Hydrophobic organic compounds (HOCs) such as polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs) are common sediment contaminants around the world. Due to their persistence and potential for bioaccumulation, these contaminants are of high concern to environmental risk assessors. Assessment of toxicity and bioaccumulation of HOCs with total extractable contaminant concentration in the sediment (bulk sediment concentration) is unreliable and often provides poor predictions 1, 2. Sediment porewater concentration of HOCs, particularly the freely dissolved porewater concentration, however, has been shown to be a good indicator of bioavailability 2–5. Predicted porewater concentration was also used to develop equilibrium partitioning benchmarks for protecting benthic organisms in PAH-contaminated sediments by the U.S. Environmental Protection Agency (U.S. EPA) 6, although this approach is limited by the ability to estimate porewater concentrations in sediments. In directly examining the pore water to which organisms are exposed, factors that affect bulk sediment bioavailability, including pH, grain size, and fraction organic carbon (fOC), are inherently taken into account 7. However, measurement of freely dissolved sediment porewater concentration represents an analytical challenge. Obstacles such as inherent method shortcomings and high detection limits have made consistent characterization of porewater concentration quite demanding. The most commonly used conventional method for porewater measurement is centrifugation, which includes sediment centrifugation or filtration, solvent extraction, solvent exchange, concentration, and analysis 8. However, because of the hydrophobicity of most HOCs and thus very low porewater concentration especially for HOCs with log KOW greater than 6.0, an impractically large volume of sample is usually needed to achieve analytically detectable concentrations. Additionally, this approach suffers from incomplete water-phase separation 8, sorption or evaporation loss during sample transition, and interference from contaminants associated with colloids and dissolved organic carbons 9. Several chemical techniques have been developed to overcome these limitations and detect freely dissolved water concentrations. These approaches include equilibrium dialysis 10, gas purging 11, alum flocculation to remove colloids 12, and passive samplers such as semipermeable membranes 13, Empore disks 14, polyoxymethylene solid phase 15, polyethylene (PE) 16, and polydimethylsiloxane (PDMS) 3, 17–19. Passive sampling has also been used as a direct biological surrogate, because tissue concentrations show a strong linear correlation with fiber concentrations 5, 20, 21. Polyoxymethylene solid phase, PE, and PDMS are sorbents with similar, but not identical, sorption capacities for PAHs and PCBs. They are typically available in different geometries: different surface area to volume ratios. This has implications for detection limits and equilibration kinetics. In the present study, PDMS is used because of its availability as a thin annular layer on a small-diameter core that provides a high surface area to volume ratio (relatively fast kinetics) and can be inserted easily into sediments during bioaccumulation studies. Solid-phase microextraction (SPME) is a partition-based, solvent-free, negligible-depletion extraction technique that was introduced by Arthur and Pawliszyn in 1990 22. Because only a very small amount of HOC is extracted, the extraction does not influence the existing equilibrium between the bound and free forms of a chemical, so only freely dissolved concentration is measured 20. Mayer et al. 17 extended the SPME technique to measure the freely dissolved porewater concentration in sediments. In this approach, SPME fibers are directly inserted into the sediment and allowed to equilibrate with the sediment–porewater system. At equilibrium, fibers are retrieved from sediment either directly injected into the analytical instrument or solvent extracted into autosampling vials. In practice, highly hydrophobic compounds will require long times to achieve equilibrium, and corrections for disequilibrium are needed 23. Porewater concentration (Cpw) is calculated from the fiber concentration (Cf; mass of contaminant absorbed by fiber/volume of PDMS) and fiber–water partition coefficient (Kf–w; volume of water/volume of PDMS) as shown by the following equation.
Our previous studies 4, 24 have shown that measurement of porewater concentrations via conventional approaches can be used to predict bioaccumulation and bioavailability of sediment-associated PAHs. In the present study, this hypothesis is further tested with a broader range of chemicals and sediments and use of matrix-SPME to measure the porewater concentration. The present study is an effort to build on previous studies that have employed SPME to predict bioaccumulation 3, 5, 20 by measuring bioaccumulation in the deposit-feeding oligochaete Ilyodrilus templetoni exposed to five different sediment treatments over 28 d. The different sediment treatments were designed specifically to contrast a field-contaminated sediment with previously demonstrated limited contaminant availability 4 (Anacostia River, Washington, DC, USA) and a laboratory-contaminated sediment (Brown Lake [BL], Vicksburg, MS, USA) prepared by redistribution of contaminants during three weeks of mixing with 3 to 25% mixtures (by weight) of a field-contaminated sediment (New Bedford Harbor [NBH], MA, USA).