Does Bitumen Sink or Float?

What the Evidence Actually Shows + Why It Matters.

The scientific consensus across major studies reveals a complex answer: bitumen initially floats, but weathering and sediment interaction can cause it to submerge or sink—especially in turbulent coastal waters.coa


Floating vs. Sinking: Why Is this a Problem?

Surface Oil: Sticky, Toxic, Manageable

  • Can be corralled, skimmed, burned off, or dispersed
  • Visible and trackable
  • Well-established response techniques
  • Significant portion can be physically removed

Sunken Bitumen: Sticky, Toxic, Catastrophic

  • Conventional tools become largely ineffective
  • Dramatically harder to detect and contain
  • Far more expensive to clean up
  • Moves underwater in plumes, settles on seafloor

Where Sunken Oil Goes

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Lodges in eelgrass meadows, kelp forests, and shellfish beds

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Contaminates underwater habitat crucial to coastal fisheries

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Damages halibut, crab, and salmon nurseries

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Cleanup nearly impossible without destroying habitat

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Bottom Line

The buoyancy of bitumen determines the overall accountability and feasibility of tanker operations. Insurers, coastal communities, and emergency planners base spill assessments on what can realistically be cleaned up — not on political assurances that 'oil floats.' In high-energy, sediment-rich waters like Hecate Strait and Dixon Entrance, diluted bitumen is far more prone to submerging or sinking.

A major spill would likely be largely unrecoverable, transforming a serious accident into a long-term ecological disaster.

The Scientific Consensus

Bitumen initially floats, because the "diluent" (light hydrocarbons added so it can flow in pipelines) makes it lighter than seawater.

But…

Once weathering begins — and once it mixes with sediment — dilbit becomes denser than seawater and can submerge or sink. This is not speculative; it has been directly observed in multiple controlled studies and real-world spills.

Key Findings Across the Literature

Government of Canada (Natural Resources Canada & DFO, 2015–2017)

Dilbit weathers quickly, losing its light components in hours to days. Without diluent, bitumen is heavier than seawater → prone to submerging, especially in areas of suspended sediment.

NOAA (U.S.)

Similar conclusions: dilbit can transition from floating → neutrally buoyant → sinking depending on turbulence and sediment load.

Royal Society of Canada Expert Panel on Oil Spill Behaviour (2015)

Bitumen becomes heavier than seawater once weathered and mixed with sediment, especially in shallow, energetic coastal environments.

Environment Canada behaviour studies (Burrard TS 2013; Gainford et al. 2017)

Under realistic ocean conditions, dilbit weathers rapidly and forms heavy oil–particle aggregates that sink or remain suspended in the water column.

Kalamazoo River spill (2010)

Real-world confirmation: after weathering, large quantities of dilbit sank to the riverbed, becoming extremely difficult and costly to recover.

What Determines Whether Bitumen Sinks or Stays Afloat?

A. Energy of the Environment (calm vs turbulent)

Low-energy, calm seas

(e.g., sheltered inlets, glassy conditions)

  • Less mixing → oil stays on surface longer
  • Lower sediment load → slower formation of oil–particle aggregates
  • Dilbit can still submerge over time, but the process is slower

Bottom line: In calm seas, dilbit may float longer, but weathering still increases density.

High-energy, turbulent seas

(e.g., Hecate Strait, Dixon Entrance, winter storms)

This is where sinking becomes much more likely.

Turbulence:

  • Rapidly accelerates weathering (loss of diluent → oil becomes heavier)
  • Mixes suspended sediment, organic matter, and sand into the oil
  • Builds oil–particle aggregates ("OPAs") that drag bitumen downward
  • Breaks slicks into droplets → easier to mix into the water column

B. Type and Amount of Suspended Sediment

Bitumen sticks easily to:

  • sand
  • silt
  • clay
  • organic detritus

Regions like Hecate Strait, Dogfish Bank, Dixon Entrance, and continental shelf edges have:

  • high turbulence
  • strong currents
  • heavy sediment loads

These are the worst possible conditions for keeping dilbit afloat.

C. Temperature

Cold temperatures slow weathering…

…but they also increase viscosity, causing oil to form thicker, heavier emulsions that can sink once mixed with sediment.

So What's the "Balance of Evidence"?

Across all reputable research:

The science overwhelmingly shows that diluted bitumen is more likely to submerge or sink than conventional crude, especially in turbulent coastal waters.

No credible study supports the idea that dilbit reliably floats in real-world Pacific conditions.

The risk of sinking increases dramatically in:

  • shallow shelf waters
  • storm conditions
  • areas with lots of sediment
  • regions with high wave energy (like Hecate Strait)

Once dilbit sinks, recovery becomes extraordinarily difficult and expensive.

NOAA calls sunken dilbit "one of the most challenging materials to recover in marine spill response."

What This Means for the BC North Coast


  • among the roughest bodies of water in the world
  • with shallow banks (Dogfish Bank, Rose Spit)
  • strong tidal currents
  • heavy sediment suspension from the shelf

Hecate Strait and Dixon Entrance provide textbook conditions for:

  • rapid weathering
  • sediment incorporation
  • submerging and sinking of dilbit

This is exactly why insurers and operators consider the route non-viable for crude tankers.

The combination of extreme weather conditions, shallow waters, heavy sediment loads, and the proven tendency of diluted bitumen to sink in such environments creates an unacceptable risk profile for marine transport along the BC North Coast.