Partitioning and Bioavailability of Polynuclear Aromatic Hydrocarbons in an Intertidal Marsh
Polycyclic aromatic hydrocarbons (PAHs) are trace organic pollutants now so ubiquitous in aquatic ecosystems that the major organic signature in urban storm runoff is that of weathered PAHs. PAHs are formed by combustion of organic matter including petroleum and wood but anthopogenically produced PAHs have increased greatly over the last century. PAHs are a problem in the environment since some are potent carcinogens to humans and wildlife alike. PAHs include the most toxic fraction of tobacco smoke, benzo(a)pyrene, naphalene, and phenanthrene and are often found in association with other organic pollutants such as chlorinated biphenyls (PCBs), and organochlorine compounds such as DDT. The persistence of PAHs in the environment and their concentration in the fat of living organisms increases the toxicity of PAHs and current regulatory standards are based on the bulk concentration of PAHs in the sediments and the amount of organic carbon in the sediments. Our research was intended to improve the understanding of PAHs in sediments, particularly the spatial and temporal variations in the bioavailabilty and bioaccumulation in shellfish and polychaete worms in the sediments of the San Francisco Bay Estuary.
We concentrated on 18 individual PAH compounds and measured their seasonal and spatial variations in the intertidal sediments and biota. We found that the sediment pore water concentration of PAHs may be more important in determining the bioconcentration of PAHs in living tissue than the simple bulk concentrations in the sediments. Similarly, we found that the kind of organic carbon present is more important in determining the bioconcentration of PAHs in shellfish and polychaete worms than the simple total amount of carbon present. The explanation is probably that silt and small organic fragments, especially soot, may be mostly composed of PAHs while other organic carbon such a decaying leaves is not. Soot contains a lot of PAHs but they are strongly bound to the carbon matrix and are paradoxically less bioavailable than lower amounts of PAH bound to naturally-occurring carbon. Winter storms carrying accumulated sooty fallout and auto crankcase drippings from the watershed and depositing them preferentially in the upper intertidal of the estuary explains the observed PAH speciation and distribution as well as the observed bioconcentration patterns in the sediment-dwelling biota. We recommend that attention be paid to the kind of organic carbon and its site in the porewaters in the setting of future regulatory standards.