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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.
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. 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.
Uncertainty Analysis: An e valuation of how: 1) definition of the physical setting; 2) model applicability and assumptions; 3) transport, fate and exposure parameter values; and 4) magnification of potential error through the steps of the risk assessment contribute to uncertainty in the results.
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.
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." (See: exempted aquifer.)
Underground Storage Tank (UST): A tank located at least partially underground and designed to hold gasoline or other petroleum products or chemicals.
Unrestricted Use: The designation of acceptable future use for a site at which the remediation levels, based on either background or standard residential exposure factors, have been attained throughout the site in all media.
Unsaturated Zone: The area above the water table where soil pores are not fully saturated, although some water may be present.
Untreated regulated medical waste: Waste that has not been treated to substantially reduce or eliminate its potential for causing disease. (See: treated medical waste, destroyed medical waste, and regulated medical waste)
Upper Detection Limit: The largest concentration that an instrument can reliably detect.
Use Cluster: A set of competing chemicals, processes, and/or technologies that can substitute for one another in performing a particular function.
Used Oil: Spent motor oil from passenger cars and trucks collected at specified locations for recycling (not included in the category of municipal solid waste).
Utility Load: The total electricity demand for a utility district.