John Wiley & Sons, Ltd.

A large‐scale field trial of thin‐layer capping of PCDD/F‐contaminated sediments: Sediment‐to‐water fluxes up to five years post‐amendment

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The longer‐term effect (3‐5 y) of thin‐layer capping on in situ sediment‐to‐surface water fluxes was monitored in a large‐scale field experiment in the polychlorinated dibenzodioxin and dibenzofuran (PCDD/F) contaminated Grenlandfjords, Norway (four trial plots of 10,000 to 40,000 m2 at 30 to 100 m water depth). Active caps (designed thickness 2.5 cm) were established in two fjords, consisting of dredged clean clay amended with powdered activated carbon (PAC) from anthracite. These active caps were compared to two non‐active caps in one of the fjords (designed thickness 5 cm) consisting of either clay only (i.e.without PAC) or crushed limestone. Sediment‐to‐water PCDD/F fluxes were measured in situ using diffusion chambers. An earlier study showed that during the first two years after thin‐layer capping, flux reductions relative to non‐capped reference fields were more extensive at the fields capped with non‐active caps (70–90%) than at the ones with PAC‐containing caps (50–60%). However, the present work shows that between three and five years after thin‐layer capping, this trend was reversed and cap effectiveness in reducing fluxes was increasing to 80–90% for the PAC caps, whereas cap effectiveness of the non‐active caps decreased to 20–60%. The increasing effectiveness over time of PAC‐containing “active” caps is explained by a combination of slow sediment‐to‐PAC mass transfer of PCDD/Fs and bioturbation by benthic organisms. The decreasing effectiveness of “non‐active” limestone and clay caps is explained by deposition of contaminated particles on top of the caps. The present field data indicate that the capping efficiency of thin active caps (i.e. enriched with PAC) can improve over time as a result of slow diffusive PCDD/F transfer from sediment to PAC particles and better mixing of the PAC by bioturbation. This article is protected by copyright. All rights reserved

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