Potential Impacts of the Deepwater Horizon Oil Spill   [back to top]  [back to main page]

Natural resources of the Gulf of Mexico (e.g., birds, sea turtles, marine mammals, fish, shellfish, plankton; and a wide variety of habitats along the shoreline and at the sea bottom, such as salt marshes and submerged aquatic vegetation) are currently being exposed to and impacted by oil from the Deepwater Horizon oil spill; as well as potentially by other materials being added to the marine environment during the response that might be toxic or change biological or chemical conditions. In addition there will be impacts on water quality near beaches, shellfish (e.g., oyster) beds, and fishery nursery grounds. The open water environment, the ongoing release of oil and the ongoing response efforts all contribute to complex, constantly-changing exposure conditions for biological resources in the offshore and near-shore environments of the northeastern Gulf of Mexico. Contributing factors to the complexity of the situation include:

1. Characteristics of released oil and other materials, which change with time due to weathering and response activities; also, there may be changes in the released material at the discharge site due to changes in materials leaving the well;
2. Volume and duration of the continued release of oil, with the oil release rate varying in time;
3. Location and nature of the release (i.e., while burning at the sea surface, from various pipe breaks on the sea floor);
4. Physical oceanographic conditions (currents, temperature, etc.), which vary in space and time;
5. Weather (winds, light exposure, air temperature), affecting the oil’s chemistry;
6. Response effectiveness to stop or slow the release of oil, as well as changes in the location, nature, and volume of the release;
7. Dispersant type, application methods (i.e., injected versus aerial or boat), volumes, effectiveness, locations and timing;
8. Exposure scenarios for biological resources (i.e. exposure duration, species, life history stages involved);
9. Location of critical habitats (live bottom, deep water corals, cold seeps; fishing grounds); and
10. Impacts of oil hydrocarbon/dispersant/contaminant mixes over time, resulting from short duration and long exposures, delayed and indirect impacts, etc.

The socioeconomic impacts of the spill will include disruption of fisheries and dependant businesses, effects on tourism and recreational uses, and potentially changes in oil industry practices.

The purpose of using dispersants on the oil is to lessen the potential impact to wildlife (birds, mammals, and sea turtles) and shoreline habitats. However, to some degree there is a tradeoff, in that the contamination in the water is increased by dispersant application. The objective is to achieve a net environmental benefit: to disperse the oil sufficiently to reduce the impact to wildlife and shorelines, but to do so in deep water where the dilution potential is high to minimize adverse effects on fisheries resources.

General Behavior and Fate of Oil   [back to top]  [back to main page]

Oils and petroleum products are generally lighter (less dense) than seawater, and so oil floats to the surface unless it is dispersed into the water directly or by turbulence. Floating oil tends to form slicks when fresh, which thin out over time to sheens, as well as collect into thick aggregations at wind rows and current convergences. The oil weathers and degrades when exposed to air and sun, such that the more volatile components evaporate off and the oil becomes tarry and sticky. Some oils form mousse, in which water becomes incorporated into the oil, making it thicker and more viscous. Eventually, floating oil breaks up into weathered tar balls, which may be transported great distances by currents. If winds are on-shore, oil will come ashore and strand on beaches and in wetlands.

If oil is dispersed in the water, it is in the form of small oil droplets or tar balls. The smaller are these particles of oil, the more readily they are dispersed throughout the water column. Oil may be dispersed from the water surface by natural turbulence from breaking waves. If dispersant is applied to oil on the water surface, this dispersion process is enhanced. Dispersants are soap-like surfactant mixtures, composed of compounds that coat the oil surface and encourage it to break into smaller particles.

Crude oils and petroleum products are composed of thousands of chemicals. In general, the hydrocarbon compounds found in crude oil are characterized by their structure. These compounds include straight-chain hydrocarbons and aromatics; aromatics include at least one benzene ring. Understanding these different compounds and their structures is important for understanding the fate and biological effects of releases of crude oil or products derived from it.

Most of the compounds in oil are not soluble in water. However, the low molecular weight aromatic compounds (such as the one-ring compounds benzene, toluene, ethylbenzene and xylenes (BTEX); and the polynuclear aromatic hydrocarbons (PAHs)) are both volatile (so evaporate from the water surface) and soluble in water. Benzene rings are very stable, and therefore persistent in the environment, and can have toxic effects on organisms. Because the BTEX and PAHs are at least semi-soluble, they can be taken up into the tissues of aquatic organisms, where they can disrupt (or poison) cellular functions. For this reason, scientists evaluate exposure of aquatic biota to these BTEX and PAH compounds derived from spilled oil, as well as the toxic effects of such exposures.

The BTEX and PAHs also are volatile, and so the evaporate off relatively rapidly when oil is exposed to the atmosphere. In addition, the smaller non-aromatic compounds (e.g., pentane, hexane, octane, etc.) evaporate rapidly. Thus, over time the oil contains less and less of both the volatile and soluble compounds, leaving a residual heavier material that can become sticky and tar-like.

Eventually oil hydrocarbons are degraded by sunlight and microbial processes (bacterial degradation), whether in the water, in bottom sediments or on shorelines. Degradation rates are generally slow, and in conditions of low oxygen, degradation can take decades because oxygen is consumed in, and so needed for, the degradation process. The largest compounds are very slow to degrade, which is why they make good road materials – they remain tarry and asphalt-like for years.

Important oil movements and processes involved in a sub-sea oil release are depicted in the cartoon figure below.

Biological Impacts of Spills   [back to top]  [back to main page]

The potential biological impacts of oil include:

• Surface smothering/coating exposure to floating and stranded oil, affecting
• Shoreline habitats (salt marshes, mangroves, sea grasses, oyster flats)
• Wildlife (birds, marine mammals, sea turtles)
• Aquatic organisms inhabiting the sea surface (called neuston)
• Toxicity from uptake of dissolved components (aromatics)
• Fish
• Shellfish and other invertebrates
• Plankton, including fish and shellfish eggs and larvae
• Subsurface suspended oil droplets
• Fish
• Shellfish and other invertebrates
• Plankton, including fish and shellfish eggs and larvae

Oil can kill marine organisms, reduce their fitness through sublethal effects, and disrupt the structure and function of marine communities and ecosystems. While such effects have been unambiguously established in laboratory studies and after well-studied spills, determining the subtler long-term effects on populations, communities and ecosystems at low doses and in the presence of other contaminants poses significant scientific challenges. Because of the high natural variability of aquatic populations, it is extremely difficult to measure the changes from before to after a spill. Thus, scientists use a variety of types of information, including past experience from other spills, field measurements, analyses of samples taken for chemistry or to count organisms, experimental tests, and biological data to estimate the impacts of a spill. We often combine such information with computer model calculations to quantify the impact.

In general, the most vulnerable species to oil spills are birds and fur-bearing marine mammals. These animals depend on their feathers or fur to maintain body heat and keep their skin relatively dry. They preen daily, and so will ingest toxic components present in oil that covers any portion of their bodies. Sea turtles, all species of which are threatened or endangered, are also highly susceptible to oil’s effects.

Shoreline habitats are very vulnerable to oil exposure. Oil stranding in wetlands or other shoreline habitats can coat small animals and plants, suffocating them. The toxic components can also impact the organisms inhabiting the habitats. These habitats require years to decades to recover from lethal-levels of oil exposure.

Because fish and invertebrates are for the most part under the water surface, and much of the oil is not soluble, their exposure to oil hydrocarbons is subject to (1) the degree to which the oil is mixed by turbulence or other means (i.e., dispersed) into the water column; (2) the degree to which the dispersed oil still contains the toxic compounds (which otherwise evaporate); and (3) the rate of dissolution of soluble aromatics into the water. Oil dispersion rate is highest in storm conditions and when large amounts of dispersants are applied to the oil. Mortality is a function of duration of exposure – the longer the duration of exposure, the lower the effects concentration. Thus, a situation where oil is largely dispersed into the water while fresh is that where the highest impacts to fish and invertebrates would be expected.

Socioeconomic Impacts of Spills   [back to top]  [back to main page]

There are many potential socioeconomic impacts that result from large oil spills, including fisheries losses, lost recreation use of beaches and waterways for boating-related activities, impacts on national parks and other protected areas, lost tourism-related business, commercial shipping disruptions, and so on.

Natural Resource Damage Assessment Process   [back to top]  [back to main page]

Natural Resource Damage Assessment (NRDA),is the process where the federal and state government agencies who are trustees for specific resources on behalf of the public may make damage claims against the responsible party. Under federal regulations of the Oil Pollution Act (OPA) of 1990, the polluter pays for restoration and replacement of services provided by natural resources. The damages are the cost of the restoration. The procedure involves assessment of an adverse impact, known as the injury, and then planning a restoration activity that is sufficient to replace the losses, including consideration of the time for recovery.

Injury Assessment

The goal of injury assessment is to determine the nature, degree, and extent of any injuries to natural resources and services. This information is necessary to provide a technical basis for evaluating the need for, type of, and scale of restoration actions. Under the OPA regulations, injury is defined as an observable or measurable adverse change in a natural resource or impairment of a natural resource service. Government trustees determine whether there is:

• Exposure, a pathway, and an adverse change to a natural resource or service as a result of an actual discharge; or
• An injury to a natural resource or impairment of a natural resource service as a result of response actions or a substantial threat of a discharge.

To proceed with restoration planning, trustees quantify the degree, and spatial and temporal extent of injuries. Injuries are quantified by comparing the condition of the injured natural resources or services to baseline, as necessary.

“Baseline means the condition of the natural resources and services that would have existed had the incident not occurred. Baseline data may be estimated using historical data, reference data, control data, or data on incremental changes (e.g., number of dead animals), alone or in combination, as appropriate.” (OPA regulations at § 990.30).

“Injury means an observable or measurable adverse change in a natural resource or impairment of a natural resource service. Injury may occur directly or indirectly to a natural resource and/or service. Injury incorporates the terms “destruction,” “loss,” and “loss of use” as provided in OPA.” (OPA regulations at § 990.30).

The Appropriate Scale of Restoration

The basic concept underlying restoration project scaling is that restoration is to be of sufficient scale to produce resources and services of the same type and quality and/or of comparable value to those that were lost. The loss is quantified from the time of injury until the resources and services return to the level they would have been at in the absence of the impact. Services include ecological and human uses of the resources. The approach used is that the restoration project is scaled to compensate for the direct kill, indirect effects and lost services from the time of the start of the incident into the future until recovery is complete.

For example, to scale a compensatory fish or shellfish restocking program, the equivalent number of eggs, larvae, or animals at the age they are stocked, is needed. The lost individuals will be replaced once that equivalent number of eggs/animals is stocked and the animals have gone through their normal life cycle to the age of the impacted animals they are to replace. The number killed by age class may be translated into an equivalent number at any age to be stocked using an age- or size-specific survival schedule.

If it is not feasible to replace a species with individuals of the same species, other options are available for restoration, such as habitat restoration or protection projects. Salt marsh and seagrass bed restoration projects are frequently considered options as compensation for injuries to marine resources. The challenge is to determine an appropriate scale for the project to be compensatory (i.e., equivalent to the loss). The approach often used is to calculate the net (e.g., fish) production gain per unit of created (or preserved) habitat. The scale of the newly-created or enhanced habitat is made such that the new production produced by created habitat is equivalent to the loss. Protection and enhancement projects are often used for restoring wildlife. For example, seabird and sea turtle nest sites might be protected from human disturbance or predation. In addition, during the spill response, extensive efforts are made to clean and rehabilitate oiled wildlife.

Restoration should not be arbitrary in scale or punitive, but should be proportional to the loss. Biological science is able to provide quantitative information that helps make this compensatory damage assessment possible. However, sufficient field- and experimental-based data are needed to make both the injury and restoration scaling assessments.

Preassessment Phase Activities

At the present time, the trustees are gathering data with which to plan for and quantify injury. The focus is on collection of ephemeral data, i.e., information that might be missed or lost if not gathered at the time of the event. The ephemeral data collections are being made in cooperation with scientists assisting the responsible party, such that as much information as possible is collected with minimal duplication of effort and maximum mutual benefit. We are organized in technical working groups to plan and execute this data collection effort. Thousands of federal and state scientists, as well as consultants and contractors, are engaged in this effort 24/7 to ensure we get the best information possible with which to assess the spill’s impacts. Clearly this monumental effort needs support from the federal government, such that a good scientific analysis of the spill’s impacts can be made.

Previous Spills as Case Examples   [back to top]  [back to main page]

Exxon Valdez Oil Spill (March 1989)

The Exxon Valdez oil spill involved 11 million gallons of crude oil. As is well understood, hundreds of thousands of seabirds and thousands of marine mammals (mostly sea otters) were oiled and killed by this spill. This large impact was due both to the nature of the Alaskan crude oil (a viscous persistent type) and the high densities of seabirds and marine mammals present in the affected area. The impacts to fish and invertebrates in open waters were relatively low in comparison because of the slow rate of dispersion into the water just after the release (winds were light at the time of the spill) and the large volume of Prince William Sound that facilitated dilution. However, impacts on and near shorelines to salmon reproduction and other resources were also considerable.

The socioeconomic impacts of the spill were largely related to disruptions to the fishing industry and subsistence uses of natural resources. The local indigenous peoples utilize nearshore and shoreline shellfish as food sources, and hold natural resources as sacred. In addition many Alaskans and Americans in general consider Alaska to be pristine, and so were outraged by the oil’s impacts.

North Cape Oil Spill (January 1996)

In January 19-20, 1996, during a severe winter storm, the barge North Cape spilled 828,000 gallons of home heating oil (No. 2 fuel oil) into the surf zone on the south coast of Rhode Island. Most of the oil was mixed into the water column by the heavy surf, resulting in high concentrations of the toxic components (PAHs) in the shallow water near the beach. It was evident that there was significant injury to marine aquatic organisms caused by the spill, in that large numbers of lobsters, surf clams, other invertebrates, and fish washed up on the beaches.

Because of the large numbers of highly valued lobsters affected, field sampling was performed to estimate the impact. Impacts to other marine organisms were estimated using computer modeling of oil fates and toxicological effects. The model assumptions and input data were based on existing literature and site-specific information.

While about 2400 birds were oiled in the North Cape spill, it was estimated that 9 million lobsters were killed, along with billions of smaller invertebrates and thousands of fish. The spill was so devastating to the local shellfish and fish populations because fresh highly-toxic oil was completely dispersed naturally into shallow water near shore by high waves.

The socioeconomic impacts of the spill were primarily related to disruptions to the fishing industry. To my knowledge, there were no claims by native Americans made against the spiller. The light oil evaporated and degraded quickly, well before the summer tourist season, so impacts on recreational uses and tourism were minimal.

Ixtoc Oil Spill

The largest spill in history was the Ixtoc blowout which began in June 1979 in Mexican waters of the Bay of Campeche. The well was not completely brought under control until late March 1980. The spill rate was estimated to be about 30,000 bbl¹/day for 5½ months until November, and then about 4,000 bbl/day for another 4 months. The impacts of this spill remain largely unknown. Shoreline-related impacts were observed to birds, sea turtles and invertebrates. However, the impact on fish and shellfish was not estimated. Because of the very large amounts of oil released in relatively shallow waters, it is likely that impacts to shrimp, other shellfish and fish in the Bay of Campeche and southern Gulf of Mexico were highly significant. The socioeconomic impacts of the spill are not documented, but likely included large disruptions of the local fisheries.

¹1 bbl (barrel) = 42 US gallons; estimates vary widely and the release may have been up to 50,000 bbl/day.

Information presented on this webpage was presented at the Commerce, Science and Transportation Senate Hearings on 18 May 2010 by Dr. Deborah French McCay. Click here to see her testimony.