to burn it as boiler fuel, or dump the tar as waste. Commonly, waste tars were disposed of in old gas holders, adits or even mine shafts (if present). Over time, the waste tars degrade with phenols, benzene (and other mono-aromatics—BTEX) and polycyclic aromatic hydrocarbons released as pollutant plumes that can escape into the surrounding environment. Other wastes included "blue billy", which is a ferroferricyanide compound—the blue colour is from Prussian blue, which was commercially used as a dye. Blue billy is typically a granular material and was sometimes sold locally with the strap line "guaranteed weed free drives". The presence of blue billy can give gasplants waste a characteristic musty/bitter almonds or marzipan smell which is associated with cyanide gas. The shift to the Carburetted Water Gas process initially resulted in a reduced output of water gas tar as compared to the volume of coal tars. The advent of automobiles reduced the availability of naphtha for carburetion oil, as that fraction was desirable as motor fuel. MGPs that shifted to heavier grades of oil often experienced problems with the production of tar-water emulsions, which were difficult, time-consuming, and costly to break. (The cause of tar change water emulsions is complex and was related to several factors, including free carbon in the carburetion oil and the substitution of bituminous coal as a feedstock instead of coke.) The production of large volumes of tar-water emulsions quickly filled up available storage capacity at MGPs and plant management often dumped the emulsions in pits, from which they may or may not have been later reclaimed. Even if the emulsions were reclaimed, the environmental damage from placing tars in unlined pits remained. The dumping of emulsions (and other tarry residues such as tar sludges, tank bottoms, and off-spec tars) into the soil and waters around MGPs is a significant factor in the pollution found at former manufactured gas plants (known as "FMGPs" in environmental remediation) today. Contaminants commonly associated with FMGPs include: BTEX Diffused out from deposits of coal/gas tars Leaks of carburetting oil/light oil Leaks from drip pots, that collected condensible hydrocarbons from the gas Coal tar waste/sludge Typically found in sumps of gas holders and decanting ponds. Coal tar sludge has no resale value and so was always dumped. Volatile organic compounds Polycyclic aromatic hydrocarbons (PAHs) Present in coal tar, gas tar, and pitch at significant concentrations. Heavy metals Leaded solder for