TRANSPORT AND DEPOSITION OF COAL TAR-CONTAMINATED SEDIMENTS IN AN URBAN-INDUSTRIAL STREAM AND FLOODPLAIN

We carried out field and laboratory studies to investigate fate and transport of PAH-contaminated sediments in Chattanooga Creek in southeastern Tennessee. Coal tar wastes from a nearby former manufactured gas/coke plant were directly dumped or channeled into a 4 km reach of this stream sometime during the period from 1918 to 1987 when the plant was in operation. Coal tar-rich deposits up to 2.5 m in thickness were found in the streambed during investigations by EPA in the early 1990s. Subsequent excavation-based cleanup operations in the late 1990s removed approximately 2000 m ³ of tar rich sediments from a 1.3 km reach of the stream and excavation of tar-rich sediments from the lower 2.7 km of stream is currently underway. Our investigations have focused on post-cleanup monitoring of the streambed sediments and on the potential for spreading of contamination from the stream to the surrounding floodplain. Preliminary studies showed that concentrations of the 16 EPA priority PAHs remain relatively high (total PAHs up to 42,000 mg/kg of sediment) in streambed sediments in much of the cleaned up zone. This is likely due to incomplete excavation of contaminated sediments or new inputs from upstream or from erosion of contaminated sediments in the banks of the stream. Coring and analysis of soil cores from the adjoining floodplain indicate that all 16 priority PAHs are present to depths of 1.2 m or more below ground surface and at distances of at least 3 km downstream of the coke plant. These PAHs were rich in 4-6 benzene ring compounds compared with 2-3 ring PAHs signifying partial weathering and biodegradation of the 2-3 ring PAHs. Age-dating of the floodplain sediments using ¹³⁷Cs and ²¹⁰Pb indicate very rapid deposition rates, on the order of approximately 1 cm/yr, with the depth of elevated PAH concentrations coinciding with the period of operations at the coke plant. However, very few fragments of tar or coke were found in the floodplain, suggesting that the PAHs were transported while sorbed to particles of clay or organic matter and subsequently deposited in the flood plain during seasonal flood events. We hypothesize that PAHs diffused from the streambed tar deposits into overlying fine-grained sediments, which were more easily eroded during floods than the underlying sticky tar deposits.