Farm families around here were greatly relieved seven years ago when the Southern Maryland Wood Treating Corp. plant closed. Gone were the stench, soot and hammering noise that regularly erupted from the wood processing facility as it sat amid corn and soybean fields above a stream that rolls across St. Mary's County to the Potomac.

But now, it seems, the company's ramshackle buildings and round metal tanks have left a legacy of chemicals tainting the fertile soil, the stream and underground reservoirs of drinking water. Traces of the chemical mutant dioxin have been found in equipment at the defunct plant, and the suspected carcinogens creosote, PCPs (pentachlorophenols) and benzene have seeped into water and soil.

Moon-suited technicians from the Environmental Protection Agency are taking hundreds of samples of water and soil to determine the extent of the contamination in this quiet community, so they can devise an emergency plan to stop its spread. But the technicians, dressed in white plastic protective jumpsuits with yellow-and-black respirators, stand in eerie contrast to the area's Amish farmers, with their plain clothes and Clydesdale-drawn wooden wagons.

"My three boys say they the technicians look like space men, and they wonder if it chemical mist from the plant hurt them when it hit their skin. They keep asking what's going on. When a grown-up can't understand it, it's hard to explain to children," said 33-year-old Wendy Cusic, who lives with her husband and children on the farm abutting the plant site.

For the Cusics and three other families who live on the 219-acre farm, life has become uneasy since they learned earlier this month that dioxin may be soiling their environment. They had learned to live with the odors, noise and chemical spray from the plant, but they said that they do not know how to cope with the lingering fears of contamination or future health risks.

It was the discovery of dioxin, an unwanted byproduct of combustion and mixing chemicals, at the plant site earlier this month that triggered the EPA's emergency cleanup efforts. Tests revealed three forms of dioxin -- hexa, hepta and octa dioxin -- all believed even in minute amounts to cause cancer and birth defects in humans. But the three types are about 250 times less toxic than TCDD, the most deadly form of dioxin that caused the federal government to resettle the entire town of Times Beach, Mo., after a 1983 flood washed the chemical through the community.

The work in St. Mary's, a county of 65,000 persons about 50 miles southeast of Washington, is similar to emergency cleanups in 52 communities across the country at waste sites posing human health and environmental dangers.

Under the $1.6 billion federal Superfund program, 425 other sites nationwide have received the EPA's urgent attention in the past five years, including nine in Maryland and two in Virginia. Immediate removal projects, such as the one in St. Mary's, can take as long as six months and cost up to $1 million. EPA has earmarked $590,000 to contain the immediate health and environmental threats at the 10-acre site in St. Mary's.

The EPA's emergency measures serve only to stanch the flow of contaminants into the environment. Thorough, long-term cleanup is required in 812 places that are the worst of the nation's hazardous waste sites. Six of these priority sites, including the Southern Maryland Wood Treating plant, are in Maryland, and nine are in Virginia. There are no Superfund sites in the District.

The work is slow and costly, often with uncertain results. Since the Superfund program got started in 1980, the long-term cleanup has been completed at only six sites, at a cost of $28 million.

The cleanup at the St. Mary's wood treatment plant marks the third time in less than two years that the EPA has come to St. Mary's County to quell an environmental emergency. In June 1983, the agency spent $460,000 to remove illegally dumped chemicals from a farm in California, Md. Six months later, at a cost of $24,000, EPA troubleshooters removed an abandoned tanker that was leaking PCBs at Chaptico.

"We just wonder where the next one's going to be," said Jack Witten, president of the county's Potomac River Association.

Talk of dioxin and other hazardous chemicals in their back yard has frightened local families.

"They're not sure how bad it is. We don't know what we're going to do. If it's bad, there's nothing we can do," said Shirley Miedzinski, whose family lives on the same farm as the Cusics. "Well, I'm glad they're cleaning it up; but if it causes cancer, what are we supposed to do about it? We don't know what's what. It scares me 'cause we're right on top of it.

Robert and Frances Larrabee built their dream house on Joy Lane in 1959. Six years later, the Southern Maryland plant opened across the road. Like other families in Hollywood who discussed their fears, the Larrabees worry that if the dioxin contamination at the site is extensive, they, like the people in Times Beach, will have to give up their homes.

"Everything I own is in this house. A lot of TLC tender loving care has been put in this house," Robert Larrabee said. "You have something like this dropped in your face and you realize, if it gets much worse I'll have to pull up stakes. But we might run into the same thing some place else."

Under court order, Maryland health officials and the owners of the plant, L.A. Clark and Son Inc., a wood treating corporation based in Fredericksburg, Va., have been trying since 1980 to clean up environmental hazards there. One of the efforts involved spreading chemical sludge from Blue Plains Sewage Treatment over the site. Officials hoped that microbes in the sludge would absorb the chemical wastes, but company officials and residents said that it failed and even may have spread the contamination.

John Curtis, company spokesman in Fredericksburg, said: "L.A. Clark developed a plan with the state of Maryland. We followed a plan and completed a plan. We just used what Blue Plains gave us -- maybe the chemicals in that made it worse. We just followed a program, put the money up and thought the program was working."

Curtis said that the company, which bought the plant in 1976, inherited much of the pollution there and has spent about $122,000 trying to purge it. The company, which has filed for bankruptcy and is reorganizing, owns a similar wood treating plant in Frederickburg, which is awaiting EPA priority cleanup.

According to John Koontz, chief of enforcement for the Waste Management Administration of the Maryland Department of Health and Mental Hygiene, L.A. Clarke did not hold up its end of the court-ordered cleanup.

"The deal is, they polluted the state of Maryland," Koontz charged last week. "It's one kind of cross to deal with hazardous waste. It's another kind of cross to deal with dioxin for two reasons: The sampling costs are high . . . and there's no place in the country to store it. There's no way to dispose of it or treat the material; the technology isn't here."

The problem of what to do with dioxin-tainted material has troubled Maryland before, Koontz said. Last August, officials discovered that leaky barrels of waste pesticides buried at a state highway facility near the Baltimore Beltway had contaminated soil with TDCC, the most toxic of the 75 forms of dioxin. State workers collected ten 18-foot containers of contaminated soil, which are still sitting at the facility because there is no other place to dispose of them, Koontz said.

After botulism and tetanus toxins, scientists classify dioxins as the most poisonous chemicals in nature. Guinea pigs, the laboratory animals most sensitive to the poison, have died from exposure to dioxin doses as small as a 10-millionth of a gram. That is roughly equivalent to one drop of water in several Olympic-sized swimming pools.

EPA has set an "action level" of one part of dioxin for every billion parts of soil.

"We clean up dirt that contains more than that. It's an EPA policy that there is no safe level of a known carcinogen," said Dr. Richard Brunker, an EPA toxicologist.

To illustrate the levels of dioxin toxicity, Brunker said that a dot of TCDD the size of the head of a pin would kill a 22-pound child. But that child would have to be exposed to a small teaspoonful of hexa or hepta dioxin, two of the compounds found at the St. Mary's site, before the child would die. The child could not survive exposure to four tablespoons of octa dioxin, the other chemical found there, Brunker said.

Six EPA samples of the St. Mary's site showed no TCDD.

But Koontz said: "It's very hard to explain to people that dioxins aren't dioxin. I don't think any dioxins are minor league. It's more prudent to go in and spend the money for . . . damage control than to study it and say, 'Gee, six months ago we should have taken action.' I certainly would not want it in my back yard, so it's certainly not something that should be in the back yard of the people of St. Mary's County."

As the EPA testing continues, the people in St. Mary's County are waiting for answers.