The Environmental Protection Agency said yesterday that the estimated 400 to 800 drums believed to be buried on a Stevensburg, Va., farm contain highly flammable materials and that the agency will secure the area and clean it up.

"Our first concern is to put a fence up around the site to keep people away. If someone carelessly lights a cigarette, it could cause a fire," EPA spokeswoman Ann Cardinal said.

This month, EPA technicians visited the farm, owned by Charles Myers, to take samples from the barrels found there and to determine whether there was contamination of a nearby stream and five wells used to provide drinking water for five families in the area.

According to Cardinal, the tests found that the drums contained xylene, toluene, ethylbenzine and several other chemicals, all used in paint thinners and solvents. Samples taken from a partly exposed barrel on the front of the property had a flash point of 39 degrees, meaning that the chemical could be ignited at that temperature.

Although the wells and stream water proved to be clean, there was some contamination in the stream-bottom sediments, Cardinal said. She said that was probably remains of contamination found there five years ago when the property was owned by James Lamphier. Stevensburg is about 70 miles southwest of Washington.

Lamphier operated Jim's Liquid Waste, a firm that disposed of septic and chemical wastes for several companies in the late 1970s. In a hazardous-waste suit settled in the 4th U.S. Circuit Court of Appeals in Richmond in August 1983, Lamphier was found to have violated federal and state Resource Conservation and Recovery Act regulations for the disposal and storage of hazardous waste.

The site was not cleaned up, however.

"We need to determine if the drums should be dug up and . . . how can the chemicals best be disposed of," Cardinal said.

EPA officials said they are contacting companies and individuals the agency believes generated the materials that were dumped at the site. They will be asked to clean it up.

If that does not happen, the EPA's Superfund program would be responsible for the cleanup and the EPA would try to recover the costs through the courts from the responsible parties, Cardinal said.

Last week, officials from Virginia Power visited the farm, but without excavating were unable to determine whether any of the drums had belonged to the electric utility, said Burton Marshall, manager of water quality for Virginia Power.

From 1975 to 1980, an engineering firm serving as an agent for Virginia Power contracted with Jim's Liquid Waste to haul septic wastes and waste solvents from the North Anna nuclear power plant construction site, about 30 miles south of Culpeper, Marshall said.

"We would like to identify what's there. We don't know if we have any liability or not. But we would like to get rid of whatever it is," Marshall said.

EPA officials said they expect to fence the property in the next two weeks, but they said they could not estimate when the cleanup will take place.

"At this point, I sincerely hope it doesn't take very long. But realistically I have no idea how long it might take," Cardinal said.