U.S. EPA Contaminated Site Cleanup Information (CLU-IN)

U.S. Environmental Protection Agency
U.S. EPA Technology Innovation and Field Services Division

Dense Nonaqueous Phase Liquids (DNAPLs)

Chemistry and Behavior

Multi-Component Waste

Coal Tars

Coal tars are produced from the distillation of coal to produce coke primarily for the steel industry and from making manufactured gas. The two processes can produce two distinctly different products. The coking process usually results in a viscous liquid or semi-solid material that is dark brown to black and has a naphthalene odor (ATSDR 2002). The coal tars from a manufactured gas plant (MGP), especially one using the carburetted water process, are reddish brown to black with the consistency of vegetable oil (NYDEC 2007). The difference in viscosity means that some MGP produced coal tars can move farther and quicker in the subsurface than their coking counterparts.

Coal tars are a very complex mixture of mono and polycyclic aromatic hydrocarbons, phenols, heterocyclic oxygen, sulfur, and nitrogen compounds (ATSDR 2002). The specific gravity of coal tars varies from about 1.18 g/cc to 1.23 g/cc (Lewis 2001) due to the inherent variability of these mixtures.

When released to the environment coal tars begin to partition by individual constituent. For example, soluble components like benzene, naphthalene, and phenol start dissolving into water while volatile chemicals like benzene, xylenes, and toluene partition to the vapor phase. Like the creosotes that are derived from coal tars, the various chemicals contained in coal tars have a wide range of Koc and Kow values that give them different fates. Some compounds in coal tar mixtures tend to bioaccumulate.

Several chemicals found in coal tars biodegrade readily and will do so if they dissolve into a surface or ground water body; however, as is evidenced by the presence of coal tars in the subsurface of several MGP sites that have been closed for 50 to 100 years, coal tars can be very long-lived.

For Further Information

Acid Tar Lagoons
Smith, C., X. Hao, S. Talbot, and N. Lawson.
CL:AIRE: Contaminated Land: Applications in Real Environments, Subr:im Bulletin SUB 7, 8 pp, 2008

Although similar to coal tars, acid tars are a unique and challenging waste product arising from petroleum refining processes (i.e., benzole refining, white oil production, and oil re-refining.) that now are largely obsolete. Depending on their composition, acid tars can behave as DNAPL. This bulletin provides a state-of-the-art update on the science, assessment, and remediation of acid tar lagoons based on research carried out from 2003 to 2007 and experience from two recent remediation projects carried out in Europe. This bulletin is available without charge on the CL:AIRE Web site to registered users (registration is also free).

Contamination at MGP Sites
New York Department of Environmental Conservation (NYDEC 2007)

This is a webpage that discusses manufactured gas plant sites in New York.

Adobe PDF LogoEbullition-Facilitated Transport of Manufactured Gas Plant Tar from Contaminated Sediment
McLinn, E.L. and T.R. Stolzenburg.
Environmental Toxicology and Chemistry 28(11):2298-2306(2009)

MGP tar and wastewater solids historically discharged into the Penobscot River, Maine, accumulated in riverbed sediment over a 5-hectare area downstream from the sewer outfall. Much of the tarry sediment is a hardened mass at the bottom of the river, but in the active zone, anaerobic biodegradation of organic matter generates methane and carbon dioxide. As gas accumulates and migrates upward, it entrains tar, eventually dragging the tar from the sediment to surface water. For tar to migrate from sediment to surface water, three conditions are necessary: the sediment must contain liquid tar, the sediment must produce gas bubbles, and the gas must come into contact with the tarry sediment. Failure to consider facilitated transport of MGP tar from sediment can cause underestimation of site risk and can lead to failure of remedial measures.

Hawley's Condensed Chemical Dictionary 14th Edition
Lewis, R.
John Wiley & Sons, 2001, 1223 pp

This book is a dictionary of chemical terms.

Adobe PDF LogoToxicological Profile for Wood Creosote, Coal Tar Creosote, Coal Tar, Coal Tar Pitch, and Coal Tar Pitch Volatiles
Agency for Toxic Substances and Disease Registry (ATSDR)
U.S. Department Of Health And Human Services, 2002, 394 pp

This profile covers coal tar creosote human health effects, chemical and physical properties, manufacturing volume data, potential for human exposure (environmental fate and transport), and analytical methods.