Sediments on the sea floor are formed by particulate matter that settles out of the water column, and may consist of anything from coarse gravel and sand to clay and organic ooze. In many areas discharges of hazardous chemicals over many years have resulted in high levels of pollution in sediments. Contaminants “stored” in such sediments may cause serious pollution problems.
A widespread problem - more information needed
The sediments in many harbours and fjords in Norway are polluted by heavy metals and persistent organic pollutants (POPs). More than 120 polluted sites were investigated during the period 1993-1996. About 90 of these were found to be heavily polluted by contaminants such as PCBs, PAHs, tributyl-triphenyl tin compounds, mercury, lead and cadmium.
Contaminated sediments are found near population centres and harbours, where there may be many sources of pollution of various kinds. In fjords where there are industrial installations close to the shore, and only one or a very few sources of pollution, contamination of sediments is also a major problem. In addition, polluted sediments may be found in areas where no obvious sources of pollution exist today. Here, landfills, contaminated ground, previous industry or dumping activity is usually the cause of the pollution.
Sediments may release pollutants for many years
Pollutants in sediments can spread to the surroundings. They may spread from the sediment to water, resuspend when sediments are disturbed, or be absorbed by benthic organisms (bioaccumulation). Because of these mechanisms contaminated sediments may continue to release hazardous chemicals to the surroundings long after the land-based sources of the pollution have been eliminated. As a consequence contaminated sediments can have serious effects on living organisms and ecosystems.
POPs and heavy metals may cause higher mortality, reduce growth or disturb reproductive processes in marine organisms. Because many contaminants accumulate in food chains, they can also affect human health. In a number of fjords and coastal areas fish and shellfish show elevated concentrations of hazardous chemicals. The Norwegian Food Safety Authority therefore advises against the consumption of seafood from these fjords.
Complex cause and effect relationships
In many areas, sediments are still being polluted by emissions from industry, urban areas, contaminated soils, landfills or by long-range pollution. In addition, the natural formation of new sediments is a slow process in Norway. Therefore contaminated sediments may release pollution for a very long time before the contaminants are immobilised by a new layer of clean sediment.
The severity of the problems associated with polluted sediments depends on a number of factors. These include the types and concentration of pollutants, and the risk of dispersal of pollutants from the sediment to living organisms or the environment. This risk varies with water currents and depth, disturbance of the sediments for instance from propelling vessels, and on how firmly bound the pollutants are to the sediment particles. The latter in turn depends both on the type of sediment and the chemical conditions on the sea floor. Abundance and composition of benthic eco-systems are also important for the release of pollution from the sediments.
Coherent planning needed to deal with the problem
The White Paper “Protecting the Riches of the Seas” (Report No. 12 (2001-2002) to the Storting) describes the government’s policy for dealing with contaminated sediments. Priority is given to three types of measures:
- Pilot projects are carried out at certain polluted locations. These are intended to provide knowledge and experience, and at the same time solve a problem by cleaning up.
- County action plans are drawn up to gain an overview and to propose specific measures in prioritized areas.
- Clean-up operations in high-risk areas where it is clear from existing knowledge that action is needed.
Before any action is taken to deal with polluted sediments, the contamination level and the risks associated with different kinds of action must be surveyed. It is also essential to ensure that the sources of pollution have been eliminated. If not, the sediments may be re-contaminated and the effect of costly clean-up operations will be short lived. Efforts to clean up contaminated sediments must therefore be in co-ordination with other measures to deal with the sources of pollution, such as:
- reducing emissions from industry
- cleaning up contaminated ground and landfills
- integrated product policy