Several well-documented events demonstrate that oil spills can greatly impact shorelines, plant and animal life, and the local economy. Even following intensive cleanup efforts, when the immediate problem appears to have been resolved, the question remains: How long does un-recovered oil remain in an ecosystem and what impact can it have?

Because oil is less dense than water, it floats on the water's surface. After a spill, a large boom, or barrier, is generally placed around a portion of the oil and its source. Specialized equipment, such as a vacuum, is then used to help remove oil from the water's surface, and chemical detergents are applied to help break up visible slicks. After human efforts have been exhausted, natural processes — in particular evaporation and the cleansing action of waves breaking on shore — deal with what remains.

Left to weather, all but the thickest grades of oil will evaporate. Just how fast this happens, however, depends on whether the oil is light, like gasoline, or heavier, like most crude oils. Knowing the amount and type of oil leaked in a spill, scientists can estimate how much will evaporate in a given time period.

What doesn't evaporate must be collected through cleanup activities — otherwise it remains in an ecosystem. In the case of the Valdez, between 13.2 and 26.5 million liters (3.5 and 7 million gallons) of oil was unaccounted for following cleanup efforts. Oil that coalesces and sinks can remain in the seabed sediment for years. What reaches shore can settle in sub-surface layers. Buried as it is, there is little if any opportunity for it to be naturally removed.

Animals that inhabit these spill zones may not be completely safe from contamination after the clean-up effort is over. Most toxic components in oil tend to rapidly evaporate, and thick deposits that can cover or suffocate shore animals soon break up, so immediate mortality is localized and on a small-scale. However, questions about non-lethal effects of exposure or ingestion — such as impaired reproduction, growth, or feeding — remain.

Sedentary animals — such as filter-feeding oysters, mussels, and clams — are most likely to accumulate oil components in their tissues. While these components may not be dangerous to the animals' own health, a strong and lasting odor may make them unsuitable for human consumption. What's more worrying, however, is that organisms that accumulate toxins may pass them down the food chain. This process, called bioaccumulation, has been linked to the persistent reproductive failure of harlequin ducks in Prince William Sound following the Exxon Valdez incident.

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