Friday, 11 February 2011

Exxon Valdez oil spill

The oil slick (blue areas) eventually extended 470 miles southwest from Bligh Reef. The spill area eventually totaled 11,000 square miles. (Source: Exxon Valdez Oil Spill Trustee Council)
The oil slick (blue areas) eventually extended 470 miles southwest from Bligh Reef. The spill area eventually totaled 11,000 square miles. (Source: Exxon Valdez Oil Spill Trustee Council)
This article has been reviewed by the following Topic Editor: Peter Saundry
Read about the BP Deepwater Horizon oil spill in the Gulf of Mexico.


On March 24, 1989, the tanker Exxon Valdez, en route from Valdez, Alaska to Los Angeles, California, ran aground on Bligh Reef in Prince William Sound, Alaska. The vessel was traveling outside normal shipping lanes in an attempt to avoid ice. Within six hours of the grounding, the Exxon Valdez spilled approximately 10.9 million gallons of its 53 million gallon cargo of Prudhoe Baycrude oil. Eight of the eleven tanks on board were damaged. The oil would eventually impact over 1,100 miles of non-continuous coastline in Alaska, making the Exxon Valdez the largest oil spill to date in U.S. waters.
The response to the Exxon Valdez involved more personnel and equipment over a longer period of time than did any other spill in U.S. history. Logistical problems in providing fuel, meals, berthing, response equipment, waste management and other resources were one of the largest challenges to response management. At the height of the response, more than 11,000 personnel, 1,400 vessels and 85 aircraft were involved in the cleanup.
The Exxon Valdez aground on Bligh Reef. (Source: NOAA)
The Exxon Valdez aground on Bligh Reef. (Source: NOAA)
Shoreline cleanup began in April of 1989 and continued until September of 1989 for the first year of the response. The response effort continued in 1990 and 1991 with cleanup in the summer months, and limited shoreline monitoring in the winter months. Fate and effects monitoring by state and Federal agencies are ongoing.
The images that the world saw on television and descriptions they heard on the radio that spring were of heavily oiled shorelines, dead and dying wildlife, and thousands of workers mobilized to clean beaches. These images reflected what many people felt was a severe environmental insult to a relatively pristine, ecologically important area that was home to many species of wildlife endangered elsewhere. In the weeks and months that followed, the oil spread over a wide area inPrince William Sound and beyond, resulting in an unprecedented response and cleanup—in fact, the largest oil spill cleanup ever mobilized; however, the scale of this spill will likely be eclipsed by the 2010 Deepwater Horizon oil spill originating in the bathypelagic zone of the Gulf of Mexico. Many local, state, federal, and private agencies and groups took part in the effort. Even today, scientists continue to study the affected shorelines to understand how an ecosystem like Prince William Sound responds to, and recovers from, an incident like the Exxon Valdez oil spill.

Events leading up to the spill

The Exxon Valdez departed from the Trans Alaska Pipeline terminal at 9:12 pm, March 23, 1989. William Murphy, an expert ship's pilot hired to maneuver the 986-foot vessel through the Valdez Narrows, was in control of the wheelhouse. At his side was the captain of the vessel, Joe Hazelwood. Helmsman Harry Claar was steering. After passing through Valdez Narrows, pilot Murphy left the vessel and Captain Hazelwood took over the wheelhouse. The Exxon Valdez encountered icebergs in the shipping lanes and Captain Hazelwood ordered Claar to take the Exxon Valdez out of the shipping lanes to go around the ice. He then handed over control of the wheelhouse to Third Mate Gregory Cousins with precise instructions to turn back into the shipping lanes when the tanker reached a certain point. At that time, Claar was replaced by Helmsman Robert Kagan. For reasons that remain unclear, Cousins and Kagan failed to make the turn back into the shipping lanes and the ship ran aground on Bligh Reef at 12:04 a.m., March 24, 1989. Captain Hazelwood was in his quarters at the time.
The National Transportation Safety Board investigated the accident and determined five probable causes of the grounding: (1) The third mate failed to properly maneuver the vessel, possibly due to fatigue and excessive workload; (2) the master failed to provide a proper navigation watch, possibly due to impairment from alcohol; (3) Exxon Shipping Company failed to supervise the master and provide a rested and sufficient crew for the Exxon Valdez; (4) the U.S. Coast Guard failed to provide an effective vessel traffic system; and (5) effective pilot and escort services were lacking.

The behavior of the oil

The oil slick (blue areas) eventually extended 470 miles southwest from Bligh Reef. The spill area eventually totaled 11,000 square miles. (Source: Exxon Valdez Oil Spill Trustee Council)
The oil slick (blue areas) eventually extended 470 miles southwest from Bligh Reef. The spill area eventually totaled 11,000 square miles. (Source: Exxon Valdez Oil Spill Trustee Council)
Prudhoe Bay crude oil has an API gravity of 27.0, and a pour point of 0 degreesCelcius. The bulk of the oil spilled from the Exxon Valdez was released within 6 hours of the ship's grounding. The general trend of the oil was south and west from the point of origin. For the first few days after the spill, most of the oil was in a large concentrated patch near Bligh Island. On March 26, a storm, which generated winds of over 70 mph in Prince William Sound, weathered much of the oil, changing it into mousse and tarballs, and distributed it over a large area. By March 30, the oil extended 90 miles from the spill site. Ultimately, from Bligh Reef, the spill stretched 470 miles southwest to the village of Chignik on the Alaska Peninsula. Approximately 1,300 miles of shoreline were oiled. 200 miles were heavily or moderately oiled (obvious impact); 1,100 miles were lightly or very lightly oiled (light sheen or occasional tarballs). The spill region contains more than 9,000 miles of shoreline.
In addition to the storm of March 26, the spill occurred at a time of year when the springtidal fluctuations were nearly 18 feet. This tended to deposit the oil onto shorelines above the normal zone of wave action. The diversity in shoreline types in the affected areas led to varied oiling conditions. In some cases, oil was present on sheer rock faces making access and cleanup difficult, or rocky beaches with grain size anywhere from coarse sand to boulders, where the oil could percolate to a sub-surface level. The spill affected both sheltered and exposed (to high wave/weather action) shorelines. Once oil landed on a shoreline it could be floated off at the next high tide, carried to and deposited in a different location, making the tracking of oil migration and shoreline impact very difficult. This migration ended by mid-summer 1989, and the remaining cleanup dealt with oiled shorelines, rather than oil in the water.
Cleanup operations continued during the summer months of 1990 and 1991. By 1990, surface oil, where it existed, had become significantly weathered. Sub-surface oil, on the other hand, was in many cases much less weathered and still in a liquid state. The liquid sub-surface oil could give off a sheen when disturbed. Cleanup in 1991 concentrated on the remaining reduced quantities of surface and sub-surface oil.

Countermeasures and Mitigation

Control of the oil spill at sea

The Alyeska Pipeline Service Company was immediately notified of the incident and sent a tug to the site to assist in stabilizing the vessel. At the time of the incident, the Alyeska spill response barge was out of service being re-outfitted. It arrived on scene by 1500 on 24 March. Alyeska was overwhelmed by the magnitude of the incident; by March 25, Exxon had assumed full responsibility for the spill and cleanup effort.
The Exxon Valdez surrounded by a containment boom. (Source: Exxon Valdez Oil Spill Trustee Council)
The Exxon Valdez surrounded by a containment boom. (Source: Exxon Valdez Oil Spill Trustee Council)
Deployment of boom around the vessel was complete within 35 hours of the grounding. Exxon conducted successful dispersant test applications on March 25 and 26 and was granted permission on March 26 to apply dispersants to the oil slick. Due to the large storm that began the evening of March 26, much of the oil turned into mousse. As dispersants aren't generally able to dissipate oil in the form of mousse, it was no longer practical to use dispersants on floating oil during this response.
On the evening of March 25, a test in-situ burn of oil on water was conducted. Approximately 15,000 to 30,000 gallons of oil were collected using 3M Fire Boom towed behind two fishing vessels in a U-shaped configuration, and ignited. The oil burned for a total of 75 minutes and was reduced to approximately 300 gallons of residue that could be collected easily. It was estimated that the efficiency of this test burn was 98 percent or better. Again, continued in-situ burning was not possible because of the change in the oil's state after the storm of March 26.
Five dispersant trials took place between March 25 and March 28, but by March 29 the Regional Response Team (RRT) decided that dispersants were no longer feasible. Because there was not enough equipment to protect all the shorelines that could be impacted, Federal, state and local agencies collaborated to establish shoreline protection priorities. The agencies decided that fish hatcheries and salmon streams had the highest priority; accordingly, containment booms were deployed to protect these areas. Five fish hatcheries in Prince William Sound and two in the Gulf of Alaska were boomed, with the largest amount of boom deployed at the Sawmill Bay hatchery in Prince William Sound.
At the height of containment efforts, it is estimated that a total of 100 miles of boom was deployed. Almost all the types of boom available on the market were used and tested during the spill response.
Due to the size of the spill, it was necessary to employ inexperienced workers to deploy and tend booms, and this led to some boom being incorrectly used or handled, and sometimes damaged. Some boom sank because of improper deployment, infrequent tending, or leakage and/or inadequacy in the buoyancy system. Other problems included fabric tears in boom due to debris, and tearing at anchorage points from wave action. In some cases, ballast chains were ripped off during boom recovery if the boom was lifted by the chain. One estimate suggests that 50 percent of the damage to larger boom came during boom recovery. For self-inflating booms, it was important to keep the inflation valves above the water during deployment so that the boom did not become filled with water and have to be replaced.
Aerial surveillance was used to direct the deployment of booms and skimmers for open water oil recovery. Visual overflight observations as well as ultraviolet/infrared (UV/IR) surveys were used by the USCG and Exxon to track the floating oil. Satellite imagery was also tested as a method to track oil but was not very useful because of the infrequency of satellite passes over Prince William Sound (every 7 to 8 days), cloud cover, and lengthy turn around time for results.
The primary means of open water oil recovery was with skimmers. In general, most skimmers became less effective once the oil had spread, emulsified and mixed with debris. To save time, it was most practical to keep skimmer offloading equipment and oil storage barges near the skimmers. The most used skimmers during the response were the Marco sorbent lifting-belt skimmers that were supplied by the U.S. Navy. Once oil became viscous, the sorbent part of the skimmer was removed and the conveyor belt alone was sufficient to pull the oil up the ramp. The pump that came with the skimmer had difficulty offloading viscous oil, so that other vacuum equipment was used to unload the collected oil. The Marco skimmers were generally not used close to shore because they draw between three and four feet. In general, the paddle belt and rope mop skimmers were the most useful for recovery of oil from the shoreline. The skimmers were placed on self-propelled barges with a shallow draft.
Sorbents were used to recover oil in cases where mechanical means were less practical. The drawback to sorbents was that they were labor intensive and generated additional solid waste. Sorbent boom was used to collect sheen between primary and secondary layers of offshore boom, and to collect sheen released from the beach during tidal flooding. Pompoms were useful for picking up small amounts of weathered oil. Towing of sorbent boom in a zigzag or circular fashion behind a boat was used to collect oil and was more efficient than towing the boom in a straight line. Sorbent booms made of rolled pads were more effective than booms made of individual particles because these absorbed less water and were stronger, and did not break into many small particles if they came apart.
Early on in the response, storage space for recovered oil was in short supply. To combat the storage space problem, water was decanted from skimmers or tanks into a boomed area before offloading. As a result, the remaining viscous oil mixture was difficult to offload, the process sometimes taking up to 6 to 8 hours. High-capacity skimmer offloading pumps, in particular grain pumps, were the most useful in transferring viscous oil.
The oil remaining on the Exxon Valdez, was completely offloaded by the end of the first week in April 1989. After offloading operations were completed, the tanker was towed to a location 25 miles from Naked Island in Prince William Sound for temporary repairs. Later in the summer of 1989, the vessel was brought to California for further repairs.

Shoreline treatment

Shoreline assessment was a prerequisite for the implementation of any beach cleanup. Assessment provided geomorphological, biological, archaeological and oiling information that was used for the development of site specific treatment strategies. Cleanup operations were scheduled around specific activities such as seal haulout activity, seal pupping, eagle nesting, fish spawning, fishing seasons, and other significant events as much as possible.
Shoreline treatment from the Exxon Valdez spill. (Source: Exxon Valdez Oil Spill Trustee Council)
Shoreline treatment from the Exxon Valdez spill. (Source: Exxon Valdez Oil Spill Trustee Council)
In 1989, hoses spraying seawater were used to flush oil from shorelines. The released oil was then trapped with offshore boom, and removed using skimmers, vacuum trucks (useful for thick layers of oil) and boom (sorbent, snare, pompoms). For hard to reach areas, or locations with weathered oil, heated seawater was used to flush oil from the shoreline.
Converted vessels and barges were used for beach washing operations. It would take several days to outfit a conventional barge with the equipment needed to heat and pump the water. Smaller vessels that were used for beach washing early in the spill were re-outfitted for bioremediation later in the response.
Along with the large-scale beach washing, manual cleanup, raking and tilling the beaches, oily debris pickup, enhanced bioremediation and spot washing were used to cleanup the oil. In some locations, oil was thick enough to be picked up with shovels and buckets. In addition, mechanical methods were used on a few sites, including the use of bulldozers to relocate or remove the contaminated beach surfaces. Mechanical rock washing machines, which were manufactured for the spill, were not used to clean contaminated rocks and return them to the beach.
Oiled storm berm was mechanically relocated in some cases so that these areas, which normally would not receive much wave action, would be more exposed and cleaned by natural processes. If the oiling in the berm was significant or persistent it was tilled to free the oil or washed to optimize the cleaning.
Recommendations were made to restrict the movement of berm to the upper third of the beach to ensure its return to the original location.
Beach washing. (Source: Exxon Valdez Oil Spill Trustee Council)
Beach washing. (Source: Exxon Valdez Oil Spill Trustee Council)
Beach applications of dispersants were tried in several locations. Corexit 7664 was applied on Ingot Island, followed by a warm water wash. No significant change in oil cover or the physical state of the oil was observed as a result of the treatment. Some ecological impacts were observed in the treated areas. It appeared that the effects were largely due to the intensive washing more than to the use of Corexit 7664, and were evident in intertidal epibenthic macrobiota.
In addition, the dispersant BP1100X was applied to a test area on Knight Island.Toxicology studies indicated that the upper and lower intertidal biota were different from pre-application communities the day after dispersant application, and returned to pre-treatment levels after seven days.
In May of 1989, the U.S. Environmental Protection Agency (EPA) and Exxon conductedbioremediation trials at two test sites on Knight Island in Prince William Sound. On the basis of these tests and other trials later in the summer, Exxon recommended the use of the bioremediation enhancement agents, Inipol (Inipol EAP22—manufactured by Elf Aquitaine of France) and Customblen (Customblen 28-8-0 —manufactured by Sierra Chemicals of California), and subsequently treated over 70 miles of shoreline in Prince William Sound with these agents.
Winter monitoring of the effects of bioremediation consisted of surveys of more than 20 beaches in Prince William Sound and theGulf of Alaska. These studies determined that oil degradation had been enhanced on the shorelines monitored, but some debate existed over whether bioremediation was solely, or even largely, responsible.
Cleanup operations in 1989 ceased by the end of September. All parties involved in the response agreed that continuation of cleanup into the Alaskan winter would jeopardize the safety of cleanup crews. In addition, it was speculated that the winter storms in Alaska could significantly remove oil from shorelines, including sub-surface oil. By the end of the 1989 cleanup, more than 25,000 tons of oiled waste and several hundred thousand barrels of oil/liquid waste were collected and disposed of in landfills.
Cleanup in 1990 began in April and ended in September. Surveys in the spring of 1990 showed that oiling conditions had been reduced or changed over the winter. Surface oil in 1990 was significantly weathered but sub-surface oil was relatively fresh in some locations. Cleanup techniques in 1990 focused more on manual methods of treatment such as hand wiping and spot washing as well as bioremediation. Mechanical equipment was used on a few sites.
Bioremediation was more extensive in 1990, with 378 of the 587 shoreline segments treated that year receiving bioremediation application. In general, Inipol was applied in cases where surface oiling existed and Customblen slow release pellets were preferred for treating beaches with sub-surface oiling. Generally, beaches were given one to three treatments over several months. Concern over the possible toxicity of Inipol led to recommendations for application of only Customblen on some sites.
By the spring of 1991, the scope of the cleanup effort was greatly reduced. Manual cleanup, bioremediation, and very limited use of mechanical equipment were employed. Cleanup took place from May of 1991 through July of 1991.
An important observation that resulted from the Exxon Valdez oil spill was that natural cleaning processes, on both sheltered and exposed beaches, were in many cases very effective at degrading oil. It took longer for some sections of shoreline to recover from some of the invasive cleaning methods (hot water flushing in particular) than from the oiling itself.

Economic impacts

The State of Alaska funded a several studies of the short term economic impact of the Exxon Valdez oil spill.
Recreational fishing in Alaska. (Source: Exxon Valdez Oil Spill Trustee Council)
Recreational fishing in Alaska. (Source: Exxon Valdez Oil Spill Trustee Council)
  1. Recreational Sport Fishing Losses. This loss was estimated based on the impacts of the spill on sport fishing activity. One must consider the impact on the number of anglers, the number of sport fishing trips, the areas fished, the species fished for, and the length of these trips. For 1989 the loss was estimated to be between $0 and $580 million dollars; for 1990 the range was $3.6 million $50.5 million dollars.
  2. Tourism Losses. The spill caused both negative and positive effects. The major negative effects were:
    1. Decreased resident and non-resident vacation/pleasure visitor traffic in the spill-affected areas due to lack of available visitor services (accommodations, charter boats, air taxis).
    2. Severe labor shortage in the visitor industry throughout the state due to traditional service industry workers seeking high-paying spill clean-up jobs.
    3. Fifty-nine percent of businesses in the most affected areas reported spill-related cancellations and 16% reported business was less than expected due to the spill.
The principle positive impact was strong spill-related business in some areas and in certain businesses such as hotels, taxis, car/RV rentals and boat charters.
  1. Existence value. Economists tried to estimate the damage to so-called non-use or existence value of the Prince William Sound region in the wake of the spill. This is an attempt top measure what cannot be observed in the market: the value to the public of a pristine Prince William Sound. They estimated existence value using contingent valuation, a survey approach designed to create the missing market for public goods by determining what people would be willing to pay (WTP) for specified changes in the quantity or quality of such goods or, more rarely, what they would be willing to accept (WTA) in compensation for well-specified degradations in the provision of these goods. The results suggest an aggrragete loss of $4.9 to $7.2 billion dollars. In effect, these amounts reflect the public's willingness to pay to prevent another Exxon Valdeztype oil spill given the scenario posed.
  2. Replacement costs of birds and mammals. These costs include the relocation, replacement and rehabilitation for some of the shorebirds, seabirds and the marine and terrestrial mammals that may have suffered injury or were destroyed in theExxon Valdez oil spill. The values range from $20,000 to $300,000 dollars per marine mammal (sea otters, whales, seal lions, seals), $125 to $500 dollars per terrestrial animal (bears, river otters, mink, deer), and $170 to $6,000 dollars for seabirds and eagles.

How much oil remains?

Based on the areas that were studied in the aftermath of the spill, scientists made estimates of the ultimate fate of the oil. A 2001 National Oceanic and Atmospheric Administration (NOAA) study surveyed 96 sites along 8,000 miles of coastline.
A pit dug on a Prince William Sound beach in 2001 revealing oil in the sediments. (Source: NOAA)
A pit dug on a Prince William Sound beach in 2001 revealing oil in the sediments. (Source: NOAA)
The survey distinguished between surface and buried oil. Buried or subsurface oil is of greater concern than surface oil. Subsurface oil can remain dormant for many years before being dispersed and is more liquid, still toxic, and may become biologically available. A disturbance event such as burrowing animals or a severe storm reworks the beach and can reintroduce unweathered oil into the water. Results of the summer shoreline survey showed that the oil remaining on the surface of beaches in Prince William Sound is weathered and mostly hardened into an asphalt-like layer. The toxic components of this type of surface oil are not as readily available to biota, although some softer forms do cause sheens in tide pools.
The survey indicates a total area of approximately 20 acres of shoreline in Prince William Sound are still contaminated with oil. Oil was found at 58 percent of the 91 sites assessed and is estimated to have the linear equivalent of 5.8 km of contaminated shoreline.
In addition to the estimated area of remaining oiled beach, several other important points were evident:
  1. Surface oil was determined to be not a good indicator of subsurface oil.
  2. Twenty subsurface pits were classified as heavily oiled. Oil saturated all of the interstitial spaces and was extremely repugnant. These “worst case” pits exhibited an oil mixture that resembled oil encountered in 1989 a few weeks after the spill—highly odiferous, lightly weathered, and very fluid.
  3. Subsurface oil was also found at a lower tide height than expected (between 0 and 6 feet), in contrast to the surface oil, which was found mostly at the highest levels of the beach. This is significant, because the pits with the most oil were found low in the intertidal zone, closest to the zone of biological production, and indicate that the survey estimates are conservative at best.

Ecosystem response to the spill

Recovery is a very difficult term to define and measure for a complex ecosystem such as Prince William Sound. If you ask a fisherman from Kodiak Island, a villager from the town of Valdez, an Exxon engineer, or a NOAA biologist, you are likely to receive such different answers that you may wonder if they heard the same question. In particular, disagreements exist between Exxon and government-funded scientists, and unknowns persist, especially in understanding how multiple processes combine to drive observed dynamics.
Despite this, there are some things known with a high degree of certainty: oil persisted beyond a decade in surprising amounts and in toxic forms, was sufficiently bioavailable to induce chronic biological exposures, and had long-term impacts at the population level. Three major pathways of long-term impacts emerge: (1) chronic persistence of oil, biological exposures, and population impacts to species closely associated with shallow sediments; (2) delayed population impacts of sublethal doses compromising health, growth, and reproduction; and (3) indirect effects of trophic and interaction cascades, all of which transmit impacts well beyond the acute-phase mortality.

Acute Mortality

Sea birds killed by the Exxon Valdez oil spill. (Source: NOAA)
Sea birds killed by the Exxon Valdez oil spill. (Source: NOAA)
Marine mammals and seabirds are at great risk from floating oil because they have routine contact with the sea surface. Oiling of fur or feathers causes loss of insulating capacity and can lead to death from hypothermia, smothering, drowning, and ingestion of toxic hydrocabons. Scientists estimate mass mortalities of 1000 to 2800 sea otters, 302 harbor seals, and unprecedented numbers of seabird deaths estimated at 250,000 in the days immediately after the oil spill. Mass mortality also occurred amongmacroalgae and benthic invertebrates on oiled shores from a combination of chemicaltoxicity, smothering, and physical displacement from the habitat by pressurized wash-water applied after the spill.

Long-term impacts

The persistent nature of oil in sediments produce chronic, long-term exposure risks from some species. For example, chronic exposures for years after the spill to oil persisting in sedimentary refuges were evident from biomarkers in fish, sea otters, and seaducks intimately associated with sediments for egg laying or foraging. These chronic exposures enhanced mortality for years.
Indirect effects can be as important as direct exposure. Cascading indirect effects are delayed in operation because they are mediated through changes in an intermediary. Perhaps the two generally most influential types of indirect interactions are: (1) trophic cascades in which predators reduce abundance of their prey, which in turn releases the prey’s food species from control; and (2) provision of biogenic habitat by organisms that serve as or create important physical structure in the environment.
A healthy stand of rockweed (Fucus gardneri) growing on a boulder in Prince William Sound. (Source: NOAA)
A healthy stand of rockweed (Fucus gardneri) growing on a boulder in Prince William Sound. (Source: NOAA)
Scientists have found that indirect interactions lengthened the recovery process on rocky shorelines for a decade or more. Dramatic initial loss of cover by the most important biogenic habitat provider, the rockweed Fucus gardneri, triggered a cascade of indirect impacts. Freeing of space on the rocks and the losses of important grazing (limpets and periwinkles) and predatory (whelks) gastropods combined to promote initial blooms of ephemeral green algae in 1989 and 1990 and an opportunistic barnacle, Chthamalus dalli, in 1991. Absence of structural algal canopy led to declines in associated invertebrates and inhibited recovery of Fucus itself, whose recruits avoid desiccation under the protective cover of the adult plants. Those Fucusplants that subsequently settled on tests of Chthamalus dalli became dislodged during storms because of the structural instability of the attachment of this opportunistic barnacle. After apparent recovery of Fucus, previously oiled shores exhibited another mass rockweed mortality in 1994, a cyclic instability probably caused by simultaneous senility of a single-aged stand. The importance of indirect interactions in rocky shore communities is well established, and the general sequence of succession on rocky intertidal shores extending over a decade after the Exxon Valdez oil spill closely resembles the dynamics after the Torrey Canyon oil spill in the UK.

State of recovery

The Exxon Valdez Oil Spill Trustee Council published a study in 2004 to assess the state of the resources injured by the spill. Fifteen years after the Exxon Valdez oil spill, it is clear that some fish and wildlife species injured by the spill have not fully recovered. It is less clear, however, what role oil plays in the inability of some populations to bounce back. An ecosystem is dynamic — ever changing — and continues its natural cycles and fluctuations at the same time that it struggles with the impacts of spilled oil. As time passes, separating natural change from oil-spill impacts becomes more and more difficult.
The Trustee Council recognizes 30 resources or species as injured by the spill. Depending on their status as of 2002, these have been placed in one of five categories:
Not Recovering
These resources are showing little or no clear improvement since spill injuries occurred: Common loon Cormorants (3 species), Harbor seal, Harlequin duck, Pacific herring, Pigeon guillemot
Recovery unknown
Limited data on life history or extent of injury is available. Current research is either inconclusive or not complete: Cutthroat trout, Dolly Varden, Kittlitz’s murrelet, Rockfish Subtidal communities
Sockeye salmon (Oncorhynchus nerka). (Source: NOAA)
Sockeye salmon (Oncorhynchus nerka). (Source: NOAA)
Recovery objectives have been met: Archaeological resources, Bald eagle, Black oystercatcher, Common murre, Pink salmon, River otter, Sockeye salmon
Clams, Wilderness Areas, Intertidal communities, Killer whale (AB pod), Marbled murrelet, Mussels, Sea otter, Sediments
Human uses
Human services that depend on natural resources were also injured by the spill. These services are each categorized as “recovering” until the resources they depend on are fully recovered: Commercial fishing, Passive use, Recreation and tourism, Subsistence
Prior to the Exxon Valdez oil spill, there was no baseline date available for the abundant number of species existing in Prince William Sound. Because of this lack of data, numbers of oil spill-related casualties and recovery rates have been difficult to determine.

Legal responsibility of ExxonMobil

The settlement among the State of Alaska, the U.S. government and Exxon was approved by the U.S. District Court on Oct. 9, 1991. It resolved various criminal charges against Exxon as well as civil claims brought by the federal and state governments for recovery of natural resource damages resulting from the oil spill. The settlement was comprised of criminal and civil settlements with Exxon, as well as a civil settlement with Alyeska Pipeline Service Company.

Criminal Settlement

Plea Agreement

Exxon was fined $150 million, the largest fine ever imposed for an environmental crime. The court forgave $125 million of that fine in recognition of Exxon's cooperation in cleaning up the spill and paying certain private claims. Of the remaining $25 million, $12 million went to the North American Wetlands Conservation Fund and $13 million went to the national Victims of Crime Fund.

Criminal Restitution

As restitution for the injuries caused to the fish, wildlife, and lands of the spill region, Exxon agreed to pay $100 million. This money was divided evenly between the federal and state governments.

Civil Settlement

Exxon agreed to pay $900 million in ten annual installments. The final payment was received in Sept. 2001. The settlement contains a “reopener window” between Sept. 1, 2002 and Sept. 1, 2006, during which the state and federal governments may make a claim for up to an additional $100 million. The funds must be used to restore resources that suffered a substantial loss or decline as a result of the oil spill, the injuries to which could not have been known or anticipated by the six trustees from any information in their possession or reasonably available to any of them at the time of the settlement (Sept. 25, 1991).

The response of ExxonMobil

Exxon logo.
Exxon logo.
ExxonMobil acknowledged that the Exxon Valdez oil spill was a tragic accident that the company deeply regrets. Exxon notes that company took immediate responsibility for the spill, cleaned it up, and voluntarily compensated those who claimed direct damages. ExxonMobil paid $300 million immediately and voluntarily to more than 11,000 Alaskans and businesses affected by the Valdez spill. In addition, the company paid $2.2 billion on the cleanup of Prince William Sound, staying with the cleanup from 1989 to 1992, when the State of Alaska and the U.S. Coast Guard declared the cleanup complete. And, as noted above, ExxonMobil also has paid $1 billion in settlements with the state and federal governments. That money is being used for environmental studies and conservation programs for Prince William Sound.
ExxonMobil hired its own scientists to study the impacts of the spill, and they come to different conclusions than many of the results published by government agencies and peer-reviewed academic journals. Exxon's scientists acknowledge the lingering pockets of oil in the sediments, but they argue that they do not pose a serious risk. It is their position that that there are now no species in Prince William Sound in trouble due to the impact of the 1989 oil spill, and that the data strongly support the position of a fully recovered Prince William Sound ecosystem.

Lessons learned from the spill

The scientists who monitored the oiled parts of Prince William Sound wanted to study the shoreline’s ecological recovery after an environmental disaster like the Exxon Valdez spill, and then use those lessons to better respond to future oil spills. Right now, their task is still incomplete. However, some of their findings have changed the way they think about cleaning up oil spills, and about how ecosystems respond to such disturbances. Following are some examples of what they have learned:
  1. Clean-up attempts can be more damaging than the oil itself, with impacts recurring as long as clean-up (including both chemical and physical methods) continues. Because of the pervasiveness of strong biological interactions in rocky intertidal and kelp forest communities, cascades of delayed, indirect impacts (especially of trophic cascades and biogenic habitat loss) expand the scope of injury well beyond the initial direct losses and thereby also delay recoveries.
  2. Oil that penetrates deeply into beaches can remain relatively fresh for years and can later come back to the surface and affect nearby animals. In addition, oil degrades at varying rates depending on environment, with subsurface sediments physically protected from disturbance, oxygenation, and photolysis retaining contamination by only partially weathered oil for years.
  3. Rocky rubble shores should be of high priority for protection and cleanup because oil tends to penetrate deep and weather very slowly in these habitats, prolonging the harmful effects of the oil when it leaches out.
  4. Oil effects to sea birds and mammals also are substantial (independent of means of insulation) over the long-term through interactions between natural environmental stressors and compromised health of exposed animals, through chronic toxic exposure from ingesting contaminated prey or during foraging around persistent sedimentary pools of oil, and through disruption of vital social functions (caregiving or reproduction) in socially organized species.
  5. Long-term exposure of fish embryos to weathered oil at parts per billion (ppb) concentrations has population consequences through indirect effects on growth, deformities, and behavior with long-term consequences on mortality and reproduction.
The Exxon Valdez also triggered major improvements in oil spill prevention and response planning.
  1. The U.S. Coast Guard now monitors fully-laden tankers via satellite as they pass through Valdez Narrows, cruise by Bligh Island, and exit Prince William Sound at Hinchinbrook Entrance. In 1989, the Coast Guard watched the tankers only through Valdez Narrows and Valdez Arm.
  2. Two escort vessels accompany each tanker while passing through the entire Sound. They not only watch over the tankers, but are capable of assisting them in the event of an emergency, such as a loss of power or loss of rudder control. Fifteen years ago, there was only one escort vessel through Valdez Narrows.
  3. Specially trained marine pilots, with considerable experience in Prince William Sound, board tankers from their new pilot station at Bligh Reef and are aboard the ship for 25 miles out of the 70-mile transit through the Sound. Weather criteria for safe navigation are firmly established.
  4. Congress enacted legislation requiring that all tankers in Prince William Sound be double-hulled by the year 2015. It is estimated that if the Exxon Valdez had had a double-hull structure, the amount of the spill would have been reduced by more than half. There are presently three double-hulled and twelve double-bottomed tankers moving oil through Prince William Sound. Two more Endeavor class tankers are under construction by ConocoPhillips, their expected induction into service is 2004 and 2005.
  5. Contingency planning for oil spills in Prince William Sound must now include a scenario for a spill of 12.6 million gallons. Drills are held in the Sound each year.
  6. The combined ability of skimming systems to remove oil from the water is now 10 times greater than it was in 1989, with equipment in place capable of recovering over 300,000 barrels of oil in 72 hours.
  7. Even if oil could have been skimmed up in 1989, there was no place to put the oil-water mix. Today, seven barges are available with a capacity to hold 818,000 barrels of recovered oil.
  8. There are now 40 miles of containment boom in Prince William Sound, seven times the amount available at the time of theExxon Valdez spill.
  9. Dispersants are now stockpiled for use and systems are in place to apply them from helicopters, airplanes, and boats.

Further Reading

Disclaimer: This article is taken wholly from, or contains information that was originally published by, the National Oceanic and Atmospheric Administration. Topic editors and authors for the Encyclopedia of Earth may have edited its content or added new information. The use of information from the National Oceanic and Atmospheric Administration should not be construed as support for or endorsement by that organization for any new information added by EoE personnel, or for any editing of the original content.


Cutler Cleveland (Lead Author);NOAA (Content Source);Peter Saundry (Topic Editor) "Exxon Valdez oil spill". In: Encyclopedia of Earth. Eds. Cutler J. Cleveland (Washington, D.C.: Environmental Information Coalition, National Council for Science and the Environment). [First published in the Encyclopedia of Earth June 9, 2010; Last revised Date December 20, 2010; Retrieved February 11, 2011 <>

The Author

Cutler ClevelandEditor-in-Chief The Encyclopedia of Earth Cutler J. Cleveland is the founding Editor-in-Chief of the Encyclopedia of Earth. Dr. Cleveland is currently a Professor in the Department of Geography and Environment at Boston University, with joint appointments in the Center for Energy and Environmental Studies and the Pardee Center for the Study of the Longer Range Future. He also is a Senior Fellow at the National Council for Science and the Environment in Washington D.C. Dr. Cleveland is als ... (Full Bio)