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Manufactured Gas Plant2 (MGP)
MGPs were operated nationwide from the early 1880s through the mid-1900s. MGPs produced gas from coal or oil for lighting, heating, and cooking. The gas manufacturing and purification processes conducted at the plants yielded residues that included tars, sludges, lampblack, light oils, spent oxide wastes, and other hydrocarbon products. Although many of the byproducts were recycled, excess residues containing polycyclic acromatic hydrocarbons (PAH), petroleum hydrocarbons, benzene, cyanide, metals, and phenols remained at MGP sites.

Mass Spectrometry
Mass spectrometry is a method of chemical analysis in which the substance to be analyzed is heated and placed in a vacuum. The resulting vapor is exposed to a beam of electrons that causes ionization to occur, either of the molecules or their fragments. The ionized atoms are separated according to their mass and can be identified on that basis.

A medium is a specific environment -- air, water, or soil -- that is the subject of regulatory concern and activities.

Mercury is a heavy metal that can accumulate in the environment and is found in thermometers, measuring devices, pharmaceutical and agricultural chemicals, chemical manufacturing, and electrical equipment.

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The term methanogenic refers to anaerobic oxidation of petroleum hydrocarbons, as well as fermentation of hydrocarbons to methane.

Methyl tertiary Butyl Ether (MtBE)
MtBE, a synthetic chemical, is a volatile, flammable, colorless liquid. MtBE has a relatively high vapor pressure and is water soluble to a significant degree. MtBE usually is produced in a refinery by mixing a feedstock of isobutylene with methanol. The isobutylene is derived by steam-cracking during production of olefin and fluid-cracking during production of gasoline. Concern about them arises from its potential contamination of groundwater as a result of releases from underground storage tanks of gasoline that contains oxygenates.

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Mine-Scarred Lands (MSL)
MSLs are lands, associated waters, and surrounding watersheds where extraction, beneficiation, or processing of ores and minerals, including coal, has occurred. MSLs have become a persistent problem in many communities because of the economic, social, and environmental challenges of cleaning up and reusing such lands. The Brownfields Law expanded the definition of brownfields to include MSLs, making these properties eligible for benefits under the Brownfields Program.

Mobile Laboratory
A mobile laboratory refers to a collection of analytical instruments contained in a vehicle that can be deployed to a project site. A mobile laboratory offers many of the advantages of a fixed laboratory, such as protection from the elements, a power supply, and climate control, while still providing the advantages of analyzing samples on site while the project is in progress. A mobile laboratory may even allow the use of laboratory-grade instruments which otherwise could not be taken into the field. Configurations can vary in sophistication from a single instrument mounted in a sampling van, to large truck trailers and recreational vehicles equipped with multiple instruments and laboratory-grade support equipment.

Monitored Natural Attenuation
The term MNA refers to the remedial approach that allows natural processes to reduce concentrations of contaminants to acceptable levels. MNA involves physical, chemical, and biological processes that act to reduce the mass, toxicity, and mobility of subsurface contamination. Physical, chemical, and biological processes involved in MNA include biodegradation, chemical stabilization, dispersion, sorption, and volatilization.

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