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U

UBIQUITOUS HYDROPHYTES—Plant species that grow either in wetland or on upland areas—e.g., red maple (Acer rubrum) as opposed to Obligate Hydrophytes, which are plant species that are found only in wetlands—e.g., cattail (Typha latifolia).

UBIQUITOUS ORGANISMS—Organisms that can tolerate a wide range of environmental conditions or variations. Also, organisms that are so active or so numerous as to seem to be present or existent in all types of environments.

UDI—See Ground Water Under the Direct Influence (UDI) of Surface Water.

UDOMETER—A Rain Gauge.

ULTRAFILTRATION—Filtration through a medium (as a semipermeable capillary wall) which allows small molecules (as of water) to pass but holds back larger ones (as of protein).

ULTRAVIOLET RADIATION (UV)—Light waves shorter than the visible blue-violet waves of the spectrum, i.e., below 3900 Angstroms (Å). UV can be used for the disinfection of water.

UNBIASED SAMPLE—(Statistics) A sample is said to be unbiased if its behavior and characteristics are representative of the total Population.

UNCONFINED—Conditions in which the upper surface of the Zone of Saturation forms a water table under atmospheric pressure.

UNCONFINED AQUIFER—An aquifer containing water that is not under pressure; the water level in a well is the same as the water table outside the well. An unconfined aquifer made up of loose material, such as sand or gravel, that has not undergone lithification (settling). In an unconfined aquifer the upper boundary is the top of the Zone of Saturation (water table).

UNCONSOLIDATED DEPOSITS (SEDIMENT)—Sediment not cemented together; may consist of sand, silt, clay, and organic material.

UNCONSOLIDATED FORMATION—Natural earth formations that have not been turned to stone, such as alluvium, soil, gravel, clay, sand and overburden.

UNDERCURRENT—A current, as of air or water, below another current or beneath the surface.

UNDERFLOW—(1) (Surface and Groundwater) The downstream flow of water through the permeable deposits underlying a stream. (2) (Water Quality) The slurry of concentrated solids or Sludge that is removed from the bottom of a Settling Tank, Clarifier, or Thickener.

UNDERDRAIN—A drain that carries away water from prepared beds or agricultural fields to which water or wastewater has been applied.

UNDERGROUND INJECTION CONTROL (UIC)—A program required in each state by a provision of the Safe Drinking Water Act (SDWA) for the regulation of Injection Wells, including a permit system. An applicant must demonstrate that the well has no reasonable chance of adversely affecting the quality of an underground source of drinking water before a permit is issued.

UNDERGROUND SOURCE OF DRINKING WATER (USDW)—An aquifer that is currently being used as a source of drinking water or those potentially 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—(Water Quality) (1) Any one or combination of underground tanks and any connecting underground pipes used to contain an accumulation of regulated substances. (2) A tank located at least partially underground and designed to hold gasoline or other petroleum products or chemicals.

UNDERGROUND WATER—Water below the surface of the ground. Also referred to as Groundwater, Ground Water, Subsurface Water, and Subterranean Water.

UNDERSTORY—Plants growing beneath the canopy of other plants. Usually refers to grasses, forbs, and low shrubs under a tree or shrub overstory.

UNIT DENSITY—A density of one gram per cubic centimeter or one gram per milliliter; the density of water at 4°C (39.2°F).

UNITED STATES ARMY CORPS OF ENGINEERS (COE)—See (United States) Army Corps of Engineers (COE). [See Appendix C-2 for the U.S. Army Corps of Engineers' organizational structure and primary missions and objectives.]

UNITED STATES BUREAU OF INDIAN AFFAIRS (BIA)—See (United States) Bureau of Indian Affairs (BIA).

UNITED STATES BUREAU OF LAND MANAGEMENT (BLM)—See (United States) Bureau of Land Management (BLM).

UNITED STATES BUREAU OF RECLAMATION (USBR)—See (United States) Bureau of Reclamation (USBR).

UNITED STATES DEPARTMENT OF THE INTERIOR (USDI) —See (United States) Department of the Interior (USDI).

UNITED STATES ENVIRONMENTAL PROTECTION AGENCY (EPA)—See (United States) Environmental Protection Agency (EPA). [See Appendix E-2 for a more complete description of the organizational structure of the U.S. Environmental Protection Agency.]

UNITED STATES FISH AND WILDLIFE SERVICE (USFWS)—See (United States) Fish and Wildlife Service (USFWS).

UNITED STATES FOREST SERVICE (USFS)—See (United States) Forest Service (USFS).

UNITED STATES GEOLOGICAL SURVEY (USGS)—See (United States) Geological Survey (USGS).

UNITED STATES NATURAL RESOURCES CONSERVATION SERVICE (NRCS)—See (United States) Natural Resources Conservation Service (NRCS). Formerly known as the Soil Conservation Service (SCS)

UNITED STATES SOIL CONSERVATION SERVICE (SCS)—Former name of the (United States) Natural Resources Conservation Service (NRCS).

UNIT HYDROGRAPH—(1) The Hydrograph of direct runoff from a storm uniformly distributed over the drainage basin during a specified unit of time; the hydrograph is reduced in vertical scale to correspond to a volume of runoff from the drainage basin of one inch. (2) The hydrograph of surface runoff (not including groundwater runoff) on a given basin due to an effective rain falling for a unit of time.

UNIT PERIOD—The time duration of a unit storm. See Unit Storm, below.

UNIT STORM—A net rainfall one-inch deep which occurs over all parts of a drainage area at a uniform rate during a specified unit period of time.

UNMEASURED SEDIMENT DISCHARGE—The difference between Total Sediment Discharge and measured Suspended-Sediment Discharge.

UNSATURATED FLOW—Movement of water in a porous medium in which the pore spaces are not filled with water and the direction of flow is from the wetter zone of higher potential to one of lower potential.

UNSATURATED ZONE—The subsurface zone between the water table (Zone of Saturation) and the land surface where some of the spaces between the soil particles are filled with air. It includes the root zone, intermediate zone, and capillary fringe. The pore spaces contain water, as well as air and other gases at less than atmospheric pressure. Saturated bodies, such as Perched Ground Water, may exist in the unsaturated zone, and water pressure within these bodies may be greater than atmospheric. Also referred to as the Vadose Zone or, less frequently, the Zone of Aeration.

UNSTEADY FLOW—Flow that is changing with respect to time.

UPGRADIENT WELL—A groundwater monitoring well, such as those required at facilities that treat, store, or dispose of hazardous waste using surface impoundments or landfills, that allows sampling and analysis of groundwater that