The federal Environmental Protection Agency has compiled a list of 35 sites in Northern Virginia where in the last 10 years the agency inspected potentially hazardous conditions, and 10 of the sites are still being cleaned up, investigated or monitored.

State health and hazardous waste officials said none of the sites appears to pose a major threat to humans or the environment. Most of the sites have low levels of toxins and are self-contained, so contaminants are not seeping into wells, public water supplies, rivers and streams, they said.

The most serious problems appear to be at military bases scattered around the region that are not, for the most part, accessible to the public.

Three of the sites that are still being monitored are in Fairfax County, six are in Prince William County and one is in Alexandria. Arlington and Loudoun counties each had four sites that were investigated, but either nothing was found or the areas have been cleaned, federal, state and local officials said.

"This is an increasingly urban area, so it's not unusual to find this many sites," said Thomas D. Modena, who heads the division that conducts preliminary assessment of potentially hazardous sites for the state Department of Waste Management.

"Are they being adequately taken care of? I think they are," he said. "I don't think anybody should be alarmed by any of these at all."

The list has sparked some concern and confusion among local officials, who were surprised to see the number of Northern Virginia sites on an EPA list of potentially hazardous areas. The list is used by the agency as a data base from which it selects projects to receive federal aid under the Superfund cleanup program. It includes more than 35,000 sites nationwide, with 500 in Virginia.

EPA officials said the purpose of the list was to allay concerns, not fire them up. "The idea was to let people at the lowest local level, the townships, have a list of places that EPA has looked into for potential hazardous waste sites," said Nanci Sinclair, an official from the agency's regional office in Philadelphia. "When the general public comes to local government and says, 'We want to build a school here,' they will know that EPA has looked to see if there is hazardous waste there."

Not all potential hazardous sites -- or even those confirmed as the most toxic -- are on the list. For example, a wetland in southern Prince William County where officials recently found fourteen 55-gallon barrels of hazardous substances is not on the list, according to Modena, "but chances are it will show up sooner or later."

Typical among sites on the list is the Richmond, Fredericksburg & Potomac Railroad yard south of National Airport in Alexandria.

Modena, reading from a report, said the site made the list after "a citizen observed some small pits containing discolored liquids and/or blackish oily-looking sludges."

State officials investigated and found three drainage ponds containing fuel oil and heavy metals, such as lead and zinc, at levels dangerous to water plants; a few PCB transformers; soil with low levels of arsenic and lead; and ash that had been dumped in a swampy area.

State and local officials said that, as part of a plan to develop the rail yard, the owners are cleaning up or preparing to act on all of the problems.

RF&P officials declined to comment.

"As far as we know, everything of concern is contained, there's no danger to anybody, and they're cleaning the whole site up," Modena said.

Other sites on the list that are being monitored, investigated or cleaned up, according to officials, are:

Fort Belvoir Army Base in southern Fairfax County, where the EPA is investigating 35 alleged violations of hazardous waste regulations, including one that workers at the Army base failed to seal three dozen toxic chemical storage or treatment areas that could leak. Army officials said 23 of the alleged violations have been fixed and the rest will be corrected by Jan. 1.

The Central Intelligence Agency in McLean, which has hired a contractor to remove an outside transformer that leaked PCBs -- an extremely toxic insulating fluid -- into the soil, which also was removed. An agency spokesman said the spill "did not contaminate water or air in the area," and that the cleanup is almost complete.

Hazelton Laboratories Inc., a research and testing laboratory about two miles north of Tysons Corner on Route 7, which has two small ponds that were used to settle effluent from the lab between 1950 and 1968, before a public sewer was extended to that area. Donald P. Nielsen, president of the company, said EPA officials found traces of heavy metals that exceeded acceptable federal levels, mostly in mud at the bottom of the ponds, which are self-contained. He said the company has hired a contractor to conduct a feasibility study on the best way to correct the problem.

Gainesville Elementary School in northern Prince William County and the Independent Hill School complex in southern Prince William, both of which have organic materials that are suspected carcinogens in their wells. A private office building near Gainesville also has the problem and is using bottled drinking water. State officials stressed that the amount of substances found in the water were below levels considered unhealthy by EPA. Nonetheless, Independent Hill has been hooked up to county water and Gainesville will be tied into the county water system before opening in the fall, officials said. They said it was unclear what action, if any, will be taken with the wells.

The old Cherry Hill Landfill, near Cockpit Point on Prince William's Potomac River shoreline, which was used by the District of Columbia as a dump until 1979. Marcus Haynes, a sanitarian with the state health department in Prince William, said the 30-acre landfill is being monitored and would be a candidate for future cleanup. He said that, according to an October 1989 report by the state Department of Waste Management, the landfill has high levels of lead in the soil, ground water with high levels of other heavy metals and runoff into the Potomac River that "very likely" could have an adverse impact on aquatic life.

Sam Jones' Junk Yard, a salvage business near Interstate 66 and Route 29 in northern Prince William, that is apparently the source of a suspected carcinogen that has been found in tests of local water, including samples taken from a mobile home park nearby. Haynes said the substance, a solvent that he said might be a dry cleaning fluid, does not appear to be at unhealthy levels. A spokesman for the junkyard could not be reached for comment. Haynes said company officials have denied being the source of problem.

The old United Fiberglass Inc. facility, in the Featherstone Industrial Park along Prince William's Potomac River shoreline. Modena said his office plans to visit the site this month to see if further action is necessary due to small amounts of fiberglass materials found in runoff at the site.

Quantico Marine Corps Base in southern Prince William County, where eight separate hazardous waste sites are being cleanedup in a project that will take several years and cost millions of dollars. The sites include a pesticide burial area, two landfills, an area where transformers leaked PCBs, a battery-acid disposal area and an arsenic disposal site.