It wasn't long after James Barnhard turned up his first duck -- reeking of fuel, its plumage matted -- that he became suspicious.

That was Friday. By yesterday, Barnhard and others scouring the banks of the Potomac River near Fort McNair in Southwest Washington had found dozens of other mallards in similar condition.

The water near the Fort McNair Yacht Basin where Barnhard lives is pungent with oil and filmed with a fine blue-green sheen. Traces of the sheen have been spotted from south of the Woodrow Wilson Bridge to the Frederick Douglass Memorial Bridge on the Anacostia River, according to boat pilots who ply the river.

For Barnhard, a former Navy airplane mechanic who now repairs marine engines, all the clues pointed in the same direction: a Delta Air Lines 727 that lost power after taking off from National Airport Wednesday dumped three tons of fuel and returned to the airport five minutes later.

The source of the pollution remains a mystery, but Coast Guard officials have begun efforts to track it down. Yesterday, technicians from the service's Marine Safety office in Baltimore took water samples for analysis.

Delta officials have described the incident, involving Flight 415 to Dallas-Fort Worth, as routine. Yesterday they insisted that the fuel was jettisoned west of the river, perhaps over Alexandria. In any event, they said, a 10- to 12-knot wind was blowing from the southeast and the fuel would have turned to vapor before falling the 1,500 to 2,000 feet to earth.

"It is virtually impossible that any of the fuel jettisoned by the Delta flight could possibly have reached the Potomac River," said Dick Jones, a Delta spokesman in Atlanta. "I'm sorry for the ducks. I just don't think that we were guilty."

But some witnesses who watched the flight of the Delta airliner took issue with the official version. Stephen D. Danzig, who at the time of the flight was aboard the Cherry Blossom, an Alexandria charter boat, said he saw the airplane dump its fuel directly over the Potomac.

Danzig, a captain in the Fairfax County Police Department in charge of the special operations division, gave this account in an interview yesterday: "Shortly after 6 p.m., I glanced up at the aircraft and saw liquid spewing from both wings . . . . When I first noticed it, it was directly over the river" next to Old Town Alexandria. Danzig added that he did not smell the fuel, or see or hear it hit the river.

The captain of the Cherry Blossom, Clayton F. Cisar, president of the Potomac River Pilots Association, described the spill as "the worst slick I've seen." He added: "And I've been a ship captain for years and years and I've seen some pretty bad spills."

As for the ducks, rescue workers said they seemed disoriented, less buoyant than usual, their feathers stripped of their natural oils.

"They walk up on shore badly irritated," said Barnard, who has been cleaning the birds with a Humane Society-approved solution of Dawn dishwashing liquid and warm water. "When they come out of the water you know there's a problem: The water is irritating them and they're sinking."

A number of groups are now trying to clean up the ducks and the river. The Humane Society and the Wild Bird Rescue League of Northern Virginia were in the act by yesterday, farming the mallards out to a coterie of volunteers specially trained to treat fuel-damaged birds.

One volunteer, Bill Taylor of Arlington, said he spent several hours Saturday and yesterday on the river trying to catch the mallards so he could treat them.

Many, he said, were barely afloat, with only their heads visible above the water. He saw one drown, and heard of others.

"We have six of them here at my place," Taylor said. "They're covered with oil. They're not lifeless, but they're weakened."

Taylor advised boaters to take sick mallards to the Arlington County Animal Shelter, where they can be cared for.

Humane Society officials and other volunteers said that unlike spills of heavy, unrefined oil that goops onto birds' feathers, the fuel in the Potomac appears highly refined and tends to be absorbed into the ducks' skin, perhaps poisoning their vital organs.

Volunteers say cleaning the birds and rehabilitating them can take days. Many are dehydrated and must be force-fed a special concoction of fluids, which may include Gatorade.