The first good news is that the 247 whooping cranes that wintered at Aransas National Wildlife Refuge on the Texas Coast—all the wild ones remaining in the world—left for their nesting area in Wood Buffalo National Park before the oil began spilling into the Gulf of Mexico. The bad news is that some of the oil will be there when they come back.
Oil is not just oil. It starts out as a complex mixture off hundreds of compounds, some of which are lighter than others. The lighter fractions are more “volatile”, meaning that they go from a liquid to a gas more easily than the heavier fractions. This is what gives fresh oil its stink. Volatilization is temperature-dependent: the hotter the day, the more it volatizes. Immediately after spilling into the Gulf of Mexico, the lighter fractions began volatizing. From 30% to 50% of the oil spilling into the Gulf will be lost to the air within a few days. Since the Gulf of Mexico oil is relatively light, a higher proportion is being lost than if it were, for example in Alberta.
The second good news is that because the lightest fractions are the most toxic, as oil ages it becomes less toxic.
What is left is heavier. It still floats, but not for long. The rainbow sheens seen from the air are composed of lighter fractions separating from the heavier mass that have not yet volatized. Sheens are only a few molecules thick: thinner than the diameter off a human hair. Every sheen seen means a heavier mass of oil is lurking unseen.
A “slick” is a layer of oil usually a few millimetres thick. It is more difficult to see from the air and contains a much larger volume of oil per unit of surface area than a sheen. Since it is heavier and has lost much of its lighter fraction, it rides lower in the water. Seawater sloshes over it, making it impossible to see from the air, although infrared sensors can still pick it up.
As it is churned by the choppy seas, water molecules get into the oil, changing its physical appearance and chemical properties. It becomes weathered. It begins to look like chocolate mousse (though far less appetizing), and clean-up specialists call it that. Mousse doesn’t form slicks as readily; it tends to coagulate into “pans”, flat blobs several centimetres thick that ride under the surface of the water, although it still gives off sheens as the lighter fractions continue to volatize. Weathered oil and mousse become progressively harder to contain in booms and to clean up.
Although pans are even heavier than slicks, they still float—at first. The churning of the waves mixes dirt, phytoplankton, and other debris into the mousse until it finally becomes heavier than water. Then it sinks to the bottom.
Clean-up companies often hasten mousse-formation by adding detergent. This is like putting dish detergent into a sink full of greasy dishes. It emulsifies the oil, making it miscible in water, so you can empty it down the sink and have clean dishes. But the oil is still somewhere. After dish-washing, it is in the sewer system. In the Gulf of Mexico, it is on the sea-bottom.
The marine life that doesn’t live suspended in the water lives on the sea bottom. Many sea-bottom organisms, from crabs to corals, have eggs or life stages that live suspended in the water, and these are food for others, including the fish and shrimp that make up the bulk of the huge commercial fisheries of the Gulf of Mexico. Oil sinking to the sea-bottom, through its toxic and physical properties (such as smothering) will continue to kill marine animals and plants and affect productivity of the whole marine ecosystem for years.
Sunken oil is rarely visible to scuba divers or to cameras in remotely-operated vehicles that clean-up crews use to look for it. It gets covered quickly with sand and debris. Eventually it forms a sort of pavement.
When slicks and pans land on a beach, they look like chocolate-brown pancakes. Pans range in size from a loony to the shadow of a Volkswagen bus; a slick can cover many kilometres of shoreline. On a flat, sandy beach, pans are easy to pick them up. Clean-up people can roll them up and put them into a garbage bag…if they get there quickly enough.
Mixed in with the oil on the beaches are the dead and dying birds that have floated in with it. They represent only a fraction of the oiled birds. Depending on circumstances, more may die and sink at sea, and others, less heavily oiled, fly off to die in some coastal thicket. Wildlife management authorities know that, on a percentage basis, it is useless to try to clean oiled birds. So few are found alive in a condition that they can be cleaned, and so few of them survive anyway, that public funds would be ill-spent to clean them. The Canadian Wildlife Service, for example, does not provide staff for oiled-bird cleaning.
Wave action breaks up the slicks and pans on the beach into smaller and smaller blobs and covers them with sand. Within a couple of days, a heavily oiled beach can look clean. In one spill I worked on, a woman on our staff had white rubber boots and we sent her onto the beaches to do a “white boot test”. If she walked across a beach and the boots stayed clean, there was no oil. But if there was invisible oil, it turned her boots black.
In a coastal marsh, which is most of the shoreline of the northern Gulf of Mexico, instead of lying there in flat blobs, the oil coats the vegetation and gets worked into the mud. There is no hope of cleaning up much of the oil in marshes. Clean-up crews can use absorbent material, but not without physically damaging the plants they are trying to protect.
Rocky shorelines are a different matter, but these are few in the Gulf of Mexico.
Like the sunken oil on the sea bottom, the heavy pans and slicks of oil on beaches and marshes eventually become a more or less permanent pavement buried in the mud.
The last good news is that the environment is amazingly resilient. Besides the physical action of waves and grinding with sand, chemical processes including photolysis (from sunlight) and oxidation continue to break down the oil. Within weeks or a few months, the pavement-like layers, although continuing to have a physical effect, will have lost their most toxic components. Plants can put roots through them, worms can tunnel into them, and crabs can walk across them. Certain microbes can consume oil, and oil-eating bacteria populations flourish. They feed other organisms. On a hot day, oil still becomes viscous and can continue to generate sheens and oil birds and other shoreline life for years. These effects diminish gradually. After a few years for a moderate-sized spill, few traces of oil can be seen on wildlife, in beach sands, or on marsh plants. Although the beaches and marshes look clean, plant and animal productivity may be depressed for years and oil in the sediments can still be detected with chemical analysis more or less indefinitely.
With luck, by the time that Whooping Cranes leave Wood Buffalo National Park and arrive back on the coast of the Gulf of Mexico next fall, they will not encounter any oil.
References
I published a series of Environment Canada technical reports and conference presentations on the fate and effects of the Nestucca (name of barge) oil spill that occurred at Grays Harbour, Washington, in late December 1988, drifted north, and oiled about 180 km of beaches on the west coast of Vancouver Island. We (Environment Canada) took the polluter to court and won a a settlement of about $3.5 million for clean-up costs and $6.5 million for environmental damages, which were used primarily to restore seabird population on Langara Island by eliminating the introduced rats that had decimated the nesting birds. Please contact me if you would like references.
