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Analysis of suspended particulate material (SPM) collected from the Columbia River and its estuary in 2007-2008 revealed the ubiquitous presence of polycyclic aromatic hydrocarbons (PAHs) from several distinct sources. The two dominant ones were: (1) a suite of non-alkylated, three to five-ringed compounds derived from high temperature combustion and (2) perylene, a compound of diagenetic origin. A particle-selective, hydrodynamic trapping process explains how both PAH types become concentrated on both a particle weight and organic carbon basis in the estuarine turbidity maximum (ETM) by as much as 10 times relative to the riverborne particle source. The ETM is a transient sedimentary feature at the land-to-sea interface of river-dominated estuarine systems which, in the case of our study region, is located remotely from the likely site of initial PAH input. Particle normalized concentrations for PAH of notable environmental concern, such as fluoranthene, chrysene, and benzo[a]pyrene, exceeded the EPA-defined threshold effects level in all cases and were typically at, or above, the probable effects level. Comparison with results from studies for other waterways around the world indicates PAH concentrations in ETM-trapped particles from the Columbia River estuary are higher than those documented for SPM in waters of many far more industrialized and populated regions. Our refined understanding of PAH behavior in the Columbia River and its estuary should prove valuable for reliably modeling the transport and dispersal mechanism that is characteristic of other hydrophobic, particle associated persistent organic pollutants prevalent in this system, and for other river-dominated estuarine systems. [ABSTRACT FROM AUTHOR]