DISTRIBUTION AND TRANSPORT OF COAL TAR DERIVED PAH COMPOUNDS IN FINE-GRAINED ALLUVIAL DEPOSITS

Field investigations at a former manufactured gas plant (FMGP) in Chattanooga, TN, indicate that all 16 PAH compounds on the EPA priority pollutant list are present at the site and have penetrated the entire 4-5 m thick layer of fine-grained alluvial deposits, which overlie the limestone bedrock. These compounds are associated with coal tar, a byproduct of manufactured gas, and are expected to be relatively immobile in soils or sediments, because of their strong tendency to sorb. However, the study at this site shows that even highly hydrophobic compounds, such as benzo[g,h,i]perylene are present throughout the silt and clay-rich sediments, at concentrations ranging from 0.01 to 50 mg/kg. Concentrations of less hydrophobic PAHs, such as naphthalene, were found in the sediments at concentration levels about a factor of 10 times higher than the highly hydrophobic compounds. Microscopic examination of thin sections of the sediments showed that fractures and biopores, which could act as pathways for PAH migration, occur throughout the depth profile, especially in the upper 1-2 m. The sediments show evidence of seasonal variations in saturation and flow, including Fe/Mn coatings, pedogenic clay linings and shrink-swell fabrics. In some cases, a tarry residue was found coating fractures/biopores, indicating that coal tar, or related mixtures, were able to penetrate as an immiscible phase. However, in the vast majority of samples in which PAHs were detected, there was no detectable immiscible residue, suggesting that much of the transport occurred in the dissolved phase. Numerous other higher solubility compounds, including chlorinated and nonchlorinated VOCs (e.g., BTEX, TCE, DCB, etc.), pesticides (BHC isomers), and semi volatile organic compounds (e.g., quinoline, furans, etc.) were detected (from previous studies) in wells in the sediments or the underlying bedrock, so it is possible that transport of the dissolved PAHs were enhanced by cosolvent effects. The study clearly indicates that fine-grained alluvial deposits cannot be assumed, a priori, to provide an effective barrier to downward migration of PAH compounds typically found in coal tar at FMGP sites. High concentrations of these compounds are expected to slowly leach out of the fine-grained sediments and could act as a long-term source of secondary contamination to the soils and the underlying bedrock.