Alexandria's Oronoco Bay Park yesterday looked like a spaceman's picnic ground as Environmental Protection Agency technicians in chemical-resistant suits tested the soil for traces of the toxic chemical dioxin.

EPA officials said Tuesday that they had identified the waterfront site that includes the park, town houses and an office complex as one possibly contaminated by dioxin. The R.H. Bogle chemical plant, built in the 1920s and torn down in 1976, used dioxin to make herbicides on the site.

While many residents of the affected area seemed to take the news in stride, city officials said they were still puzzled as to why EPA targeted this particular spot.

The city, having found high levels of arsenic in the soil, required developers to take extensive precautionary measures to seal off the ground there before allowing any new construction there after the Bogle plant was torn down.

"We want this cloud to be removed . . . we don't believe there's a health risk and we don't see how there could be because the dioxin would be deep in the ground," said James Alexander, director of environmental quality for the city's health department.

"We are obviously distressed that this beautiful part of our city has to be singled out for this kind of action," said Alexander, "but we are extremely anxious for EPA to proceed swiftly and thoroughly and we are going to do everything to cooperate with them."

The testing is part of EPA's National Dioxin Strategy to determine the extent of dioxin contamination throughout the country.

The $7.4 million program involves testing more than 600 sites for possible contamination by the chemical compound that was the deadly ingredient in "Agent Orange," a herbicide used in Vietnam.

Even EPA officials could not explain why an already sealed-off site is being tested.

"We were just given a list [from Washington] and told to go out and sample them," said Janet Luffy, acting director of public affairs in EPA's Philadelphia office, which is overseeing the testing.

"We don't know if anyone took into consideration that it had been paved over," she said.

However, tests done by the city in 1976 found that rain water flowed from the site of the former Bogle plant into the Oronoco Bay Park area, leading EPA officials to test that area first for dioxin-toxic soil.

Some property owners in the area have speculated that if any threat is found, it will be to the Potomac River in the area of the Oronoco Bay.

Yesterday, EPA technicians set up tables, chairs and equipment in the park and in white suits, black masks, yellow boots and green gloves drilled down into the soil to collect their samples.

Out in the bay, the EPA technicians worked from a boat collecting fish, which will also be tested for dioxin.

While curious onlookers gathered nearby, most of those who live in the area expressed only mild concern about the possible threat.

"I've been living with the notion that I'm sitting on the top of chemicals for years," said Neil Gearin, a consultant who has lived in a town house in the Tobacco Quay development for three years.

"I show a concern [about the threat], but I'm pleased EPA is going about it in proper way," said Jack Woolley, a resident of the Princess Street Town Houses, which are located on the block adjoining the former site of the chemical plant.

But until the results come in, "there's going to be a cloud over everyone's head as property owners and as human beings," said Tobacco Quay resident Peter H. Pattridge.

If the EPA tests of fish, soil and sediment on the periphery of the former chemical plant site show the presence of dioxin, EPA technicians will return in about four months to take samples of the soil on the exact site of the former plant.

Dalton Wharf, a complex of offices, now stands on this site.