A U.S.-based conservation group has warned that the Great Lakes could become the "next frontier" of oil transportation.

The Alliance for the Great Lakes said companies are eager to begin shipping oilsands crude to refineries, via Lake Superior.

Alliance water quality program director Lyman Welch said the plan poses a threat to the drinking water on which millions depend.

Lyman Welch

Lyman Welch, water quality program director with the Alliance for the Great Lakes. (Supplied)

"Lake Superior is our deepest Great Lake, and a spill in the deep waters of Lake Superior, or other Great Lakes, would really be devastating,” he said.

"I think everyone in the Great Lakes [area] should be concerned about a threat to the drinking water supply that serves more than 40 million people.”

Spill prevention and response requirements on both the Canadian and US sides of the Great Lakes are inadequate, he added.

Spill prevention rules need tweaking

A spill of oilsands crude would be especially bad, Welch noted.

"It's heavier than water. So if a spill occurs in water, the tar sands crude sinks to the bottom, and is extremely difficult to remove."

Welch said there’s a lot of economic pressure to bring oilsands crude into the U.S. Midwest for refining at Great Lakes refineries. There's more oil than can be transported using pipelines, so people are looking at shipping as an alternative method.

The Alliance is particularly concerned about the company Calumet Specialty Products Partners. It has proposed building a complex to begin loading crude onto ships in Superior, Wisc. Welch said that proposal could mean shipping crude on the Great Lakes as soon as 2015.

The organization recently published a report that looked at the current spill prevention and response requirements in both the U.S. and Canada.

"We found that the Great Lakes are just not ready right now to handle tar sands crude,” as there are too many holes in the spill prevention rules, he said.

The report also noted shipping fleets and ports are not designed for this purpose.

Welch said Alliance wants to raise awareness about the threat, and they are pushing the U.S. and Canada to at least begin a discussion about this in the region.

Currently no oilsands crude is shipped on the Great Lakes.