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MGP Glossary

Benzene is a common organic chemical compound made up of six carbon atoms arranged in a hexagonal ring, with hydrogen atoms attached to each corner. Benzene is a widely used chemical in industry, and is often found in gasoline. Benzene has been found to cause cancer in laboratory animals, and has been designated by USEPA as a known human carcinogen as well.

BTEX is an abbreviation for a group of chemical compounds: Benzene, Toluene, Ethylbenzene, and Xylene. BTEX compounds are commonly found in MGP wastes, and are also used as antiknock compounds in gasoline. They are commonly found as groundwater contaminants near gas stations, MGP sites, and other industrial facilities.

Cyanides are a class of chemical compounds which contain a carbon-nitrogen triple bond. Free cyanide, in which the cyanide ion is found by itself (not complexed with other ions) is highly toxic. In most cases, MGP wastes such as purifier waste contain complexed cyanides, in which other ions are tightly bound to the cyanide. This complexity appears to lower the toxicity significantly, but there are some concerns that complexed cyanide compounds may be able to decompose and release free cyanide.

DNAPL or Dense Non-Aqueous Phase Liquid means a non-aqueous phase or immiscible liquid which remains as a separate phase or layer and has a specific gravity greater than water. A DNAPL has the potential to sink through a formation until it pools on a confining unit or is immobilized as a residual. Unlike LNAPLS, DNAPLS may flow down the slope of the aquifer bottom in directions which are not the same as the direction of groundwater movement.

Emulsion refers to a physical mixture of two liquids which will not fully dissolve in each other. Some emulsions (common examples include vinegar/oil salad dressing) will readily separate when they are allowed to stand undisturbed. Other emulsions (such as raw milk) may remain mixed for considerable lengths of time.

Gas Holder (also known as a gasometer) is a large, expandable tank used to store gas at an MGP. The earliest gas holders were housed in circular brick buildings, built around a deep pit foundation which was kept full of water. A steel tank (open at the bottom) would rise and fall according to how much gas was being stored at the time. The water formed a seal at the bottom of the tank to keep the gas from escaping. Later, larger water-seal gas holders were built with an external steel frame to guide the tank as it rose and fell, but with no brick building surrounding the tank. Most MGPs contained at least one water seal holder, and many MGPs had several. As a group, these holders are referred to as pit holders or in-ground holders.

Larger MGPs often had at grade holders where the walls of the tank were fixed and did not move up and down. Instead, the roof of the tank would rise and fall like a piston to accommodate the changing volume of gas being stored. Although these holders generally did not leak tar into the subsurface as profusely as water seal holders did, they may in some cases still be sources of tar contamination. In some cases, the piston-like roof was kept lubricated by circulating tar around the edge of the roof. This tar could leak during operation, and in some cases it would be left in place when the holder was demolished.

Gas holders are typically the most contaminated structures on former MGP sites. Water seal holders often accumulated large quantities of tar, especially in relief holders where freshly manufactured gas was held and cooled prior to purification. Much of the tar held in these holders eventually leaked out the bottom. Furthermore, when MGPs ceased operations, it was a common practice to bulldoze the debris from the MGP plant buildings into the circular foundation of the water-seal holders. It is common to find tar-soaked demolition debris in these holder foundations today, decades after the plants closed down.

Groundwater means water below the land surface in a saturated zone of soil or rock. This includes perched water separated from the main body of groundwater by an unsaturated zone.

IRM or Interim Remedial Measure means a discrete set of activities to address both emergency and non-emergency site conditions, which can be undertaken without extensive investigation and evaluation, to prevent, mitigate, or remedy human exposure and/or environmental damage or the consequences of human exposure and/or environmental damage attributable to a site.

LNAPL or Light Non-aqueous Phase Liquid means a non-aqueous phase or immiscible liquid which remains as a separate phase or layer and has a specific gravity less than water. Because LNAPLs are less dense than water, they tend to float on top of the water table and are also commonly referred to as a floating product. Typically, LNAPLs will move through the subsurface in the same direction that the groundwater moves.

Monitoring wells are wells (often small-diameter wells) drilled for the purposes of measuring water levels and testing water quality. Monitoring wells are not typically used to supply water for drinking or other uses.

NAPL or Non-Aqueous Phase Liquid means an immiscible liquid which remains as a separate phase or layer in the environment.

Purifier Waste (also known as box waste) is a solid MGP waste which was produced during purification of the manufactured gas. It is typically found as a dark mixture of wood chips with a very strong, unpleasant burnt odor. Once exposed at the ground surface, the waste will often develop an iridescent blue color known as Prussian Blue. Pieces of solidified tar may be mixed in with the waste, but it is unusual to find liquid tar. Some purifier waste is made of lime instead of wood chips; however, this material has not been commonly found at New York State MGP sites.

Receptor means any humans or organisms which are, or may be expected to be, or have been, exposed to or affected by a contaminant from a site.

Sediment means soils or organic material in water, as found in lakes, rivers, streams and other water bodies and in, or in close proximity to, wetland areas.

Soil gas refers to the air and other gases found in the pore spaces of soils above the water table. (Below the water table, these pore spaces are filled with water). In some cases, vapors from MGP wastes can migrate into soil gas and enter buildings through cracks in basement walls or floors.

Tar well refers to any subsurface tank or vessel used to accumulate or store tar. It is common to find these structures, partially or totally full of tar, during MGP investigations.

Tar/water separator refers to a structure used for settling the tar/water emulsions which often accumulated in MGP gas holders. Since most MGP tars were more dense than water, the tar could often be allowed to simply settle to the bottom of the separator, where it could be drawn off for sale or disposal.

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