Oil in the sea

Issues in Science and Technology, Fall 2003 by Rabalais, Nancy

Nonpoint pollution, not oil spills, is the largest source, and reducing it will require coordinated efforts on a number of fronts.

"When it rains, it pours"-or so a motorist caught in a sudden storm might think while sliding into another vehicle. It is not merely the reduced visibility and the frenetic behavior of drivers in the rain that foster such mishaps; the streets also are slicker just after the rain begins to fall. Why? Because the oil and grease that are dripped, spewed, or otherwise inadvertently deposited by motor vehicles onto roadways are among the first materials to be lifted off by the rain, thereby literally lubricating the surface. Nor do matters end with making life miserable for motorists. The oil and grease washed off roads will most likely run into storm sewers and be discharged into the nearest body of water. From there, the oily materials often are carried to the sea, where they can cause a host of environmental problems.

These events on a rainy day in the city illustrate an important but often overlooked route by which petroleum finds its way into coastal waters. Shutting down this and other routes presents a pressing challenge. True, the nation is doing a better job than ever of keeping oil out of the marine environment. But much work remains. We need to better understand the various pathways by which oil gets into the environment, how it behaves when it gets there, what effects it has on living organisms, and, perhaps most important, what steps can be taken to further reduce the amount of petroleum that enters the nation's and the world's oceans.

Sources and problems

Approximately 75 million gallons of petroleum find their way into North America's oceans each year, according to Oil in the Sea III: Inputs, Fates, and Effects, a report issued in 2002 by the National Research Council (NRC). About 62 percent of the total-roughly 47 million gallons per year-derives naturally from seepages out of the ocean floor. The rest comes from human activities.

Contrary to common belief, the bulk of human-related inputs is not due to large-scale spills and accidents that occur during the transport of crude oil or petroleum products. Indeed, these types of releases account for only about 10 percent of the oil that reaches the sea as a result of human activity. The other 90 percent comes in the form of chronic low-level releases associated with the extraction and consumption of petroleum. Within this category, the biggest problem is nonpoint source pollution. Rivers and streams that receive runoff from a variety of landbased activities deliver roughly 16 million gallons of oil to North American coastal waters each year, more than half of the total anthropogenic load. The loads are most obvious in watersheds that drain heavily populated areas. Other sources of oil that turns up in the marine environment include jettisoned aircraft fuel, marine recreational vehicles, and operational discharges, such as cargo washings and releases from petroleum extraction.

There is at least some good news. Less oil is now entering the oceans as compared to the levels found in a previous NRC report issued in 1985. Some of this change may be attributable to differences between methodologies used in the two reports, but some decreases are due to improved regulations regarding how oil is produced and shipped. Spills from vessels in North American waters from 1990 through 1999 were down by nearly two-thirds compared to the prior decade. There also has been a dramatic decline in the amount of oil released into the environment during exploration for and production of petroleum and natural gas. Still, the recent NRC report concludes that despite such progress, the damage from oil in the marine environment is considerably more pervasive and longer-term than was previously understood.

Oil in the sea, whether from catastrophic spills or chronic releases, poses a range of environmental problems. Major spills receive considerable public attention because of the obvious attendant environmental damage, including oil-coated shorelines and dead or moribund wildlife, especially among seabirds and marine mammals. The largest oil spill in U.S. waters occurred on March 24, 1989, when the tanker Exxon Valdez, en route from Valdez, Alaska, to Los Angeles, California, ran aground on Bligh Reef in Prince William Sound, Alaska. Within six hours of the grounding, the ship spilled approximately 10.9 million gallons of crude oil, which would eventually affect more than 1,100 miles of coastline. Large numbers of animals were killed directly, including an estimated 900 bald eagles, 250,000 seabirds, 2,800 sea otters, and 300 harbor seals.

Oil pollution also can have more subtle biological effects, caused by the toxicity of many of the compounds contained in petroleum or by the toxicity of compounds that form as the petroleum degrades over time. These effects may be of short duration and limited impact, or they may span long periods and affect entire populations or communities of organisms, depending on the timing and duration of the spill and the numbers and types of organisms exposed to the oil.