Bunker C fuel oil, 29,000 litres of which poured from a tank on the Westwood Anette into the Squamish Estuary, is also known as Type 6 heating fuel oil in Canada and No. 6 fuel oil in the United States. It is a dense, sticky oil produced by blending heavy residual oils with a lighter oil to meet specific needs for viscosity and pour point.
It is inexpensive to make and is used in industrial burners worldwide, as well as in internal combustion engines of the compression-ignition type used by certain marine vessels.
According to the U.S. Coastguard, the following can occur during a bunker C fuel spill:
When spilled on water, bunker C fuel usually spreads into thick, dark colored slicks, which can contain large amounts of oil.
The most viscous bunker C oils will often break up into discrete patches and tar balls when spilled instead of forming slicks. Oil recovery by skimmers and vacuum pumps can be very effective, early in the spill.
It is a persistent oil; only 5 to 10 per cent is expected to evaporate within the first hours of a spill. Consequently, the oil can be carried hundreds of miles in the form of scattered tar balls by winds and currents. The tar balls will vary in diameter from several metres to a few centimeters and may be very difficult to detect visually or with remote sensing techniques.
The specific gravity of a particular bunker C fuel oil can vary from 0.95 to greater than 1.03. Thus, spilled oil can float, suspend in the water column, or sink. Small changes in water density may dictate whether the oil will sink or float.
Floating oil in a high sediment environment (rivers, beaches) could potentially sink once it picks up sediment, resulting in subsurface tar balls or tar mats.
These oils can occasionally form an emulsion, but usually only slowly and after a period of days.
Because of its high viscosity, beached oil tends to remain on the surface rather than penetrate sediments. Light accumulations usually form a "bathtub ring" at the high-tide line; heavy accumulations can pool on the beach.
Shoreline cleanup can be very effective, before the oil weathers, becoming stickier and even more viscous. Natural degradation rates for these heavy oils are very slow. The oil may persist on beaches for months to years.
Adverse effects of floating bunker C fuel oil are related primarily to coating of wildlife dwelling on the water surface, smothering of inter-tidal organisms, and long-term sediment contamination. Bunker C fuel oil is not expected to be as acutely toxic to water column organisms as lighter oils.
Direct mortality rates can be high for seabirds, waterfowl, and fur-bearing marine mammals, especially where populations are concentrated in small areas, such as during bird migrations or marine mammal haul-outs.
Direct mortality rates are generally less for shorebirds because they rarely enter the water. Shorebirds, which feed in inter-tidal habitats where oil strands and persists, are at higher risk of sub lethal effects from either contaminated or reduced population of prey.
The most important factors determining the impacts of bunker C fuel oil contamination on marshes are the extent of oiling on the vegetation and the degree of sediment contamination from the spill or disturbance from the cleanup. Many plants can survive partial oiling; fewer survive when all or most of the above-ground vegetation is coated with heavy oil. However, unless the substrate is heavily oiled, the roots often survive and the plants can re-grow.
