Reduced bioavailability of polycyclic aromatic hydrocarbons (PAHs) in sediments impacted by carbon manufacturing...

Potential risks of polycyclic aromatic hydrocarbons (PAHs) in sediments of a Japanese bay contaminated by carbon manufacturing plant effluent were evaluated by calculating toxicity units (TUs). TUs calculated from the measured whole-sediment concentrations (C_whole) were often higher than or close to 1, signaling a possible toxicity concern to benthic organisms. In contrast, TUs based on the freely dissolved pore water concentrations (C_free) measured by an ex-situ passive sampling method with polyethylene strips were 0.0007-0.005, much lower than 1, indicating no effect. We also found that the fractions of black carbon in sediments of the contaminated bay were significantly higher than those of reference sites. Overall, we conclude that carbon manufacturing plant effluent substantially increases C_whole of PAHs in sediments but also increases the fraction of carbonaceous particles that strongly retain PAHs. As a combined result, bioavailable concentrations (as expressed by pore water C_free) of PAHs do not increase as much as C_whole. The results of this study indicate that ecotoxicological risks of PAH contamination by carbon manufacturing plants should be evaluated by directly measuring pore water C_free instead of C_whole.