A new federal government study has found that diluted bitumen floats in sea water, except when it is mixed with some types of sediment, which can make it heavy enough to sink.

The study used laboratory simulations of two common types of crude produced from the Alberta oilsands, and salt water tanks that approximated wave action.

The behaviour of diluted bitumen in water was one of the key issues in the Northern Gateway pipeline hearings, which led to a recommendation in December that the twin pipeline from northern Alberta to oil tanker port facilities should be allowed to proceed.

The federal government has funded an ocean-based study of spilled bitumen as part of its shipping safety program, launched in the 2012 budget and now labelled the "World Class Initiative."

There are no studies or records of actual diluted bitumen spills at sea available.

The laboratory study by Environment Canada, Fisheries and Oceans Canada and Natural Resources reached two major conclusions: "Like conventional crude oil, both diluted bitumen products floated on saltwater (free of sediment) even after evaporation and exposure to light and mixing with water; "When fine sediments were suspended in the saltwater, high-energy wave action mixed the sediments with the diluted bitumen, causing the mixture to sink or be dispersed as floating tarballs."

The study also looked at the 2010 spill of diluted bitumen from an Enbridge pipeline rupture into the Kalamazoo River in Michigan, where the heavy oil was carried nearly 100 kilometres downstream and between 10 and 20 per cent of it mixed with sediments and sank.

While floating oil is easier to recover, a study commissioned by the B.C. government last year found that the existing oil spill response system for B.C.'s north coast would only be able to recover three to four per cent of it.

NDP critics seized on the finding that diluted bitumen sinks when combined with sediment.

"There is no longer any doubt that the Enbridge Northern Gateway pipeline poses an unacceptable risk to northwest communities and the environment," said NDP environment critic Spencer Chandra Herbert, who has been touring the region.