Contaminated sediments and fibre banks

A large proportion of the environmentally hazardous substances released to lakes and water bodies will eventually end up in the bottom sediments. Locally, the deposition of new sediments over the polluted sediment can have the effect of sealing in the pollutants. However, under other conditions no such sediment burial occurs, and sediments can become a pollution source long after the original discharge has ceased. Fibre banks constitute a special type of polluted sediment. These are the large amounts of fibres that have accumulated in water bodies close to paper/pulp industries. Mercury, dioxin or PCB may accumulate in these fibre banks, giving rise to serious environmental damage.

On behalf of the Swedish Environmental Protection Agency, Kemakta has developed methods to estimate the release of pollutants from polluted lake sediments and fibre banks. The methods can be used in predictions of environmental impacts and to study the effectiveness of different alternative remedial measures. The methods have been used in investigations of polluted bottom sediments and fibre banks close to pulp industries.

In order to evaluate the reduction of contaminant release achieved by alternative methods of remediation in specific cases, the processes governing the dispersion of pollutants must be understood.

The release of a pollutant from a natural sediment is strongly dependent on sedimentation, bioturbation, (mixing caused by living organisms), chemical processes and sorption. The release from fibre banks is also strongly dependent on transport due to gas formation and erosion.

The relevant processes in pollutant transport

At present, the methods available for reduction of environmental damage caused by contaminated sediments are covering at the site or dredging followed by land disposal. In-situ covering of the sediments reduces contaminant release by reducing erosion and a by the formation of a barrier to diffusion. Covering can also alter the chemical conditions in the polluted sediments which can have a stabilising impact. However, covering can be difficult to achieve in practice if the sediments are soft or if gas production is large. Dredging of sediments creates a number of problems: Discharge is temporarily increased during the dredging operation. Land disposal of polluted dredged sediment requires a carefully designed disposal site. On removal from the aquatic environment, the chemical conditions in the dredged masses are altered, for example due to oxygen access, which can result in increased mobility for some pollutants.