Crude oil is a naturally occurring substance generated by geological and geochemical processes. A variety of petroleum products, such as gasoline, diesel fuel, and heavy fuel oil are derived from this natural resource. Crude oil and petroleum products can vary greatly depending on where and how they were extracted and refined, and their unique characteristics influence how they will behave when released into water and how they will affect animals, plants, and their habitats. Because oil is typically less dense than water, oil spills on or near the surface of water will float and form slicks. An untreated slick will remain at the surface until it evaporates, disperses naturally into the water column, washes onto the shoreline, breaks up into smaller collections of oil—known as tarballs—or is recovered or removed from the water. Oil or petroleum products spilled on water undergo a series of physical and chemical processes that may cause the oil to change––known as weathering––or migrate. Some processes cause oil to be removed from the water’s surface, while others change its form on the surface. Figure 1 depicts these processes, which are further described and defined in table 1. Regardless of their physical and chemical properties, all oils will weather once spilled. The rate of weathering depends on the conditions at the time of the spill and the nature of the spilled oil. Most weathering processes are highly temperature dependent, however, and will often slow considerably as the temperature approaches freezing temperatures. A blowout is an uncontrolled release of oil or gas from a well. less dense than water, it will float toward the surface. The speed at which it rises is based on the oil’s droplet size—the larger the droplet the faster the oil rises. Once it reaches the surface, the oil forms a slick thinner than those that result from surface spills, in part because of the diffusion and dispersal of oil droplets as they rise. When an oil spill occurs, responders have several techniques for responding, including the following: Chemical dispersants—applying chemicals to help break up the oil into smaller droplets to facilitate the movement of the oil off the surface and into the water column and enhance microbial breakdown of the oil. Mechanical containment and recovery—using booms, skimmers, sorbents, and other techniques to trap and remove the oil. In-situ burning—burning spilled oil on the surface of the water. Shoreline clean-up—physically picking up oil and washing or chemically treating shorelines, or deploying bioremediation, which involves the addition of nutrients to enhance the ability of microorganisms to degrade the oil more rapidly. No action—taking no active response to the spill. Each response technique has its own operational requirements, benefits, limitations, and potential adverse impacts. Responders must evaluate which method or combination of methods to use depending on the circumstances and conditions of the oil spill, such as the weather, sea state, type and amount of oil spilled, distance of spill from shore, and potentially affected natural resources. In the United States, mechanical containment and recovery is the primary response option, since it physically removes oil from the environment. However, experience has shown that mechanical containment and recovery in open waters can be limited depending on sea conditions. Specifically, for such operations to be conducted most effectively, seas need to be relatively calm, with waves under about 3 feet, according to documents we reviewed and specialists with whom we spoke. Oil spills inevitably have environmental impacts, and response actions may only reduce these impacts or shift them. In determining which response options are best for an individual spill, agency officials said that decision makers weigh the ecological risks and consequences with the goal of minimizing adverse effects as much as possible. For example, when considering the use of chemical dispersants as a response option, the essential question asked is whether dispersing the oil into the water column offers more benefits (i.e., causes less harm) than leaving the oil on the surface if it cannot be adequately removed by mechanical means or burned. Decision makers would collect as much information as possible to assess, for example, whether the potential harm to wetlands or waterfowl that could occur if dispersants were not applied is greater than the potential harm to marine species from chemically dispersed oil entering the water column. This evaluation of these trade-offs is sometimes called a net environmental benefit analysis. Chemical dispersants function by reducing the surface tension between oil and water—similar to the way that dish detergents break up cooking oil on a skillet—and enhancing the natural process of dispersion by generating larger numbers of small droplets of oil that are mixed into the water column by wave energy. Thus, rather than having a surface slick of oil, one will have an underwater plume of chemically dispersed oil. Throughout this report we use the term “chemically dispersed oil” to discuss the mixture that results when chemical dispersants are applied to oil and facilitate the formation of oil droplets. A typical commercial dispersant contains a mixture of three types of chemicals: surfactants, solvents, and additives. Surfactants are the active agents that reduce oil- water surface tension. Surfactant compounds contain both oil-compatible and water-compatible groups on the same molecule, with the oil- compatible group interacting with oil and the water-compatible group interacting with water to make the interaction between the two easier. Solvents are added to promote the dissolution of the surfactants and additives into the dispersant mixture and then, during application, into the oil slick. Additives may be present for a number of purposes, such as improving the dissolution of the surfactants and increasing the long term stability of the dispersant formulation. Federal statutes required the development of a National Oil and Hazardous Substances Pollution Contingency Plan that, among other things, delineates the procedures for preparing for and responding to oil spills and details the roles and responsibilities of federal agencies and others involved in dispersant decision making. Specifically, the National Contingency Plan is based on a framework that brings together the functions of the federal government, the affected state governments, and the party responsible for a spill under a unified command to achieve an effective and efficient response. In response to an oil spill, the National Contingency Plan calls for a Federal On-Scene Coordinator to direct and coordinate response efforts. In the case of oil spills in the coastal zone,such as in the Deepwater Horizon incident, a representative from the Coast Guard serves as the Federal On-Scene Coordinator. EPA provides the Federal On-Scene Coordinator for spills occurring in the inland zone, and the designation of these zones is documented in the Regional Contingency Plans. As part of the National Contingency Plan, EPA maintains the National Oil and Hazardous Substances Pollution Contingency Plan Product Schedule, which lists chemical dispersants that may be authorized for use on oil discharges. Inclusion on the Product Schedule does not mean that EPA recommends the product for use; rather, it only means that certain data have been submitted to EPA and that the dispersant has a certain effectiveness. The data that a manufacturer must submit to EPA includes effectiveness and toxicity data, special handling and worker precautions for storage and application, recommended application procedures and conditions for use, and shelf life. An appendix to the regulations implementing the National Contingency Plan describes the test methods a manufacturer is to follow for measuring effectiveness and toxicity of dispersants. In terms of effectiveness, the manufacturer must demonstrate that the dispersant can disperse at least 45 percent of oil in testing. To assess toxicity, the appendix specifies the standard test for a chemical dispersant, which involves exposing two species––silverside fish (Menidia beryllina) and mysid shrimp (Mysidopsis bahia)—to varying concentrations of the dispersant, oil, and a mixture of the two, to determine mortality rates at the end of 96 hours for silversides and 48 hours for mysid shrimp. Chemical dispersant manufacturers must submit the results of effectiveness and toxicity testing to EPA, which may request further documentation or verify test results in determining whether the dispersant meets listing criteria. Both the presidential commission that investigated the Deepwater Horizon incident and the EPA Inspector General have recommended that EPA update the Product Schedule’s testing protocols and requirements for listing.Inspector General recommended, among other things, that EPA modify the Product Schedule and contingency plans to include additional information learned from the Deepwater Horizon oil spill response, such as subsurface dispersant application in deep water. EPA anticipates issuing a proposed rule in winter 2012 that would revise the requirements for listing a product on the Product Schedule and is considering changes to effectiveness and toxicity testing protocols. In addition, the EPA A National Response Team and Regional Response Teams serve as preparedness and planning organizations prior to a response and may serve as incident-specific response teams to provide support and advice to the Federal On-Scene Coordinator during a response. The National Response Team includes 20 federal departments and agencies responsible for national response and preparedness planning, for coordinating regional planning, and for providing policy guidance and support to Regional Response Teams. Regional Response Teams are composed of representatives of each National Response Team agency and representatives from relevant state and local governments (as agreed upon by the states) and may also include tribal governments. There are 13 Regional Response Teams corresponding to the 10 standard federal regions, plus separate teams for Alaska, Oceania in the Pacific, and the Caribbean. The Regional Response Teams develop Regional Contingency Plans establishing procedures for preparing for and responding to oil spills in the region. Within the regions, area committees composed of officials from federal, state, and local agencies have been designated to develop Area Contingency Plans. Regional and Area Contingency Plans may address the specific situations in which chemical dispersants should and should not be used and may preauthorize their use by the Federal On-Scene Coordinator.