Ecological Risk Assessment of Polycyclic Aromatic Hydrocarbons in Sediments: Identifying Sources and Ecological Hazard

Polycyclic aromatic hydrocarbons (PAHs) are nearly ubiquitous contaminants of freshwater and marine sediments. Sediment PAHs are derived from combustion of organic matter, fossil fuels, and biosynthesis by microbes. Pyrogenic PAHs, particularly those associated with combustion particles (soot), have a low accessibility and bioavailability in sediments. Polycyclic aromatic hydrocarbons associated with petroleum, creosote, or coal tar in sediments may have a moderate accessibility/bioavailability, particularly if the PAHs are part of a nonaqueous phase liquid (NAPL) phase that is in contact with sediment pore water. We present a method for estimating the hazard of complex PAH assemblage in sediments to benthic organisms. Concentrations of all PAHs in sediment pore water are estimated by an equilibrium partitioning model relative to concentrations in bulk sediment. Predicted log K-oc values can be used for predicting sediment/water partitioning of petrogenic PAH, but empirically derived log K-d values are needed to predict partitioning of pyrogenic PAH. A hazard quotient (HQ) for each PAH is calculated as the ratio of the estimated concentration in pore water to the chronic toxicity of the PAH determined by a log K-ow/toxicity model. Hazard quotients for all PAH in a sample are summed to produce a hazard index (HI), which is a measure of the worst-case estimated hazard of the sediment PAH to benthic organisms. The results of this study show that the integration of HI results with PAH source data provides insights into the causes of sediment toxicity that are useful in an ecological risk assessment.