vacuum Any absolute pressure less than that exerted by the atmosphere.

vacuum breaker A device that permits air into a water supply distribution line to prevent backsiphonage.

vacuum filter

vacuum pump

vacuum valve

vadose

vadose water

vadose zone The subsurface zone where pore spaces are not completely filled with water.

valence The combining power of an atom; its capacity to combine with other atoms to form a molecule, expressed in terms of the number of hydrogen atoms or their equivalent with which any atom may combine.

validity The degree to which a measurement actually measures or detects what it is supposed to measure.

valence

valve

valve box

vapor An air dispersion of molecules of a substance that is liquid or solid in its normal state, i.e., at standard temperature and pressure. Vapors of organic liquids are loosely called fumes.

vapor capture system Any combination of hoods and ventilation systems that captures or contains organic vapors so they may be directed to an abatement or recovery device.

vapor density

vapor dispersion The movement of vapor clouds in air due to wind, thermal action, gravity spreading, and mixing.

vaporization

vapor plumes Flue gases visible because they contain water droplets.

vapor pressure The pressure exerted by a vapor in equilibrium with its solid or liquid phase.

variable Any characteristic or attribute that can be measured.

variable charge The portion of a water bill that varies with water usage; also known as a commodity charge.

variable costs 1) Input costs that change as the nature of the production activity of its circumstances change; for example, as production levels vary. 2) Costs associated with water service that vary with the amount of water produced or sold.

variance 1) A State with primacy may relieve a public water system from a requirement in respect to an MCL by granting a variance if certain conditions exist. These are: a) the system cannot meet the MCL in spite of the application of best available treatment technology, treatment techniques, or other means (taking costs into consideration), due to the characteristics of the raw water sources that are reasonably available to the system and b) the variance will not result in an unreasonable public health risk. A system may also be granted a variance from a specified treatment technique if it can show that, due to the nature of the system's raw water source, such treatment is not necessary to public health. Also see exemption. 2) A conditional waiver of a specific regulation which is granted to a specific waterworks. A PMCL variance is a variance to a primary maximum contaminant level, or to a treatment technique requirement. An operational variance is a variance to an operational regulation or to a secondary maximum contaminant level. Variances for monitoring, reporting and public notification requirements will not be granted. 3) An exemption granted to a specific regulatory standard to releive unnecessary economic burden. 4) A measure of the dispersion shown by a set of observations, defined by the sum of the squares of deviations from the mean, divided by the number of degrees of freedom in the set of observations.

varied flow

varying noise Noise levels that fluctuate considerably as a function of time.

vascular Containing vessels or channels for conveying fluids, such as blood or sap; also, tissues supplied with such channels.

vector 1) An organism that carries a disease, parasite, or infection. 2) In physics, a force that has both magnitude and directionality. 3) A plasmid, phage or cosmid into which foreign dna may be inserted for cloning.

vector control The process of controlling a disease, parasite, or infection by control of the carrier.

vegetation Plants in general or the total assemblage of plants and their gross appearance as determined by the largest and most common.

vegetation type A plant community of any size, rank, or state of development.

vegetative controls Nonpoint source polltion control practices that involve plants (vegetative cover) to reduce erosion and minimize the runoff of pollutants.

vegetative stage The stage of active growth as opposed to the resting or spore stage.

vehicle An inanimate intermediary in the indirect transmission of an agent that carries the agent from a reservoir to a susceptible host.

vehicle miles traveled (VMT) A measure of the extent of motor vehicle operation; the total number of vehicle miles travelled within a specific geographic area over a given period of time.

velocity coefficient

velocity gradient (G value) A measure of the mixing energy imparted to water or wastewater intended to induce flocculation.

velocity head

ventilation The act of admitting fresh air into a space in order to replace stale or contaminated air; achieved by blowing air into the space.

venturi A specially designed channel constriction and gradient change designed to induce critical depth at flows within an established range.

venturi scrubbers Air pollution control devices that use water to remove particulate matter from emissions.

venue Place of trial. If venue improper, a court lacks jurisdiction to hear a case.

verdict The decision of the trier of fact in a civil or criminal case.

verification test A test to verify suspected coliform colonies (sheen) in the Membrane Filter Procedure.

vernal Of spring.

vertebrate Those animals possessing a spinal column or backbone.

viable Living.

vibrio A slightly-curved organism which resembles a comma. Vibrio cholerae was once called vibrio comma.

video bridge Specialized equipment that permits three locations or more to be joined together in a videoconference.

videoconference A meeting, instructional session, or conversation between people at different locations relying on video technology as the primary communication link. Communication is 2-way audio with either 1-way or 2-way video. The term, videoconference, is sometimes used to refer to conferences via compressed video, conferences via land lines, and broadcasts via satellite. To avoid confusion, using the term or phrase which uniquely describes the communication technology is recommended.

vinyl chloride A chemical compound, used in producing some plastics, that is believed to be oncogenic.

virgin materials Resources extracted from nature in their raw form, such as timber or metal ore.

viricide An agent that kills viruses.

Virulence The capacity of a microorganism to produce disease or injury.

virus An intracellular parasitic microorganism smaller than bacteria and containing either RNA or DNA. It is the smallest form of microorgansims capable of causing disease, especially a virus of fecal origin that is infectious to humans by waterborne transmission.

viscosity Internal resistance to flow of a fluid that is a function of fluid temperature.

VOC Volatile organic chemical/compound(s).

volatile Readily vaporized at relatively low temperature.

volatile acids Acids produced during digestion. Fatty acids that are soluble in water and can be steam-distilled at atmospheric pressure. Also called “organic acids.” Volatile acids are commonly reported as equivalent to acetic acid.

volatile liquids Liquids that easily evaporate or vaporize at room temperature.

volatile matter See volatile solids.

volatile organic carbon (VOC) Any organic compound that participates in atmospheric photochemical reactions except for those designated by the EPA Administrator as having negligible photochemical reactivity. One of the family of manmade organic compounds generally characterized by low molecular weight and rapid vaporization at relatively low temperatures or pressures.

volatiles 1) Material that pass into a gaseous state at ordinary temperatures and pressures. 2) In geochemistry, substances that readily move or have moved through the earth's atmosphere.

volatile solids Defined by a testing method (such as Standard Methods), but may be roughly defined as combustible solids or those solids in water or other liquids that are lost on ignition of the dry solids at 550 °C.

volatilization 1) The process of forming vapor. 2) Loss of a substance through evaporation. When manure is spread on a field, ammonia-nitrogen in the manure may volatize quickly and be lost as fertilizer unless it is incorporated into the soil.

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

volume reduction Processing, such as compacting, shredding, or incineration, waste materials to decrease the amount of space they occupy.

volumetric A measurement based on the volume of some factor. Volumetric titration is a means of measuring unknown concentrations of water quality indicators in a sample by determining the volume of titrant or liquid reagent needed to complete particular reactions.

volute The spiral-shape casing surrounding a pump impeller that collects the liquid discharged by the impeller.

vortex 1) A revolving mass of water which forms a whirlpool. This whirlpool is caused by water flowing out of a small opening in the bottom of a basin or reservoir. A funnel-shaped opening is created downward from the water surface. 2) Any circular or rotary flow in the atmosphere that possesses vorticity

vortex fix The location of the surface and/or flight level center of a tropical or subtropical cyclone obtained by reconnaissance aircraft penetration.

vorticity The measurement of the rotation of a small air parcel. It has vorticity when the parcel spins as it moves along its path. Although the axis of the rotation can extend in any direction, meteorologists are primarily concerned with the rotational motion about an axis that is perpendicular to the earth's surface. If it does not spin, it is said to have zero vorticity. In the Northern Hemisphere, the vorticity is positive when the parcel has a counterclockwise, or cyclonic, rotation. It is negative when the parcel has clockwise, or anticyclonic, rotation.

vulnerability A system is considered vulnerable if the analysis of several factors shows susceptibility to contamination.

volumetric tank test One of several tests to determine the physical integrity of a storage tank; the volume of fluid in the tank is measured directly or calculated from product-level changes. A marked drop in volume indicates a leak.

vulnerable zone An area over which the airborne concentration of a chemical accidentally released could reach the level of concern.

vulnerability analysis Assessment of elements in the community that are susceptible to damage should a release of hazardous materials occur.