A $56,000-contract has been signed to begin a more intensive search for the source of a substantial underground fuel leak contaminating a portion of downtown Whitehorse.

The Department of the Environment and the consulting firm of Gartner Lee Ltd. entered into the contract this week, with small-bore drilling around the Optometrist Building expected to begin Tuesday, Brian Levia, manager of the department's environmental monitoring branch, said today.

The leaking diesel fuel or home heating fuel first came to light in early April, when an area resident reported a sheen on the Yukon River. Officials with Environment Canada and the Yukon government determined the fuel was leaking from the Lambert Street storm drain, and the source was the sump at the Optometrist Building.

But the underground tanks at the Optometrist Building were dug up and found to be sound. When crews dug down about a metre below the tanks, however, they witnessed a gush of fuel into the hole, in the amount of 800 litres in the two hours before the hole had to be filled back in to restore integrity around the building's footings.

Together with the fuel collected from the hole and the ongoing collection from the sump, about 4,000 litres have been retrieved. The source remains a mystery.

Levia said emphasized again today that the excavation work underway at Whitehorse Esso on Second Avenue, kitty-corner to the Optometrist Building, is not related to this investigation.

Imperial Oil spokesman Pius Rolheiser explained earlier this month the work at the Esso station is part of a remediation plan approved a year ago to deal primarily with an underground leak the station experienced in the early 1990s.

Permits for Imperial Oil's remediation work involve the use of a mobile water treatment plant to treat contaminated ground water. They also allow for the removal of up to 2,284 metres of soil from the Esso yard, and areas along Second Avenue and Lambert Street.

Levia said investigating the source of the current leak will involve the use of a mobile small-bore drill of fewer than five centimetres in diametre.

The contract for Gartner Lee calls for nine holes to be drilled around the Optometrist Biulding, with hopes of establishing the depth the fuel is concentrated at, and hopefully from what direction it's flowing, Levia explained.

He acknowledged the $56,000 is not buying any guarantees in terms of identifying the source.

'But the work that is being done is always going to give us an answer, and it will always give us information that can be used, even if they find nothing,' he said. 'Nothing is an answer; it all gives us information.'

Under Yukon law, he explained, the party responsible for a fuel spill is responsible for the cost of cleaning up the contamination.

Should the source of the leak be identified, however, a legal opinion from the Yukon's Department of Justice indicates the government would not be able to recover the $56,000 or any additional cost associated with determining the source.

In the days and weeks following the report of the leak, fuel records from other buildings in the neighbourhood of the Optometrist Building and some testing of underground tanks were inconclusive in determining where the fuel was leaking from.

Levia said the drill work is expected to take three days. Though he isn't sure exactly how long it will take to analyze the data, it will likely be matter of days rather than weeks, he suspects.

The drill contains sensors that record soil types as it bores down into the ground, the location of the ground water table and any sign of contaminants.