Flood-induced transport of PAHs from streambed coal tar deposits

We assessed whether coal tar present in contaminated streambed sediments can be mobilized by flood events and be re-deposited in an adjacent floodplain. The study was conducted within a contaminated urban stream where coal tar wastes were released into a 4-km reach from a coke plant in Chattanooga, Tennessee, USA. Sediments containing visible amounts of coal tar were dredged from the streambed in 1997-98 and 2007 as part of a cleanup effort. However, post-dredging sampling indicated that very high concentrations of polycyclic aromatic hydrocarbons (PAHs) remained in streambed sediments. Sampling of sediments in the floodplain at two sites downstream of the coke plant indicated that high concentrations of PAHs were also present in the floodplain, even though no coal tar was observed in the samples. Age-dating of the floodplain sediments using ¹³⁷Cs indicated that peak PAH concentrations were contemporary with coke plant operations. While there was little or any direct contamination of the floodplain sediments by coal tar, sediment contamination was likely a result of deposition of suspended streambed sediments containing sorbed PAHs. A flood model developed to delineate the extent of flooding in various flood recurrence scenarios confirmed the potential for contaminated streambed sediments to be transported into the adjacent floodplain. It was hypothesized that coal tar, which was visibly "sticky" during dredging-based stream cleanup, may act as a binding agent for streambed sediments, decreasing mobility and transport in the stream. Therefore, coal tar is likely to remain a persistent contaminant source for downstream reaches of the stream and the adjacent floodplain during flood events. This study also showed that even after excavation of tar-rich streambed sediments, PAH contaminated non-tarry sediments may be a source of flood-related contamination in the adjacent flood plain. A conceptual framework was developed to delineate specific mechanisms that can mobilize contamination from stream sources.