Contamination at MGP Sites
Why Were Waste Materials Generated at MGPs?
Freshly-manufactured gas had to be cooled and purified before it could be used. Two principal byproducts resulted from the cooling process, coal tar and purifier waste. Both wastes present a number of environmental problems, which may persist to this day. There may also be health concerns based upon exposure to some of the constituents of these materials.
What is Coal Tar?
As the manufactured gas cooled, a number of less-volatile chemical compounds would condense to form a complex oily liquid mixture commonly called coal tar. In some cases, this tar could be burned as a fuel at the MGP itself. Some tar was sold as a roofing and road-building material. Other uses for tar developed over the years, as chemists discovered processes for making dyes and a variety of other chemicals, using coal tar as a starting point.
However, for a variety of reasons, the tar could be considered a waste. In many cases, some or all of the tars produced by the plant formed an emulsion with water, and could not be easily reused or sold. Many MGPs simply produced more tar than the owners could find buyers for. In addition, many of these plants operated for decades and significant quantities of coal tar were likely released due to spills or leaks.
Although it is common to use the phrase "coal tar" to describe this material, it is important to note that this name is somewhat misleading. Most people think of tar as a sticky, viscous material, commonly used for road building or roofing repair. MGP tars, particularly those produced by the carburetted water gas (CWG) process, were not like this. Many of the tars found at CWG MGP sites are quite fluid, with roughly the same viscosity as vegetable oil. Consequently, CWG tars are more likely to migrate through soils and appear at different locations from where they were originally leaked or disposed of.
What is Purifier Waste?
Following cooling, a number of other impurities had to be removed from the gas before it was used. Sulfur and cyanide compounds were removed by passing the gas through "purifier beds" made up of either lime or wood chips impregnated with iron filings. Chemical reactions would take place between the gas and the purifier material, removing impurities which otherwise would corrode the gas piping, stoves, and lighting fixtures where the gas was burned.
Although the beds of purifier material were recycled, they would eventually load up with tar and other materials and become unusable. At this point, the material would become a waste material requiring disposal. This purifier waste (also referred to as "box waste") required special handling: it had a very strong odor, and the wood chip mixtures had a tendency to spontaneously ignite if left uncovered on the ground surface. Most MGPs either shipped their purifier waste off site to landfills or used it to fill low-lying areas on site.
What Do These Wastes Look Like?
Coal tar is a dark reddish brown to black, oily liquid that does not readily mix with water. It has a very strong odor, which many people find similar to mothballs or driveway sealer. It is important to realize that this material does not always have the thick, sticky consistency that most people think of as "tar." Water gas tar (the most common MGP waste in New York) typically has the consistency of vegetable oil. Coal carbonization tar is somewhat more viscous, but still may act as a liquid.
Where tar has been exposed on the ground surface, it tends to partially solidify. This material typically takes on the appearance of ordinary asphalt road pavement; however, it is notably softer than asphalt, and retains its characteristic odor. In warm weather, the partially hardened tar can become liquid again and flow slowly across the ground surface.
Purifier Waste 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.
What Contaminants are Typically Present in Coal Tar?
Coal tars, from both coal carbonization and carburetted water gas processes, are complex mixtures of organic chemicals. The following two major classes of chemical compounds found in coal tar are the focus of most investigations:
Volatile organics, characterized by benzene, toluene, ethylbenzene and xylene, which are identified by their initials as the BTEX compounds. The BTEX compounds are widely associated with gasoline, and are commonly encountered as contaminants surrounding leaking gasoline tanks or spills. Together, these compounds often represent only a small percentage of the mass of MGP tar. However, of all the contaminants found in coal tar, these are the most soluble and are thus the most likely to be dissolved in groundwater and migrate off site. These are also the most volatile and are thus the most likely to migrate through subsurface soils as vapors or soil gas.
A group of semivolatile organic compounds known as polycyclic aromatic hydrocarbons or PAHs. PAHs are present in many forms in the environment (notably in asphalt and other petroleum products such as diesel fuel). Coal tar is still commonly used today as asphalt pavement sealer. Although hundreds of PAH compounds have been identified in coal tar, the most important ones are the 17 (listed below) which are recognized by the USEPA as priority pollutants. These are the PAHs about which we know the most, and data concerning their presence are commonly used to evaluate levels of PAH contamination:
- indeno(1,2,3-cd) pyrene
PAH concentrations are typically referred to as the summation of the individual PAHs listed above (i.e. total PAHs or tPAHs). The italicized PAHs are classified by the USEPA as probable human carcinogens. The summation of the italicized PAHs is referred to as carcinogenic PAHs or cPAHS.
Most of the PAH compounds do not readily dissolve in water, so they are not as easily transported in groundwater as are the BTEX compounds. With the exception of naphthalene, most of the PAHs also do not readily volatilize. Consequently, they are not as readily transported as soil gas. However, despite low solubility and volatility, PAHs may still migrate significant distances, because mobile tars or emulsions may move through the subsurface.
What Contaminants are Typically Present in Purifier Waste?
Purifier waste often contains significant quantities of chemically complexed cyanide compounds. Some cyanide can leach from the waste and contaminate groundwater, which can move through the subsurface away from the burial location. Although some cyanide compounds are highly toxic, there is evidence that the cyanide compounds typically leached from purifier waste are in a chemically complexed form which is significantly less toxic. NYSDEC and NYSDOH are continuing to collect data to evaluate this question.
In addition to containing complexed cyanide, water which comes into contact with purifier waste, is often highly acidic. Concrete structures in contact with purifier waste may be corroded to some extent by the acid. If the acidic water discharges to a stream or other surface water body, it may cause harm to fish and wildlife.
Although purifier waste typically contains coal tar residue, this contamination is generally in a solid form, which does not migrate on its own, or dissolve readily in water.