How Oil Breaks Down in Water

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Nature has its own chemical processes to minimize oil's impact in seawater—can human dispersant efforts measure up?

By Cassie Rodenberg

oil and water mix satellite view

Initiatives to scrub the Gulf of oil are moving forward. One method includes spraying chemicals to break up and sink globs of oil before the slick spreads to the shorelines—but using chemicals to clean up the ocean is controversial because the environmental consequences are unknown. The sea has its own way of handling the problem, but researchers worry that the water's processes won't act fast enough to save coastal shores from damage. Of course, science informs the debate. Here's what happens on a molecular level when oil hits ocean water.

As soon as oil hits water, the ocean begins its deconstruction. In fact, the marine environment handles oil much like a human body handles alcohol: destroying, metabolizing and depositing the excessive compounds —in oil's case, hydrocarbons—then transforming the compounds into safer substances, says Stanislav Patin, chairman of the Aquatic Toxicology Committee under the Russian Academy of Sciences and international expert on marine pollution.

Here's how it breaks down.

In a 10-minute span after spilling into the sea, 300 gallons of oil can spread to a radius of 160 feet and create a slick a fourth of an inch deep. After that, how far and fast the oil spreads depends upon the water's surface tension—how much the molecules in the water are attracted to one another—and the oil's thickness.

The day after it enters the water, chemicals in the oil begin to transform, both at the water's surface and farther into the water column. Trace elements lurking in water can speed or slow the process while the sun fuels the breakdown, decomposing even the most complex of oil's components over time. The warmer the water temperature and the more sun exposure, the faster the oil breaks down.

During the first few days after a spill, between 20 to 40 percent of oil's mass turns into gases, and the slick loses most of its water-soluble hydrocarbons—what's left are the more viscous compounds that slow down the oil's spread across the water.

When components of crude oil evaporate and its lighter fractions dissolve or are chemically transformed, oil clumps form. These sticky masses are found in all types of water environments, in open and coastal waters and on beaches. They have an uneven shape and can measure tenths of inches to 4 inches in length. The oily masses serve as a base for developing bacteria and one-celled algae, while invertebrates such as crustaceans, resistant to the impact of oil, use them as shelter. These clumps can exist from months to years in enclosed seas and for years in the open ocean—eventually, they degrade.

But not all oil lives and dies at the surface. Between 10 and 30 percent of the oil is absorbed by sediments and suspended materials and deposited on the bottom of the sea. This generally occurs in coastal zones and shallow waters where water mixes readily and more particles float. Usually, oil drifts in the same direction as the wind, though storms and water turbulence can speed up the spread of the oil. This movement in turn can degrade parts of the oil into separate fragments that spread far away from the initial spill. As these fragments move further offshore and into deeper areas, sediment absorption becomes a much slower process. While sediments latch onto oil components, so do filter feeders and plankton. Planktonic organisms absorb the oil/water mix and dump it at the bottom of the ocean when they excrete other metabolites. Once at the bottom, though, the decomposition rate of the oil halts almost completely because of lack of oxygen, and heavy oil fragments can be preserved inside sediments for years.

From deep-sea sediment to bobbing clumps, oil quickly loses its original properties and breaks off into hydrocarbon components, with many different chemical compositions and existing in different forms. These forms can prove toxic to marine life, but after a time—with weathering, feasting micro-organisms and solar decomposition—the water self-purifies, as intermediate compounds gradually disappear and water and carbon dioxide reform.

To speed the process, researchers use oil dispersants, specialized chemical agents, to change the physical and chemical composition of oil. Basically, these chemicals work to mix oil and water better than normally possible so that the oil sinks deeper into the water column. Burying the oil a little deeper means that surface slicks won't float toward shorelines as readily. However, mixing these chemicals into water has long been a controversial process, as they have proved toxic to some marine organisms.

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