Leachate is a :

Concept

Leachate is defined as the liquid that forms when water — rainfall infiltrating from above, or moisture already held in the waste — percolates downward through a mass of deposited solid waste and carries dissolved and suspended matter with it. Two properties fix the definition: the phase is liquid, and the origin is percolation through a waste deposit. It is a liquid effluent of a disposal facility, not an airborne or atmospheric species.

Application

At a landfill, rain infiltrates the buried refuse. As the water travels down through the cell, anaerobic decomposition and chemical dissolution load it with organic acids, ammonia, chlorides, sulphates and heavy metals. This contaminated water accumulates on the liner at the base of the cell and is drawn off by the leachate collection system for treatment; if the liner fails, it migrates into soil and groundwater. That is exactly the description "liquid extract from landfill site" — a liquid drawn out of the waste mass at a disposal site.

Cross-check

Contrast the definition against the other environmental categories offered:

• Greenhouse gases (carbon dioxide, methane, nitrous oxide) are gaseous atmospheric constituents that absorb outgoing infrared radiation. A landfill does emit methane, but that gas is a separate output stream from the liquid one.

• Air pollutants (particulate matter, sulphur dioxide, oxides of nitrogen) are substances dispersed in ambient air, reaching people mainly by inhalation and regulated through ambient air-quality standards.

• Ozone-depleting substances (CFCs, halons, carbon tetrachloride) are volatile halogenated compounds that release chlorine or bromine radicals in the stratosphere and are phased out under the Montreal Protocol.

• Leachate is a liquid; its receptor is soil and groundwater, so it belongs to none of the three atmospheric categories above.

Result: leachate is the liquid extract obtained from a landfill site.