Two primary types of waste materials are present at many MGP sites: coal tars and purifier waste. Coal tars are black to reddish brown, oily liquids which do not readily dissolve in water. Scientists refer to materials like these as a non-aqueous phase liquid, or NAPL. Purifier waste is a mixture of wood chips and iron filings which MGPs used to remove sulfur and other compounds from the manufactured gas before distributing the gas to the public. MGPs often used purifier waste no longer capable of removing impurities as fill material on site.
National Grid did not find any evidence of purifier waste in the fill material at the Gloucester MGP Site during the extensive site investigation, but did find evidence of coal tars. The main chemical compounds in coal tar are volatile organic compounds (VOCs) and semi-volatile organic compounds (SVOCs). The main VOC detected in groundwater and pore water at the Site is benzene, and the main SVOCs detected in soil and sediments at the Site are polycyclic aromatic hydrocarbons, referred to as PAHs. These are the compounds that make up tars and asphalt.