Marine Oil Spillsby Rob Goldberg,
Academy of Natural Sciences
May, 1991
Infrared image of oil spill on the Schuylkill River
Fact sheet on sources, fates and effects of marine oil spills, as well as methods of cleanup, economic impacts and methods of prevention.
Marine Oil Spills
by Rob Goldberg,
Academy of Natural Sciences
May, 1991
Infrared image of oil spill on the Schuylkill River
With the massive Persian Gulf War oil spill of January 1991 and the Exxon Valdez spill of March 1989, public attention has once again focused on this recurring environmental scourge. Are such massive oil spills more common
today, and just how damaging are these spills to the environment?
Catastrophic oil spills are nothing new. The recent Persian Gulf oil well spill, last estimated at 68 million gallons by the International Maritime Organization, may be smaller than a 1983 oil spill in the Gulf of approximately 80 million gallons that occurred during the Iran Iraq War. The largest spill yet occurred in 1979 when about 147 million gallons of oil
gushed from an offshore well platform in the Gulf of Mexico. In comparison, the Exxon Valdez tanker spill, although considered massive, totaled only about 7.5 percent of this amount or 11 million gallons.
Worldwide, the general trend may indeed be improving or, in U.S. waters, at least holding steady. Data from the Oil Spill Intelligence Report, which tracks oil spills worldwide, show a decline in
the total volume of oil spilled from 1978 through 1980 compared to 1981 through 1990, with occasional megaspills inflating the total. The number of megaspills (greater than 10 million gallons) has also decreased, with 15 reported during the first 3 years
that data were kept, and only 11 reported during the remaining 10 years. In U.S. waters, Coast Guard data on oil spills from 1970-1986 show no clear increase or decrease in the number of spills, and the volume spilled remains
relatively constant (see bar charts).
However, there are many sources of oil pollution in the ocean other than tanker and oil rig accidents, which collectively account for about 12.5 and 1.5 percent of all spilled oil worldwide, respectively, according to a 1985 report from the National Academy of Sciences. (See the pie chart for a full breakdown.) Many might be surprised to find that human activity on the land and natural seeps are significant sources of ocean oil pollution; municipal and industrial waste discharge and runoff account for about 37 percent and natural sources almost 8 percent of the total of oil spilled into
the ocean.
Oil spills from tankers, drilling operations, and natural seeps cause a concentrated input of oil into the ocean, often occurring in a very short period of time, and thus can have serious local effects on animal and plant life and on land cherished for its recreational value. The seriousness of an oil spill depends on several factors in addition to size, including the type of oil spilled, "fate" or final destination of the oil, and environmental factors such
as the weather and time of year.
Oil is actually a very complex mixture made up of hundreds of compounds, mostly hydrocarbons. Lighter portions of the oil, though more toxic than crude oil, are more likely to evaporate or otherwise dissipate before they
can reach the sensitive shoreline habitat. In fact, laboratory studies have shown that evaporation can reduce the volume of even a crude oil slick by 20-40 percent. Heavier oil that does not evaporate, although not as toxic as
refined oil, can wash onto rocky and sandy shores and there can cause serious short-term harm to shellfish and plant life. In the early stages of an
oil spill, large numbers of sea birds and mammals may be killed by coming in contact with the freshly spilled oil. For example, the National Oceanic and Atmospheric Administration estimates that the Exxon Valdez spill killed 350,000-390,000 waterfowl and 3,500-5,500 sea otters. The effects on finfish are less clear, but it appears that schools of fish may be able to swim away from the spill, although fish in ocean hatcheries, floating fish eggs, and larvae can be destroyed.
Eventually the least volatile portion of the oil forms tarry lumps that either float, sink, cover habitats of shallow water organisms, or foul beaches, but are not considered as serious an environmental threat as fresher oil. Marshes and tidal flats, however, can act as reservoirs of spilled oil, oozing liquid oil 10 years or more after a spill.
Oil spilled in the open ocean tends to cause less harm than oil that reaches or escapes near coasts, where the bulk of large marine plants, such as kelps, and their associated organisms live. For this reason the weather is a key factor for oil spills. Strong onshore winds can drive an oil spill shoreward. Wind can also can whip it into a "mousse," a difficult-to-pump emulsion that can last for weeks. On the other hand, severe weather can very effectively clean the shore, as was the case with heavy storms in Prince William Sound during the winter after the Exxon Valdez spill.
The condition of the existing wildlife is an important factor in the severity of an oil spill. For example, Prince William Sound's robust and diverse ecosystem has proved relatively resilient, and National Oceanic and Atmospheric Administration scientists predict close to full recovery within 2-4 years, according to NOAA spokesman Brian Gorman. In contrast, already-stressed ecosystems, such as in the Mediterranean, would not be likely to recover as quickly. Also, even in areas with healthy environments, an oil spill coinciding with the migration or breeding season could be devastating to populations of certain species.
There are a wide variety of techniques to cope with oil spills, but regardless of the technique, prompt action is crucial. The Congressional Office of Technology Assessment in its recent report, Coping With an Oiled Sea, says, "If important decisions, such as how to deploy mechanical equipment and whether to use dispersants, are not made within a few hours after a major spill, the spill may be beyond effective control." In the U.S., the current decentralized way of coping with major oil spills, which divides authority among the Coast Guard, U.S. EPA, other federal branches, oil companies, and local governments, can delay large-scale control of the spill.
The most common treatment techniques used in the U.S. are containment with large floating barriers and skimming with specialized ships that either vacuum the oil off the ocean or soak it up with absorbent material.
Dispersants, solvent and/or detergent chemicals that break up continuous spills into tiny droplets, are also sometimes used to prevent oil from fouling coastal habitats. Although dispersants can spare the shoreline if used correctly, they do not remove the oil and can make an acutely toxic concentration of oil available to marine life directly under the slick. "While pouring a detergent-type dispersant into the ocean is probably not lethally toxic to most organisms, using a kerosene-based dispersant in a small, shallow bay could be troublesome," says Michael Rucker of the Center for
Marine Conservation. Nonetheless, some countries such as the United Kingdom use dispersants as their first line of defense in battling oil spills.
Once the shoreline has been oiled, the best response, ironically, may be to do nothing. The National Oceanic and Atmospheric Administration recently stated that the use of high-pressure hot water to clean pools of oil in the
Exxon Valdez cleanup may have "done more environmental harm than good." An NOAA study comparing treated and untreated sites found that the hot spray displaced oil from upper areas on the shore to lower beaches
and tidal pools, where it damaged a wide variety of more sensitive marine plants and animals. Exxon and the EPA have experimented with a shoreline cleanup technique called bioremediation -- the use of bacteria to degrade
oil into harmless water, carbon dioxide, and fatty acids. Exxon's pilot project involved spraying fertilizers on 74 miles of the more than 900 miles of oiled shoreline in Alaska in the hopes of spurring growth of naturally occurring bacteria that eat oil. The fertilizers appear to have accelerated natural biodegradation of the oil without harmful side effects such as
eutrophication, toxicity to sensitive organisms, and release of untreated oil.
However, this method does require that the oil be spread evenly over the shore, such as occurs well after a spill or after the controversial hot-water spraying. Bioremediation can also be performed by introducing genetically engineered microbes into the water, as was done experimentally in the 1990 Mega Borg spill in the Gulf of Mexico.
Another experimental technique is the intentional burning of the oil, but the oil must be ignited before the most flammable portion evaporates, and the slick must be contained first. Also, the public may be opposed to the
technique on the grounds that it creates air pollution, even though evaporation creates a similar but invisible input of toxins into the air.
Whatever the technique, the present oil response system in the U.S. is not capable of recovering more than a maximum of about 10-15 percent of the oil from a Valdez or larger size of spill, according to the Office of
Technology Assessment. OTA's 1990 report Coping With an Oiled Sea concludes, "The country's ability to recover oil from large spills is inadequate."
The federal government has responded by passing the Oil Pollution Act of 1990, which requires a gradual phase-in of double hulls for large oil tankers by 2015, an on-scene spill coordinator, more comprehensive contingency planning for massive spills, more federal oversight, and suggests coordination with an newly formed industry oil spill cooperative
The cooperative, called the Marine Spill Response Corp., will focus on mechanical containment and recovery techniques, and plans to be ready to cope with oil spills of several million gallons by February 1993. "Current
industry cooperatives are very good at what they do but are limited to about 40,000 barrels [1,680,000 gallons] in sheltered areas and 25,000 barrels [1,050,000 gallons] in the open ocean," says Greg Rixon of the
Marine Spill Response Corp. "It is our hope to significantly increase the percentage of oil recovered."
Double-hulls are an important method of preventing oil spills. However, experts caution against relying on double hulls alone. The National Research Council's 1991 report, Tanker Spills: Prevention By Design, stresses the
importance of crew training, maintenance, and inspection in addition to a tanker design that minimizes spills.
Selected Sources
Coping With an Oiled Sea: An Analysis of Oil Spill Response Technologies
U.S. Congress
Office of Technology Assessment
Washington, DC, 1990.
Natural Recovery of Cold Water Marine Environments After an Oil Spill
J.M. Baker, R.B. Clark, P.F. Kingston, and R.H. Jenkins
presented at the Thirteenth Annual Arctic and Marine Oilspill
Program
Technical Seminar, June 1990.
EPA's Alaska Oil Spill Bioremediation Project
P. Hap Pritchard and Charles F. Costa.
Environ. Sci. Technol., 25(3):372-379, 1991.
Oil in the Ocean: The Short- and Long-Term Impacts of a Spill
Congressional Research Service
Library of Congress, Washington, DC, 1990.
Oil in the Sea: Inputs, Fates, and Effects
National Research Council, National Academy Press
Washington, DC, 1985.
Oil Spill Intelligence Report
Cutter Information Corp.
Arlington, MA, 14(8):5-6, 1991.
Summary of Effects of the Exxon Valdez Oil Spill on Natural Resources
and Archaeological Resources
National Oceanic and Atmospheric Administration
Washington, DC, March 1991.