| absorption | The incorporation of one substance into or through another of a different state (e.g., liquids in solids, gases in liquids). Unless it is certain that absorption is occurring, as opposed to adsorption, the term sorption should be used. |
| adsorption | Physical adhesion of vapor or dissolved matter to the surface of a solid. The term also refers to a method of treating wastes in which activated carbon removes organic matter from wastewater. Unless it is certain that adsorption is occurring, as opposed to absorption, the term sorption should be used. |
| advection | The process where liquids are transported at the velocity of the fluid. It is the primary transport mechanism for groundwater. |
| aeration | The act of exposing a substance to air, usually for the purpose oxidizing or volatilizing the substance. See technology description of Aeration. |
| aerobic | Living or occurring only in the presence of oxygen. |
| air-sparging | The process in which air is injected into the groundwater in a contaminated area. See technology description of Air Sparging. |
| air-stripping | A treatment process that removes or "strips" VOCs from contaminated groundwater as air is forced through the water, causing the compounds to evaporate. See technology description of Air Stripping. |
| alluvium | Unconsolidated clay, silt, sand or gravel deposited during relatively recent geological time by a stream or other water body. |
| anaerobic | Not capable of living in the presence of free oxygen |
| anion | An anion is an ion with a negative charge. |
| anoxic | Depravation of oxygen. |
| anthropogenic | Induced or altered by the activities of man. |
| aquifer | An underground rock formation composed of permeable materials such as sand, soil, or gravel that can store groundwater and supply it to wells and springs. |
| aquitard | A bed of almost impermeable material that retards, but does not prevent the movement of groundwater to or from an adjacent aquifer. |
| aromatic | Of, relating to, or containing the six-carbon ring typical of the benzene series and related organic groups. |
| augur | A large tool with a spiral cutting edge, as in a drill bit, used to bore holes into the earth. |
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