Remediation of contaminated seabed in the Oslofjord

Through decades the fjord has been exposed to industrial activities, dockyard operations and municipal wastewater. In addition the inner part of the basin has been the subject of illegal dumping of rubbish and scrap. This contamination is situated in the shallow parts of the harbour. Remobilisation of contaminants by ship traffic and leisure boats is the main mechanism in the shallow parts. The contamination of the sediments has resulted in restrictions on the consumption of fish and fishery products from the inner parts of the Oslofjord. Joint efforts The Oslofjord has been the subject of several studies during the last 15 years and different options of remediation have been evaluated. The construction of a submersed road tunnel to redirect the main traffic away from the city centre, as part of the Oslo city redevelopment plan, required dredging in the most contaminated parts of the harbour and triggered an immediate need for remediation. Through a joint effort of the Oslo Harbour Authorities, The Municipal of Oslo, The Norwegian Road Administration and private city developers a plan for the harbour of Oslo has been implemented. The Pollution Control Authorities has given financial contribution to the work. Remediation The contaminated layer vary in thickness from 0.1 to 4.5 m consisting of among others cadmium (Cd), mercury (Hg) and PCB. Most of the mud consists of contaminated organic material. The removal of the contaminated sediments (≈ 650,000 m³) in the shallow parts of the harbour will be dredged and stored in a deep water confined aquatic disposal facility (CAD) in the inner parts of the Oslofjord. The deeper zones (≈ 1,000,000 m²) will be capped using clean clay recovered from the tunnel construction project. The sediments are being dredged using a specially designed closed grab in order to minimise resuspension. The material is then transported by a barge to the site of the CAD and pumped down to 70 m using a submerged trim pipe of excess water. To improve the vertical stability of the water column in the CAD salt is added. Control and monitoring To supervise the operations at the site an extensive control and monitoring program is implemented.

• On-line monitoring of turbidity and current velocity
• Monthly sampling and analysis of water samples
• Periodic analysis of sediment traps
• Passive sampler devices at different water depth
• Physical inspection of CAD using ROV (Remotely Operated Vehicle)