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ebb tide: The stage of the tide when the water recedes to what is commonly called low tide. Contrast with flood tide. See also low tide.
ecosystem: The organisms in a community and the nonliving environment with which they interact.
eelgrass: A marine, flowering plant (Zostera marina) that grows subtidally in sand and mud. Eelgrass beds are an important habitat and nursery for fish, shellfish, and waterfowl.
effluent: The outflow of water, with or without pollutants, usually from a pipe.
Emergency Operations Center (EOC): See Incident Command Post.
Emergency Response Division (ERD): A division of NOAA's Office of Response and Restoration (OR&R) that provides scientific expertise to support oil and chemical spill responses in U.S. coastal waters. ERD specialists also offer tools, training, and information to emergency responders and planners. Formerly the Hazardous Materials Response Division (HAZMAT).
emulsification: The formation of a mixture of water and oil, which can be mixed only when energy is applied. Different oils exhibit different tendencies to emulsify, and emulsification is more likely to occur under high energy conditions (strong winds and waves). An emulsified mixture of water in oil is commonly called "mousse."
emulsion: A mechanical mixture of two liquids that do not naturally mix, such as oil and water. A water-in-oil emulsion is sometimes created when oil spills on water. See also mousse.
endangered species: A plant or animal that is in immediate danger of becoming extinct and needs protection to survive. Compare with threatened species.
endothermic: A chemical reaction that must absorb heat from its surroundings in order to proceed. Contrast with exothermic, in which heat is produced by the forward reaction.
entrain: To incorporate with and carry along.
entrainment: The loss of oil from containment when it is pulled under a boom by a strong current. Entrainment typically occurs from booms deployed perpendicular to currents greater than 1 knot (0.5 meter per second).
environmental sensitivity: The susceptibility of a local environment or area to any disturbance that might decrease its stability or result in short- or long-term adverse impact. Environmental sensitivity generally includes physical, biological, and socio-economic parameters.
environmental trade-off: The relinquishment of one environmental benefit for another regarded as more desirable.
EOC: Emergency Operations Center. See Command Post/Center.
EPA: Environmental Protection Agency (www.epa.gov). A U.S. federal agency that works to protect human health and safeguard the natural environment (air, water, and land).
EPCRA: The Emergency Planning and Community Right-to-Know Act ("Title III of SARA"). Defines 378 chemicals as Extremely Hazardous Substances (EHSs) and requires planning for and reporting of EHS releases from facilities.
ERD: See Emergency Response Division.
ERPG: Emergency Response Planning Guideline, a level of concern representing the adverse health effects of a hazardous substance on members of the general public. The ERPGs were developed by the ERPG committee of the American Industrial Hygiene Association (www.aiha.org).
ESF: Emergency Support Function. A formal means of consolidating multiple agencies that perform similar or like functions into a single, cohesive unit to allow for the better management of emergency response functions. For example, the responsibilities of Emergency Support Function 10, Hazardous Materials (ESF-10) include coordination in response to an actual or potential release of hazardous materials resulting from a natural or technological disaster. An ESF is activated only if the National Response Plan is activated.
ESI: Environmental Sensitivity Index. A system of mapping a region's shoreline types and biological and human use resources, based on their sensitivity to oiling. ESI maps help spill responders and planners identify vulnerable coastal locations before a spill happens, so that protection priorities can be established and cleanup strategies identified in advance. See also ESI maps.
ESI maps: Maps used by the On-Scene Commander and oil spill response team that designate areas of biological, social, and economic importance in a given region. These maps often prioritize sensitive areas so that in the event of an extensive spill, these areas can be protected or cleaned up first. Sensitivity maps usually contain other information useful to the response team, such as the location of shoreline access areas, landing strips, roads, communities, and the composition and steepness of shoreline areas. Maps of this type often form an integral part of local or regional contingency plans. See also ESI.
estuarine: Of, relating to, or occurring in an estuary.
estuary: The general location where a river meets the sea and freshwater mixes with saltwater. Estuaries often contain salt-marshes and other wetlands, which are important habitat for many species.
evaporation: The process by which a substance is converted from liquid to vapor. In the case of oil, the rate of evaporation depends on the volatility of various hydrocarbon constituents, temperature, wind and water turbulence, and the spreading rate of the slick. Evaporation is the most important process in the weathering of most oils.
exclusion booming: The deployment of floating booms to prevent spilled oil from entering a sensitive area. See also boom.
exothermic: A term used to describe the generation of heat from a chemical reaction. Contrast with endothermic, in which heat is taken in from the surroundings.
explosion: A chemical or physical process in which the rate at which energy is being generated exceeds its ability to escape to its surrounding environment. The hazards of an explosion can include a shock wave, especially near the point of explosion, and projectiles generated by the shock wave.
explosive: A material synthesized or mixed deliberately to allow the very rapid release of chemical energy (i.e., an explosion). Also, a chemical substance that is intrinsically unstable and liable to detonate under conditions that might reasonably be encountered.
explosive limits: Or flammability limits. The lower explosive limit is the lowest concentration of a vapor in air that catches fire when exposed to a source of ignition. The upper explosive limit is the highest concentration of the vapor in air that catches fire when exposed to a source of ignition. The flash point and explosive limits of a substance are often used as measures of its flammability hazard in a given situation.

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