One man was killed and a dozen were injured when a van carrying a group of day laborers from the District of Columbia to jobs in Northern Virgina collided head-on with a 10-ton dump truck near Herndon yesterday morning, Fairfax County police said.

The injured received nearly instantaneous medical attention at the scene from an off-duty Fairfax County firefighter, Bill Roberts, who was driving two cars behind the dump truck when the accident occurred shortly before 9 a.m.

The impact was so severe that the van, its engine and hood mashed, was spun around and came to rest half on Dranesville Road and half in a ravine sloping into the woods. The road is a two-lane strip that winds through the woods south of Rte. 7 in the northwest corner of the county.

"It was just total chaos," said Roberts. "It was like a firefight in Vietnam." Roberts, a licensed emergency medical technician, said he served in Vietnam during the Tet offensive.

With the aid of another man, John Nitzke, who had been driving behind the van, Roberts assessed the injuries, summoned emergency help and helped the dazed and shocked men who clambered out of the van.

The driver of the van, Kenneth E. Bridgewater, 32, of 1201 Allison St. NW, apparently was killed on impact, authorities said. The driver of the truck, Arthur Dokes, 41, of Stephens City, Va., was treated for minor injuries at the scene.

At least two other men remained at hospitals last night, one in fair condition at Fairfax Hospital and one in serious condition at Reston Hospital Center.

"There were people just wandering off everywhere," said Roberts. "I just tried to keep everybody together."

Nitzke declined to discuss his actions with reporters.

The van was carrying 14 laborers who had signed up to work at Northern Virginia job sites under the auspices of Tracy Labor, a manpower and temporary help firm with offices in the District. The driver of the van was a full-time employe of Tracy.

The 1982, three-axle GMC dump truck, which was not loaded, was southbound and the 1987 Dodge Ram van northbound when they collided. Police said the van was left of center at the point of collision, but they do not know why.

The truck came to rest about 60 yards down the road, its front left fender, headlight and a wheel shorn off.

Thirteen of the injured were taken to Reston Hospital Center and Access, the emergency medical facility in Reston. One man was flown by helicopter to Fairfax Hospital.

None of the men in the van was wearing a seat belt, authorities said. No charges are pending, police said.