Oslo Port Authority

The remediation of Oslo Harbor requires that approximately 780,000 m 3 of contaminated sediments be dredged and subsequently disposed. Several disposal methods have been assessed; however, due to elevated concentrations of both organic contaminants and heavy metals as well as the high content of fines, sediment treatment to reduce the disposal volume has been ruled out. Capped disposal in an anoxic deep- water basin has been selected due to its potential for safe and economical long-term storage of dredged contaminated sediments. The disposal of dredged contaminated sediments under water requires that great care is taken to avoid both short term and long term release of contaminants. This paper presents the conceptual model utilized to investigate the environmental risk associated with possible contaminant transport mechanisms during and after disposal in an anoxic deep-water basin. Experimental results from the specific site assessment for Oslo Harbor are presented as well. The results indicate that the risk of contaminant transport during the dredging and placement process is principally associated with the transport of particles or suspended solids. Particle transport can be avoided by ensuring that a minimal amount of water is added to the sediment during the dredging process, utilizing a lower pumping rate for the placement of the sediment on the sea floor and disposing the sediment as close to the sea floor as possible. Vertical mixing in the water column can be reduced by increasing the density of the pore water with the addition of a salt mixture during sediment placement. With regard to the flux of contaminants out of the deep-water basin after disposal, it is imperative that the water conditions remain anoxic.