Coast Guard officials are investigating a leak from undergound diesel fuel tanks at Bolling Air Force Base as a possible cause of a slick that drenched ducks in the Potomac River about a week ago.

The 640-acre base, where a faulty pump caused between 6,000 and 7,000 gallons of No. 2 diesel fuel to leak into a storm drain, may provide the latest clue in the search for the origins of a spill that measured about 200 yards in diameter, a mystery that Coast Guard officials say may never be solved.

"Thirty to 50 percent of oil spills, we just don't know who did," said Lt. William Diaduk, adding that numerous sources along the river could suffer such leaks. "Unless you can get to scene right after a spill is sighted, it is very difficult to determine where it came from."

Bolling is the third site investigated as the source of the spill, which local boaters reported seeing from south of the Woodrow Wilson Bridge to Frederick Douglass Memorial Bridge on the Anacostia River.

Ducks covered with fuel were first reported Aug. 14. Initial attention focused on a Delta Air Lines jet that reportedly dumped three tons of fuel over the river after losing power. Officials then investigated Piney Point Industries, where jet fuel had been transferred between a barge and an oil tank.

By midweek, officials from the Coast Guard's Marine Safety Office, the District and the Environmental Protection Agency were notified of the leak at Bolling Air Force Base.

On Aug. 15 or 16, Air Force officials say, an automatic pump serving three 20,000-gallon tanks in front of the Defense Intelligence Agency complex malfunctioned, causing one tank to overflow. Thousands of gallons of diesel fuel then seeped into a storm drain, Capt. Jim O'Brien, an Air Force spokesman, said.

"By Tuesday or Wednesday people began to smell fuel and began to investigate," O'Brien said. Air Force civil engineers and environmental officials subsequently monitored the situation, and four 3,500-gallon trucks have been pumping fuel out of the drain. Officials have also used sandbags and absorbent "booms" to help prevent leakage into the river.

Most of the fuel became trapped in the drain and did not pass through two floodgates about a half mile away, where the Anacostia and Potomac rivers meet, O'Brien said.

Capt. Douglas Anderson, an Air Force bio-environmental engineer, said the seepage during the week had been minimal, with less than 55 gallons entering the river. "That would not affect the environment," he said.

"We were fortunate that we didn't have a rain" before the leak was discovered, said Air Force Col. Joe LaFoy, chief civil engineer for the facility. Rainwater might have flooded the drain, carrying the spilled fuel into the river, he explained.

Samples from Bolling, Piney Point Industries, just south of the Frederick Douglass Memorial Bridge, and jet fuel from National Airport will be analyzed to determine if any match the slick. Results should be available in a few weeks.

Given the difficulty of finding the source of such leaks, Diaduk said he "wouldn't be suprised if none of the samples match."

If a source is identified it could be fined for violating federal law prohibiting dumping of petroleum products in navigable waters. Because the spill has dissipated by natural causes, no cleanup costs are anticipated.