OIL AND RELATED CHEMICALS - Marine Pollution: What Everyone Needs to Know - page 65

from tankers and pipelines (about 70%), while the contribution
of offshore drilling is lower. Most spills are relatively small
and result from routine operations such as loading and dis-
charging in ports or oil terminals. The largest spills arise from
accidents (groundings and collisions) involving tankers carry-
ing large amounts of oil. Catastrophic spills that release more
than 30,000 tons of oil are relatively rare but can cause the most
serious ecological damage (primarily for sea birds and mam-
mals) and produce long-term environmental damage as well
as major economic impacts. The highly publicized 1989 spill
of the Exxon Valdez in Prince William Sound, Alaska caused
unprecedented damage to the fragile Arctic ecosystem. The
other major oil disaster in recent years is the blowout of the
mile-deep BP well Deepwater Horizon in the Gulf of Mexico,
which gushed oil continually for four months.
What happens to the oil after it is spilled?
Floating oil has its greatest effects on animals that live at the
sea surface such as birds and sea otters. Over time the lighter
components evaporate, leaving behind the heavier compo-
nents. When the oil comes into shallow water and coastal
marshes it can coat and smother the resident communities. If
it arrives on rocky shores and coats rocks, the lighter compo-
nents of the oil evaporate, leaving behind the heaviest com-
ponents and turning the oil into tar that covers the rocks but
eventually erodes away due to wave action. Under those cir-
cumstances, biological communities will return rather rapidly
since the oil is removed relatively quickly. However, when the
oil reaches soft substrates like salt marshes or beaches, it can
sink below the surface and persist for many decades.
What happened with the Exxon Valdez ?
On March 24, 1989, the Exxon Valdez oil tanker struck Bligh
Reef in Prince William Sound, Alaska and spilled 11 million