eagre (eager) Same as tidal bore.

earthen (or earthfill) dam An embankment dam in which more than 50% of the total volume is formed of compacted fine-grained material. A homogeneous earthen dam is constructed of similar earthen material throughout. These are the most common type of dam because their construction involves using materials in the natural state, requiring little processing.

earth station A ground-placed antenna used to transmit or receive signals to or from satellites, typically located in geostationary orbit.

earth tide Periodic movement of the Earth's crust caused by gravitational interactions between the Sun, Moon, and Earth.

easterly wave An inverted, migratory wave-like disturbance or trough in the tropical region that moves from east to west, generally creating only a shift in winds and rain. The low-level convergence and associated convective weather occur on the eastern side of the wave axis. Normally, it moves slower than the atmospheric current in which it is embedded and is considered a weak trough of low pressure. It is often associated with possible tropical cyclone development and is also known as a tropical wave.

East Greenland Current A North Atlantic Ocean current setting southward and then southwestward along the east coast of Greenland.

ebb axis Average set of the current at ebb strength.

ebb current (ebb) The movement of a tidal current away from shore or down a tidal river or estuary. In the mixed type of reversing tidal current, the terms greater ebb and lesser ebb and are applied respectively to ebb tidal currents of greater and lesser speed each day. The terms maximum ebb and minimum ebb are applied to the maximum and minimum speeds of a current running continuously ebb, the speed alternately increasing and decreasing with coming to a slack or reversing. The expression maximum ebb is also applicable to any ebb current at the time of greatest speed. See ebb strength.

ebb interval The interval between the transit of the Moon over the meridian of a place and the time of the following ebb strength.

ebb strength (strength of ebb) Phase of the ebb tidal current at the time of maximum speed. Also, the speed at this time. See strength of current.

ebullition The act, process or state of boiling or bubbling up.

eccentricity of orbit Ratio of the distance from center to focus of orbit to the semimajor axis.

ecliptic The intersection of the plane of the Earth's orbit with celestial sphere.

EC medium The medium used for the fecal coliform test.

ecocline Gradual changes in the morphological or physiological features in organisms along an environmental gradient.

ecological amplitude Pertains to the breadth of a species’ tolerance to an environmental factor.

ecological balance Range of response normally expressed by an unperturbed ecosystem.

ecological dominance Pertains to a species’ control, competitiveness, and alteration of conditions for the remainder of the community.

ecological efficiency The defined exchange of energy and/or nutrients between trophic levels; usually the ratio between production of one level and that of a lower level in the same food chain.

ecological energy Most commonly, that portion of the visible solar radiation captured by plants and ultimately used for food by the animals in an ecosystem.

ecological equivalent Analogous species in similar environmental contexts; that is, distinctly related species displaying closely similar adaptive mechanisms, like loons and cormorants, flying squirrels and flying phalangers, etc.

ecological impact The effect that a man-made or natural activity has on living organisms and their abiotic environment.

ecological indicator Use of certain species’ tolerances to reflect or infer more general environmental characteristics; see indicator.

ecological niche The functions of the organism in its ecological setting. See niche.

ecological resilience A system’s ability to return to a prior state following environmental perturbation (stress).

ecological risk assessment The application of a formal framework, analytical process, or model to estimate the effects of human action(s) on a natural resource and to interpret the significance of those effects in light of the uncertainties identified in each component of the assessment process. Such analysis includes initial hazard identification, exposure and dose-response assessments, and risk characterization.

ecological succession See succession.

ecological system See ecosystem.

ecology 1) The relationship of living things to one another and their environment, or the study of such relationships. 2) The science which treats organisms in relation to their environment; frequently subdivided into human ecology, animal ecology, plant ecology, and bioecology.

econometrics The branch of economics that uses mathematics and statistics to build and analyze economic models, explain economic phenomena, and estimate values for economic variables. The statistical methods used are especially designed to deal with time-series data.

economic base study A study that evaluates the economic structure of a region to provide economic and demographic projections necessary for the appraisal of future water resource needs.

economic life The period determined by the estimated point in time at which the combined effect of physical depreciation, obsolescence, changing requirements for project services, and time and discount allowances will cause the cost of continuing the project to exceed the benefits to be expected from continuation. It may be equal to or greater than the amortization period and may be equal to, but generally less than the physical life.

economic poisons Chemicals used to control pests and to defoliate cash crops auch as cotton.

ecosystem A community and its living and nonliving environment considered collectively; the fundamental unit in ecology. The system may be quite small, as the ecosystem of one-celled plants, in a drop of water, or indefinitely large, as in the grassland ecosystem.

ecosystem analysis Examination of the structure, function, and control mechanisms present and operating in an ecosystem.

ecosystem dynamics The characteristic and measurable processes within an ecosystem such as 1) succession; 2) energy flow and nutrient cycling; 3) community metabolism.

ecosystem function The energy flow and material production cycling within an ecosystem.

ecosystem integrity Implications of ecosystem properties as a whole, especially of resilience.

ecosystem structure 1) The who, what, and where of an ecosystem; its functionally important and weighable components mostly organisms. 2) The pattern of organism’s interrelations and spatial arrangements.

ecosphere The envelope of the earth’s surface where biological and ecological activities occur. See biosphere.

ecotone A transition zone between two recognized communities.

ecotype A race or subdivision of species adapted to local habitat and climate. These genetic groups are broader than a biotype and narrower than a species.

edaphic Of or related to the influence of the soil especially on the plants growing upon it.

edaphic climax A self-perpetuating community where soil is limiting further succession at a stage believed to be short of climatic potential.

eddy A quasi-circular movement of water whose area is relatively small in comparison to the current with which it is associated.

edge effect Phenomena such as changed diversity and/or density of organisms that occur in the vicinity of community boundaries.

edge waves Waves moving between zones of high and low breakers along the shoreline. Edge waves contribute to changes in water level along the shoreface which helps to control the spacing of rip currents. See longshore current and rip current.

eductor A hydraulic device used to create a negative pressure (suction) by forcing a liquid through a restriction, such as a Venturi. An eductor or aspirator (the hydraulic device) may be used in the laboratory in place of a vacuum pump. As an injector, it is used to produce vacuum for chlorinators.

effective corrosion inhibitor residual 1) A concentration of corrosion inhibitor sufficient to form a protective coating on the interior walls of a pipe, reducing its corrosion. 2) For the purpose of 12 VAC 5-590-420 C 1 only, means a concentration sufficient to form a passivating film on the interior walls of a pipe.

effective opening The minimum cross-sectional area at the point of water supply discharge, measured or expressed in terms of 1) diameter of a circle or 2) if the opening is not circular, the diameter of a circle of equivalent cross-sectional area.

effective porosity The amount of interconnected pore space in a soil or rock through which fluids can pass, expressed as a percent of bulk volume. Some of the voids and pores in a rock or soil will be filled with static fluid or other material, so that effective porosity is always less than total porosity.

effective precipitation That portion of total precipitation that becomes available for plant growth.

effective range That portion of the design range (usually upper 90 percent) in which an instrument has acceptable accuracy. Also see range and span.

effective size The diameter of the particles in a granular sample (filter media) for which 10 percent of the total grains are smaller and 90 percent larger on a weight basis. Effective size is obtained by passing granular material through sieves with varying dimensions of mesh and weighing the material retained by each sieve. The effective size is also approximately the average size of the grains.

efficacy Worth, merit, value.

effluent 1) Water or some other liquid—raw, partially or completely treated, flowing from a reservoir, basin, treatment process, or treatment plant. 2) The liquid discharged from a treatment plant after completion of the treatment process.

effluent guidelines Technical, regulatory documents that set effluent limitations for given industries and pollutants.

effluent limitation Restrictions established by regulatory agencies on quantities, rates, and concentrations in wastewater discharges and set forth in a formal permit .

ejector A device used to disperse a chemical solution into water being treated.

Ekman spiral A logarithmic spiral (when projected on a horizontal plane) formed by current velocity vectors at increasing depth intervals. The current vectors become progressively smaller with depth.

electrical current An electric current is caused by the flow of electrons. However, the electric current flows in a direction opposite to the flow of electrons. (This is accepted though seemingly illogical.)

electrical resistivity Measurement of subsurface electrical resistance. Electrical resistivity is a function of the physical and mineralogical properties of the soil and rock and the chemistry of pore fluids.

electric tape gauge A gauge consisting of a graduated Monel metal tape on a metal reel (with supporting frame)., voltmeter, and battery. Heights can be measured directly by unreeling the tape into its stilling well . When contact is made with the water's surface, the circuit is completed and the voltmeter needle moves. At that moment the length of tape is read against an index mark, the mark having a known elevation relative to the bench marks.

electrochemical methods Direct corrosion monitoring method based on the electrochemical nature of corrosion in water. Measures instantaneous corrosion rates, usually in mils per year (mpy).

electrochemical reaction Chemical changes produced by electricity (electrolysis) or the production of electricity by chemical changes (galvanic action). In corrosion, a chemical reaction is accompanied by the flow of electrons through a metallic path. The electron flow may come from an external force and cause the reaction, such as electrolysis caused by a D. C. (direct current) electric railway or the electron flow may be caused by a chemical reaction as in the galvanic action of a flashlight dry cell.

electrochemical series A list of metals with the standard electrode potentials given in volts. The size and sign of the electrode potential indicates how easily these elements will take on or give up electrons, or corrode. Hydrogen is conventionally assigned a value of zero.

electrochemistry The result of an electrical and chemical reaction such as when a metal goes into solution as an ion or reacts in water with another element to form a compound resulting in a flow of electrons (electricity).

electrode A metal in contact with an electrolyte that serves as a site where an electrical current enters the metal or leaves the metal to enter the solution.

electrodialysis Separation of carged particles in solution in an electric field through a semipermeable membrane.

electrolysis The decomposition of material by an outside electrical current.

electrolyte A substance which dissociates (separates) into two or more ions when it is dissolved in water.

electrolytic cell A device in which the chemical decomposition of material causes an electric current to flow. Also, a device in which a chemical reaction occurs as a result of the flow of electric current. Chlorine and caustic (NaOH) are made from salt (NaCl) in electrolytic cells.

electromagnetics Method that measures subsurface conductivity through the use of low-frequency electromagnetic induction.

electromotive force (EMF)The electrical pressure available to cause a flow of current (amperage) when an electrical circuit is closed. See voltage.

electromotive series A list of metals and alloys presented in the order of their tendency to corrode (or go into solution). Also called the galvanic series. This is a practical application of the theoretical electrochemical series.

electron An extremely small, negatively charged particle; the part of an atom that determines its chemical properties.

electron acceptor A chemical entity that accepts electrons transferred to it from another compound. It is an oxidizing agent that, by virtue of its accepting electrons, is itself reduced in the process. See also terminal electron acceptor and oxidation-reduction.

electron donor A chemical entity that donates electrons to another compound. It is a reducing agent that, by virtue of its donating electrons, is itself oxidized in the process. (see also electron acceptor and oxidation-reduction.)

electronic bulletin boards Information services that can be reached via computers connected by modem and/or Internet. With these services users can gather information, place and read electronic messages from other users, and download available files.

electronic mail More often called E-Mail. E-mail is a fast, easy, and inexpensive way to communicate with individuals or groups on networked computers and computers equipped for Internet access. Besides basic correspondence, with some systems you can attach and send documents and other files.

electrostatic precipitator (ESP) A device that removes particles from a gas stream (smoke) after combustion occurs. The ESP imparts an electrical charge to the particles, causing them to adhere to metal plates inside the precipitator. Vibrating the plates causes the particles to fall into a hopper for disposal.

electrostatic precipitation

element A substance which cannot be separated into its constituent parts and still retain its chemical identity. For example, sodium (Na) is an element.

eligible costs The construction costs for wastewater treatment works upon which EPA grants are based.

elimination One of the final processes in the harmonic analysis of tides in which preliminary values for the harmonic constants of a number of constituents are cleared of the residual effects each other.

elution The removal of adsorbed material from an adsorbent such as the removal of a product from an enzyme bound on a column.

elutriation The washing of digested sludge in plant effluent with a suitable ration of sludge to effluent to remove the fine particulates or soluble components in the sludge.

EMAP data Environmental monitoring data collected under the auspices of the Environmental Monitoring and Assessment Program (EMAP). All EMAP data are of known quality, having been collected in the context of explicit data quality objectives and a consistent quality assurance program.

embankment Fill material, usually earth or rock, placed with sloping sides and usually with length greater than height. All dams are types of embankments.

embryo An early stage of development of animals and plants. Usually passed through by an egg after fertilization and before hatching.

emergent Aquatic plants, usually rooted, which during part of their life cycle have portions above water.

emergency 1) A situation created by an accidental release or spill of hazardous chemicals that poses a threat to the safety of workers, residents, the environment, or property.

emergency alert system (EAS) A system designed to permit government officials to issue up-to-date and continuous emergency information and instructions to the public in case of a threatened or actual emergency. It is replacing the Emergency Broadcast System.

emergency operations center (EOC) A State, county, or city emergency facility that serves as a central location for the coordination and control of all emergency preparedness and response disaster activities.

emergency planning zone (EPZ) A generic area around a commercial nuclear facility used to assist in offsite emergency planning and the development of a significant response base. For commercial nuclear power plants, EPZs of about 10 and 50 miles are delineated for the plume and ingestion exposure pathways, respectively.

emergency public information Information disseminated primarily, but not unconditionally, at the time of an emergency frequently includes actions, instructions and direct orders.

emergency public shelter Generally a public school or other such structure designated by county or city officials as a place of refuge. A volunteer group such as the American Red Cross or Salvation Army usually manages a shelter.

emergency response values Concentrations of chemicals, published by various groups, defining acceptable levels of short-term exposures in emergencies.

emission 1) The act of sending forth, to emanate. 2) Pollution discharged into the atmosphere from smokestacks, other vents, and surface areas of commercial or industrial facilities; from residential chimneys; and from motor vehicle, locomotive, or aircraft exhausts.

emission cap A limit designed to prevent projected growth in emissions from existing and future stationary sources from eroding any mandated reduction. Generally, such provisions require any emission growth from facilities under the restrictions be offset by equivalent reductions at other facilities under the same cap. See Emissions Trading.

emission factor The relationship between the amount of pollution produced and the amount of raw material processed.

emission inventory A listing, by source, of the amount of air pollutants discharged into the atmosphere of a community; used to establish emission standards.

emission standard The maximum amount of air polluting discharge legally allowed from a single source.

emissions trading The creation of surplus emission reductions at certain stacks, vents, or similar emissions sources and the use of this surplus to meet or redefine pollution requirements applicable to other emission sources. This allows one source to increase emissions when another sources reduces them, maintaining an overall constant emission level. Facilities that reduce emissions substantially may bank their credits or sell them to other industries.

empirical Relying upon or gained from experiment or observation.

emulsion A liquid mixture of two or more liquid substances, not normally dissolved in one another, but one liquid held in suspension in the other.

encapsulation The treatment of asbestos-containing material with a liquid that covers the surface with a protective coating or embeds fibers in an adhesive matrix to prevent their release into the air.

enclosed trickling filter

enclosure Putting an airtight, impermeable, permanent barrier around asbestos -containing materials to prevent the release of asbestos fibers into the air.

endangered species Species that are in danger of becoming extinct.

endangerment assessment A study to determine the nature and extent of contamination at a site on the National Priorities List and the risks posed to public health or the environment. An endangerment assessment supplements a remedial investigation.

endangerment measure A site-specific risk assessment of the actual or potential danger to human health or welfare and the environment from the release of hazardous substances or waste. The endangerment assessment document is prepared in support of enforcement actions under cerla or rcra.

end bells Devices used to hold the rotor and stator of a motor in position.

endemic Something peculiar to a particular people or locality, such as a disease which is always present in the population.

endemic disease The constant presence of a disease or infectious agent within a given geographic area or population group; may also refer to the usual prevalence of a given disease within such area or group.

endogenous A diminished level of respiration in which materials previously stored by the cell are oxidized.

endogenous respiration Utilization of internal cellular material as food under aerobic conditions when an adequate external food supply is unavailable.

endonuleases Enzymes which cleave bonds within nucleic acid molecules.

endotherms Warm blooded animals. Animals that have the facility to regulate their body temperatures over a wide range of external temperatures.

endotoxin A heat-stable lipopolysaccharide associated with the outer membrane of certain gram-negative bacteria. It is not secreted and is released only when the cells are disrupted. When injected into humans, endotoxins produce a febrile response, leading to severe clinical problems, including death.

end points Samples are titrated to the end point. This means that a chemical is added, drop by drop, to a sample until a certain color change occurs. This is called the end point of the titration. In addition to a color change, an end point may be reached by the formation of a precipitate or the reaching of a specified pH. An end point may be detected by the use of an electronic device such as a pH meter.

endrin A pesticide toxic to freshwater and marine aquatic life that produces adverse health effects in domestic water supplies.

end use Main, ultimate, or intended use for water as a result of certain process, delivery, or treatment.

end user residential, commercial, industrial, governmental, or institutional water consumer.

energy That which does or is capable of doing work and is equal to average power multiplied by the interval of time.

energy budget A quantitative account sheet of inputs, transformations, and outputs of energy in an ecosystem. It may apply to the long-wave radiation (heat) of an organism or a lake, or to the food taken in and subsequently reduced to heat by an individual or population.

energy dissipator A structure which slows fast-moving spillway flows in order to prevent erosion of the stream channel.

energy flow The one-way passage of energy (largely chemical) through the system, entering by way of photosynthesis, being exchanged through feeding interactions, and being reduced to heat at each stage.

energy grade line (EG.L) A line that represents the elevation of energy head of water flowing in a pipe, conduit, or channel. The line is drawn above the hydraulic grade line (gradient) a distance equal to the velocity head (V²/2g) of the water flowing at each section or point along the pipe or channel. Also see hydraulic gradeline.

energy recovery Obtaining energy from waste through a variety of processes.

energy subsidy The man-induced addition of energy designed to increase the production of ecological energy. For example, the use of fossil fuel energy in tractors to increase the amount of solar energy available from agricultural crops.

energy transfer process Any process that transfers energy from one component in an ecosystem to another. Photosynthesis, feeding, and bacterial break down are examples.

energy yield The total effective energy released in a nuclear explosion.

enforceable requirements Conditions or limitations in permits issued under the Clean Water Act, Section 402 or 404 that, if violated, could result in the issuance of a compliance order or initiation of a civil or criminal action under federal or applicable state laws. If a permit has not been issued, the term includes any requirement which, in the regional Administrator’s judgement, would be included in the permit when issued. Where no permit applies, the term includes any requirement which the regional Administrator determines is necessary for the best practical waste treatment technology to meet applicable criteria.

enforcement EPA, state, or local legal actions to obtain compliance with environmental laws, rules, regulations, or agreements and/or obtain penalties or criminal sanctions for violations. Enforcement procedures may vary, depending on the requirements of different environmental laws and related implementing regulations.

enforcement decision document (EDD) A document that provides an explanation to the public of the selection of the cleanup alternative at enforcement sites on the National Priorities List. Similar to a Record of Decision.

engineer's level A telescope which is attached to a spirit-tube level, all revolving around a vertical axis and ismounted on a tripod. An engineer's level is used for determining the difference in elevation between two points. The telescope on the level has a vertical cross hair and a horizontal cross hair. Once the instrument is leveled, the sighting through the horizontal cross hair represent ahorizontal plane of equal elevation.

enhanced inspection and maintenance An more stringent automobile inspection and maintenance program-aimed at reducing automobile emissions-that contains, at a minimum, more vehicle types and model years, tighter inspection, and better management practices.

enhancement An increase in value resulting from intensified use of a particular land area due to structural measures that solve some water-oriented problem in the area.

enjoin A restraint through formal court order; to cease a practice or to stop further violation of the act.

enrichment The addition of nutrients (e.g., nitrogen, phosphorus, carbon compounds) from sewage effluent or agricultural runoff to surface water that greatly increases the growth potential for algae and other aquatic plants.

ensemble (hydrologic) forecasting A process whereby a continuous hydrologic model is successively executed several times for the same forecast period by use of varied data input scenarios, or a perturbation of a key variable state for each model run. A common method employed to obtain a varied data input scenario is to use the historical meteorological record, with the assumption that several years of observed data covering the time period beginning on the current date and extending through the forecast period comprises a reasonable estimate of the possible range of future conditions.

enteric Of intestinal origin, especially applied to wastes or bacteria.

enteritis Inflammation of the intestines and human ileum.

enterobacter A fecal coliform group that includes Salmonella.

enterocolitis Enteritis affecting both the small and large intestines.

enterovirus A genus of RNA viruses with over 70 types identified in humans. They reproduce in the intestinal tract, and various members can cause a variety of human diseases, including poliomyelitis, aseptic meningitis, hepatitis, inflammatory heart disease, and rhinitis.

entrain To trap bubbles of air or other gas within a liquid either mechanically through turbulence or chemically through a reaction.

entrained Particulates or vapor transported along with flowing gas or liquid.

entropy The state of thermal disorganization of a system. In a system, entropy is proportional to the nonuseable heat produced.

entry point The place where water from the source after application of any treatment is delivered to the distribution system.

environment The sum total or the resultant of all the external conditions that act upon an organism.

environmental amenities Attractive or esthetically pleasing environments or portions of environments.

environmental assessment An environmental analysis prepared to determine whether a federal action would significantly affect the environment and thus require a more detailed environmental impact statement.

environmental audit An independent assessment of the current status of a party’s compliance with applicable environmental requirements or of a party’s environmental compliance policies, practices, and controls.

environmental criteria Standards of physical, chemical, and biological components that define a given quality of an environment.

environmental effect The result of natural or man-made perturbations of the physical, chemical, or biological components making up the environment.

environmental factor An extrinsic factor (geology, climate, insects, sanitation, health services, etc.) which affects the agent and the opportunity for exposure.

environmental equity Equal protection from environmental hazards of individuals, groups or communities regardless of race, ethnicity, or economic status.

environmental exposure Human exposure to pollutants originating from facility emissions. Threshold levels are not necessarily surpassed, but low-level chronic pollutant exposure is one of the most common form of environmental exposure.

environmental impact statement A document required of federal agencies by the National Environmental Policy Act for major projects or legislative proposals significantly affecting the environment. A tool for decision making, it describes the positive and negative effects of the undertaking and cites alternative actions.

environmental indicator A measurement, statistic, or value that provides a proximate gauge or evidence of the effects of environmental management programs or of the state or condition of the environment.

environmental inventory A listing of the components making up an environment— or a listing of types of environments.

environmental justice The fair treatment of all races, cultures, incomes, and educational levels with respect to the development, implementation, and enforcement of environmental laws, regulations, and policies. Fair treatment implies that no population of people should be forced to shoulder a disproportionate share of the negative environmental impacts of pollution or environmental hazards

environmental monitoring The simultaneous or sequential systematic measuring of various components constituting the environment.

environmental parameters Physical, chemical, or biological components and their interactions that can be stated in quantitative terms. A parameter is what is measured by a statistic.

environmental quality Human considerations of desirable ecological situations.

environmental release The introduction of a pollutant into the environment through waste-water discharge, air emission, or volatilization or leaching from soil, landfill, or other contaminated site.

environmental resistance The restrictions imposed upon the numerical increase of a species by the physical and biological factors of the environment.

environmental response team Experts who can provide around-the-clock technical assistance during all types of hazardous waste site emergencies and spills of hazardous substances.

environmental setting The environmental context.

environmental stress Perturbations likely to cause observable changes in ecosystems, usually departures from normal or the optimum. See stress.

environs The neighborhood; surroundings.

enzootic The constant presence or usual prevalence of a disease or infectious agent in animal populations of a given geographic area.

enzyme An organic compound produced within an organism that is capable of catalytic action.

EPA United States Environmental Protection Agency. Also USEPA.

epibenthos Life forms attached to and growing upon rather than within the bottoms of standing and flowing waters.

epidemic Widespread outbreak of a disease, or a large number of cases of a disease in a single community or relatively small area in excess of the usual incidence. Disease may spread from person to person and/or by the exposures of many persons to a single source, such as a water supply.

epidemic curve A histogram that shows the course of a disease outbreak or epidemic by plotting the number of cases by time of onset.

epidemic period A time period when the number of cases of disease reported is greater than expected.

epidemiologic study Study of human populations to identify causes of disease. Such studies often compare the health status of a group of persons who have been exposed to a suspect agent with that of a comparable non-exposed group.

epidemiologic triad The traditional model of infectious disease causation. Includes three components: an external agent, a susceptible host, and an environment that brings the host and agent together, so that disease occurs.

epidemiology A branch of medicine which studies epidemics (diseases which affect significant numbers of people during the same time period in the same locality). The objective of epidemiology is to determine the factors that cause epidemic diseases and how to prevent them.

epilimnion The upper layer of water in a thermally stratified lake or reservoir. This layer consists of the warmest water and has a fairly uniform (constant) temperature. The layer is readily mixed by wind action.

epiphytes Plants that grow on other plants, but which are not parasitic.

epizootic The occurrence of cases of similar nature in animal populations in a particular geographic area clearly in excess of normal incidence. Epizootics may result from common or propagated sources of infection.

epoch Angular retardation of the maximum of a constituent of the observed tide (or tidal current) behind the corresponding maximum of the same constituent of the theoretical equilibrium tide. Also known as phase lag.

equal-energy hypothesis A hypothesis stating that equal amounts of sound energy will produce equal amounts of hearing impairment, regardless of how the sound energy is distributed in time.

equation of time Difference between mean and apparent time. From the beginning of the year until near the middle of April, mean time is ahead of apparent time, the difference reaching a maximum of about 15 minutes near the middle of February. From the middle of April to the middle of June, mean time is behind apparent time but the difference is less than 5 minutes. From the middle of June to the first part of September, mean time is again ahead of apparent time with maximum difference less than 7 minutes. From the first of September until the later part of December, mean time is again behind apparent time, the difference reaching a maximum of nearly 17 minutes in the early part of November.

Equator The geographic circle at 0 degrees latitude on the earth's surface. It is equal distance from the North and South Poles and divides the Northern Hemisphere from the Southern.

Equatorial Countercurrent A current setting eastward between the North and South Equatorial Currents of the Atlantic ,Pacific, and Indian (in northern winter) Oceans. In the Atlantic and Pacific, its axis lies about latitude 7° north and in the Indian, about 7° south.

equatorial tidal currents Tidal currents occurring semimonthly as a result of the Moon being over the Equator. At these times the tendency of the Moon to produce a diurnal inequality in the tidal current is at a minimum.

equilibrate To maintain an equilibrium.

equatorial tides Tides occurring semimonthly as a result of the Moon being over the Equator. At these times the tendency of the Moon to produce a diurnal inequality in the tide is at a minimum.

Equatorial Undercurrent A subsurface current setting eastward along the Equator in the Pacific, Atlantic, and Indian Oceans.

equilibration Maintenance of an equilibrium.

equilibrium A steady state in a dynamic system, with system output directly proportional to system input about which the system ordinarily fluctuates to some small degree.

equilibrium drawdown The ultimate, constant drawdown for a steady rate of pumped discharge.

equilibrium potential The electrode potential at equilibrium.

equilibrium surface discharge The steady rate of surface discharge which results from a long-continued, steady rate of net rainfall, with discharge rate equal to net rainfall rate.

equilibrium theory A model under which it is assumed that the waters covering the face of the Earth instantly respond to the tide-producing forces of the Moon and Sun to form a surface of equilibrium under the action of these forces. The model disregards friction, inertia, and the irregular distribution of the land masses of the Earth. The theoretical tide formed under these conditions is known as the equilibrium tide.

equilibrium tide Hypothetical tide due to the tide producing forces under the equilibrium theory. Also known as gravitational tide.

equilibrium time The time when flow conditions become substantially equal to those corresponding to equilibrium discharge or equilibrium drawdown.

equinoctial The celestial equator.

equinoctial tides Tides occurring near the times of the equinoxes.

equinoxes The two points in the celestial sphere where the celestial equator intersects the ecliptic; also, the times when the Sun crosses the equator at these points. The vernal equinox is the point where the Sun crosses the Equator from south to north and it occurs about March 21. Celestial longitude is reckoned eastward from the vernal equinox. The autumnal equinox is the point where the Sun crosses the Equator from north to south and it occurs about September 23.

equipotential Lines of equal pressure; lines connecting points of equal hydraulic head.

equipotential line A line, in a field of flow, such that the total head is the same for all points on the line, and therefore the direction of flow is perpendicular to the line at all points.

equipotential surface Same as geopotential surface.

equivalent continuous noise level (Leq) Varying, intermittent or impulsive noise exposure that is equal in energy to a continuous noise level for a certain duration.

equivalent method Any method of sampling and analyzing for air pollution that has been demonstrated to be, under specific conditions, an acceptable alternative to normally used reference methods.

equivalent residential connection A volume of water used equal to a residential connection which is 400 gallons per day unless supportive data indicates otherwise.

erosion Wearing away of the soil by running water, wind, or ice; erosion is the process by which the earth's surface is shaped and occurs even in remote, uninhabited areas at a slow rate (geologic erosion); of more concern is accelerated erosion caused by people's activities.

escalation rate A percentage used to adjust an expenditure forecast to account for the increasing value of a good or service or time (apart from the discount rate and inflationary effects).

Escherichia coliform A genus of the enterobacter family known as E. coli. Some serotypes may be pathogenic.

establishment A place of business or residence, including all equipment essential to such business or resident.

establishment of the port Also known as high water, full and change (HWF&C). Average high water interval on days of the new and full Moon. This interval is also sometimes called the common or vulgar establishment to distinguish it from the corrected establishment, the latter being the mean of all the high water intervals. The latter is usually 10 to 15 minutes less than the common establishment.

ester A compound formed by the reaction between an acid and an alcohol with the elimination of a molecule of water.

estimated detection limit The minimum concentration of an analyte that can be measured and reported with confidence that the analyte concentration is greater than zero.

estuarine Of or related to the mouth region of a river affected by tides.

esturine waters Deepwater tidal habitats and tidal wetlands that are usually enclosed by land but have access to the ocean and are at least occasionally diluted by freshwater runoff from the land, such as bays, mouths of rivers, salt marshes, lagoons.

esturine zone The area near the coastline consisting of estuaries and coastal saltwater wetlands.

estuary Regions of interaction between rivers and near-shore ocean waters where tidal action and river flow mix fresh and saltwater. Such areas include bays, mouths of rivers, salt marshes, and lagoons. These brackish water ecosystems shelter and feed marine life, birds, and wildlife.

et al And others.

ethology Study of the behavior or organisms usually or preferably in their natural environment.

etiology The study or theory of the causation of disease; the sum of knowledge regarding causes.

ethylene dibromide (EDB) A chemical used as an agricultural fumigant and in certain industrial processes that has been banned for most agricultural uses in the United States.

eubacteriales An order of the class Schizomycetes. Also known as “true bacteria.”

eucaryote An organism having one or more cells with well-defined nuclei.

eucaryotic cell A type of cell possessing a well-defined nuclear membrane.

Eularian measurement Observation of a current with a device fixed relative to the flow.

euphotic Of the upper layers of the water in which sufficient light penetrates to permit growth of green plants.

euryhaline Able to live in brackish waters of marine or non-marine origin and resistant to great changes in salinity.

eurythermal Able to live through a wide range of temperature conditions.

eurytopic Adaptation of species to widely varied conditions.

eustatic sea level rate The worldwide change of sea level elevation with time. The changes are due to such causes as glacial melting or formation, thermal expansion or contraction of sea water, etc.

eutrophic Reservoirs and lakes which are rich in nutrients and very productive in terms of aquatic animal and plant life.

eutrophication The aging process of a lake in the production of organic matter. The increase in nutrients required for the growth of organisms may come about by natural processes or rapid enrichment may occur due to the introduction of sewage effluent.

eutrophy In lakes, a condition that is rich in dissolved nutrients. See eutrophic.

evacuation time The lead-time that a populated coastal area must have to safely relocates all residents of vulnerable areas from an approaching hurricane. This time can also be perceived as the necessary amount of time between the local official evacuation order and the arrival of sustained gale force winds (40 mph) and/or flooding.

evaluation A process that attempts to determine as systematically and objectively as possible the relevance, effectiveness, and impact of activities in the light of their objectives.

evaporation The process by which water passes from the liquid to the vapor state. Water loss to the atmosphere (e.g., from surface water bodies, soil, etc.).

evaporation pan A pan used to hold water during observations for the determination of the quantity of evaporation at a given location. Such pans are of varying sizes and shapes, the most commonly used being circular or square.

evaporation ponds Areas where sewage sludge is dumped and dried.

evaporation rate The quantity of water, expressed in terms of depth of liquid water, which is evaporated from a given surface per unit of time. It is usually expressed in inches depth, per day, month, or year.

evaporimeter An instrument which measures the evaporation rate of water into the atmosphere.

evapotranspiration The combined loss of water from a given area and during a specified period of time, by evaporation from the soil surface, snow, or intercepted precipitation, and by the transpiration and building of tissue by plants.

evection A perturbation of the Moon depending upon the alternate increase and decrease of the eccentricity of its orbit, which is always a maximum when the Sun is passing the Moon's line of apsides and a minimum when the Sun is at right angles to it.

exceedance Violation of the pollutant levels permitted by environmental protection standards.

exceedence frequency Percentage of values that exceed a specified magnitude.

excess activated sludge The quantity of settled biomass or activated sludge above that needed for process operation.

excess rain Effective rainfall in excess of infiltration capacity.

excess risk Percentage with hearing impairment in an occupational-noise-exposed population after subtracting the percentage who would normally incur such impairment from other causes in a population not exposed to occupational noise.

exchange rate An increment of decibels that requires the halving of exposure time, or a decrement of decibels that requires the doubling of exposure time. For examples, a 3-dB exchange rate requires that noise exposure time be halved for each 3-dB increase in noise level; likewise, a 5-dB exchange rate requires that exposure time be halved for each 5-dB increase.

exclusion In the asbestos program, one of several situations that permit a Local Education Agency (LEA) to delete one or more of the items required by the Asbestos Hazard Emergency Response Act (AHERA).

exclusionary ordinance Zoning that excludes classes of persons or businesses from a particular neighborhood or area.

exclusive flood control storage capacity The space in a reservoir reserved for the sole purpose of regulating flood inflows to abate flood damage.

excretion Elimination of waste material from the body of an organism.

excystation The release of the internal (i.e., encysted) contents (e.g., trophpzoites or sporozoites) from cysts or oocysts.

execution of decree The carrying out of the court's order.

exempted aquifer Underground bodies of water defined in the Underground Injection Control program as aquifers that are potential sources of drinking water though not being used as such and thus are exempted from regulations barring underground injection activities.

exemption A state with primacy may relieve a public water system from a requirement respecting an MCL, treatment technique, or both by granting an exemption if the system cannot comply due to compelling economic or other factors; the system was in operation on the effective date of the requirement or MCL; and the exemption will not create an unreasonable public health risk. Also see variance.

exempt solvent Specific organic compounds not subject to requirements of regulation because are deemed to be of negligible photochemical reactivity.

exfiltration Leakage from a conveyance system or storage area into the surrounding and underlying materials. This process will occur if the ambient ground-water pressure is less than the internal pressure of the conveyance system or storage area at a breach.

exhaust 1) To use up as in the ion-exchange capacity of a resin or adsorption capacity of activated carbon. 2) To let out or discharge from; the device or arrangement that performs the discharge.

exonucleases Enzymes that catalyze the removal of nucleotides from the ends of a dna molecule.

exotherms Organisms, such as fish, reptiles, and insects, that cannot regulate their own body temperature independently of the temperature of their surroundings.

exotic Of or related to any non-native or rare species, usually introduced.

exotic species A species that is not indigenous to a region.

exotoxin A toxin produced by a bacteria that is released by the bacterial cell into the culture medium (or host) and, thus, found in cell-free filtrates and cultures of intact bacteria.

ex parte On one side only (legal cases in which only one side is represented).

experimental process A nonconventional treatment operation or equipment for which performance reliability is unknown and must be verified by a controlled monitoring and testing program.

experimental study A study in which the investigator specifies the exposure category for each individual (clinical trial) or community (community trial), then follows the individuals or community to detect the effects of the exposure.

experimental use permit Obtained by manufacturers for testing new pesticides or uses of thereof whenever they conduct experimental field studies to support registration on 10 acres or more on land or one acre or more of water.

exploratory data analysis A method of data analysis that emphasizes the use of graphical and statistical techniques to isolate patterns and features in a data set, revealing these forcefully to the data analyst.

explosion The rapid release of a large amount of energy within a limited space.

explosive deepening A decrease in the minimum sea-level pressure of a tropical cyclone of 2.5 mb/hr for at least 12 hours or 5 mb/hr for at least six hours.

explosive limits The amounts of vapor in the air that form explosive mixtures; limits are expressed as lower and upper limits and give the range of vapor concentrations in air that will explode if an ignition source is present.

exposed [group] A group whose members have been exposed to a supposed cause of disease or health state of interest, or possess a characteristic that is a determinant of the health outcome of interest.

exposure 1) Contact with a chemical or physical agent through either a direct or indirect mode of transmission. An effective exposure is one in which the exposure actually results in infection. 2) A measurement of the total amount of radiation to which an individual is exposed related to the ionization produced in air by x-ray or gamma radiation. Similar to dose.

exposure assessment The determination or estimation (qualitative or quantitative) of the magnitude, frequency, duration, route, and extent (number of people) of exposure to a chemical.

exposure coefficient Term which combines information on the frequency, mode, and magnitude of contact with contaminated medium to yield a quantitative value of the amount of contaminated medium contacted per day.

exposure indicator A characteristic of the environment measured to provide evidence of the occurrence or magnitude of a response indicator’s exposure to a chemical or biological stress.

exposure level, chemical The amount (concentration) of a chemical at the absorptive surfaces of an organism.

exposure scenario A set of conditions or assumptions about sources, exposure pathways, concentrations of toxic chemicals and populations (numbers, characteristics, and habits) which aid the investigator in evaluating and quantifying exposure in a given situation.

ex situ Moved from its original place; excavated; removed or recovered from the subsurface.

extended aeration

extended-duration outage

extent [GIS] The area over which the coverage contains data.

extent of evacuation The identification of vulnerable people who must evacuate based on estimated damage and/or homes susceptible to hurricane force winds.

external validation The application of the developed model to a new data set (validation data set) from another study.

extraction procedure (E P toxic) Determining toxicity by a procedure which simulates leaching; if a certain concentration of a toxic substance can be leached from a waste, that waste is considered hazardous, i.e., E P Toxic.

extraction well A well employed to extract fluids (either water, gas, free product, or a combination of these) from the subsurface. Extraction is usually accomplished by either a pump located within the well or suction created by a vacuum pump at the ground surface.

extrapolation Estimation of unknown values by extending or projecting from known values.

extratropical A term used in advisories and tropical summaries to indicate that a cyclone has lost its "tropical" characteristics. The term implies both poleward displacement of the cyclone and the conversion of the cyclone's primary energy source from the release of latent heat of condensation to baroclinic (the temperature contrast between warm and cold air masses) processes. It is important to note that cyclones can become extratropical and still retain winds of hurricane or tropical storm force.

extratropical cyclone A cyclone in the middle and high latitudes often being 2000 kilometers in diameter and usually containing a cold front that extends toward the equator for hundreds of kilometers. These cyclones forms outside the tropics, the center of storm is colder than the surrounding air, have fronts and the strongest winds in the upper atmosphere.

extreme high water The highest elevation reached by the sea as recorded by a tide gauge during a given period. The National Ocean Service routinely documents monthly and yearly extreme high waters for its control stations.

extreme low water The lowest elevation reached by the sea as recorded by a tide gauge during a given period. The National Ocean Service routinely documents monthly and yearly extreme low water for its control stations.

extremely hazardous substances Any of 406 chemicals, subject to revision, identified by EPA as toxic and listed under SARA Title III.

eye The center of a tropical storm or hurricane characterized by a roughly circular area of light winds and rain-free skies and the lowest pressure. An eye will usually develop when the maximum sustained wind speeds exceed 78 mph. It can range in size from as small as 5 miles to up to 60 miles (20-50 km) but the average size is 20 miles. In general, when the eye begins to shrink in size, the storm is intensifying.

eye wall An organized band of convection surrounding the eye, or center, of a tropical cyclone. It contains cumulonimbus clouds, severest thunderstorms, heaviest precipitation and strongest winds.