ubiquitous Being found in many widely divergent places; able to thrive under different conditions.

ultimate BOD A measure of dissolved oxygen required for biological oxidation of carbon and nitrification of organic matter in wastewater. See BOD.

ultra clean coal (UCC) Coal that is washed, ground into fine particles, then chemically treated to remove sulfur, ash, silicone, and other substances; usually briquetted and coated with a sealant made from coal.

ultra-low-flush toilet A toilet that uses not more than 1.6 gallons per flush.

ultraviolet light Light, not visible to the eye, at the violet end of the light spectrum.

ultraviolet radiation

ultraviolet rays Radiation from the sun that can be useful or potentially harmful. UV rays from one part of the spectrum (UV-A) enhance plant life and are useful in some medical and dental procedures; UV rays from other parts of the spectrum (UV-B) can cause skin cancer or other tissue damage. The ozone layer in the atmosphere partly shields us from ultraviolet rays reaching the earth's surface.

unaccounted-for water The amount of nonaccount water less known or estimated losses and leaks.

uncertainty factor A number (equal to or greater than one) used to divide NOAEL or LOAEL values derived from measurements in animals or small groups of humans in order to estimate a NOAEL value for the whole human population.

unconfined aquifer An aquifer characterized by the absence of an aquitard above it, so that the water table forms the upper boundary of the aquifer and is free to move with atmospheric influences such as atmospheric pressure. Also referred to as a water table aquifer.

underdrain Channel used to collect flow from beneath a treatment unit using controlled spacing of inflow.

underfilm corrosion Corrosion that occurs under lacquers and other organic films in the form of randomly distributed hairlines. Also called filiform corrosion.

underground injection control The program under the Safe Drinking Water Act that regulates the use of wells to pump fluids into the ground.

underground sources of drinking water Aquifers currently being used as a source of drinking water or those capable of supplying a public water system. They have a total dissolved solids content of 10,000 milligrams per liter or less and are not “exempted aquifers.”

underground storage tank (UST) A tank located at least partially underground and designed to hold gasoline or other petroleum products or chemicals.

undersaturated solution A solution that contains less of a substance than needed to saturate it.

understory Vegetation zone lying between the forest canopy layer (overstory) and the vegetation covering the ground (ground cover).

uniform corrosion Corrosion that results in an equal amount of material loss over an entire pipe surface.

uniformity coefficient Range in particle size equal to the ratio of seive sizes in which the size that passes 60 percent of the particles divided by the size that passes ten percent of the particles.

uniform rate A pricing structure for which the dollar amount charged per unit of water (such as dollars per gallon) does not vary with the amount of water usage.

unit cancer risk Estimate of the lifetime risk caused by each unit of exposure in the low exposure region.

unit hydrograph The hydrograph of one inch of storm runoff generated by a rainstorm of fairly uniform intensity within a specific period of time, usually measured in cubic feet per second.

unit-hydrograph method.

unit hydrograph theory Surface runoff hydrographs for storm events of the same duration will have the same shape, and the ordinates of the hydrograph will proportional to the ordinates of the unit hydrograph. For example, the discharge from one-half inch of runoff will be half of that from the unit hydrograph.

universal metering Metering of all water-service connections.

universal time coordinate (UTC) One of several names for the twenty-four hour time that is used throughout the scientific and military communities. Other names for this time measurement are Greenwich Mean Time (GMT) or Zulu Time (Z). See Greenwich Mean Time for more information.

unmetered water Water delivered but not measured for accounting and billing purposes.

unreasonable risk Under FIFRA, any unreasonable risk to man or the environment after consideration of the medical, economic, social, and environmental costs and benefits of any pesticide.

unregulated contaminant (UC) A contaminant for which a monitoring requirement has been established but for which no MCL or treatment technique requirement has been established.

unsaturated zone The subsurface zone between the land surface and water table in which the pore spaces are only partially filled with water. Also called zone of aeration.

upflow Inflent moving from the bottom of a treatment unit to the surface where the effluent is withdrawn.

upland All types of land forms other than water-filled depressions or areas in close proximity to rivers, streams, flood plains, beaches, salt marshes, etc.

upland game A term describing animals.

uplink/downlink Programs are "uplinked" to the satellite transponder and "downlinked" to the ground receiving dish.

uploading The transfer of copies of a file from the user's own computer to a remote database or other computer. The reverse of downloading.

upper-bound estimate Estimate not likely to be lower than the true risk.

upset An unexpected change or disturbance in process performance.

uptake The entrance of a chemical into an organism -- such as by breathing, swallowing, or absorbing it through the skin -- without regard to its subsequent storage, metabolism, and excretion by that organism.

upwelling The process by which water rises from a lower to a higher depth, usually as a result of divergence and offshore currents.

uranium A type of atom used to fuel nuclear reactors due to its ability to undergo fission with a free neutron creating a nuclear chain reaction and resulting in heat.

urbanized area An area containing one or more cities of 50,000 inhabitants and includes that portion of the surrounding territory maintaining the criterion of a population density of at least 1,000 persons per square mile.

urban population All persons living in an urbanized area or in places of 2,500 inhabitants or more outside urbanized areas. Places may be either incorporated or unincorporated.

urban runoff Storm water from city streets and adjacent domestic or commercial properties that may carry pollutants of various kinds into the sewer systems and/or receiving waters.

URLs (Uniform Resource Locators). The address system used by the Internet to locate resources such as web sites. An URL includes the type of resource being accessed such as gopher or hypertext), the address of the server, and the location of the file. For example, the complete URL for the PHTN Web site is <http://www.cdc.gov/phtn/index.htm>. "http://" indicates the access method as Hyper Text Transfer Protocol. "www.cdc.gov" is the address of the server. "/phtn/" specifies the directory the file is located. "index.html" is the initial page of the PHTN Web site. Web browsers will assume "http://" and "index.html", so you can simply use <www.cdc.gov/phtn> as the URL.

USDA United States Department of Agriculture.

used water Any water supplied by a water purveyor from the waterworks to a consumer's water system after it has passed through the service connection.

user class See customer class.

user fee A fee which is collected only from those persons who use a particular service, as opposed to one collected from the public in general. User fees generally vary in proportion to the degree of use of the service.

UST Underground storage tank.

utility load The total electricity demand for a utility district.