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

Unsaturated Zone: The area above the water table where soil pores are not fully saturated, although some water may be present.

Upper Detection Limit: The largest concentration that an instrument can reliably detect.

Vadose Zone: The zone between land surface and the water table within which the moisture content is less than saturation (except in the capillary fringe) and pressure is less than atmospheric. Soil pore space also typically contains air or other gases. The capillary fringe is included in the vadose zone. (See: Unsaturated Zone1.)

Vapour: The gas given off by substances that are solids or liquids at ordinary atmospheric pressure and temperatures.

Vapour barrier:  An installation of polyethylene sheeting, to contain fuel oil vapour originating from the grossly contaminated area and isolates areas not impacted by spilled fuel oil.  Typically used in conjunction with vapour recovery/treatment to generate a negative air pressure within the isolated area.

Vapour Pressure: A measure of a substance's propensity to evaporate, vapor pressure is the force per unit area exerted by vapor in an equilibrium state with surroundings at a given pressure. It increases exponentially with an increase in temperature. A relative measure of chemical volatility, vapor pressure is used to calculate water partition coefficients and volatilization rate constants.

Vapour Recovery System: A system by which volatile gases are captured instead of being released into the atmosphere. (DLS uses an Air Phase Unit to accomplish this task.)

Variance: Government permission for a delay or exception in the application of a given act, guideline or regulation.

Viscosity: The molecular friction within a fluid that produces flow resistance.

Volatile: Any substance that evaporates readily.

Volatile Organic Compound (VOC): Any organic compound that participates in atmospheric photochemical reactions except those designated by EPA as having negligible photochemical reactivity.

VOC Cleaner:  A portable, trailer housed and MOE licensed system used to volatilize fuel oil vapours that have condensed on or been absorbed by some soft goods and furniture.  Once volatilized, the fuel oil is captured for treatment and removal.

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