The Environmental Protection Agency has identified an area of prime waterfront land in Alexandria as possibly contaminated by the toxic chemical dioxin and will begin testing around the site today, EPA officials said last night.

The officials stressed they have no evidence yet that the site, part of which was occupied for about 50 years by the R.H. Bogle chemical plant, is contaminated. Today the site contains town houses and offices.

There will be three days of testing this month and additional testing in January in which samples of fish, soil and sediment will be taken to determine the level of contamination, if any.

Results of the tests will not be known for several months, the officials said.

Dioxin is a compound produced in the manufacture of herbicides and pesticides. It is the element found to be so dangerous in the herbicide used in Vietnam known as "Agent Orange" and can be passed to humans by touching contaminated soil or breathing contaminated dust.

In a celebrated instance of dioxin contamination, the EPA bought properties in the town of Times Beach, Mo., to permit its residents to moved.

There have been no known reports of anyone living or working in the suspected area of about five blocks near the Alexandria waterfront becoming ill because of dioxin poisoning.

The Bogle plant manufactured herbicides until it was torn down in the late 1970s. Federal officials targeted the site because their records indicated that Bogle had reported the use of dioxin in its plant in 1976.

At a meeting in City Hall last night attended by city officials, EPA officials and residents of the Princess and Tobacco Quay town houses, which are in the area to be tested, the federal officials outlined their sampling process.

EPA technicians, dressed in chemical-resistant suits resembling those worn by astronauts, and sometimes breathing through respirators, will today begin sampling river-bottom sediment, fish and soil from Oronoco Bay.

In January they will take samples from the soil along the sidewalks of Pendleton and Oronoco streets and some soil samples in Founder's Park.

The sampling this week and in January is to be done on the periphery of the site because EPA officials believe that if any dioxin is present it is most likely to be found in these areas where it may have been carried by the runoff of rainwater from the affected site.

If these preliminary test results, available in about four months, disclose any level of dioxin, EPA technicians will return to the center of the site, now occupied by an office complex called Dalton Wharf, where they will drill through the parking lot and through several feet of fill that was placed there after the chemical plant was torn down.

The site in question and neighboring parcels of land were found to have a high content of arsenic in the 1970s as a result of the plant's operation.

The city required developers to fill over the land with 5 feet of heavy clay and then a layer of macadam to seal it off from new buildings.

Testing for dioxin was not done at the time the arsenic contamination was found because dioxin was not seen as a major threat then.

If the final tests down to the original site of the chemical plant show dioxin at unacceptable levels, EPA will do a "risk assessment" in collaboration with the federal Centers for Disease Control in Atlanta, and then take appropriate steps to deal with the contamination. The officials did not spell out last night what those steps might be.

City officials said they believe that the "sealing" of the site done in the 1970s also will be adequate protection against any dioxin contamination that may be found.