Xenobiota: Any biotum displaced from its normal habitat; a chemical foreign to a biological system.
Yard Waste: The part of solid waste composed of grass clippings, leaves, twigs, branches, and other garden refuse.
Yield: The quantity of water (expressed as a rate of flow or total quantity per year) that can be collected for a given use from surface or groundwater sources.
Zero Air: Atmospheric air purified to contain less than 0.1 ppm total hydrocarbons.
Zooplankton: Small (often microscopic) free-floating aquatic plants or animals.
Zone of Saturation: The layer beneath the surface of the land containing openings that may fill with water.