objective The lens or system of lenses in a compound microscope nearest the object being observed.

obligate aerobes Bacteria that must have molecular (dissolved) oxygen to survive.

obliquity factor A factor in an expression for a constituent tide (or tidal current) involving the angle of the inclination of the Moon's orbit to the plane of the Earth's Equator.

obliquity of the ecliptic The angle, approximately 23.45°, which the ecliptic makes with the plane of the Earth's Equator.

obliquity of the Moon's orbit The angle which the Moon's orbit makes with the plane of the Earth's Equator. Its value varies from 18.3° to 28.6°, depending upon the longitude of the Moon's ascending node; the smaller value corresponding to a longitude of 180° and the larger one, to a longitude of 0°.

observational study Epidemiological study in situations where nature is allowed to take its course. Changes or differences in one characteristic are studied in relation to changes or differences in others, without the intervention of the investigator.

observation well A non-pumping well used for observing the elevation of the water table or piezometric surface.

occluded front The front formed by a cold front overtaking a warm or stationary front and lifting the warm air above the earth's surface.

ocean discharge waiver A variance from Clean Water Act requirements for discharges into marine waters.

oceanography The science of all aspects of the oceans, in spite of its etymology.

occupational exposure limit

Occupational Safety and Health Act See OSHA. A Federal law designed to protect the health and safety of industrial workers.

odds ratio. A measure of association which quantifies the relationship between an exposure and health outcome from a comparative study; also known as the cross-product ratio.

odor threshold The minimum odor of a sample that can just be detected after successive dilutions with an odorless medium. Also called threshold odor.

Office of Drinking Water (ODW) The EPA office, under the management of the Office of Water, that is primarily responsible for implementing the Safe Drinking Water Act.

Office of Environmental Health Services (OEHS) The Virginia Department of Health’s The Office of Environmental Health Services is composed of two divisions. They are the Division of Food and Environmental Services and the Division of Onsite Sewage and Water Services. As a whole the office works to mitigate the risks which provide for the contraction and spread of diseases through unsafe and unhealthy environmental conditions.

Office of Water Programs (OWP) The Virginia Department of Health's Office of Water Programs coordinates and directs Virginia's water-based programs. A field office is responsible for each region of Virginia. The OWP has two broad divisions: the Division of Water Supply Engineering (responsible for public drinking water supply systems or waterworks) and the Division of Shellfish Sanitation (responsible for protecting consumers who eat shellfish and crustaceans).

offset 1) The difference between the actual value and the desired value (or set point); characteristic of proportional controllers that do not incorporate rest action. Also called droop. 2) A concept whereby emissions from a proposed new or modified stationary source are balanced by reductions from existing sources to stabilize total emissions. See: bubble, emissions trading, netting.

off-site facility A hazardous waste treatment, storage or disposal area that is located away from the generating site.

offstream use Water withdrawn or diverted from a ground- or surface-water source for another use, such as public-water supply, industry, irrigation, livestock, and thermoelectric power generation. Sometimes called off-channel use or withdrawal.

OHM The unit of electrical resistance. The resistance of a conductor in which one volt produces a current of one ampere.

oil desulfurization A widely used precombustion method for reducing sulfur dioxide emissions from oil-burning power plants. The oil is treated with hydrogen which removes some of the sulfur by forming hydrogen sulfide gas.

oil fingerprinting A method that identifies sources of oil and allows spills to be traced to their source.

oil spill An accidental or intentional discharge of oil which can enter a body(ies) of water or spills from tanks and pipelines can also contaminate the soil, get into sewer systems and threaten underground water sources. Can be controlled by chemical dispersion, combustion, mechanical containment, and/or adsorption.

olfactory fatigue A condition in which a person's nose, after long exposure to certain odors, is no longer able to detect the odor.

olgoaerobe Organism that thrives at low oxygen concentrations.

oligoaeorobic Conditions of low oxygen pressure or concentration.

oligonucleotides Short segments of dna or rna; a chain of a few nucleotides.

oligotrophic Reservoirs and lakes which are nutrient poor and contain little aquatic plant or animal life.

omnivorous Eating a wide variety of plant and animal food.

onboard controls Devices placed on vehicles to capture gasoline vapor during refueling and route it to the engines when the vehicle is starting so that it can be efficiently burned.

oncology Study of cancer.

one-hit model Mathematical model based on the biological theory that a single “hit” of some minimum critical amount of a carcinogen at a cellular target– namely DNA– can initiate an irreversible series of events, eventually leading to a tumor.

one-hundred year flood Flood magnitude which has a one chance in one hundred of being exceeded in any future one-year period. The occurrence of floods is assumed to be random in time, or a regularity of occurrence is implied. The exceeding of a one percent chance flood is no guarantee, therefore, that a similar size flood will not occur next week. The risk of epxerienceing a large flood within time periods longer than one year increases in a nonadditive fashion. For example, the risk of exceeding a one percent chance flood (i.e., a one hundred year flood) one or more times during a thirty-year period is 25 percent and during a seventy-year period is 50 percent.

one hundred year flood level The flood elevation which will, over a long period of time, be equaled or exceeded on the average once every 100 years.

one-way video/two-way audio An interactive conference, class, or meeting in which participants see and hear the speaker(s) at the originating site as well as hear participants at other receiving sites. Two-way audioconferencing is used for the real-time verbal interaction.

ONPG An organic chemical substrate, o-nitrophenyl-ß-D-glactopyranoside used to detect ß-D-galactoside activity in coliforms. Using this agent, a yellow color indicates the presence of coliforms.

on-scene coordinator (OSC) The predesignated regulatory who coordinates and directs Superfund removal actions or Clean Water Act oil or hazardousspill response actions.

on-site facility A hazardous waste treatment, storage or disposal area that is located on the generating site.

oocyst A microscopic, protective, reproductive form of protozoa, such as the infectious stage of Cryptosporidium parvum and some other coccidian parasites that has a protective wall that facilitates survival in water and other environments.

opacity 1) The state or quality of being opaque. 2) The amount of light obscured by particulate pollution in the air; clear window glass has zero opacity, a brick wall is 100 percent opaque. Opacity is an indicator of changes in performance of particulate control systems.

open burning Uncontrolled fires in an open dump.

open dump An uncovered site used for disposal of waste without environmental controls.

operable unit Term for each of a number of separate activities undertaken as part of a Superfund site cleanup. A typical operable unit would be removal of drums and tanks from the surface of a site.

operating conditions Conditions specified in a RCRA permit that dictate how an incinerator must operate as it burns different waste types. A trial burn is used to identify operating conditions needed to meet specified performance standards.

operation and maintenance costs The ongoing, repetitive costs of operating a treatment works; for example, employee wages and costs for treatment chemicals and periodic equipment repairs.

operator Any individual employed or appointed by any owner who is designated by such owner to be the person in responsible charge, such as a supervisor, a shift operator, or a substitute in charge, and whose duties include testing or evaluation to control waterworks operations. Not included in this definition are superintendents or directors of public works, city engineers, or other municipal or industrial officials whose duties do not include the actual operation or direct supervision of waterworks.

operator gene A gene which switches on adjacent structural gene(s).

operator-in-training An individual employed or appointed by any owner to work under the direct supervision and direction of an operator holding a valid license in the proper category and classification for the purpose of gaining experience and knowledge in the duties and responsibilities of an operator of a waterworks or wastewater works. An operator-in-training is not an operator.

operon Complete unit of bacterial gene expression consisting of a regulator gene(s), control elements (promoter and operator), and adjacent structural gene(s).

opportunity cost The value of a foregone opportunity that canot be pursued because resources are taken by a chosen activity.

optimal corrosion control treatment The corrosion control treatment that minimizes the lead and copper concentrations at users' taps while insuring that the treatment does not cause the waterworks to violate any other Waterworks Regulations.

oral Of the mouth; through or by the mouth.

oral toxicity Ability of a pesticide to cause injury when ingested.

order 1) A category of biological classification ranking above the family and below the class. 2) A formal written statement from a court, requiring action or simply stating a ruling.

ordinal scale Classification into ordered qualitative categories; e.g., social class (I, II, III, etc.), where the values have a distinct order, but their categories are qualitative in that there is no natural (numerical) distance between their positive values.

ordinary With respect to tides, the use of this nontechnical word has, for the most part, been determined to be synonymous with mean. Thus, ordinary high (low) water is the equivalent of mean high (low) water. The use of ordinary in tidal terms is discouraged.

ores A metal-bearing mineral or rock, or a native metal, that can be mined at profit. A mineral or natural product serving as a source of some nonmetallic substance, as sulfur.

organic 1) Substances that come from animal or plant sources. Organic substances always contain carbon. 2) In chemistry, any compound containing carbon. Also see inorganic.

Organic Act of 1890 The act that assigned the responsibility of river and floor forecasting for the benefit of the general welfare of the nation's people and economy to the Weather Bureau, and subsequently the National Weather Service.

organic chemical or compound A natural or synthetic chemical containing carbon.

organic deposit

organic detritus Particles or fragments of a larger living or recently dead body produced by disintegration. In aquatic systems, finely divided, settleable particles whose destruction consumes oxygen.

organic matter The material of a living organism and/or the remains or a key byproduct of a living organism.

organics 1) A term used to refer to chemical compounds made from carbon molecules. These compounds may be natural materials (such as animal or plant sources) or man-made materials (such as synthetic organics). Also see organic. 2) Any form of animal or plant life. Also see bacteria.

organic vapor analyzer A portable instrument used to detect a variety of organic compounds in air, soil, and water.

organic waste Waste material from animal or vegetable sources. Organic waste can generally be consumed by bacteria and other small organisms.

organism Any living or recently dead thing.

organophosphates Pesticides that contain phosphorus, short-lived, but some can be toxic when first applied.

organotins Chemical compounds used in anti-foulant paints to protect the hulls of boats and ships, buoys, and pilings from marine organisms such as barnacles.

orifice 1) An opening (hole) in a plate, wall, or partition. An orifice flange or plate placed in a pipe consists of a slot or a calibrated circular hole smaller than the pipe diameter. The difference in pressure in the pipe above and at the orifice may be used to determine the flow in the pipe. 2)The end of a small tube, such as a Pitot tube, piezometer, etc. 3) See stilling well and protective well.

original AHERA inspection An examination (performed by accredited inspectors) of school buildings arranged by Local Education Agencies to identify asbestos-containing materials, evaluate their condition, and take samples of materials suspected to contain asbestos.

original generation point Where regulated medical or other material first becomes waste.

originating site The site initiating the conference or meeting that is to be distributed simultaneously with technology such as audioconferencing or videoconferencing.

orographic Related to, or caused by, physical geography (such as mountains or sloping terrain).

orographic lift Lifting of air caused by its passage up and over mountains or other sloping terrain.

orographic precipitation Precipitation which is caused by hills or mountain ranges deflecting the moisture-laden air masses upward, causing them to cool and precipitate their moisture.

ORP Oxidation-Reduction Potential. The electrical potential required to transfer electrons from one compound or element (the oxidant) to another compound or element (the reductant); used as a qualitative measure of the state of oxidation in water treatment systems.

orthotolidine Orthotolidine is a colorimetric indicator of chlorine residual. If chlorine is present, a yellow-colored compound is produced. This reagent is no longer approved for chemical analysis.

OSHA The Williams-Steiger Occupational Safety and Health Act of 1970. OSHA is a law designed to protect the health and safety of industrial workers and also the operators of water supply systems and treatment plants. OSHA also refers to the federal agency that administers the OSHA regulations.

osmosis The passage of a liquid from a weak solution to a more concentrated solution across a semipermeable membrane. The membrane allows the passage of the solvent (water), but not the dissolved solids (solutes). This process tends to equalize the conditions on either side of the membrane.

outbreak The occurrence of two or more cases that are epidemiologically related.

outfall The place where effluent is discharged into receiving waters.

outflow channel A natural stream channel which transports reservoir releases.

outlet An opening through which water can be freely discharged from a reservoir.

outlet discharge structure An appurtenance that protects the downstream end of the outlet pipe from erosion and is often designed to slow down the speed of released water to prevent erosion of the stream channel.

overall efficiency, pump The combined efficiency of a pump and motor together. Also called the wire-to-water efficiency.

overburden Rock and soil cleared away before mining.

overdraft The pumping of water from a groundwater basin or aquifer in excess of the supply flowing into the basin. This pumping results in a depletion or mining of the groundwater in the basin.

overfalls Breaking waves caused by the meeting of currents or by waves moving against the current. See rip.

overfire air Air forced into the top of an incinerator or boiler to fan the flames.

overflow rate One of the loading rates for the design of settling basins and clarifiers in treatment plants. Used by operators to determine if tanks and clarifiers are hydraulically over- or under loaded (flow). Also called surface loading.

Overflow rate (gpd/sq ft) = Flow (gpd)/Surface Area (sq ft)

overland flow 1) A land application technique than treats wastewater by allowing it to flow uniformly over a sloped soil surface and through the root zone of surface vegetation. As the water flows over and through the soil certain contaminants are removed and the treated flow is collected at the bottom of the slope for reuse or further processing prior to discharge. 2) The flow of rainwater or snowmelt over the land surface toward stream channels. After it enters a watercourse it becomes runoff.

oversized regulated medical wastes Medical waste that is too large for plastic bags or standard containers.

overtide A harmonic tidal (or tidal current) constituent with a speed that is an exact multiple of the speed of one of the fundamental constituents derived from the development of the tide-producing force. The presence of overtides is usually attributed to shallow water conditions.

overturn The almost spontaneous mixing of all layers of water in a reservoir or lake when the water temperature becomes similar from top to bottom. This may occur in the fall/winter when the surface waters cool to the same temperature as the bottom waters and also in the spring when the surface waters warm after the ice melts.

owner An individual, group of individuals, partnership, firm, association, institution, corporation, governmental entity or the federal government, which supplies or proposes to supply water to any person within this Commonwealth from or by means of any waterworks. See waterworks.

oxidant A substance that contributes electrons during an oxidation-reduction usually containing oxygen that reacts chemically to produce a new substance, i.e., the primary ingredient of photochemical smog.

oxidase An enzyme which acts by oxidation.

oxidation The addition of oxygen, removal of hydrogen, or the removal of electrons from an element or compound. In the environment, organic matter is oxidized to more stable substances. The opposite of reduction.

oxidation ditch A continuous channel type of biological reactor in which flow is propelled by mixing and aeration devices past an influent point and effluent is usually withdrawn at a point prior to the flow passing by the influent. The suspended biomass is maintained in a high mass level with an extended sludge age.

oxidation pond A man-made lake or body of water in which organic wastes are consumed or oxidized by bacteria. It is used most frequently with other waste treatment processes. An oxidation pond is basically the same as a sewage lagoon.

oxidation-reduction potential (ORP) The electrical potential required to transfer electrons from one compound or element (the oxidant) to another compound or element (the reductant); used as a qualitative measure of the state of oxidation in water treatment systems.

oxidation-reduction reaction A chemical transformation in which electrons are transferred from one chemical (the reducing agent) to another chemical (the oxidizing agent). In oxidation-reduction, reactions involving the transfer of oxygen from one molecule to another, the molecule losing the oxygen is the oxidizing agent and the molecule gaining the oxygen is the reducing agent.

oxidizing agent Any substance, such as oxygen or chlorine that will readily take on (add) electrons. The opposite is a reducing agent.

oxygenated fuel Gasoline that has been blended with alcohols or ethers that contain oxygen in order to reduce carbon monoxide and other emissions.

oxygenated solvent An organic solvent containing oxygen as part of the molecular structure. Alcohols and ketones are oxygenated compounds often used as paint solvents.

oxygen concentration cell A galvanic cell caused by a difference in oxygen concentration at two points on a metal surface.

oxygen depletion Removal or exhaustion of oxygen by chemical or biological use, including the removal of dissolved oxygen from water.

oxygen meter An instrument that measures the atmospheric oxygen concentration directly by means of a galvanic cell.

oxygen sag A drop in oxygen concentration (usually at night) due to biological respiration, such as the oxidation of organic waste by bacteria in water which exceeds the rate of addition of oxygen to water through aeration.

oxygen uptake rate The rate at which oxygen is used for oxidation of organic waste as in wastewater under aeration.

oxygen utilization The oxygen consumed to support aerobic biological treatment processes.

ozonation The application of ozone to water for disinfection or for taste and odor control.

ozonator A device that applies ozone to water for disinfection and/or taste and odor control.

ozone (O₃) Ozone is a natural form of oxygen that provides a protective layer shielding the earth from ultraviolet radiation and an oxidant. Found in two layers of the atmosphere, the stratosphere and the troposphere. In the troposphere (the layer extending up 7 to 10 miles from the earth's surface and below the atratosphere), ozone is a chemical oxidant and major component of photochemical smog. Ozone in the troposphere is produced through complex chemical reactions of nitrogen oxides, which are among the primary pollutants emitted by combustion.

ozone depletion Destruction of the stratospheric ozone layer which shields the earth from ultraviolet radiation.

ozone layer The protective layer in the stratosphere, about 7-10 miles above the ground, that absorbs some of the sun's ultraviolet rays, thereby reducing the amount of potentially harmful radiation reaching the earth's surface.