Manufactured Gas Plants (MGPs) had their roots in the 1700’s with the discovery that coal carbonization was a major means of producing coal gas for illumination in many cities in England, Germany and the United States.  The first U.S. manufactured gas plant was opened in Baltimore in 1816.  It is estimated that there were several thousand MGP’s that operated in the U.S. between the early 1800’s and the 1970’s, when the last such sites were closed. The plants were closed as natural gas from pipelines became available in their service areas. There are over a dozen such sites in New Hampshire, the last of which ceased operations in the mid-1950’s.

MGPs were sometimes referred to as "gasworks" or "town gas." From each plant, a network of underground pipes brought the gas into homes and businesses where it was used for lighting, cooking, and heating - the same purposes that natural gas is used for today. The New Hampshire MGPs closed by the mid-1950’s, as the natural gas pipeline system reached the State.

Two major types of waste materials are present at many sites at which MGP associated materials are found today: coal tars and purifier waste. No purifier waste was found at the Lower Liberty Hill site. Coal tars are reddish brown to black, oily liquids which do not readily dissolve in water. Materials such as this are commonly referred to as a non-aqueous phase liquid, or NAPL. Although most tars are slightly more dense than water, the difference in density is slight. Consequently, they can either float or sink when in contact with water. Some underground tars move laterally away from the locations at which they were released. Near the ground surface, some of the tars may weather and partially solidify.  Elsewhere, the tars retain their original, oily fluid properties and may still be capable of moving slowly through the subsurface.  Purifier waste is a mixture of wood chips and iron filings which was used to remove sulfur and other compounds from the manufactured gas before the gas was distributed to the public. The main categories of contaminants associated with coal tar are volatile organic compounds and semivolatile organic compounds. The main volatile organic compound of concern in soil and groundwater is benzene. Specific semi-volatile organic compounds of concern in soil and groundwater are polycyclic aromatic hydrocarbons, referred to as PAH's. These are the compounds that make up tars and asphalt.