Glossary of Terms

Aerobic digestion – A biological treatment process that utilizes aerobic microbes to stabilize solids. In this process, bacteria thrive in an oxygen-rich environment to break down and digest the waste. Pollutants are broken down into carbon dioxide, water, and other oxidized end products, including nitrates and sulfates.

Anaerobic digestion – A biological treatment in which microorganisms break down biodegradable material in the absence of oxygen, often leading to a methane- and carbon dioxide-rich biogas suitable for energy production.

Aquifer – A subsurface, permeable rock layer that can yield significant quantities of water.

Aquitard – A subsurface rock layer that restricts water flow.

Coalbed methane (CBM) – Natural gas, primarily methane, that occurs naturally in the fractures and matrices of coal beds.

Confining bed – See aquitard.

Geographic information system (GIS) – Computer mapping system for creating, storing, analyzing, and managing spatial data and associated attributes.

Hydrogeologic system – Grouping of aquifers into one system based on the depositional age.

Lithology – The physical character of a rock or rock formation, described by depositional characteristics.

Parts per million (ppm) – A volume unit of measurement commonly employed to express the number of parts a chemical is contained within a million parts of gas (air), liquid (water), or solid (soil).

Produced water – Water that is generated during the extraction of fossil fuels.

Saline aquifer – An underground rock layer that contains water with dissolved solids (salt) concentrations higher than drinking water standards.

Softening – A process used to remove hardness as a result of calcium, magnesium, and some iron ions in a water supply.