PRINCE RUPERT, B.C. -- A dispute has erupted at the Northern Gateway hearings over whether diluted bitumen that would course through Enbridge's proposed pipeline would sink to the ocean floor.

Enbridge experts say the heavier oil product would not sink, prompting immediate protest from six First Nations and environmental groups taking part in the hearings in Prince Rupert, B.C.

Conservationists say diluted bitumen sinks, making it extremely difficult – if not impossible – to clean up.

Several Enbridge experts weighed in saying their own experience, and lab tests, found heavier oil does not sink unless mixed with a considerable amount of sand – something they say won't happen offshore.

The issue is key at the hearings, which are now considering Northern Gateway's marine oil spill plan for the section of the pipeline route where petroleum products from Alberta would be shipped overseas via tankers loaded in Kitimat, B.C.

The company has not submitted a plan that specifically deals with cleaning up oil from the ocean floor.