Exploring Bioavailability of Hydrophobic Organic Contaminants in Sediments

Due to high affinity for organic carbon phase, hydrophobic organic contaminants (HOCs) preferentially deposit in bed sediments in surface aquatic systems. Consequently, many contemporary sediment contaminants are HOCs, including DDTs, PCBs, and PBDEs. However, their accumulation potential and toxicity to benthic organisms in sediments are regulated by bioavailability, rather than bulk chemical concentrations. The overall purpose of the current project was to develop and apply chemically-based methods to explore bioavailability of HOCs in sediments, and help improve the assessment of their potential ecotoxicological risks and sometimes human health effects in the environment.A solid phase microextraction (SPME) method using disposable polydimethylsiloxane (PDMS) fibers was developed for measuring the phase distribution of flame retardant PBDEs in sediments. Results showed that PBDEs were predominantly sorbed to the sediment phase, with freely dissolved concentration (Cfree) only accounted for < 0.012% of the total chemical mass and < 0.43% of the dissolved mass. Addition of black carbon further reduced bioavailability (or Cfree) of HOCs as indicated by the matrix-SPME method. At 1% amendment rate in a sediment with low organic carbon (0.12%) content, Cfree of selected PBDEs was reduced by 47.5&ndash78.0%, 47.3&ndash77.5%, and 94.1&ndash98.3% with biochar, charcoal, and activated carbon, respectively. To overcome the shortcomings of current measurement methods, an isotope dilution method (IDM) was developed and applied for bioavailability prediction of legacy HOCs (e.g., DDT and its metabolites, PCBs) in historically-contaminated sediments from the Palos Verdes Superfund site. The IDM-based accessible concentration was shown to correlate closely with tissue residues in the exposed marine benthic polychaete Neanthes arenaceodentata (R2=0.84&ndash0.94). Bioavailability measurements from the SPME, IDM and Tenax methods were compared in amendment-based remediation. After normalizing over the unamended sediments, significant linear correlations (R2 >0.90, p <0.01, slopes=1.00&ndash1.09) were found between these methods, suggesting they are interchangeable or complementary for evaluating remediation efficiency or progress.Overall, the bioavailability estimation methods or their specific applications developed from this project will improve understanding environmental fate of HOCs and their potential toxicity to such organisms as the sediment-dwelling invertebrates. The developed methods may be readily adoptable for other HOCs as well as other matrices (e.g., soil)