardous materials.

Aberdeen is one of 20 of 207 Army posts that were added in 1984 to the national Superfund list, an inventory of the country's most life-threatening chemical dumps compiled by the EPA.

Army officials said 2,105 sites on Army posts from coast to coast are being studied for possible cleanup and that Army funds, not the $1.6 billion appropriated by Congress for cleaning up Superfund locations, will be used on the Army's chemical dumps. Some sites already have been cleaned up, they said.

Army officials said there are many unknowns about what has gone on over the years at Aberdeen because personnel are rotated often and because of "gaps in the records."

The Pentagon's Souci said top Army officials are concerned about Aberdeen. But, he added, "Headquarters is not going to sweep onto Aberdeen like some avenging angel and take care of it."