A B.C.-based mining company thinks there might be gold beneath Shoal Lake in northwestern Ontario, but Manitoba Premier Gary Doer has vowed to block any mining proposal.

"We have consistently opposed any – any – development of a gold mine in the Shoal Lake watershed," said Manitoba Premier Gary Doer on Wednesday.

Winnipeg gets its drinking water from Shoal Lake, which is on the Ontario-Manitoba border. A 150-kilometre aqueduct from the lake has fed the city's water system for 85 years.

FAST FACTS: SHOAL LAKE

Shoal Lake covers 277 square kilometres and contains 2.64 billion cubic litres of water.

Winnipeg is licensed to take up to 455 million litres of water per day from the lake.

The intake for Winnipeg's water system, located on Indian Bay at the western end of the lake, is only accessible by rail or boat.

The 135-kilometre aqueduct from Shoal Lake to Winnipeg took more than four years and $17 million to build. It opened Sept. 9, 1919.

Source: City of Winnipeg

Doer said he was concerned that extracting gold could release toxins such as cyanide and mercury into the lake water.

Halo Resources Ltd. has made offers to buy 67 mine claims on 3,000 acres near the lake.

Nick Demare, a company spokesman, said plans for mining the shores of Shoal Lake are in the preliminary stages. He said he understands the importance of the water, and he said Halo would protect the environment.

Dave Green, a Manitoba water quality specialist, said there is no specific agreement between Manitoba and Ontario to protect the lake's water.

However, there is an understanding between the two provinces that Manitoba would be informed of any potential developments on the lake.

The city is less adamant in its opposition.

"We're not opposed to a gold mine or any mining development in principle," said Diane Sacher, manager of the city's water services branch. "What we are opposed to is anything that would detrimentally affect the water quality."