The source of diesel fuel that spilled into the Yukon River Tuesday from a Whitehorse storm drain is still under investigation, says a senior investigator with Environment Canada.

Steve Arrell said today that after a complaint of a sheen on the river, it was determined Tuesday afternoon that the diesel fuel was being pumped from a sump located in the Optometrist Building. That structure stands at the corner of Lambert Street and Second Avenue.

The source of the diesel or heating fuel, however, has yet to be determined, Arrell emphasized during an interview.

He said the sump in the Optometrist Building is capturing ground water flowing through the general area of the building. The diesel fuel, he said, could be coming from anywhere in the vicinity.

Officials have ruled out a number of commercial buildings in the area with above-ground fuel tanks. They have asked the owners of other buildings with underground tanks to have their's checked, Arrell said.

'At around 12:30 p.m. (Tuesday), we got a call from a member of the public, that they had observed an oil sheen on the Yukon River,' he said.

'We came down to the Yukon River and observed that oil sheen, and followed it back to the storm drain at Lambert Street.'

Arrell said with the assistance of the city's public works department, crews worked back from the storm drain. They checked the various buildings until they found the contaminated sump in the basement of the Optometrist Building.

The sump was turned off, and a vacuum truck is being used to empty it every four or five hours.

Judging from the sheen on the river and other evidence, such as the frequency of the sump pump kicking in, Arrell suspects the spill likely involved tens of litres (or gallons), and not anything like full drums of diesel.

While he can't be certain the substance is diesel fuel until he gets back the laboratory analysis, everything in his experience tells him it is.

By the time Environment Canada officials witnessed the sheen, it had reached Main Street and beyond, he explained.

Mother Nature, said Arrell, will have to take care of it through evaporation and the fuel eventually breaking down. It would have been impossible to trap and collect it after it had spread across the moving water, he added.

'We do not like to see that pollution but there is just nothing we can do about it at this point.'

Staff from public works and the city's fire department are taking steps to clean the storm drain of any fuel residue to minimize any further contamination of the river should a storm event occur, such as a heavy spring rain.

And with the sump turned off, Arrell is confident no more fuel will be pumped directly into the storm drain.

Arrell said his office won't stop its investigation until it finds the source of the diesel fuel.

Once the source is located, it will involve a cleanup of the soil contaminated by the leak, he said.

The sump caught some of the leaking oil as a low point in the area. But he doesn't think it caught it all, and that leaking fuel has seeped into other ground water not captured by the sump.