The freely dissolved concentration (C_pw) of hydrophobic pollutants in sediments is a good predictor for exposure to aquatic organisms, bioaccumulation, toxicity, and flux across interfaces. Direct extraction and measurement of sediment porewater are typically understood to be inadequate for providing a reliable measurement of C_pw for hydrophobic pollutants like PAHs and PCBs in sediments. Equilibrium passive sampling approaches have been developed to provide accurate measurement of C_pw in sediments avoiding interference from colloids and dissolved organic carbon. The objective of this research is to compare the C_pw of hydrophobic organic compounds measured using passive sampling with concentrations obtained via direct water extraction method and with C_pw estimates from sediment concentrations and organic carbon data.
Sediment samples were collected from Indiana Harbor, Chicago, IL, which was contaminated with PAHs and PCBs. Polymeric samplers made of polyethylene (PE) and polydimethylsiloxane (PDMS) were directly inserted into sediment in the laboratory to yield measures of C_pw. The sediment exposure was conducted in parallel experiments of active (shaking table) and static conditions for 4 weeks for equilibrium. Direct porewater measurement was performed by first equilibrating the sediment with water in closed bottles for 1 month followed by settling and alum flocculation to remove colloids. The direct water extracted concentration was a combination of the freely dissolved and the dissolved organic carbon (DOC) associated concentrations. The direct porewater measurement was corrected for DOC using measured DOC concentration in porewater and estimated partition coefficients for DOC.
The direct water measurement with DOC correction provided consistent results with passive samplers for both PCBs and PAHs. Predicted bioaccumulation of PCBs and PAHs and prediction of PAH toxicity via narcosis in model benthic organisms agreed within 50% across the measurement methods. We also demonstrate that C_pw for high molecular weight compounds that were below analytical detection limits could be estimated for each of the methods by extrapolating a K_oc-K_ow correlation.
This study was supported by the US Department of Defense, ESTCP, as part of a larger study to standardize passive sampling methods through an inter-laboratory testing program. Dr. Danny Reible was also involved in this program and provided valuable inputs for the PDMS passive sampling analysis.

Songjing yan

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