Sensitive equilibrium sampling to study polychlorinated biphenyl disposition in Baltic Sea sediment

An equilibrium sampling approach using glass jars with μm thin coatings of the silicone polydimethylsiloxane (PDMS) was validated and applied to background sediment samples from a >50 km transect in the Stockholm Archipelago. Equilibrium between the sediment and the PDMS was demonstrated using different coating thicknesses. From the concentrations of polychlorinated biphenyls (PCBs) in the PDMS, we assessed (i) freely dissolved concentrations in the sediment interstitial porewater (C(Sediment_free)); (ii) the equilibrium status between sediment and water; (iii) the equilibrium status between sediment and biota; and (iv) site-specific sediment/water distribution ratios (K(D)). The results showed that (i) Stockholm was a source of PCBs to the Baltic Sea as evidenced by significantly higher C(Sediment_free) in Stockholm Harbor; (ii) the fugacity in sediment exceeded that in water (monitoring samples collected in February) by an average factor of 4.0; (iii) the fugacity in sediment exceeded that in herring by an average factor of 5.2; and (iv) K(D) near Stockholm Harbor was 0.3-1.7 log units greater than in the outer archipelago. The coated glass jar method with its high precision and built-in QA/QC opens new possibilities to study the disposition of hydrophobic chemicals at trace levels (C(Sediment_free) down to 1.06 fg/L) in background environments.