Diffusion of PAH and PCB from contaminated sediments with and without mineral capping; measurement and modelling

A novel laboratory microcosm test was developed to measure the diffusion of native PAHs and PCBs from sediments in the presence and absence of a capping layer. Diffusive flux of 15 PAHs and 7 PCBs from uncapped sediment from Oslo harbour was 3.8 ± 0.9 μg m−2 d−1 and 0.010 ± 0.003 μg m−2 d−1, respectively. The flux from sediments capped with 1 cm mineral cap (crushed limestone or crushed gneiss (0–2 mm)), observed during the first 410 d, was 3.5–7.3% of the flux from uncapped sediments. By measuring freely dissolved pore water concentrations of 10 PAHs the flux in the microcosm was modelled with steady state and transient diffusion models. The measured flux from uncapped sediment was 27–290% of modelled steady state flux. Good agreement was also found between the measured flux of pyrene from capped sediment and the flux modelled with the transient model when fitting only with the distribution coefficients for pyrene between the cap material and water (Kd_pyr). Fitted Kd_pyr, (210 and 23 l kg−1 for limestone and gneiss, respectively) was in the same order of magnitude as Kd calculated from organic carbon content in the cap materials (68 and 14 l kg−1 respectively). Calculation of the efficiency of a hypothetical cap with 10 cm diffusion path shows that the increased diffusion path length alone can yield a flux reduction >99% through a strong increase in the stagnant diffusive boundary layer from <1 to 100 mm.