Bioavailability of polycyclic aromatic hydrocarbons in two estuarine sediments: Decoupling biological, physical, and chemical processes

Polycyclic aromatic hydrocarbon (PAH) contaminated sediments from two sites in the NY/NJ Harbor estuary were fractionated and extensively characterized. PAH concentrations were greatly elevated in the low-density fractions, but PAHs in these fractions were relatively unavailable for desorption or biodegradation. Sediment desorption kinetics for more hydrophobic PAHs (e.g. five-ring compounds like benzo(a)pyrene) were well described by a one-domain diffusion model. Compound hydrophobicity and sediment specific surface area were the parameters most correlated with observed diffusivity. For less hydrophobic compounds (e.g. three- and four-ring compounds like phenanthrene or pyrene), accounting for the contributions of both a fast and a slow diffusion domain was required to describe desorption kinetics. A large and predictable fraction of PAHs may desorb via a relatively fast macro/mesopore diffusion mechanism. This fast-domain diffusivity could be estimated a priori by accounting for sediment-pore water partitioning and the pore structure of the sediment aggregates.; For less hydrophobic PAHs, differences in magnitude of the equilibrium sediment-pore water partition coefficient between the two whole sediments corresponded with the relative rates of PAH mobility from the sediments. Less hydrophobic PAHs may tend to associate with smaller molecular weight fractions of dissolved organic carbon (DOC) in pore waters compared to more hydrophobic PAHs, which tend to associate with the colloidal fraction of DOC. This preferential complexation of three- and four-ring PAHs with potentially more mobile DOC fractions may also facilitate pore diffusion of these compounds.; The extent of PAH biodegradation by Mycobacterium sp. strain PC01 can be predicted by the fraction of desorbable PAHs in the fast-diffusion regime. A closed-form model accounting for rapid PAR diffusion through large, water-filled sediment pores controlling biodegradation outside sediment aggregates effectively predicted independent biodegradation kinetics. Finally, an approach for estimating the combined effects of PAH mobility, exposure time, and compound-specific toxicity is presented. The synergistic effects of relatively low PAH mobility for compounds with relatively high toxicity may suppresses overall risk from exposure to PAHs in the environment, possibly by one hundred times or more compared to EPA default guidance.