TRANSPORT AND DEPOSITION OF COAL TAR CONTAMINATED SEDIMENTS IN AN URBAN FRESHWATER STREAM

The goal of this research is to determine whether coal tar deposited in urban streams can be mobilized by flood events and be re-deposited in adjacent floodplain where human contact with contaminated sediment deposits is likely. The study was conducted at Chattanooga Creek, an urban freshwater stream located in Chattanooga, TN. For several decades, coal tar wastes were directly dumped or channeled into a 4 km reach of the Creek by a nearby coke plant. Streambed sediment characterization determined that large quantities of coal tar-rich sediments were present and that they were distributed according to the stream channel morphology. Analysis of soil and sediment cores in adjacent floodplain did not indicate black staining typically indicative of coal tar. However, all 16 EPA priority PAHs were detected in floodplain sediments up to a depth of 1.2 m b.g.s. and at distances of over 3 km downstream of the coke plant. These PAHs were rich in 4-6 benzene ring compounds compared with 2-3 ring PAHs signifying increased weathering and biodegradation of the 2-3 ring PAHs. Preliminary age dating of floodplain sediments using ¹³⁷Cs and ²¹⁰Pb suggested that sediment deposition is on the order of approximately 1 cm/yr and that peak PAH concentrations coincided with coking operations at the coke plant. These lines of evidence suggest that there is little or any direct contamination of the floodplain soils by the coal tar per se. Instead, the source of contamination of floodplain sediments is most likely by deposition of suspended stream sediments containing sorbed PAHs. It is hypothesized that coal tar acts as 1) a binding agent to sediments making them resistant to entrainment and transport in the stream during normal flow conditions and 2) a persistent source for PAH contamination of lower reaches of the stream and the floodplain during flood events.