ZONE — (1) (Ecology) An area characterized by similar flora or fauna; a belt or area to which certain species are limited. (2) (Engineering) In earth dams, a segment of the earthfill containing similar materials; earth structures may be divided into two or more segments or zones to make the best use of available materials.

ZONE OF AERATION — The comparatively dry soil or rock located between the ground surface and the top of the Water Table. A zone immediately below the surface of the ground, in which the openings are partially filled with air, and partially with water trapped by molecular attraction. Generally subdivided into: (a) belt of soil moisture; (b) intermediate belt; and (c) capillary fringe.

ZONE OF CONTRIBUTION (ZOC) — The area surrounding a pumping well that encompasses all areas or features that supply ground-water recharge to the well.

ZONE OF ELUVIATION — The two uppermost zones in the soil profile, consisting of the A-Horizon, from which soluble Salts and Colloids are leached, and in which organic matter has accumulated and generally constitutes the most fertile soil layer, and the B-Horizon, or the lower soil zone which is enriched by the deposition or precipitation of material from the overlying zone, or A-horizon.

ZONE OF ENGINEERING CONTROL — The area occupied by a hazardous waste treatment, storage, or disposal facility which the owner or operator can readily decontaminate if a leak is detected, thus preventing hazardous waste or its constituents from entering groundwater of surface water.

ZONE OF INFLUENCE (ZOI) — (1) (Hydrologic) The area surrounding a pumping well within which the water table or Potentiometric Surfaces has been changed due to ground-water withdrawal. (2) (Environmental) The geographic area whose social, economic, and/or environmental conditions is significantly affected by changes in the study area.

ZONE OF INITIAL DILUTION (ZID) — That area within a lake or stream where the discharge from an outfall first mixes with the receiving water.

ZONE OF NET METABOLIC PRODUCTION — (Biology) The region (or depth) of a body of water in which constructive metabolism (Anabolism or Assimilation) is equal to or greater than destructive metabolism (Catabolism or Dissimilation). Metabolism is the sum of the processes concerned in the building up of protoplasm and its destruction incidental to life; the chemical changes in living cells, by which the energy is provided for the vital processes and activities, and new material is assimilated to repair the waste. Metabolism may be considered as including two aspects or processes: constructive metabolism (Anabolism) or destructive metabolism (Catabolism). Anabolism and Catabolism go on together, but one may predominate and obscure the other. Below this level (the Compensation Level), the destructive process tend to exceed the constructive processes. Also referred to as the Photic Zone.

ZONE OF SATURATION — Underground region within which all openings are filled with water. The top of the zone of saturation is called the Water Table. The water that is contained within the zone of saturation is called ground water.

ZONE OF TRANSPORT (ZOT) — The area surrounding a pumping well, bounded by an Isochrone and/or isoconcentration contour, through which a contaminant may travel and reach the well.

ZONING — The partition of a city, county, township, or other governmental unit or area by ordinance into sections reserved for different land-use purposes, such as residential, business, manufacturing, greenbelt, or agriculture.

ZOOGLEA — (Water Quality) A jelly-like matrix developed by bacteria which is abundant in activated sludge flocs and trickling filter slimes.