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.

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Anthropogenic hydrocarbons in surficial urban sediments derived from nonpoint sources (e.g., stormwater runoff, surface runoff, direct atmospheric deposition, and small but persistent discharges) are the principal characteristics of "urban background". Establishing the character and concentration of urban background helps determine the incremental impacts from point sources and develop successful remedial strategies. In this study, we compared the nature and amount of total extractable hydrocarbons (THC) and polycyclic aromatic hydrocarbons (PAHs), including alkylated PAHs, within 280 surficial (mostly 0-10 cm) sediments from nine, well-studied urban waterways on the East and West U.S. Coasts. These 280 sediments were predominantly impacted by urban background. All the sediments were analyzed by consistent preparation and analytical methods and met consistent data quality objectives, thereby minimizing variations attributable to methodology. The data demonstrate that the anthropogenic hydrocarbons comprising urban background from all locations exhibit a generally consistent nature, dominated by (1) a variably shaped unresolved complex mixture (UCM) within the residual (C20+) range and (2) a variable distribution of resolved 4- to 6-ring nonalkylated (parent) PAHs, mostly dominated by fluoranthene and pyrene (and exhibiting a FL/PY ratio of 0.9 +/- 0.2). The variable nature of both the THC and PAH distributions testifies that, while there is a general consistency to urban background, there are definite differences between (and even within) different urban settings. This indicates thatthere is no single "representative" urban background THC or PAH signature. The greatest mass of THC is reasonably attributable to heavy petroleum(s) comprising the UCM, whereas the greatest mass of PAHs is reasonably attributable to combustion-derived particulate matter. The mean concentration of THC attributable to urban background was 415 mg/kg (dry wt). The concentration of EPA 16-Priority Pollutant PAHs was less than 20 000 microg/kg (dry wt) in 96% of the sediments studied. Thus, sediments containing significantly more than 20000 microg/kg of the EPA 16 Priority Pollutant PAHs (or more the 30000 microg/kg of 43 parent and alkylated PAHs) should be suspected to contain PAHs not entirely attributable to urban background, unless site- or regional-specific survey data supports a different urban background concentration profile.