Multi-Ratio Equilibrium Passive Sampling Method to Estimate Accessible and Pore Water Concentrations of Polycyclic Aromatic Hydrocarbons and Polychlorinated Biphenyls in Sediment

The freely dissolved concentration (C-w,C-0) in the pore water and the accessible (releasable) concentration in the sediment (C-as,C-0) are important parameters for risk assessment. These parameters were determined by equilibrating contaminated sediments and passive samplers using largely differing sampler-sediment ratios. This method is based on the principle that incubations at low sampler/sediment ratios yield the concentration in the pore water (minor depletion of the sediment phase) and incubations at high sampler/sediment ratios yield the accessible concentration in the sediment (maximum depletion of the sediment phase). It is shown that equilibration was faster in dense suspensions and at high sampler/sediment ratios when compared to low sampler/sediment ratios. An equilibrium distribution model was used to estimate C-w,C-0 and C-as,C-0 by nonlinear least-squares regression. The method was evaluated three sediments (harbor, estuarine, marine). Accessible concentrations of 13 PAHs were 2 (low K-ow) to 10 (high K-ow) times lower than the total concentrations (three sediments). By contrast, the accessible concentrations of 15 PCBs were about 1.2 times lower than the total concentrations and displayed no trend with K-ow (one sediment). Implications for risk assessment and considerations for application of multi-ratio equilibrium passive sampling with other sediments are discussed.