Arbejdsrapport fra Miljøstyrelsen, 24/2005 – Eksponeringsrisici ved deponering af forurenet havnesediment

Eksponeringsrisici ved deponering af forurenet havnesediment

Summary and conclusions

The choice of method for disposal of sediment, which needs to be dredged from harbours in order to secure their normal operation, should take in to consideration the environmental impact caused by the method. Consequently, the choice of method can depend on the degree of pollution of the sediment. This project seeks to analyse the impact on the surface water recipient, or the exposure, resulting from a number of methods that have been or will be predominant depending on the legal and administrative regulations currently being prepared.

The following methods have been evaluated as part of the project:

Dumping
Dumping with limited particle transportation
Disposal according to the statutory order on landfills
Disposal in a coastal disposal site (primarily under water).

Dumping

In the case of traditional dumping the dispersion of pollutants takes place in different ways:

During the dumping itself, where particle-bound contaminants are transported by current and waves away from the dumping site to settle at other sites. Here part of the sorbed contaminants will desorb and consequently more easily be transported until they are again bound to particulate matter in the water and settle. During this process the contaminants might be taken up by living organisms.
After sedimentation at the dumping site a resuspension of the particles may take place depending on the current and wave conditions and then the particles with the sorbed contaminants will be transported from the dumping site as described above.
Contaminants in the pore water may also be removed from the dumped material due to diffusion caused by differences in concentration and/or flow of pore water caused by tide, waves, etc.
Finally, the presence of benthic fauna may increase the water flow in the sediment (bioturbation) and the direct removal of dissolved and particle bound material from the sediment.

According to the calculations carried out the removal of polluted matters during a traditional dumping is totally dependent on the local conditions. At worst, the removal can reach 100% in a relatively short time.

Dumping with limited resuspension of particles

If removal of particle bound material by resuspension is reduced or completely stopped, the removal of contaminants will be reduced considerably. In order to illustrate this possibility, an example has been set up based on a previous sedimentation basin established at the harbour of Århus. Here the entry possibilities have been reduced – and as a result also the current and waves inqw the basin – which has caused a substantial reduction of the resuspension. Consequently, the contaminants are primarily removed by the diffusion and by tidal currents moving through the sediment.

Comparing the removal of contaminants from a traditional dumping, where the resuspension is set as the annual removal of the uppermost centimetre of the sediment layer, to the removal of material from a dumping with in principle no resuspension, it can be seen that a reduction of the resuspension may reduce the overall removal of contaminants by 50->99% depending on the contaminant. The greatest reduction is seen for metals while the lowest is seen for TBT where diffusion plays a more important role.

Disposal according to the statutory order on landfills

This scenario examines which concentrations are to be expected in the leachate from a sediment disposal site compared to leachate concentrations from ordinary mixed waste. The comparison shows that the concentrations are of the same order of magnitude. In this connection is should be noted that landfills for mixed waste will probably disappear within a number of years since the new disposal regulations require degradation/stabilisation of all waste ensuring substantially reduced leaching of contaminants within a period of approximately 30 years. This means that in the future waste with a high content of organic matter is expected to undergo pre-treatment before disposal resulting in lower leachate concentrations. This may also lead to requirements as to pre-treatment of harbour sediments if the long-term reduction of the contaminant leaching is not deemed satisfactory.

Geochemical calculations have been used to estimate the leachate concentrations from oxidised sediments due to location above the water table and infiltration of rainwater. Due to insufficient data the final quantification of the leachate concentrations has been based on measured partitioning coefficients for contaminants in particulate matter in the water phase (which is assumed to be oxidised).

Disposal in a coastal disposal site

The statutory order on landfills allows the establishment of a landfill with reduced requirements with respect to leachate collection etc. if an environmental risk assessment shows that the disposal will cause no potential risk to soil, groundwater and surface water. On this basis it has been deemed relevant to carry out an assessment of the potential leaching from a coastal disposal site without membrane where the harbour sediments are placed below the water level. The assessment is based on this assumption because especially metals are often harder bound to the particulate matter under reduced conditions.

Based on values taken from literature for the equilibrium between solid phase and pore water for a number of relevant contaminants the potential leaching from the sediment disposal has been estimated and compared to quality criteria for surfacae water. It is seen that the leaching of metals will not become a problem while particularly TBT may cause leachate concentrations that exceed the quality criteria considerably. It should be noted that it is not quite realistic to maintain strict anaerobic conditions in a landfill even under water, and that oxidation can be expected to increase the leaching of metals as mentioned previously.

All assessments have been based on available data and it is in general a problem that the amount of data applicable for this type of evaluations is limited and it is often difficult to tell under which redox conditions the data have been registered, which may be of major importance. It is, however, possible to conclude that disposal methods, which reduce the resuspension and try to maintain the sediment under reduced conditions, cause the least environmental impact.