A major underground petroleum pipeline ruptured in Manassas yesterday, sending 100,000 gallons of kerosene gushing into the air and threatening to pollute the major reservoir in Northern Virginia.

Kerosene that ran off into a stream that feeds the Occoquan Reservoir, source of drinking water for more than half a million people, poses no health hazard, according to a Fairfax County Water Authority spokesman.

But drinking water could take on a "bad odor" as early as this weekend, depending on the efficacy of containment efforts and the amount of rainfall contributing to stream flows.

Fire and rescue companies were using floating barriers last night in an attempt to contain the kerosene that ran off into Bull Run.

The 32-inch diameter pipeline, a key transmission line for refined petroleum products from Texas to New Jersey, burst about 3:40 p.m., sending a five-foot geyser of kerosene onto nearby roads and blocking traffic for more than an hour.

Fire companies from Centreville, Chantilly and Clifton initially tried to construct a crude dam across Bull Run, using everything from chicken wire fences to insulation blankets.

A spokesman for Fairfax County Fire and Rescue Services said those efforts plus an attempt to build an earthen dam across the stream failed.

Then the fire and rescue workers obtained floating barriers called flotation collars, which extend a few inches below the water's surface and are designed to trap petroleum products floating on the surface. A fire spokesman said the collars were loaned by the Amoco Oil Co. and were being placed at the Bull Run Marina.

Thomas M. Schwarberg, Northern Virginia director of the State Water Control Board, said fish kills are possible because "the spill could choke off the oxygen supply."

James A. Warfield, spokesman for the Fairfax water authority, said water from the Occoquan Reservoir will continue to be safe to drink.

The reservoir supplies water to customers in Alexandria, Fairfax and Prince William counties, Manassas and Manassas Park.

"We have no idea whether or not significant amounts of the kerosene will reach" Occoquan Dam, which impounds the reservoir, but "the kerosene will wash over the top of the dam while the water intakes are completely submerged," he said.

"There will be odor problems," Warfield said, "and we will be using activated carbon near the intake of the dam" as a prevention measure, "but that might not be 100 percent successful."

It would normally take seven to 10 days for such a spill to reach the dam, Warfield said. However, with rain anticipated today and Saturday, the kerosene might reach the dam over the weekend.

Deputy Fire Marshal Edwin Lowe said kerosene has a flashpoint of 410 degrees and that there is little danger of its catching fire.

The pipeline break occurred at a fuel oil depot owned by the Colonial Pipeline Co. at Rte. 234 and Sudley Manor Drive in the northwest section of Manassas.

Police said initial reports of the break were received at 3:40 p.m., but the exact location was not determined for another 20 minutes. The pipeline was shut off at 6 p.m., according to a company spokesman.

Prince William County fire investigator Steve Small said U.S. Coast Guard officials, who are called in for cleanup measures when a spill reaches fresh water, had been notified. A private, Baltimore-based oil spill clean-up crew also was summoned.